Carbon-based electrode materials for sensor application: a review

Abstract

Due to their remarkable qualities, carbon-based electrode materials have attracted a lot of interest and are considered excellent options for a variety of uses with sensors. The review provides a summary of electrochemistry and some electrochemical measurement techniques including cyclic voltammetry (CV), chronoamperometry and chronopotentiometry, electrochemical impedance spectroscopy (EIS), and cyclic potentiodynamic polarisation (CPDP). A thorough analysis of the wide range of carbon-based materials used in sensor technologies, including their structural features, electrochemical qualities, and manufacturing processes was appraised. In addition to highlighting the special physicochemical characteristics of carbon nanotubes, graphene, carbon nanofibers, and other carbon-derived materials that make them suited for sensing applications, the survey examines the developments and contributions of these materials. Additionally, the critical role that these materials play in improving the sensitivity, selectivity, and stability of sensors is covered in the review paper. Numerous sensor applications are covered in the report, such as environmental control, biomedical diagnostics, and industrial monitoring. Finally, the appraisal highlights the potential directions for more research and innovation by providing insights into the present difficulties and future opportunities in the development of sensors using carbon-based electrode materials.

Keywords:

• Sensors
• electrochemistry
• carbon-based materials
• electrode

1. Background

With an increase in population and subsequent globalisation, many researchers in the scientific community have tried to explore alternative sources of clean energy to power homes and industries as the traditional sources of energy (fossil fuel) are not reliable and could deplete over time (Ajay and Dinesh 2018; Romanescu et al. 2013; Dubey and Guruviah 2019). Many environmentally benign sources of energy have been explored to serve as a substitute for fossil fuels such as wind and tidal but the major setbacks have been the inability to receive constant supply (Dubey and Guruviah 2019). When considering energy storage devices, three main electrochemical systems have proven to be reliable over several decades now namely batteries, electrochemical double-layer capacitors (EDLC) which are usually referred to as supercapacitors, and capacitors (Jian et al. 2016). Even though metal-based electrodes such as Cu, Pt, and Ni have been used for over a century, and are somehow still in use, it has been observed that carbon-based electrode materials are more efficient in the production and storage of electrical energy. This is mainly because, such materials possess improved, optical properties (Dong et al. 2012) specific surface area, high electrical conductivity, small size and weight with tuneable electronic properties, high porosity, good chemical and temperature stability, low cost of production compared to the conventional electrodes, availability, and exist in many allotropic forms (Wake et al. 2017). Appropriate modification of the surface chemistry of the electrode material would cause the EDLC’s performance to increase. Therefore, to obtain good capacitance, it must be ensured that the electrode material has good wettability (Chen et al. 2014).

Sensors are electronic devices with the ability to detect small differentials in input energy and transform it into the same or other types of energy as output in the form of signals (KP et al. 2020). Electrochemical sensors are widely used to identify and track a variety of everyday factors. The sensing device reacts to chemical ions or physical movement to support environmental and health inspection. A sensor’s electrical response generates a signal, which is mostly observed concerning electrochemical gradient, current, and resistance (Skoog, Holler, and Crouch 2007).

In electrochemical reactions, information about the reaction between a particular analyte and the electrodes is converted into a signal that can be interpreted. In an era where there is the need to improve in every aspect of life and facilitate technological advancement, to produce sensors, there are some basic requirements the device must meet to make it acceptable for such purposes. A few of these requirements include fast response time, higher sensitivity, relatively small size and portability, fast recovery, cost-effective, simple mode of operation and it must be reliable (Bezzon et al. 2019). Many compounds of food quality, clinical, and environmental relevance have been analysed using carbon-based electrode materials (CBEMs), such as conducting polymers, activated carbon (AC), carbon aerogel (CA), carbon nanofibers (CNF), graphene, and carbon nanotube (CNT) (Gupta, Pathak, and Semwal 2019). Due to signal amplification, electrochemical sensors based on CBEMs can identify analytes at low concentrations and respond quickly (Vashist and Vashist 2011; Vashist et al. 2011). Chemically functionalised CBEMs have been applied in DNA electroanalysis (Kato and Niwa 2013), volatile organic compounds (VOCs) and gas sensing devices (Mirica et al. 2013), glucose sensing (Kaasik et al. 2013), and real-time determination of heavy metal ions (Huang et al. 2014).

The development of these materials with remarkably unique electrochemical properties has allowed for the creation of ultrasensitive biosensors that can detect specific amino acids, cholesterol, E. coli, influenza, cancer cells, human papillomavirus, dopamine, glutamic acid, uric acid, ascorbic acid, cortisol, and cytosomes. These biosensors are fully capable of amplifying signals (Kumar, Deep, et al. 2015; Kumar, Deep, et al. 2015).

In current publications, many scientists seem to be gearing their attention towards carbon-based nanocomposite materials as electrode materials for various applications due to immense improvement in the aforementioned characteristics and advantages (Kumar, Kim, et al. 2015; He et al. 2015). Carbon-based materials have a high conductivity, a big surface area, and strong chemical stability, making them popular electrode-sensing materials for electrochemical detection. The primary shortcomings of conventional electrodes are their low sensitivity and poor selectivity. One of the most common methods for enhancing an electrochemical sensor’s performance is to functionalise the electrode with a large surface area nanomaterial. Carbon-based electrodes are ideal for electrochemical detection because of their large specific surface area, uniform and adjustable pore size distribution, long-range structural order composed of mesopore channels, and superior conductivity (Shao et al. 2010). Using ordered mesoporous carbon (OMC), a sensitive electrochemical sensor has been created to identify ultratrace nitroaromatic explosives. OMC-functionalised glassy carbon electrodes (GCEs) have a high sensitivity of 62.7 μAcm⁻² per ppb towards 2,4,6-trinitrotoluene (TNT). In comparison with other materials, including ordered mesoporous silica and carbon nanotubes, OMC's exceptional performance in sensing TNT may be due to its large specific surface area and quick electron transfer capabilities. On OMC-based electrodes, levels of as little as 0.2 ppb TNT, 1 ppb 2,4-dinitrotoluene, and 1 ppb 1,3-dinitrobenzene can be found (Zang et al. 2011). Ensuring selectivity is crucial when developing sensor materials. Zeng et al. (2014) introduced a biosensor based on chronoamperometry MCN/tyrosinase (Tyr), demonstrating notable specificity for phenol and catechol in compost samples. Chronoamperometry measurements revealed that the reduction current for phenol and catechol exhibited proportionality to their concentrations within the range of 5 × 10⁻⁸ to 9.5 × 10⁻⁶ M and 5 × 10⁻⁸ to 1.25 × 10⁻⁵ M, respectively. The correlation coefficients were 0.9991 for phenol and 0.9881 for catechol. The detection limits for catechol and phenol were determined to be 10.24 nM and 15.00 nM, respectively, with a signal-to-noise ratio of 3 (Zhou et al. 2014).

Jian et al. (2016) developed a composite film based on ordered mesoporous carbon functionalised Nafion (OMC@Nafion) to immobilise Ru(bpy)₃²⁺ for surface modification of a glass carbon electrode (GCE). The resulting Ru(bpy)₃²⁺/OMC@Nafion composite film-modified GCE exhibited excellent sensitivity for electrochemiluminescence (ECL) determination of tripropylamine (TPA) over a wide linear range (4.75 × 10⁻⁹ to 6.25 × 10⁻⁴ M) and a low detection limit (1.58 × 10⁻⁹ M) (Wu et al. 2014).

RuO₂-activated carbon composites have been applied in capacitive deionisation (CDI) processes and found to be more efficient than normal activated carbon electrode material (Ma et al. 2019). Porous titanium carbide/boron-doped diamond composite electrodes have also been synthesised to obtain ultrahigh energy densities of EDLCs (Li et al. 2010).

This article delves into the utilisation of carbon-based materials in sensor applications, in multiple fields as electrochemical sensors for DNA, proteins, pollutants, metal ions, gases, and immunosensors. We showcase our ongoing advancements aimed at enhancing these areas while also suggesting potential avenues for future exploration. While acknowledging the availability of various electrode materials, our primary focus revolves around investigating CBEMs such as conducting polymers, activated carbon (AC), carbon aerogel (CA), carbon nanofibers (CNFs), graphene, and carbon nanotube (CNTs) and reduced graphene oxide (rGO). The review also focuses on exploring various electrochemical methods as applied in sensor applications.

2. Electrochemistry

Electrochemistry is known to be one of the oldest fields of chemistry which focuses primarily on the transfer of electrons from one point to another point in the system (Chen, Feng, and Li 2012; Murray 2008). The movement of these electrons induces the production of electricity which is used in our homes to power a wide variety of devices and industries. The rapid rate of urbanisation and industrialisation couldn’t have been achieved without electrochemistry and associated electrochemical reactions leading to the production of electricity. Electrochemistry can therefore be defined as the study of the close relationship between electrical signals caused by the flow of electrons and chemical systems that are incorporated into an electrochemical cell (Oldham, Myland, and Bond 2011). It plays a very crucial role in various fields of chemistry including chemical analysis, thermodynamic studies, synthetic chemistry, molecular kinetics measurement, energy conversions and biological electron transport (BET), metallurgical studies, corrosion science semiconductors, self-assembled coatings, fuel cells, and electrochemical sensors (Rackus, Shamsi, and Wheeler 2015). Many studies have also shown that electrochemistry remains this far one of the most important fields of science. Just to mention a few; the polymerisation of conductive polymers involves an electrochemical process mainly oxidation (Ping et al. 2018; Shi, Peng, and Yu 2015), single molecule electrochemical detection in aqueous solutions and ionic liquids (Byers et al. 2015), and direct electrical detection (Hayat and Marty 2014). All these highlight the use of electrochemical studies to solve nearly ‘impossible’ problems in biological systems.

Chemical sensors are widely used devices for tracking and identifying a variety of everyday activities. The sensing device reacts to changes in chemical ions or physical movement to assist with environmental and health inspection. An electrical signal is produced by a sensor’s electrical reaction, and electrical potential, current, and resistance are commonly used to interpret signals (Skoog, Holler and Crouch 2007).

Many electroanalytical techniques have been developed since the emergence of electrochemistry to make chemical analysis easier and worthwhile (Wang, Li, et al. 2019). A few of these techniques include, voltammetry, amperometry, potentiometry, coulometry, chronopotentiometry, and measurement of impedance has been developed for such purposes. To fully appreciate this review, it is important to learn some of the often-used terms in electrochemistry.

1. Electrode: They are solid materials electric conductors that transfer current into non-metallic materials (could be solids, liquids, gases, plasma). In an electrochemical system, reduction and oxidation occur at the electrode. The anode (negative) is the electrode where oxidation occurs, while the cathode is the electrode where reduction occurs (positive). The name of the electrode is based on the function and the relative potential (Tripkovic et al. 2011).

2. Electrolyte: A material, either liquid or solid which dissociates to produce ions in an electrochemical reaction is termed as the electrolyte. The electrolyte is also known as the ionic conductor (Hänsel, Lizundia, and Kundu 2019).

3. Working Electrode (WE): The electrode in the electrochemical system where relevant redox reaction occurs (Ng, La Mantia, and Novák 2009).

4. Counter Electrode (CE): is the secondary electrode that permits the passage of current. The potentiostat regulates the working electrode potential (WEP) and current balancing to reduce variations in this electrode’s potential (Papageorgiou 2004).

5. Reference Electrode (RE): The potential reference is the RE. It enables management of the potential used at the WE (Napporn et al. 2018). Since any changes in the potential at the WE are compared with the RE, the RE is crucial when evaluating the potential at a WE in a three-electrode setup. It’s also crucial to remember that the RE is completely current-free. Figure 1 depicts the standard calomel electrode (SCE) and silver/silver chloride electrode as examples of reference electrodes.

6. Electrochemical Potential (EP): The electrochemical potential is defined as the mechanical work that is needed to transfer one (1) mole of ions from a standard state to a particular electrode potential and concentration (Malloggi 2016). The Nernst equation dictates how the electrode potential and the concentrations of the substances being reduced or oxidised (CO, CR) change (Faridbod, Norouzi, and Ganjali 2015).

Figure 1. Diagram of (a) silver/silver chloride electrode and (b) calomel electrode.

(1) $$E=\mathit{\text{EO}}+\frac{\mathit{\text{RT}}}{\mathit{\text{nF}}}\mathrm{\text{ln}}\hspace{0.17em}\frac{\mathit{\text{Co}}}{\mathit{\text{CR}}}$$(1)

The variables in this equation are the temperature (T), the number of electron transfers (n), the universal gas constant (R), the standard potential for the reaction (EO), and Faraday’s constant (F).

• Amperometry: is a form of electrochemical analysis where a constant external electric potential is applied between the WE and CE, and current is monitored. When electrons move, current starts to flow, and this current is measured as a function of time (t).

• 8. Potentiostat: A control device commonly used for measuring tiny currents and applying electric potentials.

2.1. Electrochemical measurement

Many methods and electroanalytical techniques are available to aid in the complete characterisation of electrochemical and corrosion systems. Our goal in this part is to give a summary of the most often used methods for electrochemical measurements (including corrosion analysis). Many of these methods have been used to study corrosion-prone metals mainly because electrochemistry and corrosion occur in the same way.

2.2. Voltammetry

The most used method in electrochemistry is voltammetry, in part because it can test if the system being investigated is reversible. Similar to amperometry, voltammetry applies an electrode potential (E) between the WE and CE and measures the resultant current (I). In contrast to amperometry, voltammetry varies E as a function of time (Compton and Banks 2018). Since it provides improved performance in electroanalysis, volumetric sensors with a working electrode positioned laterally are an intriguing replacement for conventional electrodes. Their advantages for measurement preparation include the elimination of gas bubbles and interferences, amplification of mass movement, and the potential for downsizing and automation. Porada et al. (2013) devised and manufactured cyclically reusable silver, gold, bismuth, glassy carbon, ceramic, and amalgam electrodes for annular bands, bi-bands, rings, and multidisc. The basic principle of voltammetry is to apply a known voltage to an electrode to assess the current generated by the redox process of the analyte. Square wave voltammetry, stripping voltammetry, and cyclic voltammetry are a few popular methods that can be used in investigations to obtain data from voltammetry (Dogan-Topal, Ozkan, and Uslu 2010; Sarıgül and İnam 2009). Conventional electrodes may find an interesting substitute in voltammetric sensors including a working electrode positioned laterally (Popescu 2018).

2.3. Cyclic voltammetry (CV)

Many techniques have been made available for the determination of the potential of various chemical systems. In the past, potentiometry which was measured under equilibrium conditions was widely used because it gives relatively accurate readings and was very easy to use but it was soon realised that the time-dependence of many reactions offered more insightful information about the kinetics and thermodynamics of many complex systems. Despite the wide array of kinetic and thermodynamic details given by cyclic voltammetry, it remains one of the complex electrochemical techniques used today (Scholz 2010) yet it is experimentally useful for estimating the standard rate constants for electron transfer reactions (Nicholson 1965). This made CV one of the most popular electroanalytical methods of determination (Heinze 1984).

Routine volumetric measurements are performed using a stationary working electrode (SWE) in a solution with no perturbation or disturbance. This electrode is subjected to a linear potential sweep, beginning at an initial potential, E_i. The sweep is reversed and the potential returns linearly to its starting value after attaining a switching potential, E_f. The reducible or oxidisable electroactive species simply diffuse mass transfer in cyclic voltammetry. Without a doubt, the most important CV metric is the sweep potential (Heinze 1984). A scan is performed on the voltage given to an electrochemical cell. The three-electrode system serves as the exclusive basis for this technique’s operation, and the cyclic potential scan’s output is a depiction of the current passing through the electrochemical cell (Figure 2) (Rusling and Suib 1994).

Figure 2. Cyclic voltammetry.

2.4. Chronoamperometry and chronopotentiometry

By observing the current flow in the cell at a single applied voltage, amperometric measurements are performed. The movement of electrons to or from the analyte is the fundamental operating component of amperometry devices, just like it is for voltammetric sensors (Bard, Faulkner, and White 2022). The working electrode, or sensor, is maintained at a constant voltage while the current is being observed during an amperometric measurement. Next, the current is connected to the analyte concentration that is present (Wang 2006). As illustrated in Figure 3, the electrochemical cell is comprised of two electrodes submerged in an appropriate electrolyte. The fundamental instrumentation necessitates controlled potential apparatus. The working electrode material has a significant impact on the performance of amperometric sensors. As a result, creating and maintaining electrodes requires a lot of work (Dickinson, Streeter, and Compton 2008).

Figure 3. Diagram of an amperometric sensor that uses two electrodes.

These are also techniques that have been utilised to a great extent in electrochemistry (Compton et al. 1992). The periodic continuous fluctuation of electrode potential in a CV experiment, for example, is more difficult because it takes into account the two key parameters in an electrochemical study; potential (E), and intensity of current (I). It is advised to modify one of these two values while maintaining the other to streamline and improve the analysis. Chronoamperometry involves holding the WE's potential constant and seeing how the current changes over time: I = f (t). In the second instance, sustaining the current and keeping track of the WE's potential’s spontaneous change is referred to as chronopotentiometry: E = f (t). These methods have been proven to be useful in many scientific domains, such as the study of metal deposits, electrolysis, and fuel cells (Lingane 1964; Napporn et al. 2018).

Due to their low background current, chemical inertness, affordability, and versatility to a broad range of sensing and detection applications, solid electrodes which include carbon, platinum, gold, silver, nickel, copper, and dimensionally stable anions are frequently used as electrode materials (Navratil and Barek 2009).

2.5. Electrochemical impedance spectroscopy (EIS)

It is well-recognized that EIS is the most effective method for corrosion and electrochemical research (Qiao and Ou 2007). The use of EIS has grown significantly in the last several years used first for alternating current (AC) polarography and double-layer capacitance assessment (Ohno et al. 2013). Since then, they have been extensively used to characterise complex interfaces (adsorption behaviour of molecules), electrode processes (anodic and cathodic reactions), batteries, and fuel cells (Randviir and Banks 2013). The EIS's strong potency primarily stems from the fact that they are fundamentally steady-state approaches that can access relaxation phenomena with relaxation periods that vary by a large order of magnitude. Because of the technique’s steady-state characteristics, signal averaging may be used to get a high degree of precision and accuracy within a single experiment. In addition, a variety of interfacial processes may be identified utilising analyzers’ broad frequency range (106–l04 Hz) (Sharifi-Asl et al. 2013).

2.6. Cyclic potentiodynamic polarization (CPDP)

CPDP is a reasonably non-destructive technique for measuring that can provide important information about the concentration limitation of the electrolyte system, the rate of corrosion, corrosion potential, and the metal’s susceptibility to pitting corrosion (Baboian and Haynes 1981). The idea behind the method is that one can predict a metal’s properties in a given environment by pushing the material out of its steady state condition and seeing how it reacts to a vastly increased force applied. The force is then gradually removed at a constant rate, and a potentiostat is used to help ensure a slow but steady force is applied (Samim and Fattah-Alhosseini 2016). Cyclic Potentiodynamic Polarizations (CPDPDs) are used to form films on metal surfaces to improve their corrosion resistance (Chang et al. 2016). Table 1 shows the advantages and disadvantages of the various electrochemical measurement techniques.

Table 1. Comparison of the advantages and disadvantages of various electrochemical measurement techniques.

Technique	Advantages	Disadvantages
CV (cyclic voltammetry)	Gives details on the kinetics, reversibility, and redox potential of processes.	Needs to choose scan rates carefully to get reliable findings.
	Enables the investigation of electroactive species and the variations in their concentration over time.	Restricted to systems having redox reactions that are reversible or nearly reversible
	A flexible method that works with a range of electrochemical systems.	Could entail difficult data interpretation, particularly for reactions requiring multiple steps.
Chronoamperometry	Easy method for calculating current vs time in systems with quick kinetics	Reaction mechanism knowledge is scarcer than for CV.
	Beneficial for researching processes regulated by diffusion.	Unsuitable for investigating intricate electrochemical systems.
	Provide measurable details on the kinetics and rates of reactions.	Exact control over the experimental setup is necessary to produce trustworthy results.
Chronopotentiometry	Determines potential against time with a steady current.	Restricted to systems where the current potential linkages are well specified.
	Helpful for researching electrode processes that require control or where the current is known.	Could not offer comprehensive details on reaction mechanisms.
	Can offer details on the dynamics of charge transport and double-layer capacitance.	Susceptible to polarisation effects and electrode fouling.
EIS (electrochemical impedance spectroscopy)	Offers thorough details on electrochemical systems, covering impedance, capacitance, and resistance.	Need specific tools and knowledge to interpret.
	Non-destructive method that works well for in-situ electrode process research.	It is frequently necessary to conduct lengthy experiments to get useful results.
	Can be applied to a wide range of electrochemical systems, including corrosion studies and battery characterisation.	Interpretation of results can be complex, requiring advanced mathematical modelling.
CPDP (cyclic potentiodynamic polarisation)	Used to investigate the behaviour of corrosion and establish important variables like corrosion potential and rate.	They are limited to systems where corrosion processes occur.
	Utilised for researching corrosion behaviour and figuring out important variables like corrosion potential and rate.	The preparation of the electrode surface and the experimental setup may have an impact on the results.

3. Carbon-based electrode materials (CBEMs) and their applications

Due to the unique properties possessed by carbon-based materials such as strong adsorption, large surface area, improved optical properties, and cheap in terms of production, they have gained much revolution in the field of sensing and other applications (Madima et al. 2020). The unique atomic structure of the carbon atom gives rise to a diversity of structures and unique properties in carbon-based materials. The grouping of carbon materials is determined by their shape, size, and dimensionality: three-dimensional (carbon nanomaterials), two-dimensional (graphene), zero-dimensional (Bulky ball and carbon dots), and one-dimensional (carbon nanotubes and carbon nanofibers) (Visakh and Morlanes 2016). Carbon nanotubes are extremely strong and stable chemically, which means there are a lot of applications for them. The single-layered graphite that makes up graphene exhibits considerable potential for application in environmental cleanup procedures. A three-dimensional carbon nanomaterial was produced by transforming a two-dimensional nanomaterial into a three-dimensional microporous structure (Gopinath et al. 2021). Three-dimensional carbon nanostructures are created using several methods. Generally, the structure of carbon nanostructures and their interactions at the interface with surrounding bulk materials dictate their physical and chemical properties (Madima et al. 2022). Several of these compounds, including carbon nanotubes, reduced graphene oxide, and activated carbon, have been widely used in the assessment of food quality, health, and environmental toxins (Wang and Lin 2008). The most prevalent and oldest type of carbon compound is graphite, which is finding increasing use in nanotechnology and nanoscience. In a range of carbonaceous compounds, the chemical, physical, and electrical characteristics of carbon are determined by its structural conformation and hybridisation state (Maiti et al. 2018). The main component of the newly produced carbon forms is the layer of sp²-connected carbon atoms, where each carbon atom is related to three other carbon atoms in the same plane and a weakly delocalised π e-cloud in the plane perpendicular to it. These materials exhibit remarkable electrical conductivity, improved charge transfer capabilities, and π-plasmon resonances due to their structure (Wang and Lin 2008).

The most widely employed carbonaceous compounds in general sensing applications are graphene and carbon nanotubes (Llobet 2013). These materials are highly suitable for detection applications because they have remarkable optical properties, quick charge transfer reactions due to their high conductivity, enhanced sensitivity because of their high surface-to-volume ratio, easy functionalization with various biomolecules and polymers to ensure selectivity in various scenarios, chemical stability, and biocompatibility. There have been several reports throughout the years of CNTs being utilised in food testing, health or biological systems, and environmental applications (Yang et al. 2010).

These carbon-based materials have diverse and specific characteristics such as dimensions, morphology, mechanical, electrical, and thermal properties as well as structure as illustrated in Table 2.

Table 2. A comparison of the diversity and specific characteristics of carbon nanotubes (CNTs), graphene, carbon nanofibers (CNFs), and other carbon-derived materials.

Property	Graphene	Carbon nanofibers (CNFs)	Carbon nanotubes (CNTs)	Carbon aerogels (CA)	Other carbon derived materials
Structure	A single layer of carbon atoms arranged in a 2D sheet	Continuous or discontinuous fibres composed of carbon atoms	Hollow cylindrical tubes of carbon atoms	Mesoporous carbon materials with an interconnected three-dimensional network structure	Fullerenes, carbon black, carbon fibres, carbon quantum dots
Morphology	Flat, planar sheet	Straight, helical, or tangled structures	Single-walled (SWCNTs), Multi-walled (MWCNTs)	Highly open-porous nanostructure	Spherical, irregular, fibrous, or crystalline structures
Surface area	The high surface area due to 2D structure	The high surface area due to fibrous morphology	High surface area due to tubular structure	Large specific surface area	Varies depending on specific material
Mechanical properties	Extremely high tensile strength	High tensile strength and modulus	Exceptionally strong and stiff	Possesses excellent mechanical properties as high as 58.45 MPa	Varies widely depending on specific material
Thermal conductivity	High thermal conductivity	High thermal conductivity	High thermal conductivity	Have high thermal conductivity	Varies depending on specific material
Electrical conductivity	Excellent electrical conductivity	Excellent electrical conductivity	Excellent electrical conductivity	Have high thermal conductivity	Varies from insulating to highly conductive
Dimensions	Typically, 1–3 atomic layers thick (nanometres)	Diameter: nanometres to micrometres, Length: micrometres to millimetres	Length: micrometres to millimetres	Most of their pores are in the range of mesopores between 2–50 nm	Varies depending on specific material

Several chemical and electrical processes have been used in the creation of sensors. However, issues with their selectivity and sensitivity have made the application of carbon-based electrode materials in sensor manufacturing more prominent. These materials’ distinct mechanical, thermal, chemical, and optical qualities have attracted a lot of attention in the field of sensing. Graphene and carbon nanotubes are widely used in sensing applications because of their high conductivity, huge surface area, and simplicity of functionalization. These properties allow electrons to transfer quickly (Norizan et al. 2020).

Graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and other carbon-based materials have high surface area-to-volume ratios that facilitate effective interaction with analytes which improve their sensitivity in sensing technologies. High-sensitivity chemical signal transduction into detectable electrical impulses is made possible by their extraordinary electrical conductivity (Nanjappa and Jayaprakash 2023). To improve their interaction with target analytes and increase sensitivity even more, carbon-based materials can be functionalised with certain groups or compounds. The development of sensors with increased sensitivity to certain analytes is made possible by the programmable electronic features of carbon materials, such as bandgap modification in graphene (Kumar et al. 2022).

By promoting particular interactions or reactions, functionalising carbon-based materials with particular chemical groups or receptors can add selectivity towards target analytes. To improve sensor selectivity, carbon-based materials’ distinct morphology and structure can be engineered to interact or adsorb particular molecules or ions in a targeted manner. Selectivity is increased overall when carbon-based materials are integrated into sensor arrays or multisensor systems to enable the simultaneous detection of several analytes. When selective recognition elements, like enzymes, antibodies, or molecularly imprinted polymers, are immobilised on carbon-based materials, the selectivity of sensors towards certain analytes is improved (Revanappa et al. 2022).

Carbon-based materials exhibit excellent mechanical robustness and chemical stability, ensuring the long-term performance of sensors under various environmental conditions. The stability of sensors is enhanced by their strong thermal conductivity and resistance to thermal degradation, especially in high-temperature applications. Because carbon-based materials are naturally biocompatible, using them in biological sensing applications lowers the risk of biofouling and deterioration. By halting oxidation or chemical deterioration, the functionalization of carbon materials with protective coatings or surface modifications might further improve their stability (Gururaj and Swamy 2013).

Table 3 lists a few characteristics of CBEMs as feasible materials for use as sensors.

Table 3. Properties of carbon-based materials as sensors.

Carbon-based material	Properties
Activated carbon electrode (ACE)	High surface area, porosity, efficient electrical and thermal conductivity, high stability, low economical cost, and availability
Graphene	High flexibility, fast heterogeneous electron-transfer rate, promising electrical conductivity, and large specific surface area
Carbon aerogels (CA)	Fine pore size, large specific surface area, high electrical and thermal conductivity, corrosion resistance, low expansion coefficient, and extremely low density
Carbon nanotubes (CNT)	Have appreciable electrical and thermal conductivity, large surface area, good chemical stability, and appropriate mechanical strength to resist extreme stress
Templated carbon (TC)	Simple to synthesise and have readily available sources, good thermal and electrical conductivity

3.1. Activated carbon electrode (ACE)

During the early 1960s, more attention was paid to the advancement of solar cells using dye sensitisation (DSSC) because it demonstrated beyond reasonable doubt that they possessed peculiar capabilities to be useful in green energy production. It was soon realised that the power conversion efficiencies were low primarily due to only the monolayer of the dye molecules which were chemically embedded in the semiconductor electrode’s surface were useful in energy conversion (Imoto et al. 2003). Since then, there has been an overwhelming interest in other materials that could be low in terms of cost and yet powerful in power conversion. Activated carbon which can be produced from coal, coke, sawdust, peat, wood char, and palm kernel shells (Lee et al. 2018) has proven to be an outstanding material for ACEs due to its immense advantages such as high porosity, surface area, effective heat and electrical conductivity, stability, affordability, and accessibility (Goldfarb et al. 2017). These unique properties reveal the application of activated carbon as a capacitive deionisation electrode (CDI). Activated carbon has been useful in many applications including aqueous electrochemical double-layer capacitors (EDLC) which are often referred to as supercapacitors (Li et al. 2011), and desalination processes including reverse osmosis, ultrafiltration, nanofiltration using selective resin. Gissawong et al. (2021) used activated carbon from waste coffee modified with gold nanoparticles for the detection of ciprofloxacin. Elmouwahidi et al. (2012) prepared activated carbon from an organ seed shell and used it as a supercapacitor. Activated carbon was prepared and doped with nitrogen and used as a supercapacitor (Xu et al. 2014). Li et al. (2010) prepared a new activated carbon doped with polyaniline which was used as a high-performance supercapacitor. Travlou et al. (2016) fabricated an activated carbon ammonia sensor doped with nitrogen. The material exhibited a great sensing performance against ammonia at a concentration range of 45–500 ppm.

A humidity gas sensor that functions well at room temperature was created using a mixture of chitosan (CS) and activated carbon (AC) as its sensitive material. Spraying the CS/AC composite, which is the humidity-sensitive layer, onto a PET substrate using an interdigitated electrode made of nickel-chromium alloy is how it is done. The humidity sensor shows a voltage variation range of 0–48 V by carefully adjusting the resistance ratio and designing an operational amplifier circuit. With excellent stability, this design provides a wide range of humidity-detecting capabilities, from 0% to 97% relative humidity (RH) (Xu et al. 2022).

Utilising a glassy carbon electrode that has been altered by the addition of monodisperse nickel and palladium alloy nanocomposites (Ni-Pd@AC/GCE NCs) and extremely sensitive AC, a novel glucose sensor was developed. The nanocomposite demonstrated impressive performance parameters, including a limit of detection of 0.014 mM, a broad linear range from 0.01 mM to 1 mM, and a high sensitivity of 90 mA mM⁻¹cm⁻² (Koskun et al. 2018).

To identify organic compounds in the air, a sensor covered in activated carbon has been developed. The sensor is a quartz crystal. The sensor’s performance was tested in a specially made air-flow system to assess its detection capabilities. Vapours of ethanol, acetone, and n-heptane were flowing through this system at a rate of 0.4 litres per minute. The sensor detection towards acetone was about 70% of that of n-heptane, and that of ethanol is four times as high as n-heptane (Kim and Choi 2002).

Heteroatom-enriched activated carbon-nickel oxide (HAC-NiO) is a composite material that can be produced using an efficient two-step synthesis technique. Furthermore, a glassy carbon electrode (NiO-HAC/GCE) was modified using the HAC-NiO nanocomposite to produce a novel glucose sensor. When its electrochemical characteristics were evaluated, they showed a low detection limit of 1 μM for glucose oxidation and a wide linear range from 10 μM to 3.3 mM (Ni et al. 2017).

A novel electrochemical glucose biosensor (GOx-PtNPs-PAA-aSPCEs) has been created using glucose oxidase (GOx) incapacitated on a surface consisting of platinum nanoparticles (PtNPs) electrodeposited on poly (Azure A) (PAA), which was initially electropolymerized on stimulated screen-printed carbon electrodes. The generated electrochemical sensor was tested for its capacity to oxidise glucose in real samples and for electrochemically monitoring H₂O₂. The electrochemical biosensor showed excellent selectivity for glucose determination, a wide linear range (20 μM–2.3 mM), a low detection limit (7.6 μM), and remarkable sensitivity (42.7 μA mM⁻¹cm⁻²) (Jiménez-Fiérrez et al. 2020).

3.2. Graphene electrode (GE)

An allotrope of carbon known as graphene is composed of a single layer of atoms arranged in a two-dimensional hexagonal honeycomb lattice (Huang et al. 2017). Graphene has gained the attention of scientists across the world due to its utility in nanotechnology and energy storage owing to its interesting physicochemical characteristics, promising electrical conductivity, large theoretical specific surface area and it is relatively easy to modify (Ke and Wang et al. 2016; Wang et al. 2016) due to its high flexibility (Wang et al. 2011), fast heterogeneous electron-transfer rate (Ping et al. 2012). They have also been found to be less sensitive to chemical oxidation, diffusion, and electro-migration (Bourrier et al. 2019). Graphene materials are usually manufactured by the electrochemical reduction of graphite oxide (Xiao et al. 2011). However, there are several drawbacks to using graphene as an electrode material. Firstly, the aggregation and restacking of graphene sheets as a result of van der Waal’s (VDW) interactions significantly reduces the electrochemically active surfaces, which consequently lowers the storage capacity (Ji et al. 2017).

Several techniques have been developed to produce graphene. One of these techniques is the mechanical cleaving of graphite. Graphite pieces are continuously removed using this technique to create a single layer (Novoselov et al. 2004). Graphene sheets are treated with a strong acid solution using an alternative approach. This process, called chemical exfoliation, adds a functional group containing oxygen to the graphene sheet to create graphene oxide (Gómez-Navarro et al. 2009). In other synthetic approaches by de Heer et al. (2007), hexagonal silicon carbide is thermally treated at high temperatures mostly above 1200 °C to evaporate silicon and form graphene. CuNPs and PANI have been added to graphene oxide (GO) to enable selective hydrazine sensing at a detection limit of roughly 480 nM (Vellaichamy, Periakaruppan, and Ponnaiah 2017).

3.3. Carbon aerogels (CA)

Carbon aerogels are one of the materials used as electrodes in electrochemical analysis. Like graphene materials, CA possesses excellent electrical and thermal conductivity, a large specific surface area, tiny pore size, corrosion resistance, low expansion coefficient, and extremely low density (Rasines et al. 2015). These characteristics make it more effective as adsorbents, chromatographic materials, and carrier membranes for transition metal catalysts (Palmre et al. 2011) CAs are synthesised using resorcinol-formaldehyde because the physiological parameters of these materials can be managed by carefully varying the temperature, pH, concentration, and reaction time (Lee and Park 2020; Chen, Xu, et al. 2011). Several researchers have reported the effectiveness of these carbon aerogels in several applications including supercapacitors and capacitive deionisation (CDI) processes. Rasines et al. (2015) and Zafra et al. (2013) have reported the textural characteristics of different CAs and their electrochemical responses using solutions of NaCl with varying concentrations. In the same work, they studied the electro-adsorption capacitance and kinetics of the aerogels synthesised. During the activation process, the micropore volume increased, which raised the sodium chloride, nitrate, and phosphate electrosorption capacitance.

Palladium nanoparticles (Pd NPs) embedded in a porous carbon aerogel (Pd/CA) have been synthesised in a very stable form by a straightforward microwave reduction method. In terms of electrochemical sensing of dopamine (DA) (ranging from 0.01 to 100 μM) and melatonin (ML) (ranging from 0.02 to 500 μM), the Pd/CA composite demonstrated outstanding electrocatalytic activity and exceptional selectivity, achieving detection limits of 0.0026 μM and 0.0071 μM, respectively (Rajkumar et al. 2017).

A simple manufacturing process was employed to produce a dual-mode sensor that is very versatile, sensitive, and flexible. These sensitivities were observed across a wide frequency spectrum spanning from 0.1 to 10 Hz. Notably, these sensitivity levels surpass those of existing flexible sensors designed for pressure and magnetism measurements (Huang et al. 2020).

3.4. Carbon nanotubes (CNTs)

A material has to possess significant electrical and thermal conductivity, a wide surface area, high chemical stability, and the right amount of mechanical strength to withstand excessive stress to be deemed suitable for use in the production of carbon-based electrodes. Carbon nanotubes (CNTs) and single-walled carbon nanotubes (SWCNTs) have received a lot of attention when it was discovered in the early 1990s by Iijima, and since then have been used extensively as an alternative material for the fabrication of electrodes in lithium-ion batteries (Liu et al. 2012; Manthiram, Fu, and Su 2013), electrochemical double-layer capacitors (EDLCs) (Xu et al. 2014), nano test tubes, hydrogen power storage system, diodes, transistors, and chromatographic material (stationary phase) (Vairavapandian, Vichchulada, and Lay 2008; Lay, Vairavapandian, and Vichchulada 2008). Wu et al. (2000) described carbon nanotubes as the ‘future material for life’ in their review because of the overwhelming attention that had been drawn to the study and application of CNTs. CNTs are essentially made of graphitic sheets that have been rolled up into tube (Trojanowicz 2006). Various modifications have been made to the tube to extend its application into biological systems such as its use as a glucose biosensor based on electrodeposition of platinum (Pt) nanoparticles onto a carbon nanotube film (Lv et al. 2022; Rakhi, Sethupathi, and Ramaprabhu 2009), and trace metals determination by anodic stripping voltammetry using a bismuth-modified carbon nanotube electrode (Hwang et al. 2008). CNTs have been at the forefront of material chemistry for the development of materials as biosensors. CNTs possess high conductivity making them suitable as small-scale electrode materials, improved semiconductor behaviours, large surface area, and mechanical strength all contribute to the use of CNTs in sensing applications (Yang et al. 2007).

Single-walled carbon nanotubes have been reported as biosensors for flavin adenine dinucleotide (FAD) and glucose oxidase (Guiseppi-Elie, Lei, and Baughman 2002). Pathak and Gupta (2021) reported the functionalization of CNT with palladium nanoparticles as a sensor towards hydrazine. The sensor demonstrated a very sensitive and high selectivity towards hydrazine at a very low detection limit of about 0.2 nM (Pathak and Gupta 2021). To include MWNTs into composite electrodes for glucose oxidase-based glucose detection, Wang, Li, et al. (2019) used Nafion, a sulphonated tetrafluoroethylene-based polymer, which involves oxidising glucose by the oxidase enzyme and measuring the resulting H₂O₂ concentration (Wang, Musameh, and Lin 2003). Further reports of glucose biosensors based on aligned CNTs covered with a conducting polymer have been made by Gao, Dai, and Wallace (2003). The polymer created a bioactive, conducting, coaxial sheath around each individually aligned CNT, enabling low-potential sensing of the H₂O₂ produced by glucose oxidase.

Qu et al. (2006) have developed a versatile and effective technique for incorporating metallic nanoparticles into carbon nanotubes (CNTs). This approach enhanced the CNTs’ electrochemical activity when the CNTs were included in functional electrodes (Qu, Dai, and Osawa 2005, 2006). Claussen et al. (2009) developed a SWNT-based electrochemical biosensor using Au-coated Pd (Au/Pd) nanocubes to increase electrochemical activity, offer selective biofunctionalization docking points, and boost biocompatibility by additionally utilising an integrated technology. The material exhibited estimated detection limits of 2.3 nM and outstanding sensitivity (2.6 mA mM − 1 cm − 2).

In a research work by Govindasami and co-workers (2017), molybdenum disulphide nanosheet (MoS₂) coating with an MWCNT surface was created for the analysis of chloramphenicol (CAP). The MoS₂/MWCNT nanocomposite showed excellent catalytic performance towards CAP with excellent electrochemical properties. The electrode modified with MoS₂/MWCNT reacted linearly within the CAP concentration range of 0.08 to 1392 μM, attaining a low limit of detection (LOD) of 0.01502 μM (Govindasamy et al. 2017).

Yin et al. (2018). prepared a molecularly imprinted polymer (MIP) electrochemical biosensor for sunset yellow by synthesising polydopamine (PDA) through monomeric self-polymerisation in water, which they then utilised to alter the surface of MWCNTs. The sensor that was produced and imprinted demonstrated an extremely sensitive and selective response to the template with a detection limit of 1.4 nM.

In recent years, Lian et al. (2013) have synthesised chitosan-silver nanocomposite/graphene-multiwall carbon nanotube decorated with gold as an electrochemical biosensor for the detection of neomycin. The material exhibited a higher sensitivity with a detection limit of 7.63 × 10⁻⁹mol/L (Lian et al. 2013). In order to test CNT-based sensors for various pollutants like NO₂, CO, and C₆H₆, (Zanolli et al. 2011) and modified the sensors using Au NPs and oxygen plasma. When contrasted with pristine CNTs modified with oxygen plasma, they demonstrated that Au modification improved detecting capacities for the detection of particular gases (NO₂ and CO), but in the instance of C₆H₆, Au NPs do not appear to be as important to the detecting action. Abdelhalim et al. (2014) investigated the reaction of a CNT-based Au-functionalised gas sensor to a range of ammonia levels (10–100 ppm). They discovered that the sensor obtained a stabilised sensitivity of 92% after being exposed to 100 ppm of the target gas (Abdelhalim et al. 2014). Huang et al. (2012) investigated an ammonia gas sensor based on reduced graphene oxide (RGO)-polyaniline (PANI) hybrids, which demonstrated a response up to 50 ppm ammonia, the unmodified RGO-based sensor changed in normalised resistance by roughly 5.2%.

3.5. Carbon nanofibres

One of the most significant components of carbon fibres, carbon nanofibre (CNF), has been studied for both practical uses and basic scientific purposes. In numerous domains, including electrical devices, electrode materials for batteries and supercapacitors, and sensors, CNF composites show great promise. Electrical conductivity is always the most important factor to take into account in these applications. The dispersion and percolation status of CNFs in matrix materials strongly influences the electrical properties of CNF composites (Feng, Xie, and Zhong 2014).

Another material that has been used as a carbon electrode material is the carbon nanofibre. These fibres are produced from a polymer slurry using a method called electrospinning; a technique which involves the application of an electrostatic force of repulsion and external electric field between two electrodes in such a way that a high voltage is supplied (Kim et al. 2004). They do not require any binders, are cheap, and easy to produce (Ferancová et al. 2010).

In recent advancements in electrochemistry, activated carbon fibres (ACFs) have been doped with iron-zirconium composite and applied in the treatment of water polluted with phosphates through an adsorption mechanism as illustrated in Figure 4 (Xiong et al. 2017).

Figure 4. Application of CNF for water treatment by adsorption.

CNFs can be prepared either by electrospinning, and after that heating, or through catalytic thermal chemical vapour deposition growth method as demonstrated by Ge and Sattler in 1994. In this method, a carbon source is usually required, mostly polyacrylonitrile. The source of carbon is usually dissolved in a suitable solvent to obtain a polymer solution. The polymer solution is then injected into an electrospinning machine using a syringe. The material is then allowed to undergo air anneal at a temperature range between 300 to 400 °C followed by calcination at a temperature of 700 °C (Yu et al. 2009; Kim et al. 2007; Xu et al. 2006).

To reflect significant mechanical deformation, Zhu et al. (2011, 2015) constructed CNF/elastomer (VM2) composites as strain sensors. Using a one-step vapour deposition polymerisation process Li and co-workers created a vapour sensor (CNF/poly(acrylate)) to detect irritating gases like NH₃ and HCl. Ji et al. (2009) also reported the synthesis of a porous electrospun PAN/SiO₂ composite as an electrode material for lithium-ion batteries.

3.6. Templated carbon (TC)

Because they are simple to synthesise and have readily available sources, carbon sphere templates have been extensively studied and used in a variety of applications, such as fuel cells, photocatalysis, batteries, and capacitors. Wang et al. (2001) effectively synthesised a unique monodispersed hard carbon sphere with equally distributed nanopores with a perfectly round shape and good surface condition. The carbon spheres were easily made by dehydrating glucose at low temperatures and carbonising it at high temperatures. This hard carbon material has a specific BET surface area of 400 m²/g for N₂ as adsorbate, and can reversibly store lithium up to 430 mAh/g.

A simpler hydrothermal process for creating micro- or nanospheres was proposed by Sun and Li (2004). Utilising an aqueous glucose solution as a precursor, carbon nano- and microspheres were directly created using the hydrothermal synthesis method at 160–180 °C. The as-prepared hard carbon spheres were adjustable in diameter, crystallinity, and chemical composition. They also had a reactive surface and received functional groups from the parent, and the synthesis procedure was simple (Sun and Li 2004). Titirici, Antonietti, and Thomas (2006) described a one-pot synthesis using a hydrothermal method to produce hollow spherical metal oxides, including Fe₂O₃, Ni₂O₃, Co₃O₄, CeO₂, MgO and CuO. Metal oxide precursors were added directly to glucose or sucrose as precursors. It was discovered that carbon spheres were also created during the synthesis technique and are used as templates. This method is simpler, but it does not affect metal oxide hollow spheres in any way.

With advancements in electroanalytical studies, several materials have been successfully employed in the fabrication of TC electrodes. Examples of these are the use of potassium chloride templated carbon for the preparation of EDLC (Cao et al. 2018), zeolite-templated carbon as an ordered

microporous electrode for aluminium batteries (Stadie et al. 2017) and organometallic block copolymer as catalyst precursor for templated carbon growth (Hinderling et al. 2004). Zeolite templated carbon chemistry has been studied mostly in high-pressure hydrogen storage (Maeda et al. 2009). Templating-structured ordered mesoporous carbon (OMC) was widely used as a substrate for metal oxides such as RuO₂ (Zhang et al. 2014) and MnO₂ (Dong et al. 2006), resulting in composite materials that were utilised in supercapacitors. These OMC-based composite materials increase the number of active sites that are accessible to pseudo-active species while also improving the metal oxide’s electrical conductivity, such as that of MnO₂. This advancement facilitates achieving a higher utilisation level of pseudo-capacitance.

Other carbon-based materials applied as sensors with their respective analytes are summarised in Table 4 and the meaning of some major abbreviations is highlighted in Table 5

Table 4. Application of carbon-based materials as sensors.

Carbon-based material	Detected material	Detection limit (nM)	Reference
AUNPs/AC/GCE	Ciprofloxacin	0.20	Gissawong et al. (2021)
MIP/SA-AC/GCE	Morphine	48	Salajegheh et al. (2019)
Ni-Pd@AC/GCE NCs	Glucose	0.014	Koskun et al. (2018)
Gox-PtNPs-PAA-aSPCEs	Glucose	7.6	Jiménez-Fiérrez et al. (2020)
CVE	Salivary Uric Acid	0.05	(Bukharinova et al. (2021)
TiO2/ACE	Hydroquinone	3.3	Hayat et al. (2016)
Aptamer/graphene gold electrode	S. aureus	0.0041	Lian et al. (2015)
Graphene FET	NH3 and CO2	0.3 and 0.4	Inaba et al. (2013)
Graphene/SnO2 NP	H2	0.1	Zhang et al. (2014)
Graphene/1-Octadecanethiol	Hg^+	0.1	Chen, Xu, et al. (2011)
GO/AnNPs	Pathogen	0.5	Jung et al. (2010)
PPy/MWCNT	NH3	0.3	Bachhav and Patil (2015)
PANI(SWCNT)	DCM	481	Chang and Yuan (2009)
PPy (Co3O4@Au/MWCNT)	DMMP	10	Ramesh et al. (2017)
(P(L-P)MCPS)	Indigo carmine (IC)	0.0187	Prinith and Manjunatha (2023)
(CeO2/ZnO/GCE)	norepinephrine	0.1	Sarbandian and Beitollahi (2023)
(PNMCPE)	catechol(CC)and hydroquinone (HQ)	1.917 and 1.902 respectively	Arpitha, Swamy, and Banu (2023)

Table 5. Meaning to some major abbreviations.

Abbreviations	Meaning
AUNPs/AC/GCE	Gold nanoparticles/Activated carbon/glassy carbon electrode
MIP/SA-AC/GCE	Molecularly Imprinted Poly L-Lysine/Sodium Alginate-activated Carbon/Glassy
Ni-Pd@AC/GCE NCs	palladium-nickel (Pd-Ni) nanoparticles supported on reduced graphene oxide (rGO) modified glassy carbon electrode
Gox-PtNPs-PAA-aSPCEs	platinum nanoparticles (PtNPs) electrodeposited on poly(Azure A) (PAA) previously electropolymerized on activated screen-printed carbon electrodes
PPy/MWCNT	Polypyrrole(PPy)–multiwalled carbonnanotubes
PANI(SWCNT)	Polyaniline-singlewalled carbonnanotube
(P(L-P)MCPS)	Poly (L-phenylalanine) modified carbon paste sensor
CeO2/ZnO/GCE	CeO2/ZnO nanocomposite-modified glassy carbon electrode
(PNMCPE)	poly(nigrosine) modified carbon paste electrode
DMMP	Dimethyl methylphosphonate

4. Limitations and directions for further research and innovation in using carbon-based electrode materials for sensors

The limitation of activated carbon to be used as an electrode material lies in its restricted energy capacity, resulting from a lower F/g (specific capacitance) in comparison to costlier carbon alternatives. This disparity arises due to the constrained ability to regulate pore structure and size during the production of activated carbon, a feature that distinguishes it from other engineered carbon forms (Chen, Xu, et al. 2011).

One significant limitation of templated carbon is its high production cost and the extensive preparation required during its manufacturing process. Templates also require double the weight in carbon, making it essential to use inexpensive template materials available in large quantities, often in the order of tons, to maintain lower production costs (Davies and Yu 2011).

Despite possessing exceptional properties, the restricted surface area of carbon nanotubes (CNTs) has limited their application in high-energy performance EDLCs (electric double-layer capacitors) (Norizan et al. 2020). Moreover, the current challenges in purification processes and the high production costs continue to impede their practical utilisation (Zhang, Lei, et al. 2010).

Graphene has several benefits as a sensor material, but it also has certain drawbacks. These include its sensitivity to temperature and humidity in the environment, its scalability in mass production, its challenges in achieving high selectivity and specificity, the complexity of surface functionalization that may change its intrinsic properties, the difficulties in maintaining a high signal-to-noise ratio, the challenges of assuring durability and long-term stability, and the effective integration of the technology with other technologies (Nag, Mitra, and Mukhopadhyay 2018; Torrinha et al. 2022).

When used as sensors, carbon nanofibers (CNFs) face several restrictions. Due to possible cross-reactivity with a variety of chemicals that can impair detection accuracy, difficulties continue to arise in establishing both high sensitivity and selectivity for specified analytes or gases (Wu et al. 2014). Although surface functionalization can improve sensing abilities, it can also change the characteristics of CNFs, which may reduce their efficacy. Due to differences in synthesis techniques, ensuring consistent and repeatable sensor performance across many CNF samples continues to be challenging. CNF-based sensors may face difficulties in achieving fast reaction times, which are essential for real-time sensing. The broad deployment of CNF sensors is hampered by the complexity of integrating them into useful products while maintaining scalability for mass production. Furthermore, persistent difficulties that affect sensor reliability over time include preserving long-term stability and durability against environmental influences or physical stresses (Shanta et al. 2017).

It is crucial to diversify and broaden the composition of carbon-based electrode materials (CBEMs) to increase their potential uses. This entails modifying surface functions to control the interfacial properties, which is essential for the creation of functional CBEMs. Furthermore, it’s critical to comprehend how the hierarchical porosity and architectures of CBEMs, including wall thickness, pore size and shape, and pore volume, relate to how well they function in diverse applications. Investigating interfacial issues needs more research (Borenstein et al. 2017). Furthermore, carbon-based multicomponent and multi-layered systems should be investigated since they can provide unanticipated effects in addition to expected features that could be useful in advanced applications. Overall, there is a lot of promise for addressing new issues in healthcare, environmental monitoring, industrial process control, and other fields if more study and creativity are put into using carbon-based electrode materials for sensors (Kumar et al. 2018). Researchers can create next-generation sensing devices with improved performance, functionality, and applicability by expanding our fundamental understanding of carbon-based materials and devising new manufacturing and sensing procedures.

5. Conclusion

In conclusion, the review emphasises the enormous potential and bright future of carbon-based electrode materials for sensor applications. Graphene, carbon nanotubes, carbon nanofibers, and mesoporous carbons are just a few of the many carbon allotropes that have remarkable qualities that make them ideal for use as electrodes in sensing applications. The distinctive characteristics of carbon nanomaterials have significantly advanced the field of electrochemical sensors and biosensors. Both novel and modified carbon-based probes often exhibit improved analytical performance compared to traditional non-nanostructured electrochemical systems. Utilising electroanalytical methods with sensing and biosensing devices incorporating carbon nanostructure-modified electrodes shows promise for real-world analytical detection applications. Specifically, carbon nanotubes (CNTs) graphene, and activated carbon materials have been utilised as electrode materials for electrochemical sensing across a wide range of analytes. The unique properties of CNTs, graphene, carbon aerogels, and carbon nanofibres have played a crucial role in the development of innovative electrochemical sensors and biosensors, leading to enhanced analytical performance compared to conventional electrochemical sensing systems. Despite some existing challenges, such as the reproducibility and scalability of current sensing devices, the properties of the carbon-based materials utilised (e.g. electrical, thermal, diameter, surface area, and chirality of carbon) significantly impact the performance of sensing systems. Moreover, more accurate estimations of certain performance characteristics and their practical application for sensing analytes in real-world samples are necessary before potential commercialisation.

In a variety of fields, their distinct structural, electrical, and surface properties present probabilities to improve sensor performance, sensitivity, and selectivity. Carbon-based materials have revolutionised modern electrochemical systems by offering superior analytical performance and unique properties, such as the electrocatalytic abilities of modified electrodes. Notably, carbon nanotubes (CNTs) and activated carbon increase the active surface area, while graphene and carbon nanofibre surfaces provide some anti-fouling capabilities.

Even with all of the amazing developments, several issues remain that require further focus, including scalability, reproducibility, selectivity, and integration with useful devices. By addressing these constraints with creative synthesis methods, surface modifications, and thorough property understanding, we can surely pave the way for the next generation of carbon-based sensors that are incredibly dependable and efficient, leading to improvements in industrial applications, environmental monitoring, and healthcare. Ongoing developments in synthesis and functionalization techniques are expected to lead to a surge in electroanalytical applications across various fields, including medical analyses, drug quality monitoring, and ensuring food and environmental security.

Acknowledgements

The authors wish to thank the Chemistry Department, Kwame Nkrumah University of Science and Technology, Kumasi-Ghana for logistics support.

Disclosure statement

The authors affirm that they have no competing interests in the publishing of this paper and have given their approval for it to appear in this journal.

Data availability

The data in this review can be obtained upon request from the corresponding author.