Biosensing based on slow plasmon nanocavities

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Biosensing based on slow plasmon nanocavities

B. Sepulveda,^1,2 Y. Alaverdyan,² B. Brian,² M. Käll²

1 Nanobiosensors and Molecular Nanobiophysics Group Research Center on Nanoscience and Nanotechnolog (CIN2)CSIC-ICN

ETSE. Campus UAB -Edificio Q, 3^rd floor 08193 Bellaterra, Barcelona, Spain

2 Department of Applied Physics, Chalmers University of Technology Origovägen 6B, SE-412 96 Göteborg, Sweden

Abstract

The Localized Surface Plasmon Resonance of metal nanostructures has attracted a remarkable interest for biosensing applications due to its high sensitivity and multiplexing capabilities.

In particular, it has been shown that nanoholes in optically thin metal films can be efficiently used in biosensing applications due to the enhancement of the electromagnetic field inside of the nanoholes.

In a recent work we have demonstrated that nanoholes in thin metal films are strongly coupled through the so-called “antisymmetric” or “slow” surface plasmon polariton (SPP). When the edge-to- edge separation distance between holes is half the wavelength of the polariton (∼200-400 nm), a large constructive plasmonic interaction is achieved, leading to a high amplification of the scattered intensity per nanohole. In this condition, the metal between nanoholes acts as a resonant cavity for the slow SPP´s. Consequently, the scattering peaks become narrower, due to the increase in the lifetime of the SPP´s inside the cavity, and the near-field inside the nanoholes presents a large enhancement. These effects, which can be induced for even two nanoholes, provide appealing characteristics for multiplexed biosensing applications.

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Keywords: Nanoplasmonics, Biosensing, Localized Surface Plasmon Resonance Introduction

The intense and confined electromagnetic fields generated by surface plasmons in metal films and nanostructures have been used for label free biosensing since 1983. In the case of metal nanostructures, the surface plasmon resonance (SPR) can be detected as a peak in their optical scattering or extinction spectra. The spectral position of the resonance is highly dependent on the shape, distribution and electrodynamic environment of the nanostructures. In particular, the strong dependence of the SPR on the permittivity of the surrounding medium constitutes the sensing principle of metal nanostructures. Thus, a local increase of the refractive index in the close proximity of the nanostructure, as the one induced by adsorbed biomolecules, produces a red-shift of the resonance position [1].

There are several advantages associated with nanoplasmonic sensing compared to conventional surface plasmon polariton (SPP) sensors employing thin metal

films, such as a much higher degree of miniaturization, only restricted by the diffraction limit, and, therefore, an unprecedented potential for high density multiplexing. In addition, light can induce plasmons in the metal nanostructures directly, without the requirement of any other optical coupling elements.

Consequently, this technology is very promising due to the high demand of high-throughput screening applications.

Attractive metal nanostructures for biosensing applications are nanoholes in thin metal layers. Unlike non-interacting metal nanoparticles, these nanostructures combine the properties of surface plasmon polaritons (SPP) in thin metal layers with the localized surface plasmon resonance (LSPR) created by the nanoholes [2]. This combination arises by the induced electric dipole excited by the incident light in the nanoholes, which acts as a source of antisymmetric SPP’s of the thin metal film. Such SPP’s interact with the nearest nanoholes modifying their net dipole moment and, therefore, drastically changing their

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radiation properties. In this work we will show how nanoholes in thin films present several interesting features for biosensing applications.

Optical properties of nanoholes in thin metal films

The dominant interaction between nanoholes in thin metal films is due to surface plasmon- polariton emission. As schematically illustrated for a nanohole pair in Figure 1, each hole in a chain acts as an efficient source of SPP’s that are emitted preferentially in the direction of the induced dipole moment P. If the holes are illuminated in phase and the edge-to-edge distance between adjacent nanoholes fulfils the criterion

d =(n+1 2)

λ

_SPP_,n=0,1,2,..., where λ_SPP is the wavelength of the antisymmetric SPP mode, then the charge induced by the incident light and the surface charge due to SPP emission adds up at the hole edges (Fig. 1). As a consequence of this matching, the induced dipolar charges at each nanohole increase, which results in a large amplification of the scattering intensity. In contrast, for d = nλ_SPP, one expects a suppression of the scattering intensity due to a partial cancellation of the primary induced dipole by the SPP charge wave. Such nanocavity effect leads as well to a large tuning of the scattering peak positions.

Fig.1. Schematic picture of the interaction between nanoholes in thin metal films via antisymmetric surface plasmon polaritons

Fabrication of nanohole samples

To study the sensing performance of nanoholes in thin metal films, linear chains of N = 3-8

circular nanoholes were milled in t = 20 nm thick gold films using a focused ion beam (FIB, FEI Strata 235 Dual Beam). The thin gold films were thermally evaporated on SiO2 glass slides using a 1 nm Ti adhesion layer. All nanoholes were D

≈ 80 nm in diameter and the edge-to-edge distance was varied from d = 100 to 300 nm. A SEM picture of the nanohole chains can be observed in Fig. 2

2

SPP/ λ

A B

2

SPP/ λSPP/2 λ

A B

Fig.2. SEM micrographs of chains of nanoholes in 20 nm gold films (A) and dark-field scattering image (B)

The optical properties of the nanohole chains have been characterized through elastic scattering spectroscopy using an inverted dark- field microscope. White light from a 100 W halogen lamp illuminates the sample from above at a fixed angle of incidence θ = 62.5°±10° that exceeds the collection angle of the microscope objective, which then channels the scattered light to a single-grating spectrometer

Biosensing features of coupled nanoholes in thin metal films

Coupled nanoholes in thin metal films present several optical features making them interesting for sensing applications as, for example: i) the significant increase of the amplitude of the electric dipole moment and red-shift of the resonance when the polarization of the incoming light is parallel to the short axis of elongated nanoholes [3]; ii) large amplification of the scattering intensity and spectral narrowing of the resonance when the distances between nanoholes are tuned to half the wavelength of the antisymmetric SPP [4]; iii) wide tunability of the resonance position of the nanohole assembly, which can be achieved by simply changing the separation distance between nanoholes. This effect can be observed in Fig.

3, which compares the scattering spectra of a non-interacting nanohole with the spectra of linear chains of only three holes. The variation of the separation distance between holes from 100 to 300 nm, induces a large red-shift of the peak position, of approximately 200 nm. Such effect has important biosensing implications since it is generally accepted that the sensitivity of the plasmonic nanoparticle systems increases as the peak position of the resonance is tuned to the NIR

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0.0 0.5 1.0 1.5

I

Scatt

150 nm Single

Hole D = 90 nm

λpeak= 755 nm

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10 20 30 40 50

d = 200 nm 8 Holes

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Wavelength (nm) 70 nm 0

2 4 6 8 10

d = 300 d = 250 d = 200

d = 100

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d = 150 0.0

0.5 1.0 1.5

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150 nm Single

Hole D = 90 nm

λpeak= 755 nm

500 600 700 800 900 1000 0

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d = 200 nm 8 Holes

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d = 300 d = 250 d = 200

d = 100

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d = 150

500 600 700 800 900 1000 0

10 20 30 40 50

d = 200 nm 8 Holes

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Scatt

2 4 6 8 10

d = 300 d = 250 d = 200

d = 100

I

Scatt

d = 150

Fig. 3. Dark-field scattering spectra of nanoholes in 20 nm Au films: a) Single nanohole of D = 90 nm; b) Three nanoholes at varying separation distance; c) Eight nanoholes at a separation distance of 200 nm

To check if nanohole systems follow the same trend as metal nanoparticles, we have analyzed the shift of the resonance peak positions when the refractive index of the external medium varies as a function of the starting peak position, i.e., as a function of their separation distance (see Fig. 4). It is evident that increasing nanohole separation (tuning the peak position to the NIR) results in a higher sensitivity to refractive index changes. However, unlike metal nanoparticles, the increase of the sensitivity is not a linear function of the peak position [1]. This behavior reflects the destructive interaction between nanoholes when their separation distance fits the wavelength of the antisymmetric SPP, i.e., ∼250-300 nm.

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A

1.0 1.1 1.2 1.3 1.4 1.5 1.6 0

20 40 60 80 100 120

Refractive index d = 100

d = 150 d = 200 d = 250 d = 300

∆λ

(nm )

650 700 750 800 850 900

100 150 200 250 300

Peak Wavelength (nm)

∆λ/∆n(nm riu-1 )

B

C

1.0 1.1 1.2 1.3 1.4 1.5 1.6 0

20 40 60 80 100 120

d = 150 d = 200 d = 250 d = 300

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(nm )

650 700 750 800 850 900

100 150 200 250 300

1.0 1.1 1.2 1.3 1.4 1.5 1.6 0

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d = 150 d = 200 d = 250 d = 300

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(nm )

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d = 150 d = 200 d = 250 d = 300

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(nm )

650 700 750 800 850 900

100 150 200 250 300

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C

1.0 1.1 1.2 1.3 1.4 1.5 1.6 0

20 40 60 80 100 120

d = 150 d = 200 d = 250 d = 300

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(nm )

650 700 750 800 850 900

100 150 200 250 300

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20 40 60 80 100 120

d = 150 d = 200 d = 250 d = 300

∆λ

(nm )

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100 150 200 250 300

B

C

B

C

Fig. 4. Shift of the scattering spectra of nanohole chains composed of eight nanoholes when the external medium is changed from air (n = 1) to water (n = 1.33) and to oil ( n

=1.52). b) and c) bulk refractive index sensitivity of chains of nanoholes at varying separation distance.

On the other hand, the constructive interaction between anti-symmetric SPP’s, leads to a much larger lifetime of the dipolar resonances and, consequently, to a very significant peak narrowing of the peaks in the scattering spectra, as Fig. 3 (c), shows. Such peak narrowing together with the increase of the scattering

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intensity up to one order of magnitude when the separation distance between nanoholes fits the condition d = λ_SPP/2 ≈ 200 nm, allows achieving a much larger signal-to-noise ratio and resolution in the biosensing measurements. The conjunction of both features will permit improving the limit of detection of nanoplasmonic biosensing systems.

0 200 400 600 800 1000

0 2 4 6 8 10

I

Scatt

Separation distance (nm)

3 Nanoholes

Fig. 5. Dark-field scattering intensity for chains of three nanoholes at varying separation distance between holes

Effect of the substrate

Despite nanohole systems have already been successfully employed in biosensing applications, we have seen that the optimization of the separation distance between nanoholes can improve their biosensing capabilities. In addition, the substrate refractive index plays a very important role in the sensitivity of these systems. Nanohole arrays are usually fabricated on glass slides and, for biosensing applications, the external medium is water. This combination of refractive indices causes that the major part of the electromagnetic field of the anti-symmetric SPP, controlling the nanohole interaction, is concentrated in the glass/metal interface, as Fig.

6 shows. This effect was reflected in a much larger sensitivity of the nanohole systems when biomolecules were attached inside of the holes than in the metal film [5].

Therefore, the control of the electromagnetic field distribution surrounding the nanohole array is crucial to optimize the sensitivity of these systems. Matching the refractive indices of the substrate and sensing medium modifies the field distribution, giving a much larger intensity of the electromagnetic field in the sensing region (see Fig. 6). Such change in the field distribution will increase the biosensing performance in the metal region between nanoholes.

Glass

Teflon Water

Water Glass

Teflon Water

Water

Fig. 6. Cross section of the electromagnetic field distribution of the antisymmetric SPP, when the substrate is glass (n

=1.52) or teflon (n = 1.31)

To test the hypothesis of the substrate refractive index influence on the field distribution, we have analyzed the spectra of samples with different substrates immersed in sensing media of varying refractive index. Since FIB fabrication technique is not suitable for practical applications, we have employed colloidal lithography to pattern gold films with short range ordered nanoholes, keeping the same mean separation distance between nanoholes.

1.0 1.1 1.2 1.3 1.4 1.5

600 650 700 850

TiO₂ TiO₂ + Al₂O₃

Teflon Teflon + Al₂O₃ Glass Glass + Al₂O₃

Peak possition (nm)

Refractive index

Fig. 7: Surface and bulk sensitivities of short-range ordered nanoholes for substrates of different refractive index.

In biosensing applications the biochemical recognition events take place at the surface of the nanostructures. Consequently, only a small fraction of the near field distribution is employed to probe the sensing interactions. To model experimentally this surface sensitivity, we have deposited a 4 nm thick dielectric layer homogenously on the samples by e-beam evaporation.

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The results of this analysis have shown that, even though the reduction of the substrate refractive index induces a blue-shift of the resonance peaks, the sensitivity to the bulk refractive index increases (see Fig. 7). In addition, the sensitivity to the local changes of refractive index close to the surface increases even more markedly. For a low refractive index substrate as Teflon, the improvement of the surface sensitivity reaches a 100%. However, the biosensing surface sensitivity could be enhanced even more with lower refractive index substrates such as porous silica (n ≈ 1.05).

Conclusions

We have shown the optical properties of strongly coupled nanoholes in thin metal films for their biosensing application. The large constructive interaction between nanoholes can be the basis of a very interesting transducer for highly sensitive and multiplexed biosensing.

This interaction allows a large amplification of the scattering intensity and a wide tunability of the resonance and field distribution around the nanostructures. Such versatility can help to fulfill the requirements of different biosensing applications, in label-free or surface enhanced fluorescence sensing schemes.

Acknowledgements

Financial support from the Swedish Research Council is gratefully acknowledged.

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