Nano Today
Volume 29, December 2019, 100797
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Review
Recent advances in upconversion nanocrystals: Expanding the kaleidoscopic toolbox for emerging applications

https://doi.org/10.1016/j.nantod.2019.100797Get rights and content

Highlights

  • Understand the fundamental principles of photon upconversion in lanthanide-doped nanocrystals.

  • Introduce the general strategies for the synthesis of lanthanide-doped upconverting nanocrystals.

  • Achieve tunable and enhanced upconversion emission through nanostructural engineering.

  • Summarize the emerging applications of lanthanide-doped nanocrystals from nanophotonics to bioscience.

Abstract

Lanthanide-doped upconversion nanoparticles enable anti-Stokes emission via nonlinear processes, where low-energy excitation photons in the near-infrared window can be upconverted into high-energy emission ones in the visible or ultraviolet regions. The past decade has seen great success in the high-quality synthesis of upconversion nanoparticles with controlled structure, crystalline phase, size, and shape. The unique capacity of upconversion nanocrystals to undertake near-infrared excitation, amalgamated with their excellent luminescent characteristics, such as massive anti-Stokes spectral shift, sharp emission band, multicolor emission, and long luminescence lifetime, makes these nanomaterials prime candidates for a plethora of applications. Herein, we review the field of upconversion nanoparticles from the perspectives of fundamental luminescence mechanisms, new synthetic routes, and current practical approaches to tuning emission color and enhancing upconversion efficiency. In particular, we highlight the recent advances in utilizing upconversion nanocrystals for bioimaging, therapy, biosensing, neuroscience, super-resolution imaging, photoswitching, and lasing applications. We also discuss the key challenges and issues that are critical for the further implementation of upconversion nanoparticles in diverse settings.

Introduction

Lanthanide ion (Ln3+)-mediated photon upconversion refers to a unique nonlinear process in which two or more near-infrared (NIR) pump photons are absorbed through real intermediate energy levels of Ln3+, leading to the emission of light at a wavelength shorter than incident radiation. In the 1960s, this phenomenon was first discovered and formulated in bulk materials by Auzel, Ovsyankin and Feofilov independently [1,2]. Since then, tremendous research efforts have been paid to Ln3+-doped upconversion luminescence owing to its remarkable optical properties such as large anti-Stokes shift, long luminescence lifetime, and tunable emission profile [[3], [4], [5], [6]]. In comparison with well-established two-photon absorption (TPA) and second-harmonic generation (SHG) where high-power density (>106 W/cm2) photon flux produced by expensive ultra-short pulsed lasers are needed to perform upconversion, Ln3+-doped photon upconversion can be activated upon irradiation with a low-cost continuous-wave (c.w.) NIR diode laser or an incoherent light source at low pumping density (∼10−1 W/cm2) [7,8]. Moreover, the lanthanides’ partially filled 4f shells are effectively shielded by outer complete 5s and 5p shells, leading to sharp emission peaks due to intra-4f electron transitions. All these unique features make Ln3+-doped upconversion materials ideal candidates for diverse applications, ranging from biomedicine and super-resolution imaging to volumetric displays and photovoltaics [[9], [10], [11], [12]].

Despite its promise, the investigation of Ln3+-doped upconversion luminescence has been mainly focused on bulk materials or glass ceramics for the first few decades after its discovery [[13], [14], [15]]. It was not until the late 1990s, when nanoscience and nanotechnology became prevalent in the field of materials science, that high-quality upconversion nanoparticles (UCNPs) with controllable composition, crystalline phase, size, and shape were routinely synthesized. The advent of these nanomaterials provides a new platform that significantly expands the repertoire of the luminescence toolbox available for advanced nanophotonic and biological research [[16], [17], [18], [19], [20]]. With the growing awareness of drawbacks associated with traditional downshifting fluorescent dyes and semiconductor quantum dots such as photobleaching, cytotoxicity and the need for high-energy ultraviolet or blue stimulation, Ln3+-doped UCNPs featuring small size, low toxicity and excellent biocompatibility alongside high photostability are particularly attractive as luminescence probes for bioimaging applications [[21], [22], [23], [24], [25]].

To date, the biggest challenge that limits the further application of Ln3+-doped UCNPs is their low quantum efficiency. Owing to the low extinction coefficients of lanthanides and small size-induced surface quenching effects, the quantum yield of Ln3+-doped UCNPs is typically less than 1% [26,27]. Although several approaches and proposals have been shown effective in enhancing the luminescence brightness of UCNPs, the fundamental mechanisms underlying the upconversion pathways are not entirely clear.

In this review, we focus on the controllable preparation, luminescence performance optimization and emerging applications of Ln3+-doped UCNPs. The review is divided into four main sections. Firstly, we briefly introduce the fundamentals governing Ln3+-doped upconversion phenomena, including different types of upconversion mechanisms and electronic transitions between energy levels. In the following section, we attempt to highlight general strategies for the controllable synthesis of UCNPs. Next, we give an overview of tuning mechanisms of upconversion luminescence and summarize current approaches to enhancing photon upconversion. Finally, we highlight the recent success of using UCNPs in developing emerging applications and provide a perspective on design strategies for the optimization of energy harvesting to maximize luminescence amplification.

Section snippets

Upconversion luminescence of lanthanide-doped nanoparticles

Upconversion luminescence generally comes from electronic transitions within lanthanide’s 4fN configuration. Although these electronic transitions are in principle forbidden by quantum mechanical selection rules, they can occur by intermixing higher electronic configurations with f states. More interestingly, the lanthanide’s 4fN electronic configuration could split into an abundance of energy sublevels as a result of strong Coulombic interactions and spin-orbit coupling as well as weak

Design and synthesis of lanthanide-doped upconversion nanoparticles

To date, many chemical techniques have been developed to synthesize Ln3+-doped upconversion nanocrystals with controllable structures, sizes, and shapes (Table 1). Among these methods, hydro(solvo)thermal synthesis, coprecipitation and thermal decomposition are the three most popular approaches for producing high-quality upconversion nanoparticles (Fig. 2). In this section, we will summarize systematically strategies used for controllable synthesis of Ln3+-doped UCNPs.

Manipulation of lanthanide-doped upconversion luminescence

Lanthanide-doped upconversion nanoparticles exhibit excellent luminescent properties. The precise control over the luminescence profiles is of vital importance for their applications in many research fields. To date, a variety of strategies have been developed to manipulate the emission profiles of Ln3+-doped UCNPs. For example, color-tunable emissions can be easily achieved by controlling the combination of dopant-host, particle size, and the concentration of dopants in upconversion

Enhancement of lanthanide-doped upconversion luminescence

Although promising results have been achieved with the precise control over size, shape, and emission profile of Ln3+-doped UCNPs, the inherent low upconversion efficiency of UCNPs still significantly restricts their practical applications. To date, a range of methods, such as host lattice manipulation, surface passivation, surface plasmon coupling, energy transfer modulation, broadband sensitization, photonic crystals mediation, and inorganic-organic hybrid, have been proposed for optimizing

Emerging applications of lanthanide-doped upconversion nanoparticles

Thanks to the rapid development of well-controlled UCNPs along with their excellent optical properties, researches on Ln3+-doped upconversion nanoparticles have made tremendous progress over the past decade, especially in the area of biomedical applications [[21], [22], [23], [24], [25]]. In addition to the aforementioned advantages of Ln3+-doped UCNPs such as multicolor emission and the absence of autofluorescence in biological specimens, the employment of near-infrared excitation light can

3D cellular imaging through upconversion

Fluorescence cellular imaging is one of the most direct tools innovated for understanding cellular dynamics. While advanced fluorescence imaging techniques have been developed over the years, there are still fundamental challenges in distinguishing intracellular activities in 2D imaging. In contrast, 3D imaging of living cells can provide a more detailed and accurate spatial visualization of the interplay of cells and their components than achieveable by conventional 2D imaging systems, in

Conclusions and outlook

Lanthanide-doped upconversion nanoparticles have received increasing attention in recent years because of their unique NIR excitation characteristics and exceptional light penetration depth, along with sharp emission bands, large anti-Stokes shifts, and remarkable photostability. In this review, we have summarized recent development of lanthanide-doped upconversion nanocrystals mainly focusing on controllable synthesis, multicolor emission tuning, luminescence enhancement, and especially the

Acknowledgements

This work is supported by the Singapore Ministry of Education (Grant MOE2017-T2-2-11), Agency for Science, Technology and Research (Grant A1883c0011), National Research Foundation, Prime Minister’s Office, Singapore under the NRF Investigatorship programme its Competitive Research Program (CRP Award No. NRF-NRFI05-2019-0003), National Natural Science Foundation of China (11774133, 21771135, and 61705137), and the Science and Technology Project of Shenzhen (JCYJ20170817093821657,

Kezhi Zheng earned his B.S. degree in physics from Harbin Normal University in 2005. He received his Ph.D. degree in Physical Electronics from Jilin University in 2011. He then joined the faculty of the Jilin University. He is currently a postdoctoral fellow in the group of Prof. Xiaogang Liu at the National University of Singapore. His current research interests focus on the design and synthesis of lanthanide-doped upconversion materials for emerging applications.

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    Kezhi Zheng earned his B.S. degree in physics from Harbin Normal University in 2005. He received his Ph.D. degree in Physical Electronics from Jilin University in 2011. He then joined the faculty of the Jilin University. He is currently a postdoctoral fellow in the group of Prof. Xiaogang Liu at the National University of Singapore. His current research interests focus on the design and synthesis of lanthanide-doped upconversion materials for emerging applications.

    Kang Yong Loh obtained his B.S. degree in Chemistry from the University of Illinois at Urbana-Champaign in 2017 under the supervision of Prof. Yi Lu. He then spent a gap year in the lab of Prof. Xiaogang Liu at the National University of Singapore and the Institute of Materials Research and Engineering (A*STAR). He is currently a Ph.D. graduate student and a Stanford ChEM-H Chemistry/Biology Interface Predoctoral Trainee at Stanford University, Department of Chemistry under the supervision of Prof. Karl Deisseroth. His research interests include developing novel chemical and protein tools to address questions in neuroscience.

    Yu Wang earned his BE degree in Jilin University. He completed his master's study in Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences in 2007 and received his Ph.D. degree from the University of Amsterdam in 2011. After working at the Hong Kong University of Science and Technology as a research fellow, he became a postdoctoral researcher in the group of Prof. Xiaogang Liu at the National University of Singapore in 2013. He joined Shenzhen University In 2016. His research interest focuses on optical spectroscopy and new applications of photon upconversion materials.

    Qiushui Chen received his B.Sc. degree in chemistry from Fuzhou University in 2009 and Ph.D. degree in Chemistry from Tsinghua University in 2014. He then joined the group of Prof. Xiaogang Liu as a research fellow in the Department of Chemistry at the National University of Singapore in 2015. His research interests are inorganic luminescence nanocrystals and their use for optical imaging.

    Jingyue Fan was born in Sichuan, China. She earned her B.S. degree in 2017 from the National University of Singapore (NUS). She is now pursuing a Ph.D. degree at NUS under the supervision of Professor Xiaogang Liu. Her research interests focus on the synthesis of optical nanomaterials and understanding of energy transport at single-particle levels.

    Taeyoung Jung was born in Gumi, Korea, in 1991. He received his B.S. at the Chungnam National University (Daejeon, Korea) in 2014. He is currently a doctorate research student at Gwangju Institute of Science and Technology (GIST) under the supervision of Prof. Kang Taek Lee. During his doctorate research, he worked at the Laboratory for Advanced Molecular Probing (LAMP) in Korea Research Institute of Chemical Technology (KRICT) for two years in 2017–2019 under the supervision of Prof. Dr. Yung Doug Suh. His research mainly focused on lanthanide-doped upconverting nanoparticles and their application in cell biology.

    Sang Hwan Nam got his B.S. (2002) in Chemistry from Kyung Hee University, Seoul, Korea and received M.S.(2004) and Ph.D. (2010) in Physical Chemistry from the same university. He joined the Laboratory for Advanced Molecular Probing (LAMP), Korea Research Institute of Chemical Technology (KRICT) as a Researcher in 2009 and worked as a Postdoc Fellow from 2010 to 2013. He was promoted to his current position, a Senior Researcher, at the same lab of KRICT in August 2013. His current research focuses on in vitro and in vivo bioimaging applications of nanomaterials, upconversion nanoparticles (UCNPs), magnetic nanoparticles (MNPs), and Au nanoparticles (AuNPs), as well as the development of high-sensitivity imaging platforms for the disease diagnosis.

    Yung Doug Suh studied at Seoul National University for his BS(1991), MS(1993), and PhD(1999) under the guidance of Prof. Seong Keun Kim in Chemistry Department, Prof. Young Kuk in Physics Department, and Dr. Dongho Kim in Korea Research Inst. of Standards and Science (KRISS) researching gas-phase molecular reaction dynamics, surface physics with UHV-STM, and laser spectroscopies, respectively. After finishing his Postdoctoral research in ETH Zurich working with Prof. Renato Zenobi in 1999–2000, where he co-invented TERS(Tip-enhanced Raman Scattering), he worked at the Pacific Northwest Nat’l Laboratory (PNNL), USA, in 2001–2002 doing single-molecule spectroscopy. He accepted a recruited principal research scientist position in 2003, to form his research group, Laboratory for Advanced Molecular Probing (LAMP), at the Korea Research Institute of Chemical Technology (KRICT) in Daejeon, Korea. He is currently a Group Leader at the Research Center for Bio Platform Technology in KRICT, and also serves as a professor at the School of Chemical Engineering in SungKyunKwan Univerisity (SKKU), Korea, since March 2013.

    Xiaogang Liu earned his B.E. degree (1996) in Chemical Engineering from Beijing Technology and Business University, P. R. China. He received his M.S. degree (1999) in Chemistry from East Carolina University under the direction of Prof. John Sibert and completed his Ph.D. (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 the National University of Singapore in 2006. Currently, he sits as an Associate Editor for Nanoscale and serves on the editorial boards of Chemistry−An Asian Journal, Advanced Optical Materials, Journal of Luminescence, and Journal of Physical Chemistry Letters. His research encompasses optical nanomaterials and energy transfer and explores the use of luminescent nanocrystals for photocatalysis, sensing, and biomedical applications.

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