Surfactant-assisted preparation of surface-enhanced Raman scattering-active substrates

Abstract

As shown in the literature, cationic surfactant hexadecyltrimethylammonium bromide (CTAB) has been predominantly employed in the shape-controlled synthesis of Au nanoparticles (NPs) in solution to produce the corresponding shape-dependent Au NP catalysts. Among the different techniques used to obtain rough metal substrates with surface-enhanced Raman scattering (SERS) activities, controllable and reproducible surface roughness can be generated using an electrochemical process. In this work, innovative SERS-active substrates with Au NPs are prepared for the first time using sonoelectrochemical deposition-dissolution cycles (SEDDCs) in CTAB-containing electrolytes. Encouragingly, the SERS intensity for both the model probe molecule Rhodamine 6G (R6G) and the general example molecule 4-mercaptopyridine (4-MPy) adsorbed onto the developed substrates are higher by more than one order of magnitude compared with R6G and 4-MPy adsorbed onto SERS-active substrates prepared in solutions in the absence of CTAB. Moreover, the limits of detection (LOD) recorded for the developed SERS-active substrates are as low as 2 × 10⁻¹⁵ and 1 × 10⁻¹⁰ M for R6G and 4-MPy, respectively.

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Introduction

Recently, surface-enhanced Raman spectroscopy (SERS) based on the well-defined localized surface plasmon resonance (LSPR) effect arising from Au, Ag and Cu nanoparticles (NPs) has been developed as a powerful analytical tool for the extremely sensitive detection of various analytes.^1,2 The effective Raman cross sections from the SERS effect are typically enhanced by 10⁶ to 10⁸ times for specific vibrational modes of molecules that are in close proximity to the resonant light-irradiated nano-sized metal NPs.^3,4 In SERS studies based on Au and Ag NPs, metal colloids in solution^5,6 and nanostructured metal surfaces on substrates^7,8 are two commonly studied systems. Among the many techniques used to obtain rough metal substrates with SERS activity,^9,10 a controllable and reproducible surface roughness can be generated using electrochemical oxidation–reduction cycling (ORC).^11,12

Au NPs are also useful for improving diagnostic imaging and in the photothermal ablation of tumors¹³ as well as catalysis.¹⁴ Most importantly, the corresponding effects of optical and electronic performance are critically dependent on the size and shape of the functionalized metal NPs.^15,16 The size and shape of NPs are often kinetically controlled via the balance of the rates of nucleation and facet-specific growth or aggregation and coarsening processes.¹⁷ Therefore, the cationic surfactants hexadecyltrimethylammonium bromide (CTAB) and hexadecyltrimethylammonium chloride (CTAC) have commonly been employed to synthesise various shapes of Au and Ag NPs.^18,19 Some typical examples are the CTAB-assisted syntheses of Au and Ag nanorods^20,21 It is thought that CTAB preferentially binds to specific crystal faces of the growing metal seeds, which leads to the anisotropic 1D growth of a small number of NPs (4–15%).^22,23 As reported by Chang et al.,²⁴ a general and facile strategy has been developed to coat hydrophilic inorganic NPs directly with mesoporous silica nanoparticles (MSNs). The cationic surfactant CTAB was adsorbed onto negatively charged CdTe quantum dots, Fe₃O₄ nanocrystals or Au NPs, introducing a bilayer CTAB coating with a positive charge. A facile and reversible method for assembling and disassembling Au nanorods using a common chelating agent, ethylenediaminetetraacetic acid (EDTA), was reported by Sreeprasad and Pradeep.²⁵ Assembly was induced by the electrostatic interaction between the CTAB bilayer present on the Au nanorods and EDTA. It was found that the SERS activity of this system could be tuned by controlling the concentration of EDTA and the metal ion, Pb(II).

As shown in the literature, CTAB has mainly been used to control the particle sizes and shapes of metal NP colloids in solution. The corresponding SERS effect based on the CTAB-assisted preparation of these metal colloids is unstable, therefore, the prepared metal colloids require aggregation onto substrates for reliable SERS measurements. However, the sizes and shapes of the metal colloids derived from the CTAB-assisted method are difficult to maintain during particle aggregation. To the best of our knowledge, the effect of a CTAB-assisted one-step preparation of SERS-active metal NPs deposited onto a substrate on the corresponding SERS performance has not been widely discussed in the literature. In our previous study,²⁶ we reported a strategy for the controllable release of Au NPs from CTAB-capped Au microparticles in solution. We have also developed a simple pathway to prepare SERS-active substrates using elemental Au NPs via sonoelectrochemical deposition-dissolution cycles (SEDDCs).²⁷

In this work, Au-containing complexes have been prepared using an electrochemical ORC procedure. Different concentrations of CTAB have then been added into Au complex-containing solutions in order to form CTAB-capped Au complexes in solution. The SERS-active substrates with elemental Au NPs are prepared using a SEDDC procedure. The effects of the addition of CTAB into the electrolyte on the improved SERS performance and corresponding trace detections based on the model probe molecule, Rhodamine 6G (R6G), and the general molecule, 4-mercaptopyridine (4-MPy), have been investigated.

Experimental section

Chemical reagents

The electrolyte (HCl), the surfactant (CTAB), the probe molecule [Rhodamine 6G (R6G)] and 4-mercaptopyridine (4-MPy) (p.a. grade) were purchased from Acros Organics and used as received without further purification. All of the solutions were prepared using deionized 18.2 MΩ cm water provided from a MilliQ system.

Preparation of the SERS-active metal substrates

For the electrochemical experiments, a sheet of gold with a bare surface area of 4 cm², a 2 × 4 cm² platinum sheet, and a KCl-saturated silver–silver chloride (Ag–AgCl) solution were employed as the working, counter and reference electrodes, respectively. All of the electrochemical experiments were performed using a three-compartment cell at room temperature (23 °C) and were controlled via a potentiostat (model PGSTAT30, Eco Chemie). In the oxidation–reduction cycling (ORC) treatment, the Au electrode was cycled in a deoxygenated 0.1 M HCl aqueous solution from −0.28 to +1.22 V vs. Ag–AgCl at a scan rate of 500 mV s⁻¹ for 200 scans under gentle stirring. The duration spent at the cathodic and anodic vertices was 10 and 5 s, respectively. After ORC treatment, AuCl₄⁻ complexes (the precursors of the Au NPs) were produced in the solution, as reported in our previous study.²⁸ Immediately, the produced AuCl₄⁻ complexes were capped with cationic surfactant by mixing CTAB into the AuCl₄⁻ complex-containing solution with gentle stirring for 30 min. The treated Au electrode was then replaced by a mechanically polished Au substrate with a bare surface area of 0.238 cm² in the same solution. A cathodic overpotential of 0.6 V and an anodic overpotential of 0.2 V from the open circuit potential (OCP) of ca. 0.81 V vs. Ag–AgCl were then applied in turn under sonication (SEDDC) in order to prepare the SERS-active Au substrate. The ratio of the reaction times for the cathodic deposition to the anodic dissolution of Au NPs is 0.2. For application of the cathodic overpotential during the pulse deposition of the Au NPs, the total accumulated deposition time is 1 min for every experiment. The ultrasonic treatment was performed using an ultrasonic generator (model XL2000, Microson) operated at 20 kHz and equipped with a 3.2 mm diameter barium titanate oscillator, delivering a power of 80 W. The distance between the barium titanate oscillator rod and the electrode was kept at 5 mm. Finally, the Au NP-deposited substrate was removed from the solution, and rinsed with deionized water and treated in an ultrasonic bath for three cycles in order to remove CTAB from the electrode. The substrate was then dried in a dark vacuum-dryer for 1 h at room temperature before subsequent testing. For comparison, a Au NPs-deposited substrate was also prepared using the same SEDDC method and the same AuCl₄⁻ complex-containing solution, but in the absence of CTAB.

SERS measurements on the SERS-active substrates

For the SERS measurements, 20 μL sample solutions were dropped onto the as-prepared SERS-active substrates. After 30 min equilibration time the sample molecule-adsorbed substrates were rinsed thoroughly with deionized water in order to remove any unbound molecules, and finally dried in a dark vacuum-dryer for 1 h at room temperature before subsequent testing.

Characterization of the SERS-active substrates

The surface morphologies of Au substrates were examined using scanning electron microscopy (SEM, model S-4700, Hitachi, Japan). For high resolution X-ray photoelectron spectroscopy (HRXPS) measurements, a ULVAC PHI Quantera SXM spectrometer was used with monochromatized Al K_α radiation and operating at a voltage of 15 kV, power of 25W and an energy resolution of 0.1 eV. To compensate for surface charging effects, all of the HRXPS spectra are referenced to the C1s neutral carbon peak at 284.8 eV. Raman spectra were obtained (Renishaw InVia Raman spectrometer) using a confocal microscope employing a diode laser operating at 785 nm with an output power of 1 mW on the sample. The excitation wavelength of 785 nm is specially chosen to avoid interference from the electronic transition of the model probe molecule R6G. A 50×, 0.75 NA Leica objective lens was used to focus the laser light onto the samples. The laser spot size is ca. 1 to 2 μm. A thermoelectrically cooled charge-coupled device (CCD) of 1024 × 256 pixels operating at −60 °C was used as the detector with 1 cm⁻¹ resolution. All of the spectra were calibrated with respect to a silicon wafer at 520 cm⁻¹. For the measurements, the laser beam was focused down to the objective lens. The backscattered Raman signal was collected using the same objective lens and passed through an adjustable confocal hole in order to filter any unexpected stray-light noise. A holographic notch filter was used to filter the excitation line from the collected light. The acquisition time for each measurement was 10 s. Measurements were replicated five times on different areas in order to verify that the spectra were a true representation of each sample. The relative standard deviation is within 5% based on the strongest band intensity of R6G in the Raman spectrum. In addition, different batches of the as-synthesised substrates were measured using the same conditions three times. The relative deviation from the average value was less than 5% for each individual sample based on the strongest intensity band of R6G in the Raman spectrum.

Results and discussion

Characterization of the SERS-active substrates with Au NPs

As reported in our previous study,²⁸ AuCl₄⁻ complexes were produced in solution after the ORC treatment. Before the deposition of Au NPs onto the substrates, the cationic surfactant CTAB was added into the solutions to cap the negatively charged AuCl₄⁻, forming micellar CTAB-capped AuCl₄⁻ in solution. The strategy used in this work aimed to prepare CTAB-confined Au NPs deposited onto metal substrates using a subsequent SEDDC procedure. As CTAB itself is a SERS-inactive compound, after complete removal of CTAB, the highly effective SERS-active substrates with confined Au island films showed promise. Fig. 1a shows an SEM image of the substrate with deposited Au NPs prepared in a CTAB-free solution. The surface morphology of this substrate exhibits typical thin metal islands and has good Raman activity, which is a result of the microstructures that are smaller than 100 nm in size.²⁹ With the addition of a small quantity of CTAB (0.001 g L⁻¹) in the electrolyte before the SEDDC procedure the type of surface morphology of the prepared substrate changes, as demonstrated in Fig. 1b. It now shows that compact thin Au islands have been deposited onto the substrate, which still exhibits a good Raman activity with microstructures smaller than 100 nm. For the SERS studies based on metal colloids, partial aggregation of the metal colloids was necessarily induced by adding a chloride solution as activating agent. The aggregated compact thin Au islands that result from the addition of CTAB may be responsible for the increased hot spots in the SERS effect. With the addition of a larger quantity of CTAB (0.01 g L⁻¹) into the electrolyte before the SEDDC procedure the surface morphology observed on the prepared substrate exhibits more compact and larger thin Au islands, as demonstrated in Fig. 1c, which also shows good Raman activity with microstructures smaller than 100 nm. As molecules located between two metallic NPs exhibit the greatest SERS enhancement^30,31 this kind of microstructure can provide a greater chance for probe molecules to be adsorbed onto hot spots. Therefore, a stronger SERS effect should be observed for this type of SERS-active substrate. An experiment based on a higher concentration of CTAB (0.1 g L⁻¹) in the preparative solution was also tried. However, in this case, less metal islands were observed on the substrate after the SEDDC procedure, as shown in Fig. 1d. The reason for this may be due to the unfavorable influence of excess micelles from the higher concentration of the cationic surfactant CTAB on the corresponding deposition-dissolution behavior during the SEDDC procedure. By further magnifying images (a–c) shown in Fig. 1, it was found that the gaps between the nanostructures are ca. 60, 30 and 15 nm for the Au NP-deposited substrates prepared using the SEDDC method in Au complex-containing solutions with 0, 0.001 and 0.01 g L⁻¹ CTAB, respectively.

Fig. 1 SEM images of Au NP-deposited substrates prepared using the SEDDC method in Au complex-containing solutions with different CTAB concentrations: (a) no CTAB for reference; (b) 0.001 g L⁻¹ CTAB; (c) 0.01 g L⁻¹ CTAB; and (d) 0.1 g L⁻¹ CTAB.

In this work, CTAB was utilized to enable the confined deposition of Au NPs into metal islands with optimal morphology onto the metal substrate in order to obtain an effective SERS effect. However, SERS-inactive CTAB must be completely removed from the final prepared SERS-active Au metal islands deposited onto the substrate. Surface composition analysis using an HRXPS survey was thus performed in order to clarify this important issue. As demonstrated in the survey spectrum shown in Fig. 2, the signals of the two characteristic components in CTAB, N and Br are not detected (0.01 g L⁻¹ CTAB was used in this synthesis). This result suggests that the prepared SERS-active Au islands on the substrate are indeed free of CTAB. The C and O signals shown in Fig. 2 are unavoidable contaminants, which are always present in HRXPS experiments, even though the samples do not contain components of carbon and oxygen. Fig. 3 shows the HRXPS N1s core-level spectrum of the SERS-active Au islands on the substrate where 0.01 g L⁻¹ CTAB was used during the preparation. As shown in this spectrum, the signal to noise ratio (S/N ratio) is too low (less than 3) and no characteristic peak of nitrogen, which is generally located at ca. 399.9 eV, can be observed. This result confirms again that CTAB has been completely removed from the final prepared SERS-active Au metal islands.

Fig. 2 HRXPS survey spectrum of Au NP-deposited substrate prepared using a SEDDC method in a Au complex-containing solution with 0.01 g L⁻¹ CTAB.

Fig. 3 HRXPS N1s core-level spectrum of Au NP-deposited substrate prepared using a SEDDC method in a Au complex-containing solution with 0.01 g L⁻¹ CTAB.

Performance of the SERS-active substrates based on R6G

First, model probe molecules were used to examine the performance of the CTAB-assisted preparation of the SERS-active substrates with different Au island morphologies. Fig. 4 shows the Raman spectra of 2 × 10⁻⁶ M R6G adsorbed onto the SERS-active substrates prepared in solutions containing different concentrations of CTAB. Comparing spectrum a (no CTAB used in the preparation) with spectrum b (small quantity of 0.001 g L⁻¹ CTAB used in the preparation), it was found that the magnitude of the SERS intensity of R6G was enhanced ca. 3-fold when CTAB was used during the preparation of the substrate. This result confirms that this innovative strategy is promising for the CTAB-assisted preparation of SERS-active substrates with Au islands that have specific microstructures. Encouragingly, as shown in spectrum c, adding a larger quantity of CTAB (0.01 g L⁻¹) during the preparation results in the SERS intensity of R6G adsorbed onto this SERS-active substrate exhibiting a higher relative intensity, which is more than one order of magnitude greater than that for the R6G adsorbed on the SERS-active substrate in spectrum a (where no CTAB was used in the preparation). This increase in intensity is significant in comparison to other reports on the SERS spectra of various other rough metal substrates.^32–34 To calculate the relative intensity, we employed the normalized Raman intensity, which can be calculated from the ratio of the strongest intensity band of the R6G (1509 cm⁻¹) adsorbed onto the SERS-active substrate where CTAB was used during the preparation to the R6G adsorbed on the SERS-active substrate where no CTAB was used in the preparation. Thus, no correction to the normal Raman scattering intensity is necessary to account for differences in the sampling geometry and scattering phenomena.³⁵ Spectrum c shown in Fig. 4 is characteristic of the Raman spectrum of R6G.^36–38 The band at ca. 611 cm⁻¹ is assigned to the C–C–C ring in-plane vibration mode. The band at ca. 769 cm⁻¹ is assigned to the C–H out-of-plane bend mode. The bands at ca. 1125 and 1181 cm⁻¹ are assigned to the C–H in-plane bend modes. The bands at ca. 1309 and 1574 cm⁻¹ are assigned to the N–H in-plane bend modes. The bands at ca. 1361, 1509 and 1649 cm⁻¹ are assigned to the C–C stretching modes. Nevertheless, with a higher concentration of CTAB (0.1 g L⁻¹) used during the preparation, less metal islands were observed on the substrate, as shown in Fig. 1d. This microstructure is reflected in the corresponding smaller SERS effect, as observed in Fig. 4d. Comparing spectrum a (no CTAB used in the preparation) with spectrum d (a large quantity of 0. 1 g L⁻¹ CTAB used in the preparation), it can be observed that the magnitude of the SERS intensity of R6G is actually decreased by ca. 40% when a large quantity of CTAB is used during substrate preparation. Therefore, the optimum quantity of CTAB to be used in this strategy in order to obtain the strongest SERS effect is around 0. 01 g L⁻¹. In addition, the value of the enhancement factor (EF) can be calculated from the definition shown in the literature.³⁹ The EF per molecule is defined as G = (N₂/N₁) × (I_SERS/I_ref), where I_SERS is the integrated intensity of the R6G band under consideration recorded at the grating and I_ref is the integrated intensity of the same Raman band obtained by focusing the laser line onto a Pt substrate immersed in a 2 × 10⁻³ M R6G solution. Based on a concentration of 2 × 10⁻³ M the normalized Raman spectrum of R6G is acceptable. N₁ is the number of molecules that comprise the first monolayer adsorbed onto the grating under the laser spot area. As the real surface area of the Au islands is difficult to obtain, a collection area based on the laser spot size (2.5 μm²) was used instead to calculate N₁ (the collection area divided by the surface area of one R6G molecule). For the calculation, a surface area of 2 × 10⁻¹⁸ m² per R6G molecule was used.⁴⁰ This value was calculated from the geometric area of length (1.37 nm) × width (1.43 nm) of one R6G molecule. Thus, the estimated value for N₁ is ca. 1.25 × 10⁶. N₂ is the number of molecules excited within the volume of the laser waist for the 2 × 10⁻³ M R6G solution. To calculate N₂ (irradiated solution volume multiplied by the concentration of analyzed molecule) the volume of the laser waist is approximated to a cylinder with a radius of 20 μm and a depth into the sample of 3 mm. The calculated volume is ca. 3.8 × 10⁻¹² m³. Thus, the estimated value for N₂ is ca. 4.5 × 10¹². Based on the ratio of (I_SERS/I_ref) being 1.8 × 10², which is calculated from the Raman intensities at ca. 1509 cm⁻¹, the prepared SERS-active substrate based on the optimum preparation conditions demonstrates a large EF of 6.5 × 10⁸.

Fig. 4 SERS spectra of 2 × 10⁻⁶ M R6G adsorbed onto Au NP-deposited substrates prepared using a SEDDC method in Au complex-containing solutions with different CTAB concentrations: (a) no CTAB for reference; (b) 0.001 g L⁻¹ CTAB; (c) 0.01 g L⁻¹ CTAB; and (d) 0.1 g L⁻¹ CTAB.

The limit of detection (LOD) of an analyte is an important index for evaluating a developed technique in the field of analytical chemistry, especially in SERS analysis. Therefore, the LOD for the probe molecule, R6G, observed using our developed CTAB-assisted SERS-active substrate was determined and compared to various other methods based on Au NPs in the literature and to our previously developed SEDDC method (no CTAB used in preparation).²⁷ As demonstrated in Fig. 5, the signal-to-noise ratios of the spectra are markedly reduced as the concentration of R6G is diluted. To determine the LOD, spectrum d (2 × 10⁻¹⁵ M R6G) was compared to spectrum a (2 × 10⁻⁶ M R6G). This showed that some of the characteristic R6G bands at ca. 611, 769, 1309, 1361 and 1509 cm⁻¹ could still be detected. The S/N ratios of these five bands are all greater than 3. Thus, the LOD for R6G adsorbed onto the proposed SERS-active substrate was determined to be ca. 2 × 10⁻¹⁵ M. Compared with LOD of R6G of 2 × 10⁻¹² M observed using our previously developed SEDDC method (no CTAB used in preparation),²⁷ the proposed strategy in this work provides a significant improvement with a LOD reduction of three orders of magnitude. Moreover, this LOD of 2 × 10⁻¹⁵ M was compared to other Au NP-based SERS studies based on the same R6G probe molecule in the literature. The LOD observed in this work is far lower than the LODs of 1 × 10⁻⁸, 1 × 10⁻⁹, 1 × 10⁻¹² and 1 × 10⁻¹² M observed for Au NPs coated onto an array of carbon nanotubes nested into a Si nanoporous pillar,⁴¹ Au-coated ZnO nanorods,⁴² Au flower-like nanoarchitectures⁴³ and a self-assembled monolayer of Au NPs on NH⁺ ion implantation modified indium tin oxide, respectively.⁴⁴ In addition, when resonating the R6G transition using laser irradiation at 532 nm, the LOD of R6G recorded on the developed SERS-active substrate is as low as 2 × 10⁻¹⁶ M, as shown in Fig. 6, which is lower than the 2 × 10⁻¹⁵ M recorded on the developed SERS-active substrate using laser irradiation at 785 nm, as discussed above.

Fig. 5 SERS spectra of different concentrations of R6G adsorbed onto Au NP-deposited substrates prepared using a SEDDC method in Au complex-containing solutions with 0.01 g L⁻¹ CTAB. Spectra a–d represent R6G concentrations of 2 × 10⁻⁶, 2 × 10⁻⁹, 2 × 10⁻¹² and 2 × 10⁻¹⁵ M, respectively.

Fig. 6 SERS spectra of different concentrations of R6G adsorbed onto Au NP-deposited substrates prepared using a SEDDC method in a Au complex-containing solutions with 0.01 g L⁻¹ CTAB. Spectra a–d represent R6G concentrations of 2 × 10⁻⁹, 2 × 10⁻¹², 2 × 10⁻¹⁵ and 2 × 10⁻¹⁶ M, respectively. For the Raman measurements, a confocal microscope employing a diode laser operating at 532 nm with an output power of 1 mW was used on the samples.

Performance of the SERS-active substrates based on 4-MPy

In SERS studies, R6G model probe molecules with a large Raman cross section have commonly been employed to examine the SERS activities of prepared substrates. Thus, the strategy proposed in this work to improve SERS activity was also applied to the detection of a general probe molecule, 4-MPy. Fig. 7 shows the corresponding results. Spectrum c of Fig. 7 shows the adsorption of 4-MPy onto the SERS-active substrate prepared with CTAB (0.01 g L⁻¹), which demonstrates the strongest SERS intensity. The bands in this spectrum, which are located at ca. 1003, 1096, 1216, 1580 and 1605 cm⁻¹, are characteristic of the Raman spectrum of 4-MPy.^45,46 Similarly, as discussed for R6G, the SERS intensity of the 4-MPy adsorbed onto the SERS-active substrate with Au islands prepared with the assistance of CTAB, when compared to the blank SERS-active substrate (spectrum a), significantly increases with an increase of the concentration of CTAB used in the preparation from 0.001 to 0.01 g L⁻¹. Nevertheless, this improved SERS effect is dramatically decreased when a higher concentration of 0.1 g L⁻¹ CTAB was used in the preparation, as shown in Fig. 7d. In addition, this phenomenon may be ascribed to the unfavorable influence of excess micelles of the cationic surfactant CTAB that are formed at a higher concentration on the corresponding deposition of Au islands onto the metal substrate.

Fig. 7 SERS spectra of 1 × 10⁻⁴ M 4-MPy adsorbed onto Au NP-deposited substrates prepared using a SEDDC method in Au complex-containing solutions with different CTAB concentrations: (a) no CTAB for reference; (b) 0.001 g L⁻¹ CTAB; (c) 0.01 g L⁻¹ CTAB; and (d) 0.1 g L⁻¹ CTAB.

Furthermore, the LOD for the 4-MPy probe molecules observed on our developed CTAB-assisted SERS-active substrate was investigated. As demonstrated in Fig. 8, the signal-to-noise ratios of the spectra are markedly reduced when the concentration of 4-MPy is diluted. In order to determine LOD, spectrum d (1 × 10⁻¹⁰ M 4-MPy) was compared to spectrum a (1 × 10⁻⁴ M 4-MPy). This showed that some of the characteristic 4-MPy bands at ca. 1003, 1096 and 1605 cm⁻¹ could still be detected. The S/N ratios of these three bands are all greater than 3. Thus, the LOD for 4-MPy adsorbed onto the proposed SERS-active substrate was determined to be ca. 1 × 10⁻¹⁰ M. In conclusion, this new strategy for the CTAB-assisted preparation of SERS-active substrates using a SEDDC procedure is effective for probe molecules with large and normal Raman cross sections.

Fig. 8 SERS spectra of 4-MPy with different concentrations adsorbed onto Au NP-deposited substrates prepared using a SEDDC method in Au complex-containing solutions with 0.01 g L⁻¹ CTAB. Spectra a–d represent the concentrations of 4-MPy used, 1 × 10⁻⁴, 1 × 10⁻⁶, 1 × 10⁻⁸ and 1 × 10⁻¹⁰ M, respectively.

Conclusions

In this work, we have successfully proposed an effective strategy for the CTAB-assisted preparation of SERS-active substrates with specific compact Au islands. This new strategy for the improvement of SERS activities is effective for both the model molecule R6G and general example molecule 4-MPy. The LOD values recorded for the developed substrates are 2 × 10⁻¹⁵ M and 1 × 10⁻¹⁰ M for R6G and 4-MPy, respectively. A detailed mechanistic investigation of the controlled synthesis of Au and Ag metal islands with specific microstructures deposited onto substrates using the latest developed SEDDC procedure in the presence of CTAB are under way.

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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