Can DAPI be excited with laser confocal λ=405nm after staining the nucleus?

Yes, DAPI can be excited using 405 nm excitation light. Although the best wavelength for excitation of DAPI is in the ultraviolet region (~350-360 nm), it can also be effectively excited with blue light at 405 nm. Using a 405 nm excitation light source can still activate DAPI and produce blue fluorescence for viewing and imaging cell nuclei.