goat igg binding of this secondary antibody to the surface was indicated by a decrease in effective optical thickness of the monolayer from 11 997 to 11 767 lowest concentration employed with a detectable signal was 10 u2212 12 m fig4 b treatment of the secondary antibody sample with anti digoxigenin mouse igg at a concentration of 10 u2212 8 m caused a further decrease in the effective optical thickness of the monolayer from 11 706 to 11 525 nm fig4 c the interaction of digoxigenin 10 u2212 6 m a steroid with molecular weight of 392 with the anti digoxigenin igg bound porous si surface was also detected with a decrease of the effective optical thickness from 11 508 to 11 346 nm fig4 d to rule out the possibility of nonspecific interaction a non biotinylated surface was subjected to the same solution and conditions as described in example 4 no measurable change in the effective optical thickness was observed on treatment with streptavidin secondary antibody primary antibody and digoxigenin detection of the relatively small biotin molecule mw 244 at concentrations as low as 10 u2212 12 m has also been demonstrated using biotin streptavidin modified porous si although the invention has been described with reference to a preferred embodiment it will be understood by those skilled in the art that various changes may be made.