Graphene-based FET biosensors

Abstract

Graphene and graphene-based materials are highly attractive for field effect transistor (FET) applications because of their elevated theoretical charge carrier mobility. Graphene FETs (GFETs) are studied for biosensing applications for their high sensitivity and fast detection times. However, beyond theory, the state-of-the-art GFETs have very low charge carrier mobilities and low ON and OFF current (ION/IOFF) ratios. The reduced electrical performance of the reported GFETs hinders their use for sensitive biosensor applications. In this work, a femtosecond laser beam was used to fabricate functional graphene-based materials and to tune their structure and electrical performance. It was found that the laser ablation process could transform two insulating two-dimensional (2D) materials, graphene oxide (GO) and hexagonal boron nitride (h-BN), into semiconductors with high ION/IOFF ratios and charge carrier mobilities. Two types of nanomaterials were fabricated using a laser ablation process. The first is B and N co-doped reduced GO (rGO) nanoflakes, and the second is B and N co-doped GO (BN-GO) gels. The co-doping concentration was controlled by changing the ratio between GO and h-BN in the precursor solutions. Greater B-doping concentrations were observed for the nanoflakes (1.8-4.1 at%) compared with the gels (0.5-1.3 at%). Similarly, greater nitrogen doping was observed for the nanoflakes (3.4-5.9 at%) compared with the gels (0.3-1 at%). Increased B and N co-doping concentrations were found to reduce the sheet resistance of the nanoflakes. Back-gated FET devices made of the BN-GO gels revealed very high ION/IOFF ratios (~106) and electron and hole mobilities in the range of 3000-9000 cm2V-1s-1 and 2000-6000 cm2V-1s-1, respectively. Several electrical performance enhancement strategies were employed, and the best mobility and ION/IOFF ratio values achieved were 440,000±200,000 cm2V-1s-1 and 107, respectively. These values were obtained when the pulse duration of the laser ablation was reduced by 3 times to approximately 10 fs. These gels demonstrated good stability over a 2-year period. The BN-GO gels were used for several sensing and biosensing applications. For gas sensing, the BN-GO gels were used as the receptor material in a membrane-type surface stress (MSS) sensor. It was found that the sensitivity of the sensor is enhanced for gels containing higher concentrations of B and N co-doping. Additionally, MSS using BN-GO gels demonstrated an improved limit of detection (LOD) for most tested compounds compared to other 2D receptors. An FET with the BN-GO gel as top-channels were used for the biosensing of brain natriuretic peptide (BNP), a heart failure (HF) biomarker, and Coronavirus disease 2019 (COVID-19) synthetic proteins in buffer. The BN-GO gel channels were covalently functionalized with BNP or COVID-19 antibodies to selectively capture the desired analytes. It was found that real-time detection of BNP biomarker in buffer could be achieved in as little as 5 seconds with an LOD of 10 fM and a detection range of 10 fM – 1 pM. The Dirac point monitoring of the same biosensor revealed an LOD of 10 aM within 2 minutes and a detection range of 10 aM – 1 µM. The biosensor demonstrated great specificity and selectivity compared with K+ OH- ions and human epidermal growth factor receptor 2 (HER2) cancer biomarker protein. The BN-GO gel FET covalently functionalized with COVID-19 antibodies was used for COVID-19 biosensing in buffer as a proof-of-concept by monitoring the shift in the Dirac point. The LOD and detection range were calculated as ~30 fg/mL and 0.01-100 pg/mL, respectively for COVID-19 protein in a 0.1x pH=7 buffer solution. The BNP and COVID-19 biosensors should be further investigated beyond the proof-of-concept stage. Additionally, other biomarker-bioreceptor pairs could also be investigated using the same BN-GO gel FET platform. Aptamers may be studied as a replacement for the antibody bioreceptors for improved sensitivity.