Investigating noise signatures of polymer adsorption using nanopores

The modification of surface properties by polymer adsorption is a widely used technique to tune interactions in molecular experiments, such as resistive pulse sensing with nanopores. Here, we investigate how variation of the ionic current noise through glass nanopores reflects the passive adsorption of short, neutral polymers to the pore surface. We find that the power spectral density of the noise shows a characteristic change upon adsorption of polymer, however the magnitude of this increase is strongly dependent on both polymer length and salt concentration. In particular, for short polymers at low salt concentrations no change in noise is observed, despite verification of comparable adsorption in these systems using quartz crystal microbalance with dissipation monitoring measurements. We propose that the characteristic noise is generated by small current changes associated with the movement of polymers on and off the nanopore surface and perform simple simulations to assess the feasibility of this model. Excellent agreement is obtained with experimental data using physically motivated parameters for the adsorption strength and range, supporting our molecular interpretation of the observed noise signatures in our data. This highlights the value of analysing system fluctuations as a method of elucidating molecular processes, and paves the way to using noise spectral analysis for in situ characterisation of functionalised nanopores.