Reversibly Manipulating the Surface Chemistry of Polymeric Nanostructures via a "Grafting To" Approach Mediated by Nucleobase Interactions.

"Grafting to" polymeric nanostructures or surfaces is a simple and versatile approach to achieve functionalization. Herein, we describe the formation of mixed polymer-grafted nanoparticles through a supramolecular "grafting to" method that exploits multiple hydrogen-bonding interactions between the thymine (T)-containing cores of preformed micelles and the complementary nucleobase adenine (A) of added diblock copolymers. To demonstrate this new "grafting to" approach, mixed-corona polymeric nanoparticles with different sizes were prepared by the addition of a series of complementary diblock copolymers containing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) to a preformed micelle with a different coronal forming block, poly(4-acryloylmorpholine) (PNAM). PNIPAM chains were distributed throughout the corona and facilitated a fast and fully reversible size change of the resulting mixed-corona micelles upon heating. Through the introduction of an environmentally sensitive fluorophore, the reversible changes in nanoparticle size and coronal composition could be easily probed. Furthermore, preparation of mixed-corona micelles also enabled ligands, such as d-mannose, to be concealed and displayed on the micelle surface. This supramolecular "grafting to" approach provides a straightforward route to fabricate highly functionalized mixed polymeric nanostructures or surfaces with potential applications in targeted diagnosis or therapy and responsive surfaces.

Introduction

Hydrogen-bonding (H-bonding) mediated assemblies have for many decades been employed as surrogates to mimic the nanostructures formed as a result of nucleobase pairing within RNA and DNA.[1−9] Hydrogen bonds are normally weaker than covalent and ionic bonds, with an energy typically between 5 and 30 kJ mol–1. Importantly, although a single hydrogen bond is weak, controlled display of multiple H-bonding groups can result in strong, specific interactions. Synthetic chemists inspired by Nature have widely utilized complementary H-bonding interactions to achieve templated polymerization,[10−14] mediate polymer tacticity,[15] and tune nanoparticle morphologies.[16−18] Selective recognition of nucleobase functionalities has also provided a new route to modify and functionalize nucleobase- or DNA-containing nanomaterials.[19−23]

Mixed polymer-grafted nanostructures and surfaces, consisting of two distinct polymers in the outer layer, enable various applications such as smart wetting surfaces,[24,25] controllable oil/water separation,[26] and cancer diagnostics.[27] Generally, mixed-corona polymeric nanostructures are made using cooperative self-assembly of two distinct diblock copolymers in a common selective solvent, and then microphase separation can occur due to differences in the properties of the two diblock copolymers.[28,29] Alternatively, two “grafting” strategies can be exploited to attach polymer chains to the surface of a nanostructure: (i) “grafting from” and (ii) “grafting to” approaches.[30] In the “grafting from” approach, polymer chains are formed from initiator-functionalized nanostructures/surfaces, which can produce thick and very dense polymer layers.[31] However, stringent reaction conditions and complicated purification steps often limit potential applications. On the other hand, the relatively simple “grafting to” strategy involves the attachment of prefabricated polymers via either physisorption[32] or covalent bond formation.[30,33−36] Typically, the “grafting to” strategy suffers from several limitations. With increasing polymer molecular weight, the reaction efficiency between the end-group and the reactive group on the surface significantly decreases.[37] Furthermore, steric repulsion between polymer chains is expected to hinder the successful fabrication of novel polymeric materials due to relatively low grafting densities.

The “grafting to” strategy also has some undoubted advantages. Tethered polymers can be thoroughly characterized prior to conjugation, which allows the fabrication of tailor-made nanostructures or surfaces.[38,39] Furthermore, the “grafting to” approach is experimentally very straightforward, unlike the “grafting from” approach. Since the development of “click” chemistry in the past decade, a range of modular conjugation strategies have breathed new life into “grafting to” approaches.[40,41] However, the relatively small and mostly buried reactive groups of long polymer chains often result in moderate grafting densities and require high reactant concentrations.[42,43]

Stimuli-responsive polymers represent a growing cadre of materials that support various applications such as responsive coatings and controlled release agents.[44] Many smart polymers have been developed to switch their various functions via external stimuli such as pH, light, temperature, etc.[45−50] Such switching behavior has been utilized to turn off/on activity or interaction of functional groups within the material. For example, switching of a cationic N,N-dimethyl-2-morpholinone (CD-Ring) film between attacking and defending against bacteria, through a change in solution pH, has been achieved in a controlled manner.[45] Furthermore, light-responsive azobenzene[46] and thermoresponsive PNIPAM[49,50] have been utilized to reversibly conceal and expose ligand for biospecific cell adhesion or targeting.

Recent work in our group has highlighted the effect of complementary multiple H-bonding interactions within the core of self-assembled nanostructures on their self-assembly behavior.[17] Indeed, the utilization of specific complementary A–T interactions within the micelle cores enabled the progressive modulation of nanostructure morphology. At high dilution (0.5 mg mL–1), efficient complementary interactions allowed for control over nanostructure morphology. To expand on the utility and application of these discoveries, we aimed to exploit the complementary interactions within the nanostructure cores to allow for the preparation of responsive mixed-corona micelles. Our previous work employed PNAM as the sole hydrophilic block and thus generated micelles with a homogeneous and non-responsive corona. In contrast, our new system uses both hydrophilic PNAM and PNIPAM in different diblock copolymers, thus forming a mixed corona micelle. Mixed-corona micelles containing both thermoresponsive PNIPAM, which can reversibly manipulate the micelle surface functionality, and a permanently hydrophilic, non-responsive PNAM, which was able to stabilize the nanoparticles above the characteristic cloud point of PNIPAM,[51] were prepared using a “grafting to” approach. This approach allowed facile control of nanostructure size through the introduction of complementary diblock copolymers, poly(N-isopropylacrylamide)-b-poly(3-(adenine-9-yl)propylacrylamide) (PNIPAM-b-PAAm) with different chain lengths, to preformed micelles with thymine-functionalized cores. Interestingly, the thermoresponsive polymeric micelles exhibited behavior consistent with single particle collapse/swelling, and the transition was fully reversible. Introduction of an environmentally sensitive fluorophore at the chain ends of the hydrophilic blocks using a “grafting to” approach allowed the hydrophobicity of the corona domain of the micelles to be probed at different temperatures. Having confirmed the reversible collapse/swelling of the thermoresponsive chains of the corona, it was then demonstrated that it was possible to selectively display or conceal a protein ligand using temperature as a responsive trigger. This work demonstrates that a “grafting to” approach, mediated by complementary nucleobase interactions, allows for the efficient functionalization of polymeric nanostructures/surfaces and represents an efficient route to fabricate tailor-made nanomaterials.

Experimental Section

Materials

2,2′-Azobis(isobutyronitrile) (AIBN) was obtained from Molekula and recrystallized from methanol. 2,2′-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044, Wako) was used without further purification. Concanavalin A (Con A) was used as purchased from Sigma-Aldrich. 4-Acryloylmorpholine (NAM) was bought from Sigma-Aldrich and was purified by vacuum distillation. N-Isopropylacrylamide (NIPAM) was purchased from Sigma-Aldrich and recrystallized from a toluene–hexane mixture prior to use. 3-(Adenine-9-yl)propylacrylamide (AAm), 3-(thymin-1-yl)propylacrylamide (TAm), 2-(((butylthio)carbonothiolyl)thio)propanoic acid, and micelle M1 were synthesized as described previously[17] and stored at 4 °C. DMF, DMSO, and other chemicals were obtained from Fisher Chemicals and used without further purification. Dry solvents were obtained by passing over a column of activated alumina using an Innovative Technologies solvent purification system. Dialysis membranes (MWCO = 3.5 kDa) were purchased from Spectra/Por.

Instrumentation

1H NMR and 13C NMR spectra were recorded on a Bruker DPX-300, DPX-400, or HD500 spectrometer with DMSO-d6 or CDCl3 as the solvent. The chemical shifts of protons were relative to tetramethylsilane (TMS) at δ = 0 ppm when using CDCl3 or solvent residues (DMSO 2.50 ppm). UV–vis spectra were recorded on a PerkinElmer Lambda 35 UV–vis instrument. Turbidimetry assays were performed as described in a previous publication.[52] Fluorescence spectra were recorded using an Agilent Cary Eclipse fluorescence spectrophotometer. The samples were incubated at the relevant temperatures for at least 2 min and then measured immediately. High-resolution mass spectrometry (HR-MS) was conducted on a Bruker UHR-Q-TOF MaXis with electrospray ionization (ESI). Size exclusion chromatography (SEC) data were obtained in HPLC grade DMF containing 5 mM NH4BF4 at 50 °C, with a flow rate of 1.0 mL min–1, on a set of two PLgel 5 μm Mixed-D columns and a guard column. SEC data were analyzed with Cirrus SEC software calibrated using poly(methyl methacrylate) (PMMA) standards.

Hydrodynamic diameters (Dh) and size distributions of the self-assemblies were determined by dynamic light scattering (DLS). The DLS instrumentation consisted of a Malvern Zetasizer NanoS instrument with a 4 mW He–Ne 633 nm laser module. Measurements were made at a detection angle of 173°, and Malvern DTS 7.03 software was used to analyze the data. Dh was calculated by fitting the apparent diffusion coefficient in the Stokes–Einstein equation Dh = kT/(3πηDapp), where k, T, and η are the Boltzmann constant, the temperature, and the viscosity of the solvent, respectively. As the measured sample is a solution of monodispersed spherical micelles, Dh coincides to the real hydrodynamic diameter as Dapp is equal to the translational diffusion coefficient (Dt). For thermoresponsive size change, all measurements were run at least three times with a minimum of 10 runs per measurement for every temperature point. For reversible size change, the samples were incubated at the relevant temperature for 2 min before measurement. Static light scattering (SLS) measurements were conducted with an ALV CGS3 (λ = 632 nm) at different temperatures. The data were collected from 50° to 150° with an interval of 10° against a toluene standard. The self-assembled solutions were filtered through 0.45 μm nylon filters prior to analysis.

TEM observations were performed on a JEOL 2000FX electron microscope at an acceleration voltage of 200 kV. TEM samples were prepared by drop deposition of an aqueous solution onto a copper grid coated with Formvar. Generally, a drop of sample (10 μL) was pipetted on a grid, left for several minutes, and then blotted away. Uranyl acetate (UA, 1%) was used for the staining of TEM samples. TEM images were analyzed using the ImageJ software package, and over 100 particles were counted for each sample to obtain number-average diameter Dn (for spherical micelles).

Synthesis of Poly(N-isopropylacrylamide) (PNIPAM) Macro-CTA via RAFT Polymerization

The typical procedure was as follows: a 10 mL ampule was charged with NIPAM (339.5 mg, 3.0 mmol), 2-(((butylthio)carbonothiolyl)thio)propanoic acid (2.4 mg, 0.01 mmol), AIBN (0.16 mg, 0.001 mmol), and 1,4-dioxane (0.68 mL). The mixture was thoroughly degassed via four freeze–pump–thaw cycles, filled with nitrogen, and then immersed in an oil bath at 70 °C for 2 h. The conversion was determined by 1H NMR spectroscopy. The polymerization solution was then precipitated three times from cold diethyl ether. The light yellow polymer was dried in a vacuum oven overnight at room temperature and analyzed by 1H NMR spectroscopy and DMF SEC.

Synthesis of PNIPAM-b-PAAm Diblock Copolymers

The typical procedure was as follows. For PNIPAM262-b-PAAm20: PNIPAM262 (75 mg, 0.0025 mmol), AAm (15.4 mg, 0.0625 mmol), and AIBN (0.04 mg, 0.00025 mmol) were dissolved in DMSO (0.3 mL). The mixture was thoroughly degassed via four freeze–pump–thaw cycles, filled with nitrogen, and then immersed in an oil bath at 70 °C overnight. An aliquot of the crude product was taken and analyzed by 1H NMR spectroscopy to calculate the conversion. The residual solution was precipitated three times from cold CH3OH. The light yellow polymer was dried in a vacuum oven overnight at room temperature and analyzed by 1H NMR spectroscopy and DMF SEC. The synthetic details and characterization of the fluorescent ABM and d-mannose containing CTAs can be found in the Supporting Information (Figures S1–S4, Schemes S1 and S2).

Self-Assembly of PNAM98-b-PTAm302P10 Diblock Copolymer in Water

The diblock PNAM98-b-PTAm302P10 was synthesized using a d-mannose containing CTA (Figure S4). For the self-assembly of P10, a solvent switch method was used. Specifically, the diblock copolymer was dissolved in DMF (at 8 mg mL–1) and stirred for 2 h. Then an excess of 18.2 MΩ·cm water was added via a syringe pump at a rate of 1 mL h–1. The final volume ratio between water and organic solvent was 8:1. The solution was then dialyzed against 18.2 MΩ·cm water (MWCO = 3.5 kDa), incorporating at least six water changes, to afford self-assemblies (M10) at a concentration of ca. 1 mg mL–1.

Preparation of Mixed-Corona Micelles Containing PNIPAM-b-PAAm and PNAM-b-PTAm

Diblock copolymers PNIPAM-b-PAAmP2–P9 were dissolved in H2O at 10 mg mL–1. This was then added to the micellar solution of M1 or M10 (0.5 mg mL–1) dropwise with stirring. The molar ratios were calculated according to the Mn determined from 1H NMR spectroscopic analyses and the polymer mass concentration. The mixture was then sealed and allowed to stir at room temperature for at least 2 h. The solutions were then analyzed by DLS, TEM, and SLS.

Results and Discussion

Synthesis of Mixed-Corona Micelles through a Supramolecular “Grafting To” Approach

A diblock copolymer, P1, with a permanently hydrophilic domain and a thymine functional hydrophobic block was prepared by RAFT polymerization. The self-assembly of this diblock copolymer was achieved via a solvent switch method as reported previously to afford micelle M1 (Dh = 69 nm, PD = 0.058) at ca. 1 mg mL–1. In order to fabricate mixed-corona micelles using a “grafting to” approach a series of PNIPAM-b-PAAm diblock copolymers with various hydrophilic block lengths were also prepared via RAFT polymerization (Table , P2–P5, Figures S5 and S6). Subsequently, 1 mol equiv. of complementary diblock copolymers P2–P5 in water was added to the solution of micelle M1 with stirring at room temperature and left for at least 2 h. In agreement with our previous work, no morphology transition or disassembly occurred when only 1 mol equiv. of complementary diblock copolymers was added to the preformed micelles.[17] Using this approach, well-defined mixed-corona micelles M2–M5 were formed through multiple H-bonding interactions between the thymine-containing micelle core of M1 and complementary adenine-containing diblock copolymers P2–P5 (Scheme and Figures S7a–S10a).

Table 1

Characterization Data of Polymers P1–P10

Polymer	Structure	Mn,NMRa (kDa)	Mn,SECb (kDa)	ĐMb
P1	PNAM96-b-PTAm301	85.1	68.4	1.29
P2	PNIPAM96-b-PAAm20	15.5	17.8	1.05
P3	PNIPAM176-b-PAAm20	25.3	23.9	1.10
P4	PNIPAM262-b-PAAm20	35.3	34.1	1.13
P5	PNIPAM438-b-PAAm20	56.7	52.9	1.18
P6	PNIPAM262	29.9	31.6	1.09
P7	PNAM295-b-PAAm20	47.0	36.8	1.17
P8	PNIPAM237-b-PAAm20c	33.0	32.8	1.17
P9	PNAM244-b-PAAm20c	40.5	33.0	1.25
P10	PNAM98-b-PTAm302d	85.8	64.2	1.32

Determined by 1H NMR spectroscopy (400 MHz) in deuterated DMSO.

Determined by DMF SEC, with poly(methyl methacrylate) (PMMA) standards.

Aminobromomaleimide-containing chain transfer agent (CTA) was used.

d-mannose containing CTA was used.

Scheme 1

Syntheses of Mixed-Corona Polymeric Nanostructures M2–M7 through a Supramolecular “Grafting To” Approach Mediated by Complementary Nucleobase Interactions

Mixed-corona micelles generally inherit the solution properties of both corona-forming blocks. Indeed, dynamic light scattering (DLS) analysis indicated that the PNIPAM chains in the mixed-corona micelles collapsed, and no aggregation between self-assembled nanostructures was observed at 60 °C (see Figures S7b–S10b). For example, Figure a shows the heating process for M4 and the resulting hydrodynamic diameter of the mixed-corona micelles, which decreased from ca. 89 to ca. 70 nm upon heating from 15 to 60 °C. Meanwhile, the polydispersity values of the mixed-corona micelles were determined to lie in a range of 0.02–0.08 during this heating process. Micelle M4 solution was dried and stained with uranyl acetate at 20 and 60 °C for TEM imaging (Figures b and 1c, respectively). TEM images further suggested that no aggregation was observed at 60 °C. Meanwhile, smaller spherical nanoparticles with diameter ca. 61 nm were observed at 60 °C compared with spherical nanoparticles with diameter ca. 70 nm at 20 °C. This further suggested that these “grafting to” mixed-corona micelles underwent intraparticle chain collapse without interparticle aggregation. In stark contrast, upon addition of 1 mol equiv. of PNIPAM P6 without a complementary nucleobase block to preformed micelle M1, large aggregates and much higher dispersity values were observed by DLS for the resultant micelle, M6, above the cloud point of the PNIPAM block (Scheme and Figure S11a). Notably, complementary diblock copolymers P2–P5 with small anchoring groups avoided the formation of frozen aggregates and facilitated efficient incorporation of the complementary diblock copolymers. Static light scattering (SLS) analyses of micelle M4 also indicated no appreciable change in the apparent molecular weight by light scattering (Mw) and aggregation number (Nagg) of the respective micelles at 15, 35, and 50 °C (Figure S12).[53] On the basis of these initial results, we proposed that the size change of the mixed-corona micelle upon heating was a result of PNIPAM collapse with retention of the micellar structure rather than disassembly and re-formation of the micellar aggregates (Scheme ). As expected, the hydrophilic PNAM chains were still capable of stabilizing mixed-corona micelles even when the PNIPAM chains were fully collapsed at elevated temperatures.

Figure 1

(a) Variation of the hydrodynamic diameter (Dh) of the mixed-corona micelles M4 with temperature as determined by DLS; TEM images (stained with uranyl acetate) and histograms of number-average diameter distribution of M4 at (b, c) 20 °C and (d, e) 60 °C. Scale bar: 200 nm.

Given that H-bonding interactions are known to be disrupted at elevated temperatures, we next investigated the effect of temperature on PNAM corona micelles prepared by a “grafting to” approach through the addition of a non-responsive diblock copolymer, P7 (PNAM295-b-PAAm20), to the preformed micelle M1 (Scheme ). The resultant micelle M7 was investigated by temperature-dependent DLS, and no appreciable size change was observed at elevated temperatures, which demonstrated the stability of the H-bonding interactions in the micellar core up to 60 °C (Figure S11b). This result also suggested that the size decrease observed in the mixed-corona micelles M2–M5 were solely caused by the intraparticle PNIPAM chain collapsing rather than the separation of complementary polymer chains at elevated temperatures.

Reversible Alteration of the Sizes of Mixed-Corona Nanoparticles

The size of a nanoparticle is an important parameter that affects the majority of applications but perhaps most notably their application in nanomedicine. In biomedical delivery applications, nanoparticle sizes greatly influence their circulation time, rate of clearance, selective tissue distribution, and intracellular fate.[54] To circumvent the need for laborious syntheses to access nanoparticles of a particular size, the “grafting to” strategy described here provides a facile method to prepare nanoparticles of controlled sizes. Indeed, we have demonstrated that a series of nanoparticles with different sizes (75–110 nm) could be easily produced by adding complementary diblock copolymers with different chain lengths to a single preformed micelle (M1 of a small size 69 nm). Moreover, the composition and properties of the newly formed nanoparticles could be adjusted by changing the chemistry of the added complementary block copolymers.

To demonstrate this approach, 1 mol equiv. of complementary diblock copolymers P2–P5 was added to separate solutions of preformed micelle M1. Hydrodynamic diameters of the resultant nanoparticles were observed to increase stepwise from ca. 69 nm for M1 to ca. 75 nm for M2 and then to ca. 110 nm for M5. We propose that this increase was due to the increase in PNIPAM chain lengths in the added complementary diblock copolymers. Upon heating of the mixed-corona micelle solutions, the PNIPAM chains appeared to gradually collapse up to their cloud point at 32 °C (Figure a). Above 40 °C, there was no further size alteration observed by DLS analysis. It is notable that mixed-corona micelles M2–M5 all collapsed to micelles of similar sizes (70–74 nm in hydrodynamic diameter) at 60 °C. We presumed that this was because the collapsed PNIPAM chains at elevated temperatures resided at the core–corona interface and therefore played a negligible role in determining the mixed-corona micelles’ hydrodynamic diameters.

Figure 2

(a) DLS analyses of the mixed-corona micelles M2–M5 with different PNIPAM chain lengths heated from 15 to 60 °C. (b) Reversible size change (Dh) of mixed-corona micelles M2–M5 at both 15 and 60 °C. Error bars show size dispersity (PD).

For thermoresponsive nanoparticles, fast and fully reversible switching behavior is of great importance.[55−57] As shown in Figure b, size switching of the mixed-corona micelles could be performed for at least five heating and cooling cycles, highlighting that this transition was fully reversible. More importantly, the transition could be achieved in just 2 min as determined by DLS analysis. We propose that the distribution of both thermoresponsive PNIPAM chains and non-responsive PNAM chains in the mixed micelle corona led to less chain entanglement and as a result a faster transition compared with micelles which have only a PNIPAM corona domain.[55,56] Therefore, the “grafting to” mixed nature of the micelle coronas resulted in cooperative effects—nanoparticles were stabilized and the speed of the coronal collapse increased. The dispersity of the nanoparticles at both high and low temperatures remained below 0.1 throughout the five cycles. Hence, we propose that at low temperatures the elongated PNIPAM chains served as the outer corona and stabilized the large mixed micelles. Then at high temperatures, the smaller mixed micelles remained stabilized by the hydrophilic PNAM chains, which were revealed as a result of the collapse of PNIPAM chains to the core–corona interface.

Altering the Coronas and Surfaces of Mixed-Corona Nanoparticles

Changes in the surface chemistry of nanostructures can be used to modulate hydrophilicity, cellular uptake, and endocytosis.[49] However, it is difficult to probe a nanoparticle surface’s chemistry and local environment. Recent work in our group has reported the development of aminobromomaleimide (ABM) and dithiolmalemide (DTM) fluorophores.[58,59] These are a new class of highly emissive compounds which can be readily incorporated into micellar constructs without noticeable effects on the particle size or dynamics.[60,61] Interestingly, these fluorophores show much lower emission in water compared with more hydrophobic environments and a concentration independent emission maximum.[62] We therefore proposed that ABMs could be used as a probe to provide a fluorescence read-out of the hydrophobicity of the fluorophore’s local environment.[63] In the present case, it was envisaged that ABMs could be used to reveal the hydration state of the coronal domain of the mixed-corona micelles.

Fluorescent diblock copolymer PNIPAM237-b-PAAm20P8 was synthesized (as described for P2–P5) and added to preformed micelle M1 to yield the mixed-corona micelle M8 (Scheme and Table ). A clear fluorescence emission peak at 488 nm (λex = 350 nm) was observed at 40 °C (when the PNIPAM coronal chain was collapsed and hydrophobic in nature), which was attributed to the fluorescence emission of the ABM fluorophore in a hydrophobic environment (Figure a). This fluorescence emission peak was no longer present at 25 °C as the elongated PNIPAM resulted in ABM being brought into a more hydrophilic environment, which led to fluorescence quenching. Note that the shoulder peak at 430 nm (in Figure a) appeared as a result of scattering of the micelle solution. A control experiment using micelle M4 (which did not contain the ABM functionality) displayed a slight decrease in the shoulder peak at 40 °C (Figure S13a), which can be rationalized by assuming that the collapsing nanostructures at 40 °C contributed to less scattering due to their smaller size. Moreover, the non-responsive corona micelle M9 consisting of preformed micelle M1 and ABM-functionalized PNAM244-b-PAAm20P9 exhibited no appreciable fluorescence change at 25 and 40 °C, as the fluorophore did not undergo any significant microenvironment change as the hydrophobicity of the coronal block did not change at elevated temperature (Figure S13b). A similar fluorescence read-out response was observed for the mixed micelle consisting of micelle M1 and ABM-attached PNIPAM40-b-PAAm20 (see Figure S13c). Hence, the fluorescence read-out of the ABM mixed micelles was proposed to be determined by the hydrophobic microenvironment where the dye lies and hence is independent of PNIPAM corona block length. We concluded that the fluorescence ON/OFF switch was triggered by changes in the microenvironment surrounding the ABM fluorophore upon collapse of the responsive block—this enabled a read-out of the surface state of the mixed corona micelles. Again, as suggested by DLS analysis and as shown in Figure b, the temperature-dependent fluorescence switch was fully reversible. The intensity of the fluorescence emission peak at 488 nm at 40 °C did not decrease, even after five cycles of heating and cooling, which indicated that collapse and expansion of the PNIPAM did not lead to any loss of micelle structure and confirmed the integrity of the nucleobase interactions within the core domain during these heat–cool cycles.

Scheme 2

Syntheses of ABM-Functionalized PNIPAM237-b-PAAm20 [P8] and Subsequent Addition to M1 To Afford Mixed Micelles M8, and d-Mannose-Functionalized PNAM98-b-PTAm302 [P10] and Subsequent Assembly To Afford Micelles M10 and Mixed Micelles, M11

Figure 3

Demonstration of a reversible fluorescence ON/OFF switch using ABM-tagged mixed-corona nanoparticles. (a) Fluorescence spectra (λex = 350 nm) of mixed-corona micelle M8 at 25 (black) and 40 °C (red). (b) Fluorescence emission intensity of M8 at 488 nm at 25 and 40 °C for five consecutive heating and cooling cycles (λex = 350 nm).

We next explored the utility of this “grafted to” system in controlling protein ligand display on the mixed micelle surface using changes in temperature. To achieve this, we utilized Concanavalin A (Con A), a tetrameric mannose-binding plant lectin, which can effectively bind to the ligand d-mannose with moderate binding constants.[64] This type of specific recognition between carbohydrates and proteins is involved in many complex cellular events, including cell adhesion, pathogen invasion, and cancer metastasis, to name but a few.[65]d-Mannose-functionalized PNAM98-b-PTAm302P10 was synthesized and self-assembled to form non-responsive micelle M10 as an experimental control (Scheme and Figure S14a). In addition, d-mannose containing mixed-corona micelles M11 were prepared by adding diblock copolymer P8 to preformed micelle M10 (Scheme and Figure S14b) as described for the previous mixed micellar systems. This afforded a micelle M11 with a mixed-corona domain, which consisted of a responsive PNIPAM block bearing the ABM probe and a non-responsive PNAM block bearing the ligand, which we proposed could be selectively revealed and buried depending on the state of the responsive block (i.e. the temperature of the system). To explore this, a Con A agglutination assay was used to measure the extent of binding between Con A and M11.[52] At 25 °C, the d-mannose ligands were expected to be fully concealed by the extended PNIPAM chains, and indeed no agglutination was observed at this temperature (Figure a). In contrast, at 40 °C significant agglutination was observed—at this temperature the PNIPAM chains were expected to collapse, revealing the ligands for binding with the Con A receptors. Notably, the binding between the receptor d-mannose and Con A is not reversible, and the formed aggregates precipitated out after longer time periods. Further analysis of the ABM-tagged mixed-corona micelle M11 using fluorescence spectroscopy confirmed the collapse of the PNIPAM above 40 °C, with the characteristic change in the emission spectrum of ABM again observed (Figure b).

Figure 4

Controlled ligand display on the mixed-corona micelle surface. (a) Agglutination assay of mixed-corona micelle M11 with Con A at 25 and 40 °C. (b) Fluorescence spectra of M11 at 25 and 40 °C (λex = 350 nm).

Conclusions

In summary, a novel supramolecular “grafting to” strategy for the formation of well-defined mixed-corona micelles has been demonstrated. This strategy involved the exploitation of multiple complementary H-bonding interactions between thymine-containing cores of preformed micelles and added adenine-containing diblock copolymers. Polymeric micelles with different sizes were easily prepared by changing the chain lengths of the added complementary diblock copolymers. Intraparticle chain collapse allowed micelle size and coronal state to be reversibly altered in response to an external stimulus—in this case temperature—and this could be coupled to the reversible display/concealment of protein ligands at the nanoparticle surface. While in this work a temperature responsive block was utilized to showcase the viability of this approach for achieving the reversible presentation of functionality at the surface of a nanoparticle, other stimuli responses could be easily utilized such as pH, light, etc. Indeed, through the synthesis of the appropriate complementary diblock copolymers, the incorporation of a diverse range of functionalities at the nanoparticle surface can be envisaged using this supramolecular “grafting to” approach. This work provides a new route and platform to fabricate mixed polymer-grafted nanostructures and surfaces, which may find potential applications as smart materials for utilization in a range of areas such as nanomedicine.