Polarization insensitive cross resonator based highly sensitive metamaterial absorber for pH level monitoring

Monitoring pH is a crucial parameter in agriculture to ensure crop health and maximize productivity, as pH directly influences nutrient availability and plant growth. This article introduces a cross-shaped resonator-based metamaterial absorber (CRMMA) for microwave pH sensing applications. The proposed CRMMA sensor offers significant advantages over conventional pH sensors with the ability to respond to a wide range of pH levels and detect even minute fluctuations. The CRMMA is engineered by utilizing a cross-shaped resonator in every quarter of a four-fold rotationally symmetric structure, achieving incident angle stability and polarization insensitivity. This compact design has a subwavelength unit cell of 0.123λ × 0.123λ × 0.018λ mm³. Additionally, the CRMMA achieves an impressive absorption rate of 99.876 % at 3.5 GHz. The permittivity, permeability, electromagnetic field distribution, and surface current distribution of the sensor were thoroughly analyzed, revealing consistent performance across TE and TM polarization modes. A 106 × 106 mm² array prototype was fabricated to validate the scattering parameters and absorption characteristics, with real-time experimental results closely aligning with simulation results. The pH sensing capability of the proposed MMA was also tested in coco peat, where the sensor demonstrated a high sensitivity of 5.14, a high Q-factor of 619.47, and a high figure of merit (FoM) of 3184.07. The results demonstrate that the proposed CRMMA is an efficient and compact solution for real-time, high-precision pH monitoring in agricultural applications.