Profiling and Modification of Silicon Nitride Based Planar Substrates and Nanopores
A nanopore—typically defined as a through-hole with dimensions <100 nm in all directions that functions as the sole path between two electrolyte>reservoirs—is a robust single molecule sensor element which has enjoyed a wealth of applications spanning genomics and proteomics, with fledgling contributions to glycomics over the past two decades1–5. Two classes of nanopores exist—biological and solid state. Biological nanopores, for example, α-hemolysin, are highly reproducible and precise—with nanopore lengths and critical constriction sizes that are well known and reproducible. This is not the case with solid state nanopores. Assuming total nanopore length is equal to the nominal thickness of the membrane provided by the manufacturer is a standard practice in the nanopore field. However, given fabrication tolerances, there is some room for error, in certain instances close to 60% of the provided nominal thickness. Any error in nanopore length will couple to errors in the radius calculation, as will be seen in the discussion of nanopore characterization. Another two key assumptions are: i) the nanopore has a cylindrical nanopore shape unless (and often even if) the shape is otherwise known and ii) a single nanopore through the membrane is formed when one is intended. We intend to address these issues by developing a framework that would subsequently be able to show errors in harboring such geometric assumptions and eventual consequences for nanopore-based sensing experiments. ^ Analytes such as DNA tend to stick on to bare silicon nitride pores. With time, this could hinder the further movement of analyte across the nanopore. Surface modification techniques, for example, hydrosilylation, silane chemistry and electroless gold plating not only tune the size (minimum radius, r 0, and total nanopore length, L) but also change the intrinsic surface chemistry. Hydrosilylation on planar silicon nitride—a less challenging and less volume-constricted environment compared to nanopore inner walls—has been shown to be possible photochemically and thermally. The photochemical approach is particularly interesting as it would allow patterns to be created—with hydrosilylation occurring only where UV light is present, and with molecules being washed away by solvent where UV light was absent. This would be advantageous to nanopore-level applications especially where removal of unreacted molecules is critical as they can cause non-specific and unpredictable interactions of analyte with the nanopore. It is also less harsh than the thermal approach, hence it would be ideally suited for fragile architectures such as free standing nanopore membranes. Thus, we extended this method (hydrosilylation) to the nanopore level—decorating inner nanopore walls in a challenging zeptoliter volume—which by careful choice of functional group could potentially overcome the analyte “sticking” problem. Choice of molecule would play a significant role—one with a reactive terminal group would allow for subsequent reactions, for example, condensation or even click reactions, which are fast and facile, thus allowing for further modification of the size and surface chemistry of the pore. ^ Electroless gold deposition—free of externally applied voltage and only depending on redox potentials and concentration-controlled kinetics—has been done on poly(ethylene terephthalate) (PET) nanopores to reduce the pore opening dimensions and to change the surface chemistry of the pore. A carefully configured electroless plating procedure has been used to deposit gold directly on silicon nitride. In both cases, the substrates are insulators, thus, conventional electroplating would be futile—hence electroless plating. Patterned solution-phase gold depositions have great promise for electronics, photonics, and sensors such as nanopores as well—especially considering augmenting nanopore function with structures such as transverse electrodes. On a more macroscopic scale, different methods have been used to fabricate conductive patterns. In brief, methods involving photolithography, silane chemistry, and (transfer) printing are used extensively. One common feature in many existing methods is the use of a mask. The smallest lateral feature that can be created is dictated by the lateral extent of the features in the mask. For nanopores and other fragile architectures, mechanical non-contact and cleaning ease (especially by simple rinsing) are key elements in designing modification and fabrication methods. Hydrosilylation meets these expectations as it can be guided and restricted to specific regions by manipulating the exposure of light (UV) to the surface. We used hydrosylilated alkanes as a suppressing layer, for metal deposition in combination with electroless deposition to create spatial patterns of gold on silicon nitride.^
Yapa Mudiyanselage Nuwan Dhananjaya Yapa Bandara,
"Profiling and Modification of Silicon Nitride Based Planar Substrates and Nanopores"
Dissertations and Master's Theses (Campus Access).