Highly Sensitive Nitrogen Dioxide Sensor Based on Enhancement of Surface Plasmon Resonance Response in Glass Waveguides by C–Ag Nanodots

Continuous monitoring of air quality and rapid detection of pollutants are highly desirable in urban planning and development of smart cities. One of the primary greenhouse gases responsible for environmental degradation and respiratory diseases is nitrogen dioxide (NO₂). Existing gas sensors for measuring NO₂ concentration suffer from drawbacks such as cross-sensitivity, limited range, and short life span. On the other hand, optical sensors, in particular, surface plasmon resonance (SPR) sensors, have emerged as a preferred alternative owing to advantages like high selectivity, immunity to electromagnetic interference, and low response time. In this work, we design and simulate a NO₂ sensor based on a glass waveguide coated with a gold film. Surface plasmons are excited at the interface by a 400–500-nm light source, incident at an angle of 43.16°. To enhance the sensitivity, we further coat the waveguide with three layers of carbon-silver (C–Ag) nanodots, which increases the surface plasmon field amplitude by nearly 7 times, in the absence of NO₂. When NO₂ concentration is varied in the range of 0–200 ppm, a corresponding change is observed in the reflected amplitude. In the absence of the C–Ag nanodots layer, the sensitivity is only 0.00042%/ppm, and on addition of C–Ag nanodots, the sensitivity increases significantly to 0.14235%/ppm which is nearly 343 times higher. These results demonstrate the efficiency of implementing nanodots in SPR sensor to detect and trace concentrations of contaminants in the gas phase.