Swab-based colorimetric determination of tear lactate using lactate oxidase-copper hybrid nanoflowers

Lactate is an important metabolic biomarker closely associated with cellular energy metabolism and oxidative stress, and abnormal lactate levels in tear fluid have been implicated in various ocular disorders. We report a swab-based colorimetric method for the noninvasive determination of tear lactate using lactate oxidase-copper hybrid nanoflowers (LOx-Cu NFs). The LOx-Cu NFs were synthesized via a biomolecule-directed coordination assembly in phosphate buffer, in which LOx guided the nucleation and hierarchical growth of copper phosphate into flower-like hybrid architectures. This enzyme-metal hybridization integrates the high substrate specificity of LOx with the intrinsic peroxidase-like activity and structural robustness of the copper phosphate matrix, resulting in enhanced catalytic stability and activity compared with the free enzyme system. In solution, the colorimetric assay exhibited a linear response to lactate concentrations ranging from 20 to 1200 µM with a detection limit of 1.2 µM. When implemented on a cotton swab, direct visual determination of tear lactate was achieved over a physiologically relevant concentration range 100-4000 µM with a detection limit of 50 µM. The analytical performance of the proposed method was further validated using real human tear samples, demonstrating good accuracy and precision in complex tear matrices. Notably, the analytical process operates under a single pH condition and requires no external instrumentation or complex reagents. By integrating durable enzyme-metal hybrid nanoflowers with a simple swab-based sampling strategy, this work provides a practical, low-cost, and noninvasive analytical approach for point-of-care determination of tear metabolites.