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Novel approach to nanopore design enhances molecule capture without compromising sensing accuracy
(Nanowerk Spotlight) Nanopore technology has emerged as a powerful tool for single-molecule sensing, offering unprecedented capabilities in fields ranging from DNA nanopore sequencing to protein analysis. These nanoscale pores, whether biological or solid-state, act as molecular gateways, allowing researchers to detect and analyze individual molecules as they pass through. The principle behind nanopore sensing is elegantly simple: as a molecule traverses the pore, it disrupts the ionic current flowing through it, creating a characteristic signal that can be used to identify and study the molecule.
However, the journey to harness the full potential of nanopores has been fraught with challenges. One of the most persistent issues has been the difficulty in capturing and controlling the movement of molecules through the pore. This is particularly problematic for weakly charged or neutral molecules, which are not readily drawn into the nanopore by electrophoretic forces. Additionally, once a molecule enters the pore, ensuring it remains there long enough to generate a meaningful signal has been a significant hurdle.
Researchers have long sought ways to enhance molecule capture and retention in nanopores. One promising approach has been the use of electroosmotic flow (EOF), a phenomenon where an applied electric field induces the movement of fluid within the nanopore. By manipulating EOF, scientists can potentially draw molecules into the pore and control their transit time, regardless of the molecule's charge.
Traditionally, EOF in nanopores has been controlled by modifying the surface charge of the pore's constriction - its narrowest point. This region is crucial for sensing, as it is where the ionic current is most sensitive to the presence of a molecule. However, altering the constriction to enhance EOF can compromise the nanopore's ability to distinguish between different molecules, creating a trade-off between improved capture and reduced sensing accuracy.
Recent research published in Advanced Materials ("Controlling Electroosmosis in Nanopores Without Altering the Nanopore Sensing Region") presents a novel approach to this longstanding challenge. The study, conducted by a team of international researchers, demonstrates that significant electroosmotic flow can be achieved in nanopores without modifying the critical constriction region. This finding could potentially revolutionize nanopore design, offering a way to independently optimize both molecule capture and sensing accuracy.
The research team employed a multi-faceted approach, combining theoretical modeling, continuum simulations, and experimental validation. They began by exploring simple cylindrical and conical nanopore geometries using continuum electrohydrodynamic simulations. These simulations revealed that adding surface charges outside the constriction could generate substantial EOF, comparable to that produced by charges at the constriction itself.
One key insight from the simulations was that the effectiveness of surface charges in generating EOF depends on their distance from the constriction. Charges placed within a few Debye lengths (a measure of the thickness of the electrical double layer near a charged surface) of the constriction were found to be particularly effective. This is because the counterion cloud associated with these charges can extend into the constriction region, where the electric field is strongest.
To validate their findings in a more realistic context, the researchers turned to biological nanopores, specifically the MspA protein pore. Using atomistic molecular dynamics simulations, they studied various mutants of MspA with charged residues placed at different locations along the pore. These simulations confirmed that significant EOF could be generated by charges placed outside the constriction, supporting the results from the continuum models.
MspA nanopore embedded in a lipid membrane
The MspA nanopore is embedded in a lipid membrane. A voltage is applied between the cis and trans reservoirs. The nanopore is cut along a plane parallel to the image. Water is reported as a blue surface while ions are not represented. (Image: Adopted from DOI:10.1002/adma.202401761 with permission by Wliey-VCH Verlag)
The team also conducted experimental studies on MspA mutants, measuring their ion selectivity through reversal potential experiments. While these experiments do not directly measure EOF, ion selectivity is considered a reliable indicator of EOF strength. The experimental results aligned well with the computational predictions, showing that charges placed near, but not directly in, the constriction could induce substantial anion selectivity and, by extension, EOF.
One particularly interesting finding was that in some cases, adding charges in larger sections of the nanopore could result in a slight increase in anion selectivity. This suggests that multiple rings of charges could be used to fine-tune the EOF without directly modifying the constriction.
The implications of this research are significant for the field of nanopore sensing. By demonstrating that EOF can be controlled independently of the constriction region, the study opens up new possibilities for nanopore design. Engineers could potentially optimize the constriction for maximum sensing accuracy while separately tuning other regions of the pore to enhance molecule capture and retention.
This approach could lead to more versatile and efficient nanopore sensors. For instance, it might enable the development of nanopores capable of capturing and analyzing a wider range of molecules, including those that are currently challenging to detect due to their neutral charge or rapid transit through the pore.
Moreover, the principles elucidated in this study could have broader applications beyond sensing. The ability to precisely control fluid flow at the nanoscale is crucial for developing advanced nanofluidic devices for applications such as energy harvesting, water purification, and drug delivery.
While the study focused primarily on biological nanopores, the researchers suggest that their findings could also be applicable to solid-state nanopores. However, they note that current fabrication technologies may pose challenges in creating the fine charge patterns required. This limitation points to potential areas for future research and development in nanopore fabrication techniques.
As with any scientific advancement, this research opens up new questions and avenues for exploration. Future studies might investigate how to optimize charge patterns for specific types of molecules or how to combine this approach with other enhancement techniques. Additionally, the development of new experimental methods to directly measure EOF in nanopores could provide further validation and insights.
This research represents a significant step forward in nanopore technology, offering a new paradigm for nanopore design that could enhance both capture efficiency and sensing accuracy. As the field continues to evolve, these insights may contribute to the development of more powerful and versatile nanopore-based devices, potentially accelerating progress in areas such as genomics, proteomics, and single-molecule analysis.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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