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Publication Title | Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals Feng Wang and Xiaogang Liu

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TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews Recent advances in the chemistry of lanthanide-doped
upconversion nanocrystals Feng Wang and Xiaogang Liu*

Received 13th October 2008

First published as an Advance Article on the web 12th February 2009 DOI: 10.1039/b809132n

Lanthanide ions exhibit unique luminescent properties, including the ability to convert near infrared long-wavelength excitation radiation into shorter visible wavelengths through a process known as photon upconversion. In recent years lanthanide-doped upconversion nanocrystals have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. These techniques offer low autofluorescence background, large anti-Stokes shifts, sharp emission bandwidths, high resistance to photobleaching, and high penetration depth and temporal resolution. Such techniques also show potential for improving the selectivity and sensitivity of conventional methods. They also pave the way for high throughput screening and miniaturization. This tutorial review focuses on the recent development of various synthetic approaches and possibilities for chemical tuning of upconversion properties, as well as giving an overview of biological applications of these luminescent nanocrystals.

1. Introduction

Upconversion (UC) refers to nonlinear optical processes characterized by the successive absorption of two or more pump photons via intermediate long-lived energy states followed by the emission of the output radiation at a shorter wavelength than the pump wavelength. This general concept was first recognized and formulated independently by Auzel, Ovsyankin, and Feofilov in the mid-1960s.1 Since then, conversion of infrared radiation into the visible has generated

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543.

E-mail: chmlx@nus.edu.sg

much of the interest in UC research, progressively generating and incorporating novel areas of investigation. The knowledge gained thus far has allowed the development of some remark- ably effective optical devices such as infrared quantum counter detectors, temperature sensors, and compact solid state lasers.

Despite their remarkable potential utility, the practical use of UC has been primarily focused on bulk glass or crystalline materials for the past 30 years but with extremely limited impact on biological sciences. These limitations are largely attributed to the difficulties in preparing small nanocrystals (sub-50 nm) that exhibit high dispersibility and strong UC emission in aqueous solutions. It was not until the late 1990s, when nanocrystal research became prevalent, that UC became more prominent in the fields of biological assays and medical

View Article Online / Journal Homepage / Table of Contents for this issue

Feng Wang

Feng Wang was born on February 16, 1979 in Shaanxi, China. He received his BE (2001) and PhD (2006) de- grees in Materials Science and Engineering from Zhejiang University. His PhD thesis was focused on the synthesis and characterization of lanthanide- doped fluoride nanomaterials under the supervision of Profs. Mingquan Wang and Xianping Fan. He joined the group of Prof. Xiaogang Liu at the National University of Singapore in 2007. His current research

Xiaogang Liu

Xiaogang Liu was born in Jiangxi, China. He earned his BE degree (1996) in Chemical Engineering from Beijing Tech- nology and Business University. He received his MS degree (1999) in Chemistry from East Carolina University under the direction of Prof. John Sibert and completed his PhD (2004) at Northwestern University under the supervision of Prof. Chad Mirkin. He then became a postdoctoral fellow in the group of Prof. Francesco Stellacci at MIT. He joined the faculty of

focuses on the synthesis, spectroscopic investigation, and appli- cation of luminescent nanomaterials.

the National University of Singapore in 2006. His research interests include nanomaterials synthesis, supramolecular chemistry, and surface science for catalysis, sensors and biomedical applications.

976 | Chem. Soc. Rev., 2009, 38, 976–989

This journal is c The Royal Society of Chemistry 2009

Published on 12 February 2009. Downloaded by National University of Singapore on 11/09/2014 11:52:48.

Image | Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals Feng Wang and Xiaogang Liu



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