Gd3+-nanoparticle-enhanced multivalent biosensing that combines magnetic relaxation switching and magnetic separation

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.

A review of nanomaterials for biosensing applications

A biosensor is a device that reacts with the analyte to be analyzed, detects its concentration, and generates readable information, which plays an important role in medical diagnosis, detection of physiological indicators, and disease prevention. Nanomaterials have received increasing attention in the fabrication and improvement of biosensors due to their unique physicochemical and optical properties. In this paper, the properties of nanomaterials such as the size effect, optical and electrical properties, and their advantages in the field of biosensing are briefly summarized, and the application of nanomaterials can effectively improve the sensitivity and reduce the detection limit of biosensors. The advantages of commonly used nanomaterials such as gold nanoparticles (AuNPs), carbon nanotubes (CNTs), quantum dots (QDs), graphene, and magnetic nanobeads for biosensor applications are also reviewed. Besides, the two main types of biosensors using nanomaterials involved in their construction and their working principles are described, and the toxicity and biocompatibility of nanomaterials and the future direction of nanomaterial biosensors are discussed.

Broad-spectrum portable magnetic relaxation switching immunosensor with gold-functionalized magnetic nanoprobes for the sensitive detection of multiple pyrethroids

At present, the most available pyrethroid (PYR) detection methods still suffer from a narrow detection spectrum, low sensitivity, and less portability. Herein, a novel magnetic relaxation switching (MRS) sensor was elaboratively designed to detect multiple PYRs, combining a novel broad-spectrum antibody CL-CN/1D2 and synthesized immune gold-functionalized magnetic nanoparticles, with the inherent response of the sensor. A series of antibodies and the immune gold-functionalized magnetic nanoparticles were designed and synthesized. The broad-spectrum antibody CL-CN/1D2 and high-performance gold-functionalized magnetic nanoprobe were further selected. The target analytes were effectively captured by the gold-functionalized magnetic nanoparticles in 20% (v/v) ethanol, resulting in the number increase of the signaling probes in the supernatant after magnetic separation. This sensor can detect multiple PYRs with a detection limit of 2.72 μg/L for cypermethrin, 3.58 μg/L for β-cypermethrin, 4.07 μg/L for cyfluthrin, 3.66 μg/L for λ-cyhalothrin, 4.42 μg/L for β-cyhalothrin, 3.51 μg/L for fenpropathrin, 4.41 μg/L for fenvalerate, and 4.12 μg/L for deltamethrin in lake water and milk within 35 min. This study not only achieves broad-spectrum PYRs detection at a trace amount but also provides an effective and universal strategy for enhancing the sensitivity and stability of the portable MRS sensor when detecting hydrophobic analytes in the environment.

Fluorescent detection of emerging virus based on nanoparticles: From synthesis to application

The spread of COVID-19 has caused huge economic losses and irreversible social impact. Therefore, to successfully prevent the spread of the virus and solve public health problems, it is urgent to develop detection methods with high sensitivity and accuracy. However, existing detection methods are time-consuming, rely on instruments, and require skilled operators, making rapid detection challenging to implement. Biosensors based on fluorescent nanoparticles have attracted interest in the field of detection because of their advantages, such as high sensitivity, low detection limit, and simple result readout. In this review, we systematically describe the synthesis, intrinsic advantages, and applications of organic dye-doped fluorescent nanoparticles, metal nanoclusters, up-conversion particles, quantum dots, carbon dots, and others for virus detection. Furthermore, future research initiatives are highlighted, including green production of fluorescent nanoparticles with high quantum yield, speedy signal reading by integrating with intelligent information, and error reduction by coupling with numerous fluorescent nanoparticles.

A chemosensing approach for the colorimetric and spectroscopic detection of Cr3+, Cu2+, Fe3+, and Gd3+ metal ions

Metal cations are present in domestic and industrial wastewater and have adverse effects on human and aqueous life. The present study describes the development of the molecular probe 9-anthracen-9-ylmethylene)hydrazineylidene)methyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (AMHMPQ) to detect Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions by using UV-visible, fluorescence, colorimetric and excitation-emission matrix (EEM) spectroscopy techniques. The interaction of Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ can be observed by the absorption maxima shift, turn-off, colour changes, and EEM shifts. In addition, fluorescence limits of detection 17.66 × 10^-6 M, 6.44 × 10^-9 M, 28.87 × 10^-8 M, and 12.49 × 10^-6 M in wide linear ranges, low limits of quantifications, high values of Stern-Volmer constant, Job's plot and Benesi-Hildebrand plot justify the 1:1 association affinity with association constants of 1.46 × 10⁴ M^-1, 1.86 × 10⁷ M^-1, 2.69 × 10⁵ M^-1, 2.13 × 10⁴ M^-1 for AMHMPQ-metal ions (Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions), respectively. Paper- and mask-based kits are developed to explore the utility of the designed chemosensor. Additionally, AMHMPQ acts as a reusable sensor for two, seven, two, and zero cycles for Cr³⁺, Cu²⁺, Fe³⁺, and Gd³⁺ ions, respectively, when checked with EDTA.

Optomagnetic biosensors: Volumetric sensing based on magnetic actuation-induced optical modulations

In comparison to alternative nanomaterials, magnetic micron/nano-sized particles show unique advantages, e.g., easy manipulation, stable signal, and high contrast. By applying magnetic actuation, magnetic particles exert forces on target objects for highly selective operation even in non-purified samples. We herein describe a subgroup of magnetic biosensors, namely optomagnetic biosensors, which employ alternating magnetic fields to generate periodic movements of magnetic labels. The optical modulation induced by the dynamics of magnetic labels is then analyzed by photodetectors, providing information of, e.g., hydrodynamic size changes of the magnetic labels. Optomagnetic sensing mechanisms can suppress the noise (by performing lock-in detection), accelerate the reaction (by magnetic force-enhanced molecular collision), and facilitate homogeneous/volumetric detection. Moreover, optomagnetic sensing can be performed using a low magnetic field (<10 mT) without sophisticated light sources or pickup coils, further enhancing its applicability for point-of-care tests. This review concentrates on optomagnetic biosensing techniques of different concepts classified by the magnetic actuation strategy, i.e., magnetic field-enhanced agglutination, rotating magnetic field-based particle rotation, and oscillating magnetic field-induced Brownian relaxation. Optomagnetic sensing principles applied with different actuation strategies are introduced as well. For each representative optomagnetic biosensor, a simple immunoassay strategy-based application is introduced (if possible) for methodological comparison. Thereafter, challenges and perspectives are discussed, including minimization of nonspecific binding, on-chip integration, and multiplex detection, all of which are key requirements in point-of-care diagnostics.

A bio-inspired plasmonic nanosensor for angiotensin-converting enzyme through peptide-mediated assembly of gold nanoparticles

Angiotensin-converting enzyme (ACE) can indicate blood pressure that relates to human health such as the cardiovascular disease. However, current methods are not competent to detect the ACE activity in a rapid and straightforward way. Plasmonic biosensors built on the modulation of metallic nanomaterials have emerged as novel tools for the detection of biomarkers. In this work, we report a bio-inspired strategy for the plasmonic detection of ACE in a rapid, sensitive, and selective way through peptide-mediated assembly of gold nanoparticles (AuNPs). In this biosensor, cysteine-angiotensin I-cysteine can assemble and aggregate AuNPs due to the Au-S bond. The presence of ACE can specifically catalyze the hydrolysis of angiotensin I, thus dissociating the cysteine-cysteine structure of the peptide that results in the disassembly and dispersion of AuNPs. This bio-inspired plasmonic nanosensor enables naked-eyed readout of ACE detection with great selectivity and high sensitivity with a LOD of 0.40 mU/mL. It also allows for the screening of ACE inhibitors and inhibitory peptides for the development of antihypertensive drugs or food. The biosensing technique developed in this work provides a new plasmonic approach that holds great promise as a point-of-care platform for biomedical diagnostics and the food industry.

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⁺.

Magnetic graphene quantum dots facilitate closed-tube one-step detection of SARS-CoV-2 with ultra-low field NMR relaxometry

The rapid and sensitive diagnosis of the highly contagious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is one of the crucial issues at the outbreak of the ongoing global pandemic that has no valid cure. Here, we propose a SARS-CoV-2 antibody conjugated magnetic graphene quantum dots (GQDs)-based magnetic relaxation switch (MRSw) that specifically recognizes the SARS-CoV-2. The probe of MRSw can be directly mixed with the test sample in a fully sealed vial without sample pretreatment, which largely reduces the testers' risk of infection during the operation. The closed-tube one-step strategy to detect SARS-CoV-2 is developed with home-made ultra-low field nuclear magnetic resonance (ULF NMR) relaxometry working at 118 μT. The magnetic GQDs-based probe shows ultra-high sensitivity in the detection of SARS-CoV-2 due to its high magnetic relaxivity, and the limit of detection is optimized to 248 Particles mL^‒1. Meanwhile, the detection time in ULF NMR system is only 2 min, which can significantly improve the efficiency of detection. In short, the magnetic GQDs-based MRSw coupled with ULF NMR can realize a rapid, safe, and sensitive detection of SARS-CoV-2.

Three-dimensional assembly and disassembly of Fe3O4-decorated porous carbon nanocomposite with enhanced transversal relaxation for magnetic resonance sensing of bisphenol A

The design and construction of a novel magnetic resonance sensor (MRS) is presented for bisphenol A (BPA) detection. The MRS has been built based on the core component of magnetic Fe₃O₄ nanoparticles (~ 40 nm), which were uniformly distributed in nanoporous carbon (abbreviated as Fe₃O₄@NPC). The synthesis was derived from the calcination of the metal organic framework (MOF) precursor of Fe-MIL-101 at high temperature. Fe₃O₄@NPC was confirmed with enhanced transversal relaxation with r₂ value of 118.2 mM^-1 s^-1, which was around 1.7 times higher than that of the naked Fe₃O₄ nanoparticle. This enhancement is attributed to the excellent proton transverse relaxation rate of Fe₃O₄@NPC caused by the reduced self-diffusion coefficient of water molecules in the vicinity of Fe₃O₄ nanoparticles in the nanoporous carbon. BPA antibody (Ab) and antigen (Ag)-ovalbumin (OVA) were immobilized onto the Fe₃O₄@NPC to form Ab-Fe₃O₄@NPC and Ag-Fe₃O₄@NPC, respectively. These two composites can cause the three-dimensional assembly of Fe₃O₄@NPC via immunological recognition. The presence of BPA can compete with antigen-OVA to combine with Ab-Fe₃O₄@NPC, thereby breaking the assembly process (disassembly). The difference in the change of the T₂ value before and after adding BPA can thus be used to monitor BPA. The proposed MRS not only revealed a wide linear range of BPA concentration from 0.05 to 50 ng mL^-1 with an extremely low detection limit of 0.012 ng mL^-1 (S/N = 3), but also displayed high selectivity towards matrix interferences. The recoveries of BPA ranged from 95.6 to 108.4% for spiked tea π, and 93.4 to 104.7% for spiked canned oranges samples, respectively, and the RSD (n = 3) was less than 4.4% for 3 successive assays. The versatility of Fe₃O₄@NPC with customized relaxation responses provides the possibility for the adaptation of magnetic resonance platforms for food safety development. The magnetic Fe3O4 nanoparticles are uniformly dispersed in the nanoporous carbon (Fe3O4@NPC), which derived from the calcinating of the metal organic framework (MOF) precursor of Fe-MIL-101. And the magnetic Fe₃O₄@NPCs are adopted for the construction of magnetic resonance sensor (MRS) for bisphenol A (BPA) detection.

Ultrasensitive determination of ractopamine based on dual catalytic signal amplification by Pd nanocubes and HRP using a flow injection chemiluminescence immunoassay

A rapid and highly sensitive flow injection chemiluminescence immunoassay was developed to detect ractopamine residues in pork products. Palladium nanocubes with excellent catalytic performance for a traditional luminol-PIP-H2O2 chemiluminescence system were used as effective carriers to connect a ractopamine antibody and horseradish peroxidase (HRP). Double amplification of a chemiluminescence signal was realized because of palladium nanocubes and HRP. Carboxyl-modified resin beads were used as suitable materials to load ractopamine-coating antigens due to their good biocompatibility and large specific surface area. Based on the principle of competitive immunity, ractopamine standard solution would compete with antigen loaded on carboxyl resin beads for limited binding sites on a ractopamine antibody. Thus, the chemiluminescence intensity of an immunosensor has a linear negative correlation with the logarithm value of ractopamine concentration. Under optimal experimental conditions, the detection range of ractopamine was 0.005-1000 ng mL-1, and the limit of detection (LOD) was 1.7 pg mL-1 (S/N = 3). The proposed immunoassay possessed acceptable accuracy, high specificity and reproducibility and RAC was examined in pork and pig feed with satisfactory results, which would provide a better prospect for the detection of small molecules in food and environment analysis.