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BY 4.0 license Open Access Published by De Gruyter Open Access December 31, 2019

A potential reusable fluorescent aptasensor based on magnetic nanoparticles for ochratoxin A analysis

  • Pinzhu Qin , Dawei Huang EMAIL logo , Zihao Xu , Ying Guan , Yongxin Bing EMAIL logo and Ang Yu
From the journal Open Chemistry


An aptasensor for the detection of ochratoxin A (OTA) in environmental samples was developed. It displayed high sensitivity and good selectivity. Factors such as specific binding between a FAM (5-carboxyfluorescein)-labeled aptamer (f-RP) and OTA, and a magnetic property of a streptavidin magbeads-modified capture probe (bm-CP) resulted in aptasensor’s linear relationship between fluorescence intensity and the concentration of OTA. This characteristic is present at the OTA concentration ranges from 0.100 μM to 25.00 μM with a LOD (limit of detection) of 0.0690 μM. The bm-CP can be reused through melting, washing and magnetic separation, which contributes to cost reduction. In addition, the proposed method is simple and detection process is fast. The aptasensor can be used in real samples.

1 Introduction

Environmental pollution and food safety have always been the focus of attention around the world [1, 2]. Mycotoxins including ochratoxin A (OTA), aflatoxin B1, aflatoxin B2, fumonisin B1 and zearalenone are toxicity secondary metabolites produced by fungi or molds [3]. Among them, OTA is widely distributed in nature and has extremely high toxicity. Humans and animals are exposed to OTA in variety of ways, for example through cereals, spices, beans, soluble coffee, dried fruit, grape juice, milk, and honey. OTA poses a great threat to human health, because it can cause toxic kidney damage, hepatotoxicity, immunosuppression or lead to cancer, teratogenicity, and mutagenesis [4, 5]. Considering the results of these severe toxic effects, several countries regulate the maximum levels of OTA. In Switzerland, the permissible amount of OTA in compound feeds for pigs and poultry should not exceed 200 μg/kg and 1000 μg/kg [6].

Traditional methods for OTA analysis include thin layer chromatography (TLC), gas chromatography (GC), liquid chromatography (LC), liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), high performance liquid chromatography-fluorescence detection (HPLC-FLD), capillary electrophoresis (CE), and immunoassay [7, 8, 9, 10, 11, 12]. Although chromatographic analysis is sensitive, it is limited by the complexity of the operation, the expensive equipment and the need for professionals to operate in practical condition. Therefore, it cannot meet the needs of an on-site rapid detection [13,14]. The methods used for on-site rapid detection are mainly immunoassay methods, including colloidal gold fast type (direct method), colloidal gold precision type (extracted reconstitution method), and Elisa kit. However, a method based on the immunological technique is costly and is easily affected by environmental factors [15, 16, 17]. Hence, a simple, low-cost and fast detection method is desirable. Fortunately, with the aptamer-specific, OTA was screened out and reported, and a number of sensors have been established using aptamers.

Aptamer is an oligonucleotide fragment which can be screened in vitro (systematic evolution of ligands by exponential enrichment, SELEX) and can bind to the corresponding ligands through various interactions such as Van der Waals forces, hydrogen bonding, electrostatic interactions, and shape matching [18, 19, 20, 21, 22]. More importantly, aptamer possess several advantages such as thermal stability, reusability, is easy to synthesize, with more sensitivity than antigen reaction [23]. Thus, aptamer-based strategies have been widely applied in food safety analysis, environmental pollution analysis and clinical diagnosis [24,25].

Fluorescence labeling is an attractive analytical method due to its stability and high detection sensitivity. Researchers developed different types of sensors such as immunolabeling molecule beacon fluorescent biosensor and biosensor fluorescence sensor with an aptamer structure switch [26, 27, 28]. Recently, the fluorescence method that involved magnetic nanoparticles has attracted the attention of researchers. Jiang et al. described the application and development of nanomaterial-enabled biosensors in OTA detection in the past years (2007-2018) [29]. Magnetic nanoparticles are widely used in the field of molecular biology because of their good biocompatibility, superparamagnetism, low toxicity and easy preparation [30], where the viruses are separated and the environment purified [31,32]. An oligonucleotide modified by magnetic nanoparticle can be prepared as capture probe, while a fluorescent labeling aptamer can be prepared as a recognition probe. After being hybridized and separated, the capture probe can be regenerated through thermal treatment for next experiment, which can reduce cost. As far as we know, fluorescence analysis methods based on magnetic nanoparticles have not been reported to detect OTA in the environment.

In this work, a fluorescent analysis method based on magnetic nanoparticles is developed to detect OTA. The OTA aptamer recognition probe was labeled with fluorescent dyes (f-RP), and the capture probe was labeled with biotin (b-CP) and then modified with magnetic nanoparticles (bm-CP). f-RP was added to the test sample to specifically bind with OTA, and bm-CP was added to hybridize with the unreacted f-RP. The fluorescence signal of the supernatant was measured after magnetic separation. Furthermore, the bm-CP was suggested to be reused after magnetic separation, cross-linking, washing, and re-magnetic separation.

2 Experimental

2.1 Reagents and chemicals

OTA was purchased from Sigma Corporation, aflatoxin B1 (AFB1) and SSC buffer powder were obtained from Yuanye Biological Technology Co., Ltd, streptavidin magbeads (10mg/mL, 100nm) were purchased from Aladdin Industrial Corporation. (Hydroxymethyl)-aminomethane (Tris), tween, sodium chloride (NaCl), potassium chloride (KCl), potassium dihydrogen phosphate (KH2PO4) and disodium phosphate (Na2HPO4) were obtained from Sinopharm Chemical Reagent Co., Ltd (Guangdong, China). Ultrapure water was used throughout the experiment (18.3 MΩ cm). The ssDNA sequences used in the experiment are as follows: aptamer (f-RP): 5’-FAM-GAT CGG GTG TGG GTG GCG TAA AGG GAG CAT CGG ACA-3’, capture probe (b-CP): 5’-biotin-TTT TTT CCG ATG CTC CCT-3’. The aptamer and capture probe were purchased from Sangon Biotech (Shanghai, China). The aptamer stock solution obtained by dissolving the oligonucleotides in 260 μL of PBS, the capture probe was dissolved in 580 μL of PBS, the two chains were stored at 4oC before usage.

2.2 Preparation of buffer solution and OTA solution

Tris binding buffer solution containing tween (TBBT): 50.0 mL of 0.10 M tris-hydroxymethane solution (0.6056 g of tris was added to 50.0 mL water) and 42.0 mL of 0.10 M hydrochloric acid (9.0 mL of concentrated hydrochloric acid was added to 1000 mL water) were diluted it to 1.0 L.

PBS buffer solution: 0.27g of KH2PO4, 1.42g of Na2HPO4, 8.00g of NaCl and 0.20g of KCl were added to 800 mL water, and then concentrated hydrochloric acid was added to adjust pH to 7.4, finally dilute it to 1.0 L.

OTA solution: 5.00 mg of OTA was added to 5.00 mL methanol and forming a 1.00 g/L standard solution. 1009 μL 1.00 g/L standard solution was shifted to 100.0 mL volumetric flask and then a 25.0 μM OTA reserve solution was obtained.

2.3 Preparation of bm-CP

48.0 µL of 10.0 mg/mL streptavidin magbeads and 8.0 μL of 25.0 μM b-CP were added to the reaction flask, the reaction was conducted at 37oC for 30 min, finally the bm-CP was obtained.

2.4 Apparatus

All fluorescence measurements were carried out on a F4600 fluorometer (Hitachi, Japan). Other instruments used during the experiment included high-speed (low-speed) centrifuges, constant-temperature digital water baths, oscillators, magnets, ultrasonic cleaners, sterilizers, and drying ovens.

2.5 The detection process for OTA and the recovery of bm-CP

After 80.0 μL of TBBT and 8.0 μL of 25.0 μM f-RP were added to the reaction flask, 8.0 μL of 25.0 μM OTA solution was added, the mixed solution was incubated at 45oC for 20 min. Then, 56.0 μL of bm-CP was added and hybridized with unreact f-RP at 30oC for 50 min; Next, the reaction flask was placed on a magnetic rack for magnetic separation, the supernatant was removed and then rinsed by TBBT. Finally, the fluorescence intensity of supernatant was measured (λex=488 nm, λem=518 nm). After the measurement, 300.0 μL of TBBT was added to the reaction flask containing magnetic nanoparticle complexes at the bottom, and the solution was shaken at a constant temperature shaker in 45oC for 30 min. After that, the supernatant was removed and the magnetic nanoparticles complex bm-CP were recovered from the bottom of the flask.

Ethical approval: The conducted research is not related to either human or animal use.

3 Results and Discussion

3.1 Experimental principle

The principle of the bioassay for OTA detection is illustrated in Figure 1. The establishment of this experiment was based on: a) the specific binding of aptamer and OTA, b) streptavidin magbeads attraction by magnets. The aptamer of OTA was labeled with FAM (f-RP), the capture probe (designed by the OTA aptamer) labeled with biotin and modified with streptavidin magbeads (bm-CP). After f-RP was combined with OTA, the bm-CP was added to hybridize with unreacted f-RP. Given enough time for hybridization, the bm-CP at the bottom of the reactor was retrieved by a magnet and the supernatant was then transferred to the cuvette and subjected to the process of magnetic separation. Therefore, the amount of OTA can be determined by measuring the fluorescence intensity of the supernatant. Three sample repeats were prepared in the experiments and all results were averaged from the three parallel experiments. In addition, the bm-CP could be recycled after melting, washing and re-magnetic separation, which saved cost.

Figure 1 The experimental principal of the bioassay for OTA detection.
Figure 1

The experimental principal of the bioassay for OTA detection.

3.2 Optimization of the aptasensor

In order to save the streptavidin magbeads and obtain ideal experiment results, the optimization of the ratio between b-CP and streptavidin magbeads was necessary. Four optimization molar ratios were tested (b-CP: streptavidin magbeads) = 1:2, 1:3, 1:4, and 1:5 with the same amounts of OTA and f-RP. As seen from Figure 2, when the ratio was 1:4, the fluorescence intensity was the weakest, indicating sufficient bm-CP and the best combination of b-CP and streptavidin magbeads. To facilitate the experiments, the ratio of 1:4 was chosen as the optimal ratio in the sensing system.

Figure 2 Effects of the ratio between b-CP and streptavidin magbeads.
Figure 2

Effects of the ratio between b-CP and streptavidin magbeads.

The different types of buffer solutions will affect the final result of the detection system [33]. Three different types of buffer solutions (tris, TBBT and SSC) were compared to determine the most suitable buffer in this bio-system. The results (Figure 3) show that the fluorescence intensity measured in the tris buffer solution was significantly higher than that measured in the SSC buffer solution, indicating that the tris buffer solution should be selected compared to SSC buffer solution. The effects of adding TBBT to tris buffer solution or not (tris) were investigated and their impact on the experiments were analyzed . The results show that TBBT can effectively enhance the fluorescence intensity. All the results indicate that TBBT is the best choice as the buffer solution.

Figure 3 Effects of different buffer solutions.
Figure 3

Effects of different buffer solutions.

Lower hybridization temperature will result in insufficient hybridization and higher temperature will cause dissolution of the oligonucleotide chain [34,35]. Temperatures such as 25, 30, 35, 40oC were selected to for testing. The results are shown in Figure 4. It was found that the highest fluorescent intensity was obtained at 30oC after 30 min of hybridization. Hybridization time was detected and it was found that the fluorescence intensity increased along with the time and reached the highest point at 50 min (the mean fluorescence intensity was found to be 565.22±19.80 (n=3), 635.57±9.19 (n=3), 716.55±12.73 (n=3), and 880.50±23.33 (n=3) for the hybridization time of 20 min, 30 min, 40 min, and 50 min, respectively). Therefore, 30oC was chosen as the optimization hybridized temperature and 50 min as the optimization hybridized time.

Figure 4 Effects of hybridized temperature.
Figure 4

Effects of hybridized temperature.

Furthermore, in order to realize the recovery of bm-CP, it is necessary to select the best temperature and time for derotation between f-RP and bm-CP. The results revealed that 45oC for 30 min was the optimum conditions (Figure 5 and Figure 6). After melting, the bm-CP was washed and magnetic particles separated. The results show that the recycled bm-CP have almost no effect on the experimental data due to its excessive amount in the proposed method.

Figure 5 Effects of derotation temperature.
Figure 5

Effects of derotation temperature.

Figure 6 Effects of derotation time.
Figure 6

Effects of derotation time.

3.3 The sensitivity of the aptasensor

Under optimized conditions, the sensitivity of the aptasensor is revealed in Figure 7. Different concentrations of OTA were selected to measure the change of fluorescent intensity. The various concentrations of OTA were 0, 0.0010, 0.010, 0.100, 1.00, 10.0, and 25.0 μM. The results indicated that higher OTA concentration results in greater fluorescence intensity (Figure 7a). Corresponding to the amplified curves in Figure 7b, a good linear relationship from 0.10 μM to 25.0 μM was obtained and the limit of detection found as 0.0690 μM (calculated by 3S+F¯bK,where S is the standard deviation of black control, K is the slope of the regression equation, F¯is the average background fluorescence intensity, b is the intercept in the linear regression equation).

Figure 7 Sensitivity of this aptasensor: (a) Plot of fluorescence intensity dependent on OTA concentrations; (b) OTA and its fluorescence intensity linear curve and linear regression equation.
Figure 7

Sensitivity of this aptasensor: (a) Plot of fluorescence intensity dependent on OTA concentrations; (b) OTA and its fluorescence intensity linear curve and linear regression equation.

3.4 The selectivity of the aptasensor

The experimental principle is based on the specific binding of the aptamer and OTA and it is necessary to define if the OTB, whose structure is similar to OTA, will interfere with the aptasensor (OTB is a dechlorination derivative of OTA). As seen from Figure 8, it is clear that fluorescent signal of OTA is much stronger than that of OTB. These phenomena might be attributed to the strong specificity and high affinity between aptamer and target, their combined stable structures such as G-tetrad and fake, the hydrogen bonding and Van der Waals forces [36, 37, 38]. This result revealed that our aptasensor was very selective for OTA detection.

Figure 8 Selectivity of our aptasensor.
Figure 8

Selectivity of our aptasensor.

3.5 Preliminary determination of OTA in real samples

In real life, a lot of OTA can be produced after the mildew from corn, rice, and bread [39]. In order to determine whether the aptasensor can be effectively applied in actual samples, an applicability test was conducted. First, 1.0 mg of moldy corn, rice, and bread material was added to a mixed solution (5.0 mL of alcohol and 5.0 mL of water), then the established aptasensor was used to determine the amount of OTA in the three samples. As shown in Figure 9, the fluorescence intensity of the system is significantly higher than the blank sample, indicating that this method can be applied to the detection of OTA in actual samples.

Figure 9 The practical application of this aptasensor.
Figure 9

The practical application of this aptasensor.

4 Conclusions

The aptasensor takes the advantage of the magnetic nanoparticles properties and it can be moved in a magnetic field. The unreacted system can be separated from the reaction system rapidly by using a magnet after a completed hybridization. As a result, a sensitive and selective method was develped with a simple and fast detection process. The streptavidin magbeads-modified capture probe (bm-CP) can be recovered by thermal analysis, achieving the goal of reusability at low cost. In addition, the process of preparing bm-CP again was eliminated, making the detection quicker and simpler. The detection limit of this method can reach nM level. Furthermore, there were no large instruments applied in the reaction steps, making the aptasensor easier more practical.


This work was financially supported by the Natural Science Foundation of Guangdong Province, China (2016A030310022), the National Natural Science Foundation of China (21507034), the Natural Science Fund for Colleges and Universities in Jiangsu Province (16KJB610003), and the Opening Foundation of Jiangsu Province Key Laboratory of Environmental Engineering (KF2014008).

  1. Conflict of interest: Authors state no conflict of interest.


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Received: 2019-05-06
Accepted: 2019-08-23
Published Online: 2019-12-31

© 2019 Pinzhu Qin et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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