Paper-based colorimetric probe for highly sensitive detection of folic acid based on open-ring form amplification of rhodamine B derivative

A simple colorimetric method was proposed for the determination of folic acid (FA) using a paper-basedsubstrate. The mechanism of the folic acid probe is based on the improvement of electron transfer fromthe rhodamine B derivative to Cu2+ in the presence of folic acid. Upon the addition of folic acid, thestructural conversion of the rhodamine B derivative, in which the spirolactam ring (colorless) was openedlittle to the open-ring form (pink color) by Cu2+, was amplified clearly. The change of color intensity canbe observed by either UV–vis spectrophotometer or naked eye. The color of the paper-based substratewas analyzed using the red, green, blue (RGB) values, which were collected from images captured witha smartphone application. The folic acid paper-based probe exhibited good linearity in the range of40–260 nM with a limit of detection (LOD) of 22 nM. The proposed probe exhibits a high potential fordetecting folic acid in real samples including drug tablets, human serum, and urine.