Salt bridge in electroanalytical chemistry: past, present, and future

Spontaneous Mesoporosity-Driven Sequestration of Ionic Liquids from Silicone-Based Reference Electrode Membranes

Nanopore-driven sequestration of ionic liquids from a silicone membrane is presented, a phenomenon that has not been reported previously. Reference electrodes with ionic liquid doped polydimethylsiloxane (PDMS) reference membranes and colloid-imprinted mesoporous carbon (CIM) as solid contact are not functional unless special attention is paid to the porosity of the solid contact. In the fabrication of such reference electrodes, a solution of a hydroxyl-terminated silicone oligomer, ionic liquid, cross-linking reagent, and polymerization catalyst is deposited on top of the carbon layer, rapidly filling the pores of the CIM carbon. The catalyzed polymerization curing of the silicone quickly results in cross-linking of the hydroxyl-terminated polydimethylsiloxane oligomers, forming structures that are too large to penetrate the CIM carbon pores. Therefore, as solvent evaporation from the top of freshly prepared membranes drives the diffusional transport of solvent toward that membrane surface, the solvent molecules that leave the CIM carbon pores can only be replaced by the ionic liquid. This depletes the ionic liquid in the reference membrane that overlies the CIM carbon solid contact and increases the membrane resistance by up to 3 orders of magnitude, rendering the devices dysfunctional. This problem can be avoided by presaturating the CIM carbon with ionic liquid prior to the deposition of the solution that contains the silicone oligomers and ionic liquid. Alternatively, a high amount of ionic liquid can be added into the membrane solution to account for the size-selective sequestration of ionic liquid into the carbon pores. Either way, a wide variety of ionic liquids can be used to prepare PDMS-based reference electrodes with CIM carbon as a solid contact. A similar depletion of the K⁺ ionophore BME-44 from ion-selective silicone membranes was observed too, highlighting that the depletion of active ingredients from polymeric ion-selective and reference membranes due to interactions with high surface area solid contacts may be a more common phenomenon that so far has been overlooked.

Physical Characteristics of von Willebrand Factor Binding with Platelet Glycoprotein Ibɑ Mutants at Residue 233 Causing Various Biological Functions

Glycoprotein (GP: HIS ¹ -PRO ²⁶⁵ ) Ibɑ is a receptor protein expressed on the surface of the platelet. Its N-terminus domain binds with the A1 domain (ASP ¹²⁶⁹ -PRO ¹⁴⁷² ) of its ligand protein von Willebrand factor (VWF) and plays a unique role in platelet adhesion under blood flow conditions. Single amino acid substitutions at residue 233 from glycine (G) to alanine (A), aspartic acid (D), or valine (V) are known to cause biochemically distinct functional alterations known as equal, loss, and gain of function, respectively. However, the underlying physical characteristics of VWF binding with GPIbɑ in wild-type and the three mutants exerting different biological functions are unclear. Here, we aimed to test the hypothesis: biological characteristics of macromolecules are influenced by small changes in physical parameters. The position coordinates and velocity vectors of all atoms and water molecules constructing the wild-type and the three mutants of GPIbɑ (G233A, G233D, and G233V) bound with VWF were calculated every 2 × 10 ^-15 seconds using the CHARMM (Chemistry at Harvard Macromolecular Mechanics) force field for 9 × 10 ^-10 seconds. Six salt bridges were detected for longer than 50% of the calculation period for the wild-type model generating noncovalent binding energy of -1096 ± 137.6 kcal/mol. In contrast, only four pairs of salt bridges were observed in G233D mutant with noncovalent binding energy of -865 ± 139 kcal/mol. For G233A and G233V, there were six and five pairs of salt bridges generating -929.8 ± 88.5 and -989.9 ± 94.0 kcal/mol of noncovalent binding energy, respectively. Our molecular dynamic simulation showing a lower probability of salt bridge formation with less noncovalent binding energy in VWF binding with the biologically loss of function G233D mutant of GPIbɑ as compared with wild-type, equal function, and gain of function mutant suggests that biological functions of macromolecules such as GPIbɑ are influenced by their small changes in physical characteristics.

Alizarin-graphene nanocomposite for calibration-free and online pH monitoring of microbial fuel cell

Microbial fuel cells (MFCs) are sensitive to acidity variations in both bioelectricity generation and biochemical digestion aspects, therefore online pH monitoring is of necessity to guarantee optimal function of MFCs. Present pH meters hardly fulfill this special need. In this work, we designed a novel voltammetric pH sensor based on electrochemically reduced graphene oxide (rGO) modified screen printed electrode. By surface doping of alizarin, good linearity of pH sensing over the range of 4.0-9.0 can be realized. Fast readout can be acquired within 15 s for each test. pH monitoring for artificial wastewater with inoculum of granular activated sludge in a MFC was successfully illustrated. Specially, it was verified that the performance was improved with alizarin doping due to the enhanced rGO surface proton diffusion. This approach provides an online, calibration-free and long stable pH monitoring method for the future MFC development.

Toward Unified pH of Saline Solutions

Fluctuations of pH in coastal systems are generally surveyed through potentiometric pH measurements. A new concept of a unified pH scale was introduced with the great advantage of enabling comparability of absolute values, pHabs, pertaining to any medium. Using water as an anchor solvent, yielding pHabsH2O, enables referencing the pHabs values to the conventional aqueous pH scale. The current work aims at contributing to implement pHabsH2O to saline solutions. To this purpose, differential potentiometric measurements, with a salt bridge of ionic liquid [N2225][NTf2], were carried out aiming at overcoming problems related to residual liquid junction potentials that affect the quality of such measurements. The ability to measure pHabsH2O with acceptable uncertainty was evaluated using Tris-Tris·HCl standard buffer solutions prepared in a background matrix close to the characteristics of estuarine systems (salinity of 20) as well as with NaCl solutions with ionic strength between 0.005 and 0.8 mol kg−1. The present study shows that for high ionic strength solutions, such as seawater, challenges remain when addressing the assessment and quantification of ocean acidification in relation to climate change. Improvements are envisaged from the eventual selection of a more adequate ionic liquid.

Reference Electrodes Based on Ionic Liquid-Doped Reference Membranes with Biocompatible Silicone Matrixes

Many reference electrodes with an ionic liquid-doped reference membrane contain a plasticizer that can gradually leach out into the sample. However, because many common plasticizers are known to be endocrine disruptors and may induce inflammatory reactions, they are preferably avoided for wearable or implantable sensors. Therefore, this work tested polymeric reference electrode membranes prepared by solvent casting from seven commercially available biocompatible silicones that are widely used in implantable devices. Only reference electrodes with membranes consisting of poly(3,3,3-trifluoropropylmethylsiloxane) (Fluorosilicone 1) and one of several 1-methyl-3-alkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids provided a stable and sample-independent potential in electrolyte solutions spanning the range of electrolyte concentrations in human blood, with more hydrophobic ionic liquids performing better. Over 8 days at 37 °C in artificial blood electrolyte solutions, the reference membranes doped with 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide exhibited a potential drift as low as 20 μV/h. In 10% animal serum, a 112 μV/h drift was observed over 5.8 days. The other six silicone materials doped with an ionic liquid either failed to form self-standing membranes or did not provide a sample-independent potential in the ionic concentration range tested. In case of the functional reference electrodes, differential scanning calorimetry confirmed good miscibility between the ionic liquid and the polymer matrix, whereas the poor miscibility of four polymer matrixes and the ionic liquids-as confirmed by differential scanning calorimetry-correlated with an undesirable sample dependence of the reference potential.

Reference Electrodes with Ionic Liquid Salt Bridge: When Will These Innovative Novel Reference Electrodes Gain Broad Acceptance?

In this paper, we raise questions that researchers have to ask if they intend to replace a conventional reference electrode with an ionic liquid-based reference electrode and try to answer these questions based on our experiences and literature data. Among these questions, the most important is which ionic liquid should be used. However, beyond the chemical composition of the ionic liquid, to realize all the potential benefits of ionic-liquid based reference electrodes, there are additional, equally important considerations. Through examples we will show the importance of the (i) purity of the ionic liquid and the consequences of deviations from its stoichiometric salt composition, (ii) form of implementation of the ionic liquid-based reference electrode membrane (free-flowing salt bridge, or ionic liquid embedded in a membrane), (iii) membrane/gelling agent material and its composition, and (iv) experimental conditions (steady state or flowing conditions) under which it will be used. Finally, we recommend methods to test the performance criteria of the ionic liquid-based reference electrodes.

A Microfluidic pH Measurement Device with a Flowing Liquid Junction

The pH values of aqueous solutions are conventionally measured with pH-sensitive electrodes such as glass electrodes or ion-sensitive field-effect transistors (ISFETs) used in conjunction with Ag/AgCl reference electrodes and KCl solutions. The speed of pH measurement with these systems can be deficient, however, as the glass electrode responds slowly during measurements of sample solutions with low buffering capacities. Our group has constructed a new pH measurement system using a microfluidic device and ISFET sensors. The device has a channel with two inlets and one outlet, with a junction connected to a Y-shaped channel on the same plane. Two ISFET sensors and an Ag/AgCl pseudo reference electrode are fitted into the channel to construct a differential measurement device. A sample solution and baseline solution supplied into the inlets by gravity-driven pumps form a flowing liquid junction during measurement. The small size and fast response of the ISFET sensors enable measurement of about 2.0 mL of sample solution over a measurement period of 120 s. The 90% response time is within 2 s. The calibrated sensor signal exhibits a wide range (pH 1.68–10.0) of linearity with a correlation factor of 0.9997. The measurement error for all solutions tested, including diluted solutions, was 0.0343 ± 0.0974 pH (average error ± standard deviation (S.D.), n = 42). The new device developed in this research will serve as an innovative technology in the field of potentiometry.

A simple method for decreasing the liquid junction potential in a flow-through-type differential pH sensor probe consisting of pH-FETs by exerting spatiotemporal control of the liquid junction

The liquid junction potential (LJP), the phenomenon that occurs when two electrolyte solutions of different composition come into contact, prevents accurate measurements in potentiometry. The effect of the LJP is usually remarkable in measurements of diluted solutions with low buffering capacities or low ion concentrations. Our group has constructed a simple method to eliminate the LJP by exerting spatiotemporal control of a liquid junction (LJ) formed between two solutions, a sample solution and a baseline solution (BLS), in a flow-through-type differential pH sensor probe. The method was contrived based on microfluidics. The sensor probe is a differential measurement system composed of two ion-sensitive field-effect transistors (ISFETs) and one Ag/AgCl electrode. With our new method, the border region of the sample solution and BLS is vibrated in order to mix solutions and suppress the overshoot after the sample solution is suctioned into the sensor probe. Compared to the conventional method without vibration, our method shortened the settling time from over two min to 15 s and reduced the measurement error by 86% to within 0.060 pH. This new method will be useful for improving the response characteristics and decreasing the measurement error of many apparatuses that use LJs.