(174m) Aluminum Thin Film Enhanced Native Fluorescence for Biosensors in the UV Spectral Region

Native fluorescence of biomolecules
such as amino acids, proteins and nucleic acids resides in the ultraviolet (UV)
region of the spectrum. However, the quantum yield and photon stability of
native fluorescence are poor compared to organic dye molecules, thus hindering
the development of biosensors based on native fluorescence. Interaction of the
fluorophores with the metal surface can improve quantum yield, photon stability
and total fluorescence yield of fluorophores. Until now, metal enhanced fluorescence
has been observed mostly on silver or gold structures. However, gold and silver
shows strong absorption in the UV range which makes them unsuitable for native
fluorescence enhancement. Aluminum (Al) has a strong plasmonic response at
wavelengths ≦ 400 nm
and here we report enhancement of native fluorescence of tryptophan molecules
on sputtered thin Al film.

We have studied UV excited fluorescence from spin-coated tryptophan on substrates
using UV spectrometer. Figure 1 shows fluorescence spectra for 1 mM tryptophan
on silicon wafer and 20-nm-thick aluminum film with a series of 0.5 sec. The highest
enhancement factor is about (26.5±3)-fold
on 20-nm-thick aluminum film (Figure 2). This enhancement factor is a product
of excitation rate enhancement (f_i) and quantum yield enhancement (f_Φ). In addition, we observed
photobleaching of tryptophan on aluminum substrates with increasing thicknesses
(Figure 3). Each data point is obtained from integrating fluorescence spectra
from 300-360 nm. Al substrates have a much larger integrated fluorescence yield
compared to the control sample (silicon substrate). Figure 4 presents the total
enhancement factor investigated by integrating total intensity according to
time. The results show a (5.2±0.5)-fold
enhancement on 20-nm-thick aluminum film. This enhancement factor is a product
of radiative rate with the inverse of photon bleaching rate. Our results have
demonstrated enhancement of fluorescent count rate and total fluorescence yield
with a sputted thin Al film, which paves way for label free Biosensing.

Figure 1. Fluorescence spectra for 1 mM tryptophan on
(a) silicon and (b) 20-nm-tick aluminum film.

Figure 2. Enhancement factor of 1 mM tryptophan
according to the aluminum thickness.

Figure 3. Photostability curves for 1 mM tryptophan on
silicon and the varied thickness of aluminum film.

Figure 4. Total enhancement factor of 1 mM tryptophan
according to the aluminum thickness.