The fabrication, characterisation and electrochemical investigation of screen-printed graphene electrodes.

We report the fabrication, characterisation (SEM, Raman spectroscopy, XPS and ATR) and electrochemical implementation of novel screen-printed graphene electrodes. Electrochemical characterisation of the fabricated graphene electrodes is undertaken using an array of electroactive redox probes and biologically relevant analytes, namely: potassium ferrocyanide(II), hexaammine-ruthenium(III) chloride, N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), ß-nicotinamide adenine dinucleotide (NADH), L-ascorbic acid (AA), uric acid (UA) and dopamine hydrochloride (DA). The electroanalytical capabilities of the fabricated electrodes are also considered towards the sensing of AA and DA. The electrochemical and (electro)analytical performances of the fabricated screen-printed graphene electrodes are considered with respect to the relative surface morphologies and material compositions (elucidated via SEM, Raman, XPS and ATR spectroscopy), the density of electronic states (% global coverage of edge-plane like sites/defects) and the specific fabrication conditions utilised. Comparisons are made between two screen-printed graphene electrodes and alternative graphite based screen-printed electrodes. The graphene electrodes are fabricated utilising two different commercially prepared 'graphene' inks, which have long screen ink lifetimes (>3 hours), thus this is the first report of a true mass-reproducible screen-printable graphene ink. Through employment of appropriate controls/comparisons we are able to report a critical assessment of these screen-printed graphene electrodes. This work is of high importance and demonstrates a proof-of-concept approach to screen-printed graphene electrodes that are highly reproducible, paving the way for mass-producible graphene sensing platforms in the future.

Fabrication of co-planar screen printed microband electrodes.

We demonstrate the first example of the fabrication of co-planar 50 µm (width) screen printed graphite microbands (length: 20 mm), fabricated entirely via screen printing which are characterised both microscopically and electrochemically via cyclic voltammetry and evaluated towards the sensing of NADH offering a competitive limit of detection (3σ) of 0.24 µM. The fabricated electrodes are also shown to be extended to gold screen printed microbands which are evaluated towards the sensing of chromium(VI) offering a limit of detection (3σ) of 2.65 µM. These microbands are seen to be the first produced entirely through screen printed technology potentially allowing disposable, mass produced microbands to be realised.

Electroanalytical sensing of chromium(III) and (VI) utilising gold screen printed macro electrodes.

We report the fabrication of gold screen printed macro electrodes which are electrochemically characterised and contrasted to polycrystalline gold macroelectrodes with their potential analytical application towards the sensing of chromium(III) and (VI) critically explored. It is found that while these gold screen printed macro electrodes have electrode kinetics typically one order of magnitude lower than polycrystalline gold macroelectrodes as is measured via a standard redox probe, in terms of analytical sensing, these gold screen printed macro electrodes mimic polycrystalline gold in terms of their analytical performance towards the sensing of chromium(III) and (VI), whilst boasting additional advantages over the macro electrode due to their disposable one-shot nature and the ease of mass production. An additional advantage of these gold screen printed macro electrodes compared to polycrystalline gold is the alleviation of the requirement to potential cycle the latter to form the required gold oxide which aids in the simplification of the analytical protocol. We demonstrate that gold screen printed macro electrodes allow the low micro-molar sensing of chromium(VI) in aqueous solutions over the range 10 to 1600 µM with a limit of detection (3σ) of 4.4 µM. The feasibility of the analytical protocol is also tested through chromium(VI) detection in environmental samples.

Screen printed graphite macroelectrodes for the direct electron transfer of cytochrome c.

We report the direct electrochemistry of cytochrome c at screen printed graphite electrodes which exhibits quasi-reversible voltammetric responses without the need for any chemical or electrochemical pre-treatment, use of mediators or nanomaterials. Through voltammetric studies and X-ray photoelectron spectroscopy (XPS) it is shown that carbonyl and carboxylic surface oxygenated species likely residing at edge plane like- sites/defects of the graphite comprising the screen printed electrodes are responsible for the favourable interaction of the cytochrome c with that of the screen printed electrochemical sensing platform.

New directions in screen printed electroanalytical sensors: an overview of recent developments.

Screen printing is widely used to fabricate disposable and economical electrochemical sensors and has helped us to establish the route from 'lab-to-market' for a plethora of sensors. We overview recent developments in the field where screen printed electrochemical sensors are utilised. Starting with their fundamental understanding, through to highlighting new developments in bulk metal and mediator modified electrodes, as well as novel advantageous electrode designs, we demonstrate the wide and diverse range of applications that sensors based on this fabrication approach have achieved.

Disposable manganese oxide screen printed electrodes for electroanalytical sensing.

Manganese dioxide screen printed graphite electrodes for electro-analytical sensing purposes have been fabricated. The prepared sensors exhibit attractive performances as electro-catalysts for the sensing of nitrite ions and ascorbic acid with detection limits comparable or lower than those obtainable with other electrochemical sensors. The manganese dioxide platforms are also explored towards the electrochemical reduction of oxygen which has importance in electrochemical energy storage. While the platforms are found to operate by a four-electron reduction producing water (rather than hydrogen peroxide which is undesirable), a large overpotential, compared to other possible electrode compositions reported in the literature, is required. Nevertheless, the promising results towards the electro-analytical sensing of nitrite ions and ascorbic acid and also given the disposable nature and scale of economies make these electrodes useful as sensor platforms.

Graphite screen printed electrodes for the electrochemical sensing of chromium(VI).

We demonstrate that graphite screen printed macroelectrodes allow the low ppb sensing of chromium(VI) in aqueous solutions over the range 100 to 1000 microg L(-1) with a limit of detection of 19 microg L(-1). The underlying electrochemical mechanism is explored indicating an indirect process involving surface oxygenated species. The drawbacks of using hydrochloric acid as a model solution to evaluate the electrochemical detection of chromium(VI) are also pointed out. The analytical protocol is shown to be applicable for the sensing of chromium(VI) in canal water samples at levels set by the World Health Organisation. The protocol is simplified over existing analytical methodologies and given its analytical performance and economical nature, holds promise for the de-centralised screening of chromium(VI).

"Cosmetic electrochemistry": the facile production of graphite microelectrode ensembles.

The facile and rapid production of microelectrode ensembles is shown to be possible using off-the-shelf cosmetic products and is exemplified with the electrochemical sensing of a toxic metal offering a novel fabrication methodology.

Gold nanoparticle ensembles allow mechanistic insights into electrochemical processes.

Gold-nanoparticle-modified electrodes find wide and diverse applications in the area of electrochemistry. We demonstrate for the first time that gold-nanoparticle-modified electrodes can provide mechanistic information and we exemplify this with the electrochemical deposition of arsenic(III). Our approach of using nanoparticle ensembles is a facile and economical methodology that provides an alternative to using expensive gold single-crystal electrodes that require careful surface preparation before each measurement.

High throughput screening of lead utilising disposable screen printed shallow recessed microelectrode arrays.

The cathodic stripping voltammetry of lead at disposable screen printed shallow recessed microelectrode arrays has been developed for the first time. The array comprises 6 microdiscs which have radii of 116 (+/-6) microns which are recessed by 4 microns and are separated by 2500 microns from their nearest neighbour in a hexagonal arrangement. The electroanalytical determination of lead was explored in 0.1 M nitric acid and found that using a 120 s deposition time, a detection limit of 3 microM is feasible which is not possible utilising a screen printed graphite macro-electrode. The sensitivity of this analytical protocol can be tailored by varying the deposition time and it is found that increasing this to 320 s facilitates a limit of detection of 39 nM. This methodology is shown to be feasible for the portable and economical screening of lead in river water samples at the levels indicated by the EC Dangerous Substances Directive (76/464/EEC).

Why 'the bigger the better' is not always the case when utilising microelectrode arrays: high density vs. low density arrays for the electroanalytical sensing of chromium(VI).

We demonstrate, with the example of the electroanalytical sensing of chromium(vi) using ultra-microelectrode arrays, that a larger number of microelectrodes comprising an array do not necessarily provide improved electroanalytical performance. Using a low density array, which consists of 256 microdiscs where each microdisc comprising the array has a radius of 10 microns in a cubic arrangement separated from their nearest neighbour by 100 microns, the electroanalytical sensing of chromium(vi) is shown to be possible over the range 13-428 microM with a limit of detection of 3.4 microM readily achievable. Using a high density microelectrode, consisting of 2597 microdiscs where each microdisc has a radius of 2.5 microns in a hexagonal pattern which are separated from their nearest neighbour by 55 microns, the electroanalytical performance, in terms of linear range and sensitivity, is considerably lower going against the misconception that a high density array should produce a superior analytical response. The reason for this disparity is discussed and it is shown that the arrangement of the microelectrodes on the array is critical due to the interaction of diffusion zones between neighbouring electrodes allowing analysts to make informed decisions on the conscientious choice of microelectrode arrays.

Enhancement by Copper(II) of the voltammetric signal of vitamin B2 applied to its determination in breakfast cereals.

Addition of copper(II) to breakfast cereal samples was shown to significantly enhance the analytical signal obtained by electrochemical reduction of vitamin B(2) using linear sweep voltammetry on disposable carbon electrodes. The enhancement was observed only when dissolved oxygen was present. In model solutions the analytical signal was linear in the concentration range 6-150 ng/mL with a calculated limit of detection of 5 ng/mL (S/N = 3). This compared favorably with earlier work using a similar measurement approach--but in the absence of copper--in which the limit of detection was calculated to be 900 ng/mL. The effects of potential interferents commonly found in cereals were examined. In addition to signal attenuation by both sugar and starch (already reported), folic acid was found to increase (+6%) and iron to decrease (-11%) the analytical signal when present in the maximum concentration ratios, with respect to vitamin B(2), that are normally found in breakfast cereals. Nevertheless, the simplicity of the approach was potentially attractive for near-line quality control applications in manufacturing. The utility of the measurement approach was demonstrated by the addition of excess copper(II) sulfate to determine vitamin B(2) in aqueous extracts of breakfast cereals. The results agreed well with those provided by the cereal manufacturer who used an established HPLC method.

Next generation screen printed electrochemical platforms: Non-enzymatic sensing of carbohydrates using screen printed electrodes.

The first example of a copper(ii) oxide screen printed electrode is reported which is characterised with microscopy and explored towards the electrochemical sensing of glucose, maltose, sucrose and fructose. It is shown that the non-enzymatic electrochemical sensing of glucose with cyclic voltammetry and amperometry is possible with low micro-molar up to milli-molar glucose readily detectable which compares competitively with nano-catalyst modified electrodes. The sensing of glucose shows a modest selectivity over maltose and sucrose while fructose is not detectable. An additional benefit of this approach is that metal oxides with known oxidation states can be incorporated into the screen printed electrodes allowing one to identify exactly the origin of the observed electro-catalytic response which is difficult when utilising metal oxide modified electrodes formed via electro-deposition techniques which result in a mixture of metal oxides/oxidation states. These next generation screen printed electrochemical sensing platforms provide a simplification over previous copper oxide systems offering a novel fabrication route for the mass production of electro-catalytic sensors for analytical and forensic applications.

Screen printed electrochemical platforms for pH sensing.

We explore the possible use of screen printing technology for fabricating disposable electrochemical platforms for the sensing of pH. These screen printed pH sensors incorporate the pH sensitive phenanthraquinone moiety which undergoes a Nernstian potential shift with pH, and the pH insensitive dimethylferrocene which acts as an internal reference. This generic approach offers a calibration-less and reproducible approach for portable pH measurements with the possibility of miniaturisation allowing incorporation into existing sensing devices. The advantages, limitations and future prospects of this fabrication approach for producing electrochemical platforms for pH sensing are also discussed.

Development of disposable bulk-modified screen-printed electrode based on bismuth oxide for stripping chronopotentiometric analysis of lead (II) and cadmium (II) in soil and water samples.

A bulk-modified screen-printed carbon electrode characterised for metal ion detection is presented. Bismuth oxide (Bi(2)O(3)) was mixed with graphite-carbon ink to obtain the modified electrode. The best composition was 2% Bi(2)O(3) (wt%) in the graphite-carbon ink. The modified electrode with onboard screen-printed carbon counter and silver-silver chloride pseudo-reference electrodes exhibited good performance in the electrochemical measurement of lead (II) and cadmium (II). The electrode displayed excellent linear behaviour in the concentration range examined (20-300 microg L(-1)) with limits of detection of 8 and 16 microg L(-1) for both lead (II) and cadmium (II), respectively. The analytical utility of the modified electrode was illustrated by the stripping chronopotentiometric determinations of lead (II) in soil extracts and wastewater samples.

Disposable sensor for measurement of vitamin B(2) in nutritional premix, cereal, and milk powder.

Methodologies for the determination of vitamin B(2) in food matrixes and a premix using simple sample conditioning steps coupled with a convenient and cheap electrochemical sensing device are presented. Electrochemical analysis based on differential pulse voltammetry (DPV) coupled to carbon electrodes gave a well-defined reduction peak at -0.42 V versus a Ag/AgCl quasi-reference electrode. Using a straightforward sample preparation step, vitamin B(2) can be measured successfully in a nutritional premix and food products. Standard additions of riboflavin were used to confirm the analyte concentrations and to provide precision data.

Development of cysteine-modified screen-printed electrode for the chronopotentiometric stripping analysis of cadmium(II) in wastewater and soil extracts.

Screen-printed carbon electrodes were fabricated with amino acid functionality by using in situ co-deposition of mercury and cysteine. The three-electrode configuration (graphite carbon working electrode, carbon counter electrode and silver/silver chloride reference electrode) incorporating a cysteine-modified working electrode exhibited good sensitivity towards cadmium(II). Several experimental variables affecting the sensor stripping response were characterised and optimised. These include cysteine and mercury concentrations, deposition time, deposition potential and stripping current. Surface analysis was also conducted using scanning electron microscopy (SEM) in order to characterize the electrode surface during cadmium analysis. The stripping chronopotentiometric response for cadmium(II) was linear in the concentration range 0.4-800 microg L(-1) when a deposition time of 2 min was used. A detection limit of 0.4 microg L(-1) was obtained using 0.025 M Tris-HCl buffer containing 0.1 M KCl (pH 7.4) as the supporting electrolyte. The analytical utility of the cysteine-modified sensor was demonstrated by applying it to cadmium analysis in various wastewater and soil samples collected from a contaminated site and extracted using acetic acid. The results obtained using the developed electrodes agreed satisfactorily with the values achieved using atomic absorption spectrometry and inductively coupled plasma mass spectrometry analysis. These results demonstrate the feasibility of using this type of sensor for cadmium analysis.

Resolving the copper interference effect on the stripping chronopotentiometric response of lead(II) obtained at bismuth film screen-printed electrode.

As copper(II) is a common ion in a variety of analytical samples, its effect on the stripping response of lead(II) at bismuth film screen-printed carbon electrode (BFSPCE) was investigated. The study was conducted using a screen-printed three-electrode system (working, counter and reference electrodes), with the carbon-working electrode plated in situ with bismuth film. Copper present at significant concentration level in samples was found to affect the sensitivity of the electrode by reducing the constant current stripping chronopotentiometric (CCSCP) response of lead(II). Recovery of the lead stripping response at the BFSPCE in the presence of copper was obtained when 0.1mM ferricyanide was added to the test solution. The ferricyanide added circumvents the detrimental effect of copper(II) by selectively masking the copper ions by forming a complex. The analytical utility of the procedure is illustrated by the stripping chronopotentiometric determinations of lead(II) in soil extracts.

Stripping chronopotentiometric measurements of lead(II) and cadmium(II) in soils extracts and wastewaters using a bismuth film screen-printed electrode assembly.

The key to remediative processes is the ability to measure toxic contaminants on-site using simple and cheap sensing devices, which are field-portable and can facilitate more rapid decision-making. A three-electrode configuration system has been fabricated using low-cost screen-printing (thick-film) technology and this coupled with a portable electrochemical instrument has provided a a relatively inexpensive on-site detector for trace levels of toxic metals. The carbon surface of the screen-printed working electrode is used as a substrate for in situ deposition of a metallic film of bismuth, which allows the electrochemical preconcentration of metal ions. Lead and cadmium were simultaneously detected using stripping chronopotentiometry at the bismuth film electrode. Detection limits of 8 and 10 ppb were obtained for cadmium(II) and lead(II), respectively, for a deposition time of 120 s. The developed method was applied to the determination of lead and cadmium in soils extracts and wastewaters obtained from polluted sites. For comparison purposes, a mercury film electrode and ICP-MS were also used for validation.