Fluorescence lifetime based characterization of active and tunable plasmonic nanostructures

Abstract

We report a non-contact method that utilizes fluorescence lifetime (FL) to characterize morphological changes of a tunable plasmonic nanostructure with nanoscale accuracy. The key component of the plasmonic nanostructure is pH-responsive polyelectrolyte multilayers (PEMs), which serve as a dynamically tunable "spacer" layer that separates the plasmonic structure and the fluorescent materials. The validity of our method is confirmed through direct comparison with ellipsometry and atomic force microscopy (AFM) measurements. Applying the FL-based approach, we find that a monolayer polycation film responds to pH changes with significantly less hysteresis than a thicker multilayer film with polyelectrolytes of both charges. Additionally, we characterize an active and tunable plasmonic nanostructure composed of self-assembled fluorescent dye (Texas Red), pH-sensitive PEMs, and gold nanospheres adsorbed on the PEM surface. Our results point towards the possibility of using stimulus-sensitive polymers to construct active and tunable plasmonic nanodevices.

Tunable gap plasmons in gold nanospheres adsorbed into a pH-responsive polymer film

“…Plot of the effective contribution of each PSS/PAH bilayer (except that "bilayer" 0 is taken to consist only of the initially deposited PAH monolayer) to the gap for films prepared at pH 9.3, extracted from the data in Figure 9(a). As comparison, previously published bilayer thicknesses in identically prepared films are shown in red, with data taken from Itano et al [1] and Ashry et al [10] The black error bars indicate errors taking into account the known index of refraction range in our PEM films, while the magenta error bars reflect errors calculated using the experimental data in Fig 6.…”

“…This is illustrated in Figure 11, where the contribution of the initial PAH layer and all four subsequent bilayers in the films prepared at pH 9.3 are plotted. It is instructive to compare these values to the bilayer thickness in unstrained films of the same type, which are indicated in red in the figure, and taken from Itano et al [1] as well as from our own previously published measurements (Ashry et al) [10]. The value from Itano comes from ellipsometry measurements on ~50 nm thick films in the dry state, while the Ashry number comes from AFM measurements on 0.5 and 1.5 bilayer films in the wet deswelled state.…”

Active molecular plasmonics: tuning surface plasmon resonances by exploiting molecular dimensions

“…One of the primary principles of AMP is to harness the externally controllable changes in the structural, electrical, and/or dielectric properties of organic materials for the dynamic tuning of SPs in terms of resonance wavelength, phase and/or amplitude, which is also the subject of this review. So far, a wide range of organic materials, including simple molecules, supramolecules, macromolecules, and polymers, has enabled AMP with various device configurations and applications [45][46][47][48][49][50][51]. It is noted that there is another important constituent part in the field of AMP, which involves the synergistic integration of plasmonic nanostructures with active gain media, such as dye molecules [52][53][54][55].…”