The formation and growth of ice crystals is considered to be a critical issue in aerospace and automotive industries as well as in cryopreservation of cells. Pure water undergoes homogenous nucleation of ice at ~ −40 °C. However, the presence of dusts, minerals, birch and conifer pollen and some species of fungus could serve as a nucleator and promotes nucleation of ice much above this temperature. Carbon nanotubes, graphene nano-flakes and carbon soots (from burning of fuels) are promising candidates to promote nucleation of ice crystals.
Base-washing has been shown to be effective in removing oxidative debris from graphene oxide (GO) and enables effective functionalization of the surface of GO with thiols, Au nanoparticles and polymers. Base-washed graphene oxide (bwGO) is a distinct graphene-like material with better qualities than the normal GO. Researchers at Department of Chemistry, Warwick Medical School and Department of Physics, University of Warwick, UK have suggested that surface modification of bwGO would offer a versatile template to evaluate the potential of 2D carbon nanomaterials as ice-nucleating agents as well as to serve as a versatile scaffold to probe the role of surface chemistry.
GO was synthesized by Hummer’s method. About 140 mg of GO was re-dispersed in 250 ml of deionized H2O by mild sonication followed by addition of 0.140 g of NaOH and heating of the solution to 70 °C for 1 h. The resultant dark brown solution was centrifuged (@12,500 rpm for 30 min). The dark brown solid was washed with water and re-centrifuged. The solid was re-protonated using 0.014 M HCl at 70 °C for 1 h, filtered, thoroughly washed with deionized H2O and dried under vacuum to yield bwGO (a black solid), which was dispersed in a H2O/CH3CN mixture via sonication. Poly(N-isopropylacrylamide), (pNIPAM) with degree of polymerization of 55 and 140 were prepared by polymerization of N-isopropylacrylamide (Fig. 1). pNIPAM hexanethiol, dodecanethiol and octadecanethiol were grafted on the surface of bwGO under Schlenk conditions in N2 atmosphere (Fig. 2).
Fig. 1 Scheme depicting polymerization of N-isopropylacrylamide
Fig. 2 Scheme depicting polymerization of N-isopropylacrylamide and grafting of polymers and thiols on the surface of base-washed graphene oxide
The ice nucleation activity of unmodified and surface modified GO was quantified by determining the average nucleation temperature to freeze a droplet (1 μL) of water. The droplets were cooled under an atmosphere of dry nitrogen, and the freezing point of each droplet was recorded by visual observation using a microscope. When tested for the nucleation activity, ultra-pure Milli-Q water nucleated at -26 °C, suggesting a heterogeneous nucleation (Fig. 3); Both bwGO and bwGO-Cyst increased the nucleation temperature by over 5 °C, to -20 and -18 °C (Fig. 4).
Fig. 3 Ice nucleation assay: No water droplet is frozen at -20 °C; At -23 °C, two water droplets (marked by red circles) are frozen while all water droplets are frozen at -30 °C.
Fig. 4 Comparison of ice nucleation activity of Milli-Q water, GO and cysteine-functionalized GO
A remarkable nucleation promotion activity is observed for bwGO surface modified with alkane thiols; octadecanethiol modified bwGO increased the nucleation temperature by > 15 °C to –12 °C (Fig. 5(a)). All the alkyl modified GOs are more active than bwGO and the cysteine modified bwGO, which suggests that the increased hydrophobicity plays a dominant role in determining the ice nucleation. The similar activity of pNIPAM-bwGO with that of bwGO (Fig. 5(b)) suggests that modification of the surface of bwGO with polymer molecules exert a very little influence on the ice nucleation temperature.
Fig. 5 Comparison of ice nucleation activity of (a) Milli Q water, GO and GO functionalized with hexanethiol, octadecanethiol and dodecanethiol; and (b) Milli Q water, GO, pNIPAM55 and pNIPAM140.
The surface modified bwGO may find application in cryopreservation and cloud seeding.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Caroline I. Biggs et al., Impact of sequential surface-modification of graphene oxide on ice nucleation, Phys. Chem. Chem. Phys., 2017,19, 21929-21932
Fig. 1 Schematic illustration of the effect of solid surfaces on ice growth; (A) introduction of AgI nanoparticles on solid surfaces to achieve ice nucleation over the entire solid surfaces in the same environment.
Fig. 2 Snapshots acquired at different time periods using an optical microscope coupled with a high-speed camera: (B, D) top-view images; and (C, E) side-view images; (B) growth process of six-leaf clover-like ice on a hydrophobic surface (θ = 107.3°); (C) Off-side growth mode; (D) growth process of sunflower-like ice on a hydrophilic surface (θ = 14.5°); (E) Along-surface growth mode (growth environment: surface temperature is −15 °C; and supersaturation is 5.16)
Fig. 3 Schematic illustration depicting that the ice crystals grown with off-surface growth mode can be easily blown away by a breeze whereas those grown with along-surface growth mode stuck to the solid surface.
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