5-Fluorouracil was dissolved in water using an ultrasonic cleaning machine for 5 min. 5-Fluorouracil is sparingly soluble in water [34]. SIS3 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 Figure 2a shows the UV-vis absorption spectrum and a typical TEM image of silver nanoparticle suspension. It can be seen from the figure that the strongest peak appears at 440 nm, and a shoulder appears at 360 nm. The absorption spectra for the 40-nm silver sphere were obtained using the Mie theory [35]. The calculated spectra for the 40-nm silver sphere shows two resonance peaks: a main dipole resonance peak at 410 nm and a weaker quadrapolar resonance at 370 nm as a shoulder. The dipole resonance BMS-907351 in vitro arises from one side of the sphere surface being positively

charged, whereas the opposite side is negatively charged, giving the particle itself a dipole moment that reverses the sign at the same

frequency as the incident light [36]. In Figure 2, it also presents a typical transmission electron microscopy image of the silver nanoparticles. It can be seen directly that the size of the nanoparticles is around tens of nanometers. Figure 2b shows the particle size distribution of 500 arbitrarily measured nanoparticles. The average particle size is around 70 nm. The larger particles shift the resonant wavelength to red [37]. Our results coincide well with the theoretical results. Figure 2 Absorption spectra and particle size distribution of nanoparticles. (a) Absorption spectra of silver nanoparticles. The inset shows the image of silver nanoparticles obtained by transmission electron microscopy; the scale of the image is 20 nm. (b) The particle science size distribution of 500 nanoparticles. Figure 3 shows the photos of silver nanoparticle film prepared with different concentrations of silver nanoparticle solution. It can be seen from Figure 3a that, at the concentration of 1 mM, only a circle pattern is formed on the edge of the solution. Because of the coffee ring effect, only a dense, ring-like deposit exists along the perimeter [23]. When the concentration is up to 10 mM, a grid-like film was formed on the surface of the wafer, as shown in Figure 3b. Continuing to increase the solution concentration in Figure 3c,d, a uniform thin film formed when the concentrations are 50 mM and 0.1 M.