Hydrophilic Indolium Cycloruthenated Complex ... - ACS Publications

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Hydrophilic Indolium Cycloruthenated Complex System for Visual Detection of Bisulfite with a Large Red Shift in Absorption Xianlong Su,†,§ Rongrong Hu,‡,§ Xianghong Li,*,† Jun Zhu,† Facheng Luo,† Xuehu Niu,† Mei Li,† and Qiang Zhao*,‡ †

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China ‡ Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, China S Supporting Information *

ABSTRACT: Bisulfite, as an important additive in foodstuffs, is one of the most widely distributed environmental pollutants. The excessive intake of bisulfite may cause asthmatic attacks and allergic reactions. Therefore, the determination and visual detection of bisulfite are very important. Herein, a newly designed hydrophilic indolium cycloruthenated complex, [Ru(mepbi)(bpy)2]+ [1; bpy = 2,2′-bipyridine and Hmepbi = 3,3-dimethyl-1-ethyl-2-[4(pyridin-2-yl)styryl]benzo[e]indolium iodide (3)], was successfully synthesized and used as a bisulfite probe. The bisulfite underwent a 1,4-addition reaction with complex 1 in PBS buffer (10 mM, pH 7.40), resulting in a dramatic change in absorption spectra with a red shift of over 100 nm and a remarkable change in solution color from yellow to pink. It is worth noting that this obvious bathochromic shift is rarely observed in the detection of bisulfite through an addition reaction. The detection limit was calculated to be as low as 0.12 μM by UV−vis absorption spectroscopy. Moreover, complex 1 was also used to detect bisulfite in sugar samples (granulated and crystal sugar) with good recovery. complexation with amines,24,25 and Michael-type additions.26−29 However, most of these probes exhibit some drawbacks. For instance, aldehyde-based probes can only be operated in acidic conditions and may be interfered with by cysteine and homocysteine.30,31 Levulinate-based probes display sensitivity under high bisulfite concentrations and are potentially cleaved by proteases and esterases.20−23 In addition, most of these probes have poor water solubility, and biologically toxic solvents or surfactants are needed to prepare homogeneous solutions for sensing,32,33 such as acetonitrile, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF). Therefore, searching for water-soluble probes that are sensitive and selective toward HSO3− is attractive and challenging. Recently, Sun et al. developed a fluorescent probe based on benzo[e]indolium that exhibited a high selectivity and sensitivity to HSO3− in PBS buffer.34 It is well-known that an appreciable change in solution or emission colors is useful for rapid visual sensing. However, a change only in the fluorescence intensity of this probe may lead to signal fluctuations by variations in the sample environment and probe concentration. Moreover, the solution of the probe displayed a color change from light yellow to colorless in the presence of HSO3−, which

1. INTRODUCTION Bisulfite has been widely used as an important additive in foods, beverages, and pharmaceutical products.1−3 Moreover, SO2 discharged by industry can be hydrated to form its derivative HSO3−. It should be noted that the excessive intake of bisulfite would cause harmful effects to tissue, cells, and biomacromolecules, causing asthmatic attacks and allergic reaction in some individuals.4−7 In view of the reported harmful effects of bisulfite on human health, its contents in food and medicine have been strictly limited in many countries. For example, the bisulfite content in sugar may not exceed 0.10 g kg−1 (calculated as SO2) in China.8 Currently, many analytical techniques for bisulfite are available, including flow-injection analysis,9 capillary electrophoresis,10 chemiluminescence,11 chromatography,12 and enzymatic techniques.13 However, most of these methods for bisulfite determination often require complicated instruments and suffer from troublesome sample pretreatment and reagent preparation, thus raising the determination time. It has become urgent to develop more convenient and faster analytical methods to determine bisulfite. Optical molecular probes have drawn much attention because of their simplicity, sensitivity, and virtues in real-time observation and visualization. To date, most of the reported molecular probes for bisulfite are based on specific chemical reactions, such as the addition reaction of bisulfite with aldehyde,14−19 selective deprotection of a levulinate group,20−23 © XXXX American Chemical Society

Received: September 25, 2015

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DOI: 10.1021/acs.inorgchem.5b02210 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthetic Routes of Complex 1

group (Scheme 1). This complex can sense bisulfite by utilizing the 1,4-addition reaction on the benzo[e]indolium moiety with an ethylene group. In PBS buffer (pH 7.40, 10 mM), emission of the probe was not observed in the presence or absence of bisulfite at room temperature. Interestingly, a remarkable redshifted metal-to-ligand charge-transfer (MLCT) absorption (above 100 nm) was observed in the presence of bisulfite, while addition reactions often lead to blue shifts of the absorptions due to the destruction of conjugated structures. The evident solution color change from yellow to red showed that complex 1 could serve as a colorimetric and visual probe for detecting HSO3− by the naked eye. Meanwhile, this probe exhibited excellent selectivity and sensitivity toward bisulfite over other common anions and relevant sulfur-containing biomolecules. Moreover, the probe could determine the bisulfite level in real sugar samples with good recovery.

was not easily observed with the naked eye at a low concentration of the probe. Compared with organic dyes, transition-metal complexes have received wide research interest as a new generation of probes for anions, 35−37 oxygen concentration,38 metal ions,39−42 etc., because their rich excited states are sensitive to external environments or stimuli.43,44 However, there have been only a few reports of bisulfite probes based on transitionmetal complexes.45−48 Cyclometalated ruthenium complexes, as a type of ruthenium oligopyridine complex, have drawn much attention in chemosensor fields for their remarkable photophysical properties with the absorption shifted to the longwavelength region by replacing a nitrogen atom in bipyridine or terpyridine with an anionic carbon center.49−51 In our recent work, we designed and synthesized a water-soluble colorimetric probe for bisulfite based on cycloruthenated complex by using the the nucleophilic addition of aldehyde,52 which exhibited a remarkable color change from deep red to light yellow. Nevertheless, the detection process was time-consuming (about 2 h). Considering the disadvantage of this complex in practical application, we hope to develop a water-soluble cyclometalated ruthenium complex that can detect bisulfite selectively and rapidly with evident changes in absorption spectra and solution color. Herein, a new hydrophilic cyclometalated ruthenium complex, [Ru(mepbi)(bpy)2]+ [1; bpy = 2,2′-bipyridine and Hmepbi = 3,3-dimethyl-1-ethyl-2-[4-(pyridin-2-yl)styryl]benzo[e]indolium iodide (3)], was successfully synthesized and characterized. It contains a benzo[e]indolium and a cycloruthenated bipyridine complex connected by an ethylene

2. EXPERIMENTAL SECTION Materials. [Ru(pba)(bpy) 2 ] + [2; Hpba = 4-(2-pyridyl)benzaldehyde]52 and 2,3,3-trimethyl-1-ethylbenz[e]indolium iodide53 were prepared by the methods described in the literature and confirmed by 1H NMR spectroscopy. All solvents and reagents were commercially available and were used without further purification unless specified. Physical Measurements. NMR spectra were performed on a Bruker Avance-400 spectrometer. Mass spectrometry (MS) spectra were obtained on an AB Scies matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectrometer. Absorption spectra were recorded using a Shimadzu UV-2550 spectrophotometer. Emission spectra were obtained on a Hitachi F-7000 fluorescence spectrometer at room temperature. All pH measurements were carried out with a model pHS-3B pH meter (Shanghai, China), and UV−vis B


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Inorganic Chemistry absorption spectra at different pH values (1−12) were carried out in aqueous solutions with a Briton−Robinson buffer and 0.1 M NaCl to keep a constant ionic strength. All spectrophotometric titration experiments involving interaction of the compounds with fresh anions (F−, Cl−, Br−, I−, SO42−, NO3−, NO2−, ClO4−, H2PO4−, HPO42−, CO32−, HCO3−, Ac−, SCN−, HS−, CN−, and HSO3−) were carried out in PBS buffer (pH 7.40, 10 mM). Caution! All of the cyanide salts are highly toxic and therefore should be caref ully handled with safety gloves. All of those used in the experiments should be treated with ferrous salts. Synthesis of 3,3-Dimethyl-1-ethyl-2-[4-(pyridin-2-yl)styryl]benzo[e]indolium Iodide (3, Hmepbi). 4-(2-Pyridyl)benzaldehyde (0.52 g, 2.8 mmol) was dissolved in 10 mL of dry ethanol, and 2,3,3-trimethyl1-ethylbenz[e]indolium iodide (1.04 g, 2.8 mmol) was subsequently added. The reaction mixture was refluxed for 24 h under argon and then cooled to room temperature. The precipitate was collected by filtration, washed with petroleum ether/ethyl acetate (3:1, v/v), and then dried in vacuo. Finally, 3 was obtained as an orange-red solid in 70% yield (1.05 g). 1H NMR (DMSO-d6, 400 MHz): δ 8.72 (d, J = 4.48 Hz, 1H), 8.58 (d, J = 16.44 Hz, 1H), 8.43 (d, J = 8.32 Hz, 1H), 8.33−8.27 (m, 5H), 8.22 (d, J = 8.12 Hz, 1H), 8.11 (t, J = 8.32 Hz, 2H), 7.96 (td, J = 7.80 Hz, J = 1.12 Hz, 1H), 7.84 (td, J = 8.04 Hz, J = 1.12 Hz, 1H), 7.78−7.72 (m, 2H), 7.45 (dd, J = 7.72 Hz, J = 4.80 Hz,1H), 4.86 (q, J = 7.20 Hz, 2H), 2.04 (s, 6H), 1.75 (t, J = 7.32 Hz, 3H). Synthesis of [Ru(mepbi)(bpy)2]+ (1). 2 (0.26 g, 0.35 mmol) and 2,3,3-trimethyl-1-ethylbenz[e]indolium iodide (0.26 g, 0.71 mmol) were dissolved in 10 mL of dry ethanol. The mixture was heated at reflux for 36 h under argon. Then the solvent was removed by evaporation. The crude product was purified by column chromatography on silica gel using CH3OH/CH2Cl2/CH3CN as an eluent to afford the complex as a claybank solid (0.18 g, 47%). 1H NMR (CD3OD, 400 MHz): δ 8.68 (d, J = 8.32 Hz, 1H), 8.61 (d, J = 8.11 Hz, 2H), 8.54 (d, J = 8.08 Hz, 1H), 8.39 (d, J = 8.62 Hz, 1H), 8.31− 8.23 (m, 3H), 8.17 (d, J = 5.56 Hz, 2H), 8.12−8.06 (m, 2H), 8.04 (t, J = 7.98 Hz, 1H), 7.99 (d, J = 8.88 Hz, 1H), 7.93−7.79 (m, 7H), 7.73 (d, J = 5.60 Hz, 2H), 7.63 (d, J = 8.15 Hz, 1H), 7.54 (t, J = 6.59 Hz, 1H), 7.44 (t, J = 7.10 Hz, 1H), 7.30−7.27 (m, 2H), 7.24 (d, J = 16.23 Hz, 1H), 7.10 (d, J = 8.47 Hz, 2H), 4.68 (q, J = 7.43 Hz, 2H), 2.01 (s, 6H), 1.61 (t, J = 7.30 Hz, 3H). 13C NMR (CD3OD, 100 MHz): δ 182.04, 166.09, 157.78, 157.15, 156.79, 155.30, 154.67, 154.13, 150.39, 149.99, 149.97, 148.75, 138.77, 137.89, 136.56, 136.07, 135.79, 135.24, 134.15, 134.03, 133.91, 133.52, 131.36, 129.91, 128.21, 127.29, 127.16, 126.98, 126.45, 126.10, 125.99, 124.19, 123.51, 123.40, 123.18, 123.07, 122.76, 120.42, 112.01, 109.98, 99.99, 53.98, 42.17, 24.97, 24.94, 12.71. MS (MALDI-TOF, CHCA). Calcd: m/z 815.24 (M − H+). Found: m/z 815.19. Sugar Sample Test. Granulated sugar and crystal sugar purchased commercially were used in the sample analysis. Sample solutions were prepared by dissolving 5.0 g of sugar in deionized water and then diluting to 10 mL. Aliquots of the sugar solutions were added directly to the solutions of 1 (20 μM) in PBS buffer (pH 7.40, 10 mM). The standard curve was determined by adding bisulfite of several given concentrations into the solution of 1 (20 μM) in PBS buffer (pH 7.40, 10 mM). Theoretical Calculations. All of the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed with the Gaussian 09 package.54 The structures in the ground state (S0) were fully optimized by Becke’s three-parameter exchange functional along with the Lee−Yang−Parr correlation functional (B3LYP) and plus polarization function 6-31G* basis set. Calculations of electronic absorption spectra were performed with the TDDFT method at the B3LYP/6-31G* level based on the optimized ground-state structures. For all of the calculations, the associated basis set was applied to describe ruthenium with the “double-ζ”-quality LANL2DZ basis set, whereas for all other atoms, the 6-31G(d) basis set was employed. The contours of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were plotted.

3. RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes of 1 are shown in Scheme 1. It is well-known that [Ru(cycme)Cl2]2 is a good ruthenium source for synthesizing cyclometalated ruthenium complexes. Herein, 3 was first prepared from 4-(2-pyridyl)benzaldehyde reacting with 2,3,3-trimethyl-1ethylbenz[e]indolium iodide in ethanol, which was characterized by 1H NMR (Figure S1). Subsequently, the cycloruthenated reaction was performed by mixing 3 with [Ru(cycme)Cl2]2 in acetonitrile in the presence of sodium hydroxide or triethylamine, followed by adding 2,2′-bipyridine to afford the final complex 1 (route i in Scheme 1). However, this synthetic route is not desirable because of the formation of many byproducts, which may be attributed to the instability of 3 under basic conditions.34,55 Therefore, the cyclometalated ruthenium complex 2 was synthesized first and then reacted with 2,3,3-trimethyl-1-ethylbenz[e]indolium iodide (route ii in Scheme 1). By this method, complex 1 was obtained finally with a reasonable yield of 47%. The structure of 1 was characterized by 1H NMR (Figure S2), 13C NMR (Figure S3), and MS (Figure S4) spectra. Absorption Properties of Ligand 3 and Complex 1. As expected, both the ligand 3 and complex 1 possess good water solubility. The UV−vis absorption spectra of 1 and 3 in PBS buffer are shown in Figure 1. Ligand 3 displayed two strong

Figure 1. UV−vis absorption spectra of 1 (20 μM) in PBS buffer (pH 7.40, 10 mM) and 3 (20 μM) in PBS buffer (pH 7.40, 10 mM, containing 2% ethanol).

absorption bands centered at 384 and 424 nm, respectively. After cycloruthenation of ligand 3, the obtained complex 1 displayed several absorption bands in the UV−vis regions. The bands occurring at 292 and 390 nm can be assigned as ligandcentered charge-transfer transitions from bipyridine and ligand 3, respectively. The broad absorption band with a maximum at 455 nm (ε = 3.16 × 104 M−1 cm−1) in the visible region can be assigned as a Ru2+ → C^N MLCT transition. This assignment is based on the large extinction coefficients and their similarity to MLCT transitions in related ruthenium(II) complexes.56−59 Then, the stability of 3 and 1 in PBS buffer was investigated by tracking the changes in the absorption intensity. The timedependent absorption spectra of 3 and 1 are shown in Figures S5 and S6, respectively. As depicted in Figure S5, the absorbances of 3 at 384 and 424 nm increased by less than C


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Inorganic Chemistry

shows the absorption intensities of complex 1 at 455 nm in the absence and presence of HSO3− in buffers over the pH range of 1.82−11.95. As shown in Figure 2, complex 1 is stable at the range of pH 3.53−8.52. However, a decrease in the absorption intensity at 455 nm was observed in the pH range from 3.53 to 1.82, which can be attributed to the effect of the carbanionic ligand.52,59 Nevertheless, the absorption intensity at 455 nm was also gradually reduced by increasing the pH values from 8.52 to 11.95. These obvious changes could be assigned to the possible OH− addition on the benzo[e]indolium moiety with an ethylene group under strong basic conditions.34,55 Upon the addition of bisulfite to the above solutions of different pH values, the probe 1 displayed a good response to HSO3− in a wide pH range of 4.50−8.52 (see Figure 2). As a result, PBS buffer at pH 7.40 was chosen as the sensing system. Optical Response of 3 to Bisulfite. In PBS buffer (pH 7.40, 10 mM, containing 2% ethanol), the absorption spectra of 3 (20 μM) after the addition of various amounts of bisulfite (0−100 μM) are shown in Figure 3. The intensities of the

10% within 4 min. After 30 min, the absorbances at 384 and 424 nm significantly increased by 30% and 46%, respectively, which may be due to hydrolysis of the indolium cations. Compared with the absorption change of 3, the absorption intensity of complex 1 was only reduced by 7% at 455 nm within 30 min (Figure S6). These results indicated that the stability of compounds containing the 2,3,3-trimethyl-1ethylbenz[e]indolium moiety could be improved by cyclometalation, which was favorable for practical application. Optimization of Sensing Conditions. Considering the impact of the aforementioned stability and the addition reaction time with bisulfite, time-dependent changes in the absorptions of 1 and 3 in the presence of bisulfite were investigated, respectively. As depicted in Figures S7 and S8, the absorption intensities of 1 and 3 in PBS buffer (pH 7.40, 10 mM) remained almost unchanged in the absence of HSO3−. After the addition of HSO3−, monitoring of the absorption changes confirmed that all of the reactions finished rapidly. As a result, the detection was delayed for 2 min to ensure that the reactions were completed. Moreover, it is worth noting that 1 with more than 5 equiv of HSO3− displayed a remarkable color change from yellow to pink within 1 min, while 3 exhibited a color change from yellow to colorless after the addition of bisulfite. Apparently, complex 1 exhibited the incomparable advantage of solution color change over ligand 3, especially at lower probe concentration. As shown in Figures S9 and S10, complex 1 can display an obvious color change even at a low concentration of 1 × 10−6 M. Subsequently, complex 1 was selected to investigate the pH effect on the interaction between bisulfite and the probe. To the best of our knowledge, the cyclometalated ruthenium complex was unstable under acidic conditions because of the possible dissociation of the Ru−C bond, which needs a period of time to reach equilibrium.52,59 Therefore, complex 1 was incubated in these buffer solutions (pH 1.82−11.95) for 2 h to exclude the extra influence on the interaction between the probe and bisulfite, which was induced by possible dissociation of the Ru−C bond in acidic conditions. The absorption changes of complex 1 in the absence or presence of bisulfite at different pH values are shown in Figure S11. Herein, Figure 2

Figure 3. Changes of absorption spectra of 3 (20 μM) in PBS buffer (pH 7.40, 10 mM, containing 2% ethanol) with increasing amounts of HSO3−. Each spectrum was recorded after a 2 min delay. Inset: Photograph of 3 in the absence (left) and presence (right) of HSO3−.

absorption bands centered at 384 and 424 nm gradually decreased, while those at 321 and 265 nm increased. The weakened absorption band centered at 384 nm and the obvious vanished absorption band centered at 424 nm led to a color change from light yellow to colorless (Figure 3, inset). These results suggested that π conjugation was broken by the addition reaction between 3 and HSO3−. Then, the detection limit for HSO3− in PBS buffer (pH 7.40, 10 mM, containing 2% ethanol) utilizing UV−vis absorption spectra was calculated to be approximately 3.0 μM (Figure S12). Furthermore, the fluorescence spectra of 3 with the addition of bisulfite were also investigated. As shown in Figure S13, 3 exhibited an emission band at 558 nm, which is attributed to the large π-conjugation skeleton. The addition of bisulfite resulted in a gradual decrease in the emission at 558 nm and an increase in the emission at 470 nm, which corresponds to that of the 2-methylenebenzo[e]indoline moiety. A well-defined isoemissive point at 548 nm was observed. The ratiometric change in the fluorescence intensity indicated that the addition reaction between 3 and bisulfite interrupted π conjugation, and fluorescence of the 2-methylenebenzo[e]indoline moiety was

Figure 2. Absorption intensities of 1 (15 μM) at 455 nm in the absence and presence of HSO3− (75 μM) under different pH conditions. D


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Inorganic Chemistry

Figure 4. 1H NMR spectral change of 3 in the absence (A) and presence (B) of 1 equiv of NaHSO3 in 7:1 (v/v) DMSO-d6/D2O.

recovered. Accordingly, the fluorescence color changed from orange to cyan (Figure S13, inset). It is worth noting that the ratio changes produced an excellent linear function with the HSO3− concentration between 0 and 10 μM (Figure S14), and the detection limit for bisulfite was calculated to be 0.089 μM. Subsequently, the addition reaction between 3 and bisulfite was identified by 1H NMR analysis. As shown in Figure 4, all of the 1H NMR signals of 3 shifted to upfield after HSO3− was added into its solution. For instance, the proton signals at δ 4.84 and 1.55, which originated from the ethyl group connected with N+, were significantly shifted upfield to δ 3.68 and 1.11, respectively. The proton signal (Hc at δ 2.04) of the two methyl groups was also shifted upfield and divided into two single signals (Hc′ at δ 1.90 and 1.54), which became nonequivalent after the formation of 3-SO3H. In addition, the protons linked to unsaturated carbon atoms displayed signals between δ 7.4 and 8.7, also shifting to upfield in the range from δ 7.0 to 8.6. The proton signal (Hd at δ 7.4−8.7) appeared at δ 5.04 after the HSO3− addition. These evident upfield shifts can be attributed to the nucleophilic attack of HSO3− toward C-4, which disturbed the π conjugation and weakened its electronwithdrawing characteristic. Therefore, the addition reaction between 3 and HSO3− was proposed as a 1,4-addition reaction rather than a 1,2-addition reaction (see Figure 4).34,55 Optical Response of 1 to Bisulfite. Next, the interaction between complex 1 and bisulfite was investigated carefully by absorption spectra. The response of 1 to different concentrations of HSO3− in PBS buffer (pH 7.40, 10 mM) is illustrated in Figure 5. Upon the addition of HSO3−, the absorption bands between 350 and 530 nm decreased remarkably, along with a red shift of the band at 455 nm to 491 nm. Moreover, the absorption intensity from 530 to 700 nm also increased, and a new absorption band at 560 nm appeared. It should be noted that the two bands occurring at around 491 and 560 nm are characteristic of ruthenium cyclometalated complexes.56−59 The band at 560 nm was assigned as Ru → bpy CT transitions. This dramatic red shift may be attributed to the interruption of π−π conjugation by the 1,4-addition of HSO3− to CC of ligand 3,34 which

Figure 5. Changes of the absorption spectra of 1 (20 μM) in PBS buffer (pH 7.40, 10 mM) with increasing amounts of HSO3−. Each spectrum was recorded after a 2 min delay. Insets: (a) UV−vis titration curve of 1 with HSO3−. (b) Photograph of 1 in the absence (left) and presence (right) of HSO3−.

reduced the molar extinction coefficients of Ru → mepbi CT transitions and resulted in an obvious Ru → bpy CT transition band at 560 nm. Furthermore, the UV−vis absorption titration curve of 1 with HSO3− was plotted and is shown in Figure 5, inset a. After the amount of HSO3− reached 20 μM (1 equiv), the value of A455 nm/A560 nm became constant, suggesting a 1:1 binding stoichiometry between 1 and HSO3−. Then, the reaction ratio between 1 and bisulfite was further identified to be 1:1 by Job’s plot analysis (Figure S15). Subsequently, the addition reaction occurring between 1 and HSO3− was also identified by 1H NMR analysis. As depicted in Figure S16, the upfield shift was also observed for the proton signals of 1 after the HSO3− addition, which is similar to that observed in ligand 3. For instance, the proton signal assigned to N+CH2 was upshifted from δ 4.62 to 3.50. Moreover, the proton signal (Hd at δ 6.7−8.9) appeared at δ 4.57. These evident upfield shifts E


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Inorganic Chemistry Scheme 2. Proposed Sensing Mechanism of Probe 1 toward HSO3−

further confirmed that the nucleophilic attack of HSO3− toward C-4 is reasonable.34,55 In addition, the formed product of 1 and bisulfite was characterized by MS analysis (Figure S17). As depicted in Figure S17, the peak occurring at m/z 919.21 is clearly observed, which should correspond to [1-SO3Na]+ (calcd m/z 919.20). Therefore, the detection mechanism for this cyclometalated ruthenium complex could be attributed to the 1,4-addition of HSO3− to the CC bond (Scheme 2). As shown in Figure 5, inset b, the dramatic red shift in the absorption spectra resulted in an obvious color change from yellow to pink upon the addition of bisulfite to the solution of 1, which indicated that 1 can serve as a promising colorimetric probe for HSO3− in water by the naked eye. Then, the detection limit for HSO3− in PBS buffer was further calculated to be approximately 0.12 μM (Figure 6), which was obviously

Table 1. Sensing Performance of Some Probes for HSO3−

benzaldehyde derivative15 BODIPY-Le derivative20 naphthalimide derivative25 coumarin hemicyanine dye27 coumarin benzimidazole dye60 benzo[e] indolium derivative34 iridium complex48 3 (this work) 1 (this work)

change of λabs,max/ nm

change of λem,max/ nm

380 → 317 510 → 620 470 → 430 545 → 410

515 → 395 553 → 647 530 ↑

330

633 → 478

445

492 →395

605 → 458

370↓

384 → 320 455 → 560

λex/ nm

solvent

LOD/μM (method)j

THF/ H2Oa DMSO/ H2Ob PBS/ DMSOc PBS/ DMFd

2 (I395/I515)

415

PBSe

0.053 (I458/ I605)

465↑

400

PBSf

0.097 (I465)

600↑

405

0.14 (I600)

558 → 470

375

HEPES/ DMSOg PBSh

538 430

450

PBS

i

58 ((Imin − I)/ (Imin − Imax)) 0.56 (I530) 0.38 (I478/I633)

0.045 (I470/ I558) 0.12 (A560/ A455)

a THF/H2O (3:7, v/v; pH 5.0). bDMSO/H2O (1:1, v/v). cPBS buffer (pH 7.20−7.40)/DMSO (9:1, v/v). dPBS buffer (pH 7.40, 10 mM, containing 30% DMF). ePBS buffer solution (1 mM CTAB, pH 7.4). f PBS buffer (pH 7.40, 10 mM, containing 0.1% DMF). gDMSO/ HEPES buffer (3:7, v/v; pH 7.50). hPBS buffer (pH 7.40, containing 2% ethanol). iPBS buffer (pH 7.40). jHerein, I is representative of the emission intensity, and A is the absorption intensity.

absorption, while most of the other probes for bisulfite exhibit blue shifts in the absorptions. The bathochromic shift is more sensitive to the naked eye than the hypochromatic shift. Generally, anion recognition is difficult and sensitivity is low in aqueous media because of the strong solvation effect of water. However, complex 1 exhibits both good solubility in PBS buffer and an appreciable detection limit of as low as 0.12 μM by using UV−vis absorption spectra. Although the emission was not observed for complex 1 under the mentioned conditions, it still demonstrates a detection limit comparable with those of some probes determined by ratiometric fluorescence spectra. Selective Optical Response of Complex 1 to Various Anions. Considering the importance of selectivity in the detection of particular analytes, the selectivity of 1 in PBS buffer (pH 7.40, 10 mM) toward bisulfite over other anions (F−, Cl−, Br−, I−, SO42−, NO3−, NO2−, ClO4−, H2PO4−, HPO42−, CO32−, HCO3−, Ac−, SCN−, HS−, and CN−), glycine (Gly), and cysteine (Cys) was investigated, respectively. The

Figure 6. Sensitivity test of 1 toward HSO3− using UV−vis absorption spectra. The limit of detection (LOD) is given by the equation LOD = 3S0/S, where 3 is the factor at the 99% confidence level, S0 is the standard deviation of the blank measurements, and S is the slope of the calibration curve. The LOD was determined to be 0.12 μM.

lower than that of ligand 3 determined by using UV−vis absorption. However, the emission of complex 1 was not observed in the presence or absence of bisulfite at room temperature. Furthermore, the sensing performance of this probe and some reported optical probe for HSO3− is summarized in Table 1. As depicted in Table 1, complex 1 displays a large red shift in F


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Inorganic Chemistry absorption spectra of 1 in the presence of these analytes were recorded. As shown in Figure 7a, anions (F−, Cl−, Br−, I−,

Figure 8. Changes of the absorption spectra of 1 (18 μM) in PBS buffer (pH 7.40, 10 mM) with increasing amounts of HS−. Each spectrum was recorded after a 2 min delay. Inset: UV−vis absorption titration curve of 1 with HS−.

the solution of 1 in PBS buffer (pH 7.40, 10 mM) resulted in a decrease of the absorption band at 455 nm. The pattern of the absorption change was almost similar to that observed for HSO3−. However, it could be seen from the inset in Figure 8 that a larger amount of HS− was necessary in comparison with the amount of HSO3−. The detection limit for HS− in PBS buffer based on UV−vis absorption spectra was calculated to be approximately 12 μM (Figure S20). Moreover, the reaction kinetics between 1 and HS− was further identified. As depicted in Figure 9, even after the addition amount of the HS− anion

Figure 7. (a) UV−vis absorption response of 1 (20 μM) upon the addition of various species in PBS buffer (pH 7.40, 10 mM). Bars represent the absorption intensity ratio A455 nm/A560 nm. Each spectrum was recorded after 2 min. Gray bars represent the blank solutions. Black bars represent the addition of various species (2 mM) to the solution of 1. White bars represent the subsequent addition of HSO3− (0.1 mM) to the above solutions. (b) Color change of 1 in PBS buffer (pH 7.40, 10 mM) after the addition of various representative anions and Cys. Herein, the concentrations of anions (containing all of the above-mentioned anions except HS−, CN−, HSO3−, and Cys), Cys, and HS− are 2 mM, while the amount of HSO3− is 0.1 mM.

SO42−, NO3−, NO2−, ClO4−, H2PO4−, HPO42−, CO32−, HCO3−, AcO−, and SCN−) and two amino acids did not induce any significant changes in absorption spectra of 1. Also, the value of A455 nm/A560 nm varied in a limited range between 4.4 and 5.4. The addition of HS− made the value of A455 nm/A560 nm vary from 5.4 to 1.7, while the treatment of 1 with HSO3− resulted in the change of A455 nm/A560 nm from 5.4 to 1.1. CN−, as a good nucleophilic reagent, did not lead to an expected significant absorption change of 1 in PBS buffer, with the value of A455 nm/ A560 nm varying from 5.4 to 2.6. However, upon the addition of small amounts of CN−, dramatic changes in the absorption of 1 in water were observed (see Figure S18). The reaction kinetics between 1 and CN− in the absence or presence of PBS buffer were further investigated and plotted in Figure S19, which indicates that the neutral pH suppresses the nucleophilic addition of CN− to 1 because of protonation of CN− [pKa(HCN) = 9.2].34,55 As a result, among these added representative anions, Gly and Cys, only HS− caused an intensity change because of the 1,2-addition reaction,31,32,61 resulting in a color change from yellow to pink (see Figure 7b). The selectivity of 1 to HSO3− was further examined by the competition experiment by adding HSO3− to the solutions of 1 in the presence of these analytes. The results indicate that the sensitivity of 1 to HSO3− is not significantly affected by these commonly coexistent analytes except HS−. Considering the possible interference of HS−, the sensing potential of 1 toward HS− was then investigated in detail. As shown in Figure 8, adding different concentrations of HS− to

Figure 9. Time-dependent absorption changes of 1 (12 μM) in the presence of different concentrations of HS− in PBS buffer (10 mM, pH 7.40).

reached 100 equiv, the completion of the reaction still needed about 10 min. By contrast, the addition of 10 equiv of bisulfite could reduce the absorption intensity at 455 nm to the minimum within 1 min (Figure S7). These results indicated that the sensitivity of 1 to HSO3− was not significantly affected by the low concentration of HS− within a relatively short time G


Article

Inorganic Chemistry Table 2. Calculated Molecular Orbital Distributions and Energy Levels of 1 and 1-SO3H

center. The LUMO is delocalized on the whole cyclometalated ligand, whereas the LUMO+1 and LUMO+2 are primarily localized over the π* system of both bpy ligands. However, the addition of HSO3− can influence the HOMO and LUMO energy levels greatly. Upon the addition of HSO3−, the HOMO primarily resides on benzo[e]indole with the methylene part of the cyclometalated ligand, whereas the HOMO−1 is delocalized on the 2-phenylpyridine part of the cyclometalated ligand and ruthenium center. In addition, the LUMOs are primarily delocalized on the bpy ligands. It is clear that a set of π* orbitals associated with the cyclometalated ligand are located at slightly lower energies than those of the polypyridyl ligands that comprise the LUMOs. Furthermore, the corresponding energy gaps between the HOMO and LUMO are calculated to be 2.14 and 1.78 eV for 1 and 1-SO3H, respectively. The marked decrease of the HOMO−LUMO gap led to the observed red shift in the absorption spectrum of 1 upon the addition of HSO3−. By comparison, complex 1 displays an intense low-energy absorption band at 455 nm. As suggested by TDDFT calculations, the lowest singlet transition responsible for the measured low-energy absorption band of 1 is assigned to the S7 state, which is mainly composed of HOMO−3 → LUMO and HOMO−1 → LUMO+2 transitions. According to the orbital distributions, this absorption band is mainly assigned to the [dπ(Ru) → π*C^N] MLCT transition and [πN^N → π*C^N] ligand-to-ligand charge-transfer (LLCT) transition, with a small contribution from the [πN^N → π*N^N] intraligand chargetransfer (ILCT) transition and [dπ(Ru) → π*N^N] MLCT transition. That is to say, the C^N ligand dominates the transitions. However, for 1-SO3H, the [dπ(Ru) → π*N^N] MLCT and [πC^N → π*N^N] LLCT transitions are responsible for the lowest-energy absorption band. That is to say, the N^N ligand dominates the transitions, which is different from complex 1. Therefore, upon the addition of HSO3− to the solution of 1, the lowest-energy states switch from the [dπ(Ru) → π*C^N] MLCT transition to the [dπ(Ru) → π*N^N] MLCT transition, which is in agreement with the remarkable red shift in the absorption spectrum of 1-SO3H compared with that of 1. Determination of the Bisulfite Concentration in Sugar. To investigate the applicability of complex 1 to real samples and the possibility of precise quantitative detection of bisulfite, the levels of bisulfite in granulated sugar and crystal sugar were examined carefully by using the proposed method. As shown in Table 4, the recoveries were good enough for practical use. The HSO3− levels in granulated sugar and crystal sugar were analyzed to be 12.1 and 13.4 mg kg−1, respectively.

period such as 2 min. Therefore, complex 1 could be used for the selective and rapid detection of bisulfite. Theoretical Calculations. As we discussed above, the conjugated structure of ligand 3 was broken by the 1,4-addition of HSO3− to the CC bond, which resulted in a blue-shifted absorption band. However, complex 1 exhibited a remarkable and unusual red-shifted absorption after the 1,4-addition reaction. For a better understanding of the influence of HSO3− on the photophysical properties of complex 1, DFT and TDDFT calculations were performed for 1 and 1-SO3H. The calculated HOMO and LUMO distributions are shown in Table 2, and the calculated low-energy transitions of 1 and 1SO3H are shown in Table 3. As shown in Table 2, the two complexes exhibited different distributions of HOMO and LUMO. For 1, the HOMO primarily resides on the 2phenylpyridine part of the cyclometalated ligand and ruthenium Table 3. Absorption of 1 and 1-SO3H in an Ethanol Solution from TDDFT Calculations complex 1

1-SO3H

state

λ/nm

E/eV

oscillator

S1

776

1.59

0.0249

S2

694

1.78

0.0069

S3

631

1.96

0.0353

S7

503

2.46

0.9559

S1

586

2.11

0.001

S2

577

2.14

0.0022

S3

527

2.35

0.0102

S5

492

2.52

0.062

main configuration (CI coeff) HOMO → LUMO (0.697) HOMO−1 → LUMO (0.697) HOMO−2 → LUMO (0.701) HOMO−3 → LUMO (0.68) HOMO−1 → LUMO+2 (0.148) HOMO−1 → LUMO+1 (0.58) HOMO−1 → LUMO (0.35) HOMO−2 → LUMO+1 (0.11) HOMO−1 → LUMO (0.58) HOMO−1 → LUMO+1 (0.34) HOMO−2 → LUMO (0.64) HOMO → LUMO (0.20) HOMO−3 → LUMO+1 (0.45) HOMO−2 → LUMO+1 (0.38) HOMO → LUMO +1 (0.27)

assignment LLCT/ MLCT LLCT/ MLCT ILCT/ MLCT LLCT/ MLCT ILCT/ MLCT MLCT/ LLCT MLCT/ LLCT LLCT/ LMCT MLCT/ LLCT MLCT/ LLCT LMCT/ LLCT LMCT/ LLCT MLCT/ LLCT LMCT/ LLCT LMCT/ LLCT H


Article

Inorganic Chemistry

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Table 4. Determination of Bisulfite Anions in Sugar bisulfite added (μmol L−1)

bisulfite found (μmol L−1)a

granulated sugar

0

3.23 ± 0.04

crystal sugar

4 8 0 4 8

sample

a

7.21 11.21 3.69 7.59 11.53

± ± ± ± ±

0.13 0.13 0.02 0.16 0.04

recovery (%)

99.5 99.8 97.5 98.0

Mean ± standard deviation (n = 5).

4. CONCLUSION In conclusion, a colorimetric probe based on the hydrophilic cycloruthenated complex 1 by using Hmepbi as the cylometalated ligand was successfully designed and synthesized. Both the ligand and complex displayed good sensitivity to bisulfite with short response time in PBS buffer. Moreover, the newly designed indolium cycloruthenated complex system exhibited several advantages over the ligand in terms of the relative stable absorption property, obvious visual color change, and improved detection limit by using UV−vis absorption spectra. Especially, 1 exhibited a dramatic color change from yellow to pink upon the addition of HSO3−, accompanied by an obvious red-shifted absorption of over 100 nm. This red shift was rarely observed in the detection of HSO3− by the 1,4addition reaction. Furthermore, the probe 1 was applied in the determination of HSO3− in granulated sugar and crystal sugar with good recovery. These results indicated that probe 1 can be potentially used in the selective recognition of bisulfite in real samples.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02210. Additional UV−vis absorption spectra data as well as fluorescence, NMR, and MALDI-TOF MS spectra (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

§ The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the National Science Foundation of China (Grants 21301196 and 21171098) and Natural Science Fundation of South-Central Univeristy for Nationalities (Grant XTZ15016).

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REFERENCES

(1) Vally, H.; Thompson, P. Thorax 2001, 56, 763−769. I


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