We have shown that an amino-propyl coating, frequently used in bioanalysis to produce activated surfaces, is sufficiently transparent to allow it to be employed in evanescent-wave cavity ring-down spectroscopy (EWCRDS). Using a quartz prism with an activated surface, it was possible to detect femtomole quantities of labeled biomolecules. The target molecules were immobilized, and thus concentrated, on the total internal reflecting surface of a simple 90 degrees prism. Initial measurements were carried out on a dye typically used for labeling biomolecules. An antigen (fibronectin) labeled with this dye, was also detected in the femtomole-range by first immobilizing the specific antibody on the activated surface. (C) 2008 Published by Elsevier B.V.