Development of a complete plasmonic grating-based sensor and its application for self-assembled monolayer detection M. Perino,1,* E. Pasqualotto,2 A. De Toni,1 D. Garoli,3,4 M. Scaramuzza,1 P. Zilio,3 T. Ongarello,4 A. Paccagnella,1 and F. Romanato3,5 1
Department of Information Engineering, University of Padova, via Gradenigo 6/B, Padova, Italy 2
Department of Biomedical Sciences, University of Padova, via Ugo Bassi 58/B, Padova, Italy 3
Department of Physics, University of Padova, via Marzolo 8, Padova, Italy
4
LaNN Laboratory for Nanofabrication of Nanodevices, CorsoStatiUniti 4, Padova, Italy
5
IOM CNR TASC Laboratory, Area Science Park, S.S. 14 Km 163.5, Basovizza Trieste, Italy *Corresponding author: [email protected] Received 4 August 2014; revised 4 August 2014; accepted 5 August 2014; posted 6 August 2014 (Doc. ID 207586); published 8 September 2014
This work presents an integrated plasmonic biosensing device consisting of a one-dimensional metallic lamellar grating designed to exploit extraordinary transmission of light toward an underlying silicon photodetector. By means of finite element simulations, the grating parameters have been optimized to maximize the light transmission variation induced by the functionalization of the gold nanostructures. An optimized grating was fabricated using an electron beam process and an optoelectronic test bench suitable for sample tests was developed. A clear difference in the grating transmitted light due to surface functionalization was observed in presence of TM polarized illumination. © 2014 Optical Society of America OCIS codes: (250.5403) Plasmonics; (280.1415) Biological sensing and sensors; (130.0250) Optoelectronics. http://dx.doi.org/10.1364/AO.53.005969
1. Introduction
The first demonstration of surface plasmon resonance (SPR) for sensing dates back to the early 1980s [1]. SPR sensors have since improved in terms of technology and performance and have a wide spectrum of potential applications [2]. During the last two decades, label-free biosensors based on SPR have been developed to detect biological analytes such pathogens, toxins, or DNA sequences [3,4]. These biosensors present high sensitivity and resolution.
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Different configurations can be used to excite SPR: prism coupling [5,6], waveguides [7–9], or periodic nanostructures [10,11]. Among these configurations, prism coupling is the most widely used. On the other hand, large area periodic structures, such as onedimensional gratings or nanohole arrays, offer specific advantages: They are easy to fabricate, may feature low-cost production, and may be successfully integrated into the microfluidic components of point-of-care diagnostic devices. Most of the sensing techniques previously listed are not suitable for point-of-care systems, since the optical setup used to detect the signal is usually cumbersome and can be operated only by a well-trained operator. Various research groups [12–15] recently 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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have proposed a way to solve these problems by integrating a photodetector into the plasmonic structure substrate. In this way, they have measured the biological coupling events through a variation of the photodetector current. Hence, the integrated photodetector may effectively sense the far field transmittance of the plasmonic structures as well as its near field component. Detecting the far field component (i.e., the zeroorder diffracted ray intensity) is the most common choice to sense the biological events [16–19]. Various authors have illuminated a plasmonic grating by a monochromatic light [20,21], measuring the zeroorder transmittance as a function of the incident light wavelength. Others have analyzed the zeroorder ray spectrum [22–26] by using a spectrophotometer, recording the transmittance variations due to surface biological events at suitable wavelengths. In this paper, we have proposed what we believe is an original solution that merges these two approaches and aims to improve the detection of surface biological layers. We directly detect the zeroorder diffracted ray intensity through a commercial photodiode array, by illuminating the plasmonic structures with a semiconductor laser. The photodiode array is in contact with the plasmonic structure substrate and no diffracted rays propagate in air, leading to a compact detection setup properly working at the chosen monochromatic laser light. More specifically, we measure the intensity of the zero-order diffracted ray, transmitted by a gold digital grating, as a function of the incoming light polarization and under normal incidence condition. To test the detection properties of our setup, we have also deposited a suitable self-assembled monolayer (SAM) over the gold grating, and measured the light intensity variations after such surface functionalization. We have chosen a gold digital grating as the plasmonic structure thanks to its ability to support different resonances [18,27–29]. High-quality gratings have been produced using a nanofabrication process based on the electron beam lithography. These devices are mounted on a simple, low-cost optoelectronic bench. Parallel to our experimental work, we have performed numerical simulations through the finite element method (FEM) to find the optimized geometrical parameters maximizing the light transmittance variation through the substrate before and after functionalization. The laser polarization state is the only varying parameter of our experimental setup. The experimental results prove the capability of our setup to effectively detect a SAM and comparison to simulated data confirms the effectiveness of our approach. 2. Materials and Methods A.
Simulation
Two-dimensional finite elements simulations allow us to predict the grating optical behavior and to find 5970
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Fig. 1. Layout of the complete detection system: (a) corresponding near field simulation of the transverse magnetic (TM) field norm map A/m (H is parallel to the z axis) for a grating with a period of 628 nm, a gold height of 290 nm, a duty cycle of 0.4, lighted with a 635 nm wavelength (b).
approximate solutions for the electromagnetic fields in the grating area. In particular, we have used the COMSOL Multiphysics software, version 3.5a. Figure 1(a) (not to scale) shows the x–y crosssection of the unit cell of our detection system. It consists of a 1D gold digital grating over an ITO/glass substrate, which is coupled to a photodiode array through optical matching oil. The grating optical response is modeled by setting proper Floquet periodic boundary conditions at the unit cell-side boundaries Fig. 1(b). We assumed the grating was infinitely extended over the z direction. Perfectly matched layers (PML) are placed above and below the grating to absorb the reflected and transmitted light. This configuration permits us to simulate both normal and oblique incident light. The model simulates only the grating near field response. The far field information (i.e., the transmittance of the diffracted orders) is calculated via a Fourier transform of the field in the substrate [28], since it can be developed into a Rayleigh expansion. The transverse magnetic (TM) and transverse electric (TE) polarization modes correspond to the magnetic field and the electric field component parallel to the z axis, respectively. The gold grating is schematized by a rectangle inside the unit’s periodic cell. The rectangle height corresponds to the height of the gold grating. A second parameter useful to describe the system (i.e., the duty cycle) corresponds to the ratio between the slits width and the period. The functionalization layer, describing the SAM absorbed on the gold grating, has been modeled as a 1.5 nm thick homogenous dielectric layer on the gold surface, with the refractive index set to 1.45 [30].
The light, transmitted by the grating propagates through the indium tin oxide layer (ITO) and the BK7 glass, which is optically matched to the photodiodes’ protective layer. The photodiodes linearly convert the transmitted light into an electrical current. We define the transmittance variation of our setup as ΔT 0 T 0;func − T 0;bare , where T 0 represents the zero-order diffracted ray intensity and the subscript indicates the grating surface condition. The bare grating condition is simulated by setting the functionalization layer refractive index equal to 1 RIU, while it is 1.45 RIU for the SAM functionalization, as previously quoted. We have analyzed the grating geometrical parameters (i.e., the period and the gold height) to optimize the figure of merit ΔT 0 , as we will later show in Section 3.A. B.
Grating Fabrication
The fabrication process has several different steps. The samples were prepared starting from ITOcoated glass substrates, spun with a 490 nm thick poly(methyl methacrylate), or PMMA, resist layer. The resist was soft baked at 180°C on a hot plate for 10 min. Then we performed an electron beam lithography (EBL) [27] step using a JEOL EB6300FS EBL system operating at 100 keV, with 2 nA current, up to a dose of 800 μC∕cm2 during the exposure. The exposed pattern was corrected to prevent proximity effects by using Layout BEAMER software from GenISys GmbH. The proximity correction enabled the pattern geometry to be uniformly transferred to a large area (3 mm × 3 mm) including the sample edges. After the exposure, the resist was developed in a deionized water (isopropyl alcohol 3∶7 solution) at room temperature for 20 s. The metallic structures (Fig. 2) were electrolytically grown on the ITO layer using the commercial solution Karatclad 265 HS, with a PH 4.5. The other deposition parameters were: temperature 36°C, metallic content Au 8 g∕l, density 15°B´e, growth current 100 mA, and applied voltage 2.9 V. C.
Test Bench and Measurements
We developed a test bench (Fig. 3) to detect the zeroorder grating transmittance T 0 as a function of the light polarization, for both the bare and the functionalized grating conditions. The light source is a
Fig. 2. Scanning electron microscope micrographs of the fabricate gratings: (a) upper view and (b) tilted view.
Fig. 3. Schematic of the optical test bench. Inset: image of the optical test bench.
635 nm wavelength laser (Edmund VHK, beam diameter of 1.1 mm, 4.9 mW output power), fixed on a suitable holder (SL 20 model, Thorlabs). The laser light polarization is controlled by a linear polarizer and by a half-wave plate mounted on a motorized rotation stage. By rotating the half-wave plate we were able to switch the polarization state between the TM and TE modes. We used a commercial photodiode array Hamamatsu S4111Q (16 elements) suitable for detecting low light levels with high sensitivity (namely, 0.43 A∕W at 635 nm light wavelength). The active area of a single photodiode is 1.305 mm2 and the spectral response ranges between 190 and 1100 nm. Each photodiode is individually addressable by a switch/control unit HP 3488A connected to a semiconductor parameter analyzer HP 4156. This instrument biases the photodiode array at −2 V and measures the corresponding current. The instrument chain is controlled by custom LabVIEW software, providing data acquisition and storage. The photodiode array was mounted on a goniometer (GNL18 model, Thorlabs) to accurately set the normal incident condition between the laser beam and the grating plane. The goniometer was mounted on a two-axis stage (ThorlabsDT25/M, Edmund Optics 56-795) to align the laser spot with respect to the grating zone. After the fabrication, we cleaned the grating by dipping it into a piraña solution (H2 O∶H2 O2 ∶ NH4 OH 5∶1∶1 at 50°C) and then carefully rinsing it with ethanol [24]. After this process, we characterized the grating bare condition at the ellipsometer (J.A. Woollam Co VASE). In this way we measured the T 0 spectrum under normal incidence using a TM polarized light, with a resolution of 2 nm. The grating was then mounted on the photodiode array via a custom socket and was optically matched to the fused silica photodiodes protective layer using an optical matching oil (Cargille Immersion Oil Type A) to avoid the refractive index variation between the two media. The T 0 bare condition 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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as a function of the polarization angle was then measured. In our experimental setup the unwanted diffracted rays were not sensed because of the structure of the measurement bench. In fact, the grating is at least 2.5 mm above the photodiode array, the diffracted rays propagates at 42° with respect to the normal of the grating plane and the cross-section between the sensitive area of the photodiode array, and the plane defined by these two diffracted rays is 1.5 mm wide. This configuration prevents the diffracted rays from reaching the photodiodes active area, only T 0 was measured. The photodiode current obtained with a direct laser illumination (without grating) was used to normalize T 0 independently of the polarization angle. After the full experimental characterization of T 0, we functionalized the grating overnight with dodecanethiol (HSCH2 CH2 10 CH3 ) in glycerol solution 10 mM [31]. The grating was then been repeatedly rinsed with ethanol and water and dried in air. After the functionalization process, we fully measured T 0 again as a function the polarization angle Finally, by using the ellipsometer, we measured the T 0 spectrum of the functionalized grating state under normal incidence for light in the TM mode.
grating parameters. The zero-order transmittance spectrum shows the typical features of a digital grating structure, and in particular two Wood–Rayleigh (WR) anomalies, the excitation of surface plasmon polaritons (SPP) at gold/air interface and a cavity mode (CM) [28]. The first WR anomaly occurs at 628 nm (see Fig. 4(a) for magnetic field norm map), when the incident light wavelength equals the grating period. The second one occurs at 942 nm and it is related to the presence of two diffracted rays propagating parallel to the grating/glass substrate interface. The transmittance minimum that occurs at 654 nm is due to the SPP excitation at the gold/air interface (Fig. 4(b)). This excitation causes absorption of the input light, corresponding to a transmittance drop. The transmittance maximum is produced by the excitation of a CM (Fig. 4(c)), consisting of light channeled through the grating slits [27,28]. Figure 5(a) shows the values of ΔT 0 as a function of the grating period and gold grating height, considering a duty cycle of 0.4 and a wavelength of 635 nm, under normal incidence and with TM polarized light. This map shows that ΔT 0 is maximized when the
3. Results and Discussion A.
Simulation Results
Figure 4 reports the simulated T 0 spectrum of the bare grating for normally impinging TM polarized light. The figure’s insets show the magnetic field norm map calculated for different wavelengths. In these simulations, a period of 628 nm, a gold grating height of 290 nm and a duty cycle of 0.4 were used as
Fig. 4. Simulated zero-order transmittance (T 0 ) spectra for a digital grating (628 nm period, 290 nm gold height and 0.4 duty cycle) under normal incidence condition and illuminated by TM polarized light. The insets show the maps of the magnetic field norm A/m of the grating calculated for different wavelengths: (a) 628 nm, (b) 654 nm, and (c) 1002 nm. 5972
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Fig. 5. (a) Values of ΔT 0 parameter for a grating having duty cycle 0.4, lighted under normal incidence condition by a 635 nm light in TM polarization mode, as a function of the grating gold height and the period. (b) Simulated T 0 spectra for bare (black curve) and functionalized (blue curve) grating condition. Red curve reports the ΔT 0 parameter as a function of the wavelength. All the curves refer to the grating having the optimized geometry and lighted under normal incidence condition and in TM polarization mode.
grating has a period of 628 nm and the gold height is 290 nm. In Fig. 5(b), we show the T 0 spectra calculated for the bare (black curve) and the functionalized (blue curve) conditions and the corresponding value of ΔT 0 (red curve) for the optimized grating geometry. The SAM deposited on the gold layer induces a 2 nm shift of the transmittance minimum related to the SPP excitation, while a maximum of ΔT 0 0.01 a.u. was found at 635 nm wavelength. Similar simulations performed under TE condition do not show any resonance; hence, we will not further consider the TE polarization in future work. Since the ΔT 0 maximum lies between the WR anomaly and the SPP excitation, we have studied this spectral region to better understand the physical behavior of our system. Figure 6 shows T 0 for the optimized grating geometry as a function of the light wavelength and incident angle θ, for the TM polarization. There are two main optical features: the WR anomalies represented by the T 0 maxima (black circles) and the SPP excitations represented by the T 0 minima (black squares). WR anomalies correspond to configurations where a diffracted order propagates parallel to the grating plane when this equation is satisfied: ηd k0 ηd k0 sin θ 2π∕Λ. Here Λ is the period of the grating and k0 2π∕λ is the light vacuum wave vector; λ is the vacuum wavelength; ηd is the refractive index of the medium surrounding the grating; and θ is the light incidence angle. The SPP resonances occur when the sum of the x components of the input light wave momentum and
the grating momentum equals the value of the SPP momentum when this equation is satisfied: kspp ηd k0 sin θ 2π∕Λ: p Here kSPP k0 εd εm ∕εd εm , where, εd η2d is the relative permittivity of the dielectric around the gold and εm is the gold relative permittivity. The T 0 map reported in Fig 6 shows that, for a fixed incident angle, WR anomalies always arise at lower wavelengths with respect to the SPP excitations. This is due to the fact that ηd k0 < kspp . The great difference between the two resonances is correlated to the different excitation natures. SPP corresponds to a y—evanescent field, since the y— momentum is described by an imaginary number, while the WR anomalies take place when two diffracted waves are radiated with the y—momentum component equal to zero. This abrupt change in the behavior of the reflected ray (that is, from the evanescent to the radiated case) produces the steep T 0 decrease observed for instance in Fig. 5(b) in the region between 630 and 640 nm. This difference in the reflected field is evident in the inset Figs. 4(a) and 4(b), where we show the magnetic field norm for wavelength of 628 and 654 nm, respectively. The functionalization process modifies the coupling constant and the SPP minimum shifts toward greater wavelengths, as illustrated in Fig. 7. Figure 7(a) shows how the spectrum minimum shifts for different thicknesses of the functionalized layer (RIU 1.45). Owing to the SPP excitation, only the spectrum minimum undergoes a shift, unlike the WR anomalies. The inset highlights the linear dependence of the spectrum minima from the overlayer thickness (black circle). The red straight line is the best fit of the reported data, featuring an angular coefficient of 1.41 nmλ ∕nmh. This coefficient permits to predict the spectrum minimum shift for our SAM overlayer (dodecanethiol), that is, 2 nm relative to a 1.5 nm SAM thickness [30,31]. Figure 7(b) shows that both resonances, WR maxima and SPP minima, shift toward greater wavelengths at an increasing dielectric refractive index. The inset illustrates the spectra minimum position as a function of the refractive index variation, featuring in this case an angular coefficient of 608 nm∕RIU. The wavelength shift due to the functionalization layer that takes place near the SPP excitation implies that the overall enhancement of transmittance variation can be approximated as: ΔT 0 ≈ −∂T 0 ∕∂λΔλ;
Fig. 6. Map of the T 0 transmittance as a function of incident angle θ and light wavelength λ, black circles show the position of Wood–Rayleigh anomalies and black squares show the position of surface plasmon polaritons excitations.
where ∂T 0 ∕∂λ is the slope of the T 0 spectra at 635 nm wavelength and Δλ is the spectrum shift due to the functionalization. 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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Fig. 7. Spectra shift increases with the functionalization layer thickness different colors represent hfunc going from 0.5 nm (blue curve) to 8.5 nm (red curve) in step of 2 nm. The inset shows the relation between the spectra minima position and the functionalization thickness hfunc (a). Spectra shift increases with the refractive index of dielectric, different colors represent Δη going from 0 RIU (blue curve) to 0.016 RIU (red curve) in step of 0.004 RIU. The inset shows the relation between the spectra minima position and the medium refractive index variation with respect to the air one Δη (b).
B.
Experimental Results
Figure 8(a) compares the transmittance spectra of the bare and functionalized gratings, relative to the experimental (dotted line) and simulated (continuous line) results. The transmittance minimum shifts by 2 nm toward longer wavelengths, while the WR anomaly does not significantly change its spectral position. The T 0 is higher in the functionalized than in the bare grating configuration at 635 nm, and generally in the spectral region between 630 and 650 nm. Figure 8(b) compares the experimental ΔT 0 versus the simulated one. Both curves exhibit a maximum for wavelengths between 634 and 638 nm. In general, Figs. 8(a) and 8(b) display good agreement between simulations and experimental data, at least for wavelengths longer than 634 nm. For wavelengths between 620 and 630 nm, some differences appear between the experimental and simulated spectra, with a maximum disagreement at 625 nm Fig. 8(a). This difference could be ascribed to some mismatches between the actual fabricated grating and the simulated one. In fact, if we used a duty cycle of 0.43 instead of 0.4, we obtained the simulated results reported in the inset of Fig. 8(a) (dashed black 5974
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Fig. 8. (a) Experimental T 0 spectra measured for the bare (dotted black) and the functionalized (dotted blue) grating and the simulated bare and functionalized condition (continuous black and blue line, respectively). The inset compares the bare experimental (dotted black) with the bare simulated results (dashed line) obtained by varying the grating parameters (duty cycle). (b) Comparison between the experimental ΔT 0 (dotted red) and the simulated one (continuous red line).
line), which are in good agreement with the experimental results (dotted black line). Figure 9 shows the grating zero-order transmittance as a function of the incoming polarization angle, experimentally measured with the optical test bench. We observed a transmittance variation of ΔT 0 0.01 a.u. between the bare and the functionalized condition only when input light polarization
Fig. 9. T 0 as a function of the polarization angle for the bare and the functionalized gratings, measured with our experimental set up. Here, 0° refers to the TM polarization mode and 90° refers to the TE polarization mode.
was in TM mode. For comparison, no substantial changes in the transmittance were measured for the TE mode. The error bars reported in Fig. 9 are mainly due to the semiconductor laser intensity drift. To compensate this drift, we acquired 50 measurements of the zero-order transmittance as a function of the polarization angle for each test. Moreover, we can estimate the sensitivity and the resolution of the developed grating-based optoelectronic system. From the simulations, we calculated that a shift of 2 nm in the SPR minimum corresponds to an effective refraction index variation of 0.0034 RIU. The sensitivity can be estimated in terms of ΔT∕RIU and a value of 2.94 a.u./RIU was correspondingly found, which is comparable with that reported by Stewart and Mack [21]. The resolution of our sensor can be estimated by considering the error bars in Fig. 9. Since the error bars value is 6.6 × 10−4 a.u., resolution is 2.2 × 10−4 RIU, which is higher than in other types of sensors, such as those prismor grating-based, where a resolution of 10−7 RIU can be usually reached [3]. Our resolution could be further improved by using a laser featuring higher stability. 4. Conclusions
A biosensing system based on a silicon photodetector coupled with a digital metallic plasmonic wire grating was developed to detect variations of the transmitted light signal due to SAM adsorption. The proposed system transduces the photons transmitted by the grating into a current signal, thus reducing the overall optical bench complexity. Fabricating the grating onto the passivation layer of photodiode sensitive area could further shrink this detection system. Through numerical simulations, we optimized the plasmonic grating geometrical parameters to maximize the transmittance variation due to the surface refractive index changes. We fabricated a gold digital grating by using electron beam lithography, and we designed and successfully assembled a compact, efficient optoelectronic test bench. We have experimentally tested the system prototype to prove its capability to detect surface variations due to a dodecanethiol SAM formation. The experimental results of transmittance variation are in good agreement with the simulations. The proposed system presents a simple measurement protocol suitable for multiplexing and miniaturization. Its ability to detect a dodecanethiol SAM, which is much thinner than proteins or a DNA layer and is normally used as a detection benchmark, encourages further work on such systems. These systems could be used for biorecognition applications, and are worth investigating for integration into a custom-developed microfluidic device. In fact, the same approach described here also could be used for a liquid dielectric medium, obtained, for instance, by fluxing liquid onto the grating through
a microfluidic channel. Structures different from the 1D digital gratings we used, such as a nanohole rectangular array, may noticeably improve the system resolution and sensitivity, as demonstrated by Dionne and co-workers [13,22], even in case of microfluidic applications. This work was supported by a grant from “Fondazione Cariparo”—Surface PLasmonics for Enhanced Nano Detectors and Innovative Devices (SPLENDID)—Progetto Eccellenza 2008. References 1. B. Liedberg, C. Nylander, and I. Lunström, “Surface plasmon resonance for gas detection and biosensing,” Sens. Actuators 4, 299–304 (1983). 2. J. Homola, “Surface plasmon resonance sensors for detection of chemical and biological species,” Chem. Rev. 108, 462–493 (2008). 3. J. Homola, S. S. Yee, and G. Gauglitz, “Surface plasmon resonance sensors: review,” Sens. Actuators B 54, 3–15 (1999). 4. X. D. Hoa, A. G. Kirk, and M. Tabrizian, “Towards integrated and sensitive surface plasmon resonance biosensors: a review of recent progress,” Biosens. Bioelectron. 23, 151–160 (2007). 5. A. Otto, “Excitation of surface plasma waves in silver by the method of frustrated total reflection,” Z. Phys. 216, 398–410 (1968). 6. E. Kretschmann and H. Raether, “Radiative decay of non radiative surface plasmons excited by light,” Zeitschrift für Naturforsch. 23a, 2135–2136 (1968). 7. R. C. Jorgenson and S. S. Yee, “A fiber-optic chemical sensor based on surface plasmon resonance,” Sens. Actuators B 12, 213–220 (1993). 8. D. Brennan, J. Justice, B. Corbett, T. McCarthy, and P. Galvin, “Emerging optofluidic technologies for point-of-care genetic analysis systems: a review,” Anal. Bioanal. Chem. 395, 621–636 (2009). 9. H. Li and X. Fan, “Characterization of sensing capability of optofluidic ring resonator biosensors,” Appl. Phys. Lett. 97, 011105 (2010). 10. H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings (Springer-Verlag, 1988). 11. W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424, 824–830 (2003). 12. F. Mazzotta, G. Wang, C. Hägglund, F. Höök, and M. P. Jonsson, “Nanoplasmonic biosensing with on-chip electrical detection,” Biosens. Bioelectron. 26, 1131–1136 (2010). 13. L. Guyot, A.-P. Blanchard-Dionne, S. Patskovsky, and M. Meunier, “Integrated silicon-based nanoplasmonic sensor,” Opt. Express 19, 9962–9967 (2011). 14. D. Sammito, D. De Salvador, P. Zilio, G. Biasiol, T. Ongarello, M. Massari, G. Ruffato, M. Morpurgo, D. Silvestri, G. Maggioni, G. Bovo, M. Gaio, and F. Romanato, “Integrated architecture for the electrical detection of plasmonic resonances based on high electron mobility photo-transistors,” Nanoscale 6, 1390–1397 (2014). 15. B. Turker, H. Guner, S. Ayas, O. O. Ekiz, H. Acar, M. O. Guler, and A. Dâna, “Grating coupler integrated photodiodes for plasmon resonance based sensing,” Lab Chip 11, 282–287 (2011). 16. J. Zhao, X. Zhang, C. R. Yonzon, A. J. Haes, and R. P. Van Duyne, “Localized surface plasmon resonance biosensors,” Nanomedicine 1, 219–228 (2006). 17. J. Ji, J. G. O’Connell, D. J. D. Carter, and D. N. Larson, “Highthroughput nanohole array based system to monitor multiple binding events in real time,” Anal. Chem. 80, 2491–2498 (2008). 18. A. Karabchevsky, O. Krasnykov, I. Abdulhalim, B. Hadad, A. Goldner, M. Auslender, and S. Hava, “Metal grating on a substrate nanostructure for sensor applications,” Photon. Nanostr. Fundam. Appl. 7, 170–175 (2009). 19. A. G. Brolo, R. Gordon, B. Leathem, and K. L. Kavanagh, “Surface plasmon sensor based on the enhanced light 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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Department of Information Engineering, University of Padova, via Gradenigo 6/B, Padova, Italy 2
Department of Biomedical Sciences, University of Padova, via Ugo Bassi 58/B, Padova, Italy 3
Department of Physics, University of Padova, via Marzolo 8, Padova, Italy
4
LaNN Laboratory for Nanofabrication of Nanodevices, CorsoStatiUniti 4, Padova, Italy
5
IOM CNR TASC Laboratory, Area Science Park, S.S. 14 Km 163.5, Basovizza Trieste, Italy *Corresponding author: [email protected] Received 4 August 2014; revised 4 August 2014; accepted 5 August 2014; posted 6 August 2014 (Doc. ID 207586); published 8 September 2014
This work presents an integrated plasmonic biosensing device consisting of a one-dimensional metallic lamellar grating designed to exploit extraordinary transmission of light toward an underlying silicon photodetector. By means of finite element simulations, the grating parameters have been optimized to maximize the light transmission variation induced by the functionalization of the gold nanostructures. An optimized grating was fabricated using an electron beam process and an optoelectronic test bench suitable for sample tests was developed. A clear difference in the grating transmitted light due to surface functionalization was observed in presence of TM polarized illumination. © 2014 Optical Society of America OCIS codes: (250.5403) Plasmonics; (280.1415) Biological sensing and sensors; (130.0250) Optoelectronics. http://dx.doi.org/10.1364/AO.53.005969
1. Introduction
The first demonstration of surface plasmon resonance (SPR) for sensing dates back to the early 1980s [1]. SPR sensors have since improved in terms of technology and performance and have a wide spectrum of potential applications [2]. During the last two decades, label-free biosensors based on SPR have been developed to detect biological analytes such pathogens, toxins, or DNA sequences [3,4]. These biosensors present high sensitivity and resolution.
1559-128X/14/265969-08$15.00/0 © 2014 Optical Society of America
Different configurations can be used to excite SPR: prism coupling [5,6], waveguides [7–9], or periodic nanostructures [10,11]. Among these configurations, prism coupling is the most widely used. On the other hand, large area periodic structures, such as onedimensional gratings or nanohole arrays, offer specific advantages: They are easy to fabricate, may feature low-cost production, and may be successfully integrated into the microfluidic components of point-of-care diagnostic devices. Most of the sensing techniques previously listed are not suitable for point-of-care systems, since the optical setup used to detect the signal is usually cumbersome and can be operated only by a well-trained operator. Various research groups [12–15] recently 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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have proposed a way to solve these problems by integrating a photodetector into the plasmonic structure substrate. In this way, they have measured the biological coupling events through a variation of the photodetector current. Hence, the integrated photodetector may effectively sense the far field transmittance of the plasmonic structures as well as its near field component. Detecting the far field component (i.e., the zeroorder diffracted ray intensity) is the most common choice to sense the biological events [16–19]. Various authors have illuminated a plasmonic grating by a monochromatic light [20,21], measuring the zeroorder transmittance as a function of the incident light wavelength. Others have analyzed the zeroorder ray spectrum [22–26] by using a spectrophotometer, recording the transmittance variations due to surface biological events at suitable wavelengths. In this paper, we have proposed what we believe is an original solution that merges these two approaches and aims to improve the detection of surface biological layers. We directly detect the zeroorder diffracted ray intensity through a commercial photodiode array, by illuminating the plasmonic structures with a semiconductor laser. The photodiode array is in contact with the plasmonic structure substrate and no diffracted rays propagate in air, leading to a compact detection setup properly working at the chosen monochromatic laser light. More specifically, we measure the intensity of the zero-order diffracted ray, transmitted by a gold digital grating, as a function of the incoming light polarization and under normal incidence condition. To test the detection properties of our setup, we have also deposited a suitable self-assembled monolayer (SAM) over the gold grating, and measured the light intensity variations after such surface functionalization. We have chosen a gold digital grating as the plasmonic structure thanks to its ability to support different resonances [18,27–29]. High-quality gratings have been produced using a nanofabrication process based on the electron beam lithography. These devices are mounted on a simple, low-cost optoelectronic bench. Parallel to our experimental work, we have performed numerical simulations through the finite element method (FEM) to find the optimized geometrical parameters maximizing the light transmittance variation through the substrate before and after functionalization. The laser polarization state is the only varying parameter of our experimental setup. The experimental results prove the capability of our setup to effectively detect a SAM and comparison to simulated data confirms the effectiveness of our approach. 2. Materials and Methods A.
Simulation
Two-dimensional finite elements simulations allow us to predict the grating optical behavior and to find 5970
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Fig. 1. Layout of the complete detection system: (a) corresponding near field simulation of the transverse magnetic (TM) field norm map A/m (H is parallel to the z axis) for a grating with a period of 628 nm, a gold height of 290 nm, a duty cycle of 0.4, lighted with a 635 nm wavelength (b).
approximate solutions for the electromagnetic fields in the grating area. In particular, we have used the COMSOL Multiphysics software, version 3.5a. Figure 1(a) (not to scale) shows the x–y crosssection of the unit cell of our detection system. It consists of a 1D gold digital grating over an ITO/glass substrate, which is coupled to a photodiode array through optical matching oil. The grating optical response is modeled by setting proper Floquet periodic boundary conditions at the unit cell-side boundaries Fig. 1(b). We assumed the grating was infinitely extended over the z direction. Perfectly matched layers (PML) are placed above and below the grating to absorb the reflected and transmitted light. This configuration permits us to simulate both normal and oblique incident light. The model simulates only the grating near field response. The far field information (i.e., the transmittance of the diffracted orders) is calculated via a Fourier transform of the field in the substrate [28], since it can be developed into a Rayleigh expansion. The transverse magnetic (TM) and transverse electric (TE) polarization modes correspond to the magnetic field and the electric field component parallel to the z axis, respectively. The gold grating is schematized by a rectangle inside the unit’s periodic cell. The rectangle height corresponds to the height of the gold grating. A second parameter useful to describe the system (i.e., the duty cycle) corresponds to the ratio between the slits width and the period. The functionalization layer, describing the SAM absorbed on the gold grating, has been modeled as a 1.5 nm thick homogenous dielectric layer on the gold surface, with the refractive index set to 1.45 [30].
The light, transmitted by the grating propagates through the indium tin oxide layer (ITO) and the BK7 glass, which is optically matched to the photodiodes’ protective layer. The photodiodes linearly convert the transmitted light into an electrical current. We define the transmittance variation of our setup as ΔT 0 T 0;func − T 0;bare , where T 0 represents the zero-order diffracted ray intensity and the subscript indicates the grating surface condition. The bare grating condition is simulated by setting the functionalization layer refractive index equal to 1 RIU, while it is 1.45 RIU for the SAM functionalization, as previously quoted. We have analyzed the grating geometrical parameters (i.e., the period and the gold height) to optimize the figure of merit ΔT 0 , as we will later show in Section 3.A. B.
Grating Fabrication
The fabrication process has several different steps. The samples were prepared starting from ITOcoated glass substrates, spun with a 490 nm thick poly(methyl methacrylate), or PMMA, resist layer. The resist was soft baked at 180°C on a hot plate for 10 min. Then we performed an electron beam lithography (EBL) [27] step using a JEOL EB6300FS EBL system operating at 100 keV, with 2 nA current, up to a dose of 800 μC∕cm2 during the exposure. The exposed pattern was corrected to prevent proximity effects by using Layout BEAMER software from GenISys GmbH. The proximity correction enabled the pattern geometry to be uniformly transferred to a large area (3 mm × 3 mm) including the sample edges. After the exposure, the resist was developed in a deionized water (isopropyl alcohol 3∶7 solution) at room temperature for 20 s. The metallic structures (Fig. 2) were electrolytically grown on the ITO layer using the commercial solution Karatclad 265 HS, with a PH 4.5. The other deposition parameters were: temperature 36°C, metallic content Au 8 g∕l, density 15°B´e, growth current 100 mA, and applied voltage 2.9 V. C.
Test Bench and Measurements
We developed a test bench (Fig. 3) to detect the zeroorder grating transmittance T 0 as a function of the light polarization, for both the bare and the functionalized grating conditions. The light source is a
Fig. 2. Scanning electron microscope micrographs of the fabricate gratings: (a) upper view and (b) tilted view.
Fig. 3. Schematic of the optical test bench. Inset: image of the optical test bench.
635 nm wavelength laser (Edmund VHK, beam diameter of 1.1 mm, 4.9 mW output power), fixed on a suitable holder (SL 20 model, Thorlabs). The laser light polarization is controlled by a linear polarizer and by a half-wave plate mounted on a motorized rotation stage. By rotating the half-wave plate we were able to switch the polarization state between the TM and TE modes. We used a commercial photodiode array Hamamatsu S4111Q (16 elements) suitable for detecting low light levels with high sensitivity (namely, 0.43 A∕W at 635 nm light wavelength). The active area of a single photodiode is 1.305 mm2 and the spectral response ranges between 190 and 1100 nm. Each photodiode is individually addressable by a switch/control unit HP 3488A connected to a semiconductor parameter analyzer HP 4156. This instrument biases the photodiode array at −2 V and measures the corresponding current. The instrument chain is controlled by custom LabVIEW software, providing data acquisition and storage. The photodiode array was mounted on a goniometer (GNL18 model, Thorlabs) to accurately set the normal incident condition between the laser beam and the grating plane. The goniometer was mounted on a two-axis stage (ThorlabsDT25/M, Edmund Optics 56-795) to align the laser spot with respect to the grating zone. After the fabrication, we cleaned the grating by dipping it into a piraña solution (H2 O∶H2 O2 ∶ NH4 OH 5∶1∶1 at 50°C) and then carefully rinsing it with ethanol [24]. After this process, we characterized the grating bare condition at the ellipsometer (J.A. Woollam Co VASE). In this way we measured the T 0 spectrum under normal incidence using a TM polarized light, with a resolution of 2 nm. The grating was then mounted on the photodiode array via a custom socket and was optically matched to the fused silica photodiodes protective layer using an optical matching oil (Cargille Immersion Oil Type A) to avoid the refractive index variation between the two media. The T 0 bare condition 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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as a function of the polarization angle was then measured. In our experimental setup the unwanted diffracted rays were not sensed because of the structure of the measurement bench. In fact, the grating is at least 2.5 mm above the photodiode array, the diffracted rays propagates at 42° with respect to the normal of the grating plane and the cross-section between the sensitive area of the photodiode array, and the plane defined by these two diffracted rays is 1.5 mm wide. This configuration prevents the diffracted rays from reaching the photodiodes active area, only T 0 was measured. The photodiode current obtained with a direct laser illumination (without grating) was used to normalize T 0 independently of the polarization angle. After the full experimental characterization of T 0, we functionalized the grating overnight with dodecanethiol (HSCH2 CH2 10 CH3 ) in glycerol solution 10 mM [31]. The grating was then been repeatedly rinsed with ethanol and water and dried in air. After the functionalization process, we fully measured T 0 again as a function the polarization angle Finally, by using the ellipsometer, we measured the T 0 spectrum of the functionalized grating state under normal incidence for light in the TM mode.
grating parameters. The zero-order transmittance spectrum shows the typical features of a digital grating structure, and in particular two Wood–Rayleigh (WR) anomalies, the excitation of surface plasmon polaritons (SPP) at gold/air interface and a cavity mode (CM) [28]. The first WR anomaly occurs at 628 nm (see Fig. 4(a) for magnetic field norm map), when the incident light wavelength equals the grating period. The second one occurs at 942 nm and it is related to the presence of two diffracted rays propagating parallel to the grating/glass substrate interface. The transmittance minimum that occurs at 654 nm is due to the SPP excitation at the gold/air interface (Fig. 4(b)). This excitation causes absorption of the input light, corresponding to a transmittance drop. The transmittance maximum is produced by the excitation of a CM (Fig. 4(c)), consisting of light channeled through the grating slits [27,28]. Figure 5(a) shows the values of ΔT 0 as a function of the grating period and gold grating height, considering a duty cycle of 0.4 and a wavelength of 635 nm, under normal incidence and with TM polarized light. This map shows that ΔT 0 is maximized when the
3. Results and Discussion A.
Simulation Results
Figure 4 reports the simulated T 0 spectrum of the bare grating for normally impinging TM polarized light. The figure’s insets show the magnetic field norm map calculated for different wavelengths. In these simulations, a period of 628 nm, a gold grating height of 290 nm and a duty cycle of 0.4 were used as
Fig. 4. Simulated zero-order transmittance (T 0 ) spectra for a digital grating (628 nm period, 290 nm gold height and 0.4 duty cycle) under normal incidence condition and illuminated by TM polarized light. The insets show the maps of the magnetic field norm A/m of the grating calculated for different wavelengths: (a) 628 nm, (b) 654 nm, and (c) 1002 nm. 5972
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Fig. 5. (a) Values of ΔT 0 parameter for a grating having duty cycle 0.4, lighted under normal incidence condition by a 635 nm light in TM polarization mode, as a function of the grating gold height and the period. (b) Simulated T 0 spectra for bare (black curve) and functionalized (blue curve) grating condition. Red curve reports the ΔT 0 parameter as a function of the wavelength. All the curves refer to the grating having the optimized geometry and lighted under normal incidence condition and in TM polarization mode.
grating has a period of 628 nm and the gold height is 290 nm. In Fig. 5(b), we show the T 0 spectra calculated for the bare (black curve) and the functionalized (blue curve) conditions and the corresponding value of ΔT 0 (red curve) for the optimized grating geometry. The SAM deposited on the gold layer induces a 2 nm shift of the transmittance minimum related to the SPP excitation, while a maximum of ΔT 0 0.01 a.u. was found at 635 nm wavelength. Similar simulations performed under TE condition do not show any resonance; hence, we will not further consider the TE polarization in future work. Since the ΔT 0 maximum lies between the WR anomaly and the SPP excitation, we have studied this spectral region to better understand the physical behavior of our system. Figure 6 shows T 0 for the optimized grating geometry as a function of the light wavelength and incident angle θ, for the TM polarization. There are two main optical features: the WR anomalies represented by the T 0 maxima (black circles) and the SPP excitations represented by the T 0 minima (black squares). WR anomalies correspond to configurations where a diffracted order propagates parallel to the grating plane when this equation is satisfied: ηd k0 ηd k0 sin θ 2π∕Λ. Here Λ is the period of the grating and k0 2π∕λ is the light vacuum wave vector; λ is the vacuum wavelength; ηd is the refractive index of the medium surrounding the grating; and θ is the light incidence angle. The SPP resonances occur when the sum of the x components of the input light wave momentum and
the grating momentum equals the value of the SPP momentum when this equation is satisfied: kspp ηd k0 sin θ 2π∕Λ: p Here kSPP k0 εd εm ∕εd εm , where, εd η2d is the relative permittivity of the dielectric around the gold and εm is the gold relative permittivity. The T 0 map reported in Fig 6 shows that, for a fixed incident angle, WR anomalies always arise at lower wavelengths with respect to the SPP excitations. This is due to the fact that ηd k0 < kspp . The great difference between the two resonances is correlated to the different excitation natures. SPP corresponds to a y—evanescent field, since the y— momentum is described by an imaginary number, while the WR anomalies take place when two diffracted waves are radiated with the y—momentum component equal to zero. This abrupt change in the behavior of the reflected ray (that is, from the evanescent to the radiated case) produces the steep T 0 decrease observed for instance in Fig. 5(b) in the region between 630 and 640 nm. This difference in the reflected field is evident in the inset Figs. 4(a) and 4(b), where we show the magnetic field norm for wavelength of 628 and 654 nm, respectively. The functionalization process modifies the coupling constant and the SPP minimum shifts toward greater wavelengths, as illustrated in Fig. 7. Figure 7(a) shows how the spectrum minimum shifts for different thicknesses of the functionalized layer (RIU 1.45). Owing to the SPP excitation, only the spectrum minimum undergoes a shift, unlike the WR anomalies. The inset highlights the linear dependence of the spectrum minima from the overlayer thickness (black circle). The red straight line is the best fit of the reported data, featuring an angular coefficient of 1.41 nmλ ∕nmh. This coefficient permits to predict the spectrum minimum shift for our SAM overlayer (dodecanethiol), that is, 2 nm relative to a 1.5 nm SAM thickness [30,31]. Figure 7(b) shows that both resonances, WR maxima and SPP minima, shift toward greater wavelengths at an increasing dielectric refractive index. The inset illustrates the spectra minimum position as a function of the refractive index variation, featuring in this case an angular coefficient of 608 nm∕RIU. The wavelength shift due to the functionalization layer that takes place near the SPP excitation implies that the overall enhancement of transmittance variation can be approximated as: ΔT 0 ≈ −∂T 0 ∕∂λΔλ;
Fig. 6. Map of the T 0 transmittance as a function of incident angle θ and light wavelength λ, black circles show the position of Wood–Rayleigh anomalies and black squares show the position of surface plasmon polaritons excitations.
where ∂T 0 ∕∂λ is the slope of the T 0 spectra at 635 nm wavelength and Δλ is the spectrum shift due to the functionalization. 10 September 2014 / Vol. 53, No. 26 / APPLIED OPTICS
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Fig. 7. Spectra shift increases with the functionalization layer thickness different colors represent hfunc going from 0.5 nm (blue curve) to 8.5 nm (red curve) in step of 2 nm. The inset shows the relation between the spectra minima position and the functionalization thickness hfunc (a). Spectra shift increases with the refractive index of dielectric, different colors represent Δη going from 0 RIU (blue curve) to 0.016 RIU (red curve) in step of 0.004 RIU. The inset shows the relation between the spectra minima position and the medium refractive index variation with respect to the air one Δη (b).
B.
Experimental Results
Figure 8(a) compares the transmittance spectra of the bare and functionalized gratings, relative to the experimental (dotted line) and simulated (continuous line) results. The transmittance minimum shifts by 2 nm toward longer wavelengths, while the WR anomaly does not significantly change its spectral position. The T 0 is higher in the functionalized than in the bare grating configuration at 635 nm, and generally in the spectral region between 630 and 650 nm. Figure 8(b) compares the experimental ΔT 0 versus the simulated one. Both curves exhibit a maximum for wavelengths between 634 and 638 nm. In general, Figs. 8(a) and 8(b) display good agreement between simulations and experimental data, at least for wavelengths longer than 634 nm. For wavelengths between 620 and 630 nm, some differences appear between the experimental and simulated spectra, with a maximum disagreement at 625 nm Fig. 8(a). This difference could be ascribed to some mismatches between the actual fabricated grating and the simulated one. In fact, if we used a duty cycle of 0.43 instead of 0.4, we obtained the simulated results reported in the inset of Fig. 8(a) (dashed black 5974
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Fig. 8. (a) Experimental T 0 spectra measured for the bare (dotted black) and the functionalized (dotted blue) grating and the simulated bare and functionalized condition (continuous black and blue line, respectively). The inset compares the bare experimental (dotted black) with the bare simulated results (dashed line) obtained by varying the grating parameters (duty cycle). (b) Comparison between the experimental ΔT 0 (dotted red) and the simulated one (continuous red line).
line), which are in good agreement with the experimental results (dotted black line). Figure 9 shows the grating zero-order transmittance as a function of the incoming polarization angle, experimentally measured with the optical test bench. We observed a transmittance variation of ΔT 0 0.01 a.u. between the bare and the functionalized condition only when input light polarization
Fig. 9. T 0 as a function of the polarization angle for the bare and the functionalized gratings, measured with our experimental set up. Here, 0° refers to the TM polarization mode and 90° refers to the TE polarization mode.
was in TM mode. For comparison, no substantial changes in the transmittance were measured for the TE mode. The error bars reported in Fig. 9 are mainly due to the semiconductor laser intensity drift. To compensate this drift, we acquired 50 measurements of the zero-order transmittance as a function of the polarization angle for each test. Moreover, we can estimate the sensitivity and the resolution of the developed grating-based optoelectronic system. From the simulations, we calculated that a shift of 2 nm in the SPR minimum corresponds to an effective refraction index variation of 0.0034 RIU. The sensitivity can be estimated in terms of ΔT∕RIU and a value of 2.94 a.u./RIU was correspondingly found, which is comparable with that reported by Stewart and Mack [21]. The resolution of our sensor can be estimated by considering the error bars in Fig. 9. Since the error bars value is 6.6 × 10−4 a.u., resolution is 2.2 × 10−4 RIU, which is higher than in other types of sensors, such as those prismor grating-based, where a resolution of 10−7 RIU can be usually reached [3]. Our resolution could be further improved by using a laser featuring higher stability. 4. Conclusions
A biosensing system based on a silicon photodetector coupled with a digital metallic plasmonic wire grating was developed to detect variations of the transmitted light signal due to SAM adsorption. The proposed system transduces the photons transmitted by the grating into a current signal, thus reducing the overall optical bench complexity. Fabricating the grating onto the passivation layer of photodiode sensitive area could further shrink this detection system. Through numerical simulations, we optimized the plasmonic grating geometrical parameters to maximize the transmittance variation due to the surface refractive index changes. We fabricated a gold digital grating by using electron beam lithography, and we designed and successfully assembled a compact, efficient optoelectronic test bench. We have experimentally tested the system prototype to prove its capability to detect surface variations due to a dodecanethiol SAM formation. The experimental results of transmittance variation are in good agreement with the simulations. The proposed system presents a simple measurement protocol suitable for multiplexing and miniaturization. Its ability to detect a dodecanethiol SAM, which is much thinner than proteins or a DNA layer and is normally used as a detection benchmark, encourages further work on such systems. These systems could be used for biorecognition applications, and are worth investigating for integration into a custom-developed microfluidic device. In fact, the same approach described here also could be used for a liquid dielectric medium, obtained, for instance, by fluxing liquid onto the grating through
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