Related Experiment Video
Updated: Jul 15, 2026

Attaching Biological Probes to Silica Optical Biosensors Using Silane Coupling Agents
Published on: May 1, 2012
Characterization of two types of silanol groups on fused-silica surfaces using evanescent-wave cavity ring-down
Hsiu-Fang Fan1, Fuping Li, Richard N Zare
1Department of Chemistry, National Taiwan University, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan.
Abstract:
Evanescent-wave cavity ring-down spectroscopy has been applied to a planar fused-silica surface covered with crystal violet (CV+) cations to characterize the silanol groups indirectly. A radiation-polarization dependence of the adsorption isotherm of CV+ at the CH3CN/silica interface is measured and fit to a two-site Langmuir equation to determine the relative populations of two different types of isolated silanol groups. CV+ binding at type I sites yields a free energy of adsorption of -29.9 +/- 0.2 kJ/mol and a saturation surface density of (7.4 +/- 0.5) x 10(12) cm(-2), whereas the values of -17.9 +/- 0.4 kJ/mol and (3.1 +/- 0.4) x 10(13) cm(-2) are obtained for the type II sites. The CV+ cations, each with a planar area of approximately 120 A2, seem to be aligned randomly while lying over the SiO- type I sites, thereby suggesting that this type of site may be surrounded by a large empty surface area (>480 A2). In contrast, the CV+ cations on a type II sites are restricted with an average angle of approximately 40 degrees tilted off the surface normal, suggesting that the CV+ cations on these sites are grouped closely together. The average tilt angle increases with increasing concentration of crystal violet so that CV+ cations may be separated from each other to minimize the repulsion of nearby CV+ and SiOH sites.
Related Concept Videos
IR and UV–Vis Spectroscopy of Aldehydes and Ketones
IR Spectroscopy: Molecular Vibration Overview
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...
UV–Vis Spectroscopy of Conjugated Systems
One of the factors influencing λmax is the extent of conjugation in the...
UV–Vis Spectroscopy: Molecular Electronic Transitions
Molecular Spectroscopy: Absorption and Emission

