Quantitative Assessment of Vapor Molecule Adsorption to Solid Surfaces by Flow Rate Monitoring in Microfluidic Channels

Abstract

Measuring parameters related to gas adsorption on the effective surfaces of solid samples is important in catalyst studies. Further attention on the subject has appeared due to the materials and methods required to concentrate the gaseous biomarkers for detection. The conventional methods are mainly based on the volumetric and gravimetric analyses, which are applicable to bulk samples. No standard method has yet been provided for such measurements on thin films, which are the most commonly used samples for material screening. Here, a novel method is presented for the adsorption coefficient measurement on thin-film samples. This method comprises coating of the inner walls of a microfluidic channel with the thin film under test. The recorded diffusion rates for a trace gas along this microchannel are compared with the solutions of the adsorption–diffusion equation of the channel for determining the adsorption coefficient of the gas molecule to the inner walls of the channel. The high ratio of surface-to-volume in such channels magnifies the gas sorption effects and improves accuracy. The method is fast, versatile, and cost-effective, allowing measurements at different temperatures and atmospheric pressures. The adsorption coefficients of different isomers of butanol on poly­(methyl methacrylate) sheets, zinc oxide thick films, and gold thin films are determined as examples.

Advancements in Microfabricated Gas Sensors and Microanalytical Tools for the Sensitive and Selective Detection of Odors

“…In recent years, microfluidic-based devices have been introduced as a very promising alternative to µGC, for the selective identification of VOCs in binary, triple, or even more complicated gas mixtures [ 252 , 253 , 254 ]. These devices provide selectivity to a general-purpose gas sensor, by fostering the natural diffusion of analytes through a specially coated microfluidic channel (see Figure 11 A) [ 254 ].…”

“…Therefore, at low analyte concentrations the “diffusion-physisorption equation” can take the following approximate form: where is defined as the adsorption coefficient ( ), which is directly related to the physical interaction between gas molecules and the channel walls. From the previous Equation (4), it can be easily deducted that the physisorption effect of gas molecules increases, either by reducing the channel’s depth or raising the adsorption coefficient [ 252 ]. For this reason, the selectivity and performance of microfluidic devices relies on the optimization of several factors, such as (i) channel’s geometry, (ii) coating of the channel walls, and (iii) environmental conditions (i.e., temperature and relative humidity) [ 254 ].…”

“…For this reason, the selectivity and performance of microfluidic devices relies on the optimization of several factors, such as (i) channel’s geometry, (ii) coating of the channel walls, and (iii) environmental conditions (i.e., temperature and relative humidity) [ 254 ]. First of all, microfluidic devices normally incorporate straight channels of a few centimeters length, with either circular or square cross-sections [ 252 , 255 ]. Similar to chromatographic columns, design and geometrical properties of the channel are strategically selected to achieve the best selectivity, while ensuring acceptable response and recovery times [ 260 ].…”

“…Hence, the performance of microfluidic devices is strongly dependent on the physisorption of gas molecules to/from the channel walls [ 254 ]. For this reason, high surface-to-volume (S/V) channels are recommended in microfluidic devices, which can be easily achieved by increasing channel’s width and reducing its depth or diameter [ 252 ]. On the other hand, nature, properties, and characteristics of the channel coating material also play a key role in the segregation power of microfluidic devices [ 253 , 261 , 262 ].…”

“…They can be deployed using well-known techniques, such as sputtering, chemical vapor deposition (CVD), or spin coating [ 262 ]. Nonetheless, other reported studies suggest different coating materials, such as metal oxides or single metal layers like gold (Au), due to its low reactivity values [ 252 ]. In addition, a common practice found in the literature is the use of coatings with multiple layers, in order to foster a better adhesion into the channel walls and increase their stability and overall performance [ 262 ].…”