Nanoporous Gold Stacked Layers as Substrates for SERS Detection in Liquids or Gases with Ultralow Detection Limits and Long-Term Stability

Abstract

Surface-enhanced Raman scattering (SERS) substrates consisting of stacked ultrathin nanoporous gold layers are used to detect very low concentrations of molecules in liquids or gases. The SERS substrates are obtained by copper chemical etching of alternative copper and gold stacked nanolayers. This process is a reliable method for fabricating uniform and reproducible SERS substrates, with a robust SERS response at extremely low detection limits. By optimizing fabrication conditions combined with a thorough analysis of the SERS mappings and using 2,2-bipyridine (BP) as probe molecules, the detection threshold corresponding to a detectable SERS response reaches a BP concentration of 10–18 mol·L–1 in water. An additional Raman mechanism is also highlighted by μ-surface enhancement spatially offset Raman spectroscopy (μ-SESORS): gold ligaments, inside nanoporous layers, act as waveguides for the incident light, leading to a significant increase in the size of the active SERS area. Moreover, these SERS substrates could be stored for several days without a significant decrease in their properties, and they can be reactivated and reused after SERS analysis. The ability to detect low BP vapor pressure is also demonstrated.

Large Area Freestanding Au Nanoporous Ultrathin Films Transfer Printed on Bendable Substrates and 3D Surfaces for Flexible Electronics

“…For the dealloyed films, the observed porosity is attributed to the removal of copper atoms from the alloy combined with the Au surface diffusion process during the dealloying, resulting in the formation of a planar gold skeleton consisting of nanopores and interconnected nanoligaments . For the transferred pure Au films, the formation of pores and ligaments for the 13 nm-Au transferred sample is attributed to the reorganization of the very thin Au layer during the etching of Cu sacrificial layer as it has already been observed in previous works. , Indeed, below a few tens of nm in thickness, the thermodynamic instability leads to a reorganization of their nanostructure . On the other hand, thicker films are thermodynamically more stable.…”

“…30 For the transferred pure Au films, the formation of pores and ligaments for the 13 nm-Au transferred sample is attributed to the reorganization of the very thin Au layer during the etching of Cu sacrificial layer as it has already been observed in previous works. 37,38 Indeed, below a few tens of nm in thickness, the thermodynamic instability leads to a reorganization of their nanostructure. 39 On the other hand, thicker films are thermodynamically more stable.…”

“…Another option is to use a pure Au layer deposited on a Cu sacrificial layer. In this case, if the pure Au layer is thin enough, a reorganization into a nanoporous material happens during the Cu etching. , In this last case, the reorganization is due to the thermodynamic instability of such an ultrathin layer …”

“…In this case, if the pure Au layer is thin enough, a reorganization into a nanoporous material happens during the Cu etching. 37,38 In this last case, the reorganization is due to the thermodynamic instability of such an ultrathin layer. 39 The oxidation of the metal to be removed, copper in this work, is usually done with acid solutions.…”

Combining physical vapor deposition structuration with dealloying for the creation of a highly efficient SERS platform

“…The behavior of the SERS response is comparable to that reported in the literature and can be related to the adsorption isotherm curve of RhB [ 49 – 50 ]. The plateau in the SERS response observed between 10 −8 and 10 −10 mol·L −1 can be assigned to either the saturation of the chemical sites (i.e., no supplementary absorption is made) or to the fact that RhB continued to be absorbed outside of the hot spot [ 6 , 51 ]. Comparing the integrated intensity of the RhB peak, it can be observed that the intensity of the signal is lower for the sample dealloyed in H 3 PO 4 than that for the sample dealloyed in HCl.…”