Our program of single metal ion sensing is a part of a collaborative project to employ SMFI chemosensors functional at the solid-gas interface as R&D toward barium tagging for the NEXT experiment. We thank ION Biosciences for supplying the IPG dyes used in these experiments and also for technical support on their best use. We acknowledge support from the US National Science Foundation under award number NSF CHE 2004111 and the Robert A Welch Foundation under award number Y-2031-20200401 (University of Texas Arlington) and the US Department of Energy under awards DE-SC0019054 and DE-SC0019223. The NEXT Collaboration acknowledges support from the following agencies and institutions: the European Research Council (ERC) under Grant Agreement No. 951281-BOLD; the European Union's Framework Programme for Research and Innovation Horizon 2020 (2014-2020) under Grant Agreement No. 957202-HIDDEN; the MCIN/AEI of Spain and ERDF A way of making Europe under grants RTI2018-095979 and PID2021-125475NB, the Severo Ochoa Program grant CEX2018-000867-S and the Ramon y Cajal program grant RYC-2015-18820; the Generalitat Valenciana of Spain under grants PROMETEO/2021/087 and CIDEGENT/2019/049; the Department of Education of the Basque Government of Spain under the predoctoral training program non-doctoral research personnel; the Portuguese FCT under project UID/FIS/04559/2020 to fund the activities of LIBPhys-UC; the Pazy Foundation (Israel) under grants 877040 and 877041; the US Department of Energy under contracts number DE-AC02-06CH11357 (Argonne National Laboratory) and DE-AC02-07CH11359 (Fermi National Accelerator Laboratory). Finally, we are grateful to the Laboratorio Subterraneo de Canfranc for hosting and supporting the NEXT experiment.