In this case study, we provide a few examples for affinity-sensors based on optical detection concepts and compare them with electronic read-out schemes. We concentrate and briefly summarize two of the most advanced versions in each category: one is a surface-plasmon field-enhanced fluorescence spectroscopic approach, while in the electronic sensing domain we concentrate on graphene-based field-effect transistors as the read-out platform. Both transduction principles are surface-sensitive and-selective, however, with penetration lengths into the analyte solution (e.g., into a flow cell attached) that are very different and that depend on totally different physical principles: while for surface-plasmons the evanescent character of the plasmon mode, propagating along the noble metal-solution interface with a penetration length in the order of 100 nm (for Au/water and a laser wavelength of = 632.8 nm), the "penetration depth" in electronic transistor-based sensing is governed by the Debye length which, for a physiological salt environment, amounts to less than 1 nm. Taking these differences into account, one can optimize the sensor read-out by the appropriate interfacial architecture used to functionalize the transducers by immobilizing one of the affinity interaction partners. We will discuss this for both concepts by giving a few examples of the achievable limit of detection for both methods. The examples discussed include a classical system, i.e., the binding of human chorionic gonadotropin (hCG) to its surface-immobilized antibodies or Fab fragments, the detection of lipopolysaccharides in a tethered bimolecular lipid membrane, and, as an example for small analyte detection by antibodies, the monitoring of aflatoxin B1, a member of the food toxin family of mycotoxins.
- Biosensor Technologies