Seeded growth of monodisperse gold nanorods using bromide-free surfactant mixtures

Interaction of Gold Nanorods with Human Dermal Fibroblasts: Cytotoxicity, Cellular Uptake, and Wound Healing

Herein, the cytotoxicity, cellular uptake and wound healing of human dermal fibroblasts were investigated upon treatment with gold nanorods (GNR) decorated with different ligands. Neutral and cationic poly ethylene glycol (PEG)-decorated GNR demonstrated the least cytotoxicity and cellular internalization, while anionic- and bovine serum albumin (BSA)-coated GNR revealed significant cytotoxicity and cellular uptake into human dermal fibroblasts. The cell scratch test demonstrated that neutral, cationic PEGylated GNR and anionic-decorated GNR have accelerated the wound healing rate in vitro after 24 h of incubation with scratched human dermal fibroblasts compared to control, while there was a drastic retardation of wound healing rate of scratched fibroblasts upon exposure to BSA-GNR accompanied with a significant release of the inflammatory cytokine; interlukin-1β (IL-1β). The cytotoxicity of GNR against the dermal cells and their ability to enhance the wound healing in vitro are greatly linked to their surface modifications.

Efficient photoredox conversion of alcohol to aldehyde and H2 by heterointerface engineering of bimetal-semiconductor hybrids

Controllable and precise design of bimetal- or multimetal-semiconductor nanostructures with efficient light absorption, charge separation and utilization is strongly desired for photoredox catalysis applications in solar energy conversion. Taking advantage of Au nanorods, Pt nanoparticles, and CdS as the plasmonic metal, nonplasmonic co-catalyst and semiconductor respectively, we report a steerable approach to engineer the heterointerface of bimetal-semiconductor hybrids. We show that the ingredient composition and spatial distribution between the bimetal and semiconductor significantly influence the redox catalytic activity. CdS deposited anisotropic Pt-tipped Au nanorods, which feature improved light absorption, structure-enhanced electric field distribution and spatially regulated multichannel charge transfer, show distinctly higher photoactivity than blank CdS and other metal-CdS hybrids for simultaneous H2 and value-added aldehyde production from one redox cycle.

Plasmon-enhanced upconversion luminescence in pyrochlore phase Yb x Er2-x Ti2O7 thin film

Pyrochlore phase Yb _x Er_2-x Ti₂O₇ (YETO) thin films have been prepared by employing a facile sol-gel method combining with spin-coating technique and post-annealing treatment at 700 °C. High concentration of Yb³⁺ ions can promote the transformation from Yb³⁺/Er³⁺ co-doped anatase phase TiO₂ to pyrochlore phase YETO at 700 °C temperature. We find that the YETO thin film with 30 mol% Yb³⁺ ions exhibits the brightest upconversion (UC) emission. Moreover, the introduction of Au nanorods (Au NRs) in the YETO thin film can further enhance the UC fluorescence. By adjusting the density of Au NRs, the UC emission intensity is increased by about 2.8-fold due to the excitation field enhancement caused by the localized surface plasmon resonance effect.

Anti-Stokes Emission from Hot Carriers in Gold Nanorods

The origin of light emission from plasmonic nanoparticles has been strongly debated lately. It is present as the background of surface-enhanced Raman scattering and, despite the low yield, has been used for novel sensing and imaging applications because of its photostability. Although the role of surface plasmons as an enhancing antenna is widely accepted, the main controversy regarding the mechanism of the emission is its assignment to either radiative recombination of hot carriers (photoluminescence) or electronic Raman scattering (inelastic light scattering). We have previously interpreted the Stokes-shifted emission from gold nanorods as the Purcell effect enhanced radiative recombination of hot carriers. Here we specifically focused on the anti-Stokes emission from single gold nanorods of varying aspect ratios with excitation wavelengths below and above the interband transition threshold while still employing continuous wave lasers. Analysis of the intensity ratios between Stokes and anti-Stokes emission yields temperatures that can only be interpreted as originating from the excited electron distribution and not a thermally equilibrated phonon population despite not using pulsed laser excitation. Consistent with this result as well as previous emission studies using ultrafast lasers, the power-dependence of the upconverted emission is nonlinear and gives the average number of participating photons as a function of emission wavelength. Our findings thus show that hot carriers and photoluminescence play a major role in the upconverted emission.

Ultrasmall gold nanorods: synthesis and glycocalyx-related permeability in human endothelial cells

Background

Clinical data show shed endothelial glycocalyx (GCX) components in blood samples of atherosclerotic patients, linking atherosclerotic development to endothelial GCX integrity. Healthy GCX has pores no >7 nm, and shed GCX has even larger pores. Therefore, we suggest targeting and treating atherosclerosis-prone blood vessels by using nanoscale vehicles to deliver drugs via the nanoscale GCX as it becomes dysfunctional.

Materials and methods

To test our idea, we investigated permeability of nanoparticles in endothelium, as related to a GCX expression. The present work involves nanorods, which are expected to interact with larger portions of endothelial cell (EC) membranes, due to surface area of the nanorod long axis. Conventional nanorod diameters are orders of magnitude larger than the GCX pore size, so we adapted conventional synthesis methods to fabricate ultrasmall gold nanorods (GNRs). Our ultrasmall GNRs have an aspect ratio of 3.4, with a length of 27.9±3.1 nm and a diameter of 8.2±1.4 nm. In addition, we produced GNRs that are biocompatible and fluorescently visible, by coating the surface with functionalized polyethylene glycol and Alexa Fluor 647. To study GNR–GCX interactions, we used human ECs, for species relevance.

Results

Under life-like flow conditions, the human ECs are densely covered with a 1.3 µm thick layer of GCX, which coincides with minimal GNR permeability. When the GCX is weakened from lack of flow (static culture) or the presence of GCX degradation enzyme in the flow stream, the GCX shows 40% and 60% decreased thickness, respectively. GCX weakness due to lack of flow only slightly increases cellular permeability to GNRs, while GCX weakness due to the presence of enzyme in the flow leads to substantial increase in GNR permeability.

Conclusion

These results clarify that the GCX structure is an avenue through which drug-carrying nanoparticles can be delivered for targeting affected blood vessels to treat atherosclerosis.

Tailoring optical cross sections of gold nanorods at a target plasmonic resonance wavelength using bromosalicylic acid

We demonstrate finely tailoring optical cross sections of AuNRs at a fixed target resonance wavelength, on the basis of AuNR overgrowth using a binary surfactant mixture consisting 5-bromosalicylic acid (BSA) and cetyltrimethylammonium bromide (CTAB).

Ultranarrow Second-Harmonic Resonances in Hybrid Plasmon-Fiber Cavities

We demonstrate second-harmonic generation with ultranarrow resonances in hybrid plasmon-fiber cavities, formed by depositing single-crystalline gold nanorods onto the surface of tapered microfibers with diameters in the range of 1.7-1.8 μm. The localized surface plasmon mode of the single gold nanorod efficiently couples with a whispering gallery mode of the fiber, resulting in a very narrow hybrid plasmon-fiber resonance with a high quality factor Q of up to 250. When illuminated with a tunable 100 fs laser, a sharp SHG peak narrower than half of the spectral width of the impinging laser emerges, superimposed on a broad multiphoton photoluminescence background. The enhancement of the SHG peak of the hybrid system is typically 1000-fold when compared to that of a single gold nanorod alone. Tuning the laser over the hybrid resonance enables second-harmonic spectroscopy and yields an ultranarrow line width as small as 6.4 nm. We determine the second-harmonic signal to rise with the square of the laser power, while the multiphoton photoluminescence background rises with powers between 4 and 6, indicating a very efficient higher-order process. A coupled anharmonic oscillator model is able to describe the linear as well as second-harmonic resonances very well. Our work will open the door to the simultaneous utilization of narrow whispering gallery resonances together with high plasmonic near-field enhancement and should allow for nonlinear sensing and extremely efficient nonlinear light generation from ultrasmall volumes.

Growing gold nanostructures for shape-selective cellular uptake

With development in the synthesis of shape- and size-dependent gold (Au) nanostructures (NSs) and their applications in nanomedicine, one of the biggest challenges is to understand the interaction of these shapes with cancer cells. Herein, we study the interaction of Au NSs of five different shapes with glioblastoma-astrocytoma cells. Three different shapes (nanorods, tetrahexahedra, and bipyramids), possessing tunable optical properties, have been synthesized by a single-step seed-mediated growth approach employing binary surfactant mixtures of CTAB and a secondary surfactant. By the use of two-step seed-mediated approach, we obtained new NSs, named nanomakura (Makura is a Japanese word used for pillow) which is reported for the first time here. Spherical Au nanoparticles were prepared by the Turkevich method. To study NS-cell interactions, we functionalized the NSs using thiolated PEG followed by 11-Mercaptoundecanoic acid. The influence of shape and concentration of NSs on the cytotoxicity were assessed with a LIVE/DEAD assay in glioblastoma-astrocytoma cells. Furthermore, the time-dependent uptake of nanomakura was studied with TEM. Our results indicate that unlike the other shapes studied here, the nanomakura were taken up both via receptor-mediated endocytosis and macropinocytosis. Thus, from our library of different NSs with similar surface functionality, the shape is found to be an important parameter for cellular uptake.

The Many "Facets" of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

Over the years, scientists have identified various synthetic "handles" while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.

Fabrication of a Flexible Gold Nanorod Polymer Metafilm via a Phase Transfer Method as a SERS Substrate for Detecting Food Contaminants

Surface enhanced Raman scattering (SERS) has been widely used in detection of food safety due to the nondestructive examination property. Here, we reported a flexible SERS film based on a polymer-immobilized gold nanorod polymer metafilm. Polystyrene-polyisoprene-polystyrene (SIS), a transparent and flexible, along with having excellent elasticity, polymer, was chosen as the main support of gold nanorods. A simple phase transfer progress was adopted to mix the gold nanorods with the polymer, which can further be used in most water-insoluble polymers. The SERS film performed satisfactorily while being tested in a series of standard Raman probes, like crystal violet (CV) and malachite green (MG). Moreover, the excellent reproducibility and elastic properties make the film a promising substrate in practical detection. Hence, the MG detection on the fish surface and trace thiram detection on orange pericarp were inspected with detection results of 1 × 10^-10 and 1 × 10^-6 M, which were below the demand of the National standard of China, exactly matching the realistic application requirements.

Polycrystallinity of Lithographically Fabricated Plasmonic Nanostructures Dominates Their Acoustic Vibrational Damping

The study of acoustic vibrations in nanoparticles provides unique and unparalleled insight into their mechanical properties. Electron-beam lithography of nanostructures allows precise manipulation of their acoustic vibration frequencies through control of nanoscale morphology. However, the dissipation of acoustic vibrations in this important class of nanostructures has not yet been examined. Here we report, using single-particle ultrafast transient extinction spectroscopy, the intrinsic damping dynamics in lithographically fabricated plasmonic nanostructures. We find that in stark contrast to chemically synthesized, monocrystalline nanoparticles, acoustic energy dissipation in lithographically fabricated nanostructures is solely dominated by intrinsic damping. A quality factor of Q = 11.3 ± 2.5 is observed for all 147 nanostructures, regardless of size, geometry, frequency, surface adhesion, and mode. This result indicates that the complex Young's modulus of this material is independent of frequency with its imaginary component being approximately 11 times smaller than its real part. Substrate-mediated acoustic vibration damping is strongly suppressed, despite strong binding between the glass substrate and Au nanostructures. We anticipate that these results, characterizing the optomechanical properties of lithographically fabricated metal nanostructures, will help inform their design for applications such as photoacoustic imaging agents, high-frequency resonators, and ultrafast optical switches.

Engineered nanomaterials growth control by monomers and micelles: From surfactants to surface active polymers

In pseudo-micellar phase, the crystal growth is primarily achieved by the surface activity of the monomers in the presence of micelles. To ensure the maximum potential of surface activity of monomers in morphology control, a micellar phase is required. This account specifically focuses on the crystal growth control by the surface active monomers of conventional surfactants and that of water soluble polymers. It also distinguishes the mechanisms involved in the shape control driven by the micellar phase of micelle forming polymers, their role as nanoreactors, micellar stability, and micellar transitions from the monomeric phase. The fundamental basis of the crystal growth control by the surface active agents holds the key of using other non-convectional surface active species like proteins, carbohydrates, and bioactive polymers to achieve morphology control bionanomaterials for their specific biological applications.

Recent advances in functional nanostructures as cancer photothermal therapy

Being a non-invasive and relatively safe technique, photothermal therapy has attracted a lot of interest in the cancer treatment field. Recently, nanostructure technology has entered the forefront of cancer therapy owing to its ability to absorb near-infrared radiation as well as efficient light to heat conversion. In this study, key nanostructures for cancer therapy including gold nanoparticles, magnetite iron oxide nanoparticles, organic nanomaterials, and novel two-dimensional nanoagents such as MXenes are discussed. Furthermore, we briefly discuss the characteristics of the nanostructures of these photothermal nanomaterial agents, while focusing on how nanostructures hold potential as cancer therapies. Finally, this review offers promising insight into new cancer therapy approaches, particularly in vivo and in vitro cancer treatments.

Silver-Assisted Synthesis of Gold Nanorods: the Relation between Silver Additive and Iodide Impurities

Seed-mediated methods employing cetyltrimethylammonium bromide (CTAB) as a surfactant, and silver salts as additives, are the most common synthetic strategies for high-yield productions of quality Au nanorods. However, the mechanism of these reactions is not yet fully understood and, importantly, significant lab-to-lab reproducibility issues still affect these protocols. In this study, the direct correlation between the hidden content of iodide impurities in CTAB reagents, which can drastically differ from different suppliers or batches, and the optimal concentration of silver required to maximize the nanorods yield is demonstrated. As a result, high-quality nanorods are obtained at different iodide contents. These results are interpreted based on the different concentrations of CTAB and cetyltrimethylammonium iodide (CTAI) complexes with Ag⁺ and Au⁺ metal ions in the growth solution, and their different binding affinity and reduction potential on distinct crystallographic planes. Notably, the exhaustive conversion of CTAI-Au⁺ to CTAI-Ag⁺ appears to be the key condition for maximizing the nanorod yield.

The evolution of size, shape, and surface morphology of gold nanorods

We investigate the transformation of single crystal gold nanorod surface morphology over extended growth times. After initial rapid anisotropic growth and disappearance of {111} bridging facets, the aspect ratios converge across AgNO₃ concentrations. The surface morphology transitions from faceted to curved. These observations imply the final aspect ratio has little dependence on the AgNO₃ concentration, consistent with primary control of the AgNO₃ over aspect ratio occurring at the symmetry breaking point.

Recent biomedical applications of gold nanoparticles: A review

Recent advances in nanotechnology are as a result of the development of engineered nanoparticles. Efficiently, metallic nanoparticles have been widely exploited for biomedical application and among them, gold nanoparticles (AuNPs) are highly remarkable. Consequent upon their significant nature, spherical and gold nanorods (Au NRs) nanoparticles attract extreme attention. Their intrinsic features such as optical, electronic, physicochemical and, surface plasmon resonance (SPR); which can be altered by changing the characterizations of particles such as shape, size, aspect ratio, or environment; ease of synthesis and functionalization properties have resulted to various applications in different fields of biomedicine such as sensing, targeted drug delivery, imaging, photothermal and photodynamic therapy as well as the modulation of two or three applications. This article reviewed the popular AuNPs synthesis methods and mentioned their established applications in various demands, especially in biological sensing.

Bacteria-like mesoporous silica-coated gold nanorods for positron emission tomography and photoacoustic imaging-guided chemo-photothermal combined therapy

Mesoporous silica nanoshell (MSN) coating has been demonstrated as a versatile surface modification strategy for various kinds of inorganic functional nanoparticles, such as gold nanorods (GNRs), to achieve not only improved nanoparticle stability but also concomitant drug loading capability. However, limited drug loading capacity and low tumor accumulation rate in vivo are two major challenges for the biomedical applications of MSN-coated GNRs (GNR@MSN). In this study, by coating uniformly sized GNRs with MSN in an oil-water biphase reaction system, we have successfully synthesized a new bacteria-like GNR@MSN (i.e., bGNR@MSN) with a significantly enlarged pore size (4-8 nm) and surface area (470 m²/g). After PEGylation and highly efficient loading of doxorubicin (DOX, 40.9%, w/w), bGNR@MSN were used for positron emission tomography (PET, via facile and chelator-free ⁸⁹Zr-labeling) and photoacoustic imaging-guided chemo-photothermal cancer therapy in vivo. PET imaging showed that ⁸⁹Zr-labeled bGNR@MSN(DOX)-PEG can passively target to the 4T1 murine breast cancer-bearing mice with high efficiency (∼10 %ID/g), based on enhanced permeability and retention effect. Significantly enhanced chemo-photothermal combination therapy was also achieved due to excellent photothermal effect and near-infrared-light-triggered drug release by bGNR@MSN(DOX)-PEG at the tumor site. The promising results indicate great potential of bGNR@MSN-PEG nanoplatforms for future cancer diagnosis and therapy.

Insight into induced charges at metal surfaces and biointerfaces using a polarizable Lennard-Jones potential

Metallic nanostructures have become popular for applications in therapeutics, catalysts, imaging, and gene delivery. Molecular dynamics simulations are gaining influence to predict nanostructure assembly and performance; however, instantaneous polarization effects due to induced charges in the free electron gas are not routinely included. Here we present a simple, compatible, and accurate polarizable potential for gold that consists of a Lennard–Jones potential and a harmonically coupled core-shell charge pair for every metal atom. The model reproduces the classical image potential of adsorbed ions as well as surface, bulk, and aqueous interfacial properties in excellent agreement with experiment. Induced charges affect the adsorption of ions onto gold surfaces in the gas phase at a strength similar to chemical bonds while ions and charged peptides in solution are influenced at a strength similar to intermolecular bonds. The proposed model can be applied to complex gold interfaces, electrode processes, and extended to other metals.

Molecular dynamics models for predicting the behavior of metallic nanostructures typically do not take into account polarization effects in metals. Here, the authors introduce a polarizable Lennard–Jones potential that provides quantitative insight into the role of induced charges at metal surfaces and related complex material interfaces.

Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection

Colloidal gold nanoparticles (Au NPs) have been used in several biological applications, which include the exploitation of size- and shape-dependent Localized Surface Plasmon Resonance (LSPR) in biosensing devices. In order to obtain functional and stable Au NPs in a physiological medium, surface modification and functionalization are crucial steps in these endeavors. Reversible addition-fragmentation chain transfer (RAFT) polymerization meets this need offering the possibility of control over the composition and architecture of polymeric shells coating Au NPs. Furthermore, playing with a careful choice of monomers, RAFT polymerization allows the possibility to design a polymer shell with the desired functional groups aiming at Au based nanocomposites suitable for biorecognition and biotargeting. This review provides important aspects concerning the synthesis and optical properties of Au NPs as well as concepts of RAFT polymerization. Understanding these concepts is crucial to appreciate the chemical strategies available towards RAFT-polymer coated Au core-shell nanostructures, which are here reviewed. Finally, examples of applications in opto-biodetection devices are provided and the potential of responsive "smart" nanomaterials based on such structures can be applied to other biological applications.

Coating Matters: Review on Colloidal Stability of Nanoparticles with Biocompatible Coatings in Biological Media, Living Cells and Organisms

Within the last two decades, the field of nanomedicine has not developed as successfully as has widely been hoped for. The main reason for this is the immense complexity of the biological systems, including the physico-chemical properties of the biological fluids as well as the biochemistry and the physiology of living systems. The nanoparticles' physicochemical properties are also highly important. These differ profoundly from those of freshly synthesized particles when applied in biological/living systems as recent research in this field reveals. The physico-chemical properties of nanoparticles are predefined by their structural and functional design (core and coating material) and are highly affected by their interaction with the environment (temperature, pH, salt, proteins, cells). Since the coating material is the first part of the particle to come in contact with the environment, it does not only provide biocompatibility, but also defines the behavior (e.g. colloidal stability) and the fate (degradation, excretion, accumulation) of nanoparticles in the living systems. Hence, the coating matters, particularly for a nanoparticle system for biomedical applications, which has to fulfill its task in the complex environment of biological fluids, cells and organisms. In this review, we evaluate the performance of different coating materials for nanoparticles concerning their ability to provide colloidal stability in biological media and living systems.

Boosting Hot Electrons in Hetero-superstructures for Plasmon-Enhanced Catalysis

Hetero-nanostructures featured with both strong plasmon absorption and high catalytic activity are believed to be ideal platforms to realize efficient light-driven catalysis. However, in reality, it remains a great challenge to acquire high-performance catalysis in such hetero-nanostructures due to poor generation and transfer of plamson-induced hot electrons. In this report, we demonstrate that Au nanorod@Pd superstructures (Au@Pd SSs), where the ordered Pd nanoarrays are precisely grown on Au nanorod surfaces via solution-based seed-mediated approach, would be an excellent solution for this challenge. Both experiment and theory disclose that the ordered arrangement of Pd on Au nanorod surfaces largely promotes hot electron generation and transfer via amplified local electromagnetic field and decreased electron-phonon coupling, respectively. Each effect is separately highlighted in experiments by the significant plasmon-enhanced catalytic activity of Au@Pd SSs in two types of important reactions with a distinct time scale of bond-dissociation event: molecular oxygen activation and carbon-carbon coupling reaction. This work opens the door to design and application of new generation photocatalysts.

Accelerating Gold Nanorod Synthesis with Nanomolar Concentrations of Poly(vinylpyrrolidone)

A novel modification for the seedless synthesis of gold nanorods (AuNRs) has been developed. Nanomolar concentrations of 10 kDa poly(vinylpyrrolidone) (PVP) can be introduced to a growth solution containing 25, 50, or 100 mM cetyltrimethylammonium bromide (CTAB) to significantly reduce the dimensions of AuNRs. We found that PVP accelerates the growth rate of AuNRs by more than two times that of nanorods grown in 50 and 100 mM CTAB solutions. Additionally, there is a time-dependent effect of adding PVP to the nanorod growth solution that can be utilized to tune their aspect ratio. Because the concentration of PVP is far below the concentration of HAuCl₄ in the reaction mixture, PVP primarily functions not as a reducing agent, but as a capping or templating ligand to stabilize the growing nanorods. Our reproducible protocol enables the synthesis of AuNRs in high yield with tunable sizes: 45 × 6.7, 28 × 5.5, and 12 × 4.5 nm for 100, 50, and 25 mM CTAB, respectively. We estimated the number of PVP chains per nanorod in growth solutions to be around 30, which suggests that the effect on the aspect ratio is caused by a direct interaction between the AuNR surface and the PVP.

Influence of Size and Shape on the Anatomical Distribution of Endotoxin-Free Gold Nanoparticles

The transport and the delivery of drugs through nanocarriers is a great challenge of pharmacology. Since the production of liposomes to reduce the toxicity of doxorubicin in patients, a plethora of nanomaterials have been produced and characterized. Although it is widely known that elementary properties of nanomaterials influence their in vivo kinetics, such interaction is often poorly investigated in many preclinical studies. The present study aims to evaluate the actual effect of size and shape on the biodistribution of a set of gold nanoparticles (GNPs) after intravenous administration in mice. To this goal, quantitative data achieved by inductively coupled plasma mass spectrometry and observational results emerging from histochemistry (autometallography and enhanced dark-field hyperspectral microscopy) were combined. Since the immune system plays a role in bionano-interaction we used healthy immune-competent mice. To keep the immune surveillance on the physiological levels we synthesized endotoxin-free GNPs to be tested in specific pathogen-free animals. Our study mainly reveals that (a) the size and the shape greatly influence the kinetics of accumulation and excretion of GNPs in filter organs; (b) spherical and star-like GNPs showed the same percentage of accumulation, but a different localization in liver; (c) only star-like GNPs are able to accumulate in lung; (d) changes in the geometry did not improve the passage of the blood brain barrier. Overall, this study can be considered as a reliable starting point to drive the synthesis and the functionalization of potential candidates for theranostic purposes in many fields of research.

Polymorphic Assembly from Beveled Gold Triangular Nanoprisms

The shape anisotropy of nanoparticle building blocks is of critical importance in determining their packing symmetry and assembly directionality. While there has been extensive research on the effect of their overall geometric shapes, the importance of nanometer morphology details is not well-recognized or understood. Here we draw on shape-anisotropic gold triangular nanoprism building blocks synthesized based on a method we recently developed; besides the "large-scale" triangular prism shape (79.8 nm in side length and 22.0 nm in thickness), the prisms are beveled with their sides convexly enclosed by two flat {100} facets. We engineer the balance between electrostatic repulsion and entropically driven depletion attraction in the system to generate self-assemblies without or with the effect of the nanoscale beveling detail. A conventional, planar honeycomb (p-honeycomb) lattice forms with the triangular basal planes packed on the same plane at low depletion attraction, whereas an unexpected interlocking honeycomb (i-honeycomb) lattice and its "supracrystal" forms are assembled with additional close-paralleling of side facets at high depletion attraction. The i-honeycomb lattice renders all the metallic surfaces in close proximity and leads to a surface-enhanced Raman scattering signal nearly 5-fold higher than that in the p-honeycomb lattice and high sensitivity for detecting the model molecule Rhodamine 6G at a concentration as low as 10^-8 M. Our study can guide future work in both nanoparticle synthesis and self-assembly; nanoscale geometrical features in anisotropic nanoparticles can be used as an important handle to control directional interactions for nonconventional ordered assemblies and to enrich diversity in self-assembly structure and function.

Efficient seed-mediated method for the large-scale synthesis of Au nanorods

Seed-mediated methods are widely followed for the synthesis of Au nanorods (NRs). However, mostly dilute concentrations of the Au precursor (HAuCl₄) are used in the growth solution, which leads to a low final concentration of NRs. Attempts of increasing the concentration of NRs by simply increasing the concentration of HAuCl₄, other reagents in the growth solution and seeds lead to a faster growth kinetics which is not favourable for NR growth. Herein, we demonstrate that the increase in growth kinetics for high concentrations of reagents in growth solution can be neutralised by decreasing the pH of the solution. The synthesis of the NRs can be scaled up by using higher concentrations of reagents and adding an optimum concentration of HCl in the growth solution. The concentration of HAuCl₄ in the growth solution can be increased up to 5 mM, and 10-20 times more NRs can be synthesised for the same reaction volume compared to that of the conventional seed-mediated method. We have also noticed that a cetyltrimethylammonium bromide (CTAB)-to-HAuCl₄ molar ratio of 50 is sufficient for obtaining high yield of NRs.

Understanding the Seed-Mediated Growth of Gold Nanorods through a Fractional Factorial Design of Experiments

Since the development of simple, aqueous protocols for the synthesis of anisotropic metal nanoparticles, research into many promising, valuable applications of gold nanorods has grown considerably, but a number of challenges remain, including gold-particle yield, robustness to minor impurities, and precise control of gold nanorod surface chemistry. Herein we present the results of a composite fractional factorial series of experiments designed to screen seven additional potential avenues of control and to understand the seed-mediated silver-assisted synthesis of gold nanorods. These synthesis variables are the amount of sodium borohydride used and the rate of stirring when producing seed nanoparticles, the age of and the amount of seeds added, the reaction temperature, the amounts of silver nitrate and ascorbic acid added, and the age of the reduced growth solution before seed nanoparticles are added to initiate rod formation. This statistical experimental design and analysis method, besides determining which experimental variables are important and which are not when synthesizing gold nanorods (and quantifying their effects), gives further insight into the mechanism of growth by measuring the degree to which variables interact with each other by mapping out their mechanistic connections. This work demonstrates that when forming gold nanorods by the reduction of auric ions by ascorbic acid onto seed nanoparticles, ascorbic acid determines how much gold is reduced, and the amount of seeds determine how it is divided, yet both influence the intrinsic growth rates, in both width and length, of the forming nanorods. Furthermore, this work shows that the reduction of gold proceeds via direct reduction on the surface of seeds and not through a disproportionation reaction. Further control over the length of gold nanorods can be achieved by tuning the amount of silver nitrate or the reaction temperature. This work shows that silver does not directly influence rod length or width, and a new primary role for silver is proposed as a catalyst promoting the reduction of gold on the ends of forming nanorods. Furthermore, this silver catalyst is removed from the reaction by adsorption onto the surface of the growing nanorod. This work also demonstrates the importance of freshly prepared silver nitrate and ascorbic acid solutions, free from even a few hours of photodegradation, in preparing gold nanorods with high shape purity and gold yield.

Detecting the shape of anisotropic gold nanoparticles in dispersion with single particle extinction and scattering

The shape and size of nanoparticles are important parameters affecting their biodistribution, bioactivity, and toxicity. The high-throughput characterisation of the nanoparticle shape in dispersion is a fundamental prerequisite for realistic in vitro and in vivo evaluation, however, with routinely available bench-top optical characterisation techniques, it remains a challenging task. Herein, we demonstrate the efficacy of a single particle extinction and scattering (SPES) technique for the in situ detection of the shape of nanoparticles in dispersion, applied to a small library of anisotropic gold particles, with a potential development for in-line detection. The use of SPES paves the way to the routine quantitative analysis of nanoparticles dispersed in biologically relevant fluids, which is of importance for the nanosafety assessment and any in vitro and in vivo administration of nanomaterials.

Rational selection of halide ions for synthesizing highly active Au@Pd nanobipyramids

Highly active Au@Pd nanobipyramids were synthesized using Br⁻ ions as an appropriate growth modifier.

Colorimetric detection of HVA by self-assembly of Au nanorods with DNA double helices to give side-by-side and end-to-end structures

Simple, less time consuming and sensitive self-assembly and colorimetric detection of HVA.

Colloidal Synthesis and Applications of Plasmonic Metal Nanoparticles

Plasmonic metal nanoparticles attract intense research attention because of their fascinating surface plasmon resonance properties and their potential applications in diverse fields. Here, some of the recent research efforts on the synthesis and applications of plasmonic metal nanoparticles are highlighted. Starting from the colloidal synthesis of metal nanoparticles, various shaped silver and gold nanostructures are discussed. The applications of plasmonic nanoparticles in photocatalysis, surface-enhanced Raman spectroscopy (SERS), and devices are used as excellent examples showcasing the advantages of these nanoparticles. The report closes with a brief summary and discussion on the challenges and future direction in this research field.

Engineering Gold Nanoparticles in Compass Shape with Broadly Tunable Plasmon Resonances and High-Performance SERS

We present the uniform and high-yield synthesis of a novel gold nanostructure of compass shape composed of a Au sphere at the central and two gradually thinning conical tips at the opposed poles. The Au compass shapes were synthesized through a seed-mediated growth approach employing a binary mixture of cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) as the structure-directing agents. Under the condition of single surfactant (CTAB), the spherical seeds tend to grow into larger spherical Au nanoparticles (NPs); while the spherical seeds favor the formation of Au compass shaped NPs using two mixed surfactants (CTAB/NaOL). The reaction kinetics clearly shows a growth mechanism of Au compass shaped NPs. Interestingly, due to their anisotropic structure, Au compass shaped NPs show two distinctive plasmonic resonances, similar to those from Au nanorods. Particularly, the longitudinal surface plasmon resonances of Au compass shaped NPs exhibit a broadly tunable range from 600 to 865 nm. In addition, the obtained Au compass shaped NPs can be self-assembled into a two-dimensional monolayer with closely packed and highly aligned NPs, which results in periodic arrays of overlapped Au tips, generating hot spots for high-performance surface-enhanced Raman scattering.

Surface Chemistry of Gold Nanorods

Gold nanorods have garnered a great deal of scientific interest because of their unique optical properties, and they have the potential to greatly impact many areas of science and technology. Understanding the structure and chemical makeup of their surfaces as well as how to tailor them is of paramount importance in the development of their successful applications. This Feature Article reviews the current understanding of the surface chemistry of as-synthesized gold nanorods, methods of tailoring the surface chemistry of gold nanorods with various inorganic and organic coatings/ligands, and the techniques employed to characterize ligands on the surface of gold nanorods as well as the associated measurement challenges. Specifically, we address the challenges of determining how thick the ligand shell is, how many ligands per nanorod are present on the surface, and where the ligands are located in regiospecific and mixed-ligand systems. We conclude with an outlook on the development of the surface chemistry of gold nanorods leading to the development of a synthetic nanoparticle surface chemistry toolbox analogous to that of synthetic organic chemistry and natural product synthesis.

Shape-Dependent Relaxivity of Nanoparticle-Based T1 Magnetic Resonance Imaging Contrast Agents

Gold nanostars functionalized with Gd(III) have shown significant promise as contrast agents for magnetic resonance imaging (MRI) because of their anisotropic, branched shape. However, the size and shape polydispersity of as-synthesized gold nanostars have precluded efforts to develop a rigorous relationship between the gold nanostar structure (e.g., number of branches) and relaxivity of surface-bound Gd(III). This paper describes the use of a centrifugal separation method that can produce structurally refined populations of gold nanostars and is compatible with Gd(III) functionalization. Combined transmission electron microscopy and relaxivity analyses revealed that the increased number of nanostar branches was correlated with enhanced relaxivity. By identifying the underlying relaxivity mechanisms for Gd(III)-functionalized gold nanostars, we can inform the design of high-performance MRI contrast agents.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Simultaneous tracking of drug molecules and carriers using aptamer-functionalized fluorescent superstable gold nanorod-carbon nanocapsules during thermo-chemotherapy

Controlling and monitoring the drug delivery process is critical to its intended therapeutic function. Many nanocarrier systems for drug delivery have been successfully developed. However, biocompatibility, stability, and simultaneously tracing drugs and nanocarriers present significant limitations. Herein, we have fabricated a multifunctional nanocomposite by coating the gold nanorod (AuNR) with a biocompatible, superstable and fluorescent carbon layer, obtaining the AuNR@carbon core-shell nanocapsule. In this system, the carbon shell, originally obtained in aqueous glucose solutions and, therefore, biocompatible in physiological environments, could be simply loaded with cell-specific aptamers and therapeutic molecules through π-π interactions, a useful tool for cancer-targeted cellular imaging and therapy. Moreover, such a stable and intrinsic fluorescence effect of the AuNR@carbon enabled simultaneous tracking of released therapeutic molecules and nanocarriers under thermo-chemotherapy. The AuNR@carbons had high surface areas and stable shells, as well as unique optical and photothermal properties, making them promising nanostructures for biomedical applications.

Facet Control of Gold Nanorods

While great success has been achieved in fine-tuning the aspect ratios and thereby the plasmon resonances of cylindrical Au nanorods, facet control with atomic level precision on the highly curved nanorod surfaces has long been a significantly more challenging task. The intrinsic structural complexity and lack of precise facet control of the nanorod surfaces remain the major obstacles for the atomic-level elucidation of the structure-property relationships that underpin the intriguing catalytic performance of Au nanorods. Here we demonstrate that the facets of single-crystalline Au nanorods can be precisely tailored using cuprous ions and cetyltrimethylammonium bromide as a unique pair of surface capping competitors to guide the particle geometry evolution during nanorod overgrowth. By deliberately maneuvering the competition between cuprous ions and cetyltrimethylammonium bromide, we have been able to create, in a highly controllable and selective manner, an entire family of nanorod-derived anisotropic multifaceted geometries whose surfaces are enclosed by specific types of well-defined high-index and low-index facets. This facet-controlled nanorod overgrowth approach also allows us to fine-tune the particle aspect ratios while well-preserving all the characteristic facets and geometric features of the faceted Au nanorods. Taking full advantage of the combined structural and plasmonic tunability, we have further studied the facet-dependent heterogeneous catalysis on well-faceted Au nanorods using surface-enhanced Raman spectroscopy as an ultrasensitive spectroscopic tool with unique time-resolving and molecular finger-printing capabilities.

Anisotropic Nanoparticles and Anisotropic Surface Chemistry

Anisotropic nanoparticles are powerful building blocks for materials engineering. Unusual properties emerge with added anisotropy-often to an extraordinary degree-enabling countless new applications. For bottom-up assembly, anisotropy is crucial for programmability; isotropic particles lack directional interactions and can self-assemble only by basic packing rules. Anisotropic particles have long fascinated scientists, and their properties and assembly behavior have been the subjects of many theoretical studies over the years. However, only recently has experiment caught up with theory. We have begun to witness tremendous diversity in the synthesis of nanoparticles with controlled anisotropy. In this Perspective, we highlight the synthetic achievements that have galvanized the field, presenting a comprehensive discussion of the mechanisms and products of both seed-mediated and alternative growth methods. We also address recent breakthroughs and challenges in regiospecific functionalization, which is the next frontier in exploiting nanoparticle anisotropy.