Calibration-Free Ionophore-Based Ion-Selective Electrodes With a Co(II)/Co(III) Redox Couple-Based Solid Contact

A high electrode-to-electrode reproducibility of the emf response of solid contact ion-selective electrodes (SC-ISEs) requires a precise control of the phase boundary potential between the ion-selective membrane (ISM) and the underlying electron conductor. To achieve this, we introduced previously ionophore-free ion exchanger membranes doped with a well controlled ratio of oxidized and reduced species of a redox couple as redox buffer and used them to make SC-ISEs that exhibited highly reproducible electrode-to-electrode potentials. Unfortunately, ionophores were found to promote the loss of insufficiently lipophilic species from the ionophore-doped ISMs into aqueous samples. Here we report on an improved redox buffer platform based on equimolar amounts of the much less hydrophilic Co­(III) and Co­(II) complexes of 4,4′-dinonyl-2,2′-bipyridyl, which makes it possible to extend the redox buffer approach to ionophore-based ISEs. For example, K⁺-selective electrodes based on the ionophore valinomycin exhibit electrode-to-electrode standard deviations as low as 0.7 mV after exposure of freshly prepared electrodes for 1 h to aqueous solutions. Exposure of freshly prepared ISE membranes to humidity prior to their first contact to electrolyte solution minimizes the initial (reproducible) emf drift. This redox buffer has also been successfully applied to sodium, potassium, calcium, hydrogen, and carbonate ion-selective electrodes, which all exhibit the high selectivity over interfering ions as expected for ionophore-doped ISE membranes

Capacitive Sensing of Glucose in Electrolytes Using Graphene Quantum Capacitance Varactors

A novel graphene-based variable capacitor (varactor) that senses glucose based on the quantum capacitance effect was successfully developed. The sensor utilizes a metal–oxide–graphene varactor device structure that is inherently compatible with passive wireless sensing, a key advantage for in vivo glucose sensing. The graphene varactors were functionalized with pyrene-1-boronic acid (PBA) by self-assembly driven by π–π interactions. Successful surface functionalization was confirmed by both Raman spectroscopy and capacitance–voltage characterization of the devices. Through glucose binding to the PBA, the glucose concentration in the buffer solutions modulates the level of electrostatic doping of the graphene surface to different degrees, which leads to capacitance changes and Dirac voltage shifts. These responses to the glucose concentration were shown to be reproducible and reversible over multiple measurement cycles, suggesting promise for eventual use in wireless glucose monitoring

Noncovalent Monolayer Modification of Graphene Using Pyrene and Cyclodextrin Receptors for Chemical Sensing

Surprisingly few details have been reported in the literature that help the experimentalist to determine the conditions necessary for the preparation of self-assembled monolayers on graphene with a high surface coverage. With a view to graphene-based sensing arrays and devices and, in particular, in view of graphene-based varactors for gas sensing, graphene was modified in this work by the π–π interaction-driven self-assembly of 10 pyrene and cyclodextrin derivatives from solution. The receptor compounds were pyrene, pyrene derivatives with hydroxyl, carboxyl, ester, ammonium, amino, diethylamino, and boronic acid groups, and perbenzylated α-, β-, and γ-cyclodextrins. Adsorption of these compounds onto graphene was quantified by contact-angle measurements and X-ray photoelectron spectroscopy. Data thus obtained were fitted with the Langmuir adsorption model to determine the equilibrium constants for surface adsorption and the concentrations of self-assembly solutions needed to form dense monolayers on graphene. The equilibrium constants of all pyrene derivatives fell into the range from 10^3.4 to 10^4.6 M^–1. For the perbenzylated α-, β-, and γ-cyclodextrins, the equilibrium constants are 10^3.24, 10^2.97, and 10^2.95 M^–1, respectively. Monolayers of 1-pyrenemethylammonium chloride on graphene were confirmed to be stable under heating to 100 °C in a high vacuum (2 × 10^–5 Torr)