Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
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A Versatile Coordinating Ligand for Coating Semiconductor, Metal, and Metal Oxide Nanocrystals Liang Du, Wentao Wang,† Chengqi Zhang, Zhicheng Jin, Goutam Palui,‡ and Hedi Mattoussi* Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States
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S Supporting Information *
ABSTRACT: We detail the design of a new set of multicoordinating polymer ligands based on the phosphonate anchoring motif and apply them for the surface coating of luminescent quantum dots, gold nanoparticles, and iron oxide nanoparticles alike. The ligand is synthesized via a nucleophilic addition reaction between poly(isobutylene-alt-maleic anhydride) and amine-modified phosphonate derivatives and short polyethylene glycol hydrophilic blocks, which allows the flexibility to tune the architecture and stoichiometry of the final compound. We find that these phosphonate-based polymers exhibit a strong coordinating affinity for ZnS-overcoated quantum dots (QDs), Au nanoparticles, and iron oxide nanoparticles, yielding nanocrystal dispersions that exhibit good colloidal stability for all three systems. The affinity of these ligands is also preserved when additional coordinating groups are introduced, such as imidazoles. Furthermore, the resulting polymer-coated nanocrystals are easily functionalized with reactive groups, introduced along the polymer chain during synthesis. The polymer coating is compact enough to allow implementation of resonance energy transfer coupling of luminescent QDs to proximal dyes. The affinity between the ligands and gold nanoparticle surfaces was compared to that of thiol groups using NaCN digestion tests.
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INTRODUCTION Bottom-up solution phase growth has been effectively exploited, over the past two decades, to prepare a wide array of inorganic colloidal nanostructures, with great control over size, shape, and stoichiometry.1−4 For instance, colloidal nanocrystals made of semiconductor cores [quantum dots (QDs)] exhibit unique photophysical properties, including broad absorption and narrow emission profiles extending from the visible to the near-infrared regions of the optical spectrum. These properties have been integrated into applications ranging from commercial light-emitting diodes and photovoltaic devices to fluorescent probes in biology.5−7 Similarly, nanocrystals made of gold cores such as nanoparticles (AuNPs), nanorods (AuNRs), and nanostars exhibit strong surface plasmon absorption properties that can be tuned via size and shape.8 They have been tested in a variety of biologically driven applications such as bioimaging, therapeutic tomography, catalysis of certain chemical reactions, and photodynamic therapy.9−13 Conversely, the strong magnetic response of iron oxide nanoparticles (IONPs) has been exploited to develop contrast agents in magnetic resonance imaging (MRI), as an alternating current energy absorber in hyperthermia, as efficient sorbents in magnetic separation, and as drug delivery nanocarriers.14−16 However, as prepared most of these nanoscale colloids either are stabilized with hydrophobic surface coating, which makes them incompatible with use in vivo and in vitro biological applications, or have weakly bound ligands making the nanocrystals unstable and prone to aggregation in biological media. Several surface modification © XXXX American Chemical Society
strategies have been developed over the past two decades to promote the transfer of these materials to buffer media and render them biocompatible.17−21 One strategy that has found success relies on exchanging the native surface cap with hydrophilic coordinating ligands. This route is easy to implement and can provide nanocrystals with a relatively small overall size. The resulting nanocolloids also exhibit longterm colloidal stability in biological media while offering tunable surface reactivity.22−31 Because ligand-to-metal coordination tends to vary from one core material to another, coating ligands have been essentially “custom-designed” to present specific coordinating groups that are adapted for different types of nanocrystals.32−37 For example, thiolappended ligands have been widely used for functionalizing gold surfaces, given the strong reported affinity between these systems. However, thiols are not effective for coating iron oxide and other magnetic nanoparticles.36,38 In comparison, dopamine was found to exhibit a high affinity for iron-rich nanoparticles but coordinates weakly onto gold or semiconductor cores. Therefore, designing a new coordinating coating platform that can potentially be applied to a wider range of core materials is of great interest. Seminal work by Peng and co-workers demonstrated that capping ligands made of phosphonate derivatives are essential to the formation of the cadmium complex required for the Received: August 19, 2018 Revised: September 13, 2018 Published: September 14, 2018 A
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 1. (A) Schematic representation of the nucleophilic addition reaction used to prepare the multicoordinating polymer ligands. (B) 31P NMR spectra of amine-PAOEt, the PAOEt-PIMA-PEG intermediate, and the final PA-PIMA-PEG (collected after hydrolysis), dissolved in DMSO-d6. (C) 1H NMR spectra of PAOEt-PIMA-PEG and PA-PIMA-PEG in DMSO-d6. Hydrolysis of the ester groups was confirmed by the absence of two peaks at 1.25 and 4.02 ppm, which designate ethyl groups of the phosphonate esters. The signature at ∼2.5 ppm shown in the 1H NMR spectra is attributed to the solvent.
growth of high-quality luminescent quantum dots made of CdSe cores.39 Subsequently, Weiss and co-workers showed that highly purified CdSe core-only QDs grown using the hot injection route are mainly capped by phosphonate molecules.40 Similarly, our group has shown in a recent study that phosphonate complexes exhibit a strong affinity for CdSeZnS core−shell QDs, even though these materials are grown in the presence of a “cocktail” of small molecule surfactants containing a small fraction of phosphonate compounds (e.g., trioctyl phosphine, trioctyl phosphine oxide, and alkyl-amines mixed with 4−5% hexyl phosphonic acid). 41 Several monodentate phosphonic acid-based short chains and oligomers (low-molecular weight polymers) have been tested as coordinating molecules for colloidal nanocrystals made of tin oxide, iron oxide, lanthanide, and metal chalcogenide cores.42−49 These findings suggest that molecular scale phosphonic acid-based ligands can interface a wider range of nanostructures with biological media. However, it has been found that nanoparticles coated with these monodentate ligands tend to exhibit limited colloidal stability in buffer media, a result that can be ascribed to the high rate of ligand desorption from the nanocrystal surfaces.46 Additionally, coordination between phosphonic acid-based ligands and Au nanocolloids is not well understood and has not been thoroughly investigated. We hereby detail the design and optimization of a set of amphiphilic multicoordinating polymers, based on a phosphonate anchoring motif, and show that such polymers can be applied for the coating of luminescent QDs, plasmonic nanoparticles, and iron oxide nanocrystals alike. This design builds on our recent work combining the use of the
nucleophilic addition reaction, a poly(isobutylene-alt-maleic anhydride) and amine-modified anchors and hydrophilic blocks, to prepare a set of high-affinity polymers. It yields multifunctional ligands that present simultaneously multiple phosphonic acid anchors and a combination of inert and reactive PEG blocks within the same chain to allow further coupling of the nanocolloids to target molecules.
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RESULTS AND DISCUSSION The chemical approach presented here builds on previous developments from our group and others, which demonstrated the benefits of the one-step nucleophilic addition reaction applied to poly(isobutylene-alt-maleic anhydride) as a means of designing multicoordinating or amphiphilic encapsulating polymer coatings.20,50,51 Here, we apply this approach to prepare polymer ligands that use phosphonic acid groups as the metal-coordinating motifs to provide enhanced affinity for ZnS-overcoated QDs, Au nanocrystals, and iron oxide nanoparticles alike. The ligands also present several polyethylene glycol (PEG) blocks to promote hydrophilicity and surface reactivity. The ligand is prepared via the addition reaction between PIMA and a mixture of amine-modified phosphonates and amine-appended PEG short chains (either NH2-PEG-OCH3 or a mixture of NH2-PEG-OCH3 and NH2PEG-NH2). This strategy is advantageous because the nucleophilic addition reaction is highly efficient and reagent free. This simplifies the purification steps and increases the product yield. It also offers an alternative route to preparing ligands via direct polymerization of precursor monomers, containing the anchoring groups and hydrophilic blocks, to form the ligands.52−54 B
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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intermediate compound prepared using a mixture of 50% amine-PAOEt and 50% amine-PEG-OCH3 yielded a polymer presenting ∼17 PAOEt and ∼21 PEG-OCH3 moieties per macromolecule; the percentage is estimated with respect to the total number of anhydride rings along the PIMA chain (∼39 units, set as 100%). Conversely, an addition reaction starting with a mixture of 30% PAOEt and 70% PEG-OCH3 precursors yielded an intermediate polymer that has ∼10 PAOEt and ∼28 PEG-OCH3 blocks in each macromolecule. The complete set of spectra is provided in Figure S1. These experimental values are consistent with the nominal molar ratios of the starting precursors. We should also note that using the same reaction route it is possible to introduce a mixed coordination in the same ligand. For instance, we found that if a fraction of the amine-PAOEt was substituted with 1-(3-aminopropyl)imidazole, the reaction yields a polymer that presents a stoichiometric mixture of PA and imidazole anchoring groups along with the PEG blocks (Table 1 and Figure S1), as reported for the polymer prepared with the lipoic acid coordinating motif.51 Mixed coordination ligands yield certain benefits. For instance, polymer ligands that combine PA and imidazole coordinating groups enhance the PL intensity of the QDs in buffer media (compared to ligands presenting PA groups only). This result can be attributed to the nature of the coordination interactions of the imidazole groups onto the surface of core−shell QDs. Like amine groups, imidazoles tend to alleviate the effects of surface defects (i.e., hole or/and electron traps) on the rate of exciton radiate recombination.51,56 This observation is consistent with previous results using LA/His-PIMA-PEG coating (see Figure S2). These results clearly prove that the synthetic route used for preparing the coordinating polymer based on the PA anchoring motif offers a few key advantages. It is highly efficient and reagent free, making purification of the product straightforward.51 Furthermore, it permits the insertion of controllable numbers of anchoring molecules, hydrophilic blocks, and reactive moieties along the same chain. Additionally, the ability to transform as many of the 39 anhydride rings (in a PIMA chain) allows several stoichiometric combinations of anchoring groups, hydrophilic moieties, and functional groups to be explored. The newly designed PA-PIMA-PEG ligands (using 50% PA and 50% PEG as a representative example) were applied to coat luminescent CdSe-ZnS QDs, AuNPs, and IONPs. All three starting materials were capped with hydrophobic ligands. Additional data using a ligand with a different stoichiometry are provided in Figure S3. Three sets of QDs, emitting at 537 nm (green), 569 nm (yellow), and 602 nm (orange), along with oleylamine-AuNPs (7.5 nm in diameter) and oleic acidcapped Fe3O4 NPs (16 nm in diameter) were tested with these polymers. A schematic of the ligand exchange is provided in Figure 2A; additional details about the phase transfer steps are provided in the Supporting Information. Ligand exchange with the new polymer has yielded homogeneous nanoparticle dispersions, as shown in the fluorescence images of the three QD dispersions as well as the white light images of the AuNPs and IONP samples. Characterization of the materials following ligand substitution involved a few analytical tests that include preservation of the native photophysical properties, colloidal stability under various conditions, and reactivity of the colloid surfaces. Panels B and C of Figure 2 show the optical absorption spectra of all three QDs dispersions side-by-side
A schematic representation of the reaction and steps involved is summarized in Figure 1A. The anhydride rings along the PIMA backbone are first reacted with a combination of amine-phosphonate ester and amine-PEG moieties, which yields the intermediate compound PAOEt-PIMA-PEG. This intermediate can be easily purified through a silica column using pure chloroform as the eluent with a high yield (see experimental section in the Supporting Information). Following purification, the intermediate PAOEt-PIMA-PEG is treated with bromotrimethylsilane (TMSBr) and methanol, which promotes hydrolysis of the phosphonate esters along the chain into phosphonic acid groups and yields the final product, PAPIMA-PEG; this hydrolysis step is crucial, as the PAOEt exhibits no metal coordination interactions. Characterization of the reaction intermediate compound and final product confirms the stoichiometric insertion of the PAOEt and PEG blocks along the PIMA chain. Figure 1B shows three 31P nuclear magnetic resonance (NMR) spectra collected from the amine-PAOEt (precursor), the intermediate compound following the addition reaction, PAOEt-PIMA-PEG, and the polymer ligand after hydrolysis, PA-PIMA-PEG. The 31P spectra show that the well-defined peak at ∼30 ppm measured for the PAOEt is preserved in the polymer compounds though with a slight upfield shift to ∼16 ppm for PAOEt-PIMA-PEG and to ∼14 ppm for PA-PIMA-PEG, which can be ascribed to changes in the surrounding conditions. Additionally, the 1H NMR spectrum collected from the polymer intermediate (i.e., before hydrolysis) shows two peaks at ∼1.25 and ∼4.02 ppm (marked with asterisks), ascribed to the methyl and ethylene protons, respectively, confirming that the phosphonate ester groups have indeed been successfully installed along the chain (see Figure 1C, top panel).55 The spectrum collected from the final product (i.e., after hydrolysis) shows that those two signatures have essentially disappeared, indicating the complete transformation of the phosphonate esters into phosphonic acid groups (see Figure 1C, bottom panel). The 1H NMR spectra in Figure 1C also show two clearly defined proton signatures: a strong peak ascribed to the PEG chain (at ∼3.52 ppm) and a sharp peak ascribed to the terminal methyl protons in the PEG-OCH3 blocks (at ∼3.25 ppm). The proton signatures mentioned above, namely, the methyl and ethylene protons of the phosphonate esters (at ∼1.25 and ∼4.02 ppm, respectively) and those of the PEG-OCH3 (at ∼3.25 ppm), have been exploited to extract quantitative information about the stoichiometry of the polymer product and thus the overall reaction efficiency from peak integration values (see Table 1).50 For instance, we found that the stoichiometry of the Table 1. Summary of the Nominal and Experimental Numbers of Different Moieties per PIMA Chain Deduced from NMR Data stoichiometry PAOEt50%-PIMAPEG50% PAOEt30%-PIMAPEG70% PAOEt20%/ API30%-PIMAPEG50%a
nominal no. of PAOEts
nominal no. of PEGs
experimental no. of PAOEts
experimental no. of PEGs
19
20
17
21
12
27
10
28
8
20
8
19
a
Nominal number of APIs (imidazole-containing anchors) = 12. Experimental number of APIs ∼ 12. C
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 2. (A) Schematic depiction of the ligand exchange using PA-PIMA-PEG (50:50 PA:PEG). Images of QDs were collected under an ultraviolet lamp, while those of AuNPs and IONPs were collected under white light exposure. The nanocrystals were transferred from hexane (top layer) to the aqueous phase (bottom layer). The concentrations of QDs, AuNPs, and IONPs used were ∼0.5 μM, 5 nM, and 10 μg/mL, respectively. (B) Absorption and photoluminescence profiles collected from dispersions of green-, yellow-, and orange-emitting QDs (GQD, YQD, and OQD, respectively) before and after ligand exchange. The solid and dashed lines designate nanocrystals before and after phase transfer, respectively. The absorption is normalized with respect to the band edge value, while the PL is normalized with respect to the peak value. The GQDs were excited at 350 nm, while the YQDs and OQDs were excited at 365 nm. (C) Absorption spectra of AuNP dispersions before and after ligand exchange.
with a spectrum collected from AuNP dispersions before and after phase transfer. The photoluminescence spectra of the QDs collected using excitation at 350 or 365 nm are also shown. The absorption and emission spectra of the QDs have been essentially unaffected by the phase transfer. Similarly, the absorption features measured for the AuNPs before and after ligation with the polymer are also fully preserved. The data included above indicate that the photophysical properties of the resulting nanocrystals were essentially unaffected by ligation with the PA-based coordinating polymer. Next, we characterized the colloidal stability of all three sets of nanocolloids under several conditions relevant for use in biology. Indeed, the colloidal stability of the nanocrystals under various conditions combined with the surface reactivity and reduced nonspecific interactions with proteins in serum and other biological media is paramount to their integration within biology and medicine.57,58 Figure 3A shows three sets of images. (1) One set is fluorescence-based acquired from dispersions of green-emitting PA-PIMA-PEG−QDs under ultraviolet (UV) light exposure (using a hand-held UV lamp)
in phosphate buffers over the pH range of 3−12, in the presence of high salt concentrations (1 M NaCl) and in the presence of 0.5 M DTT (dithiothreitol). Shown are images of freshly prepared dispersions, together with those collected after storage for 2, 4, and 6 months at 4 °C (in a fridge); a dispersion of QDs in DI water is shown as a control. (2) A set of AuNP dispersions under the same conditions and storage time. (3) Polymer-ligated IONPs dispersed in phosphate buffers at different pHs and in the presence of 1 M NaCl. Figure 3B shows the fluorescence image of a PA-PIMA-PEG QD dispersion (at 100 nM) tracked after storage for up to 6 months at room temperature and room light exposure. These data confirm that the new polymer coating promotes the colloidal stability of all three sets of nanocrystals over a broad range of conditions, where all dispersions stayed stable for extended periods of time with no sign of degradation or aggregate buildup. These stability tests were further complemented with agarose gel electrophoresis measurements applied to dispersions at various pHs and dispersions incubated with bovine D
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 3. (A) Colloidal stability tests applied to QDs, AuNPs, and IONPs capped with PA-PIMA-PEG (50:50 PA:PEG). Images of QDs were collected under UV irradiation using a hand-held UV lamp, while those of AuNPs and IONPs were collected under room light exposure. The concentrations of QDs, AuNPs, and IONPs were ∼0.5 μM, 15 nM, and 17 μg/mL, respectively. (B) Colloidal stability test of PA-PIMA-PEG−QD dispersions at a low concentration (100 nM), stored under ambient conditions (room temperature and light exposure). (C) Electrophoresis images of PA-PIMA-PEG−QDs, PA-PIMA-PEG−AuNPs, and PA-PIMA-PEG−IONPs run in a 0.6% agarose gel at 7.0 V/cm for 30 min. The images were collected from freshly prepared dispersions (top images) and after storage for 6 months (bottom images). The concentrations of QDs, AuNPs, and IONPs were ∼0.5 μM, 30 nM, and 10 μg/mL, respectively. (D) Gel electrophoresis images collected from dispersions of nanocrystal dispersions mixed with BSA. Top panels are images of nanoparticles with no BSA (control), and bottom panels are dispersions containing BSA; gels were run for 30 min at 7.0 V/cm. The gel images were collected using a cell phone camera.
of the intensity autocorrelation functions together with histograms of intensity versus RH, extracted from the Laplace transform of the autocorrelation functions, collected from dispersions of AuNPs and IONPs before and after phase transfer. Data show that RH has increased by 3−4 nm following phase transfer for both set of materials: that of the AuNPs increased from 7.2 to 11.7 nm, while that of the IONPs increased from 14.8 to 17.9 nm. Homogeneous singlepopulation nanocrystals characterize these dispersions, nonetheless, indicating the absence of aggregate formation in these samples. The measured hydrodynamic radii are also unaffected by storage time for both sets of nanocrystals, as shown in Figure 4E. The small increase in RH following substitution of the hydrophobic ligand with the PA-PIMA-PEG can be attributed to the nature of the polymer coating compared to the molecular scale native caps. Similarly, DOSY was applied to characterize the hydrodynamic radius of QDs. Though DLS was previously used to characterize such small size nanomaterials by our group,51 DOSY is more effective when the probed materials become very small and Raleigh scattering becomes rather weak compared to that of larger colloids.41 Figure 4F shows a DOSY spectrum acquired for a PA-PIMAPEG−QD dispersion in D2O. The RH value extracted from the diffusion coefficient, ∼7.4 nm, is comparable to those reported for green CdSe-ZnS QDs coated with a similar polymer coating based on the imidazole coordinating motif.50 We should note that our DOSY spectrum relied on tracking the proton signatures of the terminal methoxy and PEG methylene groups in the lateral chains. Other NMR isotopes such as 19F can be used for such measurements, as recently reported in ref 63. There is a slight advantage of relying on 1H NMR signatures, because protons are more abundant in biological and nonbiological media. They also provide larger signal-tonoise ratios. It is critically important to select a group that
serum albumin (BSA). Figure 3C shows that homogeneous and narrow migration bands with no smearing were observed for each set of nanocrystals dispersed in different pH buffers, freshly prepared (top panel) and after storage for 6 months (bottom panel). Furthermore, the images show that the mobility shifts were essentially unchanged by the pH of the medium or the storage time. We also note that the nanocrystals functionalized with the new PA-based polymer coating migrated toward the anode. Such a result reflects a homogeneous distribution in size and a net negative surface charge of the nanocrystals, attributed to the presence of several carboxylic groups along the polymer backbone freed during the ring opening reaction. Similar findings were reported for a coordinating polymer based on the lipoic acid or imidazole motif.50,51,59 The final colloidal stability test focused on the ability of the PA-polymer ligands to prevent the formation of a “protein corona” in the presence of a high concentration of BSA. This test is highly valuable as an unstable and noninert coating has been shown to compromise nanoparticle behavior in biological media (e.g., biodistribution), due to strong nonspecific interactions with native proteins.60−62 As shown in Figure 3D, the nanoparticle dispersions in phosphate buffer (pH 7, 20 mM) incubated for 1 h in the presence of 0.75 mM BSA exhibited no change in mobility after exposure to the proteins, indicating that the PA-PIMA-PEG provides a robust shielding against protein adsorption for all three sets of nanocrystals. Additional characterization of the hydrophilic nanocrystals relied on measurements of the hydrodynamic radius (RH) before and after phase transfer, using dynamic light scattering (DLS) for metallic and magnetic cores and diffusion-ordered NMR spectroscopy (DOSY) for QDs. Details on the DLS measurements and data analysis are provided in the Supporting Information. Panels A−D of Figure 4 show representative plots E
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 4. (A and B) Intensity correlation functions collected from dispersions of AuNPs and IONPs, respectively, before after phase transfer. (C and D) Histograms showing the intensity distribution vs hydrodynamic radius, extracted from the Laplace transform of the autocorrelation functions mentioned above. (E) Time progression of the hydrodynamic radius over 5 months acquired from dispersions of AuNPs and IONPs, capped with PA-PIMA-PEG dispersed in phosphate buffer (pH 8, 20 mM). (F) DOSY spectrum of QDs capped with PA-PIMA-PEG measured in D2O; the average value for RH is ∼7.4 nm. Note that this RH value is slightly smaller than that reported for LA-PIMA-PEG−QDs in ref 51. This is due to a combination of slight difference in QD core size, structure of the ligand, and the fact that the nanocrystal concentration used here is larger than that used for dynamic light scattering measurements.
indicates that PA-PIMA-PEG ligands are adapted for the stabilization and functionalization of a wide range of nanoparticle surfaces. Functionalization of the colloids is easy to realize using the present polymer design. Here, we demonstrate this by coating luminescent QDs with an NH2-appended ligand and reacting the primary amines with a fluorescing dye (Texas Red-NHS
provides a well-resolved signature, nonetheless. It is also important to make sure that only surface-bound protons are probed. The data presented above, summarized in Figures 3 and 4, confirm the effectiveness of the polymer coating based on the PA coordinating motif in imparting long-term colloidal stability and reduced nonspecific interactions onto all three sets of nanocrystals over a broad range of conditions. This F
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 5. (A) Schematic depiction of the coupling reaction between amine-functionalized QDs and Texas Red-NHS. (B) Absorption and (C) photoluminescence spectra collected from dispersions of QD−dye conjugates, QDs alone, and dye alone.
ester, TXR) to yield QD−dye conjugates, as schematically shown in Figure 5A. Coupling of the dye onto the QDs was confirmed and quantified by measuring changes in the fluorescence properties of the QDs and TXR, which accounts for the specific FRET (fluorescence resonance energy transfer) interactions, promoted by proximity. Panels B and C of Figure 5 show the absorption and emission spectra, respectively, of a QD dispersion and a dye solution (as references) side-by-side with those collected from the QD−dye conjugates. The combined absorption and emission data provide clear proof that proximity between the QD and TXR (in a conjugate construct) has been achieved by introducing a few PEG-amine moieties into the polymer structure. The measured loss in the QD PL upon reaction with NHS-TXR combined with the change in PL lifetime (see Figure S4) confirm that the proximity between the QD donor and TXR acceptor has been realized in the formed conjugates. The PL data were used to extract an estimate for the energy transfer efficiency, E, using the equation E=1−
FDA FD
developed for a centrosymmetric conjugate configuration, to extract an estimate of the number of dyes per QD.64 That value was further compared to the one extracted from the absorption data shown in Figure 5B. A good agreement was found, confirming the anticipated conjugate formation and the expected centrosymmetric arrangement of the dyes around the QD.21 We assume that FRET is the main source of the QD PL quenching measured for the present system and ignore potential non-FRET (including charge transfer or Dexter type interactions).65 Additional details about the FRET data and analysis are provided in the Supporting Information. The final characterization/test probed the resistance of the PA-PIMA-PEG−AuNPs to sodium cyanide (NaCN) digestion and furthermore compared the digestion kinetics to those measured for AuNPs coated with ligands presenting a lipoic acid motif (instead of PA), namely, LA-PEG (monomer) and LA-PIMA-PEG (polymer). This test is informative as it allows us to compare the strengths of the coordination interactions of the ligands with the Au colloids. Interactions of Au colloids with a NaCN solution and progressive digestion measured in such mixtures can be described by the chemical equation
(1)
4Au + 8NaCN + O2 + 2H 2O
where FDA and FD designate the PL intensity measured for QD−dye conjugates and QDs alone, respectively. An E of 0.65 was measured for the present dispersion. This value was combined with the FRET expression for E
→ 4NaAu(CN)2 + 4NaOH
E=
Here, the gold atoms on the surface of a nanocrystal (e.g., nanoparticle, nanorods, and other shapes) react with cyanide anions in the medium to form Au−cyanide complexes. This transformation converts the pinkish dispersion into a colorless solution (see Figure S5).66,67 Panels A−C of Figure 6 show side-by-side the progression of the UV−vis absorption spectra
nR 0 6 nR 0 6 + r 6
(3)
(2) G
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 6. (A−C) Progression of the absorption spectra acquired at 5 min intervals from dispersions of AuNPs after mixing with NaCN. Dispersions of AuNPs capped with PA-PIMA-PEG (50:50 PA:PEG), LA-PEG, and LA-PIMA-PEG (50:50 PA:PEG) at a concentration of ∼5 nM were used. (D) Normalized absorbance at 520 nm (SPR peak) vs time extracted from the three sets of data depicted in panels A−C.
PEG AuNPs), a result that can be attributed to the weaker coordination offered by the PA anchors than by the LA groups. The introduction of several PA groups along the polymer does not seem to compensate for that difference, nonetheless. We should note that molecular PA-PEG ligands failed to produce colloidally stable AuNPs, as a ligand exchange reaction routinely yielded a bluish dispersion with a broad absorption band (data not shown). Thus, overall combining multiple anchoring groups in the same ligand, using for example a polymer structure as done here, enhances the affinity of the ligand for the NP regardless of the exact nature of the groups used, thus improving the colloidal stability of the prepared nanomaterials. This can be attributed to increased entropy for the polymer-plus-NP system compared to the case in which a monomer is used for stabilization. The effects of combining higher coordination with the multidentate nature of a polymer ligand certainly yields better steric stabilization of the nanocolloids and thus better resistance to digestion by NaCN ions.59,67,70
with time for three distinct aqueous dispersions of AuNPs coated with PA-PIMA-PEG, LA-PIMA-PEG, and LA-PEG, respectively, during digestion experiments using 12.5 mM NaCN. All dispersions contained 5 nM AuNPs, and the spectra were recorded at 5 min intervals.59,67,68 The time progression of the SPR absorbance versus time (t) induced by NaCN can be fitted to a first-exponential decay function:67,69 A520 = A 0 exp( −t /td)
(4)
where A0 and td designate the initial SPR absorbance value at time zero and the decay constant, respectively. Data show that the decay constant varied with the nature and structure of the ligand coating used, with td values of 74 min for LA-PIMAPEG, 30 min for PA-PIMA-PEG, and 49 min for LA-PEG. This simple test provides insight into the nature of the affinity of the ligand for the colloid. The measured digestion kinetics reflect differences in the strength of the coordination affinity between thiol and phosphonate groups toward AuNP surfaces. It has been previously reported that coordination of thiols to Au surfaces is very strong, and in certain studies, the coordination/binding was essentially described as “covalent”. Additionally, given the fact that every lipoic acid yields two thiol groups after reduction, the overall coordination of the LA-polymer coating is ∼2 times larger than that of the PApolymer for a given ligand stoichiometry. This may explain the sizable difference in the measured td values for the two polymer coatings. Conversely, the PA-PIMA-PEG ligands exhibit a coordinating affinity higher than that of a single LA, ∼1 order of magnitude when considering that each LA yields two thiolcoordinating groups. However, faster digestion kinetics were measured for the PA-PIMA-PEG−AuNPs (compared to LA-
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CONCLUSION We have developed a multicoordinating hydrophilic ligand based on phosphonate anchoring groups with increased affinity for three different sets of inorganic nanocrystals. The ligand design and preparation benefits from the nucleophilic addition reaction between amine-modified PA and PEG blocks and anhydride rings in a poly(isobutylyene-alt maleic anhydride) chain, to prepare coordinating polymers with control over their stoichiometry, hydrophilicity, and reactivity. We found that this ligand could be equally applied for the coating of the semiconductor, metal, and magnetic nanoparticles. Our data H
DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
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showed that PA-PIMA-PEG-stabilized CdSe-ZnS QDs, AuNPs, and IONPs exhibit excellent colloidal stability for at least 6 months over a wide range of biological media. Additionally, the coating is compact as verified by data on the hydrodynamic radius. We have also shown that the polymer-capped QDs can be reacted with organic dyes to yield QD−dye constructs that engage in high degrees of energy transfer interactions. These observations pave the way for the applications of PA-PIMA-PEG-coated nanomaterials in biological research in which compact sizes and steric stability are required.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03527. Materials, instrumentation, ligand synthesis, ligand exchange, 1H NMR spectra of NH2-PAOEt and ligand compounds, FRET data and analysis, DLS characterization, gel electrophoresis, and NaCN digestion (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wentao Wang: 0000-0003-2273-4171 Hedi Mattoussi: 0000-0002-6511-9323 Present Addresses †
W.W.: DNA Electronics, Inc., 1891 Rutherford Rd., Suite 100, Carlsbad, CA 92008. ‡ G.P.: National Center for Toxicological Research, The Food and Drug Administration, 3900 NCTR Rd., Jefferson, AR 72079. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Florida State University, the National Science Foundation (NSF-CHE 150850), the National Institutes of Health (R01 DC013080), and Asahi-Kasei for financial support. The authors also thank Dinesh Mishra, Anshika Kapur, Woody Perng, Sisi Wang, and Yuya Sugiyama for fruitful discussions.
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REFERENCES
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DOI: 10.1021/acs.chemmater.8b03527 Chem. Mater. XXXX, XXX, XXX−XXX
