Chip-based biosensors for sensitive label-free detection were fabricated and tested by using Fano-type resonant nanostructures. The sensor was composed of a 190nm-thick gold nanoslit surrounded by 600-nm-period grooves. Transverse-magnetic polarized wave in these gold nanostructures generated asymmetrical resonant spectra due to the interference of broad-band cavity resonance in the single slit and narrow-band surface plasmon resonance on the periodic grooves. Compared to nanoslit arrays, such Fano-type sensor has a sharper resonance which yields a figure of merit up to 48. In addition, the crossed talk between sensing elements is reduced due to the Bragg reflection of the periodic grooves. A smaller detection separation down to 10μm width was achieved. An antigen-antibody interaction experiment in aqueous environment verified the detection sensitivity in surface binding event.
©2011 Optical Society of America
Surface plasmon resonance (SPR) sensing is a real-time and label-free detection technique to measure biomolecular interactions on a gold surface [1–3]. The most common method to excite the SPR on a gold surface is to utilize an optical prism, known as the Kretschmann configuration. Based on this SPR excitation technique, commercial BIAcoreTM SPR instruments enable real-time and label-free measurements of molecular binding affinity. In addition to the prism coupling method, the SPR can also be excited using metallic nanostructures [4–7]. Biosensors based on extraordinary transmissions of periodic gold nanohole arrays or nanoslit arrays have been proposed [8–17]. Compared to the prism-based SPR sensors, the gold nanostructures take advantages of small detection volume and normal light incidence condition. They provide a feasible way to achieve chip-based, high-throughput and label-free detections for modern DNA and protein microarrays [18, 19].
Sensors capable of high-sensitivity and high-throughput detections are desirable and beneficial to biomedical applications. In order to reach best sensing performance and high density packing of sensing elements, some issues should be addressed, including (1) a higher value of figure of merit (FoM), defined as the ratio of wavelength sensitivity to a bandwidth of resonance (a full width at half-maximum, FWHM), and (2) a lower neighbor-to-neighbor crosstalk. We propose a nanostructure of a single nanoslit surrounded by periodic grooves to fulfill the requirements of high sensitivity, multiplexed and miniaturized detections. The nanostructure consisted of a nanoslit and periodic grooves is known for its enhanced transmission and optical beaming effect [6, 20, 21]. When light is normally incident onto the grooves, there is an enhanced optical transmission through the single slit. The optimal condition for light transmission enhancement is the collaboration of three main mechanisms, slit waveguide mode (localized surface plasmon resonances (LSPR) in the nanoslit), groove cavity mode (LSPR in grooves) and in-phase grooves re-emission. When grooves are added to the output surface, a highly collimated beam at the resonant wavelength is produced. In this paper, we show that a nanoslit with periodic grooves at the outside can also be acted as a good sensor. An asymmetrical Fano-type resonance was generated due to the interference of broad-band waveguide mode in the single slit and narrow-band surface plasmon resonance on the periodic grooves. Different from the transmission enhancement by using deep grooves, a sharp resonance [22–24] was found for shallow grooves. In the case of nanostructures composed of 190-nm-thick gold nanoslits surrounded by 600-nm-period, 20-nm-wide and 20-nm-depth grooves, we achieved a Fano-type sensor with a wavelength sensitivity of 615 nm/RIU and a sharp resonance spectrum. The FoM of the nanostructure is up to 48. This value is much greater than that of nanoslit and nanohole arrays. In addition, the crossed talk between sensing elements is reduced due to the Bragg reflection of the periodic grooves. We verified that the separation between sensors was smaller than 10 μm.
2. Fabrication and measurement
2.1 Fabrication of metallic nanostructures
Metallic nanostructures of gold single nanoslit surrounded by periodic grooves were fabricated using electron beam lithography and a reactive ion etching method. Figure 1(a) depicts the flowchart for the fabrication of metallic nanostructures. A 5-nm-thick Ti film and a 190-nm-thick gold film were sequentially deposited on a sodalime glass using an electron gun evaporator. The Ti layer was used to improve the adhesion between the glass substrate and gold film. After the deposition, a 400-nm-thick PMMA resist (MicroChem Corp. Newton, MA) was spin-coated. A field emission scanning electron microscope (LEO-1530) modified with a nano-pattern generating system (NPGS) was used to write single nanoslit surrounded by narrow grooves with various depths on the PMMA resist. The patterns were then transferred to Ti/Au film by using argon sputtering in a reactive ion etching machine (Oxford Instrument, plasmalab 80plus). The power of the radiofrequency (RF) electromagnetic wave in the reaction chamber was 200 W. The chamber pressure was 1 × 10−2 torr, and the flow rate of Ar gas was 40 sccm. The PMMA resist was removed by rinsing the sample in acetone for a few hours. To make sure that no PMMA resist residue remained on the gold nanoslits, we cleaned the sample by using ozone sputtering in the RIE chamber. The sample was then put in ultrapure water, placed in an ultrasound bath for 20 min, and purged dry by nitrogen. In the experiment, patterns of single nanoslit with/without periodic grooves were fabricated. The finished patterns consist of a 90-nm-wide single nanoslit and 20-nm-wide grooves with various depths, from 20 to 190 nm. The period of the grooves was 600 nm. Figure 1(b) shows the optical and scanning electron microscope (SEM) images of a 90-nm-wide slit with 600-nm-period grooves. The samples were made with different groove depths on PMMA film. After the reactive ion etching, groove depths with about 20, 40 and 60 nm were formed on the gold film. In the optical microscope, the deep grooves showed red color due to LSPR in the grooves. The color became green for 40-nm-deep grooves due to a shorter resonant length in the grooves. No obvious LSPR in grooves was found for 20-nm-deep sample.
2.2 Optical setup for transmission spectrum measurement
The transmission spectra were measured by a simple optical setup . A 12W halogen light was spatially filtered by using an iris diaphragm and a collimation lens. Its incident polarization was controlled by a linear polarizer. The white light was focused on a single array by using a 10 × objective lens. The transmission light was collected by another 10 × objective lens and focused on a fiber cable. The transmission spectrum was taken by using a fiber-coupled linear CCD array spectrometer (BWTEK, BTC112E). In the experiment, the metallic nanostructures were mounted on a microfluidic channel made from plexiglass. The refractive index sensitivities were measured by covering purified water mixed with various fractions of glycerin over the sample surface. The refractive indexes of the mixtures (from 0 to 10% glycerin) ranged from 1.3320 to 1.3415. The biosensing experiments were conducted using bovine serum albumin (BSA) and anti-BSA assay in 10 mM phosphate-buffered saline (PBS) buffer condition.
3. Results and discussion
3.1 Optical properties
Figure 2(a) shows the measured transmission spectra of single nanoslit with 600-nm-period grooves and various groove depths in air for normally-incident TM-polarized light. The light is impinged on glass side. There are transmission peaks and dips in the spectrum due to the couplings of LSPR in the nanoslit and Bloch wave surface plasmon polariton (BW-SPP) on the periodic grooves. The purple curve is the transmission spectrum of a thin gold film. There is no resonance. The transmission drops quickly when incident wavelength increases. The green curve shows a broadband resonance due to LSPR in the single nanoslit [25, 26]. Under the presence of periodic grooves, there are two resonances in the transmission spectrum: a broad resonance at 750 nm-800 nm wavelength and an asymmetrical resonance near 630 nm. The broad resonance is the LSPR in the slit and grooves. Because the LSPR is a cavity mode, the resonance wavelength will increase with the increase of the depth. Figure 2(a) verified that peak wavelength of the cavity mode is increased with the deep of grooves. Near the 630 nm wavelength, there were anti-symmetrical resonances. These resonances are Fano-type resonances formed by the interference between spectrally overlapping broad resonance and a narrow discrete resonance . The LSPR occurring within the single nanoslit plays a role as the broad resonant state. On the other hand, the discrete state is associated with BW-SPP on the periodic grooves . The BW-SPP occurs when the Bragg condition is satisfied. For a normally incident light, the condition for a 1-D array is dominated by the period and described byEq. (1), the resonant wavelength of the BW-SPP at the air/gold interface is 628 nm (εm = −4.5815 + 2.5546i for gold at 600 nm, i = ± 1, n = 1 and P = 600 nm). The theoretical value is between the measured peak and dip wavelength. Such resonance can be fitted by Fano resonance equation:Figure 2(b) shows the theoretical fitting of Fano resonance by using the Fano formula. For the Fano-type resonance, the resonant shape was affected by the groove depth. For shallow grooves, the interference of the light in single nanoslit and the BW-SPP on the periodic grooves was weak. It formed an anti-symmetrical resonant spectrum. The Fano parameter was only 0.8. As the groove depth increased, the Fano resonance became higher and broadened. The Fano parameter was increased to 1.7 which rendered an increased transmission near the BW-SPP mode. Obviously, when the groove depths increased, the peak wavelengths of the Fano resonances and LSPR modes were red-shifted. The observed phenomenon is coincident with the theoretical predication by other group .
The observed bandwidths of Fano-type resonances decrease with the decrease of the groove depth. It implies that devices with shallow grooves are better for biosensing. Figure 3(a) shows the Fano resonance in water environment. When the outside medium changed from air to water, the Fano resonances were shifted from 625 nm to 820 nm. The observed Fano resonant wavelengths are close to the theoretical values of BW-SPP modes at the air/gold (628 nm) and water/gold (828nm) interfaces (εm = −23.9287 + 1.4978i for gold at 800 nm, i = ± 1, n = 1.3320 and P = 600 nm). Another important feature of the proposed nanoslit-grooves structure is that the crossed talk between sensing elements can be reduced using the Bragg reflection of the periodic grooves. It provides a smaller detection area and has the potential for high density packing of sensing elements. Figure 3(b) shows the transmission spectra of multiple single-nanoslits and multiple single-nanoslits with periodic grooves in air and water for normally-incident TM-polarized wave. The width of the grooves region was 10μm and the period of the grooves was 600 nm. The separation between two neighboring nanostructures was 10μm. In this figure, the red curve shows the transmission spectrum of multiple single-nanoslits in air. The original broadband resonance for a single nanoslit was superimposed with an interference signal. This extra signal was due to the neighbor-to-neighbor crosstalk and interference via the excited SPP waves. The surface between two slits behaves as a Fabry-Perot cavity for the SPP wave. When the outside medium was changed from air to water, the modified resonance was red-shifted (green curve). In contrary to single-nanoslits, the slit-grooves structure has an extra Fano resonance in the spectra in air and water environments. The Fano resonance is not affected by the interference signal (black and blue curve). It is noted that the periodic grooves couple some SPPs to the propagative field, making the SPP leaky and visible in the optical microscope as seen in Fig. 1(b). In addition to the leaky wave, some SPPs will be reflected and confined within the first few grooves when it matches the Bragg condition. Such SPP wave confinement with Bragg mirror grooves has been experimentally observed using near-field scanning optical microscope . Therefore, the interference via the excited SPPs from the adjacent slits will be reduced. In Fig. 3(b), the wavelength near the Fano resonance, which fulfills Bragg condition, is not affected by the interference signal. It accounts for that the Bragg reflection of the periodic grooves not only leads to an important contribution to the Fano resonance but also reduces the crossed-talk between neighboring sensing elements. The proposed Fano nanostructure can provide a small sensing separation down to 10μm. Compared with the reported structure, a 3-by-3 nanohole array with plasmonic Bragg mirrors , the non-cutoff transmission in the nanoslit leads to higher light transmission which results in a higher signal to noise ratio .
3.2 Bulk sensitivity test
We further tested the sensitivities for the nanostructure with 600-nm-period grooves. The refractive index sensitivities were measured by injecting purified water mixed with various ratios of glycerin into the devices. Figure 4(a) shows the transmission spectra of the gold nanostructure with various water/glycerin mixtures for normally-incident TM-polarized wave. There are Fano-like resonant profiles in the spectra. In water environment, the wavelengths of the resonant peak and dip are 810 and 828, respectively. When the concentrations of glycerin increased, the wavelengths of Fano resonances were red-shifted. Figure 4(b) shows the peak wavelength and dip wavelength against the refractive index of outside medium. The slope of the fitting curve shows the RIU (refractive index unit) sensitivities are 577 and 615 nm/RIU for the resonant peak and dip, respectively. These sensitivities are close to the theoretical sensitivity which indicates that the wavelength sensitivity (Sλ) is close to the period of the nanostructures, i.e. Sλ ~P nm/RIU. It is noted that the sensing performances are compared by the FoM value. It is necessary to take the bandwidth of the resonances into account. In this nanostructure, the line width of the resonant peak and dip was 12 and 19 nm, respectively. FoM values of 48 and 32 were obtained. Taking the line width from resonant peak to dip, the bandwidth was 18 nm and the FoM was 34. These values are better than that of gold nanohole arrays (RIU sensitivity of 331 nm/RIU and FOM of 23) , the individual nanoparticle-type LSPR sensors (from 0.6 for Au nanoshperes to 4.5 for nanobipyramids) [7, 28], and the reported plasmonic nanostructures and metamaterials with Fano-type resonances. For example, non-concentric ring/disk cavity (RIU sensitivity of 541 nm/RIU and FOM of 8.34) , plasmonic nanoparticle clusters (RIU sensitivity of 515 nm/RIU and FOM of 10.6) , an electromagnetically induced transparency (EIT) based planar metamaterial (RIU sensitivity of 588nm/RIU and FOM of 3.8) , and nanocross structures (RIU sensitivity of 1000 nm/RIU and FoM of 4.6) .
3.3 Biosensing experiment
The interactions between BSA (Sigma-Aldrich) and anti-BSA (Sigma-Aldrich) were measured using nanoslit-grooves nanostructure with 600-nm-period grooves, as shown in Fig. 5(a) . In the experiment, the buffer solution, 10 mM phosphate-buffered saline (PBS, UniRegion Bio-Tech), was first injected to the fluidic channel to clean the gold surface. The transmission spectrum of the nanostructure in buffer solution was measured. Then 500 μg/mL BSA was injected on the structure surface. Due to the physical absorption of BSA on gold surface a BSA monolayer BSA coated on the surface. The BSA was flowed for 1.5 hours to make sufficient BSA immobilized on the gold surface. Next, the sample was washed by PBS buffer to remove the unbound BSA proteins. We then measured the time-lapsed transmission spectra after injecting 375 μg/mL anti-BSA into the sample. After 80 minutes protein-protein interactions, the unbound anti-BSA was washed away by the PBS buffer. Figure 5(b) shows the peak wavelength as a function of the interaction time for the nanostructure. The measured spectra for different surface conditions are shown in the inset of Fig. 5(b). Significant changes in wavelength shift were observed when BSA and anti-BSA were bound on the gold surface. The monolayer BSA results in a 0.39-nm red shift. The wavelength shift is small because BSA is a small molecule with 66kDa in size. The wavelength shift for anti-BSA molecules is large. The 150-kDa-sized anti-BSA resulted in a 3.87-nm wavelength shift. If the wavelength resolution is 0.02 nm, the detection limit can reach a concentration of 1.94 μg/mL for the anti-BSA.
A new Fano-type resonant nanostructure by using single nanoslit with periodic grooves was fabricated, measured and tested for antigen-antibody interactions in aqueous environment. Such Fano-type sensors fulfill the requirements of high sensitivity, multiplexed and miniaturized detections. Compared to nanoslit and nanohole arrays, the Fano-type sensor has a sharper resonance which yields a figure of merit of 48. In addition, the crossed talk between sensing elements is reduced due to the plasmonic Bragg reflection of the periodic grooves. A smaller detection area down to 10μm width can be achieved. Compared to conventional SPR sensors, the proposed Fano-type sensor is better for chip-based and high-throughput detections. Currently, the wavelength sensitivity is 615 nm/RIU in the visible region. As the bulk sensitivity is proportional to the period of grooves, the sensitivity can be further improved by increasing the period to micrometer long and detected in the near-infrared region.
This work was supported by National Science Council, Taipei, Taiwan, under Contract No. 100-2120-M-007-006 and 100-2221-E-001-010-MY3.
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