Resonant excitation energy transfer from carbon dots to different sized silver nanoparticles.

The influence of size on the efficiency of the nanometal surface energy transfer (NSET) process between excited donors and different sized metal nanoparticles (NPs) is poorly explored in the literature. Here we present a systematic study by correlating the size of silver nanoparticles (Ag NPs) with the efficiency of excitation energy transfer (EET) from carbon dots (CDs) to Ag NPs. Three different sized citrate-capped Ag NPs with a mean hydrodynamic diameter of 39.91 ± 1.03, 53.12 ± 0.31 and 61.84 ± 0.77 nm have been synthesized for the present study. The estimated zeta potential of the synthesized CD is -25.45 ± 1.23 mV while that for the smallest, medium and largest sized Ag NPs are -76.24 ± 3.92, -67.60 ± 4.40, and -58.01 ± 3.10 mV, respectively. The steady-state and time-resolved PL measurements reveal a significant PL quenching of CDs as a function of Ag NP size. A control experiment with Ag NPs having a LSPR at 398 nm shows a negligible amount of PL quenching of CDs as a consequence of inadequate spectral overlap. The origin behind this PL quenching of CDs has been rationalized on the basis of the increased nonradiative decay rate due to NSET from the CDs to the Ag NP surface. Various energy transfer related parameters have been estimated from the NSET theory and it has been observed that the NSET efficiency increases with the increase in the size of Ag NPs. This phenomenon has been explained by considering a larger spectral overlap and a shorter separation distance between the CDs and larger sized Ag NPs due to reduced electrostatic repulsion. Our present results reveal that the size of NPs plays an important role in the NSET process and this phenomenon can be easily utilized to tune the efficiency of energy transfer for various applications.

Effect of compartmentalization of donor and acceptor on the ultrafast resonance energy transfer from DAPI to silver nanoclusters.

The mechanism and dynamics of excitation energy transfer (EET) from photo-excited 4',6-diamidino-2-phenylindole (DAPI) to silver nanoclusters (Ag NCs) and its subsequent modulation in the presence of cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and Calf Thymus DNA (CT-DNA) have been demonstrated using steady-state fluorescence and femtosecond fluorescence upconversion techniques. The synthesized Ag NCs were characterized using FTIR, mass spectrometry, XPS, HRTEM, DLS, UV-Vis and PL spectroscopy. Mass spectrometric analysis reveals the formation of ultrasmall Ag4 NCs with a small amount of Ag5 NCs. UV-Vis and PL spectra reveal distinct molecular-like optoelectronic behaviour of these ultrasmall Ag NCs. The dihydrolipoic acid-capped Ag NCs strongly quench the fluorescence of DAPI with concomitant increase in its photoluminescence (PL) intensity at 675 nm. This steady-state fluorescence quenching proceeds with a significant shortening of the fluorescence lifetime of DAPI in the presence of Ag NCs, signifying the nonradiative Förster resonance energy transfer (FRET) from DAPI to Ag NCs. Various energy transfer parameters have been estimated from FRET theory. The present FRET pair shows a characteristic Förster distance of 2.45 nm and can be utilized as a reporter of short-range distances in various FRET based applications. Moreover, this nonradiative FRET is completely suppressed in the presence of both 0.2 wt% PDADMAC and CT-DNA. Our results reveal selective compartmentalization of Ag NCs and DAPI in the presence of 0.2 wt% PDADMAC and CT-DNA, respectively. This selective compartmentalization of donor and acceptor and the subsequent modification of the FRET process may find application in various sensing, photovoltaic, and light harvesting applications.

Size-dependent penetration of carbon dots inside the ferritin nanocages: evidence for the quantum confinement effect in carbon dots.

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.

Concentration-dependent reversible self-oligomerization of serum albumins through intermolecular ß-sheet formation.

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 ß-sheet rich interfaces at high protein concentration. Excellent correlations have been observed between ß-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.

Microscopic evidence of "necklace and bead"-like morphology of polymer-surfactant complexes: a comparative study on poly(vinylpyrrolidone)-sodium dodecyl sulfate and poly(diallyldimethylammonium chloride)-sodium dodecyl sulfate systems.

Here, we report the microscopic evidence of "necklace and bead"-like morphology, which has long been the most widely accepted model for polymer-surfactant complexes. The lack of microscopic evidence of the initial complexation between surfactant and polymer has resulted in many contradictory reports in the literature. In this paper, we visualized these initial complexes formed between negatively charged surfactant sodium dodecyl sulfate (SDS) with neutral poly(vinylpyrrolidone) (PVP) and cationic poly(diallyldimethylammonium chloride) (PDADMAC) polymer through photoluminescence (PL) microscopy and atomic force microscopy (AFM) using silicon quantum dot (Si QD) as an external PL marker. It is observed that, for the PVP-SDS system, SDS molecules bind at the hydrophobic sites on the random-coiled PVP chain through their hydrocarbon tails, while for the PDADMAC-SDS system, SDS head groups are associated with the positively charged nitrogen centers of the polymer, where the polymer chain wraps around the surfactant head groups.