Rhodamine-pyrene conjugated chemosensors for ratiometric detection of Hg2+ ions: Different sensing behavior between a spirolactone and a spirothiolactone

Article abstract of DOI:10.1016/j.dyepig.2013.02.019

Two novel rhodamine-pyrene conjugated chemosensors were successfully designed and synthesized, which exhibited high affinity to Hg²⁺ ions. As ratiometric chemosensors, each displayed highly selective and sensitive colorimetric and fluorogenic dual-responses toward Hg²⁺. The comparison of two chemosensors indicated that the spirothiolactone containing sensor was superior to the spirolactone analog in sensing behavior when detecting Hg²⁺, presumably due to the thiophilic nature of mercury and the different sensing mechanisms in operation. Upon interaction with Hg ²⁺, the spirothiolactone showed a 1:1 stoichiometry for the Hg ²⁺ complex, accompanied with a weakened fluorescence resonance energy transfer (FRET) behavior. However, the spirolactone was hydrolyzed and consequently induced the monomer-excimer switch of the resulted pyrene in the presence of Hg²⁺ ions.

Full text of DOI:10.1016/j.dyepig.2013.02.019

Accepted Manuscript
Rhodamine-pyrene conjugate chemosensors for ratiometric detection ofHg2+ions:
Different sensing behaviorsbetween aspirolactone and aspirothiolactone
Kai-Hui Chu, Yi Zhou, Yuan Fang, Li-Hong Wang, Ju-Ying Li, Cheng Yao
PII:
S0143-7208(13)00069-7
10.1016/j.dyepig.2013.02.019
DYPI 3866
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 19 December 2012
Revised Date: 26 February 2013
Accepted Date: 26 February 2013
Please cite this article as: Chu K-H, Zhou Y, Fang Y, Wang L-H, Li J-Y, Yao C, Rhodamine-pyrene
conjugate chemosensors for ratiometric detection ofHg2+ions: Different sensing behaviorsbetween
aspirolactone and aspirothiolactone, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.02.019.
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Rhodamine-pyrene conjugate chemosensors for ratiometric detection of
Hg2+ ions: Different sensing behaviors between a spirolactone and a
spirothiolactone
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Kai-Hui Chu, Yi Zhou*, Yuan Fang, Li-Hong Wang, Ju-Ying Li, Cheng Yao*
State Key Laboratory of Materials-Oriented Chemical Engineering and College of Science, Nanjing
University of Technology, Nanjing 211816, P. R. China
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*Corresponding author. Tel: +86-25-5813-9482; Fax: +86-25-5813-9482
E-mail: yaochengnjut@126.com (C. Yao); zhouyi624@126.com (Y. Zhou)
Abstract
Two novel rhodamine-pyrene conjugated chemosensors were successfully designed and
synthesized, which exhibited high affinity to Hg2+ ions. As ratiometric chemosensors, each displayed
highly selective and sensitive colorimetric and fluorogenic dual-responses towards Hg2+. The
comparison of two chemosensors indicated that the spirothiolactone containing sensor was superior
to the spirolactone analogue in sensing behavior when detecting Hg2+, presumably due to the
thiophilic nature of mercury and the different sensing mechanisms in operation. Upon interaction
with Hg2+, the spirothiolactone showed a 1:1 stoichiometry for the Hg2+ complex, accompanied with
a weakened fluorescence resonance energy transfer (FRET) behavior. However, the spirolactone was
hydrolyzed and consequently induced the monomer-exciemer switch of the resulted pyrene in the
presence of Hg2+ ions.
Key words: rhodamine; pyrene; mercury; fluorescent chemosensors; FRET; excimer emission
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1. Introduction
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As we know, in the environment, mercury pollution can be caused by natural phenomena and
human activities, including volcanic eruptions, wind erosion, water erosion, solid waste incineration
and industrial production [1]. Mercury can gain access to the body orally or dermally, and inorganic
mercury can be converted by bacteria into methyl mercury in the environment, which subsequently
bioaccumulates through the food chain. As one of the most highly toxic and hazardous contaminants,
once absorbed in the human body, even at very low concentrations, mercury can cause serious human
health problems, which results in permanent damages to the brain, nervous system, DNA, mitosis,
kidneys, and endocrine system [2]. Accordingly, the effective and selective detection of mercury for
biochemistry, environmental science and medicine, is highly desirable and indispensable [3].
Compared with conventional analyzing/detecting methods of Hg2+, such as atomic absorption
and emission spectroscopy, inductively coupled plasma mass spectrometry, selective cold vapor
atomic fluorescence spectrometry, neutron activation analysis, electrochemical sensoring, X-ray
microanalysis and a variety of potentiometric ion-selective electrodes, etc., analytical techniques
based on UV-vis and fluorescence spectroscopy are very popular in terms of nondestructive, rapid,
highly selective and sensitive detection. As a result, developing new practical and optical chemical
sensors (optodes) that exhibit dual-responsive chromogenic and fluorogenic detection of Hg2+ ions is
still a challenge [4].
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Rhodamine-based dyes are widely used for their excellent spectroscopic properties, such as
large absorption coefficient, high fluorescence quantum yield, absorption and emission at a long
wavelength [5]. Generally, in the presence of specific metal ions, the colorless and nonfluorescent
rhodamine spirolactam structure would be converted into the colored and highly fluorescent
ring-opened amide form, which provides not only excellent enhancement in absorption and
fluorescence intensity, but also direct visual detection [6]. Moreover, the long emission wavelength
(> 550 nm) can often be preferred to serve as reporting groups for analytes to avoid the influence of
background fluorescence (< 500 nm) [7]. Recently, many rhodamine-based chromogenic and
fluorogenic chemosensors have been extensively applied to selectively sense heavy metal ions, such
as Hg2+ ions, with an “off-on” mechanism [8]. On the other hand, the pyrene moiety is one of the
most useful fluorophores in the construction of fluorogenic chemosensors for a variety of important
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chemical species [9]. Pyrene is widely employed as ratiometric fluorescent chemosensors, due to its
well-known photophysical properties in monomer/excimer emission switching [10].
Yoon et al. reported a rhodamine-pyrene sensor which was designed as a ratiometric and
“off-on” sensor for the detection of Cu2+ [11]. Subsequently, Kim et al. developed a Hg2+-selective
probe with a calix[4]arene scaffold appended rhodamine and pyrene moiety [12]. Herein, considering
the optical and configurable characteristics of rhodamine and pyrene subunits, we design novel
ratiometric chromogenic and fluorogenic chemosensors RPS (spirothiolactone) and RPO
(spirolactone) (Scheme 1) for sensitive, selective and reversible detection of Hg2+ in aqueous medium,
introducing a pyrene moiety into the rhodamine fluorophore. The two chemosensors are structurally
similar, but they show different sensing mechanisms upon the interaction with Hg2+.
<Place Scheme 1 here.>
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2. Experimental
2.1 Apparatus and reagents
UV-visible spectra and fluorescence spectra were recorded on a Perkin-Elmer 35 spectrometer
and a Perkin-Elmer LS 50B fluorescence spectrophotometer at room temperature, respectively.
1H-NMR and 13C-NMR were measured on a BrukerAV-500 or BrukerAV-300 spectrometer with
chemical shifts reported in ppm (in CDCl3; TMS as internal standard). Electrospray ionization mass
spectra (ESI-MS) were measured on a Micromass LCTTM system. All pH measurements were made
with a Sartorius basic pH-Meter PB-10. Melting points were determined on a hot-plate melting point
apparatus XT4-100A and were uncorrected. Chromatographic separations were done by column
chromatography using 200-300 mesh silicagel.
Unless otherwise noted, materials were obtained from commercial suppliers and were used as
received. All the solvents were of analytic grade. Water was re-distilled. The salts used in stock
solutions of metal ions were NaNO3, KNO3, Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, FeCl2·4H2O,
MnCl2·4H2O, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O,
AgNO3, Hg(ClO4)2·3H2O, Pb(NO3)2 and CrCl3·6H2O. Rhodamine B and 1-pyrenecarboxaldehyde
were purchased from Alfa Aesar. The other reagents were purchased from Taiyuan RHF Reagents
Ltd.
2.2 General procedures of spectra detection
Stock solutions (10-2 M) of the nitrate salts (or chlorate salts) of Na+, K+, Mg2+, Ca2+, Fe2+, Mn2+,
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Ni2+, Co2+, Cu2+, Zn2+, Cd2+, Ag+, Hg2+, Pb2+ and Cr3+ in water were prepared. Stock solutions
(1.0×10-4 M) of RPS and RPO were prepared in ethanol. Before spectroscopic measurements, the
solution of RPS and RPO was freshly prepared by diluting the high concentration stock solution to
corresponding solution. All the measurements were made according to the following procedure. To
10 mL glass tubes containing different amounts of metal ions, appropriate amounts of solution of
chemosensors RPS or RPO were added directly by pipette, then diluted with ethanol and deionized
water to 10 mL (EtOH/H2O = 1:1, v/v) and mixed, then the absorption and fluorescence sensing of
metal ions were run. In selectivity experiments, the test samples were prepared by placing the
appropriate amounts of metal ions stock solution into 10 mL solution of RPS (3 μM) or RPO (5 μM).
All samples containing RPS and RPO were prepared at room temperature and shaken for 10 s. Then
waited for 5 min and 80 min respectively before UV-vis and fluorescence spectra measurements.
Excitation wavelength was 365 nm for RPS, 365 nm and 515 nm for RPO.
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The wide pH range solutions were prepared by adjustment of HCl or NaOH solutions.
2.3 Synthesis
2.3.1 Synthesis of Rhodamine-B hydrazide 4
Rhodamine B hydrazide was synthesized according to the reported method [13]. 1H-NMR (500
MHz, CDCl3): δ = 7.93 (d, J = 5.4 Hz, 1H, Ar-H), 7.44 (d, J = 3.6 Hz, 1H, Ar-H), 7.42 (d, J = 3.6 Hz,
1H, Ar-H), 7.10 (d, J = 5.6 Hz, 1H, Ar-H), 6.46 (d, J = 8.0 Hz, 2H, Xanthene-H), 6.42(d, J = 3.2 Hz,
2H, Xanthene-H), 6.29 (m, 2H, Xanthene-H), 3.60 (s, 2H, -NH2), 3.34 (q, J = 7.0 Hz, 8H, -CH2-),
1.16 (t, J = 7.0 Hz, 12H, -CH3).
2.3.2 Synthesis of Thiooxorhodamine-B hydrazide 3
Rhodamine B hydrazide (5.0 mmol, 2.28 g) and Lawesson’s Reagent (5.0 mmol, 2.03 g) were
dissolved in dry toluene (60 mL), and the reaction mixture was refluxed at 85°C for 24 h under N2
atmosphere and evaporated under reduced pressure. The residue was added to K2CO3 saturated
solution and stirred for 2 h at room temperature, and then extracted by CH2Cl2. After removal of
CH2Cl2, the residue was purified by flash chromatography (CH2Cl2 / petroleum, 4:1, Rf = 0.4) as
eluent to afford Thiooxorhodamine B hydrazide (0.87 g, yield: 37%). M.p. 185.4~186.7 °C. Rf = 0.4
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(SiO2; CH2Cl2 / petroleum, 4:1). H-NMR (500 MHz, CDCl3): δ = 8.09 (d, J = 7.2 Hz, 1H, Ar-H),
7.46 (m, 2H, Ar-H), 7.10 (d, J = 7.2Hz, 1H, Ar-H), 6.43 (s, 2H, xanthene-H), 6.35 (d, J = 8.8Hz, 2H,
xanthene-H), 6.27 (m, 2H, xanthene-H), 4.81 (s, 2H, -NH2), 3.34 (q, J = 7.0Hz, 8H, -CH2-), 1.16 (t, J
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= 7.0Hz, 12H, -CH3). C-NMR (75 MHz, CDCl3): 182.93, 153.45, 149.26, 149.20, 136.36, 131.95,
128.50, 128.02, 124.53, 123.05, 108.02, 103.34, 97.98, 44.34, 12.55. Anal. Calcd for C28H32N4OS: C,
71.15; H, 6.82; N, 11.85. Found: C, 71.24; H, 6.86; N, 11.77. TOF-MS: m/z 473.16 [M+H]+.
2.3.3 Synthesis of chemosensor 2 (RPO)
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Rhodamine B hydrazide 4 (1.0 mmol, 0.46 g) and 1-Pyrenecarboxaldehyde (1.0 mmol, 0.23 g)
were dissolved in anhydrous boiling methanol (20 mL), the reaction mixture was refluxed at 65 °C
for 12 h under N2 atmosphere. The resulting yellow precipitate was filtered and washed with cold
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methanol to afford a yellowish solid of RPO (0.42 g, yield: 63%). M.p. 192.1~192.7 °C. H-NMR
(300 MHz, CDCl3): δ = 9.64 (s, 1H, imine-H), 8.45-8.47 (d, J = 8.1, 1H, Ar-H), 8.15-8.18 (d, J = 9.3,
1H, Ar-H), 8.07-8.09 (d, J = 7.5, 3H, Ar-H), 7.99-8.02 (d, J = 8.1, 1H, Ar-H), 7.89-7.96 (m, 4H,
Ar-H), 7.48-7.58 (m, 2H, Ar-H), 7.19-7.21 (d, J = 7.2, 1H, Ar-H), 6.63-6.66 (d, J = 9, 2H, Ar-H),
6.56 (s, 2H, Ar-H), 6.26-6.29 (d, J = 8.7, 2H, Ar-H), 3.31 (q, J = 6.6, 8H, -CH2-), 1.12 (t, J = 6.9, 12H,
-CH3). 13C-NMR (300 MHz, CDCl3): 165.00, 153.21, 151.85, 148.98, 145.54, 133.36, 132.05,
131.18, 130.55, 129.52, 129.30, 128.33, 128.23, 128.04, 127.90, 127.37, 125.81, 125.42, 125.10,
125.04, 124.78, 124.58, 124.48, 123.92, 123.38, 122.79, 108.22, 106.09, 97.96, 66.11, 44.32, 12.57.
Anal. Calcd for C45H40N4O2: C, 80.81; H, 6.03; N, 8.38. Found: C, 80.89; H, 6.08; N, 8.31. TOF-MS:
m/z 669.2 [M+H]+.
2.3.4 Synthesis of chemosensor 1 (RPS)
The synthesis of chemosensor RPS is similar to that of chemosensor RPO. Thiooxorhodamine
B hydrazide (1.0 mmol, 0.47 g) and 1-Pyrenecarboxaldehyde (1.0 mmol, 0.23 g) were dissolved in
anhydrous boiling methanol (20 mL), and the reaction mixture was refluxed at 65 °C for 12 h under
N2 atmosphere. The resulted yellow precipitate was filtered and washed with cold methanol to afford
a yellowish solid of RPS (0.40 g, yield: 58%). M.p. 211.8~213.1 °C. 1H-NMR (300 MHz, CDCl3): δ
= 1.16 (t, J = 6.9, 12H, -CH3), 3.32 (q, J = 6.6, 8H, -CH2-), 6.35 (s, 4H, Ar-H), 6.85-6.87 (d, J = 8.4,
2H, Ar-H), 7.16-7.19 (m, 1H, Ar-H), 7.46-7.48 (t, J = 3.3, 2H, Ar-H), 7.98-8.06 (m, 2H, Ar-H),
8.10-8.15 (t, J = 8.1, 2H, Ar-H), 8.19-8.22 (d, J = 9, 4H, Ar-H), 8.58-8.61 (d, J = 8.1, 1H, Ar-H),
9.05-9.08 (d, J = 9.3, 1H, Ar-H), 9.58 (s, 1H, imine-H). 13C-NMR (500 MHz, CDCl3): 158.31, 155.49,
151.90, 148.27, 135.38, 133.08, 132.19, 131.20, 130.64, 130.32, 130.15, 128.97, 128.69, 128.44,
127.86, 127.42, 127.39, 127.17, 126.99, 126.06, 125.88, 125.75, 124.91, 124.83, 124.51, 123.57,
122.37, 110.62, 108.32, 97.57, 44.35, 12.62. Anal. Calcd for C45H40N4OS: C, 78.92; H, 5.89; N, 8.18.
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Found: C, 79.01; H, 5.94; N, 8.11. TOF-MS: m/z 685.3 [M+H]+.
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3. Results and discussion
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Chemosensors RPS and RPO are Schiff bases and both contain a rhodamine uint and a pyrene
moiety. RPS was synthesized by the condensation of thiooxorhodamine-B hydrazide 3 with
1-formylpyrene in 58% yield, and the reaction between rhodamine-B hydrazide 4 and
1-formylpyrene afforded RPO in 63% yield (Scheme 1).
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<Place Figure 1 here.>
As shown in Fig. 1, the UV-vis properties of RPS and RPO were investigated towards Hg2+.
Without Hg2+, both chemosensors showed weak absorption over 550 nm, which indicated that the
spirolactam rhodamine moiety was the dominant species. Upon addition of Hg2+, a new absorption
band centered at 560 nm (RPS, ε = 9.37×104 L·mol-1·cm-1; RPO, ε = 6.36×104 L·mol-1·cm-1)
appeared and the absorption coefficient increased significantly, due to the ring-opening of the
rhodamine spirolactam. And the ring-opening mechanism was also responsible for the color changes
from faint yellow to pink (RPS) and from colorless to purplish red (RPO), which were visually
perceptible. Meanwhile, a typical pyrene absorption band appeared at 326 nm. In Fig. 1a, the
absorbance of RPS at 326 nm and 424 nm decreased gradually, however, in Fig. 1b, the absorbance
of RPO at 378 nm increased with the increasing amount of Hg2+, which might be attributed to the
influence of Hg2+ on the pyrene moiety. The two insets of Fig. 1 showed the absorbance at 560 nm as
a function of Hg2+ concentrations, respectively.
<Place Figure 2 here.>
<Place Scheme 2 here.>
The fluorescence characteristics of the two chemosensors were also investigated. Before adding
Hg2+, free RPS showed a strong emission band centered at 454 nm which might be attributed to the
pyrenyl excimer emission (Fig. 2a), but free RPO displayed no obvious emission at this wavelength
(Fig. 2b). As depicted in Fig. 2a, on the gradual addition of Hg2+ to a solution of RPS, a new
fluorescence emission band centered at 594 nm appeared, which was due to the ring-opening of the
rhodamine spirolactam. Subsequently, the emission band changed with a blue shift in the maximum
to 582 nm, which might be accounted for the intermolecular electron transfer between RPS and Hg2+.
The observed pyrenyl excimer emission at 454 nm concomitantly declined slightly, which might be a
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result of Hg2+-induced weak change of the stacked or folded conformation of the pyrene (Scheme 2).
An isosbestic point centered at 537 nm was observed, and the fluorescence intensity ratios at I582/I454
changed from 0.09 to 2.79, which suggested that RPS would be useful for quantitative determination
of Hg2+ concentrations over a large dynamic range. This ratiometric fluorescence change also
denoted that there existed a fluorescence resonance energy transfer (FRET) behavior from the pyrene
excimer to rhodamine moiety. The titration curve can be used as the calibration curve for the
detection of Hg2+, from which the LOD and LOQ were approximately inferred as 4.34 × 10-7 mol·L-1
(S/N = 3) and 1.45 × 10-6 mol·L-1 (S/N = 10) respectively, as well as the association constant (Ka) of
RPS with Hg2+ was 1.44 × 106 M-1 (error < 10%).
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In the case of RPO, remarkable changes of the fluorescence intensity at 452 nm (λex = 365 nm)
(Fig. 2b) and 576 nm (λex = 515 nm) (Fig. 2c) were observed upon the addition of Hg2+. The former
change might be interpreted as the result of Hg2+-induced monomer/excimer switch of the pyrene
moiety. The latter change was similar to that of RPS at 594 nm, both resulted from the Hg2+-induced
ring-opening of the rhodamine spirolactam. It should be noted that there was no efficient FRET
behavior observed in the RPO system on addition of Hg2+. The insets in Fig. 2b and Fig. 2c exhibited
the dependence of fluorescence intensities at 452 nm (b) and at 576 nm (c) on Hg2+ concentrations,
respectively. From the titration results, the LOD was estimated as 1.91 × 10-5 mol·L-1 (S/N = 3), as
well as the LOQ was 6.37 × 10-5 mol·L-1 (S/N = 10).
<Place Figure 3 here.>
<Place Scheme 3 here.>
The Job’s plot analysis exhibited a maximum at about 0.5 mol fraction of Hg2+ indicating a 1:1
stoichiometry for the RPS/Hg2+ complex (Fig. 3). However, on the addition of Hg2+, RPO was first
hydrolysed to compound 5, and then hydrolysed to rhodamine-B, which could be illustrated in
Scheme 3. The proposed hydrolytic mechanism was confirmed by ESI-MS (Fig. S8). In the absence
of Hg2+, the m/z 669.2 peak corresponded to [RPO+H]+ (Fig. S7), when introducing Hg2+ into the
RPO ethanol aqueous solution (EtOH/H2O, 1:1), the peak at m/z 669.2 disappeared, and new peaks
at m/z 457.1 and m/z 443.1 appeared, which were assigned to compound 5 and rhodamine-B,
respectively (Fig. S8). The hydrolysis reaction might induce the conformational changes of the
resulted pyrene moiety from the weak pyrene monomer emission to strong pyrene excimer emission.
<Place Figure 4 here.>
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The fluorescence spectra of RPS and RPO in the presence of several other metal cations,
including K+, Na+, Ca2+, Mg2+, Cr3+, Ag+, Cu2+, Co2+, Ni2+, Cd2+, Mn2+, Fe2+, Zn2+ and Pb2+, were
investigated in EtOH/H2O (1:1, v/v, pH = 7.0) (Fig. 4). While the introduction of 3 μM Hg2+ into
RPS ethanol aqueous solution (3 μM) induced a remarkable change in the fluorescence intensity, the
addition of other metal ions caused very weak changes, even at the mM level, such as alkali metal
ions (K+, Na+) and alkali-earth metal ions (Ca2+, Mg2+). In terms of RPO (1 μM), the addition of
various metal ions caused much weaker changes except Hg2+ (100 μM), which resulted in a 230-fold
enhancement of fluorescence. This change is similar to that of RPS. Competition experiments were
also conducted to corroborate a potential applicability of chemosensors RPS and RPO for the
selective detection of Hg2+ ions in EtOH/H2O (1:1, v/v, pH = 7.0) upon various cations. In the case of
RPO, no significant changes were found, in addition to the solution mixed with Mn2+, which reduced
the fluorescence intensity. Nevertheless, these results indicated that the competed ions would not
interfere with the recognition of Hg2+ by RPS and RPO, thus it was demonstrated that both RPS and
RPO could be served as potential Hg2+-selective chemosensors.
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<Place Figure 5 here.>
To examine the reversibility of the proposed RPS/Hg2+ species and RPO/Hg2+ species,
Na2S-addition experiments were conducted. The addition of 0.3 mL Na2S restored the initial value of
free RPS since the Kd value of [HgS2]2- was 10-50 M2, which was much higher than Ka [RPS/Hg2+]
[14], and the initial color pink faded (Fig. 5a). However, when RPO interacted with Hg2+ completely,
the addition of excess Na2S made no changes of the solution, neither in the fluorescence intensity nor
in the color change (Fig. 5b and Fig. 5c). The irreversibility of RPO/Hg2+ corroborated the hydrolytic
mechanism of RPO with Hg2+.
<Place Figure 6 here.>
In addition, for practical applications, the effect of pH on the fluorescence emission of RPS and
RPO in the absence and presence of Hg2+ with different pH conditions in EtOH/H2O (1:1, v/v) was
also explored (Fig. 6). The solution of RPS exhibited very weak fluorescence and elicited negligible
changes between pH 5.5 and 12.0. Compared with free RPO, the fluorescence intensity increased
dramatically in the pH range of 3.0 ~ 9.0 when interacted with Hg2+. Between pH 6.5 and 9.0, the
intensity was relatively stable, but decreased at pH > 9.0 (Fig. 6a). The solution of free RPO
displayed almost no fluorescence with the change of pH value, whereas, it exhibited apparent
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fluorescence emission in the presence of Hg2+, and the fluorescence intensity enhanced slightly with
the increase of pH value, but changed little between 6.5 and 8.0 (Fig. 6b). All these results
demonstrated that both chemosensor RPS and RPO could be applied in recognizing Hg2+ under near
neutral pH conditions.
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<Place Figure 7 here.>
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The RPS sensor exhibited a rapid fluorescence response to Hg2+. All the tests of RPS with Hg2+
were conducted in 5 min after the addition of Hg2+. Nevertheless, the response of RPO to Hg2+ was
not as quick as that of RPS to Hg2+. The time dependence of the fluorescence of RPO was illustrated
in Fig. 7. The fluorescence intensity of RPO with Hg2+ leveled off after about 80 min, which was
much slower than that of RPS. For practical application, chemosensor RPS was also employed to
detect Hg2+ by doping tap water with Hg2+ (Table 1).
<Place Table 1 here.>
Keeping all the aforementioned results in mind, we would find that chemosensor RPS is
superior to RPO with the following considerations: (i) the detection limit of RPS (4.34 × 10-7 mol·L-1)
is much lower than that of RPO (1.91 × 10-5 mol·L-1); (ii) the response time of RPS to Hg2+ is much
shorter than that of RPO; (iii) the reversibility of RPS/Hg2+ has an advantage over RPO/Hg2+; (iv) a
FRET behavior was observed in the combination of RPS with Hg2+, which could not be obtained in
the combination of RPO with Hg2+; (v) upon the interaction with Hg2+, RPS showed a 1:1
stoichiometry for the RPS/Hg2+ complex, however, RPO was hydrolyzed; (vi) chemosensor RPS can
be well applied in the water sample for the detection of Hg2+.
4. Conclusion
In summary, two novel rhodamine-pyrene derivatives RPS and RPO have been successfully
designed as ratiometric chromogenic and fluorogenic chemosensors for sensitive, selective, and
reversible detection of Hg2+ in aqueous medium. The significant color changes could be used for
direct visual detection. The stoichiometry of the RPS/Hg2+ complex is 1:1. The FRET efficiency
changed during the combination of RPS with Hg2+, whereas, a hydrolytic reaction occurred when
adding Hg2+ into the ethanol aqueous solution of RPO. To detect Hg2+, RPS is superior to RPO for
its lower detection limit, shorter response time and advantageous reversibility, presumably due to the
thiophilic nature of mercury and the different sensing mechanisms in operation. And RPS can be well
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applied in the water sample for the detection of Hg2+.
Acknowledgments
This work is supported by the Open Fund of the State Key Laboratory of Materials-Oriented
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Chemical Engineering, the postgraduate practice innovation fund of Jiangsu province
(CXZZ12-0444), and the doctor thesis innovation fund of Nanjing University of Technology.
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Figure captions
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Figure 1. (a) UV-vis spectra of RPS (3 μM) in the presence of different concentrations of Hg2+ (0-4
μM) in EtOH/H2O (1:1, v/v, pH = 7.0). (b) UV-vis spectra of RPO (5 μM) in the presence of different
concentrations of Hg2+ (0-500 μM) in EtOH/H2O (1:1, v/v, pH = 7.0). Insets: the absorbance at 560
nm as a function of Hg2+ concentrations.
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Figure 2. (a) Fluorescence titration of RPS (3 μM) in EtOH/H2O (1:1, v/v, pH = 7.0) in the presence
of different concentrations of Hg2+ (0-4 μM). Inset: the ratio of fluorescence intensity at 582 nm and
454 nm as a function of Hg2+ concentrations. Excitation wavelength was 365 nm. (b) and (c)
Fluorescence titration of RPO (1 μM) in EtOH/H2O (1:1, v/v, pH = 7.0) in the presence of different
concentrations of Hg2+ (0-300 μM). (b) The fluorescence spectra of RPO with the fluorescence
enhancement at 452 nm upon the excitation at 365 nm. Inset: fluorescence intensity at 452 nm as a
function of Hg2+ concentrations. (c) The fluorescence spectra of RPO with the fluorescence
enhancement at 576 nm upon the excitation at 515 nm. Inset: fluorescence intensity at 576 nm as a
function of Hg2+ concentrations.
Figure 3. The Job’s plot of chemosensor RPS, with a total concerntration of ([RPS] + [Hg2+]) = 10
μM. The detection wavelength was 560 nm.
Figure 4. (a) Fluorescence responses of 3 μM RPS to various 10 μM transition metal ions (1 mM for
alkali and alkali-earth metal ions). Spectra were acquired in EtOH/H2O (1:1, v/v, pH = 7.0). Bars
represent the ratio of fluorescence intensity at 582 nm (F582) over that at 454 nm (F454). The black
bars represent the fluorescence emission of 3 μM RPS solution with different competing metal ions.
The red bars represent the change of the emission that occurs on the subsequent addition of 3 μM
Hg2+ to the above solutions. (b) Fluorescence responses of 1 μM RPO to various 10 μM transition
metal ions (1 mM for alkali and alkali-earth metal ions). Spectra were acquired in EtOH/H2O (1:1,
v/v, pH = 7.0). Bars represent the final (Ff) over the initial (Fi) integrated emission. The black bars
represent the fluorescence emission of 1 μM RPO solution with different competing metal ions. The
red bars represent the change of the emission that occurs on the subsequent addition of 100 μM Hg2+
to the above solution.
Figure 5. (a) Reversibility of Hg2+ to RPS by Na2S. Red line: free RPS (3 μM), blue line: RPS + 1
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equiv of Hg2+, black line: RPS + 1 equiv of Hg2+ + 0.3 mL of Na2S (0.1 mM). (b) and (c)
Reversibility of Hg2+ to RPO by Na2S. Red line: free RPO (1 μM), blue line: RPO + 100 equiv of
Hg2+, black line: RPO + 100 equiv of Hg2+ + 0.6 mL of Na2S (0.1 mM). (b) The change of
fluorescence intensity at 452 nm, λex = 365 nm. (c) The change of fluorescence intensity at 576 nm,
λex = 515 nm.
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Figure 6. (a) Fluorescence intensity (582 nm) of RPS (3 μM) in the absence (red line) and presence
(black line) of 1 equiv Hg2+ in EtOH/H2O (1:1, v/v) at different pH. (b) Fluorescence intensity (576
nm) of RPO (1 μM) in the absence (red line) and presence (black line) of 100 equiv Hg2+ in
EtOH/H2O (1:1, v/v) at different pH.
Figure 7. Time dependence of the fluorescence of RPO with Hg2+ in EtOH/H2O (1:1, v/v, pH = 7.0).
Scheme 1. Synthesis of RPS and RPO
Scheme 2. Proposed mechanism of RPS with Hg2+.
Scheme 3. Proposed hydrolytic mechanism of RPO with the addition of Hg2+.
Table 1. Determination of the concentrations of Hg2+ in water samples by fluorescent method using
RPS.
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Table 1.doc
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Table 1. Determination of the concentrations of Hg2+ in water
samples by fluorescent method using RPS.
Addition of Hg2+
Measured value of Hg2+
I582nm / I454nm
Recovery
/10-6 M
2.4
/10-6 M
2.32
1.241
2.473
96.67%
95.62%
3.2
3.06

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Highlights.doc
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Highlights
 Two chemosensors, differentiated by a spirothiolactone and a spirolactone unit, based
on a rhodamine-pyrene platform were synthesized.
 With the addition of Hg2+, the spirothiolactone sensor exhibited a weakened FRET
behavior, while the spirolactone chemosensor was hydrolyzed inducing a large
monomer-excimer switch.
 Both chemosensors exhibited colorimetric and fluorescent dual-responses towards
Hg2+.
 Both chemosensors could be used for ratiometric detection of Hg2+.