The recovery of gold from water is an important research area. Recent reports have highlighted the ultrahigh capacity and selective extraction of gold from electronic waste using reduced graphene oxide (rGO). Here, we made a further attempt with the thermal rGO membranes and found that the thermal rGO membranes also had a similarly high adsorption efficiency (1.79 g gold per gram of rGO membranes at 1000 ppm). Furthermore, we paid special attention to the detailed selectivity between Au³⁺ and other ions by rGO membranes. The maximum adsorption capacity for Au³⁺ ions was about 16 times that of Cu²⁺ ions and 10 times that of Fe³⁺ ions in a mixture solution with equal proportions of Au³⁺/Cu²⁺ and Au³⁺/Fe³⁺. In a mixed-ion solution containing Au³⁺:Cu²⁺:Na⁺:Fe³⁺:Mg²⁺ of printed circuit board (PCB), the mass of Au³⁺:Cu²⁺:Na⁺:Fe³⁺:Mg²⁺ in rGO membranes is four orders of magnitude higher than the initial mass ratio. A theoretical analysis indicates that this selectivity may be attributed to the difference in the adsorption energy between the metal ions and the rGO membrane. The results are conducive to the usage of rGO membranes as adsorbents for Au capture from secondary metal resources in the industrial sector.

Heating affects the interfacial properties of two-dimensional nanomaterials, especially when they interact with biomolecules. Here, we theoretically studied the dynamic processes driving single-strand DNA (ssDNA) molecules from the hydrophilic to hydrophobic regions on the graphene oxide (GO) surface by heating, as reported by recent experiments. This was accomplished by using multi-sample molecular dynamics simulations in the NVT ensemble, with the temperature increasing from 300 K to 350 K. When the temperature increased, the lifetime of hydrogen bonds between water molecule and oxygen-containing groups on the GO surface decreased from 10.04 ps to 6.86 ps, and the end-to-end distance of 4-mer and 8-mer ssDNA molecules also decreased. This indicated that heating facilitated the breaking/formation of hydrogen bonds and enhanced the flexibility of ssDNA molecules. By heating, active hydrogen bonding first led to unbalanced interactions between the ssDNA molecule and GO surface, and the enhanced flexibility allowed the ssDNA molecule to release stress by moving on the GO surface and relaxing its structure. The ssDNA molecule constantly adjusted its structure by a competition between intra and inter π–π stacking structures. With dynamic cooperation of hydrogen bonding and π–π stacking, the ssDNA molecule moved from the hydrophilic to hydrophobic regions. Our results offer fundamental interfacial science insights into the effects of heating on the interactions between biomolecules and two-dimensional nanomaterials.

DNA/GO composite plays a significant role in the research field of biotechnology and nanotechnology and thus it attracts a great deal of interest. However, it is still unclear how the oxidation degree of the graphene-based surface affects the adsorption process of ssDNA. In this paper, based on the molecular dynamics simulations, we find that ssDNA molecule is absorbed on the GO surface in the most stable state with the oxidation degree around 15%. The microscopic mechanism is attributed to the van Der Walls and the electrostatic interactions between the ssDNA molecule and the graphene-based surface, which is accompanied with the π–π stacking and hydrogen bond formation. The number of π–π stacking between ssDNA and GO reaches the maximum value when the oxidation degree is around 15% among all the GO surfaces. Our simulation results also reveal the coexistence of stretched and curved configurations as well as the adsorption orientation of ssDNA on the GO surface. Furthermore, it is found that the absorbed ssDNA molecules are more likely to move on the graphene-based surface of low oxidation degree, especially on pristine graphene. Our work provides the physics picture of ssDNA's physisorption dynamics onto graphene-based surface and it is helpful in designing DNA/GO nanomaterials.

Using molecular dynamics simulations, we have revealed a novel wetting phenomenon with a droplet on composite structures formed by embedded water into (111) surface of β-cristobalite hydroxylated silica. This can be attributed to the formation of a composite structure composed of embedded water molecules and the surface hydroxyl (–OH) groups, which reduces the number of hydrogen bonds between the composite structure and the water droplet above the composite structure. Interestingly, a small uniform strain (±3%) applied to the crystal lattice of the hydroxylated silica surface can result in a notable change of the contact angles (>40°) on the surface. The finding provides new insights into the correlation between the molecular-scale interfacial water structures and the macroscopic wettability of the hydroxylated silica surface.