Colorimetric and fluorescent signaling of Au3+ by desulfurization of thiocoumarin

Article abstract of DOI:10.1021/ic202080v

We investigated the chemosignaling of Au³⁺ by the selective desulfurization of thiocoumarin. In the presence of a heavy metal ion chelator N,N,N′,N′-tetrakis-(2-pyridylmethyl)ethylenediamine, thiocoumarin was selectively converted to its oxo analogue by reaction with Au³⁺, resulting in a pronounced chromogenic and fluorescent signaling. Selective signaling of Au³⁺ was possible in the presence of common alkali, alkaline earth, and transition metal ions, as well as Au⁺ in a mixed aqueous environment. The colorimetric determination of Au³⁺ was possible by the color change from pink to yellowish green of the designed probe. The detection limit for the determination of Au³⁺ in 50% aqueous acetonitrile was 1.1 × 10^-7 M.

Full text of DOI:10.1021/ic202080v

Article
pubs.acs.org/IC
3+
Colorimetric and Fluorescent Signaling of Au by Desulfurization of
Thiocoumarin
Ji Eun Park, Myung Gil Choi, and Suk-Kyu Chang*
Department of Chemistry, Chung-Ang University, Seoul 156-756, Korea
*
S Supporting Information
ABSTRACT: We investigated the chemosignaling of Au3+ by
the selective desulfurization of thiocoumarin. In the presence
of a heavy metal ion chelator N,N,N′,N′-tetrakis-(2-
pyridylmethyl)ethylenediamine, thiocoumarin was selectively
3+
converted to its oxo analogue by reaction with Au , resulting
in a pronounced chromogenic and fluorescent signaling.
3
+
Selective signaling of Au was possible in the presence of
common alkali, alkaline earth, and transition metal ions, as well
+
as Au in a mixed aqueous environment. The colorimetric
3
+
determination of Au was possible by the color change from
pink to yellowish green of the designed probe. The detection
limit for the determination of Au3+ in 50% aqueous acetonitrile was 1.1 × 10 M.
−7
INTRODUCTION
Research on gold-related chemistry is currently one of the most
rapidly growing fields in chemistry because of its relevance to a
conjugate to yield its methylketone derivative having ratio-
metrically analyzable signals via vinylgold intermediate.
■
17
N,N,N′,N′-Tetrakis-(2-pyridylmethyl)ethylenediamine
(TPEN) is known as an efficient Zn2+ chelator and is a
potential candidate for a practical and selective complexing
18
1
number of issues in material science. Recently, a series of
thorough reviews were published on various topics of gold
1
9
2+ 20
2+ 21
2
3
4
agent for a variety of soft metal ions, such as Hg , Cd ,
chemistry, such as catalysis, selective oxidation, nanoparticles
2+
22
2+ 23
5
Cu , and Pd . It also is frequently used as a separation tool
for d- or f-block metal ions. We have reported a dual signaling
in diagnostics and detection, and photophysical perspective of
24
6
this valuable element.
2
+
Hg -selective probe based on the metal ion-induced
The determination of precious metals of gold as well as other
platinum group elements (Pt, Pd, Rh, Ir, Ru, and Os) in
biological, clinical, and industrial samples has been of great
2
5
desulfurization of thiocoumarins. On the other hand,
thioamide is known to be readily converted to its oxo amide
by Au3+ ions. On the basis of this, we devised a simple Au -
selective signaling system that uses the desulfurization of
thiocoumarin to its oxo derivative, while utilizing TPEN as a
masking agent for the possible interfering ions. In fact,
significant Hg2+ response of thiocoumarin was effectively
26
3+
7
8
interest. Inductively coupled plasma mass spectrometry has
been a key technology for the sensitive and simultaneous
determination of traces of these elements. Other techniques
9
such as atomic absorption spectrometry and nuclear and
1
0
electrochemical methods have also been reported. More
recently, quantitative analysis of widely investigated gold
27
3+
suppressed using TPEN, and efficient Au -selective signaling
was realized. The developed system exhibited a pronounced
chromogenic and fluorescence signaling behavior, which is
typical of coumarin derivatives, toward Au3+ ions in an aqueous
environment.
11
nanoparticles has also been reviewed. In spite of the ongoing
efforts to develop versatile analytical techniques for gold
1
2
3+
analysis, the optical signaling of Au is more convenient in
terms of the necessary instruments and technical skills and well
1
3
discussed in a recent report. Among the recently developed
EXPERIMENTAL METHODS
General Methods. Coumarin 6 (2), N,N,N′,N′-tetrakis-(2-
pyridylmethyl)ethylenediamine (TPEN), Lawesson’s reagent, and
3
+
■
Au -selective colorimetry and fluorescent signaling systems,
1
4
selective chemical reaction based chemosignaling approach
which has been uniquely employed for the construction of
many sophisticated signaling systems is interesting. Represen-
AuCl were purchased from Aldrich Chemical Co. AuCl was obtained
3
from Alfa Aesar Co. Other inorganic salts were in perchlorate form
and used as received. Acetonitrile was purchased from Aldrich
3
+
tative examples are Au -induced intramolecular cyclization of
the alkynes to yield a fluorescent ring-opened structure and
cyclization of propargylamide to oxazolecarbaldehyde of
1
13
Chemical Co. as “spectroscopic grade”. H NMR (300 MHz) and
C
NMR (75 MHz) spectra were obtained on a Varian gemini 2000
spectrometer and referenced to the residual solvent signal. UV−vis
1
5
3+
rhodamines. The other approach utilized the Au -mediated
hydroarylation reaction of a latent fluorophore of the alkyne
16
derivative to yield a strongly fluorescent coumarin derivative
and the Kucherov reaction of 1,8-naphthalimide-alkyne
Received: September 23, 2011
Published: February 10, 2012
©
2012 American Chemical Society
2880
dx.doi.org/10.1021/ic202080v | Inorg. Chem. 2012, 51, 2880−2884

Inorganic Chemistry
Article
spectra were recorded with a Jasco V-550 spectrophotometer equipped
with a Peltier temperature controller. Fluorescence spectra were
measured on an Aminco-Bowman Series 2 spectrophotometer. Mass
spectra were obtained on a Micromass Autospec mass spectrometer.
Elemental analysis data were obtained by Flash EA 1112 (Thermo
Electron corporation) elemental analyzer.
Detection Limit. The detection limit was estimated by plotting the
fluorescence changes of 1 in the presence of TPEN as a function of
3+
28
log[Au ] following the reported procedure. A linear regression
curve was fitted to the intermediate values of the sigmoidal plot. The
point at which this line crossed the ordinate axis was taken as the
detection limit.
pH Effect on Au3 Signaling. The pH effect on Au3+ signaling of
1 was investigated by measuring the UV−vis absorption spectra in a
series of buffered solutions between pH 4.0−9.0. The pH of the
solution was fixed by using acetate (pH 4.0−6.0), phosphate (pH 7.0−
8.0), and tris (pH 9.0) buffer solutions. Final concentrations of probe
+
Preparation of 1. Thiocoumarin 1 was prepared by the reaction of
coumarin 6 (2) with Lawesson’s reagent following the reported
25
1
procedure. Yield, 65%; H NMR (300 MHz, CDCl ) δ 8.96 (br d, J
3
=
0.6 Hz, 1H), 8.01 (ddd, J = 8.2, 1.1, and 0.7 Hz, 1H), 7.95 (ddd, J =
7
7
.9, 1.3, and 0.6 Hz, 1H), 7.53 (d, J = 9.0 Hz, 1H), 7.48 (ddd, J = 8.2,
.2, and 1.3 Hz, 1H), 7.37 (ddd, J = 8.1, 7.1, and 1.1 Hz, 1H), 6.73
3+
1, TPEN, Au , and each buffer solution were 5.0 μM, 0.20 mM, 0.10
mM, and 10 mM, respectively, under the same measurement
conditions.
(
dd, J = 9.0 and 2.5 Hz, 1H), 6.67 (br d, J = 1.9 Hz, 1H), 3.45 (q, J =
13
7
1
1
.1 Hz, 4H), 1.25 (t, J = 7.1 Hz, 6H); C NMR (75 MHz, CDCl ) δ
3
+
92.8, 164.4, 160.0, 153.1, 151.6, 139.0, 136.4, 131.4, 126.2, 124.8,
Interference from Au at Higher Temperature. The possibility
+
+
3+
24.4, 122.1, 121.6, 111.9, 111.4, 96.4, 45.5, 12.7. HRMS (DIP) m/z
of the interference from Au by the transformation of Au to Au at
higher temperature was tested. The signaling of AuCl solution by 1
was measured after treatment of the solution at 50 and 80 °C for 2 h in
a water bath. Final concentrations of probe 1, TPEN, and AuCl were
5.0 μM, 0.20 mM, and 0.10 mM, respectively, under the same
measurement conditions.
+
calcd for C H N OS [M] 366.0861, found 366.0857. Anal. Calcd
20
18
2
2
for C H N OS : C, 65.54; H, 4.95; N, 7.64; S, 17.50. Found: C,
20
18
2
2
6
5.56; H, 4.89; N, 7.50; S, 17.72.
Elucidation of Signaling Mechanism by H NMR Spectros-
1
3+
copy. To have evidence for the Au -assisted conversion from 1 to 2,
1
H NMR spectra of 1, 2, and the reaction product of 1 with AuCl in
3
the presence of TPEN were obtained. To a solution of probe 1 (10
mM, in 50% aqueous acetonitrile) in the presence of TPEN (40 mM)
RESULTS AND DISCUSSION
■
Thiocoumarin derivative 1 was prepared by the reaction of
coumarin 6 (2) with Lawesson’s reagent following a previously
was added AuCl (20 mM), and the mixture was stirred at room
3
temperature. After 30 min, the volatiles were evaporated under
25
reported procedure (Scheme 1). The structural motif of
reduced pressure, and the residue was purified by column
1
chromatography (silica gel, eluant: dichloromethane). H NMR
3
+
spectra of probe 1, compound 2, and the reaction product of 1−
Scheme 1. Preparation of Thiocumarin 1 and its Au -
Selective Signaling
3+
TPEN−Au were measured in D O/DMSO-d (1:9, v/v).
2
6
Measurements of UV−Vis and Fluorescence Spectra. Stock
solutions of probe 1 (0.50 mM) and TPEN (10 mM) were prepared in
acetonitrile. Stock solutions of metal ions (10 mM) were prepared in
+
+
+
deionized water by dissolving perchlorate salts of alkali (Li , Na , K ),
2
+
2+
2+
3+
alkaline earth (Mg , Ca , Ba ), and transition metal ions (Fe ,
2+
2+
2+
2+
+
2+
2+
2+
Co , Ni , Cu , Zn , Ag , Cd , Pb , Hg ). Solutions (10 mM) of
Au and Au were prepared from their chloride salts in deionized
water.
+
3+
The UV−vis spectrum of the probe 1 was measured in the presence
and absence of metal ions. The measuring solution was prepared by
mixing stock solutions of probe 1 (30 μL), metal ions (30 μL), and
TPEN (60 μL) in 50% aqueous acetonitrile solution. Among the
tested pH values, pronounced signaling behavior was observed around
pH 4.7; thus, the solution was fixed at pH 4.7 by using an acetate
buffer solution. Final concentrations of probe 1, metal ions, and
acetate buffer were 5.0 μM, 0.10 mM, and 10 mM in water−
acetonitrile (1:1, v/v), respectively. After 1 min of each mixing, the
UV−vis spectrum of the solution was measured. The discrimination
coumarin 6 was selected as a probe because of its prominent
chromogenic change from pink to yellowish green and its
marked off-on type fluorogenic behavior due to the Au -
induced conversion of thiocarbonyl to carbonyl function.
Compound 1 had absorption bands at 366 and 520 nm in a
3
+
1
:1 mixture of acetonitrile and acetate buffer solution (10 mM,
pH 4.7). Upon treatment with various metal ions, prominent
changes in the absorption behavior were observed exclusively
with Au3+ and Hg ions (Figure 1 and Figure S1, Supporting
between Au3 and Hg was attempted by using 2 equiv (with respect
to the metal ions) of additional chelating agents of EDTA, 8-
hydroxyquinoline, cyclam, and TPEN. To compensate the inherent
+
2+
2+
3+
2+
2
+
2+
absorbances of some transition metal ions, such as Co , Cu , and
Information). With 20 equiv of Au or Hg ions, the
absorption band of 1 at 520 nm disappeared almost completely,
and a new band at 463 nm developed. Concomitantly, the
solution color changed from pink to yellowish green. The
absorbance ratio at the two characteristic wavelengths 520 and
3+
Fe ions, in the analytes, the same amount of TPEN was added to the
reference solution. The final concentration of added chelating agent
was 0.20 mM. The fluorescence spectrum of 1 upon treatment with
various metal ions in the absence and presence of TPEN was also
obtained under the same conditions by excitation at 485 nm.
Competition Signaling Behaviors. Under the same measure-
4
63 nm (A520/A4633) of 1 significantly changed from 3.29 to
+ 2+
ment conditions, competitive signaling behavior of 1 toward Au3+ ions
in the presence of coexisting metal ions as background was studied.
about 0.01 for Au and Hg ions (Figure S2, Supporting
Information). The Hg response of 1 was previously reported
by us, but the response of thiocoumarin toward Au is
unprecedented. Other metal ions revealed almost no responses,
and the ratio A520/A463 varied in a narrow range between 3.02
2+
25
3+
3+
Final concentrations of probe 1, TPEN, Au , and all the other
competing metal ions and anions were 5.0 μM, 0.20 mM, 0.10 mM,
and 0.10 mM, respectively, in a mixture of acetate buffered water (pH
4
.7, 10 mM)−acetonitrile solution (1:1, v/v).
+
2+
(
for Au ) and 3.47 (for Ba ). One more thing to note is that
Time Course Plot of Signaling. The time course of the signaling
probe 1 could discriminate the two common oxidation states of
gold(I) and gold(III): compound 1 exhibited a significant
response to Au3+ but almost no responses toward Au . On the
of 1 for Au3 ions was followed by measuring the absorbance of the
+
measuring solution at 463 nm. The concentrations of the probe 1,
+
2+
3+
TPEN, Hg , and Au ions were 5.0 μM, 0.20 mM, 0.10 mM, and
+
0.10 mM, respectively, in a mixture of acetate buffered water (pH 4.7,
other hand, Au ions are known to be unstable to
10 mM)−acetonitrile solution (1:1, v/v).
disproportionation to Au3+ and Au. We checked the possibility
2
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Inorganic Chemistry
Article
2+
the Hg -induced signaling was still significant (Figures S7 and
S8, Supporting Information).
Thiocoumarin 1 showed a weak emission at 511 nm due to
3
0
the presence of a strongly quenching thiocarbonyl function.
3
+
After treatment with metal ions, particularly with Au and
2+
Hg ions, prominently enhanced fluorescence of 1 at 511 nm
was observed (Figure S9, Supporting Information). On the
other hand, in the presence of TPEN, the response of 1 toward
Hg ions at 511 nm completely disappeared, resulting in an
exclusive Au selectivity with a 49.7-fold fluorescence
enhancement (Figure 3). Other metal ions did not induce
2+
3
+
Figure 1. UV−vis spectra of 1 in the presence of Au and Hg2+ ions.
3+
−6
n+
−4
[
1] = 5.0 × 10 M, [M ] = 1.0 × 10 M. In a mixture of CH CN
3
and acetate buffer solution (pH 4.7, 10 mM) (1:1, v/v).
of interference from this transformation by measuring the
+
responses toward Au after treatment of the solution at 50 and
8
0 °C for 2 h. There was no noticeable change in signaling
+
behavior of 1 toward Au solution at 50 °C; however, some
responses were observed at 80 °C (Figure S3, Supporting
Information). We also tested another Au species, NaAuCl4,
and found that there was no difference in signaling behavior of
2
9
3+
1
between the two species AuCl and NaAuCl (Figure S4,
3 4
Supporting Information).
Figure 3. Fluorescence spectra of 1 in the presence of various metal
3+
−6
n+
Based on this observation, we aimed to develop an Au -
selective signaling system using an extra additive that could
provide discrimination between the two responding metal ions
ions with additional chelating agent TPEN. [1] = 5.0 × 10 M, [M ]
−
4
−4
= 1.0 × 10 M, [TPEN] = 2.0 × 10 M. In a mixture of CH CN and
3
acetate buffer solution (pH 4.7, 10 mM) (1:1, v/v). λex = 485 nm.
of Au3 and Hg . Surveyed additives include representative
+
2+
2
+
any noticeable signaling, and the fluorescence intensity ratio of
chelating agents for Hg ions, such as TPEN, EDTA, 8-
hydroxyquinoline, and cyclam. Among the tested additives,
TPEN effectively suppressed the Hg2+ response of 1 (Figure 2
1 in the presence versus absence of metal ions I/I
at 511 nm
o
2+
varied in a narrow range between 0.92 for Ni and 1.58 for
2+
Hg (Figure S10, Supporting Information). Although using a
masking agent is less desirable for the practical applications in
terms of simplicity and convenience, these observations imply
3+
that a new Au -selective fluorescence signaling system can be
achieved using the heavy metal chelator TPEN as a masking
agent for the interfering metal ions.
3
+
The observed signaling is due to the Au -assisted
desulfurization of the thiocarbonyl function of 1, as has been
2
+
previously reported for Hg ions. Desulfurization of
2
+
3+
thiocarbonyl groups with Hg and Au ions is already
31
2+
known. However, in the presence of TPEN, Hg ions are
efficiently sequestered (K for Hg −TPEN = ∼10 M) to
suppress the desulfurization of 1, and the exclusive signaling of
Au was realized. It is known that Au also forms a complex
with TPEN; however, the stability of the complex is not
sufficiently high (K for Au −TPEN = ∼10 M), and thus
gold ions could react with the thiocarbonyl group of 1 under
2+
−25
32
d
3+
3+
3+
−10
33
d
Figure 2. UV−vis spectra of 1 in the presence of various metal ions
−
6
n+
with additional chelating agent TPEN. [1] = 5.0 × 10 M, [M ] =
−4
−4
the signaling conditions.
The postulated conversion was evidenced by H NMR, UV−
vis, and fluorescence measurements. The H NMR spectrum of
1
.0 × 10 M, [TPEN] = 2.0 × 10 M. In a mixture of CH CN and
3
1
acetate buffer solution (pH 4.7, 10 mM) (1:1, v/v).
1
3+
and Figure S5, Supporting Information). In the presence of
cyclam, quite effective suppression of Hg2+ signaling of 1 was
also observed (Figure S6, Supporting Information). With other
surveyed chelating agents of EDTA and 8-hydroxyquinoline,
the desired exclusive Au3+ selectivity was not realized because
reaction product of 1 with Au in the presence of TPEN (1−
3+
TPEN−Au ) was almost the same as that of 2 under the
measurement conditions (Figure 4). In addition, UV−vis and
fluorescence measurements of 1 in the presence of TPEN and
3+
Au ions also support the postulated desulfurization of 1 to 2
as depicted in Scheme 1.
2
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Article
1
3+
−3
Figure 4. Partial H NMR spectra of 1, the purified reaction product of 1−TPEN−Au , and 2. [1] = [2] = 5.0 × 10 M. In D O/DMSO-d (1:9,
2
6
v/v).
The quantitative analytical behavior of 1 in the signaling of
h after the sample preparation. To check the pH effect on the
Au3 ions was investigated by UV−vis titration. The large
+
signaling of Au ions by the 1−TPEN system, signaling
3+
3
+
difference in absorption bands of 1 induced by Au allowed a
convenient ratiometric analysis of the concentration-dependent
signaling behavior (Figure 5). A plot of the absorbance ratio at
measurements were carried out by using a series of buffer
3+
solutions (pH 4.0−9.0). The Au -selective signaling behavior
was not significantly affected in acidic conditions (pH 4.0−6.0)
without any noticeable interference from Hg2+ ions (Figure
S12, Supporting Information) as well as other metal ions.
3
+
Finally, we performed a competition experiment for Au
signaling of 1 in the presence of TPEN with potentially
interfering metal ions and anions as background. Alkali and
alkaline earth metal ions showed no significant interference
(
Figure 6). Potential interference from transition metal ions,
Figure 5. Changes in UV−vis spectra of 1−TPEN as a function of
3+
−6
−4
Au concentration. [1] = 5.0 × 10 M, [TPEN] = 2.0 × 10 M. In a
mixture of CH CN and acetate buffer solution (pH 4.7, 10 mM) (1:1,
3
v/v). Inset shows the ratiometric (A520/A463) behavior of 1 as a
3+
function of [Au ].
3
+
5
20 and 463 nm (A520/A463) as a function of [Au ] showed a
−6
3+
prominent change up to 4 × 10 M of Au . From this
concentration dependent response, the detection limit of 1 for
Figure 6. Competitive signaling of Au3+ by 1−TPEN in the presence
3
+
−7
the determination of Au ions was estimated as 1.1 × 10
M
−6
of representative metal ions as background. [1] = 5.0 × 10 M,
28
3
+
n+
−4
−4
in 50% aqueous acetonitrile solution.
[
Au ] = [M ] = 1.0 × 10 M, [TPEN] = 2.0 × 10 M. In a mixture
3
+
Signaling of Au by 1 was completed within 1 min under the
measurement conditions (Figure S11, Supporting Information).
Under the same conditions, however, thiocoumarin 1 was
stable and did not show any significant signaling at 24 h after
of CH CN and acetate buffer solution (pH 4.7, 10 mM) (1:1, v/v).
3
particularly Hg2+ ions, also was not observed. Representative
anions also induced no noticeable interferences for the selective
2
+
3+
sample preparation. Possibly interfering Hg ions also induced
signaling of Au by 1 (Figure S13, Supporting Information).
an insignificant response to 1 in the presence of TPEN even 24
All these observations clearly suggest that the designed
2
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Inorganic Chemistry
Article
thiocoumarin−TPEN system could selectively signal the
valuable Au3 ions in the presence of commonly coexisting
metal ions.
1169. (c) Pitcairn, I. K.; Warwick, P. E.; Milton, J. A.; Teagle, D. A. H.
+
Anal. Chem. 2006, 78, 1290−1295.
(
(
13) Jang, G.-G.; Roper, D. K. Anal. Chem. 2011, 83, 1836−1842.
14) Jun, M. E.; Roy, B.; Ahn, K. H. Chem. Commun. 2011, 7583−
7
601.
CONCLUSIONS
A simple thiocoumarin derivative was developed for a gold-
selective chemosignaling system. Au -induced desulfurization
■
(
15) (a) Yang, Y.-K.; Lee, S.; Tae, J. Org. Lett. 2009, 11, 5610−5613.
(
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3
+
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3
+
on”-type fluorescence signaling of Au ions in a mixed aqueous
2
010, 12, 932−934.
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3
+
3+
(17) Dong, M.; Wang, Y.-W.; Peng, Y. Org. Lett. 2010, 12, 5310−
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5
313.
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+
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(
19) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533−3539.
2
+
environment. Interference from Hg ions was successfully
eliminated by using a chelating agent TPEN. The detection
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918.
−7
limit was 1.1 × 10 M in 50% aqueous acetonitrile. Gold(III)
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induced transformation from thiocoumarin to coumarin was
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1
confirmed by H NMR measurements. The signaling was fast
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3
+
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ASSOCIATED CONTENT
Supporting Information
■
(
24) (a) Takeshita, K.; Ishida, K.; Nakano, Y.; Matsumura, T. Chem.
*
S
Lett. 2007, 36, 1032−1033. (b) Takeshita, K.; Matsumura, T.; Nakano,
Additional chemosignaling behavior, H NMR, and 13C NMR
spectra of 1 are reported. These materials are available free of
charge via the Internet at http://pubs.acs.org.
1
Y. Prog. Nucl. Energy 2008, 50, 466−469.
(25) Choi, M. G.; Kim, Y. H.; Namgoong, J. E.; Chang, S.-K. Chem.
Commun. 2009, 3560−3562.
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AUTHOR INFORMATION
Corresponding Author
E-mail: skchang@cau.ac.kr.
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Trans. 2 1982, 971−977. (c) Micallef, J. V.; Satchell, D. P. N. J. Chem.
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■
*
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2
010, 8, 3618−3620.
ACKNOWLEDGMENTS
■
(28) Shortreed, M.; Kopelman, R.; Kuhn, M.; Hoyland, B. Anal.
This work was supported by a fund from Korea Research
Foundation of Korean Government (2011-0004870).
Chem. 1996, 68, 1414−1418.
(
29) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th
ed.; John Wiley & Sons: New York, 1988; p 938. A reviewer suggested
the possibility of probe 1 for the monitoring of this Au(I) to Au(III)
transition. In fact, after 2 h at 80 °C, there were some changes in the
UV−vis spectral behavior of 1, which might be due to the transformed
Au(III) species (Figure S3, Supporting Information). We suppose that
the selective response of probe 1 toward Au(III), in principle, could be
applicable for the monitoring of the progress of the disproportionation
of Au(I).
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