Morphine (MORPH) is natural alkaloid and mainly used as a pain reliever. Its monitoring in human body fluids is crucial for modern medicine. In this paper, we have developed an electrochemical sensor for submicromolar detection of MORPH. The sensor is based on modified carbon paste electrode (CPE) by investigating the Fe_xW_1-xO₄ ratio in iron tungstate (FeWO₄), as well as the ratio of this material in CPE. For the first time, the effect of the iron–tungsten ratio in terms of achieving the best possible electrochemical characteristics for the detection of an important molecule for humans was examined. Morphological and electrochemical characteristics of materials were studied. The best results were obtained using Fe₁W₃ and 7.5% of modifier in CPE. For MORPH detection, square wave voltammetry (SWV) was optimized. Under the optimized conditions, Fe₁W₃@CPE resulted in limit of detection (LOD) of the method of 0.58 µM and limit of quantification (LOQ) of 1.94 µM. The linear operating range between 5 and 85 µM of MORPH in the Britton–Robinson buffer solution (BRBS) at pH 8 as supporting electrolyte was obtained. The Fe₁W₃@CPE sensor resulted in good selectivity and excellent repeatability with relative standard deviation (RSD) and was applied in real-world samples of human urine. Application for direct MORPH detection, without tedious sample pretreatment procedures, suggests that developed electrochemical sensor has appeared to be a suitable competitor for efficient, precise, and accurate monitoring of the MORPH in biological fluids.

Here, we present the results of the synthesis, surface modification, and properties analysis of magnetite-based nanoparticles, specifically Co_0.047Fe_2.953O₄ (S1) and Co_0.086Fe_2.914O₄ (S2). These nanoparticles were synthesized using the co-precipitation method at 80 °C for 2 h. They exhibit a single-phase nature and crystallize in a spinel-type structure (space group Fd$\overline{\mathsf{3}}$m). Transmission electron microscopy analysis reveals that the particles are quasi-spherical in shape and approximately 11 nm in size. An observed increase in saturation magnetization, coercivity, remanence, and blocking temperature in S2 compared to S1 can be attributed to an increase in magnetocrystalline anisotropy due to the incorporation of Co ions in the crystal lattice of the parent compound (Fe₃O₄). The heating efficiency of the samples was determined by fitting the Box-Lucas equation to the acquired temperature curves. The calculated Specific Loss Power (SLP) values were 46 W/g and 23 W/g (under H_AC = 200 Oe and f = 252 kHz) for S1 and S2, respectively. Additionally, sample S1 was coated with citric acid (Co_0.047Fe_2.953O₄@CA) and poly(acrylic acid) (Co_0.047Fe_2.953O₄@PAA) to obtain stable colloids for further tests for magnetic hyperthermia applications in cancer therapy. Fits of the Box-Lucas equation provided SLP values of 21 W/g and 34 W/g for CA- and PAA-coated samples, respectively. On the other hand, X-ray photoelectron spectroscopy analysis points to the catalytically active centers Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ on the particle surface, suggesting possible applications of the samples as heterogeneous self-heating catalysts in advanced oxidation processes under an AC magnetic field.

The formation of interfacial charge transfer (ICT) complexes between phenolic ligands and metal oxide surfaces enables surface functionalization strategies with potential applications in catalysis and bioconjugation. In this study, magnetite (Fe₃O₄) nanoparticles were modified with two phenolic ligands, 5-aminosalicylic acid (5ASA) and caffeic acid (CA), to generate ICT complexes capable of covalent or non-covalent enzyme immobilization, respectively. The modified nanomaterials were structurally characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FTIR). Horseradish peroxidase (HRP) was immobilized on these functionalized supports using varying nanoparticle amounts (10–30 mg) and initial enzyme concentrations (25–250 µg mL⁻¹). Catalytic activity was evaluated using pyrogallol oxidation assays. The Fe₃O₄/5ASA–HRP system exhibited a maximum activity of 2.5 U per 20 mg of support (approximately 125 U g⁻¹), whereas Fe₃O₄/CA showed minimal activity under the same conditions. Enzyme loading studies confirmed that 5ASA-enabled covalent attachment resulted in significantly higher immobilization efficiency (up to 1068 mg g⁻¹) compared to the CA system. Reusability tests demonstrated that the Fe₃O₄/5ASA system retained high absolute catalytic activity during the initial reaction cycles and consistently outperformed the non-covalently immobilized Fe₃O₄/CA system upon repeated reuse. The magnetic properties of Fe₃O₄ allowed rapid recovery of the biocatalysts using an external magnetic field. These results highlight the effectiveness of ICT-based functionalization for enzyme immobilization, positioning Fe₃O₄/5ASA as a promising platform for robust and reusable biocatalysts in environmental and industrial applications.

The formation of interfacial charge transfer (ICT) complexes between phenolic ligands and metal oxide surfaces enables surface functionalization strategies with potential applications in catalysis and bioconjugation. In this study, magnetite (Fe₃O₄) nanoparticles were modified with two phenolic ligands, 5-aminosalicylic acid (5ASA) and caffeic acid (CA), to generate ICT complexes capable of covalent or non-covalent enzyme immobilization, respectively. The modified nanomaterials were structurally characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FTIR). Horseradish peroxidase (HRP) was immobilized on these functionalized supports. Catalytic activity was evaluated using pyrogallol oxidation assays, with systematic variations in nanoparticle mass and enzyme concentration. The Fe₃O₄/5ASA–HRP system exhibited a maximum activity of 2.5 U per 20 mg of support (approximately 125 U/g), whereas Fe₃O₄/CA showed minimal activity under the same conditions. Data from enzyme loading studies confirmed that 5ASA-enabled covalent attachment resulted in significantly higher immobilization efficiency (up to 1068 mg/g) compared to the CA system. The magnetic properties of Fe₃O₄ allowed for rapid recovery of the biocatalysts using an external magnetic field. These results highlight the effectiveness of ICT-based functionalization for enzyme immobilization, positioning Fe₃O₄/5ASA as a promising platform for robust and reusable biocatalysts in environmental and industrial applications.