In this sub-project highly sensitive sensors for new applications are developed. Due to their outstanding properties, MOF substances are very well suited as receptors. Therefore they are evaluated with respect to their performance characteristics, whereat different sensor types (chemisorption, electron-spin resonance, impedance- and IR-spectroscopy) are considered in dependence on their application.
Highly porous materials like MOFs have by nature a high sensitivity for gas and vapor detection. This is due to the fact that analyte molecules can be highly effective adsorbed and gettered in higher concentrations in those materials. While the sensitivity is to some extent dependent on the method of data transmission the binding strength between the analyte and the MOF as well as the transport dynamics in the MOF play a crucial role. Stronger bindings lead to lower detection limits. A very slow transport mechanism may lead to a very long response time. Therefore thin layers and bulk materials should be arranged in a way that the dimensions are small enough to ensure a rapid uptake of the analyte.
At Fraunhofer IWS the main focus in the sensor segment is on optically active MOFs. The adsorption of analyte molecules (e.g. H2O) leads for many MOF materials to a color change in the visible region. Due to the adsorption of guest molecules many other optical properties (e.g. refractive index) of MOF substances change in a characteristic way. In addition they are easy to analyze with optical methods. During the project different MOF materials have been applied as thin layers on a substrate (100-400 µm) and studied in-situ with spectroscopic methods (UV/Vis and NIR) at different relative humidity (100-4000 ppm).
Furthermore, a high-throughput screening method which is based on the optical infrared detection is to be developed for characterization of MOF materials. In contrast to the common characterization tools, the new optical screening method offers a quick, parallel, miniaturized and low-cost measurement.
The technology of optical calorimetry was developed by the Fraunhofer IWS Dresden (in collaboration with Rubotherm GmbH) and allows for a rapid characterization of porous materials. Therefore, the investigated sample is exposed to an adsorbing test gas. When adsorption takes place, heat of adsorption is released and warms up the adsorbent. The consequent temperature change is monitored by an optical (non-contact) heat sensor. The resulting temperature profile of the sample gives information about the amount of gas adsorbed and adsorption kinetics. The optical calorimetry allows for a screening of one sample within few minutes.
Due to the ongoing process development of the optical calorimetry method and instrumentation setup, an isotherm-like plot is obtained within a short time.
In the common dynamic experimental procedure, the sample is exposed to a flow of test gas. In contrast to the dynamic mode, the test gas is dosed in small pressure pulses (static mode) - analogous to the established volumetric measurement – to obtain an isotherm-like plot. Starting with an evacuated cell, the thermal response after each gas pulse is measured until the desired final pressure is reached.
The dynamic as well as the static procedure are not limited in the variety of the test gases. The requirement of the test gas selection is a sufficient heat effect of the adsorption process or an adequate adsorption capacity of the porous material resulting in a quantifiable temperature change.