Gold nanoparticles, negatively charged hydrophobic colloids, are among the earliest and most widely studied nanomaterials. Due to its high specific surface area, straightforward particle preparation, uniform particle size control, ability to increase catalyst size by silver deposition, and efficient surface modification by thiols or other bioligands [36,37,38]. Its unique biosensor properties and low toxicity make it a promising general-purpose platform for immobilizing antibodies (Ab), enzymes, aptamers (Apt), DNA, and other biomolecules [39]. The presence of AuNPs provides a suitable microenvironment for immobilizing biomolecules and reduces electron transfer resistance.

For the immobilization of Ab, Talan et al. developed a fluorine-doped tin-oxide electrode (FTO)-based electrochemical nanosensor for chlorpyrifos detection with AuNPs and Ab. The AuNPs were physically adsorbed on the surface of the FTO electrode to provide a homogeneous layer to obtain high conductivity. At the same time, AuNPs offered a platform to immobilize Ab through ionic or hydrophobic interactions. A specific antigen was then added for chlorpyrifos detection, resulting in significant current changes [40].

For the immobilization of enzymes, Song et al. created a carbamate pesticide sensing interface based on a citrate-capped AuNPs/(3-mercaptopropyl)-trimethoxysilane (MPS)/gold electrode (AuE). The negatively charged AuNPs/MPS/AuE interface repelled the equally charged [Fe(CN)₆]³⁻/⁴⁻, producing a negative response when ferricyanide ([Fe(CN)₆]³⁻/⁴⁻) was used as the redox probe. AChE catalyzed the hydrolysis of acetylthiocholine iodide (ATCI) to positively charged thiocholine. Thiocholine can then be fixed on AuNPs by replacing citrate with an Au–S bond, turning the negatively charged surface into a positively charged surface and attracting the negatively charged [Fe(CN)₆]³⁻/⁴⁻, producing a corresponding positive response [41]. Since carbamate pesticides can inhibit AChE activity, the sensing interface exhibited a satisfactory sensitivity. In another report, AuNPs were electrodeposited on screen-printed carbon electrodes (SPCE) to covalently immobilize AChE via the Au–S bond. After the formation of the sensor, ATCI was used as the substrate to reflect the activity of AChE. The fabricated transducer showed high potential in detecting OP pesticides [42].

Due to the unique optical properties of AuNPs, the localized surface plasmon resonance (LSPR) technology based on sensors composed of AuNPs has been gradually applied to detect biomolecules. Li et al. developed a sensor probe functionalized with graphene oxide (GO), AuNPs, molybdenum disulfide nanoparticles (MoS₂-NPs), and creatininase enzyme. The concentration of creatinine was determined by optical fiber LSPR [43]. Zhu et al. immobilized AuNPs and GO on the bare probe. Then ascorbic acid oxidase was functionalized. The great potential of ascorbic acid (AA) sensor in practical application of daily diagnosis was developed [44]. Singh et al. immobilized a layer of AuNP between the GO layer and the fiber surface to form a tapered optical fiber sensor probe. The selectivity of the sensor to uric acid (UA) was improved by functionalizing the nanomaterial-coated fiber with uricase enzyme [45]. Other biological molecules, such as AchE and heparin, can also be detected using AuNPs. Based on the competitive host guest interaction between sulfonated calix [6] arene (p-SC6A)-terminated AuNPs and rhodamine B (RhB)/acetylthiocholine, Lv et al. developed a fluorescent and colorimetric dual channel probe for rapid detection of AChE with high sensitivity and selectivity [46]. Unser et al. proposed a functional plasma protein scaffold by integrating collagen fibers with negatively charged citrate-capped gold nanoparticles. With the multiple functions of collagen, protein nanoparticle scaffold is used to detect its specific interaction with glucose and heparin through plasma coupling [47].

AgNMs are also commonly used in the manufacture of biosensors. Among all metals, silver (Ag) has the highest electrical and thermal conductivity and reflectivity. It is almost completely harmless to the human body. Representative Ag-based nanosensors include Ag nanoparticles (AgNPs), Ag nanorods (AgNRs), and Ag nanowires (AgNWs) [48].

For AChE immobilization, Lang et al. compared the performance of sensors composed of AuNRs and Au–Ag heterogeneous nanorods (AuAgNRs). Compared with AuAgNRs, AuNRs exhibited better catalytic performance and lower detection limits. The results showed that, when it comes to AChE immobilization, the silver surface did not show higher catalytic sites than gold [49].

However, AgNMs are suitable for butylcholinesterase (BChE) immobilization. The two cholinesterases have some differences in substrate binding, catalytic mechanism, and sensitivity to enzymes [50]. In order to assess the paraoxon content in multiple matrices, Turan et al. described a BChE amperometric biosensor combining AgNWs with a conducting polymer. First, a polymer film was created by electrochemical polymerization of the compound 5,6-bis(octyloxy)-4,7-bis(thiopheno[3][3,2-b]thiophene-2-yl)benzo[c][1,2,5]oxadiazole (TTBO) over the surface of a graphite electrode. Subsequently, BChE was cross-linked on the modified electrode surface via FA crosslinking. The results showed that biomolecules can maintain their activity under the best experimental conditions [51].

Magnetic nanoparticles (MNPs) are nanoscale particles that are generally composed of iron (Fe), cobalt (Co), nickel (Ni), and other metal oxides. A class of nanomaterial with distinctive physicochemical characteristics and a staggering array of uses is MNPs. MNPs can simplify complicated traditional experimental procedures and shorten the experimental time because of their distinct physical and chemical properties. Especially since iron oxides (Fe₃O₄) are biocompatible and facilitate easy preparation and functionalization, they are commonly used in biological analysis. MNPs are usually used as electrode modifiers in the detection technology integration of pesticide residue determination. When used as electrode modifiers, MNPs can significantly enhance the electron transfer between analytes and electrodes due to their very high charge transfer capability.

Unique properties of magnetic Fe₃O₄ nanoparticles include their huge surface area, excellent chemical stability, and low toxicity. Therefore, biomolecules (such as AChE) can be fixed on iron oxide nanoparticles (Fe₃O₄NP) to build a sensor for detecting OP. Fe₃O₄NP and carboxylated multi-walled carbon nanotubes (c-MWCNT) were utilized by Chauhan and Pundir to modify AuE. Following the decoration of Fe₃O₄ nanoparticles onto the surface of c-MWCNT, carboxyl groups (-COOH) that were present on the external surface of MWCNTs were free to bond with amino groups (-NH₂) on the surface of AChE. The sensor had good sensitivity, reusability, and durability, making it appropriate for the trace detection of OP pesticide residues in milk and water [52].

The aggregation of MNPs can be prevented by coating carbon materials, which also offers large-area support for potential modification. The properties of the obtained materials can be further enhanced by combining MNPs with large-area carriers (such as carbon spheres) [53]. AChE sensors were created in a study using mesoporous hollow carbon spheres (MHCS) and magnetic mesoporous hollow carbon spheres with core–shell structures (Fe₃O₄/MHCS). Glutaraldehyde (GA) was used as a cross-linker to immobilize AChE into nanocomposites without destroying the activity of AChE. The created sensor demonstrated good stability, particularly with the addition of Fe₃O₄NP, which improves the sensor’s stability [54].

Silica coating also can prevent the aggregation of MNPs and improve biocompatibility. In addition, it provides a suitable platform for further surface modification through various functional groups. Dzudzevic Cancar et al. synthesized core–shell MNPs sequentially modified with silica and -COOH. The MNPs’ silica inner shell and carboxyl outer skeleton maintain the nanoparticles in solution and offer locations for biomolecules to connect covalently. The carboxyl group of MNPs can form amide bonds with various enzymes, maintaining the enzyme’s catalytic activity and random orientation. In this regard, a graphite electrode was reported with its surface electrochemically polymerized with 4,7-bis (furan-2-yl) benzo [c] [1,2,5] thiadiazole (FBThF); the polymer surface was then modified with the -COOH and -SiO₂-functionalized magnetic nanoparticle (f-MNP) and AChE to form a biosensor for OP from tap water samples [55].

A crystalline material called a metal–organic framework (MOF) is composed of inorganic metal centers (metal ions or clusters) joined by organic ligands. It offers unparalleled tunability, a large surface area, high porosity, excellent catalytic activity, and a wealth of active sites [56]. It can be used as a highly selective platform for the development of sensors related to electrochemical detection. Numerous MOF materials have been applied to construct electrochemical sensors, including copper-based (Cu-MOF) [57], zirconium-based (Zr-MOF), nickel-based (Ni-MOF) [58], iron-based (Fe-MOF), and cobalt-based MOF (Co-MOF). In order to further endow more functions and improve its performance, it is an effective strategy to modify MOF by introducing heteroatoms, functional groups, and metal ions.

Deep et al. described the assembly of nano-MOF (NMOF) on a parathion sensing electrode. The indium tin oxide (ITO) electrode slide was modified by 2-aminobenzylamine (2-ABA) and sequentially dipped into the organic linker 2-aminoterephthalic acid and the metal ion “Cd²⁺” solutions. A derivative of aniline, 2-ABA, was electrochemically deposited on the ITO surface, freeing –NH₂ moieties to react with the –COOH groups of the organic linker, on which the metal center (Cd) subsequently assembled to grow a rod-like NMOF structure. The available –COOH groups immobilized anti-parathion antibody on the NMOF sensor, resulting in sensitive and selective analysis of parathion in a rice sample [59].

As an analog of graphene, two-dimensional (2D) transition metal dichalcogenides (TMDs), include MoS₂, WS₂, and MoSe₂. Through the interaction of van der Waals forces, these materials are composed of transition metal atomic layers sandwiched by two chalcogenide atomic layers and further stacked into multiple layers. Molybdenum disulfide (MoS₂) is a graphene analog stacked by covalently bonded S-Mo-S. In order to solve the problem that the AuNPs coating on the electrode surface easily fell off in the measurement process, monolayer MoS₂ nanosheets were reported in advanced biosensors to secure AuNPs by forming solid Au–S bonds [60]. For effective paraoxon detection, Jia et al. assembled an AuNPs/MoS₂/rGO/polyimide (PI) composite electrode. PI is a polymer with good film-forming properties. A wide potential range, high mechanical properties, flexibility, and temperature tolerance are all produced by the ideal interaction between rGO and PI [61,62]. Researchers further modified the rGO/PI electrode with MoS₂ and AuNPs, where the MoS₂ monolayer can reduce the AuNPs. Finally, the formed AuNPs/MoS₂/rGO/PI flexible film is used to immobilize AchE for paraoxon biosensing [63]. As another example, Zhao et al. utilized metallic MoS₂ nanosheets as the electrochemical platform to develop a disposable enzyme-based pesticide biosensor. In order to improve the performance of the sensor, AuNPs were electrodeposited in situ on the SPE surface. The results showed that the exfoliated thin metal MoS₂ nanosheet had suitable electron transfer properties and good immobilization effect on AchE. With ATCI as the substrate, the biosensor has satisfactory electrocatalytic activity for the electrochemical oxidation of thiocholine [64]. Representative examples of such biosensors are listed in Table 3.

The cylindrical nanostructures that build carbon nanotubes (CNTs) offer great physical properties, such as low density, significant porosity, and excellent conductivity. To create CNTs, single-layer graphite sheets are rolled into tubes, or multiple-layer graphite sheets are rolled into compressed cylinders [65]. As a promising material, they are often used to be incorporated into immunosensors in different ways. They are often coated on carbon electrodes to improve the selectivity and sensitivity of small molecule electrochemical assays. At the same time, the electrode surface area and adsorption density can be increased.

SWCNT or MWCNT needs further modification to immobilize biological molecules. In one study, MWCNTs were deposited with reducing gold salts and then treated with acid, in order to introduce hydrophilic functional groups such as -OH and -COOH. Functional groups provided anchor sites for metal nanoparticles and biological enzymes. The outer surface of MWCNT was modified by AuNPs, which is expected to improve the immobilization of enzymes and the electron transfer rate between reaction sites and electrodes [66]. In a subsequent study, SWCNT and MWCNT were used in combination. Carboxylated SWCNT surfaces were used to immobilize AchE, while MWCNTs were used to enhance the electrocatalytic activity of the electrodes. In order to create the working electrode’s core electrode, a gold wire was coated with AuNPs and MWCNT paste. AchE was immobilized on carboxylated SWCNT, then pasted onto the electrode core, and then coated with a Nafion layer which acted as binder to prevent the enzyme from leaching from the electrode. Excellent stability and reusability were displayed by this biosensor [67]. Introducing heteroatoms into SWCNTs can change their physical and chemical properties and improve their conductivity. Nitrogen is the ideal dopant for SWCNTs since its atomic radius is similar to that of carbon. As a result, C–N bonds make it simple to access SWCNTs [68].

To overcome the disorder and aggregation problem of SWCNT, an advanced version, vertical nitrogen-doped single-walled carbon nanotubes (VNSWCNTs) were developed. In one practice, VNSWCNTs were produced by spontaneous chemisorption of thiol-functionalized SWCNTs, where AuNPs were electroless plated on the surface. Subsequently, AchE was fixed on the AuNPs via Au–S bonds. The constructed AchE-immobilized biosensor was reported with excellent detection of methanol in cabbage water [69].

Graphene is a hexagonal network of covalently bonded sp² hybrid carbon atoms. It possesses unique physical and chemical properties, especially high conductivity, a sizable specific surface area, low toxicity, and high electron mobility [70,71,72,73]. In addition, GO with a variety of oxygen-containing chemical groups can bind biomolecules easily without additional activation, and it has excellent hydrophilicity and can be easily synthesized in the laboratory at very low cost [74].

For AchE immobilization, graphene nanocomposites perform better performance than pure graphene in the manufacturing of biosensors. A biosensor with AchE immobilization on CdS-decorated graphene (CdS-G) nanocomposite was reported by Wang et al. The nanocomposites had good electron transfer channels, and the immobilized AchE had high enzyme activity and affinity for ATCI. The AchE’s active site can be more easily contacted when OP was in contact with CdS-G nanocomposites, which reduced the inhibition time [75].

For Ab immobilization, Mehta et al. used the graphene sheets (GS) to modify the screen-printed carbon electrodes (SPCE). The electrode surface was electrochemically treated with 2-ABA to generate active -NH₂ functional group. The amine-functionalized graphene electrodes were biologically linked to the ferrocene (Fc) region of anti-parathion Ab through amide bonds (-CO–NH-). The biological conjugation process can be carried out directly without cross-linking agents or any other cumbersome chemicals [76]. A follow-up study replaced GS with amine-functionalized graphene quantum dots (GQD) to form electrochemical biosensors. It was simple to decorate the GQD with more homogeneity on the screen-printed surface, which improved the accuracy and precision of the sensor control. Additionally, the presence of functional groups in situ on the surface of GQD makes it easier for the 2-ABA moiety to firmly assemble and promote the amine function that modifies the anti-parathion antibody on the sensor. Compared with the previous model, the sensor composed of GQD provides a lower detection limit and a broader analysis range [77] (Table 4).

The SELEX technique, a rigorous in vitro chemical selection method, is used to create Apts, which are short nucleotide sequences of either single-stranded ribonucleic acids (ssRNAs) or single-stranded deoxyribonucleic acids (ssDNAs) [78]. Recently, efforts have been made to develop aptasensors for detecting pesticides.

Hong et al. identified four unique malathion-specific ssDNA Apts. Among them, two Apts showed high affinity to thioflavin T (ThT) and produced strong fluorescent signals. On such basis, two independent sensing strategies using fluorescent labeling and ThT displacement were designed. When malathion is combined with aptamer, it can be directly detected by fluorescence. When malathion with higher concentration is added to the sample, the fluorescence intensity decreases proportionally, indicating that malathion directly replaces the ThT dye in the determination. The selected aptamers formed a G4-tetraploid-like (G4Q) structure, which facilitated the development of a label-free assay with a detection limit of 2.01 ppb [79].

In addition, Apts can also be immobilized onto nanomaterials to form sensors for the detection of pesticides. In one study, thiol-tethered DNA-Apt-captured probes were immobilized on gold nanoparticles/polyaniline composite membrane-modified electrodes (AuNPs–PANI). Profenofos solutions containing a fixed amount of biotinylated DNA-Apt were dropped onto the aptasensor, with practically sufficient detection sensitivity. The hybridization reaction was measured using a streptavidin alkaline phosphatase conjugate that catalyzes the hydrolysis of 1-naphthyl phosphate. The 1-naphtholase product was detected by competitive binding [80]. In another study, AuNPs were electrodeposited on the surface of 2D Mo₂C/Mo₂N composite-modified electrode to connect with ferrocene (Fc) probe through Au–S bond. The Fc probe can hybridize with the Apt-probe to form a double-chain structure. The addition of chlorpyrifos (CPF) melted the double-chain, and the Fc probe was close to the electrode surface, which led to the amplification of the electrochemical response. The aptasensor was used to detect CPF in apple and cabbage samples with satisfactory recovery [81]. Xu et al. electrodeposited polydopamine-AuNPs (PDA-AuNPs) on the electrode surface and added exonuclease I (ExoI) to form a dual signal aptamer sensor for the detection of malathion. The 5‘and 3’ hairpin probes labeled with Fc and sulfhydryl groups respectively were fixed on the electrode by an Au–S bond and hybridized with the 5‘carboxylated Apt. Sulfur (Tn) with amino group is modified by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) as the second signal of Apt 5‘. After malathion was added, the aptamer malathion complex was formed by aptamer hybridization, which led to the melting of double chains and the weakening of Tn electrochemical signal. The sensor has high sensitivity and good selectivity, and was successfully applied to the practical detection of vegetable samples [82] (Table 5).