The ever-growing demand for new biosensors has boosted the materials processing and synthesis in organic electronics. As a result, different layouts of organic field-effect transistors (OFETs) are now suitable to be operated in aqueous solution. Among them, electrolyte-gated organic field-effect transistors (EGOFETs) are the ultimate example [1]. In particular, these devices rely on an electrolytic solution, which acts as a gate dielectric bridging the gate electrode and the organic semiconductor thin-film. Different examples of biosensors have been successfully demonstrated in the past few years aiming at the detection of DNA, penicillin, streptavidin and dopamine [2].

Different factors are pivotal in order to achieve an electronic biosensor, such as the electrostatic screening, the pH of the aqueous environment and the density of the reactive sites assembled on the device [3]. An additional factor has to be taken into account for an immunological detection: the antibody (Ab) orientation once it is immobilized.

Here, it has been designed and fabricated an immunosensor towards interleukin-4 (IL4) and interlekin-6 (IL6). These two proteins belong to the cytokine family, which are known to be signalling mediators of the immune system. In particular, IL4 is an anti-inflammatory biomarker, whereas IL6 is a pro-inflammatory one.

The sensing core of this electronic device is the gate electrode (i.e. polycrystalline Au). The assessment of the role exerted by the Ab orientation has been pursued by comparing two different surface treatments: i) the use of an amine-terminated SAM activated by glutaraldehyde and ii) the exploitation of the recombinant protein G. Although the former protocol yields a covalent Ab grafting, it has a poor control on the orientation of the immobilized Ab. The latter leads to a highly oriented Ab on the gate electrode, making use of the biological functionality of recombinant protein G.

These two functionalization protocols have been characterized by DC/AC electrochemical measurements (viz. cyclic voltammetry and impedance spectroscopy). Furthermore, the bio-molecular recognition between antibody and antigen has been characterized by single force spectroscopy for both protocols. Many parameters have been successfully extracted, like (i) unbinding length and force, (ii) dissociation constant and (iii) reaction length. All these results point out how the protocol based on protein G leads to a more efficient bio-molecular recognition than the other one based on glutaraldehyde. As a result, the EGOFET shows sensitivity down to nM scale only for highly oriented Abs on the gate surface. In conclusion, electrical and nano-mechanical data have showed how the control of Abs orientation is crucial for IL4 and IL6 biosensing.

References:

[1] L. Kergoat et al., Advanced Materials, 22, 2565-2569 (2010)

[2] S. Casalini et al., Organic Electronics, 14, 156-163 (2013)

[3] M. Hammock et al., ACS Nano, 7, 3970-3980 (2013)

Stefano Casalini

University of Modena and Reggio Emilia

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