Abstract
Surface-enhanced Raman spectroscopy (SERS) substrates prepared by immobilizing silver cubic nanoparticles (Ag CNPs) on titanium dioxide nanotubes (TiO2 NTs) were used for investigations of the “coffee ring” (CR) effect and its impact on spatial reproducibility of measured Raman signals in comparison with flat surfaces (Ti and Si) where the CR effect is usually significant. The immobilization of nanoparticles from drops, which is a very simple technique, usually does not permit a homogeneous distribution of deposited NPs because there is significant accumulation of the material at the boundary of the drying area. Our proposed SERS substrates effectively reduced the CR effect through the use of well-ordered nanostructures where a smaller number of Ag CNPs were transferred to the boundary region. It was not only the surface morphology that was important but also the physicochemical properties of TiO2 NTs, such as wettability. The wettability of the prepared samples was determined by measuring the static water contact angle (WCA), and the chemical composition near the boundary of the drying area was studied using Auger electron spectroscopy. The morphology of the substrates obtained was characterized using scanning electron microscopy. Our studies showed that reducing the coffee ring effect increased the spatial reproducibility of the measured SERS signal in the area of the deposited CNPs. Therefore, the platforms obtained may be very useful in commercial SERS applications.
Highlights
When nanostructures formed from metals with a negative real and small positive imaginary dielectric constant interact with electromagnetic radiation, an oscillation of the surface conduction electrons is induced, which leads to an increase in the intensity of the electric field near the illuminated nanostructures
Due to its specific morphology and physicochemical properties such as wettability, it was possible to use this substrate for the formation of films of silver nanoparticles by depositing a suspension of Ag nanoparticles and evaporating the solvent; the coffee ring” (CR) effect for films formed of Ag nanoparticles was significantly smaller than for films formed on Ti substrates before nanostructuring
Literature data.[36−39] The substrate of titanium oxide nanotubes with a contact angle of 4° has superhydrophilic properties. It seems that the surface structure, in terms of crystallinity and morphology, is the main factor affecting the hydrophilicity of the annealed nanotubes at 450 °C.36
Summary
When nanostructures formed from metals with a negative real and small positive imaginary dielectric constant (for example, silver or gold nanoparticles) interact with electromagnetic radiation, an oscillation of the surface conduction electrons is induced, which leads to an increase in the intensity of the electric field near the illuminated nanostructures. In the case of blocking, the linkers that connect the nanoparticles to the substrate are usually used.[16,17] Experimentally, the easiest method used for depositing films of plasmonic nanoparticles is to deposit a suspension of such nanoparticles on the substrate and evaporate the solvent. This very simple technique usually does not make it possible to obtain homogeneous SERS substrates because, due to the “coffee ring” (CR) effect, there is significant accumulation of the plasmonic material at the boundary of the drying area (for some SERS materials, the appearance of the CR effect is desirable; see below). The CR effect occurs when a solution containing solutes dries on the substrate[18,19] and manifests as a characteristic line surrounding the place where the drop was Received: March 26, 2020 Accepted: May 14, 2020 Published: June 3, 2020
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