Inkjet Printing of Enzymes for Glucose Sensors

Drop on demand inkjet printing is a potential method for depositing enzymes onto electrodes for sensor applications. This technology offers drop sizes in the region of picolitres and allows a production rate up to 200 mm/s. This enables not only a more rapid method of device prototyping but also a method for possible miniaturization of the sensors themselves. However, previous work [1] has indicated that inkjet printing may cause a drop in the retained activity of the enzyme. Here we assess the criticality of this drop in activity and how it may have been influenced by changes to the protein structure during printing. The enzyme used is glucose oxidase and the test methods include; protein analysis, in the form of analytical ultra-centrifugation and circular dichroism, scanning electron microscopy, atomic force microscopy and phase contrast microscopy, to analyse the surface topology of the electrodes and contact angle analysis, to assess the degree of spreading and the interactions between the drops and the electrode surface. With glucose oxidase there is no change in the conformation, structure or hydrodynamic radius of the protein after printing. The analysis of the electrode surface shows a relatively smooth surface that is made up of individual graphite flakes laid down by a screen printing method. When contact angle and spreading analysis is carried out it demonstrates reliability in the printing process as well as a drop in the sessile volume of the drop in conjunction with a growth in the base diameter of the drop as expected. It also demonstrates a fairly quick rate of evaporation of the drop. Upon the addition of surfactants to the solution the spreading is seen to be more extensive in relation to the surfactant concentration, although some initial reduction in experienced at low concentrations which may be due to the absorption into the carbon surface.