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Sensors 2020, 20, 1333 11 of 15 debris [33]. When using a lower plasma power of 20 W with a longer processing time up to 60 s of the plasma treatment, the wetting behavior of the Au ink on COC was too poor. In contrast, the direct printing of Ag on plasma-pretreated COC resulted in structures that delaminated easily when being wetted. The influence of humidity on the adhesion of silver inks on polyimide film (PI) was presented in other works before. The cause of the decreasing of the adhesion was mainly the humidity absorption of the PI [34,35]. In this work COC was used as substrate material, which has much lower moisture absorption than PI. Another possible reason could be an oxidation of the binders and capping agents of the insufficiently sintered silver layer. This could have led to swelling and delamination [34,35]. Printing the Ag ink onto Au structures led to stable Ag structures. The results show that photonic curing is a convenient alternative for sintering nanoparticle inks. The high energy of the light combined with the short process time enables the sintering of the nanoparticles even on temperature-sensitive substrate materials. The thermal sintering before the photonic curing was performed for removing the solvent from the printed structure. Performing the photonic curing of printed structures with high solvent content can lead to an explosive evaporation of the solvent and cause damage of the printed structures [36]. Photonic curing after thermal sintering at 130 ◦C yields structures that feature about 10% of the conductivity of bulk gold, which is a 60-fold higher conductivity than previously reported structures on cyclic olefin polymer [16] that were sintered in Ar plasma for 30 min. On other substrates, like polyethylene naphthalate (PEN) or SU-8 covered polytetrafluoroethylene (PTFE) conductivities of a comparable order of magnitude were achieved by thermal sintering at 150 ◦C [17] and 130 ◦C [37], respectively. However, the latter sources report the necessity of performing an electrochemical activation step prior to use. This is not required for the electrodes reported by us, as demonstrated for the 2.5D arrays, which were not pretreated prior to functionalization. Dependent on the sintering parameters, inkjet-printing can result in more or less porous metal structures [38,39]. Apart from few exceptions [40], pores are not desired, since dense structures are typically more stable and conductive. For (bio-)sensing applications, however, an increased surface can be interesting. For example, DNA detection on porous electrodes fabricated by dealloying showed an increased sensitivity [41]. The dependency of AOxide on the number of inkjet-printed layers clearly shows that the electrodes are porous (Figure 5). The determined number of immobilized capture probes also increases with the number of inkjet-printed layers, which demonstrates that the pores are accessible for capture probe immobilization. As the capture probe density, when referred to AOxide, is comparable for all the IPEs, a homogeneous distribution of the capture probes is likely. However, the amount of hybridized signal probes does not scale with the number of inkjet-printed layers. The lower layers obviously do not contribute to the signal generation within the analysis time (Figure 6b). This may be explained by the reduced accessibility of the pores for signal probes, which is possibly enhanced by electrostatic repulsion of the charged capture and signal probes. This assumption is supported by a study on pore-size restriction for hybridization [42], where pores appear to be accessible for functionalization with capture probes. Access of complementary DNA is strongly dependent on pore diameter and decreases for average pore diameters of about 65 nm. The inkjet-printed structures of this work feature pore sizes in a comparable dimension and below (Figure 2b). Further work would be required to investigate whether an optimization of capture probe density, the use of neutral charge peptide nucleic acid probes [43] or the tuning of pore size e.g., by electrochemical coarsening [42] improves accessibility and hybridization efficiency. Apart from that, the hybridization signal of IPEs is good and the performance of the IPEs comparable to the commercial SPEs (Figure 6b). In particular, SWV measurements (discussed in SI and displayed in Figure S12) are sensitive to detect and monitor hybridization from the first minute of hybridization. The good reproducibility of electrochemical signals after stripping demonstrates the good stability of the electrodes and their functionalization. Non-specific signals have not been detected, either for electrodes that were functionalized with non-complementary capture probes or for MCHPDF Image | Inkjet-Printing Nanoparticle Gold Silver Ink Cyclic Olefin
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