Amphiphilic N-Heterocyclic Carbene-Stabilized Gold Nanoparticles

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Amphiphilic N-Heterocyclic Carbene-Stabilized Gold Nanoparticles and their Self-Assembly in Polar Solvents Mina R. Narouz, Chien-Hung Li, Ali Nazemi, and Cathleen M. Crudden Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02248 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Amphiphilic N-Heterocyclic Carbene-Stabilized Gold Nanoparticles and their SelfAssembly in Polar Solvents Mina R. Narouz,† Chien-Hung Li,† Ali Nazemi,*,† and Cathleen M. Crudden*,†,‡ †

Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario,

Canada K7L 3N6 ‡

Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa,

Nagoya 464-8602, Japan

ABSTRACT: Soft matter-directed self-assembly of amphiphilic inorganic nanoparticles (NPs) has recently emerged as a promising approach to access NP ensembles with superior collective properties. While thiol-terminated molecules are primarily employed to tether the amphiphilic ligand to the metal, serious concerns remain regarding the stabilities of the resulting NPs and their corresponding aggregates. As an alternative to such ligands, we report amphiphilic N-heterocyclic carbene (NHC)-functionalized gold nanoparticles (AuNPs). To accomplish this, an amphiphilic NHC-AuI complex based on an asymmetric triethylene glycol-/dodecyl-functionalized benzimidazole was first synthesized and used to prepare the corresponding stable amphiphilic NHC-decorated AuNPs. The resulting NPs were comprehensively characterized using both solution- and solid-state-based techniques such as proton nuclear magnetic resonance spectroscopy, dynamic light scattering, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. By optimizing the self-assembly behavior of these amphiphilic AuNPs in deionized water, ethanol, and their mixtures, we were able to fine tune the plasmonic properties of the AuNPs in the wide range of 525-640 nm. Furthermore, when treated with thiols, the ensembles showed greater stability compared to their parent discrete AuNP counterparts at room temperature.

INTRODUCTION

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Inorganic nanoparticles (NPs) have received widespread attention in the scientific community due to their promising optoelectronic, magnetic, and catalytic properties.1-5 Recently, soft matter-guided self-assembly of inorganic NPs has been introduced as a powerful method to access ensembles of NPs.6-10 Such NP assemblies exhibit superior collective properties that are otherwise not accessible via their parent discrete NPs. These new properties result from plasmonic coupling, magnetic–magnetic and plasmon–exciton interactions of their NP constituents.6 In such assemblies, size of the resulting aggregates and interparticle distances within the structures govern the final collective properties of the resulting materials.11 Among various NPs, gold nanoparticles (AuNPs) are among the most commonly studied due to their excellent biocompatibility, optical properties, and the presence of wellestablished synthetic protocols to control their size and morphology.12-15 To date, the most common approach to obtain AuNP ensembles via soft matter-directed self-assembly is to anchor either mixtures of hydrophilic and hydrophobic molecules16-22 or small molecule amphiphiles23-25 onto their surface followed by self-assembly in solvents of interest. These molecules are typically attached to AuNP surfaces via sulfur-based linkages. In a seminal early report, Rotello and coworkers developed amphiphilic AuNPs by partial place exchange of octanethiol-capped NPs with 11-thioundecanoic acid.16 The authors studied the aggregation behavior of these NPs in aqueous media by adjusting the pH of the system. They observed that while at pH 10 NPs remained dispersed in solution, at neutral pH both small aggregates and individual particles were present. As expected, in acidic media (pH 4) much larger and denser aggregates were obtained.16 Subsequent to this work, numerous examples of amphiphilic AuNPs decorated with small molecule amphiphiles have been reported with a myriad of applications.18-21,24,25 In addition to small molecule amphiphiles as directing groups, thiol-terminated homopolymers and block copolymers (BCPs) have also been used to tether AuNPs and, ultimately, generate AuNP ensembles.26-33 Despite the diversity of structures accessible through self-assembly of inorganic NPs, in all the aforementioned reports, sulfur-based linkages are employed to bind amphiphilic


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ligands to gold surface. The lack of sufficient S–Au chemical stability can be a challenge for the application of the resulting nanomaterials.34-37 Following on the work of Siemeling,38 Johnson,39 and Tilley,40 our group and others have introduced N-heterocyclic carbenes (NHCs) as alternatives to thiols for the preparation of stable gold surfaces and NPs.41-49 Our group demonstrated definitively that NHC-based SAMs have higher thermal, chemical, and oxidative stability compared to thiol-based SAMs.44 Experimental determination of bond strength illustrated that typical NHCs have stronger bonds to Au surfaces by a value of approximately 25 kJ/mol.46 Herein we report the synthesis of amphiphilic NHC-functionalized AuNPs and investigate their self-assembly behavior in polar solvents. To accomplish this, we first synthesized an amphiphilic NHC-Au complex that was subsequently reduced to form the target amphiphilic AuNPs. Self-assembly behavior of these AuNPs was then investigated in polar solvents such as water and ethanol (Scheme 1). Furthermore, we compare the stability of the resulting ensembles in the presence of thiols to those of the parent AuNPs in their discrete form.

Scheme 1. General schematic representation of amphiphilic AuNP formation and their self-assembly in polar solvents.

EXPERIMENTAL SECTION General Considerations, Materials, and Methods. All the commercially available chemicals and solvents were purchased from Sigma or Alfa Aesar and used without further purification unless otherwise noted. Reactions were performed under air without the use of air- and moisture-free techniques using reagent grade solvents. Dialyses were performed using Spectra/Por regenerated cellulose membranes with a 12,000–14,000 g/mol molecular weight cutoff (MWCO). 1H and

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C NMR spectra were recorded on

Bruker Avance 300 or 500 MHz spectrometers. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane, using


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residual protonated solvent as an internal standard (1H NMR CD2Cl2: 5.32 ppm;

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C

NMR CD2Cl2: 53.84 ppm). All measurements were carried out at 298 K. thermogravimetric analysis (TGA) experiments were performed using TGA Q500 with platinum crucibles. A constant heating rate of 5°C/min and gas purging (N2) at a flow rate of 60 mL/min were used. UV–visible absorption spectroscopy was performed either on a Varian Cary 60 Bio UV–visible spectrophotometer or an OLIS (On-Line Instrument Systems, Bogart, Georgia) modified Cary 17 UV-vis spectrophotometer. Dynamic light scattering (DLS) data were obtained using a Zetasizer Nano ZS instrument from Malvern Instruments. High-resolution mass spectrometry (HRMS) was performed on a Micromass GCT (GC-EI) time-of-flight mass spectrometer by the Mass Spectroscopy and Proteomics Unit. X-ray photoelectron spectroscopy (XPS) spectra were measured on a Kratos Nova AXIS spectrometer equipped with an AlnX-ray source. The samples were dropped on the glass coverslips and the coverslips were cleaned by fresh piranha solution before use. The dried samples were mounted on the sample holder by using double-sided adhesive Cu tape and kept under high vacuum (10-9 Torr) overnight inside the preparation chamber before they were transferred into the analysis chamber (ultrahigh vacuum, 10-10 Torr) of the spectrometer. The XPS data were collected using AlKα radiation at 1486.69 eV (150 W, 15 kV), charge neutralizer and a delay-line detector (DLD) consisting of three multi-channel plates. The spectra were measured using the Vision 2 software and processed using the CasaXPS software. The binding energies of all spectra are referred to the C1s peak at 285 eV. Transmission Electron Microscopy (TEM). The samples for electron microscopy were prepared by drop casting one drop (ca. 20 µL) of the colloidal NP solution onto a Formvar carbon-coated copper grid rested on a piece of filter paper. Bright field TEM micrographs were obtained on a FEI Tecnai Osiris microscope operating at 200 kV. Images were analyzed using the ImageJ software package developed at the US National Institute of Health. For the statistical diameter analysis, minimum of 200 NPs were carefully traced by hand to determine their contour diameter. Synthesis of Compound 3. Benzimidazole (0.50 g, 4.2 mmol, 1.0 equiv.), compound 150 (1.6 g, 5.1 mmol, 1.2 equiv.), and Cs2CO3 (2.1 g, 6.4 mmol, 1.5 equiv.) were placed in a pressure tube. Acetonitrile (30 mL) was added to this mixture, the pressure tube was


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sealed, and heated at 90 °C for 24 h. At this point, the solvent was removed under reduced pressure. CH2Cl2 (10 mL) was added and the residue was filtered through a pad of celite. After the removal of CH2Cl2 on a rotary evaporator, the remaining brown residue was transferred to a 100 mL round bottom flask. 1-bromododecanethiol (3.1 g, 12 mmol, 3.0 eq) and acetonitrile (35 mL) were also added to the flask and the resulting solution was refluxed at 90 °C for 18 h. After removal of the solvent on a rotary evaporator, the residue was dissolved in minimal CH2Cl2 and compound 3 was precipitated by adding diethyl ether (50 mL). This precipitation procedure was repeated twice to give pure 3 in 72% yield. 1H NMR (500 MHz, CD2Cl2): δ 11.06 (s, 1H, NCH=N), 7.94 (m, 1H, ArH), 7.72 (m, 1H, ArH), 7.65 (m, 2H, ArH), 4.78 (t, 2H, JHH = 5.0 Hz, NCH2CH2O), 4.56 (t, 2H, JHH = 7.5 Hz, NCH2C11H23), 4.04 (t, 2H, JHH = 4.9 Hz, NCH2CH2O), 3.66 (m, 2H, OCH2CH2O), 3.52 (m, 4H, OC2H4O), 3.44 (m, 2H, OCH2CH2O), 3.29 (s, 3H, OCH3), 2.04 (m, 2H, NCH2CH2C10H21), 1.39 (m, 4H, NC2H4C2H4C8H17), 1.25 (m, 14H, NC4H8C7H14CH3), 0.86 (t, 3H, JHH = 11.8 Hz, CH2CH3). 13C{1H} NMR (75 MHz, CD2Cl2): δ 143.27, 132.64, 131.66, 127.36, 114.83, 113.19, 72.32, 71.08, 70.83, 70.80, 69.29, 59.09, 54.00, 48.17, 48.11, 32.41, 30.11, 30.03, 29.92, 29.84, 29.56, 27.04, 23.19, 14.39. HRESI-MS (m/z) for C26H45N2O3+ [M]+: 433.3442, Calc.: 433.3425. *Note: 2D 1H – 1H COSY spectrum of 3 was used to assign the protons to their corresponding chemical shifts (see Figure S3). Synthesis of Compound 4. Compound 3 (87 mg, 0.17 mmol, 1.0 equiv.), chloro(dimethylsulfide)gold(I) (50 mg, 0.17 mmol, 1.0 equiv.), and K2CO3 (72 mg, 0.52 mmol, 3.0 equiv.) were stirred in acetone (15 mL) at 60 °C for 18 h. After cooling to room temperature, the mixture was filtered through a pad of celite and acetone was removed under reduced pressure. The target amphiphilic NHC-AuI complex 4 was obtained in 88% yield as an off white sticky solid after purification by flash chromatography using CH2Cl2:methanol (25:1) as eluent (Rf = 0.5). 1H NMR (300 MHz, CD2Cl2): δ 7.65 (m, 1H, ArH), 7.45 (m, 3H, ArH), 4.65 (t, 2H, JHH = 5.1 Hz, NCH2CH2O), 4.48 (t, 2H, JHH = 7.3 Hz, NCH2C11H23), 3.96 (t, 2H, JHH = 5.2 Hz, NCH2CH2O), 3.49 (m, 8H, (OC2H4O)2), 3.29 (s, 3H, OCH3), 1.95 (m, 2H, NCH2CH2C10H21), 1.37 (m, 4H, NC2H4C2H4C8H17), 1.25 (m, 14H, NC4H8C7H14CH3),


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0.87 (t, 3H, JHH = 6.1 Hz, CH2CH3). 13C{1H} NMR (75 MHz, CD2Cl2): δ 181.98 [Au(I)C(NHC)], 134.65, 133.52, 124.96, 124.90, 113.36, 111.94, 72.55, 71.50, 71.11, 71.07, 70.90, 59.28, 54.00, 49.56, 49.51, 32.61, 30.60, 30.31, 30.25, 30.15, 30.04, 29.90, 27.44, 23.39, 14.60. HRESI-MS (m/z) for C26H45N2O3AuBr+ [M]+: 709.2306, Calc.: 709.2274. Synthesis of Amphiphilic NHC-Stabilized AuNPs. Compound 4 (26 mg, 36 µmol, 1.0 equiv.) was dissolved in CH2Cl2 (10 mL) in a 50 mL round bottom flask. In a separate vial, NaBH4 (14 mg, 0.36 mmol, 10 equiv.) was dissolved in DI-water (10 mL). NaBH4 solution was then added to the gold complex solution and the resulting mixture was vigorously stirred for 16 h at room temperature. The aqueous layer was then separated to leave a wine-red organic solution. After the removal of CH2Cl2 under a stream of air, the crude particles were dissolved in THF (2 mL) and dialyzed against HPLC grade THF for 24 h (50 mL) with two dialysate changes. Removal of THF under reduced pressure yielded the target amphiphilic NHC-stabilized AuNPs (8.3 mg). These NPs were characterized by UV-vis, DLS, TGA, and XPS as discussed in the text. Self-Assembly of Amphiphilic NHC-Stabilized AuNPs in Polar Solvents. To obtain 0.3 mg/mL solutions of the self-assembled materials, DI-water or ethanol (EtOH) (3 mL) was added to a solution of amphiphilic NHC-stabilized AuNPs in THF (4.2 mg/mL, 0.2 mL) and, after shaking the samples for 5 s, the resulting colloidal solutions were aged at room temperature for 24 h. UV-vis spectra of these AuNP ensembles were collected after 2 and 24 h of the self-assembly using a Varian Cary 60 Bio UV–visible spectrophotometer. Preparation of NP ensembles in mixtures of DI-water and EtOH follows the same procedure except mixtures of these solvents with varying contents of EtOH were used as the self-assembly media. Stability of AuNP Discrete Particles and Ensembles in the Presence of Thiols. To study the stability of the AuNP ensembles in aqueous media in the presence of glutathione (GSH), a 0.1 mg/mL solution of the assemblies (3 mL) was prepared and aged for 24 h first. GSH (0.06 M, 0.1 mL) was added to the assemblies to provide a GSH final concentration of 2 mM. UV-vis spectra were recorded immediately and then every 1 h for up to 24 h at room temperature using an OLIS (On-Line Instrument Systems, Bogart, Georgia) modified Cary 17 UV-vis spectrophotometer. The sample was placed in


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the spectrophotometer cell holder for 15 min prior to the addition of thiol to allow thermal equilibration to 23 °C. Same procedures were adapted to investigate the stability of the discrete amphiphilic NHC-stabilized AuNPs except THF was used as the solvent and 1-dodecanethiol was used as the thiol of the choice.

RESULTS AND DISCUSSION Synthesis of Amphiphilic NHC-AuI Complex. Our design of the target amphiphilic AuNPs

required

a

benzimidazolium-based

precursor

that

is

asymmetrically

functionalized with triethylene glycol (TEG) as the hydrophilic chain and a dodecyl alkyl group as the hydrophobic segment. To synthesize such a molecule, we first reacted benzimidazole with p-toluenesulfonyl (tosyl)-functionalized TEG (1) using cesium carbonate as the base in refluxing acetonitrile (ACN) overnight (Scheme 2). The crude product (2) was then reacted with 1-bromododecane in refluxing ACN without further purification to form the target amphiphilic NHC precursor 3 in 72% yield. Formation of 3 was confirmed by the appearance of a singlet peak at 11.06 ppm, corresponding to the proton in its carbenic position, in its 1H NMR (Figure S1). In the next step, we reacted 3 with chloro(dimethylsulfide)gold(I) in the presence of potassium carbonate in acetone at 60 °C overnight.51 After purification by flash chromatography using CH2Cl2:methanol (25:1) as eluent, the target amphiphilic NHC-AuI complex 4 was obtained in 88% yield as an off-white sticky solid.


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Scheme 2. Synthesis of amphiphilic NHC-AuI complex 4.

Formation of 4 was confirmed by loss of the diagnostic peak for the C2-C–H appearing at 11.06 ppm in its 1H NMR spectrum (Figure S4), indicative of successful in situ carbene formation and coordination to the gold(I) precursor. In addition, the peak at 181.27 ppm in the 13C NMR of 4 (Figure S5), corresponding to the carbene carbon atom in C–Au, is observed in good agreement with literature values.52 Synthesis and Characterization of Amphiphilic NHC-Stabilized AuNPs. With NHCAuI complex 4 in hand, we then prepared amphiphilic AuNPs, employing an adapted procedure of Prezhdo, Brutchey and coworkers for the synthesis of NHC-stabilized silver NPs.53,54 The target amphiphilic NHC-functionalized AuNPs were obtained by the reduction of complex 4 with excess NaBH4 in a water and CH2Cl2 biphasic system. Unlike monophasic solvent systems for the reduction of gold complexes, the biphasic system provides a reaction medium in which reduction of the gold complex takes place slowly, which results in the formation of good quality, highly uniform NPs. After removing deionized (DI) water, the resulting AuNPs were purified by dialysis against tetrahydrofuran (THF) to remove any unreacted starting complex 4. TEM analysis of the resulting pure NPs revealed the formation of highly uniform AuNPs with an average diameter of 4.1 ± 1.1 nm (Figure 1a and Figure S6a). TEM analysis gives a reliable measure of the AuNP core size in which the corona-forming segments of the particles are excluded due to their much lower electron contrast relative to the Au core. For this


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reason, we used DLS to measure the overall hydrodynamic radius of the amphiphilic AuNPs in solution. As shown in Figure 1b, the average diameter of the AuNPs was measured to be ~ 9 nm in THF. This increase in the average diameter of the amphiphilic AuNPs is consistent with the fact that DLS measurements take into account the added volume of the corona-forming segments. UV-vis analysis of the AuNPs in THF confirmed the presence of a plasmonic band at 525 nm (Figure S6b).

Figure 1. (a) TEM micrograph with the corresponding size histogram, and (b) DLS volume distribution of the amphiphilic AuNPs in THF. XPS spectra of (c) N 1s, and (d) C 1s for the AuNPs. XPS spectra of (e) N 1s, and (f) C 1s for compound 4.

Next we performed XPS measurements to investigate the binding of the NHCs on the AuNP surface. As shown in Figure 1c, the N 1s peak of the NHC appears at ~ 401 eV which is consistent with our previous studies on flat gold surfaces and nanoparticles.44,48 Aromatic and sp3 carbons in the hydrophobic alkyl chain were observed at 285.4 and 284.9 eV, respectively, and carbon atoms attached to nitrogen (C–N) and oxygen (C–O)


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at 286.4 and 286.8 eV, respectively, in the C 1s spectrum of these amphiphilic AuNPs (Figure 1d). As expected, the N 1s spectrum for this molecule exhibits only one peak at about 401 eV (Figure 1e). These values correspond well with what is observed in molecular complex 4 (Figure 1e and 1f).55-57 The Br 3d5/2 and 3d3/2 signals for compound 4 were found at ~ 68.8 and 69.8 eV (Figure S7a), respectively, consistent with the reported values in the literature.58 As anticipated, after the formation and purification of the AuNPs, we did not observe any Br species in their Br 3d spectrum (Figure S7b). Thermogravimetric analysis (TGA) was employed to calculate the Au:NHC ratio in the resulting NPs (Figure S8). This value was determined to be 57:43 (weight ratio). Comparing the weight percent of the NHC from TGA analysis and the diameter of AuNPs, from TEM measurements, we were able to calculate the average number of NHCs on each NP following the method employed by Johnson and coworkers.45 An average number of ~ 720 amphiphilic NHCs was present on the surface of each AuNP. This value is in excellent agreement with that reported in the literature for NHCfunctionalized AuNPs with similar core sizes.45 Self-Assembly of Amphiphilic NHC-Stabilized AuNPs in Polar Solvents. Having completed the synthesis of the amphiphilic NHC-stabilized AuNPs, we then studied their self-assembly behavior in polar solvents such as DI-water and EtOH. For this purpose, the AuNPs were first dissolved in THF, which is a good solvent for both the hydrophilic and the hydrophobic chains on the NPs. In this solvent, AuNPs exist in discrete form. In the next step, DI-water or EtOH was added to these AuNPs and the resulting mixtures were left to age for 2–24 h at room temperature. The THF content of the solutions was adjusted to a value as low as 3.2% v/v. After 2 h of selective solvent addition, a drop of each sample was drop-cast on a carbon-coated copper grid and analyzed by TEM. As shown in Figures 2a and S9a, while relatively small islands of AuNP ensembles were observed for the sample in DI-water, higher order aggregation was evident for AuNPs self-assembled in EtOH (Figure 2b and S9b). Consistent with this, UV-vis analyses of these samples revealed that the plasmonic band of the discrete NPs had red-shifted from 525 nm to 555 and 580 nm for the ensembles in DI-water and EtOH, respectively (Figure 2c).


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Figure 2. TEM micrographs of AuNP ensembles in (a) DI-water and (b) EtOH after aging the samples for 2 h. (c) UV-vis spectra of AuNP assemblies in DI-water and EtOH after 2 h.

Upon further aging for 24 h, the aqueous sample did not change noticeably (Figure 3a). On the other hand, amphiphilic AuNPs in EtOH had grown to aggregated structures considerably larger than those observed after 2 h (Figure 3b). DLS data for these assemblies were consistent with TEM data. As shown in Figure 3c, size of the assemblies formed in DI-water after 2 h (~ 220 nm) increased only slightly (~ 250 nm) upon further aging to 24 h. On the other hand, while NP assemblies of ~ 700 nm were formed within 2 h in EtOH (Figure 3d), a bimodal distribution of self-assembled AuNPs with maxima at ~ 200 nm and ~ 5 µm was observed after 24 h. The collective optical properties of these samples were further investigated by UV-vis spectroscopy. As shown in Figure 3e, the absorption spectrum of the sample in DI-water after 24 h was identical to that of 2 h with no change in the plasmonic band at 555 nm. However, for the AuNPs in EtOH, this value red-shifted from 580 nm (after 2 h) to 640 nm after 24 h. This result is consistent with higher order of aggregation observed for this sample by TEM and DLS measurements. We would like to note that this self-assembly behavior is reversible as dialyzing these samples against THF resulted in redispersion of AuNPs. This is evident by the blue shift in the plasmonic bands of the assemblies back to 525 nm (Figure S12) confirming disintegration of the aggregates and the formation of discrete amphiphilic AuNPs.


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Figure 3. TEM micrographs of AuNP ensembles in (a) DI-water and (b) EtOH after aging the samples for 24 h. DLS volume distribution of the amphiphilic AuNP ensembles in (c) DI-water and (d) EtOH after 2 and 24 h. (e) UV-vis spectra of AuNP assemblies in DI-water and EtOH after 24 h. (f) UV-vis spectra of AuNP ensembles in mixtures of DIwater and EtOH with varying EtOH content after 2 h.

Although the interparticle distance measurement in the dry state is not an accurate reflection of their actual values in solution, it can still provide insight into the processes at play. This value was measured to be 1.5 ± 0.2 nm and 1.0 ± 0.2 nm for samples exposed to DI-water and EtOH for 24 h, respectively. Thus AuNPs within the ensembles in EtOH are slightly more densely packed than in aqueous media. These observations imply that the self-assembly of these amphiphilic AuNPs in aqueous media is fast and largely irreversible, during which the AuNP aggregates are formed within the first 2 h and remain unchanged, while self-assembly in EtOH seems to be dynamic. The highly-insoluble nature of the hydrophobic segment in aqueous media likely results in facile aggregation to form the initial aggregates upon the addition of DI-


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water. After this fast process, these alkyl chains may not have enough mobility in water to enable further self-assembly. On the other hand, although EtOH is a polar organic solvent, it can partially solubilize dodecyl blocks on the amphiphilic AuNP surfaces, enabling them to reorganize and form higher aggregates. It should be noted that it is the aggregation numbers of the aggregates as well as the interparticle distances within the structures that change in this process, not the discrete AuNPs. In fact, the average diameter of the AuNPs was found to be 4.1 ± 1.3 nm and 4.3 ± 1.3 nm within the aggregates formed in DI-water and EtOH, respectively, after 24 h. In addition, the effect of NP concentration on the self-assembly behavior and optical properties of the resulting aggregates was investigated. As shown in Figure S10, no significant change in the plasmonic bands of the resulting aggregates was observed when samples with final concentrations of 0.15, 0.3, and 0.6 mg/mL were employed. Taking advantage of this difference in the solubility of dodecyl chains in DI-water and EtOH, we then attempted to tune the collective plasmonic properties of the resulting AuNP ensembles in mixtures of DI-water and EtOH. For this purpose, we prepared three samples in this solvent mixture with 20%, 50%, and 80% EtOH content (v/v). UV-vis measurements of these samples after 2 h demonstrated that the plasmonic band of the ensembles was red-shifted to 570, 600, 615 nm, respectively, as the EtOH content of the solutions increased (Figure 3f). This observation was further supported by TEM analysis of these samples. As shown in Figure S11, by increasing the EtOH content in solution, aggregates of larger size are formed that can account for the red shift of their respective plasmon band. Thus by careful adjustment of the solvent polarity we can gain control over the optical properties of AuNPs decorated with small molecule amphiphiles. We would like to note that for samples with EtOH contents higher than 50%, the resulting aggregates form macroscopic precipitates after 24 h. In addition to polar solvents, we also studied the self-assembly of the amphiphilic AuNPs in hexanes, which is a good solvent for the long alkyl chain and a relatively poor solvent for TEG in the AuNPs. Thus, the former is expected to form a corona of the resulting aggregates while the latter would avoid contact with solvent. TEM analysis of this sample showed the presence of both AuNP aggregates and their discrete building blocks (Figure S13). This is because although hexane is a selective solvent for the alkyl chain, it can also


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solubilize TEG chains to some extent. In fact, hexane has been used for the removal of unreacted TEG.59 As a result, self-assembly in nonpolar solvents is not as efficient as in polar solvents. Stability of the Amphiphilic NHC-Stabilized AuNP Ensembles against Thiols. The stability of AuNPs under physiological conditions is essential for potential biological applications. Glutathione (GSH) is a thiol-containing tripeptide found in biological media, and is often responsible for in vivo degradation of Au nanostructures as it replaces ligands on the surface of nanoparticles.60,61 In advanced biomedical applications, this can result in loss of key functionality such as targeting or stabilizing ligands. Thus far, watersoluble AuNPs with small core sizes have been found to be prone to decomposition in the presence of GSH.45,48 For instance, it has been shown that poly(ethylene glycol)functionalized AuNPs are stable for up to 3 h in the presence of GSH.45 However, for biological applications further stability may be desired. Although using AuNPs with larger core size gave slightly improved stability under the same conditions,48 systems that show high stability against thiol-induced degradation are still unknown. Thus, we chose to investigate the stability of our AuNP ensembles in DI-water (0.1 mg/mL) in the presence of excess GSH (2 mM) at room temperature for 24 h. As shown in Figure 4a, after this period of time, only a 10% decrease in the plasmonic band intensity was observed. Compared to our previously published results,48 this is a significant enhancement in the stability of AuNPs in their self-assembled form. Moreover, TEM analysis of this sample after 24 h confirms that the AuNP assemblies retain their integrity after treatment with GSH (Figure 4b). To compare this finding to discrete NPs, we examined the stability of the amphiphilic NHC-stabilized AuNPs in THF (0.1 mg/mL). Note that DI-water could not be used in this case because of the amphiphilic nature of the AuNPs which results in their self-assembly into aggregates in aqueous media. In the presence of excess 1-dodecanethiol (2 mM– employed instead of GSH due to the latter’s poor solubility in THF) at room temperature for 24 h, a 25% decrease in the plasmonic band intensity of the starting amphiphilic AuNPs was observed (Figure 4c) while the NPs remained well-dispersed in solution (Figure 4d). Thus aggregated NPs are more stable against thiol-induced decomposition than discrete NPs with identical surface functionality. It should be noted that this is not an


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ideal comparison as both the solvents and theiols used in the experiments are different. However, it provides us with the opportunity to indirectly compare the two systems. Thus, besides enabling access to superior collective properties, self-assembling NPs into higher order aggregates is an effective method to enhance their stability against thiols (and potentially other species).

Figure 4. (a) UV-vis spectra demonstrating the stability of amphiphilic AuNP ensembles (0.1 mg/mL) in aqueous GSH (2 mM) at room temperature during 24 h and (b) TEM micrograph of AuNP ensembles in aqueous GSH at room temperature after 24 h. (c) UVvis spectra demonstrating the stability of discrete amphiphilic AuNPs (0.1 mg/mL) in THF in the presence of 1-dodecanethiol (2 mM) at room temperature during 24 h and (d) TEM micrograph of the discrete AuNPs in THF in the presence of 1-dodecanethiol at room temperature after 24 h.

CONCLUSIONS


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In summary, we report herein amphiphilic NHC-stabilized AuNPs. These NPs were extensively characterized by TEM, DLS, UV-vis and TGA methods. In DI-water, selfassembly of these amphiphilic NHC-stabilized AuNPs resulted in an initial red shift in the plasmonic band of the discrete NPs from 525 nm to 555 nm within the first 2 h. TEM analysis confirmed the presence of aggregates under these conditions. Aging for an additional 22 h gave no further changes in the aggregation state as assessed by TEM, DLS and UV-vis spectroscopy. AuNP ensembles were found to be more dynamic in nature when EtOH was used as the self-assembly solvent. While an initial red shift to 580 nm was observed during the first 2 h of EtOH addition, aging for 24 h resulted in further red shifting of the plasmonic band to 640 nm. TEM and DLS measurements confirmed even greater aggregation occurring over this time frame. We were able to fine tune the optical properties of the resulting AuNP ensembles from 525 nm to 615 nm, in a modular manner, by carefully adjusting the solvent composition of the self-assembly media using different contents of EtOH in DI-water/EtOH mixtures. Finally, when tested the stabilities of the AuNPs both in their discrete and self-assembled forms, we found that the AuNP ensembles had superior resistance against etching by thiols at room temperature. Further structural modifications to gain control over the self-assembled materials’ sizes and morphologies as well as potential biomedical applications of the resulting AuNP assemblies are the subject of current investigations in our laboratory. We believe that the introduction of NHCs as anchoring ligands to attach amphiphiles onto NP surfaces, in place of the traditional thiol-based ligands, will receive significant interest among the scientific community.

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(56) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. Formation of Self-Assembled Monolayers by Chemisorption of Derivatives of Oligo(Ethylene Glycol) of Structure HS(CH2)11(OCH2CH2)MOH on Gold. J. Am. Chem. Soc. 1991, 113, 12-20. (57) We would like to note that a peak at ~289 eV was observed in Figure 1f that could result from the presence of trace impurities (< 5%) in the sample. Additionally, we note that the experimental C/N in the nanoparicle sample deviates slightly from the calculated value. We attribute this to the lower signal-to-noise ratio in the nanoparticle sample compared to that corresponding to compound 4 and concomitant higher sensitivity to aliphatic and aromatic contaminants that are often found in XPS chambers. (58) Van Attekum, P. M. T. M.; Van der Velden, J. W. A.; Trooster, J. M. X-Ray Photoelectron Spectroscopy Study of Gold Cluster and Gold(I) Phosphine Compounds. Inorg. Chem. 1980, 19, 701-704. (59) Nazemi, A.; Boott, C. E.; Lunn, D. J.; Gwyther, J.; Hayward, D. W.; Richardson, R. M.; Winnik, M. A.; Manners, I. Monodisperse Cylindrical Micelles and Block Comicelles of Controlled Length in Aqueous Media. J. Am. Chem. Soc. 2016, 138, 44844493. (60) Parida, S.; Maiti, C.; Rajesh, Y.; Dey, K. K.; Pal, I.; Parekh, A.; Patra, R.; Dibakar, D.; Dutta, P. K.; Mandal, M. Gold Nanorod Embedded Reduction Responsive Block Copolymer Micelle-Triggered Drug Delivery Combined with Photothermal Ablation for Targeted Cancer Therapy. Biochim. Biophys. Acta 2017, 1861, 3039-3052. (61) Hong, R.; Han, G.; Fernandez, J. M.; Kim, B. J.; Forbes, N. S.; Rotello, V. M. Glutathione-Mediated Delivery and Release Using Monolayer Protected Nanoparticle Carriers. J. Am. Chem. Soc. 2006, 128, 1078-1079.


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