Department of Chemistry
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Item Exploration of Electrostatic Field Force in Surface-Enhanced Raman Scattering: An Experimental Investigation Aided by Density Functional Calculations(ACS, 2008-10-27) Basu, MrinmoyeeSurface chemical properties of metal nanoparticles must be tunable to create chemical specificity and are a key prerequisite for successful sensing and imaging platforms. To relate surface enhanced Raman scattering (SERS) to electrostatic field force, a simple colloidal chemistry approach has been deliberately exploited for syntheses of gold nanoparticles with negative and positive surface charges to study their interactions with charged analytes. We took up the challenge with sulfur-containing analytes because “Au−S” interaction is well-known. Thiocyanate ion, −SCN−, a well-known SERS analyte, has been proved to be chemically ligated/anchored on positively charged gold nanoparticles surface owing to favorable electrostatic attraction. The Au−S vibrational band at ∼240 cm−1 and blue-shifting of the −C≡N stretching frequency by ∼46 cm−1 in conjunction with its intensity enhancement by an order of ∼103 in the SERS spectrum clearly illustrate a chemisorption phenomenon. In contrast, physisorption of the −SCN− ion becomes evident on negatively charged colloid. Again, methylene blue has been shown to remain engrossed on the negatively charged gold surfaces. However, the electrostatic field force could not be accounted for from fluorescence quenching while methylaminopyrene was introduced because of the distance-dependence effect. The feasibility of such coordinative/chemical attachment also has been examined theoretically by density functional theory (DFT). Moreover, employment of this DFT calculation has been performed on five different metal−molecule interaction models to fruitfully interpret the experimental SERS findings and also the orientation of the SERS analyte. The observed Raman signals have been assigned from the potential energy distributions in terms of internal coordinates of adsorbate from the output of DFT calculations. The results thus provide a benchmark illustration of the value of DFT for aiding interpretation of adsorbate vibrational spectra attainable by using SERS.Item Room Temperature Ferromagnetic Ni Nanocrystals: An Efficient Transition Metal Platform for Manifestation of Surface-Enhanced Raman Scattering(ACS, 2009) Basu, MrinmoyeeA simple solid-phase synthetic approach has been deliberately exploited for the synthesis of room temperature ferromagnetic, phase pure, fcc Ni nanocrystals on resin matrix. Self-assembly directed chainlike hierarchical nanostructures on the matrix could be engendered from magnetic dipole−dipole interaction between the nanocrystallites. Then, a practical virtue of the transition metal nanoparticle, Ni, was expressed from the rich and high-quality vibrational information of a chelating ligand, 1,10-phenanthroline (phen), onto the magnetically separated metal particles. Thus, surface-enhanced Raman scattering (SERS) has emerged exclusively from the time-dependent surface complexation of the chemically adhered probe molecule. Finally, kinetic effect has bestowed Ni(II)-phen chelate which later on demonstrates unique SERS activity on fcc Ni nanocrystals. The results provide a benchmark illustration of the value of transition metal for aiding interpretation of the vibrational signature of the adsorbate attainable from SERS studies.Item Dopamine Molecules on Aucore−Agshell Bimetallic Nanocolloids: Fourier Transform Infrared, Raman, and Surface-Enhanced Raman Spectroscopy Study Aided by Density Functional Theory(ACS, 2009-04-08) Basu, MrinmoyeeAdsorption of dopamine (DA) on a Aucore−Agshell bimetallic nanocolloidal surface has been investigated using surface-enhanced Raman spectroscopy (SERS). The normal Raman spectra (NRS) of DA molecules in bulk and in aqueous solution have been investigated in depth. The vibrational signatures, as observed from the Raman and FTIR spectra of the molecule, have been assigned from the potential energy distributions. The pH-dependent NRS of the DA molecule in aqueous solution has been recorded to elucidate the protonation effect and preferential existence of different forms of the molecule. The pH-dependent SERS spectra of the molecule adsorbed on the bimetallic Aucore−Agshell nanocolloidal surface are also reported. The enhancement of bands in the pH-dependent SERS spectra suggests that the molecules are adsorbed onto the bimetallic Aucore−Agshell surface with the molecular plane tilted with respect to the silver surface of Aucore−Agshell bimetallic nanoparticles. The model study authenticates the spectral disposition and orientation of the molecule. Thus, experiment and theory keep abreast of the variety of DA structures envisaged from SERS studies on a new substrate.