Electrochemical detection of protein binding at physiological salt concentration by planar field effect transistor (FET) platforms has yet to be documented convincingly. In fact, use of immunologically modified FET sensors (immunoFETs) for in vivo detection of proteins has been dismissed as infeasible due to a classical analysis carried out nearly 20 years ago. Despite numerous conceptual flaws, this model has taken hold as dogma in the FET sensing field and stalled the development of FET-based sensing platforms for in vivo protein detection. In this work, the flaws in this classical analysis, approaches for addressing this model and demonstrating feasibility of immunoFET sensing in physiologic environments are first discussed and proposed. Successful detection of clinically relevant levels of monokine induced by interferon γ (MIG) at physiological salt concentrations with an AlGaN heterojunction field effect transistor (HFET) are then reported and discussed. Approaches for improving sensitivity and enabling realization of a functioning immunoFET in physiologic environments are also discussed.
In planar FET sensors receptors are bound to the FET through a polymeric interface, and gross disruption of interfaces that removes a large percentage of receptors or inactivates large fractions of them diminish sensor sensitivity. Sensitivity is also determined by distance between the bound analyte and the semiconductor. Consequently, differential properties of surface polymers are design parameters for FET sensors. Thickness, surface roughness, adhesion, friction and wear properties of silane polymer layers bound to oxides (SiO2, and Al2O3, model surfaces for AlGaN HFETs) are compared. These properties of the film/substrate pairs after additional deposition of biotin and streptavidin are further compared. Systematic, consistent differences in thickness and wear resistance of silane films that can be correlated to film chemistry and deposition procedures are reported and discussed. These results provide guidance for rational interfacial design for planar AlGaN HFET sensors.
The structure and tribology of silane interfaces composed of one of two different silane monomers deposited on oxidized AlGaN were also studied. Distinct morphologies and wear properties for the interfacial films, attributable to the specific chemistries of the silane monomers used in the films were demonstrated. For each specific silane monomer, film morphologies and wear are broadly consistent on multiple oxide surfaces. A testable model of the hypothetical differential interfacial depth distribution of protein analytes on FET sensors interfaces with distinct morphologies is presented.
Improved interfacial properties of bio/immunoHFETs and incorporation of a control gate into AlGaN device architecture result in improved bio/immunoHFET sensitivity. A genuine immunoFET which detects binding of the human protein monokine induced by interferon gamma (MIG) in physiologic buffer containing high levels of salt (150 mM NaCl) is reported after incorporation of control gate and interfacial optimization approaches. The immunoFET is functionalized with polyclonal whole anti-MIG IgG antibody. The results provide an unequivocal example of a functional immunoFET operating in physiological buffer, in direct contradiction of the classical assessment of immunoFET feasibility, demonstrating not only the need for critical reconsideration of the classical assessment, but also the need for, and desirability of, consideration of immunoFETs for clinical applications.