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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  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.
Vacancy behaviors during ageing of Cu-26Zn-4A1 and Cu-14Al-4Ni alloys have been investigated and compared by means of positron annihilation (PA) and electrical resistivity measurement. For ageing in martensitic state after direct quenching, it is observed that the S parameter values of Cu-Zn- Al specimens, measured in liquid nitrogen, increase at first and then decrease, while those of Cu-Al-Ni remain unchanged. The activation energies calculated from the S parameter for increasing and decreasing stages are o.4lev and o.63ev respectively, and the former can be corresponding to the formation energy of vacancy clustering, while the latter may be regarded as the migration energy of effective vacancies. A mechanism is put forward that the clustering of quenched-in vacancies results in a decreasing of the ordering degree and a reduction of the stored energy in martensite, which is responsible for the early stage of the stabilization of martensite in Cu-Zn-Al alloys. However, the fact that Cu-Al-Ni alloy is not subject to the stabilization is assumed to be owing to the immobility of supersaturated vacancies in its martensitic state which may be associated with the strong binding force between Ni and Al atoms.
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