Carbon nanofiber-based gas sensors have emerged as a promising platform for detecting various gaseous species, including nitrogen oxides (NOx), carbon dioxide (CO2), and volatile organic compounds (VOCs). Their high surface area, tunable electrical properties, and compatibility with functionalization strategies make them particularly attractive for gas sensing applications. Unlike their carbon-based counterparts such as graphene or carbon nanotubes (CNTs), carbon nanofibers offer distinct advantages in terms of scalability and cost-effective fabrication, while maintaining competitive sensitivity and selectivity.

The performance of carbon nanofiber gas sensors heavily depends on surface functionalization, which enhances their interaction with target gas molecules. Common functionalization approaches include the deposition of metal oxides such as tin oxide (SnO2), zinc oxide (ZnO), or tungsten oxide (WO3), as well as conductive polymers like polyaniline (PANI) or polypyrrole (PPy). These modifications introduce active sites that selectively adsorb gas molecules, altering the electrical properties of the nanofibers. For instance, SnO2-functionalized carbon nanofibers exhibit high sensitivity to NOx due to the redox reactions between the metal oxide and gas molecules, leading to measurable changes in resistance. Similarly, polymer-coated nanofibers can selectively interact with VOCs through dipole-dipole interactions or hydrogen bonding, enabling discrimination between different analytes.

Transduction mechanisms in carbon nanofiber gas sensors primarily rely on resistive or capacitive changes. Resistive sensors operate by measuring the variation in electrical resistance upon gas adsorption. When target molecules bind to the nanofiber surface, charge transfer occurs, modifying the overall conductivity. For example, in the case of CO2 detection, functionalized carbon nanofibers may exhibit resistance changes due to the formation of carbonate species on metal oxide surfaces. Capacitive sensors, on the other hand, detect changes in dielectric properties caused by gas adsorption. This mechanism is particularly useful for polar gases like ethanol or acetone, where the permittivity of the sensing layer is altered upon exposure.

Comparative studies between carbon nanofiber sensors and those based on graphene or CNTs reveal distinct trade-offs. Graphene sensors, with their ultra-high carrier mobility, often demonstrate exceptional sensitivity at low gas concentrations. However, their lack of inherent selectivity necessitates complex functionalization strategies, increasing fabrication complexity. CNT-based sensors share similar challenges, as their sensitivity is highly dependent on tube chirality and defect density, leading to variability in performance. In contrast, carbon nanofibers provide a more uniform and scalable platform, as their synthesis via electrospinning or chemical vapor deposition can be easily controlled to produce consistent morphologies. Additionally, the fibrous structure of carbon nanofibers facilitates higher gas diffusion rates compared to densely packed graphene or CNT films, improving response times.

Scalability is a critical factor in the practical deployment of gas sensors, and carbon nanofibers hold an advantage in this regard. Techniques such as electrospinning allow for large-scale production of nanofiber mats, which can be directly integrated into sensor arrays. The compatibility of carbon nanofibers with roll-to-roll processing further enhances their manufacturability, making them suitable for industrial applications. In contrast, the production of high-quality graphene or aligned CNT films often requires more sophisticated and costly methods, limiting their widespread adoption.

Performance metrics such as sensitivity, response time, and recovery time are essential for evaluating carbon nanofiber gas sensors. For NOx detection, functionalized nanofibers have demonstrated detection limits in the parts-per-billion (ppb) range, with response times under 30 seconds. In the case of VOCs like benzene or formaldehyde, selectivity can be fine-tuned by adjusting the polymer coating thickness or incorporating dopants into metal oxide layers. Long-term stability remains a challenge, however, as repeated exposure to reactive gases can lead to sensor degradation. Strategies such as protective coatings or periodic thermal regeneration have been explored to mitigate this issue.

Environmental conditions, including humidity and temperature, also influence sensor performance. Carbon nanofiber sensors functionalized with hydrophobic polymers exhibit reduced interference from moisture, a common issue in real-world applications. Temperature modulation techniques, where the sensor is periodically heated to desorb contaminants, have been shown to improve reproducibility. These adaptive approaches highlight the versatility of carbon nanofiber-based systems in diverse operating environments.

In summary, carbon nanofiber gas sensors represent a robust and scalable alternative to graphene or CNT-based devices, particularly for detecting NOx, CO2, and VOCs. Their tunable surface chemistry, coupled with efficient transduction mechanisms, enables high sensitivity and selectivity. While challenges such as long-term stability and environmental interference persist, ongoing advancements in functionalization and fabrication techniques continue to enhance their practicality. As the demand for reliable and cost-effective gas sensors grows, carbon nanofibers are poised to play a pivotal role in environmental monitoring, industrial safety, and air quality assessment.