Surface-enhanced Raman spectra of medicines with large-scale self-assembled silver nanoparticle films based on the modified coffee ring effect
© Zhou et al.; licensee Springer. 2014
Received: 6 December 2013
Accepted: 22 January 2014
Published: 19 February 2014
We report here a simple and innovative method to prepare large-scale silver nanoparticle films based on the controlled coffee ring effect. It is demonstrated that the films can be used as surface-enhanced Raman scattering probes to detect low-concentration medicines. Silver nanoparticles with the average size about 70 nm were prepared by reduction of silver nitride. In our experiment, the coffee ring effect was controlled by tilting the substrates during the deposition of silver nanoparticle films. Silver nanoparticle films were spontaneously formed on the surface of silicon substrates at the temperatures about 50°C based on the solvent evaporation and the coffee ring effect. The microstructure of the films was investigated using the scanning electron microscope and atomic force microscope. The surface roughness of the films is found as small as 20 nm. Then, the films were exposed to aqueous solutions of medicine at different concentrations. A comparison with a Raman spectra measured with a conventional Raman spectrometer showed that the Raman signal can be detected in the solution with concentrations as low as 1 × 10−5 M, and the enhancement factor achieved by the silver nanoparticle film can at least reach to 1.08 × 104. Our experimental results indicate that this technique is promising in the production of large-scale silver nanoparticle films for the surface-enhanced Raman scattering. These may be utilized in biochemical and trace analytical applications.
Raman spectroscopy is an important analytical technique for chemical and biological analysis due to the wealth of information on molecular structures, surface processes, and interface reactions that can be extracted from Raman spectra . The Raman cross section of a normal Raman spectroscopy is inherently weak, thus preventing from the application of high-sensitivity analysis. Fortunately, for the last three decades, Raman techniques have experienced increasing application in many fields due to the observations of the enormous Raman enhancement of molecules adsorbed on special metallic surfaces. In 1974, it was first reported that an unusually strong enhanced Raman scattering signal occurred with pyridine molecules adsorbed on silver electrode surfaces that had been roughened electrochemically by oxidation-reduction cycles . It was discovered that this process may enhance Raman activities at a 106-fold at an appropriately prepared coinage metal surface.
Since its discovery in 1970s, surface-enhanced Raman spectroscopy (SERS) is becoming more attractive for applications, and it is fast moving from fundamental research to analytical applications in the biomedical and environmental areas . The further development of SERS is mainly limited by the reproducible preparation of clean and highly active substrates . The original substrates for SERS were electrochemically roughened metal electrodes . Metallic nanoparticle films were used shortly after the discovery of the SERS effect and became the most studied class of substrates. Up to date, the SERS probes can be arbitrarily classified in three categories: (1) metallic nanoparticles in suspension, (2) metallic nanoparticles immobilized on solid substrates, and (3) nanostructures fabricated directly on solid substrates, which include nanolithography, template synthesis of nanostructures, pulsed laser deposition, and laser lithography [5–8].
The application of dispersed and aggregated metallic nanoparticles as a SERS probe in a real analytical problem is limited due to the poor reproducibility. The reproducibility problem can be mitigated by immobilizing the metallic nanoparticles on some kind of solid support . Since the report of a SERS substrate consisting of metallic nanoparticles synthesized by a wet chemistry method and subsequently immobilized onto a solid support , the procedure gained popularity. Several works have been published based on this approach and its variations [8, 11–13]. The attempts of using self-assembled metallic nanoparticles on solid support as a SERS substrate was reported by the Natan group in 1995 . The self-assembly of metallic nanoparticles onto solid surfaces based on electrostatic attraction using polymers [14–16] and biomolecules [17, 18] has also been widely reported, such as poly(vinylpyridine) which was used to immobilize Ag nanoparticles onto continuous Ag films . Bifunctional SERS-active single microsize particles can be fabricated through the electrostatic-induced self-assembly. For example, Spuch-Calvar et al.  reported the fabrication of SERS and magnetic bifunctional spindle particles using polyelectrolyte as the linking reagent. Although the chemical and electrostatic self-assemblies are popular for fabricating SERS substrates, different approaches have also been explored. For example, capillary forces, dominant during the evaporation of a liquid droplet, can be used to drive the assembly of metallic nanoparticles [21–23]. The Halas group  used a drop-dry method to assemble a film of CTAB-capped nanoparticles on silicon wafers.
We report here a simple method to prepare large-area silver (Ag) nanoparticle films based on the coffee ring effect for the use of SERS. The ‘coffee ring effect’ is widely known as a typical evaporation-driven self-assembly and self-organization . When a droplet of solutions containing nonvolatile solutes (e.g., coffee particles) dries on a substrate, it leaves a dense, ring-like deposit of the solutes, i.e., a ‘coffee ring,’ along the perimeter. In an industrial inkjet printing [25, 26] and a biological application , a uniform pattern is usually required. The ‘coffee stain effect’ is an undesirable phenomenon. Thus, some efforts were spent to eliminate the coffee ring effect by changing the shape of the suspended particles . In this paper, we show an innovative method to control the coffee ring effect by simply tilting the substrate and thereby obtaining a large-scale silver nanoparticle film. Moreover, the film can be applied as substrates for SERS to detect medicines. 5-Fluorouracil was selected as a model drug in this experiment since 5-fluorouracil-containing solutions and creams are extensively used in human patients for the treatment of solar and actinic keratoses and some superficial skin tumors. 5-Fluorouracil, an antimetabolite, is also used in veterinary medicine for the treatment of some cancers [29, 30]. Drug content in the solution of a low concentration can be detected according to our experimental results. Our experimental results indicate that this self-assembly method shows great promise in the production of large-scale metallic films. These may be utilized in biochemical sensing and optical processing applications.
Preparation of silver nanoparticles
Silver nitrate (AgNO3), sodium citrate dehydrate, and deionized water, all in analytical grade, were used without further purification. Ag nanoparticles used in this study were prepared by an aqueous method as follows: Ag nanoparticles were synthesized through a chemical reduction reaction of AgNO3 using sodium citrate dehydrate as reducing agent. Details of the synthesis procedure have been presented in a previous study . A solution of AgNO3 (1 mM) in 250-mL ultrapure water was heated to 80°C. A volume of 10-mL aqueous solution of Na3C6H5O7 · 2H2O (0.34 mM) was then added to the AgNO3 solution. Heating was continued to 90°C for 30 min after adding the citrate solution. The color of the solution changed from the colorless water to yellow after 15 min of heating and to gray after 25 min. The resulting sol is simply silver nanoparticles coated with organic shell, dispersed in water at a concentration of 1 mM [32, 33].
Preparation of silver nanoparticle solution with different concentrations
The different concentrations of the silver nanoparticle solution were fabricated by increasing the concentration of the silver nanoparticle solution from 1 mM to 0.1 M by centrifugation. Centrifugation was conducted at 9,000 revolutions per minute (rpm) for 5 min in 10-mL centrifuge tubes. The water was extracted from the centrifuge tubes using a pipette, leaving aqueous-based Ag nanoparticle paste at the bottom. Shock the tube to make the nanoparticle paste back into suspension, then collect the rest of the solution for the next centrifugation. Repeat this process until the required concentration solution was obtained.
Preparation of silver nanoparticle films on silica substrates
The absorption spectrum of the silver colloid was obtained using a UV-vis (UV-9000S, Shanghai Metash Instruments Co., Ltd., China) spectrophotometer. The morphology of the silver nanoparticles was examined by transmission electron microscopy (TEM; JEM-2010, JEOL Ltd., Akishima, Tokyo, Japan). The silver nanoparticle films were imaged using a scanning electron microscope (SEM; XL30 S-FEG, FEI Co., Hillsboro, OR, USA). The cross-sectional profiles of the silver nanoparticle films were measured using an atomic force microscope (AFM; Pico Scan TM 2500, Scientec, Les Ulis, France) and a Veeco surface profiler (Wyko NT1100, Veeco Instruments Inc., Plainview, NY, USA).
Applications for surface-enhanced Raman spectroscopy
The silver nanoparticle film on silicon wafers was used as a substrate for surface-enhanced Raman spectroscopy. 5-Fluorouracil was dissolved in water using an ultrasonic cleaning machine for 5 min. 5-Fluorouracil is sparingly soluble in water . In our experiment, the concentration of solution 1 × 10−1 M was not obtained because of the low solubility of 5-fluorouracil at room temperature. The concentrations of the solution were prepared as 1 × 10−2 M, 1 × 10−3 M, and down to 1 × 10−6 M. Then, the solution was dropped on the substrate for Raman detection. The SERS signal was measured with a commercial Raman equipment (inVia-Reflex, Renishaw, Gloucestershire, UK) using a laser with a 532-nm wavelength as the excitation source; the measuring laser spot size was about 3 μm, and the acquisition time was 10 s.
Results and discussion
Large-scale self-assembled silver nanoparticle films formed on the substrate are based on the modified coffee ring effect. When a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in a ring-like fashion, forming a coffee ring [23, 27]. In the present study, the conditions for forming the coffee ring were modified. At the concentration of silver nanoparticle solution ranging from 50 mM to 0.1 M with an incident substrate, smooth silver nanoparticle films can be obtained. The evaporating solution features an air-water interface shaped like a spherical cap. At the perimeter, the deposition of particles will pin the contact line, and thus, the radius of the liquid surface cannot shrink . To realize this during evaporation, liquid must flow outwards. In practical, the liquid surface certainly decreases with the reduction of the solution. This results in the contact line moving inward. During the contact line movement, the capillary flow outward from the center of the solution brings suspended silver nanoparticles to the edge as evaporation proceeds . Then, the self-assembled silver nanoparticles are deposited on the solid-liquid contact line. With the solid-liquid contact line moving inward, the silver nanoparticle film will be formed. Optimizedly, the decreasing speed of the liquid surface is synchronous with the forming velocity of solid films on the edge. At a low concentration of solution, almost all of the nanoparticles were deposited on the outer ring, causing no film generated inside, as shown in Figure 4a. Increasing the concentration up to 10 mM, scattering particles are deposited inside the ring. When the concentration is high enough, such as 50 mM or 0.1 M in our experiment, the silver nanoparticles promptly will fill the solid-liquid contact line and thereby form a smooth film.
An innovative concept of preparing silver nanoparticle films based on the coffee ring effect using the surface-enhanced Raman spectroscopy for the detection of the low-concentration medicine is demonstrated. Silver nanoparticles with the average size about 70 nm were prepared by reduction of silver nitride. In our experiment, the coffee ring effect was controlled and used for preparing silver nanoparticle films. The silver nanoparticles were spontaneously formed on the surface of the silicon substrate at the temperatures about 50°C based on the coffee ring effect. The quantitative characterization of the surface characteristics shows that the average roughness of the film is from 20.24 to 27.04 nm prepared using the solution of the concentration from 50 mM to 0.1 M. It is evident that the silver nanoparticle film exhibits the remarkable surface-enhanced Raman scattering effect. Raman signal can be detected in the 5-fluorouracil solution with concentrations as low as 1 × 10−5 M, and the enhancement factor achieved by the silver nanoparticle film can be higher than 1.08 × 104. Our experimental results show great promise in the production of large-scale silver nanoparticle films for the surface-enhanced Raman scattering.
The work is partially supported by the Beijing High-level Overseas Talents project and strategic research project of Beijing Natural Science Foundation, People's Republic of China.
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