Abstract
With the increasing demand for biomarker detection in wearable electronic devices, flexible biosensors have garnered significant attention. Additionally, graphene field-effect transistors (GFETs) have emerged as key components for constructing biosensors, owing to their high sensitivity, multifunctionality, rapid response, and low cost. Leveraging the advantages of flexible substrates, such as biocompatibility, adaptability to complex environments, and fabrication flexibility, flexible GFET sensors exhibit promising prospects in detecting various biomarkers. This review provides a concise summary of design strategies for flexible GFET biosensors, including non-encapsulated gate without dielectric layer coverage and external gate designs. Furthermore, notable advancements in sensing applications of biomolecules, such as proteins, glucose, and ions, are highlighted. Finally, we discuss the future challenges and prospects in this field, aiming to inspire researchers to address these issues in their further investigations.
1 Introduction
Graphene-base biosensors, including graphene field-effect transistors (GFETs), enable continuous label-free detect of biomarkers with exceptional sensitivity and selectivity. (; ; ). GFETs also offer fast response times, making them well-suited for rapid detection of biomarkers. These features make GFET biosensors an ideal candidate for in-vitro detection of biomarkers.
Most of the GFET sensors uses a three-electrode structure, namely, drain, gate, and source electrodes. The drain and source electrodes are connected by graphene, which serves as the sensing element. Graphene holds a theoretical carrier mobility greater than 200,000 cm2 V−1 s−1 (), which translates into high electrical conductivity in GFETs and enhances their sensitivity as sensors (). Furthermore, the high mechanical flexibility of graphene enables FETs to employed in wearable applications, expanding their potential utility. Graphene also functions as the biomolecule functioning sites. When biomolecules bind to the graphene surface, they modify the local charge density and electrical properties of the graphene, resulting in changes in the conductance of the GFET. These changes can be measured as a shift in the GFET’s transfer characteristic, enabling the detection and quantification of biomolecules (). By functionalizing GFETs with various types of biomolecules such as DNA, proteins, and aptamers, a wide range of analytes including ions (; Tseliou F et al., 2019; ), proteins (; ; ), bacteria (; ) and other kinds of biomarkers (Zhan B et al., 2014; ) can be detected.
Flexible graphene field-effect transistor (F-GFET) biosensors are a new type of GFET sensor emerged recently (). Comparing with conventional GFET, which are typically fabricated on rigid and planar substrates like silicon (). F-GFETs, on the other hand, are fabricated on flexible or stretchable substrates such as polyimide (), paralyene and other polymers (). Although rigid substrate provides a stable and flat surface for graphene deposition and device fabrication, which allows for precise control of device parameters and reproducibility, they lack flexibility, making them unsuitable for applications that require the use of conformal surfaces. Flexible substrates, however, provide the necessary mechanical flexibility. F-GFET offer several advantages over conventional GFET, such as conformal contact with biological tissues and improved device biocompatibility, which are critical for implantable and wearable devices. Additionally, F-GFETs enable real-time monitoring of physiological signals like muscle activity and heart rate, as well as environmental monitoring for gas sensing and humidity detection.
Currently, there is a paucity of literature reviews on flexible graphene biosensors, prompting us to author this comprehensive review article. This work summarizes the latest advancements in flexible graphene field-effect transistors (GFETs) for biomolecular sensing and identifies their limitations. It also presents future research prospects and solutions to address the current challenges in the field.
2 F-GFET biosensor design
Existing F-GFET device design mostly uses a liquid-gate configuration and can be further categorized into two sub-groups according to the gate electrode positions: the non-external gate () and the external gate ().
In a typical flexible liquid gate GFET, graphene is used as the channel material for the source and drain. Chemical vapor deposition (CVD) is mainly used for synthesizing high-quality graphene, which is transferred to the substrates using either polymer-assisted transfer () or wet transfer methods (). The electrical properties and biomarker responses of the F-GFET biosensor are investigated by introducing biological molecules of interest into the solution. The conductivity of the graphene channel in GFET can be modulated by changing the gate voltage (Vg) at a constant drain-source voltage (Vds). Two main designs of gate electrode are commonly used: non-external and external. In the non-external design, the gate electrode is deposited onto the flexible substrate by metal deposition, the graphene channel region is directly exposed without any dielectric layer covering, making the graphene susceptible to environmental contamination, resulting in a degradation of GFET performance (Figure 1A). While in the external design, liquid electrodes such as Ag/AgCl or Pt electrodes are used for electrochemical sensing (Figure 1B).
FIGURE 1
3 Applications of F-GFET biosensors
Given graphene’s capability to undergo functionalization with different types of biomolecules, F-GFET holds significant promise for detecting an extensive array of analytes, encompassing DNA, proteins, small molecules, and various other substances. The following section categorizes the application of F-GFET by its analytes.
3.1 F-GFET for protein detection
In 2012, Oh Seok Kwon et al. firstly proposed that conducting polymers containing heteroatoms could be used to prepare doped graphene and successfully fabricated nitrogen-doped few-layer graphene (PPy-NDFLG) from polypyrrole (Figure 2A). They integrated the PPy-NDFLG with RNA aptamers specific to anti-vascular endothelial growth factor (VEGF) onto a flexible field-effect transistor (FET) platform based on a polyethylene naphthalate (PEN) substrate for electronic control. This work was the first to use nitrogen-doped graphene to manufacture a flexible FET-based aptamer sensor for the detection of VEGF as a cancer biomarker, and a detection limit of 100 fM was achieved (
FIGURE 2

(A) (i) Schematic illustration of reaction steps for the fabrication of aptasensor platforms based on PPy-NDFLG conjugated with anti-VEGF RNA aptamer. (ii) Schematic diagram of a liquid-ion gated FET using aptamer-conjugated PPy-NDFLG (Ag/AgCl reference electrode, R; platinum counter electrode, C; source and drain electrodes, S and D). (iii) Real-time responses and a calibration curve (S in the inset indicates ΔI/I0) of aptasensor with various vascular endothelial growth factor concentrations. Adapted with permission from (
In 2017, Yang et al. developed a F-GFET sensor with a sensing element composed of a graphene nanomesh (GNM) featuring 3 nm pores. The GNM is a continuous two-dimensional graphene nanostructure with a high density of holes punched in the basal plane, introducing lateral confinement and improving the on/off ratio. Additionally, the graphene carrier concentration and mobility can be adjusted to enhance the sensor’s performance. By modifying the GFET with an aptamer, the sensor successfully detected human epidermal growth actor receptor 2 with a minimum detectable level of 0.6 pM (Yang Y et al., 2017). In 2018, Hao et al. engineered a GFET sensor incorporating oligonucleotides as the functional group on a 125 µm thick PEN substrate. By using VR11 DNA aptamers with high specificity to TNF-α, they investigated the effects of substrate bending on the equilibrium dissociation constant between the oligonucleotides and the biomarker as well as the graphene transconductance. This work demonstrated that the sensor could specifically respond to changes in TNF-α concentration within 5 min and an ultra-low detection capability o 26 pM in a repeatable manner (
In 2022, another aptamer-based F-GFET sensor was engineered by Hao and colleagues to rapidly detect hemoglobin in undiluted biological fluids. The sensor uses polyethyleneimine (PEI) as a low-cost linking molecule for the immobilization of aptamers. The experimental results indicate that the graphene sensor modified with PEI can respond to changes in hemoglobin concentration within 6–8 min, with a minimal measurable quantity of 10.6 fM in 1× PBS, 14.2 fM in undiluted serum, and 11.9 fM in undiluted urine, respectively. Additionally, the optimal PEI modification concentration was determined to be 0.4 μM based on comparison experiments of hemoglobin detection in undiluted serum. (Figure 2C). Therefore, this sensor has potential to accurately monitor hemoglobin in a clinical setting (
3.2 F-GFET for other biomarkers detection
Compared to proteins, the application of F-GFET biosensors in detecting other types of biomolecules is far less common. In 2012, Yeon Hwa Kwak et al. developed a flexible glucose GFET sensor on a PET substrate, which exhibited ambipolar transfer characteristics. In its planar state, the sensor was capable of detecting glucose molecules in the range of 3.3–10.9 mM in PBS, with a detection limit of 3.3 mM. Even under deformation, the sensor was able to fit the model curve well and provide high-resolution, continuous real-time monitoring. This technology has significant potential for use in portable, wearable, and implantable glucose level monitoring applications (
In 2013, Ji Hyun An et al. reported the fabrication of a F-GFET sensor with rapid response to the heavy metal Mercury ion (Hg2+), which was designed for the detection and monitoring of the potential harm caused by Mercury ions to human health and the environment. (Figure 2D). The sensor used a 150 µm thick PEN substrate and DNA nucleic acid as the probe, achieving an ultra-low detection capability of 10 pM for Hg2+. The response time was less than 1 s and the sensitivity was 2-3 orders of magnitude higher than previous studies (
In 2022, Huang C et al. developed an ultra-flexible and transparent wearable GFET biosensor on a 1 μm thick PET substrate. The biosensor was designed to detect body fluid biomarkers, with a focus on L-cysteine and was able to detect L-cysteine in undiluted human sweat as well as artificial tears with a sensitivity threshold of 0.022 × 10−6 M, and 0.043 × 10−6 M, respectively. Considering its ultra-thin thickness and transparency, this F-GFET is expected to be used in applications such as contact lenses, which promotes the development of wearable biosensors in medical detection applications (
TABLE 1
| Ref | Sensor type | Target | Limit of detection | Sensitive range |
|---|---|---|---|---|
| PPy-GFET | VEGF | 100 fM | 10fM-10nM | |
| PDMS-GFET | AFP | 300 ng/mL | - | |
| PDMS-GFET | IFN-γ | 83 pM | 2-100 µM | |
| PEN-GFET | TNF-a | 26 pM | 50 pM-500 nM | |
| Wang et al., 2019 | Mylar-GFET | TNF-a | 5 pM | 50 pM-100 nM |
| Wang et al., 2020b | Mylar-GFET | TNF-a、INF-γ | 2.75 pM、2.89 pM | 0.2–500 nM 0.2–500 nM |
| PET-GFET | hemoglobin | 10.6 fM | 0.001nM–1000 nM | |
| Kapton-GFET | IL-6 | 10 pM | 10 pM-100 nM | |
| Yang et al., 2017 | PET-GFET | HER2 | 0.6 pM | 0.0001–200 ng/mL |
| PET-GFET | Glucose | 3.3 mM | 3.3–10.9 mM | |
| PEN-GFET | Hg2+ | 10 pM | 10 pM-100 nM | |
| PEN-GFET | HIV | 1 pM | 1 pM-10 nM | |
| PET-GFET | L-cysteine | 22 nM | 0–4.8 nM |
An overview of sensing performance of various GFET based biomolecule sensors.
4 Conclusion and future research prospects
F-GFET biosensors have gained significant attention due to their potential for highly sensitive detection of biomolecule. GFET biosensors facilitate label-free, rapid and accurate detection of biomarkers, and flexible liquid-gated GFET biosensors exhibit excellent reusability, mechanical flexibility, and durability. Consequently, biosensors utilizing F-GFET technology hold immense potential in transforming into wearable devices for continuous health monitoring. However, the flexible liquid-gated GFETs are limited by the external gate electrode and the exposed properties of graphene that are susceptible to environmental contaminations, leading to limitations in application scenarios and inaccurate experimental results. Nevertheless, by regulating the charge movement in the graphene channel region through the back gate structure, it is possible to make its application scenario unrestricted, while effectively protecting the graphene from contamination through the dielectric layer, thus enhancing the sensitivity of flexible back-gated GFET biosensors. In the future, various back-gated GFET biosensors based on ultra-thin flexible substrates will become a promising real-time application device. At the same time, paper has the potential to become a flexible substrate for GFET biosensors, which can further reduce the manufacturing cost (
Statements
Author contributions
WZ contributed to conceptualization and unified management of the work, edited the manuscript. HD performed supervision and provided funds. BH wrote the first draft of the manuscript. All authors contributed to the article and approved the submitted version.
Funding
This work was supported by the National Natural Science Foundation of China (Grant Nos 62173093, 61604042), Fujian Provincial Nat. Sci. Foundation (Grant Nos 2020Y0014, 2017J01501), Fujian Province Outstanding Youth Talent Program (Grant No. 601931). Natural Science Foundation of Top Talent of SZTU (No. 20200204); The Special Projects in Key Fields of Ordinary Universities of Guangdong Province (No. 2021ZDZX1018); Guangdong Provincial Engineering Technology Research Center for Materials for Advanced MEMS Sensor Chip (No. 2022GCZX005); University-enterprise cooperation research and development project of SZTU (Nos 20221061030010/20221064010022); Self-made experimental equipment project of SZTU (No. 20224027010009).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
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Summary
Keywords
flexible, graphene field-effect transistor, biomolecule, biosensor, biomarker
Citation
Hu B, Sun H, Tian J, Mo J, Xie W, Song QM, Zhang W and Dong H (2023) Advances in flexible graphene field-effect transistors for biomolecule sensing. Front. Bioeng. Biotechnol. 11:1218024. doi: 10.3389/fbioe.2023.1218024
Received
06 May 2023
Accepted
29 June 2023
Published
07 July 2023
Volume
11 - 2023
Edited by
Miao Wang, Xiamen University, China
Reviewed by
Trieu Nguyen, Honda Research Institute USA Mountain View, United States
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© 2023 Hu, Sun, Tian, Mo, Xie, Song, Zhang and Dong.
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*Correspondence: Qiu Ming Song, songqiuming@sztu.edu.cn; Wenwei Zhang, zhangwenwei@sztu.edu.cn; Hui Dong, hdong@fzu.edu.cn
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.