Zhenhui Kang

Ultrastable, Highly Fluorescent, and Water‐Dispersed Silicon‐Based Nanospheres as Cellular Probes

Fluorescent cellular probes are powerful tools for studying cellular morphology, behavior, and physiological functions. For optimum imaging and tracking of biological cells, these probes should be water-dispersible, antibleaching, luminescent, and biocompatible. In the last century, organic dyes and fluorescent proteins were mostly used as fluorescent probes in biological and biomedical research; however, they suffer from severe photobleaching that restricts their applications for long-term in vitro or in vivo cell imaging. This shortcoming has led to an intense interest in colloidal fluorescent semiconductor quantum dots (QDs) since II/VI QDs were shown to be promising for cell imaging in 1998. Compared to fluorescent dyes, QDs possess unique advantages such as size-tunable emission wavelengths, broad photoexcitation, narrow emission spectra, strong fluorescence, and high resistance to photobleaching. Consequently, QDs have been widely used as a new class of fluorescent probes in cellular research, for example, in vitro imaging, cell labeling, and tracking cell migration. However, recent investigations have shown that these QDs are cytotoxic, especially under harsh conditions (e.g., UV irradiation), because heavy metal ions (such as Cd or Pb ions) are often released in oxidative environments, which leads to severe cytotoxicity by conventional mechanisms of heavy metal toxicity. Surface modification of QDs, such as epitaxial growth of the ZnS shell, silica coating, or polymer coating, has been used to alleviate the cytotoxicity problem. While these techniques are viable to a certain extent, they are relatively complicated and require additional processing steps. Notably, the risk of cytotoxic problems still remains since heavy-metal ions contained in the modified QDs may invariably be released in physiological environments. Consequently, the inherent problems, that is, severe photobleaching and cytotoxicity, associated with the traditional dyes and the fluorescent II/VI QDs remain unsolved, and have fueled a continual and urgent search for new cellular probes that are more photostable and biocompatible. Silicon is the leading semiconductor material for technological applications because of its wide-ranging applications by the electronics industry. Silicon-based nanostructures, such as nanoribbons, nanowires, and nanodots, are being intensely investigated. The quantum confinement phenomenon in silicon QDs (SiQDs) is a particular focus of research since it would increase the probability of irradiative recombination by indirect-to-direct band-gap transitions, which lead to enhanced fluorescent intensity and the prospect of longawaited optical applications. The biocompatibility and noncytotoxic properties of SiQDs are much better than those of traditional II/VI QDs, although their photoluminescence (PL) intensity is weaker. As such, silicon-containing materials may be useful for biological applications such as cellular imaging and labeling because of their favorable biocompatibility, if water-dispersed silicon nanomaterials with adequate stability and strong fluorescence are properly developed. Herein we report a new variety of silicon-based nanospheres for use as cellular probes, which possess excellent water-dispersibility, strong photoluminescence, and robust photostability. Our design of these nanospheres was formulated from calculations performed by using the B3LYP/6-31G method, a classic calculation method based on hybrid density functional theory. Figure 1a shows that the calculated free energy of the products PSi O ( 15.2 kcal mol ) is higher than that of PSi C ( 33.1 kcalmol ), whereas the energy barrier of forming the Si O bond (ESi O = 41.5 kcalmol ) is much lower than that of the Si C bond (ESi C = 59.5 kcal mol ). The energy consideration implies that an acrylic acid monomer (AAc) would react with the surface of the SiQDs via the carbonyl or alkene groups by forming Si O or Si C bonds, as previously reported. Furthermore, the calculations yield the energy relation of ESi O (41.5 kcal mol )<Eblue (58.3 kcalmol )< ESi C (59.5 kcal mol )<EUV (79.4 kcalmol ). It shows that the energy value of blue light (Eblue) is larger than the energy barrier of formation ESi O of the Si O bond, but less than the [*] Prof. C.-H. Fan Shanghai Institute of Applied Physics Chinese Academy of Sciences Shanghai 201800 (P.R. China) E-mail: fchh@sinap.ac.cn

Polyhedral Organic Microcrystals: From Cubes to Rhombic Dodecahedra

Research Grants Council of Hong Kong SAR, China [CityU101608]; City University of Hong Kong [7002275]; National Basic Research Program of China [2006CB933000, 2007CB936000]; National Natural Science Foundation of China [50825304, 50903059]

High-Quality Graphenes via a Facile Quenching Method for Field-Effect Transistors

Single- and few-layer graphene sheets with sizes up to 0.1 mm were fabricated by simply quenching hot graphite in an ammonium hydrogen carbonate aqueous solution. The identity and thickness of graphene sheets were characterized with transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. In addition to its simplicity and scalability, the present synthesis can produce graphene sheets with excellent qualities in terms of sizes, purity, and crystal quality. The as-produced graphene sheets can be easily transferred to solid substrates for further processing. Field-effect transistors based on individual graphenes were fabricated and shown to have high ambipolar carrier mobilities.

Dye degradation induced by hydrogen-terminated silicon nanowires under ultrasonic agitations

A method for degradation of environmentally hazardous dyes using silicon nanowires (SiNWs) has been developed. Environmentally unfriendly methyl red was degraded with assistance of H-terminated SiNWs under ultrasonic agitation. The hydrogenated surfaces of SiNWs are shown to be responsible for the surface reaction and decay of methyl red. The rate of degradation increases with the amount of SiNWs and agitation power. SiNWs after their application can be recycled and reactivated for further uses by a simple heating in hydrogen plasmas.