An ingenious cellulose membrane sensor design strategy for colorimetric detection of Ag+/Hg2+ based on redox reaction.

This paper describes an ingenious cellulose membrane sensor design strategy for colorimetric detection of Ag+/Hg2+ based on redox reaction. The colorless 3,3',5,5'-tetramethylbenzidine (TMB) can be oxidized to blue oxidized TMB (oxTMB) when exposed to Ag+/Hg2+ that with strong oxidizing properties. Based on this phenomenon, TMB can be design as a colorimetric probe for Ag+/Hg2+, and the reaction mechanism and sensing performance of TMB as Ag+/Hg2+ were explored. In addition, the TMB probe-immobilized cellulose membranes (TMB@CMs) were developed by combining TMB with high-purity cellulose membranes (CMs) carrier with porous and polyhydroxy structures. As a platform for probe immobilization, TMB@CMs can effectively improve colorimetric sensing response and stability of TMB. The colorimetric mechanism of TMB@CMs was investigated including in situ oxidation of TMB and immediate immobilization of oxTMB. The experimental results showed that the visual detection limit (VLOD) of Ag+/Hg2+ was 10 μM when TMB was used as colorimetric probe, while the VLOD of the TMB@CMs was 1 μM. In addition, TMB@CMs had good reusability and stability. Through the analysis of SEM, EDS and XPS results, the mechanism of TMB colorimetric detection of Ag+/Hg2+ was that blue oxTMB and Ag/Hg elementals were generated by redox reaction between them. This study not only verified the feasibility of TMB as an Ag+/Hg2+ colorimetric probe, but also designed a probe-immobilized cellulose membrane model with convenient operation, uniform color development and stable color, which effectively improved the colorimetric sensing response and stability.