Subscriber access provided by Lulea University of Technology
Communication
Enhanced Photothermal Effect of Plasmonic Nanoparticles Coated with Reduced Graphene Oxide Dong-Kwon Lim, Aoune Barhoumi, Ryan Wylie, Gally Reznor, Robert Langer, and Daniel S. Kohane Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl4014315 • Publication Date (Web): 30 Jul 2013 Downloaded from http://pubs.acs.org on August 3, 2013
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Enhanced Photothermal Effect of Plasmonic Nanoparticles Coated with Reduced Graphene Oxide Dong-Kwon Lim,†,‡,# Aoune Barhoumi,†,‡ Ryan Wylie,†,‡ Gally Reznor,† Robert S. Langer,‡ and Daniel S. Kohane*,† †
Laboratory for Biomaterials and Drug Delivery, Department of Anesthesiology, Division of Critical Care Medicine, Children’s Hospital Boston, Harvard Medical School, 300 Longwood Avenue, Boston, Massachusetts 02115, United States ‡ David H. Koch Institutes for Integrative Cancer Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States # Department of BIN Fusion Technology, Graduate School of Engineering, Chonbuk National University, Jeonju, South Korea Supporting Information Placeholder ABSTRACT: We report plasmonic gold nanoshells and nanorods coated with reduced graphene oxide that produce an enhanced photothermal effect when stimulated by near-infrared (NIR). Electrostatic interactions between nanosized graphene oxide and gold nanoparticles followed by in-situ chemical reduction generated reduced graphene oxide-coated nanoparticles; coating was demonstrated using Raman and HR-TEM. Reduced graphene oxide-coated gold nanoparticles showed enhanced photothermal effect compared to non-coated or non-reduced graphene oxide-coated gold nanoparticles. Reduced graphene oxide-coated gold nanoparticles killed cells more rapidly than did non-coated or non-reduced graphene oxide-coated gold nanoparticles.
Plasmonic nanoparticles and carbon-based materials have been extensively explored for their use in photothermal therapy, stimulated by near-infrared (NIR) light.1-3 Plasmonic nanoparticles used for this purpose include gold nanoshells (AuNS),2,4 gold nanorods (AuNR),2,4 gold nanocages,1 gold stars,5 and gold-decorated silicon nanowires.6,7 Plasmonic gold nanomaterials have been used in many photothermal therapy systems because of their tunable optical properties and potential biocompatibility.8,9 However, further enhancement of the photothermal performance of gold particles generally requires increasing their dimensions or concentration2,3. Carbon-based materials such as carbon nanotubes11 and nanoscale reduced graphene oxide (r-GO)12 have recently been applied in hyperthermia therapy, as they can absorb light from the UV to NIR ends of the spectrum and convert it into heat through non-radiative decay. Although nanoscale r-GO shows a 6-fold greater NIR absorbance than non-reduced GO12, the low quantum efficiency of r-GO and its broad absorption spectrum render it less sensitive to specific wavelengths than plasmonic nanoparticles.11,13 Moreover, graphene’s usefulness as a nanoparticulate formulation is limited by its poor colloidal stability, which necessitates surface chemistry modification with stabilizing agents (e.g., PEG-lipid).12 Here, we report enhanced photothermal effects with a new hybrid material composed of AuNS or AuNR coated with nanosized sheets of reduced graphene. The AuNS or AuNR can serve as photothermal sources through non-radiative decay4, and also act as local nano-antennae to enhance the optical energy absorption of graphene at a selected plasmon frequency.1,4,14,15 The potential for interaction between plasmonic nanoparticles and carbon-based materials has been seen in the enhancement of photoACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 10
current with plasmonic nanostructures applied to the surface of graphene.16,17 Recently, ultrasmall gold nanoparticles anchored to large graphene sheets were reported to enhance photothermal effects.18 However, practical photothermal therapy requires well-defined individual nanoparticles rather than large sheet-like structures. We studied the photothermal effects of two different NIR-sensitive (700 – 800 nm) nanoparticles, AuNS (150 nm in diameter) and AuNR (10 nm width, aspect ratio = 3.5) with and without graphene coating. Those particle types were treated with different graphene oxide coating procedures, due to their different surface charges (Fig. S1). The surfaces of the AuNS were rendered positively charged by modification with cysteamine prior to exposure to negatively charged graphene oxide. No such modification was required for AuNR since they have a positively charged surface as synthesized. Negatively charged nano-sized graphene oxide was prepared through a modified Hummer’s method19 (See SI & Figs. S2S4). They were then bound (Figs. S5, S6) to AuNS (Fig. 1 A-C) or AuNR (Fig. 1 D-F). Chemical reduction of GO-coated nanoparticles (see SI for the detailed protocol) generated well-defined r-GOAuNS (Fig. 1 B) and r-GO-AuNR (Fig. 1 E). Representative high resolution-transmittance electron microscope (HR-TEM) images (Fig. 1 B and E) showed the successful formation of r-GO layers on AuNS and AuNR. A thicker graphene layer was formed on the AuNR (2.5 nm) than the AuNS (～1.0 nm). This correlates with the larger graphene peak seen at 231 nm in the UV-Vis spectrum of GO-AuNR (Fig. 1 F) than GO-AuNS (Fig. 1 C). After in-situ chemical reduction, the r-GO-AuNS and r-GO-AuNR solutions developed a peak at 270 nm (Fig. 1 C and F-red dot arrow), indicating the recovery of the electronic transition state (π π*) of the graphene layer,12,20 which is also seen in Fig. S4 A. The more positive surface charge of CTAB-capped AuNR (ζ = 35.0 ± 2.7 mV) compared to cysteamine-modified AuNS (Cys-AuNS, ζ = 16.0 ± 1.3 mV) was likely responsible for the greater graphene deposition on AuNR (Fig. 2 A). r-GO coated particles showed some sedimentation after 1-2 days at room temperature that was visible to the naked eye; it was easily reversible by gentle manually agitation. The colloidal stability of solutions of r-GO-AuNS and r-GO-AuNR at room temperature was evidenced by essentially unchanged UV-Vis spectra and dynamic light scattering measurements over time (Fig. S7), The colloidal stability was due to their slightly negative surface zeta potentials (in distilled water, pH 4-5), attributable to the many carboxylic acid and hydroxyl groups still found in r-GO due to incomplete reduction,19,20 as evidenced by the negative zeta potential at pH 7.4 in 10 mM phosphate buffer (Fig. 2 A, dotted black arrow; note that the pKa of r-GO is 7.921); the incomplete reduction is further documented by Raman analysis below (Fig. 2 B). To investigate the chemical and physical nature of the r-GO on the particles, we performed Raman and HR-TEM-based diffraction analysis. For the Raman analysis, sample solution (2 µL) was dropped on quartz slide, allowed to dry, and measured using 785 nm laser for excitation. Raman analysis of AuNS and AuNR (Fig. 2 B) did not show the peaks characteristic of graphene (at 1,355 (D band) and 1,593 cm-1 (G band))20,22 which were seen in GO-AuNS and GO-NR. r-GO-AuNS and r-GO-AuNR showed distinctive Raman shift peaks at 2,710 cm-1 (2D band) and 2,900 cm-1 (D+D’ band) indicative of successful chemical reduction of the GO layer. Due to the intrinsic limitations of chemical GO reduction 12,20 and the presence of multiple GO layers on the particles, reduction of GO was incomplete, resulting in broad Raman peaks (at 2D, D+D’) (Fig. 2 B).20,22 This incomplete reduction of the GO may have had the beneficial effect of resulting in a negative surface potential on the particle, leading to greater hydrophilicity and improved colloidal stability compared to fully reduced GO.23 The colloidal stability of fully reduced r-GO tends to be poor because of its hydrophobicity, necessitating the use of surfactants.12 The selected-area electron diffraction (SEAD) patterns obtained from r-GO sheets attached to (but slightly removed from) the r-GO-AuNS (Fig. 2 C, red circle) showed highly aligned diffraction patterns indicating a single crystalline state.24 The diffraction pattern from r-GO sheets right on the particles showed continuous circular (from the gold)25 and periodic (from the graphene) patterns, demonstrating
ACS Paragon Plus Environment
Page 3 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
the preserved crystalline state of r-GO layers on the particle (Fig. 2 C). These Raman and TEM analyses showed that the nanoparticle coatings were crystalline r-GO and not amorphous carbon.24,25 The photothermal performance of r-GO AuNS or AuNR was studied in the dry state (Fig. 3 A and B) and solution state (Fig. 3 C and D) under NIR illumination (808 nm, continuous wave (CW), power density; 3.0 W/cm2). For the dry state study, particle solution (2 µL of OD 5.0 [8.0 pM for AuNS, 0.28 nM for AuNR]) was spotted on a silica thin layer chromatography (TLC) plate and allowed to dry. TLC plates were illuminated for 30 sec (808 nm, 3.0 W/cm2), then imaged with an IR camera, allowing measurement of temperature changes. When irradiated, AuNS and GO-AuNS spots increased in temperature from 25.0 °C (∆T = Tfinal – 25 °C) by 20.0 ± 0.7 °C and 27.0 ± 1.9 °C respectively, while r-GOAuNS increased by 57.0 ± 1.2 °C (Fig. 3 A, where temperatures shown are Tfinal). Similarly, irradiated AuNR spots heated by 23.0 ± 0.9 °C, GO-AuNR to 32.0 ± 1.3 °C, and r-GO AuNR to 67.0 ± 1.7 °C (Fig. 3 B). With both particle types, r-GO coating led to a 2.9 - 3.5 fold increase in ∆T upon irradiation. For the solution state study, solutions with the same optical density (OD = 1.0) of AuNS (1.6 pM) and AuNR (0.056 nM) and sample volume (2 mL) were illuminated for 5 min at 3.0 W/cm2, CW, 808 nm. Distilled water increased in temperature by only 0.8 ± 0.2 °C (0.1 °C/min) (Fig. 3 C and D). AuNS solutions increased in temperature by 7.0 °C/min and 35.1 ± 0.2 °C over 5 min (Fig. 3 C). GO-AuNS showed a similar rate of 7.4 °C/min and 36.7 ± 0.2 °C over 5 min. Solutions of r-GO-AuNS induced greater temperature changes (rate = 8.5 °C/min, 42.3 ± 0.2 °C over 5 min). Similar observations were made with AuNR (Fig. 3 D): r-GO-AuNR heated at 9.5 °C/min, 47.5 ± 0.2 °C over 5 min which was greater than the heating rates with GO-AuNR (8.5 °C/min, 42.5 ± 0.2 °C over 5 min) and AuNR (8.3 °C/min, 41.7 ± 0.2 °C over 5 min). These independent measurements demonstrate the greater photothermal effect with particles coated with r-GO, which could be attributable to light absorption by r-GO and/or to plasmonic effects from the gold nanoparticle. To investigate the origin of this enhanced photothermal effect, we performed the same solution-based experiment as above, using r-GO coated silica nanoparticles (see SI and Fig. S6), where no surface plasmonic effect would be expected (Fig. 3 E). We used particle concentrations of 1.0 × 109 particles/mL (0.07 µg/mL of r-GO), and 3.5 × 1010 particles/mL (2.45 µg/mL of r-GO) which are equivalent to the concentrations (OD 1.0) of AuNS and AuNR used above. Despite the thick coating (> 5 nm) with r-GO on the silica particles, as seen by (Fig. 3 E), only negligible temperature changes (0.9 ± 0.15 °C, 1.6 ± 0.03 °C over 5 min at both concentrations) were observed from r-GO-silica nanoparticles from irradiation (3.0 W/cm2, CW, 808 nm) (Fig. 3 F). These results show that r-GO by itself did not cause the increase in photothermal effect seen with r-GO gold nanoparticles. Therefore, we can conclude that the interactions between the r-GO and gold plasmons were crucial to the enhanced photothermal effect seen with the r-GO-gold particles. The effectiveness of these nanomaterials for photothermal therapy was demonstrated with cultured human umbilical vein endothelial cells (HUVECs), using a MTS assay. HUVECs were used since photothermal therapy has been used to destroy normal or abnormal endothelial cells in diseases such as aged-related macular degeneration or cancer. In the absence of irradiation, HUVECs exposed to GOcoated particles for 24 hours at the concentrations used showed no significant cytotoxicity, irrespective of the oxidation state of graphene (Fig. S8). However, HUVECs incubated with graphene-coated and uncoated nanoparticles for 24 hours followed by irradiation (3.0 W/cm2, CW, 808 nm) showed a timedependent decrease in cell viability (Fig. 4 A and B). After 1 min of irradiation, a sharp decrease in cell viability (to 23%) was observed in r-GO-AuNS treated cells (Fig. 4 A), while 41 - 43% of cells treated with AuNS or GO-AuNS were still viable. After 1 min of irradiation, the viability of HUVECs incubated with r-GO-coated AuNR was reduced to 33%, while 53 - 57% of cells exposed to non-coated and GO-coated AuNR were viable. These results showed that r-GO coating on plasmonic nanoparticles accelerated cell killing. Live/dead staining showed that cell death was restricted to laser-illuminated areas (Fig. 4 C).
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 10
In summary, we report enhanced NIR-sensitive photothermal materials composed of graphene oxide and plasmonic nanoparticles. The addition of a thin layer of graphene oxide around the plasmonic nanoparticles showed enhanced photothermal properties, which may be useful in improving biomedical applications based on the photothermal effect, by increasing their efficacy and/or decreasing the duration of therapy. The increased killing of cells in contact with the particles, together with the reduced duration of exposure needed for effect, may decrease the irradiation needed, and therefore phototoxicity25.
ACS Paragon Plus Environment
Page 5 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Figure 1. Optical characterization of r-GO-AuNS and r-GO-AuNR. (A) Schematic representation of rGO-AuNS, (B) HR-TEM image of r-GO-AuNS; (C) UV-Vis spectra before and after GO layer formation on AuNS and GO reduction, (D) Schematic representation of r-GO-AuNR, (E) HR-TEM image of r-GO-AuNR; (F) UV-Vis spectra before and after GO layer formation on AuNR and GO reduction. (Red arrows in Figs. B and E indicate the r-GO layer thickness on the particles; Red dotted arrows in Figs. C and F indicate the red shift in absorption from GO reduction).
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 10
Figure 2. Effect of surface modifications and reduction on physicochemical properties. (A) Zeta potential of GO or r-GO coated particles measured in distilled water (red line: AuNS, black line: AuNR). Data are means ± standard deviations. (B) Raman spectra of AuNS and AuNR with and without GO or r-GO coatings. (C) HR-TEM images of r-GO-AuNS and SEAD patterns of the circled areas on the particle (blue) and on the graphene coating (red). (Cys-AuNS: Cysteamine modified AuNS, black dotted arrow indicates the effect on the surface potentials of r-GO AuNR and r-GO AuNS of suspension in phosphate buffer pH 7.4 rather than distilled water).
ACS Paragon Plus Environment
Page 7 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Figure 3. Dry and solution state measurements of the photothermal effect. (A, B) Photographs of coated and uncoated nanoparticles spotted on silica thin-layer-chromatography (TLC) plates and thermal images after 30 sec of irradiation at 3.0 W/cm2, CW, 808 nm. Crosses indicate the positions of the temperature measurements. (C, D) Solution temperature changes over time upon irradiation (3.0 W/cm2, CW, 808 nm) of solutions of coated and uncoated nanoparticles, and DW (distilled water). (E) HR-TEM images of SiO2 nanoparticles (120 nm) and r-GO coated SiO2 nanoparticles. Scale bar = 20 nm. (Insets: photographs of vials containing solutions of each sample, red arrows indicate the thickness of r-GO coating). (F) Temperature change of r-GO-SiO2 solutions upon irradiation (3.0 W/cm2, CW, 808 nm). In C, D, F: data are means ± standard deviations (SD), N=4, * = p < 0.05 by ANOVA, SD are shown but are too small to be seen.
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 10
Figure 4. Photothermal cell killing with r-GO-AuNS and r-GO-AuNR. (A, B) Cell viability (MTS assay) of HUVECs as a function of irradiation time in the presence of (A) no NP, AuNS, GO-AuNS, and r-GOAuNS, or (B) no NP, AuNR, GO-AuNR, and r-GO-AuNR. Data are means ± standard deviation, N=4, (* = p < 0.05). The dotted red lines indicate 50% cell viability. (C) Live/dead assay after 30 sec, 2 min, and 5 min of irradiation (808 nm, 3.0 W/cm2). The dotted white line represents the boundary of the irradiated area. Supporting Information. Materials, experimental details including synthesis, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] ACKNOWLEDGMENT This work was supported by a grant from Sanofi-Aventis. REFERENCES (1)
Skrabalak, S. E.; Chen, J.; Au, L.; Lu, X.; Li, X.; Xia, Y. N. Advanced Materials 2007, 19, 3177.
(2) Cole, J. R.; Mirin, N. A.; Knight, M. W.; Goodrich, G. P.; Halas, N. J. Journal of Physical Chemistry C 2009, 113, 12090. (3) Dreaden, E. C.; Mackey, M. A.; Huang, X. H.; Kang, B.; El-Sayed, M. A. Chemical Society Reviews 2011, 40,
ACS Paragon Plus Environment
Page 9 of 10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters 3391.
(4) American
Huschka, R.; Zuloaga, J.; Knight, M. W.; Brown, L. V.; Nordlander, P.; Halas, N. J. Journal of the Chemical Society 2011, 133, 12247.
(5) (6) Kim, Y. H.;
Yuan, H.; Fales, A. M.; Vo-Dinh, T. Journal of the American Chemical Society 2012, 134, 11358. Park, G. S.; Kwon, H.; Kwak, D. W.; Park, S. Y.; Kim, M.; Lee, J. H.; Han, H.; Heo, S.; Li, X. S.; Lee, J. G.; Yang, W.; Cho, H. Y.; Kim, S. K.; Kim, K. Nano Letters 2012, 12, 1638.
(7) Wang, X. J.; Wang, C.; Cheng, L.; Lee, S. T.; Liu, Z. Journal of the American Chemical Society 2012, 134, 7414. (8) Austin, L. A.; Kang, B.; Yen, C. W.; El-Sayed, M. A. Journal of the American Chemical Society 2011, 133, 17594. (9)
Kang, B.; Mackey, M. A.; El-Sayed, M. A. Journal of the American Chemical Society 2010, 132,
(10)
Rossella, F.; Soldano, C.; Bellani, V.; Tommasini, M. Advanced Materials 2012, 24, 2453.
1517.
(11) J. Journal
Robinson, J. T.; Tabakman, S. M.; Liang, Y. Y.; Wang, H. L.; Casalongue, H. S.; Vinh, D.; Dai, H. of the American Chemical Society 2011, 133, 6825.
(12)
Moon, H. K.; Lee, S. H.; Choi, H. C. Acs Nano 2009, 3, 3707.
(13)
Huschka, R.; Barhoumi, A.; Liu, Q.; Roth, J. A.; Ji, L.; Halas, N. J. ACS Nano 2012, 6, 7681.
(14) Angewandte
Kuo, W. S.; Chang, C. N.; Chang, Y. T.; Yang, M. H.; Chien, Y. H.; Chen, S. J.; Yeh, C. S. Chemie-International Edition 2010, 49, 2711.
(15) Geim, A.
Echtermeyer, T. J.; Britnell, L.; Jasnos, P. K.; Lombardo, A.; Gorbachev, R. V.; Grigorenko, A. N.; K.; Ferrari, A. C.; Novoselov, K. S. Nat. Commun. 2011, 2.
(16) Commun.
Liu, Y.; Cheng, R.; Liao, L.; Zhou, H. L.; Bai, J. W.; Liu, G.; Liu, L. X.; Huang, Y.; Duan, X. F. Nat. 2011, 2.
(17) (18) B.; Lu, W.;
Zedan, A. F.; Moussa, S.; Terner, J.; Atkinson, G.; El-Shall, M. S. ACS Nano 2012. Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z. Z.; Slesarev, A.; Alemany, L. Tour, J. M. Acs Nano 2010, 4, 4806.
(19)
Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nature Nanotechnology 2008, 3, 101.
(20)
Konkena, B.; Vasudevan, S. The Journal of Physical Chemistry Letters 2012, 3, 867.
(21)
Huh, S.; Park, J.; Kim, Y. S.; Kim, K. S.; Hong, B. H.; Nam, J. M. Acs Nano 2011, 5, 9799.
(22)
Tambe, D. E.; Sharma, M. M. Advances in Colloid and Interface Science 1994, 52, 1.
(23)
Zhang, Y.; Zhang, L.; Kim, P.; Ge, M.; Li, Z.; Zhou, C. Nano Letters 2012, 12, 2810.
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(24)
Sun, Y.; Mayers, B.; Xia, Y. Advanced Materials 2003, 15, 641.
(25)
Timko, B. P.; Dvir, T.; Kohane, D. S. Advanced Materials 2010, 22, 4925
ACS Paragon Plus Environment
Page 10 of 10
