Metal nanoclusters: New fluorescent probes for sensors and bioimaging

doi:10.1016/j.nantod.2014.02.010

Nano Today

Volume 9, Issue 1, February 2014, Pages 132-157

https://doi.org/10.1016/j.nantod.2014.02.010 Get rights and content

Highlights

•
Fluorescent metal nanoclusters have been developed as a new class of fluorophores with excellent properties.
•
New developments in the controllable synthesis and properties of metal nanoclusters are introduced.
•
The applications of metal nanoclusters in sensors and bioimaging are summarized.
•
Future challenges and prospects of the development of metal nanoclusters are concluded.

Summary

Fluorescent metal nanoclusters (NCs) as a new class of fluorophores have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. The size of metal NCs approaches the Fermi wavelength of electrons, between metal atoms and nanoparticles, resulting in molecule-like properties including discrete energy levels, size-dependent fluorescence, good photostability and biocompatibility. These excellent properties make them ideal fluorescent probes for biological application. Up to now, significant efforts have been devoted to the synthesis, property and application studies of gold and silver NCs. Recently, a growing number of studies on copper and other metal clusters have also been reported. In this review article, we focus on summarizing recent advances in controllable synthesis strategies, chemical and optical properties, and sensing and imaging applications of metal NCs (mainly including Au, Ag, Cu, etc.). Finally, we conclude with a look at the future challenges and prospects of the future development of metal NCs.

Graphical abstract

Introduction

Nanomaterials have been recognized as the most modish research topics in the past decades [1], [2], [3]. Nobel metal nanomaterials with interesting size-dependent electrical, optical, magnetic, and chemical properties have been intensively pursued, not only for their fundamental scientific interest, but also for their many technological applications [4], [5], [6]. Especially, metal nanoclusters (NCs) have attracted special attention due to their attractive features and molecule-like properties [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Metal NCs usually consist of a few to a hundred atoms, and the sizes are comparable to the Ferimi wavelength of electrons [7], which endows them an important role-the missing link between single metal atoms and plasmonic metal nanoparticles. In this size regime, the continuous density of states breaks up into discrete energy levels [16], [17]. Due to the electrons of metal atoms confined in molecular dimensions and the special discrete energy levels, metal NCs exhibit dramatically different optical, electronical and chemical properties, including strong photoluminescence, excellent photostability, good biocompatibility and sub-nanometer size. Such novel properties make metal NCs an ideal nanomaterial for promise applications in biological analysis and imaging, environmental monitoring, industrial catalysis and electronic devices.

Fluorescent metal NCs have been developed as a new class of fluorophores. Current fluorescence applications mostly involve organic fluorophores, semiconductor quantum dots (QDs) [18] or fluorescent proteins [19]. Organic fluorophores exist in a wide range of chemical structures and spectral properties; however, they are prone to photobleaching, which may limit their many applications. Semiconductor QDs are fluorescence-tunable and photostable; however, their large physical size may hinder their use as fluorescent reporters of binding events, which may compromise their use for in vivo applications. Fluorescent proteins are genetically encodable, which can be produced by the cells and organisms themselves. Consequently, there is no need for additional labeling and/or other chemical procedures in studies of live cells and organisms. Metal NCs show strong photoluminescence, combined with good photostability and high emission rates, and have a sub-nanometer size and size or scaffold-dependent tunable fluorescence. They have an appealing set of features that complements the conventional fluorophores. These properties establish them as a new class of ultra-small, biocompatible fluorophores for applications as biological labels or optoelectronics emitters.

It should be pointed out that several excellent review papers have been dedicated to the metal NCs [8], [9], [10], [11], [12], [13], [14]. However, these previous reviews focused on either one kind of metal or metal NCs with actually a wide size range and different focusing aspects. In this article, we will mainly summarize recent advances in the synthesis, special properties and the applications of metal NCs (Au, Ag, Cu, etc.) as new fluorescent probes for analytical sensing and biological imaging. In the final section, we will give a brief outlook on the challenges and opportunities for future metal NCs research.

Section snippets

Energy levels: from bulk metals to nanoclusters

The physical and chemical properties of metals depend greatly on their size. With the varying size, their behaviors go through several noticeable transitions (Fig. 1) [17]. Bulk metals are good optical reflectors and electrical conductors. The electronic situation in bulk metals is characterized by the existence of energy bands. They result from the combination of an infinite number of energetically very similar orbitals. The valence band contains the relevant valence electrons. The conduction

Synthesis of metal nanoclusters

The controllable synthesis of metal NCs with high quality is of paramount importance and highly desirable. To date, in order to obtain high-quality metal NCs, some key factors should be taken into consideration. (1) The ligand should have strong interaction with metal NCs. (2) The reducing condition should be strict. Generally, strong reducing agents or light irradiation or sonication should be employed to improve the quantum yield (QY) of NCs. (3) Long aging time is also important for

Absorption properties

The absorption properties of metal nanoparticles are mainly dependent on the surface plasmon resonance of conduction electrons. Generally, metal nanoparticles show intense colors due to the collective oscillation of conduction electrons upon interaction with visible light. While, as for metal NCs, the continuous density of states breaks up into discrete energy levels [16], they no longer exhibit plasmonic properties, but they still interact with light through electronic transitions between

Applications of metal nanoclusters

The unique physicochemical properties of metal NCs make them attractive for use in various fields including both fundamental science studies and technological exploration. In this section, we summarize recent advances in the application of metal NCs as new fluorescent probes for analytical sensors and biological imaging.

Conclusion and perspective

In summary, we provide recent advances of the research progress on metal NCs, from the challenging synthesis, unique properties to the promising application in analytical sensors and biological imaging. Now, metal NCs have been developed as a new class of fluorophores, their excellent properties endow them attractive fluorescent probes for biological applications. These research results reveal that metal NCs can open many good opportunities in an extremely multidisciplinary environment for

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21190040 and 91227114) and 973 Project (Nos. 2010CB33600 and 2011CB911000).

Libing Zhang was born in Shandong Province, China. He received his B.S. degree from Liaocheng University in 2007 and M.S. degree from Changchun University of Science and Technology in 2010. Then, he moved to Changchun Institute of Applied Chemistry as a Ph.D. student under the direction of Professor Erkang Wang, and received his Ph.D. degree in 2013. He is currently a Postdoctoral Research Associate in Biodesign Institute at Arizona State University. His scientific interests focus on functional

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Erkang Wang, Professor of Chemistry at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS), and an advisor in the State Key Laboratory of Electroanalytical Chemistry. He received his Ph.D. degree in 1959 from Czechoslovak Academy of Sciences directly under Professor J. Heyrovsky, the Nobel Prize Laureate. He is academician of both the CAS and the Academy of Sciences for the Developing World. He has been on the Editorial and Advisory Board of nine international journals. His research interests lie in the fields of nanomaterials/nanotechnology, biosensors, electrochemistry and electrochemiluminescence. He has published over 900 papers and monographs in international journals with the SCI over 17,000 times cited.

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Nano Today

ReviewMetal nanoclusters: New fluorescent probes for sensors and bioimaging

Highlights

Summary

Graphical abstract

Introduction

Section snippets

Energy levels: from bulk metals to nanoclusters

Synthesis of metal nanoclusters

Absorption properties

Applications of metal nanoclusters

Conclusion and perspective

Acknowledgments

Nano Today

Nano Today

Anal. Chim. Acta

Anal. Chim. Acta

Chem. Rev.

Chem. Soc. Rev.

Acc. Chem. Res.

Adv. Mater.

Chem. Soc. Rev.

Annu. Rev. Phys. Chem.

Anal. Bioanal. Chem.

Chembiochem

Chem. Soc. Rev.

Chem. Soc. Rev.

Acc. Chem. Res.

J. Mater. Chem.

Acc. Chem. Res.

Nanoscale

Advanced Fluorescence Reporters in Chemistry and Biology II

Nat. Methods

Chem. Soc. Rev.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Nanoscale

Nanoscale

J. Am. Chem. Soc.

J. Am. Chem. Soc.

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Small

J. Phys. Chem. B

J. Am. Chem. Soc.

Nanoscale

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Nano Lett.

J. Phys. Chem. B

Nanoscale

J. Am. Chem. Soc.

Phys. Rev. Lett.

Nano Res.

J. Am. Chem. Soc.

ACS Nano

J. Am. Chem. Soc.

Small

Chem. Eur. J.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Review
Metal nanoclusters: New fluorescent probes for sensors and bioimaging