Diffraction sensor for detecting binding affinity

The invention provides a diffraction sensor (1) comprising: a substrate (3); -two interdigital affinity gratings (2), a first affinity grating (20) comprising a first unit cell (200) having an affinity element (201), and a second affinity grating (21) comprising a second unit cell (210) having an affinity element (211), wherein the first unit cell (200) and the second unit cell (210) are configured and arranged such that coherent light of a predetermined wavelength generated at a predetermined beam generation location (40) and diffracted by target molecules (204, 214) bound to the affinity elements (201, 211) constructively interferes in opposite phases at a predetermined detection location (50), and wherein the first affinity grating (20) and the second affinity grating (21) are balanced to generate a bias signal at the predetermined detection position (50), the bias signal corresponding to a difference ([Delta] m) in scattering masses of the first affinity grating (20) and the second affinity grating (21),