Nonmonotonic enhancement of nanorod diffusion during 1D to quasi- 2D confinement transition

Confined nanoparticle diffusion is ubiquitous and holds potential for applications in bio/nanotechnology. Contrary to the conventional belief that confinement slows down diffusion, recent studies have shown that increased confinement can increase the orientation of anisotropic nanoparticles, thereby enhancing their diffusional transport. Despite significant independent advances under 1D or 2D confinement, comparison of different confinement dimensionalities and their transition effects on particle diffusion are largely overlooked. Here, we constructed nanochannels transitioning from 1D (flat walls) to quasi- 2D confinement (sinusoidal walls) and investigated the diffusion of a single nanorod (SNR) using molecular simulations. Our results reveal that during dimensionality transition, stronger confinement improves SNR orientation along the unconstrained direction and leads to more rapid diffusion. The distinction is that, under 1D confinement, SNR diffusion is enhanced monotonically with increasing orientation along the unconstrained direction, whereas under quasi- 2D confinement, the relationship becomes non-monotonic, with an optimal situation yielding the fastest diffusion. We further confirmed that the confinement-induced enhancement of diffusion is not limited to specific confinement shapes, as demonstrated in geometries with sawtooth and square-wave walls, thereby validating its universality. This work could provide design principles for nanochannel geometries to regulate nanoparticle transport in applications such as drug delivery and nanofluidics.