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Letter
Incorporation of a Basil-seed Based Surface Enhanced Raman Scattering Sensor with a Pipette for Detection of Melamine Ningning Zhou, Qitao Zhou, Guowen Meng, Zhulin Huang, Yan Ke, Jing Liu, and Nianqiang Wu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.6b00312 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016
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Ningning Zhou, †, ‡ Qitao Zhou, † Guowen Meng,*, † Zhulin Huang, † Yan Ke †, ‡, Jing Liu † and Nianqiang Wu*, § †
Key Laboratory of Materials Physics, CAS Center for Excellence in Nanoscience, and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031, China. ‡ University of Science and Technology of China, Hefei, 230026, China. § Department of Mechanical and Aerospace Engineering, West Virginia University, P. O. Box 6106, Morgantown, WV 26506, USA KEYWORDS: Basil-seed, surface-enhanced Raman scattering, sensor, pipette, melamine ABSTRACT: A basil-seed based surface enhanced Raman scattering (SERS) sensor has been incorporated with a transfer pipette via a plastic chamber to create an integrated portable device, in which the transfer pipette is used for flow injection. A small amount of liquid sample is loaded to the dry basil-seed based SERS sensor using the transfer pipette. The dry basil-seed can store the liquid sample like a sponge so that the plasmonic silver nanoparticles deposited on the basil-seed keep an intimate contact with the liquid sample containing the analyte, which enhances the sensitivity of the device. The excessive liquid sample is then ejected out of the plastic box by the transfer pipette, leaving the basil-seed based SERS substrate exposed to air. This reduces the interference of the opaque liquid sample on the SERS signal, and avoids the tedious procedure for extraction of melamine from the milk. As a result, the pipette-basil-seed based SERS device can be used to detect melamine in milk rapidly. This work has demonstrated a facile approach to construct a low-cost, safe, disposable, user-friendly, and field-deployable portable SERS device.
Surface-enhanced Raman scattering (SERS) provides high sensitivity, fast response, and unique “fingerprint identification” of target molecules.1-3 In the past decade, research has been focused on designing effective SERS substrates with high activity.4-7 To enable field-deployable detection of SERS substrates, the first challenge is that a SERS substrate needs to be integrated into a fluidic system to have user-friendly interface so that a lay-person can operate the sensing system without need of any professional personnel. The second challenge is that the interference of real-world complex sample matrices such as whole blood and milk on the SERS signal intensity needs to be mitigated or even eliminated. For example, when the transparency of the analyte solution is poor, the incident light is scattered, attenuating the SERS signal intensity. Currently, the common strategy for addressing the abovementioned challenges is integrating the SERS substrate to a microfluidic chip,8-15 which proves to be an effective approach. However, a microfluidic system requires complicated fabrication process. Also, most active microfluidic systems require external syringes or pumps to drive the analyte solutions in the microfluidic channel. 12-19 An alternative approach is combination of a SERS substrate with a macro-fluidic system. For example, a pipette has been used to transport the liquid sample.20-22 In the present work, a basil-seed supported SERS substrate is incorporated with a plastic transfer pipette to form a portable field-deployable device, as shown in Figure 1. In this device, the Ag nanoparticles (Ag-NPs) are deposited on a basil-seed that is composed of a three-
dimensional (3D) network of natural fibres, which serves as the SERS substrate (denoted as Ag-NPs@basil-seed). This SERS substrate is then embedded into a small polymethylmethacrylate (PMMA) box. A commercial plastic transfer pipette is cut into two segments. The two segments are then inserted to the two opposite sides of the PMMA box, respectively (Figure 1). Besides novel design, easy assembly, low cost, this pipette-basil-seed based SERS device has several unique features: First, the whole systems can be assembled easily in less than 10 min. Second, the analyte solution can be loaded into the PMMA box to wet the Ag-NPs@basil-seed SERS substrate by creating a negative pressure inside the transfer pipette without using a syringe or a pump. Third, the basil-seed based SERS substrate will be swollen 17 to cram the PMMA box after the liquid is absorbed on it. The excessive liquid sample can be easily pushed out of the PMMA box. In this case, the interference of an opaque sample matrix on the SERS signal can be reduced. This is very helpful when the device is used to measure toxic analytes such as melamine in milk because it overcomes the poor transparency of milk and avoids the tedious steps to extract melamine from the milk. Fourth, compare to the recyclable SERS substrate,23 the pipette-basil-seed based SERS sensor is a disposable substrate, and can be discarded after detecting poisonous substance. Therefore, the pipette-basil-seed based SERS assembly shows a promising potential in field-deployable detection of pollutants in the aqueous solution.
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Figure 1. Schematic illustration of preparation of the pipette-basil-seed based SERS device and SERS Measurement.
In this work, the pipette-basil-based device is used to detect melamine in milk. Melamine has the high nitrogen content. If human takes milk products and wheat gluten containing melamine, acute kidney failure, serious renal problems and even death may occur. 24 This raises a serious concern on food safety. Several laboratory-based analytical techniques have been used to measure melamine, including the enzyme-linked immunosorbent assay (ELISA),25 the gas chromatography/mass spectrometry (GC/MS),26 the high-performance liquid chromatography (HPLC).27 These methods require large-scale instruments that are operated by professional personnel, and cannot be used in-field. Recently portable devices such as an electrochemical sensor28 and a colorimetric sensor 29 have been developed for melamine detection. Nevertheless, both the electrochemical and colorimetric sensors are vulnerable to the interference of complex sample matrices. Hence the SERS technique has been developed to detect melamine in milk.27, 30-31 However, tedious processes, such as ultrasonication, extraction, centrifugation and filtration, are usually involved in extracting melamine from the milk prior to detection, which hinders the measurement in-field. It is imperative to develop a portable SERS device that is able to circumvent the abovementioned drawbacks in order to enable rapid fielddeployable detection of melamine in milk. Natural basil-seeds can absorb water and further swell into a fluffy ball owing to their pectinous fibrillar polysaccharide layers.32 Figure S1a shows the photograph of the dry basil-seeds with an average diameter of 2.05 mm. After freeze-drying, the average diameter of the soaked basil-seeds became 4.5 mm (Figure S1b). Large flakes and numerous cross-linked fibres formed a 3D network (Figure S1c and S1d), which can serve as a scaffold for loading high-density of Ag-NPs. The Ag-NPs were prepared via the following silver mirror reaction process: CH2OH(CHOH)4CHO+2Ag(NH3)2OH→CH2OH(CHOH) 4COONH 4+2Ag↓+3NH 3↑+H2O There exist a lot of hydroxyl groups on the flakes of the basil-seeds derived from the fibrillar layer. The free Ag(NH3)2+ ions in the solution tend to bind with the functional groups on the basil-seeds. Subsequently the
Ag(NH3)2+-fibre compound immediately reduces Ag + to Ag in the presence of glucose. As a result, Ag-NPs are deposited on the basil-seeds to form the so called AgNPs@basil-seeds. Figure 2a shows the freeze-dried Ag-NPs@basil-seed, in which the peculiar morphology of the Ag-NPs@basilseed was retained being swollen in the aqueous solution. Figure 2b reveals the high-density Ag-NPs that were uniformly deposited on the flakes of the basil-seeds. The enlarged TEM view further reveals that the Ag-NPs had an average diameter of about 45 nm, and the gap between the neighbouring Ag-NPs was less than 10 nm (Figure 2c). For the oven-dried Ag-NPs@basil-seed (Figure 2d), the 3D-network of the Ag-NPs@basil-seed was shrunk. The higher magnification images (Figure 2e and 2f) show that the Ag-NPs were aggregated closely on the surface of the basil-seed due to the shrinking of the basil-seed, in which the inter-particle gap was also reduced upon oven-drying. Energy dispersive X-ray (EDS) spectrum (Figure S2) further reveals that the resultant NPs were composed of Ag elements. The UV-vis absorption spectrum (Figure S3) of the Ag-NPs@basil-seeds revealed the localized surface plasmon resonance (LSPR) band at around 420 nm (red curve). In the absence of the Ag-NPs, there was obviously no any absorption band (black curve) for the bare basilseeds. The 1 μM R6G solution was used to test the SERS performance of the free-standing Ag-NPs@basil-seed. Although the basil-seeds were organic compounds, the Raman bands at different laser excitations of 532, and 633 nm were rather weak under the low laser power excitation (Figure S4.). To assess the signal reproducibility, we measured the SERS spectra of R6G at 10 random spots from the Ag-NPs@basil-seed substrate. The relative standard deviation (RSD) of the vibrational peak at 615 cm-1 taken on the Ag-NPs@basil-seed was calculated to be 14.8% (Figure S5), showing good SERS signal uniformity of the Ag-NPs@basil-seed. To further evaluate the SERS signal reproducibility of the Ag-NPs@basilseeds from batch to batch, signal variations at 615 cm -1 of R6G taken from randomly chosen 25 substrates (Figure S6) demonstrated that the RSD was about 16.5%, revealing good SERS-signal reproducibility of Ag-NPs@basil-
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Figure 2. (a) Low-magnification, and (b) High-magnification SEM images of the soaked Ag-NPs@basil-seed after freeze-drying. (c) TEM image taken from a slice of freeze-dried Ag-NPs@basil-seed. The inset is the inter-particle gap histogram, (d-f) SEM images of the soaked Ag-NPs@basil-seed after oven-drying with different magnifications.
seeds from batch to batch. Also, the detection concentra tion of R6G with the Ag-NPs@basil-seed can be down to 1 pM (Figure S7), revealing high sensitivity of the AgNPs@basil-seed substrate. R6G was also measured with the pipette-Ag-NPs@basil-seed device. The measurement process can be real-time and in-situ observed as shown in Figure S8a. The specific peaks of R6G can be distinguished from the background within less than one minute and the SERS-signal intensity increased gradually. After 5 minutes, it reached the equilibrium condition. It has been reported that the peak intensity of SERS substrate embedded inside the microfluidic channel was ~4 times lower than that of the open SERS substrate. 19 In our case, the SERS intensity obtained from the pipette-Ag-NPs@basilseed device was only 2 times lower than that of the open Ag-NPs@basil-seed substrate (Figure S8b). The pipette-Ag-NPs@basil-seed device was used to detect methyl parathion in orange juice. Methyl parathion is a widely used hazardous insecticide, which may be trans
Figure 3. SERS spectra of methyl parathion in orange juice under excitation the 633 nm laser with the pipette-Ag-NPs@basil-seed device. The inset is the chemical structure of methyl parathion.
ported into soil, groundwater and food.33 It was found that the wavelength of a 633 nm laser was more suitable for acquiring the SERS spectra of methyl parathion in orange juice (Figure S9). The rational explanation is that the orange juice should have a strong adsorption of 532 nm excitation line, resulting in the fluorescence background signals (Figure S10). Figure 3 shows the SERS spectra of methyl parathion in juice at different concentrations from 50 μM to 0.1 μM by using the pipette-Ag-NPs@basilseed SERS device. The dominated characteristic peaks were located at 856, 1107, 1142, 1348, 1402, and 1595 cm-1.34-35 The detection concentration of methyl parathion down to 0.1 μM (0.026 ppm) was beyond the limit ranges from 0.1 to 1.0 ppm, as recommended by the Collaborative International Pesticides Analytical Council.36 A further practical test of the pipette-Ag-NPs@basilseed device was carried out in the poor transparent liquid, such as milk containing melamine. SERS spectra were acquired from the milk with different concentrations of melamine from 1 mM to 1 μM (Figure 4a) after the excessive milk was ejected out of the PMMA box by the transfer pipette. Furthermore, the SERS intensity at 685 cm-1 was proportional to the logarithmic concentration of melamine (Figure 4b). The linear relationship was observed from 1 μM to 1 mM with a correlation coefficient of 96.6%. The limit of detection (LOD) was calculated to be 0.85 μM (0.107 ppm), based on a signal-to-noise ratio of 3 according to the International Union of Pure and Applied Chemistry (IUPAC) standard.37 The LOD was lower than the safety limits (2.5 ppm in USA and in Europe Union, and 1 ppm for infant formula in China).24 For the sake of comparison, Ag-NPs@basil-seed was used to obtain the SERS spectra of melamine in milk with different concentrations in the open PMMA box without a glass cover on the top, as shown in Figure S11. The designed height of the cavity of the PMMA box was constructed from 3 mm to 5 mm, in which the Ag-NPs@basil-seed can be fully soaked in milk. Even though the glass cover
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Intensity distribution; SERS sensitivity, SERS evolution of R6G and average spectra; SERS spectra of methyl parathion, UV spectra of methyl parathion and pure orange juice, SERS spectra of melamine
* *
E-mail:
[email protected] E-mail:
[email protected] The authors declare no competing financial interest.
We thank the support from National Key Basic Research Program of China (Grant 2013CB934304), the Natural Science Foundation of China (Grants 51632009, 51628202, 51201159 and 51472245), SRG-HSC and the CAS/SAFEA International Partnership Program for Creative Research Teams for the financial support.
Figure 4. (a) SERS spectra of melamine in milk under excitation of the 532 nm laser measured with the pipette-Ag-NPs@basilseed SERS device. The inset shows the chemical structure of melamine. (b) Linear correlation between the SERS intensity at 685 cm-1 and the logarithmic concentration of melamine. The red curve is collected from the pipette-Ag-NPs@basil-seed SERS device; the black curve is obtained from melamine in milk using Ag-NPs@basil-seed soaked in the milk in the open PMMA box.
was not used, the sensitivity collected from the AgNPs@basil-seed soaked in milk in the open PMMA box was worse due to the poor transparency of the milk. Our results show that the SERS intensity can be enhanced after the excessive milk was pushed out of the PMMA box with the transfer pipette. In summary, the transfer pipette can be used for flow injection in the portable pipette-Ag-NPs@basil-seed SERS device. The analyte solution can be loaded easily onto the SERS substrate by applying a pressure on the transfer pipette, significantly improving the portability of device. The pipette-AgNPs@basil-seed device was successfully used to detect methyl parathion in orange juice, and melamine in milk. The pipetteAg-NPs@basil-seed device has a great promise in fielddeployable detection of food additive or pollutants in the aquatic environment.
Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental Section, Photographs and SEM images, EDS spectrum, UV-Visible spectra, Raman spectra, SERS spectra,
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