Recently the research paper by Associate Professor Yue Hu and her team “Size-Tunable Gold Aerogels: A Durable and Misfocus-Tolerant 3D Substrate for Multiplex SERS Detection” has been published as the cover paper in the famous optical journal Advanced Optical Materials.

SERS, due to its high sensitivity (can reach single molecule detection level), has been under spotlight and is regarded as one of the most promising analytical methods. However, the hotspot of traditional 2D SERS substrate is limited to X-Y plane only, which results in limited signal strength and poor focusing tolerance. Therefore developing a 3D SERS substrate becomes a new and essential focus of research direction. Although certain achievements have been made in the study of 3D SERS substrate, the feature size of the z direction of the solid 3D substrate in existing papers is less than 10μm, which is not ideal. However, Noble metal aerogels (NMAs), due to its multi-pore, large specific surface area, and self-supporting features, is highly likely to become a high-performance 3D SERS substrate to overcome the defects of traditional 2D SERS substrate. However it’s extremely difficult to control the sizes (e.g. the diameter of nanowire), which also restricts the study of structure-activity relationship and performance optimization.

In the preliminary work conducted by an international team led by Associate Professor Yue Hu of Wenzhou University, Professor Ran Du of Beijing Institute of Technology and Professor Alexander Eychmüller of Technische Universitat Dresden, through in-depth study of the metal aerogels synthesis methodology, a range of easy-to-control preparation methods were developed, which achieved effective control of the rapid preparation of metal aerogels and a wide range of feature size, specific surface area, and pore volume (Sci. Adv.2019, Nat. Commun.2020, Matter2020). On that basis, the same team worked together again. Take the gold aerogels with network feature size of 5-100 nm as an example, they researched on the feature size of aerogels substrate and the SERS activity dependence from theoretical and experimental perspectives. Through optimizing some parameters, they not only came up with durable and misfocus-tolerant SERS substrate (>300 μm), but also achieved high-sensitivity and multi-channel detection of dyes, pesticides and carcinogens.

Some of their research findings have been published in Advanced Optical Materials (DOI: 10.1002/adom.202100352) and have been chosen as the front cover article with Wenzhou University listed as first institute, Lin Zhou, a chemistry postgraduate student enrolled in 2016 as the first author of this article, and Associate Professor Yue Hu from the School of Chemical and Material Engineering of Wenzhou University as corresponding author.

The researchers first of all prepared a range of gold aerogels with the feature size (nanowire diameter) of 5-100 nm. Research shows that SERRS enhancement factor (EF) can be affected by the feature size and excitation wavelength of gold aerogels. Under the same excitation wavelength, the enhancement factor has an optimal feature size. However along with the red shift of the excitation wavelength the corresponding optimal feature size also increases, which is also observed in many probe molecules. By adopting the finite difference time domain theory (FDTD) simulation and using gold nanorods or tetrapods as models, it can be clearly observed that with the increase of diameter, the absorption wavelength also gradually moves towards the long wave. From that we can infer that the resonant excitation wavelength of gold aerogels is positively correlated with the feature size, which can to some extent explain the mechanism of the dependence between SERS activity and the feature size of the excitation wavelength.

Compared to typical 2D SERS substrate (8 nm gold film), gold aerogels display higher enhancement factor (up to 8.9×109) against dyes, pesticides and carcinogens, which is higher than most of the 3D SERS substrates reported so far (105-107). In addition, gold aerogels are not only reusable but also extremely stable (>1 month) and has superior multi-channel detectability, which indicates that it has outstanding practical applications. Finally, gold aerogels also are misfocus-tolerant (along z direction>300 μm). It means even when it’s used in multi-channel detection, the value can still reach 150 μm, which is higher than the existing solid 3D SERS substrates.

This research systemically studied the structure-activity relationship and property features of NMAs as 3D SERS substrates, obtained genuine 3D SERS substrate, laid a solid foundation for the design of aerogel-based 3D SERS substrate, and pointed out a new direction for achieving durable misfocus-tolerance, high-stability, high-sensitivity, and multi-channel SERS detection.