Aluminum nanoparticles as substrates for metal-enhanced fluorescence in the ultraviolet for the label-free detection of biomolecules

Abstract

We use finite-difference time-domain calculations to show that aluminum nanoparticles are efficient substrates for metal-enhanced fluorescence (MEF) in the ultraviolet (UV) for the label-free detection of biomolecules. The radiated power enhancement of the fluorophores in proximity to aluminum nanoparticles is strongly dependent on the nanoparticle size, fluorophore-nanoparticle spacing, and fluorophore orientation. Additionally, the enhancement is dramatically increased when the fluorophore is between two aluminum nanoparticles of a dimer. Finally, we present experimental evidence that functionalized forms of amino acids tryptophan and tyrosine exhibit MEF when spin-coated onto aluminum nanostructures.

Figure 1

Results for a d = 20 nm aluminum nanoparticle. (a) Extinction, scattering, and absorption efficiencies. Inset: complex dielectric constant of aluminum. (b) Radiated power enhancement for dipoles spaced s = 5 and 10 nm from the nanoparticle calculated with the FDTD method (solid curves), and with analytical theory (dashed curves). The dipoles are oriented perpendicular to the metal surface.

Figure 2

Results for a d = 80 nm aluminum nanoparticle: (a) Extinction, scattering, and absorption efficiencies; (b) radiated power enhancement for dipoles spaced s = 5 and 10 nm from the nanoparticle. The dipole is oriented perpendicular to the metal surface; (c) similar to (b) but now the dipoles are oriented parallel to the metal surface.

Figure 3

Enhancement measure for emission in 300–420 nm (see text) as a function of aluminum nanoparticle size (d = 20–140 nm). Three dipole orientations are represented: (a) Perpendicular (P); (b) Parallel (L); and Orientation Averaged = (P + 2L)/3.

Figure 4

(a) Enhancement measure for emission in 300–420 nm at various distances (s = 1–20 nm) from the surface of a d = 80 nm aluminum nanoparticle. Three dipole orientations are considered, as in the caption for Figure 3. (b) Enhancement measure for a dipole located in the middle of an 80 nm aluminum nanoparticle dimer with various surface-surface particle spacings (2s = 2–40 nm). Inset: Radiated power enhancement of a perpendicularly oriented dipoledimer system with 2s = 4 nm.

Figure 5

Radiated power enhancement of d = 80 nm aluminum nanoparticle systems with perpendicular fluorophore orientation. The horizontal axis denotes the system where “M” denotes an isolated monomer and “D” denotes a dimer. The number following “M” is the fluorophore-surface spacing, s (nm), and the number after “D” is the surface-surface distance, 2s, between the particles (nm). The fluorophore is at the midpoint of the dimer.

Figure 6

Near-field intensities arising from a point dipole radiating at 350 nm: (a) Isolated dipole; (b) Dipole between two 80 nm diameter aluminum nanoparticles with surface-surface particle spacing 2s = 4 nm; (c) Near-field enhancement/quenching obtained by dividing Figure 6b by Figure 6a. Images are on a log scale, and the dipole is oscillating along the x-axis, which corresponds to the perpendicular orientation described in the text.

Figure 7

Fluorescence spectra of functionalized amino acids contained in a 15 nm thick PVA film on top of a 10 nm thick aluminum film and on a quartz control. Insets are the corresponding intensity decays: (a) Tryptophan (NATA); (b) Tyrosine (NATA-tyr). IRF is the Instrument Response Function.