Design of Physicochemical Factors for Regulating the Retention Mechanism of 4-Aminothiophenol in Surface-Enhanced Raman Scattering toward Nitrite Sensing

The extensive use of 4-aminothiophenol (4-ATP) as a Raman-active molecule is attributed to ease in its functionalization and use for potential sensing in surface-enhanced Raman scattering (SERS) research. However, its SERS spectrum is susceptible to be influenced and chemically affected by several experimental factors. Thus, the current study focuses on theoretically and experimentally investigating such physicochemical factors and its counteractive stimulus to discriminate the SERS signal arising from the analyte of interest and interferents. The sensing mechanism of nitrite-triggered 4-ATP that might induce its oxidation and/or dimerization and the conditions inhibiting or generating abnormal type b₂ bands (1142, 1388, and 1432 cm–¹) were discussed. The factors covered were as follows: (i) solvent effect; (ii) laser power; (iii) nature of the substrate; (iv) reaction media; (v) reaction kinetics; and (vi) influence of air or any oxygen (dissolved)-containing moiety. The activation of 4-ATP by any factor forms 4,4-dimercaptoazobenzene with characteristic symmetric b₂ bands in nearly all the literature, whereas, herein, the species formed between nitrite and 4-ATP was differentiated and confirmed by basic electronic assignments along with their respective binding mechanism toward 1-naphthylamine. The retention mechanism of 4-ATP was assigned and controlled at pH < 2.0, where the protonation of amine and unavailability of necessary precursors in the presence of excess H⁺ greatly resists its oxidation, making it independent of many potential factors. Moreover, a highly stable platform for nitrite sensing has been established with exceptional selectivity (in the presence of potent interferences) and sensitivity (0.3 × 10–⁸ M).