Synthesis of Absorption-Dominant Small Gold Nanorods and Their Plasmonic Properties

Tuning Optical Properties of Plasmonic Aerosols through Ligand-Solvent Interactions

Plasmonic nanoparticles are highly tunable light-harvesting materials with a wide array of applications in photonics and catalysis. More recently, there has been interest in using aerosolized plasmonic nanoparticles for cloud formation, airborne photocatalysts, and molecular sensors, all of which take advantage of the large scattering cross sections and the ability of these particles to support intense local field enhancement ("hot spots"). While extensive research has investigated properties of plasmonic particles in the solution phase, surfaces, and films, aerosolized plasmonics are relatively unexplored. Here, we demonstrate how the capping ligand, suspension solvent, and atomization conditions used for aerosol generation control the steady-state optical properties of aerosolized Silica@Au plasmonic nanoshells. Our experimental results, supported with spectral simulations, illustrate that ligand coverage and atomization conditions control the degree of solvent retention and thus the spectral characteristics and potential access to surfaces for catalysis in the aerosol phase, opening a new regime for tunable applications of plasmonic metamaterials.

Thiols Modulated Gold Nanorods Self-Assembly: Indirect Hydrophobic Effects Instead of Direct Electrostatic/Hydrogen Bonds Attraction

For nanocrystals (NCs) self-assembly, understanding the chemical and supramolecular interactions among building blocks is significant for both fundamental scientific interests and rational nanosuperstructure construction. However, it has remained an extreme challenge for many self-assembly systems due to the lack of appropriately quantitative approaches for the corresponding exploration. Herein, by combination of the proposed colorimetric method for cationic surfactant quantitation and all-atom simulations, we manage to present a clear chemical picture for the thiol molecules modulated self-assembly of gold nanorods (GNRs), one of the earliest and most convenient methods for the fabrication of freestanding GNR self-assemblies. It is revealed that the self-assembly of GNRs is driven by the hydrophobic effects of the alkyl chains of the modified cationic surfactants, as their bilayer structure is destroyed by the added thiol molecules. In other words, the actual roles of the thiol molecules for causing GNRs assembly are indirectly inductive effects instead of the previously believed direct electrostatic attraction and/or hydrogen-bond linking effects of the binding thiol molecules. Furthermore, the GNRs exhibit diameter-dependent assembly behaviors: thicker GNRs tend to adopt the end-to-end assembly mode, while thin ones prefer the side-by-side assembly mode, further demonstrating that hydrophobic effects among the build blocks are the driving force for the GNRs assembly.

A Pd-based plasmonic photocatalyst for nitrogen fixation through an antenna-reactor mechanism

Plasmonic metal nanocrystals (e.g., Au, Ag, and Cu) hold great promise for driving photocatalytic reactions, but little is known about the plasmonic properties of Pd nanocrystals. Herein, we constructed a plasmonic Pd/Ru antenna-reactor photocatalyst through the controllable growth of a Ru nanoarray 'reactor' on a Pd nano-octahedron 'antenna' and demonstrated a plasmonic Pd-driven N2 photofixation process. The plasmonic properties of Pd nano-octahedrons were verified using finite-difference time-domain (FDTD) simulations and refractive index sensitivity tests in water-glycerol mixtures. Notably, the constructed plasmonic antenna-reactor nanostructures exhibited superior photocatalytic activities during N2 photofixation, with a maximum ammonia production rate of 117.5 ± 15.0 μmol g-1 h-1 under visible and near-infrared (NIR) light illumination. The mechanism can be attributed to the ability of the plasmonic Pd nanoantennas to harvest light to generate abundant hot electrons and the Ru nanoreactors to provide active sites for adsorption and activation of N2. This work paves the way for the development of Pd-based plasmonic photocatalysts for efficient N2 photofixation and sheds new light on the optimal design and construction of antenna-reactor nanostructures.

Controlled synthesis of monodisperse gold nanorods with a small diameter of around 10 nm and largest plasmon wavelength of 1200 nm

Gold nanorods have been widely used in various fields due to their tunable anisotropic localized surface plasmon resonance (SPR) property. The facile preparation of gold nanorods with a tunable SPR wavelength extending to a near-infrared window, and at the same time, a relatively small particle size for facilitating applications especially in the biomedical field is of great value yet highly challenging. In this work, a new reducing agent, 1,6-dihydroxynaphthalene, is proposed for the synthesis of gold nanorods. The results indicate that gold nanorods with good monodispersity, high shape yield, maximum SPR wavelength of 1200 nm, and especially small diameter of around 10 nm can be acquired simultaneously. In terms of spectral and size controls, by respectively varying the experimental parameters including the amount of silver ions, reducing agents, and gold seeds not only can a good linear correlation be acquired corresponding to a SPR wavelength ranging from around 600 nm to 1200 nm, but a regular change in the particle diameter from 10.5 nm to 7.5 nm could also be observed. The structural and morphological evolutions of the particle for each changed parameter were carefully studied, and insights were gained into the growth mechanism based on the detailed analysis of particle evolution at a specific stage of the growth process.

Photothermal effects of gold nanorods in aqueous solution and gel media: Influence of particle size and excitation wavelength

Gold nanorods (GNRs) have emerged as the most efficient photothermal agent in cancer therapy and photocatalysis. Understanding the influence of the surrounding medium, particle size, and excitation wavelength is critical to optimising the photothermal conversion rate. Here, three pairs of large and small gold nanorods of different aspect ratios and their heat generation under laser radiation at on and off surface plasmon resonance wavelengths in aqueous solution and gel-like media are investigated. In the aqueous solution, the temperature rise of the large gold nanorods is more than with small gold nanorods at resonance excitation. In contrast to the large gold nanorods (LGNRs), the small gold nanorods (SGNRs) were less sensitive to excitation wavelength. At off-resonance excitation, the temperature rise of the SGNRs is larger than that of the LGNRs. In the agarose gel, the photothermal effect of the SGNRs is greater than LGNRs excited at the wavelength near their solution phase longitudinal surface plasmon resonance wavelength. The temperature increase of LGNRs in gel is significantly less than in aqueous solution. These findings suggest that SGNRs could be more beneficial than the LGNRs for photothermal applications in biological systems and provides further insight when selecting GNRs.

Optimizing Plasmonic Gold Nanorod Deposition on Glass Surfaces for High-Sensitivity Refractometric Biosensing

Owing to high surface sensitivity, gold nanorods (AuNRs) are widely used to construct surface-based nanoplasmonic biosensing platforms for label-free molecular diagnostic applications. A key fabrication step involves controlling AuNR deposition onto the target surface, which requires maximizing surface density while minimizing inter-particle aggregation, and is often achieved by surface functionalization with a self-assembled monolayer (SAM) prior to AuNR deposition. To date, existing studies have typically used a fixed concentration of SAM-forming organic molecules (0.2-10% v/v) while understanding how SAM density affects AuNR deposition and resulting sensing performance would be advantageous. Herein, we systematically investigated how controlling the (3-aminopropyl)triethoxysilane (APTES) concentration (1-30% v/v) during SAM preparation affects the fabrication of AuNR-coated glass surfaces for nanoplasmonic biosensing applications. Using scanning electron microscopy (SEM) and UV-visible spectroscopy, we identified an intermediate APTES concentration range that yielded the highest density of individually deposited AuNRs with minimal aggregation and also the highest peak wavelength in aqueous solution. Bulk refractive index sensitivity measurements indicated that the AuNR configuration had a strong effect on the sensing performance, and the corresponding wavelength-shift responses ranged from 125 to 290 nm per refractive index unit (RIU) depending on the APTES concentration used. Biosensing experiments involving protein detection and antigen-antibody interactions further demonstrated the high surface sensitivity of the optimized AuNR platform, especially in the low protein concentration range where the measurement shift was ~8-fold higher than that obtained with previously used sensing platforms.

Surface Plasmon-Enhanced NIR-II Fluorescence in a Multilayer Nanoprobe for Through-Skull Mouse Brain Imaging

Fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) holds great potential for the accurate visualization of deeply located biological structures in vivo. However, the weak fluorescence of current NIR-II fluorophores remains a long-standing challenge for the ever-growing imaging demand. Here, we describe a surface plasmon-enhanced NIR-II fluorescence strategy by incorporating silica-coated gold nanorods (GNRs) and polymer dots (Pdots) into a multilayer nanostructure. Precise manipulation of the silica spacing layer thickness signifies an optimum distance of 8.6 nm, where an enhancement factor of up to 6.4 is achieved in the NIR-II imaging window. The surface plasmon enhancement approach is successfully extended to several types of Pdots fluorophores with NIR-II emission. We finally perform outer-layer encapsulation and PEGylation for the multilayer probes and demonstrate surface plasmon-enhanced NIR-II fluorescence for mouse brain imaging through the skull, which exhibits a refined signal-to-background ratio and penetration depth as compared to the clinically approved ICG dye.

Gold Nanorods Induce Endoplasmic Reticulum Stress and Autocrine Inflammatory Activation in Human Neutrophils

Gold nanorods (AuNRs) are promising agents for diverse biomedical applications such as drug and gene delivery, bioimaging, and cancer treatment. Upon in vivo application, AuNRs quickly interact with cells of the immune system. On the basis of their strong intrinsic phagocytic activity, polymorphonuclear neutrophils (PMNs) are specifically equipped for the uptake of particulate materials such as AuNRs. Therefore, understanding the interaction of AuNRs with PMNs is key for the development of safe and efficient therapeutic applications. In this study, we investigated the uptake, intracellular processing, and cell biological response induced by AuNRs in PMNs. We show that uptake of AuNRs mainly occurs via phagocytosis and macropinocytosis with rapid deposition of AuNRs in endosomes within 5 min. Within 60 min, AuNR uptake induced an unfolded protein response (UPR) along with induction of inositol-requiring enzyme 1 α (IREα) and features of endoplasmic reticulum (ER) stress. This early response was followed by a pro-inflammatory autocrine activation loop that involves LOX1 upregulation on the cell surface and increased secretion of IL8 and MMP9. Our study provides comprehensive mechanistic insight into the interaction of AuNRs with immune cells and suggests potential targets to limit the unwanted immunopathological activation of PMNs during application of AuNRs.

The Shape of Nanostructures Encodes Immunomodulation of Carbohydrate Antigen and Vaccine Development

Gold nanoparticles (AuNPs) have shown remarkable potential for vaccine development, but the influence of the size and shape of nanoparticles modulating the T-cell-dependent carbohydrate antigen processing and immunomodulation is poorly investigated. Here, we described how different shapes and sizes of gold nanostructures carrying adjuvant modulate carbohydrate-based antigen processing in murine dendritic cells (mDCs) and subsequent T-cell activation produce a robust antibody response. As a prototype, CpG-adjuvant-coated spherical and rod- and star-shaped AuNPs were conjugated to the tripodal Tn-glycopeptide antigen to study their DC uptake and activation of T-cells in a DCs/T-cell co-culture assay. Our results showed that the spherical and star-shaped AuNPs displayed relatively weak receptor-mediated uptake and endosomal sequestration; however, they induced a high level of T helper-1 (Th1) biasing immune responses compared with rod-shaped AuNPs. Furthermore, the in vivo administration of AuNPs showed that the small spherical and star-shaped AuNPs induced an effective anti-Tn-glycopeptide immunoglobulin (IgG) antibody response compared with rod-shaped AuNPs. These results indicated that one could obtain superior carbohydrate vaccines by varying the shape and size parameters of nanostructures.

Photothermal Conversion Profiling of Large-Scaled Synthesized Gold Nanorods Using Binary Surfactant with Hydroquinone as a Reducing Agent

Photothermal application of gold nanorods (AuNRs) is widely increasing because of their good photothermal conversion efficiency (PCE) due to local surface plasmon resonance. However, the high concentration of hexadecyltrimethylammonium bromide used in the synthesis is a concern. Moreover, the mild and commonly used reducing agent-ascorbic acid does not reduce the Au(I) to A(0) entirely, resulting in a low yield of gold nanorods. Herein we report for the first time the PCE of large-scaled synthesized AuNRs using the binary surfactant seed-mediated method with hydroquinone (HQ) as the reducing agent. The temporal evolution of the optical properties and morphology was investigated by varying the Ag concentration, HQ concentration, HCl volumes, and seed solution volume. The results showed that the seed volume, HQ concentration, and HCl volume played a significant role in forming mini-AuNRs absorbing in the 800 nm region with a shape yield of 87.7%. The as-synthesized AuNRs were successfully up-scaled to a larger volume based on the optimum synthetic conditions followed by photothermal profiling. The photothermal profiling analysis showed a temperature increase of more than 54.2 °C at 2.55 W cm⁻² at a low optical density (OD) of 0.160 after 630 s irradiation, with a PCE of approximately 21%, presenting it as an ideal photothermal agent.

Gold nanostructures: synthesis, properties, and neurological applications

Recent advances in technology are expected to increase our current understanding of neuroscience. Nanotechnology and nanomaterials can alter and control neural functionality in both in vitro and in vivo experimental setups. The intersection between neuroscience and nanoscience may generate long-term neural interfaces adapted at the molecular level. Owing to their intrinsic physicochemical characteristics, gold nanostructures (GNSs) have received much attention in neuroscience, especially for combined diagnostic and therapeutic (theragnostic) purposes. GNSs have been successfully employed to stimulate and monitor neurophysiological signals. Hence, GNSs could provide a promising solution for the regeneration and recovery of neural tissue, novel neuroprotective strategies, and integrated implantable materials. This review covers the broad range of neurological applications of GNS-based materials to improve clinical diagnosis and therapy. Sub-topics include neurotoxicity, targeted delivery of therapeutics to the central nervous system (CNS), neurochemical sensing, neuromodulation, neuroimaging, neurotherapy, tissue engineering, and neural regeneration. It focuses on core concepts of GNSs in neurology, to circumvent the limitations and significant obstacles of innovative approaches in neurobiology and neurochemistry, including theragnostics. We will discuss recent advances in the use of GNSs to overcome current bottlenecks and tackle technical and conceptual challenges.

Large-Scale Synthesis of Hybrid Conductive Polymer-Gold Nanoparticles Using "Sacrificial" Weakly Binding Ligands for Printing Electronics

We describe the gram-scale synthesis of hybrid gold nanoparticles with a shell of conductive polymers. A large-scale synthesis of hexadecyltrimethylammonium bromide (CTAB)-capped gold nanoparticles (AuNP@CTAB) was followed by ligand exchange with conductive polymers based on thiophene in a 10 L reactor equipped with a jacket to ensure a constant temperature of 40 °C and a mechanical stirrer. Slow and controlled reduction of the gold precursors and the presence of small amounts of silver nitrate are revealed to be the critical synthesis variables to obtain particles with a sufficiently narrow size distribution. Batches of approximately 10 g of faceted AuNP@CTAB with tunable average particle sizes from 54 to 85 nm were obtained per batch. Ligand exchange with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) in the same reactor then yielded hybrid Au@PEDOT:PSS nanoparticles. They were used to formulate sinter-free inks for the inkjet printing of conductive structures without the need for a sintering step.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

How to Utilize Excited Plasmon Energy Efficiently

Plasmonic nanoparticles can concentrate electromagnetic fields at the nanoscale and function as a powerful intermediary to enhance light-matter interactions. They have been widely employed for solar energy harvesting, photocatalysis, medicine, sensing, imaging, spectroscopy, optics, and optoelectronics. In this Perspective, we provide a brief overview of research progress in the utilization of excited plasmon energy, with emphasis on the charge- and energy-transfer processes. We discuss important factors that affect the charge- and energy-transfer efficiencies and present open questions and major challenges in the efficient utilization of excited plasmon energy.

Optical properties and stability of small hollow gold nanoparticles

In the current work, small hollow Au nanoparticles (d ≈ 16 nm) with excellent thermal stability and high photo-thermal conversion efficiency, which have great potential for use in photo-thermal cancer therapy, were prepared through galvanic replacement reaction between Ag nano-templates and gold salt. The position of surface plasmon resonance (SPR) bands for these nanoparticles could be tuned by varying the amount of gold salt. The hydrophobic hollow nanostructures were made water-dispersible by being encapsulated with poly(maleic anhydride-alt-1-octadecene) - PMAO. The obtained nanostructures were stable in an aqueous solution of NaCl with concentration up to 280 mM. The hollow gold nanoparticles (HGNPs) were then heated using an 808 nm laser at different power densities, the obtained data showed that they are highly photo-thermal stable under a high power density laser up to 1.6 W cm^-2 after three circles of irradiation at 20 min per circle (20 min continuous irradiation for each circle). The facile synthesis of small size HGNPs with a plasmon peak in the near infrared range, colloidal and photo-thermal stability, and high capacity of conversion of photon energy into heat makes them a promising material for photo-thermal and imaging applications.

Light-driven bimorph soft actuators: design, fabrication, and properties

Soft robots that can move like living organisms and adapt to their surroundings are currently in the limelight from fundamental studies to technological applications, due to their advances in material flexibility, human-friendly interaction, and biological adaptation that surpass conventional rigid machines. Light-fueled smart actuators based on responsive soft materials are considered to be one of the most promising candidates to promote the field of untethered soft robotics, thereby attracting considerable attention amongst materials scientists and microroboticists to investigate photomechanics, photoswitch, bioinspired design, and actuation realization. In this review, we discuss the recent state-of-the-art advances in light-driven bimorph soft actuators, with the focus on bilayer strategy, i.e., integration between photoactive and passive layers within a single material system. Bilayer structures can endow soft actuators with unprecedented features such as ultrasensitivity, programmability, superior compatibility, robustness, and sophistication in controllability. We begin with an explanation about the working principle of bimorph soft actuators and introduction of a synthesis pathway toward light-responsive materials for soft robotics. Then, photothermal and photochemical bimorph soft actuators are sequentially introduced, with an emphasis on the design strategy, actuation performance, underlying mechanism, and emerging applications. Finally, this review is concluded with a perspective on the existing challenges and future opportunities in this nascent research Frontier.

Cell membrane coated smart two-dimensional supraparticle for in vivo homotypic cancer targeting and enhanced combinational theranostics

Development of intelligent and multifunctional nanoparticle for the diagnosis and treatment of cancer has drawn great attention recently. In this work, we design a smart two-dimensional (2D) supraparticle for tumor targeted magnetic resonance imaging (MRI)/photothermal imaging (PTI) and chemo/photothermal therapy (PTT).

Methods: The nanoparticle consists of a manganese dioxide (MnO₂) nanosheet coated gold nanorod (GNR) core (loading with chemotherapeutics doxorubicin (DOX)), and cancer cell membrane shell (denoted as CM-DOX-GMNPs). Decoration of cell membrane endows the nanoparticle with greatly improved colloidal stability and homotypic cancer cell targeting ability. Once the nanoparticles enter tumor cells, MnO₂ nanosheets can be etched to Mn²⁺ by glutathione (GSH) and acidic hydrogen peroxide (H₂O₂) in the cytosol, leading to the release of DOX. Meanwhile, stimuli dependent releasing of Mn²⁺ can act as MRI contrast agent for tumor diagnosis. Illumination with near-infrared (NIR) light, photothermal conversion effect of GNRs can be activated for synergistic cancer therapy.

Results:In vivo results illustrate that the CM-DOX-GMNPs display tumor specific MRI/PTI ability and excellent inhibition effect on tumor growth.

Conclusion: This bioinspired nanoparticle presents an effective and intelligent approach for tumor imaging and therapy, affording valuable guidance for the rational design of robust theranostics nanoplatform.

Photostability of Contrast Agents for Photoacoustics: The Case of Gold Nanorods

Plasmonic particles as gold nanorods have emerged as powerful contrast agents for critical applications as the photoacoustic imaging and photothermal ablation of cancer. However, their unique efficiency of photothermal conversion may turn into a practical disadvantage, and expose them to the risk of overheating and irreversible photodamage. Here, we outline the main ideas behind the technology of photoacoustic imaging and the use of relevant contrast agents, with a main focus on gold nanorods. We delve into the processes of premelting and reshaping of gold nanorods under illumination with optical pulses of a typical duration in the order of few ns, and we present different approaches to mitigate this issue. We undertake a retrospective classification of such approaches according to their underlying, often implicit, principles as: constraining the initial shape; or speeding up their thermal coupling to the environment by lowering their interfacial thermal resistance; or redistributing the input energy among more particles. We discuss advantages, disadvantages and contexts of practical interest where one solution may be more appropriate than the other.

Miniature Hollow Gold Nanorods with Enhanced Effect for In Vivo Photoacoustic Imaging in the NIR-II Window

The miniaturization of gold nanorods exhibits a bright prospect for intravital photoacoustic imaging (PAI) and the hollow structure possesses a better plasmonic property. Herein, miniature hollow gold nanorods (M-AuHNRs) (≈46 nm in length) possessing strong plasmonic absorbance in the second near-infrared (NIR-II) window (1000-1350 nm) are developed, which are considered as the most suitable range for the intravital PAI. The as-prepared M-AuHNRs exhibit 3.5 times stronger photoacoustic signal intensity than the large hollow Au nanorods (≈105 nm in length) at 0.2 optical density under 1064 nm laser irradiation. The in vivo biodistribution measurement shows that the accumulation in tumor of miniature nanorods is twofold as high as that of the large counterpart. After modifying with a tumor-targeting molecule and fluorochrome, in living tumor-bearing mice, the M-AuHNRs group gives a high fluorescence intensity in tumors, which is 3.6-fold that of the large ones with the same functionalization. Moreover, in the intravital PAI of living tumor-bearing mice, the M-AuHNRs generate longer-lasting and stronger photoacoustic signal than the large counterpart in the NIR-II window. Overall, this study presents the fabrication of M-AuHNRs as a promising contrast agent for intravital PAI.

A Review on Cancer Therapy Based on the Photothermal Effect of Gold Nanorod

Background:

Cancer causes millions of deaths and huge economic losses every year. The currently practiced methods for cancer therapy have many defects, such as side effects, low curate rate, and discomfort for patients.

Objective:

Herein, we summarize the applications of gold nanorods (AuNRs) in cancer therapy based on their photothermal effect-the conversion of light into local heat under irradiation.

Methods:

The recent advances in the synthesis and regulation of AuNRs, and facile surface functionalization further facilitate their use in cancer treatment. For cancer therapy, AuNRs need to be modified or coated with biocompatible molecules (e.g. polyethylene glycol) and materials (e.g. silicon) to reduce the cytotoxicity and increase their biocompatibility, stability, and retention time in the bloodstream. The accumulation of AuNRs in cancerous cells and tissues is due to the high leakage in tumors or the specific interaction between the cell surface and functional molecules on AuNRs such as antibodies, aptamers, and receptors.

Results:

AuNRs are employed not only as therapeutics to ablate tumors solely based on the heat produced under laser that could denature protein and activate the apoptotic pathway, but also as synergistic therapies combined with photodynamic therapy, chemotherapy, and gene therapy to kill cancer more efficiently. More importantly, other materials like TiO2, graphene oxide, and silicon, etc. are incorporated on the AuNR surface for multimodal cancer treatment with high drug loadings and improved cancer-killing efficiency. To highlight their applications in cancer treatment, examples of therapeutic effects both in vitro and in vivo are presented.

Conclusion:

AuNRs have potential applications for clinical cancer therapy.

Analyzing fidelity and reproducibility of DNA templated plasmonic nanostructures

Synthetic DNA templated nanostructures offer an excellent platform for the precise spatial and orientational positioning of organic and inorganic nanomaterials. Previous reports have shown its applicability in the organization of plasmonic nanoparticles in a number of geometries for the purpose of realizing tunable nanoscale optical devices. However, translation of nanoparticle-DNA constructs to application requires additional efforts to increase scalability, reproducibility, and formation yields. Understanding all these factors is, in turn, predicated on in-depth analysis of each structure and comparing how formation changes with complexity. Towards the latter goal, we assemble seven unique plasmonic nanostructure symmetries of increasing complexity based on assembly of gold nanorods and nanoparticles on two different DNA origami templates, a DNA triangle and rhombus, and characterize them using gel electrophoresis, atomic force- and transmission electron microscopy, as well as optical spectroscopy. In particular, we focus on how much control can be elicited over yield, reproducibility, shape, size, inter-particle angles, gaps, and plasmon shifts as compared to expectations from computer simulations as structural complexity increases. We discuss how these results can contribute to establishing process principles for creating DNA templated plasmonic nanostructures.

Synthesis of Small Gold Nanorods and Their Subsequent Functionalization with Hairpin Single Stranded DNA

Small gold nanorods have a significantly large absorption/scattering ratio and are especially beneficial in exploiting photothermal effects, for example in photothermal therapy and remote drug release. This work systematically investigates the influence of growth conditions on the size, growth yield, and stability of small gold nanorods. The silver-assisted seed-mediated growth method was optimized to synthesize stable small gold nanorods with a high growth yield (>85%). Further study on the influence of silver ions on the growth facilitates the growth of small gold nanorods with tunable longitudinal surface plasmon resonance from 613 to 912 nm, with average dimensions of 13-25 nm in length and 5-6 nm in diameter. Moreover, the small gold nanorods were successfully functionalized with thiol-modified hairpin oligonucleotides (hpDNA) labeled with Cy5. Fluorescence intensity measurements show an increase in the presence of target DNA and an enhanced signal/background ratio when the longitudinal surface plasmon resonance of small gold nanorods overlaps with the excitation and emission wavelength of Cy5. This coincides with a reduced fluorescence lifetime of Cy5 in the hairpin structure, indicating surface plasmon resonance-enhanced energy transfer to the small gold nanorods. This study may provide insight on the synthesis and functionalization of small gold nanorods in biomedical sensing and therapy.

Application of Nanoparticles and Nanomaterials in Thermal Ablation Therapy of Cancer

Cancer is one of the major health issues with increasing incidence worldwide. In spite of the existing conventional cancer treatment techniques, the cases of cancer diagnosis and death rates are rising year by year. Thus, new approaches are required to advance the traditional ways of cancer therapy. Currently, nanomedicine, employing nanoparticles and nanocomposites, offers great promise and new opportunities to increase the efficacy of cancer treatment in combination with thermal therapy. Nanomaterials can generate and specifically enhance the heating capacity at the tumor region due to optical and magnetic properties. The mentioned unique properties of nanomaterials allow inducing the heat and destroying the cancerous cells. This paper provides an overview of the utilization of nanoparticles and nanomaterials such as magnetic iron oxide nanoparticles, nanorods, nanoshells, nanocomposites, carbon nanotubes, and other nanoparticles in the thermal ablation of tumors, demonstrating their advantages over the conventional heating methods.

Virus-Sized Gold Nanorods: Plasmonic Particles for Biology

Plasmons, collective oscillations of conduction-band electrons in nanoscale metals, are well-known phenomena in colloidal gold and silver nanocrystals that produce brilliant visible colors in these materials that depend on the nanocrystal size and shape. Under illumination at or near the plasmon bands, gold and silver nanocrystals exhibit properties that enable fascinating biological applications: (i) the nanocrystals elastically scatter light, providing a straightforward way to image them in complex aqueous environments; (ii) the nanocrystals produce local electric fields that enable various surface-enhanced spectroscopies for sensing, molecular diagnostics, and boosting of bound fluorophore performance; (iii) the nanocrystals produce heat, which can lead to chemical transformations at or near the nanocrystal surface and can photothermally destroy nearby cells. While all the above-mentioned applications have already been well-demonstrated in the literature, this Account focuses on several other aspects of these nanomaterials, in particular gold nanorods that are approximately the size of viruses (diameters of ∼10 nm, lengths up to 100 nm). Absolute extinction, scattering, and absorption properties are compared for gold nanorods of various absolute dimensions, and references for how to synthesize gold nanorods with four different absolute dimensions are provided. Surface chemistry strategies for coating nanocrystals with smooth or rough shells are detailed; specific examples include mesoporous silica and metal-organic framework shells for porous (rough) coatings and polyelectrolyte layer-by-layer wrapping for "smooth" shells. For self-assembled-monolayer molecular coating ligands, the smoothest shells of all, a wide range of ligand densities have been reported from many experiments, yielding values from less than 1 to nearly 10 molecules/nm2 depending on the nanocrystal size and the nature of the ligand. Systematic studies of ligand density for one particular ligand with a bulky headgroup are highlighted, showing that the highest ligand density occurs for the smallest nanocrystals, even though these ligand headgroups are the most mobile as judged by NMR relaxation studies. Biomolecular coronas form around spherical and rod-shaped nanocrystals upon immersion into biological fluids; these proteins and lipids can be quantified, and their degree of adsorption depends on the nanocrystal surface chemistry as well as the biophysical characteristics of the adsorbing biomolecule. Photothermal adsorption and desorption of proteins on nanocrystals depend on the enthalpy of protein-nanocrystal surface interactions, leading to light-triggered alteration in protein concentrations near the nanocrystals. At the cellular scale, gold nanocrystals exert genetic changes at the mRNA level, with a variety of likely mechanisms that include alteration of local biomolecular concentration gradients, changes in mechanical properties of the extracellular matrix, and physical interruption of key cellular processes-even without plasmonic effects. Microbiomes, both organismal and environmental, are the likely first point of contact of nanomaterials with natural living systems; we see a major scientific frontier in understanding, predicting, and controlling microbe-nanocrystal interactions, which may be augmented by plasmonic effects.

Gold nanorods: from anisotropy to opportunity. An evolution update

Gold nanoparticles have drawn attention to nanomedicine for many years due to their physicochemical properties, which include: good stability; biocompatibility; easy surface chemistry and superior magnetic; and last, electronic properties. All of these properties distinguish gold nanoparticles as advantageous carriers to be exploited. The challenge to develop new gold nanostructures has led to anisotropy, a new property to exploit for various medical applications: diagnostic and imaging strategies as well as therapeutic options. Gold nanorods are the most studied anisotropic gold nanoparticles because of the presence of two absorption peaks according to their longitudinal and transversal plasmon resonances. The longitudinal surface plasmonic resonance can provide the absorption in the near-infrared region and this is an important aspect of using gold nanorods for medical purposes.

Miniature gold nanorods for photoacoustic molecular imaging in the second near-infrared optical window

In photoacoustic imaging, the second near-infrared (NIR-II) window is where tissue generates the least background signal. However, the large size of the few available contrast agents in this spectral range impedes their pharmacokinetics and decreases their thermal stability, leading to unreliable photoacoustic imaging. Here, we report the synthesis of miniaturized gold nanorods absorbing in the NIR-II that are 5-11 times smaller than regular-sized gold nanorods with a similar aspect ratio. Under nanosecond pulsed laser illumination, small nanorods are about 3 times more thermally stable and generate 3.5 times stronger photoacoustic signal than their absorption-matched larger counterparts. These unexpected findings are confirmed using theoretical and numerical analysis, showing that photoacoustic signal is not only proportional to the optical absorption of the nanoparticle solution but also to the surface-to-volume ratio of the nanoparticles. In living tumour-bearing mice, these small targeted nanorods display a 30% improvement in efficiency of agent delivery to tumours and generate 4.5 times greater photoacoustic contrast.

Disconnecting Symmetry Breaking from Seeded Growth for the Reproducible Synthesis of High Quality Gold Nanorods

One of the major difficulties hindering the widespread application of colloidal anisotropic plasmonic nanoparticles is the limited robustness and reproducibility of multistep synthetic methods. We demonstrate herein that the reproducibility and reliability of colloidal gold nanorod (AuNR) synthesis can be greatly improved by disconnecting the symmetry-breaking event from the seeded growth process. We have used a modified silver-assisted seeded growth method in the presence of the surfactant hexadecyltrimethylammonium bromide and n-decanol as a co-surfactant to prepare small AuNRs in high yield, which were then used as seeds for the growth of high quality AuNR colloids. Whereas the use of n-decanol provides a more-rigid micellar system, the growth on anisotropic seeds avoids sources of irreproducibility during the symmetry breaking step, yielding uniform AuNR colloids with narrow plasmon bands, ranging from 600 to 1270 nm, and allowing the fine-tuning of the final dimensions. This method provides a robust route for the preparation of high quality AuNR colloids with tunable morphology, size, and optical response in a reproducible and scalable manner.

Photothermal therapy and photoacoustic imaging via nanotheranostics in fighting cancer

The nonradiative conversion of light energy into heat (photothermal therapy, PTT) or sound energy (photoacoustic imaging, PAI) has been intensively investigated for the treatment and diagnosis of cancer, respectively. By taking advantage of nanocarriers, both imaging and therapeutic functions together with enhanced tumour accumulation have been thoroughly studied to improve the pre-clinical efficiency of PAI and PTT. In this review, we first summarize the development of inorganic and organic nano photothermal transduction agents (PTAs) and strategies for improving the PTT outcomes, including applying appropriate laser dosage, guiding the treatment via imaging techniques, developing PTAs with absorption in the second NIR window, increasing photothermal conversion efficiency (PCE), and also increasing the accumulation of PTAs in tumours. Second, we introduce the advantages of combining PTT with other therapies in cancer treatment. Third, the emerging applications of PAI in cancer-related research are exemplified. Finally, the perspectives and challenges of PTT and PAI for combating cancer, especially regarding their clinical translation, are discussed. We believe that PTT and PAI having noteworthy features would become promising next-generation non-invasive cancer theranostic techniques and improve our ability to combat cancers.

DNA-Decorated, Helically Twisted Nanoribbons: A Scaffold for the Fabrication of One-Dimensional, Chiral, Plasmonic Nanostructures

Crafting of chiral plasmonic nanostructures is extremely important and challenging. DNA-directed organization of nanoparticle on a chiral template is the most appealing strategy for this purpose. Herein, we report a supramolecular approach for the design of DNA-decorated, helically twisted nanoribbons through the amphiphilicity-driven self-assembly of a new class of amphiphiles derived from DNA and hexaphenylbenzene (HPB). The ribbons are self-assembled in a lamellar fashion through the hydrophobic interactions of HPB. The transfer of molecular chirality of ssDNA into the HPB core results in the bias of one of the chiral propeller conformations for HPB and induces a helical twist into the lamellar packing, and leads to the formation of DNA-wrapped nanoribbons with M-helicity. The potential of the ribbon to act as a reversible template for the 1D chiral organization of plasmonic nanomaterials through DNA hybridization is demonstrated.

Monitoring the death of single BaF3 cells under plasmonic photothermal heating induced by ultrasmall gold nanorods

Morphological changes and trypan-blue staining are temporally tracked in single cells via optical microscopy after plasmonic photothermal heating.

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).

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

Plasmonic gold nanorods (Au NRs)-copper sulfide heterostructures have recently attracted much attention owing to the synergistically enhanced photothermal properties. However, the facile synthesis and interface tailoring of Au NRs-copper sulfide heterostructures remain a formidable challenge. In this study, the rational design and synthesis of Au NRs-Cu₇ S₄ heterostructures via a one-pot hydrothermal process is reported. Specifically, core-shell and dumbbell-like Au NRs-Cu₇ S₄ heterostructures are obtained with well-controlled interfaces by employing the Au NRs with different aspect ratios. Both core-shell and dumbbell-like Au NRs-Cu₇ S₄ have proven effective as photothermal therapy agents, which offer both high photothermal stability and significant photothermal conversion efficiency up to 62%. The finite-difference time domain simulation results confirm the coupling effect that leads to the enhanced local field as well as the optical absorption at the heterostructure interface. Importantly, these Au NRs-Cu₇ S₄ heterostructures can be compatibly used as an 808 nm laser-driven photothermal therapy agents for the efficient photothermal therapy of cancer cells in vitro. This study will provide new insight into the design of other noble metal-semiconductor heterostructures for a broad range of applications utilizing surface plasmon resonance enhancement phenomena.

Mini Gold Nanorods with Tunable Plasmonic Peaks beyond 1000 nm

Gold nanorods of small sizes have larger absorption cross sections and higher photothermal efficiency compared to larger ones. However, tuning the surface plasmon resonance of small gold nanorods remains a challenge because increasing an aspect ratio usually results from increasing dimensions. We demonstrate the synthesis of mini gold nanorods with tunable longitudinal surface plasmon resonance from ~600 to >1300 nm accompanied by precise control over widths <10 nm. Two weak reducing agents, ascorbic acid and even milder hydroquinone, were applied to a seed-mediated growth method to tune the aspect ratios of mini gold nanorods from 2.2 to 10.8 corresponding to average dimensions 19.3 × 9.0 nm through 93.1 × 8.7 nm, respectively. This seed-mediated growth of mini gold nanorods results in an average 96% of rods and yields of at least 79% based on gold ion reduction. The extinction coefficients of mini gold nanorods were established based on the gold content from inductively coupled plasma mass spectrometry. The longitudinal extinction coefficients range from 1.6 × 108 to 1.4 × 109 M-1 cm-1 depending on aspect ratio. We show that liter-scale mini gold nanorod syntheses are reproducible, and the dimensions, aspect ratios, and shape percent yields are comparable to those of a small-scale synthesis.

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.

Small Gold Nanorods: Recent Advances in Synthesis, Biological Imaging, and Cancer Therapy

Over the past few decades, the synthetic development of ultra-small nanoparticles has become an important strategy in nano-medicine, where smaller-sized nanoparticles are known to be more easily excreted from the body, greatly reducing the risk caused by introducing nano-theranostic agents. Gold nanorods are one of the most important nano-theranostic agents because of their special optical and electronic properties. However, the large size (diameter > 6 nm) of most obtained gold nanorods limits their clinical application. In recent years, more and more researchers have begun to investigate the synthesis and application of small gold nanorods (diameter < 6 nm), which exhibit similar optical and electronic properties as larger gold nanorods. In this review, we summarize the recent advances of synthesis of the small gold nanorods and their application for near-infrared light-mediated bio-imaging and cancer therapy.

97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold

In this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO₂) nanowires. It is found that the randomly oriented dense TiO₂ nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO₂ random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials.

Highly Concentrated Seed-Mediated Synthesis of Monodispersed Gold Nanorods

The extremely large optical extinction coefficient of gold nanorods (Au-NRs) enables their use in a diverse array of technologies, rnging from plasmonic imaging, therapeutics and sensors, to large area coatings, filters, and optical attenuators. Development of the latter technologies has been hindered by the lack of cost-effective, large volume production. This is due in part to the low reactant concentration required for symmetry breaking in conventional seed-mediated synthesis. Direct scale up of laboratory procedures has limited viability because of excessive solvent volume, exhaustive postsynthesis purification processes, and the generation of large amounts of waste (e.g., hexadecyltrimethylammonium bromide(CTAB)). Following recent insights into the growth mechanism of Au-NRs and the role of seed development, we modify the classic seed-mediated synthesis via temporal control of seed and reactant concentration to demonstrate production of Au-NRs at more than 100-times the conventional concentration, while maintaining independent control and narrow distribution of nanoparticle dimensions, aspect ratio, and volume. Thus, gram scale synthesis of Au-NRs with prescribed aspect ratio and volume is feasible in a 100 mL reactor with 1/100th of organic waste relative to conventional approaches. Such scale-up techniques are crucial to cost-effectively meet the increased demand for large quantities of Au-NRs in emerging applications.

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.

Universal analytical modeling of plasmonic nanoparticles

Analytical expressions are applied to calculate the plasmonic spectra of nanoparticles with arbitrary morphology, in excellent agreement with experimental data.