Novel silica surface charge density mediated control of the optical properties of embedded optically active materials and its application for fiber optic pH sensing at elevated temperatures

Abstract

Silica and silica incorporated nanocomposite materials have been extensively studied for a wide range of applications. Here we demonstrate an intriguing optical effect of silica that, depending on the solution pH, amplifies or attenuates the optical absorption of a variety of embedded optically active materials with very distinct properties, such as plasmonic Au nanoparticles, non-plasmonic Pt nanoparticles, and the organic dye rhodamine B (not a pH indicator), coated on an optical fiber. Interestingly, the observed optical response to varying pH appears to follow the surface charge density of the silica matrix for all the three different optically active materials. To the best of our knowledge, this optical effect has not been previously reported and it appears universal in that it is likely that any optically active material can be incorporated into the silica matrix to respond to solution pH or surface charge density variations. A direct application of this effect is for optical pH sensing which has very attractive features that can enable minimally invasive, remote, real time and continuous distributed pH monitoring. Particularly, as demonstrated here, using highly stable metal nanoparticles embedded in an inorganic silica matrix can significantly improve the capability of pH sensing in extremely harsh environments which is of increasing importance for applications in unconventional oil and gas resource recovery, carbon sequestration, water quality monitoring, etc. Our approach opens a pathway towards possible future development of robust optical pH sensors for the most demanding environmental conditions. The newly discovered optical effect of silica also offers the potential for control of the optical properties of optically active materials for a range of other potential applications such as electrochromic devices.

The effect of ionic species on pH dependent response of silica coated optical fibers

“…As the concentration of ions forming the electric double-layer on the porous silica surface increases, the refractive index significantly increases. The increased refractive index will cause light traveling through the fiber to propagate more within the porous silica coating relative to the dense silica core (Figure 1b) and this change in the refractive index leads to a substantial change in the light transmission through the sensing element in the presence of light absorbing Diffuse Layer or scattering materials such as the Au NPs, as previously shown 12,13 . As the concentration of ionic species in the electrical double layer as well as their ability to alter the refractive index is determined by the nature of ionic species, the optical response is therefore anticipated to be highly dependent on the ionic species in the solution.…”

“…The typical means of detection ordinarily employs some reagent-based detection method that relies on the change in optical properties of a colorimetric or fluorescent dye. Another useful method relies on refractive index changes for pH sensing 11,12,13 . In one class of sensors, the mechanism for pH sensing relies on surface charging of a porous silica coating and the resulting concentration of ionic species in the porous film to change the refraction index, n, of the sensing layer.…”

LSPR and Interferometric Sensor Modalities Combined Using a Double-Clad Optical Fiber

“…Increased bulk RI, using glycerol and sucrose, induces a large blueshift of the LSPR peak wavelength, contrary to the expected redshift from the increased medium RI. This blueshift can only be explained as a result of alteration of the surface electronic structure of the GNRs donated by the anionic polymer network and the glycerol or sucrose solutions [ 17 , 18 , 19 , 20 ]. In spite of these unexpected behaviors, we do show, using biotin-functionalized GNRs binding to streptavidin, that the LSPR signal from GNRs embedded in a hydrogel can be used for biosensing.…”

Hydrogel-integrated plasmonic nanostructures on optical fiber facet for remote and real-time pH sensing

“…Integrating SPR nanostructure on optical fiber can realize more accessible and versatile plasmonic sensing applications especially in hazardous, remote and spatially limiting working space. 7,8 Among many chemical sensing applications, new sensors for pH value measurement remain to be of wide interest, especially for minimally invasive measuring need. 7 The optical fiber has merits to be a miniaturized pH sensor owing to its small cross-section and long working distance.…”