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N A N O P R O B E S     E - N E W S

Vol. 9, No. 12          December 31, 2008


Updated: December 31, 2008

In this Issue:

This monthly newsletter is to inform you about techniques to improve your immunogold labeling, highlight interesting articles and novel applications of metal nanoparticles, and answer your questions. We hope you enjoy it and find it useful, as always, let us know if we can improve anything.

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Double Labeling for EM: Silver-Enhanced Nanogold® and DAB

Immunoenzymatic labeling is a viable electron microscopic labeling method, although the contrast of the deposited DAB is sometimes less than ideal. DAB generally produces a relatively diffuse and continuous stain of medium to low contrast. This is easily differentiated from the particulate staining pattern produced by gold or silver-enhanced gold, and therefore the combination of enzymatic labeling with gold labeling provides a method for double labeling which has been used to generate significant results for a number of publications.

The basal ganglia are a group of subcortical nuclei that participate in a number of functions, including motor, cognitive and mnemonic behaviors. Interactions between glutamatergic corticostriatal afferents and dopaminergic nigrostriatal afferents are central to basal ganglia function. The thalamostriatal projection provides a glutamatergic innervation of similar magnitude to the corticostriatal projection, and in their paper in the Journal of Neuroscience, Moss and Bolam use double labeling with silver-enhanced Nanogold and avidin-biotin complex (ABC) peroxidase labeling as part of a study to test the hypotheses that thalamostriatal synapses have similar spatial relationships with dopaminergic axons as corticostriatal synapses do, and that the spatial relationships between excitatory synapses and dopaminergic axons are selective associations.

[Silver-enhanced Nanogold and enzymatic double labeling (186k)]

Left: Sequential double labeling, showing (b) labeling of first target with gold and silver enhancement, followed by (c) labeling of second target using peroxidase developed with DAB. Right: Double labeling with silver-enhanced Nanogold and immunoperoxidase. Subcellular distribution of m4R immunoreactivity in striatal cholinergic neurons of control rats (A) or rats treated with oxotremorine (B). Cholinergic neurons were identified by immunoreactivity for ChAT, detected by the immunoperoxidase method: m4R reactivity was detected by Nanogold pre-embedding labeling with HQ silver enhancement. The ChAT immunoreaction product was visible throughout the cytoplasm of the perikarya. (A) The neuron immunopositive for ChAT and m4R has a large volume of cytoplasm, typical of a striatal interneuron, with immunoparticles at the external surface of the ER (arrows), Golgi apparatus (G), and external nuclear membrane ( flat arrow). Very few are associated with the plasma membrane (arrowheads). (B) After treatment with oxotremorine, most immunoparticles are clearly associated with endoplasmic reticulum lamina (arrows). Scale bars, 1 mm.

Rat striatum was immunolabeled to reveal vesicular glutamate transporters (VGluTs) 1 and 2 - markers of corticostriatal and thalamostriatal terminals respectively - together with tyrosine hydroxylase (TH) to reveal dopaminergic axons. 65 µm sagittal brain sections were washed five times in phosphate-buffered saline (PBS) and placed in cryoprotectant (0.05 M phosphate buffer, 25% sucrose, 10% glycerol) for a minimum of 2 hours before freeze-thawing. Each section was immersed in chilled isopentane, then liquid nitrogen for a few seconds, thawed at ~25°C for ~5 minutes, then washed three times in PBS. The sections were then double-immunolabeled to reveal VGluT1 or VGluT2 (markers of cortical and thalamic terminals, respectively) and tyrosine hydroxylase (TH; this is the rate limiting enzyme in the synthesis of catecholamines) as a marker of dopaminergic axons and terminals. In contrast with previous accounts, this time the immunogold-silver staining was completed before peroxidase labeling.

Although TH immunolabeling will also reveal noradrenergic axons, these are rare in the striatum. Normal goat serum in PBS (NGS-PBS; Vector Laboratories) was used to block (10% NGS) and wash (2% NGS) sections before incubation with primary antibodies. Primary antibodies for each of the double immunolabeling experiments were added sequentially. Antibodies against VGluT1 or VGluT2 raised in rabbits were used at a dilution of 1:2000 in 2% NGS-PBS; sections were incubated overnight (15 hours), shaking at room temperature or over three nights (64 h) at 4°C. They were then washed once in 2% NGS-PBS, three times in PBS and once in 1% NGS in PBS. The sections were then incubated for 2 hours in Nanogold goat anti-rabbit antibody at a dilution of 1:100 in 1% NGS-PBS. This was followed by one wash in 1% NGS-PBS, then three washes each in PBS and acetate buffer (0.1 M sodium acetate 3-hydrate) in preparation for silver enhancement. HQ Silver was added to each section and allowed to react for 24 minutes, washed three times each in acetate buffer and PBS, and once in 2% NGS-PBS.

Sections were then incubated in a mouse monoclonal antibody raised against TH (1:1000 dilution of in 2% NGS-PBS) overnight, shaking at room temperature. They were washed once 2% NGS-PBS, three times in PBS and once in 1% NGS-PBS, then incubated with biotin-conjugated goat anti-mouse secondary antibody with shaking at room temperature, for a minimum of 2 hours before one wash in 1% NGS-PBS and three washes in PBS. They were then incubated in avidin-biotin-peroxidase complex (ABC; Vector Laboratories), with shaking for 90 min at ~25°C. After the ABC incubation, sections were washed three times in PBS and twice in Tris buffer (0.05 M, pH 7.4). Diaminobenzidine (DAB; 2.5 mL of 0.025% in Tris buffer) was added to the sections with regular mixing for 15 minutes then 40 µL of H2O2 (0.03% in H2O) was added. The reaction continued for 57 minutes until staining was revealed and the reaction was stopped by the addition of Tris buffer; then, sections were washed twice in Tris buffer and three times in PBS, then into 0.1 M phosphate buffer (PB; pH 7.4).

The primary antibodies were raised against rat VGluT1, VGluT2 and TH (amino acids 543560, 510582 and 40152, respectively). The distribution of immunolabeling at the light and electron microscopic levels was distinct for each primary antibody, and consistent with previous observations. No immunolabeling was observed after omission of the primary antibodies and omission of each of the secondary antibodies individually showed no evidence of cross-reactivity. The sections were then placed flat on the bottom of glass Petri dishes and postfixed in osmium tetroxide (1% in PB; Oxkem) for 7 minutes. They were then washed in 0.1 M PB and dehydrated in an ascending series of ethanol dilutions (15 minutes in 50% ethanol, 35 minutes in 70% ethanol which included 1% uranyl acetate; TAAB; 15 min in 95% ethanol, and twice 15 minutes in absolute ethanol). Then, sections were exposed to two changes of propylene oxide (Sigma) for 15 minutes and lifted, using modified forceps, into resin (Durcupan ACM, Fluka) in crafted foil boats and left overnight (15 hours) at room temperature. The resin was then warmed to decrease its viscosity and sections were placed on microscope slides, a coverslip applied and the resin cured at 65°C for ~70 hours.

For electron microscopic analysis, all sections were first examined in the light microscope and areas from the dorsolateral striatum were cut from the slide, glued to the top of a resin block and trimmed with razor blades. Serial 50 nm sections (silver/gray) were then cut using an ultramicrotome, collected on pioloform-coated, single-slot copper grids, then lead stained to improve contrast for electron microscopic examination.

Over 80% of VGluT-positive synapses were within 1 µm of a TH-positive axon and ~40% were within 1 µm of a TH-positive synapse. Of structures postsynaptic to VGluT1- or VGluT2-positive terminals, 21 and 27%, respectively, were apposed by a TH-positive axon and about half of these made synaptic contact. When structures postsynaptic to VGluT-positive terminals and VGluT-positive terminals themselves were normalized for length of plasma membrane, the probability of them being apposed by, or in synaptic contact with, a TH-positive axon was similar to that of randomly selected structures. Extrapolation of the experimental data to more closely reflect the distribution in 3D reveals that all structures in the striatum are within 1 µm of a TH-positive synapse. Therefore, it was concluded that thalamostriatal synapses are in a position to be influenced by released dopamine to a similar degree to corticostriatal synapses, and these associations arise from a nonselective dopaminergic axon lattice.

References:

  • Moss, J., and Bolam J. P.: A dopaminergic axon lattice in the striatum and its relationship with cortical and thalamic terminals. J. Neurosci., 28, 11221-11230 (2008).

  • Bernard, V.; Levey, A. I., and Bloch, B.: Regulation of the subcellular distribution of m4 muscarinic acetylcholine receptors in striatal neurons in vivo by the cholinergic environment: evidence for regulation of cell surface receptors by endogenous and exogenous stimulation. J. Neurosci., 19, 10237-10249 (1999).

More information:

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Nanogold® Labeling on Surfaces

In addition to their wide application to labeling biomolecules, all of our Nanogold® reagents can be used to label functional groups on surfaces of solids. They can be used to map the distribution of target functional groups on surfaces, or with biomolecular templates for the preparation of nanopatterned arrays. When Nanogold labeling is combined with silver enhancement or gold enhancement it provides a method for the preparation of silver or gold mirror surfaces. Provided the reactive groups on the surface are unobstructed, the Nanogold reagents will readily react and bind to them.

Although labeling surface features simplifies separation, since the unreacted Nanogold may be removed by washing, the interaction of Nanogold with surfaces can be different to that with biomolecules. In particular, non-specific binding needs to be carefully monitored. All our Nanogold reagents are formulated to be water-soluble, and therefore may mostly be removed by aqueous buffers. However, they do still retain an element of hydrophobic character, and this can result in non-specific interactions with some surfaces. If you find this to be a problem after thorough washing, we recommend the following:

  • Organic solvent: on surfaces, the issue of biomolecule denaturation is less likely to be a problem, so you may have more freedom to test organic solvents. First try 50% isopropanol-water, then 100% isopropanol if necessary. If isopropanol does not work, dimethylsulfoxide (DMSO) is an excellent solvent for Nanogold, and should be highly effective in removing it from surfaces. If you are labeling a surface with a biomolecule component, note that you should not use a higher proportion of DMSO than your protein can tolerate: use a DMSO-water mixture with the highest allowable DMSO content. You may need to leave the DMSO in contact with the surface for a few minutes to dissolve any tightly-bound gold.

    Other organic solvents that may be useful are acetonitrile, dimethylformamide (DMF), or N,N-dimethylacetamide (DMA). Mixtures of alcohol (methanol or ethanol) with dichloromethane or with trichloromethane (chloroform) may be useful on surfaces with a more ionic character, such as silica or alumina-based surfaces.

  • Triethylammonium bicarbonate is another excellent solvent for Nanogold. It is prepared by bubbling carbon dioxide through a mixture of degassed triethylamine and degassed water; as the carbon dioxide dissolves and the mixture hydrates, the weakly ionic solution is formed. You should prepare a 0.6 M solution in 20% isopropanol; make a more concentrated solution in water first, then dilute in water / isopropanol as appropriate. More details are given in the following reference:

    Safer, D.; Bolinger, L., and Leigh, J. S.: Undecagold clusters for site-specific labeling of biological macromolecules: simplified preparation and model applications. J. Inorg. Biochem., 26, 77-91 (1986).

If the gold is undergoing chemical changes after adhering to the surface (such as forming larger particles) you may need to protect the surface first (using a surfactant or other protective molecule). Formation of larger gold particles is evidenced by a color change of the surface to red or purple, or the formation of a gray or black precipitate.

More information:

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AuroVist: More Contrast for X-Ray Imaging

We have recently introduced AuroVist, the first gold nanoparticle X-ray contrast agent for micro CT and CT imaging in research applications. With AuroVist, contrast enhancement up to ten times that of iodine reagents is possible. You can obtain high-resolution, high-contrast images of blood vessels, organs, other anatomical structures and tumors in animals. AuroVist is highly soluble, biocompatible, and stable to the environment found in the vascular system and in tissues.

AuroVist is a stabilized 1.9 nm gold particle. It provides better contrast than iodine for both micro-CT and clinical CT applications.* At appropriate beam energies, gold achieves a contrast up to three times greater than iodine per unit mass, yielding initial blood contrast greater than 500 Hounsfeld Units (HU). Gold concentrations up to four times those of iodine can be achieved, providing a total contrast gain of up to ten times or more. In addition, AuroVist gives you these enhanced performance features:

  • Longer blood residence time than iodine agents, due to its larger size (1.9 nm gold core, ~50,000 Da).

  • High contrast (>500 HU initial in blood, kidneys >1,500 HU).

  • Clears through kidneys. Kidney fine structure may be imaged up to an hour or more after injection; concentration in the kidneys can provide contrast values as high as 1,500 HU or more.

  • Permeates angiogenic endothelium, enabling imaging of tumors.

  • Concentration >4 times that of standard iodine agents (up to 1.5 g Au/cc).

  • Can be imaged using standard microCT.
  • Low toxicity (LD50 >1.4 g Au/kg).

  • Up to 10 times the contrast of standard iodine agents (Gold absorbs ~3 times more than an equivalent weight of iodine at 20 and 100 keV and can be ~4 times more concentrated, giving more than 10-fold combined contrast enhancement.

  • Low osmolality, even at high concentrations

  • Low viscosity, similar to water; easy to inject, even into small vessels.

  • Yields enhanced radiotherapy dose.

AuroVist mouse, kidney and inferior vena cava images [(104k)]

(Upper left): Live mouse, 5 minutes after injection. (Upper right) Imaging of kidney in live mouse 1 hour after injection, showing kidney contrast and fine structure (bar = 1 mm). (Lower left): Live mouse, 2 minutes after injection showing vascular fine structure (bar = 5 mm); (Lower right) MicroCT of mouse inferior vena cava (bar = 1 mm).

* Research use only. Not approved for clinical or human use.

We have received several inquiries about whether a targeted version of this reagent is available. The preparation of a targeted gold nanoparticle reagent on a sufficient scale for X-ray contrast imaging is challenging both because of the greater complexity of the synthesis, and also because the amount of reagent required for visualization depends on the size and target density of the feature to be imaged, and how effectively the targeting mechanism can deliver a visible dose to the target. However, we are working to develop this technology, and hope to incorporate it into future AuroVist products.

AuroVist is a new product, and therefore it has not been optimized in all possible applications. However, the following guidelines may be helpful in obtaining the best results with this reagent.

Best instrument and beam settings

The absorption increases by a significant factor (jump ratio) above the gold L and K edges. The X-ray properties of gold, showing the jump ratios for these regions, are shown in the table below. It is therefore advantageous to image using these absorptions. The settings below are appropriate for the different instruments.

This must be compared this with the absorption spectrum of soft tissue. X-ray absorption spectra for elements and tissues are available from NIST:

X-ray absorption spectra

  Energy (keV) µ/p(cm2/g) jump ratio
  11.8 75.8  
L3 11.9 187.0 2.5
  13.6 128.3  
L2 13.7 176.4 1.4
  14.3 158.8  
L1 14.4 183.0 1.2
  80.6 2.1  
K 80.7 8.9 4.2
  • Mammography: These instruments are suitable for small animal imaging. Use of lower kVp, at or below the L-edge absorptions of gold at ~13 keV, or ~40 kVp (e.g., 22 kVp) is recommended to take advantage of the L edge gold absorptions. Exposures are typically 1 second or less for a mouse, so live imaging is possible. Resolution can be < 0.1 mm.

  • Clinical CT: 80 kVp gives the greatest attenuation, but higher voltages, particularly with filtering can make use of the Au K edge; beam energy can be tuned to just above gold's 80.7-keV K-edge. Imaging time is typically a few seconds, with resolution ~0.3 mm.

  • MicroCT: Here the resolution is increased (to even 2 microns), but the tube power is typically ~100 times less than a clinical unit. Fine area 2D detectors mean that many tiny pixels must each receive enough counts. This then requires a much longer imaging time (e.g., 0.5 - 2 hours) than clinical CT. Many units also slow the tube rotation such that only 1 revolution is done in the selected imaging time (e.g., 1 hour). Animal movement must be minimized during this time. One solution is to sacrifice the animal, but live imaging has been accomplished if the region can be gated or immobilized during the imaging time. Beam energy should be just above golds 12-14-keV L-edge. To enhance detection, or give the best absorptive contrast, the ideal settings are those that overlap the L-edge absorptions of gold at ~13 keV; this requires a kVp of about 3 times this or ~40 kVp - so the first choice for kVp setting would be 40 kVp, or as close as the instrument allows.

How to ensure minimal toxicity

Certain strains of mice appear to be more tolerant of this gold. For Balb/C, the LD50 is ~ 3.2 g Au/kg. Nude mice and C3H mice also seem to respond about the same. Some outbred mice, however, appear to have a lower LD50 of about > 1.4 g Au/kg. If you are using outbred mice, or a different strain to those mentioned above, it is recommended that you use this lower value. The following suggestions may be helpful:

  • Start with a moderate dose, such as 120 or 160 mg/mL. The value of 270 mg/mL reported in our paper in the British Journal of Radiology was the highest value tested; at high concentrations acute toxicities increase rapidly, and a modest reduction in dose can significantly reduce toxicity without compromising your results.

  • Centrifuge and then filter the reconstituted reagent through a sterile filter immediately before injection to ensure that no aggregates or larger particles are present. Larger particles and aggregates can impact biocompatibility, and may also have reduced stability and hence a higher tendency to deposit in tissues and organs, with negative consequences.

  • Use one of the pure-bred mouse strains mentioned above, which are known to have higher tolerance. If you are planning to conduct multiple experiments using a different strain, it may be worthwhile to test your strain first.
We encourage you to tell us how well it works in your application, or if you encounter problems; that way, you can help contribute to the knowledge base on this reagent and its applications.

References:

  • Hainfeld, J. F.; Slatkin, D. N.; Focella, T. M, and Smilowitz, H. M.: Gold nanoparticles: a new X-ray contrast agent. Br. J. Radiol., 79, 248-253 (2006).

  • Hainfeld, J. F.; Slatkin, D. N.; Focella, T. M., and Smilowitz, H. M.: In Vivo Vascular Casting. Microsc. Microanal., 11, (Suppl. 2: Proceedings); Price, R.; Kotula, P.; Marko, M.; Scott, J. H.; Vander Voort, G. F.; Nanilova, E.; Mah Lee Ng, M.; Smith, K.; Griffin, P.; Smith, P., and McKernan, S., Eds.; Cambridge University Press, New York, NY, p. 1216CD (2005).

  • Hainfeld, J. F., Slatkin, D. N., and Smilowitz, H. M.: The use of gold nanoparticles to enhance radiotherapy in mice. Phys. Med. Biol., 49, N309-N315 (2004).

More information:

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Nanogold® and GoldEnhance Track Cell Migration

Gold enhancement is an alternative to silver enhancement, developed by Nanoprobes. With gold enhancement, gold nanoparticles, gold - instead of silver - is deposited onto colloidal gold or gold cluster labels. This catalytic enlargement and enhancement process produces enlarged particles for electron microscopic observation and dark staining for light microscopy and blotting.

GoldEnhance: how it works [(37k)]

Enhancement of Nanogold® by GoldEnhance: mechanism. Final particle size is controlled by enhancement time.

Gold enhancement has important advantages over silver enhancement for several applications:

  • Cleaner signals with lower background: for light microscopy and blotting, GoldEnhance offers both high sensitivity and improved signal clarity, with lower background, than silver enhancement.

  • Osmium etch resistance for EM: gold enhancement may safely be used before any strength osmium tetroxide - gold enhanced particles are not etched as silver can be.

  • Physiological compatibility: Gold enhancement may be used in physiological buffers such as phosphate-buffered saline without the risk of precipitation (halides, such as chloride, precipitate silver).

  • Metal substrate compatibility: gold enhancement may be conducted in the presence of metals (such as metallic substrates for cell culture or biomaterials).

  • pH compatibility: the metallographic reaction is less pH sensitive than that of silver.

  • Better SEM Visualization: gold gives a much stronger backscatter signal than silver, making it better for SEM labeling applications.

  • Mild and easy to use: GoldEnhance is near neutral pH for best ultrastructural preservation, and has low viscosity so the components may be dispensed and mixed easily and accurately.

Gold enhancement has been used for in situ hybridization, double labeling by sequential enhancement of Nanogold®, and for electron microscopic visualization and tracking of cellular uptake of gold nanoparticles for drug delivery; it also provides cleaner, more sensitive detection than silver enhancement on western blots and dot blots. In their recent paper in the Journal of Cell Biology, Kriebel and co-workers show that it remains a highly effective tool for electron microscopy, using it show the cellular distribution of adenylyl cyclase (ACA) and its role in cell mobility.

Chemoattractant signaling induces the polarization and directed movement of cells, secondary to the activation of multiple effector pathways. Chemotactic signals can also be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms of these processes remain ill-defined. The ability of Dictyostelium cells to spontaneously aggregate and stream relies on the presence of a finely regulated signal relay loop that is centered on cAMP. In this organism, the detection, synthesis, and degradation of cAMP are highly regulated. Addition of chemoattractants leads to a burst in the activity of adenylyl cyclase (ACA).

Immunogold EM staining was used to localize a YFP-ACA fusion protein, thus enabling combined fluorescent and immunogold labeling of the target sites. Cells were differentiated and prepared for microscopy, and fixed at room temperature for 15 minutes in 1% formaldehyde, 0.1% glutaraldehyde, and 0.01% digitonin in 15 mM Pipes/1 mM EGTA, followed by 15 minutes in 1% formaldehyde. After blocking in 50 mM NH4Cl, 0.1% digitonin, and 1% BSA in PBS, the cells were incubated with an anti-GFP antibody (1:500; Abcam) in block solution overnight at 4°C followed by Nanogold anti-rabbit Fab fragments. Staining was enhanced for 7 minutes with GoldEnhance (Nanoprobes) made with one part component A, one part component B, one part component C, and three parts PBS. Afterward, the samples were stained with osmium and embedded in epon. Images were taken with a transmission microscope.

An asymmetrical distribution of adenylyl cyclase (ACA) was found at the back of Dictyostelium discoideum cells. This is an essential determinant of their ability to migrate in a head-to-tail fashion, and requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. The results also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. The authors propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis.

Reference:

  • Kriebel, P. W.; Barr, V. A.; Rericha, E. C.; Zhang, G., and Parent, C. A.: Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge. J. Cell Biol., 183, 949-961 (2008).

More information:

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Nanoprobes: New Year Hours - and Order Now at 2008 Prices

Nanoprobes will be closed on New Years Day, January 1, 2009, and also for Martin Luther King Day, Monday January 19. Since key staff may be taking additional time off during the Holidays, you should make sure to use the correct contact information and make sure your message reaches the right people to avoid delays in our response.

Unfortunately, our costs have increased over the last few months, and as a result, we will need to raise our prices slightly. Therefore, if you plan to order our products, you can avoid the increase by placing your orders immediately.

More information:

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Other Recent Publications

Quite apart from our combined fluorescent and gold probe, FluoroNanogold, we are occasionally asked whether gold nanoparticles can themselves fluoresce. With Nanogold®, the fact that it effectively quenches other fluorophores makes us rather doubt it. However, other researchers have found that small gold nanoparticles, prepared in an appropriate matrix, can indeed fluoresce, and have even been proposed as potentially improved quantum dots. The brightest fluorescence is obtained when gold particles are prepared within polyamidoamine (PAMAM) dendrimers, branched-chain polymers with precise molecular weight and molecular geometry. The fluorescence emission wavelength is determined by the size of the gold particles, and includes UV- (Au5), green- (Au13), red- (Au23), and near-IR- (Au31) emitting species. Although very small - possibly too small to be readily visualized by TEM - these particles provide an alternative method for visualizing targets by both fluorescence and electron microscopy in a single labeling procedure, using a particle which is considerably smaller than conventional semiconductor quantum dots.

References:

  • Zheng, J.; Zhang, C., and Dickson, R. M.: Highly fluorescent, water-soluble, size-tunable gold quantum dots. Phys. Rev. Lett., 93, 077402 (2004).

  • Zheng, J.; Nicovich, P. R., and Dickson, R. M.: Highly fluorescent noble-metal quantum dots. Annu. Rev. Phys. Chem., 58, 409-431 (2007).

Further insight into the effect of gold nanoparticles on fluorescence was provided by Reil and co-workers in the current Nano Letters. They investigated the enhancement of FRET (fluorescence resonance energy transfer) by metal nanoparticles both theoretically, and experimentally, using gold nanodisks prepared by electron-beam lithography to enhance FRET between a europium3+ complex (tris(dibenzoylmethane) mono(1,10-phenanthroline)europium(III)) donor and and Cy5 dye (1,1?,3,3,3?,3?-hexamethyl-indodicarbocyanine iodide) acceptor. By tuning the plasmonic resonance wavelength of the metal nanoparticles close to the emission wavelengths of the donor, the nanoparticle transforms part of the near-field energy of the donor into light, at the expense of the Förster transfer rate and other nonradiative processes. In addition, the emission intensity of the acceptor is strongly enhanced when the MNPs resonance wavelength approaches the acceptors emission wavelength: an enhancement by up to a factor of seven was found experimentally.

Reference:

  • Reil, F.; Hohenester, U.; Krenn, J. R., and Leitner, A.: Forster-Type Resonant Energy Transfer Influenced by Metal Nanoparticles. Nano Letters, 8, 4128-4133 (2008).

In the same issue of Nano Letters, Bigall and co-workers presented a facile, reproducible method for synthesizing monodisperse platinum (Pt) nanoparticles with sizes ranging from 10 to 100 nm in diameter. These particles were highly monodisperse, with standard deviations of 3% for the larger sizes. The reaction takes place in aqueous solution using a multistep, seed-mediated approach. Seeds were prepared by reduction of chloroplatinic acid with sodium citrate solution, followed quickly by sodium borohydride. Larger particles were prepared by addition of these seeds to solutions of hexachloroplatinic acid in sodium citrate / ascorbic acid solution. The Pt nanospheres were found to consist of several small crystallites resulting in a surface roughness of 5-10 nm. Extinction spectra are measured from particles dispersed in water and calculated for single particles which are found to be in excellent agreement. A linear correlation was obtained between the plasmon extinction maximum (from UV to the visible regions) and the particle diameter, providing a potentially useful tool for identification and monitoring of such nanoparticles.

Reference:

  • Bigall, N. C.; Härtling, T.; Klose, M.; Simon, P.; Eng, L. M., and Eychmüller, A.: Monodisperse Platinum Nanospheres with Adjustable Diameters from 10 to 100 nm: Synthesis and Distinct Optical Properties. Nano Letters, 8, 4588-4592 (2008).

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