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.

Hot-wire fiber optic anemometer is an emerging technique for wind speed measurement. Many applications have a need to know both the magnitude and the direction of the flow. However, most of previously reported fiber optic anemometers show omnidirectional response, rendering difficulty in recognizing the flow direction. Here, we propose a simple and effective fiber-optic flow sensor with directional response based on an asymmetric coating of single-wall carbon nanotubes (SWCNTs) on the surface of tilted fiber Bragg grating (TFBG), paving the way for the vector analysis of wind flow field. TFBG exhibits unique capabilities of combining the coupling and sensing function together, which significantly simplifies the sensing structure. To further realize directional sensing, wind direction dependent heat transfer mechanism was induced via simple symmetry-breaking surface layer of high efficiency photo-thermal conversion nanomaterial. FLUENT software was used to analyze the wind speed field, temperature distribution field and vortex street. The temperature field shows asymmetrical distribution directly related to the wind directions, causing directional response of the sensor to wind flow. In the experiments, with optimized 12 TFBG with 1.3 μm thick asymmetric SWCNTs coating, three wind directions at different angle (0, 180, 270) were clearly distinguished. Moreover, high sensitivity and efficiency were also achieved by investigating and optimizing the system parameters like pumping power, coating thickness. In addition to directional response, the proposed sensor exhibits many advantages such as simple structure, low power consumption, small size, and high sensitivity, which has good potential application for infield static wind field and slowly varying wind (SVW) measurement.