Carbon
dots (C-Dots) are promising new materials for the development
of biocompatible photosensitizers for solar-driven catalysis and hydrogen
production in aqueous solution. Compared to common semiconducting
quantum dots, C-Dots have good physicochemical, as well as photochemical
stability, optical brightness, stability and nontoxicity, while their
carbon based source results in tunable surface chemistry, chemical
versatility, low cost, and biocompatibility. Herein we show that doping
the C-Dots with phosphate or boron significantly influences their
excited-state dynamics, which is observed by the formation of a unique
long-lived photoproduct as a function of the different dopants. To
probe the photosensitizing capabilities of the C-Dots, we followed
the photoreduction of methyl viologen (MV2+), which acts
as a molecular redox mediator (electron acceptor) to the C-Dots (the
photosensitizer, i.e., electron donor) in aqueous solution, using
steady-state and time-resolved fluorescence and absorption spectroscopic
techniques as well as electrochemical measurements. We show that ultrafast
electron transfer to MV2+ and slow charge recombination
results in a high quantum yield of MV2+ photoreduction,
while the doping drastically influences this quantum yield of MV2+ radical. Our findings contribute to the photophysical understanding
of this intriguing and relatively new carbon-based nanoparticle and
can improve the design and development of efficient photosensitizers
over commonly used heterogeneous catalysts in photocatalytic systems
by increasing the efficiency of radical generation.
Post formation modification of protein-based materials can attenuate the proton conduction efficiency resulting from change in conduction mechanism, charge carrier mobility, carrier concentrations and inner hydration layer.
Gold nanoclusters are promising candidates as biological markers without having toxic effects like fluorescent quantum dots. Herein, bovine serum albumin (BSA) protein stabilized gold nanoclusters of two different sizes emitting at 410 and 645 nm have been synthesized. These nanoclusters have been shown to interact with molecular oxygen differentially. Spectroscopic and chemical evidences show that dioxygen molecule gets adsorbed at two different orientations on the nanoclusters. The orientation motifs have been hypothesized to be superoxo and peroxo types on the smaller and the larger gold nanoclusters, respectively. Due to the difference in attachments, the oxygen molecule shows opposite changes in fluorescence intensity for the nanoclusters. The fluorescence intensity of the blue emitting nanocluster shows a profuse enhancement whereas the red emitting species shows quenching of emission. Superoxo type adsorption of the oxygen molecule on the blue emitting gold nanoclusters induce formation of singlet oxygen that in turn enhances the fluorescence intensity of the species. This could be verified by oxidation of diaminobenzidine (DAB) by singlet oxygen. Enhancement in fluorescence intensity of the blue emitting gold nanoclusters with an increase in concentration of molecular oxygen may enable them to be good candidates in bioimaging and detection.
Coumarin 6 precipitates in water as microcrystals resulting in a considerable loss in fluorescence yield. Differential interaction of the microcrystals with cyclodextrins of different cavity size enhances the fluorescence yield of the dye by 160 fold in some cases. Highly fluorescent ultrathin lamellar entities form through hydrogen bonding. † Electronic supplementary information (ESI) available: Structure of C6, AFM and SEM images of C6 aer incubation in water (with 1% methanol) for 30 minutes at room temperature, uorescence spectra of C6 in a-, band g-CDs in aqueous environment and time resolved uorescence decay prole for C6 in presence of band g-CDs. See
Dynamic
self-assembly of nanoparticles (NPs) for the formation
of aggregates takes place out of thermodynamic equilibrium and is
sustained by external energy supply. Herein, we present light energy
driven dynamic self-assembly process of AuNPs, decorated with pH sensitive
ligands. The process is being controlled by the use of photoacids
and photobases that undergo excited state proton or hydroxide transfer,
respectively, due to their large pK
a change
between their ground and excited electronic states. The unique design
is underlined by record subsecond conversion rates between the assembled
and disassembled AuNPs states, and the ability to control the process
using only light of different wavelengths. Measurements in both aqueous
and nonaqueous solutions resulted in different self-assembly mechanisms,
hence showing the wide versatility of photoacids and photobases for
dynamic processes.
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