Mercury ion (Hg²⁺) is considered to be one of the most toxic heavy metal ions. Once the content of Hg²⁺ exceeds the quality standard in drinking water, the living environment and health of human beings will be threatened and destroyed. Therefore, the establishment of simple and efficient methods for Hg²⁺ ion detection has important practical significance. In this paper, we present a highly sensitive and selective fiber-optic surface plasmon resonance (SPR) Hg²⁺ ion chemical nanosensor by designing thymine (T)-modified gold nanoparticles (Au NPs/T) as the signal amplification tags. Thymine-1-acetic acid (T-COOH) was covalently coupled to the surface of 2-aminoethanethiol (AET)-modified Au NPs and Au film by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride/N-Hydroxysuccinimide (EDC/NHS) activation effect, respectively. In the presence of Hg²⁺ ions, the immobilized thymine combines specifically with Hg²⁺ ions, and forms an Au/thymine-Hg²⁺-thymine/Au (Au/T-Hg²⁺-T/Au) complex structure, leading to a shift in SPR wavelength due to the strong electromagnetic couple between Au NPs and Au film. Under optimal conditions, the proposed sensor was found to be highly sensitive to Hg²⁺ in the range of 80 nM–20 µM and the limit of detection (LOD) for Hg²⁺ was as low as 9.98 nM. This fiber-optic SPR sensor afforded excellent selectivity for Hg²⁺ ions against other heavy metal ions such as Fe³⁺, Cu²⁺, Ni²⁺, Ba²⁺, K⁺, Na⁺, Pb²⁺, Co²⁺, and Zn²⁺. In addition, the proposed sensor was successfully applied to Hg²⁺ assay in real environmental samples with excellent recovery. Accordingly, considering its simple advantages, this novel strategy provides a potential platform for on-site determination of Hg²⁺ ions by SPR sensor.

Plasmonic nanostructures have become the most promising candidates for biosensing applications because of their miniature sizes, ease of integration, and high-throughput detection. Both propagating and localized surface plasmon modes in the nanostructures have been used for sensing and biomolecular detection. However, to maximize the biosensing potential of nanostructures, the choice of an optimized sensing detection strategy among two plasmon modes depends on the relationship between the biomolecule sizes and field decay length of plasmon modes. Here, we propose and investigate plasmonic coupling on a single-crystalline gold film, wherein there are two distinct optical modes, a gap mode (localized surface plasmon) originating from the parallel coupling of a nanoring and a surface lattice mode (propagating surface plasmon) originating from the anti-parallel coupling of a nanoring in an array. The sensing performances of the above two modes are thoroughly investigated and compared by considering two aspects, i.e., bulk and surface sensitivities. It is demonstrated that there is a reciprocal relationship between bulk and surface sensitivities for two modes, which also illustrates that the surface sensitivity is indispensable to fully describe the sensing performance of nanostructures. Furthermore, due to their different decay lengths, the gap and surface lattice modes on a single optical substrate can achieve simultaneous detection of target analytes with various sizes. Therefore, we can provide a high performance sensing platform based on a metallic nanoring array for a broad range of biomolecules with various sizes.

Plasmonic spectrum filtering devices based on metallic nanostructures have attracted wide attention due to their good reliability, ease of fabrication, and wideband tunability. However, the presence of thick substrate significantly limits the structure’s longitudinal size for further optoelectronic integration and reduces the devices’ performance. Here we propose and demonstrate an ultra-thin plasmonic bandpass filter based on free-standing periodic metal-dielectric-metal stack geometry working in the near-infrared wavelength range. The coupling between free-space electromagnetic waves and spatially confined plasmonic modes in the designed structure is systematically investigated. As demonstrated in the calculation and experiment, the free-standing plasmonic filters have more than 90% transmission efficiency and superior angular tolerance. The experimental results are in good agreement with the theoretical calculations. These artificial nanostructured filtering devices may find potential applications in the extremely compact device architectures.

Glucose detection using surface-enhanced Raman scattering (SERS) spectroscopy has aroused considerable attention due to its potential in the prevention and diagnosis of diabetes as a result of its unique molecular fingerprinting capability, ultrahigh sensitivity and minimal interference from water. Despite numerous solutions to improve the sensitivity of glucose detection, the development of a new SERS-based strategy to detect glucose with high sensitivity and low-cost is still required. In this study, we propose a simple and sensitive SERS-based plasmonic metasurface sensing platform for a glucose sandwich assay using self-assembled p-mercapto-phenylboronic acid (PMBA) monolayers on a gold nanodisk (Au-ND) metasurface and synthesized silver nanoparticles (Ag NPs) modified with a mixture of p-aminothiophenol (PATP) and PMBA. The localized near-field of the proposed plasmonic metasurface is markedly enhanced due to the coupling between the Au-ND and Ag NPs, which greatly improves detection sensitivity. The experimental results show that SERS signals of the glucose assay are significantly enhanced by more than 8-fold, in comparison with the SERS substrate of smooth Au films and Ag NPs. Moreover, the plasmonic metasurface-based glucose sandwich assay exhibits high selectivity and sensitivity for glucose over fructose and galactose. The developed plasmonic metasurface sensing platform shows enormous potential for highly sensitive and selective SERS-based glucose detection and opens a new avenue for scalable and cost-effective biosensing applications in the future.

An omnidirectional and broadband optical absorber has long been pursued for its wide application in optics, sensing and energy fields.

Accurate and real-time detection of hydrogen (H₂) is essential for ensuring energy security. Fiber-optic H₂ sensors are gaining attention for their integration and remote sensing capabilities. However, they face challenges, including complex fabrication processes and limited response times. Here, we propose a fiber-optic H₂ sensing tip based on Tamm plasmon polariton (TPP) resonance, consisting of a multilayer metal/dielectric Bragg reflector deposited directly on the fiber end facet, simplifying the fabrication process. The fiber-optic TPP (FOTPP) tip exhibits both TPP and multiple Fabry-Perot (FP) resonances simultaneously, with the TPP employed for highly sensitive H₂ detection. Compared to FP resonance, TPP exhibits more than twice the sensitivity under the same structural dimension without cavity geometry deformation. The excellent performance is attributed to alterations in phase-matching conditions, driven by changes in penetration depth of TPP. Furthermore, the FP mode is utilized to achieve an efficient photothermal effect to catalyze the reaction between H₂ and the FOTPP structure. Consequently, the response and recovery speeds of the FOTPP tip under resonance-enhanced photothermal assistance are improved by 6.5 and 2.1 times, respectively. Our work offers a novel strategy for developing TPP-integrated fiber-optic tips, refines the theoretical framework of photothermal-assisted detection systems, and provides clear experimental evidence.