**1. Introduction**

Foodborne illness induced by pollution is a serious safety issue [1]. Pathogens in food can cause the food to spoil and rot, and some can produce toxic substances that can lead to illness. In recent years, food-borne poisoning incidents due to *Listeria monocytogenes* have aroused widespread concern [2]. *Listeria monocytogenes* can survive and grow under both aerobic and anaerobic conditions. It grows in a wide temperature range and has strong resistance to alkali and salt [3]. It is widely found in milk; meat; aquatic products; and frozen, and cold storage, and ready-to-eat foods [4]. *Listeria monocytogenes* can cause meningitis and sepsis, which have high hospitalization rates and mortality [5–7]. Various countries have formulated relevant limit standards. In China, it is stipulated that *Listeria monocytogenes* cannot be detected in meat products (GB29921-2013), as is true in the United States [8].

Traditional microbial detection methods include separation, culture, detection, and other steps. Although the detection accuracy is high, the detection time is long and the subsequent detection steps are cumbersome, which is not conducive to rapid detection [9]. In recent years, many rapid detection methods have been developed for pathogenic bacteria. These are mainly immunological detection methods based on the enzyme-linked immunosorbent method and the enzyme-linked fluorescence analysis method [10], along with molecular biological detection based on PCR technology and loop-mediated isothermal amplification technology [9,11–13]. However, all these methods need special reagents, instruments, and equipment, which are operated by professionals. There is a very important, practical need to explore more convenient, sensitive, effective, and rapid detection

**Citation:** Zhu, L.; Hao, H.; Ding, C.; Gan, H.; Jiang, S.; Zhang, G.; Bi, J.; Yan, S.; Hou, H. A Novel Photoelectrochemical Aptamer Sensor Based on CdTe Quantum Dots Enhancement and Exonuclease I-Assisted Signal Amplification for *Listeria monocytogenes* Detection. *Foods* **2021**, *10*, 2896. https://doi.org/ 10.3390/foods10122896

Academic Editors: Hong Wu and Hui Zhang

Received: 14 October 2021 Accepted: 16 November 2021 Published: 23 November 2021

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methods. Because of their good sensitivity, rapidity, and easy operation, biosensors have been developed and widely used. They have been used in medicine, environmental testing, food safety, and other fields [14–19]. The photoelectrochemical (PEC) sensor is a new type of detection technology with development potential. Because of the separation of the excitation light source and the generated photocurrent, the PEC sensor has a higher sensitivity than the electrochemical sensor and has grea<sup>t</sup> development potential [20]. PEC sensors have been used to detect DNA [21,22], miRNA [23,24], small organic molecules [25,26], heavy metal ions [27–29], proteins [30,31], and other substances, and they have attracted much attention.

Choosing optoelectronic materials with good photoelectric activity is one of the keys to improving sensor performance [32]. Graphite phase carbon nitride [33–35], bismuthbased semiconductors [36,37], and sulfides [30,38] are all considered potential materials. WO3 is a typical n-type tungsten oxide semiconductor that has good chemical stability, strong electron transmission ability, and a proper band gap (2.5–2.8 eV). It has been widely used in photocatalysis and sensors, as well as in other fields. Shen et al. [39] constructed a WO3-doped gold nanoparticle gas sensor that can detect ppb-level NO2. Lu et al. [40] prepared a core-shell WO3/CdS heterojunction, which also had excellent photocatalytic performance in the near-infrared region. Gold nanoparticles (Au NPs) have good electrical conductivity and the ability to fix identification elements. Therefore, they are often used to construct sensors and are promising intermediates [41].

Because of their characteristics of the narrow band gap, high yield, wide absorption, and adjustable size, quantum dots (QDs) have been widely used in the field of sensors [42,43]. Quantum dots are a kind of nanoscale semiconductor, such as CdS, CdSe and CdTe. In addition, QDs can be labeled with various biomolecules after chemical modification, and their good biocompatibility has unique advantages in biosensor applications. Aptamers are low cost, have high selectivity and good specificity, and are excellent substitutes for antibodies. In addition, aptamers have a variety of targets, which can be used to identify proteins, amino acids, pathogenic bacteria, cells, and viruses [44]. Exonuclease I (Exo I) can degrade single-stranded DNA in the 3→5 direction, and it only recognizes the 3 end of single-stranded DNA. Compared with other cutting enzymes, its operation is simple, so it is also widely used [45].

In this study, a PEC aptamer sensor based on WO3, CdTe QDs, and Exo I auxiliary signal amplification was constructed to detect *Listeria monocytogenes*. WO3 was used as a photosensitive material and modified with Au NPs as intermediates for connecting the complementary DNA (cDNA) of the aptamer. Then, the quantum dot-coupled aptamer (QD–Ap) is modified on the electrode by DNA hybridization. The photocurrent is significantly enhanced because of the sensitization of quantum dots. When the target bacteria and Exo I are present, the photocurrent is significantly reduced. The constructed sensing platform provides a new strategy with a high potential for the rapid and efficient detection of *Listeria monocytogenes* in food.
