Chi Him A. Tsang

Water‐Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design

Carbon nanostructures are attracting intense interest because of their many unique and novel properties. The strong and tunable luminescence of carbon materials further enhances their versatile properties; in particular, the quantum effect in carbon is extremely important both fundamentally and technologically. Recently, photoluminescent carbonbased nanoparticles have received much attention. They are usually prepared by laser ablation of graphite, electrochemical oxidation of graphite, electrochemical soaking of carbon nanotubes, thermal oxidation of suitable molecular precursors, vapor deposition of soot, proton-beam irradiation of nanodiamonds, microwave synthesis, and bottom-up methods. Although small (ca. 2 nm) graphite nanoparticles show strong blue photoluminescence (PL), definitive experimental evidence for luminescence of carbon structure arising from quantum-confinement effects and size-dependent optical properties of carbon quantum dots (CQDs) remains scarce. Herein, we report the facile one-step alkali-assisted electrochemical fabrication of CQDs with sizes of 1.2– 3.8 nm which possess size-dependent photoluminescence (PL) and excellent upconversion luminescence properties. Significantly, we demonstrate the design of photocatalysts (TiO2/CQDs and SiO2/CQDs complex system) to harness the use of the full spectrum of sunlight (based on the upconversion luminescence properties of CQDs). It can be imagined that judicious cutting of a graphite honeycomb layer into ultrasmall particles can lead to tiny fragments of graphite, yielding CQDs, which may offer a straightforward and facile strategy to prepare high-quality CQDs. Using graphite rods as both anode and cathode, and NaOH/EtOH as electrolyte, we synthesized CQDs with a current intensity of 10–200 mAcm . As a reference, a series of control experiments using acids (e.g. H2SO4/EtOH) as electrolyte yielded no formation of CQDs. This result indicates that alkaline environment is the key factor, and OH group is essential for the formation of CQDs by the electrochemical oxidation process. Figure 1a shows a trans-

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

ZnO nanowires array p-n homojunction and its application as a visible-blind ultraviolet photodetector

We demonstrated a simple and low-cost fabrication of ZnO p-n homojunction. The junction consists of n-type ZnO nanowires array by a hydrothermal method covered with p-type Al, N co-doped ZnO film by a sol-gel method. The junction exhibits good rectification characteristics, with reverse leakage current and rectification ratio of ∼5 μA and ∼150 at bias of 3 V, respectively. The junction is operated as a photodetector when light radiation is shined on the glass-side of the device. The photodetector shows a peak responsivity at 384 nm with UV-visible responsivity ratio (R384 nm/R550 nm) of ∼70 at an operating bias of −3 V.

Silicon nanowires for high-specificity and high-selectivity sensors under low-frequency scanning

The high specificity and selectivity of H–Si nanowire bundles, which are single crystalline and composed of pure Si without oxygen, for detecting water (peak at 12 Hz) and ethanol (peak at 70 Hz) in their mixture are measured by a frequency scanning test. The signal amplitude deduced between the work channel and the reference channel {[(VR-VS)/VR]×100%} is defined as the impedance recorded under different scanning frequencies.