The origin of the excitation wavelength (λex)-dependent photoluminescence (PL) of carbon dots (CDs) is poorly understood and still remains obscured. This phenomenon is often explained on the basis of surface trap/defect states, while the effect of quantum confinement is highly neglected in the literature. Here, we have shown that the λex-dependent PL of CDs is mainly due to the inhomogeneous size distribution. We have demonstrated the λex-dependent PL quenching of CDs inside the ferritin nanocages through selective optical excitation of differently sized CDs. It has been observed that Fe(3+) ions of ferritin effectively quench the PL of CDs due to static electron transfer, which is driven by favorable electrostatic interactions. However, control experiment with aqueous Fe(3+) ions in bulk medium revealed λex-independent PL quenching of CDs. The λex-dependent PL quenching of CDs by Fe(3+) ions of ferritin has been rationalized on the basis of a different extent of accessibility of Fe(3+) ions by differently sized CDs through the funnel-shaped ferritin channels. PL microscopy of individual CDs has been performed to get further information about their inherent PL properties at single dot resolution. Our results have shown that these hydrophilic CDs can be used as potential iron sensors in biological macromolecules.

The molecular origin behind the concentration-dependent intrinsic blue fluorescence of human serum albumin (HSA) is not known yet. This unusual blue fluorescence is believed to be a characteristic feature of amyloid-like fibrils of protein/peptide and originates due to the delocalization of peptide bond electrons through the extended hydrogen bond networks of cross-β-sheet structure. Herein, by combining the results of spectroscopy, size exclusion chromatography, native gel electrophoresis, and confocal microscopy, we have shown that the intrinsic blue fluorescence of HSA exclusively originates from oligomeric interfaces devoid of any amyloid-like fibrillar structure. Our study suggests that this low energy fluorescence band is not due to any particular residue/sequence, but rather it is a common feature of self-assembled peptide bonds. The present findings of intrinsic blue fluorescence from oligomeric interfaces pave the way for future applications of this unique visual phenomenon for early stage detection of various protein aggregation related human diseases.

Proteins inside a cell remain in highly crowded environments, and this often affects their structure and activity. However, most of the earlier studies involving serum albumins were performed under dilute conditions, which lack biological relevance. The effect of protein-protein interactions on the structure and properties of serum albumins at physiological conditions have not yet been explored. Here, we report for the first time the effect of protein-protein and protein-crowder interactions on the structure and stability of two homologous serum albumins, namely, human serum albumin (HSA) and bovine serum albumin (BSA), at physiological conditions by using spectroscopic techniques and scanning electron microscopy (SEM). Concentration-dependent self-oligomerization and subsequent structural alteration of serum albumins have been explored by means of fluorescence and circular dichroism spectroscopy at pH 7.4. The excitation wavelength (λex) dependence of the intrinsic fluorescence and the corresponding excitation spectra at each emission wavelength indicate the presence of various ground state oligomers of serum albumins in the concentration range 10-150 μM. Circular dichroism and thioflavin T binding assay revealed formation of intermolecular β-sheet rich interfaces at high protein concentration. Excellent correlations have been observed between β-sheet content of both the albumins and fluorescence enhancement of ThT with protein concentrations. SEM images at a concentration of 150 μM revealed large dispersed self-oligomeric states with sizes vary from 330 to 924 nm and 260 to 520 nm for BSA and HSA, respectively. The self-oligomerization of serum albumins is found to be a reversible process; upon dilution, these oligomers dissociate into a native monomeric state. It has also been observed that synthetic macromolecular crowder polyethylene glycol (PEG 200) stabilizes the self-associated state of both the albumins which is contrary to expectations that the macromolecular crowding favors compact native state of proteins.

In the present study, we have demonstrated the excitation energy transfer (EET) from silicon quantum dots (Si QDs) to silver nanoparticles (Ag NPs) and its modulation in the presence of cetyltrimethylammonium bromide (CTAB) surfactant by means of steady-state and time-resolved photoluminescence (PL) spectroscopy. Significant spectral overlap between the emission spectrum of Si QDs and localized surface plasmon resonance of Ag NPs results in a substantial amount of PL quenching of Si QDs. In addition, the PL lifetime of Si QDs is shortened in the presence of Ag NPs. The origin of this PL quenching has been rationalized on the basis of increased nonradiative decay rate due to excitation energy transfer from Si QDs to Ag NPs surface. The observed energy-transfer efficiency correlates well with the nanometal surface energy transfer theory with a 1/d 4 distance dependence rather than conventional Forster resonance energy transfer theory. It has also been observed that the EET efficiency drastically reduces in the presence of 0.5 mM CTAB. Dynamic light scattering and single-particle PL microscopy results indicate the formation of large surfactant-induced aggregates of Ag NPs. Finally, the energy-transfer efficiency values obtained from experiment have been used to calculate the distance between Si QDs and Ag NPs in the absence and presence of CTAB, which correlates well with the proposed model.