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The use of acoustic streaming as a noncontact mixing platform to accelerate mass-transport-limited diffusion processes in small-volume heterogeneous reactions has been investigated. Single-bead anion exchange of plutonium at nanomolar and subpicomolar concentrations in 20 [micro]L liquid volumes was used to demonstrate the effect of acoustic mixing. Pu uptake rates on individual ~760 [micro]m diameter AG 1 x 4 anion-exchange resin beads were determined using acoustic mixing and compared with Pu uptake rates achieved by static diffusion alone. An 82 MHz surface acoustic wave (SAW) device was placed in contact with the underside of a 384-well microplate containing flat-bottomed semiconical wells. Acoustic energy was coupled into the solution in the well, inducing acoustic streaming. Pu uptake rates were determined by the plutonium remaining in solution after specific elapsed time intervals using liquid scintillation counting (LSC) for nanomolar concentrations and thermal ionization mass spectrometry (TIMS) analysis for the subpicomolar concentration experiments. It was found that this small batch uptake reaction could be accelerated by a factor of about 5-fold or more, depending on the acoustic power applied.

A derivative of a known [Ca.sup.2+]-selective ionophore, ETH 129, was synthesized to contain a polymerizable acrylic moiety (AU-1) and covalently grafted into a methyl methacrylate-co-decyl methacrylate polymer matrix. The polymer containing AU-1 was prepared via a simple one-step homogeneous polymerization method. It exhibited mechanical properties suitable for the fabrication of plasticizer-free ion-selective membrane electrodes and bulk optode films by solvent-casting and spin-coating techniques, respectively. The segmented sandwich membrane technique was utilized to assess the binding constant of free and covalently bound ionophores to calcium and to study their diffusion coefficients in the membrane phase. Diffusion was greatly diminished for the bound ionophore. This was confirmed in ion-selective electrode membranes containing no calcium ions in the inner solution, which should normally show apparent super-Nernstian response slopes in dilute calcium solutions. The response slope was Nernstian down to submicromolar concentration levels, indicating slow mass transport of calcium in the membrane. Optical-sensing films with the new copolymer matrix, unblended and blended with PVC-DOS, also confirmed that covalently bound ionophores are fully functional for maintaining selective ion extraction and binding properties of the sensing membrane.

Recently, it has been discovered that carba-closo-dodecaborates can be used as cation exchangers in neutral carrier-based ion-selective chemical sensors. Because of their inherent chemical stability and versatile functionalization chemistries, they offer many advantages that may potentially be exploited for ion analyses that require nontraditional sample conditions, including strongly acidic media. In this work, trimethylammonium salts of undecachlorinated (UCC), undecabrominated (UBC), hexabrominated (HBC), and undecaiodinated (UIC) carborane anions were prepared and evaluated for their potential use in solvent polymeric membrane-based sensors. Computational methods including Natural population analysis and electrostatic mapping were used to predict the ion-exchanging ability of each lipophilic anion. In addition, the sandwich membrane technique was used to evaluate the ion-pairing ability of each carborane anion in situ (i.e., within bis(2-ethylhexyl) sebacate (DOS)- and 2-nitrophenyl octyl ether (o-NPOE)-plasticized ISE membranes). The results of the computational and potentiometric studies found that binding affinity of the anions followed the generalized trend HBC > UCC > UBC > UIC. PVC-DOS bulk optode thin films containing the chromoionophore ETH 5315 and a respective anion were used to determine the chemical stability/lipophilicity of the carboranes and tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (TFPB) in acidic media (0.2 M HOAc) under flowing conditions. The studies found that in terms of stability/lipophilicity UIC > UBC > TFPB [approximately equal to] UCC [much greater than] HBC. Electrodes containing a [Pb.sup.2+]-selective ionophore, tert-butylcalix[4]arene-tetrakis-(N,N-dimethylthioacetamide)(lead IV), were used to evaluate the functionality of each cation exchanger. An evaluation of response characteristics such as slope and selectivity found that UIC and UBC were quite comparable to the behavior of TFPB. Interestingly, both UIC and UBC showed a marked selectivity improvement over cadmium, with log [K.sup.pot.sub.Pb,Cd] values of -7.19 and -7.29, respectively, with TFPB giving a value of -5.89. Demonstrating excellent stability and suitable electrostatic properties, the carboranes, UIC in particular, are a very promising

Fluorescent sensing microspheres based on perhaps the most selective and practically useful chloride ionophore known, the recently reported [9]mercuracarborand-3 (MC-3), have been prepared and optimized for physiological measurements. In initial work, this ionophore was shown to yield functional optical sensing films in combination with an electrically neutral chromoionophore, ETH 5418. Unfortunately, however, these optodes suffered from unacceptably high levels of sodium interference under physiological conditions. To better understand the sensing mechanism, optical and potentiometric binding experiments were used to characterize the stoichiometry and the complex formation constants for this ionophore. It was found that the preferred stoichiometry is 1:2, rather than 1:1 as assumed earlier. The 1:2 complex is extremely stable (log [[beta].sub.2] = 13.4), but a relatively strong 1:1 complex also exists (log [K.sub.1] = 9.9). These characteristics were used to fabricate chloride optodes that make use of the stepwise ion-ionophore decomplexation equilibrium, by adding a calculated amount of lipophilic anion exchanger to the polymer film. Such optodes showed dramatically reduced sodium interference while maintaining the excellent selectivity of the traditional formulation. The optimized composition also shifted the measuring range to physiological conditions, making them useful for the assessment of chloride in undiluted and 10-fold-diluted blood at pH 7.4. After necessary alterations of the particle preparation procedure and sensor formulation, the new insights were used to fabricate mass-produced optical sensing microspheres with characteristics essentially identical to those of the optode sensing films.

The use of flow cytometry as a rapid, high-throughput diagnostic tool for the analysis of ions is described. Monodisperse, uniform microspheres, which obey bulk optode theory and are governed by bulk extraction processes rather than surface phenomena, were prepared under mild, nonreactive conditions using a sonic stream particle casting apparatus. As an initial example demonstrating the utility of this approach, microspheres that contained a [H.sup.+]-selective fluorescent chromoionophore (ETH 5294), a cation-exchanger (NaTFPB), and either a highly sodium-selective (sodium ionophore X) or a potassium-selective ionophore (BME-44) were prepared. Separate solution analysis of sodium- and potassium-selective microspheres resulted in the generation of functional response curves using peak channel fluorescence intensities. The selectivity observed for both types of particles is sufficient for the clinical determination of [Na.sup.+] and [K.sup.+]. Furthermore, sodium- and potassium-selective microspheres were analyzed in parallel using sodium sample solutions, resulting in the successful determination of sodium ion concentrations and providing important information about the selectivity of the potassium-selective sensors over sodium. This work demonstrates the potential applicability of flow cytometry as a means for developing multiplexed, rapid, high-throughput analyses for clinically relevant ions.

Submicromolar to picomolar lower detection limits have recently been obtained with various polymer membrane ion-selective electrodes by minimizing biases due to ion fluxes through the membrane. For the best performance, the compositions of the membrane and inner solution should be optimized for each application. Given the number of parameters to be adjusted, it has been difficult to find the best parameters for a target sample. In this paper, a much simplified and more practical steady-state model of zero-current ion fluxes is derived, which is based on measurable parameters. The model allows one to predict achievable lower detection limits for a membrane with given selectivities. It can also be used to predict the optimal composition of the inner filling solution for the measurement of samples with a known, typical ionic background. Selectivity coefficients of monovalent and divalent analyte ions required for desired detection limits in drinking water are calculated. As an application of the proposed general recipe, a silver-selective electrode is developed on the basis of the ionophore O,O'-bis[2-(methylthio)ethyl]-tert-butylcalix[4]arene. With the predicted optimal composition of the inner electrolyte, its lower detection limit is found to be [10.sup.-9] M or 100 ppt [Ag.sup.+] with an ionic background of [10.sup.5] M LiN[O.sub.3], which is very close to the expected value.

The 2,3,4,5,6,7,8,9,10,11,12-undecabromocarborane anion, 1-HC[B.sub.11][Br.sub.11.sup.-] (UB[C.sup.-]) has been evaluated for its suitability as an ion exchanger in solvent polymeric membrane electrodes and bulk optodes. Experiments comparing the chemical stability of the perhalogenated carborane anion to that of the best lipophilic tetraphenylborate, 3,5-[bis(trifluoromethyl)phenyl]borate (TFP[B.sup.-]), demonstrated that in the presence of 0.2 M acetic acid TFP[B.sup.-] was completely lost within 6 h, while the concentration of UB[C.sup.-] decreased by less than 10% in the same time period. Thin-film bulk optodes containing BME-44 as potassium-selective ionophore, ETH 5294 as chromoionophore, and UB[C.sup.-] as ionic sites exhibited a [K.sup.+] response similar to analogous optodes containing TFP[B.sup.-], with comparable selectivities over [Na.sup.+] and [Ca.sup.2+]. Potentiometric measurements evaluating the selectivity behavior of UB[C.sup.-] in both ionophore-free and ionophore-containing electrodes were performed. Ionophore-free PVC membranes containing UB[C.sup.-] as ion exchanger and either DOS or NPOE as plasticizer also demonstrated selectivity similar to TFP[B.sup.-]-containing membranes. Sodium-selective membranes containing the ionophore 4-tertbutylcalix[4]arenetetraacetic acid tetraethyl ester (sodium ionophore X) and UB[C.sup.-] as ionic sites showed a Nernstian response for sodium and selectivity comparable to that found in analogous electrodes containing TFP[B.sup.-].

A convenient method for the preparation of monodisperse, plasticized poly(vinyl chloride) particles based on an automated particle casting technique is described. The particles are made highly selective for a number of ions by doping them with ionophores and other active components, in complete analogy to thin-film or fiber-optic chemical sensors. The approach used here produces spheres of high monodispersity at a rate of ~20 000 particles/s. The casting process is based on a reproducible polymer drop formation and precipitation process, and the particles are formed under very mild, nonreactive conditions. This allows one to conveniently incorporate known amounts of different active components into the polymers. As an initial example, the particles are doped with three optical sensing components, the sodium ionophore tert-butylcalix[4]arene tetraethyl ester, the [H.sup.+]-chromoionophore ETH 5294, and the anionic additive sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate. The particles are found to be of spherical shape with a diameter of ~10 [micro]m. They respond individually and selectively to sodium according to classical optode theory, as determined by fluorescence microscopy. With a RSD of 1.6%, sensing reproducibility from particle to particle is excellent. This technique may allow the development of mass-produced chemically selective microspheres on the basis of bulk extraction processes.

The spectroscopic characterization and thermodynamic stability of the mixed-ligand complex, U[O.sub.2]([O.sub.2])[(C[O.sub.3]).sub.2.sup.4-], is described in both solution and solid state. The studies have shown that the stable solution species is U[O.sub.2]([O.sub.2])[(C[O.sub.3]).sub.2.sup.4-], which has a formation constant that is relative to the tris-carbonato complex.

Zero-current ion fluxes through polymer membranes of ion-selective electrodes (ISEs) may lead to biased endpoints of potentiometric titrations. The bias is eliminated, and the sensitivity of the end-point detection is improved through reducing transmembrane ion fluxes by an appropriate choice of the inner solution. Surprisingly, ISE membranes that have a significant primary ion flux toward the inner solution show much larger sensitivities than expected by titration theory; however, depending on the experimental conditions, their application may bias the endpoint. With the optimal systems, endpoint detection is now possible with total sample concentrations below [10.sup.-6] M, as demonstrated by the titration of EDTA with a [Pb.sup.2+] solution.

Microscale plasticized PVC particles doped with hydrophobic ionophores are prepared by solvent evaporation of aqueous suspensions of sensing cocktails (poly(vinyl chloride), plasticizer, active sensing components, and tetrahydrofuran) and tested as particulate microoptical sensors. The particles contain either only the chromoionophore ETH 2458 as active reagent or the potassium ionophore BME-44, chromoionophore ETH 5294, and lipophilic anionic sites NaTFPB. The former system functions according to an anion-hydrogen ion coextraction mechanism and shows a Hofmeister anion selectivity pattern, while the latter sensor containing additional ionophores represents the more complex, truly selective optical sensors based on ion-exchange equilibria. Single microspheres are simultaneously characterized spatially and spectrally by fluorescence microscopy, coupled to a spectrometer equipped with a CCD detector. The results indicate that these microspheres respond in complete analogy to traditional thin-film-based optodes previously reported in the literature. The introduction of small, spherical ionophore-based sensing particles that operate on the basis of bulk extraction principles holds the promise of significantly expanding the available chemical palette of microsphere-based analytical assays.