a, Polymer spots were visible over the first six spots in the array following a 10 min incubation with avidin-functionalized macroinitiator and a 10 min dose of 5 mW cm−2, 365 nm light. Using enzymatic amplification (horseradish peroxidase/3,3′,5,5′-tetramethylbenzidine–dextran), only the first two spots were visible. b, The maximum number of biotinylated oligonucleotides present in each spot. The bottom right spot contained only unlabelled oligonucleotides and served as a negative control. c, The minimum radiation dose that was delivered to each spot before observation of polymer formation, where the dose is a product of the light intensity and exposure time. The error bars reflect variations in the intensity output from the lamp as measured with a radiometer.