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In this work, we introduce a new perspective on the development of Localized Surface Plasmon Resonance (LSPR) optical biosensors. Computational simulations, focused on the assessment of the LSPR spectrum and spatial distribution of the electromagnetic field enhancement near a metallic nanoparticle, elucidated the behavior of crucial parameters, as figure of merit, bulk and molecular sensitivity, which governs a LSPR sensor performance. Gold and silver nanospheres were explored as starting point to assess plasmonic optical characteristics of the nanostructured sensor platform. Here, for the first time in the literature, Campbell's model was evaluated exploiting a NP size-dependence approach. The theoretical analyses indicate a nonlinear behavior of the bulk and molecular sensitivity as function of the NP size. Substantial LSPR peak shifts due to the adsorption of molecules layer on a NP surface were observed for nanoparticles with ~5 nm and ~40 nm radius. Moreover, on molecular sensing, LSPR peak shift is also determined by the thickness of adsorbed molecular shell layers. We observed that for 40 nm radius gold and silver nanospheres, significant LSPR peak shift could be induced by small (few nm) thickness change of the adsorbate shell layer. Moreover, this work provides insights on the LSPR behavior due to adsorption of molecular layer on a NP surface, establishing a new paradigm on engineering LSPR biosensor. Furthermore, the proposed approach can be extended to engineer an efficiently use of different nanostructures on molecular sensing.

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The localized surface plasmon resonance (LSPR) due to light–particle interaction and its dependence on the surrounding medium have been widely manipulated for sensing applications. The sensing efficiency is governed by the refractive index-based sensitivity (ηRIS) and the full width half maximum (FWHM) of the LSPR spectra. Thereby, a sensor with high precision must possess both requisites: an effective ηRIS and a narrow FWHM of plasmon spectrum. Moreover, complex nanostructures are used for molecular sensing applications due to their good ηRIS values but without considering the wide-band nature of the LSPR spectrum, which decreases the detection limit of the plasmonic sensor. In this article, a novel, facile and label-free solution-based LSPR immunosensor was elaborated based upon LSPR features such as extinction spectrum and localized field enhancement. We used a 3D full-wave field analysis to evaluate the optical properties and to optimize the appropriate size of spherical-shaped gold nanoparticles (Au NPs). We found a change in Au NPs’ radius from 5 nm to 50 nm, and an increase in spectral resonance peak depicted as a red-shift from 520 nm to 552 nm. Using this fact, important parameters that can be attributed to the LSPR sensor performance, namely the molecular sensitivity, FWHM, ηRIS, and figure of merit (FoM), were evaluated. Moreover, computational simulations were used to assess the optimized size (radius = 30 nm) of Au NPs with high FoM (2.3) and sharp FWHM (44 nm). On the evaluation of the platform as a label-free molecular sensor, Campbell’s model was performed, indicating an effective peak shift in the adsorption of the dielectric layer around the Au NP surface. For practical realization, we present an LSPR sensor platform for the identification of dengue NS1 antigens. The results present the system’s ability to identify dengue NS1 antigen concentrations with the limit of quantification measured to be 0.07 μg/mL (1.50 nM), evidence that the optimization approach used for the solution-based LSPR sensor provides a new paradigm for engineering immunosensor platforms.