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A novel magnesium-selective ion-channel mimetic sensor based on a well-known calcium ionophore, ETH 5234, was demonstrated. Due to different stoichiometry of the ionophore with [Mg.sup.2+] and [Ca.sup.2+], the sensor exhibits high selectivity to [Mg.sup.2+] by controlling ionophore concentration on the electrode surface, which allows the measurement of [Mg.sup.2+] in a 1 mM [Ca.sup.2+] background solution. 10.1021/ac901570u

A novel electrochemical sensor was developed for sensitive and selective detection of mercury(II), based on thymine--[Hg.sup.2+]--thymine (T--[Hg.sup.2+]--T) coordination chemistry. This strategy exploited the cooperativity of proximate poly-T oligonucleotides in coordination with [Hg.sup.2+]. Ferrocene (Fc)-tagged poly-T oligonucleotides were immobilized on the electrode surface via self-assembly of the terminal thiol moiety. In the presence of [Hg.sup.2+], a pair of poly-T oligonucleotides could cooperatively coordinate with [Hg.sup.2+], which triggered a conformational reorganization of the poly-T oligonucleotides from flexible single strands to relatively rigid duplexlike complexes, thus drawing the Fc tags away from the electrode with a substantially decreased redox current. The response characteristics of the sensor were thoroughly investigated using capillary electrophoresis and electrochemical measurements. The results revealed that the sensor showed a sensitive response to [Hg.sup.2+] in a concentration range from 1.0 nM to 2.0 [micro]M, with a detection limit of 0.5 nM. Also, this strategy afforded exquisite selectivity for [Hg.sup.2+] against a reservoir of other environmentally related metal ions, compared to existing anodic stripping voltammetry (ASV) techniques. In addition, this sensor could be implemented using minimal reagents and working steps with excellent reusability through mild regeneration procedure. It was expected that this cost-effective electrochemical sensor might hold considerable potential in on-site applications of [Hg.sup.2+] detection.

An optical sensing approach for the detection of saccharides based on reversible boronic acid--diol complexation in a lipophilic polymer membrane is presented. The complexation of saccharides with phenyl boronic acids that are immobilized in a hydrophobic polymer matrix produces a stable boronate anion and liberates a mobile hydrogen ion. The change in fluorescence intensity of the sensing films resulting from the increase in hydrogen ion concentration is thus directly related to the ambient saccharide concentration. By monitoring the pH change in a bulk optical film, we were able to detect D-glucose, D-fructose, D-galactose, and D-sorbitol with a concentration range from 0.1 to 100 mM at physiological pH 7.4.

Aptamer-based rolling circle amplification (aptamer-RCA) was developed as a novel versatile electrochemical platform for ultrasensitive detection of protein. This method utilized antibodies immobilized on the electrode surface to capture the protein target, and the surface-captured protein was then sandwiched by an aptamer-primer complex. The aptamer-primer sequence mediated an in situ RCA reaction that generated hundreds of copies of a circular DNA template. Detection of the amplified copies via enzymatic silver deposition then allowed enormous sensitivity enhancement in the assay of target protein. This novel aptamer-primer design circumvented time-consuming preparation of the antibody-DNA conjugate for the common immuno-RCA assay. Moreover, the detection strategy based on enzymatic silver deposition enabled a highly efficient readout of the RCA product as compared to a redox-labeled probe based procedure that might exhibit low detection efficiency due to RCA product distance from the electrode. With the platelet-derived growth factor B-chain (PDGF-BB) as a model target, it was demonstrated that the presented method was highly sensitive and specific with a wide detection range of 4 orders of magnitude and a detection limit as low as 10 fM. Because of the wide availability of aptamers for numerous proteins, this platform holds great promise in ultrasensitive immunoassay.

An optical microsensor array is described for the rapid analysis of silver ions at low parts per trillion levels. Because the ionophore o-xylylenebis(N,N-diisobutyldithio-carbamate) (Cu--I) was reevaluated and shown to exhibit excellent selectivity for silver ions, ion-selective electrode (ISE) membranes were optimized and found to exhibit the lowest reported detection limit so far (3 x [10.sup.10] M). A corresponding [Ag.sup.+]-selective fluorescent optical microsensor array for the rapid sensing of trace level [Ag.sup.+] was then developed. It was fabricated using plasticized PVC-based micrometer-scale fluorescent microspheres that were produced via a sonic particle casting device. They contained 156 mmol/kg Cu--I, 10 mmol/kg 9-(diethylamino)--5-[4-(15-buty1-1,13-dioxo-2,14-dioxanodecyl) phenylimino]-benzo[a]phenoxazine (chromoionophore VII, ETH 5418), 2.3 mmol/kg 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (internal reference dye), and 14 mmol/kg sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and were deposited onto the etched distal end of a 3200-[micro]m-diameter optical fiber bundle. The microarray was characterized by fluorescence spectroscopy in samples containing [10.sup.12]--[10.sup.8] M AgN[O.sub.3] at pH 7.4, with selectivity characteristics comparable to the corresponding ISEs. The response time of the microsensor array was found to be less than 15 min for [10.sup.9] M AgN[O.sub.3], which is drastically shorter than earlier data on optode films (8 h) and corresponding ISEs (30 min). A detection limit of 4 x [10.sup.11] M for [Ag.sup.+] was observed, lower than any previously reported optode or silver-selective ISE. The microsensor array was applied for measurement of free silver levels in buffered pond water samples.

Electrochemical and surface enhanced Raman scattering (SERS) spectroelectrochemical measurements were used to report the studies of the structure and anticorrosion features of two kinds of self-assembled monolayers (SAMs) of IP6 at the silver surface under various pH values, 1.27 and 13. Based on in situ SERS electrochemical measurements, a tentative explanation for the difference in corrosion potentials of two kinds of the silver surfaces in the presence of SAMs formed from completely protonated or deprotonated IP6 molecules are presented.

The spontaneous hydroxy-bridged dimer formation of metalloporphyrins in ion-selective membranes gives rise to a short sensor lifetime (typically days), triggered by solubility problems, the occurrence of a super-Nernstian response slope, and a pH cross response. This dimer formation is eliminated here by covalent attachment of the ionophore to the polymer matrix. Specifically, two different [indium.sup.III]porphyrins containing polymerizable groups, the chloride-selective chloro(3-[18-(3-acryloyloxypropyl)7,12-bis (1-methoxyethyl)-3,8,13,17-tetramethylporphy-rin-2-yl]propyl ester)indium(III) and the nitrite-selective Chloro(5-(4-acryloyloxyphenyl)-10,15,20-triphenylporphyrinato)indium(III), were synthesized and copolymerized with methyl methacrylate and decyl methacrylate. The covalent attachment of the ionophore to the polymer matrix indeed prevents the metalloporphyrin from forming dimeric species, as confirmed by UV/visible spectroscopy. The ion-selective membranes with grafted indium porphyrin showed Nernstian response slopes to chloride, nitrite, perchlorate, and thiocyanate anions, with a selectivity comparable to membranes with freely dissolved or underivatized metalloporphyrin. The membranes containing grafted ionophores showed a lifetime of at least two months, apparently since crystallization of the poorly soluble dimeric species may no longer occur. This is one of the first examples where the covalent attachment of an ionophore drastically improves on a number of important sensor characteristics.

The traditional cation exchangers used in ion-selective electrodes and optodes are tetraphenylborate derivatives, which are generally adequate for most analytical applications but may in some cases suffer from decomposition by acid hydrolysis, oxidants, and light. Recently, halogenated dodecacarboranes were found to be improved cation exchangers in terms of lipophilicity and chemical stability. This forms the basis for the convenient covalent attachment of the cation exchanger to the polymeric backbone of the sensing material. This is a challenge that has not satisfactorily been solved and which is especially important in view of developing ultraminiaturized sensing arrays. Here, a C-derivative of the closo-dodecacarborane anion (C[B.sub.11][H.sub.12.sup.-]) with a polymerizable group was synthesized as a chemically stable cation exchanger. This new derivative was copolymerized with methyl methacrylate and decyl methacrylate (MMA-DMA) to fabricate a plasticizer-free polymer with cation-exchange properties. This polymer could be conveniently blended with traditional plasticized poly(vinyl chloride) or with noncrosslinked methacrylic polymers to give solvent cast films that appear to be clear and homogeneous and that could be doped with ionophores. Optode leaching experiments supported the covalent grafting of the carborane anions. Ion-selective membranes and optode thin films were evaluated in terms of response function, response time, and selectivity. In all cases, the new material exhibited behavior similar to free tetraphenylborate derivative-based membranes. As a result of these studies, an all-polymeric plasticizer-free calcium-selective membrane was fabricated on the basis of the covalently attached carborane, a recently introduced grafted calcium ionophore, and an MMA-DMA polymer matrix. The resulting ion-selective electrodes showed Nernstian response slopes and rapid response times, demonstrating that covalent grafting of all sensing components is a feasible approach to the development of ion sensors.

The lower detection limit and the selectivity behavior of anion-selective electrodes (ISEs) are improved by using optimized inner solutions and membrane compositions. With a membrane based on the recently described ionophore [9]mercuracarborand-3, a detection limit of 2 x [10.sup.-9] M has been achieved for iodide. Nevertheless, the improvements are less pronounced than in the case of cation ISEs. This is mainly due to the fact that so far no anion ISE is known with the extremely high selectivities of cation ISEs. If the membrane does not contain an ionophore, leaching of the ion exchanger from the membrane into the sample is also a relevant limiting factor except for ion exchangers of very high lipophilicity.

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 upper detection limit of polar ionophore-based ion-selective electrode membranes is predicted by utilizing the coextraction constant of dissociated electrolyte, the stability constant of the ionophore, and the membrane composition. The coextraction constant of dissociated electrolytes into the polar poly(vinyl chloride) membrane plasticized with o-nitrophenyl octyl ether (PVC-NPOE) is here measured by a novel approach. The sandwich membrane technique is utilized, with one membrane segment containing a lipophilic cation exchanger and the other containing an anion exchanger. This yields information about the coextraction constant and the free ion concentrations of the electrolyte in the two segments. Predictions correlate quantitatively with the upper detection limit observed for ion-selective electrodes based on the ionophores valinomycin, tert-butylcalix[4]arene tetraethyl ester, and calcimycin. The difficulties of the prediction of the upper detection limit for nonpolar poly(vinyl chloride) membranes plasticized with bis(2-ethylhexyl sebacate) (PVC-DOS) due to ion association are discussed in detail. A thermodynamic cycle experiment with a series of sandwich membranes shows that the principal processes governing the upper detection limit of PVC-DOS membranes are identical to those for the PVC-NPOE membranes. However, the stability of the ion pairs between the ionophore-metal ion complexes and the extracted anion are different from that of ion pairs formed between the same anion and the lipophilic anion exchanger. This makes it difficult to quantitatively predict the upper detection limit on the basis of simple apparent coextraction and complexation data alone. The approach reported herein is useful not only for mechanistic purposes but also to shed light onto the many cases where coextraction effects need to be understood but are not directly experimentally accessible.

A new second-order calibration procedure, the coupled vectors resolution (COVER) method, has been developed. The objective of the method is to seek a couple of vectors that minimize a least-squares criterion. With the knowledge indispensable for quantitation, the method yields direct solutions to various cases of second-order calibration. Moreover, it allows a statistically plausible way to make use of multisample information. In the case of multiple calibration samples, the method uses the calibration samples to resolve the profiles of the analytes in each order, and then calculates the concentrations of the analytes. This offers the advantage that unknown mixtures newly collected can be predicted in a direct manner. In the case of one calibration sample, the method provides an effective way to utilize the information of spectral profiles of the analytes. Results of simulated experiments and a real analytical example show that the proposed method produces acceptable performance in profile resolution and concentration estimation.

Manganese–copper spinel is a kind of efficient catalyst for NO reduction by CO; however, its unsatisfactory low-temperature catalytic performance and poor N2 selectivity limit its application. Here, peanut-shaped Cu0.75Mn2.25O4 nano-hollow spinel (Cu0.75Mn2.25O4-NH) was prepared by a one-pot solvothermal method and applied in NO reduction by CO. The structure and physicochemical properties of catalysts were researched by comprehensive characterizations. Compared with Cu0.75Mn2.25O4 nanoparticles (Cu0.75Mn2.25O4-NP), Cu0.75Mn2.25O4-NH displayed excellent low-temperature catalytic performance, achieving 90% NO conversion at 200 °C, and possessed a lower apparent activation energy (36.4 kJ·mol–1). Importantly, the unique nanostructure with more exposed active sites enhanced the redox properties and oxygen mobility of the Cu0.75Mn2.25O4-NH catalyst. In addition, a synergistic effect between different metal ions in the Cu0.75Mn2.25O4-NH catalyst promoted the formation of oxygen vacancies and more low-oxidation-state species, which were conducive to the N–O bond scission at low temperatures. Combining the in situ DRIFTS results and DFT calculations, the dispersed species of Cuy+-O-Mnx+ could be reduced to the main reactive species of Cu(y–1)+-□-Mn(x–1)+. Moreover, the formation of oxygen vacancies optimized NO adsorption and activation ability, which improved the catalytic performance in NO reduction by CO. [ABSTRACT FROM AUTHOR]