Microgel-based etalon coated quartz crystal microbalances for detecting solution pH: The effect of Au overlayer thickness

• Etalon coated quartz crystal microbalances (QCM) are sensitive to pH.
• The sensitivity of the QCM depends dramatically on etalon construction.
• Sensitivity of 6.67 × 10−5 pH unit · Hz−1 is achieved over a 2 pH unit range.

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels were “painted” on the Au electrode of a quartz crystal microbalance (QCM). Another Au layer (overlayer) was subsequently deposited on the microgel layer. This structure is known as a microgel-based etalon. These devices have been shown to exhibit optical properties (i.e., color) that depend on solution pH and temperature, among other things. Previously, we measured QCM frequency shifts that are a result of solution pH changes; the frequency shifts are a direct result of the pH dependent solvation state of the microgels that make up the etalon. In fact, the shifts observed for the etalons were much greater in magnitude than just a microgel layer immobilized on the QCM crystal without the Au overlayer. We reasoned that the Au overlayer lead to an enhancement of the observed frequency change due to its mass. In this submission we investigate how the Au overlayer thickness (mass) affects the observed sensitivity to solution pH. We found that the change in QCM resonant frequency depended dramatically on the mass of the Au overlayer.

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