Colorimetric approach towards detection of heavy metal ions by gold nanorods

Abstract

Gold nanorods (GNRs) have emerged as novel chemosensors and biosensors owing to their excellent optical properties in the visible region. GNRs exhibit two plasmonic bands namely, the longitudinal and transverse surface plasmon resonance bands in the visible region whereby the former can be modulated upon addition of analytes which trigger a change in the aspect ratio of the nanorods. Consequently, the gold nanorods are used for the non-aggregation based colorimetric detection of heavy metal ions such as Cr (Ⅵ), Pb (II), Hg (II), etc. In order to investigate the effect of gold nanorods size during interaction with heavy metal ions, two different sizes of gold nanorods viz. (GNR (B) and GNR (E)) were synthesized by seed mediated growth mechanism and the changes in the longitudinal absorbance were monitored. CTAB capped seed is used instead of citrate capped seed to get rod shaped particles in the large fraction. For the desired length of gold nanorods, different silver ion content in the growth solution was used. Upon increasing the concentration of silver ions, the aspect ratio (length/width) of gold nanorods increased due to the under potential deposition (UPD). Additionally, the shift in the surface plasmon resonance (SPR) band was observed due to the changes in the concentration of cetyltrimethylammonium bromide (CTAB), aqueous solutions of heavy metal ions, and variable concentration of silver ions. Our findings revealed that the length of gold nanorods shorten significantly due to the redox etching of gold nanorods by Cr (Ⅵ) compared to that of Hg (II) and the effect was more prominent for longer GNR(E) compared to the shorter GNR(B). A linear relationship is shown by the blue shift of longitudinal surface plasmon resonance absorption (LSPA) with concentration of Cr (Ⅵ).