Nitrate ion effect on gold nanorods plasmon resonance behavior: A potential chemical sensor

Abstract

High sensitivity of surface plasmon resonances (SPR) in noble metals to the surrounding refractive index variations in the environment is a powerful bio-sensing technique. Here, we report the development of a simple colorimetric detection method for nitrate ion in water using gold nanorods (Au-NRs). Au-NRs suspension with the average aspect ratio of 3.5 was prepared by a modified seed growth method. According to the safety limit standard of nitrate amount in drinking water, different volumes of 10 mg/l nitrate solution were added to the Au-NRs suspension. Adding the nitrate group to the suspension resulted in turning to the nanorods with lower aspect ratios and finally spherical nanoparticles. The concentration of nitrate in water can be determined by monitoring with the naked eye and a UV–Vis spectrometer. It was shown that lower volumes of nitrate additive (25 μl) was a better identifier than those with higher volumes (50–150 μl). The color of Au-NRs suspension turned somehow from faint pink to purple after two days of adding 25 μl of nitrate solution. In addition, the UV–Vis absorption peak corresponding to longitudinal SPR (LSPR) blue-shifted from 814 nm to 712 nm after two days while the peak corresponding to transverse SPR (TSPR) showed a slight shift from 519 nm to 521 nm. Much more effect of nitrate group on LSPR than TSPR showed that nanorods are better identifier and biosensors than spherical nanoparticles. Decomposing and converting the Au NRs to sphere particles and lowering the initial aspect ratio of NDs in presence of nitrate ion is reported for the first time, but more works needed for talking about its mechanism.