In this work, experiments, molecular dynamics (MD) simulations, and theoretical analysis are conducted to study ion transport in thin carbon nanotubes (CNTs). Diverse nonlinear relationships between the ionic conductance (G) and the ion concentration (C) are observed. MD simulations show that the distinct GC dependences are caused by the functionalization of the CNT entrance, which affects the energy barrier for ion transport and changes the ionic conductance. The various GC relationships are also predicted using the electrokinetic theory by considering the potential generated by the functional groups at the CNT entrance. Practically, the number of functional groups at the CNT entrance is influenced by several factors, including both intrinsic and external effects, which make it difficult to regulate the ionic conductance and pose a challenge to CNT-based nanofluidic systems in practical applications.

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