Optical Materials 35 (2013) 372–382

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Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Preparation and characterization of transition metal based organo-inorganichybrid optical materials for bioassay fluorescence sensor probe

Asok K. Dikshit ⇑, Jijo LukoseFiber Optics and Photonics Division, Central Glass and Ceramic Research Institute (CGCRI), 196, Raja S.C. Mullick Road, Kolkata 700 032, India

a r t i c l e i n f o

Article history:Received 21 December 2011Received in revised form 10 September 2012Accepted 11 September 2012Available online 26 October 2012

Keywords:Transition metal ionsHigh refractive index organic dopantsNanoparticlesFluorescenceStructure and morphology

0925-3467/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.optmat.2012.09.007

⇑ Corresponding author. Present address: BRIC, Inc.61 B.L. Shah Road, New Alipore, Kolkata 700 053, Indi30 36 8928; fax: +91 33 2473 0957.

E-mail address: adikshit21@yahoo.com (A.K. Diksh

a b s t r a c t

High refractive index (H.R.I.) tetravalent transition metal ion coordinated with organic co-dopant to formcharge transfer complexes into the optical acrylic preform. They have been fabricated with different con-centrations of transition metal ions and H.R.I. organic co-dopant and characterized. The improved photophysical properties, thermal, structure, optical, and morphology of transition metal ions into the opticalacrylic preform hybrid matrix were analyzed. Transition metal ions are dispersed inside the preformmatrix with nanoparticles of mesophase structural organization through charge transfer complex forma-tion. It gives a crucial role of nano-interface properties on fluorescence bright imaging and less life time.This hybrid functional optical hybrid material has high potential use in bio-assay sensor probe.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Researchers are trying to develop good quality optical materialsto fabricate optical perform rod to obtain bio-sensor probe usingdifferent fluoro probe. They tried to explore silica nanoparticles, la-ser dyes and others rare earth elements as fluroprobe into the opti-cal matrix. The use of high refractive index transition (H.R.I.)transition metal ions of zirconium (Zr) and hafnium (Hf) has goodpotential. They have unique properties like low phonon energy,high refractive index, high thermal, chemical stability, high dielec-tric constant, improved mechanical strength, low electrical con-ductivity, low toxicity, hard coating and resistance to neutron. Ithelps to make new dimension of optical preform with microndimension optical tip for potential use in bio-sensor probe.

Dopant tetravalent transition metal ion and H.R.I. organic co-dopant are coordinated to form inorganic–organic hybrid compos-ite materials inside the functional acrylic matrix. This functionalhybrid optical material can provide more interface properties ofboth organic and inorganic by controlling their components ratio.Zr and Hf are inorganic high refractive index (H.R.I.) transition me-tal ion coordinated with co-doped H.R.I. organic dopant in the ac-rylic matrix, produces a charge transfer complex of new properties.It can be tailored for sensor probe application using luminescence

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fluorescence properties. The complexes of Zr/Hf are formed at lowpH of solution with the participation of inner orbital electron [1–3].

The potentiality of high refractive index organic co-dopant inthe acrylic matrix is due to good miscibility, higher refractive indexthan acrylic matrix itself; higher boiling point that improved fiberdrawing temperature of fabricated preform up to 200 �C, largermolecular volume of benzene ring contains in the co-dopant thanthat of the monomer MMA and secondary interaction with thefunctional acrylic preform. The diffusion of dopant molecules de-pends on mobility of chain segment into the matrix which influ-ences the glass transition temperature (Tg) and co-dopants arebehaves like plasticizer in the matrix [4].

Bally et al. [5] has reported that tetravalent Zr-metal ion canform inorganic three dimensional well organized stable networkstructure with phosphate and phosphonate ion that can bind withDNA and nucleic acid. The tetravalent Zr-metal ion binding throughnon-covalent specific interaction with biomolecules will providethree dimensional network structures for potential bio-sensing.

Plucinski et al. has [6] reported hybrid inorganic–organic com-posites of SiC nanocrystals embedded within the photopolymeroligoether acrylate matrices. They observed that it is a new prom-ising photo induced nonlinear optical effects material.

Zheng et al. [7] have showed that hollow mesoporous zirconia(hm-ZrO2) inorganic nanocapsules are highly bio-compatible andeffective carriers loading of cancer drugs, easy surface conjugationwith biomolecules and excellent stability. Hafnium (Hf) in hafniumoxide (HfO2) has wide band gap, high refractive index, resistance toirradiation, unusual ability for absorbing high-energy neutrons,

A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382 373

resistance to corrosion, and high dielectric constant tough ceramicmaterials. The important properties are belonging to high trans-mission co-efficient with almost zero losses, high laser damagethreshold and transparency with IR to UV light.

Dong et al. [8] has shown that Zr4+ ions in composite matrixinteract with both the phosphate moiety and nitrogenous basesof DNA molecules. This composite film could preserve the activityof biomolecules, and it is a good material for immobilizing biomol-ecules and can be used for constructing biological sensors.

The choice of H.R.I. organic dopants of benzyl benzoate (BZZ)and triphenyl phosphate (TPP) for the interaction with H.R.I. tran-sition metal ion zirconium and hafnium dopant into acrylic matrixforms a charge-transfer complex through coordination with theirfunctional moieties that influences the photo physical properties.

The purpose of this work is to report synthesis of new class oforganic–inorganic hybrid composite optical fluroprobe materialsthat enhances fluorescence signal. High refractive index (H.R.I.)nanodimension tetravalent transition metal ion zirconium andhafnium with organic co-dopant forms complexes into the acrylicmatrix. The interaction of transition metal ions with H.R.I. organicdopants forms charge transfer complexes through coordinatedbonds. They have been characterized in terms of thermal, struc-ture, morphology, and spectroscopy to optimize best opticalproperties.

The nanodimension transition metal ions are dispersed into theoptical preform matrix that binds through non-covalent interac-tion at specific locations on biomolecules like protein and nucleicacid. They produce three dimensional network structures thatinteracted with biomolecules. The dispersed nanodimension tran-sition metal ions are producing bright imaging luminescence forpotential bioassay sensor probe.

Acrylic Tube

MMA monomer at surface-interface of PMMA tube

Organic dopant at core

Monom

er free radical

Organic dopant

Fig. 1. Schematic representation of interfacial gel polymerization during preformfabrication inside acrylic tube.

2. Experimental

2.1. Fabrication

Interfacial-gel polymerization techniques have been used tofabricate high refractive index transition metal ions, zirconium(Zr) carboxy-ethyl acrylate 60% in (n-propanol), Zr (CH2CHCO2-

CH2CH2CO2)4 and Hf carboxy-ethyl acrylate 60% in 1-butanol, Hf(CH2CHCO2(CH2)2CO2)4 were doped with methylmethacrylate(MMA) monomer inside the acrylic tube. The preform rod wasfabricated after the polymerization of monomer. Poly methylmethacrylate (PMMA) was chosen as host optical material be-cause of its good optical quality in terms of refractive index(1.48) and good compatibility with high refractive index (H.R.I.)organic co-dopants. A solution mixture was obtained withMMA, benzoyl peroxide (0.04 wt.%) which acts as a polymeriza-tion initiator and dodecyl mercaptan (0.01 wt.%) as chain transferagent. We have used two different high refractive index (H.R.I.)organic dopants e.g. triphenyl phosphate (TPP) and benzoylbenzoate (BBZ) of various concentrations into the monomer mix-ture. Finally Zr carboxy-ethyl acrylate 60% in (n-propanol),Zr(CH2CHCO2CH2CH2CO2)4 and Hf carboxy-ethyl acrylate 60% in1-butanol, Hf (CH2CHCO2(CH2)2CO2)4 were added into the solu-tion and it was sonicated for 30 min for reducing the aggregationas well as good dispersion of Zr and Hf cations with H.R.I. organiccarboxyethyl acrylate anions ligand moiety. The anionic counteri-ons of Zr/Hf cations come from the Zr/Hf salts precursors that canco-polymerize with methylmethacrylate. The solution was pouredinto the acrylic tube of 8 mm ID to 10 mm OD and kept it in thepredetermined temperature silicon oil bath. The acrylic tubeswere kept at uniform temperature of 80 �C for 36 h to get the de-sired molecular weight matrix by completing the polymerizationprocess [9].

2.1.1. Reaction schemeThe higher molecular volumes of H.R.I. organic dopant mole-

cules are concentrated in the centre core region to form a nearlyquadratic refractive index. The schematic representation of fabri-cation mechanism of acrylic optical preform rod during interfacialgel polymerization has shown in Fig. 1. The refractive index, chem-ical structure and the boiling point of the two organic co-dopants(BBZ and TPP) as well H.R.I. organic dopants (Zr/Hf carboxy ethylacrylate) are shown in Table 1.

The schematic representation of charge transfer complex for-mation of transition metal cation of Zr/Hf ions with anionic ligandof H.R.I. organic moieties of carboxyethyl acrylate forms chargetransfer complex through co-ordinate bond formation with co-dopant BBZ and TPP as shown in Fig. 2a and b.

2.2. Characterizations

2.2.1. Thermal studyThe thermal study of different H.R.I. organic co-doped preforms

has been done in a Perkin–Elmer DSC-Diamond instrument usingpyres series software under nitrogen flow atmosphere. The sam-ples of (5–10) mg were taken in aluminum pan. They were thenheated at 10�/min scan rate. The glass transition temperature (Tg)of different H.R.I. organic co-doped optical preforms was obtainedby noting the inflection point of transition.

2.2.2. StructureX-ray diffraction (XRD) pattern of the fabricated solid samples

were obtained by focusing an X-ray beam to the central portionof the sample with a X’pert Philips (PW1710) at 40 kV and 20 mAusing CuKa (k = 0.154 nm) radiation.

2.2.3. Morphology studyTransmission electron microscopy (TEM) of the Zr and Hf solu-

tion was diluted with respective alcohol was taken in a TechnaiG-30ST made by FEI Company, Netherlands. Here the sample solu-tion was dropped on a carbon coated copper grid which was oper-ated by 300 kV that provide the size, nature and distribution oftransition metal ion (Zr/Hf) particles. The solid samples in smallsize were used for FE-SEM measurement in Carl Zeiss, Subra35VP, Germany.

2.2.4. Fluorescence studyThe preform samples were cut into small size from fabricated

preform rod for taking fluorescence measurements. The fluores-cence measurements were performed using an Edinburg Instru-ment Spectrophotometer NIR 301 using Xe-900 xenon lamp at anexcitation of 380 nm with resolution in order of 0.04 nm. The

Table 1The chemical structure, refractive index, and the boiling point of the organic dopants and co-dopants are shown.

H.R.I organic dopants/co-dopants Chemical structure Refractive index(R.I.)

Molecular weight (g/mol)

Boiling point (B.P.,�C)

Benzyl benzoate (BZZ) 1.58 212.24 323

Triphenyl phosphate (TPP) 1.563 326.28 370

Zirconium carboxyethyl acrylate, 60% in (n-propanol)

1.53 663.69 100–121

Hafnium carboxyethyl acrylate, 60% in 1-butanol 1.55 750.96 _

Fig. 2. Schematic representation of transition metal ion (M4+) complex formation with H.R.I. Organic co-dopant: (a) BBZ and (b) TPP.

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obtained solid cylindrical samples were cut in 15 mm length andpolished both ended for fluorescence measurement.

2.2.5. Life timeFluorescence life time was measured using time correlated sin-

gle photon counting (TCSPC) by excitation by Nano-LED HORIBAJOBIN YVONIBH, model – Nano-LED 560 (pd 1.4 ns) and detectorMCP PMT Hamamatsu R3809, Hamatsu using IRF of Detector(FWHM) of 45 ps using software data station v2.3 and DAS 6.

2.2.6. Absorption studyThe UV–vis absorption spectra of the Zr and Hf containing acry-

late liquid samples were obtained by Shimadzu 960 UV–vis Spec-trophotometer at a resolution of 4 cm�1 over 64 scans in the1000–2000 cm�1 range.

2.2.7. FT-IR studyFT-IR spectra of the powder samples were collected from fabri-

cated optical preform and it was recorded using KBr pellet tech-nique from (400–4000) cm�1. This was done in a Spectrum 1001Perkin–Elmer Instrument, USA.

3. Results and discussions

3.1. Thermal study

Fig. 3a shows the DSC thermo grams of fabricated Zr doped andhigh refractive index organic benzoyl benzoate (BBZ) co-dopedwith various concentrations into acrylic optical preform matrix.The variation of glass transition temperature (Tg) with variousfunctional moieties containing organic co-dopant of BBZ and TPPof different concentration has shown in Fig. 3b. The glass transitiontemperature (Tg) is lowered with increasing concentration of H.R.I.organic dopants. Here, organic co-dopants are acts as good plasti-cizer in the acrylic matrix. The addition of co-dopants into the ac-rylic matrix improves flexibility, processibility as well utility too. Itreduces the brittleness nature of amorphous acrylic polymer, evenin small quantities markedly reduces the Tg of the acrylic matrix.This effect is due to a reduction in cohesive force of attraction be-tween polymer molecular chains [10]. The dopant molecules arepenetrate into the matrix and established polar attractive forcesbetween it and chain segments mobility and hence with of the fab-ricated preform is less than the acrylic homo polymer due to thepresence of 25 wt.% 3-phenyl-1-propanol which provides great

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(b) Fig. 3. (a) DSC thermogram of Zr-doped and different concentration of H.R.I. organic BBZ co-doped optical preform and (b) different concentration of BBZ and TPP H.R.I.co-dopant vs. glass transition temperature (Tg) of acrylic preform.

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plasticization effect. Plasticization effect occurs in a polymericmaterial in the presence of some adsorbed chemicals which con-verts the acrylic material from hard brittle stage to a softer mate-rial with more flexibility. It was observed in Fig. 3b that the Tg ofthe developed optical material is lowered with the increasing con-centration of H.R.I. organic co-dopants due to organic co-dopantare also works as good plasticizer. The additions of dopants, thesechemicals are incorporate themselves in-between the polymerchains and then the free volume increases and finally results inlowering of Tg [11].

3.2. Structure

Fig. 4a and b has shown the XRD profile of Zr and Hf doped ac-rylic optical preform with H.R.I. organic co-doped. It was observedthat zirconium (Zr) ion doped into the optical preform is amor-phous in nature but there was also a small peak appear at2h = 35� as shown in Fig. 4a, that corresponds the peak for zirco-nium ion. The developed small narrow peak indicates that the fab-ricated material has developed a small Zr ion micro-crystallinity.

Fig. 4b shows hafnium (Hf) ion doped with H.R.I. co-doped acrylicpreform where a small peak was observed for hafnium ion, the fab-ricated material is also amorphous in nature. This small peak issmaller and narrow compare to zirconium (Zr) ion that reveals thatHf ion has fewer tendencies for crystallinity than zirconium ion atfabricated temperature of 80 �C. So, the developed hybrid opticalmaterials of Zr and Hf ion doped and H.R.I. organic anion co-dopedoptical preform are amorphous in nature, except few crystallitesand Hf ion are developed. This indicates that the samples are inthe mesophase structural organization. A small peak at 2h = 29�correspond for Zr peak and halos at 2h = 15� and 42� correspondsacrylic PMMA. Similar trends are also observed for Hf as shownin Fig. 4b.

The average crystallite size was measured using Sherrers equa-tion, Lhkl = Kk/b0 Cos h, where Lhkl is the mean dimension of thecrystallites perpendicular to the planes (hkl), k is X-ray wave-length of 1.542Å, b0 is the integral breadth at half-maximum inten-sity peak, K is constant and h is angle [12,13].

The measure crystallite sizes for Zr ion was 90 nm and Hf ionwas 99 nm into fabricated optical preform. Zirconia is in the oxide

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Fig. 4. XRD profile of acrylic preform rod doped with acrylic monomer derivative with H.R.I. BBZ organic co-doped; (a) Zr and (b) Hf.

376 A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382

form has three kinds of polymorphs: monoclinic, tetragonal andcubic. There was no sharp peak for Zr ion or Hf ion was observedwhich reflects either nanoparticles of Zr and Hf is amorphous innature or to very small nano-crystallites or combination of both[14,15].

The broad reflections at 30.2� (011), 35.5� (110), and 50.5�(112) are represented for tetragonal ZrO2 or HfO2. It was observedthat crystallization occurs at a temperature higher than that ex-pected for pure hafnia or zirconia [16]. This phenomenon occurswhenever zirconia (Zr) or hafnia (Hf) are homogeneously dispersedinto acrylic matrix [17]. The crystallization of stabilize tetragonalzirconia or hafnia have been observed to occur at 400 �C [18] andpartial transformation from tetragonal to monoclinic phase wasobserved at higher temperature 800 �C. The single monoclinicphase is observed at temperatures higher than (1000–1200)�C.The homogeneously dispersion of zirconia or hafnia are in a silicamatrix, the crystallization occurs through diffusion and observedsizes of grain growth.

3.3. Morphology

Fig. 5a has shown FE-SEM of Zr ion complexes are doped in ac-rylic preform matrix, which shows Zr ion particles are uniformly

distributed inside the acrylic matrix with particle size (30–40)nm. A few particles are buried inside the matrix, and the sizes ofthe particles are uniform and spherical in nature. Fig. 5b has shownFE-SEM of Hf ion doped into acrylic matrix, where particles arescattered into the matrix with few domains are aggregated.

But in most of the zones, Hf ions are distributed on the surfaceas well as buried inside the acrylic functional monomer. The sizesof the Hf ion particles are in (40–60) nm. These cation particles areembedded into the functional group of acrylic monomer ofcarboxyl ethyl acrylate anion ligand into acrylic matrix throughformation of complexes with H.R.I. organic codopants TPP andBBZ. These are coordinate with functional moiety of BBZ and TPPto form complex formations that changes the photo physical envi-ronment and enhancement of fluorescence intensity as shown inFig. 2. Microcrystalline distinct particles of ZrO2 are observed inSEM micrograph [19].

Fig. 6a and b shows the SAED pattern of zirconium (Zr) and haf-nium (Hf) acrylate doped into the acrylic optical preform rod.Fig. 6a shows few microcrystal diffraction patterns in SAED. Thisreveals the formation of small crystals into nanoparticles duringfabrication and also due to the beam stopper heating during highvoltage TEM operation. The presence of these small crystallinenanoparticles of zirconia was confirmed. The development of

Fig. 5. FE-SEM images of optical preform; (a) Zr doped and (b) Hf doped.

Fig. 6. SAED patterns of preform; (a) Zr doped and (b) Hf doped.

A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382 377

crystalline tendency of zirconium is evidence from SAED even it isamorphous at fabricated lower temperatures. The beam heating ofthe amorphous material may causes the presence of nanocrystalsduring SAED analysis [20], which were causes for development ofmicro-crystallinity. Fig. 6b indicates that the hafnium exhibitsmesophase of different structural layer, which was developedstructural organization of border line between amorphous andcrystalline phases. Henceforth, both transition metal Zr and Hf ionsare amorphous phases into the fabricated optical preform but crys-talline tendency is less for hafnium than zirconium into acrylic ma-trix during TEM experiments.

High-resolution transmission electron microscopy (HRTEM)images of Zr ion and Hf ion particles have shown in Fig. 7a and

Fig. 7. High-resolution transmission electron microscopy (

b. It was observed that that ion particle is in 4 nm size for bothZr and Hf. The particles are structurally arranged as shown inFig. 7a, whereas this arrangement is less in Hf as shown inFig. 7b. The arrangements of atoms into the particles are moreprominent in Zr samples rather than Hf samples. But it is not thefully structural organization occurring in the Zr that indicates itsmesophase structure.

Hyeon et al. [21] reported HRTEM images of highly crystallinenanoparticles of uniform size distribution and size of 4.0 nm. Theevidence of nucleation during atomic layer deposition thin filmfabrication followed by crystal growth of Zr and Hf was inducedby electron beam during HRTEM leads to surface roughnessthrough crystallite formation [22].

HR-TEM) images; (a) Zr–acrylate and (b) Hf-acrylate.

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(b)(a)Fig. 8. (a and b): shows the elemental analysis of zirconium (Zr) and hafnium (Hf) doped into optical preform.

378 A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382

Fig. 8a and b shows the elemental analysis of zirconium (Zr) andhafnium (Hf) into fabricated optical preform. It shows that therewas sufficient amount of zirconium and hafnium transition ele-ments are present into the corresponding samples. There was alsoconsiderable amount of oxygen present in both samples of zirco-nium and hafnium due to their oxide form. It was also observedthe presence of carbon and copper elements which are originatedfrom the Cu grid used in TEM experiment.

3.4. Photoluminescence study

3.4.1. Metal ion concentration effectFig. 9a and b have shown the fluorescence intensity in different

concentrations of Zr metal ion doped with H.R.I. organic TPP co-doped in Fig. 9a, whereas BBZ H.R.I. co-doped in Fig. 9b. It has beenobserved that less concentration (0.043 wt.%) of Zr ion gives muchenhanced fluorescence compared to high concentrations of Zr ionfor both TPP and BBZ H.R.I. organic co-doped samples in acrylicpreform matrix. The more metal ions dopant concentration leadsto irreversible kinetic aggregation of ions in the optical acrylic pre-form due to formation of metal ion clusters during fabrication [23].These clusters provide less intense fluorescence compared to indi-vidual metal ions. Similar trends are also observed for hafnium me-tal ion for fluorescence intensity. So, less concentration of both Zrand Hf metal ions have more free surface properties which leadto more effective to produce brighter fluorescence imaging. Therewas no peak shifted and hence no clustering effect was observedup to 15 wt.% of transition metal acrylate monomer doping into ac-rylic preform matrix. Novel zirconia-based fluorescent terbiumnanoparticles of monodisperse, spherical and uniform in size havebeen reported as a fluorescent nanoprobe for fluorescence bioassaynanoparticles [24].

The emission intensity (Iem) is a Gaussian function and has itsmaximum value at the same kem independent of kexc, Iem = Q exp[�a (kem–k0em)2], where a > 0 is a parameter which depends uponthe material property and Q is maximum fluorescence emission.The excitations of different size particles have different absorptionspectra but they have different fluorescence spectra with sameintensity [25].

3.4.2. H.R.I. organic co-dopant effectFig. 10 has showed that fluorescence intensity much enhanced

with increasing concentration of organic dopant TPP and BBZ.The fluorescence intensity is maximum for 15 wt.% of H.R.I. BBZcompared to no H.R.I., whereas a similar trend was also observed

for 15 wt.% of H.R.I. TPP. The large fluorescence intensity increasesis due to charge transfer complex formation between transitionmetal zirconium/hafnium ions with the anionic ligand of carboxylethyl acrylate copolymerizes and coordinated charge transfer com-plexes with H.R.I. organic co-dopants BBZ and TPP. Here, Zr/Hf ionforms charge complexes through the coordination linking P@Ogroup of PO4 moiety of triphenylphosphate (TPP) and COO moietyof C@O group of benzyl benzoate (BZZ) [26–29] in acrylic functionmatrix as shown in Fig. 2. The fluorescence intensity enhancementin presence of H.R.I. organic co-dopant is due to the +R effect ofPO3�

4 moiety in TPP and carbonyl oxygen moiety of BBZ. H.R.I.BBZ co-doped optical preform has more fluorescence intensitycompare to TPP doped preform because BBZ has carbonyl oxygen(AC@O) which is coordinated with Zr4+ and easily influenced by+R effect on Zr4+ whereas phosphate group PO3�

4

� �in TPP has less

+R effect. So it reveals that H.R.I. organic co-dopant influenced onZr/Hf ion electronic vibration in fluorescence spectrum.

Aromatic ring attached with BZZ and TPP H.R.I. co-dopantsforms excimer complex that also direct influences +R effect onZr/Hf ions [30], and fluorescence emission from such excimer ofZr/Hf complex, resulting higher quantum yield than monomeremission itself. It was also observed that the complexes of transi-tion metal ions make clustering after certain optimum concentra-tion which affected ion-ion interaction and causes decreases offluorescence intensity. The reasons for complex formation of tran-sition metal ions with co-dopants containing oxygen atoms in thefunctional moiety to which transition metal ions are coordinates.So, limited doping concentration of transition metal ions withH.R.I. organic co-dopant TPP and forms complexes that gives bestefficiency of fluorescence intensity. Lei et al. [31] prepared poly-mer–rare earth complex using salicylic acid-containing polysty-rene and characterized its fluorescence emission property. Theyshowed experimental that the complex SAPS–Eu(III) has finechemical stability because of the bidentate chelating effect of sali-cylic acid ligand that binary intrachain complex SAPS–Eu(III) hasthe strongest fluorescence emission. Pirozzi et al. [32] also re-ported that Zr(4+) ions are formed strong complexation with ace-tylacetonate ligands as a polymeric network of zirconium oxoclusters on the surface and these complexes are organic–inorganichybrid in nature.

In general, p–p� transition in complexes contribute good fluo-rescence since absorption coefficient, e is largest for this transitiondue to charge transfer complex formation, it is proportional withflurophore concentration and n–p� transition reduce fluorescenceby increasing the possibility of intersystem crossing. So the charge

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(a) (b)Fig. 9. Fluorescence intensity vs. different concentration of transition metal ion (Zr) in H.R.I. organic co-doped; (a) 5% TPP of 0.17 (wt.%), 0.087 (wt.%) and 0.043 (wt.%) and(b) 5% BBZ of 0.17 (wt.%), 0.087 (wt.%) and 0.043 (wt.%).

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MMA with (0.04 wt%) doped Hf with no H.R.I co-doped MMA with (0.04 wt%) doped Hf and 5wt% TPP co-doped MMA with (0.04 wt%) doped Hf and 10wt% TPP co-doped MMA with (0.04 wt%) doped Hf and 20wt% TPP co-doped

0.0

4.0x103

8.0x103

1.2x104

1.6x104

2.0x104

2.4x104

2.8x104

3.2x104

3.6x104

4.0x104

Inte

nsity

(a.u

.)

Wavelength (nm)

MMA with (0.04 wt%) doped Hf with no H.R.I co-doped MMA with (0.04 wt%) doped Hf and 5wt% BBZ co-doped MMA with (0.04 wt%) doped Hf and 10wt% BBZ co-doped MMA with (0.04 wt%) doped Hf and 20wt% BBZ co-doped

(a) (b)

400 425 450 475 500 525 550 575 600 400 425 450 475 500 525 550 575 600

400 425 450 475 500 525 550 575 600400 425 450 475 500 525 550 575 600

Fig. 10. Fluorescence intensity for transition metal Zr/Hf ion doped with different concentration of H.R.I. organic co-doped 5%, 10% and 15%: (i) Zr doped, (a) TPP and (b) BBZ;(ii) Hf doped, (a) TPP and (b) BBZ.

A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382 379

transfer complex formation provides p–p� transition which isresponsible for fluorescence enhancement.

The valence band of ZrO2 is originated from occupied O 2pstates and the conduction band is constructed of unoccupied Zr4d states, which split into two sub-bands. The 4d states of lowerlying sub-bands are Zr 4d x2 � y2 and z2 states, and higher lying

bands are composed of Zr 4d xy, yz and zx states [33]. The crystal-line phase governs conduction band of ZrO2 varies greatly uponphases. The two sub conduction bands are merges as the crystal-line phase transformed from a more symmetric cubic structureto a less symmetric monoclinic phase [34]. These monoclinicphases have showed two direct band transitions with band gaps

380 A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382

of 4.89 and 5.78 eV. The calculated bandgaps of the monoclinic,tetragonal and cubic form of ZrO2 are in the range of 3.12–5.42,4.10–13.3 and 3.25–12.3 eV, respectively. The observed peaks at410 nm (3.02 eV) and 432(2.87 eV) arises from amorphous phasesof rigid organic molecules and metal ions Zr and Hf cluster form asindirect band gap transition followed by direct band transitions. Toget best efficiency of Zr/Hf fluorescence of metal ions intensity15 wt.% H.R.I. organic co-dopants are best choice in optical preformfabrication.

Life time measurement. Time-correlated single photon counting(TCSPC) technique has been used to measure fluorescence life timeof fabricated preform samples. The average lifetime is derived byequation, s ¼

RtIðtÞdt=IðtÞdt. It is also important to mention that

this curve fitting with experimental curves to a single exponentialfunction shows deviation (s � s1) are less than (5–8)%, whichindicates the weak biexponential decays character [35,36].

It was observed that decay time is in nanosecond (ns) scale atdifferent emission wavelength doing not vary significantly at432 nm (2.87 eV) and 410 nm (3.02 eV), respectively. Zr and Hfhave similar type of fluorescence spectra regardless of their pres-ence in the host matrix. Fig. 11 shows that the temporal photoncount provided by the instrument for Hf samples with and withoutorganic co-dopants. The curve fitted lifetime for the preform sam-

180 200 220 240 260 280 300

0

1000

2000

3000

4000

5000

Phot

on C

ount

s

Decay Time (ns)

Hf-no H.R.I. Hf-BBZ

Fig. 11. Lifetime of transition metal hafnium doped in preform at 432 nm emissionWavelength: (a) no H.R.I. dopant, and (b) BBZ dopant.

200 225 250 275 300 325 350 375 400

Abso

rban

ce (a

.u.)

Wavelength (nm)

Zr-No H.R.I. Zr-TPP 155 Zr-BBZ 15%

(a)Fig. 12. UV–Vis of transition metal ion doped acrylate into

ple without organic co-dopant is obtained as 1.285 ns while the va-lue becomes 1.2767 for H.R.I. organic dopant BBZ co-dopedhafnium sample. H.R.I. organic dopant Hf have mare life time thanno H.R.I. co-doped acrylic preform.

3.4.3. Absorption studyFig. 12a and b has shown the UV–vis absorbance of Zr and Hf

containing acrylate with H.R.I. organic moiety containing co-dopant TPP and BBZ forms charge transfer complexes. It wasobserved that bandwidth increases in the presence of organic codo-pants TPP and BBZ and it was more for BBZ than TPP co-dopants.The charge transfer (CT) complex formation occurs with the C@Ogroup of carboxylic oxygen (COO�) in benzoate group in BBZ andalso oxygen (O�) of the phosphate group PO�4

� �in TPP co-dopants

[37], which is responsible for broadening of bandwidth from about(220–300) nm. So, it can be explained that absorption increaseswith the interaction of transition metal ion with the organic func-tional moieties into acrylic matrix which coordinated with co-dopant TPP and BBZ results in charge transfer complex formation.

3.4.4. FT-IR studyFig. 13i and ii shows FTIR spectra of transition metal Zr/Hf ion

with functional moieties of H.R.I. organic dopants interactions in-side acrylic matrix. The peak at 1728 cm�1 corresponds to theC@O acrylate vibration for asymmetric stretching. A peak was alsoobserved at 1636 cm�1 attributed to C@C stretching of unreactedMMA monomer [38].

The presence of P@O stretching mode was confirmed from thepeak at 1067 cm�1. ZrAO stretching was observed at 749 cm�1.The peaks are at 1591 cm�1, 1489 cm�1, and 1454 cm�1 are inter-actions of Zr/Hf ion with phosphate moiety of TPP. The presence ofCH2 rocking vibration was clearly visible in spectra at 842 cm�1.The absorption bands of PO due to PO4 groups in the 1080–1020 cm�1 region, ZrAOAP band at 1026 cm�1 and peak around600–610 cm�1 indicates the OAH vibrational mode. The observedbands or shoulders of OAH stretching at (3600–3580) cm�1[39].Benzoate vibration at 1275 cm�1 in benzyl benzoate, but1296 cm�1 vibration band was observed after coordination withZr ion. Similar results were also observed for hafnium doped inH.R.I. organic co-doped acrylic matrix samples where additional750 cm�1 stretching band was observed for HfAO bond. Fig. 13icand iic both showed that after being coordinated Zr ion with BBZH.R.I. organic functional moiety copolymerizes with carboxy ethylacrylate into acrylic matrix, the CAO stretching vibration of car-boxyl group at 1728 cm�1 of BBZ disappeared, but alternatively, a

400375350325300275250225200

Abso

rban

ce (a

.u.)

Wavelength (nm)

Hf-No H.R.I. Hf-TPP Hf-BBZ

(b) optical preform matrix; (a) Zr-doped and (b) Hf-doped.

450 600 750 900 1050 1200 1350 1500 1650 1800 1950

Tran

smitt

ance

(a.u

.)

(c)

(b)

(a)

Wavelength (cm-1)

(a) Zr no H.R.I(b) Zr TPP(c) Zr BBZ

450 600 750 900 1050 1200 1350 1500 1650 1800 1950

(c)

(b)

(a)Tran

smitt

ance

(a.u

.)

(a) Hf no H.R.I.(b) Hf TPP(c) Hf BBZ

Wavelength (cm-1)

Fig. 13. FT-IR spectra of transition metal ion Zr/Hf doped with H.R.I. organic co-dopants; (i) Zr doped; (a) no H.R.I., (b) TPP, (c) BBZ, and (ii) Hf doped: (a) no H.R.I., (b) Hf-TPP,(c) Hf-BBZ.

Fig. 14. Schematic representation of preform made with Zr/Hf doped with H.R.I.organic co-doped; (a) core with Zr or Hf metal ions, and clad with acrylic matrix,(b) cross sectional view of core region in optical preform.

A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382 381

new symmetric stretching vibration of carboxylate ion (COO�) ofcoordinated BBZ situated at about 1371 cm�1 is emerged.

3.4.5. ModelingFig. 14 has shown the schematic representation of transition

metal Zr/Hf nanoparticles dispersed into H.R.I. organic functionalmoiety copolymerizes with carboxy ethyl acrylate containing ac-rylic matrix inside the optical preform. The cross sectional areahas shown in Fig. 14b. It has shown that nanoparticles are embed-ded with functional organic moieties inside the acrylic matrix in

the centre core region of optical preform. Acrylic preform matrixcontains various functional moieties of H.R.I. organic dopants, car-boxylate (COO�) of BZZ and Phosphate PO�1

4

� �of TPP, which bind

the Zr/Hf metal ion nanoparticles inside H.R.I. organic functionalanionic ligand moieties of carboxyl ethyl acrylate copolymerizesat the core region and conjugated with the MMA monomer in cladregion into the optical preform. This fabricated optical preform hasgood potential application for bio-sensor assay sensor probe. Thecomponents into hybrid inorganic–organic composite of Zr/Hf ionsare non-toxic in nature with biological materials. Experimental andclinical studies reported that Zr compounds are bio-compatibleand exhibit low toxicity. In recent years, Zr toxicity has virtuallydisappeared from the medical literature [40]. Hf in HfO2 nanopar-ticles (NP) were found to have low cytotoxicity to human cell andthe toxicity assays decreased or increased the average particle sizeof HfO2 NPs due to dispersion or agglomeration, respectively [41].So, nanodimension of Hf particles are dispersed into acrylicmatrix will have low toxicity to biomaterials. They are compatiblewith acrylic matrix and bind with biomolecules and acts asbio-analytes.

4. Conclusions

Transition metal (M4+) ions are acts as flurophore embeddedwith organic H.R.I. anionic ligand carboxyl ethyl acrylate copoly-merizes with H.R.I. codopants of BBZ and TPP inside the acrylicoptical preform matrix through formation of charge transfer com-plex. This is new class of organic–inorganic hybrid optical fluro-phore materials enhances fluorescence signal and no-toxicity.These transition metal Zr and Hf ion particles are in boarderphase’s in-between amorphous and mesophase structure. Theinteraction with H.R.I. organic dopants forms charge transfer com-plexes with metal ions through coordinated bonds. The metal ionparticles are enhanced interface effects due to nano-trapping levelsand potential application for fluorescent nano-sensor probe in biolabeling for bioassay.

Acknowledgements

Authors wish to acknowledge Council of Scientific Industrial Re-search (CSIR), Govt. of India for financial support in this project OLP0285. Mr. Jijo Lukose is also wishes to thanks Research Fellows andTechnical staffs of Central Glass and Ceramic Research Institute

382 A.K. Dikshit, J. Lukose / Optical Materials 35 (2013) 372–382

(CGCRI), Kolkata for extending their co-operation and support dur-ing pursue MS thesis.

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