Lívia Nagy

Copper electrode based amperometric detector cell for sugar and organic acid measurements

Abstract Electrochemical behavior of sugars and organic acids at catalytic copper electrode was investigated in strongly alkaline media. Linear relationship was found for different analytes between concentration and voltammetric current wave or amperometric current. Electrode fouling was not observed neither in voltammetric nor in amperometric measurements. The number of electrons per molecule transferred during electrochemical oxidation was measured with controlled potential electrolysis and using the Koutecky–Levich function. The high number of electrons taking part in the electrode process and the stable operation of the electrode makes the catalytic copper electrode a well applicable sensor in FIA analysis and in chromatographic detection of different sugars and some organic acids. In the work a thin layer type detector cell was prepared and its function was studied.

Enrichment of Amadori products derived from the nonenzymatic glycation of proteins using microscale boronate affinity chromatography

Amadori peptides were enriched using boronate affinity tips and measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS). As demonstrated by electrochemical measurements, the tips show the highest binding efficiency for glucose at pH 8.2 employing ammonium chloride/ammonia buffer with ionic strength of 150 mM, exceeding taurine buffer at the same concentration. The bound constituents were released by sorbitol and formic acid. It was also demonstrated that elution with sorbitol at 1.2 M is superior to acidic media. Comparison of results was based on the numbers of detected peptides and their glycated sites. Using sorbitol for elution requires desalting prior to analysis. Therefore, three different sorbents were tested: fullerene-derivatized silica, ZipTip (C18), and C18 silica. Fullerene-derivatized silica and ZipTip showed the same performance regarding the numbers of glycated peptides, and sites were better than C18 silica. The elaborated off-line method was compared with liquid chromatography-tandem mass spectrometry (LC-MS/MS) measurements, by which considerable less modified peptides were detected. Affinity tips used under optimized conditions were tested for the analysis of human serum albumin (HSA) from sera of healthy and diabetic individuals. A peptide with a mass of 1783.9 Da could be detected only in samples of diabetic patients and, therefore, could be a very interesting biomarker candidate.

The electrochemical determination of phenolic derivates using multiple pulsed amperometry with graphite based electrodes.

The electrochemical determinations of 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) by chronoamperometry (CA) and multiple pulsed amperometry (MPA) using expanded graphite-epoxy composite (EG-Epoxy) and rotating spectral graphite disc (SG) electrodes are reported. The electrochemical behaviours of both electrodes in the presence of organics informed about oxidation peak potential and the electrode fouling with organics concentration increasing. Setting up the oxidation peak potential as detection potential, only SG gave good electroanalytical performance using CA. However, by MPA applying both electrodes exhibited the capability to assess electrochemically and quantitatively the pollutants from aqueous solutions. UV spectrometric method detecting 4-CP and 4-NP at lambda=280nm and lambda=398nm wavelength, respectively was used for validation and parallel determinations.

Development and study of an amperometric biosensor for the in vitro measurement of low concentration of putrescine in blood

An amperometric biosensor was developed for the in vitro determination of putrescine in blood samples because elevated level of putrescine in blood can be a diagnostic indicator of certain kinds of cancer. The electrochemical transducer consisted of a flat form, three electrode amperometric micro-cell fabricated with thin film photolithography on flexible Kapton substrate. An immobilized putrescine oxidase (PUO) layer provided the biocatalytic oxidation of the putrescine, while the generated hydrogen peroxide was detected on the platinum-working electrode. An electropolymerized poly(m-phenylenediamine) (pPDA) size-exclusion layer was used to protect the working electrode from fouling and to prevent signal generation by common electroactive interferents present in blood. The preparation of the biocatalytic enzyme- and outer protective layers was optimized for improved sensitivity and response time. A detection limit of 50 nM was achieved in pH-adjusted whole blood samples, which is below pathological levels.

Electrochemical sensors developed for gathering microscale chemical information

Abstract Miniaturization is a broad trend that has been observed in various areas of technology. As a result of the intensive high tech research efforts of the past few decades in the electronics and telecommunication industries, different tools and technologies have been developed for creating highly structured miniaturized parts and units. These tools and technologies have also been employed for producing special miniaturized units for use in chemical analysis. Miniature sensors, flow micro channels, and detectors are becoming widespread devices in analytical laboratories. The lab‐on‐a‐chip concept is gaining acceptance for a variety of applications. In many different areas of experimental science, researchers endeavor to collect chemical information with high spatial resolution about various subjects. Miniaturized electrochemical sensors and micro or ultramicro electrodes can be used for this purpose. A variety of microelectrodes have been developed and used to collect chemical information in the last few decades. Life scientists, who were studying various biochemical processes inside living tissues with microelectrodes, were pioneers in this work. Recently, a powerful new technique for collecting high resolution chemical information has been devised. This method is known as “scanning electrochemical microscopy.” Scanning electrochemical microscopy relies on the electrochemical micro‐measuring tip. In this short review, the properties of these microelectrodes are discussed briefly, and the types of ion‐selective microelectrodes and voltammetric electrodes developed by several research groups for gathering high resolution information are introduced.

On the selection and design of proteins and peptide derivatives for the production of photoluminescent, red-emitting gold quantum clusters

Novel pathways of the synthesis of photoluminescent gold quantum clusters (AuQCs) using biomolecules as reactants provide biocompatible products for biological imaging techniques. In order to rationalize the rules for the preparation of red-emitting AuQCs in aqueous phase using proteins or peptides, the role of different organic structural units was investigated. Three systems were studied: proteins, peptides, and amino acid mixtures, respectively. We have found that cysteine and tyrosine are indispensable residues. The SH/S-S ratio in a single molecule is not a critical factor in the synthesis, but on the other hand, the stoichiometry of cysteine residues and the gold precursor is crucial. These observations indicate the importance of proper chemical behavior of all species in a wide size range extending from the atomic distances (in the AuI-S semi ring) to nanometer distances covering the larger sizes of proteins assuring the hierarchical structure of the whole self-assembled system.

Thermotropic and structural effects of poly(malic acid) on fully hydrated multilamellar DPPC–water systems

The thermotropic and structural effects of low molecular weight poly(malic acid) (PMLA) on fully hydrated multilamellar dipalmitoylphosphatidylcholine (DPPC)-water systems were investigated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and freeze-fracture transmission electron microscopy (FFTEM). Systems of 20wt% DPPC concentration and 1 and 5wt% PMLA to lipid ratios were studied. The PMLA derivatives changed the thermal behavior of DPPC significantly and caused a drastic loss in correlation between lamellae in the three characteristic thermotropic states (i.e., in the gel, rippled gel and liquid crystalline phases). In the presence of PBS or NaCl, the perturbation was more moderate. The structural behavior on the atomic level was revealed by FTIR spectroscopy. The molecular interactions between DPPC and PMLA were simulated via modeling its measured infrared spectra, and their peculiar spectral features were interpreted. Through this interpretation, the poly(malic acid) is inferred to attach to the headgroups of the phospholipids through hydrogen bonds between the free hydroxil groups of PMLA and the phosphodiester groups of DPPC.

Development of Solid Contact Micropipette Zn-Ion Selective Electrode for Corrosion Studies

Micron-size ion selective micropipettes can be used in scanning electrochemical microscopy (SECM). They can provide excellent spatial resolution. Unfortunately the resistance of these small sensors is high. Their application needs special shielding and slow scanning rates. Usually their lifetime hardly exceeds a few days. Zinc layer or dispersed zinc particles containing films are often used for providing cathodic protection against corrosion in case of metal surfaces. Therefore, in corrosion studies, measurements of local zinc ion concentration can give important information about the nature of the process. For corrosion studies we needed SECM measuring tips for imaging concentration profiles of Zn2+ions involved in surface processes. Based on our earlier experience, solid contact micropipettes for selective measurements of Zn2+ion concentration were prepared with a tip size of a few micrometers. The properties of the micropipettes were investigated. They were also used in SECM imaging. In this paper, details of Zn2+ion selective microelectrode preparation are described. Data about their properties, lifetime, resistance, and ion activity response are shown. Preliminary findings in SECM imaging of zinc ion concentration profiles are shown. The improvement of the scanning rate achieved by lowering tip resistance is a main advantage in potentiometric SECM.

Periodically interrupted amperometry at membrane coated electrodes: A simplified pulsed amperometry

Amperometric detection combined with separation technique or with selective molecular recognition step can be very effective solving quantitative analytical tasks. When the amperometric working electrode surface needs cleaning or reactivation, pulsed amperometric technique can be the choice. Coating working electrodes with different sensitizing or protecting layer is quite common in the practice of voltammetric analysis. In these studies the behavior of coated electrodes using a simplified pulsed amperometric working program which can be named periodically interrupted amperometric (PIA) detection has been investigated. Rotating platinum, and carbon paste electrodes coated with dialysis film or porcine intestinal membrane were used in the experiments. The signal in case of electrochemical oxidation of hydrogen peroxide and ascorbic acid at convective conditions has been evaluated. The signal, obtained with conventional amperometry has been compared with signal collected with a periodically interrupted amperometric measuring program, allowing time for the diffusion to reload the diffusion layer at the electrode surface. The sensitivity and the lower limit of detection (4.5 x 10(-7)M for ascorbic acid and 2 x 10(-6)M for H2O2) proved superior in case of the periodically interrupted amperometry.

Total synthesis of isotopically enriched Si-29 silica NPs as potential spikes for isotope dilution quantification of natural silica NPs.

A new method was developed for the preparation of highly monodisperse isotopically enriched Si-29 silica nanoparticles ((29)Si-silica NPs) with the purpose of using them as spikes for isotope dilution mass spectrometry (IDMS) quantification of silica NPs with natural isotopic distribution. Si-29 tetraethyl orthosilicate ((29)Si-TEOS), the silica precursor was prepared in two steps starting from elementary silicon-29 pellets. In the first step Si-29 silicon tetrachloride ((29)SiCl4) was prepared by heating elementary silicon-29 in chlorine gas stream. By using a multistep cooling system and the dilution of the volatile and moisture-sensitive (29)SiCl4 in carbon tetrachloride as inert medium we managed to reduce product loss caused by evaporation. (29)Si-TEOS was obtained by treating (29)SiCl4 with absolute ethanol. Structural characterisation of (29)Si-TEOS was performed by using (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy and Fourier-transform infrared (FTIR) spectroscopy. For the NP preparation, a basic amino acid catalysis route was used and the resulting NPs were analysed using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and zeta potential measurements. Finally, the feasibility of using enriched NPs for on-line field-flow fractionation coupled with multi-angle light scattering and inductively coupled plasma mass spectrometry (FFF/MALS/ICP-MS) has been demonstrated.