2.2. Colorimetric sensor array preparation and evaluation system
2.2.1. Preparation of natural pigments and characterisation
The pigments from spinach (Spinacia oleracea), red radish (Raphanus sativus L), winter jasmine (Jasminum nudiflorum), and black rice (Oryza sativa L. indica) were extracted with ethanol. Spinach, red radish, winter jasmine, and black rice were washed with water and vacuum dried at 60 C. After crushing into fine powder using a mortar and pestle, these materials were immersed in absolute ethanol at room temperature in the dark for one week. Then the solids were filtered out, and the filtrates were concentrated at 40 C for use. The absorption spectrum of the samples was determined by using a UV-2450 UV–Vis spectrophotometer (Shimadzu Corporation, Kyoto, Japan).