Polyphosphonium-based bipolar membranes for rectification of ionic currents

Abstract

Bipolar membranes (BMs) have interesting applications within the field of bioelectronics, as they may be used to create non-linear ionic components (e.g., ion diodes and transistors), thereby extending the functionality of, otherwise linear, electrophoretic drug delivery devices. However, BM based diodes suffer from a number of limitations, such as narrow voltage operation range and/or high hysteresis. In this work, we circumvent these problems by using a novel polyphosphonium-based BM, which is shown to exhibit improved diode characteristics. We believe that this new type of BM diode will be useful for creating complex addressable ionic circuits for delivery of charged biomolecules. Combined electronic and ionic conduction makes organic electronic materials well suited for bioelectronics applications as a technological mean of translating electronic addressing signals into delivery of chemicals and ions. 1 For complex regulation of functions in cells and tissues, a chemical circuit technology is necessary in order to generate complex and dynamic signal gradients with high spatiotemporal resolution. One approach to achieve a chemical circuit technology is to use bipolar membranes (BMs), which can be used to create the ionic equivalents of diodes 2-5 and transistors. 6-8 A BM consists of a stack of a cation-and an anionselective membrane, and functions similar to the semiconductor PN-junction, i.e., it offers ionic current rectification 9,10 (Figure 1(a)). The high fixed charge concentration in a BM configuration make them more suited in bioelectronic applications as compared to other non-linear ionic devices, such as diodes constructed from surface charged nanopores 11 or nanochannels, 12 as the latter devices typically suffers from reduced performance at elevated electrolyte concentration (i.e., at physiological conditions) due to reduced Debye screening length. 13 However, a unique property of most BMs, as compared to the electronic PN-junction and other ionic diodes, is the electric field enhanced (EFE) water dissociation effect. 10,14 This occurs above a threshold reverse bias voltage, typically around À1 V, as the high electric field across the ion-depleted BM interface accelerates the forward reaction rate of the dissociation of water into H þ and OH À ions. As these ions migrate out from the BM, there will be an increase in the reverse bias current. The EFE water dissociation is a very interesting effect and is commonly used in industrial electrodialysis applications, 15 where highly efficient water dissociating BMs are being researched. 16 Also, BMs have also been utilized to generate H þ and OH À ions in lab-on-a-chip applications. 2,17 However, the EFE water dissociation effect diminishes the diode property of BMs when operated outside the 61 V window, which is unwanted in, for instance, chemical circuits and addressing matrices for delivery of complex gradients of chemical species. The effect can be suppressed by incorporating a neutral electrolyte inside the BM, 10,18 for instance, poly(ethylene glyc...

Electrochemical mechanism of ion current rectification of polyelectrolyte gel diodes

“…These results imply that the conservation of the number of counterions plays an essential role in suppressing electric currents in reverse applied voltages. Our results are effective for relatively small applied voltages (for example, the experiments in refs 8,9), where the dissociations of water molecules by electric fields at the interface between the two gels are negligible 14,15 (in practice, the onset of water dissociations is sensitive to the chemical properties of polyelectrolyte gels 16 ).…”

“…Nanochannels of asymmetric shapes were shown to rectify ion currents that are generated by Ag/AgCl electrodes [3][4][5][6] . Polyelectrolyte gel diodes (PGDs) that are the double layers of two oppositely charged polyelectrolyte gels (that are swollen in aqueous solutions), sandwiched by two symmetric metal electrodes, are another type of ionic diodes that rectify ion currents [7][8][9][10][11][12][13][14][15][16] . These diodes are designed to operate with a physical mechanism that is analogous to conventional semiconductor diodes-the asymmetry in the permeability of ions across the interfaces between the two oppositely charged gels [8][9][10][11][12] .…”

Polyphosphonium-based ion bipolar junction transistors

“…Here, we report an IBJT, in which the EFE water dissociation is avoided by the use of a novel polyphosphonium-based anion-selective material, which previously has been shown to prevent EFE water dissociation in BM diodes. 11 This allows the collector and emitter to directly contact the base without an intermediate PEG-layer. Without the need for a PEG-separator inside the BMs, the collector-emitter distance is reduced to only 2 lm.…”

“…Such non-linear iontronic components are, for example, interesting as they allow further control of ions in, for instance, electrophoretic drug delivery devices. A range of microfabricated diodes, [9][10][11] transistors, 12,13 and circuits 9,14 has been constructed using ion-selective membranes. These membranes contain fixed charges of either polarity, compensated by mobile ions of opposite charge (counter-ions).…”

Organic electronics for precise delivery of neurotransmitters

“…PSS-co-MA was bought as a sodium salt, and sodium was exchanged for protons using dialysis in HCl(aq), to obtain the proton form needed for cross-linking. The polycations qPVBC and polyphosphonium were prepared as described in [96] and [30], respectively.…”