QCM-D: The power of combination

Q-Sense Quartz Crystal Microbalance with Dissipation (QCM-D) is an acoustic surface sensitive technique, which provides simultaneous, real-time information on mass and structure of thin films.

Figure 1: The Q-Sense window module in combination with microscopy.

The sensing principle is based on tracking changes in vibration of a thin quartz disc as molecules are adsorbed, removed or changed in conformation at the sensor surface. QCM-D can thus provide information about reactions at, and in proximity of the sensor. However, as the complexity of studied systems increases, characterization by several surface sensitive techniques is required for correct and unambiguous interpretation. However, combining data from separate experimental set-ups is not always straight forward. Therefore, merging several techniques in the same set-up and on the same sensor is the desired approach. Q-Sense has therefore focused on technical solutions enabling simultaneous multi-technique monitoring. New, specially designed QCM-D modules enable simple combinations of QCM-D with, for example, optical microscopy, ellipsometry and electrochemistry.

Figure 2: QCM-D data and microscopy images of cell attachment and spreading on two different surfaces (A, B and C, D respectively). Cell attachment is followed by cell spreading as indicated by the arrow.

Figure 3: The Q-Sense ellipsometry module allows combination of QCM-D and ellipsometry.

The Q-Sense window module allows optical access to the sensor, enabling QCM-D experiments involving irradiation-induced reactions and the combination with light microscopes (Fig. 1). A system of interest can be simultaneously sensed from the surface beneath and manipulated or observed from above. Such a setup enables for example the analysis of cells at a solid support. Figure 2 shows QCM-D (Fig. 2A, 2C) and fluorescence microscopy (Fig 2B, 2D) data of cell adhesion and the spreading onto protein precoated surfaces [1]. Upon initial attachment an increase in mass and viscoelasticity of the adlayer is sensed by QCM-D. With time, QCM-D indicates a stiffening of the adlayer, which corresponds well to the cell spreading observed with microscopy. Cell spreading could be accompanied by decreased mass response (Fig. 2A), which does not reflect cell detachment as confirmed by micrographs (Fig 2B). Instead, this loss of mass is attributed to the remodeling of the local environment in proximity to the cell membrane, resulting in a decreased cell-sensor contact area. Interestingly, QCM-D trends depend on the underlying substrate (Fig 2A and 2C), even though the microscopy images are comparable (Fig 2B and 2D).

Figure 4: Adsorbed protein mass, as measured by ellipsometry (mellipsometry) and by QCM-D (mQCM-D).

Q-Sense Ellipsometry Module allows the parallel QCM-D and ellipsometry analysis on the same surface (Fig. 3). The combination of the two techniques provides a detailed insight into the behavior of thin films with the output of both mechanical and optical parameters. QCM-D and ellipsometry are highly complementary, since solvent associated with a molecular film contributes to the mass measured by QCM-D, but not by ellipsometry. Therefore, the parallel approach enables the distinction of swelling/collapse, form adsorption/desorption events, refined analysis of adsorbed film’s morphological changes, as well as the monitoring of changes in the solvent contents of thin films in real-time. An example from a combined measurement is presented in Figure 4, where the mass of a protein film is monitored during an adsorption process [2]. The mass sensed by QCM-D is typically higher compared to ellipsometry. The difference corresponds to the amount of solvent in the protein layer. QCM-D can also be combined with techniques such as cyclic voltammetry and electrochemical impedance spectroscopy using the Q-Sense Electrochemistry Module (Fig. 5).

Figure 5: The Q-Sense electrochemistry module allows combination of QCM-D with electrochemical techniques.

Figure 6: Quantification of protein adsorption with (A) QCM-D sensing total amount of protein and (B) electrochemistry sensing only the active molecules.

The module has a three-electrode configuration with the QCM-D sensor doubling as a working electrode. While QCM-D provides film mass and structural information, electrochemistry can be the stimulus of an interaction or provide information about interfacial charge transfer. Application examples include studies of electrochemically initiated growth of films, transport of species during redox chemistry, electrostatic interactions of molecules with surfaces and membrane potential measurements. Figure 5 shows an example from the combined setup. Adsorption of an electron-transfer protein (Cu(II) - Cu(I)), monitored with QCM-D (Fig. 6A) indicates a mass corresponding to a monolayer coverage. However, the quantification with cyclic voltammetry (Fig. 6B) shows that only ~30% of the protein remains electrochemically active [3]. In conclusion, the combined analytical technique approach enables real-time correlation of QCM-D analysis with complementary techniques, thus offering unique information. More information about Q-Sense combination modules, other products and applications can be found at www.q-sense.com. Please do not hesitate to contact us. References [1] Lord MS et al, Biomaterials 2006, 27, 4529-4537 [2] Ralf Richter and Stefan Stahl, CICBiomagune, San Sebastian, Spain (to be published) [3] Fleming BD et al, Langmuir 2008, 24, 323-327

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