Redox Buffer Capacity of Ion-Selective Electrode Solid Contacts Doped with Organometallic Complexes.

While ion-selective electrodes (ISEs) with inner filling solutions are used widely, solid-contact ISEs are better suited for miniaturization and mass manufacturing. Calibration-free measurements with such electrodes require the reproducible control of the phase boundary potential between the ion-selective membrane and the underlying electron conductor. The most promising approach to achieve this goal is based on redox buffers incorporated into the ion-selective membrane. Here we introduce the theory and present experimental data for Co(III), Co(II), Ru(II), Fe(II), and Os(II) compounds that show quantitatively how the phase boundary potential at a solid contact doped with redox-active compounds is affected by weighing errors, reagent impurities, and redox-active interferents. Perhaps surprisingly, theory predicts that there is only a minimal dependence of the phase boundary potential on the ratio of the concentrations of a pure oxidized and a pure reduced compounds if those two compounds are not a redox couple. However, theory predicts that even small redox-active impurities of those compounds shift the phase boundary potential drastically. Experimentally, a surprisingly good in-batch reproducibility was observed by us and others for solid contacts prepared to contain either only the reduced or only the oxidized species of a redox couple. This can be explained by redox-active impurities and is unlikely to be repeatable when different suppliers of reagents are used or long-term experiments are performed. This work confirms that the preferred approach to calibration-free sensing is based on redox buffers that comprise the reduced and oxidized species of a redox couple in well-controlled concentrations.

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.

Machine Learning-Based Rapid Detection of Volatile Organic Compounds in a Graphene Electronic Nose.

Rapid detection of volatile organic compounds (VOCs) is growing in importance in many sectors. Noninvasive medical diagnoses may be based upon particular combinations of VOCs in human breath; detecting VOCs emitted from environmental hazards such as fungal growth could prevent illness; and waste could be reduced through monitoring of gases produced during food storage. Electronic noses have been applied to such problems, however, a common limitation is in improving selectivity. Graphene is an adaptable material that can be functionalized with many chemical receptors. Here, we use this versatility to demonstrate selective and rapid detection of multiple VOCs at varying concentrations with graphene-based variable capacitor (varactor) arrays. Each array contains 108 sensors functionalized with 36 chemical receptors for cross-selectivity. Multiplexer data acquisition from 108 sensors is accomplished in tens of seconds. While this rapid measurement reduces the signal magnitude, classification using supervised machine learning (Bootstrap Aggregated Random Forest) shows excellent results of 98% accuracy between 5 analytes (ethanol, hexanal, methyl ethyl ketone, toluene, and octane) at 4 concentrations each. With the addition of 1-octene, an analyte highly similar in structure to octane, an accuracy of 89% is achieved. These results demonstrate the important role of the choice of analysis method, particularly in the presence of noisy data. This is an important step toward fully utilizing graphene-based sensor arrays for rapid gas sensing applications from environmental monitoring to disease detection in human breath.

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.