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Imagine a once crystal-clear lake transformed into a lifeless wasteland, not by visible contaminants but by imperceptible ammonia nitrogen levels that trigger algal blooms, suffocate aquatic life, and disrupt entire ecosystems. This is not alarmist rhetoric but a pressing global challenge facing water bodies worldwide. Ammonia nitrogen, a primary pollutant from agricultural, industrial, and domestic wastewater, poses direct threats to aquatic organisms and indirect risks to human health through the food chain. Consequently, developing precise and efficient ammonia nitrogen detection methods has become crucial for environmental protection and sustainable water resource management.

Ammonia nitrogen refers to the combined concentration of ammonia (NH₃) and ammonium ions (NH₄⁺) in water, with their relative proportions determined by pH levels. When pH falls below 8.75, NH₄⁺ dominates; above 9.75, NH₃ prevails. These compounds are highly water-soluble, corrosive, and potentially hazardous. Even minimal ammonia nitrogen exceedances (e.g., concentrations above 0.5 μmol/L) can significantly impact aquatic environments.

While ammonium ions typically dominate in natural waters, the far more toxic ammonia—even at nanomolar concentrations—is the primary cause of aquatic organism poisoning. Elevated ammonia nitrogen levels stimulate excessive plankton growth, triggering algal blooms, eutrophication, and ecosystem collapse. Subsequent decomposition of algae depletes dissolved oxygen, causing mass fish and benthic organism die-offs. This vicious cycle not only reduces water self-purification capacity but may also release harmful gases, further degrading environmental conditions.

Ammonia nitrogen toxicity extends to humans, fish, and crustaceans, particularly affecting juvenile aquatic life. Blood ammonia accumulation in fish causes fatalities, threatening fishery resources and ecological balance. More alarmingly, ammonia nitrogen can convert into toxic nitrites and nitrates under certain conditions, exacerbating water pollution risks. Recognizing these dangers, nations worldwide have established strict ammonia nitrogen concentration standards for drinking water and seawater, making rapid, accurate detection technologies essential for water security and ecosystem protection.

Conventional ammonia nitrogen detection relies primarily on two methods:

• Nessler's Reagent Method: This colorimetric technique measures yellow-brown complex formation between ammonia nitrogen and potassium tetraiodomercurate solution. However, it suffers from interference from water turbidity, color, and other substances, while its mercury-based reagents pose environmental hazards.
• Indophenol Blue Method (IPB): This more sensitive approach quantifies blue indophenol compounds formed by ammonia nitrogen reactions with hypochlorite and phenol reagents. While outperforming Nessler's method, IPB still faces accuracy limitations in complex water matrices like seawater and aquaculture systems.

Despite their historical utility, these methods struggle with increasingly stringent environmental requirements and low-concentration detection needs. Their labor-intensive sample preparation, lengthy procedures, and environmentally unfriendly reagents highlight the necessity for advanced alternatives.

• Optical Analysis: Spectrophotometric and fluorometric methods offer high sensitivity but require complex sample pretreatment to eliminate interference, reducing analytical efficiency.
• Electrochemical Analysis: Cost-effective and easily automated, these methods measure ammonia nitrogen's redox reactions at electrode surfaces. Nanomaterial-modified electrodes significantly enhance sensitivity and selectivity.
• Biosensing Technology: Utilizing enzymes, antibodies, or microorganisms for specific ammonia nitrogen reactions, biosensors deliver rapid, sensitive, selective detection ideal for field applications. Integration with microfluidics and gas diffusion techniques further improves performance.

• Microfluidic Chip Technology: Integrating sample processing, reaction, and detection on miniature chips enables high-throughput, automated analysis with minimal reagent use, suitable for online monitoring.
• Fiber Optic Sensing: Leveraging light-medium interactions in optical fibers, these compact, interference-resistant sensors facilitate remote monitoring in harsh environments.
• Colorimetric pH Detection: Simple and cost-effective, this method uses pH indicators' color changes from ammonia nitrogen-induced pH shifts for rapid field testing.

• Metal Nanoparticles: Gold or platinum nanoparticles catalyze redox reactions, lowering overpotential and enhancing sensitivity while increasing electrode surface area.
• Carbon Nanotubes/Graphene: These materials provide conductive, mechanically robust electrode frameworks that improve stability and lifespan, especially when combined with nanoparticles or enzymes.
• Metal Oxide Nanoparticles: Offering chemical stability and biocompatibility, they protect electrodes from corrosion while some variants catalyze ammonia nitrogen redox reactions.

• Glutamate Dehydrogenase (GLDH): Measures NADH consumption during α-ketoglutarate-to-glutamate conversion for highly sensitive, selective detection, though enzyme activity depends on temperature and pH.
• Urease: Detects urea indirectly via ammonia nitrogen production from urea hydrolysis, suitable for wastewater and urine analysis.

While enzymatic methods provide superior sensitivity, selectivity, and speed, challenges include enzyme stability, high production costs, and potential activity loss during immobilization.