Metal-Enhanced Fluorescence from Silver Nanowires with High Aspect Ratio on Glass Slides for Biosensing Applications

Abstract

High enhancement of fluorescence emission, improved fluorophore photostability, and significant reduction of fluorescence lifetimes have been obtained from high aspect ratio (>100) silver (Ag) nanowires. These quantities are found to depend on the surface loading of Ag nanowires on glass slides, where the enhancement of fluorescence emission increases with the density of nanowires. The surface loading dependence was attributed to the creation of intense electric fields around the network of Ag nanowires and to the coupling of fluorophore excited states that takes place efficiently at a distance of 10 nm from the surface of nanowires, which was confirmed by theoretical calculations. The enhancement of fluorescence emission of fluorescein isothiocyanate (FITC) was assessed by fluorescence spectroscopy and fluorescence-lifetime imaging microscopy (FLIM) to demonstrate the potential of high aspect ratio Ag nanowires. Fluorescence enhancement factors exceeding 14 were observed on Ag nanowires with high loading by FLIM. The photostability of FITC was the highest on nanowires with medium loading under continuous laser excitation for 10 min because of the significant reduction in the fluorescence lifetime of FITC on these surfaces. These results clearly demonstrate the potential of Ag nanowires in metal-enhanced fluorescence-based applications of biosensing on planar surfaces and cellular imaging.

Scheme 1. Schematic Depiction of (A) Metal-Enhanced Fluorescence from FITC-Labeled Avidin Immobilized on Ag Nanowire-Deposited Glass Slides Using b-BSA, (B) the Distance between the Ag Nanowires and FITC, and (C) Experimental Setup for Metal-Enhanced Fluorescence Studies Based on Fluorescence Emission Spectrum Measurements

Figure 1

Absorbance spectra of (A) Ag nanowires in solution (inset, real-color photograph of Ag nanowires solution before the spray coating process) and (B) blank glass slide and Ag nanowires (low, medium, and high loading) deposited onto glass slides (inset, real-color photographs of Ag nanowires on glass slides.

Figure 2

Low-resolution (A) and high-resolution (B) SEM images of as-deposited Ag nanowire networks on glass slides. (C) Real-color photographs of Ag nanowires on glass substrates to demonstrate the reproducibility of the surfaces.

Figure 3

Electric field intensity (largest predicted value) versus the distance of Ag nanowires using COMSOL Multiphysics: (A) Ag nanowires @ horizontal and vertical array format and (B) Ag nanowires @ horizontal (2 × 3) and vertical (3 × 2) array format to predict the coupling of fluorescence emission (520 nm) to translational mode of surface plasmons. (C) The electric field (E_z) density distribution over the translational cross section of Ag nanowires (the distances between the nanowires are 10 nm). In this configuration, the wavelength of the light source is 520 nm, similar to the fluorescence emission wavelength of FITC, which is expected to be ∼11 nm from the Ag nanowires in all directions, as depicted in Scheme 1

Figure 4

(A) Emission spectrum of fluorescein-labeled avidin on Ag nanowire-deposited glass slides (low, medium, and high loading) and control sample (blank glass slide). Inset: real-color photographs of fluorescence emission from surfaces prepared on the different platforms and control experiments. The measurements were the mean spectra of five separate surface locations for three different runs. (B) Fluorescence emission measurements of fluorescein-labeled avidin on Ag nanowire-deposited glass slides and control sample of 2 × 2 mm² area using FLIM with a dwell time of 1 ms. Emission, 514 ± 30 nm; objective, 20×; NA, 0.4; I, intensity, arbitrary units. Excitation for both types of measurements, 473 nm; EF, enhancement factor = intensity value of Ag nanowires divided by intensity value on blank glass.

Figure 5

(A) Photostability of fluorescein-labeled avidin on Ag nanowire-deposited glass slides (low, medium, and high loading) and control samples. Note: these values are averages of three different measurements. (B) Normalized fluorescence intensity versus time measured in panel A.

Figure 6

Fluorescence lifetime image and histogram for FITC-labeled avidin on (A) blank glass slides and Ag nanowire-deposited glass slides with (B) low loading, (C) medium loading, and (D) high loading (τ, average lifetime).