Multi-harmonic spectral phasor analysis of LAURDAN hyperspectral imaging enables the in vivo study of membrane dynamics of multiple membrane-bound organelles.

Hyperspectral imaging (HSI) is a paramount technique in biomedical science, however, unmixing and quantification of each spectral component is a challenging task. Traditional unmixing relies on algorithms that need spectroscopic parameters from the fluorescent species in the sample. The phasor-based multi-harmonic unmixing method requires only the empirical measurement of the pure species to compute the pixel-wise photon fraction of every spectral component. Using simulations, we demonstrate the feasibility of the approach for up to 5 components and explore the use of adding a 6th unknown component representing autofluorescence. The simulations show that the method can be successfully used in typical confocal imaging experiments (with pixel photon counts between 101 and 103). As a proof of concept, we tested the method in living cells, using 5 common commercial dyes for organelle labeling and we easily and accurately separate them. Finally, we challenged the method by introducing a solvatochromic probe, 6-Dodecanoyl-N,N-dimethyl-2-naphthylamine (LAURDAN), intended to measure membrane dynamics on specific subcellular membrane-bound organelles by taking advantage of the linear combination between the organelle probes and LAURDAN. We succeeded in monitoring the membrane order in the Golgi apparatus, mitochondria, and plasma membrane in the same in-vivo cell and quantitatively comparing them. The phasor-based multi-harmonic unmixing method can help expand the outreach of HSI and democratize its use by the community for it does not require specialized knowledge.