Improved ion-selective detection method using nanopipette with poly(vinyl chloride)-based membrane.

Ion-selective electrodes (ISEs) are widely used to detect targeted ions in solution selectively. Application of an ISE to a small area detection system with a nanopipette requires a special measurement method in order to avoid the enhanced background signal problem caused by a cation-rich layer near the charged inner surface of the nanopipette and the selectivity change problem due to relatively fast saturation of the ISE inside the nanopipette. We developed a novel ion-selective detection system using a nanopipette that measures an alternating current (AC) signal mediated by saturated ionophores in a poly(vinyl chloride) (PVC) membrane located at the conical shank of the nanopipette to solve the above problems. Small but reliable K(+) and Na(+) ionic current passing through a PVC membrane containing saturated bis(benzo-15-crown-5) and bis(12-crown-4) ionophore, respectively, could be selectively detected using the AC signal measurement system equipped with a lock-in amplifier.

Effect of concentration gradient on ionic current rectification in polyethyleneimine modified glass nano-pipettes.

Ion current rectification dependent on the concentration gradient of KCl solutions was systematically investigated in polyethyleneimine modified glass nano-pipettes with inner diameter of 105 nm. Peak shape dependence of the rectification factor on outer KCl solution concentration was observed when inner KCl solution with concentration from 1 mM to 500 mM was used. The peak shape dependence was also observed when the concentrations of the inner and outer KCl solutions were identically controlled. The peak shape in the ion current rectification could be explained by the ion conductance changes through the conical nano-pipette, which result from modulation of ion concentration.

Nanopipette exploring nanoworld.

Nanopipettes, with tip orifices on the order of tens to hundreds of nanometers, have been utilized in the fields of analytical chemistry and nanophysiology. Nanopipettes make nanofabrication possible at liquid/solid interfaces. Moreover, they are utilized in time-resolved measurements and for imaging biological materials, e.g., living cells, by using techniques such as scanning ion-conductance microscopy and scanning electrochemical microscopy. We have successfully fabricated ion-selective nanopipettes that can be used to identify targeted ions such as sodium and potassium in- and outside of living cells. In this review, we discuss the extent of utilization of nanopipettes in investigating the nanoworld. In addition, we discuss the potential applications of future nanopipettes.

Ion-selective detection by plasticized poly(vinyl chloride) membrane in glass nanopipette with alternating voltage modulation.

An alternating current (AC) voltage modulation was applied to ion-selective observations with plasticized poly(vinyl chloride) membranes in glass nanopipettes. The liquid confronting the membranes in the nanopipettes, the conditioning process, and AC voltage modulation play important roles in the ion-selective detection. In the AC detection system developed by us, where distilled water was used as the liquid within the nanopipettes, potassium ions were selectively detected in the sample solution of sodium and potassium ions because sodium ions were captured at the membrane containing bis(12-crown-4) ionophores, before the saturation of the ionophores. The membrane lost the selectivity after the saturation. On using sodium chloride as the liquid within the nanopipette, the membrane selectively detected potassium and sodium ions before and after the saturation of ionophores, respectively. The ion-selective detection of our system can be explained by the ion extraction-diffusion-dissolution mechanism through the bis(12-crown-4) ionophores with AC voltage modulation.

Manipulating double-decker molecules at the liquid-solid interface.

We have used a scanning tunneling microscope (STM) to manipulate heteroleptic phthalocyaninato, naphthalocyaninato, and porphyrinato double-decker (DD) molecules at the liquid-solid interface between 1-phenyloctane solvent and graphite. We employed nanografting of phthalocyanines with eight octyl chains to place these molecules into a matrix of heteroleptic DD molecules; the overlayer structure is epitaxial on graphite. We have also used nanografting to place DD molecules in matrices of single-layer phthalocyanines with octyl chains. Rectangular scans with a STM at low bias voltage resulted in the removal of the adsorbed DD molecular layer and substituted the DD molecules with bilayer-stacked phthalocyanines from phenyloctane solution. Single heteroleptic DD molecules with lutetium sandwiched between naphthalocyanine and octaethylporphyrin were decomposed with voltage pulses from the probe tip; the top octaethylporphyrin ligand was removed, and the bottom naphthalocyanine ligand remained on the surface. A domain of decomposed molecules was formed within the DD molecular domain, and the boundary of the decomposed molecular domain self-cured to become rectangular. We demonstrated a molecular "sliding block puzzle" with cascades of DD molecules on the graphite surface.

Building self-assembled molecular layers with axially substituted titanium phthalocyanines.

Adsorption on graphite (HOPG) by titanium phthalocyanine axially bonded to a catechol ligand (TiPcat), titanylphthalocyanine (TiOPc), and 1:1 mixtures of these are studied at the HOPG-octylbenzene interface. The surface structures of a two-component bilayer, and of the individual monolayers of TiOPc and TiPcat, were determined by scanning tunneling microscopy (STM). TiPcat self-segregates onto a monolayer of TiOPc when an equal molar mixture is used. The preferential formation of a TiOPc monolayer from a solution containing both molecules is attributed to the difference in adsorption energies between TiPcat and TiOPc on graphite. The transformation of the hexagonal lattice of the pure TiOPc monolayer into a pseudo-square lattice was induced by the adsorption of TiPcat molecules. DFT calculations of the catechol orientation are presented.

Changing stations in single bistable rotaxane molecules under electrochemical control.

We have directly observed electrochemically driven single-molecule station changes within bistable rotaxane molecules anchored laterally on gold surfaces. These observations were achieved by employing molecular designs that significantly reduced the mobility and enhanced the assembly of molecules in orientations conducive to direct measurement using scanning tunneling microscopy. The results reveal molecular-level details of the station changes of surface-bound bistable rotaxane molecules, correlated with their different redox states. The mechanical motions within these mechanically interlocked molecules are influenced by their interactions with the surface and with neighboring molecules, as well as by the conformations of the dumbbell component.

Reversible photo-switching of single azobenzene molecules in controlled nanoscale environments.

We drive reversible photoinduced switching of single azobenzene-functionalized molecules isolated in tailored alkanethiolate monolayer matrices on Au{111}. We designed molecular tethers to suppress excited-state quenching from the metal substrate and formed rigid assemblies of single tethered azobenezene molecules in the domains of monolayer to limit steric constraints and tip-induced and stochastic switching effects. Single molecules were reversibly photoisomerized between trans and cis conformations by cycling exposure to visible and UV light. Trans and cis conformations were imaged as high (2.1 +/- 0.3 A) and low (0.7 +/- 0.2 A) protrusions in STM images and were assigned to the on and off states of the molecule, respectively.

Tuning interactions between ligands in self-assembled double-decker phthalocyanine arrays.

Lanthanide double-decker (DD) complexes represent an interesting system to engineer intermolecular interactions at different distances from the surface. Such atomic-level understanding and control are crucial for the development of molecular devices with three-dimensional complexity. We herein demonstrate an example of how the surface arrangements of DD molecules are influenced by varying intermolecular interactions. By varying the size of a ligand, we observed significant expansion of the lattice even though the top ligands of DD molecules remain too small to interact directly with each other. We also demonstrate that DD molecules in different local environments adopt significantly different structures. Such structures suggest that subtle interactions exist between the top ligand of one molecule and the bottom ligands of its neighbors.

Two-dimensional crystal growth and stacking of bis(phthalocyaninato) rare earth sandwich complexes at the 1-phenyloctane/graphite interface.

Initial stages of two-dimensional crystal growth of the double-decker sandwich complex Lu(Pc*)2 [Pc* = 2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] have been studied by scanning tunneling microscopy at the liquid/solid interface between 1-phenyloctane and highly oriented pyrolytic graphite. High-resolution images strongly suggest alignment of the double-decker molecules into monolayers with the phthalocyanine rings parallel to the surface. Domains were observed with either hexagonal or quadrate packing motifs, and the growing interface of the layer was imaged. Molecular resolution was achieved, and the face of the phthalocyanine rings appeared as somewhat diffuse circular features. The alkyl chains are proposed to be interdigitating to maintain planar side-by-side packing.