Facile synthesis of magnetic-plasmonic nanocomposites as T ... · Facile synthesis of magnetic-plasmonic nanocomposites as T1 MRI contrast enhancing and photothermal therapeutic agents

Nano Res

1

Facile synthesis of magnetic-plasmonic nanocomposites as T1 MRI contrast enhancing and photothermal therapeutic agents

Zhongzhen Yang1,2, Xianguang Ding1,2, and Jiang Jiang1 (*) Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0958-9

http://www.thenanoresearch.com on November. 26, 2015

© Tsinghua University Press 2015

Just Accepted

This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.

Nano Research DOI 10.1007/s12274-015-0958-9

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

64 Nano Res.

Template for Preparation of Manuscripts for Nano Research

This template is to be used for preparing manuscripts for submission to Nano Research. Use of this template will save time in the review and production processes and will expedite publication. However, use of the template is not a requirement of submission. Do not modify the template in any way (delete spaces, modify font size/line height, etc.). If you need more detailed information about the preparation and submission of a manuscript to Nano Research, please see the latest version of the Instructions for Authors at http://www.thenanoresearch.com/.

TABLE OF CONTENTS (TOC)

Authors are required to submit a graphic entry for the Table of Contents (TOC) in conjunction with the manuscript title. This

graphic should capture the readers’ attention and give readers a visual impression of the essence of the paper. Labels, formulae, or

numbers within the graphic must be legible at publication size. Tables or spectra are not acceptable. Color graphics are highly

encouraged. The resolution of the figure should be at least 600 dpi. The size should be at least 50 mm × 80 mm with a rectangular

shape (ideally, the ratio of height to width should be less than 1 and larger than 5/8). One to two sentences should be written below

the figure to summarize the paper. To create the TOC, please insert your image in the template box below. Fonts, size, and spaces

should not be changed.

Facile synthesis of magnetic-plasmonic

nanocomposites as T1 MRI contrast enhancing and

photothermal therapeutic agents

Zhongzhen Yang, Xianguang Ding, and Jiang Jiang*

Key Laboratory of Nano-Bio Interface, i-lab and

Division of Nanobiomedicine, Suzhou Institute of

Nano-Tech and Nano-Bionics, Chinese Academy of

Sciences, Suzhou, China 215123.

Magnetic-plasmonic nanocomposites have been constructed using a

polypyrrole adhesion layer, showing T1 MRI contrast enhancing and

efficient photothermal effect.

Provide the authors’ webside if possible.

Author 1, webside 1

Author 2, webside 2


Zhongzhen Yang1,2, Xianguang Ding1,2, and Jiang Jiang1 (*)

1Key Laboratory of Nano-Bio Interface, i-lab and Division of Nanobiomedicine, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, China 215123 2Graduate University of Chinese Academy of Sciences, Beijing, China 100049

Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014

KEYWORDS plasmonic, iron oxide, photothermal, T1, magnetic resonance imaging (MRI), theranostics

ABSTRACT Nanocomposites which combine magnetic and plasmonic components have received widespread attention in recent years due to their potential applications in biomedical research. Herein, a facile method to construct small iron oxide nanoparticles on various plasmonic core materials with varying shapes and surfaces by utilizing a polypyrrole interlayer has been demonstrated. By focusing on Au nanorod@polyprrole@iron oxide (Au NR@PPy@FexO) nanocomposites, we have shown that they exhibit low r2/r1 ratio at 4.8, making them good T1 positive contrast enhancing agents for MRI. Moreover, the nanocomposites are demonstrated to be excellent photothermal agents in the second near infrared region with a high photothemal conversion efficiency reaching 46%. In addition, these Au NR@PPy@FexO nanocomposites show verylow cytotoxicity. Taken all together, the synthetic method and nanocomposites developed in this study can be of great potential use in T1-MRI and/or infrared thermal imaging-guided photothermal cancer therapeutic applications.

Nano Research DOI (automatically inserted by the publisher) Research Article

1 Introduction

Nanocomposites which combine magnetic (e.g., iron oxide) and optically active plasmonic (e.g., Au) components have received widespread attention in recent years due to their potential applications in biomedical research, such as in magnetic field assisted sensing, for cancer therapies, as drug-delivery vehicles and imaging contrast enhancing agents [1-4]. Such nanocomposites, possessing both magnetization and near infrared (NIR) absorption properties, have the potential to substantially improve current cancer therapeutic effect by enabling simultaneous imaging and therapy (theranostics) [5-7]. In these nanocomposites, the magnetic component can act as magnetic resonance imaging (MRI) agent for cancer diagnosis, due to its high magnetic relaxivity and contrast enhancing effect [8]; meanwhile, the optically active component with its surface plasmon resonance (SPR) characteristics has been found to be of potential use in bio-labeling and photothermal therapy applications [9-13]. For best imaging and therapeutic effect, the optical absorption of the nanocomposites needs to be tuned into the NIR region, where the light absorption and scattering is the least through the tissue [14]. Thus, a tailor-made combination of magnetic and optically active plasmonic components is urgently needed for the MRI-guided photothermal nanotheranositics applications.

MRI is one of the most powerful medical diagnosis tools because it can provide excellent anatomical details based on the soft tissue contrast and functional information in a noninvasive and real-time monitoring manner [15-21]. The MRI contrast enhancing agents are generally categorized according to their effects on protons’ longitudinal (T1) and transversal (T2) relaxations. T1 contrast enhancing agent provides a bright signal compared to the dark background in T1-weighted MRI, whereas T2 contrast enhancing agent results in a negative contrast in T2-weighted MRI. However, the intrinsic dark signal in T2-weighted MRI can sometimes mislead the clinical diagnosis, due to the labeled tumor areas that are often

confused with other hypointense areas such as bleeding, calcification, or metal deposition [16, 22].

Therefore, T1 contrast enhancing agent can be more desirable than T2 agent for the accurate high-resolution imaging. Paramagnetic compounds including Gd3+, Mn2+, and Fe3+ chelates with large number of unpaired electrons can be used as T1

contrast enhancing agents [23-32]. It has been proven that the accessibility of water molecules to the paramagnetic centers is the critical factor for enhancing the resolution and signal-to-noise ratio of T1-MRI. Although Gd3+ and Mn2+ complexes are widely used as T1 contrast enhancing agent, they have several disadvantages in clinical settings including potential cytotoxicity and short circulating time due to rapid excretion through urine [33, 34]. In addition, they are difficult to be functionalized for active targeting purposes. Therefore, more biofriendly nanoprobes with easy surface functionalization as substitutes for Gd and Mn based T1-MRI contrast enhancing agents are critically needed.

Iron oxide is more biocompatible than gadolinium- or manganese-based materials by virtue of the rich iron species present in the body. However, commonly prepared iron oxide nanoparticles are not appropriate for T1-MRI, due to the high r2/r1 ratio (>20) derived from their intrinsic high magnetic moment. Therefore, current numerous material systems such as organic, inorganic, and inorganic/organic iron oxide hybrid nanoparticles have all been developed into the T2-MRI contrast enhancing agents [35-40]. Fortunately, this problem can be solved by decreasing the physical size of the magnetic nanoparticles [41]. Recently, it has been demonstrated that iron oxide nanoparticles with ultra small sizes can be a good candidate as T1

agent with low toxicity, likely due to reduced magnetic moment and enhanced accessibility of Fe paramagnetic centers to water molecules [42-45]. However, a general method to simultaneously integrate optically active plasmonic component and iron oxide into a multifunctional theranostic platform for T1-MRI imaging guided photothermal therapy has not yet been demonstrated to our best knowledge.

Herein, we report a facile but efficient route developed for constructing an optically active

plasmonic core/iron oxide shell structured nanocomposites, in which Au nanocrystals of

Address correspondence to Jiang Jiang, email [email protected]


3Nano Res.

varying shapes or transition metal selenide semiconductors (Cu2-xSe) act as the core, an thin conjugated polymer polypyrrole (PPy) works as the mid cohesive layer, and iron oxide as an outer nanoshell (e.g., Au@PPy@FexO). For example, with Au nanorod (Au NR) as the core material, a conjugated polymer (PPy) was used to encapsulate Au NRs and obtain Au NR@PPy first by surfactant-assisted chemical oxidative polymerization. Then, a large number of iron oxide nanocrystals were formed on the surface of Au NR@PPy nanoparticles, in the presence of Fe2+ and Fe3+ under alkaline conditions. The resulting Au NR@PPy@FexO nanocomposites exhibit excellent NIR photothermal conversion efficiency (46%) and biocompatibility, making them promising theranostic agents for simultaneous T1-MR imaging and photothermal therapy.

2 Experimental Section

2.1 Materials. Cetyltrimethylammonium bromide (CTAB, 99%), polyvinyl alcohol (PVA, 98% hydrolyzed, average molecular weight 105,000), pyrrole (99%), iron (III) chloride hexahydrate (FeCl3·6H2O, 99%) were obtained from Aladdin (Shanghai, China). L-ascorbic acid, dodecyl sodium sulfate (SDS, 99%), ammonium hydroxide (NH4OH, 28.0−30.0%), sodium borohydride (NaBH4, 96%), chloroauric acid tetrahydrate (HAuCl4·4H2O), silver nitrate (AgNO3), ferrous sulfate (FeSO4·7H2O, 99%), hydrochloric acid (HCl, 37%), sodium oleate (NaOL, 99%), and dimethylsulfoxide (DMSO, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized (DI) water was used throughout the experiments. All chemicals were used as received without further purification. 2.2 Preparation of gold nanorods (Au NRs). Au NRs were synthesized by a seed-mediated method as previously described [46] with some minor modifications. In a typical synthesis, CTAB-modified gold seeds were first synthesized by chemical reduction of HAuCl4 with NaBH4. 250 µL of 10 mM HAuCl4 was added to a freshly prepared 9.75 mL of 0.1 M CTAB solution. Then, 0.6 mL of fresh ice-cold 0.01 M NaBH4 was injected to the Au (III)-CTAB solution under vigorous stirring (1200 rpm). The solution color changed from yellow to brownish yellow and the stirring was stopped after 2 min. This seed solution was aged at

room temperature for 1 h before use. To prepare the growth solution, 0.7 g of CTAB and 0.1234 g of NaOL were dissolved in 25 mL of warm water (~ 50 °C). The solution was allowed to cool down to 30 °C and 480 µL of 0.02 M AgNO3 solution was added. The mixture was kept undisturbed at 30 °C for 15 min after which 25 mL of 1 mM HAuCl4 solution was added. The solution became colorless after 90 min of stirring (700 rpm) and 300 µL of HCl (37 wt. % in water, 12.1 M) was then introduced to adjust the solution pH. After another 15 min of slow stirring at 400 rpm, 80 µL of 0.1 M ascorbic acid (AA) was added and the solution was vigorously stirred for 30 s. Finally, 40 µL of as-synthesized seeds solution was injected into the growth medium. The resultant mixture was stirred for 30 s and left undisturbed at 30°C for 12 h for NR growth. The final products were isolated by centrifugation at 9000 rpm for 20 min, followed by removal of the supernatant. After discarding the supernatant, the precipitate was dispersed in 20 mL of DI water for the next step to synthesize the polypyrrole-coated Au NRs hybrid nanomaterials. 2.3 Preparation of gold nanorod@polypyrrole (Au NR@PPy) nanocomposites. As soon as the Au NRs were synthesized, 5 mL of aqueous solution of Au NRs, 300 µL of 40 mM SDS aqueous solution, and 200 µL of pyrrole monomer were mixed together under vigorous stirring. Subsequently, 3 mL of 2 mM acidic (NH4)2S2O8 solution was added. After being stirred for 10 s, the reaction mixture was incubated at room temperature for 2 h to complete the oxidative polymerization, with the color of mixture turned to black. The PPy-coated NRs were purified by centrifugation at 9000 rpm for 10 min three times. After the removal of the supernatant, the concentrated PPy-coated Au NRs solution was collected at the bottom of the centrifuge tube. 2.4 Preparation of gold nanorod/polypyrrole@iron oxide (Au NR@PPy@FexO) nanocomposites. The Au NR@PPy nanocomposites solution obtained in the above step were diluted with 2 mL DI water and 3 mL PVA (1%) aqueous solution under N2 atmosphere with vigorous stirring for 0.5 h. Then, 15 µL of 0.214 mol/L FeCl3 and 0.107 mol/L FeCl2 mixture solution was added under N2 atmosphere with vigorous stirring. After 0.5 h, 300 µL of NH4OH (9%) solution was quickly injected to the reaction solution at 70 ºC under N2 atmosphere for another 1 h. The final products were purified by centrifuging at 9500 rpm for 10 min twice. The


4 Nano Res.

supernatant was discarded and the precipitate was redispersed in water under ambient conditions. 2.5 Synthesis of other plasmonic core nanomaterials. Au nanoparticles (d ~ 17 nm) [47], Au nanostars (d ~ 40 nm) [48], Au triangular nanoplates (d ~ 65 nm) [49], and Cu2-xSe nanoparticles (d ~ 30 nm) [50] were prepared separately following the published literature procedures. 2.6 Synthesis of other cores@PPy@FexO core−shell nanocomposites. 5 mL of the aqueous solution of Au nanoparticles, Au nanostars, Au triangular nanoplates, or Cu2-xSe nanoparticles, 300 µL aqueous solution of 40 mM SDS, and 200 µL of pyrrole monomer were mixed together under vigorous stirring. Subsequently, 3 mL of 2 mM acidic (NH4)2S2O8 solution was added. After being stirred for 10 s, the reaction mixture was incubated at room temperature for 2 h to finish the oxidative polymerization process. Then, these PPy coated nanostructures were purified by centrifugation at 9000 rpm for 10 min three times. After removing the supernatant, the concentrated PPy-coated nanocomposites solutions were used to grow iron oxide nanoshells in the same way as described in section 1.4 above. 2.7 Characterization. The morphology and size of the synthesized nanocomposites were characterized by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM), using a FEI Technai G2 S-Twin at an acceleration voltage of 200 kV. The sizes of all as-prepared nanomaterials were determined from TEM micrographs using Image J (V1.41, NIH, USA) for image analysis. The elemental components were analyzed by an EDX analyzer as TEM accessory. UV-vis-NIR spectra were recorded on a Lambda-25 spectrophotometer (PerkinElmer Inc., USA) at room temperature under ambient conditions. Au and Fe concentrations of the nanocomposite solution were measured by an Inductively Coupled Plasma Optical Emission Spectrometer (PerkinElmer ICP-OES 2100 DV). MRI relaxation properties of Au NR@PPy@FexO nanocomposites (0.054 - 0.216 mM Fe) were scanned under a 0.5 T NMR-analyzer (Shanghai Huantong Corporation, GY-PNMR-10). The measurement parameters were set as follows: repetition time (TR) = 500 ms; echo time (TE) = 30 ms; number of excitations (NEX) =1. The relaxation rates R1 and R2 were calculated by a linear fit of the

1/T1 and 1/T2 relaxation time versus the Fe concentration (mM). MTT assay was conducted using a Biotek Elx 800 Microplate Reader. 2.8 Photothermal performance of Au NR@PPy@FexO nanocomposites in aqueous solution. To evaluate the photothermal heating performances of the as-synthesized Au NR@PPy@FexO nanocomposites, 1.5 mL of aqueous dispersion of Au NR@PPy@FexO of different concentrations were put in a quartz cuvette, and exposed to a 1064 nm laser at different power densities for 5 min. A thermocouple probe with an accuracy of 0.1 °C was inserted into the nanocomposites solutions perpendicular to the laser path to avoid direct light irradiation on the probe. The temperature was recorded every 20 s by a digital thermometer. 2.9 Cytotoxicity assay of Au NR@PPy@FexO nanocomposites. The cytotoxicity of the nanocomposites was evaluated on neural stem cells (NSC) by MTT assay. Briefly, the NSC cells were seeded into 96-well plates at the density of 1×104 cells per well and cultured for 24 h to allow cells to attach at 37 °C in a humidified incubator, in which the CO2 level was maintained constant at 5%. Afterward, the original culture medium was changed to fresh medium, or Au NR@PPy@FexO dispersions in medium at the desired concentrations (50, 100, 150, and 200 µM Au, respectively) for 24 h at 37 °C. Subsequently, the medium was removed and replaced by 10 µL MTT regent in 100 µL of serum-free medium. After being incubated for additional 4 h at 37 °C, the absorbance of each well at 490 nm was measured using a microplate reader. The relative cell viability was calculated by the formula 100%´Atest/Acontrol, with three replicates carried out for each treatment group, and Atest and Acontrol are the absorbance of cell treated with and without Au NR@PPy@FexO nanocomposites, respectively.

3 Results and discussion

3.1 Synthesis Au NR@PPy@FexO nanocomposites. The strategy of constructing Au NR@PPy@FexO

nanocomposites was through surfactant-assisted chemical oxidative polymerization of pyrrole on the surface of Au NRs, and subsequent formation of iron oxide nanocrystals at the outer shell of the Au NR@PPy nanocomposite in the presence of Fe2+ and Fe3+ under alkaline conditions, as schematically


5Nano Res.

illustrated in Scheme 1. Specifically, anionic surfactant SDS was first absorbed onto the CTAB layers on Au NRs through electrostatic interactions. Then, the pyrrole monomers introduced into the aqueous solution were trapped within the surfactant layers. Followed by the addition of an oxidant, ammonium persulfate ((NH4)2S2O8), the polymerization of pyrrole occurred, resulting the coating of a PPy shell onto the surface of Au NRs. In this polymerization process, surfactant SDS served as a stabilizing agent, preventing Au NRs aggregation and allowing single NR encapsulation upon polymerization [51-53]. For the subsequent

Scheme 1 Schematic diagram illustrating the synthetic procedures of Au NR@PPy@FexO nanocomposites. experiments of forming iron oxide on Au NR@PPy, an aqueous solution containing Fe3+, PVA, and Fe2+ were added into the system under the protection of N2. The formed Au NR@PPy nanocomposites can be modified by PVA, which functioned as a capping agent to make the particles highly hydrophilic [54-56]. Then, an ammonia solution was dropped into the system with moderate heating at 70 °C to coprecipitate the Fe3+ and Fe2+ ions, forming small iron oxide nanocrystals on the Au NR@PPy nanocomposites.

Another way to coat PPy on Au NRs is by Fe3+-mediated oxidative polymerization [56-59]. Compared to the method of using ammonium persulfate as the oxidant, the polymerization process of using Fe3+ as oxidant was much faster and therefore difficult to control. Moreover, we found that the surfactant SDS has an effect on the formation of Au NR@PPy nanoparticles prepared by Fe3+-mediated oxidative polymerization, where the excess surfactant SDS can act as a soft template for free PPy formation. By increasing the added amount of SDS, the number of free PPy increased in the solution, along with a clear damping of the SPR absorption from Au NRs (Figure S1 in the Electronic Supplementary Material (ESM)). Furthermore, a large amount of ferric and ferrous ions remained in the obtained Au NR@PPy solution

from the starting Fe3+ and its reduced form. When ammonia solution was added directly into the above Au NR@PPy solution without centrifugation treatment, iron oxide nanoparticles can form in-situ both on the Au NR@PPy nanocomposites and in solution (Figure S2 in the ESM), likely due to the large amount of free PPy formed in the reaction solution. One concern on using Fe3+-mediated polymerization is the oxidative etching exerted by Fe3+ on Au NRs. To check whether the strong damping observed in Figure S1d is due to this effect, the optical extinction spectra of the prepared Au NRs before and after Fe3+ treatment have been measured and compared (Figure S3 in the ESM). Results show that there is a slight decrease in intensity and blue shift of the longitudinal SPR peak, indicating the chemical etching effect of Fe3+ on Au NRs. However, based on the magnitude of this change, the Fe3+ role on the large damping of Au NRs SPR can be ruled out.

The excess PPy in solution can be removed by centrifuging the Au NR@PPy dispersion. By performing colorimetric determination of iron species, we found there was only a small amount of ferric ions and no ferrous ions left after centrifugation (Figure S4 in the ESM). Therefore, by adding a small amount of Fe2+ under alkaline condition, the iron oxide nanocrystals can form on the outer shell of the Au NR@PPy nanocomposites (Figure S5 in the ESM), eliminating free iron oxides formation in solution. However, due to fast polymerization when Fe3+ was used as oxidant, neat PPy encapsulated Au NRs cannot be obtained (only thicker and irregular PPy).

We also investigated the effect of concentration of ferric and ferrous ions on the formation of the nanocomposites. With the concentration of ferric and ferrous ions increased, the TEM image contrast of the shell in the nanocomposites became darker, due to larger amount of deposited iron oxides. When the concentration of ferric and ferrous ions was high enough, free iron oxides became clearly evident in the reaction mixture (Figure S6 in the ESM). Therefore, the concentration of ferric and ferrous ions should be kept below an optimal concentration in order to avoid the formation of free iron oxide nanocrystals.

It has been demonstrated that PPy, a conducting polymer, can support the adhesion and growth of different cell types and does not irritate the immune system of mammalians in some previous


6 Nano Res.

studies, due to its high conductivity and good biocompatibility [59-61]. In this study, the thin PPy shell has served as a cohesive layer to facilitate small iron oxides coating on the Au NRs. In the absence of PPy layer, iron oxides can still grow along the outer contour of the Au NRs, albeit in a very uneven manner (Figure S7 in the ESM). 3.2 Characterizations of Au NR@PPy@FexO nanocomposites.

As discussed above, under optimized experimental conditions, uniform Au NR@PPy@FexO nanocomposites exhibiting a clear core-shell structure were successfully synthesized as shown in Figure 1a and b. The size of the Au NR core is 16 ´ 92 nm, PPy coating is ~18 nm, and the iron oxide nanoparticles display a very small size (difficult to identify by TEM images). STEM image clearly shows that FexO NPs are distributed along the outer rim of the polymer shell, judging from the image contrast (Figure 1c) as well as the elemental line profiles (Figure 1d). The elemental composition of the as-synthesized nanocomposites over a larger area was determined using energy dispersive X-ray spectroscopy (EDS) (Figure 1e), further confirming the existence of Fe. XRD patterns of the nanocomposites only display Au diffraction peaks (Figure S8 in the ESM), indicating either very small or amorphous iron oxide nanoparticles.

Figure 1 (a, b) TEM images at different magnifications of the as-synthesized Au NR@PPy@FexO nanocomposites. (c) STEM image of the Au NR@PPy@FexO nanocomposites and

(d) the EDS elemental line scanning profiles for Au and Fe, showing that the Fe element existed in the nanocomposites and was distributed along the outer rim of the polymer shell. (e) Bulk EDS analysis of Au NR@PPy@FexO nanocomposites showing the presence of Au and Fe (Cu from TEM grid). (f) UV-vis NIR absorption spectra of the Au NRs, Au NR@PPy, and Au NR@PPy@FexO nanocomposites.

Next, the UV-vis spectroscopy was conducted to analyze the surface plasmon resonance feature of the Au NRs, the Au NR@PPy, and the Au NR@PPy@FexO nanocomposites. As shown in Figure 1f, the LSPR of Au NRs was centered at ~980 nm. This peak was red-shifted by 100 nm after PPy shell coating as a result of changing dielectric environment. The Au NR@PPy@FexO exhibited a broader and further damped LSPR peak extending from the visible to the NIR region with its maximum at nearly 1100 nm. 3.3 MR Relaxivity Measurement.

Relaxivity of the as-synthesized Au NR@PPy@FexO nanocomposites was examined to evaluate their potential use as the MR contrast enhancing agents. Figure 2a and 2b show the

Figure 2 The T1 (a) and T2 (b) relaxation rate of the Au NR@PPy@FexO nanocomposites as a function of the iron concentration. longitudinal relaxation rates (1/T1) and the transverse relaxation rates (1/T2) of protons in Au NR@PPy@FexO aqueous solution as a function of iron concentration at 0.5 T magnetic field. The longitudinal relaxivity (r1, the longitudinal relaxation rate per mM of iron) and transverse relaxivity (r2, the transverse relaxation rate per mM of iron) of the Au NR@PPy@FexO are calculated to be 1.07 mM-1 S-1 and 5.1 mM-1 S-1, respectively. The r2/r1 ratio of the Au NR@PPy@FexO nanocomposites is 4.8. The measured relaxivity of 1.07 mM-1 S-1 is comparable to recently reported FeOOH-silica nanocomposite (0.48-2.12 mM-1 S-1) by Chou and coworkers [62]. Their relatively low r2/r1 value demonstrates that Au NR@PPy@FexO has the potential to be utilized as a good T1-weighted positive contrast enhancing agent for MRI. We also


7Nano Res.

investigated the MR relaxivity of the Au NR@PPy@FexO nanocomposites with different iron oxides densities (samples shown in Figure S6b and S6d). It was found that the r1 and r2 values do not depend on the amount of iron oxide nanocrystals in the nanocomposites (Figure S9 in the ESM). In contrast, larger FexO on Au NR@PPy showed much higher r2/r1 value (~9), making them good T2-weighted negative contrast enhancing agent for MRI, with no apparent FexO NP density dependence observed as well (Figure S10 in the ESM).

Our results are significantly different from what has been first reported by Han et al [58], and more recently by He and coworkers [57] on forming iron oxide nanocrystals on PPy NPs or shells. The iron oxide synthesized here has a much smaller dimension, making them better suited for T1-MRI contrast enhancing [42-45]. The small nanocrystal dimension is a result of the experimental protocol employed here, in particular, the use of thin PPy layer prepared by ammonium persulfate oxidative polymerization and the addition of PVA. 3.4 Photothermal performance of Au NR/PPy@FexO nanocomposites.

Photothermal therapy is an emerging technique that makes use of photothemal agents converting photons into local heat, thus causing cancer cell death at the target tumor sites. The photothermal properties of the Au NR@PPy@FexO nanocomposites were first studied by monitoring the temperature change of 1.5 mL of Au NR@PPy@FexO nanocomposites solution at various concentrations (50-200 µM Au) under the irradiation of a 1064 nm cw laser. At laser power density of 1 W/cm2, with the nanocomposites concentration increasing, the solution temperature elevation increased from 10 to 16.5 °C after 5 min laser irradiation, while pure water showed little heating effect at the same power density (Figure 3a). The final temperature rise at the end of the laser irradiation period as a function of the nanocomposites concentration is plotted in Figure 3b. Temperature elevation profile flattens out with increasing nanocomposites concentrations, due to the logarithmic absorbance dependence on the fraction of incident radiation. When the nanocomposites concentration was fixed at 200 µM, it is obvious that the photothermal effect of the nanocomposites was strongly laser power dependent with the highest temperature elevation

up to 16.5 °C obtained at 1 W/cm2 (Figure 3c). These results indicate that the Au NR@PPy@FexO nanocomposites can rapidly and efficiently convert the energy from 1064 nm laser into

Figure 3 (a) Temperature elevation curves of plain water (0 µM) and Au NR@PPy@FexO nanocomposites aqueous solutions at different concentrations (50-200 µM) under laser irradiation. (b) Plot of temperature change versus concentration of Au NR@PPy@FexO nanocomposites after exposing to laser for 5 min. (c) Temperature elevation profiles of 200 µM Au NR@PPy@FexO nanocomposites aqueous solutions under different laser power densities. (d) The monitored temperature profile of Au NR@PPy@FexO nanocomposites aqueous solution (200 µM) irradiated by laser for 600 s, followed by natural cooling with laser light turned off. Unless otherwise noted, a 1064 nm laser was used at a power densities of 1 W/cm2, and the error bars represent standard deviations (n = 3). thermal energy. In all of our solution photothermal measurements, the nanocomposites solutions were exposed to laser three times each and no large variation of the temperature change was observed, indicating good photostability of the Au NR@PPy@FexO nanocomposites.

To further quantify the photothermal transduction efficiency, we recorded the temperature change of the sample solution with the concentration of 200 µM as a function of time under the 1064 nm laser continuous irradiation for 600 s at the power density of 1 W/cm2 (Figure 3d). The nanocomposites dispersion were illuminated until reaching a steady state temperature, at which point the light source was removed, and the solution was allowed to cool down naturally. During this cooling stage, the temperature decrease was monitored to determine the rate of heat dissipation from the system to the environment, which was obtained by fitting the measured


8 Nano Res.

cooling curve (Figure S11 in the ESM). According to the obtained data and the reported method [63, 64], the photothemal conversion efficiency of the Au NR@PPy@FexO nanocomposites can reach 46%. As Au NRs are reported to have a lower photothermal transduction efficiency at ~25%, experiments on Au NR and Au NR@PPy were also conducted. They were found to exhibit lower conversion efficiency at 28.6% (Figure S12 in the ESM) and 37.5% (Figure S13 in the ESM) respectively. Furthermore, black FexO colloidal solution was also checked for its photothermal performance, and the measured conversion efficiency is 36.5% (Figure S14 in the ESM). These results clearly demonstrated that the Au NR@PPy@FexO nanocomposites are highly efficient and photostable photothemal agents. 3.5 Cytotoxicity of Au NR@PPy@FexO nanocomposites.

Au, PPy, and iron oxide are all known to be biocompatible with little cytotoxicity. The cytotoxicity of the synthesized Au NR@PPy@FexO nanocomposites was evaluated on the neural stem cells (NSC) using standard MTT assay (Figure S15 in the ESM). The NSC cells incubated with Au NR@PPy@FexO nanocomposites did not result in any obvious cytotoxicity over a wide concentration range (0-200 µM Au) for 24 h. The percentages of viable cells for NSC were 82.0 ± 8.4 %, after being exposed to the nanocomposites with the concentration at 200 µM Au for 24 h, demonstrating good biocompatibility of the Au NR@PPy@FexO nanocomposites, showing their great potential to be used as biocompatible theranostic agents. 3.6 General method for coating iron oxides on different cores.

PPy coating and subsequent growth of iron oxides shell on nanocrystals is a facile and cost-effective method, which is also independent of the geometry and composition of the core materials and surface properties. Using the thin polymeric PPy shell as cohesive layer, we extended the coating of iron oxides onto a variety of different materials. For example, we have successfully fabricated Au triangular-plate@PPy@FexO (Figure 4a, e), Au nanostar@PPy@FexO (Figure 4b, f), and Au nanoparticle@PPy@FexO (Figure 4c, g) nanocomposites by using various shapes of Au core. In addition, semiconductor Cu2-xSe was also prepared to form Cu2-xSe@PPy@FexO

nanocomposites (Figure 4d, h). These as-synthesized nanocomposites with both optically active plasmonic core and magnetic shell have the potential to be used in the future

Figure 4 TEM images of core@PPy@FexO nanocomposites with different core materials: (a, e) Au triangular nanoplates; (b, f) Au nanostars; (c, g) Au nanoparticles; (d, h) Cu2-xSe nanoparticles. application as both magnetic resonance imaging and photothemal therapy agents for realizing simultaneous imaging and therapeutic administrations.

4 Conclusions

In summary, we have developed a facile and general strategy that enables formation of small iron oxide nanocrystals on a variety of different materials cores with the help of the thin inner-layer PPy as cohesive layer. The obtained Au NR@PPy@FexO nanocomposites were found to be effective photothermal agents, exhibiting good dispersity, intense NIR absorption, photostability, and high photothermal conversion efficiency. Additionally, we demonstrated that these nanocomposites can be used as competent contrast agent for T1-MRI imaging. Furthermore, the nanocomposites show very low cytotoxicity, which can potentially be used as biocompatible multifunctional photothermal agents. Therefore, we anticipate that the nanocomposites developed in this study have great potential use in T1-MRI and/or infrared thermal imaging-guided photothermal cancer therapeutic applications.

Acknowledgements

This work is funded by the “Hundred Talents” program of Chinese Academy of Sciences, and Natural Science Foundation of China (grant no. 21175148 and 21473243).


9Nano Res.

Electronic Supplementary Material: Supplementary material (Au NR@PPy@FexO prepared under different conditions, Fe3+ etching effect on Au NR, XRD patterns, relaxivity measurements, NP cytotoxicity, results for photothermal transduction efficiencies of Au NR, Au NR@PPy and FexO) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher). References [1] Dong, W.;Li, Y.;Niu, D.;Ma, Z.;Gu, J.;Chen, Y.;Zhao,

W.;Liu, X.;Liu, C.; Shi, J. Facile Synthesis of Monodisperse Superparamagnetic Fe3O4 Core@hybrid@Au Shell Nanocomposite for Bimodal Imaging and Photothermal Therapy. Adv. Mater. 2011, 23, 5392-5397.

[2] Xu, C.;Wang, B.; Sun, S. Dumbbell-like Au-Fe3O4 Nanoparticles for Target-Specific Platin Delivery. J. Am. Chem. Soc. 2009, 131, 4216-4217.

[3] Bardhan, R.;Chen, W.;Bartels, M.;Perez-Torres, C.;Botero, M. F.;McAninch, R. W.;Contreras, A.;Schiff, R.;Pautler, R. G.;Halas, N. J., et al. Tracking of Multimodal Therapeutic Nanocomplexes Targeting Breast Cancer in Vivo. Nano Lett. 2010, 10, 4920-4928.

[4] Lal, S.;Clare, S. E.; Halas, N. J. Nanoshell-Enabled Photothermal Cancer Therapy: Impending Clinical Impact. Acc. Chem. Res. 2008, 41, 1842-1851.

[5] Wang, C.;Chen, J.;Talavage, T.; Irudayaraj, J. Gold Nanorod/Fe3O4 Nanoparticle "Nano-Pearl-Necklaces" for Simultaneous Targeting, Dual-Mode Imaging, and Photothermal Ablation of Cancer Cells. Angew. Chem. Int. Ed. 2009, 48, 2759-2763.

[6] Liong, M.;Lu, J.;Kovochich, M.;Xia, T.;Ruehm, S. G.;Nel, A. E.;Tamanoi, F.; Zink, J. I. Multifunctional inorganic nanoparticles for imaging, targeting, and drug delivery. ACS Nano 2008, 2, 889-896.

[7] Larson, T. A.;Bankson, J.;Aaron, J.; Sokolov, K. Hybrid plasmonic magnetic nanoparticles as molecular specific agents for MRI/optical imaging and photothermal therapy of cancer cells. Nanotechnology 2007, 18, 325101.

[8] Ma, L. L.;Feldman, M. D.;Tam, J. M.;Paranjape, A. S.;Cheruku, K. K.;Larson, T. A.;Tam, J. O.;Ingram, D. R.;Paramita, V.;Villard, J. W., et al. Small Multifunctional Nanoclusters (Nanoroses) for Targeted Cellular Imaging and Therapy. ACS Nano 2009, 3, 2686-2696.

[9] El-Sayed, I. H.;Huang, X. H.; El-Sayed, M. A. Surface plasmon resonance scattering and absorption of anti-EGFR antibody conjugated gold nanoparticles in cancer diagnostics: Applications in oral cancer. Nano Lett. 2005, 5, 829-834.

[10] Wu, X.;Ming, T.;Wang, X.;Wang, P.;Wang, J.; Chen, J. High-Photoluminescence-Yield Gold Nanocubes: For Cell Imaging and Photothermal Therapy. ACS Nano 2010, 4, 113-120.

[11] Novo, C.;Funston, A. M.;Pastoriza-Santos, I.;Liz-Marzan, L. M.; Mulvaney, P. Spectroscopy and high-resolution microscopy of single nanocrystals by a focused ion beam registration method. Angew. Chem. Int. Ed. 2007, 46, 3517-3520.

[12] Kennedy, L. C.;Bickford, L. R.;Lewinski, N. A.;Coughlin, A. J.;Hu, Y.;Day, E. S.;West, J. L.; Drezek, R. A. A New Era for Cancer Treatment: Gold-Nanoparticle-Mediated Thermal Therapies. Small 2011, 7, 169-183.

[13] Chen, J.;Glaus, C.;Laforest, R.;Zhang, Q.;Yang, M.;Gidding, M.;Welch, M. J.; Xia, Y. Gold Nanocages as Photothermal Transducers for Cancer Treatment. Small 2010, 6, 811-817.

[14] Hirsch, L. R.;Stafford, R. J.;Bankson, J. A.;Sershen, S. R.;Rivera, B.;Price, R. E.;Hazle, J. D.;Halas, N. J.; West, J. L. Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 13549-13554.

[15] Weissleder, R.; Pittet, M. J. Imaging in the era of molecular oncology. Nature 2008, 452, 580-589.

[16] Na, H. B.; Hyeon, T. Nanostructured T1 MRI contrast agents. J. Mater. Chem. 2009, 19, 6267-6273.

[17] Na, H. B.;Song, I. C.; Hyeon, T. Inorganic Nanoparticles for MRI Contrast Agents. Adv. Mater. 2009, 21, 2133-2148.

[18] Jun, Y. W.;Huh, Y. M.;Choi, J. S.;Lee, J. H.;Song, H. T.;Kim, S.;Yoon, S.;Kim, K. S.;Shin, J. S.;Suh, J. S., et al. Nanoscale size effect of magnetic nanocrystals and their utilization for cancer diagnosis via magnetic resonance imaging. J. Am. Chem. Soc. 2005, 127, 5732-5733.

[19] Jun, Y.-W.;Seo, J.-W.; Cheon, A. Nanoscaling laws of magnetic nanoparticles and their applicabilities in biomedical sciences. Acc. Chem. Res. 2008, 41, 179-189.

[20] Huh, Y. M.;Jun, Y. W.;Song, H. T.;Kim, S.;Choi, J. S.;Lee, J. H.;Yoon, S.;Kim, K. S.;Shin, J. S.;Suh, J. S., et al. In vivo magnetic resonance detection of cancer by using multifunctional magnetic nanocrystals. J. Am. Chem. Soc. 2005, 127, 12387-12391.

[21] Gao, J.;Liang, G.;Cheung, J. S.;Pan, Y.;Kuang, Y.;Zhao, F.;Zhang, B.;Zhang, X.;Wu, E. X.; Xu, B. Multifunctional yolk-shell nanoparticles: A potential MRI contrast and anticancer agent. J. Am. Chem. Soc. 2008, 130, 11828-11833.

[22] Lee, N.;Kim, H.;Choi, S. H.;Park, M.;Kim, D.;Kim, H.-C.;Choi, Y.;Lin, S.;Kim, B. H.;Jung, H. S., et al. Magnetosome-like ferrimagnetic iron oxide nanocubes for highly sensitive MRI of single cells and transplanted pancreatic islets. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 2662-2667.

[23] Caravan, P. Strategies for increasing the sensitivity of gadolinium based MRI contrast agents. Chem. Soc. Rev. 2006, 35, 512-523.


10 Nano Res.

[24] Hifumi, H.;Yamaoka, S.;Tanimoto, A.;Citterio, D.; Suzuki, K. Gadolinium-based hybrid nanoparticles as a positive MR contrast agent. J. Am. Chem. Soc. 2006, 128, 15090-15091.

[25] Park, Y. I.;Kim, J. H.;Lee, K. T.;Jeon, K.-S.;Bin Na, H.;Yu, J. H.;Kim, H. M.;Lee, N.;Choi, S. H.;Baik, S.-I., et al. Nonblinking and Nonbleaching Upconverting Nanoparticles as an Optical Imaging Nanoprobe and T1 Magnetic Resonance Imaging Contrast Agent. Adv. Mater. 2009, 21, 4467-4471.

[26] Bridot, J.-L.;Faure, A.-C.;Laurent, S.;Riviere, C.;Billotey, C.;Hiba, B.;Janier, M.;Josserand, V.;Coll, J.-L.;Vander Elst, L., et al. Hybrid gadolinium oxide nanoparticles: Multimodal contrast agents for in vivo imaging. J. Am. Chem. Soc. 2007, 129, 5076-5084.

[27] Rieter, W. J.;Kim, J. S.;Taylor, K. M. L.;An, H.;Lin, W.;Tarrant, T.; Lin, W. Hybrid silica nanoparticles for multimodal Imaging. Angew. Chem. Int. Ed. 2007, 46, 3680-3682.

[28] Na, H. B.;Lee, J. H.;An, K.;Park, Y. I.;Park, M.;Lee, I. S.;Nam, D.-H.;Kim, S. T.;Kim, S.-H.;Kim, S.-W., et al. Development of a T-1 contrast agent for magnetic resonance imaging using MnO nanoparticles. Angew. Chem. Int. Ed. 2007, 46, 5397-5401.

[29] Choi, S.-H.;Na, H. B.;Park, Y. I.;An, K.;Kwon, S. G.;Jang, Y.;Park, M.-h.;Moon, J.;Son, J. S.;Song, I. C., et al. Simple and Generalized Synthesis of Oxide-Metal Heterostructured Nanoparticles and their Applications in Multimodal Biomedical Probes. J. Am. Chem. Soc. 2008, 130, 15573-15580.

[30] An, K.;Kwon, S. G.;Park, M.;Na, H. B.;Baik, S.-I.;Yu, J. H.;Kim, D.;Son, J. S.;Kim, Y. W.;Song, I. C., et al. Synthesis of Uniform Hollow Oxide Nanoparticles through Nanoscale Acid Etching. Nano Lett. 2008, 8, 4252-4258.

[31] Yang, H.;Zhuang, Y.;Hu, H.;Du, X.;Zhang, C.;Shi, X.;Wu, H.; Yang, S. Silica-Coated Manganese Oxide Nanoparticles as a Platform for Targeted Magnetic Resonance and Fluorescence Imaging of Cancer Cells. Adv. Funct. Mater. 2010, 20, 1733-1741.

[32] Kim, T.;Momin, E.;Choi, J.;Yuan, K.;Zaidi, H.;Kim, J.;Park, M.;Lee, N.;McMahon, M. T.;Quinones-Hinojosa, A., et al. Mesoporous Silica-Coated Hollow Manganese Oxide Nanoparticles as Positive T-1 Contrast Agents for Labeling and MRI Tracking of Adipose-Derived Mesenchyrnal Stem Cells. J. Am. Chem. Soc. 2011, 133, 2955-2961.

[33] Penfield, J. G.; Reilly, R. F., Jr. What nephrologists need to know about gadolinium. Nat. Clin. Pract. Nephrol. 2007, 3, 654-668.

[34] Limbach, L. K.;Wick, P.;Manser, P.;Grass, R. N.;Bruinink, A.; Stark, W. J. Exposure of engineered nanoparticles to human lung epithelial cells: Influence of chemical composition and catalytic activity on oxidative stress. Environ. Sci. Technol. 2007, 41, 4158-4163.

[35] Bardhan, R.;Chen, W.;Perez-Torres, C.;Bartels, M.;Huschka, R. M.;Zhao, L. L.;Morosan, E.;Pautler, R.

G.;Joshi, A.; Halas, N. J. Nanoshells with Targeted Simultaneous Enhancement of Magnetic and Optical Imaging and Photothermal Therapeutic Response. Adv. Funct. Mater. 2009, 19, 3901-3909.

[36] Kim, J.;Park, S.;Lee, J. E.;Jin, S. M.;Lee, J. H.;Lee, I. S.;Yang, I.;Kim, J.-S.;Kim, S. K.;Cho, M.-H., et al. Designed fabrication of multifunctional magnetic gold nanoshells and their application to magnetic resonance imaging and photothermal therapy. Angew. Chem. Int. Ed. 2006, 45, 7754-7758.

[37] Wang, L.;Bai, J.;Li, Y.; Huang, Y. Multifunctional nanoparticles displaying magnetization and near-IR absorption. Angew. Chem. Int. Ed. 2008, 47, 2439-2442.

[38] Melancon, M. P.;Lu, W.;Zhong, M.;Zhou, M.;Liang, G.;Elliott, A. M.;Hazle, J. D.;Myers, J. N.;Li, C.; Stafford, R. J. Targeted multifunctional gold-based nanoshells for magnetic resonance-guided laser ablation of head and neck cancer. Biomaterials 2011, 32, 7600-7608.

[39] Ji, X.;Shao, R.;Elliott, A. M.;Stafford, R. J.;Esparza-Coss, E.;Bankson, J. A.;Liang, G.;Luo, Z.-P.;Park, K.;Markert, J. T., et al. Bifunctional gold nanoshells with a superparamagnetic iron oxide-silica core suitable for both MR imaging and photothermal therapy. J. Phys. Chem. C 2007, 111, 6245-6251.

[40] Tian, Q.;Hu, J.;Zhu, Y.;Zou, R.;Chen, Z.;Yang, S.;Li, R.;Su, Q.;Han, Y.; Liu, X. Sub-10 nm Fe3O4@Cu2-xS Core-Shell Nanoparticles for Dual-Modal Imaging and Photothermal Therapy. J. Am. Chem. Soc. 2013, 135, 8571-8577.

[41] Roch, A.;Muller, R. N.; Gillis, P. Theory of proton relaxation induced by superparamagnetic particles. J. Chem. Phys. 1999, 110, 5403-5411.

[42] Taboada, E.;Rodriguez, E.;Roig, A.;Oro, J.;Roch, A.; Muller, R. N. Relaxometric and magnetic characterization of ultrasmall iron oxide nanoparticles with high magnetization. Evaluation as potential T-1 magnetic resonance imaging contrast agents for molecular imaging. Langmuir 2007, 23, 4583-4588.

[43] Tromsdorf, U. I.;Bruns, O. T.;Salmen, S. C.;Beisiegel, U.; Weller, H. A Highly Effective, Nontoxic T-1 MR Contrast Agent Based on Ultrasmall PEGylated Iron Oxide Nanoparticles. Nano Lett. 2009, 9, 4434-4440.

[44] Hu, F.;MacRenaris, K. W.;Waters, E. A.;Liang, T.;Schultz-Sikma, E. A.;Eckermann, A. L.; Meade, T. J. Ultrasmall, Water-Soluble Magnetite Nanoparticles with High Relaxivity for Magnetic Resonance Imaging. J. Phys. Chem. C 2009, 113, 20855-20860.

[45] Shen, L.-h.;Bao, J.-f.;Wang, D.;Wang, Y.-x.;Chen, Z.-w.;Ren, L.;Zhou, X.;Ke, X.-b.;Chen, M.; Yang, A.-q. One-step synthesis of monodisperse, water-soluble ultra-small Fe3O4 nanoparticles for potential bio-application. Nanoscale 2013, 5, 2133-2141.

[46] Ye, X.;Zheng, C.;Chen, J.;Gao, Y.; Murray, C. B. Using Binary Surfactant Mixtures To Simultaneously Improve the Dimensional Tunability and Monodispersity in the Seeded Growth of Gold Nanorods. Nano Lett. 2013, 13, 765-771.


11Nano Res.

[47] Frens, G. Controlled Nucleation for Regulation of Particle-size in Monodispersed Gold Suspensions. Nature 1973, 241, 20-22.

[48] Li, J.;Wu, J.;Zhang, X.;Liu, Y.;Zhou, D.;Sun, H.;Zhang, H.; Yang, B. Controllable Synthesis of Stable Urchin-like Gold Nanoparticles Using Hydroquinone to Tune the Reactivity of Gold Chloride. J. Phys. Chem. C 2011, 115, 3630-3637.

[49] Chen, L.;Ji, F.;Xu, Y.;He, L.;Mi, Y.;Bao, F.;Sun, B.;Zhang, X.; Zhang, Q. High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching. Nano Lett. 2014, 14, 7201-7206.

[50] Lie, S. Q.;Wang, D. M.;Gao, M. X.; Huang, C. Z. Controllable copper deficiency in Cu2-xSe nanocrystals with tunable localized surface plasmon resonance and enhanced chemiluminescence. Nanoscale 2014, 6, 10289-10296.

[51] Xing, S.;Tan, L. H.;Yang, M.;Pan, M.;Lv, Y.;Tang, Q.;Yang, Y.; Chen, H. Highly controlled core/shell structures: tunable conductive polymer shells on gold nanoparticles and nanochains. J. Mater. Chem. 2009, 19, 3286-3291.

[52] Lin, M.;Guo, C.;Li, J.;Zhou, D.;Liu, K.;Zhang, X.;Xu, T.;Zhang, H.;Wang, L.; Yang, B. Polypyrrole-Coated Chainlike Gold Nanoparticle Architectures with the 808 nm Photothermal Transduction Efficiency up to 70%. ACS Appl. Mater. Interfaces 2014, 6, 5860-5868.

[53] Liu, Z.;Ye, B.;Jin, M.;Chen, H.;Zhong, H.;Wang, X.; Guo, Z. Dye-free near-infrared surface-enhanced Raman scattering nanoprobes for bioimaging and high-performance photothermal cancer therapy. Nanoscale 2015, 7, 6754-6761.

[54] Zha, Z.;Yue, X.;Ren, Q.; Dai, Z. Uniform Polypyrrole Nanoparticles with High Photothermal Conversion Efficiency for Photothermal Ablation of Cancer Cells. Adv. Mater. 2013, 25, 777-782.

[55] Wang, Q.;Wang, J.;Lv, G.;Wang, F.;Zhou, X.;Hu, J.; Wang, Q. Facile synthesis of hydrophilic polypyrrole nanoparticles for photothermal cancer therapy. J. Mater. Sci. 2014, 49, 3484-3490.

[56] Hong, J.-Y.;Yoon, H.; Jang, J. Kinetic Study of the Formation of Polypyrrole Nanoparticles in Water-Soluble Polymer/Metal Cation Systems: A Light-Scattering Analysis. Small 2010, 6, 679-686.

[57] Feng, W.;Zhou, X.;Nie, W.;Chen, L.;Qiu, K.;Zhang, Y.; He, C. Au/Polypyrrole@Fe3O4 Nanocomposites for MR/CT Dual-Modal Imaging Guided-Photothermal Therapy: An in Vitro Study. ACS Appl. Mater. Interfaces 2015, 7, 4354-4367.

[58] Tian, Q.;Wang, Q.;Yao, K. X.;Teng, B.;Zhang, J.;Yang, S.; Han, Y. Multifunctional Polypyrrole@ Fe-3 O-4 Nanoparticles for Dual-Modal Imaging and In Vivo Photothermal Cancer Therapy. Small 2014, 10, 1063-1068.

[59] Vaitkuviene, A.;Kaseta, V.;Voronovic, J.;Ramanauskaite, G.;Biziuleviciene, G.;Ramanaviciene, A.; Ramanavicius, A. Evaluation of cytotoxicity of polypyrrole nanoparticles synthesized by oxidative polymerization. J. Hazard. Mater. 2013, 250, 167-174.

[60] Stewart, E. M.;Liu, X.;Clark, G. M.;Kapsa, R. M. I.; Wallace, G. G. Inhibition of smooth muscle cell adhesion and proliferation on heparin-doped polypyrrole. Acta Biomater. 2012, 8, 194-200.

[61] Ramanaviciene, A.;Kausaite, A.;Tautkus, S.; Ramanavicius, A. Biocompatibility of polypyrrole particles: an in-vivo study in mice. J. Pharm. Pharmacol. 2007, 59, 311-315.

[62] Peng, Y.-K.;Liu, C.-L.;Chen, H.-C.;Chou, S.-W.;Tseng, W.-H.;Tseng, Y.-J.;Kang, C.-C.;Hsiao, J.-K.; Chou, P.-T. Antiferromagnetic Iron Nanocolloids: A New Generation in Vivo T1 MRI Contrast Agent. J. Am. Chem. Soc. 2013, 135, 18621-18628.

[63] Roper, D. K.;Ahn, W.; Hoepfner, M. Microscale heat transfer transduced by surface plasmon resonant gold nanoparticles. J. Phys. Chem. C 2007, 111, 3636-3641.

[64] Ding, X.;Liow, C. H.;Zhang, M.;Huang, R.;Li, C.;Shen, H.;Liu, M.;Zou, Y.;Gao, N.;Zhang, Z., et al. Surface Plasmon Resonance Enhanced Light Absorption and Photothermal Therapy in the Second Near-Infrared Window. J. Am. Chem. Soc. 2014, 136, 15684-15693.

Electronic Supplementary Material

12


Zhongzhen Yang1,2, Xianguang Ding1,2, and Jiang Jiang1 (*)

1Key Laboratory of Nano-Bio Interface, i-lab and Division of Nanobiomedicine, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, China 215123 2Graduate University of Chinese Academy of Sciences, Beijing, China 100049

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Figure S1. TEM images of the Au NR@PPy nanoparticles fabricated by iron cation-mediated oxidative polymerization by adding 300 µL (a), 350 µL (b), and 450 µL (c) of 40 mM SDS. (d) UV-vis NIR absorption spectra of the corresponding Au NR@PPy dispersion a (black line), b (blue line) , and c (red line).

Address correspondence to Jiang Jiang, email [email protected]

13

Figure S2. TEM image of the nanocomposites synthesized by dropping an ammonia solution directly into the PPy coated Au NR solution using iron cation-mediated oxidative polymerization without centrifugation treatment. The inset shows the TEM image of the corresponding individual particle.

Figure S3. The UV-vis absorption spectra of the initial gold nanorods (black) and those treated with 12.1 mM FeCl3 (red) for 4 h. The LSPR peak blue shifted from 972 nm to 951 nm, with slight decrease in peak intensity (~5.5%).

14

Figure S4. Digital photographs of (a) the Au NR@PPy dispersion fabricated by iron cation-mediated oxidative polymerization with centrifugation treatment, after being added with (b) phenanthroline solution (indicator for Fe2+), and (c) KSCN solution (indicator for Fe3+).

Figure S5. (a) TEM image of the nanocomposites synthesized by adding a small amount of Fe2+ and subsequently dropping an ammonia solution directly into the PPy coated Au NRs solution using Fe3+-mediated oxidative polymerization after centrifugation treatment. (b) The corresponding UV-vis-NIR absorption spectrum.

15

Figure S6. TEM images of the Au NR@PPy@FexO nanocomposites synthesized by tuning the amount of the added ferric and ferrous ions at the same amount of Au about 4.2×10-7 mol: (a, e) n(Fe3+) = 1.07×10-6 mol, n(Fe2+) = 0.54×10-6 mol; (b, f) n(Fe3+) = 2.14×10-6 mol, n(Fe2+) = 1.08×10-6 mol; (c, g) n(Fe3+) = 3.21×10-6 mol, n(Fe2+) = 1.62×10-6 mol; (d, h) n(Fe3+)= 4.08×10-6 mol, n(Fe2+)= 2.16×10-6 mol.

Figure S7. TEM images of Au NR@FexO nanoparticles without PPy cohesive layer between Au NR core and iron oxide shell.

16

Figure S8. XRD pattern of Au NR@FexO nanocomposite, displaying no diffraction pattern from FexO.

Figure S9. The T1 and T2 relaxation rate of the Au NR@PPy@FexO nanocomposites as a function of the Fe concentration, which were synthesized by adding different amount of ferric and ferrous ions: (a) n

(Fe3+) = 2.14×10-6 mol, n (Fe2+) = 1.08×10-6 mol; (b) n (Fe3+) = 4.08×10-6 mol, n (Fe2+) = 2.16×10-6 mol.

17

Figure S10. The relaxation time measurements of the Au NR@PPy@FexO nanocomposites fabricated by iron mediated oxidative polymerization with different amount of iron oxide. (a-c) T1 and T2 relaxation rate of the nanocomposites with ferric iron concentration of 6.07×10-3 mol/L, 12.13×10-3 mol/L, and 18.20×10-3 mol/L respectively, as a function of the Fe concentration. (d-f) The corresponding TEM images of the nanocomposites with different ferric iron concentration. The r2/r1 values of the each nanocomposite with different amount of iron oxide are 9.90, 9.45 and 8.99, respectively.

Figure S11. Determination of the time constant for heat transfer from the system using linear regression of the cooling profile shown in Figure 3d. With τs = 252 s obtained by fitting, m is 1.5 g and the C is 4.2 J/g, hS is calculated to be 25.0 mW/°C. With I = 0.785 W and A1064 = 1.71, the photothermal transduction efficiency of Au NR@PPy@FexO is determined to be 46%.

18

Figure S12. Determination of the time constant for heat transfer from the system using linear regression of the cooling profile of Au NR at Au concentration of 200 µM. With τs = 357 s obtained by fitting, m is 1.5 g and the C is 4.2 J/g, hS is calculated to be 17.6 mW/°C. With I = 0.785 W, A1064 = 1.1, the photothermal transduction efficiency is determined to be 28.6%.

Figure S13. Determination of the time constant for heat transfer from the system using linear regression of the cooling profile of Au NR@PPy at Au concentration of 200 µM. With τs = 320 s obtained by fitting, m is 1.5 g and the C is 4.2 J/g, hS is calculated to be 19.7 mW/°C. At I = 0.785 W, A1064 = 1.57, the photothermal transduction efficiency is determined to be 37.5%.

19

Figure S14. Determination of the time constant for heat transfer from the system using linear regression of the cooling profile of bare FexO at Fe concentration of 50 ug/mL. With τs = 220.5 s obtained by fitting, m is 1.5 g and the C is 4.2 J/g, hS is calculated to be 28 mW/°C. Substituting I =0.785 W, A1064 = 0.41, the photothermal transduction efficiency of the bare FexO is determined to be 36.5 %.

Figure S15. Cytotoxicity of Au NR@PPy@FexO nanocomposites, which is tested by incubating neural stem cells in 100 µL culture medium with different concentrations of the nanocomposites for 24 h, and followed by MTT assay. Data are shown as the means ± standard error of the means.

20

Silver Nanowires with Semiconducting Ligands for Low Temperature Transparent Conductors

Brion Bob,1 Ariella Machness,1 Tze-Bin Song,1 Huanping Zhou,1 Choong-Heui Chung,2 and Yang Yang1,*

1 Department of Materials Science and Engineering and California NanoSystems Institute,

University of California Los Angeles, Los Angeles, CA 90025 (USA)

2 Department of Materials Science and Engineering, Hanbat National University, Daejeon

305-719, Korea

Abstract

Metal nanowire networks represent a promising candidate for the rapid fabrication of transparent electrodes with high transmission and low sheet resistance values at very low deposition temperatures. A commonly encountered obstacle in the formation of conductive nanowire electrodes is establishing high quality electronic contact between nanowires in order to facilitate long range current transport through the network. A new system of nanowire ligand removal and replacement with a semiconducting sol-gel tin oxide matrix has enabled the fabrication of high performance transparent electrodes at dramatically reduced temperatures with minimal need for post-deposition treatments of any kind.

Keywords: Silver Nanowires, Sol-Gel, Transparent Electrodes, Nanocomposites

21

1. Introduction. Silver nanowires (AgNWs) are long, thin, and possess conductivity values on the same order of magnitude as bulk silver

(Ag) [1]. Networks of overlapping nanowires allow light to easily pass through the many gaps and spaces between nanowires, while transporting current through the metallic conduction pathways offered by the wires themselves. The high aspect ratios achievable for solution-grown AgNWs has allowed for the fabrication of transparent conductors with very promising sheet resistance and transmission values, often approaching or even surpassing the performance of vacuum-processed materials such as indium tin oxide (ITO) [2-6].

Significant electrical resistance within the metallic nanowire network is encountered only when current is required to pass between nanowires, often forcing it to pass through layers of stabilizing ligands and insulating materials that are typically used to assist with the synthesis and suspension of the nanowires [7, 8]. The resistance introduced by the insulating junctions between nanowires can be reduced through various physical and chemical means, including burning off ligands and partially melting the wires via thermal annealing [9, 10], depositing additional materials on top of the nanowire network [11-14], applying mechanical forces to enhance network morphology [15-17], or using various other post-treatments to improve the contact between adjacent wires [18-21]. Any attempt to remove insulating materials the network must be weighed against the risk of damaging the wires or blocking transmitted light, and so many such treatments must be reined in from their full effectiveness to avoid endangering the performance of the completed electrode.

We report here a process for forming inks with dramatically enhanced electrical contact between AgNWs through the use of a semiconducting ligand system consisting of tin oxide (SnO2) nanoparticles. The polyvinylpyrrolidone (PVP) ligands introduced during AgNW synthesis in order to encourage one-dimensional growth are stripped from the wire surface using ammonium ions, and are replaced with substantially more conductive SnO2, which then fills the space between wires and enhances the contact geometry in the vicinity of wire/wire junctions. The resulting transparent electrodes are highly conductive immediately upon drying, and can be effectively processed in air at virtually any temperature below 300 °C. The capacity for producing high performance transparent electrodes at room temperature may be useful in the fabrication of devices that are damaged upon significant heating or upon the application of harsh chemical or mechanical post-treatments.

2. Results and Discussion

2.1. Ink Formulation and Characterization

Dispersed AgNWs synthesized using copper chloride seeds represent a particularly challenging material system for promoting wire/wire junction formation, and often require thermal annealing at temperatures near or above 200 °C to induce long range electrical conductivity within the deposited network [22, 23]. The difficulties that these wires present regarding junction formation is potentially due to their relatively large diameters compared to nanowires synthesized using other seeding materials, which has the capacity to enhance the thermal stability of individual wires according to the Gibbs-Thomson effect. We have chosen these wires as a demonstration of pre-deposition semiconducting ligand substitution in order to best illustrate the contrast between treated and untreated wires.

Completed nanocomposite inks are formed by mixing AgNWs with SnO2 nanoparticles in the presence of a compound capable of stripping the ligands from the AgNW surface. In this work, we have found that ammonia or ammonium salts act as effective stripping agents that are able to remove the PVP layer from the AgNW surface and allow for a new stabilizing matrix to take its place. Figure 1 shows a schematic of the process, starting from the precursors used in nanowire and nanoparticle synthesis and ending with the deposition of a completed film. The SnO2 nanoparticle solution naturally contains enough ammonium ions from its own synthesis to effectively peel the insulating ligands from the AgNWs and allow the nanoparticles to replace them as a stabilizing agent. If not enough SnO2 nanoparticles are used in the mixture, then the wires will rapidly agglomerate and settle to the bottom as large clusters. Large amounts of SnO2 in the mixture gradually begin to increase the sheet resistance of the nanowire network upon deposition, but greatly enhance the uniformity, durability, and wetting properties of the resulting films. We have found that AgNW:SnO2 weight ratios ranging between 2:1 and 1:1 produce well dispersed inks that are still highly conductive when deposited as films.

The nanowires were synthesized using a polyol method that has been adapted from the recipe described by Lee et al. [22, 23] Silver nitrate dissolved in ethylene glycol via ultrasonication was used as a precursor in the presence of copper chloride and PVP to provide seeds and produce anisotropic morphologies in the reaction products. Synthetic details can be found in the experimental section. Distinct from previous recipes, we have found that repeating the synthesis two times without cooling down the reaction mixture generally produces significantly longer nanowires than a single reaction step. The lengths of nanowires produced using this method fall over a wide range from 15 to 65 microns, with diameters between 125 and 250 nm. This range of diameters is common for wires grown using copper chloride seeds, although the double reaction produces a number of wires with roughly twice their usual diameter. The morphology of the as-deposited AgNWs as determined via SEM is shown in Figure 2(a), higher magnification images are also provided in Figures 2(c) and 2(d).

22

The SnO2 nanoparticles were synthesized using a sol-gel method typical for multivalent metal oxide gelation reactions. A large excess of deionized water was added to SnCl4·5H2O dissolved in ethylene glycol along with tetramethylammonium chloride and ammonium acetate to act as surfactants. The reaction was then allowed to progress for at least one hour at near reflux conditions, after which the resulting nanoparticle dispersion can be collected, washed, and dispersed in a polar solvent of choice. The material properties of SnO2 nanoparticles formed using a similar synthesis method have been reported previously [24], although the present recipe uses excess water to ensure that the hydrolysis reaction proceeds nearly to completion.

After mixing with SnO2 nanoparticles, films deposited from AgNW/SnO2 composite inks show a largely continuous nanoparticle layer on the substrate surface with some nanowires partially buried and some sitting more or less on top of the film. Representative scanning electron microscopy (SEM) images of nanocomposite films are shown in Figure 2(b). Regardless of their position relative to the SnO2 film, all nanowires show a distinct shell on their outer surface that gives them a soft and slightly rough appearance, as is visible in the higher magnification images shown in Figure 2(e) and 2(f). The SnO2 nanoparticles do a particularly good job coating the regions near and around junctions between wires, and frequently appear in the SEM images as bulges wrapped around the wire/wire contact points.

The precise morphology of the SnO2 shell that effectively surrounded each AgNW was analyzed in more detail using transmission electron microscopy (TEM) imaging. Figures 3(a) to 3(c) show individual nanowires in the presence of different ligand systems: as-synthesized PVP in Figure 3(a), inactive SnO2 in Figure 3(b), and SnO2 activated with trace amounts of ammonium ions in Figure 3(c). The as-synthesized nanowires show sharp edges, and few surface features. In the presence of inactive SnO2, which is formed by repeatedly washing the SnO2 nanoparticles in ethanol until all traces of ammonium ions are removed, the nanowires coexist with somewhat randomly distributed nanoparticles that deposit all over the surface of the TEM grid. When AgNWs are mixed with activated SnO2, a thick and continuous SnO2 shell is formed along the nanowire surface. In when sufficiently dilute SnO2 solutions are used to form the nanocomposite ink, nearly all of the nanoparticles are consumed during shell formation and effectively no nanoparticles are left to randomly populate the rest of the image.

As the AgNWs acquire their metal oxide coatings in solution, the properties of the mixture change dramatically. Freshly synthesized AgNWs coated with residual PVP ligands slowly settle to the bottom of their vial or flask over a time period of several hours to one day, forming a dense layer at the bottom. The AgNWs with SnO2 shells do not settle to the bottom, but remain partially suspended even after many weeks at concentrations that are dependent on the amount of SnO2 present in the solution.

A comparison of the settling behavior of various AgNW and SnO2 mixtures after 24 hours is shown in Figures 3(d) and 3(e). The ratios 8:4, 8:16, and 8:8 indicate the concentrations of AgNWs and SnO2 (in mg/mL) present in each solution. The 8:8 uncoupled solution, in which the PVP is not removed from the AgNW surface with ammonia, produces a situation in which the nanowires and nanoparticles do not interact with one another, and instead the nanowires settle as in the isolated nanowire solution while the nanoparticles remain well-dispersed as in the solution of pure SnO2. The mixtures of nanowires and nanoparticles in which trace amounts of ammonia are present do not settle to the bottom, but instead concentrate themselves until repulsion between the semiconducting SnO2 clusters is able to prevent further settling.

Our current explanation for the settling behavior of the wire/particle mixtures is that the PVP coating on the surface of the as-synthesized wires is sufficient to prevent interaction with the nanoparticle solution. The addition of ammonia into the solution quickly strips off the PVP surface coating and allowing the nanoparticles to coordinate directly with the nanowire surface. This explanation is in agreement with the effects of ammonia has on a solution of pure AgNWs, which rapidly begin to agglomerate into clusters and sink to the bottom as soon as any significant quantity of ammonia is added to the ink.

We attribute the stripping ability of ammonia in these mixtures to the strong dative interactions that

occur via the lone pair on the nitrogen atom interacting with the partially filled d-orbitals of the Ag atoms

on the nanowire surface. These interactions are evidently strong enough to displace the existing

coordination of the five-membered rings and carbonyl groups contained in the original PVP ligands and

allow the ammonia to attach directly to the nanowire surface. Since ammonia is one of the original

surfactants used to stabilize the surface of the SnO2 nanoparticles, we consider it reasonable that ammonia

coordination on the nanowire surface would provide an appropriate environment for the nanoparticles to

adhere to the AgNWs.

23

Scanning Energy Dispersive X-ray (EDX) Spectroscopy was also conducted on nanoparticle-coated AgNWs in order to image the presence of Sn and Ag in the nanowire and shell layer. The line scan results are shown in Figure 3(f), having been normalized to better compare the widths of the two signals. The visible broadening of the Sn lineshape compared to that of Ag is indicative of a Sn layer along the outside of the wire. The increasing strength of the Sn signal toward the center of the AgNW is likely due to the enhanced interaction between the TEM’s electron beam and the dense AgNW, which then improves the signal originating from the SnO2 shell as well. It is also possible that there is some intermixing between the Ag and Sn x-ray signals, but we consider this to be less likely as the distance between their characteristic peaks should be larger than the detection system’s energy resolution.

2.2. Network Deposition and Device Applications

For the deposition of transparent conducting films, a weight ratio of 2:1 of AgNWs to SnO2 nanoparticles was chosen in order to obtain a balance between the dispersibility of the nanowires, the uniformity of coated films, and the sheet resistance of the resulting conductive networks. Nanocomposite films were deposited on glass by blade coating from an ethanolic solution using a scotch tape spacer, with deposited networks then being allowed to dry naturally in air over several minutes.

The as-dried nanocomposite films are highly conductive, and require only minimal thermal treatment to dry and harden the film. Without the use of activated SnO2 ligands, deposited nanowire networks are highly insulating, and become conductive only after annealing at above 200 °C. The sheet resistance values of representative films are shown in Figure 4(a). The capability to form transparent conductive networks in a single deposition step that remain useful over a wide range of processing temperatures provides a high degree of versatility for designing thin film device fabrication procedures.

Figure 5(a) shows the sheet resistance and transmission of a number of nanocomposite films deposited from inks containing different nanowire concentrations. The deposited films show excellent conductivity at transmission values up to 85%, and then rapidly increase in sheet resistance as the network begins to reach its connectivity limit. The optimum performance of these networks at low to moderate transmission values is a consequence of the relatively large nanowire diameters, which scatter a noticeable amount of light even when the conditions required for current percolation are just barely met. Nonetheless, the sheet resistance and transmission of the completed nanocomposite networks place them within an acceptable range for applications in a variety of optoelectronic devices. Figure 5(b) shows the wavelength dependent transmission spectra of several nanowire networks, which transmit light well out into the infrared region. The presence of high transmission values out to wavelengths well above 1300 nm, where ITO or other conductive oxide layers would typically begin to show parasitic absorption, is due to the use of semiconducting SnO2 ligands, which is complimentary to the broad spectrum transmission of the silver nanowire network itself.

Avoiding the use of highly doped nanoparticles has the potential to provide optical advantages, but can create difficulties when attempting to make electrical contact to neighboring device layers. In order to investigate their functionality in thin film devices, we have incorporated AgNW/SnO2 nanocomposite films as electrodes in amorphous silicon (a-Si) solar cells. Two contact structures were used during fabrication: one with the nanocomposite film directly in contact with the p-i-n absorber structure and one with a 10 nm Al:ZnO (AZO) layer present to assist in forming Ohmic contact with the device. The I-V characteristics of the resulting devices are shown in Figure 6(a).

The thin AZO contact layers typically show sheet resistance values greater than 2.5 kΩ/⧠, and so cannot be responsible for long range lateral current transport within the electrode structure. However, their presence is clearly beneficial in improving contact between the nanocomposite electrode and the absorber material, as the SnO2 matrix material is evidently not conductive enough to form a high quality contact with the p-type side of the a-Si stack. We hope that future modifications to the AgNW/SnO2 composite, or perhaps the use of islands of high conductivity material such as a discontinuous layer of doped nanoparticles will allow for the deposition of completed electrode stacks that provide both rapid fabrication and good performance.

Figure 6(b) contains the top view image of a completed device. The enhanced viscosity of the nanowire/sol-gel composite inks allows for films to be blade coated onto substrates with a variety of surface properties without reductions in network uniformity. In contrast with traditional back electrodes deposited in vacuum environments, the nanocomposite can be blade coated into place in a single pass under atmospheric conditions and dried within moments. We anticipate that the use of sol-gel mixtures to enhance wetting and dispersibility may prove useful in the formulation of other varieties of semiconducting and metallic inks for deposition onto a variety of substrate structures.

3. Conclusions

In summary, we have successfully exchanged the insulating ligands that normally surround as-synthesized AgNWs with shells of substantially more conductive SnO2 nanoparticles. The exchange of one set of ligands for the other is mediated by

24

the presence of ammonia during the mixing process, which appears to be necessary for the effective removal of the PVP ligands that initially cover the nanowire surface. The resulting nanowire/nanoparticle mixtures allow for the deposition of nanocomposite films that require no annealing or other post-treatments to function as high quality transparent conductors with transmission and sheet resistance values of 85% and 10 Ω/⧠, respectively. Networks formed in this manner can be deposited quickly and easily in open air, and have been demonstrated as an effective n-type electrode in a-Si solar cells when a thin interfacial layer is deposited first to ensure good electronic contact with the rest of the device. The ligand management strategy described here could potentially be useful in any number of material systems that presently suffer from highly insulating materials that reside on the surface of otherwise high performance nano and microstructures.

4. Experimental Details

Tin oxide nanoparticle synthesis. Tin chloride pentahydrate was dissolved in ethylene glycol by

stirring for several hours at a concentration of 10 grams per 80 mL to serve as a stock solution. In a typical

synthesis reaction, 10 mL of the SnCl4·5H2O stock solution is added to a 100 mL flask and stirred at room

temperature. Still at room temperature, 250 mg ammonium acetate and 500 mg ammonium acetate were

added in powder form to regulate the solution pH and to serve as coordinating agents for the growing

oxide nanoparticles. 30 ml of water was then added, and the flask was heated to 90 °C for 1 to 2 hours in

an oil bath, during which the solution took on a cloudy white color. The gelled nanoparticles were then

washed twice in ethanol in order to keep trace amounts of ammonia present in the solution. Additional

washing cycles would deactivate the SnO2, and then require the addition of ammonia to coordinate with

as-synthesized AgNWs.

Silver nanowire synthesis. Copper(ii) chloride dihydrate was first dissolved in ethylene glycol at

1 mg/ml to serve as a stock solution for nanowire seed formation. 20 ml of ethylene glycol was then added

into a 100 ml flask, along with 200 µL of copper chloride solution. the mixture was then heated to 150 °C

while stirring at 325 rpm, and .35g of PVP (MW 55,000) was added. In a small separate flask, .25 grams of

silver nitrate was dissolved in 10 ml ethylene glycol by sonicating for approximately 2 minutes, similar to

the method described here.22 The silver nitrate solution was then injected into the larger flask over

approximately 15 minutes, and the reaction was allowed to progress for 2 hours. After the reaction had

reached completion, the various steps were repeated without cooling down. 200 µL of copper chloride

solution and .35g PVP were added in a similar manner to the first reaction cycle, and another .25g silver

nitrate were dissolved via ultrasonics and injected over 15 minutes. The second reaction cycle was allowed

to progress for another 2 hours, before the flask was cooled and the reaction products were collected and

washed three times in ethanol.

Nanocomposite ink formation. After the synthesis of the two types of nanostructures is complete,

25

the double washed SnO2 nanoparticles and triple-washed nanowires can be combined at a variety of weight

ratios to form the completed nanocomposite ink. The dispersibility of the mixture is improved when more

SnO2 is used, although the sheet resistance of the final networks will begin to increase if they contain

excessive SnO2. AgNW agglomeration during mixing is most easily avoided if the SnO2 and AgNW

solutions are first diluted to the range of 10 to 20 mg/ml in ethanol, with the SnO2 solution being added

first to an empty vial and the AgNW solution added afterwards. The dilute mixture was then be allowed to

settle overnight, and the excess solvent removed to concentrate the wires to a concentration that is

appropriate for blade coating.

Film and electrode deposition. The completed nanocomposite ink was deposited onto any desired

substrates using a razor blade and scotch tape spacer. The majority of the substrates used in this study were

Corning soda lime glass, but the combined inks also deposited well on silicon, SiO2, and any other

substrates tested. Electrode deposition onto a-Si substrates was accomplished by masking off the desired

cell area with tape, and then depositing over the entire region. The p-i-n a-Si stacks and 10 nm AZO

contact layers were deposited using PECVD and sputtering, respectively.

ACKNOWLEDGMENTS The authors would like to acknowledge the use of the Electron Imaging Center for Nanomachines

(EICN) located in the California NanoSystems Institute at UCLA.

REFERENCES [1] Sun, Y.; Gates, B.; Mayers, B.; Xia, Y., Crystalline silver nanowires by soft solution

processing. Nano Lett. 2002, 2, 165-168.

[2] Kim, T.; Kim, Y. W.; Lee, H. S.; Kim, H.; Yang, W. S.; Suh, K. S., Uniformly

interconnected silver-nanowire networks for transparent film heaters. Adv. Funct.

Mater. 2013, 23, 1250-1255.

[3] Hu, L.; Wu, H.; Cui, Y., Metal nanogrids, nanowires, and nanofibers for transparent

electrodes. MRS Bull. 2011, 36, 760-765.

26

[4] van de Groep, J.; Spinelli, P.; Polman, A., Transparent conducting silver nanowire

networks. Nano Lett. 2012, 12, 3138-3144.

[5] Yang, L.; Zhang, T.; Zhou, H.; Price, S. C.; Wiley, B. J.; You, W., Solution-processed

flexible polymer solar cells with silver nanowire electrodes. ACS Appl. Mater.

Interfaces 2011, 3, 4075-4084.

[6] Scardaci, V.; Coull, R.; Lyons, P. E.; Rickard, D.; Coleman, J. N., Spray deposition of

highly transparent, low-resistance networks of silver nanowires over large areas. Small

2011, 7, 2621-2628.

[7] Wiley, B.; Sun, Y.; Xia, Y., Synthesis of silver nanostructures with controlled shapes

and properties. Acc. Chem. Res. 2007, 40, 1067-1076.

[8] Korte, K. E.; Skrabalak, S. E.; Xia, Y., Rapid synthesis of silver nanowires through a

cucl- or cucl2-mediated polyol process. J. Mater. Chem. 2008, 18, 437-441.

[9] Anuj, R. M.; Akshay, K.; Chongwu, Z., Large scale, highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application in

touch screens. Nanotechnology 2011, 22, 245201.

[10] Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P., Solution-processed metal nanowire

mesh transparent electrodes. Nano Lett. 2008, 8, 689-692.

[11] Zhu, R.; Chung, C.-H.; Cha, K. C.; Yang, W.; Zheng, Y. B.; Zhou, H.; Song, T.-B.;

Chen, C.-C.; Weiss, P. S.; Li, G.; Yang, Y., Fused silver nanowires with metal oxide

nanoparticles and organic polymers for highly transparent conductors. ACS Nano 2011,

5, 9877-9882.

[12] Chung, C.-H.; Song, T.-B.; Bob, B.; Zhu, R.; Duan, H.-S.; Yang, Y., Silver nanowire

composite window layers for fully solution-deposited thin-film photovoltaic devices.

Adv. Mater. 2012, 24, 5499-5504.

27

[13] Kim, A.; Won, Y.; Woo, K.; Kim, C.-H.; Moon, J., Highly transparent low resistance

zno/ag nanowire/zno composite electrode for thin film solar cells. ACS Nano 2013, 7,

1081-1091.

[14] Ajuria, J.; Ugarte, I.; Cambarau, W.; Etxebarria, I.; Tena-Zaera, R. n.; Pacios, R.,

Insights on the working principles of flexible and efficient ito-free organic solar cells

based on solution processed ag nanowire electrodes. Sol. Energy Mater. Sol. Cells

2012, 102, 148-152.

[15] Tokuno, T.; Nogi, M.; Karakawa, M.; Jiu, J.; Nge, T.; Aso, Y.; Suganuma, K.,

Fabrication of silver nanowire transparent electrodes at room temperature. Nano Res.

2011, 4, 1215-1222.

[16] Lim, J.-W.; Cho, D.-Y.; Jihoon, K.; Na, S.-I.; Kim, H.-K., Simple brush-painting of

flexible and transparent ag nanowire network electrodes as an alternative ito anode for

cost-efficient flexible organic solar cells. Sol. Energy Mater. Sol. Cells 2012, 107,

348-354.

[17] De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.;

Boland, J. J.; Coleman, J. N., Silver nanowire networks as flexible, transparent,

conducting films: Extremely high dc to optical conductivity ratios. ACS Nano 2009, 3,

1767-1774.

[18] Hu, L.; Kim, H. S.; Lee, J.-Y.; Peumans, P.; Cui, Y., Scalable coating and properties of

transparent, flexible, silver nanowire electrodes. ACS Nano 2010, 4, 2955-2963.

[19] Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Greyson Christoforo,

M.; Cui, Y.; McGehee, M. D.; Brongersma, M. L., Self-limited plasmonic welding of

silver nanowire junctions. Nat. Mater. 2012, 11, 241-249.

28

[20] Yu, Z.; Zhang, Q.; Li, L.; Chen, Q.; Niu, X.; Liu, J.; Pei, Q., Highly flexible silver

nanowire electrodes for shape-memory polymer light-emitting diodes. Adv. Mater.

2011, 23, 664-668.

[21] Song, T.-B.; Chen, Y.; Chung, C.-H.; Yang, Y.; Bob, B.; Duan, H.-S.; Li, G.; Tu,

K.-N.; Huang, Y., Nanoscale joule heating and electromigration enhanced ripening of

silver nanowire contacts. ACS Nano 2014, 8, 2804-2811.

[22] Lee, P.; Lee, J.; Lee, H.; Yeo, J.; Hong, S.; Nam, K. H.; Lee, D.; Lee, S. S.; Ko, S. H.,

Highly stretchable and highly conductive metal electrode by very long metal nanowire

percolation network. Adv. Mater. 2012, 24, 3326-3332.

[23] Lee, J. H.; Lee, P.; Lee, D.; Lee, S. S.; Ko, S. H., Large-scale synthesis and

characterization of very long silver nanowires via successive multistep growth. Cryst.

Growth Des. 2012, 12, 5598-5605.

[24] Bob, B.; Song, T.-B.; Chen, C.-C.; Xu, Z.; Yang, Y., Nanoscale dispersions of gelled

Sno2: Material properties and device applications. Chem. Mater. 2013, 25, 4725-4730.

29

Figure 1. Process flow diagram showing the synthesis of AgNWs and SnO2 nanoparticles followed

by stirring in the presence of ammonium salts to create the final nanocomposite ink. Transparent

conducting films were produced by blade coating the completed inks onto the desired substrate.

30

Figure 2. (a,c,d) SEM images of as-synthesized AgNWs at various magnifications. (b,e,f) SEM

images of nanocomposite films, showing the tendency of the SnO2 nanoparticles to coat the entire

outer surface of the AgNWs, increasing their apparent diameter and giving them a soft appearance.

31

Figure 3. Schematic diagrams and TEM images of (a) a single untreated AgNW, (b) an AgNW in the

presence of uncoupled SnO2 (all ammonium ions removed), and (c) an AgNW with a coordinating

SnO2 shell. Scale bars in images (a), (b), and (c) are 300 nm, 400 nm, and 600 nm, respectively. (d,e)

Optical images of AgNW and SnO2 nanoparticle dispersions mixed in varying amounts (d) before and

(e) after settling for 24 hours. The numbers associated with each solution represent the AgNW:SnO2

concentrations in mg/ml. The uncoupled solution contains AgNWs and non-coordinating SnO2

nanoparticles, and shows settling behavior similar to the pure AgNW and pure SnO2 solutions. (f)

Normalized Ag and Sn EDX signal mapped across the diameter of a single nanowire, with the inset

showing the scanning path across an isolated wire.

32

Figure 4. Sheet resistance versus temperature for films deposited using (red) AgNWs that have been

washed three times in ethanol and (blue) mixtures of AgNW and SnO2 with weight ratio of 2:1. The

annealing time at each temperature value was approximately 10 minutes. The large sheet resistance

values of the bare AgNWs when annealed below 200 °C is typical for nanowires fabricated using

copper chloride seeds, which clearly illustrate the impact of SnO2 coordination at low treatment

temperatures.

33

Figure 5. (a) Sheet resistance and transmission data for samples deposited from solutions of varying

nanostructure concentration. Each of these samples were fabricated starting from the same

nanocomposite ink, which was then diluted to a range of concentrations while maintaining the same

AgNW to SnO2 weight ratio. (b) Transmission spectra of several transparent conducting networks

chosen from the plot in plot (a).

34

Figure 6. (a) I-V characteristics of devices made with AgNW/SnO2 rear electrodes with (blue) and

without (red) a 10 nm AZO contact layer. The dramatic double diode effect is likely a result of a

significant barrier to charge injection at the electrode/a-Si interface. (b) Top view SEM image of the

AgNW/SnO2 composite films on top of the textured a-Si absorber. (c) Schematic cross section of the

a-Si device architecture used in solar cell fabrication. The thickness of the thin AZO contact layer is

exaggerated for clarity.

Documents

Facile synthesis of magnetic-plasmonic nanocomposites as T ... · Facile synthesis of magnetic-plasmonic nanocomposites as T1 MRI contrast enhancing and photothermal therapeutic agents