A Comprehensive Review on Filled Carbon Nanotubes: Synthesis, Properties and Applications

Carbon nanotubes (CNTs) have emerged as one of the most exciting families of carbon nanomaterials. Their hollow tubular architecture, with a nanometric inner cavity, not only defines their distinct physical and chemical behavior but also enables the encapsulation of a wide range of materials, including inorganic and organic compounds. This encapsulation capability allows CNTs to function as nanocontainers, protective hosts, and confined reaction vessels, leading to novel hybrid materials with tailored optical, electronic, catalytic, and mechanical properties. In this review, we provide a comprehensive overview of the methodologies employed for filling CNTs, including <i>in situ</i> and <i>ex situ</i> approaches. We critically examined the diverse range of materials encapsulated within CNTs, highlighting how confinement at the nanoscale influences their chemical reactivity, phase stability, and emergent quantum phenomena. Special attention is given to the wide range of applications of filled CNTs in addressing societal challenges. These include biomedicine, catalysis, energy storage, gas separation, filtration membranes, sensing technologies, and nanoelectronics. Beyond revisiting the current state-of-the-art, this review offers a critical discussion of future directions and challenges in this field.