Distance-Based Self-Powered Signal Transduction of Ion-Selective Electrodes to an Electronic Paper Display Array.

In this article, we report on the first distance-based readout self-powered potentiometric sensor. The approach is considered more user-friendly for detection by the naked eye and is less prone to optical interferences compared with a direct observation of the pixel darkening. pH-selective electrodes were chosen as a model system to demonstrate the principle in which seven bar-shaped pixels connected in series on one e-paper share one common ground. By connecting each of the pixels serially to capacitors of different capacitances, the fraction of the measurement cell voltage loaded onto the pixels becomes controllable. Consequently, the pixels give different gray values when powered by the same ion-selective electrode (ISE). As a result, the pH information on the sample is visualized as a distance-based signal and the dependence between the capacitance and 1/K (the reciprocal slope in the relationship between absorbance and pH) was constructed. In the current system, a 1 µF capacitance difference changes the value of 1/K by 4.18. With the current setup, the pH accuracy is about 0.5 when comparing the e-paper output to a color card.

Self-Powered Potentiometric Sensor Based on a Passive Signal Amplifier with Electronic Paper Display.

Self-powered potentiometric sensors are attractive because of their simple operation, low cost, fast response, and ability to be integrated with electronic components. Self-powered potentiometric sensors that give a direct colorimetric output are especially interesting, because no power supply is needed, which dramatically reduces waste. Recently reported work from our group using an electronic paper display, however, exhibits limitations, because the visualization of small pH changes is difficult. A self-powered ion-selective potentiometric sensor is introduced here that may amplify the e-paper pixel sensitivity by improving the self-powered circuit. The voltage is amplified by changing the circuit from incorporating parallel to incorporating serial capacitors. With three such capacitors, a greatly improved sensitivity is observed, amplifying the absorbance 3-fold. A portable device is realized that changes the position of the capacitors from parallel to serial through a simple mechanical sliding action. As a result, the pH information on the sample is more easily visualized with a pH uncertainty of about 0.1 when comparing the e-paper output to a color card.

Direct Energy Transfer from a pH Glass Electrode to a Liquid Crystal Display.

Self-powered sensors are attractive because the lack of a dedicated battery makes them environmentally friendly and allows them to be more easily miniaturized. Unfortunately, the development of self-powered potentiometric sensors is challenging because only very limited energy can be harvested from this measurement principle. For the first time, the potential of a high impedance glass pH electrode (130 M Ω) is shown here to be directly read out optically. This is accomplished by a liquid crystal display (LCD) as the electrochromic transducer, which changes its transmission upon imposing an external voltage in the range of 2-3 V. Importantly, owing to its low capacitance of about 50 pF, this process requires a very small transient charge on the order of 100 pC, which may be spontaneously imposable even across pH glass electrodes. For the LCD to be turned on, the cell voltage is boosted by additional Zn2+/Zn elements placed in series. The LCD is found to give a time-dependent absorbance decrease, which is mitigated by adding a high resistance element to attenuate the associated decay. The approach gives repeatable LCD absorbance values that allows one to directly visualize pH with a precision of about 0.01 pH units. The absorbance value depends inversely on pH in a much wider range (pH 1-13) than what is normally observed with optical sensors while based on the same underlying measurement as a potentiometric pH probe.

Self-Powered Signal Transduction of Ion-Selective Electrodes to an Electronic Paper Display.

Mobile integrated electrochemical sensors normally require a power supply for operation. Unfortunately, the practice of discarding batteries associated with these devices runs counter to our desire for a sustainable world. Self-powered sensing concepts that draw the energy directly from the measurement itself would overcome this limitation. Potentiometric sensors for the measurement of pH, many electrolytes, and gases are ubiquitous in analytical practice. However, in potentiometry, the voltage is acquired in the absence of current flow, making it seemingly impossible to draw power. Fortunately, it has been recently established that transient currents may be tolerated across potentiometric measurement cells to charge a capacitive or electrochromic element such as Prussian blue integrated in the measurement cell and whose absorbance then directly follows the potential changes in a reversible manner. We have shown here that commercial electronic paper (e-paper), widely used to make electronic ink and ebook readers, can directly be driven by a potentiometric measurement cell in a reversible manner at mild potentials of >100 mV typical for such sensors. The capacitance of the e-paper pixel studied here was found to be 0.53 µF mm-2, 30 times smaller than that of Prussian blue films. The colorimetric absorbance of the e-paper was also more stable (observed drift over 2 h corresponding to 0.76 mV h-1) and reproducible (corresponding to 1 mV standard deviation). The e-paper pixel was directly driven by a polymeric pH electrode as a model system. Choosing a basic inner solution (pH 12.9) behind the membrane gave sufficiently positive cell potentials for driving visible absorbance change in a sample pH range of 4-10, while a more acidic pH of 3.4 and alternating the connections to the e-paper were more suited for more basic samples of pH > 10. This convenient and cost-effective approach makes it possible to directly drive an optical display from the potentiometric measurement itself and should be suitable for moderate sensing membrane resistances of less than about 100 kΩ, depending on the area of the chosen pixel.

Colorimetric and fluorescent turn-on detection of chloride ions with ionophore and BODIPY: Evaluation with nanospheres and cellulose paper.

Here, we present the optical detection of chloride ions with co-extraction based anion-selective optodes containing a BODIPY derivative (BDP-OH) and the ionophore In(OEP)Cl. Spectroscopic studies suggested that BDP-OH and In(OEP)Cl formed an adduct BDP-O-In(OEP), which was converted to BDP-OH and In(OEP)Cl upon increasing sample Cl- concentration, and induced signal changes in both fluorescence and absorbance modes. The method was evaluated in polystyrene-graft-poly(ethylene oxide) (PS-PEO) nanospheres (ca. 40 nm in diameter) and on cellulose paper. In contrast to Cl- probes based on fluorescence quenching, the nanospheres exhibited turn-on fluorescence and ratiometric absorbance responses to a tunable Cl- concentration range (10 µM-1 M). Through fluorescence microscopy, the nanospheres were able to respond to Cl- concentration changes in HeLa cells. Cellulose paper-based Cl- optodes with colorimetric response were successfully used to measure Cl- in artificial sweat, providing a potential analytical tool for clinical diagnosis of cystic fibrosis.

Graphene Quantum Dots Integrated in Ionophore-Based Fluorescent Nanosensors for Na+ and K.

To enrich the recipes of ion-selective nanosensors, graphene quantum dots (GQDs) were integrated into ionophore-based fluorescent nanosensors with exquisite selectivity and high sensitivity for Na+ and K+. The unique property of GQDs gave the nanosensors ultrasmall size (ca. 10 nm), high brightness, good biocompatibility, and potential pH sensing possibility. At pH 7.4, the sensors exhibited a detection range from 0.1 mM to 1 M for Na+ and from 3 µM to 1 mM for K+. The nanosensors were successfully applied to blood serum and urine samples. Chemically induced intracellular sodium concentration change in HeLa cells was also qualitatively monitored.