Self-Assembled Plasmonic Pyramids from Anisotropic Nanoparticles for High-Efficient SERS
- 266 Downloads
Surface-enhanced Raman scattering (SERS) substrates play important roles for the enhancement of inelastic scattering signals. Traditional substrates such as roughened electrodes and colloidal aggregates suffer from well-known signal reproducibility issues, whereas for current dominant two-dimensional planar systems, the hot spot distributions are limited by the zero-, one- or two-dimensional plane. The introduction of a three-dimensional (3D) system such as a pyramid geometry breaks the limitation of a single Cartesian SERS-active area and extends it into the z-direction, with the tip potentially offering additional benefits of strong field enhancement and high sensitivity. However, current 3D pyramidal designs are restricted to film deposition on prepared pyramid templates or self-assembly in pyramidal molds with spherical building blocks, hence limiting their SERS effectiveness. Here, we report on the fabrication of a new class of low cost and well-defined plasmonic nanoparticle pyramid arrays from different anisotropic shaped nanoparticles using combined top-down lithography and bottom-up self-assembly approach. These pyramids exhibit novel optical scattering properties that can be exploited for the design of reproducible and sensitive SERS substrate. The SERS intensity was found to decrease drastically in accordance with a power law function as the focal planes move from the apex of the pyramid structure towards the base. In comparison to sphere-based building blocks, pyramids assembled from anisotropic rhombic dodecahedral gold nanocrystals with numerous sharp tips exhibited the strongest SERS performance.
KeywordsSelf-assembly Pyramid Plasmonics Shape anisotropy Surface-enhanced Raman scattering
Since the discovery and recognition of surface-enhanced Raman scattering (SERS), SERS has gradually emerged as a powerful analytical technique, capable of providing molecular fingerprint vibrational information with extreme sensitivity. This SERS effect originates from the confinement of light in interstitial spaces between noble metal nanoparticles, thus creating hot spots with excited and enhanced local electromagnetic fields [1, 2]. As such, SERS technology enables detection of trace amount of molecules even down to a single-molecule level, leading to a myriad of technical applications in the fields of biology, medicine, forensics, nanophotonics and optical sensors [2, 3, 4, 5, 6, 7].
Over the past decades, various SERS substrates from electrochemically roughened electrodes to colloidal suspensions, disordered aggregates and ordered assemblies have been developed with various fabrication techniques [2, 8, 9, 10, 11, 12, 13, 14]. These fabrication techniques can be categorized into two distinct approaches, namely bottom-up self-assembly and top-down lithography. Compared to random plasmonic nanostructures, patterned nanostructures often led to much more predictable and reliable signal, but are usually produced by expensive top-down lithography techniques with low throughput. In addition, traditional SERS limitations such as poor large area reproducibility and lack of fundamental plasmon understanding on SERS performance still exist, which hinder their translation from laboratory-based research to real-life everyday use. Currently, most of the substrate developments are predominantly focused on 2D planar systems such as metallic films and nanosheets. Despite their excellent performances, 2D systems also present a limitation of only a single Cartesian plane SERS-active area . In a view to increase substrate versatility, 3D pyramidal substrates have been proposed and demonstrated to increase the number and utility of hot spots (larger surface area) as a result of extension into the third dimension in the z-direction [16, 17].
Klarite is a commercially available classic SERS substrate that features a silicon surface with systematically patterned arrays of inverted square pyramid pits coated with a gold layer [18, 19, 20]. The unique property of the pyramidal structure is claimed to be responsible for controlling the propagating surface plasmons, which provides a hot spot for remarkably enhanced and uniform SERS intensity [21, 22]. As a result, this sparked concerted efforts by a number of researchers to utilize Klarite design as a starting point towards the fabrication of idealized SERS substrate. These include free-standing mesoscale metallic pyramids with well-defined ultrasharp tips , integrated nanocones on silicon micropyramid arrays , nanopillar-on-pyramid structures  and template-assisted gold nanosphere-assembled pyramidal periodical arrays  as effective SERS substrates. While these substrates show good SERS performance, they may not be the most efficient design for a pyramid substrate. For instance, majority of the structures are based on the coating of SERS-active film layers onto pyramid molds [16, 27, 28, 29, 30, 31], which may potentially lead to inhomogeneous deposition of nanoparticles. Direct self-assembly of nanoparticles in pyramid templates has also been demonstrated, but is only limited to simple isotropic spherical nanoparticles , hence limits the plasmonic tunability. Since the amplification of Raman signals are predominantly dependent on the nature of SERS substrate, inclusive of size and shape of nanoparticle building blocks, interparticle distance and arrangement patterns [1, 2, 32, 33, 34, 35], the continual development of pyramid-based substrates still remains a crucial and active area for SERS researches to overcome these existing limitations.
To address these issues, we aim to fabricate large-scale pyramidal substrates with controlled morphology with a view to investigate how shapes of constituent nanoparticle building blocks influence the optical properties of self-assembled pyramidal substrates and maximize their SERS enhancement. In particular, we utilized a combined top-down templated and bottom-up self-assembly approach to fabricate well-defined plasmonic pyramidal SERS substrate, which offers advantages of low cost, precise patterning definition, high throughput, assembly simplicity and possibility of scaling up to macroscale assemblies. This allows a thorough exploration of the plasmonic behavior stemming from the pyramid structure, morphological parameters of the pyramid building blocks and its related SERS effects, which are critical to the development of new, highly sensitive and reproducible SERS substrates.
Gold(III) chloride trihydrate (HAuCl4·3H2O, ≥99.9%), hexadecyltrimethylammonium bromide (CTAB), silver nitrate (AgNO3), sodium borohydride (NaBH4), l-ascorbic acid (AA), potassium bromide (KBr), potassium hydroxide (KOH), isopropyl alcohol (IPA), 4-aminothiophenol (4-ATP) cetylpyridinium chloride (CPC), ammonium hydroxide solution (NH4OH), hydrogen peroxide solution (H2O2) were purchased from Sigma-Aldrich. Poly(dimethylsiloxane) (PDMS) Sylgard (184) silicon elastomer, curing agent, and precursor were purchased from Dow Corning, USA. All chemicals were used as-received unless otherwise indicated. Deionized water was used in all aqueous solutions, which were further purified with a Milli-Q system (Millipore). All glassware used in the following procedures were cleaned in a bath of freshly prepared aqua regia and were rinsed thoroughly in H2O prior to use.
2.2 Synthesis of Plasmonic Nanoparticle Building Block
The synthesis of gold rhombic dodecahedral, octahedral and spherical nanocrystals are given in detail in Part S1 of the Supplementary Information.
2.3 Template Fabrication
The template was prepared according to a previously reported procedure [36, 37]. First, silicon nitride was deposited on a clean silicon wafer. Then, wafer was coated with pattern photoresist, and designed mask patterns were transferred onto the silicon substrate via photolithography means (the mask was made by Minnesota Nano Center at the University of Minnesota). The substrate was then etched in a wet etching solution containing KOH (30%) and IPA (5%) at 40 °C. The etching time depends on the size of pyramid, and it takes 30 min for complete etching of a square pyramid with 5.0 μm sides lengths. The etching angle is 54.7°, resulting in a pyramid height of around 3.5 μm. After that, the etched inverted pyramid template is washed by Milli-Q water, and cleaned in a cleaning solution (H2O:NH4OH:H2O2 = 5:1:1) to remove the bumps on the pyramid surface.
2.4 Nanoparticle Assembly
10 μL of concentrated gold nanocrystals solution was dropped on the surface of the template. The system was placed in a chamber with high humidity for slow evaporation. After 96 h, the evaporation process is complete, and the assembled pyramid is transferred to a carbon tape or PDMS for characterization and SERS measurement.
2.5 Structural and Optical Characterization
Scanning electron microscopy (SEM) images of the nanostructures were taken with a field emission SEM (JEOL 7001 F). Absorption spectra of the nanoparticle solution were recorded using an Agilent 8453 UV–vis spectrometer. The scattering spectra of the pyramid films and the single pyramid images were obtained using CytoViva hyperspectral imaging system. Dark field optical images were captured by Nikon Ti-U microscope.
2.6 SERS Measurement
The pyramid film assembled with different shapes of nanoparticles (sphere, octahedral and RD nanoparticles) were immersed in 1 mL 4-ATP (100 mM) overnight, washed with ethanol, and allowed to dry prior to SERS measurements. The SERS measurement was carried out using a WITEC 300R Raman microscope, mapping areas of 26 × 36 µm2, with a step size of 500 nm (100× objective) upon excitation with 532 nm and NIR (785 nm) laser line.
2.7 Numerical Modeling
In the experiment, gold pyramids are arranged in form of a square two-dimensional array. Therefore, to simulate the electric field distribution on the surface of a gold pyramid we first modeled the unit cell of the array of pyramids using COMSOL Multiphysics®. The dimensions of each pyramid are obtained from the experiments—each having a square base with sides of 5 µm, and height of 3.5 µm. The separation between two pyramids is considered to be 3.5 µm, measured as average from experimental data. The unit cell used in the simulation is shown using yellow dashed lines in Fig. S1.
Normally incident light from a 785 nm wavelength laser is considered for the simulation. Air is assumed to be the surrounding medium of the pyramids. Using frequency domain finite element method solver, the Floquet periodic boundary conditions are implemented in both lateral directions. This emulates an infinite two-dimensional square array of pyramids. Extremely fine physics-controlled meshing is implemented to accurately calculate the electric field distribution on the surface of a gold pyramid. The permittivity of gold is obtained from Refs. [38, 39, 40].
3 Results and Discussion
The macroscale pyramidal arrays were self-assembled by depositing a concentrated gold nanoparticle solution on the template and allowed to dry under sealed high humidity conditions (Fig. 1b). This entropy driven process compresses the nanoparticles and once a critical concentration is reached, the nanoparticles spontaneously assemble into ordered superstructures (Fig. 1e) held together by cohesive inter-structure van der Waals interactions between neighboring nanoparticles [26, 34]. Without this critical concentration of nanoparticles, the assembled pyramids are of poor quality with imperfect structuring (Fig. S4). Due to the repulsive forces between the hydrophilic template walls and hydrophobic assembled superstructures, these assemblies can be easily stamped and transferred onto flat substrates such as poly(dimethylsiloxane) (PDMS) and/or tape (Fig. 1c) . Low magnification SEM images in Fig. S3b show the homogenous RD-based pyramids sitting in line to form periodic arrays after transfer onto a PDMS surface. Zoomed-in observations on a single pyramid revealed the close-packed RD assemblies, with each pyramid intact with well-defined tip and edges (Figs. 1f, S5a–c). For a RD-based pyramid, the tip of the pyramid comprises a single RD nanocrystal, followed by a second layer of four RD nanocrystals, a third layer comprising 9 RD nanocrystals and so on. It is worth mentioning that in addition to RD, OH (Fig. S5d–f) and NS (Fig. S5g–i) have been successfully utilized as building blocks for assembly of pyramidal structure with similar configurations. This proves that the shape of building blocks has negligible influence on pyramid assembly as compared to critical concentration and template quality.
The observed plasmonic coupling effect from the intimately contacting nanoparticles that are immobilized in 3D space can lead to generation of concentrated hot spots with 3D geometry and high local electromagnetic field enhancement for amplification of Raman signals. To interrogate the Raman enhancing capability of our self-assembled pyramidal substrates, 4-aminothiophenol (4-ATP) was chosen as the model detection analyte due to its high affinity of gold and apparent characteristic fingerprints. The strong inter-structure van der Waals forces provide the pyramids with high structural stability during solvent immersion, allowing analyte adsorption on to pyramid surfaces without perturbing the assembled structure. From Fig. S7, the Raman spectra of 4-ATP can be clearly observed for all three pyramids. The observed peaks can be assigned in accordance to the literature , in which the two evident 1078 and 1388 cm−1 peaks that are enhanced by electromagnetic enhancement are assigned to the C–S stretching, and C–C stretching modes, respectively. The weaker peaks at 1141, 1392 and 1438 cm−1 are attributed to the weak charge transfer mechanism from Au–S bonds. The dominant 1078 cm−1 peak was then chosen as the main peak for subsequent studies on the effect of pyramidal structure and building block on SERS activity.
Power law fit constant values for each pyramid substrate
In addition to superior signal enhancement, signal reproducibility is also an important factor which determines the efficacy of a SERS substrate. To evaluate the homogeneity of each pyramid substrate, we collected SERS spectra from five random tips on each substrate. The relative standard deviation of Raman intensities for 1074 cm−1 peak from tip-to-tip was calculated to be 1.6, 2.6 and 3.4% for RD-, OH- and NS-based pyramidal substrates, respectively. These values are much lower than those reported for gold pyramidal arrays (22%) , Klarite (10%) , silver pyramidal arrays (9%)  and gold nanopillar films (7%) , confirming the good reproducibility of our self-assembled pyramid film substrates.
In summary, a combined top-down templated and bottom-up self-assembly approach was utilized to assemble RD, OH and NS nanocrystals into well-defined macroscale pyramidal arrays. Owing to the close-packed arrangement and nanoparticle interactions, the pyramids exhibited strong optical scattering and SERS intensities, particularly at the tips of the pyramids. All of these pyramid substrates can serve as sensitive and reproducible (<4% deviation) SERS substrates, but RD-based pyramid was found to provide the strongest Raman enhancement due to the presence of its anisotropic tips and strong molecular binding energy. This work opens up an avenue for large-scale fabrication of organized periodic pyramidal arrays with unprecedented tailorable plasmonics that can be exploited for numerous potential applications in the sensor field.
M.P., and W.L.C. acknowledge Discovery Grants DP110100713, DP140100883, DP120100170, and DP140100052. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). D. Sikdar acknowledges Engineering and Physical Sciences Research Council UK’s funding scheme EP/L02098X/1. The manuscript was written through contributions of all authors.
- 18.Alexander, T. A. In Applications of surface-enhanced Raman spectroscopy (SERs) for biosensing: an analysis of reproducible, commercially available substrates. 2005; p. 600703.Google Scholar
- 29.Xu Z, Wu H-Y, Ali SU, Jiang J, Cunningham BT, Liu GL. NANOP. 2011;5:053526-053526-053511.Google Scholar