Introduction

Precise imaging of tissues or cellular structure is extremely useful for fundamental research in biology as well as for medical applications. Optical methods are especially well suited for this purpose as they are non-invasive. Most biological exhibit a rather low optical contrast, and even if sensitive microscopy techniques such as Phase Contrast or Differential Interference Contrast can reveal some intracellular structures of live cells [fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][1], most organelles remain hidden with white light microscopes. To gain specificity and contrast, fluorescent molecules are commonly used to label particular target components and report their localization. Such labels which include organic fluorophores or autofluorescent proteins may however interfere with cellular activity. For instance, Rhodamine6G or MitoTracker Orange can interfere with mitochondrial respiration [2] [3]. In addition, the label-target complex may have different behavior from the native target and the labeling specificity and efficiency can vary according to experimental conditions [4].

In this work, we report the far-field optical imaging of mitochondria of live cells without the use of any label. High quality imaging of mitochondria is of great interest for medical applications. In particular, the shape, spatial distribution, and aggregation of mitochondria can be markers for identifying some myopathies[5] or cancer cells, as previously demonstrated with fluorescence labeling techniques [6].

Here we used an ultra-sensitive photothermal method developed recently for the detection individual absorbing nano-objects[7, 8]. This method referred as Light Induced Scattering Around a NanoAbsorber (LISNA) was used to ultimately detect gold nanoparticles as small as 1.4 nm in diameter[7, 9] and is applicable in different fields of science such as for the improvement of DNA chips[10]. Owing to its unsurpassed sensitivity in detecting absorbing nano-objects, LISNA was the natural candidate to perform high resolutions far-field optical images of mitochondria in live cells, a well-known light absorbing cellular organelle. This assumption was further encouraged by the earlier detection of extracts of mitochondria, by a thermal lens microscope [11]. A photothermal assay that is sensitive to mitochondrial morphology [12] has also been proposed more recently with potential for identifying cancer cells but still suffer from poor resolution capabilities. In both of these studies, cytochrome c, a heme highly present in mitochondria was assumed to be at the origin of the signals.

In our work, the much higher sensitivity over previous methods and confocal-like resolution allow a systematic study of mitochondria morphology as a function of cellular state. Our results rule out the main contribution of cytochrome c in the mitochondrial photothermal signals, which is in contrast to previous assumptions.

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