Anisotropic Metal Nanoparticles Research Articles

Face-to-face Assembly Strategy of Au Nanocubes: Induced Generation of Broad Hotspot Regions for SERS-Fluorescence Dual-Signal Detection of Intracellular miRNAs.

While designing anisotropic noble metal nanoparticles (NPs) can enhance the signal intensity of Raman dyes, more sensitive surface-enhanced Raman scattering (SERS) probes can be designed by oriented self-assembly of noble metal nanomaterials into dimers or higher-order nanoclusters. In this study, we engineered a self-assembly strategy in living cells for real-time fluorescence and SERS dual-channel detection of intracellular microRNAs (miRNAs), using Mg2+-dependent 8-17E DNAzyme sequences as the driving motors, gold nanocubes (AuNCs) as the driver components, and three-branched double-stranded DNA as the linking tool. The assembly selects adenine in DNA as a reporter molecule, simplifying the labeling process of Raman reporter molecules and reducing the synthesis process. In addition, adenine is stably distributed between the faces of AuNCs and the wide hotspot region gives good reproducibility of the adenine SERS signal. In this strategy, the SERS channel was consistently stable and more sensitive compared to the fluorescence channel. Among them, the detection limit of the SERS channel was 2.1 pM and the coefficient of variation was 1.26% in the in vitro liquid phase and 1.49% in MCF-7 cells. The strategy successfully achieved accurate tracking and quantification of miRNA-21 in cancer cells, showing good reproducibility in complex samples as well as cells. The reported strategy provides ideas for exploring intracellular specific triggering of nanoparticles for precise control of self-assembly.

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

One-step surfactant-free photoreduction synthesis of single-crystal silver triangular nanoprisms by surface modified chemically patterned ferroelectric crystals for SERS application

Anisotropic metal nanoparticles with plasmonic and photocatalytic action exhibit significant applications for SERS detection and green transformation. Current synthetic approaches using multiple chemicals cause substantial resource consumption and surface chemical residues that hinder analyte adsorption and thus prevent efficient action. Herein, we present a green production process using surface modified ferroelectric templates to grow triangular Ag nanoprisms for surface enhanced Raman spectroscopy (SERS). Surface modification of ferroelectric templates with positively charged ions suppresses the Ag nucleation on template surface and enables the reaction-controlled process to promote anisotropic photoreduction for the growth of Ag nanoprisms. Under optimal experimental conditions, numerous Ag nanoprisms with the (111) plane can be photoreduced with sharp tips and edges and greatly strengthen the excited electric field for Raman signal enhancement. The synthesized Ag nanoprism array presents superior SERS performance, including enhancement factor of 3.78 × 108, limit of detection as low as 4.95 × 10-9 M, and Raman signal enhancement by 8.63 times. The practical application of Ag nanoprisms is effectively demonstrated by the SERS detection of antibiotics. The reaction control by utilization of surface-modified ferroelectric templates provides a promising prospect for the green synthesis of anisotropic metal nanomaterials and the technical development of green applications.

Efficiency of Plasmon-Induced Dual-Mode Fluorescence Enhancement upon Two-Photon Excitation.

Anisotropic noble metal nanoparticles supporting more than one localized surface plasmon resonance can be tailored for efficient dual-mode fluorescence enhancement by ensuring an adequate coupling to both absorption and emission bands of fluorophores. This approach is naturally extended to two-photon excitation fluorescence, where a molecule is excited by simultaneous nonlinear absorption of two photons. However, the relative impact of plasmon coupling to excitation and emission on the overall fluorescence enhancement can be very different in this case. Here, by using the finite-difference time-domain method, we study the two-photon excitation fluorescence of near-infrared fluorescent protein (NirFP) eqFP670, which is the most red-shifted NirFP to date, in proximity to a silver nanobar. By optimizing the length and aspect ratio of the particle, we reach a fluorescence enhancement factor of . We show that the single mode coupling regime with highly tuned near-field significantly outperforms the dual-mode coupling enhancement. The plasmon-induced amplification of the fluorophore’s excitation rate becomes of utmost importance due to its quadratic dependence on light intensity, defining the fluorescence enhancement upon two-photon excitation. Our results can be used for the rational design of hybrid nanosystems based on NirFP and plasmonic nanoparticles with greatly improved brightness important for developing whole-body imaging techniques.

One-Step, DNA-Programmed, and Flash Synthesis of Anisotropic Noble Metal Nanostructures on MXene.

Precise morphological control over anisotropic noble metal nanoparticles (ANPs) is one of the key issues in the nano-research field owing to their unique optoelectronic, magnetic, mechanical, and catalytic properties. Although nanostructures fabricated by the directed assembly of adsorbate have been widely demonstrated recently, facile yet universal synthesis of nanocrystal with tunable morphologies, green templates, no seeds, and high yield remains challenging. Herein, we develop a versatile method, allowing for the rapid, one-step, seedless, surfactant-free synthesis of a noble metal nanostructure with tunable anisotropy on MXene in a sequence-dependent manner through a single-DNA molecular regulator. Based on the mild reducibility of MXene and the selective affinity of the DNA to the specific facets in the crystals, oriented aggregations and the growth of ANPs (Au, Pt, Pd) can be achieved and the resulting asymmetric morphology from polyhedrons, or flowers, or nanoplates to dendrites is observed. The ability to align such ANPs on the MXene surface is expected to lead to improved photothermal effect and surface-enhanced Raman scattering. Furthermore, our work makes the fabrication of the ANPs or ANP-MXene heterostructure easier, stimulating further explorations of physical, chemical, and biological applications.

Bottom-Up Fabrication of Large-Scale Gold Nanorod Arrays by Surface Diffusion-Mediated DNA Origami Assembly.

Self-assembly of anisotropic metal nanoparticles serves as an effective bottom-up route for the nanofabrication of novel artifacts. However, there still are many challenges to rationally manipulate anisotropic particles due to the size and geometric restrictions. To avoid the aggregation and mishybridization from DNA sticky-end-guided assembly in buffer solution, in this work, we utilized a cation-controlled surface diffusion strategy to the spatial arrangement of gold nanorods (AuNRs) into 1D and 2D arrays by using DNA origami tiles as binding frames on the solid-liquid interface through π-π stacking interactions. To facilitate the further manipulation of those patterns, a novel pattern transfer method was introduced to transfer the arrays of AuNRs from a liquid to a dry ambient environment with high yield and minor structural damage. The results demonstrated a successful strategy of DNA origami-assisted, large-scale assembly of AuNRs for constructing complex superstructures with potential applications in the nanofabrication of plasmonic and electronic devices.

Fast One-Step Synthesis of Anisotropic Silver Nanoparticles

The shape control of metal nanoparticles, along with the size, is critical for most of their applications as they control their optical properties. Anisotropic metal nanoparticles show superior performance in a number of applications compared to spherical ones. Shape control is usually achieved by a two-step process, where the first involves the formation of spherical nanoparticles and the second is about the actual shape transformation. In this paper, we report on a fast and facile synthesis of silver nanoplates in a single step, involving laser ablation of a silver target in a liquid medium while this is exposed to light irradiation and hydrogen peroxide flow. We obtained anisotropic particles with a mixture of shapes, of 70–80 nm in size and 10–20 nm in thickness, which showed a plasmon sensitivity greater than 200 nm/RIU.

The role of deep eutectic solvents and carrageenan in synthesizing biocompatible anisotropic metal nanoparticles.

Plasmonic metal nanoparticles are widely used for many applications due to their unique optical and chemical properties. Over the past decade, anisotropic metal nanoparticles have been explored for imaging, sensing, and diagnostic applications. The variations and flexibility of tuning the size and shape of the metal nanoparticles at the nanoscale made them promising candidates for biomedical applications such as therapeutics, diagnostics, and drug delivery. However, safety and risk assessment of the nanomaterials for clinical purposes are yet to be made owing to their cytotoxicity. The toxicity concern is primarily due to the conventional synthesis route that involves surfactants as a structure-directing agent and as a capping agent for nanoparticles. Wet chemical methods employ toxic auxiliary chemicals. However, the approach yields monodispersed nanoparticles, an essential criterion for their intended application and a limitation of the green synthesis of nanoparticles using plant extracts. Several biocompatible counterparts such as polymers, lipids, and chitosan-based nanoparticles have been successfully used in the synthesis of safe nanomaterials, but there were issues regarding reproducibility and yield. Enzymatic degradation was one of the factors responsible for limiting the efficacy. Hence, it is necessary to develop a safer and nontoxic route towards synthesizing biocompatible nanomaterials while retaining morphology, high yield, and monodispersity. In this regard, deep eutectic solvents (DESs) and carrageenan as capping agent for nanoparticles can ensure the safety. Carrageenan has the potential to act as antibacterial and antiviral agent, and adds enhanced stability to the nanoparticles. This leads to a multidimensional approach for utilizing safe nanomaterials for advanced biomedical and clinical applications.

Reversible isomerization of metal nanoclusters induced by intermolecular interaction

Reversible isomerization of metal nanoclusters induced by intermolecular interaction

Metal Nanoparticles-Enhanced Biosensors: Synthesis, Design and Applications in Fluorescence Enhancement and Surface-enhanced Raman Scattering.

Metal nanoparticles (NP) that exhibit localized surface plasmon resonance play an important role in metal‐enhanced fluorescence (MEF) and surface‐enhanced Raman scattering (SERS). Among the optical biosensors, MEF and SERS stand out to be the most sensitive techniques to detect a wide range of analytes from ions, biomolecules to macromolecules and microorganisms. Particularly, anisotropic metal NPs with strongly enhanced electric field at their sharp corners/edges under a wide range of excitation wavelengths are highly suitable for developing the ultrasensitive plasmon‐enhanced biosensors. In this review, we first highlight the reliable methods for the synthesis of anisotropic gold NPs and silver NPs in high yield, as well as their alloys and composites with good control of size and shape. It is followed by the discussion of different sensing mechanisms and recent advances in the MEF and SERS biosensor designs. This includes the review of surface functionalization, bioconjugation and (directed/self) assembly methods as well as the selection/screening of specific biorecognition elements such as aptamers or antibodies for the highly selective bio‐detection. The right combinations of metal nanoparticles, biorecognition element and assay design will lead to the successful development of MEF and SERS biosensors targeting different analytes both in‐vitro and in‐vivo. Finally, the prospects and challenges of metal‐enhanced biosensors for future nanomedicine in achieving ultrasensitive and fast medical diagnostics, high‐throughput drug discovery as well as effective and reliable theranostic treatment are discussed.

Dimethylenebis-(tetra-decyldimethylammonium Bromide)-Driven Metal Nanoparticles: Hg2+ Sensing a Competency.

We report an excellent anisotropic Au nanoparticle-based colorimetric probe for the detection of Hg2+ ions with higher detection ability and selectivity. The manifestation of different morphologies of Au nanoparticles including round, triangular, rectangular, pentagonal, and hexagonal has been realized by the dimethylenebis-(tetra-decyldimethylammonium bromide) (14-2-14 Gemini surfactant) assisted one-step thermal reduction method where the average size of Au nanoparticles was 54.65 ± 44.3 nm. The growth and frequency of Au nanoparticles were enhanced as a function of Gemini surfactant’s concentration. The detection limit as low as 1.8 nM was efficaciously achieved and was considerably lower than the required world standards defined the maximum allowable level of Hg2+ ions for health hazards. Notably, the Au nanoparticles showed visible detection for 100 μM Hg2+ ion by means of the change in the solution color from red to tarnish blue within 180 s followed by saturation in the absorption ratio (ALSPR/ATSPR). These results provide novel insight into the detection of the heavy metal ion using Gemini surfactant-assisted grown anisotropic metal nanoparticles. On the basis of obtained results, it is concluded that the size of metal nanoparticles is no longer critical for preparation of efficient selective chemoprobe; rather, growth of more number of edges provides a large number of sights for incoming moieties and plays an important role in improving the detection capability of the anisotropic metal nanoparticle irrespective of their large sizes. We believe that this work provides valuable insight into researchers working in the area of chemosensor applications.

1-nm-Wide Hydrated Dipole Arrays Regulate AuNW Assembly on Striped Monolayers in Nonpolar Solvent

Summary We show that striped phases of horizontally oriented phospholipids presenting 1-nm-wide orientable dipole arrays can order and straighten flexible 2-nm-diameter gold nanowires (AuNWs) with lengths (up to 1 μm) that greatly exceed the template pitch (∼7.5 nm). AuNW ordering can extend over areas > 100 μm2. Whereas wires bundle in solution, with contact between ligand shells, wires adsorbing to the template separate to much larger center-to-center distances (e.g., 12–30 nm) near multiples of the template pitch, consistent with repulsive interactions between wires emerging during interactions with dipole arrays in the template. AuNW assembly was carried out in cyclohexane, a nonpolar solvent. Interestingly, we found that wire ordering was contingent on the extent of phospholipid headgroup hydration and the presence of excess oleylamine, which assembled into hemicylindrical micelles around the hydrated headgroups. Together, these components generated a protected polar environment that enabled the phospholipid headgroups to function collectively to regulate AuNW assembly.

Polydopamine-assisted immobilization of Ag@AuNPs on cotton fabrics for sensitive and responsive SERS detection

Depositing anisotropic noble metal nanoparticles with high uniformity and yield on flexible substrates is the determining factor for surface enhanced Raman spectroscopy (SERS) detection. In this work, flexible, durable and sensitive SERS substrates were fabricated by in situ reduction of Ag nanoparticles on polydopamine templated cotton fabrics (CF) as catalytic hotspots to enhance the following deposition of Au nanoparticles. The coated CF were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy and X-ray diffraction to understand the surface morphology, chemical composition and crystalline structure, respectively. The SEM images indicate that the nanoparticles are dispersed evenly on the CF. 4-Mercaptobenzoic acid (4-MBA) has been used as the probe molecule to evaluate the sensitive and reproducible SERS properties of the as-fabricated SERS substrate. The as-prepared SERS substrates were demonstrated to detect carbaryl pesticides on a cucumber, and carbaryl with the concentration of as low as 10−6 M (0.20 ppm) could be detected to ensure food safety.

Hybrid Silver Nanocubes for Improved Plasmon-Enhanced Singlet Oxygen Production and Inactivation of Bacteria.

Plasmonic nanoparticles can strongly interact with adjacent photosensitizer molecules, resulting in a significant alteration of their singlet oxygen (1O2) production. In this work, we report the next generation of metal-enhanced 1O2 nanoplatforms exploiting the lightning rod effect, or plasmon hot spots, in anisotropic (nonspherical) metal nanoparticles. We describe the synthesis of Rose Bengal-decorated silica-coated silver nanocubes (Ag@SiO2-RB NCs) with silica shell thicknesses ranging from 5 to 50 nm based on an optimized protocol yielding highly homogeneous Ag NCs. Steady-state and time-resolved 1O2 measurements demonstrate not only the silica shell thickness dependence on the metal-enhanced 1O2 production phenomenon but also the superiority of this next generation of nanoplatforms. A maximum enhancement of 1O2 of approximately 12-fold is observed with a 10 nm silica shell, which is among the largest 1O2 production metal enhancement factors ever reported for a colloidal suspension of nanoparticles. Finally, the Ag@SiO2-RB NCs were benchmarked against the Ag@SiO2-RB nanospheres previously reported by our group, and the superior 1O2 production of Ag@SiO2-RB NCs resulted in improved antimicrobial activities in photodynamic inactivation experiments using both Gram-positive and -negative bacteria model strains.

Single-Photon-Single-Electron Transition for Interpretation of Optical Spectra of Nonspherical Metal Nanoparticles in Aqueous Colloidal Solutions

Noble metal nanoparticles—especially shape anisotropic particles—have pronounced resonances in the optical spectrum. These sensitive absorption modes attract great interest in various fields of application. For nonspherical particles, no analytic description of the absorption spectra according to the commonly used Mie theory is possible. In this work, we present a semi-empirical approach for the explanation of the optical spectra of shape anisotropic particles such as silver nanoprisms and gold nanorods. We found an interpretation of the optical absorption spectra which is based on a single-photon-single-electron transition. This model is in a better agreement with the basic assumptions of quantum mechanics than the electrodynamic model of a localized surface plasmon excitation. Based on microfluidically obtained Ag nanoprisms and Au nanorods with very high ensemble homogeneities, dependencies between the geometrical properties of the shape anisotropic noble metal nanoparticles and the spectral position of the longitudinal absorption mode could be derived, which show that the assumption of a composed relative permittivity and the inclusion of the Rydberg constant is sufficient to describe the optical properties of the shape anisotropic particles. Within the scope of the measuring accuracy, the calculations furthermore lead to the value of the refractive index of the particle-surrounding medium.

Recent advances in the rational synthesis and self-assembly of anisotropic plasmonic nanoparticles

Abstract The field of plasmonics has grown at an incredible pace in the last couple of decades, and the synthesis and self-assembly of anisotropic plasmonic materials remains highly dynamic. The engineering of nanoparticle optical and electronic properties has resulted in important consequences for several scientific fields, including energy, medicine, biosensing, and electronics. However, the full potential of plasmonics has not yet been realized due to crucial challenges that remain in the field. In particular, the development of nanoparticles with new plasmonic properties and surface chemistries could enable the rational design of more complex architectures capable of performing advanced functions, like cascade reactions, energy conversion, or signal transduction. The scope of this short review is to highlight the most recent developments in the synthesis and self-assembly of anisotropic metal nanoparticles, which are capable of bringing forward the next generation of plasmonic materials.

(Invited) The Role of Pyridine Derivatives in the Formation of Anisotropic Gold Nanoparticles

It is appealing to develop and understand new approaches to generate anisotropic metal nanoparticles (NPs) owing to their application in optical-based sensing platforms. Empirically, the formation of anisotropic NPs is typically back-rationalized by speculating that shape-directing ions or molecules preferentially adsorb on different crystallographic facets of the growing particle. An alternative strategy is described herein whereby electrochemical measurements of pyridine-derivative adsorption on different low index single crystal surfaces conclusively demonstrate preferentially adsorption on Au(100} surfaces. This serves as a starting basis for developing rational approaches to generate homogeneous and heterogeneous anisotropic NPs through chemical and electrochemical reduction of AuIII precursors. The pyridine derivatives are shown to play a critical role in the formation of Au nanopods and nanodaggers. Electrochemical evidence is provided of a two-step reduction of tetrachloroaurate involving a pyridine-stabilized AuI species which plays a key role in producing anisotropic structures. Both electrodeposited and suspended NPs have surface plasmon resonances that extend well into the near IR (λmax ≈ 1000-1350 nm). Near-IR Raman sensing applications are demonstrated using FT-Raman with 1064 nm excitation. Electrodeposited nanodaggers provide SERS enhancement factors greater than 106 for monolayers of 4-aminothiophenol (4-ATP).

Droplet microfluidics for the highly controlled synthesis of branched gold nanoparticles

The synthesis of anisotropic metallic nanoparticles (NPs) has been a field of intense and challenging research in the past decade. In this communication, we report on the reproducible and highly controllable synthesis of monodisperse branched gold nanoparticles in a droplet-based microfluidics platform. The process has been automated by adapting two different bulk synthetic strategies to microdroplets, acting as microreactors, for NP synthesis: a surfactant-free synthesis and a surfactant-assisted synthesis. Microdroplets were generated in two different microfluidic devices designed to accommodate the requirements of both bulk syntheses. The epitaxial growth of AuNSTs inside the microdroplets allowed for a fine control of reagent mixing and local concentrations during particle formation. This is the first time branched gold NPs have been synthesised in a microfluidics platform. The monodispersity of the product was comparable to the synthesis in bulk, proving the potential of this technology for the continuous synthesis of high quality anisotropic NPs with improved reproducibility.

Synthesis of Triangular Silver and Gold Nanoprisms Using Consensus Sequence Tetratricopeptide Repeat Proteins.

Anisotropic metallic nanoparticles, such as Au and Ag nanoprisms (NPSMs), have received tremendous attention for their application in catalysis, molecular sensing, signal amplification, bioimaging, and therapeutic applications due to their shape-dependent optical and physical properties. Herein, we present a protein-enabled synthetic strategy for the seeded growth of silver and gold NPSMs with low shape polydispersity, narrow size distribution, and tailored plasmonic absorbance. During the initial seed nucleation step, consensus sequence tetratricopeptide repeat (CTPR) proteins are utilized as potent stabilizers to facilitate the formation of planar-twinned Ag seeds. High yield production of well-defined Ag/Au NPSMs is achieved, respectively, by adding CTPR-stabilized Ag seeds into the growth solutions containing metal precursor, mild reducing agent, sodium halide, and additional CTPR.

Directed Assembly of Gold Nanorods by Terminal-Base Pairing of Surface-Grafted DNA.

Directed assemblies of anisotropic metal nanoparticles exhibit attractive physical and chemical properties. However, an effective methodology to prepare differently directed assemblies from the same anisotropic nanoparticles is not yet available. Gold nanorods (AuNRs) region-selectively modified with different DNA strands can form side-by-side (SBS) and end-to-end (ETE) assemblies in a non-crosslinking manner. When the complementary DNA is hybridized to the surface-bound DNA, stacking interaction between the blunt ends takes place in the designated regions. Such AuNRs assemble into highly ordered structures, assisted by capillary forces emerging on the substrate surface. Moreover, insertion of a mercury(II)-mediated thymine-thymine base pair into the periphery of the DNA layer allows selective formation of the SBS or ETE assemblies from the strictly identical AuNRs with or without mercury(II).