Frequency-Encoded Multicolor Fluorescence Imaging with Single-Photon-Counting Color-Blind Detection.

Standard fluorescence microscopy relies on filter-based detection of emitted photons after fluorophore excitation at the appropriate wavelength. Although of enormous utility to the biological community, the implementation of approaches for simultaneous multicolor fluorescence imaging is commonly challenged by the large spectral overlap between different fluorophores. Here, we describe an alternative multicolor fluorescence imaging methodology that exclusively relies on the absorption spectra of the fluorophores instead of their fluorescence emissions. The method is based on multiplexing optical excitation signals in the frequency domain and using single color-blind detection. Because the spectral information is fully encoded during excitation, the method requires minimal spectral filtering on detection. This enables the simultaneous identification of multiple color channels in a single measurement with only one color-blind detector. We demonstrate simultaneous three-color confocal imaging of individual molecules and of four-target imaging on cells with excellent discrimination. Moreover, we have implemented a non-negative matrix factorization algorithm for spectral unmixing to extend the number of color targets that can be discriminated in a single measurement. Using this algorithm, we resolve six spectrally and spatially overlapping fluorophores on fixed cells using four excitation wavelengths. The methodology is fully compatible with live imaging of biological samples and can be easily extended to other imaging modalities, including super-resolution microscopy, making simultaneous multicolor imaging more accessible to the biological research community.