Plasmonic Metals for Nanohole-Array Surface Plasmon Field-Enhanced Fluorescence Spectroscopy Biosensing

Abstract

Gold (Au) is so far the most commonly used plasmonic metal in nanohole-array-based surface plasmon resonance biosensors, due to its excellent plasmonic properties in visible light range and chemical inertness in solutions. However, Au intrinsically absorbs blue-green light heavily, leading to poor plasmonic excitation efficiency below 600 nm. This work explores the possibilities of using various kinds of plasmonic metals (Au, Ag, Cu, Al) or their composites (Ag/Au, Cu/Au, Al/Au) in nanohole-array surface plasmon field-enhanced fluorescence spectroscopy (SPFS) biosensors, aiming to excite different fluorescent dyes (working in different wavelength ranges) with satisfactory plasmonic performance. It is found that SiO2 anti-tarnish-coated Ag nanohole array is suitable and practical for exciting 400–500-nm fluorescent dyes. The Au-coated Al nanohole array supports a very good plasmon resonance for fluorescence excitation in 600–700 nm, whose field enhancement even exceeds that of pure Au. The combination of Au and Ag is found to be superior to pure Au or Ag for the 500–600- or 700–800-nm fluorescent dyes. These findings provide the guideline on selecting suitable plasmonic metals for nanohole-array SPFS biosensors.