Modification of Single-Walled Carbon Nanotubes with Halogen-Substituted Metal Phthalocyanines for Chemiresistive Ammonia Detection

The development of highly sensitive and stable gas sensors at room temperature is crucial for environmental and industrial monitoring. This study reports the synthesis and characterization of novel ammonia sensors based on covalently functionalized single-walled carbon nanotubes (SWCNT-NH2) with tetra- and octa-chlorinated zinc phthalocyanines (ZnPcCl4 and ZnPcCl8). These covalent hybrids were prepared by replacing peripheral halogen atoms with amino groups on the surface of the nanotubes, which significantly increased the loading of the macrocycles compared to non-covalently functionalized analogs, reduced nanotube aggregation, and improved electrical conductivity. The sensors exhibited fully reversible responses to NH3 at room temperature. Among the tested materials, the covalent hybrid of SWCNT-NH2/ZnPcCl8 demonstrated superior performance. It achieved a limit of detection of 0.7 ppm and rapid response and recovery times (60 and 65 s, respectively, at 10 ppm). The sensor’s response was over 10 times higher than that of its tetrasubstituted ZnPcCl4 counterpart and more than 100 times higher than for non-covalent hybrids. The sensor also showed excellent long-term stability (greater than 1 month) and high selectivity for CO2 and volatile organic compounds. Quantum chemical calculations revealed that the increased sensitivity of the octa-chlorinated derivative did not result from stronger analyte binding but rather from the strong electron-withdrawing nature of chlorine substituents, which create an electron-depleted carbon framework. This causes a constant charge transfer from adsorbed NH3, inducing a proportionally larger change in majority carrier concentration.