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Abstract
Surface-enhanced Raman scattering (SERS) is widely considered to be a fingerprint spectrum that can realize molecular identification, and it continues to receive a lot of attention due to its high sensitivity and powerful qualitative analysis capabilities. In recent years, there has been a lot of work and reports on super-sensitive SERS substrates, but often the enhanced ability of the substrate is also effective for impurities and irrelevant molecules. Therefore, a problem that still remains to be solved is how to perform effective trace detection of specific substances in a complex detection environment. Herein, a superhydrophobic Ag nanoparticle/glass microfibre filter (AgNP/GF) substrate was designed to realize the Raman detection of complex multiphase solutions. The hydrophobic three-dimensional net-like structure provides efficient Raman enhancement, making the substrate have extremely high detection limits for dye molecules and even achieving specific detection of the hexane phase component (thiram molecule) in a multiphase solution.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As one of the most powerful qualitative analysis methods, Surface-enhanced Raman scattering (SERS) has attracted a lot of attention in recent years due to a series of unique advantages [1–9]. SERS has ultra-high detection sensitivity, quick and simple detection process, non-invasion and label-free detection way, which makes it have excellent performance in various fields such as biological testing and chemical analysis, etc. [10–17] The ultra-high sensitivity of SERS is mainly achieved through surface plasmon resonance, whose main feature is to use a ‘hot spot’ area with a small volume for signal enhancement. This mechanism of realizing regional Raman signal amplification mainly by constructing precious metal micro-nano structures is called electromagnetic enhancement (EM) [18,19]. EM is the main source of enhancement for most SERS substrates, and its enhancement capability can even be as high as 106-108 so as to realize the detection of single analyte molecules [20–22].
However, the enhancement of EM acts on a specific spatial region rather than on the molecule to be tested, which is disadvantageous for the testing of complex samples. On the one hand, the enhancement effect of EM mainly exists in the hot spot area, which only occupies a very small part of the entire substrate space structure [23,24]. Taking metal nanoparticles as an example, previous studies have shown that the particle gap is a typical hot spot area with a strong enhancement effect, and the smaller the gap, that is, the smaller the volume of the hot spot area, the higher its enhancement ability. This results in that as the enhancement capability of the hot spot increases, the number of analyte molecules that can be enhanced by the hot spot will decrease, and the probability that the molecules will be covered or captured by the hot spot area will decrease, which seriously affects the accuracy of detection [25]. In addition, for analytes containing impurities, if the substrate itself does not have screening and specific adsorption capabilities, the impurities and molecules located at the hot spots will be simultaneously affected by EM. Under this premise, it is difficult to achieve specific enhancement of a single component of a complex or mixed test object by the substrate. Therefore, a large number of previously reported work based on SERS substrates is usually only applicable to research in the laboratory, because its analyte is often a single component with high purity. However, in a complex testing environment, such as sewage detection, the actual pollution source in sewage is often multiphase mixed solution with complex components [26]. Thus, realizing to use SERS for analysis or specific detection of complex samples and expanding its scope of application has increasingly become an important requirement for the development and promotion of SERS technology.
The wettability control of the SERS substrate provides a simple and effective solution. For liquid-phase analytes, the hydrophobic and hydrophilic surface of the substrate is conducive to regulating the dispersion of analyte molecules, so that can make more analyte molecules located in the hot spot area, then achieve a more effective enhancement [27,28]. In addition, a reasonable surface modification can not only enable the aggregation of molecules on the substrate, but can even achieve the directional extraction of molecules dissolved in a multiphase solution [29]. According to previous reports, the hydrophobic slippery platform was often employed as a concentrator to achieve the aggregation of analyte molecules, and the three-dimensional porous structure such as filter paper was often used for separation of multiphase solutions [30,31]. Both the two structures and uses are of great help to SERS detection in complex environments.
Herein, a superhydrophobic Ag nanoparticles/glass microfibre filter (AgNPs/GF) substrate was designed and prepared. The filter paper-like three-dimensional structure can achieve efficient oil-water separation after a hydrophobic treatment process, so as to realize the Raman detection of complex multiphase solutions. The substrate is confirmed to have excellent Raman enhancement effects for three commonly used Raman probe molecules (rhodamine 6 G, malachite green and crystal violet), which was prepared by simple magnetron sputtering and annealing. In addition, after the gas bath treatment using perfluorosilane vapor, the substrate became hydrophobic and can only allow the oil phase in the multiphase mixed solution to pass through, thereby achieving separation and selective detection of complex solution samples. A mixed solution of hexane and water was used as a simulated sewage solution to be tested, and the results proved that the substrate has almost complete separation effect for it, and its separation rate is as high as 95.6%. And it can easily realize the directional detection and concentration of thiram in the hexane phase.
2. Experimental
2.1 Materials
Acetone (CH3COCH3, AR, 99.5%), hexane (C6H14, AR, 99.5%), alcohol (C2H6O, 99.7%), Sudan I (C16H12N2O, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd., Glass microfibre filters (Silicate glass) were purchased from Shanghai Bitai Biotechnology Co., Ltd., and rhodamine 6G (C28H31N2O3Cl, AR), 4-aminothiophenol (C6H6NS, AR), malachite green (C23H25N2·C2HO4·0.5C2H2O4, AR), 1H, 1H, 2H, 2H-Perfluorodecyltriethoxysilane (C16F17H19O3Si, 96%), Crystal violet (C25H30N3Cl, AR) and tetramethyl thiuram disulfide (C6H12N2S4, 97%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All these materials were used without further purification.
2.2 Preparation of the hydrophobic AgNP/GF substrate
The preparation and processing method of the hydrophobic AgNPs/GF substrate is shown in Fig. 1. First, sputter a layer of Ag film on clean glass microfibre filter by magnetron sputtering. After that, the Ag-plated glass filter was placed in a tube furnace to be annealed under an argon atmosphere for 30 min, the temperature is set to 300°C with a rate of 10°C/min. The power of the RF magnetron sputtering process is 100 W, the target material used is a high-purity Ag target, and the sputtering time is 100s. After annealing, AgNPs/GF was obtained due to the conversion of the Ag film into nano-diameter Ag particles. Finally, place a beaker containing 3 mL of 7% perfluorosilane ethanol solution and the AgNPs/GF substrate in the same container. Seal and heat to 90 °C for a gas bath, after the solution completely evaporated, AgNPs/GF become hydrophobic.
2.3 SERS experiment
In all Raman inspections, the incident light is a laser with a wavelength of 532 nm, and its intensity is set to 0.48 mW. The integration time is set to 4 s and the grating is selected to 600 gr/mm. Each spectrum is an average of ten randomly measured spectra to ensure the rigor of the data.
3. Results and discussion
3.1 Characterization of the AgNP/GF structure
The scanning electron microscope (SEM) image of the prepared substrate is shown in Fig. 2(a). It can be seen from the image that the three-dimensionally crisscrossed distributed glass fibers and the AgNPs distributed on their surface form a hierarchical micro-nano structure, which is greatly favorable both for SERS performance and hydrophobicity. From the high-magnification SEM image Fig. 2(b), it can be observed more clearly that the AgNPs are evenly spread on the surface of the glass fiber. Randomly select 50 GFs and 100 AgNPs in the SEM image to count their diameter, and display the distribution result in the insert. From the result, it can be seen that the size of most GFs is concentrated between 0.5-2.5 μm, the size of the AgNPs is relatively uniform and concentrated between 60-100 nm. In order to verify the chemical composition of the substrate surface, the energy dispersive spectrum (EDS) was tested and shown in Fig. 2(c-f). Among all the detected elements, Si, O, Al, and Na come from the silicate of the glass fiber itself, and F comes from the F-containing functional group modified by perfluorosilane. Therefore, it can be confirmed that the assembly of AgNPs and hydrophobic modification are effective.
Fig. 2. (a, b) The SEM topographies of the AgNPs/GF substrate (The ruler in the figure represents a: 10 μm and b: 2μm); Insert: size distribution diagram of the GF and AgNPs. (c-f) The EDS spectra of the substrate.
3.2 Exploration of the SERS properties of the AgNP/GF substrate
After the substrate is prepared, a series of experiments are designed and completed in order to measure the Raman enhancement capability of the substrate. The three-dimensional network structure of the substrate itself can allow the formation of a large number of hot spots and molecular adsorption sites, which is conducive to the substrate being used as an excellent SERS enhancement platform. To confirm this, several different commonly used Raman probes (R6G, CV and MG) were selected as analytes, their ethanol solution was dropped directly on the surface of the substrate and diffused to eliminate the additional enhancement effect caused by hydrophobicity. The Raman spectra collected on the substrate dripped with different concentrations of R6G, CV and MG solutions are shown in Fig. 3. It can be confirmed from the figure that the substrate has high detection sensitivity, the detection limit of R6G, CV and MG molecules can reach 10−10 M, 10−9 M, and 10−9 M respectively. In addition, the Raman signal intensity in the spectra shows a good linear correlation with the solution concentration, which proves that the Raman detection results have high reliability. According to the commonly used formula to calculate the estimated enhancement factor (EF) to accurately evaluate the SERS performance of the substrate, there is:
Among them, ISERS and INR respectively represent the signal intensity of the Raman spectrum measured from the SERS substrate and the corresponding blank substrate under the same test parameters, which can be characterized by measuring the height of the Raman characteristic peaks. NSERS and NRS refer to the average number of molecules present in the laser spot when collecting Raman spectra on the SERS substrate and blank substrate. It is worth mentioning that when the analyte molecules are evenly distributed in the solution, the ratio of the latter can be approximated by using the ratio of the solution concentration to simplify the calculation process. The Raman spectra collected on the blank substrate which were dripped high-concentration Raman probe solution and corresponding SERS spectra are shown in Fig. 3(c). The related data of the three molecules are obtained and substituted into the above formula to calculate EFs. Calculated with the intensity of the characteristic peaks of 613 cm-1, 913 cm-1 and 1620 cm-1, the estimated EF of R6G, CV and MG can reach 1.85×106, 1.09×106 and 1.03×106, respectively.
Fig. 3. (a1) The Raman spectra and (a2) corresponding linearity analysis of R6G (b: CV, c: MG) solutions with different concentrations measured on the AgNPs/GF substrate; (a3) Raman spectrum of R6G solution (b: CV, c:MG, 10−2 M) collected on pure GF substrate.
The high EF of the substrate is attributed to the EM enhancement of AgNPs. From the ultraviolet-visible absorption spectrum, it can be observed that the substrate has strong absorption for the laser with a wavelength of 430 nm -760 nm (Fig. 4(a)). In addition, 4-aminothiophenol (4-ATP), a common pollutant in the water environment, is detected as an analyte. As shown in Fig. 4(b), 4-ATP with a concentration of 10−7 M can also be clearly detected; and no Raman peaks are observed on the bare substrate, which proves that the substrate itself does not produce impurity peaks that interfere with the test results. To improve the quality of the SERS characterization of the substrate, the uniformity and stability test is supplemented in Fig. 4(c-f) using R6G solution with a concentration of 10−7 M as the analyte. The Raman spectra on the substrates placed in the air for 1-7 days are shown in Fig. 4(c). Observing and comparing the characteristic peak intensities at 613cm-1, 774 cm-1, and 1365 cm-1, it can be found that as the storage time becomes longer, the enhancement ability of the substrate continues to decay, but the attenuation rate is gradually flat, and nearly 50% of the strength enhancement remains after being placed for a week. In addition, in the Raman mapping image measured on the AgNPs/GF substrate (step length is 2 μm, area is 20 μm×20 μm), the signal intensity varies with position is not obvious, and its relative standard deviation (RSD) is 13.63%, which showing good uniformity of the substrate.
Fig. 4. (a) Ultraviolet-visible absorption spectra of bare GF and AgNPs/GF substrate, (b) Raman spectra collected on a substrate dropping different concentrations of 4-ATP solution, (c-d) Raman spectra and characteristic peak intensity of 10−7 M R6G solution dropped on the substrate; (e-f) Raman mapping of the substrate dropping 10−7 M R6G solution.
The estimated EFs confirmed that the substrate after only simple silver-plated and annealed shows an excellent enhancement effect, which is in line with our expectations and design. For densely distributed three-dimensional hot spots, the most critical conditions that affect the Raman enhancement effect are the diameter and gap of AgNPs; and under appropriate annealing conditions, the thickness of the Ag film before annealing has a direct relationship with the morphology of the particles formed after annealing. According to previous reports, as the thickness of the Ag film increases, the grain diameter and gaps of AgNPs formed after annealing will also increase. However, too small grain diameter and too large gaps cannot achieve high Raman enhancement effects, thus there is a reasonable range of Ag film thickness, which can achieve the relatively best enhancement effect. In the exploration and control experiments, to achieve a great enhancement effect, the same glass fibers were plated with Ag films of different thicknesses and then annealed with the same parameters. The thickness of Ag film is adjusted by controlling the length of sputtering time, the substrates that have undergone the Ag sputtering process for 25 s, 50 s, 75 s, 100 s 125 s and 150 s are annealed and observed by SEM. The SEM images are shown in Fig. 5, and the SEM images of the Ag-plated and non-annealed substrate are shown as inserts for visual comparison.
Fig. 5. (a-f) The SEM morphology images of AgNPs/GF substrates after (a) 25 s, (b) 50 s, (c) 75 s, (d) 100 s, (e) 125 s and (f) 150 s of Ag sputtering and annealing. Insert: SEM image of the corresponding unannealed substrate (The ruler in the Figures and inserts represents 2 μm).
First of all, it can be seen from the illustration that the Ag after the magnetron sputtering process is originally a smooth film, and the morphology change caused by annealing is significant. It is also worth mentioning that the glass fiber itself can withstand a high temperature of 300 °C without deformation, which cannot be achieved by conventional filter paper or fiber, that is also the reason why we chose it as the substrate. In addition, for AgNPs formed after annealing, it can be seen from the figure that as the thickness of the Ag film increases, the diameter of the AgNPs which were formed after annealing gradually increases; meanwhile, the density of AgNPs decreases, which also means that the gap between the particles increases.
To verify their SERS performance, R6G solution with a concentration of 10−6 M were dropped on these substrates to be used as the analyte and the Raman spectra measured on these substrates are displayed and compared as shown in Fig. 6(a). It can be intuitively observed from the Raman spectra that as the sputtering time increases, the intensity of the Raman signal collected on the substrate after annealing continues to increase, and the substrate with a sputtering time of 100 s shows the highest Raman enhancement effect; when the sputtering time exceeds 100 s, as the thickness of the Ag film continues to increase, the SERS performance of the annealed substrate decreases instead. In addition, the diameter and gap of AgNPs on the surface of these substrates were measured and counted from the SEM images (shown in Fig. 6(c)), which also can conform and fully confirm the aforementioned assumption. Therefore, the sputtering time of the Ag film is set to 100 s as a parameter for substrate preparation process.
Fig. 6. (a) The Raman spectra comparison of R6G solution with a concentration of 10−6 M on the substrates with different sputtering time; (b) The characteristic peak intensity at 613 cm-1, 774 cm-1 and 1365 cm-1 in the corresponding spectra. (c) The size distribution diagram of particle gap and diameter.
3.3 Infiltration regulation and characterization
Commercial glass fiber membranes have strong hydrophilicity, which depends on the high surface energy and high roughness spatial structure of silicate glass. For a particular interface, its hydrophilic or hydrophobic behavior depends on its surface energy, and the high roughness only magnifies this behavior [32]. Therefore, the substrate was passed through a perfluorosilane gas bath to reduce its surface energy and then obtain a superhydrophobic surface. To characterize its hydrophobic effect, 4 μL of deionized water was dropped on the surface of the substrate to measure the contact angle (CA) of the water on the substrate. As shown in Fig. 7(a), for the pure glass fiber, water will directly infiltrate and diffuse into it, while the treated glass fibers exhibit excellent hydrophobic properties, and the contact angle of deionized water is 151.3°. For the prepared substrate, the hydrophobic effect will be further improved (CA=155.4°) because of the presence of AgNPs on the surface of the glass fiber, which further improves the surface roughness. The construction of the hydrophobic interface makes the specific selection of the substrate to the multiphase solution feasible. Here, a mixed solution of hexane and deionized water (dyed with R6G) is used as the solution to be separated, and the substrate is used as a filter to construct a simple oil-water separation system. The experimental setup and results are shown in Fig. 7(b). It can be found that hexane can pass through while water is trapped above the substrate. Calculated based on the quality of the separated water, the separation rate was as high as 95.6%, and no dyed water can be observed in the hexane clarified through the substrate. For further visual observation, the mixed solutions of two different dyeing methods were dropped on the substrates and the behavior of the solutions was observed. It can be clearly seen from Fig. 7(c) that for pure glass fiber, both water and hexane can penetrate into its interior. For hydrophobic substrates, hexane can penetrate into the substrate, while water will stay on top of the substrate. Observing the infiltration traces of solution I when the remaining water is removed, it can be seen that the color of the hexane permeation area is highly uniform, which proves that the presence of water will not affect the hexane permeation process.
Fig. 7. (a) CA and hydrophobic effect detection of different substrates; (b) Oil-water separation device and separation experiment effect; (c) Wetting behavior of simulated mixed-phase sewage solution on the substrate (before and after hydrophobic).
3.4 Raman detection of complex samples
The high Raman enhancement capability and oil-water separation capability of the substrate provide the possibility for the separation and detection of multiphase complex samples. A mixed solution containing 3 mL thiram hexane solution (10−4 M) and 3 mL R6G aqueous solution (10−5 M) was used as a simulated sewage solution, thiram (only soluble in hexane) and R6G (only soluble in water) were regarded as pollutant molecules to construct a Raman detection. The mixed solution was dropped on the back surface of the substrate and the hydrophobic substrate respectively, and after solution completely infiltrated, the Raman test was performed on the water stains oozing out of the front surface. From the measured Raman spectrum on the pure AgNPs/GF substrate (shown in Fig. 8(a)), it can be observed that on the pure substrate, only the characteristic peaks of R6G can be recognized because of its high signal intensity, and the signal of thiram is masked completely. And on the hydrophobic AgNPs/GF substrate, the characteristic peaks of Thiram are clearly displayed and the Raman signal of R6G cannot be observed, which fully proves that the substrate has the function of selective detection. In addition, the substrate can not only separate the hexane solution of the multiphase solution, but also can aggregate and concentrate it. In fact, in the actual environment sewage, the content of organic phase only accounts for a small part of the total volume, which makes effective means of aggregation become more important. To test the effect of the substrate gathering the organic phase, the process in Fig. 7(a) is repeated ten times, and the spectrum collected on the hydrophobic substrate is compared with the result of a single process (shown in Fig. 8(b)). It can be observed from the Fig. 8(b) that as the number of experiments increases, the signal intensity of thiram has increased significantly, but the Raman signal of R6G is still not observed, which makes it possible to detect trace amounts of the trace organic phase solution in the sewage.
Fig. 8. (a) Raman spectrum collected on pure AgNPs/GF substrate after filtering simulated mixed-phase sewage solution and (b) Raman Spectra collected on hydrophobic AgNPs/GF substrate. (Repeat filtering 1 time and 10 times).
4. Conclusions
This paper describes a method for preparing hydrophobic AgNPs/GF substrate, which has an excellent Raman enhancement effect and the EF of R6G, CV and MG can reach 1.85×106, 1.09×106 and 1.03×106 respectively. In addition, the excellent oil-water separation ability of the substrate allows it to perform specific detection of complex multiphase mixed solutions, thereby avoiding effective signals from being interfered or even masked. The experimental results show that this substrate can detect or even concentrate the thiram in the mixed solution. It can be proved that this solution provides a new idea for the detection task of complex samples.
Funding
State Key Laboratory of Bio-polysaccharide Fiber Formation and Eco-Textile (KF2020204); National Natural Science Foundation of China (11774208, 11904214, 11974222).
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.
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