Shedding light on the aqueous synthesis of silicon nanoparticles by reduction of silanes with citrates.

The synthesis of silicon nanoparticles in water has recently attracted a lot of attention. However, many scientists have expressed concerns on the nanomaterials obtained. We decided to explore two different routes to obtain silicon nanoparticles starting from a silane precursor. We report our findings regarding the preparation of nanomaterials under microwave irradiation and hydrothermal conditions starting from aqueous mixtures of (3-aminopropyl) triethoxysilane and sodium citrate. The microwave process, in particular, has been reported to yield silicon quantum dots bearing a surface layer of 3-aminopropyl moieties, allowing for the preparation of luminescent substrates amenable to biological-friendly amide chemistry. However, rigorous experimental design and thorough characterization of the products definitely rules out the formation of silicon nanoparticles. By highlighting the main issues linked to the proper characterization of these materials, we prove that the nanoparticles produced under both microwave and hydrothermal conditions, are a mixture of silica and carbon quantum dots.

Ultrasmall silicon nanoparticles as a promising platform for multimodal imaging.

Bimodal systems for nuclear and optical imaging are currently being intensively investigated due to their comparable detection sensitivity and the complementary information they provide. In this perspective, we have implemented both modalities on biocompatible ultrasmall silicon nanoparticles (Si NPs). Such nanoparticles are particularly interesting since they are highly biocompatible, have covalent surface functionalization and demonstrate very fast body clearance. We prepared monodisperse citrate-stabilized Si NPs (2.4 ± 0.5 nm) with more than 40 accessible terminal amino groups per particle and, for the first time, simultaneously, a near-infrared dye (IR800-CW) and a radiolabel (64Cu-NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid) have been covalently linked to the surface of such Si NPs. The obtained nanomaterials have been fully characterized using HR-TEM, XPS, UV-Vis and FT-IR spectroscopy. These dual-labelled particles do not exhibit any cytotoxicity in vitro. In vivo studies employing both positron emission tomography (PET) and optical imaging (OI) techniques revealed rapid renal clearance of dual-labelled Si NPs from mice.

Biodistribution studies of ultrasmall silicon nanoparticles and carbon dots in experimental rats and tumor mice.

Ultrasmall clearable nanoparticles possess enormous potential as cancer imaging agents. In particular, biocompatible silicon nanoparticles (Si NPs) and carbon quantum dots (CQDs) hold great potential in this regard. Their facile surface functionalization easily allows the introduction of different labels for in vivo imaging. However, to date, a thorough biodistribution study by in vivo positron emission tomography (PET) and a comparative study of Si vs. C particles of similar size are missing. In this contribution, ultrasmall (size <5 nm) Si NPs and CQDs were synthesized and characterized by high-resolution transmission electron microscopy (HR-TEM), Fourier-transform infrared (FTIR), absorption and steady-state emission spectroscopy. Subsequent functionalization of NPs with a near-infrared dye (Kodak-XS-670) or a radiolabel (64Cu) enabled a detailed in vitro and in vivo study of the particles. For radiolabeling experiments, the bifunctional chelating agent S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) was conjugated to the amino surface groups of the respective NPs. Efficient radiolabeling of NOTA-functionalized NPs with the positron emitter 64Cu was found. The biodistribution and PET studies showed a rapid renal clearance from the in vivo systems for both variants of the nanoparticles. Interestingly, the different derivatives investigated exhibited significant differences in the biodistribution and pharmacokinetic properties. This can mostly be attributed to different surface charge and hydrophilicity of the NPs, arising from the synthetic strategy used to prepare the particles.