The use of aluminum nanostructures as platforms for metal enhanced fluorescence of the intrinsic emission of biomolecules in the ultra-violet

Abstract

We consider the possibility of using aluminum nanostructures for enhancing the intrinsic emission of biomolecules. We used the finite-difference time-domain (FDTD) method to calculate the effects of aluminum nanoparticles on nearby fluorophores that emit in the ultra-violet (UV). We find that the radiated power of UV fluorophores is significantly increased when they are in close proximity to aluminum nanostructures. We show that there will be increased localized excitation near aluminum particles at wavelengths used to excite intrinsic biomolecule emission. We also examine the effect of excited-state fluorophores on the near-field around the nanoparticles. Finally we present experimental evidence showing that a thin film of amino acids and nucleotides display enhanced emission when in close proximity to aluminum nanostructured surfaces. Our results suggest that biomolecules can be detected and identified using aluminum nanostructures that enhance their intrinsic emission. We hope this study will ignite interest in the broader scientific community to take advantage of the plasmonic properties of aluminum and the potential benefits of its interaction with biomolecules to generate momentum towards implementing fluorescence-based bioassays using their intrinsic emission.

Figure 1

Schematic of the model radiating fluorophore/metal nanoparticle system studied where: the arrow represents the fluorophore, the fluorophores will be oriented along the x axis; d is the diameter of the aluminum nanoparticle, 2s is the surface to surface distance between dimers (for the monomer case – s is the space between the fluorophore and the surface of the particle), θ is the polar angle from the z-axis where 0≤ θ≤π and Φ is the azimuthal angle in the x–y plane from the x-axis with 0≤Φ<2π. Al NP = aluminum nanoparticle.

Figure 2

Wavelength dependent radiated power enhancement for dipoles spaced s = 5 from d = 20, 40 and 80 nm aluminum nanoparticles calculated using FDTD. The dipoles are oriented perpendicular to the metal surface (oriented along the x-axis).

Figure 3

Total radiated power enhancement in the λ = 300–420 nm range for dipoles at a distance of s = 5 nm from the surface of aluminum nanoparticles of varying diameters (d = 20, 40, 80, 100 and 140 nm).

Figure 4

Computed total radiated power enhancement of emission (integrated around a closed surface containing the system) for different d = 80 nm monomer and dimer aluminum nanoparticles with the perpendicular fluorophore orientation. For the dimer systems, the fluorophore is located midway between the two aluminum particles. Note the enhancement is represented in the log scale (base 10). F = fluorophore.

Figure 5

(a) Near-field image of the field enhancement around a d = 60 nm aluminum nanoparticle dimer spaced 2s = 10 nm apart by its interaction with the plane wave with wavelength 280 nm propagating along the z-axis and polarized along the x-axis; (b) Near-field image of the field enhancement around a d = 80 nm aluminum nanoparticle dimer spaced 2s = 10 nm apart by its interaction with a dipole radiating at 350 nm located in the middle of the dimer. The dipole is oriented along the x-axis. Note all images are displayed in the log scale (base 10).

Figure 6

Experimental fluorescence spectra of a 10 nm thick PVA film on top of a 10 nm thick aluminum film, and on a quartz control that contain: (a) NATA; (b) GMP.