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Abstract
Surface-enhanced Raman scattering (SERS) sensors combined with superhydrophobic/superhydrophilic (SH/SHL) surfaces have shown the ability to detect ultra-low concentrations. In this study, femtosecond laser fabricated hybrid SH/SHL surfaces with designed patterns are successfully applied to improve the SERS performances. The shape of SHL patterns can be regulated to determine the droplet evaporation process and deposition characteristics. The experimental results show that the uneven droplet evaporation along the edges of non-circular SHL patterns facilitates the enrichment of analyte molecules, thereby enhancing the SERS performance. The highly identifiable corners of SHL patterns are beneficial for capturing the enrichment area during Raman tests. The optimized 3-pointed star SH/SHL SERS substrate shows a detection limit concentration as low as 10−15 M by using only 5 µL R6G solutions, corresponding to an enhancement factor of 9.73 × 1011. Meanwhile, a relative standard deviation of 8.20% can be achieved at a concentration of 10−7 M. The research results suggest that the SH/SHL surfaces with designed patterns could be a practical approach in ultratrace molecular detections.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) is one of the most promising candidates for detecting ultra-low concentration molecular [1–4]. It has been widely used in biomedicine, chemical analysis, environmental monitoring, and surveillance [5–9]. The sensitivity of SERS detection depends on the surface nanostructure induced “hot spot”, which can produce an enhanced electric field and improve SERS intensity [10–14]. In general, the SERS-active substrate should be immersed in the target solution to functionalize the hot spots, yet most of the target solution is wasted [15–17]. Moreover, the detection of ultra-trace molecules from solutions with a limited amount is vital for precancer diagnosis, forensic analysis, and food safety [18,19]. However, on a common hydrophilic SERS substrate, the coffee ring effect tends to hinder the targeted molecules from being concentrated in a limited area.
Recently, to improve the SERS performance, superhydrophobic (SH) surfaces have been widely used to suppress the molecular diffusion caused by the coffee ring effect [20–24]. For example, Sun et al. put forward a strategy to prepare an SH surface with a rolling property, and the caused droplet continuous-rolling motion in a synergistic way can achieve a detection limit of 10−15 M [22]. George et al. fabricated flexible superhydrophobic SERS substrates by depositing redox Ag on a femtosecond laser-written hierarchical surface, and a detection limit of 8 femtomolar can be achieved [21]. However, on the pure SH surface with low adhesion, the rolling property of droplets may lead to randomness in the location of deposited molecules, which makes the enriched colorless analyte hard to be targeted by the detection laser.
To achieve the localized enrichment of analytes, surfaces with wettability differences were introduced [25–29]. By fabricating high adhesion superhydrophilic (SHL) patterns on SH SERS substrates, the analyte molecules can be concentrated to a predetermined position. Li et al. fabricated SHL Au-areoles arrays on an SH substrate using selective electrochemical deposition, and a sensitivity of 10−15 M can be achieved [28]. Yang et al. proposed a facile and scalable strategy to fabricate a hybrid SERS-active substrate by femtosecond laser, achieving a detection limit of 10−14 M [29]. These studies proved that the hybrid SH/SHL surfaces with the unique capabilities of manipulating drops had been successfully applied in ultratrace detection. Nevertheless, most preceding researches only focus on the SERS enhancement achieved by the SH/SHL surfaces with a simple circular or square shaped SHL pattern [25–28]. After the droplet evaporates, the precipitated analyte is steadily and evenly distributed ate the edge of the SHL pattern, and only part of the sediment contributes to the SERS detection. Therefore, it may provide a way to further improve the utilization of analytes by studying the deposited molecules’ distribution on different shaped SHL patterns.
To prepare SHL patterns with different sizes and shapes on SH surfaces, laser microprocessing is a simple and effective technique that has been proven successful in various fields [30]. In this study, femtosecond laser fabricated hybrid SH/SHL surfaces with designed patterns on the copper sheet were used to investigate the distribution behaviors of sediment after droplet evaporation. With femtosecond laser ablation and chemical modification, SHL patterns with different sizes and shapes, such as circle, square, equilateral triangle, and star, can be easily prepared on SH surfaces [31,32]. Via drops containing polystyrene (PS) nanoparticles (NPs), the precipitate distribution of analyte droplets on patterned SH/SHL surfaces was observed and investigated [33,34]. Via a confocal Raman microscope, the Raman signal intensity of target molecules on patterned SH/SHL surfaces with different sizes and shapes was measured and recorded. Meanwhile, the suitable shape and size on hybrid SH/SHL surfaces with designed patterns were discussed. Finally, the Raman spectra of analyte droplets with various concentrations were measured to demonstrate designed SHL patterns have an effect on the limit of detection. The SH/SHL surfaces with specific patterns may be a practical approach to improve the limit of Raman detection.
2. Experimental section
Preparation of the sample substrate: Due to the high critical pulse width for ultra-fast laser cold processing (Supplement 1, Table S1) [16–36], high-purity (99.9%) copper sheets are selected as the substrates. Copper sheets with a dimension of 1 × 30 × 30 mm3 were ultrasonically bathed in ethanol for 10 min. Then, a femtosecond pulsed laser galvo-scanning system was applied to texture the sample surface in ambient air, as shown in Fig. 1(a). A 520 nm femtosecond laser beam (Spectra-Physics Spirit HE 1040-30-SHG) with a pulse width of 300 fs at a 250 kHz pulse repetition rate is focused onto the sample surface by an F-Theta lens at a focal spot size of 16 µm in diameter.
Fig. 1. (a) Schematically illustration of the laser fabrication process for SH and patterned SH/SHL surfaces. (b) Laser scanning path of SH surface. (c) Different SHL patterns used in this experiment.
The laser processing parameters for each step are presented in Table 1. The grid structure, as shown in Pattern 1 of Fig. 1(b), was obtained using the optimized laser processing parameters: laser fluence of 6.8 J/cm2, scanning spacing of 50 µm, and scanning speed of 50 mm/s. In order to obtain high processing efficiency, the laser fluence used for preparing the SH surfaces is significantly higher than the ablation threshold of Cu (Supplement 1, Table S2) [37]. Due to the high surface energy of copper, the femtosecond laser textured samples were immersed into 0.01 M ethanol stearic acid solutions for 2 min to obtain superhydrophobicity [38–41]. Also, additional laser ablation was required to selectively remove the chemical modification layer to obtain designed SH/SHL patterns. The specific parameters are as follows: laser fluence of 6.8 J/cm2, scanning spacing of 10 µm, and scanning speed of 50 mm/s. The structural depth of the SERS substrate after laser ablation is shown in Supplement 1, Fig. S1. Different patterns used in this experiment are illustrated in Pattern 2 of Fig. 1(c). Finally, a 50 nm Ag layer (Table 2) was deposited by a high-vacuum evaporation technique (Wuhan Wakesc Technology Co., Ltd, QM-1A) on the laser-ablated Cu sheet for SERS detection. The laser-prepared SERS substrate should be stored in a vacuum and dry environment to maintain durability and stability [42,43].
Table 1. Laser processing parameters for preparing the SERS substrates
Table 2. Thermal evaporation deposition parameters for preparing the SERS substrates
Characterization: A field-emission scanning electron microscope (Hitachi, Su 8010) and a 3D measuring laser microscope (Olympus, OSL5000) were applied to characterize the morphologies of the Cu surface irradiated by the laser. The video optic contact angle instrument (Dataphysics, SL200 KS) was used to measure static contact angles (CA) of the SH copper substrate. The final CA value was averaged by measuring three different points on the Cu sheet surface.
SERS Capability Characterization: To evaluate the SERS performance of the SH/SHL surfaces with designed patterns, a 5 µL R6G solution droplet was deposited and dried on the prepared SERS substrate to allow the formation of self-assembled monolayer. A confocal Raman microscope (French, Horiba, LabRAM Odyssey) equipped with a 532 nm excitation source was used for all experimental characterization. The Raman signals were recorded through a 50× microscope lens and 0.5 mW laser power with an integration time of 3s.
3. Result and discussion
Figure 2 shows the surface morphologies of the femtosecond laser textured Cu sheet. After the first laser scanning, the material surface presents periodic micro-pillars, which are covered with plentiful laser-induced NPs (Fig. 2(c) and (d)). The depth of the laser fabricated grooves is about 28.6 µm (Supplement 1, Fig. S1). The Energy Dispersive X-Ray Analysis (EDX) mapping of the laser textured surface shows the femtosecond laser ablation process of copper is accompanied by material oxidation (Supplement 1, Fig. S2). With the subsequent stearic acid modification, this type of composite structure endows the surface with superhydrophobicity and ultralow adhesion. The inset in Fig. 2(a) shows that the CA is up to 154° and the SA is only 3°, which ensures that the drop’s contact line (CL) on this SH surface is very small and can recede freely. After the stearic acid was removed with the second laser scanning (Supplement 1, Fig. S1), the central area became superhydrophilicity, which is beneficial for localizing the droplet. The inset in Fig. 2(a) shows the CA of the SHL pattern is nearly 0°. After a 50 nm Ag layer was uniformly deposited, the laser-ablated Cu sheet was covered with a large number of Ag nanoparticles (Supplement 1, Fig. S3). The densely aggregated Ag nanoparticles provide nanoscale gaps, which can act as “hot spots” for SERS signal enhancement [23]. Meanwhile, Fig. 2(e-j) illustrates that the shapes of the deposited patterns can be controlled by the SHL patterns, which can be not only regular geometric shapes but also intricate star-shaped patterns.
Fig. 2. (a) 3D profile of the surface and (b-d) SEM images of the laser-textured SH Cu surface. (e-j) Optical images of PS NPs deposited on patterned SH/SHL surfaces.
For a drying analyte droplet, the sediment’s area and distribution determine the SERS detection accuracy. Figure 3 shows the effects of different SHL patterns on the deposition characteristics of particle-laden droplets. When a droplet is drying on the SH surface with a circular SHL pattern, the CL recedes freely on the SH surface first. Then the CL is pinned at the edge of the SHL pattern, and the sediment is also mainly distributed along the SHL pattern’s edge [29]. When the SHL pattern is not circular, such as triangle, square, star, and so on, part of the CL will take the lead in contact with the SHL pattern during the receding process (Fig. 3(a) and (d)). The CL on the SH surface continues to shrink inward, causing the droplet to evaporate asymmetrically in both radial and azimuthal directions along the CL with non-uniform curvature [30]. As shown in Fig. 3(b), on the equilateral triangle SHL pattern, the faster evaporation rate at the vertex corners causes a local high evaporation flux. This results in a large volume loss and an internal capillary flow towards the vertex is generated to replenish the lost volume, as reported in previous studies [44,45]. However, on the star-shaped SHL pattern with concave corners, the CL on the SH surface can shrink to a position closer to the center. The analyte’s enrichment effect caused by the further shrinkage of the CL is more significant than that by the internal capillary flow towards the vertex. The related phenomena can be clearly demonstrated by the sediment distribution characteristics on the star-shaped SHL patterns with the different number of corners (Fig. 2(h-j) and Fig. 3(f)). The vertex and concave corners with high identification are beneficial for capturing the enrichment area during Raman tests.
Fig. 3. (a-b) Schematics of droplet evaporation and (c) optical image of PS NPs deposited on the equilateral triangle patterned SH/SHL surface. (d-e) Schematics of droplet evaporation and (f) optical image of PS NPs deposited on the 3-pointed star patterned SH/SHL surface.
Different regions of the SH/SHL pattern can lead to various SERS performances (Fig. 4(a)). After a 5 µL R6G droplet evaporates on the silver-deposited SH/SHL surface, the characteristic Raman peaks at 611, 776, 1190, 1315, and 1366 cm-1 can be observed (Fig. 4(b)). The most prominent peak at 611 cm-1 is selected to characterize the Raman enhancement effect. Notably, the Raman signal of 10−6 M R6G solutions in region 2 is found to be significantly stronger than that in region 1 ((b)). This result is further confirmed by the Raman mapping image (Fig. 4(a)). The reason is that the target molecules concentrated at the SHL pattern’s edge are much more than that at the inner region [46]. Therefore, the subsequent Raman detections were conducted at the edges of the SHL patterns.
Fig. 4. (a) SEM and Raman mapping images of the circle patterned SH/SHL surface. (b) Raman spectra of 10−6 M R6G solutions at region 1 and 2. (c) Raman spectra of 10−6 M R6G solutions on surfaces with different SHL pattern diameters 300 µm, 400 µm, and 500 µm, respectively.
The SHL pattern’s size has proven to be a critical factor for SERS performance. As shown in Fig. 4(c), three circular SHL patterns with different sizes were fabricated on the SH surface to conduct the comparative experiment. As shown in Fig. 4(c), the Raman signal obtained on the circular pattern with a diameter of 400 µm shows a maximum intensity of 4965 counts. Increasing or decreasing the diameter would result in the attenuation of the Raman signal. This is because the reduction of the SHL pattern can enhance the analyte’s enrichment. However, the target molecules cannot be infinitely concentrated by decreasing the SHL pattern’s area. On the circular SHL pattern with a diameter of 300 µm, a portion of the target molecules is deposited on the surrounding SH surface. A similar phenomenon has been verified by previous research [29]. Therefore, an excessive reduction in the size of the SHL pattern would lead to a decrease in SERS intensity.
To further explore the influence of different SHL patterns on the SERS performances, variously shaped SHL patterns were fabricated for comparative experiments (Fig. 5). For the square, equilateral triangle, and 3-pointed star SHL patterns, the circumcircle diameters were set as 400 µm (Fig. 5(a-c)). As shown in Fig. 5(d-f), the Raman signal at the edge is much stronger than that at the center region, which can be observed on all the SHL patterns. In addition, the Raman signal detected at the vertex of the square pattern is much stronger than that at the side. A similar phenomenon can be observed on the equilateral triangle SHL pattern. The reason is that, during evaporation, the capillary flows induced by the relatively fast evaporation at the pinned CL can carry more target molecules toward the vertices [47,48]. However, on the 3-pointed star SHL pattern with concave corners (Fig. 5(c)), the SERS intensity at the concave corner is higher than that at the vertex region (Fig. 5(f)). A possible explanation is that the concave corner is closer to the center. In this case, the enrichment effect induced by the further shrinkage of the CL is stronger than that by the capillary flow moving to the vertex. Meanwhile, the Raman mapping method was used to show the molecular enrichment at different locations more clearly. The regions with strong Raman signals are basically consistent with the distribution of analyte molecules (Supplement 1, Fig. S4, and Fig. 3). For each pattern, the highest Raman data are selected to compare the SERS performance, as shown in Fig. 5(g). The non-circular SHL patterns all show stronger signals than the circular pattern, and the 3-pointed star SHL pattern possesses the maximum SERS intensity of 11521 counts.
Fig. 5. SEM images of the hybrid SH/SHL surfaces, on which the shapes of the SHL patterns are (a) a square, (b) an equilateral triangle and (c) a 3-pointed star, respectively. (d-f) Raman spectra of 10−6 M R6G solutions at different parts of the SHL pattern. (g) Raman spectra of 10−6 M R6G solutions on surfaces with different SHL pattern shapes.
To further investigate the effect of star-shaped patterns on the SERS performance, 4-pointed or 5-pointed stars are prepared on the SH surfaces (Fig. 6(a-c)). For all the star-shaped SHL patterns, the SERS intensities detected at the concave corners are stronger than those at the vertexes with 10−8 M R6G solutions (Fig. 6(d-f)). However, the increase in the number of star corners did not result in an improvement in the SERS performance. As shown in Fig. 6(g), the Raman signal detected on the 3-pointed star SHL pattern still shows the maximum intensity of 7836 counts. The reason may be that the fewer concave corners can avoid the dispersion of the target molecules. In addition, the 3-pointed star pattern with the smallest area is more favorable for the analyte’s enrichment.
Fig. 6. SEM images of the hybrid SH/SHL surfaces, on which the shapes of the SHL patterns are (a) a 3-pointed star, (b) a 4-pointed star, and (c) a 5-pointed star, respectively. (d-f) Raman spectra of 10−8 M R6G solutions at different parts of the SHL pattern. (e) Raman spectra of 10−8 M R6G solutions with different SHL patterns.
As shown in Fig. 7(a), the detection limit of the 3-pointed star SHL/SH SERS substrate is investigated with various R6G concentrations, ranging from 10−16 M to 10−7 M. By analyzing the characteristic Raman peak at 611 cm-1, it can be seen that the SERS intensity increases with the solution concentration (Fig. 7(b)). The displayed peaks of 10−15 M R6G indicate that the star-shaped SH/SHL SERS substrates are able to trace extremely low-concentration analytes. According to the calculation method, the corresponding enhancement factor (EF) is up to 9.73 × 1011 [49,50]. Such a detection limit and EF are highly impressive (Supplement 1, Table S3) [51–54]. Additionally, eight SERS substrates with the same 3-pointed star SHL pattern were prepared to explore the SERS signal uniformity (Fig. 7(c)). As shown in Fig. 7(d), a relative standard deviation (RSD) of 8.20% can be obtained from the SERS spectra of 10−7 M R6G, indicating relatively high reproducibility.
Fig. 7. (a) Raman spectra of R6G with different concentrations. (b) SERS intensities at 611 cm−1 for R6G concentrations ranging from 10−15 to 10−7 M. (c) SERS spectra of 8 SH/SHL pattern with a 3-pointed star with 10−7 M R6G analytes. (d) Comparison of SERS intensities using the eight SERS spectra extracted from (c).
4. Conclusion
In summary, a remarkably improved SERS enhancement has been successfully achieved on hybrid SH/SHL surfaces with optimized SHL patterns. The femtosecond laser is a facile and efficient tool to control the size and shape of SHL patterns on the SH surfaces. A series of experiments were conducted to investigate the droplet deposition characteristics and SERS performances on the differently patterned SH/SHL surfaces. The experimental results show that the uneven droplet evaporation at the edge of the non-circular SHL patterns is beneficial to the enrichment of the analyte molecules, resulting in improved SERS performance. Furthermore, the highly identifiable vertex and concave corners of SHL patterns can improve the convenience and efficiency of the Raman test. On a 3-pointed star SHL pattern with a 400 µm circumcircle diameter, a detection limit concentration as low as 10−15 M can be achieved by using only a 5 µL R6G droplet, corresponding to an EF of 9.73 × 1011. The RSD of the Raman signal at a concentration of 10−7 M can reach 8.20%. The research results suggest that the patterned SH/SHL SERS substrate could be a practical approach in ultratrace analysis.
Funding
Open Project Program of Wuhan National Laboratory for Optoelectronics (2022WNLOKF015); Shenzhen Key Project for Technology Development (JSGG20191129105838333); Natural Science Foundation of Top Talent of Shenzhen Technology University (2019010801005); Natural Science Foundation of Top Talent of SZTU (2020103); National Natural Science Foundation of China (72071149).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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