Paweł Krysiński

New trends in the electrochemical sensing of dopamine

Since the early 70s electrochemistry has been used as a powerful analytical technique for monitoring electroactive species in living organisms. In particular, after extremely rapid evolution of new micro and nanotechnology it has been established as an invaluable technique ranging from experiments in vivo to measurement of exocytosis during communication between cells under in vitro conditions. This review highlights recent advances in the development of electrochemical sensors for selective sensing of one of the most important neurotransmitters—dopamine. Dopamine is an electroactive catecholamine neurotransmitter, abundant in the mammalian central nervous system, affecting both cognitive and behavioral functions of living organisms. We have not attempted to cover a large time-span nor to be comprehensive in presenting the vast literature devoted to electrochemical dopamine sensing. Instead, we have focused on the last five years, describing recent progress as well as showing some problems and directions for future development.

Three-probe voltammetric characterisation of octadecanethiol self-assembled monolayer integrity on gold electrodes

Abstract The passivating behaviour of self-assembled octadecanethiol monolayers on polycrystalline gold electrodes was tested by means of the cyclic voltammetry technique in the presence of three different redox probe molecules. The three compounds chosen for this purpose, ruthenium hexamine chloride [Ru(NH 3 )Cl 3 ], potassium ferrocyanide [K 4 Fe(CN) 6 ] and benzoquinone, were selected on the basis of the differences in their heterogeneous electron transfer rate constants and their hydrophobic/hydrophilic character. Our studies show that passivating properties of octadecanethiol monolayers against these probes are different. The results obtained are discussed in terms of Tafel approach and monolayer integrity. It appears that the screening of self-assembled films with different probes is necessary in order to properly characterise the film integrity and passivating behaviour. Additionally, the interaction of octadecanethiol molecules with a polycrystalline gold surface was evaluated by means of potentiostatic pulse measurements.

A voltammetric study of monolayers and bilayers self-assembled on metal electrodes

Abstract Three types of aliphatic thiols, n-dodecanethiol, n-hexadecanethiol and n-octadecanethiol, differing in their physical state in self-assembled monolayers at ambient temperature were tested from the point of view of their integrity and usefulness as the sub-layer for the second, adjacent layer of phospholipids. These self-assembled monolayers were formed on mercury and polycrystalline gold electrodes in order to assess their electrochemical behavior as monolayers passivating the electrodes against various redox probes present in aqueous phase, such as hexaamineruthenium(III) chloride and benzoquinone, differing in their heterogeneous rate constants and hydrophobic–hydrophilic character. Subsequently, as such a type of hydrophobic alkanethiol monolayer is frequently used as a part of alkanethiol|phospholipid asymmetric bilayer membranes, chemically bound to the metallic surface, the monolayer was covered with the adjacent phosphatidylcholine monolayer, and the electrochemical behavior of such an asymmetric system was again tested. The results obtained show that the liquid-like monolayers, particularly those formed on mercury, possess better passivating properties (much smaller number of defects) as compared to crystalline thiols, also providing a better driving force for the attachment of the second liquid-crystalline phosphatidylcholine monolayer. An interesting observation was also noted that the outer phospholipid monolayer imposes a larger energy barrier to the penetration of benzoquinone that hexaamineruthenium(III) cations.

Progress in Targeting Tumor Cells by Using Drug-Magnetic Nanoparticles Conjugate

To limit cytotoxicity of anticancer drugs against healthy cells, an appropriate carrier should be synthesized to deliver the drug to the tumor tissue only. A good solution is to anchor a magnetic nanoparticle to the molecule of the drug and to use a properly directed external magnetic field. The synthesis of the conjugate of doxorubicin with magnetic nanoparticles (iron oxide) modified by us resulted in a substantial depression of the aggregation process of the nanoparticles and therefore allowed the correct examination of cytotoxicity of the modified drug. It has been shown, by performing the electrochemical microbalance measurements, that the use of magnetic field guaranteed the efficient delivery of the drug to the desired place. The change in the synthesis procedure led to an increase in the number of DOX molecules attached to one magnetic nanoparticle. The release of the drug took place at pH 5.8 (and below it), which pH characterizes the cancer cells. It has also been found that while the iron oxide magnetic nanoparticles were not cytotoxic toward human urinary bladder carcinoma cells UM-UC-3, the tumor cell sensitivity of the DOX-Np complex was slightly higher in comparison to the identical concentration of doxorubicin alone.

Photopolymerized polypyrrole microvessels.

We report on the preparation of water-filled polymer microvessels through the photopolymerization of pyrrole in a water/chloroform emulsion. The resulting structures were characterized by complementary spectroscopic and microscopic techniques, including Raman spectroscopy, XPS, SEM, and TEM. The encapsulation of fluorescent, magnetic, and ionic species within the microvessels has been demonstrated. Confocal microscopy and fluorescence anisotropy measurements revealed that the encapsulated chromophore (Rhodamine 6G) resides within voids in the capsules; however, strong interaction of the dye with polypyrrole results in a measurable decrease in its rotational dynamics. Microvessels loaded with ferrofluid exhibit magnetic properties, and their structures can be directed with an external magnetic field. TEM measurements allowed imaging of individual nanoparticles entrapped within the vessels. The application of Cu(2+)-loaded microvessels as a transducer layer in all-solid-state ion-selective electrodes was also demonstrated.

Bulk- and surface-initiated chemical in situ polymerisation of 2,5-dimethoxyaniline and 2-methoxyaniline on thiol-coated gold electrodes

Abstract We present the study of poly(2-methoxyaniline) and poly(2,5-dimethoxyaniline) chemical deposition in situ on gold electrodes coated with self-assembled thiol monolayers (SAM) containing different terminal groups: methyl, hydroxy, and carboxy. The differences in the deposition rates of polymers on various thiol monolayers were investigated with the quartz crystal microbalance (QCM) and UV–Vis spectroscopy. It was found that for poly(2,5-dimethoxyaniline) the polymer is formed in the bulk of the polymerisation solution and subsequently or simultaneously it accumulates on the substrate surface. The data obtained for poly(2-methoxyaniline) suggest substantially different mechanism of deposition. The polymerisation is initiated on SAM-modified gold, and this process is strongly influenced by the chemical nature of the monolayer. Thus, we observed large differences in the deposition rates on ‘hydrophobic’ and ‘hydrophilic’ monolayers modifying the electrode surface. The effect of the underlying thiol monolayer on the morphology of synthesised polymers was also investigated by atomic force microscopy (AFM).

Electrochemical deposition of poly(o-anisidine) and polypyrrole at octadecanethiol coated gold electrodes

Abstract The o-anisidine and pyrrole have been polymerized by the electrochemical oxidation of monomers on gold electrodes, covered with self-assembled monolayers. The obtained polymer–monolayer systems have been investigated by cyclic voltammetry in aqueous supporting electrolyte solutions containing K4Fe(CN)6 and Ru(NH3)6Cl3. The deposition of conducting polymers strongly depends on the integrity of a monolayer. In the case of a large number of SAM defects, the polymerization of o-anisidine and pyrrole leads to the formation of nuclei of the conducting polymer in the insulating matrix of the thiol monolayer. When the polymer is in the conducting (oxidized) form, the nuclei act as an array of microelectrodes. The polarogram-shaped voltammograms obtained for K4Fe(CN)6 confirm the hemispherical diffusion of redox species to the polymer nuclei. When the polymer is in the non-conducting (reduced) form, the polymer–octadecanethiol layer blocks the redox processes on the electrode. The exponential-type CV curves observed for Ru(NH3)6Cl3, when the polymer is in its non-conducting state, can be assigned to the tunnelling of electrons through the passivating layer. The use of monolayers with a low number of defects influences the mechanism of polymer growth. Thus, the polypyrrole grows on the layer of thiols, and the poly(o-anisidine) forms polymer nuclei.

Interactions of doxorubicin with self-assembled monolayer-modified electrodes: electrochemical, surface plasmon resonance (SPR), and gravimetric studies.

We present the results on the partitioning of doxorubicin (DOX), a potent anticancer drug, through the model membrane system, self-assembled monolayers (SAMs) on gold electrodes. The monolayers were formed from alkanethiols of comparable length with different ω-terminal groups facing the aqueous electrolyte: the hydrophobic -CH(3) groups for the case of dodecanethiol SAMs or hydrophilic -OH groups of mercaptoundecanol SAMs. The electrochemical experiments combined with the surface plasmon resonance (SPR) and gravimetric studies show that doxorubicin is likely adsorbed onto the surface of hydrophilic monolayer, while for the case of the hydrophobic one the drug mostly penetrates the monolayer moiety. The adsorption of the drug hinders further penetration of doxorubicin into the monolayer moiety.

Polymer sandwiches: polyaniline films deposited on thiol-coated gold by chemical in situ method

We present a study of polyaniline chemical deposition in situ on gold electrodes coated with thiol monolayers containing different terminal groups: methyl, hydroxy, and carboxy. The differences in the deposition rates of polymers on various thiol monolayers were investigated with the quartz crystal microbalance (QCM) and UV-vis spectroscopy. It was found that the polymer is formed in the bulk of the polymerization solution and subsequently precipitated on the electrode surface. The synthesized polymers were studied by cyclic voltammetry. A novel method allowing for the investigation of the inner (thiol) layer integrity was proposed. It is based on the electrochemical oxidation of the gold substrate and formation of complexes with chloride ions incorporated in the polymer films. The observation of a characteristic AuCl4− reduction peak allows for the determination whether the polymer is adjacent just to the surface of gold or it is separated from the bare metal by thiol monolayer. It was found that the deposition of polymer on the thiol monolayers with terminal methyl and carboxy groups leads to the formation of sandwich-like systems with the thiol monolayer interposed between the polymer layer and the gold surface, while the chemical polymerization on hydroxy-ended thiols induces partial desorption of the monolayer; thus, the polymer is directly adjacent to the bare gold over large areas of the electrode surface. The morphology of the synthesized polymer was also investigated by contact mode atomic force microscopy (AFM).

Photoinduced reactivity of doxorubicin: catalysis and degradation.

Doxorubicin exhibits unusual photoreactivity in aqueous solutions. Our data show that there are two distinct photoreactive pathways for doxorubicin. One is a two-step process that leads to the formation of 3-methoxysalicylic acid, a stable degradation product. The other pathway is a photoreduction of doxorubicin to form the corresponding dihydroquinone, which undergoes spontaneous oxidation mediated by dissolved oxygen to recover doxorubicin with the formation of hydrogen peroxide. Our data account for the known nonlinear dependence of doxorubicin fluorescence intensity on concentration.