Viscoelastic and Morphological Behavior of Stearic Acid Layer on Top of Polystyrene as Immobilisation Matrix for QCM Sensor

Abstract

The effect of different solvents on the acoustic property and morphology of polystyrene (PS) and the stearic acid (SA) layer was investigated in this study. The acoustic property was analyzed by using impedance analyzer and morphology of SA layer in a quartz microbalance (QCM) sensor have been studied to quantify their effects on viscoelasticity within the sensor. The polystyrene coating on a QCM sensor was created by spin-coating with various solvents, such as chloroform and toluene, which contains a 3% polystyrene solution by mass. Then, the SA coating was deposited onto the polystyrene layer using a low-vacuum evaporation method. The viscoelasticity was measured by an impedance analyzer coated with the SA layer to determine whether the material used as a coating will effectively immobilize a biomolecule and whether the material produces an acoustic load. The experimental results showed that the impedance value in the series resonant frequency was small (i.e., near 10 Ω), indicating that the deposited SA coating is rigid and that the SA coating does not produce a loading effect on the QCM sensor. Therefore, the coating technique used on the QCM sensor surface to produce the SA coating is likely to be an effective biosensor material for QCM immunosensor. Additionally, the study shows that the frequency change (Δf) of the SA layer deposited onto the polystyrene coating created with chloroform is larger than that of the coating created with toluene. This also shows that the SA layer deposited onto the polystyrene coating created with chloroform is thicker than the coating created with toluene. The Δf correlates with the mass change (Δm), according to the Sauerbrey equation, which requires that the material be rigid. The Δf value also correlates with the deposited SA mass change. From the calculation of Δf, the SA coating created with the chloroform solvent was shown to be thicker than that created with the toluene solvent. In addition, the roughness of the SA surface using a test of non-contact topography measurement system TMS TopMap-1200 showed that the SA surface roughness with the chloroform solution was 763 nm compared to that with the toluene solution, which was 424 nm.