Ultrathin Plasmene Nanosheets as Soft and Surface‐Attachable SERS Substrates with High Signal Uniformity

Abstract

COMMUNICATIONstronger than those obtained from a commercial Klarite SERS substrate, which was rigid, opaque, and could not be used for direct chemical identifi cation on banknotes or coins.A combination of polymer-ligand-based strategy and dryingmediated air-water interfacial self-assembly was utilized for the fabrication of plasmene nanosheets with Au@Ag nanocubes (NCs) as model building blocks. [7][8][9] The physical steps towards obtaining a free-standing plasmene nanosheet are demonstrated in Figure 1 a. In brief, a concentrated drop of polystyrene (PS)-capped NCs is spread and allowed to solidify into a silvery refl ection on the surface of a water droplet. Subsequent slow evaporation resulted in plasmene nanosheets that covered almost the entire grid. One striking feature of our plasmene nanosheets is their high mechanical fl exibility, which allows them to be transferred using a high-fi delity, polydimethylsiloxane (PDMS) elastomer. This enables their use as powerful "SERS adhesive" for chemical identifi cation of trace amount of chemicals on solids of different materials with complex surface structures.To prove this exciting capability, 4-aminothiophenol (4-ATP) was used as the model target analyte because of its strong affi nity to silver and its apparent, large SERS signal. For this 40 µL of 4-ATP (1 µM, corresponding to around 4 ng cm −2 ) drops were deposited and allowed to dry onto the surface of respectively a Malaysian banknote (fi brous), an Australian banknote (polymer), and an Australian coin (rough metal) before PDMS-mediated stamping of the plasmene nanosheets. The transferred sheets had intimate contact with these complex surfaces because of their high fl exibility and robustness, and they enabled a substantial signal enhancement in the SERS detection of the trace amounts of 4-ATP molecules (Figure 1 b-d). The characteristic Raman peaks for 4-ATP could not be observed on the plain and ATP-covered solid surfaces (blue curves and black curves), but they became evident for the surfaces with attached plasmene nanosheets (red curves). This demonstrates a simple non-destructive manner for chemical identifi cation on non-planer surfaces and we believe that the observed signalenhancing capability can be potentially extended to any other complex solid surfaces.We further systematically investigated how the size and shape of the constituent nanoparticles could infl uence the SERS enhancement. We deliberately synthesized six plasmonic building blocks with different geometries (Supporting Information, Section 3.1) -small-(s-), medium-(m-), large-(l-) nanocubes (NCs) and nanobricks (NBs), which were used to produce six different sheets with the same capping ligands of identical molecular lengths. As opposed to NC building blocks, which tend to pack side by side with a regularity imposed by its highly ordered symmetrical nature, isotropic NBs are aligned horizontally with random packing directions to form unique, Plasmonic nanoparticles have become a prominent research subject in the fi eld of chemical ide...

Self-Assembled Plasmonic Pyramids from Anisotropic Nanoparticles for High-Efficient SERS

“…S7, the Raman spectra of 4-ATP can be clearly observed for all three pyramids. The observed peaks can be assigned in accordance to the literature [2], in which the two evident 1078 and 1388 cm -1 peaks that are enhanced by electromagnetic enhancement are assigned to the C-S stretching, and C-C stretching modes, respectively. The weaker peaks at 1141, 1392 and 1438 cm -1 are attributed to the weak charge transfer mechanism from Au-S bonds.…”

“…5, the plot of the SERS intensity for the dominant 1078 cm -1 peak revealed that RD pyramid exhibited the strongest SERS signal, followed by OH and NS pyramid. The enhancement factors, as calculated in accordance to the classical equation [2,34], were found to be 1.8 9 10 7 , 5.3 9 10 6 and 1.9 9 10 6 for RD, OH and NS pyramids, respectively (see Supplementary Information Part SIV for calculation). This trend can be explained according to the electromagnetic theory and lightning rod effect, in which building blocks with sharp features are expected to be able to support higher electromagnetic field intensities due to the concentrated electromagnetic field within the regions of curvature [46,47].…”

“…This SERS effect originates from the confinement of light in interstitial spaces between noble metal nanoparticles, thus creating hot spots with excited and enhanced local electromagnetic fields [1,2]. As such, SERS technology enables detection of trace amount of molecules even down to a single-molecule level, leading to a myriad of technical applications in the fields of biology, medicine, forensics, nanophotonics and optical sensors [2][3][4][5][6][7].…”

“…Direct self-assembly of nanoparticles in pyramid templates has also been demonstrated, but is only limited to simple isotropic spherical nanoparticles [26], hence limits the plasmonic tunability. Since the amplification of Raman signals are predominantly dependent on the nature of SERS substrate, inclusive of size and shape of nanoparticle building blocks, interparticle distance and arrangement patterns [1,2,[32][33][34][35], the continual development of pyramid-based substrates still remains a crucial and active area for SERS researches to overcome these existing limitations.…”

“…Over the past decades, various SERS substrates from electrochemically roughened electrodes to colloidal suspensions, disordered aggregates and ordered assemblies have been developed with various fabrication techniques [2,[8][9][10][11][12][13][14]. These fabrication techniques can be categorized into two distinct approaches, namely bottom-up selfassembly and top-down lithography.…”