A highly selective and sensitive rhodamine-derived fluorescent probe for detection of Cu2?+

Article abstract of DOI:10.1016/j.saa.2017.02.053

A novel water-soluble and reversible fluorescent probe was designed and synthesized based on a rhodamine B derivative. It was used for detection of Cu^2?+ in drinking water and in living cells with high sensitivity and excellent selectivity. The tested concentration range and the limit of detection (LOD) of the probe were 0–15.00?μmol?L^??1 and 0.085?μmol?L^??1, respectively. In addition, the mode of binding and mechanism of interaction between the probe and Cu^2?+ were analyzed by density functional theory (DFT) calculations.

Full text of DOI:10.1016/j.saa.2017.02.053

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 179 (2017) 221–226
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A highly selective and sensitive rhodamine-derived fluorescent probe for
detection of Cu2+
⁎
Linlin Lv, Quanping Diao
School of Chemistry and Life Science, Anshan Normal University, Ping'an Street 43, Anshan 114005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel water-soluble and reversible fluorescent probe was designed and synthesized based on a rhodamine B
derivative. It was used for detection of Cu2+ in drinking water and in living cells with high sensitivity and excel-
lent selectivity. The tested concentration range and the limit of detection (LOD) of the probe were 0–
15.00 μmol L−1 and 0.085 μmol L−1, respectively. In addition, the mode of binding and mechanism of interaction
between the probe and Cu2+ were analyzed by density functional theory (DFT) calculations.
© 2017 Elsevier B.V. All rights reserved.
Received 23 December 2016
Received in revised form 14 February 2017
Accepted 22 February 2017
Available online 24 February 2017
Keywords:
Fluorescence
Probe
Rhodamine
Cu2+
DFT
Living cells
Drinking water
1. Introduction
28]. In addition, the development of novel fluorescent probes has also
boosted the development of diagnostic techniques for imaging live bio-
As a soft transition metal that plays pivotal roles in various physio-
logical processes, the content of copper (Cu) is lower than that of iron
and zinc in the body [1]. However, excessive accumulation of Cu leads
to severe toxicity [2,3]. For example, short-term exposure to an environ-
ment with high concentration of Cu2+ causes gastrointestinal discom-
fort in the human body, while a long-term exposure can lead to liver
and kidney damage [4]. Therefore, the World Health Organization
(WHO) and the United States Environmental Protection Agency (EPA)
have set the maximum allowable level (MAL) of Cu2+ in drinking
water at 2.0 mg L−1 and 1.3 mg L−1, respectively [5]. Currently, the
methods for detection of Cu2+ mainly include [6–8](AAS), atomic emis-
sion spectrometry (AES) [9,10], atomic fluorescence spectrometry (AFS)
[11], and inductively coupled plasma–mass spectrometry (ICP–MS) [12,
13], and so on. However, these methods require expensive equipment
and complicated sample preparation procedures, and therefore, cannot
be used for detection of Cu2+ in living tissues. Hence, it is important to
develop a simple method for rapid estimation of Cu2+ concentration in
drinking water and in the human body.
medical and medical samples [29–32]. Among the various known fluo-
rescent dyes, rhodamine and its derivatives received great attention
owing to their excellent photo-physical properties such as high fluores-
cence quantum yield, excitability by visible light, relatively long emis-
sion wavelength, and superior photo-stability [33,34]. Generally, the
molecular structure of rhodamine derivatives can be changed from the
spiro-ring to the open-ring forms by altering the chemical environment,
and hence these molecules exhibit distinct fluorescence properties.
As early as 1997, Czarnik et al. designed and synthesized the first
fluorescent probe with a rhodamine B hydrazide structure for detection
of Cu2+ [35]. It was found that the formation of coordination complexes
between rhodamine hydrazide and Cu2+ led to the ring opening of
spirolactam, which could be further hydrolyzed to produce a strong
fluorescent emission spectrum in a neutral HEPES buffer solution ac-
companied by color change of the solution. Later in 2006, Toon et al. in-
troduced a salicylaldehyde group based on the structure of rhodamine
hydrazide to synthesize the fluorescent probe salicylaldehyde rhoda-
mine B hydrazine for the detection of Cu2+ [36]. This probe greatly en-
hanced the ability of the system to complex with Cu2+, with a
complexation constant above 106 L mol−1. The lowest detection limit
of Cu2+ achieved micromolar levels in acetonitrile and Tris-HCl buffered
solution (pH = 7.0) without the interference of other ions. Thereafter,
studies on the design and synthesis of Cu2+-selective fluorescent
probes utilizing rhodamine derivatives has received increased attention
of scientific researchers. Recently, researchers have developed better
Currently, fluorescent probe sensing is one of the most rapid and de-
veloped detection methods for metal ions [14–24]. Compared to the tra-
ditional detection methods, it shows several significant advantages such
as low cost, ease of operation, high sensitivity, and fast response [25–
⁎
Corresponding author.
E-mail address: quanpingdiao@163.com (Q. Diao).
http://dx.doi.org/10.1016/j.saa.2017.02.053
1386-1425/© 2017 Elsevier B.V. All rights reserved.

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L. Lv, Q. Diao / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 179 (2017) 221–226
recognition sites by changing the structure of the probe, continuously
improved the sensitivity and selectivity of the probe, and expanded its
applications [37–48]. In this study, we designed and synthesized a
novel fluorescent probe (RBA) based on rhodamine B and 3-formyl-2-
hydroxybenzoic acid for detection of Cu2+. Compared with the reported
probe, the probe RBA presented in this work provides better selectivity
and sensitivity, and was further applied for practical samples such as
drinking water and living cells.
2.3.2. Synthesis of Compound 2
Compound 2 was synthesized from Rhodamine B according to the
reported method [36]. 1.2 g of Rhodamine B (2.5 mmol) and 50 mL of
ethanol were placed into a 100 mL bottom flask. Then 4.0 mL of hydra-
zine hydrate (excess) was added into the flask. The mixture was
refluxed for 6 h till the color of the solution was changed from dark pur-
ple to light orange. Then the mixture was cooled and the solvents were
removed under reduced pressure. 60 mL of 1 mol L−1 HCl was added to
the flask to generate a clear red solution. After that, 1 mol L−1 NaOH was
added slowly with stirring until the pH of the solution reached 9. The
precipitate produced was filtered and washed 3 times with water, and
dried over P2O5 under vacuum. The 0.78 g of compound 2 as pink
solid (yield 68%) was obtained. 1H NMR (300 MHz, d6-DMSO) δ 7.87–
7.66 (m, 1H), 7.56–7.39 (m, 2H), 7.07–6.91 (m, 1H), 6.36 (d, 6H), 3.32
(dd, 8H), 1.08 (t, 12H).
2. Experimental Details
2.1. Chemicals and Reagents
Rhodamine B (96%) and salicylic acid (99%) were purchased from
J&K Chemical Ltd. Salts of all the cations, that is, Cu(ClO4)2·H2O,
Mg(ClO4)2·6H2O, Zn(ClO4)2·6H2O, Co(ClO4)2·6H2O, Cd(ClO4)2·H2O,
Ni(ClO4)2·6H2O, Mn(ClO4)2·6H2O, Ba(ClO4)2·6H2O, Ca(ClO4)2·6H2O,
Fe(ClO4)3·6H2O, and Hg(ClO4)2·xH2O were purchased from Energy
Chemical Reagent Co., Ltd. Other reagents used here were analytical re-
agent grade and used without further purification or treatment. All
aqueous solution was prepared with ultrapure water with Milli-Q
water purification system (18.2 MΩ cm).
2.3.3. Synthesis of Compound RBA
0.17 g of compound 1 (1 mmol) was added in 25 mL of absolute eth-
anol containing 0.46 g of compound 2 (1 mmol). The resulting mixture
was refluxed and stirred for 24 h. The solvent was removed under re-
duced pressure to give a purple solid. The crude product was purified
by chromatography on silica gel using CH2Cl2:methanol (10:1, v/v) as
an eluent. The 0.46 mg of RBA as purple solid (yield 76%) was obtained.
ESI-MS: m/z 605.3 for [RBA + H]+. 1H NMR (300 MHz, CDCl3) δ 8.56 (s,
1H), 8.06 (dd, 1H), 7.60–7.44 (m, 2H), 7.27–7.08 (m, 2H), 6.73 (dd, 4H),
6.41 (s, 2H), 3.35 (dd, 8H), 1.17 (t, 12H).
2.2. Instruments
UV–Vis and fluorescence spectra were recorded on a Cary 60 spec-
trophotometer (Agilent Technologies, USA) and a Cary Eclipse
spectrofluorophotometer (Agilent Technologies, USA), respectively.
Spectra of 1H NMR (TMS as internal standard) were measured on a Mer-
cury 300BB nuclear magnetic resonance spectrometer (Varian Inc.,
USA). MS spectra were obtained by a LC/MS QTRAP spectrometer
(SCIEX Inc., USA). All pH measurements were made with a PHS-3C
pH-Meter (INESA Scientific Inc., China). Living cells were imaged by
an Olympus IX 71 inverted fluorescence microscopy (Olympus Corpora-
tion, Japan) equipped with integrated color filters, using green light ex-
citation (510–550 nm).
2.4. Spectrophotometric Experiments
The probe RBA was dissolved in DMF to get a 1 mmol L−1 standard
solution. 20 μL of this standard solution was added into 2 mL CH3CN/
H2O mixture (3:7, v/v; HEPES buffer 50 mmol L−1; pH = 7.4) contain-
ing testing cations and allowed to stand for 15 min at room temperature
before fluorescence and UV–Vis measurements. Blank solution of RBA
without cations was prepared by the same procedure. The fluorescence
emission spectra were excitation wavelength at 535 nm. Both the exci-
tation and emission slits were set at 5.0 nm.
2.3. Synthesis of Probe RBA
2.5. Preparation of Cells
The synthetic route of RBA is shown in Fig. 1.
The HeLa Cells were grown in Dulbecco's modified Eagle's medium
(DMEM) supplemented with 10% fetal bovine serum and 1% penicil-
lin–streptomycin, incubated under a humidified atmosphere of 5% CO2
and 95% air at 37 °C for 24 h. Cells were seeded on dish for fluorescence
microscopic imaging by inversion fluorescence microscope.
2.3.1. Synthesis of Compound 1
Compound 1 was synthesized from salicylic acid according to the re-
ported method [49]. 1.38 g of salicylic acid (10 mmol) and about 6.0 mL
of ethanol were put into a 100 mL bottom flask. Then 15 mL of 50%
NaOH solution, 3.0 mL of CHCl3 (40 mmol), and 50 mg of dibenzo-18-
crown-6 (catalytic amount) were added in the flask. The reaction tem-
perature was maintained at 55 °C and the mixture was stirred for
24 h. Having been cooled, the mixture was acidified with 10 mol L−1
H2SO4. The crude product was recrystallized from ethanol. Finally,
0.48 g of compound 1 as a white solid (yield 29%) was obtained. 1H
NMR (300 MHz, d6-DMSO) δ 9.88 (s, 1H), 8.35 (d, 1H), 8.00 (m, 1H),
7.12 (d, 1H).
3. Results and Discussion
3.1. Fluorescence and Absorbance Spectra
The fluorescence emission spectrum of RBA in CH3CN/H2O mixture
(3:7, v/v; HEPES buffer 50 mmol L−1; pH = 7.4) solution is shown in
Fig. 2. RBA showed very weak fluorescence emission with a low
Fig. 1. Synthetic route of probe RBA.

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223
Fig. 4. Predicted spirolactam ring-opening mechanism of RBA induced by Cu2+
.
shown in Fig. 3, the color of the solution changed from colorless to
strong pink with the addition of Cu2+ (150 μmol L−1). The reason for
this phenomenon is the same as that for the increase in the fluorescence
emission intensity discussed above. The typical absorption peak of rho-
damine at 560 nm appeared, and the intensity increased with Cu2+ con-
centration. Therefore, RBA can also be used as a colorimetric probe for
selective detection of Cu2+
.
3.2. Binding Affinity of RBA to Cu2+
Fig. 2. Fluorescence emission spectra of RBA (10 mol L−1) in CH3CN/H2O mixture (3:7, v/v;
HEPES buffer 50 mmol L−1; pH = 7.4) solution, in the absence and presence of common
metal ions (2 × 10−4 mol L−1). Excitation wavelength was 535 nm.
Job's point experiment was used to determine the stoichiometry of
coordination between RBA and Cu2+. As shown in Fig. S5, the intersec-
tion point of the fluorescence intensities was observed at the molar frac-
tion of ~0.50, indicating that the coordination ratio between RBA and
Cu2+ was 1:1. Furthermore, ESI-MS showed that the binding ratio of
RBA to Cu2+ was also 1:1. The ESI-MS spectrum of RBA revealed a
main peak at m/z 605.3 corresponding to [RBA + H]+. After the addition
of 50 μmol L−1 of Cu2+, a main peak at m/z 666.2 appeared, coinciding
exactly with the species of [RBA + Cu – 2H]+that confirming the 1:1
formation of the RBA-Cu2+ coordination. Fig. 4 shows that the predicted
mechanism of the spirolactam ring-opening of rhodamine is induced by
absolute quantum yield (Φ = 0.04) under the excitation wavelength of
535 nm. This was due to the spirolactam ring form of its molecular
structure, and RBA did not show the typical emission peak of the rhoda-
mine skeleton in the wavelength range of 400–600 nm. The influence of
metal ions on the fluorescence intensity of the probe was tested by
using 11 common metal ions, including Cu2+, Mg2+, Zn2+, Co2+
,
Cd2+, Ni2+, Mn2+, Ba2+, Ca2+, Fe2+, and Hg2+ at the concentration
of 2 × 10−4 mol L−1. As shown in Fig. 2, only Cu2+ significantly en-
hanced the fluorescence emission intensity of the RBA probe at
586 nm, with an absolute quantum yield of 0.35. This was because the
ring-opening of spirolactam within the RBA molecular structure was in-
duced by the coordination of RBA and Cu2+. This result showed that
RBA was suitable for selective detection of Cu2+ in the presence of var-
ious other ions. Fig. S4 shows that the fluorescence emission intensity of
RBA increased with an increase in Cu2+ concentration (0–
150 μmol L−1).
the coordination of RBA and Cu2+
.
To better understand of the coordination chemistry between Cu2+
and RBA, the optimized energy structures of the RBA and RBA-Cu com-
plexes (Fig. 5) were obtained using density functional theory (DFT) cal-
culation at the B3LYP level using 6–31G(d,p) basis set for simple
receptor (RBA) and LANL2DZ basis set for metal complex with the
Gaussian 09 program [50]. The spatial distribution of the electron
cloud and the orbital energy of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) of RBA
and the complex were also calculated. As shown in Fig. 5, the HOMO
The UV–visible absorption spectra of RBA in CH3CN/H2O mixture
(3:7, v/v; HEPES buffer 50 mmol L−1; pH = 7.4) were also studied. As
Fig. 3. Absorption spectra of RBA (10 mol L−1) in CH3CN/H2O mixture (3:7, v/v; HEPES
buffer 50 mmol L−1; pH = 7.4) solution with different concentration levels of Cu2+ (0–
150 mol L−1).
Fig. 5. Energy diagram of HOMO and LUMO orbitals RBA and RBA–Cu complex.

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L. Lv, Q. Diao / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 179 (2017) 221–226
Fig. 7. Fluorescence images of HeLa cells: (a) cells incubated with RBA (10 μmol L−1) for
30 min; (b) cells incubated with Cu2+ (10 μmol L−1) for μmol L−1 30 min.
3.4. Reversibility of the Binding of RBA to Cu2+
Reversibility of target ion binding is an important feature of fluores-
cent probes. Therefore, the chemical reversibility of the binding be-
tween RBA and Cu2+ was also investigated. It could be seen with the
naked eye that the pink color of the RBA-Cu solution disappeared rapid-
ly upon the addition of excess sodium salt of EDTA, and simultaneously,
the fluorescence intensity returned to its original level (Fig. S8). These
phenomena indicated that the coordination of EDTA-Cu could lead to
the dissociation of the RBA-Cu complex, and hence the coordination be-
tween RBA and Cu2+ was chemically reversible.
Fig. 6. Curves of the fluorescence intensity at 586 nm versus the concentrations of Cu2+
Inset: calibrations curve in the concentration range of 0 to 15.00 μmol L−1 of Cu2+
.
.
of RBA was distributed only at the substituted spirolactam ring, while
the LUMO was distributed throughout the molecule. The result showed
that when the C\\N bond of the spirolactam ring was broken, it could
promote the coordination between the ionophore carbonyl oxygen
and the metal ion. The π-electrons in the HOMO of RBA-Cu complex
were distributed around the metal ion, while that in the LUMO were
distributed only in the xanthene moiety. The calculated energy gap
(ΔE) between HOMO and LUMO of RBA and RBA-Cu were 3.389 eV
and 1.240 eV, respectively, which indicated that the coordination of
Cu2+ and RBA decreased the energy gap of HOMO-LUMO and stabilized
the system. Therefore, an optimal ideal coordination structure was pro-
posed according to the coordination features presented above.
3.5. Determination of Cu2+
The standard curve for Cu2+ determination was obtained by plotting
the fluorescence emission intensity at 583 nm versus the Cu2+ concen-
tration (Fig. 6). The calibration curve showed a linear fit in the concen-
tration range of 0–15.00 μmol L−1, with a correlation coefficient (r) of
0.9960. The detection limit of Cu2+ concentration was 0.085 μmol L−1
(S/N = 3), which is much lower than what has been previously reported
[51–55] and most importantly, it falls beneath the MAL of Cu2+ in drink-
ing water according to USEPA and WHO regulations. The RBA probe was
successfully applied for Cu2+ detection in drinking water samples;
however, the results showed no significant differences with those ob-
tained by atomic absorption spectroscopy (AAS) (Table 1). Nonetheless,
the method for Cu2+ detection in drinking water reported here has the
advantages of low cost and easy operation compared to AAS.
3.3. Influences of Time and pH
The effect of reaction time on the fluorescence intensity of the RBA-
Cu complexes was studied at room temperature. Fig. S6 showed that in
the presence of Cu2+ in CH3CN/H2O mixture (3:7, v/v; HEPES buffer
50 mmol L−1; pH = 7.4) solution the fluorescence intensity of RBA in-
creased significantly within 5 min, reached a maximum after 15 min
and tended to be stable. This indicated that the coordination of RBA
and Cu2+ can be completed within 15 min. Therefore, a reaction time
of 15 min was applied in the subsequent experiments.
The effect of pH on the fluorescence intensity of the RBA-Cu complex
was also investigated under various pH conditions. As shown in Fig. S7,
the fluorescence intensity of RBA increased rapidly with a decrease of
pH from 6 to 4, indicating that the spirolactam ring of RBA can be
opened in acidic solutions. At pH of 6–8, RBA exhibited a highly selective
fluorescence “turn-on” response to Cu2+. The pH range of drinking
water and living cells is usually between 6.0 and 7.6, therefore the fluo-
rescent probe RBA can be utilized for Cu2+ detection in drinking water
and living cells.
3.6. Fluorescence Imaging for Living Cells
In order to examine the practical application of RBA in biological re-
search, cytotoxicity and fluorescence imaging experiments were per-
formed in living cells. The toxicity of RBA on HeLa cells was evaluated
by the MTT assay in a concentration range of 0 to 20 μmol L−1 of RBA
(Fig. S9). The results showed that more than 90% of the cells were viable
after the incubation, indicating that RBA was non-toxic to the cells
under our experimental conditions. Next, the HeLa cells were incubated
in phosphate buffer solution containing 10 μmol L−1 RBA for 30 min at
37 °C and processed for fluorescence imaging. As shown in Fig. 7a, no
fluorescence emission was observed. The cells were then incubated in
a medium containing 10 μmol L−1 of Cu2+ for further 30 min at 37 °C
Table 1
Analytical results of Cu2+ in drinking water samples (n = 5).
Sample No.
Spiked (μmol L−1
)
AAS method
Fluorescent method
Found (μmol L−1
)
RSD (%)
Recovery (%)
Found (μmol L−1
)
RSD (%)
Recovery (%)
1
2
10.00
10.00
9.92
9.77
5.3
4.7
99.2
97.7
10.08
10.11
6.5
5.8
100.8
101.1

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225
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In this work, a novel “turn-on” fluorescent probe based on the
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.saa.2017.02.053.
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