Enzyme-free colorimetric nanosensor for the rapid detection of lactic acid in food quality analysis

Lactic acid is an essential organic chemical in food, clinical, chemical, and bioprocessing industries. We are reporting for the first time, a rapid and facile enzyme-free colorimetric sensor for the detection of lactic acid (LA) in food samples using nitrophenol (p-NP) added copper nanoparticles (CuNPs). The responses of p-NP (5 mmol/L) to lactic acid in the presence of CuNPs were reported by analysing the UV-VIS absorption spectra in an aqueous solution. CuNPs were synthesized by the chemical reduction method and were identified to be in the Cubic phase using XRD analysis. The sample characterization studies were performed using the SEM, UV for obtaining good optical and selective lactic acid-sensing properties. The sensing mechanism is due to the aggregation responses of CuNPs conjugated p-NP in the presence of lactic acid as seen in the UV-VIS absorption spectra shift studied in an aqueous solution. The absorption spectrum of p-NP exhibited one intense band at 402 nm in the presence of CuNPs. Although the addition of lactic acid may cause pi -pi* transitions led to a hypsochromic shift (blue shift) with a subtle decrease in the wavelength from 402 to 315 nm, the solution underwent from being colored (greenish-yellow) to colorless. The stoichiometric ratio of a binding event is explained using Job's plot. The naked eye colorimetric response of the sensor-enabled detection limit of lactic acid to 0.33 mM. The sensor studies were compared with standard LC-MS/MS revealing a good recovery (95%) in food samples. This method is facile and alternate to the current high-cost techniques and enzymatic sensors.