The ability to detect disease markers at the single molecule level promises the ultimate sensitivity in clinical diagnosis. Fluorescence-based single-molecule analysis, however, is limited by matrix interference and can only probe a very small detection volume, which is typically not suitable for real world analytical applications. We have developed a microtiter plate immunoassay for counting single molecules of the cancer marker prostate specific antigen (PSA) using photon-upconversion nanoparticles (UCNPs) as labels that can be detected without background fluorescence. Individual sandwich immunocomplexes consisting of (1) an anti-PSA antibody immobilized to the surface of a microtiter well, (2) PSA, and (3) an anti-PSA antibody-UCNP conjugate were counted under a wide-field epifluorescence microscope equipped with a 980 nm laser excitation source. The single-molecule (digital) upconversion-linked immunosorbent assay (ULISA) reaches a limit of detection of 1.2 pg mL (42 fM) PSA in 25% blood serum, which is about ten times more sensitive than commercial ELISAs, and covers a dynamic range of three orders of magnitude. This upconversion detection mode has the potential to pave the way for a new generation of digital immunoassays.

This review compares various types of (bio)electronic tongues. The design and principles of potentiometric and voltammetric electronic tongues are discussed together with applications in food and environmental analysis. Different approaches towards bioelectronic tongue are presented. Several methods for evaluation and interpretation of the measured data are described. Finally, the potential of such devices for analysis of wine is discussed.