Open Journal of Applied Sciences, 2013, 3, 332-336
http://dx.doi.org/10.4236/ojapps.2013.35043 Published Online September 2013 (http://www.scirp.org/journal/ojapps)
Facile, Green Synthesis of Large Single Crystal Copper
Micro and Nanoparticles with Ascorbic Acid
and Gum Arabic
Preston B. Landon*#1,2, Alexander H. Mo#3, Carlos T. Ramos4, Jose J. Gutierrez4, Ratnesh Lal*1,2,3
1Department of Mechanical Engineering, University of California, San Diego, USA
2Department of Bioengineering, University of California, San Diego, USA
3Materials Science and Engineering Program, University of California, San Diego, USA
4Department of Chemistry, The University of Texas Pan American, Edinburg, USA
Email: *firstname.lastname@example.org; email@example.com
Received June 29, 2013; revised August 5, 2012; accepted August 12, 2013
Copyright © 2013 Preston B. Landon et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Large single crystal colloidal copper particles with diameters between 0.5 - 2 μm were created using a green synthesis
process. The process used ascorbic acid to reduce Schweizer’s reagent created in situ using copper salts in the presence
of various concentrations of gum arabic. The Schweizer’s reagents were created by varying the concentrations of am-
monium hydroxide and copper nitrate solutions, copper hydroxide, or copper sulfate. The pH of the solution was con-
trolled by the addition of ascorbic acid. Particle formation was favored at high temperature using copper sulfate at pH
values ranging from 7.5 to 9, while the optimal formation occurred at a pH value of 8.5. At high concentrations, copper
particle formation was found to occur from the aggregation of smaller particles which continued to nucleate once ag-
gregated, and this resulted in the creation of globular particles and large aggregates of micron-sized particles. The addi-
tion of gum arabic resulted in the creation of large single crystal particles that did not aggregate. SEM was used to ob-
serve the effect of increasing gum arabic concentrations and EDX was used to confirm the elemental purity of the parti-
Keywords: Copper; Green Synthesis; Microparticles; Nanoparticles; Gum Arabic; Ascorbic Acid
Colloidal copper particles are of great research interest
because of their low cost, high conductivity, unique
chemical , thermal , and optical  properties. A
variety of physical [4,5] and chemical [6-12] methods
exist for the synthesis of colloidal copper. The parame-
ters under study are typically the nature of the copper
salts [10,11], reducing agents[11,12], binding agents [9,
12], temperature , pH , and catalyst . In a cost-
effective and environmentally friendly process, however,
the conditions must allow a high efficiency for the for-
mation of particles with minimal risks to people and en-
vironment and the reagents must be readily available and
inexpensive. Recently, a novel green aqueous method for
the formation of colloidal copper has been reported
[13,14]. Here, we report a process to synthesize colloidal
copper using copper sulfate and ascorbic acid. We also
report the effect of pH and introduce a way to create
large single colloidal copper particles using gum arabic.
Using the readily available copper sulfate and ammo-
nium hydroxide dissolved in deionized (DI) water, solu-
tions of Schweizer’s reagent [Cu(NH3)4(H2O)2](OH)2
were prepared and optimized for use with ascorbic acid
(vitamin C). Schweizer’s reagent is formed by the chemi-
CuSO2OHCu OHSOaqaqaq aq
CuNHH OOH2H O
The inspiration for this particular method came from
observing the reduction of Tollen’s reagent with different
agents including ascorbic acid.
#These authors contributed equally to this work. Tollen’s reagent is a silver-ammonia metal-ion com-
opyright © 2013 SciRes. OJAppS
P. B. LANDON ET AL. 333
plex that deposits a shiny, metallic silver film onto its
container upon reduction and has been used for making
mirrors for hundreds of years. Noting the similarities
between the Tollen’s and Schweizer’s reagent, initial
testing of different reducing agents found that only as-
corbic acid was effective in producing a copper film
To improve the quality of the formed crystals, a bind-
ing and capping agent, gum arabic, was used. Gum ara-
bic is derived from hardened sap of the acacia tree and
has previously been demonstrated to form well-deve-
loped gold and silver particles [15,16] and nanoparticle
functionalization . To the best of the authors’ knowl-
edge, this is the first reported usage of gum arabic in the
formation of copper micro and nanoparticles.
Schweizer’s reagent/ascorbic acid reaction solutions
have been optimized adjusting levels of copper sulfate,
ammonium hydroxide, and ascorbic acid and observing
the copper film quality on the reaction vials. Once opti-
mized, solutions were prepared with increasing amounts
of gum arabic, while heated and stirred. The supernatant
of each sample was extracted, centrifuged, and dried on a
SEM post. SEM images were then used to observe gum
arabic’s effects on copper formation.
2. Materials and Methods
2.1. Preparation of Solutions
Stock solutions of copper sulfate consisted of 12.5 g of
CuSO4·5H2O (Sigma, St Louis, MO), a certain amount of
concentrated NH4OH (Fischer Scientific, Pittsburgh, PA),
dissolved in DI water (MilliQ, Millipore, Billerica, MA)
with a resistance of 18.6 MΩ to a total volume of 500
mL. A 0.3 M reducing solution of L-ascorbic acid was
prepared with 52.8 g of L-ascorbic acid (Sigma, St. Louis,
MO) in 500 mL of DI water. Different reaction condi-
tions were tested in 20 mL glass vials by adding a certain
volume of copper sulfate solution to a certain volume of
L-ascorbic acid solution for a total of 20 mL. Solution
pH was monitored initially and after conclusion of the
reaction with a pH monitor (Accumet Excel XL60,
Fischer Scientific). Varying amounts of gum arabic were
introduced into the optimized reaction mixture in order to
observe its effects on copper crystal formation.
2.2. Preparation for Imaging
After 72 hours to ensure reaction completion, particle
suspensions were transferred in entirety to a 50 mL coni-
cal centrifuge tubes (Corning Inc., Corning, NY) and
cen- trifuged at room temperature at 3220 g for 40 min-
utes. Afterwards the supernatant was disposed. The par-
ticles were washed twice by resuspension of the pellet in
50 mL of DI water, centrifuging, disposing the super-
natant. After washing, the pellet was resuspended by
adding 5 mL of DI water. 10 µL of copper particle solu-
tion was extracted and placed onto a flat SEM post. The
posts were placed in a vacuum oven to prevent oxidation
and to evaporate the residual water.
Copper particles were imaged using scanning electron
microscopy. Samples were placed in a Phillips FEI XL30
with an FEI Sirion column that allows for resolutions up
to 1 nm at 10 kV. An Oxford energy dispersive X-ray
spectroscopy (EDX) instrument was used to perform
elemental analysis to confirm the presence of copper in
the SEM images.
3. Results and Discussion
High quality copper particles resulting from the reduction
of Schweizer’s reagent formed were desired. To that end,
three parameters were optimized: the NH3/Cu2+ molar
ratio in the precursor solution, the pH by adjusting the
amount of ascorbic acid added to the reaction mixture,
and the amount of gum arabic introduced into reaction
3.1. Optimization of the Precursor Solution
12.5 g of copper sulfate and different amounts of con-
centrated ammonium hydroxide mixed together and dis-
solved in DI water to achieve 500 mL stock solutions
with NH3/Cu2+ ratios of 0, 1.15, 2.30, 4.60, 6.90, and
9.20. 10 mL aliquots of these stock solutions were trans-
ferred to 20 mL glass vials and reduced with 10 mL of
0.3 M ascorbic acid. The reaction mixture was allowed to
deposit on the sides of the glass vials for 24 hours at
15˚C - 20˚C. Visual inspection of the film color, smooth-
ness and reflectivity was used to determine the best reac-
tion conditions for the set. A copper sulfate solution with
a NH3/Cu2+ ratio of 6.90 provided the most consistent
and highest quality plating on the vial walls.
Using the 6.90 ratio as a starting point, the precursor
solution was finely tuned with stepwise additions of 50
μL of concentrated ammonium hydroxide to a 10 mL
6.90 ratio sample. These stepwise additions were made
every 2 hours until another addition of ammonium hy-
droxide caused the solution to turn clear. This fine ad-
justment resulted in an optimal precursor solution with
NH3/Cu2+ ratio of 7.13.
3.2. Optimization of pH
After the precursor solution was optimized the best reac-
tion mixture needed to be determined. Starting with 10
mL of 7.12 ratio precursor, reaction solutions were pre-
pared with 0.3 M ascorbic acid added in volumes be-
tween 0 to 10 mL in 0.5 mL steps and the remainder
Copyright © 2013 SciRes. OJAppS
P. B. LANDON ET AL.
filled with DI water to a total reaction volume of 20 mL
(Figure 1). Visual inspection for film color, smoothness,
and reflectivity was once again used to determine film
quality. Determining the optimal pH of the solution was
based on the color of the solution. If the solution was too
green, the mixture was too acidic; if the solution were too
basic the solution would turn yellow. The amount of
ascorbic acid was adjusted carefully to turn the solution
clear and then follow through to green (Figure 1(b)).
This corresponded to a molar ratio of ascorbic acid to
copper of 2.55 at a pH of 8.5.
3.3. Optimization of Gum Arabic Concentration
A series of reaction solutions were prepared with the
1/7.13/2.55 solution and final concentration of gum ara-
bic between 0% - 2.26% w/v as listed in Table 1. Upon
SEM imaging, it was determined that a final concentra-
tion in solution above approximately 0.16% w/v induced
the formation of highly crystalline copper microparticles
(Figure 2) Higher concentrations produced relatively
regular single crystals ranging between 0.5 to 2 μm.
These solutions reacted under vigorous stirring and ele-
vated temperatures (100˚C).
It was found upon examination of the SEM images
that the gum arabic acted both as binding and capping
agent depending on the concentration. At low concentra-
tions, small droplets of gum arabic served as nucleation
centers by binding many small copper seeds and allow-
lowing an orderly crystallization around it (Figure 3(a)).
However, at higher concentrations, the gum arabic drop
Figure 1. Plating quality with varying ascorbic acid content/
pH (a) Finding the right plating in the copper after 168 hrs.
Start on the left increasing amounts of 0.5 mL L-ascorbic
acid was added to 10 mL of an Ammonium hydroxide/
Copper sulfate solution mixed to a mole ratio of NH4/Cu =
7.12. From left to right: 1, 2.5, 4, 5.5, 7 (pH 8.9), 8.5 (pH 8.5),
10 (pH 7.5), 11.5, 13, 15 mL of L-ascorbic acid were added
to solution and deionized water was used to raise the vol-
ume to 20 mL. On visual inspection, the best the condition
was found to be 8.5 mL of L-ascorbic acid with a pH 8.5.
The best sample is indicated by a black box around the
sample. (b) Time elapsed observation of the optimal condi-
tion (8.5 mL of ascorbic acid) (left to right) 5 min., 1 hr, 4.5
hr, 7.5 hr, 28 hr, 168 hr.
lets became incorporated into the copper crystal facets,
spreading out, and limiting further incorporation of cop-
per into the particle (Figure 3(b)). Energy-dispersive
X-ray (EDX) spectroscopy was used confirm that parti-
cles observed were composed of copper (Figure 4).
On the other hand, while ascorbic acid reduction of
Schwiezer’s reagent is specific to copper sulfate-ammo-
nium solutions, we explored the use of other copper salts
like copper hydroxide and copper nitrate. Preliminary
work in this area allowed the formation of copper films
on reaction vials using various NH3/Cu ratios.
Schweizer’s reagent was optimized and reduced with
ascorbic acid in the presence of gum arabic to produce
Figure 2. Transition from grainy to highly crystalline ob-
jects. SEM shows the particle formation with increasing
gum arabic concentration (w/v) (A) 0% (B) 0.014% (C)
0.12%, (D) 0.16%, (E) 0.32%, (F) 0.34%. All scale bars are
Figure 3. Proposed model for the role of gum arabic. Scale
bars are 500 nm. (a) Gum arabic acts as a binding agent
pulling in copper seeds and growing larger. (b) Below the
threshold, not enough gum arabic is around to form nice
complete copper microparticles. (c) At the threshold value
nice copper particles formed. d) Much above the threshold,
the gum arabic starts to act as a capping agent, embedding
itself in the copper crystal and limiting further inclusion of
opper from solution in the crystal. c
Copyright © 2013 SciRes. OJAppS
P. B. LANDON ET AL.
Copyright © 2013 SciRes. OJAppS
Figure 4. EDX confirmation that particles are copper. Scale bars are 1 μm. (A) SEM image of the particle under analysis. (B)
Sample EDX analysis of shows presence of copper in the particles. Other elemental peaks are ascribed to the aluminum alloy
Table 1. Concentration of reaction mixtures tested at pH 8.5 at 100˚C.
[Cu2+] (M) [Ammonium] (M) [Ascorbic Acid] (M) [Gum arabic] (% w/v)[Ammonium]/[Copper] [Ascorbic Acid]/[Cu]
0.054 0.384 0.137 0.014% 7.13 2.55
0.054 0.382 0.137 0.04% 7.13 2.55
0.053 0.376 0.135 0.12% 7.13 2.55
0.025 0.178 0.064 0.16% 7.13 2.55
0.025 0.178 0.064 0.32% 7.13 2.55
0.050 0.359 0.128 0.34% 7.13 2.55
0.049 0.347 0.124 0.50% 7.13 2.55
0.025 0.178 0.064 0.54% 7.13 2.55
0.047 0.336 0.120 0.64% 7.13 2.55
0.046 0.326 0.117 0.77% 7.13 2.55
0.049 0.347 0.124 0.99% 7.13 2.55
0.025 0.178 0.064 1.08% 7.13 2.55
0.048 0.342 0.122 1.13% 7.13 2.55
0.047 0.336 0.120 1.27% 7.13 2.55
0.046 0.326 0.117 1.54% 7.13 2.55
0.025 0.178 0.064 1.61% 7.13 2.55
0.044 0.316 0.113 1.80% 7.13 2.55
0.042 0.298 0.107 2.26% 7.13 2.55
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