Carbon nanotubes (CNTs) possess exceptional electrical properties that make them highly suitable for gas and biosensing applications. Their one-dimensional structure, high surface-to-volume ratio, and ability to undergo conductivity changes upon analyte adsorption enable sensitive detection of various chemical and biological species. The principle behind CNT-based sensors relies on charge transfer between the nanotube and adsorbed molecules, which alters the conductance of the CNT network. This change can be measured and correlated to the concentration of the target analyte.
The conductivity of CNTs is highly sensitive to surface interactions. When gas molecules or biomolecules adsorb onto the CNT surface, they act as electron donors or acceptors, modifying the charge carrier density. For instance, oxidizing gases such as nitrogen dioxide (NO₂) withdraw electrons from p-type semiconducting CNTs, increasing hole concentration and enhancing conductivity. Conversely, reducing gases like ammonia (NH₃) donate electrons, decreasing hole density and reducing conductivity. Similarly, in biosensing, charged biomolecules such as DNA, proteins, or glucose can induce measurable conductivity shifts upon binding to functionalized CNTs.
A major challenge in CNT-based sensing is achieving selectivity, as bare CNTs often respond to multiple analytes. To address this, polymer coatings are widely employed to enhance specificity. These coatings act as selective filters or recognition layers that preferentially interact with target molecules while blocking interfering species. The choice of polymer depends on the analyte of interest and the sensing environment. For example, polyethyleneimine (PEI) is commonly used for detecting acidic gases like carbon dioxide (CO₂) due to its amine-rich structure, which interacts strongly with CO₂ molecules. On the other hand, hydrophobic polymers such as polystyrene can improve selectivity for nonpolar volatile organic compounds (VOCs) like toluene by repelling polar interferents like water vapor.
Conducting polymers, including polyaniline (PANI) and polypyrrole (PPy), are particularly effective for selective CNT-based sensing. These polymers not only provide selectivity but also contribute to signal amplification due to their intrinsic conductivity. When PANI-coated CNTs are exposed to ammonia, the deprotonation of PANI leads to a conductivity decrease, which is further modulated by the underlying CNT response. Similarly, PPy-functionalized CNTs exhibit high sensitivity to humidity and certain VOCs due to the swelling and charge transfer mechanisms of the polymer matrix.
Another approach involves molecularly imprinted polymers (MIPs), which are synthesized to contain cavities complementary to the target analyte. MIP-coated CNTs offer high specificity by allowing only the template molecule to bind, minimizing cross-sensitivity. For instance, MIPs designed for glucose can selectively capture glucose molecules in complex biological fluids, enabling precise monitoring without interference from structurally similar sugars like fructose or galactose.
In biosensing, biopolymer coatings such as chitosan or Nafion are used to improve biocompatibility and selectivity. Chitosan, a polysaccharide with abundant amino groups, can immobilize enzymes like glucose oxidase on CNT surfaces, facilitating selective glucose detection through enzymatic reactions. Nafion, a sulfonated tetrafluoroethylene copolymer, is often employed for detecting neurotransmitters like dopamine by repelling negatively charged interferents such as ascorbic acid.
The performance of polymer-coated CNT sensors is influenced by several factors, including polymer thickness, morphology, and the method of deposition. Thin, uniform coatings ensure rapid analyte diffusion and fast response times, while excessively thick layers may hinder sensitivity. Techniques such as drop-casting, spin-coating, or electrochemical polymerization are used to achieve optimal film properties. Additionally, cross-linking agents or nanostructuring the polymer can enhance mechanical stability and prevent swelling-induced degradation in humid environments.
Quantitative studies have demonstrated the effectiveness of polymer-CNT composites in sensing applications. For example, PANI-coated CNT sensors have shown response times as fast as 10 seconds for ammonia detection at concentrations as low as 1 ppm. Similarly, PEI-functionalized CNT arrays exhibit a linear response to CO₂ in the range of 100–5000 ppm with minimal interference from humidity. In biosensing, glucose sensors using chitosan-enzyme-CNT hybrids achieve detection limits in the micromolar range, suitable for clinical diagnostics.
Environmental stability is another critical consideration. Polymer coatings can protect CNTs from oxidative degradation and fouling in harsh conditions. For instance, fluorinated polymers like polyvinylidene fluoride (PVDF) improve the durability of CNT sensors in high-temperature or chemically aggressive environments. However, long-term exposure to UV radiation or extreme pH levels may still degrade certain polymer-CNT interfaces, necessitating careful material selection for specific applications.
Recent advancements focus on multi-functional polymer coatings that combine selectivity with self-cleaning or regenerative properties. Stimuli-responsive polymers, such as those sensitive to pH or temperature, can release adsorbed analytes upon external triggers, resetting the sensor for repeated use. Similarly, incorporating photocatalytic nanoparticles like titanium dioxide (TiO₂) into the polymer matrix can enable light-activated regeneration by degrading adsorbed contaminants.
In summary, the integration of polymer coatings with CNTs significantly enhances the selectivity and functionality of gas and biosensors. By tailoring the chemical and physical properties of the polymer layer, researchers can design sensors with high sensitivity, rapid response, and minimal cross-interference. Continued development in polymer science and nanomaterial engineering will further expand the capabilities of CNT-based sensing platforms for diverse applications in environmental monitoring, healthcare, and industrial safety.