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Perspective
Molecularly imprinted polymer as an antibody substitution in pseudoimmunoassays for chemical contaminants in food and environmental samples Chaochao Chen, Jiaxun Luo, Chenglong Li, Mingfang Ma, Wenbo Yu, Jianzhong Shen, and Zhanhui Wang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05577 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018
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Journal of Agricultural and Food Chemistry
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Invited Perspective for the Journal of Agricultural and Food Chemistry
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Molecularly imprinted polymer as an antibody
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substitution in pseudo-immunoassays for chemical
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contaminants in food and environmental samples
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Chaochao Chen, Jiaxun Luo, Chenglong Li, Mingfang Ma, Wenbo Yu, Jianzhong Shen,
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Zhanhui Wang*
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Beijing Advanced Innovation Centre for Food Nutrition and Human Health, College of
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Veterinary Medicine, China Agricultural University, Beijing Key Laboratory of
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Detection Technology for Animal-Derived Food Safety, Beijing Laboratory for Food
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Quality and Safety, Beijing 100193, People’s Republic of China
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* Author to whom correspondence should be addressed
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Tel: +86-10-6273 4565
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Fax: +86-10-6273 1032
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E-mail:
[email protected] 19
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ABSTRACT
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The chemical contaminants in food and environment are quite harmful to food safety
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and human health. Rapid, accurate and cheap detection can effectively control the
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potential risk derived from these chemical contaminants. Among all detection methods,
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the immunoassay based on specific interaction of antibodies-analyte is one of the most
25
widely used technique in the field. However, biological antibodies employed in
26
immunoassay usually cannot tolerate extreme conditions, resulting in unstable state in
27
both physical and chemical profiles. Molecularly imprinted polymers (MIPs) are a class
28
of polymers with specific molecular recognition abilities, which are highly robust,
29
showing excellent operational stability under a wide variety of conditions. Recently,
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MIPs have been used in biomimetic immunoassays for chemical contaminants as
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antibody substitute in food and environment. Here, we reviewed these applications of
32
MIPs
33
immunosorbent assay, fluorescent immunoassay, chemiluminescent immunoassay,
34
electrochemical
35
homogeneous immunoassay, and discussed current challenges and future trends in the use
36
of MIPs in biomimetic immunoassays.
incorporated
in
different
immunoassay,
analytical
microfluidic
platforms, such
paper-based
as
enzyme-linked
immunoassay,
and
37 38
KEYWORDS: molecularly imprinted polymers, biomimetic immunoassay, chemical
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contaminants, food, environment
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INTRODUCTION
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Food safety and environmental protection is directly related to human health, which
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is a hot topic of study worldwide. Currently, the detection of chemical contaminants in
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food and environment, such as small organic molecules (e.g., phenolic substances,
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antimicrobial agents, insecticides, and illegal additives), mycotoxins (e.g., aflatoxins and
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ochratoxins), and small inorganic molecules (e.g., phosphate), is usually achieved by
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physical and chemical analysis like chromatography and chromatography-mass
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spectrometry,
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chromatography and chromatography-mass spectrometry, which needs complicated and
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expensive equipment, immunoassays that are based on antigen-antibody specific
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interaction have been widely applied due to the high sensitivity, specificity,
51
reproducibility, and analysis speed.
biological
detection,
and
the
immunoassay1.
Compared
with
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In the past decades, the immunoassay has been continuously improved and
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developed, and a variety of immunoassays format with different labels have been
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established in the following order: radioimmunoassay, enzyme-linked immunoassay,
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fluorescent immunoassay, and chemiluminescent immunoassay2. These assays have been
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effectively applied in the detection of chemical contaminants in food control and
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environment monitoring. The development of immunoassay is mainly reflected in the
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synthesis of new labeled probes and the development of highly efficient labeling
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technologies, such as the appearance of nanoprobes (e.g. quantum dots, graphene, and
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upconversion luminescent materials). Despite the advances in efficient labeling of probes,
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the sensitivity and stability of the current immunoassay methods based on traditional
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antibodies still do not meet the requirements of highly sensitive detection due to the 3
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complexity of the matrix in food and environmental samples. This is mainly because
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biological antibodies, including the receptor protein, aptamer, and other protein scaffolds,
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are susceptible to matrix interference when the immunoreaction happens, especially
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under acid, alkali, high-salt, high-temperature, and other harsh conditions, thus affecting
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recognition ability and resulting in low sensitivity and stability. The preparation of
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antibodies and bioconjugates not only takes a long time and costly, but also requires a
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large number of experimental animals. Because of the low chemical and thermal stability,
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biological antibodies is not suitable for long-term preservation. In addition, some toxic
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compounds and immunosuppressive agents have strong toxic effects on immune system,
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causing low yield of effective antibodies from host animals. All above mentioned
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disadvantages make the preparation of biological antibodies and the development of
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immunoassay faces great challenges. Consequently, materials with high affinity, high
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stability, and low cost which could act as antibody function in assay are in high demand3.
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Molecularly imprinted polymers (MIPs) have multiple binding sites on the surface,
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which can selectively recognize target molecules, appearing in response to this demand.
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At present, MIPs have applied to many fields such as solid phase extraction4, chiral
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separation5, drug controlled release6, mimetic enzyme catalysis7, environmental
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monitoring8. Compared with traditional biological molecules, MIPs can tolerate high
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temperature, acid, alkali, and organic solvents, and they do not degrade easily, which
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makes them reusable. Therefore, MIPs as biomimetic antibodies represent a stable and
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low-cost alternative to existing antibodies in immunoassays. In this perspective, we
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elucidated pseudo-immunoassays based on MIPs from the aspects of biomimetic enzyme-
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linked immunosorbent assay (ELISA), biomimetic fluoroimmunoassay, biomimetic
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chemiluminescent immunoassay, biomimetic electrochemical immunoassay, biomimetic
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microfluidic paper-based immunoassay, and biomimetic homogeneous immunoassay
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with the aim of facilitating the development and exploration in the current field for
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researchers (Figure 1).
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PSEUDO-IMMUNOASSAYS
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Biomimetic ELISA. The ELISA is the most widely used detection technology for
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rapid detection of chemical contaminants in food and environment. It has many
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advantages such as being fast, sensitive, and easy to standardize; also, its results are easy
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to evaluate and capable of detecting lots of samples. Therefore, a variety of biomimetic
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ELISAs (BELISA) have been established by combining with MIPs and applied to detect
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chemical
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sulfamethazine residue12, trichlorfon13, metolcarb residues14, and methimazole15. In
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BELISA, MIPs, instead of antibodies, are coated on the microplate, and detect the targets
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generally by competitive methods. Compared with the traditional ELISA, there is no need
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for repeated coating and blocking steps, thus assay time is reduced. In addition, MIPs can
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be reused repeatedly without loss of activity, making up for the defects of biological
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antibodies. However, compared with conventional antibody based assay, the sensitivity of
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BELISA is not significant improved. This may due to the low affinity of MIPs.
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Furthermore, MIPs are mostly prepared in organic solutions which leads the denaturation
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of enzymes and other used proteins.
contaminants,
such
as
enrofloxacin9,
carbaryl10,
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green11,
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With the development of hydrophilic MIPs, the sensitivity of BELISA has been
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greatly improved. For example, Chianella et al.16 synthesized vancomycin MIP
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nanoparticles (nanoMIPs) by solid-phase synthesis in a special reactor, using vancomycin
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covalently bound on glass beads as a template. The prepared nanoMIPs were then coated
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on a microplate, and a direct competitive BELISA was established to detect vancomycin
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in porcine plasma (Figure 2). The detection sensitivity of this method is three times
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higher than that of traditional ELISA. It can accurately detect vancomycin with the limit
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of detection (LOD) as low as 2.5 pM, and it was less affected by the plasma matrix.
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Smolinska-Kempisty et al.17 synthesized MIPs of four different small molecules to
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establish BELISA, and compared them to ELISA with antibodies as the recognition
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molecule. They found that the BELISA method could detect the target at picomolar
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concentrations with no cross-reactivity, and performed similarly to or even better than
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ELISA which is based on biological molecules. In addition, it has a high stability because
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the microtiter plate coated with nanoMIPs membrane can be stored at room temperature
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for one month without loss of activity. These findings have laid a solid foundation for
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commercial applications of MIPs in the field of biological detection.
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Biomimetic Fluoroimmunoassay. The fluorescent detection has the advantage of
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rapid, simple, and high sensitivity, thus it shows great potential for rapid detection. The
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biomimetic fluoroimmunoassay (BF), in which MIPs are integrated with fluorescent
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molecules, takes advantage of the recognition capabilities of fluorescent MIPs (fMIPs)
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for target molecules and uses a fluorescence analyzer for fluorescence signal. It combines
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the selectivity of MIPs with the high sensitivity of fluorescence detection, realizing fast
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and simple detection and analysis for targets. Based on whether the target molecule has
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the property of fluorescence, BF detection can be classified into two categories: direct
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fluorescence detection and indirect fluorescence detection18. In direct fluorescence
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detection, the target molecule should possess a fluorophore or chromophore itself. The
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fluorescence intensity changes after MIPs bind with the target molecule thereby enabling
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the detection. For instance, in the work of Ton et al.19, as enrofloxacin (ENRO) is UV-
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excited, it was used as a template to synthesize hydrophilic MIPs in order to detect
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fluoroquinolones in water and milk. However, very few targets themselves possess
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fluorescence property, the application of direct fluorescence detection is restricted. The
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most commonly used technique is indirect fluorescence detection. During the MIPs
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preparation process, fluorescent functional monomers or fluorescent materials such as
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organic dyes, quantum dots(QDs), and upconversion fluorescent molecules are
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introduced. When the target molecules bind with the MIPs, electron, charge, or energy
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transfer will occur in the fluorescent functional monomer or fluorescent material, leading
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to fluorescence quenching or enhancement by which the analytes are detected. We will
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discuss as follows: i) BF based on organic fluorescent dyes; ii) BF based on quantum dots;
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iii) BF based on metallic nanomaterials; iv) BF based on optical fiber.
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Biomimetic Fluoroimmunoassay Based on Organic Fluorescent Dyes. Organic
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fluorescent dyes are often used in the establishment of fMIPs chemosensors (fMIPcs) to
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detect contaminants, because of their high fluorescence quantum yield in water or organic
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phase and no radiation. Wu et al.20 established fMIPcs by doping dansyl methacrylate as
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a functional monomer in order to detect BPA in environmental water, which also can
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monitor the biodegradation of BPA in real time. The prepared fMIPs have dual functions
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of binding BPA and generating a fluorescent signal. Since BPA has two phenolic
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hydroxyl groups, it is weakly acidic and can interact with the amino groups of dansyl
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methacrylate which altered the electron density near the fluorophore center, resulting in
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fluorescence quenching. The fMIPcs could accurately detect BPA with a LOD of 13.14
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pM in environment. Wang et al.21 prepared fMIPs via fluorescein 5(6)-isothiocyanate and
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3-aminopropyltriethoxysilane/SiO2 particles for detection of λ-Cyhalothrin in chinese
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spirits. By fluorescence quenching mechanism, the fMIPs could detect λ-Cyhalothrin
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with a LOD of 9.17 nM, which demonstrated fMIPcs a potential tool for recognition and
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detection of analytes in food.
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Some researchers used Fe3O4@SiO2 magnetic nanoparticles in combination with
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organic dyes to prepare fMIPs, which have exhibited excellent superparamagnetism and
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stable fluorescence properties. For example, Gao’s group22 first coated SiO2 and 3-
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(methacryloxyl) propyl trimethoxysilane (MPS) on Fe3O4 to form Fe3O4/SiO2-MPS,
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which was used as core, and then synthesized fMIPs shell by copolymerization of
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acrylamide with allyl fluorescein on the surface of Fe3O4/SiO2-MPS (Figure 3A), thereby
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detecting λ-cyhalothrin in honey. Due to the unique magnetic properties of Fe3O4
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nanoparticles, the synthesized fMIPs were much easier to separate from the solution,
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which simplified the whole process. In addition, the fMIPs can be recycled by an external
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magnetic field, and there was almost no fluorescence intensity decrease after being reused
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for five times, which makes fMIPs potential substitution of antibodies.
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Biomimetic Fluoroimmunoassay Based on Quantum Dots. Besides organic
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fluorescent dyes, QDs are also widely used in BF. They have stable photochemical
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properties and high luminous efficiency; the emission spectrum is narrow and
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symmetrical, and the fluorescence signals can be obtained without background
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interference, when compared with organic fluorescent dyes. Furthermore, changing the
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conditions of QD growth in synthesis could obtain QDs with different emission
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wavelengths. Xu et al.23 used trinitrophenol as a mimicking template, 3-
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aminopropyltriethoxy silane (APTES) as the functional monomer, and coated MIPs on
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CdTe QDs by the sol-gel method to synthesize fMIPs (Figure 3B). The synthesized
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fMIPs were utilized as recognition elements to detect 2,4,6-trinitrotoluene (TNT) in soil
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on the basis of electron transfer induced fluorescence quenching. With the existence and
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increase of TNT in samples, a Meisenheimer complex can be formed between the TNT
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and the amino groups of APTES on QDs. The energy of QDs transferred to the
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Meisenheimer complex which would lead to fluorescence quenching, thereby detecting
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TNT. The detection range of this method was 0.8–30 µM with a LOD of 0.28 µM.
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Raksawong et al.24 synthesized hybrid MIP-coated QD nanocomposite to detect
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salbutamol. When salbutamol molecules bind with the recognition sites in MIP, electron
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transfer happens from QDs to salbutamol, which leads to a fluorescence quenching in the
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nanocomposite. The synthesized nanocomposite could sensitive and selective detect
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salbutamol in animal feeds and meat samples with a detection limit of 0.142 pM.
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However, QDs encapsulated into highly cross-linked MIPs often affect the fluorescence
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quenching efficiency, leading to poor detection sensitivity18. To increase the fluorescence
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quenching efficiency and fluorescent signals, mesoporous structures are used by
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researchers to synthesize MIPs with nanosized pore wall. For instance, Kim et al.25
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prepared fMIPs by using mesoporous silica for the detection of BPA. The fMIPs had a
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detection sensitivity of 0.438 nM and were highly selective. As mesoporous silica
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usually had a highly cross-linked rigid structure with a large pore volume, the reaction
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sites of CdSe QDs with mesopores were located in the mesopores. Therefore, QDs could
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stably exist in mesoporous silica to make the analyte bind to the functional monomer
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more easily.
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However, the self-quenching of QDs is prone to occur, which seriously limit its
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application in bio-analysis. The ion-doped QDs not only retain the advantages of
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traditional QDs, but also have a large Stokes shift, which can effectively avoid self-
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quenching. Therefore, ion-doped QDs have attracted more and more attention, and the
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use of them in preparing fMIPs has become a hot research topic. In a study by Ren et
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al.26, Mn2+ doped ZnS QDs were used as core, and MIPs were encapsulated on its surface
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to detect cyphenothrin in water samples. As the UV absorption of cyphenothrin was close
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to the band gap of absorption spectra of MIPs-QDs, when cyphenothrin was added, the
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conduction band electrons of QDs transited to the lowest unoccupied orbit of
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cyphenothrin, a charge transfer then occurred, resulting in fluorescence quenching. The
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LOD of this method was 9 nM with a good linearity in the concentration range of 0.1-80
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µM, which provides a new way of preparing fMIPs by encapsulating ion-doped quantum
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dots.
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Although ion-doped QDs have been used in the field of food and environmental
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detection, they are all preparation of heavy metal and are highly toxic to humans and
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environment. Thus, graphene quantum dots (GQDs), which show low toxicity, high
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fluorescence activity, chemical inertness, and excellent water solubility, have become one
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of the most popular optical elements in multi-field research. Mehrzad-Samarin et al.27
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embedded GQDs into silica MIPs, then synthesized an optical nanosensor to detect
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metronidazole in biological samples. The fluorescence of the GQD-MIPs was
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proportionally quenched in the presence of different concentrations of metronidazole, and
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a LOD of 0.15 µM was obtained. Similar to GQDs, carbon dots are also used in the field
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of biological detection as a new material because of their unique optical properties, low
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toxicity, and high biocompatibility. Yang et al.28 encapsulated carbon dots on MIPs by
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reverse microemulsion method to detect tetracycline in fish samples with a LOD of 9 nM,
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laying a foundation for the establishment of new MIPs based fluorescence biosensors.
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Biomimetic Fluoroimmunoassay Based on Metallic Nanomaterials. Some of
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metallic nanomaterials have unique luminescent properties and have become a powerful
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component in the construction of chemical sensors. For example, noble metallic
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nanoclusters have strong fluorescence due to the aggregation induced effect and are
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widely used in the preparation of fluorescent probes because of their small size, mild
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preparation conditions, and non-toxic properties29. Compared with organic fluorescent
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dyes, noble metallic nanoclusters exhibit stronger and more persistent fluorescence
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signals; also, they are less biologically toxic and have better biocompatibility than QDs.
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Therefore, they have showed great potential for practical applications in many fields. By
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combining noble metallic nanoclusters with MIPs and traditional immunoassays, the
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performances can be significantly improved in several aspects. Wu et al.30 covalently
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bound gold nanoclusters (AuNCs) with SiO2 nanoparticles to form SiO2@AuNCs and
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prepared MIPs membrane by the sol-gel method in SiO2@AuNCs using BPA as a
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template (Figure 3C). When BPA existed, it would form a Meisenheimer complex with
246
the primary amino group on the surface of AuNCs. For the principle of fluorescence
247
resonance energy transfer, the fluorescence energy of AuNCs was transferred to the
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complex, leading to fluorescence quenching. BPA was detected with the changes in
249
fluorescence intensity and a LOD of 0.963 nM was achieved. In this paper, MIPs showed
250
high selectivity for BPA which makes it a stable alternative to existing antibodies. Based
251
on the fact that silver nanoclusters (AgNCs) in the aqueous solution showed higher
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fluorescence intensity than AuNCs, which have more superiority in optical detection31,
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Deng et al.32 synthesized MIPs of BPA on the surface of AgNCs and detected BPA with a
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LOD of 87.6 pM, which was an order of magnitude higher than that of Wu’s work.
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Besides noble metallic nanomaterials, lanthanides, as emerging optical materials, are
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gradually combined with MIPs for applications in bio-analysis field. They have abundant
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emission spectra, high quantum yield and chelate stability. Liu et al.33 developed core-
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shell surface molecularly imprinted fluorescent sensor, YVO4:Eu3+@MIPs, using
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lanthanide doped YVO4:Eu3+ nanoparticles as the signal output and MIPs as recognition
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elements, while λ-cyhalothrin was employed as the molecular template. Based on the
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energy transfer principle, When λ-cyhalothrin was combined with MIPs, the donor
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YVO4:Eu3
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fluorescence quenching, thereby qualitatively and quantitatively detecting the λ-
264
cyhalothrin in environment with a LOD of 1.76 µM. Liu’s group34 also synthesized
265
fMIPs, mSiO2-Eu (TTA) 3Bpc@MIPs, by Pickering emulsion polymerization with good
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homogeneity and monodispersity, and successfully applied it to detect λ-cyhalothrin. The
+
transferred its energy to the receptor λ-cyhalothrin, resulting in static
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Eu3+-modified mesoporous silica microspheres mSiO2-Eu(TTA)3Bpc synthesized can
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replace the surfactant as a stabilizer in Pickering emulsion polymerization, making the
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dispersed droplets more stable. This is a new type of interfacial nanotechnology, which is
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suitable for the preparation of MIPs, and the final results showed that these MIPs have
271
good stability, as the fluorescence intensity remained stable for nearly a month.
272 273
Biomimetic Fluoroimmunoassay Based on Optical Fiber. Unlike MIPs based on
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organic materials and SiO2 nanoparticles, optical-fiber based MIPs sensors are light in
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weight, small in volume, and easy to work into a variety of configurations. They possess
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anti-electromagnetic interference, electrical insulation, corrosion resistance, and can be
277
measured from a long distance. Ton et al.35 prepared micro-scale MIPs at the end of an
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optical fiber by photo-polymerization, and it only took a few seconds to prepare the
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fluorescence sensor. Gold nanoparticles were embedded in the polymers to improve
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signal response. The sensor was used to detect herbicide 2,4-D with a LOD of 250 pM,
281
which is comparable to that of the LC-MS method. This kind of fluorescence
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enhancement strategy avoids the occurrence of false positives due to non-specific
283
fluorescence quenching, thus it is more advantageous than fluorescence quenching
284
method. Later, Carrasco et al.36 utilized an optical fiber bundle containing about 50,000
285
individual fibers to create microwells via chemical etching, which was then deposited
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with red-fluorescent-encoded MIPs for ENRO to detect antibiotic. It worked by red-
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fluorescently-tagged ENRO competing with untagged ENRO. In the presence of low
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concentration of ENRO, strong fluorescence would be detected. The sensor demonstrated
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excellent selectivity which makes it potential for ENRO determination in complex
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samples.
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With the developments in science and technology, the new fiber optic sensor came
292
into being, which combines the evanescent wave, BF, and optical fiber into one single
293
sensor. The evanescent wave generates when light is transmitted in the optical fiber in a
294
total reflection manner and stimulates fluorescence MIPs on the surface of the fiber core,
295
causing fluorescent changes. The target molecules attached to the fiber surface could then
296
be detected by the changes in fluorescence intensity. Ton et al.37 developed a disposable
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optical fiber sensor by coating MIP particles on 4-cm long polystyrene optical
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waveguides with N-(2-(6-4-methylpiperazin-1-yl)-1,3-dioxo-1H-benzo[de]isoquinolin-
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2(3H)-yl-ethyl)acrylamide (FIM) as a fluorescent monomer. FIM produces fluorescence
300
under the evanescent wave excitation, and the target molecule with a carboxyl group
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bound to FIM would enhance its fluorescence. The herbicides 2,4-D and citrinin were
302
detected based on this mechanism. The LOD can reach nanomolar level with high
303
sensitivity. Though the BF has been widely in the detection, the problem of low
304
sensitivity is common in MIPs-based fluorescent assays. There still exists much work to
305
be done to improve the sensitivity while maintaining the selectivity18.
306 307
Biomimetic
Chemiluminescence
Immunoassay.
Unlike
the
fluorescence
308
immunoassay, the chemiluminescence immunoassay does not require external excitation
309
and has the advantages of simplicity and wide detection range. However, the anti-
310
interference ability of the chemiluminescent assay derived from matrices is not enough
311
robust, which would lead to different luminescence signals with remarkable matrix effect
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for the same quantity in different of sample matrices.38 In recent years, researchers found
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that MIPs could effectively eliminate the matrix effect, and used MIPs to specifically
314
identify and capture target molecules, which significantly improves the selectivity of the
315
chemiluminescence immunoassay.
316 317
Flow Injection Biomimetic Chemiluminescence Immunoassay. The most significant
318
feature of flow injection chemiluminescence (FI-CL) is its sensitivity; however, it has
319
poor specificity. By combining FI-CL with MIPs, the problem of selectivity is
320
fundamentally avoided. For example, Qiu et al.39 developed a FI-CL sensor using MIPs
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as biomimetic antibodies to detect sulfamethoxazole. With high adsorption binding sites
322
of MIPs, the sulfamethoxazole could be absorbed selectively
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With the popularity of surface imprinting technology based on Fe3O4 magnetic
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beads (MBs), the amount of its combined application with FI-CL is on the rise. Lu et al.40
325
obtained core-shell magnetic molecularly imprinted polymers (MMIPs) with chrysoidine
326
acting as molecular templates by surface imprinting on Fe3O4@SiO2 MBs. The MMIPs
327
were then combined with FI-CL to detect chrysoidine with a detection limit of 0.617 µM.
328
Based on Lu’s work, Cao et al.41 introduced an oil-based magnetic fluid to successfully
329
prepare surface MIPs with oleic acid (OA) encapsulating Fe3O4 MBs. The OA@Fe3O4
330
has strong magnetic properties and can be well dispersed in the oil phase. It improved the
331
adsorption capacity and separation performance of MIPs, thus enhancing the selectivity
332
of FI-CL.
333
Core-shell imprinted microspheres and solid-core imprinted microspheres prepared
334
by surface imprinting technique only have effective recognition sites on the surface of
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MIPs. Being different with them, porous hollow imprinted microspheres form a high
336
density of binding sites not only on the surface, but also in the interior, thereby greatly
337
improving the recognition efficiency of MIPs. Zhao et al.42 prepared fenpropathrin-
338
imprinted porous hollow microspheres (FIPHMs) by using mesoporous silica particles
339
and encapsulated FIPHMs in a "Y"-channel flow cell as recognition elements, then a flow
340
injection system was connected with to construct a FI-CL-MIP sensor. The sensor could
341
achieve detection of fenpropathrin with a LOD of 87.8 pM.
342 343
Biomimetic electrogenerated chemiluminescence immunoassay. Electrogenerated
344
chemiluminescence immunoassay (ECL) based on MIPs (MIP-ECL) is a new analytical
345
method developed in recent years. It integrates the advantages of MIPs into ECL, and
346
shows great application prospects in food, environment and other fields. Li’s group43
347
obtained MIP membranes by electro-polymerization on the surface of gold electrodes
348
using isoproturon (IPU) as templates, and prepared a new type of MIP-ECL sensor
349
through competitive adsorption of IPU and glucose oxidase-labeled IPU (GOD-IPU) on
350
MIP membranes and the chemiluminescence response of luminol and H2O2 produced by
351
GOD’s hydrolysis of glucose. The sensor was successfully applied to detect IPU in the
352
water sample with a LOD of 3.78 pM. Later, Li’s group44 also established a MIP-ECL
353
method for determination of gibberellin A3 (GA3) based on direct competition (Figure
354
4A). First, the MIPs membrane was synthesized on gold electrodes, and GA3 directly
355
competed with rhodamine B-labeled GA3 (RhB-GA3) to bind with MIPs. When RhB-
356
GA3 was bound to the MIPs membrane, RhB was electrochemically oxidized to produce
357
intermediate RhBox, thereby amplifying the chemiluminescence signal of luminol in the
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solutions to achieve the determination of GA3. The results showed that the MIP-ECL
359
sensor had a good selectivity and a high sensitivity to GA3 with a LOD of 3.45 pM.
360
Recently, exploring nanomaterials as MIP-ECL emitters has become a new trend.
361
For instance, as UCNPs possess tremendous ECL performance and prolonged
362
fluorescence lifetime, Jin et al.45 employed them as luminophor in MIP-ECL for detection
363
of clenbuterol with a LOD of 6.3 nM. They first prepared reduced grapheme oxide (rGO)
364
on electrode and fabricated UCNPs; later MIPs were made upon the electrode. When
365
there was no analyte in the K2S2O8 solution, the coreactant S2O82- would react with
366
UCNPs and generate light. With the increasing concentration of clenbuterol, the
367
combination of clenbuterol with MIPs would block electron transfer channels between
368
UCNPs and S2O82-, thereby decreasing the ECL response gradually. In addition, the
369
clenbuterol mixing with S2O82- would be catalyzed into new intermediates, which could
370
quench the ECL intensity of UCNPs. Based on above mechanisms, the clenbuterol was
371
detected. But generally, chemical luminescence is the result of chemical reaction between
372
marked enzymes and substrates. The luminescence is not stable and intermittent. And it is
373
easy to fission in the reaction process, which results in unstable result. Thus, researching
374
in new luminescent materials and how to prolong the luminescence time are imperative.
375 376
Biomimetic Electrochemical Immunoassay. Electrochemical sensors are currently
377
the most sophisticated type of sensor. They selectively convert response to target
378
molecules into electrical signals, which requires the incorporated material of the sensor
379
be capable of molecular recognition ability. Based on the high selectivity of MIPs,
380
combining them with an electrochemical sensor could obtain a biomimetic
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381
electrochemical sensor, molecular imprinted electrochemical sensor (MIECS), thereby
382
achieving rapid and efficient detection of target molecules. Based on different response
383
signals, MIECS are divided into five categories: amperometric sensors, potentiometric
384
sensors, conductometric sensors, impedimetric sensors, and piezoelectric sensors.
385 386
Biomimetic Amperometric Sensors. Biomimetic amperometric sensors quantitatively
387
detect the changes of the current before and after target molecules binding to MIPs. The
388
key is that there must be certain pores in the MIPs membrane so that the target molecules
389
can penetrate through the membrane to the surface of the electrode surface, and then a
390
redox reaction would occur to generate voltammetric response. For example,
391
Amatatongchai et al.46 prepared a biomimetic amperometric sensor by electro-
392
polymerization using 4-ter-butylcalix [8] arene-carbofuran as the template, and achieved
393
detection of carbofuran with a LOD of 3.8 nM.
394
Since the sensitivity of the MIPs sensor is directly related to the effective
395
recognition ability of MIPs, in order to raise the efficiency of imprinting thereby
396
improving the sensitivity, Xie et al.47 developed PATP-AuNP-gc sensor by molecular
397
imprinting in electro-polymerized poly aminothiophenol (PATP) membranes on glassy
398
carbon electrode which was modified by AuNPs for pesticide chlorpyrifos (CPF) (Figure
399
4B). As the large surface of AuNPs can greatly increase the recognition sites on the
400
surface of the imprinting sensor and good electron transport rate can improve electron
401
conductivity of the imprinted membrane, the amperometric response was 3.2 times better
402
than that of the imprinted PATP-Au sensor, which was made by electro-polymerization
403
of PATP directly on gold electrode, and the LOD was also two orders of magnitude lower
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404
than that of the imprinted PATP-Au sensor. The biomimetic amperometric sensor can
405
resist acid, alkali, and heavy metals. Thus it has a good stability and has been widely
406
used. However, it is only suitable for the detection of targets with electrical activity. For
407
the detection of targets without electrical activity, the addition of electroactive molecules
408
such as potassium ferricyanide as an electrochemical signal probe is required, but this
409
kind of electroactive molecule is rarely available, making the use of the sensor limited.
410 411
Biomimetic Potentiometric Sensors. Compared with the amperometric sensor,
412
biomimetic potentiometric sensors achieve the detection by the potential signal produced
413
after target molecules bind with the imprinted membrane. The target molecules do not
414
need to pass through the imprinted membrane, so there is no limit to the size of the
415
imprinted template. Its conjunction with MIPs could greatly improve the selectivity of
416
molecular imprinting potentiometric sensors. For instance, Gao et al.48 created a
417
membrane electrode with atrazine-imprinted membrane for detection of atrazine. The
418
LOD was 47 nM. Other conductive materials are also being used to build the sensor.
419
Özkütük et al.49, knowing that carbon nanotubes are highly conductive, used modified
420
multiwalled carbon nanotubes and clenbuterol as the template to obtain MIPs by
421
polymerization, and then used them to synthesize a PVC membrane electrode and
422
nanocarbon paste electrode. The developed metal-chelate based MIPs selective
423
potentiometric sensors was used for detection of clenbuterol, showing a great sensitivity
424
with LODs of 4.9 nM and 9.1 nM, respectively, within only 1−2 minutes. These MIPs,
425
prepared with metal chelates, are effective for the specific recognition of the target, and
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426
potentionmetric sensors based on these MIPs are not only simple and inexpensive, but
427
also highly selective and sensitive in detecting target molecules.
428 429
Biomimetic Conductometric Sensors. Biomimetic conductometric sensors achieve
430
quantitative detection of the target molecule by measuring the changes in conductivity
431
before and after the MIPs bind with the target molecules. For example, Warwick et al.50
432
developed a conductometric sensor for measuring soluble phosphate in environmental
433
water with MIPs designed to function as the receptor. In the presence of phosphate, it
434
would bind to MIPs, leading the conductance of MIPs membrane changed reversibly, and
435
thereby measuring the phosphate in wastewater. Although the working mechanism of the
436
conductiometric sensor is simple, the preparation and elution of the MIPs membrane will
437
directly affect the performance of the sensor. In addition, its selectivity is not high, as
438
trace impurities in the solution can have an adverse effect on the determination. To some
439
extent, the popularity of MIP-based conductiometric sensor is limited.
440 441
Biomimetic Impedimetric Sensors. Biomimetic impedimetric sensors achieve
442
detection by measuring the changes in impedance before and after the sensor responds to
443
the target. They are of good specificity, high sensitivity, and labeling is not needed.
444
Furthermore, they can achieve real-time monitoring, thus have garnered a lot of attention
445
and have been widely used. Zamora-Gálvez et al.51 synthesized MIPs-Fe3O4
446
nanoparticles with screen printing electrodes, and used it as an impedimetric sensor for
447
determination of pesticide tributyltin. The results obtained from the impedance
448
measurement showed that the sensor can detect tributyltin with a LOD of 5.37 pM, which
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was selective and sensitive. In practical applications, the ability to obtain an ultra-thin
450
MIPs membrane with excellent insulation performance is the key to build the imprinting
451
impedimetric sensor, so it needs to be further studied and explored.
452 453
Biomimetic Piezoelectric Sensors. Biomimetic piezoelectric sensors are like quartz
454
crystal microbalance (QCM) sensors. The MIPs film is fixed on the surface of quartz
455
crystal electrode and the mass change before and after the target molecules binding with
456
MIPs is shown by the frequency changes of electrical signal output, thus achieving the
457
quantitative detection of the targets. They can dynamically monitor mass change at the
458
nanogram level by the quartz crystal electrode, which has led to its popularity. Gao et
459
al.52 prepared a QCM sensor for the detection of profenofos, using MIPs ultra-thin film as
460
the
461
mercaptoundecanoic acid to form a self-assembled monolayer, and then the MIPs film
462
was immobilized on the electrode by radical induced polymerization. After the molecular
463
template was eluted, a sensor with specific binding sites of profenofos was obtained. It
464
could sensitively detect profenofos in spiked water and the QCM response showed a
465
LOD of 0.535 fM. Fang et al.53 established a novel 3D biomimetic QCM sensor for the
466
detection of citrinin in cereal grains. First, mesoporous carbon CMK-3 was mixed with
467
AuNPs to obtain AuNPs@CMK-3, it was immobilized directly on Au electrode, then
468
MIP/AuNPs@CMK-3/AuE was prepared by electro-polymerization. The LOD of the
469
imprinting sensor was 1.8 nM for citrinin, and other results showed high selectivity, anti-
470
interference ability, and long-term stability.
recognition
element.
The
QCM
sensor
was
471
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pretreated
with
11-
Journal of Agricultural and Food Chemistry
472
Biomimetic
Microfluidic
Paper-based
Immunoassay.
Page 22 of 44
In
recent
years,
473
microfluidic technology has become an important analytical and detection technique and
474
it has been applied in food and environmental fields for point-of-care testing. The rise of
475
microfluidic paper-based analytical devices (µPADs), which are mainly achieved through
476
preparation of hydrophilic or hydrophobic micro-channels on papers, greatly simplified
477
microfluidic system. It combines the advantages of traditional microfluidic chip such as
478
miniaturization, automation, integration, and portability with that of paper materials such
479
as being low cost, having strong biocompatibility, and being simple to process and easy
480
to degrade. By integrating µPADs with MIPs, the performance of assays can be positively
481
influenced in several aspects, not only in the shortening of time, but also in consuming
482
less reagents and gaining greater selectivity. Wang et al.54 prepared paper-based MIP-
483
grafted multi-disk micro-disk plate(P-MIP-MMP) by a simple in-situ polymerization of
484
MIP layer on the surface of the paper for sensitive and specific competitive
485
chemiluminescence detection of pesticide 2,4-D in water. Under optimal conditions, the
486
P-MIP-MMP could detect 2,4-D at femtomolar level. Although P-MIP-MMP
487
polymerized in-situ on the paper surface can maintain sensitivity and specificity in
488
complex samples, the thickness of the MIPs layer is difficult to control. On account of
489
this, Liu et al.55 prepared MIPs with circle-shaped advice by physical adsorption on the
490
unmodified or activated paper, and a chemiluminescence method was built to detect
491
dichlorvos in vegetables. With an well-controlled depth of MIPs, the paper-based chip
492
can be specific determination for dichlorvos and LOD was 3.6 pM. However, for some
493
samples with low surface tension, if there is insufficient hydrophobicity in the
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494
hydrophobic area of the paper, it may cause the leakage of the sample, which would
495
affect the detection, thus more effort need to be paid in this area.
496 497
Biomimetic Homogeneous Immunoassay. The homogeneous immunoassay does
498
not need to be coated, washed, or processed in any other way. It simplifies the operations,
499
shortens the analysis time, and is easy to be automated. The homogeneous immunoassay
500
mainly includes three modes: fluorescence polarization immunoassay (FPIA), resonance
501
energy transfer (RET) immunoassay, and luminescent oxygen channeling immunoassay
502
(LOCI). However, the homogeneous immunoassay always suffers from matrix
503
interference and low sensitivity. Integrating MIPs with homogeneous immunoassay could
504
greatly improve its selectivity. The currently reported biomimetic homogeneous
505
immunoassays are FPIA and RET.
506 507
Biomimetic Fluorescence Polarization Immunoassay. As a rapid, simple and high-
508
throughput homogeneous fluorescence immunoassay, FPIA quantitatively determines the
509
analyte by detecting the change in the FP value before and after a fluorescently labeled
510
antigen (tracer) binds to a specific antibody. Based on the nature of MIPs that they can
511
specifically bind to particular target molecules or tracer, tracers have a larger change in
512
molecular weight before and after binding to the MIPs than antibodies, thereby creating a
513
more significant polarization signal. In 2005, Chen et al.56 firstly used anthracene as a
514
template to prepare MIPs and proved that anthracene could closely bind to MIPs through
515
conventional FPIA and time-resolved FPIA, which demonstrated that MIPs could be used
516
as receptor in biomimetic FPIA. Consequently, Ton et al.57 used the inherent fluorescence
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517
property of the FQs as the signal output and ENRO as the template to prepare a
518
hydrophilic MIPs and established a noncompetitive direct FPIA method for the detection
519
of ENRO. In milk, ENRO could be selectively measured and distinguished from other
520
antibiotics. The method required no tracer, and no separation was needed here because
521
the analyte that bound to MIPs could be distinguished in the solution. However, in FPIA,
522
the tracer may be combined with the matrix, leading to the increase of the FP value,
523
which limits its application.
524 525
Biomimetic Fluorescence Resonance Energy Transfer Immunoassay. Fluorescence
526
resonance energy transfer (FRET) is a photochemical distance dependent process. When
527
the fluorescence emission spectrum of the donor is overlapped with the absorption
528
spectrum of the acceptor in two different fluorescent molecules, and when the distance
529
between two molecules is less than 10 nm, the donor in the excited state would transfer
530
its energy to the receptor in a nonradiative manner by dipole-dipole interaction. In recent
531
years, the homogeneous assay based on the combination of FRET and MIPs has been
532
used in food inspection and environmental testing. Some researchers introduced the
533
fluorescence functional monomer in the preparation of MIPs for homogeneous
534
fluorescence analysis of target molecules20. Some reported homogeneous analysis of
535
target molecules by embedding fluorescent materials in the preparation of MIPs23.
536
Descalzo et al.58 developed a FRET-based competitive biomimetic fluorescence assay for
537
determination of ENRO. The Ru (phen)32+ coated in SiO2 nanoparticles, where the MIPs
538
are synthesized on, acted as the donor and cyanine dye-labeled ENRO (NIR-ENRO)
539
acted as the receptor. NIR-ENRO and ENRO competed for recognition sites on MIPs.
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Journal of Agricultural and Food Chemistry
540
The binding of ENRO to MIPs was determined according to the changes in the
541
fluorescence signal and the LOD reached 2 µM. Integrating MIPs into the homogeneous
542
assay greatly improved the specificity of the reaction, and the sample does not require
543
purification, thus enhancing the detection efficiency.
544
545
PROSPECTS
546
The biomimetic immunoassay based on MIPs shows good selectivity, high
547
sensitivity and perfect stability, and more importantly it is resistant to harsh conditions,
548
thus lower the requirements for developing sensor which usually work in physiological
549
conditions when biological antibodies are employed. Therefore, it has been applied in the
550
fields of food and environmental safety, and renders the MIPs-based assay a potential
551
candidate for replacement of traditional antibodies. However, it is hard for current
552
biomimetic MIP-based assay to replace traditional immunoassay. There still be some
553
challenges needing to be solved for its practical applications in real world.
554
(1) There usually are a variety of pollutants coexisting in the sample to be tested,
555
which requires that the biomimetic immunoassay achieve co-extraction of a variety of
556
analytes and their simultaneous detection. The computer-aided template molecular design
557
and computer simulation techniques may increase the possibility to obtain the MIPs with
558
broad-specificity. If multiple MIPs with different specificity are used in combination for
559
multi-analyte detection, a suitable detection signal pattern needs to be proposed to
560
distinguish different signal generated by each analyte.
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Journal of Agricultural and Food Chemistry
561
(2) The MIPs usually have relatively lower affinity than that of antibodies do,
562
resulting in low detection sensitivity of corresponding assay. New technologies may be
563
employed to achieve better performance, such as controlled free radical polymerization,
564
click chemical cycloaddition technology, and multi-template/multi-functional monomer
565
imprinting. In addition, MIPs could be used in conjunction with highly sensitive methods,
566
and a more sensitive signal output mechanism can be used to achieve highly sensitive
567
detection of the target.
568
(3) There are not enough types of functional monomers and cross-linking agents
569
used in the preparation of MIPs to meet the detection of targets with special groups.
570
Therefore, the design and preparation of new monomers and cross-linkers is urgently
571
needed.
572
(4) Template molecules are prone to leakage if they are not completely eluted in the
573
preparation of MIPs. Since the target in the sample is often present in a trace amount, a
574
very small amount of residual template leakage would cause inaccurate determination.
575
Although the use of mimicking templates and fragmented MIPs can partly solve this
576
problem, it is exactly difficult to select suitable molecules as a mimicking template for all
577
possible targets.
578
(5) So far, MIPs are mostly synthesized and exhibited excellent molecular
579
recognition performance in organic phase. The samples of small molecules used in food
580
safety inspection and environmental monitoring are usually in aqueous solution, however,
581
the MIPs that are suitable for aqueous solution are limited. Thus it is imperative to
582
develop MIPs with excellent recognition properties of small molecules in aqueous
583
solutions. Although researchers have developed and synthesized water phase MIPs for
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Journal of Agricultural and Food Chemistry
584
some small molecules, it is still far from enough. From the aspects of cost, ecology and
585
environmental protection, water phase synthesis or green synthesis methods of MIPs
586
remain to be developed.
587
(6) In addition to chemical contaminants, biological contaminants such as
588
pathogens, proteins and viruses are important challenges for the preparation of MIPs. As
589
MIPs are mostly prepared in organic solvents and protein are almost insoluble in organic
590
solvents, the preparation of MIPs for protein becomes more difficult. Therefore, MIPs
591
preparation for biological macromolecules requires more attention.
592
593
594
We thank the National Science Foundation of China (31622057) and China Agriculture
595
Research System (Grant No. CARS-36) for the financial support.
ACKNOWLEDGMENTS
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597
(1) Fawell, J. K.; Stanfield, G. Drinking Water Quality and Health. Pollution: Causes,
598
Effects and Control. 2001, vol.4, 59−81.
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(2) Li, Y.; Sun, Y.; Beier, R.; Lei, H.; Gee, S.; Hammock, B.; Wang, H.; Wang, Z.; Sun,
600
X.; She, Y.; Yang, J.; Xu, Z. Immunochemical techniques for multianalyte analysis of
601
chemical residues in food and the environment: A review. TrAC, Trends Anal. Chem.
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2017, 88, 25−40.
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(3) Wang, P.; Sun, X.; Su, X.; Wang, T. Advancements of molecularly imprinted
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polymers in the food safety field. Analyst. 2016, 141, 3540−3553.
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(4) Liu, H.; Mu, L.; Chen, X.; Wang, J.; Wang, S.; Sun, B. Core-shell metal-organic
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frameworks/molecularly imprinted nanoparticles as absorbents for the detection of
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pyrraline in milk and milk powder. J. Agric. Food. Chem. 2017, 65, 986−992.
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wave fiber optic sensor coated with a molecularly imprinted polymer as a selective
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(38) Chen, G.; Yang, L.; Jin, M.; Du, P.; Zhang, C.; Wang, J.; Shao, H.; Jin, F.; Zheng,
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oxide-molecularly
isoproturon
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carbon
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782
783
Corresponding Author
784
*E-mail:
[email protected] 785
Notes
786
The authors declare no competing financial interest.
AUTHOR INFORMATION
787
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788
FIGURE CAPTIONS
789
Figure 1. Application of molecularly imprinted polymers used in pseudo-assays for the
790
detection of chemical contaminants in food and environment.
791 792
Figure 2. Schematic diagram of molecularly imprinted polymers used in biomimetic
793
ELISA: A novel assay for vancomycin using molecularly imprinted polymers
794
nanoparticles as antibodies substitution (modified from reference 16).
795 796
Figure 3. Schematic programs of molecularly imprinted polymers used in a biomimetic
797
fluoroimmunoassay. (A) Schematic illustration of the preparation of Fe3O4/SiO2-
798
MPS/MIPs (modified from reference 22). (B) Schematic illustration for the preparation of
799
dummy MIPs-capped CdTe QDs and the sensing mechanism for TNT (modified from
800
reference 23). (C) Schematic illustration of the process for the preparation of the
801
SiO2@AuNCs-MIPs (modified from reference 30).
802 803
Figure 4. Schematic diagrams of molecularly imprinted polymers used in the
804
chemiluminescence assay and electrochemical sensors. (A) The procedure to construct a
805
MIP-ECL sensor and determination of GA3 (modified from reference 44). (B) Schematic
806
illustrations for the adsorption of the ATP molecule at the AuNP surface and the further
807
self-assembly of CPF at an ATP-modified AuNP-gc electrode (modified from reference
808
47).
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Table 1. MIPs working as biomimetic antibodies in analysis techniques Detection method
Category
Biomimetic ELISA Based on Organic Fluorescent Dyes
Biomimetic
Based on Quantum Dots
Fluoroimmunoassay Based on Metallic Nanomaterials Based on Optical Fiber Biomimetic
Flow Injection
Chemiluminescence Immunoassay
electrogenerated chemiluminescence
Synthesis
Analyte
LOD
Reference
solid-phase synthesis
vancomycin
2.5 pM
16
bisphenol A
13.14 pM
20
λ-Cyhalothrin
9.17 nM
21
sol-gel method
2,4,6-trinitrotoluene
0.28 µM
23
copolymerization
salbutamol
0.142 pM
24
surface imprinting
cyphenothrin
9 nM
26
precipitation polymerization
metronidazole
0.15 µM
27
sol-gel method
bisphenol A
0.963 nM
30
surface imprinting
bisphenol A
87.6 pM
32
precipitation polymerization
λ-cyhalothrin
1.76 µM
33
photo-polymerization
2,4-D
250 pM
35
coprecipitation
chrysoidine
0.617 µM
40
precipitation polymerization
fenpropathrin
87.8 pM
42
isoproturon
3.78 pM
43
gibberellin A3
3.45 pM
44
clenbuterol
6.3 nM
45
precipitation polymerization
electro-polymerization
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Journal of Agricultural and Food Chemistry
Table 1. continued Detection method
Category Amperometric Sensor
Biomimetic Electrochemical Immunoassay
Potentiometric Sensor Impedimetric Sensor
Synthesis
Biomimetic Microfluidic Paper-based Immunoassay Fluorescence Polarization
LOD
Reference
electro-polymerization
carbofuran
3.8 nM
46
pre-graft polymerization
atrazine
47 nM
48
electro-polymerization
clenbuterol
4.9 nM
49
surface imprinting
tributyltin
5.37 pM
51
profenofos
0.535 fM
52
electro-polymerization
citrinin
1.8 nM
53
precipitation polymerization
dichlorvos
3.6 pM
55
precipitation polymerization
enrofloxacin
0.1 nM
57
radical induced Piezoelectric Sensor
Analyte
polymerization
Biomimetic Homogeneous
Immunoassay
Immunoassay
Biomimetic FRET
precipitation polymerization
bisphenol A
13.14 pM
20
Immunoassay
surface-grafting
enrofloxacin
2 µM
58
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Figure 1. Application of molecularly imprinted polymers used in pseudo-assays for the detection of chemical contaminants in food and environment. 137x151mm (300 x 300 DPI)
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Journal of Agricultural and Food Chemistry
Figure 2. Schematic diagram of molecularly imprinted polymers used in biomimetic ELISA: A novel assay for vancomycin using molecularly imprinted polymers nanoparticles as antibodies substitution (modified from reference 22). 84x48mm (300 x 300 DPI)
ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
Schematic programs of molecularly imprinted polymers used in a biomimetic fluoroimmunoassay. (A) Schematic illustration of the preparation of Fe3O4/SiO2-MPS/MIPs (modified from reference 22). (B) Schematic illustration for the preparation of dummy MIPs-capped CdTe QDs and the sensing mechanism for TNT (modified from reference 23). (C) Schematic illustration of the process for the preparation of the SiO2@AuNCs-MIPs (modified from reference 30). 80x146mm (300 x 300 DPI)
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Page 43 of 44
Journal of Agricultural and Food Chemistry
Figure 4. Schematic diagrams of molecularly imprinted polymers used in the chemiluminescence assay and electrochemical sensors. (A) The procedure to construct a MIP-ECL sensor and determination of GA3 (modified from reference 49). (B) Schematic illustrations for the adsorption of the ATP molecule at the AuNP surface and the further self-assembly of CPF at an ATP-modified AuNP-gc electrode (modified from reference 52). 80x96mm (300 x 300 DPI)
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Journal of Agricultural and Food Chemistry
For Table of Contents only. 80x35mm (300 x 300 DPI)
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