Disposable Glucose Sensors for Flow Injection Analysis Using Substituted 1,4-Benzoquinone Mediators

Feature Article

Disposable Glucose Sensors for Flow Injection Analysis UsingSubstituted 1,4-Benzoquinone MediatorsKing-Tong Lau,a Stephane A. L. de Fortescu,c Lindy J. Murphy,*b Jonathan M. Slater*c

a National Centre for Sensor Research, Dublin City University, Dublin 9, Irelandb Seconded to Birkbeck College from Drew Scientific Group PLC, Park Road, Barrow in Furness, Cumbria LA14 4QR, UK* e-mail: [email protected]

c Electroanalytical Group, School of Biological and Chemical Sciences, Birkbeck College, University of London, 29 Gordon Square,London, WCIH OPP, UK

* e-mail: [email protected]

Received: March 21, 2002Final version: July 22, 2002

AbstractThe redox chemistry of several substituted benzoquinones was investigated by cyclic voltammetry at a glassy carbonelectrode and candidates for inclusion in a mediated biosensor for use in flow analysis were selected on the basis ofoxidation potential, electrochemical reversibility and solubility. Glucose sensors constructed by sequential depositiononto a carbon pellet of 2,6-dimethyl-1,4-benzoquinone, 2,3,5,6-tetramethyl-1,4-benzoquinone or phenyl-1,4-benzo-quinone mediator solution, followed by glucose oxidase in polyvinylalcohol-Nafion solution, were tested for responseto glucose using flow injection analysis. Sensors prepared from 2,6-dimethyl-1,4-benzoquinone gave highest sensitivity,with a linear range of response to glucose of 2.5 ± 40 mM. The use of an enzyme-free comparative electrode toeliminate the response from interferents was investigated.

Keywords: Glucose biosensor, Substituted benzoquinone, Electron mediator, Disposable, Enzyme immobilization,FIA

1. Introduction

The use of electron mediators such as substituted ferroceneor ferricyanide is well established for one-shot screen-printed glucose sensors [1, 2]. Screen-printing can producereproducible and robust sensors, with the significant advan-tages of low cost of materials and automated fabricationprocesses [3 ± 5]. However, multi-analyte cartridges do notexclusively use screen-printed electrodes due to the diffi-culty in assembly of a cartridge containing several screen-printed sensors and the constraint that sodium and potas-sium ion selective electrodes are difficult to reliably screen-print. Consequently some commercial multi-analyte car-tridges aremanufactured using a combination of fabricationtechniques, such as the ion-sensing potentiometric sensorsmanufactured by Roche. These are formed by deposition ofmembrane solution onto a screen-printed base [6]. Anotherapproach is used by i-STAT Corporation, where all sensorsare formed by sequential deposition of solutions formingmultiple layers on the underlying silicon-based conductingmatrix [7].Inclusion of ion-selective electrodes on multi-analyte

cartridges usually results in a flow measurement mode inorder to establish a stable liquid junction potential betweenthe ion selective electrode and its associated referenceelectrode. Successful operation of mediated biosensorsincorporated in a multi-analyte cartridge used in a flowsystemwill put particular constraints on the solubility of the

mediator. Sensor cartridges are stored dry, and rapidsolubilization of the mediator on wet-up of the sensor isrequired. If the mediator solubility is too low, the biosensorwill have poor sensitivity to analyte resulting in a higher lowdetection limit,whereas if themediator solubility is too high,the mediator will rapidly wash out of the sensor and noresponse to analyte will be observed. In addition, lowconcentration of mediator in the sensor can result in poorcompetition with oxygen, the physiological electron accept-or, and subsequent loss of signal, particularly at low analyteconcentrations.In the search for suitable mediators for use in sensors in

flow systems, substituted benzoquinones were investigated.The use of benzoquinone as an electron mediator forglucose oxidase is well known and has been reported byseveral workers [8 ± 11]. Benzoquinone has also beenreported to act as an efficient mediator for other oxidor-eductase enzymes, such as sulfite oxidase [12] and �-aminoacid oxidase, [13] and also for nicotinamide adeninedinucleotide (NADH) [14]. However, although substitutedbenzoquinones have been reported to be suitablemediatorsfor the study of biological redox systems, [15] there are noreported examples of substituted benzoquinones acting asmediators to glucose oxidase.The high solubility of benzoquinone limits its application

in biosensors used in flow injection mode. Althoughadsorption of benzoquinone onto the electrode surfacecan give a stable sensor response for up to 48 hours, [14] the

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limited amount of mediator is not sufficient for measure-ment of high analyte concentrations. In addition, theworking potential of mediated biosensors using benzoqui-none can be relatively high for a mediated biosensor(� 500 mV vs. SCE) [9], and interferents such as ascorbicacid and uric acid can be oxidized at appreciable rates.Substituted benzoquinones could offer significant advan-tages over the parent benzoquinone as mediators inbiosensors in flow systems. Varying the substituents on thebenzoquinones results in a range of compounds withdifferent solubilities and oxidation potentials from whichselection can bemade for different applications. Thepresentstudy describes a low-cost disposable glucose sensor car-tridge suitable for flow analysis, prepared by immobilizingglucose oxidasewithin a polymer-compositemembrane onacarbon pellet modified with a substituted benzoquinone.

2. Experimental

2.1. Materials and Reagents

2,5-Dimethyl-1,4-benzoquinone (� 98%) was obtainedfrom Fluka. All other substituted benzoquinones (97%purity or higher) were obtained from Sigma-Aldrich.Glucose oxidase (Aspergillus niger, 200 U/mg), bovineserum albumin (BSA), �-glucose, polyvinylalcohol (PVA,average molecular weight 126000), Nafion (5% solution),bis-tris buffer, ascorbic acid, uric acid and acetaminophenwere obtained from Sigma-Aldrich. Carbon pellets wereobtained from Erodex, UK. Silver/silver chloride paste wasobtained from Metech, UK.All solutions were freshly prepared prior to use, with the

exception of glucose standards that were prepared at least24 h before use to allow full mutarotation. Bis-tris buffer(40 mM, pH 6.95) containing 0.8 mM Na2EDTA, 142 mMNaCl and 4.2 mMKClwas used as the carrier solution for FIAmeasurements and for preparation of all aqueous solutions.

2.2. Instrumentation

Measurements with cartridges in the flow cell were made ina flow system comprising a peristaltic pump (WatsonMarlow) and a manual injection valve (Omnifit). The flowrate used was 90 �Lmin�1. Measurements were made usingan AutoLab PGStat 100 instrument with multichannelfacility (3 measuring channels). The instrument was con-trolled by a PC which incorporated data capture softwareand data analysis facilities provided by the manufacturer.

2.3. Cyclic Voltammetry

The cyclic voltammetry of the compounds was investigatedusing a glassy carbon working electrode (4 mm diameter) inbis-tris buffer (pH 6.95). The benzoquinones were eitherused as saturated solution in buffer (or, if the solubility was

too low, using the compound deposited as a layer frombutanone solution (8 �L of 0.5 M)) onto the electrode. Ascan rate of 25 mV s�1 was used, with the scan starting at0 mV and commencing in the positive direction, withpotential limits of �0.6 and �0.6 VThe reference electrodewas a conventional silver/silver chloride/sat. KCl electrodeand the counter electrode was a platinum gauze electrode.

2.4. Sensor Cartridges and Flow Cell

Blank cartridges (i.e., unpopulated with pellets) weresupplied by Drew Scientific, Cumbria, UK. Figure 1 showsa schematic diagram of the flow cell configuration. Theblank cartridges, of dimensions 4 cm length, 1 cm width and4 mm depth, were made of either PVC or nylon. Eachcartridge contained 4 holes of 2 mm diameter, surroundedby a recess of 3 mm diameter and 0.1 mm depth on the frontface. Carbon pellets (2 mmdiameter and 4 mm length) werewashed with methanol followed by distilled water and driedin an oven before insertion in the cartridges. The washedpellets were press-fitted into the holeswith the top face levelwith the bottom of the recess. Silver epoxy glue was appliedto the back face of the pellets to ensure a good seal. OnecarbonpelletwasmodifiedwithAg/AgCl ink to act as both apseudo-reference electrode and counter electrode.The flow cell was made in-house to accommodate the

sensor cartridge. It consisted of two parts; the top block wasmade of transparent acrylic material with a recess of 0.2 mmdepth, 4 mm width and 4 cm length surrounded by a grooveinto which a rubber O-ring was inserted. At each end of therecess, a hole was drilled (0.8 mm diameter) to act as inletand outlet for flowing solution. The bottom block of the cellwas fitted with four copper spring probes that madeelectrical connection to the back face of the pellets. The

Fig. 1. Schematic diagram of the complete flow cell, consisting ofa top block with recess (A), a sensor cartridge (B) and a bottomblock with spring contacts (C). The individual components arebiosensors (1), (2) and (3), Ag/AgCl reference electrode (4),spring contacts (5), flow inlet (6), flow outlet (7), hole for screw toclamp the cell together (8), rubber O-ring (9) and a channel of0.2 mm depth (10).

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Electroanalysis 2003, 15, No. 11

cartridgewas clampedbetween the topandbottomblocks toform the flow cell.Benzoquinone mediated glucose sensing pellets were

prepared by sequential deposition of 3 layers comprising aninner mediator layer, an enzyme layer and an outer anti-interferent and diffusion layer. First 2 ± 4 �L of benzoqui-none solution in acetone (200 mM)was deposited onto eachcarbon pellet and allowed to air dry. Enzyme solution (4 �L)containing a mixture of glucose oxidase (2 ± 4 U/�L), PVA(0.09%) and Nafion (0.31%) was then deposited onto thebenzoquinone modified pellet. The final enzyme activitywas estimated to be 8 ± 16 U per sensor. The sensors wereair-dried for 20 minutes before deposition of 2.5% Nafion(3 �L of 5%Nafion diluted 1 :1 with propanol). The sensorswere allowed to air dry before storage at 4 �C until use. Thesensors were normally calibrated within a week of prepa-ration and discarded after use. A comparator sensorcontaining deactivated enzyme was prepared by treatingone of the working sensors with 0.1 M NaOH for 1 hourprior to use.All sensor calibrations were performed by chronoamper-

ometry at aworking potential of � 200 mVunless otherwisestated. Samples were not injected until the sensor responseshad reached a stable baseline (typically 10 min).

3. Results and Discussion

3.1. Cyclic Voltammetry

The redox chemistry of substituted benzoquinones is similarto that of the unsubstituted benzoquinone, in that thesubstituted benzoquinones can undergo two one-electronreduction reactions to the corresponding phenols, shown inFigure 2.DependingonpH, a proton canbe takenup in eachreduction step [16, 17].At neutral pH, each electron transferis accompanied by a proton transfer. There is a higheractivation energy barrier for the initial reduction, and thesecond electron transfer can be rapid so that cyclicvoltammograms of benzoquinones may show only oneoxidation peak and one reduction peak.The effects of substituents on the redox potential of

simple quinones has been studied by Driebergen et al. [17]Electron-donating groups were found to shift the half wavepotential (E1/2) to more negative values, with a potentialshift of 50 mV per methyl group. Consequently benzoqui-nonederivativeswith electron-donating substituentsmaybeused as mediators in biosensors at lower working potentialscompared to the parent compound benzoquinone, provided

the kinetics of electron transfer to the enzyme are suffi-ciently fast. The addition of substituents can also improvethe chemical stability [17] and can result in lower solubilityin aqueous media, which would be an advantage for sensorsused in a flowing stream.The cyclic voltammograms of some of the substituted

benzoquinones are shown in Figure 3. The oxidation andreduction potentials of benzoquinone and the substitutedbenzoquinones obtained from the cyclic voltammogramsare given in Table 1. Solubilities of the compounds in waterare also given. With the exception of p-benzoquinone, thesolubilities were obtained from estimation software and soare approximate values.However, there is a general trend ofalkyl groups decreasing solubility relative to 1,4-benzoqui-none, with t-butyl having a greater effect than methyl, andhydroxy and methoxy groups increasing solubility.Several of the benzoquinones had lower oxidation

potentials and solubilities than the parent compoundmaking them potential candidates for use in a mediatedbiosensor in a flow system, seen fromTable 1. The reductionin oxidation potential on increasing the number of methylgroups seen in Figure 3a follows the pattern observed byDriebergen, [17] with potential shifts of an average of �70.5 mV per methyl group. The oxidation/reduction peakcurrents are significantly smaller for tetramethyl-1,4-ben-zoquinone because of its lower solubility.Several di- and tri- substituted 1,4-benzoquinones with

similar oxidation potentials are compared in Figure 3b. Themethoxy and hydroxy methyl groups are less electrondonating than the methyl group and so compounds withthese substituents have higher oxidation potentials com-pared to 2,6-dimethyl-1,4-benzoquinone. 2,5-dimethyl-1,4-benzoquinone has a higher oxidation potential compared to2,6-dimethyl-1,4-benzoquinone since two methyl groups inthe meta position to each other results in greater chargestabilization compared to the para position.The cyclic voltammograms for phenyl-1,4-benzoquinone

and 2,6-dimethoxy-1,4-benzoquinone shown in Figure 3care asymmetrical, with an oxidation peak observed on thepositive scan and a plateau observed on the negative scan.The greater magnitude of current passed in the reverse scanindicates a proportion of the reduced species is notreoxidized, possibly because of loss to solution or polymer-ization of radical anions to form oligomers which are unableto undergo further oxidation.The cyclic voltammograms of the benzoquinones with

tertiary butyl substituents are not shown since they weregenerally poor, with small currents passed due to lowsolubility, and no clear oxidation or reduction peaks. An

Fig. 2. Reaction scheme to illustrate the two step electrochemical reduction of benzoquinones to the corresponding phenols.

977Disposable Glucose Sensors for Flow Injection


exception was 3,5-di-tert-butyl-1,4-benzoquinone, whichhad well defined oxidation and reduction peaks, probablydue to the tertiary butyl groups being meta to each other.The cyclic voltammogram of 2,5-dihydroxy-1,4-benzoqui-none is also not shown, since it rapidly formed a soluble redproduct and was assumed to have undergone polymeriza-tion.

3.2. Evaluation of Mediators in Sensor Cartridges

Three of the substituted benzoquinones, 2,6-dimethyl-1,4-benzoquinone (2,6-DMBQ), phenyl-1,4-benzoquinone

(PBQ) and tetramethyl-1,4-benzoquinone (TMBQ) wereidentified as having lower oxidation potential and solubilitycompared to the unsubstituted benzoquinone. 2,5-Dimeth-yl-1,4-benzoquinone was not tested for its use in a sensorsince it had similar solubility and oxidation potential to 2,6-DMBQ. These three compounds were used to preparesensor cartridges for use in the flow system. Workingpotentials of � 200 mV for 2,6-DMBQ and TMBQ and �300 mV for PBQ were selected to be at least 60 mV morepositive than the oxidation potential observed in the cyclicvoltammetry experiments. For comparison, sensors pre-pared with 1,4-benzoquinone (1,4-BQ) were also testedusing the higher working potential of � 400 mV.The sensors were fabricated by sequential deposition of

three layers from solution. The first layer consisted of themediator deposited from solution in acetone. It was foundnecessary to place the mediator next to the electrode toensure fast electron transfer and to minimize loss ofmediator to solution. The second layer contained immobi-lized enzyme. The immobilization of glucose oxidase byNafion, a perfluorosulfonic acid polymer, has been reported[18] and was initially employed for the sensor fabrication.However, severe cracking of the membrane was observedafter storage of the dry sensors for a few days. This problemwas solved by using a PVA/Nafion composite membranethat had a much smoother appearance after drying. Thethird layer was a charge exclusion membrane of Nafion,which also acted as an outer diffusion layer. The PVA/Nafion/GOX membrane composite provided a smoothersurface onto which the outer layers of pure Nafionmembrane could be deposited and prevented cracking ofthis outer layer.Flow injection analysis showed the most promising

mediator compound was 2,6-DMBQ. Typical real timecalibration responses obtained from three sensors on thesame sensor cartridge using 2,6-DMBQ as mediator areshown inFigure 4. The glucose sensors responded to glucosein the range 2.5 ± 125 mM. Calibration plots of response to

Fig. 3. Cyclic voltammograms of benzoquinones at a glassycarbon electrode. A) 1,4-benzoquinone (solid line), methyl-1,4-benzoquinone (dashed line), 2,6-dimethyl-1,4-benzoquinone (dotand dash line) and tetramethyl-1,4-benzoquinone (dotted line); B)2,6-dimethyl-1,4-benzoquinone (solid line), 2,5-dimethyl-1,4-ben-zoquinone (dashed line), 2,3-dimethoxy-5-methyl-1,4-benzoqui-none (dot and dash line) and 2-hydroxymethyl-6-methoxy-1,4-benzoquinone (dotted line); C) phenyl-1,4-benzoquinone (solidline) and 2,6-dimethoxy-1,4-benzoquinone (dot and dash line).

Fig. 4. Current responses on injection of glucose samples forthree sensors on a single cartridge using 2,6-dimethyl-1,4-benzo-quinone mediator. Solid, dotted and dashed lines correspond tothe responses from sensors 1, 2 and 3, respectively. The flowdirection was from sensor 3 to sensor 1. The glucose concen-trations of the samples were 5, 10, 20, 40, 62.5 and 125 mM inorder of increasing time.



glucose are shown in Figure 5. The linear range of responsewas 2.5 ± 40 mM glucose. This range satisfies the require-ments for clinical diagnosis. [19] The sensors had an averagesensitivity of response to glucose of 42 nA/mM (n� 9).The responses of sensors to glucose using 1,4-BQ were

determined for comparison, using three sensors on the samesensor cartridge. The sensors had a linear response range of5 ± 20 mM glucose and higher average sensitivity of re-

sponse to glucose of 166 nA/mM (n� 3). The highersensitivity reflects the slightly better mediating ability ofthe parent compound, due to higher solubility and possiblyalso due to higher rate of electron transfer between theenzyme redox site and the mediator, since placing substitu-ents on the 1,4-benzoquinone could hinder close approach.Sensors usingTMBQhad very low sensitivities (10 nA/mM)and a lower response range of 5 ± 20 mM glucose, due to thelower solubility of this compound compared to the twoothersubstituted benzoquinones used. Sensors using PBQ had alinear response range similar to 2,6-DMBQ, but thesensitivity (20 nA/mM glucose) was also low, probablybecause of the lower solubility. In addition, PBQ wasunstable on storage, changing color from orange to black.All further work was performedwith sensors prepared from2,6-DMBQ.

3.3. Characterization of Glucose Sensors with 2,6-DMBQMediator

Figures 4 and 5 show that there was significant variation inthe sensitivity of the sensors due to the manual method ofpreparation (20%). The sensor layers were depositedmanually and it is expected that mass fabrication of the

Table 1. Solubility values and oxidation/reduction potentials for benzoquinone compounds. Solubility values are estimated values inwater at 25 �C, following the method of Meylan et al [23] using estimated octanol/water partition coefficients determined with KowWinEstimation software [24].

Benzoquinone (BQ) Solubility(104 mg dm�3)

Oxidationpotential (mV)

Reductionpotential (mV)

Notes

1,4-BQ 2.3 (1.1 [b]) � 278 � 98methyl-1,4-BQ 2.0 (1.9 [c]) � 200 � 492,5-dimethyl-1,4-BQ 0.18 � 151 � 15 Small peak

at � 220 onnegative scan

2,6-dimethyl-1,4-BQ 0.62 (0.78 [c]) � 137 � 20tetramethyl-1,4-BQ 0.029 (0.044 [c]) � 22 � 145phenyl-1,4-BQ 0.049 (0.11 [c]) � 229 wave starting

at � 146 [a]2,3-dimethoxy-5-methyl-1,4-BQ 1.98 (1.11 [c]) � 156 � 152,5-dihydroxy-1,4-BQ 31.9 wave starting

at � 313,peak at � 557

wave startingat� 288,peak at � 73

2,6-dimethoxy-1,4-BQ 5.09 (7.06 [c]) � 156 � 171 [a] Small peakat � 112 mVon negative scan

2-hydroxymethyl-6-methoxy-1,4-BQ 28.5 � 163, � 529 � 75 Shoulder onreverse scanat � 188

2-tert-butyl-1,4-BQ 0.11 plateau at � 119 Hysteresis onreverse scan

2,5-di-tert-butyl-1,4-BQ 0.0002 ± ± No peaks2,6-di-tert-butyl-1,4-BQ 0.0011 (0.0011 [c]) � 71 � 32, � 179 [a] Small negative

peaks3,5-di-tert-butyl-1,2-BQ 0.0002 � 220 � 733-tert-butyl-5-methoxy-1,2-BQ 0.14 ± ± No peaks4-tert-butyl-5-methoxy-1,2-BQ 0.090 ± ± No peaks

[a] Hysteresis on reversing scan direction at � 600 mV. [b] Experimentally determined solubility at 18 �C [25]. [c] Estimated solubility obtained from theSyracuse Research Corporation Database are given in parentheses for comparison [26].

Fig. 5. Calibration plots for the data obtained in Figure 4. Sensor1 (�), sensor 2 (�) and sensor 3 (�).



sensors should lead to greater reproducibility in sensorsensitivity. In addition, the three sensors on the samecartridge did not experience the same working potential,since the distance between each sensor and the Ag/AgClreference electrodewas greater for one sensor, resulting in alarger potential drop. The flow dynamics also played a part,since slightly different responses were obtained from eachindividual sensor when the direction of flow was reversed.Therefore comparison of sensors on different cartridges wasbest made using sensors at the same position.There was a 30% loss of sensitivity after four calibrations

(4000 s duration each), indicating the Nafion/PVA polymercomposite could retain glucose oxidase sufficiently well foruse in a disposable sensor for one shot use. Sensors storedimmersed in buffer solution at room temperature aftercalibrations were found to be active after 5 days, but only50% of the sensitivity was retained, presumably due to lossof enzyme activity. The sensors could be usedwith flow ratesup to 120 �L min�1 without measurable loss of response toglucose, indicating good retention of immobilized enzymeand mediator below this flow rate. The response becameunacceptably noisy at higher flow rates.The potential dependence of the sensor responses to

glucose, ascorbate and to amixture of glucose and ascorbateis shown in Figure 6.Above 100 mV, the response to glucoseis not significantly affected by the increase in appliedpotential. However, the response to ascorbate or a mixtureof glucose and ascorbate increased sharply with increasingpotential. The chosen working potential of � 200 mV for2,6-DMBQ mediated sensors minimizes the effect ofascorbate while maintaining good response to glucose.Initial work investigating the effect of interferents found

that the sensor response to 18 mMglucose was 5%higher inthe presence of 200 �M ascorbate compared to the absenceof ascorbate, indicating the outer Nafion film is reasonablyeffective in rejecting ascorbate. Responses to 100 �M urate

and 500 �M paracetamol were negligible, due to the lowworking potential. It was concluded the outer anti-interfer-ent film needs to be improved to attain the accuracyrequired for clinical use.An alternative approach to correct for the effect of

interferents is the use of a comparator sensor or differentialmeasurement [20 ± 22]. Two sensors are prepared, one withand one without active enzyme. If the responses due tointerferent and substrate are additive, the response of thecomparator sensor can be subtracted from that with theactive enzyme to obtain a signal corrected for the effect ofinterferents. Normalized responses can be used to compen-sate for the slight differences in response that can occurbetween two sensors due to small variations in the diffusionfilm thickness.The use of a comparator sensor for data correction is

demonstrated in Figure 7. The response to a mixed samplecontaining 17 mM glucose, the interferents ascorbate, urateand paracetamol, and BSA was compared to a samplecontaining only 17 mM glucose. The comparator sensorresponded to both solutions indicating the enzyme was notcompletely deactivated. Subtraction of the comparatorsensor response from the responses of the two glucosesensors gave corrected responses in reasonable agreementwith the sample containing only glucose (10% overestima-tion). Automated sensor fabrication should result in repro-ducible sensor responses and allow the inclusion of acomparator sensor as part of the sensing cartridge.The long-term storage stability of the sensor was inves-

tigated using a batch of sensor cartridges stored dry at 4 �Cafter preparation. A fresh cartridge was taken from thebatch at weekly intervals and tested for response to glucose.The sensitivity of response to glucose decreased by 90%after 10 weeks, although the linear range of response wasmaintained up to 30 mM. The decrease in sensitivity wasprobably due to enzyme inactivation, although loss ofmembrane permeability on storage could also have oc-curred. The storage stability of the sensors clearly needs tobe improved, such as by the use of filtered solutions during

Fig. 6. Potential dependence of the sensor response to 18 mMglucose (�), 2 mM ascorbic acid (�) and a mixed solution of18 mM glucose and 2 mM ascorbic acid (�). The responses wereobtained from one individual sensor on the cartridge.

Fig. 7. Current responses on the injection of sample containing17 mM glucose (A) and a mixture containing 17 mM glucose,0.1 mM uric acid, 0.5 mM ascorbic acid, 0.5 mM paracetamol and40 mg/mL BSA (B). The sensor cartridge consisted of two glucosesensors (dotted and dashed lines) and a comparator sensor (solidline).



sensor preparation to remove microbial species and greatercontrol of humidity during storage.

4. Conclusions

A series of benzoquinone compounds have been investi-gated for use as mediators for glucose oxidase and 2,6-dimethyl-1,4-benzoquinone has been shown to have the bestcombinationof lowoxidationpotential and solubility for usein single shot sensors. The oxidation potential comparesfavorably with existing established mediators such asferrocenes [1] and benzoquinone [8]. The reported sensorconfiguration is simple to implement and amenable to massproduction techniques.

5. Acknowledgements

K.-T. L., S.A.L.D.F. and J. M. S. wish to thank DrewScientific for financial support. This work is described inpatent application number GB 0125094.3.

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Disposable Glucose Sensors for Flow Injection Analysis Using Substituted 1,4-Benzoquinone Mediators