Novel choline selective electrochemical membrane sensor with application in milk powders and infant formulas

Choline (Ch⁺), is a vitamin-like essential water-soluble organic micronutrient. The US-FDA requires that infant formula not made from cow's milk must be supplemented with Ch⁺. Direct determination of Ch⁺ in milk powders and infant formulas is a challenging task due to the lack of a detectable chromophore, its existence in free and complexed forms as well as the presence of multi-analytes in these complex matrices. Here, an enzyme-free potentiometric ion selective electrode (ISE) with high selectivity for Ch⁺, a linear range from 0.03 μM up to 1 mM, a 0.061 μM detection limit (LOD) and a typical response time less than 5 and no greater than 60 s is developed for monitoring of Ch⁺ in infant formula and milk powders. To achieve these ISE parameters we relied on the ability of calixarenes and its derivatives to form host-guest complexes with the positively charged quaternary ammonium moiety of Ch⁺. We employed a lipophilic (membrane-compatible) calixarene as an ionophore in the sensing membrane phase to provide a molecular receptor for Ch⁺ capable of selective binding; while utilizing, hydrophilic (water-soluble) p-sulfonated calixarene as a buffering agent to optimize the inner filling solution to reduce transmembrane Ch⁺ fluxes. All the calixarene structures and their complexes with Ch⁺ were optimized at the density functional theory (DFT) level and the Gibbs free energies for the inclusion of Ch⁺ into the calixarenes were calculated. The prepared sensor was shown to selectively respond only to Ch⁺ in the presence of all other interferents in the tested matrices with results that are not statistically significantly different for either accuracy or precision relative to the much more laborious official AOAC 1999 coupled enzymatic-spectrophotometric method. The proposed method is highly selective, non-enzymatic, requires no derivatization or incubation steps, offers a fast response time, and has the potential of portability for in situ analysis, while being relatively cost effective and non-laborious.