Tailoring plasmonic response by Langmuir–Blodgett gold nanoparticle templating for the fabrication of SERS substrates

Abstract

Nanoparticle self-assembly is a robust and versatile strategy for the development of functional nanostructured materials, offering low-cost and scalable methods that can be fine-tuned for many different specific application. In this work, we demonstrate a pathway for the fabrication of tailorable quasi-two-dimensional lattices of gold nanoparticles to be used in Surface Enhanced Raman Scattering (SERS) detection of biomolecules. As a first step, nanoparticles are spread as a monolayer at the water/air interface, compressed to a target lateral density in a Langmuir-Blodgett technique, and transferred to a properly functionalized substrate surface. Once firmly adhered to the substrate, the lattice of nanoparticles can be directly used or be further processed using electroless gold deposition to let the nanoparticle grow thus tuning the plasmonic response and SERS enhancement. Compared to direct deposition or self-assembly methods, our protocol enables to obtain consistent results and much higher coverage of Au nanoparticles thanks to the active control of the surface pressure of the spread monolayer.

Optimizing Gold Nanoparticle Size and Shape for the Fabrication of SERS Substrates by Means of the Langmuir–Blodgett Technique

“…In our previous work studying LB films prepared using commercial gold NPs of 10 nm [ 12 ], we estimated the size of the hotspots after different amounts of gold electroless plating was applied on the bare NP layer by comparing the UV–vis spectrum of our substrate with that of gold NP of different sizes, as reported by the NP manufacturer ( Figure 1 ). We found that, for the optimal electroless plating time (8–9 min), the average effective hotspot size was around 50 nm.…”

“…In the present study, we used commercial spherical gold NPs of sizes 5–40 nm, and studied the performance of LB films prepared at different lateral pressures for each particle size. For the final SERS substrates, we increased the size and roughness of the hotspots by applying a common gold electroless plating time of 8 min, which was found to be near optimal in our previous study [ 12 ].…”

“…Typically, nanoparticles (NPs) are the chosen building blocks, whose size, shape, and composition are selected based on the target physical properties of the final device [ 6 ]. These strategies have also been often employed for the development of SERS substrates [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ].…”

“…This original technique has been amply used in recent years for the manufacturing of nanomaterials as well, since nanoparticles with suitable surface chemistry, rather than molecular entities, can be spread at the water/air interface, tuned for lateral packing, and further transferred onto a substrate. The LB method has also been used to prepare SERS substrates, either directly [ 7 , 9 , 10 , 11 ] or with further processing [ 8 , 18 ], where the transferred nanoparticles act as seeds for the growth of hotspots [ 12 ]. A complementary transfer technique, the Langmuir–Shaefer method, has also been demonstrated for the preparation of SERS substrates.…”

“…In a recent work [ 12 ], we demonstrated that the Langmuir–Blodgett technique is a simple and versatile approach to build SERS substrates through the tailored self-assembly of commercial spherical gold nanoparticles. In that work, we found that the signal gain produced by the plasmonic substrate strongly depended upon the lateral density and the size of the hotspots, leading to a quasi-resonance for a narrow set of experimental conditions.…”