Biosilicified nanostructured microshells from the marine diatom Coscinodiscus wailesii have been properly functionalised to bind a molecular probe which specifically recognises a target analyte. The chemical modification process has been characterised by Fourier transformed infrared spectroscopy. Fluorescence measurements demonstrate that the antibodies we used, even if linked to the amorphous silica surface of C. wailesii microshells, still efficiently recognise their antigens. These low cost and largely available natural materials can be thus used as transducers elements for optical biosensors or as targeting microcapsules for drug delivery.