High-sensitivity dual-band microwave CO₂ sensing using SWCNT/g-C₃N₄ nanocomposite-functionalized CSRR-loaded antennas

Carbon dioxide (CO₂) monitoring is essential for environmental protection, industrial process control, and indoor air-quality assessment, driving the development of compact, highly sensitive, and low-power sensing technologies. In this work, dual-band microwave CO₂ sensors based on complementary split-ring resonator (CSRR)-loaded patch antennas functionalized with ultrathin oxidized single-walled carbon nanotube/graphitic carbon nitride (o-SWCNT/g-C₃N₄) nanocomposite films are proposed. Two antenna configurations, with CSRRs integrated either into the radiating patch (A/Front/CSRR) or the ground plane (A/Ground/CSRR), were designed, fabricated, and experimentally evaluated to investigate the influence of resonator placement on sensing performance. The sensors operating simultaneously in the X- and Ku-bands (8–18 GHz) i.e. (i) CSRRs loaded antenna into the patch (ACNT/Front/CSRR) exhibiting dual resonances near 9.09 and 16.74 GHz and (ii) CSRRs integrated into the ground plane (ACNT/Ground/CSRR) exhibiting dual resonances near 10.65 and 16.0 4 GHz, were tested under vacuum assisted (ideal) and ambient conditions for CO₂ concentrations ranging from 1800 to 7200 ppm. The ground-CSRR configuration demonstrated superior performance, achieving a maximum sensitivity of 8.89 kHz ppm⁻¹ at low CO₂ concentration and complete recovery within approximately 6 min . Compared with our previous sensors employing thicker sensing layers, the optimized ultrathin nanocomposite films (727–997 nm) significantly enhanced electromagnetic field–material interaction, reduced dielectric shielding, and improved adsorption/desorption dynamics. The sensing mechanism is attributed to adsorption-induced variations in the effective dielectric permittivity of the o-SWCNT/g-C₃N₄ layer, producing measurable resonance-frequency shifts through dielectric perturbation of the localized electromagnetic field. The proposed sensors demonstrate improved sensitivity, faster response, reduced shielding effects, and stable dual-band operation, highlighting their potential for low-power real-time environmental monitoring applications.