Facile Method for Preparing Surface-Mounted Cucurbit[8] - American

Aug 29, 2014 - BP Oil UK Ltd, Whitchurch Hill, Pangbourne, Reading, Berkshire RG8 7QR, .... fabrication of CB[8]-based surface-mounted rotaxanes.12 In...
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Facile Method for Preparing Surface-Mounted Cucurbit[8]uril-Based Rotaxanes Chi Hu,† Yang Lan,† Feng Tian,† Kevin R. West,‡ and Oren A. Scherman*,† †

Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom ‡ BP Oil UK Ltd, Whitchurch Hill, Pangbourne, Reading, Berkshire RG8 7QR, United Kingdom S Supporting Information *

ABSTRACT: Surface-immobilized rotaxanes are of practical interest for myriad applications including molecular rotors and analytical sensing. Herein, we present a facile method for the preparation of cucurbit[8]uril (CB[8])-based rotaxanes on gold (Au) surfaces threaded onto a viologen (MV2+) axle. The surface-bound CB[8] rotaxanes were characterized by contact angle measurements and optical microscopy. Direct imaging of the rotaxanes was accomplished by attaching either azobenzene-functionalized silica (Si-azo) colloids or fluorescein-labeled dopamine that were bound to the Au surface through a supramolecular heteroternary (1:1:1) complex with CB[8]. The surface density of CB[8] rotaxanes was examined based on their detection of dopamine. The calculated surface density is 4.8 × 1013 molecules·cm−2, which is only slightly lower than the theoretical value of 5.0 × 1013 molecules·cm−2. Surface-functionalized rotaxanes can be reversibly switched using external stimuli to bind electron-rich second guests for CB[8], including both small molecules such as dopamine and appropriately-functionalized colloidal particles. Such controlled reversibility gives rise to potential applications including selective sensing or reusable templates for preparing well-defined colloidal arrays. The formation of the surface-bound rotaxane structure is critical for successfully anchoring CB[8] host molecules onto Au substrates, yielding an interlocked architecture and preventing the dissociation of binary host−guest complex MV2+⊂CB[8]. The MV2+⊂CB[8] rotaxane structure thus effectively maintains the material density on the Au surface and dramatically enhances the stability of the functional surface.

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INTRODUCTION With a mechanically interlocked moleclar architecture, rotaxanes have potential uses in future nanotechnology applications such as the development of molecular machines, switches, receptors, and sensors.1−4 Macrocyclic host molecules such as cyclodextrins and crown ethers are of special interest in the preparation of rotaxanes on account of their intrinsic cyclic structures and their host−guest complexation ability.5 By readily combining the properties of the rotaxane structure with additional host−guest chemistry, this supramolecular architecture exhibits unique behavior and gives rise to new applications. For example, rotaxanes constructed from cyclic hosts threaded onto guest molecules have been employed as sensors, often exhibiting high sensitivity and selectivity.6 Rotaxane sensors of this type have been developed for detecting chloride and alkali metal ions as well as changes in viscosity based on supramolecular recognition.7,8 Cucurbit[n]urils (CB[n]s) are a family of highly symmetric, barrel-shaped, synthetic macrocyclic host molecules exhibiting high binding affinities in aqueous solutions.9 CB[8] is one of the larger members of the CB family, possessing a hydrophobic cavity accessible through both identical portals and is capable of simultaneously binding two different guest molecules (e.g., methyl viologen (MV2+) and naphthol), forming stable 1:1:1 heteroternary complexes.10 With its unique structural features, © 2014 American Chemical Society

CB[8] is useful as a versatile receptor and building block for the construction of supramolecular architectures and functional chemical systems.11−13 Previously, our group has examined the formation of colloidal arrays by immobilizing monodisperse naphthalene-functionalized colloids onto Au substrates bearing viologen moieties using CB[8] as a supramolecular “handcuff”.13 In this work the MV2+⊂CB[8] binary complex forms an azimuthal rotor as shown in Figure 1A(i) which exhibits an open structure and could undergo complex dissociation, removing CB[8] from the surface. Although CB[8] exhibits extraordinary host−guest properties to simultaneously encapsulate two different guest molecules, little attention has been paid to the development of CB[8]based rotaxanes. Pseudorotaxanes containing CB[6] were reported by Kim et al. in 2002.14,15 Several years later, a CB[7]-containing rotaxane was investigated by Kaifer et al.16 Additionally, Urbach and coworkers described the first example of a CB[8]-based rotaxane, which was threaded through a viologen core and stopped by tetraphenylmethane groups to probe binding affinities in organic solvents.17 Received: July 3, 2014 Revised: August 19, 2014 Published: August 29, 2014 10926

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guests including metal ions, organic chromophores, and many other classes of molecules.9 For example, the binary MV2+⊂CB[8] host−guest complex has been shown to encapsulate different second guests selectively, including azobenzene (Azo), dopamine, and phenylalanine derivatives, with binding constants as high as 105 M−1.10 Herein, we describe a facile method for fabricating CB[8] rotaxane-functionalized Au surfaces whereby the Au surface itself acts as the stopper. The potential to exploit this supramolecular approach for the development of well-organized surface-bound materials and the construction of highly sensitive sensors is further demonstrated (Figure 1).

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RESULTS AND DISCUSSION Preparation of CB[8] Rotaxanes on Au Surfaces. The formation of CB[8] rotaxanes on Au substrates was carried out in a stepwise fashion and characterized by contact angle (CA) measurements. In this manner, complicated synthesis of a thiolfunctionalized rotaxane prior to assembly on the Au surface could be avoided. As shown in Figure 2, an amine-terminated self-assembled monolayers (SAM) was prepared by immersing a bare Au substrate in a solution of 2-aminoethanethiol, changing the CA of the substrate from 96.3° (Figure 2A) to 30.6° (Figure 2B). The substrate was then immersed in a solution of terephthalaldehyde to obtain an aldehyde-functionalized surface (S1), with the CA increasing slightly to 37.9° (Figure 2C). Subsequently, three independent experiments were carried out using S1 as the starting substrate. In the first experiment (shown in Figure 2B(i)), a monoaminoethyl viologen was reacted with the aldahydeterminated S1, leading to the formation of a viologenfunctionalized substrate (S-MV2+), this served to increase the CA of the substrate from 37.9 to 52.7°. It is important to point out why a more hydrophobic surface is generated while a doubly charged species is attached to the Au substrate. This is because the viologen moiety displays a high affinity for the gold surface and is likely to bury itself, thus exposing a more hydrophobic part of the linker, consistent with previous work.13,27 CB[8] was then assembled on the MV2+ motifs by immersing the MV2+-functionalized Au substrate in a supersaturated aqueous suspension of CB[8] for 1 h, with the CA decreasing to 31.4°; in a 1:1 complex with CB[8] the MV2+ groups do not have any particular affinity for the gold substrate and sit upright, leading to a much more hydrophilic surface. Thus, an azimuthal surface-mounted molecular rotor was prepared with its axis of rotation perpendicular to the surface, as shown in Figure 1A(i). This MV2+⊂CB[8]-functionalized substrate was then soaked in a supersaturated aqueous suspension of 1-adamantylamine (ADA). As ADA is a strong competitive guest for CB[8] and the azimuthal MV2+⊂CB[8] rotor is structurally “open”, guest exchange can readily occur, leading to the removal of CB[8] from the substrate. S-MV2+ was regenerated with an increased CA of 51.6°. A second experiment employed bis-aminoethyl viologen, which was reacted with the aldehyde functionalities on S1 as depicted in Figure 2B(ii). After soaking S1 in an aqueous solution of the bis-aminoethyl viologen, MV2+ functionalities with both ends attached to the Au surface were formed (SMVrot2+), changing the CA of the substrate to 48.4°. This CA value is intermediate between S1 and S-MV2+, as the viologen groups are not able to bury themselves completely as in the “open” structure. The S-MVrot2+ substrate was subsequently immersed in a supersaturated aqueous suspension of CB[8]

Figure 1. (A) Azimuthal (i) and altitudinal (ii) surface-mounted molecular rotors of CB[8]. (B) Redox-controlled ternary complexation of (MV2+·DA)⊂CB[8] on the Au surface. (C) Redox-controlled and photoresponsive ternary complex formation of (MV2+·Azo)⊂CB[8] on the Au surface.

Recently, efforts have been devoted to attach rotaxanes to solid substrates, such as silicates or gold, in order to optimize their ability to work in practical devices.5,18−23 Kim et al. reported the immobilization of functionalized CB[6] on a glass surface to be used as a sensor for small molecules and alkyl ammonium ions.24 Few reports, however, have focused on the fabrication of CB[8]-based surface-mounted rotaxanes.12 In 2013, Li et al. demonstrated the first surface-immobilized CB[8] rotaxane on a quartz wafer, which utilized a naphthalene second guest moiety as the threading unit.25 While the detection of a variety of neutral aromatic explosive molecules was achieved with this CB[8] rotaxane sensor in the vapor phase, the heteroternary complexation recognition properties of the CB[8]-based rotaxane remarkably remained unexplored, likely on account of synthesis difficulties. Surface-mounted rotaxanes based on host−guest chemistry have the potential to immobilize functional materials such as colloids, nanoparticles, and polymers on a surface and form well-defined monolayers for surface functionalities.13,26 The host−guest chemistry of many synthetic macrocycles is well known, and host molecules can be selected for a variety of 10927

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Figure 2. Synthesis route for preparing CB[8]-based rotaxanes on Au surfaces and corresponding CA data. (A) Preparation of aldehydefunctionalized Au substrate S1. (B) Three different experiments were carried out on functionalized substrate S1. (i) S1 was reacted with a monoaminoethyl viologen to make S-MV2+. The MV2+ moieties on S-MV2+ were then complexed with CB[8], leading to S-MV2+⊂CB[8]. Subsequently, S-MV2+ could be regenerated after immersion in a solution containing competitive guest ADA. (ii) S1 was reacted with a bisaminoethyl viologen, leading to functionalized substrate S-MVrot2+. It was subsequently immersed in a CB[8] aqueous suspension and then in an ADA aqueous suspension, without any effect on its CA. (iii) Finally, S1 was reacted with the preformed binary complex of bis-aminoethyl viologen and CB[8], leading to rotaxane-functionalized surface S-MVrot2+⊂CB[8]. It was then extensively washed with water and immersed in an ADA aqueous suspension, without any loss of CB[8].

the substrate was soaked in a supersaturated aqueous suspension of ADA for 1 h at room temperature, and again the CA did not change (43.0°) as shown in Figure 2B(iii). These results suggest that the CB[8] molecules are robustly anchored to the Au substrate and that a closed structure was formed on the surface, thus confirming the successful formation of CB[8] rotaxanes. Small-Molecule Sensing. Binary complex MV2+⊂CB[8] has been shown to bind biologically important molecules containing aromatic π-donor moieties, such as tyrosine, tryptophan, and dopamine (DA), in aqueous solution to form stable charge-transfer ternary complexes with high equilibrium constants (Ka = 103−105 M−1), driven by the release of the preencapsulated high-energy water from the cavity.28−31 Here, the heteroternary complexation of S-MVrot2+⊂CB[8] is used to bind DA, demonstrating the sensing ability of the rotaxanefunctionalized surface (Figure 1B). Additionally, the density of the CB[8] rotaxane structures on the Au surface could be directly calculated on the basis of its DA sensing ability. SAMs of alkanethiol are known to form a (31/2 × 31/2)R30° closepacked phase with a molecule-to-molecule spacing of 5 Å.32,33 The distance between two adjacent molecules on the 2-

again for 1 h. The CA (48.9°) remained the same as for SMVrot2+ prior to exposure to CB[8], suggesting that both of the amino groups in the bis-aminoethyl viologen had indeed reacted with the aldehyde surface functionalities of S1, yielding a “closed” structure and thus preventing complexation between MV2+ and CB[8]. The substrate was then soaked in a supersaturated aqueous suspension of ADA, and again the CA did not change (48.1°), demonstrating the absence of CB[8] on the surface. Finally, in the third experiment (Figure 2B(iii)), the preparation of a CB[8] rotaxane on the Au surface was accomplished by first complexing the bis-aminoethyl viologen reagent to form a 1:1 binary complex in water before its reaction with aldehyde-functionalized S1. After 1 h at 40 °C, the CA of the substrate changed to 43.4°, indicating the formation of the rotaxane structure (S-MVrot2+⊂CB[8]). To confirm that an altitudinal surface-mounted rotaxane SMVrot2+⊂CB[8] had formed with its axis of rotation parallel to the surface, two control experiments were conducted. The rotaxane-functionalized substrate was washed with copious amounts of 40 °C water for 2 days; nevertheless, the CA remained stable (44.1°) after the robust washing. Additionally, 10928

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complexation of DA by the functionalized rotaxane surface. Fluorescein-labeled dopamine (FITC-DA) was used for direct imaging of the μCP pattern as shown in Scheme 1.34 A

aminoethanethiol deposition layer was thus assumed to be 5 Å, and the height of a single CB[8] molecule between its two carbonyl portals is 9.1 Å.28 Therefore, the distance between two CB[8] rotaxanes with their carbonyl portals oriented parallel to the surface is 14.1 Å, giving a theoretical surface density of 5.0 × 1013 molecules·cm−2. CB[8] rotaxane-functionalized Au substrate S-MVrot2+⊂CB[8] was dipped into an aqueous solution containing DA (5 mM) for 30 min to form 1:1:1 heteroternary complex (MV2+· DA)⊂CB[8], resulting in a slight increase in the CA from 43.4 to 51.1° as shown in Figure 3B. Under these conditions,

Scheme 1. Synthesis of FITC-DA

patterned SAM of 2-aminoethanethiol on the Au substrate was accomplished by first protecting a portion of the Au substrate with an inert dodecanethiol via μCP using a PDMS stamp with indented areas (dots) followed by backfilling the “bare” areas with 2-aminoethanethiol. Subsequently, CB[8] rotaxanes were fabricated as described above in the third experiment only on the areas with 2-aminoethanethiol present. The patterned CB[8] rotaxane-functionalized substrate was next dipped in a solution containing DA and FITC-DA (DA/FITC-DA = 10:1) for 30 s, and a fluorescent pattern could be readily visualized on account of the formation of heteroternary complexes (MV2+· FITC-DA)⊂CB[8], as shown in Figure 3A. The substrate was then immersed in a solution containing Na2S2O4 for 1 min under nitrogen, resulting in the disappearance of the fluorescence (inset in Figure 3A), suggesting the dissociation of the DA upon formation of the viologen radical cation. Finally, the substrate was subsequently immersed in the same solution of DA and FITC-DA with a continuous flow of oxygen purging the solution, and the fluorescent pattern was fully recovered. This process was repeated for five cycles, and in each cycle, the FITC-DA was successfully anchored and removed without apparent decay. As a control, the process was also repeated for five cycles on a nonpatterned substrate with a 30 min DA soaking time in each cycle to ensure complete formation of the heteroternary complex, and the CA was observed to switch between 43.4° (CB[8] rotaxane-functionalized substrate) and 51.1° ((MV2+·DA)⊂CB[8]) as shown in Figure 3B. The sensitivity of the CB[8] rotaxane-functionalized Au surface toward DA was investigated by varying the concentrations of the analyzed DA solutions. As shown in Table 1, the functionalized surface was soaked in a DA solution with a concentration of 1 × 10−4 M for 30 min. Next, the complexed DA remaining on the substrate was extracted into 1 mL of water by reducing the MV2+ moieties with Na2S2O4 for 1 min under nitrogen. The concentration of the extracted DA could be readily calculated from a calibration curve (Supporting Information, Figure S1). The same substrate was subsequently reused after copious washing with water in air and immersed in a freshly diluted DA solution (e.g., 5 × 10−5 M). This procedure was repeated eight times, diluting the DA solution at each cycle with the data shown in Table 1, which enabled a reasonable estimation of the surface density of the CB[8] rotaxane. (See the Supporting Information for a description of the calculation.) As the UV absorbance of the extracted DA remained stable (0.0381 ± 0.0010) for DA solutions ranging in concentration from 5 × 10−8 to 0.005 M, the formation of the

Figure 3. (A) Fluorescence microscopy image (λex = 488 nm) of 50 μm (dot diameter) × 350 μm (interval length) dotted FITC-DA arrays on the CB[8] rotaxane-functionalized Au substrate. The inset image (scale bar = 200 μm) was taken after the substrate was immersed in a solution containing Na2S2O4. (B) Water CA of the unpatterned substrate with DA present.

heteroternary complex formation between DA and MV2+⊂CB[8] reaches equilibrium, and most of the CB[8] rotaxanes present are complexed with DA molecules (vide infra, see discussion regarding Table 1). DA was subsequently extracted Table 1. Concentrations of DA Solutions Analyzed and the Corresponding UV Absorbances/Densities of DAs Immobilized on the Surface concentration analyzed (M) 5 1 5 1 5 1 5 1 5

× × × × × × × × ×

10−3 10−4 10−5 10−6 10−7 10−7 10−8 10−8 10−9

UV absorbance (a.u.)a 0.0385 0.0372 0.0364 0.0391 0.0380 0.0391 0.0385 0.0212 0.0079

surface density (molecules·cm−2)b 4.8 4.6 4.5 4.9 4.7 4.9 4.8 2.3 4.5

× × × × × × × × ×

1013 1013 1013 1013 1013 1013 1013 1013 1012

a

UV absorbance of the DA extracted from the surface into 1 mL of water. bCalculated surface density of the complexed DAs on the surface.

from the substrate into 1 mL of water by reducing the viologen dication (MV2+) to its radical cation (MV+·) form with sodium dithionite (Na2S2O4) at a concentration of 5 mM. The UV absorbance of the extracted DA solution at λmax = 280 nm was then used to obtain a concentration from an experimentally derived calibration curve (Supporting Information). As the surface area examined was 157 cm2, the functionalization density of CB[8] rotaxanes on the substrate was calculated to be 4.8 × 1013 molecules·cm−2. The ease of recovery of the original CB[8] rotaxane structure is important for developing reusable sensors. To demonstrate the robust and fully reversible nature of our CB[8] rotaxanes, microcontact printing (μCP) was utilized to monitor the 10929

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1:1:1 heteroternary complex is likely quantitative under these conditions, and thus the number of extracted DA molecules represents the number of rotaxane molecules on the surface. The data suggested that the detection limit for DA using the CB[8] rotaxane-functionalized surface is 5 × 10−8 M, which is very low compared to literature values.35 Moreover, as the detection and binding of DA is fully reversible through redox control, the rotaxane-functionalized surface can act as a reusable, highly sensitive DA sensor. Reusable Template for Well-Defined Colloidal Arrays. The manipulation of colloids with high accuracy and a defined arrangement on hard substrates is of great interest for the possible integration of these building blocks into photonic devices, high-density patterned media, and chemical sensors.36 Functionalized silica colloids were employed to study the interactions between the colloidal particles and the rotaxanefunctionalized Au surface as they are easily produced with a wide variety of surface functionalities.37 The use of lightresponsive azobenzene-functionalized silica colloids (Si-azo) with Dh = 408 nm facilitates the direct imaging of μCP patterns and also gives rise to potential applications in the fields of nanophotonics and device construction.38 Controllable recognition-directed self-assembly of Si-azo colloids on the Au surface was achieved through complexation between the Azo, which acts as a good second guest, and the 1:1 MV2+⊂CB[8] rotaxane (Figure 1C).12 The μCP technique verified that the surface-mounted rotaxanes were capable of anchoring functionalized silica particles onto the Au surface through host−guest complexation as the Si-azo colloids were visibly only when bound to rotaxane-functionalized areas. After the Au substrate was dipped into an aqueous suspension of Si-azo colloids (0.5% w/v) for 30 s, colloidal monolayers were found on the dotted locations precoated with CB[8] rotaxanes, as can be seen in Figure 4A,B. An enlarged image of a dotted area with colloids present in Figure 4B shows the monolayer deposition of Si-azo colloids on the substrate (Figure 4C). On an unpatterned Au substrate, an increased CA of 94.1° was observed after dipping the substrate into the colloidal suspension, likely on account of the hydrophobic nature of the Si-azo colloids as shown in Figure 4D. To demonstrate the fully reversible nature of the colloidal assembly, immersion of the patterned substrate into a 5 mM solution of Na2S2O4 for 1 min under a nitrogen atmosphere led to the disappearance of the colloidal arrays (inset image in Figure 4A) and an increased surface hydrophilicity. Removal of the Si-azo colloids was driven by single-electron reduction of the surface-bound viologen dications (MV2+) to their radical cation (MV+·) state, breaking the heteroternary complex of (MV2+·Si-azo)⊂CB[8]. The substrate was subsequently dipped into the Si-azo colloid suspension for 30 s with oxygen bubbling through the solution, and the Si-azo colloids were immobilized on the substrate once more with the viologen radical cation (MV+·) being oxidized back to its dication (MV2+) form. Furthermore, the reversible attachment and removal of the Siazo colloids on the Au surface could also be achieved by orthogonal photochemical transformation of the azobenzene functionalities on the Si-azo colloids.11,12 UV light (350 nm, 1 min) irradiation led to the disappearance of the colloidal arrays driven by the UV light-induced trans-to-cis isomerization of Azo, resulting in the dissociation of (MV2+·Si-azo)⊂CB[8] and the regeneration of the CB[8] rotaxane-functionalized Au substrate. The CA (43.4°) of the unpatterned substrate demonstrated that the surface wettability of the substrate was

Figure 4. (A) Optical microscopy image of 50 μm (dot diameter) × 150 μm (interval length) dotted Si-azo colloidal arrays on the CB[8] rotaxane-functionalized Au substrate. The inset image (scale bar = 100 μm) was taken after the substrate was immersed in a solution containing Na2S2O4 or after UV light (λ = 350 nm) irradiation. (B) SEM image of the patterned colloidal arrays. (C) Enlarged SEM image. (D) Water CA of the substrate with unpatterned colloidal monolayers present.

also recovered with the removal of the Si-azo colloids. Finally, the colloidal pattern could be rewritten onto the substrate with a concomitant increase in the surface hydrophobicity (CA = 94.1°) after irradiating the substrate with visible light (420 nm, 2 min) while it was immersed in the same Si-azo colloidal suspension, which induced the cis-to-trans isomerization of Azo moieties on the surface of the silica colloids. While immobilization of colloids onto the substrate surface can be achieved in both an azimuthal and altitudinal manner as shown in Figure 5, altitudinal rotaxanes show higher stability compared to their azimuthal counterparts. For example, after immobilization of the Si-azo onto an azimuthal-functionalized surface, the colloids were readily removed upon addition of ADA. The CB[8] host molecules become fully occupied by ADA to form binary 1:1 complexes of ADA⊂CB[8], breaking the heteroternary complex of (MV2+·Si-azo)⊂CB[8] and releasing Si-azo colloids. In the rotaxane case, however, Si-azo colloids remained on the Au surface even upon addition of ADA on account of the closed structure of the surface-bound rotaxanes; the CB[8] molecules could not dethread to complex with ADA.

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CONCLUSIONS We report a simple, stepwise method for preparing CB[8] rotaxane-functionalized Au surfaces. Our route eliminates the problematic synthesis of thiol-functionalized pseudorotaxanes, and the chemical transformations can be easily followed by CA. The surface-bound rotaxane structure is important in terms of maintaining robust surface functionality, thus improving the reusability of the functional surface and prolonging its working lifetime. Rotaxane-functional surfaces were capable of complexing CB[8] second guests such as Azo and DA, both in the form of small molecules or when the guests were attached to another 10930

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Figure 5. Comparison between azimuthal and altitudinal surface-bound rotors. Scale bar: 200 μm. (6) Anslyn, E. V. Supramolecular Analytical Chemistry. J. Org. Chem. 2007, 72, 687−699. (7) Gassensmith, J.; Matthys, S.; Lee, J. J.; Wojcik, A.; Kamat, P.; Smith, B. Squaraine Rotaxane as a Reversible Optical Chloride Sensor. Chem.Eur. J. 2010, 16, 2916−2921. (8) Zhu, L. L.; Li, X.; Ji, F. Y.; Ma, X.; Wang, Q. C.; Tian, H. Photolockable Ratiometric Viscosity Sensitivity of Cyclodextrin Polypseudorotaxane with Light-Active Rotor Graft. Langmuir 2009, 25, 3482−3486. (9) Kim, J.; Jung, I.-S.; Kim, S.-Y.; Lee, E.; Kang, J.-K.; Sakamoto, S.; Yamaguchi, K.; Kim, K. New Cucurbituril Homologues: Syntheses, Isolation, Characterization, and X-ray Crystal Structures of Cucurbit[n]uril (n = 5, 7, and 8). J. Am. Chem. Soc. 2000, 122, 540−541. (10) Rauwald, U.; Biedermann, F.; Deroo, S.; Robinson, C. V.; Scherman, O. A. Correlating Solution Binding and ESI-MS Stabilities by Incorporating Solvation Effects in a Confined Cucurbit[8]uril System. J. Phys. Chem. B 2010, 114, 8606−8615. (11) del Barrio, J.; Horton, P. N.; Lairez, D.; Lloyd, G. O.; Toprakcioglu, C.; Scherman, O. A. Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and Supramolecular Polymers. J. Am. Chem. Soc. 2013, 135, 11760−11763. (12) Tian, F.; Jiao, D.; Biedermann, F.; Scherman, O. A. Orthogonal Switching of a Single Supramolecular Complex. Nat. Commun. 2012, 3, 1207. (13) Tian, F.; Cheng, N.; Nouvel, N.; Geng, J.; Scherman, O. A. SiteSelective Immobilization of Colloids on Au Substrates via a Noncovalent Supramolecular “Handcuff. Langmuir 2010, 26, 5323− 5328. (14) Kim, K. Mechanically Interlocked Molecules Incorporating Cucurbituril and Their Supramolecular Assemblies. Chem. Soc. Rev. 2002, 31, 96−107. (15) Ko, Y. H.; Kim, E.; Hwang, I.; Kim, K. Supramolecular Assemblies Built with Host-Stabilized Charge-Transfer Interactions. Chem. Commun. 2007, 1305−1315. (16) Sindelar, V.; Moon, K.; Kaifer, A. E. Binding Selectivity of Cucurbit[7]uril: Bis(pyridinium)-1,4-xylylene versus 4,4′-Bipyridinium Guest Sites. Org. Lett. 2004, 6, 2665−2668. (17) Ramalingam, V.; Urbach, A. R. Cucurbit[8]uril Rotaxanes. Org. Lett. 2011, 13, 4898−4901. (18) Nguyen, T. D.; Tseng, H. R.; Celestre, P. C.; Flood, A. H.; Liu, Y.; Stoddart, J. F.; Zink, J. I. A Reversible Molecular Valve. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10029−10034. (19) Liu, Y.; Flood, A. H.; Bonvallet, P. A.; Vignon, S. A.; Northrop, B. H.; Tseng, H. R.; Jeppesen, J. O.; Huang, T. J.; Brough, B.; Baller, M.; Magonov, S.; Solares, S. D.; Goddard, W. A.; Ho, C. M.; Stoddart, J. F. Linear Artificial Molecular Muscles. J. Am. Chem. Soc. 2005, 127, 9745−9759. (20) Berna, J.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.; Perez, E. M.; Rudolf, P.; Teobaldi, G.; Zerbetto, F. Macroscopic Transport by Synthetic Molecular Machines. Nat. Mater. 2005, 4, 704−710. (21) Patel, K.; Angelos, S.; Dichtel, W. R.; Coskun, A.; Yang, Y.-W.; Zink, J. I.; Stoddart, J. F. Enzyme-Responsive Snap-Top Covered Silica Nanocontainers. J. Am. Chem. Soc. 2008, 130, 2382−2383.

surface. Importantly, the functional surfaces can sense DA in solution with concentrations as low as 5 × 10−8 M without apparent decay over many cycles; this detection limit is extremely low compared to that of previously reported DA sensors.35 The closed structure of the surface-bound CB[8]based rotaxanes exhibits chemical, redox, and photoresponsivities, thus offering a versatile platform for the development of molecular devices with high complexity.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of monoaminoethyl viologen, bis-aminoethyl viologen, and the dopamine-dye conjugate and the preparation of the functional silica colloids. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(F.T.) School of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge, Massachusetts 02138, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Ziyi Yu for help with the μCP masks and SEM. C.H. is thankful to BP for supporting this work and Hughes Hall for a scholarship. F.T. is grateful for a CSC Cambridge Scholarship and a Duke of Edinburgh Scholarship.

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