Magnetic Relaxation Switching Immunoassay Based on Hydrogen Peroxide-Mediated Assembly of Ag@Au-Fe3 O4 Nanoprobe for Detection of Aflatoxin B1

A CRISPR/Cas12a-based direct transverse relaxation time biosensor via hydrogel sol-gel transition for Salmonella detection

This research developed a magnetic relaxation switching (MRS) biosensor based on hydrogel sol-gel transition and the CRISPR/Cas12a system (MRS-CRISPR) to detect Salmonella. Herein, the alkaline phosphatase (ALP) labeled with streptavidin was captured by the biotin-modified DNA on magnetic nanoparticles (MNPs) surface, which generated an acidic environment via enzymatic reaction to release Ca2+ and induced the transformation of alginate sol to hydrogels. In contrast, Salmonella activated the trans-cleavage activity of the CRISPR/Cas12a system, interrupting the capture of ALP and the subsequent sol-gel transition. Then, transverse relaxation time (T2), which was regulated by the hydrogelation process was measured for Salmonella detection. The MRS-CRISPR biosensor enables sensitive detection of Salmonella with a detection limit of 158 CFU/mL. It directly alters the state of water molecules, overcoming the disadvantages of traditional MRS sensors that rely on MNPs to produce T2 signals indirectly. This method offers innovative insights for the application of the MRS technology in food safety analysis.

Magnetic relaxation switching immunosensor based on polystyrene microcolumn and tyramine signal amplification for ultrasensitive and user-friendly detection of aflatoxin B1 in corn

Aflatoxin B1 is highly mutagenic in humans, and long-term exposure can impair immunity and increase the risk of cancer. It is imperative to develop immunoassays with convenient operation and high sensitivity to detect aflatoxin B1. This study presents a polystyrene microcolumn-mediated magnetic relaxation switching immunosensor based on a tyramine signal amplification strategy for detecting aflatoxin B1. An environmentally friendly hand-held polystyrene microcolumn was designed as an effective immunoreaction carrier, remaining 91% efficiency after 12 repeated uses. And the microcolumn provides a user-friendly procedure for rapid separation and reagent switching within 3 s by simple stirring in solution. The combination of a strong anti-interference magnetic relaxation switching biosensing and an efficient tyramine signal amplification enables the quantitative detection of aflatoxin B1 in the range of 0.01-10 ng/mL, with a limit of detection of 0.006 ng/mL. This method has potential application in the rapid detection of trace food contaminants.

CRISPR/Cas12a and Hybridization Chain Reaction-Coregulated Magnetic Relaxation Switching Biosensor for Sensitive Detection of Viable Salmonella in Animal-Derived Foods

We combined a CRISPR/Cas12a system with a hybridization chain reaction (HCR) to develop an ultrasensitive magnetic relaxation switching (MRS) biosensor for detecting viable Salmonella typhimurium (S. typhimurium). Magnetic nanoparticles of two sizes (30 and 1000 nm: MNP30 and MNP1000, respectively) were coupled through HCR. The S. typhimurium gene-activated CRISPR/Cas12a system released MNP30 from the MNP1000-HCR-MNP30 complex through a trans-cleavage reaction. After magnetic separation, released MNP30 was collected from the supernatant and served as a transverse relaxation time (T2) signal probe. Quantitative detection of S. typhimurium is achieved by establishing a linear relationship between the change in T2 and the target gene. The biosensor's limit of detection was 77 CFU/mL (LOD = 3S/M, S = 22.30, M = 0.87), and the linear range was 102-108 CFU/mL. The accuracy for detecting S. typhimurium in real samples is comparable to that of qPCR. Thus, this is a promising method for the rapid and effective detection of foodborne pathogens.

Multienzyme-Mediated Dual-Channel Magnetic Relaxation Switching Taste Biosensor (D-MRSTB) for Simultaneous Detection of Umami Compounds and Synergistic Enhancement in Food

Umami substances play a significant role in the evaluation of food quality, and their synergistic enhancement is of great importance in improving and intensifying food flavors and tastes. Current biosensors available for umami detection still confront challenges in simultaneous quantification of multiple umami substances and umami intensities. In this study, an innovative dual-channel magnetic relaxation switching taste biosensor (D-MRSTB) was developed for the quantitative detection of representative umami substances. The multienzyme signal of D-MRSTB specifically catalyzes the umami substances of interest to generate hydrogen peroxide (H2O2), which is then used to oxidate Fe2+ to Fe3+. Such a valence-state transition of paramagnetic ions was utilized as a magnetic relaxation signaling switch to influence the transverse magnetic relaxation time (T2) within the reaction milieu, thus achieving simultaneous detection of monosodium glutamate (MSG) and inosine 5'-monophosphate (IMP). The biosensor showed good linearity (R2 > 0.99) in the concentration range of 50-1000 and 10-1000 μmol/L, with limits of detection (LOD) of 0.61 and 0.09 μmol/L for MSG and IMP, respectively. Furthermore, the biosensor accurately characterized the synergistic effect of the mixed solution of IMP and MSG, where ΔT2 showed a good linear relationship with the equivalent umami concentration (EUC) of the mixed solution (R2 = 0.998). Moreover, the D-MRSTB successfully achieved the quantitative detection of umami compounds in real samples. This sensing technology provides a powerful tool for achieving the detection of synergistic enhancement among umami compounds and demonstrates its potential for application in the food industry.

Lanthanide-Assisted Nanozyme Performs Optical and Magnetic Resonance Dual-Modality Logical Signal for In Vitro Diagnosis

The iron nanozyme-based colorimetric method, which is widely applied for biosubstrate detection in in vitro diagnosis (IVD), faces some limitations. The optimal catalytic conditions of iron nanozymes necessitate a strong acidic environment, high temperature, and other restrictive factors; additionally, the colorimetric results are highly influenced by optical interferences. To address these challenges, iron nanozymes doped with various transition elements were efficiently prepared in this study, and notably, the manganese-modified one displayed a high catalytic activity owing to its electron transfer property. Furthermore, the introduction of lanthanide ions into the catalytic reactions, specifically the neodymium ion, significantly boosted the generation efficiency of hydroxyl radicals; importantly, this enhancement extended to a wide range of pH levels and temperatures, amplifying the detection signal. Moreover, the nanozyme's superparamagnetic characteristic was also employed to perform a logical optical and magnetic resonance dual-modality detection for substrates, effectively eliminating background optical interference and ensuring a reliable verification of the signal's authenticity. Based on this magnetic signal, the integration of natural glucose oxidase with the nanozyme resulted in a notable 61.5% increase in detection sensitivity, surpassing the capabilities of the traditional colorimetric approach. Consequently, the incorporation of lanthanide ions into the magnetic nanozyme enables the effective identification of physiological biomarkers through the dual-modality signal. This not only guarantees enhanced sensitivity but also demonstrates significant potential for future applications.

Ni-Pt nanozyme-mediated relaxation and colorimetric sensor for dual-modality detection of norovirus

An NiPt nanozyme-mediated relaxation and colorimetric sensor is developed for dual-modality detection of norovirus (NoV). The relaxation modality is based on the "catalase-like" activity of the NiPt nanozyme, which adjusts the hydrogen peroxide (H2O2) mediated Fe (II)/Fe(III) conversion, thereby changing the relaxation signal. Poly-γ-glutamic acid (MW ≈ 1w) can enhance the relaxivity of Fe(III) (r1 = 7.11 mM-1 s-1; r2 = 8.94 mM-1 s-1). The colorimetric modality exploits the "peroxidase-like" activity of the NiPt nanozyme, which can catalyze the oxidation of colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) to blue oxTMB in H2O2. Under optimal conditions, the relaxation modality exhibits a wide working range (1.0 × 101-1.0 × 104 fM) and a limit of detection (LOD) of 4.7 fM (equivalent to 2820 copies/μL). The spiked recoveries range from 99.593 to 106.442 %, and the relative standard deviation (RSD) is less than 5.124 %. The colorimetric modality exhibited the same working range with a lower LOD of 2.9 fM (equivalent to 1740 copies/μL) and an RSD of less than 2.611 %. Additionally, the recombinase polymerase amplification reaction enabled the detection of low NoV levels in food samples with a working range of 102-106 copies/mL and LOD of 102 copies/mL. The accuracy of the sensor in the analysis of spiked samples is consistent with the gold standard method (real-time quantitative reverse transcription-polymerase chain reaction), demonstrating the high accuracy and practical utility of the sensor.

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.

Fluorescent Selectivity-Enhanced FRET Based on 3D Photonic Crystals for Multianalyte Sensing

Fluorescence resonance energy transfer (FRET) finds widespread utility in biochemical sensing, single-molecule experiments, cell physiology, and various other domains due to its inherent simplicity and high sensitivity. Nevertheless, the efficiency of energy transfer between the FRET donor and acceptor is significantly contingent on the local photonic environment, a factor that limits its application in complex systems or multianalyte detections. Here, a fluorescent selectivity-enhanced acridine orange (AO)-aflatoxins (AFs) FRET system based on a range of 3D topological photonic crystals (PCs) was developed with the aim of enhancing the selectivity and discrimination capabilities of FRET. By exploring the angle-dependent characteristics of the photonic stopband, the stopband distribution across different 3D topological PCs pixels was investigated. This approach led to selective fluorescence enhancement in PCs that matched the stopbands, enabling the successful discrimination of six distinct aflatoxins and facilitating complex multianalysis of moldy food samples. In particular, the stopband, which was strategically positioned within the blue-purple structural color range, exhibited a strong alignment with the fluorescence peaks of both the FRET donor and acceptor. This alignment allowed the 3D three-pointed star PCs to be effectively employed for the identification of mixed samples containing six distinct aflatoxins as well as the detection of real aflatoxin samples present in moldy potatoes, bread, oats, and peanuts. Impressively, this approach achieved a remarkable accuracy rate of 100%. This innovative strategy not only presents a novel avenue for developing a multitarget discrimination analysis system but also offers a convenient pretreatment method for the quantitative detection of various aflatoxins.

SERS and MRS signals engineered dual-mode aptasensor for simultaneous distinguishment of aflatoxin subtypes

The accurate monitoring of aflatoxin subtypes is vitally important for food safety. Herein, a dual-mode aptasensor with surface-enhanced Raman scattering (SERS) and magnetic relaxation switching (MRS) signals is developed for the detection of aflatoxin B1, B2 and M1 (i.e. AFB1, AFB2 and AFM1). Au-Ag Janus NPs and Au-mushroom NPs are prepared and show intense and non-interfering SERS peaks without the additional modification of Raman molecules, and are utilized as SERS nanotags for the distinguishment of AFB1 and AFB2. Fe3O4@Au NPs functionalized by AFM1 aptamers are applied as MRS nanoprobes for the monitoring of AFM1. Aptamers engineered SERS nanotags and MRS nanoprobes are assembled, and show strong SERS performances and high transverse relaxation time (T2). AFB1, AFB2 and AFM1 induce the separation of SERS nanotags from the assemblies and the dispersion of Fe3O4@Au NPs, resulting in the decrease of SERS signals at 1278 cm-1 and 1000 cm-1 as well as the reduction of T2 values. The dual-mode but three kinds of detection signals don't interfere with each other and exhibit a significant linear relationship with the concentration of targets. This platform provides a high throughput monitoring strategy for the simultaneous analysis of different subtypes of mycotoxin.

Magnetic nanoparticle-based immunosensors and aptasensors for mycotoxin detection in foodstuffs: An update

Mycotoxin contamination of food crops is a global challenge due to their unpredictable occurrence and severe adverse health effects on humans. Therefore, it is of great importance to develop effective tools to prevent the accumulation of mycotoxins through the food chain. The use of magnetic nanoparticle (MNP)-assisted biosensors for detecting mycotoxin in complex foodstuffs has garnered great interest due to the significantly enhanced sensitivity and accuracy. Within such a context, this review includes the fundamentals and recent advances (2020-2023) in the area of mycotoxin monitoring in food matrices using MNP-based aptasensors and immunosensors. In this review, we start by providing a comprehensive introduction to the design of immunosensors (natural antibody or nanobody, random or site-oriented immobilization) and aptasensors (techniques for aptamer selection, characterization, and truncation). Meanwhile, special attention is paid to the multifunctionalities of MNPs (recoverable adsorbent, versatile carrier, and signal indicator) in preparing mycotoxin-specific biosensors. Further, the contribution of MNPs to the multiplexing determination of various mycotoxins is summarized. Finally, challenges and future perspectives for the practical applications of MNP-assisted biosensors are also discussed. The progress and updates of MNP-based biosensors shown in this review are expected to offer readers valuable insights about the design of MNP-based tools for the effective detection of mycotoxins in practical applications.

Computer vision-assisted smartphone microscope imaging digital immunosensor based on click chemistry-mediated microsphere counting technology for the detection of aflatoxin B1 in peanuts

Aflatoxin B1 is a carcinogenic contaminant in food or feed, and it poses a serious health risk to humans. Herein, a computer vision-assisted smartphone microscope imaging digital (SMID) immunosensor based on the click chemistry-mediated microsphere counting technology was designed for the detection of aflatoxin B1 in peanuts. In this SMID immunosensor, the modified polystyrene (PS) microspheres were used as the signal probes and were recorded by a smartphone microscopic imaging system after immunoreaction and click chemistry reaction. The number of PS probes is adjusted by aflatoxin B1. The customized computer vision procedure was used to efficiently identify and count the obtained PS probes. This SMID immunosensor enables sensitive detection of aflatoxin B1 with a linear range from 0.001 ng/mL to 500 ng/mL, providing a simple, sensitive, and portable tool for food safety supervision.

Design of an Antigen-Triggered Nanobody-Based Fluorescence Probe for PET Immunoassay to Detect Quinalphos in Food Samples

Photoinduced electron-transfer (PET) immunoassay based on a fluorescence site-specifically labeled nanobody, also called mini Quenchbody (Q-body), exhibits extraordinary sensitivity and saves much time in the homogeneous noncompetitive mode and is therefore regarded as a valuable method. However, limited by the efficiency of both quenching and dequenching of the fluorescence signal before and after antigen binding associated with the PET principle, not all original nanobodies can be used as candidates for mini Q-bodies. Herein, with the anti-quinalphos nanobody 11A (Nb-11A) as the model, we, for the first time, adopt a strategy by combining X-ray structural analysis with site-directed mutagenesis to design and produce a mutant Nb-R29W, and then successfully generate a mini Q-body by labeling with ATTO520 fluorescein. Based on this, a novel PET immunoassay is established, which exhibits a limit of detection of 0.007 μg/mL with a detection time of only 15 min, 25-fold improved sensitivity, and faster by 5-fold compared to the competitive immunoassay. Meanwhile, the recovery test of vegetable samples and validation by the standard ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) both demonstrated that the established PET immunoassay is a novel, sensitive, and accurate detection method for quinalphos. Ultimately, the findings of this work will provide valuable insights into the development of triggered PET fluorescence probes by using existing antibody resources.

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.

Magnetic Bead Manipulation in Microfluidic Chips for Biological Application

Magnetic beads manipulation in microfluidic chips is a promising research field for biological application, especially in the detection of biological targets. In this review, we intend to present a thorough and in-depth overview of recent magnetic beads manipulation in microfluidic chips and its biological application. First, we introduce the mechanism of magnetic manipulation in microfluidic chip, including force analysis, particle properties, and surface modification. Then, we compare some existing methods of magnetic manipulation in microfluidic chip and list their biological application. Besides, the suggestions and outlook for future developments in the magnetic manipulation system are also discussed and summarized.

Cluster-bomb type magnetic biosensor for ultrasensitive detection of Vibrio parahaemolyticus based on low field nuclear magnetic resonance

Herein, a novel cluster-bomb type signal sensing and amplification strategy in low field nuclear magnetic resonance was proposed, and a magnetic biosensor for ultrasensitive homogeneous immunoassay of Vibrio parahaemolyticus (VP) was developed. The capture unit MGO@Ab was magnetic graphene oxide (MGO) immobilized by VP antibody (Ab) to capture VP. And, the signal unit PS@Gd-CQDs@Ab was polystyrene (PS) pellets covered by Ab to recognize VP and Gd-CQDs i.e. carbon quantum dots (CQDs) containing lots of magnetic signal labels Gd³⁺. In presence of VP, the immunocomplex signal unit-VP-capture unit could be formed and separated by magnetic force conveniently from the sample matrix. With the successive introduction of disulfide threitol and hydrochloric acid, signal units were cleaved and disintegrated, resulting in a homogeneous dispersion of Gd³⁺. Thus, cluster-bomb type dual signal amplification was achieved through increasing the amount and the dispersity of signal labels simultaneously. Under optimal experimental conditions, VP could be detected in the concentration range of 5-1.0 × 10⁶ CFU/mL, with a limit of quantitation (LOQ) 4 CFU/mL. In addition, satisfactory selectivity, stability and reliability could be obtained. Therefore, this cluster-bomb type signal sensing and amplification strategy is powerful in designing magnetic biosensor and detecting pathogenic bacteria.

Colorimetric and photothermal dual-mode lateral flow immunoassay based on Au-Fe3O4 multifunctional nanoparticles for detection of Salmonella typhimurium

Au-Fe3O4 multifunctional nanoparticles (NPs) were synthesized and integrated with lateral flow immunoassay (LFIA) for dual-mode detection of Salmonella typhimurium. The Au-Fe3O4 NPs not only combined excellent local surface plasmon resonance characteristics and superparamagnetic properties, but also exhibited good photothermal effect. In the detection, antibody-conjugated Au-Fe3O4 NPs first captured S. typhimurium from complex matrix, which was then loaded on the LFIA strip and trapped by the T-line. By observing the color bands with the naked eyes, qualitative detection was performed free of instrument. By measuring the photothermal signal, quantification was achieved with a portable infrared thermal camera. The introduction of magnetic separation achieved the enrichment and purification of target bacteria, thus enhancing the detection sensitivity and reducing interference. This dual-mode LFIA achieved a visual detection limit of 5 × 105 CFU/mL and a photothermal detection limit of 5 × 104 CFU/mL. Compared with traditional Au-based LFIA, this dual-mode LFIA increased the detection sensitivity by 2 orders of magnitude and could be directly applied to unprocessed milk sample. Besides, this dual-mode LFIA showed good reproducibility and specificity. The intra-assay and inter-assay variation coefficients were 3.0% and 7.9%, and with this dual-mode LFIA, other bacteria hardly produced distinguishable signals. Thus, the Au-Fe3O4 NPs-based LFIA has potential to increase the efficiency of pandemic prevention and control. Au-Fe3O4 nanoparticle proved to be a promising alternative reporter for LFIA, achieving multifunctions: target purification, target enrichment, visual qualitation, and instrumental quantification, which improved the limitations of traditional LFIA.

Liquid Crystal Droplet-Based Biosensors: Promising for Point-of-Care Testing

The development of biosensing platforms has been impressively accelerated by advancements in liquid crystal (LC) technology. High response rate, easy operation, and good stability of the LC droplet-based biosensors are all benefits of the long-range order of LC molecules. Bioprobes emerged when LC droplets were combined with biotechnology, and these bioprobes are used extensively for disease diagnosis, food safety, and environmental monitoring. The LC droplet biosensors have high sensitivity and excellent selectivity, making them an attractive tool for the label-free, economical, and real-time detection of different targets. Portable devices work well as the accessory kits for LC droplet-based biosensors to make them easier to use by anyone for on-site monitoring of targets. Herein, we offer a review of the latest developments in the design of LC droplet-based biosensors for qualitative target monitoring and quantitative target analysis.

A CRISPR-Cas12a-powered magnetic relaxation switching biosensor for the sensitive detection of Salmonella

Magnetic relaxation switching (MRS) biosensors are attractive in the field of food safety owing to their simplicity and high signal-to-noise ratio. But they are less in sensitivity and stability caused by the insufficient crosslinking or non-specific binding of magnetic nanoparticles (MNPs) with targets. To address this problem, the CRISPR-Cas12a system was introduced into an MRS biosensor for the first time, to precisely control the binding of two types of MNPs with sizes of 130 nm (MNP₁₃₀) and 30 nm (MNP₃₀), for the sensitive detection of Salmonella. Delicately, the biosensor was designed based on the different magnetic properties of the two sizes of MNPs. The target Salmonella activated the collateral cleavage activity of the CRISPR-Cas12a system, which inhibited the binding of the two sizes of MNPs, resulting in an increase of unbound MNP₃₀. After separating MNP₁₃₀-MNP₃₀ complexes and MNP₁₃₀ from MNP₃₀, the free MNP₃₀ left in solution acted as transverse relaxation time (T₂) signal reporters for Salmonella detection. Under optimized conditions, the CRISPR-MRS biosensor presented a limit of detection of 1.3 × 10² CFU mL^-1 for Salmonella, which is lower than most MRS biosensor analogues. It also showed satisfactory specificity and performed well in spiked chicken meat samples. This biosensing strategy not only extends the reach of the CRISPR-Cas12a system in biosensors but also offers an alternative for pathogen detection with satisfactory sensitivity.