FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICA TJON
(See section 10, rule 13)
1. Title of the invention
A PROCESS TO PREPARE AN AQUEOUS SUSPENSION OF COPPER NANOPARTICLES AND AQUEOUS
SUSPENSION THERE OF
2. Applicant (s)
Name Nationality Address
TATA CHEMICALS LIMITED INDIA BOMBAY HOUSE:., 24 HOME MODI STREET. MUMBAI-
400001
3. Preamble to the description
COMPLETE SPECIFICA TION
The following specification particularly describes the invention and the manner in which it is
to be performed.

FIELD OF INVENTION
The present disclosure relates to production of stable suspension of copper nanoparticles in aqueous medium. More particularly, the present disclosure relates to a process for producing high concentration, stable, copper nanoparticles suspension in aqueous medium.
BACKGROUND
In the past few years, the synthesis of Copper (Cu) nanoparticles has attracted much attention because of its inherent properties such as high electrical and thermal conductivity, corrosion resistance, and antimicrobial efficacy in emerging applications including electronics, appliance heat exchanger, and hygienic surfaces. A number of methods have been developed to prepare copper nanoparticles in aqueous/organic medium. One of the methods of preparation of such nanoparticle is wet chemical process. The wet chemical process uses certain reducing agents to change the oxidation state of metals and form nanoparticles. Some known reducing agents are tri-sodium citrate dihydrate, ascorbic acid, sodium borohydride, sodium formaldehyde sulfoxylate, hydrazine hydrate or D-Glucose etc. However, a major problem in utilizing these copper nanoparticles is their inherent tendency to oxidize in ambient conditions to form Cu2O and CuO. This inherent tendency of copper nanoparticle to oxidize makes it further difficult to prepare stable dispersion of copper nanoparticles in an aqueous medium. The presence of copper oxides on the surface of nanoparticles has two negative consequences: it increases the required sintering temperature, and reduces the electrical conductivity. To overcome this, various protection methods have been implemented in order to prevent the oxidation of copper, which in general is based on minimizing the exposure of the copper nanoparticles to oxygen, by a protective layer of polymer, surfactant, metals etc and additionally providing

an antioxidant. However, limited published and patent literatures describe methods to obtain stable aqueous suspensions of copper nanoparticles. Further, in the field of nanotechnology. the supply of high molar concentration of copper nanoparticles in aqueous medium is still an important issue which requires immediate attention. Another important issue is the cost of production of such nanoparticles. Tomonari et al. (2008): "Size-Controlled Oxidation-Resistant Copper Fine Particles Covered by Biopolymer Nanoskin" Journal of Nanoscience and Nanotechnology Vol.8, 2468-2471, describes a method of preparing copper nanoparticle suspension in water using mercaptoacetic acid as a complex agent, hydrazine monohydride as reducing agent and gelatin as a protective agent. However, such a method is tedious and is expensive to carry out. A process for preparation of copper nanoparticles is described in Indian Patent Application No. 325/MUM/2010 which discloses a process for preparing an antimicrobial composition. The disclosed process of preparation of copper nanoparticles comprises forming the copper nanoparticles on the rice husk ash by an in situ process of reacting rice husk ash with a copper precursor and a reducing agent (sodium formaldehyde sulfoxylate) at elevated temperature in the presence of a stabilizing agent (gelatin). However, such methods are not suitable for preparing high molar concentration copper nanoparticles suspensions which are stable for long period of time at room temperature,
Thus there is a need to devise a simple and cost-effective method which can be used to prepare aqueous suspensions of copper nanoparticles that are stable for long term at room temperature. It is also desirable that such a method can also be used to prepare high molar concentration aqueous suspensions of copper nanoparticles that are stable for long periods of time at room temperature.

SUMMARY
The present invention relates to a process for preparing an aqueous suspension of copper nanoparticles. The said process comprises of dissolving a biopolymer selected from the group comprising of gelatin and chitosan in an aqueous solution of a copper precursor, followed by reducing the said copper precursor at room temperature by adding to the aqueous solution of copper precursor and biopolymer. an aqueous solution of sodium formaldehyde sulfoxylate.
DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1 shows the UV-Visible absorption spectrum of copper nanoparticles in the aqueous suspension prepared by the present method using gelatin as the stabilizing agent.
Figure 2 shows the particle size measurement of copper nanoparticles as synthesized in aqueous solution.
Figure 3 shows the UV-Visible absorption spectrum of copper nanoparticles in the aqueous suspension prepared by the present process using chitosan as the stabilizing agent.
Figure 4 shows the particle size measurement of copper nanoparticles in the aqueous suspension prepared by the present processs using chitosan as stabilizing agent.
Figure 5A indicates the zeta potential of the aqueous suspension of copper nanoparticles prepared by the present process using gelatin as stabilizing agent.
Figure 5B indicates the zeta potential of the aqueous suspension of copper nanoparticles prepared by the present process using chitosan as stabilizing agent.
Figure 6A illustrates UV-Visible spectrum of the copper nanoparticles prepared by the present process using gelatin as stabilizing agent, after 2 hours and 20 days.
Figure 6B illustrates UV-Visible spectrum of the copper nanoparticles prepared by the present process using chitosan as stabilizing agent, after 2 hours and 20 days.

DETAILED DESCRIPTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment described and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the process, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
Reference throughout the specification to an embodiment, an aspect or similar language means that a particular feature, characteristic or step in connection with the embodiment is included in at least one embodiment of the present invention. References to an aspect or an embodiment may, but do not necessarily all refer to the same embodiment.
The present disclosure provides a wet chemical process for preparing stable copper nanoparticles suspension in aqueous medium. The said process comprises of reducing a copper precursor at room temperature by using a specific reducing agent in the presence of a specific stabilizing agent. Here, the reducing agent is sodium formaldehyde sulfoxylate and the stabilizing agent is a biopolymer selected from the group comprising of gelatin and chitosan.
In accordance with an aspect, sodium formaldehyde sulfoxylate performs a dual role of a reducing agent and a stabilizing agent in the preparation of said copper nanoparticles suspension. Firstly, sodium formaldehyde sulfoxylate causes partial reduction of the copper precursor at room temperature. In addition to this, due to the reduction being carried out at room temperature, some of the sodium formaldehyde sulfoxylate molecules remain inactivated which acts as stabilizing agent and further prevents aggregation of the copper nanoparticles formed in the aqueous suspension.

Further, due to the partial reduction of copper precursor, a fraction of copper precursor remains in ionic form after the reduction process is over. The left over copper precursor in ionic form increases the zeta potential of the aqueous suspension of copper nanoparticles causing additional charge stabilization of copper nanoparticles in the aqueous suspension and hence further stabilizes the said suspension. Figure 5A and 5B shows the zeta potential of the aqueous suspension of copper nanoparticles prepared by the present process using gelatin and chitosan respectively.
In accordance with another aspect, the biopolymer selected from the group comprising of gelatin and chitosan . also performs a dual role of a stabilizing agent and an antioxidant, thereby further assisting in preventing the aggregation of copper nanoparticles in the aqueous suspension formed. Additionally, the biopolymer prevents oxidation of the copper nanoparticles in the aqueous suspension. This particular wet chemical process using sodium formaldehyde sulfoxylate as the reducing agent and the said biopolymers as the stabilizing agent at room temperature causes the formation of aqueous suspension of copper nanoparticles, which are stable for long period of time at room temperature.
The copper precursor may be any base metal salts including but not limited to CuS04, CuCI2, Cu(HCOO)2, Cu(CH3COO)2, CuCO3, Cu(NO3)2. Preferably the copper precursor is CuSO4.
In accordance with an aspect, the aqueous solution of the copper precursor may have a molar concentration between 0.1 to 2 Molar and is preferably 0.5 Molar. By way of a specific example, the molar concentration of the aqueous solution of copper precursor is 2 Molar.
In accordance with an aspect, the amount of biopolymer is 1-5 percent by volume of the aqueous solution of copper precursor and is preferably 2 percent by volume of the aqueous solution of copper precursor.

In accordance with an aspect, when gelatin is selected as the biopolymer in the present process, gelatin is preferably dissolved in the aqueous solution of the copper precursor by heating and stirring between approximately 50 to 100 degree Celsius and more preferably at 50 degree Celsius. By way of a specific example, the amount of gelatin equal to 2 percent by volume of the aqueous solution of copper precursor, is dissolved in the aqueous solution of copper precursor by heating and stirring at approximately 50 degree Celsius.
In accordance with an aspect, when chitosan is selected as the biopolymer in the present process, chitosan is dissolved in an aqueous solution containing 5 percent by volume of citric acid and this resulting solution is then added to the aqueous solution of the copper precursor. This is required as chitosan has poor solubility in neutral pH and is soluble in slightly acidic condition. By way of a specific example, 1 percent by volume of chitosan is dissolved in 100 milliliters of water containing 5 percent by volume of citric acid.
After dissolving the biopolymer in the aqueous solution of copper precursor, the resulting aqueous solution of copper precursor and the biopolymer is allowed to cool to room temperature. The aqueous solution of sodium formaldehyde sulfoxylate is then added dropwise at room temperature to the aqueous solution of copper precursor and the biopolymer, with stirring and preferably with vigorous stirring.
In accordance with an aspect, the aqueous solution of the sodium formaldehyde sulfoxylate may have a molar concentration in the range of 0.1- 10 Molar and is preferably 5 Molar. By way of a specific example, the molar concentration of aqueous solution of sodium formaldehyde sulfoxylate is 5 Molar.

In accordance with an aspect, the said process can be used to obtain stable aqueous suspension of copper nanoparticles of molar concentration up to 2 Molar which is stable for a long period of time.
In accordance with an aspect, the particle size of the copper nanoparticles formed by the present process is in the range of 5-1000 nanometers and is particularly in the range of 10-100 nanometers.
The present invention has advantages of producing stable and uniformly dispersed aqueous suspension of copper nanoparticles at room temperature. In addition, high molar concentration aqueous suspension of copper nanoparticles prepared by the present process is stable for longer periods of time.
In accordance with an aspect, an experiment was conducted to evaluate the stability of the copper nanoparticles in aqueous suspension with respect to time. The aqueous suspensions of copper nanoparticles formed in Example I and Example 2 (given below) were analyzed by UV-Visible spectrometer. Figure 6A and 6B illustrates UV-Visible spectrum of the copper nanoparticles prepared by the present process using gelatin and chitosan respectively, after 2 hours and 20 days.
The following examples are provided to explain and illustrate preferred embodiments of the process of the invention:
Example 1:
2 moles of Copper Sulphate (CUSO4.H2O) is dissolved in 1 liter of water to prepare 2 Molar aqueous solution of Copper sulphate. 2 percent by volume of gelatin is then dissolved in the aqueous solution of copper sulphate by heating and stirring at 50 degree Celsius for 20 minutes. To the resulting solution, an aqueous solution of Sodium formaldehyde sulfoxylate of 5 Molar is added dropwise with vigorous stirring, at room

tempeiature. The aqueous suspension of copper nanoparticles thus formed is very well dispersed and stable. The copper nanoparticles thus formed are analyzed by UV-Visible spectrometer and particle size analyzer and Transmission electron microscopy. Figure 1 shows the UV-Visible absorption spectrum of copper nanoparticles in the aqueous suspension prepared by the present method using gelatin as the stabilizing agent and Figure 2 shows the particle size measurement of copper nanoparticles as synthesized in aqueous solution.
Example 2:
0.01 moles of Copper sulfate (CuS04.5H2O) is dissolved in 1 liter of distilled water to prepare 0.01 Molar aqueous solution of Copper sulfate. 1% by volume of chitosan is then dissolved in 100 milliliters of water containing 5 % by volume of citric acid. 100 milliliters of this resulting solution is added to 1 liter Copper sulfate solution and stirred for 15 minutes. 0.5 M sodium formaldehyde sulfoxylate (CH3NaO3S,H2O; SFS) is prepared separately in 100 milliLiters of distilled water by stirring it for 20 minutes till it dissolves completely. To the resulting solution, 100 mL of above sodium formaldehyde sulfoxylate solution is added drop wise while stirring the solution mixture, at room temperature. The reduction of Cu2+ ions to Cu occurs during the addition of sodium formaldehyde sulfoxylate to the copper sulfate -chitosan mixture. The copper nanoparticles thus formed are very well dispersed & stable. The copper nanoparticles thus formed are analyzed by UV-Vis spectrometer & particle size analyzer. Figure 3 shows the UV-Visible absorption spectrum of copper nanoparticles in the aqueous suspension prepared by the present method using chitosan as the stabilizing agent and Figure 4 shows the particle size measurement of copper nanoparticles as synthesized in aqueous solution.

SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
A process for preparing an aqueous suspension of copper nanoparticles comprising the steps of dissolving a biopolymer in an aqueous solution of a copper precursor, the said biopolymer being selected from the group comprising of gelatin and chitosan. followed by reducing the copper precursor at room temperature by adding to the aqueous solution of copper precursor and the biopolymer an aqueous solution of sodium formaldehyde sulfoxylate.
Such a process for preparing an aqueous suspension of copper nanoparticles wherein the aqueous solution of the copper precursor has a molar concentration between 0.1 to 2 M and is preferably 0.5 M.
Such a process for preparing an aqueous suspension of copper nanoparticles wherein the biopolymer is gelatin that is dissolved in the aqueous solution of the copper precursor by heating between 50 to 100 degree Celsius.
Such a process for preparing an aqueous suspension of copper nanoparticles wherein the aqueous solution of the copper precursor is cooled to room temperature before addition of the aqueous solution of sodium formaldehyde sulfoxylate.
Such a process for preparing an aqueous suspension of copper nanoparticles wherein the biopolymer is chitosan that is dissolved in an aqueous solution containing 5 percent by volume of citric acid.
Such a process for preparing an aqueous suspension of copper nanoparticles wherein the aqueous solution of sodium formaldehyde sulfoxylate is added to the aqueous solution of copper precursor and the biopolymer dropwise.

Such a process for preparing an aqueous suspension of copper nanoparticles wherein the aqueous solution of the sodium formaldehyde sulfoxylate has a molar concentration in the range of 0.1- 10M and is preferably 5M.
Such a process for preparing an aqueous suspension of copper nanoparticles wherein the amount of the biopolymer is 1- 5 percent by volume of the aqueous solution of copper precursor and is preferably 2 percent.
FURTHER SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
An aqueous suspension of copper nanoparticles prepared by a process comprising the steps of dissolving a biopolymer in an aqueous solution of a copper precursor, the said biopolymer being selected from the group comprising of gelatin and chitosan, followed by reducing the copper precursor at room temperature by adding to the aqueous solution of copper precursor and the biopolymer an aqueous solution of sodium formaldehyde sulfoxylate.
INDUSTRIAL APPLICABILITY
The process of preparing an aqueous suspension of copper nanoparticles described above is simple and the aqueous suspensions of copper nanoparticles produced by the process are stable for long periods of time at room temperature. Additionally, the said process can be used to prepare high concentration aqueous suspensions of copper nanoparticles. which are stable for long periods of time at room temperature. The said process is also efficient for large scale production of Aqueous suspensions of copper nanoparticles. The aqueous suspension of copper nanoparticles produced finds application as anti-microbial. anti-biotic and anti-fungal (fungicide) agent when incorporated in coatings, plastics and textiles, in copper diet supplements, in the interconnect for micro

and integrated circuits: for its ability to absorb radioactive cesium and in super strong metals and alloys and in nanowire, nanofiber and and in certain alloy and catalyst applications.

WE CLAIM:
1. A process for preparing an aqueous suspension of copper nanoparticles comprising the
steps of:
dissolving a biopolymer in an aqueous solution of a copper precursor, the said biopolymer being selected from the group comprising of gelatin and chitosan; reducing the copper precursor at room temperature by adding to the aqueous solution of copper precursor and the biopolymer an aqueous solution of sodium formaldehyde sulfoxylate.
2. A process as claimed in claim 1 wherein the aqueous solution of the copper precursor has a molar concentration between 0.1 to 2 M and is preferably 0.5 M.
3. A process as claimed in claim 1 wherein the biopolymer is gelatin that is dissolved in the aqueous solution of the copper precursor by heating between 50 to 100 degree Celsius.
4. A process as claimed in claim 1 wherein the aqueous solution of the copper precursor is cooled to room temperature before addition of the aqueous solution of sodium formaldehyde sulfoxylate.
5. A process as claimed in claim 1 wherein the biopolymer is chitosan that is dissolved in an aqueous solution containing 5 percent by volume of citric acid.
6. A process as claimed in claim 1 wherein the aqueous solution of sodium formaldehyde sulfoxylate is added to the aqueous solution of copper precursor and the biopolymer dropwise.
7. A process as claimed in claim 1 wherein the aqueous solution of the sodium formaldehyde sulfoxylate has a molar concentration in the range of 0.1- 10M and is preferably 5M.

8. A process as claimed in claim 3 wherein the amount of the biopolymer is 1- 5 percent by volume of the aqueous solution of copper precursor and is preferably 2 percent.
9. An aqueous suspension of copper nanoparticles prepared by a process comprising the steps of:
dissolving a biopolymer in an aqueous solution of a copper precursor, the said biopolymer being selected from the group comprising of gelatin and chitosan; reducing the copper precursor at room temperature by adding to the aqueous solution of copper precursor and the biopolymer an aqueous solution of sodium formaldehyde sulfoxylate.
10. A process for preparing an aqueous suspension of copper nanoparticles substantially as herein described with reference to and as illustrated by the accompanying figures.
11. An aqueous suspension of copper nanoparticles substantially as herein described with reference to and as illustrated by the accompanying figures.