6.3.3.2　Cross-linking induced nano-precipitation

Cross-linking can induce nano-precipitation of polymers into nano-gels in solution.This method is mainly aimed at the multifunctional hydrophilic polymers，such as polysaccharides and poly（amino acids），particularly，chitosan.According to the different mechanisms of cross-linking，it can be further divided into physical-and chemical-cross-linking induced nano-precipitation.

Ionic cross-linking is a commonly used physical-cross-linking method.Specifically，when chitosan is dissolved in an acid solution，the amine（—NH2）is protonated to cationic—NH3+.In this case，the chitosan molecules would be cross-linked and gather into nano-gels via electrostatic interaction caused by the introduction of negatively charged anion crosslinking agent.The commonly used anionic cross-linking agents include sodium tripolyphosphate，sodium phosphate，sodium dodecyl sulfate.Among them，sodium tripolyphosphate is mostly used because of fast cross-linking speed and mild reaction conditions.Moreover，the obtained nano-gels are uniform in size，safe and non-toxic，and their surface charge are adjustable.(https://www.daowen.com)

Chemical-cross-linking such as covalent cross-linking is one of the earlier methods used in the preparation of chitosan nano-gels.Chitosan molecules are cross-linked into nano-gels by forming covalent bonds between cross-linker and the functional groups on chitosan such as hydroxy and amino groups.Sadighian et al.prepared pH-responsive and magnetic chitosan nano-gels by using glutaraldehyde as cross-linking agent and discussed the effects of physicaland chemical-cross-linking on the performance of magnetic chitosan nano-gels as adriamycin targeting drug carriers.The chemical-cross-linked nano-gels have better stability，more controllable release property，but the residual cross-linking agents could be toxic and it means more workload for post-processing purification.