Silica microparticles as guiding substrates for the fabrication of polyamide-coated stainless-steel sheets by dip-coating technique: Application to the determination of opioids in saliva samples by direct infusion mass spectrometry.

Background: Dip-coating is a simple and cost-effective technique for synthesizing thin film sorptive phases in microextraction. The success of the coating (amount and homogeneity) relies dramatically on the wettability of the substrate, thus making the covering of smooth-surface substrates challenging. In these cases, prior to the slurry (particles and binder) deposition, the smooth-surface substrate typically requires an aggressive etching step (e.g., treating the surface with strong acids or bases) to increase the roughness of the metallic substrate. This strategy has a clear environmental and operational safety impact and increases the associated costs.

Results: In this article, we explored the use of amorphous silica gel particles as a cost-effective guiding substrate to improve the wettability and increase the superficial area of the final coating. For this purpose, a double-sided adhesive tape is used to immobilize the silica particles over stainless-steel sheets, thus creating a porous substrate where a thin layer of polymer is finally deposited by dip-coating. The resulting nylon-silica gel sheets (NSG-sheet) have been evaluated for the extraction of some opioids (codeine, methadone, oxycodone, and tramadol) from saliva samples and their direct infusion mass spectrometry analysis. The developed method allows the determination of the analytes at the low μg L range with levels with intra- and inter-day precision, expressed as relative standard deviation, better than 9.5 and 13.7 %, respectively.

Significance And Novelty: This research deepens the potential of adhesive tapes for designing novel sorptive phases. It also provides a straightforward approach to synthesizing thin film microextraction devices using affordable and sustainable materials and procedures. The resulting analytical method has been thoroughly validated, and it has been demonstrated to be competitive with existing methods. Also, the potential of the devices in on-site extraction procedures has been preliminarily explored.