Johanna Suomi

Time-Resolved Detection of Hot Electron-Induced Electrochemiluminescence of Fluorescein in Aqueous Solution

Strong electrogenerated chemiluminescence (ECL) of fluorescein is generated during cathodic pulse polarization of oxide-covered aluminum electrodes and the resulting decay of emission is so sluggish that time-resolved detection of fluorescein is feasible. The present ECL in aqueous solution is based on the tunnel emission of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons and hydroxyl radicals acting as redox mediators. The successive one-electron redox steps with the primary radicals result in fluorescein in its lowest excited singlet state. The method allows the detection of fluorescein (or its derivatives containing usable linking groups to biomolecules) over several orders of magnitude of concentration with detection limits well below nanomolar concentration level. The detection limits can still be lowered, e.g., by addition of azide or bromide ions as coreactants. The results suggest that the derivatives of fluorescein, such as fluorescein isothiocyanate (FITC), can be detected by time-resolved measurements and thus be efficiently used as electrochemiluminescent labels in bioaffinity assays.

Hot Electron-Induced Electrogenerated Chemiluminescence

In this chapter, we discuss the basics of cathodic hot electron-induced electrogenerated chemiluminescence (HECL). In the applications of HECL, we discuss, e.g., the usable electrode materials and their advantages as well as the applicable solution conditions in aqueous media. We also summarize the luminophore types excitable by this method and their usability as labels in practical bioaffinity assay applications.

Electrochemiluminescence of Lanthanides

Electrogenerated luminescence of lanthanides is reviewed with emphasis on the electrochemiluminescence (ECL) of lanthanide chelates. Main application area of lanthanide chelates in this field is their use as electrochemiluminescent labels in bioaffinity assays such as in immunoassays or DNA probe assays. With lanthanide chelates as labels, hot electron-induced ECL at thin insulating film-coated cathodes outperforms ECL based on traditional electrochemistry at active metal electrodes. ECL excitation of lanthanide(III) chelates occurs normally by ligand-sensitized mechanisms wherein the ligand is first excited by redox reactions followed by energy transfer from ligand to the central ion, which finally emits by f–f transitions. Luminescent lanthanide ions can be excited at oxide-coated metal electrodes when these ions act as luminescence centers in the oxide film and/or at the oxide/electrolyte interface or as solvated in the close vicinity of the electrode surface. These ions typically show high field-induced solid-state electroluminescence when embedded inside of the oxide films and ECL at the surface of the electrode or in solution close to the electrode surface. These lanthanide-doped oxide films can be fabricated either by anodization of certain lanthanide-doped valve metals or from pure valve metals by anodization in lanthanide ion-containing anodization bath preferably with AC voltages. Some lanthanide ions can be electrically excited also in strongly acidic sulfuric acid solutions at platinum electrodes with mechanisms not known with certainty.

Miniature Tunneloxide Electrodes on Silicon for Aqueous Hot Electron Electrochemistry and Electrochemiluminecscence Studies

The basic principles of cathodic hot electron-induced electrochemiluminescence (HECL) and hot electron (HE) injection into aqueous electrolyte solution are shortly discussed. The applicability of miniaturized oxide-coated silicon electrodes as working electrodes in detection of electrochemiluminescent labels by HECL is studied. In addition, the fabrication processes of these tunnel oxide electrodes are described, and an immunoassay is used as an example of a real bioaffinity assay carried out using oxide-coated silicon electrodes.