Flexible organic electrochemical transistors for highly selective enzyme biosensing

Organic electrochemical transistors (OECTs) have attracted significant attention in recent years. Because of their inherent amplification capability, OECTs serve as high-performance sensing transducers that can efficiently convert biochemical signals into electronic ones. These devices have been successfully implemented for the detection of specific analytes in complex multi-analyte environments and physiological samples¹ (e.g., metal ions, glucose, dopamine, bacteria, protein, and cells). Typically, an OECT has a very simple structure, with a thin layer of organic semiconductor material that is directly in contact with electrolytes deposited on the channel area. These devices are therefore easily fabricated using conventional solution processes, such as inkjet printing and spin-coating techniques.² Additionally, because OECTs can be assembled on a variety of flexible substrates (e.g., polyethylene terephthalate, polyimide, polydimethylsiloxane, fiber, and even paper), they have high potential for use in futuristic applications such as wearable electronics and implantable microchips.³

Enzyme biosensors based on OECTs enable the accurate analysis of a range of substances. They therefore hold promise for a variety of sensing applications, including clinical diagnostic analysis, point-of-care healthcare testing, drug development, and environmental monitoring.⁴ However, the selectivity feature of enzyme biosensors—a significant parameter in their practical application—has rarely been investigated in OECT-based sensors.

To tackle this challenge, we have developed multilayer-functionalized enzyme sensors for highly sensitive and selective detections. We fabricated flexible OECTs with active layers of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) on thin polyethylene terephthalate substrates (∼50μm) pre-coated with platinum electrodes: see Figure 1. The resultant flexible biosensor can be attached to various deformable surfaces, including human skin and medical bandages—see Figure 2(a)—to accommodate surface movements. Our flexible device demonstrates high-level bending stability and no obvious performance degradation even after 1000 bending tests: see Figure 2(b).

Figure 1. Flexible organic-electro-chemical (OECT)-based enzyme biosensor, functionalized with multilayer modification techniques. PANI: Polyaniline. GO: Glucose oxidase. H₂O₂: Hydrogen peroxide. PEDOT:PSS: Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate).

Figure 2. (a) Flexible organic electrochemical transistors (OECTs) attached to (i) the surface of human skin and (ii) a medical bandage, under different operational conditions. (b) The transfer characteristics of an OECT measured in phosphate-buffered-saline solution after bending tests of up to 1000 times. Voltage at drain source =0:05V. I_DS: Current through the drain source. V_G: Gate voltage applied to the transistor.

Most enzyme biosensors are based on the detection of hydrogen peroxide (H₂O₂). We therefore began by fabricating highly selective OECT-based H₂O₂sensors. Modifying the gate electrodes with a bilayer functionalized film (polyaniline/nafion-graphene) causes the interference signals of other electroactive elements in the multi-analyte electrolyte to be effectively eliminated. As a result, H₂O₂ alone diffuses to the gate and induces an electrochemical reaction.

Figure 3(a) shows the changes of the effective gate voltage (ΔV_G^eff) versus the analyte concentrations of an OECT with a polyaniline/nafion-graphene/platinum gate, characterized in PBS (phosphate-buffered saline) solution. The H₂O₂ detection limit of this device is 3×10⁻⁹mol L⁻¹, which is several orders of magnitude lower than that achieved by conventional electrochemical detection platforms. Additionally, the selectivity of the bilayer-modified device was remarkably improved compared with that of unmodified devices. The dopamine (DA) and ascorbic acid (AA) detection limits (∼3×10⁻⁶mol L⁻¹) are significantly higher than that of H₂O₂, indicating that the device is not sensitive to interference.

Figure 3. (a) The changes of effective gate voltage (ΔV_G^eff) versus the concentrations (C) of hydrogen peroxide (H₂O₂), ascorbic acid (AA) and dopamine (DA). The gate electrode of the OECT was modified with PANI/nafion-graphene double layers. (b) The changes of ΔV_G^eff versus the concentrations of uric acid (UA), AA, and glucose. The gate electrode of the OECT was modified with uricase-GO/PANI/nafion-graphene layers.

We have also realized a high-performance uric acid (UA) sensor by introducing a layer of the enzyme uricase (UOx) on the surface of polyaniline/nafion-graphene bilayer films. Our results demonstrate that the strong covalent bonding that occurs between the enzyme layer and underlying polyaniline film causes the chemical immobilization of UOx, leading to a higher performance of UA detection. Figure 3(b) shows the corresponding ΔV_G^eff changes in the UA-sensitive OECT after the addition of analytes with different concentrations. The flexible UA sensor exhibits a detection limit of 10×10⁻⁹mol L⁻¹ and a ΔV_G^eff of 147mV per decade of concentration. This high sensitivity guarantees a sufficient response to a trace amount of the analyte in highly diluted biological samples. Moreover, the interference effects caused by DA, AA, and glucose are negligible, showing promise for real sensing applications.

In summary, we have developed a flexible OECT with multilayer modified gate electrodes. The functionalized enzyme sensors show excellent selectivity and sensitivity, with promise for the realization of highly selective and sensitive biological sensors for practical applications. In our future work, we plan to develop device fabrication techniques that use printing processes to enable the introduction of low cost, flexible, and disposable biosensors into the market.