Investigating the impact of metal ions and 3D printed droplet microfluidics chip geometry on the luminol‑potassium periodate chemiluminescence system for estimating total phenolic content in olive oil

The impact of the chip design and the mixing mechanisms using six different 3D printed microfluidic chips were investigated. The study was conducted using novel 3D printed droplet based microfluidics. A multi-mixing approach was utilized to enhance the CL signal of the CL system under investigation. The approach is based on droplet formation, droplet mixing and droplets merging in the 3D printed microfluidic chip. A 154% higher CL signal intensity was obtained using this approach compared to the CL signal obtained using the serpentine chip commonly used for improving the mixing inside droplet microfluidics. This chip was exploited to study the role of three metal ions: Co²⁺, Mn²⁺ and Fe²⁺ on catalyzing the luminol‑potassium periodate chemiluminescence (CL) reaction with selected phenolic compounds in basic media was carefully investigated. Furthermore, the luminol‑potassium periodate-metal ions system was optimized for all metal ions using gallic acid as the reference standard. Despite the popularity of luminol systems in estimating antioxidant activity or total phenolic content (TPC), the results of this study revealed the necessity of careful and vigilant attention when applying it to complex matrices. The only metal ion that showed quenching behavior with all 20 of the tested phenolic compounds was Fe²⁺, while Co²⁺and Mn²⁺ showed both quenching and enhancement in the CL signal. The luminol‑potassium periodate-Fe²⁺ system was applied to estimate TPC in olive oil extracts.