QCM-D和椭偏仪联用检测吸附薄膜

Biolin Scientific[ Progress Together] [Application Note] 20 Combined QCM-D/ellipsometry setup for real-timecharacterisation of thin molecular filmsSeveral surface sensitive techniques are able to monitor the formation and prop-erties of thin molecular films on a sensor surface. As the complexity of the studiedsystems increases， one technique alone can often not provide all desired insight.Multitechnique approaches provide complementary information， and thus allowfor improved interpretation and/or the identification of artifacts. Comparing datafrom several separate experimental setups， however， is not always straight forwarddue to differences in the experimental conditions. Therefore， merging several tech-niques into the same setup and monitoring events on the same surface is a promis-ing approach. This application note presents how a combined experimental setup ofQuartz Crystal Microbalance with Dissipation Monitoring (QCM-D) and ellipsometrycan augment the measurement of molecular behavior at a sensor surface. Background Both QCM-D and ellipsometry can provide information aboutadsorption events on surfaces and the properties of the resultingfilms. In particular， both techniques can quantify adsorbed massesin real-time. Ellipsometry， being an optical technique， is sensitiveto the mass of adsorbed molecules only， whereas the massdetermined by QCM-D also includes solvent that is coupled tothe film. Thus， a comparison of the masses measured by the twotechniques provides information about the amount of solventin the film. With such an approach， film swelling or collapsecan easily be distinguished from adsorption/desorption events，changes in the solvent content of thin films can be monitored overtime， and other structural or morphological changes of adsorbedfilms identified. Figure 1 shows a schematic figure of the combined QCM-D/ellipsometry setup.QCM-D data， changes in oscillation frequencyof the sensor to acquire information on mass and dissipation datato acquire data on structural properties of the film is measured.Simultaneously， polarized light is reflected at the surface ofthe QCM-D sensor and the changes in its polarization statemeasured. Here， the formation of a supported lipid bilayer fromsmall unilamellar vesicles (SUVs) containing 10% biotinylatedlipids， followed by the specific binding of streptavidin， as well asbiotinylated vesicles， were investigated. [Figure 1]： A setup combining QCM-D and ellipsometry was used to study abuild up of lipid bilayer onto a QCM-D sensor， followed by streptavidin andvesicle binding. Results & discussion Figure 2 shows the adsorbed mass upon exposure of a silica-coated QCM-D sensor to SUVs， followed by streptavidin andadditional vesicles. First， a supported lipid bilayer was formedfrom vesicles that attached to the QCM-D sensor (Fig 2A). Thebi-phasic behavior of the QCM-D mass provides a direct indicationthat vesicles initially adsorbed intact and then ruptured to forma planar lipid bilayer. This transition is not readily visible from theellipsometry data， which only shows a monotonous mass increaseduring bilayer formation. Second， streptavidin bound to the biotin groups that are present on the supported lipid bilayer， as indicatedby the mass increases in Fig 2B. Finally， biotinylated vesicles wereagain added to the streptavidin layer. The absence of a biphasicbehaviour in this step (Fig 2C) provides a first indication thatadsorbing vesicles remained intact. As expected， the massesdetermined by QCM-D(mQCM-D) were consistently higher thanthe ellipsometric masses (mopt)， because QCM-D senses thesolvent that is dynamically coupled to and trapped in the film. Arather small mass difference between QCM-D and ellipsometrywas observed for the supported lipid bilayer (Fig 2A)， consistentwith expectations for such a planar and solvent-poor structure. [Figure 3]： Amount of coupled solvent (H=1-mopt/mQCMD) for streptavidinbinding as function of the optical mass (=coverage). This can be compared to binding of the solvent filled， intactvesicles，where solvent contributes more than 50% to theQCM-D mass (Fig 2C). The amount of solvent in the adsorbed film can been defined as： The evolution of this parameter over time is shown in Fig. 3 forthe streptavidin binding step. The amount of solvent is displayedas a function of the optical mass， which is proportional to thesurface coverage of the adsorbed species. The dependency islinear over almost the entire range of binding. Conclusions Combination of QCM-D and ellipsometry gives real-time，complementary data about the amount of solvent associatedwith molecular films. In addition to the data presented in thisapplication note， QCM-D can provide information on mechanicalproperties of thin films (viscoelasticity)， while ellipsometrycan measure optical film properties (refractive index). Furthermore， the layer thickness can be determined with bothtechniques， and results compared. Thus， parallel QCM-D andellipsometry analysis in the same setup can provide a detailedunderstanding of molecular events taking place at a surface. ( References： ) Bingen， P， Wang， G， Steinmetz， N. F， Rodahl， M， Richter， R. P. Anal. Chem. inpress， and work by R. Richter and S. Stahl at CIC biomaGUNE， Spain， incollaboration with Q-Sense AB. []Q-Sense AN