Full Color Plasmonic Nanostructured Surfaces and Their Sensor

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Full Color Plasmonic Nanostructured Surfaces and Their Sensor Applications Yunfeng Li, Junhu Zhang, Tieqiang Wang, Shoujun Zhu, Huijun Yu, Liping Fang, Zhanhua Wang, Liying Cui, and Bai Yang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed: July 26, 2010; ReVised Manuscript ReceiVed: October 14, 2010

In this Article, we report on plasmonic nanostructured surfaces with almost all colors by nanosphere lithography. The feature sizes of the plasmonic nanostructures can be controlled by O2 reactive ion etching and gold evaporation conditions. The optical properties of the plasmonic nanostructured surfaces can be easily tuned in the visible light region by changing etching time, and the color of plasmonic nanostructured surfaces does not change with the viewing angle up to 45°. More importantly, such surfaces show high bulk sensitivity to the refractive index variation, and they can be used as label-free, real-time biosensors to detect biomacromolecules. Introduction Noble metal nanostructured arrays have attracted immense interest due to their potentials in label-free forms of biological and chemical sensors.1 These sensors may owe their high performance to novel optical properties of noble metal nanostructured arrays, for instance, localized surface plasmon resonance (LSPR),2 anomalous transmission and reflection of light,3 and surface-enhanced Raman spectroscopy (SERS).4 The LSPR results from a collective oscillation of surface conduction electrons of metal interacting with light.5 These oscillations lead to an evanescent electric field that extends from the metal surface to the dielectric over several hundred nanometers scale.6 The LSPR is very sensitive to the changes in refractive index occurring at a metal/dielectric interface.7 In conventional LSPR (Kretschmann configuration), light can be coupled into surface plasmon by using prisms at the metal surface.8 This configuration is difficult to integrate into portable and low-cost devices for practical applications. In contrast, in nanostructured metal arrays and metal nanoparticles, LSPR can be excited by direct illumination of light and measured by using conventional spectroscopy.1,6 Because of the recent development of nanofabrication techniques, many biological and chemical sensors based on metal nanoparticles,9 nanorods,10 nanotubes,11 nanorice,12 nanorings,13 nanodisks,14 nanoholes,15 and nanoshells16 have been reported. Top-down nanofabrication techniques such as electron beam lithography and focused ion beam lithography have been used to prepare noble metal arrays for LSPR sensors.17 These methods can precisely control the dimensions of nanostructured arrays. However, it is difficult to prepare nanostructured arrays over large enough areas in practical applications. Several techniques can overcome the limitations of electron beam lithography and focused ion beam lithography. For example, nanosphere lithography18 reported by Van Duyne and co-workers uses a hexagonal close-packed monolayer nanosphere on a substrate as a mask of deposition or etching to generate metal nanoparticles arrays or as a substrate for preparation of metal-film/nanosphere composites. Nanosphere lithography has been widely used to * Corresponding author. Tel: +86 431 85168478. Fax: +86 431 85193423. E-mail: [email protected].

fabricate functional materials due to its intrinsic properties such as low cost, time efficiency, parallel, and high-output. Using nanosphere lithography, Van Duyne and co-workers have prepared many metal nanostructured arrays for chemical and biological sensors based on LSPR and SERS.19 Alternatively, Nuzzo and co-workers have prepared plasmonic sensors based on nanowell arrays by soft nanoimprint lithography6,20 and measured chemical force of pH-responsive hydrogel;20a after that, they used similar arrays to realize submonolayer molecule imaging.20b,c Because of their novel optical properties, LSPR sensors are widely used to detect proteins, DNA, diseases, interaction of host-guest, and biomarkers. For sensors based on LSPR, due to the absorption of light of special wavelength, the sensors will show different color; however, there are few methods that can be used to prepare LSPR sensors with almost full colors. Besides, subwavelength dimensions of metal nanostructures will be demanded to realize colors; unfortunately, few protocols can be used to prepare large enough area with spatial uniformity by simple equipment at low cost and short time. In this Article, we report full color plasmonic nanostructured surfaces (PNSs) on silicon substrates using nanosphere lithography. By simply changing the time of reactive ion etching (RIE), the colors of PNSs can be changed from red to blue. Moreover, we found that the colors of PNSs did not change in the 45° viewing angle range. More importantly, the reflective peaks of PNSs were sensitive to the changes of surrounding refractive index, so such surfaces were candidates of sensor applications. Because of the ordered arrays of PNSs, these LSPR sensors can be integrated with SERS.1b We believe that these PNSs can be widely used as sensors in the fields of disease diagnostics, environment monitoring, and food safety and are vital tools for investigating biological phenomena. Experimental Section Materials. Polystyrene (PS) nanospheres were prepared by emulsion polymerization as mentioned in ref 21. The PS nanospheres used in our work were 320 nm in diameter. The silicon substrates were cut into 20 mm × 20 mm pieces, were soaked in the mixture of 98% H2SO4/30% H2O2 (volumetric ratio 7:3) for 20 min under boiling (Caution: strong oxide), then

10.1021/jp106948m  2010 American Chemical Society Published on Web 11/09/2010

Full Color Plasmonic Nanostructured Surfaces were rinsed with deionized water several times, and at last were dried with N2 stream. All the chemical reagents in our work were used as received. Preparation of Non-Close-Packed 2D PS Colloidal Crystals. The PS nanosphere monolayers were prepared by the interface method.22 In brief, 0.1 mL of 0.5% PS nanosphere dispersion in a mixture of deionized water and absolute ethanol (v/v, 1:1) was dropped onto the surface of water in a 12 cm diameter glass tank, and then 20 µL of 5% sodium lauryl sulfate solution was added. Finally, the monolayer nanospheres were lifted onto the silicon substrate, and the 2D colloidal crystals were obtained when the slides became dry. To prepare the nonclose-packed 2D colloidal crystals,23 oxygen RIE operating at 15 mTorr pressure, 20 SCCM flow rate, and RF power of 30 W, ICP power of 30 W was carried out from 0 to 210 s. The oxygen RIE was performed on a Plasmalab Oxford 80 plus (ICP 65) system (Oxford Instrument Co., UK). Fabrication of Full Color PNSs. The full color PNSs were prepared by sputtering Au film on the top of 2D PS collidal crystals and 2D non-close-packed PS colloidal crystals by vacuum deposition. The nominal thickness of Au film is from 16 to 55 nm. The thicknesses of the Au films were measured by Dektak150 surface profiler (Veeco). Characterization. SEM micrographs were taken with a JEOL FESEM 6700F electron microscope with primary electron energy of 3 kV. The photographs of samples were taken by a Canon G9 camera. The normal specular reflectance was measured by homemade equipment consisting of a collimated beam of a fiber-coupled tungsten-bromine lamp (Ocean Optics), and the spectra were obtained using a spectrometer (Ocean Optics, USB4000) from 400 to 800 nm. The angle-depended specular reflection was evaluated using homemade equipment consisting of a collimated beam of a fiber-coupled tungstenbromine lamp (Ocean Optics), variable-angle reflection sampling system (RSS-VA, Ocean Optics), and the spectra were obtained using a spectrometer (Ocean Optics, USB4000) from 400 to 800 nm. All the reflection spectra shown were normalized with respect to the silicon wafer. The chemical sensor properties of samples were recorded as they were immersed in different concentrations of glycerol dissolved in water (0-40 wt %). The refractive index of each solution was verified by using an Abbe refractometer. The label-free, real-time biosensor performance was evaluated by in situ measuring the LSPR peaks of the PNSs when they were in the 0.2 wt % bovine serum albumin (BSA). A poly(dimethylsiloxane) (PDMS) container with a hole (2 mm in height, 6 mm in diameter) was placed on the surface of the PNSs, providing enough space for the PNSs in the BSA solution. Reflective spectra were measured every 5 s over 45 min, and the LSPR peaks were recorded as a function of time. The PNSs were first immersed in phosphate buffer (pH ) 7.4) for several minutes to discriminate the optical response change induced by phosphate buffer, and then adding the BSA solution. Results and Discussion Fabrication of PNSs. A schematic representation of the fabrication process of PNSs is illustrated in Figure 1. The full color PNSs were prepared by nanosphere lithography. In brief, a 2D close-packed PS colloidal crystal was prepared on a silicon substrate by the modified interface method; then, using O2 RIE, 2D non-close-packed PS colloidal crystals were obtained; at last, the full color PNSs were prepared by sputtering Au films over the 2D close-packed or non-close-packed PS colloidal crystals. Figure 2a shows the 2D PS colloidal crystals used in our work, with PS nanospheres 320 nm in diameter. We can see that they

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Figure 1. Schematic of fabricating the full color PNSs by nanosphere lithography: (I) Changing the distance between the nanospheres by O2 RIE, (II) deposition of Au film on the top of close-packed PS colloidal crystals, and (III) deposition of Au film on the top of non-close-packed PS colloidal crystals.

are hexagonal close-packed on the silicon substrate. To prepare full color PNSs, it is a key point to precisely control the diameter of PS nanosphere. By using O2 RIE, the diameters of PS nanospheres can be continuously reduced by increasing the etching time (Figure 2b and c), and the diameter of PS nanosphere almost reduces linearly with the etching time (Figure 2d). Besides, after RIE, a mushroom-cap-like PS nanosphere array formed due to the anisotropic etching properties of RIE.24 Figure 3 shows the scanning electron microscopy (SEM) images of PNSs. We can see that ordered arrays of PS nanospheres are not destroyed after sputtering a thin film of Au. The PS nanospheres were hemispherically covered by Au (the inset of Figure 3a), and the resulting Au film consists of hexagonal close-packed or non-close-packed arrays of Au halfshells with a size by the template spheres. The nominal thickness is about 30 nm. It is noted that during the process of sputtering, the Au is inevitably deposited onto the silicon substrates through the interstices of the 2D colloidal crystals. For close-packed PS templates, there are Au triangular nanoislands on the substrates, and for non-close-packed ones, there are Au ordered circle nanovoids on the substrates (the inset of Figure 3b). The diameters of the Au half-shell can be easily controlled by changing the RIE time. In addition, due to the locally curved surface of the PS nanosphere templates, a lateral variation of the Au thickness was created on the nanospheres, with the thickest layer on the top of the nanospheres and the thinnest layer at the equator of nanospheres.3c Optical Properties of PNSs. The typical reflective spectra of full color PNSs are shown in Figure 4a. To eliminate the influence of silicon substrates on the optical properties of PNS, polished silicon substrates are used as a baseline in all reflective measurement. For close-packed PNS, a shoulder peak appears at about 603 nm. For non-close-packed PNSs, their reflective peaks blue-shift as the etching times increase. Figure 4b shows the photographs of PNSs; we can see the colors of PNSs change from red to blue, and they are homogeneous over large areas. That is to say, the reflective peaks of PNSs can be continuously tuned due to continuously changing the diameters of PS nanospheres by proper RIE conditions. The optical properties of PNSs are a superposition of scattering diffraction and light reradiation by the excitation of LSPR on the Au films.3c More importantly, due to the halfshell morphology of PNSs, the present nanostructures allow strong coupling of the LSPR with the colloidal crystals.3c In addition, except for the half-shell arrays, there are Au nanois-

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Figure 2. Typical SEM images of PS colloidal crystals after RIE 0 s (a), 120 s (b), and 210 s (c). All the scale bars are 500 nm. (d) The revolution of diameters of PS with the etching time increasing from 0 to 210 s.

Figure 3. Typical SEM images of PNSs with 30 nm Au films on the top of PS colloidal crystals before etching (a) and after 210 s etching (b). The inset of (a) shows the cross-sectional SEM images of PNSs, and the Au film shows the half-shell. The inset of (b) shows the tilted SEM images of PNSs, and we can see the Au nanovoids on the substrates.

lands or Au nanovoids arrays on the silicon substrates, and they also show LSPR. As a result, there may exist coupling between these Au nanostructures on the silicon substrates and Au halfshells due to the spatial separation below 100 nm,20,25 and they will have a positive influence on the optical properties of the full color PNSs. In addition, the blue-shiftings of reflective peaks with the etching time increasing stem from the deceasing of PS nanospheres.3c

Figure 4. (a) Optical properties of PNSs with RIE time from 0 to 210 s. (b) Photographs of PNSs; the sizes of PNSs are 20 mm × 20 mm.

The optical response of the PNSs can be tuned by changing the thickness of Au film. Figure 5 shows the reflective spectra under normal incidence of a series of samples with different Au thickness. The thickness of Au film varied from 16 to 55 nm. We can see that changing the Au thickness on the PNSs dramatically affects the width and magnitude of the peak, but only weakly shifts the position of the spectral features. The magnitude of peak decreases as the thickness of Au film increase. These phenomena are similar to conclusions reported before.20b More importantly, the PNSs show almost the same color with the incident angle from 15° to 45°. In our experi-

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Figure 5. Optical properties of 120 s PNS with different thickness Au.

Figure 6. Reflective spectra of 120 s (a) and 150 s (b) PNSs with different incident angles.

ments, the angle-dependent reflective spectra were used to evaluate the optical properties of the PNSs. The angle-resolved reflection spectra were performed by setups described detailedly in the Experimental Section. In Figure 6a and b, we can see that the intensity of spectra decreases by increasing the incident angle, but the peaks of spectra almost do not change. Sensor Applications of PNSs. To explore the LSPR sensitivity of the PNSs to bulk refractive index (nbulk) changes, we measured reflective spectra of the samples when they were immersed in water and glycerol water solution (0-40 wt %). The refractive index of each solution was verified by using an Abbe refractometer. Taking the 120 s sample as an example, Figure 7a shows its reflective spectra in which the peaks of reflection move toward the longer wavelength as the nbulk increases. To evaluate the bulk sensitivity of the PNSs, the reflective peak shifts were plotted against the change in refractive index (Figure 7b). The bulk sensitivity is defined as Sbulk ) dλ/dn and can be determined from linear fits to the experimental data points.11 As shown in Figure 7b, by changing the RIE etching time, the bulk sensitivity can be tuned. For the 90 s sample, the bulk sensitivity is about 156 nm/RIU, and the bulk sensitivity of the samples decreases as the RIE time increases with the distance increment between nanospheres after RIE. To demonstrate the potential of the PNSs as a label-free, realtime biosensor, the PNSs with bulk sensitivity about 93 nm/ RIU (120 s) were exposed to 0.2 wt % BSA in phosphate buffer

Figure 7. PNSs as sensors. (a) Reflective spectra of 120 s PNS in response to glycerol water solution (0-40 wt %), black line (in air), direction of arrow stands for concentration increment of glycerol solution; (b) the peaks of reflective spectra red-shift as a function of background refractive index; and (c) label-free, real-time detection of BSA using PNSs (120 s sample).

at room temperature. Figure 7c shows the real-time LSPR peak shifts of the PNSs after absorption of BSA on the surface of the PNSs surface. We can see that the red-shifts of the LSPR peaks were saturated at about 3.2 nm after about 240 s, indicating that the PNSs are candidates for label-free, real-time biosensors. Conclusions In summary, we have successfully fabricated full color PNSs using nanosphere lithography, and the optical properties can be tuned almost over the visible light region by simply changing the RIE time. The PNSs can realize all colors, and the colors do not change with the viewing angle up to 45°. In addition, the PNSs show good bulk sensitivity of LSPR to the refractive index variations and high-quality sensitivity to biomacromolecules. These inexpensive, easily fabricated PNSs can be integrated with SERS for quality and quantity measurements. Besides, their nanostructures, together with the high uniformity of the PNSs, make them promising candidates for the development of microarrays for label-free biosensor applications. Acknowledgment. This work was supported by the National Science Foundation of China (Grant nos. 21074048, 20921003, 20874039) and the National Basic Research Program of China (2007CB936402).

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