*Communication* **An Exonuclease I-Aided Turn-Off Fluorescent Strategy for Alkaline Phosphatase Assay Based on Terminal Protection and Copper Nanoparticles**

**Yan Wang, Ying Yan, Xinfa Liu \* and Changbei Ma \***

School of Life Sciences, Central South University, Changsha 410013, China; 2801170213@csu.edu.cn (Y.W.); Yany2018@csu.edu.cn (Y.Y.)

**\*** Correspondence: xinfaliu@163.com (X.L.); macb2012@csu.edu.cn (C.M.); Tel.: +86-731-8265-0230 (X.L. & C.M.)

**Abstract:** As an important DNA 30 -phosphatase, alkaline phosphatase can repair damaged DNA caused by replication and recombination. It is essential to measure the level of alkaline phosphatase to indicate some potential diseases, such as cancer, related to alkaline phosphatase. Here, we designed a simple and fast method to detect alkaline phosphatase quantitively. When alkaline phosphatase is present, the resulting poly T-DNA with a 30 -hydroxyl end was cleaved by exonuclease I, prohibiting the formation of fluorescent copper nanoparticles. However, the fluorescent copper nanoparticles can be monitored with the absence of alkaline phosphatase. Hence, we can detect alkaline phosphatase with this turn-off strategy. The proposed method is able to quantify the concentration of alkaline phosphatase with the LOD of 0.0098 U/L. Furthermore, we utilized this method to measure the effects of inhibitor Na3VO<sup>4</sup> on alkaline phosphatase. In addition, it was successfully applied to quantify the level of alkaline phosphatase in human serum. The proposed strategy is sensitive, selective, cost effective, and timesaving, having a great potential to detect alkaline phosphatase quantitatively in clinical diagnosis.

**Keywords:** alkaline phosphatase; terminal protection; copper nanoparticle; fluorescence
