In Situ Evaluation of Density, Viscosity, and Thickness of Adsorbed Soft Layers by Combined Surface Acoustic Wave and Surface Plasmon Resonance

Abstract

We show the theoretical and experimental combination of acoustic and optical methods for the in situ quantitative evaluation of the density, the viscosity and the thickness of soft layers adsorbed on chemically tailored metal surfaces. For the highest sensitivity and an operation in liquids, a Love mode surface acoustic wave (SAW) sensor with a hydrophobized gold coated sensing area is the acoustic method, while surface plasmon resonance (SPR) on the same gold surface as the optical method is monitored simultaneously in a single set-up for the real-time and label-free measurement of the parameters of adsorbed soft layers, which means for layers with a predominant viscous behavior. A general mathematical modeling in equivalent viscoelastic transmission lines is presented to determine the correlation between experimental SAW signal shifts and the waveguide structure including the presence of the adsorbed layer and the supporting liquid from which it segregates. A methodology is presented to identify from SAW and SPR simulations the parameters representatives of the soft layer. During the absorption of a soft layer, thickness or viscosity changes are observed in the experimental ratio of the SAW signal attenuation to the SAW signal phase and are correlated with 2 the theoretical model. As application example, the simulation method is applied to study the thermal behavior of physisorbed PNIPAAm, a polymer whose conformation is sensitive to temperature, under a cycling variation of temperature between 20 and 40• C. Under the assumption of the bulk density and the bulk refractive index of PNIPAAm, thickness and viscosity of the film are obtained from simulations; the viscosity is correlated to the solvent content of the physisorbed layer.3

Combined surface acoustic wave and surface plasmon resonance measurement of collagen and fibrinogen layer physical properties

“…While the SAW/SPR technique ( Fig. 1) and data processing with our proposed formalism was already shown [51] to be appropriate in the identification of some physical properties of a rigid adsorbed layer (S-layer) [52], we here quantitatively analyze the organization of collagen and fibrinogen adsorbed layers which are expected to possess a substantial water content. This condition is expected to lead to the largest differences between acoustic and optical signals since both techniques respond differently to a viscous (solvent containing) layer: acoustic methods tend to overestimate the bound mass due to hydrodynamic interactions, while optical methods provide an estimate of the dry bound mass after appropriate modeling of the response but cannot resolve both parameters, thickness and optical index of the layer which SPR is sensitive to.…”

Comparison of FBAR and QCM-D sensitivity dependence on adlayer thickness and viscosity

“…From these investigations it is known that the QCM responds to changes is the density, thickness, viscosity and elasticity of the adsorbent. Analogously, Francis et al (2006) investigated the influence of density, thickness and viscosity of adsorbents on surface acoustic wave (SAW) sensors. They used the temperatureinduced conformational changes of a polymer to model the SAW device behaviour.…”

Love Wave Immunosensor for the Detection of Carbaryl Pesticide

“…With the purpose of analyzing the viscoelastic behavior of the layer in touch with the sensing area of the immunosensor and explore the veracity of hypothesis (2), an additional study was performed calculating the ratio of the measured attenuation change and phase change. The amplitude to phase ratio ( acoustic ratio , ΔdB/Δrad) of LW devices is a significant indicator of rigid or viscous interactions of the layer in contact with the sensor's vibrating surface [ 15 , 16 ]. A small ratio (<0.15 dB/rad in absolute value) occurs for a rigid loading, indicating that the attenuation-shift is small compared to the phase-shifts of the signal for such loading, while a high ratio (>8.8 dB/rad in absolute value) occurs for a viscous loading [ 12 ].…”