We numerically evaluate the optical response of a Kretschmann surface plasmon resonance (SPR) biosensor featuring metallic nanogratings and patterned immobilization of surface receptors. Parameters are chosen such that the biosensor is operated near the generated bandgap of the surface plasmon dispersion. In this paper, we demonstrate that the sensitivity can be increased by concentrating the surface receptors and adsorbed analytes on regions where the field intensity is the greatest. Specifically, a surface presenting receptors on the grating mesas is shown to be twice as sensitive as that of a uniformly functionalized corrugated surface. The grating geometries are also studied; it is found that higher aspect ratio features show increased SPR response. The analysis differs from existing studies of enhanced SPR as the sensitivity improvement originating from the concentration and mapping of surface receptors to the plasmon field distribution is studied rather than the absorption or scattering enhancement effect of the nanostructures.
Current and future demands for biosensors for use in point-of-care clinical diagnosis require increasingly compact and sensitive devices. Metallic nanogratings are studied owing to their capacity to perturb a propagating surface plasmon wave; it has been shown that these perturbations create strong field gradients and modify the surface plasmon dispersion relation. The development of novel surface functionalization techniques including polymer matrices, self-assembled monolayers, and nanocolloidal particles adds further potential for sensitivity improvements [
In a traditional surface plasmon resonance (SPR) biosensor configuration, known as the Kretschmann or Attenuated Total Reflection (ATR) configuration, surface plasmon waves are excited on a metallic-dielectric interface via a high refractive index prism [
It has been shown that sensitivity of the SPR biosensor can be increased by the modification of the metallic surface with a periodic grating for both propagating and localized surface plasmon. Numerous studies have been presented in which nanoposts, nanowires, and gratings are used for enhancement [
The SPR interface consists of thin layer of gold of thickness
(a) SPR surface grating configuration, (b) single surface biomolecule active medium (dark gray), (c) active medium uniformly distributed along the grating, (d) active medium in the grating trough, (e) active medium on the grating mesa.
The simulations were carried out using a proprietary rigorous coupled wave analysis (RCWA) code. In the literature, RCWA has been extensively employed to study metallic corrugated surfaces, with significant pioneering work by Moharam and colleagues [
Figure
Angular reflectance spectrum at 820 nm and 970 nm wavelength.
The grating redistributes the field intensity of the propagating surface plasmon wave such that the highest field intensity is found near the grating edge, and with higher field strength on the grating mesa rather than in the trough as shown in Figure
(a) Field intensity (
The intensity distribution over the grating surface and consequently the selective sensitivity of the plasmon wave suggest that one can significantly enhance the SPR biosensor by guiding, selectively, the immobilization of surface receptors, such as antibodies for the detection of proteins, or oligonucleotide probes for the detection of DNA, onto the more sensitive area of the grating surface.
The grating structure is simulated with the active medium uniformly distributed along the entire grating (as in Figure
The resonance shift for the functionalized mesa is almost twice that of the uniformly covered grating and almost 3 times that of a conventional (planar) interface, while a small response is measured when the analyte is adsorbed in the trough (Figure
Resonance shift for 970 nm operating wavelength for immobilization on the mesa, trough, or uniform across the grating and also for the flat (conventional SPR) surface.
Field distribution (
The effects of the metallic grating height and the grating duty factor are also examined with the mapped immobilization of surface receptors. The plasmon field distributions are dependent on the features of the metallic gratings. For an initial binary grating with a depth of 25 nm and underlying gold thickness of 25 nm as described in the previous section, RCWA calculations of the angular resonance shift are obtained for grating duty factors from 20% to 80%. For these simulations, the immobilization of surface receptors is assumed to be exclusively on the grating mesas, where field concentrations are the greatest, at 970 nm wavelength. Furthermore, an equivalent effective refractive index change is assumed in all cases, as to model an equal adsorption of analytes. It assumes that in an actual system the surface and solution concentrations have reached equilibrium, with unsaturated surface receptors. Shown in Figure
(a) Sensitivity for grating duty factor from 20%–80%. (b) SPR curves for increasing gold thickness (
While the previous analysis has been carried out on a binary grating structure with 50% duty factor, 25 nm depth, and 25 nm underlying gold, the latter two parameters can also be optimized for a mapped immobilization. Similar to a planar SPR interface configuration, increasing the underlying metallic film narrows the angular resonance curve. A thin gold underlayer increases the loss of the propagating evanescent wave, reducing its propagation length, resulting in broad resonance responses (Figure
A higher sensitivity is generally observed for gratings with deep troughs and a thinner underlying gold layer (
Figure of merit for different grating geometries. The grating with 5 and 10 nm underlying gold layer is not included.
A number of methods have been reported for the fabrication of nanometric metallic gratings or periodic structures, generally based on a metallic lift-off process of an electron-beam patterned polymeric resist or fast-replication techniques [
In conclusion, we have demonstrated numerically that the targeted immobilization of surface receptors onto the nanostructured biointerface of a surface plasmon-based biosensor, in a Kretschmann configuration, can increase the detection sensitivity. The guided immobilization allows the concentration of the index change (surface adsorption) to be targeted to zones of high field intensity generated by the grating. Coupled with the enhancement from operating the SPR-biosensor near the generated bandgap [
The authors would like to acknowledge Le Fond Québécois de la Recherche sur la Nature et les Technology (FQRNT) and NanoQuébec for their financial support. The authors would also like to thank N. A. Nicorovici and R. C. McPhedran (CUDOS, University of Sydney, Australia) for their advice in this project.