Pore-Size-Dependent Role of Functional Elements at the Outer Surface and Inner Wall in Single-Nanochannel Biosensors

Biological nanopores feature functional elements on the outer surfaces (FE_OS) and inner walls (FE_IW), enabling precise control over ions and molecules with exceptional sensitivity and specificity. This provides valuable inspiration to scientists for the development of intelligent artificial nanochannel-based platforms, with a wide range of potential applications, including biosensors. Much effort has been dedicated to investigating the distinct contribution of FE_OS and FE_IW of multichannel membrane biosensors. However, the intricate interactions among neighboring pores in multichannel biosensors have presented challenges. This underscores the untapped potential of single nanochannels as ideal candidates in this field. Here, we employed single nanochannel membranes with different pore sizes to investigate the distinct contributions of FE_IW and FE_OS to single-nanochannel biosensors, combined with numerical simulations. Our findings revealed that alterations in the negative charges of FE_IW and FE_OS, induced by target binding, have differential effects on ion transport, contingent upon the degree of nanoconfinement. In the case of smaller pores, such as 20 nm, the ion concentration polarization driven by FE_IW can independently control ion transport through the surface's electric double layer. However, as the pore size increases to 40-60 nm, both FE_IW and FE_OS become essential for effective ion concentration polarization. When the pore size reaches 100 nm, both FE_IW and FE_OS are ineffective and thus unsuitable for biosensors. Simulations demonstrate that the observed phenomena can be attributed to the interactions between the charges of FE_IW and FE_OS within the overlapping electric double layer under confinement. These results underscore the critical role of pore size as a key parameter in governing the functionality of probes within or on nanopore-based biosensors as well as in the design of nanopore-based devices.