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This journal is©The Royal Society of Chemistry 2015 Chem. Soc. Rev., 2015, 44, 1299--1301 | 1299
Cite this: Chem. Soc. Rev., 2015,
44, 1299
Photon upconversion nanomaterials†
Xiaogang Liu,*ab Chun-Hua Yan*c and John A. Capobianco*d
Photon upconversion through the use oflanthanide-doped materials has been thefocus of a growing body of research inthe fields of materials chemistry andphysics for more than 50 years.1 Theattraction of this field has been the abilityto generate photons at shorter wavelengthsthan the excitation wavelength after laserstimulation. Despite its potential utility fora number of applications, photon upcon-version has been primarily investigated inbulk glasses or crystalline materials.2 Thatsituation dramatically changed in themid-2000s, with the widespread researchin the development of upconversion
nanomaterials.3 As a unique class ofoptical materials, upconversion nano-materials exhibit useful applicationsspanning from lighting to volumetric3D displays to photovoltaics. Particularly,nanosized upconversion nanocrystals haveproven valuable as luminescent labels forchemical and biological sensing withmarked improvements in the sensitivityand versatility of the sensors. Recentadvances in the field of upconversion nano-materials have motivated us to initiate athematic issue focusing on fundamentalprinciples, synthetic strategies, materialscharacterization, broad applications, and
a Department of Chemistry, National University of
Singapore, Singapore 117543, Singapore.
E-mail: [email protected] Institute of Materials Research and Engineering,
Agency for Science, Technology and Research,
Singapore 117602, Singaporec Beijing National Laboratory for Molecular Sciences,
State Key Laboratory of Rare Earth Materials
Chemistry and Applications, PKU-HKU Joint
Laboratory in Rare Earth Materials and Bioinorganic
Chemistry, Peking University, Beijing 100871, China.
E-mail: [email protected] Department of Chemistry and Biochemistry and
Centre for NanoScience Research, Concordia
University, Montreal, Quebec, H4B 1R6, Canada.
E-mail: [email protected]
† Part of the photon upconversion nanomaterialsthemed issue.
Xiaogang Liu
Xiaogang Liu was born in Jiangxi,China. He earned his BE degreein Chemical Engineering fromBeijing Technology and BusinessUniversity, China. He received hisMS degree in Chemistry from EastCarolina University and com-pleted his PhD at NorthwesternUniversity. He then worked as apostdoctoral fellow at MIT fortwo years before joining thefaculty of the National Universityof Singapore. He holds a jointappointment with the Institute of
Materials Research and Engineering. His interests includeluminescent materials synthesis, supramolecular chemistry,transition metal-based catalysis, and surface science for sensing,optoelectronic and biomedical applications.
Chun-Hua Yan
Chun-Hua Yan was born inShanghai, China. He receivedhis BS, MS and PhD degreesfrom Peking University (PKU),China. He became full Professorin 1992 and Cheung KongProfessor of Chemistry in 1999at PKU. He is an electedMember of Chinese Academy ofSciences (2011) and a Fellow ofthe Academy of Sciences for theDeveloping World (TWAS, 2012).He is now the Director of theState Key Laboratory of Rare
Earth Materials Chemistry and Applications at PKU; he alsoserves as the Managing Editor-in-Chief for J. Rare Earths(Elsevier) and an Associate Editor for Inorganic Chemistry (ACS).His main research fields are rare earth separation chemistry andengineering and rare earth functional materials.
DOI: 10.1039/c5cs90009c
www.rsc.org/csr
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major future opportunities and challengesfor further development of the field. Acommon theme is the exquisite control ofphoton emissions afforded by lanthanidedoping in a wide range of host lattices.
A key to the design of upconversionsystems with precisely controlled emis-sion profiles is the understanding of theelectronic structure of the materials.In his contribution, Liu (DOI: 10.1039/c4cs00187g) uses theoretical models andmethods to provide a critical analysisof nano-phenomena in upconversion.Using these approaches, it is possible toobtain accurate theoretical interpreta-tions of the spectroscopic characteristicsand luminescence dynamics of photonupconversion.
A photon upconversion process invol-ving lanthanide-doped nanomaterialstypically occurs in three steps; stepwisephoton absorption, energy transfer andthe emission of photons. Zhang andco-workers (DOI: 10.1039/c4cs00168k)survey examples of recent optical investi-gations on upconversion nanocrystals inorder to reveal the impact of excitationenergy migration dynamics on photonupconversion. Special attention is paid tothe critical role of the spatial confinementof excitation energy in the enhancement ofemission intensity. Meanwhile, Dong,Sun and Yan (DOI: 10.1039/c4cs00188e)provide a detailed overview of the funda-mental mechanisms underlying energy
transfer over short- and long-range dis-tances, and then highlight the principalguidelines for regulating emissions inlanthanide-activated nanocrystals.
A robust synthesis of deliberatelydesigned high quality upconversion nano-crystals should lay the foundation forvalidating upconversion pathways. Chan(DOI: 10.1039/c4cs00205a) highlights therecently emerged combinatorial strategiesfor preparing upconversion nanocrystalsand developing theoretical methods thatallow the optical properties of the nano-crystals to be precisely modeled.
Spatially controllable assembly of dif-ferent optical elements into a core–shellor composite upconversion nanocrystalopens the door to an efficient method forfabricating nanostructures with unprece-dented flexibility in tuning the opticalemission. Chen et al. (DOI: 10.1039/c4cs00151f) discuss the control of thecore–shell interface within upconversionnanoparticles to achieve energy transferprocesses with high efficiencies. As aparallel development, Chen, Prasad andco-workers (DOI: 10.1039/c4cs00170b)present a thorough assessment on photonupconversion arising from core–shellnanostructures. In their contribution,they describe the synthesis and characteri-zation of the core–shell nanostructures,and then they comprehensively expoundpotential applications of these nano-materials in bioimaging and light-triggered
drug release. Zhang and colleagues (DOI:10.1039/c4cs00163j) have also focused onthe production and emission tuning ofupconversion nanocrystals, with routesto the construction of multifunctionalcounterparts.
For use in biological settings, theupconversion nanomaterial must havegood water dispersibility. However, mostluminescent upconversion nanocrystalsare prepared with the coating of hydro-phobic, long alkyl chain ligands. Sedlmeierand Gorris (DOI: 10.1039/c4cs00186a)review a number of methods for convert-ing nanocrystals from hydrophobic tohydrophilic, a key criterion to considerwhen assessing the suitability of upconver-sion nanoparticles for biological applica-tions. Chen and co-workers (DOI: 10.1039/c4cs00178h) also review the progressachieved in the design and synthesis ofbiocompatible upconversion nanocrystalsfor in vitro biodetection applications.
A number of instrumentation techni-ques have been utilized to probe thenature of upconversion nanocrystals andquantify structure–property relationships,as discussed by Liu et al. (DOI: 10.1039/C4CS00356J). In their contribution, theyhighlight the rational design and combi-nation of a variety of instrumental tools forcharacterizing upconversion nanomaterials,from surface structures to intrinsic proper-ties to ultimate challenges in nanocrystalanalysis at single-particle levels.
Apart from the fascinating opticalproperties on display, upconversion nano-materials can exhibit amazing magneticand ferroelectric characteristics inheritedfrom certain lanthanide dopants. Theseattributes are likely to provide an outlookon the generation of stimuli responsiveupconversion emission. Tsang, Bai andHao (DOI: 10.1039/c4cs00171k) reviewthe efforts toward the elucidation ofstimuli responsive properties displayedby upconversion nanomaterials, as wellas potential applications of such nano-structures under different forms ofstimulus control.
In view of the tremendous potential ofupconversion nanocrystals as contrastagents for in vivo and in vitro imagingstudies, the concerns on the biosafety ofthese nanocrystals must be evaluated.There are two balanced and carefully
John A. Capobianco
John A. Capobianco is a Professor of Chemistry andConcordia University Research Chair in Nanoscience, inthe Department of Chemistry and Biochemistry,Concordia University in Montreal, Quebec, Canada. Hegraduated from McMaster University, Hamilton, Canadawith a BSc degree and l’Universite de Geneve,Switzerland with a PhD degree. He is the co-founderand co-director of the Centre for NanoScience Research,Concordia University. Over the past 30 years his researchhas focused on the optical spectroscopy, synthesis andphysiochemical characterization of single crystals,amorphous and nanocrystalline materials doped withlanthanide ions. Currently, his main research activitiesdeal with upconversion, which has attracted considerable
attention due to its potential applications in cell targeting, imaging, diagnostic medicine,photodynamic therapy, and drug delivery using photoswitching. He has held invited facultyfellowships at l’Universita di Trento, Italy, Universite d’Angers, France, UniversidadAutonoma de Madrid, Spain and l’Universita di Verona, Italy.
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considered reviews devoted to this topic.Gnach et al. (DOI: 10.1039/c4cs00177j)report a summary of the main achieve-ments in this area of research. Li andco-workers (DOI: 10.1039/c4cs00175c)discuss particle circulation-induced toxi-city in organs and tissues of small animalsafter administration.
Easy structural modification and highlevels of freedom in the functional multi-plicity of upconversion nanocrystals arecritical for their therapeutic applications.In their contribution, Zhang and co-workers(DOI: 10.1039/c4cs00158c) explore recentadvances in the development of photo-therapeutic agents activated by multi-functional upconversion nanocrystals.Li, Lin and co-workers (DOI: 10.1039/c4cs00155a) illustrate the aspects of
upconversion activation for drug deliveryand exemplify the potential impact ofthis research area on biomedical science.
With features of high photostabilityand non-autofluorescence backgroundfactored in, upconversion nanomaterialsare ideal for high-resolution bioimaging.Suh, Hyeon and co-workers (DOI: 10.1039/c4cs00173g) report on the use of upconver-sion nanocrystals as multimodal in vivocontrast agents, with particular emphasison the feasibility of using such nanocrystalsas a versatile platform for wide-field two-photon microscopy.
We wish to thank all of our contributingauthors for their dedication and thereviewers for their constructive commentsand suggestions. We owe a special debt ofgratitude to the editorial staff of Chemical
Society Reviews for invaluable assistance inassembling this issue. It is hoped that thisthematic issue provides an update on someof the best work and advances in the field ofphoton upconversion nanomaterials. At thevery least, we hope that this special issueserves as a useful reference work that couldappeal to members of the chemical as wellas the biological and medical communities.
References
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3 F. Wang and X. Liu, Chem. Soc. Rev.,2009, 38, 976.
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