Patent Publication Number: US-2005130207-A1

Title: Small organometallic probes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation of copending U.S. patent application Ser. No. 10/093,770, filed Mar. 8, 2002; which is a continuation-in-part of application Ser. No. 09/619,343, filed Jul. 19, 2000 and issued as U.S. Pat. No. 6,369,206; which is a divisional of application Ser. No. 09/039,601, filed Mar. 16, 1998 and issued as U.S. Pat. No. 6,121,425; which is a continuation-in-part of application Ser. No. 08/652,007, filed May 23, 1996 and issued as U.S. Pat. No. 5,728,590; which is a continuation-in-part of application Ser. No. 08/282,929, filed Jul. 29, 1994 and issued as U.S. Pat. No. 5,521,289. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention is directed to small organometallic probes, processes for making the small organometallic probes, and applications of the small organometallic probes. In particular, the small organometallic probes of the present invention are generally less than two microns (2 μm) across, and comprise a metal cluster compound having a solid metal core, with organic groups attached to the metal core. Alternatively, the organometallic probes may comprise a metal colloid having organic groups attached to the outer surface of the metal colloid. The metal clusters or colloids may also be functionalized with other molecules attached that can be used for targeting and detecting another substance, generally, a biologically significant substance, such as an antibody. The metal in the metal clusters or colloids is gold, platinum, silver, palladium or combinations thereof.  
      Previous work by others has described the preparation of gold and silver colloids. Such colloids do not have a fixed number of metal atoms and vary considerably in size. For example, the metal colloids could vary in size from 1 nm to 2 μm in size and could contain from about 10 metal atoms to thousands of metal atoms, depending on size. It was found that a number of proteins, such as IgG antibodies, could be adsorbed to these sol particles. Gold colloids have been most commonly described. These conjugates have been used in electron and light microscopy as well as on immunodot blots for detection of target molecules. These conjugates have many shortcomings. Since the molecules are only adsorbed onto the colloids, they also desorb to varying extents. This leads to free antibody which competes for antigen sites and lowers targeting of gold. Furthermore, the shelf life of the conjugates is compromised by this problem. The ‘sticky’ colloids also tend to aggregate. If fluorescence is used to detect the target molecules, the gold particles quench most of it. Also, the gold colloids must be stabilized against dramatic aggregation or ‘flocculation’ when salts are added by adsorbing bulky proteins, such as bovine serum albumin. Due to the effects of aggregation and bulky additives, the penetration of immunoprobes into tissues is generally &lt;0.5 μm. Accessibility of the probes to internal cell structures, e.g., nuclear proteins, or to cells deeper in a tissue sample, is impeded by these properties. Colloidal gold immunoprobes are also used in diagnosis on blots. The sensitivity of these detection schemes is again degraded by problems of aggregation, detachment of antibodies from the gold, and problems with shelf life. The gold prepared in standard ways also has low activity due to few adsorbed antibodies and denaturation of some antibodies during adsorption.  
      Various metal cluster containing organic shells have also been previously described, such as Au 11 Ph 7  (Ph=phenyl), Au 25 R (R=organic), Pd 561 , and others. These metal clusters have a fixed number of metal atoms in their metal cores which range in size from ca. 0.7-2.2 nm. Most of these metal clusters are based upon reduction of metal-triphenyl phosphines or the use of 1,10-phenanthroline.  
      For example, Barlett, P. A. et al, in “Synthesis of Water-Soluble Undecagold Cluster Compounds . . . ,” J. Am. Chem. Soc., 100, 5085 (1978), describe a metal cluster compound (Au 11 ) having a core of 11 gold atoms with a diameter of 0.8 nm. The metal core of 11 gold atoms in the undecagold metal cluster compound is surrounded by an organic shell of PAr 3  groups. This metal cluster compound has been used to form gold immunoprobes, for example, by conjugating Au 11  to Fab′ antibody fragments as well as other biological compounds.  
      Another metal cluster compound which has been used as a probe is Nanogold™ available from the assignee of the present application. Nanogold™ has a metal core with 50-70 gold atoms (the exact number not yet being known but believed to be 67 gold atoms) surrounded by a similar shell of organic groups (PAr 3 ) as undecagold. The metal core of Nanogold™ is 1.4 nm in diameter. The production of Nanogold is described in U.S. Pat. No. 5,360,895, of James F. Hainfeld and Frederic R. Furuya.  
      Although the preparation and properties vary for these metal cluster compounds having organic shells, many of these can only be synthesized in low yields, derivatization for use in coupling to biomolecules is expensive in time and effort, and again in low yields, and many of the cluster compounds are degraded rapidly by heat or various chemical reagents.  
     SUMMARY OF THE INVENTION  
      Briefly stated, the invention in a preferred form is a cluster complex or colloid including metal core particles and probe molecules covalently attached to the core particles. The probe molecules functionalize the cluster complex or colloid to target at least one other substance or to endow the particles with specified properties.  
      The core particles are less than 2 microns in size and may be composed of gold, platinum, silver, palladium, or combinations thereof.  
      The probe molecules may be covalently bonded to the core particles by a specific reaction of the probe molecules with a functional group incorporated into an outer surface of the core particle by means of chemical synthesis or modification. The functional group may be maleimide, N-hydroxysuccinimide, Sulfo-N-hydroxysuccinimide, aliphatic amine, 1,2-dihydroxy, aldehyde, nitrilotriaceitic acid (NTA) or its chelate with nickel, copper, iron, cobalt, or other transition metal.  
      The probe molecules may be antibodies, antibody fragments, avidin or streptavidin, peptides, drugs, antigens, hormones, DNA, and RNA.  
      The clusters or colloids may also include fluorescently labeled molecules covalently attached to the core particles, where the chemistry for covalently attaching the probe molecules and the fluorescently labeled molecules to the core particles controls the fluorophore of the fluorescently labeled molecules.  
      The cluster complex or colloid may also include a polymer coating the core particles, the polymer being chosen from a group containing linear or branched polymer with functional groups, polyamino acids, polyethylene derivatives, and mixtures thereof.  
      The invention is also a method for detecting a substance, where a sample suspected of containing the substance is mixed with a cluster complex or a colloid having metal core particles and probe molecules covalently attached to the core particles, the probe molecules functionalizing the cluster complex or colloid to target the substance.  
      The method may include detecting the substance with a piezoelectric crystal mass measuring device, detecting the substance by using changes in reflected light from a surface, detecting the substance using light microscopy, detecting the substance using fluorescent microscopy, detecting the substance using confocal microscopy, or detecting the substance using electron microscopy.  
      The method may include visualizing the localization or distribution of the substance within cells, tissue sections, organelles or organs using light microscopy, using fluorescent microscopy, or using confocal microscopy, using electron microscopy.  
      The method may include detecting the substance using both light, fluorescence or confocal microscopy and electron microscopy, the cluster complex or colloid also including fluorescently labeled molecules covalently attached to the core particles, where the chemistry for covalently attaching the probe molecules and the fluorescently labeled molecules to the core particles controls the fluorophore of the fluorescently labeled molecules.  
      The method may include visualizing the localization or distribution of the substance within cells, tissue sections, organelles or organs using both light, fluorescence or confocal microscopy and electron microscopy, the cluster complex or colloid also including fluorescently labeled molecules covalently attached to the core particles, where the chemistry for covalently attaching the probe molecules and the fluorescently labeled molecules to the core particles controls the fluorophore of the fluorescently labeled molecules.  
      Other objects and advantages of the invention will become apparent from the drawings and specification.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      1. “Thiol gold clusters” are novel gold clusters produced by a novel synthesis. The procedure is: form an organic-gold complex by reacting a compound containing a thiol with gold in solution. A second equivalent is also added of the thiol compound. Finally the gold organic is reduced with NaBH 4  or other reducing agents and organometallic particles are formed. These have the general formula Au n R m R′ i  . . . , where n, m, and l are integers, R and R′ are organic thiols, (e.g., alkyl thiols, aryl thiols, proteins containing thiol, peptides or nucleic acids with thiol, glutathione, cysteine, thioglucose, thiol benzoic acid, etc.) and the ellipsis indicates that one or more organic thiols may be used. With two equivalents of organic thiol compound, clusters with gold cores ˜1.4 nm are formed with many organics. The organic moiety may then be reacted by usual reactions to covalently link this particle to antibodies, lipids, carbohydrates, nucleic acids, or other molecules to form probes. Mixtures of organic thiols may be used to provide mixed functionality to the clusters. These organo-gold clusters are stable to heating at 100° C.  
      2. The thiol-gold preparation described in #1 above may be altered such that a larger molar ratio of organic thiol to gold is used. Ratios above approximately 2:1 or below 1:2 result in organic-gold colloids whose size depends on this ratio. These are useful when large gold particles are desired.  
      3. The organic thiol-gold preparations described in #&#39;s 1 and 2 above may be made using a similar process with alternatives metals to gold, e.g., platinum, silver, palladium and other metals.  
      4. The organic thiol-metal particles described in #&#39;s 1, 2, and 3 above may be made using mixtures of metal ions, e.g., gold and silver, resulting in mixed metal clusters.  
      5. A novel process has been developed for coating colloidal particles (of various types including gold, silver, and other metals) with organic moieties having groups suitable for covalently attaching additional molecules, such as antibodies, nucleic acids, lipids, peptides, and other proteins. The process consists of synthesizing the metal colloid in the presence of a suitable polymer, e.g., HAuCl 4  (0.01%) in 0.05M sodium hydrogen maleate buffer (pH 6.0), with 0.004% tannic acid. The polymer may be chosen from a linear or branched group with functional groups attached, such as polyamino acids, polyethylene derivatives, other polymers, or mixtures thereof. Optimal molecular weight of the polymer varies with the specific ones chosen. A second method is to synthesize the metal particle first, e.g., by combining 0.01% HAuCl 4  with 1% sodium citrate with heating. Once gold colloid is formed of the desired size, it is coated with one of the above polymers by mixing the two together and optionally warming to 60°-100° C. for several minutes. The polymer coating may be further stabilized by a) microwave heating, b) further chemical crosslinking, e.g., by glutaraldehyde or other linkers, or by continued polymerization adding substrate molecules for a brief period. Use of N, N′-methylene bis acrylamide, for example, can covalently further stabilize the polymer coating. Photocrosslinking may also be used.  
      The functionalized polymer coating may now be used to covalently attach proteins, peptides, antibodies, lipids, carbohydrates, nucleic acids, drugs, hormones, or other substances. This has the advantage that this step may be done mildly, in physiological buffers if desired, using standard crosslinking technology. This eliminates the usual restriction that conjugation must be performed in very low ionic strength buffers, which precludes attachment of certain molecules such as many IgM&#39;s which cannot withstand the low ionic strength requirement.  
      6. Combined (bifunctional) fluorescent and metal particle probes have been synthesized that have the formula: M n (OrF) m (Or′T) l (Or″) p , where M is a metal core consisting of multiple metal atoms (Au, Pt, Ag, Pd) that can be mixed, covalently bonded to a shell of organic groups (Or, Or′, and Or″). Or and Or″ are organic coupling moieties, (e.g., triphenyl phosphines 1,10 phenanthrolines, triphenyl phosphines or phenanthrolines containing linkable groups such as amines or carboxyls, triphenyl phosphines or 1,10 phenanthrolines containing reactive groups such as maleimides or N-hydroxysuccinimide esters, P(C 6 H 4 —CO—NH—(CH 2 ) 3 —NH 2 ) 3 , P(C 6 H 4 —CO—NH—CH 3 ) 2 (C 6 H 4 —CO—NH—(CH 2 ) 3 —NH 2 ), P(C 6 H 4 —CO—N H—(CH 2 ) 3 —NC 4 O 2 H 2 ) 3 , P(C 6 H 4 —CO—NH—(CH 2 ) 3 —NH—(CH 2 ) 6 —CO 2 —NC 4 O 2 H 4 ) 3 , etc.), Or″ is an organic group (e.g., triphenyl phosphines, 1,10 phenanthrolines, P(C 6 H 4 —CHOH—CH 2 OH) 3 , P(C 6 H 4 —CO—NH—CH 3 ) 3 , P(C 6 H 5 ) 3 , P(C 6 H 4 SO 3 ) 3 , etc.), part of the metal cluster, F is a fluorescent molecule, (e.g., fluorescein, rhodamine, aminomethyl coumarin, Texas Red, etc.) and T is an optional targeting molecule such as antibody, peptide, drug, etc. Or, Or′, and Or″ may be the same or different. The subscripts 1, n, m and p indicate that multiple copies of each moiety are possible and include mixtures of different metals, fluorescent groups, organic groups and targeting molecules for multifunctional conjugates. A specific example of this is: Au 11 [Pph(CONHCH 3 )] 6  phCONH(CH 2 ) 3  NH—F, where ph is a phenyl group, and F is a fluorescent molecule.  
      A process to produce such multifunctional metal particles is as follows: a metal cluster or metal colloid is synthesized having one or more reactive groups such as an amine or carboxyl. These are then reacted to covalently link a fluorescent molecule molecule such as fluorescein, Texas Red, rhodamine, or aminomethyl coumarin, and optionally a targeting or other molecule is covalently attached such as an antibody or antibody fragment, a peptide, streptavidin, or other proteins, a nucleic acid (RNA or DNA), drugs, hormones, or other molecules. Alternatively, the fluorescent groups may be incorporated into ligands which are then used to prepare the cluster.  
      7. Lipid molecules (fatty acids, phospholipids, or others) are covalently attached to metal particles (clusters or coated reactive colloids). This process uses a reactive lipid derivative, e.g., a sulfonyl chloride or anhydride, which is reacted with an amino group, for example, on the metal particle. Alternatively, organic groups on the metal particle or lipid may be reacted with bifunctional crosslinkers which are then reacted with the other species. Another process is to presynthesize the organic components of the metal particles (e.g., phosphines or polymers) with lipid molecules attached and then to use these organo-lipids in constructing the functionalized metal particle.  
      The general formula for the product is: 
 
M-Or-L 
 
 where M is the metal particle (either cluster or colloid of Au, Pt, Ag, Pd, and combinations), Or is an organic group of the organometallic particle (such as, phosphines containing linkable groups, polymers containing linkable groups, P(C 6 H 4 —CO—NH—(CH 2 ) 3 —NH 2 ) 3 , polyethyleneimine, polyacrylamide hydrazide, polylysine, etc.), and L is the lipid moiety. 
 
      8. A novel gel stain product is a metal (preferably gold) cluster of the form described in #1 above using appropriate organic thiols. The thiols are preferably o-thiol benzoic acid, glutathione, and thioglucose, although others may be used. The general formula describing the product is: 
 
M k (SOr) m (SOr′) n  . . . 
 
 where M is the metal, S is sulfur, Or and Or′ are organic groups (such as proteins or nucleic acids containing thiols, most other organic thiols, glucose, benzoic acid, glutathione, cholesterol, etc.), and k, m and n are integers. The ellipsis indicates that one or more different (SOr) groups (organic thiols) may be attached per metal core. 
 
      The process for a protein polyacrylamide gel is as follows: mix the organometallic particle with the protein sample with or without SDS (sodium dodecyl sulfate), for native or denaturing gels; if a reducing gel is desired, the protein is first reduced with a reductant, preferably β-mercaptoethanol, followed by a thiol blocking agent, preferably iodoacetamide, before adding the metal stain. The sample is heated briefly (e.g., one minute), loaded onto the gel and run normally. Stain development is effected by soaking the gel in a silver enhancement medium (e.g., AgNO 3  with hydroquinone) for several minutes followed by water or fixing washes.  
      9. The organometallic particles described above may be covalently incorporated into nucleic acids by several techniques. One is via synthesis of a metal particle attached to a nucleic acid base or analogue that has its other functional groups protected so that it is compatible with automated nucleic acids synthesis, such as use of phosphoramadite chemistry. A second approach is to incorporate appropriate organomettalic base analogs into nucleic acids enzymatically. A third method is to react activated organometallic clusters or colloids with functional groups incorporated into nucleic acids (such as primary amines). A fourth approach is to use organometallic particles that contain photoactive group(s) that then covalently attach to the nucleic acids when light activated.  
      The invention will further be described by reference to specific examples.  
     EXAMPLES  
      Preparation of Fluorescent and Gold Immunoprobes  
      1. Preparation of Fluorescein-Conjugated Nanoqold Using Fluorescein-Pbosphine  
      A tris (aryl) phosphine ligand bearing a single fluorescein substitutent, and a second tris (aryl) phosphine ligand bearing a single primary amine, were mixed with tris (p-N-methylcarboxamidophenyl) phosphine in the ratio 2:1:5. Ninety mg of this ligand mixture in 25 mL of methanol (an estimated twelve-fold molar excess) was added to a solution of freshly prepared Nanogold (product from 0.4 g of gold (I) triphenylphosphine chloride) in dichloromethane (25 mL) and stirred at room temperature overnight.  
      The reaction mixture was extracted with 0.02M ammonium acetate with acetic acid, pH 5.8, in 20% isopropanol/water (3×150 mL), then evaporated to dryness, redissolved in DMSO (2 mL) and 0.6M triethylammonium bicarbonate in 20% isopropanol/water. The fluorescein-substituted Nanogold was isolated by gel filtration, using a coarse gel in a large column (length=120 cm, internal diameter=2.5 cm, volume=590 mL), eluting with 0.6M triethylammonium bicarbonate in 20% isopropanol/water. The brownish-green product is the first species to be eluted. Yield was 820 mmol. UV/visible data suggested that the product incorporated 6 fluorescein groups per cluster.  
      2. Preparation of Fluorescein-Conjugated Undecagold Using Fluorescein-Phosphine  
      Gold (I) cyanide (31 mg, 0.14 mmol) was stirred with a 4:1:1 mixture of a tris (aryl) phosphine ligand bearing a single fluorescein substitutent, a second tris (aryl) phosphine ligand bearing a single primary amine, and tris (p-N-methylcarboxamidophenyl) phosphine (105 mg, 0.14 mmol) in a mixture of methanol (3 mL) and ethanol (5 mL) for 4 hours. Sodium borohydride (3 mg, 0.1 mmol) in ethanol (2.0 mL) was added dropwise over 30 minutes, then 8 drops of acetone were added to stop the reduction. The orange-brown solution was added to 10 mL of aqueous 3.0M sodium chloride, stirred for 30 minutes, then evaporated to dryness, stirred with methanol (20 mL) and filtered through a medium porosity glass frit in order to exchange the coordinated cyanide ligands for chlorides.  
      The cluster was separated from uncoordinated ligands and other smaller molecules by chromatography over a coarse gel filtration column (dimensions as described in the fluorescent Nanogold preparation) eluting with 0.6M triethylammonium bicarbonate buffer in 5% methanol/water. The pale greenish-orange cluster (yield close to 50 nmol) is eluted first, followed by uncoordinated ligands and smaller molecules. Calculations based on the UV/visible absorption spectrum suggested incorporation of 5.5 fluorescein groups per undecagold.  
      3. Preparation of Texas Red-Conjugated Nanogold  
      Nanogold was prepared as described for fluorescein-conjugated analog, except that a different mixture of phosphines was used to perform the ligand exchange on the freshly made compound. A 2:1:8 mixture of a phosphine bearing a single primary amine, a phosphine containing a single primary amine protected with a t-Boc group, and tris (p-N-methylcarboxamidophenyl) phosphine. The protected cluster was isolated in the same manner as the fluorescein-conjugated analog.  
      400 nmol of protected Nanogold was evaporated to dryness five times from methanol to remove triethylammonium bicarbonate, then dissolved in isopropanol (0.2 mL) and 0.1M sodium borate buffer, pH 9.0 (0.4 mL). This solution was added to a solution of Texas Red sulfonyl chloride (8-fold excess, 2.1 mg) in isopropanol (0.2 mL) and the mixture incubated at 4° C. for 1 hour. The products were separated by coarse gel filtration (GH25 gel, Amicon, using column with length=50 cm, internal diameter=0.66 cm, volume=16 mL) eluting with 0.6M triethylammonium bicarbonate in 50% isopropanol/water. The dark blue product eluted in the void volume: it was evaporated to dryness from methanol five times, then the t-Boc protecting group was removed with 0.1M hydrochloric acid in methanol (1 hour). The solution was neutralized with triethylammonium buffer, evaporated to dryness and rechromatographed (GH25 gel, buffer as above) to remove triethylammonium chloride. Final yield was 170 nmol (43%). Labeling calculated from the UV/visible spectrum was 1.17 Texas Red groups per cluster.  
      4. Conjugation of Fluorescein-Nanoqold or Undecaqold to Antibody Fab′ Fragments  
      F(ab′) 2  antibody fragments (1.0 mg) were reduced with 40 mM mercaptoethylamine hydrochloride in 0.1M sodium phosphate at pH 6.0, with 5 mM EDTA (1.4 mL) for 1 hour at room temperature. Then the Fab′ fragments were separated on a coarse gel filtration column (GH25, Amicon: length=50 cm, internal diameter=1.0 cm, volume=40 mL), and eluted with 0.02M sodium phosphate, pH 6.5, with 150 mM NaCl and 1 mM EDTA.  
      Fluorescein-conjugated Nanogold (250 nmol) was evaporated to dryness five times from methanol solution to remove any triethylammonium bicarbonate, then dissolved in DMSO (0.5 mL) and 0.1M sodium phosphate buffer, pH 7.5 (0.9 mL) and added to a solution of a 100-fold excess of N-methoxycarbonylmaleimide (NMCM, 8 mg) in DMSO (0.1 mL), mixed and incubated at 0° C. for 30 minutes. Maleimido-[Nanogold-fluorescein] was separated from unreacted NMCM on a coarse gel filtration column (GH25, Amicon: length=50 cm, internal diameter=1.0 cm, volume=40 mL), eluted with 0.02M sodium phosphate, pH 6.5, with 150 mM NaCl and 1 mM EDTA in 10% isopropanol/water. Maleimido-[Nanogold-fluorescein] was eluted in the excluded volume; a 5-fold excess was added to the Fab′ fragments and the mixture mixed and incubated at 4° C. overnight. The product was concentrated to 0.5 mL using centrifuge membrane concentrators with a 30,000 molecular weight cutoff (Centricon-30, Amicon), then isolated by gel filtration chromatography using a medium gel (Amicon GCL-90: Column length=50 cm, internal diameter=0.66 cm, volume=16 mL). 0.02M sodium phosphate with 150 mM sodium chloride, pH 7.40, was used as the eluent. The labeled Fab′ fragments eluted first, followed by unbound fluorescein-Nanogold. The process was repeated once for highest purity of product.  
      [Au 11 -Fluorescein] labeling was conducted in the same manner, except that the buffer used to separate the activated fluorescein gold was prepared as a solution in 40% DMSO/water.  
      5. Labeling of Streptavidin with Fluorescein-Conjugated Nanoqold  
      Fluorescein-conjugated Nanogold (250 nmol) was evaporated to dryness five times from methanol to remove any triethylammonium bicarbonate, then dissolved in DMSO (0.4 mL) and 0.02M HEPES-NaOH buffer, pH 7.5 (0.9 mL) and added to a solution of bis (sulfo-N-hydroxysuccinimidyl) suberate (BS 3 ) (250-fold excess: 38 mg) in DMSO (0.1 mL). The solution was mixed thoroughly, incubated at room temperature for 1 hour and 25 minutes, then sulfo-NHS-[Fluorescein-Nanogold] was separated from excess (BS 3 ) by chromatography on a coarse gel filtration column (GH25, Amicon: length=50 cm, internal diameter=1.0 cm, volume=40 mL), eluting with 0.02M HEPES-NaOH, pH 7.5, in 20% isopropanol/water. Activated fluorescein-Nanogold was the first species to be eluted, and was mixed in 12-fold excess to a solution of streptavidin in an aqueous solution of the same buffer. The mixture was incubated at 4° C. overnight, then reduced to 0.5 mL by membrane centrifugation (Centricon-30, Amicon) and purified twice by gel filtration (Amicon GCL-90 gel: Column length=50 cm, internal diameter=0.66 cm, volume=16 mL) eluted with 0.02M sodium phosphate buffer, pH 7.4 with 150 mM NaCl. Labeled streptavidin is the first species to be eluted.  
      Labeling of Antibody Fab′ Fragments with Texas Red-Conjugated Nanogold  
      Antibody reduction and fluorescent gold label conjugation were conducted as described for fluorescein-Nanogold. Texas Red-Nanogold (200 nmol) was converted to the maleimide form by reaction with a 100-fold excess of NMCM (3 mg) for 30 minutes at 0° C. in DMSO (0.30 mL), and 0.1M sodium phosphate buffer at pH 7.5 (0.45 mL); the activated label was isolated by chromatography over a coarse gel filtration column (GH25, Amicon: length=50 cm, internal diameter=0.66 cm, volume=16 mL), eluted with 0.02M sodium phosphate with 150 mM sodium chloride and 1 MM EDTA in isopropanol/water; gold-containing fractions were added in fivefold excess to Fab′ fragments, prepared from F(ab′) fragments as described in Example 3, and incubated at 2°-8° C. overnight. The product was isolated on a medium gel filtration column (Amicon GCL-90: 50 cm, internal diameter=0.66 cm, volume=16 mL), eluting with 0.02 P 7.4+150 mm NaCl. The blue-grey product was the first species to be eluted.  
      7. Conjugation of Hoescht-33258 to Nanogold  
      Cross-linking was performed with 1,1′-carbonyldiimidazole (CDI). Hoescht-33258 (3 mg) was dissolved in 0.5 mL DMSO with a stoichiometric amount of CDI and incubated at room temperature for 30 minutes, then polyamino-1.4 nm gold cluster (150 nmol, giving 100-fold excess of dye) in DMSO (0.4 mL) was added and incubation continued for a further 1 hour at room temperature. The reaction mixture was then diluted with deionized water to 20% DMSO and concentrated three times to minimal volume using membrane centrifugation (10,000 MW cutoff). The solution was switched to a 30,000 MW cutoff membrane concentrator and centrifuged to minimal volume a further 8 times; no 1.4 nm gold was observed to pass through the membrane. The concentration of fluorescent dye in the filtrate was determined spectrophotometrically, and by the end of the eighth centrifugation had fallen to less than half the concentration of 1.4 nm gold particles remaining in the retained solution, diluted to the same volume. The ratio of dye molecules to gold clusters in the final product was estimated spectrophotometrically to be 1.2.  
      Preparation of Gold-Labeled Dipalmitoyl Phosphatidyl Ethanolamine (DPPE)  
      8. Labeling of DPPE with Freshly Activated 1.4 nm Gold  
      Activation of the gold particles was conducted in methanol with a small amount of triethylamine added, until the reading of a pH meter inserted into the solution was between 7.5 and 8.0, to promote the reaction. 300 nmol of monoamino 1.4 nm gold, isolated by ion exchange chromatography (in 0.6M triethylammonium bicarbonate buffer in 20% isopropanol/water) was evaporated to dryness five times from methanol to remove the volatile buffer, then dissolved in 1.5 mL of triethylamine-treated methanol in which a 500-fold excess of bis(sulfo-succinimidyl) suberate (86 mg) was dissolved. This mixture was incubated at room temperature for 1 hour 30 minutes. The activated 1.4 nm gold was separated over a coarse gel filtration column (Amicon, GH25: length=50 cm, internal diameter=1.0 cm, volume=40 mL), eluted with methanol; the dark brown activated Nanogold is the first species to elute. Fractions containing uncontaminated activated Nanogold were combined to yield 180 nmol of activated gold, in 4 or 6 mL. A solution of DPPE (100-fold excess: 12 mg) in one-half this volume of trichloromethane was added and the mixture incubated at 4° C. overnight.  
      The reaction mixture was evaporated to dryness, and stirred with 0.02M ammonium acetate, pH 5.80 (50 mL) to extract any unreacted Nanogold; this suspension was extracted three times with chloroform (15 mL). The combined chloroform extracts were evaporated to dryness, dissolved in a 2:1 methanol/chloroform mixture, and separated on a column identical to that used above for the Nanogold activation, eluted with the same solvent mixture. The dark brown Nanogold-DPPE conjugate is the first species to be eluted; unconjugated DPPE is eluted later.  
      9. Labeling of DPPE with Lyophilized Mono-Sulfo-NHS-1.4 nm Gold (Nanogold)  
      60 nmol of lyophilized monofunctional sulfo-NHS-Nanogold (Nanoprobes, catalog number 2025: two vials) was redissolved in methanol (2 mL) with a drop of triethylamine, and added to a 150-fold excess of DPPE in a 2:1 methanol/chloroform mixture with a small amount of dichloromethane (4 mL), mixed thoroughly and incubated overnight at 4° C.  
      The reaction mixture was evaporated to dryness, then redissolved in a 2:1 methanol/chloroform mixture (0.8 mL), filtered, and chromatographed over a coarse gel filtration column (Amicon GH25: length=50 cm, internal diameter=0.66 cm, volume=16 mL), eluting with 2:1 methanol:trichloromethane. The brown DPPE conjugate is the first species to be eluted; colorless unreacted DPPE is eluted later.  
      10. Preparation of Undecagold-DPPE  
      Both the activation reaction of the gold, and chromatographic separation of activated gold, are conducted in a basic methanolic buffer, prepared as follows: sufficient HEPES to give 200 mL of a 0.02M solution (0.956 g) are suspended in methanol (200 mL), and triethylamine added slowly, dropwise, until all the HEPES is dissolved. A pH electrode was then inserted into the solution, and dilute triethylamine in methanol was added until the reading on the pH meter was between 7.6 and 8.0.  
      Ion exchange-isolated monoamino undecagold (150 nmol, by UV/visible absorption spectroscopy, dissolved in aqueous 0.6M triethylammonium bicarbonate buffer in 5% methanol) was evaporated to dryness five times from ethanol to remove the volatile buffer. It was then dissolved in 0.6 mL of the methanolic HEPES buffer and added to a solution containing a 200-fold excess of bis(sulfosuccinimidyl) suberate (17 mg) in methanolic HEPES buffer (0.2 mL) to give a total volume of 0.8 mL. The solution was mixed thoroughly, then incubated at room temperature in a small polyethylene vial for 1 hour and 30 minutes. Activated undecagold was separated from excess BS 3  using a coarse gel filtration column (Amicon GH25; length=50 cm, internal diameter=0.66 cm; volume=16 mL), eluting with methanolic HEPES buffer prepared earlier. The activated undecagold cluster is the first species to be eluted. Fractions containing uncontaminated activated undecagold were pooled, to yield about 80 nmol of activated undecagold in 2 or 3 mL. This was then added to a solution of a 100-fold excess of DPPE (5.4 mg) in a volume of chloroform equal to one-half the volume of the activated undecagold (1 or 1.5 mL), mixed thoroughly, and incubated overnight at 4° C.  
      The mixture was then evaporated to dryness, and shaken with a water/chloroform mixture to remove unreacted undecagold; the aqueous layer was removed and the process repeated twice. The organic layer and solid material accumulated at the phase boundary were then evaporated to dryness, dissolved in a 1:1 methanol/dichloromethane mixture (0.8 mL), and separated on a column identical to that used above to separate activated undecagold. The eluent was a 1:1 methanol/dichloromethane mixture. The Undecagold-DPPE conjugate is the first species to be eluted, and is orange in color; colorless unconjugated DPPE is eluted later.  
     Golden Lipids  
      Preparation of Alkylamido-1.4 nm Gold Cluster Derivatives  
      General:  
      Monoamino-1.4 nm gold cluster is reacted in excess with alkyl acid chloride or alkyl acid anhydride in dichloromethane. The unreacted gold cluster was extracted from the reaction mixture into aqueous buffer and the product was purified from alkyl carboxylic acids, produced from the hydrolysis of unreacted acide derivatives, by size exclusion chromatography. The alkyl carboxylic acid derivatives include nonanoyl chloride, decanoyl chloride, decanoic anhydride, lauroyl chloride, lauric anhydride, palmitoyl chloride, palmitic anhydride, heptadecanoyl chloride, stearoyl chloride, and stearic anhydride.  
      11. 300 nmol of monoamino 1.4 nm gold cluster, isolated by ion exchange chromotography and removal of volatile buffer salts by evaporation, was placed in 3 ml dichloromethane and treated with 200 nmol palmitoyl chloride. The mixture was stirred for 1 hour and then washed three times with 0.1M sodium phosphate buffer pH 6.5. The remaining reaction mixture was evaporated to dryness, redissolved in a 2:1 methanol/chloroform, and separated on a gel filtration column (Amicon GH25) eluting with 2:1 methanol/chloroform. The first to elute is the dark brown palmitamido gold cluster.  
      Preparation of Large Platinum and Palladium Cluster Immunoprobes  
      12. Preparation of Functionalized 1-2 nm Platinum Cluster  
                 
 
      From platinum (II) acetylacetonate and bathophenanthroline: A mixture of Pt 11 (acac) 2  (0.20 g, 0.5 mmol), bathophenanthroline (1:35 mg, 0.064 mmol) and 5-10:1 mixed ligand (3:5 mg, 0,016 mmol) was stirred in glacial acetic acid (20 mL) under a slow flow of nitrogen. Meanwhile sodium borohydride (2.3 g) was dissolved in 2-methoxyethyl ether (diglyme: 30 mL) and a 1:1 mixture of ethanol/water (20 mL) was added dropwise over 30 minutes; the hydrogen generated from this solution was bubbled into the reaction mixture. After 30 minutes no color change was observed: therefore, a small amount of platinum (II) chloride (˜0.05 g) suspended in acetic acid (1 mL) was added and stirring continued. H 2  bubbling was continued for a further 4 hours, during which time the solution darkened from yellow to green and a greenish-black precipitate was formed. The solution was then aerated for 2 hours by blowing air into the flask.  
      The contents of the reaction vessel were evaporated to dryness, and the greenish-black solid extracted with DMSO (0.5 mL) and 0.6M triethylammonium bicarbonate in 20% isopropanol/water (1.5 mL), filtered, then separated over a coarse gel filtration column (GH25, Amicon) eluting with 0.6M triethylammonium bicarbonate in 20% isopropanol/water. A dark greenish-brown compound was eluted in the void volume, and pale yellow species later. Electron microscopy of the greenish-brown compound showed metal particles 1-2 nm in diameter.  
      From Platinum (II) chloride and substituted phenanthrolines: A mixture of platinum (II) chloride (0.5 mmol, 126 mg), ligands 2 (14.8 mg) and 3 (5.1 mg) were stirred in glacial acetic acid (25 mL) for 6 hours and heated to 100° C. to partially dissolve the solids. After cooling to room temperature overnight, hydrogen (generated as described above) was bubbled slowly through the reaction mixture for 3 hours; the color darkened from brown to greenish-black. The reaction mixture was then poured into a glass beaker and stirred overnight while air was bubbled through the solution. All the acetic acid was removed to leave a greenish black solid. This was then extracted into DMSO (0.5 mL) and 0.6M triethylammonium bicarbonate in 20% DMSO/water (1.5 mL), filtered, and separated on a coarse gel filtration column (GH25, Amicon) eluting with 0.6M triethylammonium bicarbonate in 20% DMSO/water. A brownish-green species was eluted in the void volume.  
      13. Conjugation of Functionalized Platinum Cluster to Antibodies  
      F(ab′) 2  fragments or IgG molecules were reduced with 40 or 50 mM respectively mercaptoethylamine hydrochloride in 0.1M sodium phosphate buffer with 5 mM EDTA, pH 6.0, for 1 hour and 5 minutes at room temperature (in 1.5 mL volume). Reduced antibody was separated from excess mercaptoethylamine hydrochloride on a coarse gel filtration column (GH25, Amicon), eluting with 0.02 m sodium phosphate with 150 mM sodium chloride and 1 mM EDTA, pH 6.5; reduced antibody was eluted in the excluded volume.  
      Platinum cluster (as isolated above; approximately one-third of the total isolated) was evaporated to dryness, then reevaporated to dryness five times from methanol solution to remove triethylammonium bicarbonate. The compound was then dissolved in DMSO (0.25 mL) and 0.1 m sodium phosphate, pH 7.5 (0.48 mL), and added to a solution of N-methoxycarbonylmaleimide (NMCM: 6 mg) in DMSO (0.07 mL); the solution was mixed thoroughly and incubated at 0° C. (ice bath/refrigerator) for 30 minutes. The activated platinum compound was then separated from excess smaller molecules on a coarse gel filtration column (GH25, Amicon), eluting with 0.02 m sodium phosphate with 150 mM NaCl and 1 mM EDTA in 20% DMSO/water; platinum-containing fractions were then added to those containing reduced antibody, mixed thoroughly and the mixture was incubated at 4° C. overnight.  
      Next day, the reaction mixture was concentrated to between 0.5 and 1 mL (Centricon-30) and the products separated on a size fractionation column (Pharmacia Superose-12) eluted with 0.02 m sodium phosphate with 150 mM sodium chloride, pH 7.4; the first species to be eluted was greenish-brown in color and was in the position expected for antibody molecules. In the Fab′ preparations, fractions comprising this peak were concentrated and re-chromatographed on a Superdex-75 column in the same buffer. The first species to emerge was the antibody conjugate; some resolution of unlabeled from labeled Fab′ fragments was observed.  
      14. Preparation and Antibody Conjugation of Functionalized 2-3 nm Pd Cluster  
      Palladium (II) acetate (1.0 mmol, 227 mg), and a mixture of ligands 2 (27 mg) and 3 (8.5 mg) were stirred in glacial acetic acid (25 mL) with stirring under a slow stream of nitrogen. The mixture was warmed to 80° C. to dissolve all the components, to give an orange-brown solution. After cooling to room temperature hydrogen (generated as described above) was bubbled slowly through the mixture for 3 hours to give a near-black solution. This was stirred in a glass beaker under a stream of air until dry, then dissolved in DMSO (1.0 mL) and 0.6M triethylammonium bicarbonate in 20% DMSO/water (3.0 mL), filtered, and separated on a coarse gel filtration column (GH25, Amicon) eluting with 0.6M triethylammonium bicarbonate in 20% DMSO/water. The cluster was eluted in the exclusion volume.  
      Fab′ conjugation is conducted using the same procedure used with the platinum cluster: F(ab′) 2  is first reduced and separated from excess reducing agent in the same manner. A suitable amount of the cluster is evaporated to dryness three times from methanol, with heating to 55°-60° C. to remove DMSO. The black solid is dissolved in DMSO (0.32 mL) and 0.1M sodium phosphate, pH 7.50 (0.40 mL) and added to a solution of N-methoxycarbonylmaleimide (NMCM: 6 mg) in DMSO (0.05 mL). The mixture is vortexed, incubated at 0° C. for 30 minutes (ice bath/refrigerator) then separated on a GH25 column. The activated cluster is eluted in the void volume: cluster-containing fractions are pooled and added to the reduced antibody. The mixture is left at room temperature for 1 hour, then stored at 4° C. overnight.  
      15. Immunoblotting of Conjugates  
      Immunoblotting was conducted using a single-layer technique: mouse IgG was spotted onto a hydrated nitrocellulose membrane in serial dilutions, and detected using platinum or palladium labeled goat IgG and Fab′ anti-mouse IgG conjugates.  
                               Buffers required:                                        PBS:   0.01M sodium phosphate with 150 mm sodium chloride,           pH 7.4       WASH:   0.02M sodium phosphate with 150 mm sodium chloride,           pH 7.4           0.8% w/w bovine serum albumin, fraction V by heat shock           0.1% w/w gelatin, type B from bovine skin, approximately           60 bloom           2 mM sodium azide.       BLOCK-   0.02M sodium phosphate with 150 mm sodium chloride,       ING:   pH 7.4           4.0% w/w bovine serum albumin, fraction V by heat           shock.           0.1% w/w gelatin, type B from bovine skin, approximately           60 bloom           2 mM sodium azide.       INCUBA-   0.02M sodium phosphate with 150 mm sodium chloride,       TION:   pH 7.4           0.8% w/w bovine serum albumin, fraction V by heat           shock.           0.1% w/w gelatin, type B from bovine skin, approx. 60           bloom           1% W/W normal goat serum           2 mM sodium azide.                  
 
 Procedure: 
      1. A nitrocellulose membrane, marked with pencilled divisions for antigen concentration identification, was simmered in gently boiling water for 15 minutes.     2. 1 μl dilutions of mouse IgG were spotted onto membrane, from 10 −9  to 10 −18  g.     3. Membrane was blocked with blocking buffer for 30 minutes at 45° C.    

      4. Membrane washed 5 minutes with wash buffer. 
      5. Membrane was incubated with 5 mL of a {fraction (1/200)} dilution of the NANOGOLD™ reagent in incubation buffer for 2.5 hours at room temperature, with slow agitation.     6. Membrane was rinsed with buffer 3 (3×5 mins), then PBS (3×30 seconds).     7. Membrane postfixed with glutaraldehyde, 1% in PBS (10 minutes).     8. Rinsed with deionized water (2×5 minutes).     9. Rinse with 0.05M EDTA at pH 4.5 (2 minutes).     10. Develop with freshly mixed LI SILVER™ (Nanoprobes, Inc.), 2×30 minutes. Rinse thoroughly with deionized water between developments to remove all the silver enhancement reagent.     11. Rinse repeatedly with deionized water, then let air-dry.    

      The last visible spot in a series of decreasing concentration contained 10 pg of mouse IgG for platinum-labeled goat Fab′- and IgG-anti-mouse IgG, and also for palladium-labeled goat anti-mouse Fab′. For the IgG conjugates, this was the same level of sensitivity as was obtained from a second blot performed with the analogous Nanogold conjugate.  
      Thiol-Gold Cluster Preparation and Antibody Labelling Procedure  
      The following solutions are used in the preparation of thiol-gold clusters: 
      A. NaBH 4  solution: 0.2% solution of NaBH 4  in ethanol.     B. TEAH: 0.6M triethylammonium bicarbonate in distilled water. Note: Unless otherwise specified, all reactions are performed at 25° C. 
 
 16. Synthesis of Thiol Gold Cluster from Aurothioglucose (I): 
   

      5 mg of aurothioglucose (C 6 H 11 O 5 SAu) is dissolved in 0.5 ml of distilled H 2 O in a test tube, forming a light yellow solution. Two 10 μL aliquots of NaBH 4  solution are added to the aurothioglucose solution over 15 minutes. Upon NaBH 4  addition the solution turns dark brown in color. The cluster is purified by size exclusion chromatography using Amicon GH-25 material (MW cutoff=3000 Daltons), with TEAH as the elution buffer. The cluster is recovered from the void volume of the column. Cluster formation is verified by UV/VIS spectroscopy and electron microscopy.  
      17. Synthesis of Thiol Gold Cluster from KAuBr 4  and Glutathione  
      (II): 5 mg of KAuBr 4  (8.418×10 −3  mMol) is dissolved in 0.3 ml of distilled water. 5.17 mg of glutathione (1.684×10 −2  mMol), dissolved in 0.5 ml of distilled water is added to the KAuBr 4  solution in two 0.25 ml aliquots at 5 minute intervals. After the first addition the dark reddish brown KAuBr 4  solution turns to a clear, colorless to pale yellow solution. The reaction mixture is left for 5 minutes after the second addition of glutathione solution. The pH of the reaction mixture is adjusted to 8 using 6N NaOH solution. Two 10 μL aliquots of NaBH 4  solution are added to the reaction mixture over 15 minutes. The pH is adjusted to neutral with 1N HCl solution, and a third 10 μL aliquot of NaBH 4  solution is added. Two 10 μL aliquot of NaBH4 solution are added to the reaction mixture over 15 minutes. The pH is adjusted to neutral with 1N HCl solution, and a third 10 μL aliquot of NaBH 4  solution is added. Upon NaBH 4  addition the solution turns dark brown in color. The cluster is purified by size exclusion chromatography using Amicon GH-25 material (MW cutoff=3000 daltons), with TEAH as the elution buffer. The cluster is recovered from the void volume of the column. Cluster formation is verified by UV/VIS spectroscopy and electron microscopy.  
      18. Synthesis of a Mixed Thiol Gold cluster from KAuBr 4  and a Mixture of Glutathione and 1-thio-.beta.-D-Glucose (III):  
      5 mg of KAuBr 4  (8.418×10 −3  mMol) is dissolved in 0.3 ml of distilled water. A mixed thiol solution of 3.62 mg of glutathione (1.18×10 −2  mMol) and 1.10 mg of 1-thio-.beta.-D-Glucose (5.05×10 −3  mMol), dissolved in 0.5 ml of distilled water is added to the KAuBr 4  solution in two 0.25 ml aliquots at 5 minute intervals. After the first addition the dark reddish brown KAuBr 4  solution turns to a clear, colorless to pale yellow solution. The reaction mixture is left for 5 minutes after the second addition of the mixed thiol solution. The pH of the reaction mixture is adjusted to 8 using 6N NaOH solution. Two 10 μL aliquots of NaBH 4  solution are added to the reaction mixture over 15 minutes. The pH is adjusted to neutral with 1N HCl solution, and a third 10 μL aliquots of NaBH 4  solution is added. Upon NaBH 4  addition, the solution turns dark brown in color. The cluster is purified by size exclusion chromatography using Amicon GH-25 material (MW cutoff=3000 daltons), with TEAH as the elution buffer. The cluster is recovered from the void volume of the column. Cluster formation is verified by UV/VIS spectroscopy and electron microscopy.  
      19. Synthesis of Antibodies (Whole Molecule) Labelled with III:  
      2 mg of IgG is combined in a siliconized microcentrifuge tube with 10 mg of mercaptoethanolamine in 1.4 ml of 0.1M sodium phosphate, pH 6.0 which contains 5 mM EDTA. The reaction is allowed to proceed 1 hour at room temperature, then the reduced antibody is separated from excess MEA on a GH-25 column (Amicon), eluting with 0.1M borate buffer, pH 9.2, with 5 mM EDTA. The reduced antibody is collected in the void volume of the column. Fractions are combined and reduced in volume to 0.1 ml with centricon-30 (Amicon). The reduced antibody fraction is then incubated at 37° C. for 1 hour with a 20-fold excess of compound III, which is added in 0.1M borate buffer, pH 9.2, with 5 mM EDTA. The total volume of this reaction mixture is made up to 1.4 ml by adding 0.1M borate buffer, pH 9.2, with 5 mM EDTA. After incubation period, the reaction mixture is reduced in volume to 0.1 ml with centricon-30 (Amicon) and injected on a Superdex-75 molecular weight fractionation column (Pharmacia) to separate out the gold cluster-IgG conjugate from excess III. Antibody labelling is verified by UV/VIS spectroscopy and electron microscopy.  
      20. Synthesis of Fab′ Antibody Fragments Labelled with III:  
      2 mg of F(ab′) 2  is combined in a siliconized microcentrifuge tube with 7 mg of mercaptoethanolamine in 1.4 ml of 0.1M sodium phosphate, pH 6.0 which contains 5 mM EDTA. The reaction is allowed to proceed 1 hour at room temperature, then the reduced antibody fragment is separated from excess MEA on a GH-25 column (Amicon), eluting with 0.1M borate buffer, pH 0.2, with 5 mM EDTA. The Fab′ fragment is collected in the void volume of the column. Fractions are combined and reduced in volume to 0.1 ml with centricon-30 (Amicon). The Fab′ fraction is then incubated at 37° C. for 1 hour with a 20-fold excess of III, which is added in 0.1M borate buffer, pH 9.2, with 5 mM EDTA. The total volume of this reaction mixture is made up to 1.4 ml by adding 0.1M borate buffer, pH 9.2, with 5 mM EDTA. After the incubation period, the reaction mixture is reduced in volume to 0.1 ml with centricon-30 (Amicon) and injected on a Superdex-75 molecular weight fractionation column (Pharmacia) to separate out the gold cluster-Fab′ conjugate from excess III. Antibody labelling is verified by UV/VIS spectroscopy and electron microscopy.  
      Preparation of Thiol-Metal Clusters (Silver, Silver-Gold, Platinum, Thallium)  
      21. Preparation of Silver-Organic Thiol Clusters  
      5×10 −6  moles of silver acetate in 0.5 ml of water was mixed with 1×10 −5  moles of thioglucose and heated to 70° C. for 5 minutes. Then 5×10 −6  moles of sodium borohydride was added. Later, 5×1 01 moles of additional sodium borohydride was added. A dark brown solution resulted, and contained organo-silver clusters ˜1-3 nm in diameter. These were purified by gel filtration size exclusion chromatography using an Amicon GH25 column (cutoff 3,000 MW) in an aqueous buffer of 0.6M triethylammonium bicarbonate and 5% methanol. The product was rotary evaporated under vacuum and resuspended in water. The size was assayed by electron microscopy.  
      22. Preparation of Mixed Silver-Gold Organic Thiol Clusters.  
      2.5×10 −6  moles of silver acetate and 2.5×10 −6  moles of chloroauric acid in 0.5 ml of water were mixed with 1×10 −5  moles of thioglucose. Then, 5×10 −6  moles of sodium borohydride was added. A dark brown solution resulted, and contained organo-silver/gold clusters ˜1-3 nm in diameter. These were purified by gel filtration size exclusion chromatography using an Amicon GH25 column (cutoff 3,000 MW) in an aqueous buffer of 0.6M triethylammonium bicarbonate and 5% methanol. The product was rotary evaporated under vacuum and resuspended in water. The size was assayed by electron microscopy.  
      23. Preparation of Platinum-Organic Thiol Clusters  
      5×10 −6  moles of platinum chloride in 0.5 ml of water was mixed with 5×10 −6  moles of thioglucose and heated to 60° C. for 5 minutes. Then 2.5×10 −6  moles of sodium borohydride was added. Later, 2.5×10 6  moles of additional sodium borohydride was added. A dark brown solution resulted, and contained organo-platinum clusters ˜1-3 nm in diameter. These were purified by gel filtration size exclusion chromatography using an Amicon GH25 column (cutoff 3,000 MW) in an aqueous buffer of 0.6M triethylammonium bicarbonate and 5% methanol. The product was rotary evaporated under vacuum and resuspended in water. The size was assayed by electron microscopy.  
      24. Preparation of Thallium-Organic Thiol Clusters  
      5×10 −6  moles of thallium chloride in 0.5 ml of water was mixed with 1×10 −5  moles of thioglucose. After 10 minutes, 5×10 −6  moles of sodium borohydride was added. Later, 2.5×10 −6  moles of additional sodium borohydride was added. A dark brown solution resulted, and contained organo-thallium clusters ˜1-3 nm in diameter. These were purified by gel filtration size exclusion chromatography using an Amicon GH25 column (cutoff 3,000 MW) in an aqueous buffer of 0.6M triethylammonium bicarbonate and 5% methanol. The product was rotary evaporated under vacuum and resuspended in water. The size was assayed by electron microscopy.  
      Thiol-Gold Clusters for Protein Staining in Polyacrylamide Gel Electrophoresis (PAGE): Preparation and Application  
      The following solutions are used in the preparation of thiol-gold clusters: 
      A. NaBH 4  solution: 0.2% solution of NaBH 4  in ethanol.     B. TEAH: 0.6M triethylammonium bicarbonate in distilled water. 
 
 Note: Unless otherwise specified, all reactions are performed at 25° C. 
 
 25. Synthesis of Thiol-Gold Cluster from KAuBr 4  and o-mercaptobenzoic Acid (1): 
   

      5 mg of KAuBr 4  (8.41×10 −3  mMol) is dissolved in 0.3 ml of distilled water. 2.60 mg of o-mercaptobenzoic acid (1.68×1 0-2 mMol), dissolved in 0.5 ml of TEAH is added to the KAuBr 4  solution in two 0.25 ml aliquots at 5 minute intervals. After the first addition, the dark reddish brown KAuBr 4  solution turns to clear, colorless to pale yellow solution. The reaction mixture is left for 5 minutes after the second addition of glutathione solution. Two 10 μL aliquots of NaBH 4  solution are added to the reaction mixture over 15 minutes. Upon NaBH 4  addition the solution turns dark brown in color. The cluster is purified by size exclusion chromatography using Amicon GH25 material (MW cutoff=3000 daltons), with TEAH as the elution buffer. The cluster is recovered from the void volume of the column. The cluster containing fractions are combined in a 50 ml round bottom flask and the solution is evaporated to dryness with a rotoevaporator (Buchi). The cluster is redissolved in methanol and re-evaporated with the rotoevaporator. This methanol evaporation is done 5 times. The cluster is then dissolved in 0.1 sodium phosphate buffer, pH 8.0, with 1.0 mM EDTA for use in subsequent protein staining experiments. Cluster formation is verified by UV/VIS spectroscopy and electron microscopy. As a thiol scavenger, an excess amount of N-ethylmaleimide is added to the cluster solution.  
      26. Synthesis of a Mixed Thiol Gold Cluster from KAuBr 4  and Mixture of o-mercaptobenzoic Acid and Nonylmercaptan(II):  
      5 mg of KAuBr 4  (8.41×10 −3  mMol) is dissolved in 0.3 ml of 90% ethanol in distilled H 2 O. A mixed thiol solution of 2.34 mg of o-mercaptobenzoic acid (1.52×10 −2  mMol) and 0.27 mg of nonylmercaptan (1.68×10 −3  mMol), dissolved in 0.5 ml of 95% ethanol in 6N NaOH, is added to the KAuBr 4  solution in two 0.25 ml aliquots at 5 minute intervals. After the first addition, the dark reddish brown KAuBr 4  solution turns to a pale yellow precipitate. Upon the addition of the second thiol mixture addition, the precipitate is solubilized to give a clear, colorless to pale yellow solution. The reaction mixture is left for 5 minutes after the second addition of the mixed thiol solution. Two 10 μL aliquots of NaBH 4  solution are added to the reaction mixture over 15 minutes. The pH is adjusted to neutral with 1N HCl solution, and a third 10 μL aliquots of NaBH 4  solution is added. Upon NaBH 4  addition, the solution turns dark brown in color. The cluster is purified by size exclusion chromatography using Amicon GH-25 material (MW cutoff=3000 Daltons), with TEAH as the elution buffer. The cluster is recovered from the void volume of the column. The cluster containing fractions are combined in a 50 ml round bottom flask and the solution is evaporated to dryness with a rotoevaporator (Buchi). The cluster is redissolved in methanol and re-evaporated with the rotoevaporator. This methanol evaporation is done 5 times. The cluster is then dissolved in 0.1 sodium phosphate buffer, pH 8.0, with 1.0 mM EDTA for use in subsequent protein staining experiments. Cluster formation is verified by UV/VIS spectroscopy and electron microscopy. As a thiol scavenger, an excess amount of N-ethylmaleimide is added to the cluster solution.  
      27. Application of I and II to SDS-PAGE in Non-Reducing Conditions:  
      Protein sample (Pharmacia LMW calibration kit proteins) to be analyzed is incubated with excess I or II in 10 mM Tris/HCl, 1 mM EDTA, pH 8.0. To the sample is added Sodium Dodecyl Sulphate (SDS) to 2.5%. The sample is heated for 5 minutes at 100° C. Bromophenol Blue is added to 0.01%. Using an eight “well” sample applicator, eight 1 μL aliquots of the sample mixture are applied to a Phastgel gradient gel (8-25) and run on the Pharmacia Phastgel system using the following protocol:  
                                                      Sample Applicator down at step 1.1    1 Vh           Sample Applicator up at step 1.1   10 Vh                                                 SEP 1.1   250 V   10.0 mA   3.0 W   15° C.   65 Vh       SEP 1.2    50 V    0.1 mA   0.5 W   15° C.    0 Vh                  
 
      After electrophoresis, the gel is developed for 5-15 minutes (or until background development) with LI SILVER silver enhancement kit (Nanoprobes, Inc.). After silver development, gels are rinsed thoroughly with deionized water, and preserved by incubation in 20% glycerol in deionized water at 45° C. for 10 minutes.  
      28. Application of I and II to SDS-PAGE in Reducing Conditions:  
      Protein sample (Pharmacia LMW calibration kit proteins) to be analyzed is incubated with a 2.5% mercaptosuccinic acid solution in 10 mM Tris/HCl, 1 mM EDTA, pH 8.0. To the sample is added Sodium Dodecyl Sulphate (SDS) to 2.5%. The sample is heated for 5 minutes at 100° C. Bromophenol Blue is added to 0.01% and N-ethylmaleimide (prepared as a 20% solution in DMSO) is added to 5%. The sample is incubated at room temperature for 10 minutes. Excess amounts of I and II are added to the sample mixture. Using an eight “well” sample applicator, eight 1 μL aliquots of the sample mixture are applied to a Phastgel gradient gel (8-25) and run on the Pharmacia Phastgel system using the following protocol:  
                                                      Sample Applicator down at step 1.1    1 Vh           Sample Applicator up at step 1.1   10 Vh                                                 SEP 1.1   250 V   10.0 mA   3.0 W   15° C.   65 Vh       SEP 1.2    50 V    0.1 mA   0.5 W   15° C.    0 Vh                  
 
      After electrophoresis, the gel is developed for 5-15 minutes (or until background development) with LI SILVER silver enhancement kit (Nanoprobes, Inc.). After silver development, gels are rinsed thoroughly with deionized water, and preserved by incubation in 20% glycerol in deionized water at 45° C. for 10 minutes.  
     Nucleic Acid  
      Detection of Oligonucleotides  
      General:  
      Oligonucleotides modified to contain biotin can be detected through the use of either gold cluster labeled anti-biotin antibodies, antibody fragments, gold cluster-labeled streptavidin, or gold cluster-labeled avidin. Incorporation of the biotin into an oligonucleotide strand can occur via a commercially available biotin-NHS reagent and a primary amine that was introduced onto the oligonucleotide via a modified nucleotide or nucleotide substitute (phosphoramidite) by a DNA synthesizer. The biotin labeled oligonucleotide can then be reacted with gold cluster labeled reagent designed to react with the biotin moiety and observed via silver development.  
      29. An oligonucleotide having a primary amine attached to the 5′ end was reacted in sightly alkaine sodium phosphate buffer with a large molar excess of biotin-LC-N-hydroxysuceinimide ester (biotin-LC-NHS II; Pierce) dissolved in DMSO. After 1 hour at room temperature the reaction mixture was purified on a GH25 (nominal exclusion limit 3000 D) column and fractions measured for nucleic acid content by monitoring at 260 nm. By comparing the ratio of the 260 and 240 nm peaks to gauge biotin incorporation. Recovered nucleic acid yield 49%.  
      The biotin labeled oligonucleotides were reacted with streptavidin gold cluster conjugates in phosphate buffered saline (PBS), pH 7.4 for four hours at room temperature and separated by size exclusion chromatography (Superdex 75, nominal exclusion limit 70 kD) using the same buffer as eluant. On this column, the major peak coincided with the retention time of the cluster conjugate and showed enhanced absorption at 260 nm indicating reaction had occurred.  
      30. Detection of Biotinylated DNA: Dot Blots  
      Blot tests were conducted to test the incorporation of gold in the isolated product. In a typical run, biotin labeled oligonucleotides was serially diluted in water by factors of ten starting with a 2.5×10 −7  M solution and ending at 2.5×10 −16 6 M. 1 μL spots were applied to a nitrocellulose membrane by Drummond capillary pipettes. The membrane was allowed to dry and then subjected to a 302 nm light placed 18 cm from the membrane for ten minutes. The membrane was blocked with 4% bovine serum albumin (BSA) for 30 minutes at 37° C. The membrane was then incubated with the streptavidin gold cluster conjugate diluted to about 2 μg/ml in 0.8% BSA for 1.5 hours. After rinsing with buffer and then water, the membrane was treated with silver developer. The membrane was again rinsed with water and examined for spot generation after drying. The silver developed gold particle is seen as a dark spot. The spots appearing unambiguously above background provide a limit of detection on the order of 10 to 100 attomole (1 attomol=10 −18 8 mol) detected with more concentrated applications appearing darker than dilute ones.  
      31. Preparation and Detection of Biotinylated M13mp18  
      Another model system was the preparation of labeled complementary fragments to the single stranded phage M13mp18. Using the random primer method, biotin was introduced as modified dUTP and incorporated with Klenow. After purification using GeneClean, the biotinylated solution was applied to a nitrocellulose membrane, immobilized in a vacuum oven and detected by streptavidin gold cluster conjugate. In this procedure 10 −15  mol M13mp18 was detected. The detection limit was independent of the concentration of the gold conjugate.  
      Preparation of Gold Cluster Labeled DNA Hybridization Probes  
      32. Random Primer Extension Method  
      Gold cluster labelled nucleotide triphosphate was prepared by reaction of NHS-gold cluster with an amino modified nucleotide triphosphate. Example: Amino-7-dUTP, available from Clontech, is a dUTP analog with a primary amine covalently attached to the pyrimidine ring through a seven atom spacer arm. 50 nmol Amino-7-dUTP was reacted with 5 nmol mono-N-hydroxy-sulfosuccinimide-1.4 nm gold cluster in 20 mM HEPES-NaOH buffer pH 7.5 at 4° C. overnight. The reaction mixture was separated on a GH25 column to remove unreacted nucleotide from the conjugate. The product was further purified by ion exchange chromatography over TSK DEAE with elution at 0.3M triethylammonium hydrogencarbonate.  
      The modified nucleotide was incorporated into an oligonucleotide by enzymatic extension of random primers.  
      While the invention has been described by reference to specific embodiments, this was for purposes of illustration only. Numerous alternative embodiments will be apparent to those skilled in the art and are considered to be within the scope of the invention.