Optical Quartz Crystal Microbalance (OQCM) for Dual-Mode Analysis

Abstract

Label-free biosensors allow for real-time measurements of the target molecule, providing valuable kinetic data about the unperturbed biological system. Yet, they generally rely on a single transduction mechanism that reflects a single aspect of a system. In order to have a more complete understanding of the system, many aspects of the system need to be examined simultaneously. An integrated multi-mode label-free sensor capable of providing consistent and complementary information about multiple aspects of a system is highly desirable for biomedical research. Currently there are some hybrid sensors utilizing the optical and quartz crystal microbalance (QCM) techniques to measure both the optical and mechanical properties of a system. However, those hybrid sensors have some shortcomings in implementation and performance that limit their applicability.

In this research, we developed Optical Quartz Crystal Microbalance (OQCM) sensors - hybrid sensors utilizing the same techniques for simultaneously measuring both optical and mechanical properties, which also address these shortcomings. Two OQCM structures were designed, fabricated and explored. The first structure is an interferometric OQCM sensor (I-OQCM) with a multilayer planar optical structure. The interference between reflections at the interfaces between layers generates an interference pattern in the optical spectrum that shifts upon accumulation of additional films on the structure. The second structure is a plasmonically-enhanced grating OQCM sensor (PEG-OQCM). The theory and simulation analyses indicate that the PEG-OQCM can achieve near zero bulk refractive index sensitivity by optimizing the incidence angle. Simulation results show that at an incident angle of 47 degrees, the bulk RI sensitivity becomes near zero around bulk RI = 1.33. Experimental results for vapor deposition, water and biosensing (solution of streptavidin) match well with the simulation results. With this PEG-OQCM structure, an optical linewidth of 25 nm was obtained in air, 15 nm in water – up to 6 times narrower than that of SPR/LSPR (50-100 nm in water).

The OQCMs were characterized separately to demonstrate the operation in each mode for each structure, and tests were performed to show biosensing capability. Dual-mode tests were conducted for both the I-OQCM and PEG-OQCM to show the capability of simultaneous measurement of both optical and mechanical properties and responses of a system. The test results validate the simulation analyses and correlation between the optical and mechanical responses that would provide corroborating information for more reliable, robust cross-examination/confirmation for the evaluation of test systems.

The OQCM-A sensor with 3 single I-OQCM sensors on a single wafer was also designed, fabricated. Each I-OQCM sensor can be characterized independently of the others. Mechanical response tests performed on the OQCM-A indicate that each sensor responds independently of the other sensors and the cross-talk between on adjacent sensors is negligible.