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N A N O P R O B E S     E - N E W S

Vol. 10, No. 2          February 28, 2009


Updated: February 28, 2009

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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Nanogold® and FluoroNanogold for SEM Labeling

Scanning Electron Microscopy (SEM) is often assumed to have a resolution less than that of transmission electron microscopy (TEM), and this may suggest that smaller gold probes such as Nanogold® are less applicable to this method. In fact, though, Nanogold labeling has been used for SEM labeling even without enhancement; however, should visualization require enhancement, we offer a gold autometallography reagent, GoldEnhance, which provides the advantage of significantly higher backscatter detectability than silver enhancement.

A point resolution limit of 5 to 10 nm is frequently quoted for backscattered electron (BSE) detection, which implies an 'information resolution' limit close to 2 nm. However, significantly higher resolution has been described for SEM imaging of gold particles with backscattered electrons, due to their higher contrast. Hermann and co-workers demonstrated successful labeling of T-even phage with Fab fragments conjugated with 0.8 1 nm colloidal gold, similar in size to undecagold. specimens were frozen by plunging into liquid ethane, freeze-dried, then double axis rotary shadowed with chromium. Gold particles were clearly visualized by backscattered electron (BSE) detection. This group has also showed successful labeling of the reactive cysteines of creatine kinase with the 1.4 nm Nanogold. A requirement for this level of resolution is the diffusion of surface charge: this was achieved by chromium coating.

Elizabeth Schröder-Reiter, Gerhard Wanner and group have contributed several advances to the SEM use of Nanogold and FluoroNanogold. In 2006, they reported the first 3D SEM analysis of a nucleolus organizing region (NOR) with an atypical peg-like terminal constriction on metaphase plant chromosomes. Improvements in signal localization, labeling efficiency, and structural preservation in the SEM ISH procedure allowed 3D SEM analysis of the NOR structure and rDNA distribution for the first time. In a new paper in the Journal of Structural Biology, they describe the use of focused ion beam (FIB) milling in combination with field emission scanning electron microscopy (FESEM) for investigations of metaphase barley chromosomes, and report new insight into the chromatin packaging in the chromosome interior and 3D distribution of histone variants in the centromeric region.

In combination with immunogold labeling, the centromere-correlated distribution of histone variants (phosphorylated histone H3, CENH3) were investigated in three dimensions. Divisions of root-tip cells of sprouting barley seeds (Hordeum vulgare Steffi, 2n = 14) were synchronized, arrested at metaphase, and fixed with 3:1 (v/v) ethanol:acetic acid. Chromosomes were isolated and dropped onto glass slides using the Drop/Cryo method. The chromosome preparations were washed in phosphate buffered saline (PBS) buffer, blocked in 1% bovine serum albumin with 0.1% Tween 20 in PBS for 30 minutes, then incubated with the respective primary antibody (diluted in blocking solution) for 1 hour (1:250 rabbit anti-serine 10 phosphorylated histone H3, or 1:300 rabbit anti-OsCENH3). After washing, specimens were incubated with Nanogold-Fab' Goat anti-Rabbit fragments for 1 hour. Specimens were washed and post-fixed with 2.5% glutardialdehyde in PBS, then silver enhanced using HQ Silver. For DNA staining, chromosomes are incubated for 30 minutes at room temperature with Platinum Blue ([CH3CN]2Pt oligomer, 10 mM, pH 7.2), then washed with distilled water. Whole mount chromosomes were sectioned with FIB with thicknesses in the range of 720 nm, resulting in up to 2000 sections, which allow high resolution three-dimensional reconstruction.

For the first time, the chromosome interior could be shown to be characterized by a network of interconnected cavities, with openings to the chromosome surface. In combination with immunogold labeling, the centromere-correlated distribution of histone variants (phosphorylated histone H3, CENH3) could be investigated with FIB in three dimensions. FIB milling overcomes the limitations of classical SEM analysis of whole mount chromosomes with back-scattered electrons, which require higher accelerating voltages, such as faint and blurred interior signals. From within the chromosome, even very small labels in the range of 10 nm could be precisely visualized, which allowed direct quantification of marker molecules in a three-dimensional context. Distribution of DNA in the chromosome interior could be directly analyzed after staining with a DNA-specific platinorganic compound Platinum Blue. Higher resolution visualization of DNA distribution could be performed by preparation of FIB lamellae with the in situ lift-out technique followed by investigation in dark field with a scanning transmission electron detector (STEM) at 30 kV.

[Combined Fluorescence and SEM Labeling, and Gold Enhancement (76k)]

Left: Scanning electron micrograph of a peg-like terminal constriction of an Oziro biflora (plant, Hyacinthaceae) chromosome. The image shows both chromosome topography (secondary electron signal) and hybridized enhanced gold signals (superimposed back-scattered electron signals, yellow) labeling 45S rDNA in the nucleolus organizing region with Alexa Fluor®* 488 FluoroNanogold-Streptavidin (micrograph courtesy of Elizabeth Schröder-Reiter and Gerhard Wanner). Right: Schematic of the gold enhancement process. In the presence of a reducing agent, gold (I) ions are reduced to metallic gold and deposited onto gold nanoparticles with high selectivity, producing a strong backscatter signal for SEM observation.

GoldEnhance has several advantages for different applications in addition to its enhanced backscatter detection for SEM:

  • Gold enhancement may safely be used before any strength osmium tetroxide - it is not etched.
  • May be used in physiological buffers (including chlorides, which precipitate silver, but not gold).
  • The metallographic reaction is less pH sensitive than that of silver.
  • Gold gives a much stronger backscatter signal than silver.
  • GoldEnhance is near neutral pH for best ultrastructural preservation.
  • Low viscosity, so the components may be dispensed and mixed easily and accurately.

Together with Nanogold and FluoroNanogold, these reagents provide a set of research tools for many difficult structural and microscopical problems.

References:

  • Hermann, R.; Walther, P., and Muller, M.: High precision immuno-scanning electron microscopy using Fab fragments coupled to ultra-small colloidal gold. J. Struct. Biol., 107, 38-47 (1991).

  • Hermann, R.; Walther, P., and Muller, M.: Immunogold labeling in scanning electron microscopy. Histochem. Cell Biol., 106, 31-39 (1996).

  • Schröder-Reiter, E.; Pérez-Willard, F.; Zeile, U., and Wanner, G.: Focused ion beam (FIB) combined with high resolution scanning electron microscopy: a promising tool for 3D analysis of chromosome architecture. J. Struct. Biol., 165, 97-106 (2009).

More information:

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Nanogold® Enables Versatile Peptide Labeling

Peptides are much more difficult than antibodies to label with conventional colloidal gold. Because of their small size, they do not adsorb as readily to gold surfaces, and in addition they have a wide variety of physical properties such as charge or solubility that can make reaction impossible or render a conjugate unstable or insoluble. Nanogold® is much better for peptide labeling, because it reacts directly and specifically with functional groups - or with specific amino acid residues - in the peptide, and being non-charged, it does not contribute to charge-based aggregation or precipitation. And because Nanogold is available with different reactivities, you may have a choice of reactive site.

Label at cysteines with Monomaleimido Nanogold®

Monomaleimido Nanogold reacts selectively with thiol (-SH) groups. Use this reagent to label at cysteine residues; alternatively, you can use 2-iminothiolane (Traut's reagent) to convert primary amino- groups to longer-chain thiols if necessary. Thiols are labeled under mild conditions, at pH values between 6 and 7. Typically, Monomaleimido Nanogold is reacted with the peptide overnight at 4°C, sometimes after a brief period (30 minutes to one hour) of gentle agitation, then the conjugate product is isolated next day. If you plan to use this reagent, you should check to make sure that the cysteine is reduced and available for labeling: often, cysteines are oxidized to form disulfide bridges which must be carefully reduced before labeling.

Gregori and co-workers provided an example of cysteine labeling: they labeled a modified tau beta-amyloid peptide containing a cysteine residue with Monomaleimido Nanogold, then used the labeled 39-amino acid peptide to investigate the binding interaction with the 20S proteasome by STEM microscopy. Because of the small size of the labeled peptide, they were able to map binding sites inside the proteasome with high resolution, as shown below.

[STEM of Nanogold-labeled beta-amyloid peptide binding in the 20S proteasome (98k)]

STEM analysis of Nanogold-labeled amyloid beta-peptide binding to the 20S proteasome, showing localization of Nanogold in end (B) and side (C) views (bar = 10 nm)

Reference:

  • Gregori, L., Hainfeld, J. F., Simon, M. N., and Goldgaber, D. Binding of amyloid beta protein to the 20S proteasome. J. Biol. Chem., 272, 58-62 (1997).

Label at lysines or N-terminal amines with Mono-Sulfo-NHS-Nanogold®

Mono-Sulfo-NHS-Nanogold® reacts with primary aliphatic amines: it therefore labels specifically at the N-terminal amine or at a lysine residue in the chain (labeling at the N-terminal is usually slightly preferred, both because it is more accessible and because the N-terminal amine usually has a slightly greater reactivity). This approach is useful because all peptides have at least one reactive site.

Segond von Banchet and Heppelmann used Mono-Sulfo-NHS-Nanogold® to label the bioactive peptide, Substance P (SP); this has only 11 amino acids and a molecular weight of 1,517 Daltons. Careful controls showed that the Nanogold-labeled SP behaved similarly to the commonly used Iodine-125-labeled SP. Specific binding to target proteins on SDS-PAGE blots could be competed off with excess unlabeled SP; binding was also inhibited by a specific NK1-receptor antagonist, but not by neurokinin A, which does not target the NK1 receptor, as shown below. Nanogold alone led to no specific staining.

[Nanogold labeling of Substance P (122k)]

Top: Nanogold-labeled Substance P (SP). Above: SDS-PAGE of proteins of the rat spinal cord. (a) India ink stained showing many proteins; (b) 125I-SP staining, showing binding to 58 and 38 kid target protein bands. (c) 125I-SP and excess SP, showing SP competes for labeled protein binding. (d) SP-Nanogold (silver enhanced) showing binding to 58 and 38 kD bands, similar to 125I-SP (lane b). (e) SP-Nanogold and excess SP, showing that excess SP competes off labeled protein. This shows that the Nanogold- labeled 11-mer peptide Substance P retains its native binding properties.

Reference:

Jones and group have described the preparation and use of Nanogold-labeled alpha-bungarotoxin. The conjugate was prepared by using Mono-Sulfo-NHS-Nanogold to label the toxin; conjugation was carried our using standard procedures given in our product instructions, and the product was purified by gel filtration. In order to compare the relative dimensions of alpha-Bgt and Nanogold, the physical size of alpha-Bgt was calculated to using a modeling package; it was shown to be 4.5 times larger than the gold core of the Nanogold particle. Displacement assays showed that alpha-Bgt binding was preserved in the Nanogold conjugate: Ki values are 7.8 and 4.2 nM for gold conjugated alpha-Bgt and unconjugated alpha-Bgt, respectively.

Reference:

  • Jones; I. W.; Barik, J.; O' Neill, M. J., and Wonnacott, S.: Alpha bungarotoxin-1.4 nm gold: a novel conjugate for visualising the precise subcellular distribution of alpha 7* nicotinic acetylcholine receptors. J. Neurosci. Methods, 134, 65-74 (2004).

Label at C-terminal carboxyls with Monoamino Nanogold®

If the only functional group I have is a carboxylic acid, what should you do? You can still label with Nanogold using Monoamino Nanogold®. To carry out the labeling reaction, you need to convert the carboxylic acid group to a reactive ester, which will then react with the amine - the same reaction that occurs when Mono-Sulfo-NHS-Nanogold is used to label an amine. Appropriate reaction schemes are shown below:

[Carboxylic Acid Nanogold Labeling (13k)]

Reactions for labeling carboxylic acids, using Monoamino Nanogold with (a) EDC (1-Ethyl-3- [3-dimethylaminopropyl] carbodiimide Hydrochloride) / Sulfo-NHS, and (b) 1,1-carbonyl-diimidazole (CDI).

The reaction used in peptide synthesis usually works well - react the carboxylic acid with EDC (1-Ethyl-3- [3-dimethylaminopropyl] carbodiimide Hydrochloride) and Sulfo-NHS to convert it to a reactive Sulfo-N-hydroxysuccinimide ester. You can purchase EDC from a number of sources; its use is described by Pierce.

EDC reacts with a carboxyl group on the molecule to be labeled, forming an amine-reactive O-acylisourea intermediate. This intermediate could then react with Monoamino Nanogold; however, it is also susceptible to hydrolysis, making it unstable and short-lived in aqueous solution. The addition of Sulfo-NHS (5 mM) stabilizes the amine-reactive intermediate by converting it to an amine-reactive Sulfo-NHS ester, thus increasing the efficiency of EDC-mediated coupling reactions. The amine-reactive Sulfo-NHS ester intermediate has sufficient stability to permit two-step cross-linking procedures, which allow the carboxyl groups on one protein to remain unaltered.

Reference:

  • Staros, J. V.; Wright, R. W., and Swingle, D. M.: Enhancement by N-hydroxysulfosuccinimide of water-soluble carbodiimide-mediated coupling reactions. Anal. Biochem., 156, 220-222 (1986).

Another reagent that works well in non-aqueous systems is 1,1-carbonyl-diimidazole (CDI). The molecule to be labeled should be dissolved in a small amount of the organic solvent and a small (5-fold to 10-fold) excess of CDA added; the pH is then raised to 7.5 or higher by the addition of aqueous reaction buffer, and the Monoamino Nanogold added.

Reference:

  • Staab, H. A., and Rohr; W.; Newer Methods Prep. Org. Chem., 5, 61 (1968).

More information:

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Neuropeptide Y, Hippocampus-Dependent Seizures, and Nanogold®

Nanogold® labeling with HQ Silver enhancement provides one of the most effective methods for pre-embedding immunoelectron microscopy, and this has been extensively and successfully used in neuroscience research. Because of their small size, antibody Fab' fragments labeled with Nanogold provide an ideal combination of features for successful labeling, which is particularly important for labeling fine nerve structures at high resolution:

  • High penetration into cells and tissues.
  • High labeling density.
  • Close to quantitative labeling of antigenic sites.
  • High labeling resolution.

Together with HQ Silver, this provides the ideal combination of features to produce high-quality staining:

  • The only commercial silver enhancer to feature a protective colloid for highest size uniformity and morphologically consistent enhancement.
  • Near neutral pH and low ionic strength ensure maximum specimen integrity.
  • High proportion of gold particles are enlarged.
  • Highly selective reaction with low background.

[Nanogold-Fab' vs. colloidal gold-IgG: resolution (61k)]

Resolution advantage: size comparison of Nanogold-Fab' with conventional 5 nm colloidal gold-IgG probe, showing overall probe size and distance of gold from target. Due to its position at the hinge region, Nanogold is positioned closer to the target upon binding, yet does not hinder or interfere with binding.

This month's contribution to this field comes from Ledoux and co-workers: in their paper in the Journal of Neuroscience, they describe the use of Nanogold labeling with silver enhancement to help uncover the mechanism by which neuropeptide Y controls hippocampus-dependent seizures. In about one-third of women with epilepsy, the seizures show a catamenial pattern: they fluctuate with the menstrual cycle. Catamenial seizures occur more frequently when the ratio of circulating estradiol to progesterone is high, which suggests that the convulsions may be promoted by estradiol. The authors used adult female rats to test how estradiol-induced suppression of GABAergic inhibition in the hippocampus affects behavioral seizures induced by kainic acid, which produces highly stereotypical seizures, and light and electron microscopy to monitor the effects upon the amount and distribution of some of the key molecules.

Electron microscopic immunocytochemistry with enzymatic DAB was used in nonseizure animals to localize and quantify neuropeptide Y immunoreactivity (IR) at the ultrastructural level. Rats were deeply anesthetized with sodium pentobarbital and perfused with heparinized saline, followed by 3% paraformaldehyde/0.5% glutaraldehyde in phosphate buffer (PB). Brains were removed, blocked to contain the dorsal hippocampus, and postfixed overnight at 4°C. Blocks were then rinsed and sectioned (50 µm) through the dorsal hippocampus and treated with 1% sodium borohydride and 1% hydrogen peroxide, followed by a blocking solution containing 3% bovine serum albumin (BSA), 0.3% dimethylsulfoxide (DMSO), and 5% normal serum in Tris-buffered saline (TBS) at pH 7.4. After rinsing, sections were incubated in primary antiserum to NPY (rabbit polyclonal, 5.0 µg/mL) in 2% BSA, 0.3% DMSO, and 1% normal serum in TBS at 4µC for 48 hours. Sections were then rinsed and incubated with biotinylated anti-rabbit IgG (1:800) followed by an avidin biotin horseradish peroxidase complex (1:100) in TBS. NPY-IR was visualized with a DAB reaction. Sections were stained with 1% osmium tetroxide, then flat-embedded in Eponate resin and blocked to contain CA1, then mounted for thin sectioning. Short series of ultrathin (~80 nm) sections were cut and collected on formvar-coated slot grids, and stained with 3% uranyl acetate and 2.66% Reynolds lead citrate.

Another group of rats was used for electron microscopic visualization of estrogen receptor-alpha (ERalpha), to investigate whether large dense-core vesicles (LDCVs) contain ERalpha-IR. Tissue preparation was as described above, except that hydrogen peroxide pretreatment was omitted. Sections were incubated in primary antiserum to ERalpha (MC-20 rabbit polyclonal, 0.5 µg/mL) followed by Nanogold anti-Rabbit IgG (1:50 dilution) followed by enhancement with HQ-silver. Controls included omission of primary antiserum, and showed no nonspecific staining. The MC-20 antiserum used for ERalpha labeling does not cross-react with ERbeta, and recognizes a single band of the appropriate size on Western blots from hippocampus; all staining was eliminated by preadsorption with a blocking peptide. Because lots of antisera can vary, each lot of MC-20 was verified by Western blot before use.

As expected, a 24 hour estradiol treatment increases susceptibility to initiation of KA-induced seizures, but it was also observed that estradiol has a mitigating effect, reducing seizure severity once seizures have begun. Additional analyses showed that the decrease in seizure severity was attributable to greater release of the anticonvulsant neuropeptide, neuropeptide Y (NPY). First, blocking hippocampal NPY during seizures eliminated the estradiol-induced decrease in seizure duration. Second, light and electron microscopic studies indicated that estradiol increases the potentially releasable pool of NPY in inhibitory presynaptic boutons and facilitates the release of NPY from inhibitory boutons during seizures. Finally, the presence of estrogen receptor-alpha on large dense-core vesicles (LDCVs) in the hippocampus suggests that estradiol could facilitate neuropeptide release by acting directly on LDCVs themselves. Understanding how estradiol regulates NPY-containing LDCVs could identify potential molecular targets for novel anticonvulsant therapies.

Reference:

  • Ledoux, V. A.; Smejkalova, T.; May, R. M.; Cooke, B. M, Woolley, C. S.: Estradiol facilitates the release of neuropeptide Y to suppress hippocampus-dependent seizures. J. Neurosci., 29, 1457-1468 (2009).

More information:

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Nanogold® Helps Reveal Siliconization Mechanism

The glass sponge Monorhaphis chuni (Porifera: Hexactinellida) forms the largest bio-silica structures on Earth; their giant basal spicules reach sizes of up to 3 m, with diameters of 8.5 mm. While it is known that the thickness growth proceeds by appositional layering of individual lamellae, the mechanism for the longitudinal growth had, until recently, remained unstudied. However, this is rectified in a recent paper from Wang and co-workers in the Journal of Structural Biology, and they used one of the smallest tools available - a Nanogold® label - to do it. As an expansion of their earlier studies mapping siliconization in desmosponges, the authors used immunoelectron microscopy to investigate the role of a silicatein(-related) monomeric protein from spicules of Monorhaphis, with size of approximately 25 kDa, in the siliconization process - in particular, to test the hypothesis that that silicatein(-like) molecules exist in the extracellular space which associate with the external surface of the respective spicule.

Electron immunogold labeling for TEM [transmission electron microscopy] analysis was performed with tissue samples that had been treated in glutaraldehyde/paraformaldehyde. tissue samples treated in 0.1% glutaraldehyde / 3% paraformaldehyde buffered in 0.1 M phosphate buffer at pH 7.4; after 2 h, the material was dehydrated in ethanol and embedded in LR-White Resin. 60 nm slices were cut, and blocked with 5% bovine serum albumin (BSA) in phosphate-buffered saline (PBS), then incubated with the primary antibody PoAb-aSilic (1:1000) for 12 hours at 4°C. For the detection of the antigenantibody complexes, Nanogold anti-Rabbit IgG was applied, and after washing, silver enhancement was conducted using the Danscher procedure. In controls, pre-immune serum was used; in these sections no antigenantibody could be detected.

Microscopic examination showed that the surface of the spicules have, towards the tip, serrated relief structures that are consistent in size and form with the protrusions on the surface of the spicules. These protrusions fit into the collagen net that surrounds the spicules. The widths of the individual lamellae do not show any pronounced size tendency. Apical elongation of the spicule proceeds by piling up cone-like structural units formed from silica. As a support of the assumption that in the extracellular space silicatein(-like) molecules exist that associate with the external surface of the respective spicule immunogold electron microscopic analyses were performed. With the primmorph system from Suberites domuncula, it was demonstrated that silicatein(-like) molecules assemble as string- and net-like arrangements around the spicules. At their tips, these silicatein(-like) molecules are initially stacked, but later they are also organized into net-like structures. Silicatein(-like) molecules were also extracted from the giant basal spicule of Monorhaphis. Applying the SDSPAGE technique revealed that silicatein molecules associate to form dimers and trimers. By scanning electron microscopy, the formation of higher complexes (filaments) from silicatein(-like) molecules was visualized. In the presence of ortho-silicate, these filaments became covered with 3060 nm long small rod-like or cuboid particles of silica.

Structural (SEM analyses) and functional data (enzymic reactions) on the 25 kDa protein in Monorhaphis, together with the cloning information from the hexactinellid Crateromorpha meyeri, indicate that an enzymatically active silicatein exists also in spicules from hexactinellids which islike in demospongesinvolved in bio-silica formation. It was concluded that the apical elongation of the spicules of Monorhaphis proceeds by piling up cone-like silica structural units: the synthesis of these units is mediated by silicatein(-like) molecules.

Reference:

  • Wang X.; Boreiko A.; Schlossmacher U.; Brandt D.; Schröder H. C.; Li J.; Kaandorp J. A.; Götz H.; Duschner H, Müller W. E.: Axial growth of hexactinellid spicules: formation of cone-like structural units in the giant basal spicules of the hexactinellid Monorhaphis. J. Struct. Biol., 164, 270-280 (2008).

Reference for specimen preparation and labeling procedure:

  • Müller, W. E. G.; Rothenberger, M.; Boreiko, A.; Tremel, W.; Reiber, A., and Schröder, H. C. Formation of siliceous spicules in the marine demosponge Suberites domuncula. Cell Tissue Res., 321, 285297 (2005).

More information:

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Gold Nanoparticles, and When to Order from a Distributor

Did you know we also make gold nanoparticles for Sigma-Aldrich? These organic-soluble octanethiol- and dodecanethiol-stabilized gold nanoparticles are used for a variety of experimental work that is important for nanotechnology research and development.

If you are ordering from outside the United States, there are several important benefits to ordering through one of our distributors for your region. Our distributors are familiar with import procedures and requirements, as well as distribution within your region, it will save you time and possibly expense with customs and import duties to order through a distributor who knows how the system works in your country. Check our distributor list for a distributor in your region.

More information:

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Other Recent Publications

We have previously described the use of gold nanoparticles to enhance radiation therapy of tumors. Gold nanoparticles can also act to concentrate and deliver conventional chemotherapeutic agents, as Hosta and group demonstrate in their recent paper in Bioconjugate Chemistry. Two cysteine-containing analogues of the anticancer drug Kahalalide F were synthesized and conjugated to 20 and 40 nm gold nanoparticles (GNPs). The resulting complexes were characterized by different analytical techniques including high-resolution transmission electron microscopy, electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) to confirm the attachment of peptide to the GNPs. The degree of cytotoxicity of single peptides (P1 and P2), single GNP solutions (GNP-20 and GNP-40), and their respective conjugates was determined by the WST-1 assay in HeLa tumor cells following 24 h of incubation. A greater degree of enhancement was found for the 40 nm gold conjugates, and this was attributed to better cellular uptake and targeting to the lysozome. HPLC studies suggested as many as 60,000 peptides could be bound to the particle surface, considerably more than the calculated 2,090 and 8,373 required to cover a 20 and 40 nm gold particle respectively, suggesting other mechanisms of association besides coordination of the cysteine thiol to the gold may be present. The self-assembly capacity of a peptide dramatically influences the final ratio number of molecules per nanoparticle, saturating the nanoparticle surface and prompting multilayered capping on the surface. In such way, the nanoparticle could act as a concentrator for the delivery of drugs, thereby increasing bioactivity. The GNP sizes and the conjugation have influence on the biological activities. Kahalalide F analogues conjugated with GNPs are located subcellularly at lysosome-like bodies, which may be related to the action mechanism of Kahalalide F. The results suggest that the selective delivery and activity of Kahalalide F analogues can be improved by conjugating the peptides to GNPs.

Reference:

  • Hosta L.; Pla-Roca M.; Arbiol J.; López-Iglesias C.; Samitier J.; Cruz L. J.; Kogan M. J, and Albericio F.: Conjugation of Kahalalide F with gold nanoparticles to enhance in vitro antitumoral activity. Bioconjug. Chem., 20, 138-146 (2009).

Gold nanoparticles also have antibacterial properties, at least when used in conjunction with methylene blue. At least, this is the conclusion of Perni and colleagues, writing in Biomaterials. The paper describes the formation of polysiloxane polymers containing embedded methylene blue and gold nanoparticles incorporated by a swell-encapsulation-shrink method. These polymers show significant antimicrobial activity against methicillin-resistant Staphylococcus aureus and Escherichia coli, with up to a 3.5 log(10) reduction in the viable count when exposed for 5 minutes to light from a low power 660 nM laser. The bacterial kill is due to the light-induced production of singlet oxygen and other reactive oxygen species by the methylene blue. The presence of 2 nM gold nanoparticles significantly enhanced the ability of the methylene blue to kill bacteria, possibly because it enhanced the generation of other reactive oxygen species in addition to singlet oxygen.

Reference:

  • Perni, S.; Piccirillo, C.; Pratten, J.; Prokopovich, P.; Chrzanowski, W.; Parkin, I. P., Wilson, M.: The antimicrobial properties of light-activated polymers containing methylene blue and gold nanoparticles. Biomaterials, 30, 89-93 (2009).

Because it is easily functionalized and conjugated, and confers protection in many environments, gold is attractive as a coating for other types of nanoparticles, and a recent Chemistry of Materials paper from Goon and co-workers describes one such preparation: magnetic nanoparticles with a gold shell. An aqueous synthesis of composite 50-150 nm magnetite-gold core-shell nanoparticles is presented, which provides the ability to engineer the coverage of gold on the magnetite particle surface. This method utilizes polyethyleneimine (PEI) for the dual functions of attaching 2 nm gold nanoparticle seeds onto magnetite particles, and also to prevent the formation of large aggregates. In this method, the surface of the magnetite particles is saturated with gold seeds: these are then 'developed' by further treatment with a tetrachloroaurate-based gold development solution to form magnetically responsive core-shell particles, which also exhibit surface plasmon resonance. The gold-shell formation process was characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and inductively coupled plasma optical emission spectroscopy. Dynamic light scattering studies also showed that PEI coating of synthesized particles protected them against aggregation. The ability of the gold shell to protect the magnetite cores was tested by subjecting the particles to a magnetite-specific dissolution procedure. Elemental analysis of dissolved species revealed that the gold coating of magnetite cores imparts remarkable resistance to iron dissolution. The ability to engineer gold coverage on particle surfaces allows for controlled biofunctionalization, whereas their resistance to dissolution ensures applicability in harsh environments.

Reference:

  • Goon, I. Y.; Lai, L. M. H.; Lim, M.; Munroe, P.; Gooding, J. J., and Amal, R.: Fabrication and Dispersion of Gold-Shell-Protected Magnetite Nanoparticles: Systematic Control Using Polyethyleneimine. Chem. Mater., 21, 673-681 (2009).

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