Heteroepitaxial growth of core-shell and core-multishell nanocrystals composed of palladium and gold

The heteroepitaxial growth of the core-shell nanocrystals composed of Pd and Au is developed. Pd nanocubes or Au nano-octahedrons are utilized as the cores. The growths of the Au and Pd shells are realized under similar conditions, where the same reducing agent and stabilizing surfactant are employed. The preparation is highly controllable, and the epitaxial growth is repeated up to three times to yield Pd@Au@Pd@Au and Au@Pd@Au@Pd core-trishell nanocrystals. The thickness of each shell is readily varied by changing the amount of the metal salts used for growth. All of the nanocrystal products have narrow size distributions and are single crystalline. The plasmon resonance properties of these nanocrystals are mainly determined by the exterior shell. The plasmon of the Pd-shell-terminated nanocrystals is suppressed, while that of the Au-shell-terminated ones is recovered and is stronger when the Au shell becomes thicker. This growth method can potentially be extended to other metals for the synthetic design of more complex core-multishell metal nanostructures with desirable optical, catalytic, and magnetic properties.     

One‐Pot Synthesis of Multifunctional Au@Graphene Oxide Nanocolloid Core@Shell Nanoparticles for Raman Bioimaging,Photothermal, and Photodynamic Therapy

The paper reports a facile one‐pot synthesis of core@shell nanoparticles (NPs) composed of Au core and graphene oxide nanocolloid (GON) shell. Unique properties of Au NPs and GON can be incorporated into a single nanohybrid structure to provide desirable functions for theranosis such as localized surface plasmon resonance, Raman scattering, amphiphilic surface, and photothermal conversion. Synthesis of Au@GON NPs is achieved by simple one‐pot reaction in aqueous phase utilizing GON as a reducing and stabilizing agent without any additional reducing agent. The zinc phthalocyanine, a photosensitizer, loaded Au@GON NPs show excellent multifunctional properties for combinational treatment of photothermal and photodynamic therapy in addition to Raman bioimaging with low cytotoxicity.     

Engineering Gold Nanorod–Copper Sulfide Heterostructures with Enhanced Photothermal Conversion Efficiency and Photostability

Plasmonic gold nanorods (Au NRs)–copper sulfide heterostructures have recently attracted much attention owing to the synergistically enhanced photothermal properties. However, the facile synthesis and interface tailoring of Au NRs–copper sulfide heterostructures remain a formidable challenge. In this study, the rational design and synthesis of Au NRs–Cu₇S₄ heterostructures via a one‐pot hydrothermal process is reported. Specifically, core–shell and dumbbell‐like Au NRs–Cu₇S₄ heterostructures are obtained with well‐controlled interfaces by employing the Au NRs with different aspect ratios. Both core–shell and dumbbell‐like Au NRs–Cu₇S₄ have proven effective as photothermal therapy agents, which offer both high photothermal stability and significant photothermal conversion efficiency up to 62%. The finite‐difference time domain simulation results confirm the coupling effect that leads to the enhanced local field as well as the optical absorption at the heterostructure interface. Importantly, these Au NRs–Cu₇S₄ heterostructures can be compatibly used as an 808 nm laser‐driven photothermal therapy agents for the efficient photothermal therapy of cancer cells in vitro. This study will provide new insight into the design of other noble metal–semiconductor heterostructures for a broad range of applications utilizing surface plasmon resonance enhancement phenomena.     

Cu7S4 Nanosuperlattices with Greatly Enhanced Photothermal Efficiency

According to the simulation, the self‐assembly of Cu₇S₄ nanocrystals would enhance the photothermal conversion efficiency (PCE) because of the localized surface plasmon resonance effects, which is highly desirable for photothermal therapy (PTT). A new strategy to synthesize Cu₇S₄ nanosuperlattices with greatly enhanced PCE up to 65.7% under irradiation of 808 nm near infrared light is reported here. By tuning the surface properties of Cu₇S₄ nanocrystals during the synthesis via thermolysis of a new single precursor, dispersed nanoparticles (NPs), rod‐like alignments, and nanosuperlattices are obtained, respectively. To explore their PTT applications, these hydrophobic nanostructures are transferred into water by coating with home‐made amphiphilic polymer while maintaining their original structures. Under identical conditions, the PCE are 48.62% and 56.32% for dispersed NPs and rod‐like alignments, respectively. As expected, when the nanoparticles are self‐assembled into nanosuperlattices, the PCE is greatly enhanced up to 65.7%. This strong PCE, along with their excellent photothermal stability and good biocompatibility, renders these nanosuperlattices good candidates as PTT agents. In vitro photothermal ablation performances have undoubtedly proved the excellent PCE of our Cu₇S₄ nanosuperlattices. This research offers a versatile and effective solution to get PTT agents with high photothermal efficiency.     

Macroscale Colloidal Noble Metal Nanocrystal Arrays and Their Refractive Index‐Based Sensing Characteristics

Colloidal noble metal nanocrystals are promising for a large number of optical and biotechnological applications. Many practical applications require the formation of large‐area, high‐density, and uniformly distributed metal nanocrystal arrays on various substrates, to overcome the limitations brought by the instability of colloidal metal nanocrystal solutions and the high cost of single‐particle spectroscopy characterizations. A method is developed for directly depositing colloidal metal nanocrystals, including Au nanospheres, Au nanorods, Au nanobipyramids, and (Au core)/(Ag shell) nanorods, from their solutions onto different substrates. The resultant nanocrystal arrays are relatively uniform and dense, with the peak extinction value of a single layer reaching 0.3. Their areas are up to 10 cm by 10 cm and can be further increased if larger‐size containers are utilized. The refractive index sensitivities are studied for Au nanorod arrays supported on glass slides, mesoporous silica and titania films, and capped with different molecules. Au nanorods deposited on mesoporous titania films are found to exhibit the highest index sensitivities, comparable to those of the same nanorod sample in solutions. It is expected that this approach will greatly facilitate plasmonic applications that require large‐area arrays of noble metal nanocrystals.     

Short‐Wave Infrared Sensor by the Photothermal Effect of Colloidal Gold Nanorods

Photodetection in the short‐wave infrared (SWIR) spectrum is a challenging task achieved often by costly low bandgap compound semiconductors involving highly toxic elements. In this work, an alternative low‐cost approach is reported for SWIR sensors that rely on the plasmonic‐induced photothermal effect of solution‐processed colloidal gold nanorods (Au NRs). A series of uniform solution‐processed Au NRs of various aspect ratios are prepared exhibiting a strong and well‐defined longitudinal localized surface plasmon resonance (L‐LSPR) maximum from 900 nm to 1.3 µm. A hybrid device structure is fabricated by applying Au NRs on the surface of a thermistor. Under a monochromatic illumination, hybrid Au‐NR/thermistor devices exhibit a clear photoresponse in the form of photoinduced resistance drop in the wavelength window from 1.0 to 1.8 µm. The photoresponsivity of such hybrid devices reaches a maximum value of 4.44 × 10⁷ Ω W^?1 at λ = 1.4 µm (intensity = 0.28 mW cm^?2), a wavelength in agreement with the L‐LSPR of the Au NRs applied. Colloidal Au NRs, capable to perform fast conversion between photon absorption and thermal energy, thus open an interesting avenue for alternative low‐cost SWIR photodetection.     

A synergistically enhanced photothermal transition effect from mesoporous silica nanoparticles with gold nanorods wrapped in reduced graphene oxide

In this work, near-infrared (NIR)-responsive core–shell gold nanorods/mesoporous silica/reduced graphene oxide (Au/SiO₂/rGO) nanoparticles with synergistically enhanced photothermal stability and transition effect had been prepared via electrostatic interaction. Gold nanorods (AuNRs) and rGO were employed as the NIR-responsive components. UV–Vis–NIR extinction spectra revealed that the surface plasmon resonance peak of AuNRs from Au/SiO₂/rGO nanohybrids remained unchanged after 9 h NIR exposure. UV–Vis–NIR extinction results also showed that thin silica shell was superior to the thick ones in the photothermal stability improvement of Au/SiO₂/rGO nanoparticles. Moreover, the doxorubicin release of Au/SiO₂/rGO was more rapid than that of Au/SiO₂ upon NIR irradiation, indicating that synergistically enhanced photothermal effect between rGO and AuNRs endowed Au/SiO₂/rGO nanoparticles with excellent photothermal transition efficiency. Such novel NIR-responsive core–shell hybrid nanoparticles with enhanced photothermal stability and transition effect are well suited for further biological applications, such as photothermal therapy, bioimaging and drug delivery.     

Tetrakis(4‐sulfonatophenyl)porphyrin‐Directed Assembly of Gold Nanocrystals: Tailoring the Plasmon Coupling Through Controllable Gap Distances

It is known that universality and controllability over nanocrystal orientation must be accomplished to facilitate the potential applications of metal nanocrystals in the areas of photonics, electronics, and optics. The facile fabrication of linear chains of Au nanorods and bifurcated junctions of nanorods/nanospheres is achieved via the crosslinking of H‐type tetrakis(4‐sulfonatophenyl)porphyrin aggregates in solution. The tuning of the plasmon coupling between the Au nanocrystals is demonstrated by varying the porphyrin concentration and thus the interparticle gap distances. Finite‐difference time‐domain calculations show that the red shift of the plasmon band exhibits a nearly exponential decay with increasing interparticle gap distances, thus giving rise to a “plasmon ruler equation.” The gap distances determined according to this equation agree well with the experimental observations and further confirm the porphyrin‐directed assembly process. The interaction mechanism between the Au nanorods and porphyrins is further investigated by a biological procedure using the dark‐field light scattering technique.     

Seed‐Induced Growth of Flower‐Like Au–Ni–ZnO Metal–Semiconductor Hybrid Nanocrystals for Photocatalytic Applications

The combination of metal and semiconductor components in nanoscale to form a hybrid nanocrystal provides an important approach for achieving advanced functional materials with special optical, magnetic and photocatalytic functionalities. Here, a facile solution method is reported for the synthesis of Au–Ni–ZnO metal–semiconductor hybrid nanocrystals with a flower‐like morphology and multifunctional properties. This synthetic strategy uses noble and magnetic metal Au@Ni nanocrystal seeds formed in situ to induce the heteroepitaxial growth of semiconducting ZnO nanopyramids onto the surface of metal cores. Evidence of epitaxial growth of ZnO{0001} facets on Ni {111} facets is observed on the heterojunction, even though there is a large lattice mismatch between the semiconducting and magnetic components. Adjustment of the amount of Au and Ni precursors can control the size and composition of the metal core, and consequently modify the surface plasmon resonance (SPR) and magnetic properties. Room‐temperature superparamagnetic properties can be achieved by tuning the size of Ni core. The as‐prepared Au–Ni–ZnO nanocrystals are strongly photocatalytic and can be separated and re‐cycled by virtue of their magnetic properties. The simultaneous combination of plasmonic, semiconducting and magnetic components within a single hybrid nanocrystal furnishes it multifunctionalities that may find wide potential applications.     

Controlled synthesis and optical properties of Au and Au@PS nanoparticles

In this study, uniform gold (Au) nanoparticles (NPs) were prepared using seed-mediated growth method. The particle size was controlled by tuning the dosage of seed solution. Au@PS core–shell NPs were then synthesized by introducing a polystyrene (PS) shell (2–3 nm thick) around the core of Au NPs (115 nm). Evaluation of the surface plasmon (SP) optical properties indicated that wavelength of SP resonance of Au NPs increased gradually with increase in the particle size. This red shift was about 0.92 nm per 1 nm increase in particle size. The results also indicated that the zeta potential and optical properties of Au NPs could be adjusted by coating PS on the outside. Therefore, surface modifications and surface coating were effective ways to control the optical properties of Au NPs.     

Development of CuInSe2 nanocrystal and nanoring inks for low-cost solar cells

The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here, we present a facile solution synthesis of chalcopyrite CuInSe 2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. We also report the first solution synthesis of uniform hexagonal shaped single crystals CuInSe 2 nanorings by altering the synthesis parameter.     

Facet‐Dependent Optical Properties Revealed through Investigation of Polyhedral Au–Cu2O and Bimetallic Core–Shell Nanocrystals

The ability to prepare Au–Cu₂O core–shell nanocrystals with precise control over particle size and shape has led to the discovery of facet‐dependent optical properties in cuprous oxide crystals. The use of Au cores not only allows the successful formation of Au–Cu₂O core–shell nanocrystals with tunable sizes, but also enables the observation of facet‐dependent optical properties in these crystals through the Au localized surface plasmon resonance (LSPR) absorption band. By tuning the Cu₂O shell morphology from rhombic dodecahedral to octahedral and cubic structures, and thus the exposed facets, the Au LSPR band position can be widely tuned. Such facet‐dependent optical effects are not observed in bimetallic Au–Ag and Au–Pd core–shell nanocrystals with the same precisely tuned particle sizes and shapes. It is believed that similar facet‐dependent optical properties could be observed in other ionic solids and other metal–metal oxide systems. The unusually large degree of plasmonic band tuning covering from the visible to the near‐infrared region in this type of nanostructure should be quite useful for a range of plasmonic applications.     

Colloid‐Interface‐Assisted Laser Irradiation of Nanocrystals Superlattices to be Scalable Plasmonic Superstructures with Novel Activities

High‐efficient charge and energy transfer between nanocrystals (NCs) in a bottom‐up assembly are hard to achieve, resulting in an obstacle in application. Instead of the ligands exchange strategies, the advantage of a continuous laser is taken with optimal wavelength and power to irradiate the film‐scale NCs superlattices at solid–liquid interfaces. Owing to the Au‐based NCs' surface plasmon resonance (SPR) effect, the gentle laser irradiation leads the Au NCs or Au@CdS core/shell NCs to attach each other with controlled pattern at the interfaces between solid NCs phase and liquid ethanol/ethylene glycol. A continuous wave 532 nm laser (6.68–13.37 W cm^?2), to control Au‐based superlattices, is used to form the monolayer with uniformly reduced interparticle distance followed by welded superstructures. Considering the size effect to Au NCs' melting, when decreasing the Au NCs size to ≈5 nm, stronger welding nanostructures are obtained with diverse unprecedented shapes which cannot be achieved by normal colloidal synthesis. With the help of facile scale‐up and formation at solid–liquid interfaces, and a good connection of crystalline between NCs, the obtained plasmonic superstructured films that could be facilely transferred onto different substrates exhibit broad SPR absorption in the visible and near‐infrared regime, enhanced electric conductivities, and wide applications as surface enhanced Raman scattering (SERS)‐active substrates.     

Bacterial Killing by Light‐Triggered Release of Silver from Biomimetic Metal Nanorods

Illumination of noble metal nanoparticles at the plasmon resonance causes substantial heat generation, and the transient and highly localized temperature increases that result from this energy conversion can be exploited for photothermal therapy by plasmonically heating gold nanorods (NRs) bound to cell surfaces. Here, plasmonic heating is used for the first time to locally release silver from gold core/silver shell (Au@Ag) NRs targeted to bacterial cell walls. A novel biomimetic method of preparing Au@Ag core–shell NRs is employed, involving deposition of a thin organic polydopamine (PD) primer onto Au NR surfaces, followed by spontaneous electroless silver metallization, and conjugation of antibacterial antibodies and passivating polymers for targeting to gram‐negative and gram‐positive bacteria. Dramatic cytotoxicity of S. epidermidis and E. coli cells targeted with Au@Ag NRs is observed upon exposure to light as a result of the combined antibacterial effects of plasmonic heating and silver release. The antibacterial effect is much greater than with either plasmonic heating or silver alone, implying a strong therapeutic synergy between cell‐targeted plasmonic heating and the associated silver release upon irradiation. The findings suggest a potential antibacterial use of Au@Ag NRs when coupled with light irradiation, which has not been previously described.     

Self‐Assembled Au/CdSe Nanocrystal Clusters for Plasmon‐Mediated Photocatalytic Hydrogen Evolution

Plasmon‐mediated photocatalytic systems generally suffer from poor efficiency due to weak absorption overlap and thus limited energy transfer between the plasmonic metal and the semiconductor. Herein, a near‐ideal plasmon‐mediated photocatalyst system is developed. Au/CdSe nanocrystal clusters (NCs) are successfully fabricated through a facile emulsion‐based self‐assembly approach, containing Au nanoparticles (NPs) of size 2.8, 4.6, 7.2, or 9.0 nm and CdSe quantum dots (QDs) of size ≈3.3 nm. Under visible‐light irradiation, the Au/CdSe NCs with 7.2 nm Au NPs afford very stable operation and a remarkable H₂‐evolution rate of (10× higher than bare CdSe NCs). Plasmon resonance energy transfer from the Au NPs to the CdSe QDs, which enhances charge‐carrier generation in the semiconductor and suppresses bulk recombination, is responsible for the outstanding photocatalytic performance. The approach used here to fabricate the Au/CdSe NCs is suitable for the construction of other plasmon‐mediated photocatalysts.     

Tuning Plasmonic Enhancement of Single Nanocrystal Upconversion Luminescence by Varying Gold Nanorod Diameter

Plasmonic enhancement induced by metallic nanostructures is an effective strategy to improve the upconversion efficiency of lanthanide‐doped nanocrystals. It is demonstrated that plasmonic enhancement of the upconversion luminescence (UCL) of single NaYF₄:Yb³⁺/Er³⁺/Mn²⁺ nanocrystal can be tuned by tailoring scattering and absorption cross sections of gold nanorods, which is synthesized wet chemically. The assembly of the single gold nanorod and single upconversion nanocrystal is achieved by the atomic force microscope probe manipulation. By selecting two kinds of gold nanorods with similar longitudinal surface plasmon resonance wavelength but different diameters (27.3 and 46.7 nm), which extinction spectra are separately dominant by the absorption and scattering, the maximum UCL enhancement by a factor of 110 is achieved with the 46.7 nm‐diameter gold nanorod, while it is 19 for the nanorod with the diameter of 27.3 nm. Such strong enhancement with the larger gold nanorod is due to stronger scattering ability and greater extent of the near‐field enhancement. The enhanced UCL shows a strong dependence on the excitation polarization relative to the nanorod long axis. Time‐resolved measurements and finite‐difference time‐domain simulations unveil that both excitation and emission processes of UCL are accelerated by the nanorod plasmonic effect.     

Fabrication of Supported AuPt Alloy Nanocrystals with Enhanced Electrocatalytic Activity for Formic Acid Oxidation through Conversion Chemistry of Layer‐Deposited Pt2+ on Au Nanocrystals

The exploitation of nanoconfined conversion of Au‐ and Pt‐containing binary nanocrystals for developing a controllable synthesis of surfactant‐free AuPt nanocrystals with enhanced formic acid oxidation (FAO) activity is reported, which can be stably and evenly immobilized on various support materials to diversify and optimize their electrocatalytic performance. In this study, an atomic layer of Pt²⁺ species is discovered to be spontaneously deposited in situ on the Au nanocrystal generated from a reverse‐microemulsion solution. The resulting Au/Pt²⁺ nanocrystal thermally transforms into a reduced AuPt alloy nanocrystal during the subsequent solid‐state conversion process within the SiO₂ nanosphere. The alloy nanocrystals can be isolated from SiO₂ in a surfactant‐free form and then dispersedly loaded on the carbon sphere surface, allowing for the production of a supported electrocatalyst that exhibits much higher FAO activity than commercial Pt/C catalysts. Furthermore, by involving Fe₃O₄ nanocrystals in the conversion process, the AuPt alloy nanocrystals can be grown on the oxide surface, improving the durability of supported metal catalysts, and then uniformly loaded on a reduced graphene oxide (RGO) layer with high electroconductivity. This produces electrocatalytic AuPt/Fe₃O₄/RGO nanocomposites whose catalyst‐oxide‐graphene triple‐junction structure provides improved electrocatalytic properties in terms of both activity and durability in catalyzing FAO.     

Core and shell sizing of small silver-coated nanospheres by optical extinction spectroscopy

Silver metal nanoparticles (Nps) are extensively used in different areas of research and technology due to their interesting optical, thermal and electric properties, especially for bare core and core-shell nanostructures with sizes smaller than 10?nm. Since these properties are core-shell size-dependent, size measurement is important in manipulating their potential functionalization and applications. Bare and coated small silver Nps fabricated by physical and chemical methods present specific characteristics in their extinction spectra that are potentially useful for sizing purposes. This work presents a novel procedure to size mean core radius smaller than 10?nm and mean shell thickness of silver core-shell Nps based on a comparative study of the characteristics in their optical extinction spectra in different media as a function of core radii, shell thickness and coating refractive index. From the regularities derived from these relationships, it can be concluded that plasmon full width at half-maximum (FWHM) is sensitive to core size but not to coating thickness, while plasmon resonance wavelength (PRW) is related to shell thickness and mostly independent of core radius. These facts, which allow sizing simultaneously both mean core radius and shell thickness, can also be used to size bare silver Nps as a special case of core-shell Nps with zero shell thickness. The proposed method was applied to size experimental samples and the results show good agreement with conventional TEM microscopy.     

Self-assembly and photocatalysis of mesoporous TiO2 nanocrystal clusters

Mesoporous nanocrystal clusters of anatase TiO₂ with large surface area and enhanced photocatalytic activity have been successfully synthesized. The synthesis involves the self-assembly of hydrophobic TiO₂ nanocrystals into submicron clusters, coating of these clusters with a silica layer, thermal treatment to remove organic ligands and improve the crystallinity of the clusters, and finally removing silica to expose the mesoporous catalysts. With the help of the silica coating, the clusters not only maintain their small grain size but also keep their mesoporous structure after calcination at high temperatures (with BET surface area as high as 277 m²/g). The etching of SiO₂ also results in the clusters having high dispersity in water. We have been able to identify the optimal calcination temperature to produce TiO₂ nanocrystal clusters that possess both high crystallinity and large surface area, and therefore show excellent catalytic efficiency in the decomposition of organic molecules under illumination by UV light. Convenient doping with nitrogen converts these nanocrystal clusters into active photocatalysts in both visible light and natural sunlight. The strategy of forming well-defined mesoporous clusters using nanocrystals promises a versatile and useful method for designing photocatalysts with enhanced activity and stability.     

Thermo-responsive plasmonic nanohybrids with tunable optical properties

In this paper, we study the temperature-dependent optical properties of gold–silver core–shell (Au@Ag) nanorods coated by a thermo-responsive polymer poly (N-isopropylacrylamide) (PNIPAM). The wavelength of the plasmonic resonant absorption of the nanohybrids changes with temperature due to the combination effects of the plasmon resonance of the core and the thermal response of the shell. Using effective medium theory, we find that with increase of temperature, the absorption peak red-shifts due to the competition effects from the changes of the thickness and the effective refractive index of the polymer shell. The working wavelength can be tuned by the aspect ratio of nanorods. Moreover, the temperature sensitivity of plasmon resonance increases with the increase of the aspect ratio. Our studies provide a proof-of-concept design of thermal responsive plasmonic smart material.