Non-interacting molecules as innate structural probes in surface plasmon resonance

Area of Science:

• Surface science
• Biophysics
• Materials science

Background:

• Determining molecular layer structure in Surface Plasmon Resonance (SPR) is challenging.
• The refractive index (RI) constraint complicates SPR thickness measurements.
• Understanding molecular layer structure is crucial for biointerfaces and material science.

Purpose of the Study:

• To develop a noninvasive method for measuring the thickness of surface-tethered molecular layers using SPR.
• To bypass the limitations imposed by the refractive index in SPR measurements.
• To investigate the relationship between molecular layer thickness and function.

Main Methods:

• Utilizing noninteracting molecules as intrinsic structural probes in SPR.
• Measuring the exclusion volume of surface-tethered molecular layers.
• Employing Bovine Serum Albumin (BSA) molecules to probe polyethylene glycol (PEG) molecular brushes.
• Comparing SPR measurements with atomic force microscopy (AFM) data.

Main Results:

• Demonstrated proof-of-concept for using molecular probes in SPR to determine layer thickness.
• SPR-acquired thicknesses for PEG brushes correlated with PEG molecular weight and hydrodynamic diameter.
• Measurements showed good agreement with AFM force-distance data.
• Theoretical analysis suggests applicability for low RI layers with ±15% error.

Conclusions:

• The developed SPR method offers a noninvasive approach to measure molecular layer thickness.
• This technique overcomes the refractive index constraint, enabling better characterization of molecular layers.
• The method is suitable for in situ analysis and can be integrated into routine SPR binding assays for studying biointerfaces.