Environment-Sensitive Multifunctional Drug Delivery Systems300480/FULLTEXT01.pdf · Drug delivery systems (DDS) with multiple functionalities such as environment-sensitive drug release

Environment-Sensitive Multifunctional Drug Delivery Systems

Jian Qin

秦健

Doctoral Thesis

Stockholm 2010

Functional Materials Division

School of Information and Communication Technology

Kungliga Tekniska Högskolan

Address Functional Materials Division

School of ICT

Royal Institute of Technology

Isafjordsgatan 22

SE 164 40, Kista/Stockholm, Sweden

Supervisor

Prof. Mamoun Muhammed

TRITA-ICT/MAP AVH Report 2010:1 ISSN 1653-7610 ISRN KTH/ICT-MAP/AVH-2010:1-SE ISBN 978-91-7415-576-1 © JIAN QIN, 2010 Kista Snabbtryck AB, Stockholm 2010

Functional Materials Division, KTH, 2010 i

Abstract

Drug delivery systems (DDS) with multiple functionalities such as environment-sensitive drug release mechanisms and visualization agents have motivated the biomedical community as well as materials chemists for more than a decade. This dissertation is concerned with the development of nanoparticles for multifunctional DDS to tackle several crucial challenges in these complex systems, including polymeric nanospheres which respond to temperature change, superparamagnetic iron oxide nanoparticles/polymeric composite for magnetic resonance imaging contrast agents and drug carriers, immunoresponse of nanomaterials and injectable magnetic field sensitive ferrogels.

The biocompatible and biodegradable polylactide (PLA) was employed as matrix materials for polymeric nanosphere-based DDS. The thermosensitive polymeric nanospheres have been constructed through a “modified double-emulsion method”. The inner shell containing the thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) undergoes a “hydrophilic-to-hydrophobic” phase transition around the human body temperature. The sensitivity of the polymer to the temperature can facilitate drug release at an elevated temperature upon administration. In addition, gold nanoparticles were assembled on the dual-shell structure to form a layer of gold shell. The cell viability was found to be enhanced due to the gold layer. The immunoresponse of the gold nanoparticles has been considered even if no acute cytotoxicity was observed.

Imaging is another functionality of multifunctional DDS. This work focuses on magnetic resonance imaging (MRI) and involves synthesis and surface modification of superparamagnetic iron oxide nanoparticles (SPIONs) for contrast agents. The SPIONs have been prepared through a high temperature decomposition method. Surface modification was carried out in different ways. Poly(L,L-lactide) (PLLA) was grafted on SPIONs through surface-initiated ring-opening polymerization. The hydrophobic model drug indomethacin was loaded in the PLLA layer of the composite particles. For biomedical applications, it is essential to modify the hydrophobic particles so that they can be dispersed in physiological solutions. A series of protocols including using small charged molecules and amphiphilic polymers has been established. Pluronic F127 (PF127), a triblock copolymer was applied as a phase transfer reagent. Most interestingly, PF127@SPIONs show remarkable efficacy as T2 contrast agents. The PF127@SPIONs have been successfully applied to image the cochlea in a rat model. As another phase transfer reagent, poly(maleic anhydride-alt-octadecene)-graft-PNIPAAm (PMAO-graft-PNIPAAm) was created for surface modification of SPIONs. This new copolymer provides the modified SPIONs with thermosensitivity together with water-dispersibility.

As another form of DDS, ferrogel made of PF127 copolymer and SPIONs was developed. Gelation process depends on the temperature of the SPIONs/PF127 mixture. This property makes it possible to use the ferrogel as an injectable drug carrier. Unlike other ferrogels based on crosslinked polymeric network, the PF127 ferrogel can entrap and release hydrophobic drugs. Application of an external magnetic field is found to enhance the drug release rate. This property can find application in externally stimulated local drug release applications.

TRITA-ICT/MAP AVH Report 2010:1, ISSN 1653-7610

ISRN KTH/ICT-MAP/AVH-2010:1-SE, ISBN 978-91-7415-576-1

ii

Functional Materials Division, KTH, 2010 iii

LIST OF PAPERS

This thesis is based on following publications:

1. Qin, Jian; Jo, Yun Suk; Ihm, Jong Eun; Kim, Do Kyung; Muhammed, Mamoun "Thermosensitive Nanospheres with a Gold Layer Revealed as Low-Cytotoxic Drug Vehicles" Langmuir 2005, 21, 9346-9351

2. Vallhov, Helen; Qin, Jian; Johansson, Sara M.; Ahlborg, Niklas; Muhammed, Mamoun A.; Scheynius, Annika; Gabrielsson, Susanne "The importance of an Endotoxin-Free Environment during the Production of Nanoparticles Used in Medical Applications" Nano Lett. 2006, 6, 1682-1686

3. Qin, Jian; Laurent, Sophie; Jo, Yun Suk; Roch, Alain; Mikhaylova, Maria; Muller, Robert N.; Muhammed, Mamoun "A High-Performance Magnetic Resonance Imaging T2 Contrast Agent" Adv. Mater. 2007, 19, 1874-1878

4. Salazar-Alvarez, G.; Qin, J.; Sepelak, V.; Bergmann, I.; Vasilakaki, I. M.; Trohidou, K. N.; Ardisson, J. D.; Macedo, W. A. A.; Mikhaylova, M.; Muhammed, M.; Baro, M. D.; Nogues, J. "Cubic versus Spherical Magnetic Nanoparticles: The Role of Surface Anisotropy", J. Am. Chem. Soc., 2008, 130, 13234-13239

5. Qin, Jian; Asempah, Isaac; Laurent, Sophie; Fornara, Andrea; Muller, Robert N.; Muhammed, Mamoun "Injectable Superparamagnetic Ferrogels for Controlled Release of Hydrophobic Drugs" Adv. Mater. 2009, 21, 1354-1357

6. Qin, Jian; Jo, Yun Suk; Muhammed, Mamoun "Coating Nanocrystals with Amphiphilic Thermosensitive Copolymers" Angew. Chem. Int. Ed., 2009, 48, 7845-7849

7. Poe, Dennis; Zou, Jing; Zhang, Weikai; Qin, Jian; Ramadan, Usama Abo; Fornara, Andrea; Muhammed, Mamoun; Pyykkö, Ilrnari "MRI of the Cochlea with Superparamagnetic Iron Oxide Nanoparticles Compared to Gadolinium Chelate Contrast Agents in a Rat Model" Eur. J. Nanomed., 2009, 2, 29-36

8. Thaler, Marlene; Roy, Soumen; Qin, Jian; Bitsche, Mario; Glueckert, Rudolf; Fornara, Andrea; Muhammed, Mamoun; Salvenmoser, Willi; Rieger, Gunde; Schrott-Fischer, Anneliese "Visualization and Analysis of Superparamagnetic Ferrogels in the Inner Ear by Light Microscopy, EFTEM and Modern Analytical Electron Microscopical Imaging" 2009, Manuscript, submitted to J. Microsc.

9. Zou, Jing; Zhang, Weikai; Poe, Dennis; Qin, Jian; Fornara, Andrea; Zhang, Ya; Abo-Ramadan, Usama; Muhammed, Mamoun; Pyykkö, llmari "MRI Manifestation of Novel SPION in Rat Inner Ear" 2009, Manuscript, submitted to Nanomedicine.

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Other work not included:

1. Qin, Jian; Nogues, Josep; Mikhaylova, Maria; Roig, Anna; Munoz, Juan S.;

Muhammed, Mamoun "Differences in the Magnetic Properties of Co, Fe, and Ni 250-300 nm Wide Nanowires Electrodeposited in Amorphous Anodized Alumina Templates" Chem. Mater. 2005, 17, 1829-1834

2. Fornara, Andrea; Johansson, Petter; Petersson, Karolina; Gustafsson, Stefan; Qin, Jian; Olsson, Eva; Ilver, Dag; Krozer, Anatol; Muhammed, Mamoun; Johansson, Christer "Tailored Magnetic Nanoparticles for Direct and Sensitive Detection of Biomolecules in Biological Samples", Nano Lett. 2008, 8, 3423-3428

3. Li, Shanghua; Liang, Yibin; Qin, Jian; Toprak, Muhammet; Muhammed, Mamoun "Template electrodeposition of ordered bismuth telluride nanowire arrays" J. Nanosci. Nanotech., 2009, 9, 1543-1547

4. Li, Shanghua; Qin, Jian; Fornara, Andrea; Toprak, Muhammet; Muhammed, Mamoun; Kim, Do Kyung "Synthesis and Magnetic Properties of Bulk Transparent PMMA/Fe-oxide Nanocomposites", Nanotechnology, 2009, 20, 185607

5. Sugunan, Abhilash; Jafri, S. Hassan M.; Qin, Jian; Blom, Tobias; Toprak,

Muhammet S.; Leifer. Klaus; Muhammed, Mamoun "Low Temperature Synthesis of Photoconducting CdTe Nanotetrapods", J. Mater. Chem. 2010, 20, 1208-1214

6. Wang, Xiaodi; Ma, Ying; Qin, Jian; Toprak, Muhammet S.; Zhu, Bin; Muhammed, Mamoun "Synthesis of CeO2 mesocrystals by a novel method", Manuscript.

7. Ye, Fei; Vallhov, Helen; Qin, Jian; Daskalaki, Evangelia; Sugunan, Abhilash; Toprak, Muhammet S.; Fornara, Andrea; Gabrielsson, Susanne; Scheynius, Annika; Muhammed, Mamoun "Synthesis of High Aspect Ratio Gold Nanorods and Their Effects on Human Antigen Presenting Dendritic Cells", Manuscript, Submitted

8. Okoli, Chuka; Fornara, Andrea; Qin, Jian; Dalhammar, Gunnel; Muhammed, Mamoun; Rajarao, Gunaratna "Purification of Coagulant Protein with Superparamagnetic Iron Oxide Nanoparticles". Manuscript, Submitted

Functional Materials Division, KTH, 2010 v

Conference presentations

1. Jian Qin, Andrea Fornara, Muhammet, S. Toprak, Mamoun Muhammed, "Crosslinked Pluronic F127 Ferrogels for Magnetically Controlled Drug Release" the 34th International Conference & Exposition on Advanced Ceramics & Composites (ICACC), Jan 24-29, 2010, Daytona Beach, Florida, USA (oral)

2. Jing Zou, Ya Zhang, Weikai Zhang, Dennis Poe, Usama Abo Ramadan, Jian Qin, Thomas Perrier, Andrea Fornara, Patrick Saulnier, Mamoun Muhammed, Ilmari, Pyykkö, "Passage of Nanoparticles through Round Window Membrane into the Cochlea: Size or Surface Property-Dependant?" EuroNanoMed, Sept. 28-30, 2009, Bled, SLOVENIA (poster)

3. Fei Ye, Abhilash Sugunan, Jian Qin, Mamoun Muhammed. "A General Method for Preparation of Mesoporous Silica Coating on Gold Nanorods, Nanoparticles, and Quantum Dots" EuroNanoMed. Sept. 28-30, 2009, Bled, SLOVENIA (poster)

4. Andrea Fornara, Alberto Recalenda, Jian Qin, Jing Zou, Abhilash Sugunan, Muhammet Toprak, Ilmari Pyykkö, Mamoun Muhammed. "Multifunctional Nanoparticles for Simultaneous Targeted Drug Delivery and Visualization" EuroNanoMed, Sept. 28-30, 2009, Bled, SLOVENIA (poster)

5. Jian Qin, Andrea Fornara, Sophie Laurent, Robert N. Muller, Mamoun Muhammed, "Pluronic F127 Ferrogels for Magnetically Controlled Drug Release" EuroNanoMed. Sept. 28-30, 2009, Bled, SLOVENIA (poster)

6. Jian Qin, Andrea Fornara, Sophie Laurent, Robert N. Muller, Mamoun Muhrunmed, "Pluronic F127 Ferrogels for Magnetically Controlled Drug Release" World Molecular Imaging Congress, Sept. 23-26, 2009, Montreal, CANADA (poster)

7. Jian Qin, Mamoun Muhammed, "Multifunctional Drug Delivery System" Italian-Swedish Workshop on Nanoscience and Nanotechnologies, Oct. 13-14, 2008, Stockholm, SWEDEN (oral)

8. J. Qin, I. Asempah, S. Laurent, A. Fornara, L. Vander Elst, R. N. Muller, M. Muhammed. "Magnetic-Sensitive Ferrogels as Injectable Drug Delivery Systems" COST ACTION D38: European Conference on Metal-Based Systems for Molecular Imaging Applications, Apr. 27-29, 2008, ITN, Sacavém, PORTUGAL (poster)

9. Jian Qin, Mamoun Muhammed, "Putting Smart Clothes on Nanoparticles" 7th International Conference on the Scientific and Clinical Applications of Magnetic Carriers, May 21-24, 2008, Vancouver, CANADA (oral)

10. Jian Qin, Mengmeng Lin, Sophie Laurent, Do Kyung Kim, Mamoun Muhammed "Polymer Coated Superparamagnetic Nanoparticles for Cellular Uptake and MRI Detection" 7th International Conference on the Scientific and Clinical Applications of Magnetic Carriers, May 21-24, 2008, Vancouver, CANADA (poster)

11. G. Salazar-Alvarez, J. Qin, V. Sepelak. I. Bergmann. M. Vasilakakis. K. N. Trohidou, J. D. Ardisson, W. A. Macedo, M. Mikhaylova, M. Muhammed, M. D.

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Baró, J. Nogues. "Surface Anisotropy in Spherical and Cubic Maghemite Nanoparticles" 52nd Annual Conference on Magnetism and Magnetic Materials, Nov. 5-9, 2007, Tampa, Florida, USA (oral)

12. J. Qin, S. Laurent, A. Roch, R. N. Muller, M. Muhammed, "Monodisperse Water-Soluble Superparamagnetic Iron Oxide Nanoparticles Revealed as a High-Performance T2 Contrast Agent for MRI" 2nd International Conference-European Society for Molecular Imaging, June 14-15, 2007, Naples, ITALY (poster)

13. Jian Qin, Sophie Laurent, Robert N. Muller, Mamoun Muhammed, "A Facile Approach to Prepare Water-Soluble Monodisperse Superparamagnetic Iron Oxide Nanoparticles" 8th International Conference on Nanostructured Materials, Aug 20-25, 2006, Bangalore, INDIA (poster)

14. Andrea Fomara, Jian Qin, Mamoun Muhammed. "3-G Multifunctional Nanoparticles for Simultaneous Drug Delivery and MRI Visualization" 8th International Conference on Nanostructured Materials" Aug 20-25, 2006, Bangalore, INDIA (poster)

15. Carmen M. Marian, Andrea Fornara, Jian Qin, Mamoun Muhammed, "Optimized Magnetic Nanoparticle for High Resolution Contrast in MRI" 8th International Conference on Nanostructured Materials, Aug 20-25, 2006, Bangalore, INDIA (poster)

16. Jian Qin, Sophie Laurent, Alain Roch, Robert N. Muller, Mamoun Muhammed, "Superparamagnetic Iron Oxide Micelles Revealed as High-Perfomlance T2 Contrast Agents" 10th Special Topic Conference on Molecular Imaging, Magnetic Resonance and Intermodality Contrast Agent Research, June 14-16, 2006, Vilnius, LITHUANIA (oral)

17. Jian Qin, Sophie Laurent, Alain Roch, Robert Muller, Mamoun Muhammed, "Fe2O3@Poly(L,L-lactide) Core-Shell Nanoparticles Synthesized by Surface-Initiated In Situ Polymerization" 6th International Conference on the Scientific and Clinical Applications of Magnetic Carriers, May 17-20, 2006, Krems, AUSTRIA (poster)

18. Andrea Fornara, Jian Qin, Mamoun Muhammed, "PLLA-PEG Nanospheres Encapsulating Monodisperse Iron Oxide Nanoparticles for Simultaneous Drug Delivery and MRI Visualization" 6th International Conference on the Scientific and Clinical Applications of Magnetic Carriers, May 17-20, 2006, Krems, AUSTRIA (poster)

19. Helen Vallhov, Jian Qin, Sara Johansson, Mamoun Muhammed, Sussane Gabrielsson, Annika Scheynius, "The Effect of Gold Nanoparticles on Dendritic Cells" Nanotech 2006, May 7-1 1, 2006, Boston, Massachusetts, USA (poster)

20. Jian Qin, Yun Suk Jo, Jong Eun Ihm, Do Kyung Kim, Mamoun Muhammed, "Thermally Responsive Low-Cytotoxic Drug Delivery System" BioNanoMaT,

Functional Materials Division, KTH, 2010 vii

Bioinspired Nanomaterials for Medicine and Technologies, Nov 23-24, 2005, Marl, GERMANY (poster)

21. Jian Qin, Yun Suk Jo, Jong Eun Ihm, Do Kyung Kim, Mamoun Muhamnled, "Thermally Responsive Low-Cytotoxic Drug Delivery System" Life is Matter - Life Matters: Conversations in Bionanotechnology, Oct 25-27, 2005, Göteborg, SWEDEN (poster)

22. Helen Vallhov, Jian Qin, Mamoun Muhammed, Susanne Gabrielsson, Annika Scheynius, "Gold Nanoparticles as Potential Medical Carriers-the Effect on Human Dendritic Cells" Life is Matter - Life Matters: Conversations in Bionanotechnology, Oct 25-27, 2005, Göteborg, SWEDEN (poster)

23. Jian Qin, Yun Suk Jo, Jong Eun Ihm, Do Kyung Kim, Mamoun Muhammed, "Environmentally Responsive Drug Delivery System" Medicinteknikdagarna. Sep 27-28, 2005, Södertälje, SWEDEN (oral)

24. Raphäel Gras, Jian Qin, Maria Mikhaylova, Mamoun Muhammed, "Fabrication and Assembly of Metal Nanowires" 7th International Conference on Nanostructured Materials, June 20-24, 2004, Wiesbaden, GERMANY (poster)

25. Yun Suk Jo, Jian Qin, Do Kyung Kim and Mamoun Muhammed. "Temperature- Triggered Release of Bovine Serum Albumin from Dual-Shell Drug Delivery Systems" 7th International Conference on Nanostructured Materials, June 20-24, 2004, Wiesbaden, GERMANY (oral)

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Contributions of the author

Paper 1. Planning of experiments, performing experiments, characterization of the samples, evaluation of the results and writing main parts of the article

Paper 2. Participate in initiating the idea, preparing the particles with LPS, developing the method of preparing particles without endotoxin contamination, writing part of the manuscript

Paper 3. Planning of experiments, coordinating collaboration, performing experiments and materials characterization, evaluation of the results and writing main part of the article

Paper 4. Preparing particles and performing structural characterization of the samples and writing the part of the article

Paper 5. Planning of experiments, designing the setup and performing experiments, characterization of the samples, evaluation of the results and writing main parts of the article

Paper 6. Planning of experiments, performing experiments, characterization of the sanlples, evaluation of the results and writing main parts of the article

Paper 7. Participate in initiating the idea, preparing the materials, characterization of the samples and writing part of the manuscript



Functional Materials Division, KTH, 2010 ix

Abbreviations and symbols 1H NMR proton nuclear magnetic resonance 13C NMR carbon 13 nuclear magnetic resonance AAS atomic absorption spectroscopy AIBN 2,2'-azobis(2-methylpropionitrile) AOT aerosol OT, sodium dioctylsulphosuccinate APTMS 3-aminopropyltrimethoxysilane ATRP atom transfer radical polymerization BCA bicinchoninic acid BPO benzoyl peroxide BSA bovine serum albumin CMC critical micelle concentration [mol L-1] Cos-7 African green monkey kidney cell line CT computed tomography CTAB cetyltrimethylammonium bromide Dc superparamagnetic critical size [m] DEM double-emulsion method DMEM Dulbecco's Modified Eagle's Medium DMSA meso-2,3-dimercaptosuccinic acid DLS dynamic light scattering DNA deoxyribonucleic acid DSC differential scanning calorimetry EA anisotropy energy [eV] EO ethylene oxide FBS fetal bovine serum FC field cooling FTIR Fourier transform infrared GPC gel permeation chromatography H magnetic field [T] HeLa human cervical epithelial carcinoma cell line HLB hydrophilic-lipophilic balance HRTEM high-resolution transmission electron microscopy kB Boltzlmann constant [m2kgs-2K-1] LA L-lactic acid LBL layer-by-layer LCST lower critical solution temperature [°C] M magnetization [Am2kg-1] MDEM modified double-emulsion method MEM-α modified eagle's medium alpha MRI magnetic resonance imaging MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide MTS 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-

sulfopheny1)-2H-tetrazolium

Mw weight average molecular weight [Dalton] NP nanoparticle NIPAAm N-isopropylacrylamide NIR near infrared o/w oil-in-water

x

OA oleic acid PALA N-(phosphonacetyl)-L-aspartate PBS phosphate buffer saline PDLA poly(D, D-lactide) PEG poly(ethylene glycol) PEO poly(ethylene oxide) PF127 pluronic F127 PLLA poly(L, L-lactide) PNIPAAm poly(N-isopropylacrylamide) PO propylene oxide PPO poly(propylene oxide) PS polystyrene PtBA poly(tert-butyl acrylate) PVA poly(vinyl alcohol) PVP poly(vinylpyrrolindone) RES reticuloendothelial system RNA ribonucleic acid ROP ring-opening polymerization SDS sodium dodecylsulphate SEC size exclusion chromatography SIROP surface-initiated ring-opening polymerization SPEL superparamagnetic ferrogel SPIO superparamagnetic iron oxide SPION superparamagnetic iron oxide nanoparticle SQUID superconducting quantum interference device Tc crystalline temperature [°C] TCCV two-color cell fluorescence viability THF tetrahydrofuran TEM transmission electron microscopy Tg glass transition temperature [°C] TGA thermogravimetric analysis Tm melting temperature [°C] TMAOH tetramethylammonium hydroxide TMA-POSS trimethylammonium-polyhedral oligometric silsesquioxanes TOP trioctylphosphine TOPO trioctylphosphine oxide TSC trisodium citrate UV-Vis ultraviolet-visible VSM vibrating sample magnetometer w/o/w water-in-oil-in-water XRD X-ray diffraction ZFC zero-field cooling σ standard deviation τ relaxation time of magnetic particles [s] χm magnetic susceptibility [cm3g-1]

Functional Materials Division, KTH, 2010 xi

Table of contents

ABSTRACT .......................................................................................................................................... I

LIST OF PAPERS ............................................................................................................................. III

ABBREVIATIONS AND SYMBOLS .............................................................................................. IX

TABLE OF CONTENTS .................................................................................................................. XI

1 INTRODUCTION ........................................................................................................................ 1 1.1 DRUG DELIVERY SYSTEMS (DDS) ......................................................................................... 1

1.1.1 Biodegradable matrices ................................................................................................... 3 1.1.2 Biocompatible surfaces .................................................................................................... 4 1.1.3 Transfer nanoparticles from organic phase to physiological solutions ........................... 6 1.1.4 Targeting .......................................................................................................................... 8 1.1.5 Visualization .................................................................................................................... 8

1.1.5.1 Magnetic resonance imaging ................................................................................................ 9 1.1.5.2 MRI contrast agents ........................................................................................................... 10 1.1.5.3 Synthesis of magnetic nanoparticles .................................................................................. 11

1.1.6 Environment-sensitive components ................................................................................ 13 1.1.6.1 Thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) ............................................. 14 1.1.6.2 Magnetic field-sensitive ferrogels ...................................................................................... 15

1.2 OBJECTIVES ......................................................................................................................... 15 2 EXPERIMENTAL ..................................................................................................................... 17

2.1 THERMOSENSITIVE NANOPARTICLES ................................................................................... 17 2.1.1 Synthesis of PNIPAAm-PDLA and PLLA-PEG copolymers .......................................... 17 2.1.2 Synthesis of PMAO-graft-PNIPAAm copolymers .......................................................... 18 2.1.3 Fabrication of thermosensitive nanospheres loaded with bovine serum albumin ......... 19

2.2 SYNTHESIS OF INORGANIC NANOPARTICLES ........................................................................ 19 2.2.1 Preparation of superparamagnetic iron oxide nanoparticles (SPIONs) ....................... 19 2.2.2 Preparation of Au nanocrystals ..................................................................................... 19 2.2.3 Controlling size and morphology of SPIONs ................................................................. 20 2.2.4 Surface modification of SPIONs .................................................................................... 20

2.2.4.1 Preparation of PLLA@SPIONs ......................................................................................... 20 2.2.4.2 Ligand exchange ................................................................................................................ 21 2.2.4.3 Coating SPIONs with Pluronic F127 (PF127) ................................................................... 21 2.2.4.4 Coating SPIONs and Au nanocrystals with PMAO-graft-PNIPAAm................................ 22

2.3 CHARACTERIZATIONS .......................................................................................................... 22 2.4 DRUG LOADING TO THE FERROGEL, PLLA@SPIONS AND OLEIC ACID COATED SPIONS ... 23

2.4.1 Ferrogel ......................................................................................................................... 23 2.4.2 PLLA@SPIONs and oleic acid coated SPIONs ............................................................. 23

2.5 DRUG RELEASE .................................................................................................................... 24 2.5.1 Ferrogel ......................................................................................................................... 24 2.5.2 PLLA@SPIONs and oleic acid coated SPIONs ............................................................. 24 2.5.3 Thermosensitive nanospheres ........................................................................................ 25

2.6 CYTOTOXICITY TESTS .......................................................................................................... 25 2.6.1 Thermosensitive nanospheres ........................................................................................ 25 2.6.2 PF127@SPIONs ............................................................................................................ 26

3 RESULTS AND DISCUSSIONS .............................................................................................. 27 3.1 SUPERPARAMAGNETIC IRON OXIDE NANOPARTICLES (SPIONS) .......................................... 27

3.1.1 Morphology and structure studies ................................................................................. 27 3.1.2 Size evolution of SPIONs ............................................................................................... 29 3.1.3 Shape control of SPlONs ............................................................................................... 30 3.1.4 Shape-dependant magnetic properties ........................................................................... 31

3.2 DRUG DELIVERY SYSTEMS ................................................................................................... 33

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3.2.1 Thermosentive nanospheres ........................................................................................... 33 3.2.1.1 Microscopic structures ....................................................................................................... 33 3.2.1.2 Determination of LCST ..................................................................................................... 34 3.2.1.3 Drug release ....................................................................................................................... 36 3.2.1.4 Cytotoxicity evaluation of polymeric nanospheres ............................................................ 37

3.2.2 Ferrogels ........................................................................................................................ 39 3.2.2.1 Microscopic observation .................................................................................................... 39 3.2.2.2 Magnetic properties of ferrogels ........................................................................................ 40 3.2.2.3 Rhoelogical properties ....................................................................................................... 40 3.2.2.4 Drug release studies ........................................................................................................... 41

3.3 SURFACE MODIFICATION OF SPIONS .................................................................................. 43 3.3.1 PLLA coated SPIONs ..................................................................................................... 43

3.3.1.1 Microscopic observation .................................................................................................... 43 3.3.1.2 Characterization of surface coating .................................................................................... 44 3.3.1.3 Magnetic properties of PLLA@SPIONs ............................................................................ 46 3.3.1.4 Drug loading and release .................................................................................................... 47

3.3.2 Phase transfer of SPIONs from organic phase to aqueous phase.................................. 48 3.3.2.1 Microscopic observation .................................................................................................... 48 3.3.2.2 Magnetic properties of PF127@SPIONs ........................................................................... 50 3.3.2.3 Cytotoxicity test of PF127@SPIONs ................................................................................. 51 3.3.2.4 Magnetic resonance studies ................................................................................................ 51

3.3.3 Thermosensitive nanocrystals ........................................................................................ 54 3.3.3.1 Structure observation and magnetic properties .................................................................. 54 3.3.3.2 Temperature-sensitive properties ....................................................................................... 56 3.3.3.3 Thermosensitive Au nanocrystals ...................................................................................... 58

4 SUMMARY AND CONCLUSIONS ......................................................................................... 60

5 FUTURE WORK ....................................................................................................................... 62

6 ACKNOWLEDGEMENTS ....................................................................................................... 63

REFERENCES ................................................................................................................................... 64

Functional Materials Division, KTH, 2010 1

1 Introduction

Nanotechnology is an interdisciplinary area of research that has been growing

explosively worldwide and has been receiving significant attention in the past few

decades. Manufacturing nanomaterials such as clusters, nanoparticles, nanorods,

nanowires, nanotubes and thin films is the key component for successful development

of nanotechnology. Particles on the nanoscale exhibit extraordinary physical, chemical

and biological properties significantly different from their conventional formulation on

the microscale.1 These unique properties offer exciting opportunities and potential

benefits in a wide range of applications and some of them are close enough to

successful implementation, including visualization of cellular structures (in tissues and

organs), deoxyribonucleic acid (DNA) sensors, controlled drug delivery, tumor therapy

etc.2 Nanotechnology has demonstrated a tremendous potential in improving both

biomedical research and clinical diagnostics because the nanomaterials are generally in

the similar size range with biological entities, e.g. cells, organelles, DNA, proteins etc.

Benefiting from various properties of nanostructures, one can greatly increase the

efficiency and accuracy of medical diagnosis, monitoring and therapy.

1.1 Drug delivery systems (DDS)

Engineering of DDS through the combination of biodegradable and biocompatible

materials with various robust functions has been an ever-increasing field in

development of novel medical devices and medical care. The term "magic bullet"

describing DDS was coined in the early 20th century by Dr. Paul Ehrlich.3 DDS should

be able to control the release rate and/or amount of therapeutic agents that will allow

desired effects on target sites. A multifunctional drug carrier can be achieved by

combination of carrier materials and other active agents which functions as release

triggers and imaging markers (Figure 1.1). The carrier matrices are usually formed by

biocompatible materials such as solid lipid NPs,4-6 inorganic materials7, 8 and spheres

fabricated from biodegradable polymers.9, 10 Multifunctional DDS can effectively

reduce the chance of both underdose and overdose, thus provide better use of the

therapeutic agents. It can also alleviate disadvantages from allergy of some patients.11

Moreover, a layer of protein-resistance coating on the nanoparticles is also necessary to

reduce the opsonization, thus avoid rapid clearance by the immune system.

2

Figure 1.1 Schematic illustration of a Next Generation Nanoparticle equipped with all the possible functions

A very important function of ideal drug carriers is to regulate the release rate of the

drug during therapy in comparison with free drugs. Reducing the frequency of drug

administration enables the patients to comply with dosing instructions. Conventional

dosage forms often lead to wide swings in serum-drug concentration. Most of the drug

contents tend to be released rapidly after the administration, which may cause rapid

increase of the drug concentration in the body. The fluctuation of administered drug

might cause an alternating period of ineffectiveness and toxicity (Figure 1.2).

Controlled DDS is generally the diffusion- and dissolution-based release system

applicable to the release of drugs intended for the circulation or the localization on the

site.13 Diffusion can be defined as a mass transfer process of the individual molecules of

a substance, brought about by random molecular motion and associated with a

concentration gradient. In an individual unit, the drug is mixed with the polymeric

matrix and presented either in dissolved or dispersed form. The release model follows

Fick’s Laws of diffusion from the carriers where the drug is involved. When the drug is

dispersed in the matrix, it is released according to the square root of time until the

concentration in the matrix decreases below the saturation value, respectively.13 The

desired releasing profiles of the drugs in bulk degrading systems can be manipulated by

adjusting the molecular weight of the polymer, copolymer composition, crystallinity,

interaction between polymer and drug and loading amount of the drug, etc.


Figure 1.2 Controlled drug delivery versus immediate release with repeated administration12

1.1.1 Biodegradable matrices

Micro/nanoparticles fabricated from biodegradable polymer for DDS have become

important since such systems enable the controlled release of drugs over a prolonged

period. Polymeric spheres protect drugs from the physiological environment where

hydrophobic drugs are not soluble, some organs such as gastrointestinal tract with

severely aggressive environment like low pH or active enzyme which facilitate

hydrolysis or decomposition.

Biodegradable polymers contain active groups (amide, enamine, enol-ketone, ester

and urethane) along the main chains, which can be hydrolyzed and/or oxidized,

eventually to small molecules, under physiological action. The degraded fragments of

the polymers should not be toxic acutely as well as chronically for practical biomedical

applications.

Figure 1.3 Scheme of the esterification process

Figure 1.4 Scheme of the ring opening polymerization

4

The degradation of hydrolyzable polymers starts with the amorphous regions

degrading prior to the complete split of the crystalline and cross-linked regions.

Polymers containing one or more hydrolyzable functional groups are synthesized and

found to be biodegradable. Biodegradable polymers can be classified into three

categories: biopolymers (e.g. polypeptide, protein, DNA, and RNA), as well as

polyesters produced by microorgan (e.g. poly (3-hydroxybutyrate)); polysaccharides

(e.g. cellulose, starch, chitosan, and dextran etc.); synthetic polymers (e.g. polyester,

polyamide, and PECA). During the past decades, biodegradable polymers is widely

used, especially in the fabrication of sutures and implants to support the body's recovery

systems.14 Another development is controlled-release of drugs with diverse

characteristics, such as anticancer drug adriamycin,15 5-fluorouracil,16 cisplatin,17

Paclitaxel,18 indomethacin,19 peptides,20 and proteins.21-23 Due to their biocompatible

property, aliphatic polyesters draw a lot of interest in particle formation in order to act

as a drug carrier.

Polyester is a kind of condensation polymer which can be produced as a result of

various types of reactions between diacid + diol, or di(acid chloride) + diol, or from

transesterification processes. Polylactide can be synthesized from the monomer lactic

acid through esterification process (Figure 1.3), as well as from monomer lactide

through ROP process (Figure 1.4). Polyesters are widely used and several studies on

drug delivery have been done and are well documented.10, 24

1.1.2 Biocompatible surfaces

It is crucial to control the physical and chemical properties of the drug carrier in case

of in vivo applications. Engineered polymeric NPs should be made of biocompatible

materials, and they do not provoke any adverse reaction or hypersensitivity in the body.

A thrombus may be formed very fast when incompatible alien materials come in contact

plasma. Materials with biogenic blood-compatible surface must be used in contact with

the blood in the vessel. The biocompatible materials for drug delivery should be able to

stay with plasma and living cells without undesired active interactions.

Copolymers containing hydrophilic segments can serve as surface layer of

particulates or micelles that are considered as potential candidates for DDS.38 The

reticuloendothelial system (RES) plays an important role in impeding the circulation of

drug carriers around the body by recognizing and removing alien materials. Therefore,


effective drug delivery can not be achieved by direct injection. The function of RES in

liver is regulated by the presence and balance of two contrary blood components:

opsonins, that promote phagocytosis; and dysopsonins, that suppress this process.

Dysopsonins shows optimal performance under in a high hydrophilic milieu. Thus,

modification with hydrophilic polymers can effectively reduce the chance of uptake by

macrophages. For the sake of increasing hydrophilicity of DDS, a hydrophilic polyether,

poly(ethylene glycol) (PEG) has been applied to modify the surface of nanocarriers in

order to avoid being captured by immune system.

Polymer coating. Ligand exchange with small molecules is recognized as a simple

route for surface activation, although it often leads to colloidal instability and

agglomeration. Polymeric coating is promising due to the flexibility in the controls of

chemical compositions in order to manipulate the functions of the polymers. Various

approaches are documented to create a polymeric coating on particle surfaces.

Layer-by-layer (LBL) deposition through charge interaction between positive and

negative polyelectrolyte has been recognized as a successful method to coat large

particles (more than tens of nanometers in diameter) with polymers.25, 26 Many

approaches to create a polymeric coating fall into two main categories: "grafting to" and

"grafting from" the particle surfaces. The former involves grafting polymeric chains

directly to the particle by means of hydrophobic interactions, electrostatic forces or

covalent bonding.27-29 However, direct grafting from the surfaces of NPs through the

atom transfer radical polymerization (ATRP) is know to be another feasible method.

Different types of NPs e.g. γ-Fe2O3,30 MnFe2O4,31 SiO232 and Au33 has been coated with

various kinds of polymers such as PS,34 PNIPAAm,35 PtBA36, 37 etc via the ATRP.

Exchange of surface capping agents with ATRP initiator is the crucial step of the whole

procedure, so that the subsequent free radical polymerization can be carried out on the

surface of NPs. In this thesis, this idea was extended to ROP to produce polylactide

coated iron oxide nanoparticles.

Inorganic coating. Several kinds of inorganic materials were applied as coating

layers to enhance biocompatibility of medical devices. Oxides such as silica is used to

coat the magnetic NPs, quantum dots, metallic NPs.225 Hydroxyapatite, a mineral that

naturally exists in teech and bones, is also developed as coating materials for medical

implants.226 Gold NPs are also used to cover the surface of drug carriers due to the inert

and non-toxic nature of elemental gold.39 Though recent analysis has indicated that the

6

colloidal gold does not cause acute cytotoxicity, it is believed that the NPs could be

rapidly modified by the surrounding cellular environment.40 This process could

potentially alter the properties of the surface of NPs and it implies that proteins would

attach to the surface of gold layer and enhance the cellular uptake which will facilitate

cellular delivery of drugs. Furthermore, the immunostimulatory effect of gold NPs on

dendritic cells was investigated, indicating there was no significant impact on

maturation of dendritic cells, implying that they could be used as a safe component in

drug carriers.

1.1.3 Transfer nanoparticles from organic phase to physiological solutions

A variety of methods has been developed to synthesize inorganic NPs either in

aqueous solution or in organic solutions.41 NPs synthesized in aqueous solution usually

have broad size distribution and low stability as a result of ionic interactions, which can

be avoided by lowering the concentration of precursors and employing certain

surfactants.42-45 Contrarily, NPs synthesized in organic phase, which will be elaborated

in the following sections, can be easily obtained in a large scale with narrow size

distribution, which is favored for investigating physical properties and certain

applications such as high-density magnetic storage. However, the high temperature

decomposition method in organic solvents has a major drawback that particles are

immiscible with water, which retards further biomedical applications. The problems are

that 1) the surface capping molecules have long alkyl chains which are hydrophobic and

cannot be stable in physiological environment; 2) such capping molecules lack of

functional groups, e.g. oleic acid, oleylamine, trioctylphosphine (TOP) and

trioctylphosphine oxide (TOPO), etc. which will not allow subsequent chemical

modification and/or conjugation of other biomolecules.

Transfer NPs from organics to aqueous. It is necessary to increase

water-dispersibility of the high quality inorganic NPs prepared in organic phase for

biomedical applications. For example, CdSe, CdSe/CdS core/shell quantum dots that are

synthesized via a typical organic-based procedure, covered by TOP and oleylamine as

stabilizing agents, has been transferred to aqueous solution by incorporation of

surfactants and phospholipids with capping molecules. The optical properties of

quantum dots were retained and used for cell labeling. Ag and iron oxide NPs have been

"pulled" to water by forming a host-guest structure with α-cyclodextrin.46 Alternatively,

surface capping molecules such as oleic acid and oleylamine can be incorporated with


amphiphilic surfactant/lipid and poly(maleic anhydride-alt-1-tetradecene) with their

hydrophobic domain and the hydrophilic domain on the other side of the molecules

solubilize NPs in aqueous solution.47 Ligand exchange with TMAOH,48 DMSA,49

TSC,50 PEG-terminated dendron,51 phosphine oxide polymer and TMA-POSS

molecules52 can also achieve successful phase transfer of NPs.

Pluronic copolymers as a phase transfer reagent. Pluronic® copolymers (termed

"Poloxamer" or "Synperonic" as well) are a kind of ABA-type triblock-copolymer

consisting of ethylene oxide (EO) and proplylene oxide (PO) blocks in a structure:

EOx-POy-EOx (Figure 1.5). Such a structure results in an amphiphilic copolymer, in

which the number of hydrophilic EO and hydrophobic PO units can be altered to vary

the size, hydrophilicity and hydrophobicity of the polymer. Pluronic copolymers with

different x and y values are characterized by distinct hydrophilic-lipophilic balance

(HLB).

HOO

OO

OO

OHx y x

Figure 1.5 Structure of Pluronic copolymer

The Pluronic copolymers undergo self-assembly into micelles in aqueous solutions.

These polymer molecules form dispersion in water at the concentrations below the

critical micelle concentration (CMC). At concentrations of the block copolymer above

the CMC, the Pluronic polymer chains aggregate to form micelles. The PO blocks

self-assemble to form the inner core due to the hydrophobic interaction and the EO

blocks form a hydrophilic moiety. Pluronic micelles are depicted as spheres composed

of a PO core and an EO corona. This scenario is correct for most block copolymers,

which have EO content above 30%, especially in relatively dilute solutions at human

body temperature. However, other/non-spherical micelle morphologies, including

lamella and rods (cylinders), can also form in Pluronic systems. The hydrodynamic

diameters are in the range from about 20 nm to about 80 nm for the spherical micelles

depending on the type of the Pluronic polymers. The number of block copolymers

forming one micelle is referred to as the "aggregation number". Usually this number

ranges from several to over a hundred. Based on the core-shell structure, Pluronic

micelles can carry hydrophobic substance and this process is referred as

"solublization".53 Such properties have been actively studied because it is promising to

use Pluronic micelles for carrying hydrophobic drugs and polypeptides within the PO

8

core of the micelles.54 PF127 was selected to coat SPIONs in this work because it has

large amount of EO units, high HLB which will maintain the particles dispersed in

aqueous phase, as well as low CMC (2.8×10-6 mol/L) which will form micellar structure

with NPs encapsulated.

Poly(maleic anhydride-alt-1-octadecene) (PMAO) as phase transfer agents.

PMAO and its analogues are selected as phase-transfer reagents for

hydrophobic-ligand-coated NPs because of the many features they offer.47, 227-229 Firstly,

PMAO is inexpensive and can be obtained commercially on a large scale. Secondly, it

has large number of hydrophobic alkyl chains, which can strongly associate with the

hydrocarbon ligands on the NCs through hydrophobic van der Waals interactions.

Thirdly, each repeating unit of PMAO has a maleic anhydride group that can be readily

hydrolyzed to carboxylic groups, which not only render the NCs water dispersible, but

also serve as an anchor for further conjugation to biomolecules or ligands. Finally,

PMAO also provides possibilities to graft other functional molecules to the main

polymeric chain owing to the highly reactive anhydride groups.

1.1.4 Targeting

DDS can have passive and/or active targeting to the required pathological sites by

incorporating different components and surface modification. A variety of methods has

been developed to attach corresponding vectors such as antibodies,55, 56 peptides,57, 58

sugar moieties,59-61 folate,62-64 and other ligands,65, 66 to the carrier surface. Modification

with specific antibodies has been applied to NPs and liposomes. The routine method to

attach antibodies includes protein covalent binding to reactive groups on the NPs. Drug

carriers with saccharide moieties on the surface can target cells with selectin and lectin

receptors, for example, cerebroendothelial cells. Folate-modified nanocarriers are used

to target tumors because folate receptor expression is frequently overexpressed in many

tumor cells.67, 68 Liposomal drug carriers,63 microgels,69 iron oxide NPs,70 quantum

dots71 and polymer-DNA hybrid NPs72 with folate tagging on the surface are reported to

selectively target cancer cells.

1.1.5 Visualization

It is of great interest to localize drug carriers around the targeted pathological sites by

combining various imaging modalities (γ-scintigraphy, MRI, CT, ultra-sonography,

flurorescent, near infrared (NIR)) with DDS. In particular, magnetic NPs are used as


MRI markers,230 quantum dots as flurorescent markers231 and gold nanorods as NIR

markers.232 Among others, MRI is a powerful imaging modality due to its high

resolution for soft tissue and non-invasive nature making it indispensable for clinical

diagnosis and post-surgery examination, especially for central neuron system diseases.

Recently, MRI-detectable drug carriers have gained tremendous research interests.

Polymeric micelles encapsulating iron oxide NPs are fabricated as MRI-ultrasensitive

markers.57 Mesoporous silica coated iron oxide are synthesized for multifunctional DDS

incorporating luminescence and MRI. Fluorescent molecules and drugs can be

encapsulated in the mesoporous silica.73 The incorporation of MRI contrast agents,

photodynamic sensitizers and targeting ligands into a multifunctional nanoparticle

platform for in vivo MRI and photodynamic therapy of brain cancer has been

constructed.74 In the following section, a brief classical theory on MRI is introduced.

1.1.5.1 Magnetic resonance imaging

When a system has two discrete energy levels, the protons in such a system are in the

high or the low energy levels according to a defined probability, which is Boltzmann

distribution:

exp( )l

u

N EN kT

Δ= (1.1)

where ΔE is the energy difference between the two levels; k is the Boltzmann constant

and T is the absolute temperature. Once a strong magnetic field B0 is applied to this

system, the energy difference ΔE will increase proportionally to B0, therefore the

difference of the protons in the lower and upper levels of the energy will be increased.

Since the intensity of NMR signal is directly dependent on the population difference,

the NMR signal also increases. Strong magnetic field is commonly applied in order to

increase the signal-to-noise ratio.

In the presence of an external magnetic field, the spinning moment of proton not

only rotates around its own axis, but also precesses around the direction of the external

magnetic field. Actually, for protons, two cones of precession exist: one for the nuclei in

the state of low energy and another one in the opposite direction for the nuclei in the

high energy state. The frequency ω of this precessing motion is given by the Lamor

equation:

ω = γ B0 (1.2)

10

where ω is the Larmor frequency (unit: MHz), γ the gyromagnetic ratio, and B0 is the

strength of the magnetic field. A resonance phenomenon will occur when an

electromagnetic wave of appropriate frequency, equal to the Larmor frequency, excites

the nuclei to the higher energy level.

A radiofrequency (RF) pulse equal to Lamor frequency is applied to transfer an

equilibrium system to the unstable state of high energy. The process which the system

needs to discharge the excess energy, returning to the equilibrium state is called

relaxation. A 90o pulse is commonly applied to excite the magnetic moment of the

protons. After the excitation, the magnetic moment no longer aligns with the external

magnetic field B0, but in the plane perpendicular to B0. During the course of relaxation,

the longitudinal fraction of the magnetic moment starts to restore from zero and the

transverse component of the magnetic moment starts to diminish to zero. The T1

relaxation time, termed as spin-lattice or longitudinal relaxation time as well, is the time

required for the system to recover to 63% of its equilibrium value after it has been

exposed to a 90o pulse.

After a spin system has been excited by an RF pulse, it initially behaves like a

coherent system which all the components of the magnetization precess in phase around

the direction of the external field. As time passes, the observed signal starts to decrease

as the spins begin to diphase. The decay in the x-y plane is faster than the decay of the

magnetization along z-axis. This additional decay of the net magnetization in the x-y

plan is due to a loss of phase coherence of the microscopic components, which partially

results from the slightly different Larmor frequencies induced by small differences in

the static magnetic fields at different locations of the samples. This process is

characterized by T2, the spin-spin or transverse relaxation.

1.1.5.2 MRI contrast agents

The imaging contrast of MR arises from the different relaxation times of different

parts of the tissue. To obtain a better image with well-defined mapping, contrast agents

are utilized during the imaging procedure. The contrast agents basically decrease the T1

and/or T2 relaxation times depending on if it is a T1 or T2 contrast agent. The relaxation

times can be manipulated by the use of T1 (e.g. gadolinium and manganese chelates)

and T2 (e.g. magnetic nanoparticles) contrast agents, producing brighter (T1-weighed)

and darker (T2- weighed) images where they are accumulated.75, 76 Several types of


superparamagnetic iron oxides (SPIO) have been developed for MRI contrast enhancing

agents with appropriate surface coating.77-81 The efficiency of an MRI contrast agent is

commonly assessed in terms of its relaxivities r1 and r2, which are rates of proton

relaxations and are determined according to the following equation:

1/Ti,obs = 1/Ti,d + ri[M] (i = 1, 2) (1.3)

where l/Ti,obs is the observed solvent relaxation rate in the presence of a contrast agent,

l/Ti,d is the relaxation rate of the pure diamagnetic solvent, and [M] is the concentration

of the contrast agent.

In this thesis, nanostructured T2 contrast agents are of major interest. Magnetite

(Fe3O4) and maghemite (γ-Fe2O3) are two important ferrimagnetic minerals among all

kinds of iron oxides. Magnetite has an inverse spinel structure.82 The unit cell is

face-centered cubic based on 32 O2- ions, and the lattice parameter is 8.39 Å.

Maghemite has a similar structure with magnetite except all or most of the Fe ions are

in the trivalent state, resulting in cation vacancies that compensate for the oxidation of

Fe2+. The lattice parameter of maghemite is 8.34 Å.83

1.1.5.3 Synthesis of magnetic nanoparticles

Synthetic methods of magnetic NPs are well documented. Three most common

approaches used to prepare magnetic NPs are chemical/physical vapor deposition,84

mechanical attrition,85-87 and chemical routes from solution.41 In this thesis, chemical

routes are primarily emphasized and four typical approaches are introduced in brief.

Precipitation. Massart carried out the synthesis of magnetite NPs by using the

alkaline precipitation technique.42 The research on magnetic NPs both on fundamental

chemistry and various applications, especially in engineering and biomedical fields,

started to prosper since then. So far this technique is still dominant in producing

clinically approved iron oxide NPs.93-96 Soluble metal salts are used as precursors and

precipitation occurs when anionic counter ions (usually oxalate, carbonate and

hydroxide) are added. Nanosized precipitation is obtained as the form of insoluble metal

salts that need further calcination to obtain the corresponding oxides. High degree of

agglomeration of the product is inevitable due to the heat treatment. Magnetite and

maghemite, as exceptions, can be directly prepared by alkalizing the Fe2+/Fe3+ solutions

as shown by the equations below. This method is also used in preparation of ferrites

containing cobalt, zinc, manganese and so on.

12

Fe2+ + 2 Fe3+ + 8 OH- → Fe3O4 + 4 H2O (1.4)

The advantage of the precipitation method is that large quantity of the NPs can be

produced per batch, although difficulties in controlling the morphology and size

distribution are well known. Parameters such as pH, metal ion concentration,

temperature and surfactants have been altered to achieve better size and morphology

control.88-92 However, rather large size distribution (σ > 30%) and roughly spherical

shape can be obtained.

Microemulsion. Surfactant molecules spontaneously aggregate into assemblies in

solution. Such assemblies of nanosized in diameter can serve as suitable template for

synthesis of NPs.47, 97, 98 The micelles have the hydrophilic section of the surfactant on

the outside of the micelles while the hydrophobic parts of the surfactant appear to be on

the surface in the case of the reverse micelles. Reverse micelles are widely utilized in

synthesis of inorganic NPs. Magnetite and maghemite NPs were firstly prepared in this

manner by oxidizing Fe2+ ions within reverse micelles.99 The interior of the micelles

containing aqueous solutions of precursors can be considered as a confined

"nano-reactors". Several kinds of surfactants has been applied for such purpose, e.g.

AOT,100, 101 CTAB,102-104 SDS,105 and polyethoxylates (Tween).106, 107 The reaction of

precipitation can be carried out within reverse micelles to produce various kinds of

ferrite materials such as (Mn,Zn)Fe2O4,108 (Ni,Zn)Fe2O4,109 ZnFe2O4,110 and

BaFe12Ol9.111 The particle size can he controlled by changing the size of micelles

through tuning the ratio between the surfactant and hydrocarbon phase. Another

advantage of microemulsion method is that the "nano-reactors" are isolated with each

other which limit the ripening process during the particle growth. As a consequence,

narrow size distribution (σ < 10%) can he achieved.

Sol-gel. Sol-gel method has been used to synthesize pure, stoichiometric and

monodisperse oxides NPs and nanocomposites,112-114 but it was not applied for the

synthesis to iron oxide until Yamanobe et al. applied the sol-gel process for preparation

of maghemite and sodium-modified maghemite particles.115 This process was further

studied by da Costa and the coworkers.116 Sugimoto et al. developed and systematically

studied the sol-gel routes to prepare hematite particles.117 Magnetite and maghemite

NPs have been derived from the obtained hematite NPs by Sugimoto and his coworkers

ten years after his first work.118 Large size range from tens of nanometers to hundreds of

nanometers and relatively narrow size distribution can be obtained.


Pyrolysis of organometallic compounds. Many kinds of NPs including

semiconductor quantum dots,119, 120 metallic/alloy NPs,121-124 and iron oxide/ferrite

NPs125-127 are synthesized from organometallic compounds at an elevated temperature.

It has been shown that polymers or organic capping agents determine the cluster size

and effectively protect the particles during the nucleation and crystal growth.128, 129

Hexadecylamine, oleylamine and oleic acid have been used in the synthesis of Co NPs

by reduction of Co(η4-C8H13)(η4-C8H12) in hydrogen. By varying the composition of

ligands, the size and shape of particles could be controlled.130 CoxPt1-x alloy has been

prepared from the organometallic compounds in the presence of poly(vinylpyrrolindone)

(PVP).131

Alivisatos et al. have shown that Co NPs can be prepared through decomposing the

cobalt carbonyl in the presence of oleic acid and TOPO.132 In these processes, oleic acid

and TOPO act as the capping agents with long alkyl chains that can stabilize the

particles during the course of nucleation and growth. Spherical maghemite NPs are

synthesized when the iron cupferron complex is rapidly injected into trioctylamine at

300 °C.133 Maghemite NPs can be synthesized by oxidizing the Fe NPs formed via

decomposition of iron pentacarbonyl or through one-step decomposition of iron

pentacarbonyl in the presence of mild oxidant.125 It is hypothesized that the iron oleate

complex was the critical intennediate to form the seeds and subsequently grow into the

NPs. Peng et al. carried out the synthesis of iron oxide NPs by choosing iron oleate,

stereate, and laureate as precursors.134 Hyeon et al. improved the procedure and

suggested a large-scale synthetic route for iron oxide NPs by pyrolysis of iron oleate

complex.127 Iron oxide particles with very narrow size distribution (σ < 5%) and high

crystallinity can be obtained via pyrolysis routes. By altering the ratio between capping

molecules and precursors, size of particles can be controlled.

1.1.6 Environment-sensitive components

Nanomaterials that are sensitive to external stimuli are of emerging interest due to

the great potential in many biomedical and technical applications. For controlled

delivery of therapeutic agents, it is an advantage to make drug carriers sensitive to local

physiological stimuli such as pathology-associated changes in local pH, temperature,

and/or chemical environments for example, specific analytes, and external triggers such

as electrical, magnetic and optical stimuli.

14

Polymeric components with pH-sensitive bonds are used to produce

stimuli-responsive DDS that are usually stable at the physiological pH, however,

undergo degradation upon exposure to lowered pH in pathological sites such as tumors,

infarcts, inflammation zones or cell cytoplasm or endosomes.135-137 Various kinds of

PEGylated liposomes have been prepared for pH-sensitive drug delivery.138-141

Polymeric micelles having acid-labile bonds in the structure have also been reported as

pH-sensitive DDS.142-144 Additionally, receptors such as folate and biotin conjugated to

the surface of pH-sensitive drug carriers facilitates the specific binding to the tumor

cells.62, 145

1.1.6.1 Thermosensitive poly(N-isopropylacrylamide) (PNIPAAm)

Aqueous solution of PNIPAAm undergoes structural changes at the certain

temperature---lower critical solution temperature (LCST).146-148 PNIPAAm has this

property mainly because of the dual characters in the structure that contains both a

hydrophobic isopropyl and a hydrophilic amide group. This intrinsic phase change as a

result of temperature variation is used in copolymers which form spheres for

thermosensitive DDS. PNIPAAm undergoes sharp and reversible phase transition at

LCST.146, 149-160 Up to now, a large variety of medical implements made from

PNIPAAm has been proposed in different forms, e.g micelles,150, 151 NPs,152 hydrogel,153

and tablets.154 The application has been expanded to not only PNIPAAm homopolymer

itself, but also other PNIPAAm-derivative materials in combination with

organic/inorganic counterparts such as PNIPAAm-poly(butylmethacrylate),150

PNIPAAm-poly(D,L-lactide),151 PNIPAAm-polystyrene (PNIPAAm-PS),146 and

Ag-PNIPAAm-PS.161 Very recently, tremendous development and investigation has

been devoted to designing nanocrystals (NCs) with the surface coating layer sensitive to

variations of temperature. The thermosensitive property makes PNIPAAm most

appropriate for the temperature-sensitive coating of NCs. Therefore, several strategies

have been developed for producing thermosensitive NCs. One of the prevailing methods

is through direct graft of PNIPAAm brushes from the surface of NCs.165-169 However

this approach involves multi-step reactions and tedious separation procedures. Many

other strategies to attach thermosensitive polymers to NCs such as ligand exchange,170

electrostatic interaction,171 soft-template in situ decomposition172 among others, have

been described recently.


1.1.6.2 Magnetic field-sensitive ferrogels

Intelligent hydrogels respond to external stimuli such as temperature, pH, electric

field, magnetic field and specific analytes and enzymes.173-179 For example, microgel

responsive to the concentration of glucose has been developed for glucose-sensitive

DDS.180 Hydrogel based on poly(aspartic acid) and poly(acry1ic acid) are demonstrated

to be able to swell and shrink due to the change of salt concentration.181 It was shown

that magnetic stimulation intrinsically enhances dopamine release from nucleus

accumbens shell of morphine-sensitized rats during abstinence.182 Ferrogels consisting

of magnetic nanoparticles embedded in polymer hydrogels, are an important category of

environment-sensitive hydrogels which respond to an external magnetic field.183-190

Their unique magnetoelastic property endows ferrogels with great potential for

magnetic-controllable drug delivery system. As an artificial intelligent delivery matrix,

ferrogels triggered by DC magnetic field which show an "on-off" drug release has been

reported.191-196 Recently, hydrogel constructed with thermosensitive polymer and

magnetic NPs has been prepared and such system is of great promise in drug delivery

applications due to the dual stimuli-sensitivity.197 Examples of other

environment-sensitive DDS such as ultrasonic198 and electric199 are not so many in the

literature so far, though we should always pay attention to the exotic triggers which can

be exploited safely and simply in DDS.

1.2 Objectives

The aim of this thesis is to tailor nanoparticles for multifunctional DDS associated

with the environment-sensitive properties as well as imaging functionality. Three DDS

systems are developed in this work: polymeric nanospheres, ferrogels, and

polymer/inorganic nanoparticles composites.

1) Thermosensitive property is incorporated with polymeric nanospheres in order to

realize temperature-programmed protein release. Thermosensitive PNIPAAm is used as

a trigger and PLA is used as a biodegradable matrices.

2) For another type of DDS, PF127 copolymer is employed as a

termperature-dependant gelling material for sustained release of hydrophobic drugs.

Incorporation of magnetic NPs in the PF127 hydrogel forms a ferrogel whose drug

release behavior varies depending on the magnetic field.

16

3) Monodisperse SPIONs are prepared through high temperature decomposition

methods and used as contrast agents for MRI. Strategies to transfer NPs from organic

media to aqueous medium are developed. As a continuation work to thermosensitive

DDS, inorganic nanocrystals (NCs) is going to be coated with a new amphiphilic

copolymer, PMAO-graft-PNIPAAm to obtain thermosensitive NCs which will find

potentil applications in sensing, magnetic separation and drug delivery.


2 Experimental

This chapter includes sketches of the experimental work. For details, please refer to

the corresponding appended papers.

2.1 Thermosensitive nanoparticles

2.1.1 Synthesis of PNIPAAm-PDLA and PLLA-PEG copolymers

Synthetic pathway of diblock copolymers is depicted in Figure 2.1. Briefly,

PNIPAAm was synthesized via free radical polymerization which was initialized by

benzoyl peroxide (BPO) under anhydrous condition. Thereafter, synthesized PNIPAAm

homopolymer, D,D-lactide and stannous 2-ethylhexanoate (Sn(Oct)2) were mixed in

anhydrous toluene and the second synthesis was performed for 7 hours. PLLA-PEG

copolymer was synthesized from PEG, L,L-lactide and Sn(Oct)2 in anhydrous toluene.

Upon completion of the reaction, polymers were collected by precipitating in an excess

volume of cold diethyl ether, followed by dried in vacuo at an ambient temperature

overnight.

Figure 2.1 Synthetic pathways of PNIPAAm-PDLA and PLLA-PEG block copolymers.

18

2.1.2 Synthesis of PMAO-graft-PNIPAAm copolymers

The telechelic PNIPAAm-NH2 was synthesized through free radical polymerization

(Figure 2.2). The initiator 2,2'-Azobis(2-methylpropionitrile) (AIBN), was double

recrystallized from hot methanol. In a typical experiment, NIPAAm,

2-aminoethanethiol·HCl and AIBN was dissolved to anhydrous dimethylformamide

(DMF). The solution was degassed by means of freeze-pump-thaw circles.

Polymerization was carried out at 75 °C for 22 hours under N2. The product was

precipitated in an excess of diethyl ether and washed with acetone 3 times. The crude

product was dissolved in deionized water and subjected to dialysis (MWCO = 25 kDa)

against deionized water for 3 days with frequent water refresh. PNIPAAm-NH2 powder

was thereafter collected after lyophilization. PMAO (Mw = 30-50 kDa). In another

container, PNIPAAm-NH2·HC1 and triethylamine were dissolved in anhydrous

chloroform. The latter was added to PMAO solution dropwise under agitation (Figure

2.2). The reaction was allowed to complete at room temperature for 12 hours. The

PMAO-graft-PNIPAAm was collected under reduced pressure and the waxy solid was

dissolved in chlorofonn.

Figure 2.2 Synthesis of PMAO-graft-PNIPAAm copolymer


2.1.3 Fabrication of thermosensitive nanospheres loaded with bovine serum

albumin

A modified double emulsion method (MDEM) was used to prepare the w/o/w

emulsion. Aqueous solution of bovine serum albumin (BSA) was dropped to the

chloroform containing PNIPAAm-PDLA diblock copolymer followed by sonication.

Afterwards, the obtained emulsion was mixed with PLLA-PEG chloroform solution and

added dropwise to PVA aqueous solution followed by sonication in an ice bath.

The silanization was carried out by mixing an emulsion of

PLLA-PEG@PNIPAAm-PDLA with APTMS ethanolic solution in order to

functionalize the surface of the nanospheres with amino groups. After the silanization,

gold colloidal suspension was added dropwise to the amino-modified

PLLA-PEG@PNIPAAm-PDLA emulsion and the reaction mixture was kept for 2 hours

to complete the self-assembly.

2.2 Synthesis of inorganic nanoparticles

2.2.1 Preparation of superparamagnetic iron oxide nanoparticles (SPIONs)

High temperature decomposition of iron based organometallic precursors are

employed. A typical synthetic procedure to produce 12 nm iron oxide nanocrystals via

decomposition of iron oleate complex is given as an example.

Solution of ferric chloride and sodium oleate in a mixed solvent (ethanol, deionized

water and hexane) was refluxed for 4 hours. The waxy Fe oleate complex was separated

and dissolved in dioctyl ether in the presence of oleic acid, heated to reflux for 1.5 hours,

and SPIONs were precipitated by ethanol. Finally, the SPIONs were dispersed in

hexane in the presence of small amount of oleic acid and stored at 4°C for further use

and characterizations.

2.2.2 Preparation of Au nanocrystals

Water-based colloidal gold NCs were synthesized by reducing [AuCl4]- ions in the

presence of reductant, NaBH4. Non-polar solvent-based gold NCs were synthesized as

the following. Au acetate, 1,2- hexadecanediol, oleic acid (50 μL) and oleylamine (300

μL) were dissolved in dioctyl ether. The mixture was heated to 190 °C and kept at this

temperature for 1.5 hours. The Au NCs were washed and collected in the same way as

for SPIONs.

20

2.2.3 Controlling size and morphology of SPIONs

To investigate the size evolution, reaction was monitored at pre-determined time

intervals. In a typical study, 3 g (3.34 mmol) of iron oleate complex was dissolved in 20

mL dioctyl ether in the presence of 0.314 g (1.11 mmol) oleic acid. The reaction

mixture was heated up to 298 °C at a constant heating rate of 5.5 °C·min-1. The first 0.5

mL sample was withdrawn at the temperature of 220 °C and the following aliquots were

periodically withdrawn from the solution. The as-prepared SPIONs dispersed in dioctyl

ether were filled in capillaries for XRD without purification.

SPIONs were also prepared in different morphology. The cubic particles were

prepared by dissolving 4 mmol of the iron oleate complex and 6 mmol of oleic acid in

37.2 mL dioctyl ether at 70 °C. The mixture was heated to 290 °C (at 3 °C·min-1) and

kept for 2 h. In the case of the sample with spherical particles, the reaction mixture was

heated at heating rate of 20 °C·min-1 to 290 °C.

2.2.4 Surface modification of SPIONs

2.2.4.1 Preparation of PLLA@SPIONs

Monodisperse SPIONs have been prepared through high temperature decomposition

of iron oleate complex as previously described. The coating procedure is schematically

illustrated in Figure 2.3. Ricinoleic acid (ROA) molecules were grafted onto the surface

of SPIONs through carboxylic groups, hereafter abbreviated as ROA@SPIONs. The

appending hydroxyl groups in ROA molecules serve as initiating sites for further

polymerization. Catalyzed by Sn(Oct)2, ROP of L,L-lactide is carried out in the presence

of ROA@SPIONs. Upon completion, the PLLA@SPIONs are collected by

centrifugation after being washed with acetone.

Figure 2.3 Ligand exchange of oleic acid coated SPIONs with ricinoleic acid and surface-initiated ring-opening polymerization


2.2.4.2 Ligand exchange

The oleic acid coated SPIONs were used in phase transfer experiments. Four kinds of

small molecules tetramethyl ammounium hydroxide (TMAOH), trisodium citrate (TSC),

nitrate acid (HNO3) and meso-2,3-succinic acid (DMSA) were tested for ligand

exchange with oleic acid.

TMAOH. 10 mg powder of SPIONs were mixed with 2.5 mL 10 wt.-% TMAOH

and fully dispersed after brief shaking. The black dispersion was centrifuged at 14,000

rpm for 10 minutes and washed with water for 5 times in order to eliminate the excess

of the TMAOH. Finally, the particles were re-dispersed in a 2.5 mL 0.01 wt.-%

TMAOH solution.

TSC. To the 2.5 mL TMAOH dispersion of SPIONs obtained from the previous step

was added 20 mg of TSC followed by adding HCl dropwise to the solution to bring the

resulting mixture to pH 6.5. Precipitation caused by alteration of pH was removed by

filtration.

HNO3. 10 mg powder of SPIONs was mixed with 10 wt.-% HNO3 to produce an

acidic sol. The brownish dispersion was washed with water for 5 times and the

precipitate was dispersed in 2.5 mL 1 wt.-% HNO3 solution.

DMSA. To the 2.5 mL HNO3 dispersion of SPIONs obtained from previous step was

added 5 mg of DMSA. The mixture was shaken vigorously for 2 hours resulting in a

stable brownish sol.

2.2.4.3 Coating SPIONs with Pluronic F127 (PF127)

The method is based on the ability of the PF127 molecules that can associate with

alkyl chains of oleic acid---the surface capping ligands on the NPs---by the hydrophobic

PPO section, whilst two hydrophilic PEO tails render the particles which were

originally immiscible with water with enhanced hydrophilicity. The particles after phase

transfer are denoted as Pluronic F127@oleic acid@SPIONs, thus abbreviated as

PF127@SPIONs thereafter.

PF127 coated PLLA@SPIONs: a suspension containing as-synthesized

PLLA@Fe3O4 was dispersed in THF. The freshly prepared suspension was mixed with

aqueous solution of PF127 followed by vigorous agitation for 30 minutes. The obtained

solution was subjected to evaporation overnight to remove the organic solvent, resulting

22

in a stable aqueous solution of PF127@PLLA@SPIONs. The aqueous solution of

nanoparticles was then dialyzed against 1 L of deionized water for 48 hours to remove

unbound PF127 polymers.

2.2.4.4 Coating SPIONs and Au nanocrystals with PMAO-graft-PNIPAAm

The coating scheme is shown in Figure 2.4. The PMAO-graft-PNIPAAm solution

was added to SPIONs in chloroform and magnetically stirred for 30 minutes. Upon

completion, sodium borate buffer (pH = 11) was added to the mixture and subjected to

vigorous agitation for 1 hour. The SPIONs in water solution were thereafter obtained

after evaporation of chloroform. The buffer was exchanged to phosphate buffer (pH =

7.4) after 3 rounds of ultrafiltration at 3000 rpm. Same procedure was also applied to

the surface coating of Au NCs.

Figure 2.4 SPIONs coated with oleic acid are encapsulated in amphiphilic PMAO-graft-PNIPAAm copolymers through hydrophobic interaction

2.3 Characterizations

TEM images were taken by using JEOL JEM-2000EX and JEOL JEM-2100F. XRD

patterns of SPIONs were recorded by a PANalytical X'Pert Pro system. The capillary

XRD patterns of the particles were measured in synchrotron radiation (λ = 1.23 Å

MAX-lab, Lund University, Sweden). The concentration of SPIONs was measured by

AAS (SpectrAA-200, Varian). FTIR spectra were recorded on a Nicolet Avatar 360

E.S.P. spectrophotometer. TGA was measured by a TGA Q500 system (TA Instuments).

DSC was measured by a modulated DSC 2920 and DSC Q2000 (TA Instruments).


Surface Potential and particle size was measured by Zetasizer Nano ZS (Malvern, UK).

The magnetization measurements were carried out at room temperature using a VSM. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance-600 MHz and

Bruker DMX 500 MHz spectrometer. The particle size distribution was measured by

DLS particlesizer (BI-90, Brookhaven). Varian Cary 100 Bio UV-Vis

spectrophotometer was used to determine the LCST and quantify the concentration of

BSA by means of BCA assay. The Mw of PLLA was analyzed with a SEC. Relaxivities

r1 (20 and 60 MHz) was measured at 0.47 T with Minispec PC-20 and at 1.41 T with

Mq Series systems; r2 was measured with Minispec (20 and 60 MHz) on spectrometer

AMX-300.

2.4 Drug loading to the ferrogel, PLLA@SPIONs and oleic acid

coated SPIONs

2.4.1 Ferrogel

An aqueous suspension of SPIONs was mixed well with a PF127 solution containing

a desired amount of IMC at 4 °C to obtain a solution containing PF127 (17.5%, w/v),

SPION (4 mg Fe/mL) and IMC (1 mg/mL). The mixture was vortexed to form a

homogeneous suspension and the temperature was increased to 35 °C. Gelation occured

in a few seconds of warming. The ferrogel was cooled down again and the previous two

steps were cycled for three times.

2.4.2 PLLA@SPIONs and oleic acid coated SPIONs

To load a model drug in PF127@PLLA@SPIONs and PF127@SPIONs,

PLLA@SPIONs in THF was mixed with 1 mg IMC. The mixture was added to PF127

aqueous solution dropwise followed by 5 minutes sonication. The mixture was shaken

for 20 hours to evaporate the organic solvent and meanwhile to allow partitioning of

drugs into the PLLA layer surrounding the SPION core. The drug loaded

PF127@PLLA@SPIONs were separated from the solution using a permanent magnet.

The drug loaded nanoparticles were washed with 5 mL water twice to attach to the

surface and subsequently loaded for lyophilization for 30 hours. To determine drug

loading, 5 mg of IMC loaded PF127@PLLA@SPIONs were dispersed in 5 mL DMSO

and placed on a shaking board for 16 hours to completely extract the drug. The

concentration of IMC in the supernatant was measured using UV-Vis (λ = 319 nm). A

24

series of IMC standard aqueous solution with concentration ranging from 1 to 50 μg/mL

were used to generate a calibration curve. Drug loading in oleic acid coated SPIONs

followed the same procedure.

2.5 Drug release

The drug release tests were carried out in a Franz diffusion cell. The donor chamber

was separated from the receiving chamber by a cellulose membrane with different

molecular weight cut-of (MWCO).

Figure 2.5 Franz diffusion cell used for drug release tests (courtesy of PermeGear, Inc.)

2.5.1 Ferrogel

The IMC loaded ferrogel was injected into the donor chamber at 4 °C, and the

temperature of the solution was slowly increased to 37 °C. The receptor chamber was

filled with 10 mL of PBS. The release experiments were carried out thermostatically at

37 °C throughout. A permanent magnet with field of 300 mT was used in order to

examine the effect of the magnetic field (MF) on drug release rate. Two comparative

sets of experiments were undertaken; one without the MF as a control and one with the

MF. The IMC concentration was recorded by UV-Visible spectrophotometer (λ = 319

nm) equipped with an autosampler. The data points were collected automatically every

5 minutes.

2.5.2 PLLA@SPIONs and oleic acid coated SPIONs

The IMC loaded nanoparticles was dispersed in PBS and injected into the donor

chamber, and the temperature of the solution was slowly increased to 37 °C. The

receptor chamber was filled with 10 mL of PBS (pH 7.4, Medicago AB, Uppsala,


Sweden). The release experiments were carried out thermostatically at 37 °C throughout.

The IMC concentration was recorded by UV-Vis spectrophotometer (Varian Cary 100

Bio, λ = 319 nm) equipped with an autosampler. The data points were collected

automatically every 5 minutes.

2.5.3 Thermosensitive nanospheres

In vitro release profiles from thermosensitive polymeric nanospheres were obtained

as follows. An appropriate amount of BSA-loaded

Au@PLLA-PEG@PNIPAAm-PDLA was introduced into a dialysis membrane bag and

placed in 100 mL of PBS as a media containing 0.02 wt-.% sodium azide. Two

comparative experiments were simultaneously undertaken; one at 22 °C as a control and

the other at 37 °C. At the pre-determined time intervals, an aliquot of the aqueous

solution was withdrawn from the media and the protein quantity was analyzed by BCA

assay.201, 202

2.6 Cytotoxicity tests

2.6.1 Thermosensitive nanospheres

PLLA-PEG@PNIPAAm-PDLA and Au@PLLA-PEG@PNIPAAm-PDLA were

dialyzed with cellulose membrane (MWCO: 3 kDa) against water for 48 hours and then

lyophilized for 72 hours. Stock solutions were prepared at the concentration of 20

mg/mL. Four different concentrations of the solutions were sequentially prepared by

diluting the stock solution with the appropriate amount of cell media. HeLa cells and

Cos-7 cells were purchased from American Type Culture Collection (ATCC).

Cells were prepared at a density of 5 × 104 cells per mL of medium. The prepared

cells were seeded 200 μL in each well of a 96-well microplate. The cells were incubated

with various concentrations of polymer for 48 hours. The medium was removed and

replaced with 100 μL of growth medium prior to the addition of the 20 μL/well of

CellTiter 96 Aqueous One Solution Reagent based on MTS assay. The results were

expressed relative to results from untreated cells. Untreated cells were used as a positive

control. The medium without cells was used as a blank.

For TCCV assay, cells were seeded on the 6-well glass plates. Aspirating cell media

from the wells, they were filled with the working solution prepared by mixing 2 μL of 2

mM ethidium homodimer-1 solution diluted in 1 mL of PBS with 0.5 μL of calcein AM

26

solution. The glass plates were incubated under darkness for 30 minutes. The wells were

washed with PBS twice and filled with 1 mL of PBS per well after contained solutions

were aspirated. Images were taken by Axio Vert 200M fluorescence microscope (Carl

Zeiss Inc., Thornwood, NY).

2.6.2 PF127@SPIONs

PF127@SPIONs was lyophilized for 72 hours. Stock solutions were prepared at the

concentration of 2 mg/mL. Three different concentrations of the solutions were

sequentially prepared by diluting the stock solution with the appropriate amount of cell

media. HeLa and MCF-12A cells were purchased from American Type Culture

Collection (Rockville, MD).

Cells were prepared at a density of 5 × 104 cells per mL of medium in case of HeLa

cells and 5 x 103 cells per mL of medium for MCF-12A respectively. The prepared cells

were seeded 200 μL in each well of a 96-well microplate. Prior to the incubation with

cells, particles were diluted in cell culture medium and filtrated through 200 nm filters.

The cells were incubated with various concentrations of particles for 48 hours. Cells

without any NPs were seeded and cultivated in the same way to use as controls. The

medium was removed and replaced with 100 μL of growth medium prior to the addition

of the 20 μL/well of CellTiter 96 Aqueous One Solution Reagent (Promega) based on

MTS assay for HeLa cells. In the case of MCF-12A cells, a 10 mL of MTT dye solution

was added to each well and cells were incubated for 3 more hours. The results were

expressed relative to results from untreated cells. The medium without cells was used as

a blank. The cell viability (%) was calculated according to the following equation:

Cell viability (%) = (ODsample - ODblank) / (ODcontrol - ODblank) × 100 (2.1)


3 Results and discussions

3.1 Superparamagnetic iron oxide nanoparticles (SPIONs)

3.1.1 Morphology and structure studies

The obtained SPIONs can be readily dispersed in nonpolar or weakly polar solvents,

e.g. hexane, toluene and so on, forming a stable black suspension. XRD pattern with

characteristic peaks of inverse spinel structure indicates that iron oxide nanocrystals are

pure ferrite phase with high crystallinity (See appended paper III). It is not possible to

distinguish whether these particles are magnetite or maghemite solely from XRD results

because both materials have the inverse spinel structure with almost identical XRD

patterns. Nevertheless from the synthetic procedure, we could assume a complete

transformation to maghemite because no inert atmosphere was applied during the course

of reaction. Therefore, iron oxide species would be completely oxidized to maghemite.

It was further confirmed by iron concentration measured by AAS, [Fe] = 70.44 ± 0.67%,

almost equal to the iron concentration in maghemite, 69.94%. Since we still lack

evidence to confirm the exact phase of the obtained NPs, we refer them as iron oxide

other than magnetite or maghemite.

NPs covered with oleic acid repulse from each other due to the Van der Waals force

of the long alkyl chain of oleic acid, thus forming ordered array of particles over a short

range (ca. 200 nm). The nearly spherical NPs are clearly seen in TEM images,

assembling into hexagonal array (Figure 3.1 a and b). Very narrow size distribution has

been achieved which is shown in the histogram (Figure 3.1c) with σ = 3.8%. Insets in

Figure 3.la show an SAED pattern of SPIONs and rings are in a good agreement with

indexed peaks in the XRD pattern. The HRTEM image (inset of Figure 3.la) of a single

particle shows distinct lattice fringe, revealing the highly crystalline nature of the

SPIONs.

Table 3.1 Parameters for controlling the size of SPIONs

Fe oleate complex Oleic acid Ratio Average size Standard (mmol) (mmol) (Fe :OA) (nm) deviation 2.23 0.78 3:l 11.5 3.8% 2.22 2.23 1:1 13.0 4.8% 2.22 4.44 1:2 16.0 4.8% 2.22 5.60 1:2.5 4.0 26% 2.22 6.70 1:3 - - 2.22 8.88 1:4 - -

28

Figure 3.1 (a) A TEM image of SPIONs arranged in a 2D hexagonal array. Insets are SAED pattern and HRTEM of a single nanocrystal of SPIONs, (b) TEM of SPIONs with a larger magnification, and (c) particle size distribution.

The size of SPIONs can be manipulated by varying the molar ratio between Fe oleate

complex and oleic acid in a predetermined volume of solvent (13.75 mL). The

experimental data of size control are summarized in Table 3.1. As increasing the mole

concentration of oleic acid, particle size is increased from 11.5 to 16.0 nm. Nevertheless,

particles stop growing when the Fe to oleic acid ratio reaches 1:2.5, the particle size

drops down to 4.0 nm with broad size distribution, indicating high concentration of

oleic acid would suppress particle growth process to some extent. Even no particle can

be observed under TEM when the ratio of Fe to oleic acid is increased to 1:3 and 1:4. A

remarkable drop of particle size occurs at Fe:OA = 1:2. TEM images of the samples

with Fe:OA = 3:1, l:l and 1:2 are shown in Figure 3.2 where size increasing can be

clearly distinguished. The facets of the nanocrystals evolve as the particle size increases.

Particles from Fe:OA = 3:l can be regarded as quasi-spherical particles while Fe:OA =

1:2 becomes polyhedral morphology as a result of size increase. It was also reported

elsewhere that by increasing the concentration of OA, one could obtain larger particles

out of this procedure.134 Peng et al. reported the particle size could reach and be kept at

a certain value before Ostwald ripening.203 More recently, Lin et al. discovered a


reversible transformation process between the Co NPs and their clusters by means of

changing the concentration of OA.204 The finding implied that an excess of ligand

molecules would work in the opposite way rather than enlarge the particles as one

expected. Such process has been applied to other particulate systems, for instance Au205

and PbS,206 as reported as "digestive ripening" which facilitates the dissolution of the

large particles and grow the small ones in order to narrow the size distribution. In our

case, by increasing the concentration of OA, SPIONs could undergo a similar "digestive

ripening" during the course of particle formation, forming TEM-undetectable clusters.

Figure 3.2 TEM images of SPIONs prepared from Fe:OA is (a) 3:1, (b) 1:1 and (c) 1:2

3.1.2 Size evolution of SPIONs

Particle growth of SPIONs in the dioctyl ether has been studied by TEM in the

similar manner as previously reported.134 To further confirm the TEM observation,

particles in dioctyl ether solvent without separation were filled in capillaries to be

measured in synchrotron diffraction. Formation of NPs are not seen when the

temperature is below 290 °C after 20 minutes heating. Although no visible particle is

observed at 45 minutes, peaks with very low intensities are seen in the corresponding

synchrotron XRD patterns (Figure 3.4). Particle size calculated through Debye-Scherrer

formula (D = Kλ/Bcosθ) is rather small at this stage (Table 3.2). After this initial stage,

quasi-cubic shape was observed at 60 minutes with rather large particle size (Figure

3.3a). The particles gradually changed to nearly monodisperse spherical shape at 90

minutes at the same temperature (Figure 3.3b). The particle size slightly increases when

the solutions are held for another 45 minutes with broadened size distribution, which is

likely due to the Ostwald ripening process (Figure 3.3c). We could therefore conclude

that there is no advantage to boil the reacting solution longer than 2 hours. Grain size of

30

particles calculated according to XRD patterns are close to the size measured by TEM

(Table 3.2), indicating that the particles are single crystals.

Figure 3.3 TEM images of evolution studies. (a) to (c) are samples held for 60, 90 and 135 minutes for Fe:OA = 3:1

Figure 3.4 XRD obtained from synchrotron radiation for the sample prepared under condition of Fe:OA = 3:1, (a) to (e) are corresponding to samples taken from 30 min to 135 min

Table 3.2 Size comparison between XRD and TEM results for Fe:OA = 3:1

Time (min) 30 45 60 90 135 Temp. (°C) 293 294 294 295 294 D (nm, XRD) - 4.3 13.7 15.8 17.7 D (nm, TEM) - - 13.9 16.5 18.4

3.1.3 Shape control of SPlONs

As can be seen from the TEM images shown in Figure 3.5, the nanoparticles

synthesized in a short reaction time (i.e. large heating rate) exhibit a clear spherical

character (Figure 3.5a) with a monodispersion of particle size (d = 14.5 nm, σ = 0.8 nm).


On the other hand, the nanoparticles synthesized in a long reaction time (i.e. low heating

rate) exhibit a cubic shape with rounded edges (Figure 3.5b) with an edge length of l =

12 nm and a narrow size distribution σ = 2.5 nm (hereon we will used d to denote both

diameter and edge length). The high resolution images show that the cubic particles

consist of low-energy {100} faces, in agreement with previous reports.207, 208 The lattice

fringes of both types of particles are indexed to those of cubic spinel structure (JCPDS

Card No. 391346). The average volume of the cubic nanoparticles, Vcub ≈ 1700 nm3, is

similar to that of the spherical ones with Vsphe ≈ 1600 nm3. Interestingly, the prolonged

time needed to obtain cubic nanoparticles allows for the Ostwald ripening of the small

particles, which are then dissolved and incorporated into the larger ones contributing to

a broader size distribution.

Figure 3.5 TEM micrographs of (a) spherical and (b) cubic SPIONs. The insets show HRTEM images of the respective nanoparticles. The lattice fringes are indicated in the insets. Note that the lattice fringes corresponding to the (400) reflections (2.0 Å) are parallel to the cube edges

3.1.4 Shape-dependant magnetic properties

SPIONs in spherical and cubic morphologies exhibit some distinct differences. As

seen from the room-temperature zero-field (ZF) Mössbauer spectra, the spherical

nanoparticles appear as more magnetically ordered (i.e., with the presence of a better

resolved magnetically split component superposed to the superparamagnetic doublet)

than the cubic ones even having a similar volume (see Figure 3.6). The presence of

superparamagnetic doublet as showing that remarkable part of Fe atoms is located in the

region of the nanoparticle surface and because of broken cubic symmetry these atoms

are exposed to nonzero electric field gradient. However, zero-field Mössbauer

spectroscopy at low temperatures, where both types of nanoparticles are magnetically

blocked, shows that the basic parameters such as magnetic hyperfine fields, quadrupole

splitting, and isomer shift of both samples are identical within the experimental error,

32

indicating similar microscopic magnetic structure, in agreement with their similar

crystalline structure.

Figure 3.6 Zero field Mössbauer spectra at room temperature for (a) spherical and (b) cubic SPIONs

The zero-field-cooled (ZFC) magnetization measurements show that the average

blocking temperature, TBM(T), obtained from the maximum at MZFC, as shown in Figure

3.7a, of the spherical nanoparticles (TBM(T)(sphe) ≈ 235 ± 2 K) is larger than the cubic

ones (TBM(T)(cub) ≈ 190 ± 2 K) in agreement with the room temperature Mössbauer

spectra.209, 210 A similar result is obtained when analyzing the temperature dependence

of the coercivity (Figure 3.7b). For instance, it is found that fitting the Hc(T) curves with

the typical Stoner-Wohlfart model,211 that is, Hc(T)= Hc(0)(l –T/TBHc)1/2, where TB

Hc is

the temperature at which the magnetization curves show no coercivity or remanence, we

find for spherical and cubic particles that TBHc(sphe) = 112 ± 10 K and TB

Hc(cub) = 87 ±

10 K, respectively. TB was also assessed from the temperature dependence of the

Mössbauer spectra as the temperature at which the area of the split spectrum and the

superparamagnetic spectrum are equivalent, TBMoss. The results, TB

Moss(sphe) = 380 ± 20

K > TBMoss(cub) = 350 ± 20 K, are consistent with the M(T) and Hc(T) analyses and also

with recent Mössbauer results for monodisperse maghemite nanoparticles of similar size.

Although, as expected, the TB obtained from the different approaches are different, the

difference in TB between the cubic and spherical nanoparticles, ΔTB, is similar for the

three techniques, ΔTBM(T) = 45 ± 3 K, ΔTB

Hc = 25 ± 15 K, ΔTBMoss

= 30 ± 20 K.


Figure 3.7 (a) Temperature dependence of the zero field cooled magnetization for the spherical and cubic γ-Fe2O3 nanoparticles. The blocking temperature of the spherical [TB(sphe)] and cubic [TB(cub)] nanoparticles are indicated by arrows. (b) Temperature dependence of the coercivity, Hc, for spherical and cubic γ-Fe2O3 nanoparticles, note that the error bars are smaller than the symbols and the lines are a fit to Hc(T) = Hc(0)(1 – T/TB

Hc)1/2

3.2 Drug delivery systems

3.2.1 Thermosentive nanospheres

3.2.1.1 Microscopic structures

Figure 3.8 illustrates the evolution pathway in constructing

Au@PLLA-PEG@PNIPAAm-PDLA. The nanosphere is in a form of dual-shell vesicle

fabricated by MDEM. Hydroxyl groups on the surface of

PLLA-PEG@PNIPAAm-PDLA are modified to amino groups and Au NPs are

deposited to form a dense layer of Au shell (Step I -IV). Above the LCST, it allows

BSA molecules to be released at an elevated temperature through a single outer shell,

since the thermosensitive inner shell is designed to burst (Step V). The resulting

nanospheres are well-defined as shown in Figure 3.9 along with the information of size

and polydispersity index.

Figure 3.8 Evolution of Au@PLLA-PEG@PNIPAAm-PDLA: (I) Formation of the BSA-loaded PNIPAAm-PDLA nanospheres. (II) Construction of [email protected] dual-shell structure by an MDEM. (III) Immobilization of Au NPs on the surface of PLLA-PEG@PNIPAAm-PDLA. (V) Burst of the inner shell when the temperature is above the LCST

34

Figure 3.9 (a) TEM image of PLLA-PEG@PNIPAAm-PDLA (negatively stained with 2% of ammonium molybdate aqueous solution for 2 min), (b) particles size distribution of PLLA-PEG@PNIPAAm-PDLA nanospheres, (c) TEM image of Au@PLLA-PEG@PNIPAAm-PDLA (without staining) and (d) particles size distribution of Au@PLLA-PEG@PNIPAAm-PDLA nanospheres

The size distribution of PLLA-PEG@PNIPAAm-PDLA lies in a range of 80-170 nm,

confirmed by DLS particle sizer. Mean diameter of PLLA-PEG@PNIPAAm-PDLA is

127 nm with 1.41 as polydispersity index. A gold layer constituted by gold NPs are

clearly seen on Au@PLLA-PEG@PNIPAAm-PDLA as dark spots in Figure 3.9c. It is

noted that particle size slightly increases up to 140 nm compared to

PLLA-PEG@PNIPAAm-PDLA. This size change results from the decreased mobility

due to deposition of Au NPs which is reflected in calculating mean diameter by DLS

particle sizer.

3.2.1.2 Determination of LCST

LCST of PNIPAAm-PDLA copolymer is observed at 36.7 °C from a sharp

endothemic peak in the DSC curve (Figure 3.10). This reflects that PNIPAAm-PDLA

copolymer undergoes the phase transition at the LCST, 36.7 °C. The result is also

confirmed by the analysis by UV-Vis spectrometry. Aqueous solution of

PNIPAAm-PDLA is transparent at a temperature lower than LCST. However, as

temperature increases, the solution becomes turbid and the transmittance rate, as shown

in Figure 3.11a, decreases in a sigmoid shape owing to the predominant hydrophobicity

over the hydrophilicity driven by the phase transition. From this graph, the maximum

corresponding to the LCST of PNIPAAm-PDLA is found from the numerically

(a) (b)

(c) (d)


differentiated curve of the transmittance rate, revealing the LCST at 36.7 °C which is

human body temperature (Figure 3.11c). It is observed that transmittance rate of the

emulsion containing Au@PLLA-PEG@PNIPAAm-PDLA also decreases in the same

fashion as temperature increases (Figure 3.11b). Whilst the transparency of

PNIPAAm-PDLA solution is reversibly regained in case the solution is cooled to the

ambient temperature, the transmittance rate of Au@PLLA-PEG@PNIPAAm-PDLA is

not restored, suggesting that the thermosensitive inner shell of the dual-shell nanosphere

is irreversibly disrupted once PNIPAAm-PDLA undergoes the phase transition when

the temperature increases.

Figure 3.10 DSC data ohtained from (a) PNIPAAm-PDLA and (b) PLLA-PEG

Figure 3.11 UV-Vis spectra at 500 nm: (a) Transmittance change of the PNIPAAm-PDLA solution, (b) Transmittance change of Au@PLLA-PEG@PNIPAAm-PDLA in w/o/w emulsion, and (c) A differentiated curve of (a)

36

3.2.1.3 Drug release

As schematically shown in Figure 3.12, PLLA-PEG@PNIPAAm-PDLA w/o/w

micelles demonstrate the morphology of spheres, since the shell-forming PNIPAAm

shows the character of hydrophilicity below LCST so that the inner milieu could be

stabilized. As soon as the temperature reaches the LCST, the inner shell comprised of

PNIPAAm turns to be hydrophobic, resulting in release of the encapsulated substances.

BSA release rates are monitored at two different temperatures including ambient

temperature and human body temperature. The results are demonstrated in Figure 3.13

by plotting the released amount of BSA versus time. In Figure 3.13, two curves

represent different release rates respectively. The profile at 37 °C (Figure 3.13a) shows

a quasi-linear increase in the release rate of BSA. However the release rate at 22 °C

(Figure 3.13b) increases smoothly before 60 hours, and expedites thereafter due to

induced instability of the dual-shell structure, while the profile at 37 °C maintains a

high release rate which can be attributed to the disrupted inner shell. Therefore, the

inner shell burst above the LCST causes the main difference of the release rate, because,

as temperature is elevated above the LCST, the inner shell becomes unstable and the

dual-shell structure changes into a simple single-shell structure. In this stage, the release

rate will be dominated only by parameters related to PLLA-PEG, the component of the

outer shell. Then, the ongoing release behavior follows the release mechanism from the

single-shell structure.

Figure 3.12 Scheme represented burst of thermosensitive PLLA-PEG@PNIPAAm-PDLA w/o/w nanospheres


Figure 3.13 In vitro release profiles of BSA from Au@PLLA-PEG@PNIPPAm-PDLA in PBS solution: (a) 37 °C and (b) 22 °C

3.2.1.4 Cytotoxicity evaluation of polymeric nanospheres

A typical graph of MTS assay results is shown in Figure 3.14, indicating that neither

of the nanospheres exerts any critical influence on HeLa cells. In addition, dead/living

cells are visualized as fluorescent microscope images taken after the TCCV assay. No

remarkable effect on the cell morphology and viability is observed. As seen in Figures

3.15, the majority of the cells are alive, with no significant destructive effect in either

case: Au@PLLA-PEG@PNIPAAm-PDLA and PLLA-PEG@PNIPAAm-PDLA.

Figure 3.14 MTS assay using HeLa cell line afier incubation with (a) Au@PLLA-PEG@PNIPAAm-PDLA and (b) PLLA-PEG@PNIPAAm-PDLA for2 days. (*denotes p<0.05 by student's t test)

38

Figure 3.15 Fluorescence microscopy images of HeLa cells exposed to (a) 100 μg/mL Au@PLLA-PEG@PNIPAArn-PDLA nanospheres, (b) 1000 μg/mL PLLA-PEG@PNIPAAm-PDLA nanospheres, and (c) control, where green spots represent living cells and red ones represent dead cells

It is noticeable that, although cell viability of PLLA-PEG@PNIPAAm-PDLA is to

some extent lower than that of Au@PLLA-PEG@PNIPAAm-PDLA, excellent

biocompatibility is acquired. However, interestingly,

Au@PLLA-PEG@PNIPAAm-PDLA exhibits significantly enhanced cell viability even

at high doses such as 1000 μg/mL or 100 μg/mL in 48-hour incubation (p < 0.05). This

effect contributed by the Au layer is commonly observed in both cell lines. It suggests

that beneficial effect of Au layer is predominantly maximized especially at high doses.

In our study, no receptor-mediated endocytosis is proposed, and a large portion of

the nanospheres are unlikely to be internalized because no favorable size effect exists

for endocytosis. Consequently, there is no driving force to ameliorate or deteriorate cell

proliferation taking place in the cytoplasm. Nonetheless, PLLA-PEG diblock copolymer

as the outer shell component of PLLA-PEG@PNIPAAm-PDLA is presumed to reduce

the interfacial affinity between “foreign” nanospheres and protein molecules due to its

hydrophilicity. PEG constituent does not merely play a role to prevent the particles from

being anchored by proteins, but more importantly, it diminishes the opportunity of the

nanospheres attaching to the cell membrane which may be induced by various

interactions. In contrast, gold possesses high affinity to biomacromolecules including

various immunoglobulins, albumins, and fibrinogen residing in the in vivo condition in

comparison to hydroxylated surface.212 Likewise, the exterior gold layer of

Au@PLLA-PEG@PNIPAAm-PDLA is, at the first stage, supposedly coated with a

variety of proteins in the cell media, whereas PLLA-PEG@PNIPAAm-PDLA does not

undergo the same binding process. In the next stage, therefore,

Au@PLLA-PEG@PNIPAAm-PDLA surrounded by the proteins initiates the

interaction with the surface of the cell membrane. It is another key to explain how cell

proliferation can be promoted by accelerated transport of nutrients from media into

cytoplasm. Moreover, Au@PLLA-PEG@PNIPAAm-PDLA anchored on the cell

(a) (b) (c)


membrane may lower the potential in transmembrane uptake and facilitate the diffusion

rate of nutrient penetrating through the cell membrane.

3.2.2 Ferrogels

3.2.2.1 Microscopic observation

SPIONs are prepared through a high temperature decomposition method and are

obtained as colloidal particles dispersed in common nonpolar or weakly polar solvents

such as hexane, tetrahydrofuran, chloroform etc. Subsequently, these SPIONs are

transferred to an aqueous solution with the assistance of PF127. A representative TEM

image shows that SPIONs in tetrahydrofuran are highly monodisperse with an average

particle diameter of 10.6 ± 0.5 nm (Figure 3.16). We demonstrated that SPIONs are

coated by a hierarchical layer composed of PF127 and oleic acid. The PEO segments of

PF127 endow SPIONs with hydrophilicity. The resulting water-soluble SPIONs are

introduced to the concentrated Pluronic aqueous solution at low temperature (4 °C).

Upon gelation at elevated temperature, the PF127 micelles organize into an ordered

structure shown by TEM (Figure 3.16b), where because the specimen is not stained,

only the micelles with SPIONs encapsulated are visible. The size of an individual

SPION containing micelle is around 13 nm which is very similar to the reported

diameter of an empty micelle formed in a concentrated PF127 solution (> 20 %, w/v)

with similar concentration used in this work. The encapsulation of SPIONs inside the

micelles did not significantly perturb micellar morphology or size.

Figure 3.16 (a) A TEM image of SPIONs dried on a carbon-Formvar-coated 200 mesh copper grid; (b) A TEM image of the ferrogel dropped on the same type of grid at 4 °C and allowed to dry at 37 °C

40

3.2.2.2 Magnetic properties of ferrogels

The magnetic properties of both SPIONs and the ferrogel are examined at room

temperature by using a vibrating sample magnetometer (VSM). Magnetization

measurements have shown that iron oxide nanoparticles retain superparamagnetism

after incorporation into the ferrogel, indicating that the magnetic moments of particles

can easily reorient upon removing the magnetic field (Figure 3.17). Saturated

magnetizations (Ms) of SPIONs and the ferrogel are 54.1 and 39.4 Am2kg-1 respectively.

The lower Ms value of the ferrogel is expected due to the formation of the magnetic

dead layer on the surface of SPIONs after transfer to aqueous phase. The initial

magnetic susceptibility χ0 (obtained by measuring the slope at the M-H curve origin) of

the ferrogel is lower than that of SPIONs. This indicates a less effective coupling

between particles incorporated in ferrogel caused by increased first-neighbor distance.

Figure 3.17 Room temperature magnetization curves of SPIONs (-■-) and and ferrogel (-o-)

3.2.2.3 Rhoelogical properties

The rheological response as a function of temperature was examined on ferrogels

with typical PF127 concentration at 17.5% (w/v). Figure 3.18a shows the

temperature-dependant rheological data of the ferrogel, where elastic modulus G' and

viscous modulus G" are plotted as functions of temperature respectively. It was found at

low temperature the "sol" behaves as viscous liquid (G" > G'), while at high temperature

the behavior becomes elastic (G' > G'). The critical temperature at which the PF127

solution forms a gel depends on the concentration of the polymer solution. The

transition temperature was determined at 32.5 °C for the ferrogel containing 17.5% (w/v)

PF127 as indicated by the crossing of G' and G" curves in Figure 3.18a. The


photographs in Figure 3.18 show the reversible sol-gel transformation of an aqueous

solution containing 17.5% (w/v) PF127 and SPIONs (4 mg Fe/mL) before and after

heating above the critical transition temperature. The solution remains as a "sol" at room

temperature (Figure 3.18b), and gelation occurs when the solution is warmed to high

temperature, e.g. human body temperature, resulting in a solid gel as seen in Figure

3.18c. The fluidity is thereafter restored when the gel is cooled down (Figure 3.18d). In

addition, the ferrogel remains stable for at least 5 months when stored above the

gelation temperature.

Figure 3.18 (a) Rheological data obtained.from ferrogel containing 17.5% (w/v) PF127; Photographs of the ferrogel (b) below the transition temperature (c) above the transition temperature, and (d) taken for the same sample when chilled again.

3.2.2.4 Drug release studies

For in vitro release test, indomethacin (IMC) was selected as a model drug. IMC has

a very low solubility in water, whereas in the ferrogel containing 17.5% (w/v) PF127

the concentration of IMC can be significantly increased due to incorporation into

hydrophobic cores of micelles. The TEM observation (Figure 3.16b) shows that the

ferrogel constitutes of micelles arranged in a cubic liquid crystalline structure. As

42

depicted in Figure 3.19a, each micelle comprises a hydrophobic core (PPO) in which

the drug molecules are solubilized, and a hydrophilic corona (PEO) acting as the

scaffold of the ferrogel. SPIONs are encapsulated inside of some micelles during the

phase transfer of nanoparticles. The release profiles of IMC from ferrogel are shown in

Figure 3.20 both in the absence and presence of a magnetic field (MF). For the release

test without an MF (Figure 3.20a), the drug is continuously released to up to 95% in

7000 minutes (ca. 5 days). The drug release mainly depends on the diffusion of IMC

molecules from hydrophobic PPO cores through water channels in the ferrogel and also

to some extent dissolution of PF127 gel. The release profile was a quasi-linear curve

and no significant initial burst could be observed. The release test was also conducted in

the presence of a static MF (300 mT). We found that in the presence of an MF, the IMC

was released much faster. The half-time (t1/2) of drug release is reduced from 3195

minutes to 1500 minutes when recorded without the MF (Figure 3.20).

Figure 3.19 Schematic representation of ordered microstructure of the ferrogel: (a) before applying the MF, IMC drug molecules are encapsulated in the hydrophobic moiety of micelles and (b) when the MF is on, SPIONs orient and approach to each other, squeezing the micelles and leading to enhancement of IMC release. Corresponding photographs of the ferrogel in a capillary (c before and (d) after applying the MF

Figure 3.20 Release profiles of IMC from the ferrogel in PBS (pH 7.4) at 37 °C when (a) MF off; (b) MF on


Under the influence of an MF gradient, the release rate of conventional cross-linked

ferrogels is decreased as reported elsewhere. Such a different performance results from

the essential difference of the microstructure between the cross-linked ferrogel and the

PF127 ferrogel. The cross-linked ferrogels contain interlaced polymer chains with

magnetic particles embedded. Numerous channels and pores in the gel serve as a

reservoir and an escaping pathway for the drugs. When subjected to an MF, magnetic

particles are attracted, pulling the polymeric scaffold. As a consequence, the pore size is

reduced and the diffusion of the drug through the pores is hindered. On the other hand,

the PF127 ferrogel contains large amount of close-packed micelles and therefore no

pore opening and closure is expected as what cross-linked ferrogels behave. The

hydrophobic drugs are encapsulated in PPO cores and in between the micelles exists the

interconnecting aqueous moiety, functioning as a pathway for diffusion of drug

molecules. As depicted in Figure 3.19b, while applying an MF, hydrophobic cores in

the ferrogel are "squeezed" when SPIONs tend to orient and approach to each other, so

that the local concentration of IMC is increased, producing a large concentration

gradient between PPO cores and water channels in the PF127 ferrogel, pumping out

many more drug molecules. Therefore, under the applied field, the release rate is

enhanced due to the constriction of the PF127 ferrogel. To further clarify the release

mechanism, we introduce a small amount of PF127 ferrogel into a capillary with the

inner diameter of 1 mm and measured the length of the gel before and after applying an

MF (Figure 3.19c and d). Being exposed to the MF (300 mT) for 2.5 hours, the length

of the PF127 ferrogel decreases by 7% from 2.39 cm to 2.22 cm. Taking into account

that about 80% of the gel is incompressible water, the volume of polymeric matrix is

decreased by 35% when subjected to the MF. It is therefore expected that such a

dramatic field-dependent volume change within a few hours gives rise to the

enhancement of drug release rate at the initial period of the release test.

3.3 Surface modification of SPIONs

3.3.1 PLLA coated SPIONs


Figure 3.21a is a representative TEM image of as-synthesized SPIONs. Particles are

of spherical shape with the average diameter of 11.5 nm (σ ~ 3.8%). After ligand

44

exchange with ricinoleic acid (ROA), the size of SPIONs is 11.8 nm (σ ~ 4.6%) (Figure

3.21b). Figure 3.21c is a TEM of PLLA@SPIONs. A homogenous layer of PLLA can

be distinguished as a light contrasted region surrounding the SPION cores (inset of

Figure 3.21c). The coating layer is 1.2 nm thick and the cores size is kept constant. The

phase of SPIONs is identified by XRD pattern. The peaks are labeled with the indexed

Bragg reflections of the inverse spinel structure and the particles are confirmed to be

highly crystalline. The particles size determined by TEM agrees well with the crystallite

size calculated from XRD data using the Debye-Scherrer equation (11 nm), showing

that the particles are single crystalline.

Figure 3.21 TEM images of SPIONs coated with (a) oleic acid, (b) ricinoleic acid and (c) a thin PLLA shell

3.3.1.2 Characterization of surface coating

PLLA@SPIONs are dispersible in many organic solvents such as THF, chloroform

and even acetone which is completely incompatible with either OA@SPIONs or

ROA@SPIONs, indicating the change of surface chemistry. Such change is verified

analytically by virtue of FTIR spectroscopy. The FTIR spectra of OA@SPIONs,

ROA@SPIONs and PLLA@SPIONs are presented in Figure 3.22. Compared to the

spectrum of OA@SPIONs, a new strong absorption band appears at 1241 cm-1 in the

spectrum of ROA@SPIONs which can be attributed to the symmetrical stretching of the

dangling hydroxyl group in ROA. In the spectrum of PLLA@SPIONs a peak at 1734


cm-1 is found to suggest the presence of ester bonds in PLLA chains. To further ensure

the coating layer is actually PLLA, FTIR spectra of the purified polymer from

PLLA@SPIONs and the commercial PLLA samples are compared, where exactly the

same spectra were obtained. The composition of the surface coating PLLA is also

confirmed by 1H NMR and 13C NMR (Figure 3.23). 1H NMR (500 MHz, CDCl3):

1.59-1.62 (d, J = 8.0 Hz, 3H), 5.15-5.21 (dd, J = 8.0 Hz, 1H). 13C NMR (500 MHz,

CDCl3): d 16.6, 69.0, 169.6. The peeled-off PLLA layer has been further characterized

by means of GPC and DSC. The molecular weight and polydispersity of PLLA surface

layer were quantified using GPC. The PLLA coating layer has a molecular weight of

10.02 kDa (Mn) with a polydispersity of 1.32. DSC was used to further characterize the

properties of PLLA coating layer (Figure 3.24). The data show that the Tg of PLLA

coating layer is 54.0 °C, Tc is 107.4 °C and Tm is 147.4 °C which are very close with the

data of commercial PLLA sample from PURAC company.

Figure 3.22 FTIR spectra of SPIONs coated with (a) o1eic acid, (b) ricinoleic acid and (c) a thin layer of PLLA

Figure 3.23 (a) 1H NMR and (b) 13C NMR of PLLA separated from PLLA@SPIONs

46

Figure 3.24 DSC curve of PLLA separated from PLLA@SPIONs

Figure 3.25 TGA data of (a) oleic acid coated SPIONs and (b) PLLA@SPIONs

3.3.1.3 Magnetic properties of PLLA@SPIONs

Magnetic properties of OA@SPIONs and PLLA@SPIONs were measured at 298 K

by VSM (Figure 3.26). The values of saturated magnetization (Ms) are 45.6 and 38.6

Am2kg-1 respectively. It can be seen that the superparamagnetic property is retained

after the coating process. The decrease of saturated magnetization results from the

PLLA coating. The amount of grafted PLLA has been measured by TGA (Figure 3.25).

The weight loss of OA@SPIONs and PLLA@SPIONs are 2.7 and 15.1 % respectively.

The percentage of the magnetic core decreases by 14.6% after PLLA coating according

to TGA results, which, in turn, pulls down the saturated magnetization by 18.1%.


-1000 -500 0 500 1000

-40

-20

0

20

40

-100 -50 0 50 100

-20

0

20

Mag

netiz

atio

n / A

m2 kg

-1

Applied field / mT

Figure 3.26 Magnetization curve of oleic acid coated SPIONs and PLLA@SPIONs

3.3.1.4 Drug loading and release

The model drug IMC is encapsulated in PF127@SPIONs and

PF127@PLLA@SPIONs. The loading efficacy is determined for both of the particles,

56.5±3.4% for PF127@SPIONs and 78.1±2.8% for PF127@PLLASPIONs. Higher

loading efficacy is resulted from the PLLA coating layer which aborbs more IMC

molecules than the ROA monolayer in PF127@SPIONs. The drug release half time was

225 minutes for PF127@PLLA@SPIONs compared to that of PF127@SPIONs which

is only 100 minutes (Figure 3.27). A rapid increase of drug concentration is recorded in

the case of PF127@SPIONs. PLLA layer in PF127@PLLA@SPIONs plays a role in

retention of drug molecules., As a result, the release rate is slowed down.

Figure 3.27 Release profile of IMC loaded in (a) PFI27@SPIONs and (b) PF127@PLLA@SPIONs

48

3.3.2 Phase transfer of SPIONs from organic phase to aqueous phase


Excess of ligand molecules were removed by dialysis against 1L water for 2 days.

Upon addition of TMAOH and HNO3, charges on the NPs surface were generated,

producing electrical double-layer which generated inter-particle repulsion, as illustrated

in Figure 3.28. In the case of exchange with TSC, TMAOH were replaced with

negatively charged citrate groups which can be evidenced by the change of ζ-potential

values from +52 mV to -40 mV. DMSA, with two mercapto groups and two carboxylate

groups, can form a strong surface layer of NPs and confer to the NPs negative charges

due to the carboxylate groups.

Figure 3.28 Schematic illustration of'TMAOH and HNO3 coated SPIONs

Spherical SPIONs with 12 nm in diameter have been synthesized via the high

temperature decomposition method and have ordered hexagonal array. After ligand

exchange with TMAOH, TSC, HNO3 and DMSA, particles no longer stayed in an array

but randomly distributed with a slight aggregation (Figure 3.29). Inter-particle distance

was decreased compared to the one before phase transfer where hydrophobic interaction

between oleic acid inolecules impulse from each other. Smaller hydrophilic molecules

such as TMAOH, TSC, HNO3 and DMSA would not sufficiently protect the particles

from attaching to each other. The stability of such dispersions mainly depends on the

electrical double-layer on the surface of the particles which can be greatly affected by

pH and ionic strength of the dispersions. To meet the requirement of biomedical

applications, either in vitro or in vivo, neutral pH of particle suspension is a primary

demand. Once the electrical double-layers get perturbed, the colloidal systems may lose


the stability and the precipitation occurs. Even with a great caution when tuning the pH,

aggregation is inevitable.

Figure 3.29 TEM iniages of (a) as-synthesized SPIONs, SPIONs coated with (b) TMAOH, (c) TSC, (d) HNO3 and (e) DMSA

Figure 3.30 Schematic illustration of PF127@SPIONs

SPIONs have been successfully transferred from THF to aqueous solution by PF127.

SPION after phase transfer are schematically illustrated in Figure 3.30. The TEM image

shows that the SPIONs are monodisperse in water (Figure 3.31). Comparative

photographs (inset of Figure 3.31) show the complete phase transfer from organic to

aqueous phase. There is no significant change in the size and shape of particles as a

result of the phase transfer. The particles possess homogeneous morphology in a regular

round shape with an average diameter of 10.1 nm (σ = 4.5%). The obtained particles in

aqueous solution can stay stable in a broad pH range, indicating a great possibility for in

vitro administration.

50

Figure 3.31 SPIONs dispersed in water after phase transfer (inset is a photograph showing SPIONs have been fully transferred to water phase)

3.3.2.2 Magnetic properties of PF127@SPIONs

The magnetic properties of the SPIONs before and after phase transfer have been

studied at room temperature by using a VSM. The magnetization of as-synthesized

SPIONs, TMAOH@SPIONs, DMSA@SPIONs and PF127@SPIONs were measured as

a function of an applied magnetic field. The representitive curve of PF127@SPIONs is

shown in Figure 3.32. The magnetic cores of TMAOH and DMSA coated particles are

from the same batch, while the magnetic cores of as-synthesized SPION and

POA@SPION are from another batch. None of the samples show a hysteresis loop,

indicating NPs retain superparamagnetic properties. The obtained curves can be

described by the Langevin equation:

M(x) = Ms (cothx- (1/x)) (3.1)

where x = μH/kBT, Ms is the saturation magnetization, μ is the magnetic moment of a

single particle, H is the applied field, kB is Boltzmann’s constant, and T is the absolute

temperature.213 The Ms of as-synthesized SPIONs and PF127@SPIONs are 41.0 and

31.7 Am2kg-1 respectively. The Ms of TMAOH@SPIONs and DMSA@SPIONs are

53.5 and 51.2 Am2kg-1. Fitting the curves in Figure 3.32 to the Langevin equation, we

obtained diameters of the magnetic cores. The size can be obtained through the

following expressions:

0118

→= Hs

)dHdM(

HkTDπρ

(3.2)

where ρs is the specific magnetization of the iron oxides. The diameters of

as-synthesized SPIONs, PF127@SPIONs, TMAOH@SPIONs and DMSA@SPIONs

are 9.98, 11.1, 12.88 and 12.78 nm, which is rather close to the value obtained from

both TEM and XRD data.


-1000 -500 0 500 1000-60

-40

-20

0

20

40

60

-100 -50 0 50 100-30-15

01530

Mag

netiz

atio

n (A

m2 kg

-1)

Applied field (mT)

Figure 3.32 Room temperature magnetization curve of as-synthesized SPIONs (-●-) and PF127@SPIONs (-○-)

3.3.2.3 Cytotoxicity test of PF127@SPIONs

The PF127 block copolymer is recognized as highly biocompatible as approved by

the Food and Drug Administration214, 215 and have long been used in experimental

medicine and pharmaceutical sciences.54, 216-218 Samples of PF127@SPIONs with three

different iron concentrations, ranging from 1 to 100 μg/mL, were incubated with the

HeLa and MCF-12A cell lines for 48 hours. The viability of the cells was evaluated by

MTS and MTT assays, and the results showed no significant cytotoxicity to the

mitochondria function for both cell types (Figure 3.33).

Figure 3.33 Viability of HeLa and MCF-12A cells exposed to PF127@SPIONs at various iron concentrations for 48 hours

3.3.2.4 Magnetic resonance studies

The relaxation times of PF127@SPIONs, T1 and T2 were measured at 0.47 T (20

MHz proton Larmor frequency, 37°C) and 1.41 T (60 MHz proton Larmor frequency,

37 °C). The imaging contrast arises from the ratio between r2 and r1 values. For a T2

52

contrast agent, the higher the r2/r1 ratio the better the contrast efficacy. The relaxivities

of PF127@SPIONs are summarized in Table 3.3 and are compared to those obtained

under the same conditions using commercial T2 contrast agent Resovist® (Schering AG,

Germany). Noticeably, the r2/r1 ratios of PF127@SPIONs are 6.6 and 15.8 folds higher

than those of Resovist at 0.47 T (20 MHz) and 1.41 T (60 MHz) respectively, indicating

that PF127@SPIONs could be considered as a T2 contrast agent with high efficiency.

Because the magnetic cores of PF127@SPIONs and Resovist are produced via different

methods, it is also necessary to exclude the impact of the magnetic cores. Therefore, we

also investigated the relaxivities of SPIONs transferred to aqueous phase by coating

with small molecules e.g. TMAOH and DMSA. From Table 3.3, it is seen that the

relaxivities and r2/r1 ratios of TMAOH@SPIONs and DMSA@SPIONs are similar with

those of Resovist. Hence the enhancement of the r2/r1 ratios can be most likely

attributed to the unique surface coating of PF127@SPIONs. The surface coatings,

hydrodynamic diameters and relaxivities of PF127@SPIONs, Resovist and Feridex®

(Advanced Magnetics, Cambridge, Massachusetts) are summarized in Table 3.3. The r1

of PF127@SPIONs is lower than those of Resovist and Feridex, while the r2 of

PF127@SPIONs is higher than both commercial contrast agents. As a result, r2/r1 ratio

of PF127@SPIONs is much higher (0.47 T, 20 MHz) than the commercial contrast

agents.

Table 3.3 Surface coatings, mean hydrodynamic diameters and relaxivities ratios of SPIONs with different coatings, Resovist and Feridex measured at 20 and 60 MHz (0.47 T and 1.41 T) in water (37°C).

Sample name Surface coating Mean hydrodynamic diameter (nm)

20 MHz 60 MHz

r2/r1 r2/r1

PF127@SPIONs PF127/OA 71 47 229

TMAOH@SPIONs TMAOH - 6.98 17.96

DMSA@SPIONs DMSA - 8.46 29.40

Resovist Carboxydextran 65 7.10 14.45

Feridex* Dextran 72 4 -

Note: The unit of ri (i = 1, 2) is s-1mM-1. * From ref. 219


Figure 3.34 Scheme of surface coating of PF127@SPIONs. Out of the dashed line is the hydrophilic domain and inside of the dashed line is the hydrophobic domain

Considering that Resovist and Feridex have a thick layer of carboxydextran and

dextran respectively, it is therefore possible that the PF127/OA amphiphilic double

layer may play a dual-role so as to enhance the T2 relaxation rate and decrease the T1

relaxation rate at the same time. As shown in Figure 3.33, the hydrophilic PEO domain

would facilitates the diffusion of large quantity of water molecules close to the magnetic

core of PF127@SPIONs, which is in turn enhances the T2 relaxation rate.220

Furthermore, it is reported that T2 relaxation rate of ice is very high.221 Similarly, in the

hierarchical structure, the mobility of water molecules in the diffusing layer is restricted

due to the hydrogen bonding with hydrophilic chains of PEO blocks, turning into large

amount of “ice-like” water. Therefore, large r2 relaxivity of PF127@SPIONs is owing

to the “ice-like” water in the hydrophilic blocks. The dimension of “diffusing layer” of

PF127@SPIONs can be characterized as the mean hydrodynamic diameter which is

similar with the other contrast agents (Table 3.3). It was reported that the r2 values can

be greatly increased by clustering T2 contrast agents in reservoirs such as liposomes or

cells.222 This effect results from the intrinsic mechanism of spin-spin relaxation, which

relies on the local concentration of the contrast agents. However, PF127@SPIONs

shows no enhancement to the local Fe concentration because it forms a well-dispersed

and stable ferrofluid as been evidenced by TEM studies (Figure 3.31). Moreover, the

smaller r2 value compared with those exceedingly high ones for clustered particles in

micelles reveals additional evidence that PF127@SPIONs are not agglomerated.223 T1

54

contrast agents require close contact with the water molecules to effectively generate T1

contrast due to the inherent mechanism of spin-lattice relaxation. As dipicted in Figure

3.34, the magnetic cores of PF127@SPIONs are segregated from the exterior water

molecules by a compact hydrophobic layer comprised of PPO blocks and oleic acid

molecules. Water molecules diffused into PEO domain can hardly come into contact

with the magnetic core because of the hydrophobic nature of the inner layer. Such a

hierarchical hydrophobic-hydrophilic coating of PF127@SPIONs is of substantial

difference from the conventional T2 contrast agents whose coating layers are entirely

hydrophilic so that the diffusion of water molecules to magnetic core is not restricted.

The covalent bond between carboxylic group of oleic acid and iron oxide ensures

complete coverage of particle surface. The subsequent incorporation of PF127

molecules further increases the thickness of this hydrophobic “isolating layer”. Such a

structure comprised of PPO blocks and oleic acid molecules would effectively hinder

the external water molecules from contact with the magnetic core, which in turn

kinetically limit the spin-lattice relaxation rate.

3.3.3 Thermosensitive nanocrystals

3.3.3.1 Structure observation and magnetic properties

TEM images of SPIONs are shown in Figure 3.35. The diameter of SPIONs in

chloroform is 9.9 ± 0.2 nm (Figure 3.35a). After coating with PMAO-graft-PNIPAAm,

the SPIONs are no longer soluble in chloroform but highly soluble in water. As

expected, no size or morphology alteration can be observed for SPIONs. The hairy

PNIPAAm coating is not visible unless the TEM specimen is stained. After negatively

staining the specimen, the polymeric coating can be clearly distinguished. It is also

verified that a full coverage of polymer has been achieved (Figure 3.35b). The water

dispersions of SPIONs are very stable and no coalescence of particles can be seen in

TEM images due to steric hindrance of the polymeric coating. HRTEM images and the

corresponding Fast Fourier transform (FFT) patterns show that the SPIONs are single

crystalline and structurally uniform both before and after coated with

PMAO-graft-PNIPAAm. The interplanar distances are measured and indexed to cubic

magnetite (insets of Figure 3.35a and b).83

The magnetic properties of SPIONs are examined by VSM at room temperature.

Figure 3.35c shows the magnetization of SPIONs versus applied field by cycling the


field between -1 T and 1 T. Both the coated and uncoated SPIONs are

superparamagnetic at room temperature showing no remanence or coercivity (inset of

Figure 3.35c). After coating and transfer to the aqueous phase, the Ms value of SPIONs

drops from 46.16 Am2kg-1 to 41.05 Am2kg-1, and the initial susceptibility decreases as

well. Fitting the curve to Langevin equation, the diameter of the magnetic cores before

coating PMAO-graft-PNIPAAm can be derived as 9.9 nm which is in a good agreement

with that measured under TEM. Since magnetite has a molar magnetic moment of 4.1

MB while that of maghemite is only 2.3 MB,224 one possible explanation for the decline

of Ms is partially oxidation of magnetite to maghemite on the surface of SPIONs when

transferred to aqueous solutions.83 However, HRTEM studies cannot distinguish the

difference between the magnetite and the maghemite because they share very close

inverse spinel crystal structures.

Figure 3.35 TEM, HRTEM images and FFT pattern of HRTEM of SPIONs coated with a) oleic acid and b) PMAO-graft-PNIPPAm copolymer, negatively stained by ammonium molybdate(2 wt.-%); c) Room temperature magnetization curve of as-synthesized SPIONs (-■-) and SPIONs coated with PMAO-graft-PNIPAAm (-○-)

56

3.3.3.2 Temperature-sensitive properties

The LCST of the PNIPAAm aqueous solution was determined using DSC. A

significant endothermic peak at 31.6 °C indicates the volume phase transition of

PNIPAAm (appended paper VI). Since PMAO-graft-PNIPAAm is poorly soluble in

water, we could not measure the LCST of the aqueous solution of the copolymer

directly. We thus measured 20 μL of water suspension of SPIONs coated with

PMAO-graft-PNIPAAm and observed an endothermic peak at 28.7 °C during the

heating section of a heating-cooling cycle (Figure 3.36). An exothermic peak on the

cooling section appears at 26.5 °C. Such hysteresis in phase transition can be due to

kinetic effects. Nevertheless, a good reversibility of the phase transition is demonstrated

by five heating-cooling cycles (Figure 3.36). The LCST of SPIONs encapsulated with

PMAO-graft-PNIPAAm is lower (2.9 °C) than that of the pure PNIPAAm. This could

be attributed to the reduction of conformational freedom of PNIPAAm chains as a

consequence of grafting to PMAO backbone and further immobilization on the SPIONs.

Figure 3.36 Transition temperatures determined by five heating-cooling cycles on a sample of SPIONs coated with PMAO-graft-PNIPAAm in an aqueous solution. LCST is determined on the heating ramp as 28.7 °C.

To further show the thermosensitivity of the PMAO-graft-PNIPAAm polymer

coated SPIONs, we measured the hydrodynamic particle size versus temperature. As

shown in Figure 3.37a, at 20 °C the particle size is 26.7 nm, and at 31 °C the particle

size is reduced to 22.9 nm due to the collapse of the PNIPAAm chains. Although the

LCST determined by DSC is 28.7 °C, the lag of response in hydrodynamic size change

might be due to the slow equilibrium of the temperature in the measuring cell. As the

temperature is increased, a sharp change in the particle size is observed and the final

hydrodynamic size is 827 nm at 45 °C indicating that large aggregates are formed


owing to aroused hydrophobicity of the coating layer. This behavior is potentially useful

for magnetic separation using superparamagnetic nanoparticles whose size is usually

below 20 nm. For such small magnetic nanoparticles, the interaction with the magnetic

field is very weak. For instance, PMAO-graft-PNIPAAm coated SPIONs in water

solution can stay stable for hours when subjected to a magnetic field of 300 mT (Figure

3.37b). It is obviously not suitable for magnetic separation. To date, mainly large

magnetic particles or beads are employed for magnetic separation because of the

demand of rapid response and fast migration under a magnetic field. However, use of

large particles has a drawback of compromising the total surface area, leading to a

limited efficiency of conjugation and/or adsorption of materials that need to be

separated. Maintaining the small size, the thermosensitive SPIONs respond to the

external magnetic field rapidly when the temperature is above the LCST due to the

formation of large aggregates (Figure 3.37c). This transition is reversible so that when

the temperature is decreased, the SPIONs resume the water dispersibility.

Figure 3.37 a) Hydrodynamic diameter of SPIONs coated with PMAO-graft-PNIPAAm versus temperature. Schematic illustration shows the dispersion state of particles at low and high temperature respectively; b) Photographs of SPIONs coated with PMAO-graft-PNIPAAm suspension under the influence of a magnetic field (300 mT) taken at 20 °C and 40 °C.

58

3.3.3.3 Thermosensitive Au nanocrystals

The encapsulation of NCs using PMAO-graft-PNIPAAm is general for NCs coated

with a layer of lipophilic ligands. Here, we demonstrated its feasibility with Au NCs.

The obtained oleylamine coated Au NCs were soluble in chloroform (Figure 3.38a).

Surface modification of the Au NCs with PMAO-graft-PNIPAAm was carried out

following the same procedure as that for SPIONs. TEM images confirm the particles

still maintain the original size and shape (Figure 3.38b). UV-Vis spectra were recorded

for Au NCs coated with PMAO-graft-PNIPAAm between 24 °C and 44 °C (Figure

3.38c). The peak of surface plasmon band is 522 nm. No red shift was found as a result

of temperature change. Because the lower band absorbance increases as the temperature

is elevated, it is not possible to observe blue shift of plasmon band when the

temperature is above 30 °C either. This finding appear to be not in agreement with that

was previously reported. For instance, a red shift of 10 nm was observed for Au

nanoparticles encapsulated in PNIPAAm microgels when the temperature is

increased.163 The red shift was resulted from the increase in the local refractive index

induced by the collapse of the PNIPAAm shell. In our case, PNIPAAm chains on the

surface of Au NCs are arranged in a loosely compact hairy way in comparison with the

dense microgel. Therefore the conformational change of PNIPAAm chains is not

significant enough to alter the local refractive index in the proximity of NCs. Figure

3.38d shows that the aggregation state of these Au NCs are reversible over multiple

cycles, revealing the robust nature of the smart PMAO-graft-PNIPAAm coatings.


Figure 3.38 TEM images of Au NCs coated with a) oleylamine and b) PMAO-graft-PNIPAAm copolymer; c) UV-Vis absorption spectra of Au NCs coated with PMAO-graft-PNIPAAm at different temperature, between 24 °C and 44 °C; d) changes of transmittance at λ = 720 nm during heating and cooling cycles between 24 °C and 44 °C

60

4 Summary and conclusions

Essential components to build multifunctional drug delivery systems (DDS)

including biocompatible and biodegradable drug carriers, thermosensitive polymers,

high performance MRI T2 contrast agents, have been developed in this thesis through

different methodologies.

Biodegradable polymers were selected as the backbone to form the nanospheres.

Temperature-responsive polymer was used as a key component to fabricate the

thermosensitive DDS. The phase transition temperature, lower critical solution

temperature (LCST), could be tuned to 36.7 °C, which was very close to the human

body temperature, by controlling the ratio between two blocks in the copolymer. Drug

release performance of such thermosensitive DDS have been examined by employing

the bovine serum albumin as a model drug. The results show an enhanced releasing rate

when the temperature was elevated above LCST. Poly ethylene glycol was used to

cover the surface of drug carriers in order to prevent unspecific binding with proteins in

the plasma, thus avoiding uptake by immune system. As an alternative, nanospheres

coated with small gold NPs have shown higher cell viability compared with those

without gold coating. The result implies that gold can be a safe component in

fabrication of multifunctional DDS. As an extension of the concept, we also attempt to

synthesize a new thermosensitive copolymer for coating the inorganic NCs so that

thermosensitive NCs can be obtained in a robust way.

Monodisperse superparamagnetic iron oxide nanoparticles (SPIONs) were prepared

through high temperature decomposition method in an organic solution. The obtained

particles were highly crystalline. By changing synthetic parameters, particle size and

morphology could be readily manipulated. Particle growth was investigated by means

of transmission electron microscope and synchrotron radiation X-ray diffraction. The

optimum reaction time and temperature to obtain SPIONs with desired shape and size

were determined based on the findings. The SPIONs were then modified with

poly(L,L-lactide) (PLLA) to use as a drug carrier and an MRI contrast agent.

Nevertheless, for biomedical applications, hydrophobic particles prepared in organic

solutions should be transferred to aqueous solutions. We successfully transferred

SPIONs with assistance of small molecules such as TMAOH, TSC, HNO3 and DMSA.

The major disadvantage of these small capping agents was that the particles after phase


transfer were not stable over broad pH range, especially at physiological pH.

Amphiphilic Pluronic F127 copolymer was used as a phase transfer agent that can form

stable SPIONs suspension over large pH range. The surface of obtained particles was

covered with poly(ethylene oxide) domain to obtain hydrophilicity. Most interestingly,

besides other promising feature for DDS, Pluronic F127 coated SPIONs showed a

remarkably high r2/r1 ratio, indicating that these particles were better T2-weighted

contrast agents than the commercially available ones. The relaxometric data were

evaluated and mechanism of such enhancenlent was discussed.

On the basis of present results, this work is one step toward the ultimate goal of "the

magic bullet" in the field of multifunctional DDS.

62

5 Future work

Researchers have been trying to figure out how a future therapeutic device looks like.

It should be in a form of minimum toxicity with a low administration frequency. To

eliminate any side-effects, it should be equipped with homing devices in order to deliver

drugs to pathological sites and release the payload in a "switch-on/off" manner. Ideally,

one can even track this device with an accessible imaging modality. This thesis has

shown possibility to achieve this goal with regard to the following aspects: on-demand

release and incorporation of imaging markers. However, there are challenging problems

need to be addressed in the future.

Regarding the PF127 ferrogel delivery system, instability of the gel was noticed once

came into contact with aqueous solutions. Cross-linking the PF127 ferrogel through

acrylate modification can enhance the mechanical toughness and the gel shows

resistance to dissolution. Nevertheless, PF127 ferrogel losses the unique injectability

when the crosslinking procedure needs to be carried out before administration.

Therefore, it is desired to develop an in situ mild crosslinking approach.

Uncontrolled diffusion of the drug has been a problem for DDS development. It is

nearly inevitable in the case of simple encapsulation of the drug in either nanospheres or

gel form. To reach the ideal "on/off" manner, the drug molecule could be connected

covalently to the building blocks of DDS through a switchable molecule which

precisely undergoes a transition when subjected to external signals.

Another direction growing rapidly is metal-organic frameworks (MOFs).233 MOF is

a new class of hybrid materials composed of metal ions and organic bridging ligands,

have emerged as a promising platform for drug delivery, owing to the high drug

loadings, biodegradability, and versatile functionality.234 The vast playground of MOFs

allows to incorporate imaging markers and environment-sensitive triggers in a more

robust way.


6 Acknowledgements

At the beginning of the most important chapter, I would like to thank my supervisor, Prof. Mamoun Muhammed, for showing me the world of acedamic research. His enlightening ideas and kindness is always strong support throughout these years.

I have to say sincere thanks to Dr. Maria Mikhaylova, for her great patience, scientific discussion and professional guidance on my first step of research. I want to thank Yun Suk for his help and discussions. I wish him a successful career as a musican and hope I can listen to Lucid Fall’s live concert someday in the future. Thanks to Do Kyung for his encouragement and invaluable advices. I wish him an outstanding academic future and a happy family. I would like to thank Muhammet for his kind help from research to everyday life. I wish him a great success in the scientific career and a happy family. I am extremely grateful to Abhi for the thorough linguistic check of the thesis. I will never forget the joyful time spent with my dearest fellows in FNM (previously Matchem), Abhi, Andrea, Andrey, Carmen, Fei, Germán, Mazher, Mohsin, Nader, Othon, Rubina, Salam, Sverker, Terrance, Xiaodi, Ying, and Zhang for their priceless friendship.

I am grateful to my coworkers from Karolinska for the fruitiful collaborations: Dr. Helen Vallhov, Dr. Susanne Gabrielsson and Prof. Annika Scheynius from Department of Medicine; Dr. Erika Witasp, Jingen Shi, Neus Feliu Torres and Prof. Bengt Fadeel from Institute of Environmental Medicine.

I would also like to thank Dr. Sophie Laurent, Dr. Alain Roch, and Prof. Robert N. Muller from University of Mons-Hainaut in Belgium for the productive collaboration and great help on MR experiments. Many thanks to Johanna Öberg, Ivan Bednar, Rouslan Sitnikov and Prof. Christian Spenger at MR Centrum of Karolinska University Hospital for their help with MRI measurements. Thanks to Prof. Josep Nogues at Universitat Autonoma de Barcelona for helping on the magnetic measurements. Thanks to Dr. Tiezhen Ren and Prof. Xiaodong Zou at Stockholms Universitetet for synchrotron XRD measurements.

I would like to express my special thanks to Peter, my best friend for 10 years since the college time, for all the joyful moment and great help. I wish him a shining future and a happy family.

Last but not least, I would like to thank my parents for their endless support and encouragement, my wife Chuanmin for her deep love, and my adorable daughter Ruoshui for sharing my time playing and enjoying the life outside of the work.

64

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Environment-Sensitive Multifunctional Drug Delivery Systems300480/FULLTEXT01.pdf · Drug delivery systems (DDS) with multiple functionalities such as environment-sensitive drug release