Research Article www.acsami.org
Pentafluorophenyl Ester-Functionalized Nanoparticles as a Versatile Platform for Selective and Covalent Inter-nanoparticle Coupling Jimmy Lawrence*,† and Todd Emrick Department of Polymer Science and Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: Preparing chemically selective nanoparticle (NP) building blocks to make robust structures from different NP compositions often requires complex hetero-bifunctional ligand pairs that have limited scalability and versatility. Here we describe pentafluorophenyl ester-functionalized nanoparticles (PFP-NPs) as versatile building blocks for covalent inter-NP coupling. This approach allows for a rapid and dense grafting of PFPfunctionalized Au NPs onto several types of amine-functionalized NPs (metals, semiconductors, and insulators) and selective identification of amine-functionalized quantum dots (QDs) in solution. Such simple yet efficient inter-NP reactions suggest the suitability of PFP-NPs as a versatile functional platform for numerous NP-based applications. KEYWORDS: nanoparticle grafting, nanoparticle covalent coupling, hybrid nanocomposite, pentafluorophenyl ester, raspberry nanoparticles, selective photoluminescence quenching, amine-reactive nanoparticles, core−satellite nanostructure, raspberry-like morphology
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INTRODUCTION The availability of nanoparticles (NPs) having chemical functionality useful for forming covalently connected, hybrid NP structures is promising for labeling, sensing, and catalysis.1,2 The seminal work of Mirkin et al. on DNA-interconnected binary NP networks3 inspired reports of complementary functional ligand pairs and bifunctional DNA linkers for covalent inter-NP coupling.4,5 Methods to form inter-NP covalent linkages from brick-and-mortar3,6,7 and “sticky” NPs8 have opened paths to multifunctional hybrid nanocomposites, which exhibit enhanced energy transfer,9 nonwetting properties,10,11 photoactivity,9 and drug delivery capabilities.12 However, such methods remain limited by a lack of general versatility or scalability of the functional ligands (e.g., thiolfunctionalized DNA linkers).1 Recent alternative approaches to prepare interconnected NPs described in situ synthesis of a second NP on the surface of the first NP (e.g., heterodimers13), as well as the use of noncovalent electrostatic forces in aqueous media.14,15 Covalent coupling of chemically reactive NPs employing simple, catalyst-free conditions would be beneficial for obtaining hybrid nanostructures. Covalent NP coupling by a catalyst-free acylation, for example, using NPs functionalized with amine-reactive ligands, is attractive for achieving NP interconnections. N-Hydroxysuccinimide (NHS) esters, employed as acylating reagents in bioconjugation, are described as ligands for labeling Au clusters (Au11(P(C6H5)3)7),16 while sulfo-NHS-(4-N-maledoimidomethyl) cyclohexane-1-carboxy© 2016 American Chemical Society
late cross-linkers are reported for attaching dye-doped SiO2 NPs to magnetic NPs (mNPs) for imaging.17 However, the well-documented hydrolysis problem of NHS ester groups18 drives the implementation of alternative approaches to functionalize nanoscale materials. We and others have utilized pentafluorophenyl (PFP) esters on polymers for postpolymerization substitution reactions, finding this a versatile method for both aqueous and organosoluble polymer functionalization.19,20 PFP-esters were found superior to NHS-esters in polymer grafting with aliphatic amines.21 In this Article, we describe the synthesis of organosoluble NPs containing PFP ligands and their use in covalent inter-NP coupling chemistry. We began by preparing Au NPs decorated with a corona of pentafluorophenyl esters (PFP-Au NPs) and examined their use as building blocks to form covalently linked NP structures. PFP-esters should be useful as NP ligands, given their known enhanced hydrolytic stability relative to NHS-esters, but to our knowledge such structures have not been reported. The strategy presented here offers several advantages: (1) the simple preparation of PFP-NPs; (2) the rapid reaction of PFPNPs with amines; and (3) the demonstrated generality of this approach across numerous NP compositions. Remarkably, PFP-NPs afford well-defined covalent NP assemblies with a fidelity normally derived from more complex coupling Received: November 27, 2015 Accepted: January 5, 2016 Published: January 5, 2016 2393
DOI: 10.1021/acsami.5b11550 ACS Appl. Mater. Interfaces 2016, 8, 2393−2398
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Conjugation between PFP-NP (NP-1) and amine-functionalized NPs (NP-2); (b) TEM images of PFP-Au NP conjugation to (left-toright) cysteamine-Au NPs (NP-2Au), amine-functionalized silica NPs (NP-2SiO2), 11-aminoundecane-1-thiol-functionalized CdSe@ZnS QDs (NP2QD), and dopamine-functionalized Fe2O3 magnetic NPs (NP-2mNP). Scale bars = 10 nm.
methods.4,5,17 The effectiveness of the PFP ligand periphery for coupling was confirmed by visualization of inter-NP connectivity by reaction of amine-functionalized quantum dots (QDs) with PFP-Au NPs (Figure 1).
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2Au, 30 nm diameter),23 amine-functionalized SiO2 NPs (NP-2SiO2, 50 nm diameter),24 11-aminoundecane-1-thiol25-functionalized CdSe@ ZnS QDs26 (NP-2QD, 13 nm diameter), and dopamine-functionalized superparamagnetic Fe2O3 NPs (NP-2mNP, 18 nm diameter).27 The bright red NP-2Au samples were freshly prepared and used within 2 days. Well-dispersed NP-2SiO2 was obtained by brief sonication of the pelletized NP-2SiO2 (after purification by centrifugation), followed by a centrifugation cycle (30 s at 5 krpm) to remove any aggregates. NP2QD was obtained by exchanging the native oleic acid ligands on the CdSe@ZnS QDs with 11-aminoundecane-1-thiol. 11-Aminoundecane1-thiol was attached to CdSe@ZnS QDs by adding 5 wt equiv of ligand to the solution of QDs, followed by heating at 40 °C for 2 h. The QDs were isolated by precipitation in a hexane/CHCl3 mixture to obtain amine-functionalized QDs (NP-2QD; average size, 13 nm). Similarly, NP-2mNP was obtained following ligand exchange of the native oleic acid ligands with dopamine hydrochloride. The superparamagnetic iron oxide nanoparticles (mNPs) were dissolved in chloroform and added with dopamine hydrochloride (5 equiv weight) in DMF. The mixture was sonicated for 1 h and allowed to stand for 24 h. The resulting dopamine-functionalized mNPs (NP-2mNP) were purified by precipitation in MeOH, then centrifuged, and re-dispersed in water. The purification process was repeated twice and the highly water-soluble NP-2mNP sample was stored as an aqueous solution. For NP conjugation, NP-1 (1−3 nmol) was added to a solution of NP-2 (1 nmol) and allowed to stand for ∼10 min. In the case of aqueous NP-2 samples, a concentrated solution of NP-2 (1 nmol, 20 μL of 50 μM) was added to a DMF solution of NP-1 (1−3 nmol, 1:50
EXPERIMENTAL SECTION
Materials. Chemicals and solvents were purchased from SigmaAldrich unless specified otherwise. Mercaptoundecanoic acid (MUA)functionalized Au NPs were synthesized by a one phase method.22 Au(PPh3)Cl (0.25 mmol) and 11-mercaptoundecanoic acid (MUA, 0.125 mmol) were dissolved in THF/isopropanol mixtures (1:1 (v/v)) in a 20 mL vial. Borane tert-butylamine complex powder (2.5 mmol) was added with stirring. The mixture changed colors from light yellow to dark purple/red after 4 h. The Au NPs were precipitated in ether and collected by centrifugation. The MUA-covered Au NPs were purified by washing with mixtures of CHCl3 and ether, followed by centrifugation−re-dispersion cycles. PFP functionalization to afford NP-1 was accomplished by carbodiimide coupling of pentafluorophenol with the MUA-NPs. A solution of MUA-Au NPs in DMF (20 mg in 1 mL), averaging 5 nm in diameter, was added to a stirring DMF solution of pentafluorophenol (0.2 M) followed by addition of 1-ethyl3-(3-(dimethylamino)propyl) carbodiimide (EDC, 0.1 M) with continued stirring for 30 min. The PFP-Au NPs were purified by consecutive precipitation−centrifugation−re-dispersion cycles and stored at room temperature as solutions in DMF or CHCl3. The amine-functionalized NPs (NP-2) utilized here were prepared as reported in the literature: cysteamine-functionalized Au NPs (NP2394
DOI: 10.1021/acsami.5b11550 ACS Appl. Mater. Interfaces 2016, 8, 2393−2398
Research Article
ACS Applied Materials & Interfaces (v/v)). Stoichiometry was determined from the NP extinction coefficient, diameter, density, and concentration (mg/mL). In the case of Au NP-mNP assemblies, the product was separated from excess NP-1 using a commercial neodymium magnet. This purification process was repeated, and the magnetically responsive NP assemblies were dispersed in DMF. Experimental Techniques. UV−vis spectra of the NP solutions were recorded on a PerkinElmer Lambda 25 spectrophotometer. Photoluminescence measurements were recorded on a PerkinElmer LS-55 fluorometer. Infrared (IR) spectra were obtained on a PerkinElmer Spectrum One FTIR spectrometer equipped with an ATR accessory. 1H and 31P NMR spectra were recorded using a Bruker Spectrospin 300 spectrometer operating at the appropriate frequencies using the residual solvent peak as the internal reference. Xray photoelectron spectroscopy (XPS) was performed on a Physical Electronics Quantum 2000 spectrometer with Al Kα excitation at a spot size of 100 μm at 25 W. XPS samples were prepared by drying a drop of concentrated nanoparticle solution on a clean Si wafer. XPS spectra were obtained at a 75° takeoff angle with respect to the plane of the sample surface. Transmission electron microscopy (TEM) was performed on a JEOL 200CX microscope. TEM grids (3−4 nm amorphous carbon film supported on 400 mesh copper grids) were purchased from Ted Pella, Inc. TEM samples were prepared by drop casting the NP solution onto the carbon-coated grid and drying the grid under ambient conditions. The organic content (weight percent organic) of the NPs was determined by thermogravimetric analysis (TA Instruments; dynamic scans under N2; heating rate of 10 °C/min from 50 to 650 °C).
cally in Figure 1b for NP-1 conjugation to NP-2 (aminefunctionalized Au NPs, SiO2 NPs, CdSe@ZnSe QDs, and Fe2O3 NPs). TEM images of the NP@NP structures show the presence of multiple NP-1 on the surface of Au, SiO2, CdSe@ ZnS, and Fe2O3 NPs, forming “core−satellite” clusters with NP-2 as the core and NP-1 satellites (raspberry-like morphology). There was little evidence for the presence of ungrafted NP-2 in solution, with aggregates indicative of interNP coupling dominating the TEM images (Figure 3a). Control
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RESULTS AND DISCUSSION FTIR spectra of the PFP-Au NPs (NP-1) showed signals corresponding to the PFP group at 1004 and 1523 cm−1 (νbending of aromatic C−F and νstretching of aromatic CC (inring)) (Figure 2), and carbonyl signals corresponding to the Figure 3. (a) Dense grafting of NP-1 on NP-2Au (30 and 2.7 nm). The arrangement of NP-1 on the NP-2 surface is highlighted as a visual aid. (b) Grafting NP-1 onto NP-2mNP and the isolation of NP-1@NP-2mNP structures (PFP-NP as NP-ligands/cross-linkers). (c) TEM images of NP-1@NP-2mNP. Inset: NP-1 packing on NP-2mNP shown in color, with lines added as a visual aid. Scale bars = 10 nm for all images.
experiments, performed by mixing MUA-Au NP and NP-2Au, showed randomly dispersed NPs and NP self-aggregation (Supporting Information Figure S4).28 These results suggest that NP-1 reacts readily with the amine-rich NP-2 surface, resulting in rapid formation of NP@NP structures. TEM image analysis (assuming spherical NPs and symmetrical front−back surface grafting) showed high grafting density (calculated as the number of NP-1 on a spherical NP surface of 100 nm2) of NP-1 on NP-2Au (1.4 ± 0.5 NP/(100 nm2)), NP-2SiO2 (0.4 ± 0.3 NP/ (100 nm2)), NP-2QD (0.4 ± 0.1 NP/(100 nm2)), and NP-2mNP (1.2 ± 0.4 NP/(100 nm2)). The larger standard deviation in NP-1 grafting density on NP-2SiO2 is presumably due to the variation in the number of available amino surface functional groups on NP-2SiO 2 and steric hindrance imposed by agglomerated NP-2SiO2. NP-2Au is functionalized only with cysteamine ligands, and the 1H NMR of NP-2QD showed quantitative ligand replacement from the native oleic acid ligands to amine-thiol ligands (i.e., the absence of olefin signal of oleic acid ligands at 5.3 ppm). The high water solubility of NP-2mNP (∼20 mg/mL) suggested successful replacement of the native oleic acid ligand with dopamine hydrochloride. Irrespective of the differences in grafting density across NP
Figure 2. FTIR spectra of pentafluorophenol (cyan), PFP-Au NP (NP-1, red), 11-mercaptoundecanoic acid (MUA, blue), and MUA-Au NPs (black).
PFP ester (1790−1820 cm−1) and residual carboxylate (1700− 1730 cm−1). NP-1 displayed reactivity toward amine-containing molecules, as they were cross-linked following addition of diamines or made hydrophilic after addition of aminefunctionalized polymers (Supporting Information Figure S2a,b). The PFP-ester ligand coverage enabled successful grafting of NP-1 onto the surface of NP-2 of various compositions to form NP@NP structures by amidation. This is depicted schemati2395
DOI: 10.1021/acsami.5b11550 ACS Appl. Mater. Interfaces 2016, 8, 2393−2398
Research Article
ACS Applied Materials & Interfaces
Figure 4. Images, photographs, and PL spectra of (a) nonfunctional, oleic acid-covered QDs (OA-QD) combined with nonfunctional, dodecanethiol-covered Au NPs (DDT-Au NP); (b) amine-functionalized CdSe@ZnS QDs (NP-2QD) combined with PFP-Au NPs (NP-1). In each photograph, the tubes on the left are control samples (before adding Au NP). (c) TEM images of NP-1@NP-2QD. Scale bars = 10 nm.
grafting of NP-1 on the surface of NP-2mNP (insoluble in DMF) rendered NP-2mNP soluble in DMF, effectively exchanging organic ligands for new NP “ligands”. TEM image analysis showed NP-2mNP to be grafted extensively with NP-1 (Figure 3c, Figure 3c inset), and the near absence of unreacted NP-1 (Supporting Information Figure S5b). The efficient reaction between PFP-NPs and aminefunctionalized NPs may prove useful for labeling and optoelectronic studies. For example, rapid photoluminescence (PL) quenching of amine-functionalized QDs (NP-2QD, 0.5 nmol) was observed when combined with 2 nmol of NP-1 (Figure 4b). Using the same QD/Au NP ratio, a solution of nonfunctional QDs (i.e., covered with their native ligands) when combined with nonfunctional Au NPs did not exhibit PL quenching even after 2 h (Figure 4a shows only a slight PL decrease due to absorption of Au NPs). The combination of other nonfunctional Au NPs with QDs also led to retention of fluorescence (Supporting Information Figure S6a,b). In contrast, even a very low ratio (1:20) of NP-1 to NP-2QD led to a rapid and large reduction in PL intensity (Figure 4b and Supporting Information Figure S7). We did not observe total PL quenching at this ratio, presumably due to the presence of nongrafted QDs within the NP-1@NP-2QD suspension (Figure 4c) and the size-dependent effects on surface energy transfer (SET),31 as seen from the bimodal PL spectrum of the NP-1@ NP-2QD structures (Figure 4b). Similar PL quenching was
compositions, these experiments confirm the ability of PFP ligands to facilitate inter-NP coupling at densities equal to or greater than those enabled by more complex methods such as DNA.17 Interestingly, we noted that NP-1 is arranged neatly on the surface of NP-2 (Figure 3a). This well-defined arrangement of NP-1, presumably due to the mobility and conformation rearrangement of the surface ligands,29 allows for high density grafting of NP-1 on NP-2. As shown in Figure 3 (with dotted lines as visual aids), NP-1 packs closely on the surface of NP2Au with a few topological defects because of the surface curvature of NP-2Au, steric hindrance, and statistical random coupling of NP-1 to NP-2Au. This resembles the results of Weitz and co-workers obtained for the minimum energy configurations of particles with arbitrary repulsive interactions on curved surfaces.30 The inter-NP coupling was not influenced strongly by NP size; a similarly rapid formation of densely covered inter-NP structures was observed when using 2.7 nm 11-aminoundecane-1-thiol covered Au NPs as the NP-2 component (Figure 3a). These initial results suggest the potential use of NP-1 as a versatile starting material to examine in conjunction with numerous NP compositions. For example, addition of excess NP-1 to NP-2mNP (10 equiv) in solution afforded conjugated structures that were separable from unreacted NP-1 using a magnet (Figure 3b) and pipet/re-dispersion cycles. Such dense 2396
DOI: 10.1021/acsami.5b11550 ACS Appl. Mater. Interfaces 2016, 8, 2393−2398
Research Article
ACS Applied Materials & Interfaces
while simultaneously separating the two different types of NPs in solution. In summary, we have described the versatility of PFPfunctionalized NPs as new building blocks for inter-NP covalent coupling. PFP-NPs can be grafted onto numerous types of amine-functionalized NP building blocks (including metals, semiconductors, and insulators) with alteration of optoelectronic properties as a result of the grafting. Such a simple yet versatile NP grafting methodology will open opportunities in hybrid nanomaterials synthesis owing to the robust nature of the PFP moiety, including its excellent stability in solution combined with rapid and selective reactivity.
observed when a NP-1 sample was added to 11-aminoundecane-1-thiol-functionalized CdSe/ZnS core/shell QDs (Supporting Information Figure S6c). Our results confirmed that PL quenching is not influenced by the composition of QDs but by the proximity of Au NPs to the QD surface (
