Natural pigments occur in plants as secondary metabolites and have been used as safe colourants in food. Studies have reported that their unstable colour intensity might be related to metal ion interaction, which leads to the formation of metal–pigment complexes. This underlines the need for further investigations on the use of natural pigments in metal detection using colorimetric methods, since metals are important elements and can be hazardous when present in large amounts. This review aimed to discuss the use of natural pigments (mainly betalains, anthocyanins, curcuminoids, carotenoids, and chlorophyll) as reagents for portable metal detection based on their limits of detection, to determine which pigment is best for certain metals. Colorimetric-related articles over the last decade were gathered, including those involving methodological modifications, sensor developments, and a general overview. When considering sensitivity and portability, the results revealed that betalains are best applied for copper, using a smartphone-assisted sensor; curcuminoids are best applied for lead, using a curcumin nanofiber; and anthocyanin is best applied for mercury, using anthocyanin hydrogel. This provides a new perspective on the use of colour instability for the detection of metals with modern sensor developments. In addition, a coloured sheet representing metal concentrations may be useful as a standard to support on-site detection with trials on masking agents to improve selectivity.
Objective: This study was aimed to understand and determine the effectiveness of allopurinol extraction in herbal medicine from three extraction methods based on parameters of accuracy and precision. Methods: The study consisted of three methods including dissolving and filtering, liquid-liquid extraction, and solid-phase extraction with mixed-mode cation exchanger (SPE-MCX). The procedures were carried out using NaOH and HCl in dissolving and filtering method; methanol, HCl, and ethyl acetate in liquid-liquid extraction; and NH4OH elution solvent in SPE-MCX. Results: The results showed that extraction effectiveness based on accuracy level was the dissolving and filtering method>SPE-MCX>liquid-liquid extraction with % recovery+SD of 91.314+2.903%, 87.533+4.950%, and 54.549+3.517%, respectively. The precision level was the dissolution and filtering method>SPE-MCX>liquid-liquid extraction based on % relative standard deviations (RSD) of 3.18%, 5.226%, and 6.446%, respectively. Conclusion: It can be concluded that the allopurinol extraction method with the highest effectiveness based on accuracy and precision parameters in herbal medicine is the dissolving and filtering method.
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