Probing dynamically tunable localized surface plasmon resonances of film-coupled nanoparticles by evanescent wave excitation

Abstract

The localized surface plasmon resonance (LSPR) spectrum associated with a gold nanoparticle (NP) coupled to a gold film exhibits extreme sensitivity to the nanogap region where the fields are tightly localized. The LSPR of an ensemble of film-coupled NPs can be observed using an illumination scheme similar to that used to excite the surface plasmon resonance (SPR) of a thin metallic film; however, in the present system, the light is used to probe the highly sensitive distance-dependent LSPR of the gaps between NPs and film rather than the delocalized SPR of the film. We show that the SPR and LSPR spectral contributions can be readily distinguished, and we compare the sensitivities of both modes to displacements in the average gap between a collection of NPs and the gold film. The distance by which the NPs are suspended in solution above the gold film is fixed via a thin molecular spacer layer and can be further modulated by subjecting the NPs to a quasistatic electric field. The observed LSPR spectral shifts triggered by the applied voltage can be correlated with angstrom scale displacements of the NPs, suggesting the potential for chip-scale or flow-cell plasmonic nanoruler devices with extreme sensitivity.

Figure 1

Darkfield (DF) microscopy study of the uniformity of the localized surface plasmon resonance (LSPR) of 60nm gold NPs deposited on a 30nm gold film (5nm Cr adhesion layer), and the influence of NP density. NPs are attached to the gold film via electrostatic attraction to a single self assembled PAH molecular layer, resulting in an average gap distance of 6 Angstroms (top left panel drawing). This drawing is not to scale, as the actual gap dimension is less than 1/60 of the NP diameter. DF microscope color images of the gold NPs at reproducible standardized deposition concentrations are shown: on glass (5 μm scale bar), 0.05x on gold film, 0.1x on gold film, 0.2x on gold film, 1x on gold film. Insets highlight the doughnut image resulting from the gold film polarization effect on the NP scattering. The white circle represents approximately the aperture size through which the scattering spectra are collected. B) Representative scattering spectra acquired from film-NPs samples at concentrations of 0.05x (olive), 0.1x (maroon), 0.2x (orange), 1x (red) and for the same gold NPs on a glass slide (green) for comparison. Plot inset: Ray diagram describing the un-polarized illumination and collection conditions for DF microscope LSPR scattering characterization (S: source, D: detector).

Figure 2

Reflectivity measurements (Fig B inset: experimental geometry) of the surface of the 30nm gold film prepared with a 1x concentration 60nm gold NP surface coverage reveal the cumulative LSPR of a large number of NPs in the beam spot. A) Reflectivity curves for film-NPs samples (all at the same 1x NP concentration) with LBL assembled poly-electrolyte (PE) multi-layers used to space NPs at varying gap distances (d). There are two distinct distance-dependent trends apparent, which are shown in B and C. B) The LSPR position (calculated by centroid- center of mass of the bottom 80% of the resonance peaks shown in A) red-shifts, following a power law, with decreasing film-NPs average gap distance. A curve fit with the function y = 644.43 * d ^−0.058, (R=.99147, where y= LSPR wavelength), is shown with the data. C) The peak amplitude of the LSPR response also increases with decreasing film-NPs average gap distance. The film-NPs based LSPR nanoruler sensor we propose would operate in the regime (gray ovals) where both the amplitude and the spectral response of the signal are strongest.

Figure 3

Reflectivity measurements of the film-coupled NPs illustrate NP concentration dependence of the LSPR amplitude. All film-NPs samples shown here have a single PAH spacer layer that averages 6 Angstroms in thickness. A) Reflectivity spectra (normalized by bare gold film reflectivity) at various NP concentrations: using the same geometry (upper inset drawing top) as described in Figure 2. This illumination geometry results in excitation of the LSPR (red field lines) of the film-NPs, as drawn in the bottom of the upper inset, and cannot directly couple to the gold film SPR. B) TIR spectra from the same samples (normalized by glass slide reflectivity) using the collection geometry and 70° off-normal P-polarized sub-surface illumination described in the top right inset. In this case, we have fixed a flow cell to the top of each of the samples and filled it with water, altering the local refractive index from n=1.0 (air) to n=1.33. As represented in the lower left inset drawing: TIR geometry illumination results in evanescent component excitation of both the SPR (brown field lines) of the gold film and LSPR (red field lines) of the film-NPs. With no NPs present, there is a strong dip in the reflectivity at 850 nm associated with SPR excitation. However, as the NP concentration grows, one can observe both the appearance of an LSPR response and the red-shifting of the film SPR. Scanning electron microscope (SEM) images of the standardized film-NPs concentrations are shown at right. Particle counting reveals that the surface coverage (relative to maximum packing density) of the NPs are 1x = 2.2%, 0.2x = 0.42%, 0.1x = 0.22% and 0.05x = 0.11%.

Figure 4

The TIR spectrum of the film-NPs (prepared with 1x concentration NP surface coverage and a single PAH spacer layer on a 30nm gold film) displays an LSPR shift when the negatively charged citrate stabilized gold NPs are driven up and down by electrophoretic forces in an applied electric field. The experimental geometry of the signal collection is equivalent to Figure 3B. A) A +/− 1 Volt square wave (0.5Hz) is applied between the gold film and the top of the flow cell. We define Polarity 1 as a positive voltage at the gold film and Polarity 2 as a negative voltage at the gold film. B) Spectra acquired during Polarity 1 (1 Volt, 2.2V/mm) phase reflect a very large (~30nm) red-shift and amplitude increase of the LSPR compared to the Polarity 2 phase. This is understood to be the negatively charged NPs migrating away from the negative electrode and towards the positive gold film in Polarity 1, reducing the average film-NPs gap distance and increasing the plasmonic coupling. Conversely, for Polarity 2 the blue-shifting LSPR represents NP displacement away from the gold film. The spectral position of the SPR remains relatively unchanged.

Figure 5

The LSPR peak position of the film-NPs (prepared with 1x concentration NPs) oscillates at the frequency of the applied AC field as the gold NPs are driven up and down primarily by an electrophoretic force. The experimental geometry of the signal collection is equivalent to Figure 4. A) A +/− 500mV square wave (0.5Hz) is applied between the gold film and the top of the flow cell (here V represents the voltage at the gold film). The LSPR peak position (solid line, including data points) is plotted along with the estimated gap distance (dashed line) for film-coupled NPs attached via a single PAH layer (B, E), a self-assembled monolayer (SAM) of 11-amino-1-undecanethiol (C11 amine thiol, C, F) and a SAM of C11 EG6 amine thiol (D, G).