The CARS microscopy application for determination of the deacetylation degree in chitin and chitosan species

The raw material for the production of chitosan, which has a wide application in science and industry, is chitin, one of the most abundant natural polymers. The transformation of chitin into chitosan is carried out by the substitution of the acetamide functional group for the amine one, and is quantified by the deacetylation degree (DD). In order to determine the DD of the chitin and chitosan samples, the coherent anti-Stokes Raman spectroscopy (CARS) has been applied in this study. The changes in the intensity of the resonant bands, related to the vibrations of acetamide group, were utilised as an indicator of the DD. By measuring the chitin/chitosan samples of different DD, with the values ranging from 8% to 95%, we were able to obtain the calibration curve of the CARS response. In such a way, the CARS microscope has been prepared for the characterisation of the chitin/chitosan species of unknown origin and for their structural imaging. The shell of the larvae of the Hermetia illucens, also known as the black soldier fly, was chosen as the subject of testing. It is one of the promising sources of raw material for chitosan production. The CARS microscopy capabilities were demonstrated by imaging the black soldier fly's shell and depicting the structural distribution of the DD within the larvae's shell.