Expanding the palette of SWIR emitting nanoparticles based on Au nanoclusters for single-particle tracking microscopy

Single-molecule localization microscopy has proved very promising to unravel the dynamics and molecular architecture of thin biological samples down to the nanoscale. However, achieving meaningful results in complex, thick biological tissues requires shifting the observation wavelengths to the shortwave-infrared (SWIR) region, where biological tissues are most transparent. In consequence, nanomaterials with optical activity in the SWIR exhibiting brightness and photostability suitable for detection at the single-molecule level are needed. Currently mainly single-walled carbon nanotubes (SWCNTs) satisfy this, but are inherently 1D objects. Here we present 0D ultra-small gold nanoclusters (AuNCs, <3nm) and ~25 nm AuNC-loaded-polymeric particles that can be detected at the single-particle level in the SWIR. Thanks to their high brightness and excellent photostability, we show that the dynamics of the spherical polymeric particles can be followed at the single-particle level in solution at video rates for minutes. Analysis of the mean square displacement confirms the diameter of the particles in aqueous media, and enables us to compare their brightness with that of biocompatible SWCNTs. This extends the library of SWIR emitting nanomaterials to 0D nano-objects of variable size for single-molecule localization microscopy in the second biological window, opening unprecedented possibilities for mapping structure and dynamics of complex biological systems.