In this paper the interactions of gold nanoparticles (Au NPs) and bovin serum albumin–gold nanoconjugates (BSA–GNPs) with cadmium sulfide quantum dots (CdS QDs) are investigated by using steady-state and time-resolved spectroscopic techniques under physiological conditions (pH ∼ 7). From the analysis of the steady-state and time-resolved fluorescence quenching of CdS QDs in aqueous solution in the presence of BSA–GNPs it has been inferred that fluorescence resonance (Förster type) energy transfer (FRET) is primarily responsible for the quenching phenomenon. But in the presence of only Au NPs the fluorescence quenching of CdS NPs is primarily static in nature. Thus, it is apparent that, in the presence of BSA (in the case of the bionanoconjugate system), FRET becomes operative between CdS QDs and Au NPs present in the BSA–GNPs bionanoconjugate, whereas in the absence of this biomolecule direct contact between CdS and Au NPs facilitates the formation of ground-state complex. As shown from the high-resolution transmission electron microscopy (HRTEM) images of the bionanoconjugate, formation of a thin BSA layer around the Au NPs, situated at the core, inhibits the CdS QDs to come in contact with the Au NPs. In the CdS–bionanoconjugate system, CdS and Au NPs become separated by a distance of ∼17 ± 2 Å, as observed from HRTEM measurement. It may be presumed that when Au NPs are present in the bionanoconjugate system, the system may suffer some conformational changes which facilitates the energy transfer process to occur within the CdS QDs and the Au NPs. Further investigations with similar systems would be necessary to make unequivocal assertion of this phenomenon. From the determination of the thermodynamic parameters it is apparent that the effect of van der Waals interaction is responsible for the interaction of CdS QDs with Au NPs to form ground-state complex. The effect of CdS NPs on the conformation of BSA–GNPs has been examined by analyzing CD spectra. Though the observed results demonstrate some conformational changes in the bionanoconjugate in the presence of CdS NPs, the secondary structure of the conjugate, predominantly of the α-helix, is found to retain its identity. This type of interaction between QDs and Au NPs in a protein-conjugated form provides a new insight for design and the development of FRET-based bionanosensors.

Present study reveals the low concentrations (∼4 ppm) of pesticide sensing vis-à-vis degradation of pesticides with the help of nontoxic zinc oxide quantum dots (QD). In our study, we have taken four different pesticides viz., aldrin, tetradifon, glyphosate, and atrazine, which are widely used in agriculture and have structural dissimilarities/diversity. By using optical sensing techniques such as steady state and time-resolved fluorescence, we have analyzed the detailed exciton dynamics of QD in the presence of different pesticides. It has been found that the pesticide containing good leaving groups (-Cl) can interact with QD promptly and has high binding affinity (∼10 M). The different binding signatures of QD with different pesticides enable us to differentiate between the pesticides. Time resolved fluorescence spectroscopy provides significant variance (∼150-300 ns) for different pesticides. Furthermore, a large variation (10 Ω to 7 × 10 Ω) in the resistance of QD in the presence of different pesticides was revealed by electrochemical sensing technique. Moreover, during the interaction with pesticides, QD can also act as a photocatalyst to degrade pesticides. Present investigation explored the fact that the rate of degradation is positively affected by the binding affinity, i.e., the greater the binding, the greater is the degradation. What is more, both optical and electrochemical measurements of QD, in tandem, as described in our study could be utilized as the pattern recognition sensor for detection of several pesticides.

In this paper, the interaction between bovine serum albumin (BSA) and zinc oxide (ZnO) nanoparticles was investigated by fluorescence quenching spectra, circular dichroism (CD), and synchronous spectra under physiological conditions. From the analysis of the steady state and time resolved fluorescence quenching of BSA in aqueous solution in presence of ZnO it was observed that the nature of the quenching is of static-type quenching. The Stern–Volmer quenching constant KS at different temperatures were determined and the thermodynamic parameters ΔH, ΔG, and ΔS were computed. The experiment revealed that the electrostatic interaction was the predominant force in stabilizing the complex. The effect of ZnO on the conformation of BSA has been analyzed by synchronous spectra and CD spectrum. Although the observed results demonstrate some conformational changes in BSA in presence of ZnO nanoparticles, the secondary structure of BSA, predominantly of α-helix, is found to retain its identity.

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