*2.2. Instrumentation*

All the fluorescence measurements were recorded using a spectrofluorometer (Shimadzu, RF-6000, Kyoto, Japan). BPA adsorption data were recorded on a UV–Vis spectrophotometer (Shimadzu, Kyoto, Japan). The magnetization measurements of the FeOx MNPs, MNP/QDs and MNP/QD@MIPs were carried out using a vibrating sample magnetometer (VSM, Lake Shore 7307, Columbus, OH, USA) at room temperature. The morphology of the QDs was characterized by a JEM-2100F high resolution transmission electron microscopy (TEM, JEOL, Tokyo, Japan). The shape and structures of the magnetic nanomaterials were examined using a H-800 transmission electronic microscopy (Hitachi, Tokyo, Japan). The Fourier transform infrared (FT-IR) spectra were determined on a Vertex 70v spectrometer (Bruker, Karlsruhe, Germany).

#### *2.3. Synthesis of Amino-Modified ZnS: Mn2*<sup>+</sup> *QDs and Carboxyl-Functionalized MNPs*

The MEA-modified Mn2<sup>+</sup>-doped ZnS QDs (ZnS: Mn2<sup>+</sup> QDs@MEA) and the carboxylfunctionalized FeOx magnetic nanoparticles (FeOx@COOH MNPs) were synthesized based on previously reported methods with some modification [45–47]. The preparation procedures are presented in the Supplementary Materials.

#### *2.4. Synthesis of FeOx*/*ZnS Nanoparticles*

The FeOX/ZnS NPs was synthesized via an EDC/NHS reaction process. Briefly, 20 mg of FeOx@COOH MNPs were first dispersed in 20 mL citrate bu ffer solution (0.02 mol <sup>L</sup>−1, pH = 6.4) to prepare magnetic fluids. Then, 2 mL of magnetic fluids were added in 50 mL EDC/NHS activating agen<sup>t</sup> (1:1.2 g <sup>L</sup>−1). The above mixture was stirred for 2 h, followed by 30 mg of as-prepared amino-modified QDs being added and incubated at 30 ◦C for 20 h. The brown solutions were collected by magnetic decantation, and washed with water and ethanol to remove the residual substance, then redispersed in bu ffer solution for further use.

#### *2.5. Fabrication of FeOx*/*ZnS@MIPs*

The FeOx/ZnS@MIPs for BPA were prepared through molecular imprinting on the surface of magnetic fluorescence nanoparticle. For the fabrication of FeOx/ZnS@MIPs, 10 mL of a methanol solution containing 10 mg BPA and 60 μL APTES were first added to a 25 mL flask and stirred for 30 min. Then, 60 mg of the as-prepared FeOx/ZnS nanoparticle and 100 μL TEOS were added in sequence. After continuously stirring for another 30 min, 2.5 mL of 5% NH3·H2O (the catalyst) was added. The solution was deoxygenated by purging with nitrogen and stirred overnight. Non-imprinted polymers (FeOx/ZnS@NIPs) were prepared by the same procedure but without the addition of BPA. The resultant products were magnetically decanted and washed with a mixture of methanol and acetic acid (9:1, *v*/*v*) to remove the template molecules until the fluorescence intensity of FeOx/ZnS@MIPs was not changed and was similar to that of the FeOx/ZnS@NIPs ones.

#### *2.6. Fluorescence Sensing of BPA*

All of the fluorescence measurements were examined using the same condition. The excitation wavelength was set to be 311 nm with the fluorescence intensity recorded at 586 nm. The recording fluorescence emission spectrum was ranged from 350 to 700 nm.

The standard solution of bisphenol A was prepared first. Then, various samples in concentrations ranging from 1.0 to 80 ng mL−<sup>1</sup> were made by diluting with citrate buffer solution (0.02 mol <sup>L</sup>−1, pH = 6.4). Two milligrams of FeOx/ZnS@MIPs or FeOx/ZnS@NIPs were dispersed in 10 mL testing samples. After incubating the samples for 5 min at room temperature, the changes in the fluorescence intensity of the solutions were recorded using a spectrofluorometer. This fluorescence quenching model was in accordance with the Stern–Volmer equation [48]:

$$F\_0/F = \ 1 + K\_{\rm SV} \mathcal{C}\_{\rm BPA} \tag{1}$$

where *F*0 is the initial fluorescence intensity in the absence of the BPA, *F* is the fluorescence intensity in the presence of the BPA, *K*SV is the quenching constant, and *C*BPA is the concentration of BPA.

## *2.7. Binding Selectivity*

The selectivity experiments were carried out with PTBP, BP and BPZ as structural analogs of BPA to run a batch rebinding test. Briefly, PTBP, BP and BPZ samples in the concentration ranged from 1.0 to 80 ng mL−<sup>1</sup> and were made by diluting with citrate buffer solution. Two milligrams of FeOx/ZnS@MIPs or FeOx/ZnS@NIPs were dispersed in 10 mL testing samples and incubated for 5 min. Then the changes in the fluorescence intensity of different samples were recorded, and the imprinting factor (IF) and selectivity coefficient (SC) were used to evaluate the selectivity properties of FeOx/ZnS@MIPs and FeOx/ZnS@NIPs toward the template BPA and structural analogs [25]:

$$\text{IF } = \text{ } \lnot \text{\textquotedblleft} \lnot \text{\textquotedblright} \tag{2}$$

$$\text{SC} = \text{ IF/IF} \tag{3}$$

where *K*MIP and *K*NIP are the slopes of the linear equation of FeOx/ZnS@MIPs and FeOx/ZnS@NIPs with the target molecule, respectively, IF and IF' are the imprinting factor for template BPA and structural analogs, respectively.

#### *2.8. Analysis of Real Samples*

The prepared FeOx/ZnS@MIP was directly applied to the detection of BPA in drinking water, tap water, and lake water. The drinking water was commercial pure water purchased from the market. The tap water was collected from the laboratory. The lake water samples were collected from three different lakes (located in Beijing, China). All the samples were filtered through a 0.45 μm filter and stored at 4 ◦C.

The detection strategy was divided into two steps: (a) the enrichment and separation of BPA and (b) fluorescence detection (Figure S1). Firstly, the FeOx/ZnS@MIPs were dispersed in the water samples. The target BPA molecule was specifically bound onto the MIP layer of the FeOx/ZnS@MIPs after incubating at room temperature. Secondly, the fluorescence quench of FeOx/ZnS@MIPs in each sample was recorded and the concentration of analytes in the samples was calculated. There were

no other pretreatment procedures employed in the sample preparation. To evaluate the developed method, a recovery test was carried out by using the samples spiked with BPA standard solution.

#### **3. Results and Discussion**

#### *3.1. Synthesis of the FeOx*/*ZnS@MIPs*

The design of the FeOx/ZnS@MIPs was mainly based on coating the molecularly imprinted polymer layer on the surface of FeOx/ZnS NPs. Figure 1 illustrates the two major synthetic steps of the proposed fluorescence sensing polymer. In the first step, mercaptoethylamine was grafted onto the surface of the Mn2<sup>+</sup>-doped ZnS QDs. The mercapto of MEA was tightly bound at the surface of the bare QDs through ligand competition. The introduction of amino group to the QDs not only increases the water dispersion ability of QDs, but also provides the possibility of being combined with FeOx magnetic nanocrystals. FeOx magnetic nanocrystals were successfully synthesized by a modified solvothermal method. The –COOH was grafted onto the surface of FeOx in just one synthesis step. The carboxyl-functionalized FeOx NPs were conjugated with MEA capped QDs to prepare the multifunctional nanocomposites. The FeOx/ZnS NPs integrated the distinct properties of the optical characteristics of QDs, and the magnetic separation ability of MNPs through an EDC/NHS reaction process. In the second step, APTES was chosen as a functional monomer that had noncovalent interactions with bisphenol A [49]. The monomer (APTES) interacted with the template molecule (BPA) through a hydrogen bond to form a "pre-polymerization" complex (Figure 1). The resultant FeOx/ZnS NPs were used as substrate, TEOS and NH3·H2O were used as the crosslinker and catalyst, respectively. The pre-polymerization complex was subsequent immobilized on the surface of FeOx/ZnS NPs through a facile molecular imprinting process. The imprinting layer was coated on the FeOx/ZnS NPs to produce a "core-shell" structure. This core-shell composite provides selectivity to the template and prevents other interfering molecules from contacting the FeOx/ZnS NPs. After the removal of the template molecule BPA, the MIP layer with imprinted cavities complementary to the BPA in size, shape, and functional groups was obtained. The quantum yield (QY) of the FeOx/ZnS NPs was 20.6%, as calculated by equation S1. The resultant FeOx/ZnS@MIPs, as an ideal candidate material, was able to be used as a multifunctional sensor for high selectivity and sensitivity magnetic separation and the fluorescent detection of target BPA.

**Figure 1.** Schematic illustration of the process for the fabrication of the FeOx/ZnS@MIP-based sensor. BPA: bisphenol A.
