Silicon substrate effects on ionic current blockade in solid-state nanopores

Iontronic Nanopore Model for Artificial Neurons: The Requisites of Spiking

Brain-inspired neuromorphic computing is currently being investigated for effective artificial intelligence (AI) systems. The development of artificial neurons and synapses is imperative to creating efficient computational biomimetic networks. Here we propose the minimal configuration of an effective iontronic spiking neuron based on a conical nanofluidic pore ionic diode. The conductance is composed of a Boltzmann open channel probability and a blocking inactivation function, forming the structure of a memristor. The presence of a negative resistance and the combination of activation-deactivation dynamics cause a Hopf bifurcation. Using the characteristic frequencies of small perturbation impedance spectroscopy, we discuss the conditions of spiking, in which the system enters a limit cycle oscillation. We arrive at the conclusion that an excitable neuron-like system can be made with a single active channel instead of the more complex combination of multiple channels that occurs in the Hodgkin-Huxley neuron model.

Effects of cone angle and length of nanopores on the resistive pulse quality

Resistive pulse sensing (RPS) has proved to be a viable method for the detection and characterization of micro and nano particles. Modern fabrication methods have introduced different nanopore geometries for resistive pulse sensors. In this paper, we have numerically studied the effects of membrane thickness and the pore's cone angle, as the main geometrical parameters, on the sensing performance of the nanopores used for nanoparticle detection in the resistive pulse sensing method. To compare the sensing performance, three resistive pulse quality parameters were investigated - sensitivity, pulse duration and pulse amplitude. The thorough investigation on the relations between the geometrical parameters and the pulse quality parameters produced several interesting results, which were categorized and summarized for different nanopore structures (as different nanopore platforms) enabling the readers to more effectively compare them with one another. The results revealed that large cone angle and low aspect ratio nanopores have higher pulse amplitude and sensitivity, but their low duration could be a challenge in the process of detecting the resistive pulse. In addition, our results show small variation in sensitivity and duration of large cone angle nanopores with respect to pore length change, which is explained using the effective length concept and the definition of electric field strength and length. The findings of the present work can be used in practical applications where choosing the optimal pore geometry is of crucial significance. Furthermore, the results provide several possible ways to improve the resistive pulse quality for better sensing performance.

Back-Side Polymer-Coated Solid-State Nanopore Sensors

We systematically investigated the influence of polymer coating on temporal resolution of solid-state nanopores. We fabricated a Si₃N₄ nanopore integrated with a polyimide sheet partially covering the substrate surface. Upon detecting the nanoparticles dispersed in an electrolyte buffer by ionic current measurements, we observed a larger resistive pulse height along with a faster current decay at the tails under larger coverage of the polymeric layer, thereby suggesting a prominent role of the water-touching Si₃N₄ thin film as a significant capacitor serving to retard the ionic current response to the ion blockade by fast translocation of particles through the nanopores. From this, we came up with back-side polymer-coated chip designs and demonstrated improved pore sensor temporal resolution by developing a nanopore with a thick polymethyl-methacrylate layer laminated on the bottom surface. The present findings may be useful in developing integrated solid-state nanopore sensors with embedded nanochannels and nanoelectrodes.