*2.4. Characterization*

UV-Vis absorption spectra were measured on a Varian UV-Cary100 spectrophotometer (Varian Inc., Palo Alto, CA, USA). Infrared spectrum (IR) was determined on a Pristige IR21 FTIR spectrometer (Shimadzu, Kyoto, Japan). FL spectra were performed on a FluoroMax-P spectrophotometer (Horiba Jobin Yvon, Paris, France). Images of transmission electron microscopy (TEM) were collected by a Tecnai G2 F20 TEM characterization (FEI, Hillsboro, OR, USA). X-ray photoelectron spectra (XPS) were measured on a ESCALAB 250Xi (Thermo fisher Scientific, West Sussex, UK). The lifetime measurements were determined on an Edinburgh fls1000 FL spectrometer (Edinburgh Instruments Ltd, Livingston, UK). The absolute quantum yields were determined using an integrating sphere on a Japan Hamamatsu Quantum Yield Determination System C9920-02G (Hamamatsu Photonics Co., Ltd., Hamamatsu, Japan).

#### *2.5. Ratiometric Fluorescence Detection for DPA*

Then, 20 μL stock solution of CDs-Tb (1.0 mg·mL−1) was mixed with HCl-Tris buffer (pH 7.6) solution; after that, various concentrations of DPA (0, 0.5, 1, 1.5, 2, 2.5, 3, 3.75, 4.5, 6, 7.5, 9 μM; aqueous solution, the concentration of DPA stock solution was 0.5 mM) were added. The total volume of the mixture was 2.0 mL. FL spectra were recorded under the excitation wavelength of 270 nm. In the selectivity detection towards DPA, the concentrations of the selected interferences and the DPA were kept as 10 μM and 4 μM, respectively. The FL titrations and the selectivity detection experiments were carried out three times to achieve reliable results.

To investigate the reaction mechanism between CDs-Tb and DPA, time-resolved decay measurements were carried out. CDs-Tb solutions (0.1 mg·mL−1) containing 0 or 80 μM of DPA were tested (pH 7.6). The emission wavelengths were 445 and 545 nm, while the excitation wavelength was 270 nm.

As for analysis in a real sample, a lake water sample taken from our campus was first centrifuged to remove larger insoluble impurities. Then, small insoluble impurities were removed by filtration. Subsequently, 20 μL stock solution of CDs-Tb was mixed with 2.0 mL HCl-Tris butter (pH 7.6) solution, which was prepared by using the above filtered lake water sample. After that, DPA aqueous solutions (1, 2, 5 μM) were added into the above mixture. FL spectra were recorded (λex = 270 nm), and each test was repeated three times.
