Graphene Electric Double-Layer Transistors for Enhanced-Sensitivity Label-Free Detection of Human Serum Albumin

Abstract: Accurate detection of human serum albumin (HSA) is essential for the early diagnosis and monitoring of renal and hepatic disorders. We present a graphene-based electrolyte-gated field-effect transistor (EGFET) for label-free, real-time quantification of HSA under non-Faradaic operation. Devices exploit the high interfacial capacitance of the electric double layer (EDL) to transduce electrostatic perturbations induced by albumin adsorption into measurable conductance modulation. Negatively charged HSA molecules induce systematic modulation of the graphene channel, producing a concentration-dependent displacement of the Dirac voltage consistent with p-type doping. To establish a molecular-level interpretation of the sensing response, Brownian Dynamics simulations show that HSA adsorbs onto graphene through multiple adsorption orientations associated with heterogeneous interfacial charge distributions and variable dipole alignments relative to the surface. Adsorption is energetically stabilized by van der Waals interactions. Analysis of transfer characteristics across concentrations ranging from 0.01 to 30mgmL-1 reveals a correlation between surface charge density and carrier transport modulation within the electric double layer. Optimized devices exhibit a limit of detection of 0.0087 mg mL-1 and a linear dynamic range extending to 10 mg mL-1. The response remains non-Faradaic under sub-volt operation with reversible and reproducible behavior. The use of an inverse-mobility analytical metric highlights the role of disorder-enhanced carrier scattering in signal amplification, enabling sensitive electrostatic detection while preserving reversible device operation. These results establish liquid-gated graphene EGFETs as a promising platform for quantitative protein sensing and provide insight into disorder-mediated transport mechanisms in graphene bioelectronic devices.

• The paper introduces an inverse-mobility analytical framework in graphene EGFETs that achieves a detection limit of 0.0087 mg/mL for HSA.
• The paper employs both atomistic Brownian Dynamics simulations and detailed transfer characteristic analyses to elucidate adsorption-induced disorder in the sensor.
• The paper demonstrates robust sensor performance with a wide linear dynamic range and low hysteresis, indicating strong potential for clinical diagnostic applications.

Graphene EGFET-Based Label-Free HSA Biosensing: A Mechanistic and Analytical Study

Introduction and Background

Quantitative and early-stage detection of human serum albumin (HSA) is imperative for clinical monitoring of renal, hepatic, and nutritional disorders. Conventional methodologies—such as dye-binding spectroscopic assays, ELISA, and HPLC—impose limitations in terms of detection threshold, instrumentation complexity, and cost. Optical nanomaterial-enabled sensors, while improving sensitivity, often rely on exogenous labels and sophisticated readout. In contrast, graphene field-effect transistors (GFETs) operated under electrolyte gating leverage atomically thin, high-mobility channels to directly transduce nanoscale electrostatic perturbations from biomolecular adsorption, providing the basis for ultrasensitive, label-free biosensing.

This paper reports a systematic experimental and theoretical analysis of a pristine graphene electrolyte-gated field-effect transistor (EGFET) for non-Faradaic, label-free, real-time detection of HSA over a broad clinical concentration window. The study introduces an inverse-mobility analytical framework to precisely quantify adsorption-induced disorder and carrier scattering. This approach augments the conventional Dirac-point shift metric, enabling high-fidelity protein sensing with robust mechanism attribution.

Device Architecture and Molecular Interaction Mechanism

The sensor is realized on a monolayer graphene channel supported by Si/SiO₂, sourced and drained by evaporated Au, with no additional surface functionalization. Gating is performed using electrolyte droplets with variable HSA concentrations in ultrapure water, producing a dynamic electric double layer (EDL) at the graphene–electrolyte interface. This EDL mediates extremely high interfacial capacitance (on the order of 1–2 $\mu$F/cm²), greatly enhancing carrier density modulation capability at sub-volt biases.

Atomistic Brownian Dynamics simulations were performed to elucidate the adsorption behavior of HSA at the graphene surface. These simulations reveal multiple favorable adsorption orientations, with strong van der Waals stabilization and heterogeneous interfacial dipole and charge distributions. The interactions are dominated by several energetically competitive configurations, rather than a single motif, and adsorption is associated with both net negative charge and substantial molecular polarizability. This landscape induces spatially and temporally distributed electrostatic perturbations in the EDL, which are transduced as modifications to the graphene channel's carrier density and transport characteristics.

Electrostatic Sensing and Disorder-Mediated Transport

Electrical analysis comprises output and transfer characteristics recorded under increasing HSA concentrations (0.01–30 mg·mL⁻¹). Output $I_{DS}$–$V_{DS}$ curves retain strict ohmicity irrespective of adsorbate coverage, confirming non-Faradaic operation with negligible contact effects. Transfer ($R$–$V_G$) characteristics display prominent ambipolarity and systematic negative shifts of the Dirac point from 1.0452 V (0.01 mg·mL⁻¹) to 0.7839 V (30 mg·mL⁻¹), consistent with p-type doping induced by adsorbed, negatively charged HSA.

Beyond Dirac-point modulation, analysis of peak-to-peak transconductance separation ($\Delta V_{pp}$) and full-width at half-maximum (FWHM) of the resistance peak reveals monotonic broadening with increasing surface coverage, signaling spatially inhomogeneous disorder. Extracted transport parameters from nonlinear resistance model fitting (using a fixed effective gate capacitance, $C_{\text{eff}} = 1.65~\mu$F/cm²) demonstrate a substantial, concentration-dependent suppression of hole carrier mobility from 2506 to 151 cm²/V·s over the range, implicating disorder-enhanced Coulomb and dipole scattering as the dominant mechanism of response. The magnitude of mobility suppression notably exceeds predictions from carrier-density modulation alone.

Inverse Mobility Signal Calibration, Analytical Figures of Merit

The analytical readout is constructed using the inverse-mobility signal $S_u = 1/\mu_h$, offering amplified sensitivity to potential fluctuations and disorder relative to conventional Dirac shift analysis. In the ultra-trace (0.01–0.1 mg·mL⁻¹) regime, $S_u$ vs. log[HSA] is near-linear ($R^2 \approx 0.96$), and the limit of detection (LOD) is estimated via baseline noise ratio (3σ/slope) as 0.0087 mg·mL⁻¹—demonstrating high sensitivity and covering both pathophysiological and dialysis effluent ranges. Across the extended analytical window up to 10 mg·mL⁻¹, the sensor maintains a wide linear dynamic range, with deviation from strict log-linearity described accurately by a Freundlich (heterogeneous) adsorption isotherm model ($I_{DS}$0), consistent with interacting, randomly-oriented protein adsorbates at high coverage.

The signal-to-noise ratio (SNR) consistently exceeds unity throughout relevant concentration domains, establishing that the sensor can reliably resolve ultra-trace albumin signals, a requisite for clinical deployment in early-stage pathology and dialysis assessment.

Device Stability, Repeatability, and Operational Robustness

The sensor platform demonstrates low hysteresis, sub-volt operation with no discernible Faradaic leakage, and repeatable reversibility over multiple cycles. The use of pristine graphene and controlled electrolyte composition circumvents baseline drift and minimizes parasitic charge-noise, unlike functionalized or polymer-based FET biosensors. The analytical model's use of an effective capacitance parameter compensates for series contributions of EDL and quantum capacitance amidst variable ionic strength, ensuring robust transport parameter extraction throughout the measurement range.

Implications, Limitations, and Prospective Directions

The combination of high interfacial capacitance, strong EDL coupling, and disorder-mediated signal amplification in a pristine graphene channel yields a sensor that rivals state-of-the-art optical and electrochemical methods in sensitivity while surpassing them in operational simplicity and suitability for label-free, real-time monitoring. By revealing that adsorption-induced disorder and spatially variable carrier scattering are the principal signal transduction mechanisms, the paper provides a rigorous mechanistic basis for the interpretation of graphene bioelectronic sensors. This framework is widely generalizable to other charged macromolecule detection scenarios on 2D materials.

Key limitations remain regarding specificity and biocomplex matrix effects, since this study employs unfunctionalized graphene and purified HSA solutions. The authors note that future work will focus on biofunctionalization and antifouling strategies necessary for selectivity in serum or urine matrices, as well as integration into multiplexed or microfluidic diagnostic platforms.

Conclusion

This work establishes a systematic mechanistic and performance analysis of label-free, electrolyte-gated graphene biosensors for HSA detection. By leveraging an inverse-mobility analytical framework and rigorous molecular simulation, the study delineates the role of adsorption-induced disorder as the dominant sensing transduction pathway. The graphene EGFET sensor achieves a LOD of 0.0087 mg·mL⁻¹, linear dynamic range over three orders of magnitude, and high reproducibility, indicating strong potential for next-generation point-of-care diagnostics and dialysis monitoring.

The mechanism-centric approach advanced here provides a template for exploitation of disorder-mediated transport in biotransistor applications, with direct implications for expanding the sensitivity, stability, and multiplexability of 2D material-based biosensors in clinical, environmental, and industrial settings.

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Citation: "Graphene Electric Double-Layer Transistors for Enhanced-Sensitivity Label-Free Detection of Human Serum Albumin" (2607.03491)