A gas phase species (such as ozone, H.sub.2O.sub.2, or N.sub.2O) is photodissociated with ultraviolet light into a reactive species that is patternwise directed (e.g., through a mask) onto a surface of a material, such as an organosilicate. The reactive species reacts with the material to form a polar oxidation product such as --OH, thereby resulting in discrete hydrophilic regions separated from each other by hydrophobic regions. The degree of hydrophilicity of the discrete regions may be tailored by controlling the concentration of the reactive species, the ultraviolet light intensity, the temperature to which the material is heated, and exposure time. End products made with the methods are suitable for use in a biomolecular array.