Montserrat Rivas

Mn-ferrite NPs are explored as multifunctional reporters for tri-modal biosensing in lateral flow immunoassays. Their intrinsic optical and magnetic properties enable photothermal, magnetic, and colorimetric detection of pneumolysin.

Magnetoresistive (MR) sensors are solid-state magnetic sensors that enable highly sensitive magnetic field detection with low power consumption,complementary metal-oxide-semiconductor (CMOS) compatibility, and scalable fabrication. This review examines the device physics, architectures, and performance trade-offs of anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) sensors, highlighting their underlying spin-dependent scattering and tunneling mechanisms. Linearization strategies, noise sources, and sensitivity limits relevant to the detection of weak and low-frequency magnetic fields are discussed from a device-design perspective.The review further explores diverse and emerging application domains that demand stringent magnetic sensing performance, such as biosensing, neuromagnetic signal detection, non-contact current sensing, power grid monitoring, intelligent transportation systems. By comparing MR sensor technologies with respect to sensitivity, noise, dynamic range, and integration complexity, this work outlines key challenges and future directions for compact, low-noise, and energy-efficient magnetic sensors for next-generation electronic sensing systems.

Controlled carbonization of hybrid NiOF powders produces graphite-encapsulated Ni nanoparticles with excellent performance for acidic water remediation. The powders can be easily recovered by magnetic separation after the adsorption process.

Water contamination is a growing global concern driven mainly by industrial, agricultural, livestock, and domestic activities. Magnetic nanoparticles (MNPs) have revolutionized water purification, pollutant degradation, and environmental sensing, owing to their high surface area, tunable surface chemistry, and magnetic responsiveness. These multifunctional capabilities enable efficient contaminant removal and rapid separation under external magnetic fields, significantly reducing energy consumption compared to conventional methods. This review discusses recent advancements and future prospects of MNPs in water purification, empathizing their potential for enhanced integration into smart environmental technologies (i) as adsorbents, that can selectively capture pollutants such as heavy metals, organic dyes, and pharmaceuticals, (ii) as catalysts, in advanced oxidation processes for pollutant degradation, and (iii) in water disinfection and desalination, further expanding their applicability. Despite these advantages, challenges remain, including scalability, cost, environmental safety, and long-term stability. Addressing these concerns through interdisciplinary research and technological advancements will be key to unlocking the full potential of MNPs for sustainable water management.

This work uses a custom oscillator circuit to explore an inductive-capacitive approach to nanoparticle quantification. The development of quantitative lateral flow tests has prompted the design of devices that can assess the number of detection markers at the test line. These markers include latex, gold, silver, and magnetic nanoparticles. The aim is to provide quantification without compromising the typical advantages of lateral flow assays: ease of use, portability, and low cost. In previous work, we described a sensor based on a coil’s self-resonant frequency (SRF) for high-precision detection of both magnetic and nonmagnetic nanoparticles. The realization of such a device requires a reliable measurement of the SRF. Here, we face this challenge by introducing a new broadband L-C oscillator that transforms the sensing coil’s impedance curve into a frequency power spectrum. This allows for straightforward, fast, and robust SRF detection. The oscillator is based on an ad-hoc astable circuit supported by theoretical modeling, simulations, and experimental validation. We call this oscillator astable tuned oscillator (ATO). Coupled with a frequency counter, the system enables quantification of microgram levels of various nanoparticles. A detection limit of 650 ng was achieved for magnetite nanoparticles using a compact and cost-effective setup. Beyond biosensing, the oscillator can support inductive sensing applications where slight variations in coil response are used to detect changes in materials or environments, such as in structural monitoring, material inspection, or soft matter characterization.

Lateral flow assays are low-cost point-of-care devices that are stable, easy to use, and provide quick results. They are mostly used as qualitative screening tests to detect biomarkers for several diseases. Quantification of the biomarkers is sometimes desirable but challenging to achieve. Magnetic nanoparticles can be used as tags, providing both visual and magnetic signals that can be detected and quantified by magnetic sensors. In the present work, we synthesized superparamagnetic MnFe₂O₄ nanoparticles using the hydrothermal coprecipitation route. MnFe₂O₄ presents low magnetic anisotropy and high saturation magnetization, resulting in larger initial magnetic susceptibility, which is crucial for optimizing the signal in inductive sensors. We functionalized the coprecipitated nanoparticles with citric acid to achieve colloidal stability in a neutral pH and to provide carboxyl groups to their surface to bioconjugate with biomolecules, such as proteins and antibodies. The nanomaterials were characterized by several techniques, and we correlated their magnetic properties with their sensitivity and resolution for magnetic detection in radiofrequency inductive sensors. We considered the NeutrAvidin/biotin model of biorecognition to explore their potential as magnetic labels in lateral flow assays. Our results show that MnFe₂O₄ nanoparticles are more sensitive to inductive detection than magnetite nanoparticles, the most used nanotags in magnetic lateral flow assays. These nanoparticles present high potential as magnetic tags for the development of sensitive lateral flow immunoassays for detecting and quantifying biomarkers.

The mapping of magnetic fields is an important task for characterizing permanent magnets, their systems, and the various devices that use magnets. However, commercial devices for this task are quite expensive and difficult to acquire. In this study, a design process for an automatized magnetic mapping setup utilizing a 3D printer was proposed via a do-it-yourself approach using common parts found in a laboratory. We used a 3D printer for two purposes: to create instrument parts to fix the Hall probe instead of the printer extruder, and to use the same 3D printer as the position controller of the magnetic probe. We describe the device assembly process, including hardware and software aspects. Several tests of the assembled setup were performed on samples with different magnetic texture dimensions from several centimeters (graduated permanent magnet) to tens of micrometers (flexible magnet with a planar Halbach structure and a 5¼ inch floppy disk). The spatial resolution was in the sub-millimeter range. The parameters of the mapping, such as the number of points and data acquisition time, were optimized for such samples.

We demonstrate how resonant planar coils may be used as sensors to detect and quantify magnetic nanoparticles reliably. A coil’s resonant frequency depends on the adjacent materials’ magnetic permeability and electric permittivity. A small number of nanoparticles dispersed on a supporting matrix on top of a planar coil circuit may thus be quantified. Such nanoparticle detection has application detection to create new devices to assess biomedicine, food quality assurance, and environmental control challenges. We developed a mathematical model for the inductive sensor response at radio frequencies to obtain the nanoparticles’ mass from the self-resonance frequency of the coil. In the model, the calibration parameters only depend on the refraction index of the material around the coil, not on the separate magnetic permeability and electric permittivity. The model compares favourably with three-dimensional electromagnetic simulations and independent experimental measurements. The sensor can be scaled and automated in portable devices to measure small quantities of nanoparticles at a low cost. The resonant sensor combined with the mathematical model is a significant improvement over simple inductive sensors, which operate at smaller frequencies and do not have the required sensitivity, and oscillator-based inductive sensors, which focus on just magnetic permeability.