Size and concentration effects of gold nanoparticles on the plasmonic sensing of Cr(VI)

Abstract

It is very important to determine the presence of toxic Cr(VI) in aqueous environment. Detection of Cr(VI) using AuNPs has been carried out based on the oxidation or etching principle, which results in the change of localized surface plasmon resonance. In this work, the effects of size and concentration of colloidal AuNPs on the Cr(VI) sensing characteristics were studied by both simulation and experiment. In the simulation study using MNPBEM toolbox, the etching process was described as shrinking of AuNPs with total volume loss reflecting the concentration of Cr(VI) as oxidizing ions. The experiment was carried out by exposing colloidal AuNPs with different size and concentration to Cr(VI). The AuNPs were obtained from laser-induced photochemical process, in which aqueous solution of Au ions were subjected to femtosecond laser irradiation. The simulation results showed wider blue-shift of λLSPR per total volume loss when larger AuNPs were employed and higher intercept value of extinction decrease per total volume loss when more AuNPs were present in the system. Correspondingly, the experiment demonstrated more distinct color change and wider λLSPR per Cr(VI) concentration when larger AuNPs were used. More concentrated colloid showed still-red color after Cr(VI) etching, representing higher intercept value of extinction decrease per Cr(VI). Both simulation and experiment results show that, in etching-based Cr(VI) sensing, large AuNPs displayed wider LSPR wavelength shift compared to the small nanoparticles. On the other hand, concentration of AuNPs contributes to the intensity change and higher concentration of AuNPs offers higher Cr(VI) sensing range. This result provided an insight on how colorimetric sensor performance might be affected by the choice of nanoparticle size and concentration used in the system.