Refractometric Sensing Using Propagating versus Localized

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NANO LETTERS

Refractometric Sensing Using Propagating versus Localized Surface Plasmons: A Direct Comparison

2009 Vol. 9, No. 12 4428-4433

Mikael Svedendahl, Si Chen, Alexandre Dmitriev, and Mikael Ka¨ll* Department of Applied Physics, Chalmers UniVersity of Technology, 412 96 Go¨teborg, Sweden Received August 20, 2009; Revised Manuscript Received October 12, 2009

ABSTRACT We present a direct experimental comparison between the refractive index sensing capabilities of localized surface plasmon resonances (LSPRs) in gold nanodisks and propagating surface plasmon resonances (SPRs) on 50 nm gold films. The comparison is made using identical experimental conditions, and for the same resonance wavelength, λSP = 700 nm. Biosensing experiments with biotin-avidin coupling reveal that the two sensing platforms have very similar performance, despite a superior bulk refractive index sensing figure of merit for the SPR sensor. The results demonstrate that LSPR sensing based on simple transmission or reflection measurements is a highly competitive technique compared to the traditional SPR sensor.

Surface plasmon resonances (SPRs) have been utilized for label-free bio/chemosensing for over two decades,1 with diverse applications in areas such as medical diagnostics, environmental monitoring, food safety screening, and threat detection.2-5 Traditional SPR sensing is based on propagating plasmons, so-called surface plasmon polaritons, in optically thin metal films, usually gold layers. Propagating plasmons can be excited in a variety of illumination schemes, including grating coupling, highly focused laser beams, and near-field excitation, but the most common, simple, and efficient design is probably excitation in the Kretschmann geometry.6 Here, the metal film is deposited on a prism that is used to match the momentum of the incoming light to that of the plasmon. Monitoring of the resonance condition, either in terms of resonance angle or resonance wavelength, then allows one to determine changes in refractive index at the free side of the metal film versus time. Biosensing using SPR in Kretschmann geometry is now a mature technology, and sensors are commercially available from several companies. During the past decade, it has been argued that plasmon excitations in nanostructured metals, so-called localized SPRs (LSPRs), constitute a promising alternative to the propagating plasmon utilized in classical SPR sensors. Proof-of-principle bio/chemosensing using LSPR spectroscopy has been demonstrated for a variety of nanostructures, including triangular silver particles,7 gold nanodisks,8 nanoshells,9 nanorings,10 nanocrescents,11 and nanoholes in thin films.12 Most of this work has been motivated by the possibilities for sensor miniaturization offered by nanoscale plasmon localization * To whom correspondence should be addressed. E-mail: [email protected]. 10.1021/nl902721z CCC: $40.75 Published on Web 10/20/2009

 2009 American Chemical Society

combined with efficient microspectroscopy. Reported sensitivity levels below 100 biomolecules indicate that singlemolecule LSPR detection is feasible.8,13 However, most practical sensing applications, for example, in medical diagnostics, do not necessarily benefit from single-molecule sensitivity and extreme miniaturization, unless low limits of detection in terms of bulk concentration are simultaneously provided. In fact, the lowest noise levels, and thus highest signal-to-noise ratios, have so far been demonstrated for macro- or microscopic LSPR,8,14,15 rather than singlenanoparticle LSPR configurations. The most relevant question is thus whether LSPR sensing offers any substantial advantages over classical SPR detection schemes under circumstances optimized for high signal-to-noise. Possible advantages could be that LSPR sensing requires simpler instrumentation than SPR, that is, transmission/extinction measurements rather than excitation through prisms, and that the lower bulk refractive index sensitivity of LSPR put less stringent demands on temperature stabilization, but the relative pros and cons of the two techniques have only rarely been addressed. To the best of our knowledge, the only comparative experimental study to date comes from the Van Duyne group.16 The authors report interesting similarities and differences between LSPR and SPR biomolecular binding to Ag nanotriangles and 50 nm Au films, respectively, but it is also evident from the presented data that the signal-tonoise performance of the LSPR measurement was very inferior to angle-resolved SPR data, obtained in a commercial system. This motivated us to make a new decisive comparison between the two plasmonic sensing techniques. In

Figure 1. Scanning electron micrograph of the array of gold nanodisks on glass fabricated with HCL. The nanodisks have a diameter of 120 nm and a height of 30 nm. The inset shows a photograph of a typical sample (the diameter of the glass slide is 2.5 cm).

particular, we wanted to make a comparison that utilized the same illumination, detection, and data analysis methodology for SPR and LSPR sensing, the same metal, and the same resonance position. We present such a comparison for the case of propagating plasmons in 50 nm gold films versus dipolar localized plasmons in gold nanodisks. The results clearly demonstrate that LSPR and SPR yield very similar peak shifts and signal-to-noise characteristics, and therefore very similar sensitivities, in realistic biosensing experiments. Materials and Methods. Gold nanodisks on glass (see Figure 1) were fabricated using hole-mask colloidal lithography (HCL),17 a bottom-up technique based on selfassembly of colloidal polystyrene particles (Interfacial Dynamics Corp.) on top of a layer of polymethyl methacrylate (PMMA) spin-coated on a suitable substrate. Deposition of a thin gold film and subsequent removal of the colloids by tape-stripping results in an evaporation mask in the form of a short-range ordered array of nanoholes. An oxygen plasma treatment removes the PMMA below the holes and allows for gold deposition directly onto the substrate, in this case, glass slides (VWR International) covered with a thin adhesion layer (1-2 nm Ti or Cr). By choosing the size of the polystyrene particles and the amount of deposited gold, one can precisely control the diameter D and thickness T of the fabricated gold nanodisks. The disk aspect ratio, D/T, is the most important factor determining the plasmonic properties of the array.18 Here, we choose D/T ) 4 (T ) 30 nm, D ) 120 nm) in order to obtain a dipolar LSPR at ∼700 nm, which is sufficiently far from the interband absorption region of gold to yield a narrow resonance while still being in the visible range, thus facilitating optical experiments. We note that the excited dipolar LSPRs of the nanodisks in the array do not interact in the near-field due to sufficient interdisk spacing and have only weak far-field coupling due to the absence of long-range periodicity.18-21 For comparison with SPR excitation, we prepared 50 nm thin homogeneous gold films using the same substrate and deposition parameters as those for the nanodisks. The chosen thickness is close to Nano Lett., Vol. 9, No. 12, 2009

the optimum for SPR excitation using red light and is typical for sensor chips used in commercial SPR instruments.2 Both LSPR and SPR excitation was achieved by white light illumination from a fiber-coupled 20 W halogen lamp (HL-2000, Ocean Optics). The excitation beam was collimated to a divergence of about 0.3° and had a diameter of ∼4 mm. The samples were mounted in a home-built flow cell with a volume of about 0.5 mL. SPR excitation in Kretschmann geometry was achieved by interfacing a BK7 dove prism to the sample substrate using immersion oil. We used an incidence angle of ∼68° in order to obtain SPR excitation at ∼700 nm, that is, coinciding with the LSPR wavelength of the nanodisks. A polarizer in the beam path allowed for selection between p- and s-polarized illumination. Reflected, transmitted, or scattered light was collected with afiber-coupleddiode-arrayspectrometer(B&WTekBRC711E). Processing of spectra was done in real time using a Matlab routine, and the resonance position was retrieved by fitting a 20 degree polynomial to the spectrum of interest, as described by Dahlin et al.15 Bulk refractive index (RI) sensitivity measurements were performed using purified water (Milli-Q Synthesis A10) mixed with various fractions of ethylene glycol (EG). In order to test the actual biosensing properties of the sensor chips, experiments were performed using a biotinylated bovine serum albumin (bBSA) and streptavidin (SA) assay in HEPES buffer (150 mM NaCl, pH 7.0). All samples were cleaned in TL1 (5:1:1, milli-Q water/30% v/v H2O2/25% v/v NH4OH) at ∼80 °C before use. The biotin-avidin affinity is very high (KA ≈ 1015 M-1), which is why this system is often used as a model in biorecognition studies. According to Svedhem et al.,22 bBSA primarily adsorbs on gold under the experimental conditions used here. In order to reduce unspecific binding of SA to the regions of the sample surface not covered by bBSA, for example, the glass between the Au disks in the case of the LSPR sensing measurements, the samples were exposed to ordinary bovine serum albumin (BSA) following bBSA functionalization. All proteins were purchased from Sigma-Aldrich and used as obtained. Results and Discussion. Optical Properties of Nanodisk Samples. The LSPR properties of isolated metal disks in the size range used here are well-understood. In brief, the Au nanodisk spectrum is dominated by a single dipolar plasmon oscillation parallel to the substrate surface (“the LSPR”), while perpendicular oscillations and higher-order modes are damped out by the interband contribution to the Au dielectric function. For a given dielectric environment, the position of the LSPR is primarily governed by the disk aspect ratio, as mentioned above, but finite-size (retardation) effects contribute a red shift that determines the precise position.18 To confirm the localized character of the nanodisk plasmons, we measured transmission spectra as a function of incidence angle using p- and s-polarized incidence. As seen in Figure 2a, the resulting dispersion λres(θ), that is, the resonance wavelength versus angle of incidence θi relative to the sample surface normal, exhibits a variation below 10 nm within the investigated angular range. This can be compared, for example, to the dispersion of the SPR excited in the 4429

Figure 2. Optical characterization of nanodisk samples. (a) The dispersion λLSPR(θi) for p- and s-polarization. The inset shows transmission and reflection for p-polarized light. (b) Measured intensity versus emission angle θe for a fixed incidence angle that equals the Brewster angle θB for the air/glass interface. Note the completely dominating signal in the forward and specular directions.

Kretschmann geometry for the 50 nm Au film in water, which shifts by more than 50 nm if the incidence angle is varied by 1° (not shown). In comparison with the SPR, the nanodisk plasmon dispersion can thus be considered essentially flat. This is the hallmark of a localized mode, that is, a LSPR. The fact that we investigate rather dense layers of nanoparticles also influences the measurement strategy for sensing experiments. The optical properties of ensembles of plasmonic nanoparticles have traditionally been characterized using transmission (or extinction) measurements, while single nanoparticles most often are characterized through their scattering spectrum, which can be readily measured using dark-field optical microscopy. The SPR of flat gold films, in contrast, is evaluated using specular reflection data. In order to select the most appropriate measurement strategy for the nanodisk samples, reflection, transmission, and scattering measurements were made for various angles of incidence θi. Figure 2b shows the scattered intensity at the peak of the LSPR response versus emission angle θe. We used p-polarization and an angle of incidence θi ) θB, where θB ) 56° is the Brewster angle of the air/glass substrate interface, to minimize the reflection from the back of the sample substrate. Remarkably, the amount of scattering at angles away from the forward and specular directions was very small indeed (the direct reflection, θe ) θi, is, for example, more than 2 orders of magnitude stronger than the signal at θe ) 0). Evidently, the ensemble of nanodisks acts like a homogeneous “metamaterial” thin film rather than 4430

Figure 3. Bulk refractive index sensing. (a) Transmission and reflection spectra for Au nanodisks in contact with water/glycol mixtures of increasing refractive index n. (b) Corresponding data for propagating plasmons on a 50 nm thin gold film. (c) The bulk refractive index sensitivity comparison between the SPR and LSPR extracted from (a) and (b).

a collection of isolated dipolar scatterers. This effect is not due to a mutual near-field interaction of neighboring nanodisks but is a consequence of a coherent superposition of the scattered far fields from closeby particles (compare Huygens principle). The data implies that a dark-field measurement configuration is not favorable for this kind of arrays and that sensing experiments should be performed in reflection or transmission geometry. The same conclusion was recently reached through optical microscopy studies by Dahlin et al.,8 who showed that “micro-extinction”, was superior to dark-field scattering spectroscopy for the same type of samples as those used in this study. Bulk Sensing Experiments. The vast majority of previously published refractometric sensing studies on plasmonic nanostructures have characterized performance through the bulk RI sensitivity Sbulk ) ∂λSP/∂n, where n is the refractive index of the interchangeable medium around the nanostructure (i.e., excluding the substrate support), or through the so-called figure of merit FoM ) Sbulk/fwhm, where fwhm is the full width at half-maximum of the plasmon resonance.23-25 Figure 3 summarizes bulk RI sensitivity data for the nanodisks and the 50 nm gold film. As seen in Figure 3a, the RI sensitivity of the Au nanodisk array amounts to ∼178 nm/RIU, in good agreement with previously reported results,18,26 and there is no measurable difference between reflection and transmission data. The corresponding FoM is ∼2 since the fwhm of the resonance is about 90 nm. As described above, the bulk sensitivity of propagating surface plasmons was studied using the same measurement setup Nano Lett., Vol. 9, No. 12, 2009

and resonance wavelength as those used for the nanodisk samples, the only principle difference being that spectra have to be collected in total internal reflection. As shown in Figure 3b, and as anticipated from previous reports,2,27,28 the SPR exhibits much higher sensitivity and FoM than the LSPR; we find Sbulk ) 3300 nm/RIU and FoM ) 54 at n ) 1.33, in good agreement with previous SPR measurements on 50 nm Au films.29,30 The differences in sensitivity and FoM between the LSPR and SPR measurements can be understood from a simple but general theoretical analysis. Following Homola et al.,2 one may assume that the dispersion of the propagating plasmon is given by kSPP ) k0[εn2/(ε + n2)]1/2, where ε ) ε′ + iε′′ is the metal dielectric function. Resonance is obtained when kSPP ) kincident ) k0nprism sin θi, which occurs x at a certain wavelength λSPR for a given combination of n and kincident . The refractive index sensitivity can then be x derived by differentiating kSPP with respect to n. As shown by Miller and Lazarides,31 one may derive the LSPR sensitivity to a refractive index change in the same spirit by assuming that a localized plasmon is characterized by a dipole polarizability resonance condition of the same form as that for an ellipsoidal particle in the quasistatic limit. The sensitivity is then obtained by differentiating the resonance condition with respect to n. The results for the SPR and LSPR bulk sensitivities are ∂λSPR ∂ε′ ) -2ε′2 /n3 ∂λ ∂n ∂λLSPR ∂ε′ ) 2ε′/n ∂λ ∂n

(1)

In a similar fashion, one may derive the SPR and LSPR line widths from the dispersion relation and resonance condition, respectively, yielding fwhmSPR

) 4ε′′/

| | | | ∂ε′ ∂λ

fwhmLSPR ) 2ε′′/ ∂ε′ ∂λ

(2)

One may note that the LSPR result can also be obtained from general energy distribution considerations.32 Finally, one may combine the refractive index sensitivities and widths to obtain the FoMs for SPR and LSPR sensing according to FoMSPR FoMLSPR

ε′2 2n3ε′′ |ε′| ) nε′′ )

(3)

Equations 1-3 indicate that the bulk RI sensitivity of plasmons is determined solely by the wavelength-dependent metal dielectric function and the refractive index of the surrounding. In particular, eq 1 explains why nanostructures of different shape but with the same composition and resonance wavelength exhibit very similar bulk RI sensitiviNano Lett., Vol. 9, No. 12, 2009

ties and why a longer resonance wavelength leads to a higher sensitivity; see for example, ref 10. Using ε(λ) for gold according to Johnson and Christy,33 eq 1 predicts a SPR sensitivity that is ∼12 times larger than that for the LSPR case at 700 nm. This is similar to what is obtained experimentally from Figure 3 if one takes into account that only ∼50% of the nanodisk surfaces are in contact with the liquid environment. The theoretical FoM is in good agreement with experiment in the case of SPR (FoMtheory ≈ 55 and FoMexp. ≈ 54 @ 700 nm), but there is a large deviation for LSPR, where the theory yields FoMtheory ≈ 12 and experiment FoMexp. ≈ 2 at 700 nm. The latter difference originates in the aforementioned substrate contribution, which reduces the experimental ∂λLSPR/∂n by a factor of ∼2, and in a significantly larger fwhm for the experimental data than what is obtained theoretically in the dipole limit according to eq 2. The latter discrepancy is not surprising as the analysis does not take into account factors that are known to contribute significantly to LSPR broadening, in particular, radiative damping, inhomogeneous broadening, and defect scattering.34,35 Biosensing. From the results in the previous paragraph, in particular, the large difference in FoM, one may expect that SPR refractive index sensing using flat gold films will always be superior to using a nanostructured surface. We set out to test this for a realistic sensing application, that is, to quantify a biorecognition reaction in real time. As mentioned in the introduction, this is by far the most common and influential application area in plasmonics to date. As shown in Figure 4, the experiment starts by injecting 100 µg/mL bBSA at 7 min, followed by 1 mg/mL BSA at 102 min and a 10 µg/mL SA injection at 187 min. The flow cell was thoroughly rinsed with buffer during a period of 25 min in between these injections. From the kinetics curves, one notes a slightly faster saturation of bBSA on the nanodisks than that on the gold film but a 1.7 times smaller peak shift. BSA adsorption, on the other hand, exhibits similar peak shifts for all detection methods. The high SPR sensitivity to bulk RI changes is evident from the large peak shifts induced by the rinsing procedures; when BSA is flushed out, for example, the resonance blue shifts almost 0.5 nm at t ) 160 min while the LSPR trace exhibits a shift on the order of 0.01 nm. Importantly, the biorecognition step, that is, SA binding to bBSA, resulted in very similar responses in all three measurements; SPR shifts 0.55 nm, while extinction and reflection measurements showed 0.57 and 0.63 nm shifts, respectively. We note that the resonance wavelengths at t ) 0 min were 706 nm for SPR and 692 nm for LSPR. These values are close enough to make the wavelength dependence of the refractive index sensitivity, predicted from eq 1, unimportant in a comparison between the two sensing structures. The noise levels (standard deviation of the peak position of a blank sample) were on the same level, below 5 × 10-4 nm, in the three types of measurements, as seen from the inset in Figure 4. It is evident that the vast differences in bulk sensitivities and FoMs bear little significance for the detection of 4431

Figure 4. Biosensing experiments. The figure shows the temporal evolution of the resonance shifts for localized and propagating plasmons on Au nanodisks and a 50 nm planar gold film, respectively. The vertical lines indicate when different biomolecules have been introduced into the sample cell. The insets show the fluctuations of the resonance position for blank samples (left) and the absorbed mass after SA injection (right).

molecular adsorption. This is mainly due to the much greater confinement of the LSPR induced electromagnetic (EM) fields around the nanostructures,16,36 which leads to a more efficient utilization of the bulk sensitivity compared to the propagating plasmon case. The confinement is usually discussed in terms of a single field decay length, although this is an obvious oversimplification for a three-dimensional nanostructure. Nevertheless, on the basis of LSPR shifts in particle pairs36,37 and arrays of nanodisks14 and electrodynamical simulations, we may estimate an effective decay length for the nanodisks to roughly a quarter of the diameter, that is, ld ≈ 30 nm. In the propagating plasmon case, the decay length is uniquely determined by the complex k-vector perpendicular to the metal surface. One then obtains a decay length of ld ≈ 246 nm at 706 nm, almost 10 times larger than that for the nanodisks. By assuming an exponential decay of the induced field, it is straightforward to derive a relation between the peak shift, the decay length, and the bulk refractive index sensitivity, m, according to29 ∆λ ) m∆n(e-2d/ld - e-2(d+b)/ld)

(4)

where ∆n is the difference in refractive index between the adsorbed layer and the bulk medium and d and b are the thicknesses of the first and second layers, respectively. Further, the de Feijters formula, M ) d∆n /0.182 cm3 g-1, relates the mass density of proteins in the layers to the refractive index contrast ∆n for a given layer thickness. By using these relations, one may roughly estimate biomolecular mass that is bound during the different reaction steps. We find that the resonance shifts from bBSA injection correspond to a detected mass density of about 170 ng/cm2 on the gold surfaces of the nanodisks and about 110 ng/cm2 on the planar gold film. This is in line with the kinetics curves, which show that the SPR shift has not leveled out even after BSA adsorption, as would be the case if the surface was not completely covered in a monolayer. Accordingly, the detected mass density after the second step was 150 ng/cm2, 4432

which is still lower than the mass density level at which saturation occurred on the nanodisks. As bBSA does not absorb well on SiO2, the LSPR shift due to BSA adsorption is interpreted as BSA binding primarily to the glass substrate and perhaps in some smaller amount to the metal surfaces not previously covered by bBSA. The SA binding to biotin in the last step of the biorecognition process produced very similar resonance shifts for SPR and LSPR. These shifts correspond to 48 and 12 ng/cm2 SA bound to the biotin, corresponding to a streptavidin-bBSA ratio of 36 and 14% on the nanodisks and on the gold film, respectively. Since the gold surfaces of the nanoparticles only correspond to about one-fourth of the total sensor area, this suggests that the amount of net bound SA is similar for the two sensing platforms and, therefore, that SA accumulated on the gold nanodisks. On the other hand, the SA-bBSA ratios that correspond to the aforementioned mass densities are surprisingly low, only 36 and 14% for the nanodisk and the flat film surface, respectively. This could be due to a substantial exchange between bBSA and BSA during the second reaction step, thus reducing the surface density of biotin binding sites. Further studies of the differences between biomolecular adsorption to the flat and nanostructured sensor surface go beyond the scope of the present work but are clearly important for the widespread application of LSPR sensing technology. Summary and Concluding Remarks. We have presented a comprehensive and unbiased experimental comparison between the refractive index sensitivity of propagating surface plasmons, that is, classical wavelength resolved SPR sensing, and localized surface plasmon resonances (LSPRs) for the case of gold. The comparison was performed using essentially identical experimental conditions and based on macroscopic sampling areas. We tuned the experiment to reach the same plasmon resonance wavelength, ∼700 nm, for SPR and LSPR. This ensures that the wavelengthdependent dielectric function of the metal influences the sensing characteristics of the two methods in the same way. Nano Lett., Vol. 9, No. 12, 2009

In essence, the result of the comparison is that LSPR and SPR exhibit very similar sensing performance in terms of signal-to-noise and wavelength shifts for realistic biosensing experiments, despite a ∼20 times higher bulk RI sensitivity and FoM for SPR. This difference can be understood from the much longer field decay length of propagating plasmons compared to the localized mode.38 We should point out that the comparison was made between a more or less ideal SPR structure, that is, a 50 nm film, and gold nanodisks that exhibit a far from optimal FoM ≈ 2. It is thus possible that further optimization of the nanostructured surface, for example, with respect to the fwhm of the resonance, could substantially improve the LSPR sensing characteristics. With the results obtained here in mind, one may ask what the pros and cons of LSPR sensing are compared to the established SPR technology. One obvious disadvantage is the higher costs of nanostructuring compared to thin film deposition, although this difference can be expected to diminish with the rise of massively parallel nanofabrication technologies, such as nanoimprint lithography. Further, the fact that LSPR sensing is generally based on isolated metal structures on a substrate has several implications for sensing. One potential advantage, indicated in the present experiments, is that biomolecules are concentrated on the nanostructures, leading to a faster response for LSPR compared to that for SPR, although this requires that unspecific binding to the dielectric substrate be minimal. The latter generally requires specific substrate passivation strategies that are not needed in SPR sensing. On the other hand, the presence of a substrate can also be utilized to realize sensing structures, for example, lipid bilayers, that are difficult to achieve for a uniform gold surface. As mentioned in the introduction, a clear advantage of LSPR is the low bulk refractive index sensitivity, implying that temperature stabilization is less important, and the simple measurement technology needed, that is, straightforward transmission or, as shown here, reflection, compared to excitation through a prism. These advantages are probably utilized best in simple and cheap sensing equipment, for example, for environmental monitoring or point of care diagnostics. Acknowledgment. We are grateful to Fredrik Ho¨o¨k for a critical reading of a previous version of this paper. We acknowledge Linda Gunnarsson and Bjo¨rn Brian for stimulating discussions and experimental assistance. This work was financially supported by Vinnova, the Swedish Foundation for Strategic Research and the Swedish Research Council. References (1) Liedberg, B.; Nylander, C.; Lundstrom, I. Biosens. Bioelectron. 1995, 10 (8), R1-R9. (2) Homola, J. Surface Plasmon Resonance Based Sensors; Springer: Berlin, Germany, 2006. (3) Piliarik, M.; Parova, L.; Homola, J. Biosens. Bioelectron. 2009, 24 (5), 1399–1404. (4) Shankaran, D.; Gobi, K.; Sakai, T.; Matsumoto, K.; Imato, T.; Toko, K.; Miura, N. IEEE Sens. J. 2005, 5 (4), 616–621.

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