Hydrogel templated CdS quantum dots synthesis and their characterization

Hydrogel templated CdS quantum dots synthesis and their characterization

Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physi...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Hydrogel templated CdS quantum dots synthesis and their characterization Nurettin Sahiner a,b,∗ , Kivanc Sel b,c , Kadem Meral d , Yavuz Onganer d , Sultan Butun a , Ozgur Ozay a , Coskun Silan b,e a

Faculty of Science & Arts, Department of Chemistry, Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkey Faculty of Science & Arts, Nanoscience and Technology Research and Application Center (NANORAC), Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkey c Faculty of Science & Arts, Department of Physics, Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkey d Ataturk University, Faculty of Science, Department of Chemistry, 25240 Erzurum, Turkey e School of Medicine, Department of Pharmacology, Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkey b

a r t i c l e

i n f o

Article history: Received 16 July 2011 Received in revised form 3 September 2011 Accepted 8 September 2011 Available online 17 September 2011 Keywords: Hydrogel templates CdS Q-dots Hydrogel composites Fluorescent particles

a b s t r a c t This work reports a facile method for preparation of CdS quantum dots (Q-dots), using a crosslinked hydrophilic p(AMPS) hydrogel network by absorption of Cd(II) ions and sequential precipitation with aqueous Na2 S within the network at room temperature. The TEM images revealed that prepared CdS Q-dots were distributed throughout the p(AMPS) hydrogel network and their sizes were about 5 nm. The amount of inorganic material inside p(AMPS) hydrogel was determined by TGA measurements to be 16.9% by weight. Additionally, the Cd and S amounts inside the p(AMPS) network were determined by dissolution of particles with HCl (three treatments with 3 M HCl) and by using ICP-AES (for Cd) and an elemental analyzer (for S). From UV–visible absorbance measurements, optical energy gap values of 5.1 ± 0.1 eV for p(AMPS) and 2.4 ± 0.05 eV for p(AMPS)–CdS were determined. From the fluorescence spectrum of the p(AMPS)–CdS hydrogel, the peak energy was observed at 2.30 eV. The in situ prepared Q-dots were recovered from hydrogel matrices by placing the p(AMPS)–CdS composite in purified water. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor metallic nanocrystal Q-dots are attractive materials due to their unique physical and optical properties [1–5]. They can be used in various fields such as biological labeling [6], nanomedicine [7], energy storage, drug delivery, catalysis [8], sensing, imaging [9], optoelectronics [10], solar cells, light-emitting devices and non-linear optical devices [11]. Some of the wellstudied Q-dots, such as CdSe, CdS, ZnSe, CdTe, PbS, HgS and InAs, are a combination of II–VI, III–V and IV–VI group elements [11–15]. Amongst them, CdS have been extensively studied due to their sufficient band gap energy (2.4 eV) at room temperature [16,17]. Material synthesis in defined geometries has been well investigated employing many templates; and the shapes of these templates are very important for the consequent material in terms of their sizes, shapes, morphologies, and physical and chemical characteristics. Polymeric micelles and/or block copolymers [18],

∗ Corresponding author at: Faculty of Science & Arts, Department of Chemistry, Canakkale Onsekiz Mart University, Terzioglu Campus, 17100 Canakkale, Turkey. Tel.: +90 286 2180018x2041; fax: +90 286 2181948. E-mail address: [email protected] (N. Sahiner). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.09.006

multilayers of sequentially adsorbed polyelectrolytes or layer-bylayer deposited polyionic assemblies, DNA, peptides, surfactants [19], and reverse micelles are commonly used confined environments for unique material synthesis with different properties [20–23]. Homo- or block polymers with various architectures are generally employed in metal nanoparticle preparation [23–26]. Therefore, hydrogels as three-dimensional insoluble hydrophilic polymeric networks with additional functional groups can provide unique environments for in situ synthesis of metal nanomaterials, and provide shelter for their protection for further use. Hydrogels can absorb large quantities of water, causing them to swell, thereby offering an excellent milieu for reactions that take place in aquatic environments. The degree of hydrogel swelling is related to the extent of the hydrophilic groups, the number of crosslinkers, the ionic strength of the solution, temperature, and pH and so on. The functional hydrophilic groups such as –OH, –COOH, –NH2 , –CONH2 , –SO3 H [27,28] are the main group responsible for swelling and can also be exploited for additional purposes. Hydrogels are soft, flexible and adaptable materials. Moreover, they are biocompatible, so they can be used in many applications in biomedical fields [29] as delivery vehicles of active agents (such as drugs, proteins, and genes), as templates for tissue engineering, for wound dressing materials and artificial organs, for separation/purification [30], for

N. Sahiner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11

SO3-H+

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Time (min) Fig. 1. (a) A schematic representation of Cd(II) absorption, and (b) the absorption isotherms of p(AMPS) hydrogels for Cd(II) (loading medium 100 mL of 250 mg L−1 M Cd(II), 100 mg hydrogel).

catalysis [31–33], and for sensors and actuators responding to various stimuli [29–33]. Q-dots, also known as semiconductor metal nanoparticles, have great potential in drug delivery, bio-labeling, optoelectronic devices and sensors. Previously we reported in situ metal particle preparation in 2-methyl-1-acrylamido-2-propansulfonic acid (AMPS) hydrogels. The in situ prepared iron, nickel and cobalt nanoparticles within p(AMPS) hydrogel networks were used as catalysts in the hydrolysis of sodium borohydride, ammonia borane and in the catalytic reduction of aromatic nitro compounds [31–34]. In this study, p(AMPS) hydrogels were loaded with Cd(II) via electrostatic interactions between –SO3 − groups of p(AMPS) and Cd(II) ions in solution, and then these ions, which were held in position inside the hydrogels, were treated with Na2 S to produce CdS nanoparticles at room temperature within the hydrogels. These CdS nanoparticles were characterized by TG, ICP-AES, TEM, UV–visible absorbance and fluorescence spectroscopy analyses.

2. Experimental 2.1. Materials For the syntheses of hydrogels, 2-acrylamido-2-methyl-1propansulfonic acid as a monomer, N,N -methylenebisacrylamide (MBA) as a crosslinker, 2,2 -azobis(2-methylpropionamidine) dihydrochloride as a UV initiator were purchased from Aldrich and Acros Chemical Companies. For in situ quantum dot preparation, cadmium chloride (Sigma, 99%) was used as a metal ion source and sodium sulfide nonahydrate (Sigma–Aldrich, 98%) was used as a sulfur source. All the reagents were of analytical grade or were the highest purity available, and thus were used without further purification. The 18.2  ohm cm (Millipore Direct-Q3 UV) purified water was used for preparation and washing of hydrogels in all reactions.

2.2. Synthesis of p(AMPS) hydrogels The crosslinked p(AMPS) hydrogel was prepared as reported earlier [24,28–31]. In brief, 0.5 mole% MBA was dissolved in 50 wt% aqueous AMPS and 0.5 mole% (with respect to monomer) and UVinitiator was added to this mixture, and the mixture was then placed in plastic straws. The straws were irradiated in a photoreactor (LUZCHEM, 420 nm, Canada) for 2 h. The obtained hydrogels were placed in purified water, rinsed for 24 h to remove unreacted species, and dried in an oven at 40 ◦ C. They were stored for use in Q-dot preparation and characterization. 2.3. Preparation of p(AMPS)–CdS composites Inside the hydrogel, in situ CdS Q-dots were prepared in a similar way to that reported earlier [23]. Briefly, the cleaned and dried 100 mg p(AMPS) hydrogels were placed in 100 mL of 250 mg L−1 M Cd(II) solution. The hydrogels were kept in the metal ion solution for 24 h in a water shaker bath to load the hydrogel with Cd(II) ions. The amount of absorbed Cd(II) was determined from the metal ion solution using ICP-AES (Inductive Coupled Plasma-Atomic Spectrometry (Varian Liberty II AX Sequential). After loading the metal ions, the hydrogels were transferred to purified water for another 24 h for cleaning. Then, the metal ions inside the hydrogels were precipitated by sulfur ions by transferring p(AMPS)–Cd(II) into 100 mL of 0.05 M NaS2 solution. The reaction period was continued for 6 h in a water shaker bath followed by a washing procedure in purified water for another 6 h. Additionally, to determine the exact amount of metal ions inside the hydrogel matrices, the Q-dots were dissolved with concentrated HCl acid (three treatments with 5 M HCl) and this quantification was carried out again using ICP-AES. 2.4. Characterization of p(AMPS)–CdS composites The amount of CdS inside the p(AMPS) hydrogel was determined by thermo gravimetric analysis (TGA/DTA – SII, 6300) and

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N. Sahiner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11

Fig. 2. (a) A schematic representation of in situ formation of CdS particles in a p(AMPS) hydrogel network. Digital images of (b) bare p(AMPS), (c) p(AMPS)–CdS.

ICP-AES measurements. The thermograms of p(AMPS) and CdS particles containing p(AMPS)–CdS were recorded by heating the samples between 50 and 1000 ◦ C using a 10 ◦ C/min heating rate and 100 mL/min nitrogen flow. The weight loss against temperature was recorded. The TEM images of CdS particles within p(AMPS)–CdS composites were obtained by grinding the dried composites with a mortar and pestle to form powder, and a drop of this powder suspended in ethanol was placed on a formvar-coated TEM grid and visualized under vacuum at an operating voltage of 200 kV (JEOL 2010 TEM). To determine the carbon and sulfur amounts in the p(AMPS)–CdS composites, elemental analysis was carried out with an elemental analyzer instrument (Leco SC-144 DR). The optical properties of the CdS embedded hydrogel were analyzed by UV–visible absorbance measurements (T80+UV/VIS PG Instruments Spectrometer, Single beam), and fluorescence spectrum measurements were performed by using a Shimadzu RF-5301 PC Spectrofluorophotometer. The p(AMPS) and p(AMPS)–CdS composite hydrogel samples in cylindrical form with a radius of about 5 mm were cut into 10 mm and 1 mm slices for absorption and fluorescence measurements.

3. Results and discussion Owing to the anionic characteristic of p(AMPS) hydrogels (–SO3 H can easily be ionized in water), the metal ions such as Cd(II) can be readily absorbed by p(AMPS) hydrogels from aqueous environments. Fig. 1(a) gives a schematic representation of the Cd(II) ion absorption by p(AMPS) hydrogels from the corresponding metal ion source, while (b) of the same figure is the Cd(II) absorption isotherm by p(AMPS), which were constructed after placing cleaned and dried p(AMPS) hydrogels in Cd(II) solution. The amount of absorbed Cd(II) was determined using ICP-AES measurements from solution. As can be seen from Fig. 1(b), the p(AMSP) hydrogels can absorb 119.30 mg Cd(II) in about 600 min and further contact time does not affect the absorption capacity. Following the treatments of absorbed Cd(II) ions with Na2 S solution within p(AMPS)

Fig. 3. Digital images of CdS containing p(AMPS) hydrogels in DI water (a), and the elution CdS particles after 24 h (b). The DLS measurements of CdS particles after 24 h (c) (98 nm; in 0.1 M SDS).

N. Sahiner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11

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Fig. 4. TEM images of CdS Q-dots within p(AMPS) hydrogels (a) and (b), and a TGA thermogram of p(AMPS) and p(AMPS)–CdS composites (c).

hydrogel, CdS Q-dots were prepared in situ as illustrated in Fig. 2(a). As soon as the p(AMPS) hydrogels were placed in the Na2 S solution a light yellow color formed on the outer periphery of the transparent p(AMSP) hydrogels, as shown in Fig. 2(b). With time, a strong yellow color was formed throughout the hydrogel, as digital camera images reveal in Fig. 2(c). After 6 h, the p(AMPS)-Q-dot hydrogels were placed in purified water to wash out the excess Na2 S and Qdots. However, as illustrated in Fig. 3(a) and (b) the CdS Q-dot can elute from the hydrogel matrices. The freshly prepared washed Qdots containing hydrogels were placed in purified water (as seen in (a) of Fig. 3), and after a while the solution started to turn yellow as the Q-dots were driven out from the hydrogel networks as seen in Fig. 3(b). Although the hydrogel network provided a shelter and stabilizing environment for the CdS particles, upon precipitation of Cd ions with S ions the interactions between hydrogel –SO3 − groups and CdS particles were no longer as strong as the interactions between –SO3 − groups and Cd(II) ions that occurred earlier, and therefore, the ejection of CdS particles was anticipated. On the other hand, if required, CdS Q-dots can be kept within hydrogels by reducing the porosity of the hydrogels through using the excess amount of crosslinking agents during synthesis and/or imprisoning Q-dots by using other types of hydrogel-composites, such as hydrogel silica composite. DLS measurement of CdS particles showed particle sizes of about 98 nm due to the aggregations. Although SDS was added to the elution solution, Q-dots were still aggregated, as evidenced with TEM measurements. The TEM measurements revealed that

the CdS particles were almost homogenously distributed throughout the p(AMSP) network, as shown in Fig. 4(a), and as the high magnification of TEM images shows, the CdS particles were about 5 nm, as depicted in Fig. 4(b). To determine the thermal stability and the amount of inorganic material inside of the p(AMPS) hydrogel, the TGA of bare p(AMPS) and p(AMPS)–CdS composite were recorded and their corresponding thermograms are presented in Fig. 4(c). It can be clearly seen that the prepared p(AMPS) and CdS containing composite hydrogel show similar degradation characteristics, except that the composite materials started degrading at a temperature about 100 ◦ C higher than was the case for p(AMSP). The bare p(AMPS) start to degrade at about 185 ◦ C, whereas p(AMPS)–CdS composites start to degrade at about 287 ◦ C. The second degradation temperature for bare hydrogels was between 347 and 464 ◦ C, while this range was 339 and 448 ◦ C for composite materials. Upon further heating it was found that p(AMPS) continued to degrade up to 986 ◦ C and p(AMPS)–CdS up to 913 ◦ C. The heating continued up to 1000 ◦ C under the same conditions and the differences between them are expected to give the amount of inorganic materials (CdS). Therefore, it can be said that the CdS content of p(AMPS)–CdS composites was 16.9% by weight. It is worth mentioning that the amount of CdS can be increased by multiple loadings of Cd ions after their treatment with Na2 S. Elemental analyses confirmed that CdS particles were formed within the p(AMSP) network. The elemental analysis and ICP-AES

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N. Sahiner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11

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measurement of CdS obtained by the dissolution during three HCl treatments within p(AMPS) hydrogels were carried out. The p(AMPS) hydrogels have 38.64 wt% C and 13.91 wt% S, where S comes from the –SO3 H groups from the network from elemental analysis results, and after in situ preparation of CdS the C content reduced to 28.77 wt% C whereas the S content was increased to S 14.43 wt% (from ICP-AES measurements) for p(AMPS)–CdS composites due to the existence of CdS nanoparticles. After dissolution of CdS with HCl, the amount of Cd was found to be 10.79 wt%. It is obvious that the increase of S content of composite hydrogel is due to the formation of CdS Q-dots. It was also confirmed with TGA analyses that the 16.9 wt% of composite hydrogels was due to the inorganic part. The UV–visible absorbance measurements performed for bare p(AMPS) and p(AMPS)–CdS, and the UV–visible spectra are presented in Fig. 5(a). The absorption coefficients (˛ (cm−1 )) were calculated from the absorbance (A) by the equation: A=˛·d

(1)

where d is the sample thickness. Considering that the CdS containing p(AMPS)–CdS hydrogels have a direct band gap [35–42], the optical energy gap (Eg ) of the hydrogels were calculated from the equation [43]: (˛h2 = B(h − Eg )

(2) (˛h)2

where B is a constant, and as a function of h graph is presented in Fig. 5(b). Eg values were determined by fitting a linear function to (˛h)2 and extrapolating it to (˛h)2 is equal to zero value. The determined Eg values were 5.1 ± 0.1 eV for p(AMPS) and 2.4 ± 0.05 eV for p(AMPS)–CdS, which were compatible with the

values reported between 2.1 and 2.9 eV for CdS films in the literature [35–42]. The fluorescence spectrum of the AMPS–CdS hydrogel, obtained by applying an excitation wavelength of 366 nm, is presented in Fig. 6. The peak energy was observed at 2.30 eV, which is in agreement with the reported optical energy gap. The in situ preparation of Q-dots within a mostly biocompatible hydrogel matrix offers great opportunities in the biomedical application of these materials. For example, such hydrogels can be smart and environmentally benign materials (and can even sometimes be designed to be biodegradable) and they have already been used in biomedical fields i.e., in pharmaceuticals, cosmetics, drug delivery devices, artificial organs, tissue engineering and so on. Moreover, hydrogels can also be prepared in different sizes (micrometer and nanometer) and morphologies (core–shell, interpenetration network (IPN), layer by layer self assembled networks and capsules etc.). Additionally, their utilization together with Q-dots offers great opportunities and many advantages in biomedical fields, such as monitoring the drug carriers within the body and illuminating the various mechanisms in operation, such as carrier/particle and/or drug internalization by cells and so on. Therefore, hydrogels with different sizes and morphologies and with the capability of in situ Q-dot preparation are relevant and viable for many applications.

4. Conclusions This investigation demonstrated that hydrophilic and functional p(AMPS) can be used as a template in the preparation of CdS Q-dots. The hydrogel-assisted CdS particles were successfully prepared by the absorption of Cd(II) into hydrogel and precipitation of these absorbed ions within the hydrogel network with Na2 S. During the Q-dot synthesis the p(AMPS) network provided a stabilization effect and after the synthesis, the in situ prepared Q-dots recovered readily. As the hydrogels are hydrophilic and have a high swelling ratio, upon placing p(AMSP)–CdS composite in purified water, CdS particles were expelled from the hydrogel network. Due to the aggregation of CdS nanoparticles outside of the hydrogel, the size of these CdS particles was found to be 98 nm with DLS measurements although the TEM images revealed that the CdS particles with hydrogel matrices were a few nm (∼5 nm) in size. The method presented here is versatile and can be used for the preparation of other types of Q-dots and metal nanoparticles with different metal ions. Additionally, it is also possible to use other hydrogel matrices to prepare different Q-dots. In fact, our current research involves

N. Sahiner et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 6–11

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