To infer the nanopore size upon fabrication, we measure its ionic conductance, G, and relate it to an effective diameter, d, assuming a cylindrical geometry and accounting for access resistance [25], [26], using, where σ is the bulk conductivity of the solution. This method, practical for nanopores fabricated in liquids, provides a reasonable first order estimate of the pore size [26], [27] as confirmed by DNA translocations, and compares well with actual dimensions obtained from TEM images (see Sections S4 and S8). I-V curves are performed in a ±1 V window, where the leakage current can safely be ignored. Figure 2 h reveals an ohmic electric response in 1 M KCl. The majority of our nanopores exhibit linear I-V curves upon fabrication. The remaining nanopores that show signs of self-gating or rectification can be conditioned, by applying moderate electric field pulses [23], to slightly enlarge them until an ohmic behaviour is attained in high salt. Such I-V characteristics imply a relatively symmetric internal electric potential pore profile [28] which supports the symmetrical geometry with a uniform surface charge distribution assumed by our pore conductance model. Otherwise, one would expect significant rectification from multiple ≤1-nm fluidic paths or from a single narrow nano-crack of similar conductance, due to strong electrostatic double layer overlap. To further characterize the nanopores, we examined the noise in the ionic current by performing power spectral density measurements. Our fabrication method consistently produces nanopores with low-1/f noise levels, comparable to fully wetted TEM-drilled nanopores (see Section S5)[29], [30]. This may be attributed to the fact that nanopores are created directly in liquid rather than in vacuum. Thus far, we have successfully fabricated hundreds of individual nanopores ranging from 1 to 25-nm in size with comparable electrical characteristics that are stable for days, 66 of which are included in Figure 2. The success rate for fabricating a nanopore under the experimental parameters presented here is estimated at >99%.

In order that a single, well-defined nanopore be created each time, we postulate that the leakage current must be highly localized on the insulating membrane, since for conductive substrates (semiconductors or metals) anodic oxidation leads to an array of nanopores[31]–[33]. The leakage spot(s) must also modify the membrane at the nanoscale since an aqueous KCl solution at neutral pH is not an active etchant of SiN_x. To elucidate the mechanism leading to the formation of a nanopore, we investigate the fabrication process as a function of applied voltage, membrane thickness, electrolyte composition, concentration, and pH. Figure 3a shows the time-to-pore creation, τ, as a function of the trans-membrane potential for 30-nm-thick membranes, in 1 M KCl buffered at various pHs. Interestingly, τ scales exponentially with the applied voltage irrespective of other conditions, and can be as short as a few seconds. For a given voltage, pH has a strong effect. As seen in Figure 3b, τ can be reduced by 1,000-fold when lowering the pH from 7 to 2. We have also observed that lower salt concentrations increase the fabrication time (see Section S6). Overall, for a given fabrication condition τ is relatively consistent, though variations by a factor of 4 are common, and is uncorrelated with the size of the fabricated pore. Figure 3c shows τ for 10-nm-thick SiN_x membranes, buffered at pH10 in various 1 M Cl-based aqueous solutions. The fabrication time in these thinner membranes also decreases exponentially with potential, but the value required for forming a nanopore is now reduced by ∼1/3 compared to 30-nm-thick membranes, irrespective of the different cations (K⁺, Na⁺, Li⁺) tested. This observation indicates that the applied electric field in the membrane is the main driving force for initiating the fabrication of a single nanopore. Fields in the range of 0.4-1 V/nm are close to the dielectric breakdown strength of low-stress SiN_x films[19], and are key for intensifying the leakage current, which is thought to ultimately cause breakdown in thin insulating layers[34]. The exponential dependence of τ on potential implies the same electric field dependency, which is reminiscent of the time-to-dielectric breakdown in gate dielectrics[34]. According to the current understanding, dielectric breakdown mechanisms proceed as follows[34]–[36]: (i) probabilistic accumulation of charge traps (i.e. structural defects) by electric field-induced bond breakage or generated by charge injection from the anode or cathode, (ii) increasing up to a critical density forming a highly localized conductive path, and (iii) causing physical damage due to substantial power dissipation and the resultant heating. We propose that the process by which we fabricate a nanopore in solution is similar, though we control the damage to the nanoscale by limiting the localized leakage current, at the onset of the first, discrete breakdown event. As indicated by Figure 3, the likelihood of defect formation within the silicon nitride membrane increases with the applied voltage and the strength of the electric field. At low values, the accumulation of charge traps is accomplished with relatively low efficiency compared to the amount of charge carriers traversing the membrane, since a leakage current of tens of nanoamperes can be sustained for hours or days. Given the stochastic nature of the pore creation process, multiple simultaneous nanoscale breakdown events are unlikely. Termination of the applied voltage following the occurrence of the first breakdown event, observed by the sudden irreversible increase in I_leakage, ensures that ultimately a single nanopore is created. Moreover, the fabrication of a single nanopore may result from the fact that the formation path of a nanopore experiences increased electric field strength during growth, which locally reinforces the rate of defect generation. The process by which material is removed from the membrane remains unclear, but broken bonds could be chemically dissolved by the electrolyte or following a conversion to oxides/hydrides [37], [38]. Another possibility is shearing due to localized plasticity of the membrane as a result of heating at the breakdown spot, but the efficiency of heat dissipation at the nanoscale, resulting from high surface-area-to-volume ratios, makes this less likely [22]. We explain the pH dependency on the fabrication time, for 30-nm thick SiN_x membranes, by the fact that breakdown at low pH is amplified by impact ionization producing an avalanche, due to the increased likelihood of hole injection or H⁺ incorporation from the anode (see Section S6 for more detail). To support the general character of nanofabrication by dielectric breakdown, we created nanopores in a different material (silicon dioxide) and present the data in Section S7.

We performed DNA translocation experiments to demonstrate that these nanopores can be leveraged for the benefit of single-molecule detection. Electrophoretically driven passage of a DNA molecule across a membrane is expected to transiently block the flow of ions in a manner that reflects the molecule length, size, charge and shape. The results using a ∼6.4-nm-diameter pore, as estimated from conductance measurements, in a 10-nm thick SiN_x membrane are shown in Figure 4. The scatter plot shows event duration and average current blockage of over 2,400 single-molecule translocations events of 5-kb dsDNA. The characteristic shape of the events is indistinguishable to data obtained on TEM-drilled nanopores [26], [39]–[41]. The observed quantized current blockades strongly support the presence of a single nanopore spanning the membrane. Using dsDNA (∼2.2 nm in diameter) as a molecular-sized ruler, the value of the single-level blockage events, ΔG = 7.4 ±0.9 nS, provides an effective pore diameter of 6.0 ±0.5-nm consistent with the size extracted from the pore conductance model [26]. This result also suggests that the membrane thickness at the vicinity of the nanopores has not been significantly altered. We observed similar DNA translocation signatures from most nanopores tested (revealing >80% success rate in detecting DNA for N = 19 nanopores tested), and provide further discussion and additional translocation data in Section S8.

Nanopore fabrication by controlled dielectric breakdown in solution represents a major reduction in complexity and cost over current fabrication methods, which will greatly facilitate accessibility to the field to many researchers, and provides a path to commercialize nanopore-based technologies. While we attribute the nanopore creation process to an intrinsic property of the dielectric membrane, such that the nanopore can form anywhere on the surface, our current understanding strongly suggests that the position of the pore can be determined by locally controlling the electric field strength or the material dielectric strength. This could be achieved, for instance, by nanopatterning or locally thinning the membrane, by positioning of a nanoelectrode, or by confining the field to specific areas on the membrane via micro- or nanofluidic channel encapsulation (see Section S9). The latter would also allow for the simple integration of independently addressable nanopores in an array format on a single chip.

Silicon Nitride (SiN_x) membranes used in our experiments are commercially available as transmission electron microscope (TEM) windows (Norcada product # NT005X and NT005Z). Each membrane is made of 10-nm or 30-nm thick low-stress (<250 MPa) SiN_x, deposited on 200-μm thick lightly doped silicon (Si) substrate by low-pressure chemical vapour deposition (LPCVD). A 50-μm × 50-μm window on the backside of the Si substrate is opened by a KOH anisotropic chemical etch. Prior to mounting into liquids, SiN_x membranes can be cleaned in oxygen plasma for 30 s at 30 W to facilitate wetting of the membrane surface, though this is not a requirement. All solutions used were filtered and degassed prior to use. The absence of pre-existing structural damages (e.g. pinholes, nano-cracks) is inferred by the fact that no current (<pA) is measured across a membrane at low voltages (<±1 V) prior to nanopore fabrication. The membrane side opposing the Si etch pit is the reference point for all applied voltages in this article. Silicon dioxide membranes were also purchased from TEMWindows (product# SO100-A20Q33). Note that we have also successfully fabricated nanopores on SiN_x membranes purchased from TEMWindows, and on custom fabricated SiN_x membranes.

A schematic of the experimental setup is shown in Figure 1. A silicon chip with an intact silicon nitride membrane is sandwiched between two silicone gaskets (shown in purple on the figure). It is then positioned between the two electrolyte reservoirs in a PTFE (polytetrafluoroethylene) or a PEEK (polyether ether ketone) fluidic cell. The two reservoirs filled with liquid electrolyte are electrically connected to a current amplifier by two Ag/AgCl electrodes. The entire system is encapsulated in a grounded faraday cage to isolate from electromagnetic interference. Data acquisition and measurement automation were performed using custom-designed LabVIEW software controlling a National Instruments USB-6351 or PXIe-6366 DAQ card. The value of the trans-membrane potential is set by the DAQ card. Leakage current is digitized at 250 kHz and the signal is filtered at 10 Hz. When a current exceed a pre-set threshold, the voltage bias is immediately ceased by the software (response time is ∼100 ms). I-V measurements and ionic current signal during DNA translocations are recorded using an Axopatch 200B with a 4-pole Bessel filter set at 100 kHz, with at 250 kHz sampling rate. Data analysis was carried out using custom-designed LabVIEW software to measure the duration and depth of each current blockade events.

We performed DNA translocation studies, using dsDNA fragments of 100 bp, 5 kbp, 10 kbp purchased from Fermantas (NoLimits products) in 1 M KCl pH8 or in 3.6 M LiCl pH8 at a final concentration of 10μg/mL. Lambda DNA (48.5 kbp) purchased from NewEngland BioLabs was also used.