Antibodies fragments as enablers of cardiac troponin (cTn) detection with extended gate metal-oxide-semiconductor field effect transistors (EGFET)
Authors: Capua L , Locca D , Ionescu A 
Published: November 2020

Abstract

Background
Cardiac troponin is the gold standard biomarker used to rule in or out acute myocardial infarction (AMI) in patients. Highly sensitive and reliable detection mechanisms for this important biomarker are required for advancing the field of early detection in medical applications. Field effect transistor (FET) sensors, specifically functionalised with troponin antibodies, can perform specific detection of cardiac troponin I or T, with high sensitivity and low limit of detection (LoD), in fully scalable and low power operated intelligent sensing systems.

Purpose
To design and realize a new wearable blood or sweat on-chip sensor, able to perform a real-time reliable detection of the cardiac troponin level in a patient suffering of AMI symptoms or during the post-operative phase. The sensor exploits the voltammetric detection of biomarkers, and offers the potential for miniaturization and reduced hospitalization costs.

Methods
Electrical current modulation in field effect transistor-based sensors is used as transducing principle, as a result of the surface potential change following exposure to biological fluids containing different concentrations of antigens. The use of antibodies fragments, properly attached to the sensor surface, enhances the sensitivity. Proof of concept tests involve the detection of fluorescein.

Results
The specific antigen detection by means of an electrical measurement has been proved, with a detection limit of &lt;1 nM (24ng/mL), linear range over one decade and a sensitivity around 70mV/decade. Fluorescein antibodies fragments, commercially purchased, were attached to the gate of an EGFET device; the binding events between fluorescein molecules and the correspondent antibodies modify the electrical conduction properties of the transducer, creating a current shift whose amplitude is univocally related to the antigen concentration. The exploitation of the antibodies fragments allows the antigens to bind closer to the sensing surface, creating a more noticeable electrical perturbation. The device is made of a commercial Metal Oxide Semiconductor FET (MOSFET), whose gate is extended by a platinum electrode for sensing. The metal is then functionalised by attaching covalently antibodies fragments that act as probes for the antigens in the tested biofluids. Laboratory samples were prepared in house, using fluorescein as antigen. Concentrations between 45pM and 1uM have been tested, and the absence of non-specific binding has been successfully proved with bovine serum albumin (BSA), for which any noticeable response has been recorded.

Conclusion
We report an important step towards a real-time and reliable detection of cardiac troponin by means of nano-ISFETs, demonstrating the fluorescein antigens-antibodies specific binding. Further steps consists in correctly achieve a surface functionalisation with cardiac troponin I antibodies, and further enhance the sensor LoD.
Antigens detection with EGFET

Funding Acknowledgement
Type of funding source: Public grant(s) – National budget only. Main funding source(s): Swiss National Science Foundation: ERA-NET, FLAG-ERA CONVERGENCE