1. Critical Concentrations and Role of Ascorbic Acid (Vitamin C) in
the Crystallization of Gold Nanorods within Hexadecyltrimethyl
Ammonium Bromide (CTAB)/Tetraoctyl Ammonium Bromide
(TOAB) Micelles
Oscar R. Miranda, Norman R. Dollahon,† and Temer S. Ahmadi*
Department of Chemistry, VillanoVa UniVersity, VillanoVa, PennsylVania 19085
ReceiVed July 14, 2006; ReVised Manuscript ReceiVed September 12, 2006
ABSTRACT: Growth of gold nanorods is investigated in the presence of ascorbic acid in an aqueous mixed micelle solution of
hexadecyltrimethyl ammonium bromide (CTAB) and tetraoctyl ammonium bromide (TOAB) through photochemical reduction of
HAuCl4‚3H2O using 300 nm Hg lamps. The role of the ascorbic acid concentration, [H2A], in the formation of gold nanorods was
studied using UV-vis absorption and transmission electron microscopy. Gold nanorods were produced only if [H2A]/[Au3+] g 0.75
in the reaction cell. For 0.75 < [H2A]/[Au3+] < 6.2, the aspect ratio of nanorods increased as [H2A] increased. For [H2A]/[Au3+]
> 6.2, the aspect ratio decreased, and shorter nanorods formed as [H2A] increased. These results are indicative of a lower and upper
limit in the concentrations of ascorbic acid and its conjugate base (ascorbate anion) for the formation of gold nanorods. The roles
of ascorbic acid and ascorbate ions are discussed, and these observations support the existence of soft templates formed by CTAB/
TOAB micelles where Au nanorods grow. Evidence for parallel nucleation and growth processes is also presented.
Introduction
Optical, magnetic, electronic, and catalytic properties of many
metal nanoparticles including gold nanorods depend strongly
on and at times are fully determined by their sizes and shapes.1
Consequently, the synthesis and mechanism of formation
of anisotropic colloidal metal nanoparticles, especially Au,
Ag, and Cu, are of great interest both for scientific and
technological reasons. Control over the size of nanoparticles
using wet-chemical methods has been achieved, but such
methodologies for shape-control have been limited to a few
metals such as Pt, Cu, Co, Au, Ag, and Au-Ag alloys.2-6 One
major reason for such limitation is a lack of understanding of
the mechanisms for the formation of nanoparticles which
consequently hinders generalization of synthetic procedures to
more metals.
Surface plasmon is the electric field-induced in-phase col-
lective oscillation of free electrons of the conduction band of
metal nanoparticles and is very sensitive to size, shape, and
aspect ratio (length/width) of those particles.1,6,7 These oscil-
lations give rise to strong absorption bands that lie in the visible-
IR range for Au, Ag, and Cu colloidal nanoparticles, and
consequently extinction spectroscopy has served as a primary
method for characterizing these particles. Aqueous colloidal
dispersion of gold nanorods, the subject of this study, possesses
two absorption bands due to transverse (shorter wavelength
∼520 nm) and tunable longitudinal (longer wavelengths, 600
nm - near IR) plasmon oscillations. It has been shown that the
longitudinal plasmon band (LPB) shifts to higher wavelengths
and the transverse plasmon band (TPB) shifts to shorter
wavelengths as the aspect ratio of particles increases.11
Several synthetic methodologies for the growth of anisotropic
gold nanoparticles, in particular, Au nanorods (AuNRs), have
been developed.8-15 Such chemical methods include the use of
either preformed nanoporous channels of a membrane such as
alumina (or polycarbonates) as hard templates or micelle
solutions acting as soft templates.10 The later method finds
application in electrochemical “seeding” and photochemical
reductions of aqueous Au salt solutions. The electrochemical
method uses Au and Pt plates as electrodes with a third plate
of Ag.9 The presence of silver ions within the cetyltrimethyl-
ammonium bromide (CTAB) and tetradodecyl-ammonium
bromide (TDAB) micelle solution was established as a necessary
element in the growth of nanorods.1,8,12,13 The “seeding” method
involves the use of small (∼3 nm and presumably spherical)
gold particles to grow nanorods in a solution of (CTAB) and
has been applied by many groups for synthesis, mechanistic
studies, or both.6,8,16,17 To date, the mechanism of rod formation
is not well understood, and several papers describing similar
syntheses lead to the formation of different morphologies of
nanoparticles.18
Photochemical reduction involves UV-irradiation of metal
cations in the presence of an electron-donating reagent and was
applied to the synthesis of metal nanoparticles such as Au, Ag,
and Cu previously.4,19,20 In the case of AuNRs, Au3+, Ag+, small
amounts of acetone and cyclohexane, and CTAB/TOAB mixed
micelle solutions are irradiated with UV light. Photochemical
reduction allows more control over the experimental parameters
as compared to the aforementioned methods. The photochemical
method was modified by our group through the use of multiple
lamps rather than one single 254 nm UV lamp as was commonly
used in the past.15 We showed that the wavelength of excitation
of UV light affects both the length of Au-NRs and their yield
of formation. We found that 300 nm lamps produce a longer
and narrower size distribution of gold nanorods. It was also
found that acetone plays a major role in the mechanism of
growth through the formation of its triplet state during the
irradiation process.15
A more rapid method is a mixed chemical-photochemical
reduction of aqueous Au3+ ions with ascorbic acid followed by
UV irradiation.13 Ascorbic acid is used in both photochemical
and seeding methods of AuNR preparation. However, the role
of ascorbic acid is not apparent. It is reported by some that
* To whom correspondence should be addressed. Phone: (610) 519-
7796. Fax: (610) 519-7167. E-mail: temer.ahmadi@villanova.edu.
† Villanova Microscopy Center and Department of Biology.
CRYSTAL
GROWTH
& DESIGN
2006
VOL.6,NO.12
2747-2753
10.1021/cg060455l CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/26/2006

2. ascorbic acid reduces Au3+ to Au+ but not to Au0.14,21 Yet,
others report that ascorbic acid is a mild reducing agent that
reduces Au3+ to Au0.16 The chemical-photochemical method
allows enough control over experimental parameters for a
systematic study of the role of ascorbic acid in the growth of
AuNRs, which is adopted in this study.
We report on the role of ascorbic acid concentration on the
length and aspect ratio of gold nanorods synthesized by the
accelerated chemical-photochemical reduction method. We
show that for fixed gold and silver ion concentration, surfactant
growth solution, and UV irradiation times, the aspect ratio of
AuNRs increases as the concentration of H2A increases. In
particular, it was found that a critical concentration of H2A exists
beyond which no further elongation of gold nanorods occurs;
instead, higher concentrations of ascorbic acid beyond the
critical value leads to the formation of shorter nanorods. Also,
a threshold concentration of H2A exists below which no AuNRs
form. These observations are discussed in terms of changing
morphology of micelles and the solution properties of ascorbic
acid and its conjugate base.
Experimental Procedures
CTAB (Fluka), TOAB (Fluka), HAuCl4‚3H2O (Alfa Aeser), AgNO3
(Fisher), and L-(+)-ascorbic acid (J. T. Baker) were of analytical grade
(purity > 98%) and were used without further purification. Solutions
were prepared with Nanopure water (resistivity >18.2 MΩ‚cm). A
typical synthesis involves a 3.5 mL aqueous solution consisting of
80 mM CTAB and 7.68 mM TOAB prepared in a 4 mL quartz
fluorescence cell (1 × 1 × 4 cm), which is used as the reaction
cell. Also, 250 µL of 24 mM HAuCl4‚3H2O solution, 31.5 µL of
1.0 mM AgNO3 solution, 65 µL of acetone, 45 µL of cyclohexane,
and 50 µL of ascorbic acid (or water) were added to the reaction cell.
The reaction cell was placed in a photochemical reactor (Rayonet,
Southern New England Ultraviolet Co.) and was irradiated with 16 ×
20-W (nominal) Hg-lamps for an hour. The actual output of the lamps
at the reaction cell was determined by chemical actinometry and was
found to be 1.95 ((2%) µW/cm2
. This procedure produces spherical
nanoparticles. To form gold nanorods, 50 µL ascorbic acid was used
with initial concentrations of the solutions of 0.010, 0.050, 0.10, 0.20,
0.40, 0.60, 0.70, 0.80, 1.00, 1.50, and 2.00 M. The concentration of
the stock solution of H2A was varied, but its volume added to the
reaction cell was kept constant to retain the fixed ratios of surfactant
concentrations to that of the reaction solution. It is the total volume of
solvent (water in this case) that determines the size and concentration
of the micelles.
Optical absorption spectra were recorded using a Lambda 35 (Perkin-
Elmer) UV-vis spectrophotometer at room temperature using quartz
cells of 10-mm path length in the 200-1100 nm spectral region.
Transmission electron microscopy (TEM) images were collected using
a Hitachi H-600 transmission electron microscope. The samples were
prepared by drying a drop of the aqueous colloidal solution in air on
a copper grid coated with a thin amorphous carbon film. An acceleration
voltage of 80 kV was used. At least three typical areas of a sample
were imaged, and size distributions of samples were calculated from
enlarged digitized photographs.
Results and Discussion
It must be emphasized that the procedure followed in this
paper is different from the more popular method of seeding.16
This procedure involves the formation of AuNRs from gold ions
within CTAB/TOAB micelles. Figure 1ashows the absorption
spectrum of aqueous solution of HAuCl4 with a metal-to-ligand
charge-transfer band centered at 288 nm, which is seen at 278
nm in more dilute solutions. Individual spectra of AgNO3 and
surfactant solution of CTAB/TOAB show strong absorption
below 300 nm with peaks centered at ∼260 nm. Ionic surfactants
dissociate in water and specifically CTAB/TOAB form Br- ions
and CTA+/TOA+ as shown in eq 1.
The initial reactants (HAuCl4 + AgNO3 + CTAB/TOAB +
acetone + cyclohexane; referred to as solution A) prior to the
addition of ascorbic acid are orange because of metal-to-ligand
charge-transfer bands with maxima at 255, 294, 381, 452, and
546 nm.22 These bands are due to absorption by [AuBr4]- ions
forming as a result of ligand substitution of [AuCl4]- in the
presence of Br- ions as shown in eq 2 (Figure 1c). This
assignment is verified easily by addition of HBr to an aqueous
solution of HAuCl4.
It should be noted that these bands have been attributed to
the presence of a complex formation of the type CTA+-AuCl4
-,
but evidence for such complex formation has not been pre-
sented.14,21
Reaction 2 is a multistep ligand exchange case that col-
lectively happens in less than a minute.23 Figure 1b shows the
absorption spectrum of HAuCl4 after addition of H2A; the
charge-transfer bands disappear, and the yellow solution turns
colorless indicating that Au3+ can be reduced by H2A. When
50 µL of 0.10 M ascorbic acid is added to solution A (final
[H2A] ) 2.00 mM) the orange color of the solution with
absorption bands in the 350-600 nm region disappears as shown
in Figure 1d. The disappearance of the orange color is due to
the reduction of [AuBr4]- to Au0, which occurs according to
eq 3.
Figure 1. Absorption spectra of (a) aqueous solution of HAuCl4, (b)
HAuCl4 after H2A was added, (c) solution A, and (d) solution A after
the addition of H2A are shown. The ligand-to-metal charge-transfer
bands are due to AuBr4
-
formation and are not present for the AuCl4
-
solution in the 350-600 nm range.
CH3(CH2)15N+
(CH3)3Br f Br-
+ CH3(CH2)15N+
(CH3)3
(or Br-
+ CTA+
) (1a)
[CH3(CH2)7]4 N+
Br f Br-
+ [CH3(CH2)7]4 N+
(or Br-
+
TOA+
) (1b)
[AuCl4]-
+ 4Br-
f [AuBr4]-
+ 4Cl-
(2)
2[AuBr4]-
+ 3H2A f 2Au0
+ 8Br-
+
6H+
E ) +0.446 (3)
2748 Crystal Growth & Design, Vol. 6, No. 12, 2006 Miranda et al.

3. The two pertinent half reactions are
The standard reduction potential of [AuBr4]- is +0.858 V (v.
SHE), which is higher than that of ascorbic acid (+0.412 v.
SHE).24 For the net reaction 3, a ∆G ) -258.2 kJ/mol is
calculated. Therefore, the reduction of HAuCl4 with H2A is
favored both kinetically (instantaneous) and thermodynamically.
The ability of ascorbic acid to reduce Au3+ halides to metals
has been known for decades and was utilized in both qualitative
and quantitative determination of gold.25 HA can reduce Ag+
ions, but as long as there are Au cations, such reduction is not
favored because the reduction potential of Ag+ is lower than
all AuBr4
-, AuCl4
-, and Au3+ ions.24a Because gold atoms are
neutral, they do not dissolve in water and hence move to the
inside of CTAB/TOAB micelles where the nucleation process
initiates. It must be noted that the stepwise dissociation of
[AuCl4]- which has been proposed for the purely photochemical
reduction process is not applicable to this study because here
the reduction process is completely carried out chemically with
H2A.15,26
Photolysis of solution A and all other solutions were carried
out in a 4-mL quartz cell with 16 × 300 nm UV lamps for 1.00
h; this duration was kept constant throughout these experiments.
Figure 2ashows the absorption spectrum of solution A after 1.00
h of irradiation. The charge-transfer bands do not disappear but
slightly higher optical extinction results in the 400-800 nm
range, which is due to the scattering of light from small gold
nanoparticles formed from photoreduction of some Au3+ ions
and lack of any plasmon absorption band which characterizes
larger gold nanoparticles possessing free electrons.27 Inset of
Figure 2 shows the TEM image of these gold nanoparticles,
which are about ∼3 nm in diameter; energy dispersive analysis
of X-rays (EDAX) showed that these are indeed gold nanopar-
ticles and not a salt of gold or silver.
When H2A was added to solution A, the sample was either
kept in the dark for 24.00 h (Figure 2b) or irradiated for 1.00
h (Figure 2c). When in the dark, a mixture of Au nanospheres
and nanorods formed. However, gold nanorods with fully
developed and distinct transverse and longitudinal plasmon
absorption bands at 520 and 705 nm formed when the sample
was irradiated. Obviously, H2A (2.00 mM) accelerated the
reaction and induced AuNR formation with an aspect ratio of
3.6 in 1.00 hsa process that would otherwise take 6.00 h without
H2A.15 A TEM image of these AuNRs which have an average
size of 27 nm is shown in Figure 4a.
To further investigate the role H2A on the formation of AuNR,
the [H2A] was varied while its volume was kept the same, which
was done to avoid any phase transition in the micelle solutions
due to changes in the concentration of the solvent. Figure 3
reproduces the spectrum of solution A with 2.00 mM H2A
(solution B) after 1.00 h irradiation for reference. As [H2A] is
increased to 4.00, 8.00, and 12.00 mM, the longitudinal plasmon
shifted to 734, 770, and 807 nm, respectively (solutions C, D,
and E respectively). For 16.00 mM H2A, a maximum LPB at
940 nm is observed (solution F). Any further increase in [H2A]
led to shorter nanorods. At [H2A] of 20.00, 30.00, and 40.00
mM, LPBs were at 867, 815, and 665 nm (G, H, and I)
Figure 2. (a) Solution A after 1.00 h of irradiation, (b) solution B
after 24 h in the dark, and (c) solution B after 1.00 h of irradiation.
Inset shows the formation of small gold clusters after 1.00 h of
irradiation in the absence of H2A. Irradiation accelerates the formation
of AuNRs.
[AuBr4]-
+ 3e f Au0
+ 4Br-
(4)
A2-
+ 2H+
+ 2e f H2A (5)
Figure 3. Upper panel: photograph of AuNR solutions prepared with
(B) 2.00 mM, (C) 4.00 mM, (D) 8.00 mM, (E) 12.00 mM, (F) 16.00
mM, (G) 20.00 mM, (H) 30.00 mM, (I) 40.00 mM of ascorbic acid
and 1.00 h of irradiation showing the change in color as the longitudinal
plasmon band is shifted. Lower panels: absorption spectra of AuNR
B-I solutions. The longitudinal plasmon is shown to red shift with
increasing [H2A] (upper graph). LP is shifted back to the blue as [H2A]
> 16.00 mM (lower graph).
Role of Vitamin C in Gold Nanorod Crystallization Crystal Growth & Design, Vol. 6, No. 12, 2006 2749

4. respectively. Table 1 lists the concentration of added H2A, length
of the resulting particles, calculated aspect ratios, and [H2A]/
[Au3+] for all the solutions B-I. It is seen that the length and
aspect ratio of gold nanorods increase when [H2A] increases
up to 16.00 mM beyond which an increase in [H2A] leads to
the formation of shorter nanorods. The inset shows a photograph
of the colloidal AuNR solutions in order of increasing [H2A]
and clearly indicates that average aspect ratio (color) changes
with [H2A].
Figure 4 shows the TEM images of gold nanorod solutions
B, D, F, and H. It is worth emphasizing that these images show
the size distributions of as prepared gold nanorods; there was
no centrifugation or any other separation procedure performed
to size select the particles. Lengths and aspect ratios shown in
Table 1 were calculated by measuring more than 300 particles
from enlarged images; in cases of mixed shapes, only rods are
considered for length measurements; no corrections to the
absorption spectra were made, and hence the spectra are those
of mixed particles in some cases. The average length of particles
increased from a length of 27 to 73 nm upon addition of H2A
up to 16.00 mM (solution F) and decreased from 73 nm down
to 58, 51, and finally to 20 nm when the concentration of H2A
increased to 20.00, 30.00, and 40.00 mM, respectively (histo-
grams are provided in Figure 9 of Supporting Information). The
corresponding aspect ratios follow a similar trend with solution
F having the highest value of 6.2.
Synthesis of metal nanoparticles from their salt solutions can
be divided into three distinct processes: (1) cation reduction,
(2) nucleation, and (3) growth. In this study, the first step is
accomplished by the addition of H2A to the initial solution, and
the other two steps are left to be done by UV photons during
the 1.00 h of irradiation that was used in this study. In the
absence of micelle solution, addition of H2A to a solution of
[AuCl4]- instantaneously generates small gold nanoparticles at
room temperature. These nanoparticles are comparable to those
of the inset of Figure 1. These particles form because there is
little or no barrier (e.g., micelles) to the coming together of
gold atoms. In the presence of micelles, such Au particles do
not form immediately, although Au atoms do so, which is
indicative of an activation energy barrier for the formation of
nuclei and their subsequent growth to nanoparticles. Nanopar-
ticle growth happens within micelles, but it does so slowly (in
dark) as shown in Figure 2. Therefore, micelles neither hinder
nucleation nor growth, but they simply slow those two processes
by creating physical barriers for Au atoms to convene and form
nuclei. These results lead us to conclude that irradiation, after
reduction by H2A, accelerates the synthesis process through
facilitating and catalyzing the nucleation and growth processes
and has very little or no effect on the reduction process itself.
In the current study, because an ample supply of Au atoms is
available after H2A is added to the initial reactants, UV
photolysis is left with the task of bringing the atoms together
(nucleation and growth), and this process naturally takes less
time than having to do additional work of reducing metal
cations. Thus, the time for growth of nanoparticles decreases
from 6.00 h to just 1.00 h because the reduction process is
hastened by H2A-, a rate-determining step without which neither
nucleation nor growth could subsequently occur.
To explain the rodlike shape of the nanoparticles, the role of
the CTAB/TOAB micelles needs to be clarified. The concentra-
tion of CTAB used in this study is a hundred times larger than
the first critical micelle concentration (CMC) of CTAB (0. 8
mM) and five times the second CMC (>0.20 mM) beyond
which rodlike micelles form.28,29 We have previously shown
that CTAB/TOAB micelles are up to ∼90 nm long with an
aspect ratio of ∼5svery similar to the sizes of AuNRs
synthesized within those micelles.15 Since rodlike micelles
already exist and the shape of nanoparticles grown within them
are also rodlike, then the micelles must be acting as soft
templates. What H2A is doing is to provide enough supply of
gold atoms to ‘fill-in’ those soft templates. As [H2A] increases,
so does the supply of uncharged Au atoms leading to the
formation of longer particles.
Another factor in determining the rodlike shape of Au
nanoparticles is the nature of their surfaces and their interaction
with Ag+ and/or CTA+/TOA+ ions. CTAB and TOAB have
been used as surface stabilizers, which bind to the surface of
nanoparticles; they decrease surface energy, control growth and
evolution of shape, and further prevent nanoparticles from
coagulating.17a Gold nanorods have [110] surfaces along the
sides and {111} on the faces. It is the growth along {111}
surfaces that lead to the elongation of AuNRs. It was shown
that CTA+ ions favorably bind to the [110] surfaces and stop
growth along those directions while allowing growth along the
ends, i.e., {111} surfaces.16 It is shown experimentally that
length of AuNRs increases as the concentration of Ag+ ions
increases.12,13 It is proposed that Ag+ ion (pairing with Br- from
CTAB) bind on the [111] surfaces of growing Au nanoparticles
and catalyze growth in those directions.6b,17b When [Ag+]
increases, longer particles form as a result of more catalyzed
growth.
Another possibility for AuNR elongation is the surface-
Figure 4. TEM images of samples (a) B, (b) D, (c) F, and (d) H are
shown. The average lengths of the AuNRs were calculated to be 27 (
4, 37 ( 6, 73 ( 10, 51 ( 6 nm, respectively.
Table 1. Maximum LP (λmax), Length, and Aspect Ratio (Length/
Width), and [H2A]/[Au3+] of Gold Nanorods Synthesized by UV
Irradiation as [H2A] Was Increaseda
solution
[AA]/
10- 3M
λmax/
nm
average
length/nm
aspect
ratio [H2A]/[Au3+]
B 2.00 705 27 3.6 0.75
C 4.00 734 30 3.7 1.5
D 8.00 770 37 3.8 2.25
E 12.00 807 54 4.5 4.5
F 16.00 941 73 6.0 6.2
G 20.00 867 58 4.7 7.5
H 30.00 815 51 3.7 11.25
I 40.00 665 20 3.6 15.0
a Data show that 16.00 × 10-3 M H2A gives the longest nanorods with
the largest aspect ratio.
2750 Crystal Growth & Design, Vol. 6, No. 12, 2006 Miranda et al.

5. specific binding of H2A on the growing gold nanoparticles
within the micelles that hence prevents the growth of certain
surfaces and allows others to grow. Ascorbic acid at higher
concentrations plays two roles of reducing most of the gold ions
as well as forming complexes at the surface of metal particles.
It is known that H2A forms bidentate complexes even with
metals that it can reduce such Cu(II), Ag(I), and Au(III) ions.
However, the relatively narrow size distribution of Au
nanorods and their dependence on H2A concentration points
against surface-selective growth and supports the “soft template”
argument. If such surface-selective growth were the major mode
of shape and size control, size distribution would have been
wider because different particles would have stopped growing
at different times of their binding with CTA+ or H2A.
A highly supersaturated solution of Au atoms is required for
AuNR formation as the case is in general for any nucleation
process to occur. These supersaturated solutions need not be
homogeneously spread throughout the matrix (micelle solution)
but could be localized within some regions, i.e., within some
micelles. We observed that at [H2A] < 2.00 mM, AuNRs do
not form, and instead very small (∼4 nm) gold spherelike
particles result because of the lower concentration of Au atoms.
In contrast, in solutions where [H2A] g 2.00 mM, a burst of
Au atoms are generated quickly after the addition of H2A and
that leads to at least localized supersaturation and subsequent
nucleation. In the 1.00 h of irradiation with UV lamps, some
nucleation may also take place due to photoreduction of Au3+
ions as well, but such concentrations are not high enough to
yield “observable” results as seen in Figure 2.
A growth process follows the nucleation step after the clusters
have reached a critical size and have formed nuclei. Growth
itself may result from coagulations of small clusters (nuclei) or
direct deposition of Au atoms on the nuclei one at a time. In
either case, the presence of UV photons is essential to the
formation of AuNRs beyond the reduction step. The photons
provide the activation energy necessary for the nucleation and
growth. This is evident from the increase in the rate of those
processes when one compares the rate of formation of AuNRs
with that of particle formation in the absence of light. In the
latter, small particles of various shapes form within 24 h, while
a much larger concentration of AuNRs is produced (with the
same [H2A]) in the presence of photons (compare inset of Figure
2 with Figure 4).
The size of a micelle is strongly affected by the pH of the
solution. It is known that NaOH reduces the CTAB micelles
12-fold in diameter from 2000 to 170 nm, for example.30 In
this study, the pH of the reaction changes when different
concentrations of H2A are added, and they range between 2.3
and 2.6 for the highest to the lowest [H2A]. To understand the
effect of initial pH of the solution on the AuNR formation,
NaOH or HCl was added to the solutions, and the pH was varied
from 2.37 to 7.00. The effect on the shape of Au nanoparticles
was studied for a fixed concentration of 16.00 mM H2A, which
has a normal pH value of 2.77 in the absence of any additional
acid or base. For pH values of 2.37 and 2.50, 3.00, 3.50, 4.00,
4.50, and 7.00, one plasmon absorption peak around 550 nm
was observed, which is indicative of the formation of spherelike
Au nanoparticles (Supporting Information). We attribute forma-
tion of spherelike particles to the shape transformation of the
micelles because pH changes alter the shape of the soft
templatessthe CTAB/TOAB micelles.
If the initial pH is not changed through addition of HCl or
NaOH, the particles’ shapes do not change, but their lengths
do. This observation suggests that H2A either helps to elongate
micelles or retains the lengths of the elongated micelles that
already exist as the concentration of the surfactant is beyond
the second CMC. Only if any or both of these two conditions
are met, the resulting particles would be rods. Our results suggest
that H2A elongate micelles up to [H2A] ) 16.00 mM. After
this point, longer micelles do not form, and instead more
energetically favored shorter micelles appear as explained next.
When H2A is used in the reduction step, a fraction of it
remains unreacted when [H2A]/[Au3+] > 3/2, as required by
the balanced eq 3. This excess H2A changes the morphology
of the micelles by first increasing (up to 16.00 mM H2A) and
then decreasing their lengths. Ascorbic acid has a pKa1 of 4.2,
which decreases to 3.65 in the presence of CTAB.32 This
increase in acidity is due to the binding of the conjugate base
(ascorbate ion; HA-) to the surface of micelles causing further
dissociation of the ascorbic acid.31 It is very likely that binding
of HA- to the headgroups of CTA+/TOA+ induces a longitu-
dinal shape transition by decreasing the head-head repulsions
of the surfactant ions. When the micelles get longer and longer
due to an increase in the concentration of H2A (which may
selectively shield CTA+ headgroups and elongates the micelle)
a point is reached where energetically it is favorable for one
long micelle to split into two short ones. A similar effect would
be observed if H2A acts as a hydrotrope-facilitating the
dissolution of CTAB in water. Hydrotropes normally change
the shapes of micelles to spherelike structures and prevent
formation of more complex.32 The hydrotropic property of
ascorbic acid has been studied in the past, and it is known that
solubility of CTAB increases in water in the presence of ascorbic
acid.33 For a more soluble surfactant, shorter micelles result.
Ascorbate anion (HA-) is the conjugate base of H2A, and
these two species are the most dominant ones for pH values
below ∼6.34 H2A has four hydroxyl groups at positions 2, 3, 5,
and 6 with the most acidic one at position 3. In HA- ion, CdO
of position 1 and O- of position 3 share the negative charge,
and because of this resonance, HA- becomes as strong as citric
acid, which can reduce Au3+.30 If the shapes of the CTAB/
TOAB micelles are altered due to the presence of ascorbate
anions and HA- can reduce Au3+, then addition of HA- (as
sodium ascorbate, for example) to the initial reactions should
also give AuNRs. In addition, there should be a lower and upper
limit in the concentration of HA- in the formation of AuNRs
similar to the case of H2A. Figures 5 and 6 show the effect of
added different [HA-] on the shapes of AuNRs. It is evident
that rodlike structures form with 2.00, 4.00, 8.00, and 12.00
mM solutions of ascorbate ions with average particle sizes of
37.0, 55.0, 41.0, and 44.0 nm (Table 2, Supporting Information).
Gold nanorods also form at concentrations of 16.00, 20.00,
30.00, and 40.00 mM ascorbate ions with average sizes of 56.0,
60.0, 52.0, and 56 nm, but their particle densities are much lower
(Table 2, Supporting Information). The largest densities of
AuNRs (90%) formed with 4.00 and 8.00 mM solutions of HA-,
which indicates that at such concentrations the shapes of the
micelles are most favorable for AuNR growth. At [HA-] >
16.00 mM, a large density of shapes other than rodlike were
formed (Figure 6). Those shapes included cubic, millet-like, and
bonelike, which have been observed by others in “seed-
mediated” synthesis as well.35 These results along with those
of ascorbic acid show that the possibility of ascorbate ion
binding to CTA+ head groups and elongating the micelles up
to a limit is very likely. This evidence supports the “micelle-
splitting” and “hydro-trope” proposals as well because at higher
concentrations of HA-, a larger concentration of spherelike or
nonlinear-shaped nanoparticles form.
Role of Vitamin C in Gold Nanorod Crystallization Crystal Growth & Design, Vol. 6, No. 12, 2006 2751

6. Extreme limits in the concentration of H2A have been
observed in seed-mediated growth of silver nanorods and
nanowires on the surface of ITO from Ag seeds of ∼4 nm in
diameter. Hirao et al. observed a lower limit in concentration
of H2A below which no growth of seeds occurred.36 They also
noticed an increase in the length of nanorods and wires with an
increase in the concentration of H2A solution, supporting our
assertion of a higher supply of Au atoms leading to larger
particles. When [H2A] was further increased, even larger islands
of Ag formed, which happens because there were no physical
barriers, i.e., micelles, to stop such growth. Although the
mechanism of growth from seeds and on planar surfaces is very
different from those of nanoparticle formation within micelle
solutions, the relation between [H2A] and length/size of particles
is seemingly common.36
Growth of AuNRs with time shows that rodlike particles
emerge within the first 10 min of irradiation as is evident from
the rise of a longitudinal and transverse plasma bands (Figure
7). The absorption peak of the LPB increases in intensity as
irradiation time is increased. This sample was made with 14.00
mM of H2A (sample J in Figure 3), which produces particles
possessing a longitudinal plasmon absorption band in the middle
of the visible range, easily monitored with the UV-vis
spectrometer. There is a small blue-shift of longitudinal plasmon
after 50.00 min, which has been observed by others as well.13
It shows the transformation of AuNRs into more thermodynami-
cally stable configurationsa process similar to coarsening of
nanoparticles.13,37 These results indicate parallel nucleation and
growth processes. Nucleation and growth are sequential (nucle-
ation then growth) within a micelle but parallel between
micelles. If the nucleation process is fully completed before the
growth process initiates, then particles would have gone from
nuclei to short rods, and then to longer rods. In such a case, the
longitudinal plasmon would have shifted to the red with time
as the growth had taken place. However, the growth in this case
shows that number density of AuNRs (absorption intensity)
increases with time but with no significant shift in the
wavelength of absorption maxima, implying that more and more
rods form as time increases. This also suggests that new nuclei
are born and they grow to become nanorods, while other earlier-
formed AuNRs are still within the micelle solution and do not
grow further. In addition, this last statement provides further
evidence for the “soft templating effect” by the micelles, i.e.,
growth stops when a particle reaches the size of the micelle.
This is in utter contrast to the findings for seed-mediated growth
where the nuclei are separately prepared and then added to the
growth solution. In that case, one clearly sees the shifting of
Figure 5. Absorption spectra of AuNR samples with (K) 2.00 mM,
(L) 4.00 mM, (M) 8.00 mM, (N) 12.00 mM, (O) 16.00 mM, (P) 20.00
mM, (Q) 30.00 mM, and (R) 40.00 of ascorbate anion. AuNRs are
longest with (M) 8.00 mM, (N) 12.00 mM of sodium ascorbate having
average lengths of 55 ( 7 and 41 ( 6 nm, respectively.
Figure 6. TEM images of samples with (a) 2.00 mM, (b) 8.00 mM,
(c) 12.00 mM, and (d) 20.00 mM sodium ascorbate. The average particle
lengths were calculated to be 37 ( 10, 41 ( 6, 44 ( 12, and 60 ( 14
nm. However, a large number of non-rodlike particles resulted for
samples made with 20.00 mM ascorbate ions.
Figure 7. Absorption specta of solution A with 14.00 mM H2A taken
at 10-min intervals shows the growth of AuNRs. The intensity of both
the longitudinal and the transverse plasmon bands increase over time
indicating parallel growth process.
2752 Crystal Growth & Design, Vol. 6, No. 12, 2006 Miranda et al.

7. the longitudinal plasmon band to higher wavelengths as the size
of the AuNRs increases, which clearly indicates a sequential
process.17b
Finally, the surfaces of AuNRs are bound by both CTA+ and
TOA+ forming a bilayer structure and rendering them soluble
in water. The first layer is formed by the binding of the cationic
headgroups to the surface Au atoms and the organic tail pointing
away from the surface. The second layer is formed by the
organic tail pointing inward and the cationic headgroups pointing
out toward the solvent, H2O. Our analysis of AuNRs after
washing the as-prepared AuNRs with water several times and
isolating the nanoparticles by centrifugation showed no H2A
on their surfaces. Indeed, the results were exactly like the
experiments performed in the absence of H2A.17,15
Conclusion
The mechanism of growth of gold nanorods is better
understood if the role of each individual reactant is fully isolated
and determined. In this study, the role of ascorbic acid was
investigated, which showed evidence of soft templating by the
CTAB/TOAB micelles. It was found that both H2A and HA-
could be used to increase the length of AuNRs as long as the
rodlike shapes of the micelles are conserved. At high or low
pH values and different concentrations of H2A or HA-, the
micelle system undergoes structural changes and short and/or
non-rodlike AuNRs result.
Acknowledgment. This project was funded by a grant from
the NanoTechnology Institute (NTI) of Southeastern PA and
the Department of Chemistry, Villanova University.
Supporting Information Available: Spectra of gold solutions with
different initial pH values and statistics of size and shapes as well as
histograms of nanoparticles prepared with different concentrations of
ascorbic acid and sodium ascorbate (Table 2 and Figures 8 and 9).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257.
(2) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed,
M. A. Science 1996, 272, 1924.
(3) Puntes, V. C.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291,
2115.
(4) Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999,
11, 2310.
(5) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirikin,
C. A. Nature 2003, 425, 487.
(6) (a) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 5882.
(b) Liu, M.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 109, 22192.
(7) (a) Kerker, M. The Scattering of Light and Other Electromagnetic
Radiation; Academic Press: New York, 1969. (b) Bohren, C. F.;
Huffman, D. R. Absorption and Scattering of Light by Small Particles;
John Wiley: New York, 1983. (c) Kreibig, U.; Vollmer, M. Optical
Properties of Metal Clusters; Springer: Berlin, 1995. (d) Kreibig,
U.; Genzel, U. Surf. Sci. 1985, 156, 678.
(8) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001,
105, 4065.
(9) Chang, S-S.; Shih, C.; Chen, C.; Lai, W.; Wang, C. R. C. Langmuir
1999, 15, 701.
(10) Foss, C. A.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys.
Chem. 1994, 98, 2963.
(11) Van der Zande, B. M. I.; Bohmer, M. R.; Fokkink, L. G. J.;
Schonenberger, C. Langmuir 2000, 16, 451.
(12) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316.
(13) Niidome, Y.; Nishioha, K.; Hawasaki, H.; Yamada, S. Chem.
Commun. 2003, 2376.
(14) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285.
(15) Miranda, O. R.; Ahmadi, T. S. J. Phys. Chem. B 2005, 109, 15724.
(16) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.;
Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857.
(17) (a) Nikoobakht, B.; El-Sayed, M. A. Langmuir 2001, 17, 6368. (b)
Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957.
(18) Jiang, X. C.; Brioude, A.; Pileni, M. P. Colloids Surf. A. 2006, 277,
201.
(19) Kapoor, S.; Mukherjee, T. Chem. Phys. Lett. 2003, 370, 83.
(20) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z.
Y. AdV. Mater. 1999, 11, 850.
(21) Perez-Juste, J.; Liz-Marzan, L. M.; Carnie, S.; Chan, D. Y. C.;
Mulvaney, P. AdV. Funct. Mater. 2004, 14, 571.
(22) (a) McBryde, W. A. E.; Yoe, J. H. Anal. Chem. 1948, 20, 1094-
1099. (b) Mason, W. R.; Gray, H. B. J. Am. Chem. Soc. 1968, 90,
5721.
(23) Elding, L. i.; Groning, A. B. Acta Chem. Scand. A 1978, 32, 867.
(24) (a) Handbook of Chemistry and Physics, 57th ed., CRC Press:
Cleveland, Ohio, 1976. (b) Note: E0 decreases slightly with
pH and temperature. See Clark, W. M. Oxidation-Reduction
Potential of Organic Systems; The Williams & Wilkins, Co.:
Baltimore, 1960.
(25) Stathis, E. C.; Gatos, H. C. Ind. Eng. Chem. Anal. Ed. 1946, 18,
801.
(26) Pucci, A.; Bernabo, M.; Elvati, P.; Meza, L. I.; Galembeck, F.; de
Paula Leite, C. A; Tirelli, N.; Ruggeri, G. J. Mater. Chem. 2006,
16, 1058 and references therein.
(27) Henglein, A. Chem. ReV. 1989, 89, 1861.
(28) Rubingh, D. N.; Holland, P. M. Cationic Surfactant-Physical
Chemistry; Marcel Dekker: New York, 1991.
(29) To¨rnblom, M.; Henriksson, U. J. Phys. Chem. B 1997, 101, 6028.
(30) Pal, T.; De, S.; Jana, N. R.; Pradhan, N.; Mandal, R.; Pal, A. Langmuir
1998, 14, 4724.
(31) Jaiswal, P. V.; Ijeri, V. S.; Srivastava, A. K. Colloids Surf. B 2005,
46, 45.
(32) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John
Wiley & Sons: Hoboken, New Jersey, 2004.
(33) Yu, W.; Zou, A.; Guo, R. Colloids Surf. A 2000, 167, 293.
(34) Taqui, Khan, M. M.; Martell, A. E. J. Am. Chem. Soc. 1967, 89,
4176.
(35) Huang, C-C.; Yang, Z.; Chang, H-T. Langmuir 2004, 20, 6089.
(36) G. Chang, J. Zhang, M. Oyama, Hirao, K. J. Phys. Chem. B 2005,
109, 1204.
(37) Wang, Z. L.; Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys.
Chem. B 2000, 104, 5417.
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