Enhanced sensitivity azimuthally controlled grating-coupled surface plasmon resonance applied to the calibration of thiol-poly(ethylene oxide) grafting

Azimuthally controlled grating-coupling surface plasmon resonance (φ ≠ 0° GC-SPR) is known to exhibit refractive index sensitivity (SRI) up to one order of magnitude higher with respect to classic φ = 0° GC-SPR. Here, grated gold surfaces (period of 500 nm and amplitude of 40 nm) were covered with different amounts of thiol-poly(ethylene oxide) (PEO, Mw 5 kDa) through a 24 h functionalization process using polymer solutions at increasing concentration (from 500 nM to 2.5 mM). The GC-SPR output signal generated by each PEO layer was measured in both null and rotated configuration. The number of PEO molecules adsorbed onto each surface (cm−2) and the corresponding molecular surface densities (σ [ng/cm2]) were calculated from the angle shifts collected after PEO film formation, by combining the model shown by Jung and the theory of PEO layer formation. In the end, we obtained a calibration curve that correlates the amount of PEO tethered onto the surface with the SPR measurement output signal, that will be useful for future reference. At the PEO surface saturation condition (PEO σ = 592 ng/cm2, obtained using PEO concentration ≥2 mM), a SRI of 603°/RIU was reached (φ = 45°), which is ≈10-fold higher than what obtained through the classic GC-SPR (φ = 0°). This enhanced instrumental sensitivity allowed detecting tiny amounts of molecules adsorbed onto the surface (3.1 ng vs 59.7 ng of classic φ = 0° GC-SPR), a property that is extremely valuable for the application of SPR sensing to quantitative determinations.