Background. Noble metal nanoparticles (NPs) exhibit strong absorption in the NIR-visible-UV range, with the specific wavelength depending on their composition, size, shape and dielectric environment. The absorption is a result of the interaction of the electromagnetic radiation with collective oscillations of charge density in the particles, called plasmons. Plasmons are quasi-particles, the quantization of plasma oscillations, much like phonons are the quantization of vibrational modes. Plasmons can be divided into two main categories: (i) bulk plasmons, inside the metal, are “dark” – spectroscopically inactive; (ii) surface plasmons – charge oscillations at the interface between a metal and a dielectric (such as air). For continuous metal surfaces, surface plasmons can propagate along the interface; coupling light into these modes is not trivial, due to the mismatch of momentum between light and the plasmon.
Typically, a thin metal film is deposited directly on a glass prism, and light is shined through the prism at a specific angle, fulfilling the momentum matching condition. The dependence of the resonance frequency on the dielectric constant of their surroundings has led to plasmonic sensing, and the prism configuration is used for SPR (surface plasmon resonance) sensing. For nanoscale features (such as nanoparticles, or nanosized holes and protrusions), a localized plasmon mode arises. Light can readily couple into this mode, meaning no special geometry is required, and strong absorption is observed. The change in resonance frequency (as well as intensity) upon absorption of even a particle monolayer of ligands on the NP’s surface is considerable, leading to a sensing modality called LSPR (localized surface plasmon resonance) sensing.
Spatial response. Plasmonic (LSPR) sensors are typically characterized by their bulk sensitivity, or refractive index sensitivity (RIS) – the plasmonic particles (usually bound to a substrate) are placed in solvents of different refractive indices, and the change in their absorption peak is noted, per refractive index unit. However, this scenario is very different from a (bio-)sensing application – first, a recognition layer is deposited on the particles, such as specific antibodies; second, the particles are exposed to the sample solution. If the solution contains the specific analyte, it binds to the recognition layer, and its proximity to the plasmonic particles effect a change in their absorption peak. So, in the sensing scenario, we are dealing with response to a change of the refractive index in a thin film, not a bulk solvent. We therefore decided to spatially map the response of plasmonic NPs to adsorbed layers. The response is due to an evanescent field, extending from the NPs into the medium. As the field intensity decays away from the particles, so does the response to adsorbed layers. We used a polyelectrolyte layer-by-layer deposition method, allowing us to build consecutive layers of highly-controlled thickness, measuring the optical response at every step. We found that the exponential decay lengths range from 3 to ~20 nm for particles about 20 to 100 nm in diameter. We discovered the decay length of the response is correlated with the bulk sensitivity (RIS), a finding with serious implications for the optimization of plasmonic sensors. The conclusions of this work guided future work in our lab and elsewhere, both for designing bio-sensors, and for further understanding the physical phenomena involved (Kedem et al., ACS Nano, 2011).
Sensing mode. The optical response of LSPR transducers can be measured in the transmission (absorption) or reflection geometries. In this short work, we characterized the bulk sensitivity (RIS) in either case, and showed that the reflection geometry offers significantly higher sensitivity compared to transmission, by 42% to ~180%, for different systems (Kedem et al., J. Phys. Chem. Lett., 2011).
Comparison to interference-based sensors. Typically, there are multiple, competing ways of sensing an analyte. To meaningfully advance the field, different techniques must be compared and assessed critically. In this invited paper, we quantitatively compared LSPR sensors, to those employing thin-film interference in a Fabry-Pérot interferometer configuration. Employing a combination of experimental and simulated data, we concluded that for nm-scale recognition and analyte layers, LSPR transducers offer higher sensitivities, especially in wet conditions; for thicker layers, interference transducers become favorable (Kedem et al., Ann. Phys. (Berlin), 2012).
Long-range oscillatory response. In our earlier work, we have shown that the response decays within a few tens of nanometers. However, several literature reports presented oscillations of the response (the resonance wavelength) of NP films for layers hundreds of nanometers thick, far beyond what would be theoretically plausible for a localized plasmon response. We replicated these findings with thick layers of polyelectrolytes or vacuum-deposited silica, and collaborated with Takumi Sannomiya, a theorist at the Tokyo Institute of Technology, to solve the riddle. We found that the response is a convolution of a short-range plasmonic response, with a long-range interference effect – the air/silica interface at the top, and the gold NP/silica interface at the bottom are both partially reflective, and form a Fabry-Pérot interferometer, or etalon. The thickness of the dielectric layer controls the distribution of light energy in the different modes (absorption, transmission and reflection), thus modulating the observed spectrum. Though the bottom surface is composed of individual NPs, which do not couple in the near-field (their plasmonic responses for thin film are independent of one another, rather than extended plasmon modes), they do couple in the far-field, forming a partially reflective mirror. This mirror differs from simple metallic films in its spectral response, and in imparting a phase-shift on the reflected light that is different from that of a full mirror (Kedem et al., J. Phys. Chem. C, 2012)
Review. In this invited Feature Article, we reviewed several topics in LSPR sensing with relation to our own work. We chose topics we felt were under-covered in previous reviews, but that are of critical importance to the field. We discussed the spatial response, attempts by our group and others to measure it, and its importance to constructing optimal sensors; sensing modes, that is, reflection and transmission geometries; comparisons to other sensing methods; sandwich configurations (e.g., attaching small NPs to the analyte, after it binds to the NP film); and alternative plasmonic materials, such as copper. We also analyzed data from a paper measuring the decay length of Au nanorods of different shapes and sizes, and found a possibly-linear correlation between decay length and RIS (bulk sensitivity), in agreement with our earlier findings (Kedem et al., J. Phys. Chem. C, 2014).
Metal-enhanced fluorescence. Metallic surfaces and particles can interact with fluorescent species, and modify their properties. Here, we used Au NP films, coated with dielectric layers of tunable thicknesses, and topped with a monolayer of the fluorophore tris(bipyridine)ruthenium(II). We found multiple distance-dependent effects, with a long-range interaction, mediated by thin-film interference, as we explored in previous work. We found that the NP film can quench the emission for very close NP-fluorophore separations (~2 nm), but enhances its magnitude (by a factor of 4-5) for longer separations, up to hundreds on nanometers.
This is a result of the NPs interacting and acting as a far-field mirror; individual NPs only interact with fluorophores up to a distance of a few tens of nanometers. The radiative lifetime of the fluorophore is shortened due to the interaction. We also observed an effect previously predicted in theory studies, but not shown experimentally – the emission wavelength and peak widths were modified by the interaction (Kedem et al., Nanoscale, 2014; further explored in a book chapter: Kedem. In Surface Plasmon Enhanced, Coupled and Controlled Fluorescence, Ed: Chris D. Geddes, Wiley & Sons, 2017).
Stabilization of Ag plasmonic NPs. There are multiple techniques used to produce plasmonic NP films – NPs can be directly patterned on surfaces using e-beam lithography; deposited through a plastic microsphere film; a thin (sub-percolation) layer of metal can be deposited and then thermally annealed, with de-wetting producing individual particles. The studies above all used this latter, deposition and annealing technique, producing Au nano-islands. Solution-phase synthesis of particles offers fine control over particle shape and size, and they can then be deposited on a surface. However, when that surface is dried, particles tend to aggregate, producing undesirable clusters and assemblies. In this study, led by my colleague Yulia Chaikin, we developed a sol-gel technique to stabilize solution-deposited Ag or Au NP films, preserving their morphology as individual particles. The technique coats the particles in a thin (3.0-3.5 nm) film of silica, which stabilizes them, and also protects them from corrosion (relevant for Ag). The coating is thin enough to allow sensing applications, as the decay length of the particles is significantly longer (Chaikin et al., Anal. Chem., 2013).