Plasmonic versus all-dielectric nanoantennas for refractometric sensing

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Plasmonic versus all-dielectric nanoantennas for refractometric sensing: a direct comparison Noemi Bosio, Hana Šípová-Jungová, Nils Odebo Länk, Tomasz J. Antosiewicz, Ruggero Verre, and Mikael Käll ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00434 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Plasmonic versus all-dielectric nanoantennas for refractometric sensing: a direct comparison Noemi Bosio,1, Hana Šípová-Jungová,1 Nils Odebo Länk,1 Tomasz J. Antosiewicz,1,2 Ruggero Verre,1# Mikael Käll1#

1Department

of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden.

2Faculty

of Physics, University of Warsaw, Pasteura 5, 02-093 Warsaw, Poland.

*Corresponding authors: [email protected]; [email protected]

ABSTRACT: In comparison to nanoplasmonic structures, resonant high-index dielectric nanoantennas hold several advantages that may benefit nanophotonic applications, including CMOS compatibility and low ohmic losses. One such application area might be label-free refractometric sensing, where changes in individual antenna resonance properties are used to quantify changes in surrounding refractive index, for example due to biomolecular binding. Here, we analyze and compare the sensing performance of silicon and gold nanodisks using a common and unbiased testing framework. We find that the all-dielectric system is fully capable of effectively monitoring small changes in bulk refractive index and biomolecular coverage, but the sensitivity is 5-10 times lower than the plasmonic counterpart. However, this drawback is partly compensated for by a more linear response to adsorbate layer thickness changes and an

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approximately four times smaller susceptibility to photothermal heating. Finally, dielectric sensors may show promise if certain strategies are employed to improve their performance, which could thus bridge the gap between the two systems. KEYWORDS: Si nanoresonators, plasmonic, refractometric sensing, high-index dielectric, heating effects

Chemical sensors and biosensors are becoming ubiquitous in todays advanced societies and are key ingredients in activities ranging from analysis of contamination levels in food1 and environment2 to novel point-of-care medical diagnostics3. Among the many different sensing schemes available, label-free optical biosensors based on surface plasmon resonance (SPR) transduction have gained particular importance because this method offers high sensitivity and simultaneous real-time monitoring capabilities.4 However, SPR typically requires bulky or complicated illumination schemes and the technique cannot be easily miniaturized. To overcome this limitation, the possibility of utilizing localized surface plasmon resonances (LSPR) in, primarily, gold nanostructures has been intensively investigated since more than 20 years.5 The advantage of LSPR over conventional SPR is that resonance excitation does not require any particular phase matching condition, which enables miniaturization of the sensor footprint and cost-effective devices. Moreover, in contrast to SPR, LSPR sensors have been shown to be able to resolve even single-molecule binding events by tracking spectral changes of individual nanoparticles over time.6-7 However, to reach the high signal-to-noise ratio required for single-molecule detection and other high-end applications, one typically needs to utilize intense illumination. This may in turn lead to photo-chemical or photo-thermal degradation of the analyte or the sensor itself because of ohmic losses in the LSPR nanostructure, which thus ultimately limit the signal read-out and sensor limit of detection

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9,10.

One solution to this dilemma could be to instead utilize resonant nanoantennas made from

dielectric materials.11 All-dielectric nanophotonics is rapidly becoming established as a mature field for advanced light management. In particular, dielectric nanostructures that exhibit well-defined multipolar optical resonances can be used as optical nanoantennas for application in, for example, flat lenses,12 metasurfaces13-14 and lasing.15 Similar to plasmonic nanoantennas, resonant all-dielectric nanoantennas are able to convert propagating fields into localized nearfields,16 and they can exhibit far-field optical cross sections several times larger than their physical dimensions. In the context of sensing, reports of detection using all-dielectric nanoantennas have started to appear in recent years.11, 17-22 However, most of these reports are based on macroscopic nanoantenna arrays that exhibit spatially extended resonance features, such as bound states in the continuum21 or lattice resonances17, 19 , and the question of how useful the intrinsic resonance modes of an all-dielectric nanoantenna is in detecting small changes in surrounding refractive index in comparison to a morphologically similar plasmonic system is still open. We address this issue by investigating short-range ordered Si and Au nanodisk arrays, whose behavior is essentially equivalent to those of the corresponding isolated nanoparticles,23-24 using identical experimental methodologies.

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RESULTS AND DISCUSSION

Figure 1. Resonance properties of isolated and short-range ordered layers of Au and Si nanodisks. (a,b) Simulated extinction cross-section spectra for isolated Au (a) and Si (b) nanodisks on glass in air and in water. The magnetic dipole (MD) and electric dipole (ED) modes are indicated. (c,d) experimental extinction spectra for the corresponding short-range ordered particle layers shown in the SEM images (e,f). The scale bars correspond to 1 μm.

Sample spectra. Most of the work in the previous literature in this area focuses on using the lowest energy dipolar nanoantenna resonance for sensing because this mode is typically spectrally isolated from other resonances and therefore well suited for spectral monitoring. The lowest energy resonance has magnetic dipole character for low aspect ratio (diameter/height) Si nanodisks25 and electric dipole character for plasmonic nanoparticles. Moreover, the magnetic dipole resonance in Si nanoparticles typically has a quality-factor similar to dipolar LSPRs (Q~15).26 In the following, we compare the sensing characteristics of the magnetic dipole (MD, indicated in Fig. 1B) mode in cylindrical Si nanoparticles and the electric dipole LSPR in gold nanodisks. Based on numerical simulations, the nanodisk heights and diameters of the two systems (𝐻𝐴𝑢= 30 nm, 𝐷𝐴𝑢= 150 nm and 𝐻𝑆𝑖= 144 nm, 𝐷𝑆𝑖= 170 nm,) were chosen such that the main

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particle resonances (electric and magnetic dipole modes for Au and Si, respectively) occurred at approximately the same wavelength, at around 𝜆 = 750 nm. This ensures that the system response of the measurement apparatus does not bias the comparison between the two systems. Calculated single particle extinction spectra for Au and Si nanodisks on a SiO2 substrate are shown in Figure 1a and b, respectively, while Figure 1c-d shows corresponding experimental extinction spectra for the two systems. By changing the surrounding refractive index from n = 1 (air) to n = 1.33 (water), a red-shift and a broadening, respectively, of the particle resonances for the two systems occurs, as mentioned above. The samples were made using the hole-mask colloidal lithography (HCL) technique and have similar particle densities, of the order of 10 particles/μm2 (Fig. 1e,f). HCL produces arrays of nanoparticles with well-defined nearestneighbor distances but no long-range translational order.27-29 Hence, lattice resonances do not occur and interparticle coupling is weak. The refractive index sensitivity of the arrays can therefore be expected to be representative of the corresponding isolated particles. This assumption is supported by the close similarity the calculated and measured spectra shown in Fig. 1. Bulk sensing. We first tested the bulk refractive index (RI) sensitivity of the Si nanodisks shown in Fig. 1f. Extinction spectra, here defined as 𝐸(𝜆) = 1 ― 𝑇(𝜆)/𝑇𝑟𝑒𝑓(𝜆), were 𝑇(𝜆) is the transmission through the sample and 𝑇𝑟𝑒𝑓(𝜆) the transmission through a region of the sample without nanodisks, were measured at normal incidence using a sampling area of ~1 mm2. The sample was interfaced to a fluidic cell to facilitate liquid exchange and data were collected in real-time while changing the RI from 1.33 (H2O) to 1.34 (H2O:Ethylene glycol, 9:1) in steps of 𝛥𝑛 = 0.0025 every 10 minutes. To verify the absence of hysteresis in the measurements, both an increase and a decrease of the RI were monitored. A plot of the change in extinction ∆𝐸(𝜆,𝑡) = 𝐸(𝜆,𝑡) ―𝐸(𝜆,0) versus time (Fig. 2b) during liquid exchange reveals small but consistent intensity changes, on the 1 % level, compared to initial spectrum measured

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in pure water (Fig. 2a). The observed intensity changes occur across the whole visible spectrum due to the multipolar response of the Si nanodisks. To facilitate a comparison with plasmonic systems, we quantify the shift of the intense magnetic dipole mode using a centroid algorithm (black lines in Fig. 2b-c). As expected, only a small, but clearly measurable, change in the resonance wavelength is observed. Additional analysis of Si particles of different sizes and resonances properties confirmed that the bulk RI sensitivity is largely independent of the exact sample geometry and resonance wavelength in the visible and NIR region (see Supporting Information, Fig. S1-S2 and Table S1).

Figure 2. Bulk sensing using resonant Si nanoparticles. (a) Extinction spectrum of silicon nanodisks in water. (b) Difference in extinction as a function of time when the surrounding medium is changed from pure water (n = 1.33) to 10% ethylene glycol in water (n = 1.34, t = 50 minutes) and then back to pure water; (c) Magnified portion of the data in b). The black lines in (b,c) traces the centroid wavelength of the magnetic dipole resonance.

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The results shown in Figure 2 clearly demonstrate that the intrinsic geometrical modes of Si nanoantennas can be used as facile real-time sensors of bulk RI changes. However, a comparison with the well-established Au system is required to contextualize their performance. The results of this comparative analysis are shown in Fig. 3 and Fig. S3. The resonance peak position red-shifts linearly with increasing RI for both the Au and the Si particles, but the Au system clearly outperforms Si in absolute numbers (216± 30 nm/RI for Au compared to 42±2 nm/RI for Si, errors estimated within the 95% confidence level). This result does not come as a surprise and it is in line with the simulations for the two systems (Fig.S4). One may also compare the two systems in terms of the change in absolute extinction amplitude per refractive index unit. Figure 3 e-f show that the maximum extinction amplitude sensitivity |∂𝐸/∂𝑛|𝑚𝑎𝑥, is approximately twice as high for the gold system. However, the Si sample shows a more complex spectral variation, ∂𝐸(𝜆)/∂𝑛, occurring over a broader spectral range due to the presence of several multipolar resonances. It is possible that this spectral complexity could be utilized to improve sensitivity further by applying more advanced analysis algorithms, such as principal component analysis, that correlate changes across the whole spectrum with changes in refractive index, but this goes beyond the scope of this work.

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Figure 3. Bulk refractive index sensitivity comparison between Si and Au nanodisk antennas. Resonance wavelength shift of the Si magnetic dipole mode (a,b) and the Au electric dipole mode (c,d) as a function of time (a,c) and refractive index change (b,d) for different concentrations of ethylene glycol (EG) in water; (e,f) shows the numerical derivatives ∂𝑬(𝝀)/∂𝒏 obtained from the Si and Au measurements. The data refer to the Si and Au samples shown in Figure 1.

Thin layer biosensing. For most applications, biosensing in particular, bulk sensitivity is not the most relevant quality measure. For example, while SPR biosensors typically exhibit at least an order of magnitude higher bulk sensitivity compared to LSPR, the two platforms show comparable performance in analysis of molecular biorecognition reactions occurring close to the metal surface.30 The differences are in this case due to the different spatial extension of the optical near-fields associated with plasmon excitation on planar metal surfaces and in nanoparticles. Following the same argument, it is clearly important to compare how plasmonic

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and all-dielectric nanoantennas respond to local RI changes induced by the same type of thin biomolecular layer. To avoid issues associated with using different covalent coupling chemistries (e.g. thiols on Au and silanes on Si), we compared the two systems using a protocol based on consecutive electrostatic adsorption of protein multilayers31. This method relies on adsorption of bovine serum albumin (BSA) layers, followed by rinsing and charging using dextran sulfate (DS) as sketched in Fig. 4a (see Methods for details). Negatively charged DS adsorbs electrostatically to the positively charged BSA monolayer and enables the adsorption of a second layer of BSA. By repeating this procedure, one can thus build up successive protein/DS layers with welldefined thickness on both gold and silicon nanoantennas. Similar to the bulk sensing case, we tracked the extinction spectral evolution during this process as a function of time using a microfluidic system (Fig. 4b, c). Both the Si and the Au nanoantennas clearly enable monitoring of the deposition of the different layers with high signal-to-noise ratio. In the following, we focus on changes in resonance peak position. Both DS and BSA red shift the resonances, but the effect of DS is smaller, which is due to a smaller effective molecular mass and volume compared to BSA. We assume that each BSA+DS bilayer corresponds to a deposited shell thickness of 𝑑 = 5 nm

32

and that the

effective refractive index is 𝑛𝑠 = 1.45 33. In Fig. 4c, we have plotted the peak shift versus number of deposited layers for the two systems. Note that this analysis has been performed starting from the second BSA layer since one cannot ensure perfect homogeneous coverage of the first BSA layer due to material heterogeneity of the substrate. The Si data refers to a sample with height H = 200 nm (sample 4 discussed in the SI). In agreement with our expectations and a large number of previous reports, the plasmonic system shows a progressively smaller peak shift as the molecular layers extends out from the metal surface. This trend is well captured by an exponential decay: 34

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∆𝜆𝐴𝑢 𝑙𝑜𝑐 =

∆𝜆 ∆𝑛

(𝑛 ― 𝑛𝐻20) (1 ― 𝑒 ―𝑑(𝑚 ― 1)/𝑙𝑑).

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(1)

∆𝜆

Here, ∆𝑛 is the LSPR bulk refractive index sensitivity, 𝑛𝑠 = 1.45 is the layer RI, 𝑛𝐻20 = 1.33 is the RI of the surrounding medium, 𝑑 ≈ 5 𝑛𝑚 is the layer thickness, 𝑚 is the number of deposited layers, and 𝑙𝑑 is the sensing decay length. By fitting the data to Eq. (1), one obtains a decay length of the order of 𝑙𝑑 ≈ 20 nm for the Au antennas (Fig. 4e, red line). The Si nanoantenna RI sensitivity, in contrast, show no tendency to saturate with increasing layer thickness and instead shows a linear trend ∆𝜆𝑆𝑖 𝑙𝑜𝑐𝑎𝑙 ≈ 𝑚(1.1 ± 0.05) nm. The linear Si sensor response is clearly advantageous for quantitative analysis and may be particularly valuable in measurements of large biomolecular interaction pairs (e.g. antigen-antibody), in which case the analyte might extend outside the sensing range of the LSPR. However, the actual sensor response is still several times smaller for the Si compared to the Au nanoantennas (by a factor 3-6, depending on layer number) and similar to the difference in bulk RI sensitivity.

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Figure 4. Sensing of thin biomolecular layers using Si and Au nanodisk antennas. (a) Sketch of the functionalization method based on successive BSA+DS layer deposition. Each layer is ~5 nm thick. (b,c) Change in resonance wavelength versus time for Si (b) and Au (c) nanoantennas. The different steps correspond to the injection of alternating BSA and DS solutions through the microfluidic chamber. (d,e) Resonance shift versus layer number 𝑚 for Si (d) and Au (e) nanoantennas. The resonance shift is counted from the first BSA layer (m=1) and the solid lines are obtained by linear (Si) and exponential fitting using Eq. 1 (Au), respectively.

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Thin-layer sensing mechanism. The basic physical mechanism behind refractometric sensing using nanoplasmonic systems is well known. The free electron LSPR oscillations polarize the displacement charges in the surrounding medium, which screens the restoring forces between electrons and positive ion cores. This effect increases with increasing RI of the surrounding medium and results in a lowering of the resonance energy of the localized plasmon and a red shift of the resonance peak. Even though the response to a RI change is superficially similar for the Si nanoantennas studied here, the origin of the effect is qualitatively different because the dielectric antenna resonances are of geometric nature and they are localized to the interior of the particle. By increasing the RI around a high-index dielectric nanoantenna, one decreases the RI contrast to the antenna interior. This can be expected to result in increasing leakage radiation and a broadening of the geometric resonances (c.f. Fig. 1b), but it is not obvious why the resonance wavelength should change. To understand this phenomenon, in particular the thin layer sensing results in some more depth, we turn to numerical simulations of isolated Si nanoantennas embedded in water and covered by a thin dielectric layer of refractive index nS and thickness 𝑠. Fig. 5a shows the resonance shift of isolated nanoantennas as a function of the shell thickness, similar to the experimental case discussed in the preceding paragraph. The numerical data confirm that the Si magnetic dipole resonance shifts almost linearly with shell thickness 𝑠, while the trend is exponential for the Au LSPR. It is also clear that the Si nanoantennas are much less sensitive to the thin layer perturbation. As mentioned above, these differences are linked to spatial differences in electromagnetic near-fields induced at resonance (Fig. 5b): The LSPR surface charge oscillation drives an exponentially decaying evanescent field while the dielectric nanoantenna acts as a geometrical cavity that stores the electromagnetic field inside the particle. The linear variation with shell thickness for the Si antenna holds for RIs from nS = 1.35 (i.e. close to the background RI nH2O = 1.33) up the RI of the resonator itself (nSi ≈ nS

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= 3.58), see Fig. 5c. This indicates that the red shift should be understood in terms of a change in the effective optical size of the resonator. It is indeed well known that dielectric antenna resonance wavelengths scale with cavity size D and RI as λres~ n𝐷.29 However, as is shown in Fig. 5d, it turns out that the peak shift is not linear with the refractive index of the layer, but rather with the change in permittivity with respect to the background, ∆εS = nS2, according to: ∆𝜆𝑆𝑖 𝑙𝑜𝑐 = 𝑘 𝑠 ∆𝜀𝑆,

(2)

where 𝑘 is a constant and 𝑠 is the shell thickness. We note that the proportionality factor 𝑘 depends on particle size and shape, indicating that it can be engineered to improve the sample sensitivity. The dependency of the resonance on the permittivity rather than the refractive index of the layer is caused by the fact that, as mentioned above, the resonance position is dictated by the optical size of the resonator. A Si particle in water is covered by a thin molecular layer can be approximated as a core-shell composite particle with a Si core of diameter 𝐷 and a molecular shell of thickness 𝑠 with permittivity 𝜀𝑚𝑜𝑙 = (𝑛𝐻20)2 +∆𝜀𝑆. The Si particle and the molecular shell together form a single bigger antenna, which can be described by an effective refractive index 𝑛𝑒𝑓𝑓 = 𝜀𝑒𝑓𝑓 =

(𝐷𝜀𝑆𝑖 + 2𝑠 𝜀𝑚𝑜𝑙)/(𝐷 + 2𝑠) , if one assumes that the effective

permittivity is given by a size-weighted average of the core and shell permittivities. Expanding the resonance position in a Taylor series around the background permittivity yields that the peak shift is linearly proportional to the increase of the permittivity of the shell over that of the background as described by Eq. (2). By using the exact solutions of a coated sphere calculated using Mie theory, we also show in Figure S5 that this behavior is general, and not restricted to the nanodisk case.

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Figure 5. Electrodynamics simulation of thin layer refractive index sensitivity of silicon and gold nanodisk antennas. (a) Changes in resonance wavelength for Si (green) and Au (red) as a function of a layer thickness (𝒏𝑺 = 1.45). (b) Plots of local intensity enhancement factors for Si and Au nanoparticles at resonance. (c) Magnetic dipole resonance peak shift versus shell thickness for various shell refractive indices 𝑛𝑆. (d) Magnetic dipole resonance refractive index sensitivity ∂𝝀𝒍𝒐𝒄 ∂𝒔 versus normalized change of refractive index (∆𝒏𝑺/∆ 𝒏𝒎𝒂𝒙 = (𝒏𝑺 ― 𝒏𝑯𝟐𝑶)/(𝒏𝑺𝒊 ― 𝒏𝑯𝟐𝑶), bottom axis) and permittivity (∆𝝐𝑺/∆𝝐𝒎𝒂𝒙 = (𝝐𝑺 ― 𝝐𝑯𝟐𝑶)/(𝝐𝑺𝒊 ― 𝝐𝑯𝟐𝑶), top axis).

Thermal simulations. As mentioned in the introduction, one of the main arguments behind investigating all-dielectric nanoantennas for sensing is that such systems can be

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expected to be less susceptible to photothermal heating than plasmonic nanoantennas. The absolute temperature increase critically depends on the sample parameters, illumination conditions and thermal conductivities around the sensing structures. In the case of uniform circular illumination over an area with diameter D covered by nanoantennas, the temperature increase is approximately given by9, 35 ∆𝑇(𝜆) =

𝜎𝑎𝑏𝑠(𝜆)𝑃(𝜆) 1 𝜅𝑒𝑓𝑓

𝜋𝐷𝐴 (1 ―

2 𝐴 𝜋𝐷

(3)

)

where 𝜎𝑎𝑏𝑠 is the antenna absorption cross section, 𝜅𝑒𝑓𝑓 is the average thermal conductivity surrounding the particle and A is the inverse of the number of particles per unit area. In Fig. 6 we estimate the heating generated by Au and Si nanoantennas similar to the ones studied experimentally in the case of laser illumination with 𝑃(𝜆) = 10 mW covering an area 50 μm in diameter and assuming 10 particles/μm2 at the interface between glass and water [𝜅𝑒𝑓𝑓 = 𝜅𝐻2𝑂 + 𝜅𝑆𝑖𝑂2 2

≈ 1 W/(m K)]. These conditions could emulate an experiment performed in an optical

microscope. We observe that the Au particle array exhibits about three times higher heating than the Si sample close to resonance (𝜆 ≈ 750 nm). Similar heating could be observed also in the case of broadband illumination. As a simple estimate, we assume a lamp spectrum 𝑃(𝜆) generated by blackbody radiation at 4000 K with a total power of 4.5 W loosely focused on a 1x1 mm2 area, as in a macroscopic sensing experiment. As shown in Figure 6, absorption in Si increases dramatically for 𝜆 < 600 nm, so we make the further assumption that a longpass filter at 600 nm is added before illuminating the sample. With these assumptions, Eq. (3) yields a total temperature increase of ~52 K and ~12 K for Au and Si respectively, that is a factor of ~4 difference. As a comparison, many proteins can denature at ~20 K above room temperature, so these numbers could be used as a quick estimate of which power levels could be used in a biosensing experiments without causing harm to the sample.

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Figure 6. Temperature increase for nanoparticle arrays made of Si and Au assuming a uniform and circular illumination (see text for details).

Conclusions. We have demonstrated that Si nanodisk antennas can be used as effective optical transducing platforms for refractometric sensing and we have compared their performance to plasmonic Au nanodisk resonators. The results show that the magnetic dipole resonance wavelength of the Si nanoantennas is five to ten times less sensitive to changes in bulk and thin film refractive index in comparison to the electric dipole resonance of the plasmonic system, irrespective to the exact nanodisk size or resonance wavelength. However, the lower sensitivity of the dielectric antennas is somewhat compensated for by their linear response to thin layer adsorption and their lower susceptibility to photothermal heating. In addition to the experimental results, we have performed electrodynamic simulations aimed to elucidate the physical origin of the thin film induced peak shift in dielectric resonators. The results show that the effect is due the change in effective optical size of the resonator, which is different from the plasmonic case where dielectric screening is the main cause. 16


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By utilizing more complex antenna morphologies than the nanodisk structure used here, one could potentially significantly increase the refractive index sensitivity of all-dielectric nanoantennas by inducing sharper or more sensitive resonant features. For example, recent reports point out that by opening up holes inside a dielectric resonator, or by using porous alldielectric materials, the field enhancement inside the resonator would be made accessible to analyte binding, which could boost sensitivity.36 Similarly, dark-field spectroscopy, phasesensitive detection,37-38 directional effects,39 collective enhancement in dimer antennas,40 periodic lattices,17,

41-42

or spectral (Fano) interferences between different geometric

resonances, might bring significant advantages in specific cases. Indeed, considering the rapid development in the field of all-dielectric nanophotonics, new strategies and effects are likely to increase the potential of these systems also in the area of refractive index sensing.

METHODS. Sample fabrication. The Au and Si nanodisks samples were produced using the colloidal hole-mask lithography technique according to ref 38 and 24, respectively. In brief, polystyrene beads are dispersed on a substrate, followed by evaporation of 10 nm Au mask and tape stripping. The hole mask is then used to evaporate particles at specific positions determined by the initial polystyrene beads position. For the gold samples, a Ti (1 nm) adhesion layer and Au (30 nm) were deposited while for Si nanoparticles a Ni mask (30nm) was deposited and then transferred to the poly-Si thin film on the substrate using Cl-etching. The Au samples were also annealed at 150°C for 10 minutes to reduce solvent induced reshaping and improve the stability during sensing measurements. The Si microfluidic master stamps were defined using a UV resist exposed by a Heidelberg laser writer, followed by a cryo etch using a PlasmaPro 100 Cobra ICP 180 (Oxford

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Instruments). The stamp was made of polydimethylsiloxane (PDMS) (SYLGARD 184 silicone elastomer kit, Galindberg) with 10:1 ratio between the base and curing agent. After cutting of the samples, the PDMS microfluidic channels were attached on the previously fabricated samples by means of an oxygen plasma treatment and the sample was baked in an oven at 80oC for two hours. Optical

measurements.

Transmission

spectra

were

recorded

in

a

home-built

spectrophotometer set-up based on a fiber coupled illumination source (HL-2000, Ocean Optics), a fiber coupled spectrometer (BRC711E, B&W Tek) and appropriate optics. Nanodisk spectra were corrected for system response by collecting reference spectra from regions of the sample without particles. Bulk and thin-layer refractometric sensing. Experimental settings, such as integration time etc., were kept constant through all the experiments. Various solutions were delivered to the sample though a fluidic cell by a peristaltic pump operating at a flow rate of 50 μl/min. Bulk sensing measurements were performed using mixtures of water and ethylene glycol (Sigma Aldrich). For the local RI sensing experiments, the sample was first stabilized in a flow of water followed by citrate buffer (10 mM sodium citrate, 1 mM sodium hydroxide, pH 4 at 25oC). The first protein layer was created by flowing a BSA solution (500 μg/ml, Sigma Aldrich) over the nanodisks for approximatively 10 minutes. The sample was then washed in a flow of buffer solution for 5 minutes and activated in a flow of dextran sulfate (1 mg/ml, Sigma Aldrich) for 10 minutes. This procedure was repeated several times equal to the number of layers to be deposited. Optical simulations. Optical simulations based on the finite-difference time-domain (FDTD) method were performed using commercial software (FDTD Solutions, Lumerical Inc., Canada). The nanodisks were discretized using a mesh resolution of 0.5 nm for Au and 1 nm for Si, and the 18


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complex permittivity data was taken from ref43 and from in-house ellipsometric measurements, respectively. Simulations were performed for single nanodisk on SiO2 (n = 1.51) at normal incidence and using perfectly matched layer boundary conditions. Acknowledgements. This work was supported by the Knut and Alice Wallenberg Foundation and the Swedish Foundation for Strategic Reseach. TJA thanks the Polish National Science Center for support via the project 2017/25/B/ST3/00744. Supporting information. Extinction and bulk sensitivity for Si nanodisk arrays of different sizes and for the Au sample. Simulated bulk sensitivity for the Au and Si sample and Mie theory calculations of a core-shell sphere made of Si covered by a dielectric shell.

TOC

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Plasmonic versus all-dielectric nanoantennas for refractometric sensing

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