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Carbon nanotubes (CNTs) possess extraordinary mechanical, thermal, and electrical properties due to their atomically perfect structure and sp2-hybridization. Recently, Single-wall CNTs (SWCNTs) have become an interesting host for the nanoscale confinement of fluids, with a wealth of surprising phenomena appearing: spontaneous filling, frictionless mass transport, unusual phase diagram, etc[1]. Most of these phenomena are still under debate and call for experimental confirmation. But the field is struggling with finding experimental approaches sensitive enough to carry out measurements at the level of individual nanotubes. SWCNT field effect transistors (SWCNT-FETs) showed that the electronic properties of SWCNTs are sensitive to their diameter, defects, doping, adsorbates, and environment [2], [3].In this contribution, we demonstrate that individual carbon nanotube field effect transistors (CNTFET) are excellent tools for this aim, for the first time allowing to precisely identify water confined inside the nanotube. By studying the electrical performances of several unopened and opened CNTFETs submitted to various atmosphere and temperature treatments, i.e. dry air, humidity, secondary vacuum, and current annealing, we show that it is possible to distinguish water being outside and inside the nanotube, just outside, or the nanotube free from water. We thus observed that for opened SWCNT both secondary vacuum and current annealing move threshold gate voltage towards more negative values, while for closed SWCNT secondary vacuum had no effect compared to current annealing. To sum up, the current annealing treatment is essential to distinguish water adsorbed outside from water confined inside. We show that this behavior is universal, as all devices' metallicities behave similarly, provided that the surface of the nanotube is pre-cleaned by current annealing. We will also discuss the mechanisms behind the coupling of electronic transport and the presence of water.Our results open up the possibility to use CNTFET for instance reliable, selective, and sensitive chemical and biological sensors, and also, to resolve the long-standing questions in the nanofluidic community about the behavior of water under nanoscale confinement.Reference:[1] T. A. Pascal, W. A. Goddard, and Y. Jung, “Entropy and the driving force for the filling of carbon nanotubes with water,” Proc. Natl. Acad. Sci., vol. 108, no. 29, pp. 11794–11798, 2011, doi: 10.1073/pnas.1108073108.[2] D. Cao et al., “Electronic sensitivity of carbon nanotubes to internal water wetting,” ACS Nano, vol. 5, no. 4, pp. 3113–3119, 2011, DOI: 10.1021/nn200251z.[3] I. Heller, A. M. Janssens, J. Männik, E. D. Minot, S. G. Lemay, and C. Dekker, “Identifying the mechanism of biosensing with carbon nanotube transistors,” Nano Lett., vol. 8, no. 2, pp. 591–595, 2008, DOI: 10.1021/nl072996i.