It was recently demonstrated that under environmentally relevant conditions the Rayleigh equation is valid to describe the enantiomeric enrichment - conversion relationship, yielding a proportional constant called the enantiomeric enrichment factor, εER. In the present study we demonstrate a quantitative structure-activity relationship model (QSAR) that describes well the dependence of εER on molecular structure. The enantiomeric enrichment factor can be predicted by the linear Hansch model, which correlates biological activity with physicochemical properties. Enantioselective hydrolysis of sixteen derivatives of 2-(phenoxy)propionate (PPMs) have been analyzed during enzymatic degradation by lipases from Pseudomonas fluorescens (PFL), Pseudomonas cepacia (PCL), and Candida rugosa (CRL). In all cases the QSAR relationships were significant with R(2) values of 0.90-0.93, and showed high predictive abilities with internal and external validations providing QLOO(2) values of 0.85-0.87 and QExt(2) values of 0.8-0.91. Moreover, it is demonstrated that this model enables differentiation between enzymes with different binding site shapes. The enantioselectivity of PFL and PCL was dictated by electronic properties, whereas the enantioselectivity of CRL was determined by lipophilicity and steric factors. The predictive ability of the QSAR model demonstrated in the present study may serve as a helpful tool in environmental studies, assisting in source tracking of unstudied chiral compounds belonging to a well-studied homologous series.

Probing electrochemical processes by mass spectrometry (EC/MS) is a developing field that benefits from the unmatched identification power of mass spectrometry and from the rapid transfer of electrochemical cell products to the mass spectrometer. Most of the current EC/MS efforts are directed towards the development of differential electrochemical mass spectrometry (DEMS) using the electro-ionization source for identification of volatile compounds, and towards the application of electrospray mass spectrometers for determination of semivolatile and nonvolatile products. The challenges in coupling mass spectrometry and electrochemistry are described, and different approaches to using the coupled system for diverse applications are reviewed, with emphasis on electrospray mass spectrometry. Reaction mechanism studies, diagnostic applications, and activity imaging of electrodes are demonstrated based on approaches that were devised in our laboratory.

A new family of copper ligand-exchange selectors, L- or D-β-amino alcohols, is employed for the chiral separation of D,L-dansyl-amino acids, unmodified amino acid racemates, phenylalanine and tryptophan, and β-blocker L,D-propranolol by SDS-micellar electrokinetic chromatography and by electrophoretic chromatography in a low molecular weight organogel (LMOG)-filled capillary. The LMOG comprised a self-assembled fibrillar gel of trans-(1S,2S)-1,2-bis-(dodecylamido) cyclohexane in methanol. The di-L-valinol-copper complex exhibited the best performance on LMOG-CE compared with all other β-amino alcohol-copper selectors. The dependence of chiral resolution on the pH*, the ratio between the copper and the L-valinol ligand and the concentration of added selector complex in the run buffer were investigated revealing a marked difference between the activity of the copper-valinol and the previously studied copper-valine selector. The optimal separation conditions were achieved using a 2:1 valinol/copper ratio, in accordance with the 2:1 structure of the complex, which was proven by single crystal and powder X-ray diffractions and by elemental analysis. Unlike the copper-valine selectors that could be used only under acidic conditions (pH* 3.5), the copper-valinol selectors could be used under near-neutral conditions and even at pH* 9.1. A comparison between SDS-micellar electrokinetic chromatography and LMOG-CE under otherwise identical conditions revealed a significant superior separation on the LMOG-filled capillaries.

A low molecular weight hydrogel which exhibits electroosmotic flow is described, and its use for separation and biocatalytic applications that require passage of a solvent stream through the gel is demonstrated.

A generic family of low molecular weight binary gels comprising beta-cyclodextrin (beta-CD) and one of a large variety of polyaromatic hydrocarbons (PAHs) in dimethylformamide (DMF), pyridine, and other polar solvents is described. The system is rather general and robust. It tolerates large changes in each of the major ingredients without losing gelation ability. alpha- and gamma-CD, and negatively or positively modified beta-CD (e.g., sulfate-, phosphate-, or amine-tethered beta-CD) as well as methylated beta-CD are all effective gelators. The cogelators encompass a similarly large variety of compounds characterized by the ability to form an ovular inclusion complex with the CD molecules and a capability to stack outside the CD cap to give long-range order far from the CD cap. Despite the low ratio between the CD and the cogelators, we show that most of the CD molecules are retained in the liquid phase and do not participate directly in the actual construction of the gel network. In fact, most of the sulfated and phosphated beta-CDs can be cleaned off the gel structure by electrophoresis, leaving an intact gel porous structure. The nonstoichiometric nature of the gel is underscored by the fact that one molecule of beta-CD can combine with as few as three molecules of chrysene or as many as 450 molecules of chrysene to gelate an additional 35,000-40,000 molecules of the solvent.

This review provides an overview of some exciting, new as well as somewhat older, directions in sol-gel electrochemical applications of silicates and composite silicate electrodes. Rather than provide an exhaustive account of all the many papers that have been published on sol-gel electrochemistry and composite sol-gel electrodes, we prefer to illustrate the versatility of sol-gel chemistry by a few select examples which on the one hand illustrate the power entailed in sol-gel technology for electrochemical applications, and on the other hand point to hot electrochemical fields in which more research is due and exciting developments are to be expected. We start this review with a brief historical perspective. The inorganic sol-gel and silicone electrochemistry fields are both rather old though never extensively dealt with particularly when it comes to sensing applications. In contrast, the sol-gel electrochemistry of inorganic-organic hybrids is a relatively young field whose importance was recognized only in the last 30 years, and despite, or maybe even owing to the late recognition it is being very extensively studied nowadays. The use of composite electrodes for sensing and other applications is emphasized in this review, and the fast evolving electrodriven deposition techniques are reviewed.

Chiral electroseparation is demonstrated, for the first time, by a low molecular weight organogel filled capillary. Five pairs of dansylated amino acids were separated by copper ligand exchange on a trans-(1S,2S)-1,2-bis-(dodecylamido) cyclohexane (1) gel in methanol. Low molecular weight organogels are emerging materials that form stable, fibrillar, thermoreversible and thixotropic gels without covalent bonding of their monomeric building blocks. The dependence of chiral resolution and complex formation stability on the pH*, the ratio between copper and the D-valine selector, as well as other parameters were investigated revealing trends that were unparalleled in previously reports on copper ligand exchange of dansylated amino acids. These observations were explained in view of a simple stacking model of (1) and the difference in axial ligation of the amide carbonyl backbone of the gel to the dansyl D- or L-amino acid:D-valine:copper ternary complexes.

EIS (electrolyte-insulator-semiconductor) sensors based on the functionalization of uncoated gold nanoparticles supported on a Si/SiO2 structure are presented. Oxygen plasma etching at moderate power (<200 W) provides a convenient and efficient way to remove organic capping agents from the gold nanoparticles without significant damage. Higher power intensities destroy the linkage between the SiO2 and the gold nanoparticles, and some of the gold nanoparticles are removed from the surface. The flat-band potential shift, i.e. the pH dependence of the gold-coated EIS sensors is similar (33 mV/pH) to the uncoated EIS pH-sensor. Lead, penicillin and glucose sensors were prepared by immobilization of β-cyclodextrin, penicillinase and glucose oxidase by various immobilization techniques.

Polysulfides are abundant form of reduced sulfur compounds whose distribution in aquatic systems continues to pose environmental challenge. The Gibbs free-energy of formation, enthalpy of formation, and standard entropy of inorganic polysulfides were derived based on measurements of the temperature-dependent distribution of inorganic polysulfides in supersaturated aqueous polysulfide solutions. The data complements the relevant Gibbs free-energy data that were derived in our recent publication. The thermodynamic data show that the average polysulfide length is increased and polysulfides dissolve better at elevated temperatures, though the extent of this increase is pH dependent. At high pH (pH > 10) increasing the temperature from 25 to 80 degrees C results in a 5.6% increase in the concentration of polysulfide bound sulfur (i.e., dissolved zerovalent sulfur) and increases the average chain length (n) by 0.2 sulfur atoms, whereas at pH 8.2 the n increases by 0.25, and the dissolved polysulfide sulfur increases threefold.

Low molecular mass organogels are nonconventional polymeric structures in which a minute amount of low molecular weight compound can reversibly gelify the whole solution without forming covalent bonds between the monomers. In this article, we demonstrate that certain electron acceptors (taking dinitrobenzoates as model compounds) that are incapable of gelifying the solvent on their own can assemble as much as a 15-16-fold larger amount of polyaromatic hydrocarbons (PAHs) and form two-component donor-acceptor organogels in different solvents. At the core of the long-range order stand donor-acceptor pairs. We assess our claims by detailed 1H NMR, spectrophotometry, fluorescence, and time-resolved fluorescence methods. The thermodynamics of the gelation process is described on the basis of temperature dependent 1H NMR studies. We believe that, in this case, 1H NMR provides direct quantification of the dissolved concentrations of the different species and therefore provides a direct way to measure the enthalpy, entropy, and free energy associated with gel formation.

OCS formation by the reaction of inorganic polysulfides with carbon monoxide, which are both abundant in natural aquatic systems, was studied as a model abiotic route for OCS formation in the dark. The net OCS accumulation rate was a function of a bimolecular formation reaction and simultaneous OCS hydrolysis kinetics. The reaction of polysulfides with CO in the dark was found to be first order with respect to CO concentration and first order with respect to the molar sum of the polysulfide species generated by the disproportionation of the dissolved polysulfide precursors. The pH dependence of the OCS production rate was controlled by the pH-dependent disproportionation of polysulfide precursors. Lower temperatures, intermediate redox potentials, and moderately basic pH conditions increase the steady-state concentration of OCS. The speciation of polysulfides in aqueous solutions is still disputed. Some authors claim that hexasulfide is one of the dominant species while others believe that pentasulfide is the largest sulfide species in aqueous systems. Despite the disagreement between different models for speciation of polysulfides, the proposed rate law agreed very well with the thermodynamic data based on four and on five polysulfide species, with only minor differences in the preexponential kinetic coefficients.

In-house reference materials (IHRMs) for determination of inorganic polysulfides in water have been developed. The determination is based on the polysulfides’ derivatization with a methylation agent followed by GC/MS or HPLC analysis of the difunctionalized polysulfides. Therefore, the IHRMs are synthesized in the form of dimethylated polysulfides containing from four to eight atoms of sulfur. The composition of the compounds is confirmed by NMR and by the dependence of the HPLC retention time of the dimethyl polysulfides on the number of sulfur atoms in the molecule. Stability of the IHRMs is studied by HPLC with UV detection at 230 nm. Carbon tetrachloride solutions of the dimethyl polysulfides are stable at −20°C for 2 weeks, while their solutions in a mixture of acetonitrile and formic acid, 1:1, are stable even at +5°C for 3 weeks. The total sulfur content is controlled by the IHRMs’ oxidation with perchloric acid in high-pressure vessels (bombes) followed by determination of the formed sulfate using ICP-AES. Certified values of the dimethyl polysulfide concentrations are in the range of 416–3,327 ppm.

A new approach based on rapid, chemical derivatization in a single phase was used to determine the disproportionation constants and the underlying thermodynamics of inorganic polysulfides in aqueous solutions. This method resolves the dispute over the existence of hexasulfide in aqueous solutions and establishes the presence of even higher polysulfide chains in water. The Gibbs free energies of formation (G(Sn)(o)2-) for the polysulfide species are 77.4, 71.6, 67.4, 66.1, 67.2, 70.5, and 73.6 kJ/mol for n = 2-8, respectively. Our approach is based on single phase, fast methylation of polysulfides with methyl trifluoromethanesulfonate (methyl triflate) and subsequent determination of the dimethylpolysulfides by HPLC. Two independent methods were used in order to confirm quantitative equivalence between the observed distribution of dimethylpolysulfides and the polysulfide distribution in the water: (i) Kinetic studies of each competing reaction step showed that the kinetics of the derivatization are faster than each of the competing reactions that may lead to disproportionation and deviation of the observed distribution of dimethylpolysulfides from that of the aqueous polysulfides. (ii) Determination of isotope mixing during the derivatization of a mixture of two solutions, one containing polysulfide of natural isotopic distribution and the second containing 34S-rich polysulfide revealed that polysulfide mixing during derivatization is rather low. The systematic error due to redistribution of pentasulfide during derivatization is 3% based on isotope dilution tests and less than 5% of total zero-valent sulfur based on kinetic considerations.

The rate of the reversible homogeneous disproportionation of polysulfides was studied by following the optical absorbance of polysulfide solutions in a continuous plug flow reactor equipped with an on-line photometric detector. In order to avoid heterogeneous slow reactions involving sulfur colloids or precipitate, the reaction was initiated by an abrupt pH change from an undersaturated solution containing predominantly tetrasulfide species to a pH where pentasulfide is the dominant species. The disproportionation was found to follow first order reversible reaction dynamics. At environmentally relevant conditions the characteristic time of the disproportionation reaction is of the order of 10 s. This characteristic time implies that necessary conditions for speciation of the different polysulfide species by chromatography or another separation and subsequent quantification scheme should be of the order of 1 s.