New insights from X-ray photoelectron spectroscopy into the chemistry of covalent enzyme immobilization, with glutamate dehydrogenase (GDH) on silicon dioxide as an example

A three-step process for immobilization of glutamate dehydrogenase (GDH) on the surface of silicon dioxide has been studied by X-ray photoelectron spectroscopy (XPS). The enzyme layer was deposited on the silicon dioxide surface after first exposing the surface to 3-aminopropyltriethoxysilane (3-APTS) and reacting the silylated surface with glutaraldehyde (GA). Fine XPS analysis, performed after each step of the chemical procedure, revealed unknown details of the step-by-step construction of the enzyme layer under different experimental conditions.     

Electrochemical and spectroscopic behavior of iron(III) porphyrazines in Langmuir-Schafer films

Thin films of a newly synthesized iron(III) porphyrazine, LFeOESPz ( L = ClEtO, OESPz = ethylsulfanylporphyrazine), have been deposited by the Langmuir-Schafer (LS) technique (horizontal lifting) on ITO or gold substrates. Before deposition, the floating films have been investigated at the air-water interface by pressure/area per molecule (pi/ A) experiments, Brewster angle microscopy (BAM) and UV-vis reflection spectroscopy (RefSpec). The complex reacts with water subphase (pH 6.2) forming the mu-oxo dimer, which becomes the predominant component of the LS films ( LS-Fe) as indicated by optical, IR, XPS, and electrochemical data. LS-Fe multilayers exhibit, between open circuit potential (OCP) and +0.90 V (vs SCE), two independent peak pairs with formal potentials, E surf (I) and E surf(II) of +0.56 V and +0.78 V, respectively. According to dynamic voltammetric and coulometric experiments the peak pair at +0.56 V is attributed to one-electron process at the iron(III) centers on the monomer, while the peak pair at +0.78 V is associated to a four-electron process involving mu-oxo-dimer oligomers. LS-Fe films prove to be quite stable electrochemically between OCP and +0.90 V. The electrochemical stability decreases, however, when the potential range is extended both anodically and cathodically outside these limits, due to formation of new species. Upon incubation with TCA solutions, LS-Fe films show remarkable changes in the UV-vis spectra, which are consistent with a significant mu-oxo dimer --> monomer conversion. Addition of TCA to the electrochemical cell using a LS-Fe film as working electrode, results in a linear increase of a cathodic current peak near -0.40 V as the TCA concentration varies in the 0.1-2.0 mM range. This behavior is interpreted in terms of TCA inducing a progressive change in the composition of the LS-Fe films in favor of the monomeric iron(III) porphyrazine, which is responsible for the observed increase in the cathodic current near -0.40 V.     

MIP sensors – the electrochemical approach

This review highlights the importance of coupling molecular imprinting technology with methodology based on electrochemical techniques for the development of advanced sensing devices. In recent years, growing interest in molecularly imprinted polymers (MIPs) in the preparation of recognition elements has led researchers to design novel formats for improvement of MIP sensors. Among possible approaches proposed in the literature on this topic, we will focus on the electrosynthesis of MIPs and on less common hybrid technology (e.g. based on electrochemistry and classical MIPs, or nanotechnology). Starting from the early work reported in this field, an overview of the most innovative and successful examples will be reviewed.     

All-electrochemical approach for the assembly of platinum nanoparticles/polypyrrole nanowire composite with electrocatalytic effect on dopamine oxidation

In the present study, a composite material consisting of polypyrrole nanowires (PPyNWs) and platinum nanoparticles (PtNPs) has been developed by an all-electrochemical approach and proved to be highly effective for electrochemical determination of dopamine (DA). PPyNWs are electropolymerized by a template-free method, and PtNPs are subsequently electrodeposited by cyclic voltammetry. Chemical characterization by X-ray photoelectron spectroscopy showed the effective PtNP immobilization on polymer nanowires discriminating at the same time Pt species deposited and revealing the occurrence of polypyrrole-PtNP interaction. The morphology of the composite material was characterized using scanning electron microscopy that showed spherical Pt nanoparticles well distributed within PPy-NW network. DA detection was performed by differential pulse voltammetry technique obtaining satisfactory performances in terms of linear range (1–77 μM), sensitivity, reproducibility (RSD 2.7%), and detection limit (0.6 μM). The electrocatalytic role of PtNPs in DA electroxidation process is clearly demonstrated by the comparison with PPyNWs only. Moreover, no significant response is observed in the presence of common interference as ascorbic acid and uric acid, which may coexist with DA in biological fluids, demonstrating a good selectivity toward DA. Moreover, DA was detected in human serum samples spiked obtaining a satisfactory recovery of 94%. A synergistic effect involving both PtNPs and PPyNWs is invoked for explaining the observed electrocatalytic activity.     

Interference-free glucose sensor based on glucose-oxidase immobilized in an overoxidized non-conducting polypyrrole film

Summary An amperometric glucose sensor is described; it is based on glucose oxidase immobilized in an overoxidized non-conducting polypyrrole membrane. The overoxidation process of polypyrrole produces a permselective, antifouling membrane capable of rejecting ascorbate, urate, acetaminophen and cysteine, as well as proteins and other surface active components typically present in serum. The amperometric assay of glucose in serum correlates well with an established routine procedure based on an enzymatic-colorimetric method.     

TRMC, XPS, and EPR characterizations of polycrystalline TiO2 porphyrin impregnated powders and their catalytic activity for 4-nitrophenol photodegradation in aqueous suspension

Characterization of polycrystalline TiO(2) bare or porphyrin impregnated powders, used as photocatalysts for the degradation of 4-nitrophenol (4-NP) in aqueous suspension, was performed by time-resolved microwave conductivity (TRMC) measurements and electronic paramagnetic resonance (EPR) and X-ray photoelectron (XPS) spectroscopies. The presence of porphyrin sensitizers, as the metal-free or Cu [5,10,15,20-tetra (4-tert-butylphenyl)] porphyrin, impregnated onto the TiO(2) surface improved the photocatalytic activity of the bare TiO(2). TRMC measurements indicate that the number and lifetime of the photoinduced excess charge carriers increase in the presence of the macrocycles, and EPR and XPS spectroscopies support the mechanistic hypotheses based on the photoreactivity experiments.     

Microfluidic Setup for Simultaneous Separation and Electrochemical Determination of Hg2+ and Ag+ Ions in Water

Here is presented a setup made of a combination of an injection valve, a hand-made chromatographic microcolumn and an electrochemical detector for the simultaneous separation of Hg²⁺ and Ag⁺ ions in water. The microcolumns were packed with exchanger resin and used for the separation of Hg²⁺ and Ag⁺ ions, whereas a screen-printed carbon electrode (SPCE) was the amperometric detector. The performances of the SPCE towards ions were firstly studied in a FIA setup. The efficiency of ion separation was then evaluated. The reproducibility, stability, and the regeneration of the obtained microcolumns were also studied and discussed.     

Ti and Fe speciation by X-ray photoelectron spectroscopy(XPS) and Mössbauer spectroscopy for a full crystal chemical characterisation of Ti-garnets from Colli Albani (Italy)

Different analytical and structural methods (Electron Probe Micro-Analysis, Single Crystal X-ray diffraction, X-ray Photoelectron Spectroscopy, M?ssbauer spectroscopy) were combined to fully characterise the crystal chemistry of natural Ti-bearing garnets from Colli Albani (Lazio, Italy). The study of the relevant complex crystal chemistry ( large number of cation substitutions affecting the three independent X(8-fold), Y(6-fold) and Z(4-fold) crystallographic sites and Fe and Ti transition elements exhibiting several oxidation states and coordination environments) benefited from the multi-technique approach. Electron probe microanalysis provided elemental composition of the analysed samples, which have low Ti-content (TiO2 in the range 1.99 - 3.48 wt %) and slightly different Fe/Al ratios. For all samples, two doublets were fitted to room temperature M?ssbauer spectra and assigned to Fe3+(Y) (approximately 95%) and Fe2+(X) (approximately 5%). Up to three doublets (Ti3+(Y), Ti4+(Y), Ti4+(Z)) were fitted to XPS spectra and yielded direct evaluation of Ti site population. XPS technique confirmed its potential for the study of speciation of Ti in minerals.     

Templateless synthesis of polypyrrole nanowires by non-static solution-surface electropolymerization

Polypyrrole nanowires (PPy-NWs) are synthesized by a novel templateless approach based on non-static solution-surface (NSSS) electropolymerization. The mechanism responsible for PPy-NW formation is the simultaneous oxidation of pyrrole and water, with concomitant formation of hydroxyl radicals and dioxygen nanobubbles. In particular, a localized PPy-NW deposition at the solution-air interface is enabled by solution-surface electropolymerization due to the surface excess of the monomer at the interface favored by the large surface tension of the solvent. In the proposed approach, solution-surface electropolymerization is performed in non-static conditions (NSSS), as the solution-air interface is shifted by flowing the electrolyte solution over the electrode surface. This allows a PPy-NW homogeneous deposition on whatever large area electrode to be rapidly achieved. Parameters influencing the morphology of PPy-NWs are studied, particularly focusing on flow rate, pH of the electrolyte solution, and electropolymerization time. The growth process of PPy-NWs is examined and the way of tuning their resulting morphology is discussed. Morphological investigation by scanning electron microscopy and chemical/electrochemical characterization of PPy-NWs by X-ray photoelectron spectroscopy and cyclic voltammetry, respectively, further support the proposed nanowire formation mechanism. Nanowires with diameter in the range of 40–300 nm are obtained, and the possibility of depositing differently sized nanowires with a predetermined spatial distribution on the same substrate is also demonstrated.