Tuning the electrical and optical properties of dinuclear ruthenium complexes for near infrared optical sensing

Article abstract of DOI:10.1021/ol060344f

Redox-active dinuclear ruthenium complexes with various 1,2-dicarbonylhydrazido (DCH) ligands are designed and prepared to have intense absorption in the near-infrared region for potential optical sensing in aqueous media, as demonstrated for sensing hydrogen peroxide in this study.

Full text of DOI:10.1021/ol060344f

ORGANIC
LETTERS
2
006
Vol. 8, No. 8
697-1700
Tuning the Electrical and Optical
Properties of Dinuclear Ruthenium
Complexes for Near Infrared Optical
Sensing
1
†
†
‡
†
†
Shidi Xun, Gaetan LeClair, Jidong Zhang, Xin Chen, Jian Ping Gao, and
,
†,‡
Zhi Yuan Wang*
Department of Chemistry, Carleton UniVersity, Ottawa, Canada K1S 5B6, and
Chinese Academy of Sciences, Changchun Institute of Applied Chemistry,
Changchun 130022, PRC
wangw@ccs.carleton.ca
Received February 9, 2006
ABSTRACT
Redox-active dinuclear ruthenium complexes with various 1,2-dicarbonylhydrazido (DCH) ligands are designed and prepared to have intense
absorption in the near-infrared region for potential optical sensing in aqueous media, as demonstrated for sensing hydrogen peroxide in this
study.
Research on new sensors for detecting biologically important
organic substances continues to advance, in part because of
a need for better environmental monitoring and a vast
potential in clinical applications and personal medical
devices. Such a sensor mainly utilizes the electrochemical
or optical properties of redox-active materials. One is an
amperometric sensor, in which ruthenium complexes are used
fabricate because of the electric and magnetic interferences.
Another is an optical sensor, which is simple in design, easy
2
to operate, and free from electric and magnetic interference.
Ruthenium complexes used so far in this type of sensor are
only active, either absorbing or fluorescent, in the visible
region (e.g., 400-750 nm). However, in this spectral
window, endogenous chromophores either absorb light or
1
as electron mediators, and is complicated to design and
(
2) (a) Rosenzweig, Z.; Kopelman, R. Anal. Chem. 1996, 68, 1408-
†
Carleton University.
Changchun Institute of Applied Chemistry.
1413. (b) Wu, X. J.; Choi, M. M. F.; Xiao, D. Analyst 2000, 125, 157-
162. (c) Cary, D. R.; Darrow, C. B.; Lane, S. M.; Peyser, T. A.; Satcher,
J. H., Jr.; Antwerp, W. P. V.; Nelson, A. J.; Reynolds, J. G. Sens. Actuators-
Chem. 2002, 87, 25-32. (d) Bakir, M.; Gyles, C. J. Mol. Struct. 2005,
753, 35-39.
‡
(1) (a) Zakeeruddin, S. M.; Fraser, D. M.; Nazeeruddin, M.-K.; Gr a¨ tzel,
M. J. Electroanal. Chem. 1992, 337, 253-283. (b) Yamamoto, K.; Zeng,
H.; Shen, Y.; Ahmed, M. M.; Kato, T. Talanta 2005, 66, 1175-1180.
1
0.1021/ol060344f CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/18/2006

3
autofluoresce. Thus, near-infrared (NIR) chromophores are
As shown in Figure 1, four DCH-Ru complexes are
designed to have different substituting R and R′ groups that
desirable in biomedical studies because tissue autofluores-
cence and light absorption by tissue and water are low
between 900 and 1300 nm. Accordingly, we intend to
4
demonstrate the use of redox-active, NIR-absorbing ruthe-
nium complexes for sensing chemically and biologically
important substances (e.g., hydrogen peroxide or glucose)
in water or under physiological conditions.
It is known that dinuclear ruthenium complexes display
intense and broad absorption in the NIR spectral region when
II
III
in the mixed-valence state (Ru /Ru ) because of intervalence
charge-transfer transitions between the two metal nuclei or
metal to metal charge transfer (MMCT). Oxidation to the
mixed-valence state can be done by electrochemical means
or with various oxidizing agents including hydrogen peroxide
Figure 1. Structure of dinuclear DCH-Ru complexes.
can affect the electronic states of the coordinating sites (N
and O atoms), which in turn influences the optical and
electrochemical properties of the complexes. All four DCH
ligands were synthesized by reaction of an acyl chloride,
chloroformate, or isocyanate with the corresponding hy-
(H
2
O
2
). Therefore, in this study, we will demonstrate NIR
optical sensing in water with H . Because H is also an
O
2 2
2 2
O
intermediate product from enzymatic glucose oxidation, it
is conceivable that glucose can be detected and quantified
as well by a NIR optical means under physiological condi-
tions (Scheme 1). The sensing principle is simple; glucose
5
drazide or hydrazine by a similar reported method. The
ruthenium complexes were prepared by exchange with Ru-
6
(bpy)
2
Cl
2
as previously reported. These complexes were
fully characterized by spectroscopic means (see Supporting
Information).
Scheme 1. Detection of Glucose on the Basis of the Redox
Reaction of the Dinuclear Ruthenium Complex with
Hydrogen Peroxidea
To correlate the substituent effect with the spectroscopic
and electrochemical data, the Hammett parameters as a direct
7
measure of electron-donor potentials were used. The elec-
trochemical and optical data for the DCH-Ru complexes
are given in Table 1, and the cyclic voltammogram of
Table 1. Electrochemical Data, Hammett Substituent
Parameters, and MMCT Band Energies for DCH-Ru
Complexes
a
Cuvettes shown are acetonitrile solutions of the complex in
their respective oxidation states.
complex
1E1/2a
2E1/2a
∆Eb
σ*
λmaxc
MMCTd
is oxidized by glucose oxidase (GOX) and produces H
2 2
O
1
2
3
4
50
160
240
500
580
700
800
530
540
560
600
-1.10
-0.79
-0.48
-0.02
1150
1270
1410
1600
0.8352
0.7133
0.6889
0.6448
as a byproduct, which then oxidizes the ruthenium complex
II
III
to its Ru /Ru state, and NIR absorbance of the complex is
monitored. Furthermore, the complex changes color from
purple to yellow, although this visual effect is slow to appear
to the naked eye.
1100
a Cyclic voltammetry performed at a 100 mV/s scan rate in acetonitrile/
.1 M tetraethylammonium perchlorate. Potentials E in millivolts vs NHE.
E ) E1/2 - E1/2 in millivolts. For the Ru /Ru state of complexes
in nanometers. MMCT band energies in electronvolts.
0
b
2
1
c
II
III
∆
The basic requirements for the materials used in such a
NIR optical sensor include: (1) intense absorption upon
d
2 2
action with the analyte (e.g., H O ) within the 900-1300
nm region, (2) adequate redox potentials, and (3) an ability
to form transparent thin films on an electrode. In this paper,
four ruthenium complexes with the 1,2-dicarbonylhydrazido
complex 1 is shown in Figure 2. There is a linear dependence
of electrochemical properties of DCH-Ru complexes on the
Hammett parameters. As the donor strength of the substituent
(DCH) ligand are presented and their band-gap energies are
1
increases, there is a shift of both the first ( E1/2) and second
systematically tuned by varying the electron-withdrawing or
electron-donating ability of the substituents on the DCH
ligand. Among them, one was selected and used for sensing
2
oxidation potentials ( E1/2) to lower values, which can be
(5) (a) Mallakpour, S.; Hajipour, A. R.; Taghizadeh, H. A. Molecules
2
003, 8, 359-362. (b) Kauer, J. C. In Organic Syntheses; Rabjohn, N.,
Ed.; John Wiley & Sons: New York; Vol. IV, pp 411-415.
6) (a) Qi, Y. H.; Dejardins, P.; Wang, Z. Y. J. Opt. A: Pure Appl. Opt.
2 2
H O in aqueous solution and as a thin film.
(
(
3) (a) Patonay, G.; Antoine, M. D. Anal. Chem. 1991, 63, 321A-327A.
2002, 4, S273-S277. (b) Qi, Y. H.; Wang, Z. Y. Macromolecules 2003,
36, 3146-3151. (c) Rastegar, M. F.; Todd, E. K.; Tang, H. D.; Wang, Z.
Y. Org. Lett. 2004, 6, 4519-4522.
(7) (a) Evans, C. E. B.; Yap, G. P. A.; Crutchley, R. J. Inorg. Chem.
1998, 37, 6161-6167. (b) Richter, M. M.; Brewer, K. J. Inorg. Chem. 1993,
32, 5762-5768. (c) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991,
91, 165-195.
(
b) Stoyanov, S. Pract. Spectrosc. 2001, 25, 35-93. (c) Thompson, R. B.
In Topics in Fluorescence Spectroscopy, 1st ed.; Lakowicz, J. R., Ed.;
Plenum Press: New York, 1994; Vol. 4, Chapter 6, pp 151-181.
(4) (a) Barone, P. W.; Baik, S.; Heller, D. A.; Strano, M. S. Nat. Mater.
2
005, 4, 86-92. (b) Jensen, P. S.; Bak, J.; Andersson-Engels, S. Appl.
Spectrosc. 2003, 57, 28-36.
1698
Org. Lett., Vol. 8, No. 8, 2006

A feasibility test was first carried out in solution using
-
4
H O
2 2
directly. To complex 1 in acetonitrile (ACN) (1 × 10
M) in a cuvette was introduced H with the desired
2 2
O
concentration up to 10 mM. The absorbance was monitored
at 1150 nm and recorded at different times up to 10 min
(Figure 4). For the glucose detection, because there is a 1:1
Figure 2. Cyclic voltammogram of complex 1 in acetonitrile/0.1
M tetraethylammonium perchlorate (trace a) and a film containing
complex 1 on indium tin oxide coated glass (trace b) at a scan rate
-
1
of 100 mV s
.
attributed to the improvement of the stability of the ruthenium
6
d-orbitals when the ligand has poor σ-donor properties.
-4
1
2
Figure 4. Absorbance (1150 nm) of 1 (1 × 10 M in 1:1
Compared with complex 4, E1/2 and E1/2 of complex 1 with
II II
acetonitrile/Tris buffer) in the Ru /Ru state by oxidation with
different concentrations of H recorded at different times.
the strong electron-donating NHPr group decreased by 450
and 520 mV, respectively, and 1 has a E1/2 as low as 50
2 2
O
1
mV, which implies ease in oxidation by H
2
O
2
only to its
II
III
NIR-absorbing Ru /Ru state rather than to the NIR-inactive
2 2
molar equivalent correlation between glucose and H O in
III
III
Ru /Ru state, and thus a better sensitivity in sensing. The
substituents also affect the optical properties of these
complexes significantly. Figure 3 shows intense NIR absorp-
8
the enzymatic oxidation of glucose, we can regard the H
2 2
O
concentration as the same as that of glucose.
Figure 4 shows clearly that complex 1 is sensitive enough
to detect H at concentrations as low as 0.1 mM, which
is much lower than the normal glucose levels in blood
typically in a range of 6-11 mM). There is a linear response
of absorbance with increasing peroxide concentration after
and 7 min of reaction time. The response is also linear at
2 2
O
(
2
other times, even after 30 s. Our spectrometer is not equipped
with a stirring system; therefore, the rate of oxidation depends
on diffusion. These conditions were not optimized, and
addition of a stirring mechanism or flow cell should increase
response time. Even in unoptimized conditions, 1 can be used
2 2
to monitor the presence of H O .
To demonstrate the usefulness of complex 1 further as a
sensoring material in a sensor device, a similar experiment
was done using a cross-linked polyurethane film doped with
9
Figure 3. Absorption spectra of complexes in the RuII/RuIII state
1. A thin film (0.58 µm) coated on a slide of indium tin
in acetonitrile/0.1 M tetraethylammonium perchlorate obtained with
an optically transparent thin-layer electrochemical cell.
oxide (ITO) glass, to act as an electrode, exhibited the two
broad redox couples at potentials similar to those of complex
1
in solution (Figure 2). The film-coated ITO plate was
placed in a Tris buffer solution (pH ) 7.4) (2 mL), which
had the same pH as human blood, in a cuvette, and 50 µL
of H O (35 wt % solution in water) was introduced to
2 2
tion bands of four DCH-Ru complexes in their Ru /RuIII
states. The MMCT band is blue-shifted upon increasing the
donor strength of the substituents. With a strong electron-
donating NHPr group, complex 1 has a maximum MMCT
absorption at 1150 nm, which falls in an ideal biowindow
II
(
8) Tolosa, L.; Szmacinski, H.; Rao, G.; Lakowicz, J. R. Anal. Biochem.
1997, 250, 102-108.
9) Complex 1 (7.4 wt %), trimethylolpropane, carbamate with xylylene
diisocyanate (available from Aldrich Chemicals Inc., catalog number
08778), trimethylolpropane (available from Aldrich Chemicals Inc., catalog
(
2 2
for sensing in an aqueous medium. Because H O should
readily oxidize complex 1 from the Ru /Ru state to the Ru /
Ru state, by monitoring the change in absorbance in the
2 2
NIR region or at 1150 nm, one can detect and quantify H O
II
II
II
4
III
number 239747), and tetraethylammonium hexafluorophosphate (9.2 wt %)
were mixed in acetonitrile (1 mL) and sonicated for 1 h and then spin coated
on an ITO plate (0.8 × 2.5 cm). After thermal curing at 50 °C under vacuum
for 1 h, a film with a thickness of 0.58 µm was formed.
directly or glucose indirectly.
Org. Lett., Vol. 8, No. 8, 2006
1699

These cycling experiments were repeated nine times with
excellent reproducibility (Figure 5). The slight variation of
the absorbance change most probably arose from the unop-
timized test technique. The cross-linked film tightly adhered
to the ITO electrode and was stable to prolonged soaking in
acetonitrile and Tris buffer solution. There was no noticeable
damage to the film on the ITO plate after many cycles of
2 2
treatment of H O and electrical reduction, demonstrating a
potentially robust sensor device.
In conclusion, we have demonstrated the working principle
of a new NIR optical sensor, based on the NIR-absorbing
ruthenium complexes, for chemically and biologically im-
portant substances in aqueous media under physiological
conditions.
Figure 5. Normalized absorbance (1150 nm), recorded in real time,
II
II
of a film containing 1 in the Ru /Ru state oxidized with H
0.3 M) in Tris buffer solution. The film was reduced electrochemi-
cally (-0.5 V). The cycle was repeated nine times.
2 2
O
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada and the National
Natural Science Foundation of China (Grant No. 20510174)
for financial support.
(
oxidize complex 1 doped in the film to its Ru /RuIII state,
while monitoring the increase in absorbance at 1150 nm
simultaneously. To prove the reusability of the device, we
II
Supporting Information Available: Synthesis and char-
acterization (including NMR and ESI-MS spectra) of DCH-
Ru ligands and complexes 1-4. This material is available
free of charge via the Internet at http://pubs.acs.org.
used a much higher concentration of H
experiment and did not wait for the absorbance to peak. After
min, a bias (-500 mV) was applied to the ITO plate after
placing it in a tetraethylammonium perchlorate/ acetonitrile
2 2
O (0.3 M) in this
8
II
II
solution (0.1 M) to reduce the complex to its Ru /Ru state.
OL060344F
1700
Org. Lett., Vol. 8, No. 8, 2006