Herein, we developed a concise, time-efficient, and high selective assay for detecting Fe2+ through its triggered surface plasmon-catalyzed reduction reaction of p-nitrothiophenol (PNTP) to p,p′-dimercaptoazobenzene (DMAB) based on surface-enhanced Raman scattering (SERS) spectroscopy. When Fe2+ was added to the PNTP-AuNPs system, the appearance of three characteristic peaks at 1142, 1392, and 1140 cm− 1 attributed to DMAB demonstrated that Fe2+ induced the catalytic coupling reaction of PNTP. The Raman intensity ratio of the peak at 1142 cm− 1 to the peak at 1336 cm− 1 and the concentration of Fe2+ presented a good linear response from 10 to 100 µM with a limit of detection (LOD) of 0.35 µM. Besides, the probable reaction process has been put forward. More importantly, the entire detection process can be completed within 2 min and was used successfully for the detection of Fe2+ in river water.
Research Article
Fe2+-induced surface plasmon-catalyzed reduction reaction for the detection of Fe2+
https://doi.org/10.21203/rs.3.rs-1615696/v1
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surface-enhanced Raman scattering spectroscopy
plasmon-catalyzed reduction reaction
Fe2+
Iron is an indispensable trace element in the human body and participates in many physiological and pathological processes, such as oxygen transport[1], cell metabolism[2], enzyme exchange reactions[3], and electron transfer[4]. Excessive iron ions can cause cancer, Parkinson's disease and other diseases[5]; too few iron ions can produce various symptoms such as anaemia and syncope[6]. 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[7]. 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[17], and inductively coupled plasma mass spectrometry[18]. 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[21], photocatalysis[22], medical diagnosis[23], and particularly in the field of trace analysis of various pollutants such as heavy metal ions[24], polycyclic aromatic hydrocarbons[25], pesticides[26], and bacteria[27]. 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[28]. Xu et al. prepared a two-dimensional Au@Ag nanorods array by the self-assembly method for the sensitive detection of thiram in apples[29]. Li’s group used cetyltrimethylammonium bromide-induced aggregation of silver nanoparticles (AgNPs) to successfully detect hydroxylated polycyclic aromatic hydrocarbons in urine[30]. 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−[33]. Further, Kneipp et al. found that Ag+, Au3+, Pt4+, and Hg2+ induced the conversion of PATP to DMAB[34]. 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[35]. 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+.
2.1 Materials
P-nitrothiophenol (PNTP) was purchased from aladdin, Chloroauric Acid (HAuCl4), sodium citrate, CaCl2, Co(NO3)2·6H2O, MgSO4, AgNO3, ZnCl2, KCl, H2PtCl6·6H2O, Pb(NO3)2, MnCl2, BaCl2·2H2O, FeCl3, Cr(NO3)3·9H2O, Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, NH4Cl, FeSO4·7H2O, H2SO4 were purchased from Sinopharm Chemical Reagent Co., LTD.
2.2 Apparatus
Scanning electron microscopic (SEM) characterization was used an FE-SEM Su-8010 system (Hitachi, Tokyo, Japan). Transmission electron microscopy (TEM) image was obtained by JEM-2100 ultra-high-resolution transmission electron microscope (JEOL, Japan). Ultraviolet–visible (UV–vis) spectroscopy was obtained a PerkinElmer Lambda 35 spectrophotometer (Norwalk, CT, USA). Inductive coupled plasma emission spectrometer (ICP-OES) test was performed on Optina 800 (PerkinElmer, USA). SERS spectra were recorded by a confocal Raman microspectrometer (Renishaw, UK) equipped with a × 50 objective lens, under the excitation of a 633 nm laser.
2.3 Fabrication of AuNPs
The AuNPs were synthesized according to previous literature[36]. 1.5 mL sodium citrate (1% by weight) solution were injected quickly into the 250 mL round-bottom flask contained 100 mL boiling HAuCl4 solution (0.01% by weight) and stirred magnetically for 20 min. Cooled the solution to room temperature and stored at 4 oC for the further use.
2.4 AuNPs modified with PNTP
Mixing 5 mL AuNPs and 5 mL PNTP (in ethanol) solution in a round flask and stirring at room temperature for 2 h. The prepared solution was stored at 4 oC for the next step.
2.5 SERS Measurement
The SERS spectra were taken from a Renishaw inVia Reflex confocal Raman system which the objective lens is 50× (NA = 0.75) and the diameter of the laser spot is 1 µm. The laser wavelength is 633 nm, the exposure time is 10 s and laser power is 17 mW. 100 µL AuNPs modified with PNTP were mixed with 100 µL Fe2+ or other cation solutions (100 µM) for 1 min, respectively. Afterward, The mixture was drawn into a capillary and tested.
2.6 Preparation of water samples
The river water was from the Hun river, Shenyang, Liaoning, China. The river water samples were filtered using 0.45 µm membrane and then were spiked with Fe2+ (20, 60 and 100 µM).
3.1 The characterization of AuNPs
We characterized the synthesized AuNPs by SEM, TEM and UV–vis absorption spectroscopy firstly. As can be seen in SEM and TEM photographs (Fig. 1A and Fig. 1B), AuNPs are spherical, uniform in size with a diameter of about 30 nm, and uniformly dispersed. The maximal UV–vis absorbance of AuNPs is at 526 nm (Fig. 1C), which is assigned to the surface plasma resonance of Au. The above characterization proved the successful synthesis of AuNPs.
3.2 The plasmon-catalyzed reduction reaction of PNTP to DMAB
Next, we measured the SERS spectrum of PNTP (Fig. 2a), and there was a typical peak at 1336 cm− 1 that is attributed to the vibration of the nitro group. When Fe2+ ions were added to the system, three new peaks appeared at 1142, 1392, and 1140 cm− 1 (Fig. 2c), suggesting that surface plasmon-catalyzed reduction reaction of PNTP to DMAB was triggered by Fe2+. Since H2SO4 solution was used in the process of preparation of the Fe2+ standard solution, the SERS spectrum of PNTP was tested in the presence of H2SO4 to exclude the influence of H2SO4. The result was present in Fig. 2b. When only H2SO4 solution was introduced into the PNTP-AuNPs system, it would not induce the conversion of PNTP to DMAB and even inhibited the combination of PNTP and AuNPs to a certain extent which is because of the significant decrease of the Raman intensity at 1336 cm− 1. The obtained results indicated that H2SO4 does not effect the PNTP-AuNPs-Fe2+ system.
According to previous reports[37], the PNTP-AgNPs system can be reduced to DMAB under the condition of irradiation. Therefore, we further explored whether the laser irradiation time can promote the conversion of PNTP to DMAB in the PNTP-AuNPs system. As shown in Fig. 3A, the characteristic peaks of PNTP hardly changed with the extension of illumination time from 10 to 60 s, suggesting that it is hard to convert PNTP to DMAB by laser illumination alone in this condition. Upon the addition of Fe2+, the reduction process was completed quickly and accompanied by the appearance of new peaks. Figure 3B represented the Raman intensity ratios of the peaks at 1142 and 1392 cm− 1 to the peak at 1336 cm− 1, defined as I1142/I1336 and I1392/I1336 (I represents the Raman intensity), which enhanced dramatically in the presence of Fe2+. The above mentioned fully illustrated the important role of Fe2+ in the conversion from PNTP to DMAB, which could be considered as the theoretical basis for qualitative and semi-quantitative detection of Fe2+.
3.3 Optimization of reaction condition
To obtain the best probe performance, we optimized necessary experimental conditions such as the concentration of PNTP and laser power. First, the final reaction concentration of PNTP was determined by fixing the Fe2+ concentration (100 µM) and changing the PNTP concentration. The experimental results are shown in Fig. S1. When the concentration of PNTP was 10− 4 or 10− 6 M, there were no characteristic peaks of DMAB generated. When the concentration was 10− 3 M, the SERS intensity at 1141 cm− 1 was weak. However, when the PNTP concentration was selected 10− 5 M, there were obvious characteristic peaks at 1142, 1392 and 1440 cm− 1, which illustrated that PNTPs are more likely to undergo conversion to DMAB on AuNPs at this concentration (Fig. S1A). Therefore, the concentration of PNTP was determined to be 10− 5 M in subsequent experiments. The laser power is also an important factor affecting the experimental results. The laser power is too weak to realize the Fe2+-induced surface plasmon-catalyzed reduction reaction. Too strong power will cause the SERS intensity to exceed the range. It can be seen from Fig. S1B that the effect of converting PNTP to DMAB was the best with the laser power of 17 mW (100%), which was selected for the optimal laser power. higher than other cations, indicating the PNTP-AuNPs system has excellent selectivity towards Fe2+.
3.4 Quantitatively SERS detection of Fe2+
To explore the potential of SERS for quantitative detection of Fe2+, we measured the SERS spectra of the PNTP-AuNPs system with the different concentration of Fe2+ (from 0 to 100 µM) and plotted the linear standard curves between the intensity ratio of I1142/I1336, I1392/I1336 and Fe2+ concentration under the optimal conditions, which are presented in Fig. 4 and Fig. S2. With the increase of the concentration of Fe2+, the SERS intensity at 1142 and 1392 cm− 1 gradually increased (Fig. 4A). The SERS intensity ratio of I1142/I1336 displayed a good linear relationship with Fe2+ concentration varying from 10 to 100 µM (R2 = 0.99248) (Fig. 4B), while a worse value (R2 = 0.97367) was obtained through the relationship between the SERS intensity ratio of I1392/I1336 and the Fe2+ concentration (Fig. S2). Therefore, we chose the intensity ratio of I1142/I1336 as the basis for quantitative detection of Fe2+. In addition, the equation of the regression line was y = 0.14041 + 0.00586x (y represents the Raman intensity ratio of I1142/I1336 and x represents the concentration of Fe2+). The limit of detection (LOD) was calculated to be 0.35 µM according to 3σ/k, where σ is the standard deviation of the background and k is the slope of the calibration curve.
3.5 Selectivity of the PNTP-AuNPs system
To further evaluate the specific selectivity of the system to Fe2+, we examined the various environmentally relevant cations. Figure 5A exhibited the SERS responses of the PNTP-AuNPs system to Ca2+, Au3+, Co2+, Mg2+, Ag+, Zn2+, K+, Pt4+, Pb2+, Mn2+, Ba2+, Fe3+, Cr3+, Ni2+, Cu2+, NH4+ and Fe2+ at the identical concentration. It was obvious that other cations cannot induce the formation of DMAB, only Fe2+ ions were able to trigger the surface plasmon-catalyzed reduction reaction of PNTP. Figure 5B intuitively revealed that the SERS intensity ratio of I1142/I1336, I1392/I1336 for Fe2+ is significantly higher than other cations, indicating the PNTP-AuNPs system has excellent selectivity towards Fe2+.
3.6 Practical application
| Samples | Spiked amount (µM) | SERS amount (µM) | SERS Recovery (%) | RSD (%) (n = 6) | Spiked Amount (µM) | ICP-OES Amount (µM) | ICP-OES Recovery(%) |
|---|---|---|---|---|---|---|---|
| River water | 0 | Not detected | -- | -- | 0 | Not detected | -- |
| 20 | 16.13 | 80.65 | 4.27 | 20 | 20.30 | 101.50 | |
| 60 | 52.81 | 88.02 | 4.05 | 60 | 54.69 | 91.15 | |
| 100 | 102.72 | 102.72 | 3.77 | 100 | 91.66 | 91.66 |
To evaluate whether the proposed SERS platform could be used for actual sample detection, we selected river water for the spiked test. The SERS spectra of river water with different Fe2+ concentration was shown in Fig S3. It can be observed that the three new peaks of DMAB were generated when river water samples were spiked with Fe2+ (20, 60, and 100 µM), and the new peaks were absent without Fe2+, which indicated that there was almost no Fe2+ in the river water samples. In addition, the comparison of the SERS method and ICP-OES is present in Table 1. Good recoveries ranged from 80.65–102.72% and a satisfactory relative standard deviation (RSD) was obtained, and the results were similar to ICP-OES method, which illustrated the proposed SERS sensor could be effectively applied to the detection of Fe2+ in river water. To further highlight the merit of this method, several strategies for detecting Fe2+ are presented in Table S2. It is obvious that the proposed SERS sensor had certain advantages in detection time, detection limit and linear range.
3.7 The possible reaction process
When the laser is irradiated on the AuNPs, pairs of "holes" and "electrons" are generated on the surface of the AuNPs[38]. With the addition of a reducing agent or oxidant, the corresponding "hole" or "electron" will be consumed, leading to the formation of isolated "electrons" or "holes", which causes the system to display reducibility or oxidizability. According to previous reports, we inferred the possible reaction process (Fig. 6). Specifically, the Fe2+ ions reacted with the oxidative "holes" generated on the surface of AuNPs and were oxidized to Fe3+, so that a large number of isolated "electrons" existed which, in turn, reduced the two PNTP molecules to DMAB. Thus, the transformation of PNTP to DMAB can be expressed by the equation:
In conclusion, we reported a new strategy for the detection of Fe2+ by inducing surface plasmon reduction reaction of PNTP to DMAB based on SERS technology. The proposed method had a wide linear range from 10 to 100 µM with a LOD of 0.35 µM. The SERS platform had high selectivity to Fe2+. In addition, the probable reaction mechanism was put forward. More importantly, the designed method had been successfully applied to detect Fe2+ in river water, which may achieve on-site detection of Fe2+ in the future.
Funding
This work was supported by the National Natural Science Foundation of China (Grant No. 21671089), the Scientific Research Projects of Liaoning Provincial Department of Education (L2020002), the Liaoning Provincial Natural Science Foundation Joint Fund Project (2020-YKLH-22).
Competing Interests
The authors have no relevant financial or non-financial interests to disclose.
Author Contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Ye Sun, Yue Wang, Peng Song, Yao Zhang and Lixin Xia. The first draft of the manuscript was written by Guangda Xu, Yao Zhang and Lixin Xia and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Data Availability
All data generated or analysed during this study are included in this published article.
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No competing interests reported.