Ultrahigh sensitive refractive index nanosensors based on nanoshells, nanocages and nanoframes: effects of plasmon hybridization and restoring force

www.nature.com/scientificreports OPEN Ultrahigh sensitive refractive index nanosensors based on nanoshells, nanocages and nanoframes: effects of plasmon hybridization and restoring force MirKazem Omrani1*, Hamidreza Mohammadi1,2* & Hamidreza Fallah1,2 In this study, the effect of the plasmon hybridization mechanism on the performance and refractive index (RI) sensitivity of nanoshell, nanocage and nanoframe structures is investigated using the finitedifference time-domain simulation. To create nanocage structure, we textured the cubic nanoshell surfaces and examined the impact of its key parameters (such as array of cavities, size of cavities and wall thickness) on the nanocage’s RI-sensitivity. Synthesis of the designed nanocages is a challenging process in practice, but here the goal is to understand the physics lied behind it and try to answer the question “Why nanoframes are more sensitive than nanocages?”. Our obtained results show that the RI-sensitivity of nanocage structures increases continuously by decreasing the array of cavities. Transforming the nanocage to the nanoframe structure by reducing the array of cavities to a single cavity significantly increases the RI-sensitivity of the nanostructure. This phenomenon can be related to the simultaneous presence of symmetric and asymmetric plasmon oscillations in the nanocage structure and low restoring force of nanoframe compared to nanocage. As the optimized case shows, the proposed single nanoframe with aspect ratio (wall length/wall thickness) of 12.5 shows RI-sensitivity of 1460 nm/RIU, the sensitivity of which is ~ 5.5 times more than its solid counterpart. Noble metallic nanoparticles have received vast applications in the fields of s ensors1,2, photodetectors3, plasmonic solar cells4–6, cancer treatment and t herapy7,8, etc. due to one of their capabilities; localized surface plasmon resonances (LSPRs) generation, and ability of light localization in nanoscale9. LSPR is the result of collective oscillations of conduction electrons on the surface of metallic nanoparticles which are induced by electromagnetic fields of the incident light10. Generation of LSPR enables the strengthening of electromagnetic fields, absorption and scattering of light, which depends on the shape, size, and chemical composition and environment of the nanoparticles1,11–13. The dependence of the LSPR properties of metallic nanoparticles on their surrounding medium is indeed the basic principle for the use of nanoparticles for refractive index (RI) nanosensors. A red- or blue-shift phenomenon may occur in LSPR wavelength when the refractive index of the local environment is changed. This feature of metallic nanoparticles allows us to design optical nanosensors for the detection of the chemical changes such as protein i nteractions14, antibodies15 in molecular dimensions for applications involved in biomarker for Alzheimer’s d isease16 and so on. The RI-sensitivity can potentially be tuned and controlled by key parameters of nanoparticles such as shape and size to achieve high sensitivity n anosensors11,12. In this regard, chen et al. investigated the solid gold nanostructures including nano-rods, nano-cubes, nano-spheres, nano-bipyramides and nano-branches and reported their RI-sensitivity in the range of 44–703 nm/RIU; the lower sensitivity is for 50 nm nano-sphere and the upper bound is reached by 80 nm nano-branch11. Khan et al. introduced aspect ratio (R) as a key parameter that controls the solid nanoparticle sensitivity (S) following an empirical equation, S = 46.87 × R + 109.37. They believe that the correlation between shape and sensitivity is much weaker than that between aspect ratio and s ensitivity12. Reviews show that there are two methods to increase the RI-sensitivity of solid nanoparticles: lengthen the nanoparticles and sharpen its a pexes11,12. 1 Department of Physics, University of Isfahan, P.O. Box 81746‑7344, Isfahan, Iran. 2Quantum Optics Research Group, University of Isfahan, Isfahan, Iran. *email: ; Scientific Reports | (2021) 11:2065 | https://doi.org/10.1038/s41598-021-81578-w 1 Vol.:(0123456789) www.nature.com/scientificreports/ On the other hand, hollow nanostructures showed that they can achieve ultra-high sensitivities thanks to their better plasmonic properties, based on the plasmon hybridization mechanism17,18. To describe the plasmon hybridization mechanism, Prodan et al. considered a nanoshell including an inner cavity and an outer spherical surface, having different resonance f requencies19. The cavity plasmons interact with the sphere plasmons thanks to finite thickness of the nanoshell. The strength of this interaction could be adjusted by manipulating the nanoshell thickness. Due to this interaction, the plasmonic oscillations of the nanoshell is split into symmetric and asymmetric oscillations, symmetric oscillation occurs at smaller frequencies and hence has lower energy than the asymmetric ones. Unlike asymmetric oscillation, which is considered as a dark mode and does not couple to the far-field radiation, the symmetric oscillations are coupled with the external optical fields and have greater RI-sensitivity than asymmetric plasmonic oscillations19. The hybridization model has a significant role in the RI-sensitivity improvement of metallic nanoparticles and its validity is tested by quantum mechanical calculations and also by FDTD s imulations18,20. The influence of nanoshell thickness on the plasmonic hybridization mechanism is studied by H alas19. Her results show that the energy gap between two modes of hybrid surface plasmons (symmetric and asymmetric modes) increases as the nanoshell thickness decreases and hence frequency shift (with respect to solid nanoparticle) is larger for thinner nanoshells. Accordingly, the spherical nanoshells are the simplest structures in complex hollow structures which hybridization model could describe. Progressive developments in the nanoparticle s ynthesis21,22 have introduced complex nanostructures with high degree of RI-sensitivity23,24. Among them, high potential structures of nanocage and nanoframe can be mentioned. In this way, Yugang Sun and Younan Xia have recorded a sensitivity of 408.8 nm/RIU for the nanocage structure with a 50 nm wall length and a 4.5 nm wall t hickness25. Also, Mahmoud A. Mahmoud and Mostafa A. El-Sayed synthesized the gold nanoframes with different wall thicknesses and reported RI-sensitivity of 620 nm/ RIU for nanoframes with 51 nm wall length and 10 nm wall t hickness24. They develop an equation for estimating the sensitivity of nanoframes as function of aspect ratio (ratio of wall length to wall thickness) in order to make it possible to compare nanoframes and nanocages synthesized by Sun and his colleague in the same aspect ratio. The results showed a ~ threefold sensitivity of nanoframes compared to nanocages, but the reason for the superiority of nanoframes over nanocages has remained unanswered so far. In (...truncated)