The synthesis of controlled shape nanoplasmonic silver-silica structures by combining sol-gel technique and direct silver reduction

Ramanauskaite and Snitka Nanoscale Research Letters The synthesis of controlled shape nanoplasmonic silver-silica structures by combining sol-gel technique and direct silver reduction Lina Ramanauskaite 0 Valentinas Snitka 0 0 Research Center for Microsystems and Nanotechnology, Kaunas University of Technology , Studentu 65, LT-51369 Kaunas , Lithuania In this work, we have obtained nanoplasmonic silver structures deposited on the glass substrates by combining sol-gel technology and direct silver ion reduction on the film surfaces. The key point of the work was the usage of polyethylene glycol 400 (PEG 400) both as the pore former and reducing agent for silver ions. We have investigated the influence of PEG 400 amount on the formation of silver nanoparticles on the film surface. It was found that control of PEG 400 amount in the sols allows the creation of porous films with specific organized silver nanoparticles or clusters on the surface. Optical, morphological and structural characteristics of the structures were measured and studied. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used for nanostructure size and shape characterization. We were able to form a 40- to 200-nm-diameter ring-type, spherical and self-assembled nanoparticles on the film surface. The results of UV-vis absorbance spectra have shown the high quality of plasmonic structures with plasmon resonance wavelength in the region between 470 and 480 nm. The synthesized silica films decorated with silver nanoparticles were tested as substrates for the surface-enhanced Raman spectroscopy (SERS) and showed an enhancement relative to micro-Raman of more than 200 times. Sol-gel; Silver nanoparticles; SERS; PEG 400; Nanoplasmonic silver structures - Background Since their discovery in 1957, surface plasmons have been exploited in applications as diverse as singlemolecule surface-enhanced Raman spectroscopy (SERS), nanoscale optical modulators, high-efficiency solar cells, nanoscale lasers, biological rulers and electromagnetic meta-materials for invisibility and sub-diffraction-limited optical microscopy [1]. Noble metal nanoparticles supporting plasmonic resonances behave as efficient nanosources of light, heat and energetic electrons. Owing to these properties, they offer a unique playground to trigger chemical reactions at the nanoscale [2]. Therefore, the emergence of the field of plasmonics - the science and engineering of electromagnetic field interactions with metallic nanostructures - has a broad scope. However, the precise understanding of the various types of plasmon resonances has a great interest in development and application of plasmonic substrates. Most of the plasmonic substrates are 2D planar systems which limit the active area to a single Cartesian plane. The fabrication of 3D plasmonic substrates with the aim to extend the SERS hot spots into the third dimension along the z-axis is a way to open new applications of plasmonic structures. There are various methods such as lithography [3-5], laser ablation [6,7], sputter coating [8] or chemical synthesis [9,10] used for the preparation of plasmonic structures. All these methods allow obtaining nanostructures of desirable sizes, shapes or arrangements which are very important for high-sensitivity Raman measurements. One of the most important parameters determining the sensitivity of SERS substrates is the shape of nanoparticles. For example, sharp-edged silver nanotriangles or nanocubes produce high electromagnetic field at the edges of the nanoparticles and result in a strongly enhanced Raman signal [11,12]. Another example is a dimer-type plasmonic nanostructure separated by the nanogaps. Electromagnetic field generated in the nanosized gaps is one of the main enhancement mechanisms to realize a single-molecule SERS [13]. Figure 1 Illustration of the synthesis of the hybrid silica films. The chemical preparation of SERS substrates often includes classical sol-gel technology [14-16] attractable for most authors because of its simplicity and inexpensiveness. This method allows the formation of the films on various surfaces directly from the solution, control the porosity by varying synthesis parameters or using organic templates, functionalization of the films with noble metals nanoparticles as well as the formation of self-assembled structures. In this paper, we propose a new chemical route for the synthesis of hybrid silica films using polyethylene glycol 400 (PEG 400) as porogen and reducing agent. Within a few years, a number of publications represented polyethylene glycol as a perfect substance for green reduction of silver ions and stabilization of silver nanoparticles [17-21]. It also became attractive to its solubility in aqueous media, low toxicity and wide selection of molecular weights [22]. In our work, we present a novel methodology for the preparation of 3D silica films decorated with silver nanostructures by combining sol-gel technology and direct silver ion reduction and demonstrate the possibility to control the shape of synthesized plasmonic structures from the ring up to the networked nanoparticles. The efficiency of the fabricated SERS substrates for the enhancement of Raman signal was tested using crystal violet dye. Methods Materials For the synthesis of hybrid silica films, ethanol, water, hydrochloric acid, PEG 400 and silver nitrate were purchased from Sigma-Aldrich (St. Louis, MO, USA) and tetraethylortosilicate (TEOS) were from Acros Organics (Geel, Belgium). All the reagents were analytical grades and used without further purification. Figure 2 Proposed mechanism of PEG 400 interaction with silane network. Figure 3 Raman spectrum of PEG 400-modified silica films after thermal treatment at 300C. (a) A film prepared at a volume ratio of sol: PEG 1:0.05 (1); a film prepared at a volume ratio of sol:PEG 1:0.10 (2); a film prepared at a volume ratio of sol:PEG 1:0.15 (3); (b) a film prepared at a volume ratio of sol:PEG 1:0.10 before the deposition of silver nanoparticles showing a band of -OH vibrations responsible for the reduction of silver ions (1) and after the deposition of silver nanoparticles showing the absence of these vibrations and suggesting the complete reduction reaction (2). Preparation of silver nanoparticle-decorated silica films The sol was prepared by mixing TEOS, ethanol and water in the flask with ratios 0.2:0.4:1, respectively. Reaction was carried out under the acidic conditions: hydrochloric acid was used to reach the pH value of 2.3. The solution was mixed for 1 h maintaining a constant temperature of 60C and finally divided into three flasks. Pure PEG 400 was added to reach sol/PEG ratios of 1:0.05 (v:v), 1:0.10 (v:v) and 1:0.15 (v:v) in each flask, respectively. All PEG 400-modified sols were aged for 48 h at room temperature. The films were formed on the ethanol-cleaned microscopic glasses by spin-coating method with a spin speed of 1,500 rpm and a spin (...truncated)