Fig. 1

Preparation of PEI-GOS-PEG-FA. a The preparation process of small GOS. The rectangles represent the chemicals, and the circles represent the steps. b Schematic illustration of the preparation of small carboxylated GOS. The OH groups were oxidized to COOH groups on GOS by NaOH. c The preparation process of ultrasmall carboxylated GOS. The rectangles represent the chemicals, and the circles represent the steps. d Schematic illustration of the preparation of PEG-FA. Only the structure of the main product, the active carboxyl-linked conjugate, is shown. e FTIR spectra of FA (black) and PEG-FA (blue). Absorbance bands for PEG-FA are observed at 1690 cm^− 1 (−NH-CO-), 2872 cm^− 1 (C-H) and 1100 cm^− 1 (C-O-C). In addition, a peak at approximately 1536 cm^− 1 characteristic of FA appears in the spectrum of PEG-FA. f The preparation process of PEI-GO-PEG-FA solution. The rectangles represent the chemicals, and the circles represent the steps. g Schematic illustration of the preparation of PEI-GOS-PEG-FA. PEG-FA and PEI were both connected to GO via an amide linkage between the amino groups on PEG-FA and PEI and the carboxylic groups on GO