Quartz Crystal Microbalance for Selenite Sensing Based on Growth of Cadmium Selenite Crystals Immobilized on a Monolayer of Phosphorylated 11-Mercapto-1-Undecanol

 A quartz crystal microbalance (QCM) sensor for selenite ions in aqueous solution was constructed based on crystal formation of cadmium selenite, immobilized with a self-assembly monolayer (SAM) of phosphorylated 11-mercapto-1-undecanol (MUD) on a QCM gold electrode surface. The mass change caused by the selective adsorption of selenite ions on the cadmium selenite crystals at the solid/solution interface was detected by the QCM. The response (−ΔF) of the modified QCM oscillator increased with increasing selenite ion concentrations in sample solutions, ranging from 9.7×10⁻⁵ to 9.0×10⁻⁴ M at pH 7.4. The synthetic process of anchoring cadmium selenite crystals on the phosphorylated MUD organic film was also followed by using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The atomic concentrations measured by XPS confirmed the crystal growth of cadmium selenite on the phosphorylated MUD SAM at the QCM gold electrode surface. From the AFM images, changes in surface topographic features were followed: the MUD SAM and phosphorylated MUD on the QCM gold electrode had similar surface roughness; however the difference for the cadmium selenite film on the phosphorylated MUD SAM was clearly seen. The observed QCM frequency change of the modified QCM oscillator per unit time was found to be proportional to the square of the supersaturation of cadmium selenite, indicating the crystal growth of cadmium selenite at the solid/solution interface. The modified QCM oscillator exhibited selectively strong QCM response to SeO₃ ²⁻ ion. In contrast, the responses to tested interfering anions were almost negligible. The order of anion selectivities of the present modified QCM sensor was SeO₃ ²⁻≫CO₃ ²⁻>SeO²⁻ ₄, SO₄ ²⁻, Br⁻, I⁻, NO₃ ⁻. These selectivities were basically attributable to the differences in solubility products and solubilities for the salts of each anion with cadmium (II) ion. Received May 12, 1998. Revision December 29, 1998.     

pH sensor based on polyaniline and aniline–anthranilic acid copolymer films using quartz crystal microbalance and electronic absorption spectroscopy

The pH sensitivity based on conducting polyaniline (PANI) and copolymer of aniline and o‐anthranilic acid (AA) films were studied using quartz crystal microbalance (QCM) technique and UV–Vis spectroscopy. The sensor was constructed from these polymer films coated on the electrode of the QCM. The resonant frequency changes as a function of pH in the range of 2–12 were measured. These changes are quantitative indication of the degree of dedoping or redoping of the polymer films upon the subsequent exposure of the electrode to 0.25 M sulfuric acid and different pH solutions. There are two linear regressions between the frequency change and pH with two different and opposite slopes in the regions from 2 to 9 and 9 to 12. The pH sensitivity of the copolymer film was found to be less than using the PANI film. Thin films of PANI and copolymer, which were chemically polymerized in a sulfuric acid solution, were deposited onto the inner walls of the quartz cuvettes. The UV–Vis absorption spectra of these films were measured in different pH solutions. Relations between the maximum absorption and its wavelength versus pH were constructed. The copolymer film shows some advantages over the PANI film. The difference between the PANI and copolymer films as pH sensors using the QCM and electronic absorption extends from the determination of pK_a for both films. Copyright © 2008 John Wiley & Sons, Ltd.     

Steric effects on the singlet-triplet energy gaps of five-membered ring R<Subscript>2</Subscript>C<Subscript>4</Subscript>H<Subscript>2</Subscript>M

Thermal internal energy gaps, ΔE _s−t; enthalpy gaps, ΔH _s−t; Gibbs free energy gaps, ΔD _s−t, between singlet (s) and triplet (t) states of R₂C₄H₂M (M = C, Si, and Ge) were calculated at B3LYP/6-311++G** level of theory. The ΔG _s−t of R₂C₄H₂C was increased in the order (in kcal/mol): R = −CH₃ (−10.51) > −H (−9.59) > i-Pr (−9.51) > t-Bu (−8.98). While, the ΔG _s−t of R₂C₄H₂Si and R₂C₄H₂Ge were increased in the order (in kcal/mol): −CH₃ (17.01) > i-Pr (15.30) > −H (15.26) > t-Bu (14.35) and -H (22.79) > −CH₃ (22.69) > i-Pr (21.66) > t-Bu (21.01), respectively.     

A Novel QCM-based Biosensor for Detection of Microorganisms Producing Hydrogen Sulfide

Abstract

A novel piezoelectric quartz crystal microbalance (QCM) device with gas permeable membrane is proposed for the detection of microorganisms producing hydrogen sulfide (H₂S). The detection theory is based on the adsorption of hydrogen sulfide onto the silver electrode of the piezoelectric crystal sensor, which causes a dramatic decrease in the resonant frequency of QCM. A 100 Hz frequency shift is chosen as the criteria value to judge the presence of microorganisms producing H₂S. Factors affecting detection were investigated. Desiccant is of great practical importance in sensor response. This new biosensor can be a potential candidate for detecting bacteria which produce hydrogen sulfide.     

Photoreduction ofortho-benzoquinones in the presence ofpara-substitutedN,N-dimethylanilines

Photoreduction ofo-benzoquinones irradiated at the wavelengths λ_max ≈ 400 and 600 nm corresponding to the S(π → π*) and S(n → π*) electron transitions in the >C=0 groups, respectively, in the presence ofN,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (k _H) ofo-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcitedo-quinone molecule (ΔG _e.t). The ΔG _e.t. values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state ofo-quinones, the reduction energies ofo-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔG _e.t≈0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1515–1521, September, 2000.     

Detecting solution pH changes using poly (N-isopropylacrylamide)-co-acrylic acid microgel-based etalon modified quartz crystal microbalances

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel-based etalons have been shown to have visible color and unique spectral properties, which both depend on solution temperature and pH. In this investigation, pNIPAm-co-AAc microgel-based etalons were fabricated on the Au electrode of a quartz crystal microbalance (QCM), and the resonant frequency of the QCM monitored as a function of temperature, at pH 3.0. Furthermore, the resonant frequency at either pH 3.0 or 7.0 was monitored while keeping the solution temperature constant at various temperatures. In all cases, when the solution temperature was below the collapse transition for the microgels (∼32 °C), the resonant frequency at pH 3.0 was lower than at pH 7.0, which we attribute to the film transitioning from a deswollen to swollen state, respectively. It was observed that the magnitude of the resonant frequency change increased as the solution temperature approached the collapse temperature for the microgels. The overall sensitivity to pH was determined to be 1.3 × 10⁻⁸ M [H⁺] Hz⁻¹ and a theoretical detection limit of 390 nM was obtained. This sensitivity will be exploited further for future biosensing applications.     

Silver Electrodeposition from Cyanide Solutions: Effect of Lead Ions

Effect of lead hydroxy compounds on the process of electrodeposition of silver from cyanide electrolytes is studied on an electrode whose surface is renewed in solution by cutting off a thin layer of metal. This permitted to perform the study on both the freshly renewed electrode and at controlled values of the time of the electrode contact with solution t. Shown is that on the freshly renewed electrode (t<1 s) the presence in the solution of lead ions in concentrations c ₁ on the order of 10^–5 M leads to the process depolarization only in the initial portion of a polarization curve. With c ₁ increased to 10^–4 M the effect of depolarization extends on the entire polarization curve. Keeping the electrode in solution after the renewal of the metal surface magnifies depolarization, and the greater the concentration c ₁, the shorter the time period t required to achieve the same effect. These regularities are attributed to catalytic influence of lead adatoms, whose surface concentration depends on c ₁ and t, as well as on the intensity of their incorporation in the silver deposit.     

Hyphenation of liquid chromatography to ion trap mass spectrometry to identify minor components in polypeptide antibiotics

The application of liquid chromatography–ion trap mass spectrometry for the characterization of linear and cyclic polypeptide antibiotics was investigated. The aim was on-line identification of impurities in those antibiotic complexes without recourse to time-consuming isolation and purification procedures. Hyphenated techniques, such as liquid chromatography coupled to mass spectrometry, are ideally suited for this purpose. Characterization was performed with an ion trap mass spectrometer offering MSⁿ capability; this enables more structural information to be obtained. Liquid chromatography in combination with ion trap mass spectrometry was successfully applied for the characterization of impurities in gramicidin, polymyxin B, polymyxin E, and bacitracin and the study of the degradation products of polymyxins B and E.     

Electrochemical decomposition of layer-by-layer thin films composed of TEMPO-modified poly(acrylic acid) and poly(ethyleneimine)

The decomposition of layer-by-layer (LbL) thin films composed of 2,2,6,6-tetramethylpiperidine-1-oxyl free radical-appended poly(acrylic acid) (TEMPO-PAA) and poly(ethylenimine) (PEI) was studied by using a quartz crystal microbalance (QCM) and cyclic voltammetry. The electrode potential of the (PEI/TEMPO-PAA)₄/PEI film-coated Au resonator was scanned from +0.2 to +0.8 V vs Ag/AgCl. The CV showed that the oxidation peak current decreased as the number of scans increased. The change in the resonance frequency of the QCM increased after electrolysis, indicating that the film was decomposed by electrolysis. The positive charges originating from the oxoammonium ions probably destabilized the (PEI/TEMPO-PAA)₄/PEI film. Furthermore, the release of 5,10,15,20-tetraphenyl-21H,23H-porphine tetrasulfonic acid (TPPS) from TPPS-loaded (PEI/TEMPO-PAA)₄/PEI-coated ITO electrodes was investigated. TPPS was released at electrode potentials greater than +0.6 V by the decomposition of the film. The results suggest that TEMPO-PAA/PEI LbL films are suitable for electrochemically controlled drug delivery systems.     

Dynamics of cyclodimerization and viscoelasticity of photo‐crosslinkable PVA

We investigated the interfacial properties of poly(vinyl alcohol) carrying UV‐crosslinkable pendant quaternized stilbazole (styrylpyridinium), PVA‐SbQ. The extent and dynamics of PVA‐SbQ cyclodimerization reactions and crosslinking induced by UV irradiation were monitored in situ and in real time by quartz crystal microgravimetry (QCM). Sensograms reflecting time‐dependent changes in density and viscoelasticity of crosslinking films followed a Boltzmann sigmoidal model, depending on precursor film composition and irradiation power. The shifts in QCM frequency and energy dissipation upon PVA‐SbQ cyclodimerization correlated with three photo‐crosslinking phases involving soft‐to‐rigid transitions, namely, induction (initiation), main crosslinking (interaction), and termination. PVA‐SbQ films crosslinked to different degree were used as protein carriers and a slower release profile was determined for the films that underwent more extensive crosslinking. Overall, this study demonstrates for the first time the dynamics of PVA‐SbQ crosslinking and its impact in system viscoelasticity and protein release. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 345–355     

Study of the adsorption of cytochrome c on a gold nanoparticle – modified gold electrode by using cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry

Adsorption of cytochrome c (Cyt c) on a gold nanoparticle – modified gold electrode was studied by using cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry in a phosphate buffer solution. It is shown that the charge transfer resistance is directly proportional to the amount of adsorbed Cyt c. The effects of temperature and time on the course of adsorption were also studied. The trends obtained in ΔG_ADS showed that Cyt c was found to have a smaller affinity for the modified electrode as indicated by their smaller negative ΔG_ADS values.     

Quartz crystal microbalance monitoring of density changes in mesoporous TiO2 phytate films during redox and ion exchange processes

Nanofilm deposits of TiO₂ nanoparticle phytates are formed on gold electrode surfaces by ‘directed assembly’ methods. Alternate exposure of a 3-mercapto-propionic acid modified gold surface to (i) a TiO₂ sol and (ii) an aqueous phytic acid solution (pH 3) results in layer-by-layer formation of a mesoporous film. Ru(NH₃)₆³⁺ is shown to strongly adsorb/accumulate into the mesoporous structure whilst remaining electrochemically active. Scanning the electrode potential into a sufficiently negative potential range allows the Ru(NH₃)₆³⁺ complex to be reduced to Ru(NH₃)₆²⁺ which undergoes immediate desorption. When applied to a gold coated quartz crystal microbalance (QCM) sensor, electrochemically driven adsorption and desorption processes in the mesoporous structure become directly detectable as a frequency response, which corresponds directly to a mass or density change in the membrane. The frequency response (at least for thin films) is proportional to the thickness of the mass-responsive film, which suggests good mechanical coupling between electrode and film. Based on this observation, a method for the amplified QCM detection of small mass/density changes is proposed by conducting measurements in rigid mesoporous structures.     

Enthalpies of mixing of some nitriles aqueous solutions with dodecylsurfactants micellar solutions

The enthalpies of mixing of some n-nitriles (from acetonitrile to valeronitrile) aqueous solutions with dodecyltzimethylammonium bromide, sodium dodecylsulfate and dodecyltzimethylammonium oxide micellar solutions were determined. The measurements were performed by systematically changing the surfactant concentration at a given solute concentration. The experimental enthalpies were rationalized in terms of the standard enthalpy of transfer of solute from the aqueous to the micellar phase and of the distribution constant between the two phase. Information on the effect of the nature of the surfactant on the standard thermodynamic quantities of transfer(G _t ^o , H _t ^o , TS _t ^o ) is reported. The present data are compared to those previously reported for primary alcohols and the solubilizing properties shown by the different types of micelles are discussed.     

Poly‐ε‐Lysine Modified Nanocarbon Film Electrodes for LPS Detection

We developed an electrochemical system for detecting lipopolysaccharide (LPS) that uses an ultraflat nanocarbon film electrode modified with poly‐ε‐lysine with a high affinity to LPS. LPS was captured on the modified electrode, and then ferrocene labeled polymyxin B (FcPMB) was captured on the LPS adsorbed electrode via the LPS‐PMB affinity interaction. The adsorbed FcPMB provided an amplified response with Fe²⁺ ions, and the current response was dependent on the amount of captured LPS (LOD=2.0 ng/mL). This was due to the efficient accumulation of the obtained current for LPS and the very low noise made possible by the ultraflat surface.     

Isolation and structural characterization of polymyxin B components

Polymyxin B is a peptide antibiotic complex present as sulphate. The components were separated preparatively on a poly(styrene-divinylbenzene) (PLRP-S), 1000 A, 8 microm, 250 x 12.5 mm I.D. stationary phase maintained at 60 degrees C and using 215 nm detection. Elution was carried out with acetonitrile-sodium sulphate solution (0.7%, m/v; pH adjusted to 2.5 with trifluoroacetic acid)-water (18:50:32, v/v) at a flow-rate of 4.0 ml/min. Seven polymyxin B components were isolated and characterized using 1H and 13C NMR. The molecular masses were confirmed by mass spectrometry. The structures of two components were determined for the first time. Polymyxins B5 and B6 were identified as having the same composition as polymyxin B1 except that the fatty acid moiety was nonanoic acid and 3-hydroxy-6-methyloctanoic acid, respectively.     

Kinetics of mixed-controlled oxygen reduction at nafion-impregnated Pt-alloy-dispersed carbon electrode by analysis of cathodic current transients

The effects of Co alloying to Pt catalyst and Nafion pretreatment by NaClO₄ solution on the rate-determining step (RDS) of oxygen reduction at Nafion-impregnated Pt-dispersed carbon (Pt/C) electrode were investigated as a function of the potential step ΔE employing potentiostatic current transient (PCT) technique. For this purpose, the cathodic PCTs were measured on the pure Nafion-impregnated and partially Na⁺-doped Nafion-impregnated Pt/C and PtCo/C electrodes in an oxygen-saturated 1 M H₂SO₄ solution and analyzed. From the shape of the cathodic PCTs and the dependence of the instantaneous current on the value of ΔE, it was confirmed that oxygen reduction at the pure Nafion-impregnated electrodes is controlled by charge transfer at the electrode surface mixed with oxygen diffusion in the solution below the transition potential step |ΔE _tr| in absolute value, whereas oxygen reduction is purely governed by oxygen diffusion above |ΔE _tr|. On the other hand, the RDS of oxygen reduction at the partially Na⁺-doped Nafion-impregnated electrodes below |ΔE _tr| is charge transfer coupled with proton migration, whereas above |ΔE _tr|, it becomes proton migration in the Nafion electrolyte instead of oxygen diffusion. Consequently, it is expected in real fuel cell system that the cell performance is improved by Co alloying since the electrode reaches the maximum diffusion (migration) current even at small value of |ΔE|, whereas the cell performance is aggravated by Nafion pretreatment due to the decrease in the maximum diffusion (migration) current.     

Determination of levofloxacin hydrochloride with multiwalled carbon nanotubes-polymeric alizarin film modified electrode

Multiwalled carbon nanotubes-polymeric alizarin film modified electrode was made. The electrochemical behavior of levofloxacin hydrochloride on modified electrode was studied with cyclic voltammetry, linear sweep voltammetry and chronopotentiometry. The results indicated that the electrical oxidation of levofloxacin hydrochloride on MWNT-PAR electrode, in HAc-NaAc buffer solution at pH 4.2 was irreversible and was controlled by diffusion. Some important parameters m, n, D, E _D, ΔS _rc and ΔH _rc of the electrochemical process were evaluated. Good linearity relationship between peak current and its concentration in the range of 5.0 × 10⁻⁶–1.0 × 10⁻⁴ mol l⁻¹ was found, of which the equation was I _p(A) = −5.456 × 10⁻⁶ 0.2667c, the correlative coefficient r = −0.9976 and detect limitation was 4.0 × 10⁻⁷ mol l⁻¹. The recovery of levofloxacin hydrochloride in levofloxacin hydrochloride injection was between 94.6 and 104.7%.     

Singlet-triplet splitting of divalent five-membered fused rings C<Subscript>12</Subscript>H<Subscript>8</Subscript>M (M = C,Si, Ge,Sn, and Pb): Application in nanocomplex synthesis

Energy differences, ΔX _s−t (X = E, H, and G) (ΔX _s−t = X(singlet) − X(triplet)) between singlet (s) and triplet (t) states of C₁₂H₈M were calculated at B3LYP/6-311+G*. The DFT calculations indicated that the ΔG _s−t between singlet (s) and triplet (t) states of C₁₂H₈M were increased from M = C to M = Pb. The ΔG _s−t of C₁₂H₈M was compared with its analogue C₄H₄M through replacement of heavy atoms from M = C to M = Pb. Configurations of the electrons in orbitals (σ² or π²) for the singlet state of C₁₂H₈M were discussed.     

Thermal Decompositions of the Molybdotellurates of Nickel(II) and Cobalt(II)

Molybdotellurates [M(H₂O)₆]₃·[TeMo₆O₂₄], with M=Ni(II) and Co(II), were synthesized and characterized by single-crystal X-ray diffraction for compound 1 and X-ray powder diffraction for compound 2, EDAX, IR, electronic spectra in the solid phase and in solution, and magnetic properties. Thermogravimetry and differential scanning calorimetry of both compounds revealed a loss of 11 water molecules through an endothermal process with ΔH=800 kJ mol⁻¹ for the nickel compound and ΔH=833 kJ mol⁻¹ for the cobalt compound. The residual compounds were characterized by chemical analysis, IR and XPS spectroscopy This revised version was published online in July 2006 with corrections to the Cover Date.     

1H NMR study of self-association of actinomycin D in an aqueous solution

This paper presents the results of a proton magnetic resonance study (500 MHz) of self-association of actinomycin D (AMD) antibiotic in an aqueous solution. The equailibrium constants and thermo-dynamic parameters (ΔH, ΔS) of molecular association as well as the limiting values of proton chemical shifts of associate molecules were determined from the concentration and temperature dependences of¹H NMR chemical shifts of AMD. The results were analyzed using dimeric and infinite-dimensional cooperative models of molecular self-association. The value of the cooperativity parameter indicates that AMD self-association is anticooperative, i.e., formation of aggregates larger than dimers is energetically unfavorable. The values of induced proton chemical shifts were used to determine the most probable mutual orientation of chromophores in AMD structure. Sevastopol State Technical University. Berkbeck College, London University. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 1, pp. 81–88, January–February, 1995. Translated by L. Smolina