Development of Solid SERS Substrates Based on Metallic Nanostructures

In addition to immobilized colloids, researchers have developed a variety of solid surface-based SERS substrates that are produced entirely from solid materials, as depicted schematically in Fig. 1. In contrast to immobilized colloids, the solid SERS-based probes described subsequently exhibit a high degree of reproducibility. In addition, nanoshells with a dielectric core and a metallic shell (e.g., Au/SiO2), or a metallic core and a dielectric shell (e.g., Au core/SiO2 shell), or a metallic core and a metallic shell (e.g., Au core/ Ag shell, Ag core/Ag shell, and Ag core/Au shell), provide a tunable geometry in which the magnitude of the local electromagnetic field at the nanoparticle surface can be precisely controlled (48-51).

2.3.1. Metal Nanoparticle Island Films

Metallic nanostructured SERS substrates based on metal island films (Fig. 1A) are among the most easily prepared surface-based media, granted the availability of a vacuum evaporation system. Such systems are commonly equipped with crystal microbalances for monitoring metal film thickness. Metal island films can be produced by depositing a thin (<10 nm) layer of a metal directly onto a smooth solid base support via sputter deposition (40) or vacuum evaporation. At such a small thickness, the metal layer forms as aggregated, isolated metal islands, the size and shape of which can be influenced largely by the metal thickness, deposition rate, geometry, and temperature, as well as postdeposition annealing. To optimize the production of SERS-active metal island films, various diagnostic techniques have been used, including optical absorption, atomic force microscopy, SERS, or combinations thereof (52,53). More recently, silver island films have been characterized by combining NSOM with Raman spectroscopy, thereby achieving <70-nm resolution (54). A disadvantage of metal island films is that they are easily disturbed by solvents encountered in typical biomedical analyses (55,56). To minimize this disadvantage, buffer metal layers, (3-mercaptopropyl)-trimethoxysilane layers, and organometallic paint layers have been applied to glass supports to stabilize gold island films (57).

Metal Island Optical Coating
Fig. 1. Various types of SERS-active metallic nanostructures: (A) metal island films; (B) metal-coated nanospheres (seminanoshells); (C) metal-coated random nanostructures; (D) polymer coatings embedded with metal nanoparticles.

2.3.2. Metal-Coated Nanosphere Substrates

In our laboratory, we have developed a very dependable solid surface-based SERS substrate technology that can be generally described as metal-coated dielectric nanospheres (Fig. 1B) supported by various planar support media. Nanospheres within a specific size range (e.g., 50-500 nm) are spin coated on a solid support in order to produce the roughness required to induce the SERS effect. The resulting nanostructured plate is then coated with a layer of silver (50-150 nm), which provides the conduction electrons required for the surface plasmon mechanisms. All factors of surface morphology are easily controlled, enabling high batch-to-batch reproducibility. An additional advantage is that the relatively thick layer of silver is less vulnerable to air oxidation than silver islands. Furthermore, the surface is highly resistant to disturbance by sample solvents, making this type of SERS substrate very practical for biomedical applications. Teflon and latex are particularly well suited for SERS substrates because they are commercially available in a wide variety of sizes, which can be selected for optimal enhancement.

Preparation of a nanosphere substrate is relatively easy. A 50-^L aliquot of a suspension of nanospheres is deposited evenly over the surface of a dielectric planar support medium, such as filter paper, cellulosic membrane, glass, or quartz (58-62). Using a photoresist to produce uniform monolayer coverage, the substrate is then spun at 800 to 2000 rpm for about 20 s. The spheres adhere to the glass surface, providing uniform coverage. The nanosphere-coated support is then placed in a vacuum evaporator, where silver is deposited on the roughened surface at a rate of 0.15 to 0.2 nm/s.

2.3.3. Metal-Coated Nanoparticle Substrates

Nanoparticles with irregular shapes (Fig. 1C) can also be used in place of regularly shaped nanospheres in the production of dependable, cost-effective SERS substrates. Dielectric nanoparticle materials investigated in our laboratory have included alumina (63), titanium dioxide (64), and fumed silica (65). The production of irregular nanoparticle-based substrates is achieved with an ease equivalent to that for nanosphere-based substrates described in Subheading 2.3.2. Generally, 5 to 10% (w/v) aqueous suspensions of the nanoparticles are spin coated onto solid support media, then coated by 75 to 150 nm of silver via vacuum evaporation. As an alternative to the vacuum evaporation process, other groups have investigated silver coating via chemical processes (66,67). Nevertheless, substrates prepared via vacuum evaporation yield exceptional reproducibility.

Alumina-based substrates produced by vacuum evaporation have proven to be among the most dependable, with a batch-to-batch variability of typically <10% for induced signals of selected model compounds. The surface of an alumina-based substrate consists of randomly distributed surface agglomerates and protrusions in the 10- to 100-nm range. These structures produce large electromagnetic fields on the surface when the incident photon energy is in resonance with the localized surface plasmons. Alumina-based substrates have numerous applications (68-75).

Silver-coated titanium dioxide (64) and fumed silica (65) surfaces also provide efficient SERS-active substrates. Titanium dioxide provides the necessary nanosize surface roughness for the SERS effect; the nominal particle diameter of TiO2 used in our probes is 0.2 ^m. When coated with 50 to 100 nm of silver, this type of probe can yield exceptional SERS enhancement with limits of detection of various compounds in the parts-per-billion range. Fumed silica substrates have allowed trace detection of polynuclear aromatic hydrocarbons and pesticides (64,76).

2.3.4. SERS Substrates Based on Metal-Coated Quartz Posts

Silver-coated, regularly spaced submicron posts formed in quartz substrates have proven to be a dependable, yet labor-intensive, SERS substrate. Lithographic techniques have been used to control the surface roughness to a degree suitable for testing the electromagnetic model of SERS (77,78). Although these surfaces produce a Raman enhancement on the order of 107, they are difficult to produce with a large surface area. However, an alternative etching procedure for producing quartz posts overcomes this limitation by using an island film as an etching mask on an SiO2 substrate (79-83). The preparation of SiO2 prolate posts is a multistep operation, as depicted in Fig. 2. A 500-nm layer of SiO2 is first thermally evaporated onto a fused quartz base support at a rate of 0.1 to 0.2 nm/s. The resulting thermally deposited crystalline quartz is annealed at 950°C to the fused quartz for 45 min. A 5-nm silver layer is then evaporated onto the thermal SiO2 layer, and the substrate is flash heated (500°C) for 20 s, causing the thin silver layer to bead up in small globules. These isolated silver globules act as etch masks when the substrate is subsequently etched for 30 to 60 min in a CHF3 plasma. This etching produces submicron prolate SiO2 posts under the silver globules. Since fused quartz is etched much more slowly than is thermally deposited quartz, the fused quartz base survives the etching process. The posts are then cleaned to remove the silver etch mask and coated with either a continuous 80-nm silver layer (83) or a discontinuous layer with the silver deposition restricted to the tips of the quartz posts (80,81). A disadvantage of quartz post-based substrates is the difficult, time-consuming etching procedure required for post fabrication. Nevertheless, optimized quartz posts can serve as the base for a reusable SERS substrate, provided that the silver coating is replaced between uses.

2.3.5. SERS Substrates Based on Metal Nanoparticle-Embedded Media

Silver nanoparticles embedded in various solid porous media (Fig. 1D) have recently been investigated as stable SERS substrates with the potential for selective detection. The in situ production of these nanoparticles in solid matrices provides several advantages. For example, a solid matrix not only spatially stabilizes but also physically protects the colloids. In addition, the porosity of such materials as sol-gels, cellulose acetate gels, and polycarbonate films permits interaction of analyte compounds with the embedded metal nanoparticles.

Plasma Etching Fused Quartz
Fig. 2. Schematic diagram of production of SERS-active quartz post: (A) after thermal deposition of quartz on fused quartz base; (B) after deposition and annealing of silver island film; (C) after plasma etching; (D) after removal of silver etch mask.

Furthermore, control of pore size and matrix polarity through simple chemical means (particularly when using the sol-gel technique) can impart selectivity. Finally, the chemical processes generally used to prepare such substrates make the SERS technique accessible for general analytical laboratories.

A silver nanoparticle-embedded sol-gel substrate produced through the chemical reduction of silver halide particles distributed in the sol-gel matrix has been reported for the detection of neurotransmitters and dopamine (84,85). In situ precipitation of silver chloride nanoparticles via reaction of silver nitrate with trichloroacetic acid throughout the sol-gel matrix was performed before curing. Immediately prior to use, the silver chloride particles were reduced to elemental silver nanoparticles with FeSO47H2O. An advantage of this technique is that the sol-gel can be stored for long periods in the nonreduced form, precluding vulnerability to air oxidation.

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