Mid-infrared Chemical Nano-imaging for Intra-cellular Drug Localisation

In the past two decades a range of fluorescence cell microscopy techniques have been developed which can achieve ~10 nm spatial resolution, i.e. substantially beating the usual limits set by optical diffraction. However, these methods rely on specialised labelling. This limits the applicability, risks perturbing the biology, and it also makes them so-called discovery techniques that can only be used when there is prior knowledge about the biological problem. The alternative, electron microscopy (EM), requires complex and time-consuming sample preparation, that risks compromising the samples integrity. Samples have to withstand vacuum, and staining with heavy metals to make them conductive, and give usable electron-contrast. None of these techniques can directly map out drug distributions at a sub-cellular level. Recently infrared light-based scanning probe techniques have demonstrated a capability for ~1 nm spatial resolution. However, they need samples that are flat, dry and dimensionally stable and they only probe down to a depth commensurate with the spatial resolution, so they yield essentially surface chemical information. Thus far they have been applied only to artificially produced test samples, e.g. gold particles, or isolated proteins on silicon. Here we show how these probe-based techniques can be adapted for use with routinely prepared general biological specimens. This allows for Mid-infrared Chemical Nano-imaging (MICHNI) that delivers chemical analysis at a ~10 nm spatial resolution, suitable for studying cellular ultrastructure. We demonstrate its utility by performing label-free mapping of the anti-cancer drug Bortezomib (BTZ) within a single human myeloma cell. We believe that this MICHNI technique has the potential to become a widely applicable adjunct to EM across the bio-sciences.