Modified Screen-Printed Potentiometric Sensors based on Man-Tailored Biomimetics for Diquat Herbicide Determination

Screen-printed platforms integrated with molecularly imprinted polymers (MIP) were fabricated and characterized as potentiometric sensors for diquat (DQ). The synthesized MIP beads were studied as sensory carriers in plasticized poly(vinyl chloride) membranes. The sensors were constructed by using poly(3,4-ethylenedioxythiophene) (PEDOT) as solid-contact material to diminish charge-transfer resistance and water layer potential. Conventional ion-selective electrodes (ISEs) with internal filling solution were used for comparison. The designed electrodes showed near Nernstian slopes of 28.2 ± 0.7 (r² = 0.999) over the concentration range of 1.0 × 10−6–1.0 × 10−2 M with the limit of detection 0.026 µg/mL over the pH range 4.2–9.0. The electrode exhibited good selectivity for diquat cations over a large number of organic and inorganic cations. The sensor was successfully introduced for direct measurement of diquat content in commercial pesticide preparations and different spiked potato samples. The results showed that the proposed electrode has a fast and stable response, good reproducibility, and applicability for direct assessment of diquat content. The proposed potentiometric method is simple and accurate in comparison with the reported HPLC methods. Besides, it is applicable to turbid and colored sample solutions.


Introduction
Pesticides and herbicides are extensively used in agriculture, forestry, and domestic activities for controlling pests. This rapid increase in their use can cause a real threat to the environment and human health. So, a highly restricted control must be followed to avoid unacceptable levels of these contaminants from entering the water environment, thus influencing the food chain of humans and animals [1].
Quats, a group of quaternary ammonium salts, is considered a particularly uneasy type of herbicide [2], attributed to their physico-chemical properties, which lags the known multi-residue methods from their quantification. Diquat (1,1 -dimethyl-4,4 -bipyridilium dibromide) is one of the In this work, we present miniaturized planar potentiometric ISEs modified with poly(3,4-ethylenedioxythiophene) (PEDOT) as solid-contact material for selective detection of diquat (DQ) herbicide. The addition of PEDOT/PSS into the diquat-selective membrane enhanced the hydrophobicity and capacitance with considerable potential stability, which was tested by electrochemical impedance spectroscopy (EIS) and constant-current chronopotentiometry techniques. For comparison, liquid-contact ISEs were also prepared and characterized, then compared with the solid-contact ISEs. The proposed ISEs revealed a high sensitivity and selectivity for potentiometric monitoring of diquat in soil samples and determination in commercial herbicidal formulations.
A (10 −2 M) stock solution of DQ was prepared by dissolving 0.362 g of pure diquat dibromide monohydrate in 100 mL distilled water. Diluents (10 −2 -10 −8 M) of DQ were preserved in brown bottles.

Apparatus
All potentiometric measurements were carried out at ±25 • C using an Orion pH/mV meter (model SA 720, Cambridge, MA, USA). Selectivity measurements were carried out using the so called "modified separate solution method (MSSM)" [42]. Fourier-transform infrared spectroscopy (FT-IR) measurements were carried out using FT-IR spectrometer (Alpha II, Bruker ABCO, Cairo, Egypt) using the attenuated total reflection (ATR) technique. Chronopotentiometry measurements of the screen-printed electrodes (SPE) were measured using Metrohom potentiostat/galvanostat (Autolab, model 204) purchased from Metrohom Instruments (Herisau, Switzerland). These tests were carried out in 10 −4 M diquat solution using a conventional three electrode system including an ISE working electrode, Ag/AgCl (3 M) as the reference electrode, and a Pt wire as the counter electrode.

Man-Taillored MIPs Synthesis
In a 25 mL glass capped vial, diquat (DQ) (temblate), methacrylic acid (MAA) (monomer), and ethylene glycol dimethacrylate (EGDMA) (cross-linker) (in the ratio of 0.5:3:3 mmol) were dissolved in the porogenic solvent acetonitrile (15 mL). Free radical initiator benzoyl peroxide (BPO, 60 mg) was added, followed by passing a flow of N 2 gas in to the mixture for 10 min to remove any dissolved oxygen. The solution is then sonicated for further 10 min to ensure solution homogeneity. The glass-capped vial was then immersed in a constant temperature oil bath for 18 h, preset at 75 • C. The control non-imprinted polymer (NIP) was prepared in a similar way as mentioned above, without involvement of the template molecule. The MIP was rendered void of the template by means of soxhlet extraction using methanol/acetic acid (8/2, v/v) and methanol, ascertained by the zero absorption of diaquat using a Shimadzu UV/VIS spectrophotometer (Model UV-1601), and by the Fourier-transform infrared spectroscopy (FT-IR). A 24 h period at ambient temperature was a sufficient time for the polymer to dry.

Screen-Printed Design and Sensor Fabrication
The screen-printed platforms (5 × 30 mm) were designed and fabricated manually using a polyester sheet (~200 µm thick) as a substrate for electrode printing. Firstly, silver conductive ink was used to print the conducting track for the working electrode. Secondly, printing of carbon conductive ink at the end of the conducting track is done to form the sensing area of the working electrode. The polyester film, after the printing step, was heat cured at 150 • C for 15 min in a pre-heated oven. Finally, the electrodes were covered with an insulating tape leaving a rectangular area (3 × 3 mm) for defining the electrode sensing area as well as the connecter leads, forming a protective layer over the electrode tracking during analysis. Figure 1 shows the final fabricated screen-printed platforms which were further used for electrochemical analysis. To the carbon screen-printed platform (C/SPE), 10 µL of PEDOT/PSS was added to the conductive carbon using drop casting method and left to dry. This acts as the solid contact between the ion-sensing membrane (ISM) and the carbon conductor. The sensing membrane was prepared by dissolving 100 mg of the components in 2 mL THF as follow:

Characterization of MIP Particles
The imprinting process of diquat was examined using Fourier-transform infrared spectroscopy (FT-IR) technique and the measurements were carried out using the FT-IR spectrometer (Alpha II, Bruker ABCO, Cairo, Egypt) using the attenuated total reflection (ATR) technique. The FTIR spectra of diquat dibromide, non-washed MIP, washed/MIP and NIP nano-beads were shown in Figure 2. As mentioned in Figure 2a, a υO-H stretch vibration peak appeared at 3390 and 3432 cm −1 , arising from the water molecule in the hydrated diquat dibromide molecule. υC-H stretch vibration peaks appeared at 3049-3006 and 2949 cm −1 assigned for aromatic C-H. υC=C stretch vibration peaks appeared at 1572 and 1526 cm −1 assigned for aromatic C-C. υC=N stretch vibration peak appeared at 1618 cm −1 . Further, almost all of these peaks already present in the FT-IR spectrum of the non-washed MIP were either in the same position or shifted as presented in Figure 2b. As shown in Figure 2c, the washed/MIP exhibited an O-H peak at 3528 cm −1 . This is very close to the peak assigned to the O-H present in NIP beads (3543 cm −1 ) ( Figure 2d). All FT-IR data were tabulated in Table 1. From the above-mentioned data, the imprinting of diquat was carried out successfully and the complete removal of it from the backbone of MIP was also achieved.

Diquat Determination in Real Samples
Analysis of real samples using the pre-designed screen-printed electrodes was checked towards their applicability as a diagnostic device for diquat monitoring in different commercial formulations and potato samples. Then, 10 g of a homogenized potato samples was placed into a 50 mL Teflon centrifuge tube. Fortified samples were prepared by spiking aqueous standards into the pre-weighed sample followed by 30 min equilibration. Then, 10 mL of 50:50 methanol/0.1 M HCl in water as an extraction solution was added to the sample. The mixture was shaken vigorously for 2 min by hand and heated in a water bath at 80 • C for 15 min. The supernatant was then analyzed using the proposed potentiometric method. Prior to HPLC analysis, the supernatant was filtered using a 45 micron PTFE syringe filter. A 500 µL portion of the filtered sample was diluted to 1.0 mL with acetonitrile prior to HPLC analysis.
In addition, DQ was analyzed in commercial agricultural formulations. Then, 0.5-1.0 mL of (Reglone 200 SL, Syngenta Company (Cairo, Egypt), 31.8 w/v% soluble liquid (SL) of diquat dibromide) formulations were diluted to 100-mL and their potential readings were used to look for their corresponding concentration along a calibration plot prepared from (10 −2 to 10 −7 M) standard diquat dibromide solutions.

Characterization of MIP Particles
The imprinting process of diquat was examined using Fourier-transform infrared spectroscopy (FT-IR) technique and the measurements were carried out using the FT-IR spectrometer (Alpha II, Bruker ABCO, Cairo, Egypt) using the attenuated total reflection (ATR) technique. The FTIR spectra of diquat dibromide, non-washed MIP, washed/MIP and NIP nano-beads were shown in Figure 2. As mentioned in Figure 2a, a υO-H stretch vibration peak appeared at 3390 and 3432 cm −1 , arising from the water molecule in the hydrated diquat dibromide molecule. υC-H stretch vibration peaks appeared at 3049-3006 and 2949 cm −1 assigned for aromatic C-H. υC=C stretch vibration peaks appeared at 1572 and 1526 cm −1 assigned for aromatic C-C. υC=N stretch vibration peak appeared at 1618 cm −1 . Further, almost all of these peaks already present in the FT-IR spectrum of the non-washed MIP were either in the same position or shifted as presented in Figure 2b. As shown in Figure 2c, the washed/MIP exhibited an O-H peak at 3528 cm −1 . This is very close to the peak assigned to the O-H present in NIP beads (3543 cm −1 ) (Figure 2d). All FT-IR data were tabulated in Table 1. From the above-mentioned data, the imprinting of diquat was carried out successfully and the complete removal of it from the backbone of MIP was also achieved.

Potentiometric Characteristics of the Proposed Sensors
Sensitivity and linearity of PEDOT:PSS-based diquat-ISE were evaluated by measuring the potential in aqueous diquat solutions of different concentrations varying from 10 −8 to 10 −2 M. As shown in Figure 3, by increasing the concentration of diquat, the potential of PEDOT:PSS-based diquat-ISE was recorded as a function of time. The sensor revealed a Nernstian response across a linear range of 10 −6 -10 −2 M with a slope of 28.2 ± 0.7 mV/decade (R 2 = 0.999) and a detection limit of 0.026 µg/mL. For contrast as a control, diquat-ISEs based on NIP beads were also tested. These sensors possessed a linear range from 5.0 × 10 −6 -1.0 × 10 −2 M of diquat solution, and the slope of calibration curve is computed to be 12.1 ± 0.5 mV/decade (R 2 = 0.997) with a detection limit 0.05 µg/mL. All performance characteristics of the proposed sensors are presented in Table 2.
pesticides and inorganic ions. Evidently, PEDOT:PSS/MIP+NaTFPB exhibited excellent selectivity towards diquat over paraquat, cyromazine, dinotefuran, acetamipride, Na + , K + , and Ca 2+ ions. The obtained results reflected an enhanced selectivity for the proposed sensor and offered a great potential for trace-level monitoring of diquat in environmental samples.     The dynamic time response of the proposed sensors was tested over the concentration range 10 −8 -10 −2 M as shown in Figure 3. It was noticed that at concentrations below 10 −5 M, the response time of PEDOT:PSS/MIP-ISE is lower than 10 s. At concentrations higher than 10 −5 M, the sensors attain the equilibrium after 15 s. All these suggest that PEDOT:PSS/MIP-ISE possesses a better analytical performance, which is attributed to the function of PEDOT:PSS substrate for effectively transferring the ionic signal generating at the diquat-ISM into the electronic signal of the electrode.
Influence of the pH on the potential response of PEDOT:PSS/MIP-ISE and PEDOT:PSS/NIP-ISE was tested for (1 × 10 −4 and 1 × 10 −3 M) diquat solutions. It can be seen that the sensors revealed a stable potential over the pH range 4.2-9.0 and 4.5-9.0 for PEDOT:PSS/MIP-ISE and PEDOT:PSS/NIP-ISE, respectively. At pH <4, there is an observed potential drift due to H + ion interference. On the other hand, the potential declined gradually at pH values >9.5. This can be attributed to the hydrolysis of diquat at this alkaline solution [43].
The selectivity coefficient (K Pot I,J ) for PEDOT:PSS/MIP+NaTFPB was evaluated using the modified separate solution method (MSSM) [42]. The selectivity values are tabulated as the negative logarithm in Table 3. The obtained small selectivity coefficient values can be explained on the basis of the high selectivity of the proposed sensor towards diquat against the studied interfering pesticides and inorganic ions. Evidently, PEDOT:PSS/MIP+NaTFPB exhibited excellent selectivity towards diquat over paraquat, cyromazine, dinotefuran, acetamipride, Na + , K + , and Ca 2+ ions. The obtained results reflected an enhanced selectivity for the proposed sensor and offered a great potential for trace-level monitoring of diquat in environmental samples. The influence of light on the potential stability was measured by immersing the proposed PEDOT:PSS/MIP+NaTFPB ISE in 10 −5 M diquat while turning on and off the ambient light. No obvious potential drifts were observed, demonstrating that the PEDOT:PSS-based solid contact has no light sensitivity. This result confirms that the PEDOT:PSS as the solid contact is superior to conducting polymers which is very sensitive to light.

Long-Term Potential Stability
The presence of the water-layer between the transducer layer PEDOT:PSS and the ion-sensing membrane (ISM) could generate a severe potential drift for the ISE. So, the water-layer test is performed to distinguish the existence of this layer. This test was carried out by recording the potential of the proposed electrode when a solution of 10 −5 M diquat was replaced by 10 −2 M NaCl, then changed back to 10 −5 M diquat again. As shown in Figure 4, PEDOT:PSS/MIP+NaTFPB ISE exhibits the characteristic EMF overshoot, which is usually seen when going back to the diquat solution. This suggests the absence of water-layer in the PEDOT:PSS/MIP+NaTFPB ISE. Moreover, long-term stability of PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs was evaluated via the potential response at the final part of the water-ayer test (Figure 4). The potential drift obtained was 0.11 and 3.3 mV/h for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs, respectively. This indicates that addition of the PEDOT:PSS layer could enhance the potential stability of the proposed electrode and reflects its high lipophilicity character.  The influence of light on the potential stability was measured by immersing the proposed PEDOT:PSS/MIP+NaTFPB ISE in 10 −5 M diquat while turning on and off the ambient light. No obvious potential drifts were observed, demonstrating that the PEDOT:PSS-based solid contact has no light sensitivity. This result confirms that the PEDOT:PSS as the solid contact is superior to conducting polymers which is very sensitive to light.

Long-Term Potential Stability
The presence of the water-layer between the transducer layer PEDOT:PSS and the ion-sensing membrane (ISM) could generate a severe potential drift for the ISE. So, the water-layer test is performed to distinguish the existence of this layer. This test was carried out by recording the potential of the proposed electrode when a solution of 10 −5 M diquat was replaced by 10 −2 M NaCl, then changed back to 10 −5 M diquat again. As shown in Figure 4, PEDOT:PSS/MIP+NaTFPB ISE exhibits the characteristic EMF overshoot, which is usually seen when going back to the diquat solution. This suggests the absence of water-layer in the PEDOT:PSS/MIP+NaTFPB ISE. Moreover, long-term stability of PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs was evaluated via the potential response at the final part of the water -ayer test (Figure 4). The potential drift obtained was 0.11 and 3.3 mV/h for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs, respectively. This indicates that addition of the PEDOT:PSS layer could enhance the potential stability of the proposed electrode and reflects its high lipophilicity character.

Short-Term Potential Stability
Short-term potential stability was checked and estimated using the reverse chronopotentiometric technique. Typical chronopotentiograms for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs were presented in Figure 5. After applying a ±1 nA current to the tree electrode

Short-Term Potential Stability
Short-term potential stability was checked and estimated using the reverse chronopotentiometric technique. Typical chronopotentiograms for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs were presented in Figure 5. After applying a ±1 nA current to the tree electrode cell, the overall resistance (R t ) of the electrode was calculated using the potential shift in the response in accordance to Ohm's law R=E/I. The total resistances were found to be 0.5 and 0.62 MΩ for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs, respectively. Furthermore, the short-term potential stability was calculated through calculating the ratio of ∆E/∆t. The potential drift value was estimated to be 53.7 and 225.4 µV/s for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs, respectively. The capacitance (C) of the fabricated electrodes was calculated via the fundamental capacitor equation, i.e., I = C × ∆E/∆t. The capacitance value for PEDOT:PSS/MIP+NaTFPB and MIP+NaTFPB ISEs was calculated as 18.8 ± 0.6 and 4.44 ± 0.3 µF, respectively. This reflects the significant importance of adding the PEDOT:PSS layer as a successful ion-to-electron transducer between the ISM and electronic conductor substrate.

Diquat Assessment
The proposed ISEs were introduced to assess diquat in commercial agriculture formulations containing diquat dibromide in addition to different potato samples. Reglone 200 SL, Syngenta Company (Cairo, Egypt), containing 31.8 w/v% soluble liquid (SL) of diquat dibromide) was taken as a commercial agriculture formulation for diquat assessment. Each sample was analyzed in triplicates. The results of analysis are shown in Table 4. These data were compared with results obtained by measuring diquat using the HPLC method as a comparative technique [44,45].  For the assessment of diquat in potato samples, the results obtained by both the proposed potentiometric method and the HPLC method as a comparative method were tabulated in Table 5. An F-test showed no significant difference at 95% confidence level between means and variances of the potentiometric and HPLC sets of results. The calculated F-values (n = 5) were found to be in the range of 1.6-4.8 compared with the tabulated value (6.39) at 95% confidence limit.

Diquat Assessment
The proposed ISEs were introduced to assess diquat in commercial agriculture formulations containing diquat dibromide in addition to different potato samples. Reglone 200 SL, Syngenta Company (Cairo, Egypt), containing 31.8 w/v% soluble liquid (SL) of diquat dibromide) was taken as a commercial agriculture formulation for diquat assessment. Each sample was analyzed in triplicates. The results of analysis are shown in Table 4. These data were compared with results obtained by measuring diquat using the HPLC method as a comparative technique [44,45]. For the assessment of diquat in potato samples, the results obtained by both the proposed potentiometric method and the HPLC method as a comparative method were tabulated in Table 5. An F-test showed no significant difference at 95% confidence level between means and variances of the potentiometric and HPLC sets of results. The calculated F-values (n = 5) were found to be in the range of 1.6-4.8 compared with the tabulated value (6.39) at 95% confidence limit.

Conclusions
The present work entails the fabrication of a disposable, point of use screen-printed potentiometric ISEs for diquat detection. The trace level detection could be achieved by integrating a molecularly imprinted polymers (MIP) for diquat as a sensory recognition receptors and PEDOT:PSS as a solid contact transducer. This study reinforced the excellent characteristic of PEDOT:PSS as a good ion-to electron transducer for fabricating solid contact-ISEs. The sensors revealed fast response towards diquat with a potentiometric Nernstian slope and reasonable sub-Nernstian slope of 28.2 ± 0.7 mV/decade over the linear range of 1.0 × 10 −6 -1.0 × 10 −2 M with a detection limit of 0.026 µg/mL. The sensor offered the advantages of reasonable selectivity over different common herbicides, good accuracy, and possible interfacing with automated systems. Advantages and limitations of many of the previously suggested potentiometric membrane diquat sensors are given in Table 6, for comparison. It can be seen that the sensors suggested in the present work have several inherent advantages over many of those previously described. The sensor was successfully applied in monitoring diquat in commercial agriculture formulations containing diquat in addition to different potato samples.