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.

Photoacoustic Chemical Imaging Sodium Nano-Sensor Utilizing a Solvatochromic Dye Transducer for In Vivo Application

Sodium has many vital and diverse roles in the human body, including maintaining the cellular pH, generating action potential, and regulating osmotic pressure. In cancer, sodium dysregulation has been correlated with tumor growth, metastasis, and immune cell inhibition. However, most in vivo sodium measurements are performed via Na23 NMR, which is handicapped by slow acquisition times, a low spatial resolution (in mm), and low signal-to-noise ratios. We present here a plasticizer-free, ionophore-based sodium-sensing nanoparticle that utilizes a solvatochromic dye transducer to circumvent the pH cross-sensitivity of most previously reported sodium nano-sensors. We demonstrate that this nano-sensor is non-toxic, boasts a 200 μM detection limit, and is over 1000 times more selective for sodium than potassium. Further, the in vitro photoacoustic calibration curve presented demonstrates the potential of this nano-sensor for performing the in vivo chemical imaging of sodium over the entire physiologically relevant concentration range.

Ion-modulated interfacial fluorescence in droplet microfluidics using an ionophore-doped oil

Fluorescence at the oil-water interface is used for chemical sensing in droplet microfluidics. Potassium ions in aqueous droplets are extracted into oil segments doped with an ionophore, a cation exchanger, and a cationic dye to expel the dye. When a low concentration of dye with a balanced solubility is used, it actively accumulates at the thin interface between oil and water instead of getting dissolved in the aqueous phase. The interfacial fluorescence is monitored distinct from the fluorescence in the oil sensor and the aqueous sample, allowing for highly sensitive and selective turn-on fluorescence sensing of ions.

Perspective on fluorescence cell imaging with ionophore-based ion-selective nano-optodes

Inorganic ions are ubiquitous in all kinds of cells with highly dynamic spatial and temporal distribution. Taking advantage of different types of fluorescent probes, fluorescence microscopic imaging and quantitative analysis of ion concentrations in cells have rapidly advanced. A family of fluorescent nanoprobes based on ionophores has emerged in recent years with the potential to establish a unique platform for the analysis of common biological ions including Na⁺, K⁺, Ca²⁺, Cl^-, and so on. This article aims at providing a retrospect and outlook of ionophore-based ion-selective nanoprobes and the applications in cell imaging.

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.

Ionophore-Based Potassium Selective Fluorescent Organosilica Nano-Optodes Containing Covalently Attached Solvatochromic Dyes

Fluorescent nanoprobes containing ionophores and solvatochromic dyes (SDs) were previously reported as an alternative to chromoionophore-based nano-optodes. However, the small-molecular SDs are prone to leakage and sequestration in complex samples. Here, we chemically attached the SDs to the surface of organosilica nanospheres through copper-catalyzed Click chemistry to prevent dye leakage. The nano-optodes remained well responsive to K+ even after exposure to a large amount of cation-exchange resin, which acted as a sink of the SDs. The potassium nanoprobes exhibited a dynamic range between 1 μM to 10 mM and a good selectivity thanks to valinomycin. Preliminary sensing device based on a nylon filter paper and agarose hydrogel was demonstrated. The results indicate that the covalent anchoring of SDs on nanospheres is promising for developing ionophore-based nanoprobes.

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.

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.

Colorimetric ratiometry with ion optodes for spatially resolved concentration analysis

The deprotonation degree of the lipophilic pH indicator dye (chromoionophore) in ionophore-based ion optodes (so-called bulk optodes) has traditionally been measured spectrophotometrically. This makes it difficult to obtain spatially resolved concentration information, for example in the study of heterogenous systems. This article reports on a new colorimetric method that relies on a ratiometric image analysis. The acquision of image data allows one to map the deprotonation degree in two dimensions, which in turn is used to obtain the spatially-resolved ion concentration of the image. Using the detection of potassium as an example, the deprotonation degree data calculated on the basis of image analysis correlate quantitatively with those from spectrophotometry. They showed no dependence on the type of camera used in spite of their different gamma correction values and spectral sensitivities, as expected from theory. As an example, the method is successfully applied to the pixel level analysis of an ensemble of pictures acquired at different times to spatially and temporally observe potassium ion diffusion into an agarose gel containing a potassium-selective optical sensor microemulsion.

Ionophore-Based Ion-Selective Nanospheres Based on Monomer-Dimer Conversion in the Near-Infrared Region

Here, we report ion-selective nanospheres with readout in the near-infrared (NIR) region in both fluorescence and absorbance modes. The nanospheres rely on an ionophore-mediated monomer-dimer conversion of an NIR transducer, DTTC. The DTTC monomer in the nanospheres emits fluorescence around 820 nm, while the dimer in the aqueous environment generates strong blue-shifted emission around 660 nm. With a lead ionophore, an unprecedented lower detection limit of 3 pM for Pb²⁺ was achieved, allowing us to determine Pb²⁺ levels in river water without diluting the sample. Also, the Cu²⁺-selective nanospheres showed a detection limit of 5 nM. Taking advantage of the biologically desired NIR window, blood potassium concentrations were also determined without a complicated sample pretreatment. The sensing process was explained with a theoretical model. The detection range was found finely adjustable by the amount of nanospheres used. Therefore, the nanospheres formed a highly selective, sensitive, versatile, and rapid analytical platform for metal-ion sensing.

Two-Dimensional Disposable Graphene Sensor to Detect Na+ Ions

The monitoring of Na⁺ ions distributed in the body has been indirectly calculated by the detection of Na⁺ ions in urine. We fabricated a two–dimensional (2D) Na⁺ ion sensor using a graphene ion–sensitive field–effect transistor (G–ISFET) and used fluorinated graphene as a reference electrode (FG–RE). We integrated G–ISFET and FG on a printed circuit board (PCB) designed in the form of a secure digital (SD) card to fabricate a disposable Na⁺ ion sensor. The sensitivity of the PCB tip to Na⁺ ions was determined to be −55.4 mV/dec. The sensor exhibited good linearity despite the presence of interfering ions in the buffer solution. We expanded the evaluation of the PCB tip to real human patient urine samples. The PCB tip exhibited a sensitivity of −0.36 mV/mM and linearly detected Na⁺ ions in human patient urine without any dilution process. We expect that G–ISFET with FG–RE can be used to realize a disposable Na⁺ ion sensor by serving as an alternative to Ag/AgCl reference electrodes.

Influence of Electrolyte Coextraction on the Response of Indicator-Based Cation-Selective Optodes

Here, we report on systematic investigation of the impact of coextraction of the aqueous electrolyte and anion interference on the response of cation-selective bulk optodes. It is evident that to deliberately manage the properties of chemical sensors and to apply them in routine analysis, one should have exhaustive insight into their operation mechanism. Despite the extensive research in the field of ionophore-based optodes and numerous attempts for their practical application, the understanding of how coextraction of an aqueous electrolyte influences its response characteristics has not been developed thus far. Meanwhile, the electrolyte coextraction determines the detection limits of analogous ion-selective electrodes. A theoretical model based on phase distribution equilibrium is proposed to quantitatively describe the effect of Donnan exclusion failure on the response of polymeric plasticized optodes. The theoretical conclusions are confirmed by the results obtained with Na⁺/pH-selective optodes based on a neutral chromoionophore as a model system in solutions containing anions of various lipophilicities (Cl^-, NO₃^-, I^-, SCN^-, and ClO₄^-). For the first time, it is shown that coextraction leads to a significant shift of the response range of the optodes as well as to nonmonotonic response curves due to the transition from cationic to anionic response. An approach to estimate the coextraction constants of electrolytes from the optode response curves is proposed. The limitations in the applicability of optodes due to co-ion interference are explored. It is found that neglecting anion interference can cause dramatic errors in the results of analyses with optical sensors.

Ionophore-based pH independent detection of ions utilizing aggregation-induced effects

Ionophores have been integrated into various electrochemical and optical sensing platforms for the selective detection of ions. Previous ionophore-based optical sensors rely on a H⁺ chromoionophore as the signal transducer and consequently, suffered from a pH cross-response. pH independent methods were proposed very recently by utilizing the solvatochromic dyes or the exhaustive mode. Here, we report a pH independent sensing principle based on nanospheres containing ionophores. As the ion-exchange occurs, the signal transducer undergoes aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ), leading to a dramatic change in fluorescence intensity. The principle was evaluated on different ionophores including those selective for K⁺, Na⁺, Ca²⁺, and Pb²⁺. The nanospheres were also introduced into microfluidic chips and successfully applied for the determination of sodium and potassium ion concentrations in diluted blood serum and urine samples.