Experimental comparison of direct and indirect aptamer-based biochemical functionalization of electrolyte-gated graphene field-effect transistors for biosensing applications.

Aptamer-based electrolyte-gated graphene field-effect transistor (EGFET) biosensors have gained considerable attention because of their rapidity and accuracy in terms of quantification of a wide range of biomarkers. Functionalization of the graphene channel of EGFETs with aptamer biorecognition elements (BREs) is a crucial step in fabrication of EGFET aptasensors. This paper presents a comprehensive comparison of commonly used biochemical functionalization approaches applied for preparation of sensing films in EGFET aptasensors, namely indirect and direct immobilization of BREs. This study is the first of its kind to experimentally compare the two BREs immobilization approaches in terms of their effects on the carrier mobility of the monolayer graphene channel and their suitability for sensing applications. Both approaches can preserve and even improve the carrier mobility of bare graphene channel and hence the sensitivity of the EGFET; however, the direct BREs immobilization method was selected to develop an aptameric EGFET biosensor as this method enables simpler and more efficient preparation of the graphene-based aptameric sensing film. The utility of the prepared EGFET aptasensor is demonstrated through detection of tumor necrosis factor-α (TNF-α), an important inflammatory biomarker. The direct BREs immobilization approach is applied to develop an EGFET aptasensor to measure TNF-α in a detection range from 10 pg/ml to 10 ng/ml, representative of its physiological level in human sweat, as a non-invasively accessible biofluid. The outstanding sensing performance of the developed TNF-α EGFET aptasensor based on direct BREs immobilization can pave the way for development of graphene biosensors.