Monodisperse micron-sized crosslinked polystyrene particles. IV. Effect of network structure on polymerization procedure

Monodisperse micron-sized polystyrene particles crosslinked using urethane acrylate were produced by dispersion polymerization in ethanol solution and the effect of the crosslinked network structure on the polymerization procedure was studied. The influences of the concentrations of the initiator and urethane acrylate on the particle diameter (D _n), the particle number density (N _p), and the polymerization rate (R _p) were found to obey the approximate relationships D _n ∝ [initiator]^0.43 [urethane acrylate]^0.05, N _p ∝ [initiator]^−1.30 [urethane acrylate]^0.19, and R _p ∝ [initiator]^{0.24 ± 0.02}. The power-law dependence of D _n and N _p on the initiator concentration showed a similar trend to that of linear polystyrene reported in the literature. Especially, it was found that urethane acrylate does not have a serious effect on D _n and N _p of the particles produced. The dependence of R _p on the initiator concentration was observed to be higher than that of linear polystyrene, suggesting that there is still competition between heterogeneous polymerization and solution polymerization because of the crosslinked network structure of the primary particle. Received: 1 April 1999 Accepted in revised form: 29 June 1999     

Improvement in the sensitivity of microfluidic ELISA through field amplified stacking of the enzyme reaction product

In this article, we demonstrate a novel approach to enhancing the sensitivity of enzyme-linked immunosorbent assays (ELISA) through pre-concentration of the enzyme reaction product (resorufin/4-methylumbelliferone) in free solution. The reported pre-concentration was accomplished by transporting the resorufin/4-methylumbelliferone molecules produced in the ELISA process towards a high ionic-strength buffer stream in a microfluidic channel while applying a voltage drop across this merging region. A sharp change in the electric field around the junction of the two liquid streams was observed to abruptly slow down the negatively charged resorufin/4-methylumbelliferone species leading to the reported pre-concentration effect based on the field amplified stacking (FAS) technique. It has been shown that the resulting enhancement in the detectability of the enzyme reaction product significantly improves the signal-to-noise ratio in the system thereby reducing the smallest detectable analyte concentration in the ELISA method. Applying the above-described approach, we were able to detect mouse anti-BSA and human TNF-α at concentrations nearly 60-fold smaller than that possible on commercial microwell plates. For the human TNF-α sample, this improvement in assay sensitivity corresponded to a limit of detection (LOD) of 0.102 pg mL⁻¹ using the FAS based microfluidic ELISA method as compared to 7.03 pg mL⁻¹ obtained with the traditional microwell plate based approach. Moreover, because our ELISAs were performed in micrometer sized channels, they required sample volumes about two orders of magnitude smaller than that consumed in the latter case (1 μL versus 100 μL).     

Production and analytical characterization of neopterin immunoreagents for biosensor developments

Neopterin is a valuable biomarker of cellular immunity associated with various pathological situations such as viral and bacterial infections, autoimmune, cardiovascular, neurodegenerative and malignant disorders. To produce specific antibodies against neopterin for a rapid multi-biomarker-based diagnosis, a novel hapten derivative was synthesized and attached to carrier proteins. The thoroughly characterized conjugates were used for immunization of BALB/c mice and rabbits. The produced monoclonal antibody reached in both direct and indirect enzyme-linked immunosorbent assay (ELISA) format LoD of 0.18 and 0.45 μg L⁻¹, respectively, and was a superior immunoreagent for further biosensor developments with regard to assay sensitivity and material availability. The best polyclonal antibody was somewhat more sensitive in direct ELISA with LoD of 0.05 μg L⁻¹. The optimized ELISA method was evaluated with blood samples collected from patients with renal insufficiency, patients with sepsis, patients without confirmed clinical diagnosis, and healthy volunteers. In plasma samples, neopterin concentrations ranging from 3.2 to 103 μg L⁻¹ could be determined with the monoclonal ELISA whereas twofold lower results were obtained with the polyclonal ELISA. A satisfactory correlation of results was found between the polyclonal ELISA and IBL Neopterin ELISA kit within the concentration range 0.5–16 μg L⁻¹ (R = 0.874; n = 40), and slightly lower correlation was found for monoclonal-based ELISA (R = 0.819; n = 40). These data show that the generated antibodies may be used as functional analytical reagents for the integration into multianalyte biochip detection systems.     

The effect of the electrolyte concentration in the solution on the voltammetric response of insertion electrodes

A solid-state redox reaction involving an insertion of ions is analyzed with respect to the influence of the concentration of inserting ions in the solution phase. The voltammetric response is independent of the mass transfer in the solution provided that z = (D _ss/D _aq)^1/2 ρ/[C⁺]* is smaller than 0.1 (D _ss: diffusion coefficient of the cation C⁺ in the crystal; D _aq: diffusion coefficient of the cation C⁺ in the solution; ρ: density of the solid compound; [C⁺]*: concentration of cations in the bulk of the solution). In real cases this condition will be satisfied at solution concentrations above 1 mol/l. Received: 15 December 1997 / Accepted: 5 March 1998     

The saddle point of the nucleophilic substitution reaction Cl− + CH3Cl: results of large-scale coupled cluster calculations

On the basis of large-scale coupled cluster calculations including connectedz triple substitutions in a perturbative way, the geometrical parameters of the D _{3 h} saddle point of the Walden inversion reaction Cl⁻ + CH₃Cl′→ ClCH₃ + Cl′⁻ are predicted to be R _s (C—Cl) = 2.301 ? and r _s (C—H) = 1.069 ?. The barrier height with respect to the reactants is recommended to be 11.5 ± 1.0 kJ mol⁻¹. Connected triple substitutions lower the barrier height by almost a factor of 2, but have very little influence on the geometric structure of the saddle point. Received: 26 June 1998 / Accepted: 15 July 1998 / Published online: 28 September 1998     

Thiol-stabilized luminescent CdTe quantum dot as biological fluorescent probe for sensitive detection of methyl parathion by a fluoroimmunochromatographic technique

Quantum dots (QDs) are preferred as high-resolution biological fluorescent probes because of their inherent optical properties compared with organic dyes. This intrinsic property of QDs has been made use of for sensitive detection of methylparathion (MP) at picogramme levels. The specificity of the assay was attributed to highly specific immunological reactions. Competitive binding between free MP and CdTe QD bioconjugated MP (MP-BSA-CdTe) with immobilized anti-MP IgY antibodies was monitored in a flow-injection system. The fluorescence intensity of MP-BSA-CdTe bioconjugate eluted from the column was found to be directly proportional to the free MP concentration. Hence, it was possible to detect MP in a linear range of 0.1–1 ng mL⁻¹ with a regression coefficient R ² = 0.9905. In this investigation, IgY proved advantageous over IgG class immunoglobulins in terms of yield, stability, cost effectiveness, and enhancement of assay sensitivity. The photo-absorption spectrum of bioconjugated CdTe QD (λ _max = 310 nm) confirmed nano-biomolecular interactions. The results suggest the potential application of bioconjugation and nano-biomolecular interactions of QDs for biological labeling and target analyte detection with high sensitivity.     

Investigation of calcium antagonist–L-type calcium channel interactions by a vascular smooth muscle cell membrane chromatography method

The dissociation equilibrium constant (K _D) is an important affinity parameter for studying drug–receptor interactions. A vascular smooth muscle (VSM) cell membrane chromatography (CMC) method was developed for determination of the K _D values for calcium antagonist–L-type calcium channel (L-CC) interactions. VSM cells, by means of primary culture with rat thoracic aortas, were used for preparation of the cell membrane stationary phase in the VSM/CMC model. All measurements were performed with spectrophotometric detection (237 nm) at 37 °C. The K _D values obtained using frontal analysis were 3.36 × 10⁻⁶ M for nifedipine, 1.34 × 10⁻⁶ M for nimodipine, 6.83 × 10⁻⁷ M for nitrendipine, 1.23 × 10⁻⁷ M for nicardipine, 1.09 × 10⁻⁷ M for amlodipine, and 8.51 × 10⁻⁸ M for verapamil. This affinity rank order obtained from the VSM/CMC method had a strong positive correlation with that obtained from radioligand binding assay. The location of the binding region was examined by displacement experiments using nitrendipine as a mobile-phase additive. It was found that verapamil occupied a class of binding sites on L-CCs different from those occupied by nitrendipine. In addition, nicardipine, amlodipine, and nitrendipine had direct competition at a single common binding site. The studies showed that CMC can be applied to the investigation of drug–receptor interactions.     

Micellization of triblock copoly(ethyleneoxide/tetrahydrofuran/ ethylene oxide)

 The association behaviour of triblock copoly(ethylene oxide/tetrahydrofuran/ethylene oxide), in particular E₁₀₀T₂₇E₁₀₀, in aqueous solutions has been investigated by means of static and dynamic light scattering, nuclear magnetic reso-nance (NMR) and surface tension techniques. On raising the polymer concentration at room temperature, the copolymer aggregates to form micelles with an aggregation number of about 105 (R _G, mic≈15 nm and R _H, mic≈13 nm, as revealed by light scattering and FT-PGSE NMR measurements, respectively). The micelles are kinetically quite stable, the micellar lifetime is shown to be more than 1 h. The residence time of a single unimer in a micelle is more than 140 ms. The apparent radius of gyration R _G, mic is fairly independent of concentration, but large effects are observed on varying the temperature. Raising the temperature initially results in an increase of the apparent micellar size, followed by a maximum at an intermediate temperature (≈45 °C). At higher temperatures a contraction of the micelles is observed. The shape of the micelles also appear to vary in this temperature interval. The interactions responsible for these phenomena are discussed in terms of, e.g., the temperature-dependent solubility of the alkylene oxide segments in water and polydispersity effects. Received: 29 January 1996 acccepted : 4 November 1996     

Chemiluminescent imaging analysis of interferon alpha in serum samples

Enzyme-linked immunosorbent assay (ELISA), horseradish peroxidase (HRP)-catalyzed fluorescent reaction, and oxalate chemiluminescence imaging analysis have been combined to develop a sensitive, simple, and rapid method for analysis of interferon alpha (α-IFN) in human serum samples. A typical “sandwich type” immunoassay was used. Reaction of o-phenylenediamine (OPD) with hydrogen peroxide (H₂O₂), catalyzed by HRP, produced 2,3-diaminophenazine (PDA), which was detected by chemiluminescence imaging analysis with the bis(2,4,6-trichlorophenyl)oxalate (TCPO)–H₂O₂–glyoxaline–PDA chemiluminescent system. The TCPO chemiluminescent imaging system is more sensitive and the chemiluminescence quantum yield is at least five times higher than for the luminol–H₂O₂–HRP–PIP (p-iodophenol) chemiluminescent imaging system. The results showed there was a very good linear correlation between response and amount of α-IFN in the range 1.3–156.0 pg mL⁻¹ (R = 0.9991) and the detection limit was 0.8 pg mL⁻¹ (S/N=3). The relative standard deviation (n = 9) was 4.7%. The proposed method has been used for successful analysis of the amount of α-IFN in human serum. The results obtained compared well with those obtained by conventional colorimetric ELISA and luminol chemiluminescent ELISA. Figure Procedures of the proposed method     

A study of dimethylferrocene diffusion and electrode kinetics using microelectrode voltammetry in poly(ethylene glycol) solvent

The heterogeneous electron transfer rate constant (k _s) of dimethylferrocene (DMFc) was estimated using cyclic voltammetric peak potential separations taken typically in a mixed diffusion geometry regime in a polyelectrolyte, and the diffusion coefficient (D) of DMFc was obtained using a steady-state voltammogram. The heterogeneous electron transfer rate constant and diffusion coefficient are both smaller by about 100-fold in the polymeric solvent than in the monomeric solvent. The results are in agreement with the difference of longitudinal dielectric relaxation time (τ_L) in the two kinds of solvents, poly(ethylene glycol) (PEG) and CH₃CN, indicating that k _s varies inversely with τ_L; k _s is proportional to D of DMFc. Both D and k _s of DMFc in PEG containing different supporting electrolytes and at different temperatures have been estimated. These results show that D and k _s of DMFc increase with increasing temperature in the polyelectrolyte, whereas they vary only slightly with changing the supporting electrolyte. Received: 5 February 1998 / Accepted: 23 July 1998     

A Selective Conversion of Benzylic Alcohols to the Corresponding Carbonyl Compounds by Means of an Ag(III) Complex

 Carbonyl compounds of the type XPhCOR (R = H, Me, Ph; X = H, Me, Cl, Br) are prepared in high yields by reaction of the corresponding benzylic alcohols XPhCHOHR with KNa₄ [Ag(HIO₆)₂]×12H₂O in alkaline solution. This method allows the selective oxidation of benzylic alcohols in compounds containing other types of alcoholic functional groups.     

Feasibility study for the rapid screening of target molecules using translational diffusion coefficients: diffusion-ordered NMR spectroscopy of biological toxins

A panel of 15 biological toxins ranging between ~60–28,000 g/mol was used to evaluate the feasibility of screening aqueous samples for toxin analytes based on their translational diffusion coefficients, D _t. Toxin D _t values were measured by pulsed-field gradient ¹H NMR spectroscopy using a bipolar pulse pair, longitudinal eddy current delay pulse sequence incorporating water suppression to achieve the maximum dynamic range for toxin signals. To collect data for an effective screening protocol, reference D _t values were determined from five independent measurements at both 25 and 37 °C for all toxins in the panel. In the protocol, D _t values are measured at both temperatures for a suspected toxin target in a sample, and for assignment as a potential toxin analyte, the measurements are required to fall within ±0.25 × 10⁻⁶ cm²/s of both reference D _t values for at least one toxin in the panel. Only solution viscosity was found to influence sample D _t measurements appreciably; however, the measurements are easily corrected for viscosity effects by calculating the D _t value of the suspected toxin at infinite dilution. In conclusion, the protocol provides a rapid and effective means for screening aqueous samples for all toxins in the panel, narrowing toxin identification to ≤2 possibilities in virtually all cases.     

Oleic acid-assisted preparation of LiMnPO<Subscript>4</Subscript> and its improved electrochemical performance by Co doping

LiMnPO₄, with a particle size of 50–150 nm, was prepared by oleic acid-assisted solid-state reaction. The materials were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The electrochemical properties of the materials were investigated by galvanostatic cycling. It was found that the introduction of oleic acid in the precursor led to smaller particle size and more homogeneous size distribution in the final products, resulting in improved electrochemical performance. The electrochemical performance of the sample could be further enhanced by Co doping. The mechanism for the improvement of the electrochemical performance was investigated by Li-ion chemical diffusion coefficient ( [(D)\tilde]_\textLi ) \left( {{{\tilde{D}}_{\text{Li}}}} \right) and electrochemical impedance spectroscopy measurements. The results revealed that the [(D)\tilde]_\textLi {\tilde{D}_{\text{Li}}} values of LiMnPO₄ measured by cyclic voltammetry method increase from 9.2 × 10⁻¹⁸ to 3.0 × 10⁻¹⁷ cm² s⁻¹ after Co doping, while the charge transfer resistance (R _ct) can be decreased by Co doping.     

A Selective Conversion of Benzylic Alcohols to the Corresponding Carbonyl Compounds by Means of an Ag(III) Complex

Summary.  Carbonyl compounds of the type XPhCOR (R = H, Me, Ph; X = H, Me, Cl, Br) are prepared in high yields by reaction of the corresponding benzylic alcohols XPhCHOHR with KNa₄ [Ag(HIO₆)₂]×12H₂O in alkaline solution. This method allows the selective oxidation of benzylic alcohols in compounds containing other types of alcoholic functional groups. Received January 18, 2000. Accepted (revised) February 23, 2000     

Quasielastic light scattering study of chemically crosslinked gelatin solutions and gels

Quasielastic light scattering measurements are reported for experiments performed on mixtures of gelatin and glutaraldehyde (GA) in the aqueous phase, where the gelatin concentration was fixed at 5 (w/v) and the GA concentration was varied from 1×10⁻⁵ to 1×10⁻³ (w/v). The dynamic structure factor, S(q,t), was deduced from the measured intensity autocorrelation function, g ₂(τ), with appropriate allowance for heterodyning detection in the gel phase. The S(q,t) data could be fitted to S(q,t)=Aexp(−D _f q ² t)+Bexp(−t/τ_c)^β, both in the sol (50 and 60 ^∘C) and gel states (25 and 40 ^∘C). The fast-mode diffusion coefficient, D _f showed almost negligible dependence on the concentration of the crosslinker GA; however, the resultant mesh size, ξ, of the crosslinked network exhibited strong temperature dependence, ξ∼(0.5−χ)^1/5exp(−A/RT) implying shrinkage of the network as the gel phase was approached. The slow-mode relaxation was characterized by the stretched exponential factor exp(−t/τ_c)^β. β was found to be independent of GA concentration but strongly dependent on the temperature as β=β₀+β₁ T+β₂ T ². The slow-mode relaxation time, τ_c, exhibited a maximum GA concentration dependence in the gel phase and at a given temperature we found τ_c(c)=τ₀+τ₁ c+τ₂ c ². Our results agree with the predictions of the Zimm model in the gel case but differ significantly for the sol state. Received: 25 May 1999 /Accepted in revised form: 27 July 1999     

Characterising latex particles and fractal aggregates using image analysis

The fractal nature of latex particles and their aggregates was characterised by image analysis in terms of fractal dimensions. The one- and two-dimensional fractal dimensions, D ₁ and D ₂, were estimated for polystyrene latex aggregates formed by flocculation in citric acid/phosphate buffer solutions. The dimensional analysis method was used, which is based on power law correlations between aggregate perimeter, projected area and maximum length. These aggregate characteristics were measured by image analysis. A two-slopes method using cumulative size distributions of aggregate length and solid volume has been developed to determine the three-dimensional fractal dimension (D ₃) for the latex aggregates. The fractal dimensions D ₁, D ₂ and D ₃ measured for single latex particles in distilled water agreed well with D ₁ = 1, D ₂ = 2 and D ₃ = 3 expected for Euclidean spherical objects. For the aggregates, the fractal dimension D ₂ of about 1.67 ± 0.04 (±standard deviation) was comparable to the fractal dimension D ₃ of approximately 1.72 ± 0.13 (±standard deviation), taking the standard deviations into account. The measured three-dimensional fractal dimension for latex aggregates is within the fractal dimension range 1.6–2.2 expected for aggregates formed through a cluster-cluster mechanism, and is close to the D ₃ value of about 1.8 indicated for cluster formation via diffusion-limited colloidal aggregation. Received: 28 September 1998 Accepted: 29 October 1998     

Determination of enzyme activity inhibition by FTIR spectroscopy on the example of fructose bisphosphatase

A mid-infrared enzymatic assay for label-free monitoring of the enzymatic reaction of fructose-1,6-bisphosphatase with fructose 1,6-bisphosphate has been proposed. The whole procedure was done in an automated way operating in the stopped flow mode by incorporating a temperature-controlled flow cell in a sequential injection manifold. Fourier transform infrared difference spectra were evaluated for kinetic parameters, like the Michaelis–Menten constant (K _M) of the enzyme and V _max of the reaction. The obtained K _M of the reaction was 14 ± 3 g L⁻¹ (41 μM). Furthermore, inhibition by adenosine 5′-monophosphate (AMP) was evaluated, and the K _M^App value was determined to be 12 ± 2 g L⁻¹ (35 μM) for 7.5 and 15 μM AMP, respectively, with V _max decreasing from 0.1 ± 0.03 to 0.05 ± 0.01 g L⁻¹ min⁻¹. Therefore, AMP exerted a non-competitive inhibition.     

Cyclodextrins as a chiral mobile phase additive in nano-liquid chromatography: comparison of reversed-phase silica monolithic and particulate capillary columns

Enantioseparations of racemic nonsteroidal anti-inflammatory drugs (naproxen, ibuprofen, ketoprofen, flurbiprofen, suprofen, indoprofen, cicloprofen, and carprofen) were performed by nano-liquid chromatography, employing achiral capillary columns and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) or hydroxylpropyl-β-cyclodextrin (HP-β-CD) as a chiral mobile phase additive (CMPA). Working under the same experimental conditions (in terms of mobile phase and linear velocity), the performance of a RP-C18 monolithic column was compared with that of a RP-C18 packed column of the same dimensions (100 μm i.d. × 10 cm). Utilizing a mobile phase composed of 30% ACN (v/v) buffered with 50 mM sodium acetate at pH 3, and containing 30 mM TM-β-CD, the monolithic column provided faster analysis but lower resolution than the packed column. This behavior was ascribed to the high permeability of the monolithic column, as well as to its minor selectivity. HP-β-CD was chosen as an alternative to TM-β-CD. Employing the monolithic column, the effects of different parameters such as HP-β-CD concentration, mobile phase composition, and pH on the retention factor and the chiral resolution of the analytes were studied. For the most of the analytes, enantioresolution (which ranged from R _s = 1.80 for naproxen to R _s = 0.86 for flurbiprofen) was obtained with a mobile phase consisting of sodium acetate buffer (25 mM, pH 3), 10% MeOH, and 15 mM HP-β-CD. When the same experimental conditions were used with the packed column, no compound eluted within 1 h. Upon increasing the percentage of organic modifier to favor analyte elution, only suprofen eluted within 30 min, with an R _s value of 1.14 (20% MeOH). Replacing MeOH with ACN resulted in a loss of enantioresolution, except for naproxen (R _s = 0.89).     

Solid state voltammetric characterization of iron hexacyanoferrate encapsulated in silica

We describe a sol-gel approach by which iron hexacyanoferrate is immobilized in silica in a manner suited to investigation by electrochemistry in the absence of a contacting liquid phase. Such physicochemical parameters as concentration of redox sites (C _o) and apparent (effective) diffusion coefficient (D _app) are estimated by performing cyclic voltammetric and potential step experiments in two time regimes, which are characterized by linear and spherical diffusional patterns, respectively. Values of D _app and C _o thereby obtained are 2.0 × 10⁻⁶ cm² s⁻¹ and 1.4 × 10⁻² mol dm⁻³. The D _app value is larger than expected for a typical solid redox-conducting material. Analogous measurements done in iron(III) hexacyanoferrate(III) solutions of comparable concentrations, 1.0 × 10⁻² and 5.0 × 10⁻³ mol dm⁻³, yield D _app on the level of 5–6 × 10⁻⁶ cm² s⁻¹. Thus, the dynamics of charge propagation in this sol-gel material is almost as high as in the liquid phase. The residual water in the silica, along with the pore structure, are important to the overall mechanism of charge transport, which apparently is limited by physical diffusion rather than electron self-exchange. Under conditions of a solid state voltammetric experiment which utilizes an ultramicroelectrode, encapsulated iron hexacyanoferrate redox centers seem to be in the dispersed colloidal state rather than in a form of the rigid polymeric film. Received: 8 April 1999 / Accepted: 13 August 1999     

Optimization of Chlorophyllase-catalyzed Hydrolysis of Chlorophyll in Monophasic Organic Solvent Media

The effects of selected reaction parameters, including solvent hydrophobicity, initial water activity, agitation speed, temperature and enzyme concentration, on the biocatalytic efficiency of a chlorophyllase enzymatic extract from Phaeodactylum tricornutum in neat organic solvent media were investigated. The highest chlorophyllase specific activity of 322 nmol hydrolyzed chlorophyll per gram of protein per minute and bioconversion yield of 91% were obtained in the reaction mixture of hexane/2-octanone (98.3:1.7, v/v), at a controlled initial water activity of 0.90. R _O/A value, which is the ratio of the specific activity in the organic solvent to that in the aqueous/miscible organic solvent medium, was 1.5 × 10⁻³. To reduce the substrate diffusional limitations, the appropriate agitation speed and enzyme concentration were determined. The optimum temperature for maximal enzymatic activity and activation energy were 35°C and 105.0 kJ/mol, respectively. Although the catalytic efficiency of chlorphyllase in the neat organic solvent mixture was lower than that in the aqueous medium, its half-life time in the first environment at temperature ranging from 35 to 50°C was increased by 5.0 to 15.0 times.