3.2.2. Electrochemical Sensors Based on MIPs
Molecularly imprinted polymers (MIPs) are tailor-made synthetic materials with high-affinity binding sites for a specific target molecule
[73][40]. MIPs are prepared by mixing a target molecule as a template with a cross-linking agent and an initiator. After polymerization, the template is removed, leaving the hole exactly the same as the target molecule. The forming hole can rebind perfectly with the target molecule, allowing it to be specifically recognized and detected target molecule. Therefore, sensors based on MIPs have been designed and applied in various fields, such as environmental analysis
[74][41], pharmaceutical analysis
[75][42], nucleic acid assay
[76][43], and food safety
[77][44].
3.2.3. Electrochemical Sensor Based on Biometric Molecules
Except for MIP, some biometric recognition elements such as antibodies, enzymes, and aptamers have been used to improve the selectivity of sensors. Timur et al.
[82][45] found an aptamer of IMI by the Systematic Evolution of Ligands by Exponential Enrichment (SELEX) process, and an aptasensor was constructed for IMI detection in a range of 0.1–50 ng mL
−1 with a LOD of 0.19 ng mL
−1. Pérez-Fernández et al.
[83][46] reported a competitive immunosensor for IMI detection based on AuNPs-modified screen-printed carbon electrodes (AuNPs-SPCE). In th
ise work, a monoclonal antibody to IMD (mAb-IMD) was immobilized on AuNPs-SPCE, and a competitive assay was performed between free IMI and IMI labeled with horseradish peroxidase (HRP). The electrochemical reduction signal of oxidized 3,3’,5,5’-tetramethylbenzidine (TMB) was associated with IMI concentration, avoiding the use of secondary antibodies. This sensor exhibited excellent performance for IMI determination, with a satisfactory low LOD, high selectivity, and stability. Furthermore, this sensor was successfully applied in IMI analysis on a real sample, and the reliability was also validated by HPLC-MS/MS.
3.2.4. Ratiometric Electrochemical Sensor
The classical electrochemical sensor contains only a single electrochemical signal of the target molecule, and its reproducibility is easily influenced by electrode properties or the complex detection system. To overcome this limitation, ratiometric electrochemical sensors involving the simultaneous measurement of two electrochemical signals at different potentials have been developed. By introducing a built-in correction for the analyte’s signal, the ratio electrochemical sensor greatly improves the reproducibility and reliability of electrochemical detection. So far, ratio electrochemical sensors have been used to detect metal ions, nucleic acids, proteins, biological small molecules, etc.
[86,87][47][48]. The researchers also applied the ratio sensing strategy for IMI detection. The Kan group
[88][49] constructed a ratio electrochemical sensor by electropolymerization of thionine and β-CD composite on GCE for IMI determination, in which thionine as an internal reference element provides a built-in correction. The current ratio of IMI and thionine was employed as the signal for IMI detection, and it exhibited a good linear relationship in the concentration range of 0.04–10 μM.
To sum up, researchers have developed a variety of electrochemical sensing strategies based on nanomaterials for IMI detection. On the one hand, designing and synthesizing nanomaterials with controlled morphology or preparing hybrid materials to further improve the electrocatalytic reduction of IMI is highly necessary. On the other hand, combining the advantages of easy miniaturization of electrochemical sensors with specific recognition elements, the establishment of hand-held electrochemical sensing devices with high selectivity and sensitivity has great prospects.
3.3. Optical Sensors
Over the past decades, researchers have devoted intensive efforts to developing various optical sensors due to their advantages, including simplicity, ultra-sensitivity, and high selectivity. The principle of an optical sensor is based on the shift of the characteristic signal caused by the interaction of the analyst with the substrate or with other optical molecules. With the development of nanotechnology, many optical detection platforms have been born. According to the different detection signals, multiple optical sensors, including fluorescence, colorimetry, surface plasmon resonance (SPR), and surface enhanced Raman spectroscopy (SERS), have been applied to detect IMI.
3.3.1. Fluorescent Method
Recently, with the significant development of optical nanomaterials, fluorescence sensing has dramatically benefited from various luminescent nanoparticles. For instance, Tian et al.
[90][50] developed two kinds of lateral flow immunoassay (LFIA) for IMI determination based on time-resolved fluorescent nanobeads and colloidal gold, respectively. The proposed LFIAs achieved high accuracy and a low LOD for IMI analysis. Guo et al.
[12][10] established a competitive fluorescence resonance energy transfer (FRET) immunoassay for IMI detection. FRET occurred through the specific immunoreaction between antigen/GO and mAb/up-converting nanoparticles, (UCNPs). The fluorescence intensity of UCNPs was weakened by GO, and the florescent recovery of UCNPs is associated with IMI concentration through the competitive mechanism. This sensor showed a wide range of 0.08–50 ng/mL for IMI in the presence of other interferences.
3.3.2. Colorimetric and Surface Plasmon Resonance (SPR) Sensors
A colorimetric sensor has the merits of visualization, simplicity, low cost, and being a powerful tool for high-throughput analysis. In recent years, various noble metal nanoparticles, such as AuNPs and AgNPs, have been applied for the construction of colorimetric sensors owing to their strong localized surface plasmon resonance (LSPR) effect. Further, plasmonic colorimetric sensors based on metal nanoparticles have been applied to detect various analytes, including metal ions, pesticides, proteins, DNA, pathogens, and so on
[92,93,94,95,96][51][52][53][54][55].
3.3.3. Surface-Enhanced Raman Spectroscopy
The Surface Enhance Raman Scattering (SERS) technique has received much attention in the field of analysis due to its non-invasive and unique fingerprint characteristics. In particular, the Raman signal can be enhanced with the use of nanomaterials with plasmonic properties, greatly improving the sensitivity of detection. For example, the O’Riordan group
[100][56] proposed a SERS sensor for two neonicotinoids, including clothianidin and IMI, using Ag nanoparticles coated Polyvinylidene fluoride (PVDF) substrates. The developed SERS can sense 1 ng/mL IMI with a LOD of 4 nM.