Red emissive cross-linked chitosan and their nanoparticles for imaging the nucleoli of living cells

•Glutaraldehyde-cross-linked chitosan of different states (solution, gel, nanoparticle, etc.) with luminesce at ca. 670nm were prepared for the first time.•They are quite inert and anti-photobleaching.•Their nanoparticles (5.6nm) could image the nucleoli of living HeLa cells with low cytotoxicity. Biocompatible glutaraldehyde-cross-linked chitosan with new red fluorescence were prepared for the first time and were shaped into nanoparticles via inverse-microemulsion method. They could luminesce at ca. 670nm either as powders and nanoparticles or in real and gelling solutions or suspensions, having a lifetime of 1.353ns and a quantum yield of 0.08 in solution or 0.01 in solid state. The new-formed pyridinium structures and the intramolecular charge transfer effect are considered to be responsible for the new red emission, which have been proved by FTIR, 13C NMR, and some calculation using Gaussian 09, respectively. Strikingly, they are quite inert and anti-photobleaching, with only