Site-specific growth of a Pt shell on Au nanoplates: tailoring their surface plasmonic behavior

A review of recent advances in the use of complex metal nanostructures for biomedical applications from diagnosis to treatment

Complex metal nanostructures represent an exceptional category of materials characterized by distinct morphologies and physicochemical properties. Nanostructures with shape anisotropies, such as nanorods, nanostars, nanocages, and nanoprisms, are particularly appealing due to their tunable surface plasmon resonances, controllable surface chemistries, and effective targeting capabilities. These complex nanostructures can absorb light in the near-infrared, enabling noteworthy applications in nanomedicine, molecular imaging, and biology. The engineering of targeting abilities through surface modifications involving ligands, antibodies, peptides, and other agents potentiates their effects. Recent years have witnessed the development of innovative structures with diverse compositions, expanding their applications in biomedicine. These applications encompass targeted imaging, surface-enhanced Raman spectroscopy, near-infrared II imaging, catalytic therapy, photothermal therapy, and cancer treatment. This review seeks to provide the nanomedicine community with a thorough and informative overview of the evolving landscape of complex metal nanoparticle research, with a specific emphasis on their roles in imaging, cancer therapy, infectious diseases, and biofilm treatment. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Diagnostic Tools > Diagnostic Nanodevices.

Morphology-Controlled Growth of Crystalline Ag-Pt-Alloyed Shells onto Au Nanotriangles and Their Plasmonic Properties

The surface plasmon resonance of noble-metal nanoparticles depends on nanoscale size, morphology, and composition, and provides great opportunities for applications in biomedicine, optoelectronics, (photo)catalysis, photovoltaics, and sensing. Here, we present the results of synthesizing ternary metallic or trimetallic nanoparticles, Au nanotriangles (Au NTs) with crystalline Ag-Pt alloyed shells, the morphology of which can be adjusted from a yolk-shell to a core-shell structure by changing the concentration of AgNO3 or the concentration of Au NT seeds, while the shell thickness can be precisely controlled by adjusting the concentration of K2PtCl4. By monitoring the growth process with UV-vis spectra and scanning transmission electron microscopy (STEM), the shells on the Au NT-Ag-Pt yolk-shell nanoparticles were found to grow via a galvanic replacement synergistic route. The plasmonic properties of the as-synthesized nanoparticles were investigated by optical absorbance measurements.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Multiple Stepwise Synthetic Pathways toward Complex Plasmonic 2D and 3D Nanoframes for Generation of Electromagnetic Hot Zones in a Single Entity

ConspectusRational design of nanocrystals with high controllability via wet chemistry is of critical importance in all areas of nanoscience and nanotechnology research. Specifically, morphologically complex plasmonic nanoparticles have received considerable attention because light-matter interactions are strongly associated with the size and shape of nanoparticles. Among many types of nanostructures, plasmonic nanoframes (NFs) with controllable structural intricacy could be excellent candidates as strong light-entrappers with inner voids as well as high surface area, leading to highly effective interaction with light and analytes compared to their solid counterparts. However, so far studies on single-rim-based NFs have suffered from insufficient near-field focusing capability due to their structural simplicity (e.g., a single rim or NF molded from simple platonic solids), which necessitates a conceptually new NF architecture. If one considers a stereoscopic nanostructure with dual, triple, and multiple resonant intra-nanogaps on each crystallographic facet of nanocrystals, unprecedented physicochemical properties could be expected. Realizing such complex multiple NFs with intraparticle surface plasmon coupling via localized surface plasmon resonance is very challenging due to the lack of synthetic strategic principles with systematic structural control, all of which require a deep understanding of surface chemistry. Moreover, realizing those complex architectures with high homogeneity in size and shape via a bottom-up method where diverse particle interactions are involved is more challenging. Although there have been several reports on NFs used for catalysis, techniques for production of structurally complex NFs with high uniformity and an understanding of the correlation between such complexity in a single plasmonic entity and electromagnetic near-field focusing have remained highly elusive.In this Account, we will summarize and highlight the rational synthetic pathways for the design of complex two-dimensional (2D) and three-dimensional (3D) NFs with unique inner rim structures and characterize their optical properties. This systematic strategy is based on publications from our group during the last 10 years. First, we will introduce a chemical step of shape transformation of triangular Au nanoplates to circular and hexagonal plates, which are used as sacrificial layers for the formation of NFs. Then, we will describe the methods on how to synthesize monorim-based plasmonic NFs using Pt scaffolds with different shapes and correlate with their electromagnetic near-field. Then, we will describe a multiple stepwise synthetic method for the formation of 2D complex NFs wherein different starting Au nanocrystals evolved from systematic shape transformation are used to produce circular, triangular, hexagonal, crescent, and Y-shaped inner hot zones. Then, we will discuss how one can synthesize NFs with multiple rims wherein rims with different diameters are concentrically connected, by exploiting chemical toolkits such as eccentric and concentric growth of Au, borrowing the concept of total synthesis that is frequently adopted in organic chemistry. We then introduce dual-rim-faceted NFs and frame-in-frame 3D matryoshka NF geometries via well-faceted growth of Au with high control of intra-nanogaps. Finally, and importantly, we will provide examples of more advanced hierarchical NF architectures produced by controlling geometrical shapes of nanoparticles, number of rims, and different components, leading to the expansion of the NF library.

Plasmonic Cyclic Au Nanosphere Hexamers

Rational design of plasmonic colloidal assemblies via bottom-up synthesis is challenging but would show unprecedented optical properties that strongly relate to the assembly's shape and spatial arrangement. Herein, the synthesis of plasmonic cyclic Au nanosphere hexamers (PCHs) is reported, wherein six Au nanospheres (Au NSs) are connected via thin metal ligaments. By tuning Au reduction, six dangling Au NSs are interconnected with a core hexagon nanoplate (NPL). Then, Pt atoms are selectively deposited on the edges of the spheres. After etching of the core, necklace-like nanostructures of Pt framework are obtained. Deposition of Au is followed, leading to PCHs in high yield (≈90%). Notably, PCHs exhibit the combinatorial plasmonic characteristics of individual Au NSs and the in-plane coupling of the six linked Au NSs. They yield highly uniform, reproducible, and polarization-independent single-particle surface-enhanced Raman scattering signals, which are attributed to the 2-dimensional isotropic alignment of the Au NSs. Those are applied to a SERS-based immunoassay as quantitative and qualitative single particle SERS nanoprobes. This assay shows a low limit-of-detection, down to 100 pm, which is orders of magnitude lower than those based on Au NSs, and one order of magnitude lower than an assay using analogous particles of smooth Au nanorings.

Au nanolenses for near-field focusing

We report a novel strategy for the synthesis of Pt@Au nanorings possessing near-field focusing capabilities at the center through which single-particle surface enhanced Raman scattering could be readily observed. We utilized Pt@Au nanorings as a light-absorber; the absorbed light could be focused at the center with the aid of a Au nanoporous structure. We synthesized the Au nanolens structure through a Galvanic exchange process between Au ions and Ag block at the inner domain of the Pt@Au nanoring. For this step, Ag was selectively pre-deposited at the inner domain of the Pt@Au nanorings through electrochemical potential-tuned growth control and different surface energies with regard to the inner and outer boundaries of the nanoring. Then, the central nanoporous architecture was fabricated through the Galvanic exchange of sacrificial Ag with Au ions leading to the resulting Au nanoring with a Au nanoporous structure at the center. We monitored the shape-transformation by observing their corresponding localized surface plasmon resonance (LSPR) profiles. By varying the rim thickness of the starting Pt@Au nanorings, the inner diameter of the nanolens was accordingly tuned to maximize near-field focusing, which enabled us to obtain the reproducible and light-polarization independent measurements of single-particle SERS. Through theoretical simulation, the near-field electromagnetic field focusing capability was visualized and confirmed through single-particle SERS measurement showing an enhancement factor of 1.9 × 10⁸ to 1.0 × 10⁹.

We synthesized a Au nanolens with electromagnetic near-field focusing capability by integrating a Au nanoporous structure at the center of the Pt@Au nanoring via synthetic steps of eccentric growth of Ag and nanoscale Galvanic exchange reaction.

Scattering Fourier Transform Biosensor: Binary Mixture Consisting of Magnetic Ni Nanorings and Plasmonic Au Nanorods

We report a biosensing platform based on a binary mixture comprised of Au nanorods (plasmonic nanoparticles, Au NRs) and magnetically responsive Pt@Ni nanorings (magnetic nanostirrers, MN-rings). The mixture of Au NRs and MN-rings was modulated with an external rotating magnetic field (a dynamic assay with magnetic perturbation), which led to fluctuating extinction in the UV-vis spectroscopy measurement. As the surfaces of Au NRs were modified with antigens and antibodies, their periodic profile of extinction changed in accordance with surface modification of the Au NRs. The obtained periodic extinction with time could be converted to a frequency domain function where the signal-to-noise ratios of the peaks were evaluated to monitor surface biorecognitions on Au NRs, which is in contrast to conventional biosensors (a stagnant assay without perturbation) that use only the peak shift of localized surface plasmon resonance of Au nanoparticles.

Synthesis of Monolayer Gold Nanorings Sandwich Film and Its Higher Surface-Enhanced Raman Scattering Intensity

Most previous studies relating to surface-enhanced Raman spectroscopy (SERS) signal enhancement were focused on the interaction between the light and the substrate in the x-y axis. 3D SERS substrates reported in the most of previous papers could contribute partial SERS enhancement via z axis, but the increases of the surface area were the main target for those reports. However, the z axis is also useful in achieving improved SERS intensity. In this work, hot spots along the z axis were specifically created in a sandwich nanofilm. Sandwich nanofilms were prepared with self-assembly and Langmuir-Blodgett techniques, and comprised of monolayer Au nanorings sandwiched between bottom Ag mirror and top Ag cover films. Monolayer Au nanorings were formed by self-assembly at the interface of water and hexane, followed by Langmuir-Blodgett transfer to a substrate with sputtered Ag mirror film. Their hollow property allows the light transmitted through a cover film. The use of a Ag cover layer of tens nanometers in thickness was critical, which allowed light access to the middle Au nanorings and the bottom Ag mirror, resulting in more plasmonic resonance and coupling along perpendicular interfaces (z-axis). The as-designed sandwich nanofilms could achieve an overall ~8 times SERS signals amplification compared to only the Au nanorings layer, which was principally attributed to enhanced electromagnetic fields along the created z-axis. Theoretical simulations based on finite-difference time-domain (FDTD) method showed consistent results with the experimental ones. This study points out a new direction to enhance the SERS intensity by involving more hot spots in z-axis in a designer nanostructure for high-performance molecular recognition and detection.

Synthesis of water-dispersible, plate-like perovskites and their core-shell nanocrystals

Shape-controlled halide perovskite nanocrystals are attractive as an emerging functional material; however, these nanocrystals are prepared using organic solvents containing alkylamines and there are few reports on the synthesis of water-dispersible halide perovskite nanocrystals. We report a simple method to prepare water-dispersible, plate-like perovskite nanocrystals by mixing a long-chain amidoamine derivative (C18AA) and potassium tetrachloropalladate (K₂PdCl₄) in water. The obtained nanocrystals have a 2D layered perovskite structure represented by the chemical formula (C18AAH₂)PdCl₄. Furthermore, because seed-mediated growth is useful for preparing shape-controlled nanocrystals, such as rods, plates, wires and cubes, we used the water-dispersible (C18AAH₂)PdCl₄ nanocrystals as seeds to grow (C18AAH₂)PdCl₄@Pt core–shell nanocrystals. The core–shell nanocrystals have rough surfaces due to the deposition of Pt on the (C18AAH₂)PdCl₄ seeds. In addition, plate-like (C18AAH₂)PdCl₄@Au core–shell nanocrystals were easily obtained using this seed-mediated growth method.

Water-dispersible, plate-like perovskite nanocrystals were prepared using a long-chain amidoamine derivative (C18AA) and perovskite@Pt or Au core–shell nanocrystals were synthesized using the plate-like perovskite nanocrystals as seeds.

Silver-mediated temperature-controlled selective deposition of Pt on hexoctahedral Au nanoparticles and the high performance of Au@AgPt NPs in catalysis and SERS

Nanomaterials with high catalytic activity and good SERS properties can be used for sensitive and real-time in situ tracking of a catalytic process via SERS, which can be a powerful tool for investigating the products and mechanisms of the catalytic reaction. In the present work, Au@AgPt NPs with a {431}-faceted hexoctahedral Au core and an AgPt alloy shell exhibiting enhanced catalysis and good SERS activity were prepared by a facile silver-mediated temperature-controlled selective deposition of Pt. The complex hexoctahedral Au nanoparticles were synthesized first as nano-templates, followed by coating with a thin layer of Ag. Then, a temperature-controlled synthesis method for preferably depositing Pt on the hexoctahedral Au NPs was proposed to prepare Au@AgPt NPs. With the increase of the synthesis temperature, the Pt atoms were controlled to selectively deposit on the tips, edges or the entire surface of the nano-templates. By systematically investigating the effects of temperature, precursor consumption and synthesis time on the morphology, composition, optical properties, catalysis and SERS properties of the Au@AgPt NPs, the kinetic and thermodynamic mechanisms of the deposition of Pt on hexoctahedral Au nanoparticles were explored. The performance of the Au@AgPt NPs in SERS-based real-time in situ monitoring of the catalytic reaction was also investigated and verified. Besides, it is easy to regulate and control their SERS and catalytic performances through the selective deposition of Pt, according to the demand of the catalytic reaction and SERS monitoring. This work not only presents a new Au@AgPt nanostructure with good catalytic and SERS properties, but also develops a facile, universal and controllable method for selective deposition of Pt on Au nano-templates with a variety of morphologies.

Ultrasound assisted one-step synthesis of Au@Pt dendritic nanoparticles with enhanced NIR absorption for photothermal cancer therapy

Near-infrared (NIR) light-mediated non-invasive photothermal therapy (PTT) has attracted considerable attention for cancer treatment. Strong optical absorption located in the NIR region and high performance in converting light to heat should be emphasized for the development of ideal photothermal agents. In this report, Au@Pt bimetallic nanoparticles (Au@Pt NPs) with dendritic structure were synthesized through an ultrasound assisted one-step method in aqueous solution. The absorption of Au@Pt NPs at 808 nm was obviously enhanced compared to that of Au NPs and could be easily manipulated via the amount of Pt NPs. Au@Pt NPs exhibited excellent photostability with a photothermal conversion efficiency of 44.2%, which is significantly higher than those in most reported studies. Au@Pt NPs with thiol PEG functionalization presented improved cellular killing capacity upon NIR laser irradiation. Moreover, the potential application of Au@Pt NPs was also investigated in xenograft tumor mouse model. Overall, the remarkable therapeutic characteristics of PEGylated Au@Pt NPs provide them with great potential for future cancer treatment.

Colloidal Gold Nanorings and Their Plasmon Coupling with Gold Nanospheres

Gold nanorings are attractive as plasmonic metal nanocrystals because they have a hollow inner cavity. Their enhanced electric field inside the ring cavity is accessible, which is highly desirable for assembling with other optical components and studying their plasmon-coupling behaviors. However, the lack of robust methods for synthesizing size-controllable and uniform Au nanorings severely impedes the study of their attractive plasmonic properties and plasmon-driven applications. Herein, an improved wet-chemistry method is reported for the synthesis of monodisperse colloidal Au nanorings. Using circular Au nanodisks with different thicknesses and diameters as templates, Au nanorings are synthesized with thicknesses varied from ≈30 to ≈50 nm and cavity sizes varied from ≈90 to ≈40 nm. The produced Au nanorings are assembled with colloidal Au nanospheres to yield Au nanoring-nanosphere heterodimers in sphere-in-ring and sphere-on-ring configurations on substrates. The sphere-in-ring heterodimers exhibit the interesting feature of plasmonic Fano resonance upon the excitation of the dark quadrupolar plasmon mode of the Au nanorings. The open cavity in a nanoring holds a great promise for studying plasmon-coupled systems, which will facilitate the construction of advanced metamaterials and high-performance Fano-based devices.

Selective Functionalization of High-Resolution Cu₂O Nanopatterns via Galvanic Replacement for Highly Enhanced Gas Sensing Performance

Recently, high-resolution patterned metal oxide semiconductors (MOS) have gained considerable attention for enhanced gas sensing performance due to their polycrystalline nature, ultrasmall grain size (~5 nm), patternable properties, and high surface-to-volume ratio. Herein, we significantly enhanced the sensing performance of that patterned MOS by galvanic replacement, which allows for selective functionalization on ultrathin Cu₂O nanopatterns. Based on the reduction potential energy difference between the base channel material (Cu₂O) and the decorated metal ion (Pt²⁺), Pt could be selectively and precisely decorated onto the desired area of the Cu₂O nanochannel array. Overall, the Pt-decorated Cu₂O exhibited 11-fold higher NO₂ (100 ppm) sensing sensitivity as compared to the non-decorated sensing channel, the while the channel device with excessive Pt doping showed complete loss of sensing properties.

Preparation of bimetallic Au/Pt nanotriangles with tunable plasmonic properties and improved photocatalytic activity

Bimetallic nanoparticles are widely used in chemical catalysis and energy conversion. Their practical performance can be better exploited through morphological control by adjusting the synthetic strategy. Herein, an aqueous phase route is used to achieve the controlled preparation of bimetallic Au/Pt and hollow Au/Pt/Au nanotriangles with tunable plasmonic properties and superior photocatalytic activity. By continuously adjusting the concentration of surfactant solution, the gradual growth orientation of Pt nanoparticles on Au nanotriangles is observed, which occurs first on the tips, then on the edges, and then on the facets. Three types of Au/Pt nanotriangles (including Pt on the tips (Au/Pt (tips)), Pt on the edges (Au/Pt (edges)), and Pt covering Au (Au@Pt)) with tunable plasmon resonance are obtained. Then, Au/Pt/Au nanotriangles with a hollow structure are synthesized based on Au/Pt (edges). By evaluating the reduction rate of p-nitrophenol under visible light irradiation, hollow Au/Pt/Au nanotriangles exhibit the best photocatalytic activity compared with Au and Au/Pt (edges). The hollow structure, high visible light absorption and a strong tip- and center-focused local electric field of Au/Pt/Au are thought to be responsible for their superior photocatalytic activity.

Controlling Shape and Plasmon Resonance of Pt-Etched Au@Ag Nanorods

Pt-based catalysts with novel structure have attracted great attention due to their outstanding performance. In this work, H₂PtCl₆ was used as both precursor and etching agent to realize the shape-controlled synthesis of Pt-modified Au@Ag nanorods (NRs). During the synthesis, the as-prepared Ag shell played a crucial role in both protecting the Au NRs from being etched away by PtCl₆^2- and leading to an unusual growth mode of Pt component. The site-specified etching and/or growth depended on the concentration of H₂PtCl₆, where high-yield core-shell structure or dumbbell-like structure could be obtained. The shape-controlled synthesis also led to a tunable longitudinal surface plasmon resonance from ca. 649 to 900 nm. Meanwhile, the core-shell Pt-modified Au@Ag NRs showed approximately 4-fold enhancement in catalytic reduction reaction of p-nitrophenol than that of the Au NRs, suggesting the great potential for photocatalytic reaction.

Insight into growth of Au-Pt bimetallic nanoparticles: an in situ XAS study

Au-Pt bimetallic nanoparticles have been synthesized through a one-pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X-ray absorption spectroscopy (XAS) and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at Indus-2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X-ray near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L₃-edges. While the XANES spectra of the precursors provide real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core-shell-type configuration of a bimetallic alloy in a one-pot synthesis method. The core-shell-type structure of the nanoparticles has also been confirmed by ex situ energy-dispersive spectroscopy line-scan and X-ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.

General and programmable synthesis of hybrid liposome/metal nanoparticles

Hybrid liposome/metal nanoparticles are promising candidate materials for biomedical applications. However, the poor selectivity and low yield of the desired hybrid during synthesis pose a challenge. We designed a programmable liposome by selective encoding of a reducing agent, which allows self-crystallization of metal nanoparticles within the liposome to produce stable liposome/metal nanoparticles alone. We synthesized seven types of liposome/monometallic and more complex liposome/bimetallic hybrids. The resulting nanoparticles are tunable in size and metal composition, and their surface plasmon resonance bands are controllable in visible and near infrared. Owing to outer lipid bilayer, our liposome/Au nanoparticle shows better colloidal stability in biologically relevant solutions as well as higher endocytosis efficiency than gold nanoparticles without the liposome. We used this hybrid in intracellular imaging of living cells via surface-enhanced Raman spectroscopy, taking advantage of its improved physicochemical properties. We believe that our method greatly increases the utility of metal nanoparticles in in vivo applications.

Fabrication of shape-controlled reduced graphene oxide nanorings by Au@Pt nanoring lithography

We fabricated a variety of reduced graphene oxide (RGO) nanoring arrays using Au@Pt nanoplates as a pattern mask. RGO nanoflakes were assembled into a 2-dimensional assembly at the water-oil interface, and then various shapes of Au@Pt nanoplates were utilized as a pattern mask in order to convert the RGO into circular, triangular, and hexagonal RGO nanorings.