A quartz crystal microbalance-based molecular ruler for biopolymers

Abstract

We developed a quartz crystal microbalance (QCM) based molecular ruler that measures the length of surface immobilized, hydrated biopolymers (DNA and proteins). These biopolymers acted as stakes that solidified surrounding liquid at high frequency vibration in QCM measurements, which led to a simple linear frequency-thickness relation.

The change in thickness of the solidified liquid layer rather than the immobilized mass determines the frequency response of a quartz crystal microbalance

“…At the first glance, for the same surface chemistry (i.e., EG-SAM), QCM indicated a larger IgG immobilization capacity than that given by SPR: 726 ng cm 2 (122 Hz) vs. 76 ng cm 2 (1151 RU). This contradiction can be resolved using the "solidified liquid layer" model: SPR gave the true immobilization capacity (i.e., the dry mass of immobilized IgG) while QCM gave a thickness change of 6.6 nm of the "solidified liquid layer" due to IgG immobilization, where the entrapped PBS buffer contributed to 90% of the wet mass sensed by QCM [21]. For the subsequent capture of goat anti rabbit IgG, SPR results indicated a ratio between anti-IgG and IgG close to 2 to 1, in agreement with the finding in the literature that one rabbit IgG has two sites for goat anti rabbit IgG recognition (Scheme 3(a)).…”

“…Previously, we have proposed a "solidified liquid layer" model, which enabled QCM to be used as a molecular ruler [21,22]. In that model, the thickness of layer II (also called the "solidified liquid layer", Scheme 1) was determined by the thickness of a swollen biomolecule layer, such as the thickness of immobilized proteins, DNA or polymer chains.…”

Solidified Liquid Layer Model Expands the Application Fields of Quartz Crystal Microbalance

“…where ρ SLL and T SLL are the density and thickness of SLL, respectively, and α is a constant and is 5.7 × 10 6 m 2 kg −1 s −1 for a 5 MHz AT‐cut QCM chip. In previous reports we assumed that the SLL has the same density as the solution so that T SLL could be obtained, which made QCM a nice molecular ruler 4, 20, 21. Herein, we provide missing pieces of evidence that support the SLL model with surface‐ tethered polymer chains.…”

“…Here we use a surface tethered polymer chain as a demonstration. Biopolymers, such as DNA and proteins are similarly applicable 20, 21. The frequency change between state 2 and 4 is defined as Δ f n ii , which contains frequency changes contributed by both mass and viscoelasticity changes.…”

“…After immobilizing a stem‐loop structured ssDNA on a gold‐coated QCM chip, one can use a sequence‐specific complementary ssDNA to open the loop, and observe a thickness doubling behaviour. By the SLL model, QCM can quantitatively monitor this de‐looping process 21. As shown in Figure 3, when the stem‐loop ssDNA is adsorbed on the surface, T SLL was 4.4 nm.…”