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Silica nanospheres functionalized by a rhodamine-based Hg(II)-sensing probe having two sensing channels: Preparation, characterization and sensing performance
Journal of Luminescence 145 (2014) 760–766
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Silica nanospheres functionalized by a rhodamine-based Hg(II)-sensing probe having two sensing channels: Preparation, characterization and sensing performance Yitong Chen n, Shuyong Mu Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences (CAS), Urumqi 830011, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 April 2013 Received in revised form 28 August 2013 Accepted 30 August 2013 Available online 12 September 2013
In this paper, we designed and synthesized a rhodamine derived probe of (E)-2-(3′,6′-bis(ethylamino)2′,7′-dimethyl-3-oxospiro[isoindoline-1,9′-xanthene]-2-ylimino)acetaldehyde (denoted as Rb–CO). The photophysical measurement on this probe suggested that its absorption and fluorescence increased with the increasing Hg(II) concentration, showing two sensing channels of colorimetric and fluorescence sensing. In addition, Rb–CO owned high selectivity and good linear response towards Hg(II). Rb–CO was then grafted onto a supporting matrix of silica nanospheres, resulting in a Hg(II) sensing system of Rb@SiO2 which was identified and characterized by electron microscopy, IR spectrum and thermogravimetry. The corresponding photophysical measurements confirmed that the absorption and fluorescence of Rb@SiO2 also increased with the increasing Hg(II) concentration. Both sensing channels had shown good selectivity, linear response and high photostability towards Hg(II). & 2013 Elsevier B.V. All rights reserved.
Keywords: Hg(II) Silica nanospheres Sensor Rhodamine derivative
1. Introduction The identification and quantification of hydrargyrum (Hg) have been widely recognized as an important issue since Hg is a wellknown heath threat to human beings and many animal species [1]. Both elemental and ionic hydrargyrum, as well as its complexes, can participate in many vital biological processes, leading to splanchnic damage and a series of diseases such as nosebleed, headache, perforation of stomach, nerve disorder, intestines septum and acute renal failure [2]. Hydrargyrum pollutions can be traced in a diverse range of natural and anthropogenic activities, such as volcanic eruption, metal mining, incineration and combustion of fossil fuels [3]. The elemental and ionic hydrargyrum can also be transformed into organometallic compounds by microbes and bacterias which are even more dangerous due to their high affinity for thiol group in proteins and enzymes in human body [4]. What is worse, those pollutions can be accumulated through food chains, threatening human health. Thus, the strong appeal for developing reliable and easy-to-go methods for Hg detection has pushed a continuous progress in analytical techniques. Although modern analytical techniques have been proved to be talented in giving accurate and reliable results [5–7], they generally need sophisticated equipments and complicated pretreatments, making them unsuitable for on-line and in-field detections.
n
Corresponding author. Tel.: þ 86 13999808688; fax: þ 86 9917885320. E-mail address: [email protected] (Y. Chen).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.08.068
Owing to the virtues of simple, quick and non-destructive characters, optical sensing has been recently developed as a promising detection technique [8–10]. Optical sensors need no sophisticated instrumental implementation and sample pretreatments compared with traditional detecting methods. In addition, they offer the advantages in terms of size, electrical safety, low cost, needing no reference element and the fact that the sensing signals can be transmitted over a long distance with no interference from electromagnetic field [8–11]. Some precursive efforts have been devoted to optical sensors for Hg(II) detection, and the corresponding results have sparked their application [10,11]. However, most of the probes in these sensors are luminescence “on–off” ones. In other words, the luminescence intensity from the probes decreases with the increasing Hg(II) concentration, and the whole sensing procedure is based on the measurement of luminescence decrease. Since other ions and emission killers in the sensing environment can also quench the luminescence, those “on–off” sensors suffer from limited accuracy and selectivity towards specific analyte, which means that novel probes should be developed for high accuracy and good selectivity. “Off–on” probes are then proposed to meet this requirement. In this case, the luminescence of the probes increases upon the presence of specific analyte, and thus eliminates the interference from other ions and emission killers, offering high accuracy and good selectivity. Rhodamine and its derivatives are a class of representative “off–on” probes [12,13]. Free rhodamine molecules usually take the nonemissive spirolactam structure. Upon the presence of metal ions, rhodamine molecules coordinate with them and take the highly
Y. Chen, S. Mu / Journal of Luminescence 145 (2014) 760–766
emissive xanthene structure. Correspondingly, the luminescence of the probe increases with the increasing concentration of metal ions, showing the “off–on” effect. In addition, a color change may be accompanied during the structural transformation, providing another sensing channel of colorimetry. For practical application in optical sensing systems, the probes need to be embedded into solid medium acting as a supporting matrix, allowing analyte transportation from surroundings and enabling further functionalization. Silica-based materials such as silica nanospheres, silica shells and molecular sieves have been proved to be promising supporting matrixes owing to the compatibility in aqueous and biological systems, along with their feasibility of connecting with other functional groups [14]. Guided by above results, in this paper, we design and synthesize a rhodamine based “off–on” probe for Hg(II) detection. By embedding this probe onto the surface of silica nanospheres, a Hg (II)-sensing system is constructed and then fully characterized. Its sensing performance towards (II) is also investigated.
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6.16 (s, 2H, xanthene–H), 6.42 (s, 2H, xanthene–H), 7.17 (dd, 1H, Ar–H), 7.52 (dd, 2H, Ar–H), 8.22 (dd, 1H, Ar–H). MS m/z: [1þH] þ calc. for C26H28N4O2, 428.2; found, 429.2. 2.2. Synthesis of Rb–CO (E)-2-(3′,6′-bis(ethylamino)-2′,7′-dimethyl-3-oxospiro[isoindoline-1,9′-xanthene]-2-ylimino)acetaldehyde (denoted as Rb–CO) was synthesized according to a procedure described as follows [15]. The mixture of 5 mmol of Rb–NH2, 5 mL of glyoxal and 20 mL of ethanol was stirred at room temperature overnight. Then the solution was poured into 500 mL of saturated NaCl aqueous solution. The resulting solid product was collected, washed with ethanol and dried at 45 1C. 1HNMR (CDCl3), δ (ppm): 1.32–1.36 (t, 6H, NCH2CH3), 1.98 (s, 6H, xanthene–CH3), 3.24–3.29 (q, 4H, NCH2CH3), 5.35 (s, NHCH2CH3), 6.17 (s, 2H, xanthene–H), 6.43 (s, 2H, xanthene–H), 7.18 (dd, 1H, Ar–H), 7.39 (d, 1H, CH ¼ N), 7.54 (dd, 2H, Ar–H), 8.25 (dd, 1H, Ar–H), 9.47 (d, 1H, CH ¼O). MS m/z: [1þ H] þ calc. for C28H28N4O3, 468.2; found, 469.2.
2. Experimental details 2.3. Synthesis of Rb–Si Scheme 1 depicts the synthetic route for the Hg(II)-sensing system (denoted as Rb@SiO2) with a rhodamine derivative as the probe and silica nanospheres as the supporting matrix. The starting reagent of rhodamine 6G and the siliane coupling reagent of 3-aminopropyltrimethoxysilane (denoted as APS) were purchased from Aldrich Chemical Company and used as received. The other chemicals and salts, including anhydrous ethanol, concentrated ammonia aqueous solution (28 wt%), glyoxal (40%), tetraethoxysilane (denoted as TEOS), anhydrous hydrazine (80 wt%), acetonitrile and toluene, were commercially obtained from Shanghai Chemical Company (Shanghai, China). The organic solvents used in this work, including toluene, n-hexane and tetrahydrofuran (THF), were purified using standard procedures. The solvent waster was deionized. 2.1. Synthesis of Rb–NH2 2-amino-3′,6′-bis(ethylamino)-2′,7′-dimethylspiro[isoindoline1,9′-xanthen]-3-one (denoted as Rb–NH2) was synthesized according to a literature procedure described as follows [15]. 10 mmol of rhodamine 6G was added into the mixed solvent of 3 mL of NH2NH2 H2O and 30 mL of ethanol. The mixture was stirred at 85 1C for 12 h under N2 protection and then poured into 500 mL of cold water. The solid product was collected and recrystallized in the mixed solvent of ethanol/water (v:v¼2:8). 1HNMR (CDCl3), δ (ppm): 1.32–1.35 (t, 6H, NCH2CH3), 1.97 (s, 6H, xanthene–CH3), 3.23–3.28 (q, 4H, NCH2CH3), 4.89 (s, N–NH2), 5.33 (s, NHCH2CH3),
3′,6′-bis(ethylamino)-2′,7′-dimethyl-2-((E)-((E)-2-(3-(triethoxysilyl)propylimino)ethylidene)amino)spiro[isoindoline-1,9′xanthen]-3-one (denoted as Rb–Si) was synthesized according to a literature procedure described as follows [16]. 1 mmol of Rb–CO was dissolved in 30 mL of anhydrous THF. Then the solution of 2 mL of APS in 10 mL of anhydrous THF was dropwise added into Rb–CO solution under N2 atmosphere. The mixture was stirred at 45 1C for 24 h. Then the solvent was removed by rotary evaporation under reduced pressure. The yellow residue was dispersed in 50 mL of cold n-hexane (0 1C). The resulting yellow solid was filtered off and purified by the recrystallization from CHCl3: n-hexane (v:v¼ 2:8). 1HNMR (CDCl3), δ (ppm): 1.14–1.17 (t, 9H, OCH2CH3), 1.24 (t, 6H, OCH2CH3), 1.32–1.36 (t, 6H, NCH2CH3), 1.98 (s, 6H, xanthene–CH3), 3.24–3.29 (q, 4H, NCH2CH3), 3.56–3.64 (q, 6H, Si(CH2)3), 5.35 (s, NHCH2CH3), 6.17 (s, 2H, xanthene–H), 6.43 (s, 2H, xanthene–H), 7.18 (dd, 1H, Ar–H), 7.39 (d, 1H, CH ¼N), 9.42 (d, 1H, CH ¼ N), 7.54 (dd, 2H, Ar–H), 8.25 (dd, 1H, Ar–H). MS m/z: [m] þ calc. for C37H49N5O5Si, 671.4; found, 471.4. 2.4. Preparation of Rb@SiO2 The silica nanospheres were firstly prepared according to the known Stöber method described as follows [17]. The mixture of 1.9 mL of H2O, 49 mL of NH3 H2O, 4.2 mL of TEOS and 45 mL of ethanol was stirred at room temperature for 5 h. Then the
Scheme 1. The synthetic procedure for Rb–CO and Rb@SiO2.
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resulting solid product was centrifugalized and collected. After being washed by pure water and ethanol, the product was used directly for the next procedure. The mixture of 0.65 g of silica nanospheres, 0.8 mL of TEOS, 0.08 g of Rb–Si, 1.2 mL of NH3 H2O and 40 mL of deionized water was stirred at room temperature for 4 h. The solid product was separated by centrifugation and washed with plenty of ethanol and water. 2.5. Measurements and apparatus The sample separation and centrifugation were finished by the combination of a TGL-16G (Shanghai Anting Scientific Instrument Factory) centrifuge and a SHZ-D(III) Vacuum Pump (Shanghai Dongxi refrigeration equipment Co., Ltd.). The NMR and the mass spectra were measured on a Varian INOVA 300 spectrometer and a Agilent 1100 MS series/AXIMA CFR MALDI/TOF (matrix assisted laser desorption ionization/time-of-flight) MS (COMPACT), respectively. The IR spectra were taken on a Model Brukerequinox-55 FTIR in the range of 400–7800 cm 1 (KBr pellet technique, 74 cm 1). The photophysical measurements, including emission and absorption spectra, were finished on a Hitachi F-4500 fluorescence spectrophotometer and a HP 8453 UV–vis–NIR diode array spectrophotometer, respectively. For all photophysical measurements, the excitation wavelength was set as 520 nm, with excitation and emission slits adjusted for data reading convenience. The pH values were adjusted by diluted nitric acid or sodium hydroxide solution. Field-emission scanning electron microscopy (SEM) images were finished on a Hitachi S-4800 microscope and LEO Model 1430VP microscope. Thermogravimetric analysis (TGA) data were collected on a Netzsch STA449F3 thermal analyzer. All measurements were carried out in the air at room temperature without being specified.
3. Results and discussion 3.1. Sensing performance of the probe Rb–CO towards Hg(II) As shown by Scheme 1, the Hg(II)-sensing system of Rb@SiO2 is constructed by embedding a rhodamine based “off–on” probe onto the surface of supporting matrix (silica nanospheres). As a start, we decide to explore the sensing performance of the probe Rb–CO towards Hg(II). 3.1.1. Sensing channel 1: colorimetric sensing Fig. 1a shows the absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations ranging from 0 to 30 μM, with interval of 2 μM. In the absence of Hg(II), Rb–CO renders a rather weak absorption in the region from 450 nm to 600 nm. Upon the presence of Hg(II) ion, Rb–CO gives a wide absorption ranging from 480 nm to 580 nm. The maximum absorption intensity at 555 nm gradually increases with the increasing Hg(II) concentration. When the Hg(II) concentration is as high as 30 μM, the absorption intensity at 555 nm is 806 times higher than its initial value. The absorbance vs. [Hg(II)] characteristic can be described by Formula (1) as follows, where A is absorbance value, the subscript “0” stands for the absence of Hg (II), and [Hg(II)] means the concentration of Hg(II) [18]: A=A0 ¼ A þ B½CuðIIÞ=BDHO
ð1Þ
The work plot for Formula (1) is then fitted to be A/A0 ¼ 89.2 þ 30.4[Hg(II)], which shows a good linearity when [Hg(II)] varies from 4 μM to 30 μM, as shown in Fig. 1b. Since the wide absorption falls in visible region, there is an obvious color change from colorless to deep pink during the addition of Hg(II), as shown
by the inset of Fig. 1b. The obvious color change makes Rb–CO a potential colorimetric sensor for Hg(II) naked-eye detection. The color change and the linearity of the work plot are comparable, or even better than literature ones [19]. 3.1.2. Sensing channel 2: fluorescence sensing Besides the colorimetric sensing mentioned above, Rb–CO also owns another sensing channel for Hg(II), which is the fluorescence sensing. Since Hg(II) has high thermodydanic affinity for N,Ochelate ligands and fast metal-to-ligand binding kinetics [19,20], it can make the rhodamine-based molecules transfer from the nonemissive spirolactam structure to the highly emissive xanthene structure. This expectation can be confirmed by the fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations ranging from 0 to 30 μM with interval of 2 μM. As shown in Fig. 2a, Rb–CO emits nearly no emission in the absence of Hg(II), which means that most of Rb–CO molecules take the non-emissive spirolactam structure. With the increasing Hg(II) concentration, there shows an emission peaking at 575 nm whose intensity increases with the increasing Hg(II) concentration. When the Hg(II) concentration is as high as 30 μM, the emission intensity at 575 nm is 442 times higher than its initial value. The fluorescence vs. [Hg(II)] characteristic is shown by Fig. 2b. It can be observed that the entire curve is composed of two linear components, with a turning point at [Hg (II)] of 20 μM. In the [Hg(II)] region of 0 to 20 μM, the emission variation towards [Hg(II)] can be expressed by Formula (2), where I means fluorescence intensity, the subscript “0” stands for the absence of Hg(II), and [Hg(II)] means the concentration of Hg(II) [18]. In the higher [Hg(II)] region of 20–30 μM, the emission variation towards [Hg(II)] can be expressed by a similar formula of Formula (3). I=I 0 ¼ C þ D½HgðIIÞ
f0 o ½HgðIIÞ o20 μMg
ð2Þ
I=I 0 ¼ E þ F½HgðIIÞ
f20 μM o ½HgðIIÞ o 30 μMg
ð3Þ
The work plots for Formulas (2) and (3) are then fitted to be I/I0 ¼ 5.1 þ8.1[Hg(II)] and I/I0 ¼ 301.3þ 25.5[Hg(II)], respectively. It can be seen that the emission gradually increases in [Hg (II)] region of 0–20 μM, then the emission greatly increases in [Hg (II)] region of 20–30 μM. The intense fluorescence enhancement can be designed as a warning signal for Hg(II) concentration of hazard level. This sensing region is much wider than literature ones, with comparable sensitivity and similar warning signal [21]. 3.1.3. Response towards other metal ions We have above expected that the recognizing and sensing procedure between Rb–CO and Hg(II) ion is based on the structural transformation from spirolactam structure to xanthene structure with the help of Hg(II) ion. However, if this transformation can also be done in the presence of other metal ions, the corresponding fluorescence enhancement will also be observed, leading to the poor recognizing ability towards specific metal ions, namely the bad selectivity. In order to testify the selectivity of Rb–CO towards Hg(II), the absorption and fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) under the presence of various metal ions (20 μM), including Na(I), K(I), Mg(II), Ca(II), Zn(II), Cd(II), Ni(II), Fe(II), Cu(II), Ag(I) and Hg(II), are shown in Fig. 3. It can be seen that only the presence of Hg(II) can result in the increased absorption (555 nm) and enhanced fluorescence (575 nm). As for the other metal ions, most of the them, including Na(I), K(I), Mg (II), Ca(II), Zn(II), Cd(II), Ni(II) and Ag(I), are powerless to increase the absorption at 555 nm or to enhance the fluorescence at 575 nm neither, which means that they can not initiate the structural transformation from spirolactam structure to xanthene
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Fig. 1. a. The absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations ranging from 0 to 30 μM, with interval of 2 μM. b. The work plot for the absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations. Inset: the photos for pure Rb–CO without Hg(II) ion (left) and Rb–CO with 30 μM of Hg(II) ion (right).
Fig. 2. a. The fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM), upon various Hg(II) concentrations ranging from 0 to 30 μM with interval of 2 μM. b. The work plot for the fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations. Inset: the photos for pure Rb–CO without Hg(II) ion (left), Rb–CO with 20 μM of Hg(II) ion (middle) and Rb–CO with 30 μM of Hg(II) ion (right).
Fig. 3. a. The absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) under the presence of various metal ions (20 μM). b. The fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) under the presence of various metal ions (20 μM).
structure. Their improper ionic charges and radiuses, which makes them unsuitable for the binding cave provided by Rb–CO, should be responsible for Rb–CO's resistance towards them. As for the other two metal ions of Fe(II) and Cu(II), they can slightly increase the absorption intensity at 555 nm, which means that the structural transformation happens in the presence of Fe(II) and Cu(II) ions. But the corresponding fluorescence enhancement effect is neglectable. Their partly-filled d ortbials may make them fluorescence killers, neutralizing the fluorescence enhancement. As a consequence, the absorption increase and the fluorescence enhancement phenomena can be observed at the same time only in the presence of Hg(II), which means that both sensing channels of Rb–
CO, colorimetric and fluorescence sensing, own good selectivity towards Hg(II), making Rb–CO a promising Hg(II) sensing probe. 3.2. Characterization on Rb@SiO2 Above results have shown that Rb–CO is a potential Hg(II) probe with good selectivity and dual sensing channels. To explore its practical sensing performance, we decide to construct a Hg(II) sensing system by grafting Rb–CO onto a supporting matrix of silica nanospheres owing to their compatibility in aqueous and biological systems and the feasibility of connecting with other functional groups [14].
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3.2.1. Morphology The silica nanospheres were prepared according to a literature procedure, then the probe Rb–CO was covalently grafted onto the surface of silica spheres through the siliane coupling reagent APS. The SEM images of the silica nanospheres and the sensing system Rb@SiO2 are shown as Fig. 4. Not surprisingly, the obtained silica nanospheres are monodispersed and uniformly spherical in morphology, which is consistent with literature report [17]. The mean diameter is as wide as 220 nm. After being functionalized by the probe, Rb@SiO2 grows slightly bigger than the original silica nanospheres, with mean diameter of 250 nm. The monodispsal and spherical morphology is well preserved, which guarantees the compatibility in aqueous and biological systems. 3.2.2. IR spectra and TGA curve The successful connection between the rhodamine based probe and the silica nanospheres can be analyzed and confirmed by the IR spectra of Rb–CO, Rb–Si and Rb@SiO2, as shown in Fig. 5a. As for the IR spectrum of Rb–CO, the strong absorption at 3431 cm 1 is attributed to the vibration of –NH group. The absorption bands at 1615 cm 1 and 1700 cm 1 can be assigned to the stretching vibration of –CHQN group and –CHO group, respectively [16]. After being reacted with APS, the absorption at 1700 cm 1 is rather weak in the IR spectrum of Rb–Si, with the absorption at 1615 cm 1 increased, which means that the –CHO from Rb–CO has reacted with the –NH2 group from APS. The sharp absorption at 1098 cm 1 can be attributed to the symmetric stretching of Si–O bond in Rb–Si. In addition, Rb–Si shows a series of absorption bands ranging from 2870 cm 1 to 2990 cm 1 which are attributed to the vibration of _(CH2)3_ group in APS [16], confirming the successful connection between Rb–CO and APS. As for the IR spectrum of Rb–SiO2, the –NH group vibration at 3431 cm 1 can be easily found [18]. While the _(CH2)3_ group vibration has been largely suppressed. Combined with the absorption bands at
465 cm 1, 800 cm 1 and 1098 cm 1, which can be attributed to νas(Si–O), νs(Si–O) and δ(Si–O–Si) (ν represents stretching, δ in plane bending, s symmetric, and as asymmetric vibrations) [16], respectively, we come to the conclusion that the rhodamine based probe has been successfully grated onto the surface of silica spheres through the siliane coupling reagent of APS. The doping content of the probe in Rb-SiO2 is then determined by TGA analysis. The corresponding DTG (derivative thermogravimetry) curve is also given to assist the assignment of weight loss. As shown in Fig. 5b, there are three gradual weight loss regions of 40–290 1C, 290–515 1C and 515–600 1C, respectively. The first one with DTGmax ¼45 1C is responsible for 5.2% of weight loss, which can be assigned to the thermal evaporation of physically absorbed water and residual solvent molecules. The second one with DTGmax ¼315 1C causes 6.8% of weight loss, which can be attributed to the thermal decomposition of the rhodamine based probe. That is to say that the doping content of the rhodamine based probe in Rb–SiO2 is as high as 6.8%. As for the third weight loss region, it should be attributed to the thermal degradation and decomposition of the organosilicate framework in Rb–SiO2, such as Si–C, C–C and C–N bonds cleavage [16,18]. 3.3. Sensing performance of Rb@SiO2 It has been above confirmed that the Hg(II) sensing probe has been successfully grafted onto the silica supporting matrix, resulting in the Hg(II) sensing system of Rb@SiO2. Its sensing performance, including both colorimetric and fluorescence sensing, is then analyzed as follows. 3.3.1. Absorption spectra and their Stern–Volmer plot Fig. 6 shows the absorption spectra of Rb@SiO2 in ethanol/ water (7:3,v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of
Fig. 4. The SEM images of the silica nanospheres (left) and Rb@SiO2 (right).
Fig. 5. a. The IR spectra of Rb–CO, Rb–Si and Rb@SiO2. b. The TGA and DTG curves of Rb@SiO2.
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2 μM. It can be seen that the absorption intensity at 555 nm increases with the increasing Hg(II) concentration, which is consistent with the case for Rb–CO in the presence of various Hg(II) concentrations. That is to say, after being grated onto the surface of silica spheres, that the sensing character of the probe is well preserved. The absorbance vs. [Hg(II)] characteristic can also be described with Formula (1) [18], and the work plot is then fitted to be A/A0 ¼ 0.62 þ 1.08[Hg(II)], which shows a good linearity when [Hg(II)] varies from 2 μM to 24 μM, as shown by the inset of Fig. 6. 3.3.2. Fluorescence spectra and their Stern–Volmer plot As for the other sensing channel of fluorescence sensing, Fig. 7 shows the fluorescence spectra of Rb@SiO2 in ethanol/water (7:3,
Fig. 6. The absorption spectra of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of 2 μM. Inset: the work plot for these absorption spectra.
Fig. 7. The fluorescence spectra of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of 2 μM. Inset: the work plot for these fluorescence spectra.
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v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of 2 μM. Correspondingly, the emission intensity at 575 nm increases with the increasing Hg(II) concentration. The emission vs. [Hg(II)] characteristic can be described with Formula (2), and the work plot is fitted to be I/I0 ¼0.46 þ 0.41[Hg(II)], which shows a good linearity when [Hg(II)] varies from 2 μM to 24 μM, as shown by the inset of Fig. 7. The sensitivity is comparable with literature values [21]. There is something different between the sensing work plots of Rb@SiO2 and Rb–CO, though. The latter one is composed of two linear components, with a turning point at [Hg(II)] of 20 μM, as above mentioned. The work plot for Rb@SiO2 shows a good linearity. It seems that the immobilization on the surface of silica nanospheres can increase the linearity of the work plot. Here, we tentatively attribute the causation for the non-linear work plot of pure Rb–CO to its aggregation under high Hg(II) concentration. 3.3.3. Selectivity and photostability The probe of Rb–CO has shown good selectivity towards Hg(II), correspondingly, the selectivity of Rb@SiO2 should also be evaluated. Fig. 8a shows the absorption intensity ratios of A/A0 for Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various metal ions (20 μM), including Na(I), K(I), Mg(II), Ca(II), Zn (II), Cd(II), Ni(II), Fe(II), Cu(II), Ag(I) and Hg(II). Here A0 stands for the absorption intensity value (555 nm) of Rb@SiO2 in the absence of any metal ions, A stands for the absorption intensity value (555 nm) of Rb@SiO2 in the presence of each metal ion, respectively. Similar to the case for Rb–CO, it is observed that only the addition of Hg(II) into the sensing system causes an obvious absorption increase. The other metal ions are nearly ineffective on increasing the absorption intensity. The corresponding fluorescence intensity ratios of I/I0 upon various metal ions (20 μM) shown by Fig. 8B have shown the same case, where I0 stands for the fluorescence intensity value (575 nm) of Rb@SiO2 in the absence of any metal ions, I stands for the fluorescence intensity value (575 nm) of Rb@SiO2 in the presence of each metal ion, respectively. It is thus concluded that the fluorescence sensing also owns a high selectivity towards Hg(II). Considering that practical examination may be taken under various pH values, the stability of the sensing signals should be investigated. Fig. 9 shows the absorption (555 nm) and fluorescence (575 nm) variations of Rb@SiO2 under various pH values. It can be seen that both colorimetric and fluorescence sensing channels are sensitive towards various pH values. When protons are added into the sensing system (pH o7.0), the intensity values of both absorption and fluorescence tend to increase. This is because protons can also initiate the structural transformation as Hg(II) ion can do. Under base condition (pH 4 7.0), the intensity values of both absorption and fluorescence tend to decrease,
Fig. 8. a. The absorption intensity ratios of A/A0 for Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various metal ions (20 μM). b. The fluorescence intensity ratios of I/I0 for Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various metal ions (20 μM).
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Fig. 9. a. The absorption variations of Rb@SiO2 under various pH values in the presence (20 μM) or absence of Hg(II) ion. b. The fluorescence variations of Rb@SiO2 under various pH values in the presence (20 μM) or absence of Hg(II) ion.
work plot had been improved. Those characteristics were comparable with literature ones. Although the selectivity and the photostability of Rb@SiO2 were good enough, the sensing signals of Rb@SiO2 could be affected by various pH values. For further improvement, the molecular structure of the probe should be further modified to overcome its response towards various pH values.
Acknowledgments
Fig. 10. The absorption (555 nm) and fluorescence (575 nm) intensity values of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon Hg(II) concentration of 20 μM.
which may be caused by the precipitation of HgO activated under base condition. To obtain precise sensing signals, the samples should be dissolved using pure water or NaAc–HAc buffer solution (pH¼7.0). Since the sensing procedure is based on the intensity values of absorption and fluorescence, the photostability of Rb@SiO2 should also be explored. It can be seen in Fig. 10 that the absorption (555 nm) and fluorescence (575 nm) intensity values of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon Hg(II) concentration of 20 μM are stable within experimental time, which means that the photostability of Rb@SiO2 is stable enough to finish general sensing procedures. 4. Conclusion Over all, we reported and studied Rb–CO as a Hg(II) sensing probe. Pure Rb–CO showed two sensing channels of colorimetric and fluorescence sensing towards Hg(II) with high selectivity and good linear response. By grafting Rb–CO onto the surface of silica nanospheres, which was confirmed by IR spectra and TGA analysis, the resulting nanocomposite of Rb@SiO2 also showed promising sensing performance towards Hg(II). Especially, the linearity of the
This work was supported by the Doctoral Fund of Xinjiang University (No. BS090118), Science and Technology Key Project of Chinese Ministry of Education (No. 210245). The authors gratefully acknowledge the assistance and advice of Hui Yin and Xinghua Zhao. We would like to thank Mr. Xiangyang Tan and Dr. Wenzhong Ma for them help in revising the paper in English. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
A. Benzoni, F. Zino, E. Franchi, Environ. Res. 77 (1998) 68. I. Hoyle, R.D. Handy, Aquat. Toxicol. 72 (2005) 147. P. Grandjean, P. Weihe, R.F. White, F. Debes, Environ. Res. 77 (1998) 165. M. Nendza, T. Herbst, C. Kussatz, A. Gies, Chemosphere 35 (1997) 1875. Y.A. Vil’pan, I.L. Grinshtein, A.A. Akatove, S. Gucer, J. Anal. Chem. 60 (2005) 45. E. Kopysc, K. Pyrzynska, S. Garbos, E. Bulska, Anal. Sci. 16 (2000) 1309. L. Yu, X. Yan, At. Spectrom. 25 (2004) 145. G. Zhang, J. Chen, S.J. Payne, S.E. Kooi, J.N. Demas, C.L. Fraser, J. Am. Chem. Soc. 129 (2007) 15728. A. Gulino, S. Giuffrida, P. Mineo, M. Purrazzo, E. Scamporrino, G. Ventimiglia, M.E. Van der Boom, I. Fragala, J. Phys. Chem. B 110 (2006) 16781. L. Zhang, B. Li, Z. Su, S. Yue, Sensor Actuat. B Chem. 143 (2010) 595. L. Zhang, B. Li, Spectrochim. Acta A 74 (2009) 1060. Y. Wang, B. Li, L. Zhang, P. Li, L. Wang, J. Zhang, Langmuir 28 (2012) 1657. Y. Wang, B. Li, L. Zhang, L. Liu, Q. Zuo, P. Li, N. J. Chem. 34 (2010) 1946. Q. Lu, F.Y. Guo, L. Sun, A.H. Li, L.C. Zhao, J. Appl. Phys. 103 (2008) 123533. V. Dujols, F. Ford, A.W. Czarnik, J. Am.Chem. Soc. 119 (1997) 7386. B. Lei, B. Li, H. Zhang, S. Lu, Z. Zheng, W. Li, Y. Wang, Adv. Funct. Mater. 16 (2006) 1883. W. Stöber, A. Fink, J. Colloid Interface Sci. 26 (1968) 62. L.P. Singh, J.M. Bhatnagar, Talanta 64 (2004) 313. Y. Gao, W. Ma, Opt. Mater. 35 (2012) 211. R. Krämer, Angew. Chem. Int. Ed. 37 (1998) 772. Y. Wang, Y. Huang, B. Li, L. Zhang, H. Song, H. Jiang, J. Gao, RSC Adv. 1 (2011) 1294.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Silica nanospheres functionalized by a rhodamine-based Hg(II)-sensing probe having two sensing channels: Preparation, characterization and sensing performance Yitong Chen n, Shuyong Mu Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences (CAS), Urumqi 830011, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 April 2013 Received in revised form 28 August 2013 Accepted 30 August 2013 Available online 12 September 2013
In this paper, we designed and synthesized a rhodamine derived probe of (E)-2-(3′,6′-bis(ethylamino)2′,7′-dimethyl-3-oxospiro[isoindoline-1,9′-xanthene]-2-ylimino)acetaldehyde (denoted as Rb–CO). The photophysical measurement on this probe suggested that its absorption and fluorescence increased with the increasing Hg(II) concentration, showing two sensing channels of colorimetric and fluorescence sensing. In addition, Rb–CO owned high selectivity and good linear response towards Hg(II). Rb–CO was then grafted onto a supporting matrix of silica nanospheres, resulting in a Hg(II) sensing system of Rb@SiO2 which was identified and characterized by electron microscopy, IR spectrum and thermogravimetry. The corresponding photophysical measurements confirmed that the absorption and fluorescence of Rb@SiO2 also increased with the increasing Hg(II) concentration. Both sensing channels had shown good selectivity, linear response and high photostability towards Hg(II). & 2013 Elsevier B.V. All rights reserved.
Keywords: Hg(II) Silica nanospheres Sensor Rhodamine derivative
1. Introduction The identification and quantification of hydrargyrum (Hg) have been widely recognized as an important issue since Hg is a wellknown heath threat to human beings and many animal species [1]. Both elemental and ionic hydrargyrum, as well as its complexes, can participate in many vital biological processes, leading to splanchnic damage and a series of diseases such as nosebleed, headache, perforation of stomach, nerve disorder, intestines septum and acute renal failure [2]. Hydrargyrum pollutions can be traced in a diverse range of natural and anthropogenic activities, such as volcanic eruption, metal mining, incineration and combustion of fossil fuels [3]. The elemental and ionic hydrargyrum can also be transformed into organometallic compounds by microbes and bacterias which are even more dangerous due to their high affinity for thiol group in proteins and enzymes in human body [4]. What is worse, those pollutions can be accumulated through food chains, threatening human health. Thus, the strong appeal for developing reliable and easy-to-go methods for Hg detection has pushed a continuous progress in analytical techniques. Although modern analytical techniques have been proved to be talented in giving accurate and reliable results [5–7], they generally need sophisticated equipments and complicated pretreatments, making them unsuitable for on-line and in-field detections.
n
Corresponding author. Tel.: þ 86 13999808688; fax: þ 86 9917885320. E-mail address: [email protected] (Y. Chen).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.08.068
Owing to the virtues of simple, quick and non-destructive characters, optical sensing has been recently developed as a promising detection technique [8–10]. Optical sensors need no sophisticated instrumental implementation and sample pretreatments compared with traditional detecting methods. In addition, they offer the advantages in terms of size, electrical safety, low cost, needing no reference element and the fact that the sensing signals can be transmitted over a long distance with no interference from electromagnetic field [8–11]. Some precursive efforts have been devoted to optical sensors for Hg(II) detection, and the corresponding results have sparked their application [10,11]. However, most of the probes in these sensors are luminescence “on–off” ones. In other words, the luminescence intensity from the probes decreases with the increasing Hg(II) concentration, and the whole sensing procedure is based on the measurement of luminescence decrease. Since other ions and emission killers in the sensing environment can also quench the luminescence, those “on–off” sensors suffer from limited accuracy and selectivity towards specific analyte, which means that novel probes should be developed for high accuracy and good selectivity. “Off–on” probes are then proposed to meet this requirement. In this case, the luminescence of the probes increases upon the presence of specific analyte, and thus eliminates the interference from other ions and emission killers, offering high accuracy and good selectivity. Rhodamine and its derivatives are a class of representative “off–on” probes [12,13]. Free rhodamine molecules usually take the nonemissive spirolactam structure. Upon the presence of metal ions, rhodamine molecules coordinate with them and take the highly
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emissive xanthene structure. Correspondingly, the luminescence of the probe increases with the increasing concentration of metal ions, showing the “off–on” effect. In addition, a color change may be accompanied during the structural transformation, providing another sensing channel of colorimetry. For practical application in optical sensing systems, the probes need to be embedded into solid medium acting as a supporting matrix, allowing analyte transportation from surroundings and enabling further functionalization. Silica-based materials such as silica nanospheres, silica shells and molecular sieves have been proved to be promising supporting matrixes owing to the compatibility in aqueous and biological systems, along with their feasibility of connecting with other functional groups [14]. Guided by above results, in this paper, we design and synthesize a rhodamine based “off–on” probe for Hg(II) detection. By embedding this probe onto the surface of silica nanospheres, a Hg (II)-sensing system is constructed and then fully characterized. Its sensing performance towards (II) is also investigated.
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6.16 (s, 2H, xanthene–H), 6.42 (s, 2H, xanthene–H), 7.17 (dd, 1H, Ar–H), 7.52 (dd, 2H, Ar–H), 8.22 (dd, 1H, Ar–H). MS m/z: [1þH] þ calc. for C26H28N4O2, 428.2; found, 429.2. 2.2. Synthesis of Rb–CO (E)-2-(3′,6′-bis(ethylamino)-2′,7′-dimethyl-3-oxospiro[isoindoline-1,9′-xanthene]-2-ylimino)acetaldehyde (denoted as Rb–CO) was synthesized according to a procedure described as follows [15]. The mixture of 5 mmol of Rb–NH2, 5 mL of glyoxal and 20 mL of ethanol was stirred at room temperature overnight. Then the solution was poured into 500 mL of saturated NaCl aqueous solution. The resulting solid product was collected, washed with ethanol and dried at 45 1C. 1HNMR (CDCl3), δ (ppm): 1.32–1.36 (t, 6H, NCH2CH3), 1.98 (s, 6H, xanthene–CH3), 3.24–3.29 (q, 4H, NCH2CH3), 5.35 (s, NHCH2CH3), 6.17 (s, 2H, xanthene–H), 6.43 (s, 2H, xanthene–H), 7.18 (dd, 1H, Ar–H), 7.39 (d, 1H, CH ¼ N), 7.54 (dd, 2H, Ar–H), 8.25 (dd, 1H, Ar–H), 9.47 (d, 1H, CH ¼O). MS m/z: [1þ H] þ calc. for C28H28N4O3, 468.2; found, 469.2.
2. Experimental details 2.3. Synthesis of Rb–Si Scheme 1 depicts the synthetic route for the Hg(II)-sensing system (denoted as Rb@SiO2) with a rhodamine derivative as the probe and silica nanospheres as the supporting matrix. The starting reagent of rhodamine 6G and the siliane coupling reagent of 3-aminopropyltrimethoxysilane (denoted as APS) were purchased from Aldrich Chemical Company and used as received. The other chemicals and salts, including anhydrous ethanol, concentrated ammonia aqueous solution (28 wt%), glyoxal (40%), tetraethoxysilane (denoted as TEOS), anhydrous hydrazine (80 wt%), acetonitrile and toluene, were commercially obtained from Shanghai Chemical Company (Shanghai, China). The organic solvents used in this work, including toluene, n-hexane and tetrahydrofuran (THF), were purified using standard procedures. The solvent waster was deionized. 2.1. Synthesis of Rb–NH2 2-amino-3′,6′-bis(ethylamino)-2′,7′-dimethylspiro[isoindoline1,9′-xanthen]-3-one (denoted as Rb–NH2) was synthesized according to a literature procedure described as follows [15]. 10 mmol of rhodamine 6G was added into the mixed solvent of 3 mL of NH2NH2 H2O and 30 mL of ethanol. The mixture was stirred at 85 1C for 12 h under N2 protection and then poured into 500 mL of cold water. The solid product was collected and recrystallized in the mixed solvent of ethanol/water (v:v¼2:8). 1HNMR (CDCl3), δ (ppm): 1.32–1.35 (t, 6H, NCH2CH3), 1.97 (s, 6H, xanthene–CH3), 3.23–3.28 (q, 4H, NCH2CH3), 4.89 (s, N–NH2), 5.33 (s, NHCH2CH3),
3′,6′-bis(ethylamino)-2′,7′-dimethyl-2-((E)-((E)-2-(3-(triethoxysilyl)propylimino)ethylidene)amino)spiro[isoindoline-1,9′xanthen]-3-one (denoted as Rb–Si) was synthesized according to a literature procedure described as follows [16]. 1 mmol of Rb–CO was dissolved in 30 mL of anhydrous THF. Then the solution of 2 mL of APS in 10 mL of anhydrous THF was dropwise added into Rb–CO solution under N2 atmosphere. The mixture was stirred at 45 1C for 24 h. Then the solvent was removed by rotary evaporation under reduced pressure. The yellow residue was dispersed in 50 mL of cold n-hexane (0 1C). The resulting yellow solid was filtered off and purified by the recrystallization from CHCl3: n-hexane (v:v¼ 2:8). 1HNMR (CDCl3), δ (ppm): 1.14–1.17 (t, 9H, OCH2CH3), 1.24 (t, 6H, OCH2CH3), 1.32–1.36 (t, 6H, NCH2CH3), 1.98 (s, 6H, xanthene–CH3), 3.24–3.29 (q, 4H, NCH2CH3), 3.56–3.64 (q, 6H, Si(CH2)3), 5.35 (s, NHCH2CH3), 6.17 (s, 2H, xanthene–H), 6.43 (s, 2H, xanthene–H), 7.18 (dd, 1H, Ar–H), 7.39 (d, 1H, CH ¼N), 9.42 (d, 1H, CH ¼ N), 7.54 (dd, 2H, Ar–H), 8.25 (dd, 1H, Ar–H). MS m/z: [m] þ calc. for C37H49N5O5Si, 671.4; found, 471.4. 2.4. Preparation of Rb@SiO2 The silica nanospheres were firstly prepared according to the known Stöber method described as follows [17]. The mixture of 1.9 mL of H2O, 49 mL of NH3 H2O, 4.2 mL of TEOS and 45 mL of ethanol was stirred at room temperature for 5 h. Then the
Scheme 1. The synthetic procedure for Rb–CO and Rb@SiO2.
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resulting solid product was centrifugalized and collected. After being washed by pure water and ethanol, the product was used directly for the next procedure. The mixture of 0.65 g of silica nanospheres, 0.8 mL of TEOS, 0.08 g of Rb–Si, 1.2 mL of NH3 H2O and 40 mL of deionized water was stirred at room temperature for 4 h. The solid product was separated by centrifugation and washed with plenty of ethanol and water. 2.5. Measurements and apparatus The sample separation and centrifugation were finished by the combination of a TGL-16G (Shanghai Anting Scientific Instrument Factory) centrifuge and a SHZ-D(III) Vacuum Pump (Shanghai Dongxi refrigeration equipment Co., Ltd.). The NMR and the mass spectra were measured on a Varian INOVA 300 spectrometer and a Agilent 1100 MS series/AXIMA CFR MALDI/TOF (matrix assisted laser desorption ionization/time-of-flight) MS (COMPACT), respectively. The IR spectra were taken on a Model Brukerequinox-55 FTIR in the range of 400–7800 cm 1 (KBr pellet technique, 74 cm 1). The photophysical measurements, including emission and absorption spectra, were finished on a Hitachi F-4500 fluorescence spectrophotometer and a HP 8453 UV–vis–NIR diode array spectrophotometer, respectively. For all photophysical measurements, the excitation wavelength was set as 520 nm, with excitation and emission slits adjusted for data reading convenience. The pH values were adjusted by diluted nitric acid or sodium hydroxide solution. Field-emission scanning electron microscopy (SEM) images were finished on a Hitachi S-4800 microscope and LEO Model 1430VP microscope. Thermogravimetric analysis (TGA) data were collected on a Netzsch STA449F3 thermal analyzer. All measurements were carried out in the air at room temperature without being specified.
3. Results and discussion 3.1. Sensing performance of the probe Rb–CO towards Hg(II) As shown by Scheme 1, the Hg(II)-sensing system of Rb@SiO2 is constructed by embedding a rhodamine based “off–on” probe onto the surface of supporting matrix (silica nanospheres). As a start, we decide to explore the sensing performance of the probe Rb–CO towards Hg(II). 3.1.1. Sensing channel 1: colorimetric sensing Fig. 1a shows the absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations ranging from 0 to 30 μM, with interval of 2 μM. In the absence of Hg(II), Rb–CO renders a rather weak absorption in the region from 450 nm to 600 nm. Upon the presence of Hg(II) ion, Rb–CO gives a wide absorption ranging from 480 nm to 580 nm. The maximum absorption intensity at 555 nm gradually increases with the increasing Hg(II) concentration. When the Hg(II) concentration is as high as 30 μM, the absorption intensity at 555 nm is 806 times higher than its initial value. The absorbance vs. [Hg(II)] characteristic can be described by Formula (1) as follows, where A is absorbance value, the subscript “0” stands for the absence of Hg (II), and [Hg(II)] means the concentration of Hg(II) [18]: A=A0 ¼ A þ B½CuðIIÞ=BDHO
ð1Þ
The work plot for Formula (1) is then fitted to be A/A0 ¼ 89.2 þ 30.4[Hg(II)], which shows a good linearity when [Hg(II)] varies from 4 μM to 30 μM, as shown in Fig. 1b. Since the wide absorption falls in visible region, there is an obvious color change from colorless to deep pink during the addition of Hg(II), as shown
by the inset of Fig. 1b. The obvious color change makes Rb–CO a potential colorimetric sensor for Hg(II) naked-eye detection. The color change and the linearity of the work plot are comparable, or even better than literature ones [19]. 3.1.2. Sensing channel 2: fluorescence sensing Besides the colorimetric sensing mentioned above, Rb–CO also owns another sensing channel for Hg(II), which is the fluorescence sensing. Since Hg(II) has high thermodydanic affinity for N,Ochelate ligands and fast metal-to-ligand binding kinetics [19,20], it can make the rhodamine-based molecules transfer from the nonemissive spirolactam structure to the highly emissive xanthene structure. This expectation can be confirmed by the fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations ranging from 0 to 30 μM with interval of 2 μM. As shown in Fig. 2a, Rb–CO emits nearly no emission in the absence of Hg(II), which means that most of Rb–CO molecules take the non-emissive spirolactam structure. With the increasing Hg(II) concentration, there shows an emission peaking at 575 nm whose intensity increases with the increasing Hg(II) concentration. When the Hg(II) concentration is as high as 30 μM, the emission intensity at 575 nm is 442 times higher than its initial value. The fluorescence vs. [Hg(II)] characteristic is shown by Fig. 2b. It can be observed that the entire curve is composed of two linear components, with a turning point at [Hg (II)] of 20 μM. In the [Hg(II)] region of 0 to 20 μM, the emission variation towards [Hg(II)] can be expressed by Formula (2), where I means fluorescence intensity, the subscript “0” stands for the absence of Hg(II), and [Hg(II)] means the concentration of Hg(II) [18]. In the higher [Hg(II)] region of 20–30 μM, the emission variation towards [Hg(II)] can be expressed by a similar formula of Formula (3). I=I 0 ¼ C þ D½HgðIIÞ
f0 o ½HgðIIÞ o20 μMg
ð2Þ
I=I 0 ¼ E þ F½HgðIIÞ
f20 μM o ½HgðIIÞ o 30 μMg
ð3Þ
The work plots for Formulas (2) and (3) are then fitted to be I/I0 ¼ 5.1 þ8.1[Hg(II)] and I/I0 ¼ 301.3þ 25.5[Hg(II)], respectively. It can be seen that the emission gradually increases in [Hg (II)] region of 0–20 μM, then the emission greatly increases in [Hg (II)] region of 20–30 μM. The intense fluorescence enhancement can be designed as a warning signal for Hg(II) concentration of hazard level. This sensing region is much wider than literature ones, with comparable sensitivity and similar warning signal [21]. 3.1.3. Response towards other metal ions We have above expected that the recognizing and sensing procedure between Rb–CO and Hg(II) ion is based on the structural transformation from spirolactam structure to xanthene structure with the help of Hg(II) ion. However, if this transformation can also be done in the presence of other metal ions, the corresponding fluorescence enhancement will also be observed, leading to the poor recognizing ability towards specific metal ions, namely the bad selectivity. In order to testify the selectivity of Rb–CO towards Hg(II), the absorption and fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) under the presence of various metal ions (20 μM), including Na(I), K(I), Mg(II), Ca(II), Zn(II), Cd(II), Ni(II), Fe(II), Cu(II), Ag(I) and Hg(II), are shown in Fig. 3. It can be seen that only the presence of Hg(II) can result in the increased absorption (555 nm) and enhanced fluorescence (575 nm). As for the other metal ions, most of the them, including Na(I), K(I), Mg (II), Ca(II), Zn(II), Cd(II), Ni(II) and Ag(I), are powerless to increase the absorption at 555 nm or to enhance the fluorescence at 575 nm neither, which means that they can not initiate the structural transformation from spirolactam structure to xanthene
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Fig. 1. a. The absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations ranging from 0 to 30 μM, with interval of 2 μM. b. The work plot for the absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations. Inset: the photos for pure Rb–CO without Hg(II) ion (left) and Rb–CO with 30 μM of Hg(II) ion (right).
Fig. 2. a. The fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM), upon various Hg(II) concentrations ranging from 0 to 30 μM with interval of 2 μM. b. The work plot for the fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) upon various Hg(II) concentrations. Inset: the photos for pure Rb–CO without Hg(II) ion (left), Rb–CO with 20 μM of Hg(II) ion (middle) and Rb–CO with 30 μM of Hg(II) ion (right).
Fig. 3. a. The absorption spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) under the presence of various metal ions (20 μM). b. The fluorescence spectra of Rb–CO in ethanol/water (7:3,v/v) solution (10 μM) under the presence of various metal ions (20 μM).
structure. Their improper ionic charges and radiuses, which makes them unsuitable for the binding cave provided by Rb–CO, should be responsible for Rb–CO's resistance towards them. As for the other two metal ions of Fe(II) and Cu(II), they can slightly increase the absorption intensity at 555 nm, which means that the structural transformation happens in the presence of Fe(II) and Cu(II) ions. But the corresponding fluorescence enhancement effect is neglectable. Their partly-filled d ortbials may make them fluorescence killers, neutralizing the fluorescence enhancement. As a consequence, the absorption increase and the fluorescence enhancement phenomena can be observed at the same time only in the presence of Hg(II), which means that both sensing channels of Rb–
CO, colorimetric and fluorescence sensing, own good selectivity towards Hg(II), making Rb–CO a promising Hg(II) sensing probe. 3.2. Characterization on Rb@SiO2 Above results have shown that Rb–CO is a potential Hg(II) probe with good selectivity and dual sensing channels. To explore its practical sensing performance, we decide to construct a Hg(II) sensing system by grafting Rb–CO onto a supporting matrix of silica nanospheres owing to their compatibility in aqueous and biological systems and the feasibility of connecting with other functional groups [14].
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3.2.1. Morphology The silica nanospheres were prepared according to a literature procedure, then the probe Rb–CO was covalently grafted onto the surface of silica spheres through the siliane coupling reagent APS. The SEM images of the silica nanospheres and the sensing system Rb@SiO2 are shown as Fig. 4. Not surprisingly, the obtained silica nanospheres are monodispersed and uniformly spherical in morphology, which is consistent with literature report [17]. The mean diameter is as wide as 220 nm. After being functionalized by the probe, Rb@SiO2 grows slightly bigger than the original silica nanospheres, with mean diameter of 250 nm. The monodispsal and spherical morphology is well preserved, which guarantees the compatibility in aqueous and biological systems. 3.2.2. IR spectra and TGA curve The successful connection between the rhodamine based probe and the silica nanospheres can be analyzed and confirmed by the IR spectra of Rb–CO, Rb–Si and Rb@SiO2, as shown in Fig. 5a. As for the IR spectrum of Rb–CO, the strong absorption at 3431 cm 1 is attributed to the vibration of –NH group. The absorption bands at 1615 cm 1 and 1700 cm 1 can be assigned to the stretching vibration of –CHQN group and –CHO group, respectively [16]. After being reacted with APS, the absorption at 1700 cm 1 is rather weak in the IR spectrum of Rb–Si, with the absorption at 1615 cm 1 increased, which means that the –CHO from Rb–CO has reacted with the –NH2 group from APS. The sharp absorption at 1098 cm 1 can be attributed to the symmetric stretching of Si–O bond in Rb–Si. In addition, Rb–Si shows a series of absorption bands ranging from 2870 cm 1 to 2990 cm 1 which are attributed to the vibration of _(CH2)3_ group in APS [16], confirming the successful connection between Rb–CO and APS. As for the IR spectrum of Rb–SiO2, the –NH group vibration at 3431 cm 1 can be easily found [18]. While the _(CH2)3_ group vibration has been largely suppressed. Combined with the absorption bands at
465 cm 1, 800 cm 1 and 1098 cm 1, which can be attributed to νas(Si–O), νs(Si–O) and δ(Si–O–Si) (ν represents stretching, δ in plane bending, s symmetric, and as asymmetric vibrations) [16], respectively, we come to the conclusion that the rhodamine based probe has been successfully grated onto the surface of silica spheres through the siliane coupling reagent of APS. The doping content of the probe in Rb-SiO2 is then determined by TGA analysis. The corresponding DTG (derivative thermogravimetry) curve is also given to assist the assignment of weight loss. As shown in Fig. 5b, there are three gradual weight loss regions of 40–290 1C, 290–515 1C and 515–600 1C, respectively. The first one with DTGmax ¼45 1C is responsible for 5.2% of weight loss, which can be assigned to the thermal evaporation of physically absorbed water and residual solvent molecules. The second one with DTGmax ¼315 1C causes 6.8% of weight loss, which can be attributed to the thermal decomposition of the rhodamine based probe. That is to say that the doping content of the rhodamine based probe in Rb–SiO2 is as high as 6.8%. As for the third weight loss region, it should be attributed to the thermal degradation and decomposition of the organosilicate framework in Rb–SiO2, such as Si–C, C–C and C–N bonds cleavage [16,18]. 3.3. Sensing performance of Rb@SiO2 It has been above confirmed that the Hg(II) sensing probe has been successfully grafted onto the silica supporting matrix, resulting in the Hg(II) sensing system of Rb@SiO2. Its sensing performance, including both colorimetric and fluorescence sensing, is then analyzed as follows. 3.3.1. Absorption spectra and their Stern–Volmer plot Fig. 6 shows the absorption spectra of Rb@SiO2 in ethanol/ water (7:3,v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of
Fig. 4. The SEM images of the silica nanospheres (left) and Rb@SiO2 (right).
Fig. 5. a. The IR spectra of Rb–CO, Rb–Si and Rb@SiO2. b. The TGA and DTG curves of Rb@SiO2.
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2 μM. It can be seen that the absorption intensity at 555 nm increases with the increasing Hg(II) concentration, which is consistent with the case for Rb–CO in the presence of various Hg(II) concentrations. That is to say, after being grated onto the surface of silica spheres, that the sensing character of the probe is well preserved. The absorbance vs. [Hg(II)] characteristic can also be described with Formula (1) [18], and the work plot is then fitted to be A/A0 ¼ 0.62 þ 1.08[Hg(II)], which shows a good linearity when [Hg(II)] varies from 2 μM to 24 μM, as shown by the inset of Fig. 6. 3.3.2. Fluorescence spectra and their Stern–Volmer plot As for the other sensing channel of fluorescence sensing, Fig. 7 shows the fluorescence spectra of Rb@SiO2 in ethanol/water (7:3,
Fig. 6. The absorption spectra of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of 2 μM. Inset: the work plot for these absorption spectra.
Fig. 7. The fluorescence spectra of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of 2 μM. Inset: the work plot for these fluorescence spectra.
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v/v) solution (10 mg in 50 mL) upon various Hg(II) concentrations ranging from 0 μM to 22 μM, with interval of 2 μM. Correspondingly, the emission intensity at 575 nm increases with the increasing Hg(II) concentration. The emission vs. [Hg(II)] characteristic can be described with Formula (2), and the work plot is fitted to be I/I0 ¼0.46 þ 0.41[Hg(II)], which shows a good linearity when [Hg(II)] varies from 2 μM to 24 μM, as shown by the inset of Fig. 7. The sensitivity is comparable with literature values [21]. There is something different between the sensing work plots of Rb@SiO2 and Rb–CO, though. The latter one is composed of two linear components, with a turning point at [Hg(II)] of 20 μM, as above mentioned. The work plot for Rb@SiO2 shows a good linearity. It seems that the immobilization on the surface of silica nanospheres can increase the linearity of the work plot. Here, we tentatively attribute the causation for the non-linear work plot of pure Rb–CO to its aggregation under high Hg(II) concentration. 3.3.3. Selectivity and photostability The probe of Rb–CO has shown good selectivity towards Hg(II), correspondingly, the selectivity of Rb@SiO2 should also be evaluated. Fig. 8a shows the absorption intensity ratios of A/A0 for Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various metal ions (20 μM), including Na(I), K(I), Mg(II), Ca(II), Zn (II), Cd(II), Ni(II), Fe(II), Cu(II), Ag(I) and Hg(II). Here A0 stands for the absorption intensity value (555 nm) of Rb@SiO2 in the absence of any metal ions, A stands for the absorption intensity value (555 nm) of Rb@SiO2 in the presence of each metal ion, respectively. Similar to the case for Rb–CO, it is observed that only the addition of Hg(II) into the sensing system causes an obvious absorption increase. The other metal ions are nearly ineffective on increasing the absorption intensity. The corresponding fluorescence intensity ratios of I/I0 upon various metal ions (20 μM) shown by Fig. 8B have shown the same case, where I0 stands for the fluorescence intensity value (575 nm) of Rb@SiO2 in the absence of any metal ions, I stands for the fluorescence intensity value (575 nm) of Rb@SiO2 in the presence of each metal ion, respectively. It is thus concluded that the fluorescence sensing also owns a high selectivity towards Hg(II). Considering that practical examination may be taken under various pH values, the stability of the sensing signals should be investigated. Fig. 9 shows the absorption (555 nm) and fluorescence (575 nm) variations of Rb@SiO2 under various pH values. It can be seen that both colorimetric and fluorescence sensing channels are sensitive towards various pH values. When protons are added into the sensing system (pH o7.0), the intensity values of both absorption and fluorescence tend to increase. This is because protons can also initiate the structural transformation as Hg(II) ion can do. Under base condition (pH 4 7.0), the intensity values of both absorption and fluorescence tend to decrease,
Fig. 8. a. The absorption intensity ratios of A/A0 for Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various metal ions (20 μM). b. The fluorescence intensity ratios of I/I0 for Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon various metal ions (20 μM).
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Fig. 9. a. The absorption variations of Rb@SiO2 under various pH values in the presence (20 μM) or absence of Hg(II) ion. b. The fluorescence variations of Rb@SiO2 under various pH values in the presence (20 μM) or absence of Hg(II) ion.
work plot had been improved. Those characteristics were comparable with literature ones. Although the selectivity and the photostability of Rb@SiO2 were good enough, the sensing signals of Rb@SiO2 could be affected by various pH values. For further improvement, the molecular structure of the probe should be further modified to overcome its response towards various pH values.
Acknowledgments
Fig. 10. The absorption (555 nm) and fluorescence (575 nm) intensity values of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon Hg(II) concentration of 20 μM.
which may be caused by the precipitation of HgO activated under base condition. To obtain precise sensing signals, the samples should be dissolved using pure water or NaAc–HAc buffer solution (pH¼7.0). Since the sensing procedure is based on the intensity values of absorption and fluorescence, the photostability of Rb@SiO2 should also be explored. It can be seen in Fig. 10 that the absorption (555 nm) and fluorescence (575 nm) intensity values of Rb@SiO2 in ethanol/water (7:3,v/v) solution (10 mg in 50 mL) upon Hg(II) concentration of 20 μM are stable within experimental time, which means that the photostability of Rb@SiO2 is stable enough to finish general sensing procedures. 4. Conclusion Over all, we reported and studied Rb–CO as a Hg(II) sensing probe. Pure Rb–CO showed two sensing channels of colorimetric and fluorescence sensing towards Hg(II) with high selectivity and good linear response. By grafting Rb–CO onto the surface of silica nanospheres, which was confirmed by IR spectra and TGA analysis, the resulting nanocomposite of Rb@SiO2 also showed promising sensing performance towards Hg(II). Especially, the linearity of the
This work was supported by the Doctoral Fund of Xinjiang University (No. BS090118), Science and Technology Key Project of Chinese Ministry of Education (No. 210245). The authors gratefully acknowledge the assistance and advice of Hui Yin and Xinghua Zhao. We would like to thank Mr. Xiangyang Tan and Dr. Wenzhong Ma for them help in revising the paper in English. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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