Microporous and Mesoporous Materials 122 (2009) 283–287
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Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso
Preparation of coral-like porous gold for metal ion detection
Hero Kim a, Younghun Kim a,*, Ji Bong Joo b, Jae Wook Ko a, Jongheop Yi b
a
b
Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea
School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 8 February 2009
Received in revised form 9 March 2009
Accepted 10 March 2009
Available online 16 March 2009
Keywords:
Porous gold
Template
Mercury ion
Sensor
Metal ion detection
a b s t r a c t
Coral-like porous gold (PAu) as a sensing substrate for metal ion was prepared using a templating method
with an aluminum precursor and stearic acid salts. After selective etching of the nanoporous alumina in
bicontinuous phase, free-standing pure gold on the ITO substrate was obtained. The resulting materials
showed submicron-sized pores in a pure gold network and high electro-conductivity as bulk gold. In a
proof-of-concept test, we found that SH-modifed PAu/ITO electrode was good applicable sensing materials in low concentration of Hg(II).
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
With interest increasing in nano/bio-sensors, metallic porous
materials are increasingly being looked at for use in electro(chemical) applications due to their biocompatibility, conductivity, stability, and high surface-to-volume ratio [1]. Porous gold (PAu) in
particular is believed to be a good candidate as a substrate for batteries, sensors, and catalysts [2–7], while gold-conjugated protein
has been proposed for use in sensing electrode systems [8]. As
compared with gold nanoparticles (AuNPs)-based electrodes, porous gold electrodes possess a much higher surface area and better
electron transport, which leads to better performance in electronic
tests. In addition, the porous structure of gold electrodes offers lots
of active sites for biomaterials, which would make porous gold
attractive for use in nano/biosensors [7]. The polished gold electrodes show poor catalytic activity due to their smooth surface,
whereas porous gold overcomes this disadvantage with characteristics such as low resource-consuming materials and enhancement
of mass transport through a pore network.
To prepare porous gold, two strategies are proposed for the control of porosity and the adjustment of morphology. The former
method uses sacrificial inorganic or organic materials as templates
to generate pore sizes ranging on a scale from nm to lm. This
method has some advantages of easy removal of template and
adjustment of pore structure, but it generally is a time-consuming
process [9]. For example, macroporous gold of above 50 nm can be
* Corresponding author. Tel.: +82 2 940 5768; fax: +82 2 941 5769.
E-mail address: korea1@kw.ac.kr (Y. Kim).
1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2009.03.012
prepared by colloidal crystal templating [10]. In the latter method,
porous gold materials are prepared by a dealloying process, which
is a kind of corrosion process to selectively remove the least noble
element within an alloy, resulting in a porous material of the more
noble element [2–7]. For example, immersion of a commercially
available white-gold alloy (Ag/Au) in nitric acid selectively removes the silver atoms leaving behind the free-standing porous
gold network. This method has the merit of easy fabrication of
an ultrathin, porous gold membrane made by hammering, but
the dealloying step generally is a resource-consuming process. Recently, the combination of templating and dealloying techniques
has been proposed in the preparation of hierarchical porous gold
[6]. Namely, a core-shell approach has been used to obtain hollow
Ag/Au shells with a polystyrene (PS) core. In this process, a Au/Ag–
PS bead is casted to form thin film, followed by removal of PS bead
and the successful dealloying of Ag. Finally, the resultant materials
have a bimodal porous gold structure, which is of great interest for
electronics and sensor applications because larger pores in the
structure facilitate mass transport while smaller pores increase
the surface area [11].
In this study, to obtain submicron-sized porous material, porous
gold was prepared by the templating method using both stearic
acid (or its salt) and aluminum alkoxide for the template. While
stearic acid is usually used as porogen in the preparation of nanoporous alumina [12], herein it acted as a reinforcement agent for
the reverse network (i.e., alumina structure) of porous gold. For
electrochemical sensing application, PAu/ITO substrate was modified with functional materials (1,6-hexanedithiol, HDT) and then
evaluated by measuring current at fixed potential after successive
addition of target metal ion.
284
H. Kim et al. / Microporous and Mesoporous Materials 122 (2009) 283–287
2. Experimental
2.3. Characterization
2.1. Preparation of porous gold and HDT/PAu/ITO electrode
Scanning electron microscopy (SEM) and energy dispersive
spectroscopy (EDS) were performed using a JSM-6700F (Jeol). A
N2 adsorption/desorption test was carried out using an ASAP
2010 (Micromeritics), and pore size distributions were calculated
using the Barret–Joyner–Halenda (BJH) model on the desorption
branch. The porosity of etched material was characterized by mercury intrusion porosimeter (Autopore IV 9500, Micromeritics). To
analyze the crystallinity of as-made materials, powder X-ray diffraction (XRD, M18XHF-SRA, MAC/Science) patterns were recorded
using Cu Ka radiation at 50 kV and 100 mA.
Scheme 1 represents a simplified preparation method for porous gold. AuNPs easily formed the aggregated AuNPs that were
used as primary materials for the porous gold material. Through
a 24 h aging process, phase-separated domains, namely, a bicontinuous structure, were obtained by inducing phase separation in parallel to the gelation of aluminum hydroxide and aggregated gold
particles. A similar phase separation was found in the preparation
of the monolithic TiO2 via a template-free sol–gel process [13]. The
surfactant was easily removed in the calcinations step, and the
resulting material had a nanoporous alumina structure with a sintered gold network. The alumina network with nanopores was
etched selectively, then we obtained porous gold, namely, the reverse phase of the alumina network.
The aluminum precursor (alumium sec-butoxide) and surfactants (stearic acid and magnesium stearate) were separately dissolved in sec-butyl alcohol. A gold precursor (HAuCl4) was added
to a solution of dissolving surfactant. The two solutions were
mixed, followed by slow addition of water at the rate of 1 ml/
min. NaBH4 was used as reducing agent. The resulting mixture
showed a dark brown color. Stirring was continued for 24 h. The
material was dried at 80 °C and calcined at 550 °C in air, followed
by etching with acid etchants (mixture of 11.8 M H3PO4 and 0.6 M
HNO3). Finally, pure porous gold with a brown color was obtained.
The molar ratio of this reaction mixture was 1 Al(sec-BuO)3: 0.09
HAuCl4: 0.2 surfactant: 10 sec-BuOH:7 H2O. To obtain HDT/PAu/
ITO electrode, ITO glass was immersed in the bottom of beaker
with calcined materials during etching process. After etching, thin
film of porous gold was deposited on the ITO glass, and then additional heating process was conducted at 150 °C for 20 min to remove the defect of PAu thin film. PAu/ITO electrode was dipping
into 1 mM HDT for 24 h, and finally we obtained HDT/PAu/ITO
electrode for Hg ion sensing.
2.2. Electrochemical sensing application
Current–time response were recorded with potentiostate
(WPG-100, WonA Tech) at 50 mV after successive addition of
0.1 mM mercury ion with 20 ll with continuous stirring of 0.5 M
H2SO4 solution. Pt substrate and Ag/AgCl are used as counter and
reference electrodes, respectively.
3. Results and discussion
3.1. Preparation of porous gold and HDT/PAu/ITO electrode
As shown in Fig. 1, an SEM image of porous gold prepared using
magnesium stearate as a surfactant was similar to coral structure.
In the magnification of Fig. 1a, the primary branch with a ca. 2–
3 lm size has the shape of a bumpy branch that resembles a
one-directional link of spherical particles (ca. 200–400 nm), which
are made up of aggregated AuNPs (ca. 10–30 nm). In the calcination process, primary AuNPs sintered with others, followed by
the production of spherical particles 200–400 nm in size. Therefore, the secondary spherical gold particles sintered sequentially
and formed a branched gold network, which resembled a kidney
bean in its pod, as shown in inset of Fig. 1a. The sintering temperature of AuNPs was decreased with decreasing particle size [14],
thus the calcination temperature (550 °C) in Scheme 1 was sufficient to sinter the AuNPs. When direct etching without calcination
was performed in as-made material, the resulting materials had
particulates of gold and revealed an unstable network (Fig. 1b). It
is noted that the calcination process is helpful for the enhancement
of structural stability via sintering of gold particles as well as for
the removal of surfactant.
To confirm effect of surfactant, three different materials were
prepared, as shown in Fig. 2. One sample (Fig. 2a) was prepared
without surfactant, and two samples were prepared using stearic
acid (Fig. 2b) and its salts (Fig. 2c). The first sample, prepared without surfactant, showed a coarse morphology and only a textural
porosity, which was formed by the aggregation of primary particles. By comparison, the second sample, prepared using stearic
acid, had a dense and smooth surface, and thus had a well-devel-
Scheme 1. Schematic diagram of porous gold prepared using the templating method. Both stearic acid and aluminum sec-butoxide were used as templates for the porous
gold. In the calcinations step, the surfactant was easily removed, and the aggregated gold particles were sintered to form the gold network. The alumina in the resulting
material was etched selectively using an acid etchant (11.8 M H3PO4 and 0.6 M HNO3), and finally the porous gold material was obtained.
ID
75960
Title
Preparationofcoral-likeporousgoldformetaliondetection
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