Adsorption of Crystal Violet to the Silica−Water Interface Monitored by Evanescent Wave Cavity Ring-Down Spectroscopy

Abstract

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been used to investigate the adsorption of crystal violet (CV + ) to a charged silica-water interface as a function of bulk pH by the direct measurement of the absorbance of the CV + chromophore. Absolute absorbances of order 10 -4 have been routinely detected, showing significant variation in the structure of the silica-water interface. At low ionic strength, the interfacial absorbance of CV + shows a monotonic increase with increasing pH. A simple competitive Langmuir adsorption model, which provides values for the silica surface parameters that are in broad agreement with the existing literature values, has been fit to the data. In addition, interfacial absorbance has been monitored as a function of pH for CV + solutions maintained at high ionic strength with NaCl, KCl, and CaCl 2 . As pH increases, the CV + interfacial absorbance exhibits a pronounced maximum, which occurs at pH 8.7 for Na + and K + and at pH 7.9 for Ca 2+ , followed by a sharp decrease. This trend is attributed to competitive binding between the metal cations and CV + to the silica surface binding site, and it has not been observed in previous measurements using second-harmonic generation. The simple Langmuir model, however, does not accurately describe the high ionic strength behavior.

Absolute surface coverage measurement using a vibrational overtone

“…While most CRDS studies have probed gas-phase species using a conventional linear optical resonator, novel extensions of the technique have included studies of a submonolayer adsorbate, [22][23][24][25] thin films, 26 -33 a thin liquid channel, 34 bulk liquids, 35,36 fibers, [37][38][39][40] and nanoparticles, 41 often employing unconventional resonator configurations. Guided by the Fresnel equations, some configurations have utilized the Brewster angle 24,26,27,36,41 or total-internal reflection ͑TIR͒ [22][23][24][25]39,42 to admit intracavity elements or interfaces that facilitate sampling while maintaining a low intrinsic loss. In principal, the Brewster angle provides unit transmission through intracavity interfaces for modes with transverse magnetic ͑TM͒ polarization, although high losses are incurred for transverse electric ͑TE͒ modes, precluding determination of absorber orientation or absolute coverage on the element surface.…”

“…Having an enhanced surface electric field with a tunable direction, the associated evanescent wave provides a well-suited probe of adsorbates, films, or particles as employed in evanescent-wave CRDS ͑EW-CRDS͒. [22][23][24][25] EW-CRDS has been implemented through several resonator designs including direct extensions of the linear resonator with intracavity elements, 24,25 a monolithic folded resonator, 22 and a monolithic TIR-ring resonator, 23,42 while whispering-gallery-mode spherical resonators 3 could also be employed in a similar mode. Readily miniaturized with no intracavity interfaces, the monolithic resonators 3,22,23,42 provide the lowest intrinsic loss ͑highest sensitivity͒, which is typically bulk-loss limited.…”

Total internal reflection spectroscopy for studying soft matter

“…The most successful experimental design is the "folded-resonator", where the entry and exit faces of the IRE are reflectively coated and the only interface within the cavity is the one where total internal reflection takes place ( figure 13). Alternatively a dove prism with anti-reflection coatings on the input and output faces can be used, 64 at the cost of reduced ring-down times due to reflection losses. Evanescent-wave CDRS uses UV, visible and occasionally near-IR light sources.…”

Liquid-Phase and Evanescent-Wave Cavity Ring-Down Spectroscopy in Analytical Chemistry

“…As with liquid-phase CRDS, the development of EW-CRDS has only recently begun. EW-CRDS has shown its potential in various studies of interactions of analytes with bare silica surfaces (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Modification of the TIR surface (19) may become a useful tool for studying (bio)molecular interactions and in label-free biosensing, analogous to, for example, surface plasmon resonance spectroscopy.…”