Abstract:In this review, we describe the synthesis, physical properties, surface functionalization, and biological applications of silicon nanoparticles (also known as quantum dots). We compare them against current technologies, such as fl uorescent organic dyes and heavy metal chalcogenide-based quantum dots. In particular, we examine the many different methods that can be used to both create and modify these nanoparticles and the advantages they may have over current technologies that have stimulated research into designing silicon nanoparticles for in vitro and in vivo applications.

The hydrogen-terminated surface of porous silicon (PS) is sufficiently reactive for the uncatalyzed hydrosilation of alkenes and alkynes. These modifications produce dramatic changes to both the physical and chemical properties of the PS.

The synthesis of supramolecular conducting nanowires can be achieved by using DNA and pyrrole. Oxidation of pyrrole in DNA-containing solutions yields a material that contains both the cationic polypyrrole (PPy) and the anionic DNA polymers. Intimate interaction of the two polymer chains in the self-assembled nanowires is indicated by FTIR spectroscopy. AFM imaging shows individual nanowires to be continuous, approximately 5 nm high and conformationally flexible. This feature allows them to be aligned by molecular combing in a similar manner to bare DNA and provides a convenient method for fabricating a simple electrical device by stretching DNA/PPy strands across an electrode gap. Current-voltage measurements confirm that the nanowires are conducting, with values typical for a polypyrrole-based material. In contrast to polymerisation of pyrrole on a DNA template in bulk solution, attempts to form similar wires by polymerisation at surface-immobilised DNA do not give a continuous coverage; instead, a beads-on-a-string appearance is observed suggesting that immobilisation inhibits the assembly process.

Polyindole (PIn) nanowires were formed on a lambda-DNA template by chemical oxidation of indole using aqueous FeCl3. The resulting nanowires are smooth, regular, conductive and had diameters in the range of 5-30 nm. These features allow them to be aligned by molecular combing and studied by scanned conductance microscopy, conductive AFM, and two-terminal I-V measurements. Using this combination of measurements, we find that the conductivity of PIn/DNA nanowires is between 2.5 and 40 S cm(-1) at room temperature, which is substantially greater than that in previous reports on the bulk polyindole conductivity (typically 10(-2)-10(-1) S cm(-1)). The conductance at zero bias shows an Arrhenius-type of dependence on temperature over the range of 233 to 373 K, and the values observed upon heating and cooling are repeatable within 5%; this behavior is consistent with a hopping mechanism of conductivity.

The reactions of alcohols and alkenes with hydrogen-terminated silicon surfaces have been investigated using infrared spectroscopy and deuterium labeling of the reagents and the surface termination. Transmission FTIR spectra were obtained on samples of electrochemically grown porous silicon or mechanically abraded silicon wafers to obtain a sufficient signal-to-noise ratio. The spectral assignments are supported by ab initio calculations on small molecule models at the MP2/6-311++G(d,p) level of theory. A convenient method for the preparation of fully deuterated (D-terminated) silicon wafers is reported; however, fully deuterated porous silicon could not be prepared this way. The spectrum of partially deuterated porous silicon could be assigned on the basis of the computed harmonic vibration frequencies for Q 3 Si-SiH 2 -SiQ 3 and Q 3 Si-SiHD-SiQ 3 where Q is a pseudo-hydrogen atom with the atomic mass of Si. The reaction of O-deuterated alcohols and water on porous silicon produced Si-D stretching and Si-HD scissor modes in the infrared spectrum. The kinetics were consistent with either a dissociative adsorption or an electrochemical corrosion mechanism for this reaction. However, in all cases a net decrease of Si-H/D species on the surface was observed. The magnitude of this decrease is consistent with hydrogen evolution from a hydridic reactivity of the surface termination analogous to the formation of SiO 2 via hydrolysis of molecular hydrosilanes. The Si-H/D, O x Si-H, and Si-O vibrations could be assigned using small molecule models of the form QOSiH 2 SiQ 3 , QOSiH 2 OQ, and (QO) 3 SiH. Significant amounts of silicon alkoxide species are formed even in the presence of water, but the major process in wet solvents is hydrogen evolution and oxide formation. The currently accepted mechanism for the hydrosilylation of alkenes by hydrogen-terminated silicon surfaces involves the attack of a silyl radical on the double bond to produce a Si-C bond and a carbon-centered radical. In principle, this carbon radical may abstract a hydrogen atom from the surface and propagate a chain; however, using deuterated silicon wafers no C-D stretching vibrations could be detected. This indicates that under the conditions employed (1 M alkene solutions in refluxing toluene) the carbon radical abstracts a hydrogen atom from the solvent or another alkene molecule. Ab initio calculations on small molecule models were used to investigate theoretically the shift to low frequency in the Si-H vibrations on the formation of Si-C bonded species at the surface and this effect is attributed to the replacement of Si-H 2 with C-Si-H functionality at the surface.