Encapsulation and controlled release characteristics of ethylene gas in cucurbit[n]urils

Abstract

Ethylene was introduced into cucurbit[n]urils (CB[n]s, n = 5–7) by molecular encapsulation, for comparison with V-type crystalline starch (V-starch) and α-cyclodextrin (α-CD). The encapsulation capacity of inclusion complexes (ICs) followed the order of CB[5] > V-starch > CB[6] > CB[7] > α-CD. X-ray diffractometry, and ¹³C solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy confirmed the presence of ethylene gas in the solid matrices. The controlled release kinetics of the ICs were further studied under different storage temperature (4 and 25 °C) and relative humidity (52.9 and 75.5%) conditions. The Avrami equation was adapted to analyze the controlled release kinetics of ethylene gas, showing a diffusion limited mechanism for CB[n]-ICs under all the storage conditions. The ethylene release rate from CB[n]-ICs was accelerated by increasing ambient temperature, but was not sensitive to relative humidity. CB[5] showed the slowest ethylene release rate among the five solid matrices, up to 700 h under 4 °C storage conditions. Molecular simulation suggested two stable molecular conformations for ethylene molecules within the matrices, i.e., type I, two ethylene molecules in the cavity; and type II, one ethylene molecule in the cavity and the other one outside the cavity. CB[5] and CB[6] had a stable type II molecular conformation of ethylene, whereas CB[7] showed similar stability to type I and type II molecular conformations. For the first time, we demonstrated that CB[n]s (particularly CB[5]) are desirable gas storage and release systems for ethylene. This technology can enable precisely controlled and targeted applications of ethylene for food processing and agricultural applications.