Graphene and its sensor-based applications: A review

Highlights

• Review on the fundamentals of graphene: preparation, properties and application.

• The preparation procedure Chemical Vapor Deposition, Mechanical Exfoliation and Hummer’s method.

• The applications include sensors with electrodes having graphene in its pure and nanocomposite form.

• Three types of applications namely electrochemical, strain and electrical sensors are explained, based on the operating principle of the sensors.

• The strength, limitations of the current graphene sensors along with their future opportunities.

Abstract

This paper presents an overview of the work done on graphene in recent years. It explains the preparation techniques, the properties of graphene related to its physio-chemical structure and some key applications. Graphene, due to its outstanding electrical, mechanical and thermal properties, has been one of the most popular choices to develop the electrodes of a sensor. It has been used in different forms including nanoparticle and oxide forms. Along with the preparation and properties of graphene, the categorization of the applications has been done based on the type of sensors. Comparisons between different research studies for each type have been made to highlight their performances. The challenges faced by the current graphene-based sensors along with some of the probable solutions and their future opportunities are also briefly explained in this paper.

Introduction

After the advent of smart sensors around two decades back [1], their applications in daily life have been ever increasing. Nowadays, almost every industrial, domestic and environmental sector utilizes sensors for improving the quality of life [[2], [3], [4], [5], [6], [7]]. Among the sensorial parts, electrodes constitute the most important section as they allow the monitoring unit to receive and analyze the sensed data. Research work has been going on continuously to develop the materials used for electrodes. Existing materials have been improved based on their mechanical, electrical and thermal properties. Gold, silver, aluminum and carbon are some of the commonly used materials used to develop electrodes [[8], [9], [10]]. Out of them, graphene has always been a popular choice due to its distinct advantage of exhibiting excellent electrical and crystal qualities. Graphene, in simple words, can be defined as a single layer of carbon atoms that are tightly packed to form a 2D honeycomb crystal lattice structure [11].

It is the basic component for the carbon allotropes which can be modified into other forms like 0D fullerenes, 1D CNTs, and 3D graphite as shown in Fig. 1. The work on graphene has been going on for the last sixty years, but earlier, it was mostly described as an allotrope of carbon to explain different carbon-based materials. It was just in recent times, around a decade back, the free-standing 2D model of graphene was experimentally proved [12,13]. In the applied field of research, among its wide range of applications, graphene has been mostly used in batteries and cells as anodes, and in supercapacitors due to its low charging time, high strength to weight ratio, and large surface area. It has also found a large number of applications in areas like sensors, biomedical engineering, nano and flexible electronics and catalysis due to its unique properties which include a distinctive nanopore structure, high mechanical strength and high electrical and thermal conductivity. The functionalization of graphene, to reduce the cohesive force between the graphene molecules in different forms, has caused significant changes in its physicochemical properties, thus increasing their end applications [[14], [15], [16], [17], [18], [19], [20]]. Two different forms of functionalization, covalent and non-covalent, are achieved for graphene molecules when the materials are chemically treated by different techniques like spin-coating, filtration, layer-by-layer assembly to cause surface modification but maintaining its intrinsic properties [21]. Even though a lot of research articles have been published based on the different techniques for the preparation of graphene and its utilization in sensors, a thorough research work on the combination of all these aspects is yet to be done. The motivation of this review article is to present a general overview of the importance of graphene as a material, its preparation, and utilization in some of the significant aspects. It showcases the different properties of graphene which makes it a superior element that can be considered for dynamic applications. It also shows a comparative study in the form of tables to highlight the performance of different research studies done on graphene-based electrochemical, strain and electrical sensors. The paper also depicts the strength and limitations of the current sensors along with the challenges and future opportunities of graphene-based sensors.

The involvement of graphene to develop sensors is attributed to some of the distinct advantages like the large surface-to-volume ratio, unique optical properties, excellent carrier mobility and exceptional electrical and thermal properties compared to the other allotropes of carbon. These properties are constant for the double and multi-layered graphene structures. Apart from the difference in the structure, working conditions, the use of these advantages in graphene sensors lie mainly in their capability to adjust according to the application. For example, in strain sensors, properties like the detection limit, maximum sensing range, signal response and reproducibility of the response hold a pivotal role to determine the quality of the sensor. These characteristics are attributed to the electrical and mechanical properties of graphene. In electrochemical sensors, its large surface area helps the loading of the desired biomolecules, resulting in the interaction between the analyte molecule and electrode surface due to the high ballistic transport capability and the very small band gap. Another advantage of graphene lies in its low environmental impact, making it more popular for sensing purposes than other metals [22,23]. Table 1, Table 2 show the comparison of the performance of some of the electrochemical and strain sensors developed with graphene with that of Carbon Nanotubes (CNTs) and silver. It is seen from Table 1 that the Gauge Factors (G.F.) and maximum attainable strain are mostly highest for the graphene-based sensors. One disadvantage of these sensors is the variation in the linearity in their response. For the other two types of sensors, the GFs are much less than graphene sensors, even though most of them attain a fairly high amount of detectable strain. It is seen from Table 2 that, even though the limit of detection achieved by all the three different types of sensors is the same, most of the graphene based sensors are able to achieve high sensitivity with a wider linear range.