From limit of detection to effective sensitivity in charge-modulated organic-transistor biosensors

Abstract

Organic-transistor biosensors are a promising platform for early diagnostics, and their reported detection limits now dwell in the femto- to attomolar range. However, the metric that dominates the literature, namely the static limit of detection (LoD), captures only one half of what governs the usable performance, and the second half is still not clearly addressed. Through a comparative analysis of organic charge-modulated field-effect transistors (OCM-FETs) and their interfacial (ISOFET) and volumetric (OECT) counterparts, we show that the achievable detection limit factorizes, to first order, into two engineering quantities, namely a capacitive gain term and a dielectric drift-floor term, so that the LoD is governed by their product. We find that the first term has been driven to its geometric minimum through deliberate capacitive-divider and channel down-scaling, whereas the second, the device-side drift floor – which in the OCM-FET reduces to the bulk Maxwell–Wagner relaxation time – has, for the affinity OCM-FET sensors in which the architecture’s advantage should matter most, scarcely been isolated as an independent quantity, even though the coupled foils quantify it routinely. Hence, the defining claim of this architecture, that its detection ceiling is an engineering rather than a thermodynamic limit, remains only half-demonstrated. We make the drift factor explicit through an effective-sensitivity decomposition, place the architectures on a single degree-of-coupling axis, and specify the time-domain measurement required to render the engineering-ceiling claim testable. We further show that, although the response time and the drift floor are set by the same bulk relaxation, they separate by orders of magnitude in affinity sensors, so that the operability number applies there without modification while saturating on a measurable capacitance ratio in the fast-transduction limit.