title: Voltammetric pH Nanosensor.
abstract: Nanoscale pH evaluation is a prerequisite for understanding the processes and phenomena occurring at solid-liquid, liquid-liquid, and liquid-gas interfaces, e.g., heterogeneous catalysis, extraction, partitioning, and corrosion. Research on the homogeneous processes within small volumes such as intracellular fluids, microdroplets, and microfluidic chips also requires nanometer scale pH assessment. Due to the opacity of numerous systems, optical methods are useless and, if applicable, require addition of a pH-sensitive dye. Potentiometric probes suffer from many drawbacks such as potential drift and lack of selectivity. Here, we present a voltammetric nanosensor for reliable pH assessment between pH 2 and 12 with high spatial resolution. It consists of a pyrolytic carbon nanoelectrode obtained by chemical vapor deposition (CVD) inside a quartz nanopipette. The carbon is modified by adsorption of syringaldazine from its ethanolic solution. It exhibits a stable quasi-reversible cyclic voltammogram with nearly Nernstian dependency of midpeak potentials (-54 mV/pH). This sensor was applied as a probe for scanning electrochemical microscopy (SECM) in order to map pH over a platinum ultramicroelectrode (UME), generating hydroxide ions (OH(-)) by the oxygen reduction reaction (ORR) at a diffusion-controlled rate in aerated phosphate buffered saline (PBS). The results reveal the alkalization of the electrolyte close to the oxygen reducing electrode, showing the insufficient buffer capacity of PBS to maintain a stable pH at the given conditions.
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.
authors.forename: Magdalena
authors.initials: M
authors.lastname: Michalak
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.
authors.forename: Malgorzata
authors.initials: M
authors.lastname: Kurel
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.
authors.forename: Justyna
authors.initials: J
authors.lastname: Jedraszko
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.
authors.forename: Diana
authors.initials: D
authors.lastname: Toczydlowska
authors.affiliation: Institute of Chemistry, Center of Interface Science, Faculty of Mathematics and Science, Carl von Ossietzky University of Oldenburg , D-26111 Oldenburg, Germany.
authors.forename: Gunther
authors.initials: G
authors.lastname: Wittstock
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.
authors.forename: Marcin
authors.initials: M
authors.lastname: Opallo
authors.affiliation: Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Nogala