Broad-Range Magnetic Relaxation Switching Bioassays Using Click Chemistry-Mediated Assembly of Polystyrene Beads and Magnetic Nanoparticles

CRISPR/Cas-based colorimetric biosensors: a promising tool for the diagnosis of bacterial foodborne pathogens in food products

Some physical phenomena and various chemical substances newly introduced in nanotechnology have allowed scientists to develop valuable devices in the field of food sciences. Regarding such progress, the identification of foodborne pathogenic microorganisms is an imperative subject nowadays. These bacterial species have been found to cause severe health impacts after food ingestion and can result in high mortality in acute cases. The rapid detection of foodborne bacterial species at low concentrations is in high demand in recent diagnostics. CRISPR/Cas-mediated biosensors possess the potential to overcome several challenges in classical assays such as complex pretreatments, long turnaround time, and insensitivity. Among them, colorimetric nanoprobes based on the CRISPR strategy afford promising devices for POCT (point-of-care testing) since they can be visualized with the naked eye and do not require diagnostic apparatus. In this study, we briefly classify and discuss the working principles of the different CRISPR/Cas protein agents that have been employed in biosensors so far. We assess the current status of the CRISPR system, specifically focusing on colorimetric biosensing platforms. We discuss the utilization of each Cas effector in the detection of foodborne pathogens and examine the restrictions of the existing technology. The challenges and future opportunities are also indicated and addressed.

A sensitive Ag+-mediated magnetic relaxation and colorimetry dual-mode sensing platform

To address the relatively low sensitivity of current redox reagent-mediated magnetic relaxation sensing methods, we present a novel Ag+-mediated magnetic sensing platform that enhances the sensitivity by three orders of magnitude. The new sensing platform is based on Ag+-catalyzed oxidation of Mn2+ to KMnO4, accompanied by a distinct color change, which facilitates colorimetric detection. In the case of insufficient Ag+ ions, MnO2 is an additional oxidation product and the KMnO4/MnO2 ratio is dependent on the concentration of Ag+. When combined with a specific quantity of reducing agent, both KMnO4 and MnO2 are reduced to Mn2+ with a large relaxivity, and the concentration of Mn2+ in the resultant solution inversely correlates with the amount of KMnO4 since KMnO4 consumes more reductant during reduction. Consequently, the transverse relaxation rate of the solution exhibits a negative correlation with the Ag+ concentration. Thus, by coupling this Ag+-mediated Mn2+ to KMnO4 transformation with reactions that modulate Ag+ concentration, a dual-mode sensing platform for magnetic relaxation and colorimetry can be realized. Herein, we take H2O2 as an example to verify the detection performance of this sensing platform since H2O2 can oxidize Ag0 in Ag@Fe3O4 nanoparticles to Ag+. Experimental findings demonstrate detection limits of 10 nM and 20 nM for the magnetic relaxation and colorimetry modes, respectively, affirming the excellent sensitivity and the potential practical application of this strategy.

Advancements in magnetic nanoparticle-based biosensors for point-of-care testing

The significance of point-of-care testing (POCT) in early clinical diagnosis and personalized patient care is increasingly recognized as a crucial tool in reducing disease outbreaks and improving patient survival rates. Within the realm of POCT, biosensors utilizing magnetic nanoparticles (MNPs) have emerged as a subject of substantial interest. This review aims to provide a comprehensive evaluation of the current landscape of POCT, emphasizing its growing significance within clinical practice. Subsequently, the current status of the combination of MNPs in the Biological detection has been presented. Furthermore, it delves into the specific domain of MNP-based biosensors, assessing their potential impact on POCT. By combining existing research and spotlighting pivotal discoveries, this review enhances our comprehension of the advancements and promising prospects offered by MNP-based biosensors in the context of POCT. It seeks to facilitate informed decision-making among healthcare professionals and researchers while also promoting further exploration in this promising field of study.

Preparation of magnetic amphiphilic resin microspheres via the one-step polymerization method and extraction of four glucocorticoids for HPLC-MS analysis

Amphiphilic materials can be used for sample preparation of chromatography or mass spectrometry. Amphiphilic materials with magnetic properties in combination with magnetic suction devices allow for automated sample preparation. However, conventional synthesis methods are cumbersome and not suitable for the mass production of the material. In this study, a micro-suspension polymerization method was developed to synthesize magnetic amphiphilic resin microspheres (MARMs), providing new ideas for the preparation of amphiphilic microspheres. MARMs with particle sizes ranging from 3 to 6 μm were successfully prepared, with BET surface area up to 653.2 m2/g. A magnetic solid-phase extraction method based on MARM-5 was developed for the extraction of four glucocorticoids including Cortisone, Hydrocortisone, Cortodoxone, and Corticosterone. This method had a very short adsorption time of 0.5 min and a total extraction time of only 13 min. The limit of detection for the four glucocorticoids ranged from 0.22 to 0.82 ng/L. There was a good linear relationship between sample concentration and peak area in the range of 25∼500 ng/L. Relative recovery of 98 %∼108 % and internal standard normalized matrix effect factors of 95∼114 % were obtained, and the relative standard deviation was between 2.3 % and 6.3 %. The MARMs would be used as excellent solid extraction material for glucocorticoids.

Well plate-based LF-NMR/colorimetric dual-mode homogeneous immunosensor for Vibrio parahaemolyticus detection

A 96-well plate-based low-field nuclear magnetic resonance (LF-NMR)/colorimetric dual-mode homogeneous immunosensor was developed for the detection of pathogen bacteria, using Vibrio parahaemolyticus (VP) as a detection template. The signal unit MNS@Ab2 is graphene oxide (GO) simultaneously loaded with VP antibody and Fe3O4 nanoparticles. A 96-well plate coated with VP antibody captures the target VP, which then binds the signal unit to form the immunocomplex. After acidolysed, Fe3O4 nanoparticles are transformed into Fe3+ and Fe2+, so the non-homogeneous system is transformed into a homogeneous one. The addition of KMnO4 can not only convert Fe2+ into Fe3+ but also provide Mn2+, improving the detection sensitivity. And, colorimetric analysis can be achieved by the quantitative reduction of KMnO4. Under the optimal experimental conditions, the limit of detection was 60 CFU/mL with good selectivity, stability, precision, accuracy, and consistency, providing a simple and reliable detection platform for pathogenic bacteria in food.

An LF-NMR homogeneous immunoassay for Vibrio parahaemolyticus based on superparamagnetic 2D nanomaterials

In this work, a sensitive low field nuclear magnetic resonance (LF-NMR) homogeneous immunoassay, also called magnetic resonance switch (MRSw) sensor, for Vibrio parahaemolyticus (VP) was developed. Superparamagnetic 2D nanomaterial was designed and used as the magnetic probe of MRSw sensor. It was GO@SPIONs&Ab, a composite nanomaterial with many superparamagnetic Fe3O4 nanoparticles (SPIONs) providing a magnetic signal and VP antibody (Ab) specifically recognizing the target VP evenly distributed on the surface of GO. The presence of VP controllably changed the aggregation state of the probe, eliminating the uncertainty of MRSw sensor type, and thus then achieving a regular variation of transverse relaxation time T2 and ensuing quantitative detection of VP. Triple signal enhancement of the MRSw sensor was obtained due to the application of the designed 2D probe, by increasing the number of SPIONs, improving the magnetic intensity and susceptibility, and forming a synergistic effect. Under optimized experimental conditions, VP could be detected with satisfied sensitivity, selectivity, precision, accuracy, and stability, even in turbid real samples. LOQ for VP was 10 CFU/mL. This detection principle is widely applicable, providing an idea for the construction of highly sensitive MRSw sensors.

Magnetic Relaxation Switching Assay Using IFNα-2b-Conjugated Superparamagnetic Nanoparticles for Anti-Interferon Antibody Detection

Type I interferons, particularly IFNα-2b, play essential roles in eliciting adaptive and innate immune responses, being implicated in the pathogenesis of various diseases, including cancer, and autoimmune and infectious diseases. Therefore, the development of a highly sensitive platform for analysis of either IFNα-2b or anti-IFNα-2b antibodies is of high importance to improve the diagnosis of various pathologies associated with the IFNα-2b disbalance. For evaluation of the anti-IFNα-2b antibody level, we have synthesized superparamagnetic iron oxide nanoparticles (SPIONs) coupled with the recombinant human IFNα-2b protein (SPIONs@IFNα-2b). Employing a magnetic relaxation switching assay (MRSw)-based nanosensor, we detected picomolar concentrations (0.36 pg/mL) of anti-INFα-2b antibodies. The high sensitivity of the real-time antibodies' detection was ensured by the specificity of immune responses and the maintenance of resonance conditions for water spins by choosing a high-frequency filling of short radio-frequency pulses of the generator. The formation of a complex of the SPIONs@IFNα-2b nanoparticles with the anti-INFα-2b antibodies led to a cascade process of the formation of nanoparticle clusters, which was further enhanced by exposure to a strong (7.1 T) homogenous magnetic field. Obtained magnetic conjugates exhibited high negative MR contrast-enhancing properties (as shown by NMR studies) that were also preserved when particles were administered in vivo. Thus, we observed a 1.2-fold decrease of the T2 relaxation time in the liver following administration of magnetic conjugates as compared to the control. In conclusion, the developed MRSw assay based on SPIONs@IFNα-2b nanoparticles represents an alternative immunological probe for the estimation of anti-IFNα-2b antibodies that could be further employed in clinical studies.

A facile and sensitive magnetic relaxation sensing strategy based on the conversion of Fe3+ ions to Prussian blue precipitates for the detection of alkaline phosphatase and ascorbic acid oxidase

Herein, a novel magnetic relaxation sensing strategy based on the change in Fe³⁺ content has been proposed by utilizing the conversion of Fe³⁺ ions to Prussian blue (PB) precipitates. Compared with the common detection approach based on the valence state change of Fe³⁺ ions, our strategy can cause a larger change in the relaxation time of water protons and higher detection sensitivity since PB precipitate can induce a larger change in the Fe³⁺ ion concentration and has a weaker effect on the relaxation process of water protons relative to Fe²⁺ ions. Then, we employ alkaline phosphatase (ALP) as a model target to verify the feasibility and detection performance of the as-proposed strategy. Actually, ascorbic acid (AA) generated from the ALP-catalyzed L-ascorbyl-2-phosphate hydrolysis reaction can reduce potassium ferricyanide into potassium ferrocyanide, and potassium ferrocyanide reacts with Fe³⁺ to form PB precipitates, leading to a higher relaxation time. Under optimum conditions, the method for ALP detection has a wide linear range from 5 to 230 mU/mL, and the detection limit is 0.28 mU/mL, sufficiently demonstrating the feasibility and satisfactory analysis performance of this strategy, which opens up a new path for the construction of magnetic relaxation sensors. Furthermore, this strategy has also been successfully applied to ascorbic acid oxidase detection, suggesting its expansibility in magnetic relaxation detection.

CRISPR/Cas12a-based magnetic relaxation switching biosensor for nucleic acid amplification-free and ultrasensitive detection of methicillin-resistant Staphylococcus aureus

Herein, we develop a CRISPR/Cas12a-based magnetic relaxation switching (C-MRS) biosensor for ultrasensitive and nucleic acid amplification-free detection of methicillin-resistant Staphylococcus aureus (MRSA) in food. In this biosensor, mecA gene in MRSA was recognized by CRISPR-RNA, which will activate the trans-cleavage activity of Cas12a and release the fastened alkaline phosphatase (ALP) on the particle. The freed ALP can then use to hydrolyze substrate to produce ascorbic acid that trigger the click reaction between magnetic probe. The transverse relaxation time of the unbound magnetic probe can be measured for signal readout. By incorporating collateral activity of CRISPR/Cas12a, on-particle rolling circle amplification, and ALP-triggered click chemistry into background-free MRS, as low as 16 CFU/mL MRSA can be detected without any nucleic acid pre-amplification, which avoids carryover contamination, but without compromising sensitivity. Moreover, this C-MRS biosensor could distinguish 0.01% target DNA from the single-base mutant. Recovery in eggs, milk and pork ranged from 75% to 112%, 82%-104%, and 81%-91%, respectively, revealing its satisfactory accuracy and applicability in the complex food matrix. The developed C-MRS biosensor fleshes out the CRISPR toolbox for food safety and provides a new approach for the sensitive and accurate detection of foodborne drug-resistant bacteria.

DNA Tetrahedron-Based Valency Controlled Signal Probes for Tunable Protein Detection

Combined detection of multiple markers related to the same disease could improve the accuracy of disease diagnosis. However, the abundance levels of multiple markers of the same disease varied widely in real samples, making it difficult for the traditional detection method to meet the requirements of a wide detection range. Herein, three kinds of cardiac biomarkers, cardiac troponin I (cTnI), myoglobin (Myo), and C-reaction protein (CRP), which were from the pM level to the μM level in real samples, were selected as model targets. Valency-controlled signal probes based on DNA tetrahedron nanostructures (DTNs) and platinum nanoparticles (PtNPs) were constructed for tunable cardiac biomarker detection. PtNPs with high horseradish peroxidase-like activity and stability served as signal molecules, and DTNs with unique spatial structure and sequence specificity were used for precisely controlling the number of connected PtNPs. By controlling the number of PtNPs connected to DTNs, monovalent, bivalent, and trivalent signal probes were obtained and were used for the detection of cardiac markers in different concentration ranges. The limit of detection of cTnI, Myo, and CRP was 3.0 pM, 0.4 nM, and 6.7 nM, respectively. Furthermore, it performed satisfactorily for the detection of cardiac markers in 10% human serum. It was anticipated that the design of valency-controlled signal probes based on DTNs and nanozymes could be extended to the construction of other multi-target detection platforms, thus providing a basis for the development of a new precision medical detection platform.

Magnetic Relaxation Switching Immunosensors via a Click Chemistry-Mediated Controllable Aggregation Strategy for Direct Detection of Chlorpyrifos

Chlorpyrifos (CPF) is the most frequently found organophosphate pesticide residue in solid food samples and can cause increasing public concerns about potential risks to human health. Traditional detection signals of such small molecules are mostly generated by target-mediated indirect conversion, which tends to be detrimental to sensitivity and accuracy. Herein, a novel magnetic relaxation switching detection platform was developed for target-mediated direct and sensitive detection of CPF with a controllable aggregation strategy based on a bioorthogonal ligation reaction between tetrazine (Tz) and trans-cyclooctene (TCO) ligands. Under optimal conditions, this sensor can achieve a detection limit of 37 pg/mL with a broad linear range of 0.1-500 ng/mL in 45 min, which is approximately 51-fold lower than that of the gas chromatography analysis and 13-fold lower than that of the enzyme-linked immunosorbent assay. The proposed click chemistry-mediated controllable aggregation strategy is direct, rapid, and sensitive, indicating great potential for residue screening in food matrices.

Polydopamine nanoparticle-mediated, click chemistry triggered, microparticle-counting immunosensor for the sensitive detection of ochratoxin A

A rapid and accurate detection method is needed for the quantitation of ochratoxin A in agricultural products due to its high toxicity. A microparticle-counting immunosensor based on polydopamine nanoparticle-mediated click chemistry was established for the highly-sensitive detection of ochratoxin A. Polydopamine nanoparticles with good biocompatibility and a strong metal-chelating ability were synthesized and conjugated with the antibody. The Coupled compounds were then used as an immune carrier to change the Cu²⁺ concentration via an immuno-reaction. Some of the remaining Cu²⁺ ions were reduced to Cu⁺ ions, which caused azide-polystyrene microspheres and alkyne-polystyrene microspheres to aggregate via a Cu⁺ ion-mediated click reaction. Particle counting was used to distinguish changes in the sizes of the polystyrene microspheres from dispersed to aggregated to detect ochratoxin A. It showed a wide linear detection range of 0.5-800 ng/mL, and a detection limit of 0.2 ng/mL. This assay provides an attractive analytical tool for the accurate detection of trace targets in complex samples.

An Overview for the Nanoparticles-Based Quantitative Lateral Flow Assay

The development of the lateral flow assay (LFA) has received much attention in both academia and industry because of their broad applications to food safety, environmental monitoring, clinical diagnosis, and so forth. The user friendliness, low cost, and easy operation are the most attractive advantages of the LFA. In recent years, quantitative detection has become another focus of LFA development. Here, the most recent studies of quantitative LFAs are reviewed. First, the principles and corresponding formats of quantitative LFAs are introduced. In the biomaterial and nanomaterial sections, the detection, capture, and signal amplification biomolecules and the optical, fluorescent, luminescent, and magnetic labels used in LFAs are described. The invention of dedicated strip readers has drawn further interest in exploiting the better performance of LFAs. Therefore, next, the development of dedicated reader devices is described and the usefulness and specifications of these devices for LFAs are discussed. Finally, the applications of LFAs in the detection of metal ions, biotoxins, pathogenic microorganisms, veterinary drugs, and pesticides in the fields of food safety and environmental health and the detection of nucleic acids, biomarkers, and viruses in clinical analyses are summarized.

Magnetic relaxation switching assay based on three-dimensional assembly of Fe3O4@ZIF-8 for detection of cadmium ions

The design and construction of a novel magnetic resonance switch (MRS) sensor for cadmium ion (Cd²⁺) detection is described. Fe₃O₄@ZIF-8 was synthesized through seed-mediated growth of dimercaptosuccinic acid-coated Fe₃O₄. Fe₃O₄@ZIF-8 with high relaxation value (163.086 mM⁻¹ s⁻¹) and large negative zeta potential (−20.69 mV) exhibited good magnetic relaxation performance and water solubility. The successfully synthesized Fe₃O₄@ZIF-8 was used to develop an immune recognition-based MOFs-MRS sensor for highly sensitive detection of Cd²⁺. The proposed MRS detected a wide linear range of Cd²⁺ concentration from 2 to 200 ng mL⁻¹ with a low limit of detection of 0.65 ng mL⁻¹ (S/N = 3), and displayed high selectivity towards matrix interference. The robust sensing system was effective even in a complex sample matrix, enabling the quantitative analysis of Cd²⁺ content in rice samples and drinking water samples with good reliability. Recoveries of Cd²⁺ ranged from 91.50 to 112.05% for spiked drinking water and from 95.86 to 110.45% for spiked rice samples. The versatility of Fe₃O₄@ZIF-8 with customized relaxation responses could allow the adaptation of magnetic resonance platforms for food safety purposes.

A sensitive immune recognition-based MOF-MRS sensor for the detection of Cd²⁺.

Amphiphilic copolymers grafted on monodisperse magnetic microspheres as an efficient adsorbent for the extraction of safrole in the plasma

Polystyrene (PS) microsphere is a kind of attractive extracting medium due to its high stability in different matrices and its particle size can be controlled. The attachment of amphiphilic copolymers to the PS microsphere surface can overcome the drawback of PS relevant to its hydrophobic nature and low wettability. In our previous work, the magnetic composite based on PS microsphere (5 µm) and poly (divinylbenzene-co-N-vinylpyrrolidone, DVB-co-NVP) shell was successfully fabricated and applied for the extraction of safrole in cola drinks. However, the large size and ease of sedimentation limited its application in the enrichment of safrole from blood samples. Considering the adjustability of PS microsphere size, we synthesized the porous PS microspheres with the uniform size of 3 µm and then functionalized with Fe₃O₄ nanoparticles and poly (DVB-co-NVP) layer in this work. Using the proposed material as extraction sorbent, a simple and fast magnetic solid phase extraction (MSPE) method coupled with HPLC was developed for quantification of safrole in the plasma. Under the optimized conditions, the response to safrole was linear in the range of 0.02-12 µg mL^-1, and the limit of detection (LOD) was 0.006 µg mL^-1. Satisfactory recoveries were obtained between 85.67% and 97.74% (spiked at 0.05, 0.2, 2 µg mL^-1) and the relative standard deviations (RSDs) for the above spiked levels of the analyte were below 3.9% (n = 6). The adsorbent can be reused for 6 cycles without a significant loss of extraction capability.

Overview of Rapid Detection Methods for Salmonella in Foods: Progress and Challenges

Salmonella contamination in food production and processing is a serious threat to consumer health. More and more rapid detection methods have been proposed to compensate for the inefficiency of traditional bacterial cultures to suppress the high prevalence of Salmonella more efficiently. The contamination of Salmonella in foods can be identified by recognition elements and screened using rapid detection methods with different measurable signals (optical, electrical, etc.). Therefore, the different signal transduction mechanisms and Salmonella recognition elements are the key of the sensitivity, accuracy and specificity for the rapid detection methods. In this review, the bioreceptors for Salmonella were firstly summarized and described, then the current promising Salmonella rapid detection methods in foodstuffs with different signal transduction were objectively summarized and evaluated. Moreover, the challenges faced by these methods in practical monitoring and the development prospect were also emphasized to shed light on a new perspective for the Salmonella rapid detection methods applications.

Tailored Magnetic Multicore Nanoparticles for Use as Blood Pool MPI Tracers

For the preclinical development of magnetic particle imaging (MPI) in general, and the exploration of possible new clinical applications of MPI in particular, tailored MPI tracers with surface properties optimized for the intended use are needed. Here we present the synthesis of magnetic multicore particles (MCPs) modified with polyethylene glycol (PEG) for use as blood pool MPI tracers. To achieve the stealth effect the carboxylic groups of the parent MCP were activated and coupled with pegylated amines (mPEG-amines) with different PEG-chain lengths from 2 to 20 kDa. The resulting MCP-PEG variants with PEG-chain lengths of 10 kDa (MCP-PEG10K after one pegylation step and MCP-PEG10K2 after a second pegylation step) formed stable dispersions and showed strong evidence of a successful reaction of MCP and MCP-PEG10K with mPEG-amine with 10 kDa, while maintaining their magnetic properties. In rats, the mean blood half-lives, surprisingly, were 2 and 62 min, respectively, and therefore, for MCP-PEG10K2, dramatically extended compared to the parent MCP, presumably due to the higher PEG density on the particle surface, which may lead to a lower phagocytosis rate. Because of their significantly extended blood half-life, MCP-PEG10K2 are very promising as blood pool tracers for future in vivo cardiovascular MPI.

Recent Applications of Magnetic Nanoparticles in Food Analysis

Nanotechnology has become a topic of interest due to the outstanding advantages that the use of nanomaterials offers in many fields. Among them, magnetic nanoparticles (m-NPs) have been one of the most widely applied in recent years. In addition to the unique features of nanomaterials in general, which exclusively appear at nanoscale, these present magnetic or paramagnetic properties that result of great interest in many applications. In particular, in the area of food analysis, the use of these nanomaterials has undergone a considerable increase since they can be easily separated from the matrix in sorbent-based extractions, providing a considerable simplification of the procedures. This allows reducing cost and giving fast responses, which is essential in the food trade to guarantee consumer safety. These materials can also be easily tunable, providing higher selectivity. Moreover, their particular electrical, thermal and optical characteristics allow enhancing sensor signals, increasing the sensitivity of the approaches based on this type of device. The aim of this review article is to summarise the most remarkable applications of m-NPs in food analysis in the last five years (2016–2020) showing a general view of the use of such materials in the field.

Magnetic-Separation-Assisted Magnetic Relaxation Switching Assay for Mercury Ion Based on the Concentration Change of Oligonucleotide-Functionalized Magnetic Nanoparticle

Magnetic-separation-assisted magnetic relaxation switching (MRS) assay based on the concentration change of magnetic nanoparticles switches has been designed for bacteria, biological macromolecules, and small molecules detection because of its better analysis performance. As one of the most hazardous pollutants and highly dangerous elements, mercury ion (Hg²⁺) was employed as a model to further investigate the applicability of nanoparticle switches concentration change-based MRS assay mode for detecting metal ions in this study. The principle is based on the specific and strong interaction between mercury ion with the thymine-thymine(T-T) mismatch in double-stranded DNA duplexes by employing oligonucleotide functionalized magnetic nanoparticle as magnetic capture probe and MRS signal probe, respectively. The result shows that magnetic nanoparticles concentration-dependent MRS sensing mode could be facile applied to detect metal ion of Hg²⁺ in tap water, lake water and serum with wider detection range and higher accuracy. The as-presented magnetic-separation-assisted MRS assay of Hg²⁺ in complicated samples shows potential application values for Hg²⁺ assay in clinical and environmental monitoring, which broadens its application.

A low-field nuclear magnetic resonance DNA-hydrogel nanoprobe for bisphenol A determination in drinking water

A low-field nuclear magnetic resonance (LF-NMR) DNA-hydrogel (LNDH) nanoprobe was designed for bisphenol A (BPA) determination. It consists of Fe₃O₄ superparamagnetic iron oxide nanoparticles (SPIONs) and a DNA-hydrogel technology. Fe₃O₄ SPIONs were encapsulated in the DNA-hydrogel to form an aggregated state. After adding BPA, the gel system transformed into a sol gel due to the target-aptamer specific binding. The coated gathered particles dispersed and thus, the relaxation time T₂ declined. The LNDH nanoprobe was developed to realize a simple, sensitive, and effective BPA determination method without repeated magnetic separation steps. Under the optimal experimental conditions, the determination range of the LNDH biosensor was 10^-2~10² ng mL^-1 and the limit of determination was 0.07 ng mL^-1. The LNDH nanoprobe was applied to two kinds of water samples (tap water and bottled water). The recovery ranged from 87.85 to approximately 97.87%. This strategy offered a new method to detect BPA by LF-NMR. It is also expected to be applicable in related fields of food safety determination, environmental monitoring, and clinical diagnosis. Graphical abstract Schematic presentation of LNDH biosensor. Acrydite-modified ssDNA was copolymerized with acrylamide to form linear conjugates PS-A/B, adding aptamer and SPIONs to form DNA-hydrogel. When aptamer captured the target, the hydrogel was destroyed to disperse the coated SPIONs. T₂ relaxation time declined.

A colorimetric immunosensor for determination of foodborne bacteria using rotating immunomagnetic separation, gold nanorod indication, and click chemistry amplification

A colorimetric immunosensor was developed for the determination of Salmonella Typhimurium using rotating magnetic separation, gold nanorod (GNR) indication, and click chemistry amplification. The target bacteria were first separated from large-volume sample using a rotating magnetic field and a small amount (50 μg) of immunomagnetic nanoparticles (MNPs), resulting in the forming of magnetic bacteria. Then, the magnetic bacteria were conjugated with catalase (CAT)-labeled antibodies, which were synthesized using trans-cyclooctene/1,2,4,5-tetrazine click chemistry reaction, resulting in the forming of enzymatic bacteria. Then the CATs on the enzymatic bacteria were used to decompose an excessive amount of hydrogen peroxide (H₂O₂), the remaining H₂O₂ was mixed with horseradish peroxidase to etch the GNRs, resulting in color change and absorbance peak shift of the GNRs. Finally, the peak shift was measured and analyzed for the quantitative determination of target bacteria. This immunosensor was able to detect Salmonella Typhimurium with a linear range of 10¹-10⁵ CFU mL^-1 in 3 h with a low detection limit of 35 CFU mL^-1. The mean recovery for Salmonella Typhimurium in spiked chicken samples was 109%. Graphical abstractSchematic representation of a colorimetric immunosensor for the determination of Salmonella Typhimurium as low as 35 CFU mL^-1 using rotating magnetic separation of Salmonella from a large-volume sample, click chemistry reaction of catalase with antibodies for signal amplification, and HRP-mediated gold nanorod etching for result indication.

Bioorthogonal Reactions Amplify Magnetic Nanoparticles Binding and Assembly for Ultrasensitive Magnetic Resonance Sensing

Conventional transverse relaxation time (T₂)-mediated magnetic resonance sensors (MRS) that utilizing the target-induces state change of magnetic nanoparticles (MNPs) mainly suffer from low sensitivity. Recent T₂-MRS that based on target-induced amount change of MNPs can achieve a higher sensitivity, but these sensors can hardly accommodate small molecules. We herein develop an ultrasensitive T₂-MRS that enable the detection of small molecules based on cascade bioorthogonal reactions (BRs)-realized MNPs binding and assembly. Benefiting from rapid and highly selective cascade BRs, a single small molecule target can not only increase MNPs binding but also assembly MNPs, which greatly amplifies T₂ signal for sensing based on both the state and amount change of MNPs for the first time. Our strategy is capable of sensing chlorpyrifos with a liner range of 0.1 ng/mL to 1000 ng/mL. We justify the practicability of our assay by detecting chlorpyrifos in apple and cabbage samples, whose accuracy is higher than that of enzyme linked immunosorbent assay. Our assay provides a cascade BRs-mediated MRS that can greatly broaden the use of T₂-based MRS for ultrasensitive sensing trace small molecules in complex samples.

A competent green methodology for the synthesis of aryl thioethers and 1H-tetrazole over magnetically retrievable novel CoFe2O4@l-asparagine anchored Cu, Ni nanocatalyst

The current work describes the successful synthesis of a magnetic CoFe₂O₄ centered asparagine functionalized noble metal (M = Cu, Ni) anchored nanocomposite. The novel materials, synthesized by post-functionalization approach, have been characterized by XRD, SEM, EDX, X-ray elemental mapping, FT-IR and VSM studies. The materials are proved to be efficient heterogeneous catalyst in the synthesis of diarylthioethers by C-S cross coupling reaction and 5-substituted 1H-tetrazoles by azide-alkyne cycloaddition reaction under green conditions. The current methodology is advantageous in terms of simplicity of procedure, facile synthesis, high yield in short reaction time, easy magnetic isolation and reusability of catalysts in consecutive runs with insignificant change in catalytic activity.