Direct Optical Carbon Dioxide Sensing Based on a Polymeric Film Doped with a Selective Molecular Tweezer-Type Ionophore

Reversible Hydrophobic Deep Eutectic Solvent-Based Uranyl-Sensing Optode Film in Aqueous Streams: Color Transformation and Reusability

A hydrophobic deep eutectic solvent (HDES)-based optode was designed for the preconcentration and determination of the UO22+ ion in aqueous media using spectroscopic techniques [energy-dispersive X-ray fluorescence (EDXRF) and solid-state absorption]. The optode was developed by incorporation of HDES (tri-n-octyl phosphine oxide and decanoic acid in an equimolar ratio), tri-(2-ethylhexyl) phosphate, and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol into a cellulose triacetate matrix. Characterization studies were carried out using different techniques to understand the roles of HDES as a plasticizer, UO22+ extractant, and Br-PADAP immobilizer. Uptake studies revealed that the optimal pH was 3 and sorption followed the type II adsorption isotherm. Uranium in the U-sorbed optode can be directly analyzed over a large concentration range of 0.021 × 10-3-2.1 × 10-3 Mol L-1 using EDXRF. The optode film exhibited a linear dynamic range of 0.84 × 10-6-84 × 10-6 Mol L-1 for uranium, with a lowest limit of detection of 0.084 × 10-6 Mol L-1 by colorimetric analysis. This optode-based method was employed for seawater analysis for its UO22+ concentration without any matrix separation, and the concentration was found to be 1.30 ± 0.06 × 10-8 Mol L-1. The optode exhibited better selectivity for UO22+ in the presence of various cations including Sr2+ and Cs+ in an aqueous medium. Compared to other prevailing optical sensors, this optode performed better in terms of key factors like pH, equilibration time, reusability, and detection limit.

Direct potentiometric bicarbonate/carbon dioxide sensing based on polymeric membranes doped with selective meso-bisubstituted calix[4]pyrrole ionophores

We evaluated in this work the properties of a promising class of HCO3- ionophores, which have not been recognized previously. Three types of neutral or charged calix[4]pyrroles with meso-bisubstituted groups were evaluated as ionophores for polymeric membrane HCO3- selective electrodes. Optimizing membrane components, such as ionophores, lipophilic additives and plasticizers, yielded ISEs exhibiting Nernstian response to HCO3- with improved linear range and detection limits, while the selectivity sequence differs significantly from the Hofmeister series. Interference from important biological and environmental species was reduced significantly, especially that from SCN-, NO3-, Br- and Cl-, which are always at high concentrations in related samples. In order to provide more insights into the properties of the ionophores and performance of the proposed ISEs, the stability constants of anion-ionophore complexes in the membrane phase were determined. Studies on the influence of the sample solution pH demonstrated that the proposed ISEs can be employed in a wide pH range of 3.0-9.0 with fast response (<30 s), good reversibility and long shelf life. Moreover, the proposed ISEs were used to quantify the concentration of HCO3- and dissolved CO2 in mineral and beverage samples with good recoveries.

Nanoscale Carbonate Ion-Selective Amperometric/Voltammetric Probes Based on Ion-Ionophore Recognition at the Organic/Water Interface: Hidden Pieces of the Puzzle in the Nanoscale Phase

Here, we report on the successful demonstration and application of carbonate (CO₃^2-) ion-selective amperometric/voltammetric nanoprobes based on facilitated ion transfer (IT) at the nanoscale interface between two immiscible electrolyte solutions. This electrochemical study reveals critical factors to govern CO₃^2--selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO₃^2-, i.e., slow dissolution of lipophilic ionophores in the organic phase, activation of hydrated ionophores, peculiar solubility of a hydrated ion-ionophore complex near the interface, and cleanness at the nanoscale interface. These factors are experimentally confirmed by nanopipet voltammetry, where a facilitated CO₃^2- IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO₃^2-ionophore (CO₃^2-ionophore VII) by voltammetrically and amperometrically sensing CO₃^2- in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO₃^2- ionophore VII-facilitated ITs (FITs) follows the one-step electrochemical (E) mechanism controlled by both water-finger formation/dissociation and ion-ionophore complexation/dissociation during interfacial ITs. The yielded rate constant, k⁰ = 0.048 cm/s, is very similar to the reported values of other FIT reactions using ionophores forming non-covalent bonds with ions, implying that a weak binding between CO₃^2- ion-ionophore enables us to observe FITs by fast nanopipet voltammetry regardless of the nature of bondings between the ion and ionophore. The analytical utility of CO₃^2--selective amperometric nanoprobes is further demonstrated by measuring the CO₃^2- concentration produced by metal-reducing bacteria Shewanella oneidensis MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as H₂PO₄^-, Cl^-, and SO₄^2-.

Recent improvements to the selectivity of extraction-based optical ion sensors

Optical sensors continue to demonstrate tremendous potential across a wide range of applications due to their high versatility and low cost. This feature article will focus on a number of recent advances made in improving the performance of extraction-based optical ion sensors within our group. This includes the progress of anchored solvatochromic transduction to provide pH and sample volume independent optical responses in nanoemulsion-based sensors. A recent breakthough is in polyion sensing in biological fluids that uses a novel indirect transduction mechanism that significantly improves the selectivity of dinonylnaphthalenesulfonate-based protamine sensors and its potential applications beyond polyion sensing. The role of particle stabilizers in relation to the response of emulsified sensors is shown to be important. Current challenges in the field and possible opportunities are also discussed.

Selectivity remains a constant challenge in the development of optical extraction-based sensors. Fortunately, there are several mechanistic and compositional changes with the potential to improve selectivity without developing new ionophores.

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.

Synthesis and application of a new class of D–π–A type charge transfer probe containing imidazole – naphthalene units for detection of F− and CO2

A new class of D–π–A type charge transfer probe, 3 and 4, containing imidazole – naphthalene moieties as donor and acceptor, respectively, has been synthesized via a Suzuki coupling reaction.

An Artificial CO2 -Driven Ionic Gate Inspired by Olfactory Sensory Neurons in Mosquitoes

A novel CO₂ -driven ionic gate, mimicking the function of olfactory sensory neurons of mosquitoes, is successfully developed by functionalizing the walls of the nanochannels using 1-(4-amino-phenyl)-2,2,2-trifluoro-ethanone. This artificial nanochannel can switch between the ON-state and OFF-state in the presence and absence of CO₂ , with an ultrahigh gating ratio of up to 1250, and has potential applications in CO₂ -related sensing, gating, and nanofluidic systems.

Off-On-Off fluorescence behavior of an intramolecular charge transfer probe toward anions and CO2

The photophysical behavior of a newly developed fluorescent probe, tricyanoethylphenyl phenanthroimidazole (TCPPI) has been studied. Upon interaction of different class of anions TCPPI displayed naked-eye sensitive fluorescence "turn-on" response to detect selectively F(-) (0.98μM, 18.62ppb) and CN(-) (1.12μM, 29.12ppb) anions in acetonitrile (MeCN). Job's plot analysis revealed a 1:1 binding stoichiometry between probe and anions. The spectral data analysis and 1H NMR titration studies suggested about the affinity of F(-) and CN(-) anions with moderately acidic -NH fragment of imidazolyl unit of probe through deprotonation and H-bonding interaction. Moreover, the anion activated probe upon interaction with CO2 revived photophysical properties of probe, "On-Off-On" type fluorescence and enabled anion-induced CO2 sensing in the medium.

An efficient ICT based fluorescent turn-on dyad for selective detection of fluoride and carbon dioxide

A new intramolecular charge transfer based fluorescent turn-on ratiometric probe exhibiting high sensitivity for F⁻ and CO₂.

Bis[alkynylplatinum(ii)] terpyridine molecular tweezer with conformationally-rigid spacer: modulating the binding selectivity in a three-component supramolecular recognition system

Modulation of binding selectivity in a three-component molecular tweezer/guest recognition system.

Ionophore-based ion-selective optical nanosensors operating in exhaustive sensing mode

Ion selective optical sensors are typically interrogated under conditions where the sample concentration is not altered during measurement. We describe here an alternative exhaustive detection mode for ion selective optical sensors. This exhaustive sensor concept is demonstrated with ionophore-based nanooptodes either selective for calcium or the polycationic heparin antidote protamine. In agreement with a theoretical treatment presented here, linear calibration curves were obtained in the exhaustive detection mode instead of the sigmoidal curves for equilibrium-based sensors. The response range can be tuned by adjusting the nanosensor loading. The nanosensors showed average diameters of below 100 nm and the sensor response was found to be dramatically faster than that for film-based optodes. Due to the strong binding affinity of the exhaustive nanosensors, total calcium concentration in human blood plasma was successfully determined. Optical determination of protamine in human blood plasma using the exhaustive nanosensors was attempted, but was found to be less successful.

Ionophore-based optical sensors

This review provides an overview of the key aspects of designing ionophore-based optical sensors (IBOS). Exact response functions are developed and compared with a simplified, generalized equation. We also provide a brief introduction into less established but promising working principles, namely dynamic response and exhaustive exchange. Absorbance and fluorescence are the main optical readout strategies used in the evaluation of a sensor response, but they usually require a robust referencing technique for real-world applications. Established referencing schemes using IBOS as well as those from other optical sensors are also discussed. Finally, the power of recently developed photoresponsive ion extraction/release systems is outlined and discussed in view of dynamically switchable IBOS or regenerative exhaustive exchange IBOS.

A simple BODIPY-aniline-based fluorescent chemosensor as multiple logic operations for the detection of pH and CO2gas

4-Aniline BODIPY dye was developed as a highly sensitive fluorescent chemosensor for the detection of pH and CO₂gas.