Biosensing with Surface-Charge-Modulated Graphene Field-Effect Transistors beyond Nonlinear Electrolytic Screening.

In field-effect transistor (FET) biosensors, charge screening in electrolyte solutions limits the sensitivity, thereby restricting the applicability of FET sensors. This is particularly pronounced in graphene FET (GFET) biosensors, where the bare graphene surface possesses a strongly negative charge, which impedes the high sensitivity of GFETs owing to nonlinear electrolytic screening at the interfaces between graphene and liquid. In this study, we counteracted the negative surface charge of graphene by decorating positively charged compounds and demonstrated the sensing of C-reactive protein (CRP) with surface-charge-modulated GFETs (SCM-GFETs). We integrated multiple SCM-GFETs with anti-CRP antibodies and nonfunctionalized GFETs into a chip and measured differentials to eliminate background changes to improve measurement reliability. The FET response corresponded to the fluorescence images, which visualized the specific adsorption of CRP. The estimated dissociation constant was consistent with previously reported values; this supports the conclusion that the results are attributed to specific adsorption. Conversely, the signal in GFETs without decoration was obscured by noise because of nonlinear electrolytic screening, further emphasizing the significance of surface-charge modulation. The limit of detection of the system was determined to be 2.9 nM. This value has the potential to be improved through further optimization of the surface charges to align with specific applications. Our devices effectively circumvent nonlinear electrolytic screening, opening the door for further advancements in GFET biosensor technology.

Ionic strength-sensitive and pH-insensitive interactions between C-reactive protein (CRP) and an anti-CRP antibody.

C-reactive protein (CRP) is an important biomarker of infection and inflammation, as CRP is one of the most prominent acute-phase proteins. CRP is usually detected using anti-CRP antibodies (Abs), where the intermolecular interactions between CRP and the anti-CRP Ab are largely affected by the pH and ionic strength of environmental solutions. Therefore, it is important to understand the environmental effects of CRP-anti-CRP Ab interactions when designing highly sensitive biosensors. Here, we investigated the efficiency of fluorescently labeled CRP-anti-CRP monoclonal antibody (mAb) interactions at different pHs and ionic strengths. Our results indicate that the affinity was insensitive to pH changes in the range of 5.9 to 8.1, while it was significantly sensitive to ionic strength changes. The binding affinity decreased by 55% at an ionic strength of 1.6 mM, when compared to that under a physiological condition (~150 mM). Based on the isoelectric focusing results, both the labeled CRP and anti-CRP mAb were negatively charged in the studied pH range, which rendered the system insensitive to pH changes, but sensitive to ionic strength changes. The decreased ionic strength led to a significant enhancement of the repulsive force between CRP and the anti-CRP mAb. Although the versality of the findings is not fully studied yet, the results provide insights into designing highly sensitive CRP sensors, especially field-effect transistor-based sensors.

Drift Suppression of Solution-Gated Graphene Field-Effect Transistors by Cation Doping for Sensing Platforms.

Solution-gated graphene field-effect transistors (SG-GFETs) provide an ideal platform for sensing biomolecules owing to their high electron/hole mobilities and 2D nature. However, the transfer curve often drifts in an electrolyte solution during measurements, making it difficult to accurately estimate the analyte concentration. One possible reason for this drift is that p-doping of GFETs is gradually countered by cations in the solution, because the cations can permeate into the polymer residue and/or between graphene and SiO2 substrates. Therefore, we propose doping sufficient cations to counter p-doping of GFETs prior to the measurements. For the pre-treatment, GFETs were immersed in a 15 mM sodium chloride aqueous solution for 25 h. The pretreated GFETs showed that the charge neutrality point (CNP) drifted by less than 3 mV during 1 h of measurement in a phosphate buffer, while the non-treated GFETs showed that the CNP was severely drifted by approximately 50 mV, demonstrating a 96% reduction of the drift by the pre-treatment. X-ray photoelectron spectroscopy analysis revealed the accumulation of sodium ions in the GFETs through pre-treatment. Our method is useful for suppressing drift, thus allowing accurate estimation of the target analyte concentration.