3D plasmonic crystal metamaterials for ultra-sensitive biosensing

Having a variety of critically important applications in biomedical research, plasmonic biosensors form the core of current label-free biosensing technology to study biomolecular binding events between a target analyte from an aqueous solution and its corresponding receptor immobilized on a solid/liquid interface1,2. Plasmonic transduction relies on the refractive index (RI) control of the binding and profits from the difference of refractive indices between biological molecules and the aqueous environment (nbio =  1.45–1.55 Refractive Index Units (RIU) compared to nwat =  1.33 RIU). Confined to a metal/liquid interface, surface plasmons are critically dependent on the RI of the liquid medium within the penetration depth of the evanescent field and can be used to follow biomolecular interactions on the metal surface. In this case, one can avoid expensive, time-consuming and precision-interfering labelling steps to mark analytes and control binding/recognition events in real-time2,3,4, while the high sensitivity of this transduction is due to the strong plasmon-mediated electric field probing the molecules5. Conventional plasmonic biosensors employ Kretschmann-Raether prism geometry to excite surface plasmon polaritons over a thin 50-nm gold film in conditions of Surface Plasmon Resonance (SPR)3,4. Such SPR biosensors can provide relatively high sensitivity of the order of 2000–4000 nm of spectral resonance shift per refractive index unit (RIU) change4, but this geometry cannot satisfy many modern trends of biosensing advancing towards new biochemical nano-architectures, resolution beyond the diffraction limit, size-based selectivity etc6. In contrast, plasmons excited over nanostructures, including localized plasmon resonances (LPR) over 2D periodic nanoparticle arrays7,8, diffraction-coupled LPR (DC-LPR) over 2D nanoparticle arrays9,10,11,12,13, or surface plasmon polaritons over 2D nanohole arrays (NHA)14,15 can offer a series of promising biosensing nanoarchitectures and novel attractive functionalities. However, spectral sensitivity of 2D nanoscale geometries to refractive index variations is much lower: 200–500 nm/RIU for LPR and DC-LPR using nanoparticle arrays7,9,11,13, 300–400 nm/RIU for nanohole arrays14,15. The sensitivity problem of 2D periodic plasmonic arrays is related to diffractive nature of coupling light to plasmons, which links the sensitivity to structure periodicity Δ λ/Δ n ~ d16, which is typically of the order of hundreds of nm. The above-stated spectral sensitivities condition a detection limit of 1 pg/mm2 of surface coverage by biomaterial for SPR3,4 and of about 1 ng/mm2 for LPR7,8 and NHA17, which makes possible label-free studies of many interactions, but still inferior to labeling methods by more than 2 orders of magnitude. The sensitivity of plasmonic transduction can be improved by employing the phase characteristics of light instead of the amplitude ones, to follow biomolecular interactions18,5,19, although such result can be achieved for a relatively narrow dynamic range of measurements19. Phase-sensitive schemes profit from a sharp phase jump, which always occurs with a sudden drop of light intensity (light darkness) at the point of the resonance20,21. The sharpness of this jump and corresponding phase sensitivity do not follow the periodicity-related limitation rule, but are determined by the quality of resonances and first of all their depth (light darkness in the resonance)11,21. As an example, due to a much lower light intensity under DC-LPR compared to SPR in the Kretschmann-Raether prism arrangement, phase sensitivity of DC-LPR can much exceed the relevant parameter for SPR promising potential improvement of plasmonic biosensing technology down to single molecule detection level21. The combination of ultrahigh point sensitivity (provided e.g., by the phase interrogation) and fairly high sensitivity for a wider dynamic range of measurements (provided by the spectral or angular interrogation) looks like a very attractive basis for the development of next generation plasmonic biosensors. However, this combination is hardly possible with current 2D nanoperiodic structures due to the periodicity limitation rule for the spectral sensitivity. Here, we explore 3D geometries of plasmon excitation by employing designed plasmon crystal metamaterials. We show that such a transition to 3D nanoarchitectures leads to the excitation of a novel plasmon mode, resulting in very high sensitivities in both spectral (>2600 nm/RIU) and phase (>3*104 deg. of phase per RIU) interrogations. On the other hand, such structures can offer a high surface-to-volume ratio for bioimmobilization, in order to increase the resulting sensitivity, as well as enable novel promising sensing functionalities.