Abstract
Precise quantification of small‐molecule biomarkers in complex biological fluids remains an analytical challenge. Conventional sensing platforms often suffer from insufficient trace‐level sensitivity, significant signal drift, and a reliance on bulky instrumentation. Advanced field‐effect transistors (FETs) utilizing pristine chemical vapor‐deposited graphene can overcome these performance limitations. Nevertheless, high material cost and tedious substrate transfer protocols prevent scaling up. Here, we report a solution‐processed, thin‐film reduced graphene oxide (RGO) FET for highly sensitive serotonin detection. The RGO channel was functionalized with a specific amine‐modified DNA aptamer as a biorecognition element. Upon binding, the aptamer undergoes a local conformational rearrangement and redistribution of associated charge near the RGO surface, enabling measurable molecular recognition. Our device delivers a sensitivity of 9.5 mV/dec across a concentration range from 100 fM to 10 nM. The sensor reliably detected serotonin at 100 fM, the lowest concentration experimentally tested, with an S/N ≥ 3. The sensor demonstrates excellent specificity against interferents such as cortisol, glucose, and dopamine. The predictable chronological Dirac voltage drift of only 0.02 V over 5 days facilitates systematic baseline recalibration.