In this paper, a tilted fiber Bragg grating (TFBG) plasmonic sensor based on thymine (T) - Hg - thymine (T) (T-Hg-T) base asymmetric pairing and Au nanoparticles (AuNPs), which can be used for highly sensitive and selective ultra-low-concentration Hg2+ detection, has been demonstrated. As is known, TFBG can provide an effective mean to easily excite the surface plasmon resonance (SPR) phenomenon while the fiber is coated with metal layer. By measuring the evolution of spectra, the SPR properties hence the surface perturbation could be detected accurately. To achieve highly sensitive and selective Hg2+ detection, the two specially designed single-strand probe DNAs (ssDNAs) with rich T base that can specifically forming stable T-Hg-T asymmetric pairing are served as a binding element for AuNPs labelled-target DNA conjugates. In the present of Hg2+, these two ssDNAs that modified respectively on the surface of the sensor and the AuNPs will create a double helix T-Hg-T structure between the two DNA oligonucleotides and result in the surface refractive index perturbation amplified by the AuNPs. To overcome the low reusability of the AuNPs based DNA biosensor, by using iodide ions that can form more stable compounds with mercury ions over the T-Hg2+-T structure, a regenerable DNA biosensor that can repeatedly detect mercury ions is developed. The experimental results demonstrate that this sensor shows an excellent ultra-low-concentration and selectivity detection capability for Hg2+ with the limit of detection (LOD) of ${3.073 \times 10^{-12} M \ (3.073 pM)}$ as well as a large dynamic range of 8 order of magnitude from ${1.0\times 10^{11}\rm \ M \ to \ {1.0\times 10^{3}} M}$ simultaneously. Thus, the experimental results in various actual sample from ultrapure/tap water to even clinical human serum, also reveal that it has potential application for Hg2+ detection in environmental samples and in clinic biological samples.

Localized surface plasmon resonance (LSPR) biosensors, which enable nanoscale confinement and manipulation of light, offer the enhanced sensitivity and electromagnetic energy localization. The integration of LSPR with the fiber-optic technology has led to the development of compact and versatile sensors for miniaturization and remote sensing. This comprehensive review explores various sensor configurations, fiber types, and geometric shapes, highlighting their benefits in terms of sensitivity, integration, and performance improvement. Fabrication techniques such as focused non-chemical bonding strategies and self-assembly of nanoparticles are discussed, providing control over nanostructure morphology and enhancing sensor performance. Bio-applications of fiber-optic LSPR (FOLSPR) sensors are detailed, specifically in biomolecular interactions and analysis of proteins, pathogens and cells, nucleic acids (DNA and RNA), and other small molecules (organic compounds and heavy metal ions). Surface modification and detection schemes are emphasized for their potential for label-free and real-time biosensing. The challenges and prospects of FOLSPR sensors are addressed, including the developments in sensitivity, fabrication techniques, and measurement reliability. Integration with emerging technologies such as nanomaterials is highlighted as a promising direction for future research. Overall, this review provides insights into the advancements and potential applications of FOLSPR sensors, paving the way for sensitive and versatile optical biosensing platforms in various fields.

Herein, we report the simultaneously application of surface plasmon resonance (SPR) spectroscopy and electrochemistry (EC) technology for the real-time quantitative investigation of heavy metal ions, with high selectivity and sensitivity. A surface of optical fiber coated over with nanometric gold film and then modified with a dithiol self-assembled monolayer of 1,6-hexanedithiol (HDT) was developed as the dual mode electrochemical-surface plasmon resonance (EC-SPR) optical fiber sensor. The limit of detection of the proposed sensor exposed in Hg ²⁺ ions solution without and with the multiple cyclic voltammetry (CV) scans to activate the HDT monolayer can be improved from 0.47 μM to 0.22 μM. More importantly, the CV scans can activate the redox reaction of Hg ²⁺ ions, and then easily strip the Hg ²⁺ ions from the HDT monolayer to restore the active sites, and endow the designed sensor the characteristics of repeated use. Under simultaneous SPR monitoring, the differential pulse voltammetry (DPV) currents of HDT-functionalized optical fiber can also act as a microelectrode to develop electrochemical sensor platform for monitoring Cu ²⁺ , with a limit of detection of 94 nM and a dynamic range up to 90 μM. As a comparison, the EC-SPR sensor still has good analysis performance for both Hg ²⁺ and Cu ²⁺ ions in presence of Ni ²⁺ , Fe ²⁺ , Co ²⁺ , Pb ²⁺ , K ⁺ , among others. We therefore demonstrated that the proposed fiber-based EC-SPR sensor has the advantages of real-time, miniaturized, and cost-effective sensing, paving the way for environmental monitoring applications for on site inspection.

To overcome low surface coverage and aggregation of particles, which usually restricts the sensitivity and resolution of conventional localized surface plasmon resonance (LSPR) fiber-optic sensors, we propose a simple self-assembled templating technique that uses a nanometer thickness block copolymer (BCP) layer of poly(styrene-b-4-vinylpyridine) to form a 33 nm gold nanoparticle (AuNP) monolayer with high uniformity and density for LSPR sensing. The LSPR resonance wavelength for this PS-b-P4VP templated methodology is 592 nm and its refractive index sensitivity is up to 386.36 nm/RIU, both of which are significantly improved compared to those of conventional LSPR techniques. Calibrated by a layer-by-layer polyelectrolyte deposition procedure, the decay length of this LSPR sensor is calculated to be 78 nm, which is lower than other traditional self-assembled LSPR sensors. Furthermore, hybridization between target ssDNA, which is linked with capture ssDNA on the LSPR biosensor and DNA–AuNP conjugates, leads to a low detection limit of 67 pM. These enhanced performances are significant and valuable for high-sensitivity and cost-effective LSPR biosensing applications.