A reflectometric ion sensor for potassium based on acrylic microspheres

A K+ ion-sensitive optical sensor based on Nile Blue (chromoionophore ETH5294) incorporated into poly(n-butyl acrylate) [poly(nBA)] microspheres was successfully developed. A combination of acrylic spheres and optical reflectometry had enhanced the sensing performance of the optical K+ ion sensor, as the former provides large surface area for better loading capacity and higher reaction rate, while the latter offers simple and rapid measurements. The sensing mechanism involves ion-exchange reaction, which occurs at the surface of the microspheres where influx of K+ ions and efflux of H+ ions take place. The sensor response was a result of the total deprotonation of chromoionophore immobilized within poly(nBA) microspheres in response to the formation of K+ ion–ionophore complex. The change of the response in the presence of K+ ions was studied via reflectance spectroscopy using an optical fibre probe. The K+ ion sensor based on these microspheres exhibited sensitivity slope of 43.15 ΔIntensity/decade (R2 = 0.961) in a range 0.01–1000 mM of KCl with a lower detection limit of 0.01 mM. Sensor response time was 10 min with good reproducibility (relative standard deviation, %RSD = 5.71). Analysis of urine samples for K+ ions gave 87–115% recovery and the results were comparable with established method for K+ ions analysis. Leaching tests and life time study revealed that acrylic microspheres worked well as an immobilization matrix for hydrophobic chromoionophore.