Aspect Ratio Of Gold Nanorods Research Articles

Dual-functional nano-photosensitizers: Eosin-Y decorated gold nanorods for plasmon-enhanced fluorescence and singlet oxygen generation.

Photosensitizer (PS) with enhanced fluorescence is attractive for image-guided photodynamic therapy (PDT) due to its dual functional role in Singlet Oxygen Generation (SOG) and producing high fluorescence signals. Here, Eosin-Y (Ey) decorated polymer coated gold nanorods (GNRs) of different aspect ratios are synthesized and introduced as novel plasmon-enhanced nano-photosensitizers for this purpose. We show, upon excitation at 519 nm, simultaneous enhancement in fluorescence and SOG was achieved for the hybrid nanostructure. The best enhancement factors of 110 and 18 for metal-enhanced fluorescence and metal-enhanced SOG, respectively, are obtained with GNRs of length 133 nm and width 45 nm, where Ey is positioned at 12.6 nm from the metal core using layer-by-layer assembly of oppositely charged polymers. The observed plasmonic effect is critically analysed by comparing the near field damping rate along with decay length, far field scattering and nonradiative energy transfer of the nanohybrids.

Au nanorod core in an AgPt cage: Synthesis of Au@AgPt core/cage nanoframes with rough surface and controllable geometry by galvanic replacement

Metallic nanoparticles with intricate nanoframe structures have provoked extensive attention for improving optical and catalytic properties owing to their open structures. However, how to precisely control the geometry shape and surface details of the nanoframes remains a daunting challenge. Herein, we report a novel structure of Au@AgPt core/cage nanoframes with rough surfaces and controllable geometry and surface details based on the galvanic replacement reaction between Ag shells wrapped around gold nanorods and Pt4+ ion. With the increase of Pt4+ ion, the six faces of Ag shell firstly undergo a continuous concaving process to form the concave surface, following by an asymmetric and continuous hollowing-out process started from one side of Ag shell, and in this whole process accompanied by re-deposition process of both silver and platinum on the edges and vertices. We also verify that the prepared Au@AgPt nanoframes have the characteristics of adjustable size, aspect ratio, roughness, skeleton diameter and composition ratio by changing these reaction conditions such as temperature, thickness of Ag layer, and aspect ratio of gold nanorod. This controlled shape evolution also results in abundant localized surface plasmon resonance (LSPR) modes. Multiple absorption bands could be tuned simultaneously from blue (400 nm) to near infrared (1300 nm) range.

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.

Post-synthesis heating, a key step to tune the LPR band of gold nanorods covered with CTAB or embedded in a silica shell

The transitioning of nanotechnology from laboratory to industrial-scale manufacturing poses various challenges in nanoparticle realization. From this perspective, beside the conventional synthetic procedure, based on the seed-mediated growth approach, a reshaping thermal strategy has been investigated to improve the control on gold nanorods aspect ratio, with the aim to point out a potential and encouraging way to better manage the scalability and reproducibility of nanoparticles. For this purpose, nanorods covered with CTAB and nanorods enclosed within a silica shell of tuned thickness have been synthesized and submitted to a post-thermal treatment at various temperatures, up to 300 °C for CTAB recovered gold nanorods (AuNR@CTAB), and up to 500 °C for silica-shell embedded gold nanorods (AuNR@SiO2). For AuNR@CTAB, through accurate temperature control, the longitudinal plasmonic band can be moved very close to the transversal one upon slight reduction of their length. Instead, for AuNR@SiO2, owing to the fully inorganic shell, a higher temperature of treatment can be reached leading to the possibility of reshaping the nanorods into spheres without the observation of any by-products.

Size effects in plasmonic gold nanorod based Pd-rGO hybrid catalyst for promoting visible-light-driven Suzuki-Miyaura coupling reaction

The studies on the phenomenon of surface plasmon resonance effect by noble metal-based nanoparticles (NPs) have largely been focused on the quantitative enhancement of reaction rates and investigating the mechanistic pathway of the reaction. However, the detailed analysis of the structure-property relation of plasmonic NPs and catalytic performance along with the contribution of absorption vs scattering remains limited. This report focuses on the comparison of the catalytic performance of different aspect ratio of gold nanorods (Au NRs) coupled with Pd-rGO nanocomposite for boosting the product yield of Suzuki-Miyaura coupling reaction under visible light irradiation. The confirmation of the successful synthesis of the Pd/Au NRs@rGO catalyst was done by multiple characterization techniques such as XRD, UV–vis, TEM, XAS, XPS and ICP. Amongst the three different aspect ratios of Au NRs, the superior catalytic performance of medium-sized NRs (length: 56 nm; diameter: 18 nm) with a TON of 50 was achieved under visible light irradiation conditions. A drastic enhancement by a factor of 4.1 was shown by the medium-sized NRs when the reaction results were compared in dark reaction conditions. The obtained enhancement has been ascribed to the higher efficiency of hot electron generation by the SPR effect of Au NRs.

Plasmonic photothermal biosensor for visual detection of tyrosinase and dopamine based on manganese dioxide nanosheets-mediated etching of gold nanorods

As a crucial enzyme involved in melanogenesis, tyrosinase plays an important role in the diagnosis of melanoma. Herein, a novel and simple photothermal quantitative method based on manganese dioxide nanosheets-mediated etching of gold nanorods was established for tyrosinase activity assay with a simple household thermometer as the readout. Gold nanorods were etched by manganese dioxide nanosheets in acid condition at room temperature with a fast and stable etching efficiency. With the etching of gold nanorods, the photothermal conversion efficiency decreased gradually with the change of the aspect ratio of gold nanorods, leading to the decrease of the temperature increment of the solution by degrees. The gold nanorods have an unparalleled photothermal conversion efficiency, and the increment of temperature caused by the photothermal effect of gold nanorods has a span as wide as nearly 17 °C within one minute of laser irradiation. A household thermometer, instead of a spectrometer or any other professional and costly instruments, was used as a signal reader to achieve the visual quantitative detection, which makes this method possible for point-of-care testing. The photothermal platform was used successfully for the determination of tyrosinase and its substrate dopamine with wider linear ranges from 1.0 to 70 U/mL and 2.0–150 μM and detection limits as low as 0.34 U/mL and 0.92 μM, respectively. Moreover, the inhibition efficiency of kojic acid on tyrosinase activity was investigated, proving the photothermal quantitative method will be a potential platform for screening inhibitors of tyrosinase.

Gold Nanorods Exhibit Intrinsic Therapeutic Activity via Controlling N6-Methyladenosine-Based Epitranscriptomics in Acute Myeloid Leukemia.

Reprograming the N6-methyladenosine (m6A) landscape is a promising therapeutic strategy against recalcitrant leukemia. In this study, we synthesized gold nanorods (GNRs) of different aspect ratios using a binary surfactant mixture of hexadecyltrimethylammonium bromide and sodium oleate. Following surface functionalization with chitosan and a 12-mer peptide, GNRa-CSP12 measuring 130 × 21 nm2 was selectively taken up by leukemia cells via targeted endocytosis. Low doses of GNRa-CSP12 inhibited the growth of leukemia cells by disrupting the redox balance and inducing ferroptosis. Mechanistically, GNRa-CSP12 abrogated endogenous Fe2+-dependent m6A demethylase activity, which led to global m6A hypomethylation and post-transcriptional regulation of downstream genes that are involved in glycolysis, hypoxia, and immune checkpoint pathways. In addition, combination treatment with GNRa-CSP12 and tyrosine kinases inhibitors (TKIs) synergistically obviated the m6A-mediated TKI resistance phenotype. Finally, GNRa-CSP12 as a potential immunotherapeutic agent could enhance immunotherapy outcome in leukemia. Our preclinical findings provide the proof-of-concept for targeting m6A-methylation-based epitranscriptomics using nanoparticle as an "epigenetic drug" for cancer therapy.

Optimization of Gold Nanorod Features for the Enhanced Performance of Plasmonic Nanocavity Arrays.

Nanoplasmonic biosensors incorporating noble metal nanocavity arrays are widely used for the detection of various biomarkers. Gold nanorods (GNRs) have unique properties that can enhance spectroscopic detection capabilities of such nanocavity-based biosensors. However, the contribution of the physical properties of multiple GNRs to resonance enhancement of gold nanocavity arrays requires further characterization and elucidation. In this work, we study how GNR aspect ratio (AR) and surface area (SA) modify the plasmonic resonance spectrum of a gold triangular nanocavity array by both simulations and experiments. The finite integration technique (FIT) simulated the extinction spectrum of the gold nanocavity array with 300 nm periodicity onto which the GNRs of different ARs and SAs are placed. Simulations showed that matching of the GNRs longitudinal peak, which is affected by AR, to the nanocavity array’s spectrum minima can optimize signal suppression and shifting. Moreover, increasing SA of the matched GNRs increased the spectral variations of the array. Experiments confirmed that GNRs conjugated to a gold triangular nanocavity array of 300 nm periodicity caused spectrum suppression and redshift. Our findings demonstrate that tailoring of the GNR AR and SA parameters to nanoplasmonic arrays has the potential to greatly improve spectral variations for enhanced plasmonic biosensing.

Establishing Surface Charge as a Key Control Parameter for Linker-Driven Tip-Specific Ordering of Anisotropic Gold Nanoparticles

Here, we show that nanoparticles’ surface charge is a key factor that determines their tip-specificity in a covalently linked assembly of anisotropic plasmonic nanoparticles. We developed a strategy to controllably tune the surface charge of gold nanoparticles over a broad ζ potential range between −5 and −35 mV using simple acid–base chemistry and showed that dithiol-driven, end-to-end linked dimers of gold nanorods were formed reproducibly within a ζ potential range between −10 and −17 mV in acetonitrile medium. Below this ζ potential range, nanoparticles collapse together to form large clusters without any tip-specificity. For ζ potentials above this range, electrostatic repulsion prevents them from binding to each other. Our approach of using the surface charge of nanoparticles as a key control parameter for achieving tip-specificity is quite versatile and works for different anisotropic nanoparticles including gold nanorods of different aspect ratios and gold nanobipyramids.

Synthesis of Asymmetric One-Dimensional Pd on Au Bimetallic Nanostructures.

Nanostructures composed of a gold nanorod (AuNR) core and a Pd/Pt shell are of great interest due to their potential application as plasmon resonance-enhanced catalysts. However, the synthesis of well-defined one-dimensional bimetallic nanostructures with precise control over shell thickness and length remains a challenge. In this study, we report a detailed and systematic study on the chemical synthesis of a uniform Pd shell on single crystalline and pentahedrally twinned (PHT) AuNRs of various lengths. AuNRs were used as a template, and the slow and controlled reduction of Pd(II) ions on preformed AuNRs was carried out for the formation of rectangular-shaped Au@Pd bimetallic nanorods. The Pd shell thickness around the AuNRs was controlled by the supply of Pd(II) ions in the growth solution. We were able to grow a ∼20 nm uniform Pd shell around the AuNR, keeping the rod-like morphology intact without local nucleation to form irregular shapes and randomly overgrown nanostructures. The formation of bimetallic nanorods was also extended beyond typical single crystalline nanorods to PHT high aspect ratio gold nanorods and nanowires, using them as templates. To our surprise, unusually curved asymmetric nanorods were formed when the Pd deposition was carried out on AuNRs longer than ∼800 nm which could be possibly due to a Pd and Au lattice mismatch at the interface and higher flexibility of the nanorods when they exceeded certain lengths.

Morphological Changes of Silica Shells Deposited on Gold Nanorods: Implications for Nanoscale Photocatalysts

Gold nanorods (GNRs) support strong localized surface plasmon resonances that can be exploited to enhance the fluorescence or catalytic properties of adjacent molecules. Coating GNRs with silica is frequently used to functionalize their surfaces, but full encapsulation limits the ability to control the spatial distribution of molecules and their related reactivity. For example, locating molecules near the ends of GNRs would enable their strong longitudinal plasmon resonance to be exploited, but such selectivity is challenging. So far, studies of anisotropic coating have been limited and the mechanism of the adsorption onto GNRs remains unclear. Here, we systematically investigated the anisotropic coating of the ends of GNRs with silica and the influence of growth conditions on the formation of silica shells. Three types of nanostructures have been observed and their origins are described: fully encapsulated core-shell GNRs, GNRs with only one end coated with silica, and dumbbell-like GNRs (dGNRs) with both ends coated. The study was performed at around room temperature, where the solubility and micellization of the surfactant cetyltrimethylammonium bromide (CTAB) can be tuned to affect the morphology, stability and density of resulting silica shells. Optimized parameters, in combination with an appropriate GNR aspect ratio, are shown to significantly improve the growth yield of dGNRs, which become the dominant product. A protocol for a high yield synthesis of dGNRs was developed, with a maximum yield exceeding 90 %. This study advances our understanding of the growth mechanism of anisotropic coating of GNRs and sheds light on the optimization of site-selective coating processes. The development of anisotropic GNR-based nanostructures for fluorescence/scattering amplifiers will be important in applications such as metal-enhanced fluorescence, surface-enhanced Raman scattering or plasmon-enhanced catalysis.

Reversible isomerization of metal nanoclusters induced by intermolecular interaction

Reversible isomerization of metal nanoclusters induced by intermolecular interaction

Oxidase-like Nanozyme-Mediated Altering of the Aspect Ratio of Gold Nanorods for Breaking through H2O2-Supported Multicolor Colorimetric Assay: Application in the Detection of Acetylcholinesterase Activity and Its Inhibitors

A convenient, fast, and colorful colorimetric platform with high resolution for acetylcholinesterase (AChE) activity and its inhibitors detection based on the regulation of oxidase-like nanozyme-mediated etching of gold nanorods (AuNRs) has been proposed in this work. MnO2 nanosheets are selected as the nanozyme. Their excellent oxidase-like activity enables the etching process to proceed smoothly without the usage of unstable H2O2. When AChE is present, it catalytically hydrolyzes acetylthiocholine (ATCh) to thiocholine (TCh). With high reducing ability, TCh induces the decomposition of MnO2 nanosheets, causing them to lose their oxidase-like activity. Thus, the etching of AuNRs is hampered. Consequently, with the increasing concentration of AChE, an apparent change in the AuNRs solution color is observed. The proposed platform achieves high-sensitivity detection of AChE (limit of detection = 0.18 mU/mL). Furthermore, the proposed platform also has been demonstrated its applicability for its inhibitors detection. Benefiting from the advantages of convenient and high resolution of visual readout, the proposed platform holds great potential for the detection of AChE and its inhibitors in clinical diagnosis.

The aspect ratio of gold nanorods as a cytotoxicity factor on Raphidocelis subcaptata

Gold nanorods (AuNRs) are promising nanoscale materials for several technological and biomedical applications. The physicochemical properties of AuNRs, including size, shape and surface features, are crucial factors affecting their cytotoxicity. In this study, we investigated the effects of different aspect ratios of AuNRs (1.90, 2.35, 3.25 and 3.50) at concentrations of 2 and 10 μg mL−1 on their cytotoxicity and cellular uptake in green algae Raphidocelis subcaptata. The experiment was performed in oligotrophic freshwater medium in a growth chamber with constant agitation of 80 rpm under controlled conditions (120 μEm−2s−1 illumination; 12:12h light dark cycle and constant temperature of 22 ± 2 °C). The algal growth was monitored daily for 96 h via electronic absorbance scanning at 600–750 nm. Oxidative stress, cell viability and autofluorescence were evaluated using a flow cytometer. Oxidative stress quantified by loading cultures with the fluorescent dye 2′, 7′-dichlorofluorescein diacetate. To assess algal cell viability, propidium iodide was selected as the fluorescent probe. Our results indicated that the aspect ratio of AuNRs mediates their biological effects in green algae R. subcaptata. A positive correlation between oxidative stress and increase of aspect ratio was found at concentration of 10 μg mL−1. Higher cytotoxicity and mortality were observed for algae incubated with higher aspect ratios AuNRs (3.50). These findings may be useful to understand the impact of the AuNRs in aquatic environments, contributing to ecosystem management and nanomaterials regulation.

Thiol-suppressed I2-etching of AuNRs: acetylcholinesterase-mediated colorimetric detection of organophosphorus pesticides.

For the first time it isdemonstrated that sulfhydryl compounds can suppress longitudinal etching of gold nanorods via consuming oxidizers, which provides a new signaling mechanism for colorimetric sensing. As a proof of concept, a colorimetric assay is developed for detecting organophosphorus pesticides, which are most widely used in modern agriculture to improve food production but with high toxicity to animals and the ecological environment. Triazophos was selected as a model organophosphorus pesticide. In the absence of triazophos, the active acetylcholinesterase can catalyze the conversion of acetylthiocholine iodide to thiocholine whose thiol group can suppress the I2-induced etching of gold nanorods. When triazophos is present, the activity of AchE is inhibited, and I2-induced etching of gold nanorods results in triazophos concentration-dependent color change from brown to blue, pink, and red. The aspect ratio of gold nanorods reduced with gradually blue-shifted longitudinal absorption. There was a linear detection range from 0 to 117nM (R2 = 0.9908), the detection limit was 4.69nM, and a good application potential was demonstrated by theassay of real water samples. This method will not only contribute to public monitoring of organophosphorus pesticides but also has verified a new signaling mechanism which will open up a new path to develop colorimetric detection methods. It has been first found that sulfhydryl compounds can suppress longitudinal etching of gold nanorods (AuNRs) via consuming oxidizers, which provides a new signaling mechanism for colorimetric sensing. As a proof of concept, a colorimetric assay is developed for sensitively detecting organophosphorus pesticides (OPs). It will not only contribute to public monitoring of OPs but also has verified a new signaling mechanism which will open up a new path to develop multicolor colorimetric methods.

Controlled wet chemical synthesis of gold nanorods for triclopyr butotyl herbicide detection based-plasmonic sensor

Gold nanorods (GNRs) have a unique optical property of metallic nanoparticles due to the localised surface plasmon resonance (LSPR) effect, which depends on the size, shape and dielectric property of the surrounding medium. Anisotropic GNRs optical behaviours are approximated based on Mie-Gans theory by predicting the presence of dual absorption bands, which are the transverse and longitudinal bands. This study discussed the investigation of the GNRs formation by controlling three process parameters; centrifugation speed, seed solution concentration and growth solution ageing period to overcome the limitations of rod-shaped nanoparticles due to inhomogeneity, uncontrollable shape and aspect ratio. Bottom-up synthesis has been used to produce fine-tuning gold nanostructures. High yield and aspect ratio of GNRs are achieved at 20 h with 10 μl seed solution concentration and 5000 rpm centrifugation speed for particles separation; a longer growth ageing period caused the uncontrollable formation of GNRs with stacking nanoparticles. Therefore, the optimized GNRs with an aspect ratio of 4.23 ± 0.36 and the yields 74.81% are used as sensing material to detect the targeted herbicide analyte, namely triclopyr butotyl. The concentration of triclopyr butotyl can be detected as low as 3% ~ 1.03 mg/l using the sensing mechanism that was based on the plasmonic effect; which was resulting from the collective oscillations of conduction band electrons in a rod nanoparticle.

Bromide Causes Facet-Selective Atomic Addition in Gold Nanorod Syntheses

The aspect ratio-dependent properties of gold nanorods are used in a variety of applications, but the cause of anisotropic nanorod growth remains unclear. Measurements utilizing single-crystal electrodes were collected to determine what additive(s) in pentatwinned gold nanorod syntheses are responsible for facet-selective atomic addition. With cetyltrimethylammonium in the absence of bromide, the rate of atomic addition to Au(100) and Au(111) single crystals was the same, and isotropic nanoparticles were produced. The addition of increasing concentrations of bromide suppressed the rate of atomic addition to Au(100) relative to Au(111) and increased the aspect ratio of gold nanorods. Bromide was a more effective passivator of Au(100) in the absence of cetyltrimethylammonium, indicating cetyltrimethylammonium does not cause facet-selective atomic addition. Cetyltrimethylammonium surfactant is still necessary for gold nanorod growth because it reduces the rate of gold ion reduction and stabilizes suspended nanoparticles against aggregation.

Cellular Uptake and Cytotoxicity of Varying Aspect Ratios of Gold Nanorods in HeLa Cells.

Gold nanorods (GNRs) have received broad attention due to their tunable surface plasmonic resonance modes, which make them an attractive candidate for biomedical applications in drug and gene delivery, biological imaging, and cancer treatment. In this study, we highlight the interactions of four different aspect ratios (ARs), 2.6, 3.2, 5.4 and 11.5, of GNRs with HeLa cells with respect to cellular uptake and cellular viability while also considering the effects of other parameters such as the surface stabilizer, supernatant, and serum proteins present in the medium. From this work, it was determined that the cell viability depended on the chemical composition of the supernatant, especially the amount of excess surfactant, hexadecyltrimethylammonium bromide (CTAB), used for the synthesis of GNRs; thus, the effect of aspect ratio of GNRs on endocytosis could not be directly discerned. For example, when exposed to GNRs of aspect ratio 11.5, HeLa cells showed higher cellular uptake compared to the shorter aspect ratios and a lower cytotoxicity simply because of the lower concentration of CTAB used during the synthesis, and a gentler purification method, such as sedimentation, also appeared to be a factor. Overall, the synthesis protocols, functionalization, purification processes, and the stability of GNRs in media have a major effect on cellular uptake and viability. Our results suggest that perhaps AR does play a role in endocytosis although an overall trend could not be unequivocally established, and sedimentation, contact time, and internalization kinetics need to be probed further using more biocompatible ligands in future studies.

The impact of size and surface ligand of gold nanorods on liver cancer accumulation and photothermal therapy in the second near-infrared window

Gold nanorods (GNRs) with longitudinal surface plasmon resonance (LSPR) peaks in second near-infrared (NIR-II) window have attracted a great amount of attention as photothermal transducer because of their inherently excellent photothermal transition efficiency, high biocompatibility and versatile surface functionalization. One key question for the application of these GNRs against tumors in vivo is which size/shape and surface ligand conjugation are promising for circulation and tumor targeting. In this study, we prepared a series of gold nanorods (GNRs) of similar aspect ratio and LSPR peaks, and thus similar photothermal transfer efficiency under irradiation of 980 nm laser, but with tunable size in width and length. The obtained GNRs were subjected to surface modification with PEG and tumor targeting ligand lactoferrin. With these tailor-designed GNRs in hand, we have the chance to study the impact of dimension and surface property of the GNRs on their internalization via tumor cells, photothermal cytotoxicity in vitro, blood circulation and tissue distribution pattern in vivo. As a result, the GNRs with medium size (70 nm in length and 11.5 nm in width) and surface PEG/LF modification (GNR70@PEG-LF) exhibit the fastest cell internalization via HepG2 cells and best photothermal outcome in vitro. The GNR70@PEG-LF also display long circulation time and the highest tumor accumulation in vivo, due to the synergetic effect of surface coating and dimension. Finally, tumor ablation ability of the GNRs under irradiation of 980 nm light were validated on mice xenograft model, suggesting their potential photothermal therapy against cancer in NIR-II window.

Photoacoustic Signal of Nanobubbles Induced in PEGylated Gold-Nanorod Colloid

We have developed a photoacoustic (PA) measurement system using a broadband ultrasonic transducer to detect the PA signals induced in a gold nanorod (GNRs) colloid irradiated by the nanosecond-pulsed laser beam. This study will investigate the PA signals of PEG-GNRs at longitudinal surface plasmon resonance. Several factors including aspect ratio of GNR, pulse energy, concentration, and wavelength are varied to investigate these effects on the PA signal. We show that at the certain condition the generated PA signal of PEG-GNRs stronger than uncoated GNRs. Moreover, the PA signals of PEGGNRs are more stable than uncoated GNRs in the same condition. Keywords