Macroscopic single-crystal gold microflakes and their devices

Understanding Biomineralization Mechanisms to Produce Size-Controlled, Tailored Nanocrystals for Optoelectronic and Catalytic Applications: A Review

Biomineralization, the use of biological systems to produce inorganic materials, has recently become an attractive approach for the sustainable manufacturing of functional nanomaterials. Relying on proteins or other biomolecules, biomineralization occurs under ambient temperatures and pressures, which presents an easily scalable, economical, and environmentally friendly method for nanoparticle synthesis. Biomineralized nanocrystals are quickly approaching a quality applicable for catalytic and optoelectronic applications, replacing materials synthesized using expensive traditional routes. Here, we review the current state of development for producing functional nanocrystals using biomineralization and distill the wide variety of biosynthetic pathways into two main approaches: templating and catalysis. Throughout, we compare and contrast biomineralization and traditional syntheses, highlighting optimizations from traditional syntheses that can be implemented to improve biomineralized nanocrystal properties such as size and morphology, making them competitive with chemically synthesized state-of-the-art functional nanomaterials.

Gold Recovery from E-Waste by Food-Waste Amyloid Aerogels

Demand for gold recovery from e-waste grows steadily due to its pervasive use in the most diverse technical applications. Current methods of gold recovery are resource-intensive, necessitating the development of more efficient extraction materials. This study explores protein amyloid nanofibrils (AF) derived from whey, a dairy industry side-stream, as a novel adsorbent for gold recovery from e-waste. To do so, AF aerogels are prepared and assessed against gold adsorption capacity and selectivity over other metals present in waste electrical and electronic equipment (e-waste). The results demonstrate that AF aerogel has a remarkable gold adsorption capacity (166.7 mg g-1) and selectivity, making it efficient and an adsorbent for gold recovery. Moreover, AF aerogels are efficient templates to convert gold ions into single crystalline flakes due to Au growth along the (111) plane. When used as templates to recover gold from e-waste solutions obtained by dissolving computer motherboards in suitable solvents, the process yields high-purity gold nuggets, constituted by ≈90.8 wt% gold (21-22 carats), with trace amounts of other metals. Life cycle assessment and techno-economic analysis of the process finally consolidate the potential of protein nanofibril aerogels from food side-streams as an environmentally friendly and economically viable approach for gold recovery from e-waste.

Gold nanoparticle-coated apoferritin conductive nanowires

Gold-metallic nanofibrils were prepared from three different iso-apoferritin (APO) proteins with different Light/Heavy (L/H) subunit ratios (from 0% up to 100% L-subunits). We show that APO protein fibrils have the ability to in situ nucleate and grow gold nanoparticles (AuNPs) simultaneously assembled on opposite strands of the fibrils, forming hybrid inorganic-organic metallic nanowires. The AuNPs are arranged following the pitch of the helical APO protein fiber. The mean size of the AuNPs was similar in the three different APO protein fibrils studied in this work. The AuNPs retained their optical properties in these hybrid systems. Conductivity measurements showed ohmic behavior like that of a continuous metallic structure.

Substrate-Free Fabrication of Single-Crystal Two-Dimensional Gold Nanoplates for Catalytic Application

Two-dimensional (2D) gold nanoplates (AuNPLs) have shown potential in catalysis, photonics, electronics, sensing, and biomedicine fields due to their high aspect ratio, fascinating surface chemistry, and quantum-size effect. Therefore, the synthesis of substrate-free, size-controlled single-crystal gold (Au) nanoplates is highly desirable for the development of catalysis and optical near-field enhancement applications. EDTA and hydroxide anions were used in this study to stimulate the formation of microscale single-crystal gold nanoplates under hydrothermal conditions. The reaction temperature, amount of EDTA, and hydroxyl anions all have a significant effect on the morphologies and size distributions of the gold nanoplates. The gold nanoplates had an average side length of between 3 and 11 μm. The application of the microscale single-crystal gold nanoplates as a nanocatalyst proved their excellent catalytic activity and recyclability for the catalysis of 4-nitrophenol to 4-aminophenol, implying that the large-size gold nanoplates were promising in heterogeneous catalysis applications.

Biofibrous nanomaterials for extracting strategic metal ions from water

Strategic metals play an indispensable role in the related industries. Their extraction and recovery from water are of great significance due to both their rapid consumption and environmental concern. Biofibrous nanomaterials have shown great advantages in capturing metal ions from water. Recent progress in extraction of typical strategic metal ions such as noble metal ions, nuclear metal ions, and Li-battery related metal ions is reviewed here using typical biological nanofibrils like cellulose nanofibrils, chitin nanofibrils, and protein nanofibrils, as well as their assembly forms like fibers, aerogels/hydrogels, and membranes. An overview of advances in material design and preparation, extraction mechanism, dynamics/thermodynamics, and performance improvement in the last decade is provided. And at last, we propose the current challenges and future perspectives for promoting biological nanofibrous materials toward extracting strategic metal ions in practical conditions of natural seawater, brine, and wastewater.

Facile and Scalable Synthesis of Whiskered Gold Nanosheets for Stretchable, Conductive, and Biocompatible Nanocomposites

Noble metal nanomaterials have been studied as conductive fillers for stretchable, conductive, and biocompatible nanocomposites. However, their performance as conductive filler materials is far from ideal because of their high percolation threshold and low intrinsic conductivity. Moreover, the difficulty in large-scale production is another critical hurdle in their practical applications. Here we report a method for the facile and scalable synthesis of whiskered gold nanosheets (W-AuNSs) for stretchable, conductive, and biocompatible nanocomposites and their application to stretchable bioelectrodes. W-AuNSs show a lower percolation threshold (1.56 vol %) than those of gold nanoparticles (5.02 vol %) and gold nanosheets (2.74 vol %), which enables the fabrication of W-AuNS-based stretchable nanocomposites with superior conductivity and high stretchability. Addition of platinum-coated W-AuNSs (W-AuNSs@Pt) to the prepared nanocomposite significantly reduces the impedance and improved charge storage capacity. Such enhanced performance of the stretchable nanocomposite enables us to fabricate stretchable bioelectrodes whose performance is demonstrated through animal experiments including electrophysiological recording and electrical stimulation in vivo.

Smooth Sidewalls on Crystalline Gold through Facet-Selective Anisotropic Reactive Ion Etching: Toward Low-Loss Plasmonic Devices

Quantum plasmonics aims to harness the deeply subwavelength confinement provided by plasmonic devices to engineer more efficient interfaces to quantum systems in particular single emitters. Realizing this vision is hampered by the roughness-induced scattering and loss inherent in most nanofabricated devices. In this work, we show evidence of a reactive ion etching process to selectively etch gold along select crystalline facets. Since the etch is facet selective, the sidewalls of fabricated devices are smoother than the lithography induced line-edge roughness with the prospect of achieving atomic smoothness by further optimization of the etch chemistry. This opens up a route toward fabricating integrated plasmonic circuits that can achieve loss metrics close to fundamental bounds.

Metal-Protein Hybrid Materials with Desired Functions and Potential Applications

Metal nanohybrids are fast emerging functional nanomaterials with advanced structures, intriguing physicochemical properties, and a broad range of important applications in current nanoscience research. Significant efforts have been devoted toward design and develop versatile metal nanohybrid systems. Among numerous biological components, diverse proteins offer avenues for making advanced multifunctional systems with unusual properties, desired functions, and potential applications. This review discusses the rational design, properties, and applications of metal-protein nanohybrid materials fabricated from proteins and inorganic components. The construction of functional biomimetic nanohybrid materials is first briefly introduced. The properties and functions of these hybrid materials are then discussed. After that, an overview of promising application of biomimetic metal-protein nanohybrid materials is provided. Finally, the key challenges and outlooks related to this fascinating research area are also outlined.

β-Lactoglobulin amyloid fibril-templated gold nanoclusters for cellular multicolor fluorescence imaging and colorimetric blood glucose assay

β-Lactoglobulin amyloid fibril (BLGF)-capped gold nanoclusters (Au NCs) with red, green and blue emissions were fabricated via pH-dependent reduction strategy. The BLGF-Au NCs exhibited 3.2 times enhancement of fluorescence (λex = 500 nm, λem = 684 nm), a significant 42 nm red shift, a 11.57% quantum yield and a 1.4 μs decay time compared with native β-lactoglobulin (BLG)-stabilized Au NCs. Meanwhile, the multicolor Au NCs were employed for cell imaging via incubation with A549 cells for 14 h. According to the Michaelis-Menten equation, the kinetic parameters of the BLGF-Au NCs showed a lower Km value (66 μmol L-1) for 3,3,5,5-tetramethylbenzidine (TMB) and a higher vmax (3.74 × 10-8 M s-1) for H2O2, which are comparable with other artificial nanoenzymes and natural peroxidases. Based on the highly intrinsic peroxidase-like activity of the BLGF-Au NCs, a colorimetric method was developed for glucose determination with a detection limit of 1.5 μmol L-1 by determining the variation of the absorption at 652 nm, ranging from 5 to 100 μmol L-1. In addition, the glucose assay method also revealed a 101.02 to 104.16% recovery in a real human serum sample.

Effect of thiolate-ligand passivation on the electronic structure and optical absorption properties of ultrathin one and two-dimensional gold nanocrystals

Gold nanomaterials, including one-dimensional (1D) gold nanorods (AuNRs) and nanowires (AuNWs) and two-dimensional (2D) gold nanoprisms with a large surface area and stability, have attracted widespread research interest. A large number of experimental and theoretical studies have shown that the properties of low dimensional gold nanomaterials depend on their anisotropic shape. In this work, we theoretically conceived a new type of gold nanomaterial, namely, thiolate (SR) monolayer passivated quasi-1D and quasi-2D gold nanocrystals and infinite superstructures, which were formed by the fusion of seed clusters Au₂₈(SR)₂₀, Au₃₆(SR)₂₄, Au₄₄(SR)₂₈ and Au₅₂(SR)₃₂ or the layer by layer growth of gold atoms along the [100] and/or [010] directions. By means of DFT and TD-DFT calculations, the structure and properties of these model gold nanocrystals and superstructures are studied in depth. It is found that the passivation of the monolayer of thiolate leads to significantly improved near-infrared absorption properties in comparison with the ligand free gold nanocrystals. Upon passivating the thiolate-monolayer, the ultrathin 1D gold nanowire and 2D gold nanosheet demonstrate a metal to semiconductor transition. The novel electronic structures, optical absorption and semiconductor-to-metal transition found in these thiolate-protected low-dimensional gold nanocrystals suggest that the passivation of the SR ligand is a promising way to tailor the properties of gold nanomaterials.

Noncloggingly Sieving Sub-6 nm Nanoparticles of Noble Metals into Conductive Mesoporous Foams with Biological Nanofibrils

Porous metal foams have been one of the most sought-after materials owing to their combination of bulk metallic characteristics (e.g., thermal/electrical conductivity and ductility) and nanometric size-effect properties (e.g., catalytic reactivity, plasmonic behavior, and high surface area). Traditional sol-gel approaches, though one of the most frequently used method to produce mesoporous metal foams, were hindered for scalable production and wide applications because of its tedious multistep procedure, time-consuming gelation time, and polydisperse pore sizes. Herein, by depositing biological nanofibrils (chitin, cellulose, and silk) on commercial filtration membranes, we report a facile approach to sieve and recycle sub-6 nm nanoparticles of noble metals (Au and Pt) via nonclogging filtration into three-dimensional (3D) networks with interconnected mesopores. The porous networks could withstand air-drying, in contrast to freezing/supercritical drying conventionally used for mesoporous foams preparation. This approach was also applicable to both mesoporous monometallic (Au, Pt) and bimetallic (Au-Pt) foams. Moreover, the resultant mesoporous metallic foams show high porosity up to 90%, homogeneous mesoporous structure, and metallic conductivity up to 10⁴ S/cm. Thus, this rapid and scalable sieving procedure not only offers a possibility of sieving noncloggingly for efficient recovery of metal nanoparticles but also starts a pathway to produce conductive and flexible mesoporous foams applicable in broad fields such as continuous flow catalysis and smart actuating.

The synthesis of high-aspect-ratio Au microwires with a biomolecule for electrochemical sensing

Gold (Au) crystalline microwires with an unprecedented diameter of >500 nm and an aspect ratio >400 were synthesized using l-tyrosine as a reducing and capping agent. The Au microwires possessed high conductivity and electrocatalytic activities towards glucose and Hg(ii). Their large diameters and aspect ratios also offered maneuverability, and it was easy to produce Au microelectrodes.

Nano oxide intermediate layer assisted room temperature sintering of ink-jet printed silver nanoparticles pattern

Sintering of metallic nanoparticles (NPs) at low temperature is highly wanted in the manufacturing of flexible electronics. And for ink-jet printing, the metallic NPs after printing usually need thermal or chemical post-treatment to remove stabilizing agents and achieve conductivity. Here, we reported a facile method to realize one-step printed sintering of silver nanoparticle (AgNP) ink at room temperature by using intermediate coated layers composed of oxide NPs and polyvinyl alcohol (PVA) mixture. We found that the detachment of the stabilizer (citrate) from the AgNPs was caused by hydroxyl groups on the surface of the oxide NPs, which enabled the coalescence and sintering of the AgNPs. With the aid of SiO₂ NPs based intermediate layer, the patterns showed resistivity as low as 3.45 μΩ cm after sintering. Moreover, the mixed PVA could ensure the forming quality of patterns owing to its adsorption of ink and the high adhesiveness of PVA with substrates. So, we envision that this approach could serve as an adaptive method for sintering of AgNPs based conductive patterns on various substrates at room temperature and promote the manufacture of printed electronics.

Electrical Conductivity, Selective Adhesion, and Biocompatibility in Bacteria-Inspired Peptide-Metal Self-Supporting Nanocomposites

Bacterial type IV pili (T4P) are polymeric protein nanofibers that have diverse biological roles. Their unique physicochemical properties mark them as a candidate biomaterial for various applications, yet difficulties in producing native T4P hinder their utilization. Recent effort to mimic the T4P of the metal-reducing Geobacter sulfurreducens bacterium led to the design of synthetic peptide building blocks, which self-assemble into T4P-like nanofibers. Here, it is reported that the T4P-like peptide nanofibers efficiently bind metal oxide particles and reduce Au ions analogously to their native counterparts, and thus give rise to versatile and multifunctional peptide-metal nanocomposites. Focusing on the interaction with Au ions, a combination of experimental and computational methods provides mechanistic insight into the formation of an exceptionally dense Au nanoparticle (AuNP) decoration of the nanofibers. Characterization of the thus-formed peptide-AuNPs nanocomposite reveals enhanced thermal stability, electrical conductivity from the single-fiber level up, and substrate-selective adhesion. Exploring its potential applications, it is demonstrated that the peptide-AuNPs nanocomposite can act as a reusable catalytic coating or form self-supporting immersible films of desired shapes. The films scaffold the assembly of cardiac cells into synchronized patches, and present static charge detection capabilities at the macroscale. The study presents a novel T4P-inspired biometallic material.

Sheet-like and tubular aggregates of protein nanofibril–phosphate hybrids

Nanofibrils assembled by bovine serum albumin aligned into microtubes and nanosheets upon heating and cooling its solution in phosphate buffer.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

Direct manipulation of metallic nanosheets by shear force microscopy

Micro/nanomanipulation is a rapidly growing technology and holds promising applications in various fields, including photonic/electronic devices, chemical/biosensors etc. In this work, we present that shear force microscopy (ShFM) can be exploited to manipulate metallic nanosheets besides imaging. The manipulation is realized via controlling the shear force sensor probe position and shear force magnitude based on our homemade ShFM system under an optical microscopy for in situ observation. The main feature of the ShFM system is usage of a piezoelectric bimorph sensor, which has the ability of self-excitation and detection. Moreover, the shear force magnitude as a function of the spring constant of the sensor and setpoint is obtained, which indicates that operation modes can be switched between imaging and manipulation through designing the spring constant before experiment and changing the setpoint during manipulation process, respectively. We believe that this alternative manipulation technique could be used to assemble other nanostructures with different shapes, sizes and compositions for new properties and wider applications.

Crab Chitin-Based 2D Soft Nanomaterials for Fully Biobased Electric Devices

2D nanomaterials have various size/morphology-dependent properties applicable in electronics, optics, sensing, and actuating. However, intensively studied inorganic 2D nanomaterials are frequently hindered to apply in some particular and industrial fields, owing to harsh synthesis, high-cost, cytotoxicity, and nondegradability. Endeavor has been made to search for biobased 2D nanomaterials with biocompatibility, sustainability, and biodegradability. A method of hydrophobization-induced interfacial-assembly is reported to produce an unprecedented type of nanosheets from marine chitin. During this process, two layers of chitin aggregations assemble into nanosheets with high aspect ratio. With super stability and amphiphilicity, these nanosheets have super ability in creating highly stable Pickering emulsions with internal phase up to 83.4% and droplet size up to 140 μm, in analogue to graphene oxide. Combining emulsifying and carbonization can further convert these 2D precursors to carbon nanosheets with thickness as low as ≈3.8 nm. Having biologic origin, conductivity, and dispersibility in various solvents, resultant carbon nanosheets start a new scenario of exploiting marine resources for fully biobased electric devices with sustainability and biodegradability, e.g., supercapacitor, flexible circuits, and electronic sensors. Hybrid films of chitin and carbon nanosheets also offer low-cost and environment-friendly alternative of conductive components desirable in green electronics, wearable electronics, biodegradable circuits, and biologic devices.

Guiding growth orientation of two-dimensional Au nanocrystals with marine chitin nanofibrils for ultrasensitive and ultrafast sensing hybrids

Chitin nanofibrils are able to modulate two-dimensional Au nanocrystals from nanoribbons, nanokites to nanosheets, showing remarkable application in wearable sensing devices.

Catalytic Membrane Reactor Immobilized with Alloy Nanoparticle-Loaded Protein Fibrils for Continuous Reduction of 4-Nitrophenol

A catalytic membrane reactor, which contains a membrane matrix and a catalytic film of alloy nanoparticle-loaded β-lactoglobulin fibrils (NPs@β-LGF), was developed for the continuous-flow reduction of 4-nitrophenol (4-NP). The Cu-Ag and Cu-Ag-Au alloy NPs were synthesized using β-LGF as a scaffold and stabilizing agent. In this process, the Cu nanoclusters were formed in the initial stage and were able to promote the synthesis of Ag⁰, which acts as a reducing agent for the rapid formation of Au⁰. Furthermore, a catalytic membrane reactor was constructed by depositing the NPs@β-LGFs on a membrane matrix. The catalytic activity of the Cu-Ag-Au alloy NPs was higher than that of the Cu-Ag alloy NPs, using the reduction of 4-NP to 4-AP as a model reaction. The observed rate constant in the continuous-flow system is also higher than that in the batch system. In addition, these catalytic membrane reactors had good operating stability and antibacterial activity.

Enhancing reducing ability of α-zein by fibrillation for synthesis of Au nanocrystals with continuous flow catalysis

Green low-cost synthesis and efficient recyclability are two major hindrances for Au nanocrystals as catalysts applying in diverse industrial reaction processes. By the use of low-cost α-zein (i.e. a major storage protein of corn) as the reductant, capping agent and stabilizer, Au nanocrystals with tunable catalytic activity were synthesized in a wet-chemical approach. Fibrillation of α-zein further enhanced its reducing ability due to larger specific surface area and more hydrophilic groups exposed on the surfaces. The obtained Au nanocrystals had biocompatibility, high stability in various solvents, unique solubility in aqueous alcohol and high catalytic ability, being able to detect ethanol composition in aqueous ethanol as well as H₂O₂ for diagnosis of diabetes mellitus. These advantages also enable efficient recyclability of Au nanocrystals with continuous flow catalysis in different solvents and environments. Thus, the use of α-zein offered Au nanocrystals not only with green low-cost synthesis, but also with tunable catalytic activities, ethanol-responsiveness and efficient recyclability, which may be applicable in diverse fields.

Self-assembled chiral helical nanofibers by amphiphilic dipeptide derived from d- or l-threonine and application as a template for the synthesis of Au and Ag nanoparticles

The discovery of a class of self-assembling peptides that spontaneously undergo self-organization into well-ordered structures opened a new avenue for molecular fabrication of biological materials. In this paper, the structure controlled helical nanofibers were prepared by two artificial β-sheet dipeptides with long alkyl chains derived from l- and d-threonine (Thr) and sodium hydroxide (NaOH). These helical nanofibers have been characterized using transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), circular dichroism (CD), Fourier transform infrared (FT-IR) spectroscopy, and X-ray powder diffraction (XRD). It was demonstrated that the helicity of the nanofibers could be easily controlled by changing the chirality of the constituent amino acids in the peptide species (d- or l-threonine). Moreover, the hydrogen bonding interactions between the amide groups as well as the hydrophobic interactions among the alkyl chains play important roles in the self-assembly process. It also can be observed that with the passage of time, the hydrogen bonding interactions between the individual nanofiber induced the conversion from nanofibers to nanobelts. Particularly, gold and silver nanoparticles performed good catalytic ability were synthesized using the assembled nanofibers as template.

Amyloid Fibrils as Building Blocks for Natural and Artificial Functional Materials

Proteinaceous materials based on the amyloid core structure have recently been discovered at the origin of biological functionality in a remarkably diverse set of roles, and attention is increasingly turning towards such structures as the basis of artificial self-assembling materials. These roles contrast markedly with the original picture of amyloid fibrils as inherently pathological structures. Here we outline the salient features of this class of functional materials, both in the context of the functional roles that have been revealed for amyloid fibrils in nature, as well as in relation to their potential as artificial materials. We discuss how amyloid materials exemplify the emergence of function from protein self-assembly at multiple length scales. We focus on the connections between mesoscale structure and material function, and demonstrate how the natural examples of functional amyloids illuminate the potential applications for future artificial protein based materials.

Growth of metal-semiconductor core-multishell nanorods with optimized field confinement and nonlinear enhancement

This paper describes a facile method for the synthesis of Au/AuAg/Ag2S/PbS core-multishell nanorods with double trapping layers. The synthesis, in sequence, involved deposition of Ag shells onto the surfaces of Au nanorod seeds, formation of AuAg shells by a galvanic replacement reaction, and overgrowth of the Ag2S shells and PbS shells. The resulting core-multishell nanorod possesses an air gap between the Au core and the AuAg shell. Together with the Ag2S shell, the air gap can efficiently trap light, causing strong field confinement and nonlinear enhancement. The as-prepared Au/AuAg/Ag2S/PbS core-multishell nanorods display distinct localized surface plasmon resonance and nonlinear optical properties, demonstrating an effective pathway for maneuvering the optical properties of nanocavities.

Vapor-phase preparation of single-crystalline thin gold microplates using HAuCl4 as the precursor for plasmonic applications

Vapor-phase preparation of single-crystalline thin gold microplates and their application in defining high-quality plasmonic nanostructures.

Soy protein-directed one-pot synthesis of gold nanomaterials and their functional conductive devices

Gold nanomaterials were synthesized via a facile and green method, using soy protein isolate as reductant, template, and capping agent.

Directed Self-Assembly of Micron-Sized Gold Nanoplatelets into Oriented Flexible Stacks with Tunable Interplate Distance

A growing demand for control over the interparticle spacing and the orientation of anisotropic metallic particles into self-assembled structures is fuelled by their use in potential applications such as in plasmonics, catalysis, sensing, and optoelectronics. Here, we present an improved high yield synthesis method to fabricate micron- and submicron-sized gold nanoplatelets with a thickness less than 20 nm using silver nanoplatelets as seeds. By tuning the depth of the secondary minimum in the DLVO interaction potential between these particles, we are able to assemble the platelets into dynamic and flexible stacks containing thousands of platelets arranged face-to-face with well-defined spacing. Moreover, we demonstrate that the length of the stacks, and the interplate distance can be controlled between tens and hundreds of nm with the ionic strength. We use a high frequency external electric field to control the orientation of the stacks and direct the stacks into highly organized 2D and 3D assemblies that strongly polarize light.