A METHOD OF PREPARING BIPYRAMID NANO-GOLD PARTICLES

Abstract

The present disclosure relates to a method of preparing bipyramid nano-gold particles comprising the steps of (1) preparation of seeds a). mixing Gold (III) chloride (HAuCl4) solution with aqueous cetyl trimethyl ammonium chloride (CTAC) and citric acid to obtain a solution; b) adding two-third volumes of aqueous sodium borohydride (NaBH4) solution rapidly with continuous stirring to the solution; c) adding one-third volume of aqueous sodium borohydride solution drop by drop to the solution obtained in step b) to obtain a brown solution; and d) heating the brown solution at a temperature of 50°C to 100°C with stirring at 1000 rpm to 2000 rpm to obtain seed solution, (2) growth of seeds to obtain bipyramid nano-gold particles e) adding HAuCl4 solution into cetyl trimethyl ammonium bromide (CTAB); f) adding hydrochloric acid, silver nitrate, and ascorbic acid to the solution obtained in step e); g) adding the seed solution obtained in step (d), and stirring at 1000 rpm to 2000 rpm for 1.5 hrs to 3 hrs; and h) centrifuging the solution at 7000 to 9000 rpm for 10 min to 15 min to obtain the bipyramid nano-gold particles.

Specification

Description:FIELD OF THE INVENTION:
[0001] The present disclosure relates to nanotechnology and the production of nanoparticles. More particularly, this disclosure relates to the method of preparing bipyramid nano-gold particles.

BACKGROUND OF THE INVENTION
[0002] The most common type of nanoparticles that are synthesized using any method, say its bottom-up or top-down approach, is the spherical nanoparticles. the spherical nanoparticles are easy to produce in large scale and is widely used in multiple applications. But certain applications involve the specific shape of nano particles. For certain optical applications it is very essential to produce nano particles with sharp edges.
[0003] Shape control of metal nanoparticles has received considerable attention in recent years because of the strong correlation between the shape and the chemical, physical, electronic, optical, magnetic, and catalytic properties of a nanoparticle. Silver and gold, in particular, have been intensively studied due to their numerous applications that may include surface plasmonics, surface-enhanced Raman scattering (SERS), as well as chemical and biological sensing. A wealth of chemical methods have been developed for synthesizing silver and gold nanostructures having well-controlled shapes, and typical examples include triangular plates, cubes, belts, wires, rods, and branched multipods.
[0004] The polyol process is well known by those in the art as a means to synthesize silver and gold nanoparticles with controllable shapes in relatively large quantities. (See, e.g. Y. Sun, Y. Xia, Science 2002, 298, 2176; R. Jin, S. Egusa, N. F. Scherer, J. Am. Chem. Soc. 2004, 126, 9900; T. K. Sau, C. J. Murphy, J. Am. Chem. Soc. 2004, 126, 8648; F. Kim, S. Connor, H. Song, T. Kuykendall, P. Yang, Angew. Chem. Int. Ed. 2004, 43, 3673; Y. Sun, B. Mayers, Y. Xia, Nano. Lett. 2003, 5, 675; Y. Sun, B. Mayers, T. Herricks, Y. Xia, Nano Lett. 2003, 3, 955; Y. Sun, Y. Yin, B. Mayers, T. Herricks, Y. Xia, Chem. Mater. 2002, 14, 4736; and Y. Sun, B. Mayers, Y. Xia, Adv. Mater. 2003, 15, 641, each of which are incorporated in their entirety by reference herein). The following experimental examples disclose various embodiments of silver nanostructure production.
A. Sánchez-iglesias, N. Winckelmans, S. Bals, M. Grzelczak, and L. M. Liz-marzán, "High YieldSeeded Growth of Monodisperse Pentatwinned Gold Nanoparticles Through Thermally-Induced Seed Twinning High Yield Seeded Growth of Monodisperse Pentatwinned Gold Nanoparticles Through Thermally-Induced Seed Twin-," 2016.
[0005] The bipyramid nanoparticle synthesis is a two step process. The first step is to synthesise gold nano seeds. In the next step the seeds are put in growth solution where they grow to bipyramid nanoparticles.
[0006] The problem encountered while adopting this synthesis method is the repeatability of high-quality seed synthesis.
[0007] Thus, most of the methods in the existing art require improvement in terms of yield, purity, monodispersity, repeatability, and scale of synthesis before they will find use in commercial applications.
[0008] Therefore, there is a need for the method of preparing bipyramid nano gold particles with better quality, repeatability and high yield.

OBJECTS OF THE INVENTION
[0009] Some of the objectives of the present disclosure, with at least one embodiment herein satisfied, are listed herein below:
[0010] It is the primary objective of the present disclosure to provide a method of preparing bipyramid nanoparticles with better quality, repeatability and high yield.

SUMMARY OF INVENTION
[0011] The present disclosure relates to a method of preparing a bipyramid nano-gold particles (AuNBP) comprising the steps of
(1) preparation of seeds
a). mixing Gold (III) chloride (HAuCl4) solution with aqueous cetyl trimethyl ammonium chloride (CTAC) and citric acid to obtain a solution;
b) adding two-third volumes of aqueous sodium borohydride (NaBH4) solution rapidly with continuous stirring to the solution;
c) adding one-third volume of aqueous sodium borohydride solution drop by drop to the solution obtained in step b) to obtain a brown solution; and
d) heating the brown solution at a temperature of 70°C to 100°C with stirring at 1000 rpm to 2000 rpm to obtain seed solution,
(2) growth of seeds to obtain bipyramid nano-gold particles
e) adding HAuCl4 solution into cetyl trimethyl ammonium bromide (CTAB);
f) adding hydrochloric acid, silver nitrate, and ascorbic acid to the solution obtained in step e);
g) adding the seed solution obtained in step (d), and stirring at 1000 rpm to 2000 rpm for 1.5 hrs to 3 hrs; and
h) centrifuging the solution at 7000 to 9000 rpm for 10 min to 15 min to obtain the bipyramid nano-gold particles.

BRIEF DESCRIPTION OF DRAWINGS
[0012] The present disclosure contains the following drawings that simply illustrates certain selected embodiments of the nanocarrier composition and processes that are consistent with the subject matter as claimed herein, wherein:
[0013] Figure 1 depicts flow chart of chemical addition for seed synthesis
[0014] Figure 2 depicts (a)SEM image of different batches of AuNBP synthesized with single step sodium borohydride (NaBH4) addition (b) SEM image of different batches of AuNBP particles synthesized with two step NaBH4 addition taken at 100000X magnification.
[0015] Figure 3 depicts scatter plot showing the longitudinal LSPR of AuNBPs. The red dots show the longitudinal LSPR of AuNBPs synthesized with seeds of single step addition of NaBH4. The blue dots show the longitudinal LSPR of AuNBPs synthesized with seeds of two step addition of NaBH4. The green zone in the graph shows the region above absorbance value 2.
[0016] Figure 4 depicts the Transmission Electron Microscopy (TEM) of the AuNBP particles.

DESCRIPTION OF THE INVENTION:
[0017] A detailed description of various exemplary embodiments of the disclosure is described herein. It should be noted that the embodiments are described herein in such detail as to communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure.
[0018] The terminology used herein is to describe particular embodiments only and is not intended to be limiting to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising", or "includes" and/or "including" or "has" and/or "having" when used in this specification specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
[0019] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0020] In an embodiment, the present disclosure relates to a method of preparing a bipyramid nano-gold particles (AuNBP) comprising the steps of
(1) preparation of seeds
a) mixing Gold (III) chloride (HAuCl4) solution with aqueous cetyl trimethyl ammonium chloride (CTAC) and citric acid to obtain a solution;
b) adding two-third volumes of aqueous sodium borohydride (NaBH4) solution rapidly with continuous stirring to the solution;
c) adding one-third volume of aqueous sodium borohydride solution drop by drop to the solution obtained in step b) to obtain a brown solution; and
d) heating the brown solution at a temperature of 70°C to 100°C with stirring at 1000 rpm to 2000 rpm to obtain seed solution,
(2) growth of seeds to obtain bipyramid nano-gold particles
e) adding HAuCl4 solution into cetyl trimethyl ammonium bromide (CTAB);
f) adding hydrochloric acid, silver nitrate, and ascorbic acid to the solution obtained in step e);
g) adding the seed solution obtained in step (d), and stirring at 1000 rpm to 2000 rpm for 1.5 hrs to 3 hrs; and
h) centrifuging the solution at 7000 to 9000 rpm for 10 min to 15 min to obtain the bipyramid nano-gold particles.
[0021] In an embodiment of the present disclosure, for a synthesis of 10 mL seed volume the concentration of the chemicals are as follows.
[0022] In an embodiment of the present disclosure, concentration of the HAuCl4 is 10 mM and volume 250 µL
[0023] In another embodiment of the present disclosure, concentration of the CTAC and CTAB is 200 mM and volume 2.5 mL.
[0024] In another embodiment of the present disclosure, concentration of the citric acid is 100 mM and volume 0.5 mL.
[0025] In yet another embodiment of the present disclosure, amount of the aqueous 10 mM NaBH4 solution added rapidly is 200 µL.
[0026] In further embodiment of the present disclosure, amount of the aqueous 10 mM NaBH4 solution added drop by drop is 100 µL
[0027] In further embodiment of the present disclosure, the seeds volume added to the growth solution controls the size of the final bipyramid particles formed. By varying the seed volume in growth synthesis AuNBPs of different sizes are obtained. In an embodiment, the volume of seed used for the growth of bipyramid nano-gold particles is 250µL.
[0028] In an embodiment, the bipyramid nano-gold particles are in the range of 5-20nm width and 40 to 60 nm roughly in length.
[0029] In an embodiment of the present disclosure, the process of synthesis of anisotropic nano particles generally involves two or more steps. The first step is generally used for seed synthesis. The following steps involve the growth of these seeds into the desired shapes in a different solution. Both steps are very important as the nature of the seed synthesized helps determine the final shape of the particle. The shape in discussion in this context is gold nano bipyramid (AuNBP).
[0030] The present disclosure also relates to bipyramid nano-gold particles. As the name suggests, AuNBP particles have a unique shape of two pyramid structures attached back to back. Like a pyramid it also has facets on its surfaces. The AuNBP particles also follow a complex process of two step synthesis. The first step involves the synthesis of penta-twinned seeds. In the second step these penta-twinned seeds are injected into the growth solution. In the growth solution facet selective growth happens which leads to the synthesis of AuNBP particles.

ADVANTAGES OF THE PRESENT INVENTION
[0031] In accordance with the present disclosure, the method of preparing AuNBP particles using two step addition of NaBH4 has the following advantages:
• High repeatability;
• Better quality of seeds
• High yield
[0032] The present disclosure will be explained using the following examples:

EXAMPLE
Example 1: Synthesis of the bipyramid AuNBP particles
[0033] The synthesis process of AuNBP is a two-step seed mediated growth process. The first one and the most important process is the synthesis of gold nano seeds. In this process we have modified the rate of addition of NaBH4 and the volume for more repeatable output as shown in figure 1.
(1) preparation of seeds
[0034] For the seed synthesis (final volume of 10 mL), 0.25 mL HAuCL4 (10 mM) was added to a mixture of 2.5 mL CTAC (200 mM) and 0.5 mL Citric acid (100 mM). The rest of the volume was balanced with water. To induce seed growth, 300 mL of NaBH4 at room temperature was added to the mixture in two steps while stirring at 500 rpm. In the first step, 200 µL NaBH4 was added rapidly, followed by a dropwise addition of 100 µL in the second step. The colour of the solution changed from pale yellow to brown. The speed of the addition can be controlled by using a syringe pump or electronic micro-pipet. It can be done manually also. Just the duration of the addition of NaBH4 to the solution in two steps should be completed within 10 seconds. The cap of the glass vial was kept open during this process to facilitate the release of hydrogen gas formed due to the breaking down of NaBH4. After 2 minutes the solution was transferred to a hotplate at 75°C, 1500 rpm and kept undisturbed for 90 minutes. The colour of the solution changed from brown to red. The cap of the container was closed in order to avoid any losses due to evaporation.
(2) growth of seeds to obtain bipyramid nano-gold particles
[0035] This crucial seed synthesis step is followed by the growth stage, where the as-prepared seeds in desired volumes are added to the growth solution. 30 mL CTAB (100 mM) was taken in a glass vial and 1.5 mL HAuCl4 (10 mM) was added. The colour of the solution immediately changed to yellow. 0.9 mL HCl (1M) and 0.6 mL AgNO3 (10 mM) was to the solution. Further addition of 0.48 mL ascorbic acid (100 mM) to the solution lead to the solution becoming colourless. Finally, 1.8 mL of freshly prepared seeds were added to the solution. The solution was stirred at 1500 rpm at room temperature for 2 hrs and the colour gradually changed from colourless to dark violet.
[0036] The obtained bipyramid nano-gold particles are then purified by centrifugation at 8000 rpm for 10 min to separate out residual chemicals and surfactants.

Comparative Example 1:
[0037] The bipyramid nano gold particles are prepared by the conventional process by the one step addition of NaBH4 in the seed synthesis method adopted from a previously published work by Ana Sánchez-iglesias, N. Winckelmans, S. Bals, M. Grzelczak, and L. M. Liz-marzán, "High Yield Seeded Growth of Monodisperse Pentatwinned Gold Nanoparticles Through Thermally-Induced Seed Twinning High Yield Seeded Growth of Monodisperse Pentatwinned Gold Nanoparticles Through Thermally-Induced Seed Twin-," 2016.

Example 2:
Repeatability study of bipyramid nano gold particles
[0038] Different batches of AuNBP are prepared by following single step addition of NaBH4 and by two step addition of NaBH4 following Comparative Example 1 and Example 1 respectively.
[0039] The yield, quality and characterization of AuNBP prepared in different batches is given in the table below:

[0040] Table 1: Characterisation of bipyramid nano-gold particles and synthesis details
Figno. Sample No. Condition of adding NaBH4 Result Longitudinal LSPR Transverse LSPR
Single step addition of NaBH4
2(a) (i) G7 Single step addition good nano bipyramid yield and many spherical particles 829 nm, 1.59 Abs 540 nm, 0.81 Abs
2(a) (ii) G8 Single step addition mostly spherical or random shaped particles 825 nm, 0.75 Abs 543 nm, 0.97 Abs
2(a) (iii) G9 Single step addition mostly spherical or random shaped particles with a very few nano bipyramids 866 nm, 0.38 Abs 549 nm, 0.94 Abs
2(a) (iv) G10 Single step addition good yield of nano bipyramid particles along with many random-shaped particles and nanorods 806 nm, 1.58 Abs 556 nm, 0.78 Abs
2(a) (v) G11 Single step addition few nano bipyramid particles along with many randomly shaped particles 779 nm, 0.53 Abs 536 nm, 0.94 Abs
2(a) (vi) G12 Single step addition very few nano bipyramid particles along with randomly shaped particles 833 nm, 0.27 Abs 541 nm, 1.07 Abs
2(a) (vii) G13 Single step addition very high yield of nano bipyramid particles along with very few randomly shaped particles 854 nm, 2.5 Abs 533 nm, 0.6 Abs
2(a) (viii) G14 Single step addition low yield of nano bipyramid particles along with randomly shaped particles 907 nm, 0.68 Abs 545 nm, 0.93 Abs
(1) J1 Single step addition very high yield of nano bipyramid particles along with few randomly shaped particles 817 nm, 1.5 Abs 533 nm, 0.65 Abs
Two step addition of NaBH4
2(b) (i) S77 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles 762 nm, 2.62 Abs 522 nm, 0.67 Abs
2(b) (ii) S78 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles 758 nm, 2.76 Abs 519 nm, 0.6 Abs
2(b) (iii) S81 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles 753 nm, 2.36 Abs 520 nm, 0.65 Abs
2(b) (iv) S82 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles 777 nm, 2.11 Abs 528 nm, 0.68 Abs
2(b) (v) S83 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles 765 nm, 2.3 Abs 523 nm, 0.64 Abs
2(b) (vi) S85 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles 796 nm, 2.76 Abs 521 nm, 0.6 Abs
2(b) (vii) S91 2/3 Fast + 1/3 drop very high yield of nano bipyramid particles along with few randomly shaped particles and self-assembly. 800 nm, 2.78 Abs 520 nm, 0.58 Abs

[0041] As can be seen from the table above the AuNBP particles prepared by single step addition of NaBH4 showed mostly spherical or random shaped particles thus lesser yield of AuNBP.

A. Scanning Electron Microscope (SEM) Analysis
[0042] The purified seeds prepared in Example 1 and comparative example 1 were then observed under a scanning electron microscope of JEOL JIB4700. to see the high yield of the AuNBP particles.
[0043] In figure 2(a) all the SEM image of AuNBP sample synthesized using single step addition of NaBH4 dose not produce good quality penta twined seeds. The seed when put in growth solution does not give high yield of high quality AuNBPs. The SEM images shows that most of the sample has lots of spherical or random shaped particles with very low bypiramid particles. Only a handful of synthesis shows high yield of AuNBP particles (Ref. Table 1). This shows that the single step addition of NaBH4 does not produce good quality seeds thus resulting in low yield of AuNBP particles.
[0044] In figure 2(b) all the SEM image of AuNBP sample synthesized using two step addition of NaBH4 in the seed synthesis process produces high quality penta twinned seeds which gives high yield of AuNBPs. In most of the cases, a very high yield of AuNBP particles with a few spherical or random shaped particles were seen.

B. Scatter Plot of UV-Visible Spectroscopy
[0045] The UV vis data shown in the Table 1 shows both the longitudinal and transverse Localized surface plasmon resonance (LSPR) values. The Longitudinal LSPR are plotted in the scatter plot.
[0046] As shown in Fig:3 Scatter plot shows the longitudinal LSPR of AuNBPs. The red dots shows the longitudinal LSPR of AuNBPs synthesized with seeds of single step addition of NaBH4. The blue dots shows the longitudinal LSPR of AuNBPs synthesized with seeds of two step addition of NaBH4. The green zone in the graph shows the region above absorbance value 2. Particles with longitudinal LSPR absorbance value more than 2 is good synthesis and having high yield. As can be seen all the seeds synthesized with two step addition of NaBH4 are giving high quality yield where as single step seed are inconsistent.

Example 3
Transmission Electron Microscopy (TEM) Analysis
[0047] The TEM images of AuNBP particles are captured in scanning mode. The length and width of randomly selected 100 particles was measured using Fiji (an image processing software). The data from that analysis is plotted as a bar graph in Figure 4. The blue colour bars shows the width of the particles and we do not see much variation in the width. The average width comes to 16.5 ± 1.3 nm. Where as the red colour bars shows the length of the bipyramid particles. The bipyramid particles mostly show sizes greater than 40 nm with an average of 52.6 nm ± 4.9 nm




SPECIFIC EMBODIMENTS OF THE PRESENT INVENTION
[0048] The present disclosure relates to a method of preparing a bipyramid nano-gold particles comprising the steps of
(1) preparation of seeds
a) mixing Gold (III) chloride (HAuCl4) solution with aqueous cetyl trimethyl ammonium chloride (CTAC) and citric acid to obtain a solution;
b) adding two-third volumes of aqueous sodium borohydride (NaBH4) solution rapidly with continuous stirring to the solution;
c) adding one-third volume of aqueous sodium borohydride solution drop by drop to the solution obtained in step b) to obtain a brown solution; and
d) heating the brown solution at a temperature of 70°C to 100°C with stirring at 1000 rpm to 2000 rpm to obtain seed solution,
(2) growth of seeds to obtain bipyramid nano-gold particles
e) adding HAuCl4 solution into cetyl trimethyl ammonium bromide (CTAB);
f) adding hydrochloric acid, silver nitrate, and ascorbic acid to the solution obtained in step e);
g) adding the seed solution obtained in step (d), and stirring at 1000 rpm to 2000 rpm for 1.5 hrs to 3 hrs; and
h) centrifuging the solution at 7000 to 9000 rpm for 10 min to 15 min to obtain the bipyramid nano-gold particles.
[0049] Such a method, wherein a concentration of the HAuCl4 is 10 mM.
[0050] Such a method, wherein a concentration of the CTAC is 200 mM.
[0051] Such a method, wherein a concentration of CTAB is 100 mM.
[0052] Such a method, wherein a concentration of the citric acid and ascorbic acid is 100 mM.
[0053] Such a method, wherein the amount of aqueous NaBH4 solution added rapidly is 200 µL.
[0054] Such a method, wherein an amount of the aqueous 10 mM NaBH4 solution added drop by drop is 100 µL.
[0055] Such a method, wherein the size of bipyramid nano-gold particles are in range of 5-20nm width and 40 to 60 nm in length
[0056] The present disclosure relates to A bipyramid nano-gold particles obtained by a method of adding NaBH4 in two steps.
, Claims:We Claim:
1. A method of preparing a bipyramid nano-gold particles comprising the steps of
(1) preparation of seeds
a). mixing Gold (III) chloride (HAuCl4) solution with aqueous cetyl trimethyl ammonium chloride (CTAC) and citric acid to obtain a solution;
b) adding two-third volumes of aqueous sodium borohydride (NaBH4) solution rapidly with continuous stirring to the solution;
c) adding one-third volume of aqueous sodium borohydride solution drop by drop to the solution obtained in step b) to obtain a brown solution; and
d) heating the brown solution at a temperature of 70°C to 100°C with stirring at 1000 rpm to 2000 rpm to obtain seed solution,
(2) growth of seeds to obtain bipyramid nano-gold particles
e) adding HAuCl4 solution into cetyl trimethyl ammonium bromide (CTAB);
f) adding hydrochloric acid, silver nitrate, and ascorbic acid to the solution obtained in step e);
g) adding the seed solution obtained in step (d), and stirring at 1000 rpm to 2000 rpm for 1.5 hrs to 3 hrs; and
h) centrifuging the solution at 7000 to 9000 rpm for 10 min to 15 min to obtain the bipyramid nano-gold particles.
2. The method as claimed in claim 1, wherein a concentration of the HAuCl4 is 10 mM.
3. The method as claimed in claim 1, wherein a concentration of the CTAC is 200 mM.
4. The method as claimed in claim 1, wherein a concentration of CTAB is 100 mM.
5. The method as claimed in claim 1, wherein a concentration of the citric acid and ascorbic acid is 100 mM.
6. The method as claimed in claim 1, wherein the amount of aqueous NaBH4 solution added rapidly is 200 µL.
7. The method as claimed in claim 1, wherein an amount of the aqueous 10 mM NaBH4 solution added drop by drop is 100 µL.
8. The method as claimed in claim 1, wherein the size of bipyramid nano-gold particles are in range of 5-20nm width and 40 to 60 nm in length
9. A bipyramid nano-gold particles obtained by a method as claimed in claim 1.

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