The fragmentation of gold nanoparticles induced by small biomolecules

Article abstract of DOI:10.1039/b809736d

Spherical gold nanoparticles (3-5 nm) undergo a surprising fragmentation without extra energy imput and are converted into ultrasmall particles (less than 1.5 nm), which is a direct result of electron transfer between gold nanoparticles and cysteine. The Royal Society of Chemistry.

Full text of DOI:10.1039/b809736d

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The fragmentation of gold nanoparticles induced by small biomoleculesw
Tie Wang,ab Xiaoge Huab and Shaojun Dong*ab
Received (in Cambridge, UK) 10th June 2008, Accepted 23rd June 2008
First published as an Advance Article on the web 4th August 2008
DOI: 10.1039/b809736d
Spherical gold nanoparticles (3–5 nm) undergo a surprising
fragmentation without extra energy imput and are converted
into ultrasmall particles (less than 1.5 nm), which is a direct
result of electron transfer between gold nanoparticles and
cysteine.
Many of the physical and chemical properties of nanoscale
materials, including luminescence, conductivity and catalytic
activity, are under control of the size of nanoparticles.1 It is
generally accepted that metallic nanostructures are easier to be
Fig. 1 TEM images of (A) gold colloid at the initial stages and (B) the
damaged by heat and light compared to their bulk solids.2 The
same colloidal suspension after aging for 6 h. The inset in image (A) is
surface oscillations affected by heat energy induces mechanical
an electron diffraction of image of large gold nanoparticles.
instability of nanoscale particles, resulting in their fragmenta-
tion. The photoinduced fragmentation of nanoparticles has
Briefly, 10.0 mL of distilled water, 0.2 mL of an aqueous
25 mM HAuCl4 solution, and 1.0 mL of an aqueous 200 mM
cysteine solution were mixed (cysteine : Au = 40 : 1). Next,
excess NaBH4 was added with stirring to prevent the presence
of unreduced gold ions. No additional cleaning step was
performed in subsequent aging process at room temperature.
At the initial stage (o5 min), the TEM image shows that
3–5 nm gold nanoparticles are present (Fig. 1A). The asso-
ciated UV-Vis spectrum shows an absorption peak at 506 nm
(Fig. 2), which is the surface plasmon absorption of small gold
nanoparticles. Subsequently, we noticed that the red color of
gold colloid suspension gradually faded (Fig. 2). Finally,
significant optical features of the gold nanoparticles disappear
after an elapsed time of 6 h. The suspension becomes trans-
parent, which indicates that the fading of the solution is not
due to the agglomeration of nanoparticles. Under TEM
observation, these initial gold nanoparticles have been con-
verted into o1.5 nm ultrasmall particles, and just few of initial
gold nanoparticles remained as shown in Fig. 1B. The num-
bers of ultrasmall particles in a same limited area is greatly
increased compared to initial stage, which can be attributed to
the fact that one large nanoparticle has successfully been
fragmented into several small ones.
been employed to change the morphology of Ag nanoparti-
cles.3 A similar mechanism is operative in that case because
thermal and photochemical changes continue to propagate.4
In all of these cases, the changes in nanostructures are induced
by thermal fluctuations. Additionally, gold nanorods have
been successfully shortened by mild oxidation with O2.5
Recently, Novo’s group demonstrated that NaBH4 induced
the fragmentation of gold nanorods.6 They stated that they
believe the fragmentation is due to electron injection. But, up
to now, such an effect with biomolecules has not been reported
for gold nanoparticles. The interaction between nanoparticles
and biomolecules is significant in chemistry, biology and
material sciences, because there has been growing interest in
the application of nanotechnology in biomedicine and life
sciences.
In our experiments, we found the biomolecule cysteine is
able to fragment gold nanoparticles. During aging in cysteine
solution, 3–5 nm gold nanoparticles spontaneously fragment
into ultrasmall particles (less than 1.5 nm) without external
energy induction. The fragmentation of gold nanoparticles is
accompanied by cysteine oxidation and electron injection. The
decrease in surface stress is caused by the increased surface
charge density. When the surface charge density is beyond the
Rayleigh limit,7 the gold nanoparticles will undergo size
changes.
The fragmentation process of gold nanoparticles was
further monitored by the UV-Vis absorption method. Fig. 2
shows the time-dependent absorption spectra during the whole
reaction process. The gold colloid suspension exhibits a slight
red shift in the absorption peak and continuous decrease in the
magnitude of absorption with the elapsed time.
a State Key Laboratory of Electroanalytical Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, Jilin 130022, People’s Republic of China.
E-mail: dongsj@ciac.jl.cn; Fax: +86-431-85689711;
Tel: +86-431-85262101
The color of metal nanoparticles originates from an absorp-
tion band of the characteristic surface plasmon.8 When the size
of gold colloids decreases to o1.5 nm, they do not possess a
plasmon absorption band in the visible range.9 The absence of
the plasmon band for ultrasmall metallic particles is attributed
to quantum-size effects which lead to the formation of
b Graduate School of the Chinese Academy of Sciences, Beijing,
100039, People’s Republic of China
w Electronic supplementary information (ESI) available: FT-IR, UV-
Vis and EI data for cysteine, and TEM images of the nanoparticle
fragments. See DOI: 10.1039/b809736d
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Chem. Commun., 2008, 4625–4627 | 4625

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Fig. 2 Photographs of aging of the gold colloid suspension at room
temperature for the indicated times, and time-dependent UV-Vis
spectra of the gold colloid suspension during the aging process.
Scheme 1 (A) The structures of cysteine, cystine and related mole-
cules. (B) Schematic presentation of surface complexation by cysteine
molecules and charge distribution on gold nanoparticles.
quantized energy states.9 The complete loss of color in the
experiments reported rather indicates that the nanoparticles
completely disintegrate to molecular sized species. While con-
ceivably particle fragments observed by TEM could be a result
of Au reduction under the electron beam, taking into account
the fact that the final suspension is colorless and the absor-
bance at the plasmon peak is reduced by 496%, the amount
of initial nanoparticles that do not fragment are a very small
portion of initial gold particles.
colloid suspension, a new band appearing at 3364 cmꢁ1 may
be attributed to B–OH vibrations, which is originated from the
product of decomposed borohydride solution.11 After aging
for 8 h, the S–H vibrational band disappears suggesting the
formation of Au–S or S–S linkages. Moreover, electron impact
(EI, Fig. S2 in ESIw) mass spectra provides strong evidence
that all the cysteine (M = 121, formula: Scheme 1A) has been
oxidized with the aid of gold nanoparticles and the main final
product is cystine (M = 240, formula, Scheme 1A). As is well
known, free mercapto (R–SH) groups are unstable in the oxo-
aqueous environment. They are easily oxidized by reactive
oxygen species such as H2O2 to form disulfide (R–S–S–R0) and
other organic sulfur oxidation products.12 However, in our
experiments, we observed that the fragmentation of gold
nanoparticles and the oxidization of cysteine still occured in
deaerated aqueous solution, oxygen not being necessary for
the oxidization of cysteine. Gold nanoparticles instead of
active oxygen species remove electrons from cysteine, and
result in the oxidization of cysteine. It has been shown
previously that colloidal particles can store a great number
of electrons.13 Pakiari et al. also reported that the gold
nanoparticles tend to oxidize amino acids and excess electrons
exist in the gold nanoparticles.14 Therefore, in the present
system, it is reasonable to conclude that the nanoparticle
fragmentation is a direct result of electron transfer between
gold nanoparticles and cysteine.
The initial gold nanoparticles exhibit a polycrystalline
structure, as determined by the microscopic electron diffrac-
tion (ED) pattern (inset of Fig. 1A). However, for the ultra-
small particles, no ED pattern was observed. The change in
crystal structure was confirmed by powder XRD (Fig. 3). The
initial gold nanoparticles have a typical diffraction peak
corresponding to (111) planes of face-centered cubic (fcc) gold.
However, the (111) diffraction peak of fcc gold in the XRD
pattern of the nanoparticle fragments disappear, which is
similar to that of noble metals from large nanoparticles to
atomic-scale clusters.10
What is the driving force for the fragmentation of the gold
nanoparticles? In FT-IR spectroscopy experiments, we ob-
served that gold nanoparticles catalyze cysteine oxidation
(Fig. S1, ESIw), which is identified by comparable experiments
between gold nanoparticles suspensions and pure cysteine
solutions. As for the cysteine solution, the small peak at
2549 cmꢁ1 corresponding to the S–H stretching vibration
modes is still seen after aging for 8 h. In the case of the gold
The surface charges of gold nanoparticles play a predomi-
nant role in stabilizing the nanoparticles. A droplet becomes
unstable when the charge on the droplet reaches a critical
value.15 This condition is known as ‘‘Rayleigh instability’’,16
of the critical value of charges, QR, that exist on a droplet of
radius, R, when electrostatic repulsion is balanced by surface
tension. This is given by eqn (1), where e0 is the permittivity of
the surroundings and g is the surface tension:
QR = 8p(e0gR3)1/2
(1)
QR
4pR2
1=2
sR
¼
¼ 2ðe0gRꢁ1
Þ
ð2Þ
Here sR is surface charge density corresponding to a particle
with charges QR. During oxidization of cysteine, the gold
Fig. 3 X-Ray diffraction patterns of (A) initial gold nanoparticles
and (B) nanoparticle fragments.
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colloids gradually collected electrons from cysteine. The co-
hesive force provided by the surface tension is offset by the
electrical stress created by charge distribution on the nano-
particles.6 As the charges approach and exceed this critical
value (QR), the disturbances due to shape oscillations grow,
leading to nanoparticle fragmentation. Eqn (2) predicts smal-
ler size nanoparticles can suffer from higher surface charge
density. Higher surface charge density produces larger repul-
sion for electrons and makes electrons injection difficult.
Thereby, the nanoparticle fragments are more stable than
large nanoparticles.
injection into gold nanoparticles from cysteine, as evidenced
by the formation of S–S bonds. The accumulating charges in
the nanoparticles are a prerequisite for the efficient fragmenta-
tion of the gold nanoparticles. Because particles with ultra-
small dimensions show fluorescence,19 this procedure can and
will be used to prepare fluorescent metallic particles with
potential applications in biological labels and light emitting
sources in nanoscale electronics.
This work was supported by the National Natural Science
Foundation of China (Nos. 20675076 and 20427003).
To further verify the above-mentioned mechanism of the
nanoparticle fragmentation, a series of control experiments
were performed. First, the effect of NaBH4 was assessed. The
freshly prepared cysteine-capped gold nanoparticles were puri-
fied by four cycles of centrifuging the suspension and discard-
ing the supernatant. The purified gold nanoparticles were
separately redispersed in NaBH4 and cysteine aqueous solu-
tion. The fading phenomenon only occurred in cysteine rather
than NaBH4 solution. The performance of cysteine was also
found valid for gold nanorods and for gold nanoparticles as
large as B20 nm (Fig. S3, ESIw), suggesting the fragmentation
was independent of the morphology and promising a suitable
range of nanoparticle size. Second, for alanine (formula,
Scheme 1A) aqueous solutions, the nanoparticles remained
intact. That indicates the mercapto (–SH) group is necessary in
the nanoparticle fragmentation. Third, cysteamine (formula,
Scheme 1A), an –SH bearing molecule, was employed to
replace cysteine during the experimental process, and again
no fragmentation of the gold nanoparticles was observed. The
only difference between cysteine and cysteamine is that cys-
teine possesses an additional carboxyl group, which is favored
for the formation of hydrogen bonding. The hydrogen bond-
ing between the cysteine adsorbed on the nanoparticle surfaces
and the free cysteine in suspension can increase the local
concentration around the gold nanoparticles.17 The distance
between the free cysteine and gold nanoparticles is distinctly
shortened, which is beneficial for free cysteine to inject elec-
trons into gold nanoparticles.18 Because cysteamine lacks a
carboxylic group, free cysteamine is far away from gold
nanoparticle surfaces. Thereby, it is difficult to transfer elec-
trons between gold nanoparticles and molecules. Based on the
same reason, the strong reducing agent NaBH4 can not inject
electrons into gold nanoparticles either. Hence, NaBH4 does
not participate in the gold nanoparticle fragmentation. All the
results indicate that not only the mercapto group but also the
carboxyl group in the cysteine molecule are key elements for
the gold nanoparticles fragmentation.
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In conclusion, we present an initial example of gold nano-
particle fragmentation induced by the small cysteine biomole-
cule. An observed increase in the rate of the oxidation of
cysteine in the gold colloid suspension leads to excess electron
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 4625–4627 | 4627