A new water-soluble and colorimetric fluorescent probe for highly sensitive detection of organophosphorus pesticides

Abstract

A new water-soluble fluorescent probe bearing 1,8-naphthalimide dye, a quaternary ammonium salt and a boronate group was developed for the detection of organophosphorus pesticides. The detection assay was composed of the probe, choline oxidase (ChOx) and acetylcholinesterase (AChE), which involved ChOx and AChE catalyze acetylcholine chloride (ACh) to produce H₂O₂ that increases the fluorescence of the probe. In the presence of pesticides, the activity of AChE was inhibited and the enzyme-generated H₂O₂ was decreased, which results in a decrease in the fluorescence of the probe. The probe displays sensitive and rapid colorimetric fluorescence towards pesticides. The fluorescence intensity was proportional to the logarithm concentration of acephate, parathion-methyl and trichlorfon over a range of 1.0 × 10⁻⁸ to 1.0 × 10⁻⁴ g L⁻¹ (R² = 0.9908), 1.0 × 10⁻⁹ to 1.0 × 10^−5.5 g L⁻¹ (R² = 0.9938), 1.0 × 10⁻⁸ to 1.0 × 10^−4.5 g L⁻¹ (R² = 0.9932), respectively. The detection limits for acephate, parathion-methyl and trichlorfon were 1.16 × 10⁻⁹, 3.36 × 10⁻¹⁰ and 4.72 × 10⁻⁹ g L⁻¹ (S/N = 3), respectively. Moreover, this method has been used for the determination of practical samples with satisfactory results, which further demonstrates its value in practical applications.

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1. Introduction

Organophosphorus pesticides (OPs) have been extensively used in agriculture as insecticides because of their high effectiveness for insect and disease eradication.¹ However, they have caused widespread residues in food products and contamination of the environment.² According to statistics, about one billion pounds of OPs are released globally into the environment and food and water supplies each year, posing great danger to public health.³ OPs are well-known to inhibit acetylcholinesterase (AChE) and there is a risk of accumulation of the neurotransmitter acetylcholine in the human body, which can lead to organ failure and eventual death.^3,4 Therefore, it is of great importance to quantitatively detect OPs in the environment or public places to avoid sudden OPs contamination or potential terrorist attacks.

Over the past few decades, several analytical techniques have been used for the direct detection of pesticides in water and food, which include mass spectrometry (MS),^4,5 high-performance liquid chromatography (HPLC),⁶ enzyme-linked immunosorbent assays⁷ and gas chromatography (GC).⁸ Although these methods have adequate sensitivity and high selectivity, the disadvantages of these techniques are that they are time-consuming, expensive, and display inadequate detection limits.³

Enzyme-based analytical methods are an indirect way to detect pesticides, which can inhibit the enzymatic activity of acetylcholinesterase (AChE) and result in the production of less H₂O₂. AChE and choline oxidase (ChOx) are often used to catalyze ACh producing an electrochemically active product (H₂O₂) or spectral sensing products for absorbance and fluorescence detection. Thus, enzyme-based methods are usually involved in electrochemical and optical detection. However, measuring the electrochemical response of enzyme-generated H₂O₂ is not sensitive enough and complex treatment of the electrode is involved.⁹ Optical methods, such as colorimetric and fluorescence assays, are widely used due to their simple operation. Colorimetric assays based on enzyme-generated H₂O₂ always depend on the use of peroxidase or an oxidase-like enzyme, such as horseradish peroxidase (HRP) or various types of nanoparticles (NPs) and various substrates including 3,3,5,5-tetrame-thylbenzidine (TMB), o-phenylenediamine (OPD), pyrogallolbv and 2,2′-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS).^3,10 However, these methods are not sensitive enough because the detection is based on a color change read by naked eye or UV-visible absorption. Alternatively, more and more attention has been paid to the fluorescence detection of OPs in recent years since this method is sensitive enough.^8a,11 The fluorescence approaches used for detection of OPs based on nanoparticles have been developed such as Mn-doped ZnSe dots,¹² CdTe quantum dots,¹³ silicon quantum dots,¹⁴ and gold nanoparticles.¹⁵ These methods are highly sensitive and selective. However, they always suffer from the size of the nanoparticles, potential toxicity and chemical instability. Therefore, the design and development of highly sensitive and selective, simple fluorescent probes for the detection of OPs is required.

At present, many assays are focused on synthesis of new fluorescent probes, such as dichlorodihydrofluorescein diacetate (DCFH-DA), 10-acetyl-3,7-dihydroxyphenoxazine (Amplex red), hydroethidine (HE), Mito-SOX and boronate, which are commonly used for measuring hydrogen peroxide, superoxide and peroxynitrite in biological systems or the detection of enzyme-generated H₂O₂. Unfortunately, the assays based on DCFH-DA, HE and Mito-SOX have high backgrounds, produced by their reaction with reactive oxygen species (ROS) when detecting H₂O₂. To the best of our knowledge, only the Amplex red ACh/AChE assay kit can provide an ultrasensitive and selective measurement for AChE activity based on enzyme-generated H₂O₂.¹⁶ However, HRP is an indispensable factor for catalyzing the oxidation of Amplex red by enzyme-generated H₂O₂. However, the maximum absorption and fluorescence emission of its oxidation product are extraordinary close (excitation at 571 and emission at 585 nm), which may display spectral interference.^16,17 Fortunately, boronate derivatives can react with H₂O₂ to produce a single product. In addition, the reaction cannot be interfered by ROS, which has great advantages over other H₂O₂-responsive probes.

Based on the advantages of boronate probes, we designed a new water-soluble and boronate-tagged 1,8-naphthalimide probe used for detection of H₂O₂. 1,8-Naphthalimide fluorophores are largely conjugated and coplanar and have the advantages of a large Stokes shift, moderate fluorescence emission wavelength and high fluorescence quantum yield.¹⁸ The probe used 1,8-naphthalimide as the fluorophore, boronic ester as a recognition unit and a quaternary ammonium salt as a hydrophilic group. Upon reaction with H₂O₂, the boronate moiety was attached to the fluorophore of 1,8-naphthalimide and hydrolytic deprotection of the boronate results in an increase in the fluorescence. The quaternary ammonium salt group can make the designed probe operate in a pure water solution. Therefore, the probe can detect enzyme-generated H₂O₂ via an intramolecular charge-transfer (ICT) in a pure water solution. It is well-known that enzyme-generated H₂O₂ is produced from AChE enzyme catalyzing ACh. However, the activity of AChE can be inhibited by pesticides. Therefore, the indirect detection of pesticide was realized. This novel strategy was based on the fluorescence of the probe, which changes with the activity of AChE inhibited by pesticides. To the best of our knowledge, there are few organic fluorescence probes combining with an enzyme used to detect the OPs. The probe does not require complex modification and enzyme immobilization, and the assay results can be read in a short time. Moreover, it can be noted that our designed probe can be used for the colorimetric and fluorescence detection of pesticides in a pure water solution. A dual-signaling chemodosimeter can combine the sensitivity of fluorescence with the convenience and esthetic appeal of a colorimetric assay¹⁹ and is very suitable for real life applications, which will expand the new methods used for monitoring organophosphates pesticides in emergency cases.

2. Experimental

2.1 Reagents and chemicals

4-Bromo-1,8-naphthalic anhydride, 2-picolylamine, bis(pinacolato)diboron and [1,1′-bis(diphenylphosphino)ferrocene] palladium(II) dichloride dichloromethane were purchased from Sigma-Aldrich company. All the other chemicals used in this work were of analytical grade, purchased from Sinopharm chemical reagent company and used without further purification. The synthesis of compound 2 was similar to the procedures reported by Minyong Li et al.²⁰ Thin-layer chromatography (TLC) was carried out on silica gel plates (60F-254) using UV-light to monitor the reaction and silica gel (200–300 mesh) was used for column chromatography. Milli-Q ultrapure water (Millipore, ≥18 M cm) was used throughout.

2.2 Instrumentation

UV-vis absorption spectra were obtained on a UV2450 (Shimadzu Co., Japan). Fluorescence measurements were carried out on an F-4500 FL spectrophotometer (Hitachi Ltd, Japan) with the excitation slit set at 5 nm and emission at 10 nm. All pH measurements were carried out using a Sartorius basic pH-meter PB-10. ¹H NMR measurements were performed on a Bruker AVB-500 MHz NMR spectrometer (Bruker biospin, Switzerland). MS analysis was performed on an electrospray ionization high-resolution mass spectra (Bruke P-SIMS-GlyFT-ICR).

2.3 Synthesis of compound 1

Compound 1 was synthesized according to the synthetic route outlined in Scheme 1. Under the protection of an argon atmosphere, a mixture of 2 (0.41 g, 1 mmol) and CH₃CH₂I (1 mL) was stirred at 80 °C for 12 h in sealed glass tube. The resulting mixture was cooled to room temperature and filtered to afford a yellow solid in 84% yield (0.48 g). ¹H NMR (500 MHz, DMSO-d₆) δ (ppm): 9.18 (d, 1H, J = 6.5), 9.10 (d, 1H, J = 8.5), 8.54 (d, 2H, J = 7.0), 8.44 (t, 1H, J = 7.5), 8.28 (d, 1H, J = 7.0), 8.13 (d, 1H, J = 7.0), 8.08 (t, 1H, J = 7.0), 7.98 (t, 1H, J = 7.5), 5.71 (s, 2H), 4.88 (m, 2H), 1.68 (t, 3H), 1.44 (s, 12H). ¹³C NMR (125 MHz, DMSO-d₆) δ (ppm): 164.2, 164.2, 153.3, 146.3, 146.1, 136.1, 135.3, 135.1, 131.4, 130.3, 128.2, 128.1, 127.1, 126.7, 124.7, 124.6, 85.1, 53.9, 40.1, 25.2, 15.9. MS calcd for C₂₆H₂₈BIN₂O₄ [M]⁺ 570.2270, found 570.0000.

Scheme 1 The synthetic route used to prepare compound 1.

2.4 Measurement procedure

PBS buffer solutions (10 mM, pH = 7.4) were prepared in deionized water. Compound 1 was dissolved in the requisite amount of PBS buffer solution (10 mM, pH = 7.4) as a stock solution (200 μM). The solutions of various biologically species were diluted with PBS buffer solution (10 mM, pH = 7.4). The pH solutions were obtained by adjustment of the PBS buffer solution (10 mM, pH = 7.4) with 1 mM HCl or 1 mM NaOH. The measurements for pesticides were carried out as follows: 12 μL of AChE (50 U mL⁻¹) was first incubated with 20 μL of PBS buffer solution (10 mM, pH = 7.4) containing various concentrations of parathion-methyl, phorate and acephate for 15 min at room temperature. The resulting solution was then added to the assay solution prepared with 84 μL of PBS (10 mM, pH = 7.4), which contained ChOx (4 μL, 50 U mL⁻¹), ACh (40 μL, 1.5 mM) and compound 1 (40 μL, 200 μM). The mixed solution was incubated for another 15 min in the dark at 37 °C. When the excitation wavelength was set at 445 nm, the fluorescence emission intensity at 562 nm was used for the quantitative analysis of the OPs. The experiments were repeated three times to ensure the accuracy of the measurements.

2.5 Determination of pesticides residues in samples

To evaluate the parathion-methyl residue levels in real applications for pesticide assays, we selected celery cabbage, apple and dipterex as the sample matrix. An atomizer was used to spray a certain volume of standard parathion-methyl solution (0.1 g L⁻¹) onto the food samples. First, the samples were chopped. Then, edible parts of the samples were crushed into a homogenate using a juice extractor, 5 g of each homogenate was dissolved in 10 mL of CH₃CN, and then, to remove the insoluble materials, filtered through a 0.22 μm membrane. The obtained filtrate was dried using a water bath. After 4 h, the remaining solid substance was diluted with methanol to a final volume of 1 mL. The pesticide residues in aliquots of the samples were collected every other day for 10 days. Thereafter, the final pesticide was detected using the method mentioned above.

3. Results and discussion

3.1 Spectra properties of compound 1

As expected, compound 1 was soluble in water and a PBS buffer solution (10 mM, pH = 7.4) of compound 1 (5 μM) can be easily prepared. Moreover, H₂O₂ will convert the boronic ester of the fluorescence sensor to phenol in a water solution. Thus, we first examined the reaction of the probe with H₂O₂ (Fig. 1). As seen, compound 1 itself has a strong absorption band at 355 nm. However, in the presence of H₂O₂, the absorption peak at 355 nm decreased and a new absorption peak at 445 nm appeared. Upon increasing the H₂O₂ concentration, the color of the compound 1 solution changed from colorless to yellow. Therefore, compound 1 can serve as a naked eye indicator for H₂O₂. Moreover, this phenomenon has been used in detecting organophosphorus pesticides (Fig. S1, see ESI†). As shown in Fig. 1B, upon excitation at 445 nm, compound 1 itself has a weak fluorescence at 562 nm and has a strong emission at 562 nm in the presence of H₂O₂. Moreover, the normalized fluorescence intensity at 562 nm was obviously increased upon an increase in the H₂O₂ concentration. The results suggest that compound 1 is an ideal fluorescent probe for the detection of H₂O₂.

Fig. 1 The (A) UV-vis absorption spectra and (B) normalized fluorescence spectra of 5 μM compound 1 upon the addition of an increasing concentration of H₂O₂ in PBS buffered solution (10 mM, pH = 7.4). λ_ex = 445 nm.

3.2 Mechanism of the detection of pesticides

The assay was composed of compound 1, AChE and ChOx. The indirect fluorescence biosensing route was based on the change in fluorescence of compound 1 and the inhibition of enzyme activity. AChE and ChOx catalyze the formation of H₂O₂ in the presence of acetylcholine:

The resultant H₂O₂ then reacts with compound 1 to produce the fluorescent product. After incubation with OPs, the enzymatic activity of AChE was inhibited and less H₂O₂ was produced, which results in a decrease in reaction of compound 1 and was accompanied with a drop in the fluorescence intensity. Therefore, the fluorescence intensity of compound 1 coupled with the enzymatic activity of AChE was expected to be a feasible measurement of OPs. The general process for the detection of OPs based on compound 1 is shown in Scheme 2.

Scheme 2 A schematic of the sensing strategy using compound 1 to detect organophosphorus pesticides.

To confirm the interaction of compound 1 with enzyme-generated H₂O₂, the normalized fluorescence of compound 1 containing Ach, AChE and ChOx in the absence and presence of OPs was investigated in PBS. As shown in Fig. 2, the fluorescence intensity of compound 1 at 562 nm was obviously increased in the absence of OPs in PBS. This suggests that the enzyme-generated H₂O₂ causes the hydrolytic deprotection of the boronate and elicits a significant increase in the fluorescence at 562 nm. However, in the presence of OPs, the activity of AChE was inhibited and lead to a decrease in the amount of enzyme-generated H₂O₂. Hence, a relatively small fluorescence of compound 1 was obtained. To further clarify the sensing mechanism of compound 1, the purified product of the reaction of compound 1 with H₂O₂ was then characterized by ¹H NMR and mass spectroscopy (Fig. S2 and S3, see ESI†). From the ¹H NMR spectrum, the peak of the boronic ester group (δ = 1.44 ppm, 12H) completely disappeared and from MS and a new mass-to-charge ratio at m/z = 459.0000 [M]⁺ appeared. The results indicate that the boronic ester group was converted to phenol group by H₂O₂.

Fig. 2 The normalized fluorescence spectra of a PBS buffered solution (10 mM, pH = 7.4) of (a) compound 1 + AchE + ChOx, (b) compound 1 + AchE + ChOx + ACh and (c) compound 1 + AchE + ChOx + parathion-methyl + ACh, respectively. λ_ex = 445 nm.

3.3 Optimization of the detection conditions

To effectively detect pesticides, it is necessary to optimize the effect of pH, incubation time, concentrations of AChE and ChOx, temperature and obtain the best concentration of ACh. Thus, we firstly investigated the effect of pH. As shown in Fig. 3, the relative fluorescence intensity of the distinct fluorescence of compound 1 changes under different pH conditions. When the solution pH was between 6.0 and 7.4 or in the range of 7.4–9.0, a small increase in the fluorescence of compound 1 was observed. At pH 7.4, the normalized fluorescence intensity was the highest. In addition, a high pH will lead to AChE and ChOx becoming denatured and results in a decrease in the activity of the enzymes. It is reported that the optimum pH values for ChOx and AChE were 7.0–8.0 and 8.0–9.0, respectively.¹² Therefore, 10 mM PBS (pH = 7.4) was selected as the detection medium in our subsequent experiments.

Fig. 3 The effect of various pH on the normalized fluorescence intensity of 40 μM compound 1 in PBS buffered solution (10 mM, pH = 7.4) containing 100 μM ACh, 3.0 U mL⁻¹ AChE and 1.0 U mL⁻¹ ChOx. λ_ex = 445 nm.

The reaction time is also an important factor for reaction-based probes. Thus, we also investigated the time-dependent fluorescence spectra of compound 1. As shown in Fig. 4, the relative fluorescence intensity at 562 nm was enhanced upon increasing the reaction time and the fluorescence intensity reached equilibrium within 15 minutes. Therefore, 15 minutes was chosen as the optimal reaction time in this detection system.

Fig. 4 The time-dependent normalized fluorescence changes of 40 μM compound 1 in the absence (a) and presence (b) of ACh in PBS buffered solution (10 mM, pH = 7.4) containing 3.0 U mL⁻¹ AChE and 1.0 U mL⁻¹ ChOx. λ_ex = 445 nm.

To confirm the best reaction temperature for the system, different reaction temperatures were investigated. As shown in Fig. 5, the relative fluorescence intensity of compound 1 changes with temperature. It was found that the fluorescence intensity reached a maximum when the temperature was 37 °C. The results can be correlated to the decomposition rate of H₂O₂ and with the enzyme kinetics. Thus, 37 °C was selected as the reaction temperature in our subsequent experiments.

Fig. 5 The effect of various temperature on the normalized fluorescence intensity of 40 μM compound 1 in PBS buffered solution (10 mM, pH = 7.4) containing 100 μM ACh, 3.0 U mL⁻¹ AChE and 1.0 U mL⁻¹ ChOx. λ_ex = 445 nm.

In order to establish a better relationship between the fluorescence intensity of compound 1 and the concentration of the pesticides, it is important to optimize the concentrations of ChOx and AChE. Fig. 6C shows the normalized fluorescence intensity of compound 1 upon interaction with 300 μM ACh and different ratios of ChOx/AChE for a fixed time interval of 15 minutes. It can be seen that a gradual increase in the fluorescence was observed when the ratio of ChOx/AChE was between 1.0 and 3.0, and the fluorescence intensity was decreased in the range of 3.0–5.0 at 562 nm. Therefore, 1.0 U mL⁻¹ ChOx and 3.0 U mL⁻¹ AChE were used in the subsequent experiments. Moreover, when the concentration of ChOx and AChE was 1.0 U mL⁻¹ and 3.0 U mL⁻¹, the fluorescence intensity showed linearity with the ACh concentration (Fig. 6A and B).

Fig. 6 (A) The normalized fluorescence spectra of 40 μM compound 1 with different concentrations of ACh in PBS (10 mM, pH = 7.4) containing 3.0 U mL⁻¹ AChE and 1.0 U mL⁻¹ ChOx thermostated at 37 °C for 15 min; (B) the linearity of the expression normalized intensity of the 40 μM probe against the concentration of ACh in PBS (10 mM, pH = 7.4) containing 3.0 U mL⁻¹ AChE and 1.0 U mL⁻¹ ChOx, and (C) the relationship between normalized intensity and the different ratios of AChE/ChOx in PBS (10 mM, pH = 7.4) containing 100 μM ACh, 40 μM compound 1. λ_ex = 445 nm.

3.4 Anti-interference capability of compound 1

To selectively detect pesticide residues, the common existing substances found in food samples, such as glucide, metal ions, organic acids and amine acids, were investigated for their interference characteristics. Fig. 7 indicates that the common existing substances found in food samples showed almost no interference even at concentrations 100 times higher than that of acephate, parathion-methyl and trichlorfon. This demonstrates that the probe has excellent specificity for pesticide detection and good anti-interference ability. Therefore, it was possible to use the developed method to detect pesticide residues in real life samples.

Fig. 7 The effects of interference compounds in the analytes on the inhibition efficiency of acephate, parathion-methyl and trichlorfon compound 1–AChE–ChOx–ACh system. λ_ex = 445 nm.

3.5 Detection of pesticides

Under the optimal conditions, the concentrations of three conventional pesticides including acephate, parathion-methyl and trichlorfon, were investigated. The relationship between inhibition efficiency and the pesticide concentrations are shown in Fig. 8. As shown in Fig. 8, upon increasing the pesticide concentration, the relative fluorescence intensity at 562 nm gradually decreased and the inhibition efficiency was linearly dependent on the logarithm concentration of the pesticides. The linear concentration ranges for acephate, parathion-methyl and trichlorfon were 1.0 × 10⁻⁸ to 1.0 × 10⁻⁴ g L⁻¹ (R² = 0.9908), 1.0 × 10⁻⁹ to 1.0 × 10^−5.5 g L⁻¹ (R² = 0.9938) and 1.0 × 10⁻⁸ to 1.0 × 10^−4.5 g L⁻¹ (R² = 0.9932), respectively. The corresponding linear equations were IE (inhibition efficiency, %) = −21.0778 log[acephate (g L⁻¹)] + 169.4987, IE = −20.2116 log[parathion-methyl (g L⁻¹)] + 191.5452 and IE (inhibition efficiency, %) = −20.8852 log[trichlorfon (g L⁻¹)] + 170.6614, respectively. The detection limits for acephate, parathion-methyl and trichlorfon were 1.16 × 10⁻⁹, 3.36 × 10⁻¹⁰ and 4.72 × 10⁻⁹ g L⁻¹ (S/N = 3), respectively. In comparison with other methods (Table 1), compound 1 has its advantages over the other reported pesticide determination methods. Therefore, the fluorescent probe has great potential for practical applications.

Fig. 8 The normalized fluorescent spectra of the compound 1–AChE–ChOx–ACh mixed solution with various concentrations of acephate (A), parathion-methyl (C) and trichlorfon (E). The concentrations of acephate, parathion-methyl and trichlorfon (from top to bottom) are 0 to 1.0 × 10⁻³ g L⁻¹. The linear fitting of the inhibition efficiency vs. concentrations of acephate (B), parathion-methyl (D) and trichlorfon (F), respectively. λ_ex = 445 nm.

Table 1 A comparison of the various approaches used for the detection of paraoxon-methyl

Method	System	Linear range (ng mL^−1)	LOD (ng mL^−1)	Ref.
Electrochemistry	UCNPs–AuNPs	2 × 10^3 to 2 × 10^4	0.6	21
Electrochemistry	Carbon nanotube/ionic liquid	1 × 10^2 to 2.5 × 10^3	8 × 10^3	22
Electrochemistry	CdTe/Au/MWCNT	5.0 to 2 × 10^2	1.0	23
Electrochemistry	Fe3O4/Au	0.5 to 1 × 10^3	0.1	24
Fluorometry	18-RF-QDs	4 × 10^−2 to 4 × 10^2	1.8 × 10^−2	25
Fluorometry	F–ZnSe QDs	0.13 to 1.3 × 10^5	1.0	26
Fluorometry	AuNPs	0.13 to 1.32 × 10^2	2.6 × 10^−2	27
Fluorometry	CdTe QDs/CTAB	25 to 3 × 10^3	18.0	28
Fluorometry	Upconversion	2 × 10^−3 to 20	6.7 × 10^−4	29
Fluorometry	Naphthalimide	1 × 10^−3 to 3.16	3.36 × 10^−4	This work

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3.6 Real life sample analysis

To validate the practicality of the present assay, the residues of pesticides in fruit and vegetable samples, such as celery cabbage, apple and cayenne pepper, were further investigated. In order to discuss parathion-methyl residue in cabbage, apple and cayenne pepper, we compared the novel sensing strategy based on the compound 1-AChE–ChOx system and the traditional method using HPLC. As shown in Fig. 9, the two approaches did not provide significantly different results. Therefore, the result revealed our method was applicable for OP detection in real life samples.

Fig. 9 The analytical results obtained for parathion-methyl in cabbages (A), apples (B) and cayenne pepper (C) using the compound 1-based method (black) and using HPLC (red). All the measurements were performed in PBS (pH = 7.4) and thermostated at 37 °C.

4. Conclusions

In summary, a new water soluble and colorimetric fluorescence probe based on a naphthalimide derivative was designed and synthesized. The probe coupled with two-enzymes (AChE and ChOx) can be used for the highly sensitive detection of pesticides. According to the fluorescence change of the probe, the amount of enzyme generated H₂O₂ can be evaluated and reflects the enzymes activity level. The inhibition effect of pesticides on the enzyme activity was linearly dependent on the logarithm concentration of the pesticides. In addition, the assay strategy has been applied successfully to detect parathion-methyl in food and vegetables, in which the accuracy coincided well with the traditional HPLC method. Furthermore, when compared with chemical sensors previously reported for OPs, the probe has the advantages of being a highly sensitive, simple and rapid colorimetric/fluorescence detection method.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21342012 and 21507028), the Hunan Provincial Natural Science Foundation of China (15JJ3094), the State Key Laboratory of Chemo/Biosensing and Chemometrics Foundation (KLCBTC MR 2011-05), the Scientific Research Fund of Hunan Provincial Education Department (16C0954) and the Startup Foundation for Doctors of Hunan University of Arts and Science (14BSQD01).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16509e

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