Concentration-Polarization-Induced Precipitation and Ionic Current Oscillations with Tunable Frequency

Abstract

At the nanoscale, charges present at the surfaces of liquid–solid interfaces greatly influence the properties of ions and molecules present in the solution, and can lead to nanoscale effects such as ion selectivity, ion current rectification, and modulation of local ionic concentrations. Concentration polarization is another nanoscale phenomenon whereby ion concentrations are enriched at one opening of an ion-selective nanopore and depleted at the other. We show that when a nanopore is in contact with a weakly soluble salt present at a concentration below its solubility product, concentration polarization can lead to locally enhanced ionic concentrations and precipitation of the salt. Formed precipitates partially or fully occlude the nanopore’s opening as indicated by a measured transient decrease of the nanopore’s conductance. We have identified experimental conditions at which the locally created precipitate is either pushed through or dissolved, clearing the pore entrance and allowing the precipitation reaction to occur again. The dynamic process of precipitate formation and dissolution is observed as ion current fluctuations and oscillations with frequencies reaching 200 Hz. The frequency of the system operation exceeds other nanopore-based oscillators by 2 orders of magnitude, which we believe stems from the 30 nm length of the pores examined here, versus ∼10 μm long pores reported before.

Biomimetic potassium-selective nanopores

“…In the dielectric breakdown process, both chambers of a custom-made polydimethylsiloxane conductivity cell were filled with a strongly acidic solution of 1 M KCl (pH 1.6) ( 32 , 33 ). The dielectric breakdown was performed with 12 V applied with two Ag/AgCl pellet electrodes.…”

“…The pore diameter was estimated on the basis of the pore resistance found from the I - V curve recorded in 1 M KCl in the voltage range between −1 and +1 V. Note that the total resistance is a sum of the geometrical pore resistance and access resistance ( 41 ). The opening diameter ( D ) was then estimated using the following equation ( 33 )italicD=G2normalσtrue⌈1+1+16normalσitalicLnormalπitalicGtrue⌉where G is the slope of the I - V curve, σ stands for the solution conductivity, and L is the pore length (30 nm). The shape of the pores was assumed cylindrical; thus, the calculated diameters are considered effective/average diameters.…”

“…The pores used in this study were formed in 30 nm thick films of silicon nitride by dielectric breakdown. (30)(31)(32) Transport characteristics of the as-prepared pores were measured in 1 M and 100 mM solutions of KCl and NaCl. The pore walls and the membrane surfaces were then modified with triethoxysilylpropylmaleamic acid (TESPMA), which led to the attachment of carboxyl groups and the reduction of the pore diameter by a few nanometers.…”

Nonlinear Ion Transport through Ultrathin Metal–Organic Framework Nanosheet

“…Ionic transport through artificial solid-state nanopores has generated great research interest due to its importance in the simulation of protein nanopores on the biological cell membrane, such as the mimic of sodium ion channel or potassium ion channel. The current solid-state nanopores of silicon nitride, [1][2][3] poly(ethyleneterephthalate) (PET), [4][5][6] molybdenum disulfide, [7][8][9] and graphene [10][11][12][13][14][15] not only establish the good mimic of existing biological nanopore but also reveal distinguished mechanisms with new ion transport phenomena. [16][17][18][19][20] The simple ion transport through regular nanopores results in ohmic behaviors from linear current-voltage (I−V) curves, while the specific nonohmic behaviors reflect ion transport mechanisms in nanopores with different Herein, we developed an electrophoretic method for fabricating Zr-1,3,5-(4-carboxylphenyl)-benzene (Zr-BTB) solid-state nanopores and discovered the voltage-activated nonlinear ion transport through the 2D ultrathin MOF nanosheets.…”

Nanoscale kinetics of amorphous calcium carbonate precipitation in H2O and D2O

“…11,17,29 The use of nanopores, however, allows for convenient analysis of precipitation at the nanoscale by using an applied potential to mix solutions inside a confined volume and monitor the resulting changes in the ion current. Previously, the precipitation of weakly soluble phosphate salts of Zn, 30 Ca, 31 Co, 31 as well as CoCl 2 , 32 and even organic crystals 33 have been studied. We have used this approach to measure induction times for ACC precipitation in the presence of various scale inhibitors.…”