Evaluation of a planar optic waveguide as a platform for evanescent field chemical sensor development

Abstract

A Ag$\sp+$-exchange planar optic waveguide was fabricated with the intent of taking advantage of the significant interfacial sensitivity of the device for evanescent field chemical sensor development in the visible spectral region. However, it was discovered that selective prism and grating coupling to discrete waveguide modes was not possible due to strong crosstalk between modes in multiple mode Ag$\sp+$-exchange waveguide structures. Thus, sensor development was delayed and research was redirected towards evaluation of a multiple mode Ag$\sp+$-exchange planar waveguide device. To carry out the evaluation, the Ag$\sp+$-exchange waveguide surface was exposed to transient impulses of a series of model chemical analytes in a flowing liquid phase system. The complex multiple mode response of the waveguide was captured at relevant time intervals using a linear array detector.Model chemical systems were chosen to characterize the Ag$\sp+$-exchange waveguide liquid phase response to bulk refractive index (glycerol), bulk absorbance (bromocresol green), and to simultaneous variations in bulk refractive index and absorbance. Cationic molecular systems were chosen to test the waveguide response to analytes which partition into or adsorb to the waveguide surface giving rise to a refractive index perturbation (Pb$\sp{2+$) or modulations in both refractive index and absorbance (methylene blue). As temperature effects are superimposed on any measurement made with the Ag$\sp+$-exchange waveguide, the multiple mode temperature response was characterized using a thermoelectric cooler assembly to control the device temperature.The waveguide multiple mode response is potentially multivariate to even single component analyte systems as the device response function may include concentration dependent mode coupling variations, modulations in mode crosstalk, and changes in mode outcoupling position on the array detector. Thus, chemometrics are necessarily employed to waveguide multiple mode response data to aid in interpretation and response calibration. The application of chemometrics to the Ag$\sp+$-exchange waveguide response data provided for the deconvolution of simultaneous liquid phase refractive index and absorbance changes. The potential also exists to separate waveguide response contributions from bulk compositional changes and interfacial partitioning or binding effects.