Limits of the Effective Medium Theory in Particle Amplified Surface Plasmon Resonance Spectroscopy Biosensors

Abstract

The resonant wave modes in monomodal and multimodal planar Surface Plasmon Resonance (SPR) sensors and their response to a bidimensional array of gold nanoparticles (AuNPs) are analyzed both theoretically and experimentally, to investigate the parameters that rule the correct nanoparticle counting in the emerging metal nanoparticle-amplified surface plasmon resonance (PA-SPR) spectroscopy. With numerical simulations based on the Finite Element Method (FEM), we evaluate the error performed in the determination of the surface density of nanoparticles σ when the Maxwell-Garnett effective medium theory is used for fast data processing of the SPR reflectivity curves upon nanoparticle detection. The deviation increases directly with the manifestations of non-negligible scattering cross-section of the single nanoparticle, dipole-dipole interactions between adjacent AuNPs and dipolar interactions with the metal substrate. Near field simulations show clearly the set-up of dipolar interactions when the dielectric thickness is smaller than 10 nm and confirm that the anomalous dispersion usually observed experimentally is due to the failure of the effective medium theories. Using citrate stabilized AuNPs with a nominal diameter of about 15 nm, we demonstrate experimentally that Dielectric Loaded Waveguides (DLWGs) can be used as accurate nanocounters in the range of surface density between 20 and 200 NP/µm2, opening the way to the use of PA-SPR spectroscopy on systems mimicking the physiological cell membranes on SiO2 supports.

Electromagnetic model of a nanodipole array above a double‐layer graphene by periodic green's function

“…In the analysis of SPR structures, the emergence of dominant plasmonic modes is necessary, as these will be directly associated with the electromagnetic response of these devices. In addition, in practical applications of plasmonic sensors, the sample containing the set of active nanoparticles has a low concentration of analytes, and therefore it is plausible to consider electromagnetic interactions of a few wavelengths [13].…”

“…A conventional SPR sensor based on the surface plasmon coupled emission (SPCE) configuration consists of a multilayer structure (prism/metal/dielectric) coupled to a microfluidic channel where immobilised active particles are excited by an external electromagnetic source [12–14]. Due to the nanometric size of these particles, the radiation‐matter coupling is typically dominated by radiation in the dipolar mode [15], thus, these active particles re‐radiate waves that are coupled at the metal/dielectric interface, exciting surface plasmon polaritons (SPPs) waves, which in turn couple the dipolar radiation through the metallic material in the form of highly polarised directive TM waves in the region of the dielectric prism [16].…”

Metamaterial Lensing Devices

“…In recent years, surface plasmons and related devices [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] have been thoroughly investigated due to their potentially wide applications in nanophotonics [24,25,26,27,28,29,30,31,32,33,34,35,36,37,38], biology [39,40,41,42,43,44,45], spectroscopy [46,47,48,49,50,51], and so on. They are capable of manipulating electromagnetic waves [52,53,54,55,56,57,58,59,60,61,62] at the nanometer scale to achieve all-optical integration, providing an effective way to develop smaller, faster and more efficient devices.…”

Surface Plasmon Resonance Spectroscopy for the Characterization of Nanoparticles, Organic Thin Film and 2-D Materials

“…The detection of nanostructured material by SPR spectroscopy in the Kretschmann configuration was boosted in the last years due the utility of PA-SPR spectroscopy in optical sensors [31,40,41]. Anyway, excluding the results presented in this dissertation, there are no reports in literature about the use of SPR spectroscopy in the accurate characterization of nanomaterials, commonly conducted by expensive techniques such as Dynamic Light Scattering (DLS) [42],…”

“…In the first case, the enhancement of the sensitivity of the optical sensors is attributed to the equilibrium between the work functions of the metal and the SLG (not oxidized), which induces an electron transfer between the two structures, changing the polarizability of the hybrid thin film supporting the plasma wave [28]. In the case of PA-SPR spectroscopy, the enhancement in the sensitivity is due to the big mass that any metal nanoparticle has in comparison with small molecular analites, and its range of applicability is limited only by difficulties associated to the accurate counting the number of nanoparticles deposited over the external surface of the sensor [31]. Interestingly, SPR spectroscopy has been rarely used for the ellipsometric characterization of thin films, and its use has been limited to the simultaneous measurements of the thickness and refractive index of transparent organic ultra-thin films deposited by self-assembling techniques [21].…”