A Au-Ga alloy layer is synthesized on a Au sheet substrate at low temperature and a new nanoporous Au material is then prepared based on Ga removal from the Au-Ga alloy by a electrochemical method. X-ray diffraction analysis confirms that the grains of the newly formed Au layer are refined and scanning electron microscopy suggests that the newly formed Au layer has a nanoporous structure with a pore size of∼16 nm. The cyclic voltammetry curve of the prepared nanoporous Au in a 1 M KCl solution presents a square structure of a supercapacitor with a specific capacitance of mF·cm-2, which is 86 times that of the smooth Au electrode. The 4-mercaptobenzoic acid (4-MBA) modified nanoporous Au material is used as a Raman substrate to detect Hg2+ with surface-enhanced Raman scattering. from 1.0×10−10 mol·L-1 to 1.0×10−5 mol·L-1), and the limit of detection is 1.0×10−10 mol·L-1.
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Hg is a liquid metal that has been widely used in thermometers, barometers and batteries. However, Hg is a human neurotoxin, and it can be bioaccumulated in water and soil environments. It may be transmitted through the food chain and cause serious adverse health effects to animals and humans. In order to study the impact of Hg in water and soil on living organisms, trace Hg detection technology remains an essential research topic that requires further development.
Common methods for the detection of Hg ions in water include atomic absorption spectrometry (AAS) [1,2], colorimetry [3,4], fluorescent probe [5–7], and electrochemical sensor methods [8,9] as well as surface enhanced Raman scattering (SERS) [10–12].
Each method has its own characteristics, for example, SERS has the characteristics of high sensitivity, so it has been widely studied in chemical and environmental fields. Hg2+ ions cannot be directly detected by SERS because they lack vibrational signatures . In order to detect Hg2+ with SERS, selective SERS probes have been used [14–16], including 4-mercaptobenzoic acid (4-MBA)-modified active metal nanoparticle sol substrates. In these analyses, the SERS intensities of 4-MBA decreased with increasing Hg2+ concentration. The SERS effect of the sol substrate is good, but the disposable substrate of noble metal nanoparticles causes a waste of resources. Therefore, new SERS substrates need to be explored and used.
The sensitivity of SERS depends on localized surface plasmon resonance characteristics, which are determined by the size, morphology, and structure of the SERS substrate . Nanoparticle film and nanoporous materials are effective substrates for SERS measurements [8,10–12], and it is of great significance that nanoporous materials can be used as reusable SERS substrates [17,18]. As far as we know, these substrates have not been used for the SERS detection of Hg2+.
Nanoporous materials can be prepared using self-assembly , electrochemical oxidation , chemical reduction , electrodeposition , displacement reaction  and dealloying methods [24,25]. Among these methods, dealloying technology has the particularly important advantage of high efficiency.
Ga is a low melting point metal that is liquid at room temperature. When Ga is removed by chemical dealloying from varying alloys with Au, Pd and Pt formed at low temperature , the corresponding nanoporous materials can be prepared on the surface of the dense substrate .
The electrochemical corrosion is rapid with controllable voltage and current. Therefore, in this study, a low-temperature alloying method is used to synthesize a Ga-Au alloy and electrochemical dealloying is then used for the successful derivation of nanoporous Au. 4-MBA modified nanoporous Au (4-MBA/NpsAu) was used as a SERS substrate to detect Hg2+ in an ethanol solution.
Ga metal (99.99%, Macklin), Au sheet (99.99%, 5 mm ×5 mm × 0.2 mm, Tianjin Yinhai Metal Products Co., Ltd.), 4-MBA (95%, Macklin), HgCl2 (99%, Aladdin) and ethanol (99.7%, Tianjin Bai Chemical Co. Ltd.).
2.2 Preparation of nanoporous Au
Smooth Au sheets were ultrasonically degreased in absolute ethanol for 10 min, followed by rinsing with deionized water and drying in an oven at 100 °C for 20 min. At ∼35 °C, the liquid Ga was evenly coated with a brush on the Au sheet  (the thickness of the Ga film was ∼100 µm). Heat treatment in an oven at 100 °C for 300 min was performed to obtain the Au-Ga alloy.
In a three-electrode system in a 1 M NaOH solution, with a smooth Au sheet as the working electrode, Pt wire as the auxiliary electrode and Hg/HgO as the reference electrode, an Epsilon electrochemical analyzer was used for cyclic voltammetry testing. Figure 1 shows the cyclic voltammetry curve of smooth Au sheets in a 1 M NaOH solution. When the electrode potential was between -0.6 and 0.2 V (vs. Hg/HgO) in anodic process, no oxidation reaction on the Au electrode was observed, suggesting that the electrode was electrochemically stable. When the electrode potential was greater than -1.0 V, Ga could be electrochemically oxidized and dissolved in the 1 M NaOH solution . Therefore, in this study 0.12 V(vs Hg/HgO) was used as the specific electrode potential for Ga dealloying.
The Ga in the alloy was removed by constant potential electrolysis at 0.12 V(vs Hg/HgO) until the electrolysis current was less than 2 µA. The surface of the electrode was then rinsed with deionized water and dried in an oven at 100 °C for 20 min before use.
2.3 X-ray diffraction and scanning electron microscopy analysis
The phase composition was determined by X-ray diffraction (XRD) performed using an X-ray powder diffractometer (D/Max-3c, Rigaku, Japan) operated at 40 kV and 40 mA with a Cu-Kα radiation (λ = 1.5406 Å) source. Diffraction angles for 2θ ranging from 20 to 85 ° (scanning speed: 5°·min−1) were taken to determine the crystalline phase. A field-emission scanning electron microscope (SEM; Quanta 200, FEI, Netherlands) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the microstructure and composition of the material at 20–30 kV.
2.4 Electrochemical performance of nanoporous Au
In 1 M KCl, a three-electrode system was used with the nanoporous Au electrode as the working electrode, Pt wire as the auxiliary electrode and a saturated calomel electrode as the reference electrode. The cyclic voltammetry test was performed in the range of -0.3 to +0.5 V (vs SCE) with a scanning speed of 10 mV/s.
2.5 SERS characteristics
Chemical modification of nanoporous Au material [28,29]: At room temperature, the prepared nanoporous Au sheet was placed in a 0.1 M 4-MBA/ethanol solution for adsorption for 3 h to form the 4-MBA/NpsAu. The nanoporous Au sheet was then rinsed with deionized water.
Detection of Hg2+ SERS: The SERS characteristics were determined by means of Raman spectroscopy (Renishaw inVia) using a spectrometer equipped with a multi-channel charge-coupled device detector and a confocal microscope (DM2500M, Leica). The intensity of the 785 nm laser was 1 mW and the integration time of the SERS spectra was and 30 s. The corresponding SERS spectra were then recorded with a 50 × objective. Prior to the measurements, 10 µL of the Hg2+ solution (HgCl2 ethanol solution) was dropped on the 4-MBA/NpsAu substrate, and the ethanol was naturally evaporated in the air after 15 min. To reduce the error caused by changes in the substrate position, the 4-MBA/NpsAu was immobilized. Afterward, the objective lens was removed, 10 µL of the subsequent Hg2+ concentration solution was dropped on the substrate, the lens was moved back to the original position, and the SERS spectrum associated with the concentration was then recorded .
3. Results and analysis
3.1 XRD analysis of phase structure
The phase structures of the Au-Ga alloy before and after the electrochemical Ga dealloying were scanned and determined by XRD.
Figure 2 shows the XRD patterns of the smooth Au sheets, Au-Ga alloys and synthetic Au materials. The results in Fig. 2(a) show that five diffraction peaks were observed in the XRD pattern of the smooth Au sheets at diffraction angles 2θ of 38.10°, 44.37°,64.60°, 77.59° and 81.73°, representing the (111), (200), (220), (311) and (222) diffraction surfaces of Au respectively (PDF No. 04-0784).
Figure 2(b) presents an XRD pattern of a sample prepared by coating a Ga film on an Au sheet after heat-treating it at 100 °C for 300 min. The results showed that the characteristic diffraction peaks of Au disappeared, but five new peaks were observed at 2θ of 25.27°, 41.59°, 49.59°, 60.76° and 66.96°. These new peaks were attributed to the the (111), (220), (311), (400) and (331) diffraction planes of AuGa2, respectively (PDF No. 03-0969). This suggested that when the Ga film on the Au sheet was heat-treated at 100 °C, the Au surface was completely transformed into a Au-Ga alloy layer. This was slightly different from previous reports: In addition to the Au-Ga phase structure observed by Wang et al. , they still observed the original Au phase structure coated with the Ga film on an Au sheet heat-treated at 100 °C. This is likely due to the Ga and AuGa2 layers coated by Wang et al. being thinner, meaning they could be penetrated by X-rays to detect the phase structure of the Au substrate.
As shown in Fig. 2(c), the five typical peaks of AuGa2 disappeared after the dealloying of Ga, whereas the five XRD typical diffraction peaks of Au that once disappeared were observed, indicating that the Ga in AuGa2 was electrochemically removed and a new Au layer was formed. According to the Scherrer equation, the grain size of the newly formed Au material was ∼20 nm, smaller than the 30 nm of the Au substrate, indicating that the grain size of the newly formed Au layer has been refined. This suggested that the use of electrochemical methods can completely remove Ga out of AuGa2 alloy layer and that a new Au layer was formed during the Ga removal process. The grain refinement during the regeneration of the new Au layer led to a broader XRD diffraction peak.
3.2 SEM analysis of nanoporous Au
As shown in Fig. 3(a), the surface of the newly formed Au layer was arranged to form a net-like nanoporous structure (pore size of 16 nm) different from the nanoporous structure obtained by chemical etching. The nanoporous Au sheet was accumulated by small Au particles when the chemical etching method was used to remove Ga from the AuGa2 alloy , while a flat and uniform nanoporous Au with a thickness of ∼30 µm was obtained when the electrochemical method was used (Fig. 3(b)).
The EDX spectrum of the nanoporous Au is shown in Fig. 3(c). Except for C and N, which can be considered as contaminated elements, Ga was not detected, indicating that the content of Ga in the material after electrochemical dealloying was removed.
3.3 Electrochemical properties of nanoporous Au
In order to study the electrochemical activity of the electrodes, the cyclic voltammetry of the smooth Au sheet and the prepared nanoporous Au sheet and the prepared nanoporous Au sheet were measured at a scan rate of 10 mv/s, with the electrode potential in the range of -0.3–0.5 V. The electrochemical test was carried out in a traditional three-electrode electrochemical cell using a 1 M KCl solution as the electrolyte (Fig. 4). As shown in Fig. 4(a), the cathode and anode current densities of the smooth Au electrode were both small because the specific surface area of the smooth Au sheet was significantly small. This suggested that the activity of the smooth Au electrode in the 1 M KCl solution was very low.
The cyclic voltammetry curve of the prepared nanoporous Au electrode suggested that the nanoporous Au electrode formed an electric double-layer region (Faraday current) and paired redox peaks. This was likely attributable to the widespread reaction hot spots on the rough surface of the nanoporous Au sheet . The crystal structure of the Au nanoparticles can form a single layer of Au hydrated oxide under a lower positive potential scan, which was reduced to nano Au particles under a negative potential scan, thereby forming a pair of redox peaks . Due to the sharp increase in the specific surface area of the nanoporous Au electrode, the current density of the cathode and anode in the cyclic voltammetry curve increased accordingly, presenting a better square shape of the cyclic voltammetry curve with the typical characteristics of a supercapacitor [29,31] (Fig. 4(b)). The area capacitance was 187 mF·cm-2, and it was 86 times that of the smooth Au electrode, which indicated that the synthesized nanoporous Au exhibited higher electrochemical activity than the polished Au.
3.4 SERS of trace Hg2+ on a 4-MBA/NpsAu
Figure 5(A) shows the SERS spectra of Hg2+ adsorbed on a solid substrate of 4- MBA/NpsAu at different Hg2+ concentrations. Figure 5(A) (a) reveals two characteristic Raman peaks at 1083 and 1598 cm-1 for 4-MBA absorbed on the nanoporous Au substrate in the absence of Hg2+, which are attributed to the two distinct ring breathing vibrational modes . The intensity of the 4-MBA characteristic Raman peaks decreases while the 4-MBA/NpsAu Raman substrate is treated in presence of Hg2+, which are attributed to the partition layer formed to draw the 4-MBA away from the Au nanoparticles surface in the presence of Hg2+ [16,33]. The thickness of the partition layer increases with increasing concentration of Hg2+ and the intensity of the 4-MBA characteristic Raman peaks continually decreases.
Figure 5(B) shows the relationship between the concentration (from 1.0×10−10 to 1.0×10−5 mol·L-1) of Hg2+ and the intensity of the 4-MBA characteristic Raman peaks. As shown in the figure, the intensity of the Raman peak was linearly dependent on the logarithm of the Hg2+ concentration ranging from 1.0×10−10 mol·L-1 to 1.0×10−5 mol·L-1(estimated linear correlation coefficient R value: (a) 0.95 at 1083 cm−1 and (b) 0.99 at 1598 cm-1). The SERS detection sensitivity of Hg2+ on the 4-MBA/NpsAu was 1.0×10−10 mol·L-1, which was similar to that of the 4-MBA modified Au nanoparticles . This study provides some research basis for reusable Raman substrates.
This study developed a SERS substrate based on a 4-MBA/NpsAu layer, and achieved the sensitive detection of Hg2+ in ethanol solutions: A nanoporous Au layer has been prepared on a Au substrate by electrochemically removing the Ga out of the pre-synthesized Au-Ga alloy. A layer of 4-MBA was assembled on the surface of nanoporous Au as a SERS probe for Hg2+ detection. And the SERS intensity of the 4-MBA characteristic Raman peaks decreased with the increase of Hg2+ concentration when the 4-MBA/NpsAu was used as a SERS substrate to detect Hg2+.The SERS intensity of which decreased linearly with the logarithm of Hg2+ concentration in the range from 1.0×10−10 to 1.0×10−5 mol·L-1. The 4-MBA/NpsAu showed high SERS sensitivity for Hg2+ and might serve as a reusable Raman substrate.
Natural Science Foundation of Guangxi Province (2018GXNSFAA138178); Major Science and Technology Projects in Guangxi (AA17204067, AA18118044); Key Laboratory of Ecology of Rare and Endangered Species and Environmental Protection (Guangxi Normal University), Ministry of Education, China; National Natural Science Foundation of China (NSFC) (21861004, 31360158).
The authors declare that there are no conflicts of interest related to this article.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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