Ultrasensitive and label-free molecular-level detection enabled by light phase control in magnetoplasmonic nanoantennas

Abstract

Systems allowing label-free molecular detection are expected to have enormous impact on biochemical sciences. Research focuses on materials and technologies based on exploiting localized surface plasmon resonances in metallic nanostructures. The reason for this focused attention is their suitability for single-molecule sensing, arising from intrinsically nanoscopic sensing volume and the high sensitivity to the local environment. Here we propose an alternative route, which enables radically improved sensitivity compared with recently reported plasmon-based sensors. Such high sensitivity is achieved by exploiting the control of the phase of light in magnetoplasmonic nanoantennas. We demonstrate a manifold improvement of refractometric sensing figure-of-merit. Most remarkably, we show a raw surface sensitivity (that is, without applying fitting procedures) of two orders of magnitude higher than the current values reported for nanoplasmonic sensors. Such sensitivity corresponds to a mass of ~ 0.8 ag per nanoantenna of polyamide-6.6 (n=1.51), which is representative for a large variety of polymers, peptides and proteins.

Figure 1. LPR phase-sensitivity in the transmitted light polarization

a. When an incident light beam hits a ferromagnetic nanoantenna, the conduction electrons inside the nanostructure oscillate driven by the electric field E_i. These E_i-driven oscillations can be modeled as a damped spring-mass harmonic oscillator. A LPR is induced at a specific photon wavelength λ*, yielding a peak in the extinction spectrum (I₀−I_t)/I₀ = 1-(E_t/E_i)², displayed in the top panel. b. If the nanoantenna is magnetized perpendicularly to the surface plane, a MO-activity is turned on inducing a second MO-coupled LPR (MO-LPR) orthogonal to that directly driven by E_i. In a circular nanoantenna the MO-LPR resonates at the same λ*. The simultaneous excitation of LPR and MO-LPR induces an elliptical polarization ε of the transmitted field E_t^,. The null condition ε = 0 is generated at a desired λ_ε (in general λ_ε ≠ λ*) simply through engineering of the size of the circular nanoantenna^,,. Measurement of λ_ε provides a precise phase sensitive detection of the LPR position. The top panel displays typical Δε spectrum (red-line), as well as the 1/|Δε| spectrum (blue-line) and its resonance at λ_ε. The close-up view of the 1/|Δε| spectrum around λ_ε shown in the inset features a very narrow FWHM (1.7 nm). c. Similarly to the case described in (b), the concerted action of the simultaneously excitated LPR and MO-LPR can be exploited to actively manipulate the reflected light’s polarization inducing the condition ε = 0 at a desired λ′_ε. In general λ′_ε ≠ λ_ε since in this case also the additional phase introduced by the substrate reflectivity contributes to the polarization of the reflected field E_r. As in transmission geometry, the detection of λ′_ε provides precise phase sensitive detection of the LPR position. The top panel displays typical Δε spectrum (red-line), as well as the 1/|Δε| spectrum (blue-line) and its resonance at λ′_ε. The close-up view of the 1/|Δε| spectrum around λ′_ε shown in the inset features a very narrow FWHM (< 1.7 nm). Both in transmission and reflection, the sensitivity increases further by measuring the magnetic field- induced variation Δε as ε reverses its sign upon inverting H (see Supplementary Fig. 2).

Figure 2. Refractive index sensitivity of Ni magnetoplasmonic nanoantennas

a 3D AFM profile of Ni magnetoplasmonic nanoantennas on glass, with lateral dimensions of 103 ± 5 nm (diameter) and 30 ± 0.5 nm (thickness). b (Top panel) extinction spectra of Ni cylindrical nanoantennas for different values of the embedding refractive index (clean n=1, water n=1.33, 50% Vol. glycerol n=1.41, and glycerol n=1.47; the inset shows a zoom of the resonance peaks in the spectral region 450-600 nm); (bottom panel) plot of the inverse of transmitted light ellipticity variation 1/|Δε| for the same values of the embedding refractive index as above. c Comparison between the bulk sensitivities of Au (Δλ*/Δn) (red-dashed line) and Ni (both Δλ*/Δn and Δλ_ε/Δn) (blue and green markers, respectively) cylindrical nanoantennas on glass. d Comparison between the bulk figure-of-merit of Au [(Δλ*/Δn)/FWHM] (purple dashed line) and Ni [(Δλ_ε/Δn)/FWHM] (blue markers are experimental data, blue dotted line is a guide for eyes) cylindrical nanoantennas, and Au surface plasmon resonance (red dashed line), in the spectral range 420-750 nm.

Figure 3. Surface sensitivity assessment combining polarimetry and AFM measurements

a Plot of the inverse of the transmitted (left panel) and reflected (right panel) light ellipticity 1/|Δε| spectra as a function of molecular layer deposition (MLD) cycles (at steps of 20 cycles). b 1/|Δε| resonance wavelength λ_ε as a function of the MLD cycles for the two measurements geometries. In both cases, a linear dependence of λ_ε vs. number of MLD cycles is observed (the black dashed lines are guide for eyes). The shift of λ_ε saturates for a number of MLD cycles equal to 120, corresponding to a PA-6.6 thickness of ~35 nm, as shown in Supplementary Fig. 7 for transmission geometry case. Such PA-6.6 thickness agrees well with the near-field spatial extension (see Supplementary Fig. 4). c AFM images taken from the same sample region (total area imaged 2.7 × 2.7 μm²) after different numbers of MLD cycles. The AFM images show that PA-6.6 grows only on top of the nanoantennas. The colors of the frames refer to the corresponding colored polarimetry (b) and thickness (d) data points. The length of the white scale bars in the images corresponds to 1 μm. d PA-6.6 average thickness as function of the MLD cycles after AFM topography image analysis. Surface sensitivities (spectral variation of λ_ε divided by the average nylon thickness) of ~3 (transmission) and ~5.3 (reflection) are found combining plots (b) and (d). The error bars indicate the standard deviation from the average thicknesses measured analyzing the AFM images.

Figure 4. Surface sensitivity in the first few cycles of PA-6.6 MLD

a Schematic of one cycle of the MLD process for PA-6.6. A substrate with –OH surface groups is exposed to a pulse of adipoyl chloride (AC). The AC reacts with these–OH groups creating the by-product HCl, which is purged away along with any unreacted AC. Next a pulse of 1.6-hexamethylenediamine (HD) is introduced to the reaction chamber and reacts with the available –Cl groups. Again the byproduct is HCl, which is purged away along with any un-reacted HD. This process is repeated until the desired thickness is achieved. Nominally the process has a growth rate of ~0.8 nm/cycle. b AFM images taken from the same sample region (total area imaged 1.2 ×1.2 μm²) before and after PA-6.6 MLD. The colors of the frames refer to the corresponding colored thickness (c) and polarimetry (d) data points. c PA-6.6 average thickness as function of the MLD cycles after AFM topography image analysis. The error bars indicate the standard deviation from the average thicknesses shown in the inset, which shows the line profiles of all the disks included in the images in (b). The line profiles are taken along two orthogonal directions, which are shown as white dashed lines only in the AFM image of the clean sample in (b). d Plot of the inverse of transmitted and reflected light ellipticity λ_ε as a function of MLD cycles (black dashed lines are guide for eyes). Surface sensitivities of ~3.1 (transmission) and ~5.4 (reflection) are found combining plots (c) and (d), in excellent agreement with the results presented in Fig. 3. The horizontal error bars indicate the standard deviation from the average thicknesses shown in the inset in (c). The vertical error bars indicate the experimental error in the magneto-optical measurements. The insets show the corresponding 1/|Δε| spectra for the two measurement geometries (reflection – top-left inset, and transmission – bottom-right inset).