Hybrid inorganic-organic fluorescent silica nanoparticles—influence of dye binding modes on dye leaching

Silica nanoparticles with embedded fluorescent dyes represent an important class of markers for example in biological imaging. We systematically studied the various incorporation mechanisms of fluorescent xanthene dyes in 30–40 nm silica nanoparticles. An important parameter was the interaction of the dye with the matrix material, either by weak electrostatic or strong covalent interactions, which also has implications on the stability of fluorescence and brightness of the dyes. Factors that can influence leaching of dyes such as the position of the dyes in particles and the intensity of the particle-dye interaction were investigated by using the solvatochromic effect of xanthene dyes and by stationary fluorescence anisotropy measurements. We compared uranine and rhodamine B, which were physically embedded, with modified fluorescein isothiocyanate and rhodamine B isothiocyanate, which were covalently bound to the silica matrix within a usual Stöber synthesis. Systematic leaching studies of time spans up to 4 days revealed that covalent bonding of dyes like fluorescein isothiocyanate or rhodamine B isothiocyanate is necessary for fluorescence stability, since dyes bound by physical interaction tend to leach out of porous silica networks. Coverage of silica particles with hydrophobic protection layers of alkyltrialkoxysilanes or hydrophilic polyethylene glycol (PEG) groups resulted in a better retention of physisorbed dyes and provides the possibility to adapt the particles to the polarity of the medium. Best results were archived with PEG groups, but even small trimethylsilyl (TMS) groups already reduce leaching

Solvent-Controlled Intermolecular Proton-Transfer Follows an Irreversible Eigen-Weller Model from fs to ns

Intermolecular Proton Transfer (PT) dynamics can be best studied by optical spectroscopy, which can cover the vast timescale spanned by the process. PT in a hydrogen bonding complex between a pyranine-based photoacid and a trialkyl-phosphine oxide is addressed. The photoreaction is traced with the help of femtosecond transient absorption and picosecond-resolved fluorescence. Characteristic kinetics and spectra of the intervening species are isolated by global analysis and spectral decomposition of time-resolved fluorescence. It is found that the shared proton shifts towards the phosphine site upon photoexcitation in acetonitrile. The process occurs on the sub-picosecond timescale, essentially, under solvent control. Despite the ultrafast rate, an equilibrium between the complex and the hydrogen-bonded ion pair (HBIP) is established. Further reaction steps are delayed to the nanosecond timescale, where formation of the excited deprotonated form is observed. The far-reaching consistency between the various methods supports an irreversible Eigen-Weller mechanism in the excited state

Solvent‐Controlled Intermolecular Proton‐Transfer Follows an Irreversible Eigen‐Weller Model from fs to ns

Intermolecular Proton Transfer (PT) dynamics can be best studied by optical spectroscopy, which can cover the vast timescale spanned by the process. PT in a hydrogen bonding complex between a pyranine-based photoacid and a trialkyl-phosphine oxide is addressed. The photoreaction is traced with the help of femtosecond transient absorption and picosecond-resolved fluorescence. Characteristic kinetics and spectra of the intervening species are isolated by global analysis and spectral decomposition of time-resolved fluorescence. It is found that the shared proton shifts towards the phosphine site upon photoexcitation in acetonitrile. The process occurs on the sub-picosecond timescale, essentially, under solvent control. Despite the ultrafast rate, an equilibrium between the complex and the hydrogen-bonded ion pair (HBIP) is established. Further reaction steps are delayed to the nanosecond timescale, where formation of the excited deprotonated form is observed. The far-reaching consistency between the various methods supports an irreversible Eigen-Weller mechanism in the excited state

Kinetics of Palladium(0)‐Allyl Interactions in the Tsuji‐Trost Reaction, derived from Single‐Molecule Fluorescence Microscopy

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(PPh3)4 with our recently designed two‐color fluorescent probes. Photochemical, metal‐free bypass reactions were found and taken into account by the combination of spectrally separated single‐molecule TIRF‐microscopy and state‐of‐the art analysis procedures. Unselective π‐complex formation (KD≈103 M−1) precedes the insertion of the active catalyst into the C−OR bond (RO−=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

Progress Report on pH-Influenced Photocatalysis for Active Motion

Living systems use catalysis to achieve chemical transformations to comply with their needs in terms of energy and building blocks. The pH is a powerful means to regulate such processes, which also influences synthetic systems. In fact, the pH sensitivity of artificial photocatalysts, such as bismuth vanadate, bears the strong potential of flexibly influencing both the motion pattern and the speed of catalytic microswimmers, but it has rarely been investigated to date. In this work, we first present a comprehensive view of the motion behavior of differently shaped bismuth vanadate microswimmers, discuss influences, such as shape, pH, and conductivity of the solutions, and find that the motion pattern of the swimmers switches between upright and horizontal at their point of zero charge. We then apply an immobilizable hydroxypyrene derivative to our substrates to locally influence the pH of the solution by excited-state proton transfer. We find that the motion pattern of our swimmers is strongly influenced by this functionalization and a third motion mode, called tumbling, is introduced. Taking other effects, such as an increased surface roughness of the modified substrates, into account, we critically discuss possible future developments

Embedding Photoacids into Polymer Opal Structures: Synergistic Effects on Optical and Stimuli-Responsive Features

Opal films with their vivid structural colors represent a field of tremendous interest and obtained materials offer the possibility for many applications, such as optical sensors or anti counterfeiting materials. A convenient method for the generation of opal structures relies on the tailored design of core-interlayer-shell (CIS) particles. Within the present study, elastomeric opal films were combined with stimuli-responsive photoacids to further influence the optical properties of structurally colored materials. Starting from cross-linked polystyrene (PS) core particles featuring a hydroxy-rich and polar soft shell, opal films were prepared by application of the melt-shear organization technique. The photoacid tris(2,2,2-trifluoroethyl) 8-hydroxypyrene-1,3,6-trisulfonate (TFEHTS) could be conveniently incorporated during freeze-drying the particle dispersion and prior to the melt-shear organization. Furthermore, the polar opal matrix featuring hydroxylic moieties enabled excited-state proton transfer (ESPT), which is proved by spectroscopic evaluation. Finally, the influence of the photoacid on the optical properties of the 3-dimensional colloidal crystals were investigated within different experimental conditions. The angle dependence of the emission spectra unambiguously shows the selective suppression of the photoacid’s fluorescence in its deprotonated state

Luminescent Symmetrically and Unsymmetrically Substituted Diboranes(4)

A series of 4‐(dimethylamino)phenyl and pentafluorophenyl‐substituted 1,2‐bis(dimethylamino)diboranes(4) of type A, benzo‐fused cyclic 1,4‐diaza‐2,3‐diborinanes of type B, and 1,2‐diduryldiboranes(4) of type C were synthesized and structurally characterized. Spectroscopic studies revealed that the substitution pattern is a decisive factor for the observation of fluorescence in most of the derivatives A to C. For diboranes(4) of type A, unsymmetrical substitution with electron‐donating and ‐withdrawing groups at the boron centers is crucial to invoke fluorescence, albeit weak. Substitution at the boron atoms of 1,4‐diaza‐2,3‐diborinane species B leads to a modified skeletal structure. Finally, the grafting of 4‐(dimethylamino)phenyl groups to diboranes(4) of type C results in extraordinary Stokes shifts in nonpolar solvents

NIR-Emitting Gold Nanoclusters-Modified Gelatin Nanoparticles as a Bioimaging Agent in Tissue

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