The enzyme-linked immunosorbent assay (ELISA) coupled with surface-enhanced Raman spectroscopy (SERS) detection is a growing area of analytical chemistry emerged as a solution to a problem of sensitivity improvement. SERS-based ELISA with horseradish peroxidase as an enzymatic label, o-phenylenediamine (oPD) substrate, and 2,3-diaminophenazine (DAP) enzymatic product was one of the first examples of such systems. Surprisingly, the full potential of this long-known approach has not been revealed yet. The current study addresses a previously neglected problem of SERS detection stage performance. Using silver nanoparticles and model mixtures of oPD and DAP, the effects of pH, concentration of aggregating agent, and particle surface chloride stabilizer were extensively evaluated. At optimal mildly acidic pH of 3, 0.93 to 1 M citrate buffer, and AgNPs stabilized with 20 mM chloride, at least two orders of magnitude advantages in the detection limits of SERS over colorimetry both for DAP and HRP were demonstrated. We suppose that this improved detection system could become a useful tool for the development of SERS-based ELISA protocols.

Immunosorbent turnip vein clearing virus (TVCV) particles displaying the IgG-binding domains D and E of Staphylococcus aureus protein A (PA) on every coat protein (CP) subunit (TVCVPA) were purified from plants via optimized and new protocols. The latter used polyethylene glycol (PEG) raw precipitates, from which virions were selectively re-solubilized in reverse PEG concentration gradients. This procedure improved the integrity of both TVCVPA and the wildtype subgroup 3 tobamovirus. TVCVPA could be loaded with more than 500 IgGs per virion, which mediated the immunocapture of fluorescent dyes, GFP and active enzymes. Bi-enzyme ensembles of cooperating glucose oxidase and horseradish peroxidase were tethered together on the TVCVPA carriers by a single antibody type, with one enzyme conjugated chemically to its Fc region, and the other one bound as target, yielding synthetic multienzyme-complexes. In microtiter plates, the TVCVPA-displayed sugar-sensing system possessed a considerably increased reusability upon repeated testing, compared to the IgG-bound enzyme pair in the absence of the virus. A high coverage of the viral adapters was achieved also on Ta2O5 sensor chip surfaces coated with a polyelectrolyte interlayer, as a prerequisite for durable, TVCVPA-assisted electrochemical biosensing through modularly IgG-assembled sensor enzymes.