Glutathione (GSH)-coated gold nanoclusters (Au NCs) were synthesized in aqueous acidic medium. On deprotonation of the carboxyl groups of the GSH molecules under alkaline condition, the anionic ends react with the added cationic surfactant molecules to convert the Au NCs hydrophobic, resulting in loss of fluorescence due to apparent insolubility in water. The fluorescence is revived by adding cyclodextrins (CDs) that encapsulate the protruding hydrophobic tails of the surfactant molecules surrounding the GSH-coated Au NCs. While addition of β-CD showed maximum revival of the Au NC fluorescence, that by adding α-CD was lesser. Interestingly, on adding γ-CD, there was no increase in fluorescence of Au NCs at all. The size of CDs varies as γ- > β- > α-. It appears that the cavity size of the CD-hosts controls the fluorescence from the Au NCs abruptly, and the reason behind that was found to be formation of suprastructures, the shapes of which varied from spherical to cubic. The work shows the production of Au NC-grafted CD suprastructures that develop fluorescence on–off composites on the basis of their overall shapes.

Photocatalysis using polluted water where organic pollutants act as electron and hole transferring agents and carbon dots (C-Dots) act as a photosensitizer is a new game-changing approach for next generation photocatalysis. We demonstrate here an excellent approach for photocatalysis using water-soluble pollutants as a sacrificial electron acceptor (benzoquinone, BQ) and a sacrificial hole acceptor (pyrazine, PYZ), creating a mimic of untreated wastewater. BQ and PYZ are two common pollutants which are found in industrial wastewater. Here, charge separation and recombination processes are investigated to find out the exact mechanism for an efficient homogeneous photocatalysis process with myriad uses of steady state, time-resolved photoluminescence and ultrafast transient absorption spectroscopy. Furthermore, we demonstrate the proof of concept in charge separation using wastewater pollutants which may provide a new cost-efficient approach for improving the efficiency of photocatalysis, where we found that the efficiency of methylene blue and rhodamine B degradation is improved in the presence of pollutants with a photosensitizer (C-Dots). This finding will create a new avenue for the design and development of cost-efficient solar-driven hydrogen production and improvement of photocatalytic activity using untreated wastewater.

Ligand-protected gold nanoclusters (Au NCs), generally being less fluorescent, are occasionally used as energy donors in Forster resonance energy-transfer (FRET) processes. Although several recent reports have stated methods to enhance the fluorescence quantum yield (QY) of Au NCs, these have limited applications. Ultrasmall Au NCs are reportedly nontoxic to biological cells limited to the protecting ligands. Herein, we have used a testified protocol to substantially enhance the fluorescence QY of the ligand-protected water-soluble Au NCs by rigidifying the protecting ligands using arginine, a physiologically essential amino acid. This has enabled us to use the Au NCs as energy donors to do FRET with a hemicyanine dye belonging to a family that is prospectively used in neuronal and mitochondrial centers in biological cells. We could hardly find applications of FRET to transfer photon energy to hemicyanine dyes using Au NCs or any metal NCs in the literature. This biofriendly FRET-based approach, creating an intrinsic antenna effect, could be useful in controlling the fluorescence emission from nanomaterials and conversion to energy useful for cellular functioning.

Proteins possess various domains and subdomain pockets with varying hydrophobicity/hydrophilicity. The local polarities of these domains play a major role in oxidation–reduction-based biological processes. Herein, we have synthesized ultrasmall fluorescent copper nanoclusters (Cu NCs) that are directed to bind to the different domain-specific pockets of the model protein bovine serum albumins (BSA). Potential electron acceptors, methyl viologen (MV) derivatives, were chosen such that they specifically reach the various domains following their hydrophobicity/hydrophilicity. Here, we have used MV 2+ , HMV + , and DHMV 2+ , possessing hydrophilic, intermediate, and hydrophobic specificities. Being electron acceptors, these derivatives draw electrons from the Cu NCs through photoinduced electron transfer (PET). The rate of PET varies at the different domains of BSA based on the local environment which has been analyzed. Here, PET is confirmed by steady state as well as time-resolved fluorescence spectroscopy. This study would provide a measurable way to identify the location of the different domains of a protein which is scalable by changing the superficial conditions without unfolding the protein.

Electron transfer (ET) across proteins is ubiquitous in nature, such as the notable photosynthesis example, where light-induced charge separation takes place within the reaction center, followed by sequential ET via...