A High-Sensitivity Microfluidic Nitrite Sensor with Synchronous Oscillatory Spiral Micromixer for In Situ Seawater Monitoring

Authors:
  • JZJincheng Zhao
    Jincheng Zhao
  • CLCai Li
    Cai Li
  • ZYZeming Yang
    Zeming Yang
  • DTDejing Tang
    Dejing Tang
  • XZXianqing Zhang
    Xianqing Zhang
  • CLCong Liu
    Cong Liu
  • SDShikun Dai
    Shikun Dai
  • WZWen Zhou
    Wen Zhou
  • ZXZhantang Xu
    Zhantang Xu
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Abstract
Colorimetry is a well-established technique for detecting physicochemical parameters in aquatic environments, yet conventional methods are hindered by bulky instrumentation, labor-intensive procedures, high reagent consumption, and lengthy assay times. Microfluidics offers a promising alternative, but existing passive micromixers often face a trade-off among mixing efficiency, pressure drop, and long-term deployability.This study presents a microfluidic colorimetric sensor for in situ nitrite monitoring in aquatic environments, incorporating a synchronous oscillatory spiral micromixer with sinusoidal channel walls to enhance Dean vortex formation while maintaining smooth flow. The micromixer achieves complete mixing at a Reynolds number of 50 with substantially reduced pressure drop. Integrated with an optical absorbance cell, the lab‐on‐a‐chip sensor provides a detection limit of 6.12 nM, a linear range up to 60 µM, and a measurement time of 140 s. Two proposed sensors were deployed in Daya Bay for over one week at two‐hour intervals, producing 164 valid data points. Intercomparison of nitrite concentrations measured by the proposed sensors and a benchtop UV‐2600 spectrophotometer, using concurrently collected grab samples, showed strong agreement, with principal component analysis yielding a near 1:1:1 ratio among the three datasets.Observed nitrite concentrations varied coherently with salinity, tidal level, chlorophyll‐a, and dissolved oxygen, reflecting tide‐driven water exchange and biological activity. With a micromixer footprint under 5 mm², the proposed platform is suitable for chip‐level integration and multi‐parameter analysis, demonstrating a high‐sensitivity, low‐power, high‐temporal‐resolution solution for in situ nutrient monitoring with potential for multiplexed oceanographic applications

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