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.

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.

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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 costeffective 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.

We report here a chemical nose utilizing the nonspecificity of a hemicyanine dye (probe P) containing three acetates and one deprotonatable phenol groups. Unlike conventional pattern-based recognition that requires a combination of different probes, probe P alone is able to differentiate 9 different metals, generating distinctive absorption spectra and various patterns upon principle component analysis (PCA). River water samples and commercial mineral water samples were evaluated by the probe and were successfully distinguished.