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This work mainly describes an electrochemical microsensor based on the reduction of molybdophosphate complex for total phosphorus (TP) determinations in water. The microelectrode was fabricated using microelectromechanical systems (MEMS) techniques and porous, branching gold nanoparticles (AuNPs) were electrodeposited on the microelectrode to improve its sensitivity. Calibration of the microsensor was performed with standard phosphate solutions prepared with KH2 PO4 with pH adjusted to 1.0. The experimental results showed that both sensitivity and current response are improved effectively using this modified microelectrodes: The limit of detection of the microsensor is 1.2 × 10−7 mol/l and linear range is 3 × 10−7 to 3 × 10−4 mol/l. The sensitivity of unmodified electrode is −0.27 nA/μmol·l−1 (R2 = 0.994), whereas the sensitivity of AuNPs modified electrode is −0.89 nA/μmol l−1 (R2 = 0.98). The current response of modified electrode is 6 times larger than that of unmodified electrode in average. Detection of TP was also carried out with digested TP standard solutions for both modified and unmodified electrodes, and results were consistent with the nominal value of phosphorus concentration. This microsensor provides the probability of combining TP detection device with micro‐digesting device to form a TP detection system, which can realise the automotive and on‐line monitoring of TP in surface water. [ABSTRACT FROM AUTHOR]