12348
Langmuir 2005, 21, 12348-12356
Investigation of the Core-Shell Interface in Gold@Silica
Nanoparticles: A Silica Imprinting Approach
Saran Poovarodom,† John D. Bass,† Son-Jong Hwang,‡ and Alexander Katz*,†
Department of Chemical Engineering, University of California at Berkeley, Berkeley,
California 94720-1462, and Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
Received July 23, 2005. In Final Form: September 17, 2005
The nature of the self-assembled core-shell interface in gold@silica nanoparticles synthesized via a
3-aminopropyltrimethoxysilane (APTMS) route is investigated using materials synthesis as a sensitive
tool for elucidating interfacial composition and organization. Our approach involves condensation of the
gold@silica nanoparticles within a silica framework for synthesis of a composite gold-silica material
containing ∼30 wt % gold. This material contains one of the highest gold loadings reported, but maintains
gold core isolation as ascertained via a single surface plasmon resonance absorption band frequency
corresponding to that of gold nanoparticles in dilute aqueous solution. The immobilized gold cores are
subsequently etched using cyanide anion for the synthesis of templated porosity, which corresponds to the
space that was occupied by the gold. Characterization of immobilized amines is performed using probe
molecule binding experiments, which demonstrate a lack of accessible amines after gold removal. Solidstate 13C CPMAS NMR spectroscopy on these materials demonstrates that the amount of amine
immobilization must be less than 10% of the expected yield, assuming that all of the APTMS becomes
bound to the gold nanoparticle template. These results require a core-shell interface in the gold@silica
nanoparticles that is predominantly occupied by inorganic silicate species, such as Si-O-Si and Si-OH,
rather than primary amines. Such a result is likely a consequence of the weak interaction between primary
amines and gold in aqueous solution. Our method for investigating the core-shell interface of gold@silica
nanoparticles is generalizable for other interfacial structures and enables the synthesis of bulk imprinted
silica using colloidal templates.
Introduction
Gold@silica nanoparticles are versatile colloidal building
blocks for the synthesis of advanced materials.1,2 The
isolated nature of colloidal gold in these nanoparticles
has been exploited in a variety of technological applications
including nonlinear optical materials,3 optical filters,4,5
and single electron capacitors.6 Gold isolation within these
nanoparticles is directly monitored via the surface plasmon
resonance absorption band4,5 and has permitted the study
of the surface plasmon resonance temperature dependence7 and of the size dependence of colloidal gold melting
temperature.8
Our goal is to investigate organosilane organization at
the core-shell interface of these nanoparticles and, by
doing so, evaluate colloidal gold as a functional template
for the imprinting of bulk silica with primary amines.9-11
†
‡
University of California at Berkeley.
California Institute of Technology.
(1) Mulvaney, P.; Liz-Marzan, L. M. Top. Curr. Chem. 2003, 226,
225-246. Hall, S. R.; Davis, S. A.; Mann, S. Langmuir 2000, 16, 14541456.
(2) Mulvaney, P.; Liz-Marzan, L. M.; Giersig, M.; Ung, T. J. Mater.
Chem. 2000, 10, 1259-1270.
(3) Selvan, S. T.; Hayakawa, T.; Nogami, M.; Kobayashi, Y.; LizMarzan, L. M.; Hamanaka, Y.; Nakamura, A. J. Phys. Chem. B 2002,
106, 10157-10162. Hamanaka, Y.; Fukuta, K.; Nakamura, A.; LizMarzan, L. M.; Mulvaney, P. J. Lumin. 2004, 108, 365-369. Anija, M.;
Thomas, J.; Singh, N.; Nair, A. S.; Tom, R. T.; Pradeep, T.; Philip, R.
Chem. Phys. Lett. 2003, 380, 223-229.
(4) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. J. Phys. Chem. B 2001,
105, 3441-3452.
(5) Salgueirino-Maceira, V.; Caruso, F.; Liz-Marzan, L. M. J. Phys.
Chem. B 2003, 107, 10990-10994.
(6) Yau, S. T.; Mulvaney, P.; Xu, W.; Spinks, G. M. Phys. Rev. B 1998,
57, R15124-R15127.
(7) Liz-Marzan, L. M.; Mulvaney, P. New J. Chem. 1998, 22, 12851288.
The objective of bulk silica imprinting is the specific
organization of chemical functional groups within pores
of a size and shape that are controlled by the template.
These pores have heretofore been microporous (less than
2 nm) and have contained up to three chemical functional
groups per imprinted site.9-11 While colloids have been
used routinely as templates for creating porosity in
materials via lost-wax types of approaches,12 imprinting
aims to selectively organize chemical functional groups
within the templated pore space. An imprinted material
containing mesoporous templated porosity and chemical
functional group organization contained therein represents a new advanced material, which can be used for the
specific nucleation of matter in porous solids, with
applications ranging from photoluminescent materials13
to tailored catalysts14 and media for large-molecule
separations.15
(8) Dick, K.; Dhanasekaran, T.; Zhang, Z. Y.; Meisel, D. J. Am. Chem.
Soc. 2002, 124, 2312-2317.
(9) Bass, J. D.; Anderson, S. L.; Katz, A. Angew. Chem., Int. Ed.
2003, 42, 5219-5222.
(10) Bass, J. D.; Katz, A. Chem. Mater. 2003, 15, 2757-2763.
(11) Katz, A.; Davis, M. E. Nature 2000, 403, 286-289.
(12) Imhof, A.; Pine, D. J. Nature 1997, 389, 948-951. Johnson, S.
A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963-965. Goltner,
C. G. Angew. Chem., Int. Ed. 1999, 38, 3155-3156. Tessier, P. M.;
Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E.
W. J. Am. Chem. Soc. 2000, 122, 9554-9555. Jiang, P.; Bertone, J. F.;
Colvin, V. L. Science 2001, 291, 453-457. Tessier, P.; Velev, O. D.;
Kalambur, A. T.; Lenhoff, A. M.; Rabolt, J. F.; Kaler, E. W. Adv. Mater.
2001, 13, 396-400. Cornelissen, J. J. L. M.; Connor, E. F.; Kim, H. C.;
Lee, V. Y.; Magibitang, T.; Rice, P. M.; Volksen, W.; Sundberg, L. K.;
Miller, R. D. Chem. Commun. 2003, 1010-1011. Li, Z. J.; Jaroniec, M.
J. Am. Chem. Soc. 2001, 123, 9208-9209. Li, Z. J.; Jaroniec, M. Chem.
Mater. 2003, 15, 1327-1333.
(13) Yun, F.; Hinds, B. J.; Hatatani, S.; Oda, S.; Zhao, Q. X.; Willander,
M. Thin Solid Films 2000, 375, 137-141.
10.1021/la052006d CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/24/2005
Core-Shell Interface in Gold@Silica Nanoparticles
We use gold@silica nanoparticles that are synthesized
via a 3-aminopropyltrimethoxysilane (APTMS) route as
the imprint for our approach because this type of coreshell nanoparticle has been widely used as a building block
for advanced materials. The synthesis of these gold@silica
nanoparticles was originally described in 1996 by LizMarzán, Giersig, and Mulvaney and relies on treatment
of aqueous colloidal gold sol with APTMS, which has been
demonstrated to facilitate silica shell nucleation and
growth. Upon addition of silica, usually as sodium silicate,
condensation around the metal core ensues and uniform
core-shell nanoparticles are synthesized. These can be
further grown to larger shell thicknesses under Stöberlike conditions.16
A bulk silica imprinting approach using gold@silica
nanoparticles synthesized via an APTMS route is schematically represented in Figure 1a,b, relying on condensation of the nanoparticles for synthesis of composite goldsilica solid 1. Material 1 consists of a high density (∼30
wt %) of colloidal gold immobilized within a silicate
framework with an average pore size considerably smaller
than the nanoparticle diameter. This framework serves
to keep the colloidal gold isolated by preventing direct
contact between adjacent gold cores. The gold cores in
this material are subsequently etched using cyanide anion
for the synthesis of templated porosity, with the possibility
of organized amino groups therein, which is schematically
represented as 2 in Figure 1b.
The silica imprinting approach depicted in Figure 1a,b
requires the specific organization of primary amines on
the colloidal gold surface in the core-shell interface of
the gold@silica nanoparticle imprint. Aminopropyl functionality has been thought to occupy this interface;16
however, this has remained as a hypothesis. There has
yet to be a rigorous demonstration of primary amines at
the core-shell interface within these nanoparticles. The
first step in the gold@silica nanoparticle synthesis has
been suggested to be adsorption of APTMS on the gold
surface followed by alkoxide hydrolysis and condensation.16 This is similar to the process when thioesters and
thiocarbonates are used for gold@silica nanoparticle
synthesis,17 which were shown to have an association
constant for the colloidal gold surface in excess of 5 × 107
M-1 in aqueous solution.18
However, unlike thiols and protected thiols, the interaction between primary amines and both planar and
colloidal gold surfaces is relatively weak. Xu et al. reported
that C10 alkylamine monolayers on planar gold surfaces
are stable in the gas phase, but decompose in polar
condensed phases such as alcohols and water,19 which are
present for gold@silica synthesis. The latter observation
is supported by a study of a C18 alkylamine adsorption on
a planar gold substrate by Bain et al., who were unable
to synthesize stable amine monolayers on gold using
ethanol as solvent.20 Kurth and Bein investigated 3-aminopropyltriethoxysilane (APTES) adsorption on a planar
gold substrate. They concluded that in the strict absence
of water there was no coordination between the APTES
(14) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc.
2002, 124, 7642-7643.
(15) Nykypanchuk, D.; Strey, H. H.; Hoagland, D. A. Science 2002,
297, 987-990.
(16) Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Chem. Commun.
1996, 731-732. Liz-Marzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir
1996, 12, 4329-4335.
(17) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 8566-8572.
(18) Chen, M. M. Y.; Katz, A. Langmuir 2002, 18, 2413-2420.
(19) Xu, C. J.; Sun, L.; Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal.
Chem. 1993, 65, 2102-2107.
(20) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989,
111, 7155-7164.
Langmuir, Vol. 21, No. 26, 2005 12349
Figure 1. Two opposing scenarios schematically representing
material 1, consisting of isolated gold nanoparticles that are
immobilized in silica, and its use in bulk silica imprinting.
Materials in (a) and (b) contain ∼600 aminopropyl functional
groups organized around the interfacial region occupied by the
colloidal gold, whereas in (c) and (d) there is no aminosilane in
this interface. Etching of the gold cores in 1 synthesizes material
2. Legend: silica (white), colloidal gold (light gray), and interface
under investigation (dark gray). Double arrows represent a
distance of 12.5 nm.
and gold surfaces.21 An adsorbed layer of APTES on gold
could only be formed by first preadsorbing water on the
gold surface, which presumably causes hydrolysis and
condensation of the adsorbing APTES, in a preferred
orientation having the primary amino groups point away
from the gold surface, toward the top surface of the film,
and the hydrolyzed Si-OH and condensed Si-O-Si
moieties interacting with the metal surface.21 The structure of monolayers reported in this study is reminiscent
of C18 alkyl trichlorosilane monolayers on a planar gold
(21) Kurth, D. G.; Bein, T. Langmuir 1995, 11, 3061-3067.
12350
Langmuir, Vol. 21, No. 26, 2005
substrate, as investigated by Finklea et al., who also report
that a thin film of water on gold is necessary for alkylsilane
monolayer formation.22 This monolayer was shown to
consist of a silyl moiety adjacent to the gold surface and
all-trans conformation of the hydrocarbon chain extending
vertically from the surface with a small average tilt. This
is exactly the opposite of the orientation for 3-mercaptopropyltrimethoxysilane adsorption on gold, which forms
monolayers with the sulfur atom adjacent to the gold
surface and methoxy headgroups oriented parallel and
away from the surface.23 More recently, using colloidal
rather than planar gold as a substrate, Thomas et al.
investigated the interaction between primary amines and
colloidal gold in toluene, and calculated an association
constant of ∼5 × 104 M-1 for primary amines to gold at
room temperature.24 The aqueous stability of primary
amine monolayers on colloidal gold has been investigated
by Heath et al., who showed that, although again stable
in toluene, these monolayers were unstable in water and
resulted in an aggregated gold film when in contact with
aqueous solution.25
The weak binding between primary amines and gold in
water suggests that siloxy functionality, which is present
during gold@silica nanoparticle synthesis in large excess,
may competitively adsorb on gold surface sites during the
core-shell nanoparticle synthesis. This could lead to the
type of siloxy interfacial composition depicted as 1 in
Figure 1c for the material after gold@silica nanoparticle
condensation, resulting in the absence of imprinted amines
as in 2 represented in Figure 1d after gold core etching.
The primary amine-gold interface in Figure 1a and
the siloxy-gold interface in Figure 1c are both examples
of extreme possible cases. Despite the weak interaction
between isolated primary amines and gold surfaces in
water, it is possible that a condensed polyamine species
adsorbs to the gold surface during the gold@silica nanoparticle synthesis. There are several examples involving
this type of interaction and resulting in the adsorption of
gold nanoparticles onto aminopropyl-functionalized glass
surfaces from aqueous solution.26 There is also the
possibility for the mixed case, having both some organized
amino groups at the gold surface as in Figure 1a, while
permitting some randomly oriented amino groups in the
bulk silica network away from the gold, as proposed by
Bharathi and Lev.27 Rigorous proof remains unavailable
on which one is most accurate of the three possible
scenarios: either the structure shown in Figure 1a, the
structure shown in Figure 1c, or the mixed case between
these two extremes, as proposed by Bharathi and Lev.
(22) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara,
D.; Bright, T. Langmuir 1986, 2, 239-244.
(23) Thompson, W. R.; Cai, M.; Ho, M. K.; Pemberton, J. E. Langmuir
1997, 13, 2291-2302.
(24) Thomas, K. G.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18,
3722-3727.
(25) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 47234730.
(26) Caruso, F.; Spasova, M.; Saigueirino-Maceira, V.; Liz-Marzan,
L. M. Adv. Mater. 2001, 13, 1090. Fan, H. Y.; Zhou, Y. Q.; Lopez, G. P.
Adv. Mater. 1997, 9, 728-731. Rubin, S.; Bar, G.; Taylor, T. N.; Cutts,
R. W.; Zawodzinski, T. A. J. Vac. Sci. Technol., A 1996, 14, 1870-1877.
Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J.
A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter,
D. G.; Natan, M. J. Science 1995, 267, 1629-1632. Grabar, K. C.; Allison,
K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox,
A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12,
2353-2361. Fleming, M. S.; Walt, D. R. Langmuir 2001, 17, 48364843. Pham, T.; Jackson, J. B.; Halas, N. J.; Lee, T. R. Langmuir 2002,
18, 4915-4920. Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N.
J. Langmuir 1998, 14, 5396-5401. Oldenburg, S. J.; Averitt, R. D.;
Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243-247.
(27) Bharathi, S.; Lev, O. Chem. Commun. 1997, 2303-2304.
Bharathi, S.; Fishelson, N.; Lev, O. Langmuir 1999, 15, 1929-1937.
Poovarodom et al.
Our objective is to differentiate between the types of
functional group organization at the gold-silica interface
in 1 and 2. Our approach is to remove the gold cores from
1 and characterize the number density and organization
of the immobilized primary amines. We use spectroscopic
signatures of covalently bound probe molecules, which
we have previously used for studying amines in imprinted
silica,9,10 as well as solid-state nuclear magnetic resonance
(NMR) spectroscopy for characterizing local environment
and quantifying amine functional group number density
in materials after gold core template removal.10,11
To the best of our knowledge, aside from elucidating
organosilane organization in the gold@silica nanoparticles
under investigation, the experiments described herein are
the first to demonstrate a materials synthesis methodology
that can be used to enable the imprinting of silica using
colloidal templates. This has been challenging to accomplish in part by the difficulty of removing the colloidal
template from bulk silica under conditions that preserve
the integrity and attachment of anchored organic functional groups on silica.28 In addition, the gold-silica
composite materials synthesized here represent one of
the highest loadings of isolated colloidal gold in bulk silica
synthesized to-date.27,29-31 This high loading of gold in
silica is necessary in order to provide enough sensitivity
for characterizing the number density of amine functionality after gold core template removal.
Experimental Section
General. UV-vis spectroscopy was performed on a Varian
Cary 400 Bio UV-vis spectrophotometer equipped with a Harric
Praying Mantis accessory for diffuse-reflectance measurements
on solids at room temperature. Optical microscopy was performed
at the Biological Imaging Facility at University of California at
Berkeley (UCB) using a Zeiss Axiophot optical microscope
equipped with a grayscale Photometrics Quantix camera (12 bit
KAF1401E CCD Kodak chip). Transmission electron microscopy
(TEM) samples were prepared by drying a drop of solution on
carbon-coated copper grids. Solid-state NMR spectroscopy was
performed at the Caltech Solid-State NMR Facility. 13C crosspolarization magic-angle-spinning (CPMAS) NMR spectra were
collected using a Bruker DSX-500 spectrometer operating at 125.4
MHz for the 13C nucleus, and using a Bruker 4 mm CPMAS
probe. A contact time of 1.0 ms was used for all CP experiments
and samples were spun at 6.0 kHz. The chemical shifts were
referenced externally to tetramethylsilane. Gas chromatography
was performed on an Agilent 6890 system equipped with a flame
ionization detector. Thermogravimetric analysis was performed
on a TA Instruments 2950 system. Nitrogen physisorption was
performed on a Quantachrome Autosorb-1 using samples that
had been degassed for at least 24 h at a temperature of 120 °C.
Measurement of gold concentration in materials was performed
using inductively coupled plasma (ICP) by QTI Inc. (Whitehouse,
NJ). All vacuum environments refer to a pressure of 50 mTorr
or less.
(28) Makarova, O. V.; Ostafin, A. E.; Miyoshi, H.; Norris, J. R.; Meisel,
D. J. Phys. Chem. B 1999, 103, 9080-9084.
(29) Zhu, H. G.; Lee, B.; Dai, S.; Overbury, S. H. Langmuir 2003, 19,
3974-3980. Khushalani, D.; Hasenzahl, S.; Mann, S. J. Nanosci.
Nanotechnol. 2001, 1, 129-132.
(30) Madler, L.; Stark, W. J.; Pratsinis, S. E. J. Mater. Res. 2003, 18,
115-120. Shi, H. Z.; Zhang, L. D.; Cai, W. P. Mater. Res. Bull. 2000,
35, 1689-1695. Nooney, R. I.; Dhanasekaran, T.; Chen, Y. M.; Josephs,
R.; Ostafin, A. E. Adv. Mater. 2002, 14, 529. Kozuka, H. In Sol-Gel
Processing of Advanced Materials; Pope, E. J. A., Ed.; American Ceramic
Society: Westerville, OH, 1998; Vol. 81, pp 263-270. Konya, Z.; Puntes,
V. F.; Kiricsi, I.; Zhu, J.; Ager, J. W.; Ko, M. K.; Frei, H.; Alivisatos, P.;
Somorjai, G. A. Chem. Mater. 2003, 15, 1242-1248. Martino, A.;
Yamanaka, S. A.; Kawola, J. S.; Loy, D. A. Chem. Mater. 1997, 9, 423429. Cheng, S.; Wei, Y.; Feng, Q. W.; Qiu, K. Y.; Pang, J. B.; Jansen,
S. A.; Yin, R.; Ong, K. Chem. Mater. 2003, 15, 1560-1566. Nooney, R.
I.; Thirunavukkarasu, D.; Chen, Y. M.; Josephs, R.; Ostafin, A. E.
Langmuir 2003, 19, 7628-7637.
(31) Kobayashi, Y.; Correa-Duarte, M. A.; Liz-Marzan, L. M. Langmuir 2001, 17, 6375-6379.
Core-Shell Interface in Gold@Silica Nanoparticles
Langmuir, Vol. 21, No. 26, 2005 12351
Materials. Tetrachloroauric acid was purchased from Acros.
All other chemicals were purchased at the highest possible level
of purity from Aldrich and were used as received unless stated
otherwise. The purity of 3-aminopropyltrimethoxysilane (APTMS) was verified via 1H NMR and Fourier transform infrared
spectroscopies. Silica used for the synthesis of all control materials
was Selecto silica gel (average pore diameter 60 Å and surface
area 500 m2/g). Water used in these experiments was distilled
at least once and treated with a Barnstead Nanopure Infinity
system to possess at least 18 MΩ purity.
Synthesis of Colloidal Gold. Citrate-stabilized colloidal gold
was synthesized according to previously published procedures.32
The gold sol (concentration of 7.8 × 10-4 M Au; 13 nM gold colloid)
diluted with four parts of water per part of colloid had a measured
λmax of 518 nm (516-520 nm literature specification) and a full
peak width at half-maximum of 84 nm (80-90 nm literature
specification). TEM was performed on a random sampling of
particles and verified the presence of spherical 12.5 nm colloidal
gold particles.
Synthesis of Gold-Silica Core-Shell Nanoparticles.
Synthesis of gold@silica nanoparticles via the APTMS route was
performed according to previously published procedures.16 Two
liters of colloidal gold sol was diluted with an equal volume of
water to synthesize 4 L of stock solution. Twenty milliliters of
a freshly prepared 1 mM aqueous solution of APTMS was added
to the 4 L of stock solution. The mixture was vigorously stirred
for 30 min. A silica solution was separately prepared by diluting
sodium silicate solution (27 wt % SiO2) with water to synthesize
a 0.54 wt % SiO2 solution. The silica solution (160 mL) was added
to the stock solution containing APTMS. Two different pHs (high
and low) of the 0.54 wt % SiO2 solution were used for the silica
coating process: a high pH silica coating solution, which led to
a pH of 9.4 upon addition to the stock solution, and a lower pH
silica coating solution, which led to a pH of 7.2 upon addition to
the stock solution. The higher pH solution was synthesized by
adding 0.54 wt % SiO2 solution at a pH of 11.0-11.2 to the stock
solution. The lower pH solution was synthesized by addition of
Dowex 50WX8 ion-exchange resin (H form) to the silica solution
immediately prior to its addition to stock solution. The resulting
mixture was vigorously stirred for 1 week and then left to stand
at room temperature for at least 4 weeks for silica coating, after
which time the synthesis of gold@silica composite material via
gelation procedure was begun. During silica coating, the pH of
the two stock solutions dropped from 9.4 to 8.5 and from 7.2 to
6.3 after 4 weeks for the high and low pH silica-coated colloids,
respectively. Gold@silica nanoparticles were characterized via
TEM (see Supporting Information).
Synthesis of Gold-Silica Composite Material. Immediately before the gelation procedure below, the high pH silicacoated colloid was adjusted from a pH of 8.5 to a pH of 6.0-6.5
by adding ∼0.15 mL per 500 mL of gold@silica colloid solution
of a 1 M aqueous citric acid solution. The low pH coated colloid
was not pH adjusted prior to gelation. In batches of 500 mL, the
gold@silica colloid was concentrated to between 25 and 50 mL
using a rotary evaporator operating at a temperature between
40 and 50 °C. The concentrated colloid was allowed to stand at
room temperature until gelation occurred during the course of
1 week. After gelation, a deep purple gold-silica composite gel
settled under a head of clear liquid. The resulting solution
containing gel and liquid was centrifuged at 1000-2000 rpm for
15 min. The liquid was decanted and replaced with water.
Centrifugation and water rinsing was repeated at least five times
before the solid gel was collected via filtration. Removal of all
soluble salts (citrate) from the gel was followed via thermogravimetric analysis. The resulting gel was dried in an oven at
105 °C and under vacuum at room temperature for 24 h before
storage in a desiccator. Typical silica yields were approximately
75% of the theoretical amount (∼0.16 g of composite gold-silica
material per 500 mL of coated colloid solution was recovered).
Gold Core Removal via Etching. Gels were crushed into
a powder (40-200 µm particle size as determined via optical
microscopy) using a granite mortar and pestle. The gold-silica
material was etched with freshly made 0.1 M aqueous KCN
One of the key features of an imprinting strategy is
ensuring site isolation.33 For colloidal gold, this can be
probed directly via the surface plasmon resonance absorption band, which is known to red shift as the degree
of gold core aggregation increases.34 We investigated the
degree of imprint isolation in bulk gold-silica composite
material 1 using diffuse-reflectance UV-vis spectroscopy.
Results are shown in Figure 2 for materials before (1) and
after gold etching (2). For 1 derived from both the low pH
(silica coating final pH 6.3) and high pH (silica coating
final pH 8.5) silica coating procedures, a maximum in the
absorbance spectrum at 518 nm is observed that is
coincident with the maximum in the surface plasmon
resonance band of isolated 12.5 nm gold nanoparticles in
aqueous solution. Upon gold core etching with cyanide
from 1 for the synthesis of 2, the surface plasmon band
disappears, and the featureless spectra shown in Figure
2d (material derived from low pH silica coating) and Figure
2e (material derived from high pH silica coating) result.
(32) Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J.
Chem. Educ. 1999, 76, 949-955.
(33) Katz, A.; Davis, M. E. Macromolecules 1999, 32, 4113-4121.
(34) Quinten, M. Appl. Phys. B: Lasers Opt. 2001, 73, 317-326.
solution. The pH was maintained at 9.0 to reduce the solubility
of silica in water, by addition of Dowex 50WX8 ion-exchange
resin (H form) to KCN solution prior to addition of gold-silica
composite material.
CAUTION: This procedure should only be conducted in a wellventilated fume hood because toxic HCN may be produced.
Approximately 50 mL of the etchant was used per gram of the
gold-silica composite material (3-fold excess of cyanide anion
per gold atom). The etching process was conducted for up to 6
days under magnetic stirring. The resulting material was filtered
and washed with excess water for removing residual KCN (wash
with 3 L of water was per gram of material). It was subsequently
dried in an oven at 105 °C and placed under vacuum at room
temperature for 24 h before storage in a desiccator. The same
etching procedure was performed with a positive control material
(synthesized as described below).
Synthesis of Surface-Functionalized Positive Control
Material. A control material containing approximately 0.023
mmol/g of aminopropyl functionality on the surface of silica was
prepared as follows. A 7.0 g sample of Selecto silica gel was
suspended in 300 mL of benzene, and 0.5 mL of 0.35 M
aminopropyltriethoxysilane in benzene was added to this suspension. The mixture was stirred for 24 h. The material was
collected via filtration and Soxhlet extracted in dry acetonitrile
for 24 h. It was subsequently dried under vacuum at room
temperature and stored in a desiccator. A portion of this material
was subjected to the etching procedure detailed above.
Salicylaldehyde Binding. A 1.0 mL volume of a 2.22 mM
solution of salicylaldehyde in acetonitrile was added to 50 mg of
silica. This represents a minimum molar ratio of salicylaldehyde
to anchored amine of 2.0. After an equilibration time of 4 days,
the materials were collected via filtration and washed with a
combination of 100 mL of acetonitrile, 100 mL of chloroform, and
50 mL of pentane. The materials were subsequently Soxhlet
extracted in chloroform for at least 16 h and dried under vacuum
at room temperature. The amount of salicylaldehyde covalently
bound was quantified using gas chromatography on syringefiltered samples and 1,3,5-trimethoxybenzene as an internal
standard.
TNBS Binding. Materials were treated with a 0.04 wt %
solution of 2,4,6-trinitrobenzenesulfonic acid (TNBS) in dimethylformamide at room temperature for 1 week. In a typical
procedure, 40 mg of sample was treated with 1.50 mL of TNBS
solution, representing a minimum of 2 equivalents of TNBS per
anchored amine. The materials were filtered, washed with
dimethylformamide (20 mL) and chloroform (30 mL), and Soxhlet
extracted in chloroform for 24 h prior to drying under vacuum
at room temperature.
Results
12352
Langmuir, Vol. 21, No. 26, 2005
Figure 2. Solid-state UV-vis spectra of (a) 1 derived from low
pH silica coating procedure, (b) 1 derived from high pH silica
coating procedure, (c) isolated gold nanoparticles in aqueous
solution that were used as precursors for 1, (d) 2 derived from
low pH silica coating procedure, (e) 2 derived from high pH
silica coating procedure.
Figure 3. Physical adsorption/desorption isotherms of nitrogen
at 77 K on materials derived from low pH silica coating
procedure (s) and high pH silica coating procedure (‚‚‚) for (a)
1 and (b) 2 (materials in (a) following gold core etching). Inset
shows the corresponding BJH pore-size distribution based on
the desorption branch of the isotherm derived from low pH
silica coating procedure (s) and high pH silica coating procedure
(‚‚‚).
Nitrogen porosimetry was used to investigate the
porosity in 1 and 2, as it is this porosity that ultimately
permits transport that is necessary for etching of gold
and access to imprinted sites in 2. Nitrogen adsorption/
desorption isotherms at 77 K are shown in Figure 3 for
1 and 2 derived from low and high pH silica coatings.
Included within the inset is a pore-size distribution based
on the Barrett-Joyner-Halenda (BJH) model of the
desorption branch of the isotherm,35 which shows a mean
mesoporosity for 1 that is significantly smaller than the
gold core diameter (12.5 nm), along with some microporosity. Material 1 derived from the low pH silica
(35) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc.
1951, 73, 373-380.
Poovarodom et al.
coating procedure has a slightly smaller average mesopore
diameter of 7.4 nm compared with the 8.8 nm diameter
for materials derived from the high pH silica coating
procedure. It is not possible to reliably subtract isotherms
for 1 and 2 for calculating the volume of templated porosity
synthesized upon gold etching, as previously performed
for templated porosity in hydrophobic bulk imprinted
silica,11 due to its small fraction relative to the total
mesopore volume in the material (this fraction is less than
0.025). The relatively small changes to the bulk network
porosity as a result of aqueous conditions employed here
for gold core etching outweigh the expected volume change
due to synthesis of templated porosity. BrunauerEmmett-Teller (BET)37 surface areas for the respective
low and high pH silica coating procedures were measured
to be 330 and 353 m2/g in 1 and 369 and 339 m2/g in 2.
Figure 4 shows snapshots of the mass-transport limited
gold etching process, as viewed on a single gold-silica
composite material particle (∼250 µm in diameter) using
an optical microscope. Figure 4a shows the particle before
etching, which is opaque due to the distributed colloidal
gold within the composite gold-silica material. Figure 4b
shows the same particle approximately 6.5 h later,
consisting of an unetched central region and an optically
clear etched silica shell surrounding it. Finally, Figure 4c
shows the same particle after 22 h of etching without
visible colloid gold. The etching process was also followed
by powder X-ray diffraction, which shows the disappearance of peaks associated with metallic gold upon etching
(Supporting Information).
Au ICP analysis was used to quantify the gold mass
fraction in these materials. The mass fraction for 1 was
measured to be 0.31 and 0.33 for materials derived from
silica coating procedures at low pH and high pH, respectively. The gold mass fraction remaining in 2 after etching
was below the 0.05% detection limit of Au ICP, indicating
a removal of greater than 99.99% of gold in 1 during
synthesis of 2.
The number density of amine functional groups in 2
was investigated both quantitatively and qualitatively
using probe molecule binding experiments. Both positive
and negative control materials were used for comparison
purposes. The negative control material consists of a
mesoporous silica scaffold without immobilized amine
functional groups that had undergone the same washing
and treatments, including etching, as 2. The positive
control material contains the maximum number density
of amine functional groups assuming that all of the amine
added during synthesis of 1 becomes covalently incorporated into the material (∼0.023 mmol/g). A portion of this
control material was subjected to the same etching
treatment as 2 in order to study the effect of etching on
the attachment of anchored aminopropyl functionality on
silica.
The covalent binding of salicylaldehyde as a probe
molecule for primary amines produces a chromophore that
can be characterized using solid-state UV-vis spectroscopy.9,10 The binding of salicylaldehyde is also used to
determine the number density of immobilized amines by
monitoring the change in concentration before and after
binding via gas chromatography.10 These data are provided
for all materials studied in Table 1. Salicylaldehydetreated materials visibly turned yellow upon salicylaldehyde binding to primary amines. The UV-vis bands
(36) Lecloux, A. J.; Bronckart, J.; Noville, F.; Dodet, C.; Marchot, P.;
Pirard, J. P. Colloids Surf. 1986, 19, 359-374.
(37) Brunauer, S. E.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc.
1938, 60, 309-319.
Core-Shell Interface in Gold@Silica Nanoparticles
Langmuir, Vol. 21, No. 26, 2005 12353
Figure 5. Solid-state UV-vis spectra of (a) positive control
silica after etching procedure and treatment with salicylaldehyde, (b) 2 derived from low pH silica coating procedure after
treatment with salicylaldehyde, (c) 2 derived from high pH
silica coating procedure after treatment with salicylaldehyde,
(d) negative control silica after treatment with salicylaldehyde,
(e) positive control silica after etching procedure and treatment
with TNBS, (f) 2 derived from low pH silica coating procedure
after treatment with TNBS, (g) 2 derived from high pH silica
coating procedure after treatment with TNBS, and (h) negative
control silica after treatment with TNBS.
Figure 4. Optical micrographs of gold etching dynamics as
viewed on a single macroscopic gold-silica composite material
particle. The scale bar represents a distance of 50 µm. The
particle before etching (a) consists of a single opaque region
representative of the colloidal gold. The concentration profile
of colloidal gold during etching follows a classical shrinking
core model, as shown in (b) after 6.5 h of etching, even though
the gold cores remaining in (b) consist of a continuum of isolated
colloidal gold particles that are immobilized within a porous
silica matrix. The particle is completely etched as shown in (c)
after a period of 22 h.
did not diminish in intensity following Soxhlet extraction
in chloroform, confirming the covalent nature of salicylaldehyde attachment.9,10 Corresponding solid-state UVvis spectra of salicylaldehyde-treated materials after
etching are shown in Figure 5a-d.
To further corroborate the salicylaldehyde binding
results, another probe molecule that is known to specifically bind to amines, TNBS, was also used. This compound
arylates amines in proteins (lysine residues) and imprinted
silica containing a silanol-rich silica framework environment forming a chromophore that has an absorbance
between 340 and 410 nm.10 Figure 5e-h shows the solidstate UV-vis spectra of materials treated with TNBS.
Whereas the probe molecule experiments described
above are sensitive to the presence of accessible amino
functional groups immobilized on silica, they do not provide
information on the absolute presence or absence of
immobilized aminopropyl functionality, because some
amines may be inaccessible. To address the question of
the absolute presence or absence of immobilized aminopropyl functionality in 2, we performed 13C CPMAS solidstate NMR spectroscopy on a positive control material
containing the theoretical number of primary amines after
the etching procedure as well as material 2 derived from
low and high pH silica coating procedures. These spectra
are represented in Figure 6.
Table 1. Salicyaldehyde Binding Results on Several Materials as Measured via Gas Chromatography
material
2 derived from low pH silica coating
2 derived from high pH silica coating
control materials
negative control silica
positive control silica before etching procedure
positive control silica after etching procedure
expected amine
density (mmol/g)
salicyaldehyde
bound (mmol/g)
0.023
0.023
0.000 ( 0.002
0.000 ( 0.002
0.000
0.025
0.025
0.000 ( 0.002
0.027 ( 0.002
0.026 ( 0.002
12354
Langmuir, Vol. 21, No. 26, 2005
Figure 6. Solid-state 13C CPMAS NMR spectra of (a) positive
control silica after etching procedure, (b) 2 derived from low pH
silica coating procedure, and (c) 2 derived from high pH silica
coating procedure. Data were recorded over 100 000 transients
with a 2 s recycle delay for each sample and normalized to the
sample weights. The contact time was 1.0 ms and the MAS
spinning speed was 6 kHz.
Discussion
Achieving colloidal gold isolation during synthesis of
gold-silica composite materials has proven to be difficult
to accomplish even at smaller gold mass fractions.29 Our
procedure for the synthesis of 1 employs gold@silica
nanoparticles as building blocks for maintaining isolation
of colloidal gold. Similar methods have been previously
used for the synthesis of bulk gold-silica composite
materialssalbeit at smaller gold mass fractions of up to
∼0.006.31 For these materials,synthesis conditions were
chosen in order to optimize the formation of isolated gold
colloid while avoiding conditions in which multiple metal
cores can agglomerate. Previous detailed studies of the
effect of pH on the adsorption of SiO3- in metal@silica
nanoparticles show that a pH range of 5-10 during silica
shell growth is desirable for maintaining isolated metal
cores during the silica coating process.38 In synthesizing
1, we chose a high (8.5) and a low (6.3) pH silica coating
procedure during silica shell formation, that were within
the narrow window above.
APTMS is known to be important as a primer during
silica coating of colloidal gold to maintain sol stability.16,31
When synthesis of 1 was conducted in the absence of
APTMS, we observed the onset of colloidal instability
marked by visible gold nanoparticle aggregation during
the concentrating step using a rotary evaporator. This
suggests that APTMS facilitates the growth of a silica
shell thicker than 2-4 nm, which is known to increase
the robustness and ability of gold@silica nanoparticles to
withstand the high ionic strengths present in solution
during silica gelation.16
The single surface plasmon resonance band frequency
observed for 1 (Figure 2) corresponds to that of isolated
gold nanoparticles in solution. Aggregation of gold cores
would result in a red shift due to different plasmon
resonances in the longitudinal and transverse directions.39
Both low and high pH variants of 1 contain a slight
broadening in their corresponding spectrum in Figure 2
relative to isolated gold nanoparticles in solution. It is
highly unlikely that this broadening is due to gold
nanoparticle aggregation, because TEM micrographs
(Supporting Information) fail to show evidence for this
during synthesis of 1. Moreover, the observed broadening
in the spectrum for 1 in Figure 2 is very similar to that
(38) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14,
3740-3748.
(39) Norman, T. J.; Grant, C. D.; Magana, D.; Zhang, J. Z.; Liu, J.;
Cao, D. L.; Bridges, F.; Van Buuren, A. J. Phys. Chem. B 2002, 106,
7005-7012.
Poovarodom et al.
observed in the solid-state UV-visible spectra of other
gold-silica composite materials that have been synthesized at much lower gold mass fractions, for which gold
nanoparticle aggregation should be even less likely than
in 1.1,30,31 Ung et al. demonstrate that the gold volume
fraction in materials synthesized from gold@silica building
blocks made from 13.2 nm diameter colloidal gold must
be below about 0.15 in order to prevent interactions
between gold cores.4 For 1, the gold volume fraction is less
than 0.025, well below the threshold predicted by Ung et
al. for shifts in the surface plasmon resonance band
frequency due to interactions between gold cores. This
suggests that the gold mass fraction in 1 can still be
increased significantly before this threshold is met.
The silica shell porosity shown in the nitrogen physisorption data in Figure 3 is similar to the expected porosity
of gold@silica nanoparticles, which has been analyzed
previously via TEM and has been suggested to be bimodal,
consisting mostly of mesopores along with some microporosity.2,16,40 The isotherms in Figure 3 exhibit a sharp
uptake of nitrogen at relative pressures of less than 0.01,
which is indicative of microporosity, and hysteresis
between adsorption and desorption branches that is
indicative of cylinder-shaped mesoporosity.36 Most
mesoporosity in 1 likely arises as the result of voids
between silica shells on adjacent gold@silica nanoparticles
comprising the bulk material network, in much the
same manner that voids between nanometer-sized colloidal silica particles are responsible for mesoporosity in
base-catalyzed silicates.36 The insets of Figure 3 clearly
represent the bimodal pore size distributions in 1
and 2.
Etching of gold cores in 1 for the synthesis of 2 was
performed using cyanide anion treatment at room temperature. We have taken care to avoid a high pH during
cyanide etching in order to minimize silica dissolution, by
balancing the pH of the etching solution with acid ionexchange resin at values of about 9.0-9.5, which corresponds to significantly reduced silica solubility relative to
pH values above 10.0.41 Cyanide anion treatment has been
previously used for etching ∼5 nm diameter surfacesupported gold nanoparticles on ceria,42 and for the
preparation of hollow shells from individual core-shell
nanoparticles,2,28,40,43,44 but it has not been used previously
for removing gold cores from gold@silica nanoparticles
immobilized in a bulk material. The dynamic concentration
profile of colloidal gold within the macroscopic gold-silica
particle of Figure 4 follows a classical shrinking core type
of profile. This is consistent with a mass-transfer-limited
process of cyanide and oxygen delivery to the gold cores
in 1 followed by a fast gold etching reaction. The diffusion
time scale of the etching process can be estimated from
Figure 4b by the ∼40 µm penetration depth that has been
achieved via etching during the course of the first 6.5 h
of etching. This corresponds to a diffusivity during etching
of ∼10-10 cm2/s, which is approximately 5 orders of
(40) Giersig, M.; Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Adv. Mater.
1997, 9, 570-&.
(41) Brinker, C. J. S.; G. W. Sol-Gel Science: The Physics and
Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA,
1990.
(42) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003,
301, 935-938.
(43) Ostafin, A. E.; Siegel, M.; Wang, Q.; Mizukami, H. Microporous
Mesoporous Mater. 2003, 57, 47-55. Rosemary, M. J.; Suryanarayanan,
V.; Reddy, P. G.; Maclaren, I.; Baskaran, S.; Pradeep, T. Proc. Indian
Acad. Sci., Chem. Sci. 2003, 115, 703-709. Gittins, D. I.; Caruso, F. J.
Phys. Chem. B 2001, 105, 6846-6852.
(44) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.;
Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999,
121, 8518-8522.
Core-Shell Interface in Gold@Silica Nanoparticles
magnitude smaller than the expected liquid-phase diffusivity for cyanide and oxygen in bulk aqueous solution
of ∼10-5 cm2/s.45 This significantly depressed value of the
diffusion coefficient during etching of 1 is consistent with
previous studies that are based on TEM observations on
single gold@silica nanoparticles and UV-vis studies of
etching of gold@poly(pyrrole) nanoparticles.2,44 We suggest
that the slow rate of gold etching in 1, as well as the
observed shrinking core behavior in Figure 4, is governed
by the slow diffusion of gold-cyanide complex out of the
material and into bulk solution.
Turning attention now to the characterization of amines
after etching in materials, Figure 5 shows solid-state UVvis spectra of materials that have been treated with
salicylaldehyde. The 392 nm band of the positive control
material is consistent with amines surrounded by a silica
framework that contains free acidic silanols, which shifts
the equilibrium of the bound salicylaldehyde to the
zwitterionic tautomer, as observed previously on studies
of imprinted amines on silica.9,10 In contrast, 2 that was
treated with salicylaldehyde did not show bands corresponding to covalently bound probe, exhibiting neither
the neutral phenolic form (320 nm band expected)10 nor
the zwitterionic form of the bound salen. The quantitative
salicylaldehyde binding data in Table 1 correspond to the
expected number of amines in the control materials and
the absence of salicylaldehyde accessible amines in 2. The
etching treatment changed the amine number density
negligibly in the positive control (less than 10% difference
between before and after etching and within error of
measurement), and this result is consistent with the
general stability of covalently immobilized amines on silica
surfaces in aqueous solution within the pH range used
here for postsynthesis materials processing.46
The spectrum of the TNBS-treated positive control after
etching procedure in Figure 5e exhibits the expected bands
in the range of 340 and 410 nm for amines surrounded by
a hydrophilic silica framework.10 However, the data in
Figure 5e-h do not show evidence of covalent binding of
TNBS to amines in 2, pointing to the same qualitative
conclusions as in the salicylaldehyde study.
There are multiple potential reasons for a lack of probe
molecule binding in 2. Imprinted primary amine inaccessibility is one possibility, despite our careful pH control
during etching and lack of evidence for significant pore
volume collapse40 in materials after etching as judged by
the surface area and nitrogen physisorption isotherm of
2 in Figure 3b. There could also be unfavorable probe
molecule partitioning in the amine-containing active-site
regions of 2, as observed previously with TNBS.10 Both of
these possibilities do not rule out the presence of imprinted
amines as schematically represented by 2 in Figure 1b.
An alternative possibility, however, is that there is an
entire lack of amines, with 2 having an interfacial
structure as schematically represented in Figure 1d. Such
an outcome would preclude the possibility of organized
aminosilane at the colloidal gold surface during gold@silica
nanoparticle synthesis.
To further discriminate between the two opposing
outcomes represented by Figure 1b,d, we acquired the 13C
CPMAS NMR spectra of 2 for assessing an absolute
measure of primary amine density. The cross-polarization
technique allows us to amplify resonances of atoms with
attached protons, thus allowing imprinted aminopropyl
(45) Sun, X. W.; Guan, Y. C.; Han, K. N. Metall. Mater. Trans. B
1996, 27, 355-361.
(46) Strother, T.; Hamers, R. J.; Smith, L. M. Nucleic Acids Res.
2000, 28, 3535-3541.
Langmuir, Vol. 21, No. 26, 2005 12355
functional groups to be observed with greater sensitivity
than what would be otherwise possible. The spectra in
Figure 6a clearly show the three expected resonances of
the propyl tether in the positive control material after
etching procedure. However, the spectra in Figure 6b,c,
which were taken under conditions identical to those in
Figure 6a and correspond to 2 show very weak signal, if
any at all, from aminopropyl resonances. If aminopropyl
functionality is present in 2, it must be present in amounts
less than 10% of the maximum potential yield and within
the noise of spectra in Figure 6b,c in order to be consistent
with the NMR results.
Our results suggest that there is a preponderance of
silanol functionality at the gold-silica interface in 1, and
are much more consistent with the scenario schematically
represented in Figure 1c,d rather than Figure 1a,b, with
the latter scenario accounting for at most 10% of aminopropyl functionality. This result is supported by more
recent syntheses of gold@silica nanoparticles, which have
successfully replaced APTMS with ammonia, thus demonstrating that an amine polysiloxane layer is not required
at the core-shell interface for nanoparticle synthesis.47
The results above do not provide an answer for the fate
of APTMS in gold@silica nanoparticle synthesis, which
expressed another way requires closing of the amine
material balance. To obtain further insight into this
question, we have used silica gel as a silica source instead
of the sodium silicate used for the synthesis of 1, at the
same sodium citrate ion concentration and pH as used for
silica coating the gold nanoparticles during the synthesis
of 1. The results of this experiment demonstrate that,
within experimental error, all of the APTMS covalently
attaches to the silica gel surface, as ascertained via
salicylaldehyde binding experiments on the silica gel after
treatment. This result requires any remaining aminosilane in solution after silica gelation to be attached to the
composite gold-silica gel material.
Since most (>90%) of the aminosilane was not found in
this material, it requires that at some point before silica
gelation, presumably during gold@silica nanoparticle
synthesis, APTMS must covalently bind to colloidal
particles of silica in solution. Because the silica yield for
the synthesis of 1 is 75%, it is possible that these colloidal
APTMS-functionalized silica particles remain dispersed
in water and function as a catalyst for silica condensation
during gold@silica nanoparticle synthesis, since polyamine
cations are known to serve in this role.48 This would explain
the experimentally observed function of APTMS as serving
to facilitate the formation of a silica shell around the gold
nanoparticle. These APTMS-functionalized silica particles
may have been separated from the silica gel 1 after
filtration and washing.
An additional question that needs to be considered is
the following: if the APTMS forms a colloidal polyamine
species on a silica particle as suggested above, then why
does this polyamine not interact and cause adsorption of
the colloidal gold to the polyamine surface? Polysiloxane
layers derived from APTMS, as well as a variety of other
organosilanes, adsorb gold nanoparticles from aqueous
solution.26 While not eliminating this possibility entirely,
we suggest that this inevitably reduces to a problem of
(47) Mine, E.; Yamada, A.; Kobayashi, Y.; Konno, M.; Liz-Marzan,
L. M. J. Colloid Interface Sci. 2003, 264, 385-390. Lu, Y.; Yin, Y. D.;
Li, Z. Y.; Xia, Y. A. Nano Lett. 2002, 2, 785-788.
(48) Delak, K. M.; Sahai, N. Chem. Mater. 2005, 17, 3221-3227.
Menzel, H.; Horstmann, S.; Behrens, P.; Barnreuther, B.; Krueger, I.;
Jahns, M. Chem. Commun. 2003, 2994-2995. Rhodes, K. H.; Davis, S.
A.; Caruso, F.; Zhang, B. J.; Mann, S. Chem. Mater. 2000, 12, 2832.
Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111-1114.
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Langmuir, Vol. 21, No. 26, 2005
competitive kinetics and adsorption. The colloidal silicate
species present in solution during the silica coating step,
by virtue of their larger number density and stronger
interaction with the polyamine layer (strong electrostatic
bond) relative to colloidal gold, may act to competitively
screen the polyamine layer away from the gold nanoparticle surface. The silica thus nucleated would subsequently
bind to the gold and produce the silica shell of the
gold@silica nanoparticle for synthesis of 1 as per Figure
1c.
Poovarodom et al.
gold@silica nanoparticles synthesized via an APTMS
route. We propose that functional groups having stronger
interactions with gold, such as thiols49 and protected
thiols,18 which have been previously used for synthesizing
gold@silica nanoparticles with controlled interfacial composition and structure,17 be implemented for imprinting
silica with colloidal gold. The materials synthesis methods
described here can be used to investigate organic functional
group organization in other metal@silica nanoparticles,
and enable the synthesis of a diverse new class of imprinted
materials based on colloidal templates.
Conclusion
We have investigated the organization of aminosilane
in gold@silica nanoparticles synthesized via an APTMS
route, while evaluating these nanoparticles as functional
templates for the synthesis of bulk imprinted silica. Goldsilica composite material 1 has been synthesized with
upwards of 30 wt % gold in silica using gold@silica
nanoparticles that have been synthesized via an APTMS
route as building blocks. Material 1, while having some
of the highest gold loadings reported in the literature to
date, still maintains gold core isolation. The gold cores
have been successfully removed from 1 via cyanide etching
at a pH that significantly reduces silica solubility during
gold removal. The resulting materials have been characterized using physisorption and a variety of spectroscopic
methods. Probe molecule binding experiments, coupled
with results from 13C CPMAS NMR spectroscopy, suggest
that covalent amine incorporation into the composite goldsilica materials synthesized has a relatively low yield of
less than 10%. This result requires that siloxy (Si-O-Si
and Si-OH) functionality and not organized aminosilane
functionality is present at the core-shell interface in
Acknowledgment. The authors are grateful to Ms.
Rina Zalpuri at the EML facility at UCB for providing
helpful assistance with TEM experiments, and to Mr.
Steven Ruzin at the Light Microscopy Facility at UCB for
technical assistance with optical microscopy. The authors
acknowledge the National Science Foundation (DMR
0444761) and a 3M Untenured Faculty Award for funding.
Supporting Information Available: Additional photographs of materials synthesis process (demonstrating necessity
of APTMS for maintaining colloid stability), X-ray diffraction
powder patterns for materials 1 and 2, nitrogen physisorption
isotherms for material 2 (derived from low and high pH silica
coating) at 77 K, TEM micrographs of gold@silica nanoparticle
building blocks, and miscellaneous experimental conditions and
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
LA052006D
(49) Hasan, M.; Bethell, D.; Brust, M. J. Am. Chem. Soc. 2002, 124,
1132-1133. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman,
R. J. Chem. Soc., Chem. Commun. 1994, 801-802.