Achieving economically feasible and eco-friendly gold recovery from electronic waste (e-waste) remains a critical challenge in the field of resource circularity. Spent coffee grounds (SCGs), a globally abundant agricultural waste, are routinely discarded without effective utilization, resulting in resource waste and ecological burden. To address the dual resource waste issue of e-waste and SCGs, this study adopted SCGs as functional adsorbents for exceptional gold extraction capacity and enable eco-friendly recovery of gold from e-waste. SCGs exhibits an optimum adsorption capacity of 4.29 g of gold per gram of SCGs (4.29 g/g) at pH 1 and a high selectivity for Au(III) in the presence of competing ions. Such excellent adsorption performance is attributed to the aromatic structure of SCGs which act as active sites and key electron donors. These structures facilitate gold ion adsorption and simultaneously provide electrons for Au(III) reduction, enabling the spontaneous transformation of gold ions into metallic gold. Technoeconomic analysis verifies the outstanding economic viability of this strategy, with a high input-output ratio of 1386% and considerable net profit. Compared with conventional gold recovery technologies, the proposed approach achieves synergistic valorization of two types of waste, providing a cost-effective and environmentally sustainable solution for e-waste recycling and SCGs upcycling.

The table salt we consume daily, sodium chloride crystal (NaCl), consists of one sodium atom for every chlorine atom. In fact, NaCl is the only crystal composed solely of sodium and chlorine elements that exists under normal temperature and pressure conditions. Recently, novel two-dimensional crystalline materials with unconventional stoichiometries, such as Na2Cl and Na3Cl, have successfully fabricated at ambient conditions. These two-dimensional (2D) crystals' unique electronic structures endow them with novel attributes, which differ from those of conventional three-dimensional crystals. This review summarizes the recent progress made in the fabrication and analysis of the structures, distinctive features, and applications of these 2D unconventional-stoichiometry crystals Na2Cl,NaCl2,CaCl,KxCl and Li2Cl on graphene surfaces in ambient conditions. Their special properties, including their piezoelectricity, metallicity, heterojunction, and room-temperature ferromagnetism, are paid particularly close attention. Finally, some significant prospects and further developments in this exciting interdisciplinary field are proposed.

Graphene-based laminar membranes open new avenues for recovering gold from electronic waste. In this work, we used reduced graphene oxide membranes (rGOMs) as a model system to elucidate how thermal reduction-induced structural changes enhance the gold extraction performance. Interestingly, the rGOM prepared at 180 degrees C (rGOM-180) exhibits dual interlayer spacings (7.20 and 3.82 & Aring;) and high-density structural defects, leading to optimal gold adsorption capacity. The dual interlayer spacings of rGOM-180 effectively lower the activation energy required for ion migration into narrow interlayer spacings. The high-density structural defects not only strengthen the cation-pi interactions between gold ions and rGOM sheets but also act as electron donors, reducing Au3+ to Au0. These facilitate the adsorption of more gold ions on both the inner and outer surfaces of the rGOM. Our work offers important insights into manipulating membrane structures for enhanced gold extraction performance at the nanoscale.

Three-dimensional covalent organic frameworks (3D COFs), a class of highly porous crystalline polymers, have exhibited great potentials in many applications. However, the reported topologies of 3D COFs have been limited to high-symmetry crystal systems, which significantly hindered the development of such functional materials. Herein, we demonstrate the first construction of four highly crystalline orthorhombic 3D COFs with an unprecedented fmj topology, based on judiciously choosing rotatable monomers. Notably, the square monomers in the unit cell of the fmj topological network adopt three different conformations, resulting in a highly complicated 3D network. Moreover, an isomeric pair (3DCOF-CN and 3DCOF-NC), differing only in the orientations of −C=N-bonds, exhibit distinct optoelectronic properties, protonation abilities, and photocatalytic activities, which is the first time to reveal such isomeric effects in 3D COFs. Particularly, a difference of 32-fold in photocatalytic hydrogen evolution rate was observed for the two isomers, with one achieving a superb rate up to ~31.1 mmol h −1  g −1 . This work achieves the first construction of complex orthorhombic 3D COFs, and offers new insights for the development of 3D COF-based high-performance photocatalysts.

Two-dimensional (2D) crystals which present unconventional stoichiometries on graphene surfaces in ambient conditions, such as Na2Cl, Na3Cl, and CaCl, have attracted significant attention in recent years due to their electronic structures and abnormal cation–anion ratios, which differ from those of conventional three-dimensional crystals. This unconventional crystallization is attributed to the cation–π interaction between ions and the π-conjugated system of the graphene surface. Consequently, their physical and chemical properties—including their electrical, optical, magnetic, and mechanical characteristics—often differ markedly from those of conventional crystals. This review summarizes the recent progress made in the fabrication and analysis of the structures, distinctive features, and applications of these 2D unconventional stoichiometry crystals on graphene surfaces in ambient conditions. Their special properties, including their piezoelectricity, metallicity, heterojunction, and room-temperature ferromagnetism, are given particularly close attention. Finally, some significant prospects and further developments in this exciting interdisciplinary field are proposed.

The interaction between DNA and two-dimensional materials, such as graphene oxide (GO), has aroused significant research interest due to its potential applications, including biosensors, drug delivery, and gene therapy. However, the difference in interaction between DNA and oxygen functional groups on GO remains unclear, and direct observation at the experimental level is still challenging. In this work, we investigated the adsorption process of a single-stranded DNA (ssDNA) onto GO exhibiting a series of oxidation degrees by molecular dynamics simulations. We found that the ssDNA preferentially binds to hydroxyl groups (-OH) over epoxy groups (-O-) on the GO surface. This preferential adsorption feature may be attributed to the stronger tendency of ssDNA to form hydrogen bonds (HBs) with hydroxyl groups compared to epoxy groups in aqueous solutions. Further analysis indicates that the affinity interaction between ssDNA and hydroxyl groups presumably increases the oxidation degree of GO, thus suggesting a better binding between ssDNA and GO. This work is not only expected to provide the underlying mechanism of ssDNA onto graphene-based interfaces but also offers a deeper understanding of the structures of DNA-two-dimensional complexes, which may potentially contribute to designing new molecular structures for bio-sensing-related nano-devices and nanostructures.

Traditional mining practices not only cause severe environmental issues, but also face the problem of insufficient production capacity of gold to meet its growing demand. The proposed alternative strategies for gold production, such as the extraction of gold from seawater, still keep a formidable challenge due to their strong dependence on adsorbent materials with high capacity, selectivity, and sensitivity, while also needing to meet the demands of being environmentally friendly and cost-effective. In practice, the direct extraction of gold from seawater is limited by its extremely low yield and high energy expenditure. However, if the combination of gold extraction techniques with seawater desalination can substantially reduce the energy consumption, the extraction of gold from seawater will become economical and feasible. In this paper, we evaluate the feasibility of marine gold extraction using reduced graphene oxide membranes (rGOM) during the seawater desalination process. The rGOM can adsorb almost all Au3+ from the solutions with trace concentrations of Au3+ ranging from 10 ppb to 200 ppb. The adsorption quantity is linearly related to the concentration, indicating that the adsorption capacity of rGOM is much higher than the total amount of Au3+ in the solution. Additionally, the rGOM can selectively adsorb 99 % of Au3+ in the mixed solution while hardly adsorbing other common elements in seawater. More importantly, the rGOM exhibits the long-term stability over 30 days when being immersed in the solution, making it directly compatible with the existing seawater desalination processes. These specific properties allow the rGOM to be an ideal candidate for combining the extraction of gold from seawater with seawater desalination processes. Our findings provide a methodology for enhancing the economic efficiency of the extraction of gold from seawater and hold promise for addressing the problem of gold scarcity.

The aggregation of monomeric amyloid beta protein (A beta) into oligomers and amyloid plaque in the brain is associated with Alzheimer's disease. The hydrophobic central core A beta(16-22) has been widely studied due to its essential role in the fibrillization of full-length A beta peptides. Compared to the homogeneous antiparallel structure of A beta(16-22) at the late stage, the early-stage prefibrillar aggregates contain varying proportions of different beta structures. In this work, we studied the appearance probabilities of various self-assembly structures of A beta(16-22) and the effects of Zn2+ on these probabilities by replica exchange molecular dynamics simulations. It was found that at room temperature, A beta(16-22) can readily form assembled beta-sheet structures in pure water, where a typical antiparallel arrangement dominates (24.8% of all sampled trimer structures). The addition of Zn2+ to the A beta(16-22) solution will dramatically decrease the appearance probability of antiparallel trimer structures to 12.5% by disrupting the formation of the Lys16-Glu22 salt bridge. Meanwhile, the probabilities of hybrid antiparallel/parallel structures increase. Our simulation results not only reveal the competition between antiparallel and parallel structures in the A beta(16-22) oligomers but also show that Zn2+ can affect the oligomer structures. The results also provide insights into the role of metal ions in the self-assembly of short peptides.

The adsorption dynamics of double-stranded DNA (dsDNA) molecules on a graphene oxide (GO) surface are important for applications of DNA/GO functional structures in biosensors, biomedicine and materials science. In this work, molecular dynamics simulations were used to examine the adsorption of different length dsDNA molecules (from 4 bp to 24 bp) on the GO surface. The dsDNA molecules could be adsorbed on the GO surface through the terminal bases and stand on the GO surface. For short dsDNA (4 bp) molecules, the double-helix structure was partially or totally broken and the adsorption dynamics was affected by the structural fluctuation of short dsDNA and the distribution of the oxidized groups on the GO surface. For long dsDNA molecules (from 8 bp to 24 bp) adsorption is stable. By nonlinear fitting of the contact angle between the axis of the dsDNA molecule and the GO surface, we found that a dsDNA molecule adsorbed on a GO surface has the chance of orienting parallel to the GO surface if the length of the dsDNA molecule is longer than 54 bp. We attributed this behavior to the flexibility of dsDNA molecules. With increasing length, the flexibility of dsDNA molecules also increases, and this increasing flexibility gives an adsorbed dsDNA molecule more chance of reaching the GO surface with the free terminal. This work provides a whole picture of adsorption of dsDNA molecules on the GO surface and should be of benefit for the design of DNA/GO based biosensors.

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.

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.

Hydrated cation-pi interaction, as a kind of non-covalent interaction, is essential to the study of soft condensed matter. This paper reviews the recent progress in the two-dimensional crystals of unconventional stoichiometries induced by the hydrated cation-pi interaction, and their distinguished features. These crystals include Na2Cl, Na3Cl and CaCl at ambient conditions. These crystals have abnormal cation-anion ratios different from those of normal three-dimensional crystals and unique electronic structures. Consequently, their physical and chemical properties are usually different from those of normal three-dimensional crystals, including the room temperature ferromagnetism. These ferromagnetic materials of unconventional stoichiometries may provide new insight into biomagnetism, medicine.related magnetism and the design of low.dimensional ferromagnetic materials.

Under ambient conditions, the only known valence state of calcium ions is +2, and the corresponding crystals with calcium ions are insulating and nonferromagnetic. Here, using cryo-electron microscopy, we report direct observation of two-dimensional (2D) CaCl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only monovalent (i.e. +1). Remarkably, metallic rather than insulating properties are displayed by those CaCl crystals. More interestingly, room-temperature ferromagnetism, graphene-CaCl heterojunction, coexistence of piezoelectricity-like property and metallicity, as well as the distinct hydrogen storage and release capability of the CaCl crystals in rGO membranes are experimentally demonstrated. We note that such CaCl crystals are obtained by simply incubating rGO membranes in salt solutions below the saturated concentration, under ambient conditions. Theoretical studies suggest that the formation of those abnormal crystals is attributed to the strong cation-π interactions of the Ca cations with the aromatic rings in the graphene surfaces. The findings highlight the realistic potential applications of such abnormal CaCl material with unusual electronic properties in designing novel transistors and magnetic devices, hydrogen storage, catalyzers, high-performance conducting electrodes and sensors, with a size down to atomic scale.

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.

DNA/GO composite plays a significant role in the research field of biotechnology and nanotechnology, and 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 single-strand DNA（ss DNA）. In this paper, based on the molecular dynamics simulations, we find that ss DNA 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 ss DNA 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 ss DNA 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 ss DNA on the GO surface. Furthermore, it is found that the absorbed ss DNA 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 ss DNA’s physisorption dynamics onto graphene-based surface and it is helpful in designing DNA/GO nanomaterials.

The sequence features of single-stranded DNA (ssDNA) adsorbed on a graphene oxide (GO) surface are important for applications of the DNA/GO functional structure in biosensors, biomedicine, and materials science.

We experimentally observed the enhanced contact angle hysteresis (CAH) of dilute aqueous salt solution on graphite surface, i.e., 40.6$^\circ$, 34.6$^\circ$, and 27.8$^\circ$, for LiCl, NaCl, and KCl, indicating the effective tuning of the CAHs by cations. Molecular dynamics simulations reveal that the preferential adsorption of cations on the HOPG surface due to the cation-{\pi} interaction pins the water at the backward liquid-gas-solid interfaces, reducing the receding contact angle and hence enhancing the CAH. This finding provides a simple method to control the contact angle and the CAH of aqueous drops on graphitic surfaces such as graphene, carbon nanotube, biomolecules, and airborne pollutants.

We theoretically and experimentally show that, with water being adsorbed, the graphene oxide (GO) is converted to a spontaneously dynamic covalent material under ambient conditions, where the dominated epoxy and hydroxyl groups are mediated by water molecules to spontaneously break/reform their C–O bonds to achieve dynamic oxygen migration. This dynamic material presents structural adaptivity for response to biomolecule adsorption. Both density functional theory calculations and ab initio molecular dynamics simulations demonstrate that this spontaneously dynamic characteristics is attributed to the adsorption of water molecules, which sharply reduces the barriers of these oxygen migration reactions on GO to the level less than or comparable to the hydrogen bonding energy in liquid water.

Recently,there are great efforts that have been taken to suppressing/controlling the coffee ring effect,but it is of challenge to achieve inexpensive and efficient control with less disturbance,suitable for scalable production and highly enhancing the printing/dyeing color fastness.By only adding trace amounts of salt into the suspensions,here we experimentally achieve the facile and highly efficient control of the coffee ring effect of suspended matter on substrates of graphene,natural graphite,and polyethylene terephthalate surfaces.Notably,friction force measurements show that ion-controlled uniform patterns also greatly enhance color fastness.Molecular dynamics simulations reveal that,due to strong hydrated cation-π interactions between hydrated cations and aromatic rings in the substrate surface,the suspended matters are adsorbed on the surfaces mediated by cations so that the suspended matters are uniformly distributed.These findings will open new avenues for fabricating functional patterns on graphene substrates and will benefit practical applications including printing,coating,and dyeing.

DNA/GO functional structures have been widely used in biosensors, biomedicine and materials science. However, most studies about DNA/GO functional structures do not take into account the coexistence of both large unoxidized and oxidized regions on GO sheets. This special local structure provides the boundary region, which is the junction area between unoxidized and oxidized regions, and exhibits a special amphiphilic property of the GO sheets. Here based on molecular dynamics simulations, our results predict that the adsorption efficiency of long strand ssDNA molecules adsorbed on GO is 43%. Further analysis has shown that the ssDNA adsorption behaviors on the GO surface are more likely to start in the boundary region, even for 20 mer ssDNA molecules. Looking into the adsorption dynamic process we can see that the hydrogen bonds between ssDNA and GO are very active and easily broken and formed, especially for the boundary region of the GO surface, resulting in easy capture and adsorption of the ssDNA molecules on this region. The result provides insightful understanding of the adsorption behavior of ssDNA molecules on this amphiphilic GO surface and is helpful in the design of DNA/GO functional structure-based biosensors.