Developing SECM apparatus and using them in different studies
The group intensively takes part in the research work dealing development and application of scanning electrochemical microscopy (SECM). This powerful measuring technique has been pioneered by Bard and Engström. It is a version of the so called probe microscopy methods. It is based on combined application of ultramicro electrochemical sensors, precision positioning device and computer controlled measurement, and evaluating programs.
Our group successfully constructed SECM instruments, worked out their working programs and applied the technique solving different problems.
Development of new microsensors for application in SECM
In practice of SECM the overwhelming part of the measurements are done with amperometric micro tip sensors . However, potentiometric sensors can provide higher selectivity, good spatial resolution. Unfortunately earlier developed ion selective electrodes of the size needed for effective SECM application are fragile sensors with high electric resistance and short life time. Their high resistance is a draw back considering application in scanning electrochemical microscopy (SECM) and in life sciences. In our work ion selective mictropipettes with lowered resistance and extended life times have been developed. It proved well applicable in SECM, and in vivo biologic experiments. Up till now new type NH4+, K+, Mg2+ and Zn2+ micropipettes were prepared and tested. Several reports have been published about their properties and applications.
Electrochemical cell for in vivo H2S measurement
Recently the involvements of H2S in numerous physiological processes have been proved. This generated a rapidly increasing interest in studying its interaction with enzymes, with hemoproteins. Its role as a signaling molecule of the inflammatory and nervous systems, and in cardiovascular system has been proved. It regulates vascular tone. It is considered as member of the group of physiological signaling gases together with the NO and CO.
The H2S in living tissues takes place in different redox processes and interacts with several endogenous species. Therefore its local concentration is changing rapidly. Furthermore the concentration is very small, it stays in the micromolar range.
Some of the hydrogen sulfide containing thermal mineral waters proved very effective in balneotherapy of different diseases. Question arose about extent of trans dermal absorption of the H2S content of this waters during balneotherapic treatments.
In our laboratory a new type of amperometric microcell was developed for investigation of transdermal H2S transport. The microcell was implanted into subdermal area of anesthetized experimental mice. Our experiments proved that considerable amount of the hydrogen sulfide absorbs through dermal membrane of the experimental animal upon bathing in hydrogen sulfide containing natural waters. Further work with employing and further developing the sensor is in progress.
Time of flight (TOF) method for measuring diffusion coefficient
Taking advantage on the special feature of the scanning electrochemical microscope a simple and fast method for measuring diffusion coefficient of different molecules in different media has been worked out. The advantage of the method has been proved experimentally. A few reports have been published about the results obtained with this technique. The technique is used in studying effect of different parameters on diffusion properties.
Periodically interrupted amperometry (PIA) for enhancing sensitivity of membrane coated electrodes
In case of amperometric biosensors a quiescent reaction layer coats the electrode surface. The amperometric base sensing element detects the change of the species involved in enzyme catalyzed reaction. In conventional amperometry the detector continuously operates. Since the detected species are taking part continuously in the electrode process, their concentration is small at the electrode surface. In the method – developed in our work – the electrolysis is periodically interrupted, allowing time for reloading the surface layer with the electroactive species. The signal, according to this is substantially increased resulting in higher sensitivity of the amperometric biosensor.
The applicability of the method has been proved experimentally.
The biosensors prepared with native enzyme sources owing to the low enzyme activity possess small sensitivity. This limits their applicability in analyzing real samples. In our work the sensitivity of a native tyrosinase based enzyme sensor was enhanced using periodically interrupted amperometric detection and optimized reaction layer thickness. Research work are in progress with applications of PIA technique.
Using SECM in corrosion studies
Ion-selective microelectrodes can be employed as tips in scanning electrochemical microscopy (SECM) for chemical imaging of corrosion processes. They present higher chemical selectivity than conventional amperometric microdisks, and may be the only effective option to visualize the dissolution of metals with negative redox potentials in aqueous environments when the use of Pt microelectrodes is limited by the onset of oxygen reduction and hydrogen evolution reactions. In international cooperation studies about detailed examination of corrosion processes have been carried out. In this work SECM measurements are done using measuring tips developed in our work. The local pH, oxygen concentration and the ionic fluxes are followed over corroding metal surfaces. Effects of different treatments and coatings on the corrosion intensity are studied. Up till now local ionic current density measurements were done by scanning electrochemical microscopy over the surface of an iron-magnesium and iron-zinc galvanic pairs immersed in aqueous chloride-containing solution.
SECM measurements with gas phase scanning
In most of the SECM measurements the tip is scanned in liquid or in gel phases. However, in certain cases gases are evolved over the surface layer of different samples like microorganism colonies or heterogenic surface bound catalysts. Therefore gas phase scanning can gain application in practice of SECM. In our laboratory experiments are in progress with gas phase SECM studies.
Miniaturized version of Severinghaus type carbon dioxide cell was prepared and was used as measuring tip in scanning electrochemical microscopy. While the conventional Severinghause cells contain gas permeable membrane for providing selectivity, the microcell presented here is prepared without membrane. It is an air gap type carbon dioxide sensor. This structure substantially decreases response time. Horizontal line scans were made in gas phase at different vertical distances over a small size, surface confined yeast colony without disturbing it. The collected data were used for preparing CO2 concentration – distance plots. Two methods were employed for estimating the carbon dioxide flux from experimental yeast colony. In practice of substrate generating – tip detecting (SG/TD) mode of SECM the delayed response, that is the long response time can bring in distortions. The air gap construction with shorter response time can provide more realistic concentration profile with higher scanning rate than a slower membrane coated tip.
The results already presented proves that potentiometric SECM can be done scanning in the gas phase over a target without disturbing its conditions. Estimating fluxes of gases from undisturbed surface confined microbial colonies can be a fruitful application of SECM in the future.
Investigation of electrochemical effects of host- guest interaction
The analytical signal generating function of certain chemical sensors is based on host –guest interaction between the sensing (host) species and the analyte (guest ) species. The interaction can change transport properties, electrode potential, electrocatalytic activity, or other voltammetric behavior. The group investigates the effects of interactions on the electrochemical properties. Properties of molecules newly synthesized by partners are studied. Membrane potential response, cyclic voltammetric behavior, electrocatalytic and biocatalytic activities, and transport rate are detected in this work.
Development and application of chemically modified electrodes, among them biosensors
The group has been working successfully in developing and application of chemically modified electrodes. In this line:
Developing and using electrochemical sensors and methods for detecting local concentration of reactive oxidizing species (ROS)
Cooperating with research groups working in the field of experimental life sciences and in the field of biosensor developments, efforts are done for working out microsensors and methods applicable for detecting ROS species like hydrogen peroxide, nitrogen oxide, peroxide nitrite, superoxide and following their concentration changes in different media.
Working out ion selective electrodes for applications in biotechnology
Micro algal culturing techniques represent a fast growing branch of biotechnology. It is successfully used in producing fine chemicals as well as just growing algae as source of renewable fuels. The ionic composition of he media is a crucial factor influencing the yield in micro algal culturing techniques. Therefore continuous in situ measurement of concentration and control of different ionic species in the media is highly beneficial. Application of ion selective electrodes as transducer for the analysis is an obvious choice, however their adaptation for this can be a challenging task because of the highly variable concentrations of interfering chemical species in biotechnologic media. Cooperating with groups involved in biotechnology research, the group takes part in working out electrochemical sensors well applicable in the complex media of biotechnology.