Analysis of dopamine and tyrosinase activity on ion-sensitive field-effect transistor (ISFET) devices

Dopamine (1) and tyrosinase (TR) activities were analyzed by using chemically modified ion-sensitive field-effect transistor (ISFET) devices. In one configuration, a phenylboronic acid functionalized ISFET was used to analyze 1 or TR. The formation of the boronate-1 complex on the surface of the gate altered the electrical potential associated with the gate, and thus enabled 1 to be analyzed with a detection limit of 7x10(-5) M. Similarly, the TR-induced formation of 1, and its association with the boronic acid ligand allowed a quantitative assay of TR to be performed. In another configuration, the surface of the ISFET gate was modified with tyramine or 1 to form functional surfaces for analyzing TR activities. The TR-induced oxidation of the tyramine- or 1-functionalized ISFETs resulted in the formation of the redox-active dopaquinone units. The control of the gate potential by the redox-active dopaquinone units allowed a quantitative assay of TR to be performed. The dopaquinone-functionalized ISFETs could be regenerated to give the 1-modified sensing devices by treatment with ascorbic acid.     

Photocurable ISFET for anionic surfactants. Monitoring of photodegradation processes

The preparation of a new ion-selective field-effect transistor (ISFET) based on a photocurable membrane sensitive to anionic surfactants is described. The membrane is formed by an urethane-acrylate matrix with 2-cyanophenyl octyl ether as the plasticiser. When compared to conventional ion-selective electrodes, the prepared ISFETs do not show significant differences in sensitivity and reproducibility (P=0.05). When calibrating with dodecylbenzenesulfonate (DBS(-)) the prepared ISFETs show a nernstian behaviour, with a slope of 57.5 mV per decade. The linear working range is 1.0x10(-3) to 3.0x10(-6) M DBS(-) and the detection limit is 1.2x10(-6) M. The response times were below 0.7 min in all cases (95% of the step change). As the application, photodegradation processes using titanium dioxide dispersions, were monitored for two common anionic surfactants: DBS(-), being aromatic, and the more alkylic dodecylsulfate, DS(-). The determination of surfactant concentration was performed following a standard addition methodology, using ISFETs as the sensors, and without any previous separation stages. The degradation kinetics in both cases are first-order processes, with half-life times (t(0.5)) of 31.5 min for DBS(-) and 52.0 min for DS(-).     

Surface Complexation Modeling of K(+), NO(3)(-), SO(4)(2-), Ca(2+), F(-), Co(2+), and Cr(3+) Ion Adsorption on Silica Gel

Surface complex formation of K(+), NO(3)(-), SO(4)(2-), Ca(2+), F(-), Co(2+), and Cr(3+) ions was determined on the surface of silica gel. Experimental data obtained by acid-base titration of suspensions were interpreted in terms of the triple-layer model. The value of the deprotonation constant of surface OH could be determined precisely but the protonation constant was rather uncertain. The logarithms of ion pair formation constants for K(+), NO(3)(-), Ca(2+), and SO(4)(2-) adsorbed in the beta-plane are log K(ipM,X) approximately 0, therefore these species can be considered inert ions in the investigated pH range. F(-), Co(2+), and Cr(3+) ions were found to be strongly sorbed in the o-plane. In order to provide a good fit and to obtain parameters independent of their initial values, all possible equilibrium must be accounted for in the models. Copyright 2001 Academic Press.     

Electrochemical behavior of pyrroloquinoline quinone immobilized on silica gel modified with zirconium oxide

Pyrroloquinoline quinone (PQQ) was immobilized on the silica gel surface modified with zirconium oxide, designated as Si:Zr, by the carboxylic groups of the PQQ molecule and the zirconium oxide on the silica surface. The electrochemistry of PQQ immobilized on the Si:Zr matrix, incorporated in a carbon paste electrode, was evaluated using the cyclic voltammetry technique. The Si:Zr:PQQ-modified electrode showed a redox couple at E(m)=(E(pa1)+E(pc))/2=-0.150 V vs SCE at pH 7, close to that observed in aqueous solution, and another oxidation peak, E(pa2)=-0.100 V vs SCE. Studies in different pH solutions in the range of 3-7 showed that the first oxidation peak, E(pa1), is highly dependent on the solution pH shifting from to -0.175 to 0.100 V vs SCE, while E(pa2) remains practically constant at 0.100 V as the pH decreases from 7 to 3. The immobilized PQQ electrode presented the property to electrocatalyze the NADH at 150 mV vs SCE. The effect of addition of Ca(2+) ions on the electrode electroactivity for the NADH oxidation was also verified. Different from that observed for the PQQ immobilized on other electrode materials, the Ca(2+) ions did not influence the electrocatalytical response; however, the electrode stability was considerably improved in the presence of Ca(2+) ions, indicating that the matrix surface has a great influence on the electrochemical behavior of PQQ.     

Borazon-gate pH-sensitive field effect transistor

A borazon-gate ISFET is used as a pH sensor. Boron nitride was deposited by the reactive-pulse plasma method and electron diffraction served for membrane identification. The borazon-gate sensors responded linearly to pH in the range 1.8–10; the slope was about 52 mV pH^?1. Selectivity for H⁺ ions over K⁺, Na⁺ or Ca²⁺ ions was better than that of silicon nitride-gate ISFETs.     

Preparation of an ion-imprinted fiber for the selective removal of Cu2+

A novel Cu(2+)-imprinted fiber (IIF) was prepared by grafting acrylic acid (AA) onto the surface of a polypropylene (PP) fiber and subsequently modified with polyethylenimine (PEI). An examination by infrared spectroscopy and scanning electron microscopy confirmed that the ion-imprinted polymer was successfully introduced onto the surface of a PP fiber. The modification of PP fibers with AA was beneficial to the grafting of PEI onto the fibers. The highest grafting degree of PEI could reach 120 wt % under optimal grafting conditions. This IIF showed excellent tensile and chemical stability in acid solution, which qualified the IIF for practical applications. Besides having a high adsorption capacity for Cu(2+) (120 mg/g), the IIF adsorbent showed a high selectivity for Cu(2+) as compared with that of the non-ion-imprinted fiber (NIF). The dynamic adsorption results indicated that IIF can thoroughly remove Cu(2+) from the solution in a relatively short contact time. The effective treatment volume was about 910 bed volumes. The selectivity coefficient of IIF for Cu(2+) with respect to Zn(2+) could reach 76.4. IIF also has good regeneration performance and could maintain almost the same adsorption capacity for copper ions after 10 adsorption-desorption cycles.     

Determination of hardness in tapwater and upland soil extracts using a long-term stable divalent cation selective electrode based on a lipophilic acrylate resin as a membrane matrix

A divalent cation-selective electrode, which utilizes a lipophilic resin as a matrix for the sensing membrane, and which has long-term stability has been developed. The sensing membrane is a lipophilic acrylate resin which is impregnated with a solution of 1-decylalcohol and the calcium salt of bis[4-(1,1,3,3-tetramethylbutyl) phenyl] phosphate at concentrations of 0.08 g ml(-1) each. The electrode exhibited nearly equal selectivity to Ca(2+) and Mg(2+) ions and could be used as a water hardness sensor. The electrode shows a Nernstian response with a slope of 29 mV decade(-1) to both Ca(2+) and Mg(2+) ions in the concentration range from 10(-5) M to 10(-1) M and could be used in the pH range from 3 to 10 for the determination of 10(-3) M Ca(2+) and Mg(2+) solutions. The initial performance of the electrode could be maintained for 1 year, since the lifetime test of the electrode was conducted in tapwater at a continuous flow rate of 4 ml min(-1). The hardnesses of tapwater and upland soil extracts were determined using the developed electrode and the analytical results were in good agreement with those obtained by chelatometric titration using an EDTA solution as the titrant. A coefficient factor of correlation 0.998 was obtained between the electrode method and titrimetry. The long-term stability of the electrode was found to be due to strong affinity of 1-decylalcohol to the lipophilic acrylate resin.     

Evolution of nickel speciation during preparation of Ni-SiO(2) catalysts: effect of the number of chelating ligands in [Ni(en)x(H2O)6-2x]2+ precursor complexes

The evolution of nickel speciation during the successive preparation steps of Ni-SiO(2) catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)(x)(H(2)O)((6-2x))](2+) (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H(2)O)(4)](2+) (64% of all Ni complexes), [Ni(en)(2)(H(2)O)(2)](2+) (81%) and [Ni(en)(3)](2+) (61%), respectively. Equilibrium adsorption of [Ni(en)(x)(H(2)O)((6-2x))](2+) on SiO(2) results in the selective grafting of [Ni(en)(H(2)O)(4)](2+) and [Ni(en)(2)(H(2)O)(2)](2+), through the substitution of two labile H(2)O ligands by two surface SiO(-) groups. The surface [Ni(en)(H(2)O)(2)(SiO)(2)] complex formed by the grafting of [Ni(en)(H(2)O)(4)](2+) onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)(2)(SiO)(2)], resulting from the grafting of [Ni(en)(2)(H(2)O)(2)](2+) onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni(2+) ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.     

Silica-coated Ln3+-Doped LaF3 nanoparticles as robust down- and upconverting biolabels

The preparation of nearly monodisperse (40 nm), silica-coated LaF(3):Ln(3+) nanoparticles and their bioconjugation to FITC-avidin (FITC=fluorescein isothiocyanate) is described in this report. Doping of the LaF(3) core with selected luminescent Ln(3+) ions allows the particles to display a range of emission lines from the visible to the near-infrared region (lambda=450-1650 nm). First, the use of Tb(3+) and Eu(3+) ions resulted in green (lambda=541 nm) and red (lambda=591 and 612 nm) emissions, respectively, by energy downconversion processes. Second, the use of Nd(3+) gave emission lines at lambda=870, 1070 and 1350 nm and Er(3+) gave an emission line at lambda=1540 nm by energy downconversion processes. Additionally, the Er(3+) ions gave green and red emissions and Tm(3+) ions gave an emission at lambda=800 nm by upconversion processes when codoped with Yb(3+) (lambda(ex)=980 nm). Bioconjugation of avidin, which has a bound fluorophore (FITC) as the reporter, was carried out by means of surface modification of the silica particles with 3-aminopropyltrimethoxysilane, followed by reaction with the biotin-N-hydroxysuccinimide activated ester to form an amide bond, imparting biological activity to the particles. A 25-fold or better increase in the FITC signal relative to the non-biotinylated silica particles indicated that there is minimal nonspecific binding of FITC-avidin to the silica particles.     

Competitive adsorption of Ca2+ and Zn(II) ions at monodispersed SiO2/electrolyte solution interface

A study of competitive adsorption of Ca(2+) and Zn(II) ions at the monodispersed SiO(2)/electrolyte solution interface is presented. Influence of ionic strength, pH, and presence of other ions on adsorption of Ca(2+) and Zn(II) in the mentioned system are investigated. zeta potential, surface charge density, adsorption density, pH(50%), and DeltapH(10-90%) parameters for different concentrations of carrying electrolyte and adsorbed ions are also presented. A high concentration of zinc ions shifts the adsorption edge of Ca(2+) ions adsorbed from solutions with a low initial concentration at the SiO(2)/NaClO(4) solution interface to the higher pH values. This effect disappears with a concentration increase of calcium ions. The presence of Ca(2+) ions in the system slightly affects the adsorption of zinc ions on SiO(2), shifting the adsorption edge toward lower pH values and thereby increasing the adsorption slope.     

ToF-SIMS depth profiling analysis of the uptake of Ba2+ and Co2+ ions by natural kaolinite clay

The sorption behavior of Ba(2+) and Co(2+) ions on a natural clay sample rich in kaolinite was studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Depth profiling at 10-A steps was performed up to a 70-A matrix depth of the clay prior to and following sorption. The results showed that Co(2+) is sorbed in slightly larger quantities than Ba(2+), with significant numbers of ions fixed on the outermost surface of the clay. Depletion of the ions K(+), Mg(2+), and Ca(2+) from the clay lattice was observed to accompany enrichment with Co(2+) and Ba(2+) ions. The data obtained using X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM) indicated insignificant structural and morphological changes in the lattice of the clay upon sorption of both Ba(2+) and Co(2+) ions. Analysis using energy dispersive X-ray spectroscopy (EDS) showed that the average atomic percentage (+/-S.D.) of Ba and Co on kaolinite surface were 0.49 +/- 0.11 and 0.61 +/- 0.19 , respectively, indicating a limited uptake capacity of natural kaolinite for both ions.     

A simple and controlled method of preparing uniform Ag midnanoparticles on Tollens-soaked silica spheres

Ag midnanoparticles (midnanoparticles are those particles whose diameters are in the range from 20 to 80 nm) with average size of 30-50 nm and tunable packing densities were formed on the surface of preformed Tollens-soaked silica spheres by a simple and controlled method. The process mainly involved two steps. In the first step the absorption of Ag(NH3)2(+) ions occurred on the silica spheres and in the second step Ag(NH3)2(+) ions on the silica spheres were reduced to Ag midnanoparticles in the presence of glucose solution. The amount of Ag midnanoparticles on the silica spheres could easily be tuned by varying the washing times in the process of preparing the Tollens-soaked silica spheres. The washing process also effectively avoided the reduction of Ag(NH3)2(+) ions and the nucleation of Ag particles in solution and easily produced more uniform Ag midnanoparticles on the silica spheres. Attributing to the uniform Ag midnanoparticles, the Ag midnanoparticle-coated silica spheres show unique optical properties in the UV-vis absorption spectra. The resulting Ag midnanoparticle-coated silica spheres were characterized with transmission electron microscopy, UV-vis-IR recording spectrophotometry, and X-ray photoelectron spectroscopy.     

Grafting of polyethylene glycols onto nanometer silica surface by 1,4‐phenylene diisocyanate

In this paper, polyethylene glycol (PEG) molecules have been grafted onto the surface of nanometer silica in toluene by using 1,4‐phenylene diisocyanate (PPDI) as a coupling agent, and dibutyltion dilaurate (DBTDL) as a catalyst. This process was executed by using a one‐step procedure involving a first reaction of PPDI with silica and a subsequent reaction of isocyanate‐bound silica with PEG. The PEG‐grafted silica has been characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and SEM analyses. The effects of reaction time, temperature and molar ratio of reactant on the effectiveness of the surface grafting were also investigated. Optimum grafting conditions of PEG were obtained at the temperature of 80 °C for 8 h. Maximum grafting of PEG molecules ratio was 22.6%, and maximum overall grafting ratio was 35%, as determined by TGA. Copyright © 2010 John Wiley & Sons, Ltd.     

Role of macrocyclic effect in complex formation of palladium(II) with ligands anchored on a solid support

The method for grafting dithiacrown ether and its linear analogue to the silica surface using preliminary prepared organosilicon derivative is developed. The importance of the acylation step in the process of grafting is underlined. The acylation is required to prevent sorption of platinum metals complex anions by the anion exchange mechanism, the probability of which increases due to protonation of the spacer in an acidic medium. The interaction of palladium(II) with anchored ligands in hydrochloric acid media was studied in details. The comparative study of two sorbents revealed that the macrocyclic effect plays a negligible role in binding of palladi um(II) ions. Therefore, linear ligands are preferred for the development of sorbents for molecular recognition of platinum metals ions.     

Development of microbiosensors

Semiconductor fabrication technology was used for development of ion sensitive field effect transistor (ISFET) and micro-electrodes which have been utilized as transducers of enzyme-based microbiosensors. A urea sensor consisted of two ISFETs; one ISFET is urease-coated ISFET and the other ISFET is reference ISFET. A linear relationship was obtained between the initial rate of voltage change and the logarithm of urea concentration over the range 1.3 to 16.7 mM. ATP and hypoxanthine sensors were also developed utilizing ISFET as a transducer. Furthermore, microelectrodes such as hydrogen peroxide and oxygen sensors were prepared by the silicone fabrication technology. A glucose sensor consisted of a hydrogen peroxide electrode and immobilized glucose oxidase membrane. A linear relationship was observed between the current increase and the concentration of glucose (1–100 mg dl⁻¹). A microoxygen electrode was constructed from Au electrodes, polymer matrix containing alkaline electrolyte and a photocross-linkable polymer membrane. This electrode was used as a transducer in microglucose sensor. A microglutamic acid sensor is also described.     

Reducing the interference from CO2 or organic acids in ion-selective polymer membrane sensors having a field-effect transistor as internal reference element

Changes in sample concentrations of CO₂ or organic acids cause potential instabilities when polymer membranes are directly applied to the surface of ion-selective field-effect transistors (ISFETs). Currently used designs avoid this well-documented effect by placing a layer of aqueous buffer between polymeric membrane and ISFET serving as internal reference element. Here, we propose another solution to the problem. In order to compensate for the effect of pH changes on the ISFET threshold voltage, a double membrane is applied whose inner layer is pH-sensitive, while the outer layer exposed to the sample is a conventional ion-selective membrane. It is shown that this approach strongly reduces the earlier-mentioned interference effects.     

Kinetic Stability Modelling of Keratinolytic Protease P45: Influence of Temperature and Metal Ions

The activity and kinetic stability of a keratinolytic subtilisin-like protease from Bacillus sp. P45 was investigated in 100 mM Tris-HCl buffer (pH 8.0; control) and in buffer with addition of Ca(2+) or Mg(2+) (1-10 mM), at different temperatures. Addition of 3 mM Ca(2+) or 4 mM Mg(2+) resulted in a 26% increment on enzyme activity towards azocasein when compared to the control (100%; without added Ca(2+) or Mg(2+)) at 55 °C. Optimal temperature for activity in the control (55 °C) was similar with Mg(2+); however, temperature optimum was increased to 60 °C with 3 mM Ca(2+), displaying an enhancement of 42% in comparison to the control at 55 °C. Stability of protease P45 in control buffer and with Mg(2+) addition was assayed at 40-50 °C, and at 55-62 °C with Ca(2+) addition. Data were fitted to six kinetic inactivation models, and a first-order equation was accepted as the best model to describe the inactivation of protease P45 with and without metal ions. The kinetic and thermodynamic parameters obtained showed the crucial role of calcium ions for enzyme stability. As biocatalyst stability is fundamental for commercial/industrial purposes, the stabilising effect of calcium could be exploited aiming the application of protease P45 in protein hydrolysis.     

Molecular dynamics study of the effect of calcium ions on the monolayer of SDC and SDSn surfactants at the vapor/liquid interface

The effect of Ca(2+) ions on the hydration shell of sodium dodecyl carboxylate (SDC) and sodium dodecyl sulfonate (SDSn) monolayer at vapor/liquid interfaces was studied using molecular dynamics simulations. For each surfactant, two different surface concentrations were used to perform the simulations, and the aggregation morphologies and structural details have been reported. The results showed that the aggregation structures relate to both the surface coverage and the calcium ions. The divalent ions can screen the interaction between the polar head and Na(+) ions. Thus, Ca(2+) ions locate near the vapor/liquid interface to bind to the headgroup, making the aggregations much more compact via the salt bridge. The potential of mean force (PMF) between Ca(2+) and the headgroups shows that the interaction is decided by a stabilizing solvent-separated minimum in the PMF. To bind to the headgroup, Ca(2+) should overcome the energy barrier. Among contributions to the PMF, the major repulsive interaction was due to the rearrangement of the hydration shell after the calcium ions entered into the hydration shell of the headgroup. The PMFs between the headgroup and Ca(2+) in the SDSn systems showed higher energy barriers than those in the SDC systems. This result indicated that SDSn binds the divalent ions with more difficulty compared with SDC, so the ions have a strong effect on the hydration shell of SDC. That is why sulfonate surfactants have better efficiency in salt solutions with Ca(2+) ions for enhanced oil recovery.     

The characterization for the binding of calcium and terbium to Euplotes octocarinatus centrin

Centrin is a member of the EF-hand superfamily that plays critical role in the centrosome duplication and separation. In the present paper, we characterized properties of metal ions binding to Euplotes octocarinatus centrin (EoCen) by fluorescence spectra and circular dichroism (CD) spectra. Changes of fluorescence spectra and alpha-helix contents of EoCen proved that Tb(3+) and Ca(2+) induced great conformational changes of EoCen resulting in exposing hydrophobic surfaces. At pH 7.4, Ca(2+) (and Tb(3+)) bond with EoCen at the ratio of 4:1. Equilibrium experiment indicated that Ca(2+) and Tb(3+) exhibited different binding capabilities for C- and N-terminal domains of protein. C-terminal domain bond with Ca(2+) or Tb(3+) approximately 100-fold more strongly than N-terminal. Aromatic residue-sensitized Tb(3+) energy transfer suggested that site IV bond to Tb(3+) or Ca(2+) more strongly than site III. Based on fluorescence titration curves, we reckoned the conditional binding constants of EoCen site IV quantitatively to be K(IV)=(1.23+/-0.51)x10(8)M(-1) and K(IV)=(6.82+/-0.33)x10(5)M(-1) with Tb(3+) and Ca(2+), respectively. Metal ions bond to EoCen in the order of IV>III>II, I.     

New membrane materials for potassium-selective ion-sensitive field-effect transistors

Several polymeric materials were studied as membrane materials for potassium-selective ion-sensitive field-effect transistors (ISFETs) to overcome the problems related with the use of conventional plasticized poly(vinyl chloride) membranes casted on ISFET gate surfaces. Several acrylate materials, such as ACE, Epocryl and derivatives, showed no reproducible results. Three room-temperature vulcanizing (RTV)-type silicone rubbers were tested. The addition-type RTV-2 silicone rubber was not suitable as a membrane material, but the condensation-type RTV-1 and especially the RTV-2 silicone rubber showed good results. ISFETs with a Silopren membrane showed a durability of at least 2 months.