Rapid Fabrication of a Flexible and Transparent Ag Nanocubes@PDMS Film as a SERS Substrate with High Performance

Flexible and transparent surface-enhanced Raman scattering (SERS) substrates have long been sought-after for nondestructive detection of analytes on nonplanar surfaces, but there is still a lack of one convenient and robust way to fabricate such SERS substrates rapidly. Herein, we demonstrate the fabrication of a high-performance SERS substrate consisting of plasmonic Ag nanocube (Ag NC) arrays anchored onto a flexible transparent poly(dimethylsiloxane) (PDMS) membrane. Through a simple organic/water interfacial self-assembly method, arrays of presynthesized Ag NCs are obtained and directly retrieved onto the PDMS membrane without the aid of rigid substrates (e.g., Si wafers or glass slides). The plasmonic Ag NC arrays can produce strong electromagnetic enhancement, achieving high SERS enhancement factor (∼3.43 × 10⁶) and ideal detection capability for methylene blue (MB) and Rhodamine 6G (R6G) at respective trace amounts of 10^-10 and 10^-9 M. Moreover, without the need to transfer from substrate to substrate, the regular Ag NC arrays are kept intact, thereby yielding a good reproducibility (RSD ∼12%). We demonstrate further that our as-fabricated SERS substrate displays ideal selectivity toward different kinds of analyte molecules (R6G, crystal violet (CV), and MB) based on principal component analysis. The PDMS membrane owns excellent transparency and flexibility; thus, the substrate enables the conformal contact with nonplanar surfaces and allows the penetration of a laser to reach the analytes from the reverse side of the substrate. This thus facilitates in situ SERS detection of trace residual crystal violet on fish skin, with limit of detection (LOD) reaching 0.6 ppm. This fabrication method reported here is economical and easily implemented. The robust Ag NCs@PDMS could be readily prepared and stored to meet diverse SERS sensing applications, especially for in situ detection of analytes on irregular nonplanar surfaces.

Highly monodisperse Au@Ag nanospheres: synthesis by controlled etching route and size-dependent SERS performance of their surperlattices

For accurate experimental and theoretical research, the preparation of nanocrystrals with regular morphology is of significant importance. In this work, we investigated a facile and effective route for generating highly spherical Au@Ag nanospheres (NSs) with tuneable size and uniform morphology at room temperature. The aqueous synthesis mainly involved seed-mediated growth method together with oxidation etching employing sodium hypochlorite (NaClO) as etching agent. The termination of the etching reaction with NaClO as etching agent was simply related to the amount of NaClO and had no connection with time. Thus Au@Ag NSs with controllable diameters in range from 24 to 87 nm were prepared only by varying the amount of NaClO added into the solutions of Au@Ag nanocubes. Additionally, combined with interface self-assembly technique, Au@Ag NSs were assembled into densely arranged two-dimensional (2D) monolayer film. Moreover, the SERS performance of these monolayers were evaluated by calculating the analytical enhancement factor using crystal violet as probe molecule. The AEF increased obviously as the diameter of Au@Ag NSs went up, and the maximum AEF could reach to 0.94 × 10⁷ at the laser excitation wavelength of 785 nm. Besides, the electromagnetic field distribution for Au@Ag NSs array were also confirmed by Mie theory and FDTD solutions software and the results revealed the similar trend with the experimental results. In general, this 2D assemblies in term of high quality Au@Ag NSs have broad prospects to act as promising candidates for SERS analytical sensor and other applications.

Formation of Uniform and High-Coverage Monolayer Colloidal Films of Midnanometer-Sized Gold Particles over the Entire Surfaces of 1.5-in. Substrates

We report a simple and facile method for fabricating monolayer colloidal films of alkanethiol-capped gold nanoparticles (AuNPs) on glass substrates. The new method consists of two sequential sonication processes. The first sonication is performed to obtain a well-dispersed state of alkanethiol-capped AuNPs in hexane/acetone in the presence of a substrate. After additional static immersion in the colloidal solution for 5 min, the substrate is subjected to sonication in hexane. By using this method, we succeeded in forming uniform and stable assemblies of midnanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10-mm square glass substrates in a short processing time of less than 10 min. It was also demonstrated that this method can be applied to a 1.5-in. octagonal glass substrate. The mechanism of monolayer colloidal film formation was discussed based on scanning electron microscopy observations at each preparation step. We found that the second sonication was the key process for uniform and high-surface-coverage colloidal film formation of midnanometer-sized AuNPs. The second sonication promotes the migration of AuNPs on top of the monolayer in contact with the substrate surface, decreasing both the multilayer region and the bare surface area. Eventually, a nearly perfect monolayer colloidal film is formed.