Markas A. T. Gilmartin

Fabrication and characterization of a screen-printed, disposable, amperometric cholesterol biosensor

A single-use, reagentless, screen-printed biosensor strip for the measurement of free cholesterol was fabricated and its performance characteristics were evaluated by cyclic voltammetry and under steady-state conditions. Cholesterol oxidase was adsorbed on the surface of an H2O2-sensing cobalt phthalocyanine screen-printed carbon electrode and retained at the sensory interface with a solvent-cast cellulose acetate membrane. The device, operated in the amperometric mode, provides an inexpensive, reliable and rapid means of quantifying this clinically significant sterol. Its analytical utility is further augmented by its ease of fabrication and favourable storage properties. The biosensor exhibits a wide functional range (6 × 10–5–5.0 × 10–3 mol dm–3) and the sensitivities of the devices (slopes of the calibration graphs) were used to derive optimum solution conditions. Several analytical parameters were also investigated to optimize and enhance the response time and sensitivity of the sensor and these are also discussed.

Voltammetric and amperometric behaviour of uric acid at bare and surface-modified screen-printed electrodes: studies towards a disposable uric acid sensor

Systematic voltammetric and amperometric studies have been undertaken to examine the electrochemical behaviour of uric acid at bare and surface-modified screen-printed electrodes. The precision of the electrode manufacture was determined by cyclic voltammetry with a 1.0 × 10–4 mol dm–3 uric acid solution and was calculated to be 6.0%(n= 5). Several strategies were investigated in an attempt to eliminate interference from ascorbic acid. These involved coating the electrode surface with Nafion, or the enzyme L-ascorbic acid oxidase. The latter was immobilized using one of two methods: either by a simple adsorption process, or by cross-linking with bovine serum albumin and glutaraldehyde. The amperometric response at the surface-adsorbed enzyme electrode for uric acid was linear over the concentration range from 5.08 × 10–6 to 1.51 × 10–4 mol dm–3; the limit of detection was 2.54 × 10–6 mol dm–3 using a full-scale deflection of 0.5 µA. This modified electrode exhibited no response to ascorbic acid at levels up to 0.53 mmol dm–3. The electrode modified by cross-linking the enzyme to the surface showed no response to ascorbic acid concentrations of up to 0.093 mmol dm–3.

Development of amperometric sensors for uric acid based on chemically modified graphite–epoxy resin and screen-printed electrodes containing cobalt phthalocyanine

Cyclic voltammetry, hydrodynamic voltammetry (HDV) and amperometry were used to investigate the behaviour of uric acid at carbon(graphite)–epoxy resin electrodes modified with the following electrocatalysts: ferrocenes, phthalocyanines, phenoxazines, phenazines and hexacyanoferrate species. As no direct electrocatalytic oxidation of uric acid was apparent, enzyme-coupled mediation was studied using uricase with electrodes modified with ferrocene, cobalt phthalocyanine (CoPC) and Meldolas Blue, in the presence and absence of oxygen. Again, no electrocatalysis was observed so we exploited the specificity of the enzyme uricase, which is known to produce hydrogen peroxide from uric acid. This product has been shown to undergo electrocatalysis at modified electrodes, and preferentially traverses certain membranes, so this rationale was adopted for the ensuing studies. Cyclic voltammetry and amperometry were used to study the electrochemical behaviour of H2O2 in 0.05 mol dm–3phosphate buffer (pH 9.3). Hydrogen peroxide was found to be electroactive at substantially lower potentials (<600 mV) at CoPC-modified electrodes than at unmodified electrodes. The precision of surface–surface reproducibility of epoxy resin electrodes was calculated by cyclic voltammetry to be 7.3%(n= 5). Further studies were performed with and without cellulose acetate (CA) membranes using amperometry in stirred solution. Epoxy resin electrodes were simply dropcoated with CA. The membrane functioned as an effective barrier against a range of potential interferences, but allowed the selective detection of enzymically generated H2O2 from urate. This methodology was then translated to screen-printed electrodes (SPEs). Cyclic voltammetry was first used to establish the precision of electrode manufacture; this was 4.9%(n= 5). The SPEs were then modified by placing each inside a tube, one end of which was covered with a solvent cast (CA) membrance. This electrode–polymer configuration furnished similar permselective properties as the drop-coated devices. The solvent cast membranes were treated with various concentrations of uricase; when the membrane was loaded with 0.48 U of enzyme a measurable H2O2 signal was obtained at the CoPC–SPE for a solution containing 0.18 mmol dm–3 uric acid.

Prototype, solid-phase, glucose biosensor

A single-shot reagentless biosensor for the amperometric determination of glucose is described. The device has been produced in a novel dual-electrode configuration, whereby the working and reference/counter electrodes are printed alongside one another in a disposable format. The biosensor is based on a cobalt phthalocyanine-containing screen-printed carbon electrode coated with cellulose acetate and glucose oxidase (GOD). The organometallic redox mediator permits the detection of GOD-evolved H2O2 at markedly reduced (<0.5 V, Ag/AgCl) overvoltages. The selectivity is further enhanced by the production of a solvent-cast, H2O2-permeable film directly over the sensor assembly, serving to screen out interferents and electrode-fouling agents. Voltammetry and amperometry have been used to optimize the sensors analytical performance and SEM to investigate the efficacy of producing the chemical and biological layers.

Rapid detection of paracetamol using a disposable, surface-modified screen-printed carbon electrode

A method for the rapid and simple determination of paracetamol in urine is described. The transducer is a reagentless, amperometric sensor that was developed through the combined use of screen-printing and permselective membrane technologies. The sensor operates selectively by means of an anti-interference barrier, cellulose acetate, which is drop-coated directly on to the screen-printed carbon electrodes. The disposable, amperometric sensor require no further modification procedures to enhance selectivity, i.e., no enzymes are involved, and hence their fabrication is simple and extremely economical. Given the rapid response times, 13 × 10–6 mol dm–3 limit of detection and wide functional range, linear up to 2 × 10–3 mol dm–3, they represent an attractive alternative for paracetamol monitoring. The surface-modified strips were applied to the detection of paracetamol in urine, correlating well with a standard enzyme–colour reagent kit.

Comparative study of the voltammetric behaviour of guanine at carbon paste and glassy carbon electrodes and its determination in purine mixtures by differential-pulse voltammetry

Cyclic voltammetry and differential-pulse voltammetry (DPV) were used in a comparative investigation into the electrochemical oxidation of guanine at a carbon paste electrode (CPE) and a planar glassy carbon electrode (GCE). The DPV peak potential and peak current were found to be dependent on the pH of the 0.5 mol dm–3 phosphate buffer over the range 3.0–9.0 for both electrodes. The effect of ionic strength was investigated over the concentration range 0.05–0.5 mol dm–3(pH 5.0). Slightly larger peak currents were obtained with 0.5 mol dm–3 supporting electrolyte at both electrodes; however, an approximate 2.5-fold increase in the anodic response was observed with the CPE. The electro-oxidation of guanine was found to occur with the production of one voltammetric peak; the product probably undergoes further oxidation to yield two peaks on the second anodic scan. Under these conditions the electrode reaction was identified as being irreversible. Systematic studies into the possible adsorption of guanine were performed on a 1 × 10–5 mol dm–3 guanine solution in 0.5 mol dm–3 phosphate buffer by DPV, utilizing CPEs and GCEs over the pH range 5.0–9.0. The peak currents did not increase with various accumulation potentials and times, indicating that the parent compound does not adsorb at either of the electrode surfaces studied. The optimum medium for quantitative determination at a CPE or a GCE by DPV was found to be 0.5 mol dm–3 phosphate buffer (pH 5.0). Anodic peak currents were found to be linearly related to concentration over the range 1 × 10–5–1 × 10–7 mol dm–3 and 1 × 10–5–7.5 × 10–7 mol dm–3 for the CPE and GCE, respectively. Guanine was successfully determined in a deoxyribonucleic acid sample, following acid hydrolysis, with use of DPV. The developed method is more sensitive than those of previous voltammetric studies.

Efficacy of organometallic-containing screen-printed carbon strips as catalysts for the oxidation of hydrogen peroxide : a voltammetric and X-ray photoelectron spectroscopic investigation

Abstract Votammetric and amperometric techniques have been used for the systematic optimization of chemically modified screen-printed carbon electrodes (SPCEs) with regard to the determination of H2O2. A variety of metallophthalocyanines were incorporated into the base carbon matrix with a view to selecting the most efficient redox catalyst for H2O2 oxidation. Sensor tailoring with organometallics accelerated the rate of this process; typical reductions in overvoltage were in the 700, 800 and 900 mV (vs. saturated calomel electrode (SCE)) regions for cobalt-, manganese- and iron-containing phthalocyanines respectively, when compared with their unmodified counterparts. The considerable faradaic activity of the redox mediators was further demonstrated by the rapid, sensitive and reliable steady state responses to concentrations of H2O2 over an analytically useful range. A mechanism for the catalytic processes has been postulated on the basis of voltammetric, amperometric and X-ray photoelectron spectroscopic evidence. The precision of sensor fabrication and stability were also investigated.

Development of one-shot biosensors for the measurement of uric acid and cholesterol

This paper is a case study in the design, fabrication and characterization of screen-printed biosensors for the measurement of uric acid and cholesterol. Four different detection strategies, namely direct oxidation, direct mediation, direct enzyme-linked mediation and indirect electrochemical oxidation are discussed with their accompanying selectivity profiles. The work culminates in the production of disposable, printed biosensors for uric acid and cholesterol; the merits and demerits of each system are outlined. Such biosensors are based on novel solid-phase biochemistry and are likely to enable simple, reagentless and convenient near-patient testing. The potentialities of such a decentralized approach to analysis are also described.