Iron is an indispensable trace element in the human body and participates in many physiological and pathological processes, such as oxygen transport, cell metabolism, enzyme exchange reactions, and electron transfer. Excessive iron ions can cause cancer, Parkinson's disease and other diseases; too few iron ions can produce various symptoms such as anaemia and syncope. Thus, it is important to maintain an iron concentration within the normal range. The maximum allowable concentration in drinking water is 0.3 mg/L (about 5.4 µM) according to the World Health Organization recommendation. As such, the detection and monitoring of iron is a topic of great significance. Traditional methods for detecting Fe3+/Fe2+ mainly include colourimetry[8, 9], fluorescence analysis[10–12], the electrochemical method[13–16], atomic absorption spectroscopy, and inductively coupled plasma mass spectrometry. Although great progress has been made, these approaches still have some limitations, such as low sensitivity, complex preprocessing, and long detection time. Therefore, the development of a fast, concise, and inexpensive method is of great significance to human health and environmental monitoring.
In recent years, surface-enhanced Raman scattering (SERS) spectroscopy has been favoured by researchers due to its high sensitivity, non-destructive detection, fingerprint recognition ability, and short detection time[19, 20]. It has been widely applied in reaction mechanism monitoring, photocatalysis, medical diagnosis, and particularly in the field of trace analysis of various pollutants such as heavy metal ions, polycyclic aromatic hydrocarbons, pesticides, and bacteria. For example, we proposed a label-free, fast, and highly sensitive assay for detecting Fe2+ using the surface-enhanced resonance Raman scattering (SERRS) technique with 2,2′-bipyridine as the probe. Xu et al. prepared a two-dimensional Au@Ag nanorods array by the self-assembly method for the sensitive detection of thiram in apples. Li’s group used cetyltrimethylammonium bromide-induced aggregation of silver nanoparticles (AgNPs) to successfully detect hydroxylated polycyclic aromatic hydrocarbons in urine. As typical examples of photo-induced plasmon-catalyzed reactions, the conversion of p-aminothiophenol (PATP) and p-nitrothiophenol (PNTP) to p,p′-dimercaptoazobenzene (DMAB) has attracted increasing attention, and a large number of related studies have been published[31, 32]. PATP and PNTP can be converted into DMAB on AgNPs under laser induction, however, it is necessary to add extra oxidizing or reducing agents to achieve the similar conversion on gold nanoparticles (AuNPs). For instance, Liu et al. reported the catalytic coupling reaction of PATP to DMAB triggered by NO2−. Further, Kneipp et al. found that Ag+, Au3+, Pt4+, and Hg2+ induced the conversion of PATP to DMAB. However, there are few reports of the surface plasma reduction reaction from PNTP to DMAB.
In this paper, we first proposed an Fe2+-induced surface plasmon-catalyzed reduction reaction from PNTP to DMAB for the determination of Fe2+. Upon the addition of Fe2+, the PNTP-AuNPs system generated three new peaks at 1142, 1392, and 1140 cm− 1, indicating the formation of DMAB. The designed SERS platform had a wide linear range from 10 to 100 µM with an LOD of 0.35 µM and excellent selectivity. More importantly, a possible reaction mechanism is described in this paper and the method was successfully applied to the detection of Fe2+ in river water, validating this new approach for the detection of Fe2+.