Ag nanoparticles on ZnO nanoplates as a hybrid SERS-active substrate for trace detection of methylene blue

Abstract

Decorating two-dimensional (2D) nanomaterials with nanoparticles provides an effective method to integrate their physicochemical properties. In this work, we present the hydrothermal growth process of 2D zinc oxide nanoplates (ZnO NPls), then silver nanoparticles (AgNPs) were uniformly distributed on the surface of ZnO NPls through the reduction procedure of silver nitrate with sodium borohydride to create a metal-semiconductor hybrid. The amount of AgNPs on the ZnO NPls' surface was carefully controlled by varying the volume of silver nitrate (AgNO₃) solution. Moreover, the effect of AgNPs on the surface-enhanced Raman scattering (SERS) property of ZnO NPls was thoroughly investigated by using methylene blue (MB) as the target molecule. After calculation, the maximum enhancement factor value for 10^-4 M of MB reached 6.2 × 10⁶ for the peak at 1436 cm^-1 and the limit of detection was 10^-9 M. In addition, the hybrid nanosystem could distinguish MB with good reproducibility over a wide range of concentrations, from 10^-9 to 10^-4 M. The SERS mechanism is well elucidated based on the chemical and electromagnetic mechanisms related to the synergism of ZnO and Ag in the enhancement of Raman signal. Abundant hot spots located at the gap between adjacent separate Ag nanoparticles and ZnO nanoplates which formed a strong local electromagnetic field and electron transfer between ZnO and Ag are considered to be the key factors affecting the SERS performance of our prepared ZnO/Ag substrates. In this research, we found high sensitivity of ZnO nanoplates/Ag nanoparticles in detecting MB molecules. This unique metal-semiconductor hybrid nanosystem is advantageous for the formation of Raman signals and is thus suitable for the trace detection of methylene blue.

Scheme 1. Schematic illustration of ZnO/Ag hybrid preparation process.

Fig. 1. SEM images of ZnO nanoplates (a) and ZnO/Ag1 (b), ZnO/Ag2 (c), ZnO/Ag3 (d), ZnO/Ag4 (e), and ZnO/Ag5 (f) grown with different amounts of AgNO₃.

Fig. 2. XRD pattern from AgNPs (a), ZnO NPls (b), ZnO/Ag with different Ag contents (c), ZnO/Ag5 (d) and crystalline size of these samples (e).

Fig. 3. (a–c) Raman spectroscopy of ZnO NPls and ZnO/Ag with varying Ag amounts, (d) FTIR spectra of the ZnO NPls (red line), pure MB molecules (black line), ZnO/Ag3 without MB (blue line), MB adsorbed on ZnO/Ag3 (green line).

Scheme 2. Energy band diagram of ZnO/Ag interface showing the electron transfer between Ag and ZnO.

Fig. 4. (a) Typical TEM and (b) HRTEM images of ZnO, (d) TEM and (e) HRTEM images of ZnO/Ag3 and corresponding spot-profile EDS spectra of (c) pure ZnO, (f) ZnO/Ag3.

Fig. 5. (a) The UV-Vis absorption spectra of ZnO and ZnO/Agx samples, (b) plot of (αhν)²versus hν, (c) the band-gap from 2.5 to 3.2 eV, (d) absorption spectra of MB alone (black line), of MB adsorbed on ZnO (red line) and ZnO/Ag3 (dash line).

Fig. 6. Photoluminescence (PL) spectra of ZnO and ZnO/Ag excited at 345 nm.

Fig. 7. Raman spectra of MB on Si substrate (a), MB adsorbed on AgNPs, ZnO NPls, ZnO/Ag (b), MB concentration in all experiments as 10⁻⁴ M.

Fig. 8. (a) Raman spectra of MB adsorbed on ZnO/Ag3 at concentrations ranging from 10⁻⁹ M to 10⁻⁴ M, (b) the linear relationship between the log I and log C of the 1436 cm⁻¹ peak, where I is the SERS intensity and C is the MB concentration, (c) Raman spectra of MB adsorbed on ZnO/Ag3 with different concentrations from 10⁻⁸ M to 2.2 × 10⁻⁶ M, (d) the linear relationship between SERS intensity and MB concentration.

Fig. 9. SERS spectra of varied MB concentrations ((a) 0.01 ppm, (b) 0.2 ppm, (c) 0.8 ppm, and (d) 6 ppm) were gathered at 10 randomly selected positions.