Abhijit Mandal

High Raman Enhancing Shape-Tunable Ag Nanoplates in Alumina: A Reliable and Efficient SERS Technique

Shape-tunable Ag nanoplates in alumina enable very strong SERS performances showing Raman enhancement factor >1 × 10(9), and allows for easy detection of analyte methylene blue having a concentration in the picomolar range. Raman enhancements have been systematically studied during the Ag nanoparticle-shape evolution from spherical nanoparticles to hexagonal nanoplates with sharp corners to truncated triangular nanoplates in alumina sol. Large SERS enhancement has been observed because of the uniform dispersion and embedment of Ag nanoplates in a relatively high dielectric alumina network where analyte molecules are held. This new approach gives uniform and strong SERS signal with reproducibility.

A single source-precursor route for the one-pot synthesis of highly luminescent CdS quantum dots as ultra-sensitive and selective photoluminescence sensor for Co2+ and Ni2+ ions

In this study, we have demonstrated a facile, simple one-pot and low cost method for the synthesis of 3-mercaptopropionic acid (MPA)-capped, water-soluble CdS quantum dots (QDs) with highly tunable optical properties. Initially Cd2+ coordinates with MPA at about pH 5, and the CdS QDs were then formed at a higher pH (7–12) under refluxing conditions through the disruption of coordination interaction with the release of sulfur. Here MPA played a dual role, as both, a source of sulfur and as a stabilizer. The particle size and the optical properties of the as-prepared CdS QDs were found to be dependent on the refluxing time for a given concentration ratio of the reactants and pH of the initial mixture. The broadness and large Stokes shift of emission of MPA–CdS QDs are due to the surface-trap state photoluminescence (PL). The PL peak around 510 nm–650 nm is due to the recombination of shallow trapped electrons in sulfur vacancy defect states with holes in the valence band, and a ∼665 nm peak (shoulder) arises from deep-trap states. The origin of the longer lifetime is presumed to be due to the involvement of surface-trap states and their environment. Use of MPA as a capping agent eventually enhances the water solubility as well as the stability of CdS QDs, which makes them useful for the ultra-sensitive detection of Co2+ and Ni2+. The selective coordination interaction of Co2+ and Ni2+ with MPA–CdS QDs through the carboxyl group of MPA provides a turn-off photoluminescence-based assay for sensitive detection of these metal ions without any interference of other commonly coexisting metal ions. The limit of detection (LOD) is 10 nM for Co2+ ions and 50 nM for Ni2+ ions. Co2+-induced color (from colorless to yellow) and UV-vis spectral change of MPA–CdS QDs is the simple way to distinguish Co2+ from Ni2+ in a higher concentration range (more than 5 µM). On the other hand the lower stability of the Co(II)–MPA complex than the Ni(II)–MPA complex provides a disodium salt of ethylenediaminetetraacetic acid (EDTA)-induced, time dependent turn-on photoluminescence-based technique to distinguish Co2+ from Ni2+ in the entire range of concentrations. EDTA-induced time dependent PL recovery of MPA–CdS QDs occurs via rapid dissociation of Co2+ ions from the surface of QDs than that of Ni2+. Thus our synthesized MPA–CdS QDs offer a very simple, rapid, cost-effective, turn-off–on photoluminescence-based technique for ultra-sensitive and selective detection of either Co2+ or Ni2+ in aqueous solution without interference of other common metal ions.

Spectral Signatures of Intramolecular Charge Transfer Process in β-Enaminones: A Combined Experimental and Theoretical Analysis

In this paper, we present spectroscopic signatures of intramolecular charge transfer (ICT) and effects of solvent on the ICT process in 3-(phenylamino)-2-cyclohexen-1-one (PACO), a member of the well-known molecular family, the beta-enaminones. The dual fluorescence in the steady state emission spectra of the molecule in polar solvents indicates the occurrence of ICT, which is further supported by time-resolved studies, using time correlated single photon counting technique with picosecond resolution. To understand the nature of the charge transfer, pH dependent studies of the probe in water were performed, where a quenching of fluorescence was observed even in the presence of very low concentrations of acids. Solvent induced fluorescence quenching was observed in ethanol and methanol. The ICT process was also investigated by quantum chemical calculations. To understand the role of solvents in the ICT process, we have theoretically studied the macroscopic and microscopic solvation of the probe in water. The absorption spectra of the molecule in the gas phase as well as in water were simulated using time dependent density functional theory with cc-pVTZ basis set and self-consistent reaction field theory that models macroscopic solvation. The possibility of microscopic solvation in water was probed theoretically and the formation of 1:3 molecular clusters by PACO with water molecules has been confirmed. Our findings could have a bearing on pH sensing applications of the probe.

Spectral Response of 4-Methyl-2,6-dicarbomethoxyphenol, an Excited-State Intramolecular Proton-Transfer Probe in Cyclohexane–Ethanol Mixtures: Signatures of Medium Microheterogeneity

In this paper, we explore the role of microscopic heterogeneity of the medium on the spectral response of an excited-state proton-transfer (ESIPT) probe, namely, 4-methyl-2,6-dicarbomethoxyphenol (CMOH) using steady-state and time-resolved emission spectroscopy. The mixtures of two solvents with widely different properties, viz., cyclohexane, a nonpolar, and ethanol, a polar protic solvent, were used as microheterogeneous media for spectroscopic studies. Dual fluorescence (normal and tautomer fluorescence) is observed in the nonpolar solvent (cyclohexane), while only a single peak is observed in the protic solvent, ethanol. The spectral responses of CMOH in the binary mixtures have been found to be dependent on the solvent composition and excitation wavelength. The emission spectral properties of CMOH in the cyclohexane-ethanol mixture have been seen to be superposition of spectral properties in their bulk counterparts, indicating the presence of microscopic heterogeneity in the system. A zwitterionic species of CMOH appears to have been detected in binary solvent mixtures with higher ethanol content only through low-energy excitations. The species is converted into an anionic species as excitation energy increases. Density functional theory calculations indicate that two intramolecularly hydrogen bonded rotamers of CMOH have a small energy difference. The formation of a hydrogen bonded 1:1 molecular cluster of CMOH with ethanol has been investigated in the ground state at the same level of theory. Our findings are expected to shed light on the mechanism of many acid-base reactions occurring in microscopically inhomogeneous media that often mimic many biologically relevant processes.

Nanopools Governing Proton Transfer in Diametrical Ways in the Ground and Excited State

We present here the effects of geometrically constrained environments on the proton transfer reaction of 4-methyl 2,6-diformyl phenol (MFOH) both in the ground and excited states by employing steady-state and time-resolved fluorescence spectroscopy having picosecond and femtosecond resolutions. The nanometer-sized water pools formed in the ternary microemulsion of n-heptane-aerosol OT-water promote reprotonation of the probe. As we go on increasing the water content up to a certain value in the ground state whereas deprotonation is favored in the excited state. The emission intensity has a complex behavior as the water content is changed in the system. The lower fluidity of confined water within the reverse micelle with respect to the normal bulk water alters the related dynamics of the H-bonded network. These observations are rationalized on the basis of altered ionic water activity in the confined surroundings, i.e., on dielectric constant, ionic mobility, pH, and the favorable orientation of dipoles in the medium. Our observations might be helpful to infer about the characteristics of nanoreactors, which often mimic many biological hydrophilic pockets.

Thermoluminescence mechanism in rare-earth-doped magnesium tetra borate phosphors

Magnesium tetra borate (MTB) doped with rare earths (REs) was prepared by the solid state sintering technique. Among the different RE dopants studied in this phosphor, gadolinium-doped phosphors resulted in a dosimetric peak at a relatively higher temperature. The thermoluminescence (TL) emission spectra of RE-doped MTB showed characteristic RE 3+ emissions. Electron paramagnetic resonance measurements were carried out in these phosphors to identify the defect centers formed during gamma irradiation and to establish a mechanism for the TL process. Signals corresponding to (BO 3)2−, O v− were seen upon irradiation which vanished on annealing at 250 °C, showing the role of these centers in the TL process. The thermal activation energies calculated based on the decay of these signals matched well with those calculated on the basis of the usual conventional method showing the validity of the mechanism of TL.

Suppressed blinking behavior of thioglycolic acid capped CdTe quantum dot by amine functionalization

Prepared water soluble thioglycolic acid capped CdTe quantum dots (QDs) were further surface functionalized by ethylene diamine (EDA). Amine functionalized CdTe QDs demonstrate enhanced luminescence intensity at ensemble measurements and suppressed luminescence intermittency behavior at the single molecule level. A clear decrease in the power law exponent for “on” time behavior is observed in amine modified CdTe QDs. Our results show that surface of CdTe QDs modified by EDA can lead to an important physical mechanism to enhance fluorescence intensity, reduce blinking, and increase photostability.

Radiation-induced defects in manganese-doped lithium tetraborate phosphor

Lithium tetraborate doped with manganese synthesised by solid-state sintering technique exhibits a dosimetric peak at 280°C. The high-temperature glow curve results in no fading for three months. The sensitivity of Li2B4O7:Mn is determined to be 0.9 times that of TLD-100. The infrared spectrum of this phosphor indicates the presence of bond vibrations corresponding to BO4 tetrahedral and BO3 triangles. The mechanism for thermoluminescence in this phosphor was proposed based on the thermoluminescence (TL) emission spectra, kinetic analysis of TL glow curves and electron paramagnetic resonance (EPR) measurements on non-irradiated and gamma-irradiated phosphors. It was identified that oxygen vacancies and Boron oxygen hole centre (BOHC) are the electron and hole trap centres for TL in this phosphor. When the phosphor is heated, the electrons are released from the electron trap and recombine with the trapped holes. The excitation energy during the recombination is transferred to the nearby Mn(2+) ions, which emit light at 580 nm.

Improved photoluminescence properties of sol-gel derived Er3+ doped silica films

Silica films (amorphous and crystalline) doped with erbium were fabricated on silica glass substrate and characterized. The inorganic-organic hybrid sol-gel method was used to prepare the films and the Na codoping induced the crystallization of silica film. Photoluminescence (PL) measurements revealed that the Er3+ ions can be excited from the ground state through an energy transfer process mediated by active defective sites in SiO2 film matrix. The annealing temperature and atmospheres have large effects on the local environment of Er3+ and the 1.54 μm PL intensity can be improved significantly by suitable heating treatments. We could correlate Er3+ sensitization effect due to the presence of carbon related species in the films. The PL intensity at nonresonant (476.5 nm) condition can be made as intense as the resonant (488 nm) one, for particular annealing conditions. Noticeable changes in PL emission intensities have not been observed whether the matrix silica film is amorphous or crystalline in nature...