The real-time sensing of metal ions at point of care requires integrated sensors with low energy and sample consumption, reversibility, and rapid recovery. Here, we report a photonic nanosensor that reversibly and quantitatively reports on variation in the concentrations of Pb(2+) and Cu(2+) ions in aqueous solutions (<500 μL) in the visible region of the spectrum (λ(max) ≈ 400-700 nm). A single 6 ns laser pulse (λ = 532 nm) was used to pattern an ∼10 μm thick photosensitive recording medium. This formed periodic AgBr nanocrystal (ø ∼ 5-20 nm) concentrated regions, which produced Bragg diffraction upon illumination with a white light source. The sensor functionalized with 8-hydroxyquinoline allowed sensing through inducing Donnan osmotic pressure and tuning its lattice spacing. The sensor quantitatively measured Pb(2+) and Cu(2+) ion concentrations within the dynamic range of 0.1-10.0 mM with limits of detection of 11.4 and 18.6 μM in under 10 min. The sensor could be reset in 3 min and was reused at least 100 times without compromising its accuracy. The plasmonic nanosensor represents a simple and label-free analytical platform with potential scalability for applications in medical diagnostics and environmental monitoring.