Submicromolar imaging of intrinsic chromophores by two-photon photothermal microscopy captures mitochondrial response to chemotherapy

Intracellular chromophores {e.g., NADH [reduced form of nicotinamide adenine dinucleotide (oxidized form)] and FAD (flavin adenine dinucleotide)} play a central role in regulation of cellular metabolism. Although autofluorescence has been extensively used for label-free mapping of chromophores inside a cell, its sensitivity and molecular specificity are constrained by the low quantum yield and the fluorescence spectral overlap. Here, we address these challenges by using a photothermal approach to measure the optical absorption of chromophores rather than its autofluorescence. Our two-photon photothermal (2PPT) microscope exploits localized thermal transients generated through two-photon absorption, enabling detection of chromophore-specific signatures beyond the reach of autofluorescence. We demonstrate submicromolar limits of detection for the metabolic coenzymes NADH and FAD of 0.87 and 0.99 μM, respectively. Such high sensitivity enables differentiating the influence of mitochondrial shapes on metabolism. 2PPT can identify the biomolecular source of contrast from cellular mitochondria in a label-free manner on the basis of spectroscopy. 2PPT microscopy is used to study metabolic alterations of mitochondria in cancer under chemotherapy at the single-organelle level.