Rosamine-based fluorescent chemosensor for selective detection of silver(I) in an aqueous solution

Article abstract of DOI:10.1021/ic800442y

The synthesis and photophysical properties of a rosamine-based fluorescent chemosensor, RosAg, for detecting Ag ion in an aqueous solution are described. This fluorescent sensor has a negligible quantum yield (<0.005) in the absence of Ag⁺, whereas a significant increase in fluorescence is observed upon complexation with Ag⁺ under physiological conditions. The crystal structure of the silver complex with the chelator moiety of RosAg reveals a trigonal-planar coordination geometry in which three S atoms occupy the metal center. Although a strong coordinative interaction of Ag-N is not observed in the crystal structure, the ¹H NMR experiments suggest that aniline nitrogen is likely to be associated with the Ag⁺ center in the solution state. This may inhibit the photoinduced electron transfer process and result in the enhancement of fluorescence.

Full text of DOI:10.1021/ic800442y

Inorg. Chem. 2008, 47, 3946-3948
Rosamine-Based Fluorescent Chemosensor for Selective Detection of
Silver(I) in an Aqueous Solution
Shohei Iyoshi,† Masayasu Taki,*,†,‡ and Yukio Yamamoto†
Graduate School of Human & EnVironmental Studies and Graduate School of Global
EnVironmental Studies, Kyoto UniVersity, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Received March 11, 2008
The synthesis and photophysical properties of a rosamine-based
fluorescent chemosensor, RosAg, for detecting Ag ion in an
aqueous solution are described. This fluorescent sensor has a
negligible quantum yield (<0.005) in the absence of Ag+, whereas
a significant increase in fluorescence is observed upon complex-
ation with Ag+ under physiological conditions. The crystal structure
of the silver complex with the chelator moiety of RosAg reveals a
trigonal-planar coordination geometry in which three S atoms
occupy the metal center. Although a strong coordinative interaction
able attention because of its antimicrobial activities.4
Although several possible roles of Ag+ in biological systems
have been proposed, such as (i) interaction and inactivation
of vital enzymes,4e,5 (ii) binding to DNA,6 (iii) interaction
with the cell membrane,7 and (iv) interference with electron
transport,4a the mechanism of the antimicrobial activity of
Ag+ has not been clarified because of a lack of suitable
detection and imaging agents.8 Ratiometric fluorescent
sensors based on pyrene8d or BODIPY,8e for which a
fluorescence shift is observed upon complexation with Ag+,
have been developed; however, they have poor water
solubility. Recently, Schmittel et al. reported a luminescent
iridium complex capable of detecting Ag+ in aqueous media;
however, this complex also must be used with a mixed
solvent of CH3CN and water.8g
1
of Ag-N is not observed in the crystal structure, the H NMR
experiments suggest that aniline nitrogen is likely to be associated
with the Ag+ center in the solution state. This may inhibit the
photoinduced electron transfer process and result in the enhance-
ment of fluorescence.
In this context, we demonstrate a novel water-soluble
fluorescent sensor for Ag+, RosAg, based on tetramethyl-
Selective and sensitive fluorescent sensors for metal ions
have been essential tools not only in the field of biology but
also in clinical and environmental studies.1,2 In particular,
these fluorescent sensors have been used in bioinorganic
chemistry to understand the effects of metal ions, which often
have important functions or produce toxic effects in cells,
on the human body.2,3 Among such biologically important
metal ions, the silver ion (Ag+) has long received consider-
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370. (b) Melaiye, A.; Sun, Z. H.; Hindi, K.; Milsted, A.; Ely, D.;
Reneker, D. H.; Tessier, C. A.; Youngs, W. J. J. Am. Chem. Soc.
2005, 127, 2285–2291. (c) Kascatan-Nebioglu, A.; Melaiye, A.; Hindi,
K.; Durmus, S.; Panzner, M. J.; Hogue, L. A.; Mallett, R. J.; Hovis,
C. E.; Coughenour, M.; Crosby, S. D.; Milsted, A.; Ely, D. L.; Tessier,
C. A.; Cannon, C. L.; Youngs, W. J. J. Med. Chem. 2006, 49, 6811–
6818. (d) Nomiya, K.; Tsuda, K.; Sudoh, T.; Oda, M. J. Inorg.
Biochem. 1997, 68, 39–44. (e) Holt, K. B.; Bard, A. J. Biochemistry
2005, 44, 13214–13223. (f) Melaiye, A.; Simons, R. S.; Milsted, A.;
Pingitore, F.; Wesdemiotis, C.; Tessier, C. A.; Youngs, W. J. J. Med.
Chem. 2004, 47, 973–977.
* To whom correspondence should be addressed. E-mail: taki@
chem.mbox.media.kyoto-u.ac.jp.
(5) (a) Liau, S. Y.; Read, D. C.; Pugh, W. J.; Furr, J. R.; Russell, A. D.
Lett. Appl. Microbiol. 1997, 25, 279–283. (b) Flemming, C. A.; Ferris,
F. G.; Beveridge, T. J.; Bailey, G. W. Appl. EnViron. Microbiol. 1990,
56, 3191–3203.
†
Graduate School of Human & Environmental Studies.
Graduate School of Global Environmental Studies.
‡
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Recognition; American Chemical Society: Washington, DC, 1993. (b)
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3946 Inorganic Chemistry, Vol. 47, No. 10, 2008
10.1021/ic800442y CCC: $40.75  2008 American Chemical Society
Published on Web 04/24/2008

COMMUNICATION
Scheme 1. Synthesis of RosAg
rosamine, which has been employed as a fluorophore in
intracellular Ca2+ imaging agents such as Rhod-2.9 In order
to achieve specific binding to Ag+, we utilized an
o-aminophenol-N,N,O-triacetic acid derivative consisting of
three thioethers instead of the carboxyl group as the chelator
and attached it to xanthene at the 9 position. Similar to all
Rhod dyes, it is expected that RosAg itself (apo form) will
not exhibit fluorescence because of the efficient photoinduced
electron transfer (PET) quenching of the xanthene dye by
the aniline nitrogen of the chelator moiety.10
Figure 1. Fluorescence emission spectra (λex ) 540 nm) of RosAg (5 µM)
as a function of added AgNO3 in 50 mM HEPES (pH 7.20 and 0.1 M
KNO3). The inset shows the fluorescence response at 574 nm, substracting
the baseline (F0) spectrum and normalizing. The obtained data were analyzed
by a nonlinear least-squares fitting to determine a dissociation constant of
2.0 µM.
The synthesis of RosAg is outlined in Scheme 1. Triethyl
ester 111 was reduced by LiAlH4 to yield triol 2, and
subsequent bromination using CBr4 and PPh3 yielded tri-
bromide 3. Treatment of tribromide 3 with NaSCH3 produced
trithioether 4, which is the Ag+ ion binding moiety of the
sensor. Successive formylation under Vilsmeier conditions,
Friedel-Crafts acylation with 3-(dimethylamino)phenol,
chloranil oxidation, and counteranion exchange by KPF6
yielded RosAg as a dark-purple powder. The structure of
1
the final product was identified by H and 13C NMR, IR
spectroscopy, and mass spectroscopy; its purity was con-
firmed by reversed-phase high-performance liquid chroma-
tography.
Figure 2. Fluorescence response of RosAg (5 µM) at 574 nm as a function
of various added metal cations (5 mM for Na+, K+, Ca2+, and Mg2+ and
50 µM for all other cations) in 50 mM HEPES (pH 7.20 and 0.1 M KNO3):
1, no metal; 2, Ag+; 3, Na+; 4, K+; 5, Mg2+; 6, Ca2+; 7, Mn2+; 8, Fe2+; 9,
Under physiological conditions (50 mM HEPES, pH 7.2,
and 0.1 M KNO3), RosAg exhibited an excitation maximum
at 551 nm (ꢀ ) 5.7 × 104 M-1 cm-1) and had a negligible
fluorescence quantum yield (<0.005) in the absence of Ag+.
Upon the addition of Ag+, however, the fluorescence
intensity of RosAg showed a ca. 35-fold increase (Φ ) 0.13;
Figure 1) without a significant change in the excitation and
emission maxima (λex ) 553 nm; λem ) 574 nm). The low
background fluorescence of RosAg and the large enhance-
ment in the fluorescence of the Ag+-bound form of this
sensor would make it more advantageous than reported
on-off-type fluorescent silver sensors, for which the fluo-
rescence intensities show only a 3-4-fold increase.8g,h
Moreover, the apo and Ag-bound forms of RosAg have stable
Co2+; 10, Ni2+; 11, Cu+; 12, Cu2+, 13, Zn2+; 14, Cd2+; 15, Pb2+; 16, Hg2+
;
17, Ag+ + Na+; 18, Ag+ + K+; 19, Ag+ + Mg2+; 20, Ag+ + Ca2+
.
fluorescence at the biological pH. The fluorescence of RosAg
in the absence of Ag+ did not change above pH 5 and
increased as the pH decreased, suggesting that the PET
process was inhibited by protonation of the aniline nitrogen
of the electron donor.2a The fluorescence intensity of RosAg
in the presence of Ag+ was also unchanged in a wide range
of pH’s (from 2 to 12). We then examined the fluorescence
responses of 5 µM RosAg to various biologically relevant
metal ions, as shown in Figure 2. Ca2+, Mg2+, Na+, and K+,
which exist in high concentrations in cells, did not cause an
enhancement of the fluorescence even at 5 mM and did not
interfere with Ag+ binding. Surprisingly, a slight enhance-
ment in the fluorescence intensity (ca. 7-8-fold) was
observed in the presence of Cu+ and Cu2+, which are known
to be effective fluorescence quenchers.2a,8a,12 Other transition
(9) Minta, A.; Kao, J. P. Y.; Tsien, R. Y. J. Biol. Chem. 1989, 264, 8171–
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Mallet, J. M. Org. Lett. 2007, 9, 2629–2632. (c) Martin, V. V.; Rothe,
A.; Diwu, Z.; Gee, K. R. Bioorg. Med. Chem. Lett. 2004, 14, 5313–
5316. (d) Gee, K. R.; Zhou, Z. L.; Ton-That, D.; Sensi, S. L.; Weiss,
J. H. Cell Calcium 2002, 31, 245–251.
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5248–5256.
Inorganic Chemistry, Vol. 47, No. 10, 2008 3947

COMMUNICATION
occupied by S atoms.15 The metal center lies slightly above
the plane formed with the three S atoms (F ) 0.1343 Å).
The nitrate anion of [Ag(4)]NO3 was considered to be
noninteracting because the shortest Ag-ONO2 distance is
3.24 Å. Surprisingly, we found that there was no direct
coordinative interaction between the Ag and N atom of the
ligand [the distance is 2.7254(9) Å]. This result may
contradict the observation of the enhancement in fluorescence
caused by Ag+. Therefore, the Ag+ complexation behavior
of 4 in solution was further examined by 1H NMR measure-
ment to confirm whether Ag+ can approach more closely
and interact electrostatically with the N atom of the
ligand.8i,16 The spectrum of [Ag(4)]NO3 in DMSO-d6
exhibited large downfield shifts of the aromatic protons,
particularly for the ortho and para positions toward the amino
group, as well as terminal methyl protons adjacent to the S
atoms. These shifts are indicative of the strong interactions
of Ag-N and Ag-S in the solution. The results obtained
are consistent with the increase in the fluorescence intensity
upon complexation with Ag+ because the PET quenching
process would be inhibited by the decrease of the electron
density of the donor.
Figure 3. ORTEP diagram of [Ag(4)]NO3. Thermal ellipsoids are shown
at 50% probability. H atoms and the nitrate anion are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ag1-S1 ) 2.5570(4), Ag1-S2
) 2.5369(3), Ag1-S3 ) 2.5421(3), S1-Ag1-S2 ) 121.944(12),
S1-Ag1-S3 ) 115.802(11), S2-Ag1-S3 ) 121.424(12).
metals including Fe2+, Mn2+, Pb2+, Cd2+, Co2+, Zn2+, Ni2+,
and Hg2+ had no effect on the fluorescence spectrum. Such
photophysical properties in a neutral aqueous solution and
the Ag+ specificity of RosAg suggest that this sensor may
be useful in biological applications.
Job’s plot monitored by fluorescence intensities indicated
the formation of a 1:1 complex. Thus, the dissociation
constant (Kd ) 2.0 µM) for Ag+ was determined by plotting
the fluorescence intensity (F - F0) against [Ag+] and fitting
these data13 (inset of Figure 1). This Kd value is comparable
to the affinities of the reported compounds containing a
tetrathia receptor.8e,i The Ag+ affinity and fluorescent proper-
ties of RosAg indicate that the aniline nitrogen of the chelator
as well as three thioether groups is likely to coordinate to
the metal ion in the aqueous solution.
In order to investigate the structural properties of the Ag+
complex of RosAg, we synthesized the silver complex using
a truncated part of the sensor that included its metal binding
moiety (trithioether 4) and determined its crystal structure
by X-ray diffraction analysis.14 As observed in the titration
for the Ag+ complexation to RosAg, [Ag(4)]NO3 was a
monomeric silver complex containing a single nitrate anion
(Figure 3). The coordination geometry of the Ag atom is
almost an ideal trigonal-planar structure (the sum of the
angles around Ag is 359.2°) with three coordination sites
In summary, we have developed a new rosamine-based
fluorescent sensor (RosAg) for Ag+. This sensor is soluble
in water and can be excited with visible light. It also has
pH-independent fluorescence at the biological pH and a large
signal-to-noise ratio. To evaluate the suitability of RosAg
for biological applications, intracellular imaging of the Ag
ion using this sensor is in progress.
Acknowledgment. This work was financially supported
in part by Grants-in-Aid for Scientific Research (No. 19750139
to M.T.; No. 19350084 to Y.Y.) from the Japan Society for
the Promotion of Science (JSPS) and the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT).
S.I. thanks the Kyoto University Venture Business Labora-
tory (KU-VBL) for support.
Supporting Information Available: Experimental details for
synthetic procedures, Job’s plot, fluorescence pH sensitivity, X-ray
1
diffraction analysis, and H NMR spectra of 4 and [Ag(4)]NO3
(PDF) and crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC800442Y
(13) See the Supporting Information.
(15) The trigonal-planar coordination of Ag+ occupied by S atoms has been
rarely reported. For example, see. Mascal, M.; Kerdelhue, J. L.; Blake,
A. J.; Cooke, P. A. Angew. Chem., Int. Ed. 1999, 38, 1968–1971.
Fujisawa, K.; Imai, S.; Suzuki, S.; Moro-oka, Y.; Miyashita, Y.;
Yamada, Y.; Okamoto, K. J. Inorg. Biochem. 2000, 82, 229–238.
(16) Ishikawa, J.; Sakamoto, H.; Nakamura, M.; Doi, K.; Wada, H. J. Chem.
Soc., Dalton Trans. 1999, 191–199.
(14) Crystal data for [Ag(4)]NO3: C15H25O4N2S3Ag1; monoclinic; P21/c
(No. 14); a ) 9.3704(8) Å, b ) 10.1330(9) Å, c ) 20.4772(17) Å, ꢀ
) 90.3451(18)°, V ) 1944.3(3) Å3; Z ) 4; T ) 203 K; λ ) 0.710 75
Å; µ ) 13.781 cm-1; Dcalcd ) 1.713 g cm-3; R1 ) 0.0312 [I > 2σ(I)];
wR2 ) 0.0859; GOF (on F2) ) 1.014. Additional crystallographic
details are contained in the Supporting Information.
3948 Inorganic Chemistry, Vol. 47, No. 10, 2008