Balanced Maximum Entropy m-Sequence Modulation Based Highly Sensitive In-Situ CDOM Sensor

Chromophoric dissolved organic matter (CDOM) is an important component of the global carbon pool. Highly sensitive and high-stability in-situ CDOM monitoring is of great significance for in-depth research into biogeochemical cycles of aquatic ecosystems, marine primary productivity, and other scientific issues. This paper proposes a novel maximum entropy construction (MEC) method, designed as a universal approach to optimize sequence properties for enhanced robustness. This method is then applied to develop the balanced maximum entropy m-sequence (BMEm) for CDOM sensors. This application significantly improves the instrument’s anti-interference ability, reducing the detection limit of the sensor to 2.1 ng/L Quinine Sulfate Equivalent (QSE), which is one-fifth of the detection limit of the best commercially available CDOM sensor. The sensor achieves a wide dynamic detection range of 0 to 4000 μg/L QSE, with a coefficient of determination of 0.9994. Furthermore, it demonstrates low power consumption (36 mA), and a fast response time (T90 < 15 s). Additionally, the sensor demonstrates excellent stability over 24-hour operation, with its noise level consistently maintained at a low level near the detection limit. When paired with the CDOM Monitor software we developed, the sensor can operate in self-contained or online mode, enabling mobile observations within a depth of 2000 meters in the sea. This sensor was deployed on the human-occupied vehicle “Shenhaiyongshi” and successfully completed sea trials in South China Sea. The performance and reliability of the sensor were verified by comparing the similar sea trial data, conductivity, temperature, and depth data with the obtained CDOM data. Therefore, the sensor is expected to become a reliable in-situ device for monitoring high-frequency changes in CDOM and understanding the biogeochemical process of CDOM in the short term, which can provide more precise information for the biogeochemical cycles and energy flow research of aquatic ecosystems.