The glucose sensor is a billion-dollar per annum global market which has its origins in screen printing and electrochemistry. Screen printing can produce economical one-shot disposable sensors which can be used for the rapid, sensitive and portable analysis of many target analytes in a plethora of areas. However, for new sensors based on this process to be developed and implemented commercially, next generation screen printed electrodes need to be designed. The Banks Group has realised the enormous potential applications of screen printed electrodes within science and thus described and continue to propose novel new applications for screen printed macro and micro electrodes.
Fundamental understanding of screen printed electroanalytical sensors
The inks selected for use within screen printed electrodes consist of graphite particles, polymer binder and other additives which are utilised for dispersion, printing and adhesion tasks. The exact ink formulation is regarded by the manufacturer as proprietary information and it has been shown that differences in ink composition e.g. type, size or loading of graphite particles and in the printing and curing conditions can strongly affect the electron transfer reactivity and overall analytical performance of the resulting carbon sensors.
We have has for the first time explored the role of the polymeric binder used in electrode fabrication via screen printing where it was demonstrated that a dramatic effect on the electrode’s morphology is evident as the amount of polymeric binder is dramatically increased. The effect on the voltammetric response from increasing the amount of binder was evaluated using an inner-sphere electron transfer redox probe. This work was found to demonstrate that the heterogeneous electron transfer rate (koedge) constant decreased as the polymer binder increased, which is represented as %MB/MI, where MB and MI are the mass of the binder and the ink, respectively. The global coverage of edge plane sites of the electrode surface was found to decrease from 5.5% to 0.3% for the range of 0 - 80% (MB/MI) respectively, indicating that the electron transfer characteristics can be tailored from that of edge plane-like, to that of basal plane-like in nature. This has significant impact in fundamental electrochemistry for example, when one is studying the electron transfer dynamics of say, metal nano-particles, there is the need to have no contributions from the underlying electrode in order to study the electrochemical response of the metal nano-particles which is usually achieved with a basal plane electrode fabricated from highly ordered pyrolytic graphite (which consequently has slow electron transfer kinetics) but has the drawbacks of being expensive and requiring renewal each time; the use of a basal plane like electrode which is disposable and cost effective therefore has clear advantages.
Other work within the Banks Group has shown that the bespoke screen printed electrodes could be electrolytically modified with palladium which is electro-catalytic towards the electrochemical oxidation of hydrazine where the underlying (unmodified) electrode exhibits slow electron transfer. An average palladium particle size of 2.7µm was found possible where the screen printed electrode substrates act as a template for the deposition of the target metal. This equates to a global coverage of only 0.27 (where 1.0 indicates a fully covered electrode surface). The electrochemical performance towards hydrazine was explored where a limit of detection of ~ 9 µM was found to be possible which compared well with nano-particle modified electrodes, even though only 27% of the surface was modified! Clearly the mass transfer from employing a micro-domain electrode, over that of a macroelectrode is highly beneficial and acts akin to a nano-particle modified electrode.
More recently we have shown that when an electrochemical modification designed for producing nickel nano-particles on boron-doped diamond is explored using a standard screen printed electrodes, the observed resulting nickel morphology is actually microrod like. The production of such structures requires complicated fabrication strategies (non-electrochemical) and this is the first example of producing such structures electrochemically; the change in deposit morphology from using a boron-doped diamond electrode to a screen printed electrode is due to the differing nucleation dynamics, allowing new and exciting structures to be derived. These nickel microrods were shown to be analytically useful towards the sensing of alcohols.
Metal oxide modified screen printed electrodes
Mediator bulk modified screen printed electrodes have been the backbone of sensors towards portable and de-centralised testing which are also easily mass produced and consequently have scales of economy. Modification of the electrode surface with electrocatalytic metals is a common approach such as decoration or through the use of metallic screen printed inks producing film modified screen printed electrodes, while another approach is to incorporate metallic electrocatalysts into a carbon paste electrode which have been extensively explored. Wu et al have utilised ruthenium oxide which is commonly used in resistive pastes for screen printing and demonstrated this towards the sensing of ascorbic acid which provided an analytical output of 0 – 4 mM with little interference from uric acid and hydrogen peroxide This work has been extended for example for the determination of hydrogen peroxide and with enzymatic modification for sensing hypoxanthine and glucose, whilst also being applied to food analysis. Along these lines we have reported the bulk modification of screen printed electrodes with copper oxide for carbohydrate sensing, nickel oxide for hydroxide detection, manganese oxide for nitrite, oxygen and ascorbic acid and finally bismuth oxide.
In the majority of cases, bulk modified screen printed electrodes, where micron sized particles of the electrocatalytic metal are incorporated into the electrode, we find that these modified screen printed electrodes act analytically similar to that of a nano-particle modified carbon electrode. The pertinent question here is, why is this the case?
The reason can be found by considering the diffusion zones at each electrode surface viz either the nano- or micron- particles. Figure 2 depicts a schematic representation of the diffusional zones at microelectrode and nano-particle ensembles. It is clear that the same parameters apply here as in the case of microelectrode arrays such that the distance between the particles is the key factor.
Again, diffusion zones will build up at each electrode, in this case the micro- or nano- particles and due to no regular spacing, diffusional overlap occurs at a substantial degree at modest scan rates resulting in essentially the same voltammetric profiles observed at the microelectrode arrays. The degree of overlap is important, and if the overlap is not significant the ensemble has faster mass transport and hence improves electro-analytical performance over that of a heavily overlapping diffusional regime. The response voltammetric response observed is identical due to the fact that the diffusional profiles are similar indicating that a nano-particle modified electrode for use in analysis does not always confer enhancements in electroanalysis. This is it clear that in going from a micro- to nano-particle size ensemble is not always beneficial since close neighbouring nano- particles will heavily overlap acting akin to a micro-particle of the same geometric area. The advantageous approach of our methodology, as described above, is that any metallic oxide can be readily incorporated, allowing a true platform technology.
Improving mass transport through advantageous designs
Microelectrodes are exploited in electrochemistry due to their increased temporal resolution and current densities, reduced ohmic drop and charging currents and high Faradaic to capacitive current ratios. However, micro electrodes have current ranges in the nanoamp to picoamp range, and are usually engulfed beneath electrochemical noise precluding useful measurements. The solution to this problem lies in using an array of microelectrodes where single microelectrodes are wired in parallel, with each electrode independent; radial diffusion dominates the mass transport, generating a signal which is many orders of magnitude larger. Microelectrode arrays are increasingly used in electrochemistry and especially in electroanalytical applications whether they facilitate lower detection limits and exhibit greater sensitivities in comparison to macroelectrodes and consequently, research is dedicated to designing new types and variations.
Recently we have reported the fabrication of disposable and flexible screen printed microelectrodes characterized with microscopy and cyclic voltammetry. The advantages presented by the fabricated electrodes included reduced expenditure and cleaning processes as each of the microelectrodes are designed to be disposable. The removal of the requirement of cleaning stages, but also pre-treatment between analyses allows for much more efficient and rapid analysis of samples. The work also demonstrates exceptional detection limits with the screen printed electrodes providing comparable detection limits to that obtained in the literature at insonated boron-doped diamond electrodes.
Screen printed electrodes used as sensing platforms
Highly sensitive detection of cadmium (II) has been achieved allowing biomonitoring within artificial and diluted oral (saliva) fluid. Electrochemical analysis with the application of screen printed electrodes enabled detection limits in the low µg L-1 with anodic stripping voltammetry utilising the presence of the bismuth film which provides enhanced sensitivity for the electroanalytical deposition of cadmium (II) coupled with a simple change in pH of the oral (saliva) fluid sample permitting quantitative measurements. Such work highlights the efficiency and flexibility of screen printed electrodes, even in such electrochemically challenging media.
The low ppb sensing of chromium (VI) utilising graphite screen printed macroelectrodes has been elegantly demonstrated by our group Sensing was carried out in aqueous solutions over the range 100 to 1000 µg L−1 with a limit of detection of 19 µg L−1. The underlying electrochemical mechanism was 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 by the group. Canal water samples are used to demonstrate the feasibility of the analytical protocol for the sensing of chromium (VI) 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).
J. P. Metters, R. O. Kadara, C. E. Banks, Analyst, DOI: 10.1039/C0AN00894J
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