Research Article www.acsami.org
Cup-Shaped Nanoantenna Arrays for Zeptoliter Volume Biochemistry and Plasmonic Sensing in the Visible Wavelength Range Rokas Drevinskas,†,⊥,# Tomas Rakickas,†,# Algirdas Selskis,‡ Lorenzo Rosa,§,∥ and Ramunas Valiokas*,† ̅ †
Department of Nanoengineering and ‡Department of Characterization of Materials Structure, Center for Physical Sciences and Technology, Savanorių 231, Vilnius LT-02300, Lithuania § Department of Information Engineering, University of Parma, Parco Area delle Scienze 181/A, 43124 Parma, Italy ∥ Applied Plasmonics Laboratory, Centre for Micro-Photonics, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia S Supporting Information *
ABSTRACT: Although three-dimensional shaping of metallic nanostructures is an important strategy for control and manipulation of localized surface plasmon resonance (LSPR), its implementation in high-throughput, on-chip fabrication of plasmonic devices remains challenging. Here, we demonstrate nanocontact-based large-area fabrication of a novel, LSPR-active Au architecture consisting of periodic arrays of reduced-symmetry nanoantennas having sub-50 nm, out-of-plane features. Namely, by combining nanosphere and molecular self-assembly processes, we have patterned evaporated polycrystalline Au films for chemical etching of nanocups with controlled aspect ratios (outer diameter d = 100 nm and void volumes = 18 or 39 zL). The resulting nanoantennas were highly ordered, forming a hexagonal lattice structure over centimetersized glass substrates, and they displayed characteristic LSPR absorption in the visible/near-infrared spectral range. Theoretical simulations indicated electric field confinement and enhancement patterns located not only around the rims but also inside the nanocups. We also explored how these patterns and the overall spectral characteristics depended on the nanocup aspect ratio as well as on electric field coupling in the arrays. We have successfully tested the fabricated architecture for detection of stepwise immobilization and interactions of proteins, thus demonstrating its potential for both nanoscopic scaffolding and sensing of biomolecular assemblies. KEYWORDS: Directed self-assembly, nanopatterning, localized surface plasmon resonance, reduced symmetry, nanocup, plasmonic engineering, biomolecular interactions
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INTRODUCTION Localized surface plasmon resonance (LSPR) generated by light in metallic nanostructures can give rise to many intriguing physical phenomena, such as extraordinary optical transmission,1 sub-wavelength focusing,2 and controlled photoluminescence.3 The physics of the nonpropagating modes involved in LSPR is of high interest because they can be excited by light directly, without special phase-matching optical components and expensive instrumentation. Instead, the optical properties of the material are fully controlled by manipulating the feature size, geometry, and dielectric environment. For example, changes in the aspect ratio of a metal nanoparticle can result in an increased polarizability and more sensitive optical responses to the refractive index of the ambient,4 whereas plasmon hybridization in complex nanostructures allows additional tuning of the resonance frequency through positioning of the interacting entities.5 Furthermore, lateral and threedimensional (3D) arraying can determine coupling between the electric fields of individual nanoantennas causing collective plasmon modes.6,7 © 2017 American Chemical Society
However, surface geometry and chemistry control remain among the major challenges in the field of plasmonics.8 Although the past 2 decades saw a significant progress in developing new techniques for fabrication and integration of plasmonic architectures (e.g., different forms of colloidal lithography9), the predominant approaches are still based on chemical synthesis and the self-assembly of metallic particles in solution.10−12 Therefore, for reproducible on-chip fabrication of surface-supported plasmonic devices, especially those based on LSPR in 3D metallic nanofeatures, the realistic options remain limited to expensive and low-throughput tools, such as electron beam and phase shift lithography or focused ion beam milling. Herein, we demonstrate that bottom-up and top-down strategies can be combined into an efficient and precise process for fabrication of highly ordered arrays of reduced-symmetry nanoantennas that display aspect-ratio-dependent LSPR activity Received: February 24, 2017 Accepted: May 19, 2017 Published: May 19, 2017 19082
DOI: 10.1021/acsami.7b02749 ACS Appl. Mater. Interfaces 2017, 9, 19082−19091
Research Article
ACS Applied Materials & Interfaces
forming a crystalline PS deposit. Depending on the PS concentration and the sample moving speed, mono- or multilayer PS deposits were obtained. The PS deposition process was monitored via an optical microscope. After the PS deposit formation, the sample was immersed into ultrapure water for 10 min and gently rinsed to remove the excess of the PSs and then dried in air. The approximate number of PS layers was estimated with an upright optical microscope (BX51; Olympus, Japan), equipped with a 10× NA 0.3 objective, by inspecting the color of the interfering light reflected by the PS layers of different thicknesses. For most of the experiments, samples with 4−10 layers of the PSs were used. The gold substrates with the formed crystalline PS deposits were exposed to the 1-hexadecanethiol (HDT; Sigma-Aldrich) vapor in a desiccator, under low vacuum. The sample was placed ∼3 cm above a coverslip covered with 1.4−1.7 mM HDT solution in ethanol, with the PS layer facing the coverslip. The desiccator was evacuated for 10 min and left for 1 h for HDT evaporation and formation of a self-assembled monolayer (SAM) on Au. After this procedure, the sample was treated in an ultrasound bath for 5 min in ethanol to remove both the PSs and excess HDT. Afterward, wet chemical etching of Au was performed in a mixture containing 20 mM Fe(NO3)3·9H2O (Fluka, Switzerland), 30 mM thiourea (Fluka, Switzerland), and 1 mM HCl (Sigma-Aldrich) dissolved in ultrapure water saturated with octanol (SigmaAldrich).20,21 Etching of Au was monitored visually and was stopped once the sample changed the color and became characteristically bluish. The typical etching time was 10−30 min. The resulting Au patterns were additionally treated with UV radiation (254 nm wavelength) for 30 min and rinsed in ethanol for 10 min to remove the products of HDT resist degradation.22 Characterization. The morphology of the nanofabricated samples was analyzed with an optical upright Olympus BX51 microscope, SEM JSM 6490 LV (JEOL, Japan), and Helios NanoLab 650 (FEI, The Netherlands). For topography measurements, a NanoWizard 3 AFM (JPK Instruments AG, Germany) mounted onto an Olympus IX81 (Olympus, Japan) inverted optical microscope was employed, and it allowed combined optical and AFM imaging of the same region of interest. Sample topography images were obtained in the contact mode using SNL-10 (Bruker) probes. Optical spectra were recorded with a visible/NIR spectrometer (AvaSpec-ULS2048XL; Avantes BV, The Netherlands), with a 600 μm diameter fiber connected to the optical microscope, Olympus BX51, equipped with a 10× objective lens. A microscope halogen bulb without the IR filter was used as a light source. The sample and reference (a SC-1 cleaned microscopy glass slide) were mounted onto a home-made poly(dimethylsiloxane) (PDMS; Dow Corning) flow cell. Reference spectra on a blank glass slide were recorded before each measurement, and 10 000 spectra were averaged. The acquired transmission spectra were smoothed using the Savitzky−Golay method and fitted for the definition of the peak position using Origin data processing software (OriginLab Corp.). The sensitivity of the fabricated Au nanocup samples to ambient refractive index changes was analyzed by sequential injections of ultrapure water and solutions of 10, 20, 30, 60, and 86% v/v glycerol (Carl Roth GmbH, Germany) in ultrapure water. The solution refractive index values were taken from the literature.23 For protein binding experiments, the running buffer and all protein solutions were prepared in a 20 mM HEPES (Carl Roth GmbH, Germany) solution with 150 mM NaCl (Carl Roth GmbH, Germany), pH = 7.5, in ultrapure water. The sample in the flow cell was rinsed with ultrapure water and the buffer solution and then the following incubations in protein solutions were made: 70 min in 100 μg ml−1 biotinylated bovine serum albumin (bBSA; Sigma-Aldrich) and 60 min in 1 mg ml−1 BSA (Sigma-Aldrich) and 10 μg ml−1 streptavidin (SA; SERVA Electrophoresis GmbH, Germany). After each protein incubation, the sample was rinsed with buffer at least for 10 min. Optical transmission spectra were measured after each step using the setup described above. Theoretical Modeling. The model was implemented using the 3D finite-difference time-domain (3D-FDTD) method provided in the Lumerical FDTD software (FDTD Solutions). A subset of the entire
in the visible/near-infrared (NIR) wavelength range. Previously, other groups have reported on nanocontact-based techniques such as edge spreading lithography13,14 for Au surface patterning via alkylthiolate nanochemistry. However, the fabricated submicrometer-sized features (Au rings) were too large for LSPR in the visible spectral range and they were not practical for further engineering (e.g., via biomolecular scaffolding) due to the substrate material (SiO2) exposed inside the plasmonic structure. On the other hand, various cup-shaped Au (or bimetallic Au/Ag) nanoantennas can be fabricated via metal deposition on silica or polymer beads15−17 as well as by ion milling of Au nanoshells.18 Despite that, in most of the cases they can be hardly organized, stabilized, and functionalized over the larger substrate areas for efficient sensor chip applications. In the present work, we have combined the directed selfassembly of particles and molecules into a nanopatterning process that yields centimeter-sized sensor chips covered by a novel, robust, and reproducible plasmonic architecture. It consists of oriented and ordered cuplike Au nanoantennas with sub-50 nm sized cavities and rims of controlled aspect ratios, a 3D geometry that is convenient for docking of biomolecular entities and further manipulation of LSPR in the visible/NIR spectral range. Contrary to the previously reported similar plasmonic systems, the obtained hexagonal arrays of the nanocups display a long-range order. We have characterized the morphology of the obtained Au nanoarchitecture by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Also, we have studied spectroscopically and theoretically the influence of the aspect ratio and coupling between the arrayed nanocups on the electric field localization, enhancement, and LSPR sensitivity to protein interactions in such zeptoliter volume “test tubes”.
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EXPERIMENTAL METHODS
Substrate Preparation. Two types of polycrystalline Au coatings on glass were used as substrates for fabrication of nanocup arrays. Glass chips with a 20 nm thick Au film and 2 nm Ti adhesion layer were purchased from Ssens BV (The Netherlands). Microscopy glass slides (Gerhard Menzel GmbH, Germany) coated with a 30 nm thick Au film were prepared in a custom-built electron-beam vacuum evaporation system (kindly provided by Prof. Bo Liedberg’s laboratory, Linköping University, Sweden) with the base pressure
