(SMSPE-SERS): From Sample Pretreatment to Detection All-in-One

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Fe3O4@Graphene Oxide@Ag Particles for Surface Magnet SolidPhase Extraction Surface-Enhanced Raman Scattering (SMSPESERS): from Sample Pretreatment to Detection All-in-One Zhigang Liu, Yi Wang, Rong Deng, Liyuan Yang, Shihua Yu, Shuping Xu, and Weiqing Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02944 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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ACS Applied Materials & Interfaces

Fe3O4@Graphene Oxide@Ag Particles for Surface Magnet Solid-Phase Extraction Surface-Enhanced Raman Scattering (SMSPE-SERS): from Sample Pretreatment to Detection All-in-One Zhigang Liu,a, b Yi Wang, a Rong Deng, a Liyuan Yang,a Shihua Yu,a ,b Shuping Xu,a and Weiqing Xua,*

a. State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012 China

b. Centre of Analysis and Measurement, Jilin Institute of Chemical Technology, Jilin 132022 China

ABSTRACT. A multifunctional magnetic graphene surface-enhanced Raman scattering (SERS) substrate was fabricated successfully by the layer-by-layer assembly of silver and graphene oxide (GO) nanoparticles (NPs) on the magnetic ferroferric oxide particles (Fe3O4@GO@Ag). This ternary particle possesses magnetic property, SERS activity and adsorption ability simultaneously. Owing to the multifunction of this Fe3O4@GO@Ag ternary complex, we put forward a new method called surface magnetic solid-phase extraction (SMSPE) technique, for the SERS detections of pesticide residues on the fruit peels. SMSPE integrates many 1 ACS Paragon Plus Environment


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sample pretreatment procedures, such as surface extraction, separation sample and detection, all-in-one. So this method shows great superiority in simplicity, rapidity and high efficiency above other standard methods. The whole detection process can be finished within 20 minutes including the sample pretreatment and SERS detection. Owing to the high density of Ag NPs, the detection sensitivity is high enough that the lowest detectable concentrations are 0.48 and 40 ng/cm2 for thiram and thiabendazole, which are much lower than the maximal residue limits in fruit prescribed by the U.S. Environmental Protection Agency. This multifunctional ternary particle and its corresponding analytical method have been proved to be applicable for practical samples and also valuable for other surface analysis.

KEYWORDS: Fe3O4@GO@Ag, Multifunctional, Surface Magnetic Solid-Phase Extraction, Sample Pretreatment, SERS Substrate, Pesticide Residues INTRODUCTION

In recent years, pesticide residues, food illegally added agents and abused antibiotics have influenced deeply on public health. It is imperative and eager to establish a food monitoring system and develop the corresponding detection methods. Surface-enhanced Raman scattering (SERS) as a promising technique that possesses the

advantages

of

high

sensitivity

and

time-saving compared

with

gas

chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (HPLC-MS) has been applied for the food inspection and the diagnosis of illegally added agents.1,

2

Nowadays, SERS measurements can be finished in

several minutes and its sensitivity has reached the single molecule level.3, 4 However, 2 ACS Paragon Plus Environment

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the time-consuming sample pretreatment involving the sample separation, enrichment and combination with SERS-active substrates, etc. extremely restrict SERS to be a practical technique in the field of food monitoring. So, to develop a simple, effective sample pretreatment approach that can integrate with the SERS measurement procedure is greatly needed. 5 Solid-phase extraction (SPE) is a very important pretreatment technology in analytical chemistry and it has been widely used for the sample separation and enrichment of analytes.6 Many nanomaterials possessing their large specific surface area have been used as the SPE materials, e.g., molecularly-imprinted polymers (MIPs),7 metal-organic frameworks (MOFs),8 and graphene oxide (GO),9, 10 etc. The progress of the novel SPE materials advances the SPE techniques to access the goal of simplicity, high efficiency, rapidity and sustainability. GO gains great attention in recent years. It is a single atomic layer of carbon and has a very large specific surface area of 2630 m2/g, 11 which supports a high adsorption capacity and a fast adsorption equilibrium12 toward aromatic compounds through the electrostatic bonding or the π–π cooperative interactions. The GO-based magnetic materials (Fe3O4@GO) 13 that combine the high adsorption capacity of GO and the separation convenience based on magnetic materials have been used as the adsorbents for the separation and preconcentration of fungicides,14 drug delivery,15 heavy metal removal,16, 17 cancer diagnosis,18, 19 and pesticide residues.20, 21 Moreover, GO can be applied as a substrate to improve raman signal (called GERS) due to its chemical charge transfer property.22 In these designs, the GO/Ag nanohybrid structures as a 3 ACS Paragon Plus Environment


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SERS substrate is better than individual Ag or Au, while GO is suggested to serve as many roles involving a chemical enhancer,23 a fluorescence quencher,24 a molecule enricher,25 avoiding agglomeration to increase the stability of NPs and deposition of monolayer NPs with high dispersity. So the fabricated substrate shows not only improved stability but also presents more sensitive SERS response toward aromatic organic molecules compared to the bare Au or Ag nanostars.26 Magnetic Fe3O4 combined with Ag/Au nanomaterials have been reported frequently, which serve as magnetic SERS substrates that are able to separate the target molecules from samples without centrifugation or filtering.27-37 One of the important applications of these magnetic SERS substrates is to detect pesticide residues on vegetables and fruit peels due to the fingerprint and high sensitivity provided by SERS.38 However, in order to enrich analytes, pesticide molecules that have penetrated into the peels are preliminary extracted out of peel surface by ethanol for 1-2 h,39-41 which is far away from the requirement of rapid detections and lies in the same time stage as HPLC (or HPLC-MS). Herein, in order to shorten the pretreatment time, we fabricated a multifunctional magnetic

GO-based

SERS-active

particles

(Fe3O4@GO@Ag),

which

takes

advantages of magnetic GO, GERS and magnetic SERS, all-in-one. Owing to the characteristics of magnetic GO, we developed a novel method that combines SPE and SERS, named as surface magnetic solid-phase extraction SERS (SMSPE-SERS) technique for rapidly measuring pesticide residues on fruit peels. SMSPE-SERS refers to a dynamic process including the surface SPE and the SERS detection using these 4 ACS Paragon Plus Environment

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Fe3O4@GO@Ag particles (Scheme 1). Sample pretreatment procedures can be simplified and the whole process of detection is finished within 15 min. Owing to the high capacity and surface area of GO, the loading of Ag NPs increases compared with Fe3O4@Ag particles, which allows for more chances for forming SERS hot spots and touching the probed analysts. Thus, the detection sensitivity can be greatly improved. Pesticide

Peel Fe3O4@GO@Ag NPs in ethanol

1

2

3

4

Glass slide

a Magnetic film

b

Magnet 2

Magnet 1

Magnet 1

c Magnet 1

d

Magnet 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


e

5

6

f

Scheme 1. Detection procedures for pesticides on a fruit peel by the SMSPE-SERS method. (a, 1) A piece of fruit peel with a disc shape (16.5 mm in diameter) is laid on a glass slide. (b, 2) Fe3O4@GO@Ag particles in ethanol (50 µL, 10 mg/mL) are dropped on the fruit peel. (c, 3) By using a magnetic bar laid on the other side of the glass slide, a magnetic GO extraction membrane forms above the fruit peel. (d) Pesticides that dissolved in ethanol diffuse to the magnetic GO membrane, which is a pesticide extraction process. (e, 4, 5) A magnetic separation process. A glass slide covers the drop and a magnetic bar moves to the top of the second glass slide to 5 ACS Paragon Plus Environment


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separate the pesticide adsorbed Fe3O4@GO@Ag particles from the solvent. (f, 6) SERS measurement of pesticide using a BWTEK Raman spectrometer with a portable optical fiber probe and 532 nm excitation wavelength. EXPERIMENTAL SECTION Materials.

Iron(III) chloride hexahydrate (FeCl3·6H2O), ethylene glycol (EG), sodium

acetate anhydrous (NaAc), silver nitrate (AgNO3 AR, 99.8%), sodium citrate (C6H5Na3O7·2H2O), anhydrous ethanol(CH3CH2OH), concentrated ammonium aqueous

solution

(NH3·H2O

25

wt%),

tetraethyl

orthosilicate

(TEOS),

polyethylenimine (PEI, M.W.: 10,000, 99%), thriam, thiabendazole, parathion-methyl, mancozeb, methanol (Chromatographic grade) and (3-aminopropyl) triethoxysilane (APTES, analytical grade) were obtained from Aladdin Reagent Co. Ltd. (Shanghai, China). Ethylenethiourea was obtained from Energy-Chemical Reagent Co. Ltd. (Shanghai, China). GO were obtained from Prof. Haolong Li group (Jilin Univ.). Deionized water (Millipore) with a resistivity of 18 MΩ cm was used in all experiments. Preparation of Fe3O4@GO@Ag Particles. Magnetic

Fe3O4 particles were prepared through

a solvothermal reaction following a reported method in order to obtain a superparamagnetic iron oxide sample.42 We chose this method to prepare iron oxide NPs because the iron oxide NPs using this method can support a highly magnetic property (~70 emu/g),42 which is extremely important for the rapid magnetic separation process. Briefly, 2.7 g of FeCl3·6H2O and 7.2 g of sodium acetate were dissolved in 80 mL of ethylene glycol under magnetic stirring. The obtained 6 ACS Paragon Plus Environment

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homogeneous yellow solution was transferred to a Teflon-lined stainless-steel autoclave and sealed to heat at 200 ℃ for 8 h. The obtained black magnetite particles were washed with ethanol for 6 times, and then dried in vacuum at 60°C for 12 h. To prepare magnetic GO composite, the surface of Fe3O4 was first modified with APTES that supplies amide group for further binding graphene oxide via the electrostatic action (Fig. 1a). 0.1 g of Fe3O4 particles were mixed of ethanol (80 mL), deioned water (20 mL) and concentrated ammonia aqueous solution (1.0 mL, 28 wt. %) under ultrasonication, followed by the addition of 67 µL of TEOS and 67 µL of APTES.43, 44 After stirring at 35℃ for 3 h, the Fe3O4 spheres were functionalized with amino groups (Fe3O4@NH2). They were separated and washed with ethanol and water by an external magnetic field. The Fe3O4@NH2 microspheres were redispersed in the GO aqueous solution (1.0 mg/mL) to prepare the magnetic graphene (Fe3O4@GO) under vigorous stirring at 75°C for 1 h.45 In order to remove the remaining GO, the obtained Fe3O4@GO particles were washed with ultrapure water for 3 times. Then, Fe3O4@GO particles were dispersed in 100 mL of AgNO3 aqueous solution (10 mM) under mechanical stirring for 30 min to load silver ions on the surface of GO. Silver ions then were reduced to Ag NPs by adding 100 mL of a sodium citrate solution (10 mM) as a reducing agent into the above solution.46, 47 The solution was heated to 60 ℃ for 6 h under mechanical stirring to obtain the Fe3O4@GO@Ag microspheres. The as-prepared Fe3O4@GO@Ag particles were washed with water for three times to remove individual Ag NPs with the help of a magnetic bar, and they were redispersed in 20 mL of ethanol before SERS detection. 7 ACS Paragon Plus Environment


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Preparation of Fe3O4@Ag Particles.

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Ag NPs with the size of about 80 nm were

synthesized using a method of Lee and Meisel.48 Briefly, 27 mg of AgNO3 was dissolved in 150 mL of ultrapure water and brought to boiling. Then, 3.0 mL of 1% trisodium citrate was added and kept boiling for 40 min. The obtained Ag NP colloid was used for preparing the Fe3O4@Ag particles. The Fe3O4@Ag particles were syntheized according to reported method.41 First, 50 mg of as-prepared Fe3O4 were functionalized with PEI by suspending them in 100 mL of 5.0 mg/mL positively charged PEI solution. PEI supplies amino groups around a Fe3O4 particle. Then AgNPs were mixed with the amino-functionalized Fe3O4 to produce the Fe3O4@Ag particles. Characterizations.

X-Ray diffraction (XRD) data were taken from a Bruker D8

FOCUS with Cu Kα radiation (k = 1.5406 A˚). The morphology and EDX elemental mapping of the Fe3O4@GO@Ag was characterized using a JEOL JSM-6700F field emission scanning electron microscope (FE-SEM) operating at 3.0 kV. Transmission electron microscopic (TEM) images were obtained by a JEM-2100F feldemission transmission electron microscope (JEOL, Tokyo, Japan) at accelerating voltages of 200kV. High performance liquid chromatography (HPLC) analysis was carried out on a Shimadzu LC-20AB system (Kyoto, Japan) equipped with a CTO-10A Scolumn oven and a SPD-M20A detector. The chromatographic separation of analysts was performed on a VP-ODS C18 column (5 µm, 4.6 mm × 250 mm i.d.). The mobile phase used was mathenol/water (50: 50). Its flow rate was 1.0 mL/min and the

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detection wavelength was monitored at 273 and 299 nm for thiram and TBZ respectively. The injection volume was 20 µL. Sample preparation and SERS measurements.

Apple peels were prepared following a

reported method.49, 50 The cleaned apples peels were cut into a 1 cm2 uniform discal piece by a knife (the diameter is about 16.5 mm). The discal piece was washed with deionized water and ethanol, dried by blowing N2 and then put on a glass slide. Then, 20 µL of the pesticide solutions with various concentrations were dropped onto the apple peels and natural dried. In order to measure the pesticides by using the SMSPE-SERS method, firstly 50 µL of the Fe3O4@GO@Ag particles (10 mg/mL) were firstly dropped on the surface of artificially contaminated fruit peels. A magnetic GO extraction membrane forms above the fruit peel by using a magnetic bar laid on the other side of the glass slide. Pesticides that dissolved in ethanol diffuse to the magnetic GO membrane in a certain extraction time. Then, a magnetic separation process is as follows: a glass slide covers the drop and a magnetic bar moves to the top of this glass slide to separate the pesticide adsorbed Fe3O4@GO@Ag particles from the solvent. Finally, SERS spectra of analytes on the magnetic film were recorded using a BWTEK Raman spectrometer with a portable optical fiber probe 532 nm excitation wavelength. The integration time was 5 s. The laser power reaching the samples was 5.01 mW. RESULTS AND DISCUSSION Preparation and characterization of Fe3O4@GO@Ag and Fe3O4@Ag particles.

The ternary

particles (Fe3O4@GO@Ag) were prepared by the layer-by-layer assembly over Fe3O4 9 ACS Paragon Plus Environment


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particles (as shown in Fig. 1a). Fe3O4 particles were first chemically modified with APTES, which allows for the immobilization of a layer of GO by amine groups. It should be noted that the assembled GO is a single layer GO according to the characterization result of AFM (Fig. S2a). Next, Ag NPs were in situ prepared over the GO by an in situ reduction growth method, in which the Fe3O4@GO particles were dispersed in a AgNO3 and citrate mixed solution in a 60 ℃ water bath for the nucleation and in situ growth of Ag NPs over the GO.46, 47, 51 Fig. 1b-d show the morphologies of the Fe3O4, Fe3O4@GO, and Fe3O4@GO@Ag particles taken by SEM. It can be found that the as-prepared Fe3O4 particles are monodisperse and in a spherical shape with an average diameter of about 300 nm (Fig. 1b). Fig. 1c is the SEM image of Fe3O4@GO, showing the presence of the folded and crinkled GO on the Fe3O4 particle’s surface. The high loading of GO maintains a high surface area, which is beneficial to the further assembly of Ag NPs and the extraction of probed targets.52, 53 The SEM image of Fig. 1d proves that Ag NPs have been grown and fixed on these crinkled GO. The average size of Ag NPs prepared by this in situ growth is about 85 nm. Thus, the Fe3O4@GO@Ag particles have been obtained.


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NH2NH2

a Fe3O4

APTES 1

O

Si Si O O O O

b

2 GO

OC NH

OC NH

AgNO3 3

OH

O O

OH

OC OC NH NH

c

d

Figure 1. (a) Schematic illustration of the surface modification of Fe3O4 particle to prepare Fe3O4@GO@Ag. (b)-(d) SEM images of Fe3O4, Fe3O4@GO and Fe3O4@GO@Ag particles.



Fe

Ag

Ag

Fe

O

C

Ag

a

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

b

Intensity (a.u.)

6000

4000

1600 cm-1

8000

1350 cm-1

Energy (keV)

Fe 3O 4@GO@Ag

2000

Fe O 3 4 0 500

6000

1000

1500

Raman Shift (cm-1 )

c

8000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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□□□□

2500

※ -Fe3O4 □ -Ag

※

Fe3O4@GO@Ag ※

2000

□ ※

※

※□

4000

Fe3O4@GO 2000

Fe3O4 0 10

20

30

40

50

60

70

Angle (° )

Figure 2. (a) EDX spectrum of Fe3O4@GO@Ag. (b) Raman spectra of Fe3O4@GO@Ag and Fe3O4 particles. (c) XRD patterns of Fe3O4, Fe3O4@GO and Fe3O4@GO@Ag particles, respectively. The produced Fe3O4@GO@Ag particles were also characterized by the energy dispersive X-ray (EDX) spectroscopy via a casting film and the result of Fig. 2a show the existence of Fe, C, O, and Ag elements (Fig. 2a). Further area mapping analysis confirms the distribution C, Fe and Ag are relatively uniform (Fig. S2b, c and d). 12 ACS Paragon Plus Environment

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Fe3O4@GO@Ag particles were further characterized with Raman and XRD spectroscopies. Fig. 2b shows the Raman spectra of the Fe3O4 and Fe3O4@GO@Ag particle films. The Fe3O4@GO@Ag particle film shows two additional Raman peaks at about 1350 and 1600 cm-1, which are asigned to the D and G bands of GO, while the D band is related to the vibration of sp3 carbon atoms of the disordered GO nanosheets and the G band corresponds to the vibration of sp2 carbon atom domains of graphite.47 Fig. 2c is the XRD spectra of the as-synthesized Fe3O4, Fe3O4@GO and Fe3O4@GO@Ag particle films. The peaks of Fe3O4 can be clearly observed at 2θ values of 30.0, 35.5, 43.1, 57.1 and 62.7°, which are assigned to the (220), (311), (400), (511) and (440) planes of cubic spinel structured magnetite (JCPDS card no. 19-0629), respectively. The top curve shows the 2θ values of 38.1, 44.3 and 64.4°, corresponding to the reflections of the (111), (200) and (220) crystalline planes of cubic Ag (JCPDS 04-0783). When the GO and Ag NPs are both immobilized on the Fe3O4 particles, the peaks of the cubic spinel structured Fe3O4 can still be identified in the composite particles in comparison with Fig. 2b and c. It should be noted that the phase of the modified Fe3O4 particles almost has no changes during the synthesis of GO and Ag NPs. The XRD analysis is in accordance with the EDX results above. For comparison, we also prepared Fe3O4@Ag particles according to the literature28 and they were characterized by the XRD and TEM. Fig. S1a and b show that Ag NPs were successfully assembled Fe3O4 surface.



a

Fe3O4@GO@Ag

2000

Intensity (a.u.)

Fe3O4@Ag

500

1000

1500

2000

2500

Raman Shift (cm-1) Fe3O4@GO@Ag

4800

500

b

Fe3O4@Ag

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1000

1500

2000

2500

Raman Shift (cm-1) Figure 3. SERS spectra of 1.0×10-5 M of thiram (a) and 1.0×10-4 M of TBZ (b) on the Fe3O4@GO@Ag and Fe3O4@Ag particles, respectively.

SERS activity of Fe3O4@GO@Ag and Fe3O4@Ag particles.

The SERS activities of these two

particles were evaluated by the detections of thiram and TBZ on the fruit peels (Scheme 1). Fig. 3 shows the SERS spectra of thiram and TBZ on the Fe3O4@GO@Ag particles (top curves) and Fe3O4@Ag particles (bottom curves). The SERS signal on Fe3O4@GO@Ag particles has 2.5 times (at 1139 cm-1) improvement in the detection of thiram and 3.5 times (at 1005 cm-1) improvement in the detection of TBZ, respectively. Through the comparison, we can make clear the role of GO in 14 ACS Paragon Plus Environment

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this configuration is to improve the efficiency of extraction of analytes by means of increasing the loading of Ag NPs on the surface under the premise of the interaction between analytes and Ag. In addition, the high density of Ag NPs due to the large specific surface area of the immobilized GO can generate more hot spots and increase the loading of analytes, which are beneficial to the high SERS signal.

To prove this, we first recorded the XRD spectra of two kinds of particles as shown in Fig. S1a. The intensity of the diffraction peak of Ag on Fe3O4@GO@Ag is 5 times higher than that on the Fe3O4@Ag particles, evidencing that the density of Ag NPs is much higher on Fe3O4@GO@Ag than that on Fe3O4@Ag particles. The SEM of Fe3O4@GO@Ag (Fig. 1d) and the TEM of Fe3O4@Ag (Fig. S1b) are also in accordance with the XRD conclusion that a higher loading of Ag particles was achieved on the Fe3O4@GO@Ag.

Adsorption experiments can tell the adsorption amount of thiram and TBZ on two kinds of particles. The adsorption conditions are provided in SI and results are shown in Fig. S3 and S4. The adsorption capacities of the Fe3O4@GO@Ag for thiram and TBZ are 0.52 and 0.58 mg/g, respectively, which are 3.7 and 9.6 times higher than those of the Fe3O4@Ag. These data reflect that the Fe3O4@GO@Ag particle has higher adsorption ability than Fe3O4@Ag when being a SERS substrate.

Adsorption kinetics of pesticides by SMSPE.

The SMSPE combined with SERS detection

was applied for the determination of pesticide residues on the fruit peels. As shown in Scheme 1, we first dropped the as-prepared Fe3O4@GO@Ag particles (dispersed in 15 ACS Paragon Plus Environment


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ethanol) onto the fruit peels that had been artificially contaminated by pesticide (Scheme 1a, b). Then, those particles were collected and aggregated on the fruit peel’s surface by forming an extraction membrane under an external magnetic field (Scheme 1c). The pesticides would be dissolved and they will diffuse upward in ethanol based on the molecular diffusion theory. In a very short time, they were adsorbed on the Fe3O4@GO@Ag particles (Scheme 1d). When arriving the adsorption equilibrium, we covered another glass slide and collected all particles into one point using a small magnetic bar (Scheme 1e). The increased particle density can enhance the Raman signals of target species by the so-called “hot spots” effect54 and avoid fluorescence caused by fruit peels.55, 56 Finally, the magnetic film with extracted pesticides was measured with a Raman spectrometer (Scheme 1f). A contrast experiment was carried out in the absence of the magnetic bar with different extraction times in order to evaluate the extraction efficiency of the SMSPE method. The whole detection process is less than 20 minutes from sample pretreatment to SERS detection.

Table 1. Kinetics fitting results SPSME and contrast experiments from two kinetic models. R2

Model

Equation

Parameters

Seudo-second-order

t 1 t = + qt k2 qe2 qe

k2: 3.44 × 10-5

qe: 3082

0.962

Intraparticle diffusion

qt = kid t 2 +C

kid:328

C: -431

0.935

1

qt is the solid-phase loading in the adsorbent at time t, qe is the adsorption capacity at equilibrium, R2 is the coefficients of determination. k2 is the rate constant of 16 ACS Paragon Plus Environment

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adsorption. kid indicates the diffusion rate constant, and C provides the thickness of the boundary layer.

0.024

5000

SMSPE

0.020

2000

Contrast

4000

0.012

I

t/I

0.016

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

0.008 0.004 0

3000

10 20 30 40 50 60

0

2

4

6

8

t1/2

t

SMSPE 2000

Contrast 1000

0 0

10

20

30

40

50

60

Time (min) Figure 4. The SERS intensities of thiram at 1139 cm−1 along with the extraction time using the SMSPE method with and without the formation of a magnetic film. Inserts are their kinetics fitting lines.

The adsorption kinetics is an important factor for understanding the fast extraction process from fruit peels to Fe3O4@GO@Ag particle. The adsorption kinetics of analytes was examined by measuring the SERS peak intensity (the 1139 cm-1 band of thiram) at different extraction time (Fig. 4). A pseudo-second-ordermodel was assumed to fit the experimental data of the SMSPE adsorption process. The model can be expressed in Table 1, in which we can find that the adsorption process using the SMSPE agrees with the pseudo-second-order kinetic model. The equilibrium was achieved at 16 min while the calculated qe values from the pseudo-second order 17 ACS Paragon Plus Environment


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model well fit the experimental data. A contrast experiment was carried out without forming a magnetic film (Step a, b, e and f in Scheme 1, no Step c and d), which shows that SERS signal increases slowly within the extraction time of 60 min (Fig. 4). A much longer equilibrium time was observed (over 60 min) and the SERS intensity is about one-fifth of that using SMSPE (comparison from the SERS intensities of 16 min adsorption time). This is because the use of magnetic bar can cause a higher density of Fe3O4@GO@Ag particles on the fruit peel. The pesticide residues dissolved in ethanol can easily reach to and be absorbed on the Fe3O4@GO@Ag particle film to quickly arrive in the adsorption equilibrium. However, in the absence of magnetic bar, a great number of Fe3O4@GO@Ag particles are dispersed stably in the ethanol drop due to the hydrophilic properties of magnetic GO.57 Only a part of them can reach the fruit peel’s surface. Thus, the adsorption of pesticide is a static adsorption and only a few dissolved pesticide molecules can diffuse to bond the Fe3O4@GO@Ag particles in the ethanol drop. So, the process agrees on the intraparticle diffusion model. Correspondingly, the latter strategy is time-consuming and the detection displays a low sensitivity. On the other hand, the SMSPE is a very simple and rapid pretreatment technique, which accelerates the complicate pretreatment process and avoids the long-time preliminary extraction and the complete evaporation in the other approaches.39, 40 SERS measurements of pesticide residues.

By using the Fe3O4@GO@Ag particles, we

developed a new detection strategy for the pesticide residues over the fruit peels (Step a-f in Scheme 1). As shown in Fig. 5, the SERS detections of two pesticides, thiram 18 ACS Paragon Plus Environment

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and TBZ, were completed by the proposed SMSPE-SERS technique. The main characteristic peaks of thiram are located at 554, 1139, 1375 and 1501 cm-1, which are attributed toυ(S-S), ρ(CH3)or υ(C–N), ρ(CH3) and υ(C–N), respectively, due to the previously reported studies.58 Moreover, the enhanced peaks of TBZ at 781, 1005, 1274 and 1547 cm-1 are attributed to υ(C–S), υ(C-N), δ(N-H) and υ(C=C), respectively.59 Fig. 5b and 5d show the calibration curves by plotting the SERS intensities of thiram at 1139 cm-1 and TBZ at 1005 cm-1. The lowest detectable concentrations of thiram on the peels are 0.48 ng/cm2 and the linear range is from 0.48 ng/cm2 to 48 µg/cm2. However, we have not detected SERS signial of TBZ at 0.48 ng/cm2, so the linear range for TBZ is from 4 ng/cm2 to 40 µg/cm2. The reason may be depends on the differences of molecular structure. We think Fe3O4@GO@Ag SERS substrates have different sensitivity toward thiram and TBZ. According to a report in the literature,

60

thiram molecule easily forms a resonated radical structure

when interacting with a metal surface such as Ag NPs, leading to the S–S bond cleavage of thiram, which may support higher SERS signal. So, the intensity of thiram and TBZ showed different response to the analytical concentrations. So the lowest detectable concentrations of thiram and TBZ on the peels are ∼0.48 ng/cm2 and ∼40 ng/cm2 using the SMSPE-SERS method, which are both much lower than the maximum permitted residues of thiram (equal to ∼140.0 µg/cm2)61 and TBZ (equal to ∼100.05 µg/cm2)62 on pome fruits stated by U.S. Environmental Protection Agency.



b Intensity (a.u.)

1501 1139

554

1000

500

1000

2000

2500

3000

1E-9

3500

1547

1000

1500

2500

Raman Shift (cm-1)

B B

Intercept Slope

1E-7

0.98667 0.9669

1E-6

Value Standard Err 5.1444 0.12857 0.2712 0.02237

1E-5

10000

d

3000

Intensity (a. u.)

2

40 μ g/cm 2 4.0 μ g/cm 2 0.40μ g/cm 2 40 ng/cm 2 4.0 ng/cm Blank

2000

y = a + b*x Instrument 21.49091

Concentration (g/cm )

Intensity (a.u.)

781

1005

4800

1E-8

Equation Weight Residual Sum of Squares Pearson's r Adj. R-Square

2

Raman Shift (cm-1)

c

500

1500

1274

Intensity (a.u.)

7700

10000

2

48 μ g/cm 2 4.8 μ g/cm 2 0.48μ g/cm 2 48 ng/cm 2 4.8 ng/cm 2 0.48 ng/cm Blank

1375

a

Intensity (a.u.)

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3500

y = a + b*x Equation Instrument Weight Residual Sum 71.81504 of Squares 0.96217 Pearson's r 0.90104 Adj. R-Square

1000

-8

10

-7

10

Value Standard Err 4.8189 0.18811 0.2045 0.03343

Intercept Slope

B B -6

10

-5

10

-4

10

2

Concentration (g/cm )

Figure 5. (a) Raman spectra of thiram on the apple peels with the concentrations. (b) The plot of the Raman intensity of thiram peaks at 1139 cm−1 with the concentration. (c) Raman spectra of TBZ on the apple peels with the concentrations. (d) The plot of the Raman intensity of TBZ peaks at 1005 cm−1 with the concentration.

To test the specificity of the method, parathion-methyl and mancozeb were detected separately by the same detection procedure for thiram. As shown in Fig. S7, the added two analytes show no identifiable SERS signals at the concentration of 0.48 µg/cm2 in addition to thiram (also 0.48 µg/cm2). At the same time, we tested the selectivity toward ethylenethiourea, a main metabolites of dimethyl dithiocarbamates pesticides (ferbam, thiram, mancozeb and ziram). Fig. S7 shows that ethylenethiourea cannot be detectable, which demonstrates that the Fe3O4@GO@Ag SERS substrates exhibits 20 ACS Paragon Plus Environment

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high selectivity for thiram. The selectivity of the proposed SMSPE-SERS technique toward thiram can be explained by a higher binding affinity between thiram and Ag/GO rather than other molecules. SERS detection of real sample:

We adopted the spike-and-recovery experiment to show

the practicality of this SMSPE-SERS method. Three different peels from apple, pear and grapes based on the steps illustrated in Scheme 1 were measured and the results are listed in Fig. S8 and Table 2. For the peel surfaces without thiram, the recoveries are in the range of 87.5–118.7 % for the addition of 0.48 ng/cm2 of thiram. These results reveal that the SERS substrate could be applied to the rapid detection of thiram on fruit peels. Table 2. Detection value of real samples based on the spike-and-recovery experiment. Measured value Measured value spiked value Recovery Sample before spiking after spiking 2 (ng/cm ) (%) (ng/cm2) (ng/cm2) Apple N/A 0.48 0.42 87.5 Pear N/A 0.48 0.57 118.7 Grape N/A 0.48 0.52 108.3

CONCLUSION

A multifunctional Fe3O4@GO@Ag particle was synthesized and a new detection strategy for the pesticide residues over the fruit peels was developed. The results show that the SMSPE process is in accordance with the adsorption ability of pseudo-second order kinetic model and the adsorption equilibrium time is only about 15 min. Compared with other methods for pesticide residue on the fruits peels surface, this method integrates and simplifies many sample pretreatment processes such as 21 ACS Paragon Plus Environment


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preextraction, dry, and SERS-active substrate combination. These Fe3O4@GO@Ag particle exhibits acceptable SERS detection reproducibility and stability (see SI). And the SERS detection results show a high sensitivity due to the magnet force-induced “hot spot” effect and a low detection limit. Owing to many advantages of this method, these Fe3O4@GO@Ag particles can also be used in other surface analysis systems and bioanalytical assays.

ASSOCIATED CONTENT

Supporting Information. Supporting Information includes (1) The characterization of Fe3O4@Ag particles, (2) The characterization of GO and Fe3O4@GO@Ag particles, (3) Adsorption experiments, (4) SERS performance of Fe3O4@GO@Ag particles, (5) SERS spectra of real samples. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, 2699 Qianjin Ave., Changchun 130012, China Email: [email protected] Fax: 86-431-85193421. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was supported by the National Natural Science Foundation of China NSFC Grant Nos. 21373096, 91441105, 21573087 and 21573092, and National Instrumentation Program (NIP) of the Ministry of Science and Technology of China No. 2011YQ03012408.

ACKNOWLEDGMENT We thank Prof. Haolong Li (Jilin Univ.) for providing us the graphene oxide samples. ABBREVIATIONS SMSPE, surface magnet solid-Phase extraction; SERS, surface-enhanced raman scattering; GO, graphene oxide; GC-MS, gas chromatography-mass spectrometry; HPLC-MS, liquid chromatography-mass spectrometry; SPE, solid-phase extraction; MIPs, molecularly-imprinted polymers; MOFs, organic frameworks; XRD, X-ray diffraction; FE-SEM, field emission scanning electron microscope; EDX, Energy Dispersive X-ray Detector; TBZ, thiabendazole.

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TOC Ternary Fe3O4@GO@Ag particles were designed and prepared for being a magnetic graphene SERS substrate with the functions of sample enrichment and fast separation. By using these Fe3O4@GO@Ag particles, we put forward a new method, called surface magnetic solid-phase extraction (SMSPE) technique, for the rapid SERS detections of pesticide residues.

NH

N

N S

Fe3O4@GO@Ag


(SMSPE-SERS): From Sample Pretreatment to Detection All-in-One

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