Voltammetric detection of organohalides by redox catalysis: improved sensitivity by immobilisation within a cubic phase liquid crystal

Abstract

Two combined strategies are reported for improving the sensitivity of organohalide detection by redox catalysis. These are, improvement of the second order rate constant (k) for catalytic reduction of the organohalide, and improvement of the rate of substrate diffusion. Values of k are calculated for both alkyl and aryl halides, from slow scan rate cyclic voltammograms in homogeneous solution. It is shown that a Zn(II) porphyrin exhibits higher catalytic rates than the previously used Co(II) porphyrin or Co(I) salen. Amperometric and rotating disk electrode studies of electropolymerised films of the Zn(II) porphyrin, reveal that at optimum thickness, mediator–substrate reaction and substrate diffusion are the rate limiting steps. Hence, immobilisation of the Zn(II) porphyrin within the more open structure of a cubic phase liquid crystal produces an increase in sensitivity of approx. 10 times, and lowers the limit of detection by one order of magnitude. The optimised sensor responds linearly to seven organohalides in the range 0.1 µM to 1.0 µM with a sensitivity of 6.95 A M⁻¹ cm⁻². Chronoamperometric experiments with a microdisk electrode show that the rate of charge transport in the cubic phase films (apparent diffusion coefficient, D_E = 5.65 × 10⁻¹⁰ ± 0.11 × 10⁻¹⁰ cm² s⁻¹) is faster than in the electropolymerised films (D_E = 3.64 × 10⁻¹¹ ± 0.02 × 10⁻¹¹ cm² s⁻¹). The variation of D_E with the concentration of Zn(II) in the cubic phase suggests that diffusion of charge is predominantly by electron self-exchange, rather than by physical movement.

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Introduction

Organohalides are an important source of water pollution, due to their use both as biocides in agriculture and as solvents in a number of chemical industries. Hence, there is a need for organohalide monitoring of environmental samples, to prevent long-term toxic effects. The traditional form of this monitoring has been either liquid¹ or gas chromatography.² However, a drawback to these methods is that they require expensive equipment and are not suitable for field use. For this reason, there has been interest in the detection of organohalides by electrochemistry, as this is one possible route to simple hand-held sensors. Reported methods have involved potentiometric detection following organohalide breakdown by microorganisms,^3,4 impedance-based detection of sorption into a conducting polymer,⁵ and voltammetric detection by accumulation in a sol-gel,⁶ as part of a collector-generator process,⁷ or by redox catalysis with an immobilised metal complex.^8–14 This last method has been the most intensively studied technique. The reaction between metalloporphyrins and organohalide compounds is well-established,^15–18 and in the case of Co(II) and Fe(III) porphyrins, has been applied to sensor construction, by drop coating porphyrin solutions onto glassy carbon⁹ or graphite foil electrodes,⁸ or by electropolymerisation of standard porphyrins¹⁰ or a Co(II) porphyrin-modified pyrrole monomer.¹² The Fe(II/III) couple present in hemoglobin and myoglobin has also been used for redox catalysis of organohalide reduction, following incorporation into surfactant films.^19–22

Electropolymerisation is a promising immobilisation method, as it allows the relatively reproducible deposition of controlled amounts of film. However, when considering electrode modifications, attention must also be paid to the resulting sensor characteristics, in terms of sensitivity and linear range. Sensitivity is especially important in the case of organohalide detection, as the EPA guidelines for many organohalides are at the ppb level.²³

The sensitivity of a voltammetric sensor may be controlled by one or more of: film partitioning, rates of diffusion of substrate, product and charge, rate of catalytic reaction, and rate of electrode reaction, depending on their relative magnitudes. Hence, the elucidation of rate limiting step(s) in relation to a particular electrode modification, is an important aid in constructing sensors of required sensitivity. To the best of our knowledge, this has not yet been performed for any of the reported organohalide sensors. Based on such a process, in this paper we wish to report two combined strategies for improving the sensitivity of organohalide detection by redox catalysis. The first technique is to improve the rate of organohalide–catalyst reaction by choice of the redox couple. The second technique is to improve the rate of substrate diffusion by use of a cubic phase liquid crystal, as has previously been applied to bio²⁴ and chemical sensors^25,26 for carbon dioxide.

Experimental

Materials

All chemicals were provided by Aldrich unless otherwise stated. Organohalides were of analytical grade. Monoolein (MO, 1-monooleoyl-rac-glycerol, 99%), cobalt 5,10,15,20-tetraphenyl porphyrin (CoTPP), N,N′-bis(salicylidene)ethylene-diamino cobalt(II) (cobalt(II) salen) and zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine tetrakis(methochloride) (ZnTPP) were all used as received. Acetonitrile (CH₃CN) and dimethyl formamide (DMF) were of HPLC grade. Tetrabutylammonium toluene-4-suphonate (TBATS) was electrochemical grade and was purchased from Fluka.

Apparatus

Electrochemical experiments were performed with an Autolab PGSTAT 10 (Eco Chemie) using GPES software version 4.3, with a three-electrode cell. Potentials are quoted relative to the Ag/AgCl reference (BAS). The working electrode was either planar glassy carbon (3 mm diameter) or a carbon microdisk electrode (7 µm diameter), both from BAS. The counter electrode was a platinum disk (5 mm diameter). Before each experiment, the working electrode was polished with an alumina–water slurry on cotton wool and then sonicated in distilled water. The solution in the electrochemical cell was deaerated by purging with nitrogen.

Procedures

All electrochemical experiments used 0.1 M TBATS as supporting electrolyte. Electropolymerisation was performed in DMF containing the relevant metal complex at a concentration of 2 mM. All other experiments were performed in a 3 ∶ 2 mixture of acetonitrile ∶ water. The potential was cycled from −1.4 V to 0 V at 0.5 V s⁻¹ for typically 1 to 40 scans. The cubic phase was prepared by mixing 63.6 mg of monoolein, 35.4 ml of water and 0.5 mg to 1.0 mg of Zn porphyrin, followed by centrifugation for 30 min at 3000 × g. This ratio of components was chosen on the basis of phase diagrams of the monoolein–water system.²⁷ After centrifugation, a transparent and highly viscous phase could be observed. A sample of this was spread on the electrode under a microscope (magnification × 100) using a spatula. The electrode was weighed before and after application to determine the exact amount of cubic phase transferred.

Results and discussion

Reaction in homogeneous solution

The overall electrochemical reduction of an organohalide can be written as

R–X + H⁺ + 2e⁻ → R–H + X⁻ (1)

although the mechanism of the overall reaction is different for aryl and alkyl halides, e.g. for aryl halides a radical anion (ArX˙⁻) is formed and then undergoes cleavage to give the halide ion,²⁸ while for alkyl halides there is no such intermediate.²⁹ When the reaction is catalysed by a metal complex, that complex is first reduced at the electrode, and then proceeds to be oxidised by the organohalide in a series of steps, often involving inner sphere electron transfers, following which, the reduced form of the complex is electro-regenerated. Hence, the process can be followed by slow scan rate cyclic voltammetry. Fig. 1 shows voltammograms (2 mV s⁻¹) of ZnTPP, CoTPP and Co salen, each present in solution at 2 mM, in the presence and absence of 120 mM CCl₄. The same concentration of CCl₄ did not significantly react at bare glassy carbon. Therefore the increased reduction peaks in the presence of organohalide can be attributed to catalytic reduction by the metal complex. The same experiments were performed using CHCl₃, chlorobenzene and chloronapthalene as the analyte. In each case, it was ensured that organohalide was present at a saturating concentration relative to that of the metal complex, i.e. that increasing the organohalide concentration from 100 mM to 120 mM did not increase the peak height. Hence, the ratio of the intensity of the cathodic peak in the absence (i_d) and presence (i_k) of analyte (i_k ∶ i_d) could be used to determine the second order rate constant of the reaction, based on the theory of Nicholson and Shain,³⁰ having replotted the working curve in Fig. 14 of ref. 30, using the data in Table XII of this reference. The results are given in Table 1. It should be noted that these are multi-step reactions, in which the value of k quoted will correspond to the rate determining step. It can be seen that the reactivity of the metal complexes to organohalide is in the order ZnTPP > Co salen > CoTPP, and that the reactivity of the analytes is in the order CCl₄ > CHCl₃ > chlorobenzene > chloronapthalene. The range of values for Co salen (2.1 to 0.5 × 10⁵ mol⁻¹ cm³ s⁻¹) for the organohalides here can be compared with considerably higher values of k for the reaction with iodoethane (5 × 10⁹ mol⁻¹ cm³ s⁻¹),³¹n-butyl bromide (2.1 × 10⁶ mol⁻¹ cm³ s⁻¹)³² or n-dodecyl bromide (7.91 × 10⁶ mol⁻¹ cm³ s⁻¹),³² as determined by rotating disk electrode,³¹ or digital simulation of cyclic voltammograms.³² This trend seems reasonable, since the ease of fission of the carbon–halogen bond will be directly proportional to bond length, and will therefore decrease in the order C–I > C–Br > C–Cl. Also, aryl halides are less reactive than alkyl halides, due to the halogen-holding carbon being sp²-hybridised in the aryl case and thus bonded to the halogen by a shorter and stronger bond than in the sp³-hybridised alkyl case.³³

Fig. 1 Cyclic voltammograms of 2 mM CoTPP, Co salen and ZnTPP in a 3 ∶ 2 mixture of acetonitrile ∶ water containing 0.1 M TBATS in the absence (dashed line) and presence (solid line) of 120 mM CCl₄. Scan rate = 2 mV s⁻¹.

Table 1 Second order rate constants for the catalytic reduction of some organohalides, determined by cyclic voltammetry in homogeneous solution (conditions as given in Fig. 1) using the method of Nicholson and Shain³⁰

Redox catalyst	k × 10^−5/mol^−1 cm^3 s^−1
	CCl4	CHCl3	Chlorobenzene	Chloronapthalene
a Cobalt 5,10,15,20-tetraphenyl porphyrin. b N,N′-Bis(salicylidene)ethylene-diamino cobalt(II). c Zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine tetrakis(methochloride).
CoTPP^a	1.1	1.0	0.7	0.2
Co salen^b	2.1	1.3	1.0	0.5
ZnTPP^c	2.7	2.4	1.5	1.3

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Reaction at electropolymerised films

Fig. 2 shows the amperometric response to 2 mM CCl₄ at −1.2 V at a series of electropolymerized films of ZnTPP, CoTPP and Co salen, where the thickness of the deposited film was varied by changing the number of scans. It can be seen that there is some increase in current for a given CCl₄ concentration, following the same trend as the rate constant values, i.e. ZnTPP > Co salen > CoTPP. Hence, this is due partly to the difference in reaction rate.

Fig. 2 Amperometric reduction current recorded at −1.2 V to 2 mM CCl₄ at films of electropolymerised CoTPP (●), Co salen (○) and ZnTPP (▼) of varying thickness. Inset: reciprocal plot of an amperometric response at RDE (−1.2 V) modified by ZnTPP (15 polymerisation scans, Γ = 2.51 × 10⁻⁹ mol cm⁻²) to 1 mM CCl₄ at varying rotation rates.

All three metal complexes show the same variation of response with thickness: the response initially increases linearly with thickness and then becomes thickness independent at deposition by ≥15 scans. In the case of ZnTPP, from integration of the current under the cathodic peak, the loading at 15 scans represents a coverage of 2.1 × 10⁻⁹ mol cm⁻² of monomer units.

In the region in which the organohalide current increases linearly with thickness, the analyte must be reacting throughout the film. For the thickness-independent response, the reaction must be localised at either the electrode–film interface (with diffusion of charge, and k as the rate controlling factor), or the film–solution interface (with diffusion of substrate, and k as the rate controlling factor). The kinetic modeling of redox-modified electrodes has been considered in detail by Andrieux et al.³⁴ They have shown that the two previous cases can be distinguished by the change in the response with varying rates of convection. If substrate diffusion (through the thin layer of film at the solution interface) is the slowest rate of transport (denoted the ‘SR’ case) then when using a rotating disk electrode, a plot of reciprocal current vs. reciprocal (rotation rate, ω)^1/2 should be linear. However, if charge diffusion (at the thin layer of film at the electrode interface) is the slowest transport (denoted the ‘ER’ case) then the same plot should show the current to be independent of rotation rate. An example of this type of plot for a ZnTPP electropolymerised film of optimum thickness (Γ = 2.5 × 10⁻⁹ mol cm⁻²) is shown in the inset to Fig. 2. Since there is a linear relationship between 1/I and 1/ω^1/2, we may conclude the response is of the ‘SR’ type. Hence, the reaction is located at the film–solution interface. An amperometric calibration to CCl₄ at −1.2 V, using a film of this type is given in Fig. 3. The response varies with CCl₄ concentration in the approx. range 1.0 µM to 40.0 µM, with a linear range of 1.0 µM to 10.0 µM and a sensitivity of 0.706 A M⁻¹ cm⁻². As this electrode operates in the SR region, substrate diffusion/partition and k are the parameters controlling the current response. Therefore, to improve the sensitivity for a given metal complex (i.e. a given value of k), we must seek to manipulate substrate diffusion/partition. This is considered below.

Fig. 3 Amperometric calibration of CCl₄ at −1.2 V at electropolymerised ZnTPP. Inset: linear range shown with expanded scale. Sensitivity = 0.706 A M⁻¹ cm⁻².

Reaction at cubic phase liquid crystal

Lyotropic liquid crystals are formed by solvents containing amphiphilic molecules which, under particular conditions of temperature and composition, orient themselves in a manner that creates long range structure. Of the three possible lyotropic phases—lamellar, hexagonal and cubic—the cubic phase (micelles ordered in a cubic arrangement) is the most viscous, and as demonstrated by Rowinski et al., this viscosity can enable stable immobilisation on an electrode surface.^24–26 Since the structure comprises of oil-in-water micelles in a cubic formation, a hydrophobic mediator can be co-immobilised by inclusion within the micelle centres.^25,26 In the case of the monoolein (MO) cubic phase, Rowinski et al.³⁵ have shown that hydrophilic compounds can move through this electrode coating with diffusion coefficients of the same order of magnitude as in the solution phase. At the same time, charge transfer through this coating, for an immobilised Ni²⁺ complex, was shown to be relatively facile.²⁵ Hence, the immobilisation of ZnTPP within the monoolein cubic phase has potential as a technique to improve substrate diffusion/partition.

Slow scan rate cyclic voltammetry (5 mV s⁻¹) of a glassy carbon electrode modified by cubic phase MO containing ZnTPP showed an increase in the ZnTPP reduction peak in the presence of 0.1 mM CCl₄ (see supplementary information†), indicating that charge transfer from the Zn complex could be realised, and that electrocatalytic reduction of the organohalide had occurred.

To compare the rate of charge transport in the monoolein films, relative to that in the electropolymerised ZnTPP, each type of film was coated onto a carbon microdisk electrode, and after steady state electroactivity was confirmed, as observed from sigmoidal-shaped voltammograms (Figs. 4 and 5), chronoamperometry was performed by stepping the potential from −0.2 V (the fully oxidised state, maintained for 3.0 s) to −1.16 V (fully reduced state, maintained for 0.5 s). Plots of I vs. 1/t^1/2 for the −1.16 V region are shown in the insets to Figs. 4 and 5. Until the oxidised redox species in the film is depleted, the variation of current with time at a microdisk, if approximated to hemispherical diffusion, is expressed by³⁶

I(t) = nFAD_Ec[1/(πD_Et)^1/2 + 1/r] (2)

where A is electrode area, c is concentration of redox species, r is electrode radius and D_E is the apparent diffusion coefficient, which may represent the contributions of actual mass transport and/or the propagation of charge by electron hopping,³⁷ as considered below. Hence from a plot of Ivs. 1/t^1/2, the ratio of gradient/y-intercept will have a value of r/(πD_E)^1/2, from which the value of D_E may be calculated. Note that in the case of the electropolymerised film, the gradient of this plot changes at longer times, which we interpret as depletion of Zn(II) species in the film. Therefore gradients were determined from the shorter times, as indicated by the solid line in the inset. In the case of the cubic phase, the gradient remained linear through the sampling time, which is probably due to the monoolein films being thicker (Zn(II) coverages were one order of magnitude greater in the cubic phase, i.e. in the order of 10⁻⁸ mol cm⁻²) and therefore not being depleted of Zn(II) over this period. Three determinations of each gave mean values of D_E of 3.64 × 10⁻¹¹ ± 0.02 × 10⁻¹¹ cm² s⁻¹ and 5.65 × 10⁻¹⁰ ± 0.11 × 10⁻¹⁰ cm² s⁻¹ for the electropolymerised and cubic phase films, respectively. As far as we know, values of D_E have not been reported for standard porphyrins. However, D_E for the Co(II)/(I) transition of a cobalt porphyrin-modified polypyrrole film was reported to be 1.46 × 10⁻¹¹ cm² s⁻¹,³⁸ hence of the same order of magnitude as found here. In the case of the electropolymerised films, we may reasonably expect the main mechanism of charge transport to be electron self-exchange, rather than physical movement of redox centres. Whereas, in the case of immobilised cubic phase, the mode of transport has not so far been determined and could potentially be either. As a preliminary investigation of this, we examined the concentration dependence of the diffusion coefficient.

Fig. 4 Cyclic voltammogram of a 7 µm diameter carbon microdisk modified by electropolymerised ZnTPP film. Scan rate = 50 mV s⁻¹. Inset: Cottrell plot of electrode in main figure for −0.2 V to −1.16 V potential step.

Fig. 5 Cyclic voltammogram of a 7 µm diameter carbon microdisk modified by MO cubic phase containing ZnTPP. Scan rate = 20 mV s⁻¹. Inset: Cottrell plot of electrode in main figure for −0.2 V to −1.16 V potential step.

In the case of electron self-exchange, D_E increases with the total redox concentration, C_T, as expressed by³⁹

D_E = δ²k_exC_T/6 (3)

where δ is the distance between species at reaction and k_ex is the electron self-exchange rate constant. In contrast, when diffusion occurs by physical movement, D_E either does not exhibit a concentration dependence, or in the case of movement between fixed sites in a polymer matrix (such as the ion-cluster regions of Nafion), may be observed to decrease with increasing concentration, as the availability of sites decreases.⁴⁰

A series of MO cubic phase liquid crystals were prepared, incorporating varying quantities of ZnTPP, and the same microdisk potential step experiment was performed for each. There was a pronounced increase in the value of D_E with ZnTPP concentration (see supplementary information†). Hence, it appears that electron self-exchange is the main mode of transport here. The value of D_E recorded was less than that found by Rowinski et al.²⁵ for a MO cubic phase entrapping a Ni(II) cyclam complex (1.90 × 10⁻⁸ cm² s⁻¹). The difference is probably due, in the most part, to the wide range of k_ex values which may be exhibited by different redox compounds (e.g. orders of magnitude of k_ex from 10⁵ M⁻¹ s⁻¹ (ref. 41) to 10⁹ M⁻¹ s⁻¹ (ref. 42) have been reported).

The thickness of the monoolein film was optimised by varying the mass of cubic phase applied to the electrode, and measuring the amperometric response to CCl₄, as shown in Fig. 6. Similar to the electropolymerised film, a thickness-independent region is reached, and as in the electropolymerised case, a plot of 1/Ivs. 1/ω^1/2 for a film within this region (inset to Fig. 6) is linear, indicating that k and substrate diffusion/partition are the rate controlling factors.

Fig. 6 Amperometric reduction current recorded at −1.0 V to 0.4 µM CCl₄ at an electrode modified by ZnTPP-containing MO cubic phase of varying thickness. Inset: reciprocal plot of an amperometric response at RDE (−1.0 V) modified by ZnTPP-containing MO cubic phase (10 mg on electrode, Γ = 2.72 × 10⁻⁸ mol cm⁻²) to 1 mM CCl₄ at varying rotation rates.

Using a film of the optimised thickness, 1-chloropropane, chloroform, iodoethane, bromoethane, carbon tetrachloride, chlorobenzene and chloronapthalene were calibrated by amperometry at −1.0 V. As shown in Fig. 7, the cathodic current varied with organohalide concentration in the approximate range 0.1 µM to 6.0 µM, and was linear across the range 0.1 µM to 1.0 µM, with all seven organohalides showing a common sensitivity in this region of 6.95 A M⁻¹ cm⁻². Hence, relative to electropolymerisation, the limit of detection was lowered by one order of magnitude, and the sensitivity increased by a factor of approx. 10. To the best of our knowledge, the limit of detection reported here is lower than the previous detection limits (in those papers where they were reported) for organohalides using catalysis by either a metal complex^8,10,11 or hemoglobin/myoglobin.^21,22 However, given a typical molar mass of approx. 150 g mol⁻¹ for the organohalides to be detected, this still leaves a limit of detection of approx. 15 ppb, which would be high for groundwater contamination. A lower limit of detection could be reached by finding a redox catalyst that exhibits a higher value of k for the organohalide reaction. We previously reported a two-stage collector-generator method with a lower limit of detection (0.1 nM),⁷ which was achieved by a preconcentration step and the use of differential pulse voltammetry, rather than amperometry. However, it requires a longer time for measurement, as a 600 s generator step is followed by a 600 s preconcentration step for each sample.

Fig. 7 Amperometric calibration of 1-chloropropane (●), chloroform (○), iodoethane (▼), bromoethane (▽), carbon tetrachloride (■), chlorobenzene (□) and chloronapthalene (♦) at −1.0 V at electrode modified by ZnTPP-containing MO cubic phase. Within the linear range all six sets of symbols overlap. Beyond the linear range, the symbols for chloroform, iodoethane, bromoethane and carbon tetrachloride overlap. Inset: Linear range shown with expanded scale, units as in main figure. Sensitivity = 6.95 A M⁻¹ cm⁻².

It should be emphasised that the sensor configuration described here will not provide speciation in the detection, since the electrical signal is always the re-reduction of the metal complex, irrespective of analyte. However, the sensor may provide a value for total organohalogen, as indicated by the detection of the seven analytes given here at a common sensitivity within the linear range. Given this aspect of the measurement technique, it could most usefully by employed to construct portable sensors for immediate on-site measurement in cases of suspected environmental contamination. Such sensors would ideally use disposable electrodes produced by a method such as screen printing. Since the cubic phase MO has a high viscosity, this may possibly be screen-printed also. Hence, a future direction of this work will be to examine MO-ZnTPP-modification of electrodes by mass-fabrication techniques.

The stability of the cubic phase layer was examined by recording repeated amperometric responses to 0.4 µM CCl₄ at −1.0 V (see supplementary information†). The electrode was stable for approx. 35 uses before a drop of signal. Given the relatively rapid fall-off in response which occurred after this, it may be that the film desorbed from the electrode.

Conclusions

We have described two combined strategies for improving the sensitivity of organohalide detection by redox catalysis. The first technique was to increase the second order rate constant of the organohalide reduction by replacing Co(II) with Zn(II) as the catalyst redox centre. The second technique was to improve substrate diffusion by replacing electropolymerisation as the electrode-modifying method with entrapment within a cubic phase liquid crystal. Since diffusion of substrate was one of the rate controlling steps, the sensitivity of the response was significantly increased. The resulting sensors exhibited the lowest limit of detection thus far reported for organohalide measurement by redox catalysis.

Acknowledgements

W.W. gratefully acknowledges a PhD scholarship from the Royal Golden Jubilee Project of the Thailand Research Fund. M.S. is an employee of BIOTEC. The authors would like to thank Chatuporn Phanthong for producing the contents list diagram.

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Footnote

† Electronic supplementary information (ESI) available: Voltammogram showing the performance of a glassy carbon electrode modified by ZnTPP-containing cubic phase; plot illustrating the relationship between D_E and ZnTPP concentration; and an examination of the stability of the cubic phase layer. See http://www.rsc.org/suppdata/an/b5/b500444f/

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