Controlled synthesis of highly monodispersed gold nanoparticles

Abstract

A method of synthesizing highly monodispersed Au nanoparticles having diameters in the range of 30-90 nm. Seed nanoparticles in a controlled concentration are combined with a precursor, also in a controlled concentration, a reducing and capping agent (e.g., sodium acrylate) in aqueous solution. Under controlled conditions of pH, temperature, and time, highly monodispersed nanoparticles having diameters in the range of 30-100 nm are produced. A relative size standard deviation of the size distribution of the resulting nanoparticles is as low as 2%.

Description

FIELD OF THE INVENTION

The invention pertains to processing nanoparticles and, more particularly to a method of growing highly monodispersed gold nanoparticles having readily controlled sizes and shapes.

BACKGROUND OF THE INVENTION

Gold nanoparticles are one of the most widely used classes of nanomaterials for chemical, bioanalytical, biomedical, optical and nanotechnological applications. While numerous methods are known for the synthesis of gold nanoparticles, the ability to control the size, shape and monodispersity of such gold nanoparticles is one of the most important areas for the targeted applications. This is because the electronic, optical, and chemical/biological properties exploited in these applications are highly dependent on the size, shape and size monodispersity of the nanoparticles. Few standard protocols have been established to allow preparation of gold nanoparticles of the desired sizes, shapes and high monodispersity in a systematic way. Such ability is critical for the targeted applications.

One method of the prior art for providing size monodispersed nanoparticles involves forming polydispersed nanoparticles using a variety of techniques known to those of skill in the art. Nanoparticles of a selected size range may then be selected from the polydispersed population using an instrument such as a differential mobility analyzer (DMA). Because the size resolution of a typical DMA is only about 10%, the degree of size monodispersity of the selected nanoparticles is similarly low.

Other techniques are known to those skilled in the art. Prior art techniques generally provide populations of gold nanoparticles in the size range of 2 to 30 nm diameter having best case size monodisperities of approximately 5-10%.

The present invention, however, provides a nanoparticle production technique that involves seeded growth of gold nanoparticles to form almost any desired size in the range of approximately 30-90 nm diameters. Nanoparticles formed in accordance with the inventive method exhibit size monodispersity having as small as a 2% relative standard deviation. This is significantly better than methods of the prior art for at least two reasons. First, highly size monodispersed nanoparticles having any desired diameter in the range of approximately 30-90 nm may be repeatably formed. This provides size control not heretofore available. Second, as previously mentioned, size monodisperity with relative standard deviations as low as 2% are achieved using the method of the invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method for synthesizing highly size monodispersed Au nanoparticles having diameters in the range of 30-90 nm. The novel technique uses a seed nanoparticle and then grows larger sized gold nanoparticles on the seed. Both the seeding and the seeded growth typically involves the use of the same reducing and capping agent (e.g., acrylate) in aqueous solution under controlled conditions. The inventive method is highly effective for the preparation of 30-90 nm sized gold nanoparticles with controllable sizes and high monodispersity.

Typically, Au nanoparticles having small diameters (e.g., 30 nm) are first synthesized using acrylate (A) as both a reducing and capping agent and AuCl₄⁻ as an Au-precursor, as is well known in the art. The smaller sized particles produced in this manner are used as seeds to grow larger-sized nanoparticles. This is achieved by mixing a controlled quantity of seeds with an AuCl₄⁻-A solution, the concentrations which are controlled, as is the pH and temperature. By varying parameters including concentration, pH, temperature, and reaction time, highly size monodispersed Au nanoparticles having desired size and shape are fabricated.

It is, therefore, an object of the invention to provide a method of fabricating highly size monodispersed nanoparticles having a predetermined nominal size.

It is another object of the invention to provide a method of fabricating highly size monodispersed gold nanoparticles having a predetermined nominal size.

It is an additional object of the invention to provide a method of fabricating highly size monodispersed gold nanoparticles having a relative standard deviation of no more than approximately 2%.

It is a further object of the invention to provide a method of fabricating highly size monodispersed gold nanoparticles grown from gold seed nanoparticles of approximately 30 nm diameters.

It is a still further object of the invention to provide a method of fabricating highly size monodispersed gold nanoparticles having controlled shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention may be obtained by reference to the accompanying drawings when considered in conjunction with the subsequent detailed description, in which:

FIG. 1 is a schematic representation of the process of Au nanoparticle seed formation followed by seed-mediated growth into larger sized Au nanoparticles in accordance with the method of the invention;

FIG. 2 a is a schematic, cross-sectional view of a nanoparticle formed in accordance with the method of the invention;

FIG. 2 b is a graph showing differences between theoretical and achieved diameters of nanoparticles formed in accordance with the invention;

FIGS. 3 a and 3b are a TEM micrograph and a size distribution, respectively, of 35.7 nm diameter seed nanoparticles formed using a method of the prior art;

FIGS. 4 a and 4b are a TEM micrograph and a size distribution, respectively, of approximately 54.4-nm diameter nanoparticles formed using the seed nanoparticles of FIGS. 3a and 3b;

FIGS. 5 a and 5b are a TEM micrograph and a size distribution, respectively, of approximately 62 nm diameter nanoparticles formed using the seed nanoparticles of FIGS. 3a and 3b;

FIGS. 6 a and 6b are a TEM micrograph and a size distribution, respectively, of approximately 82.4 nm diameter nanoparticles formed using the seed nanoparticles of FIGS. 5a and 5b; and

FIG. 7 is a set of UV/Visible spectrophotometric spectra image monitoring the growth of nanoparticle formed in accordance with the inventive method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides methods for controllably forming highly size monodispersed gold (Au) nanoparticles of any desired size and shape. Sizes in the range of approximately 30-90 nm or larger may be formed using the inventive method. It will be recognized that extensions of the novel method may be expected to produce nanoparticles outside the range of 30-90 nm, so the inventive method is not limited to the particular size range of nanoparticles chosen for purposes of disclosure. It will also be recognized that while the method chosen for purposes of disclosure are directed to Au nanoparticles, it is reasonable to expect that the inventive method may be applied to other compositions of nanoparticles. Consequently, the invention is not considered limited to gold or gold alloy nanoparticles but encompasses nanoparticles of any composition.

One embodiment of the inventive method uses Au nanoparticles of a smaller size (e.g., 30 nm diameter) as seed particles. Au nanoparticles of approximately 30 nm diameter may be synthesized by a number of methods. One such method is described in the papers: Jana, N. R.; Gearheart, L.; Murphy, C. J., Seeding Growth for Size Control of 5-40 nm Diameter Gold Nanoparticles, Langmuir 2001, Volume 17, No. 22, pages 6782-6786 (hereinafter JANA et al.); and Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I., Preparation of Acrylate-Stabilized Gold and Silver Hydrosols and Gold-Polymer Composite Films, Langmuir 2003, Volume 19, No. 11, pages 4831-4835 (hereinafter HUSSAIN et al.). Both these references are included herein by reference. It will be understood, however, that the formation of the seed Au nanoparticles forms no part of the instant invention and seed nanoparticles formed using different methods may also be used to practice the invention.

The synthesis of Au nanoparticles as described by JANA et al. uses trisodium citrate as a capping agent and AuCl₄⁻ is reduced using sodium borohydride. However, pH adjustment is not performed. A stabilizing agent, cetyltrimethylammonium bromide (CTAB) is also used. Particles produced using the JANA et al. method are smaller in size compared to those synthesized in accordance with the method of the invention. In the JANA et al. method, first, seeds having a diameters of approximately 3.5±0.7 nm are prepared. These seeds are then used to synthesize larger particles with diameters in the range of approximately 5.5±0.6 nm and 8.0±0.8 nm by varying CTAB and seeds (3.5±0.7 nm) quantities. Then, using 8.0±0.8 nm particles as seeds, particles having a diameter of approximately 17.0±2.5 nm are formed. Finally, using these 17 nm particles again as seeds, larger particles of approximately 35±5 nm are obtained.

In the synthesis of Au nanoparticles method described by HUSSAIN et al., sodium acrylate is used both as a capping and as a reduced agent. An aqueous solution of HAuCl₄ is refluxed (100° C. ) for 5-10 min, and a warm (50-60° C.) aqueous solution of sodium acrylate is then added. Reflux is continued for another 30 min until a deep-red solution is observed. No pH adjustment step is used. No seeding based growth was reported in this work.

Referring first to FIG. 1, there is shown a schematic representation 100 of the first embodiment of the method of the invention. Seed Au gold nanoparticles 102 are first synthesized using sodium acrylate (A) 104 as both reducing agent and the capping agent and AuCl₄⁻106 as Au-precursor using a modified method as described by HUSSAIN et al and/or JANA et al. In the modification, the pH was adjusted to 7 and the reaction was carried out at room temperature. The production of the seed Au nanoparticles 102 forms no part of the present invention. The small sized seed nanoparticles 102 are then used as seeds to grow larger-sized nanoparticles 112. This growth is achieved by mixing a controlled quantity of seeds 102 with an AuCl₄⁻ precursor solution 108 with acrylate (A) 110. Other possible reducing and/or reducing agents include sodium citrate and sodium borohydride.

The reducing and capping agents maybe different materials, like the synthesis by Jana et al. as a reducing and capping solution 108 at a controlled concentration and under controlled pH and temperatures. The pH may be controlled by addition of dilute aqueous sodium hydroxide solution and/or dilute hydrochloric acid. This process yields larger diameter, highly size monodispersed nanoparticles.

Providing the preformed seeds (as nucleation centers) and by controlling the growth condition, the resulting nanoparticle size may be controlled by varying both the concentration of the seeds and the concentration of the metal precursor (AuCl₄⁻). In addition to controlling the concentrations, controlling both the pH of the reaction solution and the reaction temperature is essential for controlled nanoparticle growth. The resulting Au nanoparticles exhibit the predetermined, desired average size and have high size monodispersity. If necessary, particles having different sizes may be separated from excess reducing/capping agents by centrifugation or any other suitable process.

Table 1 provides specific experimental conditions for the synthesis of the Au seeds and the subsequent growth of the seeds into larger sized Au nanoparticles of several different sizes.

TABLE 1
Experimental conditions for the synthesis of Au seeds and the subsequent growth
SEEDED Au NANOPARTICLES IN AQUEOUS SOLUTION
			Seed
	AuCL4−	Acrylate	Vol:Total	Seed Conc.
Seeds	(mM)	(M)	Vol	(Particles/mL)	Stirring	size (nm)
To make 30.5	0.1705	0.01024	250	—	Yes	30.5 ± 1.2
nm seeds
Using 30.5	0.05	0.01024	25:125	1.17 × 1011	Yes	45.5 ± 1.6
nm seeds
Using 30.5	0.0853	0.01024	25:125	1.17 × 1011	Yes	53.7 ± 1.7
nm seeds
Using 30.5	0.1705	0.01024	25:125	1.17 × 1011	Yes	61.9 ± 2.1
nm seeds
Using 62 nm	0.1705	0.01024	25:125	1.39 × 1010	No	82.4 ± 2.6
seeds
Using 62 nm	0.1705	0.01024	25:125	1.39 × 1010	Yes	92.5 ± 2.8
seeds
1) The pH is adjusted to 7 before adding Sodium acrylate (A)
2) The reaction performed at room temperature i.e. 18-30° C.
3) The reaction time is 1-3 days

As may be seen from the data of Table 1, depending on the size of the seeds, larger sized particles can be produced under the indicated conditions.

Referring now to FIG. 2a, there is shown a schematic, cross-sectional view of a nanoparticle 120 indicating the seed and the grown portions 122, 126, respectively. The core 122 of nanoparticle 120 (i.e., seed) has a radius r 124. The growth portion 126 of nanoparticle 120 has a thickness d 128.

The theoretical growth thickness d 128 may be calculated based on a spherical model for an r (radius)-sized seed and the density value for bulk gold. Equation 1 predicts growth thickness d as a function of the concentration of AuCl₄⁻ (C_mM). For the first seeded growth, the growth thickness (d₁) may be expressed as: $\begin{array}{cc}{d}_1=r\left(\sqrt[3]{1+197\times {10}^{-6}\times C_{\mathrm{mM}}\frac{V_{G_1}\times V_{S_1}}{M_{{\mathrm{Au}}_1}\times V_{S_1}^{\prime }}}-1\right)& \left(1\right)\end{array}$

By substituting experimental parameters, such as r, V_G1, V_S1, V′_S1 and M_Au1, Equation 1 may be simplified: d₁=r(³√ 1+29.3C_mM−1)   (2) where:

• d₁=growth thickness (nm) for the first-seeding growth;
• r=radius of Au nanoparticle seeds (e.g., 15.2 nm);
• V_G1=growth solution total volume (mL);
• V_S1=total volume of the seed stock solution (mL);
• V′_S1=volume removed from the seed stock solution (mL)
• M_Au1=mass of Au used to synthesize Au 30.5 nm seeds (g); and
• C_mM=concentration of HAuCl4 in the growth solution (mM).

For the second seeded growth, the growth thickness (d₂) may be expressed as: $\begin{array}{cc}{d}_2=r\left(\sqrt[3]{1+197\times {10}^{-6}\times C_{\mathrm{mM}}\frac{V_{G2}\times V_{S2}}{V_{S2}^{\prime }\left(M_{\mathrm{Au}1}\frac{V_{S1}}{V_{S1}^{\prime }}+M_{\mathrm{Au}2}\right)}}-1\right)& \left(3\right)\end{array}$

By substituting the experimental parameters such as r, V_G1, V_S1, V′_S1 and M_Au1, the Equation 3 may be simplified: d₂=r(³√ 1+4.9C_mM−1)   (4) where:

• d₂=growth thickness (nm) for the second-seeding growth;
• r=radius of Au nanoparticle seeds (e.g., 31 nm);
• V_G2=growth solution total volume (mL);
• V_S2=seed 62 nm synthesis total solution volume (stock seed solution) (mL);
• V′_S2=volume taken out from the 62 nm seed stock solution (mL);
• M_Au2=mass of Au used to synthesize Au 62 nm seeds (g); and
• C_mM=concentration of HAuCl4 in the growth solution (mM).

The spherical model used for deriving Equations 1 and 3 assume 100% conversion efficiency.

Referring now to FIG. 2b, there is shown a plot 140 of theoretical vs. experimental data of the growth thicknesses vs. the concentration of AuCl₄⁻. Curve 142 (d₁) is derived from Equation 2 and represents 30 nm seeds. Curve 146 (d₂) is derived from Equation 4 and represents 62 nm seeds. Data points 144 and 148 represent actual test results from experiments such as those provided in Table 1. As may readily be seen, there is generally good agreement between the theoretical and the measured results within the indicated range of AuCl₄⁻ concentrations, assuming an approximately 65% conversion efficiency for the formation of the 30 nm diameter gold nanoparticle seeds.

In principle, the particle sizes ranging from 30 nm to 100 nm diameters may be produced in a quantitative way by controlling the concentration of AuCl₄⁻ and the size of the seeds.

Several examples are provided herein to demonstrate the simplicity and effectiveness of the inventive technique. In these examples, 35.0 nm diameter gold particles were used as seeds to synthesize Au nanoparticles of sizes 46 nm, 54 nm, 62 nm and 82 nm that are highly monodispersed in size. As used herein, the term highly monodispersed size is intended to represent a size distribution of nanoparticle sizes wherein a relative standard deviation of the distribution is approximately 2%-5%. The morphology and size of the gold nanoparticles were examined using a transmissive electron microscope (TEM).

FIGS. 3 a and 3b show, respectively, a TEM micrograph and a size distribution, respectively, of a first sample of Au nanoparticle seeds obtained in the laboratory using the procedures of JANA et al. and/or HUSSAIN et al. The preparation steps for the 35.7 nm particles are:

Conditions for preparing 35.7-nm
NANOPARTICLE SEEDS IN AQUEOUS SOLUTION
	AuCL4−	Acrylate	Total	Seed Conc.		Final size
Seeds/nm	(mM)	(M)	Vol	(Particles/mL)	Stirring	(nm)
Synthesis of	0.1705	0.01024	250	—	No	35.7 ± 1.6
35.7 nm seeds
Notes:
1) The pH is adjusted to 7 before adding Sodium acrylate (A);
2) The reaction performed at room temperature i.e. 20-24° C.; and
3) The reaction time is 3-4 days.

The TEM data for the particle seeds display an average diameter of 35.7±1.6 nm.

FIGS. 4 a and 4b show a TEM micrograph and a size distribution, respectively, for nanoparticles synthesized using the 35.7 nm particles of FIGS. 3a and 3b as seeds. The preparation steps for the 54.4 nm particles are:

Conditions for preparing 54.4 nm
NANOPARTICLES IN AQUEOUS SOLUTION
			Seed
	AuCL4−	Acrylate	Vol:Total	Seed Conc.
Seeds	(mM)	(M)	Vol	(Particles/mL)	Stirring	size (nm)
35.7-nm seeds	0.1705	0.01024	25:125	1.17 × 1011	No	54.4 ± 2.6
Notes:
1) The pH is adjusted to 7 before adding Sodium acrylate (A);
2) The reaction performed at 4° C.; and
3) The reaction time is 7-10 days.

The average size for the resulting nanoparticles (FIGS. 4a, 4b) was 54.4±2.6 nm, the relative standard deviation being approximately 5%.

FIGS. 5 a and 5b show a second example of TEM data for nanoparticles synthesized using the 35 nm sized particles (FIGS. 3a, 3b) as seeds. The preparation steps for the 62 nm particles are:

Conditions for preparing 62 nm
NANOPARTICLES IN AQUEOUS SOLUTION
			Seed
Seed	AuCL4−	Acrylate	Vol:Total	Seed Conc.
Size	(mM)	(M)	Vol	(Particles/mL)	Stirring	size (nm)
35.7 nm	0.1705	0.01024	25:125	1.17 × 1011	No	61.9 ± 2.1
Notes:
1) The pH is adjusted to 7 before adding Sodium acrylate;
2) The reaction temperature is 20-24° C.; and
3) The reaction time is 2 to 4 days.

It should be recognized that nanoparticles synthesized in a first stage may be used as seed particles for a subsequent synthesis process. FIGS. 6a and 6b are a TEM micrograph and size distribution, respectively, for nanoparticles synthesized using the 62 nm sized particles (FIGS. 5a, 5b) as seeds. The preparation steps for the 82 nm particles are:

Conditions for preparing 82.4 nm
NANOPARTICLES IN AQUEOUS SOLUTION
			Seed
Seed	AuCL4−	Acrylate	Vol:Total	Seed Conc.
size	(mM)	(M)	Vol	(Particles/mL)	Stirring	size (nm)
62 nm	0.1705	0.01024	25:125	1.39 × 1010	No	82.4 ± 2.6
Notes:
1) The pH is adjusted to 7 before adding Sodium acrylate;
2) the reaction temperature is 20-24° C.; and
3) the reaction time is 2 to 4 days.

The average size for the resulting nanoparticles (FIGS. 6a, 6b) was 82.4±2.6 nm. The relative standard deviation is approximately 2%.

In addition to the above examples, a variety of highly monodispersed Au nanoparticles of several other sizes between approximately 30 and 90 nm diameters have been produced by manipulating the relative concentrations of the Au seed, Au-precursor and capping agents under controlled conditions.

The particles below 80 nm were well suspended in the aqueous solution for months without precipitation, whereas the suspension of particles of sizes above 80 nm was stable for at least 3 days before precipitation occurred. The precipitated particles can be easily re-dispersed in the solution by brief sonication.

It was also noted that the seeded growth of Au nanoparticles can be easily monitored by UV/Visible spectrophotometric measurement.

Referring now to FIG. 7, the increase of the absorbance for the surface plasmon resonance band at 530 nm is consistent with the growth of the Au nanoparticles observed in the TEM micrographs.

In summary, the method of the present invention provides a simple and highly effective technique for synthesizing monodispersed gold nanoparticles in the size range of 30-90 nm diameters. This technique is shown to produce highly monodispersed Au nanoparticles of almost any sizes between 30 and 100 nm diameters. The size monodispersity is much higher than most commercial Au nanoparticles. For example, Au nanoparticles of ˜80 nm core sizes obtained in accordance with the method of the invention were compared with Au particles from a well known commercial nanoparticle source. The Au particles from a sample of the Au 80 nm particle solution from the commercial source showed an average size of 76.9±4.6 nm and an SP band at 550 nm. The Au nanoparticles prepared in accordance with the invention showed an average size of 82.4±2.6 nm and an SP band at 554 nm. The particle concentrations of the nanoparticles prepared in accordance with the inventive method are on the order of 1011 or 1010 particles per mL, comparable with the concentration of commercially available products.

Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.

Claims

1. A method of preparing highly monodispersed nanoparticles, the steps comprising: a) providing seed nanoparticles at a predetermined concentration, said seed particles having a predetermined diameter and; b) combining said seed nanoparticles with a precursor at a predetermined concentration; c) combining at least one of a reducing agent and a capping agent with said combination of seed nanoparticles and said precursor; and d) controlling at least one of the parameters: pH, temperature, reaction time, and amount of stirring; whereby highly monodispersed resultant nanoparticles having a predetermined size are grown.

2. The method of preparing highly monodispersed nanoparticles as recited in claim 1, wherein said seed nanoparticles and said resultant nanoparticles are Au nanoparticles.

3. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said precursor comprises AuCl4−.

4. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said at least one of a reducing agent and a capping agent comprises acrylate.

5. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said seed nanoparticles have a diameter in the range of approximately 30 to 62 nm.

6. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said resultant nanoparticles have diameters in the range of approximately 30-100 nm.

7. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said concentration of seed nanoparticles is within the range of between approximately 1×1010 to 2×1011 particles/mL.

8. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said concentration of precursor is within the range of approximately 0.05 to 0.2 M.

9. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said temperature is controlled within the limits of approximately 18° C. to 30° C.

10. The method of preparing highly monodispersed nanoparticles as recited in claim 2, wherein said pH is controlled within the limits of approximately 6.5 to 7.5.

11. The method of preparing highly monodispersed nanoparticles as recited in claim 2, the steps further comprising: d) centrifuging a solution containing said resultant nanoparticles.

12. A method of preparing highly monodispersed nanoparticles, the steps comprising: a) providing seed nanoparticles at a predetermined concentration, said seed nanoparticles having a predetermined diameter; b) combining said seed nanoparticles with a precursor at a predetermined concentration; c) combining at least one of a reducing agent and a capping agent with said combination of seed nanoparticles and said precursor; d) controlling at least one of the parameters: pH, temperature, reaction time, and amount of stirring; and e) using resulting grown nanoparticles having predetermined diameters as seed nanoparticles, repeating steps (b), (c), and (d); whereby highly monodispersed resultant nanoparticles having a predetermined size are grown.

13. The method of preparing highly monodispersed nanoparticles as recited in claim 12, wherein said seed nanoparticles and said resultant nanoparticles are Au nanoparticles.

14. The method of preparing highly monodispersed nanoparticles as recited in claim 12, wherein said at least one of a reducing agent and a capping agent comprises acrylate.

15. The method of preparing highly monodispersed nanoparticles as recited in claim 13, wherein said precursor comprises AuCl4−.

16. The method of preparing highly monodispersed nanoparticles as recited in claim 13, wherein said seed nanoparticles have a diameter in the range of approximately 30 to 62 nm.

17. The method of preparing highly monodispersed nanoparticles as recited in claim 13, wherein said resultant nanoparticles have diameters in the range of approximately 30-100 nm.

18. The method of preparing highly monodispersed nanoparticles as recited in claim 13, wherein said concentration of seed nanoparticles is within the range of approximately 1×1010 to 2×1011 particles/mL.

19. The method of preparing highly monodispersed nanoparticles as recited in claim 13, wherein said concentration of precursor is within the range of approximately 0.05 to 0.2 M.

20. The method of preparing highly monodispersed nanoparticles as recited in claim 12, wherein said temperature is controlled within the limits of approximately 18° C. to 30° C.

21. The method of preparing highly monodispersed nanoparticles as recited in claim 12, wherein said pH is controlled within the limits of approximately 6.5 to 7.5.

22. The method of preparing highly monodispersed nanoparticles as recited in claim 13, the steps further comprising: d) centrifuging a solution containing said resultant nanoparticles.