Exploring the Bottom-Up Growth of Anisotropic Gold Nanoparticles from Substrate-Bound Seeds in Microfluidic Reactors

Abstract

We developed an unconventional seed-mediated in situ synthetic method, whereby gold nanostars are formed directly on the internal walls of microfluidic reactors. The dense plasmonic substrate coatings were grown in microfluidic channels with different geometries to elucidate the impacts of flow rate and profile on reagent consumption, product morphology, and density. Nanostar growth was found to occur in the flow-limited regime and our results highlight the possibility of creating shape gradients or incorporating multiple morphologies in the same microreactor, which is challenging to achieve with traditional self-assembly. The plasmonic–microfluidic platforms developed herein have implications for a broad range of applications, including cell culture/sorting, catalysis, sensing, and drug/gene delivery.

Direct Bottom-Up In Situ Growth: A Paradigm Shift for Studies in Wet-Chemical Synthesis of Gold Nanoparticles

“…Then, the seeds were overgrown into gold nanostars by flowing a growth solution into the channel (Figure A). We found that flow rate, growth time, and flow profile play important roles in the in situ growth, affecting the development of branches. , …”

“…We note some reports of more uniform particle substrate attachment with APTES/APTMS mixed coatings . Demonstrations of in situ shape control from colloidal seeds have been performed with cubes, concave structures, rods, and stars. , The colloidal seeds can also be patterned, e.g. , by soft lithography. ,, One benefit of performing in situ growth in this way is that it can improve the density of the overgrown nanostructures, which is useful for fabricating sensors …”

“…(H, I) Digital photographs of the macroscale color gradient in the featureless and herringbone channels, respectively, caused by a depletion of growth reagents as the solution flows through the channel. (J–L) Scanning electron microscopy images showing the shape gradient along the channels with each image taken at the inlet, center, and outlet positions, respectively [Adapted with permission from ref . Copyright 2023 American Chemical Society].…”

“…Note that mass transport for substrate growth differs from 3D systems, so these differences must be taken into consideration when developing models for these systems. The use of different flow profiles for mass transport to the substrate also offers an interesting avenue for controlling in situ growth. ,, Competition between in situ growth and colloidal nucleation unpredictably consumes reagents and creates nuclei that can deposit in unintended locations. Some strategies for preventing or reducing homogeneous or secondary nucleation include: Implementing only weak reductants in the growth solution and using substrate bound or native strong reductants. Using overgrowth solutions with slower growth kinetics. Reducing interactions of the growth solution with other surfaces ( e.g ., glass). Performing overgrowth from smaller (higher surface energy) seeds. Using clean water sources. …”

“…We found that flow rate, growth time, and flow profile play important roles in the in situ growth, affecting the development of branches. 6,304 In this work, both "featureless" channels and "herringbone" micromixers were applied. The featureless channels are characterized by laminar flow profiles, where mass exchange occurs by diffusion, rather than advection, whereas the 3D features in the herringbone channels (Figure 14B,C) promote solution mixing.…”