**4. Conclusions**

In summary, a new ratiometric fluorescent nanoprobe, CDs-Tb, was synthesized for DPA detection by grafting Tb3+ onto the surface of CDs. The schizochytrium-based CDs with blue fluorescence were facilely synthesized via a hydrothermal process and served as a FL reference and a supporting substrate for coordination with Tb3+, while green emission peaks of Tb3+ served as the fluorescence response signal owing to the AETE. This nanoprobe displays high selectivity and sensitivity for DPA detection and can realize the monitoring of DPA in lake water samples.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/9/1234/s1, Table S1. Comparison of representative fluorescence probes for measuring DPA, Figure S1: FL emission spectra of CDs under di fferent excitation wavelength (λex = 265–395 nm), Figure S2: UV-Vis absorption of CDs-Tb, Figure S3: FL emission spectra of CDs-Tb under different excitation wavelength (λex = 265–375 nm), Figure S4: Influence of pH on the ratio FL intensity *F*545 /*F*445 of CDs-Tb upon addition of 5 μM DPA, λex = 270 nm.

**Author Contributions:** Conceptualization, L.Z. and B.L.; Formal analysis, Z.W., J.Z. and D.Z.; Investigation, Z.W., J.Z. and X.S.; Writing–original draft, L.Z. and C.S.; Writing–review & editing, L.Z.

**Funding:** This work was supported by the National Natural Science Foundation of China (21805071, U1804135, 51671080); the Key Scientific Research Project of Henan Province (16A150052); the Fundamental Research Funds for the Universities of Henan Province (NSFRF170911) and Dr. Funds of Henan polytechnic university (B2016-47).

**Conflicts of Interest:** The authors declare no conflict of interest.
