The sensor is the core of the QCM measurements, and the sensor coating must of course represent the material intended to be studied. QSense provides a wide range of sensors, and it is also possible to order custom sensor materials, but there could still be situations where the system to be studied requires a sensor coating to be added ex-situ. In this post, we guide you on how to set up such an ex-situ coating process, including QCM-D data capture and analysis.
Steps for QCM sensor Ex-Situ Coating
Any QCM-D study starts with selecting an appropriate sensor that represents the surface material intended to be studied. If the desired coating is not available in the QSense sensor range, or if you want to extract information about the coated layer and/or layer behavior, such as thickness, swelling, or layer removal, then you may need to coat the sensor ex-situ. There are several coating methods that can be used, such as spin-coating, dip-coating, spray-coating, and electrodeposition. Regardless of the chosen method, there are a few key steps that must be included in the coating process to allow for proper subsequent data analysis. To be able to extract information about the coated layer, such as thickness, swelling, or removal, crystal reference data representing the uncoated bare sensor need to be collected prior to any coating step. The very same sensor must be characterized both before and after coating, and the collected data stitched, so that all relevant information is included in the same measurement file.
This is the process that we recommend.
Ex situ coating step-by-step
- Set the temperature in the QSense instrument and equilibrate
- Mount the sensor that you plan to coat
- Measure the stable baseline in air for at least five minutes and save the measurement, “part 1”
- Remove the sensor from the instrument and coat it externally using the method of choice
- Put the same, now coated, sensor back in the instrument, and measure for at least five minutes if you are only interested in the layer thickness and/or properties
- If you are interested in the layer behavior at specific conditions, for example, layer swelling or removal, prime and proceed to perform the measurement as normal.
- Save the measurement file, “part 2”.
- Take out the sensor and clean the system
- Use QSoft to stitch the two data files, part 1 and part 2, that were captured before and after sensor coating
- Analyze the stitched file in Dfind with the preferred analysis approach
Concluding remarks
Every QCM-D measurement starts with a selection of a sensor that accurately represents the surface or material to be analyzed. In some cases, the situation requires that the sensor is coated ex-situ using for example spin-coating. To ensure accurate analysis of the coated layer, enabling extraction of for example layer thickness, swelling, or layer removal, it is important to use proper data collection before and after coating, using the bare sensor as a reference, and stitching of the measurement files. Notably, if a thin coating is applied, the sensor mounting error needs to be considered. In general, the sensor mounting error should be much smaller than the applied coating. Mounting and re-mounting of the sensor to average out the sensor mounting error should perhaps be considered. If a thick coating is applied, the loss of higher harmonics might occur, and this will have an impact on the ability to model the collected data.
