Sequential Surface-Modification of Graphene Oxide

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

Self-folding 3D graphene

Graphene finds widespread application in energy storage, sensors and flexible electronics. For most of the applications, a planar geometry of graphene would suffice while applications such as wearable electronics, biological or dispersible sensors, and actuators demand a curved and folded architectures. Since pristine graphene is highly chemically inert, achieving controlled self-folding in response to external environmental stimuli is very difficult.

Researchers at the Johns Hopkins University and Massachusetts Institute of Technology, USA have developed a method to fold and unfold monolayer graphene into ordered 3D structures so that they can be designed and fabricated in accordance with a predictable shape (Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084). The processing involves various stages including surface functionalization of graphene, transfer of the functionalized graphene on patterned Al coated on Si, shape design by photolithography, removal of unwanted graphene by oxygen plasma etching, removal of Al by chemical etching and folding of the functionalized graphene by an increase in solution temperature.

Surface functionalization of graphene

Immersion of monolayer graphene in a dilute aqueous solution of dopamine (2.0 mg/ml) at pH of 8.5 (10 mM tris-HCl) for 2 – 4 h promoted self-polymerization of dopamine, that lead to the formation of a thin layer (~5 nm) of polydopamine (PD) on the surface of graphene. Subsequently, the PD-coated graphene was immersed in a dilute aqueous solution containing amine-terminated poly(N-isopropylacrylamide) (PNIPAM) (2.0 mg/ml) at pH 8.5 (10 mM tris-HCl) at 60 °C for 3 h (Fig. 1(a)). The PD served as an intermediate active layer to graft PNIPAM on PD-coated graphene. The thermoresponsive properties of PNIPAM enables the surface functionalized graphene to behave as an ultrathin shape-changing material.

Fabrication of self-folding microstructures

A patterned sacrificial Al layer was deposited on Si. Subsequently, the PD-PNIPAM functionalized graphene was transferred onto the substrate. The functionalized graphene was patterned into various shapes by photolithography, and the graphene in unwanted areas was removed by oxygen plasma etching. The underlying Al layer was dissolved using 5 mM NaOH + 3 mM sodium dodecyl sulphate. Folding of the functionalized graphene was induced by heating the solution to 45 °C using a hot plate. Selective pinning prevented the folded structures from being washed away (Fig. 1(b)).

Fig. 1 Schematic illustration of surface functionalization, patterning, fabrication and folding process of graphene microstructures

Grafting of the thermoresponsive PNIPAM to the surface of graphene is necessary for folding; neither the pristine graphene nor the PD-graphene exhibit the self-folding behavior with an increase in temperature. Different 3D shapes, including flower, dumbbell, and box can be obtained after folding (Fig. 2(a-c)). The reversible switching behavior of PNIPAM also helps to unfold the structure by reversing the temperature from 45 °C to 25 °C (Fig. 2(d-f). Addition of a rigid polymer layer to the petals (increase in thickness up to 100 nm) reduces the adhesion between the petals and favours easy reversibility (Fig. 2(g-i)).

The flower shaped folding tends to fold its free petals toward the center and go from an open to a closed state, it is possible to encapsulate live cells within the self-folding flower. The temperature employed for cell culture (37 °C) is sufficient to induce folding of the functionalized graphene flowers to encapsulate the cells inside its petals. It is confirmed that the cells are alive after encapsulation, which suggests that the self-folding process is biocompatible and can be used to capture biological cargo.

The process is highly tunable and offer control over the self-folding nature of graphene. The extent of folding is increased with an increase in temperature from 25 °C to 45 °C while reversing the temperature enabled unfolding of the structure. Different 3D shapes such as flower, dumbbell, box, etc., can be achieved. A variety of applications such as encapsulation and delivery of cells, design and fabrication of novel electrical devices and field-effect transistors, formation of a variety of Origami and Kirigami shape-changing structures, etc. are envisioned.

Fig. 2 (a-c) Snapshots of temperature-induced self-folding of ultrathin graphene microstructures with a flower geometry; (d-i) Reversibility of the temperature-induced self-folding. The sequence of images (d, e f) shows folding and unfolding of functionalized graphene flower; The sequence of images (g, h, i) shows folding and unfolding of a flower with rigid SU8 polymer petals, with better stability and reversibility but with increased thickness (100 nm). Scale bars: (a-c) 100 µm; and (d-i) 50 µm.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Weinan Xu et al., Ultrathin thermoresponsive self-folding 3D graphene, Science Advances  06 Oct 2017: Vol. 3, no. 10, e1701084; 10.1126/sciadv.1701084