*2.2. Apparatus*

The X-ray diffraction (XRD) patterns of samples were measured using a 7000S device (Shimadzu, Japan). The test conditions were set with a Cu-Ka XRD source, a scan rate of 5◦/min, and a scan range of 10–70 degrees at room temperature. Surface morphologies of the prepared materials were observed using field emission scanning electron microscopy (JSM-7800F, Tokyo, Japan). Transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 UHR (Tokyo, Japan). The UV–visible (UV–vis) absorption spectra were recorded using a TU-1901 UV–visible spectrophotometer (Beijing, China). Photoluminescence (PL) spectroscopy was characterized by an F-7000 fluorescence spectrometer (Hitachi, Japan) at room temperature. A xenon lamp was applied as the excitation source for the photoelectrochemical tests. Photocurrent and electrochemical characterizations were conducted with a CHI 660C electrochemical workstation (Shanghai, China) with a typical three-electrode system.

## *2.3. Preparation of WO3 Nanoplate*

First, 0.4 g of Na2WO4·2H2O and 0.17 g of (NH4)2C2O4·H2O was dissolved in 33 mL deionized water. After stirring for 10 min, 9 mL of HCl solution (37%) was added. After stirring for another 10 min, 8 mL of H2O2 (30%) was added. Stirring was continued for 20 min. Then, 30 mL of absolute ethanol was added, and the solution was stirred for 30 min. The conductive surface of the pretreated SnO2 transparent conductive glass doped with fluorine conductive glass (FTO) was facing downward, inclined at 45◦ close to the beaker wall, placed in the above solution. This was placed in a constant temperature bath at 85 ◦C for 200 min and was then washed with deionized water and dried at 60 ◦C. Finally, the WO3/FTO electrode was prepared by calcining at 500 ◦C for two hours in a muffle furnace, cooling to room temperature, washing with deionized water, and drying.

## *2.4. Preparation of Au NPs*

Two milliliters of 50 mmol/L HAuCl4 solutions were added into a double-necked flask with 98 mL deionized water. Stirring and refluxing were performed in an oil bath at 120 ◦C. When reflux began, the sodium citrate solution was added (10 mL, 38.8 Mm). When the solution turned wine-red, after refluxing for 20 min, the solution was cooled to room temperature and stored in a refrigerator.

## *2.5. Preparation of CdTe QDs*

CdCl2·2.5H2O (0.0457 g) was dissolved in 50 mL of deionized water; then, 18 μL of TGA (thioglycolic acid) was added, and the solution was stirred for 10 min. The pH was then adjusted to 10.7 with 1 M NaOH. K2TeO3 (0.010 g) was dissolved in deionized water (50 mL) and added to the above solution after fully stirring. After 5 min, 80 mg NaBH4 was added. Then, the flask was connected to the condenser and condensed at 100 ◦C for 3 h. After cooling to room temperature, the TGA-coated CdTe QDs were centrifugally washed with ethanol. Then, the same amount of deionized water was added, and the solution was stored in a refrigerator at 4 ◦C.

#### *2.6. Preparation of the CdTe QD–Ap Conjugates*

Quantum dots (QDs) were coupled with aptamers (Ap) through an EDC–NHS coupling reaction. CdTe QDs (400 μL) were activated by 30 μL of 40 mM EDC and 30 μL of 15 mM NHS for 1 h. Then, 40 μL of 2 μM Ap was added, and the solution was placed in 4 ◦C shakers for 12 h. Finally, the solution was purified to obtain the QD–Ap conjugate.

#### *2.7. The Culture Process of Listeria Monocytogenes*

First, 1 g tryptone, 0.5 g yeas<sup>t</sup> extract powder, and 1 g NaCl were added to 100 mL of deionized water, and the pH was adjusted to 7.5 with NaOH. After sterilization, the configuration of the LB liquid medium was completed. The solid medium was prepared with 1 g tryptone, 0.5 g yeas<sup>t</sup> extract powder, 1 g NaCl, 1.5 g agar powder, and 100 mL deionized water. The pH was adjusted to 7.5, and the medium was sterilized with highpressure steam. The purchased *Listeria monocytogenes* lyophilized powder was stored with glycerin. Next, 20% glycerin was added to the bacterial solution, and the mixture was evenly mixed, divided into cryogenic tubes, and stored in a refrigerator at −80 ◦C. Before the experiment, an ultra-clean worktable was sterilized by ultraviolet irradiation for 30 min, and the *Listeria monocytogenes* frozen solution was thawed naturally at room temperature. After sterilization, 100 μL of thawing solution was added to the liquid medium, and then it was shaken in a shaking table at 37 ◦C for 15 h activation. The configured solid medium was poured into a Petri dish plate next to an alcohol lamp on the ultra-clean workbench to allow it to solidify naturally. After the plate was completely solidified, the secondary activation solution was picked up with the inoculation ring and streaked on the plate. Then, the plate was placed in a 37 ◦C incubator for 12 h. A single *Listeria monocytogenes* colony was added into the liquid medium, which was shaken on a shaking table at 37 ◦C for 15 h. Thus, the *Listeria monocytogenes* stock solution culture was completed. A series of 10 times gradient dilutions of *Listeria monocytogenes* stock solution was carried out.

#### *2.8. Fabrication of the Aptamer Sensor*

The working electrode was constructed as illustrated in Figure 1. First, 30 μL of Au NPs was dropped onto the surface of the WO3/FTO electrode. Then, 30 μL of cDNA was mixed with TCEP (0.6 μL, 10 mM) and dropped onto the surface of the Au/WO3/FTO electrode and incubated. After that, the electrode was washed with Tris HCl to remove the unconnected cDNA. Next, 30 μL of 1 mM 6-mercapto-1-hexanol (MCH) was added dropwise to the electrode, sealed for 1 h, and washed with Tris HCl to obtain an MCH/cDNA/Au/WO3/FTO electrode. Next, 30 μL of CdTe Quantum dot–aptamers (QD–Aps) was added to the MCH/cDNA/Au/WO3/FTO electrode and incubated at 4 ◦C for 1 h to hybridize the cDNA with Ap. When the QD–Ap/MCH/cDNA/Au/WO3/FTO electrode was obtained, photocurrent detection was carried out, and the detection value was recorded as a. After rinsing with Tris HCl, 30 μL of pathogenic bacteria solutions of different concentrations containing 20 U of Exonuclease I (Exo I) was dropped onto the QD–AP/MCH/cDNA/Au/WO3/FTO electrode and incubated for 60 min. After rinsing with Tris HCl, the electrode was placed in a refrigerator at 4 ◦C for photocurrent detection in the next step, and the photocurrent was recorded as b. The current change, detected twice, was recorded as ΔI = a–b. Finally, the relationship between ΔI and the concentration of *Listeria monocytogenes* was calculated and analyzed.

**Figure 1.** Construction of working electrode and working mechanism of the sensor.
