The compound 3,3',5,5'-tetramethylbenzidine (TMB) is a low-toxic, non-carcinogenic, chromogenic substrate which can be oxidized resulting in a color change. TMB has been used in clinical testing, forensic testing, criminal detection, and environmental monitoring. In particular, in clinical biochemical tests, TMB is widely used as a substrate for peroxidase in enzyme immunoassay (EIA) and enzyme-linked immunosorbent assay (ELISA). Recently, research on TMB has mainly focused on its use as an indicator for highly sensitive and selective colorimetric sensors that detect Ag+[1] and Fe3+ [2]. Studies have shown that TMB can react with Ag+ and Fe3+ to form a blue complex, and in actual testing, these two ions will interfere with each other, that is, TMB has no specific selectivity for these two ions. Therefore, in colorimetric detection, TMB is not suitable as a probe to detect a mixture of Ag+ and Fe3+.

In order to solve the above problems, Fu et al. prepared a novel electrochemical sensor for the specific detection of Ag+ [3]. The sensor is based on a sulfur-doped graphene (S-Gr) and TMB composite (S-Gr-TMB) modified glassy carbon electrode (GCE). The S-Gr-TMB composite was prepared via the electrostatic interaction between TMB and S-Gr and then coated on the glassy carbon electrode surface. The experimental results show that the S-Gr-TMB/GCE electrode has a significant voltammetric response to Ag+ at 0.3V vs. Ag/AgCl. The sensor detects Ag+ with a minimum detection limit of 2.15 mM and shows good linearity from 50 mM to 400 mM. In addition, Fe3+ and other metal ions are added to the detection system, and the current signal at the same concentration remains unchanged. This shows that the electrochemical sensor has a high selectivity for Ag+, which solves the problem of non-specific selectivity of TMB as a spectral probe.

The sensor based on S-Gr-TMB prepared by Fu et al. solves the problem of specific selectivity of Ag+, and will be widely used in real environmental and living sample systems in the future.

References:

[1] S. Lu, J. Q. Tian, L. Wang, X. P. Sun, Sens. Actuators, B, 2012, 165, 44–47.

[2] L. F. Zhang, J. X. Du, Spectrochim. Acta, Part A, 2016,158, 24–28.

[3] Y. Fu, Y. Yang, et al., Analyst, 2018, 143, 2076-2082.

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