An optical sensor for label-independent detection, having improved spatial resolution and reduced angular sensitivity, the sensor including: a substrate; a waveguide grating adjacent the substrate; and a waveguide coat layer adjacent or over the waveguide grating, the waveguide coat layer having a thickness (W) of from 30 nm to 300 nm, the waveguide grating having a teeth height (H) of from 0.2×W to 1×W, and for example, a waveguide core thickness (Wcore=W−H) from 5 nm to 50 nm. Also disclosed is a well-plate article, a well-plate reader system, and methods of using the well-plate and sensor articles, as defined herein.