Anal Bioanal Chem (2015) 407:6031–6039 DOI 10.1007/s00216-015-8776-1
RESEARCH PAPER
Silver-nanoparticle-based surface-enhanced Raman scattering wiper for the detection of dye adulteration of medicinal herbs Dan Li 1,2 & Qingxia Zhu 3 & Diya Lv 1 & Binxing Zheng 1 & Yanhua Liu 1 & Yifeng Chai 1 & Feng Lu 1,2
Received: 24 March 2015 / Revised: 6 May 2015 / Accepted: 8 May 2015 / Published online: 5 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract By using a silver nanoparticle wiper as a surfaceenhanced Raman scattering substrate, a highly sensitive, convenient, and rapid platform for detecting dye adulteration of medicinal herbs was obtained. Commercially available filter paper was functionalized with silver nanoparticles to transform it into the flexible wiper. This device was found to collect dye molecules with unprecedented ease. Experiments were performed to optimize various factors such as the type of wiper used, the wetting reagent, and the wetting/wiping mode and time. Excellent wiper performance was observed in the detection of the simulated adulteration of samples with dyes at various concentrations. The limits of detection for nine dyes, including 10−6 g/mL for malachite green, 10−7 g/mL for Rhodamine 6G, and 5 ×10−8 g/mL for methylene blue, were discerned. The results of this investigation show that this proposed method is potentially highly advantageous for fieldbased applications. Keywords Surface-enhanced Raman scattering . Silver nanoparticle wiper . Dye adulteration . Medicinal herbs Electronic supplementary material The online version of this article (doi:10.1007/s00216-015-8776-1) contains supplementary material, which is available to authorized users. * Feng Lu [email protected] 1
Department of Pharmaceutical Analysis, School of Pharmacy, Second Military Medical University, 325 Guohe Road, Shanghai 200433, China
2
School of Pharmacy, Fujian University of Traditional Chinese Medicine, 1 Huatuo Road, Fuzhou 350108, China
3
Department of Pharmacy, Shanghai Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
Introduction There has recently been a rapid increase in the number of cases of dye adulteration of food and drugs around the world. The phenomenon of medicinal herb dyeing is also becoming more prevalent. Some of the dyes used for this adulteration, including Rhodamine B, malachite green and Auramine O, are harmful to human health, so their use as food additives is prohibited. However, due to their wide availability and low cost, these dyes have been illegally used to improve product color in order to cover up illegal behavior such as undeclared premarket extraction of effective components. Therefore, monitoring dye adulteration of medicinal herbs is becoming an important and necessary task. Over the past few years, a number of techniques—such as HPLC [1], MS [2], NMR [3], UV [4], FT-Raman [5], and fluorescence spectroscopy [6]—have been applied in the detection of dyes. However, despite their excellent performance in the research lab setting, the sampling, extraction, and preconcentration process that is employed before analysis using these techniques limits their applicability to fieldbased applications. Surface-enhanced Raman scattering (SERS) is emerging as a powerful technique for the trace-level detection of various dyes [7–10]. In real-world applications, such as complex samples, the efficiency of dye molecule collection is a decisive factor. Also, transferring the dye to a simple device before SERS detection is considered to reduce the difficulties associated with SERS analysis, especially in situ analysis. However, conventional SERS substrates based on silicon, glass, and porous alumina are not efficient materials for sample collection due to their non-conformal, rigid, and brittle nature [11]. Compared with those substrates, a flexible paper device is more suitable for the demands of practical SERS substrates given the portability and simplicity of paper [12]. Paper-based
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SERS substrates have been fabricated using various approaches, including inkjet printing [13–15], soaking [11, 16, 17], and chemical reduction [12, 18–21]. Although the use of nanoparticle-treated paper as a swab or rag for smooth surfaces has been demonstrated [11–14], crucial parameters such as the optimal wetting and wiping times have been overlooked. Meanwhile, there are still unparalleled challenges associated with selectively but also effectively collecting analyte molecules from the rough surfaces of complex samples, such as medicinal herbs. In the work reported in the present paper, a highly sensitive, convenient, and rapid platform for detecting trace levels of dyes on the surfaces of medicinal herbs was developed that combines a silver nanoparticle wiper (BAg NPs wiper^) with SERS technology. To create the wiper, silver nanoparticles were trapped in filter paper to form a SERS-active substrate. This was found to be a highly practical and efficient tool for collecting dye molecules by merely wiping the wetted herb. Four types of Ag NPs wiper, two modes of wetting/wiping, three kinds of wetting reagents, and 16 combinations of wetting and wiping times were tested, evaluated, and optimized. The resulting optimal Ag NPs wiper showed high sensitivity and reproducibility in the detection of nine dyes. Moreover, wiping tests were performed for different samples that had undergone simulated dye adulteration. To the best of our knowledge, this paper is the first to report a SERS substrate based on filter paper, which was found to be highly flexible and efficient at collecting dye molecules present at trace levels on the surfaces of herbs. We also believe that this wiper would be easy to use in field-based applications. The whole processing procedure employed in this work is shown in Fig. 1.
Apparatus and instruments
Experimental
Ag NPs-PVP wiper (wiper 2) As per a reported process [23], the filter paper was dipped into an ethanol solution of 2 % PVP for 5 h and then dried after a certain amount of time. The filter paper was dipped in silver colloid for 0.5 h.
Reagents and materials All chemicals were reagent grade and used as received. Silver nitrate, sodium citrate, glucose, sodium hydroxide, sodium borohydride, polyvinylpyrrolidone (PVP), and all organic reagents (methanol, ethanol, and acetic acid) were purchased from Shanghai Chemical Reagent Company (Shanghai, China). Ultrapure water was obtained from a Barnstead 1800 filter (Thermo Scientific, Waltham, MA, USA). Common filter paper was obtained from the Hangzhou Whatman-Xinhua Filter Paper Company (Hangzhou, China). Nine dyes—Rhodamine 6G (R6G), methylene blue (MB), malachite green (MG), crystal violet (CV), Rhodamine B (RB), bromophenol blue (BB), erythrosine B sodium salt (EB), crocein scarlet 3B (CS), and Auramine O (AO)—were obtained from Aladdin (Shanghai, China). The medicinal herbs Ganoderma (whole plate), medlar (fruit), honeysuckle (flower), and prickly ash peel (peel) were purchased from a pharmacy.
Ultraviolet–visible (UV–vis) absorption spectra were recorded on a Varian Cary 100 Conc spectrometer (Varian, Palo Alto, CA, USA). Scanning electron microscope (SEM) images were captured using a Zeiss EVO MA-10 (Carl Zeiss AG, Oberkochen, Germany). Raman spectra were recorded using two Raman spectrometers: a portable spectrometer (B&W Tek, Newark, DE, USA) with an excitation wavelength of 785 nm and a benchtop spectrometer (HORIBA Jobin Yvon XploRA-FDU, Stanmore, UK) with an excitation wavelength of 532 nm. The laser power of each system was approximately 10–50 and 0.05 mW, respectively, and the reported spectrum accumulation time was 5 and 0.1 s, respectively. Fabrication of the Ag NPs wipers Soaking method Silver nanoparticles were synthesized using the classic method of Lee and Meisel [22]. As described in a previous report, silver nitrate (90 mg) was dissolved in 500 mL distilled water and the solution was boiled. A solution of 1 % sodium citrate (10 mL) was added and the mixture was then boiled for 1 h. After the solution had turned greenish brown—the typical color of a silver colloid—it was removed from the heat. Ag NPs wiper (wiper 1) Similar to the approach used in previous studies [11, 16, 17], a piece of filter paper that had not been pretreated was immersed in a culture dish containing 10 mL of silver colloid for 48 h.
Chemical reduction method Two reducing agents, sodium borohydride and glucose, were selected. Ag NPs-NaBH4 wiper (wiper 3) The filter paper was immersed in 0.1 M silver nitrate solution and allowed to dry for a few seconds [18]. The wet filter paper containing silver ions was then sprayed with 0.2 M sodium borohydride solution for 30 s. Ag NPs-glucose wiper (wiper 4) The procedure for preparation was as follows [21]. Silver nanoparticles were grown by immersing the filter paper into Tollens’ reagent. After that, the container was moved into a water bath at 55 °C for 6 min.
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Fig. 1 Schematic showing the fabrication of the paper-based wiper and SERS examination of dyes obtained from the surfaces of medicinal herbs
Prior to use, the prepared Ag NPs wiper was dried in the air at room temperature and then cut into 1×2 cm rectangles. The wiper was kept at 40 % relative humidity and 18 °C until use.
Results and discussion Properties of the SERS device Characteristics of the SERS substrates
Sample preparation A series of dye solutions with different concentrations were prepared by dissolving the relevant powders in water or by dilution. The adulterated samples were obtained by immersing the herbs in dye solution and then drying the herbs. A photo of stained herbs is provided in Fig. S1a in the BElectronic supplementary material^ (ESM).
Wetting and wiping the dyed medicinal herbs Two approaches, wetting the wiper or wetting the herbs before wiping them with a dry wiper, were trialled after selecting the most appropriate wipe device. Afterwards, factors such as the wetting reagent and the wetting and wiping times were systematically analyzed.
Both UV–vis and SEM analyses of the substrates were conducted. Absorption clearly occurred at 431 nm, corresponding to a typical plasmon band, which suggests that the nanoparticle diameter is approximately 45–50 nm [23]. The change in color of the dried Ag-NPs-treated paper is shown in Fig. 2. The filter paper turned from white to light yellow after being exposed to silver colloid. However, a significant difference was observed when the SERS device was synthesized by chemical reduction. Scanning electron micrographs of the substrates are presented in Fig. S3 (see the ESM). The images were used to evaluate the quantity of Ag NPs adsorbed onto the paper. The morphology of the paper was that of uncoated cellulose filter fibers and an uneven surface resulting from the presence of deposited silver nanoparticles. As shown in Figs. S3b and S3c in the ESM (images of wiper 1 and wiper 2), the Ag NPs were well dispersed and adsorbed onto the surface of the paper. The randomly distributed aggregation
SERS detection using the Ag NPs wipers In order to obtain standard dye spectra, a 0.4-μL droplet of dye solution (10−4 g/mL) was deposited onto the wiper before performing SERS measurements (see Fig. S1b in the ESM). High-quality spectra were obtained when the wiper zones were still wet. Each measurement was repeated three times.
Fig. 2 Photos of the filter paper and wipers. From left to right: wipers 1– 4 and the filter paper
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of nanostructures on the paper illustrated the potential of the soaking method to generate a device with high SERS activity. However, the images of wiper 3 and wiper 4 (see Figs. S3d and S3e) revealed a high degree of large-scale aggregation of silver nanoparticles, which was not beneficial for obtaining stable Bhotspots^ for SERS analysis. Ag NPs wiper performance First, in order to verify the influence of the signal from the paper on our detection technique, the filter paper and the four types of wipers were tested using the two Raman spectrometers. The corresponding spectra are plotted in Fig. S4 in the ESM and Fig. 3. Because of the low laser power and short accumulation time of benchtop spectrometers, the spectra shown in Fig. S4a in the ESM were relatively flat and Fig. 3 a Background signals from the filter paper and wipers recorded by a portable Raman spectrometer. b SERS spectra of 10−4 g/mL R6G detected on (a) wiper 1, (b) wiper 2, (c) wiper 3, (d) wiper 4, and (e) bare filter paper
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featureless. However, in the spectra obtained using the portable spectrometer, two characteristic peaks were seen at 1122 and 1160 cm−1 (see lines a, b, and e in Fig. 3a), which can be attributed to the C–O–C bending modes in cellulose fiber structures [24, 25]. It was observed that the dark wipers (wiper 3 and wiper 4) are much easier to burn using the laser than the other device. Moreover, these spectra exhibited very weak features (see lines c and d in Fig. 3a) which indicated that they may not be suitable for detection. To perform a more direct comparison, we investigated the enhancement factor, which is generally defined as the improvement in the Raman signal with SERS as compared to without SERS. Here, R6G (10−4 g/mL) was employed as the probe to evaluate the performance of the SERS device: a 0.4-μL droplet was spotted onto the filter paper and four wipers. The portable spectrometer was used for immediate
Detection of dye adulteration by SERS-wiper
detection. When using the benchtop spectrometer, 1 μL R6G was pipetted onto the surface of the paper or wiper and the Raman spectrum was recorded after the droplet had dried. The resulting SERS spectra are shown in Fig. 3b and Fig. S4b in the ESM, respectively. The results show that a featureless spectrum was obtained after R6G was deposited on the filter paper. However, the typical spectrum of R6G can clearly be seen for the wipers [26]. The height of the most prominent peak in the Raman spectrum was calculated (at 1511 cm−1) in order to work out the EF. Mathematically, the EF is determined as follows: E F ¼ I S N R=I R N S ; where the intensity I is the height of the Raman peak at 1511 cm−1 and N represents the total number of molecules deposited onto the substrate, while the subscripts S and R represent BSERS^ and BRaman,^ respectively. By analyzing the data in Fig. 3b, the EFs for wipers 1–4 were found to be 7.46×103, 1.16×103, 5.07×102, and 1.17×102, respectively. When the data in Fig. S4b in the ESM were used, the values were found to be 3.45×102, 1.54×102, 3.75×101, and 5.83× 101, respectively. In other words, wiper 1 showed excellent sensitivity and could be used as a SERS substrate, regardless of which Raman spectrometer is used. However, compared with the benchtop spectrometer, the portable one is more widely available, portable, and suitable for field-based testing, which makes it more compatible with our paper device and thus crucial to our analytical method. Detection of dye adulteration of medicinal herbs using Ag NPs wipers Selection of the wetting and wiping mode In order to evaluate the influence of the wetting and wiping mode, two methods were devised. Firstly, the wiper was dipped into the wetting reagent for a few seconds. After that, the wetted device was used to wipe the dyed herb. Secondly, 20 μL of wetting reagent were added dropwise to the dyed herb, and then the dried SERS device was wiped gently but firmly over the surface of the herb. Unlike glasses or fruit peels, the surface of a herb is relatively rough, which hinders the effective collection of dye molecules. Also, the wiper can easily tear if it is exposed to the reagent for a long period. Therefore, the latter mode was selected. Selection of the wetting reagent When an organic solution is used, it is important to consider that the scattered light from the reagent can interfere with the signal from the analyte. Thus, prior to detection, 20 μL of 25 % v/v solution (methanol, ethanol, or acetic acid) and water
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were first added to the unstained Ganoderma for about 2 s. The presence of water prevented the fast evaporation of the wetting reagent, and a piece of the wiper was continuously swabbed across the surface of the herb for about 10 s. After that, the wiper was immediately analyzed. The spectra in Fig. S5 (see the ESM) show no signal aside from that for cellulose fiber. We can therefore conclude that wetting reagents do not produce noticeable spectral interferences. We measured the scattered light from the abovementioned wetting reagents in order to select the most suitable one. The dyed Ganoderma (stained by 10−5, 10−4, and 10−3 g/mL dye solutions) were wetted by the wetting reagent (the same amount was used in each case), and then the wiper was applied. The SERS spectra obtained using these reagents are shown in Fig. S6 in the ESM (in the list, A–C are methanol, ethanol, and acetic acid, respectively). We observed that the choice of the solvent used as the wetting reagent can have an effect on detection. When compared with the standard spectrum, most of the dyes were clearly identifiable. When acetic acid was used, the signals from the dyes BB, EB, and CS were weak at a concentration of 10−3 g/mL, as can be seen in Fig. S6c (f–h) in the ESM. In addition, the spectra for the dyes were barely visible at concentrations of 10−4 g/mL or lower, suggesting that acetic acid is not suitable for use as a wetting agent. On the contrary, high-quality spectra were seen when using either methanol or ethanol. Meanwhile, the toxicity of the solvent should also be considered. For this reason, ethanol solution was chosen as the wetting agent. Optimization of the wetting and wiping time Ethanol is widely used in the extraction and dissociation of the matrix in medicinal herbs. It is conceivable that wetting or wiping for a long time may cause the extraction of complex intrinsic ingredients from medicinal herbs, which will inevitably interfere with SERS detection. For example, when the unstained Ganoderma was wetted for 20 s and then wiped for 10 s, the SERS spectra obtained from three different areas of the wiper exhibited some unidentified peaks (see Fig. S7 in the ESM). Thus, in order to eliminate the influence of ingredients and identify the optimal time needed to obtain a high density of dye on the surface of wiper, four levels of time (2, 5, 10, and 15 s) were set up in an orthogonal experimental design. Further details on the experimental design are given in Table S1 (see the ESM). Ganoderma stained with RB were then selected for further study. A similar wet-and-wipe operation was performed, and then the wiper was analyzed with a portable spectrometer. In order to compare the experimental results, we noted the Raman intensity of the band at 1357 cm−1 following different wetting and wiping times at three dye concentrations (10−5, 10−4, and 10−3 g/mL). In Fig. 4, the height of a bar represents the mean value of three separate detection results, and each set
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Fig. 4 Bars showing the intensity of the 1357 cm−1 Raman peak for various wetting and wiping times (wetting × wiping). Each group of three bars represents a combination of wetting time and wiping time
parameters, ranging from 2 s × 2 s to 15 s× 15 s. The black bars represent a dye concentration of 10−3 g/mL, red bars 10−4 g/mL, and blue bars 10−5 g/mL, respectively
of three bars represents the results for a particular wetting and wiping time combination. Each error bar represents the standard deviation of the signal intensity. The intensities of all of the characteristic bands of RB decreased monotonously with decreasing concentration, regardless of the values of the wetting time and wiping time. The data in Fig. 4 demonstrate that the intensity was strongest when the wetting time was 2 or 5 s. When the wetting time was longer than this, the reagent was able to permeate deeply into the Ganoderma or evaporate into the air, resulting in a low surface coverage of dye after wiping. The strongest intensity was obtained after wiping for about 10 s. It was difficult for the wiper to collect dye molecules if the wiping time was shorter than this. Meanwhile, a low SERS activity was noted when the wiping time was 15 s. There are a number of reasons for this observation, with the most likely being the evaporation of the wetting reagent during wiping. Based on a D-SERS description [12], solvent can act as a protective agent and the dried wiper can easily be burnt under high laser power. Thus, compromises between the signal intensity and specificity and between wetting for 2 s and wetting for 10 s were selected for the experimental design.
reports, EB (red), MG (green), and RB (pink) solutions were selected to stain medlar (red), honeysuckle (green), and prickly ash peel (pink), respectively. Measurements of Raman intensity were obtained over a range of concentrations. The results of applying the SERS technique to the detection of dyes with concentrations ranging from 10−3 to 10−5 g/mL on the three different herbs are shown in Fig. 5. The data illustrate that the characteristic bands of the dyes are clearly distinguishable down to 10−5 g/mL. The high signal-to-noise ratios for these bands indicate that we can detect a low concentration of the analyte, which demonstrates that the paper device can be used for the detection of simulated similar-color samples.
Detection of dyes in herbs that have undergone simulated dyeing (Bsimulated samples^) Detection of dyes in simulated similar-color samples In order to confirm that the SERS device can be used in conjunction with wiper-based sampling, we further tested three kinds of herbs: medlar, honeysuckle, and prickly ash peel, which were stained by similarly colored dyes. Based on news
Minimum concentration required for detection Under the abovementioned optimized conditions, the limit of detection (LOD) for each dye was calculated as the concentration that gives a signal-to-noise ratio (S/N) of 3. It can be seen that the LODs for the nine dyes were 10−6 g/mL or lower. In addition, for the Bcolorless^ medicinal herbs such as Ganoderma, the minimum concentrations of the nine dyes that were needed to visibly dye the herbs, as well as the LODs, are shown in Table S2 (see the ESM). The results indicate that the LODs are relatively low, suggesting that the combination of the flexible wiper with SERS detection can satisfy the needs of real sample detection. Rapid identification of nine dyes The main features of the standard SERS spectra of the nine dyes (see Fig. S1b in the ESM), along with their
Detection of dye adulteration by SERS-wiper Fig. 5a–c Wiping tests on simulated samples: a medlar; b honeysuckle; c prickly ash peel. The blue lines represent a dye concentration of 10−5 g/mL, red 10−4 g/mL, and black 10−3 g/mL, respectively
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assignments, are summarized in Table S3 (in the ESM). This table facilitates the simple identification of even some structurally similar dyes, such as R6G and RB or CV and MG. The structures and SERS spectra of those four dyes are shown in Fig. S9 in the ESM. R6G yielded a sharp band at 774 cm−1, along with another feature at 1310 cm−1. These bands were not present, however, in the SERS spectrum of RB, with the latter presenting bands at 735 and 1280 cm−1. The spectrum of MG revealed a band at 1218 cm−1, while this band did not appear in the spectrum of CV. Similar results were also obtained for other dyes; see Table S3 in the ESM. Comparisons of these characteristic bands provide an extremely effective method of identifying dyes. Several other dyes will be examined in future work in order to explore the broader application of this method to the simple identification of dye adulteration.
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Conclusion
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In summary, this work explored the practical application of an Ag NPs wiper to the sensitive SERS detection of dyes on the surfaces of medicinal herbs. Silver nanoparticles were adsorbed onto filter paper simply by soaking. Dye molecules can be transferred onto the resulting substrate by merely wiping it over the wetted medicinal herb. Factors including the wetting reagent, wetting/wiping mode, and wiping time were systematically studied in order to optimize the use of the wiper. This method was shown to perform well in the detection of nine dyes, with detection limits ranging from 10−6 to 5×10−8 g/mL, which are lower than the minimum concentrations needed to visibly dye the Bcolorless^ herbs. More importantly, the excellent performance of the Ag NPs wiper was also demonstrated by detecting EB, MG, and RB on the surfaces of herbs that had undergone simulated dye adulteration; the results clearly verified that this method has the potential for field-based applications. Additionally, there are expected to be numerous opportunities to integrate this simple SERS-based approach with other analytical platforms, such as chromatographic and chemometric techniques, in order to undertake more complex analyses of mixtures of dyes. Therefore, further research along these lines would be worthwhile.
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18. Acknowledgments The authors gratefully acknowledge Professor Lu Jian from the Second Military Medical University for assisting with our study. This work was supported by the Ministry of Science and Technology of the People’s Republic of China (grant no.2012YQ180132) and the Shanghai Municipal Science and Technology Commission (grant no. 11431922502).
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RESEARCH PAPER
Silver-nanoparticle-based surface-enhanced Raman scattering wiper for the detection of dye adulteration of medicinal herbs Dan Li 1,2 & Qingxia Zhu 3 & Diya Lv 1 & Binxing Zheng 1 & Yanhua Liu 1 & Yifeng Chai 1 & Feng Lu 1,2
Received: 24 March 2015 / Revised: 6 May 2015 / Accepted: 8 May 2015 / Published online: 5 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract By using a silver nanoparticle wiper as a surfaceenhanced Raman scattering substrate, a highly sensitive, convenient, and rapid platform for detecting dye adulteration of medicinal herbs was obtained. Commercially available filter paper was functionalized with silver nanoparticles to transform it into the flexible wiper. This device was found to collect dye molecules with unprecedented ease. Experiments were performed to optimize various factors such as the type of wiper used, the wetting reagent, and the wetting/wiping mode and time. Excellent wiper performance was observed in the detection of the simulated adulteration of samples with dyes at various concentrations. The limits of detection for nine dyes, including 10−6 g/mL for malachite green, 10−7 g/mL for Rhodamine 6G, and 5 ×10−8 g/mL for methylene blue, were discerned. The results of this investigation show that this proposed method is potentially highly advantageous for fieldbased applications. Keywords Surface-enhanced Raman scattering . Silver nanoparticle wiper . Dye adulteration . Medicinal herbs Electronic supplementary material The online version of this article (doi:10.1007/s00216-015-8776-1) contains supplementary material, which is available to authorized users. * Feng Lu [email protected] 1
Department of Pharmaceutical Analysis, School of Pharmacy, Second Military Medical University, 325 Guohe Road, Shanghai 200433, China
2
School of Pharmacy, Fujian University of Traditional Chinese Medicine, 1 Huatuo Road, Fuzhou 350108, China
3
Department of Pharmacy, Shanghai Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
Introduction There has recently been a rapid increase in the number of cases of dye adulteration of food and drugs around the world. The phenomenon of medicinal herb dyeing is also becoming more prevalent. Some of the dyes used for this adulteration, including Rhodamine B, malachite green and Auramine O, are harmful to human health, so their use as food additives is prohibited. However, due to their wide availability and low cost, these dyes have been illegally used to improve product color in order to cover up illegal behavior such as undeclared premarket extraction of effective components. Therefore, monitoring dye adulteration of medicinal herbs is becoming an important and necessary task. Over the past few years, a number of techniques—such as HPLC [1], MS [2], NMR [3], UV [4], FT-Raman [5], and fluorescence spectroscopy [6]—have been applied in the detection of dyes. However, despite their excellent performance in the research lab setting, the sampling, extraction, and preconcentration process that is employed before analysis using these techniques limits their applicability to fieldbased applications. Surface-enhanced Raman scattering (SERS) is emerging as a powerful technique for the trace-level detection of various dyes [7–10]. In real-world applications, such as complex samples, the efficiency of dye molecule collection is a decisive factor. Also, transferring the dye to a simple device before SERS detection is considered to reduce the difficulties associated with SERS analysis, especially in situ analysis. However, conventional SERS substrates based on silicon, glass, and porous alumina are not efficient materials for sample collection due to their non-conformal, rigid, and brittle nature [11]. Compared with those substrates, a flexible paper device is more suitable for the demands of practical SERS substrates given the portability and simplicity of paper [12]. Paper-based
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SERS substrates have been fabricated using various approaches, including inkjet printing [13–15], soaking [11, 16, 17], and chemical reduction [12, 18–21]. Although the use of nanoparticle-treated paper as a swab or rag for smooth surfaces has been demonstrated [11–14], crucial parameters such as the optimal wetting and wiping times have been overlooked. Meanwhile, there are still unparalleled challenges associated with selectively but also effectively collecting analyte molecules from the rough surfaces of complex samples, such as medicinal herbs. In the work reported in the present paper, a highly sensitive, convenient, and rapid platform for detecting trace levels of dyes on the surfaces of medicinal herbs was developed that combines a silver nanoparticle wiper (BAg NPs wiper^) with SERS technology. To create the wiper, silver nanoparticles were trapped in filter paper to form a SERS-active substrate. This was found to be a highly practical and efficient tool for collecting dye molecules by merely wiping the wetted herb. Four types of Ag NPs wiper, two modes of wetting/wiping, three kinds of wetting reagents, and 16 combinations of wetting and wiping times were tested, evaluated, and optimized. The resulting optimal Ag NPs wiper showed high sensitivity and reproducibility in the detection of nine dyes. Moreover, wiping tests were performed for different samples that had undergone simulated dye adulteration. To the best of our knowledge, this paper is the first to report a SERS substrate based on filter paper, which was found to be highly flexible and efficient at collecting dye molecules present at trace levels on the surfaces of herbs. We also believe that this wiper would be easy to use in field-based applications. The whole processing procedure employed in this work is shown in Fig. 1.
Apparatus and instruments
Experimental
Ag NPs-PVP wiper (wiper 2) As per a reported process [23], the filter paper was dipped into an ethanol solution of 2 % PVP for 5 h and then dried after a certain amount of time. The filter paper was dipped in silver colloid for 0.5 h.
Reagents and materials All chemicals were reagent grade and used as received. Silver nitrate, sodium citrate, glucose, sodium hydroxide, sodium borohydride, polyvinylpyrrolidone (PVP), and all organic reagents (methanol, ethanol, and acetic acid) were purchased from Shanghai Chemical Reagent Company (Shanghai, China). Ultrapure water was obtained from a Barnstead 1800 filter (Thermo Scientific, Waltham, MA, USA). Common filter paper was obtained from the Hangzhou Whatman-Xinhua Filter Paper Company (Hangzhou, China). Nine dyes—Rhodamine 6G (R6G), methylene blue (MB), malachite green (MG), crystal violet (CV), Rhodamine B (RB), bromophenol blue (BB), erythrosine B sodium salt (EB), crocein scarlet 3B (CS), and Auramine O (AO)—were obtained from Aladdin (Shanghai, China). The medicinal herbs Ganoderma (whole plate), medlar (fruit), honeysuckle (flower), and prickly ash peel (peel) were purchased from a pharmacy.
Ultraviolet–visible (UV–vis) absorption spectra were recorded on a Varian Cary 100 Conc spectrometer (Varian, Palo Alto, CA, USA). Scanning electron microscope (SEM) images were captured using a Zeiss EVO MA-10 (Carl Zeiss AG, Oberkochen, Germany). Raman spectra were recorded using two Raman spectrometers: a portable spectrometer (B&W Tek, Newark, DE, USA) with an excitation wavelength of 785 nm and a benchtop spectrometer (HORIBA Jobin Yvon XploRA-FDU, Stanmore, UK) with an excitation wavelength of 532 nm. The laser power of each system was approximately 10–50 and 0.05 mW, respectively, and the reported spectrum accumulation time was 5 and 0.1 s, respectively. Fabrication of the Ag NPs wipers Soaking method Silver nanoparticles were synthesized using the classic method of Lee and Meisel [22]. As described in a previous report, silver nitrate (90 mg) was dissolved in 500 mL distilled water and the solution was boiled. A solution of 1 % sodium citrate (10 mL) was added and the mixture was then boiled for 1 h. After the solution had turned greenish brown—the typical color of a silver colloid—it was removed from the heat. Ag NPs wiper (wiper 1) Similar to the approach used in previous studies [11, 16, 17], a piece of filter paper that had not been pretreated was immersed in a culture dish containing 10 mL of silver colloid for 48 h.
Chemical reduction method Two reducing agents, sodium borohydride and glucose, were selected. Ag NPs-NaBH4 wiper (wiper 3) The filter paper was immersed in 0.1 M silver nitrate solution and allowed to dry for a few seconds [18]. The wet filter paper containing silver ions was then sprayed with 0.2 M sodium borohydride solution for 30 s. Ag NPs-glucose wiper (wiper 4) The procedure for preparation was as follows [21]. Silver nanoparticles were grown by immersing the filter paper into Tollens’ reagent. After that, the container was moved into a water bath at 55 °C for 6 min.
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Fig. 1 Schematic showing the fabrication of the paper-based wiper and SERS examination of dyes obtained from the surfaces of medicinal herbs
Prior to use, the prepared Ag NPs wiper was dried in the air at room temperature and then cut into 1×2 cm rectangles. The wiper was kept at 40 % relative humidity and 18 °C until use.
Results and discussion Properties of the SERS device Characteristics of the SERS substrates
Sample preparation A series of dye solutions with different concentrations were prepared by dissolving the relevant powders in water or by dilution. The adulterated samples were obtained by immersing the herbs in dye solution and then drying the herbs. A photo of stained herbs is provided in Fig. S1a in the BElectronic supplementary material^ (ESM).
Wetting and wiping the dyed medicinal herbs Two approaches, wetting the wiper or wetting the herbs before wiping them with a dry wiper, were trialled after selecting the most appropriate wipe device. Afterwards, factors such as the wetting reagent and the wetting and wiping times were systematically analyzed.
Both UV–vis and SEM analyses of the substrates were conducted. Absorption clearly occurred at 431 nm, corresponding to a typical plasmon band, which suggests that the nanoparticle diameter is approximately 45–50 nm [23]. The change in color of the dried Ag-NPs-treated paper is shown in Fig. 2. The filter paper turned from white to light yellow after being exposed to silver colloid. However, a significant difference was observed when the SERS device was synthesized by chemical reduction. Scanning electron micrographs of the substrates are presented in Fig. S3 (see the ESM). The images were used to evaluate the quantity of Ag NPs adsorbed onto the paper. The morphology of the paper was that of uncoated cellulose filter fibers and an uneven surface resulting from the presence of deposited silver nanoparticles. As shown in Figs. S3b and S3c in the ESM (images of wiper 1 and wiper 2), the Ag NPs were well dispersed and adsorbed onto the surface of the paper. The randomly distributed aggregation
SERS detection using the Ag NPs wipers In order to obtain standard dye spectra, a 0.4-μL droplet of dye solution (10−4 g/mL) was deposited onto the wiper before performing SERS measurements (see Fig. S1b in the ESM). High-quality spectra were obtained when the wiper zones were still wet. Each measurement was repeated three times.
Fig. 2 Photos of the filter paper and wipers. From left to right: wipers 1– 4 and the filter paper
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of nanostructures on the paper illustrated the potential of the soaking method to generate a device with high SERS activity. However, the images of wiper 3 and wiper 4 (see Figs. S3d and S3e) revealed a high degree of large-scale aggregation of silver nanoparticles, which was not beneficial for obtaining stable Bhotspots^ for SERS analysis. Ag NPs wiper performance First, in order to verify the influence of the signal from the paper on our detection technique, the filter paper and the four types of wipers were tested using the two Raman spectrometers. The corresponding spectra are plotted in Fig. S4 in the ESM and Fig. 3. Because of the low laser power and short accumulation time of benchtop spectrometers, the spectra shown in Fig. S4a in the ESM were relatively flat and Fig. 3 a Background signals from the filter paper and wipers recorded by a portable Raman spectrometer. b SERS spectra of 10−4 g/mL R6G detected on (a) wiper 1, (b) wiper 2, (c) wiper 3, (d) wiper 4, and (e) bare filter paper
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featureless. However, in the spectra obtained using the portable spectrometer, two characteristic peaks were seen at 1122 and 1160 cm−1 (see lines a, b, and e in Fig. 3a), which can be attributed to the C–O–C bending modes in cellulose fiber structures [24, 25]. It was observed that the dark wipers (wiper 3 and wiper 4) are much easier to burn using the laser than the other device. Moreover, these spectra exhibited very weak features (see lines c and d in Fig. 3a) which indicated that they may not be suitable for detection. To perform a more direct comparison, we investigated the enhancement factor, which is generally defined as the improvement in the Raman signal with SERS as compared to without SERS. Here, R6G (10−4 g/mL) was employed as the probe to evaluate the performance of the SERS device: a 0.4-μL droplet was spotted onto the filter paper and four wipers. The portable spectrometer was used for immediate
Detection of dye adulteration by SERS-wiper
detection. When using the benchtop spectrometer, 1 μL R6G was pipetted onto the surface of the paper or wiper and the Raman spectrum was recorded after the droplet had dried. The resulting SERS spectra are shown in Fig. 3b and Fig. S4b in the ESM, respectively. The results show that a featureless spectrum was obtained after R6G was deposited on the filter paper. However, the typical spectrum of R6G can clearly be seen for the wipers [26]. The height of the most prominent peak in the Raman spectrum was calculated (at 1511 cm−1) in order to work out the EF. Mathematically, the EF is determined as follows: E F ¼ I S N R=I R N S ; where the intensity I is the height of the Raman peak at 1511 cm−1 and N represents the total number of molecules deposited onto the substrate, while the subscripts S and R represent BSERS^ and BRaman,^ respectively. By analyzing the data in Fig. 3b, the EFs for wipers 1–4 were found to be 7.46×103, 1.16×103, 5.07×102, and 1.17×102, respectively. When the data in Fig. S4b in the ESM were used, the values were found to be 3.45×102, 1.54×102, 3.75×101, and 5.83× 101, respectively. In other words, wiper 1 showed excellent sensitivity and could be used as a SERS substrate, regardless of which Raman spectrometer is used. However, compared with the benchtop spectrometer, the portable one is more widely available, portable, and suitable for field-based testing, which makes it more compatible with our paper device and thus crucial to our analytical method. Detection of dye adulteration of medicinal herbs using Ag NPs wipers Selection of the wetting and wiping mode In order to evaluate the influence of the wetting and wiping mode, two methods were devised. Firstly, the wiper was dipped into the wetting reagent for a few seconds. After that, the wetted device was used to wipe the dyed herb. Secondly, 20 μL of wetting reagent were added dropwise to the dyed herb, and then the dried SERS device was wiped gently but firmly over the surface of the herb. Unlike glasses or fruit peels, the surface of a herb is relatively rough, which hinders the effective collection of dye molecules. Also, the wiper can easily tear if it is exposed to the reagent for a long period. Therefore, the latter mode was selected. Selection of the wetting reagent When an organic solution is used, it is important to consider that the scattered light from the reagent can interfere with the signal from the analyte. Thus, prior to detection, 20 μL of 25 % v/v solution (methanol, ethanol, or acetic acid) and water
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were first added to the unstained Ganoderma for about 2 s. The presence of water prevented the fast evaporation of the wetting reagent, and a piece of the wiper was continuously swabbed across the surface of the herb for about 10 s. After that, the wiper was immediately analyzed. The spectra in Fig. S5 (see the ESM) show no signal aside from that for cellulose fiber. We can therefore conclude that wetting reagents do not produce noticeable spectral interferences. We measured the scattered light from the abovementioned wetting reagents in order to select the most suitable one. The dyed Ganoderma (stained by 10−5, 10−4, and 10−3 g/mL dye solutions) were wetted by the wetting reagent (the same amount was used in each case), and then the wiper was applied. The SERS spectra obtained using these reagents are shown in Fig. S6 in the ESM (in the list, A–C are methanol, ethanol, and acetic acid, respectively). We observed that the choice of the solvent used as the wetting reagent can have an effect on detection. When compared with the standard spectrum, most of the dyes were clearly identifiable. When acetic acid was used, the signals from the dyes BB, EB, and CS were weak at a concentration of 10−3 g/mL, as can be seen in Fig. S6c (f–h) in the ESM. In addition, the spectra for the dyes were barely visible at concentrations of 10−4 g/mL or lower, suggesting that acetic acid is not suitable for use as a wetting agent. On the contrary, high-quality spectra were seen when using either methanol or ethanol. Meanwhile, the toxicity of the solvent should also be considered. For this reason, ethanol solution was chosen as the wetting agent. Optimization of the wetting and wiping time Ethanol is widely used in the extraction and dissociation of the matrix in medicinal herbs. It is conceivable that wetting or wiping for a long time may cause the extraction of complex intrinsic ingredients from medicinal herbs, which will inevitably interfere with SERS detection. For example, when the unstained Ganoderma was wetted for 20 s and then wiped for 10 s, the SERS spectra obtained from three different areas of the wiper exhibited some unidentified peaks (see Fig. S7 in the ESM). Thus, in order to eliminate the influence of ingredients and identify the optimal time needed to obtain a high density of dye on the surface of wiper, four levels of time (2, 5, 10, and 15 s) were set up in an orthogonal experimental design. Further details on the experimental design are given in Table S1 (see the ESM). Ganoderma stained with RB were then selected for further study. A similar wet-and-wipe operation was performed, and then the wiper was analyzed with a portable spectrometer. In order to compare the experimental results, we noted the Raman intensity of the band at 1357 cm−1 following different wetting and wiping times at three dye concentrations (10−5, 10−4, and 10−3 g/mL). In Fig. 4, the height of a bar represents the mean value of three separate detection results, and each set
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Fig. 4 Bars showing the intensity of the 1357 cm−1 Raman peak for various wetting and wiping times (wetting × wiping). Each group of three bars represents a combination of wetting time and wiping time
parameters, ranging from 2 s × 2 s to 15 s× 15 s. The black bars represent a dye concentration of 10−3 g/mL, red bars 10−4 g/mL, and blue bars 10−5 g/mL, respectively
of three bars represents the results for a particular wetting and wiping time combination. Each error bar represents the standard deviation of the signal intensity. The intensities of all of the characteristic bands of RB decreased monotonously with decreasing concentration, regardless of the values of the wetting time and wiping time. The data in Fig. 4 demonstrate that the intensity was strongest when the wetting time was 2 or 5 s. When the wetting time was longer than this, the reagent was able to permeate deeply into the Ganoderma or evaporate into the air, resulting in a low surface coverage of dye after wiping. The strongest intensity was obtained after wiping for about 10 s. It was difficult for the wiper to collect dye molecules if the wiping time was shorter than this. Meanwhile, a low SERS activity was noted when the wiping time was 15 s. There are a number of reasons for this observation, with the most likely being the evaporation of the wetting reagent during wiping. Based on a D-SERS description [12], solvent can act as a protective agent and the dried wiper can easily be burnt under high laser power. Thus, compromises between the signal intensity and specificity and between wetting for 2 s and wetting for 10 s were selected for the experimental design.
reports, EB (red), MG (green), and RB (pink) solutions were selected to stain medlar (red), honeysuckle (green), and prickly ash peel (pink), respectively. Measurements of Raman intensity were obtained over a range of concentrations. The results of applying the SERS technique to the detection of dyes with concentrations ranging from 10−3 to 10−5 g/mL on the three different herbs are shown in Fig. 5. The data illustrate that the characteristic bands of the dyes are clearly distinguishable down to 10−5 g/mL. The high signal-to-noise ratios for these bands indicate that we can detect a low concentration of the analyte, which demonstrates that the paper device can be used for the detection of simulated similar-color samples.
Detection of dyes in herbs that have undergone simulated dyeing (Bsimulated samples^) Detection of dyes in simulated similar-color samples In order to confirm that the SERS device can be used in conjunction with wiper-based sampling, we further tested three kinds of herbs: medlar, honeysuckle, and prickly ash peel, which were stained by similarly colored dyes. Based on news
Minimum concentration required for detection Under the abovementioned optimized conditions, the limit of detection (LOD) for each dye was calculated as the concentration that gives a signal-to-noise ratio (S/N) of 3. It can be seen that the LODs for the nine dyes were 10−6 g/mL or lower. In addition, for the Bcolorless^ medicinal herbs such as Ganoderma, the minimum concentrations of the nine dyes that were needed to visibly dye the herbs, as well as the LODs, are shown in Table S2 (see the ESM). The results indicate that the LODs are relatively low, suggesting that the combination of the flexible wiper with SERS detection can satisfy the needs of real sample detection. Rapid identification of nine dyes The main features of the standard SERS spectra of the nine dyes (see Fig. S1b in the ESM), along with their
Detection of dye adulteration by SERS-wiper Fig. 5a–c Wiping tests on simulated samples: a medlar; b honeysuckle; c prickly ash peel. The blue lines represent a dye concentration of 10−5 g/mL, red 10−4 g/mL, and black 10−3 g/mL, respectively
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assignments, are summarized in Table S3 (in the ESM). This table facilitates the simple identification of even some structurally similar dyes, such as R6G and RB or CV and MG. The structures and SERS spectra of those four dyes are shown in Fig. S9 in the ESM. R6G yielded a sharp band at 774 cm−1, along with another feature at 1310 cm−1. These bands were not present, however, in the SERS spectrum of RB, with the latter presenting bands at 735 and 1280 cm−1. The spectrum of MG revealed a band at 1218 cm−1, while this band did not appear in the spectrum of CV. Similar results were also obtained for other dyes; see Table S3 in the ESM. Comparisons of these characteristic bands provide an extremely effective method of identifying dyes. Several other dyes will be examined in future work in order to explore the broader application of this method to the simple identification of dye adulteration.
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Conclusion
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In summary, this work explored the practical application of an Ag NPs wiper to the sensitive SERS detection of dyes on the surfaces of medicinal herbs. Silver nanoparticles were adsorbed onto filter paper simply by soaking. Dye molecules can be transferred onto the resulting substrate by merely wiping it over the wetted medicinal herb. Factors including the wetting reagent, wetting/wiping mode, and wiping time were systematically studied in order to optimize the use of the wiper. This method was shown to perform well in the detection of nine dyes, with detection limits ranging from 10−6 to 5×10−8 g/mL, which are lower than the minimum concentrations needed to visibly dye the Bcolorless^ herbs. More importantly, the excellent performance of the Ag NPs wiper was also demonstrated by detecting EB, MG, and RB on the surfaces of herbs that had undergone simulated dye adulteration; the results clearly verified that this method has the potential for field-based applications. Additionally, there are expected to be numerous opportunities to integrate this simple SERS-based approach with other analytical platforms, such as chromatographic and chemometric techniques, in order to undertake more complex analyses of mixtures of dyes. Therefore, further research along these lines would be worthwhile.
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18. Acknowledgments The authors gratefully acknowledge Professor Lu Jian from the Second Military Medical University for assisting with our study. This work was supported by the Ministry of Science and Technology of the People’s Republic of China (grant no.2012YQ180132) and the Shanghai Municipal Science and Technology Commission (grant no. 11431922502).
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