Green curcumin extract from Curcuma longa L. for downscaling boron assay: optimization and application with a portable spectrometer

Due to the low water solubility of curcumin, in this work, coconut oil in addition to ethanol was employed as
green solvent to extract curcumin from Curcuma longa L. For all extraction solvents, the obtained curcumin
extracts served as a natural colorimetric reagent for boron assay, which resulted in a change of colour from
yellow to a reddish-brown due to the formation of curcumin‑boron complexes, rosocyanine, prior to spectrometric detection using the PiCOEXPLORER spectrometer. By using ethanol, the ethanolic curcumin extracts were operated for boron assay at a volume ratio of the curcumin extract: sulfuric solution (6 mol L^-1): boron solution of 1:10:30, respectively, obtaining a linear calibration y = 0.0065x + 0.0092 (R² = 0.9949). Coconut oil was employed as an alternative green solvent to extract curcumin for boron assay by mixing with the boron solution (pH 10) at volume ratio of 1:1 for 2 min. Under this condition, a linearity equation, y = 0.0162x + 0.0399 (R² = 0.9637) was obtained. The developed boron assay approaches using ethanolic and coconut oil curcumin extracts were demonstrated to boron fertilizer and dietary boron supplementary samples, respectively. The results obtained agree well with the ICP-OES reference method. Finally, the proposed procedures were evaluated for greenness and sustainability compared to that ICP-OES reference method.