Aerosol-into-liquid capture and detection of atmospheric soluble metals across the gas-liquid interface using Janus-membrane electrodes.

The soluble fraction of atmospheric transition metals is particularly associated with health effects such as reactive oxygen species compared to total metals. However, direct measurements of the soluble fraction are restricted to sampling and detection units in sequence burdened with a compromise between time resolution and system bulkiness. Here, we propose the concept of aerosol-into-liquid capture and detection, which allowed one-step particle capture and detection via the Janus-membrane electrode at the gas-liquid interface, enabling active enrichment and enhanced mass transport of metal ions. The integrated aerodynamic/electrochemical system was capable of capturing airborne particles with a cutoff size down to 50 nm and detecting Pb(II) with a limit of detection of 95.7 ng. The proposed concept can pave the way for cost-effective and miniaturized systems, for the capture and detection of airborne soluble metals in air quality monitoring, especially for abrupt air pollution events with high airborne metal concentrations (e.g., wildfires and fireworks).

Increasing the Sensitivity of pH Glass Electrodes with Constant Potential Coulometry at Zero Current.

It has recently become possible to increase the sensitivity of ion-selective electrodes (ISEs) by imposing a constant cell potential, allowing one to record current spikes with a capacitor placed in series in the circuit. The approach requires a transient current to pass through the measurement cell, which unfortunately may introduce measurement errors and additionally excludes the use of high-impedance indicator electrodes, such as pH glass electrodes. We present here an electronic circuit that overcomes these limitations, where the cell is measured at zero current in combination with a voltage follower, and the current spike and capacitor charging occur entirely within the instrument. The approach avoids the need for a counter electrode, and one may use any electrode useful in potentiometry regardless of its impedance. The characteristics of the circuit were found to approach ideality when evaluated with either an external potential source or an Ag/AgCl electrode. The current may be linearized and extrapolated to further reduce the measurement time. The circuit is further tested with the most common yet very challenging electrode, the pH glass electrode. A precision of 64 µpH was obtained for 0.01 pH change up to 0.05 from a reference solution. Similar pH changes were also measured reliably further away from the reference solution (0.5-0.55) and resulted in a precision of 377 µpH. The limitations of this experimental setup were explored by performing pH calibrations within the measuring range of the probe.

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.

On-Chip Antifouling Gel-Integrated Microelectrode Arrays for In Situ High-Resolution Quantification of the Nickel Fraction Available for Bio-Uptake in Natural Waters.

We aimed to monitor in situ nickel (Ni(II)) concentrations in aquatic systems in the nanomolar range. To achieve this, we investigated whether an analytical protocol for the direct quantification of cobalt (Co(II)) using adsorptive cathodic sweep voltammetry (Ad-CSV) on antifouling gel-integrated microelectrode arrays (GIME) we recently developed is also suitable for direct Ni(II) quantification. The proposed protocol consists of the reduction of the complex formed between Ni(II) (or Ni(II) and Co(II)) and nioxime adsorbed on the surface of the GIME-sensing element. The GIME enables to (i) avoid fouling, (ii) control the metal complex mass transport and, when interrogated by Ad-CSV, (iii) selectively determine the dynamic (kinetically labile Ni-nioxime) fraction that is potentially bioavailable. The nioxime concentration and pH were optimized. A temperature correction factor was determined. The limit of detection established for 90 s of accumulation time was 0.43 ± 0.06 in freshwater and 0.34 ± 0.02 nM in seawater. The sensor was integrated in a submersible probe in which the nioxime-containing buffer and the sample were mixed automatically. In situ field measurements at high resolution were successfully achieved in Lake Geneva during a diurnal cycle. The determination of the kinetically labile Ni-nioxime fraction allows one to estimate the potential ecotoxicological impact of Ni(II) in Lake Geneva. Additional Ni fractions were measured by ICP-MS and coupled to the in situ Ad-CSV data to determine the temporal Ni(II) speciation.

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.

Solid-Contact Potentiometric Cell with Symmetry.

By its nature, a traditional potentiometric cell composed of an Ag/AgCl-based reference electrode and a solid-contact indicating electrode is not symmetric. This results in undesirable potential drifts in response to a common perturbation such as a temperature change of the sample. We propose here an approach to restore symmetry by constructing a cell with two identical solid-contact ISEs used as reference and indicating electrodes. In this arrangement, the reference electrode is immersed in a compartment containing a constant background of an ion of interest, while the indicating electrode is directly immersed in the sample solution. This approach was successfully demonstrated for a cell composed of nitrate-selective electrodes with the hydrophobic derivative of poly(3,4-ethylenedioxythiophene) as a transducer layer. In particular, the symmetric setup is shown to lower by 4-5 times the observed potential drift resulting from temperature changes between +25 and +5 °C.

Unconditioned Symmetric Solid-Contact Electrodes for Potentiometric Sensing.

In potentiometric sensing, the preparation of the electrodes preceding a measurement is often the most time-consuming step. Eliminating the conditioning process can significantly speed up the preparation procedure, but it can also compromise the need for proper pre-equilibration of the membrane. We propose here a symmetric setup to address this challenge with an identical indicator and reference elements measured against each other, thereby compensating for potential drift. This strategy allows one to achieve potentiometric measurements using non-conditioned all-solid-state ion-selective electrodes for the detection of nitrate and potassium ions with Nernstian response slopes and detection ranges identical to those of conventional systems. To establish symmetry, a set of solid-contact ion-selective electrodes placed in a reference cell is measured against a set of identical electrodes in a sample cell. By subtracting the potentials between the two cells, potential instabilities not directly relevant to the measuring sample are eliminated, giving minimal potential drifts and stable 5-day potential responses. The E0 value of the nitrate-selective electrodes in the symmetric setup had a standard deviation of just 3 mV for the 5-day period in contrast to 19 mV in the asymmetric system, clearly demonstrating the influence of the conditioning step which is almost eliminated in the former system. During the 20 h potential monitoring experiments, the drift dropped to below 0.3 mV/min in less than 6 min, as opposed to an average time of 35 min for the asymmetric system. The applicability of the proposed setup was successfully demonstrated with the measurement of nitrate in a river water sample, where a potential drift lower than 0.1 mV/min was reached in less than 5 min of first contact with solution.

Speciation of Cu, Cd, Pb and Zn in a contaminated harbor and comparison to environmental quality standards.

The water column of harbors contains significant amounts of (priority) hazardous trace metals that may be released into coastal areas of high societal and economic interests where they may disturb their fragile equilibria. To deepen our understanding of the processes that influence the transport of the various metal fractions and allow for a more rigorous environmental risk assessment, it is important to spatially monitor the relevant chemical speciation of these metals. It is of particular interest to assess their so-called dynamic fraction, which comprises the dissolved chemical forms that are potentially bioavailable to living organisms. In this study this was achieved in the Genoa Harbor (NW Italy) for copper (Cu), lead (Pb), cadmium (Cd) and zinc (Zn) by applying a multi-method approach. For the first time in this system the dynamic fractions of the target metals (CuDyn, CdDyn, PbDyn, ZnDyn) were observed in real-time on-board by voltammetry using innovative electrochemical sensing devices. Trace metals in the operationally defined dissolved <0.2 µm and <0.02 µm fractions were equally quantified through sampling/laboratory-based techniques. The obtained results showed a clear spatial trend for all studied metals from the enclosed contaminated part of the harbor towards the open part. The highest CuDyn and CdDyn fractions were found in the inner part of the harbor while the highest PbDyn fraction was found in the open part. The proportion of ZnDyn was negligible in the sampled area. Small and coarse colloids were involved in Cu, Cd and Zn partitioning while only coarse colloids played an important role in Pb partitioning. The determined concentrations were compared to the Environmental Quality Standards (EQS) established by the EU and those determined by the Australia and New Zealand to trigger for 99 and 95% species protection values. The results of this work allow us to highlight gaps in the EQS for which metal concentration thresholds are excessively high or non-existent and should urgently be revised. They also reflect the need to quantify the potentially bioavailable fraction of hazardous trace metals instead of just their total dissolved concentrations. The data support the establishment of environmental quality standards and guidelines based on realistic risk assessment to protect aquatic life and resources and ultimately human health.

Hyperpolarized Solvatochromic Nanosensors towards Heparin Sensing in Blood.

Heparin quantification at the point of care has been of medical interest for years but a suitable point of care measurement method for whole blood is still elusive. Our group has recently developed a nanoparticle-based optical sensor for protamine that allows for heparin quantification in plasma. This work discusses the effect of the transducing-dye structure and the promise of embedding the sensors in an agarose gel for avoiding red blood cell interference.

The risk of tooth loss in patients with diabetes: A systematic review and meta-analysis.

AIM: The aim of this systematic review was to comprehensively and critically summarize and synthesize the risk of losing teeth among with diabetes mellitus (DM) compared to those without DM, as established in observational studies. MATERIALS AND METHODS: MEDLINE-PubMed and Cochrane databases were searched through a period from their inception through October 2020 to identify eligible studies. Papers that primarily evaluate the number of teeth in DM patients compared to non-DM individuals were included. A descriptive analysis of the selected studies was conducted, and when feasible, a meta-analysis was performed. The quality of the studies was assessed. RESULTS: A total of 1087 references were generated, and screening of the papers resulted in 10 eligible publications. A descriptive analysis demonstrated that six of these studies indicate a significantly higher risk of tooth loss in DM patients. This was confirmed by the meta-analysis risk ratio of 1.63 95% CI (1.33; 2.00, p < 0.00001). Subgroup analysis illustrates that this is irrespective of the risk-of-bias assessment. The higher risk of tooth loss in DM patients was also higher when only DM type II patients or studies with a cross-sectional design were considered. Patients with a poor DM control status presented a significantly increased risk of tooth loss. When the data were separated by the world continent where the study was performed, Asia and South America had numerically higher risks and a 95% CI that did not overlap with Europe and North America. CONCLUSION: There is moderate certainty for a small but significantly higher risk of tooth loss in DM patients as compared to those without DM.

Chemo- and Regioselective Multiple C(sp2 )-H Insertions of Malonate Metal Carbenes for Late-Stage Functionalizations of Azahelicenes.

Chiral quinacridines react up to four times, step-by-step, with α-diazomalonates under RuII and RhII catalysis. By selecting the catalyst, [CpRu(CH3 CN)3 ][PF6 ] (Cp=cyclopentadienyl) or Rh2 (oct)4 , chemo and regioselective insertions of derived metal carbenes are achieved in favor of mono- or bis-functionalized malonate derivatives, respectively, (r.r.>49 : 1, up to 77 % yield, 12 examples). This multi-introduction of malonate groups is particularly useful to tune optical and chemical properties such as absorption, emission or Brønsted acidity but also cellular bioimaging. Density-functional theory further elucidates the origin of the carbene insertion selectivity and also showcases the importance of conformations in the optical response.

Self-Powered Electrochromic Readout of Potentiometric pH Electrodes.

An absorbance-based colorimetric sensor array that is self-powered by an ion-selective electrode (ISE) in a short-circuited cell is presented. As the cell voltage is maintained at zero, the potential at the ISE serves as the power generator to directly transfer its power to a potential-dependent Prussian blue (PB) film in contact with an electrolyte solution in a separate detection compartment. This allows one to activate the color change of the PB film without the need for an external power supply. The potential of the PB detection element is optimized to change color between 50 and 250 mV (vs Ag/AgCl). Because the potential originates at the ISE, it is proportional to the ion activity in the sample in agreement with the Nernst equation. In this way, a higher cation activity in the sample generates a more positive potential, which enhances the PB absorbance that serves as the analytical signal. A self-powered optical sensor array coupled to poly(vinyl-chloride)-based pH electrodes based on two different ionophores is utilized here as a model. The measuring range is tuned chemically by varying the pH of the inner filling solution of each ISE, giving a measuring range from pH 2 to 10.5. As the optical sensor is driven by a potentiometric probe, the sensor output is independent of solution ionic strength. It is successfully applied for quantitative analysis in unmodified turbid/colored samples that included red wine, coke, coffee, baking soda, and antacid. The colorimetric output correlates well with the reference method, a calibrated pH electrode. Compared to earlier systems where the cell potential is dictated by an external power source, the PB film exhibits excellent reproducibility and a rapid response time of about 44 s.

Surfactants for Optode Emulsion Stabilization without Sacrificing Selectivity or Binding Constants.

We compare here the effect of surfactants on ion-selective membranes measured via voltammetry and optode emulsions measured optically. Cyclic voltammetry on a thin-film ion-selective membrane is shown to be a useful screening technique for the estimation of effective complex formation constants and selectivity coefficients for different surfactants with various cations. This technique is particularly useful for its ability to identify separate ion-transfer events (free, surfactant complexed, ionophore complexed) for a specific membrane. However, we also caution against the over-reliance on this technique as changes in membrane characteristics are observed following surfactant partitioning. Of the surfactants explored here, a zwitterionic sulfobetaine-based surfactant was found to stabilize sensors without reducing effective binding constants and selectivity, with greatly superior characteristics to other commonly utilized surfactants. Those include Brij-35, F-127, and Triton X-100, all of which showed significant binding to so-called free ions in the membrane, resulting in peak potential shifts of 199 ± 10, 180 ± 24, 278 ± 11 mV, respectively, for potassium following the subtraction of transducing layer effects. This peak shift translated to a much larger undesired free ion response in optode emulsions. The selectivity in emulsion-based systems was also shown to decrease in the presence of nonionic surfactants compared to that containing the zwitterion.

Ion-to-electron capacitance of single-walled carbon nanotube layers before and after ion-selective membrane deposition.

The capacitance of the ion-to-electron transducer layer helps to maintain a high potential stability of solid-contact ion-selective electrodes (SC-ISEs), and its estimation is therefore an essential step of SC-ISE characterization. The established chronopotentiometric protocol used to evaluate the capacitance of the single-walled carbon nanotube transducer layer was revised in order to obtain more reliable and better reproducible values and also to allow capacitance to be measured before membrane deposition for electrode manufacturing quality control purposes. The capacitance values measured with the revised method increased linearly with the number of deposited carbon nanotube-based transducer layers and were also found to correlate linearly before and after ion-selective membrane deposition, with correlation slopes close to 1 for nitrate-selective electrodes, to 0.7 and to 0.5 for potassium- and calcium-selective electrodes.

Potentiometric Sensor Array with Multi-Nernstian Slope.

The sensitivity of potentiometric sensors functioning in equilibrium mode is limited by the value predicted according to the Nernst equation and inversely proportional to the charge in the analyte ion. Therefore, an increased ion charge results in a dramatic decrease in the sensor sensitivity. We propose an approach to allow one to increase the sensitivity of the potentiometric measurements by using a combined electrochemical cell composed of several identical ion-selective electrodes immersed into separate sample solutions of equal composition. The combination of n electrodes, demonstrating individually a Nernstian slope in one electrochemical cell allows to amplify the signal and associated response slope by n times. The proposed approach is shown to provide a double and triple Nernstian slope for potassium-, calcium-, nitrate-, and carbonate-selective electrodes by combining two or three identical electrodes, correspondingly. Each ion-selective electrode functions in an equilibrium mode, hence, ensuring response stability and reproducibility.

Optical Sensing with a Potentiometric Sensing Array by Prussian Blue Film Integrated Closed Bipolar Electrodes.

The simultaneous optical readout of a potentiometric sensor array of ion-selective electrodes (ISEs) based on PVC membranes is described here for the first time. The optical array consists of electrochromic Prussian Blue (PB) films in multiple closed ion-selective bipolar electrodes (BPEs), which gives a physical separation between the optical detection and sample compartments. The potential-dependent turnover of PB generates Prussian White (PW). A near-Nernstian response of the PB film is confirmed by colorimetric absorbance experiments as a function of applied potential. In the combined bipolar electrode cell, the overall potential is kept constant with a single potentiostat over the entire array where each PB spot indicates the potential change of an individual connected potentiometric probe. For cation-selective electrodes, the absorbance or blue intensity of the connected PB film is enhanced with increasing target cation activity. The colorimetric absorbance changes are simultaneously followed by a digital camera and analyzed by Mathematica software. A multiple cation-BPE array allows one to achieve simultaneous quantitative analysis of potassium, sodium, and calcium ions, demonstrated here in highly colored fruit juices. Mass transport at the PB thin film is shown not to be rate-limiting. The measuring ranges can be tuned in a wide range by potential control. The PB film exhibits greatly improved reproducibility and stability as compared to previous work with a ferroin redox probe confined in a thin solution layer.

Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor.

A constant potential capacitive readout of solid-contact ion-selective electrodes (SC-ISE) allows one to obtain easily identifiable current transients that can be integrated to obtain a charge vs logarithmic activity relationship. The resulting readout can therefore be much more sensitive than traditional open-circuit potentiometry. Unfortunately, however, comparatively long measurement times and significant baseline current drifts make it currently difficult to fully realize the promise of this technique. We show here that this challenge is overcome by placing the SC-ISE in series with an electronic capacitor, with pH probes as examples. Kirchhoff's law is shown to be useful to choose an adequate range of added capacitances so that it dominates the overall cell value. Two different ion-to-electron transducing materials, functionalized single-wall carbon nanotubes (f-SWCNTs) and poly(3-octylthiophene) (POT), were explored as solid-contact transducing layers. The established SC-ISE-based f-SWCNT transducer is found to be compatible with a wide range of external capacitances up to 100 µF, while POT layers require a narrower range of 1-4.7 µF. Importantly, the time for a charging transient to reach equilibrium was found to be less than 10 s, which is dramatically faster than without added electronic component. Owing to the ideal behavior of capacitor, the response current decays rapidly to zero, making the determination of the integrated charge practically applicable.

Emulsion Doping of Ionophores and Ion-Exchangers into Ion-Selective Electrode Membranes.

Ion-selective electrodes (ISEs) are widely used analytical devices to selectively measure ionic species. Despite significant advances in recent years, ion-selective membranes are still mostly prepared in the same manner, by preloading the selective components into a solvent that is subsequently cast into a membrane or film. This paper describes an alternative method to prepare ISE membranes by mass transfer of the sensing components from an emulsion phase. Specifically, blank (undoped) plasticized poly(vinyl chloride) (PVC) membranes mounted into an electrode body are immersed into an aqueous solution containing analyte ions and an appropriate emulsion of the desired sensing components to allow their transfer into the membrane. The concept is demonstrated with conventional membrane electrodes containing an inner solution as well as all-solid-state electrodes. It is shown to be universally useful for the realization of ISEs for K+, Na+, Ca2+, and NO3-.

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design.

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading. Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities. We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion-ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO2 in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane. In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.