Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation

Quartz crystal microbalance-based method to study adsorption of endocrine disruptor compounds on zeolite

Endocrine disrupting chemicals (EDCs) can be present as trace-level organic pollutants in aquatic environments and are difficult to measure and remove. In this study, a method was developed using a modified quartz crystal microbalance (QCM) to investigate the adsorption of EDCs by zeolite filter. Bisphenol A (BPA), oestrone (E1), oestradiol (E2), and sulfamethoxazole (SMZ) were selected as four representative endocrine disruptors in a water environment and their adsorption on zeolite was measured by QCM in real-time. The adsorption results were well described by a pseudo-first-order kinetic model and by a Sips isotherms model. The adsorption of the four adsorbents is related to their molecular structure, molecular polarity, and chargeability. The removal rate of EDCs by zeolite for different initial concentrations appeared to plateau, with the removal rates of the four selected EDCs all above 80% except for the maximum initial concentration. Changes of pH and ionic strength had no effect on the adsorption capacity of the four EDCs, with a removal rate of about 90%. However, the response time at pH 5.50 was about 300 s faster than that at pH 8.50 and the addition of electrolyte shortened the mass response time of several organic compounds on QCM.

Immunoassay of Glomalin by Quartz Crystal Microbalance Biosensor Containing Iron Oxide Nanoparticles

Glomalin is a soil protein resembling heat shock protein (HSP) 60 and exerting high affinity to metals, causing retention of water in the environment and improving mechanical stability of soil. Currently, glomalin is determined in the soil or other samples by combination of autoclaving extraction and total protein determination typically by the Bradford method. In this paper, a piezoelectric biosensor was prepared to determine glomalin in a label-free measurement. The biosensor contained antibodies immobilized on quartz crystal microbalance (QCM), and the recognition layer was stabilized by iron oxide nanoparticles. The assay was tested on real soil samples and compared with the standard Bradford assay. Limit of detection of the assay was equal to 2.4 µg/g for a soil extract with a volume of 50 µl. The assay takes approximately half of an hour and was fully correlated to the Bradford assay. The biosensor had significant advantages than the other methods: it worked in a label-free mode and was fully applicable for practical samples.

Molecular-Scale Structure of Electrode-Electrolyte Interfaces: The Case of Platinum in Aqueous Sulfuric Acid

Knowledge of the molecular composition and electronic structure of electrified solid-liquid interfaces is key to understanding elemental processes in heterogeneous reactions. Using X-ray absorption spectroscopy in the interface-sensitive electron yield mode (EY-XAS), first-principles electronic structure calculations, and multiscale simulations, we determined the chemical composition of the interfacial region of a polycrystalline platinum electrode in contact with aqueous sulfuric acid solution at potentials between the hydrogen and oxygen evolution reactions. We found that between 0.7 and 1.3 V vs Ag/AgCl the electrical double layer (EDL) region comprises adsorbed sulfate ions with hydrated hydronium ions in the next layer. No evidence was found for bisulfate or Pt-O/Pt-OH species, which have very distinctive spectral signatures. In addition to resolving the long-standing issue of the EDL structure, our work establishes interface- and element-sensitive EY-XAS as a powerful spectroscopic tool for studying condensed phase, buried solid-liquid interfaces relevant to various electrochemical processes and devices.

Influence of the Surface Roughness of Platinum Electrodes on the Calibration of the Electrochemical Quartz-Crystal Nanobalance

The electrochemical quartz-crystal nanobalance (EQCN) is an in situ technique that measures mass changes (Δm) associated with interfacial phenomena. Analysis of Δm sheds light on the mass balance (in addition to the charge and energy balances) and provides new insight into the nature of electrochemical processes. The EQCN measures changes in frequency (Δf) of a quartz-crystal resonator, which are converted into Δm using the Sauerbrey equation containing the characteristic constant (C_f). The value of C_f is determined by physical parameters of the crystal and refers to an atomically smooth surface. However, real resonators are not smooth and electrodes have their intrinsic roughness. Thus, the conversion of Δf to Δm should be done using an experimentally determined characteristic constant (C_f,exp) for a given value of the surface roughness factor (R). Here, we calibrate the system using Ag electrodeposition on Pt electrodes of gradually increasing R; the latter is adjusted through Pt electrodeposition. The surface morphology of the Pt substrates prior to and after Ag electrodeposition is examined using atomic force microscopy. The values of C_f,exp are determined by analyzing the slopes of charge density versus Δf plots for the Ag electrodeposition. They are different than C_f and increase logarithmically with R. The C_f and C_f,exp values are used in a comparative analysis of the mass changes (δΔm) for complete cyclic voltammetry profiles covering the 0.05-1.40 V range. This reveals that the employment of C_f instead of C_f,exp provides inaccurate values of δΔm, and the magnitude of the discrepancy increases with R.