Surface plasmon enhanced fluorescence measurement on flat and constructed gold surfaces

The fluorescence properties of porphyrin self-assembly monolayers (SAMs) and FITC labeled polylysine (PL) on a gold surface under surface plasmon (SP) excitation and the effect of the surface structure were studied for the effective application of SP excitation to fluorescence analysis on a gold surface. The spacer between the chromophores and the gold surface effectively mitigates the fluorescence quenching by gold, which is an unavoidable disadvantage of fluorescence analysis on a gold surface. The decay of the SP field intensity by the spacer was negligible and the chromophores were effectively excited to give enhanced fluorescence. SP excitation provided considerable advantages over the conventional direct photoirradiation. The forbidden Q-bands were clearly identified in the fluorescence excitation spectrum of the porphyrin SAM under SP excitation, but identification by direct photoirradiation was impossible. The background scattered excitation light was effectively eliminated by the SP excitation, which was particularly effective for fluorescein iosthiocyanate labeled PL having a small Stokes shift. The advantages of SP excitation were remarkably enhanced by a two-axis grating structure of the gold surface. Moreover, direct excitation of the porphyrin SAM prepared on the gold-coated two-axis grating and gold microdots on Si showed enhanced fluorescence similarly to the SP excitation, suggesting induction of the edge and/or particle plasmons.