*2.1. Materials and Instruments*

Indium (III) iodide (InI3) (99.998%), zinc (II) chloride (ZnCl2) (≥98%), tris(diethylamino)- phosphine (C12H30N3P) (97%), 100 mesh selenium powder (99.99%), hydrochloric acid (HCI) (mass fraction, 36.46%), zinc stearate (C36H70O4Zn) (technical grade, 65%), trioctylphosphine (C24H51P) (>97%), octadecene (C18H36) (technical grade, 90%), chloroform (CHCI3) (≥99%), 3-mercaptopropionic acid (C3H6O2S) (MPA) (≥99%), tetramethylammonium hydroxide pentahydrate ((CH3)4N-OH·5H2O) (TMAH) (≥97%), sulfur powder, oleylamine (C18H37N) (mass fraction, 80–90%), citric acid (C6H8O7) (≥99.5%), disodium hydrogen phosphate (Na2HPO4) (≥99%), and all of the soluble metal salts (NaCl, MgCl2, AlCl3, KCl, CaCl2, MnCl2, FeCl3, CoCl2, CuCl2, PbCl2, CdCl2, BaCl2, and AgNO3) were analytically pure and purchased from Sigma–Aldrich (St. Louis, MO, USA). All chemical reagents were directly used without further purification. The distilled water used in all experiments had a conductivity of 1.37 (μs/cm) and a resistivity of 0.73 (MΩ/cm).

Transmission electron microscopy (TEM) observations were carried out by using a transmission electron microscope (JEOL, JEM-2100, Akishima, Japan). Fourier-transform infrared spectra (FT-IR) were obtained by FT-IR spectroscopy (Tianjin Jiangdong Company, FT-IR-650, Tianjin, China). X-ray photoelectron spectroscopy (XPS) was recorded by using an XPS spectrometer (Japan Electronics, D/MAX, Tokyo, Japan). Additionally, the quantum yield (QY) was measured by using a fluorescence spectrometer (Horiba Jobin Yvon, Nanolog FL3-2 IHR, Pairs, France). The fluorescence spectra of the probes were recorded by using a fluorescence spectrometer (Edinburgh Instruments Ltd., Edinburgh FS5, Livingston, UK) at an excitation wavelength (λex) of 250 nm and an emission wavelength (λem) of 520 nm with an integration time of 0.1 s. Both the excitation and emission slits were

set to a width of 5 nm. A time-resolved fluorescence spectrometer (Horiba Jobinyvon IBH Inc, Deltaflex 77-500K, Pairs, France) was used to measure the fluorescence lifetime of the probes with λex = 250 nm and λem = 520 nm. The ultraviolet-visible (UV-Vis) spectrum was scanned in the wavelength range of 190–1100 nm with a UV-Vis spectrophotometer (Hach Company, DR6000, Loveland, USA) with a step size of 5 nm. A refrigerated centrifuge (Sigma, 3k15, Landkreis Osterode, Germany) was used for the pretreatment of samples. In addition, the pH was adjusted by an automatic acid-base titration apparatus (Mettler Toledo, Titration Excellence T9, Zurich, Switzerland), and the sample was sonicated by an ultrasonic cleaner (Kunshan Shumei, KQ-250DE, Kunshan, China).

#### *2.2. Synthesis of MPA-InP/ZnS QDs*

The MPA-InP/ZnS QDs were synthesized according to a previously reported procedure [21]. The solvothermal method was used to proceed with the MPA-InP/ZnS QDs. First, 100 mg of InI3 (0.45 mM) and 300 mg of ZnCl2 (2.2 mM) were added to 5.0 mL (15 mM) of technical oleylamine, which is a coordinating solvent at 120 ◦C, and stirred and degassed to react for one hour; then, 0.45 mL (1.6 mM) of tris(diethylamino)-phosphine (phosphorus:indium ratio = 3.6:1) was injected into the above mixture at 180 ◦C under inert atmosphere for 20 min to synthesize the InP nanocrystals. Subsequently, 1 mL of saturated TOP-S (2.2 M) was slowly injected into the solution of InP nanocrystals, and the temperature was raised to 200 ◦C after 40 min; 4 mL of ODE with 1 g Zn(stearate)2 was injected into the solution, and the temperature was increased to 220 ◦C after 60 min; 0.7 mL saturated TOP-S (2.2 M) was slowly injected into the solution, and the temperature was increased to 240 ◦C, and 2 mL of ODE with 0.5 g Zn(stearate)2 was injected into the solution in turn, and temperature was increased to 260 ◦C after 30 min and 60 min, respectively. At the end, the solution reacted at 260 ◦C for 30 min to form trioctyl phosphine oxide (TOPO)-capped InP/ZnS QDs.

To improve the monodispersion and optical properties in aqueous media, the InP/ZnS QDs were capped by MPA. One hundred milligrams of the organic base TMAH were mixed well vigorous shaking with 50 μL MPA in 1 mL of chloroform and allowed to stand for 1 h for MPA deprotonation. Then, the bottom organic phase containing deprotonated MPA was transferred into a polypropylene tube. Then, 100 μL of the TOPO-capped InP/ZnS QDs (0.1 μM in chloroform) was added to the polypropylene tube and mixed at room temperature for 40 h for the ligand-exchange reaction between TOPO and MPA. Finally, the product was washed with chloroform and ethanol 3 times to obtain MPA-InP/ZnS QDs.

#### *2.3. Detection of Cu2+ Based on MPA-InP/ZnS QDs*

Next, 100 μL of different concentrations of Cu2+ (0 nM, 3 nM, 5 nM, 10 nM, 15 nM, 20 nM, 30 nM, 50 nM, 100 nM, 150 nM, 200 nM, 250 nM, 300 nM, 400 nM, 600 nM, and 1000 nM) were added to 2 mL of MPA-InP/ZnS QDs PBS solution (14 nM, pH = 8.0) for 12 min incubation at room temperature, and the fluorescence spectra were recorded (λex = 250 nm). The calibration curves of the fluorescence intensity at the emission peak of 520 nm vs. the concentrations of Cu2+ were plotted. The stability and selectivity of MPA-InP/ZnS QDs were investigated. The fluorescence intensity was recorded at indoor environment every day for 7 days at room temperature; different metal ions, including Na+, Mg2+, Al3+, K+, Ca2+, Co2+, Mn2+, Fe3+, Ba2+, Cd2+, Pb2+, and Ag+ (500 nM for each), were added to the MPA-InP/ZnS QDs solution, and the fluorescence intensity of each solution was recorded.

The probes were used for the detection of Cu2+ in the environmental water samples and drinking samples. In the spiked and real samples detection, 100 μL of samples were added to 2 mL of MPA-InP/ZnS QDs PBS solution (14 nM, pH = 8.0) for 12 min incubation at room temperature, and the fluorescence spectra were recorded (λex = 250 nm).

Samples 1–5 were mineral water of different brands, including Sample 1 (Uni-President; Tainan, China), Sample 2 (Jingtian Food & Beverage Co., Ltd.; Shenzhen, China), Sample 3 (Evergrande Mineral Water Group Co., Ltd.; Guangzhou, China), Sample 4 (Voss Beverage Co., Ltd.; Voss, Norway), and Sample 5 (Danone Co., Ltd.; Pairs, France); the carbonated

drinks included Sample 6 (Coca-Cola; Atlanta, GA, USA) and Sample 7 (Pepsi; New York, NY, USA), and the drinking water Sample 8 (Beijing Xiangshan Tianquan Beverage Co., Ltd.; Beijing, China), all of which were purchased from the local market in Haidian District of Beijing. The river water samples (Sample 9) were obtained from the Jingmi Diversion Canal in Beijing. Tap water (Sample 10) was obtained in Haidian District of Beijing. The river water samples were centrifuged at 5000 rpm for 3 min to obtain the supernatant, which was filtered through a 0.45-μm filter membrane. The carbonated drink samples were pretreated by ultrasonic oscillation for 5 min. The Cu2+ of the samples were detected by ICP-MS method as comparisons [22] and all experiments were performed in triplicate. Unless otherwise specified, all experiments in this article were performed at room temperature.
