Ion-Selective Electrodes Based on Hydrophilic Ionophore-Modified Nanopores.

We report the synthesis and analytical application of the first Cu2+ -selective synthetic ion channel based on peptide-modified gold nanopores. A Cu2+ -binding peptide motif (Gly-Gly-His) along with two additional functional thiol derivatives inferring cation-permselectivity and hydrophobicity was self-assembled on the surface of gold nanoporous membranes comprising of about 5 nm diameter pores. These membranes were used to construct ion-selective electrodes (ISEs) with extraordinary Cu2+ selectivities, approaching six orders of magnitude over certain ions. Since all constituents are immobilized to a supporting nanoporous membrane, their leaching, that is a ubiquitous problem of conventional ionophore-based ISEs was effectively suppressed.

Potentiometric sensing of nucleic acids using chemically modified nanopores.

Unlike the overwhelming majority of nanopore sensors that are based on the measurement of a transpore ionic current, here we introduce a potentiometric sensing scheme and demonstrate its application for the selective detection of nucleic acids. The sensing concept uses the charge inversion that occurs in the sensing zone of a nanopore upon binding of negatively charged microRNA strands to positively charged peptide-nucleic acid (PNA) modified nanopores. The initial anionic permselectivity of PNA-modified nanopores is thus gradually changed to cationic permselectivity, which can be detected simply by measuring the nanoporous membrane potential. A quantitative theoretical treatment of the potentiometric microRNA response is provided based on the Nernst-Planck/Poisson model for the nanopore system assuming first order kinetics for the nucleic acid hybridization. An excellent correlation between the theoretical and experimental results was observed, which revealed that the binding process is focused at the nanopore entrance with contributions from both in pore and out of pore sections of the nanoporous membrane. The theoretical treatment is able to give clear guidelines for further optimization of potentiometric nanopore-based nucleic acid sensors by predicting the effect of the most important experimental parameters on the potential response.

Hyphenated FT-IR-attenuated total reflection and electrochemical impedance spectroscopy technique to study the water uptake and potential stability of polymeric solid-contact ion-selective electrodes.

A new hyphenated method utilizing FT-IR-attenuated total reflection (ATR) and electrochemical impedance spectroscopy (EIS) is presented to correlate the water uptake with concomitant potential and impedance changes of polymeric coated-wire electrodes (CWEs) and solid-contact ion-selective electrodes (SCISEs). The Ca(2+)-selective silicone rubber (RTV 3140) based SCISEs with poly(3-octylthiophene) (POT) as the solid-contact (SC) showed good correlation between a very low water content at the Pt-coated ZnSe substrate/SC interface and a superior potential stability. This is due to the hydrophobicity of both RTV 3140 and POT and the approximately 2 orders of magnitude lower water diffusion coefficients in POT compared to RTV 3140. Practically no potential drift could be observed during 24 h when unconditioned CaSCISEs were contacted with 10(-3) M CaCl(2), in contrast to the Ca(2+)-selective CWEs with considerably higher water uptake and potential drift. The CaSCISEs had a fast Nernstian response with a detection limit of 8 × 10(-9) M Ca(2+) and a good reproducibility and stability of the standard potential, which indicates that the CaSCISEs does not require any conditioning prior to use.

Ionophore-gold nanoparticle conjugates for Ag(+)-selective sensors with nanomolar detection limit.

We report on the synthesis and trace analytical application of ionophore-gold nanoparticle conjugates as a basis of a new concept in confining ionophores to ion-selective membranes using inert nanoparticle carriers.

Hybridization-modulated ion fluxes through peptide-nucleic-acid- functionalized gold nanotubes. A new approach to quantitative label-free DNA analysis.

The inner walls of gold nanotubes, prepared by template synthesis in the nanopores of polycarbonate track etch membranes, have been chemically modified with peptide nucleic acid (PNA) and used for label-free quantification of complementary DNA sequences. Selective binding of DNA to the PNA-modified nanotubes is shown to decrease the flux of optically detected anionic markers through the nanotubes in a concentration-dependent manner. The strong dependence of the biorecognition-modulated ion transport through the nanopores on the ionic strength suggests a dominantly electrostatic exclusion mechanism of the ion flux decrease as a result of DNA binding to the PNA-modified nanopores.

Chemical imaging of biological systems with the scanning electrochemical microscope.

A brief overview on recent advances in the application of scanning electrochemical microscopy (SECM) to the investigation of biological systems is presented. Special emphasis is given to the mapping of local enzyme activity by SECM, which is exemplified by relevant original systems.