New electrochemical method to deposit surface-enhanced Raman scattering-active silver nanoparticles on metal substrates

Abstract

As shown in the literature, many methods have been developed to improve the signal, reproducibility and stability of surface-enhanced Raman scattering (SERS) for its reliable application. However, the fabrication needed is laborious. In this work, we propose an effective method to prepare SERS-active substrates with Ag nanoparticles (NPs) by a new electrochemical strategy of deposition–dissolution cycles (DDCs). This electrochemical method is based on a cathodic potential of 0.25 V and an anodic potential of 1.05 V vs.Ag/AgCl, which are applied in turn under sonication. The prepared SERS-active substrate demonstrates a large Raman scattering enhancement for adsorbed Rhodamine 6G (R6G). The practical detection limit for polypyrrole deposited on this substrate is 0.1 μC cm⁻². Moreover, the prepared SERS-active substrates exhibit satisfactory reproducibility and thermal stability. The SERS spectrum of R6G on the newly developed substrate still performs well at a high temperature of 250 °C.

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Introduction

Surface-enhanced Raman scattering (SERS) occurring on rough metal substrates of Ag, Au and Cu is an effective spectroscopy technique for obtaining vibrational information on adsorbate-surface interactions due to its unique sensitivity and excellent frequency resolution from the large increase in scattering.^1,2 The mechanism of SERS consists of two major components. They are electromagnetic (EM) enhancement³ and chemical (CHEM) enhancement.⁴EM enhancement results from the enhancement of local electromagnetic fields at the surface of a metal which can support surface plasma/optical conduction resonances. CHEM enhancement is associated with the charge transfer between the metal and adsorbate at atomic-scale roughness features. In contrast to the well known EM, the CHEM remains much more enigmatic and hard to ascertain. As shown in the literature, the electrochemical method is the oldest method of fabricating SERS substrates. Actually, almost all SERS-active substrates were made by an electrochemical method in the 1980's. Today, it is still widely used, especially in the electrochemist community to probe surface reactions.⁵ As reported by Tsai, et al.,⁶ when they presented a novel SERS-active surface prepared by the electrochemical anodic deposition of silver nanoparticles (NPs) in a multiwalled carbon nanotube (MWCNT)-alumina-coated silica (ACS) nanocomposite. The Ag-MWCNT-ACS-coated indium tin oxide substrate has a considerable effect on the Raman spectra with improvements of more than four times of magnitude, compared to Ag-coated indium tin oxide substrates. As reported by Prokopec, et al.,⁷ different SERS-active Au and Ag substrates suitable for spectral mapping were prepared using procedures which consisted of the electrochemical deposition of the metal layer and further roughening with oxidation–reduction cycles (ORCs). In their work, monolayers formed both by covalent and noncovalent linkages to the metal surface were detected and Raman spectral maps were then measured. Yang, et al.,⁸ demonstrated a facile fabrication of gold nanostructures including Au nanoplates, Au nanothorns (NTs), and Au nanowires (NWs) on indium tin oxide substrates via electrochemical growth for SERS study. A simple two-electrode electrochemical deposition system was applied for the fabrication process. Dense Au nanostructures were grown directly on an Au seeding layer on the substrate. With the Au NWs, Rhodamine 6G (R6G) can be detected at a concentration as low as 10⁻⁹ M.

Recently, Ag NP arrays with high SERS activity and improved uniformity were reported.^9,10 However, the fabrication process is laborious. To improve the reproducibility and stability of SERS-active substrates, Liao et al.¹¹ have attempted to develop SERS substrates by the use of two-dimensional (2D) Au nanorod arrays and to characterize the SERS-active sites of the Au nanostructures. They prepared two different types of 2D Au nanorod arrays by means of anodic aluminum oxide (AAO) template-assisted nanofabrication. The strongest SERS effect was observed for both types of substrates with an Au nanorod diameter of ca. 66 nm. Van Duyne et al.¹² reported a nanosphere lithography method to fabricate nanoparticle arrays with a tunable localized surface plasmon resonance for the analysis of single-molecules. In the report by Liu et al.,¹³Ag nanorod arrays with different lengths fabricated by oblique angle deposition at various vapor deposition angles were studied systematically on their morphologies, optical reflections and SERS responses. It was found that the optical reflectance from these substrates depends not only on the length of the Ag nanorods but also on their deposition angle. Also the SERS enhancement factor decreases nearly monotonically with the increase of the reflectance at the SERS excitation wavelength, and the highest SERS enhancement factor can reach close to 10⁹. However, the preparation procedures for obtaining reliable SERS-active substrates are exceedingly complicated.

Generally, there are two methods that can be used to evaluate the SERS effects. One is to directly measure the SERS intensities and compare them with each other.^14,15 The other is to investigate the enhancement factors (EFs).^12,13 Bell and Sirimuthu¹⁶ suggested that the simplest way to reduce the error was to take multiple points on the surface of SERS-active solid substrates. Similarly, three measurements were used to evaluate the experimental errors in the study of SERS-active Ag films prepared from the photoreduction of Ag ions on TiO₂.¹⁷ Therefore, we adopt the former method to evaluate the SERS effects in the following discussions. Meanwhile, multiple points on the surface were measured to verify satisfactory reproducibility of the obtained SERS-active substrates proposed in this work.

As shown in the literature, solid systems of SERS-active silver and gold substrates^18,19 were more commonly employed than liquid systems of SERS-active silver and gold colloids²⁰ because the solid systems are relatively stable and reliable. Cheng et al.²¹ reported the results of an investigation of SERS spectroscopy based ultrasensitive detection of dipicolinic acid using a gold nanoparticle/polyvinylpyrrolidone/gold substrate (AuNPs/PVP/Au). A detection limit of 0.1 ppb was obtained from the substrates with 60 nm sized Au NPs. In the report by Zhu et al.,²² arrays of Au hierarchical micro/nanotowers were achieved on Au-coated silicon planar substrate viaelectrochemical deposition. The Au hierarchical micro/nanotower arrays have exhibited a distinct SERS effect due to the enhanced local electromagnetic field in the vicinity of the sharp nanotips of the towers and the gaps between the neighboring nanotowers. More importantly, the SERS effect has been further significantly improved via decorating Ag NPs onto the surfaces of the Au hierarchical micro/nanotowers due to both the Ag NP hot spots themselves and the hot spots formed at the junctions between the Ag NPs and the Au micro/nanotowers.

In this work, we propose a simple pathway to prepare SERS-active metal substrates with Ag NPs by an innovative electrochemical strategy of deposition–dissolution cycles (DDCs). The corresponding SERS performances of signal reliability and thermal stability based on probe molecules of adsorbed R6G were investigated in detail. Moreover, the prepared SERS-active substrates were examined by detecting trace molecules of deposited polypyrrole (PPy).

Experimental section

Chemical reagents

Electrolytes and R6G reagents (p.a. grade) were purchased from Acros Organics and used as received without further purification. All of the solutions were prepared by using deionized 18.2 MΩ cm water provided from a MilliQ system.

Electrochemical preparation of Ag-containing complexes in solution

All the electrochemical experiments were performed in a three-compartment cell at room temperature (24 °C) and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). A sheet of silver with a bare surface area of 4 cm², a 2 × 4 cm²platinum sheet and KCl-saturated silver-silver chloride (Ag/AgCl) were employed as the working, counter and reference electrodes, respectively. Before the treatment of the oxidation–reduction cycles (ORCs), the Ag electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 μm of alumina slurry to a mirror finish. Then the electrode was cycled in a deoxygenated 0.1 M HNO₃ aqueous solution (40 mL), in which the pH was adjusted to 7 by adding 1 M NaOH, from −0.3 to +1.0 V vs.Ag/AgCl at 50 mV s⁻¹ for 100 scans under slight stirring. The durations at the cathodic and anodic vertices are 5 and 5 s, respectively. After the ORCs treatment, Ag- and N-containing complexes were produced in the solution, as revealed by the analysis of high resolution X-ray photoelectron spectroscopy (HRXPS). Further inductively coupled plasma-mass spectrometer (ICP-MS) analysis indicates that the concentration of the produced Ag ions in solution is ca. 120 ppm.

Sonoelectrochemical preparation of SERS-active Ag NPs on a Pt substrate

Immediately, the working electrode of Ag was replaced by a Pt substrate with a bare surface area of 0.238 cm² in the same solution. Here the Pt substrate was chosen because it is inert in the following experiments. Then a cathodic potential of 0.25 V and different anodic potentials vs.Ag/AgCl were applied in turn under sonication to effectively prepare SERS-active Ag NPs on the Pt substrate. Also, the ratio of reaction times of cathodic deposition to anodic dissolution of Ag NPs was investigated to obtain the most effective SERS effect. In applying the cathodic potential of 0.25 V vs.Ag/AgCl for the pulse deposition of Ag NPs, the total accumulated deposition time is 2 min for every experiment (including a blank experiment without applying an anodic potential for dissolving the deposited Ag from Pt substrate). After the deposition of SERS-active Ag NPs, the Pt substrate was rinsed thoroughly with deionized water, and finally dried in a dark vacuum-dryer for 1 h at room temperature for subsequent use. The ultrasonic treatment was performed by using an ultrasonic generator (model XL2000, Microson) and operated at 20 kHz with a barium titanate oscillator of 3.2 mm diameter to deliver a power of 80 W. The distance between the barium titanate oscillator rod and the electrode is kept at 5 mm.

Adsorption of R6G on SERS-active Ag NPs deposited on a Pt substrate

For the SERS measurements, the prepared SERS-active Ag NP-containing Pt substrates were incubated in 2 × 10⁻⁵ M R6G aqueous solutions for 30 min. Then the substrates were rinsed thoroughly with deionized water and finally dried in a dark vacuum-dryer for 1 h at room temperature for the subsequent test.

Deposition of polypyrrole on SERS-active Ag NPs deposited on a Pt substrate

For other SERS measurements, electrochemical depositions of conductive polymers of PPy on the prepared SERS-active Ag NP-containing Pt substrates were carried out using a constant anodic potential of 0.80 V vs.Ag/AgCl in deoxygenated aqueous solutions containing 0.1 M pyrrole monomers and 0.1 M LiClO₄. Then the substrates were rinsed thoroughly with deionized water, and finally dried in a dark vacuum-dryer for 1 h at room temperature for subsequent test.

Characterization of SERS-active Ag NPs deposited on a Pt substrate

Experiments regarding SERS-active substrates at different temperatures were performed by mounting the samples on a thermal heater (THMS 600, Linkam Scientific Instruments, UK) at a heating rate of 1 °C min⁻¹ in air. The surface morphologies of the Ag NP-containing platinum substrates were examined by scanning electron microscopy (SEM, model S-4700, Hitachi). For HRXPS measurements, a ULVAC PHI Quantera SXM spectrometer with monochromatized Al K_α radiation, 15 kV and 25 W, and an energy resolution of 0.1 eV was used. To compensate for the surface charging effects, all HRXPS spectra are referred to the C 1 s neutral carbon peak at 284.8 eV. Surface chemical compositions were determined from peak-area ratios corrected with the approximate instrument sensitivity factors. Raman spectra were obtained (Renishaw InVia Raman spectrometer) by a confocal microscope employing a diode laser operating at 785 nm with an output power of 1 mW on the sample. A 50×, 0.75 NA Leica objective was used to focus the laser light on the samples. The laser spot size is ca. 1∼2 μm. A thermoelectrically cooled charge-coupled device (CCD) 1024 × 256 pixels operating at −60 °C was used as the detector with 1 cm⁻¹ resolution. All spectra were calibrated with respect to silicon wafer at 520 cm⁻¹. In measurements, a 90° geometry was used to collect the scattered radiation. A holographic notch filter was used to filter the excitation line from the collected light. The acquisition time for each measurement was 10 s. As explained before, multiple points on the surface of a substrate and different batches of the as synthesised substrates were measured to verify satisfactory reproducibility of the obtained SERS-active substrates proposed in this work. In calculation, the strongest band intensity of R6G on the Raman spectrum was used. In the measurements of multiple points on one substrate, the error is defined as a general rule of the difference between the Raman band intensity on one point and the average of the Raman band intensities on four points divided by the average of the Raman band intensities on four points. The error is within 5%. In the measurements of different batches of the as synthesised substrates, the error is defined as the difference between the average of the Raman band intensities on one substrate and the average of the Raman band intensities on three similar substrates divided by the average of the Raman band intensities on three similar substrates. The error is also within 5%.

Results and discussion

Optimization of SERS performances using the DDCs method

As shown in our previous study,²³ a pathway was proposed to prepare SERS-Active Au/TiO₂ nanocomposites (NCs)-containing substrates by sonoelectrochemical methods. First, the Au substrate was treated by electrochemical ORCs in a solution containing 0.1 M HCl and 1 mM TiO₂ NPs. Subsequently, the Au substrate was immediately replaced by a Pt substrate and a cathodic potential of 0.25 V vs.Ag/AgCl was applied under sonication to prepare SERS-active Au/TiO₂ NCs on the Pt substrate. In this work, the proposed preparation procedure for obtaining SERS-active NPs is further modified to significantly increase the SERS enhancement by providing an additional anodic potential for partially dissolving the deposited NPs, as discussed below. In the ORC treatment, the HNO₃ electrolyte was selected since this facilitates the metal dissolution–deposition process that can produce a considerable amount of Ag- and NO₃⁻-containing complexes in the solution.²⁴ Then these produced complexes were cathodically deposited onto the Pt substrates by the DDCs method to prepare SERS-active Ag NPs. Since the molecules located between two metallic NPs display the greatest SERS enhancement,^25,26 this new strategy of a fixed cathodic potential for Ag deposition and different anodic potentials for Ag dissolution were applied in turn under sonication to effectively prepare SERS-active Ag NPs on the Pt substrate in this work.

First, we examine the influence of pH in solution on the corresponding SERS performances observed on substrates prepared by the DDCs method. Spectrum a of Fig. 1, shows a Raman spectra characteristic of R6G.^27–29 The band at ca. 613 cm⁻¹ is assigned to the C–C–C ring in-plane vibration mode. The band at ca. 774 cm⁻¹ is assigned to the C–H out-of-plane bend mode. The bands at ca. 1128 and 1184 cm⁻¹ are assigned to the C–H in-plane bend modes. The bands at ca. 1313 and 1578 cm⁻¹ are assigned to the N–H in-plane bend modes. The bands at ca. 1363, 1512 and 1651 cm⁻¹ are assigned to the C–C stretching modes. Clearly, the SERS intensity of R6G observed on the Ag NP-deposited Pt substrate prepared in a neutral solution (spectrum a) exhibits a higher relative intensity by more than a 6-fold magnitude, compared with that observed for the Ag NP-deposited Pt substrate prepared in an acidic solution (spectrum b). This increase in intensity is significant in comparison with the reports of SERS spectra on various rough metals.^15,30,31 In calculating the relative intensity, we employ the normalized Raman intensity, which is calculated from the ratio of the strongest intensity of R6G observed in spectrum a of Fig. 1, to that of R6G observed on spectrum b of Fig. 1 (representing a blank experiment). Thus, no correction to the normal Raman scattering intensity is necessary to account for the differences in sampling geometry and scattering phenomena.³² The effect of pH on the corresponding performances can be ascribed to different types of Ag-containing complexes, as precursors of SERS-active Ag NPs are formed in solutions with different pH values after the ORC treatments.³³Spectra a and b of Fig. 2 display the HRXPS Ag 3d_5/2-3/2 core-level spectra of the Ag- and NO₃⁻-containing complexes prepared by the electrochemical treatment of ORCs in neutral and acidic NO₃⁻-containing solutions. For comparison, spectrum c of Fig. 2 also displays the HRXPS Ag 3d_5/2-3/2 core-level spectrum of the polished Ag substrate before the treatment of ORCs. In spectrum c, the doublet peaks located at 367.9 and 373.9 eV can be assigned to Ag(0), according to the XPS handbook and a previous study.³⁴ As shown in spectra a and b of Fig. 2, positive shifts of 0.3 and 0.1 eV of binding energy of the Ag 3d_5/2-3/2 doublet region based on complexes prepared in neutral and acidic solutions, respectively, were observed. These positively charged Ag ions can be assigned to Ag⁺ and Ag₂⁺ for Ag ions prepared in neutral and acidic solutions, respectively, according to the XPS handbook and a previous study.³⁴ To balance the electropositive Ag, negatively charged NO₃⁻ ions, which were used as electrolytes, should be the main anions in the anion-containing complexes of Ag. Therefore, the Ag- and NO₃⁻-containing complexes prepared in neutral and acidic solutions can be assigned to AgNO₃ and Ag₂NO₃, respectively. The correspondingly increased SERS effect can be ascribed to EM enhancement because closer packing and even Ag NPs islands can be deposited on the Pt substrate by the neutral solution, as shown in the SEM images. The following discussions are based on the Ag NP-deposited Pt substrate prepared in a neutral solution, as described in the experimental section.

Fig. 1 SERS spectra of 2 × 10⁻⁵ M R6G adsorbed on Ag NP-deposited Pt substrates prepared by sonoelectrochemical deposition–dissolution cycles in different solutions under a cathodic potential of 0.25 V and an anodic potential of 0.85 V vs.Ag/AgCl with a ratio of reaction times of deposition to dissolution of Ag NPs of 0.1: (a) in neutral 0.1 M HNO₃ (solution pH adjusted to 7 by adding 1 M NaOH); (b) in acidic 0.1 M HNO₃.

Fig. 2 HRXPS Ag 3d_5/2-3/2 core-level spectra of different Ag- and NO₃⁻-containing complexes prepared by the electrochemical treatment of ORCs in different solutions: (a) in neutral 0.1 M HNO₃ (solution pH adjusted to 7 by adding 1 M NaOH); (b) in acidic 0.1 M HNO₃. Spectrum c represents a blank polished Ag substrate before the treatment of ORCs.

Fig. 3 demonstrates the effects of the ratio of cathodic deposition time to anodic dissolution time for Ag NPs deposited on Pt substrates by the DDCs method on the corresponding SERS performances. By comparing spectra a, b and c, it was found that the optimal ratio for obtaining the strongest SERS effect at fixed cathodic and anodic potentials is 0.1. Namely, when applying a cathodic potential of 0.25 V vs.Ag/AgCl for 1 s, an anodic potential of 0.85 V vs.Ag/AgCl should be applied for 10 s to obtain the strongest SERS effect. Encouragingly, in this case, the SERS intensity of R6G observed on the Ag NP-deposited Pt substrate prepared by the DDCs method (spectrum b) exhibits a markedly high relative intensity by more than a 50-fold magnitude, compared to that observed for the Ag NP-deposited Pt substrate prepared by a single procedure of cathodic deposition (spectrum d). Moreover, the SERS intensity of R6G adsorbed on the Ag NP-deposited Pt substrate can be significantly increased by more than one order of magnitude by using the DDCs method developed in this work instead of a single cathodic deposition proposed in a previous work.²³ Moreover, the effects of the applied anodic potentials at a fixed ratio of 0.1 for the cathodic deposition time to the anodic dissolution time for Ag NPs deposited on Pt substrates by the DDCs method on the corresponding SERS performances were examined, as shown in Fig. 4. Obviously, a slight dissolution of the deposited Ag NPs by applying an anodic potential of 1.05 V vs.Ag/AgCl was suggested for obtaining the strongest SERS effect. However, the deposited Ag NPs are dissolved with difficultly from the substrates into the solutions under a more negatively anodic potential of 0.65 V vs.Ag/AgCl to effectively modify the Ag NP-deposited Pt substrate prepared by a single procedure of cathodic deposition (without applying the procedure of anodic dissolution). Thus the effect of the supplemental procedure of anodic dissolution on the increased SERS enhancement is markedly reduced. As demonstrated in Fig. 4, the SERS intensity of R6G observed on the Ag NP-deposited Pt substrate prepared by an anodic potential of 1.05 V vs.Ag/AgCl exhibits a higher relative intensity of more than a 2-fold magnitude, compared to that observed on the Ag NP-deposited Pt substrate prepared by an anodic potential of 0.85 V vs.Ag/AgCl (the same spectrum as spectrum b of Fig. 3, demonstrating the strongest SERS effect observed on Fig. 3). Therefore, comparing the result of Fig. 4 (the Ag NP-deposited Pt substrate prepared by the DDCs method with the optimal preparation conditions) with blank spectrum d of Fig. 3 (representing the Ag NP-deposited Pt substrate prepared by a single cathodic deposition), it was clear that the SERS intensity of the probe molecules of R6G can be markedly increased by more than two orders of magnitude based on the DDCs method proposed in this work.

Fig. 3 SERS spectra of 2 × 10⁻⁵ M R6G adsorbed on different Ag NP-deposited Pt substrates prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V and an anodic potential of 0.85 V vs.Ag/AgCl with different ratios of reaction times of deposition to dissolution of Ag NPs: (a) 1; (b) 0.1; (c) 0.05; spectrum d represents a blank experiment without applying an anodic potential to dissolve the deposited Ag from Pt substrate.

Fig. 4 Normalized Raman intensity of 2 × 10⁻⁵ M R6G adsorbed on different Ag NP-deposited Pt substrates prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V vs.Ag/AgCl and different anodic potentials vs.Ag/AgCl with a ratio of reaction times of deposition to dissolution of Ag NPs of 0.1.

Characterization of Ag NPs deposited on a Pt substrate by the DDCs method

Fig. 5 demonstrates the microstructures of Ag NPs-deposited Pt substrates prepared by applying different anodic potentials at a fixed ratio of 0.1 of cathodic deposition time to anodic dissolution time. Comparing images a–c with image e (which represents a blank experiment without applying an anodic potential for the dissolution of the deposited Ag from the Pt substrate), it was found that closer packing and even Ag NP islands with stereoscopic structures can be deposited on the Pt substrates using the supplemental procedure of anodic dissolution. These phenomena are responsible for the significant increases in SERS effects observed on the Ag NP-deposited Pt substrates prepared by the DDCs method proposed in this work. However, this effect is reduced under the application of a more positively anodic potential. It means that suitable dissolution of the deposited Ag NPs is favorable for obtaining the strongest SERS effect based on the idea of the DDCs method. On the other hand, the deposited Ag NPs are dissolved with difficulty from the substrates into the solutions under a more negatively anodic potential of 0.65 V vs.Ag/AgCl to effectively modify the Ag NP-deposited Pt substrate prepared by a single procedure of cathodic deposition, as inspecting image d and comparing this image with image e. Thus the effect of the supplemental procedure of anodic dissolution on the increased SERS enhancement is markedly reduced. It is well known that molecules located between two metallic NPs display the greatest SERS enhancement.^25,26 As demonstrated in image c of Fig. 5, which corresponds to the strongest SERS effect obtained, the closest packing and the most even Ag NPs islands with distinctly stereoscopic structures of ca. 50 nm with Raman activity were observed. This microstructure provide more chances for the probe molecules to be adsorbed on two metallic NPs. Therefore, a stronger SERS effect was observed.

Fig. 5 SEM images of different Ag NP-deposited Pt substrates prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V vs.Ag/AgCl and different anodic potentials from OCP with a ratio of the deposition to dissolution reaction times of the Ag NPs of 0.1: (a) anodic potential of 1.25 V vs.Ag/AgCl; (b) anodic potential of 1.05 V vs.Ag/AgCl; (c) anodic potential of 0.85 V vs.Ag/AgCl; (d) anodic potential of 0.65 V vs.Ag/AgCl. Image e represents a blank experiment without applying an anodic potential to dissolve the deposited Ag from the Pt substrate.

As shown in the literature,^35,36 there are two kinds of nucleation, namely instantaneous and progressive, and two types of growth, such as two-dimensional (2D) and three-dimensional (3D). The number of nuclei in the instantaneous nucleation mechanism is constant, and they grow on their former positions on the bare substrate surface without the formation of new nuclei. Hence the radii of the nuclei are larger and the surface morphology is rougher. In progressive nucleation, the nuclei not only grow on their original positions on the bare substrate surface but also on new nuclei which form smaller nuclei particles and the surface morphology is flatter. The current maxima (I_m) for the electrodeposition of Ag NPs on Pt substrates prepared by applying a supplemental anodic potential of 1.05 V vs.Ag/AgCl and by a single procedure of cathodic deposition (blank experiment) obtained from the chronoamperometric curves are compared with the theoretical curves of 2D and 3D nucleation and growth obtained from those equations derived by Harrison and Thirsk³⁶ for current–time relation, as shown in Fig. 6. It is clear that before and after the nuclei overlap (I_m), the experimental curves obtained by applying a supplemental anodic potential of 1.05 V vs.Ag/AgCl, and by a single procedure of cathodic deposition are consistent with the theoretical curves of the 3D instantaneous and progressive nucleations, respectively. These results are also consistent with the observations from the corresponding SEM images, as shown in Fig. 5. In electrodepositing Ag NPs by a single procedure of cathodic deposition, the electrodeposition model is 3D progressive nucleation. By applying a supplemental anodic potential of 1.05 V vs.Ag/AgCl to suitably dissolve the deposited Ag NPs, the electrodeposition model in the cathodic deposition becomes 3D instantaneous nucleation. Correspondingly, the surface with the deposition of Ag NPs becomes rougher. This is responsible for obtaining a stronger SERS effect.

Fig. 6 Dimensionless plot of an I–t curve for an Ag NP-deposited Pt substrate prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V and an anodic potential of 1.05 V vs.Ag/AgCl with a ratio of reaction times of deposition to dissolution of Ag NPs of 0.1 (hollow triangles), compared with the theoretical models for nucleation. Curves a and b represent the 3D instantaneous and progressive models (dashed lines), respectively. Curves c and d represent the 2D instantaneous and progressive models (solid lines), respectively. Solid triangles represent a blank experiment without applying an anodic potential to dissolve the deposited Ag from the Pt substrate.

To further realize the influences of the preparation conditions in the DDCs method on the corresponding SERS performances, HRXPS analyses were performed to examine the surface components formed on the Ag NPs-deposited Pt substrates. Fig. 7 shows the HRXPS survey spectra of Ag NP-deposited Pt substrates prepared by applying a supplemental anodic potential of 1.05 V vs.Ag/AgCl and by a single procedure of cathodic deposition (blank experiment). The main signals of Ag and N from Ag NPs and incorporated anions, respectively, are clearly shown in these spectra. Generally, the signals of C and O are unavoidably present in the HRXPS spectra even though the samples don't contain these two components. Basically, these two spectra are similar. The only noticeable phenomena are the different intensities of the N signals. Further analyses of surface chemical compositions based on their individual detailed spectra show that the contents of N are 8.2 and 4.7 mol% (content of N divided by the contents of N and Ag) for the Ag NP-deposited Pt substrates prepared by applying a supplemental anodic potential of 1.05 V vs.Ag/AgCl and by a single procedure of cathodic deposition, respectively. Also, these contents of N from NO₃⁻ are 4.5 and 2.9 mol% for the Ag NP-deposited Pt substrates prepared by applying a supplemental anodic potential of 1.05 V vs.Ag/AgCl and by a single procedure of cathodic deposition, respectively. Interestingly, these contents of N are proportional to their corresponding SERS intensities. This phenomenon was also observed in our other SERS study.³⁷ As reported in the literature,^38–40 anion containing complexes of metals, which were distinguishable from bulk metals, were easily formed during the electrochemical deposition. It was also found that the SERS activity is much more stable when the constituents of the complex are simultaneously present.⁴¹ Furthermore, it was believed that the SERS effect chiefly comes from the corresponding complex formed at the interface of roughened metal.³⁹ In this electrochemical DDCs method, negatively charged NO₃⁻ ions are the main anions in the anions-contained complexes of Ag and contribute to the increased SERS effects.

Fig. 7 HRXPS survey spectrum of Ag NPs-deposited Pt substrate prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V and an anodic potential of 1.05 V vs.Ag/AgCl with a reaction time ratio of deposition to dissolution of Ag NPs of 0.1 (spectrum a). Spectrum b represents a blank experiment without applying an anodic potential for dissolving the deposited Ag from the Pt substrate.

Application of the SERS-active Ag NPs deposited on a Pt substrate by the DDCs method

In the measurement of Raman spectra, an acquisition time of less than one minute is generally used. However, acquisition times of two hours are also used for obtaining satisfactory spectroscopy. Moreover, the thermal degradation of conducting polymers can be investigated by in situSERS spectroscopy, as reported in our previous study.⁴² Thus, the destructively thermal influence on the SERS enhancement capability should be carefully considered under long-term laser irradiation. For protecting the objective from damage when heating the sample, the thermal stability of the substrates in SERS experiments was examined under 250 °C. Meanwhile, the analyte molecules of R6G used in this work are stable at 250 °C and the SERS spectrum of R6G on the roughened Ag substrate prepared by ORCs treatment is featureless when the temperature is raised to 150 °C, as reported in our previous study.⁴³ This issue can be overcome using the newly developed SERS-active Ag NPs-deposited Pt substrate prepared in this work. Fig. 8 shows the satisfactory thermal stability based on the SERS-active Ag NP-deposited Pt substrate prepared under optimal conditions. Raising the substrate temperatures from 25 °C to 150, 200, 225 and 250 °C, the characteristic Raman bands of R6G are still clear and well resolved. Conclusively, the prepared SERS-active substrates based on the DDCs method demonstrate a large Raman scattering enhancement and satisfactory thermal stability.

Fig. 8 SERS spectra of 2 × 10⁻⁵ M R6G adsorbed on the same Ag NPs-deposited Pt substrate, which was prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V and an anodic potential of 1.05 V vs.Ag/AgCl with a reaction time ratio of deposition to dissolution of Ag NPs of 0.1, at different temperatures. Spectra a–e represent experiments performed at 25, 150, 200, 225, and 250 °C, respectively.

Since SERS technologies are generally applied to two systems of adsorbates⁴⁴ and deposited species³¹ the strategy proposed in this work to improve the SERS performances was investigated using other probe molecules of a conductive polymer of PPy. The result also demonstrates a positive effect. As shown in spectrum a of Fig. 9, the peaks are characteristic of PPy on a Raman spectrum.⁴⁵ The peak at 937 cm⁻¹ is assigned to the symmetric stretching mode of the ClO₄⁻ dopants. The broader Raman peaks of PPy appearing in the range of 1000–1150 cm⁻¹ and 1250–1420 cm⁻¹ are assigned to the C–H in-plane deformation and the ring stretching, respectively. The peak at 1605 cm⁻¹ is assigned to the C=C backbone stretching of PPy. The SERS intensity of 1 mC cm⁻²PPy electrodeposited on the SERS-active substrate prepared with the optimal condition proposed in this work can be significantly increased by more than 50-fold of magnitude, compared with that of PPy electrodeposited on the Ag NP-deposited Pt substrate prepared by a single procedure of cathodic deposition (spectrum e of Fig. 9). Moreover, the other important contribution in this work is to significantly reduce the practical detection limit (PDL) of PPy deposited on the Ag NP-deposited Pt substrate prepared by the DDCs method under optimal preparation conditions. As shown in Fig. 9, the spectra resolutions are reduced when diluting the concentrations of the target analytes, as expected. Spectrum d of Fig. 9 represents the SERS spectrum of PPy with an extremely small quantity of 0.1 μC cm⁻² deposited on the substrate. Comparing this spectrum with spectrum a (PPy with a normal quantity of 1 mC cm⁻² deposited on the substrate), it was found that the characteristic PPy peaks still clearly appear at ca. 937 (symmetric stretching mode of ClO₄⁻ dopants) and 1605 cm⁻¹ (C=C backbone stretching). Thus, the PDL for PPy electrodeposited on the Ag NPs-deposited Pt substrate proposed in this work was decided to be ca. 0.1 μC cm⁻².

Fig. 9 SERS spectra of PPy with different charges deposited on Ag NP-deposited Pt substrates prepared by sonoelectrochemical deposition–dissolution cycles under a cathodic potential of 0.25 V and an anodic potential of 1.05 V vs.Ag/AgCl with a reaction time ratio of deposition to dissolution of Ag NPs of 0.1: (a) 1 mC cm⁻²; (b) 0.01 mC cm⁻²; (c) 1μ mC cm⁻²; (d) 0.1 μC cm⁻². Spectrum e represents a blank experiment without applying an anodic potential for dissolving the deposited Ag from Pt substrate (deposition charge of PPy being 1 mC cm⁻²).

Conclusion

In this work, the previously proposed preparation procedure for obtaining SERS-active NPs is further modified by a new electrochemical strategy of the DDCs method for partially dissolving the deposited NPs to significantly increase the SERS enhancement. The prepared SERS-active Ag NPs-deposited substrates were investigated in two general probe molecules of adsorbates and deposited species. They demonstrate a large Raman scattering enhancement for adsorbed probe molecules of R6G. The practical detection limit for PPy deposited on the substrate is 0.1 μC cm⁻². Moreover, the prepared SERS-active substrates exhibit satisfactory reproducibility and thermal stability.

Acknowledgements

The authors thank the National Science Council of the Republic of China and Taipei Medical University for their financial support.

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