Formation of an emissive telluroxide promoted by Hg2+ in aqueous environment: A new naked-eye and ratiometric rhodamine dimer fluorescent mercury(II) probe

Article abstract of DOI:10.1016/j.dyepig.2018.06.005

A new chemosensor based on tellurium-rhodamine B (TRB) for the fast and selective detection of Hg²⁺ in aqueous medium was designed and synthetized in only two steps from a symmetrical organic telluride and rhodamine B. The structure of TRB was determined by RMN, HRMS and IR and its photophysical properties were determined in MeCN/H₂O 75:25 v/v. TRB is able to detect Hg²⁺ in solutions diluted until 1.71 μmol/L and containing until 75% of water in it. In the presence of Hg²⁺, TRB presents an intense pink color with maximum absorption in 558 nm (log ε = 4.93) and a strong fluorescence emission with maximum intensity in 582 nm (Φ = 0.17). The detection of Hg²⁺ by TRB occurs by the oxidation of the tellurium promoted by Hg²⁺, where a redox mechanism was proposed. Even the detection of Hg²⁺ occurring by the oxidation of tellurium, TRB presented a high selectivity for Hg²⁺, even when several ROS (H₂O₂, KNO₂, NaOCl, NaONOO e KO₂) were submitted to the screening.

Full text of DOI:10.1016/j.dyepig.2018.06.005

Accepted Manuscript
2+
Formation of an emissive telluroxide promoted by Hg in aqueous environment: A
new naked-eye and ratiometric rhodamine dimer fluorescent Mercury(II) probe
Antônio A. Soares-Paulino, Lilian Giroldo, Gizele Celante, Carlos Lodeiro, Alcindo A.
Dos Santos
PII:
S0143-7208(18)31097-0
10.1016/j.dyepig.2018.06.005
DYPI 6810
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 May 2018
Revised Date: 3 June 2018
Accepted Date: 4 June 2018
Please cite this article as: Soares-Paulino AntôA, Giroldo L, Celante G, Lodeiro C, Dos Santos AA,
2+
Formation of an emissive telluroxide promoted by Hg in aqueous environment: A new naked-eye and
ratiometric rhodamine dimer fluorescent Mercury(II) probe, Dyes and Pigments (2018), doi: 10.1016/
j.dyepig.2018.06.005.
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Full Title: Formation of an Emissive Telluroxide Promoted by Hg2+ in aqueous
environment: A New Naked-eye and Ratiometric Rhodamine Dimer Fluorescent
Mercury(II) Probe
Author Names: Antônio A. Soares-Paulino,[1,2] Lilian Giroldo,[1] Gizele Celante,[1]
Carlos Lodeiro,[2,3] and Alcindo A. Dos Santos*[1]
Affiliations:
[1] Department of Fundamental Chemistry, Institute of Chemistry, University of São
Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil.
[2] BIOSCOPE Group, LAQV-REQUIMTE, Departamento de Química, Faculdade de
Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Portugal.
[3] Proteomass Scientific Society, Rua dos Inventores, Madan Park, 2829-516 Caparica,
Portugal.
Contact Information for the Corresponding Author:
[*]Email: cle@fct.unl.pt
[**]Email: alcindo@iq.usp.br

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1. Introduction
Traditional analytical methods as plasma emission spectroscopy, high performance liquid
chromatography, X-ray fluorescence, atomic absorption spectrometry and mass spectroscopy are
techniques that need equipment that have a high cost of acquisition, with complex analytical
protocols, and request high qualified researchers to operate them. Alternatively, fluorescent and
colorimetric methods are preferable because they are inexpensive, the equipment are easy to
handle and regularly shows high sensitivity.[1,2] The development of fluorescent and colorimetric
chemosensor for the detection of analytes of biologic interest has been gaining more and more
prominence, mainly for the detection of heavy metals in several mediums.[3–5] Specially,
techniques to detect mercury has been explored due to its high toxicity.[6] Mercury can be found in
environment in different forms, as elemental metal, organometallic complex and inorganic salt.
There are substantial differences in its toxicity and it varies according to its form, many times being
related to its solubility.[7] Typically, mercury (II) compounds are more toxic when orally taken due
to its higher solubility in water.[7] Low concentrations of this metal can induce some diseases like
digestive, renal and mainly neurologic ones.[8] The toxicity of mercury is associated to an
inactivation of proteins by the bonding among sulfhydryl/disulfide and mercury.[9,10] This strong
interaction between mercury/protein is deeply related to the chalcogenophilicity that mercury has.
[11,12] Due to this, the development of new analytical tools to detect toxic substances in a selective,
sensitive and robust way is always a constant interest for the researchers.
The concept of chalcogenophilicity to detect mercury allows creating more efficient
chemosensors for Hg (II). From this knowledge, many chemosensors were developed based on the
formation of the chalcogen-mercury bond.[13–17] All these chemosensors have a mechanism of
detection based on the formation of complexes or on the dechalcogenation of the fluorescent core.
Even being well explored in organic synthesis,[18] organic tellurides are little explored as ligands in
the synthesis of new chemosensors. Besides the lack of affinity for mercury, organic compounds of
tellurium have several other characteristics, among them, a high susceptibility to redox
processes.[19] This enables that new fluorescent chemosensors are developed for both oxidant
species and metals, in special, mercury.[20]
After an extensive searching on the literature, we realized there are a very small number of
papers concerning fluorescent chemosensors based on tellurium, having mainly applications on the
detection of oxidant species.[20–22] As a fluorescent probe for metal ion, this number is even
smaller.[23–25] As it is a field poorly explored, we have developed a new naked-eye and

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ratiometric fluorescent chemosensor containing a dimmer of tellurium-rhodamine (TRB) units to
the fast and selective detection of Hg2+ ions in water, where the mechanism of detection occurs via
the oxidation of tellurium promoted by the mercury ion (Hg2+).
2. Experimental
2.1 Chemicals and starting material
4-iodoaniline, dimethyl sulfoxide (DMSO), potassium hydroxide, elemental tellurium (200
mesh
powder),
rhodamine
B,
4-methylbenzene-1-sulfonyl
chloride
(TsCl),
4-
dimethylaminopyridine (DMAP) were purchased from Sigma-Aldrich. Metal salts: Al(NO3)3·9H2O,
Cr(NO3)3·9H2O, Co(NO3)2·6H2O, Pb(ClO4)2·3H2O, Ni(ClO4)2·6H2O, Zn(ClO4)·6H2O, Cd(ClO4)2·6H2O,
Hg(ClO4)2·XH2O, Hg2Cl2, Fe(ClO4)3·XH2O, Mn(ClO4)2·6H2O, AgClO4, Fe(ClO4)2·XH2O, Co(ClO4)2·6H2O
and KCl were purchased from Alpha Aesar and used as received. All salts and KO2 and KNO2 were
dissolved in distilled water. NaOCl was standardized at pH 12 (ε292 nm = 362 M-1cm-1).[26] H2O2 Was
standardized using ε240 nm = 43.6 M-1cm-1.[27] Peroxynitrite was prepared and standardized (ε302 nm
= 1670 M-1cm-1) by nitrosation of H2O2 at pH ≥ 12.5 by potassium nitrite.[28,29] CAUTION! All
compounds (intermediates, reagents and final products) were manipulated in a fume hood and
avoiding contact with skin.
2.2 Instrumentations
1
13
NMR spectra were recorded at Bruker Avance III 200 (200 MHz, H; 50 MHz, C; 63MHz,
127Te) using CDCl3 as a solvent and tetramethylsilane (0 ppm) and diphenyl ditelluride (422 ppm)
as internal standard. NMR spectra were processed using software MestReNova 6.0. Data are
reported as follows: chemical shifts in ppm (δ), multiplicity (s = singlet, d = doublet, q = quartet, m =
multiplet, dd = doublet of doublet), coupling constants (Hz), integration, and interpretation. High-
resolution mass spectra were recorded at micrOTOF-QII Bruker mass spectra (by electron spray
time-of-flight (ESI-qTOF)), with the spray gas pressure (4.0 bar, dry gas to 8.3 l/h, drying
temperature = 180 °C, capillary voltage 4.5 kV, collision energy 12 eV, collision energy for MS/MS
18 eV, the mass flow to 180 μl/h, calibration reference Na Formate). Mass spectra were processed
using software Data Analysis 4.3. The theoretical mass was calculated using software Isotope
Pattern. Other mass spectra were recorded on a GC/MS-QP2010 (70 eV) Shimadzu
(chromatography column Restek Rtx-5MS, flow to 1.30 mL/min and gas pressure 69,3 kPa). Other

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mass spectra were recorded on a Shimadzu LCMS-8030 quadrupole mass spectrometer operating in
the electrospray ionization (ESI) mode (with the nebulizing gas flow 3.0 L/min, drying gas flow
15L/min, interface voltage 4.5 kV, interface current 45.4 uA, DL temperature 250 °C, heat block
temperature 300 °C, CID gas 230 kPa) and a Shimadzu Nexera HPLC system. Elemental analysis was
performed using a Perkin Elmer 2400 series ii. The infrared spectra were recorded on a FTIR-
Frontier-PerkinElmer.
2.3 Spectrophotometric and Spectrofluorimetric Measurements
UV-vis spectra were recorded on a JASCO V-650 spectrophotometer and the fluorescence
emission spectra on a HORIBA-JOBIN-YVON Fluoromax-4 (Proteomass Scientific Society Facilit). A
correction for the absorbed light was performed when necessary. The spectroscopic
characterizations and titrations were performed by preparing stock solutions of compound TRB in
MeCN (ca. 10-3 mol/L) in a 1 mL volumetric flask. The studied solutions were prepared by
appropriate dilution of the stock solutions in MeCN/H2O (75:25, v/v). All the measurements were
performed at 298 K.
For Detection (LOD) and quantification (LOQ) limits, ten different analyses for the selected receptor
were performed in order to obtain the LOD and LOQ. The LOD and LOD were obtained by applying
the formula:
LOD = yblank + 3 std
LOQ = yblank + 10 std;
(1)
(2)
Where yblank = signal detection limit and std = standard deviation.
2.4 Synthesis of 4,4'-tellurodianiline (2).
Compound 2 was synthesized according to the reference.[30]
In a sealed tube were, sequentially added, elemental tellurium (powder 200 mesh, 263 mg; 2.06
mmol), KOH (224 mg; 4.0 mmol), 4-iodoaniline (438 mg; 2.0 mmol) and DMSO (4 mL). The tube
was closed and the mixture was heated at 110 ° C under stirring for 12 h. After complete
consumption of the reactants, the organic phase was extracted with CH2Cl2 (3x, 20 mL), washed
with sat. NH4Cl (3x, 25 mL), dried with MgSO4, filtered and concentrated under vacuum. The
reaction mixture was purified by column chromatography on silica gel with an eluent mixture of

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hexane:EtOAc (1:1). The compound 2 was obtained as a yellow solid; Yield = 26% (211 mg); Rf:
0.31 (Hexane:EtOAc 1:1); EIMS m/z: M+ 314.
2.5 Synthesis of 3',6'-bis(diethylamino)-2-(4-((4-(3'-(diethylamino)-6'-(diethylamino)-3-
oxospiro[isoindoline-1,9'-xanthen]-2-yl)phenyl)tellanyl)phenyl)spiro[isoindoline-1,9'-
xanthen]-3-one (TRB).
Compound TRB was synthesized according to the reference with adaptation.[31]
To a stirred solution of rhodamine B (958 mg, 2.0 mmol) in anhydrous CH2Cl2 (20 mL) was added
TsCl (686 mg, 3.6 mmol). The mixture was stirred for 15 min. After that, was added DMAP (611 mg,
5.0 mmol). The mixture was stirred for 15 min. Following, an anhydrous CH2Cl2 (15 mL) solution of
compound 2 (160 mg, 0.515 mmol) was added. The reaction mixture was stirred until consumption
of compound 2 (3 h; monitored by TLC – hexane:EtOAc, 7:3). The mixture was quenched with
saturated aqueous sodium bicarbonate solution (10 mL); the mixture was extracted with ethyl
acetate (3 x, 20 mL). The resulting organic phase was dried over anhydrous MgSO4, filtered and
evaporated. The crude product was purified by silica-gel flash chromatograph (hexane: EtOAc, 7:3).
The product TRB was obtained as a light yellow solid; yield = 66% (395 mg); mp. 173-178°C; Rf:
0.40 (Hexane: EtOAc, 1:1); FTIR (neat, cm-1) 2969, 2928, 2898, 2870, 1701, 1635, 1615, 1542, 1515,
1490, 1330, 1306, 1266, 1220, 1119, 1013, 820, 785. 1H NMR (200 MHz, CDCl3) δ 8.01 – 7.95 (m,
1H, HA), 7.50 – 7.44 (m, 2H, HB), 7.36 – 7.29 (m, 2H, HC), 7.14 – 7.08 (m, 1H, HD), 6.75 – 6.68 (m, 2H,
HE), 6.62 – 6.54 (m, 2H, HF), 6.31 – 6.23 (m, 4H, HG), 3.30 (q, J = 7.0 Hz, 9H, HH), 1.13 (t, J = 7.0 Hz,
13
13H, HI). C NMR (50 MHz, CDCl3) δ 167.8, 153.6, 153.0, 148.8, 137.7, 136.8, 133.0, 130.5, 128.6,
128.1, 127.5, 124.0, 123.4, 112.1, 108.1, 106.2, 97.9, 67.4, 44.4, 12.7. 125Te NMR (63 MHz, CDCl3) δ
658.96. UV-Vis (MeCN) λmax (lg ε) 277 (4.73), 320 (4.43) and cut off at 420 nm; HRMS (m/z): [M +
H]+ calcd for C68H68N6O4Te: 1163.4448, found 1163.4430.
3. Results and discussion
3.1 Design, synthesis and structure characterization of the TRB dimer
The design of the compound TRB was done based on the high susceptibility of tellurium to
redox processes,[32,33] on the facility to functionalize rhodamine from its spirolactamic ring [34–
36] and, mainly, because its excellent photophysical.[4] In order to join all these characteristics,
compound TRB was synthetized from a symmetric organic telluride (2) and rhodamine B. The
compound TRB was synthetized in only two steps as shown in scheme 1. The symmetry of the

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compound containing two units of rhodamine has the objective to obtain a more sensitive probe.
Initially, telluride 2 was synthetized using the methodology developed by Jin and co-workers,
which is based on the synthesis of symmetrical diaryl tellurides with Te0/KOH.[30] In this reaction,
the nucleophilic tellurium specie K2Te (formed by reacting elemental Te0 with KOH), reacts with 4-
iodoaniline (1) by a SNAr supposed mechanism for the formation of compound 2 within 26% yield.
Through the product analysis by EI-MS (Fig. S1), it is possible to detect a signal with mass of 314
m/z and with a cluster with isotopic pattern compatible with the structure of the compound 2. In
the step where the TRB is formed, the insertion of 4,4'-tellurodianiline 2 occurs via activation of
carboxylic group of rhodamine by TsCl and DMAP in anhydrous medium.[31] The product TRB was
obtained within 66% yield in the last step and 17% overall yield.
1
13
The compound TRB was characterized by H, C and 125Te NMR, HRMS and FTIR (Fig. S2-
S6). The analysis of 1H NMR spectrum (fig. S2) shows signals compatible with the TRB structure as
a triplet in 1.17 ppm and a quartet in 3.33 ppm referent to the ethyl groups (-NEt2); two signals
with pattern of para-disubstituted ring in 7.32 ppm and 6.71 ppm referring to HC and HE,
respectively, and the signals of the aromatic hydrogens from the southern rhodamine hemisphere
in 6.58 ppm (HF) and 6.27 ppm (HG). Besides, all the integrals perfectly correspond to the signals of
the spectrum. In the analysis of 13C NMR spectrum (Fig. S3) all signals regarding to the TRB
structure are observed. The carboxyl carbon [(NC=O) in 167.8 ppm], the signal in 67.4 ppm,
correspondent to the quaternary carbon sp3 (indicating that the spirolactamic ring is in its closed
form) and the signals in 44.3 ppm and 12.7 ppm refers to the ethyl group (-NEt2), are present in the
spectra. In the 125Te NMR spectrum (fig. S4), it is possible to observe a single signal in 659.0 ppm,
typical of diaryl tellurides, indicating that our system contains a single tellurium atom. The analysis
of TRB by high resolution mass spectrometry (ESI-qTOF-MS; Fig. S5) supports the NMR analyses,
being possible to observe a signal in 1163.4430 m/z relative to the protonated adduct [TRB + H+]
with a relative error of 0.6 ppm and having an isotopic pattern completely compatible to the
structure proposed as TRB. Moreover, the analysis of IR spectrum (Fig. S6) shows some of the
signals compatible with TRB as a medium stretching (C-H) related to the ethyl groups at 2969-
2870 cm-1 and a strong stretching (C=O) of the carboxyl of the lactone ring at 1701 cm-1.

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Scheme 1. Synthesis of TRB
3.2 Photophysical characterization of TRB
The photophysical properties of compound TRB were investigated in MeCN/H2O (75/25,
v/v). The absorption, emission and excitation spectra are shown in Fig. 1. The mainly photophysical
data are reported in table S1. Two transition bands can be observed at 277 nm (log ε = 4.76) and
320 nm (log ε = 4.45) in the absorption spectrum and attributed to the high energy π-π* transitions
of the rhodamine chromophore. The compound TRB in solution does not present any absorption
band in the range 400-700 nm, making the solution completely colorless. Associated to this, it is
observed a weak intensity in the fluorescence emission band, centered at 550 nm followed by a low
quantum yield of Φ < 0.01 and having a high Stokes shift of 230 nm. These conditions of absorption
and emission indicate that the spirolactamic ring is in its closed form, what causes the
intramolecular charge transfer (ICT) to be impaired, being the main cause for the fluorescence
quenching system. The low ICT process is directly associated to the orthogonal geometry among the
electron-donating (Et2N-) and -receptor (-NCOPhTe-) groups, causing a minimum overlap among
the p orbitals, impairing the π-π* transitions.[37] Moreover, it is possible to observe that the system
is free from any emissive impurities or precursors due to the similarity between the absorption and
excitation spectrums.

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Fig. 1. Absorption, corrected normalized emission and excitation spectra of compound TRB.
Condition: CH3CN/H2O 75/25 (v/v); T = 298 K; [TRB] = 5.0 x 10-6 mol/L; λem = 500 nm; λexc = 320
nm.
3.3 pH response of TRB
Xanthenes, compounds possessing a spiro ring able to suffer closure/opening, as for
example, rhodamine and fluorescein, have its fluorescence emission easily modified according to
the pH variation of the medium.[4] In the case of rhodamines, generally presents fluorescence
emission in acid solution. So, from this information, it was monitored the fluorescence intensity of
TRB in a pH range of 4-7 in MeCN/H2O 75:25 (fig. S7). The pH variation was made by the addition
of MeSO3H. As it can be clearly seen, the system does not present measurable fluorescence in a
medium with pH > 5. So, a system with a neutral pH, like MeCN/H2O, was used in the subsequent
experiments.
3.4 Metallic ion sensing studies and time-dependent response of TRB
Based on such results, the sensorial ability of TRB was investigated towards several metal
ions (K+, Ni2+, Mn2+, Zn2+, Co2+, Cd2+, Pb2+, Cu2+, Ag+, Al3+, Cr3+, Fe2+, Fe3+, Hg+ and Hg2+) in
CH3CN/H2O (75:25 v/v), followed by absorption and fluorescence emission spectroscopy (fig. 2). As
it can be seen, the chemosensor TRB presented only a strong absorption and an intense
fluorescence emission when Hg2+ is added to the solution. It is also observed a small response when

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the system is treated with Fe3+. The changes observed by absorption and emission are attributed to
the opening of the spirolactam ring. For all the other analyzed metals, less significant responses
were obtained. It was observed that TRB has an interaction response faster for the Hg2+ then for all
other metals. The TRB kinetics was monitored by the absorption of Hg2+ and Fe3+ (fig. S8). Right
after the addition of Hg2+, it is already possible to observe a pink coloration of the solution, taking
only 10 minutes to the stabilization of the absorption. However, the response for Fe3+ is around 6
times slower, taking 60 minutes to the stabilization of the absorption band. This selectivity
observed in TRB to detect Hg2+ in a short time is attributed to the high chalcogenophilicity of this
metal.[11]
Fig. 2. Absorption (A) and fluorescence (B) spectra of TRB in response to the metal ions at 2 min.
Condition: T= 298 K; MeCN/H2O 75/25 (v/v); [TRB] = 5.0 x 10-6 mol/L; [Metal] = 1.7 x 10-3 mol/L;
λexc = 515 nm; slit = 3.
3.5 Spectrophotometric and Spectrofluorimetric titrations of TRB with Hg2+
Once knowing the parameters, spectrophotometric and spectrofluorimetric titrations of the
chemosensor TRB with Hg2+ (0 - 160 eq) solutions were carried out (fig. 3). The photophysical data
are summarized in table S2. The analysis of the product formed TRB-O by UV-vis reveals the
appearance of a new band with maximum absorption at 558 nm with log ε = 4.93 accompanied by a
naked-eye color change from colorless to pink. Besides, TRB-O shows a band with maximum

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fluorescence emission at 582 nm, with a fluorescent quantum yield of Φ = 0.17 and Stokes’ Shift of
24 nm accompanied by a bathochromic shift of 32 nm. The raise in the fluorescence signal of the
system after the addition of Hg2+ (Φ < 0.01 to Φ = 0.17) is caused by the intensification of the ICT
process, that is increased due to the formation of a conjugated system between the electron-
donating (Et2N-) and -receptor (-NCOPhTe-) groups. The fact of TRB-O presenting a bathochromic
shift indicates that Hg2+ reacted with the electron-receptor group.[2] From the titrations, it was
possible to observe a linear relation between the absorption intensity and the concentration of Hg2+
(fig. S9), enabling the chemosensor TRB to be used as a ratiometric probe for Hg2+. Moreover, it
was determined the limits of detection (LOD = 1.71 μmol/L) and quantification (LOQ = 2.90
μmol/L) for Hg2+ using to the methodology described in the experimental section (table 1).
Through the continuous variation method Job’s plot[2,38] (fig. S10), it was determined the
stoichiometric relation between TRB and Hg2+ as 1 : 1. The detection of Hg2+ by TRB was
investigated by absorption and fluorescence emission in different proportions of MeCN/H2O (fig.
S11). As it can be seen, the chemosensor TRB can detect Hg2+ in an environment containing until
75% of water. Interestingly, it was observed that the reaction between TRB and Hg2+ lead to a
diminish in the pH of the solution, indicating the formation of H+ ions.
Fig. 3. Spectrofotometric (A) and spectrofluorimetric (B) titrations of TRB with increasing amounts
of Hg2+. Inset: Absorption at 558 nm or emission at 582 nm, as a function of [Hg2+]/[TRB] and
visual changes in the (A) visible color and (B) fluorescence color upon addition of Hg2+. Condition:
T= 298 K; MeCN/H2O 75/25 (v/v); [TRB] = 5.0 x 10-6 mol/L; [Metal] = 1.7 x 10-3 mol/L; λexc = 515
nm; slit = 3.

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Table 1. Minimal detectable (LOD) and quantified (LOQ) of for TRB-O in MeCN/H2O 75:25 (v/v).
Relative standard deviation (RSD) of the values measured was below 10%, n = 6.
Absorbance at 558 nm
Fluorescence at 582 nm
Limit
(μmol/L)
4.05
(μmol/L)
1.71
Detectable
Quantified
4.27
2.90
3.6 Study by mass spectrometry and proposed sensing mechanism
In the best of our knowledge, all reported mechanism detection of the chemosensors
described in the literature, based on the interaction of Chalcogen-Metal occur via
complexation[15,24,25,39] or dechalcogenation[16,23,40]. So, with the objective of discovering
which is the mechanism involved in the detection of Hg2+ by TRB, some experiments by mass
spectrometry were done. Initially, the product formed from the treatment of a solution of TRB (in
MeCN/H2O 75:25 v/v) with 1 eq. of Hg2+ was analyzed by ESI-qTOF-MS (fig. S12). To our surprise, it
was observed that the main signals in the TRB-O spectrum refers to an oxidation product of TRB.
These signals were attributed to [TRB + O + 2H]2+ = 590. 2211 m/z, [TRB + O + H]+ = 1179.4366
m/z and [TRB + O + Na]+ = 1201.4224 m/z, having all the values a relative error in the range of 1-4
ppm. All the signals present an isotopic pattern compatible to the proposed product. Additionally, it
was performed a titration of TRB with Hg2+ (0.25 eq., 0.50 eq., 0.75 eq. and 1.00 eq.) accompanying
by LC-ESI-MS (fig. S13). As it can be seen, it was necessary 1 eq. of Hg2+ for the complete oxidation
of TRB, confirming the stoichiometric 1 : 1 (TRB : Hg2+) relation, the same one obtained from the
Job’s Plot experiment.
In order to determine which region of the TRB structure is occurring the oxidative process,
it was made an experiment of structural fragmentation in gas phase by mass spectrometry (MS/MS)
from the ion 590.2213 m/z, that was attributed to TRB-O (fig. 4). It is possible to identify in the
spectrum three signals, which were attributed to [TRB-O fragment 1] = 664.1806 m/z, [TRB-O
fragment 2] = 634.1337 m/z and [TRB-O fragment 3] = 516.2657 m/z, where all the signals

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present a relative error lower than 3 ppm. Initially, it is possible to identify the presence of
tellurium in the signals related to the fragments 1 and 2 due to the isotopic pattern, typical of
tellurium (fig. S14). The mass values for these fragments indicate that the fragmentation of TRB-O
occurs in the symmetry point of the molecule, or else, between the tellurium atom and the phenyl
group. The fragments 1 and 2 correspond exactly to the half size of the TRB molecule containing
tellurium increased of one oxygen atom. The fact of TRB present a structural symmetry and the
fragment 3 corresponding to the second half of TRB without the addition of oxygen, is evident that
the oxidation of TRB occurs directly in the tellurium atom, or else, the detection process of Hg2+ is
accompanied by the formation of telluroxide.
Rationalizing all the results obtained about the detection process of Hg2+ by TRB
(stoichiometry of 1 : 1, pH diminishing, oxidation of the tellurium atom and opening of the
spirolactamic ring) and knowing that oxidized processes are usually accompanied by the transfer of
2 electrons with the generation of 2 H+, we propose a plausible mechanism for the detection of Hg2+
by TRB (Scheme 2). Initially, an electron is transferred from the telluride TRB to the Hg2+, which is
converted to Hg+ and the radical cation Te+• TRB-1 is generated. In the following step, TRB-2 is
generated by the nucleophilic attack of water at the tellurium atom of the TRB-1 intermediate,
eliminating one H+ and forming the corresponding radical tellanol TRB-3. By the transfer of a
second electron to Hg+, TRB-3 is converted to the cation tellanol TRB-4. In the next step,
telluroxide TRB-5 is formed through the stabilization of the electronic structure of tellurium
through the electrons of the hydroxyl group, followed by the release of a second H+ for the
formation of TRB-6. In the last step, due to the electronegativity of Te=O and the pH diminishing of
the medium, the spirolactamic ring opens for the formation of TRB-O.
Knowing the mechanism of detection of Hg2+ by TRB, associated to the fact that TRB
demonstrated some response to Fe3+ it, was investigated if TRB also respond to this cation via
oxidation of the tellurium atom. It was used mass spectrometry to monitor the treatment of a
solution of TRB with 10 eq. of (fig. S15). It was not possible to detect the signal related to the
oxidation of TRB, being observed only the signal of TRB itself. This result suggests that the
mechanism of the detection of Fe3+ occurs by a simple interaction Lewis’ acid/base between the
lactam and Fe3+.
As the detection of Hg2+ is based on the oxidation of tellurium, it was investigated if
biologically relevant endogenous oxidants (reactive oxygen species - ROS), traditionally used to
oxidize chalcogens (Se/Te), could be detected by TRB.[41–43] For this end, aqueous solutions of
H2O2, KNO2, NaOCl, NaONOO and KO2 were prepared and 10 eq. of each ROS were added to a

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solution of TRB. The assays were monitored by fluorescence emission (fig. S16). As it can be seen,
none of the ROS analyzed was able to induce the oxidation of TRB that results in fluorescence
emission or visible change in the color, which indicate that TRB has a high selectivity for Hg2+, even
when some endogenous oxidants could be present.
It was also investigated the reversibility of the oxidative reaction of TRB by Hg2+, using the
endogenous reduction agent, glutathione (GSH). This experiment was also monitored by mass
spectrometry (fig. S17). GSH was chosen because its properties as reductive agent in the conversion
of telluroxide (Te4+) to telluride (Te2+) are well known.[44,45] After the addition of 1 eq. of GSH
almost all the telluroxide TRB-O was converted to the original, reduced telluride TRB-7. This result
suggests that the chemosensor TRB can be used in more than one cycle to detect Hg2+.
Fig. 4. MS/MS spectrum of compound TRB after addition of 1 equiv Hg2+ in MeCN/H2O. Spectrum
obtained from fragmentation of the ion [TRB-O]2+ = 590.2213 m/z. Spectrum obtained by ESI-
qTOF-MS. Inset: Structure of TRB-O and their fragments.

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Scheme 2. A plausible mechanism for the oxidation of telluride (TRB) to the corresponding
telluroxide, mediated by Hg2+ in aqueous environment.
4. Conclusions
A new rhodamine dimmer derivative containing tellurium TRB was designed and
synthetized in only two steps; its structure was determined by RMN, HRMS and IR and its
photophysical properties were measured. This new compound was applied as a naked-eye and
ratiometric fluorescence chemosensor for fast and selective detection of Hg2+ by a unique and
analyte-discriminatory mechanism. From de mass spectrometry and other additional analytical
investigations, a new mechanism to detect Hg2+ was discovered. Differently from other
chemosensors developed and related in the literature where the mechanism of detection of the
metallic ions occurs by the complexation or defunctionalization, TRB detects Hg2+ by the oxidation
of tellurium. The treatment of TRB with some ROS (H2O2, KNO2, NaOCl, NaONOO e KO2) did not
result in any color change or fluorescent emission, indicating that this sensor is highly selective to
Hg2+ via a new and unconventional mechanism. The selectivity of metal recognition was also

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demonstrated by differentiation between Fe3+ and Hg2+ proving that in the first case the interaction
probably is a result of a conventional complexation and in the last case a unique oxidation of
tellurium by Hg2+ takes place.

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Formation of an Emissive Telluroxide Promoted by Hg2+ in aqueous
environment: A New Naked-eye and Ratiometric Rhodamine Dimer
Fluorescent Mercury(II) Probe
Antônio A. Soares-Paulino,1,2 Lilian Giroldo,1 Gizele Celante,1 Carlos Lodeiro,*2,3 and Alcindo
A. Dos Santos**1
1Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes, 748, 05508-
000, São Paulo, SP, Brazil.
2BIOSCOPE Group, LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova
de Lisboa, 2829-516 Portugal
3Proteomass Scientific Society, Rua dos Inventores, Madan Park, 2829-516 Caparica, Portugal
Corresponding Authors: *E-mail: cle@fct.unl.pt (CL) / **E-mail: alcindo@iq.usp.br (AS)

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Abstract
A new chemosensor based on tellurium-rhodamine B (TRB) for the fast and selective
detection of Hg2+ in aqueous medium was designed and synthetized in only two steps from a
symmetrical organic telluride and rhodamine B. The structure of TRB was determined by RMN,
HRMS and IR and its photophysical properties were determined in MeCN/H2O 75:25 v/v. TRB is
able to detect Hg2+ in solutions diluted until 1.71 μmol/L and containing until 75% of water in it. In
the presence of Hg2+, TRB presents an intense pink color with maximum absorption in 558 nm (log
ε = 4.93) and a strong fluorescence emission with maximum intensity in 582 nm (Φ = 0.17). The
detection of Hg2+ by TRB occurs by the oxidation of the tellurium promoted by Hg2+, where a redox
mechanism was proposed. Even the detection of Hg2+ occurring by the oxidation of tellurium, TRB
presented a high selectivity for Hg2+, even when several ROS (H2O2, KNO2, NaOCl, NaONOO e KO2)
were submitted to the screening.
Keywords: tellurium, telluroxide, fluorescent sensor, mercury, nacked-eye.

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Acknowledgments
A. A. Dos Santos and A. A. Soares-Paulino (CNPq scholarship holder – Brazil) are thankful to
CNPq (141454/2013-0, 207311/2015-3 and 153759/2016-0), FAPESP (2016/09579-0) and also
to the financial and structural support offered by the University of São Paulo through the NAP-
CatSinQ (Research Core in Catalysis and Chemical Synthesis). A. A. Dos Santos and C. Lodeiro are
also thanks to the CNPq program, Science without borders 2014-2016 for the “Special Guest
Researcher” grant (Brazil). C. Lodeiro thanks The Associate Laboratory Research Unit for Green
Chemistry - Clean Processes and Technologies - LAQV which is financed by national funds from
FCT/MEC (UID/QUI/50006/2013) and co-financed by the ERDF under the PT2020 Partnership
Agreement (POCI-01-0145-FEDER - 007265) and the Scientific Society PROTEOMASS (Portugal) for
funding support (General Funding Grant).

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Supplementary Material
A New Naked-eye and Ratiometric Fluorescent Chemosensor for
Detection of Mercury: A Special Case for Formation of Telluroxide
Promoted by Hg2+ in aqueous environment
Antônio A. Soares-Paulino,1,2 Lilian Giroldo,1 Gizele Celante,1 Carlos Lodeiro,*2,3 and Alcindo
A. Dos Santos**1
1Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes, 748, 05508-
000, São Paulo, SP, Brazil.
2BIOSCOPE Group, UCIBIO-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova
de Lisboa, 2829-516 Portugal.
3Proteomass Scientific Society, Rua dos Inventores, Madan Park, 2829-516 Caparica, Portugal
Corresponding Authors: * E-mail: cle@fct.unl.pt (CL) / **E-mail: alcindo@iq.usp.br (AS)

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Summary
1. Determination of fluorescence quantum yields ..................................................................................... 24
2. Data Spectra............................................................................................................................................................... 24
Figure S1. Compound 2 EIMS................................................................................................................................... 24
Figure S2. Compound TRB 1H NMR (200 MHz) CDCl3................................................................................... 25
Figure S3. Compound TRB 13C NMR (50 MHz) CDCl3. ................................................................................... 25
Figure S4. Compound TRB 125Te NMR (63MHz, CDCl3). ............................................................................... 26
Figure S5. Compound TRB HRMS (M + H+)........................................................................................................ 26
Figure S6. Compound TRB FTIR............................................................................................................................. 27
3. Photophysical Studies........................................................................................................................................... 27
4. Spectroscopic Mass Studies ............................................................................................................................... 31
Figure S14. Theoretical isotopic standard for the TRB-O fragments...................................................... 33
5. Reference..................................................................................................................................................................... 36

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1. Determination of fluorescence quantum yields
Fluorescence quantum yields of TRB were measured using a solution of rhodamine B in absolute
ethanol as standard (
Φ
= 0.70)[1] and is calculated using the equation S1 described in the
supporting information.
ꢖ
ꢌꢍꢎ
ꢕ
ꢈꢁꢂꢃꢄꢅ
ꢇ
ꢏꢐꢑꢒꢓꢔ
ꢖ
Фꢀꢁꢂꢃꢄꢅ
ꢆ
ꢋ
ꢋ Фꢀꢉꢊ
(S1)
ꢗ
ꢚ
ꢈꢉꢊ
ꢍꢎ
ꢏꢘꢙ
ꢇ
Φsample and Φstd are the radiative quantum yields of sample and the standard respectively, Risample
and Ristd are the refractive indices of the sample and standard solutions (pure solvents were
assumed respectively), ꢛꢜꢝꢞꢟꢠ ꢜꢡꢢ ꢛꢣꢢ the respective areas of emission for sample and
ꢇ
ꢇ
standard. The sample and the standard were excited at the same wavelength, maintaining equal
absorbance.
2. Data Spectra
Figure S1. Compound 2 EIMS.

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Figure S2. Compound TRB 1H NMR (200 MHz) CDCl3.
Figure S3. Compound TRB 13C NMR (50 MHz) CDCl3.

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Figure S4. Compound TRB 125Te NMR (63MHz, CDCl3).
Figure S5. Compound TRB HRMS (M + H+).

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Figure S6. Compound TRB FTIR.
3. Photophysical Studies
Table S1. Photophysical data of compound TRB in MeCN/H2O 75:25 (v/v) at 298 K.
Absorption data
Emission data
λabs (nm)
277
Log
ε
λem (nm)
Stokes shift (nm)
Φ
4.76
4.45
550
230
<0.01
320