Screen-printed electrodes (SPEs) are electrochemical measurement devices that are manufactured by printing different types of ink on plastic or ceramic substrates, allowing quick in-situ analysis with high reproducibility, sensitivity and accuracy. The composition of the different inks (carbon, silver, gold, platinum) used in the manufacture of the electrode determines its selectivity and sensitivity. This fact allows the analyst to design the most optimal device according to its purpose. The evolution of these electrochemical cells arises from the need to reduce the size of the devices, that implies a decrease of the sample volume required in each experiment. In addition, the development of SPEs has enable the reduction of the production costs. One of the principal advantages is the possibility of modifying the screen-printed electrodes, modifying the composition of its inks by adding different metals, enzymes, complexing agents, polymers, etc., which is useful for the preparation of multitude electrochemical analyses.
Description Screen printing is one of the oldest methods of reproduction. The screen-printed electrodes (SPEs) are presented as a single device in which there are three different electrodes:
Working electrode. Their response is sensitive to the analyte concentration. Reference electrode. It allows the application of a known potential, which is independent of the analyte and other ions concentration. Its potential is constant, and the working electrode potential is measured against it. Auxiliary or counter electrode. It is the electrode that completes the circuit of the three-electrode cell, as it allows the passage of current. It enables the analysis of processes in which electronic transfer takes place.
The three electrodes could be printed on different types of substrates (plastic or ceramic) and could be manufactured with a great variety of inks. The most common inks are those composed of silver and carbon, however, they can be based on other metals such as platinum, gold, palladium or copper. In addition, the electrodes can be modified with enzymes, metallic nanoparticles, carbon nanotubes, polymers or complexing agents. The electrode ink composition is chosen according to the final application and the selectivity and sensitivity required for the analysis. The electrode manufacturing process involves the sequential deposition of different layers of conductive and/or insulating inks on the substrates of interest. The process consists of several stages:
Film deposition usually on plastic or ceramic. Drying of the printed films, thus eliminating possible organic solvents needed to produce a proper adhesion. Drying can be done in an oven at temperatures between 300 and 1200 °C, or in cold cured ink with a subsequent UV light photocuring process. The process can be repeated if complex structures are required using the appropriate material for the specific design. On the other hand, as mentioned above, the most commonly used inks are silver and carbon, therefore, their printing and manufacturing characteristics should be highlighted:
Silver ink. This ink acts as a conductor, while the working electrodes are printed mainly with graphite inks, although gold, platinum and silver inks are also used. Some ink components induce differences in detection and analysis. Silver/silver chloride ink. Silver/silver chloride is an industry preferred reference electrode because it has stable electrochemical potential under numerous measurement conditions. This makes silver/silver chloride ink a good choice for a variety of medical and industrial applications that require conductive ink, such as biometric monitoring or heavy metal detection. The properties of the ink can be adjusted by changing the ratio of silver to silver chloride. Carbon ink. The electrodes composition is usually confidential information from the manufacturing company, however, there are key elements for the electrodes composition such as binders, used to improve the affinity of the substrate and ink, and solvents employed to improve the viscosity for the printing process. The type, size or charge of the graphite particles and the printing and drying conditions could affect the electron transfer and the analytical yield of the carbon sensors. Gold ink. Gold ink is currently generating more interest due to the formation of self-assembling monolayers (SAM) by means of strong Au-S bonds.
Advantages and applications Screen-printed electrodes offer several advantages such as low cost, flexibility of their design, great reproducibility of the process and of the electrodes obtained, the possibility of manufacturing them with different materials and the wide capacity of modification of the work surface. Another advantage is the possibility of connection to a portable instrumentation allowing the in-situ determination of specific analytes. In addition, screen-printed electrodes avoid tedious cleaning processes. Currently, they are used as a support to produce portable electrochemical biosensors for environmental analysis. Some applications are:
… excerpt ends here. Continue reading the full article.
