A method for patterning flow cell substrates using photo-initiated chemical reactions that includes fabricating a planar waveguide flow cell by forming a layer of light coupling gratings on a glass substrate layer; depositing a core layer on the layer of light coupling gratings; depositing a cladding layer on the core layer; and forming nanowells in the cladding layer; silanizing the cladding layer; coating the silanized cladding layer and nanowells with a first group of reactants; introducing a second group of reactants into the nanowells, wherein the second group of reactants includes a target reactant and a light-sensitive photoinitiator system; coupling a light source to the light coupling gratings and directing light internally within the planar waveguide flow cell for photo-initiating a chemical reaction between the first and second groups of reactants, wherein the photo-initiated chemical reaction covalently binds the target reactant to only the bottom portion of each nanowell.