Chemical Imaging of Mass Transport Near the No-Slip Interface of a Microfluidic Device using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy.

Mass transport in geometrically confined environments is fundamental to microfluidic applications. Measuring the distribution of chemical species on flow requires the use of spatially resolved analytical tools compatible with microfluidic materials and designs. Here, the implementation of an attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) imaging (macro-ATR) approach for chemical mapping of species in microfluidic devices is described. The imaging method is configurable between a large field of view, single-frame imaging, and the use of image stitching to build composite chemical maps. Macro-ATR is used to quantify transverse diffusion in the laminar streams of coflowing fluids in dedicated microfluidic test devices. It is demonstrated that the ATR evanescent wave, which primarily probes the fluid within ∼500 nm of the channel surface, provides accurate quantification of the spatial distribution of species in the entire microfluidic device cross section. This is the case when flow and channel conditions promote vertical concentration contours in the channel as verified by three-dimensional numeric simulations of mass transport. Furthermore, the validity of treating the mass transport problem in a simplified and faster approach using reduced dimensionality numeric simulations is described. Simplified one-dimensional simulations, for the specific parameters used herein, overestimate diffusion coefficients by a factor of approximately 2, whereas full three-dimensional simulations accurately agree with experimental results.

Surface sensitive infrared spectroelectrochemistry using palladium electrodeposited on ITO-modified internal reflection elements.

Palladium nanoparticles have been electrodeposited on the surfaces of conductive indium tin oxide (ITO) modified silicon internal reflection elements. The resulting films are shown to be excellent platforms for attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies of palladium surfaces. Monitoring the mid-infrared reflectivity of the interface during the constant potential electrodepostion of a Pd2+ precursor reveals a distinct and reproducible minimum that corresponds to the onset of the electronic percolation threshold of the deposited metal islands as confirmed by scanning electron microscopy. Effective medium theory (EMT) is used to model the reflectivity of the Si/ITO/Pd interface as a function of the volume fraction of the deposited metal and to calculate the ATR-SEIRA spectra of an adsorbed monolayer of organic molecules. EMT calculations are in qualitative agreement with most aspects of the experimental spectra which show that the intensities and spectral line shapes are highly dependent on the amount of deposited palladium. The methodology is applied to the potential dependent adsorption of 4-methoxypyridine on palladium. The experimental results show that the pyridine derivative adopts an edge-tilted orientation on oxide covered Pd and undergoes an orientation change in the hydrogen adsorption region that increases both the degree of edge-tilt and the extent of π-bonding between the pyridine ring and the metal.