Novel plasmonic field-enhanced nanoassay for trace detection of proteins

• New gated fluorescence resonance energy transfer (GRET) method has been developed.
• By modulating of plasmonic field-enhanced RET, high sensitivities are attained.
• Cyt c and other proteins can be detected by GRET using 5 nm gold nanoparticles.
• Low limit of detection of 370 pM Cyt c has been achieved for dye:Cyt c ratios up to 30:1 (over-labeling).

Recently discovered effects of plasmonic field on molecular fluorescence offer new insights into the optical interactions at the nanoscale which can help solving problems encountered in widely applied fluorescent labeling of biomolecules for studying life processes in biomedicine and pharmacy. In this work, we have focused on exploring a novel sensitivity-enhancing phenomenon based on protein modulation of plasmon-controlled fluorescence. We have demonstrated that a protein (cytochrome c (Cytc c) or bovine serum albumin (BSA)) can be employed to gate fluorescence resonance energy transfer occurring from a fluorescein isothiocyanate fluorescent dye to plasmonic citrate-capped gold nanoparticles. By applying plasmonic field gated by protein, facilitated by the formation of multi-shell nanoparticles, (AuNP@Cit/Cytc-FITC or AuNP@Cit/BSA-FITC), low limits of detection for Cyt c (LOD=370 pM) and for BSA (LOD=1.8 nM) have been achieved even for large fluorophore:protein ratios, up to 30:1 (over-labeling), normally plagued with energy migration and background fluorescence problems. Control experiments confirming adsorption of proteins on AuNPs have been performed using light scattering and piezometric techniques. The proposed nanoassay may be applied in microanalysis of trace amounts of proteins, e.g. in microfluidic devices.