Gold and silver nanoparticle monomers are non-SERS-active: a negative experimental study with silica-encapsulated Raman-reporter-coated metal colloids

Abstract

Noble metal nanoparticles (NPs) are the most commonly employed plasmonic substrates in surface-enhanced Raman scattering (SERS) experiments. Computer simulations show that monomers of Ag and Au nanocrystals ("spherical" NPs) do not exhibit a notable plasmonic enhancement, i.e., they are essentially non-SERS-active. However, in experiments, SERS enhanced by spherical NP colloids has been frequently reported. This implies that the monomers do not have strong SERS activity, but detectable enhancement should more or less be there. Because of the gap between theory and practice, it is important to demonstrate experimentally how SERS-active the metal colloid actually is and, in case a SERS signal is observed, where it originates from. In particular the aggregation of the colloid, induced by high centrifugal forces in washing steps or due to a harsh ionic environment of the suspension medium, should be controlled since it is the very high SERS activity of NP clusters which dominates the overall SERS signal of the colloid. We report here the experimental evaluation of the SERS activity of 80 nm Au and Ag NP monomers. Instead of showing fancy nanostructures and super SERS enhancement, we present the method on how to obtain negative experimental data. In this approach, no SERS signal was obtained from the colloid with a Raman reporter on the metal surface when the NPs were encapsulated carefully within a thick silica shell. Without silica encapsulation, if a very low centrifugation speed is used for the washing steps, only a negligible SERS signal can be detected even at very high NP concentrations. In contrast, strong SERS signals can be detected when the NPs are suspended in acidic solutions. These results indicate that Au and Ag NP monomers essentially exhibit no SERS activity of practical relevance.

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“…Moreover, the corresponding SERS performance of these two nanoparticle morphologies was tested in solutions containing CV at a concentration of 1 μ m (Figure D). This experiment confirmed that SERS of even the highly Raman‐active CV, was below the detection threshold when using Au nanospheres, due to the moderate electromagnetic field enhancement, but SERS signals were clearly observed when anisotropic AuNTs were used. Thus, AuNTs were selected and applied in the subsequent experiments.…”

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“…SERS signals are recorded, if our spherical, monodispersed AgNP aggregate and induce plasmon coupling between the NPs resulting in hot spots [107]. SERS signals are not detected in Cinn-AgNP-PI because NP clusters did not form [107]. Cinn-AgNP-PI has the smallest particle size compared to our other compounds TCA-AgNP, TCA-AgNP-PI, and Cinn-AgNP.…”

“…The nanocolloidal sample Cinn-AgNP-PI is SERS-inactive ( Figure 9d). SERS signals are recorded, if our spherical, monodispersed AgNP aggregate and induce plasmon coupling between the NPs resulting in hot spots [107]. SERS signals are not detected in Cinn-AgNP-PI because NP clusters did not form [107].…”

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“…The challenge has recently been mastered for individual spherical particles that were in fact proven to be inactive in terms of Raman enhancement. [24] Based on our simulations, we also consider the SERS effects of SDN as the result of agglomeration and do not assume that the signal arises from individual particles. We also hypothesize that the ability of Nanorice and SDN, namely, to transfer the large particle size into large areas of nearfield enhancement, is an advantage over a spherical approach in case of larger biological structures.…”