Laser-Ablative Synthesis of Silicon-Iron Composite Nanoparticles for Theranostic Applications.

The combination of photothermal and magnetic functionalities in one biocompatible nanoformulation forms an attractive basis for developing multifunctional agents for biomedical theranostics. Here, we report the fabrication of silicon-iron (Si-Fe) composite nanoparticles (NPs) for theranostic applications by using a method of femtosecond laser ablation in acetone from a mixed target combining silicon and iron. The NPs were then transferred to water for subsequent biological use. From structural analyses, it was shown that the formed Si-Fe NPs have a spherical shape and sizes ranging from 5 to 150 nm, with the presence of two characteristic maxima around 20 nm and 90 nm in the size distribution. They are mostly composed of silicon with the presence of a significant iron silicide content and iron oxide inclusions. Our studies also show that the NPs exhibit magnetic properties due to the presence of iron ions in their composition, which makes the formation of contrast in magnetic resonance imaging (MRI) possible, as it is verified by magnetic resonance relaxometry at the proton resonance frequency. In addition, the Si-Fe NPs are characterized by strong optical absorption in the window of relative transparency of bio-tissue (650-950 nm). Benefiting from such absorption, the Si-Fe NPs provide strong photoheating in their aqueous suspensions under continuous wave laser excitation at 808 nm. The NP-induced photoheating is described by a photothermal conversion efficiency of 33-42%, which is approximately 3.0-3.3 times larger than that for pure laser-synthesized Si NPs, and it is explained by the presence of iron silicide in the NP composition. Combining the strong photothermal effect and MRI functionality, the synthesized Si-Fe NPs promise a major advancement of modalities for cancer theranostics, including MRI-guided photothermal therapy and surgery.

Effect of Silicate Additive on Structural and Electrical Properties of Germanium Nanowires Formed by Electrochemical Reduction from Aqueous Solutions.

Layers of germanium (Ge) nanowires (NWs) on titanium foils were grown by metal-assisted electrochemical reduction of germanium oxide in aqueous electrolytes based on germanium oxide without and with addition of sodium silicate. Structural properties and composition of Ge NWs were studied by means of the scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. When sodium silicate was added to the electrolyte, Ge NWs consisted of 1-2 at.% of silicon (Si) and exhibited smaller mean diameter and improved crystallinity. Additionally, samples of Ge NW films were prepared by ultrasonic removal of Ge NWs from titanium foils followed with redeposition on corundum substrates with platinum electrodes. The electrical conductivity of Ge NW films was studied at different temperatures from 25 to 300 °C and an effect of the silicon impurity on the thermally activated electrical conductivity was revealed. Furthermore, the electrical conductivity of Ge NW films on corundum substrates exhibited a strong sensor response on the presence of saturated vapors of different liquids (water, acetone, ethanol, and isopropanol) in air and the response was dependent on the presence of Si impurities in the nanowires. The results obtained indicate the possibility of controlling the structure and electrical properties of Ge NWs by introducing silicate additives during their formation, which is of interest for applications in printed electronics and molecular sensorics.

Germanium Nanoparticles Prepared by Laser Ablation in Low Pressure Helium and Nitrogen Atmosphere for Biophotonic Applications.

Due to particular physico-chemical characteristics and prominent optical properties, nanostructured germanium (Ge) appears as a promising material for biomedical applications, but its use in biological systems has been limited so far due to the difficulty of preparation of Ge nanostructures in a pure, uncontaminated state. Here, we explored the fabrication of Ge nanoparticles (NPs) using methods of pulsed laser ablation in ambient gas (He or He-N2 mixtures) maintained at low residual pressures (1-5 Torr). We show that the ablated material can be deposited on a substrate (silicon wafer in our case) to form a nanostructured thin film, which can then be ground in ethanol by ultrasound to form a stable suspension of Ge NPs. It was found that these formed NPs have a wide size dispersion, with sizes between a few nm and hundreds of nm, while a subsequent centrifugation step renders possible the selection of one or another NP size fraction. Structural characterization of NPs showed that they are composed of aggregations of Ge crystals, covered by an oxide shell. Solutions of the prepared NPs exhibited largely dominating photoluminescence (PL) around 450 nm, attributed to defects in the germanium oxide shell, while a separated fraction of relatively small (5-10 nm) NPs exhibited a red-shifted PL band around 725 nm under 633 nm excitation, which could be attributed to quantum confinement effects. It was also found that the formed NPs exhibit high absorption in the visible and near-IR spectral ranges and can be strongly heated under photoexcitation in the region of relative tissue transparency, which opens access to phototherapy functionality. Combining imaging and therapy functionalities in the biological transparency window, laser-synthesized Ge NPs present a novel promising object for cancer theranostics.

Tailoring Photoluminescence from Si-Based Nanocrystals Prepared by Pulsed Laser Ablation in He-N2 Gas Mixtures.

Using methods of pulsed laser ablation from a silicon target in helium (He)-nitrogen (N2) gas mixtures maintained at reduced pressures (0.5-5 Torr), we fabricated substrate-supported silicon (Si) nanocrystal-based films exhibiting a strong photoluminescence (PL) emission, which depended on the He/N2 ratio. We show that, in the case of ablation in pure He gas, Si nanocrystals exhibit PL bands centered in the "red - near infrared" (maximum at 760 nm) and "green" (centered at 550 nm) spectral regions, which can be attributed to quantum-confined excitonic states in small Si nanocrystals and to local electronic states in amorphous silicon suboxide (a-SiOx) coating, respectively, while the addition of N2 leads to the generation of an intense "green-yellow" PL band centered at 580 nm. The origin of the latter band is attributed to a radiative recombination in amorphous oxynitride (a-SiNxOy) coating of Si nanocrystals. PL transients of Si nanocrystals with SiOx and a-SiNxOy coatings demonstrate nonexponential decays in the micro- and submicrosecond time scales with rates depending on nitrogen content in the mixture. After milling by ultrasound and dispersing in water, Si nanocrystals can be used as efficient non-toxic markers for bioimaging, while the observed spectral tailoring effect makes possible an adjustment of the PL emission of such markers to a concrete bioimaging task.

Enhanced photoluminescence of porous silicon nanoparticles coated by bioresorbable polymers.

A significant enhancement of the photoluminescence (PL) efficiency is observed for aqueous suspensions of porous silicon nanoparticles (PSiNPs) coated by bioresorbable polymers, i.e., polylactic-co-glycolic acid (PLGA) and polyvinyl alcohol (PVA). PSiNPs with average size about 100 nm prepared by mechanical grinding of electrochemically etched porous silicon were dispersed in water to prepare the stable suspension. The inner hydrophobic PLGA layer prevents the PSiNPs from the dissolution in water, while the outer PVA layer makes the PSiNPs hydrophilic. The PL quantum yield of PLGA/PVA-coated PSiNPs was found to increase by three times for 2 weeks of the storage in water. The observed effect is explained by taking into account both suppression of the dissolution of PSiNPs in water and a process of the passivation of nonradiative defects in PSiNPs. The obtained results are interesting in view of the potential applications of PSiNPs in bioimaging.