Calcium-selective optodes for the management of plant nutrient solutions

Abstract Robust, real-time, ion-selective sensors are much needed by the greenhouse and controlled environment plant production sectors. Ion-selective sensors will provide growers with the capability to minimize nutrient and water requirements while improving crop yields. Such considerations are more critical requirements for space-based plant production systems such as those proposed for future long-duration space missions. This work has developed ion-selective bulk optodes tailored specifically for calcium monitoring in hydroponic nutrient solution. Lack of control of calcium levels within nutrient solution can result in reduced plant growth levels and undesired conditions such as blossom-end rot. This work explores both theoretically and experimentally the fabrication of calcium-selective membranes based upon the calcium ionophore ETH 1001 and hydrogen ion chromoionophore ETH 5350 incorporated into highly plasticized PVC membranes. Membranes incorporating several different ratios of these active components are analyzed and a resultant calcium-selective membrane chosen and characterized. The resultant optode has been shown to exhibit a calcium activity measuring range from 0.022 to 105.9 mM at pH 6.0. The demonstrated full scale response times on the order of several minutes are more than sufficient for monitoring in operational plant growth systems. Sensor selectivity has been tested and the sensor meets worst case selectivity requirements for monitoring calcium in half strength Hoagland solution. When continuously immersed in nutrient solution the optode membrane demonstrated lifetimes on the order of 30 h. In addition to confirming sensor repeatability and characterizing the sensor response to varied temperature, this work investigates at least one mechanism to operationally extended ion-selective membrane lifetime by drying the optode between measurements. The developed membranes have been tested within a portable fiber optic instrument that could be directly incorporated into an operational plant growth system to selectively measure calcium.