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 (${\eta }_{RIS}$) and the full width half maximum (FWHM) of the LSPR spectra. Thereby, a sensor with high precision must possess both requisites: an effective ${\eta }_{RIS}$ and a narrow FWHM of plasmon spectrum. Moreover, complex nanostructures are used for molecular sensing applications due to their good ${\eta }_{RIS}$ 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, ${\eta }_{RIS}$, 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.

A metal dimeric nanostructure is an alternative design for a Localized Surface Plasmon Resonance sensor. At the dimer nanojunction, a high E-field enhancement (hot spot) is observed. In this work, nanostructure size parameters such as aspect ratio, junction, and length of gold dimeric nanorods (AuDNR) linked end-to-end by a thin dielectric junction were evaluated numerically, optimizing the sensing parameters of the plasmonic nanoplatform. The computational approach focused on the assessment of the LSPR extinction spectral peak shift to their local surrounding medium in order to estimate the behavior of sensing parameters such as the figure of merit (FoM), the bulk sensitivity ( η _B ) and the η _B × FoM. The nanostructure aspect ratio, junction, and rod length were evaluated, optimizing the plasmonic nanoplatforms sensing parameters. The optimized AuDNR achieved a high sensing performance with η _B = 544 nm/RIU and FoM = 12.4 RIU ^-1 . Moreover, the obtained η _B × FoM = 6745.6 nm/RIU ² is the highest reported value in the literature. For molecular sensing, Campbell's model was explored to evaluate the wavelength shift as a result of molecular adsorption on the nanoparticle surface. We demonstrate that the dimeric nanostructure exhibit twice LSPR peak shift due to a thin molecular adsorption layer rather than a monomer of equivalent size. Our results show that optimized dimeric nanostructure is highly attractive for molecular identification, opening the possibility of establishing new nanoplatforms for medical diagnostic.

In this work, we establish a new paradigm on identifying optimal arbitrarily shaped metallic nanostructures for photothermal applications. Crucial thermo-optical parameters that rule plasmonic heating are appraised, exploring a nanoparticle size-dependence approach. Our results indicate two distinct figures of merit for the optimization of metallic nanoheaters, under both non-cumulative femtosecond and continuum laser excitation. As a case study, gold nanorods are evaluated for infrared photothermal conversion in water, and the influence of the particle length and diameter are depicted. For non-cumulative femtosecond pulses, efficient photothermal conversion is observed for gold nanorods of small volumes. For continuous wave (CW) excitation at 800 nm and 1064 nm, the optimal gold nanorod dimensions (in water) are, respectively, 90 × 25nm and 150 × 30 nm. Figure of Merit (FoM) variations up to 700% were found considering structures with the same peak wavelength. The effect of collective heating is also appraised. The designing of high-performance plasmonic nanoparticles, based on quantifying FoM, allows a rational use of nanoheaters for localized photothermal applications.

A modified spatial filtering method that improves the sensitivity of single-beam and mode-mismatched thermal lens spectroscopy (TLS) for fluorescence quantum yield measurement is presented. The method is based on the detection of the external part of a laser beam transmitted by the fluorescent sample (eclipsing detection mode). The experimental results show that the signal/noise (S/N) ratio of the absolute quantum yield of Rh6G can be enhanced up to ~1400% using the eclipsing detection mode on the TLS experimental setup. The method was evaluated by measuring the fluorescence quantum yield of varying concentration of ethanolic solutions of Rhodamine 6G.