β-CD was oxidized by sodium periodate to obtain β-CD-CHO [21]. Briefly, 7.93 mmol β-CD was dissolved in 200 mL deionized water at 60 °C. 24.03 mmol sodium periodate was added when the solution cooled to room temperature. The reaction was carried out in a water bath at 20 °C and keep magnetically stirred for 1 h in the dark. Then, 5 mL ethylene glycol and 27.03 mmol calcium chloride were added respectively to the mixed solution to stop the reaction. The calcium iodate precipitate formed by reaction with iodate was removed by suction filtration. The resulted mixture was dialyzed against distilled water for 6 h, and finally β-CD-CHO powder obtained by lyophilization.

The aldehyde content of β-CD-CHO was determined by hydroxylamine hydrochloride-potentiometric titration. The aldehyde reacts with hydroxylamine hydrochloride to produce oxime and hydrochloric acid (HCl). The release of HCl was titrated with sodium hydroxide to calculate the aldehyde content. Concretely, 0.1 g β-CD-CHO was added to 25 mL of hydroxylamine hydrochloride solution at 0.25 mol/L. The mixture was stirred at room temperature for 2 h and titrated. The amount of HCl in the mixed solution was measured by potentiometric titration method with standard NaOH solution at 0.05 mol/L. The volume (V, mL) of NaOH solution consumed and the corresponding pH of the solution were recorded, and the titration was stopped when the pH of the solution reached 5.0. The pH-V titration curve was plotted with the volume of the NaOH solution as the abscissa and the pH as the ordinate. Then, the curve was further differentiated, and the peak value of the obtained differential curve was the consumption volume of the NaOH solution. The aldehyde content of β-CD-CHO was calculated by Equation (1).

where [-CHO] (mmol/g) is the aldehyde content of β-CD-CHO; VNaOH (mL) is the peak value of differential curve; c (mol/L) is the concentration of NaOH; m (g) is the weight of β-CD-CHO. The experiments were done in triplicate.

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