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Abstract
The use of surface-enhanced Raman scattering (SERS) spectroscopy for the detection of substances in non-volatile systems, such as edible oil and biological cells, is an important issue in the fields of food safety and biomedicine. However, traditional dry-state SERS detection with planar SERS substrates is not suitable for highly sensitive and rapid SERS detection in non-volatile liquid-phase systems. In this paper, we take contaminant in edible oil as an example and propose an in situ SERS detection method for non-volatile complex liquid-phase systems with high-performance optical fiber SERS probes. Au-nanorod clusters are successfully prepared on optical fiber facet by a laboratory-developed laser-induced dynamic dip-coating method, and relatively high detection sensitivity (LOD of 2.4 × 10−6 mol/L for Sudan red and 3.6 × 10−7 mol/L for thiram in sunflower oil) and good reproducibility (RSD less than 10%) are achieved with a portable Raman spectrometer and short spectral integration time of 10 s even in complex edible oil systems. Additionally, the recovery rate experiment indicates the reliability and capability of this method for quantitative detection applications. This work provides a new insight for highly sensitive and rapid SERS detection in non-volatile liquid-phase systems with optical fiber SERS probes and may find important practical applications in food safety and biomedicine.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful tool for molecule detection due to its significant advantages such as fingerprint spectral characteristics, high sensitivity, simple sample pretreatment and short detection time [1,2]. The use of SERS spectroscopy for the detection of substances in non-volatile systems, such as edible oil [3] and biological cell [4,5], is an important issue and has potential prospects in the fields of food safety and biomedicine. For SERS detection with traditional planar SERS substrates, dry-state [6,7] or dynamic SERS detection methods [8,9] are usually adopted, where a droplet of tested solution is dripped onto planar SERS substrates, and SERS detection is performed when the solvent is completely or partially evaporated. Obviously, these detection methods are not suitable for direct SERS detection in non-volatile liquid-phase systems. Up to now, in order to realize SERS detection in non-volatile systems, extraction with a volatile extractant to separate the analyte from the non-volatile solvent [10] or marking with antigen-antibody or fluorescent molecules [11,12] are usually required. However, the extraction or marking process increases the complexity of the experiments, and incomplete extraction or marking of analytes may affect detection accuracy.
Optical fiber SERS probes [13–15], combining the advantages of low-transmission loss of optical fiber and highly sensitivity of SERS technique, provide a feasible way for in situ SERS detection in non-volatile liquid-phase systems. By fabricating noble metal nanoparticles or nanostructures onto the surface of optical fibers [16–18] and coupling both Raman excitation light and SERS signal light into the optical fiber for transmission [19,20], in situ SERS detection can be achieved by simply dipping the modified end of an optical fiber into the liquid-phase system under test. However, because of the cylindrical surface, small cross-section, and chemical inertness of quartz optical fiber [21], the mature nano-fabrication methods for planar SERS substrates are difficult to be adopted directly for the fabrication of optical fiber SERS probes. Thus many nanostructures with high local electric field enhancement factor, such as nanoclusters, nanostars, nanoflowers, are difficult to be prepared onto optical fiber surfaces, which is not conducive to achieve high detection sensitivity with optical fiber SERS probes. Additionally, for the in situ SERS detection in complex liquid-phase systems, the relatively weak molecule adsorption behavior [22,23] and the “hotspot occupation effect” [24–26] furtherly reduce the SERS detection sensitivity. Therefore, it is urgent to improve the detection sensitivity of optical fiber SERS probes for satisfying the requirement of in situ SERS detection in non-volatile liquid-phase systems.
As is known, the cluster structures can provide large SERS enhancement factor due to the near-field optical coupling in tiny gaps [27,28]. Recently, template-guided self-assembly method [29] or lithography method [16,30] are used to fabricate nanoparticle clusters on optical fiber surfaces. However, these methods are usually time-consuming, complicated, and difficult to realize rapid, low-cost and batch preparations of cluster-patterned optical fiber SERS probes. In this article, a simple low-cost laser-induced dynamic dip-coating method was proposed to realize reproducible preparation of nanoparticle-cluster-patterned optical fiber SERS probes. By optimizing the experimental parameters of the inducing laser power and dipping-pulling cycles, lots of Au-nanorod clusters were successfully prepared on the facet of 105/125 µm multi-mode optical fiber and thus provided numerous SERS hotspots. On one hand, the strong localized optical field at the hotspot region near clusters can provide large optical gradient force, which is helpful for better capture of target molecules into the hotspot region; on the other hand, the numerous SERS hotspots can effectively reduce the influence of “hotspot occupation effect” as there are enough hotspots for tested molecules, and thus improve the in situ detection sensitivity significantly. Take contaminant in sunflower oil as an example, we perform in situ SERS detection in complex non-volatile liquid-phase system by the nanoparticle-cluster-patterned optical fiber SERS probes. High SERS detection sensitivity and good spectral reproducibility were achieved with a portable Raman spectrometer and a short spectral integration time of 10 s. This work may provide a new method for highly sensitive SERS detection in non-volatile liquid-phase systems, and find important applications in food safety and biomedicine.
2. Experimental
2.1 Chemical and materials
Rhodamine 6G (R6G), Sudan red and thiram with analytical grade were purchased from Sigma-Aldrich (Shanghai, China). Sunflower oil was purchased from a local supermarket in Dongguan, China. Multimode quartz optical fibers with its core and cladding diameter of 105 µm and 125 µm and numerical aperture of NA=0.22 were purchased from Yangtze Optical Fiber and Cable Company (YOFC, China), and Au-nanorod colloid with the type of NR-20-750 and optical density of OD=10 was purchased from NanoSeedz company (HongKong, China).
2.2 Preparations of optical fiber SERS probes
The optical fiber SERS probes were prepared by a laboratory-developed laser-induced dynamic dip-coating method. A 20 cm-long multimode quartz optical fiber was used for probe preparation. Both ends of the optical fiber were cleaved by an optical fiber cutter to obtain flat facets. One end of the fiber was connected to a laser ($785 \pm 0.3$ nm) with an optical fiber pigtail, and the other end was dipped into a presynthesized noble metal nanoparticle colloid. The middle of the optical fiber was fixed to the movable rod of a programmed dip-coater by a vertical optical fiber clamp for subsequent automatic dipping and pulling processes. Initially, we set the coordinate origin of the z-axis at a position approximately 0.2-0.3 mm above the colloid surface, where the optical fiber was just off the meniscus when it was pulled upward from the colloid. The dip-coating parameters, such as dipping speed of 1000 µm/s, upward pulling speed of 100 µm/s, residence time in air or in colloid of 2 s and total displacement of 1 mm, were controlled by a computer. After setting these parameters, the laser was turned on, and optical fiber SERS probes were fabricated after several dipping-pulling cycles near the colloid surface.
2.3 Characterization and measurements
High-resolution field-emission scanning electron microscopy (FESEM, GeminiSEM, ZEISS, Germany) was used to characterize the surface topographies of prepared optical fiber SERS probes. For SERS spectral collection, a 785 nm commercial portable Raman spectrometer (i-Raman Pro, BWTEK, USA) and dip-in optical-fiber detection mode were adopted, in which the modified end of optical fiber SERS probe was completely immersed in the solution under test. The SERS signals were obtained under the experimental parameters as Raman excitation laser power of 30 mW and spectral integration time of 2 s for R6G aqueous solutions and 10 s for edible oil samples.
2.4 Calculation of limit of detection (LOD)
The LOD was calculated with “$3 \sigma$ method” [31,32]: $I_{\text {dl}}=I_{\text {blank}}+3\sigma$, where $I_{\text {dl}}$ was the lowest detectable signal intensity, $I_{\text {blank}}$ was the mean signal intensity of blank spectra (containing no analytes) and $\sigma$ was the standard deviation of signal intensity for the Raman peaks of the analyte solution at its lowest detectable concentration. During the calculation, SERS spectra at different concentrations were collected to obtain the calibration curve between the SERS intensity of strongest Raman peak and the concentration of analyte. By substituting the value of $I_{\text {dl}}$ into the calibration curve equation, the LOD value was obtained.
3. Results and discussion
3.1 Nanoparticle-cluster-patterned optical fiber SERS probes
The experimental setup for the laser-induced dynamic dip-coating method is shown in Fig. 1. In the experiments, a low-cost and conventional multimode quartz optical fiber with its core and cladding diameters of 105 µm and 125 µm (NA=0.22, YOFC, China), and the Au-nanorod colloid (NR-20-750, OD=10) with its longitudinal surface plasmonic resonant wavelength located around 750 nm were used for probe preparation. It should be pointed out that in our previous works in Refs. [14,20] and other reported literature [13,18], the multi-mode optical fiber with larger core diameter (such as 200 µm) were usually adopted for probe fabrication. This is because larger cross-section indicates larger SERS interaction area and provides higher SERS detection sensitivity. However, as is known, when the cladding diameter of optical fiber is larger than 125 µm, some specialized fiber processing equipment (such as large-diameter fiber fusion splicer or cutter) is needed in the experiments, which is usually expensive and not conducive to the practical application of optical fiber SERS probes. So, in the following, we will demonstrates the feasibility of fabricating highly sensitive optical fiber SERS probes using conventional 105/125 µm multi-mode optical fiber.
Fig. 1. Experimental arrangement for fabrication of optical fiber SERS probes by laser-induced dynamic dip-coating method.
For the fabrication of optical fiber SERS probes by laser-induced dynamic dip-coating method, the optical fiber facet underwent several dipping-pulling cycles near the colloid surface under the irradiation of laser, and each dipping-pulling cycle contained one laser-induced evaporation self-assembly process. The detailed preparation mechanisms of the laser-induced dynamic dip-coating method can be found in Ref [20]. Briefly, the noble metal nanoparticles adsorbed the laser energy and converted it to heat, which caused obvious increase in the local temperature near the nanoparticles. This local high temperature brought a non-uniform evaporation inside a colloid droplet adhering to the optical fiber facet, and promoted the nanoparticles in the colloid to migrate towards previously deposited nanoparticles, forming nanoparticle clusters. Obviously, the inducing laser power, the residence time in air, and the number of dipping-pulling cycles were critical for the formation of nanoparticle clusters on optical fiber facet. According to our experiments, when the inducing laser power was between 40 mW to 50 mW, cluster structures were observed on the facet of a 105/125 µm optical fiber under different dipping-pulling cycles. It should be pointed out that the inducing laser power mentioned here refers to the output laser power from the end of optical fiber, and it was set to be 45 mW for probe preparations. Then, in order to set a suitable residence time in air, we observed under a microscope the evaporation process of a colloid droplet adhered to the fiber facet under the irradiation of 45 mW laser power. It was found that the droplet was completely evaporated in about 1.5 s. Therefore, the residence time in air during the probe fabrication was set to 2 s, which was slightly longer than the time for droplet evaporation completely. At last, the dipping-pulling cycle was optimized for the best SERS performances. Optical fiber SERS probes with different numbers of dipping-pulling cycles ranging from 5 to 25 were prepared by the laser-induced dynamic dip-coating method, and then used to measure the SERS spectrum of $1.0 \times 10^{-7}$ M R6G aqueous solution by a portable Raman spectrometer with 30 mW laser power and 2 s spectral integration time. The SERS spectra were background subtracted by the Raman spectrum of the optical fiber itself. For each case, five optical fiber SERS probes were prepared sequentially, and the averaged SERS spectra are shown in Fig. 2(a). The Raman peaks for R6G at 1126 cm$^{-1}$, 1181 cm$^{-1}$, 1310 cm$^{-1}$, 1362 cm$^{-1}$, 1509 cm$^{-1}$ and 1650 cm$^{-1}$ were clearly observed. For comparison, the SERS intensities for the strongest Raman peak at 1509 cm$^{-1}$ versus the numbers of dipping-pulling cycles are shown in Fig. 2(b). The SERS intensity first increased and then decreased, and the strongest SERS intensity occurred with approximately 13 dipping-pulling cycles.
Fig. 2. Optimization of the number of dipping-pulling cycles for the laser-induced dynamic dip-coating method. (a) SERS spectra of a $10^{-7}$ mol/L R6G aqueous solution measured by optical fiber SERS probes fabricated with different numbers of dipping-pulling cycles. Each spectrum was the average of measurements made with five optical fiber SERS probes. (b) SERS intensity of the strongest Raman peak at 1509 cm$^{-1}$ versus the number of dipping-pulling cycles, extracted from Fig. 2(a).
Figure 3(a-c) displays the SEM images of optimized optical fiber SERS probes. Many Au-nanorod clusters were almost uniformly distributed on the optical fiber facet, and each cluster contained dozens of Au-nanorods. The stable adsorption of Au-nanorods on optical fiber facet may origin from the Van der Waals force and the thermal-assisted adsorption process [33]. Different from the single-layered distribution of nanoclusters on previous 200/220 µm optical fiber facet [20], multi-layered distribution of nanoclusters were clearly observed in Fig. 3(c), which provided more hotspots for highly sensitive SERS detection. Furtherly, the SERS spectra of R6G with different concentrations were measured with the optimized optical fiber SERS probes, and the results are shown in Fig. 3(d). The measured detection limit as low as $1.0 \times 10^{-10}$ mol/L was achieved, which was almost the same as that in our previous work [20] though with only 1/4 cross-section area, and much lower than most of the reported results by other optical fiber SERS probes [18,34,35].
Fig. 3. (a-c) SEM images of optimized optical fiber SERS probes with different scale bars representing (a) 40 µm, (b) 3 µm, and (c) 250 nm.(d) The measured SERS spectra of R6G aqueous solutions with different concentrations by the optimized optical fiber SERS probes. The concentration unit of “M” means “mol/L”.
3.2 In situ SERS detection in a non-volatile liquid-phase system with optical fiber SERS probes
As mentioned above, the optimized optical fiber SERS probe prepared by laser-induced dynamic dip-coating method can provide numerous SERS hotspots due to the formation of nanoparticle clusters on optical fiber facet. It is expected that this kind of optical fiber SERS probes possess good SERS detection capability in non-volatile liquid-phase systems. Herein, without losing generality, we take edible oil as an example of non-volatile solvent, and attempt to detect contaminants in edible oil for illustrating the in situ SERS detection capability in non-volatile liquid-phase systems with cluster-patterned optical fiber SERS probes. As is known, the contaminants in edible oil can be divided into two main source categories. One involves impurities originating from the plant seeds themselves, such as pesticide residue or spoilage [36]; the other category involves impurities originating from processing contaminants imparted during production, transportation or storage, such as bacteria or illegal additives [37]. In our study, two kinds of contaminants, which were Sudan red and thiram as examples of illegal additives and pesticide residues respectively, were adopted as the target molecules, and sunflower oil bought from a local supermarket was used as the oil solvent. Stock solutions of $1.0 \times 10^{-3}$ M Sudan red and thiram in sunflower oil were first prepared and then gradually diluted to lower concentrations for SERS detection.
A series of optical fiber SERS probes made with optimized experimental conditions involving 45 mW inducing laser power, 2 s residence time in air and 13 dipping-pulling cycles were prepared sequentially by the laser-induced dynamic dip-coating method for subsequent usage. Figure 4 shows the SERS spectra of $1.0 \times 10^{-3}$ mol/L Sudan red and thiram in sunflower oil measured by different optical fiber SERS probes. During SERS measurements, the optical fiber SERS probes were directly immersed into the oil-phase solutions for in situ SERS detection. All SERS spectra for oil-phase solutions were acquired with 30 mW Raman excitation laser power and 10 s integration time, and the Raman background of sunflower oil itself were subtracted. From the figures, characteristic Raman peaks at 878 cm$^{-1}$, 1002 cm$^{-1}$, 1076 cm$^{-1}$, 1352 cm$^{-1}$, and 1586 cm$^{-1}$ for Sudan red [38] and at 1149 cm$^{-1}$ and 1380 cm$^{-1}$ for thiram [39] were observed; the SERS spectra measured with different probes exhibited good reproducibility, as the relative standard deviations (RSD) of the strongest peaks at 1352 cm$^{-1}$ for Sudan red and 1380 cm$^{-1}$ for thiram were approximately 6.76% and 7.23%, respectively (see Fig. 4(c) and (d)). This superior reproducibility for in situ SERS detection in sunflower oil may originate from the overall integration of SERS signals within the entire SERS-active area on the optical fiber facet as well as the consistent preparation of optical fiber SERS probes by the laser-induced evaporation self-assembly method.
Fig. 4. SERS spectra of (a) $1.0 \times 10^{-3}$ mol/L Sudan red in sunflower oil and (b) $1.0 \times 10^{-3}$ mol/L thiram in sunflower oil with five independent measurements using sequentially fabricated optical fiber SERS probes; and fluctuations in SERS intensities with different measurements at the strongest Raman peak of (c) 1352 cm$^{-1}$ for Sudan red and (d) 1380 cm$^{-1}$ for thiram.
3.3 Sensitivity and quantitativity of in situ SERS detection in a non-volatile liquid-phase system
The SERS spectra for the two oil solutions with different concentrations were measured and shown in Fig. 5(a-b). The intensity of the SERS signal became weaker with decreasing concentration. Furtherly, Fig. 5(c-d) show log-log plots of the relationships between SERS intensity and concentration for the strongest Raman peaks at 1352 cm$^{-1}$ for Sudan red and 1380 cm$^{-1}$ for thiram. The discrete points corresponding to the SERS intensities were measured at different concentrations, and the error bars were calculated from six independent measurements using different optical fiber SERS probes fabricated with the optimized parameters. Very good linearity between the logarithm of SERS intensity $I$ and the logarithm of concentration $C$ were observed in the concentration range $1.0 \times 10^{-3}$ mol/L to $1.0 \times 10^{-6}$ mol/L for both cases, and the linear equations for the two calibration curves were $\log I=0.364\times \log C+5.091$ and $\log I=0.267\times \log C+4.607$ corresponding to the linear correlation coefficients ($R^2$) are 0.981 and 0.988, respectively. This linear relationship in the log-log plots origins from the adsorption kinetic equilibrium of analyte molecules on the surface of Au-nanorods [40].
Fig. 5. SERS spectra measured with different concentrations of (a) Sudan red in sunflower oil and (b) thiram in sunflower oil. The log-log plot of (c) SERS intensity at 1352 cm$^{-1}$ vs. Sudan red concentration and (d) SERS intensity at 1380 cm$^{-1}$ vs. thiram concentration. The concentration unit of “M” means “mol/L”.
Herein, the widely adopted “$3 \sigma$ method” [31] was used to evaluate the limit of detection (LOD) of Sudan red and thiram in sunflower oil. In the experiments, twenty optical fiber SERS probes were repeatably prepared under the optimized experimental conditions. These probes were randomly divided into two groups for the detection of Sudan red and thiram, respectively. With each group, ten SERS spectra without analytes (blank spectra) and with $1.0\times 10^{-5}$ mol/L Sudan or thiram in sunflower oil were acquired. By extracting the signal intensities at the strongest Raman characteristic peaks of 1352 cm$^{-1}$ for Sudan and 1380 cm$^{-1}$ for thiram from the blank spectra and the SERS spectra, the lowest detectable intensity with the “$3 \sigma$ method” were obtained and shown in Fig. 5(c-d) with blue dashed lines. And the LODs of $2.4 \times 10^{-6}$ mol/L for Sudan red and $3.6 \times 10^{-7}$ mol/L for thiram in sunflower oil were obtained. We compared our results with some recent works regarding to edible oil [32,41–43], and showed them in Table 1. It can be clearly seen that our method does not require complicated sample pretreatment, and the SERS spectra of analytes in edible oil can be easily acquired by directly dipping the SERS probes into oil. As sunflower oil is a complex mixture that contains unsaturated fatty acids, fat-soluble vitamins and many other substances, during the in situ SERS detection in such complex liquid environment, the "hotspot occupancy effect" (i.e. the impurity molecules in complex liquid environments may occupy the SERS hotspots and preventing the target molecules from being located in the hotspot region) significantly decreases the sensitivity of SERS detection [25]. Benefit from the numerous SERS hotspots provided by the cluster-patterned optical fiber SERS probes, though the interfering molecules occupy some of the hotspots, there are still enough hotspots available for the target molecules. Therefore, the LODs in our case are comparable with that reported in recent literature, even with a portable Raman spectrometer and short integration time of 10 s.
Table 1. The comparisons of SERS detection in edible oil.
The recovery rate experiments were performed to verify the reliability and accuracy of contaminant concentrations in edible oil determined by the in situ SERS detection method. Edible oil samples of both Sudan red and thiram spiked with concentrations of $2.0 \times 10^{-4}$ mol/L, $4.0 \times 10^{-5}$ mol/L and $7.0 \times 10^{-6}$ mol/L were prepared. For each concentration, replicate SERS measurements were performed with three different optical fiber SERS probes. By comparing the measured SERS intensities for characteristic Raman peaks at 1352 cm$^{-1}$ for Sudan red and 1380 cm$^{-1}$ for thiram with the calibration curves shown in Fig. 5(c) and (d), the corresponding concentrations of contaminants in edible oil were predicted and are shown in Table 2 and 3. The recovery rate, defined as the ratio of the predicted concentration to the spiked concentration, and the relative standard deviation (RSD) for each spiked concentration were also calculated and are listed in the tables. The results indicate that the recovery rates were between 89.6% and 114.8% for Sudan red in sunflower oil and between 89.3% and 106.4% for thiram in sunflower oil, and the RSD values were all less than 10%. These results clearly confirmed the very good preparation reproducibility of optical fiber SERS probes by laser-induced dynamic dip-coating method as well as the viability of rapid and quantitative SERS detection in complex non-volatile liquid-phase systems with our optical fiber SERS probes, which may find important practical applications in food safety and biomedicine.
Table 2. Concentration, recovery, and RSD for Sudan red in edible oil samples with different spiked concentrations.
Table 3. Concentration, recovery, and RSD for thiram in edible oil samples with different spiked concentrations.
4. Conclusion
In conclusion, this article presents a simple and efficient method for in situ SERS detection in non-volatile liquid-phase system with the help of highly sensitive optical fiber SERS probes. Numerous Au-nanorod clusters were successfully prepared on conventional 105 µm/125 µm multimode optical fibers by a laboratory-developed laser-induced dynamic dip-coating method, with optimized experimental parameters of 45 mW inducing laser power, 2 s residence time in air and 13 dipping-pulling cycles. Take edible oil as an example of non-volatile liquid-phase system, we show the excellent capability for in situ SERS detection in oil-phase environments with the optimized optical fiber SERS probes. It was demonstrated that by directly dipping the optical fiber SERS probes into the sunflower oil to be tested, high detection sensitivity (LODs lower than $2.4 \times 10^{-6}$ mol/L for Sudan red and $3.6 \times 10^{-7}$ mol/L thiram in sunflower oil) as well as good spectral reproducibility (the RSDs for SERS intensity were less than $10\%$ even in the complex oil environment) could be achieved with a portable Raman spectrometer and short spectral integration time of 10 s.
A good linear relationship between $\log I$ and $\log C$ was observed in our experiments, where $I$ is the Raman peak intensity and $C$ is the concentration of Sudan red or thiram in sunflower oil. The linear calibration curves were $\log I=0.364\times \log C+5.091$ with a linear correlation coefficient of 0.981 for Sudan red in sunflower oil and $\log I=0.267\times \log C+4.607$ with a linear correlation coefficient of 0.988 for thiram in sunflower oil. Furthermore, the recovery rates ranged from 89% to 115% confirmed the efficiency and accuracy of excellent capability for in situ SERS detection in edible oil with optical fiber SERS probes, which indicates the possibility of quantitative substance detection in non-volatile liquid-phase system. Since there is no additional sample pretreatment required and the processes for preparation of optical fiber SERS probes and SERS detection are very simple and fast, this in situ SERS detection method may have great potential in rapid detection of substances in various non-volatile liquid-phase environments, and find practical applications in the field of food safety and biomedicine.
Funding
National Natural Science Foundation of China (11874111, 51771189); National Key Research and Development Program of China (2019YFC1511001); Key Laboratory of Robotics and Intelligent Equipment of Guangdong Regular Institutions of Higher Education (2017KSYS009); Key research platforms and projects of universities in Guangdong Province (2019KZDXM016); Dongguan Social Science and Technology Development Project (2019507140172); Dongguan Science and Technology Special Project (20201800500302).
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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