From nanopatterning to functionality—surface and bulk imprinting for analytical purposes

Abstract Combining the appropriate transducer with the optimised sensitive layer is the fundamental challenge in sensor development. Acoustic devices, such as quartz crystal microbalances (QCMs) respond according to the Sauerbrey equation as long as the analyte is rigidly attached and moving with the transducer surface. This is true for example for automotive engine oil degradation products incorporated into bulk-imprinted TiO2-layers produced by a sol–gel-process. In the case of larger particles, such as yeast cells or viruses, the Sauerbrey condition is only met if the analyte is firmly engulfed by surface pores of the sensitive layer. In contrast to this, non-Sauerbrey effects dominate (e.g. for yeasts adhered to flat surfaces) as long as the cells are free to move over the transducer. At lower microorganism concentrations a frequency increase is observed, whereas at higher concentrations aggregation leads to the opposite effect. Interdigital capacities (IDC) and surface plasmon resonators (SPR) show a different behaviour: the field lines between the finger electrodes of the IDC or the evanescent field of the SPR reach into the medium surrounding the device. Therefore, the analytes need not tightly adhere to the surface to yield coherent responses with these sensors.