Silver nanospirals via autoaccelerated oriented assembly.

Abstract

The unusual geometry of spiral nanocrystals gives rise to unique optical, electronic, and mechanical properties. In addition, noble metal nanoparticles, such as gold and silver, exhibit strong plasmonic responses, making them highly attractive for applications in sensing, photonics, and biomedicine. Spiral growth of noble metals was demonstrated for gold already over 70 years ago, yet its formation mechanism remains elusive. Here, we extend spiral morphology to silver, with yields of over 90%, that likely originates from the stochastic mismatch of dendrite arms during planar autoaccelerated oriented assembly (AOA). Silver nanoplates (AgNpt) synthesized via H₂O₂ with 3-mercaptopropionic acid (MPA) ligand form dendrites, leading to nanoribbons, nanospirals or nanoflowers, depending on MPA concentration. Dendrite formation results from MPA coverage-dependent conformational change and decreasing total nanoparticle surface area during H₂O₂ addition. MPA redistributes preferentially to AgNpt edges due to higher reactivity and surface area decrease, triggering interparticle attraction at a conformation transition coverage, which can be traced by pH profile during MPA adsorption. Rapid 2D oriented assembly launches diffusion-limited AOA, forming dendritic morphology. Occasionally, random arm overlap, caused by building blocks mismatch, represents the point of spiral formation. Necessity of ligand coverage-dependent conformation swich is confirmed by formation of nanospirals using 2-mercaptobenzoic acid (2MBA) ligand, and absence of their formation for all other tested mercaptans. The tailorable AOA onset, controlled by the MPA (or 2MBA) coverage, provides a bottom-up route for twisted nanocrystal synthesis within minutes. Further in-situ experiments would elucidate details of nanospiral formation on atomic scale.