Visual and Label-Free Detection of Cadmium Ions Based on Oscillatory Reaction

Abstract

A method for ultrasensitive visual determination of Cd2+ was developed based on the oscillatory reaction of poly(toluidine blue O)-gold (PTBO–Au). PTBO–Au was fabricated using HAuCl4 as the oxidizing reagent to polymerize toluidine blue O (TBO), a dye utilized as an indicator for colorimetric analysis and detection of Cd2+. Thiosulfate was used to leach Au nanoparticles onto PTBO–Au to form Au+ ions in the presence of (1-hexadecyl)trimethylammonium bromide. After addition of NaBH4, PTBO–Au, Au+, and Cd2+ were reduced to form reduced PTBO–Au (rPTBO–Au) and Au–Cd nanoalloy. The color of the reaction solution changed from blue to red (the color of the Au–Cd nanoalloy). When rPTBO–Au was oxidized by dissolved oxygen, a critical point of color change occurred, and the solution color returned to blue as the reduction rate of PTBO–Au became lower than that of oxidating rPTBO–Au (determined by the concentration of NaBH4). The Au–Cd nanoalloy was found to have minimal catalytic effect on PTBO–Au reduction in the o...