(Invited) Engineering of Nanostructured Aluminum-Based and Polymer-Based Platforms for SERS Sensing
Authors: Ijaz Dar Gohar , Xifre-Perez Elisabet , Marsal Lluis F. 
Published: August 2024

There has been great hope for ultrasensitive detection methods with the application of Surface Enhanced Raman Spectroscopy (SERS) because of its impressive advancements. Nevertheless, a major challenge is the creation of large-scale, controllable, and reproducible substrates with high SERS activity [1]. As a basis for SERS detection, we report here the fabrication of different types of aluminum-based and polymer-based SERS platforms with the synthesis of gold nanoparticle (AuNP) arrays on them. The tunability of the AuNPs size and distribution is achieved by methodically controlling the aluminum substrates nanostructuration and the subsequent processes for the synthesis of the AuNP. This research presents a scalable and dependable approach for creating ordered Au patterns on nanostructured aluminum and polymer substrates.
For both aluminum-based and polymer-based platforms, the initial formation of nanoporous anodic alumina (NAA) on an aluminum substrate by electrochemical anodization [2] is essential. The electrolytes used (sulphuric, oxalic, or phosphoric acid) and its respective concentration determines the ideal voltage for anodization and the electrolyte temperature [3,4]. The formed NAA layer is chemically removed obtaining a nanostructured aluminum substrate with a hexagonally ordered distribution of nanoconcavities [5,6]. The distance between nanocavities ranges from tens to hundreds of nanometers depending on the electrolyte used [7]. The polymer-based nanostructured substrates are formed with the deposition of the selected polymer on the obtained aluminum nanoconcavities substrates and peeled off when dried. Instead of nanoconcavities, it presents a distribution of nanosemispheres that corresponds to the imprinting of the aluminum nanoconcavities on the polymer.
The final SERS platforms are obtained with the gold sputtering on the nanoconcavities aluminum substrates and the polymer substrates, followed by a thermal annealing process [8]. The result is a scalable, ordered and controlled distribution of AuNPs on the nanostructured substrates (Fig. 1). An evaluation of the sputtering time and the thermal annealing paramenters (time and temperature) is presented. Over a large surface area of several square centimeters, the AuNPs size, shape, and interparticle spacing may be controlled by using the flexible manufacturing method discussed in this work.
All these AuNPs nanostructured platforms have shown high and interesting amplifications of the Raman signal demonstrating its suitability for SERS detection of several molecules like 4-Mercaptopyridine, rhodamine, streptavidine, melamine, etc.

Acknowledgments

This work was supported by the Spanish Ministerio de Ciencia e Innovación (MICINN/FEDER) PDI2021-128342OB-I00, by the Agency for Management of University and Research Grants (AGAUR) ref. 2021-SGR-00739 and by the Catalan Institution for Research and Advanced Studies (ICREA) under the ICREA Academia Award. Has also received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 945413.

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Figure 1