Bioelectroanalysis with nanoelectrode ensembles and arrays

This review deals with recent advances in bioelectroanalytical applications of nanostructured electrodes, in particular nanoelectrode ensembles (NEEs) and arrays (NEAs). First, nanofabrication techniques, principles of function, and specific advantages and limits of NEEs and NEAs are critically discussed. In the second part, some recent examples of bioelectroanalytical applications are presented. These include use of nanoelectrode arrays and/or ensembles for direct electrochemical analysis of pharmacologically active organic compounds or redox proteins, and the development of functionalized nanoelectrode systems and their use as catalytic or affinity electrochemical biosensors.     

Nanoelectrodes, nanoelectrode arrays and their applications

This review deals with the topic of ultrasmall electrodes, namely nanoelectrodes, arrays of these and discusses possible applications, including to analytical science. It deals exclusively with the use of nanoelectrodes in an electrochemical context. Benefits that accrue from use of very small working electrodes within electrochemical cells are discussed, followed by a review of methods for the preparation of such electrodes. Individual nanoelectrodes and arrays or ensembles of these are addressed, as are nanopore systems which seek to emulate biological transmembrane ion transport processes. Applications within physical electrochemistry, imaging science and analytical science are summarised.     

Fabrication and electrochemical characterization of interdigitated nanoelectrode arrays

Interdigitated nanoelectrode arrays with controlled electrode bandwidth and gap geometries ranging from 30 nm to 1 μm were fabricated on glass substrates by a planar process involving high resolution electron beam lithography and lift-off, and their characteristic electrochemical responses to an aqueous ferrocene derivative solution were examined using fundamental electrochemical techniques. Despite the comparatively large electrode area of electrode arrays containing 10 bands to a single band electrode, quasi-steady-state currents with high current density were obtained at a slow potential sweep rate in cyclic voltammograms of ferrocene derivative since the lateral dimension of the nanoelectrode arrays was considerably less than the scale of the diffusion layer of redox species. Additionally, it was demonstrated that the electrode thickness influenced limiting currents of voltammograms in the case of nanoelectrode arrays. In generation-collection mode experiments, furthermore, a collection efficiency as high as ∼99% was attained by 100 nm wide electrode arrays with a gap dimension of 30 nm.     

Solute-solvent interactions probed by intermolecular NOEs

Nuclear Overhauser effects arising from the interactions of spins of solvent molecules with spins of a solute should reveal the "exposure" of solute spins to collisions with solvent. Such intermolecular NOEs could, therefore, provide information regarding conformation or structure of the solute. Determinations of solute-solvent NOEs of 1,3-di-tert-butylbenzene in solvents composed of perfluoro-tert-butyl alcohol, tetramethylsilane, and carbon tetrachloride have been carried out. A crude, but apparently reliable, method for prediction of intermolecular solvent-solute NOEs based on hard (noninteracting) spheres was developed. Comparison of experimental to predicted NOEs indicates that tetramethylsilane interacts with the solute according to the model. By contrast, intermolecular NOE data indicate attractive interactions between the solute and perfluoro-tert-butyl alcohol. All NOE results and the corresponding predictions confirm that proton H2 of the solute is protected by the flanking tert-butyl groups from interactions with solvent molecules.     

Dielectrophoretic trapping of single bacteria at carbon nanofiber nanoelectrode arrays

We present an ac dielectrophoretic (DEP) technique for single-cell trapping using embedded carbon nanofiber (CNF) nanoelectrode arrays (NEAs). NEAs fabricated by inlaying vertically aligned carbon nanofibers in SiO2 matrix are applied as "points-and-lid" DEP devices in aqueous solution. The miniaturization of the electrode size provides a highly focused electrical field with the gradient enhanced by orders of magnitude. This generates extremely large positive DEP forces near the electrode surface and traps small bioparticles against strong hydrodynamic forces. This technology promises new capabilities to perform novel cell biology experiments at the nanoscale. We anticipate that the bottom-up approach of such nano-DEP devices allows the integration of millions of nanolectrodes deterministically in lab-on-a-chip devices and will be generally useful for manipulating submicron particles.     

The intermolecular interactions in binaries

The recapitulation of systematical investigations of excess enthalpy of mixing in binary mixtures: pyridine base +n-alkane or some of arenes is presented. On the base of experimental results as well as model calculations (PFP, ERAS) the discussion of intermolecular interactions in pyridine bases is given.     

Thermodynamic functions characterizing intermolecular interactions

It is shown that the Gibbs vaporization potential G^* is additive with respect to molecular groups at all temperatures and it most completely characterizes the intermolecular interactions. The excess entropy of vaporization is identical for all spherical molecules (30 J/mole·K and does not depend on the size of the molecule or the temperature. In long-chain molecules it is additive with respect to the number of links in the chain, varies with temperature, and is equal to the difference between the heat capacities of the gas and liquid and exceeds 30 J/mole·K.Leningrad State Scientific Institute of Industrial Chemistry. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 1, pp. 66–70, January–February, 1991. Original article submitted April 27, 1988.     

Label-free electrochemical impedance detection of kinase and phosphatase activities using carbon nanofiber nanoelectrode arrays

We demonstrate the feasibility of a label-free electrochemical method to detect the kinetics of phosphorylation and dephosphorylation of surface-attached peptides catalyzed by kinase and phosphatase, respectively. The peptides with a sequence specific to c-Src tyrosine kinase and protein tyrosine phosphatase 1B (PTP1B) were first validated with ELISA-based protein tyrosine kinase assay and then functionalized on vertically aligned carbon nanofiber (VACNF) nanoelectrode arrays (NEAs). Real-time electrochemical impedance spectroscopy (REIS) measurements showed reversible impedance changes upon the addition of c-Src kinase and PTP1B phosphatase. Only a small and unreliable impedance variation was observed during the peptide phosphorylation, but a large and fast impedance decrease was observed during the peptide dephosphorylation at different PTP1B concentrations. The REIS data of dephosphorylation displayed a well-defined exponential decay following the Michaelis–Menten heterogeneous enzymatic model with a specific constant, k_cat/K_m, of (2.1 ± 0.1) × 10⁷ M⁻¹ s⁻¹. Consistent values of the specific constant was measured at PTP1B concentration varying from 1.2 to 2.4 nM with the corresponding electrochemical signal decay constant varying from 38.5 to 19.1 s. This electrochemical method can be potentially used as a label-free method for profiling enzyme activities in fast reactions.     

Compact method for calculating intermolecular interactions

The development of efficient methods for calculating intermolecular interactions (which are responsible for the existence of stable molecular associates, solvation shells, etc.) is a pressing problem of quantum chemistry. We propose a new method for correct calculations of intermolecular interactions, which is based on the solution of SCF equations with fractional occupation numbers. Calculating intermolecular interactions by this method does not require the use of exchange potentials in an explicit form. The method is intended primarily to describe the charge transfer between interacting subsystems. The calculations by this method are compact since the dimensions of matrix problems remain unchanged in the course of the numerical procedure. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 401–405, May–June, 1995. Translated by I. Izvekova     

Retarded intermolecular interactions involving diamagnetic molecules

Second‐ and third‐order time‐dependent perturbation theory within the multipolar framework of nonrelativistic quantum electrodynamics is used to calculate the retarded dispersion interaction between two diamagnetic molecules, a diamagnetic molecule and a magnetic‐dipole susceptible molecule, and a diamagnetic molecule and an electric‐quadrupole polarizable molecule. New expressions for the energy shift valid for all intermolecular separation distances, R, beyond the region of overlap of molecular electronic wave functions and applicable to a pair of randomly oriented molecules in the ground electronic state are given. The R‐dependent behavior of the far‐zone limit of the interaction energies is also examined. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 437–442, 2000     

Study of intermolecular interactions in aqueous solutions by millimeter spectroscopy

The absorption of millimeter electromagnetic radiation (v=1.4, 1.71, and 5 cm⁻¹) by aqueous solutions of glycine (pH 6.1–6.2) in the concentration range of 0.5–2.5 mol L⁻¹ was measured. It was found that the absorbing ability of the water present in the solutions, is higher than that of pure water. This phenomenon is explained by the presence of a center of negative hydration in the structure of the glycine zwitterion, which results in an increase in the rotational mobility of water molecules immobilized in the hydrate shell of the glycine zwitterion. For Part 5, see Ref. 1. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1305–1307, July, 1997.     

SCF perturbation theory and intermolecular interactions

A self-consistent perturbation theory is derived in the framework of Roothaan's MOLCAO procedure for closed shell systems. Contrary to previous investigations which have considered only one particle perturbations, two particle perturbation operators are considered. Expressions for the first-order density matrix and first- and second-order energy corrections are obtained. A diagram formulation of the complete perturbation expansion is presented. The results are applied to the treatment of the intermolecular interaction problem. The interaction energy is represented as a sum of several contributions: Coulomb, exchange, resonance, polarization and exchange repulsion. A semi-empirical version of the theory is suggested which explicitly involves all the physically significant energy terms and may be useful for the investigation of complex systems.     

The intermolecular interactions in the aminonitromethylbenzenes

The intermolecular non-covalent interactions in aminonitromethylbenzenes namely 2-methyl-4-nitroaniline, 4-methyl-3-nitroaniline, 2-methyl-6-nitroaniline, 4-amino-2,6-dinitrotoluene, 2-methyl-5-nitroaniline, 4-methyl-2-nitroaniline, 2,3-dimethyl-6-nitroaniline, 4,5-dimethyl-2-nitroaniline and 2-methyl-3,5-dinitroaniline were studied by quantum mechanical calculations at RHF/311++G(3df,2p) and B3LYP/311++G(3df,2p) level of theory. The calculations prove that solely geometrical study of hydrogen bonding can be very misleading because not all short distances (classified as hydrogen bonds on the basis of interaction geometry) are bonding in character. For studied compounds interaction energy ranges from 0.23 kcal mol⁻¹ to 5.59 kcal mol⁻¹. The creation of intermolecular hydrogen bonds leads to charge redistribution in donors and acceptors. The Natural Bonding Orbitals analysis shows that hydrogen bonds are created by transfer of electron density from the lone pair orbitals of the H-bond acceptor to the antibonding molecular orbitals of the H-bond donor and Rydberg orbitals of the hydrogen atom. The stacking interactions are the interactions of delocalized molecular π-orbitals of the one molecule with delocalized antibonding molecular π-orbitals and the antibonding molecular σ-orbital created between the carbon atoms of the second aromatic ring and vice versa.      

A post-Hartree-Fock model of intermolecular interactions

Intermolecular interactions are of great importance in chemistry but are difficult to model accurately with computational methods. In particular, Hartree-Fock and standard density-functional approximations do not include the physics necessary to properly describe dispersion. These methods are sometimes corrected to account for dispersion by adding a pairwise C6R6 term, with C6 dispersion coefficients dependent on the atoms involved. We present a post-Hartree-Fock model in which C6 coefficients are generated by the instantaneous dipole moment of the exchange hole. This model relies on occupied orbitals only, and involves only one, universal, empirical parameter to limit the dispersion energy at small interatomic separations. The model is extensively tested on isotropic C6 coefficients of 178 intermolecular pairs. It is also applied to the calculation of the geometries and binding energies of 20 intermolecular complexes involving dispersion, dipole-induced dipole, dipole-dipole, and hydrogen-bonding interactions, with remarkably good results.     

Hierarchical carbon nanotube assemblies created by sugar-boric or boronic acid interactions

We previously found that polysaccharide "schizophyllan (SPG)" can entrap as-grown and cut single-walled carbon nanotubes (as-SWNTs and c-SWNTs, respectively): we here reported that the c-SWNT-s-SPG (single stranded SPG) composites thus obtained can be aligned regularly using the covalent bond formation between boric acid or boronic acid derivatives and the 4,6-dihydroxyl group of the glucose side-chain unit.     

Study of intermolecular interactions in liquid nitrobenzene by depolarized hyper-Rayleigh scattering

We employed depolarized hyper-Rayleigh scattering (HRS) to investigate the intermolecular interactions in liquid nitrobenzene. By comparing the depolarization ratios of the second-harmonic scattered light from neat nitrobenzene and mixtures of nitrobenzene and methanol of varying mixing ratios, we demonstrated the existence of a coherent component of HRS in liquid nitrobenzene. The coherent component was found to essentially disappear at a sufficiently high dilution of the nitrobenzene liquid. We also observed that both localized orientational correlation and delocalized libron excitation contribute to coherent HRS in liquid nitrobenzene. The delocalized contribution to coherent HRS was found to diminish much more readily with the introduction of interstitial foreign molecules than the localized contribution.