EPR and Structural Characterization of Water-Soluble Mn2+-Doped Si Nanoparticles.

Water-soluble poly(allylamine) Mn2+-doped Si (SiMn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble SiMn NPs showed a strong photoluminescence with a quantum yield up to 13%. The SiMn NPs had significant T1 contrast with an r1 relaxivity of 11.1 ± 1.5 mM-1 s-1 and r2 relaxivity of 32.7 ± 4.7 mM-1 s-1 where the concentration is in mM of Mn2+. Dextran-coated poly(allylamine) SiMn NPs produced NPs with T1 and T2 contrast with a r1 relaxivity of 27.1 ± 2.8 mM-1 s-1 and r2 relaxivity of 1078.5 ± 1.9 mM-1 s-1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn2+ in these NP's. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP's.

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.

Chemical insight into the origin of red and blue photoluminescence arising from freestanding silicon nanocrystals.

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms and are biocompatible. Numerous methods have been developed to realize size-controlled Si NC synthesis. While these procedures produce Si NCs that appear identical, their optical responses can differ dramatically. Si NCs prepared using high-temperature methods routinely exhibit photoluminescence agreeing with the effective mass approximation (EMA), while those prepared via solution methods exhibit blue emission that is somewhat independent of particle size. Despite many proposals, a definitive explanation for this difference has been elusive for no less than a decade. This apparent dichotomy brings into question our understanding of Si NC properties and potentially limits the scope of their application. The present contribution takes a substantial step forward toward identifying the origin of the blue emission that is not expected based upon EMA predictions. It describes a detailed comparison of Si NCs obtained from three of the most widely cited procedures as well as the conversion of red-emitting Si NCs to blue emitters upon exposure to nitrogen-containing reagents. Analysis of the evidence is consistent with the hypothesis that the presence of trace nitrogen and oxygen even at the parts per million level in Si NCs gives rise to the blue emission.

Development of iron-doped silicon nanoparticles as bimodal imaging agents.

We demonstrate the synthesis of water-soluble allylamine-terminated Fe-doped Si (Si(xFe)) nanoparticles as bimodal agents for optical and magnetic imaging. The preparation involves the synthesis of a single-source iron-containing precursor, Na(4)Si(4) with x% Fe (x = 1, 5, 10), and its subsequent reaction with NH(4)Br to produce hydrogen-terminated Si(xFe) nanoparticles. The hydrogen-capped nanoparticles are further terminated with allylamine via thermal hydrosilylation. Transmission electron microscopy indicates that the average particle diameter is ∼3.0 ± 1.0 nm. The Si(5Fe) nanoparticles show strong photoluminescence quantum yield in water (∼10%) with significant T(2) contrast (r(2)/r(1) value of 4.31). Electron paramagnetic resonance and Mössbauer spectroscopies indicate that iron in the nanoparticles is in the +3 oxidation state. Analysis of cytotoxicity using the resazurin assay on HepG2 liver cells indicates that the particles have minimal toxicity.