Preparation of carboxyl group-modified palladium nanoparticles in an
aqueous solution and their conjugation with DNA
Zhifei Wang,
*
a
Hongying Li,
a
Shuang Zhen
a
and Nongyue He
b
Received 18th March 2012, Accepted 5th April 2012
DOI: 10.1039/c2nr30649b
The use of nanomaterials in biomolecular labeling and their corresponding detection has been
attracting much attention, recently. There are currently very few studies on palladium nanoparticles
(Pd NPs) due to their lack of appropriate surface functionalities for conjugation with DNA. In this
paper, we thus firstly present an approach to prepare carboxyl group-modified Pd NPs (with an average
size of 6 nm) by the use of 11-mercaptoundecanoic acid (MUDA) as a stabilizer in the aqueous solution.
The effect of the various reducing reaction conditions on the morphology of the Pd NPs was
investigated. The particles were further characterized by TEM, UV–vis, FT-IR and XPS techniques.
DNA was finally covalently conjugated to the surface of the Pd NPs through the activation of the
carboxyl group, which was confirmed by agarose gel electrophoresis and fluorescence analysis. The
resulting Pd NPs–DNA conjugates show high single base pair mismatch discrimination capabilities.
This work therefore sets a good foundation for further applications of Pd NPs in bio-analytical
research.
1. Introduction
The development of a variety of metal nanoparticle-based tech-
niques for DNA detection is an interesting alternative to the
standard fluorescence technique due to their unique physico-
chemical properties, such as their large extinction and scattering
coefficients, catalytic activity, surface electronics and efficient
Brownian motion in solution.
1–5
For example, gold nanoparticles
(Au NPs) have been used extensively as biomolecular labels in
the past decades due to the ease of their synthesis and func-
tionalization, their limited toxicity and the ease of detection.
Consequently, no special requirements of the detection equip-
ment are required and the particles exhibit higher stability and
lower costs during bio-analysis applications.
6–10
However, in
contrast to the big advancement in Au NPs research, up to now,
there have been few reports on the applications of other noble
nanoparticles, such as Pd, in the biomedical field. As a new kind
of functional material, Pd NPs have been developed in recent
years and have shown excellent performance in catalysis appli-
cations.
11–14
In DNA sensor applications, such properties can
also be used to substantially amplify the read-out signal through
the use of suitable reaction systems, such as electroless plating,
which present Pd NPs as an ideal potential label with a similar
behavior to the horseradish peroxidase (HRP) enzyme used in
enzyme-linked immunosorbent assays (ELISA).
15,16
One of the
reasons why Pd NPs have been poorly studied in biomolecular
detection is that Pd NPs with appropriate surface properties for
the conjugation of DNA are not currently available. Conse-
quently, the widely adopted Au NPs–oligonucleotide conjugate
chemistry,
1
such as ligand exchange reactions between thiol-
modified oligonucleotides and other pre-existing ligands on the
Au NPs’ surface, such as sodium citrate, can not be applied
for Pd NPs.
Up to now, many methods have been exploited for the
synthesis of Pd NPs with various shapes, sizes and surface
properties. These methods include wet and dry approaches, such
as gas-evaporation, the sol–gel method, hydrothermal
approaches and water-in-oil (w/o) microemulsions.
10–14,17–21
However, because the motivation for these syntheses have mainly
been driven by the catalytic applications of Pd NPs in organic
reactions, for example, to prevent the aggregation of Pd NPs in
organic solutions and/or to facilitate recycling of the catalyst,
such Pd NPs have been generally functionalized with a variety of
organic stabilizers, such as tetraoctylammonium bromide,
22
polyvinyl pyrrolidone (PVP),
23
dendrimers,
21
alkanethiols,
20
and
so on. Therefore, it has been difficult to attach DNA to the
surfaces of PD NPs through simple ligand exchange reactions or
other strategies employed in biomolecular conjugation. Very
recently, Cargnello et al.
20
presented a versatile approach to
prepare functionalized thiol-protected Pd NPs. In their work,
they claimed that the obtained MUDA–Pd NPs had potential
applications in biology due to their high water solubility, which
was achieved through the conversion of surface carboxylic
groups to their carboxylate forms. However, this methodology
still remains complicated with regards to the post-synthesis
a
School of Chemistry and Chemical Engineering, Southeast University,
Nanjing 211189, China. E-mail: zfwang@seu.edu.cn; Fax: +86 (25)
83790885; Tel: +86 (25) 83790885
b
State Key Laboratory of Bioelectronics, Southeast University, Nanjing
210096, China
3536 | Nanoscale, 2012, 4, 3536–3542 This journal is ª The Royal Society of Chemistry 2012
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