- Email: [email protected]
Self-assembly of PbS hollow sphere quantum dots via gas–bubble technique for early cancer diagnosis
Journal of Luminescence 133 (2013) 188–193
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Self-assembly of PbS hollow sphere quantum dots via gas–bubble technique for early cancer diagnosis Masoud Mozafari a,b,n, Fathollah Moztarzadeh b, Alexander M. Seifalian c,d, Lobat Tayebi a a
Helmerich Advanced Technology Research Center, School of Material Science and Engineering, Oklahoma State University, OK 74106, USA Biomaterials Group, Faculty of Biomedical Engineering (Center of Excellence), Amirkabir University of Technology, P.O Box 15875-4413, Tehran, Iran c Centre for Nanotechnology & Regenerative Medicine, UCL Division of Surgery & Interventional Science, University College London, London, UK d Royal Free Hampstead NHS Trust Hospital, London, UK b
a r t i c l e i n f o
a b s t r a c t
Available online 30 December 2011
Quantum dots (QDs) with their unique optical properties have attracted widespread interest in cancer diagnosis and therapy. Due to their ability to absorb and emit light very efficiently, lead sulfide (PbS) hollow spheres with nanometer-to-micrometer dimensions having tailored structural, optical, and surface properties represent an important class of QDs that are potentially useful for early cancer detection. In this study, PbS hollow sphere QDs have been successfully synthesized using a templatefree and green method. The formation of hollow structures was explained by a gas–liquid interface aggregation mechanism, in which the formation of SO2 gas bubbles plays a key role. The synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analyzer (EDX), photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). The results demonstrate that the PbS hollow spheres possess good optical quality with strong luminescence properties, which indicate their capabilities for the simultaneous detection of multiple cancer biomarkers in blood assays and cancer tissue biopsies. & 2011 Elsevier B.V. All rights reserved.
Keywords: Lead sulfide Quantum dot Hollow sphere Gas–liquid interface aggregation mechanism Cancer detection
1. Introduction Early screening of cancer is desirable as most tumors are detectable only when they reach to a certain size containing millions of cells already in metastatic stage. Currently employed diagnostic techniques are insufficiently sensitive and specific to detect most types of early-stage cancers. Moreover, these assays are labor intensive, time consuming, expensive and without multiplexing capability. From other point of view, quantum dot (QD) based detection is rapid, easy, cost effective and quick pointof-care screening of cancer markers. In the past few years, QD nanocrystals (NCs) have attracted broad attention since they have represented quantum confinement effects and size-dependent characteristics in contrast with the bulk counterparts [1]. Therefore, shape and size of inorganic NCs are well known to have important influence on their widely varying electrical and optical properties. Nowadays, metal sulfide semiconductor NCs, particularly PbS QDs are of great interest because of their wide applications. By carefully designing and controlling of different n Corresponding author at: Helmerich Advanced Technology Research Center, School of Material Science and Engineering, Oklahoma State University, OK 74106, USA. Tel.: þ1 918 594 8634; fax: þ 1 270 897 1179. E-mail address: [email protected] (M. Mozafari).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.12.054
parameters, it is possible to manipulate the optical and electronic properties of these materials for specific technological applications. In principle, the electronic and optical properties of semiconductor NCs are tunable by varying their morphology and size. Thus, rational control over the configuration of semiconductors has become a hot topic in recent material research field. Among all semiconductor NCs, PbS is important due to its small direct energy gap (i.e. 0.41 eV at 300 K for bulk PbS) and a large excitation Bohr radius of 18 nm. The band gap can be blue shifted to the spectral region of 0.7–1.5 mm (1.77–0.82 eV) upon lowering the diameter of the PbS NCs below the size of the excitonic Bohr radius, due to quantum confinement effect. This fact makes lead sulfide appropriate for telecommunication requirements and optical switches [2]. In nano and micro-scale morphologies, hollow spheres are particularly important because of their specific structures and potential applications. The synthesis and characterization of such materials, especially of inorganic materials hollow spheres, has aroused research interests lately [3–6]. They show special structures, optical and surface properties with wide potential applications in transportation systems of carriers, photonic crystals, filling agents, catalysts, microreactors and fuel cells [7–11]. Different preparation methods of hollow spheres have been reported, including the templating method using silicon dioxide
M. Mozafari et al. / Journal of Luminescence 133 (2013) 188–193
[12] or polystyrene spheres [13], inducing with tri-block copolymers [14,15], preparing in emulsion/microemulsion [16,17], solvothermal [18] and gas bubble [11] methods. Despite achieving successes in these methods, disadvantages such as troublesome removal of the spherical templates [12], difficulty in recycling the templates in emulsion/microemulsion [16,17], long reaction time under ultrasonic by inducing tri-block copolymers [15], and high temperature in autoclave for solvothermal method [19] still need to be overcome. Thus, the control synthesis of inorganic materials with specific size and shape is still difficult to achieve and remains a great challenge for future. Among all preparation methods of inorganic hollow spheres, gas bubble method seems more simple, effective and suitable. There are some important advantages which indicate that gas bubble method could be the best way to synthesis semiconductor hollow spheres. In other template methods for preparing inorganic hollow spheres, the template cores are needed to be removed by calcinations or solvent etching after the procedure. However, we do not have this problem with gas bubble method. In addition, having simple and clean procedures, short reaction time and low temperature synthesis are other advantages of this method comparing with the other ones. In this study, we report a simple, clean and template-free synthesis route of well-defined PbS hollow sphere particles with strong photoluminescence properties via gas–liquid interface aggregation mechanism. To our knowledge, well-defined PbS hollow spheres obtained by a surfactant-free and simple method with mild conditions have not been reported yet.
2. Experimental procedures 2.1. Materials and methods All the reagents were purchased from Merck Inc. without any further purification. The de-ionized water that used in all synthesis process was in high purity grade with a conductivity of 18.2 MO cm. In a typical procedure, the PbS hollow spheres were synthesized in water at room temperature. In brief, 20 ml of 0.1 M Pb(NO3)2 (lead nitrate) diluted with de-ionized water and stirred for 30 min at 500 rpm. Also, 20 ml of 0.1 M sodium thiosulfate (Na2S2O3 2H2O) was diluted with de-ionized water to obtain S2O2 (thiosulfate) 3 [20]. It should also be considered that thiosulfate is an oxyanion of sulfur produced by the reaction of sulfite ions with elemental sulfur in water. Finally, the second solution containing thiosulfate ions was slowly injected drop-wise to the lead nitrate solution to form PbS hollow spheres. The mixture was stirred for about 2 h at 130 1C. After ultrasonic agitating, the product filtered out and washed with absolute alcohol and de-ionized water several times and dried under vacuum at 40 1C.
2.2.3. EDX analysis Energy dispersive X-ray analyzer (EDX, Rontec, Germany) directly connected to SEM was used to investigate the semiquantitatively chemical composition of the synthesized PbS hollow spheres. 2.2.4. TEM analysis TEM observations were performed on a Philips CM120 electron microscope at an accelerating voltage of 100 kV. The morphology and size of the synthesized PbS NCs assessed using TEM by dispersing in ethanol (0.1 g/10 ml) and ultrasound for 20 min. Finally, the samples were prepared by placing one drop of nanoparticles’ dispersion on a carbon-coated grid and allowing them to dry in the air for 24 h. 2.2.5. FTIR analysis The synthesized sample was examined by FTIR (Bomem-MB 100, USA) spectrometer. For this purpose, at first 1 mg of the sample were carefully mixed with 300 mg of KBr (infrared grade) and palletized under vacuum. Then the pellet was analyzed in the range 500–4000 cm 1 at a scan speed of 23 scan/min with 4 cm 1 resolution. 2.2.6. PL analysis The fluorescence excitation and emission spectra of the samples were acquired at right angle on a Perkin–Elmer LS-55 fluorescence spectrophotometer. Excitation and emission fluorescence spectra were measured using original reacting mixtures at room temperature. The instrument parameters affecting photoluminescence intensity are as follows. Measurement type: wavelength scan, scan mode: excitation/emission, data mode: fluorescence, ex. slit: 5.0 nm, em. slit: 5.0 nm, PMT voltage: 950 V.
3. Results and discussion 3.1. XRD analysis of PbS hollow spheres The XRD pattern of the synthesized PbS hollow spheres is shown in Fig. 1 which shows the phase purity and phase structure of the PbS NCs. It is notable that all the diffraction peaks were corresponded to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0) and (4 2 2) reflections of cubic PbS (JCPDS No. 5-592), and no obvious characteristic reflection peaks could be detected in the XRD pattern. Also, the strong and sharp reflection peaks in the XRD pattern indicated that PbS products were well crystallized. Among all peaks, the intensity of the (2 0 0) peak was much higher than that
2.2. Characterization 2.2.1. XRD analysis The synthesized samples were analyzed by XRD (Rigaku-Dmax 2500 diffractometer) which works with voltage and current settings of 40 kV and 200 mA, respectively and uses Cu-Ka ˚ For qualitative analysis, XRD diagrams were radiation (1.5405 A). recorded in the interval 201r2y r801 at scan speed of 21/min. 2.2.2. SEM analysis The samples were coated with a thin layer of gold (Au) by sputtering technique (EMITECH K450X, England) to promote electrical conductivity and then their morphology and microstructure were observed using SEM (Seron Technology, model AIS-2100) at an acceleration voltage of 30 kV.
189
Fig. 1. XRD pattern of the synthesized PbS hollow sphere QDs.
190
M. Mozafari et al. / Journal of Luminescence 133 (2013) 188–193
of the (1 1 1) peak, which suggested a higher growth rate on the {1 0 0} facets in comparison to the {1 1 1} facets. The average nanocrystallite size was determined from the half-width of diffraction peak of (2 0 0) using the Debye–Scherrer’s formula (1): D ¼ kl=b cos y
ð1Þ
where D is the crystallite diameter, k is a constant (shape factor, ˚ b is the full width about 0.9), l is the X-ray wavelength (1.5405 A), at half maximum (FWHM) of the diffraction line, and y is the diffraction angle. The average crystallite size was estimated approximately 13.5 nm. The lattice constant parameter a was found by calculation of interplanar crystal spacing dhkl and using Bragg’s equation as follows (2) and (3): dh k l ¼ l=2 sin y
ð2Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 a ¼ dh l k h þ k þ l
ð3Þ
The measurements showed that the lattice constant parameter a was 0.5946 nm at room temperature, which is very close to the value of PbS in cubic phase in the standard card 0.5936 nm (JCPDS No. 5-592). In fact, the formation of PbS NCs structures involves the formation and growth processes of PbS nuclei. Then the as-formed nuclei grow along different directions to form various PbS structures. According to Wang [21], the shape of an FCC (face center cubic) NC was mainly determined by the ratio of the growth rate in the /1 0 0S direction to that in the /1 1 1S direction. And balancing the relative growth rates between these crystalline faces is a key to control the morphology of the products. When the ratio is relatively high, PbS cubes bounded by the six {1 0 0} planes will be formed [22]. 3.2. SEM and TEM observations The morphology and the particle size of the synthesized PbS hollow spheres were characterized by SEM. Fig. 2 shows the typical SEM micrographs of the synthesized PbS micro- and nanoparticles, which suggested that almost all of the synthesized PbS particles exhibited uniform sphere-shaped particles. According to the observations, particles were made within the first few minutes, but after few minutes those particles were aggregated and their size got larger because of their large surface to volume ratio. In fact, without coating of surfactant on the particles, due to the increase in the surface area to volume ratio, the attractive force between the nanoparticles will increase, and some of the particles will agglomerate. Thus, the particle size became larger, and it is worth mentioning that the PbS aggregated particles were composed of much smaller crystallites. This figure also presents typical TEM micrographs of the synthesized PbS hollow spheres in nano scale. In these micrographs the average diameter of the particles is about 20–40 nm and the shell thickness is about 10 nm. From the TEM micrograph of a single PbS hollow sphere shown in Fig. 3(b), we realize the surface of the shell is coarse, suggesting that the shell could consist of primary PbS nanoparticles. In addition, the strong contrast between the dark edges and the pale centers of the spherical particles evidence their hollow nature. 3.3. EDX analysis Fig. 3 shows the EDX spectra of the synthesized PbS hollow sphere particles. The EDX spectrum clearly shows the peaks of S and Pb which are the main components of the samples. EDX analysis also indicates the presence of gold (Au) on the surface of PbS particles which is related to the coating layer in SEM analysis.
Fig. 2. Typical SEM micrographs of the synthesized PbS hollow sphere QDs (a) 4.0 K, (b) 5.0 K and (c) 12 K (Typical TEM micrograph in the corner).
The strength of the peaks of Pb and S in the spectrum confirmed the formation of the PbS NCs without any impurity. 3.4. Photoluminescence study of the synthesized PbS hollow spheres We have investigated the photoluminescence properties of the synthesized PbS hollow sphere QDs in room-temperature. Fig. 4 presents the emission spectra of the sample. When the sample
M. Mozafari et al. / Journal of Luminescence 133 (2013) 188–193
was excited at 338.5 nm, a strong luminescence emission peak at 645.5 nm was appeared that comes from the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes. This luminescence peak did not occur when the sample was excited at other spectra. Generally, the photoluminescence study of PbS NCs is intricate because it is sensitive to the synthesis conditions, crystallite sizes and shapes. The appeared emission spectrum with a strong narrow band and high intensity indicates the high photoluminescence potential of the synthesized PbS hollow spheres. These observations suggest the immense potential of these particles as efficient optical imaging nano probes in diagnostic imaging, particularly for early detection of cancer tumors.
191
3.5. FTIR analysis With the aim of identification or determination of the compounds, structure and new compounds FTIR analysis was employed. Also, the bond of Pb–S is mainly an electrovalent bond and the FTIR spectra of the samples do not show strong bands associated with Pb–S stretching and bending vibrations. Fig. 5 shows a representative example of the FTIR spectrum of the synthesized PbS hollow spheres before heat-treatment. In this spectrum two extra weak peaks were detected at 832 cm 1 and 1115 cm 1 were the characteristic peaks for Pb–S bond [23,24]. Moreover, the peak at about 1365 cm 1 represented aliphatic nitro compounds or nitrate ions, and the peak at about 3500 cm 1 was related to the O–H group [25].
3.6. The formation mechanism of PbS hollow sphere
Fig. 3. EDX spectra of the synthesized PbS hollow sphere QDs.
Generally, interfaces are formed between two different phases with common boundaries. The boundary region is the characteristic of forces acting on it by the particles involved. As a result of interfacial interactions, atoms at the interfaces are more prone to react with the surrounding species. In our experiment, sodium thiosulfate was used as one of the main precursors due to its capability to form stable complexes with many metals ions. Sodium thiosulfate forms strong complexes with monovalent ions Cu þ , Ag þ and Au þ . Alkaline thiosulfate solution dissolves many insoluble salts of Pb2 þ , Hg2 þ , Cu þ and Ag þ . The monovalent complexes are mostly S-bonded, while the divalent ones may be S or O bonded [20]. According to Lukashin et al. [26], nearly all methods for synthesizing nanocrystalline sulfides which involve the sulfidizing (for example, with hydrogen or sodium sulfide) of metal compounds are exothermic which can result in the aggregation of separate nanoparticles and cause nanosystems with a wide size distribution of particles. In this context, the special interest is the method in which PbS NCs are formed in the course of decomposition of a sulfur-containing metal complex such as a thiosulfate complex. To obtain nanocrystalline PbS hallow spheres, an anionic lead complex that decomposes to produce sulfide is required. The thiosulfate complex [Pb(S2O3)2]2 suits this requirement. This complex is prepared by reacting lead nitrate with sodium thiosulfate after filtration and washing with de-ionized water by the following reaction (4): 2þ
Pb
Fig. 4. Emission fluorescence spectra for the synthesized PbS hollow sphere QDs.
2 þ 2S2 O2 3 -½PbðS2 O3 Þ2
ð4Þ
In the experiment process, it is suggested that the [Pb(S2O3)2]2 complex with Pb2 þ and S2O2 could decompose to form PbS, S, 3 SO2 and SO24 . Thus, the formation of PbS can be formulated as
Fig. 5. FTIR spectrum of the synthesized PbS hollow sphere QDs.
192
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Fig. 6. Schematic illustration for the fabrication of PbS hollow sphere QDs.
follows (5) ½PbðS2 O3 Þ2 2 -PbSþS þ SO2 m þ SO2 4
ð5Þ
In the current study with the aim of synthesis PbS hollow spheres, the produced SO2 plays the main role in the growth of PbS hollow spheres because SO2 could act as an appropriate soft template. Driven by the minimization of interfacial energy, small PbS NCs may be aggregated around the gas–liquid interface between SO2 and the solvent, and finally well-defined PbS hollow spheres are formed. According to the SEM and TEM micrographs, our system had a high efficiency of the PbS hollow spheres production, which indicates the rapid formation of PbS NCs around the gas bubbles right after the reaction. Herein, the SO2 gases generated in the liquid phase chemical reaction were released as bubbles which acted as free-templates, these bubbles created numerous gas–liquid interfaces inside the solution phase. In the gas–liquid interfaces mechanisms [27–32], the gas–liquid interfaces in the solution may serve as the nucleation or agglomeration centers for the PbS NCs. Especially, in our aqueous solution, since the system was surfactant-free, the surface of the in situ generated PbS NCs was not protected with foreign species like surfactants, so they had the tendency to aggregate together to release the high surface energy of the PbS NCs. The formation of the bubbles in the reaction system may enable this agglomeration process to proceed in a controllable way. With the introduction of nano and micro-bubbles into the reaction system, the agglomeration process occurred around the bubbles. This process is apparently thermodynamically favorable. On one hand, the forces on the solvent molecules at the concave interfaces (between the bubbles and the bulk liquid phase) are asymmetric so that these molecules need other species (e.g., NCs) to stabilize. On the other hand, the high surface tension of PbS NCs should be released to reach a stable state. As a result, the in situ-generated PbS NCs tend to move into the interface region. When the concentration of PbS NCs in this region is sufficiently high, the tiny PbS NCs will interact with each other to form PbS hollow spheres with the bubbles as soft physical templates. This process has been named a ‘‘gas–liquid interface aggregation mechanism’’. Fig. 6 shows the schematic formation of the PbS hollow spheres via this mechanism [11]. As illustrated, right after the initial nucleation, the monomers will form into NCs with tendency for aggregation (first step). At the same time, large quantities of SO2 gas bubbles produced in the reaction might serve as the aggregation centers. Due to the minimization of interfacial energy, small PbS NCs may aggregate at the gas–liquid interface of SO2 bubbles and water (second step). Finally, PbS hollow spheres are formed (third step).
The use of gas bubbles generated during the reaction to provide aggregation centers is a new and effective method for fabricating inorganic hollow nano and micro-spheres. Compared to the other template-synthetic methods, this simple soft template method avoids the introduction of impurities and introduces a suitable technique for modern chemical synthesis. This idea can be extended to other solution systems in which aggregated monodispersed NCs are produced during the reaction and the gas bubbles provides pseudo-templates for NCs to form hollow spheres.
4. Conclusion In this research, PbS hollow sphere QDs have been successfully synthesized using a template-free and green method. Lattice constant parameter of the samples was calculated 0.5946 nm at room temperature which is close to the related standard card. Also, the obtained results demonstrated that these particles possess good photoluminescence quality with strong luminescence properties which indicated their high potential in detection of cancer biomarkers for early diagnosis and possible early treatment. References [1] X. Zhang, C. An, S. Wang, Z. Wang, D. Xia, J. Cryst. Growth 311 (2009) 3775. [2] T. Saraidarov, R. Reisfeld, A. Sashchiuk, E. Lifshitz, J. Non-Cryst. Solids 345–346 (2004) 698. [3] M.Q. Zhao, L. Sun, R.M. Crooks, J. Am. Chem. Soc. 120 (1998) 4877. [4] L.L. Beecroft, C.K. Ober, Chem. Mater 9 (1997) 1302. [5] H. Huang, E.E. Resen, J. Am. Chem. Soc. 121 (1999) 3805. [6] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56. [7] G.M. Gratson, F. Garcia-Santamaria, V. Lousse, M.J. Xu, S.H. Fan, J.A. Lewis, P.V. Braun, Adv. Mater 18 (2006) 461. [8] M. Yang, J. Ma, Z.W. Niu, X. Dong, H.F. Xu, Z.K. Meng, Z.G. Jin, Y.F. Lu, Z.B. Hu, Z.Z. Yang, Adv. Funct. Mater. 15 (2005) 1523. [9] G.F. Zou, Z.P. Liu, D.B. Wang, C.L. Jiang, Y.T. Qian, Eur. J. Inorg. Chem. 22 (2004) 4521. [10] X.M. Sun, Y.D. Li, X. Sun, Y. Li, Angew. Chem. 43 (2004) 3827. [11] Q. Peng, Y.J. Dong, Y.D. Li, Angew. Chem. Int. Ed. 42 (2003) 3027. [12] S.W. Kim, M. Kim, W.Y. Lee, T.H. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642. [13] M.L. Breen, A.D. Donsmore, R.H. Pink, S.B. Qadri, B.R. Ratna, Langmuir 17 (2001) 903. [14] Y.R. Ma, L.M. Qi, J.M. Ma, H.M. Cheng, Langmuir 19 (2003) 4040. [15] Y.R. Ma, L.M. Qi, J.M. Ma, H.M. Cheng, W. Shen, Langmuir 19 (2003) 9079. [16] Y.J. He, Y. He, Mater Res. Bull 40 (2005) 629. [17] J.C. Bao, Y.Y. Liang, Z. Xu, L. Si, Adv. Mater 15 (2003) 1832. [18] Z. Hu, L. Li, X. Zhou, X. Fu, G. Gu, J. Colloid Interf. Sci. 294 (2006) 328. [19] S.J. Lei, K.B. Tang, Q. Yang, H.G. Zheng, Eur. J. Inorg. Chem. 20 (2005) 4124. [20] S.M. Bulatovic, Handbook of Flotation Reagents, Chemistry, Theory and Practice: vol. 1: Flotation of Sulfide Ores, Elsevier, Ontario, Canada, 2007., pp. 60. [21] Z.L. Wang, J. Phys. Chem. B. 104 (2000) 1153. [22] S.M. Lee, S.N. Cho, Cheon, J. Adv. Mater 15 (2003) 441. [23] M. Mozafari, F. Moztarzadeh, Micro. & Nano. Lett. 6 (2011) 161.
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[24] M. Mozafari, F. Moztarzadeh, J. Colloid Inter. Sci. 351 (2010) 442. [25] Mozafari M., Moztarzadeh F., ‘‘Green Synthesis of Well-defined Spherical PbS Quantum dots and its Potential in Biomedical Imaging Research and Biosensing’’, 1st Middle East Conference on Biomedical Engineering, February 21– 24, 2011, Sharjah, UAE. DOI:10.1109/MECBME.2011.5752075, pp. 100–103. [26] A.V. Lukashin, A.A. Eliseev, N.G. Zhuravleva, S.V. Kalinin, A.A. Vergetel, Y.D. Tret’yakov, Dokl. Chem. 383 (2002) 93.
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Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Self-assembly of PbS hollow sphere quantum dots via gas–bubble technique for early cancer diagnosis Masoud Mozafari a,b,n, Fathollah Moztarzadeh b, Alexander M. Seifalian c,d, Lobat Tayebi a a
Helmerich Advanced Technology Research Center, School of Material Science and Engineering, Oklahoma State University, OK 74106, USA Biomaterials Group, Faculty of Biomedical Engineering (Center of Excellence), Amirkabir University of Technology, P.O Box 15875-4413, Tehran, Iran c Centre for Nanotechnology & Regenerative Medicine, UCL Division of Surgery & Interventional Science, University College London, London, UK d Royal Free Hampstead NHS Trust Hospital, London, UK b
a r t i c l e i n f o
a b s t r a c t
Available online 30 December 2011
Quantum dots (QDs) with their unique optical properties have attracted widespread interest in cancer diagnosis and therapy. Due to their ability to absorb and emit light very efficiently, lead sulfide (PbS) hollow spheres with nanometer-to-micrometer dimensions having tailored structural, optical, and surface properties represent an important class of QDs that are potentially useful for early cancer detection. In this study, PbS hollow sphere QDs have been successfully synthesized using a templatefree and green method. The formation of hollow structures was explained by a gas–liquid interface aggregation mechanism, in which the formation of SO2 gas bubbles plays a key role. The synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analyzer (EDX), photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). The results demonstrate that the PbS hollow spheres possess good optical quality with strong luminescence properties, which indicate their capabilities for the simultaneous detection of multiple cancer biomarkers in blood assays and cancer tissue biopsies. & 2011 Elsevier B.V. All rights reserved.
Keywords: Lead sulfide Quantum dot Hollow sphere Gas–liquid interface aggregation mechanism Cancer detection
1. Introduction Early screening of cancer is desirable as most tumors are detectable only when they reach to a certain size containing millions of cells already in metastatic stage. Currently employed diagnostic techniques are insufficiently sensitive and specific to detect most types of early-stage cancers. Moreover, these assays are labor intensive, time consuming, expensive and without multiplexing capability. From other point of view, quantum dot (QD) based detection is rapid, easy, cost effective and quick pointof-care screening of cancer markers. In the past few years, QD nanocrystals (NCs) have attracted broad attention since they have represented quantum confinement effects and size-dependent characteristics in contrast with the bulk counterparts [1]. Therefore, shape and size of inorganic NCs are well known to have important influence on their widely varying electrical and optical properties. Nowadays, metal sulfide semiconductor NCs, particularly PbS QDs are of great interest because of their wide applications. By carefully designing and controlling of different n Corresponding author at: Helmerich Advanced Technology Research Center, School of Material Science and Engineering, Oklahoma State University, OK 74106, USA. Tel.: þ1 918 594 8634; fax: þ 1 270 897 1179. E-mail address: [email protected] (M. Mozafari).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.12.054
parameters, it is possible to manipulate the optical and electronic properties of these materials for specific technological applications. In principle, the electronic and optical properties of semiconductor NCs are tunable by varying their morphology and size. Thus, rational control over the configuration of semiconductors has become a hot topic in recent material research field. Among all semiconductor NCs, PbS is important due to its small direct energy gap (i.e. 0.41 eV at 300 K for bulk PbS) and a large excitation Bohr radius of 18 nm. The band gap can be blue shifted to the spectral region of 0.7–1.5 mm (1.77–0.82 eV) upon lowering the diameter of the PbS NCs below the size of the excitonic Bohr radius, due to quantum confinement effect. This fact makes lead sulfide appropriate for telecommunication requirements and optical switches [2]. In nano and micro-scale morphologies, hollow spheres are particularly important because of their specific structures and potential applications. The synthesis and characterization of such materials, especially of inorganic materials hollow spheres, has aroused research interests lately [3–6]. They show special structures, optical and surface properties with wide potential applications in transportation systems of carriers, photonic crystals, filling agents, catalysts, microreactors and fuel cells [7–11]. Different preparation methods of hollow spheres have been reported, including the templating method using silicon dioxide
M. Mozafari et al. / Journal of Luminescence 133 (2013) 188–193
[12] or polystyrene spheres [13], inducing with tri-block copolymers [14,15], preparing in emulsion/microemulsion [16,17], solvothermal [18] and gas bubble [11] methods. Despite achieving successes in these methods, disadvantages such as troublesome removal of the spherical templates [12], difficulty in recycling the templates in emulsion/microemulsion [16,17], long reaction time under ultrasonic by inducing tri-block copolymers [15], and high temperature in autoclave for solvothermal method [19] still need to be overcome. Thus, the control synthesis of inorganic materials with specific size and shape is still difficult to achieve and remains a great challenge for future. Among all preparation methods of inorganic hollow spheres, gas bubble method seems more simple, effective and suitable. There are some important advantages which indicate that gas bubble method could be the best way to synthesis semiconductor hollow spheres. In other template methods for preparing inorganic hollow spheres, the template cores are needed to be removed by calcinations or solvent etching after the procedure. However, we do not have this problem with gas bubble method. In addition, having simple and clean procedures, short reaction time and low temperature synthesis are other advantages of this method comparing with the other ones. In this study, we report a simple, clean and template-free synthesis route of well-defined PbS hollow sphere particles with strong photoluminescence properties via gas–liquid interface aggregation mechanism. To our knowledge, well-defined PbS hollow spheres obtained by a surfactant-free and simple method with mild conditions have not been reported yet.
2. Experimental procedures 2.1. Materials and methods All the reagents were purchased from Merck Inc. without any further purification. The de-ionized water that used in all synthesis process was in high purity grade with a conductivity of 18.2 MO cm. In a typical procedure, the PbS hollow spheres were synthesized in water at room temperature. In brief, 20 ml of 0.1 M Pb(NO3)2 (lead nitrate) diluted with de-ionized water and stirred for 30 min at 500 rpm. Also, 20 ml of 0.1 M sodium thiosulfate (Na2S2O3 2H2O) was diluted with de-ionized water to obtain S2O2 (thiosulfate) 3 [20]. It should also be considered that thiosulfate is an oxyanion of sulfur produced by the reaction of sulfite ions with elemental sulfur in water. Finally, the second solution containing thiosulfate ions was slowly injected drop-wise to the lead nitrate solution to form PbS hollow spheres. The mixture was stirred for about 2 h at 130 1C. After ultrasonic agitating, the product filtered out and washed with absolute alcohol and de-ionized water several times and dried under vacuum at 40 1C.
2.2.3. EDX analysis Energy dispersive X-ray analyzer (EDX, Rontec, Germany) directly connected to SEM was used to investigate the semiquantitatively chemical composition of the synthesized PbS hollow spheres. 2.2.4. TEM analysis TEM observations were performed on a Philips CM120 electron microscope at an accelerating voltage of 100 kV. The morphology and size of the synthesized PbS NCs assessed using TEM by dispersing in ethanol (0.1 g/10 ml) and ultrasound for 20 min. Finally, the samples were prepared by placing one drop of nanoparticles’ dispersion on a carbon-coated grid and allowing them to dry in the air for 24 h. 2.2.5. FTIR analysis The synthesized sample was examined by FTIR (Bomem-MB 100, USA) spectrometer. For this purpose, at first 1 mg of the sample were carefully mixed with 300 mg of KBr (infrared grade) and palletized under vacuum. Then the pellet was analyzed in the range 500–4000 cm 1 at a scan speed of 23 scan/min with 4 cm 1 resolution. 2.2.6. PL analysis The fluorescence excitation and emission spectra of the samples were acquired at right angle on a Perkin–Elmer LS-55 fluorescence spectrophotometer. Excitation and emission fluorescence spectra were measured using original reacting mixtures at room temperature. The instrument parameters affecting photoluminescence intensity are as follows. Measurement type: wavelength scan, scan mode: excitation/emission, data mode: fluorescence, ex. slit: 5.0 nm, em. slit: 5.0 nm, PMT voltage: 950 V.
3. Results and discussion 3.1. XRD analysis of PbS hollow spheres The XRD pattern of the synthesized PbS hollow spheres is shown in Fig. 1 which shows the phase purity and phase structure of the PbS NCs. It is notable that all the diffraction peaks were corresponded to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0) and (4 2 2) reflections of cubic PbS (JCPDS No. 5-592), and no obvious characteristic reflection peaks could be detected in the XRD pattern. Also, the strong and sharp reflection peaks in the XRD pattern indicated that PbS products were well crystallized. Among all peaks, the intensity of the (2 0 0) peak was much higher than that
2.2. Characterization 2.2.1. XRD analysis The synthesized samples were analyzed by XRD (Rigaku-Dmax 2500 diffractometer) which works with voltage and current settings of 40 kV and 200 mA, respectively and uses Cu-Ka ˚ For qualitative analysis, XRD diagrams were radiation (1.5405 A). recorded in the interval 201r2y r801 at scan speed of 21/min. 2.2.2. SEM analysis The samples were coated with a thin layer of gold (Au) by sputtering technique (EMITECH K450X, England) to promote electrical conductivity and then their morphology and microstructure were observed using SEM (Seron Technology, model AIS-2100) at an acceleration voltage of 30 kV.
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Fig. 1. XRD pattern of the synthesized PbS hollow sphere QDs.
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of the (1 1 1) peak, which suggested a higher growth rate on the {1 0 0} facets in comparison to the {1 1 1} facets. The average nanocrystallite size was determined from the half-width of diffraction peak of (2 0 0) using the Debye–Scherrer’s formula (1): D ¼ kl=b cos y
ð1Þ
where D is the crystallite diameter, k is a constant (shape factor, ˚ b is the full width about 0.9), l is the X-ray wavelength (1.5405 A), at half maximum (FWHM) of the diffraction line, and y is the diffraction angle. The average crystallite size was estimated approximately 13.5 nm. The lattice constant parameter a was found by calculation of interplanar crystal spacing dhkl and using Bragg’s equation as follows (2) and (3): dh k l ¼ l=2 sin y
ð2Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 a ¼ dh l k h þ k þ l
ð3Þ
The measurements showed that the lattice constant parameter a was 0.5946 nm at room temperature, which is very close to the value of PbS in cubic phase in the standard card 0.5936 nm (JCPDS No. 5-592). In fact, the formation of PbS NCs structures involves the formation and growth processes of PbS nuclei. Then the as-formed nuclei grow along different directions to form various PbS structures. According to Wang [21], the shape of an FCC (face center cubic) NC was mainly determined by the ratio of the growth rate in the /1 0 0S direction to that in the /1 1 1S direction. And balancing the relative growth rates between these crystalline faces is a key to control the morphology of the products. When the ratio is relatively high, PbS cubes bounded by the six {1 0 0} planes will be formed [22]. 3.2. SEM and TEM observations The morphology and the particle size of the synthesized PbS hollow spheres were characterized by SEM. Fig. 2 shows the typical SEM micrographs of the synthesized PbS micro- and nanoparticles, which suggested that almost all of the synthesized PbS particles exhibited uniform sphere-shaped particles. According to the observations, particles were made within the first few minutes, but after few minutes those particles were aggregated and their size got larger because of their large surface to volume ratio. In fact, without coating of surfactant on the particles, due to the increase in the surface area to volume ratio, the attractive force between the nanoparticles will increase, and some of the particles will agglomerate. Thus, the particle size became larger, and it is worth mentioning that the PbS aggregated particles were composed of much smaller crystallites. This figure also presents typical TEM micrographs of the synthesized PbS hollow spheres in nano scale. In these micrographs the average diameter of the particles is about 20–40 nm and the shell thickness is about 10 nm. From the TEM micrograph of a single PbS hollow sphere shown in Fig. 3(b), we realize the surface of the shell is coarse, suggesting that the shell could consist of primary PbS nanoparticles. In addition, the strong contrast between the dark edges and the pale centers of the spherical particles evidence their hollow nature. 3.3. EDX analysis Fig. 3 shows the EDX spectra of the synthesized PbS hollow sphere particles. The EDX spectrum clearly shows the peaks of S and Pb which are the main components of the samples. EDX analysis also indicates the presence of gold (Au) on the surface of PbS particles which is related to the coating layer in SEM analysis.
Fig. 2. Typical SEM micrographs of the synthesized PbS hollow sphere QDs (a) 4.0 K, (b) 5.0 K and (c) 12 K (Typical TEM micrograph in the corner).
The strength of the peaks of Pb and S in the spectrum confirmed the formation of the PbS NCs without any impurity. 3.4. Photoluminescence study of the synthesized PbS hollow spheres We have investigated the photoluminescence properties of the synthesized PbS hollow sphere QDs in room-temperature. Fig. 4 presents the emission spectra of the sample. When the sample
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was excited at 338.5 nm, a strong luminescence emission peak at 645.5 nm was appeared that comes from the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes. This luminescence peak did not occur when the sample was excited at other spectra. Generally, the photoluminescence study of PbS NCs is intricate because it is sensitive to the synthesis conditions, crystallite sizes and shapes. The appeared emission spectrum with a strong narrow band and high intensity indicates the high photoluminescence potential of the synthesized PbS hollow spheres. These observations suggest the immense potential of these particles as efficient optical imaging nano probes in diagnostic imaging, particularly for early detection of cancer tumors.
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3.5. FTIR analysis With the aim of identification or determination of the compounds, structure and new compounds FTIR analysis was employed. Also, the bond of Pb–S is mainly an electrovalent bond and the FTIR spectra of the samples do not show strong bands associated with Pb–S stretching and bending vibrations. Fig. 5 shows a representative example of the FTIR spectrum of the synthesized PbS hollow spheres before heat-treatment. In this spectrum two extra weak peaks were detected at 832 cm 1 and 1115 cm 1 were the characteristic peaks for Pb–S bond [23,24]. Moreover, the peak at about 1365 cm 1 represented aliphatic nitro compounds or nitrate ions, and the peak at about 3500 cm 1 was related to the O–H group [25].
3.6. The formation mechanism of PbS hollow sphere
Fig. 3. EDX spectra of the synthesized PbS hollow sphere QDs.
Generally, interfaces are formed between two different phases with common boundaries. The boundary region is the characteristic of forces acting on it by the particles involved. As a result of interfacial interactions, atoms at the interfaces are more prone to react with the surrounding species. In our experiment, sodium thiosulfate was used as one of the main precursors due to its capability to form stable complexes with many metals ions. Sodium thiosulfate forms strong complexes with monovalent ions Cu þ , Ag þ and Au þ . Alkaline thiosulfate solution dissolves many insoluble salts of Pb2 þ , Hg2 þ , Cu þ and Ag þ . The monovalent complexes are mostly S-bonded, while the divalent ones may be S or O bonded [20]. According to Lukashin et al. [26], nearly all methods for synthesizing nanocrystalline sulfides which involve the sulfidizing (for example, with hydrogen or sodium sulfide) of metal compounds are exothermic which can result in the aggregation of separate nanoparticles and cause nanosystems with a wide size distribution of particles. In this context, the special interest is the method in which PbS NCs are formed in the course of decomposition of a sulfur-containing metal complex such as a thiosulfate complex. To obtain nanocrystalline PbS hallow spheres, an anionic lead complex that decomposes to produce sulfide is required. The thiosulfate complex [Pb(S2O3)2]2 suits this requirement. This complex is prepared by reacting lead nitrate with sodium thiosulfate after filtration and washing with de-ionized water by the following reaction (4): 2þ
Pb
Fig. 4. Emission fluorescence spectra for the synthesized PbS hollow sphere QDs.
2 þ 2S2 O2 3 -½PbðS2 O3 Þ2
ð4Þ
In the experiment process, it is suggested that the [Pb(S2O3)2]2 complex with Pb2 þ and S2O2 could decompose to form PbS, S, 3 SO2 and SO24 . Thus, the formation of PbS can be formulated as
Fig. 5. FTIR spectrum of the synthesized PbS hollow sphere QDs.
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Fig. 6. Schematic illustration for the fabrication of PbS hollow sphere QDs.
follows (5) ½PbðS2 O3 Þ2 2 -PbSþS þ SO2 m þ SO2 4
ð5Þ
In the current study with the aim of synthesis PbS hollow spheres, the produced SO2 plays the main role in the growth of PbS hollow spheres because SO2 could act as an appropriate soft template. Driven by the minimization of interfacial energy, small PbS NCs may be aggregated around the gas–liquid interface between SO2 and the solvent, and finally well-defined PbS hollow spheres are formed. According to the SEM and TEM micrographs, our system had a high efficiency of the PbS hollow spheres production, which indicates the rapid formation of PbS NCs around the gas bubbles right after the reaction. Herein, the SO2 gases generated in the liquid phase chemical reaction were released as bubbles which acted as free-templates, these bubbles created numerous gas–liquid interfaces inside the solution phase. In the gas–liquid interfaces mechanisms [27–32], the gas–liquid interfaces in the solution may serve as the nucleation or agglomeration centers for the PbS NCs. Especially, in our aqueous solution, since the system was surfactant-free, the surface of the in situ generated PbS NCs was not protected with foreign species like surfactants, so they had the tendency to aggregate together to release the high surface energy of the PbS NCs. The formation of the bubbles in the reaction system may enable this agglomeration process to proceed in a controllable way. With the introduction of nano and micro-bubbles into the reaction system, the agglomeration process occurred around the bubbles. This process is apparently thermodynamically favorable. On one hand, the forces on the solvent molecules at the concave interfaces (between the bubbles and the bulk liquid phase) are asymmetric so that these molecules need other species (e.g., NCs) to stabilize. On the other hand, the high surface tension of PbS NCs should be released to reach a stable state. As a result, the in situ-generated PbS NCs tend to move into the interface region. When the concentration of PbS NCs in this region is sufficiently high, the tiny PbS NCs will interact with each other to form PbS hollow spheres with the bubbles as soft physical templates. This process has been named a ‘‘gas–liquid interface aggregation mechanism’’. Fig. 6 shows the schematic formation of the PbS hollow spheres via this mechanism [11]. As illustrated, right after the initial nucleation, the monomers will form into NCs with tendency for aggregation (first step). At the same time, large quantities of SO2 gas bubbles produced in the reaction might serve as the aggregation centers. Due to the minimization of interfacial energy, small PbS NCs may aggregate at the gas–liquid interface of SO2 bubbles and water (second step). Finally, PbS hollow spheres are formed (third step).
The use of gas bubbles generated during the reaction to provide aggregation centers is a new and effective method for fabricating inorganic hollow nano and micro-spheres. Compared to the other template-synthetic methods, this simple soft template method avoids the introduction of impurities and introduces a suitable technique for modern chemical synthesis. This idea can be extended to other solution systems in which aggregated monodispersed NCs are produced during the reaction and the gas bubbles provides pseudo-templates for NCs to form hollow spheres.
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