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Colloidal gold nanoparticles have been successfully synthesized using electro-dissolution-reduction process that consists of a simple two-electrode cells connected to a DC power supply. Throughout the process, bulk gold at the anode was oxidized into gold cations which then reacted with the chloride ions to form aurochloride complex. The complex ions were then reduced by the citrate ion to form colloidal gold nanoparticles. The size and shape of the nanoparticles were modulated by varying the citrate concentration. The colloidal gold nanoparticles obtained were characterized by field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and ultraviolet-visible spectrophotometer (UV-Vis). From FESEM analysis, it was found that by increasing the citrate concentration, the size of colloidal gold nanoparticles produced marginally increased. The mean sizes of gold nanoparticles were roughly about 18.7 nm, 19.3 nm, 20.5 nm and 21.3 nm for citrate concentrations of 0.05 M, 0.10 M, 0.15 M and 0.20 M, respectively. However, sample prepared without the addition of citrate, remained colorless indicating that aurochloride complex ions were not reduced to colloidal gold nanoparticles. TEM micrographs showed that the shape of gold nanoparticles obtained is almost spherical. The characteristic peaks of UV-Vis spectra revealed that the suspension was indeed colloidal gold nanoparticles.Keywords: Gold, Nanoparticles, Electro-dissolution-reduction

Gold nanoparticles were fabricated by reduction of highly concentrated Au(III) ions (200 mM) with casein proteins from milk. The gold nanoparticles were converted to nanoparticle-powders after washing and subsequent vacuum drying without aggregation. The nanoparticle-powders completely re-dispersed in aqueous solution, and stable colloidal gold nanoparticles were obtained. UV-vis extinction spectra and dynamic light scattering (DLS) measurements revealed that large assemblies (size, ca. 3 μm) and subaggregates (size,0.5 μm) composed of gold nanoparticle-casein protein chain-Au(III) ion were dynamically formed and disintegrated over the course of the growth of the gold nanoparticles. Fourier transform infrared (FT-IR) spectra indicated conformational changes of casein proteins induced by the interaction of casein protein-Au(III) ion and -gold nanoparticle. Finally, rapid, one-pot, and highly concentrated synthetic procedures of gold and silver nanoparticle powders protected by casein (mean diameters below 10 nm) were successfully developed using 3-amino-1-propanol aqueous solutions as reaction media. Dense colloidal gold (40 g L(-1)) and silver (22 g L(-1)) nanoparticle aqueous solutions were obtained by re-dispersing the metal nanoparticle powders.

A solution containing ssDNA was detected by using colloidal gold nanoparticles upon a color change of solution. First of all, we prepared colloidal gold nanoparticles with a size of less than 20 nm by means of citrate reduction of HAuCl 4 . Since colloidal gold nanoparticles modified with citrate anions have a negative charge, they are very well dispersed in the solution due to the negative charge repulsion, showing red color. If the electrostatic repulsion is screened by additives such as NaCl, the nanoparticles start to aggregate, leading to a color alteration. We found that the color alteration is retarded when the solution contains ssDNA, which plays a role in preventing the nanoparticles from aggregation. This allows the determination of whether or not ssDNA is present in a solution. However, the color alteration is not retarded when the solution contains dsDNA.

An electrostatic interaction is responsible for the attachment of gold seeds of 1-3 nm onto APTMS (3-aminopropyl trimethoxysilane)-coated silica cores in the formation of gold clusters. A surface plasmon resonance and morphology of gold clusters were significantly affected by the pH of gold colloids prepared by THPC reducing agent. Gold colloids of alkaline pH induced the heterogeneous deposition of gold seeds onto the silica nanoparticles, probably due to the continuous reduction of residual gold ions during the attachment process. Gold colloids of acidic pH induced the monodisperse deposition of gold seeds, consequently leading to the formation of smooth gold layer on the silica nanoparticles surface. The gold nanoshells (core radius = 80 nm) prepared by gold colloids of pH 3.1 exhibited the more red-shift and relatively stronger intensity of plasmon absorption bands, compared with gold nanoshells prepared by alkaline gold colloids of pH 9.7.

Synthesis of nanometer-sized particles with new physical properties is an area of tremendous interest. In metal particles, the changes in size modify the electron density in the particles, which shifts the plasmon band. The most significant size effects occur when the particles are ultrafine (size is

Surface modification of citrate-reduced gold nanoparticles with 2-mercaptosuccinic acid (MSA) was carried out in the aqueous phase. This provides a way to obtain carboxylic acid functionalized gold nanoparticles with diameter above 10 nm. The influence of the protecting MSA layer on the behavior of the modified gold nanoparticles in comparison with that of the traditional citrate-reduced gold colloid was evaluated by HCl titration, cyanide etching, and seeded growth tests. The modified gold nanoparticles show an improved stability against pH changes and cyanide etching. They do show further growth, which appears to be more homogeneous than for the unmodified particles.

Gold and silver nanoparticles were prepared by reducing aqueous solutions of the respective metal salts with sodium acrylate (SA), which also acts as capping agent. Gold nanoparticles were also prepared using poly(sodium acrylate) (PSA) with various molecular weights (2100, 5100, 15 000, and 30 000 g/mol). These polymers act as both reducing and stabilizing agents for the gold nanoparticles. Gold nanoparticles formed using SA are nearly spherical and have a narrow size distribution (11−17 nm). These particles are similar in size and stability to those formed by the conventional citrate reduction method. PSA-stabilized gold nanoparticles are somewhat less uniform in size but also highly stable. The polymer-stabilized gold particles can be used to form composite polymer films with optical properties that depend on the volume fraction of gold.

The gold nanoparticles were prepared in many methods, differed in reducing and dispersion agents and stability. The first solution was prepared by using potassium bitartrate as reducing agent and Polyethylene glycol as a dispersion factor. We got a solution of colloid gold with wine-red color that has been measured the particle size; it was between 50 - 200 nanometers. The second gold colloidal solution was prepared by using sodium citrate as a reducing agent and without adding a dispersion agent, the color of the final solution is dark grey, but this solution did not remain stable, deposition happened quickly and crystals separated from the solution. The third one was prepared by using sodium citrate as a reducing agent and the solution was heated to a certain temperature, and the color of the solution became dark red after several minutes. Either the fourth method has been prepared gold colloid solution by using potassium bitartrate as a reductant for the gold ion. We have noted that no reaction under normal conditions, while the reaction by heating the solution to high temperature, and when added poly vinyl pyrrolidone will lead to a solution of colloild gold with dark purple-red color and stable under normal conditions for a long time. We measured the particles size in this experiment; it was less than 100 nanometers. We conclude from the above that all the reducing agent needs to appropriate conditions to complete the process of reduction of gold ion, and to get the nano gold particles, therefore; these particles in the colloid solution need to dispersion factor to the survival of the particles widespread so that the solutions with certain concentrations of poly vinyl pyrrolidone can be proven in the process of stability of the gold colloid solution.

A new method to determine the number concentration of gold nanoparticles in colloidal solutions is based on silver electrodeposition on the particles adsorbed on a carbon paste electrode. When colloidal gold was attached to a carbon paste electrode surface and then oxidised in a suitable media, the overpotential for cathodic deposition of silver was lowered relative to that of the carbon paste. Thus a range of potentials exists where the direct reduction of silver ions on the carbon paste surface could be avoided whilst the process did proceed on the surface of the adsorbed gold particles. A pretreated carbon paste electrode was immersed in colloidal gold solutions for 10 min. The modified electrode was then activated in a suitable media (0.1 M NaOH and 0.1 M H 2 SO 4 ) to form oxides on the surface of gold particles. Then, silver electrodeposition at −0.12 V for 45 s followed by anodic stripping voltammetry were carried out in aqueous 1.0 M NH 3 –2.0×10 −4 M AgNO 3 . A linear relationship was found between the area of the anodic stripping peak and the number of colloidal gold particles cm −3 (from 3.62×10 7 to 9.06×10 8 particles cm −3 ) and a detection limit of 1.25×10 7 gold particles cm −3 were obtained. In addition, colloids of gold with molecules of protein (albumin) adsorbed on their particles could be determined. In this last case, a detection limit of 1.80×10 7 gold particles cm −3 was obtained.

Gold nanoparticles stabilized by a water-soluble mercapto ligand (sodium 3-mercaptopropionate) were produced by a modified citrate reduction of HAuCl 4 . No rigorous cleanliness is needed and the reductant is decomposed to CO 2 . Stable aqueous dispersions of gold nanoparticles without any precipitation for several months could be obtained. The particle size can be controlled by the stabilizer/gold ratios. These gold nanoparticles were incorporated successfully into the interlayers of cast films of cationic bilayers by immersion of the film in gold nanoparticle dispersions.

Monodispersed spherical gold particles ranging in modal diameter from 80 nm to 5 μm, were prepared by reducing tetrachloroauric(III) acid with iso-ascorbic acid in aqueous solutions at 20°C. The particle size was altered by changing the pH, which affected the composition of gold(III) solute complexes. The latter controlled the redox potential of the system, essential to the formation of the initial nanosize gold dispersions. Depending on the experimental conditions, the resulting primary particles remained either stable or they aggregated to form much larger uniform spheres. The mechanisms of the precipitation of the precursors (primary) particles and of their mutual interactions to yield the final dispersions are discussed.

The formation of colloidal gold produced from hydrogen tetrachloroaurate by three kinds of the reducing agents is studied by using a spectrophotometer. The absorption due to gold sol formation appeared in the 500 to 600nm wavelength. When tri-sodium citrate is used as a reducing agent, the wavelength of maximum absorbance due to colloidal gold shifts from high to low values and approaches to a constant value of 525nm. It apparently indicates that the particle size decreases over the course of the reaction. The reaction rate of the formation of gold sol decreases with the increase in the tri-sodium citrate concentration. Howerer, when citric acid or 1, 3-acetone dicarboxylic acid is used, the wavelength of maximum absorbance is almost constant at 525nm, and the reaction rate increases as a function of the reducing agent. The reason of the difference in these results could not be found in this work, so it might be necessary to study the citrate anion absorption of the surface of gold sol or the complex formation between citrate and gold ions.

For an ideal solution, an analytical expression for the macromolecule concentration, electrolyte concentration, and solution osmotic pressure is obtained on the basis of the van't Hoff equation and the Donnan equilibrium. The expression was further applied to a colloid solution of about 3 nm glutathione-stabilized gold nanoparticles. The concentration of the colloid solution and the average net ion charge number for each gold nanoparticle were determined with the measured osmotic pressure data. Meanwhile, the gold contents of the solutions were analyzed by means of atomic absorption spectrophotometry, and the results were combined with the determined concentration of gold nanoparticle colloids to determine that the average number of gold atoms per 3 nm gold nanoparticle is 479, which is 1/1.7 times the number of atoms in bulk metallic gold of the same size. The same proportion also occurred in the 2 nm 4-mercaptobenzoic acid monolayer-protected gold nanoparticles prepared by Ackerson et al., who utilized the quantitative high-angle annular dark-field scanning transmission electron microscope to determine the average number of gold atoms per nanoparticle (Ackerson, C. J.; Jadzinsky, P. D.; Sexton J. Z.; Bushnell, D. A.; Kornberg, R. D. Synthesis and Bioconjugation of 2 and 3 nm-Diameter Gold Nanoparticles. Bioconjugate Chem. 2010, 21, 214-218).

Nanosized colloidal gold was prepared by the reduction of chloroauric acid with trisodium citrate in aqueous solution with microwave heating method. The diameters of gold nanoparticles were from 13 to 70 nm with the ratio of the trisodium citrate and chloroauric acid. With rhodamine 6G (R6G) as the detection molecules, the SERS properties of gold colloid were studied. Experimental results show that the gold colloids with 50–60nm particle diameters are excellent SERS substrates and sodium chloride (NaCl) can improve gold colloid's surface-enhancement effect.

Stable dispersions of colloidal gold in n-hexane and cyclohexane have been prepared by transfer of colloidal gold particles from an aqueous phase to a n-hexane phase and to a cyclohexane phase, respectively. The phase transfer of gold particles occurred on addition of magnesium chloride to the emulsion of hexane in the aqueous suspension of gold with sodium oleate. The size distributions of the gold particles in the resulting colloidal gold in hexane suspensions were almost the same as that (mean diameter 14 nm, standard deviation 1.5 nm) of the original aqueous suspension of gold. The critical phase-transfer concentrations of various salts were determined. The phase-transfer powers of cations were larger than those of unions. Those of divalent and trivalent cations were exceedingly larger than that of the monovalent cation.

Lateritic gold enrichment can be associated with the formation of colloidal gold and its interaction with the iron-oxide-rich medium. Adsorption of colloidal gold on synthetic goethite and hematite was measured at 25°C. The observed high affinity between the oppositely charged particles depends on the medium composition. The presence of citrate anions can inhibit mutual coagulation, while inert electrolytes enhance this effect.

Stable sols of gold nanoparticles with an average diameter of about 13 nm were prepared from hydrogen tetrachloroaurate (prepared from bulk metallic gold) and trisodium citrate dihydrate. The organic salt acts as reducing as well as stabilizing agent for the gold nanoparticles, having an average diameter of 13±4 nm. The deep purple colloidal suspension is stable for several weeks. UV-Vis absorption spectra shows the typical surface plasmon resonance band of nanosized gold with an absorption maximum at 521.5 ± 0.5 nm.

Colloidal gold probes have become widely used for immunocytochemical staining at the electron microscopic level. Gold sols are producing by boiling a solution of tetrachloroauric acid with a reducing agent. The type of reducing agent and the concentration of components determine the final particle size. Gold sols that have a particle size ranging from 2 to 40 nm can be made in the laboratory, depending on the type and concentration of the reducing agent. This chapter details methods for producing various sizes of gold. The methods are relatively simple and very reproducible from batch to batch.

We have developed a simple method for the preparation of nearly mono-dispersed stable gold colloids with a fairly high concentration using a two step procedure. First we synthesize citrate capped gold nanoparticles and then exchange the citrate ions with triethyleneglycolmono-11-mercaptoundecylether (EGMUDE). This leads to the immediate precipitation and formation of composite assemblies. The gold nanoparticles were successfully self-redispersed after a few days. The prepared gold colloid can be easily concentrated up to 20 times by separation of the flocculated part. UV-visible spectra, transmission electron microscopy (TEM), and dynamic light scattering (DLS) were used to characterize the products thus formed.

Gold(III) (tetrachloroaurate(III)) was reduced in an aqueous solution containing only a small amount of 2-propanol to form colloidal gold nanoparticles in a standing wave system generated by a 200 kHz ultrasonic generator. The rates of gold(III) reduction and the sizes of the formed gold particles could be sonochemically controlled by controlling the irradiation parameters such as the temperature of the solution, the intensity of the ultrasound, and the positioning of the reactor. The size of gold particles strongly depended on the rate of gold(III) reduction, suggesting that the rate of gold(III) reduction affects the initial nucleation of the gold particles.

Two synthetic techniques for colloidal gold particles was improved by using SDS. And colloidal Au particles of mean diameters between 5 and 14 nm are synthesized, that exhibit improved monodispersity relative to previously published methods. According to the particular molecular structure of surfactants and different electrons distribution arising from colloidal small sizes and high surface/ volume ratios, it is found that there are a complex between SDS and Au3+ (and Au atoms ) during synthesizing colloidal gold nanoparticles and as a stablizer for Au particles, SDS can prevent their further growth. So the colloidal gold particles is monodispersize and more steady. But other surfactants don't affect the process of synthesizing gold nanoparticles because of their structures and properties different from SDS. Gold nanoparticles have considerable bioaffinity and can be applied to study the adsorption of proteins or polypeptides.

The preparation of well-size-controlled colloidal gold nanoparticles in organic solvent is presented. After the preparation of well-size-controlled aqueous colloidal gold particles, we changed the solvent to an organic one. This technique is required to enable a chemical reaction between gold particles and hydrophobic molecules, since a colloidal gold solution is typically prepared in water using a reduction process. We also investigated the stability of the gold particle suspension, and found that the stability decreases in the sequence of water, ethanol, chloroform, and benzene solution.

A new method of preparing metal sols, by the use of a continuous high frequency electrical discharge of sufficient intensity to produce a sinusoidal spark, is described. Stable yellow colloidal silver and purple colloidal gold have been prepared, without a protective colloid, by this method. Chemical determinations have failed to indicate any silver oxide in the yellow silver sols.

1. A slight modification of Zsigmondy's method of preparation of colloidal gold is described, wherein conductivity, instead of triply distilled water is used, in addition to an increased amount of alkali. 2. The titration of the solution is accomplished with phenal red as an indicator, and thePh is determined with permanent inorganic standards.

In this paper I should like to emphasise the technique of the colloidal gold reaction rather than the results I have obtained, for the cerebro-spinal fluids I have been so far able to examine have been mostly from cases of dementia paralytica, and, though the results are interesting and suggestive, the number of cases is too limited to allow one to dogmatise on the findings.

The synthesis of gold nanoshells involves the preparation of precursor seed particles consisting of nanoparticulate gold loaded on a dielectric core as scaffolds on which the layer of gold shell can be grown. The common route in preparing these seed particles involves a two-step technique of synthesizing colloidal gold particles followed by attaching them to amine functionalized dielectric core. In this paper, we present the use of a singlestep deposition-precipitation (DP) process, commonly used in the catalytic field, as a feasible alternative route to seeding gold hydroxide nanoparticles onto a silica core and growing a layer of gold shell around the core without the need for prior synthesis of colloidal gold. The various factors such as pH, temperature and time of reaction, as well as the effect of amine functionalization of silica on the deposition of gold were investigated. Compared to bare silica nanoparticles whose low isoelectric point render the DP process of seeding nanoparticulate gold unfavorable, amine functionalization of the silica surface is able to alter its isoelectric point to facilitate the deposition of gold hydroxide nanoparticles and increase the uniformity and density of seeding. We have also established that by varying the pH and time of reaction, it is possible to control the size of the gold hydroxide nanoparticles and density of seeding. The highest seeding density was achieved at pH 8 where the surface charge on the amine terminated silica surface favored the attraction of complex gold anions and the hydrolysis of these complex anions at elevated temperature produced the insoluble gold hydroxide precipitate which was readily deposited on the silica. As the time of reaction was increased from 3 min to 60 min, these gold hydroxide nanoparticles also increased in size from 2 nm to 7 nm. We have also shown the progressive growth of the gold hydroxide seeds to eventually form gold nanoshells with a complete layer of gold shell with tunable optical response.

Gold nanoshells are new constructional units consisting of dielectric core encapsulated by gold nano- layer that exhibits a plasmon-derived optical resonance typically shifted to near-infrared (NIR) wavelengths (800∼1200 nm). The fabrication of gold nanoshell was optimized based on the relative contributions of the aging of gold colloids, concentration of reducible gold salts, and dosages of formaldehyde reducing agents. One-week-aged gold colloids produced a relatively monodisperse deposition of gold seeds onto silica nano- particles, whereas one-day-aged gold colloids produced a heterogeneous deposition of gold seeds on the silica nanoparticle surfaces. At the optimal dosages of reducing agents, the thickness of the gold layer increased with the increment of the concentrations of the reducible gold salts, consequently leading to the smoother morphol- ogies and stronger plasmon resonances of the gold nanoshells. The excessive addition of gold salts and/or re- ducing agents, however, resulted in a significant reduction of the plasmon resonance because of the increased aggregation between the gold nanoparticles.

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With the application of ultrasonic energy a colloidal gold may be easily prepared with a particle diameter of less than 10 nm and suitable for use as an immunological marker for TEM-studies. This new approach replaces the use of phosphorus, traditionally used for producing gold sols of the smallest diameter.

(1910). COLLOIDAL GOLD AND “PURPLE OF CASSIUS.”. Transactions of the Royal Society of South Africa: Vol. 2, No. 1, pp. 203-204.

Colloidal silver and gold have been formed by irradiation of AgClO4 and HAuCl4 solutions with 253.7 nm light in the presence of protective agents such as sodium dodecylsulphate, sodium alginate and colloidal silica. Absorption maxima of colloidal silver and gold have been observed at 390–420 and 540–560 nm, respectively. Colloidal metals have been purified by dialysis and their amounts analysed quantitatively. The quantum yields of metal deposition are 0.053–0.15 for colloidal silver and 0.030–0.039 for colloidal gold. The quantum yield is increased several times by the use of protective agents, with the exception of a AgClO4–sodium alginate solution.

SOME time ago, in a communication from this Department1, it was shown that a steadily increasing exposure to X-radiation produced alternate increases and decreases in the electrokinetic potential of certain colloid particles. The colloid used at the time (a dilution of 'Aquadag' in water) is peculiarly stable, and at no stage were there any signs of coagulation. It was thought possible that with a more sensitive colloid the decrease in potential at the minima of the curve might be sufficient to bring the colloid to its flocculation point, and at my suggestion Dr. Liebmann and Mr. Jones have been working with certain pure gold sols.