Elsevier

Biosensors and Bioelectronics

Volume 77, 15 March 2016, Pages 950-956
Biosensors and Bioelectronics

One-pot synthesis of carbon dots-embedded molecularly imprinted polymer for specific recognition of sterigmatocystin in grains

https://doi.org/10.1016/j.bios.2015.10.072Get rights and content

Highlights

  • DT was selected as an alternative template of ST to synthesize CDs@MIP composite.

  • The optosensing material (CDs@MIP) was prepared in a one-pot NHSG process.

  • The synthesized CDs@MIP was applied to construct FL optosensor for ST.

  • The FL sensor exhibited high sensitivity and selectivity towards ST.

Abstract

A novel sensitive fluorescent sensor for determination of sterigmatocystin (ST), which was based on carbon dots-embedded molecularly imprinted polymer (CDs@MIP), was prepared by an efficient one-pot reaction. First, highly blue luminescent CDs were synthesized via a one-step reaction. Then, through a non-hydrolytic sol–gel process, MIP was formed on the CDs surface in the presence of 1,8-dihydroxyanthraquinone as an alternative template molecule to obtain CDs@MIP. The CDs acted as antennas for signal amplification and optical readout, and the MIP coated on the CDs surface provided specific binding sites for ST. The performance of CDs@MIP was compared with that of CDs embedded in non-imprinted polymer (CDs@NIP). CDs@MIP exhibited high selectivity and sensitivity toward ST. Under optimized conditions, the relative fluorescence intensity of CDs@MIP decreased linearly with the concentration of ST from 0.05 to 2.0 mg L−1 with a detection limit of 0.019 mg L−1 (S/N=3) and the precision for five replicate detections of 0.10 mg L−1 ST was 2.31%. The sensor was also used to determine the content of ST in grains with satisfactory results.

Introduction

Sterigmatocystin (ST), a fungal secondary metabolite produced by many different Aspergillus species (Atalla et al., 2003), is a precursor of aflatoxin B1 (AFB1). ST has a similar 8,9-double bond structure to that of AFB1, as shown in Fig. S1 (Georgianna and Payne, 2009). ST is widely distributed in a variety of food and feed sources (Flores-Flores et al., 2015, Veršilovskis et al., 2008), especially in grains such as wheat, corn and millet, and can pose a serious health risk to humans and livestock that consume contaminated foods (Marin et al., 2013). A large number of studies have indicated that ST affects liver and kidney function. In addition, ST has been closely correlated with the occurrence of esophagus, gastric, and hepatocellular carcinomas (Purchase and van der Watt, 1970, Steyn, 1995). The International Agency for Research on Cancer has classified ST as a group 2B carcinogen (Castegnaro and Wild, 1995) because of its potential carcinogenicity, mutagenicity and teratogenicity to humans (Mori et al., 1986, Ueda et al., 1984). However, very little data concerning monitoring of ST is available at present. Therefore, an accurate and reliable analytical method is needed to facilitate the risk assessment of human exposure to ST, and more importantly, to monitor food products for existing or future legislation.

In the past few decades, a variety of methods, including the enzyme-linked immunosorbent assay (ELISA) (Li et al., 1996, Morgan et al., 1986), thin-layer chromatography (TLC) (Gimeno, 1979, Josefsson and Moller, 1977, Lee et al., 1980), and high-performance liquid chromatography (HPLC) (Frisvad, 1989, Stack et al., 1976), as well as liquid chromatography coupled with mass spectrometry (LC–MS) and tandem mass spectrometry (LC–MS/MS) (Arroyo-Manzanares et al., 2013, Jia et al., 2014, Malachova et al., 2014, Njumbe Ediage et al., 2015, Rubert et al., 2012, Veršilovskis et al., 2007) have been developed for the detection of ST. TLC methods are the most frequently used, but suffer from insufficient selectivity in complex food matrices, and thus are usually employed as qualitative screening tools (Turner et al., 2009). Modern analysis of ST heavily relies on HPLC with ultraviolet–visible (UV–vis) or fluorescence (FL) detectors, which generally requires tedious sample pretreatment or complicated derivatization. LC–MS/MS methods with high sensitivity and selectivity allow qualitative screening and quantitative analysis of ST (Veršilovskis and De Saeger, 2010) but need expensive equipment and highly trained operators, which limits their application. These aforementioned methods have made great progress in the detection of ST; however, when faced with complex food matrices together with low concentrations of contaminants, they are limited by factors like slow response, high cost, and laborious pretreatment. Thus, a simple, selective and sensitive analytical method for detection of trace ST is still required.

Molecular imprinting is an effective strategy to form recognition sites with a predetermined selectivity for template molecules (Ma et al., 2013) and their analogs in complex samples. The traditional technology for preparing molecularly imprinted polymers (MIPs) uses an organic polymer-based system because of the availability of a variety of monomers and excellent stability of such systems at different pH. However, the system may swell or shrink when exposed to different solvents, which may considerably change the morphology of the polymer network (size and shape) and relative positions of the functional groups that are essential for recognition (Yu and Mosbach, 2000). An interesting methodology to prepare silica-based hybrid MIPs based on the non-hydrolytic sol–gel (NHSG) process offers the potential to overcome the above shortcomings, and decrease nonselective absorbance, as well as subtly increase selectivity (Wang et al., 2006, Wang et al., 2008).

Carbon dots (CDs) are an emerging type of carbon nanomaterial of quantum size (Himaja et al., 2015). Since their serendipitous discovery in 2004 (Xu et al., 2004), CDs have attracted intense interest because of their fascinating properties such as green synthesis (Wang et al., 2010, Yu et al., 2015), stable photoluminescence (PL) (Hu et al., 2015), high electrochemical activity, excellent aqueous solubility (Mosconi et al., 2015), easy functionalization, low toxicity (Baker and Baker, 2010) and good biocompatibility (Weng et al., 2015). CDs are benign alternatives to conventional semiconducting quantum dots, so they have been widely applied in bioimaging (Li et al., 2012a, Li et al., 2012b, Zhao et al., 2015), but have rarely been used in sensing (Esteves da Silva and Gonçalves, 2011). Although some FL sensors based on CDs have been reported, these sensors are mainly for detection of metal ions and inorganic anions like Cu2+, Hg2+, Fe3+, Pb2+, Cr6+, Ag+, CN, F, and NO2- (Dong et al., 2012, Gogoi et al., 2015, Liu et al., 2015, Qu et al., 2013, Wang et al., 2015, Yu et al., 2015, Zhu et al., 2012).

The main obstacles in the development of CDs-based sensors are associated with immobilization of CDs in a suitable matrix that allows the CDs to retain their PL properties and analyte permeation, and prevents CDs leaching. The selectivity and sensitivity of CDs sensors also remain underdeveloped. Silica-based MIPs may be an ideal matrix to efficiently protect the fluorescent CDs and also selectively bind template molecules. However, to strengthen the interaction between CDs and a silica-based matrix, the CDs need to be modified with organosilane before polymerization, which means that polymer preparation requires at least two steps.

In this paper, we introduce a novel methodology based on the NHSG process and molecular imprinting technology to fabricate CDs directly encapsulated in a silica MIP in a one-pot reaction. The high selectivity of the MIP is combined with the stable FL of CDs to realize simple, sensitive and selective detection of ST. Considering the high price and toxicity of ST, we use 1,8-dihydroxyanthraquinone as an alternative template to ST in the imprinting process. The synthesis process of CDs@MIP, is outlined in Fig. 1. CDs in the center of the sensor can be treated as antennas for selective recognition, signal amplification and optical readout, while the MIP coated on the surface of the CDs provides specific binding sites for ST. The FL of CDs@MIP is quenched when ST is present, and the mechanism of FL quenching is discussed. The extent of FL quenching is proportional to the concentration of ST in the sample. The properties of CDs@MIP are evaluated by a series of experiments, and it is also used to analyze the content of ST in grain samples.

Section snippets

Materials

N-(β-aminoethyl)-γ-aminopropyl methyldimethoxysilane (AEAPMS, 97%), γ-methacryloxypropyltrimethoxysilane (MPTMS, 99%), quercetin (QC, 98%) and 2,2-azobisisobutyronitrile (AIBN, 99%) were purchased from TCI (Shanghai) Development Co., Ltd., Shanghai, China. Methacrylic acid (MAA, 99%) was obtained from Tianjin Kermel Chemical Reagents Co., Ltd., Tianjin, China. 1,8-Dihydroxyanthraquinone (DT, 96%), ST (98%), AFB1 (98%), ochratoxin A (OTA, 99%) and zearalenone (ZEN, 98%) were purchased from

Preparation and characterization of CDs

As illustrated in Fig. 1A, highly luminescent CDs were synthesized by an efficient one-step reaction using multi-aminosilane AEAPMS as a coordinating solvent. The synthesized organosilane-functionalized CDs were characterized with FT-IR and FL spectroscopies, the results of which are presented in Fig. 2. The synthesis of organosilane-functionalized CDs involved the decomposition and pyrolysis of anhydrous citric acid, which was accompanied by the formation of gas and condensable vapors.

Conclusion

We constructed a highly sensitive and selective FL sensor by an efficient one-pot reaction. The sensor integrated the advantages of both CDs and MIPs, so it had the functions of signal amplification and optical readout, as well as specific recognition. When the sensor was used to determine the content of ST in real samples, satisfactory results were obtained. This study provides a new possible modality for analysis of mycotoxins, and also furthers the application of CDs in optical and chemical

Acknowledgments

This work was supported by the Ministry of Science and Technology of People's Republic of China, China (Project no. 2012AA101602), National Science Foundation for Distinguished Young Scholars of China (Project no. 31225021), Special Fund for Agro-scientific Research in the Public Interest, China (Project no. 201203069), and Innovation Talents of Science and Technology Plan of Yunnan Province (Project no. 2012HA009).

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