Optical and acoustic biosensors can monitor dynamic surface processes, e.g., the adsorption of proteins on surfaces, in a real-time, label-free in situ fashion. While each technique has its own merits, current needs demand more powerful systems in order to increase the amount of derived information and complex data interpretation. Surface plasmon resonance (SPR) is an optical detection technique that measures refractive index changes caused by the binding of biomolecules on a metal surface. Love wave (LW) surface acoustic wave (SAW) sensors employ acoustic wave phase and amplitude changes as the detection mechanism.
Figure: Schematic representation of the newly developed SPR/LW-SAW sensor. The acoustic wave geometry comprises a piezoelectric substrate where a set of interdigitated electrodes is deposited, covered by a polymer layer for acoustic waveguiding. The optical SPR device uses the gold layer deposited on top of the polymer to create a SPR response through a light beam reaching the gold sensing area from the opposite site. Binding of biomolecules on the gold surface can be detected by measuring simultaneously the amplitude, phase and SPR response.
In the present study, we developed an integrated SPR/LW-SAW platform for combined optical and acoustic measurements. The hybrid sensor device was designed and fabricated using photolithography. The sensor was optimized in terms of materials and structures, incorporating a polymethylmethacrylate (PMMA) acoustic waveguide and a Au sensing layer. A 3D-printed device holder integrated with a PDMS microfluidic chip was built to allow for continuous flow-through experiments. The set-up was calibrated by introducing a series of glycerol/water solutions on the sensor surface, in order to support quantitative analysis. Finally, the SPR/LW-SAW platform was used to investigate the adsorption behaviour of a protein (bovine serum albumin- BSA) on the Au surface. Through the combined measurements, we estimated simultaneously the optical ‘dry’ mass and acoustic ‘wet’ mass of the adsorbed protein layer, leading to the estimation of the degree of hydration of the film. While hybrid acoustic/optical devices have been constructed in the past, this is the first time that a combined system is manufactured allowing simultaneously refractive index and acoustic wave phase and energy measurements in a real-time flow-through experiment. The use of 3D-printing for the manufacturing of the system also demonstrates the capability of additive manufacturing for fast and reliable prototyping.
Our future goal is to employ the combined system for the investigation of more complex binding processes focusing on DNA–protein interactions and membrane-binding events. The latter, in particular, is the focus of our HFSP grant entitled ‘Self-organization and biomechanical properties of the endosomal membrane’, and will benefit significantly from such an advanced biosensing method. We anticipate to be able to derive simultaneously information related to the binding kinetics and structure of membrane-tethered proteins in terms of their (mechanical) properties when behaving as a polymer brush.
The HFSP grant contributed to the development, in our lab, of this new biophysical tool for the study of biomolecular binding events on membranes and other inorganic surfaces.