Progress towards Interior Mass Measurements in Carbon Nanotube Single-digit Nanopores

Carbon nanotubes (CNTs) have emerged as a unique class of single-digit nanopores, or pores with diameter <10 nm, that serve as powerful testbeds for studying confined fluid thermodynamics. Recent optical spectroscopy studies of water confined within isolated CNTs suggest remarkable phase behavior, including strongly non-monotonic melting points that vary with nanotube diameter and can exceed bulk water's boiling point. Such diameter-dependent phase transitions have been predicted by theoretical studies and molecular dynamics simulations, though quantitative understanding remains incomplete. While optical techniques have provided valuable insights, additional experimental probes are needed to fully characterize these confined phases. Here, we report progress in developing a complementary approach to directly measure the mass of confined water within individual CNTs. We adapt an established electromechanical approach to sensitively determine changes in mass of an isolated carbon nanotube due to filling with water. We present a novel implementation using ultralong carbon nanotubes as well as their structural characterization using resonance Raman spectroscopy. We will discuss the mass sensitivity of our platform and its utility for the determination of fluid densities and their changes inside carbon nanotubes. Our work introduces an important new tool that will advance our understanding of matter under extreme nanoscale confinement.