Ion-selective Optical Sensors Research Articles

Response Patterns of Chromoionophore-Based Bulk Optodes Containing Lipophilic Electrolytes: Toward Background-Independent pH-Sensing.

Despite a number of advantages of ion-selective optical sensors (optodes), their practical application is limited by their response mechanism, which leads to the dependence of the signal on the activity of two ions (analyte ion and the so-called reference ion) in the solution at the same time. Here, we show that the introduction of a lipophilic electrolyte into the polymeric optode membrane allows assessing the ionic activity of H+ cations regardless of the concentration of the background electrolyte containing a hydrophilic cation, with NaCl as an example of such an electrolyte. For the first time, the applicability of this approach is proven theoretically utilizing the numerical simulation of optode response. A correlation between the interfacial potential stability and the single-ion optical response is established. The predicted optical response is independent of background cation concentration to a significant extent. Theoretical conclusions are supported by experimental data obtained with chromoionophore-based optodes doped with various lipophilic electrolytes, including ionic liquids, by thin-film spectrophotometry and macrophotography coupled with digital color analysis. Most of the experimental sensor characteristics, such as the response range and its median, as well as its independence from the background electrolyte concentration are in quantitative agreement with the proposed theoretical description.

Probe anchored porous organic polymer monolithic architectures as optical sensor for ultra-trace analysis of Hg2+ in water samples

In this article, we have demonstrated an effective strategy to detect and remove the toxic Hg2+ by an ion-selective optical sensor using chromoionophoric receptor that is homogenously dispersed onto the surface of porous organic polymer (POP) template. Herein, we present a detailed summation of the synthesis of different POPs by employing various kinds of monomers and crosslinkers to discover excellent structural property materials that could cater to the needs of an optical sensor. The resultant POP from the bulk polymerization of N,N-dimethyl allylamine (DMA), and trimethylolpropane triacrylate (TMPTA), i.e., poly(DMA-co-TMPTA), manifested a higher surface area with pore size, thus has been utilized as the substrate material for the fabrication of the sensor. Further, the chromoionophoric coordinating ligand, i.e., 7-4(diethylamino)phenyldiazenyl quinolin-8-ol (DPDQ), is immobilized as receptor molecules onto the POP template for ultra-trace detection of Hg2+. The surface dispersed receptor forms stable 1:1 charge-transfer complexes [Hg2+-DPDQ] with a striking naked-eye color transition from pale orange to bright reddish-brown. The developed optical sensor offers an ultra-fast, selective response for Hg2+ with a lower detection limit of 0.1 μg/L (ppb) in the linear range of 0–100 μg/L. The regeneration of the sensor is envisaged with 0.02 M HCl and can be reused for up to six sensing cycles. Actual water samples from different geographical locations of India are tested using the developed sensor, and the method proved effective for real-time monitoring of Hg2+.

Lipophilic Fluorescent Dye Liquids: Förster Resonance Energy Transfer-Based Fluorescence Amplification for Ion Selective Optical Sensors Based on a Solvent Polymeric Membrane.

Optical sensors based on solvent polymeric membranes have the potential to measure analytes present in an aqueous solution through the development of a tailored method for a specific target. However, limits in the concentrations of the component dyes have prevented improvements in sensitivity. We propose a Förster resonance energy transfer (FRET)-based fluorescence amplification system for ion-selective optical sensors using a highly fluorescent liquid material composed of a lipophilic phosphonium cation and a pyrene modifying sulfonate anion ([P66614][HP-SO3]), as both the plasticizer and donor, in addition to a combination of the lipophilic phosphonium cation and the fluorescein dodecyl ester anion ([P66614][12-FL]) as the fluorescent sensing dye acceptor. For ion extraction-based sensing, the donor and acceptor were retained in the plasticized PVC membrane with negligible leaching upon exposure to acidic and basic aqueous solutions. Systematic investigation of the donor and acceptor ratios clarified the effect of the amplification factor and the sensitivity of the sensor. At an acceptor doping level of 0.5 mol % (vs donor), an approximately 22-fold higher sensitivity was obtained compared to that of a conventional PVC membrane optical sensor. During ion measurement based on the coextraction of protons and anions, selectivity following the Hofmeister order was observed, which was controlled by the addition of ionophores. The proposed FRET system based on a lipophilic fluorescent liquid material has the potential to significantly improve the sensitivities of optical sensors using solvent polymeric membranes with high selectivities for various target analytes.

PH-dependent distribution of the indicator dye tetrabromophenolphthalein ethyl ester between aqueous solution and plasticized polymeric phase: Predicting the lifetime of ion-selective optical sensors

The pH-dependent distribution of the indicator dye tetrabromophenolphthalein ethyl ester (TBPE, chromoionophore VIII) between plasticized polymeric phase and aqueous solution is studied. The commonly utilized algorithms for partition properties calculation are evaluated experimentally in terms of their suitability for the ionized species. The physical-chemical characteristics of TBPE that determine its applicability for use in chemical sensors, namely, the lipophilicity and acidity constant are estimated in the hydrophobic polymeric phase for the first time. The distribution coefficients of the neutral (protonated) and ionized (deprotonated) forms of TBPE are determined in plasticized PVC/water biphase system as 7.2 and 3.8, respectively (logk, ionic strength 1 M, 25 °C). The acidity constant of TBPE in plasticized PVC membrane, pKaPVC−DOS, is estimated spectrophotometrically as 6.1 ± 0.2 log units. The results can be used for developing optical chemical sensors based on TBPE (chromoionophore VIII), in particular for predicting the sensor lifetime, optimization of the sensor operating range and measuring conditions.

Simplified Fabrication for Ion-Selective Optical Emulsion Sensor with Hydrophobic Solvatochromic Dye Transducer: A Cautionary Tale.

It has recently been reported that polystyrene microbeads may be modified to realize plasticizer-free ion-selective optical sensors (optodes) on the basis of solvatochromic dye transducers. We show here that the functionalized microbeads, individually isolated by flow cytometry, exhibit unexpectedly poor fluorescent properties and that the sensor response is instead attributed to the supernatant. A more thorough study reveals that such optical microemulsion sensors can be made operationally functional and chemically selective, seemingly in the absence of any solvent matrix or added surfactant. Instead, it is shown that residual THF used in the fabrication of the emulsified sensors may solubilize the sensing components and give a functional optode response. To evaluate this further, the number of sensing components was stepwise simplified to assess their need. Variation of residual THF levels has no effect on the ion optode response when plasticizer is present, in support of established results. Lipophilic solvatochromic dye transducers are also shown not to require an added surfactant as their nature already endows the emulsified sensors with a stabilizing ionic surface charge. The ionophores are shown to exhibit much larger stability constants in the surfactant-free formulations than surfactant-based ones (valionomycin, log β > 9.2 compared to 6.1; Na+-ionophore X, 6.7 vs 4.7), which is attributed to a less polar solvent environment for the ionophore. Potassium-, sodium-, and calcium-selective sensors were used as model systems in this study.

Renovating the chromoionophores and detection modes in carrier-based ion-selective optical sensors.

Ion-selective optical sensing is an important branch of analytical and bioanalytical chemistry. Conventional ion-selective optodes are based on H(+) chromoionophores. These sensors are known to be pH dependent and usually operated in a passive mode. In view of the applications in complex real samples, the sensors must exhibit not only excellent chemical selectivity but also the ability to eliminate the optical background interference such as autofluorescence and light scattering. In this article, recent advances to renovate the chromoionophores and detection modes to overcome the pH cross-response and to eliminate the background optical interference are summarized. Topics include sensors based on solvatochromic dyes, alternative chromoionophores, photoswitchable sensors, upconverting nanoparticles, luminescence decay time, and others.

Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles.

We present here a new family of pH insensitive ion-selective optical sensors based on emulsified nanospheres containing densely functionalized 15-, 16-, 18- and 20-membered pyreneamide derivatives. These compounds were successfully synthesized by the reaction of α-diazo-β-ketoesters with cyclic ethers of the desired size in the presence of dirhodium complexes followed by a stereo-selective tandem amidation-transposition process and characterized by 1H-NMR, 13C-NMR, IR, HR-ESI-MS, UV-VIS and fluorescence spectroscopy and potentiometry. Their unique structure consisting of a crown ether ring linked to pyrene moieties through amide groups exhibits on-off switchable behavior upon binding of specific cations and allows one to incorporate these chemosensors as fluorescent ionophores into ion-exchange nanospheres. The nanosphere matrix is composed of bis(2-ethylhexyl)sebacate (DOS), poly(ethylene glycol) (PEG), sodium tetrakis 3,5-bis(trifluoromethyl)phenyl borate and pyreneamide functionalized 18-crown-6 ether (18C6). These optode nanoparticles exhibit a strong affinity to the potassium cation over other metal ions up to the millimolar concentration range in an exhaustive detection mode. The logarithmic complex formation constant was determined using the segmented sandwich membrane method and was found to be 6.5 ± 0.3 (SD) in PVC membrane plasticized with NPOE and 5.3 ± 0.3 (SD) in DOS with a 1 : 1 complex stoichiometry. The nanosensors were characterized in broad range of pH from 4 to 10 and the same linear calibration curves were obtained in the concentration range from 10-7 M to 10-5 M and thus the pH dependent response was largely overcome. These nanosensors are sufficiently stable, simple to prepare, exhibit a rapid response and their nanoscale size makes them suitable for sensing purposes in samples of limited dimensions.

Development and characterization of a copper ion-selective optical sensor based on a novel calix[4]dicyano-diimidazole thin film

A novel optical sensor for sensitive and selective determination of copper ions in aqueous solution based on a novel calix[4]dicyano-diimidazole ionophore was prepared. The newly synthesized calix[4]dicyano-diimidazole exhibited obvious absorbance enhancement at 284 nm in the presence of copper ions. At pH 6.8, the measuring range of the optode membrane for Cu2+ ions was from 1.0 × 10−8 to 1.0 × 10−4 M with a detection limit of 7.0 × 10−9 M. Furthermore, the response time of the proposed optical device was within 10 min. With the optimum condition described, the optical sensor revealed good selectivity toward Cu2+ ions in comparison with common coexisting cations (Hg2+, Pb2+, Cd2+, Ni2+ and Zn2+). Finally, the reproducibility, regeneration, reversibility and repeatability of this optical sensor were discussed.

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.

PH Independent Nano-Optode Sensors Based on Exhaustive Ion-Selective Nanospheres

Bulk optode-based ion selective optical sensors work on the basis of extraction equilibria, and their response toward the analyte ion is known to dependent on the sample pH. This pH dependence has been one of the major disadvantages that have hampered the broad acceptance of bulk optodes in chemical sensing. We present here for the first time the use of exhaustive Ca(2+)-selective nanosensors that may overcome this pH dependent response. The nanosensors were characterized at different pH and the same linear calibration was obtained in the Ca(2+) concentration range from 10(-7) M to 10(-5) M.

Oxazinoindolines as Fluorescent H+ Turn-On Chromoionophores For Optical and Electrochemical Ion Sensors

We propose here a new class of fluorescent H(+) turn-on oxazinoindoline (Ox) dyes as chromoionophores suitable for the development of polymeric ion-selective membranes. Through rational design, oxazinoindolines with varying basicities and tunable absorption bands that extend to the near-infrared region were prepared. The basicities of the compounds were evaluated by potentiometry and fluorescence spectroscopy. By comparing the potentiometric lower detection limit with chromoionophore I (pKa = 14.80 ± 0.03 in PVC-NPOE), pKa values were determined for Ox Y, Ox R, and Ox B to be 9.80 ± 0.03, 12.85 ± 0.03, and 12.95 ± 0.03, respectively. Unlike conventional pH indicators that involve only a protonated and deprotonated form, a third species, arising from the thermal isomerization of the Ox compounds in polar solvents, may influence the overall protonation equilibrium. Interestingly, this results in an apparent pKa increase in the fluorescence mode when more polar PVC-NPOE membranes were utilized. The isomerization is suppressed in nonpolar PVC-DOS membranes. Na(+)-selective optical sensors were successfully prepared using the Ox dyes as an early application of the compounds in ion-selective optical sensors. The compounds were also successfully evaluated in a dynamic electrochemistry (chronopotentiometry) sensing mode for the detection of total alkalinity.

Studies of aluminum (III) ion-selective optical sensor based on a chromogenic calix[4]arene derivative

An optical chemical sensor based on azo-calix[4]arene derivative is developed for the determination of aluminum (III) ions in aqueous solutions. The complex formation ability of azo-calix[4]arene toward metal cations such as Li(I), Cs(I), K(I), Sn(II), Pb(II), Cu(II), Eu(III), Er(III) and Al(III) is investigated by UV–vis spectroscopy. Assessments of results reveal that the azo-calix[4]arene derivative has high affinity to Al(III). The stoichiometric ratio and the association constant were determined as 1:1 and 1.24×104M−1, respectively for the complex between Al3+ and the azo-calix[4]arene. The sensing film is fabricated by spin coating on glass plates. Under the optimized conditions, at pH 6.8, the proposed optical sensor displays a linear response to Al3+ over 10−7 to 10−5M range with response time of 12min. The optical sensor can be regenerated with HNO3 solution. In addition to its high stability and reproducibility, the sensor shows good selectivity for Al3+ ion.

Novel optical bulk membrane sensor and its application for determination of iron in plant and cereal samples

A new ion selective optical bulk membrane sensor is fabricated for the highly sensitive and selective determination of iron(III) ion that is ion concentrations based on plasticized poly vinyl chloride containing 3% 1,1′-(iminobis(methan-1-yl-1-ylidene))dinaphthalen-2-ol as a chromo-ionophore, 3% sodium tetraphenylborate as an anionic additive and 64% tris(2-ethylhexyl)phosphate as a plasticizer. The proposed sensor displays a wide linear dynamic range from 1.0×10−9 to 1.0×10−3molL−1 with a low limit of detection of 6.2×10−10molL−1 at pH 4.7. The sensor has a fast response time of less than 80s. In addition to its high stability, reproducibility and stability over long life time period, the optical sensor shows a unique selectivity toward Fe(III) ions over a large number of organic and inorganic substances such as metal ions, sugars, amino acids and vitamins. The proposed optical sensor could readily be regenerated by exposure to 1.0×10−1molL−1 sodium thiourea solution and is also applied successfully determination of the iron contents in plant and cereal samples with satisfactory results.

Photoresponsive Ion-Selective Optical Sensor

A novel concept of photoresponsive ion-selective optical sensor is described. Photochemical reactions can be utilized to generate and control ion fluxes in an ion-selective optode in the same manner as nonequilibrium electrochemical methods have been used in ion-selective electrodes. In contrast to their equilibrium counterparts, the photoresponsive pH-selective ion optodes are sensitive to both the buffer capacity of the sample and activity of hydrogen ions. Active optical probes are especially attractive for intracellular applications because they can be fabricated as submicron-sized beads. Common optical techniques, such as fluorescence microscopy and flow cytometry, can be combined with active ion probes with only minor modification of the existing experimental setup.

Molecularly Imprinted Ionomers

AbstractIonomers have been defined as copolymers that have a certain proportion of ionic groups. The ionic groups have a significant effect on the mechanical properties of the copolymers. This is generally due to aggregation of ions in a low dielectric medium. The primary result is to restrict chain motion and raise the glass transition temperature. These attributes have relevance to molecular imprinting, since restricted chain motion should help preserve the integrity of the binding site. The connection between ionomers and molecular imprinting has come from the production of metal ion imprinted resins. Metal ions are used in the production of molecularly imprinted polymer ion exchange resins and ionically permeable membranes. The polymers have applications as separations media, sequestering media and as ion selective sensors. Metal ions are also being used to form imprinted polymers based on metal mediated imprint binding. We have prepared ion exchange resins, selectively permeable polymer membranes, ion selective electrodes and ion selective optical sensors using a modified version of the molecular imprinting technique. The modification is a reduction in the amount of covalent crosslinking used to form the polymers. This reduction may be justified by the presence of residual metal ion crosslinking in the immediate region of the imprinted binding site. The effects of metal ions on the thermal and mechanical properties of the polymers, as well their impact on binding selectivity are critical variables.

Development of a mercury ion-selective optical sensor based on fluorescence quenching of 5,10,15,20-tetraphenylporphyrin

A selective optical chemical sensor for mercury ion based on a lipophilized sensing material (5,10,15,20-tetraphenylporphyrin, H2tpp) dissolved in a plasticized poly(vinyl chloride) (PVC) membrane has been developed. H2tpp immobilized in the PVC membrane acts not only as a selective host molecule for Hg 2+, but also as a fluorescing transducer. The sensing mechanism involves the extraction of Hg 2+ from aqueous sample solution to organic membrane phase and the formation of a metalloporphyrin complex between H2tpp and Hg 2+, which results in the fluorescence quenching of H2tpp. The optode membrane containing H2tpp reversibly responds to Hg 2+ and shows extremely high selectivity to Li +, Na +, K +, Mg 2+, Cd 2+, Cu 2+, Fe 3+, Ag + and Pb 2+. The detection limit for Hg 2+ is 4.0×10 −8 mol/l at pH 8.0. The response characteristics of the sensor including dynamic range, reversibility, reproducibility, response time and lifetime are discussed in detail. This sensor has been used for the determination of mercury ion in water samples containing potential interferents with satisfactory recovery.

Cross-linked dodecyl acrylate microspheres: novel matrices for plasticizer-free optical ion sensing

Hexanedioldiacrylate (HDDA) cross-linked dodecyl acrylate (DDA) microspheres ranging from approximately 1 to 10 μm in diameter have been prepared via photoinitiated dispersion polymerization and used as polymer matrices in the development of a new class of “plasticizer-free” ion-selective optical sensors (optodes). Incorporation of a fluorescent Nile Blue derivative (ETH 5294) into the microspheres produced independent self-referencing optodes capable of measuring the activities of various anions in aqueous media. In order to increase sensitivity and signal reproducibility, changes in the fluorescence intensity of two emission peaks, corresponding to the protonated and deprotonated forms of the chromoionophore, were used to perform ratiometric analysis. Spatial and 3D spectral resolution of individual sensing microspheres is reported, providing analytically useful information about the local sensing environment. Physical properties of the polymeric microspheres that affect sensor function such as size, morphology, surface topography, dispersity, and glass transition temperature ( T g) are also reported here.

Characterization of Chromogenic Calix[4]arene Derivative Based Ion-Selective Optical Sensors

The analytical characterization of a novel chromogenic calix[4]arene derivative based sodium selective, direct optode membranes has been investigated in detail. The dynamic concentration range, ion-selectivity, response time and reproducibility are discussed. The effect of the nature of the membrane matrix on the dynamic concentration range and the operation pH of the relevant optode membranes are explained. The sodium ion response is interpreted on the basis of the ion-exchange theory, which is underlined by the results of calixarene mediated ion-transport experiments.

Lithium ion selective optical fiber sensor based on a novel neutral ionophore and a lipophilic anionic dye

Lithium ion selective optical fiber sensor based on a novel neutral ionophore and a lipophilic anionic dye

Ion‐Selective Hydrazone–Azine Tautomerism of a 14‐Membered Macrocylic Ligand

A distinct change in the absorption characteristics of the hexaaza macrocycle 1 is observed upon complexation of alkali- and alkaline-earth-metal ions. This effect, which is due to the tautomerization of the hydrazone form (left) to the azine form (right) during complexation, is especially pronounced for complexes of Li, Mg, and Ca. 1 is thus targeted as a possible component of ion-selective optical sensors.