electrical current, potential, conductance, impedance, and resonant frequency. For instance,

a bioelectronic system can be functionalized when linker molecules are covalently con­

nected to 2D materials–based sensors. The linker molecules are responsible for the identi­

fication of specificity, improving the transduction of signals, and amplifying the signals

received from sensing elements. Though the covalent bonds between linker molecules and

graphene alter the electrophysical properties of graphene from sp² to sp³ bond and reduce

carrier mobility [47]. In addition, non-covalent routes employ electrostatic, van der Waals,

hydrophobic interactions between linker molecules and graphene. This route not only

permits shallow functionalization via adsorption of different molecules but also results in a

yield of non-specific adsorption. The problem of non-specific adsorption can be resolved

through passivation that restricts non-functional sites through surfactants and stabilizing

biomolecules [9].

3.4.1 Mechanism for Field-Effect Transistors

2D materials–based FETs possess configuration analogous to a solution-gated FET bio­

sensor. In these devices, efficient gating could be achieved through an electrolyte. The

conductance in such devices is controlled via the potential difference between a grounded

electrode (drain) and a reference electrode. The measurement of source-drain current is

determined in terms of gate voltage that shows the least value of source-drain current at a

finite gate voltage, which is also known as the Dirac point. Conceptually, the sensing

mechanism is governed by the combination of the electrostatic gating effect and the

Schottky barrier [48,49]. The electrostatic gating effect could change transistor conductance

due to an electrostatic disturbance caused by biomolecule adsorption. This also results in

doping in graphene and alters the Dirac point [47]. In the case of the Schottky barrier

mechanism, the adsorbed biomolecules at metal contact alter the variation between the

work functions and conductive channel. This changes an asymmetric conductance in p- and

n-branches of a source-drain current (ISD) – gate voltage (VG) plot. When the passivation on

the conductive channel-metal contact occurs then the electrostatic gating effect dominates

the Schottky barrier effect. The other mechanism involves variation in carrier mobility and

decreased gate performance [50]. At the sensing site, adsorbed molecules adversely affect

the mobility of the charge carrier. Moreover, less permittivity of adsorbed biomolecules in

comparison to electrolytes led to a decrease in gate conductance and reduces the efficacy of

gate. Compared to the electrostatic gating effects, these changes in carrier mobility and gate

coupling are minimal. To develop 2D material-based FET devices for biomedical applica­

tions, the Debye length must be considered [51]. The Debye length is defined as a distance

where surplus ions in an electrolyte screen the target and probe biomolecules. This length

depends on the ionic composition of buffer solution, temperature, and dielectric constant.

The larger distance from the surface of the FET device as compared to the Debye length,

seldom affect the mobile charges. Consequently, the target and probe interaction must

occur within the Debye length and the probe size should be lower than the Debye length.

The limit of detection in FET devices depends on the buffer solution ionic strength and size

of interacting biomolecules. This mechanism enables electrochemical and electrical sensing

of DNA, biomolecules, and cells.

3.4.2 Mechanism for Nanopore-Based Bioelectronics

A nano-sized aperture on a thin film is used for sensing in nanopore-based bioelectronics.

This mechanism is broadly used to sense DNA. In this mechanism, the electric potential is

2D Materials for Bioelectronics

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