− the presence of small air bubbles (due to the manually insertion of the solution by a micropipette, which could contain air bubbles, or due to the insertion of some air between the solution and the top covering of the electrolytic chamber).
Furthermore, the static measurements involve an already-assembled gate electrode: the direct effect of the bond between antibodies and antigens is not visible, because it occurs before the measurement step. Even increasing concentrations of antigen cannot be measured in sequence through this system.
For all these reasons, real-time measurements were promoted, allowing the analysis in a hermetic structure, with an automatic pumping of the solutions and a non-stop monitoring of the current over time.
Chapter 4 Results and discussion 111 The complete RTP (Fig. 4.23) was assembled and filled by the automatic pump.
Before the starting of the RTP characterization, the current must achieve the stabilization (an example is reported in Fig. 4.24).
The characterisation is useful for the individuation of the best values of Vgs and Vds, that will be applied during the current measurements over time.
Fig. 4.23 – The RTP before and after its complete installation and a zoom on the positioning of the gate electrode in the chamber covering.
I
ds(m A )
Fig. 4.24 – An example of current stabilization just after the solution pumping (before the beginning of this measurement the gate electrode was not completely covered by the electrolyte solution). Vds=-0.5 V,
Vgs=0V.
An exemplar output curve is reported in Fig. 4.26. It presents a very different shape, thus a totally different characteristic curve, than the one shown in Fig.
4.22. In this situation the current decreased only of about 30 mA. The gold gate electrode modified the transfer curve of the device, not allowing a proper working of the device and thus an easy choice of the best Vgs value for following measurements. Possible liquid leakages are the most probable reason of these difficulties during the measurements. Further study towards the resolution of this problem are still in process.
Fig. 4.26 – The transfer characteristic curve of the RTP at Vd= -0.5V in PBS.
5.
Conclusions and future works
The two improvements were successful, since the sulfuric acid post-treatment achieved higher values of current and transconductance and the GOPS addition maintained a preserved surface of the polymeric channel, even after the characterisation measurements. The ink-jet printed and acid-treated samples exhibited the maximum values of transconductance (52.9 mS), while the spin-coated ones achieved 19.6 mS. In both cases, they exceeded the highest value of transconductance (ca. 4 mS) of untreated-Clevios devices. After 15 days from the fabrication, the same devices have still maintained a maximum transconductance of 47 mS and 10.7 mS, for the ink-jet printing and the spin-coating, respectively.
The GOPS devices showed under the optical microscope a preserved surface, without cracks or damages, which appeared on the acid-treated device instead.
Concerning the geometries, the ink-jet printed channels reported a comparable thickness with respect to the spin-coated ones, but only in the samples with GOPS addition. Since the volumetric dimension of the channel is directly connected to the current flow through the channel, the acid-treated devices resulted not really comparable in their electrical performances.
For all these reasons, a deeper investigation of the morphology of the surfaces and a further optimization of the printing process for Clevios are clearly needed.
Furthermore, a greater number of devices might to be fabricated for a more solid statistical analysis. After these first necessary steps, different thicknesses of both spin-coated and ink-jet printed channels might be investigated and compared with respect of their electrical behaviour and also during real-time biosensing. A deeper study of the ink-jet printing technique can be conducted, testing different and/or more complex geometries for the pattern and different bio-applications, for example the employment of bio-inks for cell cultures or the nanoparticles deposition for flexible electronics.
Concerning the first detection measurements, the devices showed an evident reduction in the channel current with the StpHRP-functionalized gate electrode in acid-treated devices, fabricated by spin-coating. In the ink-jet printed ones, the variation was less consistent. The antiVEGF-functionalization and the subsequent bond with the VEGF led to a difficult interpretation of the current variation, mainly due to the static nature of these first measurements.
In conclusion, a better comprehension of the effects connected to the changing of the gate voltage after the surface modification and the solution of the problems related to the real-time platform are strictly necessary to continue and evaluate the measurement operations. After this, the functionalization with other more specific antibodies and antigens (for example biomarkers) can be explored. The analysis of more complex samples, such as saliva or serum, might be the last step of the whole design process of this kind of biosensors.
Appendix
Easy guide to Jetlab 4 ink-jet printer
During the thesis work an easy guide to the Microfab Jetlab 4 printing system was written. Here the most relevant issues are reported.
General description
For the usage of Jetlab, the following equipment is necessary:
− a white-light lamp, in order to obtain the right illumination for the cameras;
− a pressure pump, to flux the fluid toward the nozzle or to hold it;
− a filter with pore size of 0.45 µm, to remove particulates, also from DI water, which is used for testing and cleaning;
− water sensitive paper, if preliminary tests with water are necessary.
The printer is mainly constituted by the printer block and the front panel.
The main elements of the printer block (Fig. A1) are briefly described:
1- Metallic holder: it provides the cylindric slots for the piezoelectric nozzles,