Plasticizer-based and polymer-free ion-selective optodes on cellulose paper

Paper-based ion-selective optodes (ISOs) allow for low-cost ion measurements using widely accessible optical detectors such as smartphones, representing an attractive analytical platform for medical testing and environmental monitoring. Previously reported paper-based ISOs rely on plasticized PVC membranes or PVC/plasticizer-free adsorption layers containing hydrophobic sensing chemicals. In this paper, we studied a new format of paper-based ISOs, in which a plasticizer phase containing sensing chemicals is deposited onto cellulose paper without using a hydrophobic polymer like PVC. Compared to ISOs without plasticizers and polymers, the plasticizer-based ISOs show much higher pH sensitivity when using chromoionophore I and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the sensing chemicals for pH. When the sensing layer further contains a solid ionophore with high molecular rigidity, the plasticizer-based ISOs for cations also exhibit enhanced colorimetric responses than the plasticizer-free ISOs. However, when the ionophore is a liquid at room temperature or has a long alkyl chain, the plasticizer-based and plasticizer-free ISOs have comparable responses because the ionophore behaves as a pseudo-plasticizer. Compared to ISOs with both plasticizers and polymers, the elimination of hydrophobic polymers like PVC makes inkjet printing more efficient and preserves the capillary action of the porous paper. By using the newly launched Samba cartridge with a smaller nozzle diameter, the plasticizer-based and polymer-free optodes for sodium, potassium, and calcium ions are prepared by inkjet printing for the first time. In an example application, the inkjet-printed plasticizer-based and polymer-free ISOs are used for colorimetric measurements of urine calcium.