WO2012074355A1

WO2012074355A1 - Preparation of silver nanoparticles

Info

Publication number: WO2012074355A1
Application number: PCT/MY2011/000094
Authority: WO
Inventors: Dr. Mohd Rais Ahmad
Original assignee: Mimos Bhd
Current assignee: Mimos Bhd
Priority date: 2010-12-03
Filing date: 2011-06-15
Publication date: 2012-06-07
Anticipated expiration: 2013-06-03
Also published as: MY155724A

Abstract

The present invention relates to a method for preparing silver nanoparticles. This method comprising steps of mixing silver salt and pentaerythritol in polar solvent, reacting the mixture with catalytic amount of reducing agent to catalyze the reduction process of silver ion clusters, precipitating the silver nanoparticles, washing the silver nanoparticles with series of solution and drying the silver nanoparticles.

Description

PREPARATION OF SILVER NANOPARTICLES

FIELD OF THE INVENTION

The present invention relates to a method for preparing silver nanoparticles.

BACKGROUND OF THE INVENTION

Silver nanoparticles are widely applied in electrode preparation and manufacturing of chemical and biosensors. Due to its exceptional conductivity, silver is among the best material for transferring chemical and electrical signals. Silver wire or screen printed silver trace are widely used in sensing devices. When the sensors are exposed to aqueous media for extended durations, silver trace makes better electrical conductor compared to copper because the former has less tendency to oxidize and corrode.

Silver-silver chloride electrode immersed in electrolyte with constant chloride activity provides constant electrical potential and can be used as a reference cell.

Nanocomposites of silver nanoparticles with graphite, conductive polymers, metal nanoparticles, ceramic, acrylate olymers are commonly used as working electrodes for chemical and biosensors.

Selective voltammetry of biomolecules using silver nanocomposites is of special interest to the inventor because it allows the manufacture and development of non-enzymatic biosensor strips and systems.

Large quantities of silver nanoparticles are manufactured for other valuable applications such as:

i. Silver-silver halide electrochemical transducer;

II Silver paste for screen printing;

iii. Silver ink for inkjet printing;

IV. Reference electrode;

v. Microelectrodes for amperometry; vi. Modified electrodes for catalytic oxidation of biomolecules; and vii. Stripping voltammetry detection of trace levels of heavy metals.

Silver nanoparticles demonstrate distinct physical and chemical

characteristics compared to its bulk metal counterpart, wherein bulk silver metal gives one of the most reflective surface while different sizes of silver nanoparticles absorb light at different wavelengths thus resulting in different colors of the dispersed colloidal silver particles,

High purity silver nanoparticles with consistent particle size are highly desired for accurate electrochemical measurements. However, silver powders which are commercially available in the market are not only prohibitively

expensive, the quality of the resulted silver nanoparticles are also relatively low. The variation of particle size and formation of lumps are all due to the fast formation of silver nanoparticles, insufficient purification and undesirably different sizes of silver aggregate formations which usually happened in the known methods for producing silver nanoparticles.

The major requirement for the manufacture of silver nanoparticles is the cost factor. The proposed method should provide a low cost and high yield, wherein the determining cost component should only be the source of silver ions i.e. silver nitrate. The rest of the reaction reagents such as reducing agent, stabilizing agent and solvent should only contribute minimally to the overall cost.

Reproducibility of the process is another main requirement for the manufacture of silver nanoparticles. The proposed method should consistently give average particle size and acceptable physical and chemical properties of silver nanoparticles.

Other requirements are good processability and ease of handling of the silver materials. The prepared and purified silver nanoparticles should be capable of being conveniently stored before use and easily mixed with other materials to give homogenous mixture or composite. The invention disclosed herein is a method for preparation of silver nanoparticles in large quantities by controlled reduction of stabilized silver ions in aqueous dispersion by using pentaerythritol as both stabilizing and reducing agent and catalyzed by hydroquinone. Thus, the present invention affords an economical benefit and consistent quality of silver nanoparticles for chemical and biosensor applications.

SUMMARY OF THE INVENTION

The present invention relates to a method for preparing silver nanoparticles, wherein the method comprises the steps of mixing silver salt and pentaerythritol in polar solvent, adding catalytic amount of reducing agent into the mixture, precipitating the silver nanoparticles, washing the silver nanoparticles with a series of solution and drying the silver nanoparticles.

A silver salt (e.g. silver nitrate, silver acetate, silver carbonate) is mixed with pentaerythritol in polar solvent (e.g. water, ethanol, methanol, tetrahydrofuran, diethylether, acetone, dimethylsulfoxide, dimethylformamide).

A catalytic amount of reducing agent (e.g sodium borohydride, trisodium citrate, ascorbic acid, hydroquinone, glucose, sorbitol, mannitol, sodium

hypophosphite, uric acid) is then added to the mixture obtained from the previous step.

The treated mixture is further precipitated, washed and dried to give average particle size of silver nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings for a better

understanding of an embodiment disclosed herein and not for the purposes of limiting the same.

Figure 1 shows the reaction of silver nanoparticles stabilized and reduced from silver ion by pentaerythritol; Figure 2 shows the silver ion clusters stabilized by pentaerythritol;

Figure 3 shows the silver nanoparticles stabilized by pentaerythritol;

Figure 4 shows the flow chart of silver nanoparticles preparation with pentaerythritol;

DETAILED DESCRIPTION

The present invention discloses a method for preparing silver nanoparticles. The silver nanoparticles are prepared by first adding a silver salt and

pentaerythritol in a polar solvent and then reacting with a catalytic amount of reducing agent, finally followed by a series of precipitation, washing and drying steps.

Silver ions can be reduced to give bulk metallic silver or particles of metallic silver (silver nanoparticles), depending on the reaction conditions. A stabilizer is: essential to stabilize the silver ions and particles during the reduction process in order to produce consistent sized silver particles, instead of bulk silver mirror. Moreover, the stabilizer has the function of preventing the encapsulated

nanoparticles from forming aggregates and thus helps to define the size of the synthesized particles.

In the disclosed method, pentaerythritol functions as both a stabilizing and reducing agent. Figure 1 illustrates the reduction process by using pentaerythritol. The positively charged silver ions and electropositive metallic silver nanoparticles are stabilized by the pentaerythritol molecules by forming a circular cage.

In this method, pentaerythritol is first added to a polar solvent. Exemplary polar solvents include water, ethanol, methanol, tetrahydrofuran, diethyl ether, acetone, dimethylsulfoxide, dimethylformamide, and the like, or mixtures thereof.

The mixture of pentaerythritol and polar solvent is stirred at room

temperature/ This step is conducted for 20 to 60 minutes, more preferably 30 minutes, to allow a complete dissolution of all solid materials. In this method, the reaction mixture is further heated to 60 °C over 20 minutes and stirred for an additional 30 minutes at this temperature. Next, a silver salt is added drop-wise into the reaction mixture of

pentaerythritol and polar solvent. Exemplary silver salts include silver nitrate, silver acetate, silver carbonate, and the like, or mixtures thereof. The mixture is stirred and heated. This step is conducted, preferably at 50 to 60 °C.

Scheme 1: Overall reaction for preparation of silver nanoparticles with

Pentaerythritol - Hydroquinone.

Silver ion is a mild oxidizing agent but sufficiently strong to oxidize primary alcohol to the corresponding aldehyde. In the above reaction, one of the hydroxyl group in pentaerythritol is presumably oxidized to aldehyde.

A catalytic amount of hydroquinone is then subjected to the mixture in order to accelerate the rate of the reduction of silver ions. The reaction mixture of silver salt, pentaerythritol, polar solvent and hydroquinone is heated and refluxed. This step is conducted at 50 to 90 °C for a few hours. The reaction temperature and reflux time are among the best ways to ensure the completion of the reaction and the size of the resulted silver nanoparticles. In this method, the reaction mixture is heated to 70 to 90 °C and refluxed for an additional 2 to 8 hours.

Formation of silver nanoparticles can be visually inspected by the formation of a brown colloid a few minutes after the gentle reflux started. The solvent e.g. water is partially removed the total liquid volume, and the silver- nanoparticles are precipitated. Preferably, the precipitation step is conducted by using a centrifuge. Alternatively, a salt solution and polar solvent may be added to the colloidal mixture for precipitation purpose. The precipitated silver nanoparticles are further subjected to a series of washing steps with solutions such as sodium bicarbonate solution, brine solution, polar solvents, and the like, or mixtures thereof.

The silver nanoparticles obtained from the precipitation step are dried. Preferably, the drying step is conducted by using an oven at temperature of 50 to 70 °C. In order to measure the sizes of the particles and to deduce the average particle size of the samples, Scanning Electron Microscope can be used after the method for preparing silver nanoparticles is completed.

The present invention is further described but not limited to the following example:

Example 1

Preparation of Silver Nanoparticles with Pentaerythritol Pentaerythritol, hydroquinone and silver nitrate may be used as received from Sigma-Aldrich. Deionized water was prepared in-house and used as a solvent and for dilution purposes throughout this work. Pentaerythritol (0.8 g) was added into a 100-mL three-neck round bottom flask and 40 mL of deionized water was added into the reaction vessel. The mixture was magnetically stirred at room temperature for 30 minutes until all the solid materials were completely dissolved. The reaction mixture were gradually heated to 60 °C over 20 minutes and stirred for additional 30 minutes at this temperature. Silver nitrate (0.5 g) in 10 mL of deionized water was added dropwise into the reaction mixture with gentle stirring. The heating and stirring were continued at 60 °C for 20 minutes and the colorless solution started to turn brownish grey. The temperature of the reaction mixture was gradually increased to 90 °C and heating and stirring were continued for another hour when tiny beads of shiny silver particles appeared from the greyish solution. The heating was discontinued and the hot reaction mixture was allowed to gradually cool to room temperature. The silver nanoparticles formed a layer of shiny beads covering the surface of the liquid phase. Most of water solvent was distilled off to leave only about 15 mL of the original solution. The remaining liquid was centrifuged and brown silver nanoparticles were precipitated at the bottom of the centrifuge tube. The brown wet particles were washed three times each, consecutively with deionized water, ethanol, acetone and diethyl ether. The washed nanoparticles were dried in the oven at 50 °C for 1 hour.

While specific features have been disclosed and described in this

specification, it should be understood that other alternatives, modifications, substitutions, variations and substantial equivalents may be apparent to those skilled in the art. Such changes and modifications can be made without departing from the scope of the invention to adapt it to various usages and conditions. Thus, the appended claims as filed are intended to embrace all embodiments which fall within the scope of the following claims. :

Claims

1. A method for preparing silver nanoparticles comprising the steps of: .

i) mixing a silver salt and pentaerythritol in polar solvent;

ii) increasing the temperature of mixture of silver salt and

pentaerythritol in polar solvent;

iii) adding a catalytic amount of reducing agent into the mixture of silver salt and pentaerythritol in polar solvent;

iv) increasing the temperature of mixture of silver salt, pentaerythritol and catalytic amount of reducing agent and refluxing the mixture; v) precipitating the silver nanoparticles from the homogeneous

dispersed colloidal solution;

vi) washing the silver nanoparticles; and

vii) drying the silver nanoparticles.

2. A method as claimed in claim 1, wherein mixing a silver salt and pentaerythritol in polar solvent and stirring the mixture is conducted at room temperature for about 20 to 60 minutes.

3. A method as claimed in claim 1, wherein the temperature of the mixture of silver salt and pentaerythritol in polar solvent is increased to a range of about 50 to 60 °C.

4. A method as claimed in claim 1, wherein the temperature of the mixture of silver salt, pentaerythritol and catalytic amount of reducing agent is increased to a range of about 70 to 90 °C and the mixture is refluxed for about an additional 2 to 8 hours.

5. A method as claimed in claim 1, wherein the step of precipitating the silver nanoparticles from the homogeneous dispersed colloidal solution is conducted by at least one or a combination of the following methods:

a. reducing the amount of solvent in the dispersed mixture followed by centrifuge;

b. adding salt solution and polar solvent to the colloidal mixture.

6. A method as claimed in claim 1, wherein the silver nanoparticles are washed with solutions including sodium bicarbonate solution, brine solution and polar solvents.

7. A method as claimed in claim 1, wherein the silver nanoparticles are dried in the oven at a range of about 50 to 70 °C.

8. A method as claimed in claim 1, wherein the silver salt is at least one or combination of the following salts: silver nitrate, silver acetate, silver carbonate.

9. A method as claimed in claim 1, wherein the polar solvent is at least one or combination of the following solvent: water, ethanol, methanol, tetrahydrofuran, diethylether, acetone, dimethylsulfoxide, dimethylformamide.

10. A method as claimed in claim 1, wherein the reducing agent used in catalytic amount is at least one or a combination of the following chemicals:

sodium borohydride, trisodium citrate, ascorbic acid, hydroquinone, glucose, sorbitol, mannitol, sodium hypophosphite, uric acid.

11. A method as claimed in claim 1, wherein the salt solution is at least one or combination of the following salts: lithium halide, sodium halide, potassium halide.

12. A use of the silver nanoparticles prepared as claimed in claim 1 as a paste for screen printed electrode.

13. A use of the silver nanoparticles prepared as claimed in claim 1 in voltammetric working electrode.

14. A use of the silver nanoparticles prepared as claimed in claim 1 in solid state reference electrode.