Jurgis Barkauskas

Investigation of conductometric humidity sensors

Sensors for determining humidity in air have been described and investigated. Sensing film of the devices was prepared from polyvinylalcohol and graphitized carbon black disperse phase. The composition, thermal treatment and design of sensing films were investigated and optimized. An optimized humidity sensor has better metrological parameters as compared with its prototype (response time approximately 45 s, detection limit 0.17%, slope 6.25 +/- 0.05 Theta/R.H., standard deviation of measurement 0.15%, standard deviation of analytical signal in the graduation equation 8.29Theta). Such construction of sensors have prospects in analytical practice.

Modified graphitized carbon black as transducing material for reagentless H2O2 and enzyme sensors

Direct electron transfer between redox enzymes and electrodes is the basis for the third generation biosensors. We established direct electron transfer between quinohemoprotein alcohol dehydrogenase (PQQ-ADH) and modified carbon black (CBs) electrodes. Furthermore, for the first time, this phenomenon was observed for pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase (PQQ-GDH). Reagentless enzyme biosensors suitable for the determination of ethanol, glucose and sensors for hydrogen peroxide were designed using CB electrodes and screen-printing technique. Aiming to create an optimal transducing material for biosensors, a set of CB batches was synthesized using the matrix of Plackett-Burman experimental design. Depending on the obtained surface functional groups as well as the nano-scale carbon structures in CBs batches, the maximal direct electron transfer current of glucose and ethanol biosensors can vary from 20 to 300 nA and from 30 to 6300 nA for glucose and ethanol, respectively. Using modified CB electrodes, an electrocatalytic oxidation of H(2)O(2) takes place at more negative potentials (0.1-0.4V versus Ag/AgCl). Moreover, H(2)O(2) oxidation efficiency depends on the amount and morphology of fine fraction in the modified CBs.

Investigation of electroconductive films composed of polyvinyl alcohol and graphitized carbon black

Films consisting of the polymer (polyvinyl alcohol (PVA)) matrix and the filler (graphitized carbon black (CB)) were prepared and examined using TMA, contact angle, and conductivity measurements. Existence of weak interaction between the molecules of PVA and the surface of CB was established. The changes occurring in the process of the thermal treatment and the chlorination of composite films were evaluated using the contact angle measurements. The results of DC conductivity measurements were approached using a model of filler cluster formation in the polymer matrix. The model of this concept is based on the percolation theory and the fractal structure of the filler network. The films exposed to environment of a certain relative humidity and the chlorinated films were examined this way. The size of the clusters, which were formed in the matrix of PVA was determined. It was established that the change of conductivity in the process of moisture adsorption runs through the aggregation of clusters.

Nanocomposite films and coatings produced by interaction between graphite oxide and Congo red

Nanocomposite films and coatings were produced from the aqueous solutions containing different proportions of graphite oxide (GO) and Congo red by filtering through a polycarbonate membrane filter into alkaline media. They were examined by electron microscopy, Raman and FTIR spectroscopy, XRD, contact angle, and electrical conductivity measurements. It was established that the Congo red is able to interact through its amino groups with different functional groups of GO to form larger moieties composed of the nanoplatelets of GO. Raman spectroscopy revealed quinoid-like ring structure for dye adhering to the GO. In the case when the interaction occurs with the terminal functional groups located on the edges of the nanoplateletes of GO, larger crystallites in the nanocomposite are formed. The interaction between the Congo red and functional groups of GO situated in a basal plane leads to more compact structure of the nanocomposite. Pulsed laser treatment was used to reduce GO to graphene. Raman spectra of laser treated areas show positive effect of addition of the Congo red on the graphene yield in nanocomposite coatings after the laser treatment.

Thermally reduced graphene oxide: The study and use for reagentless amperometric D-fructose biosensors.

Aiming to create reagentless amperometric D-fructose biosensor, graphene based electrode materials have been synthesized by newly proposed thermal reduction of graphene oxide. The method allowed to separate and collect different fractions of thermally reduced graphene oxide (TRGO) with different physicochemical properties. The structural characteristics and surface morphologies of TRGO fractions were evaluated using SEM, XRD, TGA analysis, Raman spectroscopy and BET measurements. Three different fractions of TRGO were tested as electrode materials for D-fructose amperometric biosensors. The direct electron transfer (DET) from the active site of D-fructose dehydrogenase (FDH) to the electrode was achieved with all TRGO fractions. High values of the sensitivity (up to 14.5 μA mM(-1) cm(-2)) are of the same order as these for other D-fructose sensors based on the synergistic mediated processes. The relationships between the structure of TRGO fractions and the molecular processes determining the effect of DET in bioelectrocatalysis by FDH have been studied. Stability of the D-fructose biosensors was also assessed. The best results were achieved when immobilization of FDH was performed using a crosslinking with glutaraldehyde. For the best group, after a period of 5 days the sensitivity of the biosensor for D-fructose determination decreased by less than 20%.

Residual content of inorganic ions in activated carbons prepared from wood

Abstract The content and redistribution of inorganic ions (calcium, magnesium, potassium and sodium) among the activated carbon (AC) samples, produced from wood in the laboratory, was determined by means of atomic absorption spectrometry. Three of those elements (Ca, Mg and K) are considered as biogenic. Calcium and Na might be introduced during the process of AC preparation. Content of surface functional groups was determined using a potentiometric titration. Effect of preparation conditions on the distribution of inorganic ions was evaluated. It is established that the high temperature raising rates in temperature zone above 500 K most probably favors the formation of closed pores, which are inaccessible to the leaching reagents. At the same time, both high temperature raising rates below 500 K and the thermal shock are favorable to the formation of open-pore structure. Close correlation between K and Na residual content in ACs is established. It is also found that residual amount of Mg stands in inverse correlation with amount of lactone and phenol functional groups on the surface of AC samples.

INTERACTION BETWEEN Fe AND Cl2 IN THE BED OF GRAPHITIZED CARBON BLACK

A possible route to remove Fe catalyst from graphitized carbon black synthesized in Boudouards reaction is employment of gaseous chlorine in role of carrier. This process was explored by means of DTA method using the equipment designed in the laboratory. Obtained results demonstrate the complexity of processes occurring in systems containing Fe, C and Cl2. Reactions in the system are highly influenced by the geometry of reacting solids. Process of FeCl3 intercalation between graphite layers was observed analyzing DTA curves. The amount of Fe in the raw product of Boudouards reaction was determined during the process of chlorination. Obtained results indicate that Cl2 stream does not remove all the Fe even at high temperatures and prolonged chlorinating time.

Investigation of Distribution of Heavy Metals between Blood Plasma and Blood Cells

The physiological importance of metals in human organism has been shown by many publications. The toxic doses of metals and their compounds can lead to serious health problems.1–4 Different metals present in the composition of blood can form different complexes with many organic compounds and biomolecules which could be found in the body fluids.5–7 Depending on the concentration of metals in the parts of body, different metal-ligand equilibriums could be established in the system. These changes could cause changes in global bioprocesses, or different clinical symptoms and metabolic stresses in human organism could occur. The distribution of metals between blood plasma and blood cells could represent an important clinical index.8–10 The aim of the present study was to investigate, for the first time to our knowledge, the distribution of heavy metals between blood plasma and cells in the blood samples from the infected by hepatitis C and non-infected patients.

Graphene oxide-dye nanocomposites: effect of molecular structure on the quality of laser-induced graphene

The nanocomposite coatings made using graphene oxide (GO) and six different organic dyes were used to produce the laser-induced graphene (LIG) coatings by means of near infrared picosecond laser irradiation. The coatings were investigated by means of contact angle measurement with three liquids (1-bromonaphtalene, glycerol and water), Raman spectroscopy, scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis. It was found that the more hydrophilic is the precursor surface the more hydrophobic LIG surface is produced after the laser treatment. Contact angle values obtained on LIG produced from pure GO reached 143°. FTIR spectra have shown that the interaction between GO and dye molecules is realized through the nitrogen atoms. Raman spectra have shown that the best quality LIG coating is obtained using a GO-neutral red nanocomposite precursor. A correlation among contact angle, Raman spectra and topological indices of dye molecules was found, and will serve for the further investigation of the mechanism of LIG production and development of low-defect coatings.

Tailoring of graphite oxide electrical properties using laser irradiation

In this work, the experimental results of the graphite oxide (GO) reduction with laser irradiation are presented. GO films on a flexible polycarbonate substrate were produced using a modified Hummers method. Experiments were conducted using a picosecond laser. The pulse energy and beam scanning speed were varied during the reduction experiments. All experiments were performed in air. Raman spectroscopy measurements and electrical resistance measurements were implemented on the laser treated GO samples. Scanning electron microscopy (SEM) was used for morphology inspection. Experiments results showed that for certain range of laser microfabrication parameters, electrical properties, suitable for electronics applications can be achieved in reduced GO films. Such laser-modified GO films are intended to be used as contacts for flexible supercapacitors.