A novel optical probe for Hg2+ in aqueous media based on mono-thiosemicarbazone Schiff base

Accepted Manuscript Title: A novel optical probe for Hg2+ in aqueous media based on mono-thiosemicarbazone Schiff base Authors: Yuqin Li, Wei Shi, Junchi Ma, Xin Wang, Xiangjuan Kong, Yipeng Zhang, Lei Feng, Yonghai Hui, Zhengfeng Xie PII: DOI: Reference:

S1010-6030(16)30999-6 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.01.026 JPC 10516

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

6-11-2016 19-1-2017 23-1-2017

Please cite this article as: Yuqin Li, Wei Shi, Junchi Ma, Xin Wang, Xiangjuan Kong, Yipeng Zhang, Lei Feng, Yonghai Hui, Zhengfeng Xie, A novel optical probe for Hg2+ in aqueous media based on mono-thiosemicarbazone Schiff base, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2017.01.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A

novel

optical

probe

for

Hg2+

in

aqueous

media

based

on

mono-thiosemicarbazone Schiff base

Yuqin Lia

Wei Shib

Junchi Mab Xin Wanga

Lei Fenga Yonghai Hui*a

a

Xiangjuan Konga Yipeng Zhanga

Zhengfeng Xie*b

Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, College of

Chemistry and Chemical Engineering, Xinjiang University, Urumqi 83004, China b

Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, School of

Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, China

* Corresponding author: fax: +86 028 83037305 E-mail address: [email protected] (Z. Xie); [email protected]

Graphic Abstract The probably sensing mechanism may attributed to the

photo-induced

electron-transfer (PET), and further induced a chelation-enhanced fluorescence quenching (CHEQ) effect. 1

Highlights



A novel mono-thiosemicarbazone Schiff base chemosensor M1 was easily prepared by a one-step reaction.



M1 showed great selectivity, sensitivity and anti-interference to Hg2+ ion.



The detection limit and the association constant were 3.11×10-8 M and 7.485×107 M-1, respectively.



M1 can be served as solid test strips to detect Hg2+conveniently.

Abstract: A novel mono-thiosemicarbazone Schiff base chemosensor M1 (N’-(4-diphenylamino)benzylidene)hydrazinecarbothiohydrazide) was designed and synthesized as a highly selective and sensitive probe for Hg2+ detecting in aqueous 2

media. A remarkable fluorescence quenching at 487 nm was observed in the presence of Hg2+, accompanied by the red-shift of the absorption spectrum. This sensing system displays remarkable specificity to Hg2+ against other possible competing ions. The detection limit and the association constant were 3.11×10-8 M and 7.485×107 M-1, respectively. Moreover, the disposable test strip will be a promising candidate for point-of-use monitoring of Hg2+ in environmental and biological samples with the advantages of cost-effectiveness, portability, and convenience. Key words: mono-thiosemicarbazones; Fluorescence chemosensors; Mercury; Solid state sensor; 1,3-diaminothiourea; Introduction Mercury (II), the most stable inorganic form of mercury with difficult biodegradability, is among the most hazardous and ubiquitous environmental contaminants with the feature of bioaccumulation and high toxic properties.[1-5] It can accumulate in the human body and affects a wide variety of diseases even in a low concentration, such as prenatal brain damage, serious cognitive and motion disorders, and Minamata disease.[6-9] When it was absorbed in the human body, ionic mercury can be converted into methyl mercury by bacteria in the environment, which enters the food chain and accumulates in higher organisms, owing to its greater affinity toward the SH groups of biomolecules.[10] Consequently, selective quantitative sensing of Hg2+ is highly imperative for environmental, biological, and clinical purposes. Though several techniques are currently available for detecting mercury and its 3

derivatives, such as spectrophotometry, neutron activation analysis, anodic stripping voltammetry, X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry. However, these methods require expensive equipment and involve time-consuming and laborious procedures that can only be conducted by trained professionals.[11] Optical fiber sensors, taking advantage of their fascinating characteristics, compact in size and light in weight, suitability of use in harsh environment, remote sensing capability, convenience and fast response time, have attracted great attention of many scientists.[12-14] Various luminescent chemosensors for Hg2+ with high sensitivity, versatility, and relatively simple handling have been described.[15-20] Most of the current Hg2+ probes are based on organic or inorganic molecules with sulfur donor centers utilizing the highly thiophilic nature of Hg2+ and some mercury-mediated reactions such as mercury-deselenation reactions,[21] mercury-catalyzed

hydrolysis,[22-26]

and

the

mercuration

reaction.[27]

Hydrazinecarbothioamide, a type of well-known ion-coordinative functional groups owing to the presence of C=S, –NH–, and –NH2 fractions in its chemical structure, has been introduced into various optical motifs to realize the optical detection of metal ions.[28-34] However,

it

is

noteworthy

that

probes

based

on

mono

aromatic-aldehyde-thiosemicarbazones are rarely reported so far. Considering our group’s

former

work

mono-thiosemicarbazone

for

Hg2+

and

Schiff

Ag+ base

detecting,[35] chemosensor

a

novel M1

(N’-(4-diphenylamino)benzylidene)hydrazinecarbothiohydrazide) was designed and 4

synthesized, which could be further condensed with other different aldehydes to obtain asymmetrical dithiosemicarbazones due to the existence of another free –NH2 group. In this work, we incorporated TPA (4-(diphenylamino)benzaldehyde) core as a fluorescent signaling unit, along with thiocarbonyl units and imine moieties as a suitable ionophore to coordinate Hg2+ selectively. The obtained probe showed more immediate and excellent selectivity, and the detection limits were lower than the former work. In addition, the solid test strips increased the potentially practical applications of M1. 1. Experimental section 1.1 Apparatus Melting point was determined on a Beijing X-4 microscopic melting point apparatus. IR spectra were recorded on an EQUINOX 55 FT-IR spectrometer with KBr pellets. 1

H NMR and

13

C NMR spectra were collected on a VARIAN INOVA-400

spectrometer operating respectively at 400 MHz (for 1H) and 100 MHz (for

13

C)

using TMS as the internal standard in DMSO-d6 solvent. Absorption spectra were recorded using a Metash UV-6000PC UV-Vis spectrometer. Emission spectra were recorded on a F-320 (GangdongTechnology) fluorescence spectrometer. HR-MS (TOF-MS) were recorded on a Waters Q-TOF Premier. 1.2 Materials All chemical reagents, unless otherwise specified, were purchased from Adamas and Acros Chemical Co. and used without further purification. The solution of various metal ions, with the exception of Hg(ClO4)2, were prepared from their nitrate. 5

Receptor M1 (0.0036 g) was dissolved in DMSO (10 mL) to obtain 1mM stock solutions and were freshly prepared by diluting the stock solutions to the final concentration of 10 μM before the spectroscopic measurements. Tris-HCl buffer solutions of different pH were prepared by combining proper amounts of Tris and HCl under adjustment by a pH meter. 1.3 Synthesis of M1 The synthesis route of sensor M1 was demonstrated in Scheme 1. Using the symmetrical diamine-thiocarbohydrazine as the parent reactants, by controlling the molar ratio of 4-(diphenylamino)benzaldehyde to diamine-thiocarbohydrazine (1:1). The mono-condensation product was obtained through slow addition of the former to an excess of the later reactant, five drops of glacial acetic acid and 15 mL ethanol was served as catalysts and reaction media, respectively. Crude product was purified by recrystallization from anhydrous ethanol and dried under vacuum to afford the pale yellow compound in 83.9% yield. M.p.: 208-209 oC. FT-IR (cm-1): 3283.9, 3246.5, 3159.3, 1586.8, 1276.1, 1170.2 1

H NMR (400 MHz, DMSO-d6, ppm) δ 11.33 (s, 1H), 9.66 (s, 1H), 7.93 (s, 1H), 7.70

(d, J = 8.6 Hz, 2H), 7.34 (t, J = 7.8 Hz, 4H), 7.20 – 6.99 (m, 6H), 6.90 (d, J = 8.6 Hz, 2H), 4.83 (s, 2H). C NMR (100MHz, DMSO-d6, ppm) δ 175.92, 148.79, 146.79, 141.99, 129.88,

13

128.72, 127.90, 125.07, 124.95, 124.04, 121.71. HR-MS: [M1+H+], m/z = 362.1416, calcd for: 362.1434. [M1+Na], m/z = 384.1242, calcd for: 384.1259. 6

2. Results and discussions 2.1. Synthesis and characterization The target compound M1 was synthesized by the condensation reaction between 4-(diphenylamino)benzaldehyde and symmetrical diamine-thiocarbohydrazine under mild condition (Scheme 1), and was characterized by NMR, FT-IR, and MS analyses (Fig. S7–S10). 2.2 Spectroscopic experiments A series of host-guest recognition experiments were performed to investigate the Hg2+ recognition ability of M1 in aqueous medium by UV–Vis absorption and fluorescence at physiological pH (50 mmol L−1, Tris-HCl buffer, pH=7.0) under ambient conditions. The fluorescence emission spectra were recorded in the wavelength range of 400–650 nm excited by 375 nm. Both the excitation and emission slits were set at 5 nm/5 nm. 2.3 Fixing of experimental condition for probing process An optimized experimental parameter is of importance for all the analytical performance, in this respect, preliminary studies were focused on the fixing of detecting environment firstly. In order to accomplish the best selectivity of M1, different ratio of DMSO and water mixed systems, pH value and time response were investigated. As shown in Fig. 1, fluorescence intensity of M1 achieved the maximum when the fraction of DMSO in co-solvent is 70%, which is rather appropriate for the “turn-off” Hg2+ sensing, but it may suffer from some selectivity problems such as Ag+ and Cu2+. Based on the well-known strong chelating ability of three hydroxyl 7

methylamine to metal ions, Tris-HCl was employed as the masking agents in the following detecting. Intriguingly, the fluorescence quenching brought by Cu2+ and the enhancement resulted by Ag+ were significantly blocked when the proportion of DMSO and Tris-HCl solution was 8:2 (Fig. S1), and the quantum yield of M1 was calculated to be 0.173. (Fig. S12) In addition, PH dependence of sensor M1 both in the absence and the presence of Hg2+ was investigated and almost no obvious fluorescence change of M1 could be observed when the pH ranged from 6.5 to 10.0 (Fig. S2), therefore, pH value of the probing medium was controlled at 7.0 which can be performed in biological system. Furthermore, time response of M1 to detect Hg2+ was also monitored (Fig. S3). Evidently, the fluorescence quenched remarkably and fixed constantly within 10 s when treated with Hg2+ (10 equiv.), indicating the immediacy of probe M1. According to the above experimental results, DMSO/Tris-HCl (VDMSO/VTris-HCl = 8/2, PH=7.0), 20 s time response were selected as the detecting conditions of all the subsequent experiments. 2.4. Selectivity over metal ions The UV-Vis absorption and the Fluorescence spectra properties of M1 in DMSO/Tris-HCl (8/2, v/v) have been investigated and corresponding results are displayed in Fig. 2 and Fig. 3, respectively. In the UV-Vis spectrum of M1, upon addition of 10 equiv. various metal ions (Ag+, Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Pb2+, Ni2+, Mg2+, Na+, K+, Zn2+ and Hg2+), clearly, Hg2+ induced a distinctive bathochromic shift on the absorption band at 375−400 nm, except that, the maximum absorption peak of M1 red-shifted and decreased concomitantly after the addition of 8

Cu2+, which provides an effective method to distinct these two ions from others. In the fluorescence spectra, the introduction of Hg2+ to the mixture solution of M1 leads to a drastically pronounced fluorescence quenching (quenching efficiency (I0 − I) / I0 × 100 = 95.5%, ΦM1-Hg = 0.014), and a marginal red-shift resulted by Ag+ was observed, while the rest of the metal cations exerted a negligible or very small influence under the identical conditions, which was further evidenced by the illumination taken under 365 nm portable UV lamp (Fig. 3.b). The surprisingly high selectivity of M1 for the Hg2+ over the other used cations, most probably arises from the strong coordination tendency of Hg2+ to sulphur and nitrogen-donor ligands. 2.5 Titration experiments Quantitative behaviors of M1 were assessed by fluorescence titration to further understand the binding affinity of M1 with Hg2+. Notably, upon progressive addition of Hg2+, the initial absorption band at 375 nm decreased constantly accompanying the appearance of a new isosbestic point at 385 nm (Fig. 4.a). A red-shift of about 10 nm in the absorption spectra resulted in a slight color change from clear to light-yellow, which is perceptible to the naked-eyes. Meanwhile, it was observed that an increasing amount of Hg2+ induced a gradual decrease in the fluorescence, and a saturation point was attained at 10 equivalents of Hg2+ ion concentration (Fig. 4.b). Corresponding calibration curve of the fluorescence intensities at 487 nm was presented as a function of the Hg2+ concentration over a range of 0–10 equiv. (Fig. 4.c). The normalized fluorescence intensities were linearly proportional to the concentrations of Hg2+ from 5×10-7 to 4×10-6 M, suggesting the potential application for quantitative determination 9

of Hg2+. The detection limit for Hg2+ of M1 was estimated to be as low as 3.11×10-8 M according to 3σ criteria,[36, 37] which is sensitive enough for the detection of the sub-millimolar Hg2+ concentrations found in many chemical and biological systems. 2.6 Anti-interference performance To further evaluate the utility of M1 as an Hg2+ selective chemosensor, competition experiments were conducted by using a dual metal system under the same circumstances (Fig. 5). Apparently, the remarkably quenched signal of probe M1 toward Hg2+ was not substantially perturbed by the background of other coexisting species. In addition, the counter anions (Br-, H2PO4-, HPO42-, PO43-, P2O74- , CN-, Cl-, S2-, SCN-, HSO3-, OH-, CO32-, F-, CH3COO-, NO3-, NO2-, I-) failed to exhibit any significant alteration in the photophysical behavior (Fig. S4), demonstrating the excellent specificity of the probe for Hg2+ in aqueous media. 3. Applications of M1 To facilitate the feasible application of M1 for the detection of Hg2+, test strips were prepared by immersing filter papers into a binary solution DMSO/H2O (v/v = 8:2) of M1 (0.1 mM) and then dried under vacuum condition. As shown in Fig. 6, when the dried test strips were drop-casted with different concentration solutions of Hg2+ and dried again for 15 minutes, more and more obvious dark spots can be observed along with the incremental concentration of Hg2+ under UV irradiation, which is convenient to handle at any moment for Hg2+ detection. The discernible concentration of Hg2+ could be as low as 1.0 × 10-4 M, indicating the potentially convenient and practical application of M1 to Hg2+ sensing. 10

We further investigated the detection ability of probe M1 toward Hg2+ in different environmental water samples by collecting the lake water, tap water and distilled water. The fluorescence responses of M1 toward these water samples were examined directly or spiked with different concentrations of Hg2+ (0, 5, 10, and 15 equivalents), respectively. As shown in Fig. 7, the similar response behavior to Hg2+ was obtained. Therefore, probe M1 was credible and practically feasible for detecting Hg2+ in real environmental samples. 4. 1H NMR titration experiments 1

H NMR titration experiments were executed in DMSO-d6 by the gradual addition of

Hg2+ to elucidate the plausible intermolecular interactions between probe M1 and Hg2+. (Fig. 8) Signal of proton NHd was found farther downfield than NHc as this proton was deshielded by anisotropic effect of adjacent imine bond, corresponding to the characteristic peaks at 11.33 ppm and 9.66 ppm, respectively.[38, 39] Upon addition of 0.5 equiv. Hg2+, proton Hd and Hc exhibited significant downfield shifts and appeared at 12.88 ppm and 11.58 ppm, respectively. Meanwhile, the signal He of imine downfield shifted from 7.93 ppm to 8.14 ppm, indicating that the thiosemicarbazide segment of M1 was involved in the coordination process.[40, 41] [42] When treated with 1 equiv. Hg2+, He weakened to a large extent, and was kept unchanged with the addition of 2 equiv. Hg2+, suggesting that the association between M1 and Hg2+ was saturated by the addition of 1 eq. Hg2+ [43-45], which is in good agreement with the finding reflected by relevant Job’s plot (Fig. S5). The charge density of mono-thiosemicarbazones-Hg2+ moiety increased after the coordination 11

between M1 and Hg2+, and further induced a chelation-enhanced fluorescence quenching (CHEQ) effect[46] due to the photo-induced electron-transfer (PET).[35, 47, 48] The overall association constant was calculated to be 7.485×107 M-1 based on the Benesi–Hildebrand equation (R2 = 0.9824),[49] (Fig. S6). Above all, the HR-MS spectra provides a solid evidence for the formation of the M1-Hg2+ complex, the prominent peak appeared at m/z 681.2692 is assignable to the species of [M1-Hg2+ + ClO4- + H2O + H+] (Fig. S11). Based on all the above obtained findings, we proposed that the reaction in this system may proceed according to the route depicted in 5. Conclusion In summary, a novel fluorescent probe M1 based on mono-thiosemicarbazone Schiff base was designed and synthesized, which is capable of selectively detecting Hg2+. The obtained probe displayed preeminent selectivity and superior sensitivity toward Hg2+. Moreover, M1 could be served as a practical fluorescent chemosensor for rapid monitoring of Hg2+ ion by virtue of test strips, suggesting the potentially practical application of M1. We expect this fluorescent probe can extend its potential applications in environment and biological systems.

Acknowledgement The authors are greatly appreciating the financial support from the National Natural Science Foundation of China (Project No. 21262034, 21364013).

12

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García-Tasende,

Synthesis

of

[2-(pyridin-2′-yl)phenyl]-mercury(II)

arylthiosemicarbazonates: An unusual coordination mode of a deprotonated 2-formyl-(2-hydroxy-benzene)-thiosemicarbazone, Polyhedron, 17 (1998) 3701-3709. [45] Y. Yu, L.-R. Lin, K.-B. Yang, X. Zhong, R.-B. Huang, L.-S. Zheng, p-Dimethylaminobenzaldehyde thiosemicarbazone: a simple novel selective and sensitive fluorescent sensor for mercury(II) in aqueous solution, Talanta, 69 (2006) 103-106. [46] L. Tang, P. Zhou, Q. Zhang, Z. Huang, J. Zhao, M. Cai, A simple quinoline derivatized thiosemicarbazone as a colorimetic and fluorescent sensor for relay recognition of Cu2+ and sulfide in aqueous solution, Inorg. Chem. Commun. 36 (2013) 100-104. [47] L. Feng, W. Shi, J. Ma, Y. Chen, F. Kui, Y. Hui, Z. Xie, A novel thiosemicarbazone Schiff base derivative with aggregation-induced emission enhancement characteristics and its application in Hg2+ detection, Sens. Actuators B Chem. 237 (2016) 563-569. [48] Y. Fang, Y. Zhou, J.-Y. Li, Q.-Q. Rui, C. Yao, Naphthalimide–Rhodamine based chemosensors for colorimetric and fluorescent sensing Hg2+ through different signaling mechanisms in corresponding solvent systems, Sens. Actuators B Chem. 215 (2015) 350-359. [49] C. Wu, J.-L. Zhao, X.-K. Jiang, C.-Z. Wang, X.-L. Ni, X. Zeng, C. Redshaw, T. Yamato, A novel fluorescence "on-off-on" chemosensor for Hg2+via a water-assistant blocking heavy atom effect, Dalton Trans. 45 (2016) 14948-14953. 19

20

2500

DMSO:H2O=8:2

DMSO:H2O=7:3 DMSO DMSO:H2O=9:1

2000

DMSO:H2O=8:2

PL intensity

DMSO:H2O=7:3 DMSO:H2O=6:4

1500

DMSO:H2O=5:5 DMSO:H2O=4:6 DMSO:H2O=3:7

DMSO

1000

DMSO:H2O=2:8 DMSO:H2O=1:9 DMSO:H2O=20:1980

99% water

500

0 400

500

600

λmax/nm

Fig. 1 Emission spectra of M1 (~1.0 × 10-5 M) in DMSO and water mixtures with different solvents ratios. Fe

0.6

0.5

Abs

0.4

Hg

2+

0.3

0.2

Cu

2+

0.1

350

400

450

wavelength/nm

Fig.2 UV–Vis spectra of M1 (1.0 × 10-5 M) in DMSO/Tris-HCl (8/2, v/v, pH=7.0) upon the addition of 10 eq. various metal ions... a

2500

PL intensity

2000

1500

Ag

+

1000

500

Hg

2+

0 400

500

λmax/nm

600

b

Fig. 3 (a) Fluorescence emission spectra of M1 in DMSO/Tris-HCl (8/2, v/v, pH=7.0) 21

upon addition of 10 eq. various metal ions, (excited by 375 nm). (b) Under portable UV lamp (365 nm). a

0.6

b

0.5

2000

0 0.4

Hg

Hg

0

0.3

2+

PL intensity

Abs

1500

10 eq

2+

10 eq 1000

0.2 500

0.1

0.0

0

350

400

450

400

500

600

wavelength/nm

wavelength/nm

c 2000 2200

Equation

y = a + b*x

Weight Residual Sum of Squares Pearson's r

Instrument

Adj. R-Squar

2.13315 -0.99684 0.99158 Value

PL intensity

2000

1000

Standard Err

B

Intercept

2180.9110

13.72397

B

Slope

-127.5160

5.8674

1800

1600 0

1

2

3

4

0 0

100

200

[Hg2+]×10-6

Fig. 4 UV–Vis (a) and fluorescence (b) spectra of M1 (1.0 × 10-5 M) towards different concentrations of Hg2+ in DMSO/ Tris-HCl mixture (8/2, v/v, pH=7.0). (c) The linear relationship between fluorescence intensity of M1 and the different concentrations of Hg2+ (inset is the corresponding enlarged range with Hg2+ from 5×10-7 to 4×10-6 M.) (excited by 375 nm) metal ions metal ions + Hg2+

2000

PL intensity

1500

1000

500

0 Ag

+

3+ 2+ 2+ 2+ 2+ 2+ + 3+ 3+ + 2+ 2+ 2+ 2+ 2+ Al Ba Ca Cd Pb Mg Na Fe Cr K Co Cu Ni Zn Hg

Fig. 5 Fluorescent intensity of the competition experiments between Hg2+and other 22

metal ions. (excited by 375 nm)

e

d

c

b

a

Fig. 6 Test-strips of M1 (0.1 mM) for Hg2+ after addition increasing concentrations of Hg2+ (a) M1 (b) 1×10-5 M. (c) 1×10-4 M . (d) 5×10-4 M. (e) 1×10-3 M under 365 nm illumination.

2500

PL intensity

2000

distilled water tap water lake water

1500

1000

500

0

only water

5equiv. Hg2+

2+

10 equiv. Hg

2+

15equiv. Hg

Fig. 7 Fluorescent detection of different equiv. of Hg2+ in distilled water, tap water, and lake water by M1. Scheme 2. M1+2.0 eq Hg Hd

Hc

Hd

Hc

Hd

Hc

2+

He M1+1.0 eq Hg

He

2+

He M1+0.5 eq Hg

14

12

Ha,Hb

He

Hd

Hc 10

2+

M1 8

6

4

Fig.8 1H NMR spectral patterns upon adding 0 to 2 equiv. of Hg(ClO4)2 in DMSO-d6

23

Scheme 1. The synthetic procedure for receptor M1

Scheme 2. The proposed mechanism for detection of Hg2+ by M1.

24