Laccase-mimicking Mn-Cu hybrid nanoflowers for paper-based visual detection of phenolic neurotransmitters and rapid degradation of dyes.

Background: Laccase-based biosensors are efficient for detecting phenolic compounds. However, the instability and high cost of laccases have hindered their practical utilization.

Results: In this study, we developed hierarchical manganese dioxide-copper phosphate hybrid nanoflowers (H-Mn-Cu NFs) as excellent laccase-mimicking nanozymes. To synthesize the H-Mn-Cu NFs, manganese dioxide nanoflowers (MnO NFs) were first synthesized by rapidly reducing potassium permanganate using citric acid. The MnO NFs were then functionalized with amine groups, followed by incubation with copper sulfate for three days at room temperature to drive the coordination interaction between the amine moieties and copper ions and to induce anisotropic growth of the petals composed of copper phosphate crystals, consequently yielding H-Mn-Cu NFs. Compared with those of free laccase, at the same mass concentration, H-Mn-Cu NFs exhibited lower K (~ 85%) and considerably higher V (~ 400%), as well as significantly enhanced stability in the ranges of pH, temperature, ionic strength, and incubation periods evaluated. H-Mn-Cu NFs also catalyzed the decolorization of diverse dyes considerably faster than the free laccase. Based on these advantageous features, a paper microfluidic device incorporating H-Mn-Cu NFs was constructed for the convenient visual detection of phenolic neurotransmitters, including dopamine and epinephrine. The device enabled rapid and sensitive quantification of target neurotransmitters using an image acquired using a smartphone.

Conclusions: These results clearly show that H-Mn-Cu NFs could be potential candidates to replace natural laccases for a wide range of applications in biosensing, environmental protection, and biotechnology.