Upconversion: road to El Dorado of the fluorescence world Li Ching Ong, a† Muthu Kumara Gnanasammandhan, a† Sounderya Nagarajan a† and Yong Zhang a,b * ABSTRACT: Upconversion nanoparticles (UCNs), in the recent times have attracted attention due to their unique properties, which makes them ideal fluorophores for use in biological applications. There have been various reports on their use for targeted cell imaging, drug and gene delivery and also for diffuse optical tomography. Here we give a brief introduction on what are UCNs and the mechanism of upconversion, followed by a discussion on the biological applications of UCNs and further on what the future holds for UCNs. Copyright © 2010 John Wiley & Sons, Ltd. Keywords: Upconversion; nanoparticles; UCN; biological applications; imaging Introduction In the world of fluorophores, upconversion nanoparticles (UCNs) are one of the newer members. Despite their relatively short history, however, UCNs have found applications in many diverse areas. The feature page shows the versatility of these nanomate- rials and some of their current applications. The optical proper- ties exhibited by these materials make them preferable to existing conventional fluorophores. These properties are due to the composition of these nanoparticles and the phenomenon of upconversion exhibited by them. The conventional fluorophores display the phenomenon of Stokes shift, in which higher energy photons are absorbed and lower energy ones are emitted (1). This single photon process is illustrated in Fig. 1(a). In this case, the absorption of a higher energy photon excites the electrons from the ground state, to a higher electronic state, State 2. Owing to some internal conversion, the electrons quickly lose some of their energy to reside in the lowest vibrational energy state, State 1. Finally, the electrons return to the ground state with the emission of a photon which is of lower energy due to the loss of some energy during the internal conversion to the lowest vibrational state (State 1). In contrast, upconversion is a phenomenon in which the lower energy photons are absorbed and photons of energy higher than the single photon absorbed are emitted. This is made possible due to the presence of multiple metastable electronic states for the material. More importantly, there exists a lowest metastable state, Metastable State 1 (Fig. 1b) that absorbs in the near-infrared (NIR) region. This lowest metastable state acts as the energy store, enabling the subsequent absorption of the second NIR photon to further promote the electrons to a much higher state (2) (for instance, from Metastable State 1 to Metastable State 3 as shown in Fig. 1b). Similar to the single photon excitation, some internal conversion may occur and the electrons can lose some energy to finally reside in a slightly lower excited state (Metastable State 2). Finally, the return to the ground state also results in the emission of a photon but this time it is of higher energy than that of a single photon absorbed. What makes an upconverting fluorescent material? A typical upconverting fluorescent material (as illustrated in Fig. 2) consists of two components, an inorganic host lattice and dopant ions that are capable of upconversion. Usually, these upconverting materials are made by doping trivalent lanthanides that have multiple metastable excited states into a transparent crystalline host lattice that can accommodate these dopants. Even though a single type of dopant ion is sufficient to get the upconversion effect, co-doping with ytterbium (Yb 3+ ) is often done since Yb 3+ has a high absorption coefficient and can increase the upconversion efficiency. In this type of co-doped system, Yb 3+ acts as the absorber, taking in the lower energy photons from the excitation source while the other dopant acts as emitter, using the absorbed energy to emit the higher energy photons (3). As for the choice of the host lattice, there are some criteria that need to be fulfilled. One important requirement is the close match of the lattice of the host with the dopant ions. Secondly, the hosts should also have low lattice phonon energies so that the energy losses can be minimized and the radiative emissions can be maximized. Among the choices for the host lattices, halide-based lattices typically have the desired low phonon energies but the hygroscopic nature of the heavier * Correspondence to:Yong Zhang, Division of Bioengineering, Faculty of Engi- neering, Block EA-03-12, National University of Singapore, 7 Engineering Drive 1, Singapore 117574. E-mail: biezy@nus.edu.sg † Li Ching Ong, Muthu Kumara Gnanasammandhan and Sounderya Nagarajan contributed equally to this work. a Division of Bioengineering, Faculty of Engineering, National University of Singapore, 117576, Singapore b Nanoscience and Nanotechnology Initiative, National University of Sin- gapore, 117576, Singapore Special Feature: Review Article Received: 01 May 2010, Revised: 17 June 2010, Accepted: 21 June 2010, Published online in Wiley Interscience (www.interscience.wiley.com) DOI 10.1002/bio.1229 290 Luminescence 2010; 25: 290–293 Copyright © 2010 John Wiley & Sons, Ltd.