Magnetic tweezers principles and promises.

Magnetic tweezers have become popular with the outbreak of single molecule micromanipulation: catching a single molecule of DNA, RNA or a single protein and applying mechanical constrains using micron-size magnetic beads and magnets turn out to be easy. Various factors have made this possible: the fact that manufacturers have been preparing these beads to catch various biological entities-the ease of use provided by magnets which apply a force or a torque at a distance thus inside a flow cell-some chance: since the forces so generated are in the right range to stretch a single molecule. This is a little less true for torque. Finally, one feature which also appears very important is the simplicity of their calibration using Brownian motion. Here we start by describing magnetic tweezers used routinely in our laboratory where we have tried to develop a device as simple as possible so that the experimentalist can really focus on the biological aspect of the biomolecules that he/she is interested in. We discuss the implications of the various components and their important features. Next, we summarize what is easy to achieve and what is less easy. Then we refer to contributions by other groups who have brought valuable insights to improve magnetic tweezers.

Construction and operation of high-resolution magnetic tape head tweezers for measuring single-protein dynamics under force.

Mechanical forces are critical to protein function across many biological contexts-from bacterial adhesion to muscle mechanics and mechanotransduction processes. Hence, understanding how mechanical forces govern protein activity has developed into a central scientific question. In this context, single-molecule magnetic tweezers has recently emerged as a valuable experimental tool, offering the capability to measure single proteins over physiologically relevant forces and timescales. In this chapter, we present a detailed protocol for the assembly and operation of our magnetic tape head tweezers instrument, specifically tailored to investigate protein dynamics. Our instrument boasts a simplified microscope design and incorporates a magnetic tape head as the force-generating apparatus, facilitating precise force control and enhancing its temporal stability, enabling the study of single protein mechanics over extended timescales spanning several hours or even days. Moreover, its straightforward and cost-effective design ensures its accessibility to the wider scientific community. We anticipate that this technique will attract widespread interest within the growing field of mechanobiology and expect that this chapter will provide facilitated accessibility to this technology.

Fabrication of magnetic Fe3O4 doped ß-cyclodextrin microporous organic network for the efficient extraction of endocrine disrupting chemicals from food takeaway boxes.

Endocrine disrupting chemicals (EDCs) are a typical class of natural or man-made endogenous hormone agonists or antagonists that can directly or potentially interfere with human endocrine system. However, it is still difficult to analyze trace EDCs directly from complex environment and food matrices. Therefore, the proper sample pretreatment is highly desired and the preparation of efficient adsorbents is of great challenge and importance. Herein, we report the facile one-pot solvothermal synthesis of Fe3O4 nanoparticle doped magnetic ß-cyclodextrin microporous organic network composites (MCD-MONs) for the magnetic solid phase extraction (MSPE) of four phenolic EDCs in water and food takeaway boxes prior to the high-performance liquid chromatography analysis. The sheet-like Fe3O4 doped MCD-MONs offered good magnetic property (16.5 emu g-1) and stability, and provided numerous hydrogen bonding, hydrophobic, π-π, and host-guest interaction sites for EDCs. Under the optimal experimental conditions, the established method was successfully verified with wide linear range (2.0-1000 µg L-1), low limits of detection (0.6-1.0 µg L-1), good precisions (intra-day and inter-day RSDs < 5.2 %, n = 3), large enrichment factors (88-98) and adsorption capacity (90.3-255.8 mg g-1), short extraction time (6 min), less adsorbent consumption (3 mg), and good reusability (at least 8 times) for EDCs. The proposed method was successfully applied to detect the trace EDCs in real samples with the recovery of 84.0-99.7 %. This work demonstrated the great potential of MCD-MONs for the efficient MSPE of trace EDCs from complex food takeaway boxes and water samples and uncovered the prospect of CD-based MONs in sample pretreatment.

Magnetic Nanoparticles: A Review on Synthesis, Characterization, Functionalization, and Biomedical Applications.

Nowadays, magnetic nanoparticles (MNPs) are applied in numerous fields, especially in biomedical applications. Since biofluidic samples and biological tissues are nonmagnetic, negligible background signals can interfere with the magnetic signals from MNPs in magnetic biosensing and imaging applications. In addition, the MNPs can be remotely controlled by magnetic fields, which make it possible for magnetic separation and targeted drug delivery. Furthermore, due to the unique dynamic magnetizations of MNPs when subjected to alternating magnetic fields, MNPs are also proposed as a key tool in cancer treatment, an example is magnetic hyperthermia therapy. Due to their distinct surface chemistry, good biocompatibility, and inducible magnetic moments, the material and morphological structure design of MNPs has attracted enormous interest from a variety of scientific domains. Herein, a thorough review of the chemical synthesis strategies of MNPs, the methodologies to modify the MNPs surface for better biocompatibility, the physicochemical characterization techniques for MNPs, as well as some representative applications of MNPs in disease diagnosis and treatment are provided. Further portions of the review go into the diagnostic and therapeutic uses of composite MNPs with core/shell structures as well as a deeper analysis of MNP properties to learn about potential biomedical applications.

High-Speed Magnetic Tweezers for Nanomechanical Measurements on Force-Sensitive Elements.

Single-molecule magnetic tweezers (MTs) have served as powerful tools to forcefully interrogate biomolecules, such as nucleic acids and proteins, and are therefore poised to be useful in the field of mechanobiology. Since the method commonly relies on image-based tracking of magnetic beads, the speed limit in recording and analyzing images, as well as the thermal fluctuations of the beads, has long hampered its application in observing small and fast structural changes in target molecules. This article describes detailed methods for the construction and operation of a high-resolution MT setup that can resolve nanoscale, millisecond dynamics of biomolecules and their complexes. As application examples, experiments with DNA hairpins and SNARE complexes (membrane-fusion machinery) are demonstrated, focusing on how their transient states and transitions can be detected in the presence of piconewton-scale forces. We expect that high-speed MTs will continue to enable high-precision nanomechanical measurements on molecules that sense, transmit, and generate forces in cells, and thereby deepen our molecular-level understanding of mechanobiology.

Thoughtful design of a covalent organic framework with tailor-made polarity and pore size for the enrichment of bisphenols and their derivatives: Extraction performance, adsorption mechanism and toxicity evaluation.

A stable, reusable and cost-effective covalent organic framework (COF) with medium polarity was successfully decorated on Fe3O4. The Fe3O4@COF contained tailor-made polarity and pore size that fitted well with bisphenols and their derivatives (BPs). When coupling magnetic solid-phase extraction (MSPE) with high-performance liquid chromatography (HPLC) detection, the Fe3O4@COF featured efficient recognition and enrichment for BPs due to π-π stacking, C-H⋯π interactions, pore-filling effect, dispersion force and hydrophobic interactions. Under optimized conditions, calibration plots exhibited good linearity (5-1000 ng mL-1), and limits of detection (LOD) ranged from 0.15 to 0.39 ng mL-1. The method was successfully employed in quantifying BPs in authentic lake and river water samples with satisfactory recoveries ranging from 81.4% to 120%. Molecular dynamics simulation revealed extraction mechanisms, and a microscopic behavior related to the clustering property of the emerging brominated compounds was first discovered. Ecotoxicological assessments of target pollutants were conducted from multiple aspects, highlighting the harmfulness of the chemicals and the significance of the analytical method. The proposed methodology offered sensitive detection and quantification, which was beneficial for the timely tracking of the concentration, transportation and distribution of BPs to better explore their environmental behavior and tackle contamination problems in complex environmental matrices.

Fabrication of a core-shell porphyrin-based magnetic covalent organic framework for effective extraction of PCPs in a wide polarity range.

A novel porphyrin-based magnetic covalent organic framework (PCOF) was first reported by using a facile synthetic procedure. The Fe3O4@NH2@PCOF nanospheres were utilized to effectively extract personal care products in a wide polarity range (log Kow values from 1.96 to 7.60). The successful magnetic solid-phase extraction (MSPE) of target analytes could be ascribed to the sufficient oxygen-, nitrogen- and phenyl-containing functional groups of the COF layer, which are demonstrated to be of good compatibility with pollutants exhibiting different polarities by using molecular dynamics simulations, independent gradient model analysis and various characterizations. The MSPE extraction efficiency was enhanced by optimizing key parameters. The findings indicated that the method had a wide linearity range (1-500 ng mL-1 for parabens and UV filters) and low detection limits (0.4-0.9 ng mL-1 for parabens and 0.2-0.6 ng mL-1 for UV filters). The accuracy was reflected by recoveries ranging from 74% to 114%. Satisfactory intra- and inter-day precisions from 3.0% to 9.8% and 0.5%-9.1% were obtained. Overall, the proposed MSPE-HPLC method is accurate and reliable for identifying parabens as well as UV filters in wastewater and swimming pool water. The potential of the method for evaluating human exposure risk was unfolded.

Magnetic metal-organic frameworks as adsorbents for the detection of azo pigments in food matrices.

Azo pigments with azo functional groups (-NN-) are commonly used in foods. Through rational design, here we report synthesis of a novel magnetic metal-organic framework (MOF-545) and the use of Fe3O4@MOF-545 for the selective separation of azo pigments from beverages. We find that Fe3O4@MOF-545 exhibits excellent adsorption/removal capacities for new coccine (NC, 459 mg g-1) and lemon yellow (LY, 476 mg g-1) individually. The results indicated that the adsorption capacities of Fe3O4@MOF-545 were 5 times higher than reported previously. The large surface area (120.4 m2/g) and the suitable pore size (1.6 nm) of Fe3O4@MOF-545 provided multiple accessible channels for azo pigment adsorption. The adsorption kinetics, stability, recovery, and reusability of Fe3O4@MOF-545 were all reported. The recoveries ranged from 92.9 to 104.5%, reflecting the capacity of MOF-545 to efficiently separate NC and LY. Fe3O4@MOF-545 is suitable for azo pigment determination and separation in food matrices.

Tailored novel multifunctional benzyl-functionalized magnetic ionic liquid for rapid and efficient monitoring of trace fluoroquinolones in food samples.

The novel hydrophobic cobalt anion-based benzyl-functionalized magnetic ionic liquid (MIL) was first designed, synthesized, and employed as a multifunctional extractant (for extraction, visual observation, and magnetic separation) for fast dispersive liquid-liquid microextraction of fluoroquinolones (FQs). According to the aromatic structure of FQs, three benzyl-functionalized MILs were customized by introducing various aromatic functional groups (benzyl, paracetyl phenyl, and m-methoxyphenyl) to justify the extraction efficiency and selectivity. In particular, the paracetyl phenyl functionalized MIL showed superior extraction efficiency among others due to its hydrophobicity and strong interactions of hydrogen bonding and π-π stacking. Moreover, under optimal conditions, the limits of detection were as low as 0.47-0.56 µg/L, and the enhancement factors were as high as 88-95. The recoveries were between 90.8 % and 107.8 %, and the relative standard deviation was less than 5.4 %. Ascribed to these outstanding experimental results, the benzyl-functionalized MIL is a potential magnetic extractant for detecting trace FQs in food samples.

A phage-based magnetic relaxation switching biosensor using bioorthogonal reaction signal amplification for Salmonella detection in foods.

Phages are uniquely suited for bacterial detection due to their low cost and ability to recognize live bacteria. Herein, our work establishes the proof-of-concept detection of Salmonella in orange juice based on a phage-mediated portable magnetic relaxation switching (MRS) biosensor. The limit of quantification (LOQ) could reach 5 CFU/mL (95 % confidence interval [CI]: 4-7, N = 4) with a linear range of 102-108 CFU/mL, which has improved 10-fold than that without bioorthogonal signal amplification. The recovery rate of the phage-based MRS biosensor was 95.0 % (95 % confidence interval [CI]: 89.0 %-100.9 %, N = 6). The specificity of the phage-based MRS biosensor was 100 % without false-positive results. In addition, this sensor was able to detect <10 CFU per 25 mL of Salmonella in orange juice with 4-h pre-enrichment. The result from the phage-based MRS biosensor is consistent with that from the standard plate count method. This sensor provides a reliable and ultrasensitive detection platform for pathogens.

Biosensors and Drug Delivery in Oncotheranostics Using Inorganic Synthetic and Biogenic Magnetic Nanoparticles.

Magnetic nanocarriers have attracted attention in translational oncology due to their ability to be employed both for tumor diagnostics and therapy. This review summarizes data on applications of synthetic and biogenic magnetic nanoparticles (MNPs) in oncological theranostics and related areas. The basics of both types of MNPs including synthesis approaches, structure, and physicochemical properties are discussed. The properties of synthetic MNPs and biogenic MNPs are compared with regard to their antitumor therapeutic efficiency, diagnostic potential, biocompatibility, and cellular toxicity. The comparative analysis demonstrates that both synthetic and biogenic MNPs could be efficiently used for cancer theranostics, including biosensorics and drug delivery. At the same time, reduced toxicity of biogenic particles was noted, which makes them advantageous for in vivo applications, such as drug delivery, or MRI imaging of tumors. Adaptability to surface modification based on natural biochemical processes is also noted, as well as good compatibility with tumor cells and proliferation in them. Advances in the bionanotechnology field should lead to the implementation of MNPs in clinical trials.

Diamond-Based Nanoscale Quantum Relaxometry for Sensing Free Radical Production in Cells.

Diamond magnetometry makes use of fluorescent defects in diamonds to convert magnetic resonance signals into fluorescence. Because optical photons can be detected much more sensitively, this technique currently holds several sensitivity world records for room temperature magnetic measurements. It is orders of magnitude more sensitive than conventional magnetic resonance imaging (MRI) for detecting magnetic resonances. Here, the use of diamond magnetometry to detect free radical production in single living cells with nanometer resolution is experimentally demonstrated. This measuring system is first optimized and calibrated with chemicals at known concentrations. These measurements serve as benchmarks for future experiments. While conventional MRI typically has millimeter resolution, measurements are performed on individual cells to detect nitric oxide signaling at the nanoscale, within 10-20 nm from the internalized particles localized with a diffraction limited optical resolution. This level of detail is inaccessible to the state-of-the-art techniques. Nitric oxide is detected and the dynamics of its production and inhibition in the intra- and extracellular environment are followed.

Core-shell-structured magnetic covalent organic frameworks for effective extraction of parabens prior to their determination by HPLC.

Covalent organic framework (COF)-decorated magnetic nanoparticles (Fe3O4@DhaTab) with core-shell structure have been synthesized by one-pot method. The prepared Fe3O4@DhaTab was well characterized, and parameters of magnetic solid-phase extraction (MSPE) for parabens were also investigated in detail. Under optimized conditions, the adsorbent dosage was only 3 mg and extraction time was 10 min. The developed Fe3O4@DhaTab-based MSPE-HPLC analysis method offered good linearity (0.01-20 µg mL-1) with R2 (0.999) and low limits of detection (3.3-6.5 µg L-1) using UV detector at 254 nm. The proposed method was applied to determine four parabens in environmental water samples with recoveries in the range 64.0-105% and relative standard deviations of 0.16-7.8%. The adsorption mechanism was explored and indicated that porous DhaTab shell provided π-π, hydrophobic, and hydrogen bonding interactions in the MSPE process. The results revealed the potential of magnetic-functionalized COFs in determination of environmental contaminants.

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.

Direct Measurement of the Stall Torque of the Flagellar Motor in Escherichia coli with Magnetic Tweezers.

The flagellar motor drives the rotation of flagellar filaments, propelling the swimming of flagellated bacteria. The maximum torque the motor generates, the stall torque, is a key characteristic of the motor function. Direct measurements of the stall torque carried out 3 decades ago suffered from large experimental uncertainties, and subsequently there were only indirect measurements. Here, we applied magnetic tweezers to directly measure the stall torque in E. coli. We precisely calibrated the torsional stiffness of the magnetic tweezers and performed motor resurrection experiments at stall, accomplishing a precise determination of the stall torque per torque-generating unit (stator unit). From our measurements, each stator passes 2 protons per step, indicating a tight coupling between motor rotation and proton flux. IMPORTANCE The maximum torque the bacterial flagellar motor generates, the stall torque, is a critical parameter that describes the motor energetics. As the motor operates in equilibrium near stall, from the stall torque one can determine how many protons each torque-generating unit (stator) of the motor passes per revolution and then test whether motor rotation and proton flux are tightly or loosely coupled, which has been controversial in recent years. Direct measurements performed 3 decades ago suffered from large uncertainties, and subsequently, only indirect measurements were attempted, obtaining a range of values inconsistent with the previous direct measurements. Here, we developed a method that used magnetic tweezers to perform motor resurrection experiments at stall, resulting in a direct precise measurement of the stall torque per stator. Our study resolved the previous inconsistencies and provided direct experimental support for the tight coupling mechanism between motor rotation and proton flux.

Magnetic supercluster particles for highly sensitive magnetic biosensing of proteins.

A strategy is reported to improve the detection limits of current giant magnetoresistance (GMR) biosensors by augmenting the effective magnetic moment that the magnetic tags on the biosensors can exert. Magnetic supercluster particles (MSPs), each of which consists of ~ 1000 superparamagnetic cores, are prepared by a wet-chemical technique and are utilized to improve the limit of detection of GMR biosensors down to 17.6 zmol for biotin as a target molecule. This value is more than four orders of magnitude lower than that of the conventional colorimetric assay performed using the same set of reagents except for the signal transducer. The applicability of MSPs in immunoassay is further demonstrated by simultaneously detecting vascular endothelial growth factor (VEGF) and C-reactive protein (CRP) in a duplex assay format. MSPs outperform commercially available magnetic nanoparticles in terms of signal intensity and detection limit.

Transverse Magnetic Tweezers Allowing Coincident Epi-Fluorescence Microscopy on Horizontally Extended DNA.

Longitudinal magnetic tweezers (L-MT) have seen wide-scale adoption as the tool of choice for stretching and twisting a single DNA molecule. They are also used to probe topological changes in DNA as a result of protein binding and enzymatic activity. However, in the longitudinal configuration, the DNA molecule is extended perpendicular to the imaging plane. As a result, it is only possible to infer biological activity from the motion of the tethered paramagnetic microsphere. Described here is a "transverse" magnetic tweezers (T-MT) geometry featuring simultaneous control of DNA extension and spatially coincident video-rate epi-fluorescence imaging. Unlike in L-MT, DNA tethers in T-MT are extended parallel to the imaging plane between two micron-sized spheres, and importantly protein targets on the DNA can be localized using fluorescent nanoparticles. The T-MT can manipulate a long DNA construct at molecular extensions approaching the contour length defined by B-DNA helical geometry, and the measured entropic elasticity agrees with the wormlike chain model (force <35 pN). By incorporating a torsionally constrained DNA tether, the T-MT would allow both the relative extension and twist of the tether to be manipulated, while viewing far-red emitting fluorophore-labeled targets. This T-MT design has the potential to enable the study of DNA binding and remodeling processes under conditions of constant force and defined torsional stress.

The First Evaluation of Remote Magnetic Navigation-Guided Pediatric Ventricular Arrhythmia Ablation.

Catheter ablation (CA) is an important treatment option for ventricular arrhythmias (VA) in pediatric cardiology. Currently, various CA techniques are available, including remote magnetic navigation (RMN)-guided radiofrequency (RF) ablation. However, no studies evaluate RMN-guided ablative therapy outcomes in children with VA yet. This study aimed to compare procedural and long-term outcomes between RMN-guided and manual (MAN)-guided VA ablation in children. This single-center, retrospective study included all CA procedures for VA performed in children with or without structural heart disease from 2008 until 2020. Two study groups were defined by CA technique: RMN or MAN. Primary outcome was recurrence of VA. Baseline clinical, procedural and safety data were also evaluated. This study included 22 patients, who underwent 30 procedures, with a median age of 15 (IQR 14-17; range 1-17) years and a mean weight of 57 ± 20 kg. In total, 14 procedures were performed using RMN and 16 using MAN (22 first and 8 redo procedures). Regarding first procedures, recurrence rates were significantly lower in RMN compared to MAN (20% versus 67%, P = 0.029), at a mean follow-up of 5.2 ± 3.0 years. Moreover, fluoroscopy dosages were significantly lower in RMN compared to MAN [20 (IQR 14-54) versus 48 (IQR 38-62) mGy, P = 0.043]. In total, 20 patients (91%) were free of VA following their final ablation procedure. This is the first study to investigate the use of RMN in pediatric VA ablation. RMN showed improved outcomes compared to MAN, resulting in lower VA recurrence and reduced fluoroscopy exposure.

Magnetic particles in motion: magneto-motive imaging and sensing.

Superparamagnetic nanoparticles have become an important tool in biomedicine. Their biocompatibility, controllable small size, and magnetic properties allow manipulation with an external magnetic field for a variety of diagnostic and therapeutic applications. Recently, the magnetically-induced motion of superparamagnetic nanoparticles has been investigated as a new source of imaging contrast. In magneto-motive imaging, an external, time-varying magnetic field is applied to move a magnetically labeled subject, such as labeled cells or tissue. Several major imaging modalities such as ultrasound, photoacoustic imaging, optical coherence tomography, and laser speckle tracking can utilize magneto-motive contrast to monitor biological events at smaller scales with enhanced contrast and sensitivity. In this review article, an overview of magneto-motive imaging techniques is presented, including synthesis of superparamagnetic nanoparticles, fundamental principles of magneto-motive force and its utility to excite labeled tissue within a viscoelastic medium, current capabilities of magneto-motive imaging modalities, and a discussion of the challenges and future outlook in the magneto-motive imaging domain.

Magnetic particle imaging: tracer development and the biomedical applications of a radiation-free, sensitive, and quantitative imaging modality.

Magnetic particle imaging (MPI) is an emerging tracer-based modality that enables real-time three-dimensional imaging of the non-linear magnetisation produced by superparamagnetic iron oxide nanoparticles (SPIONs), in the presence of an external oscillating magnetic field. As a technique, it produces highly sensitive radiation-free tomographic images with absolute quantitation. Coupled with a high contrast, as well as zero signal attenuation at-depth, there are essentially no limitations to where that can be imaged within the body. These characteristics enable various biomedical applications of clinical interest. In the opening sections of this review, the principles of image generation are introduced, along with a detailed comparison of the fundamental properties of this technique with other common imaging modalities. The main feature is a presentation on the up-to-date literature for the development of SPIONs tailored for improved imaging performance, and developments in the current and promising biomedical applications of this emerging technique, with a specific focus on theranostics, cell tracking and perfusion imaging. Finally, we will discuss recent progress in the clinical translation of MPI. As signal detection in MPI is almost entirely dependent on the properties of the SPION employed, this work emphasises the importance of tailoring the synthetic process to produce SPIONs demonstrating specific properties and how this impacts imaging in particular applications and MPI's overall performance.