Development of a colorimetric sensor based on nanofiber cellulose film modified with ninhydrin to measure the formalin index of fruit juice.

In this research, a color sensor based on nanofiber cellulose film modified with ninhydrin was designed to measure amino acids and formalin index in fruit juice. For this purpose, three types of cellulose films with porosity of 5, 30 and 125 μm were used. These films were treated with standard solution of ninhydrin. The characteristics of modified films were investigated using Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and X-ray Diffraction (XRD) tests. The color factors of the sensors (a and b) changed in the presence of amino acids and juice with different levels of formalin index. Therefore, the modified films with ninhydrin as a colorimetric sensor were calibrated using 7 types of amino acids and based on the formalin index of 4 types of juice. Then the sensors were used to measure the formalin index in 4 types of juice. The results showed that the sensors have relative selectivity towards methionine amino acid. The formalin index values calculated in the juices by the sensor were compared with the titration method as a reference method. All three types of sensors were able to detect formalin index. The results of the sensor performance verification showed that the sensors can measure formalin index in different juices with 95-98 % accuracy. These sensors showed fast sensitivity and selectivity to the amino acids in juice, also these sensors are safe and the measurement method is fast and simple.