Electrochemical detection of protein binding in physiological salt concentration environments by planar field effect transistor (FET) platforms has not previously been convincingly presented. Historically, the use of immunologically modified FET sensors (immunoFETs) for in vivo detection of proteins has been dismissed as infeasible due to assumptions about charge shielding and Debye length constraints. These assumptions, and others, are misconceptions and do not accurately represent what is known immunologically about proteins and antibodies. In this work, the flaws of the classical analysis, approaches for addressing this model and demonstrating feasibility of immunoFET sensing in physiologic environments are discussed and proposed. Successful detection of multiple, distinct analytes by an AlGaN heterojunction immunoFET are then reported and discussed. Approached for improving sensitivity of the device are also discussed.
A key parameter in device sensitivity and function is the overall distance between bound analyte and the semiconductor. Consequently, differential properties of surface polymers and surface receptors are design parameters for FET sensors. Thickness, adhesion and wear of silane polymer layers bound to surface Al2O3 oxides are compared. These properties of the film after additional deposition of biotin and streptavidin are further compared. Consistent differences in thickness and wear resistance of silane films that can be correlated to film chemistry and deposition procedures are reported and discussed.
Improved interfacial properties of immunoHFETs and incorporation of a control gate into AlGaN device architecture result in improved immunoHFET sensitivity. Genuine immunoFETs which detect binding of multiple, distinct analytes (huCXCL9, muCXCL9, CXCL10, CCL5 and streptavidin) in physiologic buffer containing high levels of salt (150mM NaCl) are reported. Additionally detection of huCXCL9 and huCXCL10 in murine serum is presented using the immunoHFET platform. The immunoHFETs reported are functionalized with polyclonal intact anti-analyte IgG antibodies. These results provide unequivocal proof of the feasibility of immunoHFETs operating in physiological environments, in direct contradiction of the classical assessment of immunoFET feasibility. This demonstrates the need for critical reconsideration of the classical assessment, but also the need and desirability of developing immunoFETs for clinical applications.