Puspal Mukherjee

Optical property characterization of novel graphene-X (X=Ag, Au and Cu) nanoparticle hybrids

The present investigation reports new results on optical properties of graphene-metal nanocomposites. These composites were prepared by a solution-based chemical approach. Graphene has been prepared by thermal reduction of graphene oxide (GO) at 90°C by hydrazine hydrate in an ammoniacal medium. This ammoniacal solution acts as a solvent as well as a basic medium where agglomeration of graphene can be prevented. This graphene solution has further been used for functionalization with Ag, Au, and Cu nanoparticles (NPs). The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-Vis spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to reveal the nature and type of interaction of metal nanoparticles with graphene. The results indicate distinct shift of graphene bands both in Raman and UV-Vis spectroscopies due to the presence of the metal nanoparticles. Raman spectroscopic analysis indicates blue shift of D and G bands in Raman spectra of graphene due to the presence of metal nanoparticles except for the G band of Cu-G, which undergoes red shift, reflecting the charge transfer interaction between graphene sheets and metal nanoparticles. UV-Vis spectroscopic analysis also indicates blue shift of graphene absorption peak in the hybrids. The plasmon peak position undergoes blue shift in Ag-G, whereas red shift is observed in Au-G and Cu-G.

A chemosensor for Al3+ ions in aqueous ethanol media: photophysical and live cell imaging studies

A novel fluorescent chemosensor L has been designed and synthesized based on the rhodamine-B moiety. The dye L exhibits selective fluorescence enhancement towards the Al3+ ion over other biologically relevant metal ions in aqueous ethanolic media (EtOH–H2O, 2 : 3, v/v). The structure of the probe L has been established by 1H and 13C-NMR spectroscopy, single crystal X-ray diffraction, ESI-mass spectrometry and elemental analysis. The cleavage of the spirolactam bond of the rhodamine moiety induced by the Al3+ ion generates the delocalized xanthene fluorophore that is responsible for the emission enhancement of the probe L. The recognition behavior of the receptor L has been investigated experimentally with supports from theoretical DFT studies. Furthermore, the efficacy of L in cell imaging studies is also probed by confocal microscopy.

Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate

Fluorescence quenching studies through steady-state and time-resolved measurements are inadequate to quantify the bimolecular electron transfer rate in bulk homogeneous solution due to constraints from diffusion. To nullify the effect of diffusion, direct evaluation of the rate of formation of a transient intermediate produced upon the electron transfer is essential. Methyl viologen, a well-known electron acceptor, produces a radical cation after accepting an electron, which has a characteristic strong and broad absorption band centered at 600 nm. Hence it is a good choice to evaluate the rate of photoinduced electron transfer reaction employing femtosecond broadband transient absorption spectroscopy. The time constant of the aforementioned process between pyrene and methyl viologen in methanol has been estimated to be 2.5 ± 0.4 ps using the same technique. The time constant for the backward reaction was found to be 14 ± 1 ps. These values did not change with variation of concentration of quencher, i.e., methyl viologen. Hence, we can infer that diffusion has no contribution in the estimation of rate constants. However, on changing the solvent from methanol to ethanol, the time constant of the electron transfer reaction has been found to increase and has accounted for the change in solvent reorganization energy.

Graphene-Metal Nanoparticle Hybrids: Electronic Interaction Between Graphene and Nanoparticles

In the present investigation, graphene (G) and graphene decorated with Ag (Ag-G), Au (Au-G) metal nanoparticles as well as AuAg alloy (AuAg-G) nanoparticles (NPs) have been successfully synthesized via solution chemical route. The specimens have been characterized by using XRD, Raman, UV–Vis spectroscopy and TEM. TEM study confirms the formation of few-layered graphene sheets decorated with metal and alloy NPs. The relative change in the positions of D and G bands in Raman spectra of Ag-G, Au-G and AuAg-G as compared to graphene reveals a significant electronic interaction between graphene and metal or alloy NPs. A better electronic interaction between AuAg alloy NPs and graphene as compared to that of pure metal NPs has been shown by an enhancement in the intensities of G and D bands in Raman spectrum of AuAg-G as compared to those of Ag-G and Au-G. UV–Vis spectroscopic analysis also indicates, a larger blue shift in the absorption peak of the graphene in AuAg-G than that of metal decorated graphene, suggesting a better interaction between graphene and alloy NPs. These results indicate that alloy NPs can tune the electronic structure of graphene better than metal NPs. Such changes in electronic interaction will lead to modification of the electronic behaviour of graphene, making these materials suitable for many electronic applications.

Graphene: a self-reducing template for synthesis of graphene–nanoparticles hybrids

The integration of graphene with certain metallic nanoparticles, such as Au, Ag, Pt, Pd, Cu, etc., to produce a new generation of hybrid materials is a field of intense research nowadays. Graphene, being a single atom thick layer sheet of hexagonally arranged sp2 carbon atoms, has a prodigious number of free electrons, which can be used to reduce metallic ions to produce a hybrid material consisting of metal nanoparticles on the 2D fabric of graphene. Efforts were made to explore such property of the virgin graphene by careful in situ study using UV-visible (UV-vis) spectroscopy and transmission electron microscopy (TEM). The results indicate that it is possible to use surface potential of graphene to reduce Au3+, Ag+, Pt2+, Pd2+ and Cu2+ ions to prepare graphene–metal nanoparticle hybrids. The extensive TEM studies substantiate the finding of the formation of graphene decorated with metal nanoparticles.

Bimolecular Photoinduced Electron Transfer in Static Quenching Regime: Illustration of Marcus Inversion in Micelle

Ultrafast bimolecular photoinduced electron transfer (PET) between six coumarin dyes and four viologen molecules in the stern layer of sodium dodecyl sulfate micelle have been studied using femtosecond broadband transient absorption spectroscopy and femtosecond fluorescence up-conversion spectroscopy over a broad reaction exergonicity (ΔG0). Emanating the formation of radical cation intermediates of viologen molecules using the transient absorption and the fast decay component of coumarins using the fluorescence up-conversion studies the forward bimolecular electron transfer rate (ket) have been measured with high accuracy. The relationship of ket with ΔG0 found to follow a Marcus type bell-shaped dependence with an inversion at -1.10 eV. In this report, we have studied PET reaction using ultrafast spectroscopy at the quencher concentration where static quenching regime prevails. Moreover, the incompetency of Stern-Volmer experiments in studying ultrafast PET has been revealed. In contrary to previous claims, here we found that the ket is lower for lower lifetime coumarins, indicating that static, nonstationary and stationary regime of quenching have the minimal role to play to in the bimolecular electron transfer process. By far, this report is believed to be the most efficient and immaculate way of approaching Marcus inverted region problem in the case of bimolecular PET and settles the long-lasting debate of whether the same can be observed in micellar systems.

Dynamical response in methanol–chloroform binary solvent mixture over fs–μs time regime

ABSTRACT The present study elucidates the extended nature of the weak interaction between methanol and chloroform in its mixture using steady-state, ultrafast and single molecule spectroscopic methods. UV–vis absorption spectroscopy using a solvatochromic dye indicates a synergistic solvation in the methanol–chloroform binary solvent mixture, which causes the solvatochromic dye to sense increased polarity compared to the bulk counterparts. Such synergism was not observed in the emission study and is explained by the weak nature of the interaction between methanol and chloroform. Fluorescence anisotropy and single molecule fluorescence spectroscopy are employed to understand the nature of such weak interaction on femtosecond to microsecond time scale. Retardation of both the rotational and translational diffusion of the reporter molecule indicates that the observed weak interaction is extended over large dimension in the condensed phase.