Abstract: Plasmonic gold nanoparticle size and shape can be engineered via chemical synthesis to achieve strong light-matter interactions at specific wavelengths. However, incorporation of tailored nanoparticles into functional devices is often limited by multistep, time-consuming processes specific to the type of substrate and nanoparticles. In situ growth is an unconventional bottom-up approach where nanoparticles are grown directly on the target substrate, thereby circumventing colloidal synthesis and surface attachment. Despite these advantages, there remains a considerable gap in synthetic control and product quality between particles grown in situ and their counterparts prepared in colloidal dispersion. Herein, we present optimized in situ strategies for the growth of anisotropic gold nanoparticles directly on the internal walls of microfluidic channels, with no need for any top-down lithography or colloidal synthesis steps. Careful optimization enabled us to obtain shape-yields for gold nanorods around 90% (with size dispersion down to 15%)-a significant improvement compared to previous approaches, which have reported a maximum yield <37%. This result relied on identifying synthetic conditions that largely favor surface growth over colloidal growth in the flowing solution, which could be tuned through surface chemistry and flow rate. Overall, these results emphasize the potential of in situ growth as a viable alternative to colloidal synthesis, with high control over the synthesis of anisotropic products directly on a substrate surface. We propose that microfluidic systems containing in situ prepared nanostructures are promising for sensing applications, offering full control over nanoparticle crystallography, morphology, and surface chemistry.

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