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.

Developing a convenient detection method for dopamine holds a significant incentive due to its high clinical significance. Herein, we synthesize crystalline MnOOH nanowires (MNWs) via a simple solvothermal treatment of KMnO₄ and demonstrate that they possess excellent oxidase-like activity owing to the presence of pure Mn³⁺ sites on the MNWs. Particularly, MNWs catalyze the rapid oxidation of dopamine into aminochromes, which show a vivid brown color. The dopamine oxidase-like activity of MNWs follows the typical Michaelis–Mentenkinetics with excellent storage stability. Based on the affirmative catalytic features, a paper dipstick incorporating MNWs in the detection zone is constructed for the one-step colorimetric detection of target dopamine. By immersing the dipstick into the sample solution for 30 min, the sample spontaneously moves to the detection zone due to capillary force, yielding a brown color proportional to the amount of dopamine, which is quantified from an image acquired using a smartphone. With the MNW-containing solution-based assay and MNW-incorporated paper dipstick, dopamine is successfully determined with high selectivity, sensitivity, and detection precision when using spiked human serum and pharmaceutical dopamine injection samples, respectively. Successful analytical values such as the dynamic linear ranges of 3–60 μM and 0.05–7 mM are achieved with the solution-based assay and paper dipstick, respectively, along with excellent detection accuracy (95–99%) and precision (1.0–3.1%). Hence, we developed a simple and efficient nanozyme-based paper dipstick biosensor for dopamine that can be used in point-of-care testing environments.