What is limiting the applications of far-red and near-infrared fluorophores in conventional flow cytometry?

Far-red and near-infrared fluorophores tend to have broader emission spectra with reduced brightness in comparison to the fluorophores emitting light in the visible range, resulting in the requirement of detectors with ultra-high sensitivity which hinders their applications in flow cytometry. The amplification ability of individual PMT, which is the common detector on a conventional flow cytometer, may not be good enough to generate measurable electronic data points. Broad filter ranges may improve the detection of far-red and near-infrared signals, but the complexity caused by spectral spillover may increase. Therefore, introducing molecules in the far-red areas with high brightness and narrow wavelengths are greatly demanding.