Investigations of ultrathin polypyrrole films: Formation and effects of doping/dedoping processes on its optical properties by electrochemical surface plasmon resonance (ESPR)

In the present work an investigation of the effects of the electropolymerization mode on the optical properties associated to the doping/dedoping processes of nanometric films of polypyrrole (PPy) is reported, monitoring in situ and in real time using simultaneously surface plasmon resonance and electrochemical techniques (ESPR). The electropolymerization of pyrrole was performed by potentiostatic, potentiodynamic and galvanostatic methods and the use of the ESPR technique showed that the electropolymerization mode is essential to the stability of polymer and the reversibility of its optical properties during the doping and dedoping processes. Thus, the optical properties of oxidized and reduced film were obtained by nonlinear least square fitting using Fresnel equations for a four-layer system. Then, the values of the real and imaginary parts of the complex dielectric constant for PPy fims were correlated with the polymer doping level. Finally, quartz crystal microbalance measurements were also applied to obtain correlation between doping/dedoping processes and the changes in the real and imaginary parts of the dielectric constant of the polypyrrole film, showing that the doping and dedoping processes in the polypyrrole film can act directly on its optical properties while the ESPR technique can give the same information indirectly.     

Electropolymerization of ethylene dioxythiophene (EDOT) in micellar aqueous solutions studied by electrochemical quartz crystal microbalance and surface plasmon resonance

Electropolymerization of 3,4-ethylene dioxythiophene (EDOT) on gold electrodes was studied in situ by means of an electrochemical quartz crystal microbalance (EQCM) and by surface plasmon resonance spectroscopy (SPR) using aqueous micellar monomer solutions. Electrodeposition from micellar solutions performed by cyclic voltammetry (CV) displayed an enhanced rate as compared to surfactant concentrations lower than the critical micelle concentration (cmc) or in surfactant-free monomer solutions. EQCM data indicate the occurrence of additional film growth during the reductive scan to large negative potentials which does not occur in electropolymerization experiments using potentiostatic pulses or cyclovoltammetric cycles with less negative potential scans. Our results further suggest that in the case of the anionic surfactant the oxidation potential of the monomer is lowered even at surfactant concentrations below the cmc. AFM investigations show a slightly enhanced roughness of the films obtained from micellar monomer solutions.     

An electrochemical quartz crystal microbalance study into the deposition of manganese dioxide

An electrochemical quartz crystal microbalance (EQCM) has been used to study the effects of electrolyte composition (MnSO₄ and H₂SO₄ concentrations) and anodic current density on the electrodeposition of manganese dioxide. The EQCM, in tandem with the electrode voltage during deposition, has been used to characterize features of the deposition mechanism such as the relative lifetime of the Mn(III) intermediate, the rate of soluble Mn(III) hydrolysis to form MnOOH, and the porosity of the resultant manganese dioxide deposit as a function of crystal nucleation. The connection between the results obtained here and commercial electrodeposition of manganese dioxide were also discussed.     

Interaction of BSA protein with copper evaluated by electrochemical impedance spectroscopy and quartz crystal microbalance

The interaction of bovine serum albumin (BSA) protein with copper in phosphate buffer solution has been studied by a combination of electrochemical impedance spectroscopy (EIS) close to the open circuit potential, with simultaneous monitoring by the electrochemical quartz crystal microbalance (EQCM), in order to throw light on BSA adsorption. Copper films were electroplated onto gold quartz crystals and mounted in the EQCM. Experiments were conducted in the presence and absence of dissolved oxygen and of BSA and the results show the influence of O₂ on the protein/metal interaction and also show specific interactions between BSA and copper. The good reproducibility obtained in these experiments suggests future application to other systems and which should lead to a better understanding of the use of such types of protein as corrosion inhibitors.     

Investigation of cobalt deposition using the electrochemical quartz crystal microbalance

Electrodeposition of cobalt from sulfate solutions at different pH values was investigated using the EQCM technique coupled with cyclic voltammetry. The results show that cobalt hydroxide is formed simultaneously with cobalt deposition during the early stages of reduction due to the pH variation near the electrode surface caused by the parallel hydrogen evolution reaction (HER). This result was confirmed using M/z values calculated using the Sauerbrey equation and Faraday's law, which showed the presence of cobalt hydroxide in the electrodeposits. A flux model was developed and it assumes a direct reduction of cobalt, simultaneous HER and the formation of cobalt hydroxide during the early stages of deposition at pH 4.10. When the solution pH is decreased to 3.33 only the direct cobalt reduction is observed without any hydroxylated species formation.     

The influence of lead upd on the oxidation of glucose at platinum electrodes an electrochemical quartz crystal microbalance study

The oxidation of glucose at Pt electrodes with upd deposits of Pb in acid media has been investigated using the electrochemical quartz crystal microbalance (EQCM). Glucose alone has a small effect on the mass response but changes in upd Pb coverage have a large effect. This allows changes in upd Pb coverage to be followed easily despite the fact that the associated current is largely obscured in the cyclic voltammogram. Cyclic voltammetry at low glucose concentrations shows a multiple peak structure in the double layer region of potential at low upd coverages that is charged to the more familiar single peak as upd coverage increases. Mass responses also reveal some blockage of Pb upd by glucose or species derived from it during cyclic voltammetry. Data from mass transients show that the deliberate formation of poisons prior to addition of Pb²⁺ ions to the electrolyte results in a substantial suppression of upd coverage especially between −0.1 and 0.1 V (sce). Finally, mass transients accompanying injections of Pb²⁺ in the presence of glucose in the electrolyte reveal the suppression of adsorption of Pb²⁺ cations on an oxidised Pt surface by species derived from glucose.     

Monitoring of formation and redox transformations of poly(Methylene blue) films using an electrochemical quartz crystal microbalance

An electrochemical quartz crystal microbalance and cyclic voltammetry have been used to monitor the growth of poly(methylene blue) films on gold during potential cycling between −0.6V and 1.0V vs. sce in sodium phosphate buffer (pH 8.2) and Na₂SO₄ solutions. Both the adsorption/desorption of monomer methylene blue and the formation of poly(methylene blue) film can be followed by microgravimetry. The response of the polymer films is significantly separated from that of the monomer species in solution. It has been proved that anion sorption/desorption occur during the redox transformation of the polymer, while cations play no role. Preliminary results on the complex protonation/deprotonation equilibria are also discussed.     

Electrochemical quartz crystal microbalance study of formation and redox transformations of poly(diphenylamine)

The electropolymerization of diphenylamine (DPA) dissolved in 2 mol dm⁻³ H₂SO₄ has been studied on a gold electrode by cyclic electrochemical quartz crystal microbalance measurements. Similar experiments have been accomplished with DPA microcrystals attached to a gold electrode, in the presence of aqueous acidic media. In both cases the redox transformations of the poly(diphenylamine) films formed on Au have been investigated in solutions of different pHs. The cyclic voltammograms and the mass changes refer to two closely spaced redox reactions resulting in the formation of cation radicals and dications, respectively. The irregular, broad shape of the voltammetric waves may be explained by presence of other electrochemically active sites beside the linear diphenylbenzidine structure. The results indicate a pH and potential-dependent relative contribution of anions and hydrogen ions to the overall ion exchange process. The expulsion of protons is the predominant ion transport process at the beginning of the oxidation and the incorporation of anions occurs at a later stage of oxidation. The sorption/desorption of solvent molecules also contributes to the mass change observed during the redox transformations of the polymer.     

石英晶体微天平(QCM)在超临界CO2过程中的应用

超临界流体技术是绿色化学的一个重要领域,由于高压的限制,传统检测手段很难应用于过程中涉及的物理和化学性质数据的测定,严重影响了超临界技术的研究与应用。石英晶体微天平(QCM)作为一种新型的介观级微称重技术,适用于高温高压等极端条件,具有测量精度高的优点。本文介绍了石英晶体微天平技术应用的基本理论及其特点,并重点回顾和展望了该技术在超临界CO2中的应用。     

Arsenic species interactions with a porous carbon electrode as determined with an electrochemical quartz crystal microbalance

The interactions of arsenic species with platinum and porous carbon electrodes were investigated with an electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry in alkaline solutions. It is shown that the redox reactions in arsenic-containing solutions, due to arsenic reduction/deposition, oxidation/desorption, and electrocatalyzed oxidation by Pt can be readily distinguished with the EQCM. This approach was used to show that the arsenic redox reactions on the carbon electrode are mechanistically similar to that on the bare Pt electrode. This could not be concluded with just classical cyclic voltammetry alone due to the obfuscation of the faradaic features by the large capacitative effects of the carbon double layer.For the porous carbon electrode, a continual mass loss was always observed during potential cycling, with or without arsenic in the solution. This was attributed to electrogasification of the carbon. The apparent mass loss per cycle was observed to decrease with increasing arsenic concentration due to a net mass increase in adsorbed arsenic per cycle that increased with arsenic concentration, offsetting the carbon mass loss. Additional carbon adsorption sites involved in arsenic species interactions are created during electrogasification, thereby augmenting the net uptake of arsenic per cycle.It is demonstrated that EQCM, and in particular the information given by the behavior of the time derivative of the mass vs. potential, or massogram, is very useful for distinguishing arsenic species interactions with carbon electrodes. It may also prove to be effective for investigating redox/adsorption/desorption behavior of other species in solution with carbon materials as well.     

利用石英晶体微量天平测定紫杉醇在二氧化碳中的溶解度

A quartz crystal microbalance (QCM) is used to determine the phase equilibrium of paclitaxel-carbon dioxide system in the pressure range of 0-11 MPa and at temperatures of 35 °C,40 °C and 45 °C.The experimental results indicated that gaseous CO2 could be absorbed poorly into paclitaxel.The swelling of paclitaxel film in CO2 was observed before paclitaxel dissolved into supercritical carbon dioxide (ScCO2) with the increase of pressure.It was found that ScCO2 was not a good solvent for paclitaxel.The mole fraction of paclitaxel in ScCO2 was in the range of (4.5×10-9)-(7.8×10-9) under all our experimental conditions.Therefore,a much higher pressure than the CO2 supercritical point and/or a cosolvent must be used in any processes wherever paclitaxel dissolution in ScCO2 is required.     

Electrochemical quartz crystal microbalance study of redox transformations of TCNQ microcrystals in concentrated LiCl solutions

Microcrystals of 7,7,8,8-tetracyanoquinodimethane (TCNQ) immobilized on gold surface have been investigated by combined cyclic voltammetry/chronoamperometry and piezoelectric nanogravimetry at an electrochemical quartz crystal microbalance (EQCM) in dilute and concentrated LiCl solutions. It is proved that TCNQ microcrystals even when TCNQ is reduced to its highly soluble Li⁺TCNQ⁻ salt can be studied in aqueous solutions when the water activity is decreased to a low level. At high electrolyte concentrations the voltammetric and chronoamperometric responses obtained for TCNQ microcrystals show the theoretically predicted pattern characteristic to the solid-solid phase transformation under the control of the nucleation and growth kinetics. The utilization of the idea presented in this work, i.e., using electrolyte of high concentrations (or decreasing the water activity by adding an indifferent component to the liquid phase in high concentration) can open up new vistas in the study of redox transformations of immobilized microcrystals which are soluble in water.     

Ion exchange properties of polypyrrole studied by electrochemical quartz crystal microbalance (EQCM)

The electrochemical polymerization of pyrrole in aqueous solution was studied as a function of a variety of parameters, such as magnitude of current density and monomer concentration. The possibility of quantitative replacement of counterions by solution anions was studied in detail using the electrochemical quartz crystal microbalance (EQCM) technique. It was found that the polymer structure can be manipulated, even after synthesis. The manipulation was revealed to be dependent on various factors, including the nature of the counterion and of the electrolyte concentration. On the basis of these observations, a two-layer structure is suggested for polypyrrole-based polymers. © 1999 Society of Chemical Industry     

Electrochemical behavior of thin polycrystalline rhodium layers studied by cyclic voltammetry and quartz crystal microbalance

Electrochemical behavior of thin polycrystalline Rh layers has been studied in 0.5 M H₂SO₄ solution by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). The properties of surface oxide formed on freshly electrodeposited Rh are different than on electrochemically aged Rh. The analysis of frequency changes in both hydrogen and oxygen regions is presented. It is suggested that hydrogen desorption occurs simultaneously with the adsorption of HSO₄⁻ ions, whose maximum surface coverage reaches ca. 8%. The EQCM results indicate that RhO is the main species formed during Rh surface oxidation. Metal dissolution proceeds during electrode cycling to sufficiently high potentials. The amount of dissolved metal increases with an increase in potential and a decrease in scan rate.     

Application of the electrochemical quartz crystal microbalance technique to copper sonoelectrochemistry: Part 1. Sulfate-based electrolytes

The applicability of the electrochemical quartz crystal microbalance technique in an ultrasonic field created by an ultrasound probe is demonstrated for the electrodeposition of copper. Cyclic voltammetry and potentiostatic depositions in acidic sulfate-based copper electrolytes were performed at different ultrasonic intensities. The electrochemical quartz crystal microbalance was operated in ultrasonic fields with intensities up to 30 W cm⁻². For cyclic voltammetry, potential resolved and averaged (apparent) current efficiencies were calculated from mass and charge data in function of the amplitude of the ultrasonic horn. Ultrasound slightly affected the current efficiencies during copper deposition in cyclic voltammetry, but did not change the efficiencies during dissolution. During potentiostatic depositions the current efficiency increased from 84% to almost 100% upon application of ultrasound. Morphology of deposits prepared by potentiostatic depositions was analyzed by scanning electron microscopy, and found to be different at high ultrasonic intensities.     

Application of polymer‐coated quartz crystal microbalance (QCM) as a sensor for BTEX compounds vapors

A sensor based on the technique of a quartz crystal microbalance (QCM) was developed for the detection of organic vapors such as benzene, toluene, ethylbenzene, and xylenes (BTEX compounds). Detection was based on the adsorption of organic vapors on a thin layer of polydimethylsiloxane (PDMS) coated at the surface of AT‐cut gold‐coated quartz crystal electrodes. The frequency shifts due to the sorption of BTEX compounds were measured. Calibration graphs were constructed by plotting the frequency changes (ΔF/Hz) against the concentration of organic compounds. Using this method, the detection of these organic vapors was carried out at the ppm level. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1062–1066, 2001     

Electrodeposition of CdS from acidic aqueous thiosulfate solution—Invesitigation of the mechanism by electrochemical quartz microbalance technique

Electrochemical quartz crystal microbalance was used to study mechanism of cathodic electrodeposition of CdS from acidic aqueous solutions containing 0.01 M Cd(ClO₄)₂ and 0.1 M Na₂S₂O₃ as a source of sulfur. Experiments were performed by means of cyclic voltammetry and potentiostatic method. A comparison of gravimetric and current responses at pH 3 and 4 allowed for determination of the potential range in which side reactions of reduction of SO₃²⁻ and H⁺ ions compete most strongly with formation of CdS. The film thickness was determined by means of two methods: from AFM profiles and EQCM measurements.     

Unobstructed electron transfer on porous polyelectrolyte nanostructures and its characterization by electrochemical surface plasmon resonance

Thin organic films with desirable redox properties have long been sought in biosensor research. We report here the development of a polymer thin film interface with well-defined hierarchical nanostructure and electrochemical behavior, and its characterization by electrochemical surface plasmon resonance (ESPR) spectroscopy. The nano-architecture build-up is monitored in real time with SPR, while the redox response is characterized by cyclic voltammetry in the same flow cell. The multilayer assembly is built on a self-assembled monolayer (SAM) of 1:1 (molar ratio) 11-ferrocenyl-1-undecanethiolate (FUT) and mercaptoundecanoic acid (MUA), and constructed using a layer-by-layer deposition of cationic poly(allylamine hydrochloride) (PAH) and anionic poly(sodium 4-styrenesulfonate) (PSS). Electron transfer (ET) on the mixed surface and the effect of the layer structures on ET are systematically studied. Under careful control, multiple layers can be deposited onto the 1:1 FUT/MUA SAM that presents unobstructed redox chemistry, indicating a highly ordered, extensively porous structure obtained under this condition. The use of SPR to trace the minute change during the electrochemical process offers neat characterization of local environment at the interface, in particular double layer region, allowing for better control over the redox functionality of the multilayers. The 1:1 SAM has a surface coverage of 4.1 ± 0.3 × 10⁻¹⁰ mol cm⁻² for ferrocene molecules and demonstrates unperturbed electrochemistry activity even in the presence of a 13 nm polymer film adhered to the electrode surface. This thin layer possesses some desirable properties similar to those on a SAM while presenting ∼15 nm exceedingly porous structure for high loading capacity. The high porosity allows perchlorate to freely partition into the film, leading to high current density that is useful for sensitive electrochemical measurements.     

Preparation of the molecularly imprinted polymers-based capacitive sensor specific for tegafur and its characterization by electrochemical impedance and piezoelectric quartz crystal microbalance

Molecularly imprinted polymers (MIPs)-based capacitive sensor specific for tegafur was constructed by electropolymerization of m-aminophenol onto the surface of gold electrode and the Au-coated quartz crystal electrodes. Electrochemical impedance (EI) and quartz crystal microbalance (QCM) were employed to characterize the modified gold electrodes and the modified Au-coated quartz crystals, respectively. Unlike the capacitive sensors reported in the previous literatures, the present sensors were not treated with alkanethiol after electropolymerization and show even more satisfactory performance. QCM measurements also confirmed the imprinting effect of the polymer layers.     

Electrochemical quartz crystal microbalance study on carbon oxides adsorption in the presence of electrosorbed hydrogen on Pd alloys with Pt and Rh

CO₂ and CO adsorption on Pd-Pt and Pd-Rh alloys has been studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). Adsorbed CO₂ inhibits partially hydrogen adsorption on Pt and Rh surface atoms but does not block significantly hydrogen absorption into alloy bulk. In the presence of adsorbed CO both hydrogen adsorption and absorption are strongly suppressed. On electrodes covered with adsorbed CO the oxidation of previously absorbed hydrogen is significantly shifted into higher potentials. The EQCM response in CO₂/CO adsorption experiments is affected by both the effects connected with the changes in mass attached to the resonator and the non-mass effects including changes in metal-solution interactions and variation of solution density and viscosity in the vicinity of the electrode. Differences in the EQCM behavior suggest that the products of CO₂ and CO adsorption on the alloys studied are not totally identical.