Mechanochemical green synthesis of hyper-crosslinked cyclodextrin polymers

Abstract

Cyclodextrin nanosponges (CD-NS) are nanostructured crosslinked polymers made up of cyclodextrins. The reactive hydroxy groups of CDs allow them to act as multifunctional monomers capable of crosslinking to bi- or multifunctional chemicals. The most common NS synthetic pathway consists in dissolving the chosen CD and an appropriate crosslinker in organic polar aprotic liquids (e.g., N,N-dimethylformamide or dimethyl sulfoxide), which affect the final result, especially for potential biomedical applications. This article describes a new, green synthetic pathway through mechanochemistry, in particular via ball milling and using 1,1-carbonyldiimidazole as the crosslinker. The polymer obtained exhibited the same characteristics as a CD-based carbonate NS synthesized in a solvent. Moreover, after the synthesis, the polymer was easily functionalized through the reaction of the nucleophilic carboxylic group with three different organic dyes (fluorescein, methyl red, and rhodamine B) and the still reactive imidazoyl carbonyl group of the NS.

Keywords: ball-milling; crosslinking; green chemistry; mechanochemistry; nanosponges; β-cyclodextrin.

Figure 1

FTIR analysis of βNS-CDI 1:4, before and after treatment for 4 h in H₂O at 40 °C, synthesized with and without solvent. The band of interest at around 1750 cm⁻¹ assignable to the carbonyl group of the carbonate bond, was visible in all samples, even after treating for hours in H₂O at 40 °C.

Figure 2

Thermogravimetric analysis of β-CD-based carbonate nanosponges, obtained through solution (DMF) and mechanochemical (ball mill) synthesis. Conditions: nitrogen flow, ramp rate 10 °C/min, rt to 700 °C.

Figure 3

Thermogravimetric analysis of α, β and γ-CD-based carbonate nanosponges, obtained through ball-mill synthesis. Conditions: nitrogen flow, ramp rate 10 °C/min.

Figure 4

Adsorption of organic dyes by ball-mill synthesized β-CD-based carbonate nanosponges. Conditions: a small amount of NS (50 mg) was added to a clear solution containing the dye (10 mL). After the adsorption and the deposition of the NSs powder containing the organic dye, a clear solution was obtained.

Figure 5

ζ-Potential of bm cyclodextrin nanosponges with relative STDev (mV).

Figure 6

Hydrolysis of the imidazoyl carbonyl group in water at 40 °C.

Figure 7

Nitrogen content in weight % in cyclodextrins NS-CDI from ball mill synthesis. a) comparison between different monomers with the same CD/CDI ratio and b) between the same CD crosslinked with different ratios.

Figure 8

Simplified schematic reaction and procedure for obtaining the dye-functionalized βNS-CDI. Surface zeta potential of the plain and functionalized βNS-CDI 1:8.