Ferrocene-Decorated Nanocrystalline Cellulose with Charge Carrier

Apr 10, 2012 - Process and Environmental Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7...
0 downloads 0 Views 3MB Size
Download PDF
Letter pubs.acs.org/Langmuir

Ferrocene-Decorated Nanocrystalline Cellulose with Charge Carrier Mobility Samuel Eyley,† Sara Shariki,‡ Sara E.C. Dale,‡ Simon Bending,§ Frank Marken,*,‡ and Wim Thielemans*,†,∥ †

School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom § Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom ∥ Process and Environmental Research Division, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom ‡

S Supporting Information *

ABSTRACT: Ferrocene-decorated cellulose nanowhiskers were prepared by the grafting of ethynylferrocene onto azide functionalized cotton-derived cellulose nanowhiskers using azide−alkyne cycloaddition. Successful surface modification and retention of the crystalline morphology of the nanocrystals was confirmed by elemental analysis, inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The coverage with ferrocenyl is high (approximately 1.14 × 10−3 mol g−1 or 4.6 × 1013 mol cm−2 corresponding to a specific area of 61 Å2 per ferrocene). Cyclic voltammetry measurements of films formed by deposition of ferrocene-decorated nanowhiskers showed that this small spacing of redox centers along the nanowhisker surface allowed conduction hopping of electrons. The apparent diffusion coefficient for electron (or hole) hopping via Fe(III/II) surface sites is estimated as Dapp = 10−19 m2s−1 via impedance methods, a value significantly less than nonsolvated ferrocene polymers, which would be expected as the 1,2,3-triazole ring forms a rigid linker tethering the ferrocene to the nanowhisker surface. In part, this is believed to be also due to “bottleneck” diffusion of charges across contact points where individual cellulose nanowhiskers contact each other. However, the charge-communication across the nanocrystal surface opens up the potential for use of cellulose nanocrystals as a charge percolation template for the preparation of conducting films via covalent surface modification (with applications similar to those using adsorbed conducting polymers), for use in bioelectrochemical devices to gently transfer and remove electrons without the need for a solution-soluble redox mediator, or for the fabrication of three-dimensional self-assembled conducting networks.

■

INTRODUCTION Ferrocene (FcH) is a widely used redox mediator in sensors and bioelectrochemical systems because of its fast electron transfer, low oxidation potential, and two stable redox states. Ferrocenyl (Fc) assemblies have been envisioned for chemiresistors, chemically sensitive field-effect transistors, and charge storage components, while surface tethering also reduces potential leaching.1 Semiconducting surfaces are commonly used to immobilize Fc, enabling direct electron transfer from the surface to the tethered Fc moieties.1−3 In these devices, the rate of electron communication between Fc groups is governed by the surface coverage of Fc,4 while electron hopping was prevented at inter-Fc distances above 18 Å on the surface of nonconductive Cowpea mosaic virus.5 Compton et al. also showed the possibility to use phase separation to create ferrocene redox-active surfaces by modifying graphene electrodes with microdroplets of n-butylferrocene in aqueous electrolytes.6 The preparation of three-dimensional Fc assemblies on well-defined structured crystalline surfaces is the obvious next step, but methods need to be developed to © 2012 American Chemical Society

control the position of the Fc groups and the resulting conductive pathways. Cellulose nanowhiskers have received much attention as reinforcement for composite materials7−9 and as building block for porous structures and membranes as the high aspect ratio nanoparticles easily form percolated networks.10−14 They also display ample reactive surface hydroxyl groups which have been used to introduce other functionalities such as cationic groups15,16 and fluorophores.17,18 However, these modifications do not use the inherent surface order of the hydroxyl groups resulting from the crystalline structure of cellulose nanowhiskers. Highly ordered crystal surfaces (in contrast to gels or polymers) provide ideal substrates for functionalization in nanodevices, for example, responsive membranes, where charge propagation is important and positioning of (distance between) the charge carrying grafts needs to be controlled. Received: January 9, 2012 Revised: April 1, 2012 Published: April 10, 2012 6514

dx.doi.org/10.1021/la3001224 | Langmuir 2012, 28, 6514−6519

Langmuir

Letter

ICP-OES was performed on a Perkin-Elmer Optima 2000 DV optical emission spectrometer. Samples were prepared by digestion in 7 M Aristar grade nitric acid at 190 °C for 6 h in a sealed Parr Instruments 4749 acid digestion bomb, before dilution to 3% v/v nitric acid for analysis. Iron content was calculated from the 238.204 and 239.562 nm emission lines, with calibration using single element standards. For voltammetric studies, an Autolab PGSTAT30 or a microAutolab III potentiostat system (EcoChemie, NL) were employed with a Pt gauze counter electrode and a saturated calomel (SCE) reference electrode (REF401, Radiometer, Copenhagen). Impedance data were obtained and analyzed with FRA software from Autolab. The working electrode was a 1 cm2 tin-doped indium oxide (ITO) electrode (Image Optics, Basildon, 15 Ω square−1). ITO electrodes were cleaned by washing in ethanol and distilled water followed by heating at 500 °C in air for 30 min and re-equilibration to ambient conditions. Atomic force microscopy (AFM) images were obtained with an Asylum Research MFP-3D stand alone microscope (tips were AC240TS from Olympus, images were obtained in tapping mode). Transmission electron microscopy (TEM) was performed on a JEOL 200FX operating at 100 kV. A dilute sample dispersion (water/DMF 1:1) was deposited onto a holey carbon coated copper grid and left for 3 min after which excess liquid was removed. One sample was used immediately after drying, while a second sample was stained using 2% uranyl acetate in water solution (exposure to staining solution 3 min). Preparation of Ferrocene-Modified Cellulose Nanowhiskers. Synthesis of 6-Azidodeoxycellulose Nanocrystals. The preparation of 6-azidodeoxycellulose nanocrystals was performed following an earlier published procedure through azidation of 6-chlorodeoxycellulose nanocrystals.15 In short, purified cotton-extracted cellulose nanowhiskers10 (2.5 g) were suspended in a solution of pyridine (25 mL, 310 mmol) and dry toluene (200 mL) under argon. A thionyl chloride (12.5 mL, 170 mmol) in dry toluene (25 mL) solution was added dropwise followed by heating at 65 °C for 16 h. The resulting light brown suspension was filtered, and the product was washed with dichloromethane (20 mL), deionized water (50 mL), and acetone (20 mL) before Soxhlet extraction with dichloromethane (24 h) and ethanol (48 h). The light brown chlorinated nanowhiskers were isolated by drying in vacuo. IR (KBr): ν̃ = 715 cm−1 (C−Cl). Found (%): C, 36.1; H, 5.70; Cl, 8.00. One gram of the obtained and dried 6-chlorodeoxycellulose nanowhiskers was then suspended in dimethylformamide (50 mL) containing sodium azide (1.5 g, 23 mmol) and heated to 100 °C for 24 h. The solid product was separated by filtration, washed with deionized water (5 × 10 mL), dispersed in deionized water, and dialyzed against deionized water for 4 days. The product was subsequently isolated by freeze-drying. IR (KBr): ν̃ = 2114 cm−1 (N3). Found (%): C, 40.15; H, 5.28; N, 5.27. Synthesis of 6-(4-Ferrocenyl-1,2,3-triazol-1-yl)deoxycellulose Nanocrystals (Fc-g-CNXLs). 6-Azidodeoxycellulose nanocrystals (270 mg) and ethynylferrocene (100 mg) were suspended in tetrahydrofuran (30 mL) under argon, and degassed 0.1 M copper(II) sulfate (2 mL, 0.2 mmol) and 0.1 M sodium ascorbate (5 mL, 0.5 mmol) were added before heating the suspension at 70 °C for 2 days. The solid product was isolated by filtration, washed with THF (10 mL) and ethanol (10 mL), then dialyzed against deionized water for 5 days. Found (wt %): Fe 6.4.

Crystalline materials offer highly defined chemical properties as a function of crystal orientation and the inherent order to surface functionalities. Therefore, surface chemistry becomes selective and properties better defined compared to amorphous surfaces. Nanoscale crystalline whisker materials are not easy to find and cellulose nanowhiskers offer an exceptionally nice case to study functionalization and function. With the primary hydroxyl groups on the nanowhisker surface spaced 6−10 Å apart (vide infra), chemical modification of these groups with charge-communicating groups may lead to rigid chargecommunicating ribbonlike structures. The rate of charge diffusion can then be varied by varying surface grafts, method of linking (rigid or flexible linker group), and may even be dependent on the environment, all of which have been shown on flat surfaces,1,19 thereby opening up a wide field of applications such as smart membranes, sensors, bioelectrochemical devices, and three-dimensional conducting systems. This work describes the first step in this investigation to prove that sufficient grafting of cellulose nanowhiskers with ferrocene groups leads to charge communication along the lateral nanowhisker surface. To achieve this, we have placed Fc grafts along the length of the nanowhisker surface at the primary hydroxyl groups. The reaction conditions were chosen to make sure the cellulose nanocrystals retained their crystallinity and morphology unlike previous work where cellulose was dissolved, followed by grafting of ferrocene containing chromophores using a long linker molecule.20 The inherent order of modifiable surface groups on the nanowhisker surface and high grafting density results in controlled and short distances between the Fc grafts (6−10 Å), enabling electron (or hole) hopping between adjacent Fc grafts.

■

MATERIALS AND METHODS

Reagents. All chemicals were purchased from Sigma-Aldrich, Alfa Aesar, or Fisher Scientific and used without further purification unless otherwise stated. Demineralized and filtered water was taken from an Elgastat water purification system (Elga, High Wycombe, Bucks) with a resistivity of not less than 18 MΩ cm−1. Potassium nitrate electrolyte salt was purchased from Aldrich in analytical grade purity and used without further purification. Aristar grade nitric acid was purchased from VWR international and used without further purification. FTIR grade potassium bromide was purchased from Fisher Scientific and oven-dried at 150 °C prior to use. Cellulose nanocrystals, chlorinated cellulose nanocrystals, and azidated cellulose nanocrystals were all prepared according to the previously established procedure.15 Instrumentation. Infrared spectra were recorded from a 1 wt % sample in oven-dried FTIR grade potassium bromide using a Thermo Nicolet 380 FTIR spectrometer in transmission mode. Powder X-ray diffraction data were recorded on a PANalytical X’Pert Pro MPD in Bragg−Brentano geometry, with monochromated Cu Kα1 (λ = 1.5406 Å, 40 kV, 40 mA) radiation, automated divergence and receiving slits (10 mm illuminated length), 10 mm beam mask, 0.04 rad soller slits, and a step size of 0.08°. The temperature during experiments was 20 ± 2 °C. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Ultra X-ray photoelectron spectrometer employing a monochromated Al Kα (hν = 1486.6 eV, 120 W) X-ray source, hybrid (magnetic/ electrostatic) optics (300 × 700 μm aperture), hemispherical analyzer, multichannel plate, and delay line detector (DLD) with a takeoff angle of 90° and an acceptance angle of 30°. All scans were acquired under charge neutralization conditions using a low energy electron gun within the field of the magnetic lens. Survey scans were taken with a pass energy of 80 eV and high resolution scans with a pass energy of 20 eV. Binding energies were referenced to adventitious carbon at 284.8 eV.

Procedure for Cellulose Film Voltammetry and Impedance Measurements. For electrochemical studies, a stable film of ferrocene-functionalized cellulose is required. Initial experiments with aqueous-ethanolic (1:1) dispersions of the cellulose (0.68 wt %) evaporated onto the ITO electrode surface proved unsatisfactory. Dimethylformamide (DMF) as a less volatile solvent component substantially improved film formation, and good films were obtained by evaporation of water/DMF (1:1) dispersions of cellulose (0.68 wt %) in an oven at 80 °C. 6515

dx.doi.org/10.1021/la3001224 | Langmuir 2012, 28, 6514−6519

Langmuir

Letter

and development of a broad peak at 401 eV which could be modeled by three closely related nitrogen environments from the triazole ring, represented by component peaks at 399.8, 400.7, and 401.8 eV (Figure 1). XPS Fe 2p high resolution

Experiments were carried out in aqueous 0.1 M KNO3. While many ferrocenes are sensitive to nucleophilic attack when oxidized and exposed to moisture,21 the ferrocene used here has a substituent that makes this nucleophilic attack insignificant in aqueous 0.1 M KNO3. Furthermore, there was no evidence of decay in signals and repeat scans and scan rate dependence investigation was possible, experimentally showing the absence of any measurable nucleophilic attack. Since we envisage important applications in conjunction with biomolecules and in bioelectronic devices which all operate under aqueous conditions, it seemed appropriate to utilize water as a solvent of first choice, rather than other common ferrocene redox solvents used in the literature such as acetonitrile. In order to extract a reliable charge diffusion coefficient, data fitting was performed (χ2 = 2 × 10−2) employing the following elements:22 R1 = 5000 Ω

R2 = 13 000 Ω

C = 1 nF σ (1 − i) with ω RT 2 σ= 2 2 = 35 kΩ n F Ac Dapp

Z (W ) =

The frequency ω = 2πf and the constants R (gas constant) and F (Faraday constant) have their usual meaning. With the absolute temperature T = 293 K, the number of transferred electrons per charge carrier arriving at the electrode, n = 1, and the area A = 10−4 m2.

Figure 1. High resolution N 1s X-ray photoelectron spectra of (a) azidocellulose nanocrystals and (b) ferrocenyl cellulose nanocrystals. (c) High resolution Fe 2p X-ray photoelectron spectrum of ferrocenyl cellulose nanocrystals.

■

RESULTS AND DISCUSSION Cellulose nanowhiskers were modified at the primary hydroxyl groups using a recently published azide−alkyne cycloaddition approach15 with Fc grafts (Scheme 1). Successful modification

scans reveal the intact Fc structure through the binding energy of the Fe 2p3/2 peak at 708.0 eV, which corresponds nicely with the literature value for Fc.24 The nanocrystal structure and morphology are not affected by the reactions. The rodlike structure of the nanoparticles is retained as is shown by TEM imaging (Figure 2). The extensive surface modification with Fe did not provide sufficient contrast for clear imaging, indicating grafting of individual molecules (Supporting Information), and uranyl acetate staining needed to be used. The crystallinity of cellulose nanowhiskers remained around 86% as determined by XRD powder diffraction. Furthermore, using the Scherrer equation to fit the (110) and (110̅ ) peaks, similar values for the dimensions of the crystalline core were found as in earlier published reports for cross-sectional nanocrystal dimensions of 6 × 6.1 nm.25 These dimensions give 1.18 mmol/g primary surface hydroxyl groups, in agreement with results obtained by selectively oxidizing these hydroxyl groups to carboxylic acids and performing a conductometric titration.14 Cellulose nanowhiskers derived from cotton exist in the Iβ polymorph with a monoclinic unit cell with lattice parameters a = 7.78 Å, b = 8.20 Å, c = 10.38 Å, and γ = 96.5°.26 The crystal planes (110) and (11̅0) correspond to the perpendicular faces of the nanowhisker (Figure 3). Using the crystal structure lattice parameters,26 we calculated the separation between the reactive hydroxyl groups on the nanocrystal surfaces. Figure 3b shows the geometrical location of the primary hydroxyl groups and interhydroxyl distances on the (110) and (11̅0) surfaces. It

Scheme 1. Reaction Scheme for the Preparation of Ferrocenyl Cellulosea

a

Part (a) has been published earlier.15 Reproduced by permission of The Royal Society of Chemistry (RSC).

of azidated cellulose nanocrystals was initially confirmed by loss of the azide stretching vibration (2114 cm−1) and small Fc vibration and deformation bands (∼ 815 and 870 cm−1) in the infrared spectrum of the cellulose nanocrystals (Supporting Information). Grafting is further corroborated by the N 1s high resolution X-ray photoelectron spectra, showing loss of the characteristic peaks of the azide species23 at 400.8 and 404.5 eV 6516

dx.doi.org/10.1021/la3001224 | Langmuir 2012, 28, 6514−6519

Langmuir

Letter

Figure 2. Transmission electron micrograph of uranyl acetate stained unmodified cotton cellulose nanocrystals (left) and ferrocenyl cellulose nanocrystals (right).

properties.5 The distance between Fc grafts on the surface of our nanoparticle is on the order of 6−10 Å, with a surface concentration of 2.84 × 10−10 mol cm−2. This is inferred from the position of the primary hydroxyl groups on the nanocrystal surface (converted to azides) and the high grafting efficiency resulting in 97% conversion of azides to Fc grafts. Electron hopping is thus expected to be possible parallel and transverse to the length longitudinal axis of the nanowhisker. When deposited from 0.68 wt % ferrocenyl-cellulose in water/DMF (1:1) onto tin-doped indium oxide (ITO) electrodes, reproducible peak currents were observed (Figure 4A). The midpoint potential for the reversible oxidation

Figure 3. (a) Cellulose nanocrystal structure where the gray rectangles depict cellulose chains going into the paper and (b) schematic of the geometrical location of primary hydroxyl groups (dots) on the (110) and (11̅0) crystal planes based on the crystallographic data from ref 26. Shaded area shows the molecular footprint of the primary hydroxyl groups.

can be noted that they are not perfectly staggered as is commonly assumed. As only the primary hydroxyl groups are converted to azides,15 these equal the Fc positions at complete conversion of the surface modification, with the smallest achievable molecular footprint of 59 Å2. The actual surface coverage of the nanowhiskers with Fc moieties is 1.14 mmol g−1of nanowhiskers (through Fe determination by ICP) corresponding to near-complete conversion of primary hydroxyl groups (1.18 mmol g−1, DS = 0.5). Using the cotton nanowhisker cross-sectional dimensions of 6 nm × 6.1 nm, the specific area per Fc graft is determined to be 61 Å2. This compares to the ball-like shape of Fc molecules with a diameter of 6.6 Å covering a specific surface of 34−43 Å2, depending on whether the surface projection is taken as a circle (excluding the intercircle voids more or less possible for grafts on a flexible tether) or as a square with sides equal to the sphere diameter (for grafts directly tethered to the surface). The latter is expected to be more correct in this work as the linking triazole can be considered to be rigid enough. Close-packed Fc monolayers tethered to silicon using flexible linkers with surface concentration 4.8 × 10−10 mol cm−2 showed lateral charge hopping between adjacent Fc groups.1,19 Nonideal electrochemical behavior is noticed for singlecomponent Fc monolayers with surface concentrations below 5 × 10−11 mol cm−2.1 For example, Fc separation distances higher than 18 Å on the surface of Cowpea mosaic virus showed electronic isolation with apparent homogeneous redox

Figure 4. (A) Cyclic voltammograms (scan rate 10 mV s−1) for the oxidation of (i) 70 μg and (ii) 0 μg ferrocenyl-cellulose deposited from water/DMF onto ITO (ca. 1 cm2) and immersed in aqueous 0.1 M KNO3. (B) As before, but deposited from water/DMF and (i) 30, (ii) 70, (iii) 110, and (iv) 140 μg deposit. (C) As before but 70 μg deposited from water/DMF and scan rate (i) 0.5, (ii) 1, (iii) 5, and (iv) 10 mV s−1. (D) Plot of anodic peak current (scan rate 10 mV s−1) versus deposition amount deposited from water/DMF. (E) Plot of the anodic peak current (70 μg deposit) versus scan rate.

process is ca. 0.50 V vs SCE. AFM images of the Fc-cellulose deposit formed from water/DMF (Supporting Information) show cellulose whiskers deposited flat onto the surface and in a random fashion. These images are essentially identical to previous images obtained for unmodified cellulose nanowhisker films.11 Some nanowhisker bundles exist which will contain Fcs buried within the bundle as well as exposed surfaces with Fcs pointing outward. The peak current for ferrocenyl-cellulose oxidation increases approximately linearly up to a 70 μg deposit of ferrocenyl cellulose on 1 cm2 ITO (see Figure 4D). The shape of these 6517

dx.doi.org/10.1021/la3001224 | Langmuir 2012, 28, 6514−6519

Langmuir

Letter

5 ± 1 kΩ, the cell resistance dominated probably by film resistance, R2 = 13 ± 1 kΩ, the average charge transfer resistance for electron transfer to ferrocenes in the vicinity of the working electrode surface, and C = 1 ± 0.1 nF, the interfacial capacitance lowered by the presence of the cellulose. From the parameter σ = 35 kΩ for the Warburg element, the diffusion coefficient Dapp = 10−19 m2 s−1 can be estimated (assuming 10% packing density and an apparent ferrocene concentration of 0.3 mol dm−3). An apparent diffusion coefficient of 10−19 m2 s−1 appears incredibly low but can be rationalized with “bottleneck” diffusion of charges across contact points where individual cellulose nanowhiskers contact each other. It is likely that the true diffusion coefficient is closer to 10−15 m2 s−1 in line with values reported for metal complexes at TiO2 nanoparticle surfaces27 and nonsolvated vinylferrocene (∼10−15 m2 s−1),28 but smaller than values reported for flexible solvated systems (10−10−10−12 m2 s−1).29,30 This is expected due to the rigidity of the 1,2,3-triazole and the imobilisation of the Fc close to the nanocrystals surface, both of which reduce mobility and thus the rate of electron transfer. On silicon surfaces, Fabre showed slower redox communication for this setup as well, with significant increases in redox communication speed with increasing tether length and flexibility.1

voltammetric peaks (Figure 4B) reveals a transition from a thin film deposit to a thick film with more drawn out signals. The effect of scan rate (see Figure 4E) suggests a nonlinear trend indicative of a contribution from charge transport/diffusion within the film deposit even at very low scan rates. The charge under the oxidation peak estimated at low scan rate and for a 70 μg deposit is approximately 300 μC. For cellulose whisker material 6 nm × 6.1 nm and a density of 1.59 g cm−3, a surface area of 2.9 × 10−2 m2 for the 70 μg deposit is calculated, giving a lower estimate for the ferrocenyl charge per area of 1 μC cm−2. This corresponds to an estimated molecular footprint of 16 nm2 or lower. Comparing the molecular footprint from ferrocenyl oxidation with the extent of surface modification (61 Å2 per Fc) indicates that only ca. 4% of the film close to the electrode surface is electrochemically active. This is believed to be due to difficulty of interparticle charge transfer (in contrast to the single crystal surface transport). The effect could be useful in pressure-responsive films. Alternatively, the effect of the deposit conformation and structure on the electrochemical activity of the deposited film could be used for further optimization. However, despite this relatively low value, enough activity of the film is achieved to investigate the redox communication between ferrocene grafts on the active fraction of the film, the main aim of this work. Impedance measurements at the reversible potential of the sample show a typical Nyquist plot for data recorded at the equilibrium potential (ca. 0.50 V vs SCE, see Materials and Methods) for a frequency range of 0.1 Hz to 10 kHz (Figure 5A). A deposit of 140 μg was chosen consistent with the “thick film” limit demonstrated in Figure 4D. The semicircle for electron transfer and at lower frequencies the typical 45° line for charge diffusion are readily observed, and a conventional Randles-Ersher circuit (see inset) is employed for data analysis. The physical interpretation of the circuit elements are for R1 =

■

CONCLUSIONS Rodlike cellulose nanowhiskers with redox communicating Fc centers were prepared, and we have shown for the first time that the ordered positions of surface hydroxyl groups on cellulose nanowhiskers can be used to place these redox grafts at positions capable of charge-communication. The molecular footprint of these hydroxyl groups makes them ideally suited for the preparation of surface-charge-communicating rodlike nanoparticles which can then be assembled in three-dimensional structures for application in electronics, computing, sensors, and bioelectrochemical assemblies. Current work to improve interparticle charge hopping in three dimensions by controlled deposition is in progress. We are also investigating increasing the flexibility of the tether which has shown on flat surface to increase the charge diffusion rate.

■

ASSOCIATED CONTENT

S Supporting Information *

Additional figures showing FTIR spectra, TEM image, and AFM image. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.M.); Wim.Thielemans@ nottingham.ac.uk (W.T.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.S. would like to thank Gharib Shariki and Maryam Shirmardi for financial support for this work. S.E. and W.T. thank Emily Smith for XPS acquisition and Mark Guyler for ICP-OES acquisition. W.T. also thanks EPSRC for funding under the Driving Innovation in Chemistry and Chemical Engineering (DICE) Science and Innovation Award (Grant Number EP/ D501229/1).

Figure 5. (A) Nyquist plot for the oxidation and back-reduction of ferrocenyl-cellulose (140 μg deposited onto ITO, area 1 cm2) immersed in aqueous 0.1 M KNO3 at 0.53 V vs SCE applied potential with 10 mV amplitude (frequency range 0.1 Hz to 10 kHz, 50 points). (B) Bode plot for phase angle versus frequency. (C) Bode plot for impedance versus frequency (data points = experiment, lines = fit, χ2 = 2 × 10−2). 6518

dx.doi.org/10.1021/la3001224 | Langmuir 2012, 28, 6514−6519

Langmuir

■

Letter

(20) Zhang, F.; Vill, V.; Heck, J. Cellulose-Based Polymers with Long-Chain Pendant Ferrocene Derivatives as Organometallic Chromophores†. Organometallics 2004, 23 (16), 3853−3864. (21) Connelly, N. G.; Geiger, W. E. Chemical redox agents for organometallic chemistry. Chem. Rev. 1996, 96 (2), 877−910. (22) Girault, H. H. Electrochemical Impedance. In Analytical and Physical Electrochemistry; EFPL Press: Lausanne, Switzerland, 2004; pp 339−374. (23) Devadoss, A.; Chidsey, C. E. D. Azide-Modified Graphitic Surfaces for Covalent Attachment of Alkyne-Terminated Molecules by “Click” Chemistry. J. Am. Chem. Soc. 2007, 129 (17), 5370−5371. (24) Connor, J. A.; Derrick, L. M. R.; Hillier, I. H. High energy photoelectron spectroscopy of transition metal complexes. Part 4.Bis(arene) and related complexes of chromium, manganese and iron. J. Chem. Soc., Faraday Trans. 2 1974, 70, 941−944. (25) Elazzouzi-Hafraoui, S.; Nishiyama, Y.; Putaux, J.-L.; Heux, L.; Dubreuil, F. d. r.; Rochas, C. The Shape and Size Distribution of Crystalline Nanoparticles Prepared by Acid Hydrolysis of Native Cellulose. Biomacromolecules 2007, 9 (1), 57−65. (26) Nishiyama, Y.; Langan, P.; Chanzy, H. Crystal Structure and Hydrogen-Bonding System in Cellulose Iβ from Synchrotron X-ray and Neutron Fiber Diffraction. J. Am. Chem. Soc. 2002, 124 (31), 9074−9082. (27) Cummings, C. Y.; Wadhawan, J. D.; Nakabayashi, T.; Haga, M.a.; Rassaei, L.; Dale, S. E. C.; Bending, S.; Pumera, M.; Parker, S. C.; Marken, F. Electron hopping rate measurements in ITO junctions: Charge diffusion in a layer-by-layer deposited ruthenium(II)-bis(benzimidazolyl)pyridine-phosphonate-TiO2 film. J. Electroanal. Chem. 2011, 657 (1−2), 196−201. (28) Watanabe, M.; Nagasaka, H.; Ogata, N. Redox Activity of Vinylferrocene Copolymers by Electron Hopping Reaction in the Absence of Fluid Solvents. J. Phys. Chem. 1995, 99 (32), 12294− 12300. (29) Merchant, S. A.; Glatzhofer, D. T.; Schmidtke, D. W. Effects of Electrolyte and pH on the Behavior of Cross-Linked Films of Ferrocene-Modified Poly(ethylenimine). Langmuir 2007, 23 (22), 11295−11302. (30) Nagarale, R. K.; Lee, J. M.; Shin, W. Electrochemical properties of ferrocene modified polysiloxane/chitosan nanocomposite and its application to glucose sensor. Electrochim. Acta 2009, 54 (26), 6508− 6514.

REFERENCES

(1) Fabre, B. Ferrocene-Terminated Monolayers Covalently Bound to Hydrogen-Terminated Silicon Surfaces. Toward the Development of Charge Storage and Communication Devices. Acc. Chem. Res. 2010, 43 (12), 1509−1518. (2) Evrard, D.; Lambert, F.; Policar, C.; Balland, V.; Limoges, B. Electrochemical Functionalization of Carbon Surfaces by Aromatic Azide or Alkyne Molecules: A Versatile Platform for Click Chemistry. Chem.Eur. J. 2008, 14 (30), 9286−9291. (3) Huang, K.; Duclairoir, F.; Pro, T.; Buckley, J.; Marchand, G.; Martinez, E.; Marchon, J.-C.; De Salvo, B.; Delapierre, G.; Vinet, F. Ferrocene and Porphyrin Monolayers on Si(100) Surfaces: Preparation and Effect of Linker Length on Electron Transfer. ChemPhysChem 2009, 10 (6), 963−971. (4) Zigah, D.; Herrier, C.; Scheres, L.; Giesbers, M.; Fabre, B.; Hapiot, P.; Zuilhof, H. Tuning the Electronic Communication between Redox Centers Bound to Insulating Surfaces. Angew. Chem., Int. Ed. 2010, 49 (18), 3157−3160. (5) Aljabali, A. A. A.; Barclay, J. E.; Butt, J. N.; Lomonossoff, G. P.; Evans, D. J. Redox-active ferrocene-modified Cowpea mosaic virus nanoparticles. Dalton Trans. 2010, 39 (32), 7569−7574. (6) Wadhawan, J. D.; Evans, R. G.; Compton, R. G. Voltammetric characteristics of graphite electrodes modified with microdroplets of nbutylferrocene. J. Electroanal. Chem. 2002, 533 (1−2), 71−84. (7) Eichhorn, S. J. Cellulose nanowhiskers: promising materials for advanced applications. Soft Matter 2011, 7 (2), 303−315. (8) Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. Review: current international research into cellulose nanofibres and nanocomposites. J. Mater. Sci. 2010, 45 (1), 1−33. (9) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40 (7), 3941−3994. (10) Heath, L.; Thielemans, W. Cellulose nanowhisker aerogels. Green Chem. 2010, 12 (8), 1448−1453. (11) Thielemans, W.; Warbey, C. R.; Walsh, D. A. Permselective nanostructured membranes based on cellulose nanowhiskers. Green Chem. 2009, 11 (4), 531−537. (12) Bonne, M. J.; Galbraith, E.; James, T. D.; Wasbrough, M. J.; Edler, K. J.; Jenkins, A. T. A.; Helton, M.; McKee, A.; Thielemans, W.; Psillakis, E.; Marken, F. Boronic acid dendrimer receptor modified nanofibrillar cellulose membranes. J. Mater. Chem. 2010, 20 (3), 588− 594. (13) Tsourounaki, K.; Bonne, M. J.; Thielemans, W.; Psillakis, E.; Helton, M.; McKee, A.; Marken, F. Nanofibrillar cellulose-chitosan composite film electrodes: competitive binding of triclosan, Fe(CN)63-/4-, and SDS surfactant. Electroanalysis 2008, 20 (22), 2395−2402. (14) Liew, S. Y.; Thielemans, W.; Walsh, D. A. Electrochemical Capacitance of Nanocomposite Polypyrrole/Cellulose Films. J. Phys. Chem. C 2010, 114 (41), 17926−17933. (15) Eyley, S.; Thielemans, W. Imidazolium grafted cellulose nanocrystals for ion exchange applications. Chem. Commun. 2011, 47 (14), 4177−4179. (16) Hasani, M.; Cranston, E. D.; Westman, G.; Gray, D. G. Cationic surface functionalization of cellulose nanocrystals. Soft Matter 2008, 4 (11), 2238−2244. (17) Nielsen, L. J.; Eyley, S.; Thielemans, W.; Aylott, J. W. Dual fluorescent labelling of cellulose nanocrystals for pH sensing. Chem. Commun. 2010, 46 (47), 8929−8931. (18) Dong, S. P.; Roman, M. Fluorescently labeled cellulose nanocrystals for bioimaging applications. J. Am. Chem. Soc. 2007, 129 (45), 13810−13811. (19) Hauquier, F.; Ghilane, J.; Fabre, B.; Hapiot, P. Conducting Ferrocene Monolayers on Nonconducting Surfaces. J. Am. Chem. Soc. 2008, 130 (9), 2748−2749. 6519

dx.doi.org/10.1021/la3001224 | Langmuir 2012, 28, 6514−6519