High Frequency Sensing with Enhanced Sensitivity in Graphene Nanogrid FET Immunosensor
Authors: Basu Joyeeta , Roychaudhuri Chirasree 
Published: February 2020

Abstract:-FET based charge detection sensing mechanism with various nanostructures has many advantages like label free detection, femtomolar sensitivity and compatibility with electronic interface. However detecting charges in high ionic strength solution is a challenge due to the inherent problem of ionic screening beyond the Debye length.To overcome this effect, high frequency signal has been applied in single walled carbon nanotube structures which has enabled detection of biotin in presence of 100mM buffer solution [1]. But the limit of detection achieved is not satisfactory. In this work we superimpose a high frequency ac signal in the range of 100KHz to 10MHz across the drain and source terminal in a graphene nanogrid structure and applied it for detection of IgG-protein A binding in sub femtomolar regime.  

The graphene nanogrid structure has been developed on a nanoporous silicon oxide template as shown in Fig.1 and is an interconnected grid structure of bilayer reduced graphene oxide(RGO) which is composed of long and narrow strips alternating with series combination of short and narrow strips on planar regions and within pores.The long and short strips are interconnected during the deposition stage and hence the structure is expected to have fewer edges. Such a structure is expected to possess certain advantages in addition to the improved binding efficiency of the biomolecules due to large surface area to volume ratio. Firstly, in the nanopores, the biomolecules usually reside within a distance shorter than the original pore radius which statistically raises the charge transfer probability between the biomolecules and the surface even at high ionic strength of buffer. As a consequence, the heterogeneous charge transfer gets facilitated and the electrode potential reaches equilibrium faster for a given electrochemical system [2]. Secondly, such a graphene nanostructure has a size dependent effective energy gap which increases the transconductance of the device [3]. Due to the absence of edges, the conductance of the fabricated smooth nanogrid structure will be contributed by every section of the device and hence all the antigen molecules that get captured at any location contribute towards the change in conductance. Also there is a probable localization effect of the charge modulation after antigen capture within the corresponding quantum dots, which can result in an enhanced change of the overall carrier transport.

Fabrication of the nanoporous silicon oxide template has been carried out by anodic etching of p-type silicon<100> wafers of 10–20 Ω-cm resistivity in a double pond electrochemical bath for 30 minute using a constant current density of 2.35 mA/cm2 with an electrolyte mixture of 48% hydrofluoric acid and dimethyl sulfoxide in the ratio of 1:9 by volume. The nanopores formed are of about 30nm thickness and 100 nm length. The silicon substrates have been oxidized for subsequent graphene deposition. Graphene has been deposited by electrophoretic method[4] and after that antibody has been immobilized through poly-l-lysine (PLL). 

Raman spectroscopic measurements have been performed for characterization of deposited graphene[Fig:2] and the spectra of the graphene shows three Raman bands at 1195,1590 and 2670 cm-1that assigned to the well-documented D,G, and 2D bands respectively in RGO. The bilayer thickness has been confirmed by the presence of a symmetric 2D band and 2D/G intensity ratio of around 1.4. The two lateral electrodes used for RGO deposition act as the drain and source electrodes. Graphene based FET has been realized with a platinum top gate electrode suspended in buffer. Protein A has been selected as the antigen and its different concentration has been listed using 100mM buffer in the range of 100KHz to 10MHz. An ac voltage of amplitude 20 mV has been fed between the drain-source terminal. The gate potential has been fixed at 0.2 V. It has been observed [Fig:3] that the drain current (Id
) decreases with frequency for 100mM which may be attributed to the slow decay of the surface potential resulting in effective increase of the Debye length and hence the capacitive impedance. We further plotted the sensitivity [Fig.:4], calculated as the fractional change in Id
before and after Protein A attachment. A peak in sensitivity has been observed at a frequency of around 8MHz which corresponds to the optimum interaction of the antigen molecules with the graphene nanostructure. The intrinsic advantages of the graphene nanogrids(as discussed above) enable  detection down to 0.1 fg/ml antigen in a high ionic strength buffer which has significant impact in point-of care applications.

Figure 1