Dual electrode micro-channel flow cell for redox titrations: Kinetics and analysis of homogeneous ascorbic acid oxidation

Journal of Electroanalytical Chemistry 692 (2013) 72–79

Contents lists available at SciVerse ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Dual electrode micro-channel flow cell for redox titrations: Kinetics and analysis of homogeneous ascorbic acid oxidation Eleni Bitziou a, Michael E. Snowden a, Maxim B. Joseph b, Simon J. Leigh b, James A. Covington b, Julie V. Macpherson a, Patrick R. Unwin a,⇑ a b

Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK School of Engineering, University of Warwick, Coventry CV4 7AL, UK

a r t i c l e

i n f o

Article history: Received 14 November 2012 Received in revised form 18 December 2012 Accepted 20 December 2012 Available online 7 January 2013 Keywords: Channel electrode Dual band electrodes Catalytic EC0 reaction Redox titration Flow cell Simulations

a b s t r a c t A channel flow cell, fabricated using microsterolithography (MSL), has been employed to determine the rate constant for the homogenous oxidation of ascorbic acid (AA) by an electrogenerated oxidant in aqueous solution under physiological pH conditions using a dual gold microband electrode format. Specifically, the oxidation of ferrocenylmethyl trimethyl-ammonium (FcTMA+) at an upstream electrode produce FcTMA2+, which oxidises AA in solution, yielding FcTMA+ which can undergo further heterogeneous oxidation at the upstream electrode. In addition, by setting the downstream electrode potential to a value where FcTMA2+ is detected at a transport-limited rate, collection efficiency measurements are also possible in a redox titration format. The benefits of studying homogeneous kinetics, and particularly this catalytic EC0 process, in a flow through system using a dual electrode format are highlighted. Collection efficiency measurements are shown to be particularly attractive for the detection of low levels of AA and are much more sensitive than measurements at a single electrode, particularly as a range of device parameters, such as flow rate, electrode size and electrode separation can be tuned to optimise detection. Various electrode sizes operating in the generation/collection (GC) mode have been assessed, with the optimum geometry of those studied (for the highest collection efficiency in the absence of homogenous reactions) being a small upstream generator electrode (25 lM long), a large downstream collector electrode (400 lM long), separated by a few tens of microns, in a channel of 200 lm height and 4 mm width. A full model for the cell hydrodynamics and mass transport has been developed using finite element modelling and extensive experiments are reported assessing the collection efficiency for different electrode geometries and AA concentrations. The homogeneous rate constant for the reaction between FcTMA2+ and AA is 5.5 (±1.6)  105 M1 s1. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Dual electrode hydrodynamic systems are attractive for elucidating complex reaction mechanisms, studying heterogeneous and homogeneous reaction rates, and for quantitative analysis under flow conditions [1–5]. The rotating ring-disc electrode (RRDE) [6,7] and double channel electrode have proven particularly effective in this regard. Typically, dual electrode formats [8–14] operate by driving an electrochemical reaction at an upstream generator electrode and the resulting product is transported and detected at a downstream collector electrode. If the upstream product (reagent) undergoes heterogeneous [15,16] or homogeneous reaction [11,17–21] during transit to the downstream electrode, the resulting downstream electrode current is modified, as reflected in the

⇑ Corresponding author. Tel.: +44 24 76523264. E-mail address: [email protected] (P.R. Unwin). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.12.014

collection efficiency, from which kinetic measurements can be made. Dual microband electrodes have been used similarly for electrochemical titrations in quiescent conditions [9,22,23]. Theory for the collection efficiency of dual electrode systems has been developed for both stationary [24–27] and hydrodynamic conditions [11,16,18,21,28–35] for a wide range of electrochemical systems. The focus of this study is on the kinetics of a complex catalytic EC0 mechanism and how this can be used for redox titrations in a dual microband electrode format under hydrodynamic conditions. In the catalytic EC0 reaction, a redox-active species (A) undergoes heterogeneous electron transfer at a generator electrode, considering the illustrative example of oxidation in Eq. (1) below. The electrode product (species B) oxidises a target species (Z) in solution which (nominally) does not undergo electron transfer at either electrode at the potential of interest. This process regenerates A in solution which, in turn, can undergo further reaction at the generator electrode.

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79

Generator electrode : A  ne ¢ B k

Solution : B þ Z ! A þ Product

ð1Þ ð2Þ

where k is the rate constant for the homogeneous reaction in solution. In practice, this mechanism is usually studied with the reactant (Z) being present in large excess over the mediator species (A) so that the homogeneous chemical step obeys pseudo-first-order kinetics [36–38]. The focus herein is to explore the possibility of indirectly detecting low concentrations of a solution species, Z, and we consider it particularly for the case of a dual band electrode in a micro-channel flow. With other electrode configurations, the EC0 format has been used to determine thiosulfates, halides, and L-ascorbic acid [25,36], as well as being used for the detection of various drugs [39], and tetramethylthiuram disulfide [40]. Herein, we study the detection of L-ascorbic acid (AA) at physiological pH conditions where the target species (AA) is present in low concentrations compared to the redox species and show that dual band electrodes in a hydrodynamic channel format can be used to both detect Z (AA) with high sensitivity and determine k for the homogeneous reaction. AA is a well-known component of biological media, needed in intracellular redox processes where it acts as an antioxidant [41]. Its detection, as well as its effectiveness as a reducing agent by means of measuring its homogeneous kinetics, in any given environment, is of great interest [21]. Previously, Williams and co-workers [25] reported the electrochemical titration of AA using ferricyanide at a dual microband electrode in quiescent solution through the oxidation of AA to dehydroascorbate (DHA):

2½FeðCNÞ6 3 þ AA ! 2½FeðCNÞ6 4 þ 2Hþ þ DHA

ð3Þ

Detection of AA in the mM range was reported, with the need for manual motion of the electrodes in order to renew the concentration boundaries before each measurement. The present work demonstrates greatly enhanced sensitivity to the lM level through the use of continuous micro-channel flow. In this study, a microfluidic channel containing two parallel gold microband electrodes operating in generation/collection (G/ C) mode has been used in combination with quantitative simulations to obtain kinetic parameters for the homogeneous process in solution as well as quantitatively titrating AA, the oxidation of which is otherwise kinetically hindered on gold electrodes at physiological pH values. A channel flow cell, fabricated using microstereolithography (MSL) has been employed to produce a microchannel over a dual-microband system for on-line electrochemical analysis. In previous studies from our group we have demonstrated the ease of use of such cells in single electrode formats [42,43], as well as for the study of complex surface processes [44,45]. Herein, we demonstrate how dual electrode formats can readily be deployed in such cells and we optimise the device by increasing the collector size significantly compared to the generator electrode, as well as considering the additional benefits of a short distance between the two microband electrodes in order to achieve higher collection rates [46,47]. The devices are thus more sensitive than some recent reports which have tended to use generator and collector electrodes of the same size, leading to rather low collection efficiencies [8,21,48].

73

[42,43]. Briefly, the channel electrode cell consisted of two separate parts: one part was a flat substrate containing the electrodes and the second part was the MSL flow cell unit which sat over the substrate (see Fig. 1a). The channel flow unit and substrate were held together tightly, simply by using a cotton thread that provided a leak-free seal, even at flow rates of up to 1.0 cm3 s1 [42]. The flow cell unit was designed using SolidWorks (Dassault Systémes, France) to give an internal channel length of 10.0 mm, channel width (w) of 4.0 mm, and channel height (2h) of 200 lm (see Fig. 1b). Two chambers were incorporated into the design upstream and downstream from the working electrodes to house counter and reference electrodes (1.0 mm diameter each). The device was manufactured on an MSL workstation (Envisiontec Perfactory Mini Multi-Lens, Germany) in which the 3D channel unit was built up in a layer-by-layer photopolymerisation process using a photocurable orange PMMA resin (R11, Envisiontec) [49]. The fabrication process took 6–8 h with a minimum of two units produced in each batch. Non-contact interferometry measurements (WYKO NT-2000 optical profiler) were performed on each flow unit produced, to accurately characterise the channel height. The substrate comprised lithographically-fabricated gold microband electrodes on glass. The electrodes were made using a shadow mask to form a 100 nm thick layer of Au (including a 5 nm Cr underlayer) thermally evaporated onto glass substrates, following a metal lift-off process using S1818 photoresist (Chestech, UK). A range of different band lengths and separations were fabricated (see Section 3 for details). In brief, generator band lengths (xg) of 25, 50, or 100 lm and collector band lengths (xc) of 100, 300, or 400 lm (see Fig. 1c) were used, with separations (d) of 25, 50, or 100 lm between the two electrodes. The width of the bands, w, was defined by the channel width (i.e. 4.0 mm). The substrate also contained two Au place markers, to aid reproducible positioning of the flow cell over the microbands (see Fig. 1a and c). Inlet and outlet tubing was connected directly to the flow cell to provide continuous flow of analyte, using either a syringe pump (KD Scientific) or a Gilson HPLC pump (Model 305, Villiers-le-Bel, France) with a model 806 manometric module and pulse dampener. 2.2. Electrochemical measurements All electrochemical measurements were made using a 3- or 4-electrode set-up with one or both of the channel microbands operating as working electrodes, together with a coated silver– silver chloride wire (Ag|AgCl) which served as a quasi-reference electrode, and a Pt wire as the counter electrode. Electrochemical measurements were carried out with an Ivium portable bipotentiostat (CompactStat, Alvatek Ltd., UK) which was operated in either linear sweep voltammetry (LSV) mode (one active working electrode band) or G/C mode (both working electrode bands active), in which LSVs were recorded at the generator electrode at 10 mV s1 with the collector electrode potential fixed (typically at 0 V vs. Ag|AgCl) to detect any electrogenerated FcTMA2+ reaching its surface by mass transport-limited reduction. From such measurements it was possible to elucidate the collection efficiency, CE, as the ratio of the steady-state collector current, ic, to the mass transport-limited steady-state generator current, ig (CE = ic/ig). 2.3. Solutions and chemicals

2. Experimental section 2.1. Fabrication of flow cell and electrodes The fabrication procedures and basic design of the channel flow cell used in this study have been described in detail elsewhere

Solutions containing 1  104 M ferrocenylmethyl trimethylammonium (FcTMA+) hexafluorophosphate (synthesised in house [50]) in 0.1 M KNO3 were used for the characterisation of single microband electrodes. For EC0 redox titrations, 1  104 M FcTMA+ was used in 1  102 M PBS buffer (pH 7.4) with L(+)-ascorbic acid

74

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79

(a)

(b)

(c)

(d)

Fig. 1. (a) Schematic of the MSL flow cell (cut down the centre to show the inner structure) positioned over generator/collector electrodes. (b) Design and key dimensions (mm) of the MSL flow cell used. (c) Schematic cross-section of the dual band electrode geometry within a flow channel of height 2h containing a generator and collector electrode of length xg, and xc, respectively, separated by a distance, d. Also showing the EC0 reaction scheme for the FcTMA+/2+/AA system. (d) 2D scheme of the channel geometry used for the simulations showing the three subdomains with the numbered boundaries used in the simulations (see text for details).

sodium salt (Sigma Aldrich, UK) at concentrations of 5, 10, 20 and 30 lM. Solutions were freshly prepared before each experiment using Milli-Q-reagent water (Millipore Corp., resistivity ca 18.2 MX cm at 25 °C). 2.4. Numerical simulations of mass transport and kinetics Simulations were carried out on a PC equipped with an Intel Pentium III Xeon quad core 2.5 GHz processor and 8 GB of RAM running Windows XP professional. The finite element method (FEM) modelling package, Comsol Multiphysics 3.5a (Comsol AB,

Sweden) together with the Matlab interface (Release, 2009a) (MathWorks Inc., Cambridge, UK) was used herein. FEM has proven accurate and robust as a means of tackling a variety of mass transport and coupled reaction problems in hydrodynamic electrode systems [42,51,52]. The experimental channel system was represented as a 2D cross-section, as shown in Fig. 1d. Hence, the model assumes that perturbations in the hydrodynamics due to the channel edges are relatively negligible, as validated previously [42]. We also note that in the model the band electrodes were not co-planar with the substrate, presenting a raised step like-geometry with a height of 100 nm (determined by atomic force microscopy (AFM)).

75

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79

The channel domain was split into three subdomains as shown in Fig. 1d. Following our previous work [42,43], the incompressible Navier–Stokes equations for momentum balance (Eq. (4)) and continuity (Eq. (5)) were solved under steady-state conditions for subdomains A–C, T

qV  rV ¼ rðq þ gðrV þ ðrVÞ ÞÞ

ð4Þ

rV ¼0

ð5Þ

where k is the homogeneous rate constant (M1 s1) for the oxidation of AA. The boundary conditions for the convection–diffusion model (subdomain 2, Fig. 1d) for FcTMA2+ and AA were:

boundary 6 : inlet; cFcTMA2þ ¼ 0;

cAA ¼ cAA

boundaries 7; 8; 12; 16 : n  Ni ¼ 0 where q is the density of water (1.00 g cm3), V is the velocity vector (u and t are components in the x and y directions, respectively), p is pressure, T is the matrix transpose operator and g is the dynamic viscosity of water (1.00 mPa s for the conditions of our experiments). The following boundary conditions were used:

boundaries 1; 2; 4; 5; 7  16; 18  20; and 22 : u ¼ 0; v ¼ 0 ð6Þ boundary 3 : u ¼ 0;

v ¼

Vf wch xch

ð7Þ

boundary 21 : p ¼ p0 ; gðrV þ ðrVÞT Þ  n ¼ 0

ð8Þ

where wch and xch are the channel width (in this case 4.0 mm in the w direction) and duct length (length of boundary 3, 0.5 mm), respectively. p and p0 are the local pressure and the pressure of the system, respectively, and n is the unit normal vector. A no-slip boundary condition (Eq. (6)) was applied to boundaries 1, 2, 4, 5, 7– 16, 18–20, and 22 which represent the walls of the flow cell, substrate, and electrodes. A normal flow was introduced in the inlet through boundary 3, as described in Eq. (7), with the standard pressure and no shear stress (Eq. (8)) applied to the outlet boundary 21. Note that lengths of 2 mm were chosen for the inlet and outlet in practice and in the simulation, with the expectation that Poiseuille flow would develop prior to solution reaching the main channel and that the outlet would not distort flow at the downstream section of the main channel [42]. Once the velocity profile had been obtained, the resulting velocities (u and t) were used to solve the steady-state convection– diffusion equation (Eq. (9)) for species within the channel,

Di

@ 2 ci @ 2 ci þ @x2 @y2

!

  @ci @ci þv ¼ Ri þ u @x @y

ð9Þ

where Di is the diffusion coefficient (6  106 cm2 s1 for FcTMA+ and AA [42]), ci the concentration, and Ri is the rate of formation or loss of species i. It was reasonable to only solve the convection–diffusion equation within subdomain B (Fig. 1d). The simulation considered the mass-transport limited oxidation of FcTMA+ (present in bulk solution) to FcTMA2+ at the generator electrode and the mass-transport limited detection of FcTMA2+ at the downstream electrode (by reduction to FcTMA+). By assuming the mass balance, [FcTMA2+] = [FcTMA+]⁄  [FcTMA+], where [FcTMA+]⁄ denotes the bulk concentration, we were able to condense this aspect of the simulation to FcTMA2+ alone. This was reasonable, because the diffusion coefficients of FcTMA+ and FcTMA2+ are similar [50]. We consider the following homogeneous reaction when ascorbic acid (AA: C6H8O6) was present in solution at a concentration of cAA :

2FcTMA2þ þ C6 H8 O6 ! 2FcTMAþ þ C6 H6 O6 þ 2Hþ

ð10Þ 2+

with the following kinetic terms [21] for AA and FcTMA

RFcTMA2þ ¼ 2kcFcTMA2þ cAA

ð11Þ

RAA ¼ kcFcTMA2þ cAA

ð12Þ

ð14Þ

boundaries 9  11 : cFcTMA2þ ¼ cFcTMAþ ; boundaries 13  15 : cFcTMA2þ ¼ 0;

ð13Þ

n  NAA ¼ 0

n  NAA ¼ 0

boundary 17 : outlet; n  ðDi rci Þ ¼ 0

ð15Þ ð16Þ ð17Þ

where Ni is the flux of species i. Boundaries 7, 8, 12, and 16 (Eq. (14)) are the insulating walls of the channel and substrate, and boundary 6 is the inlet where fresh solution enters at the bulk concentration. Boundaries 9–11 (Eq. (15)) define the generator electrode surface (including two edges of height = 100 nm) where transport-limited electrolysis of the analyte occurs, boundaries 13–15 (Eq. (16)) correspondingly define the collector electrode surface, and boundary 17 is the convective flux. The limiting current response at the generator (iG) and the collector (iC) were calculated by summing the normal flux for FcTMA2+ at boundaries 9–11 (generator) and boundaries 13–15 (collector) and multiplying by the electrode width (w), the number of electrons transferred per redox event (n = 1), and the Faraday constant, F (96 485 C mol1). 3. Results and discussion 3.1. Mass transport at single electrodes The hydrodynamics of similar flow cells to that used herein has been modelled previously [42]. The flow is well-defined and predictable with essentially fully developed Poiseuille velocity profile achieved over the microband electrodes located mid-way in the channel even at high flow rates. However, although mass transport has been treated to large electrodes in this device [42], there has been no assessment of the small electrodes of the type of interest here, and so this was an initial goal. Under fully developed laminar flow, and for conditions where concentration changes are confined close to the channel wall, such that the Poiseuille velocity profile can be linearised by the so-called Lévêque approximation [5], the current response for the steady-state limiting current at a co-planar channel electrode obeys the Levich equation:

ilim ¼ 1:165nFcb D2=3



Vf 2hwch

1=3 h

1=3

wxe2=3

ð18Þ

As we detail below, this provides a useful limit to which to compare simulation for real devices. Initially, fabricated microbands were characterised in the channel flow cell using FcTMA+ oxidation at various volume flow rates (Vf). Typical LSVs for the one-electron oxidation of 1  104 M FcTMA+ in 0.1 M KNO3 (vs. Ag|AgCl) at a single microband electrode (xe = 100 lm, wch = w = 4.0 mm, 2h = 200 lm) over the Vf range 1.66  104  0.0667 cm3 s1 are shown as an insert in Fig. 2. The voltammograms are well-defined and show a clear limiting current region. This is in accord with expectations; although voltammetry in thin channel cells can be impacted by solution resistance, the resistance of the channel (which is the main contribution) is a few kO for the conditions herein and this gives rise to an ohmic drop of just a few mV. Fig. 2 itself shows the experimental (points) steady-state limiting-currents vs. 1=3 V f for different sized microbands of (i) 25 lm, (ii) 50 lm, (iii) 100 lm, (iv) 300 lm, and (v) 400 lm. Also included are the currents

76

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79

0.8

1.8 (v)

1.6

(iv)

0.2 µA

(iv)

ilim / µA

1.2 1.0 -0.1

0.0

0.1

0.2

0.3

0.4

0.8 (iii) 0.6 (ii)

0.4

Collection Efficiency

1.4

0.6 (iii)

0.4

(ii)

0.2

(i)

(i) 0.2 0.05

0.10

0.15

0.20

[Vf /

0.25

cm3

0.30

0.35

0.40

0.0 0.000

0.45

0.005

0.010

Fig. 2. Experimental limiting currents (points) as a function of V 1=3 obtained using a f channel cell (2h = 200 lm), together the theoretical Levich estimation (solid lines) and COMSOL simulations (dotted lines) for gold band electrodes with lengths, xe, of (i) 25 lm, (ii) 50 lm, (iii) 100 lm, (iv) 300 lm, and (v) 400 lm. Insert: Typical LSVs (scan rate 10 mV s1) for the oxidation of 1  104 M FcTMA+ in 0.1 M KNO3 at a gold band electrode (w = 4.0 mm, xe = 100 lm) at Vf values from 1.66  104 cm3 s1 to 0.0667 cm3 s1 with the precise values defined in the ilim  V 1=3 plot. f

predicted by the Levich equation (solid lines) and the simulated limiting currents (dotted lines). It can be seen that the experimental observations agree very well with: (a) Levich theory for a co-planar electrode geometry; and (b) the simulated response for the full model that included a 100 nm step height on the electrode, which evidently had little effect on the steady-state current response. This is due to the large channel height (200 lm) compared to the electrode step height. Additionally, the agreement of the Levich equation with the full model implies that diffusional edge effects are negligible for the range of flow rates and electrode sizes used, in accordance with previous theory [11,35]. 3.2. Collection efficiency measurements For the case where two consecutive microbands were active in the G/C mode, CEs were determined at different flow rates, for various band widths and band distances. Fig. 3 shows a plot of CE vs. Vf with experimental (points) and simulated responses (lines) for the oxidation of 1  104 M FcTMA+ (0.1 M KNO3) at the generator electrode and reduction of FcTMA2+ at the collector electrode for the following G/C geometries: (i) xg = 100 lm, d = 50 lm, xc = 25 lm; (ii) xg = 25 lm, d = 50 lm, xc = 100 lm; (iii) xg = 100 lm, d = 100 lm, xc = 400 lm; and (iv) xg = 25 lm, d = 25 lm, xc = 400 lm. There is a good fit of the experimental observations with simulations for each case over the full range of geometries investigated. The CE had a constant value at volume flow rates in excess of 0.0083 cm3 s1 for all electrode geometries considered and the CE is largely independent of flow rate as expected from double electrode hydrodynamic systems [1,5]. At slower flow rates edge diffusion effects manifest in a very slight enhancement in the CE of the electrodes. The lowest steady-state CE observed in this study (0.15) was for the largest band generator electrode (xg = 100 lm) and the smallest collector (xc = 25 lm), with a separation of 50 lm, whereas a CE of 0.45 was achieved by significantly increasing the size of the collector to 400 lm for a generator length of 100 lm and a separation of 100 lm. The important effects of the collector size and gap size were evident when comparing geometries (ii) and (iv), where a 4-fold increase of xc and decrease in the gap from

0.015

0.020

0.025

0.030

0.035

Vf / cm3 s-1

s-1]1/3

Fig. 3. Experimental (points) and simulated collection efficiency (lines) responses as a function of flow rate (Vf) for generation/collection measurements using 1  104 M FcTMA+ (0.1 M KNO3) with different generator/collector geometries: (i) xg = 100 lm, d = 50 lm, xc = 25 lm; (ii) xg = 25 lm, d = 50 lm, xc = 100 lm; (iii) xg = 100 lm, d = 100 lm, xc = 400 lm; and (iv) xg = 25 lm, d = 25 lm, xc = 400 lm.

50 lm to 25 lm achieved an almost double CE value. The highest steady-state CE reported herein is 0.62 for case (iv) where a small generator and a much larger collector electrode are separated by a small gap (xg = 25 lm, d = 25 lm, xc = 400 lm). These CE value trends are as expected based on earlier double channel electrode studies [28], and are comparable with those obtained in a thinlayer channel system [23,53], and it follows that if the electrodes defined herein were deployed in a thin-layer cell configuration, by reducing the channel height, the CE responses could be further enhanced.

3.3. Redox titrations of ascorbic acid (AA) Fig. 4 shows the LSV responses of a single microband electrode (xg = 25 lm), at a scan rate of 10 mV s1, with Vf = 0.0166 cm3 s1, for the following solutions: (i) PBS buffer (pH 7.4) as the background electrolyte; (ii) 3  105 M AA (in PBS); (iii) 1  104 M

0.30 20 nA

0.25

Current / µA

0.0 0.00

(iv)

0.1 V

0.20 0.2

0.4

(iii)

0.6

0.15 0.10 0.05 0.00 0.1

(ii) (i) 0.2

0.3

0.4

0.5

0.6

E / V (vs. Ag|AgCl) Fig. 4. LSVs at a gold microband channel electrode (we = 4.0 mm, xe = 25 lm, 2h = 200 lm) using a 10 mV s1 scan rate, at a flow rate (Vf) of 0.0167 cm3 s1, for aqueous solutions of: (i) only PBS buffer (pH 7.4); (ii) 3  105 M AA (in PBS); (iii) 1  104 M FcTMA+ (in PBS); and (iv) 1  104 M FcTMA+ and 3  105 M AA (in PBS). Insert shows gold microband response in 3  105 M AA (PBS) at 4 flow rates of 0.0016, 0.0083, 0.0167, and 0.033 cm3 s1.

77

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79

(a)

0.50 0.45

ilim,G / µA

0.40 0.35

k / (M-1 s-1) 0 4 1.0 x 10 5 1.0 x 10 6 1.0 x 10

0.30 0.25 0.20

(b)

0.55 0.50 0.45

Collection Efficiency

FcTMA+ (in PBS); and (iv) 1  104 M FcTMA+ and 3  105 M AA (in PBS). The inset of Fig. 4 shows that there is no effect of increasing flow rate (Vf = 0.0016, 0.0083, 0.0167, and 0.033 cm3 s1) to the AA oxidation response (oxidative current 15 nA), indicating that this heterogeneous reaction is kinetically-limited and negligible at these potentials for this electrode material. Furthermore, no reverse reaction (current) was detected for any products of AA oxidation when the downstream collector electrode was held at 0 V vs. Ag|AgCl (the potential for the detection of FcTMA2+ in G/C measurements). There have been reports on the direct oxidation of AA to DHA in acidic conditions; however these reports are mostly based on palladium modified gold electrodes [54,55] to catalyse the reaction and to avoid fouling effects. Such processes are not significant on the gold electrodes used herein. The LSV of 1  104 M FcTMA+ with 3  105 M AA (solution (iv)) shows a noticeable increase in the limiting-current region of the voltammogram compared to the case where no AA is present (solution (iii)). This elevated current response on the single microband is mostly due to a fast homogeneous solution reaction between FcTMA2+ and AA, catalytically regenerating FcTMA+ which undergoes re-oxidation at the generator electrode. In order to evaluate the G/C response of FcTMA+/2+ system in the presence of AA, and to assess this format for redox titrations generally, we carried out simulations for a range of homogeneous rate constants, k, for a hypothetical reaction between electrogenerated FcTMA2+ and AA. Fig. 5a and b shows the simulated results for the limiting current of the generator electrode, ilim,G (lA), and CE, respectively, vs. Vf, for k values covering a wide range. For illustrative purposes, we considered a geometry defined by xg = 50 lm, d = 100 lm, xc = 400 lm for a 10:1 ratio of FcTMA+ (1  104 M) to AA (1  105 M). As mentioned earlier, this concentration ratio is different from the usual conditions for EC0 reactions where the target species is present in high excess. Fig. 5a and b clearly shows that ilim,G is much less sensitive to k than CE measurements. The generator current is enhanced slightly at all flow rates, and the enhancement increases with k, but is relatively small. In contrast, the CE is much more sensitive to k and not the flow rate. Hence, CE appears to be a particularly sensitive means to elucidate k. Fig. 5b shows the high sensitivity of this G/C geometry to rate constants over the range 104 M1 s1 up to 106 M1 s1. Furthermore, it should be noted that the kinetic range open to study can be further tuned by changing the electrode geometry, as pointed out above. Fig. 6a and b shows experimental CE vs. Vf plots for two G/C geometries: (a) xg = 50 lm, d = 100 lm, xc = 400 lm; and (b) xg = 25 lm, d = 25 lm, xc = 400 lm for FcTMA+ concentrations of 0.1 mM and concentrations of AA (points) varying from 5 lM to 30 lM ((i) 0 M; (ii) 5  106 M; (iii) 1  105 M; (iv) 2  105 M; and (v) 3  105 M) together with the simulated CE responses for k values that best fit the experimental data. The following k values were determined using the COMSOL model: (a) (i) 0 M1 s1, (a) (ii) 2.0  105 M1 s1, (a) (iii) 5.0  105 M1 s1, (a) (iv) 6.0  105 M1 s1, (a) (v) 7.4  105 M1 s1; (b) (i) 0 M1 s1, (b) (ii) 4.7  105 M1 s1, (b) (iii) 5.7  105 M1 s1, (b) (iv) 6.5  105 M1 s1, (b) (v) 6.3  105 M1 s1. There is a good correlation between the experimental data and simulations, with the main effect being a decrease of the CE at slow flow rate. This is due to the enhanced influence of AA that converts FcTMA2+ back to FcTMA+ in solution before it reaches the collector. At high flow rates a higher proportion of FcTMA2+ reaches the collector rather than being consumed by AA in solution. The data in Fig. 6 also illustrate how the electrode geometry influences CE at particular AA concentrations, and how this can be tuned to make the CE more sensitive. Thus, it is noted that at the higher flow rates (greater than 0.05 cm3 s1) for the 10:3 concentration ratio of FcTMA+ to AA (case (v)) the CE for geometry (a) was reduced by 75%, whereas for geometry (b) the CE was reduced by

0.40 0.35 0.30 0.25

-1

0.15 0.10 0.05 0.00 0.00

-1

k / (M s ) 0 4 1.0 x 10 5 1.0 x 10 6 1.0 x 10

0.20

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Vf / cm3 s-1 Fig. 5. Theoretical plots for the EC0 reaction under hydrodynamic conditions showing: (a) the dependence of the ilim,G on flow rate for different k values; and (b) the effect of different k on the CE as a function of flow rate. Data were simulated for 1  104 M FcTMA+ and 1  105 M AA with the following typical geometry: xg = 50 lm, d = 100 lm, xc = 400 lm.

only 50%. This demonstrates the significant effect of d (in particular) on the CE of the hydrodynamic double band electrode system. In general, a bigger d value (geometry (b)) leads to a lower CE. The value for the rate constant of the homogeneous reaction obtained by averaging the individual k values in the geometries depicted in Fig. 6a and b is 5.5 (±1.6)  105 M1 s1. It is particularly note worthy that a concentration of AA as low as 5 lM has a detectable effect on CE for a concentration of FcTMA+ of 0.1 mM. Having set out the basic principles and provided proof of concept of G/C redox titrations in a microchannel format it should be possible to implement this approach in future work for in vivo microdialysis biosensing systems. For example, AA is a major interferent for the detection of glucose and lactate at physiological pH conditions [56] and the ability to remove this molecule by selective redox titration is worthy of exploration. Furthermore, the online method presented in this paper could find further applications in potency studies of drugs containing quinone-active compound that play a major role as bioreductive drugs [57]. 4. Conclusions A dual microband electrode format in a channel flow regime has been developed and employed for redox titrations based on the catalytic EC0 system. The system has shown well defined hydrodynamics and mass transport. The optimal device geometry (within practical limits) has been assessed and identified by, in particular, the effect of the generator and collector electrode sizes and their separation (d).

78

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79

als Project 2), with support from Advantage West Midlands (AWM) and partially funded by the European Regional Development Fund (ERDF).

(a) 0.6 Collection Efficiency

0.5

(i) (ii)

0.4

References (iii)

0.3

(iv) 0.2

(v) 0.1 0.0 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Vf / cm3 s-1

Collection Efficiency

(b) 0.7 0.6

(i) (ii)

0.5

(iii)

0.4

(iv) (v)

0.3 0.2 0.1 0.0 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Vf / cm3 s-1 Fig. 6. Experimental CE responses (points) and best simulated k fits (lines) for G/C geometry: (a) xg = 50 lm, d = 100 lm, xc = 400 lm; and (b) xg = 25 lm, d = 25 lm, xc = 400 lm. Experiments used 1  104 M FcTMA+ (PBS) with AA concentrations of: (i) 0 M; (ii) 5  106 M; (iii) 1  105 M; (iv) 2  105 M; and (v) 3  105 M. The following k values provided the best fits: (a) (i) 0 M1 s1, (a) (ii) 2.0  105 M1 s1, (a) (iii) 5.0  105 M1 s1, (a) (iv) 6.0  105 M1 s1, (a) (v) 7.4  105 M1 s1; (b) (i) 0 M1 s1, (b) (ii) 4.7  105 M1 s1, (b) (iii) 5.7  105 M1 s1, (b) (iv) 6.5  105 M1 s1, (b) (v) 6.3  105 M1 s1.

A particular focus has been to examine the detection of analyte in solution at low concentration, using the analysis of AA as an exemplar system. The homogeneous reaction between AA and the electro-generated FcTMA2+, at physiological pH conditions has demonstrated the capabilities of the device for determining kinetic information and achieving the detection of AA at low concentrations. Importantly, this work has extended kinetic studies of EC0 processes in a dual channel electrode format to second order kinetics, and opened up the possibilities of on-line analysis. The versatility of the device to achieve indirect detection of a solution reactive species at relatively low concentrations, by adjusting parameters like electrode size, distance between electrodes, flow rate, and ultimately channel height (to achieve a thin-layer format) are attractive for further applications in real-samples and should be compatible with a range of techniques such as flow injection analysis and microdialysis. Acknowledgements Part of the equipment used in this study was obtained through Birmingham Science City: Innovative Uses for Advances Materials in the Modern World (West Midlands Centre for Advanced Materi-

[1] W.J. Albery, M.L. Hitchman, Ring-Disc Electrodes, Oxford University Press, 1971. [2] J.A. Alden, R.G. Compton, J. Electroanal. Chem. 404 (1996) 27–35. [3] A.J. Bard, L.R. Faulkner, Electrochemical Methods – Fundamentals and Applications, second ed., John Wiley & Sons, 2001. [4] J.A. Cooper, R.G. Compton, Electroanalysis 10 (1998) 141–155. [5] R.G. Compton, P.R. Unwin, J. Electroanal. Chem. 205 (1986) 1–20. [6] L.N. Nekrasov, Faraday Discuss. Chem. Soc. 56 (1973) 308–316. [7] A. Frumkin, L. Nekrasov, B. Levich, J. Ivanov, J. Electroanal. Chem. 1 (1959) 84– 90. [8] C. Amatore, N. Da Mota, C. Lemmer, C. Pebay, C. Sella, L. Thouin, Anal. Chem. 80 (2008) 9483–9490. [9] T. Paixao, R.C. Matos, M. Bertotti, Electroanalysis 15 (2003) 1884–1889. [10] A.C. Fisher, R.G. Compton, J. Appl. Electrochem. 21 (1991) 208–212. [11] K. Aoki, K. Tokuda, H. Matsuda, J. Electroanal. Chem. 76 (1977) 217–233. [12] E.O. Barnes, G.E.M. Lewis, S.E.C. Dale, F. Marken, R.G. Compton, Analyst 137 (2012) 1068–1081. [13] I. Dumitrescu, D.F. Yancey, R.M. Crooks, Lab Chip 12 (2012) 986–993. [14] P.R. Unwin, J. Electroanal. Chem. 297 (1991) 103–124. [15] R.G. Compton, K.L. Pritchard, P.R. Unwin, Freshwater Biol. 22 (1989) 285–288. [16] R.G. Compton, G.M. Stearn, P.R. Unwin, A.J. Barwise, J. Appl. Electrochem. 18 (1988) 657–665. [17] W.J. Albery, S. Bruckenstein, Trans. Faraday Soc. 62 (1966) 1946–1954. [18] K. Aoki, H. Matsuda, J. Electroanal. Chem. 90 (1978) 333–346. [19] W.J. Albery, C.M.A. Brett, J. Electroanal. Chem. 148 (1983) 211–220. [20] J. Galceran, S.L. Taylor, P.N. Bartlett, J. Electroanal. Chem. 476 (1999) 132–147. [21] M. Thompson, O.V. Klymenko, R.G. Compton, J. Electroanal. Chem. 576 (2005) 333–338. [22] T. Paixao, E.M. Richter, J.G.A. Brito-Neto, M. Bertotti, J. Electroanal. Chem. 596 (2006) 101–108. [23] T.R.L.C. Paixao, R.C. Matos, M. Bertotti, Electrochim. Acta 48 (2003) 691–698. [24] I.B. Svir, A.I. Oleinick, R.G. Compton, J. Electroanal. Chem. 560 (2003) 117–126. [25] H. Rajantie, J. Strutwolf, D.E. Williams, J. Electroanal. Chem. 500 (2001) 108– 120. [26] B. Fosset, C.A. Amatore, J.E. Bartelt, A.C. Michael, R.M. Wightman, Anal. Chem. 63 (1991) 306–314. [27] B. Fosset, C. Amatore, J. Bartelt, R.M. Wightman, Anal. Chem. 63 (1991) 1403– 1408. [28] K. Aoki, K. Tokuda, H. Matsuda, J. Electroanal. Chem. 79 (1977) 49–78. [29] F. Marken, W.M. Leslie, R.G. Compton, M.G. Moloney, E. Sanders, S.G. Davies, S.D. Bull, J. Electroanal. Chem. 424 (1997) 25–34. [30] R.G. Compton, B.A. Coles, A.C. Fisher, J. Phys. Chem. 98 (1994) 2441–2445. [31] J.A. Alden, R.G. Compton, J. Electroanal. Chem. 415 (1996) 1–12. [32] J.A. Alden, M.A. Feldman, E. Hill, F. Prieto, M. Oyama, B.A. Coles, R.G. Compton, P.J. Dobson, P.A. Leigh, Anal. Chem. 70 (1998) 1707–1720. [33] M.J. Bidwell, J.A. Alden, R.G. Compton, Electroanalysis 9 (1997) 383–389. [34] R. Ferrigno, P.F. Brevet, H.H. Girault, J. Electroanal. Chem. 430 (1997) 235–242. [35] K. Tokuda, K. Aoki, H. Matsuda, J. Electroanal. Chem. 80 (1977) 211–222. [36] H. Rajantie, D.E. Williams, Analyst 126 (2001) 1882–1887. [37] P.R. Unwin, R.G. Compton, Comprehensive Chemical Kinetics, Elsevier, 1989 (Chapter 6). [38] G.L. Che, S.J. Dong, Electrochim. Acta 37 (1992) 2701–2705. [39] P. Tomcík, M. Krajcíková, D. Bustin, Talanta 55 (2001) 1065–1070. [40] P. Tomcík, M. Krajcíková, D. Bustin, I. Skacani, Electrochem. Commun. 3 (2001) 191–194. [41] N.S. Lawrence, E.L. Beckett, J. Davis, R.G. Compton, Anal. Biochem. 303 (2002) 1–16. [42] M.E. Snowden, P.H. King, J.A. Covington, J.V. Macpherson, P.R. Unwin, Anal. Chem. 82 (2010) 3124–3131. [43] M.E. Snowden, P.R. Unwin, J.V. Macpherson, Electrochem. Commun. 13 (2011) 186–189. [44] M.M. Mbogoro, M.E. Snowden, M.A. Edwards, M. Peruffo, P.R. Unwin, J. Phys. Chem. C 115 (2011) 10147–10154. [45] R.D. Fisher, M.M. Mbogoro, M.E. Snowden, M.B. Joseph, J.A. Covington, P.R. Unwin, R.I. Walton, ACS Appl. Mater. Interfaces 3 (2011) 3528–3537. [46] C. Amatore, O.V. Klymenko, I. Svir, Chem. Phys. Chem. 7 (2006) 482–487. [47] F. Bjorefors, C. Strandman, L. Nyholm, Electroanalysis 12 (2000) 255–261. [48] C. Amatore, M. Belotti, Y. Chen, E. Roy, C. Sella, L. Thouin, J. Electroanal. Chem. 573 (2004) 333–343. [49] S. Jacesko, J.K. Abraham, T. Ji, V.K. Varadan, M. Cole, J.W. Gardner, Smart Mater. Struct. 14 (2005) 1010–1016. [50] P. Bertoncello, I. Ciani, D. Marenduzzo, P.R. Unwin, J. Phys. Chem. C 111 (2007) 294–302. [51] E. Bitziou, N.C. Rudd, M.A. Edwards, P.R. Unwin, Anal. Chem. 78 (2006) 1435– 1443. [52] E. Bitziou, N.C. Rudd, P.R. Unwin, J. Electroanal. Chem. 602 (2007) 263–274. [53] T. Paixao, R.C. Matos, M. Bertotti, Talanta 61 (2003) 725–732.

E. Bitziou et al. / Journal of Electroanalytical Chemistry 692 (2013) 72–79 [54] R.C. Matos, M.A. Augelli, J.J. Pedrotti, C.L. Lago, L. Angnes, Electroanalysis 10 (1998) 887–890. [55] R.C. Matos, L. Angnes, M.C.U. Araujo, T.C.B. Saldanha, Analyst 125 (2000) 2011– 2015.

79

[56] M.C. Parkin, S.E. Hopwood, A.J. Strong, M.G. Boutelle, Trac-Trends Anal. Chem. 22 (2003) 487–497. [57] E.A. Hillard, F.C. de Abreu, D.C.M. Ferreira, G. Jaouen, M.O.F. Goulart, C. Amatore, Chem. Commun. 23 (2008) 2612–2628.