Modified biopolymers, such as, charge-modified biopolymers, cross-linked biopolymers, and cross-linked, charge-modified biopolymers are provided along with methods of producing and using the same.

Disclosed are a M-cell targeting and pH-responsive starch-based carrier material, and a preparation method and application thereof. The starch-based carrier material has a molecular structure as follows, a molecular weight of 7.0410.sup.4 to 2.1110.sup.6 g/mol, a degree of substitution of carboxymethyl groups of 0.04 to 0.28, and a grafting amount of targeting peptide GRGDS of 0.01% to 1.12% (calculated based on the content of N). The material is not dissolved in water when protonated at pH (pH=1.2) of the stomach, and is slowly dissolved in water when deprotonated at pH (pH=6.8) of the small intestine, therefore it has good responsiveness to pH of the gastrointestinal tract. The material can efficiently encapsulate a positively charged active substance by means of electrostatic interaction, therefore it can protect the active substances from being destroyed and inactivated in the gastrointestinal tract. Meanwhile, the material can target M cells to improve the transport efficiency of the active substance by M cells, thereby improving the bioavailability of the active substance.