Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF METAL NANO PARTICLES

Abstract

A process for the preparation of metal nano particles: The invention provides a rapid miniscale isoelectric focusing process that separates nanocrystals of noble metals preferably gold, silver or metal sulfides preferable cadmium sulfide of various sizes that fall in between 2 nm to 20 nm. The nanocrystals of gold and silver are separated from their mixture in this invention. The separated metal clusters are monodispersed indicating their possible applications in various fields.

Full Text

This invention relates to a process for the preparation of metal nano particle . More particularly it relates to a rapid miniscale isoelectric focusing process that separates nanocrystals of noble metals preferably gold, silver or metal sulfides preferable cadmium sulfide of various sizes that fall in between 2 nm to 20 nm. The nanocrystals of gold and silver are separated from their mixture in this invention. The separated metal clusters are monodispersed indicating their possible applications in various fields. Colloidal dispersions of metals exhibit absorption bands or broad regions of absorption in the ultraviolet-visible range. These are due to the collective excitations of electrons known as plasma oscillations or interband transitions and they are very characteristic property of metal colloids. Nanoparticles are entities which lie in between molecules and bulk. Their optical, electronic and thermodynamic properties are completely different from that of the bulk. At a particular size of the crystallite the properties start to change. This effect is called quantum size effect. Many other properties that the theory predicts cannot be proved experimentally due to lack of size and shape uniformity. The optical properties of small metallic particles are unique and are of practical importance. These colloids can be used in the manufacturing of optical filters as labels for biomacromolecules as well as in the production of reversible photosensitive monochromatic glasses [Freeman, R. G. et al (1995) Science 267,1629]; [Encyclopedia of chemical Technol. (1994) 12, 569-571]. They are also useful for enhancement of intensity in Raman Spectroscopy. Gold and/or silver particles consisting of 70 to 800 atoms are known to exhibit optical properties of special importance. Some of these properties are demonstrated by conventional optical spectrophotometric methods. The optical absorption spectra of the nanometer scale metal particles can indicate the nature of their electronic structure. For the systematic study of nanocrystals of gold/silver obviously require high purity samples of different sizes. The special optical properties of metallic clusters are characterized by a prominent broad absorption band that is not observed in the bulk spectra.[Alvarez M.M (1997), J.Phys.Chem, 101, 3706-3712]. The presence of this band in the visible region of the spectrum is responsible for the striking colors of dilute colloidal solutions of noble metals like gold, silver and metal sulfides. The colloids that are synthesized normally are polydisperse. For the determination of size dependent properties it is prerequisite to minimize the effect of size distribution. Therefore, several methods were attempted to control the size and size distribution of the particles by some investigators. In the prior art process for the separation of cadmium sulfide (CdS) particles with a mean diameter between 40 and 60 Å and a relatively narrow size distribution are available. The separation is achieved by controlled precipitation at a definite pH [Spanhel L. et al (1987) J. Am. Chem. Soc. 109, 5649-5655]. A method for incorporating quantized particles of colloidal semiconductors in transparent silicate glasses has also been described [Rajh, T. et al. (1988), Chem. Phys. Lett. 143, 305-308]. The utility of inverted micelles in producing CdS (5 Å) particles of very low aggregation number has been described by a few investigators [Lianos. P. and Thomas J.K. (1986) Chem. Phy. Lett. 125, 299-308]. CdS and PbS clusters could be encapsulated to study their optical properties. Since zeolites have well-defined crystalline structures with internal porous open framework of molecular dimensions of 3-13 Å [Wang,Y. and Herron, N.(1987) J. Phys. Chem. 91, 257-260]. Isolation of polydisperse CdS colloidal particles into fractions of narrower size distribution by low and high pressure size exclusion chromatography has also been described in the literature [Fischer, Ch.-H. et al. (1989) 5, 429-432]. Laponite, porous vycor glass and molecular sieves, faujasite X and sodalite, have been used to prepare small particles of CdS with limited dimensions [Stramel, R. D. et al (1988) J. Chem. Soc. Faraday. Trans. I 84, 1287-1300]. Preparative gel electrophoresis was applied to separate Q-size CdS colloids into fractions containing highly monodisperse particles [Eychmuller, A. et al (1990) Langmuir 6, 1605-1608]. These methods to obtain refined Q-sized nanocrystal molecules are mainly described for CdS colloids some of them are time consuming and low yielding. The major drawbacks of the earlier methods for obtaining pure forms of nanometer- scale metal particles are:- (1) the colloids are not obtained in a refined form. (2) they are time consuming and costly. (3) the yields are low. On the contarary, the present process is rapid and the products are obtained in a short period of 5 hours. The inventors of the present invention have, during their course of research, invented that the miniscale isoelectric focusing techniques can be used to purify the nanocrystals of noble-metals/metal sulfide colloids in refined condition within a short time. The object of the present invention is to provide an improved process for the preparation of sulphide nano metal particles exemplified but not limited by gold, silver and cadmium sulfide colloids from their mixtures with different shapes, sizes, electrical charges and electrokinetic properties. Another object is to increase monodispersity of the metal colloids and the refined nanocrystals of metals preferably gold or silver to obtain rapid separation by fabricating a U - tube assembly or miniscale single column isoelectric focusing unit. Accordingly the present invention provides a process for the preparation of metal nanoparticle preferably nanocrystals of noble metal gold or silver or metal - sulfide preferably cadmium sulfide which comprises preparing the colloidal particles of the metals as defined above by reduction of aqueous solution of metal salts selected from gold, silver, cadmium salt by a conventional reducing agent such as herein described, and inparting charge on the metal particles by surface derivatization by a capping agent preferably 4 - carboxy thio phenol having concentration 10-5 M , separating colloidal metal particles using an isoelectric focusing unit applying a 4mA current at 400 V for a period ranging between 5 to 18 hrs., obtaining separated bands of nano dimensional particles and setting the fraction of purified metal-sulfide nano particles using the side arm of the said isoelectric focusing unit by conventional methods such as herein described to get the purified nanocrystal of noble metals/ metal - sulfide. In one of the embodiments of the present invention the metals of which the colloids could be separated may be the metals which could be obtained in colloidal form exemplified but not limited by gold, silver or cadmium sulfide. In another embodiment of the present invention , gold colloids were prepared by the reduction of 100 ml [ 10-4 M chloroauric acid , HAuCl4 ] using sodium borohydride ( 0.01 g NaBH4) and surface derivatized by the molecule 4 - carboxy thio phenol (4-CTP). This preparation is negatively charged. It was dialyzed against water in a collodione bag for 3 hours at 25 °C. This preparation is consisting of different types of gold colloids. In yet another embodiment, silver colloids were prepared by the reduction of silver sulfate (Ag2SO4) using sodium borohydride and surface derivatized by 4-carboxy thio phenol (4-CTP).This preparation was dialyzed against water in a collodione bag for 1 hour at 25 °C. This preparation is consisting of different types of silver colloids. EXAMPLE 1 This example illustrates the separation of nanometer-scale metal particles using miniscale U-tube isoelectric foucusing unit. This unit was consisting of two hollow tubes with 42 cm and 43 cm diameter joined by a rubber tubing. A side arm was fixed with a small cock attached to the right arm of U-tube which facilitates easy collection of nanometer scale metal colloids directly in a fraction collector without disturbing the pH and density gradient. The U-tube assembly was placed in a cold room (5° C + 2). The gradient solutions were prepared as follows: Light density solution: Water, 3.0 ml; ampholyte pH 3.0 - 10.0 (40 %), 0.2 ml; and colloidal solution 2 ml. Separation solution: Glycerol, 1.5 ml; and water, 1 ml. The electrode solutions were prepared as follows: Cathode solution: 0.1 M NaOH, 2.5 ml and water, 7.5 ml. Anode solution: 0.1 M o-Phosphoric acid, 4.0 ml and sucrose, 15 g; diluted to 25 ml by adding water. Small gold colloids (average size 20 Å) and big gold colloids (average size 130 Å) were prepared separately and then loaded in 1:1 ratio. Preparation of small gold colloids: Reduction of 100 ml chloroauric acid (HAuCl4, 1.25 X 10-4) was carried out by 0.01 grams of sodium borohydride (NaBH4) in presence of the capping agent 4-carboxy-thio-phenol (4-CTP). The concentration of the capping agent used for surface derivatization was 10-5 M. The gold colloids formed after complete reduction showed a maximum absorbance at 510 nm. Preparation of big gold colloids: Chloroauric acid (100 ml, 1.25 X 10-4 M, HAuCl4) was heated slowly on a magnetic stirrer. When the temperature reaches 100 °C, reduction was carried out by slowly adding 10 ml, 1 % citric acid. After continuing to boil for some more time the color changes to dark blue. The colloidal solution so formed was cooled for half an hour. After increasing the pH to 8.5, surface derivatization was done by adding 4-CTP (10-5 M). The gold colloids formed after complete capping showed a maximum absorbance at 526 nm. The 1:1 ratio of small and big colloids was dialyzed against water in a collodione bag for 3 h at 25°C. The heavy electrode solution was filled in the left arm and a pinch cock which was fitted at the center of flexible tubing was closed. One millilitre of separation solution was layered above the heavy electrode solution in the right arm. This was followed by introducing the gradient solutions in the right arm using a gradient mixer. Cathode solution was layered gently above the gradient with the help of a syringe. Then the left arm was filled with heavy electrode solution 1 cm less than right arm to balance the level of solutions in the both arms of unit. The pinch cock at the center of U-tube was released. Different layers of the solution are shown in schematic diagram. The platinum electrodes were dipped into respective solutions. A direct current 4 mA at 400 V was applied for 12 h. The electrofocusing was discontinued and the cock (at the center) was closed.. Fractions each of 0.2 ml were collected by opening the cock of the side arm attached to the right arm electrofocusing column. Purity of the gold colloids (that was observed as a single sharp band) was ascertained spectrophotometrically at a wave length of 514 nm. Thus, a separation of two types of gold colloids was obtained within 12 h and the purity was ascertained spectrophotometrically. The recovery of the refined colloids is more than 90%. EXAMPLE 2 Small gold colloids (average size 20 Å) and big gold colloids (average size 130 Å) were prepared separately as shown in example 1 and then loaded in 1:1 ratio. The 1:1 ratio of small and big colloids was dialyzed against water in a collodione bag for 3 h at 25°C. The separation of these two types of gold colloids was achieved within a period of 5 h in a single tube minscale isoelctric focusing unit with some modifications.(fig. 1). In this case the carrier electrolyte solution used was 300 µl in the pH range 3.5 to 10. The unit consists of a single main column (43 X 0.8 cm) provided with a side arm for easy collection of various fractions. The unit is provided with an ion conducting polymer containing ethyl hydroxy groups at the anodic end of the unit. That has got two functions, it separates the anodic solutions from the rest of the solutions and it provides an easy path for charge transport. Thus two forms of gold colloids from their mixture was separated in a single column isoelectric focusing unit in 5 h. The purity of the separated samples was checked spectrophotometrically. EXAMPLE 3 Small gold colloids (average size 20 Å) and big gold colloids (average size 130 Å) were prepared separately and then loaded in 1:1 ratio. Small and big gold colloids were synthesized as in example 2. The 1:1 ratio of small and big colloids was dialyzed against water in a collodione bag for 3h at 25° C. The separation of these two types of gold colloids was achieved in a single tube miniscale isoelectric focusing unit using a mixture of ampholytes of the pH range 6 to 8 and 8 to 10.5 within 5 h. The two forms of gold colloids was separated and the purity was ascertained spectrophotometrically. EXAMPLE 4 Medium sized gold colloids (average size 40 Å) and big gold colloids (average size 130 Å) were prepared separately and then loaded in 1:1 ratio. Big gold colloids were synthesized as in example 2. The medium sized gold colloids were synthesized as follows: Reduction of 100 ml, 1.25 X 10-4 M chloroauric acid was carried out by 0.01 g sodium borohydride. The gold colloids formed after complete reduction showed a maximum absorbance at 504 nm. After reaching this value of absorbance, surface derivatization was carried out by adding 10-5 M capping agent, 4-CTP. After complete surface derivatization, the absorbance maxima red shifts to 518 nm. The 1:1 ratio of medium and big colloids was dialyzed against water in a collodione bag for 3 h at 25° C. The separation of these two types of gold colloids was achieved in a single tube miniscale isoelectric focusing unit using a mixture of ampholytes of the pH range 6 to 8 and 8 to 10.5 within 5 h. The two forms of gold colloids were separated and the purity was ascertained spectrophotometrically. EXAMPLE 5 In this example, the separation of small gold colloids (average size 20 A) and silver colloids (average size 70 A) is shown.. Silver colloids were synthesized as follows : Silver sulfate (100 ml, 2 X 10-4 M, Ag2SO4) was reduced by using 0.01 g sodium borohydride. Uncapped silver sol is yellowish in color and shows maximum absorbance at 388 nm. After complete reduction, 1 ml 10-5 M, 4-CTP was used for surface derivatization. After complete surface derivatization the absorbance maxima red shifts to 398 nm. The 1:1 ratio of Au and Ag colloids was dialyzed against water in a collodione bag for 3 h at 25 °C. The separation of these two types of sols was achieved within 6 h in a single tube miniscale isoelectric focusing unit using a mixture of carrier electrolyte in the pH range 6 to 8 and 8 to 10.5. The purity of the separated sols was ascertained spectrophotometrically. Example-6 In this example, the size separation of cadmium sulfide (CdS) colloids is shown. The mixtures of different sizes of CdS colloids were synthesized as follows:- Cadmium sulfate ( 100 ml, 10-4M, 3 CdSO4, 8 H2O) was reduced by 0.01g sodium sulfide (Na2S) in the presence of 4 CTP (10-5 M). After complete reduction the cadmium sulfide so formed showed the absorption band edge at 476 nm. These colloids were dialyzed against water in a collodione bag for 3h at 25 °C. The separation of CdS colloids according to their different sizes was achieved at a pI of 7.7 and 9.6 in a single tube miniscale isoelectric focusing unit using a mixture of ampholytes of the pH range 6 to 8 and 8 to 10.5 within 5 h. The purity of separated colloids was ascertained spectrophotometrically. The size difference was evident by the fact that different sizes show different absorbance band edges in the UV-VIS spectra. We Claim: 1. A process for the preparation of metal nanoparticle preferably nanocrystals of noble metal gold or silver or metal - sulfide preferably cadmium sulfide which comprises preparing the colloidal particles of the metals as defined above by reduction of aqueous solution of metal salts selected from gold, silver, cadmium salt by a conventional reducing agent such as herein described, and inparting charge on the metal particles by surface derivatization by a capping agent preferably 4 - carboxy thio phenol having concentration 10-5 M , separating colloidal metal particles using an isoelectric focusing unit applying a 4mA current at 400 V for a period ranging between 5 to 18 hrs., obtaining separated bands of nano dimensional particles and setting the fraction of purified metal- sulfide nano particles using the side arm of the said isoelectric focusing unit by conventional methods such as herein described to get the purified nanocrystal of noble metals/ metal - sulfide. 2. A process as claimed in claim 1 , wherein the reducing agent is selected from sodium borohydride, citric acid and sodium sulfide. 3. A process for the preparation of metal nanoparticle substantially as herein defined with reference to examples.

Documents:

1577-del-1999-abstract.pdf

1577-del-1999-claims.pdf

1577-del-1999-correspondence-others.pdf

1577-del-1999-correspondence-po.pdf

1577-del-1999-description (complete).pdf

1577-del-1999-drawings.pdf

1577-del-1999-form-1.pdf

1577-del-1999-form-19.pdf

1577-del-1999-form-2.pdf

1577-del-1999-form-3.pdf