Abstract
Lactic acid is a relevant analyte in the food industry, since it affects the flavor, freshness, and storage quality of several products, such as milk and dairy products, juices, or wines. It is the product of lactose or malo-lactic fermentation. In this work, we developed a lactate biosensor based on the immobilization of lactate oxidase (LOx) onto N,N′-Bis(3,4-dihydroxybenzylidene) -1,2-diaminobenzene Schiff base tetradentate ligand-modified gold nanoparticles (3,4DHS–AuNPs) deposited onto screen-printed carbon electrodes, which exhibit a potent electrocatalytic effect towards hydrogen peroxide oxidation/reduction. 3,4DHS–AuNPs were synthesized within a unique reaction step, in which 3,4DHS acts as reducing/capping/modifier agent for the generation of stable colloidal suspensions of Schiff base ligand–AuNPs assemblies of controlled size. The ligand—in addition to its reduction action—provides a robust coating to gold nanoparticles and a catalytic function. Lactate oxidase (LOx) catalyzes the conversion of l-lactate to pyruvate in the presence of oxygen, producing hydrogen peroxide, which is catalytically oxidized at 3,4DHS–AuNPs modified screen-printed carbon electrodes at +0.2 V. The measured electrocatalytic current is directly proportional to the concentration of peroxide, which is related to the amount of lactate present in the sample. The developed biosensor shows a detection limit of 2.6 μM lactate and a sensitivity of 5.1 ± 0.1 μA·mM−1. The utility of the device has been demonstrated by the determination of the lactate content in different matrixes (white wine, beer, and yogurt). The obtained results compare well to those obtained using a standard enzymatic-spectrophotometric assay kit.
Highlights
L-lactate is a metabolite generated during anaerobic glucose metabolism
The cell pH decreases, and this may result in cell lactic acidosis, which disrupts the performance of the muscles [2]
We reported the multi-tasking N,N 0 -Bis(3,4-dihydroxybenzylidene)1,2-diaminobenzene Schiff base tetradentate ligand (3,4DHS) as reductant, stabilizer, and catalyst in a new concept of AuNPs synthesis
Summary
Its production involves an increase in the proton concentration inside the cells. 1.5 mM in normal people at rest, and can increase to 12.0 mM during exercise [1]. Under extreme conditions of excessive exercise, lactate levels can become as high as 25 mM. In this case, the cellular proton buffering may be exceeded. The cell pH decreases, and this may result in cell lactic acidosis, which disrupts the performance of the muscles [2]. Knowledge of personal lactic threshold is very important to perform any physical activity, and this is the reason why sport medicine requires the monitoring of L-lactic levels in order to evaluate the so-called lactic threshold [3], which
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