Curvature-Insensitive Transparent Surface-Enhanced Raman Scattering Substrate Based on Large-Area Ag Nanoparticle-Coated Wrinkled Polystyrene/Polydimethylsiloxane Film for Reliable In Situ Detection.

Flexible and transparent surface-enhanced Raman scattering (SERS) substrates have attracted considerable attention for their ability to enable the direct in situ detection of analytes on curved surfaces. However, the curvature of an object can impact the signal enhancement of SERS during the measurement process. Herein, we propose a simple approach for fabricating a curvature-insensitive transparent SERS substrate by depositing silver nanoparticles (Ag NPs) onto a large-area wrinkled polystyrene/polydimethylsiloxane (Ag NP@W-PS/PDMS) bilayer film. Using rhodamine 6G (R6G) as a probe molecule, the optimized Ag NP@W-PS/PDMS film demonstrates a high analytical enhancement factor (AEF) of 4.83 × 105, excellent uniformity (RSD = 7.85%) and reproducibility (RSD = 3.09%), as well as superior mechanical flexibility. Additionally, in situ measurements of malachite green (MG) on objects with diverse curvatures, including fish, apple, and blueberry, are conducted using a portable Raman system, revealing a consistent SERS enhancement. Furthermore, a robust linear relationship (R2 ≥ 0.990) between Raman intensity and the logarithmic concentration of MG detected from these objects is achieved. These results demonstrate the tremendous potential of the developed curvature-insensitive SERS substrate as a point-of-care testing (POCT) platform for identifying analytes on irregular objects.

Rapid identification and quantitative analysis of malachite green in fish via SERS and 1D convolutional neural network.

Rapid and quantitative detection of malachite green (MG) in aquaculture products is very important for safety assurance in food supply. Here, we develop a point-of-care testing (POCT) platform that combines a flexible and transparent surface-enhanced Raman scattering (SERS) substrate with deep learning network for achieving rapid and quantitative detection of MG in fish. The flexible and transparent SERS substrate was prepared by depositing silver (Ag) film on the polydimethylsiloxane (PDMS) film using laser molecular beam epitaxy (LMBE) technique. The wrinkled Ag NPs@PDMS film exhibits high SERS activity, excellent reproducibility and good mechanical stability. Additionally, the fast in situ detection of MG residues onfishscales was achieved by using the wrinkled Ag NPs/PDMS film and a portable Raman spectrometer, with a minimum detectable concentration of 10-6 M. Subsequently, a one-dimensional convolutional neural network (1D CNN) model was constructed for rapid quantification of MG concentration. The results demonstrated that the 1D CNN quantitative analysis model possessed superior predictive performance, with a coefficient of determination (R2) of 0.9947 and a mean squared error (MSE) of 0.0104. The proposed POCT platform, integrating a transparent flexible SERS substrate, a portable Raman spectrometer and a 1D CNN model, provides an efficient strategy for rapid identification and quantitative analysis of MG in fish.

Multi-function sensing applications based on high Q-factor multi-Fano resonances in an all-dielectric metastructure.

A multi-function sensor based on an all-dielectric metastructure for temperature and refractive index sensing simultaneously is designed and analyzed in this paper. The structure is composed of a periodic array of silicon dimers placed on the silicon dioxide substrate. By breaking the symmetry of the structure, the ideal bound states in the continuum can be converted to the quasi-bound states in the continuum, and three Fano resonances are excited in the near-infrared wavelength. Combining with the electromagnetic field distributions, the resonant modes of three Fano resonances are analyzed as magnetic dipole, magnetic toroidal dipole, and electric toroidal dipole, respectively. The proposed sensor exhibits an impressive maximal Q-factor of 9352, with a modulation depth approaching 100%. Our investigation into temperature and refractive index sensing properties reveals a maximum temperature sensitivity of 60 pm/K. Regarding refractive index sensing, the sensitivity and figure of merit are determined to be 279.5 nm/RIU and 2055.1 RIU-1, respectively. These findings underscore the potential of the all-dielectric metastructure for simultaneous multi-parameter measurements. The sensor's versatility suggests promising applications in biological and chemical sensing.

High-Q Fano resonances in all-dielectric metastructures for enhanced optical biosensing applications.

Fano resonance with high Q-factor is considered to play an important role in the field of refractive index sensing. In this paper, we theoretically and experimentally investigate a refractive index sensor with high performance, realizing a new approach to excite multiple Fano resonances of high Q-factor by introducing an asymmetric parameter to generate a quasi-bound state in the continuum (BIC). Combined with the electromagnetic properties, the formation mechanism of Fano resonances in multiple different excitation modes is analyzed and the resonant modes of the three resonant peaks are analyzed as toroidal dipole (TD), magnetic quadrupole (MQ), and magnetic dipole (MD), respectively. The simulation results show that the proposed metastructure has excellent sensing properties with a Q-factor of 3668, sensitivity of 350 nm/RIU, and figure of merit (FOM) of 1000. Furthermore, the metastructure has been fabricated and investigated experimentally, and the result shows that its maximum Q-factor, sensitivity and FOM can reach 634, 233 nm/RIU and 115, respectively. The proposed metastructure is believed to further contribute to the development of biosensors, nonlinear optics, and lasers.

High Q-factor Fano resonances based on a permittivity-asymmetric dielectric pea-shaped cylinder.

We numerically investigate two Fano resonances with high Q-factors based on a permittivity-asymmetric metastructure composed of two pea-shaped cylinders. By employing different materials to break the permittivity-asymmetry, the quasi-bound state of the continuum spectrum (BIC) resonance at 982.87 nm is excited, showing the Q-factor as high as 8183.7. The electromagnetic fields and vectors are analyzed by using the finite-difference time-domain (FDTD) method, and the resonance modes are identified as magnetic dipole (MD) responses and MDs by multipole decomposition in Cartesian coordinates, displaying that the light is confined within a pea-shaped cylinder to achieve localized field enhancement. In addition, the sensing performances of the metastructure are evaluated, and an optical refractive index sensor can be obtained with the sensitivity of 152 nm/RIU and maximum figure of merit (FOM) of 832.6. This proposed structure offers a new, to the best of our knowledge, way to achieve Fano resonant excitation on all-dielectric metastructures and can be used in nonlinear optics, biosensing, optical switches, and lasers.

Excitation of multiple Fano resonances on all-dielectric nanoparticle arrays.

In this paper, an all-dielectric metasurface consisting of a unit cell containing a nanocube array and organized periodically on a silicon dioxide substrate is designed and analyzed. By introducing asymmetric parameters that can excite the quasi-bound states in the continuum, three Fano resonances with high Q-factor and high modulation depth may be produced in the near-infrared range. Three Fano resonance peaks are excited by magnetic dipole and toroidal dipole, respectively, in conjunction with the distributive features of electromagnetism. The simulation results indicate that the discussed structure can be utilized as a refractive index sensor with a sensitivity of around 434 nm/RIU, a maximum Q factor of 3327, and a modulation depth equal to 100%. The proposed structure has been designed and experimentally investigated, and its maximum sensitivity is 227 nm/RIU. At the same time, the modulation depth of the resonance peak at λ = 1185.81 nm is nearly 100% when the polarization angle of the incident light is 0 °. Therefore, the suggested metasurface has applications in optical switches, nonlinear optics, and biological sensors.

Multiple Fano resonances on the metastructure of all-dielectric nanopore arrays excited by breaking two-different-dimensional symmetries.

We have designed and analyzed the high quality-factor (Q-factor), multiple Fano resonances device on the basis of the all-dielectric metastructure. The unit structure consists of two rectangular air holes etched within a silicon cube and periodically aligns on the substrate of silicon dioxide. The results demonstrate that four Fano resonances are achieved by integrating the theory of bound states in the continuum (BIC)and breaking the symmetry (width symmetry or depth symmetry) of two rectangle air holes, and the resonant wavelength can be modified by altering structural parameters. The sensing characteristics of the presented structure are studied. The sensitivity(S) of 304 nm/RIU, the maximal Q-factor of 2142 and the figure of merit (FOM) of 515.3 are achieved while width symmetry is broken. Meanwhile, the sensitivity of 280 nm/RIU, the maximal Q-factor of 2517 and the FOM of 560 are gotted through breaking depth symmetry. The proposed metastructures can be used for the lasers, biosensing and nonlinear optics.