Smartphone RGB camera-based colorimetric platform with double-layer spherical Ca (II)-alginate/ZnONPs hydrogel liquid-core curcumin emulsion for rapid and selective detection of pyridoxine (Vitamin B6) in functional beverages

A portable smartphone-based RGB colorimetric sensor was developed for the quantitative detection of pyridoxine (vitamin B6, VB-6) in functional beverages, employing a novel core–shell hydrogel probe (CURHydrogel), engineered with a calcium alginate matrix encapsulating a liquid-phase curcumin emulsion and externally layered by poly(diallyl dimethyl ammonium chloride)-functionalized ZnO nanoparticles (ZnONPs/PDADMAC). The CURHydrogel spheres were fabricated via a molecular self-assembly reverse spherification and demonstrated high mechanical stability (stiffness: 6.45 × 10⁴ N/m, compressive strength: 1.01 × 10⁶ N/m²) along with excellent UV-blocking photostability for up to 28 days. The CURHydrogel probe was applied for detecting VB-6 via a smartphone-based sensing platform. The colorimetric assay was based on a two-step strategy involving initially the formation of the colorless pyridoxine-boron complex by VB-6 and boric acid, after which the remaining boric acid binds with curcumin to produce a red rosocyanine dye. A higher VB-6 concentration yields less rosocyanine and a discernible color shift from orange to yellow, which is quantified via smartphone RGB analysis. The RGB values were assessed via a smartphone application, providing the linearity of VB-6 detection of 10–125 mg L⁻¹ with LOD and LOQ of 2.93 mg L⁻¹ and 9.77 mg L⁻¹. The CURHydrogel exhibited excellent precision with relative standard deviations (RSDs) ranging from 0.20 to 0.27 %, while recoveries ranged from 99.84 % to 103.03 %. Particularly, the results of this method were also validated by comparing with HPLC and UV–vis spectrophotometry. The smartphone RGB camera-based colorimetric sensor of CURHydrogel has great potential application prospects for detecting VB-6 in functional beverage samples.