Identifying molecular signatures in parts-per-billion level by anomalous diffusion kinetics on metal organic frameworks

Abstract Biology achieves parts-per-billion (ppb) sensitivity in gas speciation by tracking chemical gradients over time. Replicating this precision in engineered sensors is challenging due to the small differences in adsorption activation energy (\(\:{E}_{a}\)) between molecular species. \(\:{E}_{a}\) imposes exponentially growing long steady-state response times (\(\:{E}_{a}/{k}_{B}T)\) relative to the Brownian thermal fluctuation scale (\(\:{k}_{B}T\)), limiting time-based molecular discernibility. Diffusion kinetics, which are often overlooked, can alter adsorption behavior over time. We demonstrate that the diffusion of volatiles on a thin \(\:\sim200\:nm\) nanoporous metal-organic framework matrix varies anomalously when synergized by strain from shear vibrations of a quartz crystal microbalance. This generates unique adsorption timescale signatures, enabling us to decouple a second fluctuational timescale, highlighting a constant interplay of steady-state and fluctuational adsorption behaviors. This distinct timescale offers a new selectivity metric, achieving ppb-level sensitivity that closely mimics biological standards. Our results showcase the potential of using fluctuational kinetics at a nanoporous interface for real-time molecular identification. *Balasubramanian Srinivasan and Arindam Phani contributed equally to this work and will share the first authorship.