Journal of Hazardous Materials 285 (2015) 11–17

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Highly sensitive SERS-based immunoassay of aflatoxin B1 using silica-encapsulated hollow gold nanoparticles Juhui Ko a , Chankil Lee b,∗ , Jaebum Choo a,∗ a b

Department of Bionano Technology, Hanyang University, Ansan 426791, South Korea Department of Electronics and Communication Engineering, Hanyang University, Ansan 426791, South Korea

h i g h l i g h t s • • • •

A SERS-based sandwich immunoassay was used for aflatoxin B1 detection. Silica-encapsulated hollow gold nanoparticles were used as SERS nano tags. The SERS-based assay has advantages in terms of sensitivity and detection time. This assay technique provides great promises for the trace analysis of toxins.

a r t i c l e

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Article history: Received 14 August 2014 Received in revised form 6 November 2014 Accepted 14 November 2014 Available online 17 November 2014 Keywords: Aflatoxin B1 Mycotoxin Surface-enhanced Raman scattering Silica-encapsulation Magnetic bead

a b s t r a c t Aflatoxin B1 (AFB1) is a well-known carcinogenic contaminant in foods. It is classified as an extremely hazardous compound because of its potential toxicity to the human nervous system. AFB1 has also been extensively used as a biochemical marker to evaluate the degree of food spoilage. In this study, a novel surface-enhanced Raman scattering (SERS)-based immunoassay platform using silica-encapsulated hollow gold nanoparticles (SEHGNs) and magnetic beads was developed for highly sensitive detection of AFB1. SEHGNs were used as highly stable SERS-encoding nano tags, and magnetic beads were used as supporting substrates for the high-density loading of immunocomplexes. Quantitative analysis of AFB1 was performed by monitoring the intensity change of the characteristic peaks of Raman reporter molecules. The limit of detection (LOD) of AFB1, determined by this SERS-based immunoassay, was determined to be 0.1 ng/mL. This method has some advantages over other analytical methods with respect to rapid analysis (less than 30 min), good selectivity, and reproducibility. The proposed method is expected to be a new analytical tool for the trace analysis of various mycotoxins. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Toxins produced by the secondary metabolites of fungi are called mycotoxins. Mycotoxins have become a serious problem in the food and agricultural industries because they are commonly found in human food and animal feed. Among various types of mycotoxins, aflatoxins are known to be genotoxic and carcinogenic to humans and animals [1–3]. Thus, the exposure through food should be kept as low as possible. Aflatoxins can be found in foods such as, nuts, maize, rice, vegetable oils, and other dried foods as a result of fungal contamination before and after harvest.

∗ Corresponding authors. Tel.: +82 31 400 5201; fax: +83 31 416 3836. E-mail addresses: [email protected] (C. Lee), [email protected] (J. Choo). http://dx.doi.org/10.1016/j.jhazmat.2014.11.018 0304-3894/© 2014 Elsevier B.V. All rights reserved.

Several types of aflatoxins are produced in nature, but aflatoxin B1 (AFB1) is the most common in food and amongst the most potent genotoxic and carcinogenic aflatoxins [4,5]. This aflatoxin is produced by two species of Aspergillus (Aspergillus flavus and Aspergillus paraciticus), which are generally found in areas with hot and humid climates. AFB1 has drawn increasing attention because of its frequent occurrence in human foods. In addition, maximum levels of AFB1 in grains and oilseeds are regulated by the Food and Drug Administration (FDA) and European Commission (EC) [6,7]. Thus, it is very important to develop a highly sensitive detection method for monitoring trace amounts of AFB1 in human food. Currently, several analytical methods, including high-performance liquid chromatography (HPLC) [8,9], liquid chromatography/mass spectrometry (LC/MS) [10,11], enzyme-linked immunosorbent assay (ELISA) [12], surface plasmon resonance (SPR) [13], and

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electrochemical detection [14], have been applied for the trace analysis of AFB1. However, the problems associated with these analytical methods, such as, length measurement time, a long sample preparation time, poor detection limit, and indirect measurements, make them less attractive. In particular, the rapid and accurate identification of AFB1 in the field is important for prompt prevention of spreading. Therefore, a method employing a portable detection system and real-time analysis of AFB1 in the field is needed. Surface-enhanced Raman scattering (SERS)-based immunoassay using antibody-conjugated metal nanoparticles is considered as a promising alternative for monitoring trace levels of hazardous materials because of its rapid and highly sensitive detection [15,16]. Wu et al. [17] measured and analyzed the SERS spectra of four different aflatoxins directly adsorbed on the surface of silver substrates through the electrostatic interaction. The SERS spectra of four different aflatoxins are expected to be distinctive because of their individual molecular structures, but their spectral disparity is small since the majority of their molecular structures are similar. To resolve this problem, authors used specific statistical data analysis method, principal component analysis (PCA). Xiaoyan et al. [18] synthesized bipyramid gold nanocrystal–gold nanoclusters (BPNG/GNC) and applied them as dual modal probes for high throughput bio-detection. This SERS-fluorescence joint optical encoding method was used for simultaneous detection of multiple aflatoxins with high sensitivity and specificity. In this work, a two-dimensional planar gold substrate was used for the SERS-based immunoassay platform. However, the SERS-based immunoassay using planar gold substrates has several limitations. First, an extended incubation time was required for each binding step because the restriction on molecular diffusion near the surface slows the kinetics of protein–protein assays. Second, the repetition of washing steps to remove nonspecific binding proteins is inconvenient. Third, all of the immune-reagent components should be immobilized to a surface of a solid substrate in air. In many cases, the exposure of proteins to air seriously reduces their biological activity. Recently, our research group developed a novel SERS-based immunoassay platform using SERS nano tags and magnetic beads [19–24]. This method does not use immobilization on a planar substrate. Instead, it uses magnetic beads as supporting materials and Raman reporter-labeled SERS nano tags as detection probes. In this system, immunocomplexes are sandwiched between magnetic beads and SERS nano tags and are immobilized on the wall of a microtube using a small magnetic bar for washing and SERS detection. In this work, we explore the feasibility of the proposed SERSbased immuno-analytical detection platform using magnetic beads and silica-encapsulated hollow gold nanoparticles (SEHGNs) for rapid and reproducible trace analysis of AFB1. Here, HGNs were used as highly reproducible sensing probes for quantitative analysis of AFB1, and they were encapsulated with silica to prevent desorption of Raman reporter molecules and adsorption of external species. A novel synthetic method for the preparation of SEHGNs has been recently reported elsewhere [25–29]. In the immuno-analytical method using magnetic beads, the loading density of antibodies was greatly increased because three-dimensional microspheres were used instead of two-dimensional planar substrates. In addition, the assay time was greatly reduced because the system overcomes the slow diffusion-limited kinetics that occurs on a planar substrate. Furthermore, a commercially available portable Raman system can be easily integrated with the SERSbased immuno-analytical platform for real-time analysis in the field [30,31]. Herein, we report a novel SERS-based analytical platform that uses SEHGNs and magnetic beads for highly sensitive detection of AFB1.

2. Materials and methods 2.1. Materials and reagents Cobalt chloride hexahydrate, gold(III) chloride trihydrate (>99.9%), sodium borohydride (99%), sodium citrate dehydrate (99%), 3-aminopropyltriethoxysilane (APTES, 99%), ethanolamine, mercaptopropyltrimethoxysilane (MPTMS), bovine serum albumin (BSA), tetraethyl orthosilicate (TEOS), and aflatoxin B1 (AFB1, >98%) were purchased from Sigma–Aldrich (St. Louis, MO). Antibodies for aflatoxin B1 (anti-AFB1 and anti-ATB1) were also purchased from Sigma–Aldrich. Fumonisin B (FMB), ochratoxin A (OTA), 1␮m-diameter streptavidin-coated magnetic beads, and malachite green isothiocyanate (MGITC) were purchased from Invitrogen. Biotin-PEG-NHS 5000, used as a linker onto the magnetic beads, was purchased from Santa Cruz Biotechnology. PBS solutions with 0.05% Tween-20 (v/v) at pH 7.4 were prepared using the standard method. All aqueous solutions were prepared using ultrapure deionized water (18 M) obtained from a Milli-Q water purification system (Millipore Corporation, Billerica, MA, USA). 2.2. Preparation of SERS nanoprobes The preparation of silica-encapsulated HGNs (SEHGNs) has been reported in our previous papers [20,21]. Cobalt nanoparticles were synthesized by reducing CoCl2 with NaBH4 under N2 purging conditions and were then used as templates for HGNs. Here, MGITC was used as a Raman reporter molecule, with 15 ␮L of 10−5 M MGITC added to 3 mL of HGN nanocolloids. To this solution, 6 ␮L of 1 mM APTES was added, and the mixture was allowed to shake for 10 min. After 10 min, the mixture was rinsed by centrifugation at 3000 rpm for 12 min, and the precipitates were resuspended in 3 mL of deionized (DI) water. Under stirring, 3 mL of SERS-active HGNs was diluted with 9 mL of isopropanol, followed by the addition of 60 ␮L of 28% NH4 OH and 30 ␮L of 0.5 mM TEOS. The reaction was allowed to proceed for 24 h at room temperature. SERSactive SEHGNs were collected by centrifugation at 3000 rpm for 12 min and resuspended in 12 mL of isopropanol. SEHGNs (1.5 mL) were mixed with 10 ␮L of 50% TEOS/MPTMS solution and were reacted for 2 h at room temperature. The thiol-modified SEHGNs were mixed with 10 ␮L of 50 mM phosphate buffer (pH 7.0). Then, 5 ␮L of 5 mg/mL monoclonal anti-AFB1 and 5 ␮L of freshly prepared sulfosuccinimidyl 4-[N-maleimidomethyl]cyclohexane1-carboxylate (sulfo-SMCC, 5 mg/mL in H2 O) were added to the tube, and the solutions were reacted with gentle mixing for 2.5 h at room temperature. After incubation, 5 ␮L of a solution consisting of 10% BSA was added to the reaction tube and incubated at room temperature for 45 min. To purify the conjugates, the reaction mixture was centrifuged at 1000 rpm for 15 min. After removing the supernatant, the pellet was resuspended in 1.5 mL phosphate buffer (20 mM, pH 7/0.1% BSA). The centrifugation/resuspension steps were repeated three times, and the particles were then resuspended to the desired concentration. To find the optimal concentration of sulfo-SMCC for maximum anti-AFB1 loading on SEHGN surface, fluorescence dye-labeled anti-AFB1s were immobilized on SEHGNs under various concentrations of sulfo-SMCC. Fluorescence signals for unbound antibodies in supernatant solution were measured, and the optimum concentration of solfo-SMCC was determined from their fluorescence intensity changes. 2.3. Preparation of magnetic capture probes Anti-ATB1 antibody-conjugated magnetic beads were prepared and used as capture probes. This bio-conjugation process was performed in two sequential steps. First, 0.1 mL of 4.0 mg/mL Biotin-PEG-NHS was mixed with 1 mL of 2.0 mg/mL anti-AT-B1

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Fig. 1. Schematic illustration of SERS-based immunoassay platform for AFB1 detection using SEHGNs and magnetic beads.

antibody and was reacted overnight at 4 ◦ C. Second, 0.2 mL of 1.0 mg/mL streptavidin-coated magnetic beads was added to this solution, and the mixture was shaken for 1 h at room temperature. After incubation, the tube was placed on a magnet for 4 min, and then the supernatant solution was removed using a micropipette. Remaining magnetic beads were washed a total of three times and were dissolved in 0.2 mL of PBS buffer.

a capillary tube, and a 20× objective lens was used to focus the laser spot on the glass tube. Here, the spot size was estimated to be 1 ␮m. The Raman spectra reported here were collected for 10 s of exposure time. In addition, a two-slit confocal arrangement was used to reduce background Raman scattering from non-focused laser beams. All spectra were analyzed using GRAMS software.

2.4. Nanoparticle characterizations and SERS measurements

3. Results and discussion

A Cary 100 spectrophotometer (Varian, USA) was used to acquire UV–vis absorption spectra. High-magnification transmission electron micrograph (TEM) images were produced using a JEOL JEM 2100F instrument at an accelerating voltage of 200 kV. Scanning electron microscope (SEM) images were produced using a TESCAN (MIRA3) instrument at an accelerating voltage of 20 kV. Fluorescence spectra of the NPs were recorded on a Fluorolog 3-11 spectrofluorometer (Jobin Yvon-Spex, Instruments S.A., Inc., Edison, NJ). Dynamic light scattering (DLS) data of the NPs were obtained using a Nano-ZS90 (Malvern). Raman measurements were performed using a Renishaw 2000 Raman microscope system. A Melles Griot He–Ne laser operating at  = 633 nm was used as the excitation source with a laser power of 30 mW. The Rayleigh line was removed from the collected Raman scattering using a holographic notch filter located in the collection path. Raman scattering was detected using a charge-coupled device (CCD) camera at a spectral resolution of 4 cm−1 . An additional CCD camera was fitted to an optical microscope to obtain optical images. Sandwich immunocomplexes in a microtube were collected using

Fig. 1 shows a schematic illustration of the formation of a sandwich immunocomplex using two different metal particles. AFB1 is a small molecule with two different types of binding epitopes [32]. Thus, two different antibodies can be used for the formation of a sandwich immunocomplex. As shown in the left box of Fig. 1, antiAFB1s and anti-ATB1 were conjugated onto the surfaces of SEHGNs and magnetic beads, respectively. Fig. S1 displays the detailed antibody conjugation processes. To selectively attach SEHGNs to AFB1 targets, the silica shell surface was modified with amine groups by reacting APTES with SEHGNs. Then, the solution was reacted with sulfo-SMCC to induce cross-linking of amine groups. Finally, thiolmodified anti-AFB1s were loaded onto the silica surface, as shown in Fig. S1(a). The resulting anti-AFB1-conjugated SEHGNs were used as SERS nano tags for detecting specific AFB1 targets immobilized on magnetic beads. On the other hand, the anti-AFB1 was biotinylated using biotin-PEG-NHS, and then biotinylated anti-ATB1s were immobilized on the surface of a streptavidin-bound magnetic bead through a biotin–streptavidin interaction, as shown in Fig. S1(b).

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Fig. 2. SEM images of sandwich immunocomplexes with (a) 1.0 ␮g/mL of AFB1 and (b) without AFB1. TEM images of sandwich immunocomplexes with (c) 1.0 ␮g/mL of AFB1 and (d) without AFB1.

When AFB1 is present, anti-AFB1-conjugated SEHGNs and ATB1-conjugated magnetic beads form a sandwich immunocomplex through antibody–antigen–antibody interaction; however, this immunocomplex is not formed in the absence of AFB1, as shown in the right figure of Fig. 1. Here, HGNs were utilized as single-particle SERS probes. In other words, HGNs display strong enhancement effects from individual particles due to their capability to localize surface electromagnetic fields through the pinholes in the hollow surfaces. Consequently, they were used as a highly reproducible sensing probe for the quantitative analysis of AFB1 because strongly enhanced SERS signals can be achieved without production of uncontrollable particle aggregations [20,21]. HGNs also exhibit strong enhancement effects regardless of silica encapsulation of individual particles. In addition, SERS-active HGNs were encapsulated with silica to prevent the aggregation of HGNs and competing adsorption between Raman reporter molecules and antibodies. SEHGNs have been considered to be a useful bio-sensing and labelling platform because they provide greatly improved chemical and optical stability as well as a maximum surface coverage of Raman reporters and receptor antibodies [22–26]. Fig. S2(a) displays the TEM images of HGNs and SEHGNs. The diameter and wall thickness of the HGNs were estimated to be 45 ± 5 nm and 15 ± 3 nm, respectively. On the other hand, the optimum thickness of the silica shell was estimated to be 50 nm. Fig. S2(b) shows the results of DLS measurements. Both the HGNs and SEHGNs show homogeneous size distributions, and the average diameter was increased when HGNs were encapsulated with silica. One of the important issues for SERS nano tags is their stability under different salt conditions. The stability of SERS nano tags in saline conditions is crucial for detection applications in human fluid diagnostics. The stability effect of silica encapsulation was clearly observed, as shown in Fig. S3. In a 1.0 M salt condition, the absorption band intensity of unencapsulated HGNs was decreased, as in Fig. S3(a), indicating particle aggregation with increased salt concentration. In contrast, SEHGNs showed no critical intensity change,

as shown in Fig. S3(b), signifying that SEHGNs are stable in the presence of salt. Fig. 2 shows the SEM ((a) and (b)) TEM and ((c) and (d)) images of magnetic beads used for immunoassay, respectively. In the left images, SEHGNs are bound on the surface of the magnetic beads by the antibody–antigen–antibody interactions that occur when AFB1 molecules are present (concentration: 1 ␮g/mL). On the other hand, the right images show that SEHGNs are not bound on the surface because AFB1 is not present. AFB1 sandwich immunocomplexes were collected using three different capillary tubes from the solution, and the SERS spectrum for each capillary tube was measured. Fig. S4 displays the SERS spectra for three different concentrations of AFB1. On the basis of SERS intensity variation, it can be concluded that reproducible measurements are possible because the SERS signals are measured for the average nanoparticle ensembles taken by the capillary tube. To evaluate the selectivity performance of our SERS-based immunosensing platform, the responses to negative control samples were tested with anti-AFB1-conjugated SEHGNs. Two toxins, fumonisin B (FMB) and ochratoxin A (OTA), were selected as negative control targets because they are the most commonly found mycotoxins in barley and corn foods [1–3]. Their chemical structures are displayed in Fig. 3(a). The SERS spectra of AFB1, FMB, OTA, and some of the mixtures (concentration of each toxin: 1 ng/mL) in distilled water are shown in Fig. 3(b). As shown in Fig. 3(c), an increase in characteristic Raman signal intensities was only observed for AFB1 and its mixtures. In contrast, no obvious Raman intensity changes were noted for FMB and OTA. Thus, our SERS-based assay system only responds to AFB1, revealing the high selectivity inherent in our detection system. In addition, different concentrations of AFB1 and negative control targets were spiked into tap water to mimic contaminated sample conditions, and corresponding SERS spectra were analysed. Fig. S5 displays these SERS spectra of AFB1, FMB, OTA, and the mixtures in tap water. The SERS signal enhancement in tap water shows the

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Fig. 3. (a) Molecular structures of AFB1, FMB, and OTA. (b) SERS spectra for AFB1, FMB, OTA, and some of their mixtures in distilled water. (c) Comparison of the relative Raman peak intensities at 1616 cm−1 .

same trend as the experimental data in distilled water, shown in Fig. 3(b). To assess the highly sensitive detection capability of our SERSbased assay technique for AFB1, we compared the assay results to those obtained from HPLC analysis. For this comparison, 10 different concentrations of AFB1 solutions in the 10−2 –105 ng/mL range were prepared. Fig. 4 compares the analytical results of AFB1 using our SERS-based immunosensing platform with those obtained by HPLC for all these concentrations. Average intensities have been determined from three measurements, and the error bars indicate their standard deviations. In the SERS-based analysis, the Raman peak intensity at 1616 cm−1 was monitored for

quantitative evaluation. The values obtained by our proposed SERSbased assay showed good agreement with those from the HPLC analysis in the higher concentration range greater than 10 ng/mL. However, it is interesting that our SERS-based assay results are more consistent in the lower concentration range (10–10−2 ng/mL) than those achieved by HPLC. Here, the red threshold lines in the figures were determined as a function of noise in the measurement process. Although measurements are produced above and below this threshold, all data points observed below this threshold may be reported as ‘not detected’ [33]. The threshold values for HPLC and SERS-based assay measurements are 10 ng/mL and 0.1 ng/mL, respectively. This indicates that more sensitive

Fig. 4. Comparison of the analytical results for ten different concentrations of AFB1 using (a) HPLC and (b) SERS-based immunoassay methods. Red line indicates the threshold for each measurement. Error bars indicate standard deviations of three measurements. (For interpretation of the references to color in this figure legend and text, the reader is referred to the web version of this article.)

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Fig. 5. (a) SERS spectra for decreasing concentrations of AFB1, and (b) the corresponding calibration curve of the SERS signal centered at 1616 cm−1 as a function of AFB1 concentration. Error bars indicate standard deviations of three measurements.

quantification for AFB1 is possible using this SERS-based sandwich immunoassay technique, demonstrating its strong potential as a new analytical tool for highly sensitive analysis of mycotoxins. Finally, the application of our SERS-based immunoassay platform was evaluated with AFB1-spiked tap water. Nine different concentrations of AFB1 were spiked into tap water, and corresponding SERS spectra were measured and analysed. Fig. 5(a) displays the SERS spectra of the antibody-conjugated SEHGN immunocomplexes in the presence of various concentrations of AFB1. Fig. 5(b) portrays the SERS intensity change of the peak centered at 1616 cm−1 as a function of AFB1 concentration. The Raman intensity gradually increased with the increase in AFB1 concentration over a range of 10−2 –105 ng/mL. Here, the error bars are the standard deviations from a total of three measurements. The curve in the figure exhibits a sigmoidal behavior similar to the immunoassay data of ELISA. A four-parameter logistic fitting model was used to obtain the fitting parameters. In addition, a good linear relationship was achieved in the higher concentration range from 1 to 105 ng/mL. This indicates that highly precise quantitative evaluation of AFB1 occurred around the limit of detection (LOD) concentration level. On the basis of our experimental data, it can be concluded that the SERS-based assay technique for AFB1 sandwich immunocomplexes can be successfully used for rapid and sensitive trace analysis of AFB1 in water. The LOD was estimated to be 0.1 ng/mL.

4. Conclusions In this study, a novel SERS-based sandwich immunoassay platform using anti-AFB1-conjugated SEHGNs and anti-ATB1conjugated magnetic beads was developed for highly sensitive detection of AFB1 toxin. Here, the SEHGNs were used as highly reproducible SERS-encoding nanoprobes, and magnetic beads were used as SERS substrates. Target-specific antibodies (antiAFB1 and anti-ATB1) form the sandwich immunocomplexes by antibody–antigen interactions when AFB1 molecules are present in water. Nonspecific binding of SEHGNs was eliminated by washing with a micropipette. The SERS peak intensity of Raman reporter molecules at 1616 cm−1 was monitored for the quantitative evaluation of AFB1. The LOD was determined to be 0.1 ng/mL from the 3 value (standard deviation for the reagent blank’s Raman scattering intensity). Our SERS-based immunoassay technique using SEHGNs and magnetic beads has some advantages over the conventional HPLC method with respect to rapid detection time (less than 30 min) and good reproducibility. The results demonstrate that this SERS-based immunoassay strategy is a viable new sensing

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Highly sensitive SERS-based immunoassay of aflatoxin B1 using silica-encapsulated hollow gold nanoparticles.

Aflatoxin B1 (AFB1) is a well-known carcinogenic contaminant in foods. It is classified as an extremely hazardous compound because of its potential to...
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