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Mesoporous silica thin films for improved electrochemical detection of paraquat Tauqir Nasir, Gregoire Herzog, Marc Hebrant, Christelle DESPAS, Liang Liu, and Alain Walcarius ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00920 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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Mesoporous Silica Thin Films for Improved Electrochemical Detection of Paraquat Tauqir Nasir, Grégoire Herzog*, Marc Hébrant, Christelle Despas, Liang Liu, Alain Walcarius Laboratoire de Chimie Physique et Microbiologie pour l’Environnement (LCPME), UMR 7564, CNRS – Université de Lorraine, 405 Rue de Vandoeuvre, 54600 Villers-lès-Nancy, France *Corresponding author: [email protected]

Abstract An electrochemical method was developed for rapid and sensitive detection of the herbicide paraquat in aqueous samples using mesoporous silica thin film modified glassy carbon electrodes (GCE). Vertically-aligned mesoporous silica thin films were deposited onto GCE by electrochemically assisted self-assembly (EASA). Cyclic voltammetry revealed effective response to the cationic analyte (while rejecting anions) thanks to the charge selectivity exhibited by the negatively-charged mesoporous channels. Square wave voltametry (SWV) was then used to detect paraquat via its one electron reduction process. Influence of various experimental parameters (i.e. pH, electrolyte concentration and nature of electrolyte anions) on sensitivity was investigated and discussed with respect to the mesopores characteristics and accumulation efficiency, pointing out the key role of charge distribution in such confined spaces on these processes. Calibration plots for paraquat concentration ranging from 10 nM - 10 µM were constructed at mesoporous silica modified GCE which were linear with increasing paraquat concentration, showing dramatically enhanced sensitivity (almost 30 times) as compared to non-modified electrodes. Finally, real samples from Meuse River (France) spiked with paraquat, without any pretreatment (except filtration) were analyzed by SWV, revealing the possible detection of paraquat at very low concentration (10-50 nM). Limit of detection (LOD) calculated from real samples analysis was found to be 12 nM, which is well below the permissible limits of paraquat in drinking water (40-400 nM) in various countries.

Keywords:

Electrochemical

sensor,

pesticide,

nanostructured

electrode,

river

water,

preconcentration, Debye length, square wave voltammetry

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Recent years have seen the emergence of electrodes modified with mesoporous and nanostructured materials, with promising applications in the fields of electroanalysis and energy.1 Silica-based mesoporous materials have unique characteristics which make them excellent choice to be used for electrochemical sensing2 or biosensing.3 These characteristics include (i) open and ordered mesopore structure exhibiting high surface area which can be fully accessible to external reagents; (ii) ease of derivatization with various organo-functional groups; (iii) hosting and support to different compounds into the mesostructures such as absorbed species, catalysts and biomolecules.

1–3

Vertically oriented mesoporous thin films on electrodes are of particular interest as they enable fast transport features from the solution to the underlying electrode surface,3 leading to efficient electroanalytical performance.4 Several methods have been developed to form mesochannels perpendicular to the electrode surface, which include epitaxy growth5, block co-polymerization,6 high magnetic field,7 oil induced coassembly process,8 Stöber-solution growth9 and electrochemically assisted self-assembly (EASA).10 Stöber-solution growth and EASA are two mostly used and more efficient techniques, and they are the only ones that have been reproduced by independent groups. Both approaches are operating under kinetic control, yielding the thin films with pore diameter of 2-3 nm by using cetyltrimethylammonium bromide (CTAB) as a template molecule, which constitutes an improvement in comparison to physical methods (ion beam milling,11 electron beam lithography and nanoimprint lithographies,12 or integrated circuit fabrication techniques13). They are restricted to the production of membranes with mesopore diameters greater than 10 nm range. Stöber-solution growth method involves the formation of mono or multilayered silica thin films by gradual transformation (1-3 days and 60-100 °C) of silica precursor and CTAB in the presence of ammonia.9 The EASA technique is much faster (5-30 seconds for film deposition) and operates at room temperature. In the EASA process, the electrode is immersed in a sol containing hydrolysed silica precursor (tetraalkoxysilane) and CTAB. A reductive potential is applied resulting in hydroxyl ions formation near the electrode surface. This local pH increase triggers the silica condensation and hence the thin film formation. Silica molecules are aligned around the surfactant micelles and grow perpendicular to the electrode surface.10 Highly ordered thin films offer significant advantages in electrochemical sensing.2 Mesoporous silica thin films are very effective in the detection of small redox active molecules,14–16 while keeping away larger molecules that could passivate the underlying electrode surface (anti-biofouling properties15). Oriented mesoporous silica films have proven to be a successful choice for sensing applications such as detection of drugs,14 aromatic explosives and pesticides,16 polyphenolic compounds,17 and heavy metals.18,19 Moreover, ease of functionalization of the silica films generated by EASA process provide extra advantage for their application in the field of sensing and electrocatalysis.20,21 2 ACS Paragon Plus Environment

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The use of pesticides and herbicides is increasing in agricultural practices to meet the global demand of food security. As a consequence, these chemical compounds are a major cause of pollution to the environment mainly to aquatic systems. Paraquat (1,1’-dimethyl-4,4’-bipyridinium) also known as methylviologen is non-selective contact herbicide being used in agricultural practices to control broad leaf weeds since 1962.22 Paraquat is highly toxic for humans and animals, it causes serious damage to liver, lungs, heart, kidneys23 and is involved in development of Parkinson’s disease24 with thousands of deaths reported in the last few decades.24,25 Paraquat is soluble in water with LD50 for humans 40-60 mg/kg. It is banned in the European Union but still widely used in more than 100 countries, China being the biggest producer with more than 100,000 tons of paraquat every year and its production is increasing every year worldwide.25 Maximum permissible levels for paraquat residues in drinking water vary in different countries from 40-400 nM. 26 Standard methods of paraquat determination include gas chromatography mass spectrometry (GC/MS)27, radioimmunoassay28, fluorescent probe titration29, spectrophotometry30, and more recently flow injection colorimetric assay31 or surface-enhanced Raman spectroscopy.32 Some of these methods have certain disadvantages as they require bulky instrumentation and are mainly laboratory-based. In contrast, electrochemical techniques and instruments are easier to handle, less expensive and robust for sensing applications. Several reports are available for the electrochemical detection of paraquat, based either on bare electrodes (hanging mercury drop electrode,33 boron doped diamond electrodes,34,35 or gold ultramicroelectrodes36,37), on copper electrodes with electrodeposited Bi film,38,39 or on electrodes modified with metal nanoparticles,40–43 carbon nanomaterials (nanotubes or graphene),44–46 organic and polymeric compounds,47–51 and inorganic materials41,52,53. These methods can have the advantages of applicability to real samples and also in most studies the detection limits are below the internationally permissible limits of paraquat in drinking water (Table SI-1). On the other hand, some of the methods reported lean on complex electrode modification, for which there is a lack of understanding of the sensing mechanism. In our study, we combined the richness of mesoporous silica with the electrochemistry at carbon electrodes to design a simple and versatile method for electrochemical detection of paraquat. Glassy carbon electrodes (GCE) modified with vertically oriented mesoporous silica film presented an interest for the improved detection of paraquat.54 We have here investigated the reasons of the observed dramatic increase in sensitivity compared to bare GCE by varying the experimental parameters (i.e. pH, electrolyte concentration and nature of electrolyte anions), close to real sample conditions. Based on this set of experiments, we were able to determine that the nanoscale of the pores contributed to a concentration enrichment of cationic paraquat linked to a potential distribution


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inside the silica mesochannel. This concentration enrichment was then exploited for the sensitive detection of paraquat in river waters, without requiring any additional chemical pretreatment.

Materials and Methods Reagents 3-aminopropyltriethoxysilane (APTES, 99%, Sigma-Aldrich), tetrabutylammonium tetrafluoroborate (Bu4N+BF4−, 99%, Sigma-Aldrich), and acetonitrile (99.9%, Sigma-Aldrich) were used for the electrografting of APTES onto the working electrode surface. Tetraethoxysilane (TEOS, 98%, Alfa Aesar), ethanol (95−96%, Merck), NaNO3 (98%, Prolabo), HCl (1 M solution, Riedel Haen), and cetyltrimethylammonium bromide (CTAB, 99%, Acros) were used for film synthesis. Ruthenium hexamine chloride (Ru(NH3)6Cl3, 98%, Aldrich) and KCl (99%, Prolabo) were used respectively as redox probe and supporting electrolyte for the electrochemical characterization of the modified glassy carbon

electrodes.

Paraquat

(1-1’-dimethyl-4-4’-bipyridinium

dichloride

(also

known

as

methylviologen), 98%, Sigma-Aldrich), and various electrolytes: NaCl (99 %, Prolabo), Na2SO4 (98%, Prolabo), NaClO4 (99%, Merck)) were used for square wave voltammetry analyses. All solutions were prepared with high-purity water (18.2 MΩ cm) obtained from a Purelab Option-Q from ELGA. Real samples were collected in the Meuse River (France). They were stored at 4°C after filtration (Durapore Membrane filters, HVLP 0.45µm, Millipore). The anionic composition was determined by ion chromatography using a Metrohm 882 Compact IC plus instrument equipped with a high pressure pump, sequential (Metrohm CO2 suppressor MCS) and chemical (Metrohm suppressor MSM II for chemical) suppression modules and a conductivity detector. The separation was performed on a Metrosep A Supp 5-250 column packed with polyvinyl alcohol particles functionalized with quaternary ammonium group (5 µm particles diameter) and preceded by a guard column (Metrosep A supp 4/5 guard) and an RP 2Guard column to remove traces of organic compounds. The mobile phase consisted of a solution of Na2CO3 (3.2 mmol L-1) and NaHCO3 (1 mmol L-1) in ultrapure water (18.2 MΩ cm at 293 K). The flow rate was 0.7 mL min-1 and the sample loop volume was 20 µL. The MagIC Net™ 2.3 Professional chromatography software controlled the 882 Compact IC plus instrument and its peripherals. The anion concentrations were measured at 0.66, 1.5, 0.45 and 0.73 mM with retention times of 10, 16.3, 22 and 25.4 minutes for Cl-, NO3-, H2PO4- and SO42-, respectively. The pH of river water was found to be 7.8. The ionic strength of the real samples was estimated by electrochemical impedance spectroscopy. Modification of GCE with mesoporous silica thin film


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The preparation protocol is summarised in Figure 1. Glassy carbon plates (SIGRADUR G plates, 20 mm × 10 mm × 1 mm) were purchased from HTW (Germany). The plates were polished with alumina (3, 1, and 0.05 μm) on a polishing cloth and were then rinsed thoroughly with ultrapure water and sonicated for 5 min before use. 3-aminopropyltriethoxysilane (APTES) was electrografted on GCE according to the method previously described54 (Figure 1, step (1)). The role of electrografted APTES is to anchor the electrogenerated silica film onto the carbon surface through the formation of Si-O-Si bonds. For film generation, a sol was prepared by mixing ethanol and aqueous NaNO3 (0.1 M) in a 1/1 v/v ratio. TEOS and CTAB (100 and 32 mM, respectively) were added to this solution. The pH of the sol was adjusted to 3 by the addition of HCl, and the sol was hydrolyzed under stirring at room temperature for 2.5 h prior to use as an electrodeposition medium (Figure 1, step (2)). Film deposition was achieved under galvanostatic conditions by immersing the electrodes in the hydrolyzed sol and applying a current density, j, of −0.74 mA cm−2 for 30 s (using a silver wire as a pseudoreference electrode and a stainless steel rod as a counter electrode) (Figure 1, step (3)). The surface area of glassy carbon modified with a film was 0.5 cm2. After film formation, the electrodes were treated overnight at 130 °C. Template removal was achieved using 0.1 M HCl in ethanol by stirring moderately for 1 hour (Figure 1, step (4)). Modified electrodes were characterised by cyclic voltammetry of 0.5 mM RuNH in 0.1 M KCl; ν = 100 mV s-1 (Figure 1, step (4’ and 5)) and by transmission electron microscopy (Figure 1, step (6)). For control experiments, non-mesoporous silica films were prepared using a 50:50 water: ethanol sol with 0.1 M TEOS + 0.1 M NaNO3 at pH 3. The sol was hydrolyzed under stirring at room temperature for 2.5 h prior electrodeposition applying a current density of −0.74 mA cm−2 for 30 s. A full characterisation of the modified electrodes is provided in our previous report.54


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Figure 1: Experimental protocol for the electrode preparation and characterisation: APTES is electrografted onto a carbon electrode (1); the modified carbon substrate is then immersed into the sol (2) and mesoporous silica thin film is formed by electrochemically assisted self-assembly (3); cross-linking of the silica matrix is ensured by heat treatment and the template is extracted (4); the


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quality of the silica thin film (4’) and the efficiency of the extraction is checked electrochemically (5); orientation and order of the mesoporous silica was verified by transmission electron microscopy (6).

Electrochemical methods All galvanostatic experiments and electrochemical impedance spectroscopy were carried out with a PGSTAT 100 apparatus from Ecochemie (Metrohm Autolab, Switzerland). Cyclic and square wave voltammetry measurements were carried out using either an EMStat2 or a PalmSens potentiostat (PalmSens, Netherlands) in the absence of oxygen (achieved through N2 purging for 20 minutes). All experiments were performed with a three-electrode system. For APTES electrografting and galvanostatic silica film deposition, a silver wire was used as pseudoreference and stainless steel rod as counter electrode. For voltammetric measurements in aqueous solutions, the reference electrode was Ag/AgCl/1 M KCl (purchased from Metrohm, Switzerland) and a stainless steel rod served as counter electrode; the modified GCE was used as working electrode by exposing only 0.125 cm2 of its surface area to ensure investigating a portion fully covered by the film (and avoiding any border effect). Conditions for square wave voltammetry measurements were the following: 5 s equilibration time, 3 mV potential step, 50 mV amplitude, and 10 Hz frequency. Electrochemical impedance spectroscopy experiments were carried out on both bare and mesoporous silica modified GCE (Figures SI-1&2) with a potential applied of 0 V, AC amplitude was 0.01 V at a frequency ranging from 0.1 Hz to 100 kHz with 10 frequencies per decade. Electrochemical impedance spectroscopy data were fitted with a Randles electrical equivalent circuit (with a constant phase element to simulate the non-ideal capacitor behaviour of the electrode)55 using the analysis tool of the Nova 2.1 software and allowed to determine the resistance of the solutions and of real samples (Table SI-2).

Results and Discussion Cyclic voltammetry characterization Mesoporous silica thin films present a high number of mesochannels per unit area with a diameter of 2.3 nm and a length of 150 nm (corresponding also to the film thickness) as it was determined by TEM (Figure 1). The mesochannels are densely packed in a regular hexagonal arrangement (approximately 1013 mesochannels per cm2) and all oriented perpendicular to the electrode surface, as it was reported in our previous reports.10,54 Prior to surfactant extraction, the film is impermeable to a cationic probe such as Ru(NH3)63+ (see curve 4’ in Figure 1). After template removal, the film becomes highly porous and the vertical orientation of mesopore channels ensures fast mass 7 ACS Paragon Plus Environment

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transport of chemical species toward the electrode surface (see well-defined voltammetric signal for Ru(NH3)63+ on curve 5 in Figure 1). The narrow diameter of the mesochannels and the anionic nature of the silica walls at pH higher than 2 (originating from surface silanolate groups) strongly influence the electrochemical behaviour of target analytes according to their charge. We have investigated the cyclic voltammetry of three organic species: paraquat (cationic), catechol (neutral) and diclofenac (anionic) before and after modification of the electrode surface with a mesoporous silica thin film (Figure 2). Although paraquat can be reduced in two successive one-electron transfers at -0.7 and -1.02 V,56 we will consider here only the first reaction as the analytical signal (Figure 2A), which is sufficient to obtain adequate quantification of this analyte in solution.50 The reduction peak current at a bare GCE was measured equal to 26 µA (blue curve on Figure 2A), while the peak current increased to 55 µA in the presence of a mesoporous silica thin film on the electrode surface (red curve on Figure 2A). In the case of neutral catechol, the peak current recorded at the modified electrode is 70 % of the peak current obtained at the bare electrode (Figure 2B). Finally, the irreversible oxidation of anionic diclofenac is not observed at a modified electrode while it is clearly visible at a bare electrode (Figure 2C). Electrostatic interactions between the anionic walls of the mesochannels and the charge of the species studied play an important role on the electrochemical response. While the voltammetric signal of the cationic analyte doubles at the modified electrode, the one corresponding to the anionic species is suppressed, due to charge selectivity (cation accumulation and anion rejection at such mesoporous silica film modified electrodes were already described in the literature,10,16,57–59 but the permselective effects observed with the quite large organic ions used here are really dramatic). The optimized structure of diclofenac, determined by X-ray diffraction, showed that the molecule larger dimension is around 1.1 nm.60 Although the size of diclofenac molecules may vary slightly when dissolved, we can assume that they are small enough to penetrate the mesochannels, which have a diameter of 2.3 nm. The absence of signal is not due to size exclusion effect and can then be attributed to electrostatic repulsions (i.e. charge exclusion). The presence of anionic silica mesochannels on the electrode surface did not influence the electrochemistry of the neutral catechol species. Note that catechol can be also detected before surfactant extraction as a result of its solubilisation into the liquid crystalline phase (as previously observed for other neutral species10,16,57– 59

), showing however a shift of peak potentials explained the excess of energy required for charge

compensation to maintain the electric charge balance after catechol oxidation. Only a small decrease in peak currents is observed due to some resistance to mass transport (compare blue and red curves in Figure 2B). In the following we will exploit the charge selectivity properties for sensitive detection of cationic paraquat species. 8 ACS Paragon Plus Environment

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Figure 2: Cyclic voltammetry of 0.5 mM of (A) Paraquat, (B) catechol and (C) diclofenac at a bare GCE (blue), and at GCE modified with the mesoporous silica film before (black) and after (red) template extraction. Background electrolyte: 0.1 M NaCl, ν = 100 mV s-1.


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Square wave analysis and factors affecting paraquat detection Paraquat detection is first investigated by square wave voltammetry (SWV) for concentrations increasing from 0.5 to 10 µM (Figure 3). The cathodic peak currents measured at an electrode modified with a silica thin film are higher by more than one order of magnitude compared to the ones obtained at a bare GCE, due to favourable electrostatic interactions between the cationic analyte and the anionic silica walls, as discussed above. In both cases, the peak currents increase linearly with the concentration showing a sensitivity of (-7.79 ± 0.27) A M-1 for the modified electrode against a sensitivity of (-0.32 ± 0.01) A M-1 for the bare electrode. The gain in sensitivity achieved with the modified electrode should allow us to push further the limits of the electrochemical detection of paraquat in water, but experimental factors likely to affect the detection process (such as ionic strength, pH, and nature of anions) should be considered beforehand.

Figure 3: Square wave voltammograms of 0.5 – 10 µM paraquat at bare (top, blue curves) and modified (bottom, red curves) GCE. Inset shows calibration curves. Background electrolyte: 0.1 M NaCl, pH 6. The influence of the solution ionic strength (between 10 and 500 mM) on the SWV response is investigated at both bare and modified electrodes (Figure 4). At the modified electrode, as the ionic strength decreases from 500 to 70 mM, the reduction peak current for paraquat increases. After reaching a maximum at 70 mM of supporting electrolyte, the peak current decreases continuously with the ionic strength at lower values. The particular V shape observed in the insert of Figure 4 should be explained by two antagonistic effects of the ionic strength. In the presence of large amounts of electrolyte, the accumulation of paraquat might undergo competition with the electrolyte cation (but this is not necessarily the main reason); at I < 70 mM, the paraquat response drop is even less obvious. A first hypothesis could be the increase in solution resistance. We have investigated the solution at different ionic strengths by electrochemical impedance spectroscopy at


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both bare and modified electrodes (Figure SI-1). The solution resistance extracted from the impedance data ranges between 64 (for I = 500 mM) to 2300 Ω (for I = 10 mM) and does not differ much whether the bare electrode or the modified one. As no variation in the reduction peak current with ionic strength is observed at bare GCE, for which the peak current remains almost constant throughout the ionic strength range (suggesting that the solution resistance does not impact paraquat reduction), the variation with ionic strength observed at the modified electrode can be related to the presence of the mesoporous silica film on the electrode surface.

Figure 4: Square wave voltammograms of 10 µM paraquat at various NaCl concentrations (from 10 to 500 mM). Inset shows the variation of peak current values obtained for each investigate electrolyte concentration at bare (blue dots) and modified (red squares) electrodes. pH was kept constant at 6. In order to further investigate this behaviour, the reduction peak current for paraquat is measured as a function of pH (in the range 1.2-10), while the ionic strength is maintained constant at 70 mM (Figure 5). At pH < 4, silica walls are either neutral or cationic and hence do not favour adsorption of cationic paraquat. For pH > 4, cationic paraquat can interact with the anionic walls of silica and an increase of the reduction peak current is observed. Maximum peak height is recorded at pH 6, whereas the reduction peak currents decay beyond this optimal pH value. As for ionic strength studies, the voltammetric signal of paraquat at the bare electrode does not vary noticeably in the pH range studied. We could therefore attribute the strong variation of peak current intensity at the modified electrode (Figures 4 & 5) to a dramatic influence of both pH and ionic strength on the state of mesochannels (surface charge density),61,62 and hence the ability of cationic species to adsorb onto the silica walls of the mesoporous film. In addition, nanoscale dimensions of the channels allow the occurrence of transport phenomena, which do not happen in channels of greater dimensions.63 Under certain conditions of ionic strength and pH, nanochannels with negatively-charged walls lead to the further enrichment of cationic species and to the exclusion of their counter anions as it has been described earlier by Plecis et al.64


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Figure 5: Square wave voltammograms of 10 µM paraquat at a modified electrode for pH values of 1.2, 6 and 10. NaNO3 is used to maintain the ionic strength at 0.07 M. HNO3 and NaOH solutions were sued to adjust the pH. Inset: peak current as a function of pH for modified electrodes (red) and bare electrode (blue). The pale blue zone represents the pH range generally encountered for real samples. When not visible, error bars are smaller than the symbols. It should be noted that the counter anion is different from Figure 4 (see Figure 6 and discussion related). On the other hand, when considering the accumulation (and detection) of paraquat, charge compensating anions should also be taken into account. In meso and nanochannels, the level of exclusion of counter anions and the degree of enrichment with cationic species are dependent on the Debye length, λD, which can be calculated according to equation (1), where ε0 is the permittivity of free space (8.85 × 10-12 F m-1), εr is the permittivity of water (6.95 × 10-10 F m-1 at 20°C), kB is the Boltzmann constant (1.38 × 10-23 J K-1), T is the temperature (293 K), NA is the Avogadro number (6.02

× 1023 mol-1), e is the elementary charge (1.60 × 10-19 C) and I the ionic strength (expressed in mol m3

).

λD

Eq. (1)

Under the experimental conditions for the formation of the mesoporous silica thin film, the pore radius, r, is measured at 1.15 nm, which is between the Debye lengths calculated for the highest and the lowest ionic strengths (Figure SI-2). When the ionic strength I > 70 mM, then r > λD, we can divide the channel in two zones: (i) a Debye layer in contact with the silica walls and (ii) a ‘free’ space in the centre of the mesochannel. As the ionic strength decreases, the Debye layer is increasing, reducing the ‘free’ space until it fully disappears when r < λD (i.e. I < 70 mM), leading to an overlap of the electrical double layers. We consider here the second case (i.e. when r < λD); the potential distribution across the mesochannel width is not homogeneous. It varies according to the z position


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(z direction is defined here as the width of the mesochannel) in the mesochannel, ψ(z), and it is calculated according to the following equation:64

ψ

ζ ! "#$%⁄λ& ! "# ⁄λ(

Eq. (2)

Where ζ is the zeta potential of the silica walls. The absolute potential value is the highest close to the charged silica walls and is equal to the zeta potential. It drops to reach a minimum in the centre of the mesochannel (when z = r) (Figure 6). Based on such a potential distribution, the mesochannel is enriched in cationic species as the concentration C(z) can be calculated from the bulk concentration C0 according to the Poisson-Boltzmann equation (3):64

) )* +,- .−

ψ%

0

Eq. (3)

Equations (2) and (3) show that the zeta potential at the diffuse layer of the mesochannel determines the overall enrichment in cationic species and hence the improved electrochemical signal. When ψ is negative, paraquat is accumulating within the mesochannel and thus leading to a higher voltammetric signal than at bare electrodes. Zeta potential values for silica surface depend on the pH and ionic strength.61,62,65 As the solution pH increases, silica surfaces become more negatively charged and the absolute zeta potential increases, leading therefore in an increase of the paraquat concentration within the mesochannels. However, we observe a drop in the reduction peak current for pH > 6 (Figure 5), which can be related to some degradation of the film mesostructure due to partial dissolution of the silica.66 Similarly, the drop of reduction peak current for I < 70 mM (Figure 4) can be linked to the overlap of the double layers, leading to the exclusion of anions from the mesochannels.62 Equations (2) and (3) show that paraquat concentration is higher in the Debye layer than in the centre of the mesochannel. As the ionic strength decreases from 500 mM to 70 mM, the Debye layer thickens, allowing the paraquat concentration within the mesochannels to rise, contributing to higher peak current for I = 70 mM than for I = 500 mM. Equation (3) also shows that this accumulation of cations in the channels leads to a higher ionic strength in the mesochannels as in the bulk, as both Na+ and paraquat are concerned. As the ionic strength decreases, the zone from which anions are excluded (marked in grey in Figure 6) increases until they are excluded when r < λD (i.e. I < 70 mM).


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Figure 6: Distribution of the potential ψ (dashed black lines) and the paraquat concentration ratio (C/C0) (solid red lines) along the mesochannel width, z, for an ionic strength, I, of (A) 0.5 and (B) 0.07 M. The grey area represent the Debye layer; ψ and C/C0 are calculated based on equations (2) and (3). (A) ζ = -6 mV;62,65 λD = 0.35 nm for I = 0.5 M and (B) ζ = -23.1 mV;62,65 1.13 nm for I = 0.07 M; r = 1.15 nm in all cases. The movement of anions to maintain the electroneutrality during the electrochemical experiments is hindered, resulting in a drop of the electrochemical signal. Paraquat reduction is investigated (Figure 7) for a series of background electrolyte anions (Cl-, NO3-, SO42-, H2PO4- and BF4-). Chloride, nitrate, sulfate and dihydrogen phosphate are selected for their occurrence in environmental samples while BF4- is selected for its solvated ionic radius (ri = 11.5 Å)67 close to the radius of the mesochannel (r = 1.15 nm). The reduction peak currents varies greatly with the different anions of the background electrolyte although the ionic strength is maintained constant (I = 70 mM, except for Na+BF4-, where I = 50 mM due to solubility constraints). Even when counter anions such as Cl-, NO3-, SO42-, H2PO4-, are partially excluded by the thickness of the Debye layer, the electrochemical signal remains greater at a modified than at a bare electrode. For BF4-, the electrochemical signal is totally blocked as BF4- do not penetrate in the mesochannels, regardless of the ionic strength.68 The intensity of reduction peak currents followed this order: NO3-> ClO4- > Cl- > SO42- > H2PO4- > BF4-. It should be noted that no correlation is found between the ionic radius, the ionic conductivity of the anions or the Hofmeister series and the reduction peak current observed.


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Figure 7: Square wave voltammograms of 10 µM paraquat at a modified electrode using different background electrolyte anions. Ionic strength was 70 mM for all anions except BF4- for which the ionic strength was 50mM; pH was kept constant at 6 for all experiments.

From the above results, we conclude that the best electroanalytical response of paraquat at mesoporous silica modified GCE is recorded with 0.07 M NaNO3 as supporting electrolyte and when the pH is 6. In these optimized conditions, paraquat reduction peak current shows linear response at concentrations in the 10 – 50 nM range with a detection limit of 4 nM (Figure SI-4). For comparison purposes, the voltammograms obtained for a bare GCE and an electrode modified with a nonmesoporous silica thin film (i.e. electrogenerated in the absence of surfactant) do not show any reduction peak, confirming that the mesochannels are responsible for the improved detection of paraquat. The limit of detection is within the same concentration range of the best methods reported in Table SI-1 and is achieved with a preconcentration time of 3 minutes, which remains lower than most of methods reported (Table SI-1).

Analysis in real medium River samples from the Meuse River (France) are spiked with paraquat and analysed by square wave voltammetry at both bare and modified electrodes (Figure 8). Electrochemical impedance spectroscopy (Figure SI-2) shows that the solution resistance (Rs = 4300 Ω) is relatively high and corresponds approximately to the resistance of a solution with an ionic strength lower than 10 mM NaCl solution Figure SI-3). Ion chromatography analysis shows that chloride, nitrate, sulfate and phosphate anions are present in the river water samples for a total concentration of 3.34 mM. These two values suggest an ionic strength in the same order of magnitude. A calibration curve for paraquat is built at the modified electrode between 10 and 40 nM with a sensitivity of 0.021 µA nM-1 (R2 = 0.946; N = 4). At the bare electrode, paraquat reduction is not observed for any of the four 15 ACS Paragon Plus Environment

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paraquat concentrations tested (10, 20, 30 and 40 nM). These results demonstrate that the presence of the mesoporous silica thin film improves the sensitivity reaching concentration levels in real samples that cannot be detected at a bare electrode. The limit of detection of paraquat at an electrode modified with a mesoporous silica thin film is estimated at 12 nM (in a spiked real sample solution), which is much lower than the maximum concentration of paraquat in drinking water recommended in guidelines and regulatory limits by different countries.26 This example shows that electrochemical analysis with a mesoporous silica modified GCE can also be used for analyzing real water samples with low detection limit. The quantification may be further improved by fixing the type and concentration of anions in the solution.

Figure 8: Square wave voltammograms recorded in Meuse River water samples at bare (blue) and modified (red) electrodes, respectively after spiking with of 0, 10, 20, 30, and 40 nM paraquat. Insert: Peak current for paraquat added concentrations of 10, 20, 30 and 40 nM at the modified electrode.

Conclusions Mesoporous silica thin films favour the electrochemical detection of cationic paraquat in aqueous samples. The nanoscale dimension of the oriented and organised channels contributes to concentration enrichment within the mesochannels, which varies with experimental conditions such as ionic strength, pH and nature of the supporting electrolyte anion due to local potential variations. Limit of detection (LOD) in real samples spiked with paraquat by this method is 12 nM which is a more than 3 times below the strictest permissible limit of 40 nM established for paraquat in drinking water.

Acknowledgements


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The authors thank the French PIA project “Lorraine Université d'Excellence” (Reference No. ANR-15IDEX-04-LUE). T.N. is grateful to the Higher Education Commission of Pakistan for funding his Ph.D. We also thank Prof. Christian Ruby for the provision of river samples and Mrs Claire Genois for the ion chromatography and ICP-AES determination of ion concentrations.

Supporting information The Supporting Information is available free of charge on the ACS Publications Website at DOI:10.1021/acssensors.xxx. Electrochemical methods for paraquat detection reported in the literature, Electrochemical impedance spectroscopy, solution resistance measurements, Debye length calculations, Electro chemical paraquat detection at three different kind of electrodes.

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