Abhinandan Makhal

Photoselective excited state dynamics in ZnO-Au nanocomposites and their implications in photocatalysis and dye-sensitized solar cells.

Improving the performance of photoactive solid-state devices begins with systematic studies of the metal-semiconductor nanocomposites (NCs) upon which such devices are based. Here, we report the photo-dependent excitonic mechanism and the charge migration kinetics in a colloidal ZnO-Au NC system. By using a picosecond-resolved Förster resonance energy transfer (FRET) technique, we have demonstrated that excited ZnO nanoparticles (NPs) resonantly transfer visible optical radiation to the Au NPs, and the quenching of defect-mediated visible emission depends solely on the excitation level of the semiconductor. The role of the gold layer in promoting photolytic charge transfer, the activity of which is dependent upon the degree of excitation, was probed using methylene blue (MB) reduction at the semiconductor interface. Incident photon-to-current efficiency measurements show improved charge injection from a sensitizing dye to a semiconductor electrode in the presence of gold in the visible region. Furthermore, the short-circuit current density and the energy conversion efficiency of the ZnO-Au NP based dye-sensitized solar cell (DSSC) are much higher than those of a DSSC comprised of only ZnO NP. Our results represent a new paradigm for understanding the mechanism of defect-state passivation and photolytic activity of the metal component in metal-semiconductor nanocomposite systems.

Hematoporphyrin-ZnO nanohybrids : Twin applications in efficient visible-light photocatalysis and dye-sensitized solar cells

Light-harvesting nanohybrids (LHNs) are systems composed of an inorganic nanostructure associated with an organic pigment that have been exploited to improve the light-harvesting performance over individual components. The present study is focused on developing a potential LHN, attained by the functionalization of dense arrays of ZnO nanorods (NRs) with a biologically important organic pigment hematoporphyrin (HP), which is an integral part of red blood cells (hemoglobin). Application of spectroscopic techniques, namely, Fourier transform infrared spectroscopy (FTIR) and Raman scattering, confirm successful monodentate binding of HP carboxylic groups to Zn(2+) located at the surface of ZnO NRs. Picosecond-resolved fluorescence studies on the resulting HP-ZnO nanohybrid show efficient electron migration from photoexcited HP to the host ZnO NRs. This essential photoinduced event activates the LHN under sunlight, which ultimately leads to the realization of visible-light photocatalysis (VLP) of a model contaminant Methylene Blue (MB) in aqueous solution. A control experiment in an inert gas atmosphere clearly reveals that the photocatalytic activity is influenced by the formation of reactive oxygen species (ROS) in the media. Furthermore, the stable LHNs prepared by optimized dye loading have also been used as an active layer in dye-sensitized solar cells (DSSCs). We believe these promising LHNs to find their dual applications in organic electronics and for the treatment of contaminant wastewater.

Modulation of defect-mediated energy transfer from ZnO nanoparticles for the photocatalytic degradation of bilirubin

Summary In recent years, nanotechnology has gained significant interest for applications in the medical field. In this regard, a utilization of the ZnO nanoparticles for the efficient degradation of bilirubin (BR) through photocatalysis was explored. BR is a water insoluble byproduct of the heme catabolism that can cause jaundice when its excretion is impaired. The photocatalytic degradation of BR activated by ZnO nanoparticles through a non-radiative energy transfer pathway can be influenced by the surface defect-states (mainly the oxygen vacancies) of the catalyst nanoparticles. These were modulated by applying a simple annealing in an oxygen-rich atmosphere. The mechanism of the energy transfer process between the ZnO nanoparticles and the BR molecules adsorbed at the surface was studied by using steady-state and picosecond-resolved fluorescence spectroscopy. A correlation of photocatalytic degradation and time-correlated single photon counting studies revealed that the defect-engineered ZnO nanoparticles that were obtained through post-annealing treatments led to an efficient decomposition of BR molecules that was enabled by Förster resonance energy transfer.

Dynamics of light harvesting in ZnO nanoparticles

We have explored light harvesting of the complex of ZnO nanoparticles with the biological probe Oxazine 1 in the near-infrared region using picosecond-time-resolved fluorescence decay studies. We have used ZnO nanoparticles and Oxazine 1 as a model donor and acceptor, respectively, to explore the efficacy of the Förster resonance energy transfer (FRET) in the nanoparticle-dye system. It has been shown that FRET from the states localized near the surface and those in the bulk of the ZnO nanoparticles can be resolved by measuring the resonance efficiency for various wavelengths of the emission spectrum. It has been observed that the states located near the surface for the nanoparticles (contributing to visible emission at lambda approximately 550 nm) can contribute to very high efficiency (>90%) FRET. The efficiency of light harvesting dynamics of the ZnO nanorods has also been explored in this study and they were found to have much less efficiency (approximately 40%) for energy transfer compared to the nanoparticles. The possibility of an electron transfer reaction has been ruled out from the picosecond-resolved fluorescence decay of the acceptor dye at the ZnO surface.

Toward an Alternative Intrinsic Probe for Spectroscopic Characterization of a Protein

The intrinsic fluorescent amino acid tryptophan is the unanimous choice for the spectroscopic investigation of proteins. However, several complicacies in the interpretation of tryptophan fluorescence in a protein are inevitable and an alternative intrinsic protein probe is a longstanding demand. In this contribution, we report an electron-transfer reaction in a human transporter protein (HSA) cavity which causes the tryptophan residue (Trp214) to undergo chemical modification to form one of its metabolites kynurenine (Kyn214). Structural integrity upon modification of the native protein is confirmed by dynamic light scattering (DLS) as well as near and far circular dichroism (CD) spectroscopy. Femtosecond-resolved fluorescence transients of the modified protein describe the dynamics of solvent molecules in the protein cavity in both the native and denatured states. In order to establish general use of the probe, we have studied the dipolar interaction of Kyn214 with a surface-bound ligand (crystal violet, CV) of the protein. By using the sensitivity of FRET, we have determined the distance between Kyn214 (donor) and CV (acceptor). Our study is an attempt to explore an alternative intrinsic fluorescence probe for the spectroscopic investigation of a protein. In order to establish the efficacy of the modification technique we have converted the tryptophan residues of other proteins (bovine serum albumin, chymotrypsin and subtilisin Carlsberg) to kynurenine and confirmed their structural integrity. We have also shown that catalytic activity of the enzymes remains intact upon the modification.

Functionalization of manganite nanoparticles and their interaction with biologically relevant small ligands: Picosecond time-resolved FRET studies

We report molecular functionalization of the promising manganite nanoparticles La0.67Sr0.33MnO3 (LSMO) for their solubilization in aqueous environments. The functionalization of individual NPs with the biocompatible citrate ligand, as confirmed by Fourier transform infrared (FTIR) spectroscopy, reveals that citrates are covalently attached to the surface of the NPs. UV-VIS spectroscopic studies on the citrate functionalized NPs reveals an optical band in the visible region. Uniform size selectivity (2.6 nm) of the functionalization process is confirmed from high resolution transmission electron microscope (HRTEM). In the present study we have used the optical band of the functionalized NPs to monitor their interaction with other biologically important ligands. Förster resonance energy transfer (FRET) of a covalently attached probe 4-nitrophenylanthranilate (NPA) with the capped NPs confirm the attachment of the NPA ligands to the surface functional group (-OH) of the citrate ligand. The FRET of a DNA base mimic, 2-aminopurine (2AP), with the NPs confirms the surface adsorption of 2AP. Our study may find relevance in the study of the interaction of individual manganite NPs with drug/ligand molecules.

Simultaneous binding of anti-tuberculosis and anti-thrombosis drugs to a human transporter protein: A FRET study

Although rifampicin (Rf) is one of the most effective antibiotics against infection caused by Mycobacterium tuberculosis, interaction of the drug with universal carrier protein in human blood plasma is not fully understood. Reduction of medicinal efficacy of other drugs, including anti-thrombosis drug warfarin (Wf), to the patients on Rf therapy also needs molecular understanding. In the present work we have studied interaction of Rf with one of the model carrier protein (human serum albumin). By using circular dichroism (CD) spectroscopy we have characterized the change in the secondary structure of the protein. The consequence of the simultaneous binding of the two drugs, Rf and Wf, on the structure of the protein has also been explored. Picosecond resolved Förster resonance energy transfer (FRET) from Wf to Rf explores possible binding sites of the anti-tuberculosis drug on the protein. In this report, we have discussed the potential problem of using the single tryptophan of the protein (Trp 214) as energy donor in FRET experiment for the characterization of the binding site of the drug Rf on the protein.

Interaction of an Antituberculosis Drug with a Nanoscopic Macromolecular Assembly: Temperature-Dependent Förster Resonance Energy Transfer Studies on Rifampicin in an Anionic Sodium Dodecyl Sulfate Micelle

In this contribution, we report studies on the nature of binding of a potent antituberculosis drug, Rifampicin (RF) with a model drug delivery system, sodium dodecyl sulfate (SDS) micelle. Temperature dependent dynamic light scattering (DLS), conductometry, and circular dichroism (CD) spectroscopy have been employed to study the binding interaction of the drug with the micelle. The absorption spectrum of the drug RF in the visible region has been employed to study Förster resonance energy transfer (FRET) from another fluorescent drug Hoechst 33258 (H33258), bound to the micelle. Picosecond-resolved FRET studies at room temperature confirm the simultaneous binding of the two drugs to the micelle and the distance between the donor-acceptor pair is found to be 34 Å. The temperature dependent FRET study also confirms that the location and efficiency of drug binding to the micelle changes significantly at the elevated temperature. The energy transfer efficiency of the donor H33258, as measured from time-resolved studies, decreases significantly from 76% at 20 °C to 60% at 55 °C. This reveals detachment of some amount of the drug molecules from the micelles and increased donor-acceptor distance at elevated temperatures. The estimated donor-acceptor distance increases from a value of 33 Å at 20 °C to 37 Å at 55 °C. The picosecond resolved FRET studies on a synthesized DNA bound H33258 in RF solution have been performed to explore the interaction between the two. Our studies are expected to find relevance in the exploration of a potential vehicle for the vital drug rifampicin.

A versatile fiber-optic coupled system for sensitive optical spectroscopy in strong ambient light

In this work we describe design and use of a fiber-optic based optical system for the spectroscopic studies on the samples under the presence of strong ambient light. The system is tested to monitor absorption, emission, and picosecond-resolved fluorescence transients simultaneously with a time interval of 500 ms for several hours on a biologically important sample (vitamin B2) under strong UV light. An efficient stray-light rejection ratio of the setup is achieved by the confocal geometry of the excitation and detection channels. It is demonstrated using this setup that even low optical signal from a liquid sample under strong UV-exposure for the picosecond-resolved fluorescence transient measurement can reliably be detected by ultrasensitive microchannel plate photomultiplier tube solid state detector. The kinetics of photodeterioration of vitamin B2 measured using our setup is consistent with that reported in the literature. Our present studies also justify the usage of tungsten light than the fluorescent light for the healthy preservation of food with vitamin B2.

Ultrafast dynamics of excitons in semiconductor quantum dots on a plasmonically active nano-structured silver film.

The excited state dynamics of core-shell type semiconductor quantum dots (QDs) of various sizes in close contact with a plasmonically active silver thin film has been demonstrated by using picosecond resolved fluorescence spectroscopy. The non-radiative energy transfer from the QDs to the metal surface is found to be of Förster resonance energy transfer (FRET) type rather than the widely expected nano-surface energy transfer (NSET) type. The slower rate of energy transfer processes compared to that of the electron transfer from the excited QDs to an organic molecule benzoquinone reveals an insignificant possibility of charge migration from the QDs to the metallic film.