Chalcones isothiocyanate anchored into mesoporous silicate: Synthesis, characterization and metal ions sensing response

Microporous and Mesoporous Materials 198 (2014) 144–152

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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Chalcones isothiocyanate anchored into mesoporous silicate: Synthesis, characterization and metal ions sensing response Tarek A. Fayed, Mohamed H. Shaaban, Marwa N. El-Nahass ⇑, Fathy M. Hassan Chemistry Department, Faculty of Science, Tanta University, 31527, Egypt

a r t i c l e

i n f o

Article history: Received 2 March 2014 Received in revised form 1 July 2014 Accepted 16 July 2014 Available online 24 July 2014 Keywords: Fluorescent nanosensor Mesoporous silicates Chalcone isothiocyanate Toxic metals

a b s t r a c t Fluorescent nanosensors are one of the most efficient techniques used for detection of toxic metal ions, based on ligand embedded into mesoporous materials. Here, novel ligands of chalcone isothiocyanate analogue namely; phenyl chalcone isothiocyanate (PCITC), naphthyl chalcone isothiocyanate (NCITC) and anthryl chalcone isothiocyanate (ACITC) were synthesized. Also, the 3D mesoporous silicate, KIT-6, was prepared through surfactant-template method. The design of fluorescent nanosensors was achieved by anchoring of CITCs into KIT-6 via the coupling agent (3-aminopropyl) triethoxysilane (APS). The obtained mesoporous silicate and CITCs anchored forms were characterized using different spectroscopic techniques. Among of metal ions, the most widely toxins, Pb(II), Cu(II) and Pd(II) were studied. Sensing of these metal ions was performed using steady-state absorption and emission techniques. A gradual increase in the absorption spectra was observed upon increasing the concentrations of the used metal ions ranging from 5 ppb to 1 ppm, with instantaneous color change. Also, a great enhancement in the fluorescence intensity was clearly observed, confirming the formation of [M(CITCs-KIT-6)n]2+ complexes. The calculated binding constants show the high efficiency of PCITC-KIT-6 nanosensor in binding and detection of these metal ions. Moreover, Pb(II) is the strongest binded metal ion with the investigated fluorescent nanosensors. Therefore, CITCs fluorescent nanosensors are suitable for the on-line analysis and remote determination of these toxic metal ions, specially Pb(II), at low concentration levels. Ó 2014 Published by Elsevier Inc.

1. Introduction Heavy metals pollution is a serious global problem because of its widespread occurrence in environment [1,2]. Also, these metals are not biodegradable and can be accumulated in living tissues, causing various diseases and disorders [3]. Among of toxic metal ions, Pb(II) and Cu(II) are common hazardous pollutant [4]. These factors demonstrate the continuous need for the development of rapid and simple means for detection of these toxic metals at low concentration levels in potable and environmental water samples [5]. On the other hand, the precious metals, particularly Pd(II) are widely used in many industries such as jewellery, electronics, medicine and aerospace [6]. Therefore, increasing demand of Pd(II) led to increase interest in the recovery of this precious metal due to deplete its natural sources [7]. In addition, the excessive Pd(II) is highly toxic and carcinogenic to humans, causing skin and eye irritations [8]. Therefore, an effective removal and recovery of Pd(II) from wastes and extraction from natural ores has become very important [9]. ⇑ Corresponding author. http://dx.doi.org/10.1016/j.micromeso.2014.07.031 1387-1811/Ó 2014 Published by Elsevier Inc.

Several analytical techniques were applied for metal ions detection at a low concentration level, including; atomic absorption spectrometer electrothermal atomization (AAS-ETA), inductively coupled plasma atomic emission spectrometer (ICP-AES) and X-ray fluorescence [10–12]. These techniques have low detection limits and highly sensitive, however, the complicated operation, the costly instrumentations, handling and time-consuming makes them unsuitable for on-line or field monitoring [13,14]. Therefore, fluorescent nanosensors were designed for the naked eye detection of these toxic metal ions at low concentration levels [15]. They introduce accurate detection in terms of sensitivity and selectivity with fast kinetic response that required today to analyze ultra-trace levels of environmental pollutants [16]. Recently, the design of fluorescent nanosensors which based on mesoporous materials is promising as highly sensitive solid-state sensors [17]. Mesoporous materials are nanostructured materials, resulting in highly ordered arrays with large surface area and narrow pore size distribution in the mesoscale range (2–50 nm in diameter) [18]. Ordered mesoporous silicates are a special class of synthetically modified colloidal silica, which have unique properties, including; good monodispersity, controlled pore size in two and three dimensions, extremely high surface areas (>1000 m2/g), large

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pore volume (1.5 cm3/g), high mechanical and thermal stability (up to 1000 °C) [19,20]. Thus, providing a confined space for intrapore inclusion chemistry [21]. Therefore, these materials have recently received much attention for their practical applications in catalysis, adsorption, separation, sensing, medicine, lasers and drug/gene delivery [22,23]. The synthesis strategy of mesoporous silicates depends on acidic hydrolysis of various kinds of silica source such as tetraethoxysilane (TEOS) in the presence of surfactant as a template, forming an amorphous silica wall around the template micelle assemblies, and then followed by template removal using appropriate methods such as calcination to leave the open ended mesoporous structure [24]. So, the surfactant type and synthesis conditions control the pore structure i.e., the pore diameter and dimensions of the resulting mesoporous materials [25]. Among of these hybrid materials, the Korean Advanced Institute of Science and Technology (KIT-6) mesoporous silicate was synthesized. Hence, butanol was used with triblock copolymers P123 (PEO20PPO70PEO20) as a co-surfactant and a swelling agent to expand the mesopore sizes and dimensions of silica monoliths, to synthesize 3D gyroid-cubic mesoporous silicates [26]. Therefore, KIT-6 was provided a high potential as drug delivery media and host for large molecules such as chalcones isothiocyanate (CITCs). CITC is the new class of chalcones analogue which have wide applications in laser dyes, photopolymer imaging systems, metal ions sensing and photoactive materials in molecular electronics [27]. In our a previous work, we report the selective anchoring of a novel tolyl chalcone isothiocyanate (TCITC) into the host of mesoporous silicates through post-synthetic incorporation techniques using two methods; (i) via the coupling agent (3-aminopropyl) trimethoxysilane (APS) into calcinated mesoporous silicates, (ii) Alkaline loading into amino-functionalized mesoporous silicates. Our final results revealed that, the high efficiency of the anchoring into calcinated mesoporous silicates via the coupling agent APS. This was attributed to the covalently attachments of APS by the addition of the amino group to the isothiocyanate group firstly, then loading into mesoporous framework. In this work, novel ligands of CITC analogue namely; phenyl chalcone isothiocyanate (PCITC), naphthyl chalcone isothiocyanate (NCITC) and anthryl chalcone isothiocyanate (ACITC) were synthesized. Our choice is to explore the effect of the molecular structure and the ring size on the loading process as well as the binding with the studied metal ions and sensing efficiency. KIT-6 mesoporous silicate was prepared through surfactant template method. Fluorescent nanosensors were designed by the anchoring of PCITC, NCITC and ACITC molecules into KIT-6 via APS loading method. Different spectroscopic techniques were used to characterize the investigated fluorescent nanosensors. Metal ions sensing response was studied using steady state absorption and emission techniques. Also, the assessment of investigated fluorescent nanosensors for the detection and sensitive sensing to various metal ions such as Pb(II), Cu(II) and Pd(II) was performed. The binding constants of the formed [M(CITCs-KIT-6)n]2+ complexes were determined from the emission spectra according to the modified Benesi–Hildebrand equation. In addition to, the naked-eye detection of the investigated metal ions was performed. Finally, the effect of ionic radius of the used metal ions on the binding and detection process was discussed.

incorporated) and antimony (III) chloride (nacalai tesque) were used. Benzaldehyde, a-naphthaldehyde, 9-anthraldehyde and triblock copolymer Pluronic P123 (EO20PO70EO20, molecular weight = 5800, where EO = ethyleneoxide, PO = propyleneoxide) were purchased from Sigma–Aldrich. 3-Amino propyl triethoxy silane (APS, MP biomedicals), dimethyl amine (40% Oxford Laboratory), methanol (HPLC, Fisher), toluene (HPLC, Tedia) and benzene (BHD Laboratory) were used without further purification. Potassium tetrachloro palatinate (II), copper (II) chloride, palladium (II) chloride and lead (II) acetate were purchased from Wako Pure Chemicals, Osaka, Japan. Potassium chloride, sodium hydroxide, hydrochloric acid (35%) and acetic anhydride (98%) were purchased from Beijing Chemical Int. Distilled water was used for the preparation of all aqueous solutions. 2.2. Synthesis 2.2.1. Synthesis of PCITC, NCITC and ACITC Chalcone was synthesized via aldol condensation in the presence of alcoholic sodium hydroxide. Therefore, the hydroxyl ion reacted directly with the isothiocyanate group to form an unstable intermediate, which captured a proton immediately from the water, forming unwanted stable thiourea, Scheme 1 [28]. This prompted us to suggest a novel method for the synthesis of CITC through protection technique. Therefore, isothiocyanate group in 4-acetyl phenyl isothiocyanate was protected by using dimethyl amine to form 4-N,N-dimethylthioureido-acetophenone. This was achieved as follows; a mixture of 4-acetyl phenyl isothiocyanate (1.1 g; 0.002 mol) and dimethylamine (0.42 g; 0.003 mol) in 20 ml ethanol were refluxed for 2 h. After cooling, the formed precipitate was filtered off and then washed with distilled water [29]. The obtained product was reacted with the corresponding aldehyde (benzaldehyde, a-naphthaldehyde and 9-anthraldehyde in case of PCITC, NCITC and ACITC, respectively) in ethanolic sodium hydroxide (6% NaOH/EtOH) to form protected chalcone. 1 g of the protected chalcone was added to 0.22 g (2.15 mmol) of acetic anhydride in 4 ml of benzene [30]. The mixture was refluxed for 2 h, then the formed precipitate was filtrated and the solvent from filtrate was removed by vacuum distillation to get chalcone isothiocyanate as shown in Scheme 2. 2.2.2. Synthesis of KIT-6 6.0 g of triblock copolymer (Pluronic P123) were dissolved in 10.5 g of HCl (35%) and 60 g H2O under vigorous stirring. After complete dissolution, 6.0 g of butanol as a swelling agent were added and stirred at 35 °C for 1 h. Then 12.48 g of tetraethyl orthosilicate TEOS was added to the clear solution. This mixture was left stirring at 35 °C for 24 h, followed by heating at 130 °C for 24 h under a static condition. The solid product was filtered, washed with distilled water, dried and subsequently calcinated at 550 °C for 6 h to remove the surfactant [31]. 2.3. Anchoring of PCITC, NCITC and ACITC into KIT-6 The amino-reactive CITCs was covalently attached to the coupling agent APS by the addition of the amino group to the isothiocyanate group to form the APS-CTIC precursor [32]. Then, APS-CTIC was attached to the calcinated mesoporous silica particles as illustrated in Scheme 3. The color of the KIT-6 changed from white to pale yellow, reveals the incorporation of CITCs.

2. Experimental section 2.1. Materials and reagents Tetraethyl orthosilicate (TEOS, 98%) was purchased from Across. 4-acetylphenyl isothiocyanate (97% trans world chemicals

R-N=C=S

OH-

[R-N-C-OH] S

H2 O

R-NH-C-OH + OH S

Scheme 1. The reaction of isothiocyanate group with hydroxyl ion in water.

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O

O

CH3 C

C

N

C

CH3

R eflux in E tO H

CH3

S

NH

+

C

4-N , N -dim ethylthioureido acetophenone (Protected isothiocyanate) H

N

dim ethyl am ine

S

N

CH3

4-acetyl phenyl isothiocyanate

CH3

N aO H /EtO H / Stir over night O

H

S

H

N C

N

H 3C

N C N H 3C

O

C

H 3C

O

H 3C

C

N

S

C

C

N

S

H

C

N

Protected anthryl chalcone isothiocyanate

C

S

C

N

O

O P henyl chalcone isothiocyanate (PC IT C )

C O

benzene/reflux

A cetic anhydride/

S

C

N

Protected naphthyl chalcone isothiocyanate

Protected phenyl chalcone isothiocyanate

O

H

C

H

S

H 3C C

H 3C

O

H

C

N aphthyl chalcone isothiocyanate (N C IT C )

C O

A nthryl chalcone isothiocyanate (A C IT C )

Scheme 2. Synthesis of chalcones isothiocyanate, PCITC, NCITC and ACITC.

of CITC to form N-1-(3-trimethoxy-silylpropyl)-N0 -chalconylthiourea (APS-CITC) precursor. The resulting slurry was dissolved in 1.30 ml of ethanol. The obtained solution of APS-CITC was refluxed with KIT-6 in toluene for 24 h, then filtrated and washed with ethanol to yield pale yellow CITCs/KIT-6 fluorescent nanosensor.

H 3 CO Si

H 3 CO

S

NH 2 +

C

N

C

Ar

H 3 CO

O

(3-aminopropyl) trimethoxysilane (APS)

Chalcone isothiocyanate (CITCs)

Stirr in For EtOH 24 hrs

H 3CO H 3CO

H

S

H

N

C

N

Si

2.4. Colorimetric detection of toxic metal ions C

Ar

H 3CO

O

(APS-TCITC) Precursor

Reflux in toluene for 24 hrs

M esoporous silicates (KIT-6)

H N

S

H

C

C

N

Ar O N

N

C

H

O

C H

S

Ar

CITCs anchored form into mesoporous silicates.

Scheme 3. Anchoring of CITCs, via the coupling agent APS into KIT-6, where; Ar = phenyl, naphthyl and anthryl moiety in case of PCITC, NCITC, ACITC fluorescent nanosensor, respectively.

The pH of the solution plays an important role in the entire detection process .The detection efficiency of metal ions was found to be a maximum around pH 3–4. This phenomenon may be attributed to competition of protons in strong acidic medium with the metal ions to react with the active sites. Also, further increase in pH value decreased the detection efficiency of metals due to precipitation of metal ions as metal hydroxides M(OH)n in slightly alkaline conditions [33]. Thus, pH 3–4 was selected as the proper pH value to achieve sensitive sensing of these metal ions from aqueous solution, typically as follow: different concentrations of Pb(II), Cu(II) and Pd(II) ranging from 5 ppb to 1 ppm were adjusted to a pH value of 3.0 (using 0.2 M of KCl/HCl) then added to 5 mg of PCITC, NCITC and ACITC solid sensors at constant volume (20 ml) at room temperature. After 5 min in an ultrasonic bath, the different concentrations of each metal ion with each of the three solid sensors were estimated quantitatively using absorption and emission techniques. The solid sensors were collected by suction using 25-mm cellulose acetate filter paper (Sibata filter holder) and identified qualitatively for naked-eye color assessment [26]. 2.5. Instruments and analysis

For typical loading achievement; 0.05 g of CITC (in each case of PCITC, NCITC and ACITC) was dissolved in 0.1827 g (0.8252 mmol) of 3-(aminopropyl)trimethoxysilane (APS). The reactants were allowed to stir for 24 h in a dried flask. The amino group of the silane coupling agent APS reacted with the isothiocyanate group

The FT-IR spectra of CITCs and their anchored forms into mesoporous silicates were recorded by BRUCKER spectrophotometer using KBr Pellets within the wavenumber range 4000–400 cm1. The X-ray diffraction (XRD) patterns of these nanostructured

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materials were recorded at room temperature using GNR, APD 2000 PRO step scans X-ray diffractometer, Cu-Ka radiation (40 kV, 30 mA), and scanning range of 2h (0–8°) with a step of 0.02°. The surface morphology of mesostructured materials were investigated (SEM) JEOL, JSM-52500LV scanning electron microscope. The pore shape and ordering of mesoporous materials were examined and photographed using (TEM) JEOL-JEM-100 SX transmission electron microscopy. Thermogravimetric analysis of the KIT-6 and its loaded forms were analyzed using the Simultaneous Thermal Analyzer (TGA/DTA) Perkin Elmer STA 6000 under nitrogen with heating rate of 15 deg/min. Nitrogen adsorption–desorption experiments were carried out at 77.40 K on Nova 3200e surface area and pore size analyzer, Quantachrome instrument. The Brunauer–Emmett–Teller (BET) surface area (SBET) was calculated from the linearity of the BET equation. The volume and pore radius were calculated from the pore size distribution curves using the Density Functional Theory (DFT) method. The UV–visible absorption spectra were done to confirm the loading process by following the uptake of CITC solution, also, to study metal ions sensing response of the investigated fluorescent nanosensors using a Shimadzu double beam UV–Vis-NIR scanning spectrophotometer (UV-3101 PC). The fluorescence spectra were recorded using a Perkin–Elmer LS 50B scanning spectrofluorometer using matched quartz cuvettes where, the excitation was performed at 260 nm with scan speed = 100 nm/min. The pH measurements were carried out using a digital Hana pH meter with a glass electrode.

1.4

(a)

Absorbance

1.2 1.0 0.8

-5

PCITC (2x10 M) 0.6

PCITC-APS before loading

0.4

PCITC-APS after loaing

0.2 0.0 250

300

350

400

450

500

550

600

650

wave length (nm) 1.6

(b)

Absorbance

1.4 1.2 1.0 -5

NCITC (2x10 M)

0.8

NCITC-APS before loading

0.6

NCITC-APS after loaing

0.4 0.2 0.0 300

350

400

450

500

550

600

650

700

wave length (nm) 3. Results and discussion 2.0

(c)

3.1. Characterization of KIT-6 and CITCs 1.6

Absorbance

In order to characterize the structure of KIT-6 mesoporous silicate, different spectroscopic investigations were performed, Figs. 1–8. These techniques confirmed the basic unit structure, morphology, pore size, surface area, uniform distribution of highly parallel channel and high thermal stability of these mesoporous silicates [24]. Also, the structures of CITCs were characterized using FT-IR spectra as shown in Fig. 2.

1.2

-5

ACITC (2x10 M) ACITC-APS before loading

0.8

ACITC-APS after loaing

0.4 0.0

300

3.2. Anchoring process and characterization of the investigated fluorescent nanosensors 3.2.1. UV–Vis spectra Anchoring of CITCs guest into mesoporous silicates cavities was followed as a function of the CITCs concentration using steady state absorption technique. The absorption bands of PCITC, NCITC and ACITC (at 340, 360 and 400 nm) were shifted to shorter wavelengths (at 300, 350 and 360 nm) upon addition of APS, respectively, Fig. 1 (a–c). This was attributed to the interaction between APS and CITCs, forming (APS + CITCs) precursor. As can be seen, the absorption spectra of the filtrate show a decrease in the intensity, indicating the uptake of CITCs into the mesoporous cavity, which confirms the anchoring process. 3.2.2. FT-IR analysis In an attempted to get an assignment for the characteristic function groups of CITCs, KIT-6 and their loaded forms, the FT-IR spectra were recorded (Fig. 2). As can be seen, the spectrum of PCITC (as an example for CITC analogue) showed sharp bands at vibrational frequencies 2950 cm1, 2052 cm1, 1700 cm1 and 1590 cm1, characteristic for the aromatic system, AC@CA, N@C@S and AC@O stretching vibrations, respectively. Also, the spectrum of KIT-6 showed a strong peak within the range 3500– 3200 cm1 which attributed to silanol groups (Si–OH). In addition, a shoulder at 1630 cm1 was observed due to the O–H of the

400

500

600

700

800

wave length (nm) Fig. 1. UV–Vis spectra for CITCs solutions before and after loading process, where, (a) PCITC, (b) NCITC and (c) ACITC.

(PCITC)

KIT-6 PCITC/KIT-6 loaded form NCITC/KIT-6 loaded form ACITC/KIT-6 loaded form

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber [Cm] Fig. 2. FT-IR spectra of PCITC (As an example of CITCs), KIT-6, and their PCITC, NCITC, ACITC/KIT-6 loaded forms.

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AC@O stretching vibration bands of CITCs, respectively, revealing the presence of CITCs molecules deposited on the surface of KIT-6. Additionally, the disappearance of the sharp peak at 2052 cm1 (characteristic for AN@C@SA stretching vibrational band), indicates the covalent interaction between an isothiocyanate group of CITCs and the amino group of APS. These results indicate the anchoring of CITCs into KIT-6 mesoporous silicates. 3.2.3. TGA analysis Mesoporous materials have high thermal stability (up to 1000 °C). Fig. 3 shows one step around 100 °C in the TGA curve of KIT-6 with the temperature range up to 800 °C, attributed to humidity losses. On the other hand, the TGA thermograms of PCITC, NCITC and ACITC loaded forms, show three decomposition steps. Therefore, the other two steps within the range (140– 380 °C) and (400–700 °C) are due to CITC and APS coupling agent decomposition, confirming the anchoring of CITCs into KIT-6 mesoporous silicate.

Fig. 3. TGA curves of KIT-6 and their CITCs loaded forms.

adsorbed water. Moreover, the observed strong peaks at 1100 cm1, 804 cm1 and 480 cm1 refer to symmetric, asymmetric Si–O–Si stretching and Si–O–Si binding vibrational frequencies, respectively. However, the spectra of CITCs loaded forms relative to that of KIT-6, showed new shoulders at 2950 cm1, 1700 cm1 and 1590 cm1, corresponding to the aromatic system, AC@CA and

3.2.5. SEM analysis The surface morphologies of mesoporous silicates were characterized using scanning electron microscope (SEM). Fig. 5 shows the SEM images of KIT-6 and its PCITC, NCITC and ACITC loaded forms. Image (a) shows that KIT-6 has cubic 3D mesoporous with uniform phase. While, the images (b–d) show more than one shape and different phases, indicating that CITCs are deposited on the surface of

Intensity (a. u)

Fig. 4. XRD pattern of KIT-6 and their CITCs loaded forms.

3.2.4. XRD analysis Amorphous materials, such as mesoporous silicates, have no periodic atomic planes compared to ordered materials. So, they give reflections at low angles (2h < 3°), causing difficulty in reflections detection. Since, some of the X-rays beam can go straight into the detector and cause high background radiation which makes it difficult to identify the peaks. Therefore, the full structural characterization of mesoporous materials by XRD is difficult [34]. However, Fig. 4 shows a small peak, at 2h = 2.34 ° in case of KIT-6, which disappears in the cases of PCITC, NCITC and ACITC/KIT-6 loaded forms, revealing the anchoring of CITCs into KIT-6.

KIT-6 PCITC/KIT-6 loaded form NCITC/KIT-6 loaded form ACITC/KIT-6 loaded form

2

3

4

5

6

7

8

2 Theta (deg)

Fig. 5. SEM images of mesoporous silicate and their loaded forms; (a) KIT-6, (b) PCITC/KIT-6, (c) NCICT/KIT-6 and (d) ACITC/KIT-6 loaded forms, Scale bar = 10 lm. Arrows indicate the two shapes of KIT-6 and their CITCs loaded forms.

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149

Fig. 6. Transmission electron micrographs of mesoporous silicate and their loaded forms, where (a) KIT-6, (b) PCITC/KIT-6, (c) NCICT/KIT-6 and (d) ACITC/KIT-6 loaded forms, Scale bar = 50 nm. Arrows indicate the slight distortion and disappearance of mesoporous pores after CITCs anchoring process.

S BET/m2/g V/cm3/g Dv(r)nm

700 (a) (b) (c) (d)

V/ ml (STP).g

-1

600 500

1096 152 280 412

0.958 0.342 0.446 0.521

10.4 6.2 7.4 7.9

400 300 200 100 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

P/Po Fig. 7. N2 adsorption–desorption isotherms of (a) KIT-6, (b) PCITC/KIT-6, (c) NCICT/ KIT-6 and (d) ACITC/KIT-6 loaded forms. BET surface area (SBET/m2/g), total pore volumes (V/cm3/g) and pore radius (Dv(r)nm) are also shown.

mesoporous silicates. This confirms the anchoring of CITCs into KIT-6. 3.2.6. TEM analysis Anchoring of CITCs into mesoporous silicates was also investigated using TEM analysis, as it is expected that loading of CITCs into KIT-6 leads to distortion and disappearance of KIT-6 pores in TEM images. As can be seen in Fig. 6 image (a) represent KIT-6 has ordered pore structure with highly parallel channel-like pores. While, images (b–d) represent PCITC, NCICT and ACITC/KIT-6 loaded forms, respectively and show the disappearance of some mesoporous pores confirms anchoring of CITCs into KIT-6. 3.2.7. Nitrogen adsorption–desorption Nitrogen adsorption–desorption measurements yielded Brunauer–Emmett–Teller (BET) surface area (SBET/m2/g), total pore volumes (V/cm3/g) and pore radius (Dv(r)nm). Fig. 7 shows the nitrogen adsorption–desorption isotherms for KIT-6 and its CITCs

anchored forms, where all curves represent Type-IV in the IUPAC classification. This indicates monolayer/multilayer adsorption isotherm with a broad hysteresis loop characteristic for nanoporous materials. A complete adsorption of N2 as a monolayer onto the surface of the sorbent material was shown by the plateau of the adsorption isotherm [35]. After anchoring of PCITC, NCITC and ACITC into KIT-6, branches of the isotherms were significantly shifted toward lower relative pressure (P/Po). Additionally, a decrease in the surface area (SBET/ m2/g), total pore volumes (V/cm3/g) and pore radius (Dv(r)nm) were observed, confirming the presence of CITCs on the surface and into the pores of mesoporous silicates which blocks partially adsorption of nitrogen molecules. The data in Fig. 7 show a decrease in BET surface area of KIT-6 from 1196 m2/g to 152, 280 and 412 m2/g for PCITC, NCITC and ACITC/KIT-6 anchored forms, respectively. Also, it indicates that the total pore volumes decreased from 0.958 cm3/g to 0.342, 0.446 and 0.521 cm3/g, with a parallel decrease in the pore radius from 10.4 nm to 6.2, 7.4 and 7.9 nm respectively. The decrease of these values in case of PCITC anchored form is greater than those for NCITC and ACITC. These results revealed the high loading efficiency of PCITC into KIT-6 cavities compared to NCITC and ACITC which could be explained on the basis of the steric hindrance as the ring size increases from phenyl to naphthyl and anthryl moiety, respectively. 3.3. CITCs/KIT-6 as fluorescent nanosensors for detection of some toxic metal ions Metal ions sensing response of the investigated PCITC, NCITC and ACITC/KIT-6 nanosensors was estimated quantitatively using steady-state absorption and emission techniques, and qualitatively via naked eyes color change. Fig. 8 shows a gradual increase in the absorption and the emission intensities upon adding different concentrations of Pb(II), Cu(II) and Pd(II) (ranging from 5 ppb to 1 ppm), with instantaneous color changes. The enhancement in the absorption intensity confirms the formation of donor–acceptor complexes with the participation of the lone electron pairs of nitrogen, resulting in [M(CITCs/KIT-6)n]2+ complexes. However, the

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Fig. 9 shows the relation between the fluorescence intensities and the concentrations of the used metal ions within the range 5 ppb to 1 ppm. It is evident that, the fluorescence intensity is more sensitive to the nature, concentration and the ionic radius of the used metal ions as well as the molecular structure of the investigated nanosensor. The great enhancement of the fluorescence intensity in case of PCITC compared to NCITC and ACITC/KIT-6, reveals the efficiency of PCITC/KIT-6 fluorescent nanosensor over NCITC and ACITC/KIT-6 in the detection of all of the used metal ions. The high efficiency of PCITC/KIT-6 was due to the steric hindrance of naphthyl and anthryl rings in NCITC and ACITC/KIT-6, respectively, which impedes the chelation of metal ions with the binding centers of the nanosensor. In order to study the effect of dimension and molecular structure of the CITCs on the investigated nanosensors, the optimized geometrical structures were calculated using the portable Hyperchem 8.07 software. It was found that, the long axis of PCITC, NCITC and ACITC molecules are 13.9, 15.8 and 14.6 Å, respectively. The full optimized geometry of the PCITC shows planner structure, while NCITC and ACITC show some deviation from planarity due to the small twisting of naphthy and anthryl moieties. This result reflects the steric hindrance of naphthyl and anthryl moieties of NCITC and ACITC nanosensors, confirming the efficiency of PCITC/ KIT-6 in anchoring and detection of metal ions. In addition, the charge density of the binding center was calculated using PM3 semiemperical calculations. The high charge density, 0.194 and 0.192e, of the two nitrogen atoms compared to the low charge density of sulfur atom, 0.044e, revealing the ability of the two nitrogen atoms to bind to the metal ions via coordination bonds to form [M(CITCs/KIT-6)n]2+ complex, as illustrated in Scheme 4. The binding constants of the formed [M(CITCs/KIT-6)n]2+ complexes were calculated from the emission spectra according to the modified Benesi–Hildebrand double reciprocal equation [36]:

   1 1 1 1 ¼ þ K½C DF max DF DF max

Fig. 8. UV–Vis absorption spectra of PCITC fluorescent nanosensor upon adding different concentrations of the used metal ions. Inset; the emission spectra and the color change of (a) Pb(II), (b) Cu(II) and (c) Pd(II) toxic metal ions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fluorescence spectra of the investigated CITCs/KIT-6 nanosensors showed a very weak fluorescence around 405 nm (inset of Fig. 8). The intensity was strongly enhanced with hypsochromic shift (ca.10 nm) after addition of Pb(II), Cu(II) and Pd(II) ions. The continuous enhancement of the fluorescence intensity is due to cation-induced restriction of the ligand free rotation, stemming from the binding of these metal ions with the investigated nanosensors. Naked-eye detection of these metal ions was also observed, where the color changes from pale yellow (free solid CITCs/KIT6) to faint pink, cyan and yellow for [Pb(CITCs/KIT-6)n]2+, [Cu(CITCs/KIT-6)n]2+ and [Pd (CITCs/KIT-6)n]2+ complexes, respectively (inset Fig. 8).

where DF = Fx  Fo and DFmax = F1  Fo, and Fo, Fx and F1 denote the fluorescence intensities of CITCs-KIT-6 in the absence of metal ions, at certain metal ion concentration, and at the concentration of complete interaction, respectively. [Mn+] is the concentration of the metal ions and K is the binding constant of the formed complexes which is determined from the plots of 1/DF against 1/[Mn+]. While, typical linear reciprocal plots were established in the concentration range 5 ppb to 1 ppm of the used metal ions, Fig. 10 shows the plots of 1/DF against 1/[Mn+] for PCITC/KIT-6 fluorescent nanosensor with Pb(II), Cu(II) and Pd(II). The calculated binding constants, K (M1) were collected in Table 1. The reported data indicates that PCITC-KIT-6 fluorescent nanosensor has the greater K values with all the investigated metal ions compared to those of NCITC and ACITC/KIT-6 nanosensors. These results confirm the efficiency of PCITC/KIT-6 over NCITC and ACITC/KIT-6, due to the vital role of the molecular structure and ring size in the binding process. Replacement of phenyl ring in PCITC/KIT-6 with larger polycyclic, naphthyl moiety in NCITC/ KIT-6 and anthranyl moiety in ACITC/KIT-6, hinders the chelation process. Therefore, the steric hindrance over the whole molecular skeleton leads to the lower value of the binding constant in the larger polycyclic chalcones isothiocyanate. Also, Pb(II) is the strongest binding metal ion with the three optical nanosensors compared to Cu(II) and Pd(II). This could be explained on the basis of the large ionic radius of Pb(II) (0.98 Å) relative to Cu(II) and Pd(II) (0.57 and 0.64 Å, respectively) [37]. Therefore, as the ionic radius of the used metal ion increases, the binding constant is also increased. The charge distribution of the larger size ion is near to the electron donation of the binding center of CITCs fluorescent

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Fig. 9. Fluorescence intensities results after addition of different concentrations of Pb(II), Cu(II) and Pd(II) to the investigated fluorescent nanosensors.

0.009

(a)

S C N

Pb(II) Cu(II) Pd(II)

0.008

H

H

0.007

C

N

0.006

O

1/ΔF

0.005

N

N

C

H

O

C

H

0.003 0.002

S

0.001

(b)

0.000 0.0

S H

H

4.0x107

6.0x107

8.0x107

1/[M ] mole/L

C C

N

Fig. 10. Plots of 1/DF against [M+2]1 for Pb(II), Cu(II) and Pd(II) with PCITC fluorescent nanosensor.

O

N

C

H

O

N C

H

Table 1 The values of binding constant, K (M1) of Pb(II), Cu(II), and Pd(II) with PCITC, NCITC and ACITC/KIT-6 optical nanosensors.

S

(c)

Optical nanosensors

PCITC/KIT-6 NCITC/KIT-6 ACITC/KIT-6

S N

2.0x107

2+

N

H

0.004

C

H C

N

Binding constant of metal ions, K (M1) Pb (II)

Cu (II)

Pd (II)

10.3  107 6.3  107 0.75  107

3  107 2.5  107 0.6  107

2.6  107 0.9  107 0.18  107

O

nanosensors which in turn leads to strong overlap between the orbitals and encapsulation of metal ions inside the pores which leads to a high probability of the formation of [M(CITCs/ KIT-6)n]2+ complexes. N H

C

N

C

H

O

4. Conclusions

S

Scheme 4. Binding of the investigated nanosensors, (a) PCITC, (b) NCITC and (c) ACITC/KIT-6 with M2+ ion.

The design of fluorescent nanosensors was achieved by the anchoring of novel CITCs into KIT-6 via APS coupling agent. The results indicated the anchoring efficiency of PCITC into KIT-6

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compared NCITC and ACITC which could be explained on the basis of the steric hindrance of the naphthyl and anthryl rings, respectively. Also, a gradual increase in the absorption and emission spectra of the fluorescent nanosensors was observed upon adding different concentrations of the used metal ions, indicating the binding of these metal ions with the investigated fluorescent nanosensors, resulting [M(CITCs/KIT-6)n]2+ complexes with instantaneous color change. The calculated binding constants show the efficiency of PCITC/KIT-6 nanosensor in binding and detection of these metal ions. Also, Pb(II) is the strongest binding metal ion with the three optical nanosensors compared to Cu(II) and Pd(II). Therefore, as the ionic radius of metal ion increases, the binding constant is increased. Among the studied metal ions, Pd(II) and Cu(II) give clear colored (faint pink and cyan, respectively), which suggest the investigated PCITC fluorescent nanosensor as a sensitive and selective sensing systems for the detection of these toxic metal ions. These results suggested that, these fluorescent nanosensors are suitable for simple, low cost, on-line analysis and remote determination of these toxic metal ions with fast kinetic responses. References [1] J. Wang, S. Chu, F. Kong, L. Luo, Y. Wang, Z. Zou, Sens. Actuators B 150 (2010) 25–35. [2] J. Wang, L. Huang, M. Xue, L. Liu, Y. Wang, L. Gao, J. Zhu, Z. Zou, Appl. Surf. Sci. 254 (2008) 5329–5335. [3] Md.R. Awual, I.M.M. Rahman, T. Yaita, Md.A. Khaleque, M. Ferdows, pH dependent Cu(II) and Pd(II) ions detection and removal from aqueous media by an efficient mesoporous adsorbent, Chem. Eng. J. 236 (2014) 100–109. [4] A. Hill, M. Friedman, S. Reiber, G. Korshin, R. Valentine, Am. Water Works Assoc. 102 (2010) 107–118. [5] A.A. Ismail, J. Colloid Interface Sci. 317 (2008) 288–297. [6] Md.R. Awual, T. Yaita, S.A. El-Safty, H. Shiwaku, Y. Okamoto, S. Suzuki, Investigation of palladium(II) detection and recovery using ligand modified conjugate adsorbent, Chem. Eng. J. 222 (2013) 172–179. [7] M. Marafi, A. Stanislaus, Resour. Conserv. Recycl. 52 (2008) 859–873. [8] J. Kielhorn, C. Melber, D. Keller, I. Mangelsdorf, Int. J. Hyg. Environ. Health 205 (2002) 417–432. [9] A. Troupis, A. Hiskia, E. Papaconstantinou, Selective photocatalytic reductionrecovery of palladium using polyoxometalate, Appl. Catal. B 52 (2004) 41–48.

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