Novel coherent multiphoton microscopy of living cells with nanodiamonds

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The purpose of this research is to develop a new imaging modality which overcomes a number of severe limitations in currently available methods to observe living cells.Optical microscopy is an indispensable tool in cell biology, and continuous effort is devoted to develop new techniques with improved performances. Two main approaches can be distinguished: Label-free microscopy techniques versus imaging methods which rely on optical labels. Label-free techniques have the major advantage of looking at unstained cellular and subcellular structures without unwanted artefacts from the labelling procedure. Coherent Antistokes Raman Scattering (CARS) has recently emerged as a powerful label-free method to distinguish biomolecules based on their intrinsic molecular vibrations. However, the benefit of CARS relies on the constructive interference from a large number of identical bonds, hence so far has been mostly successful in distinguishing concentrated lipids in living cells. In order to visualise proteins and DNA with high sensitivity, specificity and at speeds compatible with live cell imaging, optical-labelling is still the only option. In this respect, the most widely utilised labels are fluorescent organic dyes or fluorescent proteins. However, all organic fluorophores are prone to photo-bleaching, an irreversible photo-chemical degradation process severely limiting long time course observations and accompanied by cell toxicity effects. Alternative to organic fluorophores, solid state inorganic nanoparticles hold a great promise as optical labels in the quest for superior photostability and reduced toxicity. Recently, nanodiamonds (NDs) have gained world-wide attention due to their inexpensive large scale synthesis based on the detonation of carbon containing explosives. They offer particle sizes down to few nm, high biocompatibility and low cytotoxicity and the simple and versatile surface bioconjugation of organic chemistry while keeping the structural integrity of diamond. Their application in optical microscopy of living cells is still at an early stage, with most promising results having been obtained from the fluorescence emission of nitrogen vacancy (NV) centres in diamond. This method is however limited by the efficiency and costs in producing NV centres in NDs. Reports so far have shown that small (<20nm) NDs have a very low probability to have even a single NV center, and that NV centres close to the ND surface are not stable. In this project, we propose a pilot study to develop a novel way of imaging nanodiamonds in cells which does not rely on (and hence is not limited by) their fluorescence properties. The method is based on the coherent nonlinear light-matter interaction response of NDs and has the added benefit of a superior three-dimensional spatial resolution owing to the nonlinearity of the response. We will explore two types of coherent nonlinearities of NDs: electronically resonant four-wave mixing (FWM) and vibrationally resonant CARS of diamond. The long term vision is the realisation of a new imaging technology that will tackle biological and biomedical problems virtually impossible to address with currently available techniques. As an example, we will follow quantitatively the coherent optical signal of nanodiamonds over time after being internalised in living cells. Our ability to quantify the number of NDs within the cell over time based on the optical signal strength without photobleaching will be a key tool in the study of complicated intracellular pathways.