1. pH effect on colloidal silver nano particle bound 1H-2(phenylazo) imidazole (PaiH) molecules. Dilip K Debnath1 Washim Hossain2, Chittaranjan Sinha3 and Uttam K Sarkar2,
1Department of Chemistry, 2 Department of Physics, Malda College, Malda & 3 Department of Chemistry, Jadavpur University, Jadavpur
Introduction:
Absorption spectroscopic technique bears an immense importance in understanding of molecular behaviour, particularly, in understanding of relative position of Molecular Orbitals (MO), molecular conformation and interactions.
1H-2(phenylazo) imidazole (PaiH) molecule comprises four nitrogen donor centers two at azo-group and two at immidazole ring. PaiH bind to colloidal silver nano particles in the wide concentration range of 7.5x10-5 to 5x10-6 (M) through the nitrogen donor center(s) and exhibit huge spectral chnges as well as Surface Enhanced Raman Spectra (SERS). Here we have showed that shifting of medium pH from acidic to basic, PaiH showed huge red shifting and that is pronounced in presnece of soilver sol.
Result and Discussion:
Colloidal silver nano particles bound PaiH shows characteristic π-π* absorption band with a absorption maxima at 340nm. PaiH at the lower concentration shows an additional absorption band at 460nm upon interaction with silver nano particles (Fig. 1). The n-π* specific second UV-Visible absorption band of PaiH shows an enormous red shifting in the basic pH. The red shifting occurs in absence of colloidal silver particle but very much pronounced in presence of the silver nano article (fig 2). Binding of PaiH to silver nano particle equires specific orientation at the surface. Probably, at the basic range of pH, PaiH molecule loses imidazole proton and binding mode to the sol particle changes.
The cumilitive effect of proton loss and change of bindng mode alters the relative positions molecular orbital and exhibit the red shift.
Surface enhanced Raman spectroscopy (SERS) studies of silver nano particle bound PaiH shows an enhanced peak intensity at 1017 cm-1 due to first layer effect [1], but this decays down with increasing pH. (fig.3)
Methodology
Colloidal silver nano particles were prepared by reducing aqueous solution of AgNO3 by NaBH4. Spex double monochromator (model 1403), fitted with holographic gratings of 1800 grooves/mm and a cooled photomultiplier tube (model R 928/1150) of Hammatsu Photonics, Japan, was used to record Raman spectra. The samples were excited by the nm line from a Spectra Physics Ar+ ion laser (model ) at a power of 200 mW. The electronic absorption spectra were recorded in Shimadzu spectrometer (model UV-Vis 2010 PC).
Fig. 1 plot of absorption maxima as a function of silver sol concentration
References:
Uttam K. Sarkar, Chemicl Phyical letter , vol 190, Issues 1-2, 28 February, 1992, pp 59-63
Acknowledgements:
Authors sincerely acknowledges UGC for funding and IACS, Kolkata for providing Raman Instrument