High sensitivity and specificity of two modified ssDNA aptamers capable of photocross-linking recombinant human basic fibroblast growth factor (bFGF((155))) were demonstrated. The aptamers were identified through a novel, covalent, in vitro selection methodology called photochemical systematic evolution of ligands by exponential enrichment (PhotoSELEX). The aptamers exhibited high sensitivity for bFGF((155)) comparable with commercially available ELISA monoclonal antibodies with an absolute sensitivity of at least 0.058 ppt bFGF((155)) under prevailing test conditions. The aptamers exquisitely distinguished bFGF((155)) from consanguine proteins, vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF). A commercially viable diagnostic system incorporating PhotoSELEX-evolved aptamers capable of simultaneous quantification of a large number of analyte molecules is also described. Such a system benefits from covalent bonding of aptamer to target protein allowing vigorous washing with denaturants to improve signal to noise.