In situ organization of gold nanorods on mixed self-assembled-monolayer substrates

A method is described for assembling gold nanorods, end-to-end, into long chains attached on top of a mixed self-assembled monolayer that has been functionalized with streptavidin. Methods to prepare chains of nanorods in colloidal suspension have been reported by others, but our protocol offers a way to directly form such structures on a substrate. The rods are spaced approximately 5 nm apart in the resulting chains, which extend for over a micrometer in length. The assembly and morphology of the nanorod structures were characterized by surface plasmon resonance spectroscopy, as well as by scanning electron microscopy and scanning probe microscopy. Structures of this type could conceivably serve as plasmonic waveguides in future nanodevices.     

Directed assembly of gold nanorods using aligned electrospun polymer nanofibers for highly efficient SERS substrates

Nonspherical metal nanoparticles are very attractive plasmonic nanostructures owing to the facile tunability of the plasmonic properties and the presence of sharp corners and edges, which act as electromagnetic hot spots for surface enhanced Raman scattering (SERS). However, such anisotropic nanostructures exhibit strong polarization dependence in their plasmonic properties, exhibiting significantly higher SERS intensity in certain orientations. In this paper, we demonstrate a facile strategy to achieve directed assembly of aligned gold nanorods using highly aligned electrospun nanofibers. We believe that the interstices between the nanofibers act as micro-and nanochannels, resulting in hydrodynamic drag forces on the gold nanorods, thus inducing massive alignment of the same on the nanofibers. Apart from exhibiting nearly 50 times higher SERS intensity compared to a planar SERS substrate with randomly oriented nanorods, our results highlight the importance of the orientation of anisotropic nanostructures. Finite difference time domain (FDTD) simulations employed to understand the electromagnetic field distribution around an aligned nanorod array showed excellent agreement with the experimental observations.     

Preparation of gold nanorods with different aspect ratio and the optical response to solution refractive index

In this paper, we report that gold nanorods with different aspect ratios are controllably synthesised by a facile modified seed-mediated growth procedure. The growth rate of gold nanorods is lowered by adding H₂SO₄, and the yield of gold nanorods is improved. Increasing the concentration of AgNO₃ yields gold nanorods with different aspect ratios that differed from 1.83 to 5.04. To prevent aggregation of cetyltrimethylammonium bromide (CTAB)-capped gold nanorods and investigate the sensitivity to the solution refractive index, surface modification is carried out. After assembling poly (sodium-p-styrensulfonate) onto the surface of gold nanorods, the samples can disperse well in water and dimethyl sulfoxide. The localised surface plasmon resonance (LSPR) wavelength of the as-prepared gold nanorods redshifts with increasing the refractive index of the solvent and a good linear relationship between LSPR peaks and refractive index is achieved.     

DNA‐Origami‐Based Assembly of Anisotropic Plasmonic Gold Nanostructures

Precise control over the assembly of anisotropic plasmonic gold nanostructures with relative spatial directionality and sequence asymmetry remains a major challenge and offers great fundamental insight and optical application possibilities. Here, a novel strategy is developed to anisotropically functionalize gold nanorods (AuNRs) by using a DNA‐origami‐based precise machine to transfer essential DNA sequence configurations to the surface of the AuNRs through an intentionally designed toehold‐initiated displacement reaction. Different AuNR products are examined via hybridization with DNA‐AuNPs that display distinct elements of regiospecificity. These assembled anisotropic plasmonic gold nanostructures based on the DNA‐origami precise machine inherit the encoded information from the parent platform with high fidelity and show fixed orientation and bonding anisotropy, thereby generating discrete plasmonic nanostructures with enhanced Raman resonance.     

Controlled assemblies of gold nanorods in PVA nanofiber matrix as flexible free-standing SERS substrates by electrospinning

Under control: Controlled assemblies of gold nanorods in a poly(vinyl alcohol) (PVA) nanofiber matrix with tunable optical properties can be achieved by using electrospinning. The resultant assemblies can be used as substrates for surface-enhanced Raman spectroscopy (SERS). This work provides a facile way to control alignment of anisotropic nanostructures in a polymer nanofiber matrix and generates new assemblies with interesting properties.     

Effect of the polyelectrolyte coating on the photothermal efficiency of gold nanorods and the photothermal induced cancer cell damage

Coating gold nanorods (GNRs) with polyelectrolytes is an effective approach to make them biocompatible for potential use in photothermal treatment (PTT) of cancer. The authors report the effect of coating of the GNRs with polystyrene sulphonate (PSS‐GNRs) and PSS plus poly di‐allyl di‐methyl ammonium chloride (PDDAC‐GNRs) on its photothermal conversion efficiency (PTE), cellular uptake and subsequently the photothermal induced cytotoxicity in human oral cancer cells (NT8e). Coating of GNRs with PSS led to decrease in PTE by ∼30% and further coating it with PDDAC led to its increase to similar level, with respect to as‐ prepared GNRs. The cellular uptake of PDDAC‐GNRs in cancer cells was double than that for PSS‐GNRs. PTT of cancer cells after treatment with 60 pM of either PDDAC‐GNRs or PSS‐GNRs resulted in cytotoxicty of ∼90%. At higher concentration of 120 pM, while PSS‐GNRs showed no further change, for PDDAC‐GNR the photothermal induced cytotoxicity decreased to ∼50%. The broadening of longitudinal surface plasmon peak of PDDAC‐GNRs and appearance of dark clusters in cells under bright‐field microscope suggested intracellular clustering of PDDAC‐GNRs. In conclusion, despite high PTE and cellular uptake of PDDAC‐GNRs, its intracellular clustering (due to acidic pH) adversely affect the PTT of cancer cells.Inspec keywords: polymer electrolytes, gold, nanorods, cancer, nanomedicine, cellular biophysics, toxicology, radiation therapy, polymer filmsOther keywords: polyelectrolyte coating, photothermal efficiency, gold nanorods, photothermal induced cancer cell damage, cancer photothermal treatment, polystyrene sulphonate, poly di‐allyl di‐methyl ammonium chloride, photothermal conversion efficiency, cellular uptake, photothermal‐induced cytotoxicity, human oral cancer cells, dark clusters, bright‐field microscope, PDDAC‐GNR intracellular clustering, acidic pH, intracellular compartment, Au     

How to Control AFM Nanoxerography for the Templated Monolayered Assembly of 2 nm Colloidal Gold Nanoparticles

This paper reports on the directed monolayered assembly of 2 nm colloidal gold nanoparticles onto charge patterns written by atomic force microscopy (AFM) on poly(methylmethacrylate) thin films with a sub-100-nm spatial resolution. The impact of key experimental parameters (surface potential of charge patterns, immersion time in the colloidal solution, nanoparticle concentration, rinsing time) on the nanoparticle assembly was quantified for the first time. This study reveals that the high level of control of this so-called AFM nanoxerography process will allow one to construct promising colloid-based devices integrating localized nanoparticle monolayers of desired density.     

ACPI Conjugated Gold Nanorods as Nanoplatform for Dual Image Guided Activatable Photodynamic and Photothermal Combined Therapy In Vivo

The nanoplatform GNR‐ACPP‐PpIX (designated as GNR‐ACPI) is designed for dual image guided combined activatable photodynamic therapy (PDT) and photothermal therapy (PTT). In GNR‐ACPI, gold nanorods (GNRs) are modified with a protoporphyrin (PpIX, a PDT agent) conjugated activatable cell penetrating peptide (ACPP), which consists of the matrix metalloproteinases‐2 (MMP‐2) sensitive peptide sequence GPLGLAG. First, the photoactivity of PpIX is effectively quenched by GNRs due to the strong near infrared region light absorption of GNR and the special “U type” structure of ACPP induced close contact between PpIX and GNR. However, once arriving at the tumor site, the GPLGLAG sequence is hydrolyzed by the MMP‐2 overexpressed by tumor cells, resulting in the release of the residual cell membrane penetrating peptide (CPP) attached PpIX (CPP‐PpIX) with the recovery of photoactivity of PpIX. In addition, with the help of CPP, more efficient cellular uptake of PpIX by tumor cells can be achieved, which will greatly improve the PDT efficacy. Moreover, the GNR can also be utilized for photothermic imaging as well as PTT for tumors. It is found that the combination of PTT and PDT under the guidance of dual‐mode imaging greatly enhances the antitumor effects, while possessing negligible systematic toxicity.     

Stable and reproducible synthesis of gold nanorods for biomedical applications: a comprehensive study

Gold nanorods (GNRs) are ideal choice in biomedical research due to their amenability of synthesis, tunable plasmonic properties, less toxicity and ease of detection but their diverse biological applications necessitate stable structure. Despite two decades'' efforts made towards reproducible anisotropic structures synthesis, still the kinetic control during GNRs growth has not been achieved. This study is an attempt to apprehend thermodynamic and kinetic parameters for synthesising mono‐disperse, reproducible and highly stable GNRs with desired aspect ratios. Effects of various growth parameters and assay steps on the facile and reproducible synthesis of GNRs are analysed. GNRs'' environmental and biological colloidal stability is studied through UV–Vis spectroscopy based particle instability parameter (PIP < 0.1). The authors hereby report GNRs with tunable longitudinal surface plasmon resonance (682–906 nm) having different aspect ratios (2.5–4.6) that are stable at 28–60°C; however, prolonged high temperature ( > 60°C) and alkaline pH can trigger colloidal instability. GNRs remain stable at higher salt concentration, physiological and slightly acidic pH. GNRs can be stored in 0.001 M cetyltrimethylammonium bromide for 3 months without compromising their stability. PEGylated GNRs are quite stable in cellular media solution (PIP < 0.1). With current optimised growth conditions, no aggregation at physiological pH and stability at high temperatures make GNRs an ideal candidate in biomedical applications.Inspec keywords: gold, nanorods, nanofabrication, nanomedicine, toxicology, thermodynamics, ultraviolet spectra, visible spectra, surface plasmon resonance, pH, cellular biophysicsOther keywords: nanorod synthesis, biomedical applications, tunable plasmonic properties, toxicity, reproducible anisotropic structures synthesis, thermodynamic parameters, kinetic parameters, GNR, biological colloidal stability, UV‐Vis spectroscopy, particle instability parameter, tunable longitudinal surface plasmon resonance, aspect ratios, alkaline pH, physiological pH, slightly acidic pH, cetyltrimethylammonium bromide, cellular media solution, optimised growth conditions, aggregation, wavelength 682 nm to 906 nm, time 3 month, Au     

二硫苏糖醇引导的金纳米棒二聚体的制备与表征

为了制备一种形状可控的金纳米棒(AuNR)二聚体结构,在十二烷基磺酸钠(SDS)表面活性剂存在的情况下,通过控制二硫苏糖醇(DTT)与单巯基脱氧核糖核酸(DNADNA)的分子比,并选择在不同的时间点修饰DNA,再共同混合作用于单分散的金纳米棒来实现。琼脂糖凝胶电泳(AGE)结果表明,胶图上能看到理想的二聚体条带。通过紫外-可见分光光度计和低压透射电镜对产物的进一步分析,表明在DTT与DNA同时作用金纳米棒时,组装产物为肩并肩型的金纳米棒二聚体结构;在DTT先与金纳米棒作用,再加盐老化修饰DNA时,组装产物为头碰头型的金纳米棒二聚体结构。表征结果充分证明了实验策略的可行性。     

Hybrid PdAg alloy-Au nanorods: Controlled growth, optical properties and electrochemical catalysis

Controllable growth of high-quality hybrid nanostructures is highly desirable for the fabrication of hierarchical, complex and multifunctional devices. Here, PdAg alloys have been controllably grown at different locations on gold nanorods, producing dumbbell-like nanostructures with PdAg at the ends of the gold nanorods or branched nanostructures with PdAg grown almost perpendicular to the gold nanorods. The nucleation sites of PdAg alloys on the gold nanorods can be effectively tuned by varying the concentrations of H2PdC14, AgNO3 and cetyltrimethylammonium bromide （CTAB）. The dumbbell-like and branched nanostructures were characterized by transmission electron microscopy （TEM）, high-resolution TEM （HRTEM）, line-scanning energy-dispersive X-ray spectros-copy （EDS）, X-ray photoelectron spectroscopy （XPS） and UV-Vis absorption spectroscopy. Their electrocatalytic performance was evaluated using ethanol oxidation as a probe reaction. The dumbbelMike nanostructures show a better anti-poisoning performance, but a worse electrochemical activity than the branched ones. The results provide guidelines for the controlled growth of complicated nanostructures for either fundamental studies or potential applications.     

pH‐Dependent Galvanic Replacement of Supported and Colloidal Cu2O Nanocrystals with Gold and Palladium

Galvanic replacement reactions (GRRs) on nanoparticles (NPs) are typically performed between two metals, i.e., a solid metal NP and a replacing salt solution of a more noble metal. The solution pH in GRRs is commonly considered an irrelevant parameter. Yet, the solution pH plays a major role in GRRs involving metal oxide NPs. Here, Cu₂O nanocrystals (NCs) are studied as galvanic replacement (GR) precursors, undergoing replacement by gold and palladium, with the resulting nanostructures showing a strong dependence on the pH of the replacing metal salt solution. GRRs are reported for the first time on supported (chemically deposited) oxide NCs and the results are compared with those obtained with corresponding colloidal systems. Control of the pH enables production of different nanostructures, from metal‐decorated Cu₂O NCs to uniformly coated Cu₂O‐in‐metal (Cu₂O@Me) core–shell nanoarchitectures. Improved metal nucleation efficiencies at low pHs are attributed to changes in the Cu₂O surface charge resulting from protonation of the oxide surface. GR followed by etching of the Cu₂O cores provides metal nanocages that collapse upon drying; the latter is prevented using a sol–gel silica overlayer stabilizing the metal nanocages. Metal‐replaced Cu₂O NCs and their corresponding stabilized nanostructures may be useful as photocatalysts, electrocatalysts, and nanosensors.     

High-yield assembly of soluble and stable gold nanorod pairs for high-temperature plasmonics

Colloidal synthetic approaches to discrete, soluble plasmonic architectures, such as nanorod pairs, offer numerous advantages relative to lithographic techniques, including compositionally asymmetric structures, atomically smooth surfaces, and continuous fabrication. Density‐driven colloidal assembly, such as by solvent evaporation, produces some intriguing structures, e.g., particle chains; however, controllability and post‐processibility of the final architecture is inadequate. Also the limited quantity of product nominally comprises a broad distribution of assembly size and type. Herein, the high‐yield formation of soluble, stable, and compositionally discrete gold nanorod (Au NR) architectures by inducing—then arresting—flocculation is demonstrated using bifunctional nanorods and reversible modulation of solvent quality to deplete and reassemble an electrostatic stabilization layer, thereby eliminating the need for an additional encapsulant. Analogous to dimer formation during step‐growth polymerization, the initial yield of Au nanorod side‐by‐side pairs can be greater than 50%. The high solubility and stability of the assembly enable purification, scale‐up of nanomolarity solutions, and subsequent chemical modification of the assembled product. As an example, in situ silica deposition via Stöber synthesis onto the assembled pair produces highly processable nanostructures with a single pair of embedded Au NRs at their center, which exhibit thermal stability at temperatures in excess of 700 °C.     

Fabrication of Micropatterned Dipeptide Hydrogels by Acoustic Trapping of Stimulus‐Responsive Coacervate Droplets

Acoustic standing waves offer an excellent opportunity to trap and spatially manipulate colloidal objects. This noncontact technique is used for the in situ formation and patterning in aqueous solution of 1D or 2D arrays of pH‐responsive coacervate microdroplets comprising poly(diallyldimethylammonium) chloride and the dipeptide N‐fluorenyl‐9‐methoxy‐carbonyl‐D‐alanine‐D‐alanine. Decreasing the pH of the preformed droplet arrays results in dipeptide nanofilament self‐assembly and subsequent formation of a micropatterned supramolecular hydrogel that can be removed as a self‐supporting monolith. Guest molecules such as molecular dyes, proteins, and oligonucleotides are sequestered specifically within the coacervate droplets during acoustic processing to produce micropatterned hydrogels containing spatially organized functional components. Using this strategy, the site‐specific isolation of multiple enzymes to drive a catalytic cascade within the micropatterned hydrogel films is exploited.     

Self-assembly of hydrophilic homopolymers: a matter of RAFT end groups

Unusual self-assembly behavior is observed for a range of hydrophilic homopolymers. This self-assembly behavior is contrary to the expected behavior of such hydrophilic polymers and instead mimics more commonly reported amphiphilic block copolymers. It is proposed that the unique combination of hydrophobic end groups at both the α and ω chain end accounts for this unusual self-assembly behavior. Complex internal polymer micelles are spontaneously formed when hydrophilic homopolymer polyelectrolytes and neutral polymers (with a weight fraction of the hydrophobic end groups <10 wt%) are directly dissolved in water. The homopolymers, poly[2-(diethylamino)ethyl methacrylate], poly(N-isopropylacrylamide), and poly(ethoxyethylacrylate) are synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization using S'-1-dodecyl-(S')-(α,α'-dimethyl-α″-acetic acid) trithiocarbonate (DDMAT) and its derivatives as chain transfer agents (CTAs). A range of polyelectrolyte homopolymers with different terminal groups are designed and synthesized, which under acidic aqueous solution direct the self-assembly to form well-defined nanostructures. This assembly behavior was also observed for neutral polymers, and it was determined that the structure of the hydrophobic end groups (and thus choice of RAFT CTA) are very important in facilitating this unusual self-assembly behavior of hydrophilic homopolymers. It is proposed that the functionality of commonly used CTAs such as DDMAT, can affect the solution association of the resultant homopolymers and can in fact afford ABA' type polymers, which can undergo self-assembly to form higher-order nanostructures.     

Bimetallic Nanostructures as Active Raman Markers: Gold‐Nanoparticle Assembly on 1D and 2D Silver Nanostructure Surfaces

It is demonstrated that bimetallic silver–gold anisotropic nanostructures can be easily assembled from various nanoparticle building blocks with well‐defined geometries by means of electrostatic interactions. One‐dimensional (1D) silver nanowires, two‐dimensional (2D) silver nanoplates, and spherical gold nanoparticles are used as representative building blocks for bottom‐up assembly. The gold nanoparticles are electrostatically bound onto the 1D silver nanowires and the 2D silver nanoplates to give bimetallic nanostructures. The unique feature of the resulting nanostructures is the particle‐to‐particle interaction that subjects absorbed analytes to an enhanced electromagnetic field with strong polarization dependence. The Raman activity of the bimetallic nanostructures is compared with that of the individual nanoparticle blocks by using rhodamine 6G solution as the model analyte. The Raman intensity of the best‐performing silver–gold nanostructure is comparable with the dense array of silver nanowires and silver nanoplates that were prepared by means of the Langmuir–Blodgett technique. An optimized design of a single‐nanostructure substrate for surface‐enhanced Raman spectroscopy (SERS), based on a wet‐assembly technique proposed here, can serve as a compact and low‐cost alternative to fabricated nanoparticle arrays.     

Nanostructured biointerfaces

Colloidal lithography is used to create nanostructured interfaces suitable for studying and interacting with cellular biosystems. Large areas of patterned surface can be produced. We investigate the use of plasma etching for transfer of the pattern of individual colloidal particles into the substrates to create short-range ordered arrays of topographic and/or chemical nanostructures. Colloidal masks perform differently to traditional photoresist masks and local redeposition of material around the particle has significant impact on the resultant structures. The colloidal particles can be reshaped to allow the fabrication of flat-topped structures. Topographic and chemical nanostructures can be used to template the self assembly of macromolecules. The phase separation of thin films of PS-PMMA symmetric block copolymers above nanostructure sites is aligned at the surface nanostructure by topographic features. PLL-PEG assembly at alkanethiol-modified nanoscale chemical patterns of gold/silicon allows the production of nanoscale protein patterns. Patterns of ferritin 100 nm in diameter are demonstrated.     

Core-shell and segmented polymer-metal composite nanostructures

Composite nanostructures (approximately 200 nm wide and several micrometers long) of metal and polyaniline (PANI) in two new variations of core-shell (PANI-Au) and segmented (Au-PANI and Ni-Au-PANI) architectures were fabricated electrochemically within anodized aluminum oxide (AAO) membranes. Control over the structure of these composites (including the length of the gold shells in the core-shell structures) was accomplished by adjusting the time and rate of electrodeposition and the pH of the solution from which the materials were grown. Exposure of the core-shell structures to oxygen plasma removed the PANI and yielded aligned gold nanotubes. In the segmented structures, a self-assembled monolayer (SAM) of thioaniline nucleated the growth of PANI on top of metal nanorods and acted as an adhesion layer between the metal and PANI components.     

Hierarchical nanostructures of PbTiO3 through mesocrystal formation

Novel hierarchical self-assembled structures; bur-like PbTiO3 nanostructures were made by self-assembly of PbTiO3 nanocrystals under hydrothermal conditions using sodium dodecylbenzene sulfonate surfactant. The bur-like nanostructures exhibit a unique geometrical shape with cores of agglomerated nanocrystals and outershells of nanorods. The nanorods were between 30 nm and 100 nm in diameter and from several hundred nm up to 2 microm in length. We demonstrate that these nanostructures are formed in a two step process where agglomeration of PbTiO3 nanoparticles into microspheres occurs in a first step, followed by assembly of cube-shaped nanoparticle building blocks into PbTiO3 mesocrystals in a second step. The mesocrystals continuously grow into nanorods from the surface of the microspheres acting as a substrate.     

Instabilities in nanoporous media

Rayleigh instabilities in thin polymer films confined within nanoporous alumina membranes were studied. Thin films of poly(methyl methacrylate) (PMMA) were prepared by filling cylindrical nanopores in an anodic aluminum oxide (AAO) membrane with a PMMA solution in chloroform followed by solvent evaporation. When the PMMA nanotubes were annealed above the glass transition temperature (Tg), undulations in the film thickness were observed that were induced by a Rayleigh instability. The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodic encapsulated holes. A similar behavior was observed when PMMA films were confined within carbon nanotubes (CNTs). The Rayleigh instabilities in these confined geometries offer a novel means of controlling and fabricating the polymer nanostructures. These compartmentalized nanorods may have potential applications as delivery devices.