Solution Titration by Wall Deprotonation during Capillary Filling of Silicon Oxide Nanochannels

Abstract

This paper describes a fundamental challenge when using silicon oxide nanochannels for analytical systems, namely the occurrence of a strong proton release or proton uptake from the walls in any transient situation such as channel filling. Experimentally, when fluorescein solutions were introduced into silicon oxide nanochannels through capillary pressure, a distinct bisection of the fluorescence was observed, the zone of the fluid near the entrance fluoresced, while the zone near the meniscus, was dark. The ratio between the zones was found to be constant in time and to depend on ionic strength, pH, and the presence of a buffer and its characteristics. Theoretically, using the Gouy-Chapman-Stern model of the electrochemical double layer, we demonstrate that this phenomenon can be effectively modeled as a titration of the solution by protons released from silanol groups on the walls, as a function of the pH and ionic strength of the introduced solution. The results demonstrate the dominant influence of the surface on the fluid composition in nanofluidic experiments, in transient situations such as filling, and changes in solvent properties such as the pH or ionic strength. The implications of these fundamental properties of silicon oxide nanochannels are important for analytical strategies and in particular the analysis of complex biological samples.

Hydronium-dominated ion transport in carbon-dioxide-saturated electrolytes at low salt concentrations in nanochannels

“…Of particular interest is the intricate interplay among surface chemistry, electrokinetics, and fluid dynamics spanning over molecular and continuum macroscopic length scales [4,5]. It has been demonstrated that the electrokinetic properties at this scale have enabled a range of innovations including those for chemical sensing and bioanalytics [6][7][8][9][10][11][12], energy harvesting systems, [13][14][15][16][17][18], and nanofluidic ion transport [19][20][21][22][23][24][25][26], including enrichment, depletion, and rectification effects [27][28][29][30][31][32].…”

The influence of energetic surface heterogeneity on proton desorption during capillary filling of silica nanochannels

“…In experiments performed by (Janssen et al 2008) nanochannels were 40-50 nm deep and 20 lm wide. They used two types of filling solutions: unbuffered KCl solution (mean pH = 7.2) and KCl solution buffered with TRIS (mean pH = 8.5).…”

“…The lines were plotted assuming various degrees of surface heterogeneity (kT/c het = 1.0, homogenous surface, solid line; kT/c het = 0.8, dashed line, kT/c het = 0.6, finely dashed line). The data were taken from (Janssen et al 2008) (Sverjensky, 2005) for amorphous silica. This is quite recent, and the most reliable collection of parameters for the 2-pK TLM determined for different oxides and various electrolytes.…”

“…According to (Janssen et al 2008) the balance of hydrogen ions in the investigated system can be calculated as follows.…”

“…In this case the key parameter is pH which has a dominant influence on electric charge of channel walls and the properties of molecules in the solution (Eijkel and Berg 2010;Napoli et al 2010). (Janssen et al 2008) investigated the filling of silicon nanochannels with fluorescein solutions. They observed two zones in the fluid introducing into nanochannels.…”