Silica-Stabilized Gold Island Films for Transmission Localized Surface Plasmon Sensing

Abstract

Ultrathin gold films prepared by evaporation of sub-percolation layers (typically up to 10 nm nominal thickness) onto transparent substrates form arrays of well-defined metal islands. Such films display a characteristic surface plasmon (SP) absorption band, conveniently measured by transmission spectroscopy. The SP band intensity and position are sensitive to the film morphology (island shape and inter-island separation) and the effective dielectric constant of the surrounding medium. The latter has been exploited for chemical and biological sensing in the transmission localized surface plasmon resonance (T-LSPR) mode. A major concern in the development of T-LSPR sensors based on Au island films is instability, manifested as change in the SP absorbance following immersion in organic solvents and aqueous solutions. The latter may present a problem in the use of Au island-based transducers for biological sensing, usually carried out in aqueous media. Here, we describe a facile method for stabilizing Au island films while maintaining a high sensitivity of the SP absorbance to analyte binding. Stabilization is achieved by coating the Au islands with an ultrathin silica layer, ca. 1.5 nm thick, deposited by a sol-gel procedure on an intermediate mercaptosilane monolayer. The silica coating is prepared using a modified literature procedure, where a change in the reaction conditions from room temperature to 90 degrees C shortened the deposition time from days to hours. The system was characterized by UV-vis spectroscopy, ellipsometry, XPS, HRSEM, AFM, and cyclic voltammetry. The ultrathin silica coating stabilizes the optical properties of the Au island films toward immersion in water, phosphate buffer saline (PBS), and various organic solvents, thus providing proper conditions where the optical response is sensitive only to changes in the effective dielectric constant of the immediate environment. The silica layer is thin enough to afford high T-LSPR sensitivity, while the hydroxyl groups on its surface enable chemical modification for binding of receptor molecules. The use of silica-encapsulated Au island films as a stable and effective platform for T-LSPR sensing is demonstrated.

Preparation of Gold Nanoisland Arrays from Layer-by-Layer Assembled Nanoparticle Multilayer Films

“…Different gold nanoisland films were generated from Au NPs-PAH multilayer films with two, five or eight layers of gold nanoparticles after annealing at ~ 500 o C. The optical properties of the films were examined using transmission UV-vis spectroscopy (Figure 3), showing strong SP bands around 533, 541, and 542 nm for nanoisland films generated from Au NPs-PAH 2-, 5-, and 8-nanoparticle layer films, respectively. Similar SP bands have been observed from Rubinstein's vacuum evaporated films after annealing at 200 -250 o C. 19,21 Their study showed that the increase in the optical intensity was observed upon increasing the nominal thickness of the evaporated island films. UV-vis spectra of our nanoisland films generated from Au NPs-PAH multilayer films with 2-, 5-, or 8-layers of gold nanoparticles also showed a strong dependence of the localized gold SP band intensity on the composition of gold island films.…”

“…[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] In LSPR sensing, the spot size can be minimized to a small number of individual sensing elements with an in-plane width of less than ~ 100 nm. Therefore, LSPR-based nanosensors can be extremely simple, small, and light.…”

“…In addition, the LSPR nanosensors can be used in both transmission mode and reflection mode. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] When UV-vis extinction spectroscopy in transmission geometry is used for sensing, it is known as transmission localized surface plasmon resonance (T-LSPR) sensing. The major advantage of T-LSPR sensing over conventional SPR sensing lies in the low-cost samples and simple experimental setup, which can be reduced to measurements at a single wavelength in a transmission configuration.…”

“…Rubinstein's method involves a preparation of ultrathin (less than 10 nm) semi-transparent gold island films by vacuum evaporation onto inert transparent substrates. [15][16][17][18][19][20][21][22] Van Duyne's nanosphere lithographic techniques involve the fabrication of arrays of triangular silver nanoparticles in the interstices between the elements of the nanosphere deposition mask. [8][9][10][11][12][13][14] The removal of the deposition mask following thermal deposition, pulsed laser deposition, or e-beam deposition of metal leaves triangular nanoisland arrays on the solid substrates.…”

Capturing Irradiation with Nanoantennae: Plasmon‐Induced Enhancement of Photoelectrolysis

“…As a linking primer, (3-aminopropyl)trimethoxysilane (APTMS) is crucial to anchor the Au surface to the silica coating, where one monolayer is adsorbed to the Au driven by the large complexation of amine groups, which is sufficient for homogenous coating 152,153 . The SSVs were modified for 15 minutes with a 1 mM solution of APTMS, in parallel to a bare Au slide as control sample.…”