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Direct Synthesis and Stabilization of Bi-Sized Cysteine-Derived Gold Nanoparticles: Reduction Catalyst for Methylene Blue

Sirajuddin^a,*, A. Nafady^b, H.I. Afridi^a, S. Sara^a, A. Shah^c and A. Niaz^c

^aNational Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro, 76080, Pakistan
^bDepartment of Chemistry, Suhag University, Egypt
^cUniversity of Science and Technology, Bannu, NWFP, Pakistan

Bi-sized and separable gold nanoparticles (AuNps) with smaller sizes were synthesized by treatment of cysteine with tetrachloroauric acid solution. These nanoparticles were made stable with further reduction in size after adding very small volume of NaOH solution before controlled heating. In NaOH treated solution the AuNps stopped precipitation as compared to untreated solution. Various methods such as UV-Vis spectroscopy, Transmission Electron Microscopy (TEM), X-ray Diffractometry (XRD), FTIR and Raman spectroscopy were used for characterization of surface properties and interaction between AuNps and cysteine. The results showed good correlation with literature regarding the interaction but interesting differences in size distribution as evident from high resolution TEM micrograph. Zeta potential studies revealed that these particles are negatively charged. Surprisingly, the Raman of the cysteine-derived AuNps without NaOH showed tremendous enhancement as compared to pure cysteine and cysteine-derived AuNps treated with NaOH. The NaOH treated Cyst-AuNps sol proved as excellent homogeneous catalyst for reduction of methylene blue (MB).

Keywords: Bi-sized gold nanoparticles, NaOH, Stabilization, SERS activity, Reduction of MB