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Numerous efforts have been undertaken to mitigate the Debye screening effect of FET biosensors for achieving higher sensitivity. There are few reports that show sub-femtomolar detection of biomolecules by FET mechanisms but they either suffer from significant background noise or lack robust control. In this aspect, deformed/crumpled graphene has been recently deployed by other researchers for various biomolecule detection like DNA, COVID-19 spike proteins and immunity markers like IL-6 at sub-femtomolar levels. However, the chemical vapor deposition (CVD) approach for graphene fabrication suffers from various surface contamination while the transfer process induces structural defects. In this paper, an alternative fabrication methodology has been proposed where glass substrate has been initially texturized by wet chemical etching through the sacrificial layer of synthesized silver nanoparticles, obtained by annealing of thin silver films leading to solid state dewetting. Graphene has been subsequently deposited by thermal reduction technique from graphene oxide solution. The resulting deformed graphene structure exhibits higher sensor response towards glial fibrillary acidic protein (GFAP) detection with respect to flat graphene owing to the combined effect of reduced Debye screening and higher surface area for receptor immobilization. Additionally, another interesting aspect of the reported work lies in the biomolecule capture by dielectrophoretic (DEP) transport on the crests of the convex surfaces of graphene in a coplanar gated topology structure which has resulted in 10 aM and 28 aM detection limits of GFAP in buffer and undiluted plasma respectively, within 15 min of application of analyte. The detection limit in buffer is almost four decades lower than that documented for GFAP using biosensors which is is expected to pave way for advancing graphene FET based sensors towards ultrasensitive point-of-care diagnosis of GFAP, a biomarker for traumatic brain injury. [ABSTRACT FROM AUTHOR]

Recently, graphene nanogrid sensor has been reported to be capable of sub-femtomolar sensing of Hepatitis B (Hep-B) surface antigen in buffer. However, for such low concentration of Hep-B in serum, it has been observed during real-time operation that there is an overlap of around 50% in the drain–source current sensitivity values between different concentrations of the target biomolecule, in the range from 0.1 to 100 fM. This has been attributed to the fact that the concentration of non-specific antigen in serum being significantly higher than that of the target antigen, there is a considerable deviation in the number of captured target antigen for the same concentration. Further, this degree of overlap varies from one set to another set of sensor, depending on the statistical variations in the sensor fabrication process. This phenomenon challenges the quantification of target antigen for ultralow limit in physiological analyte. In this paper, we introduce probabilistic neural network (PNN) for quantification of Hep-B down to 0.1 fM in serum using graphene nanogrids field-effect transistor biosensor. The sensor has been operated in heterodyne mode in the frequency range of 100 kHz to 1 MHz applied between drain and source to overcome the problem of Debye screening effect. The application of PNN limits the quantification error within 10% in the range of 0.1 to 100 fM in contrast to 77% and 66% using polynomial fit and static neural network models, respectively. Further, the proposed methodology lowers the detection limit of Hep-B in serum by more than three orders of magnitude compared with the state-of-the-art, real-time, label-free sensors. [ABSTRACT FROM AUTHOR]

Reduced graphene oxide, in various configurations has been extensively researched as field effect transistor (FET) biosensor, but they usually exhibit low signal to noise ratio. In this paper, we report the fabrication of low noise FET biosensors with electrochemically reduced graphene oxide(ERGO) on fluorine doped tin oxide(FTO) glass, directly by electrochemical reduction of GO in solution. It has been observed that the sensor is capable of detecting down to1 fM concentration of Hep-B in serum with a dynamic range from 1 fM to 20 pM, linearity of around 95% and current sensitivity of 1.21 mA. Moreover, the signal to noise ratio is around 80 in serum, which is enhanced by more than one order of magnitude compared to the most sensitive reports. Further, the presence of Hep-B can be confirmed by the Lorentzian nature of noise spectra, which makes the detection more precise. The combined advantages of low cost and clean fabrication coupled with sensitive and reliable performance renders the device suitable for point of care diagnostics. [ABSTRACT FROM AUTHOR]

A smooth and reliable reduced graphene oxide (RGO) nanogrid structure has been developed using a nanoporous silicon oxide template, which resulted in the attomolar sensitivity of a field effect transistor biosensor. Scanning electron microscopy, Raman, surface profilometry, and conductance measurements verify the nanogrid structure of RGO, its bilayer thickness, and smoothness. The conductance of the biosensor reliably increases with the increasing concentration of hepatitis B virus molecules. The combined effects of insignificant line edges, quantum dot like transport behaviors, and improved interaction of the biomolecules within the nanopores with the pore walls have been attributed to the ultrasensitivity of the biosensor. [ABSTRACT FROM AUTHOR]

The authors would like to make corrections to the above paper [1, Fig. 2(b) and 4(b)] due to some post-processing carried out in these figures. After capturing the SEM image-cross-section [1, Fig. 2(b)], we wanted to measure the pore lengths, pore diameters at different positions along the lengths and also the spacing between the pores, to evaluate the uniformity of the electrochemical etching process. To do so, vertical and horizontal lines had to be constructed. For this purpose, we were trying to select different textured lines in the image processing software whose texture would match closely the original texture of the SEM image. But exact matching was not possible. Initially we tried to measure with a textured line, a part of which is actually the white region above the “nm” in 30 nm word in [1, Fig. 2(b)]. Later, we shifted to different textured lines which are visible above the “100 nm” word and also within the pores in [1, Fig. 2(b)]. Within the pores these lines look grayish. These lines are visible in most of the regions since they were a better match with the original SEM figure than the whitish line. However, all these lines were only for our own measurements and should have been deleted before submitting the final paper. [ABSTRACT FROM PUBLISHER]

In this paper we report the sensing of aflatoxin B1(AFB1) by field effect capacitive method using electrophoretically deposited reduced graphene oxide (RGO) films for the first time. The RGO film has been characterized using SEM, surface profilometer and Raman spectroscopy. It has been observed that both quantum capacitance of RGO ( C q ) and effective electrical double layer capacitance ( C EDL ) contribute significantly towards the overall sensitivity for molar concentration in the range of 20–50 mM. As C q and C EDL changes in opposite direction after AFB1 capture and the nature of frequency dependence of C q and C EDL are different, the sensitivity shows a minima at a particular frequency. Interestingly, the sensitivity minima is also dependent on AFB1 concentration. Further, the maximum sensitivity obtained is around 30% for 10 −4 ppt (0.1 fg/ml) AFB1 which is greater than 1.5 times that of previous reports. This has been possible through the enhanced biomolecule immobilization capability of RGO. Thus the RGO based field effect capacitive sensor provides a combined advantage of both a high sensitivity and concentration dependent frequency behavior. [ABSTRACT FROM AUTHOR]