Synthetic nanopores provide benefits over biological nanopores in sensing applications; they are stable over a broader range of environmental conditions and offer greater flexibility with regard to pore size and surface chemistry. Micromachining processes have been well developed over the past five decades with the growth of IC technology, which can be employed to fabricate synthetic nanopores. For example, nanopores may be milled straightforwardly in thin, solid-state membranes using a focused ion or electron beam. Since nanopore-based sensing devices rely on changes in conductance as analytes block the nanopore or are translocated through it, there have been many attempts to reduce signal noise to improve sensitivity. For example, insulating material (e.g., PDMS, silicon dioxide or silicon nitride) is commonly deposited on the membrane surface to decrease noise. Although most micromachined nanopores are silicon based, it has been reported that glass capillary nanopores show lower noise and a higher signal-to-noise ratio for DNA translocation compared to nanopores in silicon nitride membranes. However, the pulled capillary glass nanopore exhibits a smaller current due to its long conical shape and is not amenable to high-throughput manufacturing. Recently, a low-noise, solid-state nanopore based on an insulating substrate has been introduced in which a thin silicon nitride membrane is attached to a quartz substrate to decrease low frequency noise by several orders of magnitude.
In this work, the creation and characterization of a glass nanopore is presented. Both the membrane and the substrate are derived from a single glass wafer using batch micromachining. By thinning the substrate with a dilute mixture of hydrofluoric and hydrochloric acid, submicron-thick membranes are successfully produced. In addition, a comparison of the glass nanopore versus the silicon-based nanopore is investigated based on the stability of the base current, the measured noise-level (e.g., RMS ionic current noise and current power spectral density in frequency domain), and the presence of undesired peaks upon changing applied voltage polarity. A relation between noise level and pore sizes is also established. To test the nanopore as a sensor, the glass chip with membrane and nanopore was sandwiched between two buffer-filled chambers. Carboxylate-terminated polystyrene beads were placed in one of the buffer-filled chambers, and a constant electric field was applied across the pore. Due to its negative charge, the bead migrates to the pore mouth and blocks it giving rise to a decrease in conductance. Using this test, the performance of the nanopores in glass and those in silicon nitride are compared.
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