Comparative SPR performance and optical constant of carbon nanomaterial-enhanced gold thin films for sensitive Sb³⁺ detection

This study explores the performance of surface plasmon resonance (SPR) sensors based on bare gold (Au) and carbon-enhanced thin films (Au/graphene oxide (GO), Au/carbon quantum dots (CQDs), and Au/graphene quantum dots (GQDs)) for detecting antimony ion (Sb³⁺). Optical constants were extracted via theoretical fitting and correlated with thickness variations at each concentration, enabling quantitative, point-by-point analysis of sensor behaviour. Bare Au and Au/GO showed negligible angle shifts at low concentrations, indicating poor sensitivity, while Au/CQDs and Au/GQDs exhibited measurable shifts due to enhanced surface interactions. The real part of the refractive index and the film thickness increased, whereas the imaginary part (k) decreased with higher Sb³⁺ concentrations. Linear regression revealed sensitivity of 0.011° ppm^− 1 for Au/CQDs and 0.0335° ppm^− 1 for Au/GQDs. Au/GQDs demonstrated the highest sensitivity (0.0335° ppm^− 1) and detection accuracy, whereas Au/CQDs showed the strongest binding affinity (K_s = 2.513 ppm^− 1) and the lowest limit of detection (LOD = 3.94 ppm), making it more effective for trace-level detection. Despite early saturation, Au/CQDs outperformed Au/GQDs (LOD = 10.22 ppm) in detecting low Sb³⁺ concentrations, highlighting a trade-off between sensitivity and detection threshold.