Tapas K. Purkait

Cytotoxicity of surface-functionalized silicon and germanium nanoparticles: the dominant role of surface charges.

Although it is frequently hypothesized that surface (like surface charge) and physical characteristics (like particle size) play important roles in cellular interactions of nanoparticles (NPs), a systematic study probing this issue is missing. Hence, a comparative cytotoxicity study, quantifying nine different cellular endpoints, was performed with a broad series of monodisperse, well characterized silicon (Si) and germanium (Ge) NPs with various surface functionalizations. Human colonic adenocarcinoma Caco-2 and rat alveolar macrophage NR8383 cells were used to clarify the toxicity of this series of NPs. The surface coatings on the NPs appeared to dominate the cytotoxicity: the cationic NPs exhibited cytotoxicity, whereas the carboxylic acid-terminated and hydrophilic PEG- or dextran-terminated NPs did not. Within the cationic Si NPs, smaller Si NPs were more toxic than bigger ones. Manganese-doped (1% Mn) Si NPs did not show any added toxicity, which favors their further development for bioimaging. Iron-doped (1% Fe) Si NPs showed some added toxicity, which may be due to the leaching of Fe(3+) ions from the core. A silica coating seemed to impart toxicity, in line with the reported toxicity of silica. Intracellular mitochondria seem to be the target for the toxic NPs since a dose-, surface charge- and size-dependent imbalance of the mitochondrial membrane potential was observed. Such an imbalance led to a series of other cellular events for cationic NPs, like decreased mitochondrial membrane potential (ΔΨm) and ATP production, induction of ROS generation, increased cytoplasmic Ca(2+) content, production of TNF-α and enhanced caspase-3 activity. Taken together, the results explain the toxicity of Si NPs/Ge NPs largely by their surface characteristics, provide insight into the mode of action underlying the observed cytotoxicity, and give directions on synthesizing biocompatible Si and Ge NPs, as this is crucial for bioimaging and other applications in for example the field of medicine.

Borane-catalyzed room-temperature hydrosilylation of alkenes/alkynes on silicon nanocrystal surfaces.

Room-temperature borane-catalyzed functionalization of hydride-terminated silicon nanocrystals (H-SiNCs) with alkenes/alkynes is reported. This new methodology affords formation of alkyl and alkynyl surface monolayers of varied chain lengths (i.e., C5-C12). The present study also indicates alkynes react more readily with H-SiNC surfaces than equivalent alkenes. Unlike other toxic transition-metal catalysts, borane or related byproducts can be readily removed from the functionalized SiNCs. The new method affords stable luminescent alkyl/alkenyl-functionalized SiNCs.

Water-soluble photoluminescent D-mannose and L-alanine functionalized silicon nanocrystals and their application to cancer cell imaging

Herein, we report the straightforward synthesis, photoluminescent properties, and cell imaging studies of d-mannose and l-alanine functionalized silicon nanocrystals (SiNCs). Tailoring nanocrystal surface functionalization is essential to interfacing SiNCs with their environment and rendering them stable - surface modification also offers the opportunity to target specific cell types for imaging. A simple and versatile surface modification procedure was developed to tether biomolecules onto the SiNC surfaces and render them water-soluble. The presented approach is precious metal-catalyst free, straightforward, and provides carbohydrate and amino acid functionalized SiNCs. The functionalized SiNCs have been investigated by fluorescence microscopy and our results indicate that they can be internalized by MCF-7 human breast cancer cells as shown in the cell imaging studies. The obtained SiNCs were characterized using FTIR, XPS, PL, and TEM.

Direct Evaluation of the Quantum Confinement Effect in Single Isolated Ge Nanocrystals

To address the yet open question regarding the nature of quantum confinement in Ge nanocrystals (Ge NCs) we employed scanning tunneling spectroscopy to monitor the electronic structure of individual isolated Ge NCs as a function of their size. The (single-particle) band gaps extracted from the tunneling spectra increase monotonically with decreasing nanocrystal size, irrespective of the capping ligands, manifesting the effect of quantum confinement. Band-gap widening of ∼1 eV with respect to the bulk value was observed for Ge-NCs 3 nm in diameter. The picture emerging from comparison with theoretical calculations and other experimental results is discussed.

Alkoxy-Terminated Si Surfaces: A New Reactive Platform for the Functionalization and Derivatization of Silicon Quantum Dots

Alkoxy-terminated silicon quantum dots (SiQDs) were synthesized via hydrosilylation of aliphatic ketones on hydride-terminated SiQD (H-SiQD) surfaces under microwave-irradiation. Aromatic ketones undergo hydrosilylation on H-SiQD surfaces at room temperature without requiring any catalyst. The alkoxy-terminated SiQDs are soluble in organic solvents, colloidally stable, and show bright and size dependent photoluminescence (PL). The alkoxy-functionalized silicon surfaces were used as reactive platform for further functionalization via unprecedented ligand exchange of the alkoxy-surface groups with alkyl or alkenyl-surface groups in the presence of BH3·THF. Proton nuclear magnetic resonance ((1)H NMR), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) spectroscopy confirmed alkoxy-terminated surfaces and their ligand exchange reactions in the presence of various alkenes and alkynes.

Chloride surface terminated silicon nanocrystal mediated synthesis of poly(3-hexylthiophene).

Abundant and environmentally benign metal-free silicon-based reagents, including chloride surface-terminated silicon nanocrystals (Cl-SiNCs) and silicon wafers as well as molecular chlorosilanes, were explored as catalysts for the synthesis of poly-3-hexylthiophene (P3HT) at room temperature. Cl-SiNC catalysts exhibit the highest activity of those investigated, and systems based upon single-crystal silicon wafers provide convenient, straightforward purification. The as-prepared P3HT exhibits moderate molecular weights and bears H/Br or Br/Br end groups; these properties will allow direct application and also facilitate their use as macroinitiators in the syntheses of block and/or telechelic polymers. The silicon-based systems are expected to provide an efficient metal-free catalytic preparation of functional polymers.

Phosphorus Pentachloride Initiated Functionalization of Silicon Nanocrystals

Phosphorus pentachloride (PCl5) has long been used to chlorinate hydrocarbons. It has also been applied in silicon surface chemistry to facilitate alkylation via a two-step halogenation/Grignard route. Here we report a study of the reaction of PCl5 with hydride-terminated silicon nanocrystals (H-SiNCs). An examination of the reaction mechanism has allowed us to establish a functionalization protocol that uses PCl5 as a surface radical initiator to introduce alkyl and alkenyl moieties to the surface of H-SiNCs. The reaction proceeds quickly in a single step, at room temperature and the functionalized silicon nanocrystals retained their morphology and crystallinity. The resulting materials exhibited size-dependent photoluminescence that was approximately 3× as bright as that observed for thermally hydrosilylated SiNCs. Furthermore, the absolute PL quantum yield (AQY) was more than double. The high AQY is expected to enable SiNCs to compete with chalcogenide-based quantum dots in various applications.

Grafting Poly(3-hexylthiophene) from Silicon Nanocrystal Surfaces: Synthesis and Properties of a Functional Hybrid Material with Direct Interfacial Contact

Hybrid functional materials (HFMs) comprised of semiconductor nanoparticles and conjugated polymers offer the potential of synergetic photophysical properties. We have developed HFMs based upon silicon nanocrystals (SiNCs) and the conductive polymer poly(3-hexylthiophene) (SiNC@P3HT) by applying surface-initiated Kumada catalyst transfer polycondensation (SI-KCTP). One unique characteristic of the developed SiNC@P3HT is the formation of a direct covalent bonding between SiNCs and P3HT. The presented method for obtaining direct interfacial attachment, which is not accessible using other methods, may allow for the development of materials with efficient electronic communication at the donor-acceptor interfaces. Systematic characterization provides evidence of a core-shell structure, enhanced interfacial electron and/or energy transfer between the P3HT and SiNC components, as well as formation of a type-II heterostructure.

Application of Engineered Si Nanoparticles in Light-Induced Advanced Oxidation Remediation of a Water-Borne Model Contaminant

Surface-engineered amphiphilic polymer-coated silicon nanoparticles (SiNPs) were employed as photocatalysts to capture and degrade a model organic contaminant (methanol) in water. This study represents the first time SiNPs have been employed in the initiation of advanced oxidation processes that are commonly used to degrade organic constituents in industrial wastewaters. The quantum yield of photocatalytic methanol oxidation and the corresponding yield factor for the generation of active OH radicals are reported. The size and surface defect dependent photocatalytic activity of SiNPs was investigated. The yield factors (η) decreased with increasing particle size and reached impressive values that exceeded that of equivalent TiO2 nanoparticle systems by 3-4 times and are comparable to the robust UV/Cl2 and UV/H2O2 systems. The higher photocatalytic efficiency of SiNPs is attributed to the combined effects of quantum confinement, effective band gap, and surface states, among which surface states play a dominant role. SiNPs provide a potentially tunable, biologically inert, and robust nanoparticle system for photocatalytic oxidation of wastewater contaminants.

Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces

Remarkable advances in surface hydrosilylation reactions of C=C and C=O bonds on hydride-terminated silicon have revolutionized silicon surface functionalization. However, existing methods for functionalizing hydride-terminated Si nanocrystals (H-SiNCs) require long reaction times and elevated temperatures. Herein, we report a room-temperature method for functionalizing H-SiNC surfaces within seconds by stripping outermost atoms on H-SiNC surfaces with xenon difluoride (XeF2 ). Detailed analysis of the reaction byproducts by in situ NMR spectroscopy and GC-MS provided unprecedented insight into NC surface composition and reactivity as well as the complex reaction mechanism of XeF2 activated hydrosilylation.