Surface engineering of nanozymes has been recognized as a potent strategy to improve their catalytic activity and specificity. We synthesized polydopamine-coated Co₃O₄ nanoparticles (PDA@Co₃O₄ NPs) through simple dopamine-induced self-assembly and demonstrated that these NPs exhibit catalase-like activity by decomposing H₂O₂ into oxygen and water. The activity of PDA@Co₃O₄ NPs was approximately fourfold higher than that of Co₃O₄ NPs without PDA, possibly due to the additional radical scavenging activity of the PDA shell. In addition, PDA@Co₃O₄ NPs were more stable than natural catalase under a wide range of pH, temperature, and storage time conditions. Upon the addition of a sample containing sulfide ion, the activity of PDA@Co₃O₄ NPs was significantly inhibited, possibly because of increased mass transfer limitations via the absorption of the sulfide ion on the PDA@Co₃O₄ NP surface, along with NP aggregation which reduced their surface area. The reduced catalase-like activity was used to determine the levels of sulfide ion by measuring the increased fluorescence of the oxidized terephthalic acid, generated from the added H₂O₂. Using this strategy, the target sulfide ion was sensitively determined to a lower limit of 4.3 µM and dynamic linear range of up to 200 µM. The fluorescence-based sulfide ion assay based on PDA@Co₃O₄ NPs was highly precise when applied to real tap water samples, validating its potential for conveniently monitoring toxic elements in the environment.

Microcystin-LR (MC-LR) is a hepatotoxin generated by the excessive proliferation of cyanobacteria, which is a threat to humans and wildlife. Therefore, rapid detection of MC-LR is an important challenge. This study describes a rapid electrochemical biosensor comprising nanozymes and aptamers. Alternating current electrothermal flow (ACEF) significantly reduced the MC-LR detection period to 10 min. We also used MnO₂/MC-LR aptamer conjugates to improve the sensitivity to MC-LR detection. Here, MnO₂ amplified the electrochemical signal and the aptamer showed high selectivity for MC-LR. Under the optimal conditions, the limit of detection (LOD) and selectivity in freshwater were detected using cyclic voltammetry and differential pulse voltammetry. As a result, an LOD of 3.36 pg mL⁻¹ was observed in the linear concentration range of 10 pg mL⁻¹ to 1 μg mL⁻¹. This study quickly and sensitively detected MC-LR in a situation where it causes serious damage worldwide. In addition, the ACEF technology introduction is the first example of MC-LR detection, suggesting a wide range of possibilities for MC-LR biosensors.

We report a fluorescent assay for the determination of vitamin B₁₂ (VB₁₂) based on the inner filter effect (IFE) of 1,3-propanedithiol-functionalized silver nanoparticles (PDT-AgNPs). PDT was simply functionalized on the surface of AgNPs through Ag–thiol interaction, which leads to significantly enhanced fluorescence, with excitation and emission at 360 and 410 nm, respectively, via their thiol-mediated aggregation. Since target VB₁₂ has strong absorption centered at 360 nm, which is almost completely overlapping with the excitation spectra of PDT-AgNPs, the VB₁₂ induced strong quenching of the fluorescence of PDT-AgNPs via IFE. The IFE-based mechanism for the fluorescence quenching of PDT-AgNPs in the presence of VB₁₂ was confirmed by the analyses of Stern–Volmer plots at different temperatures and fluorescence decay curves. The fluorescence-quenching efficiency of PDT-AgNPs was linearly proportional to the concentration of VB₁₂ in a wide range of 1 to 50 μM, with a lower detection limit of 0.5 μM, while preserving excellent selectivity toward target VB₁₂ among possible interfering molecules. Furthermore, the PDT-AgNPs-mediated assay succeeded in quantitatively detecting VB₁₂ in drug tablets, indicating that PDT-AgNPs can serve as an IFE-based fluorescent probe in pharmaceutical preparations by taking advantages of its ease of use, rapidity, and affordability.

Intracellular reactive oxygen species (ROS) are associated with various inflammatory physiological processes and diseases, highlighting the need for their regulation to mitigate the detrimental effects of oxidative stress and to reduce cellular damage and disease progression. Here, we demonstrate cerium oxide (ceria) nanorods synthesized using a sol–gel method followed by heat treatment, called “AHT-CeNRs”, as an efficient intracellular ROS scavenger. The synthesized AHT-CeNRs exhibited exceptional superoxide dismutase (SOD) and catalase (CAT)-like activities, both of which are crucial for converting ROS into harmless products. This was attributed to their high crystallinity, large surface area, numerous defects including oxygen vacancies, and abundant Ce³⁺ species. AHT-CeNRs exhibited higher CAT-like activities than natural CAT and conventional nanozymes, with a more than five-fold lower K_m. When tested on HaCaT human keratinocyte cells, AHT-CeNRs primarily localized to the membrane but effectively scavenged intracellular ROS, potentially through their transmembrane catalytic action without disrupting the membrane. This contrasts with conventional antioxidant nanoparticles that act within the cytosol after penetrating the plasma membrane. AHT-CeNRs maintained cell viability by efficiently scavenging ROS, resulting in approximately 4-fold and 2-fold lower levels of inducible nitric oxide synthase (iNOS) and lactate dehydrogenase (LDH) compared to those in ROS-induced inflammation-stimulator lipopolysaccharide (LPS)-treated control groups, respectively. This simple yet effective method for intracellular ROS scavenging using AHT-CeNRs holds great potential for applications in cell and in vivo therapeutics to regulate intracellular ROS levels.