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Surface-enhanced Raman scattering (SERS)-based detection of suspension-phase analytes holds great promise for a variety of applications; however, plasmonic colloidal SERS substrates are not stable in many solution conditions unless they are protected by a stabilizing agent. Mesoporous silica shells on plasmonic nanoparticle cores have been demonstrated to perform well in a variety of liquid matrices. However, this silica shell can be seen as barrier from the perspective of the analyte, as the analyte molecules need to reach the plasmonic core after they pass through the shell. In this work, mesoporous silica-coated gold nanorods have been synthesized and characterized as aqueous colloidal SERS substrates systematically considering how SERS performance is impacted by three different factors: adsorbed molecules, the silica shell, and bulk solvent media. The results show that SERS signal intensities from the model hydrophobic analyte, trans-1,2-bis(4-pyridyl) ethylene (BPE), are enhanced when the pore size, hydrophobicity of the shell, and ionic strength are increased, indicating more favorable interaction between the substrates and the analyte. The silica shell presented herein facilitates efficient adsorption of the analyte to the gold core and enhanced sensitivity to environmental refractive index changes. This efficient adsorption can be further enhanced by controlling the incubation temperature. Overall, this work reveals how substrate exposure conditions can be tuned to maximize analyte SERS signals without compromising the silica shell that protects the plasmonic properties of the SERS-enhancing core.
Bibliographical noteFunding Information:
We would like to acknowledge 3M for their collaboration and funding. This work was supported by National Science Foundation under the Center for Sustainable Nanotechnology (CSN), CHE-1503408. The CSN is part of the Centers for Chemical Innovation Program. We acknowledge helpful discussion with Prof. Theresa M. Reineke and Victoria Szlag. We thank Joshua G. Hinman and Catherine J. Murphy for providing Au nanorods used in preliminary experiments for this work. Parts of this work, especially TEM characterization, were carried out in the Characterization Facility at the University of Minnesota, which receives partial support from NSF through the MRSEC program (DMR-1420013).
© 2019 American Chemical Society.
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