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

• New simple paper-based colorimetric method for sensing folic acid. • Sensitive sensing was conducted through the images captured with a smartphone. • Good analytical performance with good linearity and low LOD of 22 nM for folic acid. A simple colorimetric method was proposed for the determination of folic acid (FA) using a paper-based substrate. The mechanism of the folic acid probe is based on the improvement of electron transfer from the rhodamine B derivative to Cu²⁺ in the presence of folic acid. Upon the addition of folic acid, the structural conversion of the rhodamine B derivative, in which the spirolactam ring (colorless) was opened little to the open-ring form (pink color) by Cu²⁺, was amplified clearly. The change of color intensity can be observed by either UV–vis spectrophotometer or naked eye. The color of the paper-based substrate was analyzed using the red, green, blue (RGB) values, which were collected from images captured with a smartphone application. The folic acid paper-based probe exhibited good linearity in the range of 40–260 nM with a limit of detection (LOD) of 22 nM. The proposed probe exhibits a high potential for detecting folic acid in real samples including drug tablets, human serum, and urine. [ABSTRACT FROM AUTHOR]