In recent years, food safety issues have become a major concern globally. The widespread utilization of pesticides has resulted in soil pollution, which has subsequently led to the detection of pesticide residues in numerous agricultural products, including those consumed by humans. Furthermore, these residues have even been found in human blood samples [1]. Food allergies are a significant public health burden in developed countries, drawing considerable attention. Notably, Australia has emerged with the highest prevalence of food allergies among developed nations, reaching an alarming rate of 10%. In contrast, other developed countries observe an incidence varying from 1% to 5% [2]. Pathogenic microorganisms have the potential to degrade seafood, leading to its spoilage. In the unfortunate event of consuming spoiled seafood, when these pathogenic microorganisms enter the body, they can pose significant food safety risks [3]. The presence of heavy metals not only hampers the growth and compromises the quality of crops but also leads to their accumulation within crops, ultimately finding their way into the human body through the food chain. Furthermore, it is worth noting that most heavy metals have been linked to genetic damage and an elevated risk of cancer development [4]. Therefore, it is essential to assess the quality of food to protect human health. Pesticide residues, illegal additives, allergens, pathogenic microorganisms, heavy metals, herbicides, and other risk factors present in food can significantly threaten human health [5]. Given the direct impact of food safety on human health, consumers are increasingly vigilant about the safety of the food they consume [6]. Therefore, the detection of food contaminants is of utmost importance. Inspectors analyze food for certain contaminants and allergens to determine whether they comply with local laws and regulations [7]. Traditional detection methods such as mass spectroscopy (MS), high-performance liquid chromatography (HPLC), enzyme-linked immunosorbent assay (ELISA), and gas chromatography (GC) have relatively high sensitivity and selectivity. However, these methods require expensive equipment and experienced operators, and the detection process is time-consuming, making them unsuitable for widespread use [8]. Thus, there is a critical need to develop methods that allow simple, fast, convenient, and highly sensitive detection of food contaminants [9].

Biosensors are analytical devices that are sensitive to biological substances and convert their concentrations into optical or electrical signals for detection. They are widely used to detect chemical substances containing biological components, such as enzymes, antibodies, and microorganisms [10]. Biosensors play a crucial role in various fields, including drug development, food processing, food safety testing, and environmental pollution detection [11]. Typically, biosensors are designed to perform rapid tests and generate a detectable signal when the molecular probe in the detector interacts with the desired analyte. This characteristic makes biosensors valuable tools for detecting a broad range of analytes, with high sensitivity and specificity [12]. Biosensors can be classified into several types, including electrochemical, piezoelectric, optical, mechanical, and thermal sensors. This categorization is based on the specific divisions of transducer types utilized in biosensing applications [13]. Sensors used for food detection are generally electrochemical biosensors and optical biosensors. The principle of electrochemical biosensors is based on the measurement of electrical signals generated by biochemical reactions occurring at the sensing interface. These biosensors typically consist of three main components: a recognition element (such as enzymes, antibodies, or DNA probes) that selectively interacts with the target analyte, a transducer that converts the biochemical signal into an electrical signal, and an electronic system for signal processing and analysis. The electrochemical biosensor operates by detecting changes in electrical properties, such as current or potential, resulting from the biochemical reaction between the recognition element and the target analyte. This interaction leads to the generation of a measurable electrical signal, which can be correlated with the concentration of the analyte of interest [14,15]. Optical biosensors typically consist of three main components: a recognition element (such as antibodies, enzymes, or nucleic acids) that selectively binds to the target analyte, a transducer that converts the biochemical signal into an optical signal, and a detector that measures the optical signal and provides the corresponding output. The principle of optical biosensors is based on the detection and analysis of changes in light signals resulting from the interaction between a biological recognition element and a target analyte. These biosensors utilize the properties of light, such as absorption, reflection, fluorescence, or refraction, to quantitatively or qualitatively measure the presence and concentration of specific analytes [16,17].

In the past decades, a variety of materials including metal nanoparticles, inorganic nanoparticles, quantum dots, and polymers have been employed to develop biosensors for on-site detection of food contaminants. Among them, polymers have shown great potential as candidates for developing biosensors that can be useful for food safety detection because polymers have many advantages for biosensing applications, including their ability to be easily functionalized with different biological or chemical sensing elements and their ability to be easily fabricated into a variety of shapes and sizes [18]. Polymers can be used as the sensing material in different types of biosensors, including optical, electrochemical, and mechanical biosensors [19]. In optical biosensors, polymers can be used as a matrix material for immobilizing fluorescent dyes or other chromophores that change their optical properties in response to the presence of a target analyte. In electrochemical biosensors, polymers can be used as the sensing material for detecting the electrochemical changes that occur when a target analyte interacts with the sensing material. Conductive polymers possess remarkable electrical conductivity, making them highly advantageous for augmenting the sensitivity of biosensors. In addition, specific types of conductive polymers demonstrate the unique ability to self-heal, consequently prolonging the operational lifespan of the sensor [20]. In mechanical biosensors, polymers can be used as the sensing material for detecting changes in the mechanical properties of the polymer when it interacts with a target analyte. In conjunction, polymer-based hydrogels offer commendable flexibility, thereby mitigating the potential for skin irritation [21]. Therefore, polymer-based biosensors have been widely used for a wide range of applications, including medical diagnosis, environmental monitoring, and food safety testing.

Heavy metals are present in humans in extremely low levels. However, an increase in the concentration of heavy metals in the body can pose a serious threat to human life. Unfortunately, due to escalating environmental and industrial pollution, heavy metals have become ubiquitous in daily food such as vegetables, fruits, and water. Their consumption can lead to severe health problems and can even induce cancer, renal dysfunction, immune system imbalance, and other debilitating conditions [22]. Hence, it is imperative to detect and monitor the levels of heavy metals in food. Table 1 summarized the applications of polymer-based biosensors in detection of heavy metals.

Lead (Pb) is one of the most common heavy metals, which can easily cause central nervous system damage in the human body, such as affecting human behavior, hearing impairment, etc. In severe cases, it may cause brain damage. Pb also interferes with the form of heme, leading to anemia [31]. Ghosh et al. employed long-period grating (LPG) and fiber Bragg grating (FBG) to create a fiber biosensor for heavy metal ion detection [23]. Graphene oxide, cross-linked chitosan composite, and poly-propionic acid were coated on the surface of the optical fiber to give the biosensor a better selectivity for Pb²⁺ detection. The attenuation bands of LPG and FBG were optimized to adapt to the optical S-band. The biosensor can detect Pb²⁺ through the selective adsorption of heavy metals, which can realize the interaction between the high-order cladding mode fading field and the surrounding refraction. Deshmukh et al. modified polypyrrole/single-walled carbon nanotubes, synthesized by electrochemical means, with ethylenediaminetetraacetic acid to create a nanocomposite platform for biosensors [24]. The biosensor can detect Pb²⁺ ions with high sensitivity, with a detection limit of 0.07 μM.

Mercury (Hg) is another common heavy metal, which can also easily accumulate in the human body and threaten human life and health. High levels of Hg in the human body can cause damage to the brain, heart, lungs, and other organs [32]. Therefore, it is very important to detect the content of Hg in food. Sadani et al. developed an optical biosensor based on localized surface plasmon resonance [25]. The biosensor immobilizes chitosan-coated gold nanoparticles onto fiber functionalized with bovine serum albumin for detecting Hg²⁺ ions. The biosensor exhibited high sensitivity, with a detection limit of 0.1 ppb for tap water. Yang et al. synthesized a three-dimensional reduced graphene oxide and polyaniline (3DrGO@PANI) by oxidative polymerization, using DNA as the adsorbent for detecting Hg²⁺ ions in aqueous solutions [26]. The biosensor exhibits high selectivity for Hg²⁺ ions in the range of 0.1 nM to 100 nM, with a detection limit of 0.035 nM. Raril et al. modified the graphene electrochemical biosensor by poly-glycine and tested Hg²⁺ through voltammetry cycling technology [27]. The biosensor also demonstrates high selectivity, stability, and sensitivity.

The accumulation of other heavy metals such as copper (Cu), chromium (Cr), and zinc (Zn) in the human body can also cause damage to the human body. Excess Cu may lead to neurological diseases such as Alzheimer’s disease [33]. A large amount of Cr in the human body will also have bad effects, which may cause human lung cancer and mitochondrial damage [34]. Small amounts of Zn are important for the body, boosting DNA and protein synthesis, but too much Zn in the body can cause diarrhea and headaches [35]. Wong et al. utilized polyurethane-wrapped β-carotene to create a biosensor capable of detecting heavy metals, such as Al, Cu, Pb, and Zn [28]. The biosensor operates on the principle that the reaction between different heavy metals and β-carotene leads to a change in optical density values. Heavy metals can be detected by measuring and observing the change in optical density values at λ = 450 nm, using a simple spectrophotometric instrument. Alizadeh et al. developed an innovative sensing platform for the detection of chromium utilizing carbon composite electrodes [29]. In this study, the researchers synthesized Cr³⁺ molecularly imprinted polymer (MIP) nanoparticles by employing methylene succinic acid as the monomer and ethylene glycol dimethacrylate as the crosslinker. Remarkably, the biosensor exhibits a low detection limit of 17.6 n mol/L for Cr³⁺ without the requirement of converting it to metallic Cr. Lu et al. have developed a biosensor to detect Cu²⁺ in drinking water using fabric materials coated with nylon 6 nanofibers, multi-walled carbon nanotubes, and 2,2′:5′,2″-terthiophene molecules [30]. The biosensor operates on the principle of current obstruction, generated by the adsorption of Cu²⁺ ions onto the biosensor. Remarkably, the biosensor can detect Cu²⁺ ions in the range of 0.65–39 ppm, exhibiting the advantages of sensitivity, flexibility, and portability.