Much more advanced sensors have been used to greatly promote intelligent production with the development of smart fishery. Harsh requirements can be found for the accuracy and biosafety of sensors in practical applications. Wearable electronic devices can be expected to serve as one of the core components with great application prospects. Among them, various sensors can pose an important impact on the design and function of future wearable electronic devices. However, most traditional sensors cannot fully meet the application requirements, due mainly to their rigid characteristics. For example, the invasive detection behavior of rigid sensors can cause fish damage, fish growth and activity, easily leading to data loss. Particularly, the conventional rigid sensors cannot be suitable for the modern fishery, such as large volume and weight, as well as biological incompatibility. Fortunately, flexible wearable sensors have attracted extensive attention and research interest in the fisheries field in recent years, due to the lightweight, portability, high integration, and excellent electrical performance. The scope of sensor applications has also been broadened in fisheries, due to their excellent stretchability and biocompatibility. In this review, a systematic summary was proposed of the materials, processes, energy supply, and communication systems required for the preparation of flexible sensors. Flexible materials were divided into the substrate, function and packaging, materials, according to their functions. These materials were needed to meet the particularity of the fishery application environment, in terms of biocompatibility, air permeability and light transmittance. A comparison was made between the preparation materials and properties of existing flexible sensors. The core process of sensor preparation was introduced, together with better energy supply and communication systems. The emerging nano-friction power generation technology was proposed for fishery flexible electronic equipment. And then, the application of flexible wearable sensing technology was reviewed on fish motion monitoring, water environment monitoring and aquatic product quality detection. The quality detection of aquatic products was divided into biogenic amine, antibacterial drug, and pH detection. Flexible electronic technology was dominated in production cost, measurement accuracy, biocompatibility and application range. At the same time, some recommendations were also given in the future. Fish motion monitoring was necessary to capture the sensing information under sensor application scenarios and biomarker monitoring. The much more flexible sensors can be expected to detect the ammonia nitrogen and dissolved oxygen in the water environment, in order to use environmentally friendly materials during preparation. The challenges and development trends of flexible wearable devices in fishery applications were as follows: 1) To improve the stability and reliability of wearable device sensing under complex conditions, in order to ensure the sensing function of fishery special application environment. 2) To design a micro-sensor system for the multi-parameter integrated detection for better sensing function and application range. 3) To develop reliable self-powered modules for wearable devices, such as nano-friction power generation with excellent biocompatibility and flexibility. 4) To promote the application scenarios of flexible sensors in fisheries, such as fish health management, fish disease prediction and research, and testing medical drugs.

Ammonia nitrogen (NH₃-N) can serve as one of the key indicators of water quality. However, the NH₃-N level can rapidly increase over a short period in intensive aquaculture, due to farming density, feeding modes, and water renewal frequency. The high-level NH₃-N-induced stress can cause eutrophication damage to aquatic animals and even mortality. Therefore, it is crucial to detect the NH₃-N in water, in order to prevent water quality deterioration for the safety of aquatic life. Particularly, the current detection of NH₃-N is often complex, costly, time-consuming, and limited in detection range. This study aims to propose an accurate and rapid detection of high NH₃-N concentrations in water. A nitrogen-doped graphene quantum dots (N-GQDs) fluorescent probe was then constructed to detect the NH₃-N content in water. Firstly, the morphology and size distribution of the N-GQDs were examined using transmission electron microscopy (TEM). The lattice spacing of the N-GQDs was observed to be 0.20 nm, corresponding to the (100) lattice plane of the graphite structure. X-ray diffraction (XRD) analysis showed the diffraction peaks at 19° and 29° were attributed to the (101) and (002) crystal planes of graphene, respectively. Fourier transform infrared spectroscopy (FT-IR) analysis demonstrated the presence of carboxyl groups (-COOH) on the surface of the N-GQDs. The N1s spectrum of X-ray photoelectron spectroscopy (XPS) indicated that the nitrogen was doped in the GQDs in the form of pyrrole nitrogen. The characterization results from TEM, FT-IR, XPS, and XRD confirmed that the N-GQDs with a graphene crystal structure were successfully synthesized. Additionally, the optical properties were performed on the N-GQDs fluorescent probe. Specifically, the N-GQDs exhibited an absorption peak at 320 nm in the UV-Vis spectrum, due to the π→π* transition within the C=C structure. Meanwhile, the fluorescence spectra showed that the optimum excitation and emission wavelengths of the N-GQDs were 350 and 450 nm, respectively. The aqueous solution of N-GQDs shared a pale yellow under daylight and then emitted blue fluorescence under a 360 nm UV lamp. The N-GQDs fluorescent probe showed a good linear response (R²=0.99) to NH₃-N concentration in the range of 0-9.0 mmol/L under the optimal experimental conditions (pH=7, N-GQDs concentration 0.8 mg/mL), with the detection limit concentration of 43.8 µmol/L and response time of 2 min. Furthermore, the stability, repeatability, and anti-interference properties of the fluorescent probe were carried out. The probe exhibited the excellent stability, and anti-interference properties against Na⁺, Hg⁺, Ag⁺, K⁺, Pb²⁺, Ca²⁺, Cd²⁺, and Cu²⁺. The fluorescence intensity of the probe still recovered to 77% of the initial value after three uses, indicating that the probe has good reproducibility. The response mechanism of the N-GQDs fluorescent probe to NH₃-N was analyzed by UV-Vis, FT-IR, and fluorescence lifetime decay analysis and Stern-Volmmer equation. The carboxyl groups on the surface of the N-GQDs exhibited electron-accepting properties. The pyrrole nitrogen doped into the GQDs carried a slight positive charge. Ammonia acted as a typical electron donor, due to the lone pair of electrons on the nitrogen atom. Therefore, it inferred that the photo-induced electron transfer occurred between ammonia and the N-GQDs, resulting in the dynamic quenching of N-GQDs fluorescence. Finally, the N-GQDs fluorescent probe was used to determine the NH₃-N in drinking, tap, and aquaculture water, with the recoveries of the spiked assay ranging from 75.03% to 128.16% and relative standard deviations (RSD) (n=3) lower than 13.53%. Consequently, the N-GQDs fluorescent probe also shared the potential applications in the detection of NH₃-N in water.