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

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 oscillatory reaction, which is indicative of the slowest consumption of NaBH4 and the longest cycle time. Further, the time of the transition from blue -> red -> blue has positive correlation with the concentration of Cd2+. The color transition can be effectively used as an indicative method to quantitatively detect Cd2+ with the naked eye in the concentration range 1 mu M to 1 nM without expensive instrumentation. This highly sensitive and selective method was applied for the determination of Cd2+ in spiked river water samples and obtained satisfactory recoveries, which suggests its potential other environmental applications.