Solution synthesis, optical properties, and bioimaging applications of silicon nanocrystals.

Understanding and unlocking the potential of semiconductor nanocrystals (NCs) is important for future applications ranging from biomedical imaging contrast agents to the next generation of solar cells and LEDs. Silicon NCs (Si NCs) have key advantages compared with other semiconductor NCs due to silicon's high natural abundance, low toxicity and strong biocompatibility, and unique size, and surface dependent optical properties. In this Account, we review and discuss the synthesis, surface modification, purification, optical properties, and applications of Si NCs. The synthetic methods used to make Si NCs have improved considerably in the last 5-10 years; highly monodisperse Si NCs can now be produced on the near gram scale. Scaled-up syntheses have allowed scientists to drive further toward the commercial utilization of Si NCs. The synthesis of doped Si NCs, through addition of a simple elemental precursor to a reaction mixture or by the production of a single source precursor, has shown great promise. Doped Si NCs have demonstrated unique or enhanced properties compared with pure Si NCs, for example, magnetism due to the presence of magnetic metals like Fe and Mn. Surface reactions have reached a new level of sophistication where organic (epoxidation and diol formation) and click (thiol based) chemical reactions can be carried out on attached surface molecules. This has led to a wide range of biocompatible functional groups as well as a degree of emission tuneability. The purification of Si NCs has been improved through the use of size separation columns and size selective precipitation. These purification approaches have yielded highly monodisperse and pure Si NCs previously unachieved. This has allowed scientists to study the size and surface dependent properties and toxicity and enabled the use of Si NCs in biomedical applications. The optical properties of Si NCs are complex. Using a combination of characterization techniques, researchers have explored the relation between the optical properties and the size, surface functionalization, and preparation method. This work has led to a greater fundamental understanding of the unique optical properties of Si NCs. Si NCs are being studied for a wide range of important applications, including LEDS with tunable electroluminescence ranging from NIR to yellow, the encapsulation of Si NCs within micelles terminated with proteins to allow targeted in vivo imaging of cells, Si NC-polymer hybrid solar cells, and the use of Si NCs in battery anodes with high theoretical capacity and good charge retention.

Cd3P2/Zn3P2 Core-Shell Nanocrystals: Synthesis and Optical Properties.

II-V semiconductor nanocrystals such as Cd3P2 and Zn3P2 have enormous potential as materials in next-generation optoelectronic devices requiring active optical properties across the visible and infrared range. To date, this potential has been unfulfilled due to their inherent instability with respect to air and moisture. Core-shell system Cd3P2/Zn3P2 is synthesized and studied from structural (morphology, crystallinity, shell diameter), chemical (composition of core, shell, and ligand sphere), and optical perspectives (absorbance, emission-steady state and time resolved, quantum yield, and air stability). The improvements achieved by coating with Zn3P2 are likely due to its identical crystal structure to Cd3P2 (tetragonal), highlighting the key role crystallographic concerns play in creating cutting edge core-shell NCs.

Effect of nanoparticles on spontaneous Ouzo emulsification.

Particles stabilize fluid interfaces. In particular, oil/water Pickering emulsions undergo limited coalescence, yielding droplets of smaller size as the amount of particles is increased. Herein, we studied the effect of hydrophobic nanoparticles (<10 nm, alkyl-coated) on submicronic droplets (ca 100 nm) formed in an Ouzo system. We investigated thoroughly the water/tetrahydrofuran (THF)/butylated hydroxytoluene (BHT) reference diagram, in the absence and in the presence of nanoparticles, using the Nanoparticle Tracking Analysis (NTA) technique. This allowed us to characterize the size distributions in a much finer way than what is usually obtained using conventional Dynamic Light Scattering (DLS). Both a Surfactant-Free Microemulsion (SFME, thermodynamically stable) and an Ouzo (metastable spontaneous emulsion) domains were identified and the transition from one to the other could be characterized by specific features of the droplet size distributions. We found that the presence of the nanoparticles limits coalescence in the metastable domain. We also show that the alkyl-coated nanoparticles are irreversibly attached to the liquid-liquid interface.

Sustainable quantum dot chemistry: effects of precursor, solvent, and surface chemistry on the synthesis of Zn3P2 nanocrystals.

The quest of exploring alternative materials for the replacement of toxic cadmium- and lead-based quantum dots (QDs) is necessary for envisaging a sustainable future but remains highly challenging. Tackling this issue, we present the synthesis of Zn3P2 nanocrystals (NCs) of unprecedented quality. New, reactive zinc precursors yield highly crystalline, colloidally stable particles, exhibiting oxide-free surfaces, size tunability and outstanding optical properties relative to previous reports of zinc phosphide QDs.

Solution Synthesis, Surface Passivation, Optical Properties, Biomedical Applications, and Cytotoxicity of Silicon and Germanium Nanocrystals.

Silicon and germanium nanocrystals (NCs) are attractive materials owing to their unique size and surface-dependent optical properties. The optical properties of silicon and germanium NCs make them highly suitable for a range of applications, including bioimaging, light-emitting diodes, and solar cells. In this review, the solution synthesis, surface passivation, optical properties, biomedical applications, and cytotoxicity of silicon and germanium NCs are compared and contrasted. Over the last 10 years, synthetic protocols have improved considerably, with size control readily achieved. Investigations have begun into a range of silicon and germanium nanostructures, including doped, alloy, and metal-semiconductor hybrid NCs, which represent the next generation of silicon and germanium nanomaterials. Silicon and germanium NCs are actively researched for a wide array of biomedical applications, including, long-term in vivo cellular imaging, fluorescent nanocarriers for drug delivery, and as contrast agents for magnetic resonance imaging (MRI). Cytotoxicity studies have shown the low toxicity of Si NCs, while demonstrating that Ge NCs are less toxic than CdSe NCs at similar concentrations, giving these materials a strong future in nanomedicine applications.

Protease sensing using nontoxic silicon quantum dots.

Herein is presented a proof-of-concept study of protease sensing that combines nontoxic silicon quantum dots (SiQDs) with Förster resonance energy transfer (FRET). The SiQDs serve as the donor and an organic dye as the acceptor. The dye is covalently attached to the SiQDs using a peptide linker. Enzymatic cleavage of the peptide leads to changes in FRET efficiency. The combination of interfacial design and optical imaging presented in this work opens opportunities for use of nontoxic SiQDs relevant to intracellular sensing and imaging.

Solution Synthesis and Optical Properties of Transition-Metal-Doped Silicon Nanocrystals.

A new synthetic method was developed to produce a range of transition-metal (Mn, Ni, and Cu) doped silicon nanocrystals (Si NCs). The synthesis produces monodisperse undoped and doped Si NCs with comparable average sizes as shown by transmission electron microscopy (TEM). Dopant composition was confirmed by EDX (energy dispersive X-ray spectroscopy). The optical properties of undoped and doped were compared and contrasted using absorption (steady-state and transient) and photoluminescence spectroscopy. Doped Si NCs demonstrated unique dopant-dependent optical properties compared to undoped Si NCs such as enhanced subgap absorption, and 40 nm shifts in the emission. Transient absorption (TA) measurements showed that photoexcitations in doped Si NCs relaxed via dopant states not present in undoped Si NCs.

How to choose a precursor for decomposition solution-phase synthesis: the case of iron nanoparticles.

The decomposition of organometallic compounds as precursors has revolutionized the synthesis of nanoparticles in solution. However, effective control of size and size distribution of iron nanoparticles has remained challenging due to the high reactivity of iron towards oxygen or oxygen-containing materials. Reported is a decomposition study that shows how metal to ligand bonding and symmetry of the compound can be manipulated to control the size and size distribution of iron nanoparticles in the 6-16 nm range. [Fe(η(5)-C6H3Me4)2] was found to be the optimal precursor with a narrow decomposition temperature range due to its symmetry and the low bond dissociation energy of the ligands from the Fe(ii) center. The precise control of nanoparticle size has enabled the tuning of magnetic properties from superparamagnetic to soft-ferromagnetic desirable for a wide range of biomedical applications.