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Pande S.,Indian Institute of Technology Kharagpur | Chowdhury J.,Sammilani Mahavidyalaya | Pal T.,Indian Institute of Technology Kharagpur
Journal of Physical Chemistry C | Year: 2011

Adsorption of 1,10-phenanthroline (PHEN) on the Aucore-Ag shell (Au@Ag) bimetallic nanocolloid surface has been investigated. Bimetallic nanocolloids have been synthesized using β-cyclodextrin (β-CD) in alkaline solution. Nanocolloids of different Au core:Agshell ratio were synthesized keeping the same Aucore size. Again, for the preparation, the total metal concentrations were always kept constant. The structure and composition of the bimetallic particles were characterized by UV-vis and high-resolution transmission electron microscopy. Detailed normal Raman (NRS) and surface-enhanced Raman spectra (SERS) of PHEN in aqueous solution have been studied. The relative contributions of the electromagnetic (EM) and charge-transfer (CT) mechanism to the overall enhancement of the SERS bands of the PHEN molecule have been estimated. PHEN molecules are adsorbed on the Agshell surface through both the nitrogen atoms with the molecular plane almost perpendicular to the surface, which has been confirmed from Ag-N stretching vibration. The three-dimensional finite difference time domain (3D-FDTD) method has been applied to simulate the local electric field on the spherical Au@Ag nanocolloid for various core/shell ratios. Comparative spectral information revealed the highest SERS effect from the 1:4 Aucore- Agshell bimetallic nanocolloid of 29 nm size. This observation has also been supported by theory. Thus, experiment and theory keep abreast the attachment of PHEN molecules on a new substrate from SERS studies. © 2011 American Chemical Society. Source


Chandra S.,P.A. College | Chowdhury J.,Sammilani Mahavidyalaya | Ghosh M.,Indian Association for The Cultivation of Science | Talapatra G.B.,P.A. College | Talapatra G.B.,Indian Association for The Cultivation of Science
Journal of Physical Chemistry C | Year: 2011

The concentration and pH dependent surface-enhanced Raman scattering (SERS) study of the industrially and biologically significant 3-thiophene carboxylic acid (3-TCA) molecule has been investigated. The SERS spectra of the molecule at different C Ad values are compared with its FTIR spectrum and normal Raman spectra (NRS) in varied environments. The optimized structural parameters and computed vibrational wavenumbers of the neutral and the anionic (3-TCA -) forms of the molecule have been estimated from density functional theory (DFT) calculations. The vibrational signatures of the molecule have been assigned from the potential energy distributions (PEDs). The concomitance of the Raman bands representing vibrational signatures emanating from the neutral and the anionic forms of the molecule signify the presence of both the forms of the molecule in the solid state and in acetonitrile (ACN) solution. However, detailed vibrational analysis reveals that 54% of 3-TCA is prevalent in the solid state while 63% of 3-TCA - is predominate in ACN solution at neutral pH. Concentration dependent SERS spectra reveal that the anionic form of the molecule is adsorbed on the nanocolloidal silver surface with the molecular plane tilted or nearly flat with respect to the surface. The genesis of selective enhancements of the Raman bands in the SERS spectra of the molecule has been unveiled from the view of Albretchts A contribution and Herzberg-Teller's charge transfer contribution. © 2011 American Chemical Society. Source


Pradhan M.,Indian Institute of Technology Kharagpur | Chowdhury J.,Sammilani Mahavidyalaya | Sarkar S.,Indian Institute of Technology Kharagpur | Sinha A.K.,Indian Institute of Technology Kharagpur | Pal T.,Indian Institute of Technology Kharagpur
Journal of Physical Chemistry C | Year: 2012

Highly branched Au flowers (AuFs) with sharp tips have been synthesized in high yield by controlling the kinetics of nanocrystal growth via galvanic replacement reaction. Controlled galvanic replacement takes place on appropriately chosen commercially available polystyrene bead supported Cu nanoparticles with HAuCl4. Such a reaction is thermodynamically driven through the exploitation of a Cu(II)/Cu(0) redox couple during nanocrystal formation. The hierarchical morphology of the resulting naked Au flowers (AuFs) depends on the galvanic exchange rate, which increases with increasing HAuCl4 concentration but over a specific concentration range. The formation of AuF is carefully studied, and a spontaneous assembly mechanism is proposed. The time-course experimental results show that the influence of electrostatic field force (EFF) of the charged resin beads is held responsible for prickly tipped AuF formation. Then the fabrication of AuF morphology becomes a large-scale free-standing synthetic protocol for a chemically stable substrate for surface-enhanced Raman scattering (SERS) studies down to the single molecular level. The SERS results guarantee that the as-prepared naked AuF is an excellent and stable SERS substrate. The prickly tips of the gold bearing enhanced field with a large number of embedded "hot spots" hidden within the oriented petals and absence of surfactant or capping agent invites a probe molecule to show the enhancement even for a concentration of 4-mercaptopyridine (4-MPy) down to 10-12 mol dm-3. The electric field distribution around the hot spot has been estimated from 3D-FDTD simulation studies. The selective enhancements of SER bands of the 4-MPy molecule have been unveiled from the view of Herzberg-Teller (HT) charge transfer (CT) mechanism. © 2012 American Chemical Society. Source


Chandra S.,P.A. College | Chowdhury J.,Sammilani Mahavidyalaya | Ghosh M.,Indian Association for The Cultivation of Science | Talapatra G.B.,Indian Association for The Cultivation of Science
Journal of Colloid and Interface Science | Year: 2013

The pH dependent surface-enhanced Raman scattering (SERS) spectra of biologically and industrially significant, 2-mercaptoimidazole (2-MI) molecule at 1.0×10-10M concentration have been investigated. The pH dependent SERS spectra are compared with the corresponding normal Raman spectra (NRS) of the molecule. The vibrational analyses of the pH dependent NRS spectra of the molecule reveal that in the acidic pH medium, the cationic form of the molecule is preponderant, while in the neutral pH, the existence of the cationic, normal, ylidic, and anionic forms of the molecule are all prevalent. However, in the alkaline pH medium, the anionic form of the molecule is estimated to be dominant. The SERS spectral analyses suggest the predominant adsorption of the cationic and the normal forms of the molecule on the nanocolloidal silver surface at acidic pH of the medium. However, at neutral pH, the cationic and/or normal, ylidic forms of the molecule take active part in the adsorption process, while considerable interactions of the normal, ylidic and/or anionic forms of the molecule with the nanocolloidal silver surface are presaged at alkaline pH of the medium. The genesis of selective enhancements of the Raman bands in the SERS spectra of the molecule recorded at various pH values of the medium has been unveiled from the view of the Albretcht's "A" and Herzberg-Teller (HT) charge transfer (CT) contribution. © 2013 Elsevier Inc. Source


Chowdhury J.,Sammilani Mahavidyalaya | Chandra S.,P.A. College | Ghosh M.,Indian Association for The Cultivation of Science
Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy | Year: 2015

The surface enhanced Raman scattering (SERS) spectra of biologically and industrially significant 5-methylthio-1, 3, 4-thiadiazole 2-thiol molecule have been investigated. The SERS spectra at various concentrations of the adsorbate are compared with the Fourier transform Infrared (FTIR) and normal Raman spectra (NRS) of the probe molecule recorded in different environmental conditions. The optimized molecular structures of the most probable thione and the thiol forms of the molecule have been estimated from the density functional theory (DFT) calculations. The vibrational signatures of the molecule have been assigned from the potential energy distributions (PEDs). The detail vibrational analyses reveal that ∼54% of the thione form of the molecule is prevalent in the solid state and its population increases to ∼65% in ACN solvent medium. Concentration dependent SERS, together with the 2-dimensional correlation spectra (2D-COS), corroborate the presence of both the thione and the thiol forms of the molecule even in the surface adsorbed state. The orientations of the thione and the thiol forms of the molecule on the nanocolloidal silver surface have been predicted from the surface selection rule. The selective enhancement of Raman bands in the SERS spectra have been explored from the view of the Albrecht's "A" and Herzberg-Teller (HT) charge transfer (CT) contribution. © 2014 Elsevier B.V. All rights reserved. Source

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