Enhanced electrochemiluminescence efficiency of Ru(ii) derivative covalently linked carbon nanotubes hybrid

Article abstract of DOI:10.1039/b916007h

The synthesized derivative Ru(bpy)₃ covalently linked CNTs hybrid shows good electrochemical activity and ca. 17 times higher luminescence quantum efficiency than the adsorbed derivative Ru(bpy)₃. The Ru-CNTs based ECL sensor exhibits high stability toward determination of TPA with a detection limit as low as 8.75 pM.

Full text of DOI:10.1039/b916007h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Enhanced electrochemiluminescence efficiency of Ru(II) derivative
covalently linked carbon nanotubes hybridw
Jing Li,a Li-Rong Guo,b Wei Gao,a Xing-Hua Xia*a and Li-Min Zheng*b
Received (in Cambridge, UK) 5th August 2009, Accepted 20th October 2009
First published as an Advance Article on the web 6th November 2009
DOI: 10.1039/b916007h
The synthesized derivative Ru(bpy)3 covalently linked CNTs
hybrid shows good electrochemical activity and ca. 17 times
higher luminescence quantum efficiency than the adsorbed
derivative Ru(bpy)3. The Ru-CNTs based ECL sensor exhibits
high stability toward determination of TPA with a detection
limit as low as 8.75 pM.
Ru(bpy)3 derivative on CNTs. The present Ru-CNTs hybrid
could be used as both electrochemical and luminescent labels
for ultrasensitive bioanalysis.
The ruthenium(II) bipyridyl derivative, [Ru(bpy)2(AMbpy)]Cl2
(AMbpy = 4,40-aminomethyl-2,20-bipyridine) (Ru(II)) was
synthesized using a modified method referring to literature.7
Products from each synthesis process were characterized
by 1H NMR and electrospray ionization mass spectrometry
(ESI-MS). The results were in good agreement with those
reported in the literature (see the ESIw for detail, Fig. S1 and S2).
The Ru(II) was then covalently linked onto functionalized
CNTs, forming the Ru-CNTs hybrid (illustrated in Fig. 1A).
The absorption spectrum of Ru-CNTs shows obvious red-
shifts from 245 to 270 nm and from 454 to 472 nm (Fig. 1B)
compared with those of free Ru(bpy)3Cl2,8 which is similar to
the case of Ru(bpy)3Cl2 immobilized silica nanoparticles.9
This phenomenon can be ascribed to the strong covalent force
and electrostatic interaction between Ru(II) and the negatively
charged carbon nanostructures. The maximum emission peak
of the Ru-CNTs appears at about 640 nm (Fig. 1B), resulting
from the triplet metal-to-ligand charge-transfer (MLCT)
excited to the ground state.8a This band exhibits an obvious
red-shift by about 40 nm compared with the free Ru(II). The
Ru-CNTs also shows distinct fluorescence (inset in Fig. 1B).
The FTIR spectrum of the oxidized CNTs shows two peaks
at 1718 and 1567 cmꢀ1 due to the carbonyl stretching modes of
COOH and COOꢀ, respectively (Fig. S3,w curve a). After the
Ru(II) was covalently bonded, the Ru-CNTs hybrid displays
two new bands at 1650 and 1568 cmꢀ1 (Fig. S3,w curve b)
corresponding to the C–O stretching modes (amide I) and a
combination of the N–H bending and C–N stretching (amide II),
respectively. In addition, another two peaks resulting from the
stretching vibrations of C–H and N–H bonds in Ru(II) appear
at 2920 and 3415 cmꢀ1, respectively. The decrease in the band
intensity corresponding to the carbonyl stretching mode also
indicates the effective reaction between –COOH and –NH2.
Nowdays, one-dimensional nanostructured materials have
been widely studied due to their potential applications in
nano-scaled optical and electrical devices. As one of the
representative materials, carbon nanotubes (CNTs) with
unique structural, superior mechanical and excellent electronical
properties, have been extensively explored for numerous
applications in field emission, molecular electronics, and
biomedicine.1 The applications can be further broadened by
functionalizing the surface properties of CNTs.2 On the other
hand, electrochemiluminescence (ECL) has been proven to be
a powerful detection technique. The ECL signal may be
significantly improved by immobilizing luminophores on
electrode surfaces, forming solid state ECL detectors. A
number of approaches including physical incorporation of
luminophores into various sol–gels3 and nanomaterials-based
composite films, have been attempted.4 However, such ECL
detectors usually lack long-term stability due to the leakage of
the luminophores. Covalent bonding of ruthenium complex
luminophore onto an electrode surface or in silica nano-
particles could solve these problems.5 This strategy could
enhance the ECL signal, however, poor conductivity of silica
materials may limit their further applications.6 Up to now, the
2+
Ru(bpy)3 complex exhibits the highest ECL efficiency, and
has been extensively used in construction of solid state ECL
sensors.3–6 In this communication, the ruthenium complex was
used as a model molecule for constructing high performance
solid state ECL sensors. In order to covalently bind this
luminophore to CNTs, a Ru(bpy)3 derivative was synthesized.
The covalently bounded Ru(bpy)3 derivative at CNTs
(Ru-CNTs) shows good electrochemical activity and ca. 17 times
higher luminescence quantum efficiency than the adsorbed
a Key Laboratory of Analytical Chemistry for Life Science,
Chemistry School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China.
E-mail: xhxia@nju.edu.cn; Fax: +86-25-83597436;
Tel: +86-25-83597436
b State Key Laboratory of Coordination Chemistry, Chemistry School
of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, China. E-mail: lmzheng@nju.edu.cn
w Electronic supplementary information (ESI) available: Detailed
experimental section, synthesis, absorption and fluorescence spectra,
fluorescence image, IR, ESI-MS, and electrodes preparation
procedure. See DOI: 10.1039/b916007h
Fig. 1 (A) Schematic illustration of the synthesis procedure of
Ru-CNTs. (B) UV-Vis absorbance spectra of Ru(II) complex
(curve a) and Ru-CNTs (curve b), and the fluorescence spectra of
Ru(II) complex (curve c) and Ru-CNTs (curve d) (excitation at 450 nm).
Inset shows the fluorescence photo of the Ru-CNTs.
ꢁc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 7545–7547 | 7545

View Article Online
These results demonstrate that the ruthenium complex has
been successfully covalently linked to the carbon nanotubes.
According to the decrease of the UV-Vis absorption band
intensity and element analysis, 0.19 mmol (0.133 g) Ru(II) per
gram of oxidized CNTs with a loading density of 13.3% was
calculated. This hybrid was modified on a glassy carbon
electrode (GCE) for electrochemical and ECL characterizations
as follows: the hybrid modified GCE (Ru-CNTs/GCE) was
prepared by casting a 5 mL of 1 mg mLꢀ1 Ru-CNTs aqueous
suspension on a cleaned GCE followed by a 5 mL 1.0% Nafion
solution coating. The amount of Ru(II) on the electrode was
calculated as 9.5 ꢂ 10ꢀ9 mol (6.7 ꢂ 10ꢀ8 g). A pure CNTs
modified electrode (CNTs/GCE) was prepared by the same
procedures. The electrochemical results show that the GCE
and the CNTs/GCE do not exhibit any Faradic signal in the
potential region from 0.6 to 1.35 V (Fig. 2A, curves a, b). The
increased current for the CNTs/GCE compared with the GCE
could be due to the significant charging current of the CNTs
matrix. While the Ru-CNTs/GCE shows a quasi-reversible
redox wave with an anodic peak at ca. 1.22 V and a cathodic
peak at ca. 1.13 V due to the electrochemical transition
between Ru(III) and Ru(II) (Fig. 2A, curve d). This redox wave
is similar to that of Ru(bpy)32+. The result demonstrates that
the Ru-CNTs has been successfully modified on the GCE and
shows excellent electrochemical activity ascribed to the good
conductivity of CNTs. Such electrochemical profiles could
not be observed if the ruthenium complex was loaded in
silica particles.5a,8c,9 For comparison, a modified electrode
(Ru&CNTs/GCE) was prepared by immersing the CNTs/GCE
in 4 mM Ru(II) solution for 17 h (saturation adsorption,
Fig. 2A, curve c). In this case, the redox peak currents for
Ru(II) are much smaller than those of the Ru-CNTs/GCE
(compare Fig. 2A, curves c and d), demonstrating that
covalent linking can increase the loading density of Ru(II)
on CNTs.
which could be due to the enhanced quantum emission
efficiency of the Ru(II) covalently linked on CNTs.
The ECL quantum efficiency (jECL) for an annihilation
system is commonly expressed as the number of photons
emitted per redox event. However, the direct measurement
of the ECL efficiency for the present coreactant system is
difficult because of the complexity of the system, the irrever-
sible nature of coreactant electrochemistry, and the high
concentration of coreactant in solution.10 In the present case,
we used the coulometric efficiency (jcoul, photons generated
per electron injected) for comparing the ECL efficiency of
the Ru-CNTs (jRu-CNTs) with that of Ru&CNTs (jRu&CNTs).
The integrated charges were 1.85 ꢂ 10ꢀ5 and 1.11 ꢂ 10ꢀ5 C for
the Ru-CNTs and Ru&CNTs modified electrodes (from
Fig. 2A, curves c and d), respectively, and the integrated
emission intensities were 1526.6 and 54.6 au (Fig. 2B). The
ECL efficiency ratio (jRu-CNTs/jRu&CNTs) was estimated by
comparing the ratio of integrated emission intensity to the
total coulombs passed for both systems at the GC electrodes.
Under the same conditions, the integrated ECL intensity of the
(Ru-CNTs)-TPA system was found to be about 28 times larger
than that observed from the (Ru&CNTs)-TPA system.
However, the oxidation charge passed on the modified
electrode was 1.67 times for the former system larger than
the latter one. Thus, it can be estimated that the coulometric
ECL efficiency for the (Ru-CNTs)-TPA system is ca. 17 times
higher than that of the (Ru&CNTs)-TPA. This demonstrates
that the covalently conjugated Ru(II) on CNTs exhibits much
higher ECL emission efficiency than the physically adsorbed
Ru(II) in the CNTs matrix, which may indicate that possible
electron transfer between the Ru(II) and CNTs occurs in the
covalently conjugated hybrid.
The ECL of the (Ru-CNTs)-TPA system also shows good
stability during continuous potential scans. Fig. 3A shows that
the ECL signal decreases only by 1.6% in 22 potential cycles.
The calibration curve displayed in Fig. 3B shows that the ECL
signal varies linearly with the concentration of TPA from
0.05 nM to 50 mM and 50 mM to 57 mM. The detection limit
of the present sensor is as low as 8.75 pM for TPA at a S/N
of 3, which is 3 orders of magnitude lower than that of
Nafion–Ru&CNTs film.2b
Fig. 2B shows the ECL of the Ru-CNTs/GCE (curve a) and
the Ru&CNTs/GCE (curve b) in the presence of a coreactant
of tri-n-propylamine (TPA). As expected, both modified
electrodes show significant ECL signals starting at potentials
positive to 1.0 V which is consistent with the oxidation of
Ru(II). The ECL peak intensity rises steeply until reaching a
maximum at ca. 1.22 V. Interestingly, the ECL peak intensity
of the Ru-CNTs/GCE is ca. 18 times larger than that of the
adsorbed Ru(II). This increased ratio of ECL is much larger
than that of the electrochemical currents (see next paragraph),
In summary, a novel ruthenium(II) bipyridyl derivative,
[Ru(bpy)2(AMbpy)]Cl2 (Ru(II)), was synthesized and then
covalently conjugated onto CNTs, which can significantly
increase the loading density of functional units on a single
carbon nanotube. Due to the high conductivity, the Ru-CNTs
modified electrode shows excellent electrochemistry. Meanwhile,
Fig. 2 (A) Cyclic voltammograms of a bare GCE (a), CNTs/GCE
(b), Ru&CNTs/GCE (c) and Ru-CNTs/GCE (d) in 100 mM PBS
(pH 7.4) containing 50 mM NaCl at a scan rate of 100 mV sꢀ1. (B)
ECL-potential profiles of the Ru-CNTs/GCE (a) and Ru&CNTs/
GCE (b) in 100 mM PBS (pH 7.4) containing 50 mM NaCl in the
Fig. 3 (A) ECL emission of Ru-CNTs/GCE in a 100 mM PBS
(pH 7.4) containing 50 mM NaCl with 35 mM TPA at a scan rate of
100 mV sꢀ1. (B) Calibration curve of TPA at the Ru-CNTs/GCE in
100 mM PBS (pH 7.4) containing 50 mM NaCl.
presence of 35 mM TPA at a scan rate of 100 mV sꢀ1
.
ꢁc
This journal is The Royal Society of Chemistry 2009
7546 | Chem. Commun., 2009, 7545–7547

View Article Online
the Ru-CNTs hybrid possesses unique electrochemiluminescence
efficiency which is ca. 17 times larger than that of the
physically adsorbed Ru(II) in the CNTs matrix, which is
possibly due to efficient electron transfer between the
ruthenium complex and the carbon nanotubes in the
covalently conjugated system. Therefore, the Ru-CNTs hybrid
based solid state ECL sensor exhibits excellent sensitivity and
high stability toward TPA determination and the detection
limit can be as low as 8.75 pM. The present Ru-CNTs hybrid
could be used as both electrochemical and luminescent labels
for ultrasensitive bioanalysis.
2683–2688; (c) N. Sardesai, S. Pana and J. Rusling, Chem.
Commun., 2009, 4968–4970.
3 (a) G. Greenway, A. Greenwood, P. Watts and C. Wiles, Chem.
Commun., 2006, 85–87; (b) W. Miao, Chem. Rev., 2008, 108,
2506–2553; (c) H. Choi, S. Cho and W. Lee, Anal. Chem., 2003,
75, 4250–4256; (d) M. Richter, Chem. Rev., 2004, 104, 3003–3036.
4 (a) W. Gao, X. Xia, J. Xu and H. Chen, J. Phys. Chem. C, 2007,
111, 12213–12219; (b) L. Zhang and S. Dong, Anal. Chem.,
2006, 78, 5119–5123; (c) Y. Bae, D. Lee, E. Rhogojina,
D. Jurbergs, B. Korgel and A. Bard, Nanotechnology, 2006, 17,
3791–3797.
5 (a) S. Zanarini, E. Rampazzo, L. Ciana, M. Marcaccio,
E. Marzocchi, M. Montalti, F. Paolucci and L. Prodi, J. Am.
Chem. Soc., 2009, 131, 2260–2267; (b) M. Rosso-Vasic, L. DeCola
and H. Zuilhof, J. Phys. Chem. C, 2009, 113, 2235–2240.
6 S. Guo and E. Wang, Electrochem. Commun., 2007, 9, 1252–1257.
7 (a) I. Gillaizeau-Gauthier, F. Odobel, M. Alebbi, R. Argazzi,
E. Costa, C. Bignozzi, P. Qu and G. Meyer, Inorg. Chem., 2001,
40, 6073–6079; (b) S. Fukuda, A. Satake and Y. Kobuke, Thin
Solid Films, 2006, 499, 263–268.
This work was financially supported by the National Basic
Research Program (2007CB714501, 2007CB936404), the
National Natural Science Foundation of China (NSFC:
20535010, 20775035, 20828006, 20821063) and the Natural
Science Foundation of Jiangsu Province (BK2007147).
8 (a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and
A. Zelewsky, Coord. Chem. Rev., 1988, 84, 85–277; (b) P. Pearson,
A. Bond, G. Deacon, C. Forsyth and L. Spiccia, Inorg. Chim. Acta,
2008, 361, 601–612; (c) M. Li, Z. Chen, V. Yam and Y. Zu,
ACS Nano, 2008, 2, 905–912.
Notes and references
1 L. Lacerda, G. Pastorin, W. Wu, M. Prato, A. Bianco and
K. Kostarelos, Adv. Funct. Mater., 2006, 16, 1839–1846.
2 (a) D. Tasis, N. Tagmatarchis and A. Bianco, Chem. Rev., 2006,
106, 1105–1136; (b) Z. Guo and S. Dong, Anal. Chem., 2004, 76,
9 L. Qian and X. Yang, Adv. Funct. Mater., 2007, 17, 1353–1358.
10 H. White and A. Bard, J. Am. Chem. Soc., 1982, 104, 6891–6895.
ꢁc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 7545–7547 | 7547