Cadmium Sulfide Silver Nanoplate Hybrid Structure: Synthesis and

ARTICLE pubs.acs.org/JPCC

Cadmium Sulfide Silver Nanoplate Hybrid Structure: Synthesis and Fluorescence Enhancement Shangxin Lin,† Matthew Man-Kin Wong,‡ Pak-Kei Pat,‡ Chun-Yuen Wong,*,‡ Sung-Kay Chiu,*,‡ and Edwin Yue-Bun Pun*,† † ‡

Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR

bS Supporting Information ABSTRACT: Semiconductor
noble metal hybrid nanostructures are of current interest in the development of bioimaging agents. To enhance photoluminescence of cadmium sulfide nanoparticles via surface plasmon mode coupling, cadmium sulfide conjugated silver nanoplates (denoted as CdS-Ag NP) have been prepared by a forward
reverse cation exchange method. The morphology, crystallinity, and atomic composition of the CdS-Ag NP have been investigated by energy dispersive X-ray spectroscopy and highresolution transmission electron spectroscopy, and the CdS-Ag NP can be described as Cd2+-rich CdS nanoparticles covalently bonded to the surfactant of Ag nanoplates. Photoluminescence enhancement of CdS measured is attributed to the matching between the emission bands of cadmium sulfide and the tailor-made localized surface plasmons resonance bands provided by the Ag NP. Application of the CdS-Ag NP in cell imaging has also been demonstrated.

’ INTRODUCTION Surface plasmons (SPs) and their corresponding optical properties in noble metal nanoparticles have received considerable attention.1
9 In recent years, there are numerous reports on the application of SPs in the development of molecular sensors,10
13 organic solar cells,14 and light-emitting diodes.15
19 Localized surface plasmons resonance (LSPR), a type of surface plasmon resonances, is an optical resonance phenomenon of nonpropagating excitation of conduction electrons in metallic nanostructures coupled to electromagnetic field.20 LSPR can affect the optical properties of materials nearby, resulting in either enhancement15,16,21
23 or quenching24
26 of the photoluminescence (PL) property of adjacent fluorophore. In noble metallic nanoparticles, LSPR presents strong absorption and scattering cross-section and is sensitive to the local refractive index, size, shape, and chemical compositions of the nanostructures.27,28 This unique characteristic provides a convenient means to manipulate the PL properties, such as modifying the LSPR bands, by controlling the shape and size of the metal nanoparticles. Recently, researchers have put significant efforts into controlling the shape of noble metal nanoparticles including nanosphere (NS), nanoprism,29,30 nanocube,31,32 polyhedron,33
35 nanorods, and nanowires.36
39 Silver and gold nanoparticles have been considered as desirable candidates for applications like enhancing the performance of solar cells40 and biosensing agents13,41
47 because their LSPR bands are within the visible to near-infrared (NIR) region. Enhancing the emission performance of cadmium chalocogenides (CdX, X = S, Se, or Te) nanoparticles via the assistance of r 2011 American Chemical Society

LSPR is of current interest.48,49 Theoretically, it is possible to enhance the emission efficiency of CdX nanoparticles by LSPR because both the emission bands of CdX and LSPR bands of the noble metallic nanoparticles are tunable. However, practical issues exist regarding the synthetic procedure and the design of CdX-metal hybrid structure: (1) forward cation exchange occurs when noble metallic ion precursors exist in CdX solution;40
52 (2) PL quenching may occur once the CdX nanoparticles and the noble metallic nanoparticles are too close.17 In this work, we have prepared cadmium sulfide conjugated silver nanoplates (denoted as CdS-Ag NP) which exhibit PL enhancement of CdS nanoparticles via LSPR. Forward
reverse cation exchange mechanisms seem to be involved in the preparation of the CdS-Ag NP. Moreover, the CdS-Ag NP have been used in cell imaging, and no apoptosis has been observed. This reveals that the CdS-Ag NP have great potential in bioimaging application.

’ EXPERIMENTAL METHODS Materials. Silver nitrate (AgNO3, 99%) and cadmium nitrate tetrahydrate (Cd(NO3)2 3 4H2O, 99%) were purchased from International Laboratory USA. Trisodium citrate (TSC, 99%), sodium borohydride (NaBH4) and polyvinylpyrrolidone (PVP, MW = 10,000) were purchased from Sigma Aldrich. Tri-n-butylphosphine Received: June 24, 2011 Revised: September 27, 2011 Published: September 30, 2011 21604

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Scheme 1. Schematic of Conjugating CdS NS to Ag NP with a Forward
Reverse Cation Exchange Methoda

a Step I (forward cation exchange): CdS NSs were converted to Ag2S NS and linked to PVP ligand to give Ag2S-Ag NPs. Step II (reverse cation exchange): the Ag2S NSs on Ag2S-Ag NPs were converted to CdS NS to give CdS-Ag NP.

(TBP, 50% in ethyl acetate) was purchased from AccuChem Industrial Cleaning Chemicals. Sodium nonahydrate (Na2S 3 9H2O, 96%) was purchased from Xilong Chemicals. Phosphate buffer saline (PBS 1X), Dulbecco’s modified of Eagles’ Medium (DMEM), and fetal bovine serum (FBS) were purchased from Invitrogen. All materials were used without further purification. The glass scintillation vials were purchased from Perkin-Elmer. Synthesis. Silver Nanoplate (Ag NP) and Cadmium Sulfide Nanosphere (CdS NS). 0.2 mM Ag NP was synthesized by a modified photoinduced conversion method reported in the literature.29,30 15 mL of 0.2 mM Ag nanosphere (Ag NS) was prepared from reduction of Ag+ by NaBH4 aqueous solution and stabilized by 30 mg PVP and 0.6 mM TSC. The yellow Ag NS solution was kept stable for 30 min and was then transferred to a glass scintillation vial and exposed under halogen lamp at 40 W for 48 h until the solution gave a dark blue color, indicating that the Ag NP was formed. 0.2 mM CdS NS was synthesized by mixing 0.2 mM Cd(NO3)2 and 0.2 mM Na2S in the presence of 0.2 mM TSC as stabilizer, and the resultant yellow CdS NS solution was kept for 1 day before being used. Silver Sulfide Conjugated Ag NP (Ag2S-Ag NP). Ten mL of the synthesized Ag NP was centrifuged and washed with DI water twice and then dispersed into 40 mL DI water containing 60 μL of 10 mM AgNO3 aqueous solution. 0.5 mL of the 0.2 mM CdS NS solution was then added to the mixture at a rate of 30 μL min
1 with vigorous stirring. The disappearance of emission centering at 670 nm from CdS NS indicated the completion of the forward cation exchange. Cadmium Sulfide Conjugated Ag NP (CdS-Ag NP). Before the reverse cation exchange, the Ag2S-Ag NP was centrifuged and washed with methanol (MeOH) three times and then dispersed into 10 mL MeOH. 80 mg PVP, and 50 μL of 10 mM Cd(NO3)2 methanolic solution were then added into the Ag2S-Ag NP solution. The mixture was deaerated and protected by argon gas and stirred for 10 min. The solution was then heated at 60
70 °C in an oil bath, and 0.2 mL of a TBP methanolic solution (ca. 1% v/v) was added dropwise.48,49,51
53 After the addition of TBP, 0.2 mL of 10 mM of methanolic solution of Cd(NO3)2 was added, and the mixture was kept at this temperature for 2 h. The resultant CdS-Ag NP was washed twice with MeOH by centrifugation and dispersed in 3 mL MeOH with the assistance of ultrasonic bath. Cell Culture and Cell Imaging with CdS-Ag NP. The washed CdS-Ag NP methanolic solution was added into DMEM with 5% FBS and stored at 4 °C before use. HeLa cancer cells were cultured in DMEM at 37 °C under 5% CO2 until reaching 70%

confluent. Then, the dish was rinsed with PBS twice. CdS-Ag NP in DMEM solution (2 mL, ca. 30 nM) was added into the culture dish, which was then incubated at 37 °C for one hour. The culture dish was rinsed twice with PBS to remove excessive CdSAg NP. Imaging of CdS-Ag NP in HeLa cells was performed with Leica TSC SP5 confocal fluorescence microscope using laser excitation wavelength at 405 nm. The emission from the CdS-Ag NP in cells was captured through a 488 nm filter to a 8-bit CCD camera in 256 step of gray. Characterizations. The absorption and emission spectra of the nanocrystals were measured by Shimadzu UV-1700 UV
vis spectrometer and Horiba Jobin Yvon FluoMax-4 spectrophotometer, respectively. Energy-dispersive X-ray studies (EDAX) were performed using Oxford INCA system. Both high and low magnification morphological characterizations were performed by JEOL JEM2100F transmission electron microscope.

’ RESULTS AND DISCUSSION A. Synthesis of CdS-Ag NP. In this work, we aimed to prepare CdS-conjugated Ag nanoparticles which show luminescent enhancement of CdS with the assistance of the LSPR from Ag nanoparticles. Regarding the design of the CdS/Ag hybrid nanostructures it is important that (1) the LSPR bands of Ag nanoparticles match the emission band of CdS; (2) CdS and Ag must be separated to avoid emission quenching by electron transfer mechanism.17 With these two considerations in mind, we developed a method to prepare the targeted CdS-Ag NP in which a forward
reverse cation exchange mechanism is likely to be involved (Scheme 1).51
54 First of all, CdS NSs were added into a mixture of Ag NP (stabilized by PVP and citrate ions) and Ag+ ions (Step I). In this step, the CdS NS were converted into Ag2S NS in the presence of Ag+ ions (regarded as forward cation exchange), and the Ag2S NS were linked to the surfactant (PVP) on the Ag NP to give Ag2S-conjugated Ag NP (Ag2S-Ag NP).54 The completion of the forward cation exchange was confirmed by the disappearance of the CdS NS emission band centering at 670 nm. The Ag2S-Ag NPs were then converted into CdSconjugated Ag NP (CdS-Ag NP) by the addition of Cd2+ (Cd(NO3)2) and TBP (regarded as reverse cation exchange, Step II).52,53 The resultant CdS-Ag NPs were dispersed in MeOH for storage after the removal of excessive salts and TBP, and they were stable for 2 months under ambient conditions. Although the linkage between the CdS NS and Ag NP could not be probed by transmission electron microscopy (TEM), the product obtained in this work was unlikely a mixture 21605

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Figure 1. (A) Optical extinction spectra of aqueous Ag NP (dashed blue), aqueous Ag2S-Ag NP (blue), washed Ag2S-Ag NP in MeOH (black), and washed CdS-Ag NP in MeOH (red). (B) Normalized extinction spectra of aqueous Ag NP (dashed blue), washed Ag2S-Ag NP in MeOH (black), and washed CdS-Ag NP in MeOH (red). A 60 nm (from 670 to 730 nm) red-shift was observed in different solvents which matched well with the theoretical reported value.13

of Ag NP and CdS NS because attempts to separate the CdS NS from the Ag NP by centrifugation at various angular speeds were not successful, even the CdS NS and the Ag NP have a large difference in size. As observed in the TEM image, the distribution of the Ag NP and CdS NS were not isotropic in the product obtained: all the CdS NS were found to be closely associated with Ag NP. There are two important features in this synthetic scheme: (1) the synthesized Ag NP was only mildly truncated throughout the synthetic steps, and this should in principle permit emission enhancement of CdS via LSPR as the shape, size, and thus the LSPR bands of the Ag NP were tailor-made to match the emission band of CdS (see discussion below); (2) the surfactant on the Ag NP acted as a physical barrier between CdS NS and Ag NP, thus preventing the emission of CdS quenched by electron transfer mechanism.17 The forward
reverse cation exchange approach was the key to the success in the synthesis of CdS-Ag NP. Attempts had been made to prepare CdS-Ag NP by conjugating CdS to Ag NP directly via introducing CdS NS into an Ag NP solution or by successive addition of S2
and Cd2+ ions into an Ag NP solution, but the Ag NP were converted into Ag2S with an irregular shape, presumably due to the following reactions55 4Ag þ 2S2
þ O2 þ 2H2 O f 2Ag2 S þ 4OH


ð1Þ

Moreover, preparing Ag2S-Ag NP by directly reacting Ag2S NP with Ag NP was found to be unsuccessful since Ag2S was not linked to the Ag NP. B. Characterizations of CdS-Ag NP. As mentioned above, the LSPR bands of the Ag NP were tailor-made to match the emission bands of CdS NS by varying the size of the Ag NP. According to the theoretical calculations by Schatz and coworkers, the LSPR bands of the truncated Ag NP centering at 670, 470, 410, and 340 nm could be assigned to the in-plane

Figure 2. (A) TEM image of Ag NP before CdS conjugation. The scale bar is 50 nm. (B) TEM image of CdS-Ag NP; the dots surrounding the Ag NP are CdS NS (see parts D and E). The scale bar is 50 nm. (C) HRTEM image of CdS-Ag NP. The small CdS NS (left ones) are not in contact with the Ag NP on the right-hand side. The scale bar is 10 nm. (D,E) The d spacings of the CdS NS surrounding the Ag NP were clearly identified under HRTEM. The scale bars are 10 and 2 nm, respectively. (F) Content analysis of CdS-Ag NP by EDAX in a 120 nm  120 nm hot-spot zone.

dipole plasmon resonance, in-plane quadrupole plasmon resonance, out-of-plane dipole plasmon resonance, and out-of plane quadrupole plasmon resonance, respectively.29,30 The former two LSPR bands match well with the emission bands of the CdS NS at 640 and 465 nm (attributed to the emissions of the Cd2+rich CdS nanoparticles);56
58 thus the Ag NP could in principle enhance the emissions from the CdS NS, provided that the dimension and shape of the Ag NP were maintained throughout the forward
reverse cation exchange reactions. A simple way to keep track on the dimension and shape of the Ag NP was to monitor the LSPR bands of the Ag NP via optical extinction spectroscopy. As depicted in Figure 1, the LSPR band profile of the Ag2S-Ag NP and CdS-Ag NP were the same as those for Ag NP, indicating that the structural deformation of the Ag NP was subtle during the cation exchange (Figure 1B). The TEM images of Ag NP and CdS-Ag NP are depicted in parts A and B of Figure 2, which show that the Ag nanostructures were still maintained in the form of nanoplate throughout the forward
reverse cation exchange. Moreover, the small spherical nanoparticles (2
5 nm) surrounding the Ag NP in Figure 2B were confirmed to be CdS nanoparticles by high-resolution TEM (HRTEM) (parts C
E of Figure 2). These CdS NS had a zinc blende structure with a lattice constant of 5.832 Å,52 and the measured d spacings of ∼2.930 Å and 2.023 Å correspond to the (001) and (011) lattice planes of CdS, respectively (see Supporting Information). Figure 2F shows the coexistence of CdS NS without structural defects (right one) and with stacking faults (left one), which is typical in forward
reverse cation exchange experiments.52 The EDAX analysis on the sample suggested that the atomic ratio of cadmium to sulfur was not stoichiometric 21606

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Figure 3. (A) a, b: CdS NS aqueous solution (yellow, 0.2 mM) and CdS-Ag NP methanolic solution (pale blue, 0.55 μM) without photoexcitation; a0 , b0 : CdS NS and CdS-Ag NP excited by UV lamp (365 nm). (B) Normalized PL spectra of CdS NS solution (dashed line,  3) and CdS-Ag NP (solid line) excited at 405 nm (red), 375 nm (blue), and 350 nm (black).

Figure 4. (A) A representative photomicrograph of HeLa cells incubated with CdS-Ag NP. (B) Emission image of CdS-Ag NP in HeLa cell. This emission image was captured through a 488-nm filter to a CCD camera in 256 step of gray and was pseudocolored as red for best illustration in print format. (C) Superposition image of image A and image B.

(Cd:S ≈ 1.5), and these Cd2+-rich CdS NSs are known to give two emission bands centering at 465 and 640 nm.56 PL measurements on a methanolic solution of CdS-Ag NP and an aqueous solution of CdS NS (as reference, 0.2 mM) were performed at room temperature; Figure 3A shows the emission from both CdS-Ag NPs and CdS NSs excited by a UV lamp, and Figure 3B depicts the PL spectra of CdS NS and CdS-Ag NPs which have been normalized by their concentration. It is documented that CdS NS can emit at ∼460 nm (blue emission) and ∼670 nm (red emission). The blue emission is attributed to the recombination of the photogenerated excitons in the structured region (interior) of the CdS NS, whereas the origin of the red

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emission has been assigned to the emission from less organized regions on the nanoparticles (defects on surface).56
58 Thus, the resultant emission from CdS-Ag NP was violet in color. To quantify the LSPR assisted luminescence enhancement, the PL spectra of the CdS-Ag NPs in Figure 3B have been normalized by the concentration and compared with that of the CdS NSs (see Supporting Information). In this work, the concentration of the nanocrystals in a given sample was determined by the number of sulfur (S) atom contained in the sample. Comparing the PL peak intensity of the CdS-Ag NP with that of the CdS NS at emission wavelength at 670 nm, the PL enhancement factors were 12-, 9-, and 6-fold when using excitation wavelength at 350, 375, and 405 nm, respectively. We investigated the anisotropic emission spectral properties of CdSAg NP at the first (460 nm) and the second (670 nm) emission peaks under different excitation wavelengths. The emission anisotropy of CdS-Ag NP was almost the same when the excitation wavelengths were 350 and 375 nm, whereas CdS-Ag NP displayed notably high anisotropy when the excitation wavelength was 405 nm (Figure 3B). Moreover, the PL enhancement effect at the excitation wavelength at 350 nm was more pronounced than the one at 375 nm, presumably because of the more pronounced field enhancement when the excitation wavelength was closer to the out-of-plane quadrupole resonance (340 nm). The high emission anisotropy of Cd2+-rich CdS nanoparticles was in accordance with previous report.56 c. Cancer Cell Imaging Using CdS-Ag NP. To be qualified as a biocompatible cell imaging agent, many aspects of the chemical agents need to be considered, such as their stability, optical performance, biocompatibility, presence of linkage group to the biomolecules, and cytotoxicity. On optical performance, the fwhm of emission spectrum of CdS-Ag NPs is ca. 50 nm centering at 460 nm (Figure 3B), which is narrower than that of the commercial nucleic acid-specific dye Hoechst 33258 (ca. 200 nm centering at 460 nm). Cancer cell imaging experiments using CdS-Ag NPs were performed by incubating the CdS-Ag NP with HeLa cells for 1 h followed by observing cell morphology for another 1 h. The CdS-Ag NPs were observed in the cytoplasm displaying blue emission in the corresponding position of the nanocrystals under confocal fluorescence microscope (parts A and B of Figure 4); the emission band was found to be 460 ( 30 nm with excitation wavelength at 405 nm, consistent with our previous PL studies. There was no observable change in the cell morphology and viability in 1 h. Importantly, there was no sign of apoptosis, including blebbing and nuclear fragmentation, indicating that the level of cytotoxicity of CdS-Ag NP was negligible. This may be attributed to the LSPR-assisted PL enhancement in the CdS-Ag NP, which allows minimal usage of CdS (at nanomolar concentration range) for imaging purpose.

’ CONCLUSIONS CdS NSs have been successfully conjugated to Ag NPs via a forward
reverse cation exchange method to give CdS-Ag NPs. The morphology, crystallinity, and atomic composition of the CdS-Ag NPs have been investigated by EDAX and HRTEM, and the CdS-Ag NP can be described as Cd2+-rich CdS NSs covalently bonded to the surfactant of Ag NP. Since the LSPR bands of the Ag NP match the emission band of CdS NP, PL enhancement ranging from 6- to 12-fold has been observed. Application of the CdS-Ag NP in HeLa cell imaging has also been demonstrated; no sign of apoptosis has been observed, indicating that the level of cytotoxicity of CdS-Ag NP was negligible. 21607

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The Journal of Physical Chemistry C Importantly, the concentrations of the CdS-Ag NPs used for the cell imaging experiments were only at nanomolar level. This work suggests that careful design of hybrid cadmium chalocogenides
noble metal nanostructures may open new opportunities for the development of bioimaging materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. The characterization of CdSAg NPs and the approach of normalizing the PL spectra. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: (852) 3442 6831. Fax: (852) 3442 0522.

’ ACKNOWLEDGMENT The work described in this paper was supported by grants from City University of Hong Kong (Project Nos. 7008034 (CYW), and 7002490 (SKC)). ’ REFERENCES (1) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (2) Wiley, B.; Sun, Y.; Chen, J.; Cang, H.; Li, Z.-Y.; Li, X.; Xia, Y. MRS Bull. 2005, 30, 356–361. (3) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60–103. (4) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (5) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (6) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Langmuir 2009, 25, 13840–13851. (7) P
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