Green synthesis of AuNPs (gold nanoparticles) is increasingly gaining importance for producing nanoparticles with high stability and strong optical activity, as current research focuses not only on feasibility but also on sustainability. In this study, AuNPs were synthesized using Citrus aurantifolia extract. Optimization was performed by varying extract concentration, precursor concentration, extract pH, synthesis time, and temperature. The optimum conditions were 10% extract, 0.6 mM precursor, pH 7, and a 10-min synthesis under boiling conditions. These conditions produced homogeneous nanomaterials, characterized by a high, narrow surface plasmon resonance (SPR) peak at ~530 nm. The characterization results revealed predominantly spherical AuNP morphology via field emission scanning electron microscopy (FESEM), an average particle size of 35.51 nm (PSA–Size), and a negative surface charge of –23.29 mV (PSA–Zeta Potential), all of which contribute to the stability of the AuNP colloid. Fourier transform infrared spectroscopy (FTIR) analysis further indicated the presence of secondary metabolite functional groups from C. aurantifolia extract on the nanoparticle surface. The synthesized AuNPs were then applied for microplastic detection using a precipitation-based method, enabling simple and direct visual identification. The system successfully detected PS microplastics at concentrations as low as 5 mg/L. Additionally, this study assessed the potential of AuNPs for pesticide detection via surface interaction mechanisms that induce SPR peak shifts. Overall, AuNPs-CA offers a sustainable synthesis route while providing opportunities for the development of low-cost, rapid, easy-to-apply, and environmentally friendly chemical sensors for environmental pollutant monitoring.