Light can be used to spatially resolve electrochemical measurements on a semiconductor electrode. This phenomenon has been explored to detect DNA hybridization with light-addressable potentiometric sensors and, more recently, with light-addressable amperometric sensors based on organic-monolayer-protected Si(100). Here, a contribution to the field is presented by comparing sensing performances when bovine serum albumin (BSA) and hexaethylene glycol (OEG(6)) are employed as antifouling layers that resist nonspecific adsorption to the DNA-modified interface on Si(100) devices. What is observed is that both sensors based on BSA or OEG(6) initially allow electrochemical distinction among complementary, noncomplementary, and mismatched DNA targets. However, only surfaces based on OEG(6) can sustain electroactivity over time. Our results suggest that this relates to accelerated SiOx formation occasioned by BSA proteins adsorbing on monolayer-protected Si(100) surfaces. Therefore, DNA biosensors were analytically explored on low-doped Si(100) electrodes modified on the molecular level with OEG(6) as an antifouling layer. First, light-activated electrochemical responses were recorded over a range of complementary DNA target concentrations. A linear semilog relation was obtained from 1.0 X 10(-11) to 1.0 X 10(-6) mol L-1 with a correlation coefficient of 0.942. Then, measurements with three independent surfaces indicated a relative standard deviation of 4.5%. Finally, selectivity tests were successfully performed in complex samples consisting of a cocktail mixture of four different DNA sequences. Together, these results indicate that reliable and stable light-activated amperometric DNA sensors can be achieved on Si(100) by employing OEG(6) as an antifouling layer.