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carbon nanocomposites for sensitive amperometric detection of nitrite
Accepted Manuscript Title: Polyoxometalate [PMo11 O39 ]7− /carbon nanocomposites for sensitive amperometric detection of nitrite Author: Feriel Boussema Raoudha Haddad Yassine Ghandour Mohamed Salah Belkhiria Michael Holzinger Abderrazak Maaref Serge Cosnier PII: DOI: Reference:
S0013-4686(16)32309-X http://dx.doi.org/doi:10.1016/j.electacta.2016.10.192 EA 28280
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
28-6-2016 27-10-2016 29-10-2016
Please cite this article as: Feriel Boussema, Raoudha Haddad, Yassine Ghandour, Mohamed Salah Belkhiria, Michael Holzinger, Abderrazak Maaref, Serge Cosnier, Polyoxometalate [PMo11O39]7−/carbon nanocomposites for sensitive amperometric detection of nitrite, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.10.192 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyoxometalate [PMo11O39]7− / carbon nanocomposites for sensitive amperometric detection of nitrite
Feriel Boussema[a], Raoudha Haddad[b], Yassine Ghandour [c], Mohamed Salah Belkhiria[c], Michael Holzinger*[b], Abderrazak Maaref[a], and Serge Cosnier[b]
[a] Laboratoire de Physique et Chimie des Interfaces, Faculté des sciences de Monastir, 5000 Monastir, Tunisia [b] Département de Chimie Moléculaire UMR 5250, Univ. Grenoble Alpes - CNRS, 38041, Grenoble, France [c] Laboratoire de Physico-chimie des Matériaux, Université de Monastir, Faculté des Sciences, 5000 Monastir, Tunisia. Corresponding author: Michael Holzinger, e-mail: [email protected]
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Graphical Abstract
The polyoxometalate [PMo11O39]7− confined in nanostructured carbon matrices was evaluated as catalyst for the selective electrochemical detection of nitrite ions.
Abstract The polyoxometalate [PMo11O39]7− was synthesized and used as catalyst for the reduction of nitrite ions. This mono-lacunary keggin anion, cited as PMo 11, was confined within a matrix of either oxidized single-walled carbon nanotubes (ox-SWCNTs) or reduced graphene oxide (rGO) on glassy carbon (GC) electrodes to improve the electron transfers. Five different configurations were characterized using cyclic voltammetry and evaluated in an amperometric sensor setup for nitrite detection where the amount of the different components were optimized. The configuration using only ox-SWCNTs and PMo11 on GCE showed best sensitivities of up to 44.41 mA Lmol-1 and a satisfying reproducibility (RSD: ∼2.64 % for four identic electrodes). Furthermore, a linear range between 3.0 10-5 molL-1 and 1.6 10-2 molL-1 with a detection limit of 3.0 10-5 molL-1 was obtained. The final sensor setup also showed good selectivity towards interfering ions. Keywords: Nitrite sensor; Single-walled Carbon Nanotubes; Reduced
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Graphene Oxide; Monolacunary Keggin anion; polyoxometalate 1. Introduction
Nitrite is found in soil, water, food and physiological systems [1] and its determination is very important due to its role in environmental processes and biomarkers, but also to its toxicity for humans at higher concentrations [2-4]. One danger after nitrite ingestion is that it converts oxyhemoglobin to methemoglobin in the bloods stream, thereby interfering with oxygen transport [5]. The elaboration of sensors with high sensitivities to nitrite is an important challenge for medical and environmental applications [6, 7]. Many methods have been developed for nitrite determination including spectrophotometry [8, 9], chromatography [10, 11] and electrochemical methods [12-14]. However, spectrophotometry and chromatography methods require expensive equipment, which are not suitable for the development of portable devices. The electroreduction of nitrite ion requires high overpotentials on most electrode surfaces. Therefore, enzyme electrodes were developed for the specific catalytic reduction of nitrites. However, although, electrochemical nitrite biosensors are extremely sensitive, they suffer from a low stability due to the use of proteins, are pH dependent and most of them use viologen derivatives for the enzyme wiring and hence must be used under argon atmosphere [15-18]. Another alternative consists in the development of chemically modified electrodes with suitable catalyst immobilized on conventional electrode materials, decreasing the overpotential for nitrite detection [19, 20]. Polyoxometalates (POMs), a class of structurally well-defined anionic clusters with an enormous disparity in size, composition, and function, attract more and more attention. Its inherent metal-oxygen framework can possess reversible and gradual multi-electron reactions [21-23]. This class of inorganic compounds is also resistant to oxidative degradation due to 3
the fact that their fundamental elements are already in high oxidation states [24]. These properties make them very attractive in many fields such as analytical chemistry, catalysis and medicine [24-26]. For these reasons, POMs became appealing candidates for electrocatalytic applications [27]. In particular, the development of nitrite sensors using POMs became a vast research field where excellent performances in terms of sensitivity, detection limits and selectivity could be obtained [28-32]. Practical applications of POMs for the preparation of chemically modified electrodes depend on the successful immobilization of these compounds. Efficient immobilization of POMs on electrodes simplifies their electrochemical studies and facilitates their applications in electroanalysis. So far, many strategies have been developed to prepare chemically modified electrodes with a variety of POMs on common solid electrode substrates such as electrochemical deposition [20, 33], adsorption [34, 35] and entrapment in polymer matrices [36, 37]. Entrapping in a positively charged polymer matrix on the electrode surface using i.e. chitosan (CS) or polyallylamines provides strong interaction via electrostatic binding between the cationic polymer and the heteropolyanions but is also accompanied with negative effects such as an insulating effect of the polymer. Over the past few years, carbon materials have widely been used as suitable matrices owing to their good chemical stability and strong affinity for POMs. Various carbon-based materials such as carbon nanotubes [38, 39] or graphene oxide [40, 41] have been used as support materials to obtain conductive POM composites. Graphene has received considerable attention thanks to its high surface area and excellent conducting nature [42]. In the field of electrochemistry, it is suggested that graphene is very suitable to be used as the support material for the manufacture of POM-based electrode to improve electrocatalysis [29, 41].
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Recently, Weihua Guo et al [43] have developed a composite film modified electrode based on electrochemically reduced graphene oxide (rGO) and Keggin-type heteropolymolybdate H4SiMo12O40 (SiMo12) for the detection of nitrite. The composite film was prepared by electrochemical reduction of GO on an ITO electrode. The developed electrochemical nitrite sensor showed a linear range between 33.3–632.7 μmol.L−1 with a detection limit of 7.73 μmol.L−1, and a sensitivity of 0.651 mA.Lmol−1. These competitive performances make POM/graphene nanocomposite films quite promising candidates for the design of electrochemical sensors. Single-wall carbon nanotubes (SWNTs) also showed their appropriateness to form stable assemblies with POMs for electrochemical applications [44]. Weihua Guo et al. [45] reported a SWCNT- SiMo12 modified electrode for the detection of nitrite but did not quantify the performances of the nitrite sensor. Here, we evaluated the slightly different kegging-type polyoxometalate [PMo 11O39]7− using five different configurations of nanocarbon / PMo 11 assemblies with and without intermittent CS layers for the amperometric detection of nitrite. We have chosen ox-SWCNTs and rGO as conductive matrices for optimized electron transfer between the electrocatalytic PMo 11 and the electrode surface. Even when the intrinsic resistance of ox-SWCNTs [46] and rGO [47] is clearly increased compared to their unmodified counterparts, the surface defects of these materials provide lower intertube [48] or intersheet [49] contact resistances in bulk samples and should therefore provide excellent conditions for enhanced electron transfers. 2. Experimental 2.1.
Reagents and apparatus. Low molecular weight chitosan (75-85% deacetylated, 50-
190 kDa), sulfuric acid (H2SO4, ACS reagent), acetic acid (reagent grade), sodium nitrite (NaNO2, (reagent grade)), were purchased from Sigma-Aldrich and used without further 5
purification. Distilled ultra-pure water (18.2 Ω cm resistivity) was produced using an ELGA Purelab flex. rGO was purchased from NanoInnova Technologies and SWCNTs in purified grade from Unidym TM and oxidized with 69% nitric acid (ACS reagent, 10.8 mol L-1) by stirring under reflux at 110°C for 2h. The reaction mixture was then filtered over a cellulose membrane filter and washed several times with distilled water. The solid was re-dispersed in distilled water and filtered again to remove entrapped or intercalated acid. Further washing steps were carried out with distilled water until the pH of the filtrate becomes neutral. The resulting black solid was re-dissolved in slightly basic sodium carbonate (Na2CO3) water (pH 9) to disperse homogeneously oxidized nanotubes and to dissolve a large amount of the formed amorphous carbon around the nanotubes. Not dispersed material was allowed to deposit. The supernatant was decanted off and filtered over regenerated cellulose. The resulting oxidized SWCNTs were washed with distilled water and then dried in vacuum. Tetrabutylammonium (TBA) salts of the α-Keggin phosphomolybdates [(C4H9)4N)4H3] [PMo11O39] were prepared based on the procedure described in literature [50]. Briefly, Phosphomolybdic acid (H3PMo12O40) (4 mmol) was dissolved in 40 ml of distilled water. The pH of the solution was adjusted to 4.3 with Li2 CO3 and a precipitate was formed after adding solid [(C4H9)4N)4] Br (56 mmol) to the solution under vigorous stirring. The crude product was collected on a medium frit, washed with distilled H2O and allowed to dry in air overnight at room temperature. Crystallization under nonprotic condition involved dissolving the crude [(C4H9)4N)4H3] [PMo11O39] in 200 ml CH3CN by stirring at room temperature followed by a slow evaporation of the solvent under ambient conditions . Electrochemical measurements were performed in an electrochemical cell with a conventional three-electrode configuration by using an Autolab pgstat100 potentiostat. Before each experiment, the surface of the glassy carbon (GC) electrode was polished with 2 mm diameter 6
diamond paste (MECAPREX Press PM) and was then rinsed with ultra-pure water to remove any residual diamond paste. Finally, the electrodes were sonicated for 5 min in ethanol and 5 min in acetone followed by a thorough washing step with distilled water and ethanol. A GC working electrode with a diameter of 3 mm was used together with a platinum wire as counter-electrode and a saturated calomel reference electrode (SCE). IR spectra were recorded using a Nicolet 470 FT-IR spectrophotometer with pressed KBr pellets. FE-SEM images were recorded using a FEI QUANTA-FEG 250 microscope with an Everhardt Thornley SED (secondary electron detector), an accelerating voltage of 3 kV and a working distance of 8.0 mm.
2.2.
Characterization of tetrabutylammonium salts of the α-Keggin
phosphomolybdates. Within all available types of polyoxometalates [33], the Keggin-type POM [PMo11O39]7− (PMo11) was used for these studies. It has a quite well-known structure which is composed of a tetrahedrally coordinated central hetero atom surrounded by four Mo3O13 groups (triads) that are connected in a corner-sharing fashion via oxygen linkers [51] as represented in Figure 1. The Lacunary Keggin structures (PMo 11), used in this study, are derived from the parent Keggin type by the removal of specific MoO 6 moieties, followed by an optional rotation of the remaining MoO6 octahedra [25]. As a consequence, (PMo11) is characterized by the presence of an enhanced amount of terminal oxygen atoms, which thus provides more negative surface charges than other POM structures. Figure 1. The prepared compound shows in the IR spectra characteristic vibration bands (Figure 2) of the α-Keggin type structure: (P-Oa), υ(Mo-Od), υ(Mo-Ob-Mo) and υ(Mo-Oc-Mo) at 1051 cm-1; 945 cm-1; 883 cm-1 and 794 cm-1, respectively. In addition, the bands at 2965 cm-1 and 1480 cm-1 are assigned to υ(CH3) asymmetric stretching of the tetrabutylammonium (TBA) counter ions. 7
Figure 2.
2.3.
Preparation of different POM assembly based catalytic electrodes PMo11/CS/GCE: 12 mg chitosan (CS) was dissolved in 10 mL of 0.1M acetic acid.
Then, 6 µL of the CS solution was dropped on the cleaned GC. 10 µL [(C4H9)4N)4H3] [PMo11O39] (2 mmol L-1 in acetonitrile) was added on the surface of the CS modified electrode (CS/GC) by drop-casting where the solvent was removed under vacuum and the electrode was then washed with acetonitrile (ACN). PMo11/ox-SWCNT/ GCE: A dispersion of oxidized single walled carbon nanotubes (ox-SWCNTs) in DMF were prepared by a 30 min sonication of 5 mg of carbon nanotubes in 1 mL of DMF until a homogenous black suspension was obtained. Then, 20 µL of the oxSWCNTs solution were drop-casted on a GC electrode and the solvent was removed under vacuum. The PMo11 layer was formed by casting 10µL of the [(C4H9)4N)4H3] [PMo11O39] solution on the ox-SWCNTs deposit followed by the evaporation of the solvent under vacuum. PMo11/rGO/ GCE: Dispersions of reduced graphene oxide in DMF were prepared by a 30 min sonication of 5 mg of reduced graphene oxide in 1 mL of DMF until a homogenous black suspension was obtained. rGO solution (20 μl) was drop casted onto the pre-cleaned GCE. The electrodes were then modified with a PMo11 layer as described for the PMo11/oxSWCNTs/GC electrodes. The fabrication of the PMo11/CS/rGO/GC and PMo11/CS/ox-SWCNT/GC electrodes was similar to that of the two preceding electrodes where an additional CS layer was formed between the carbon and PMo 11 layer.
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3. Results and discussion 3.1.
Electrochemical behavior of the different POM based electrodes
The cyclic voltammetric responses of PMo 11/rGO, PMo11/CS/rGO, PMo11/CS, PMo11/ oxSWCNTs and PMo11/CS/ox-SWCNTs modified GCE were examined in 0.1 mol L -1 H2SO4 at pH 1. Figure 3A illustrates the cyclic voltammograms (CVs) of PMo 11/rGO, PMo11/CS/rGO, PMo11/CS modified GC. In the potential range between 0.4 and -0.1 V (vs. SCE), the CV curves exhibit two redox waves which show a two-step reduction processes for PMo 11. The redox couples I–I’ at E1/2 = 0.23 V vs SCE and II–II’ at E1/2 = 0.12 V vs SCE are assigned to two consecutive processes of Mo centers within PMo 11 (MoVI MoV) [52]. Referring to a similar Mono-Lacunary Keggin type anion [32], these reversible redox systems can be attributed to following equations: PMo11VI O397- + 2e- + 2H+ H2PMo2V Mo9VI O397- (I’)
(1)
H2PMo2V Mo9VI O397- + 2e- + 2H+ H4PMo4V Mo7VIO397- (II’).
(2)
In comparison with the PMo 11/CS/rGO and PMo11/CS electrodes (curve b and c respectively), a significant current increase is observed for PMo 11/rGO modified electrodes (curve a). Furthermore, the intermittent CS layer, necessary for PMo 11 adhesion on GC electrodes, led to a slight reduction of the peak current intensities compared to pure rGO deposits. The higher currents and the shape of the CVs of the PMo11/rGO setup are mainly due to an enhanced capacitance and surface area of rGO, and to an improved electron transfer of the electrode to PMo11 [29, 53, 54].
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Using ox-SWCNTs instead of rGO under identic conditions, the electrochemistry of the formed composites clearly changed (Figure 3-B, curve e, f). Further waves were recorded and the measured currents increased in average by factor ten. Concerning the additional redox couples, a shoulder, indicated as system III–III’, can be seen in the curves e and f at 0.41 V vs SCE for the anodic scan and at 0.32 V for the cathodic scan which is attributed to the redox active carbon oxide species on ox-SWCNTs [55] (curve d). The redox waves at E1/2 = 0.23 V and at E1/2 = 0.12 V are the two processes of the couples I– I’ and II-II’ of equations (1) and (2). The redox system at E1/2 = -0.15 V, labeled as peaks IVIV’, might be a further redox systems following equation (3): H4PMo4V Mo7VIO397- + 2e- + 2H+ H6PMo6V Mo5VIO397- (IV’)
(3)
A further small redox peak (labeled as * and *’) is observed at E 1/2 = 0.07 V and corresponds to an intermediate redox process IV-IV’ of PMo11 in close proximity to the ox-SWCNT surface. This is in line with previous electrochemical studies of polyoxometalate composites [56, 57]. The fact that this third redox wave could not be recorded for the PMo 11/CS and PMo11/rGO samples is due to the fact that it is covered by a large irreversible reduction peak at potentials < -0.05 V vs SCE which prevented us to scan a larger potential window. This irreversible reduction appears for the ox-SWCNT/ PMo11 composites at lower potentials (around -0.3 V vs SCE). We attribute this to the reduction of protons [58] where the formed hydrogen is partly dissolved in solution and partly adsorbed at the electrode. The adsorbed hydrogen is then re-oxidized to protons at the cathodic scan at -0.25 V (marked with °). The current increase is to one extent due to the capacity of the composite. As shown in Figure 3B, curve d, pure ox-SWCNTs show clearly lower capacitances than assembled with PMo 11.
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This synergetic effect of polyoxometalates and SWCNTs also opens the possible application of such composites as supercapacitors [59]. To the other extent, ox-SWCNTs provide improved bulk conductivity [60] leading to improved electron transfers compared to rGO. In addition, we could observe a decrease in the current response for the electrodes with a CS layer (PMo11/CS/ox-SWCNTs/GC and PMo 11/CS/rGO/GC), which indicates that the electron transfer rate is hindered by the CS layer. It further seems that PMo 11 is well adsorbed on rGO and ox-SWCNTs layers even when the carbon oxide functions on this material provide a negatively charged environment such as PMo11. The phenomenon that negatively charged polyoxometalates and negatively charged oxidized carbon materials form stable layers was studied by Schwegler et al. [61] using oxidized activated carbon. He showed that the hetero polyanion clusters interact with oxygen containing functions such as hydroxyls, carbonyls, and ether groups via acid-base interactions thus forming hydrogen bonds between protonated and anionic oxygen. Similar results were obtained by Choi et al. [62] studying the adsorption properties of different polyoxometalates on HOPG and glassy carbon electrodes and Kang et al. [63] even could attach polyoxometalate nanoparticles on oxidized CNTs just by using ultrasound. In summary, the incorporation of rGO or ox-SWCNTs in the PMo11 film not only improves the conductivity of the film and enhances the electron transfer, but also avoids the necessity of an additional CS layer. It can further be concluded that these advantages are much more pronounced for ox-SWCNTs than for rGO. Figure 3 3.2.
Morphological studies of the composites.
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To get more insight in the observed electrochemical behavior of the ox-SWCNTs/PMo11 and rGO/PMo11 composites, we performed morphological studies using SEM. Figure 4 shows representative SEM images of ox-SWCNT and rGO before and after modification with PMo11. As produced ox-SWCNTs appear as solid as undulating aggregates due to defect group related agglomeration and stacking behavior [64] (Figure 4A). After adsorption of the PMo11 layer (figure 4B), it seems that the polyoxometalate crystallizes on the ox-SWCNT forming branched sheet structures around the nanotube aggregates of about 1 µm thickness. Some more cubic structures can also be seen on the right side of figure 4B. We assume that crystallization is induced after adsorption of small clusters or nanoparticles as proposed by Chen et al. [59] and continues to grow during time encasing the ox-SWCNT shapes until the formation of 3D crystals. We also observed large vacancies where no PMo 11 is adsorbed on ox-SWCNT. This inhomogeneous coating can be explained by the irregular density of carboxylates and other defect groups on ox-SWCNTs [65] which prevent a complete coverage.
For the rGO samples, the SEM investigations are more consistent. Figure 4C shows the used rGO samples before the deposition of PMo 11. Untreated rGO deposits show laminated structures which a typical for such materials. After adsorption of PMo 11, these rGO structures are homogeneously coated filling the interstices of the flakes as shown in Figure 4D. Obviously, the absence of a dense amount of negative charges on the carbon surface allows the polyanion to form a uniform layer around this material where no bigger aggregates or crystals were observed. Figure 4
3.3.
Evaluation of the PMo11 composites for nitrite reduction 12
In order to evaluate the appropriateness of PMo 11 in its rGO and ox-SWCNT composites for the amperometric detection of nitrite, the fabricated electrodes were tested under sensing conditions in presence of nitrites at different concentrations. For the rGO based PMo 11 electrodes, the optimized potential for the electrocatalytic reduction of NO2- was determined to be 0.10 V vs. SCE, while for the ox-SWCNTs based electrodes the potential of -0.15 V vs. SCE was chosen according the third reduction wave (equation (3)) of PMo11 in the composite since it provided the highest catalytic activity [56]. The catalytic reduction mechanism of nitrite to NO using polyoxometalates at low pH values was intensively studied and validated for different Keggin- or Dawson-type polyanions [32, 66]. As NO2- was added to the stirred 0.1 molL-1 H2SO4 solution, a well-defined steady-state current response was obtained at the applied potential. The current decreased along with every successive addition of NO2- (Figure 5A). At high NO2- concentrations, the noise of the amperometric signal increases and the current stabilization takes longer and longer until no straight line can be obtained anymore. The details about the origin of such parasitic current still have to be revealed but reliable measurements could be done until a total nitrite concentration of 16 mmolL-1. Figure 5B shows the current increase as a function of nitrite concentration. The sensitivity, determined by the slope of the calibration curve, is much higher for the setup using the PMo 11/ox-SWCNTs composite than for the PMo 11/rGO electrodes where both performances largely overpass the one obtained for the PMo 11/CS setup. The characteristics of the different sensor configurations for nitrite detection are summarized in table 1 and confirm the superiority of the PMo 11/ox-SWCNTs system. Figure 5 Compared to literature [19], the obtained performances with a sensitivity of 44.41 mA. Lmol-1 (555 mA. Lmol-1 cm-2), a linear range of 3.0 10-5-1.6 10-2 molL-1 and a detection limit of 3.0 10-5 molL-1 are within the best values reported for nitrite detection [30, 67, 68] using
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polyoxometalates as sensitive layer and is also competitive with the performances of some enzymatic nitrite biosensors [69-71]. Moreover, the sensor showed the relative standard deviation (RSD) of ∼2.64% for four different electrodes prepared under the same technique. The results show that the ox-SWCNTs/PMo11 assembly may be an excellent platform for the electrochemical detection of nitrite. It can be concluded that small molecules have a rapid diffusion from the solution to the nanohybrid modified electrode and have a better electron transfer rate using ox-SWCNTs than rGO. However, considering long term stability, the ox-SWCNT/PMo11 setup showed only 25% of its initial performances after one month. Further studies will reveal the origins of such performance loss. One reason might be the mechanic stability of the PMo 11 layer on the ox-SWCNT deposit. Beside the simple loss of the sensing layer, a slight desorption also provokes reduced electron transfers even when the PMo11 layer remains.
3.4.
Selectivity.
Nonetheless, the sensor setup giving the best performance (PMo 11/ox-SWCNTs/GCE) was also examined for its selectivity towards classic interfering ions such as NO 3−, ClO4 -, and PO43– in 0.1 mol L-1 (H2SO4) (pH 1) solution (Figure 6). No significant responses were observed for each addition of NO3−, ClO4 - , PO43- , but a notable amperometric response can be recorded when nitrite was added to the same solution. This result further confirms the beneficial properties of PMo11/ox-SWCNTs composites for high sensitive detection of nitrites in complex media containing nitrates, chlorates and phosphates. Figure 6 4. Conclusion. A high performant electrochemical nitrite sensor was developed using the polyoxometalate [PMo11O39]7− as electrocatalyst. To establish an efficient electron transfers between the catalyst and the external circuit, a layer of oxidized ox-SWCNTs 14
showed to be the best material since it also ensured a satisfying stability of the catalytic layer which allowed omitting the often necessary chitosan layer. The selected setup further showed very promising selectivity at the given potential toward possible interfering ions. All these beneficial properties are promising for high performance nitrite sensors. However, the reasons for the unsatisfying long term stability still have to be revealed and eliminated. Furthermore, the electrochemical properties of polyoxometalates can be tuned and adjusted for the electrochemical detection of other compounds.
Acknowledgements We gratefully acknowledge funding from the Agence Nationale de la Recherche with the project CAROUCELL (ANR-13-BIOME-0003-02). The authors wish also to acknowledge the support from the LabEx ARCANE (ANR-11-LABX-0003-01), the PHC Utique program n° 14G1206 (CMCU), n° 30583QD (Campus France), and the ICMG Chemistry Nanobio Platform, Grenoble for providing facilities (PCN-ICMG).
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Figure Captions: Figure 1: Representation of the Polyhedral [PMo11O39]7− structure. Figure 2: IR spectrum of the TBA4H3[PMo11O39] salt. Figure 3: (A): CVs obtained using (a): PMo 11/CS/rGO/GC, (b): PMo11/rGO/GC, (c): PMo11/CS/GC and a zoom of (c) as inset. (B): CVs obtained using. (d): ox-SWCNTs / GC, (e): PMo11/ox-SWCNTs/GC (f): PMo11/CS/ox-SWCNTs/GC. (g): bare GC electrode Electrolyte: H2SO4 (0.1 mol L-1). Scan rate: 20 mV.s-1
Figure 4: Representative FE-SEM images of A) ox-SWCNTs, B) PMo11/ox-SWCNTs, C) rGO, and D) PMo 11/rGO.
Figure 5: A) - Current–time responses at -0.150 V with an increasing nitrite concentration for the PMo11/ox-SWCNTs/GC. B)-Cathodic catalytic current as a function of NO2 concentration recorded at the (a) PMo11/CS/GC, (b) PMo11/rGO/GC and (c) PMo11/ox-SWCNTs/GC. Chronoamperometric experiments were performed at a fixed potential of 0.1 V vs. SCE for (PMo11/CS/GC, PMo11/rGO/GC) and -0.150 V vs. SCE for PMo 11/ox-SWCNTs/GC in H2SO4 (0.1 mol L-1), pH 1 Figure 6: Amperometric response of the (Ox-SWCNTs/PMo11) to successive injections of nitrates, chlorates, and phosphates, all at concentrations of 10 -3 mol L-1, in 0.1 mol L-1 (H2SO4, Na2SO4), applied potential: -0.15 V vs SCE.
26
Figure1
Figure 2
Figure 3
Figure4
Figure 5
Figure 6
Table I. Characteristics of the prepared electrodes for NO2– detection
Modified electrodes
CS/PMo11
rGO/PMo11
rGO/CS/PMo11
ox-SWCNTs /PMo11
ox-SWCNTs / CS /PMo11
Imax (µA)
-0.21±0.032
-80.15±2.6
-64.76±2.78
-522.6±13.34
-470.8±11.32
Sensitivity (mA. L mol-1)
0.0045±0.00015
3.12±0.087
2.13±0.065
44.41±1.16
37.96±0.981
detection limit (mol.L-1)
10-6
10-6
2.5 10-6
3 10-5
1.5 10-4
linear range (mol.L-1)
2.5 10-6 - 1.6 10-3 2.5 10-6 - 1,6 10-2 2.5 10-6 - 1.8 10-3 3 10-5 - 1.6 10-2
1.5 10-4 - 1.4 10-2
RSD (%)
3.5
2.65
2.8
2.9
27
2.64
S0013-4686(16)32309-X http://dx.doi.org/doi:10.1016/j.electacta.2016.10.192 EA 28280
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
28-6-2016 27-10-2016 29-10-2016
Please cite this article as: Feriel Boussema, Raoudha Haddad, Yassine Ghandour, Mohamed Salah Belkhiria, Michael Holzinger, Abderrazak Maaref, Serge Cosnier, Polyoxometalate [PMo11O39]7−/carbon nanocomposites for sensitive amperometric detection of nitrite, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.10.192 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polyoxometalate [PMo11O39]7− / carbon nanocomposites for sensitive amperometric detection of nitrite
Feriel Boussema[a], Raoudha Haddad[b], Yassine Ghandour [c], Mohamed Salah Belkhiria[c], Michael Holzinger*[b], Abderrazak Maaref[a], and Serge Cosnier[b]
[a] Laboratoire de Physique et Chimie des Interfaces, Faculté des sciences de Monastir, 5000 Monastir, Tunisia [b] Département de Chimie Moléculaire UMR 5250, Univ. Grenoble Alpes - CNRS, 38041, Grenoble, France [c] Laboratoire de Physico-chimie des Matériaux, Université de Monastir, Faculté des Sciences, 5000 Monastir, Tunisia. Corresponding author: Michael Holzinger, e-mail: [email protected]
1
Graphical Abstract
The polyoxometalate [PMo11O39]7− confined in nanostructured carbon matrices was evaluated as catalyst for the selective electrochemical detection of nitrite ions.
Abstract The polyoxometalate [PMo11O39]7− was synthesized and used as catalyst for the reduction of nitrite ions. This mono-lacunary keggin anion, cited as PMo 11, was confined within a matrix of either oxidized single-walled carbon nanotubes (ox-SWCNTs) or reduced graphene oxide (rGO) on glassy carbon (GC) electrodes to improve the electron transfers. Five different configurations were characterized using cyclic voltammetry and evaluated in an amperometric sensor setup for nitrite detection where the amount of the different components were optimized. The configuration using only ox-SWCNTs and PMo11 on GCE showed best sensitivities of up to 44.41 mA Lmol-1 and a satisfying reproducibility (RSD: ∼2.64 % for four identic electrodes). Furthermore, a linear range between 3.0 10-5 molL-1 and 1.6 10-2 molL-1 with a detection limit of 3.0 10-5 molL-1 was obtained. The final sensor setup also showed good selectivity towards interfering ions. Keywords: Nitrite sensor; Single-walled Carbon Nanotubes; Reduced
2
Graphene Oxide; Monolacunary Keggin anion; polyoxometalate 1. Introduction
Nitrite is found in soil, water, food and physiological systems [1] and its determination is very important due to its role in environmental processes and biomarkers, but also to its toxicity for humans at higher concentrations [2-4]. One danger after nitrite ingestion is that it converts oxyhemoglobin to methemoglobin in the bloods stream, thereby interfering with oxygen transport [5]. The elaboration of sensors with high sensitivities to nitrite is an important challenge for medical and environmental applications [6, 7]. Many methods have been developed for nitrite determination including spectrophotometry [8, 9], chromatography [10, 11] and electrochemical methods [12-14]. However, spectrophotometry and chromatography methods require expensive equipment, which are not suitable for the development of portable devices. The electroreduction of nitrite ion requires high overpotentials on most electrode surfaces. Therefore, enzyme electrodes were developed for the specific catalytic reduction of nitrites. However, although, electrochemical nitrite biosensors are extremely sensitive, they suffer from a low stability due to the use of proteins, are pH dependent and most of them use viologen derivatives for the enzyme wiring and hence must be used under argon atmosphere [15-18]. Another alternative consists in the development of chemically modified electrodes with suitable catalyst immobilized on conventional electrode materials, decreasing the overpotential for nitrite detection [19, 20]. Polyoxometalates (POMs), a class of structurally well-defined anionic clusters with an enormous disparity in size, composition, and function, attract more and more attention. Its inherent metal-oxygen framework can possess reversible and gradual multi-electron reactions [21-23]. This class of inorganic compounds is also resistant to oxidative degradation due to 3
the fact that their fundamental elements are already in high oxidation states [24]. These properties make them very attractive in many fields such as analytical chemistry, catalysis and medicine [24-26]. For these reasons, POMs became appealing candidates for electrocatalytic applications [27]. In particular, the development of nitrite sensors using POMs became a vast research field where excellent performances in terms of sensitivity, detection limits and selectivity could be obtained [28-32]. Practical applications of POMs for the preparation of chemically modified electrodes depend on the successful immobilization of these compounds. Efficient immobilization of POMs on electrodes simplifies their electrochemical studies and facilitates their applications in electroanalysis. So far, many strategies have been developed to prepare chemically modified electrodes with a variety of POMs on common solid electrode substrates such as electrochemical deposition [20, 33], adsorption [34, 35] and entrapment in polymer matrices [36, 37]. Entrapping in a positively charged polymer matrix on the electrode surface using i.e. chitosan (CS) or polyallylamines provides strong interaction via electrostatic binding between the cationic polymer and the heteropolyanions but is also accompanied with negative effects such as an insulating effect of the polymer. Over the past few years, carbon materials have widely been used as suitable matrices owing to their good chemical stability and strong affinity for POMs. Various carbon-based materials such as carbon nanotubes [38, 39] or graphene oxide [40, 41] have been used as support materials to obtain conductive POM composites. Graphene has received considerable attention thanks to its high surface area and excellent conducting nature [42]. In the field of electrochemistry, it is suggested that graphene is very suitable to be used as the support material for the manufacture of POM-based electrode to improve electrocatalysis [29, 41].
4
Recently, Weihua Guo et al [43] have developed a composite film modified electrode based on electrochemically reduced graphene oxide (rGO) and Keggin-type heteropolymolybdate H4SiMo12O40 (SiMo12) for the detection of nitrite. The composite film was prepared by electrochemical reduction of GO on an ITO electrode. The developed electrochemical nitrite sensor showed a linear range between 33.3–632.7 μmol.L−1 with a detection limit of 7.73 μmol.L−1, and a sensitivity of 0.651 mA.Lmol−1. These competitive performances make POM/graphene nanocomposite films quite promising candidates for the design of electrochemical sensors. Single-wall carbon nanotubes (SWNTs) also showed their appropriateness to form stable assemblies with POMs for electrochemical applications [44]. Weihua Guo et al. [45] reported a SWCNT- SiMo12 modified electrode for the detection of nitrite but did not quantify the performances of the nitrite sensor. Here, we evaluated the slightly different kegging-type polyoxometalate [PMo 11O39]7− using five different configurations of nanocarbon / PMo 11 assemblies with and without intermittent CS layers for the amperometric detection of nitrite. We have chosen ox-SWCNTs and rGO as conductive matrices for optimized electron transfer between the electrocatalytic PMo 11 and the electrode surface. Even when the intrinsic resistance of ox-SWCNTs [46] and rGO [47] is clearly increased compared to their unmodified counterparts, the surface defects of these materials provide lower intertube [48] or intersheet [49] contact resistances in bulk samples and should therefore provide excellent conditions for enhanced electron transfers. 2. Experimental 2.1.
Reagents and apparatus. Low molecular weight chitosan (75-85% deacetylated, 50-
190 kDa), sulfuric acid (H2SO4, ACS reagent), acetic acid (reagent grade), sodium nitrite (NaNO2, (reagent grade)), were purchased from Sigma-Aldrich and used without further 5
purification. Distilled ultra-pure water (18.2 Ω cm resistivity) was produced using an ELGA Purelab flex. rGO was purchased from NanoInnova Technologies and SWCNTs in purified grade from Unidym TM and oxidized with 69% nitric acid (ACS reagent, 10.8 mol L-1) by stirring under reflux at 110°C for 2h. The reaction mixture was then filtered over a cellulose membrane filter and washed several times with distilled water. The solid was re-dispersed in distilled water and filtered again to remove entrapped or intercalated acid. Further washing steps were carried out with distilled water until the pH of the filtrate becomes neutral. The resulting black solid was re-dissolved in slightly basic sodium carbonate (Na2CO3) water (pH 9) to disperse homogeneously oxidized nanotubes and to dissolve a large amount of the formed amorphous carbon around the nanotubes. Not dispersed material was allowed to deposit. The supernatant was decanted off and filtered over regenerated cellulose. The resulting oxidized SWCNTs were washed with distilled water and then dried in vacuum. Tetrabutylammonium (TBA) salts of the α-Keggin phosphomolybdates [(C4H9)4N)4H3] [PMo11O39] were prepared based on the procedure described in literature [50]. Briefly, Phosphomolybdic acid (H3PMo12O40) (4 mmol) was dissolved in 40 ml of distilled water. The pH of the solution was adjusted to 4.3 with Li2 CO3 and a precipitate was formed after adding solid [(C4H9)4N)4] Br (56 mmol) to the solution under vigorous stirring. The crude product was collected on a medium frit, washed with distilled H2O and allowed to dry in air overnight at room temperature. Crystallization under nonprotic condition involved dissolving the crude [(C4H9)4N)4H3] [PMo11O39] in 200 ml CH3CN by stirring at room temperature followed by a slow evaporation of the solvent under ambient conditions . Electrochemical measurements were performed in an electrochemical cell with a conventional three-electrode configuration by using an Autolab pgstat100 potentiostat. Before each experiment, the surface of the glassy carbon (GC) electrode was polished with 2 mm diameter 6
diamond paste (MECAPREX Press PM) and was then rinsed with ultra-pure water to remove any residual diamond paste. Finally, the electrodes were sonicated for 5 min in ethanol and 5 min in acetone followed by a thorough washing step with distilled water and ethanol. A GC working electrode with a diameter of 3 mm was used together with a platinum wire as counter-electrode and a saturated calomel reference electrode (SCE). IR spectra were recorded using a Nicolet 470 FT-IR spectrophotometer with pressed KBr pellets. FE-SEM images were recorded using a FEI QUANTA-FEG 250 microscope with an Everhardt Thornley SED (secondary electron detector), an accelerating voltage of 3 kV and a working distance of 8.0 mm.
2.2.
Characterization of tetrabutylammonium salts of the α-Keggin
phosphomolybdates. Within all available types of polyoxometalates [33], the Keggin-type POM [PMo11O39]7− (PMo11) was used for these studies. It has a quite well-known structure which is composed of a tetrahedrally coordinated central hetero atom surrounded by four Mo3O13 groups (triads) that are connected in a corner-sharing fashion via oxygen linkers [51] as represented in Figure 1. The Lacunary Keggin structures (PMo 11), used in this study, are derived from the parent Keggin type by the removal of specific MoO 6 moieties, followed by an optional rotation of the remaining MoO6 octahedra [25]. As a consequence, (PMo11) is characterized by the presence of an enhanced amount of terminal oxygen atoms, which thus provides more negative surface charges than other POM structures. Figure 1. The prepared compound shows in the IR spectra characteristic vibration bands (Figure 2) of the α-Keggin type structure: (P-Oa), υ(Mo-Od), υ(Mo-Ob-Mo) and υ(Mo-Oc-Mo) at 1051 cm-1; 945 cm-1; 883 cm-1 and 794 cm-1, respectively. In addition, the bands at 2965 cm-1 and 1480 cm-1 are assigned to υ(CH3) asymmetric stretching of the tetrabutylammonium (TBA) counter ions. 7
Figure 2.
2.3.
Preparation of different POM assembly based catalytic electrodes PMo11/CS/GCE: 12 mg chitosan (CS) was dissolved in 10 mL of 0.1M acetic acid.
Then, 6 µL of the CS solution was dropped on the cleaned GC. 10 µL [(C4H9)4N)4H3] [PMo11O39] (2 mmol L-1 in acetonitrile) was added on the surface of the CS modified electrode (CS/GC) by drop-casting where the solvent was removed under vacuum and the electrode was then washed with acetonitrile (ACN). PMo11/ox-SWCNT/ GCE: A dispersion of oxidized single walled carbon nanotubes (ox-SWCNTs) in DMF were prepared by a 30 min sonication of 5 mg of carbon nanotubes in 1 mL of DMF until a homogenous black suspension was obtained. Then, 20 µL of the oxSWCNTs solution were drop-casted on a GC electrode and the solvent was removed under vacuum. The PMo11 layer was formed by casting 10µL of the [(C4H9)4N)4H3] [PMo11O39] solution on the ox-SWCNTs deposit followed by the evaporation of the solvent under vacuum. PMo11/rGO/ GCE: Dispersions of reduced graphene oxide in DMF were prepared by a 30 min sonication of 5 mg of reduced graphene oxide in 1 mL of DMF until a homogenous black suspension was obtained. rGO solution (20 μl) was drop casted onto the pre-cleaned GCE. The electrodes were then modified with a PMo11 layer as described for the PMo11/oxSWCNTs/GC electrodes. The fabrication of the PMo11/CS/rGO/GC and PMo11/CS/ox-SWCNT/GC electrodes was similar to that of the two preceding electrodes where an additional CS layer was formed between the carbon and PMo 11 layer.
8
3. Results and discussion 3.1.
Electrochemical behavior of the different POM based electrodes
The cyclic voltammetric responses of PMo 11/rGO, PMo11/CS/rGO, PMo11/CS, PMo11/ oxSWCNTs and PMo11/CS/ox-SWCNTs modified GCE were examined in 0.1 mol L -1 H2SO4 at pH 1. Figure 3A illustrates the cyclic voltammograms (CVs) of PMo 11/rGO, PMo11/CS/rGO, PMo11/CS modified GC. In the potential range between 0.4 and -0.1 V (vs. SCE), the CV curves exhibit two redox waves which show a two-step reduction processes for PMo 11. The redox couples I–I’ at E1/2 = 0.23 V vs SCE and II–II’ at E1/2 = 0.12 V vs SCE are assigned to two consecutive processes of Mo centers within PMo 11 (MoVI MoV) [52]. Referring to a similar Mono-Lacunary Keggin type anion [32], these reversible redox systems can be attributed to following equations: PMo11VI O397- + 2e- + 2H+ H2PMo2V Mo9VI O397- (I’)
(1)
H2PMo2V Mo9VI O397- + 2e- + 2H+ H4PMo4V Mo7VIO397- (II’).
(2)
In comparison with the PMo 11/CS/rGO and PMo11/CS electrodes (curve b and c respectively), a significant current increase is observed for PMo 11/rGO modified electrodes (curve a). Furthermore, the intermittent CS layer, necessary for PMo 11 adhesion on GC electrodes, led to a slight reduction of the peak current intensities compared to pure rGO deposits. The higher currents and the shape of the CVs of the PMo11/rGO setup are mainly due to an enhanced capacitance and surface area of rGO, and to an improved electron transfer of the electrode to PMo11 [29, 53, 54].
9
Using ox-SWCNTs instead of rGO under identic conditions, the electrochemistry of the formed composites clearly changed (Figure 3-B, curve e, f). Further waves were recorded and the measured currents increased in average by factor ten. Concerning the additional redox couples, a shoulder, indicated as system III–III’, can be seen in the curves e and f at 0.41 V vs SCE for the anodic scan and at 0.32 V for the cathodic scan which is attributed to the redox active carbon oxide species on ox-SWCNTs [55] (curve d). The redox waves at E1/2 = 0.23 V and at E1/2 = 0.12 V are the two processes of the couples I– I’ and II-II’ of equations (1) and (2). The redox system at E1/2 = -0.15 V, labeled as peaks IVIV’, might be a further redox systems following equation (3): H4PMo4V Mo7VIO397- + 2e- + 2H+ H6PMo6V Mo5VIO397- (IV’)
(3)
A further small redox peak (labeled as * and *’) is observed at E 1/2 = 0.07 V and corresponds to an intermediate redox process IV-IV’ of PMo11 in close proximity to the ox-SWCNT surface. This is in line with previous electrochemical studies of polyoxometalate composites [56, 57]. The fact that this third redox wave could not be recorded for the PMo 11/CS and PMo11/rGO samples is due to the fact that it is covered by a large irreversible reduction peak at potentials < -0.05 V vs SCE which prevented us to scan a larger potential window. This irreversible reduction appears for the ox-SWCNT/ PMo11 composites at lower potentials (around -0.3 V vs SCE). We attribute this to the reduction of protons [58] where the formed hydrogen is partly dissolved in solution and partly adsorbed at the electrode. The adsorbed hydrogen is then re-oxidized to protons at the cathodic scan at -0.25 V (marked with °). The current increase is to one extent due to the capacity of the composite. As shown in Figure 3B, curve d, pure ox-SWCNTs show clearly lower capacitances than assembled with PMo 11.
10
This synergetic effect of polyoxometalates and SWCNTs also opens the possible application of such composites as supercapacitors [59]. To the other extent, ox-SWCNTs provide improved bulk conductivity [60] leading to improved electron transfers compared to rGO. In addition, we could observe a decrease in the current response for the electrodes with a CS layer (PMo11/CS/ox-SWCNTs/GC and PMo 11/CS/rGO/GC), which indicates that the electron transfer rate is hindered by the CS layer. It further seems that PMo 11 is well adsorbed on rGO and ox-SWCNTs layers even when the carbon oxide functions on this material provide a negatively charged environment such as PMo11. The phenomenon that negatively charged polyoxometalates and negatively charged oxidized carbon materials form stable layers was studied by Schwegler et al. [61] using oxidized activated carbon. He showed that the hetero polyanion clusters interact with oxygen containing functions such as hydroxyls, carbonyls, and ether groups via acid-base interactions thus forming hydrogen bonds between protonated and anionic oxygen. Similar results were obtained by Choi et al. [62] studying the adsorption properties of different polyoxometalates on HOPG and glassy carbon electrodes and Kang et al. [63] even could attach polyoxometalate nanoparticles on oxidized CNTs just by using ultrasound. In summary, the incorporation of rGO or ox-SWCNTs in the PMo11 film not only improves the conductivity of the film and enhances the electron transfer, but also avoids the necessity of an additional CS layer. It can further be concluded that these advantages are much more pronounced for ox-SWCNTs than for rGO. Figure 3 3.2.
Morphological studies of the composites.
11
To get more insight in the observed electrochemical behavior of the ox-SWCNTs/PMo11 and rGO/PMo11 composites, we performed morphological studies using SEM. Figure 4 shows representative SEM images of ox-SWCNT and rGO before and after modification with PMo11. As produced ox-SWCNTs appear as solid as undulating aggregates due to defect group related agglomeration and stacking behavior [64] (Figure 4A). After adsorption of the PMo11 layer (figure 4B), it seems that the polyoxometalate crystallizes on the ox-SWCNT forming branched sheet structures around the nanotube aggregates of about 1 µm thickness. Some more cubic structures can also be seen on the right side of figure 4B. We assume that crystallization is induced after adsorption of small clusters or nanoparticles as proposed by Chen et al. [59] and continues to grow during time encasing the ox-SWCNT shapes until the formation of 3D crystals. We also observed large vacancies where no PMo 11 is adsorbed on ox-SWCNT. This inhomogeneous coating can be explained by the irregular density of carboxylates and other defect groups on ox-SWCNTs [65] which prevent a complete coverage.
For the rGO samples, the SEM investigations are more consistent. Figure 4C shows the used rGO samples before the deposition of PMo 11. Untreated rGO deposits show laminated structures which a typical for such materials. After adsorption of PMo 11, these rGO structures are homogeneously coated filling the interstices of the flakes as shown in Figure 4D. Obviously, the absence of a dense amount of negative charges on the carbon surface allows the polyanion to form a uniform layer around this material where no bigger aggregates or crystals were observed. Figure 4
3.3.
Evaluation of the PMo11 composites for nitrite reduction 12
In order to evaluate the appropriateness of PMo 11 in its rGO and ox-SWCNT composites for the amperometric detection of nitrite, the fabricated electrodes were tested under sensing conditions in presence of nitrites at different concentrations. For the rGO based PMo 11 electrodes, the optimized potential for the electrocatalytic reduction of NO2- was determined to be 0.10 V vs. SCE, while for the ox-SWCNTs based electrodes the potential of -0.15 V vs. SCE was chosen according the third reduction wave (equation (3)) of PMo11 in the composite since it provided the highest catalytic activity [56]. The catalytic reduction mechanism of nitrite to NO using polyoxometalates at low pH values was intensively studied and validated for different Keggin- or Dawson-type polyanions [32, 66]. As NO2- was added to the stirred 0.1 molL-1 H2SO4 solution, a well-defined steady-state current response was obtained at the applied potential. The current decreased along with every successive addition of NO2- (Figure 5A). At high NO2- concentrations, the noise of the amperometric signal increases and the current stabilization takes longer and longer until no straight line can be obtained anymore. The details about the origin of such parasitic current still have to be revealed but reliable measurements could be done until a total nitrite concentration of 16 mmolL-1. Figure 5B shows the current increase as a function of nitrite concentration. The sensitivity, determined by the slope of the calibration curve, is much higher for the setup using the PMo 11/ox-SWCNTs composite than for the PMo 11/rGO electrodes where both performances largely overpass the one obtained for the PMo 11/CS setup. The characteristics of the different sensor configurations for nitrite detection are summarized in table 1 and confirm the superiority of the PMo 11/ox-SWCNTs system. Figure 5 Compared to literature [19], the obtained performances with a sensitivity of 44.41 mA. Lmol-1 (555 mA. Lmol-1 cm-2), a linear range of 3.0 10-5-1.6 10-2 molL-1 and a detection limit of 3.0 10-5 molL-1 are within the best values reported for nitrite detection [30, 67, 68] using
13
polyoxometalates as sensitive layer and is also competitive with the performances of some enzymatic nitrite biosensors [69-71]. Moreover, the sensor showed the relative standard deviation (RSD) of ∼2.64% for four different electrodes prepared under the same technique. The results show that the ox-SWCNTs/PMo11 assembly may be an excellent platform for the electrochemical detection of nitrite. It can be concluded that small molecules have a rapid diffusion from the solution to the nanohybrid modified electrode and have a better electron transfer rate using ox-SWCNTs than rGO. However, considering long term stability, the ox-SWCNT/PMo11 setup showed only 25% of its initial performances after one month. Further studies will reveal the origins of such performance loss. One reason might be the mechanic stability of the PMo 11 layer on the ox-SWCNT deposit. Beside the simple loss of the sensing layer, a slight desorption also provokes reduced electron transfers even when the PMo11 layer remains.
3.4.
Selectivity.
Nonetheless, the sensor setup giving the best performance (PMo 11/ox-SWCNTs/GCE) was also examined for its selectivity towards classic interfering ions such as NO 3−, ClO4 -, and PO43– in 0.1 mol L-1 (H2SO4) (pH 1) solution (Figure 6). No significant responses were observed for each addition of NO3−, ClO4 - , PO43- , but a notable amperometric response can be recorded when nitrite was added to the same solution. This result further confirms the beneficial properties of PMo11/ox-SWCNTs composites for high sensitive detection of nitrites in complex media containing nitrates, chlorates and phosphates. Figure 6 4. Conclusion. A high performant electrochemical nitrite sensor was developed using the polyoxometalate [PMo11O39]7− as electrocatalyst. To establish an efficient electron transfers between the catalyst and the external circuit, a layer of oxidized ox-SWCNTs 14
showed to be the best material since it also ensured a satisfying stability of the catalytic layer which allowed omitting the often necessary chitosan layer. The selected setup further showed very promising selectivity at the given potential toward possible interfering ions. All these beneficial properties are promising for high performance nitrite sensors. However, the reasons for the unsatisfying long term stability still have to be revealed and eliminated. Furthermore, the electrochemical properties of polyoxometalates can be tuned and adjusted for the electrochemical detection of other compounds.
Acknowledgements We gratefully acknowledge funding from the Agence Nationale de la Recherche with the project CAROUCELL (ANR-13-BIOME-0003-02). The authors wish also to acknowledge the support from the LabEx ARCANE (ANR-11-LABX-0003-01), the PHC Utique program n° 14G1206 (CMCU), n° 30583QD (Campus France), and the ICMG Chemistry Nanobio Platform, Grenoble for providing facilities (PCN-ICMG).
15
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Figure Captions: Figure 1: Representation of the Polyhedral [PMo11O39]7− structure. Figure 2: IR spectrum of the TBA4H3[PMo11O39] salt. Figure 3: (A): CVs obtained using (a): PMo 11/CS/rGO/GC, (b): PMo11/rGO/GC, (c): PMo11/CS/GC and a zoom of (c) as inset. (B): CVs obtained using. (d): ox-SWCNTs / GC, (e): PMo11/ox-SWCNTs/GC (f): PMo11/CS/ox-SWCNTs/GC. (g): bare GC electrode Electrolyte: H2SO4 (0.1 mol L-1). Scan rate: 20 mV.s-1
Figure 4: Representative FE-SEM images of A) ox-SWCNTs, B) PMo11/ox-SWCNTs, C) rGO, and D) PMo 11/rGO.
Figure 5: A) - Current–time responses at -0.150 V with an increasing nitrite concentration for the PMo11/ox-SWCNTs/GC. B)-Cathodic catalytic current as a function of NO2 concentration recorded at the (a) PMo11/CS/GC, (b) PMo11/rGO/GC and (c) PMo11/ox-SWCNTs/GC. Chronoamperometric experiments were performed at a fixed potential of 0.1 V vs. SCE for (PMo11/CS/GC, PMo11/rGO/GC) and -0.150 V vs. SCE for PMo 11/ox-SWCNTs/GC in H2SO4 (0.1 mol L-1), pH 1 Figure 6: Amperometric response of the (Ox-SWCNTs/PMo11) to successive injections of nitrates, chlorates, and phosphates, all at concentrations of 10 -3 mol L-1, in 0.1 mol L-1 (H2SO4, Na2SO4), applied potential: -0.15 V vs SCE.
26
Figure1
Figure 2
Figure 3
Figure4
Figure 5
Figure 6
Table I. Characteristics of the prepared electrodes for NO2– detection
Modified electrodes
CS/PMo11
rGO/PMo11
rGO/CS/PMo11
ox-SWCNTs /PMo11
ox-SWCNTs / CS /PMo11
Imax (µA)
-0.21±0.032
-80.15±2.6
-64.76±2.78
-522.6±13.34
-470.8±11.32
Sensitivity (mA. L mol-1)
0.0045±0.00015
3.12±0.087
2.13±0.065
44.41±1.16
37.96±0.981
detection limit (mol.L-1)
10-6
10-6
2.5 10-6
3 10-5
1.5 10-4
linear range (mol.L-1)
2.5 10-6 - 1.6 10-3 2.5 10-6 - 1,6 10-2 2.5 10-6 - 1.8 10-3 3 10-5 - 1.6 10-2
1.5 10-4 - 1.4 10-2
RSD (%)
3.5
2.65
2.8
2.9
27
2.64