Plasmonic Nanochips Development and Applications

Introduction

The phenomenon of plasmonics is widely used in optical devices [1], imaging microscopy [2], biosensing [3-5], and medical diagnostics [6-8]. Improvement in sensitivity, even down to single-molecule detection limits, is needed in many applications, and this problem demands a solution at the present moment. One of the possible ways to obtain general sensitivity enhancement for multiple applications is to fabricate nanopatterned plasmonic substrates (nanochips) capable of generating strong local electromagnetic fields or, in other words, offering significant plasmonic enhancement (PE), due to the occurrence of localized surface plasmon resonance (LSPR) phenomenon in highly conductive metal nanoparticles. It was shown both theoretically and experimentally that enhanced local field provides signal amplification for LSPR [9-11], surface-enhanced Raman scattering (SERS) [10, 12], surface-enhanced fluorescence (SEF) [13-15], and surface-enhanced infrared absorption (SEIRA) [16,17] techniques. The peculiarity of PE accompanying LSPRis that the enhanced field is concentrated in confined space with nanometer dimensions ("hot spots") [18]—a phenomenon that depends on nanostructure size, shape, and material properties [19, 20].

In this chapter, different approaches for the fabrication of plasmonic nanochips based on noble metal thin films and

Molecular Plasmonics: Theory and Applications Volodymyr I. Chegel and Andrii M. Lopatynskyi Copyright © 2021 Jenny Stanford Publishing Pte. Ltd.

ISBN 978-981-4800-65-5 (Hardcover), 978-0-429-29511-9 (eBook) www.jennystanford.com nanostructure arrays are discussed, including a method based on gold island film deposition with subsequent thermal annealing and nanoimprint lithography (NIL) technique. The application of latter techniques for the preparation of plasmonic nanochips is comparatively described in more detail by analyzing their structural, spectral, sensing, and plasmonic enhancement properties. Additionally, this chapter presents the results of theoretical and experimental studies on application of plasmonic nanochips for SEIRA- and SEF-based research.

Fabrication of Plasmonic Nanochips Based on Noble Metal Thin Films and Nanostructure Arrays

Fabrication of Thin Films with Surface Roughness

As it was mentioned before, continuous thin films of high-conductive metals can be used both as a surface exhibiting plasmonic enhancement phenomenon and a sensitive element of surface plasmon resonance (SPR) sensor. However, the optimal parameters of metal films in terms of film thickness and surface roughness are quite different for these applications.

For plasmonic enhancement applications, thin and rough metal films are optimal. For example, gold films with a thickness of 20-40 nm have been exploited in SEIRA experiments [17, 21]. To fabricate such substrates, the protocol based on vacuum deposition of 99.999 pure Au upon glass supports (TF-1 glass, 20x20 mm) via an intermediate adhesive Cr layer was used. Before Au deposition, glass surface was cleaned by NH₄0H:H₂0₂:H₂0 and HC1:H₂0₂:H₂0 solution, subsequently, both 1:2:2 by volume concentration during 5 min at boiling temperature. Then it was rinsed in bidistilled water and dried in a flow of pure nitrogen. The gold was evaporated from molybdenum heater and deposited at a rate of 1.0-1.5 nm/s on room temperature substrate. The Cr interlayer thickness did not exceed 1-1.5 nm. The gold surface just after deposition looked like hydrophobic surface with wetting angle close to 80° and random roughness about 5 nm (Fig. 3.1a).

Figure 3.1 Atomic force microscopy (AFM) images of gold film surface (a) before and (b) after chemical polishing by piranha solution. Adapted by permission from Ref. [21], Copyright 2004, Springer Nature.

For SPR sensor application, larger thickness and lower surface roughness of metal film are preferable. Gold films, 45-50 nm thick, produced by thermal vacuum evaporation method are usually exploited as plasmon-polariton oscillation bearers in SPR sensors [22-24]. To enhance the adhesion, a thin (up to 5 nm) chromium layer was evaporated on the glass slide before the evaporation of gold. Thermal evaporation was performed in vacuum (10~⁶ mmHg) on the substrates at the room temperature. Annealing of films for 30 min at 120°C took place after the evaporation to decrease the surface roughness [22]. To further improve the surface evenness, chemical etching by piranha solution was applied (Fig. 3.1b). This is a crucial stage in order to obtain a surface suitable for biomodification relying on an oriented immobilization of biomolecules, for example, immunoglonulins (see Section 4.2 in Chapter 4).

Fabrication of Random Nanostructure Arrays

Surface nanopartening of random-fashion nanostructures using colloidal nanoparticles

Among several approaches, colloidal Au and Ag nanoparticles are used for the preparation of chip-based structures in array format by immobilization of nanoparticles on the solid support. Several articles from different groups have presented chip-based format optical biosensors, in which gold or silver nanostructures are immobilized in a random fashion on an optically transparent substrate [25-28]. This approach opens a way for high-throughput biosensor control and multiplexed analysis of biomolecular interactions with minimal consumption of reagents. There are several different approaches to fabrication of chip-based structures based on colloidal plasmonic nanoparticles. Liu et al. [27] demonstrated simple fabrication of large-area gold nanostructures using thiol-stabilized gold nanoparticles without complicated lithography and vacuum evaporation techniques involved in the fabrication process. Gold nanoparticles having a mean size of about 5 nm with a distribution range from 2 to 8 nm were obtained using a modernized method of Murray et al. [29]. In this work, hexanethiol was used as the stabilizing functional ligand to cover the gold nanoparticles to reach good dispersion in organic solvents such as xylene and toluene. The Au nanoparticle colloidal solution was spin-coated onto the 10 mm x 10 mm indium-tin-oxide glass substrate at a speed of 2000 rpm for 30 s and then annealed at temperatures in the range of 200-550°C on air. As a result of the melting of Au nanoparticles, nanostructures with different morphologies were formed (Fig. 3.2). The high limit of temperature (550°C) was found due to the observation of undesired structural changes in the transparent glass support. The regulation of temperature during the annealing process and changing the concentration of the Au nanoparticle colloids allow the tunability of the optical response. In addition, a concentration ranging from 40 to 120 mg/ml was recommended by authors for the large-area fabrication of size-optimized gold nano-island structures. It should be noted that a disadvantage of this method is a relatively wide LSPR absorbance peak of prepared structures, which is explained by large size dispersity of nanostructures.

Malynych and Chumanov [28] proposed the method of forming stable random-fashion monolayers of colloidal Ag nanoparticles on the glass substrate with a layer of polymer maintaining only the lower part of the nano particles. This enables LSPR oscillations at the top part of particles, which are free from the polymer. Importantly, arrays of Ag nanoparticles prepared in such a way exhibit extra narrow peaks in the extinction spectra, which can be used to develop high-sensitive biosensors. To produce such a nanoparticle array, a self-assembly of 100 nm silver particles on glass or silicon

Figure 3.2 (a)-(f) Scanning electron microscopy (SEM) images of the samples of gold nanostructures that have been fabricated using an annealing temperature of 200, 250, 300, 350, 450, and SS0°C, respectively, (g) The corresponding optical extinction spectra. Adapted from Ref. [27], Copyright 2010, with permission from Elsevier.

substrates coated with poly(vinylpyridine) (PVP) was created. PVP acts as an efficient surface modifier for the immobilization of metal nanoparticles due to its capability of simultaneous attachment to different substrates via hydrogen bonding and interaction with metal particles due to metal-ligand interactions of the nitrogen atom of the pyridyl group [30]. The PVP-treated substrates were immersed into a colloidal solution of silver nanoparticles in deionized water at low ionic strength. Low ionic strength is needed to support long- range electrostatic repulsion between silver nanoparticles to yield two-dimensional arrays of non-touching particles. By adjusting the exposure time, arrays with various surface densities can be produced. If the average interparticle distance for such arrays becomes of the same order as the nanoparticle diameter, the light extinction spectrum changes drastically with the appearance of a new sharp peak located at 436 nm (Fig. 3.3, black curve).

Figure 3.3 Extinction spectra of 100 nm Ag particles in water (red curve) and the same particles assembled into a closely spaced 2D array imbedded in poly(dimethylsiloxane) (PDMS) (black curve). Inset: electron microscopy image of this 2D array. Reprinted with permission from Ref. [28]. Copyright (2003) American Chemical Society.

In several papers, techniques of utilizing colloidal nanoparticles were extended to obtain nanorod chip-based structures. Two chip-based methods (slow and rapid) have been proposed by Geddes' group [31] for the deposition of silver nanorods onto glass substrates using colloidal Ag nanoparticles. Before nanorod deposition, the surface of glass slides was modified in several steps.

At first, glass slides were treated with "piranha solution” (3:7 30% hydrogen peroxide/concentrated sulfuric acid) for at least 2 h. After that, the substrates were rinsed copiously with deionized water and dried under a stream of dry N₂ gas. The pretreated slides were silanized by dipping into a 2% ethanolic solution of 3-(aminopropyl) triethoxysilane (APS) for 2 h. Then, the APS-coated glass substrates were rinsed in ethanol and water with further sonication in ethanol for 30 s. Subsequently, the glass slides were rinsed with water and dried under a stream of dry N₂ gas. Furthermore, the technology developed by Murphy et al. [32] for the preparation of silver nanorods in solution was modernized for chip-based variant. Briefly, Murphy proposed to use a prepared beforehand silver seed to stimulate growth in silver nanorods by chemical reduction of a silver salt. To fabricate nanorods and nanowires of different aspect ratio, AgN0₃ salt was reduced by ascorbic acid in the presence of silver seed, cetyltrimethylammoniun bromide (СТАВ) surfactant, and NaOH. The rod-like surfactant micelles in solution promoted silver nanorod growth. As a result, it became possible to reproducibly fabricate silver nanorods having varying aspect ratio of 2.5-15 (with 10-15 nm short axes) and nanowires 1-4 micrometer long with 12-18 nm short axes. Subsequently, in the slow method by Geddes, silver nanorods were precipitated onto APS-coated glass slides by ordinary immersion into the silver nanorod solution. The adsorption of silver nanorods on the surface of glass slides from the solution continued for a few days, and the light absorption at 550 nm reached only 20% that of the silver nanorods solution. In the rapid method, spherical silver seeds chemically bound to the glass surface were grown into silver nanorods due to a cationic surfactant and silver ions present in the solution. The length of nanorods was determined by the number and duration of immersions of silver-seed-coated glass slides into a growth solution and ranged from tens of nanometers to several micrometers. The formation of silver nanorods on the glass substrates was evident after 10 min of immersion due to the color change (clear to green) on the glass slide and in the solution. To increase the concentration of silver nanorods on the surface, the silver nanorod-coated glass slides were immersed in similar fresh growth solution containing СТАВ, AgN0₃, ascorbic acid, and NaOH again. This procedure can be repeated till the needed loading of the silver nanorods onto the glass substrate is reached (Fig. 3.4). Interestingly, that minimal change in the process of preparation (immersing the silver-seed-coated glass slides in 40 mL of 0.80 M СТАВ solution for 1-3 min [33] instead of 5 min) results in the growth of triangular structures. These properties of surfactant-based technology are of importance, because the shape of nanostructures sometimes plays a crucial role in their further applications in biosensors. For instance, silver triangles are more suitable for SERS applications, whereas silver nanorods are preferable for biomolecular sensing [34]. The major limitations of the above-mentioned technologies are related to nanoparticle shape, disordering, monodispersity, and reproducibility, which negatively influence their extensive application.

Figure 3.4 Rapid deposition of silver nanorods on a glass substrate. Reprinted with permission from Ref. [31]. Copyright (2005) American Chemical Society.