3.3.1.5 Liquid Exfoliation Via Etching

This approach is successfully used to prepare 2D MXene via selective etching of their 3D

counterparts, i.e., MAX phases [43]. The general formula for MAX phases in Mn+1AXn, here

M is transition metal, A is group IIIA or IVA metal, X is C or N and n is the integer. In this

process, the bulk MAX phase is immersed in an etching agent to remove the A layer. The

etching agents frequently used are hydrofluoric acid, hydrochloric acid, and lithium fluoride.

Firstly, the MAX phases are etched to form MXenes attached with different functional groups

(O, OH, F, H) depending upon the etching agent used. Later, the MXenes are intercalated by

using different salts and organic compounds to increase the interlayer spacing. Finally,

mono- or few-layered MXenes are obtained via sonication and centrifugation.

3.3.2 Bottom-Up Synthesis Protocols

2D materials can be frequently synthesized by two bottom-up approaches such as che­

mical vapor deposition (CVD) and wet-chemical synthesis methods.

3.3.2.1 Chemical Vapor Deposition

Thin films of 2D materials are widely prepared through the CVD technique [44]. In this

method, a precursor in gas or vapor form reacts or decompose on a substrate at elevated

temperature under vacuum condition. 2D sheets could be grown on the substrate without

the aid of a catalyst. There are a variety of 2D materials are developed through the CVD

technique including graphene, h-BN, TMDs, metal carbides, borophenes, antimonene,

and silicene [44]. Somani and coworkers [45] were the first to propose a CVD technique

for the synthesis of graphene. CVD method also offers ease to control the synthesis

parameters to obtain 2D materials as compared to other techniques. In addition, a high

yield of a 2D material with high purity and the least defect can be accomplished through

the CVD technique. The major drawback of this technique is the higher production cost.

3.3.2.2 Wet-Chemical Synthesis

In this method, 2D materials are obtained via chemical reactions in a solution medium

using a precursor at a particular condition. The wet-chemical route is highly preferable to

develop 2D materials that one cannot develop through a top-down approach. A higher

degree of control and excellent repeatability are the two key features of the wet-chemical

synthesis route. There is a variety of wet-chemical synthesis methods such as sol­

vothermal, hydrothermal, template synthesis, self-assembly, oriented attachment, hot-

injection, and interface-mediated synthesis to prepare 2D materials [46].

3.4 Mechanism of Bioelectronic System

The transduction and sensing mechanisms of various 2D materials–based bioelectronic

devices are addressed in this section. These devices include field-effect transistors (FETs),

electrodes and electrode arrays, optical resonators, and multifunctional sensors [1]. In

general, the biological elements provide signals related to the alternation of interfacial

properties through biological activities. These signals are transduced to the readouts via

46

Bioelectronics