Silicon‐Vacancy Color Centers in Nanodiamonds: Cathodoluminescence Imaging Markers in the Near Infrared

Abstract

We recently demonstrated [ 18 ] multi-color cathodoluminescence (CL) of nanodiamonds as a powerful tool for nanoscale imaging of biological structures. CL is the emission of light by matter as the result of electron bombardment. CL imaging of bulk matter is typically carried out in an electron microscope outfi tted with an optical detector, and is widely used in materials characterization. [ 19 ] However, application of CL to imaging biological structures has been hindered by low photon count rates and rapid signal degradation due to the destruction of biomolecules and organic fl uorophores under electron beam irradiation. [ 20 ] These problems may be overcome with correlated CL and secondary electron (SE) imaging of samples tagged with surface-functionalizable nanoparticles containing defects that are robust under electron beam illumination and emit stable, spectrally distinct CL: e.g., A-band defects and NV centers in nanodiamonds, as well as Ce:LuAG nanophosphors. [ 18 ] In this approach, the CLemitting particles function as color-distinguishable nanoscale markers of targeted epitopes, while the correlated SE image provides high-resolution information about the cellular structure. The combination of nanoscale molecular localization and structural imaging acquired simultaneously and in the same instrument constitutes a uniquely powerful new imaging modality. For applications with large intrinsic CL background, e.g., correlated CL and SE imaging of unfi xed/ living cells in an environmental chamber in a scanning electron microscope (SEM), [ 21 ] it is desirable to use spectrally narrow CL markers with emission peaks at wavelengths distinct from the CL background (such as from proteins, nucleic acid, and fl uorophore-conjugated antibodies, which usually emit CL at short optical wavelengths). [ 22 ] However, many of the nanoparticle species investigated to date have relatively broad (∼100 nm) CL emission spectra at room temperature. [ 20 , 23,24 ] Here, we show that silicon-vacancy (Si-V) color centers [ 25 ] in nanodiamonds provide a promising solution to this challenge. Specifi cally, we demonstrate experimentally that nanodiamonds fabricated to incorporate Si-V color centers provide bright, spectrally narrow (∼5 nm) CL emission in the near-infrared (∼740 nm), which lies within the near-infrared transmission window of biological tissue and is DOI: 10.1002/smll.201303582 Nanodiamonds