Nano Today
ReviewSilicon nanostructures for bioapplications
Introduction
Nano-biotechnology is a promising and interdisciplinary research involving chemistry, physics, biology and medicine [1], [2]. Particularly, the utilization of nanostructured materials in biology and biomedicine is recognized as one of the fast moving and exciting interfaces in nano-biotechnology, and has been extensively explored [3], [4], [5], [6], [7]. It is critically important to develop and fabricate novel functional nanomaterials with well-defined structures for nano-biotechnology [5], [6], [7]. During the past decade, a great number of nanomaterials and nanofabrication techniques have been elegantly developed, imparting momentum to the development of nano-biotechnology and opening up new avenues in biological and biomedical studies [1], [2], [3], [4]. To date, many interesting nanomaterials, such as fluorescent quantum dots [5], magnetic nanoparticles [6] and carbon nanotubes [7], have been studied and utilized in widespread nano-bioapplications (e.g., biological imaging, biosensor, disease diagnosis and therapy). Their attractive optical, magnetic and mechanical properties provide new platforms for studying complicated biological processes that are difficult to access with conventional techniques [3], [4], [5], [6], [7].
Silicon nanomaterials are a type of important nanomaterials with attractive properties including excellent electronic/mechanical properties, favorable biocompatibility, huge surface-to-volume ratios, surface tailorability, improved multifunctionality, as well as their compatibility with conventional silicon technology [8], [9], [10], [11]. Consequently, there has been great interest in developing functional silicon nanomaterials for various applications ranging from electronics to biology. To meet increasing demands of silicon-based applications, silicon materials of various nanostructures (e.g., nanodot [12], [13], nanowire [14], [15], nanorod [16], [17] and nanoribbon [18], [19]) have been developed, among which silicon quantum dots (SiQDs) and silicon nanowire (SiNWs) are of our primary interest [12], [13], [14], [15], [20], [21], [22], [23], [24]. Quantum confinement phenomenon in SiQDs can increase the probability of irradiative recombination via direct band gap transition [15], [26], leading to improvement of fluorescent intensity and the prospect of optical applications. Due to their excellent biocompatibility and noncytotoxic property, SiQDs are also considered as promising fluorescent biological probes for in vivo and in vitro imaging [27], [28], [29], [30], [31]. On the other hand, SiNWs could act as a general platform for greatly enhanced surface-enhanced Raman scattering (SERS) studies. Significantly, SiNWs decorated with metal nanoparticles, e.g. silver nanoparticles, possess high enhancement factor (EF) of up to 107–109. Such SiNWs-based highly efficient SERS-active substrates could be utilized as biosensors for ultrasensitive detection of biomolecules (e.g., DNA and protein) [32], [33], [34], [35], [36].
In this review article, by paying particular attention to the SiQDs and SiNWs, we summarize recent research progress in synthesis and bioapplications of these low-dimensional silicon nanomaterials, based particularly on recent progress of our laboratory. In brief, in the following sections, we first introduce methods for preparing SiQDs and SiNWs, and categorize representative synthesis strategies. Then we summarize typical examples of SiQDs/SiNWs-based bioapplications that were reported recently, and describe in detail about the biological applications of SiQDs or SiNWs as fluorescent bioprobes or biosensors, respectively. In the final section, we discuss challenges and perspectives for the silicon-relative biological applications in the future. This review intends to take SiQDs and SiNWs as models and discuss topics ranging from their synthesis to their biological applications, with the hope to outline these exciting achievements as the starting points in the realm of silicon-based nano-biotechnology. Due to page limitation, this review will not discuss the applications of silicon nanostructures in other important areas, such as energy, catalysis and optoelectronics.
Section snippets
Synthesis of SiQDs
Bulk silicon is considered as one of the most important materials in semiconductor microelectronics industry; nevertheless, its optical properties are poor because of its indirect band gap, leading to the slow electron-hole radiative recombination [37]. Notably, intense room temperature photoluminescence could be observed from nanocrystalline silicon, which is generally believed to result from a combination of quantum confinement effects. For examples, recombination rates of electrons and holes
Bioapplications of SiQDs and SiNWs
SiQDs and SiNWs are attractive low-dimensional silicon nanostructures that have found important applications in biology, particularly fluorescence bioimaging and ultrasensitive biosensing. We will summarize their interesting bioapplications in this section (Fig. 4).
SiNWs biosensor
One-dimensional nanomaterials (e.g., carbon nanotubes (CNTs) and SiNWs) have been employed as an effective substrate for various sensing applications because of their favorable biocompatibility, convenient surface modification, huge surface-to-volume ratios, fast response and good reproducibility [7], [14], [15], [79]. These unique properties make CNTs and SiNWs attractive in the development of field-effect transistor (FET), chemical and biological sensors [80], [81], [82], [83], [84], [85],
Conclusion and perspectives
In the past few years, there have been considerable advances in the synthesis and bioapplications of silicon quantum dots and silicon nanowires. Several efficient synthetic strategies including solution-phase reduction, plasma-assisted aerosol precipitation, microemulsion, mechanochemical and chemical etching synthesis have been developed to synthesize highly fluorescent SiQDs so far. Importantly, SiQDs become well water-dispersed after appropriate surface modification with either hydrophilic
Acknowledgements
This work was supported by the Research Grants Council of HKSAR (CityU5/CRF/08 and N_CityU108/08), Ministry of Health (2009ZX10004-301), National Basic Research Program of China (2007CB936000) and NSFC (30900338, 20725516, 90913014) 973 (2010CB934502). The authors would like to acknowledge significant contributions from Dr. K.Q. Peng, Dr. M.W. Shao, Dr. Z.H. Kang, Dr. J. Antonio Zapien, Dr. Y.Y. Su, Dr. M.L. Zhang, Dr. X. Fan, Dr. S. Su, Dr. T.T. Xu and Mr. C.W.C. Michael.
Yao HE received his bachelor degree (2003) in chemistry from Fudan University (China), and continued the graduate study (Master and Doctor) in Fudan University (China) in 2003–2007. He was a visiting scholar for the joint program in Shanghai Institute of Applied Physics, Chinese Academy of Sciences (2005–2007). Afterwards, he worked in the Center of Super-Diamond and Advanced Films (COSDAF) in the City University of Hong Kong, HKSAR, China, as a research fellow. He became an associate professor
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Yao HE received his bachelor degree (2003) in chemistry from Fudan University (China), and continued the graduate study (Master and Doctor) in Fudan University (China) in 2003–2007. He was a visiting scholar for the joint program in Shanghai Institute of Applied Physics, Chinese Academy of Sciences (2005–2007). Afterwards, he worked in the Center of Super-Diamond and Advanced Films (COSDAF) in the City University of Hong Kong, HKSAR, China, as a research fellow. He became an associate professor in Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University (China, 2009). His research interest focuses on development of functional nanomaterials and exploration of nanomaterials-based bioapplications (e.g., bioimaging and biosensors).
Chunhai Fan received his BS (1996) and PhD (2000) degrees in biochemistry and molecular biology from Nanjing University (China). After postdoctoral research at the University of California, Santa Barbara (USA), he became a professor in Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences in 2004. He is now the director of the Division of Physical Biology of SINAP. He was the recipient of the Chinese Chemical Society Prize for Young Scientists (2006) and NSFC Outstanding Young Investigators (2007). His research interests focus on electrochemical and optical biosensors and the design of nanoprobes for biological systems.
Shuit-Tong Lee received his BS (1969) from The Chinese University of Hong Kong, HKSAR, China, MS (1971) from University of Rochester (USA), PhD (1974) from the University of British Columbia (Canada), and postdoctoral training (1974–1976) in University of California, Berkeley, all in Chemistry. He worked in the Research Laboratories of Eastman Kodak Co. (USA) in 1976–1994 as Senior Scientist and Project Manager. He joined City University of Hong Kong in 1994 as a Senior Lecturer, and became Chair Professor of Materials Science in 1996 and Founding Director of the Center of Super-Diamond and Advanced Films (COSDAF) in 1998. He is Founding Director of Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University, Suzhou (China). He was elected a Member of Chinese Academy of Sciences in 2005 and a Fellow of the Academy of Sciences for the Developing World in 2006. His research interests are in nanomaterials and nanotechnology, organic electronics, diamond and superhard coatings, and surface science. He has published over 650 peer-reviewed SCI articles, six book chapters and 20 US patents. He was bestowed the Humboldt Senior Research Award (Germany, 2001), Croucher Senior Research Fellowship (Hong Kong, 2002), two National Natural Science Awards of China (second rank, 2003 and 2005), Scientific and Technological Progress Award of Ho Leung Ho Lee Foundation (Hong Kong, 2008) and Hans Fischer Senior Fellowship of Institute of Advanced Study of Technical University of Munich (Germany, 2009). He is the Associate Editor of Applied Physics Letters and Diamond and Related Materials.