Institute of Technical and Macromolecular Chemistry, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany.
Anal Chem. 2010 Mar 15;82(6):2237-42. doi: 10.1021/ac902012e.
By specifically binding derivatized colloidal particles and physisorbing nonderivatized particles to the surface of a quartz crystal microbalance (QCM), we have observed positive shifts of frequency, Deltaf, in contrast to the negative frequency shifts typically found in adsorption experiments. Evidently, the Sauerbrey relation does not apply to this situation. A comparison of frequencies shifts and bandwidths on different overtones reveals a coupled resonance: at low overtones, Deltaf is negative, whereas it is positive at high overtones, with maximal resonance bandwidth observed at the crossover point. As predicted by the Dybwad model, the spheres bound to the surface form resonating systems on their own. A composite resonator is formed, consisting of a large crystal with resonance frequency omega and the adsorbed spheres with resonance frequency omega(S). In the case in which the resonance frequency of the small spheres (firmly attached to crystal), omega(S), is higher than the resonance frequency of the crystal, omega, Deltaf of the composite system is negative (leading to the Sauerbrey limit). In the opposite limit (that is, in the case of large adsorbed particles bound to the sensor surface via a sufficiently weak bridge) Deltaf is positive. Such a behavior is known from sphere-plate contacts in the dry state. Finite element calculation demonstrates that this phenomena is also plausible in liquid phase media, with Deltaf critically dependent on the strength of the sphere-plate contact. Operated in this mode, the QCM most likely probes the contact strength, rather than the mass of the particle.
通过将衍生胶体颗粒特异性地结合到石英晶体微天平 (QCM) 的表面，并物理吸附非衍生颗粒，我们观察到频率的正移（Δf），与通常在吸附实验中发现的负频率移动形成对比。显然，Sauerbrey 关系不适用于这种情况。对不同泛音的频率移动和带宽的比较揭示了耦合共振：在低泛音时，Δf 为负，而在高泛音时为正，在交叉点处观察到最大的共振带宽。正如 Dybwad 模型所预测的那样，表面结合的球体本身形成了共振系统。形成了一个复合谐振器，由具有谐振频率ω的大晶体和具有谐振频率ω(S)的吸附球体组成。在小球体（牢固地附着在晶体上）的谐振频率ω(S)高于晶体的谐振频率ω的情况下，复合系统的Δf 为负（导致 Sauerbrey 极限）。在相反的极限情况下（即，在通过足够弱的桥结合到传感器表面的大吸附颗粒的情况下），Δf 为正。这种行为在干燥状态下的球体-板接触中是已知的。有限元计算表明，这种现象在液相介质中也是合理的，Δf 关键取决于球体-板接触的强度。以这种模式操作时，QCM 最有可能探测到接触强度，而不是颗粒的质量。
