Synthesis of HgS nanocrystals in the Lysozyme aqueous solution through biomimetic method

Applied Surface Science 258 (2012) 8185–8191

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Synthesis of HgS nanocrystals in the Lysozyme aqueous solution through biomimetic method Li Zhang a , Guangrui Yang b , Guoxu He a , Li Wang a , Qiaoru Liu a , Qiuxia Zhang a , Dezhi Qin a,∗ a b

College of Chemistry and Chemical Engineering, Pingdingshan University, Pingdingshan 467000, PR China Institute of Environmental and Municipal Engineering, North China University of Water Conservancy and Electric Power, Zhengzhou 450011, PR China

a r t i c l e

i n f o

Article history: Received 1 April 2012 Received in revised form 3 May 2012 Accepted 4 May 2012 Available online 11 May 2012 Keywords: HgS nanocrystals Lysozyme Biomimetic Quantum effect Optical properties

a b s t r a c t In the present work, it is reported for Lysozyme-conjugated HgS nanocrystals with tunable sizes prepared at Lysozyme (Lyso) aqueous solutions by using biomimetic method. The obtained HgS nanoparticles with good dispersibility have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission microscopy (HRTEM) and energy-dispersive X-ray spectrum (EDS). The Lysozyme molecules can control nucleation and growth of HgS crystals by binding on the surface of nanocrystals to stabilize protein-capped nanoparticles. Quantum confinement effect of Lyso-conjugated HgS nanocrystals has been confirmed by UV–vis spectra. The nanoparticles exhibit a well-defined emission feature at about 470 nm. Fourier transform infrared (FT-IR) data are used to envisage the binding of nanoparticles with functional groups of Lysozyme. The results of circular dichroism (CD) spectra indicated that the formation of HgS nanocrystals can lead to conformational change of Lysozyme. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor chalcogenides nanocrystals have attracted extensive interest not only because of their size-dependent characteristics but also because of their novel electronic and optical properties arising out of quantum confinement effects compared with the corresponding bulk compounds [1–6]. II–VI semiconductor materials play the central role in many areas of modern science and technology, such as photodetectors, photovoltaic cells, multicolor light-emitting diodes (LEDs), catalysis, electroluminescence devices and sensors [7–11]. Most importantly, the spectral properties of these nanocrystals can be controlled effectively by tuning the size, composition, surface properties and crystal structure. HgS is one of the most important II–VI semiconductor compounds possessing excellent optoelectronic properties in the infra-red region and it can be widely used in ultrasonic transducers, electrostatic image materials and photoelectric conversion devices etc. [4,6,10–14]. In China, the ancients believed cinnabar mercury sulfide can be made into invigorating pills called “Zhusha” through heating. While II–VI semiconductors CdS, CdSe and ZnS nanocrystals have been prepared and studied extensively due to its relatively easy synthesis and distinct particle size dependent on optical properties [3,5,9,15–17], very little work exists to date on

∗ Corresponding author. E-mail address: [email protected] (D. Qin). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.018

HgS nanocrystals owing to the difficulties in handling the materials during synthesis and the toxicity of mercury. Recently, there are many methods for synthesizing II–VI semiconductors nanocrystals, such as sol–gel processing [18], reverse micelles [11,16], chemical bath deposition [19], sonochemical method [8], microwave irradiation [20], hydrothermal and solvothermal routes [21–23] etc. However, traditional methods have some disadvantages involving high synthetic temperature and pressure, requirement of toxic organic solvents or ligands such as thiols or polyphosphates as stabilizing agents. The development of simple, effect, economically, less energy consuming, and environmental friendly synthetic routes is of importance to nanotechnology and remains a key research challenge. In the last decade, biomimetic synthesis has become a hot topic [24–30]. A challenge in chemical engineering and materials science today is the design and construction of self-assembled nanoscale structures in biological system [31]. Suitable watersoluble bio-macromolecules such as polysaccharides, amino acids, peptides, proteins, biopolymers, virus, DNA and RNA can be chosen as matrix to synthesize inorganic materials. For instance, Huang and co-workers [32] have prepared self-assembly gold nanoparticles in polyaldehyde dextran solutions which acting as both a reducing agent and a stabilizer. Saha and co-workers [33,34] have prepared amino acid- and protein-conjugated CdS nanocrystals. Dujardin et al. [35] have used tobacco mosaic virus as organic template for the controlled deposition and organization of Pt, Au and Ag nanoparticles. Mann and co-workers [36] have prepared CdS nanocrystals using self-assembled bacterial S-layers. Li and Mann

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[37] have prepared DNA-directed assembly of multifunctional nanoparticle networks using metallic and bioinorganic building blocks. In addition, Yang et al. [38] and Keshari et al. [39] have reported biomimetic method for the synthesis of pepsin-coated CdS nanoparticles. These nanoarchitectures are of importance and can be used in life sciences for luminescence tagging, biological labeling, drug delivery, and many other aspects because of their non-toxicity and bio-compatibility. Yang et al. [40] have utilized protein-conjugated sulfides nanocrystals to investigate their inhibition of tumor cell viability. Among these bio-macromolecules, proteins are important biomolecules in bodies and play crucial roles in many biological activities, which have been adopted to synthesize various chalcogenide nanocrystals. For example, Cui and co-workers [41,42] have prepared Ag2 Se, CdSe, PbSe and CuSe nanocrystals in the bovine serum albumin (BSA) solutions and studied the interaction between protein and inorganic nanocrystals. Yang and co-workers [12,38,40,43] have reported the biomimetic synthesis of CdS, Ag2 S, PbS, HgS nanocrystals using proteins as matrices. These researches suggest that proteins are capable of controlling inorganic crystals nucleation and growth to a remarkable degree through biomineralization. The growth of nanocrystals is controlled by the conformation, surface coverage, superstructure, thermodynamic and kinetic properties of proteins [31]. The Lysozyme (Lyso) is an enzyme with 129 amino acid residues which exists in mucosal secretion such as saliva, tears, human milk, mucus and egg-white. It kills bacteria by breaking down the cell walls of bacteria and has been widely used in pharmaceutical, biotechnology and food industries. Recently, Lysozyme has been utilized as template in fields of fabrication of bio-nanocomposites. For example, gold and silver nanoparticles capped with Lysozyme have been prepared with water solubility and potential biocompatibility [44,45]. Herein, we have used the Lysozyme as a capping as well as stabilizing agent for the synthesis of HgS nanoparticles through biomimetic method and studied the interaction of HgS nanoparticles and the Lysozyme. To the best of our knowledge, the preparation of HgS nanocrystals directly conjugated with Lysozyme has not been reported before. 2. Experimental 2.1. Chemicals and materials Mercury (II) chloride (HgCl2 , >99%) and thioacetamide (TAA, >99%) were purchased from Sinopharm Chemical Regent Co., Ltd., People’s Republic of China. All chemicals were of analytical reagent grade and can be used without further purification. Lysozyme was of electrophoretic purity, purchased from Zhengzhou Creatlife Company, People’s Republic of China. Water was used after purification through double distillation. 2.2. Synthesis of HgS nanocrystals The synthesis of HgS nanocrystals was performed by two-step procedure. The first step was the generation of the Hg (II)–Lyso complex by mixing of the mercury ions and Lysozyme solution. The second step was the formation of HgS nanocrystals by adding TAA into the above mixing solution at 37 ◦ C. TAA was comparatively unstable and slowly hydrolyzed to release S2− into the reaction solution. A typical synthesis was carried out as follows: 100 mL of different concentrations Lyso solution and 50 mL of mercury chloride (c = 50 mM) were mixed and added into the 250 mL roundbottom flask reacting vessel at 37 ◦ C. The mixed solution of Hg (II)–Lyso was kept static under N2 atmosphere for 24 h at 37 ◦ C. Then 50 mL of 50 mM TAA solution was added into as-prepared solution with vigorous stirring for 30 min. After TAA addition, the

Fig. 1. XRD patterns of HgS nanocrystals synthesized in different concentration of Lysozyme solutions (a) 5 mg/mL, (b) 3 mg/mL and (c) 1 mg/mL.

color of solution changed from colorless to gray immediately and finally to dark brown. The mixed reaction solution was again maintained under a static condition for 7d, then separately by high speed centrifuging at 12,000 rpm. The collected black solid-state product was washed with double distilled de-ionized water and absolute ethanol, and finally dried in a vacuum at room temperature for 48 h. 2.3. Characterization of as-prepared Lyso–HgS nanocrystals Samples for transmission electron microscopy (TEM) and highresolution transmission microscopy (HRTEM) characterizations were prepared by depositing a drop of the as-prepared nanocrystals solution on a carbon-coated copper grid and then drying at room temperature. TEM and HRTEM measurements were made on a JEOL JEL-2010 transmission electron microscope. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8Advance X-ray powder diffractometer with graphite monochromatized Cu K␣ ( = 0.15406 nm). A scanning rate of 0.05 deg/s was applied to record the pattern in the 2 range of 20–60◦ . The UV–vis spectra of samples were acquired with a Shimadzu UV-2550 spectrophotometer. Photoluminescence spectra were obtained with Hitachi F-7000 FL spectrophotometer with a 450 W xenon lamp as excitation source. All the optical measurements were performed at room temperature under ambient conditions. The interaction of Lysozyme and HgS nanocrystals was studied through Fourier transform infrared (FT-IR) and circular dichroism (CD) spectroscopy characterizations. Infrared spectra were taken on a Bruker Tensor-37 spectrophotometer in the wave number range of 4000–600 cm−1 ; the spectra were collected at 2 cm−1 resolution with 128 scans by preparing KBr pellets with a 3:100 “sample-toKBr” ratio. The CD spectra of reaction systems were measured at 10 ± 1 ◦ C with a Jasco-810 spectropolarimeter. The same samples were repeated three times. This instrument had been calibrated previously for wavelength with benzene vapor, and for optical rotation with d10 -camphorsulfonic acid. A cell with a path length of 0.1 cm was used. The parameters used were as follows: bandwidth, 1 nm; step resolution, 0.1 nm; scan speed, 50 nm/min; response time, 0.25 s. Each spectrum was obtained after an average of six scans. 3. Results and discussion 3.1. Structure and morphology of HgS nanocrystals Wide-angle XRD patterns of the as-prepared HgS nanocrystals with different amounts of Lysozyme (1–5 mg/mL) are shown in Fig. 1. The XRD patterns of samples synthesized in Lysozyme

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Fig. 2. Typical TEM images of HgS nanocrystals synthesized in different concentrations of Lysozyme solutions (a. 5 mg/mL, b. 3 mg/mL, c. 1 mg/mL), (d) HRTEM image of HgS nanocrystals (cLyso = 3 mg/mL), (e) EDS spectrum of HgS nanocrystals.

solutions exhibit prominent broad peaks at 2 values of 26.2◦ , 30.3◦ , 43.9◦ , 51.8◦ and 53.7◦ which could be indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) direction of the cubic zinc blende phase (ˇ-HgS) in close agreement with JCPDS (no. 06-0621) data. The structure of HgS synthesized in water without Lysozyme confirmed by XRD pattern is cinnabar ˛-HgS. It can be clearly noted that, as changing the content of Lysozyme, there are significant changes in width and intensity of XRD peaks. Decreasing the content of Lysozyme from 5 to 1 mg/mL shows an increase of diffraction peak intensity and a decrease of diffraction peak full width at halfmaximum (FWHM) due to the growth of grain. The average grain size of the samples is determined using full width at FWHM of XRD peak with the help of Scherre’s formula: D=

k ˇ cos

where D is the mean crystalline size, k is the constant (k = 0.9),  is the X-ray wavelength (1.54056 A˚ for Cu K␣), ˇ is the FWHM of the diffraction peak and  is the Bragg diffraction angle. The grain sizes of HgS nanocrystals are found to be 7.54 nm, 4.36 nm and 4.11 nm while the corresponding concentrations of Lysozyme are 1.0, 3.0 and 5.0 mg/mL. From the XRD results of samples it is apparent that Lysozyme is able to control the nucleation, growth, size, crystallographic orientation of HgS nanocrystals and play a crucial role in the formation of nanoparticles. Fig. 2 shows TEM images of the HgS nanocrystals at various Lysozyme concentrations. The samples are approximately spherical in shape. According to size distributions of HgS nanoparticles

(Fig. 3), as-obtained nanoparticles have mean diameters of approximately 14, 19 and 27 nm as concentrations of Lysozyme are 5.0, 3.0 and 1.0 mg/mL, respectively. It is becoming clear that the smaller nanoparticles associated with greater amounts of Lysozyme. The size of nanoparticles estimated from the TEM pictures is much larger than that obtained from Scherre’s formula, which suggests that the obtained products are polycrystalline particles. From HRTEM image (Fig. 2d), we can also observe polycrystalline structure of HgS nanoparticles. Because of sensitiveness of HgS to electron beams, we have not obtained clearly detailed image in Fig. 2d. The energy-dispersive X-ray spectrum (EDS) of HgS nanocrystals shows strong peaks attributed to Hg and S can be found in Fig. 2e. The composition is 48.7:51.3 atomic percent for Hg and S, respectively, nearly in agreement with the stoichiometric molar ratio of mercury (II) sulfide.

3.2. Optical properties of HgS nanocrystals UV–vis absorption spectra have been proven to be very sensitive to the formation of nanoparticles [34,44,46,47]. Both the position and the intensity of absorption are related to the size of semiconductor particles [48]. Fig. 4 shows the UV–vis absorption spectra of HgS nanocrystals in the wavelength range of 300–600 nm synthesized in various concentration of Lysozyme. The UV–vis spectra of the HgS–Lyso suspension are broad and structureless curves with gradually increasing absorbance toward shorter wavelengths due to quantum size effect. The theoretical band

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Fig. 3. The size distribution of HgS nanoparticles in different concentrations of Lysozyme solutions (a) 5 mg/mL, (b) 3 mg/mL and (c) 1 mg/mL.

Fig. 4. UV–vis adsorption spectra of HgS nanocrystals synthesized in various concentrations of Lysozyme solutions (a) 5 mg/mL, (b) 3 mg/mL and (c) 1 mg/mL.

gap energy of semiconductors can be calculated by the followed formula:

 hv

˛(v) = A

2

− Eg

m/2

where A is constant, ˛(v) is the absorption coefficient and Eg is the band gap. For a direct transition m = 1, a plot of (˛Ephot )2 vs. Ephot was constructed and the value of Ephot extrapolated to a = 0 gives the band gap, Eg . The band gaps of samples calculated from formula are 2.49 eV, 2.53 eV and 2.66 eV, while the corresponding concentrations Lysozyme solutions are 1, 3 and 5 mg/mL, respectively, which are blue shifted from that of bulk HgS (2.0 eV) because of quantum confinement effect. Fig. 5 shows the UV–vis spectra of the reaction system after TAA precursor was added to the Hg2+ –Lyso

solutions at different stages. It can be clearly noted that, the intensity of absorbance gradually increased with prolonged reaction time. Such trend associated with the spectra alterations (red-shift) can be attributed to the formation and growth of HgS nanocrystals. The PL spectra of the pure Lysozyme and Lyso-conjugated HgS nanocrystals prepared in different concentration of Lysozyme are determined with excitation wavelength of 290 nm (see in Fig. 6). It exhibits the emission peak of the pure Lysozyme centered at about 362 nm associated with intrinsically fluorescent of proteins such as Trp, Tyr and Phe residues [46,49]. The PL spectra of Lyso-conjugated HgS nanocrystals appear the emission peak at about 469 nm and have no characteristic peak of Lysozyme, indicating that the fluorescence of protein is quenched by HgS nanocrystals. This result shows that the formation of HgS nanocrystals has influence on micro-environment of the Trp and Tyr residues in Lysozyme. It might be worthwhile to note here that the increase of concentration of Lysozyme leads to an increase in the emission intensity of HgS nanocrystals. Emissions from semiconductor nanoparticles originate from electrons in the conduction band, excitonic states and trap states. It is well known that emission is very sensitive to the nature of surface of nanoparticles [50]. The PL spectra of HgS nanocrystals are observed to be broad and occurred at a lower energy value than that corresponding to the excitonic emission band. So it can be attributed to the recombination of the charge carrier trapped in the surface states. XRD and TEM characterizations indicate there is descending trend in particle size with the increase of concentration of Lysozyme. The smaller nanoparticles can result in more surface defects of crystals which enhance the intensity of photoluminescence peaks. 3.3. Investigation on the interaction of HgS nanocrystals and Lysozyme To explore the effect of Lysozyme on the formation of HgS nanocrystals, FT-IR and CD spectroscopy measurements were performed to study the interaction of HgS nanocrystals and Lysozyme. FT-IR spectroscopy technique can be used to research the structural changes of protein and investigate protein–ligand complex formation [12,51]. The FT-IR spectra of pure Lysozyme, Hg2+ –Lyso and HgS–Lyso are shown in Fig. 7. The FT-IR data of main peaks are presented in Table 1. The IR spectrum peaks of pure Lysozyme at

Table 1 The main peaks of pure Lysozyme, Hg2+ –Lyso and HgS–Lyso nanocrystals from the FT-IR spectra. Assignment Fig. 5. UV–vis adsorption spectra of HgS nanocrystals synthesized in Lysozyme solution (cLyso = 3 mg/mL) with different reaction time (a) 5 min, (b) 30 min and (c) 3 h.

Pure Lysozyme Hg2+ –Lyso HgS–Lyso

OH (cm−1 ) 3434 3432 3430

Amide I (cm−1 )

Amide II (cm−1 )

1649 1647 1647

1530 1508, 1533 1508, 1541

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Fig. 6. Photoluminescence (PL) spectra of (a) pure Lysozyme, HgS nanocrystals synthesized in various concentrations of Lysozyme solutions (b) 1 mg/mL, (c) 3 mg/mL and (d) 5 mg/mL.

3434, 1649, and 1530 cm−1 are assigned to the stretching vibration of OH, amide I (mainly C O stretching vibrations), and amide II (the coupling of bending vibrations of NH and stretching vibrations of CN) bands [42,51], respectively. From Table 1, the difference between amide II of pure Lysozyme and that of Hg2+ –Lyso and HgS–Lyso is obvious, suggesting that there might be coordination interaction between Hg2+ /HgS and NH group of Lysozyme, which may play an important role in the formation of nanoparticles. And there are negligible variations in the characteristic peak of OH group and amide I band because that Hg2+ as a soft Lewis acid has low affinity for the hard base. These results confirm that Lysozyme molecules remain attached to the nanoparticles surface in the separated samples through multiple binding sites such as NH, OH, C O groups for Hg2+ and HgS, which greatly inhibit the fusion connection of nanoparticles themselves. CD is one of the most powerful techniques to study protein conformations in aqueous solution. It can provide the information of secondary structures changes in proteins. For example, Liu and co-workers [49] have studied the conformation change of bovine hemoglobin (BHb) after interacting with oxytetracycline (OTC) by CD spectroscopy. Saha and co-workers have prepared BSA-conjugated CdS nanocrystals and investigated the difference of CD curves between pure BSA and BSA-CdS [34]. Many other researchers also have studied secondary structure of proteins through CD characterization [12,41–43]. To further ascertain the possible influence of Hg2+ /HgS binding on the secondary structure of Lysozyme, CD measurements of Lysozyme were performed in the presence of Hg2+ and HgS nanocrystals (Fig. 8). The CD spectroscopy is observed when molecules absorb left and right circularly polarized light to different extent. The CD spectrum of pure Lysozyme has intensive negative bands in the far UV region at 208 nm and

Fig. 7. FT-IR spectra of pure Lysozyme, Hg2+ –Lyso and HgS–Lyso reaction systems.

222 nm, which are characteristic bands of ␣-helix structure of proteins. These bands corresponds to ␲–␲* transition of ␣-helix due to strong hydrogen bonding environment of conformation. From the figure, it can be seen the CD curve of pure Lysozyme is different from that of Hg2+ –Lyso and HgS–Lyso. The results indicate that the characteristic CD patterns of progressively fade after interacting with Hg2+ and HgS nanocrystals. It is well known that the hydrogen bond of ␣-helix forms between the oxygen atom of the (i) carboxylic group and the hydrogen atom of the (i + 4) amino group. The decrease of ␣-helix content suggests that metal ions and nanoparticles bind with the amino acid residues of the main polypeptide chains of Lysozyme and destroy their hydrogen bond networks, which leads to the loosening and unfolding of the protein skeleton, decreasing the hydrophobicity of the microenvironment of Lysozyme and changing its secondary structure. According to discussion above, the scheme of the HgS nanoparticles formation in the Lysozyme solution is illustrated in Fig. 9. At the first step of the preparation, by mixing the Lysozyme solution and Hg2+ complexes, there is a cooperative interaction between the functional groups such as OH, NH of the Lysozyme and Hg2+ ; the concentration of the Hg2+ around the Lysozyme molecule is very high because of this interaction. When the TAA is added to Hg2+ –Lyso solution, S2− from hydrolysis gradually of TAA combines with Hg2+ to form HgS nuclei. In this step, the Lysozyme plays very important role in reducing activation energy of nucleation as capping agent. The growth of nuclei can result in HgS nanostructures in biomimetic mineralization.

Fig. 8. CD spectra of (a) pure Lysozyme, (b) Hg2+ –Lyso and (c) HgS–Lyso nanocrystals solutions.

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Fig. 9. Scheme of HgS nanoparticles formation in the Lysozyme solution.

4. Conclusions Protein-conjugated HgS nanoparticles were synthesized in the Lysozyme solution using biomimetic method. The synthesis process of Lyso-conjugated HgS nanocrystals is facile, effective and environment friendly. XRD, TEM and HRTEM characterizations indicate that the obtained nanocrystals have cubic zinc blende phase and tunable sizes associated concentration of Lysozyme. UV–vis absorption spectra show the blue shift of band gap compared to bulk HgS, thus confirming quantum confinement effects. The prepared HgS nanocrystals exhibit good photoluminescence, which may not only have potential applications in biomedical engineering fields, but also provide new inspiration for nanotechnology. Hg2+ /HgS can react with functional groups of Lysozyme and the formation of nanocrystals has influence on secondary structure of Lysozyme. During the formation of HgS nanoparticles, the nucleation and growth of nanocrystals will be affected by Lysozyme through electrostatic matching, structural and stereochemical complementarity (interfacial molecular recognition). References [1] C.C. Shen, W.L. Tseng, One-step synthesis of white-light-emitting quantum dots at low temperature, Inorganic Chemistry 48 (2009) 8689–8694. [2] J. Zhang, Y. Tang, K. Lee, M. Quyang, Nonepitaxial growth of hybrid core-shell nanostructures with large lattice mismatches, Science 327 (2010) 1634–1638. [3] Z. Gao, N. Liu, D. Wu, W. Tao, F. Xu, K. Jiang, Graphene–CdS composite synthesis and enhanced photocatalytic activity, Applied Surface Science 258 (2012) 2473–2478. [4] M. Kristl, M. Drofenik, Sonochemical synthesis of nanocrystalline mercury sulfide, selenide and telluride in aqueous solutions, Ultrasonics Sonochemistry 15 (2008) 695–699. [5] K. Ravichandran, P. Philominathan, Comparative study on structural and optical properties of CdS films fabricated by three different low-cost techniques, Applied Surface Science 255 (2009) 5736–5741. [6] B.K. Patel, S. Rath, S.N. Sarangi, S.N. Sahu, HgS nanoparticles: structure and optical properties, Applied Physics A 86 (2007) 447–450. [7] R. Viswanatha, D.M. Battaglia, M.E. Curtis, T.D. Mishima, M.B. Johnson, X. Peng, Shape control of doped semiconductor nanocrystals (d-dots), Nano Research 1 (2008) 138–144. [8] M.L. Breen, A.D. Dinsmore, R.H. Pink, S.B. Qadri, B.R. Ratna, Sonochemically produced ZnS-coated polystyrene core-shell particles for use in photonic crystals, Langmuir 17 (2001) 903–907. [9] H. Zhu, R. Jiang, Y. Guan, Y. Fu, L. Xiao, G. Zeng, Effect of key operational factors on decolourization of methyl orange during H2 O2 assisted nanosized CdS/TiO2 /polymer composite thin films under simulated solar light irradiation, Separation and Purification Technology 74 (2010) 187–194. [10] W. Wichiansee, M.N. Nordin, M. Green, R.J. Curry, Synthesis and optical characterization of infra-red emitting mercury sulfide (HgS) quantum dots, Journal of Materials Chemistry 21 (2011) 7331–7336. [11] I. Chakraborty, D. Mitra, S.P. Moulik, Spectroscopic studies on nanodispersions of CdS, HgS, their core-shells and composites prepared in micellar medium, Journal of Nanoparticle Research 7 (2005) 227–236. [12] D.Z. Qin, X.M. Ma, L. Yang, L. Zhang, Z.J. Ma, J. Zhang, Biomimetic synthesis of HgS nanoparticles in the bovine serum albumin solution, Journal of Nanoparticle Research 10 (2008) 559–566. [13] P.S. Nair, T. Radhakrishnan, N. Revaprasadu, G.A. Kolawole, P. O’Brien, The synthesis of HgS nanoparticles in polystyrene matrix, Journal of Materials Chemistry 14 (2004) 581–584. [14] K. Singh, M.L. Srivastava, S.S.D. Mishra, Electrochemical deposition and photoelectrochemical characterization of colloidal HgS containing CdSe composites, Solar Energy Materials & Solar Cells 90 (2006) 923–932. [15] A. Priyam, A. Chatterjee, S.K. Das, A. Saha, Size dependent interaction of biofunctionalized CdS nanoparticles with tyrosine at different pH, Chemical Communications 32 (2005) 4122–4124.

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[41]

[42]

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