In situ synthesis of reagents in paper-based analytical devices using paper stacking

This article demonstrates a technique for the in situ synthesis of an insoluble analytical reagent in paper analytical devices, using paper stacking. Previously, our group has demonstrated that stacking a fast-wicking paper membrane on top of a slow-wicking paper membrane containing dried reagents enables the uniform rehydration of the dried reagents over large areas. This technique is utilized here to fabricate distance-based sweat chloride quantification strips, which requires the synthesis of insoluble silver chromate as an analytical reagent in paper. The in situ generation of silver chromate for sweat chloride detection was previously accomplished by manually dipping a hydrophobically patterned paper channel into multiple precursor solutions with intermittent washing and drying. Compared to the previous technique, the stacking method obviates the need for (i) patterning hydrophobic barriers in paper for creation of flow channels, and (ii) multiple dipping steps that need large reagent volumes. The method is amenable to large scale manufacturing as the insoluble reagent can be synthesized uniformly over large paper areas and can then be cut into multiple sensing strips. The developed sensor has a limit of detection of ∼0.3 mM and a wide linear dynamic range of 0-120 mM for the detection of chloride ions, which enables the diagnosis of cystic fibrosis, characterized by sweat chloride levels greater than 60 mM. This simple technique of in situ synthesis of insoluble analytical reagents in paper could enable expanding the range of analytical chemistries that may be performed in paper-based analytical devices.