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Abstract
Noble metal nanoparticles (NMNPs) assembly substrates with strongly enhanced local electromagnetic fields provide new possibilities for surface-enhanced Raman spectroscopy (SERS) sensing. Although the external-electric-field-based self-assembly (EEFSA) strategy for decreasing NMNP gap in liquid phase is relatively developed, it is rarely described in solid phase. Here, by combining corona discharge technique (CDT) as a simple EEFSA approach on flexible substrate surface modification, a flexible SERS substrate medicated with gold nanospheres (AuNSs) is produced. Because of the CDT’s peculiar discharge event, makes AuNSs aggregation simply achieved. The modified flexible SERS substrate is sensitive to the detection limit of ∼10−5 mM for Rhodamine 6G (R6G), with a maximum enhancement factor of 2.79×106. Furthermore, finite-difference time-domain (FDTD) simulation confirms the SERS enhancement impact of AuNSs-based substrate. This study not only provides a low-cost, simple-to-process, high-yield, high sensitivity, and activity flexible SERS substrate, but also suggests a more practical and adaptable NMNPs self-assembly approach.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Because of the benefits of short measurement time, high sensitivity, and chemical specificity [1,2], surface-enhanced Raman spectroscopy (SERS) substrates have been widely employed in public health and healthcare, food safety, and the ecological environment in recent years [3–6]. Aggregating colloids to form stable field enhancement hotspots is a fundamental problem in attaining a higher SERS enhancement factor [7–9]. Unlike traditional Raman spectroscopy, SERS technology examines not only the interaction of light with matter, but also the interaction of light with substrates [10–13]. The fabrication of SERS substrates may be done in two ways: top-down and bottom-up [14–18]. Multiple processing stages are frequently required for top-down techniques like lithography [19,20]. However, colloidal chemical synthesis, as a typical bottom-up strategy, is very easy, affordable, and appropriate for large-scale production [21–24].
According to data, gold nanoparticles (AuNPs) prepared using a wet colloidal chemical synthesis technique are attracting a lot of attention and play an important role in SERS signal enhancement [25,26]. Such a strong enhancement effect is primarily attributed to the localized surface plasmon resonance (LSPR) [27,28], which can be provided by hot spots formed by shortening the gap distances between AuNPs [29–32]. Furthermore, the amplification of the local electromagnetic (EM) field of AuNPs has been demonstrated to be well accomplished by external-electric-field-based self-assembly (EEFSA) due to its good controllability, relative simplicity, and low equipment requirement [33,34]. EEFSA is quite mature in the liquid phase, but it is extremely inconvenient in the solid phase, particularly on the surface of the flexible SERS substrate [35–37]. Corona discharge treatment (CDT), a technique that generates a strong electric field near the needle tip to ionize the air, has recently attracted a lot of attention for the flexible substrate polish and wettability, adhesion, and roughness improvement [38–41]. Here a simple EEFSA approach to assemble the AuNSs and form a special arrangement has been explored, which benefits from the charged ion stream produced by CDT.
In this paper, the homogeneity gold colloid solutions in this research were made using a modified hydro-thermal technique, and filter papers doped with AuNSs were modified using CDT. Because of the self-assembly of AuNSs with CDT, the filter paper-based substrate was able to exhibit a very strong SERS signal with high sensitivity, good repeatability, and low cost. The maximal enhancement factor (EF) is 2.79×106, and the detection limit for Rhodamine 6G (R6G) is 10−5 mol/L. The self-assembly of nanospheres by CDT achieved here is more practicable and promising for practical SERS applications.
2. Preparation of flexible paper-based SERS substrate
2.1 Chemicals
The AuNSs were prepared via the silver ion-assisted seed mediated method [42]. The chemicals, Gold (III) chloride trihydrate (HAuCl4·3H2O), trisodium citrate (C6H5O7Na3), R6G were analytical grade, which were purchased from Sigma-Aldrich. Deionized water with a resistivity higher than 18.2 MΩ/cm was used in the synthesis process.
2.2 Characterization
Scanning electron microscopy (SEM) images were taken from the Hitachi S4800 at voltage of 30 kV. The atomic force microscopy (AFM, Bruker Dimension Icon 3100) was used in tapping mode to evaluate the sample thickness and radius. High-resolution transmission electron microscopy (TEM) imaging was performed on an electron microscopy instrument (Tecnai G2 F20 S-TWIN). Analysis of zeta potential and particle size were performed on Malvern Zetasizer Nano ZS. The ultraviolet spectra were measured by an ultraviolet-visible (UV-Vis) spectrophotometer (Pgeneral, T6S). The SERS spectra of the samples were collected by a home-build confocal Raman microscopy, which composed of Ocean Raman spectrometer (IDR-MICRO-785), Raman laser (SPL-laser-532, Spectral linewidth: < 0.2 nm), Raman Probe (SPL-RPB-532, optical density: > 6), and optical microscope (50× microscope objective, spot size: 40 µm, numerical aperture: 0.55). The laser power was set as 20 mW with an integration time of 10 s. The excitation power of CDT setup was a high voltage DC power supply (Teslaman, TCM600). The high-voltage equipment included a high-voltage generator with positive/negative polarity and a grounding system. The high-voltage ranged from 0 kV to 50 kV, and the accuracy of the ammeter was 0.1 mA.
2.3 Synthesis and characterization of AuNSs
Gold colloid solutions were prepared by a modified hydro-thermal method. The HAuCl4·3H2O powder was dissolved in deionized water to make a 2.5 mM solution. After several times diluting, the HAuCl4·3H2O solution was obtained with concentration of 0.25 mM. Heat the HAuCl4·3H2O solution and add 1.5 ml of 1% sodium citrate solution immediately at the beginning of the boiling state. Continue heating until the color becomes transparent wine red, with obvious Tyndall effect. Finally, the gold colloid solution was prepared by natural clarification. Remove the solution from the magnetic heating jacket (CLT-1A 100-200 ml) and naturally cool down for further low-temperature preservation. AFM was used to roughly observe the morphology of the prepared gold colloid solution, as shown in Fig. S1.
TEM was used to characterize the morphology of AuNSs dispersed on TEM grids (Figs. 1(a)–1(c)). As demonstrated in Fig. 1(a), the AuNSs were evenly disseminated and hardly no overlapped nanoparticles. The average diameter of AuNSs is ∼23.5 nm, which is confirmed by statistical analysis of the TEM measurements. Meanwhile, the high-resolution image was used to examine the crystalline phase of typical AuNSs, as depicted in Fig. 1(c). The lattice fringe of AuNSs can be observed clearly, and they were wrapped by a ∼1.5 nm thin organic layer, which may be identified as trisodium citrate left during the gold colloid manufacturing procedure.
Fig. 1. Synthesis and characterization of AuNSs. (a) TEM image of AuNSs. (b) Enlarge TEM image of AuNSs. (c) High resolution TEM image of typical AuNSs. Wrapping layer is about 1.5 nm. (d) Extinction spectra of as-synthesized AuNSs in solution. As-synthesized AuNSs show an ensemble LSPR wavelength at 525 nm. (e) Particle size distribution of ligand-free AuNSs illustrating a mean size of 23.5 nm. (f) Zeta potential measurements of AuNSs solution.
Adding different concentrations of HAuCl4·3H2O solution during the synthesis process will affect the particle size of the gold nanospheres. As the concentration of HAuCl4·3H2O solution increases, the particle size of AuNSs also gradually increases. Figure 1(d) shows the extinction spectra of as-synthesized AuNSs in solution prepared with different HAuCl4·3H2O solution concentrations of 0.25 mM and 0.35 mM were also measured by UV-Vis spectrophotometer, respectively. A slight red-shift of the extinction spectrum peak of AuNSs in solution occurs near 525 nm. However, as the particle size of AuNSs increases, significant aggregation will occur in the solution, thereby affecting the stability of the gold colloid solution, thus the HAuCl4·3H2O solution with concentration of 0.25 mM was used in the subsequent preparation process.
The particle size distributions of AuNSs were analyzed by counting more than 200 particles in the TEM images (Fig. 1(e)). The particle size ranges from 20 nm to 25 nm, and the average size is 23.5 ± 2.5 nm, which further confirmed the uniformity of the as-prepared AuNSs.
Before the CDT processing program, the charged state of the AuNSs is a key factor for selecting the appropriate electrode polarity. As shown in Fig. 1(f), the charge states of AuNSs were quantified by the zeta potential test. 59.8% of the AuNSs have a zeta potential of −37.6 mV, whereas 40.2% have a zeta potential of −22 mV. Therefore, the average zeta potential of all AuNSs is −31.6 mV, which means the net charges of as-prepared citrate-coated AuNSs are negative.
It is worth mentioning that, the prepared gold colloidal solutions did not change color even after one month, as shown in Fig. S2. So, the gold colloid solution prepared here has strong stability, good uniformity, and determined electrical properties. Meanwhile, this synthesis method has the advantage of low cost, simple operability, and high repeatability.
2.4 AuNSs assembling on flexible substrate by corona discharge
Figure 2(a) shows the manufacturing procedure of the flexible paper-based SERS substrate. Cut the qualitative filter paper into a 1 cm×1 cm square, then submerge it fully in the prepared gold colloid solution and lift it after ∼5 minutes. After that, the filter paper was dried in an oven at 35°C. Place the dried substrate on the self-built discharge platform, stimulate corona discharge with high power sources, and perform positive CDT (p-CDT) or negative CDT (n-CDT) with variable time intervals on the surface to prepare the flexible paper-based SERS substrate.
Fig. 2. Preparation and characterization of flexible SERS substrate. (a) Protocol of the flow chart of substrate treatment with corona discharge at positive and negative high voltage. (b) Schematic diagram of p-CDT and n-CDT. (c) Digital camera image of the experimental corona discharge. (d–f) SEM image of AuNSs doped filter paper substrate before CDT. (g–i) SEM image of the doped filter paper substrate after p-CDT, the gully-shaped surface appeared on the filter paper substrate. Subimages show enlarged typical p-CDT assembly.
As the CDT process is the research focus in the whole experiment, the mechanism of AuNSs assembling on the flexible substrate is deeply discussed. As is known to all, corona discharge is an electrical discharge caused by the ionization of air surrounding the needle electrode. The polarity of corona discharged depends on the polarity of the voltage on the needle electrode. As shown in Fig. 2(b), the p-CDT process can be described as follows, that the negative ions produced by air ionization are attracted by the positive needle electrode and move upwards. On the contrary, the positive ions move to the flexible substrate below due to the attracting of the negative plate electrode. Therefore, the negatively charged AuNSs on the substrate would be attracted by the positive ions and aggregated. In an actual discharge process (Fig. 2(c)), the streamer corona discharge is clearly shown between the needle and the home-build discharge platform.
Figures 2(d)–2(i) show the SEM images of the AuNSs doped filter paper substrates before and after p-CDT. In view of Fig. 2(d), the AuNSs are uniformly distributed on each cotton fiber in filter paper before p-CDT. When enlarging the fibers, it can be observed that AuNSs are not only on the surface, but also inside the paper substrate, as shown in Fig. 2(e). After zoom in (Fig. 2(f)), the shape of an individual particle can be clearly observed. It was confirmed that most AuNSs were uniformly dispersed on the substrate and only a few AuNSs are stacked and aggregated in some areas.
Meanwhile, the assembled state of AuNSs after CDT was also characterized. Figure 2(g) shows the SEM image focused on one cotton fiber. Compare with Figs. 2(d)–2(f), it can be clearly seen that the AuNSs after CDT no longer present independently distributed, but are aggregated in filamentary distribution, which is due to the streamer corona discharge as shown in Fig. 2(c). Higher-resolution SEM images (Figs. 2(h)–2(i)) show that the surface of filter paper is no longer relatively flat, but gully-shaped surface appeared. These SEM images prove that filter papers doped with AuNSs simply modified through p-CDT, the AuNSs can be aggregated and assembled into a special arrangement.
Compared with rigid SERS substrates, flexible SERS substrates can exhibit excellent flexibility on the detection of irregular surface. In order to more conveniently assemble AuNSs on a flexible substrate, here an innovative preparation approach combined with CDT has been explored.
3. SERS results and discussion
3.1 Raman detection of R6G on the flexible SERS substrate
In order to prove the SERS activity of the fabricated flexible substrate, the relationship between the CDT assembly state and the Raman spectra enhancement are investigated. Raman detection of R6G solution (10−5 mM) was carried out under p/n-CDT with different treatment times, as shown in Fig. 3. The discharge parameters such the current, voltage, and discharge distance were set as 15 µA, ±15 kV, and 1.5 cm, respectively. For p-CDT, Fig. 3(a) shows the Raman spectra of R6G on prepared SERS substrates of different treatment time from 20 minutes to 60 minutes with a 10 minutes’ interval. The locations of R6G characteristic peaks are 610 cm−1 (C-C-C aromatic ring bending), 774 cm−1 and 1190 cm−1 (C-H aromatic ring bending), 1602 cm−1 (C = C stretching) and 1308 cm−1, 1360 cm−1, 1505 cm−1, and 1644 cm−1 (C-C stretching), respectively, which have a little red-shift with the standard characteristic peaks, as shown in Fig. 3(b). All characteristic peaks of R6G are enhanced via increasing the p-CDT time compared with the spectrum on the flexible substrate without CDT, which prove great SERS enhancement ability due to the self-assembly of AuNSs. The intensity of R6G fingerprint peaks reach the maximum when the p-CDT time is 40 minutes, and it is great than twice that when the p-CDT time is 20 minutes. Thereafter, the Raman enhancement effect gradually deteriorated. Furthermore, to evaluate the substrate-to-substrate reproducibility of flexible SERS substrate, 3 flexible SERS substrates via the same preparation process were investigated. Figure 3(c) shows a fairly consistent trend of change in SERS intensity for the main characteristic peaks of R6G with different p-CDT time.
Fig. 3. Raman spectra of SERS substrate after CDT. (a) Raman spectra of R6G on flexible substrate with time-dependent p-CDT. (b) The red-shift and (c) intensity of R6G fingerprint peak under different length of p-CDT time. The error bars are a standard deviation from the mean. (d) Raman spectra of R6G on flexible substrate with time-dependent n-CDT. (e) The red-shift and (f) intensity of R6G fingerprint peak under different length of n-CDT time.
Combining with the prior SEM and TEM images in Fig. 1 and Fig. 2, it is possible to deduce that increasing the CDT duration causes AuNSs to cluster in a filamentary pattern and shortens the distance between AuNSs, resulting in the local EM field amplication. However, when the CDT duration time further increases, the AuNSs appear coalesce, the local EM field gradually diminishes, and the Raman enhancement effects of the SERS substrates deteriorate.
Therefore, even at a very low concentration, R6G molecules can still produce a strong Raman signal on the as-prepared flexible SERS substrate with p-CDT. In addition, EF is one of the most important parameters for characterizing the ability of a SERS substrate in Raman applications, which can be defined as the following equation [43]
Table 1. Raman enhancement effect of R6G characteristic peaks under different discharge time.
According to the previous discussion, for n-CDT, the negative ions produced by air ionization move to the flexible substrate, resulting in AuNSs repelling on the substrate. As expected, the substrates after n-CDT process exhibited very low Raman signal, as shown in Figs. 3(d)–3(f). With the same treatment time, the substrates with the p-CDT exhibit a much better performance. Therefore, our experiments show an effectively approach for flexible SERS substrates preparation by simply assembling of AuNSs with p-CDT.
3.2 Numerical verification of CDT based self-assembly principle
Furthermore, the FDTD method-based simulation was employed to verify the above analysis, as shown in Fig. 4. AuNSs were modeled as spheres with experimental dimensions, and the relative permittivity values for gold are taken from the experimental measurements [44]. Due to the small particle size, a minimum mesh size of 0.2 nm was applied to the structure.
Fig. 4. FDTD simulations of the electromagnetic properties of AuNSs. (a) The extinction cross sections of AuNSs with radius from 20 to 28 nm. (b–d) Calculated logarithmic |E/E0|2 profiles with different gap distances at 532 nm excitation wavelength, where E and E0 are the total and incident electric fields, respectively. (e) Log calculated EF of double AuNSs as a function of the gap distances from −2 to 10 nm. (f–i) Calculated logarithmic |E/E0|2 profiles with different numbers of AuNSs at 532 nm excitation wavelength. The gap distance of AuNSs is 1 nm. (j) Normalized |E/E0|2 density as a function of different particle numbers from 1 to 20.
The extinction properties of individual AuNSs were investigated first. The extinction cross-section of the AuNSs can be can be defined as [45]:
The characteristic of the SERS substrate prepared in this paper is using p-CDT to assemble AuNSs. In this case, the local electric field enhancement is considered to be the main reason for the substantial increase in the Raman signal. To reveal this enhancement mechanism, single and double AuNSs excited by a 532 nm laser were considered.
Figures 4(b)–4(d) show the electric field enhancement |E/E0|2 profiles on the logarithmic scale with different gap distances, while the more detailed are provided in Fig. S3. The coupling enhancement electric field of double AuNSs is significantly stronger than that of single AuNSs.
The magnitude of EM enhancement can be approximately evaluated by [45]:
where E and E0 are the total and incident electric fields, respectively. In Fig. 4(e), the calculated EFEM values of double AuNSs reach a maximum at the gap distance of 1 nm. It can be seen that using p-CDT to assemble AuNSs on the flexible substrate will shorten the distance between AuNSs, thereby building hot spots and generating local electromagnetic field enhancements, which can greatly increase the SERS signal strength and increase the Raman detection sensitivity. In particular, the AuNSs appear coalesce with further increase of the p-CDT duration time, so the spacing of the double AuNSs was set to −1 or −2 nm to indicate that they have overlapped. But such coalesce will reduce the enhanced intensity of the local field, which is consistent with the gradual weakening of the Raman signal as the p-CDT time exceeds 40 minutes in Fig. 3(a).As mentioned before, filamentary distribution of AuNSs formed due to the streamer corona discharge. In this case, AuNSs were modeled as a row, and considering the minimum thickness of the sodium citrate coated on AuNSs is ∼0.5 nm [22,46,47], so the gap distance was set to 1 nm. Figures 4(f)–4(i) show that the filamentary distribution of AuNSs can significantly increase the density of hot spots in a certain volume, and more R6G molecules can be adsorbed in the hot spots’ areas, so such a distribution can effectively increase the SERS signal intensity. The more detailed distribution images are provided in Fig. S4. Such a distribution can also effectively increase the SERS signal intensity. Figure 4(j) shows the normalized curve of the electric field energy density of different particle numbers. As the number of AuNSs increases, the electric field energy density also gradually increases.
In general, AuNSs assembly on flexible substrate through p-CDT will bring two effects. One is to reduce the gap distance of AuNSs, and the other is to make AuNSs arranged in filament. And both of these phenomena will cause Raman signal enhancement.
4. Conclusions
In summary, we experimentally demonstrated corona discharge induced self-assembly of AuNSs on a flexible SERS substrate. A simplified hydro-thermal technique was used to create AuNSs with great homogeneity. AuNSs may self-assemble into filament configurations directly on filter paper using CDT, effectively improving SERS signal intensity before and after construction. The highest EF for detection R6G may be 2.79×106. Simulation findings have also demonstrated the SERS enhancing impact. The CDT based self-assembly method makes it feasible and convenient to provides low-cost, easy-to-process, high-yield, high sensitivity and activity flexible SERS substrate, which holds potential applications of SERS in the detection of various chemicals, biological molecules, and environmental contaminants.
Funding
National Natural Science Foundation of China (61705058); Fundamental Research Funds for the Central Universities (B210202150); Changzhou Science and Technology Program (CJ20200073); Nantong Science and Technology Program (JC2021071).
Acknowledgments
Wei Su and Jian Wu supervised the project.
Disclosures
The authors declare no conflicts of interest.
Data Availability
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.
Supplemental document
See Supplement 1 for supporting content.
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