Longitudinal Surface Plasmon Resonance Research Articles

Gold Nanoearbuds: Seed-Mediated Synthesis and the Emergence of Three Plasmonic Peaks.

We demonstrate the first successful synthesis of reasonably monodisperse and single-crystalline gold nanoearbuds (Au NEBs) using a binary surfactant mixture of cetyltrimethylammonium chloride (CTAC) and benzyldimethylhexadecylammonium chloride (BDAC) in seed-mediated growth method. We have focused on the key chemical parameters behind the formation and growth of Au NEBs to result in tunable dimensions (length, 37-77 nm; width, 4-6 nm; aspect ratio, 7-19), as a consequence of which the longitudinal surface plasmon resonance (LSPR) peak could be tuned beyond 1200 nm. The achievement of LSPR beyond 1200 nm while maintaining the dimension well below 100 nm is a challenging accomplishment in the realm of one-dimensional (1D) Au nanostructures. This earbud-like morphology additionally exhibits three plasmonic peaks, rather uncommon for 1D nanostructures, which were analyzed theoretically based on the finite element method. The new resonance peak of the Au NEB was assigned as an additional longitudinal mode intensified by the bulbous ends as well as the high aspect ratio, thereby providing conclusive evidence that it is indeed a new morphology.

The morphology regulation and plasmonic spectral properties of Au@AuAg yolk-shell nanorods with controlled interior gap.

Au@AuAg yolk-shell nanorods with tunable and uniform interior gap were synthesized through galvanic replacement reaction, where Au@Ag core-shell nanorods served as sacrificial templates and HAuCl4 solution served as reductant. The effects of HAuCl4, Ag shell thickness and aspect ratio (AR) of Au nanorods on the morphology of Au@AuAg yolk-shell nanorods had been investigated systemically. The results clearly indicated that AuAg alloy shell thickness of Au@AuAg yolk-shell nanorods could be increased from 3.6 to 10.0nm by varying the amount of HAuCl4. Meanwhile, the shape of AuAg alloy shell could be tuned by changing the shape of Ag coating. With the increasing of Ag coating thickness, the interior gap could be finely tuned in the range from 2.6 to 8.1nm. The uniformity of interior gap could be improved by increasing the AR of Au nanorods. All these tunable geometries can further affect the plasmonic spectral properties of Au@AuAg yolk-shell nanorods. Because of the appearance of interior gap, the longitudinal localized surface plasmon resonance (LSPR) peak of Au@AuAg yolk-shell nanorods was located between that of bare Au nanorods and Au@Ag core-shell nanorods without interior gap. The increase of outer AuAg shell thickness can weaken the coupling between the inner and outer surface of the AuAg shell and lead to the decrease of AR, so the transverse and longitudinal LSPR bands gather together. The decrease of Ag coating thickness can enhance the coupling between inner Au nanorod and outer AuAg shell, which results in the red shift of the longitudinal LSPR band. This paper provides a method for studying the plasmonic coupling between two metal surfaces with a metal layer or a dielectric layer, which is also a new approach for regulating the plasmonic spectral properties of bimetallic nanoparticles. The controllability of Au@AuAg yolk-shell nanorods in both the interior gap and outer alloy shells makes them have potential applications in biomedicine, catalysis, nanoreactors, and energy storage.

Multicolor colorimetric detection of ochratoxin A via structure-switching aptamer and enzyme-induced metallization of gold nanorods

Colorimetric aptasensors have been intensively studied for the ochratoxin A (OTA) detection, but they mostly exhibit just one-color change, resulting in poor visual resolution and limited use for semi-quantitative analysis. Thus, we designed a high-resolution colorimetric assay on the basis of aptamer structural switching and enzyme-induced metallization of gold nanorods (AuNRs). DNA-alkaline phosphatase (ALP)-immobilized magnetic beads were prepared. The aptamer bounded to OTA to form G-quadruplexes, releasing ALP-labelled complementary DNA (cDNA-ALP). After magnetic separation, cDNA-ALP catalyzed the decomposition of ascorbic acid 2-phosphate to ascorbic acid that reduced Ag+, forming an Ag shell on the surface of AuNRs. This caused a blue-shift of the longitudinal local surface plasmon resonance peak of the AuNRs and a naked eye visible multicolor change. Under optimal conditions, the assay exhibited a 9.0 nM detection limit for OTA, with high specificity. This method is promising for the on-site visual semi-quantitative detection of mycotoxins in foods.

Surface-Enhanced Infrared Absorption of Ligands on Colloidal Gold Nanowires through Resonant Coupling.

Pushing the detection limit of infrared absorption (IR) through surface-enhanced (SEIRA) approaches have far-reaching prospect for related applications in molecular analysis and detection. Specifically engineered Au nanowires (NWs) can be applied as the surface-enhancing substrates in colloidal solution, given their longitudinal surface plasmon resonance (SPR) being aspect-ratio dependent and extendable into the infrared region. Through carefully designed control experiments, we realized resonant coupling between the longitudinal surface plasmons of Au NWs and the vibration modes of the bonded oleylamine (OA) ligands. In our system, after deliberately tuning thickness of the OA ligands and ratio of the detached/attached ligands in the solution, the apparent enhancement factor of IR signal from ligands around Au NWs could be pushed up to 5.29 × 104. Given the facile tuning of SPR properties of Au NWs in the colloidal solution and the performance demonstrated in the report, our work could be an intriguing platform for SEIRA implementations in a broad spectrum of circumstances.

In situ formation of Ag/Au nanorods as a platform to design a non-aggregation colorimetric assay for uric acid detection in biological fluids

Herein, we report on a sensitive probe for colorimetric detection of uric acid based on the formation of Ag shell on Au nanorods (Ag/AuNRs) in the presence of uric acid. Uric acid can reduce Ag+ to Ag atoms which deposit on the surface of AuNRs and form Ag shell. As a result of this phenomenon, the dielectric environment around AuNRs changes and the longitudinal surface plasmon resonance (SPR) peak of AuNRs blue-shifts, leading to the change of solution color from purple to green. Based on these facts, a colorimetric sensor for the detection of uric acid in the concentration range of 0.1–1.0 µM with a detection limit of 0.065 µM was developed. The method was applied for analysis of uric acid in human plasma and urine samples with satisfactory results. The sensor can be used for uric acid detection even with bare eyes.

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.

Precisely tuning the longitudinal localized surface plasmon resonance of gold nanorods via additive-regulated overgrowth.

Gold nanorods (GNRs) with desired longitudinal localized surface plasmon resonance (LLSPR) and strong scattering intensity are important for extending their practical applications in bioimaging and sensing. Herein, a simple additive (HCl and Na2S)-regulated overgrowth approach has been proposed for preparing GNRs with tunable LLSPR. In this approach, HCl is used to slow down the growth reaction rate by changing chemical equilibrium, while Na2S is utilized to halt the reaction when LLSPR is reaching the expected wavelength under monitoring by a UV-Vis spectrometer. Under optimal conditions, GNRs with an LLSPR range from 850 to 650 nm could be facilely prepared with a high precision of 3 nm deviation. The TEM images reveal that GNRs have high monodispersity, displaying an increase in both length and diameter but a decrease in the aspect ratio. With the increase in size, the produced GNRs show enhanced scattering intensity and are applicable for single nanoparticle imaging due to the enlarged absorption and scattering cross-section and improved matching efficiency toward the CCD response.

Modeling the hardness of the titanium alloy subjected to laser processing

The article considers the influence of the radiation power W, the longitudinal feed Spr of the laser beam and the distance L from the protective glass of the laser focusing head to the workpiece on the hardness Hv of the titanium alloy TiTaN. A multi-factor model is established that relates the surface layer hardness to the input factors of laser processing and allows you to quickly assign a laser treatment mode and to control it in order to improve the quality of the processed layer. The power W has the greatest effect on the hardness of the surface layer. With increasing W, the hardness increases. Increasing the feed Spr leads to a decrease in the Hv parameter. The effect of the distance L is similar to the radiation power, but the degree of influence of L is more than half as small as W. The developed method of operational assignment of the laser processing mode allows to reduce the auxiliary time for performing the technological operation. The research results are relevant for manufacturing enterprises that implement laser processing processes, as well as for design organizations that develop modern laser equipment.

Formation of anisotropic gold nanoparticles on indium tin oxide substrates as a plasmonic sensing material

A simple technique of seed-mediated growth has been successfully performed to grow anisotropy gold nanoparticles on solid substrates. The growth of the gold nanoparticles has been carried out in the presence of a binary surfactant mixture of hexadecyltrimethylammonium bromide with two different molecular weights of a capping agent, namely polyvinylpyrrolidone: 40,000 and 55,000. In this study, the effect of process parameters, growth time and molecular weight of capping agent was investigated. The growth time shows a significant impact on the shape and size of nanoparticles. The shorter growth time produced small spherical to square-like shape particles, whereas bigger particles including nanorods, nanosquares and nanotriangles were formed with longer growth time. The shape controlling agent, polyvinylpyrrolidone, was used to synthesis gold nanoparticles. It was found that monodisperse gold nanoparticles with uniform shape and size are hardly obtained when polyvinylpyrrolidone 40,000 was used as capping agent. Polyvinylpyrrolidone 55,000 produced more uniform shape and size of gold nanoparticles. Thus, these process parameters were found affected to the size, shape, surface density and uniformity of gold nanoparticles. This sample was further applied as a sensing material in the detection of toxic fungicide, namely chlorothalonil. The sensitivity of the sensor system was determined by the changes in peak positions and intensities of the transverse and longitudinal surface plasmon resonance peaks on different medium, that is, air, deionized water and chlorothalonil solution. The sensor response of gold nanoparticles thin film in 30 mM chlorothalonil showed two resonance peaks in comparison to the control experiment without gold nanoparticle thin film. The gold nanoparticles thin film sensor was successfully synthesized and potentially useful as a sensing material for fungicide detection.

Modification of the optical characteristics of Rhodamine B dye with different concentrations using gold nanorods

ABSTRACT The optical absorption and fluorescence of Rhodamine B dye solution with different dye concentrations, from 0.1 to 40 µM, in the presence of gold nanorods were studied. Colloidal gold nanorods were characterized by UV–Visible spectrophotometry and Transmission Electron Microscopy. Rhodamine B dye absorbance with and without gold nanorods was shown to be approximately constant, due to the dominance of the absorbance term of longitudinal surface plasmon resonance of gold nanorods. The fluorescence peak of dye-nanorods mixture with respect to that of pure dye was blue shifted. By increasing the dye concentrations, the energy transfer efficiency between the dye-nanorod pair was increased and, as a result, the fluorescence quantum yield was decreased. The results of the spectral overlapping of the absorption and the fluorescence spectra of RhB dye solution, with and without gold nanorods, matched well with the calculated energy transfer efficiency.

Tailoring Fluorescence and Singlet Oxygen Generation of a Chlorophyll Derivative and Gold Nanorods via a Silica Shell

Gold nanorods deserve special attention as they exhibit tunable longitudinal localized surface plasmon resonances (LSPRs). In our study, gold nanorods of the aspect ratio of 2.25 (maximum of LSPR b...

Precisely Tuning LSPR Property via "Peptide-Encoded" Morphological Evolution of Gold Nanorods for Quantitative Visualization of Enzyme Activity.

Longitudinal surface plasmon resonance (LSPR)-based optical signals possess unique advantages in biomolecular sensing and detection which can be attributed to their ultrahigh sensitivity and signal-to-noise ratio. However, the lack of effective strategies for morphological control of gold nanorods (GNRs) complicates the precise tuning of their LSPR property. Herein, a "peptide-encoded" strategy was first developed to precisely control the morphologies of GNRs via overgrowth of GNR seeds in the presence of thiol-containing peptides. Significantly, the "peptide-encoded" GNRs exhibit a tunable LSPR peak ranging from 685 to 877 nm by altering the amount of peptide. A few obvious colorimetric changes were accompanied from pink to purple and even to blue. Other parameters, e.g., pH, temperature, and Ag+ concentration, could also be utilized to regulate the morphologies of the "peptide-encoded" GNRs. The ultrasensitive detection of tumor-related protease activities based on LSPR peak shifts was further successfully performed without the need for labeling or instrumental aid, achieving a limit of detection of 60 fM. It is much lower than traditional absorption-based analysis (1 nM) and enzyme-linked immunosorbent assay (ELISA) method (1 pM), indicating the great potential of this peptide-encoded strategy in the application of ultrasensitive biomarker assay and clinical diagnosis.

Sensitive Multicolor Visual Detection of Exosomes via Dual Signal Amplification Strategy of Enzyme-Catalyzed Metallization of Au Nanorods and Hybridization Chain Reaction.

Exosomes as nanosized vesicles have been recognized as potential noninvasive biomarkers for early cancer diagnosis. Herein, we presented a sensitive multicolor visual method for exosome detection based on enzyme-induced silver deposition on gold nanorods (Au NRs). To achieve highly sensitive determination of exosomes, hybridization chain reaction (HCR) was employed to introduce more alkaline phosphatase (ALP) for signal amplification. First, exosomes were captured by magnetic bead-labeled CD63 aptamer, and, then, cholesterol-modified DNA probes were spontaneously inserted into the exosomal lipid membrane. The ends of the DNA probes act as the initiator to trigger the HCR for signal amplification. Finally, with the help of HCR, increased sites led to enhanced ALP loading and thus boosted the ascorbic acid generation. Silver ions were reduced by ascorbic acid, and silver shells were formed on Au NRs, giving rise to the blue shift of the longitudinal localized surface plasmon resonance peak. Correspondingly, the concentration of exosomes can be obviously distinguished with naked eyes via the vivid color variation. Due to the dual signal amplification of HCR and metallization of Au NRs, highly sensitive detection for exosomes were realized with detection limits as low as 1.6 × 102 particles/μL by UV-vis spectroscopy and 9 × 103 particles/μL by naked eyes. Compared to the reported colorimetric methods for exosome quantification, visualization based on plentiful color tonalities is the most captivating merit of our approach, and HCR-induced signal amplification highlights the virtue of the strategy. The applicability of the method was validated by the analysis of clinical samples.

Kinetic and plasmonic properties of gold nanorods adsorbed on glass substrates

Monodisperse gold nanorods with different sizes were synthesized and adsorbed on chemically modified glass substrates. Influence of surfactant molar concentration on nanorod adsorption was studied and the optimum range was determined. During substrate coverages we monitored the growth of longitudinal localized surface plasmon resonances at short times due to density increase of isolated nanorods and, at longer times, their subsequent decrease and a concurrent growth of coupling resonances owing to nanoparticle surface mobility and aggregation. Temporal evolution of amplitudes of resonance peaks in extinction spectra and nanorod counting statistics in electron micrographs were used to model both coverage and aggregation processes, as exponential-like functions of time. Their characteristic times and saturation values were analyzed and related to kinetic parameters, nanorod dimensions and extinction coefficients. This work can be used as a predictive tool to prepare plasmonic substrates with desired optical resonances.

Development of thickness-tunable gold nanorods for anti-oxidant detection

Here, in this study, we strategically utilized low-energy mediated epitaxial deposition of Au0 atoms reduced by gallic acid over preformed gold nanorods (GNRs) seeds. It can be suggested that GNRs seed/Au3+ ratio influences the directional attachment of Au0 atoms to the GNRs. Alteration in the thickness of the GNRs upon deposition of Au0 in presence micromolar levels of antioxidant reduces the aspect ratio of the nanorods. Change in the aspect ratio altogether induces a blue-shift in the longitudinal surface plasmon resonance (LSPR) of the GNRs from the NIR region of the spectrum to the shorter wavelength. TEM imaging, DLS and zeta potential analyses confirms the morphological and surface-charge alterations after interaction with antioxidant. Based on the relation between blue-shift of the LSPR band and the concentration of gallic acid, the sensing platform achieves a linear detectable range of 1.25–35 μM with detection limit as low as 90 nM and the limit of quantification as 300 nM. The method has high selectivity against tested interferents and was found to be reproducible. The potential application of the developed sensor was validated by quantifying gallic acid in commercially available apple juice. High recovery (99.46–100.4%) was obtained, suggesting that the established assay which is reliable and facile can be successfully used for gallic acid detection real food samples. The developed method of tuning the aspect ratio of nanomaterial can be further extended for detection other anti-oxidant molecules.

Gold nanorods/g-C3N4 heterostructures for plasmon-enhanced photocatalytic H2 evolution in visible and near-infrared light

Recently, broad spectrum (visible and near-infrared (NIR)) light utilization has aroused widespread attention in the research of photocatalysis. While g-C3N4, highly stable, cheap and easily synthesized, shows H2 evolution activity under visible light irradiation, it doesn’t perform under NIR light irradiation. Here we report an Au nanorods (NRs)/g-C3N4 heterostructure with Au nanorods on g-C3N4’s surface. The most exciting feature of designed Au NRs/g-C3N4 heterostructures is that Au nanorods themselves are excited by visible and NIR light, which produce hot electrons and inject into g-C3N4. The photocatalytic H2 evolution rate of Au NRs/g-C3N4 heterostructures (350.6 μmol g−1 h−1) is nearly 4 times higher than that of g-C3N4 with Pt as cocatalyst (68.9 μmol g−1 h−1) under visible light illumination. The improved photocatalytic activity is ascribed to the increasing visible light-absorbing capacity of transverse surface plasmon resonance (TSPR) of Au nanorods and improved charge separation of Au NRs/g-C3N4 heterostructure. Even more important, Au NRs/g-C3N4 heterostructures achieve NIR photocatalytic H2 evolution performance (63.1 μmol g−1 h−1), owing to the longitudinal SPR (LSPR) effect of Au nanorods induced NIR light harvesting ability.

Dichroic Plasmon Superstructures of Au Nanorods over Macroscopic Areas via Directed Self‐Assemblies of Diblock Copolymers

AbstractThe ability to organize metal nanoparticles over the macroscopic length scale has fundamental implications for advanced plasmonic applications. In particular, linear assemblies of anisotropic nanorods (NRs) may represent a dichroic superstructure, in which plasmonic extinction is inherently dependent on the direction of light polarization. As plasmonic properties are interconnected with optics, electronics, catalysis, and sensing, dichroism in plasmonic systems can stimulate interesting opportunities in many fields of chemistry. However, the preparation of dichroic structures with a single orientation at the centimeter scale remains a significant challenge. Herein, directed self‐assemblies of polystyrene‐block‐poly(methyl methacrylate) (PS‐b‐PMMA) diblock copolymers on lithographical patterns are applied to produce a linear pattern of Au NRs. Periodic nanogrooves from plasma‐etched PS‐b‐PMMA nanodomains facilitate the dense packing of Au NRs into end‐to‐end alignment, which entail strong plasmonic coupling among longitudinal plasmonic modes. Because the resulting NR assemblies have uniform orientation, their dichroic functions can be examined by conventional UV–vis spectroscopy without the aid of single‐particle measurements. Interestingly, both transverse and longitudinal localized surface plasmonic resonance peaks can be turned on and off under two polarized lights in orthogonal directions. Moreover, the resulting superstructures can be transferred onto various substrates, allowing the implantation of dichroic functions for numerous plasmonic applications.

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

Computer Simulations of the Optical Properties of Triangular Gold Nanoparticle Dimers

In this paper the properties of a “bowtie” nanostructure formed by a pair of distorted triangular nanoparticles are investigated using computer simulation. It is shown that the electromagnetic field enhancement upon excitation of the longitudinal surface plasmon resonance mode is localized in the gap between the particles and reaches the value $${{\left| E \right|} \mathord{\left/ {\vphantom {{\left| E \right|} {\left| {{{E}_{0}}} \right|}}} \right. \kern-0em} {\left| {{{E}_{0}}} \right|}} = 58.$$

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.