A Highly Sensitive In Situ CDOM Sensor Based on Frequency Spectrum Shifted m-Sequence Modulation

Abstract

An in situ colored dissolved organic matter sensor based on a frequency spectrum shifted maximum-length sequence (FSSM) modulation method was proposed in this paper. The energy behavior resulting from the direct maximum-length sequence (m-sequence) modulation method was analyzed. We found a kind of twice modulation method, which optimizes m-sequence frequency spectrum structures. This novel method can effectively reduce the energy loss caused by filtering and reduce the noise introduced by demodulation. The sensitivity of the developed sensor was improved to an unprecedented level by optimizing the modulation/demodulation scheme without increasing the light-emitting diode intensity. Subsequently, we developed a cost-saving hardware sensor which can meet the requirements of the in situ measuring system with the FSSM modulation method. The equivalent input noise current of the electronic system can be as low as 12.20 fA. The limit of detection (LOD) of the sensor reached a maximum of 0.0168 μg/L within a concentration range of 0-3000 μg/L, and with an R-squared value of 0.9967. The results demonstrated that the LOD of our sensors was fivefold higher than the LOD of commercially available chromophoric dissolved organic matter (CDOM) sensors and doubled its normal detection range. The sensor also exhibited excellent anti-interference ability and it can attenuate sunlight and electromagnetic interference by up to ten times while improving the stability of the system. The improvement of the CDOM sensor's performance is of great significance to the marine environmental information monitoring field.