Abstract: The invention provides a method for determining the density of functional molecules attached to a substrate used for analysis of biological samples. A dye molecule responding to near infrared radiation at a wavelength of at least 600 nm is attached to the substrate and used to indicate the number of the functional molecules attached to the substrate by comparing the infrared absorption of the dye molecules with the ultraviolet absorption of the functional molecules. Such substrates may be employed in immunoassays and in vivo diagnostics.