The fluorescence behavior of molecular dyes at discrete distances from 1.5 nm diameter gold nanoparticles as a function of distance and energy is investigated. Photoluminescence and luminescence lifetime measurements both demonstrate quenching behavior consistent with 1/d(4) separation distance from dye to the surface of the nanoparticle. In agreement with the model of Persson and Lang, all experimental data show that energy transfer to the metal surface is the dominant quenching mechanism, and the radiative rate is unchanged throughout the experiment.

Silicon nanocrystals (Si NCs) are attractive functional materials. They are compatible with standard electronics and communications platforms as well being 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 ppm level in Si NCs gives rise to the blue emission.

Nanometal surface energy transfer (NSET) is a molecular ruler technique that has been utilized to optically probe long distances in biomolecular structures. We investigate the useful spectral range of donor dyes and the importance of overlap between the localized surface plasmon resonance (LSPR) and the donor photoluminescence (520-780 nm) and provide a comprehensive study of the R(0) values for the NSET processes from dyes to 2 nm Au NP (gold nanoparticle). The distance-dependent quenching results provide experimental evidence that the efficiency curve slope, R(0) value, and distance of quenching is best modeled as a surface-mediated NSET process analogous to the predictions of Persson-Lang and Chance-Prock-Silbey (CPS). The results show that the LSPR plays a very important role in the observed quenching of excited-state donors at the surface of the nanometal, and the correlation to the NSET model allows a compilation of the necessary biophysical constants for application within the toolbox of biophysics.

Nanometal surface energy transfer (NSET), which describes an energy transfer event from optically excited organic fluorophores to small metal nanoparticles, may be used as a molecular beacon/ruler similar to FRET, but with advantages over this classical technique. Here we use NSET to measure Mg(2+)-induced conformational changes for a hammerhead ribozyme and confirm these measurements using FRET. These optical experiments enhance our understanding of the different kinetic pathways for this ribozyme.

Although it is 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 Fe3+ ions from the core. A silica coating seemed to impart toxicity, in line with the reported toxicity of silica. Intracellular mitochondria seem to be a target organ for the toxic NPs since a dose-, surface charge- and size-dependent imbalance of the mitochondrial membrane potential was observed. Such 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 Ca2+ 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 in 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.