Electrochemically Controlled Assembly and Logic Gates Operations of

Jan 7, 2012 - The reversible assembly of β-cyclodextrin-functionalized gold NPs is studied on mixed self-assembled monolayer (SAM), formed by ...
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Electrochemically Controlled Assembly and Logic Gates Operations of Gold Nanoparticle Arrays Marco Frasconi*,† and Franco Mazzei* Department of Chemistry and Drug Technology, “Sapienza” University of Rome, Piazzale Aldo Moro 5, Rome 00185, Italy S Supporting Information *

ABSTRACT: The reversible assembly of β-cyclodextrinfunctionalized gold NPs (β-CD Au NPs) is studied on mixed self-assembled monolayer (SAM), formed by coadsorption of redox-active ferrocenylalkylthiols and n-alkanethiols on gold surfaces. The surface coverage and spatial distribution of the β-CD Au NPs monolayer on the gold substrate are tuned by the self-assembled monolayer composition. The binding and release of β-CD Au NPs to and from the SAMs modified surface are followed by surface plasmon resonance (SPR) spectroscopy. The redox state of the tethered ferrocene in binary SAMs controls the formation of the supramolecular interaction between ferrocene moieties and β-CD-capped Au NPs. As a result, the potential-induced uptake and release of β-CD Au NPs to and from the surface is accomplished. The competitive binding of β-CD Au NPs with guest molecules in solution shifted the equilibrium of the complexation−decomplexation process involving the supramolecular interaction with the Fc-functionalized surface. The dual controlled assembly of β-CD Au NPs on the surface enabled to use two stimuli as inputs for logic gate activation; the coupling between the localized surface plasmon, associated with the Au NP, and the surface plasmon wave, associated with the thin metal surface, is implemented as readout signal for “AND” logic gate operations.

1. INTRODUCTION The controlled release and uptake of molecules or nanoparticles (NPs) is an important tool for a wide range of applications in science and engineering. Precise manipulation of nanoparticle assemblies would enable one to exploit the features of available NPs such as electronic, magnetic, or optical properties1 so as to generate functional devices, ranging from information storage and delivery systems2,3 to sensor, plasmonic, and other microelectronic devices.4,5 Various routes to direct nanoparticle assemblies have been explored.1,6 One of the most popular strategies to form monolayer of Au NPs on a substrate was achieved by modifying a surface with a self-assembled monolayer (SAM), where Au NPs can be attached via the affinity of the free functionality on the SAM distal end for nanoparticulate gold.7 For instance, Au NPs were assembled on carboxylic,8 amines,9 or dithiols10 functionalized SAMs through electrostatic or covalent interactions. Albeit challenging, further control using external stimuli to direct the ordering and local environmental of nanoparticles would be ideal for the design of responsive functional nanocomposites. Surfaces with stimuli-responsive properties, also know as smart surfaces, have attracted substantial research interest in the past few years.11 Functional, stimuli-responsive small molecules can be readily substituted to fine-tune or incorporate specific redox, photonic, or catalytic properties. By taking advantage of © 2012 American Chemical Society

noncovalent interactions, nanocomposites responsive to pH, light, heat, and redox potential were developed, where the spatial distribution of the nanoparticles can be varied by changing the local environment.11,12 Directional hydrogen-bonding and host−guest interactions are particularly attractive to organize collections of nanoparticles into controlled architectures. Indeed, intermolecular interactions in the nanocomposite can be strengthened or broken by external stimuli, resulting in changes of the local environment and in the spatial distribution of the nanoparticles.12 Therefore, the local environment and the spatial arrangement of the nanoparticles can be tailored, opening further routes to manipulate the properties of these nanostructures. For example, Au NPs can be capped with host molecules, such as β-cyclodextrins (β-CDs), and a nanoparticle network can be created via supramolecular interactions.13 In addition, electrochemically controlled attachment/detachment of threedimensional structure was achieved by multivalent supramolecular interaction between ferrocene-functionalized dendrimers and β-CD-capped gold and silica NPs.14 Anchoring of functionalized guest molecules by multiple specific interactions is key to the development of molecular printboards for nanopatterning.15 Indeed, the redox behavior of ferrocene Received: October 11, 2011 Revised: December 28, 2011 Published: January 7, 2012 3322

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Scheme 1. Redox-Controlled Reversible Uptake and Release of β-CD-Capped Au NPs to and from Gold Substrate Modified with Ferrocenylalkylthiolates and Competitive Binding with Guests Molecules in Solution

gates operations. The basic criteria for the operation of molecule-based logic gates are fast responses (output) that result from external stimuli (input), and adequate reversibility for resetting. Most of the studies reported on chemical and biochemical systems that are capable to perform logic operations employ small molecules or biomolecules as inputs and a fluorescence signal as output.19−21 Some of these devices find application in medical diagnostic and material science.19 More recent advances have relied on functional integration methods based on supramolecular chemistry as a means of designing the active elements of logic gates. For instance, a voltage-activated supramolecular-plasmonic device based on resonance surface-enhanced Raman scattering of laser light was recently reported.22 To the best of our knowledge, the light/ electrical driven assembly of Au NPs via supramolecular interaction and the application of surface plasmon coupling between Au NPs and the gold surface readout signal for logic gate operations, have not been devised to date; therefore, we describe a proof-of-principle system that can be considered as an active molecular plasmonic device for elementary computing. The system is triggered by photonic and electronic signals to yield different states exhibiting variable binding/release capacities for NPs. Since the system is activated by two inputs (electrical and optical) that generate four distinct states, one may consider the system as readout logic gate. This principle

derivatives is well-known for its reversibility in host−guest systems.16 In general, the neutral ferrocene residues form stable inclusion complexes with β-CD, whereas the oxidized, positively charged ferricinium ions are not bound by β-CDs.17 Here, we report the electrochemically controlled assembly of β-CD-functionalized Au NPs (β-CD Au NPs) on ferrocenylalkylthiolates self-assembled monolayer (Fc-SAM). The surface coverage and spatial distribution of β-CD Au NPs monolayer on the surface can be tuned by the self-assembled monolayer composition. The reversible binding of β-CD Au NPs onto the Fcfunctionalized surface was followed by probing the surface plasmon resonance (SPR) angle shift, amplified by the coupling between the localized surface plasmon (LSP) associated with the Au NP and the surface plasmon polariton (SPP) associated with the thin metal surface.5,18 The impact of Au NPs surface coverage and interparticle distance on the SPR response was also investigated. The competitive binding with guest molecules in solution shifts the equilibrium of the complexation−decomplexation process involving the supramolecular interaction with the Fcfunctionalized surface (see Scheme 1). Here, the combination of electrochemical switching of the ferrocenyl groups in the SAM and photochemical switching of a competitive azobenzene derivative was employed to demonstrate molecular-based logic 3323

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High-resolution transmission electron microscopy (HRTEM) was performed using a JEOL 2010 electron microscope (JEOL USA Inc., Peabody, MA). 2.3. Synthesis of Nanoparticles. Au NPs functionalized with perthiolated β-CD (β-CD Au NPs) were prepared by mixing a 10 mL solution containing 25 mg of HAuCl4 in DMSO and a 5.0 mL solution containing 5 mg of per-6-thio-β-cyclodextrin in DMSO.13a Subsequently, 5 mL of DMSO solution containing 40 mg of sodium borohydride (NaBH4) was added dropwise. The reaction mixture turned dark-colored. The reaction was allowed to continue for 24 h, and then 20 mL of CH3CN was added to precipitate the Au NPs. The particles were washed and centrifuged with 30 mL of 1:1 (v/v) CH3CN:DMSO mixture and then 30 mL of ethanol, isolated by centrifugation, and dried under vacuum for 24 h. A mean particle size of ca. 3.0 nm was estimated using HR-TEM (see Supporting Information, Figure S1). The UV/vis spectrum of β-CD Au NPs dissolved in aqueous solution showed plasmon resonance absorption maximum in the range 510−520 nm. 2.4. Surface Modification. Glass supported gold substrates for SPR spectroscopy, composed of a gold sensing surface (thickness 50 nm) deposited onto a glass microscope slide with a titanium adhesion layer (1.5 nm), were purchased from Xantec Bioanalytics (Muenster, Germany). Prior to the deposition of a thiol monolayer, the gold surface was cleaned by a repetitive electrochemical sweep from −0.7 to 1.2 V (vs SCE) in 50 mM KClO4 at the scan rate of 20 mV s−1 until a reproducible voltammogram was obtained. The effective surface area was evaluated from the differential capacitance in 50 mM KClO4. The mixed self-assembled monolayer was formed from 2 mM ethanolic solutions containing 12-ferrocenyl-7-oxo-1-dodecanethiol (Fc(CH2)5O(CH2)6SH) and dodecanethiol (CH3(CH2)11SH) at different mole fraction for one night at 4 °C. After the adsorption period, the modified gold surfaces were rinsed with ethanol and water and dried under a nitrogen flow. For the morphological characterization, the Au(111) surface was prepared by thermal evaporation on a mica substrate in vacuum at 1 × 10−7 mbar. The mica (Ruby Muscovite mica, S&J Trading) was outgassed at 563 K for 5 h and was kept at the same temperature during the evaporation. Typically, 1000 Å of gold (CERAC, 99.999% purity) was deposited on freshly cleaved mica, and after metallization, it was allowed to cool down to room temperature. The chamber was then filled with nitrogen, and the sample was taken out and immediately immersed into mixed SAMs. For the QCM experiments, chips were cleaned prior to each experiment in a 1:1:3 NH4OH 28%:H2O2 30%:H2O mixture at 70 °C for 15 min, washed several times with water and ethanol, and dried under a stream of nitrogen. All experiments were performed at a temperature of 25.00 ± 0.05 °C.

can be extended also to create delicate supramolecular organizations that reversibly assemble and disassembly by different type of stimuli.

2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals were purchased from Aldrich and used without any other purification. β-Cyclodextrin (β-CD) was dried in a vacuum with P2O5 at 80 °C for 5 h before used. Solvents were purified according to standard laboratory procedures. 12-Ferrocenyl-7-oxo-1-dodecanethiol (Fc(CH2)5O(CH2)6SH) was prepared by a procedure analogous to that reported in the literature.23 Briefly, Friedel−Crafts acylation of 12-bromo-6-oxo-1-dodecanoyl chloride with AlCl3 in dichloromethane yields an α-keto alkyl chain. The ketone group was reduced to bromide under Clemmensen conditions (Zn/HCl) and then converted to thiol by treatment with thiourea in ethanol and basic hydrolysis (NaOH) yielding the 12ferrocenyl-7-oxo-1-dodocanthiol as a yellow-orange powder. Purification was achieved by column chromatography on silica and a 95:5 (v/ v) mixture of hexane and ethanol as an eluent and recrystallization from hexane solution by layered addition of methanol. Alkyl-substituted hydroxyphenylazo compounds (Az, Scheme 1) were synthesized by the azo coupling method.24 The compounds were purified on a cellulose (Whatman CC31) column using a mixture of 1butanol, NH3 (2 mol L−1 aqueous solution), and ethanol 60:20:20 (v/ v/v) as eluent. The synthesis of per-6-thio-β-cyclodextrin (β-CD) was performed according to a literature procedure.25 All aqueous solutions were prepared using deionized water (specific resistivity ≥18.2 MΩ cm) obtained from a Direct-Q 3 UV apparatus (Millipore, France). 2.2. Instrumentation. SPR experiments were performed by an Eco Chemie Autolab SPR Kretschmann-type system (Ecochemie, The Netherlands) that works with a laser diode fixed at a wavelength of 670 nm. In this configuration, the intensity of the reflected light is minimum at the resonance angle. The glass-supported SPR substrates at the prism were clamped against a Teflon cuvette with O-rings (diameter 3 mm), providing liquid-tight seals. The SPR instrument is complemented by a μAutolab electrochemical analyzer (Eco Chemie, Utrecht, The Netherlands) in a threeelectrode setup. The gold SPR substrate at the prism mounted against the Teflon cell with use of an O-ring was used as a working electrode (0.8 mm2 area exposed to the solution). The Teflon cell allowed to simultaneously record the SPR and electrochemical experiments. Auxiliary Pt and quasi-reference Ag electrodes wires (0.5 nm diameter) were part of the cell. The Ag quasi-reference electrode was calibrated according to the potential of dimethyl viologen (N,N′-dimethyl-4,4′bipyridinium dichloride),26 E0 = −0.687 V versus standard calomel electrode (SCE), measured by cyclic voltammetry, and the potentials are versus SCE. All measurements were performed in 0.1 M phosphate buffer solution (PBS), pH 7.0. For irradiation of the samples, the light for the photoreaction was generated by a Shark OTLH-040-UV-LED (365 nm, Laser Components GmbH, Olching, Germany) and an LED435-66-60 (435 nm, Roithner Lasertechnik GmbH, Vienna, Austria) emitting diodes. During the photoirradiation experiments, the sample was irradiated via a quartz glass window. Quartz crystal microbalance (QCM) measurements were performed with a Q-sense E4 instrument (Q-Sense, Sweden) using gold “chips” that consisted of a thin quartz crystal coated with a gold layer (QSense, f = 5 MHz, C = 17.7 ng cm−2 Hz−1, geometric area of gold working surface = 0.78 cm2). Change in fundamental frequency, Δf, was converted into a mass change, Δm (g cm−2), via the Sauerbrey equation. The morphological characterization of SAMs and β-CD Au NPs modified surface was performed by atomic force microscopy (AFM) ex situ, in air, tapping mode, with a Digital Instruments MultiMode AFM (Santa Barbara, CA) with a Nanoscope IIIa controller.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Characterization of Tethered Ferrocenes in Binary SAMs. The reversible assembly of βCD Au NPs on ferrocenylalkylthiolates SAM is depicted in Scheme 1. Binary SAMs on gold surfaces were prepared by coadsorption of 12-ferrocenyl-7-oxo-1-dodecanethiol (Fc(CH2)5O(CH2)6SH) and dodecanethiol (CH3(CH2)11SH) in ethanolic solutions at the stated mole fraction, χFcsoln (Fc(CH2)5O(CH2)6SH mole fraction in solution). The use of ether-based link in placed of methylene-based link, in the ferrocenylalkylthiol chains, reduces steric constraints and increases the stability of the monolayer film. The introduction of an ether linkage provides better spatial distribution of the ferrocene over the surface; the molecular design allows the ferrocene moiety to protrude from the alkanethiols matrix (Scheme 1), minimizing the steric hindrance of the ferrocene molecules. Figure 1 shows cyclic voltammograms of mixed SAMs, Fc(CH2)5O(CH2)6SH/CH3(CH2)11SH, formed by 3324

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mol cm−2 could be estimated for χFcsoln = 0.1. This value is about 35% of the theoretical value (4.5 × 10−10 mol cm−2) calculated for an hexagonal close-packing of ferrocene moiety, assumed as a sphere of diameter 6.6 Å (or the experimental maximum value of 4.4 × 10−10 mol cm−2 obtained here).29 The relationship between the Fc moles fractions in solution used for obtaining SAMs vs the corresponding surface coverage of Fc on the gold surface is plotted in Figure 1, inset. The Fc surface coverage obtained in the cases of χFcsoln ≥ 0.6 is only slightly larger than the χFcsoln = 0.3 case. Furthermore, an increase in the peak width broadening is nonetheless observed for the highest ferrocene coverages. Such behaviors are compatibles with an increase in the interactions between the Fc moieties, as the redox probe surface concentration is raised.30 3.2. Redox-Controlled Uptake and Release of β-CDCapped Nanoparticles on Fc-SAM. The assembly of β-CD Au NPs (diameter ca. 3.0 nm) onto the ferrocenylalkylthiolates SAM was studied. Figure 2 shows the SPR curves of surfaces

Figure 1. Cyclic voltammograms recorded at 20 mV s−1 for a binary SAMs modified electrode formed from 2 mM ethanolic solutions containing increasing Fc(CH2)5O(CH2)6SH/CH3(CH2)11SH ratios (χFcsoln): 0.01, 0.05, 0.1, 0.3, 0.6, and 1.0. Inset: plot of the ferrocene surface coverage (ΓFc) as a function of incubation solution mole fraction of Fc(CH2)5O(CH2)6SH (χFcsoln). All measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0).

incubation of ferrocenylalkylthiolates at different mole fractions, ranging from 0.01 to 1.0. When the SAM is formed from χFcsoln ≤ 0.1, a remarkably well-defined symmetrical pair of redox peaks, characteristic of the Fc/Fc+ couple, is observed at formal potential (E0′) of 316 ± 5 mV (vs SCE). Both anodic and cathodic peak currents show a linear dependence on the scan rate (v), as expected for surface-confined electroactive centers. The low peak-to-peak potential separation value, less than 18 mV at potential scan rate of 20 mV s−1, and the ΔEfwhm for each peak close to the theoretical value of 89 mV, reported for an ideal Nernstian redox species adsorbed on electrode surface, suggest reduced interactions between the immobilized ferrocene/ferricinium molecules and a highly homogeneous environment around the redox centers. This result reflects somewhat the high degree of self-organization afforded by the selected immobilization strategy. At high scan rates (v ≥ 200 mV s−1), the peak potential difference between the anodic and cathodic waves increases, reflecting progressive kinetic control by the rate of electron transfer of the ferrocene units (Figure S2, Supporting Information). The electron transfer rate constant (ket) was then estimated using Laviron’s method.27 The best fit of the data with the classical Butler−Volmer equations for a surface confined-redox reaction gives a ket of 21 s−1, a value that agrees well with previous measurements of ket for analogous systems.23 However, to take into account the slight interactions between the immobilized redox probes, the voltammograms were also simulated with the Laviron’s equation for a surface-confined redox reaction with interaction between the immobilized molecules.28 In this simulation (see Supporting Information for more details), the same value of ket was assumed and the following values of the interaction parameters were determined in order to gain the best fit: β = −0.4, γ = 0.6, μ = 0, λ = 0. An improved fitting of the simulated curves was observed (Figure S2). From the integration of the anodic and cathodic peak, an equilibrium Fc(CH2)5O(CH2)6S- coverage (ΓFc) of 1.6 × 10−10

Figure 2. SPR spectra recorded before (a) and after addition of β-CD Au NPs (0.40 mM) onto mixed SAM modified gold surfaces with different ferrocene surface coverage (10−10 mol cm−2): 0.0 (b), 0.04 (c), 0.09 (d), 0.16 (e), 0.32 (f), 0.44 (g). Inset: dependence of the resonance angle shift on the composition of the SAM as ΓFc. All measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0). Error bars are standard errors of the mean with N = 5.

modified with different Fc coverages upon their interaction with a fixed concentration of β-CD Au NPs (0.40 mM). A low density of Fc moiety on the surface (low ΓFc) induces a small amount of β-CD Au NPs to interact with the modified surface, as evidenced from the small shift of the SPR curve. As the mole fraction of Fc on the binary SAM increases, a significant higher shift in plasmon angle as well as an increase in the minimum reflectance and noticeable broadening of the SPR curve is observed. The shift of the plasmon angle, which is mainly dependent on the NPs coverage and the steric hindrance effect between neighboring NPs on the surface, attains the largest change for a Fc surface coverage close to 3.2 × 10−10 mol cm−2 (Figure 2, inset). These perturbations in the SPR curve are most likely the results of the coupling between the localized surface plasmons associated with the Au NPs and the surface plasmon wave of the thin metal surface.18 Microgravimetric quartz crystal microbalance (QCM) analysis allowed the determination of the surface coverage of β-CD Au NPs. The frequency changes (Δf) occurring upon assembly of β-CD Au NPs on Fc-SAM modified surface (ΓFc = 3325

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1.6 × 10−10 mol cm−2) was followed at different time intervals. After ∼10 min of assembly, the frequency changes reaches a constant value that translates to a weight coverage of 2.7 × 10−6 g cm−2, corresponding to an average coverage of a β-CD Au NPs monolayer (∼3.9 × 1012 particles cm−2). Figure 3A depicts the sensorgrams, after blank subtraction, for the binding interaction between β-CD Au NPs, at various

adsorption of β-CD Au NPs onto the modified surface was evaluated by recording the SPR angle shift at different concentrations of β-CD Au NPs onto the alkanethiol-modified surface (Figure S4, Supporting Information). The association phase resulted in a slight increase in the SPR signal that drop back to its original value when the NPs solution is replaced by buffer, suggesting the absence of nonspecific adsorption of NPs on SAMs modified surface without ferrocene. Kinetic analysis of the host−guest interaction between β-CD Au NP and Fc-SAM was performed with a 1:1 binding model (Figure S5, Supporting Information) according to the equations31 k tr

β‐CD Au NP bulk XooooY β‐CD Au NPsurface k−tr

(1a)

kon

β‐CD Au NPsurface + Fc XooooY Fc·β‐CD Au NP koff

(1b)

Taking into account the mass transport limitation (MTL) of the NPs to the surface, the association constant (Ka = kon/koff) and diffusion rate constant (ktr) of β-CD Au NPs to the FcSAMs are derived to be 1.9 × 103 M−1 and 1.27 × 10−5 m s−1, respectively. The dissociation of β-CD Au NPs from the Fc-SAMs modified surface is controlled electrochemically by tuning the redox state of ferrocene moieties. When the surface is subjected to the oxidation potential of E = 0.5 V, the ferrocene groups are transformed to the positively charged ferricinium ions. The removal of the stabilized host−guest interaction results in the release of β-CD Au NPs from the modified surface, as shown from the topographic images (Figure 4A). It is apparent from the AFM images that the nanoparticles assembled onto the FcSAMs modified surface as well-organized isolated particles, with average thickness of ∼5 nm. Upon the oxidation, the resulting AFM image shows a smooth surface. The microgravimetric analysis of the surface subjected to the reduction potential of E = 0.1 V revealed a surface coverage of 6.8 × 1012 particles cm−2, a value that falls very close to the theoretical surface coverage of a densely packed monolayer of NPs (7.1 × 1012 particles cm−2), calculated from the surface area and the size of β-CD Au NP (3 nm diameter). The value of surface coverage, taken together with the AFM image, suggests that a densely packed monolayer of β-CD Au NPs is assembled on the reduced Fc-SAMs, as a consequence of the higher affinity between β-CD units and the fully reduced ferrocene moieties. The concomitant SPR angle changes of the Fc-SAM modified surface, subjected to series of applied potential cycles, are depicted in Figure 4B, curve a. The application of reductive potential results to an increase of the SPR angle toward high value, consistent with the uptake of β-CD Au NPs onto the surface. When the Fc-SAM modified surface is subjected to an oxidation potential of E = 0.5 V, a fast drop of θ back to its original value is observed as result of the release of β-CD Au NPs from the surface. As evidenced, the process is fully reversible and the electrochemical reduction of ferricinium ion to neutral ferrocene, at E = 0.1 V, re-forms the complex and the β-CD Au NPs assemble on the neutral surface. In a control experiment, no SPR angle changes are observed upon the application of the potential cycles on SAMs modified surface without ferrocene (Figure 4B, curve b).

Figure 3. (A) SPR sensorgrams obtained after blank subtraction corresponding to the interaction between Fc-SAM modified gold surface (ΓFc = 1.6 × 10−10 mol cm−2) and variable concentrations of βCD Au NPs (mM): 0.03 (a), 0.07 (b), 0.15 (c), 0.25 (d), 0.40 (e), 0.70 (f), and 1.3 (g). (B) Calibration curve corresponding to the resonance angle shifts at different concentrations of added β-CD Au NPs on (a) Fc-SAM modified surface (ΓFc = 1.6 × 10−10 mol cm−2) and (b) SAMs modified surface without ferrocene. All measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0). Error bars are standard errors of the mean with N = 5.

concentrations, and Fc-modified surface (ΓFc = 1.6 × 10−10 mol cm−2). The shift in the SPR signal indicates the association of β-CD Au NPs to Fc-SAMs up to equilibrium value. The resonance angle increases as the concentration of β-CD Au NPs is raised, and it levels off at 0.70 mM (Figure 3B, curve a). For comparison, the SPR angle shifts corresponding to the interaction of various concentrations of β-CDs, prior to functionalization with Au NPs, to the Fc-modified surface are recorded (Figure S3, Supporting Information), resulting in a minute SPR angle change (Figure 3B, curve b). To confirm the formation of a host−guest inclusion complex between β-CD Au NPs and the Fc moieties, the nonspecific 3326

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Figure 4. (A) AFM images (topography) corresponding to the modulated loading (at E = 0.1 V) and unloading (at E = 0.5 V) cycle of β-CD Au NPs on Fc-SAM modified gold surface (ΓFc = 1.6 × 10−10 mol cm−2). (B) SPR angle changes corresponding to the cyclic electrochemically controlled uptake and release of β-CD Au NPs (0.40 mM) on the (a) Fc-SAM modified gold surface (ΓFc = 1.6 × 10−10 mol cm−2) and (b) on SAM-modified surface (without ferrocene). All measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0).

3.3. Competitive Selective Binding with Guest Molecules. In addition to reversibly binding on Fcfunctionalized surface, β-CD Au NPs can also provide a vehicle for the noncovalent encapsulation of dye molecules and drugs into the cavity of β-CD.32 The equilibrium established by the displacement of ferrocene and the competitive association of guest molecules to the β-CD cavity can be controlled by the concentration of the guest in solution. Figure 5A shows the SPR spectrum of β-CD Au NPs assembled on Fc-SAM modified gold surface (ΓFc = 1.6 × 10−10 mol cm−2) prior to the displacement with 1-adamantanecarboxylic acid (1) (curve a) and after equilibration with 1.5 mM 1 (curve b). The shift of the SPR spectrum indicates the release of β-CD Au NPs from the surface. Figure 5B depicts the sensorgrams obtained when solutions at variable concentration of 1 (0.05−3.00 mM), containing a fixed concentration of βCD Au NPs (1.5 mM), were passed over the Fc-functionalized surface. The presence of 1 in the solution of β-CD Au NPs reduces the concentration of free β-CD Au NPs. Correspondingly, the SPR angle shift and the initial rate of binding of β-CD Au NPs onto the Fc-SAM modified surface decreases as the concentration of 1 increases; when the concentration of 1 is 1.5

Figure 5. (A) SPR spectrum corresponding to the Fc-SAM modified gold surface (ΓFc = 1.6 × 10−10 mol cm−2) in the presence of 0.15 mM β-CD Au NPs (a) in the absence of 1-adamantanecarboxylic acid (1) as competitive guest and (b) upon interaction with 1, 1.5 mM. (B) Sensorgrams corresponding to the interaction of β-CD Au NPs (1.5 mM) over the Fc-SAM modified surface (ΓFc = 1.6 × 10−10 mol cm−2), upon the addition of variable concentrations of 1 (mM): 0.00 (a), 0.05 (b), 0.10 (c), 0.20 (d), 0.35 (e), 0.70 (f), 1.50 (g), and 3.00 (h). The SPR responses were obtained after blank subtraction. (C) SPR angle shifts to the binding of β-CD Au NPs (0.15 mM) over the Fc-SAMs functionalized surface in the presence of variable concentrations of 1. Inset: Scatchard plot used for the determination of affinity constant for the interaction of β-CD Au NPs with 1 in solution. All measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0). Error bars are standard errors of the mean with N = 5. 3327

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mM, all β-CD cavities are filled with 1 and the β-CD Au NPs are unable to bind to the surface (Figure 5C). Similar results are observed upon interaction with 4-hydroxytoluene (2) as competitive guest molecule (Figure S6, Supporting Information). The detection limits for analyzing 1 and 2 corresponded to 3 and 10 μM, respectively. The guest molecules 1 and 2 were selected as model compounds due to their well-established association to β-CD.33 We can estimated the association constant for the interaction between β-CD and guest molecules in solution (1 or 2) based on the decrease in the initial rate of binding of β-CD Au NPs to immobilized ferrocene in the presence of 1 or 2 as a competitive guest (a Scatchard plot, see Supporting Information for more details). The derived association constants of 1 and 2 to the β-CD were 3.3 × 104 and 8.5 × 103 M−1, respectively. The obtained constants falls very close to the values reported in the literature for the interaction of the compounds 1 and 2 with β-CD.33 The quantitative analysis of the complexation−decomplexation process involving the supramolecular interactions can be implemented for many substrates. It should be noted that under conditions where the analyzed substrate associates to βCD with a binding constant that allows the displacement of the ferrocene probing molecule the reported system can be employed as sensing platform for several substrates. The proposed system was further applied to study the competitive inclusion of alkyl-substituted hydroxyphenylazo derivatives (Az) to the β-CD cavity. Azobenzene and its derivatives exist in two configurations (trans and cis) and can be interconverted from the more stable trans to the less stable cis configuration upon adsorption of UV light. Previous investigations demonstrated a high binding affinity in aqueous solution between β-CD and trans-Az derivatives and a low, if any, binding between β-CD and cis-derivatives.34 Addition of increasing concentration of Az compounds (0.05−3.00 mM) in the presence of fixed concentration of βCD Au NPs (1.5 mM), over the Fc-functionalized surface, yielded the SPR angle changes depicted in Figure 6. Az derivatives associate competitively to the cavity of β-CD, a process that is reflected by the decrease of SPR angle shifts as the concentration of guest molecules increases. The derived association constants for the inclusion equilibria with β-CD disclose an interesting correlation with the steric effects of various alkyl substituents in the phenol moiety of the guests. From the competitive binding curves, we estimated for 3 an association constant of 2.2 × 103 M−1. The introduction of a methyl substituent in the 3-position of the phenol side does not appreciably affect the stability of the complex. Considerable increase in the association constant is observed on introducing one ethyl group, Ka = 6.0 × 103 M−1. This stabilization can be ascribed to the size of the phenol side of the guest molecule 4, about 7.5 Å, which is almost the same as the diameter of the βCD cavity. On the other hand, the introduction of one tertbutyl group in the 3-position (5) leads to a lower value of association constant, Ka = 2.6 × 103 M−1. Considerable decrease in stability of the inclusion complex is observed on introducing two tert-butyl groups into the 3- and 5-positions, Ka = 1.4 × 103 M−1. This destabilization can be attributed to the steric repulsion between the rim of β-CD and the bulky tertbutyl groups in the guest. The relationship between the association constants and the steric effects of the alkyl substituents demonstrates the regioselective binding of the βCDs on the Au NPs with the guest molecules.

Figure 6. SPR angle shifts corresponding to the binding of β-CD Au NPs (0.15 mM) over the Fc-SAMs functionalized surface in the presence of variable concentration of alkyl-substituted hydroxyphenylazo derivatives (Az): 6 (a), 3 (b), 5 (c), 4 (d). Inset: Scatchard plot used for the determination of affinity constant for the interaction of βCD Au NPs with Az derivatives. All measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0). Error bars are standard errors of the mean with N = 5.

3.4. Logic Gate Operations. A further aspect to consider relates to the effect of the light-induced isomerization of azo compounds on the uptake and release of β-CD Au NPs to and from the Fc-functionalized surface. Figure 7 shows the time-

Figure 7. SPR angle changes corresponding to the light-controlled uptake and release of β-CD Au NPs (0.15 mM) on the Fc-SAM modified gold surface (ΓFc = 1.6 × 10−10 mol cm−2) upon (a) reduction potential (E = 0.1 V) and (b) oxidation potential (E = 0.5 V). The measurements were performed in a 0.1 M phosphate buffer solution (pH = 7.0), in the presence of 4 (9 mM), upon the cyclic irradiation with UV (λ = 365 nm) and visible (λ > 365 nm) light.

dependent angle shifts of the Fc-SAMs modified surface in the presence of β-CD Au NPs and 4, upon the cyclic irradiation of the system with UV (λ = 365 nm) and visible (λ > 365 nm) light. When the surface was subjected to the reduction potential (E = 0.1 V) and the solution was irradiated by UV light, a high SPR signal was observed (curve a). This observation suggests that the photoisomerization of 4 to the cis configuration destabilized the inclusion complex of 4 with β-CD, which favors 3328

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Figure 8. Truth table for an “AND” logic gate system. Input 1 is the irradiation of the solution by UV light (λ = 365 nm). Input 2 is the application of reduction potential (E = 0.1 V) onto Fc-SAMs. The output is the assembly of β-CD Au NPs onto Fc-SAMs modified gold surface.

the assembly of β-CD Au NPs on the surface. The photoisomerization of 4 in the trans configuration results in a decrease in the SPR angle shift as a consequence of a competitive association of 4 with β-CD and, thus, the release of

β-CD Au NPs from the surface. These results clearly demonstrate that upon the cyclic irradiation with UV and visible light the inclusion and exclusion of Az with the β-CD cavity in solution are reversibly cycled, and the uptake and 3329

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release of the β-CD Au NPs onto the Fc-functionalized surface, and from it, can be reversible stimulated. To confirm that the uptake and release of β-CD Au NPs on Fc-SAMs is controlled by the conformation change of azobenzene in solution, control experiments were performed (curve b). The application of UV/vis light cycles when an oxidation potential is applied on the Fc-SAMs modified surface does not yield an observable assembly of β-CD Au NPs on the surface, indicating that the oxidation of ferrocene groups to the positively charged ferricinium ions destabilized the host−guest interaction with the β-CD and thus the assembly of β-CD Au NPs on the surface. The dual-controlled uptake and release of β-CD Au NPs was further examined for logic gate operation. In particular, the uptake of β-CD Au NPs onto Fc-SAMs yielded an “AND” gate upon application of UV light (input 1) and reductive potential (input 2). The SPR angle shift as a function of time was reported to follow the assembly process (Figure 8). When UV light or reductive potential stimuli alone was applied, the β-CD cavity was unable to form an inclusion complex with the ferrocene moiety on SAM, and no assembly of the β-CD Au NPs on the surface occurred. Only in the presence of the two inputs was assembly observed. The present system allowed the digital conversion, in term of “True/False” (1/0), of the final output signal to the initial redox and light inputs by the logic gate.

association onto Fc-SAM, Scatchard analysis of equilibrium competitive binding. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected] (M.F.), franco.mazzei@ uniroma1.it (F.M.); Tel: 00390649913225; Fax: 00390649913133. Present Address †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113.

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ACKNOWLEDGMENTS The authors thank “Sapienza” University of Rome for the financial support.

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REFERENCES

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4. CONCLUSION In the present study the organization of nanoparticles in twodimensional arrays with controlled interparticle separation and ordering is achieved. The electrochemically controlled reduction and oxidation of the ferrocene redox probe-modified SAMs controls the formation of the supramolecular interaction between ferrocene moieties and β-CD-capped Au NPs. This enables a well-regulated and reversible electrochemically stimulated uptake and release of β-CD Au NPs. We further demonstrated that the equilibrium established by the displacement of ferrocene and the competitive association of a guest molecule to the β-CD cavity can be controlled by the concentration of the guest in solution, thus enabling the development of sensing devices and drug delivery systems. Beside the electrochemical stimuli, the light-induced isomerization of azo-compounds to direct the uptake and release of βCD Au NPs to and from the Fc-functionalized surface was introduced. The stimuli-driven assembly of β-CD Au NPs enabled us to use light and redox potential as inputs that activate logic gates and implement the coupling between the localized surface plasmon, associated with the Au NP, and the surface plasmon wave, associated with the thin metal surface, as readout signal for “AND” logic gate operations. Further variation of the system could be achieved by changing the guest with a pH- or thermo-induced isomerizable compound, thus enabling the design of devices that could perform other operations. The proposed dual stimuli programmable NP release system prompted research focus toward the development of implantable drug carriers for therapeutic applications.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

HR-TEM image of β-CD Au NPs, electron transfer kinetic of ferrocenylalkylthiol, association of β-CD on Fc-SAM, nonspecific binding controls, kinetic analysis of β-CD Au NP 3330

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