Gemma Cepriá

Detection, characterization and quantification of inorganic engineered nanomaterials: A review of techniques and methodological approaches for the analysis of complex samples

The increasing demand of analytical information related to inorganic engineered nanomaterials requires the adaptation of existing techniques and methods, or the development of new ones. The challenge for the analytical sciences has been to consider the nanoparticles as a new sort of analytes, involving both chemical (composition, mass and number concentration) and physical information (e.g. size, shape, aggregation). Moreover, information about the species derived from the nanoparticles themselves and their transformations must also be supplied. Whereas techniques commonly used for nanoparticle characterization, such as light scattering techniques, show serious limitations when applied to complex samples, other well-established techniques, like electron microscopy and atomic spectrometry, can provide useful information in most cases. Furthermore, separation techniques, including flow field flow fractionation, capillary electrophoresis and hydrodynamic chromatography, are moving to the nano domain, mostly hyphenated to inductively coupled plasma mass spectrometry as element specific detector. Emerging techniques based on the detection of single nanoparticles by using ICP-MS, but also coulometry, are in their way to gain a position. Chemical sensors selective to nanoparticles are in their early stages, but they are very promising considering their portability and simplicity. Although the field is in continuous evolution, at this moment it is moving from proofs-of-concept in simple matrices to methods dealing with matrices of higher complexity and relevant analyte concentrations. To achieve this goal, sample preparation methods are essential to manage such complex situations. Apart from size fractionation methods, matrix digestion, extraction and concentration methods capable of preserving the nature of the nanoparticles are being developed. This review presents and discusses the state-of-the-art analytical techniques and sample preparation methods suitable for dealing with complex samples. Single- and multi-method approaches applied to solve the nanometrological challenges posed by a variety of stakeholders are also presented.

Voltammetric analysis of iron oxide pigments

Eighteen earthy and four pure synthetic pigments containing alpha-Fe2O3 (hematite), alpha-FeOOH (goethite) and poorly crystalline Fe and Mn oxide species were analyzed by voltammetry of microparticles. Three natural samples were subjected to an interlaboratory test to evaluate the reproducibility of the voltammetric peak potentials and peak shapes. The results confirmed that linear-sweep voltammetry is able to distinguish between poorly crystalline, ferrihydrite-like oxides and well-crystalline hematite and goethite and to detect XRD-amorphous Mn(III,IV) oxides via the peak occurrence. Voltammetry is further able to distinguish between pigments containing well-crystalline goethite (according to its structural features) and hematite (according to its particle size). The microsamples of primers from two baroque paintings were also analyzed by XRD and voltammetry and shown to be analogous to common clayey ochres.

Electrochemical Behavior of Silver-Copper Alloys in Voltammetry of Microparticles: A Simple Method for Screening Purposes

Voltammetry of microparticles (VMP) is used as a tool to identify Ag/Cu alloys. The identification process is based on the position of the voltammetric peaks obtained in different media: ammonia buffer 0.1 M, oxalic acid 0.4 M and thiocyanate 0.4 M, for samples of Ag, Cu and Ag/Cu alloys, immobilized on a graphite electrode. The phases and structure of four Ag/Cu alloys (91 : 9, 70 : 30, 40 : 60, 10 : 90) were studied by scanning electron microscopy (SEM) and then related to the voltammograms to establish a relationship between structure and peak shape and position.

Identification of iron(III) oxides and hydroxy-oxides by voltammetry of immobilised microparticles

In this paper, we propose the use of an electroanalytical technique based on the direct oxidation or reduction of the electroactive components of the sample on the surface of the working electrode, called voltammetry of immobilised microparticles (VMPs). The sample is easily deposited on the electrode by abrasion and then the electrode is transferred to the electrochemical cell where the square wave potential scan is performed. Electroactive species showed peaks whose peak potential is related to the standard formal potentials. We applied this technique to the identification of iron oxides and hydroxy-oxides in cosmetics. To characterise and identify the iron(III) oxides and hydroxy-oxides VMP was performed in two different media: oxalic acid and hydrochloric acid, that is, a complexing and a slightly complexing media. Two electrode processes were observed. They were influenced by the media and the synthesis procedure of the oxides. The reduction peak at negative potentials ( −0.50 V in hydrochloric acid and −0.60 V in oxalic acid) is related to the direct reduction of the iron(III) oxide and it does not appear in the case of the more reactive phases (hydroxy-oxides). The peak at positive potentials (0.90 V in hydrochloric acid and 0.60 V in oxalic acid) involves the reduction of iron(III) in solution. The same electrode process were observed for binary mixtures but the peak potentials are shifted from the pure components peak potentials. This allowed us to distinguish between their mixtures. Finally, VMP was used to characterise iron oxides in cosmetic powders.

Models for studying the binding capacity of albumin to zinc by stripping voltammetry

Abstract The complexation capacity of albumin to zinc was studied by different voltammetric methods. Cyclic voltammetry was used with a hanging mercury drop electrode (HMDE) to diagnose qualitatively the electrode mechanisms occuring at a mercury electrode and to evaluate the charge-transfer rate constant of the electrochemcial reaction of zinc in the presence of different concentrations of albumin. The conditional stability constant of the zinc-albumin complex was determined pseudo-polarographically by differential-pulse anodic stripping voltammetry (DP-ASV) with a Nafion-coated mercury film electrode (Nf-MFE). This electrode permits the determination of free (labile) zinc and rejection of the interfering adsorption of free albumin on the glassy coarbon surface. A mean conditional stability constant of log β′=6.10±0.16 was obtained by this method. DP-ASV was used to calculate β′ by titrating an albumin solution with ZN(II) and by Ruzic data treatment. In this instance a mean of log β′=5.49±0.28 was found with the HMDE and Nf-MFE. The results are compared and discussed.

Electroanalytical Determination of Arsenic(III) and Total Arsenic in 1 mol L−1 HCl Using a Carbonaceous Electrode Without a Reducing Agent

Abstract Arsenic(V) [As(V)] was reduced to As(0) at pH 0.0 and As(III) at pH 4.5 on a carbon-paste electrode modified with hematite, which allowed their selective determination. Arsenic(V) suffered interference from copper (Cu) and bismuth (Bi). Arsenic(III) was almost free of them. Humic acid did not affect the signal of As(V) but increased the signal of As(III). Arsenic was preconcentrated at −0.8 V for 100 s. The response was linear up to 70 µg L−1 for As(V) and 50 µg L−1 for As(III). The limits of detection were 2 µg L−1 and 5 µg L−1 respectively. This method was applied to drinking water and compost lixiviate.

Voltammetry of immobilised microparticles: a powerful analytical technique to study the physical and chemical composition of brass

Voltammetry of immobilised microparticles with a square wave potential scan (SQW) is used to study the phases composition of brass. Several electrolytes are considered in order to obtain peaks representing all the brass phases as well as to achieve the best resolution between peaks. Cyclic voltammetry was also used to characterise the electrode processes and the results obtained were in agreement with those from SQW voltammetry. The position and shape of the peaks changed with the media and concentration used to perform the experiment. Thiocyanate medium offered the richest information.

Silver nanoparticle detection and characterization in silver colloidal products using screen printed electrodes

Silver nanoparticles (AgNPs) were detected and characterized in several silver colloidal products available on the market. The relationship of the diameter of the nanoparticle with the corresponding peak potential was used to determine the dimensions of AgNPs in real samples. Quantitative analysis was carried out by voltammetry of immobilized particles on screen printed and glassy carbon electrodes. Screen printed electrodes were used prior to modification of the reference electrode to attain stable readings of peak potential. The repeatability of the modification as well as the quantitative results was checked and found satisfactory.

Analytical study of the behaviour of some ingredients used in lustre ceramic decorations following different recipes

Ceramic lustre is a fine decoration obtained by a complex technical process. Although the general production technique is more or less known, it is much more difficult to specify the ingredients mixed in the ancient recipes used to produce lustre and their function during the process, especially components as mercury sulphide and iron oxide. To understand this point, mixtures of the components were previously characterised and tested to see which mixtures resulted in the desired lustre. Synchrotron-Radiation-Source X-ray Diffraction (SRS–XRD) of the fired mixtures was performed to check the changes produced in the ingredients. Also Voltammetry of immobilised Microparticles was applied to the raw pigment powders to obtain information about the redox behaviour. Square-wave voltammetry demonstrates the possibility of different deposits that can be formed in the first steps of the reduction process in the pigment mixture for lustre production, especially those related to solid-form reactions and pigment-electrolyte interactions. Moreover, SR–XRD shows the formation of different crystalline phases at the end of the process, in this case probably also related to gas–solid interactions and to a strong reduction atmosphere.

Quick, Easy, and Inexpensive Way to Detect Small Metallic Particles in Suspension Using Voltammetry of Immobilized Microparticles

Abstract Voltammetry of immobilized microparticles on the surface of a glassy carbon electrode was used to detect the presence of metallic copper particles in suspension. The solution sample containing copper particles was placed on the electrode surface and allowed to dry, then square wave voltammetry was performed. Two voltammetric peaks at −0.076 and 0.365 V vs. Ag/AgCl/KCl sat. in HCl 0.5 M were obtained, which correspond to electrooxidation of Cu particles deposited on the electrode surface.