Reusable Plasmon-Decorated Chestnut-like Copper Oxide Nanostructures for SERS Sensing
Authors: Shvalya Vasyl , Filipič Gregor , Vengust Damjan , Zavašnik Janez , Modic Martina , Abdulhalim Ibrahim , Cvelbar Uros 
Published: May 2020

Reusable plasmonic substrates are crucial for the development of biosensing applications using surface-enhanced Raman scattering (SERS), as they can provide unique advantages for ultrafast and accurate single-molecule recognition of different species. In this research, we employed thermally annealed cupric CuO and cuprous oxide Cu2O heterostructures to serve as highly stable nanotextured surfaces for designing robust 3D plasmonic biochips.
Figure 1: Schematic representation of a research outline. Step 1 – controllable surface texturing by thermal annealing at oxygen-enriched atmosphere. Step 2 – plasmonic activation by ion spattering of Au/Pd alloy film. Step 3 – SERS performance monitoring by means of R6G dye fingerprint detection. Step 4 – Recovery test under ultra-fast mild cleaning in reactive oxygen/argon plasma. Step 5 – Simplified scheme of Rhodamine degradation inside plasma followed by the vanishing of the characteristic peaks in the Raman spectrum.
Figure 2: (A-E) – SEM images showing the cuprous oxide Cu2O surface morphology altering with Au/Pd alloy thickening. (F) – Broadband Raman spectra representing vibrational features of chestnut Cu2O structures decorated by different plasmonic thicknesses together with ethanol diluted Rhodamine 10-5 M absorbed on the surface. (G) – Normalized intensity of the in-plane C-H bending vibrations of R6G 10-5 M as a function of bimetallic film thickness.
The influence of bimetallic layer thickness on SERS performance has been studied for the chestnut patterned substrate. Figure 2 F depicts Raman spectra of Rhodamine used as a model biomarker (600-1800 cm-1) complemented by a signal originated from the Cu2O layers (100-800 cm-1). During Au/Pd film thickness reduction from 80 nm down to 20 nm, the R6G peak intensities undergo a gradual decrease, simultaneously, acoustic bands originated from Cu2O nanoparticles located in a range of 100-800 cm-1 become dominant. In accordance with Figure 2 G, that displays a signal intensity dropping for in-plane C-H bending vibrations as a function of bimetallic layer thicknesses, the analytical enhancement factor AEF acquires the following values: for 60 nm ~ 9.8×104, for 40 nm ~ 8.4×104 and for 20 nm ~ 1.1×104, respectively. The analogous finding was observed for Au decorated CuO nanoflakes, where EF increases with a noble metal film thickening [1].
Due to prominent anti-reflection properties, CuO/Cu2O heterostructures [2] appear to be an extremely suitable material for designing SERS-active substrates. Moreover, the high chemical inertness of Au/Pd alloy plasmonic layer allows a non-destructive substrate recovery with the assistance of reactive plasma species. Furthermore, the generation of reactive oxygen species-rich plasma by introducing oxygen gas into argon plasma drastically reduces cleaning session to less than 1 minute. For our best knowledge, this is the shortest recovery time for any 3D plasmonic SERS substrate reported in the literature. Additionally, results indicate that the nano-roughness is more important for achieving optimal plasmonic activity than micro-roughness. Overall, the designed chestnut-like Au/Pd@/Cu2O substrate is a ready-to-use chip that possesses a decent signal enhancement ~ 105 and reveals remarkable robustness under multiple plasma treatment showing nearly 100% of self-recovery with no degradation of a plasmonic layer. These findings are of great significance for the development of novel reliable SERS substrates with low-cost manufacture which can be utilised with great success in a wide area of plasmonic biosensing.

REFERENCES

[1] A. Balčytis, M. Ryu, G. Seniutinas, J. Juodkazyte, B. C. C. Cowie, P. R. Stoddart, M. Zamengo, J. Morikawa, S. Juodkazis, “Black-CuO: Surface-enhanced Raman scattering and infrared properties”, Nanoscale, 7, 43, October 2015, 18299-18304.
[2] J. Yu, M. Yang, C. Zhang, S. Yang, Q. Sun, M. Liu, Q. Peng, X. Xu, B. Man, F. Lei, “Capillarity-Assistant Assembly: A Fast Preparation of 3D Pomegranate-Like Ag Nanoparticle Clusters on CuO Nanowires and Its Applications in SERS”, Advanced Materials Interfaces, 5, 19, July 2018, 1-8.

Figure 1