This paper proposes an all-fiber micro-nano (μ-n) plastic sensing system utilizing the surface plasmon resonance (SPR) phenomenon. The sensor can effectively identify the targeted μ-n plastics in the water bodies through the refractive index changes. The sensing element is a tailored nanostructured optical quantum-metamaterial with noble metals (planer silver (Ag) and 3D-grated gold (Au)) and topmost graphite coating. The optimized thickness of combination layers was designed to maximize the absorption of the light at the SPR site. The optimal layering sequence and corresponding thicknesses were calculated analytically and numerically through transfer matrix and finite-difference time-domain methods respectively. Consequently, the layers’ thickness was obtained as Cgr= 0.69 nm, Au= 6 nm, and Ag= 45 nm. The comparative reflected intensity dip in the output SPR spectrum is found to be 100% steeper concerning the corresponding planer array for angular and wavelength shifts. The sensor additionally promises to increase the adsorption of the μ-n plastics due to the large surface area of nano-cuboidal structures.