The NEXT collaboration seeks the neutrino-less double beta decay 136Xe → 136Ba + 2e- , as an unambiguous proof of the Majorana nature of neutrinos. The detection of the daughter atom in the decay, Ba2+ would constitute a positive evidence of the reaction occurrence, since no other known radioactive decay produces such ion in coincidence with two electrons. I will show recent results towards the development of an organic monolayer (ML) which works as an ion sensor for barium. For this we use a combination of surface science techniques (XPS and STM-STS) in ultra-high vacuum (UHV).More information in [1] P. Herrero et al. arXiv:2201.09099 “Ba2+ ion trapping by organic submonolayer: toward an ultra-low background 0νββ decay detector.”