Designing nanoscale devices, such as gas sensors and thermal dissipators, is challenging at multiple levels. Exploring their properties through combined experimental and theoretical collaborations is a valuable approach that expands the understanding of their peculiarities and allows for the optimization of the design process. In order to select the most relevant functional molecules for carbon-based gas sensors, and provide the best sensitivity and selectivity possible, we study the electronic transport properties of functionalized carbon nanotubes (CNTs), both through experiments and theoretical calculations. The measurements are carried out both in argon and synthetic air, using CO, NO, and H2S as test cases, with carboxyl-functionalized CNTs. The calculations, performed in the framework of density functional theory, consider both metallic and semi-conducting prototype CNTs, with respective chiralities (6,6) and (7,0), exploring a broader range of functional molecules and gases. The behavior of individual carboxyl-functionalized CNTs deduced from the multiscale results consistently reflect what happens at a larger scale and provides useful insights regarding the experimental results. CNTs are excellent thermal conductors as well and show much promise as heat dissipators in microelectronics. However, in practice, thermal properties of CNTs are affected due to the unavoidable presence of defects and interface with the environment. We investigated these limitations using a multiscale approach. Using molecular dynamics simulations, here we investigate the heat flow across the interface of a (10,10) CNT with various substances, including air and water. We also analyzed computationally the impact of CNT defects on its thermal transport properties using first principles calculations.