Synchronizing the chemical composition of silver nanoparticle surfaces

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Non-technical Summary:Silver nanoparticles (~1 – 100 nm in size, more than 1000x smaller than the width of a human hair) are used in a wide range of applications, including antibacterial coatings, cosmetics, electronics, chemical and biological sensors, and catalysis. However, silver is a highly reactive material, particularly when shrunk down to nanoscale dimensions, and is susceptible to unwanted side reactions, such as the well-known tarnish that forms when silver jewelry or utensils are exposed to the ambient environment. These unwanted side reactions can impact the performance and reproducibility of silver nanomaterials in critical applications. Even silver nanoparticles taken from the same bottle will perform differently on day 1 vs. day 100, and there can be significant batch-to-batch heterogeneity, due to the extent and chemical nature of the reactions that have happened on the surface. The objective of this proposal is to determine whether it is possible to clean the surface of silver nanoparticles that have undergone these unwanted reactions and restore them to a more uniform, reproducible silver surface. To accomplish this, silver nanoparticles will be exposed to a variety of chemical and electrochemical conditions, and the change in their surface will be monitored by following time-dependent changes in their optical properties. By tracking the behavior of large numbers of single nanoparticles, these studies will determine specific conditions that produce a shift from significant surface heterogeneity to improved homogeneity across a nanoparticle population. The proposal will also support the training of graduate and undergraduate students, all of whom participate in community outreach with the Willets lab (including the Adventures in Silver high school chemistry lab developed by the group), as well as support the ongoing professional development efforts of the PI, which focus on improving scientific communication as well as creating a more equitable and inclusive environment within chemistry.Technical Summary:Metal nanoparticles are widely used across a number of applications, including sensing, nanomedicine, and catalysis, yet dynamic changes in their surface chemistry can affect their reproducibility. Rather than attempt to control the surface of nanoparticles during synthesis, this proposal seeks to develop strategies to synchronize the surface chemistry of nanoparticles post-synthesis, thereby creating more reproducible behaviors when integrated into devices and used in applications. To synchronize the surface chemistry across a population of silver nanoparticles, sacrificial shells will be introduced, either chemically or electrochemically, which outcompete unwanted contaminants on the surface of the nanoparticles. The shells will then be removed via electrochemical stripping, ideally recovering pristine silver surfaces. This proposal will test a variety of chemical and electrochemical shells, in order to assess their ability to improve the surface homogeneity within a nanoparticle population. To quantify the success of the approach, dark field scattering microscopy will be used to track the scattering intensity, spectral profile, and spatial origin of single nanoparticles throughout the shell growth and stripping cycle. The kinetics of single nanoparticle electrodissolution will be used as a metric to confirm the quality of the resulting silver surfaces, with fast electrodissolution indicating near-pristine silver and sluggish electrodissolution kinetics suggesting high levels of surface impurities. These studies will not only yield strategies for improving the post-synthesis/storage surface homogeneity in silver nanoparticles, which will improve reproducibility in critical applications, but will also allow for direct comparisons in how the kinetics and interfacial behaviors of nanoparticles change when exposed to various perturbations. Outreach projects introducing high school students from a local minority-serving school to silver nanoparticle synthesis and spectroscopy complement the proposed work, along with PI-led professional development activities for graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.