An apparatus comprising one or more piezoelectric mass sensors for use in diagnostic and analytic processes, in particular for immunochemical detection of diagnostically relevant analytes in real time, is described. Each piezoelectric mass sensor comprises a piezoelectric crystal with a receptor surface which has immobilized thereon a lawn of recombinant antibodies comprising single V.sub.H chain or single-chain Fv (scFv) polypeptides specific for a particular antigen. Binding of antigen to the recombinant antibodies results in a change in mass on the receptor surface which is detected as a change in resonant frequency. In a preferred embodiment, the receptor layer is a precious metal such as gold which facilitates self-assembly of the recombinant antibodies into a lawn on the receptor surface via a cysteine residue at the carboxy terminus of the attachment polypeptide.