MOF-composite sensors to eliminate the QCM positive frequency shift

Abstract

In a quartz crystal microbalance (QCM) Sauerbrey equation can usually be applied in the ideal scenario and when the QCM frequency shift goes to the negative direction. In some cases, however, the requirements to achieve the ideal condition are not fulfilled and thus leading to QCM positive frequency shift phenomenon. Herein we have also observed that a porphyrin-based metal organic frameworks (MOF), namely MOF-525(Zn), QCM sensor also exhibits the positive frequency shift phenomenon when exposed to xylene isomers, which could be caused by the loose attachment of the MOF nanoparticles on the QCM surface. Therefore, in the presence of the analytes, the bonding between the MOF and the QCM surface is disturbed and higher analyte concentration also leads to the increase of the sensitive layer stiffness, resulting in higher positive frequency shift. However, such a phenomenon can be completely eliminated by simply adding polymer to MOF nanoparticle sensors to strengthen the bonding of MOF nanoparticles to the QCM surface. Through this study we have shown that adjusting the amount of polymer is very crucial to completely reverse the positive frequency response. Otherwise, it appears a critical analyte concentration of gas analytes where the QCM frequency shift changes its direction from negative to positive response. Lastly, the same strategy has also been used to other MOF-based QCM sensor systems where their sensitive layer is built from other MOFs, namely MOF-525 and NU-902, and ethanol is used as the analyte to prove the generalizability of this simple approach.