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SERS-based pesticide detection by using nanofinger sensors

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Nanotechnology Nanotechnology 26 (2015) 015502 (7pp)

doi:10.1088/0957-4484/26/1/015502

SERS-based pesticide detection by using nanofinger sensors Ansoon Kim1, Steven J Barcelo2 and Zhiyong Li2 1

Center for New Functional Materials Metrology, Korea Research Institute of Standards and Science, Daejeon, 305-340, Korea 2 Systems Research Lab, Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 94304, USA E-mail: [email protected] and [email protected] Received 11 July 2014, revised 12 October 2014 Accepted for publication 16 October 2014 Published 9 December 2014 Abstract

Simple, sensitive, and rapid detection of trace levels of extensively used and highly toxic pesticides are in urgent demand for public health. Surface-enhanced Raman scattering (SERS)based sensor was designed to achieve ultrasensitive and simple pesticide sensing. We developed a portable sensor system composed of high performance and reliable gold nanofinger sensor strips and a custom-built portable Raman spectrometer. Compared to the general procedure and previously reported studies that are limited to laboratory settings, our analytical method is simple, sensitive, rapid, and cost-effective. Based on the SERS results, the chemical interaction of two pesticides, chlorpyrifos (CPF) and thiabendazole (TBZ), with gold nanofingers was studied to determine a fingerprint for each pesticide. The portable SERS-sensor system was successfully demonstrated to detect CPF and TBZ pesticides within 15 min with a detection limit of 35 ppt in drinking water and 7 ppb on apple skin, respectively. Keywords: SERS, pesticide sensing, portable detector (Some figures may appear in colour only in the online journal) Introduction

simple, low-cost, and sensitive method of detecting trace level of two pesticides, chlorpyrifos (CPF) and thiabendazole (TBZ), by using the gold nanofinger chips based on SERS technique. CPF is one of the most widely used pesticides, which plays a critical role in protecting fruit or crops. Research has indicated CPF as a neurotoxin, carcinogen and suspected endocrine disruptor, and it has been associated with asthma, reproductive and developmental toxicity, and acute toxicity [17]. Nevertheless, the CPF residue was found on 51 foods including fruits, vegetables, grains, beans, and even butter in a report by US Department of Agriculture [18]. The US Environmental Protection Agency (EPA) also analyzed National Water Quality Assessment (NAWQA data) for surface water contamination. A total of 1530 agricultural streams and 604 urban streams were tested. Of the streams tested, 15% of the agricultural streams and 26% of the urban streams contained CPF pesticide at concentrations ranging from 26 ppt to 400 ppt [19, 20]. According to the EPA’s guideline, the acute drinking water level of concern for children ages 1–6 years is 900 ppt; the chronic is 150 ppt. In addition, the

In agriculture, numerous pesticides are commonly used to protect crops and seeds. Pesticide residues may remain on or in food through air, water, and soil after they are applied to food crops. The levels of these residues in foods may cause environmental impact and public health risks. Pesticides can be carcinogenic, mutagenic, or cytotoxic. Due to the common use of various pesticides and their impact, surface-enhanced Raman scattering (SERS) has long been projected as a powerful analytical technique for chemical and biological sensing applications [1–11] because SERS technique can be used for highly sensitive detection of targets over practical time scales. Pairing with portable Raman spectrometers makes the technique extremely appealing as real-time sensors for field application. However, the lack of reliable, low cost and easy to use SERS enhancement structures has prevented the wide adoption of this technique for general applications. Previously, we reported a novel hybrid structure based on the high-density arrays of gold nanofingers over a large surface area for SERS applications [12–16]. We report here a rapid, 0957-4484/15/015502+07$33.00

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© 2015 IOP Publishing Ltd Printed in the UK

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Figure 1. Schematic illustrations of portable pesticide sensing using pentamer gold nanofingers. The representative SEM images (scale

bar = 500 nm) of open (a) and closed (b) pentamer gold nanofingers before and after treatment with the pesticide solution, respectively, are shown along with the illustration of how the gold nanofinger chips were used for pesticide sensing. The photographs show the prototype portable sensor system, which include (a) the nanofinger sensor strip and (c) the custom-built portable reader [16].

enhancement for trans-1,2-bis(4-pyridyl)-ethylene (BPE) [14], are shown in figure 1. In this paper, we demonstrate simple, rapid and sensitive detection of CPF and TBZ pesticides using pentamer nanofinger structures and a portable spectrometer (shown in figure 1). The pesticide sample solution is exposed directly onto the SERS strip with periodic nanofinger structures, followed by rinsing with ethanol and drying in air. Finally, Raman spectra can be measured using a custom-built portable spectrometer (figure 1). The entire process can be completed within less than 15 min, which makes it highly practical for field deployable sensing applications. We also present here the characterization of CPF and TBZ bonding on our gold nanofinger surfaces and compare the binding affinity of CPF and TBZ with the surface.

European Union has strictly limited emissions, discharges and losses of CPF, and thus established the tolerance level of CPF below 100 ppt in drinking water [21]. TBZ is a fungicide also used on many agricultural products and found in many animal products. Even though the TBZ has low toxicity to humans overall, it is likely carcinogenic at high exposure, and highly toxic to freshwater and estuarine fish and invertebrates [10]. The US Department of Agriculture Pesticide Data Program demonstrates that the TBZ is the most frequently detected pesticide on bananas and detected in more than 60% of the apple samples over the entire sampling period [15]. The established tolerance level of TBZ by the US EPA is 5 ppm on pome fruits [10]. Previously, we reported that the flexibility and high aspect ratio of gold nanofingers enable the fingers to undergo a self-closing process during evaporation of solution (figure 1) [12–16]. During the closing process, the self-limited sub-nm gaps are created between the finger tips to trap molecules. The molecules thus being trapped among the touching gold nanofinger tips will experience greatly amplified electromagnetic fields under incident laser illumination, and hence generate uniform, strong Raman signals for molecule identification and quantification. Free standing pentamer nanofingers arranged in five-fold symmetry, which was identified previously as providing the highest signal

Experimental details Preparation of gold nanofinger chips

As described in previous reports [12, 14], the polymer nanofinger structures were fabricated on Si wafers using nanoimprint lithography (NIL). The typical diameter of each nanofinger was 140 nm and the height was 530 nm. Gold with nominal thickness of 70 nm was deposited over the polymer nanofingers by e-beam evaporation to produce the gold 2

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nanofinger substrates. The substrates were mounted onto strips and evaluated using a custom-built portable spectrometer [11]. Sampling procedures

Chlorpyrifos (CPF, Sigma-Aldrich) and thiabendazole (TBZ, Sigma-Aldrich) were dissolved in deionized water (DI) to prepare a 1000 ppm stock solution. Each solution was further diluted in DI water for sensing. Bottled water and organic apples were purchased from local grocery stores for testing with each pesticide. For TBZ sensing on apple skin, we dipped three different whole apples in 400 mL of TBZ solution for 2 s, and monitored the surface uptake of the solution by calculating the volume difference of the solution before and after dipping [22]. For example, the surface uptake by dipping 230 g of apple was 0.45 mL. Taking the 1 ppm contamination for example, the concentration of TBZ solution for dipping should be 511.11 μg mL−1 (230 g/ 0.45 mL × 1 μg g−1 = 511.11 μg mL−1). In order to mimic real applications for TBZ sensing, the 1 ppm apple was dipped in 400 mL of water with shaking for 1 min to release the contaminated TBZ from apple skin. If all of the TBZ is recovered, the TBZ concentration in the rinsed solution should be 0.575 μg mL−1 (511.11 μg mL−1 × 0.45 mL/ 400 mL = 0.575 μg mL−1). Because the real-world application by dipping a whole piece of apple in large amount of water is difficult to handle the whole apple and such large amount of water, we developed the following simplified procedure. First, we calculated TBZ concentration of 20 μL solution exposed onto apple skin to mimic the above dipping procedure. Assuming that all the TBZ molecules exposed onto apple skin are recovered by rinsing with 2 mL of water, the TBZ concentration in the 20 μL solution should be 57.5 μg mL−1 to prepare the 1ppm apple sample (0.575 μg mL−1 × 2 mL/ 20 μL = 57.5 μg mL−1). We verified our procedure by comparing the Raman spectra obtained from both sampling procedures, where the Raman intensity of all the TBZ peaks were similar for the three tested apples. Figure 6 were obtained by using this simplified sampling procedure. A pentamer gold nanofinger chip was exposed to the pesticide sample solution for 10 min and then air-dried. In order to ensure gold nanofinger closing, the chips were rinsed with pure ethanol before Raman measurement.

Figure 2. Raman spectra of CPF powder (black) and CPF-trapped

nanofingers (red). The molecular structure of CPF is depicted in the inset. The intensity of the powder spectrum was magnified ten times. Measurements from the powder and nanofingers were collected with ∼3 mW and ∼0.3 mW laser power, respectively.

300 μW at the powder and nanofinger sample surfaces, respectively. All spectra obtained from this spectrometer were collected with the same setup through a 100× objective lens and recorded with a 5 s accumulation time. The custom-built portable Raman spectrometer (13 cm × 11 cm × 6 cm), shown in figure 1, is equipped with a 785 nm diode laser for illumination over an area about 100 μm in diameter on the sample surface. The portable spectrometer consists of a diode laser excitation source, collimating lens, a beam splitter, focusing lens, a grating and CCD detector. Pesticide sensing was achieved using a laser power of 10 mW with 1 s integration time. The spectral resolution of this instrument is 10−12 cm−1. Results and discussion Previously, we have discovered the high-density and uniform arrays of gold nanofingers fabricated over a large surface area by NIL for SERS applications [12–16]. As shown in figure 1, free standing pentamer gold nanofingers arranged in five-fold symmetry were exposed to pesticide sample solution because the pentamer nanofingers were identified previously as providing the highest signal enhancement for trans-1,2-bis(4pyridyl)-ethylene (BPE) [14]. The exposure to the sample solution enables the fingers to collapse into well-defined groups and form self-limiting sub-nm gaps between the gold finger tips [13]. At the same time, the pesticide molecules thus can be trapped among the touching finger tips (Raman hot spot). For pesticide sensing, we prepared different concentrations of CPF solution in deionized (DI) water. The freestanding nanofingers mounted on the black strip, shown in figure 1(a), were dipped in the CPF solution, and then dried in air to trap the CPF molecules in the Raman hot spot. In order to form fully-closed nanofinger (figure 1(b)), the CPFexposed chips were then rinsed with ethanol and subsequently dried in air [16]. The SERS strips were then simply inserted

Raman measurements

Raman measurements were performed either using an upright confocal Raman microscope (Horiba Jobin Yvon T64000) equipped with a nitrogen-cooled multichannel CCD detector or a custom-built portable Raman spectrometer. The laboratory Raman spectrometer was used to characterize the interaction of CPF pesticide with the gold nanofinger surface (figure 2). All other spectra for CPF and TBZ sensing were collected from the custom-built portable Raman spectrometer described in the previous reports [16]. For the laboratory Raman spectrometer, a 784.6 nm solid state laser was used as the excitation source with a measured power of 3 mW and 3

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Table 1. Raman peak assignments of powder CPF and CPF-trapped

nanofingers. Frequency (cm−1) Powder CPF

CPF on nanofinger chips

969 1100 1241 1276

951 1066 1147, 1183 1205 1327

1407 1451 1569

1435 1503 1569

Assignments Cl-ring ν(P−O−C)1 Ring breathing Cl-ring Cl-ring modes, δ(C−H) Cl-ring modes Cl-ring modes Ring stretching

into the sample slit in the portable reader (shown in figure 1(c)), and Raman spectra were acquired with 5 s integration time. Compared to a laboratory Raman system, which is equipped with bulk laser sources, a series of lenses and mirrors defining the optical path, liquid nitrogen cooling CCD detector with a microscope attachment, the portable spectrometer delivers similar performance when used with our SERS strips [16]. By comparing the Raman spectra measured from pesticide powder [23–28] and pesticide-trapped nanofingers, we can understand the interaction of pesticide molecules with gold nanofingers. Figure 2 shows the comparison of the Raman spectra measured from CPF powder and CPF-trapped nanofinger surfaces. The Raman spectrum of the CPF powder (black) is in good agreement with the data from previous literature [29, 30]. Table 1 summarizes the peak assignments of unenhanced CPF Raman and SERS spectra. The most intense peak in the powder spectrum was observed at 1569 cm−1, and assigned to the ring stretching mode, which involves the pyridine ring of CPF molecule [8, 17]. Compared to the powder spectrum, the Raman spectrum of the trappedCPF nanofingers shows no peak shift of the pyridine ring stretching mode. The peak at 1100 cm−1 was previously assigned to P−O−C stretching mode [31], while the peaks at 1241, 1276, 1407 and 1451 cm−1 are due to Cl-ring modes [8, 18]. The Raman spectra of CPF-trapped nanofingers show the peak shifts of both P−O−C stretching and Cl-ring modes (shaded in table 1) compared to the spectrum of the powder. It is expected that CPF molecules may interact with gold nanofingers through either phosphate or chlorine groups rather than the pyridine ring of CPF. Despite the peak shifts in the SERS spectra compared to the powder spectrum, the SERS spectra obtained from the finger substrate can be used as a fingerprint for CPF sensing. In order to perform quantitative CPF sensing using the nanofinger chips, different CPF water solutions with concentrations from 350 ppm to 35 ppt were introduced to the periodic pentamer nanofinger chips for SERS measurements. Figure 3(a) shows the SERS spectra of pentamer nanofingers exposed to the different concentrations of CPF. The spectra indicate that the characteristic CPF peaks are clearly observed

Figure 3. Quantitative data of CPF sensing using gold nanofinger

chips. (a) SERS spectra of CPF in water at different concentrations. (b) Quantitative curves of CPF sensing. The Raman intensity measured at 1327 cm−1 was plotted as a function of CPF concentration. The error bars were obtained from multiple measurements on different surface areas and chips. The red curve is a fit to the experimental data using Hill’s equation.

at a concentration of 35 ppt, and saturate at around 3.5 ppm. The intensity of the strongest CPF characteristic peak at 1327 cm−1 was plotted as a function of the CPF concentration in figure 3(b). The error bars were obtained from at least two measurements on three different locations on the same chip as well as on three different chips. The quantitative plot was fitted using the Hill equation [32] to further understand the interaction of CPF molecule with the gold nanofingers. According to our previous result [16], the SERS intensity (ISERS) is generally proportional to the surface coverage of adsorbed molecules on hot spots (Θ), assuming all hot spots are uniform over the surface. The Hill equation (1) can be expressed as shown below: ISERS = VΘ = V

cn , k n + cn

(1)

where V is a constant, c is the concentration of the molecular solution, k is the equilibrium constant for dissociation, and n 4

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Figure 4. SERS spectra of CPF measured in bottled mineral water at different concentrations.

is a cooperative constant. Based on the fitting results, we obtained n = 0.41, which indicates that surface-bound CPF molecule decreases the affinity of the incoming molecules to the surface (negatively cooperative binding), and k = 1.64 ppb. At concentrations above 3.5 ppm, the SERS intensity reached a plateau indicating the adsorption of the molecules saturated the surfaces. At low concentrations, where c ≪ k, a log–log plot of ISERS versus c revealed a linear relationship ( log ISERS ≈ n log c + const), as shown in the inset of figure 3(b) for the experimental data at concentrations below 1 ppb. This suggests that pentamer nanofinger SERS chips can be used as reliable sensor for quantitative analysis of CPF at low concentrations in water solution. Therefore, we achieved the limit of detection (LOD) of 35 ppt, well below the CPF tolerance level for drinking water by the European Union restriction (100 ppt). After demonstrating the sensitivity of nanofinger sensor for CPF detection in DI water, we also analyzed CPF in bottled mineral water samples. We spiked bottled mineral water samples purchased from a local grocery store with different CPF concentrations from 0 ppt to 350 ppb, and tested the samples with the same method as used for the DI water solutions. Figure 4 shows the SERS spectra obtained from the mineral water exposed to different CPF concentrations. Considering the high signal to noise ratios in the spectra, the results depict clear differences between the spectra of the control (0 ppb) and the water samples spiked with low concentrations of CPF (from 35 ppt to 3.5 ppb). The characteristic CPF peaks at 1205, 1327, 1435, 1503, and 1569 cm−1 also appear in the bottled water samples spiked with CPF, but no CPF peaks appear in the pure water sample. The peaks from the pure sample are possibly associated with various minerals or salts. We found that the detection limit for CPF sensing both in mineral water and DI is same, whereas the CPF peaks obtained from mineral water samples are a little less intense.

Figure 5. Quantitative data of TBZ sensing on nanofinger chips. (a) SERS spectra of TBZ dissolved in water at different concentrations. (b) Quantitative curves of TBZ sensing. The Raman intensity measured at 1011 cm−1 was plotted as a function of TBZ concentration. The error bars were obtained from multiple measurements on different surface areas and chips. The red curve is a fit to the experimental data using Hill’s equation.

Similar to the CPF sensing, the TBZ pesticide was detected using the periodic pentamer gold nanofinger chips. The preparation method of the SERS sensor chip for pesticide sensing is the same as that for CPF sensing, shown in figure 1. Quantitative TBZ sensing was performed by introducing different TBZ water solutions with concentrations from 100 ppm to 1 ppb to free-standing periodic pentamer nanofinger chips. Figure 5(a) shows the SERS spectra of the pentamer nanofingers exposed to different concentrations of TBZ. Similar to the previous literature [22] studied on the TBZ interaction with silver substrate, the SERS spectra show the characteristic TBZ peaks measured at 781, 1011, 1274, and 1547 cm−1. The strong Raman peaks observed at 781 and 1011 cm−1 are associated with out-of-plane vibration modes of delocalized π electrons in planar TBZ rings. Based on the previous literature, the observation of strong Raman intensities related to the out-of-plane vibration modes suggests that the main adsorption configuration is the parallel orientation of TBZ molecules to the gold nanofinger surface. The SERS spectra indicate that the characteristic TBZ peaks are clearly observed at a concentration of 1 ppb. The intensity of the 5

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the organic apple control sample, which are different from the TBZ characteristic peaks (781, 1011, 1274, and 1547 cm−1). The peaks from the control sample are possibly associated with contaminants on the apple skin such as wax coating materials. Considering the high signal to noise ratios in the spectra, the results depict clear differences between the spectrum of the control (0 ppb) and the apple samples (from 7 ppb to 140 ppb) spiked with low concentrations of TBZ. The characteristic TBZ peaks appear in the apple sample spiked with 7 ppb TBZ, but no such peaks appear in the control sample. We found that our gold nanofinger sensor can detect the TBZ concentration on apple skin as low as 7 ppb, which is three orders of magnitude lower than the EPA’s regulation level.

Conclusion

Figure 6. TBZ sensing on the organic apple skin spiked with

different concentrations of TBZ. The black Raman spectrum was obtained from the gold nanofinger exposed to 100 ppb of TBZ solution for the comparison purpose. The concentration-dependent SERS spectra (red) were measured from organic apple skin spiked with different concentrations of TBZ solutions.

We demonstrated simple, rapid, and field-deployable sensing of a trace amount of pesticides, CPF and TBZ, based on SERS with pentamer gold nanofinger structures. According to the fitting results obtained from the quantitative curves, we found that the interaction of CPF binding with the gold nanofinger surface is stronger than the TBZ interaction. Using the high performance and reliable gold nanofinger SERS chips, we have achieved the LOD of 35 ppt for CPF in drinking mineral water, and 7 ppb for TBZ on apple skin. For both pesticides sensing, we have successfully developed a simple and rapid (within 15 min) sensing technique to detect trace level pesticides below the regulated levels using our portable sensor system. The demonstration of the high performance of our portable sensor system opens new opportunities for simple, rapid, and inexpensive chemical and biological sensing.

most intense characteristic peak of TBZ at 1011 cm−1 was plotted as a function of the TBZ concentration in figure 5(b). The error bars were obtained from at least two measurements on three different locations on the same chip as well as on three different chips. The quantitative plot (figure 5(b)) shows the TBZ sensor is saturated at around 10 ppm. Based on the quantitative plot fitted with Hill equation [32], we obtained n = 0.63, which indicates the negatively cooperative binding of TBZ, and k = 92.57 ppb. Compared to the CPF interaction (k = 1.64 ppb) with gold nanofingers, it was found that the TBZ molecule interacts with the gold naonfinger with about two orders of magnitude lower binding affinity. The higher sensitivity of CPF (35 ppt) than TBZ (1 ppb) is expected due to the higher binding affinity of CPF on gold nanofingers. After analyzing the sensitivity of TBZ sensor in DI water samples, we detected TBZ on apple skin. We spiked an organic apple purchased from a local grocery store with different concentrations of TBZ solution dissolved in DI water. The spiked apple was rinsed with DI water, and then the pentamer nanofinger chip was dipped into the rinsed solutions for 10 min. Before measuring SERS, the chip was rinsed with ethanol and dried in air in order to close the nanofingers. The spiked TBZ concentration on apple skin was calculated by using the apple weight and spiked TBZ mass as described in the Experimental section. Figure 6 shows the SERS spectrum (black) of 100 ppb TBZ in water and the concentrationdependent SERS spectra (red) measured from organic apple skin spiked with different concentrations of TBZ solutions. Compared to the SERS spectrum measured from 100 ppb TBZ solution, all the characteristic TBZ peaks were observed from the spiked apple samples. As shown in the spectra, the Raman intensity of the TBZ peaks increased with TBZ concentrations between 0 ppb and 140 ppb. We also measured the SERS peaks at 985, 1222, 1263, 1358, 1434 and 1547 cm−1 in

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SERS-based pesticide detection by using nanofinger sensors.

Simple, sensitive, and rapid detection of trace levels of extensively used and highly toxic pesticides are in urgent demand for public health. Surface...
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