Biosensors are analytical devices that use biological interactions to detect and quantify single molecules, clinical biomarkers, contaminants, allergens, and microorganisms. By coupling bioreceptors with transducers, such as nucleic acids or proteins, biosensors convert biological interactions into electrical signals. Electrochemical and optical transductions are the most widely used methods due to their high detection capability and compatibility with miniaturization. Biosensors are valuable in analytical chemistry, especially for health diagnostics, as they offer simplicity and sensitivity. Despite their usefulness, challenges persist in immobilizing biorecognition elements on the transducer surface, leading to issues such as loss of sensitivity and selectivity. To address these problems, the introduction of nanomaterials, in particular magnetic nanoparticles (MNPs) and magnetic beads, has been implemented. MNPs combine their magnetic properties with other interesting characteristics, such as their small size, high surface-to-volume ratio, easy handling, and excellent biocompatibility, resulting in improved specificity and sensitivity and reduced matrix effects. They can be tailored to specific applications and have been extensively used in various fields, including biosensing and clinical diagnosis. In addition, MNPs simplify sample preparation by isolating the target analytes via magnetic separation, thus reducing the analysis time and interference phenomena and improving the analytical performance of detection. The synthesis and modification of MNPs play a crucial role in adjusting their properties for different applications. This review presents an overview of the synthesis and surface modifications of magnetic nanoparticles and their contributions to the development of biosensors and bioassays for their applications across different areas. The future challenges of MNP synthesis and integration in assays are focused on their stability, multiplex detection, simplification and portability of test platforms, and in vivo applications, among other areas of development.

Bisphenol A (BPA) has been classified as an endocrine-disrupting substance that may cause adverse effects on human health and the environment. The development of simple and sensitive electrochemical biosensors is crucial for the rapid and effective quantitative determination of BPA. This work presents a study on electrochemical sensors utilizing gold nanoparticle-modified multi-walled carbon nanotubes (CNT/AuNPs). Glassy carbon electrodes (GCEs) and screen-printed electrodes (SPEs) were conveniently modified and used for BPA detection. AuNPs were electrodeposited onto the CNT-modified electrodes using the galvanostatic method. The electrodes were properly modified and characterized by using Raman spectroscopy, cyclic voltammetry (CV), and electrochemical impedance analysis (EIS). The electrochemical response of the sensors was studied using differential pulse voltammetry (DPV) and constant potential amperometry (CPA) for modified GCE and SPE electrodes, respectively, and the main analytical parameters were studied and optimized. Problems encountered with the use of GCEs, such as sensor degradation and high limit of detection (LOD), were overcome by using modified SPEs and a flow injection device for the measurements. Under this approach, an LOD as low as 5 nM (S/N = 3) was achieved and presented a linear range up to 20 μM. Finally, our investigation addressed interference, reproducibility, and reusability aspects, successfully identifying BPA in both spiked and authentic samples, including commercial and tap waters. These findings underscore the practical applicability of our method for accurate BPA detection in real-world scenarios. Notably, the integration of SPEs and a flow injection device facilitated simplified automation, offering an exceptionally efficient and reliable solution for precise BPA detection in water analysis laboratories.

A novel material composed of Au@SiO₂-(3-Aminopropyl Triethoxysilane) (Au@SiO₂-APTES) was successfully synthesised using the sol–gel method, and was used to modify glassy carbon electrodes. Its effectiveness as a molecular recognition element is evaluated in the design of an electrochemical sensor for the precise detection of dopamine. The Au@SiO₂-APTES composite was analysed using Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Elemental analysis verified the presence of oxygen, silicon, and gold, with atomic percentages of around 77.19%, 21.12%, and 1.65%, respectively. The corresponding elemental mapping for Au@SiO₂-APTES composite showed that the spatial distribution of all the elements was fairly homogeneous throughout the composite, indicating that the Au NPs are embedded in the silica structures. Traces of dopamine were detected by differential pulse voltammetry with a low limit of detection (S/N = 3) and quantification (S/N = 10) of 1.4 × 10⁻⁸ molL⁻¹ and 4.7 × 10⁻⁸ molL⁻¹, respectively. The Au@SiO₂-APTES composite had two linear ranges: from 4.7 × 10⁻⁸ to 1 × 10⁻⁷ molL⁻¹ and 1.25 × 10⁻⁷ to 8.75 × 10⁻⁷ molL⁻¹. Moreover, the sensor showed outstanding selectivity even in the presence of various potential interfering species. It also demonstrated good reusability and signal recovery when tested in human urine and plasma samples spiked with different dopamine concentrations. The electrochemical sensor, constructed using this novel composite material, shows great promise in the selective and sensitive detection of dopamine in the biological matrix. These results underscore the sensor’s capability for practical application in analysing real-world samples.