DOI: 10.1002/chem.201406670
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In Situ Formation of Dual-Phase Thermosensitive Ultrasmall Gold Nanoparticles Zidong He, Aiqing Zhong, Hui Zhang, Linfeng Xiong, Yang Xu, Tianqi Wang, Minghong Zhou, and Kun Huang*[a] Abstract: A novel method for the in situ synthesis of dualphase thermosensitive ultrasmall gold nanoparticles (USGNPs) with diameters in the range of 1–3 nm was developed by using poly(N-isopropylacrylamide)-block-poly(Nphenylethylenediamine methacrylamide) (PNIPAM-b-PNPEDMA) amphiphilic diblock copolymers as ligands. The PNPEDMA block promotes the in situ reduction of gold precursors to zero-valent gold and subsequently binds to the surface of
Introduction Gold nanoparticles (GNPs) have attracted considerable attention in recent years due to their potential applications in a wide range of areas, such as catalysis,[1] biosensing,[2] drug delivery,[3] molecular electronics,[4] and surface patterning.[5] In particular, ultrasmall gold nanoparticles (USGNPs) with sizes in the 1–3 nm range are of great interest because of their unique properties compared to those larger than 5 nm.[6] To date, many synthetic strategies for preparing USGNPs have been developed. A typical method is the reduction chloroauric acid (HAuCl4) by sodium borohydride (NaBH4) in the presence of ligands containing thiol groups, which can significantly stabilize nanoparticles.[7] More recently, the use of dendrimers,[8] ionic liquids,[9] nucleotides,[10] organic molecular cages,[11] and thiotethered polymers[12] as templates or stabilizers for preparing USGNPs has been reported. Despite the progress achieved, developing a facile and general method to prepare stabilized USGNPs, particularly with ultrasmall size and tunable surface chemistry, still retains a significant challenge. On the other hand, it has been recently demonstrated that colloidally stable gold nanoparticles can be conveniently prepared in a single step without any external reducing agents by designing certain copolymers that can act as both a reducing agent and a stabilizer simultaneously. Polymers such as polymethylhydrosiloxane,[13] poly(N-vinyl-2-pyrrolidone),[14] poly(so[a] Z. D. He, A. Q. Zhong, H. Zhang, L. F. Xiong, Y. Xu, T. Q. Wang, M. H. Zhou, Prof. K. Huang School of Chemistry and Molecular Engineering East China Normal University 500 N, Dongchuan Road, Shanghai, 200241 (P. R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406670. Chem. Eur. J. 2015, 21, 10220 – 10225
gold nanoparticles, while PNIPAM acts as a stabilizing and thermosensitive block. The as-synthesized USGNPs stabilized by a thermosensitive PNIPAM layer exhibit a sharp, reversible, clear–opaque transition in aqueous solution between 30 and 38 8C. An unprecedented finding is that these USGNPs also show a reversible soluble–precipitate transition in nonpolar organic solvents such as chloroform at around 0 8C under acidic conditions.
dium acrylate),[15] poly(ethylene oxide),[16] polyallylamine,[17] poly[2-(N,N-dimethylamino)ethyl methacrylate],[18] and polyethylenimine[19]-based homopolymers or copolymers can fulfill the required dual role of reducing AuCl4¢ counterions to zerovalent gold and at the same time stabilizing the resulting gold nanoparticles. However, in syntheses with these polymers, large gold nanoparticles (> 3 nm) were usually produced. Reports of tailoring the surface chemistry of gold nanoparticles with sizes in the 1–3 nm range by this facile in situ synthesis approach remain rare. Herein, we demonstrate a novel method for the in situ synthesis of dual-phase thermosensitive USGNPs with diameters in the range of 1–3 nm stabilized by poly(N-isopropylacrylamide)block-poly(N-phenylethylenediamine methacrylamide) (PNIPAM-b-PNPEDMA) diblock copolymers, which were prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization and contain a thermoresponsive PNIPAM block and a reductive PNPEDMA block. By simply mixing the polymer solution with a solution of HAuCl4 at room temperature, thermosensitive USGNPs can be obtained. This procedure does not require the addition of an external reducing agent and results in highly stabilized USGNPs that show thermally responsive behavior not only in aqueous solution, but also in organic solution.
Results and Discussion Amphiphilic PNIPAM-b-PNPEDMA diblock copolymers with narrow molecular weight distributions were prepared by RAFT polymerization with S-1-dodecyl-S’-(a,a’-dimethyl-a’’acetic acid) trithiocarbonate as the RAFT chain-transfer agent (CTA). The structure and synthetic pathway of the target copolymer precursor are shown in Scheme 1. First, long-chain PNIPAM macroCTA with an average of 180 units was prepared by RAFT
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Scheme 1. Synthetic pathway to amphiphilic PNIPAM-b-PNPEDMA diblock copolymer by RAFT polymerization.
polymerization. The polymer had an extremely narrow molecular weight distribution (Mw/Mn = 1.09). The resulting macroCTA was then chain-extended with N-phenylethylenediamine methacrylamide (NPEDMA) to yield a well-defined PNIPAM180-bPNPEDMA4 diblock copolymer with Mn and Mw/Mn ratio of 14.2 kDa and 1.10, respectively (see the Supporting Information, Figures S1 and S2). In a typical USGNP synthesis, PNIPAM180-b-PNPEDMA4 copolymer (5 mg) was dissolved in water (1 mL) with vigorous stirring, and then HAuCl4 (82 mL, 0.02 mm) was quickly added to the polymer solution. On mixing, the solution immediately changed color from orange to dark brown without any precipitate, indicating efficient AuCl4¢ reduction and stabilization of the resulting GNPs by PNIPAM180-b-PNPEDMA4. After 5 h reaction time, the Au@ PNIPAM180-b-PNPEDMA4 complex was further purified by dialysis. The morphology and size of the USGNPs were investigated by TEM and dynamic light scattering (DLS). The TEM image (Figure 1 A) showed well dispersed GNPs with an average size of 1.7 0.5 nm. However, the average size of Au@PNIPAM180-bPNPEDMA4 measured by DLS experiment was approximately 100 nm, which is much larger than the 1.7 nm size of the gold nanoparticle cores observed by TEM (see the Supporting Information, Figure S3). This difference is due to the fact that the PNIPAM180-b-PNPEDMA4 corona, which has poor contrast, cannot be observed by TEM. It has been reported earlier that GNPs larger than 2 nm typically have a plasmon band in the range of 500–550 nm,[20] but when the particle size is less than 2 nm, the distinctive plasmon band is replaced by a featureless absorbance, which increases monotonically toward higher energies.[21] The UV/Vis spectrum of the synthesized USGNPs (Figure 1 B) does not show an intense plasmon absorption band close to 525 nm, which indicates that most particles are distributed around 2 nm in size. The result is also consistent with the conclusion made from the TEM image. Moreover, UV/Vis absorbance measurements showed that the USGNPs have excellent stability in aqueous solution with no evidence of agglomeration and without noticeable color change over periods of several months. More importantly, the solution can be evaporated to dryness under high vacuum for storage for several months and the USGNPs then redissolved in water (see the Supporting Information, Figure S4) or common organic solvents with no sign of aggregation. The difunctional monomer NPEDMA combines an aniline group and methacrylamide functionality. Therefore, it is possible to prepare a PNIPAM-b-PNPEDMA diblock copolymer by free-radical polymerization through the methacrylamide group, Chem. Eur. J. 2015, 21, 10220 – 10225
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Figure 1. A) TEM image of Au@PNIPAM180-b-PNPEDMA4 and histogram of gold particle sizes (inset). B) UV/Vis absorption spectrum of Au@PNIPAM180-bPNPEDMA4 in water.
leaving the aniline group exposed for further redox reaction with HAuCl4. The present method of synthesizing USGNPs is similar to other in situ synthesis routes with the exception of using NPEDMA as reducing reagent. However, the products described herein have a unique ultrasmall structure that is very different to the previously reported morphologies. Although the formation mechanism of the USGNPs is not fully clear at this stage, the above-mentioned results allow us to suggest a possible process responsible for the formation of such unusual particles. We propose that Au nanoclusters are possibly preorganized with monodisperse micelles formed by the amphiphilic diblock copolymer, which are composed of a hydrophilic PNIPAM block and a hydrophobic PNPEDMA block with
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Scheme 2. In situ formation of dual-phase thermosensitive USGNPs from PNIPAM180-b-PNPEDMA4 diblock copolymers.
a C12 hydrophobic trithioester head group (Scheme 2). First, the hydrophobic section containing NPEDMA and trithioester group self-assembles into the cores of micelles in the aqueous solution. On addition of HAuCl4, the PhNH group easily reacts with HAuCl4, to form a protonated PhNH-/AuCl4¢ salt through a base/acid reaction. At the same time, the AuCl4¢ anion acts as an oxidant and is capable of oxidizing the PhNH group,[22] which results in the formation of gold nanoparticles, which can be stabilized by the PNPEDMA block and trithioester head group.[23] This mechanism was further proved by determining the critical micelle concentration (CMC) of the diblock copolymer (CMC = 0.06 mg mL¢1), which was well above the concentrations used in the majority of the experiments (see the Supporting Information, Figure S5). Second, in this system, the hydrophobic trithioester head group of the polymer also plays an important role in the formation of USGNPs. An additional control experiment showed that an identical PNIPAM-b-PNPEDMA copolymer in which the terminal trithioester was removed by addition of excess diazo initiator cannot form USGNPs under the conditions reported here. Only poorly controlled particles were observed in the TEM image. This demonstrates, as supported by the TEM image and UV/Vis spectrum (see the Supporting Information, Figure S6), that the trithioester head group is necessary for formation of USGNPs. Furthermore, the hydrophobic trithioester head group of the RAFT polymer, which is packed inside the micelle, can also further stabilize gold nanoparticles. According to the proposed formation mechanism of gold nanoparticles, an increase in PNPEDMA chain length (molecular weight) should favor the reduction of AuCl4¢ and affect the size and shape of gold nanoparticles. To confirm this hypothesis, three PNIPAM-b-PNPEDMA diblock copolymers, namely, PNIPAM180-b-PNPEDMA4, PNIPAM180-b-PNPEDMA10, and PNIPAM180-b-PNPEDMA28, were synthesized to examine the effect of block composition on gold nanoparticle formation. At the same NPEDMA/AuCl4¢ ratio, a change in shape from ultrasmall to big to nanoplate took place when the number of PNPEDMA units in PNIPAM-b-PNPEDMA copolymers changed from 4 to 10 to 28, respectively (Figure 2). These results were also verified by UV/Vis spectra, which clearly showed intense plasmon absorption bands close to 550 and 500 nm for Au@ Chem. Eur. J. 2015, 21, 10220 – 10225
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PNIPAM146-b-PNPEDMA10, and Au@PNIPAM180-b-PNPEDMA28, respectively, whereas no peak around 525 nm was observed for Au@PNIPAM180-b-PNPEDMA4 (Figure 2 D). The above behavior may be attributed to differences in reactivity caused by the amount of aniline groups in the micelles, which is directly related to the content of PNPEDMA in PNIPAM-b-PNPEDMA copolymers. The size of the gold nanoparticles produced tends to increase with increasing PNPEDMA block length. However, many gold nanoplates were produced when PNIPAM180-bPNPEDMA28 copolymers were used as reducing agents. The formation mechanism of gold nanoplates is very complex. Recently, Shankar et al. proposed that formation of platelike structures involves rapid reduction, assembly, and sintering of “liquidlike” spherical gold nanoparticles formed in the first stages of the reaction.[24] Therefore, the longer PNPEDMA chain in PNIPAM180-b-PNPEDMA28 copolymer may offer high reactivity to rapidly reduce the AuCl4¢ ions to form gold nanoplates, which means that if the PNPEDMA block is too long, it is not well suited for the preparation of high-quality USGNPs. The effect of the concentration of HAuCl4 on formation of gold nanoparticles was also taken into consideration. Table S1 (see the Supporting Information), which lists four preparations with different NPEDMA/HAuCl4 ratios, indicates that the sizes of the gold nanoparticles were distributed between 1 and 3 nm when the ratio of [NPEDMA]/[HAuCl4] was held below 1/ 2.5 (see the Supporting Information, Figure S7 A and B). However, when the ratio increased to 1/3.25 or higher, ultrasmall gold nanoparticles accompanied by some larger gold nanoparticles (> 20 nm) were observed (see the Supporting Information, Figure S7C and D). This was also confirmed by UV/Vis spectra (see the Supporting Information, Figure S8), which showed no intense plasmon absorption band close to 520 nm for ratios of 1/1.75 and 1/2.5, which suggests that the average size of the nanoparticles is smaller than 3 nm. The intense plasmon absorbance bands close to 560 nm, observed for ratios of 1:3.25 and 1:7.0, are ascribed to the appearance of large gold nanoparticles. These results suggest that the concentration of HAuCl4 has only a small influence on the size of the USGNPs but greatly influences their polydispersity. PNIPAM is a typical thermoresponsive polymer with a low critical solution temperature (LCST) of about 34 8C in water.[25]
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Figure 2. TEM images of gold nanoparticles prepared with A) PNIPAM180-b-PNPEDMA4, B) PNIPAM180-b-PNPEDMA10, and C) PNIPAM180-b-PNPEDMA28. D) UV/Vis absorption spectra recorded of the three gold sols. The reaction conditions were as follows: [NPEDMA]/[HAuCl4] molar ratio = 1/1.75; [HAuCl4] = 1.6 Õ 10¢6 mol for PNIPAM180-b-PNPEDMA4 (5 mg mL¢1), 3.8 Õ 10¢6 moles for PNIPAM180-b-PNPEDMA10 (2 mg mL¢1), and 9.2 Õ 10¢6 mol for PNIPAM180-b-PNPEDMA28 (1 mg mL¢1). The synthesis was conducted for 5 h at 20 8C.
PNIPAM is hydrophilic below the LCST but becomes hydrophobic above that temperature, because a strong hydrogen bond between water and the amide group is established when the temperature is below the LCST, but this interaction is broken above that temperature. As expected, the USGNPs coated by PNIPAM180-b-PNPEDMA4 diblock copolymer show remarkable temperature sensitivity, as manifested in their optical transmittance switching at 450 nm. Figure 3 A shows the typical temperature-dependent optical transmittance of thermosensitive USGNPs. The transition temperature or the inflection point of the transmittance–temperature curve of USGNPs is Tt = 32.4 8C. The thermosensitive response of Au@PNIPAM180-b-PNPEDMA4 at 32.4 8C can be ascribed to the thermosensitive change of the outer PNIPAM layer. To prove this, we performed a control experiment by measuring the thermosensitive property of pure PNIPAM180-b-PNPEDMA4 under the same conditions in aqueous solution. We found that the pure PNIPAM180-b-PNPEDMA4 copolymers showed a phase-transition temperature at 32.6 8C, similar to what was seen for Au@PNIPAM180-b-PNPEDMA4. Moreover, this temperature-dependent clear–opaque transition of thermosensitive USGNPs is completely reversible. Figure 3 B shows the response of optical transparency as the thermosensitive Au@PNIPAM180-b-PNPEDMA4 suspension undergoes several heating– cooling cycles between 30 and 38 8C. When the temperature is below 30 8C, the solution is transparent, but it becomes Chem. Eur. J. 2015, 21, 10220 – 10225
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opaque above 38 8C. The optical transparency changes reversibly between about 60 % and about 2 % transmittance. The PNIPAM180-b-PNPEDMA4-stabilized USGNPs can be easily dissolved in nonpolar organic solvents such as chloroform after drying under high vacuum overnight. The size and size distribution of Au@PNIPAM180-b-PNPEDMA4 in chloroform were very similar to those in water (Figure 4 A). The UV/Vis spectrum further confirmed that the redispersion process did not cause a significant change in particle size (Figure 4 A, inset). These results indicated that Au@PNIPAM180-b-PNPEDMA4 has excellent stability not only in solution, but also in the dried state. Most importantly, these USGNPs show an unprecedented reversible soluble–precipitate transition in chloroform around 0 8C under acidic conditions. They can be dissolved in chloroform very well at temperatures above 0 8C and precipitated at temperatures below 0 8C (Figure 4 B and C). However, the origin and mechanism of this transition are currently unclear and detailed studies are in progress.
Conclusion A facile and general method for preparing highly stabilized gold nanoparticles with ultrasmall size (1–3 nm) and dualphase thermosensitive properties was developed. Well-controlled amphiphilic diblock copolymer PNIPAM180-b-PNPEDMA4, constructed from thermosensitive PNIPAM and reductive
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Figure 3. Thermoresponsive changes in transmittance of Au@PNIPAM180-bPNPEDMA4 (squares) and PNIPAM180-b-PNPEDMA4 (circles) on switching from transparent to opaque (A and inset). The solution switches reversibly (B) from a transparent solution to a slightly white opaque suspension between 30 8C (squares) and 38 8C (circles).
PNPEDMA, can act as efficient reductant and stabilizer in the single-step synthesis of USGNPs from AuCl4¢ ions in aqueous solution without any additional reductants. The roles of the chemical composition of the copolymer, the nature of the end groups, and the concentration of HAuCl4 in the preparation of high-quality USGNPs were examined. The resulting USGNPs coated by a thermosensitive PNIPAM layer exhibited a reversible clear–opaque transition in aqueous solution between 30 and 38 8C. A reversible, soluble–precipitate transition in organic solution around 0 8C under acidic conditions was observed for the first time. We envision that this approach can be further developed into a general design principle for preparing functional USGNPs, which would have potential application in catalysis or as thermosensitive switches.
Experiment Section Materials All reagents were used as received unless stated otherwise. 2,2Azoisobutyronitrile (AIBN) and N-isopropylacrylamide (NIPAM) were purified by recrystallization from methanol and hexane, respectively. N-Phenylethylenediamine methacrylamide (NPEDMA)[26] and S-1Chem. Eur. J. 2015, 21, 10220 – 10225
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Figure 4. A) TEM image and UV/Vis spectrum (inset) of Au@PNIPAM180-bPNPEDMA4 dissolved in chloroform. Photographs of Au@PNIPAM180-bPNPEDMA4 in chloroform at temperature above (B) and below 0 8C (C) after acidification with HCl gas.
Dodecyl-S’-(a,a’-dimethyl-a’’-acetic acid) trithiocarbonate (TC)[27] were synthesized as described in the literature.
Measurements All H1 NMR spectra were recorded on a Bruker AVANCE IIITM 500 spectrometer (500 MHz) by using CDCl3 or [D6]DMSO as solvent. GPC data were obtained from a Waters GPC system equipped with a Waters 2414 refractive index (RI) detector, a 1515 isocratic HPLC pump, and two Waters HPLC columns. THF (HPLC grade) was used as the solvent for polymers and eluent for GPC with a flow rate of 1 mL min¢1 at 30 8C. The GPC instrument was calibrated with narrowly dispersed linear polystyrene standards. TEM images were obtained by using a JEM-2100F TEM instrument. Samples were prepared by dip-coating a 400 mesh carbon-coated copper grid in the dilute sample solution and allowing the solvent to evaporate. UV/ Vis spectra were recorded on dilute aqueous or organic phase dispersions of gold nanoparticles in a 1 cm quartz cuvette by using a SOPTOP UV2400 spectrophotometer. Optical spectroscopy and transmittance measurements were carried out on a Cary 60 instrument (Agilent Technologies) with temperature-controlled cell holders. Temperature was controlled within 0.1 8C. Fluorescence spectra were recorded with a TECAN infinite M200PRO spectrofluorimeter. Particle size distributions of the USGNPs in aqueous solution were measured with a Zetasizer Nano ZS (Malvern Instruments, U.K.).
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Full Paper Synthesis Synthesis of PNIPAM180 : NIPAM (1 g, 8.8 mmol), TC (16 mg, 44 mmol), AIBN (0.72 mg, 44 mmol) and N,N-dimethylformamide (DMF; 1.5 mL) were mixed in a reaction vessel and degassed by three freeze–pump–thaw cycles. Polymerization was then conducted at 60 8C for 4 h. The polymer was precipitated from THF into diethyl ether three times and dried under vacuum for 24 h. Yield: 890 mg (89 %). GPC: Mn = 14 kg mol¢1, Mw/Mn = 1.09; 1H NMR: n(NIPAM) = 180. PNIPAM180-b-PNPEDMA4 : The following is a general procedure for the synthesis of PNIPAMm-b-PNPEDMAn copolymers. Typically, PNIPAM (180 mg, 8.7 mmol), NPEDMA (90 mg, 44 mmol), AIBN (0.15 mg, 90 mmol), and 1,4-dioxane (1.4 mL) were mixed in a reaction vessel and degassed by three freeze–pump–thaw cycles. Polymerization was then conducted at 60 8C for 6 h. The polymer was precipitated from THF into ethyl ether three times and dried under vacuum for 24 h. Yield: 153 mg (57 %). GPC: Mn = 14 kg mol¢1, Mw/ Mn = 1.10; 1H NMR: n(NPEDMA) = 4. Typical synthesis of Au@PNIPAM-b-PNPEDMA complex: All of the Au nanoparticles were prepared by similar procedures. Typically, the block copolymers were completely dispersed in deionized water at room temperature. Then, an aqueous solution of HAuCl4 was added to the solution with vigorous stirring. After stirring for 5 h, the solution was dialyzed for 48 h to remove free copolymer. The residual solution was dried under vacuum at 50 8C for 24 h to give Au@PNIPAM-b-PNPEDMA complex. Removing the trithioester end group of PNIPAM180-b-PNPEDMA4 : The literature method for removing trithioester groups was used[28] but with some modifications. PNIPAM180-b-PNPEDMA4 copolymers (60 mg, 2.9 mmol) were dissolved in 1,4-dioxane (0.5 mL), and AIBN (13 mg, 80 mmol) was added to the solution. After degassing by three freeze-pump-thaw cycles, the reaction was then conducted at 80 8C for 3 h. The polymer was then precipitated from THF into diethyl ether three times and dried under vacuum for 24 h.
Determination of CMC of PNIPAM180-b-PNPEDMA4 Nile Red is a hydrophobic dye that itself is not soluble in water, as can be discerned from the lack of absorption or emission spectral intensity in aqueous solution. However, this dye can be sequestered inside the hydrophobic pocket generated by micelles. To determine the CMC of the copolymers, the PNIPAM180-b-PNPEDMA4 copolymer (6 mg) was dissolved in THF (6 mL) to give a final concentration of 1 mg mL¢1. A solution of Nile Red in THF (5 mL, 1 mm) and certain amounts of polymer solution were added to each bottle. The mixtures were dried under vacuum to remove THF, and then redissolved in water (1 mL) to create polymer solutions of various concentrations. The solutions were all stirred for 1 d and then centrifuged to remove unloaded Nile Red. The fluorescence intensity of each solution was monitored with an excitation wavelength of 510 nm and an emission wavelength of 620 nm.
Acknowledgements We gratefully acknowledge Zhiai Xu for assistance with optical spectra and transmittance measurements. This work is supported by National Natural Science Foundation of China grant 51273066, Shanghai Pujiang Program grant 13J1402300 and Large Instruments Open Foundation of East China Normal University (No. 2014-15).
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Keywords: block copolymers thermoresponsive materials
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gold
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nanoparticles
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& Nanoparticles
In Situ Formation of Dual-Phase Thermosensitive Ultrasmall Gold Nanoparticles Zidong He, Aiqing Zhong, Hui Zhang, Linfeng Xiong, Yang Xu, Tianqi Wang, Minghong Zhou, and Kun Huang*[a] Abstract: A novel method for the in situ synthesis of dualphase thermosensitive ultrasmall gold nanoparticles (USGNPs) with diameters in the range of 1–3 nm was developed by using poly(N-isopropylacrylamide)-block-poly(Nphenylethylenediamine methacrylamide) (PNIPAM-b-PNPEDMA) amphiphilic diblock copolymers as ligands. The PNPEDMA block promotes the in situ reduction of gold precursors to zero-valent gold and subsequently binds to the surface of
Introduction Gold nanoparticles (GNPs) have attracted considerable attention in recent years due to their potential applications in a wide range of areas, such as catalysis,[1] biosensing,[2] drug delivery,[3] molecular electronics,[4] and surface patterning.[5] In particular, ultrasmall gold nanoparticles (USGNPs) with sizes in the 1–3 nm range are of great interest because of their unique properties compared to those larger than 5 nm.[6] To date, many synthetic strategies for preparing USGNPs have been developed. A typical method is the reduction chloroauric acid (HAuCl4) by sodium borohydride (NaBH4) in the presence of ligands containing thiol groups, which can significantly stabilize nanoparticles.[7] More recently, the use of dendrimers,[8] ionic liquids,[9] nucleotides,[10] organic molecular cages,[11] and thiotethered polymers[12] as templates or stabilizers for preparing USGNPs has been reported. Despite the progress achieved, developing a facile and general method to prepare stabilized USGNPs, particularly with ultrasmall size and tunable surface chemistry, still retains a significant challenge. On the other hand, it has been recently demonstrated that colloidally stable gold nanoparticles can be conveniently prepared in a single step without any external reducing agents by designing certain copolymers that can act as both a reducing agent and a stabilizer simultaneously. Polymers such as polymethylhydrosiloxane,[13] poly(N-vinyl-2-pyrrolidone),[14] poly(so[a] Z. D. He, A. Q. Zhong, H. Zhang, L. F. Xiong, Y. Xu, T. Q. Wang, M. H. Zhou, Prof. K. Huang School of Chemistry and Molecular Engineering East China Normal University 500 N, Dongchuan Road, Shanghai, 200241 (P. R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406670. Chem. Eur. J. 2015, 21, 10220 – 10225
gold nanoparticles, while PNIPAM acts as a stabilizing and thermosensitive block. The as-synthesized USGNPs stabilized by a thermosensitive PNIPAM layer exhibit a sharp, reversible, clear–opaque transition in aqueous solution between 30 and 38 8C. An unprecedented finding is that these USGNPs also show a reversible soluble–precipitate transition in nonpolar organic solvents such as chloroform at around 0 8C under acidic conditions.
dium acrylate),[15] poly(ethylene oxide),[16] polyallylamine,[17] poly[2-(N,N-dimethylamino)ethyl methacrylate],[18] and polyethylenimine[19]-based homopolymers or copolymers can fulfill the required dual role of reducing AuCl4¢ counterions to zerovalent gold and at the same time stabilizing the resulting gold nanoparticles. However, in syntheses with these polymers, large gold nanoparticles (> 3 nm) were usually produced. Reports of tailoring the surface chemistry of gold nanoparticles with sizes in the 1–3 nm range by this facile in situ synthesis approach remain rare. Herein, we demonstrate a novel method for the in situ synthesis of dual-phase thermosensitive USGNPs with diameters in the range of 1–3 nm stabilized by poly(N-isopropylacrylamide)block-poly(N-phenylethylenediamine methacrylamide) (PNIPAM-b-PNPEDMA) diblock copolymers, which were prepared by reversible addition–fragmentation chain-transfer (RAFT) polymerization and contain a thermoresponsive PNIPAM block and a reductive PNPEDMA block. By simply mixing the polymer solution with a solution of HAuCl4 at room temperature, thermosensitive USGNPs can be obtained. This procedure does not require the addition of an external reducing agent and results in highly stabilized USGNPs that show thermally responsive behavior not only in aqueous solution, but also in organic solution.
Results and Discussion Amphiphilic PNIPAM-b-PNPEDMA diblock copolymers with narrow molecular weight distributions were prepared by RAFT polymerization with S-1-dodecyl-S’-(a,a’-dimethyl-a’’acetic acid) trithiocarbonate as the RAFT chain-transfer agent (CTA). The structure and synthetic pathway of the target copolymer precursor are shown in Scheme 1. First, long-chain PNIPAM macroCTA with an average of 180 units was prepared by RAFT
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Scheme 1. Synthetic pathway to amphiphilic PNIPAM-b-PNPEDMA diblock copolymer by RAFT polymerization.
polymerization. The polymer had an extremely narrow molecular weight distribution (Mw/Mn = 1.09). The resulting macroCTA was then chain-extended with N-phenylethylenediamine methacrylamide (NPEDMA) to yield a well-defined PNIPAM180-bPNPEDMA4 diblock copolymer with Mn and Mw/Mn ratio of 14.2 kDa and 1.10, respectively (see the Supporting Information, Figures S1 and S2). In a typical USGNP synthesis, PNIPAM180-b-PNPEDMA4 copolymer (5 mg) was dissolved in water (1 mL) with vigorous stirring, and then HAuCl4 (82 mL, 0.02 mm) was quickly added to the polymer solution. On mixing, the solution immediately changed color from orange to dark brown without any precipitate, indicating efficient AuCl4¢ reduction and stabilization of the resulting GNPs by PNIPAM180-b-PNPEDMA4. After 5 h reaction time, the Au@ PNIPAM180-b-PNPEDMA4 complex was further purified by dialysis. The morphology and size of the USGNPs were investigated by TEM and dynamic light scattering (DLS). The TEM image (Figure 1 A) showed well dispersed GNPs with an average size of 1.7 0.5 nm. However, the average size of Au@PNIPAM180-bPNPEDMA4 measured by DLS experiment was approximately 100 nm, which is much larger than the 1.7 nm size of the gold nanoparticle cores observed by TEM (see the Supporting Information, Figure S3). This difference is due to the fact that the PNIPAM180-b-PNPEDMA4 corona, which has poor contrast, cannot be observed by TEM. It has been reported earlier that GNPs larger than 2 nm typically have a plasmon band in the range of 500–550 nm,[20] but when the particle size is less than 2 nm, the distinctive plasmon band is replaced by a featureless absorbance, which increases monotonically toward higher energies.[21] The UV/Vis spectrum of the synthesized USGNPs (Figure 1 B) does not show an intense plasmon absorption band close to 525 nm, which indicates that most particles are distributed around 2 nm in size. The result is also consistent with the conclusion made from the TEM image. Moreover, UV/Vis absorbance measurements showed that the USGNPs have excellent stability in aqueous solution with no evidence of agglomeration and without noticeable color change over periods of several months. More importantly, the solution can be evaporated to dryness under high vacuum for storage for several months and the USGNPs then redissolved in water (see the Supporting Information, Figure S4) or common organic solvents with no sign of aggregation. The difunctional monomer NPEDMA combines an aniline group and methacrylamide functionality. Therefore, it is possible to prepare a PNIPAM-b-PNPEDMA diblock copolymer by free-radical polymerization through the methacrylamide group, Chem. Eur. J. 2015, 21, 10220 – 10225
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Figure 1. A) TEM image of Au@PNIPAM180-b-PNPEDMA4 and histogram of gold particle sizes (inset). B) UV/Vis absorption spectrum of Au@PNIPAM180-bPNPEDMA4 in water.
leaving the aniline group exposed for further redox reaction with HAuCl4. The present method of synthesizing USGNPs is similar to other in situ synthesis routes with the exception of using NPEDMA as reducing reagent. However, the products described herein have a unique ultrasmall structure that is very different to the previously reported morphologies. Although the formation mechanism of the USGNPs is not fully clear at this stage, the above-mentioned results allow us to suggest a possible process responsible for the formation of such unusual particles. We propose that Au nanoclusters are possibly preorganized with monodisperse micelles formed by the amphiphilic diblock copolymer, which are composed of a hydrophilic PNIPAM block and a hydrophobic PNPEDMA block with
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Scheme 2. In situ formation of dual-phase thermosensitive USGNPs from PNIPAM180-b-PNPEDMA4 diblock copolymers.
a C12 hydrophobic trithioester head group (Scheme 2). First, the hydrophobic section containing NPEDMA and trithioester group self-assembles into the cores of micelles in the aqueous solution. On addition of HAuCl4, the PhNH group easily reacts with HAuCl4, to form a protonated PhNH-/AuCl4¢ salt through a base/acid reaction. At the same time, the AuCl4¢ anion acts as an oxidant and is capable of oxidizing the PhNH group,[22] which results in the formation of gold nanoparticles, which can be stabilized by the PNPEDMA block and trithioester head group.[23] This mechanism was further proved by determining the critical micelle concentration (CMC) of the diblock copolymer (CMC = 0.06 mg mL¢1), which was well above the concentrations used in the majority of the experiments (see the Supporting Information, Figure S5). Second, in this system, the hydrophobic trithioester head group of the polymer also plays an important role in the formation of USGNPs. An additional control experiment showed that an identical PNIPAM-b-PNPEDMA copolymer in which the terminal trithioester was removed by addition of excess diazo initiator cannot form USGNPs under the conditions reported here. Only poorly controlled particles were observed in the TEM image. This demonstrates, as supported by the TEM image and UV/Vis spectrum (see the Supporting Information, Figure S6), that the trithioester head group is necessary for formation of USGNPs. Furthermore, the hydrophobic trithioester head group of the RAFT polymer, which is packed inside the micelle, can also further stabilize gold nanoparticles. According to the proposed formation mechanism of gold nanoparticles, an increase in PNPEDMA chain length (molecular weight) should favor the reduction of AuCl4¢ and affect the size and shape of gold nanoparticles. To confirm this hypothesis, three PNIPAM-b-PNPEDMA diblock copolymers, namely, PNIPAM180-b-PNPEDMA4, PNIPAM180-b-PNPEDMA10, and PNIPAM180-b-PNPEDMA28, were synthesized to examine the effect of block composition on gold nanoparticle formation. At the same NPEDMA/AuCl4¢ ratio, a change in shape from ultrasmall to big to nanoplate took place when the number of PNPEDMA units in PNIPAM-b-PNPEDMA copolymers changed from 4 to 10 to 28, respectively (Figure 2). These results were also verified by UV/Vis spectra, which clearly showed intense plasmon absorption bands close to 550 and 500 nm for Au@ Chem. Eur. J. 2015, 21, 10220 – 10225
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PNIPAM146-b-PNPEDMA10, and Au@PNIPAM180-b-PNPEDMA28, respectively, whereas no peak around 525 nm was observed for Au@PNIPAM180-b-PNPEDMA4 (Figure 2 D). The above behavior may be attributed to differences in reactivity caused by the amount of aniline groups in the micelles, which is directly related to the content of PNPEDMA in PNIPAM-b-PNPEDMA copolymers. The size of the gold nanoparticles produced tends to increase with increasing PNPEDMA block length. However, many gold nanoplates were produced when PNIPAM180-bPNPEDMA28 copolymers were used as reducing agents. The formation mechanism of gold nanoplates is very complex. Recently, Shankar et al. proposed that formation of platelike structures involves rapid reduction, assembly, and sintering of “liquidlike” spherical gold nanoparticles formed in the first stages of the reaction.[24] Therefore, the longer PNPEDMA chain in PNIPAM180-b-PNPEDMA28 copolymer may offer high reactivity to rapidly reduce the AuCl4¢ ions to form gold nanoplates, which means that if the PNPEDMA block is too long, it is not well suited for the preparation of high-quality USGNPs. The effect of the concentration of HAuCl4 on formation of gold nanoparticles was also taken into consideration. Table S1 (see the Supporting Information), which lists four preparations with different NPEDMA/HAuCl4 ratios, indicates that the sizes of the gold nanoparticles were distributed between 1 and 3 nm when the ratio of [NPEDMA]/[HAuCl4] was held below 1/ 2.5 (see the Supporting Information, Figure S7 A and B). However, when the ratio increased to 1/3.25 or higher, ultrasmall gold nanoparticles accompanied by some larger gold nanoparticles (> 20 nm) were observed (see the Supporting Information, Figure S7C and D). This was also confirmed by UV/Vis spectra (see the Supporting Information, Figure S8), which showed no intense plasmon absorption band close to 520 nm for ratios of 1/1.75 and 1/2.5, which suggests that the average size of the nanoparticles is smaller than 3 nm. The intense plasmon absorbance bands close to 560 nm, observed for ratios of 1:3.25 and 1:7.0, are ascribed to the appearance of large gold nanoparticles. These results suggest that the concentration of HAuCl4 has only a small influence on the size of the USGNPs but greatly influences their polydispersity. PNIPAM is a typical thermoresponsive polymer with a low critical solution temperature (LCST) of about 34 8C in water.[25]
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Figure 2. TEM images of gold nanoparticles prepared with A) PNIPAM180-b-PNPEDMA4, B) PNIPAM180-b-PNPEDMA10, and C) PNIPAM180-b-PNPEDMA28. D) UV/Vis absorption spectra recorded of the three gold sols. The reaction conditions were as follows: [NPEDMA]/[HAuCl4] molar ratio = 1/1.75; [HAuCl4] = 1.6 Õ 10¢6 mol for PNIPAM180-b-PNPEDMA4 (5 mg mL¢1), 3.8 Õ 10¢6 moles for PNIPAM180-b-PNPEDMA10 (2 mg mL¢1), and 9.2 Õ 10¢6 mol for PNIPAM180-b-PNPEDMA28 (1 mg mL¢1). The synthesis was conducted for 5 h at 20 8C.
PNIPAM is hydrophilic below the LCST but becomes hydrophobic above that temperature, because a strong hydrogen bond between water and the amide group is established when the temperature is below the LCST, but this interaction is broken above that temperature. As expected, the USGNPs coated by PNIPAM180-b-PNPEDMA4 diblock copolymer show remarkable temperature sensitivity, as manifested in their optical transmittance switching at 450 nm. Figure 3 A shows the typical temperature-dependent optical transmittance of thermosensitive USGNPs. The transition temperature or the inflection point of the transmittance–temperature curve of USGNPs is Tt = 32.4 8C. The thermosensitive response of Au@PNIPAM180-b-PNPEDMA4 at 32.4 8C can be ascribed to the thermosensitive change of the outer PNIPAM layer. To prove this, we performed a control experiment by measuring the thermosensitive property of pure PNIPAM180-b-PNPEDMA4 under the same conditions in aqueous solution. We found that the pure PNIPAM180-b-PNPEDMA4 copolymers showed a phase-transition temperature at 32.6 8C, similar to what was seen for Au@PNIPAM180-b-PNPEDMA4. Moreover, this temperature-dependent clear–opaque transition of thermosensitive USGNPs is completely reversible. Figure 3 B shows the response of optical transparency as the thermosensitive Au@PNIPAM180-b-PNPEDMA4 suspension undergoes several heating– cooling cycles between 30 and 38 8C. When the temperature is below 30 8C, the solution is transparent, but it becomes Chem. Eur. J. 2015, 21, 10220 – 10225
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opaque above 38 8C. The optical transparency changes reversibly between about 60 % and about 2 % transmittance. The PNIPAM180-b-PNPEDMA4-stabilized USGNPs can be easily dissolved in nonpolar organic solvents such as chloroform after drying under high vacuum overnight. The size and size distribution of Au@PNIPAM180-b-PNPEDMA4 in chloroform were very similar to those in water (Figure 4 A). The UV/Vis spectrum further confirmed that the redispersion process did not cause a significant change in particle size (Figure 4 A, inset). These results indicated that Au@PNIPAM180-b-PNPEDMA4 has excellent stability not only in solution, but also in the dried state. Most importantly, these USGNPs show an unprecedented reversible soluble–precipitate transition in chloroform around 0 8C under acidic conditions. They can be dissolved in chloroform very well at temperatures above 0 8C and precipitated at temperatures below 0 8C (Figure 4 B and C). However, the origin and mechanism of this transition are currently unclear and detailed studies are in progress.
Conclusion A facile and general method for preparing highly stabilized gold nanoparticles with ultrasmall size (1–3 nm) and dualphase thermosensitive properties was developed. Well-controlled amphiphilic diblock copolymer PNIPAM180-b-PNPEDMA4, constructed from thermosensitive PNIPAM and reductive
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Figure 3. Thermoresponsive changes in transmittance of Au@PNIPAM180-bPNPEDMA4 (squares) and PNIPAM180-b-PNPEDMA4 (circles) on switching from transparent to opaque (A and inset). The solution switches reversibly (B) from a transparent solution to a slightly white opaque suspension between 30 8C (squares) and 38 8C (circles).
PNPEDMA, can act as efficient reductant and stabilizer in the single-step synthesis of USGNPs from AuCl4¢ ions in aqueous solution without any additional reductants. The roles of the chemical composition of the copolymer, the nature of the end groups, and the concentration of HAuCl4 in the preparation of high-quality USGNPs were examined. The resulting USGNPs coated by a thermosensitive PNIPAM layer exhibited a reversible clear–opaque transition in aqueous solution between 30 and 38 8C. A reversible, soluble–precipitate transition in organic solution around 0 8C under acidic conditions was observed for the first time. We envision that this approach can be further developed into a general design principle for preparing functional USGNPs, which would have potential application in catalysis or as thermosensitive switches.
Experiment Section Materials All reagents were used as received unless stated otherwise. 2,2Azoisobutyronitrile (AIBN) and N-isopropylacrylamide (NIPAM) were purified by recrystallization from methanol and hexane, respectively. N-Phenylethylenediamine methacrylamide (NPEDMA)[26] and S-1Chem. Eur. J. 2015, 21, 10220 – 10225
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Figure 4. A) TEM image and UV/Vis spectrum (inset) of Au@PNIPAM180-bPNPEDMA4 dissolved in chloroform. Photographs of Au@PNIPAM180-bPNPEDMA4 in chloroform at temperature above (B) and below 0 8C (C) after acidification with HCl gas.
Dodecyl-S’-(a,a’-dimethyl-a’’-acetic acid) trithiocarbonate (TC)[27] were synthesized as described in the literature.
Measurements All H1 NMR spectra were recorded on a Bruker AVANCE IIITM 500 spectrometer (500 MHz) by using CDCl3 or [D6]DMSO as solvent. GPC data were obtained from a Waters GPC system equipped with a Waters 2414 refractive index (RI) detector, a 1515 isocratic HPLC pump, and two Waters HPLC columns. THF (HPLC grade) was used as the solvent for polymers and eluent for GPC with a flow rate of 1 mL min¢1 at 30 8C. The GPC instrument was calibrated with narrowly dispersed linear polystyrene standards. TEM images were obtained by using a JEM-2100F TEM instrument. Samples were prepared by dip-coating a 400 mesh carbon-coated copper grid in the dilute sample solution and allowing the solvent to evaporate. UV/ Vis spectra were recorded on dilute aqueous or organic phase dispersions of gold nanoparticles in a 1 cm quartz cuvette by using a SOPTOP UV2400 spectrophotometer. Optical spectroscopy and transmittance measurements were carried out on a Cary 60 instrument (Agilent Technologies) with temperature-controlled cell holders. Temperature was controlled within 0.1 8C. Fluorescence spectra were recorded with a TECAN infinite M200PRO spectrofluorimeter. Particle size distributions of the USGNPs in aqueous solution were measured with a Zetasizer Nano ZS (Malvern Instruments, U.K.).
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Full Paper Synthesis Synthesis of PNIPAM180 : NIPAM (1 g, 8.8 mmol), TC (16 mg, 44 mmol), AIBN (0.72 mg, 44 mmol) and N,N-dimethylformamide (DMF; 1.5 mL) were mixed in a reaction vessel and degassed by three freeze–pump–thaw cycles. Polymerization was then conducted at 60 8C for 4 h. The polymer was precipitated from THF into diethyl ether three times and dried under vacuum for 24 h. Yield: 890 mg (89 %). GPC: Mn = 14 kg mol¢1, Mw/Mn = 1.09; 1H NMR: n(NIPAM) = 180. PNIPAM180-b-PNPEDMA4 : The following is a general procedure for the synthesis of PNIPAMm-b-PNPEDMAn copolymers. Typically, PNIPAM (180 mg, 8.7 mmol), NPEDMA (90 mg, 44 mmol), AIBN (0.15 mg, 90 mmol), and 1,4-dioxane (1.4 mL) were mixed in a reaction vessel and degassed by three freeze–pump–thaw cycles. Polymerization was then conducted at 60 8C for 6 h. The polymer was precipitated from THF into ethyl ether three times and dried under vacuum for 24 h. Yield: 153 mg (57 %). GPC: Mn = 14 kg mol¢1, Mw/ Mn = 1.10; 1H NMR: n(NPEDMA) = 4. Typical synthesis of Au@PNIPAM-b-PNPEDMA complex: All of the Au nanoparticles were prepared by similar procedures. Typically, the block copolymers were completely dispersed in deionized water at room temperature. Then, an aqueous solution of HAuCl4 was added to the solution with vigorous stirring. After stirring for 5 h, the solution was dialyzed for 48 h to remove free copolymer. The residual solution was dried under vacuum at 50 8C for 24 h to give Au@PNIPAM-b-PNPEDMA complex. Removing the trithioester end group of PNIPAM180-b-PNPEDMA4 : The literature method for removing trithioester groups was used[28] but with some modifications. PNIPAM180-b-PNPEDMA4 copolymers (60 mg, 2.9 mmol) were dissolved in 1,4-dioxane (0.5 mL), and AIBN (13 mg, 80 mmol) was added to the solution. After degassing by three freeze-pump-thaw cycles, the reaction was then conducted at 80 8C for 3 h. The polymer was then precipitated from THF into diethyl ether three times and dried under vacuum for 24 h.
Determination of CMC of PNIPAM180-b-PNPEDMA4 Nile Red is a hydrophobic dye that itself is not soluble in water, as can be discerned from the lack of absorption or emission spectral intensity in aqueous solution. However, this dye can be sequestered inside the hydrophobic pocket generated by micelles. To determine the CMC of the copolymers, the PNIPAM180-b-PNPEDMA4 copolymer (6 mg) was dissolved in THF (6 mL) to give a final concentration of 1 mg mL¢1. A solution of Nile Red in THF (5 mL, 1 mm) and certain amounts of polymer solution were added to each bottle. The mixtures were dried under vacuum to remove THF, and then redissolved in water (1 mL) to create polymer solutions of various concentrations. The solutions were all stirred for 1 d and then centrifuged to remove unloaded Nile Red. The fluorescence intensity of each solution was monitored with an excitation wavelength of 510 nm and an emission wavelength of 620 nm.
Acknowledgements We gratefully acknowledge Zhiai Xu for assistance with optical spectra and transmittance measurements. This work is supported by National Natural Science Foundation of China grant 51273066, Shanghai Pujiang Program grant 13J1402300 and Large Instruments Open Foundation of East China Normal University (No. 2014-15).
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Keywords: block copolymers thermoresponsive materials
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gold
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nanoparticles
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