Introduction

Organophosphorus pesticides (OPs) are widely used in agriculture [1] to enhance production and quality currently. However, even trace contamination of organophosphorus pesticides in the natural aquatic systems creates a lot of pollution problems due to their toxicity and bioaccumulation property [2]. Methyl parathion (MP) is an effective OP insecticide against many pests in important crops [3, 4]. But it may be converted in vivo into the active metabolite, methyl-paraoxon, which acts as an acetylcholinesterase inhibitor [5] and can be harmful and fatal. Therefore it is considered extremely toxic [6]. Due to high toxicity of OPs, quantitative analysis of these toxicants is the persistent purpose for environment control and food security [7].

Many methods for organophosphorus pesticide determination have been developed in recent years, for example, spectrometry, gas chromatography, high-performance liquid chromatography and chromatography-mass spectrometry [8–11], etc. Although these methods are sensitive and accurate, these analytical instruments are available only in centralized laboratories [12]. Moreover, these analytical techniques are difficult to be applied under field conditions or on-line monitoring. Some drawbacks of these methods mainly are complex separation processes in the samples pretreatment, expensive equipments [6], time consuming, complex and using toxic organic solvent, etc. So, we need convenient, fast, sensitive and accurate methods for their detection.

Electroanalytical sensors are alternative and viable methods because of their compact nature, easy handling in field trials, low cost and low power requirements. They can also produce selective responses within a very short time [13] with respect to those methods described above. In recent years, various enzyme-based electrochemical biosensors have been fabricated for detecting OPs. For instance, Gong et al. [14] reported a nanomaterial based acetylcholinesterase (AChE) biosensor, where they immobilized AChE onto Au nanoparticles (AuNPs)-polypyrrole nanowires composite film modified GC electrode (labeled as AChE-AuNPs-PPy/GC electrode). Though these enzyme-based biosensors usually exhibit high sensitivity, the stability of enzyme is limited since they tend to denature during immobilization and storage [15].

Nitroaromatic organophosphorus pesticides, such as methyl parathion (MP) exhibit good redox activities at the electrode surface [16] and thus very suitable for electrochemical detection. Recently, in order to improve the performance of the electrode for methyl parathion sensing, e.g. their electrocatalysis properties, selectivity, sensitivity and foul resistance with respect to non-modified electrodes, some work has been done by applying suitable modifiers such as nanosilver/nafion composite [17], mesoporous zirconia [2], ex-situ depositing bismuth-film [18] on the electrode surface.

Over the past decades, carbon-based materials, e.g. carbon nanotubes (CNTs) have been exploited for the electrochemical determination of methyl parathion [19]. The experiment results indicate the electrocatalytic activity of CNTs. Besides CNTs mentioned above, there has been significant interest in the development of ordered mesoporous carbon (OMC). Since the discovery of OMC in 1999 [20], the OMC has been receiving much attention owing to its large surface area, well defined pore size, high thermal stability, flexible framework composition and chemical inertness [20–24]. For these reasons, OMC is a particularly promising carbon material and can be widely applied in the field of molecule separation, adsorption, catalysis, sensors, energy storage and capacitors [25]. It has recently been reported that, compared to CNTs, OMC exhibits remarkably strong and stable electrocatalytic response towards some compounds [26]. To verify that OMC can promote the electron-transfer reactions, electrocatalysis of some important molecules by OMC have been investigated, such as morphine, hydroquinone, nitrobenzene [27–29], which has made OMC be a good model for constructing various electrochemical sensors for further electrochemical detection of other molecules.

Nevertheless, to the best of our knowledge, the detection of methyl parathion at an OMC modified electrode has not yet been reported. In this work, an OMC/GC electrode was fabricated in purpose of improving the analytical characteristics of detecting MP, e.g. preconcentration capacity, linear range, detection limit and stability. An obvious decrease in the overvoltage and a marked enhancement in the current response of the MP reduction reaction (compared with the GC electrode) were observed. This OMC/GC electrode also exhibited excellent accumulative effect on MP. These make OMC/GC electrode a promising candidate for efficient electrochemical sensors for the detection of MP.

Experimental

Reagents

OMC was synthesized according to the previous reported work [24]. Methyl parathion (MP) was purchased from Dr. Ehrenstorfer Company (Germany, http://www.ehrenstorfer.com/). MP stock solutions of 0.01 M were prepared in ethanol and kept at 4 °C until used. Other reagents not mentioned here are of analytical grade and used as received. Double distilled water was prepared through a quartz apparatus and used throughout. The 0.1 M phosphate buffer solution, which was made up from Na2HPO4, NaH2PO4 and H3PO4, was employed as a supporting electrolyte.

Apparatus

All electrochemical measurements were performed on a CHI830b electrochemical workstation (Shanghai Chenhua Instrument Co., China, http://www.chistruments.com/). The working electrode is a modified GC electrode (3 mm diameter), whereas an Ag/AgCl (in saturated KCl solution) and a platinum wire served as the reference and counter electrodes, respectively. All solutions were purged with high-purity nitrogen gas before the experiments. All experiments were carried out at room temperature.

Small angle X-ray diffraction (XRD) patterns were obtained on an X-ray D/max-2200vpc (Rigaku Corporation, Japan, http://www.rigaku.com/) instrument operated at 40 kV and 20 mA and using Cu Kα radiation (λ = 0.15406 nm). TEM images were obtained using a JEM-2100 F transmission electron microscope (JEOL, Japan, http://www.jeol.cn/) operating at 200 V.

Preparation of OMC modified electrode

SBA-15, as the template, was prepared as discussed in the literature [30] and detailed information on the preparation and properties of OMC can be found in [24, 30]. Prior to the modification, the GC electrode was hand polished with 1, 0.3 and 0.05 μm alumina powder, respectively, rinsed with water and then sonicated successively in alcohol and water. Approximately 60 min of ultrasonication was necessary to disperse 1.5 mg OMC into 3 mL N, N-dimethylformamide (DMF). Then 5 μL of OMC suspensions was placed onto the surface of GC electrode, and the solvent was evaporated under an infrared lamp. Thus, OMC/GC electrode was obtained.

Results and discussion

Characterization of OMC

The morphology of OMC was characterized using TEM. TEM images of OMC clearly show highly ordered carbon nanowires viewed from the [0 0 1] (Fig. 1a) and [1 0 0] (Fig. 1b) directions. The structure of OMC is exactly an inverse replica of their parent templates SBA-15. The ordered arrangement gives rise to the well-resolved XRD peaks, as shown in inset of Fig. 1a, which can be assigned to three (1 0 0), (1 1 0) and (2 0 0) diffractions of the 2D hexagonal space group p6mm.

Fig. 1
figure 1

(a) TEM image of OMCs viewed from [0 0 1]; (b) TEM image of OMCs viewed from [1 0 0] directions. Inset of Fig. 1a shows small-angle XRD of OMCs

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Electrochemical catalysis of MP at the OMC/GC electrode

The successive cyclic voltammogram (CV) of 10 μM methyl parathion in 0.1 M phosphate buffer solution at pH 6.0 at the OMC/GC electrode is shown in Fig. 2. In the first cycle, a sharp irreversible reduction peak (E pc1 , –0.531 V) can be observed, which corresponds to the reduction of the nitro group to hydroxylamine group via a four-electron process [31] (reaction 1). Then it was oxidized to the nitroso group (E pa , 0.064 V, reaction 2). In the following second cycle, the appearance of another reduction peak (E pc , –0.002 V) is ascribed to reverse process (reaction 3) of reaction 2. The pair of reversible redox peaks should be attributed to a two-electron-transfer redox process. The reduction peak Pc1 almost disappears, which should be due to the reaction exhaustion of methyl parathion at the surface of OMC/GC electrode. The mechanism above accords with the described electrochemical reactions of nitroaromatic compounds [6, 18].

Fig. 2
figure 2

Cyclic voltammograms at a scan rate of 100 mV·s–1 for 10 μM MP at the bare GC electrode (a) and the OMC/GC electrode (b) in 0.1 M phosphate buffer solution at pH 6.0. Inset figure: the magnified image of (a)

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figure b

However, at the bare GC electrode (inset of Fig. 2), the redox peaks (E pa , 0.144 V and E pc , 0.007 V) are not obvious, and a shoulder peak with low current response is observed for the reduction of methyl parathion at –0.750 V (E pc1 ). Whereas at the OMC/GC electrode, it was interesting to note that the peak potential (E pc1 ) shifts 219 mV positively and the peak current (i pc1 ) increases by 76 times, when compares with the response of MP at the bare GC electrode under identical conditions. So far as we know, there has been no report for such a drastic current increase for the reduction of methyl parathion.

The experimental results indicate the presence of OMC makes the electron transfer easier. We think the unique structure of the OMC may offer a favorite microenvironment which is beneficial for accelerating electron transfer between the electrode and species in solution [26]. Additionally, OMC is hydrophobic and can extract MP via hydrophobic interaction [32], also making the response of MP increase. Accordingly, OMC has excellent electrocatalytic towards methyl parathion.

Influence of scan rate on electrochemical behavior

For further discussion of redox process of MP at the OMC/GC electrode, we investigated the relationship between the peak currents of the second cycle and scan rate by cyclic voltammetry.

As can be known from the CVs (Fig. 3a), both I pa and I pc become larger with the increase of scan rate. It is also found that the anodic and cathodic peak currents of the OMC/GC electrode in MP solution are both linearly proportional to the scan rates ranging from 10 to 130 mV·s–1 (Fig. S1a, Electronic Supplementary Material), and the linear regression equations are I pa (μA) = –6.85 – 0.48 v (mV·s–1) (R2 = 0.9928) and I pc (μA) = 7.01 + 0.49 v (mV·s–1) (R2 = 0.9910). It manifests that the redox reaction of MP occurring at the OMC/GC electrode is a surface-controlled process in the range from 10 to 130 mV·s–1. Furthermore, we have already observed the peak currents of MP at the OMC/GC electrode become larger with the increase of dipping time in the electrolyte, indicating the occurrence of adsorption at the OMC/GC electrode. At higher scan rates, the peak currents increases linearly with the square root of the scan rates from 140 to 600 mV·s–1 (Fig. S1b, Electronic Supplementary Material), which indicates a diffusion-controlled behavior. The linear regression equations, I pa (μA) = 6.80 – 6.78 v 1/2 (mV·s–1) and I pc (μA) = –22.51 + 8.03 v 1/2 (mV·s–1), with correlation coefficient (R2) of 0.9948 and 0.9992 were obtained, respectively.

Fig. 3
figure 3

(a) Cyclic voltammograms of 10 μM MP at the OMC/GC electrode in 0.1 M phosphate buffer solution (pH 6.0) at scan rates of 10, 50, 100, 150, 200, 240, 300, 400, 500, 600 mV·s–1 (from inter to outer). (b) Plots of peak potential as function of lnν (ν: V s–1). Inset figure: the plots of E p versus lnν at high scan rate

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The dependence of the anodic peak potential (E pa ) and the cathodic peak potential (E pc ) on the lnv in pH 6.0 phosphate buffer solution is represented in Fig. 3b. At lower scan rates, E pa and E pc remain practically unchanged with the increase of the scan rate, but E pa and E pc shift slightly to positive and negative potential at higher scan rates, respectively. The peak separation (ΔE p ) is increased, suggesting the electrode reaction becomes electrochemically irreversible. At higher scan rates, the plots of E p versus lnv were linear (inset of Fig. 3b), and obeys the following Nicholson’s equation [33]:

$$ {E_{{pa}}} = E′ + \frac{{RT}}{{\left( {1 - \alpha } \right)nF}}\left\{ {0.780 + ln\left( {\frac{{D_R^{{1/2}}}}{{{k^0}}}} \right) + ln{{\left[ {\frac{{\left( {1 - \alpha } \right)nFv}}{{RT}}} \right]}^{{1/2}}}} \right\} $$

Where α is the electron transfer coefficient, n is the electron transfer number, v is the scan rate. The calibration equations are as follows: E pa (V) = 0.122 + 0.069 lnv (V·s–1) (R2 = 0.9975) (anodic peak is used to study), E pc (V) = –0.171 – 0.074 lnv (V·s–1) (R2 = 0.9962). According to the literature, we know that n = 2 [2] (reaction 2 and 3), and from the equations above and the slope of linear curve of E p -lnv, α was calculated to be 0.82 at 288 K.

Optimization of some experimental conditions

In Fig. 4 is represented the evolution of the CV curve when solution pH was varied from 3.0 to 8.0. As one can notice that the anodic peak current of 10 μM MP first reaches maximum at pH 6.0. It indicates that OMC has the maximum adsorption to methyl parathion at mild acid environment. In the acidic medium, the substantial quantity of H+ may somewhat prevent the access of MP to the adsorption sites of the OMC, and whereas basic medium can result in the hydrolyzation of OP compounds [18].

Fig. 4
figure 4

Cyclic voltammograms of 10 μM MP at the OMC/GC electrode in 0.1 M phosphate buffer solution at a pH value of 3.0–8.0 (from left to right). Scan rate: 100 mV·s–1

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And it is also found that the increase of the solution pH leads to the negative shift of both peak potentials, which is a consequence of the deprotonation involved in the oxidation process that is facilitated at higher pH values [28]. The peak potential and the pH display a linear relationship (Fig. S2, Electronic Supplementary Material) with the regression equation E pa (V) = 0.350 – 0.056 pH (R2 = 0.9925), which is approximately consistent with that given by the Nernstian equation for a two protons, two electrons process [34]. CVs of three support electrolytes (inset of Fig. S2) indicate that there is the biggest current response in phosphate buffer solution. To sum up, a pH value of 6.0 phosphate buffer solution was employed for further measurements.

The anodic peak current of MP is almost independent of accumulation potential. However, the peak current increases remarkably within 5 min and scarcely changes with longer accumulation time (Fig. S3, Electronic Supplementary Material), meaning that the adsorption of MP on the OMC/GC electrode became saturated. Thus, 5 min of accumulation time under open-circuit conditions was selected.

Chronocoulometric response of MP

Chronocoulometry was used to determine the saturating adsorption capacity for MP on the surface of the electrode. The GC or OMC/GC electrode was immersed in a 10 μM MP solution under stirring for 5 min to achieve saturated adsorption. Then, a step potential from –0.6 V to 0.4 V was applied. The typical Q-t curves are recorded in Fig. 5. For the OMC/GC electrode, the charge amount increases rapidly and then slows down (curve b). After subtracting the background charge, we obtained the plot of Q versus t 1/2 (inset of Fig. 5). The linear regression equations are as follows: Q (C) = 8.02 × 10–6 + 1.07 × 10–6 t 1/2 (s) (R2 = 0.9947) (curve c) and Q (C) = 1.24 × 10–4 + 7.19 × 10–6 t 1/2 (s) (R2 = 0.9952) (curve d).

Fig. 5
figure 5

Chronocoulometric response of bare GC electrode (a) and OMC/GC electrode (b) in 0.1 M phosphate buffer solution (pH 6.0) containing 10 μM MP. Inset figure: The plots of Q versus t 1/2, bare GC electrode (c) and OMC/GC electrode (d) in 0.1 M phosphate buffer solution (pH 6.0) containing 10 μM MP (background charge subtracted)

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According to the Cottrell equation [35]:

$$ Q = \frac{{2nFA{D^{{1/2}}}C{t^{{1/2}}}}}{{{\pi^{{1/2}}}}} + {Q_{{dl}}} + {Q_{{ads}}} $$

Q dl is the double-layer charge, and Q ads is the adsorption charge of the electrochemical reaction of adsorbed species. The corresponding apparent diffusion coefficients (D) were calculated 2.92 × 10−6 cm2·s−1 (for the GC electrode) and 1.20 × 10−5 cm2·s−1 (for the OMC/GC electrode), respectively. The difference between them may be due to the slightly faster diffusion of MP in the OMC film around the GC electrode.

Using Laviron’s [36] theory of Q ads  = nFAГ and intercepts for curves c and d, a Г (surface coverage) value of 2.34 × 10−9 mol·cm−2 (for the OMC/GC electrode) was calculated, which was almost 8.5 times larger than 2.47 × 10−10 mol·cm−2 (for the GC electrode).

Obviously, OMC has excellent accumulative ability towards methyl parathion. The bulky adsorption of MP on the surface of the OMC/GC electrode is attributed to the unique properties of OMC, especially the presence of the oxygen-containing functional groups on the surface of OMC, e.g. –OH、–COOH and C = O [21, 27], which can attract pesticides molecules containing special groups. On the other hand, MP is slightly soluble, and has the higher interfacial activity, which enables MP to move to the electrode surface easily and adsorb largely.

Analytical performance

LSV was recorded for MP determination with the OMC/GC electrode under optimized conditions. As shown in Fig. 6, the anodic peak currents (I pa ) increase with the concentration of MP. It can be observed that I pa is linearly related to MP concentration ranging from 0.09 to 61 μM with an equation of I pa (μA) = –18.74 – 1.31c (μM) (R2 = 0.9936), and the detection limit is 7.6 nM (S/N = 3). A comparison of linear range, detection limit for OMC modified electrode with other MP sensors reported in the literature [7, 12, 19, 37] is shown in Table S1 (Electronic Supplementary Material). All the data reveal the analytical parameters for OMC/GC electrode are comparable and even better than those obtained at several electrodes reported recently.

Fig. 6
figure 6

Linear sweep voltammograms of 10 μM MP at the OMC/GC electrode (from a to j: 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.10, 0.30, 5, 11, 21, 31, 41, 51 μM). Inset figure: a linear correlation between peak current vs. concentration of MP

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Interferences from electroactive aromatic compounds and inorganic ions were tested. The results show voltammetric determination of 10 μM MP is not affected by 10-fold of phenol, nitrophenol and nitrobenzene, 100-fold of \( {\hbox{CO}}_3^{{2 - }} \) and \( {\hbox{C}}{{\hbox{H}}_{{3}}}{\hbox{CO}}{{\hbox{O}}^{ - }} \), 500-fold of Mg2+, Ca2+, \( {\hbox{PO}}_4^{{3 - }} \) and \( {\hbox{SO}}_4^{{2 - }} \), 1000-fold of \( {\hbox{NH}}_4^{ + } \), \( {\hbox{NO}}_3^{ - } \) and Cl-.

We also investigated the stability and reproducibility of the OMC/GC electrode. By performing eight parallel experiments with the same electrode, the relative standard deviation (RSD) 1.36% was obtained. After the OMC/GC electrode was stored at 277 K for 2 weeks, the oxidation peak current remained 91.3% of the initial response, suggesting a good stability of the electrode.

Application to real sample

The practicality of the method was evaluated by applying it to the determination of MP in water sample collected from Jing Lake. The real water samples were filtered with a 0.45 μm filter to eliminate particulate matters. No current response of MP was observed in the pretreated sample solution. Thus, in recovery tests, we adopted the standard-additions approach to estimate the reliability. Different amounts of MP were spiked into the lake water samples respectively, and adjusted to a certain concentration. The results obtained were summarized in Table 1. The recoveries were from 98.5% to 100.6%. So the method outlined in this work for MP sensing is promising for direct analysis of environmental samples.

Table 1 Recovery tests of MP in lake samples (n = 4)
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Conclusion

In summary, we have confirmed that the OMC/GC electrode performed an enhanced electrocatalytic activity and accumulative effect towards MP. Furthermore, the electrode also exhibited wider linear range, lower detection limit and better electrochemical stability. Therefore this electrochemical sensing methodology is promising for determing OPs in environment samples.