Optical Properties Of Gold Nanoparticles Research Articles

Gold Nanoparticle‐Based Optical Probe for Quick Colorimetric Visualization of Cysteine

AbstractIn this work, a highly sensitive and selective optical probe for cysteine was developed using unmodified gold nanoparticles. Quick visual detection depended on the distance‐dependent optical properties of gold nanoparticles, the self‐assembly of cysteine on the surface of gold nanoparticles, and the chelation in the presence of lead ions. The form of complex could rapidly induced the aggregation of gold nanoparticles, thereby resulting in red‐to‐blue (or purple) color change. The concentration of cysteine was determined with a limit of detection (LOD) lower than 1 × 10−7 M by the naked eye and could further be monitored by a UV‐vis spectrometer easily. The highest selectivity was proved in comparison with other seventeen amino acids and the relative mechanism was detailedly investigated by the addition of ethylenediaminetetraacetate (EDTA).

Colorimetric Detection of Nucleic Acid Signature of Shiga Toxin Producing <I>Escherichia coli</I> Using Gold Nanoparticles

Enterohemorrhagic E. coil (EHEC) serotype O157:H7 is one of the major pathogens, responsible for the severe disease outbreaks. EHEC causes diseases in humans through production of shiga-like toxin leading to bloody diarrhea. The toxin is encoded by stx2 gene in E. coli. The current methodology for detection of EHEC relies on fluorogenic-substrate based culture media or nucleic acid amplification based Real-Time Polymerase Chain Reaction assays that are either time consuming or need expensive instrumentation. In this study, the optical properties of gold nanoparticles (GNPs) have been exploited for detection of nucleic acid of Escherichia coli O157:H7. The stx2 gene representing EHEC signature has been targeted using the gold nanoparticle probes. Gold nanoparticles (GNPs) of 20 +/- 0.2 nm were synthesised by citrate reduction method and characterised by spectroscopy and transmission electron microscopy. The GNPs were functionalised with 19 and 22 bp of thiolated single stranded DNA complementary to target highly conserved 149 bp region of stx2 gene. Transmission Electron Microscopy revealed the hybridization, aggregation and reduction in the interparticle distances of the GNP probes in the presence of target DNA. The aggregation and the spectral shift in the plasmon band observed with 10(6) copies of target DNA indicates feasibility of a simple and quick colorimetric 'spot and read' test in contrast to amplification based detection methods.

Differential Pathlength Spectroscopy for the Quantitation of Optical Properties of Gold Nanoparticles

An accurate estimation of optical absorption coefficient (microabs) and scattering coefficient (microsca) is important in characterizing nanoparticles for identifying or optimizing applications such as photothermal therapy and photoacoustic imaging. In this exciting period where several fascinating methods have been unveiled for the synthesis of various nanoparticles, the field is still lacking in the availability of efficient characterization methods. We introduce an accurate and simple methodology to optically characterize nanoparticles which could fill the gap. This is based on differential pathlength spectroscopy (DPS), a dual optical fiber approach, originally developed to detect cancer endoscopically by measuring the optical properties of tissue in small interrogation volumes. We expand its use to nanoparticles in a method that allows us to resolve the effects of microabs and microsca in the extinction coefficient of low concentration samples. We outline the measurement protocol using the DPS system and describe the analysis of the data taking additional inputs from electron microscopy and discrete dipole approximation (DDA) simulations. The DPS signal from the sample is first translated into the backscattering coefficient using a calibration constant. Further, the backscattering coefficient is converted via the simulated scattering phase function into the scattering coefficient. With this knowledge and extinction coefficient measured using a conventional photospectrometer, the absorption coefficient is calculated. We prove the validity of the method using spherical and rod-shaped gold nanoparticles, comparing the results with outputs from DDA simulations. We also briefly touch upon the dilemma of the choice of the appropriate dielectric function for gold at the nanoscale.

Visual detection of melamine in raw milk using gold nanoparticles as colorimetric probe

We report the development of a simple and rapid colorimetric detection method for melamine in raw milk using gold nanoparticles as probe. This assay relies upon the distance-dependent optical properties of gold nanoparticles. In neutral media, melamine could rapidly induce the aggregation of gold nanoparticles, thereby resulting in red-to-blue (or purple) colour change. The concentration of melamine in raw milk can be determined by monitoring with the naked eye or a UV–vis spectrometer. The present limit of detection for melamine is 0.4 mg/L. The method is rather simple, and the whole process including sample pretreatment takes only 12 min at room temperature. The merits (such as simplicity, rapidity, low cost and visual colorimetry) make the proposed method specially useful for on-site screening melamine levels well below the current safety limit in raw milk.

Accuracy of the discrete dipole approximation for simulation of optical properties of gold nanoparticles

We studied the accuracy of the discrete dipole approximation (DDA) for simulations of absorption and scattering spectra by gold nanoparticles (spheres, cubes, and rods ranging in size from 10 to 100 nm). We varied the dipole resolution and applied two DDA formulations, employing the standard lattice dispersion relation (LDR) and the relatively new filtered coupled dipoles (FCD) approach. The DDA with moderate dipole resolutions is sufficiently accurate for scattering efficiencies or positions of spectral peaks, but very inaccurate for e.g. values of absorption efficiencies in the near-IR. To keep relative errors of the latter within 10% about 107 dipoles per sphere are required. Surprisingly, errors for cubes are about 10 times smaller than that for spheres or rods, which we explain in terms of shape errors. The FCD is generally more accurate and leads to up to 2 times faster computations than the LDR. Therefore, we recommend FCD as the DDA formulation of choice for gold and other metallic nanoparticles.

Temperature-Dependent Optical Properties of Gold Nanoparticles Coated with a Charged Diblock Copolymer and an Uncharged Triblock Copolymer

We demonstrate that the optical properties of gold nanoparticles can be used to detect and follow stimuli-induced changes in adsorbed macromolecules. Specifically, we investigate thermal response of anionic diblock and uncharged triblock copolymers based on poly(N-isopropylacrylamide) (PNIPAAM) blocks adsorbed onto gold nanoparticles and planar gold surfaces in a temperature range between 25 and 60 degrees C. By employing a palette of analytical probes, including UV-visible spectroscopy, dynamic light scattering, fluorescence, and quartz crystal microbalance with dissipation monitoring, we establish that while the anionic copolymer forms monolayers at both low and high temperature, the neutral copolymer adsorbs as a monolayer at low temperatures and forms multilayers above the cloud point (T(C)). Raising the temperature above T(C) severely affects the optical properties of the gold particle/polymer composites, expelling associated water and altering the immediate surroundings of the gold nanoparticles. This effect, stronger for the uncharged polymer, is related to the amount of polymer adsorbed on the surface, where a denser shell influences the surface plasmon band to a greater degree. This is corroborated with light scattering experiments, which reveal that flocculation of the neutral polymer-coated particles occurs at high temperatures. The flocculation behavior of the neutral copolymer on planar gold surfaces results in multilayer formation. The observed effects are discussed within the framework of the Mie-Drude theory.

金ナノ粒子を用いたバイオイメージングおよびフォトサーマル治療

Such unique optical properties of gold nanoparticles as surface plasmon and photothermal effects have

Visual detection of ascorbic acid via alkyne–azide click reaction using gold nanoparticles as a colorimetric probe

An approach for visual sensing of ascorbic acid is presented. This method is based on the distance-dependent optical property of gold nanoparticles and the Cu(+)-catalyzed alkyne-azide click reaction. We prepared the terminal azide- and alkyne-functionalized gold nanoparticle probes. In the presence of Cu(2+), ascorbic acid could rapidly induce the aggregation of the functionalized gold nanoparticles, thereby resulting in a red-to-purple (or pink) color change. Ascorbic acid can be quantified visually or using a UV-vis spectrometer. The present limit of detection for ascorbic acid is 3 nM. This method exhibits excellent selectivity over other common organic reducing compounds (such as glucose, cysteine, dopamine, thiamine and uric acid). Without the aid of any advanced instruments, the assay of ascorbic acid could be performed with one step at room temperature. This method provides a potentially useful tool for the on-site detection of ascorbic acid.

Gold Nanowires: Their Synthesis and Surface Plasmon Resonances

The objective of the project was to fabricate gold nano-rods and study the optical properties of gold nano-particles when coupled to Indium Arsenide quantum dots. The gold nano-rods were synthesized by a seed-mediated growth method using CTAB and BDAC as the surfactants, and the feasibility of changing the aspect ratio of the rods and their Plasmon resonance frequency was studied by varying the concentrations of these two surfactants. Finally, gold nano-rods with longitudinal Plasmon resonance of 940 nm were synthesized. Next, we studied the feasibility of coupling gold nano-wires to indium arsenide quantum dots for investigating their optical properties and studying the spontaneous emission enhancement of InAs QDs in the presence of the plasmon resonances of gold nano-wires. The sample containing nano-wires coupled to quantum dots was excited by red laser, and the emission was passed through a spectrometer and the spectrum was obtained.

The influence of mechanical strain on the optical properties of spherical gold nanoparticles

Abstract We utilize classical Mie scattering theory to investigate the effects of tensile and compressive mechanical strain on both the far field (absorption, scattering and extinction efficiencies) and near field (surface enhanced Raman scattering) optical properties of spherical gold nanoparticles with diameters ranging from 10 to 100 nm. By accounting for the strain effects on both the ionic core (bound) and conduction (free) electrons through appropriate modifications of the bulk dielectric functions, we find that gold nanoparticles are relatively sensitive to the effects of mechanical strain due to the fact that the plasmon resonance wavelength for spherical gold particles, which occurs around λ = 520 nm , is nearly coincident with the interband transitions of the core electrons. Specifically, we find that tensile strain leads to significant enhancements ranging from 60% to 120% in the far field optical efficiencies, while compressive strain leads to similar decreases, and that the plasmon resonance wavelength can be red or blueshifted up to 100 nm due to the applied strain. Finally, we find that tensile strain also strongly enhances the local electric ( E )-field at the surface of the nanoparticles, which is of considerable interest for surface-enhanced Raman scattering applications; 5% tensile strain is found to enhance the | E | 4 intensity by 63%. The present results demonstrate the potential of mechanical strain, and specifically that of tensile mechanical strain in enhancing and tailoring the optical properties of gold nanoparticles.

Aptamer−Nanoparticle Strip Biosensor for Sensitive Detection of Cancer Cells

We report an aptamer-nanoparticle strip biosensor (ANSB) for the rapid, specific, sensitive, and low-cost detection of circulating cancer cells. Known for their high specificity and affinity, aptamers were first selected from live cells by the cell-SELEX (systematic evolution of ligands by exponential enrichment) process. When next combined with the unique optical properties of gold nanoparticles (Au-NPs), ANSBs were prepared on a lateral flow device. Ramos cells were used as a model target cell to demonstrate proof of principle. Under optimal conditions, the ANSB was capable of detecting a minimum of 4000 Ramos cells without instrumentation (visual judgment) and 800 Ramos cells with a portable strip reader within 15 min. Importantly, ANSB has successfully detected Ramos cells in human blood, thus providing a rapid, sensitive, and low-cost quantitative tool for the detection of circulating cancer cells. ANSB therefore shows great promise for in-field and point-of-care cancer diagnosis and therapy.

Coating Time Effect on Surface Structures of Silica-Encapsulated Gold Nanoparticles

This paper reports the silica density, surface structures and optical properties of gold nanoparticles coated with different thickness of silica shells. The gold nanoparticles encapsulated with amorphous silica shells were prepared in a slight modification of Stǒber method. The silica-shell thickness could be varied from 20 to 50 nm by controlling the experimental conditions, such as reaction time. Transmission Electron Microscopy (TEM) and UV-Visible absorption spectroscopy were employed to characterize the size, shell density, surface structures and the optical properties of these silica-coated gold nanoparticles. The TEM images demonstrated that the density of the silica shell were depended on the reaction time, and the surface morphology was changed from porous structures in the initial coating to the final continuous and smooth silica surface. With the increasing of the reaction time, the silica-coated gold nanoparticles became more and more round and monodispersed. UV-Vis spectra showed that surface plasmon absorption peak had a red-shifted of 3~12 nm on increasing the thickness of silica shell from 20 to 50 nm. A possible mechanism of silica formation on gold nanoparticles was proposed on the basis of silica shell density and the shift of absorption peak of coated gold nanoparticles.

Gold Nanoparticles: Past, Present, and Future

This perspective reviews recent developments in the synthesis, electrochemistry, and optical properties of gold nanoparticles, with emphasis on papers initiating the developments and with an eye to their consequences. Key aspects of Au nanoparticle synthesis have included the two-phase synthesis of thiolated nanoparticles, the sequestration and reduction of Au salts within dendrimers, the controlled growth of larger particles of well-defined shapes via the seeded approach, and the assembling of a variety of nanoparticle networks and nanostructures. The electrochemistry of thiolated Au nanoparticles is systemized as regions of bulk-continuum voltammetry, voltammetry reflective of quantized double-layer charging, and molecule-like voltammetry reflective of molecular energy gaps. These features are principally determined by the nanoparticle core. Interesting multielectron Au nanoparticle voltammetry is observed when the thiolate ligand shell has been decorated with redox groupings. Another development is that Au nanoparticles were discovered to exhibit unanticipated properties as heterogeneous catalysts, starting with the low-temperature oxidation of CO. Substantial progress has also been made in understanding the surface plasmon spectroscopy of Au nanoparticles and nanorods. The need to investigate the optical properties of metal particles of a single, well-defined shape and size has motivated the development of a number of new techniques, leading to the study of electron transfer and redox catalysis on single nanoparticles.

Self-Assembled Two-Dimensional Array of Gold Nanoparticles with Different Size for the Sensing Application

Self-assembled two-dimensional array of gold nanoparticles (GNPs) on the glass substrate was systematically investigated in terms of glass cleaning, K2CO3 addition, GNP size, and pH of gold colloids. An ambient-air plasma treatment produced a highly-activated glass surface with the lowest air/water contact angles and K2CO3 addition is very effective to preserve the optical properties of gold nanoparticles for a long time. Small GNPs (≤40 nm) was uniformly arrayed on the amine-functionalized glass through the optimization process of electrostatic attractions between positively-charged glass and negatively-charged gold nanoparticles. For large GNPs (≥50 nm) that resulted in discrete (or loosely-packed) array on the glass substrate, pH adjustment of gold colloids (from pH 11 to 9) produced more densely-packed array of GNPs with less void areas, probably due to the reduction of electrostatic repulsion forces between large gold nanoparticles.

Biospecific protein immobilization for rapid analysis of weak protein interactions using self‐interaction nanoparticle spectroscopy

"Reversible" protein interactions govern diverse biological behavior ranging from intracellular transport and toxic protein aggregation to protein crystallization and inactivation of protein therapeutics. Much less is known about weak protein interactions than their stronger counterparts since they are difficult to characterize, especially in a parallel format (in contrast to a sequential format) necessary for high-throughput screening. We have recently introduced a highly efficient approach of characterizing protein self-association, namely self-interaction nanoparticle spectroscopy (SINS; Tessier et al., 2008; J Am Chem Soc 130:3106-3112). This approach exploits the separation-dependent optical properties of gold nanoparticles to detect weak self-interactions between proteins immobilized on nanoparticles. A limitation of our previous work is that differences in the sequence and structure of proteins can lead to significant differences in their affinity to adsorb to nanoparticle surfaces, which complicates analysis of the corresponding protein self-association behavior. In this work we demonstrate a highly specific approach for coating nanoparticles with proteins using biotin-avidin interactions to generate protein-nanoparticle conjugates that report protein self-interactions through changes in their optical properties. Using lysozyme as a model protein that is refractory to characterization by conventional SINS, we demonstrate that surface Plasmon wavelengths for gold-avidin-lysozyme conjugates over a range of solution conditions (i.e., pH and ionic strength) are well correlated with lysozyme osmotic second virial coefficient measurements. Since SINS requires orders of magnitude less protein and time than conventional methods (e.g., static light scattering), we envision this approach will find application in large screens of protein self-association aimed at either preventing (e.g., protein aggregation) or promoting (e.g., protein crystallization) these interactions.

Disposable Nucleic Acid Biosensors Based on Gold Nanoparticle Probes and Lateral Flow Strip

In this article, we describe a disposable nucleic acid biosensor (DNAB) for low-cost and sensitive detection of nucleic acid samples in 15 min. Combining the unique optical properties of gold nanoparticles (Au-NP) and the high efficiency of chromatographic separation, sandwich-type DNA hybridization reactions were realized on the lateral flow strips, which avoid multiple incubation, separation, and washing steps in the conventional nucleic acid biosensors. The captured Au-NP probes on the test zone and control zone of the biosensor produced the characteristic red bands, enabling visual detection of nucleic acid samples without instrumentation. The quantitative detection was performed by reading the intensities of the produced red bands with a portable strip reader. The parameters (e.g., the concentration of reporter probe, the size of Au-NP, the amount of Au-NP-DNA probe, lateral flow membranes, and the concentration of running buffer) that govern the sensitivity and reproducibility of the sensor were optimized. The response of the optimized device is highly linear over the range of 1-100 nM target DNA, and the limit of detection is estimated to be 0.5 nM in association with a 15 min assay time. The sensitivity of the biosensor was further enhanced by using horseradish peroxidase (HRP)-Au-NP dual labels which ensure a quite low detection limit of 50 pM. The DNAB has been applied for the detection of human genomic DNA directly with a detection limit of 2.5 microg/mL (1.25 fM) by adopting well-designed DNA probes. The new nucleic acid biosensor thus provides a rapid, sensitive, low cost, and quantitative tool for the detection of nucleic acid samples. It shows great promise for in-field and point-of-care diagnosis of genetic diseases and detection of infectious agents or warning against biowarfare agents.

Molecular beacon-functionalized gold nanoparticles as probes in dry-reagent strip biosensor for DNA analysis

The highly specific molecule recognition properties of molecular beacons (MB) are combined with the unique optical properties of gold nanoparticles (Au-NPs) for the development of a dry-reagent strip-type nucleic acid biosensor (DSNAB) that enables sensitive and low-cost detection of nucleic acid samples within 15 min.

Nonlinear optical properties of gold nanoparticles dispersed in different optically transparent matrices

The nonlinear optical properties of gold nanoparticles dispersed in optically transparent matrices Al2O3, ZnO, and SiO2 are investigated using the classical and off-axis techniques of the Z-scan method at a wavelength of 532 nm (radiation from a nanosecond Nd: YAG laser). The experimental data on the nonlinear refraction in composite materials are obtained. The nonlinear refractive indices and the light absorption coefficients are determined, and the real and imaginary parts of the third-order nonlinear susceptibility for the structures under investigation are calculated. It is demonstrated that, for the composite materials under consideration, the nonlinear properties of the medium under the chosen conditions of laser irradiation are predominantly determined by the Kerr effect and the contribution of this effect exceeds the contribution of the thermal lens.

Sensitive and selective detection of cysteine using gold nanoparticles as colorimetric probes

We report herein the development of a highly sensitive and selective colorimetric detection method for cysteine using gold nanoparticles probes. This assay relies upon the distance-dependent optical properties of gold nanoparticles, the self-assembly of cysteine on gold nanoparticles, and the interaction of a 2:1 cysteine/Cu2+ complex. In the presence of Cu2+, cysteine could rapidly induce the aggregation of gold nanoparticles, thereby resulting in red-to-blue (or purple) color change. The concentration of cysteine can be determined by monitoring with the naked eye or a UV-vis spectrometer. The present limit of detection for cysteine is 10 nM. This method exhibits excellent selectivity for cysteine over other alpha-amino acids, glutathione, thioglycolic acid and mercaptoethyl alcohol.

Aptamer-Functionalized Gold Nanoparticles as Probes in a Dry-Reagent Strip Biosensor for Protein Analysis

The highly specific molecular recognition properties of aptamers are combined with the unique optical properties of gold nanoparticles for the development of a dry-reagent strip biosensor that enables qualitative (visual)/quantitative detection of protein within minutes. A model system comprising thrombin as an analyte and a pair of aptamer probes is used to demonstrate the proof-of-concept on the conventional lateral flow test strip. The assay avoids the multiple incubation and washing steps performed in most current aptamer-based protein analyses. Although qualitative tests are realized by observing the color change of the test zone, quantitative data are obtained by recording the optical responses of the test zone with a portable "strip reader". The response of the biosensor is linear over the range of 5-100 nM of thrombin with a detection limit of 2.5 nM (S/N = 3). By comparing the analytical performances of the aptamer-based strip biosensor with the antibody-based strip sensor, we can demonstrate that aptamers are equivalent or superior to antibodies in terms of specificity and sensitivity, respectively. The sensor was used successfully for detection of thrombin in human plasma samples. It shows great promise for use of aptamer-functionalized gold nanoparticle probes in dry-reagent strip biosensors for point-of-care or in-field detection of proteins.