U.S. patent application number 16/653389 was filed with the patent office on 2020-04-30 for photochemical synthesis of dendritic silver particles.
The applicant listed for this patent is Zhi Wang Zhao. Invention is credited to Chao Wang, Zhi Zhao.
Application Number | 20200130066 16/653389 |
Document ID | / |
Family ID | 70326343 |
Filed Date | 2020-04-30 |
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United States Patent
Application |
20200130066 |
Kind Code |
A1 |
Zhao; Zhi ; et al. |
April 30, 2020 |
PHOTOCHEMICAL SYNTHESIS OF DENDRITIC SILVER PARTICLES
Abstract
Forming dendritic silver particles by combining silver ions, a
reducing agent, and a polymer comprising amine groups in an aqueous
solution to yield a precursor solution, and irradiating the
precursor solution with ultraviolet radiation to form a
multiplicity of dendritic silver particles. A desired morphology of
the dendritic particles, including branch and junction density, may
be achieved by selecting growth parameters, such as molar ratio of
amine groups to silver ions, a length of time of irradiating, or
both.
Inventors: |
Zhao; Zhi; (Mesa, AZ)
; Wang; Chao; (Chandler, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zhao; Zhi
Wang; Chao |
Mesa
Chandler |
AZ
AZ |
US
US |
|
|
Family ID: |
70326343 |
Appl. No.: |
16/653389 |
Filed: |
October 15, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62750615 |
Oct 25, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 2999/00 20130101;
C22C 5/06 20130101; C30B 29/00 20130101; B22F 2301/255 20130101;
B22F 2202/11 20130101; B22F 1/004 20130101; B22F 1/0011 20130101;
B22F 1/0044 20130101; C22C 1/0466 20130101; B22F 9/24 20130101;
B22F 2999/00 20130101; C22C 1/0466 20130101; B22F 9/24 20130101;
B22F 2202/11 20130101 |
International
Class: |
B22F 9/24 20060101
B22F009/24 |
Claims
1. A method of forming dendritic silver particles, the method
comprising: combining silver ions, a reducing agent, and a polymer
comprising amine groups in an aqueous solution to yield a precursor
solution; and irradiating the precursor solution with ultraviolet
radiation to form a multiplicity of dendritic silver particles.
2. The method of claim 1, wherein the reducing agent comprises an
organic acid.
3. The method of claim 1, wherein the reducing agent comprises
citric acid or ascorbic acid.
4. The method of claim 1, wherein a molar ratio of silver ions to
the conjugate base of the weak acid in the precursor solution is in
a range of about 3 to about 3.5.
5. The method of claim 1, wherein the polymer comprising amine
groups is poly(allylamine).
6. The method of claim 1, wherein a pH of the precursor solution is
in a range of about 12 to about 13.
7. The method of claim 1, wherein a molar ratio of amine groups to
silver in the precursor solution is between about 6 and about
12.
8. The method of claim 1, wherein irradiating the precursor
solution occurs under ambient conditions.
9. The method of claim 1, wherein the precursor solution is
irradiated with ultraviolet radiation for at least 3 minutes.
10. The method of claim 9, wherein the precursor solution is
irradiated with ultraviolet radiation for up to 20 minutes.
11. The method of claim 1, wherein the wavelength of the
ultraviolet radiation is in a range of about 320 nm to about 400
nm.
12. The method of claim 1, wherein the ultraviolet radiation has an
output power in a range of about 1.5 W/cm.sup.2 to about 4
W/cm.sup.2.
13. The method of claim 1, wherein the dendritic silver particles
have a linear dimension up to about 100 microns.
14. The method of claim 1, wherein the dendritic silver particles
are dendritic silver nanoparticles.
15. The method of claim 1, wherein the dendritic silver particles
comprise at least 95 wt % silver.
16. The method of claim 1, wherein a branch density of the
dendritic silver particles is in a range of about
0.1.times.10.sup.5 branch/mm.sup.2 to about 11.times.10.sup.5
branch/mm.sup.2.
17. The method of claim 1, wherein a junction density of the
dendritic silver particles is in a range of about 1.times.10.sup.4
junction/mm.sup.2 to about 36.times.10.sup.4 junction/mm.sup.2.
18. The method of claim 1, wherein a fractal dimension of the
dendritic silver particles is in a range of about 1.4 to about
1.9.
19. The method of claim 1, wherein each dendritic silver particle
of the multiplicity has a unique structure.
20. The method of claim 1, further comprising selecting a molar
ratio of amine groups to silver, a length of time of the
irradiating, or both to achieve a desired morphology of the
multiplicity of dendritic silver particles.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Patent
Application No. 62/750,615 entitled "PHOTOCHEMICAL SYNTHESIS OF
DENDRITIC SILVER PARTICLES" and filed on Oct. 25, 2018, which is
incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] This invention relates to photochemical synthesis of
dendritic silver particles.
BACKGROUND
[0003] Over the last several years, the global value of counterfeit
goods has exceeded half a trillion dollars. In addition to direct
economic losses to manufacturers, counterfeit materials, parts and
assemblies typically provide inferior performance and poor
reliability, which can cause security issues, such as security
risks for national defense. There is an increasing demand for high
trust, high reliability tagging methodologies, in which genuine
articles manufactured in a legitimate facility carry "trust
elements" incapable of being cloned. Current physical tagging
technologies include holograms, coded tags, DNA signatures,
mechanical deformation, and fabricated nanostructures. However,
such techniques have several disadvantages, including difficulties
in manufacture, lack of structural stability and reliability, and
complicated readout procedures.
[0004] Fractal structures, such as synthetic dendritic silver
particles, are promising candidates for physical identifiers to
combat counterfeiting. For example, dendritic silver particles can
be applied to an item and decoded to yield a large exclusive
integer, which can be mapped to the item in a secure database.
Traditionally, dendritic silver particles are prepared using
organic reducing agents, ultrasonically assisted templated
synthesis, direct replacement reactions, photoreduction, plating,
.gamma.-irradiation, magnetic field assisted growth, or pulsed
sonoelectro-chemical methods. These methods, however, typically
require a long preparation time and/or precisely controlled
environmental conditions that are not conducive to mass
manufacturing. In addition, the morphology of dendritic silver
particles obtained by these methods can lack natural diversity.
SUMMARY
[0005] Dendritic silver particles are synthesized by a
photochemical process of irradiating an aqueous precursor solution
containing silver ions, a conjugate base of a weak acid, and a
polymer comprising amine groups. This process yields dendritic
silver particles after 20 minutes or less of UV irradiation under
ambient conditions. The size and shape of the particles can be
altered by varying experimental parameters, such as length of
irradiation and local chemical environment. Unique dendritic
structures obtained by this process have distinctive morphological
characteristics suitable for tagging and securing manufactured
items. The dendritic silver particles synthesized by this process
can reach sizes of up to about 100 which allows structural
information about the particles to be quickly read and analyzed by
optical microscopy, thereby facilitating the use of the particles
as anti-counterfeiting labels in supply chains.
[0006] In a general aspect, dendritic silver particles are formed
by combining silver ions, a reducing agent, and a polymer
comprising amine groups in an aqueous solution to yield a precursor
solution, and irradiating the precursor solution with ultraviolet
radiation to form a multiplicity of dendritic silver particles.
[0007] Implementations may include one or more of the following
features.
[0008] The reducing agent may include an organic acid (e.g., citric
acid or ascorbic acid). A molar ratio of silver ions to the
conjugate base of the weak acid in the precursor solution is
typically in a range of about 3 to about 3.5. In one example, the
polymer is poly(allylamine). A pH of the precursor solution is in a
range of about 12 to about 13. A molar ratio of amine groups to
silver in the precursor solution is between about 6 and about 12.
Irradiating the precursor solution occurs under ambient conditions.
The precursor solution is typically irradiated with ultraviolet
radiation for at least 3 minutes, up to 20 minutes, or both. The
wavelength of the ultraviolet radiation is typically in a range of
about 320 nm to about 400 nm. The ultraviolet radiation has an
output power in a range of about 1.5 W/cm.sup.2 to about 4
W/cm.sup.2.
[0009] The dendritic silver particles typically have a linear
dimension up to about 100 microns. In some cases, the dendritic
silver particles are dendritic silver nanoparticles. The dendritic
silver particles comprise at least 95 wt % silver. A branch density
of the dendritic silver particles is in a range of about
0.1.times.10.sup.5 branch/mm.sup.2 to about 11.times.10.sup.5
branch/mm.sup.2. A junction density of the dendritic silver
particles is in a range of about 1.times.10.sup.4 junction/mm.sup.2
to about 36.times.10.sup.4 junction/mm.sup.2. A fractal dimension
of the dendritic silver particles is in a range of about 1.4 to
about 1.9. Each dendritic silver particle of the multiplicity has a
unique structure. In some cases, selecting a molar ratio of amine
groups to silver, a length of time of the irradiating, or both is
selected to achieve a desired morphology of the multiplicity of
dendritic silver particles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 depicts an exemplary scheme for synthesis of
dendritic silver particles.
[0011] FIGS. 2A-2B show samples of precursor solution before and
after the solution is subjected to UV irradiation,
respectively.
[0012] FIGS. 3A-3C show electron microscopy images of the structure
of dendritic silver particles.
[0013] FIG. 4 shows the energy-dispersive X-ray spectrometry
spectrum for an exemplary branch of a dendritic silver
particle.
[0014] FIGS. 5A-5C show transmission electron microscopy images of
dendritic silver particles at 30 seconds, 60 seconds, and 3 minutes
of ultraviolet (UV) irradiation, respectively.
[0015] FIG. 6A depicts the growth mechanism of dendritic silver
particles. FIG. 6B depicts a proposed structure of a silver
particle-poly(allylamine) ((AgNP)-PAAm) complex that enables the
formation of dendritic particles.
[0016] FIG. 7 shows dendritic silver particles synthesized from
precursor solutions containing various amine/silver ratios. The
scale bars represent 500 nm.
[0017] FIGS. 8A-8E show patterns of dendritic silver particles
formed after 20 minutes of UV irradiation.
[0018] FIG. 9 depicts an exemplary skeleton analysis for
identification of dendritic branch levels.
[0019] FIGS. 10A-10E show branch length distribution plots for five
types of dendritic silver particle patterns.
[0020] FIG. 11 shows a plot of dendritic silver particle branch
densities as a function of fractal dimension.
[0021] FIG. 12 shows molecular structures of silver particles
during the synthesis of dendritic silver particles using a
precursor solution containing allylamine (AAm).
[0022] FIG. 13 depicts molecular structural changes that occur
during synthesis of dendritic silver particles.
[0023] FIG. 14 shows a plot of the average branch length of
dendritic silver particles as a function of the amine/silver ratio
of the precursor solution.
[0024] FIGS. 15A-15E show distributions of the weighted number of
branches of a specific length (w) as a function of branch length
(L) for five types of dendritic silver particle patterns.
DETAILED DESCRIPTION
[0025] A method for synthesizing dendritic silver particles is
described. This method includes steps of combining silver ions, a
reducing agent, and a polymer comprising amine groups in an aqueous
solution to yield a precursor solution, and irradiating the
precursor solution with ultraviolet radiation to form a
multiplicity of dendritic silver particles. Examples of suitable
reducing agents for the precursor solution include organic acids,
such as citric acid and ascorbic acid. An example of a suitable
polymer comprising amine groups in the precursor solution is
poly(allylamine) (PAAm). The pH of the precursor solution is
typically in a range of about 12 to about 13. The molar ratio of
silver ions to weak acid in the precursor solution is typically in
a range of about 3 to about 3.5. The molar ratio of amine groups to
silver in the precursor solution is typically in a range of about 6
to about 12. The described method of synthesizing dendritic silver
particles may be conducted under ambient conditions. As used
herein, "ambient conditions" generally refers to a combination of
common or prevailing temperature, pressure, and relative humidity
found in a laboratory or manufacturing setting.
[0026] Irradiation of the precursor solution to synthesize
dendritic silver particles is performed using ultraviolet (UV)
radiation. The precursor solution may be irradiated for a length of
time between about 3 minutes and about 20 minutes or more to
achieve a desired variation in the size and patterns of the
dendritic silver particles. A wavelength of the UV radiation is
typically from about 320 nm to about 400 nm (e.g., UVA), and the
output power of the UV radiation is in typically in a range of
about 1.5 W/cm.sup.2 to about 4 W/cm.sup.2.
[0027] The dendritic silver particles synthesized by the described
method have defined geometric features, including a core and
dendritic branches that extend from the core. The dendritic silver
particles also include junctions (or nodes) at which the dendritic
branches meet. These geometric features, or minutiae, of the
dendritic silver particles create unique patterns and structures
that enable individual particles to be identified out of the
multiplicity of particles. The branch density of the dendritic
silver particles synthesized by the described method ranges from
about 0.23.times.10.sup.5 branch/mm.sup.2 to about
10.4.times.10.sup.5 branch/mm.sup.2. The junction density of the
dendritic silver particles synthesized by the described method
ranges from about 1.0.times.10.sup.4 junction/mm.sup.2 to about
35.8.times.10.sup.4 junction/mm.sup.2. Individual particles may be
distinguished from the multiplicity of particles by determining the
fractal dimension of the particle. The fractal dimension of the
dendritic silver particles synthesized by the described method
ranges from about 1.4 to about 1.8.
[0028] The dendritic silver particles synthesized by the described
method can have a size of up to about 100 microns. As used herein,
"particle size" refers to the linear dimension from the end of one
dendrite (i.e., branch) of the particle to the end of an opposing
dendrite of the particle. In some implementations, the dendritic
silver particles created by the described method are nanoparticles.
As used herein, "nanoparticle" refers to particles with a dimension
in a range of about 20 nm to about 1000 nm. The dendritic silver
particles created by the described process are substantially pure
silver (e.g., at least 95 wt % silver).
[0029] FIG. 1 depicts an exemplary setup 100 for synthesis of
dendritic silver particles using a process of polymer-assisted
photolysis. A precursor solution 102 for synthesis of dendritic
silver particles may be obtained by combining silver nitrate,
sodium citrate, and a poly(allylamine) (PAAm) solution. In one
example, a precursor solution was prepared by combining silver
nitrate and sodium citrate at a 2:1 molar ratio with a
poly(allylamine) (PAAm) solution containing a .about.1 M amine
group. A photo-reduction of nitrate and a polymeric ligand was
selected for the precursor solution to achieve synthesis of
dendritic silver particles under ambient conditions. The precursor
solution was exposed to UV radiation. In one example, the precursor
solution was irradiated by a 365 nm UV light for 3 minutes. As a
result of UV radiation, small dendritic silver particles were
formed. A change in color of the precursor solution 200 and 202 in
FIGS. 2A and 2B, respectively, was observed as a result of the UV
radiation, indicating the presence of dendritic silver particles.
The dendritic silver particles bound to the PAAm and served as
seeds for further growth. The bounded particles connected via
crystal growth under the presence of citrate and free amine groups.
Following irradiation, polymeric ligands formed a polymer backbone
to create a chain tethering together the particles. Some amine
groups fixed the relative positions of dendritic silver particles
by forming N--Ag coordination bonds. Free amine groups served as a
reducing agent that facilitated Ag growth, especially
inter-particle growth.
[0030] As shown in FIGS. 3A and 3B, presence of dendritic silver
particles 300 was confirmed using transmission electron microscopy
(TEM). The dendritic silver particles 300 contained a dense core
302 and extended branches 304. Each branch was composed of silver
particles approximately 50 nm in size. The composition of the
dendritic silver particles 300 was further analyzed using scanning
electron microscope (SEM) (as shown in FIG. 3C). As shown in FIG.
4, energy-dispersive X-ray spectrometry of the particles revealed a
high abundance of Ag (39.14%), indicating that the particles
obtained from the precursor solution were substantially pure
silver. The remaining elements are believed to be from the
substrate (e.g., carbon coated copper TEM grid). Silver
distribution matched the morphology of the observed dendritic
particles.
[0031] As shown in FIGS. 5A and 5B, as radiation time increased to
60 seconds, a densely packed Ag core 500 emerged, with newly formed
particles 502 extending from the edges. As shown in FIG. 5C,
further increasing the UV exposure time to 3 min resulted in the
formation of dendritic particles 504. As indicated by FIGS. 5A-5C
and 6A, the formation of dendrites, rather than simple isotropic
growth, is preferred due to the local chemical gradient created by
PAAm. The chemical concentration gradient around the growth front
of crystals is vital in the formation of dendritic particles. As
shown in FIG. 6A, the presence of PAAm provides a nanoscale
chemical gradient for formation of dendritic silver particles.
Fixation of a portion of the silver particles by PAAm creates a
steric effect that results in the fixed silver particles having
improved accessibility to reagents compared to non-fixed silver
particles, creating a first chemical gradient. In addition, the
distribution of free amine groups on PAAm is anisotropic with
regard to individual silver particles, resulting in a second degree
of chemical gradient.
[0032] The dendritic AgNPs are believed to grow in a two-step
process. First, during nucleation, small AgNPs form after UV
illumination through the photoreduction of silver nitrate with
sodium citrate as the reducing agent, and further bind PAAm to
yield clusters. Next, during growth, the clusters serve as seeds to
guide the further growth of Ag crystals under the presence of
reducing agents (e.g., citrate and PAAm). FIG. 6B depicts a
proposed structure of a silver particle (AgNP)-PAAm complex 600
that enables the formation of dendritic particles. The amine group
between AgNPs 602 indicated by dashed curves 604 facilitates the
anisotropic growth of silver.
[0033] Alteration of the local chemical environment changes the
relative reaction rate of Ag nucleation and seeded growth, which
results in significant changes in the morphology of the silver
particles. The amine/Ag.sup.+ ratio (N/Ag) can be adjusted to alter
the local chemical environment. In one example, precursor solutions
with N/Ag ratios of 1, 2, 4, 6, 10, 12, and 20 were tested and the
corresponding products obtained after 3 minutes of UV irradiation
were analyzed. As shown in FIG. 7, five distinctive dendritic
morphologies 700, 702, 704, 706, and 708 were observed. For
precursor solutions with N/Ag<6, only random silver particle
aggregates formed as a result of insufficient amine groups.
Dendritic silver particles possessing long major branches and short
sub-branches started to form at N/Ag ratios equal to 6. Some
individual silver particles that did not have a chance to grow can
be observed. For precursor solutions with N/Ag ratios equal to 6-8
(700 and 702), the sub-branches were longer, and a well-branched
dendritic structure was observed. Further increasing the N/Ag ratio
to 8-10 (704 and 706) led to the formation of medium and long
sub-branches, and the apparent number of branches started to
reduce. When the N/Ag ratio of the precursor solution was equal to
10 (706), a further reduction in branch number was observed and the
as-formed dendritic structures featured very long branches and
sub-branches. Increasing the N/Ag ratio to 12 (708) led to
formation of small particles with very thick branches. Only
individual silver particles were observed when the N/Ag ratio of
the precursor was above 12.
[0034] The size of dendritic particles can be adjusted by
increasing or decreasing the irradiation time. In one example, the
irradiation time was increased to 20 minutes. As shown in FIGS.
8A-8E, the size of the Ag particles (type I-type V, or 800, 802,
804, 806, and 808, respectively) increased by approximately 1-2
orders of magnitude when the irradiation time was increased from 3
minutes to about 20 minutes. Dendritic patterns as large as 50-100
.mu.m can be formed by adjusting irradiation times to about 20
minutes.
[0035] Mathematical analysis of the dendritic Ag patterns may be
performed to reveal their unique structures, as well as their
potential as information carriers. The type and position of
minutiae, geometric features of the particles, confer uniqueness on
a dendritic pattern and distinguish one pattern from all others.
For dendritic silver particles, the junctions (or nodes) of the
dendrites are the relevant minutiae. A measurable parameter in each
of the nodes may be used to represent a value of modulus B such
that the total number of possible patterns is given by B.sup.no,
where n.sub.o is the number of junctions measured. For example, if
the position of each junction was read as being in either an even
(0) or odd (1) numbered location in a Cartesian grid overlay, then
B=2. When the junction density is 10.sup.5 per mm.sup.2 and the
reading resolution is 3 .mu.m, the total number of possible
patterns in a 50 .mu.m.times.50 .mu.m dendrite area is in the order
of 10.sup.75, which is more than enough to tag every manufactured
item. Considering the junction density and the branch length
distributions shown in Table 1 below, type III patterns may be most
suitable for tagging purposes as they possesses a high junction
density and a greater portion of branches over 3 .mu.m in
length.
TABLE-US-00001 TABLE 1 Exemplary branch and junction densities for
five pattern types of dendritic silver particles Pattern type I II
III IV V Branches per 2.7 .+-. 3.9 .+-. 2.0 .+-. 0.35 .+-. 8.0 .+-.
mm.sup.2 0.50 .times. 0.09 .times. 0.78 .times. 0.12 .times. 2.4
.times. 10.sup.5 10.sup.5 10.sup.5 10.sup.5 10.sup.5 Junctions per
9.7 .+-. 15.7 .+-. 8.1 .+-. 1.5 .+-. 26.2 .+-. mm.sup.2 2.9 .times.
0.5 .times. 3.6 .times. 0.5 .times. 9.6 .times. 10.sup.4 10.sup.4
10.sup.4 10.sup.4 10.sup.4
[0036] In one example, image analysis was performed using ImageJ.
Skeleton analysis of a dendrite may be conducted to reveal branch
and junction information, such as first, second, and third branches
900 and junctions 902 depicted in FIG. 9. FIGS. 10A-10E show
changes in the N/Ag ratio of the precursor solution resulted in
variations in branch/junction densities and distinctive branch
length distributions. As shown in Table 1, when N/Ag ratio of the
precursor solution increased, the branch/junction density also
increased (type II), and then dropped until the N/Ag ratio of the
precursor solution reached 10. Type V patterns possessed the
highest branch/junction density. This trend was verified using
optical imaging (FIGS. 8A-8E). When the N/Ag ratio of the precursor
solution was relatively low, increasing the N/Ag ratio resulted in
increased branching. As N/Ag ratio of the precursor solution
further increased, longer but fewer branches were favored. At very
high N/Ag ratios, small and highly dense branches were formed.
[0037] As shown in Table 2, essentially all of the dendritic silver
particle patterns were found to be perfect dendritic structures and
were distinguishable according to their fractal dimension (FD). As
shown in FIG. 11, formation of fractal structure is driven by
reaction dynamics. Branch density increased as a power of FD.
TABLE-US-00002 TABLE 2 Exemplary fractal dimension of five pattern
types of dendritic silver particles Pattern type I II III IV V FD
1.716 .+-. 1.671 .+-. 1.571 .+-. 1.41 .+-. 1.821 .+-. 0.013 0.017
0.046 0.058 0.015 R.sup.2 0.998 0.999 0.999 0.998 0.999
EXAMPLE
[0038] Silver nitrate (ACS reagent, .gtoreq.99.0%), sodium citrate
dehydrate (.gtoreq.99.0%), allylamine (.gtoreq.99.0%) and
poly(allylamine) solution (Mw.about.17,000, 20 wt. % in H2O) were
purchased from Sigma-Aldrich. 400 mesh ultra-thin carbon coated TEM
grids were purchased from Ted Pella.
[0039] Dendritic silver particles were synthesized via
polymer-assisted photolysis. A precursor solution for silver
particle synthesis was obtained by first combining 204 mg silver
nitrate and 134 mg sodium citrate dehydrate in 200 mL DI water. A
poly(allylamine) (PAAm) solution containing .about.1 M amine group
was obtained by diluting 20% PAAm solution. 1 mL of the silver
nitrate/sodium citrate solution was mixed with the PAAm solution,
with the final precursor solution having an equivalent molar ratio
of amine group and Ag.sup.+ ions (N/Ag) of 10:1. For samples with
different N/Ag values, the amount of PAAm solution added was
adjusted accordingly to mix with 1 mL silver nitrate/sodium citrate
solution to achieve a final precursor solution having an equivalent
molar ratio of amine group and Ag.sup.+ ions (N/Ag) of 10:1. For
the purpose of comparison, the synthetic process was also repeated
by replacing PAAm with allylamine (AAm).
[0040] The final precursor solution was subjected to UV radiation.
UV radiation was applied using a BlueWave.RTM. 200 UV curing spot
lamp. The output power of the UVA band was adjusted to 3.0
W/cm.sup.2. The wavelength of the UV radiation was about 365
nm.
[0041] The reaction products were deposited on a TEM grid following
irradiation for TEM and SEM analysis. TEM images were captured
using a Philips CM 12 TEM. SEM and energy-dispersive X-ray
spectrometry (EDX) data was obtained using a Hitachi S4700
FESEM.
[0042] Microscopic images of the reaction products were obtained
using an Olympus BX53 microscope. For optical imaging, reaction
products were drop-casted onto a glass slide cleaned with Harrick
plasma cleaner. The reaction products were allowed to dry in air
overnight before imaging.
[0043] As shown in FIG. 12, for mixtures containing AAm, rather
than PAAm, as the ligand and reducing agent, no dendritic particles
were formed. In mixtures containing AAm, silver particles 1200 were
bound to the small molecule ligands and remained subject to random
movement in the aqueous solution. As a result, particles could not
be connected efficiently through crystal growth. By comparison, in
mixtures containing PAAm, the polymeric ligand was able to fix the
relative positions of adjacent silver particles, so that
inter-particle Ag growth could occur. As depicted in FIG. 13, the
relative position of the initially formed silver particles 1300 was
fixed by PAAm chains 1302 through chelation, which resulted in
variation of the accessibility of individual silver particles to
citrate ions 1304. For example, silver particles 1300 located at
the edge of polymer chains 1302 had a greater chance to continue
growing under the presence of a reducing agent. In addition, the
local distribution of free amine groups varied for individual
silver particles 1300, leading to an anisotropic growth of Ag under
continuous UV irradiation.
[0044] Optical images of dendritic silver patterns were analyzed
using ImageJ (Fiji version). Fractal box analysis was conducted by
converting images to 8 bit and using the "fractal box count"
function. The box size was chosen to be 2, 3, 4, 6, 8, 12, 16, 32
and 64. The number of boxes containing a fraction of the image was
counted. All of the patterns observed were perfect dendritic
structures, demonstrated by the high R values in linear regression
shown in Table 2. The FD value for various pattern types varied
from 1.41 to 1.82 (Table 2). The standard deviation of FD for a
specific type of pattern was much smaller than the difference in FD
between the various pattern types, making it possible to readily
distinguish pattern types according to FD.
[0045] In order to count the number of branches and junctions of
the dendritic silver particles, the 8 bit image was first
skeletonized and then analyzed using the "analyze skeleton"
function. The skeletonization function classified the distance
between two adjacent junctions (or one junction and one end) as a
branch. The junction/end was defined as a voxel that had more than
two neighbors or only one neighbor, respectively. The shortest
branch method was used to prune the ends to eliminate loops and
end-points. For accuracy, at least three images were analyzed for
each type of pattern.
[0046] As shown in FIG. 9, different levels of branches may be
observed in dendritic patterns corresponding to the order of silver
growth, with the smallest n.sup.th level branch the result of Ag
particles that attached to the Ag nanowire network. The number of
branches increased as the order of silver growth increased. Branch
density increased following a near-exponential trend as the FD
increased, as shown in FIG. 11. This is explained by the fact that
FD is a measurement of surface coverage and follows a scaling
rule:
N= .sup.-FD
where the variable N stands for the number of segments, and
.epsilon. is the scaling factor. The only deviation from the
scaling rule observed was in type I patterns, which may be the
result of increased branch thickness compared to the other pattern
types.
[0047] Changes in the N/Ag ratio resulted in an alteration in the
reaction rate, which lead to variations in branch/junction
densities and distinctive branch length distributions (FIGS.
10A-10E). For type I and type II patterns, the branch length
distribution was fitted by a single Gaussian function with peak
centers located at 1.3 .mu.m and 1.5 .mu.m, respectively. Compared
with type I patterns, the branches in the type II pattern had a
narrower distribution. Additional Gaussian peaks were observed in
type III patterns at 4.2 .mu.m and type IV patterns at 5.4 .mu.m,
which corresponds to longer branches and indicates that the growth
speed of the Ag crystals exceeded the seeding speed under those
conditions. The longest branches only exist as first level branches
due to steric effects (FIG. 9). As a result, the major portion of
branches still featured a shorter length. The average branch length
dropped to 0.8 .mu.m for type V patterns. The drop in branch length
may be due to the insufficiency of free silver ions, as most silver
ions were chelated by the amine groups on PAAm, which hindered
crystal growth.
[0048] As shown in FIG. 14, the average branch length (L) generally
reflected the same findings. To analyze the branching characters in
dendritic Ag patterns, a parameter w was defined as the weighted
number of branches at a specific length. The value of w represents
the total length of Ag that grows into a specific type of branch.
By definition, w=N.sub.L.times.L, where N.sub.L is the number of
branches with length L. Each type of dendritic pattern can be
distinguished by the distribution of w as a function of branch
length (FIGS. 15A-15E). In the w-L plot, several linear trends can
be observed for each type of dendritic structure. The slope of the
linear trends represents NL, and for each linear trend NL is the
same. In addition, the linear trend with the lower slope typically
possesses a greater average length. Each linear trend represents
one level of branching. The trend with the smallest slope comes
from level 1 branching, which is the major branch that first forms
in the solution. Similarly, the trend with the second, third, . . .
, m.sup.th smallest slope will be level 2, 3, . . . , m branches.
The number of distinguishable linear trends indicates the number of
times the structure has copied itself. For type II, III and IV
patterns, four levels of branches are identified, demonstrating a
significant complexity for these pattern types. For type I and V,
three levels are observed, indicating a lower complexity for these
pattern types.
[0049] Application. Two tags were made using the dendrites, and a
series of tests were performed to verify if it is possible to
discriminate features from different tags. Two rectangular regions
of 90 .mu.m.times.50 .mu.m were located under the microscope, and
denoted as tag I and tag II. The microscopic features of them were
recorded to establish a database. Tag I was assumed to be the label
of an authentic or wanted object, while tag II was a control. Five
square regions 25 .mu.m.times.25 .mu.m were randomly chosen from
tag I and tag II, which were set as keys to be identified. A
scale-invariant feature transform (SIFT) analysis was performed to
identify those keys via comparing feature points. It was found that
all the keys selected from tag I could be readily identified. Tens
to hundreds of matching feature points were found between tag I and
key 1-4, the positions of which accurately matched the regions that
the keys were selected from. On the other hand, key 5 selected from
tag II didn't show any match to tag I, although both tags were
generated from the same batch of Ag dendrites. The results showed
that the dendritic features possessed great ability to form unique
taggants. Moreover, the information contained in a tiny region was
already sufficient for identification, which has several
advantages. First, the cost for a single tag could be readily
reduced. For example, tags with a dimension of 100 .mu.m.times.100
.mu.m would be sufficiently large for encryption, which only cost 2
nL of the Ag dendrite suspension. Second, it is possible to produce
a vast number of tags from a single batch of product
(5.times.10.sup.5 tags per mL). Third, a tag could still be
accurately identified even if most of it were damaged, which makes
the tag highly durable and reliable.
[0050] In summary, a photochemical method to synthesize various
types of dendritic AgNPs has been demonstrated. Experimental
parameters (e.g., N/Ag ratio and illumination time) were found to
affect the morphologies of the dendrite AgNPs. Moreover, the size
and morphology of those particles can be uniquely generated and
readily tuned by choosing appropriate growth parameters (e.g., N/Ag
and illumination time). Optical imaging and mathematical analysis
revealed that dendritic particles grown under the different
conditions could be well distinguished based on their
branch/junction densities and branch lengths. Further, the superior
ability of the as-prepared dendrites to produce vast numbers of
unique patterns makes it perfectly suitable for physical tagging
for anti-counterfeiting and security purposes.
[0051] A number of embodiments have been described. Nevertheless,
it will be understood that various modifications may be made
without departing from the spirit and scope of the disclosure.
Accordingly, other embodiments are within the scope of the
following claims.
* * * * *