Electrochemical sensors and biosensors have gained widespread attention due to their various applications in pharmaceutical analysis, drug detection, cancer diagnosis, and monitoring contaminants in drinking water [1]. These sensors provide multiple benefits compared to traditional analytical methods, such as mobility, affordability, simplicity, selectivity, sensitivity, and, specifically for whole cell biosensors, the capacity to evaluate the toxicity of polluted water [2]. Fig. 1 schematically illustrates the three-electrode setup of an electrochemical sensor, consisting of a working electrode (WE), a counter electrode (CE), and a reference electrode (RE). An electrocatalytic process takes place in the working electrode and can be enhanced by the incorporation of diverse nanomaterials. Panel A and panel B details a specific application, likely stripping voltammetry, where toxic chemical ions or bio enzymes are pre-concentrated onto the working electrode surface via a magnetic stirrer before being “stripped” off to generate a quantifiable electrical signal for highly sensitive trace analysis.

Further, gold based sensors [4] are often used in electrochemical biosensors because they easily bond with biomolecules that contain sulfur for sulfur containing proteins, enzymes, thiolated DNA and biomolecules. However, the usual ways of making gold electrodes are expensive and need specialized equipment.

However, carbon-based sensors are extensively utilized due to their low cost, excellent electron mobility, chemical stability, and biocompatibility. Carbon fibres, glassy carbon electrodes, and pyrolytic graphite are examples of conventional carbon sensors. As a result of their greater surface area, enhanced adsorption capabilities, enhanced electrocatalytic activity, high biocompatibility, and quicker electron transfer than conventional materials, carbon nanomaterials, ranging in size from 1 to 100 nm, have recently found application in sensors [5,6]. Rolling up sheets of graphene, carbon nanotubes (CNTs) can be single-walled (SWCNT), double-walled (DWCNT), or multi-walled (MWCNT). The electrical characteristics and thickness of these varieties vary. To provide defect sites and oxygen functional groups that improve adsorption and electron transport, CNTs are frequently acid treated to remove end caps. They can be applied by dip method on electrode surfaces.

On the other hand, graphene is the basic building block of graphitic materials and is commonly utilized in electrochemical investigations, mostly in the form of inexpensive and readily available reduced graphene oxide. Enhancing the electron transport between graphene and biological molecules can be achieved by oxidizing the sp3 hybridized carbons and defects. Nitrogen doping of graphene or carbon nanotubes (CNTs) generates flaws and improves biocompatibility. Sensors use graphene in a variety of shapes, including graphene flowers, carbon nano-horns, foams, and nanoribbon [7].

Biological sensors can be integrated with carbon nanomaterials by several methods such as direct growth on a substrate, drop casting, polymer embedding, co-depositing with metal nanoparticles, or utilization in field-effect transistor (FET) devices to enhance conductivity. For batch production, direct growth of nanomaterials on electrodes yields a more consistent coating. Coatings made of polymers can help disperse carbon nanomaterials but may restrict diffusion, slowing temporal resolution and decreasing conductivity [8].

Several carbon-based structures, discovered decades ago, are now used in technology including energy storage and conversion, nanoelectronics, chemical and biosensing, environmental monitoring, catalysis, and biomedical applications due to their physical, chemical and electronic properties. Nanodiamonds (NDs) from the 1960s are 5-100 nm carbon structures with unique properties due to dopants and surface defects. Fullerenes (FLNs), discovered in 1985, are 3D closed-cage carbon molecules. Carbon nanotubes (CNTs), tubular structures part of the FLN family, gained attention after Iijima’s 1991 study. Graphene (GPN), a single layer of carbon atoms in a honeycomb lattice, was the first discovered two-dimensional atomic crystal [9]. Fig. 2 shows various carbon-based structures used in sensor applications.

The above materials of carbon are crucial in the roles of modifiers in electrochemical sensors and as working electrode materials [10]. Screen-printed carbon electrodes and glassy-carbon electrodes are the most frequently utilized carbon-based electrodes for the detection of environmental contaminants. These variety of porous carbon materials, including carbon materials, quantum dots, graphene sheets, fullerenes, reduced graphene oxide (rGO), graphene oxide (GO), carbon nanotubes (CNTs), graphitic carbon nitrides (both multiwalled and single), three-dimensional porous carbon, and diamonds, have been effectively utilized as modifiers in sensor fabrication [11–13]. Hybrid materials frequently function as substrates for supplementary modifier materials, including metal oxides, metals, and conductive polymers [14]. This review aims to examine the significant function of carbon materials as both modifiers and components of working electrodes for electrochemical sensors.

Carbon nanotubes (CNTs) are remarkable materials known for their very high surface to volume ratio and unique hollow cylindrical structure [15]. These features give them outstanding physical and chemical characteristics, making them excellent candidates for a wide range of sensing and electronic applications. CNTs can absorb many molecules on their surface through electronic interactions, enabling efficient chemical and heavy metal detection. Their electrical behavior is equally impressive, showing high carrier mobility, nearly perfect quantum efficiency, and an extremely thin body structure. Such properties allow CNTs to function effectively at the sub-8 nanometer scale, which approaches the practical limits of conventional semiconductor technology.

With the successful fabrication of CNT field effect transistors in 1998, researchers began viewing them as potential alternatives to traditional metal oxide semiconductor transistors [16]. Beyond electronics, CNTs are now widely studied in environmental and biomedical fields, especially for detecting gases and toxic metals in water. Trace amounts of metals like lead, cadmium, mercury, zinc, and copper are necessary for many biological and metabolic processes, but at higher concentrations these elements can cause severe toxicity to humans and the environment.

Building upon these capabilities, researchers have developed a wide variety of CNT-based sensors for gas detection. Chen et al. produced a lithium-doped CNT film whose electrical conductivity changed upon exposure to methane gas. The lithium ions enhanced the binding strength between methane and the CNT surface and induced dipole interactions, causing a transition from sp² to sp³ bonding and altering resistance. This design achieved a sensitivity of 14.5 percent at 500 ppm of methane, demonstrating strong potential for gas detection [17]. Similarly, Algadri et al. created a hydrogen gas sensor using multiwalled CNTs (MWCNTs) deposited between palladium electrodes through a dielectrophoretic process. Their device exhibited a maximum sensitivity of 239 percent when tested with hydrogen in a nitrogen mixture, showing that CNTs can promote rapid charge transfer and efficient gas adsorption [18]. Song et al. also proposed a novel ionized-gas temperature sensor with a triple-electrode system capable of identifying both gas type and temperature [19], while Menacer et al. used simulations to show that CNT-based acetone sensors experience a decrease in resonance frequency as more acetone molecules interact, with shorter and more chiral CNTs displaying greater sensitivity [20].

In addition to gas sensing, CNTs have proven highly effective in biomedical and pharmaceutical applications. Farias et al. designed a functionalized MWCNT-based sensor for detecting the anticancer drug flutamide, which is monitored to prevent overdose-related liver toxicity. The MWCNTs were functionalized with hydroxyl, carboxyl, and carbonyl groups and deposited on a glassy carbon electrode. The resulting device achieved a detection limit of 0.03 micromole per liter and a broad linear range, indicating strong catalytic activity toward flutamide oxidation and reduction [21]. Likewise, Heidari and Masrournia et al. developed modified carbon paste electrodes for chromium detection in water using MWCNTs functionalized with acyl chloride and organic ligands. Their electrodes achieved a low detection limit of 5 × 10^-8 M and maintained high accuracy across a wide pH range, highlighting the benefit of CNT modification for metal-ion detection in complex environments [22].

Research has also advanced significantly in the area of heavy metal-ion detection. Wanekaya et al. employed cysteine-modified MWCNTs to detect trace levels of lead and copper, achieving detection limits of 1 ppb and 15 ppb respectively. The cysteine functional groups provided selective binding sites for metal ions without interference from other common ions [23]. Kim et al. developed a selective mercury-ion sensor using single-walled CNTs functionalized with poly-azomethine, where conformational changes in the polymer upon metal binding improved selectivity [24]. In a related approach, Forzani et al. incorporated peptide-functionalized polymers onto single-walled CNT field-effect transistors. Peptide sequences such as His6 and Gly-Gly-His displayed selective affinity toward nickel and copper ions, enabling precise metal-ion recognition [25].

Researchers have also explored CNT composites for enhanced sensing and removal capabilities. Alijani et al. fabricated single-walled CNT nanocomposites with magnetite cobalt sulfide that achieved mercury-removal rates above 99 percent within seven minutes, compared with about 45 percent for CNTs alone [26]. Similarly, Bagheri et al. combined triphenylphosphine-modified CNTs with a room-temperature ionic liquid binder on a carbon-paste electrode to enable the simultaneous detection of cadmium, lead, and mercury ions using square-wave voltammetry. Their optimized system reached detection limits in the range of 10^-5 M for all metals [27]. Wei et al. treated MWCNTs with ammonia plasma to introduce amino groups that served as cation traps, enabling ultra-trace detection of zinc, cadmium, copper, and mercury ions with detection limits below one nanomolar [28]. Wu et al. further expanded the detection range by combining fluorinated MWCNTs with magnetite nanoparticles to form a composite capable of detecting multiple heavy metals with high sensitivity, achieving detection limits as low as 0.02 nanomolar [29].

In contrast, the next study focused on electrochemical sensing, Hwang et al. (2008) developed a bismuth modified CNT electrode (Bi-CNT) using a screen-printing method that emphasized electrochemical enhancement rather than composite formation. The Bi-CNT electrode showed superior performance compared with traditional glassy-carbon, activated-carbon, and graphite electrodes because of its higher hydrogen evolution potential. Using square-wave anodic stripping voltammetry, it simultaneously detected lead, cadmium, and zinc ions with sharp and reproducible stripping peaks. The peak current response increased linearly with metal-ion concentrations from 2 to 100 micrograms per liter, and the limits of detection were 1.3 micrograms per liter for lead, 0.7 for cadmium, and 12 for zinc, confirming high analytical precision and repeatability [30].

Graphene and its derivatives (GPNs) have gained significant attention as materials for sensor development due to their exceptional electrical and thermal conductivity, large surface area, rapid electron transfer ability, and strong mechanical strength. These properties make them highly attractive for detecting gases, heavy metals, and biomolecules across environmental and biomedical fields [31,32]. Studies have reported excellent sensitivity, low fabrication cost, simple operation, short response and recovery time, and good selectivity when using graphene-based sensing platforms [33–35]. However, certain factors still require optimization, including determining the ideal number of graphene layers, establishing reliable response-measurement methods, and improving scalable production techniques. Among their many applications, one of the most common uses of graphene-based sensors is in gas sensing and heavy-metal detection [36]. Initial progress in nitrate sensing was reported by Alahi et al., who designed a low-cost nitrate sensor with high porosity and corrosion resistance, resulting in an increased active sensing surface. The sensor achieved less than five percent error, required minimal sample preparation, and provided real-time readings validated through UV spectroscopy. Although promising, further work was recommended to improve reproducibility and long-term stability [37]. In a similar direction, Chen et al. developed a nitrate sensor using reduced graphene oxide (rGO) combined with benzyl triethylammonium chloride. The system demonstrated a low detection limit of 1.1 microgram per liter, a rapid response time of five to seven seconds, and selectivity against chloride ions. The main limitation was difficulty in achieving uniform immobilization of rGO layers, which can lead to variability between devices [17].

Further, advances in heavy metal sensing followed quickly. Yoshii et al. created graphene and chelatormodified graphene sensors capable of detecting cadmium compounds at concentrations between 50 and 1000 micromolar in under five minutes. Their platform was able to differentiate between different cadmium species rather than simply detecting free cadmium ions, and by integrating a sensor array on the same substrate, the researchers successfully identified distinct compound types [38]. Building on this concept, Zhu et al. prepared a gold nanoparticle graphene-cysteine composite (Au-GN-Cys). When coated onto a bismuth film glassy-carbon electrode, the composite improved electron transfer efficiency and enhanced selectivity toward target metal ions such as Cd²⁺ and Pb²⁺ due to the binding capability of cysteine [39]. Moreover, improvement in graphene-based detection was demonstrated by Lee et al., who combined ferrous oxide nanoparticles with graphene to fabricate an electrochemical sensor for detecting zinc, cadmium, and lead ions. The synergistic effect between graphene and Fe₂O₃ nanoparticles led to strong catalytic activity and high sensitivity, with detection limits of 0.11, 0.08, and 0.07 microgram per liter for Zn²⁺, Cd²⁺, and Pb²⁺ respectively [40]. Expanding this structural engineering approach, Zhu et al. employed ion-beam sputtering depositions to create three-dimensional porous gold electrodes, which significantly improved adsorption and electron-transfer properties. Their sensor showed broad linear detection ranges for copper (0.009 to 4 micromolar) and lead (0.006 to 2.5 micromolar), along with strong reusability, stability, and resistance to interference [41].

A simplified one-step hydrothermal method was used by Vajedi et al. to synthesize TiO₂/rGO nanocomposites capable of simultaneously reducing graphene oxide and depositing TiO₂. The resulting material showed strong adsorption affinity for metal ions and excellent electrochemical performance during square wave anodic stripping voltammetry, confirming both high sensitivity and removal capability [42]. In a detailed mechanistic investigation, Theeazen Al-Gahouari et al. examined how the degree of electrochemical reduction of reduced graphene oxide (rGO) films influences heavy-metal detection. They fabricated rGO films with different reduction levels via controlled voltammetry cycles and characterized structural changes using UV-visible spectroscopy, infrared spectroscopy, X-ray diffraction, atomic-force microscopy, and scanning- electron microscopy [43]. The rGO film optimized through cyclic voltammetry delivered the best analytical response, achieving detection limits of 1.2 microgram per liter for Cd²⁺ and 0.2 microgram per liter for Pb²⁺. The coating mechanism and structural evolution are illustrated in Fig. 3, showing how progressive reduction improves electron transfer and sensing performance.

The development of advanced graphene composites has greatly expanded the performance and range of electrochemical sensors. Lu et al. demonstrated this potential by synthesizing graphene quantum dots with uniform size and stable surface properties and incorporating them into vertically ordered mesoporous silica nanochannel membranes connected to indium tin oxide electrodes. By depositing the quantum dots inside the nanochannels through electrochemical deposition, the authors achieved enhanced enrichment of positively charged analytes and improved electron transport efficiency. This approach illustrated how tuning graphene at the quantum scale can significantly strengthen charge-transfer dynamics and sensing precision [44].

Building upon the idea of hybrid nanostructures, Gong et al. designed a gold nanoparticle decorated graphene platform for ultra-trace mercury detection. In this work, uniformly dispersed gold nanoparticles were anchored across two-dimensional graphene nanosheets, forming a conductive and catalytically active composite. The combination of graphene’s high conductivity with the catalytic properties of gold nanoparticles resulted in a strong electrochemical response and efficient electron transfer. The sensor achieved outstanding sensitivity and selectivity, with a detection limit of only six parts per trillion, an order of magnitude below the World Health Organization’s safety threshold for mercury in drinking water. This study established gold-decorated graphene as a highly effective framework for trace-level metal ion detection [45].

Cui et al. explored the use of bismuth nanoparticles supported on graphene sheets within a nanoporous carbon matrix produced through pyrolytic deposition. The resulting BiNPs@NPCGS composite featured large surface area, interconnected porosity, and high electrical conductivity, all of which enhanced activesite availability and signal response. When evaluated under optimized square wave anodic stripping voltammetry conditions, the sensor achieved simultaneous detection of lead and cadmium with detection limits of 3.2 and 4.1 nanomolar, respectively. This work demonstrated that incorporating bismuth into graphene scaffolds can yield stable and highly sensitive electrodes suitable for practical trace-metal monitoring [46].

As research shifted toward improving structural complexity and environmental robustness, Hao et al. developed a nanocomposite electrode consisting of cobalt oxide nanowires integrated with nitrogen-doped graphene aerogel on a glassy carbon substrate. The hierarchical structure and strong interaction between cobalt oxide and the graphene aerogel facilitated efficient electron transfer and rapid ion adsorption. Tested in pond water samples, the sensor delivered detection limits of 2.90 and 2.14 microgram per liter for lead and cadmium, respectively, while maintaining wide linear dynamic ranges. This study highlighted how three-dimensional graphene frameworks combined with transition metal oxides can produce durable and highly responsive sensing platforms suitable for real environmental conditions [47].

Continuing the movement toward field-deployable systems, Zhao et al. introduced a compact and lowcost electrochemical device for multiplexed detection of lead and cadmium in water using square wave anodic stripping voltammetry. Their system integrated nanocomposite-modified screen-printed electrodes into a three-dimensional printed flow cell coated with a bismuth oxide reduced graphene oxide Nafion composite. This combination enhanced both selectivity and sensitivity while enabling portability and ease of use. The system achieved detection limits of 0.8 microgram per liter for cadmium and 1.2 microgram per liter for lead, with excellent reproducibility (less than 5.5 percent variation) and long-term stability after one week of air exposure. Recovery tests in simulated river water confirmed 95 to 101 percent recovery, demonstrating the practical reliability of this technology for on-site environmental monitoring [48].

Fig. 4 illustrates the square-wave anodic stripping voltammetry (SWASV) response of the modified electrode toward Cd²⁺ and Pb²⁺ ions in aqueous solution. In panel (A), two well-defined oxidation peaks appear at approximately –1.0 V and –0.8 V (vs. Ag/AgCl), corresponding to the stripping of Cd and Pb, respectively. As the concentrations of both metal ions increase from 0 to 50 μg L^–1, the peak currents rise proportionally, indicating efficient preconcentration and oxidation processes at the electrode surface. This linear dependence reflects the strong affinity of the nanocomposite-modified electrode for these metal ions and its excellent electron-transfer capability. Panels (B) and (C) present the corresponding calibration curves for Pb²⁺ and Cd²⁺, respectively. The linear regressions exhibit coefficients of determination (R²) of 0.97 for Pb²⁺ and 0.98 for Cd²⁺, demonstrating outstanding linearity over the tested concentration range. These high R² values confirm the reproducibility and quantitative reliability of the developed electrochemical sensor.

Extending this progress, Yasri et al. proposed an innovative electrochemical sensor for mercury detection based on a glassy carbon electrode modified with a nanocomposite film composed of partially oxidized graphene and the conductive polymer PEDOT: PSS. The partially oxidized graphene was synthesized through electrochemical exfoliation of graphite in a perchloric acid and sodium chloride electrolyte using a two-electrode system. Differential pulse stripping voltammetry was then employed for the quantification of mercury ions. The sensor displayed excellent repeatability, with a relative standard deviation of approximately 0.93 percent over ten replicates and achieved a detection limit of 0.19 micromole. It also maintained high selectivity in the presence of interfering ions such as copper and iron, confirming its robustness and reliability for real environmental monitoring applications [49].

Taken together, these studies clearly demonstrate the versatility and promise of graphene-based sensors for the detection of toxic metal ions. Through careful manipulation of graphene’s structure, ranging from quantum dots and gold nanoparticle hybrids to polymer composites and three-dimensional frameworks. Researchers have achieved remarkable improvements in sensitivity, selectivity, and operational stability. The consistent advancement from laboratory prototypes to portable, field-ready devices underscores graphene’s growing role as a core material in electrochemical sensing platforms for environmental and industrial safety.

Yasri et al. focused the differential pulse stripping voltammetry (DPSV) responses recorded for successive additions of Hg²⁺ in 0.05 M HNO₃ utilizing a po-Gr/PEDOT: PSS-modified glassy carbon electrode (GCE). With the increase in Hg²⁺ concentration from 0 μM to 14 μM, the oxidation peak around 0.25 V becomes more pronounced, demonstrating a clear relationship between the electrochemical signal and the concentration of mercury ions [49]. The rise in peak current indicates effective preconcentration of Hg²⁺ ions on the nanocomposite surface, succeeded by swift oxidation during the stripping step, as illustrated in Fig. 5.

The incorporation of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) with graphene significantly enhances electron transfer kinetics and provides abundant adsorption sites for mercury ions, improving both sensitivity and reproducibility. The linear calibration curve in the inset (R²=0.9911) confirms excellent analytical linearity over the studied concentration range, with a sensitivity of 8.72 μA per 10^-7 M of Hg²⁺.

Such performance demonstrates that the po-Gr/PEDOT: PSS nanocomposite efficiently facilitates charge transfer and ion accumulation, key processes for trace-level heavy-metal detection. Moreover, the use of DPSV allows discrimination of the faradaic signal from capacitive currents, providing better sensitivity than conventional linear sweep voltammetry.

Fullerenes are unique carbon structures known for their excellent electrical conductivity, high charge mobility, and remarkable photophysical characteristics. These properties make them valuable materials for developing sensors capable of detecting a wide range of physical and chemical changes [50]. Because of their spherical shape and ability to transfer electrons efficiently, fullerenes are often used to create sensitive interfaces for electrochemical, gas, optical, and strain sensors. Their surface can undergo electrophilic substitution with electron donating groups such as amines, allowing the formation of immobilized C₆₀ enzyme systems that improve catalytic performance and signal transduction [51]. A typical sensor includes a receptor that interacts with the analyte and a transducer that converts the resulting interaction into a measurable signal [52].

Mazloum-Ardakani et al. developed a label-free electrochemical immunosensor for the detection of tumor necrosis factor α, utilizing fullerene-functionalized carbon nanotubes and ionic liquid (C₆₀–CNTs–IL) as depicted in Fig. 6. The biosensor exhibits a linear range from 5.0 pg·mL⁻¹ to 75 pg·mL⁻¹, demonstrating a low detection limit of 2.0 pg·mL⁻¹ for TNF-α. Furthermore, the developed label-free electrochemical immunosensor was effectively utilized for serum samples.

Building upon this foundation, Zhong et al. developed an enzyme free glucose sensor that demonstrated the multifunctionality of fullerenes in biosensing systems. The researchers combined fullerene with cysteine and palladium nanoparticles to form a composite material known as Pd@Cys C₆₀, illustrated in Fig. 7. When applied to a glassy carbon electrode, this material provided excellent electrocatalytic activity toward glucose oxidation. The device operated effectively across a broad concentration range from one micromole to one millimole and showed strong selectivity in the presence of interfering compounds such as fructose and ascorbic acid. This work highlighted the potential of fullerene composites to replace traditional enzyme based glucose sensors by offering both durability and precision [54].

Expanding upon these findings, Shetti et al. created an electrochemical biosensor using a fullerene modified glassy carbon electrode for detecting acyclovir, an antiviral drug. The electrode was fabricated by coating the carbon surface with a fullerene solution in dichloromethane, which was then reduced with potassium hydroxide. When exposed to acyclovir, the sensor demonstrated an impressive detection range from ninety nanomoles to six micromoles, with a low detection limit of 1.48 nanomoles. The fullerene modification significantly improved electron transfer and sensitivity compared to unmodified electrodes, while maintaining a minimal matrix effect in real samples [56].

Further advancing multifunctional carbon nanomaterial designs, Pan et al. developed a ternary hybrid sensor that combined carbon nanotubes, graphene, and fullerenes embedded within a styrene ethylene butylene styrene polymer matrix. The polymer provided strong adhesion and flexibility, enabling the sensor to maintain performance under mechanical strain. The hybrid material exhibited a conductivity of 5.179 siemens per meter, a gauge factor of fifteen, an optimal stretching range of two hundred three percent, and a linear response with an R² value of 0.998. This study demonstrated the feasibility of combining different carbon nanostructures to create durable, stretchable, and highly responsive strain sensors [57].

The electrochemical potential of fullerene composites was further explored by Brahman et al., who prepared a C₆₀ multiwalled carbon nanotube nanocomposite on a glassy carbon electrode for pyruvic acid detection. The material displayed strong electrocatalytic activity, reducing pyruvic acid within five minutes while lowering the overpotential. In related work, the same group developed a ternary composite electrode composed of copper nanoparticles, C₆₀, and multiwalled carbon nanotubes on a pretreated carbon paste substrate. This sensor showed excellent activity for paracetamol detection, demonstrating how the integration of metallic and carbon nanostructures can enhance both sensitivity and selectivity [58].

Beyond electrochemical applications, several researchers have explored the environmental and photocatalytic capabilities of fullerenes. Brunet et al. reported that hydrophilic functionalized C molecules can efficiently eliminate pathogenic microorganisms in water through photocatalytic oxidation. Their study also emphasized that fullerenes represent clean and renewable materials capable of hydrogen storage because of their ability to form C-H bonds from C-C bonds under mild conditions [59]. Similarly, Wu et al. synthesized water-soluble fullerene derivatives that acted as light activated sensitizers to generate reactive oxygen species under visible and ultraviolet irradiation. These species were effective in degrading organic contaminants in water, and the fullerols themselves acted as antioxidants that could be easily removed after use, minimizing secondary pollution [60].

In another environmental application, Alekseeva et al. performed comparative studies on copper ion removal using fullerene and nanocomposite polystyrene films. Their findings revealed that fullerenes displayed superior adsorption capacity and followed the Langmuir model of monolayer adsorption. This confirmed the efficiency of fullerenes in metal ion capture and suggested their potential use in wastewater treatment and remediation systems [61].

Diamond has gained significant attention as a material for electrochemical sensors because of its outstanding biocompatibility, broad potential window, low background current, and remarkable chemical stability. In electrolyte solutions, diamond exhibits superior corrosion resistance and reproducibility compared with silicon, silicon dioxide, gold, glassy carbon, tin oxide, and zinc oxide [62]. Its large overpotentials for hydrogen and oxygen evolution provide a wide operational range of about 3.25 volts or more, while the low background current ensures accurate electrochemical signals in phosphate buffer solutions [63]. Through controlled doping, diamond can be modified to behave as a metallic, semiconducting, or insulating material. Surface terminations also influence their behavior: hydrogen termination produces a hydrophobic surface, while oxygen termination yields hydrophilicity [63,64]. Diamond’s ability to form stable bonds with biomolecules such as DNA, combined with its chemical inertness, makes it ideal for biomedical applications [62]. Although its use was once limited by high production costs, the advent of nanocrystalline diamond synthesis has substantially reduced expenses, allowing for widespread adoption in sensors, microelectronics, and life science technologies.

The development of diamond-based gas sensors first demonstrated the material’s potential for robust sensing under harsh conditions. Kang et al. designed a microelectronic hydrogen detector composed of polycrystalline diamond film combined with a catalytic metal. The layered structure of palladium, intrinsic diamond, and p-type diamond formed a metal insulator semiconductor Schottky diode on a tungsten substrate. When tested between twenty-seven and three hundred degrees Celsius, the sensor displayed consistent hydrogen detection, with the response attributed to variations in barrier height within the diamond layer. This study provided early evidence that diamond could outperform conventional semiconductor materials in high-temperature gas detection [65].

Upon these early insights, Gurbuz et al. broadened the application of diamond sensors by creating devices capable of detecting benzene and toluene vapors, two pollutants of significant environmental concern. Their design incorporated catalytic metal, intrinsic diamond, and doped diamond layers, and they observed that increasing the thickness of the intrinsic diamond layer to approximately 0.25 micrometer improved both sensitivity and current voltage response. Further refinement using platinum and tin oxide films enhanced selectivity for oxygen, carbon monoxide, and hydrogen. For toluene detection, a palladiumgated diamond configuration generated higher currents with rising temperature and concentration, confirming the sensor’s strong responsiveness. Together, these advancements demonstrated that diamond film technology could serve as a foundation for reliable, wide-range gas sensors suited for demanding environments [66].

Following the progress in environmental sensing, researchers began applying diamond materials to biomedical diagnostics. Kim et al. introduced a diamond like carbon microelectrode compatible with standard enzyme linked immunosorbent assay kits for detecting viral antigens. Using boron doped diamonds like carbon electrodes and cyclic voltammetry, they quantified oxidized 3,3′,5,5,5,5-tetramethylbenzidine without the conventional sulfuric acid quenching step. The resulting electrochemical profiles were clean, and step shaped, enabling more precise identification of oxidation peaks. When tested with commercial HIV, HBV, and HCV antigen kits, the electrode provided consistent and sensitive readings, underscoring the suitability of diamond- like carbon materials for medical diagnostics requiring chemical stability and repeatable signal output [67].

Extending this research to enzymatic systems, Maalouf et al. designed a glucose biosensor by immobilizing glucose oxidase on both glassy carbon and diamond like carbon electrodes. The device detected hydrogen peroxide formed during the enzyme catalyzed oxidation of glucose by oxygen. The glucose oxidase glassy carbon electrode achieved a detection limit of twenty micromoles, while the diamond-based electrode reached fifty micromoles. Although slightly less sensitive, the diamond electrode offered superior stability, chemical resistance, and durability, which are critical for long-term biomedical use [68].

As research has expanded, the exceptional mechanical and structural characteristics of diamond and diamond like carbon films have found uses far beyond electrochemistry. Their high hardness, tensile strength, low friction, minimal thermal expansion, and outstanding wear resistance make them ideal materials for microelectromechanical systems. They are employed as structural components in high frequency resonators, comb drives, and sensing elements, as well as surface coatings that reduce wear and friction on moving parts. In nanomedicine, diamonds like carbon coatings are preferred in devices requiring electrical insulation, whereas pure diamond surfaces are used in orthopedic prostheses due to their biocompatibility and stability.

Collectively, these studies illustrate the transformation of diamonds and their amorphous counterparts from laboratory curiosities into practical materials for advanced sensing systems. By adjusting doping levels, microstructure, and surface termination, researchers have achieved materials that combine electrochemical precision with mechanical endurance. Diamonds like carbon continue to bridge disciplines spanning environmental monitoring, medical diagnostics, and microdevice engineering and remain central to the next generation of multifunctional sensor platforms.

Carbon nanocomposites demonstrate remarkable properties such as increased surface area, enhanced electrical conductivity, superior thermal conductivity, and improved mechanical characteristics [69]. Numerous carbon nanomaterials and their derivatives, such as graphene, carbon nanotubes (CNTs), reduced graphene oxide (rGO) and carbon nanofibers (CNF), have been employed in electrochemical nanosensors for the identification of trace metals in water. The synthesis, post-treatment, optimization, and characterization of carbon-based nanomaterials have been examined, along with the influence of their structural properties on electrochemical sensors [70]. Development of sensors that demonstrate high sensitivity and selectivity, alongside efficient drug delivery and release control, as well as enhanced tissue regeneration capabilities. The current research emphasis on the initiatives that are underway in the development of carbon-based drug delivery systems and the formulation of applicable nanomaterials for tissue regeneration, employing carbon supports that exhibit superior biocompatibility [71]. Karthika et al. [72] created a nanocomposite of graphene carbon nitride decorated with silver molybdate through a solvothermal method aimed at the electrochemical detection of chromium (VI) in water samples. The amperometric electrochemical method demonstrated remarkable sensitivity of 65.8 μA μM⁻¹ cm⁻², exhibiting good linear ranges from 0.1 to 0.7 μM, along with a lower limit of detection of 0.0016 μM, which can be linked to the increased surface area and availability of adsorption sites.

Carboxylated SWCNTs were integrated onto carbon-based screen-printed electrodes (SPEs) to create an electrochemical biosensor aimed at detecting point-of-care antibodies (Anti-S protein) for SARS-CoV-2 [73]. In this study, following the introduction of SWCNT on the SPEs, p-phenylenediamine underwent modification to enable the amine groups for the immobilization of the SARS-CoV-2 Spike protein, facilitating the capture of SARS-CoV-2 antibodies. The developed biosensor, utilizing a highly conductive electrode made of SWCNTs, demonstrated a sensitive limit of detection at 0.7 pg mL⁻¹ and showcased a broad linear response from 1.0 pg mL⁻¹ to 10 ng mL⁻¹, as assessed through electrochemical impedance spectroscopy (EIS). The detection of SARS-CoV-2 antibodies in human positive sera was achieved at a level of 2.3 μg mL⁻¹.

Also, gold nanoparticle-decorated multiwalled carbon nanotubes (Au–MWCNTs) have demonstrated remarkable sensitivity in arsenic detection applications. Xiao et al. achieved a sensitivity of approximately 1985 μA μM^–1 for As(III) via square-wave anodic stripping voltammetry (SWASV), with a limit of detection (LOD) of roughly 0.1 μg L^–1 (≈1.3 nM) when employing a 120 s deposition period. The CNT network functioned as a high-surface-area conductive scaffold, and the ~10 nm gold nanoparticles acted as the principal electrocatalytic sites [74], leading to stripping peak currents exceeding those observed with unmodified carbon electrodes by more than an order of magnitude.

Graphene-based hybrid nanocomposites have also shown notable advancements in the detection of heavy metals. In particular, Fe₃O₄–rGO nanocomposites, distinguished by the uniform attachment of approximately 20 nm Fe₃O₄ nanoparticles to rGO sheets, have exhibited improved charge-transfer kinetics and metal ion adsorption characteristics. Hu et al. attained a sensitivity of approximately 2.15 μA ppb^–1 and a limit of detection (LOD) of roughly 1.19 ppb for As(III) through the application of square-wave voltammetry under optimized conditions [75]. Moreover, electrochemical impedance spectroscopy revealed a significant reduction in charge-transfer resistance compared to bare glassy carbon electrodes, thus confirming the role of rGO in facilitating rapid electron transport; simultaneously, Fe₃O₄ enhanced analyte preconcentration and selectivity.

In addition, surface engineering of electrodes coupled with hybrid nanocomposites, presents a strategy for augmenting sensing performance beyond the intrinsic characteristics of the constituent materials. Carbon screen-printed electrodes (cSPEs), which underwent electrochemical polishing followed by modification with bismuth-rGO nanocomposites, exhibited a voltammetric current enhancement of approximately 41%, a reduction in charge-transfer resistance of roughly 88%, and sensitivities of roughly 5.0 μA ppb^–1 cm^–2 for Cd²⁺ and 2.7 μA ppb^–1 cm^–2 for Pb²⁺, alongside sub-ppb detection limits [76]. These results, taken together, suggest that hybrid carbon nanocomposites, when incorporated into optimized electrode designs, enable ultra-low detection limits, high sensitivity, and multiplexed detection capabilities, while concurrently highlighting persistent issues related to fabrication complexity, reproducibility, and long-term operational stability.

Like Fe₃O₄-rGO and bismuth-rGO nanocomposites, rGO-MoS₂ nanocomposites also stems from a synergistic interfacial mechanism that shows enhanced sensing capabilities. Within this mechanism, rGO forms a highly conductive percolation network, which in turn accelerates electron transfer kinetics. Concurrently, MoS₂ provides edge-rich sulfur sites, which function as preferred adsorption centers for heavy metal ions through metal-sulfur coordination. This dual functionality improves the effective surface coverage of Pb²⁺ at the electrode-electrolyte interface and reduces the energy barrier for electrochemical reduction. As a result, a rGO/MoS₂/chitosan-modified glassy carbon electrode exhibited sensitive Pb²⁺ detection across a concentration range of 0.005-2 μM, with a limit of detection of 0.0016 μM [77], attributable to enhanced interfacial accumulation and efficient charge transport. These mechanistic advantages underscore the potential of rGO-MoS₂ nanocomposites for detecting trace levels of heavy metals in water matrices.

Therefore, a detailed comparative analysis of various applications has been provided in below, focusing on different carbon nanomaterials to enhance the reader’s understanding of these materials.

Future research in carbon-based biomedical sensing should focus on integrating multifunctional nanocomposites that merge the exceptional conductivity, mechanical flexibility, and biocompatibility of carbon materials with the catalytic and selective features of metals, metal oxides, and polymers. Emerging studies demonstrate that hybrid structures such as graphene-MoS₂, CNT-metal oxide, and MXene–graphene composites can dramatically enhance electron transfer kinetics and bioreceptor immobilization efficiency [82,83]. The development of surface functionalization chemistries, including covalent coupling via EDC/ NHS, π–π stacking using pyrene linkers, and bio-inspired coatings, can yield highly stable and selective biointerfaces for enzymes, aptamers, and antibodies [85]. The incorporation of conductive hydrogels, polypyrrole, and PEDOT:PSS has further improved the flexibility, anti-fouling resistance, and signal reproducibility necessary for wearable or implantable biosensors [78,79]. Future work should also prioritize scalable fabrication methods such as laser-induced graphene (LIG), inkjet printing, and 3D micro-extrusion to achieve cost-effective, disposable sensor platforms [80]. Integrating electrochemical sensing with optical or piezoresistive modalities, as well as microfluidic control, will enable multiplexed and real-time diagnostics at the point of care. To ensure translational success, upcoming studies must also focus on cytotoxicity, long-term stability, sterilization compatibility, and real biofluid testing to meet regulatory and clinical standards. In parallel, combining carbon-based biosensors with artificial intelligence and data analytics could revolutionize personalized healthcare by enabling continuous, non-invasive monitoring of biomarkers such as glucose, cortisol, or neurotransmitters [87].

In environmental and toxic chemical sensing, the next generation of carbon-based electrochemical sensors should emphasize improved selectivity, reproducibility, and sustainability to meet real-world analytical needs. Hybrid electrodes that combine rGO, CNTs, or biochar-derived carbon with metal/metal oxide nanoparticles such as Au, Bi, CuO, Fe₃O₄, and ZnO have shown promising results for trace-level detection of heavy metals, pesticides, and dyes [33,81].

Future research should aim to fine-tune the surface chemistry of carbon nanostructures through nitrogen, sulfur, and thiol functionalization to improve binding affinity toward specific contaminants such as Pb²⁺, Cd²⁺, and Hg²⁺ [60]. Sustainable synthesis and printing methods including green chemical reduction, laser scribing, and biomass-derived carbons will be crucial for low-cost, eco-friendly mass production of disposable screen-printed carbon electrodes [86]. Real matrix validation, interference studies, and anti-fouling coating remain essential to ensure sensor reliability in complex water or soil samples. Additionally, coupling electrochemical data with IoT and cloud-based monitoring systems could enable distributed, autonomous water-quality surveillance networks [81]. Machine learning models should also be incorporated to correct matrix interferences and identify multi-analyte contamination signatures. The long-term vision involves integrating these portable carbon-based platforms with advanced data analytics, forming self-calibrating, sustainable, and intelligent environmental sensing systems capable of on-site, real-time toxic chemical detection for public health and environmental protection.

Carbon-based sensors have greatly evolved with time from conventional carbon structure into highly tailored materials with exceptional electrochemical performance, especially those that use nanomaterials such as graphene and carbon nanotubes (CNTs). Conventional carbon sensors, like glassy carbon electrodes and carbon fibers, are prized for their superior electron transmission, chemical stability, and low cost. On the other hand, carbon nanostructured forms such as carbon nanotubes, graphene, fullerenes and nanodiamonds possess enhanced sensitivity, large surface area, better adsorption, rapid electron transfer kinetics, collectively providing improved sensing capabilities and analytical precision. Because of their superior electrical qualities and great sensitivity, carbon nanotubes (CNTs) and their single and multi-walled counterparts are useful for detecting a wide range of substances, including gases and heavy metals. Despite being less prevalent, fullerenes and nanodiamonds also contribute because of their special electrical and photophysical qualities as well as their biocompatibility. It is necessary to handle issues like device variability and susceptibility to surface adsorbates. These properties suit CNTs for various electrochemical sensors, including amperometric enzyme electrodes and medical biosensors. Future research should concentrate on the advancement of hybrid nanocomposites that integrate carbon materials with metal oxides or polymer to improve performance. The fusion of these sensors with artificial intelligence (AI), internet and portable technologies will further support real time and intelligent diagnostic. These advances will support cost-effective, sustainable and scalable detection platforms for healthcare, environmental and industrial applications.