Surface passivation of the photoelectric sensors as a label-free modality to detect target molecules

Label-free sensing is a highly promising detection method since the extra labeling step that imposes a high manufacturing cost as well as a lengthy testing time is no longer needed. Here, we describe a novel label-free modality for detecting molecules of interest present on a silicon surface in an aqueous solution via monitoring the change in the photocurrent as a response to surface passivation induced by the ionic state of the functional group at the surface. The experiments presented in this paper entail studying the influence of the protonation state of the 3-aminopropyltriethoxysilane (APTES) surface-deposited layer on the sensor operation by characterizing the change in the photocurrent as well as external quantum efficiency (EQE) at a p-n junction. We also investigate the improvement of the detection signal resulting from an electric potential applied to the solution via an electrode that ideally perturbs the counterions on the device surface, thereby reduces charge screening. A significant increase in the current when the amino groups of APTES were protonated (16.6 ± 5.3%) and a higher response to a change in the ionic state compared to the control silane that lacks a charged moiety (16.6% vs 3.5%) when bias was applied indicated the sensors were sensitive to the positive charge of the molecules of interest. In addition, a better surface passivation, as indicated via EQE, confirms surface sensitivity.