Shell Thickness-Dependent Raman Enhancement for Rapid

Article pubs.acs.org/ac

Shell Thickness-Dependent Raman Enhancement for Rapid Identification and Detection of Pesticide Residues at Fruit Peels Bianhua Liu,† Guangmei Han,†,‡ Zhongping Zhang,*,† Renyong Liu,† Changlong Jiang,† Suhua Wang,† and Ming-Yong Han*,†,§ †

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui 230031, China University of Science & Technology of China, Hefei, Anhui 230026, China § Institute of Materials Research and Engineering, A-STAR, 3 Research Link, 117602, Singapore ‡

S Supporting Information *

ABSTRACT: Here, we report the shell thickness-dependent Raman enhancement of silver-coated gold nanoparticles (Au@ Ag NPs) for the identification and detection of pesticide residues at various fruit peels. The Raman enhancement of Au@Ag NPs to a large family of sulfur-containing pesticides is ∼2 orders of magnitude stronger than those of bare Au and Ag NPs, and there is a strong dependence of the Raman enhancement on the Ag shell thickness. It has been shown for the first time that the huge Raman enhancement is contributed by individual Au@Ag NPs rather than aggregated Au@Ag NPs with “hot spots” among the neighboring NPs. Therefore, the Au@Ag NPs with excellent individual-particle enhancement can be exploited as stand-alone-particle Raman amplifiers for the surface identification and detection of pesticide residues at various peels of fruits, such as apple, grape, mango, pear, and peach. By casting the particle sensors onto fruit peels, several types of pesticide residues (e.g., thiocarbamate and organophosphorous compounds) have been reliably/rapidly detected, for example, 1.5 nanograms of thiram per square centimeter at apple peel under the current unoptimized condition. The surfacelifting spectroscopic technique offers great practical potentials for the on-site assessment and identification of pesticide residues in agricultural products.

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the determination of surface molecules at materials/living cells/ organisms,14,22 the probe of explosive particulates at luggages,23 and the detection of pesticide/herbicide residues in agricultural products.24,25 The currently employed near-field or tipenhanced SERS has many limitations in application due to rather weak Raman signals for the use only in a very flat solid surface. Meanwhile, highly SERS-active metal NPs in their aggregates can produce interparticle hot spots, but the surface targets are very difficult to enter the conjunctions among the aggregated NPs. In addition to the ineffective contact with targets, the inconsistency of NPs aggregation (e.g., size, shape, geometry, and so on) will also greatly affect the localized plasmon resonance,26 and this makes the reliable and quantitative measurements of targets almost impossible. In contrast, individual metal NPs with high SERS activity are very desirable for intimate surface adsorption of targets so as to achieve their sensitive and quantitative analysis;27 for example, metal NPs isolating each other by thin silica shells have been successfully applied to probe target molecules at surfaces.28

ith the integration of high sensitivity, unique spectroscopic fingerprint, and nondestructive data acquisition,1−8 surface-enhanced Raman scattering (SERS) technique has become one of the most widely pursued spectroscopic tools for the identification and detection of chemical and biological species2,3 and the molecular imaging and monitoring at cellular,4,5 tissue,6 and animal levels.7,8 In the last decades, tremendous efforts have been made to develop highly SERSactive metal nanoparticles (NPs; e.g., Ag and Au) in colloids9,10 or films;11−13 in particular, single-molecule detection has been realized by achieving enormous Raman enhancement from aggregated metal NPs.1,14,15 The conjunctions among metal NPs in their aggregates (hot spots) can maximize the localized electric field to greatly amplify the Raman signals of target molecules.15 This strategy has greatly stimulated an intense interest to prepare metal NPs into different configurations such as dimers,16,17 trimers,18 and regular arrays,19,20 which further optimize the localized surface plasmon resonance to achieve strong and stable SERS readout. In addition to the demonstration for SERS assay of chemical/biological samples using various metal nanostructures,21 there has been a strong driving force to expand the SERS technique to nontraditional but important surface analysis, which is greatly needed in material science, environmental/biological inspection, and public/food safety22−25 for © 2011 American Chemical Society

Received: September 19, 2011 Accepted: November 28, 2011 Published: November 28, 2011 255

dx.doi.org/10.1021/ac202452t | Anal. Chem. 2012, 84, 255−261

Analytical Chemistry

Article

Here, we report our investigations on the Raman enhancement behaviors of individual silver-coated gold nanoparticles (Au@Ag NPs) with different shell thickness for the identification and detection of pesticide residues that are annually used in several million tons in agriculture and have caused intense public concerns by directly threatening human/ animal’s health due to their residues in agricultural products. The Au@Ag NPs can obtain the similar high Raman enhancement29,30 to that of large Ag NPs which exhibit stronger Raman enhancement than other metal NPs,10,31 and the Au@Ag NPs can also be synthesized to have uniform size and shape using Au NPs as seeds, which is difficult to be obtained for Ag NPs. The current work reveals the shell thickness-dependent Raman enhancement of individual Au@ Ag NPs for analyzing sulfur-containing pesticides at various fruit peels with sensitivity of a few nanograms per square centimeter under the currently unoptimized condition. The better understanding of underlying enhancement mechanism in the individual metal NPs can provide the useful strategy for the improvements on their Raman enhancement and practical applications. The individual particle-based SERS technique is simple, fast, low-cost, and supersensitive and thus can be used for the on-site analysis of trace pesticide residues to improve food safety.32,33 The corresponding chromatography or mass spectroscopy-based techniques require sophisticated sampling and beforehand separation/extraction and preconcentrated procedures, and thus, they are complicated, have high-cost, and are time-consuming for the off-site pesticide assay.

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RESULTS AND DISCUSSION Characterizations of Au@Ag NPs. Uniform Au@Ag NPs were synthesized in aqueous solution via seed growth through consecutive two-step reactions. (See the experimental section in Supporting Information.)34 First, Au NPs colloid with a size of 30 nm was prepared by the chemical reduction of chloroauric acid with sodium citrate. Subsequently, ascorbic acid solution as reductive agent was mixed into the Au NPs colloid, followed by the addition of AgNO3 solution under vigorous stirring. Due to the match of crystalline lattice between Ag and Au, the resultant Ag metal was selectively grown on the Au cores to form the core−shell Au@Ag NPs, accompanying an obvious color change from wine red to orange. By casting the NPs colloid onto a silicon substrate, the overview scanning electron microscopy (SEM) observation in Figure 1A shows the Au@ Ag NPs are highly monodispersive and have an average particle size of ∼45 nm, whose core−shell structure with an average shell thickness of ∼7 nm was clearly revealed by the transmission electron microscopy (TEM) image (the inset of Figure 1A). Both pure Ag and bare Au NPs were not found as carefully examined by TEM, suggesting the highly selective growth of Ag on the Au cores. The thickness of Ag shells increased with the amount of AgNO3, and they are tunable from 1 to 11 nm for a fixed 30 nm Au core (Figure 1B,C). Meanwhile, it is clear that the core−shell NPs become larger and rounder with the increase of Ag shell thickness. Shell Thickness-Dependent Sensitivity to Pesticides. Figure S1 (Supporting Information) shows the SERS spectrum of 1 × 10−6 M thiram (a typical sulfur-containing pesticide molecule) in Au@Ag colloid and the normal Raman spectrum of thiram powder. The main Raman bands include 564 cm−1 attributed to υ(S−S), 1150 cm−1 to ρ(CH3) or υ(C−N), and 1386 and 1514 cm−1 to ρ(CH3) or υ(C−N), respectively.35 The SERS spectrum greatly differs from normal Raman

Figure 1. (A) SEM image of Au@Ag NPs (inset: the corresponding TEM image). (B) The high-magnification TEM images of Au@Ag NPs with Ag shell thickness from 1, 3, 5, 7, 9, and 11 nm, respectively. (C) UV−vis absorbance spectra of Au NPs (red line) and Au@Ag NPs (black lines) with different Ag shell thickness.

spectrum of thiram powder, for example, the peak at 1514 cm−1 was greatly enhanced by Au@Ag NPs while the peak was not observable in thiram powder. On the other hand, the strongest band at 564 cm−1 from thiram powder becomes much weaker in the SERS spectrum. The characteristics suggest that the thiram undergoes a breakdown in the S−S bond to the formation of two dimethylthiocarbamate fragments due to the laser irradiation and the possible catalysis effect of metal NPs.36 As reported in the literature,35 thiram molecule easily forms the resonated radical structure when interacting with the metal surface such as Ag NPs, leading to the S−S bond cleavage of thiram, which give rise to two dimethyl residues that are strongly adsorbed onto Ag shells by replacing the original citrate ligands at the nanoshells (Figure 2A). The strong adsorption takes place via the SCS group through a bidentate complex with Ag atoms.36 The adsorption of sulfur-containing pesticide at the Au@Ag NPs provides the best possibility for their identification and detection by SERS techniques. A series of experimental observations on the enhancement sensitivity of the core−shell NPs to pesticides were carried out (Figure 2B). We first changed the concentration of thiram from 256



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Figure 2. (A) Representative structure of sulfur-containing pesticide, thiram, and its chemical coordination adsorption at Ag shells. (B) SERS spectra of thiram in Au@Ag NPs colloid with increase of thiram amount (shell thickness: 7 nm). (C) Variation of thiram Raman intensity (at 1386 cm−1) with Ag shell thickness (thiram concentration is 1 × 10−6 M in Au@Ag NPs colloid).

1 nM to 1 μM in the Au@Ag colloid with 7 nm shell thickness. As shown in Figure 2B, the intensity of Raman signals of thiram increases with the thiram concentration. Even at 1 nM level, the strongest signal at 1386 cm−1 can also clearly be detected (the red circle in Figure 2B, also see Figure S2 in the Supporting Information). These confirm that Au@Ag NPs exhibit a huge Raman enhancement effect to the sulfur-containing pesticide by the surface adsorption and plasmonic resonance enhancement. Furthermore, we carried out a systematic study on the Raman scattering intensity as a function of the Ag shell thickness from 1 to 11 nm for fixed 30 nm Au core. Figure 2C shows that the enhancement effect of Au@Ag NPs is closely related to the thickness of Ag shells. When the thickness of shell is smaller than 7 nm, the enhancement effect significantly increases with the shell thickness, in which the Raman intensity for 7 nm Ag shell is about 30-fold that of 1 nm Ag shell. The further increase in thickness leads to the decrease of enhancement effect. On the other hand, detailed experiments reveal that the further increment of shell thickness may make the Au@Ag NPs unstable and easily precipitate due to the larger weight of the NPs, and together with the change of absorption spectra (see the following text), their enhancement effects are thus reduced. Therefore, the Au@Ag NPs with 7 nm Ag shell exhibit the highest Raman enhancement effects that can be used for the Raman detections of pesticide residues at fruit surfaces and in fruit juices. Raman Enhancement Mechanism of Au@Ag NPs. In order to obtain a better understanding on the enhancement mechanism, we first compared the enhancement effects of Au@ Ag NPs (7 nm shell) with those of pure Au and Ag NPs under the identical conditions. The spectra (a) in Figure 3A shows that 1 × 10−6 M thiram in the Au@Ag colloid displays very strong Raman scattering signals. However, the SERS spectra of 1 × 10−6 M thiram in Ag and Au colloids only exhibit very weak Raman intensity, in which the strongest vibrating band at 1386 cm−1 are just only monitored as shown in spectra (b) and (c), respectively. We can thus assess the enhancement sensitivities (Es) of individual particles to thiram by the intensity of the most strong band 1386 cm−1 (I) and the concentration of metal

Figure 3. (A) SERS spectra of 1 × 10−6 M thiram in (a) Au@Ag, (b) Ag, and (c) Au NPs colloids, and the comparison of enhancement effects of the three metal particle colloids. (The Au@Ag NPs have the Au core of 30 nm and Ag shell of 7 nm and the sizes of Ag and Au NPs are 30 nm; the used concentrations of Au@Ag, Ag, and Au NPs are 0.16, 0.12, and 0.24 nM, respectively.) (B) The normalized absorption spectra of (a) Au, (b) Au@Ag, and (c) Ag NPs and the colors of corresponding NPs colloids.

NPs (M) using the equation: Es = I/(C × M), in which C is the concentration of thiram (1 × 10−6 M) and the concentrations (M) of Au@Ag, Ag, and Au NPs are 0.16, 0.12, and 0.24 nM, respectively. It can be estimated that the enhancement effect of Au@Ag NPs is ∼280- and 65-fold those of pure Au and Ag NPs. This suggests that the Au@Ag NPs exhibit an excellent individual-particle Raman activity by 2 orders of magnitude higher than the corresponding Au and Ag NPs. It should be 257



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Figure 4. (A) Evolutions of Au@Ag NPs absorbance and thiram Raman intensity (at 1386 cm−1) with the increase of thiram concentration and (B) TEM images of Au@Ag NPs states at three typical concentrations of thiram. (The inset images show the colors of Au@Ag NPs colloid before and after aggregation. The concentration of Au@Ag NPs is 0.16 nM.)

achieved by the addition of very concentrated salts (e.g., sodium chloride).40 However, this makes the accurate evaluation of the aggregation-based Raman enhancements from colloidal metal NPs almost impossible, due to severe influence on the adsorption of analyte on metal NPs. Here, we found that the aggregations of Au@Ag, Ag, and Au NPs can directly be induced by the addition of target analyte, without the need of additional aggregating agents. With the excellent convenience, the effect of particle states on the Raman enhancement can accurately be estimated by synchronically monitoring the Raman spectra and optical absorption (Figure 4). Here, the peak at 1386 cm−1 is used as the assessment standard of intensity of Raman signals, while the strongest absorbances of Au@Ag, Ag, and Au NPs (at 392, 405, and 526 nm, respectively) are the indicator of particle-aggregating degree. As shown in Figure 4A, the Raman signal of thiram in Au@Ag colloid can be clearly detected in the presence of 1 nM thiram and slowly becomes stronger with the increase of thiram concentration up to 50 nM. Meanwhile, the UV−visible absorbance and color of Au@Ag NPs keep unchanged, suggesting that no aggregation of particles occurs. The observations indicate that the Raman signals from an ultratrace amount of thiram are due to the individual particle resonance enhancement. In contrast, when the thiram concentration is larger than 50 nM, the absorbance of Au@Ag NPs sharply reduces, suggesting the occurrence of Au@Ag NPs aggregation. Figure S3 (Supporting Information) also shows the appearance of a new absorption peak at 780 nm. Furthermore, the TEM images in Figure 4B reveal that the monodispersive state of particles transforms into the aggregating state, and a visible color change from orange to green is also observed (inset images). These confirm that 50 nM is a crucial concentration for the aggregation of Au@Ag NPs. At the same time, the Raman signals of thiram become much stronger as well once the aggregation occurs. The enhancement sensitivity can accurately be evaluated by the ratio of Raman intensity to analyte concentration (I1386/Cthiram) because there is not any additional aggregating agent added. As listed in Table 1, surprisingly, the I1386/Cthiram values decrease when the concentration of thiram is higher than the crucial value of 50 nM for the occurrence of particle aggregation. Therefore, the aggregation of Au@Ag NPs does not enhance their Raman sensitivity to thiram analyte and, on the contrary, leads to the decrease of enhancement effects. These observations suggest that the huge Raman enhancement results from the individual particle resonance rather than the Raman “hot spots” produced

noted that the Raman signals of thiram at the concentration lower than 5 × 10−7 M cannot be monitored using Ag or Au NPs. Au and Ag NPs of ∼30 nm size exhibit a strong plasmonic absorption at 526 and 405 nm, respectively (Figure 3B, a and c). The optical properties of the core−shell Au@Ag NPs are more complex involving the two different plasmon resonance frequencies of Ag and Au interaction in the hybrid particles. As a general observation, the plasmon resonance of the core particles is rapidly masked or attenuated by that of the growing shell, and after passing through a regime, in which two plasmon resonances are present, the shell resonance dominates. For the Au@Ag NPs, the core resonance is rapidly attenuated and blueshifted until the two resonances merge into a single peak at a rather similar wavelength to that which would have been exhibited by Ag NPs. As the interband transitions for Ag NPs are restricted to the UV region, the plasmon resonance damping is minimized,37,38 which means the stronger surface plasmon resonance. When the coating thickness is increased up to 3 nm, the Au core resonance is blue-shifted and a new resonance starts to be observed as well (Figure 1C). When the shell thickness reaches to 7 nm, Ag shell resonance becomes stronger while the Au core resonance is attenuated. As a result, a wide range of plasmon resonance from 320 to 560 nm is obtained (Figure 3B, b). The Au@Ag particles with 7 nm shell thickness display the bright orange that is different from the gray green of Ag NPs and wine red of Au NPs (inset images of Figure 3B). The wide and strong plasmon resonance of Au@Ag NPs is responsible for a higher enhancement effect than those of monometallic Ag and Au NPs. With the further increase of shell thickness to 11 nm, Ag shell resonance start to become dominant, exhibiting a greater blue-shift (Figure 1C). The plasmon resonance at laser excitation of 532 nm becomes weaker, and the smoother Ag shells with the increase of thickness can also reduce their SERS activity or active sites (Figure 1B). Here, the characteristics of plasmon resonance and SERS enhancement can be explained in terms of the interactions between the plasmons of metallic geometrical shapes.37 The plasmon resonance of core−shell particles can be understood from the coupling between the plasmon modes of a nanoshell and a sphere. In theory, the coupling intensity is maximized when the outer metal shell has a thickness in the range of 5−10 nm.39 Raman Enhancement for Individual NPs. It is wellknown that the particle aggregation can greatly enhance the Raman signals of target species located at the conjunction of particles by the so-called “hot spots” effect, which is usually 258



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Table 1. Raman Enhancement Sensitivity of Au@Ag NPs to Thiram at Different Thiram Concentrations (before and after the Aggregation of Au@Ag NPs)a

a

The Au@Ag NPs begin to aggregate at the thiram concentration of 5 × 10−8 M. The data are the average results of three measurements.

by aggregation, which is similar to the Raman behaviors observed on the Ag or Au shells on silica cores.27 Moreover, the aggregation of particles may reduce the adsorption of analyte at their surfaces, with comparison with the colloidal Au@Ag NPs before aggregation, which in turn leads to the decrease of enhancement effects. Moreover, the similar observations were also done on the pure Ag and Au NPs, respectively. (See the detailed results in Figure S4, Supporting Information.) At the concentration of thiram lower than 1 μM, Ag NPs remain intact without undergoing any aggregation as observed by absorption spectra, and thus, there is not any Raman signal to be detected as revealed by Raman spectra. Upon the aggregation of Ag NPs occurring at the thiram concentration higher than 1 μM, the Raman signals are remarkably enhanced due to the formation of hot spots but are still much weaker than that of Au@Ag NPs. Different from Ag NPs (once aggregated, strong SERS signal can be observed immediately), the highly aggregated Au NPs can only give very weak Raman signals of thiram. These above observations further suggest the high individual-particle activity of Au@Ag NPs by the comparison with pure Ag and Au NPs. Detection of Pesticides at Various Fruit Peels. The high Raman activity of Au@Ag NPs is particularly suitable for the particle sensors to probe the residual molecules at various surfaces.28 We now present the unusual but important applications of Au@Ag NPs to the identification and detection of pesticide residues at fruit peels, in which the other techniques either are undetectable or need complicated sample pretreatments. Figure 5A illustrates the principle to detect pesticide residues at various fruit peels by directly casting the “smart” particles onto the surface of peels that are to be probed. In general, the pesticide residues either adsorb at the surface of fruit peels or permeate into the inner of peels. A droplet of ethanol is first added onto the peels and can thus disassociate the pesticide molecules from peel matrix, which is equal to a simple extraction to increase the analyte concentration at the outer surface of peels and the interactions between the Au@Ag NPs and the targets. After the solvent completely evaporates at room temperature, a droplet of Au@Ag NPs colloid is then added to this site and remains until nearly dry. Au@Ag NPs will be located at the surface of peels and closely be in contact with or adsorb the target species. If Raman spectrum is collected from these sites by confocal laser, the fingerprint information of target molecules can be lifted due to the effective enhancement. The Raman technique for the detection of several pesticides at five fruit peels (apple, grape, mango, pear, and peach) was demonstrated here. The thin peels were taken from fruits using a knife and formatted to a uniform disk piece with a standard

Figure 5. (A) Schematic drawing for the direct detection of pesticide residues at fruit peels by the SERS spectroscopy using Au@Ag NPs as Raman amplifier, and the SERS spectra of (B) apple and (C) mango peels spiked with thiram by the enhancement of Au@Ag NPs colloid that was cast onto the surfaces of the fruit peels. All Raman spectra were recorded with a 532 nm laser with 5 mW power and 10× objective.

metal punch of ∼0.32 cm radius. The spiked peels were prepared by the addition of 20 μL of pesticide solution with different concentrations to cover the overall surface of disk peels and were completely dried at room temperature. Before used, the dilute Au@Ag NPs colloid was concentrated ∼15-fold for achieving a higher particle density at fruit peels. (See the experimental section in Supporting Information.) Although individual particles exhibit a stronger SERS effect than aggregated ones due to the possible desorption of analytes in the case of particle aggregation, the intensity of Raman signals for the surface detection is dependent on the number of individual particles under one incident laser spot and the amount of target molecules contacted by the particle sensors.41,42 Detailed experiments reveal that original Au@Ag colloid did not detect the Raman signals of trace pesticides due to the rather dilute particle concentration. With the TEM observation and adsorption spectra, the concentrated colloid can keep stable at a monodispersive state for at least 1 day that is long enough for further use in real detection. When the concentrated colloid is cast onto the spiked peels, the number 259



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Table 2. SERS Detection Limits of Three Pesticides Residues at the Peels of Five Fruitsa thiram fruit peels

ng/cm2

apple grape pear mango peach

1.46 1.46 1.46 7.23 1.46

chlorpyrifos I1386 cm−1

μg/cm2

± ± ± ± ±

0.14 0.07 0.70 0.70 0.14

421 94 110 84 165

15 5 7 4 9

methyl parathion I618 cm−1

μg/cm2

± ± ± ± ±

0.10 0.025 0.50 0.10 0.10

85 100 197 96 151

5 4 11 6 7

I1345 cm−1 211 340 273 437 752

± ± ± ± ±

11 18 14 23 32

a I1386 cm−1, I618 cm−1, and I1345 cm−1 are the intensities of the strongest peaks in the SERS spectra of thiram, chlorpyrifos, and methyl parathion, respectively. The data are the average results of three measurements.

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CONCLUSIONS In summary, the core−shell Au@Ag nanoparticles with an optimized shell thickness exhibit a wide and strong plasmonic resonance absorption, and the resulting much higher Raman enhancement than the corresponding gold and silver nanoparticle is mainly from the surface plasmon resonance of individual particles rather than the hot spots in neighboring nanoparticles. Meanwhile, the particle aggregation that was directly induced by the analytes at relatively high concentration does not improve the Raman enhancement effect, which is very different from the other Raman-enhanced systems reported previously. On the basis of the high enhancement from individual nanoparticles and strong surface adsorption of sulfurcontaining pesticides to them, we have successfully demonstrated the “smart” nanoparticle sensors for the rapid detection and identification of pesticide residues at various fruit peels. The simple, rapid, and ultrasensitive surface detection technique using core−shell nanoparticles as Raman amplifier exhibits great practical potentials for the on-site assessments of food/environment safety and the spectroscopic identification of molecules adsorbed at various surfaces when portable Raman spectrometers are applied successfully.

of individual particles is greatly increased. The greater the number of particles at the fruit peels, the stronger are the Raman signals of pesticide residues. Before detection of pesticides, 20 μL of ethanol was first dropped to the surface of spiked peel and naturally evaporated to dryness. Subsequently, 20 μL of concentrated Au@Ag colloid was added to the above peel and left until the colloid nearly dried. Figure 5B,C shows the typical Raman spectra collected from the peels of apple and mango. The Raman signals of blank fruit peels can clearly be detected by the Au@Ag enhancement, as indicated with red circles. (The enhanced Raman spectra of several fruit peels were detailed in Supporting Information Figure S5.) Meanwhile, the characteristic vibrating bands of thiram molecules at the spiked peels appeared and became stronger and stronger with the increase of thiram residue amount at the peels. However, the weak signals of fruit peel were covered with the background of strong Raman spectra of thiram. The limits of detection of thiram residues at apple and mango peels are ∼1.5 and 7.2 ng/cm2, respectively, which is much lower than the maximum permitted residue of thiram at apple (1.6 mg/kg, equal to ∼2 μg/cm2 at peels). The ultrahigh sensitivity of Au@ Ag NPs to the surface residue of thiram confirms its unusual ability in surface detection. On the other hand, Figure 5 also shows that the Raman sensitivity varies at different fruit peels. More tests at other fruit peels indicate that the Raman signal intensities from the same pesticide dosage at different peels are distinctive. (See the Supporting Information Figure S6.) The variations in sensitivity should be attributed to the differences in the surface properties of fruit peels and the interaction strength between pesticide and fruit peel. The similar detections of organophosphorous pesticides including chlorpyrifos and methyl parathion at five fruit peels were also performed, and Raman spectra were detailed in the Supporting Information (Figures S7, S8). The data of detection and the intensities of Raman are listed in the Table 2. Most of organophosphorous pesticides also contain S element in the form of either P−S single bond or PS double bond, which can greatly improve the pest-killing and fungicide effects, in comparison with nonsulfur organophosphorous compounds. These sulfur-containing organophosphorous molecules usually exhibit very strong coordinative ability with many metal ions and are thus widely used in floatation reagents of noble metals,43 and their coordination interaction at the surface of nanoparticles was documented in our recent work.24 The adsorbing ability is strong enough to replace the surface citrate ligand at the Ag shell of Au@Ag NPs and thus provides the best possibility of identifying and detecting them by SERS techniques. In addition, as another very convenient application, the particle sensors can also be employed to directly detect the trace pesticide residues in fruit juices (See the Supporting Information Figure S9.)

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis absorption spectra; SERS spectra of thiram, chlorpyrifos, and methyl parathion residues at fruit peels; SERS spectra of several fruit peels; SERS detection of thiram in juices. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z.Z.); [email protected] (M.-Y.H.).

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ACKNOWLEDGMENTS The authors are thankful for the support from China-Singapore Joint Research Project (2009DFA51810), Natural Science Foundation of China (Nos. 20807042, 21077108, 61071055, 20925518), and Innovation Project of Chinese Academy of Sciences (KSCX2-YW-G-058).

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