Determination of pKa of dyes by electrical impedance spectroscopy

In this work, we analyze the electrical response of aqueous solutions of dyes as a function of pH. When the acid-base equilibrium is reached, it is possible to detect structural transitions related with changes in molecular conformation. To this, it is necessary the introduction of parallel plates into the solution and from analysis of Nyquist diagrams of samples, it is possible to determine the values of pK_a. The advantage is the absence of redox markers in solution, thus avoiding charge transfer stimulated by the diffusion processes. The effects measured on the electrical response are function of the dye molecular aggregation state, which is structurally dependent on the solution protonation conditions. The detection of aggregated molecules such as dimers, trimers, and oligomers defines Electrical Impedance Spectroscopy (EIS) as a fundamental tool for the analysis of pK_a of dyes.     

Electrochemical evaluation of lectin-sugar interaction on gold electrode modified with colloidal gold and polyvinyl butyral

In this work, ConA and CramoLL lectins were immobilized on gold nanoparticles (AuNp) with polyvinyl butyral (PVB), and adsorbed on the surface of gold (Au) electrodes. Electrochemical impedance spectroscopy (EIS), in the frequency range from 100mHz to 100KHz, and cyclic voltammetry (CV), from -0.2 to 0.7V, were performed on these electrodes, in phosphate buffer (PBS) solution containing 10mM K(3)[Fe(CN)(6)]/K(4)[Fe(CN)(6)] (1:1) mixture as a redox probe. EIS and CV measurements showed that redox probe reactions on the modified Au electrodes were partially blocked due to the adsorption of AuNp-ConA-PVB and AuNp-CramoLL-PVB. SEM images showed the presence of aggregates of AuNp-ConA on PVB spherules in a tridimensional structure on the surface of the Au electrode. Bovine serum albumin (BSA) was adsorbed on the AuNp-Lectin-PVB modified electrode in order to block the remaining free gold sites. Both EIS and CV techniques yielded results that confirm positive responses of the lectins to ovalbumin agglutination. These results indicate an improvement of the sensitivity for detection of sugars that can be applicable to construction of a biosensor sensitive to glycoproteins in solution.     

Adsorption of concanavalin A and lentil lectin on platinum electrodes followed by electrochemical impedance spectroscopy: effect of protein state

Adsorption of concanavalin A and lentil lectin on platinum electrode was investigated through electrochemical impedance spectroscopy and cyclic voltammetry. By using ferro/ferricyanide system to probe the electrochemical interface it was possible to model the EIS data with a simple equivalent circuit. The blocking effect for electron transfer reactions observed with these proteins, indicated that they readily adsorb on platinum surface and that the degree of adsorption is related to the state of the proteins. When the proteins are in the presence of divalent cations (Ca²⁺ and Mn²⁺) they adsorb less strongly than in their absence. There is also evidence that at least convanavalin A retains its biological activity in the adsorbed state.     

Aggregational processes of low molecular weight polyethylene-b-polyethylene oxide diblock copolymer probed by electrical impedance spectroscopy

In this paper, we analyze the use of electrical impedance spectroscopy (EIS) to investigate the aggregational processes of low molecular weight polyethylene-b-polyethylene oxide copolymer (PE-b-PEO (50% PEO)) induced by concentration. The real and imaginary part of impedance have been compared to theoretical results based on equivalent circuits, providing information pertaining to the properties of the bulk and contributions due to the interactions at the electrolyte/electrode interface. The bulk resistance is used as a convenient tool for identifying structures such as spheres (S), hexagonally packed cylinders (HPC) and lamellar phases. To characterize the transition mechanisms involved in micellization we applied the Phillips criteria, associating the maximum change in the gradient of bulk resistance with the formation of micelles. The transition from micelles to HPC structures is characterized from the minimum in the bulk resistance, while the electrical signature relative to the formation of lamellar structures is characterized by the decrease in the diffusion (reduction in the polarization effects) and the inversion in the slope of bulk resistance as a function of copolymer concentration. Thus, the EIS may be defined as a convenient tool for characterizing the conformation and phase transitions in amphiphilic block copolymers.     

Redox capacitance of poly-3,4-ethylenedioxythiophene studied by cyclic voltammetry and faradaic impedance spectroscopy

The paper presents results of studying the electrochemical properties of poly-3,4-ethylenedioxythiophene films using the methods of cyclic voltammetry and faradaic impedance spectroscopy in acetonitrile and propylene carbonate solutions of different electrolytes: LiClO₄, Bu₄NBF₄, Bu₄NPF₆. The effect of the film synthesis conditions, the nature of anion and solvent, and the supporting electrolyte concentration on the film redox capacitance is discussed. Main attention is paid to the comparison of values characterizing the redox capacitance of a poly-3,4-ethylenedioxythiophene film C _lf (determined using the faradaic impedance method) and C _cv (determined using the cyclic voltammetry method) and the studies of their dependence on the varied experimental factors. The experimental C _lf vs. E curves are analyzed using the relationships of a model of a uniform film.     

Electrical impedance spectroscopy of epoxy systems: The case of 1,4-butanediol diglycidyl ether/cis-1,2-cyclohexanedicarboxylic anhydride and triethylamine as initiator

A series of epoxy cured samples, with different molar composition of 1,4-butanediol diglycidyl ether (EP), cis-1,2-cyclohexanedicarboxylic anhydride (CH) and triethylamine (TEA) as initiator, was studied by Electrical Impedance Spectroscopy (EIS) in the frequency range 10⁻²-10⁵ Hz. The resistivity of the system was evaluated by using the complex impedance analysis method. Resistivity values in the range between 10⁷ and 10¹³ Ω cm were obtained and related to the composition (molar fraction x_EP or x_CH) of the epoxy system. The gel formation characteristics of the samples were obtained as theoretically described by Flory.     

Interfacial properties of self-organized TiO<Subscript>2</Subscript> nanotubes studied by impedance spectroscopy

The semiconductor properties of the interface TiO₂/electrolyte in high organized porous oxide structures were analyzed by means of impedance spectroscopy near the flat band potential. The impedance and capacitance studies performed on the as-anodized and thermally treated samples (anatase) indicate the presence of a duplex structure formed by (1) the oxide at the bottom of the pores and (2) the walls of pores with different donor densities and surface state concentrations.     

Rapid analysis of Radix puerariae by near-infrared spectroscopy

A new, rapid analytical method using near-infrared spectroscopy (NIRS) was developed to differentiate two species of Radix puerariae (GG), Pueraria lobata (YG) and Pueraria thomsonii (FG), and to determine the contents of puerarin, daidzin and total isoflavonoid in the samples. Five isoflavonoids, puerarin, daidzin, daidzein, genistin and genistein were analyzed simultaneously by high-performance liquid chromatography-diode array detection (HPLC-DAD). The total isoflavonoid content was exploited as critical parameter for successful discrimination of the two species. Scattering effect and baseline shift in the NIR spectra were corrected and the spectral features were enhanced by several pre-processing methods. By using linear discriminant analysis (LDA) and soft independent modeling class analogy (SIMCA), samples were separated successfully into two different clusters corresponding to the two GG species. Furthermore, sensitivity and specificity of the classification models were determined to evaluate the performance. Finally, partial least squares (PLS) regression was used to build the correlation models. The results showed that the correlation coefficients of the prediction models are R = 0.970 for the puerarin, R = 0.939 for daidzin and R = 0.969 for total isoflavonoid. The outcome showed that NIRS can serve as routine screening in the quality control of Chinese herbal medicine (CHM).     

Study of fibrinogen adsorption on hydroxyapatite and TiO2 surfaces by electrochemical piezoelectric quartz crystal impedance and FTIR-ATR spectroscopy

The electrochemical piezoelectric quartz crystal impedance (EQCI), a combined technique of piezoelectric quartz crystal impedance (PQCI), electrochemical impedance (EI), and Fourier transform infrared spectroscopy-attenuated total internal reflectance spectroscopy (FTIR-ATR) were used to in situ study the adsorption process of fibrinogen onto the surface of biomaterials—TiO₂ and hydroxyapatite (Ca₅(PO₄)₃OH, HAP). The equivalent circuit parameters, the resonance frequencies and the half peak width of the conductance spectrum of the two biomaterial-modified piezoelectric quartz crystal (PQC) resonances as well as the FTIR-ATR spectra of fibrinogen during fibrinogen adsorption on TiO₂ and HAP particles modified electrode surface were obtained. The adsorption kinetics and mechanism of fibrinogen were investigated and discussed as well. The results suggested that two consecutive steps occurred during the adsorption of fibrinogen onto TiO₂ and hydroxyapatite (HAP) surface. The fibrinogen molecules were firstly adsorbed onto the surface, and then the rearrangement of adsorbed fibrinogen or multi-layered adsorption occurred. The FTIR-ATR spectroscopy investigations showed that the secondary structure of fibrinogen molecules was altered during the adsorption and the adsorption kinetics of fibrinogen related with the variety of biomaterials. These experimental results suggest a way for enriching biological analytical science and developing new applications of analytical techniques, such as PQCI, EI, and FTIR-ATR.     

A study of the electrical behaviour of isolated tomato cuticular membranes and cutin by impedance spectroscopy measurements

Different isolated tomato fruit cuticular membranes (ripe and green tomato cuticles and the cutin of these membranes) were studied by impedance spectroscopy measurements when the membranes were in contact with NaCl solutions at different concentrations. Remarkable differences in the impedance plots and the equivalent circuits associated to each membrane sample were obtained: the ripe tomato cuticle and the cutin, only present a relaxation process, but for the green tomato cuticle two relaxation processes were obtained. Using the equivalent circuits as models, electrical and electrochemical parameters for each membrane were determined. These results permit us to assign the relaxation processes to the different components of the tomato membrane (polyester matrix, carbohydrates and pigments), obtaining in this way a detailed picture of the different environments of the plant interface. Variation with NaCl concentration for the different electrical parameters was also studied, and the electrical resistance of the biopolymer matrix was obtained.     

Electrical impedance spectroscopy of epoxy systems II: Molar fraction variation, resistivity, capacitance and relaxation processes of 1,4-butanediol diglycidyl ether/succinic anhydride and triethylamine as initiator

The Electric Impedance Spectroscopy (EIS) was used to evaluate resistivity, capacitance and relaxation processes of a cured epoxy system with different molar composition of 1,4-butanediol diglycidyl ether (EP), succinic anhydride (SA) and triethylamine (TEA) as initiator. The measurements were done over range frequencies between 10⁻¹ and 10⁵ Hz. The systematic change of the molar fraction composition affected the resistivity and capacitance changes indicating gelation critical compositions according to the Flory’s aggregation theory. The complex electric functions, dielectric constant ε^•, electric modulus M^•, impedance Z^•, admittance Y^• and the loss factor tan δ were utilized in order to investigate the relaxation processes. Relaxation peaks were observed for different molar fraction composition only in the imaginary impedance, the electric modulus and the tan δ as frequency functions. The relaxation frequencies obtained by Z″, M″ and tan δ evaluation are distinct and have been discussed in terms of resin components molar fraction dependence.     

Determination of critical micelle concentrations and aggregation numbers by fluorescence correlation spectroscopy: aggregation of a lipopolysaccharide

Fluorescence correlation spectroscopy (FCS) is often used to determine the mass or radius of a particle by using the dependence of the diffusion coefficient on the mass and shape. In this article we discuss how the particle size of aggregates can be measured by using the concentration dependence of the amplitude of the autocorrelation function (ACF) instead of the temporal decay. We titrate a solution of aggregates or micelles with a fluorescent label that possesses a high affinity for these structures and measure the changes in the amplitude of the ACF. We develop the theory describing the change of the ACF amplitude with increasing concentrations of labels and use it to fit experimental data. It is shown how this method can determine the aggregation number and critical micelle concentration of a standard detergent nonaethylene glycol monododecyl ether (C₁₂E₉) and a lipopolysaccharide (LPS: Escherichia coli 0111:B4).     

Electrochemical copolymerization of pyrrole and 2,2-bithiophene and semiconducting characterization of the resulting copolymer films by electrochemical impedance spectroscopy and photoelectrochemistry

The electrochemical copolymerization of pyrrole and bithiophene was studied at a polymerization potential of 1.1 V for various monomer ratios. The cyclic voltammograms showed that the electrochemical properties of the resulting copolymer films changed gradually from those of polypyrrole to polybithiophene with an increase in concentration of bithiophene in the initial electrolyte. The evidence for copolymer formation is based on the analytical results of electrospray ionization mass spectroscopy, thermoanalysis, and Raman spectroscopy. The results showed that cooligomers and homooligomers were found in the electrolyte after copolymerization. The difference between the morphology of a copolymer of pyrrole and bithiophene and a polymer mixture of polypyrrole and polythiophene was demonstrated by scanning electron microscopy. Electrochemical impedance and photocurrent measurements were carried out in order to achieve information on the semiconducting properties of the homopolymers and copolymers obtained. A model of a very thin layer of polypyrrole formed immediately on the electrode surface covered by a thicker copolymer film was developed to explain the results.Dedicated to Zbigniew Galus on the occasion of his 70^th birthday.     

Study on the interaction between salvianic acid A sodium and bovine serum albumin by spectroscopic methods

The interaction between salvianic acid A sodium (SAS) and bovine serum albumin (BSA) was investigated using fluorescence and ultraviolet spectroscopy at different temperatures under imitated physiological conditions. The experimental results showed that the fluorescence of BSA was quenched by SAS through a static quenching procedure. The binding constants of SAS with BSA were 2.03, 1.17 and 0.71×10(5) L mol(-1) at 291, 298 and 305 K, respectively. Negative values of ΔG, ΔH, and ΔS indicate that the interaction between SAS and BSA is driven by hydrogen bonds and van der Waals forces. According to F?rster non-radiation energy transfer theory, the binding distance between BSA and SAS was calculated to be about 2.92 nm. The effect of SAS on the conformation of BSA was analyzed using synchronous fluorescence spectroscopy. In addition, the effect of some metal ions Cu(2+), Ca(2+), Mg(2+), and Zn(2+) on the binding constant between SAS and BSA was examined.     

Discrimination of Rhizoma Corydalis from two sources by near-infrared spectroscopy supported by the wavelet transform and least-squares support vector machine methods

Near-infrared spectroscopy (NIRS) was applied for direct and rapid collection of characteristic spectra from Rhizoma Corydalis, a common traditional Chinese medicine (TCM), with the aim of developing a method for the classification of such substances according to their geographical origin. The powdered form of the TCM was collected from two such different sources, and their NIR spectra were pretreated by the wavelet transform (WT) method. A training set of such Rhizoma Corydalis spectral objects was modeled with the use of the least-squares support vector machines (LS-SVM), radial basis function artificial neural networks (RBF-ANN), partial least-squares discriminant analysis (PLS-DA) and K-nearest neighbors (KNN) methods. All the four chemometrics models performed reasonably on the basis of spectral recognition and prediction criteria, and the LS-SVM method performed best with over 95% success on both criteria. Generally, there are no statistically significant differences in all these four methods. Thus, the NIR spectroscopic method supported by all the four chemometrics models, especially the LS-SVM, are recommended for application to classify TCM, Rhizoma Corydalis, samples according to their geographical origin.     

Electroreductive polymerization of trans-[RuCl 2(vpy) 4] on Nd-Fe-B magnets: electrochemical impedance spectroscopy interpretation, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy analysis

The electropolymerization of trans-[RuCl₂(vpy)₄] (vpy=4-vinylpyridine) monomer on Nd-Fe-B magnets was studied by electrochemical impedance spectroscopy (EIS). Impedance diagrams obtained during the polymerization process were used to monitor film formation. The EIS results gave insight into the electrochemical phenomena occurring at the magnet surface as the polymerization process progressed. The film structure and morphology were also studied by X-ray photoelectron spectroscopy, scanning electron microscopy and Raman spectroscopy. The Raman spectroscopy results showed that the polymerization takes place at the vinyl groups of the monomer and also that the redox polymer structure is very similar to that of the monomer. The ratio of the intensity of the XPS peaks for fluorine (from the electrolyte PF₆ ^–) and ruthenium present in the film showed that the polymer on Nd-Fe-B contained an equal proportion of Ru²⁺ and Ru³⁺, indicating that part of the film is positively charged, i.e. {[RuCl₂(vpy)₄]⁺} _n .     

Chemometric analysis of multiple species of Bacillus bacterial endospores using infrared spectroscopy: Discrimination to the strain level

Previous work using infrared spectroscopy has shown potential for rapid discrimination between bacteria in either their sporulated or vegetative states, as well as between bacteria and other common interferents. For species within one physiological state, however, distinction is far more challenging, and requires chemometrics. In the current study, we have narrowed the field of study by eliminating the confounding issues of vegetative cells as well as growth media and focused on using IR spectra to distinguish only between different species all in the sporulated state. Using principal component analysis (PCA) and a classification method based upon similarity measurements, we demonstrate a successful identification rate to the species level of 85% for Bacillus spores grown and sporulated in a glucose broth medium.     

Study on the interaction between an ionic liquid and L-tryptophan by fluorescence spectroscopic technique

In this work, the interaction of an ionic liquid (IL) 1-octyl-3-methylimidazolium chloride ([Omim]Cl) and L-tryptophan was investigated with fluorescence spectroscopic method. The quenching mechanism and binding characteristics of [Omim]Cl with L-tryptophan at different temperatures were studied from fluorescence measurement. The thermodynamic parameters such as free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), were calculated by thermodynamic equations. The experimental results indicated that [Omim]Cl and L-tryptophan had weak interaction and the mechanism of quenching belonged to static quenching. Hydrogen bonds and van der Waals interactions played important roles in the binding of [Omim]Cl with L-tryptophan.