Introduction

Organophosphorous pesticides (OPs), as a wide series of toxic compounds, prevent the activity of acetylcholinesterase, which play an important role in nerve functions of insects, humans, and other animals [1]. OPs have been widely applied to control pests [2]. The usage of pesticides leads to the introducing of their residues in environmental water and soils. This can also result in contamination of vegetables and fruits [3] and finally, OPs can entry human body and cause serious side effects. It is demonstrated that pesticides can have mutagenic and carcinogenic effect and cause many diseases [3]. So, to provide a healthful and safe environment for humans, we need a sensitive and efficient screening technique for the recognition of trace pesticides in environmental samples. Dimethoate (O,O-dimethyl-S-methylcarbamoylmethyl phosphorodithioate, DM) is one of the important pesticides, which is applied for destroying an extensive variety of insects and acari in agriculture products. The chemical structure of DM is showed in Fig. 1. The maximum acceptable limit of DM in environment is in the range of few μg/kg [2]. DM and other pesticides is commonly quantified by chromatographic based techniques or immunoassays [2, 4–8] which are more expensive, complicated and laborious. Thus, developing a simple, cost-effective and rapid method for the determination of pesticides is a challenge for researchers.

Fig. 1
figure 1

Chemical structure of dimethoate

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Fluorescence techniques with high sensitivity can be assigned as a useful case for this purpose. They have unique interesting features such as simplicity, high speed, and low-cost. Fluorescence sensors based on quantum dots (QDs), have been recently reported for many of important analytes which have several advantages to other developed methods. High luminescent QDs have widely applied in analytical areas to sense several organic and inorganic compounds [9] that is because of their unique features [10, 11] such as high photo-stability, narrow emission with a high quantum efficiency, broad absorption spectrum and large Stokes shift [11–13]. They have quanum-size effect that’s mean the emission wavelength is dependent to the diameter of nanomaterial [14–16]. These unique advantages make QDs superior to common organic fluorescent materials. More specific, ZnO QDs with high quantum yield, broad excitation spectrum and a narrow emission spectrum have been exploited by many researchers in chemosensors [16–21]. According to the Fonoberov and Balandin studies [14, 22], ZnO QDs with a size of less than 7 nm can exhibit a tunable fluorescence emission [14, 15]. Furthermore, they are non-toxic, bio-compatible, cheap, eco-friendly and can be synthesized by a simple and cost-effective process and so, they are superior to other QDs [14, 15, 23–26]. These properties make ZnO QDs as an interesting material in fluorescence-based probes. They have been applied in many studies leading to excellent analytical features.

Molecular imprinting as a potent procedure for the synthesis of particular polymeric receptors with high tendency to specific analytes, is interesting in sensor designing owing to its great selectivity, high physical strength and thermal stability along with an easy and low-cost preparation method [2, 11, 27–32]. Molecularly imprinted polymers (MIPs) are usually created based on the co-polymerization of certain monomers and cross-linkers in the presence of specific template molecules. The presence of template in the polymerization process leads to its trapping in polymer matrix, which can be removed by suitable washing methods. The created molecularly imprinted polymers have specific sites for rebinding template molecules [27]. The binding between MIP sites and template molecules is based on the key-lock principle, providing high selectivity and specificity for template. The high selectivity of MIPs-based methods along with a simple preparation process make them preferable to immunosorbents, increasing their applications area. The combination of the specific recognition features of MIPs and the outstanding advantages of QDs-based fluorescence detection methods can be an attracting analysis technique for the selective detection of important analytes [33–43].

To the best of our knowledge, there is no report for the determination of DM based on MIPs-functionalized ZnO QDs. Here, a selective probe based on MIP-coated ZnO QDs was developed for the rapid recognition of DM in environmental samples. ZnO QDs were synthesized via a simple mixing method and applied as an efficient support for MIPs due to their high luminescence and other aforementioned advantages. The MIP shell are produced on the surface of ZnO QDs by self-assembly of 3-aminopropyl triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) in the presence of DM as template. The created MIP-coated ZnO QDs composite could specifically bind to DM molecules, and in this format, the fluorescence emission of ZnO QDs undergoes a diminishing effect proportional to DM concentration. So, it is applicable for the accurate and sensitive determination of DM which provided a low detection limit. The offered probe was used to the determination of DM in some environmental water samples. The method is more economical and sensitive in comparison with common chromatographic and immuno-based techniques. Figure 2 shows the schematic design for developed MIP-based probe for the determination of DM.

Fig. 2
figure 2

Schematic design of offered probe for the determination of DM, based on MIP-coated ZnO QDs

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Experimental

Reagents and Apparatus

Dimethoate (DM), Zinc acetate dihydrate (Zn(OAc)2.2H2O), 3-aminopropyl triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) were all from Sigma Aldrich (Germany) and were applied without any purification. Ethanol and KOH were obtained from Merck (Germany). All the used materials were of analytical grade. All solutions were prepared in deionized water (Kasra CO., Iran).

Fluorescence (FL) experiments were accomplished by using a Shimadzu RF-5301 PC spectrofluorometer. UV-visible absorption spectra were achieved by UV-1800 spectrophotometer (Shimadzu). The size and morphology of synthesized QDs were investigated by Transmission electron microscopy (TEM, Leo 906, Zeiss, Germany). A Tensor 27 FTIR spectrometer (Bruker, Germany) was used to achieve Fourier transform infrared (FTIR) spectra.

Synthesis of ZnO Quantum Dots

Preparation of ZnO quantum dots was performed by a simple and rapid method [26]. Briefly, a certain amount of Zn(OAc)2.2H2O (0.001 mol) was added to 100 mL ethanol and ultra-sonication was used to its dissolving. 10 mL ethanol containing 2.8 mmol KOH was added slowly to Zn(OAc)2 solution at room temperature. The solution was vigorously stirred during the KOH addition. ZnO QDs was gradually formed as a white suspension after about 1 h, which was then precipitated by its centrifugation (10,000 rpm) for 15 min. The obtained QDs were dispersed in ethanol and then were centrifuged to eliminate unreacted reagents. The rinsing process was repeated for several times. In order to increase the stability of obtained QDs in aqueous solution, they were modified by using APTES. ZnO QDs (0.04 g) were dispersed in 50 mL ethanol and then APTES solution (0.5 g in 5 mL ethanol) was drop-wisely added, followed by addition of 0.2 mL distilled water. The mixture was stirred for another 1 h without its heating and then, coated QDs were separated by centrifugation (10,000 rpm, 10 min). Again, to remove excess precursors, QDs were twice redisposed and then precipitated by their centrifugation. The yellow emission which was observed from obtained solution under UV radiation, confirmed the success synthesis of ZnO QDs. The QDs were completely washed and dispersed in deionized water for future experiments.

Synthesis of MIP-Coated ZnO Quantum Dots

In order to prepare the MIP coating layer on the surface of ZnO QDs, 500 μL of APTES (as the main monomer) and 20 mL of DM (as the template molecules) solution (containing 250 mg DM in ethanol) were mixed and stirred for 30 min. Then, TEOS (2 mL) was added as the cross-linking monomer. After 5 min stirring, certain amounts of prepared ZnO QDs (600 mg) as well as 2 mL of 8% NH3·H2O were added. The solution was stirred for another 16 h. Also, for control experiments, non-imprinted polymer (NIP)-coated ZnO QDs was obtained by the same way, but in the absence of template. Finally, the produced modified ZnO QDs were centrifuged and rinsed with ethanol for three times to remove template molecules and get approximately the identical FL emission intensity from NIP and MIP-QDs.

Fluorescence Determinations

Fluorescence studies for the determination of DM were performed at room temperature in a batch system. The synthesized MIP-capped ZnO QDs (500 μL, 100 mg L−1) were added to certain volumes of DM standard or sample solutions in a 5 mL volumetric flask. The pH of solution was adjusted to about 7 by adding 500 μL Tris buffer. The solution was effectively mixed and its final volume was increased to 5 mL by deionized water. Finally, the fluorescence emission intensity of solution was measured at maximum emission wavelength of ZnO QDs (λem = 520 nm and λex = 374 nm).

Sample Preparation

To assess the practical applicability, some environmental water samples were used for the determination of DM by developed probe. No further pretreatments was performed for samples. Just, each sample was cleaned by 220 nm micro-porous filter to remove suspended particles. Also, to study the recovery of probe, certain volumes of DM standard solution were spiked into samples before any pretreatment. The fluorescence intensity of DM solutions can be related to its concentration using a calibration graph plotted for standard solutions.

Results and Discussion

Characterization of Synthesized ZnO QDs

Surface structure of synthesized ZnO QDs and MIP-ZnO QDs was investigated by FTIR spectra (Fig. 3a). FTIR spectrum of ZnO QDs contains clear peaks at about 3412 and 1522 cm−1 which are related to N-H bonds. The peak at 2948 cm−1 is for well-known symmetric C-H vibrations. Two peaks at 1110 and 1021 cm−1 show Si-O bonds, and a characteristic peak at 505 cm−1 indicates the Zn-O bonds [44]. In FTIR spectrum of MIP-capped ZnO QDs, the peaks of Si-O and N-H were strengthened and a peak at 1611 cm−1 was appeared which is related to C = C bonds. Also, the peak of Zn-O bonds at about 500 cm-1 was weakened because of the coating of a polymeric layer on its surface. These changes confirm the MIP placing on the surface of ZnO QDs.

Fig. 3
figure 3

a FT-IR spectra for ZnO QDs and MIP-ZnO QDs; b, c TEM images for ZnO QDs (b) and MIP-ZnO QDs (c), the insets show the size distribution of QDs; and d) XRD pattern for ZnO QDs

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TEM images of ZnO QDs and MIP-coated ZnO QDs were indicated in Fig. 3b and c. As it is clear, mono-dispersed spherical ZnO QDs with a diameter size of about 2–8 nm were obtained. Also, the average size of MIP-coated QDs was about 35 nm and so, it can be considered an about 20 nm layer of MIP on the QDs. Also, XRD analysis was applied to investigate the crystallinity of QDs, which indicated some clear intense peaks showed in Fig. 3d. The observed peaks confirmed the crystalline structure of obtained QDs with the size in nanometer range. Also, there isn’t any additional peak which indicates that the synthesized ZnO QDs are free from any impurity. The size of obtained particles was calculated to be 5.93 nm by using Debye Scherrer formula [44].

Furthermore, UV-visible absorption spectrum of prepared ZnO QDs is indicated in Fig. 4a. The spectrum showed a shoulder at about 315 nm. Comparing to bulk ZnO, there is a clear shift to shorter wavelengths in absorption spectrum of QDs which can be related to their nanometer size [44]. On the other hand, the synthesized QDs showed an intense fluorescence emission at 536 nm (λex = 389 nm). The stability and quantum yield of the fluorescence emission were investigated for prepared MIP-coated QDs. The results showed a stable emission (Fig. 4b) with a quantum yield of 13%. It can be observed from Fig. 3b that the florescence emission intensity of MIP-coated ZnO QDs remains constant during 50 day after their synthesis. The relative standard deviation (RSD) of 0.48% was acquired by 20 repetitive determination of the emission intensity for the aqueous solution of MIP-coated ZnO QDs every 10 min. The silica coating of ZnO QDs is the chief cause of the stable emission.

Fig. 4
figure 4

a Absorption spectrum of MIP-coated ZnO QDs; b Emission spectra for MIP-coated ZnO QDs recorded in different times after their synthesis (10 min, 10 h, 3 day and 15 day)

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Basis of Developed Probe

MIP-capped ZnO QDs were used as recognition element for a specific pesticide. ZnO QDs can act as optical readout and lead to intensification of obtained signal. The stability of synthesized QDs was enhanced by their coating with APTES hydrolysis reaction which leads to reduction of surface defects of QDs. Furthermore, in order to increase selectivity for DM and remove the interfering effect of other compounds, the MIP recipient was employed on the surface of ZnO QDs. It was done by self-assembling of APTES monomers on the amino capped-ZnO QDs in the presence of TEOS as cross-linking agent and DM as template molecules. The amino groups on the surface of the APTES-modified QDs can support the template molecules to enter the designed MIP shell during the imprinting polymerization stage. The template molecules were non-covalently entrapped into the polymer and after eliminating the template, specific sites with a shape and functional groups matching to the DM molecules were shaped in the polymer matrix. Also, NIP-coated ZnO QDs were synthesized to further investigation of selectivity. Study of the fluorescence emission of MIP-coated ZnO QDs showed a relatively strong emission peak at about 536 nm with an excitation wavelength of 389 nm. In the presence of templates, MIP-coated QDs had a weaker emission intensity which was remarkably restored after template removal (Fig. 5). The fluorescence intensity of the MIP-coated QDs without templates was almost equal to that of the NIP-coated QDs. This indicate the complete removal of template molecules from the MIP sites on the surface of QDs.

Fig. 5
figure 5

Fluorescence emission spectra for (a) NIP-coated ZnO QDs and (b-d) MIP coated ZnO QDs before (b) and after (c) template removal; (d) Fluorescence emission spectrum for template removed MIP-coated ZnO QDs in the presence of 2.5 mg L−1 DM

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Observations showed that the MIP-coated QDs can be practically applied to facile and rapid recognition of DM in aqueous media without any preconcentration stage. Decrease in fluorescence intensity of MIP-coated QDs in the presence of DM used as the probe signal, and concentration of DM in a sample can be obtain by using standard addition method.

Optimization of Experimental Condition for DM Determination

In order to obtain a high sensitive probe for DM, the effect of some important factors including the pH of determination media and incubation time were investigated.

The pH effect on the fluorescence response of MIP-coated QDs was studied in the range of 4–12 (Fig. 6a). As the results showed, the fluorescence signal decreased in very low or high pHs and a pH between 5 and 8 is suitable for determination process. The low fluorescence response outside of this range can be explained as follows: The high concentration of H+ or OH- can affect the functional group of MIP sites and therefore, decrease their tendency for template molecules. Also, ZnO QDs in the center of probe, may be affected in very low or high pH values. In result, their fluorescence emission would be changed. Experiments showed that Tris buffer in a relatively low concentration (0.5 mmol L−1) was useful for pH adjusting. Other buffers such as phosphate quenched the fluorescence of ZnO QDs and were not suitable.

Fig. 6
figure 6

a Fluorescence response of MIP-coated ZnO QDs for 1.2 mg L−1 DM in different pH values, b Fluorescence intensity of MIP-coated ZnO QDs after their exposing to 1.2 mg L−1 DM in different times

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In order to study the effect of incubation time on the response of probe, MIP-coated ZnO QDs were exposed to DM in a constant concentration for different time scales and then, the fluorescence intensity was recorded (Fig. 6b). A rapid quenching effect was observed in the emission of QDs by addition of DM. The fluorescence intensity was decreased in first 5 min and then remained constant. This result show that the interaction of DM molecules with created specific sites on MIP matrix occur with high speed. So, an incubation time of 7 min was considered for all experiments.

Effect of DM Concentration on the FL of MIP-Coated ZnO QDs

The fluorescence of MIP-coated QDs showed a sensible decrease in the presence of DM in trace concentrations (Fig. 7a). Based on this observation, a sensitive and selective method was designed for the determination of DM in aqueous media. The calibration graph was obtained as the response of QDs to DM, F0/F, against to the concentration of DM (C) in mg L−1 (F0 and F show the fluorescence intensity of MIP-coated ZnO QDs in the absence and presence of DM, respectively). The obtained linear range was 0.02–3.2 mg L−1 with a regression equation of F0/F = 0.551C + 0.983 (R2 = 0.9995) (Fig. 7b). The limit of diction (LOD) for method was achieved 0.006 mg L−1. On the other hand, the reducing effect of analyte on the fluorescence emission of MIP-coated ZnO QDs can be described by electron transfer process [44]. Analyte molecules on the MIP sites near the QDs can trap the excited electron leading to decrease of excited QDs. Figure 2 explains the entire procedure for the determination of DM.

Fig. 7
figure 7

a Fluorescence intensity of MIP-coated ZnO QDs in the presence of different concentrations of DM and b corresponding calibration graph

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The reproducibility of the developed method was investigated by calculating relative standard deviation (RSD) for seven replicate determinations of 0.1 and 1 mg L−1 DM which were obtained 2.31 and 1.67%, respectively.

Selectivity

To investigate the probable interfering effect of some species present in the real samples on the offered determination probe, increasing quantities of them were added into a DM solution (0.2 mg L−1) and determination process was accomplished according the general procedure. The results in Table 1 showed that the most of the examined species have not considerable interfering effect on the signal of probe. The tolerable concentration ratios for interferences mentioned in Table are for relative error of <5%. The amounts of most of tested substances in real samples are below their tolerable levels and so, no interferences were observed in DM determination.

Table 1 Interference of different species on the determination of 1 mg L−1 DM (in optimum condition)
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Furthermore, the effect of some similar compounds with DM for example other organophosphates have been studied. Figure 8a shows the response of developed probe for the 10 mg L−1 solutions of these compounds. No considerable effect was observed on the fluorescence of MIP-coated ZnO QDs from examined compounds. It can be explained by specific sites produced in the presence of DM as template. The functional groups in these sites are matching to the DM chemical structure. This shows the great selectivity of present sensor for DM. The ZnO QDs without MIP shell on their surfaces show a sensible response for about all of examined organophosphates.

Fig. 8
figure 8

a Effect of different organophosphates on the signal of MIP-coated ZnO QDs developed for DM; b fluorescence emission of MIP-coated ZnO QDs in the presence of 1 mg L−1 DM [a: absence of DM, b: DM in deionized water, c: DM in tap water and d: DM in river water]

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Also, three different water samples (including deionized water, tap water and river water) have been applied to prepare the analyte solution with constant concentration (1 mg L−1). The response of probe for the prepared solutions was recorded (Fig. 8b) which showed no obvious difference between three examined solution. So, developed method is applicable for the determination of pesticides in real water samples.

Analysis of Real Samples

The presented system was applied for DM quantification in environmental water samples (Table 2). For more validation of method, certain quantities of DM standard were added into samples prior to preparation stages, and then DM was followed according to the described method in Experimental section. As can be seen from the results (Table 2), the obtained recoveries were acceptable.

Table 2 Results of DM determination in the water samples by established system
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Conclusions

The ZnO QDs coated with MIP have been applied to recognize a specific pesticide in water samples. The MIP created on the surface of ZnO QDs using the co-plymezation of APTES and TEOS in the presence of DM as template molecules. So, recipient sites molecules can be created in the polymer matrix which is specific for DM providing the selective interaction of MIP-coated QDs with DM. Adsorption of DM molecules on the MIP sites leads to an efficient electron transfer from ZnO QDs to DM which quench the FL emission of ZnO QDs. The developed technique has more advantages such as high selectivity and sensitivity. This provide a rapid, cost-effective, simple and safe system for the detection of DM in the linear range of 0.02–40 mg L−1 with a LOD of 0.006 mg L−1. The method displays good specificity for DM in the presence of other similar compounds.