Abstract
In the present study, zinc oxide (ZnO) nanorods (NRs) with a hexagonal structure have been synthesized via a hydrothermal method assisted by microwave radiation, using specialized cardboard materials as substrates. Cardboard-type substrates are cost-efficient and robust paper-based platforms that can be integrated into several opto-electronic applications for medical diagnostics, analysis and/or quality control devices. This class of substrates also enables highly-sensitive Raman molecular detection, amiable to several different operational environments and target surfaces. The structural characterization of the ZnO NR arrays has been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical measurements. The effects of the synthesis time (5–30 min) and temperature (70–130 °C) of the ZnO NR arrays decorated with silver nanoparticles (AgNPs) have been investigated in view of their application for surface-enhanced Raman scattering (SERS) molecular detection. The size and density of the ZnO NRs, as well as those of the AgNPs, are shown to play a central role in the final SERS response. A Raman enhancement factor of 7 × 105 was obtained using rhodamine 6 G (R6G) as the test analyte; a ZnO NR array was produced for only 5 min at 70 °C. This condition presents higher ZnO NR and AgNP densities, thereby increasing the total number of plasmonic “hot-spots”, their volume coverage and the number of analyte molecules that are subject to enhanced sensing.
Highlights
In the last decade, several efforts have been made to develop inexpensive opto-electronic devices with unique properties, such as flexibility, portability and/or disposability
The cardboard packaging substrates used in this study consist of several compressed layers of cellulose fibres, polymeric coatings and evaporated aluminium
Two sets of experimental conditions were tested for a microwave power input of 50 W: (1) The variation of the synthesis time over 5, 10, 20 and 30 min with a constant temperature of 70 °C, which is the minimum temperature required for zinc oxide (ZnO) NR formation
Summary
Several efforts have been made to develop inexpensive opto-electronic devices with unique properties, such as flexibility, portability and/or disposability. To the best of our knowledge, this is the first report demonstrating the direct growth of ZnO NRs on cardboard substrates using a low-cost and ultra-fast synthesis method (requiring a few minutes) via microwave-assisted hydrothermal synthesis. After complete characterization, these structures were tested as platforms for SERS application by evaporating an ultra-thin silver (Ag) film on the ZnO NRs and allowing the formation of Ag nanoparticles (AgNPs) by a dewetting method [7,42]. Several reports have already demonstrated different applications for ZnO nanostructures decorated with Ag, demonstrating it to be a versatile material [64,65,66]
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