Immobilization procedures, intended to enable prolonged observation of single molecules by fluorescence microscopy, may generate heterogeneous microenvironments, thus inducing heterogeneity in the molecular behavior. On that account, we propose a straightforward surface preparation procedure for studying chemical reactions on the single-molecule level. Sensor fluorophores were developed, which exhibit dual-emissive characteristics in a homogeneously catalyzed showcase reaction. These molecules undergo a shift of fluorescence wavelength of about 100 nm upon Pd(0)-induced deallylation in the Tsuji–Trost reaction, allowing for separate visualization of the starting material and product. Whereas a simultaneous immobilization of dye and inert silane leads to strongly polydisperse reaction kinetics, a consecutive immobilization routine with deposition of dye molecules as the last step provides substrates underlying the kinetics of ensemble experiments. Also, the found kinetics are unaffected by the chemical variation of inert silanes, nearly uniform, and therefore well reproducible. Additional parameters like photostability, signal-to-noise ratio, dye-molecule density, and spatial distribution of dye molecules are, as well, hardly affected by surface modification in the successive immobilization scheme.
Gold nanocluster (AuNC) synthesis using a well‐distinguished polymer for nanoparticle‐mediated drug delivery paves the way for developing efficient theranostics based on pharmaceutically accepted materials. Gelatin‐stabilized AuNCs are synthesized and modified by glutathione for tuning the emission spectra. Addition of silver ions enhances the fluorescence, reaching also high quantum yield (26.7%). A simplified model can be proposed describing the nanoclusters' properties–structure relationship based on X‐ray photoelectron spectroscopy data and synthesis sequence. Furthermore, these modifications improve fluorescence stability toward pH changes and enzymatic degradation, offering different AuNCs for various applications. The impact of nanocluster formation on gelatin structure integrity is investigated by Fourier transform infrared spectrometry and matrix‐assisted laser desorption/ionization time of flight mass spectroscopy, being important to further formulate gelatin nanoparticles (GNPs). The 218 nm‐sized NPs show no cytotoxicity up to 600 µg mL−1 and are imaged in skin, as a challenging autofluorescent tissue, by confocal microscopy, when transcutaneously delivered using dissolving microneedles. Linear unmixing allows simultaneous imaging of AuNCs–GNPs and skin with accurate signal separation. This underlines the great potential for bioimaging of this system to better understand nanomaterials' behavior in tissue. Additionally, it is drug delivery system also potentially serving as a theranostic system.
Self-calibrating, fluorescent nanoparticles with diameter far below 50 nm are synthesized with embedding a new ratiometric and pH sensitive indicator dye. The prompt response of the fluorophore allows for determining the intracellular pH.
Single-molecule (SM) chemistry is devoted to unravel reaction steps which are hidden in cuvette experiments. Controversies about the substrate activation during the Tsuji-Trost deallylation motivated us to study, on the single-molecule level, the kinetics of the catalyst precursor Pd(PPh 3 ) 4 with our recently designed two-color fluorescent probes. Photochemical, metal-free bypass reactions were found and taken into account by the combina-tion of spectrally separated single-molecule TIRF-microscopy and state-of-the art analysis procedures. Unselective π-complex formation (K D � 10 3 M À 1 ) precedes the insertion of the active catalyst into the CÀ OR bond (RO À = leaving group), indicated by the lacking immediate change of fluorescence color. The formed intermediate then decomposes on a time scale of � 2 -3 s to the deallylated product.
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