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J. Phys. Chem. C 2008, 112, 15562–15569
Reading Oxidation States of Encapsulated Ferrocenium in Calix[4]arene Heterodimers Immobilized on Gold Dots by Means of a Scanning Electrochemical Microscopy Probe Renkang Zhu,† Songbo Xu,‡ Ganna Podoprygorina,§ Volker Bo¨hmer,§ Silvia Mittler,‡ and Zhifeng Ding*,† Department of Chemistry, The UniVersity of Western Ontario, London, Ontario N6A 5B7, Canada, Department of Physics and Astronomy, The UniVersity of Western Ontario, London, Ontario N6A 3K7, Canada, and Fachbereich Chemie, Pharmazie und Geowissenschaften, Johannes Gutenberg-UniVersita¨t Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany ReceiVed: June 18, 2008; ReVised Manuscript ReceiVed: July 29, 2008
The oxidation states of encapsulated ferrocenium (Fc+ filled in calix[4]arene heterodimers) in the form of a self-assembled monolayer (SAM) immobilized on a gold surface were recognized by a 2.2 µm diameter ultramicroelectrode (tip) in combination with scanning electrochemical microscopy (SECM). By approaching the SECM tip biased at -0.10 V to the vicinity of the encapsulated Fc+ moieties, an electrochemical current at the tip was observed, indicating the reduction of the encapsulated Fc+ to ferrocene (Fc). On the other hand, no Faradaic current was observed at the tip if the immobilized SAM with encapsulated Fc was beneath the same tip. When the immobilized Fc+ is alternated between the two oxidation states by scanning the electrochemical potential applied to the SAM between -0.20 and 0.50 V, an oxidation current appeared and disappeared periodically at the tip biased at 0.40 V, corresponding to the oxidation state exchange of the Fc+ beneath it. In this way, the SECM tip can be used to monitor or change the oxidation states of the encapsulated Fc+ in the SAM on the gold surface. Two different open circuit potential-time curves were observed for the two oxidation states in the SAM on the gold surface, which confirms that both oxidation states of the encapsulated Fc+ are stable. This system might be developed to a molecular memory device with a small SECM tip used as the reading and writing probe. Introduction Semiconductors have been the basic materials in electronics for more than 50 years. The demand for high density, large capacity, high speed, and low power in integrated circuits leads to continuous miniaturization of electronic components in the industry. When the feature sizes of the conventional semiconductor materials reach nanoscale dimensions, it is uncertain whether they can still function as required. As an alternative, molecular-based electronic materials1,2 have been demonstrated to function as memories, diodes, and switches since Aviram and Ratner’s pioneering work in 1974.3 Compared to the conventional semiconductor materials, molecular electronics can easily reach molecule scales. Normally less than 10 nm, the molecules can be synthesized and tailored to obtain desirable specific properties and many molecules have distinctly stable states to be easily identified. As a result, the bright future of molecular electronics has been attracting lots of attention in the past 10 years.4-6 For a molecular memory, one needs a molecule with two states to be defined as 1 and 0. There are several ways to show that the bistability of molecules is possible, e.g., in the form of resistance, geometric position, and charge storage. For example, a redox active molecule possesses two oxidation states, one is the oxidized state and the second is the reduced state, which are suitable for an application as a data storage medium. * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (519) 661-2111 ext. 86161. Fax: (519) 661-3022. † Department of Chemistry, The University of Western Ontario. ‡ Department of Physics and Astronomy, The University of Western Ontario. § Gutenberg-Universita ¨ t.
We have focused on an encapsulated ferrocenium (Fc+) system, which is a redox species that has potential for information storage purposes according to its electrochemical behavior.7 In general, each molecular capsule host was formed by two beaker-like shaped calix[4]arenes (4 units of p-tert-butylphenol condensed with formaldehyde8) via hydrogen bonds of four urea residues at each of their wide rims. The inclusion of guests, such as dichloromethane or Fc+, can be realized during the formation of the capsules that can be either homodimers or heterodimers. We have designed dialkylsulfide substituted tetraarylurea calix[4]arene (1) and tetratosylurea calyx[4]arene (2) whose molecular structures are shown in Figure 1.9 The calyx[4]arene heterodimers (1 · 2) were found to encapsulate the Fc+ permanently7,9 and keep its electrochemical activity both in solutions and on the gold surface.7,10 The Fc+ cation was trapped in the capsule by the interaction of the cation and the π-conjugation of Fc+ with the capsule. By attaching di-n-alkyl sulfide groups to the calyx[4]arenes, the capsules were securely anchored to a gold surface stably to form self-assembled monolayers (SAMs). In this way, an immobilized encapsulated Fc+ was formed on the gold surface. Due to the attachment of di-n-dodecyl sulfide groups to the upper part of the calix[4]arene capsule, as shown in Figure 2, such immobilized capsules should be hard to dissociate even if the hydrogen bonds in the capsules are broken. The reversible electrochemical reaction of ferrocene offers two oxidation states, Fc+ and ferrocene (Fc). The encapsulated Fc+ in the SAM can be easily reduced by using an electrochemical method.7,9 The voltammetric behavior of Fc encapsulated inside hemicarcerand hosts was studied by Mendoza et al.10 The encapsulated Fc showed a more positive half-wave
10.1021/jp805349m CCC: $40.75 2008 American Chemical Society Published on Web 09/09/2008
Reading Oxidation States of Encapsulated Ferrocenium
Figure 1. Chemical structures of dialkyl sulfide substituted tetraarylurea calix[4]arene (1) and tetratosylurea calix[4]arene (2).
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15563 SECM11-14 has been developed for a wide range of applications in the areas of chemical and biochemical kinetics,15-17 chemical activity imaging,18-29 and micrometer scale structuring12,23,30,31 at liquid/liquid, liquid/solid, and liquid/membrane interfaces. Shiku et al.32 fabricated and characterized diaphorasepatterned surfaces using SECM. They generated Cl2 and/or Br2 on tip to deactivate the localized enzyme molecules on the substrate. Yasukawa et al.33 fabricated and characterized a microvial for electrochemical measurements on the picoliter level. They also studied the photosynthetic activity of a single protoplast by using this apparatus. Ufheil et al.34 prepared the patterns of adsorption sites for alkanethiols with high lateral resolution. Turyan et al.31 locally deposited Au particles on Si wafers followed by monolayer formation and functionalization, which was used as microsubstrates for assembling cystamine monolayers for covalently attaching fluorescein isothiocyanate or glucose oxidase. In this work, a small platinum tip (radius: 1.1 µm) is used to reduce the encapsulated Fc+ in the calix[4]arene capsule and to read the redox states of encapsulated Fc+ on small gold dots (0.5 µm in diameter) without adding extra redox media in the solution for the first time. These measurements mimic the potential application for reading and writing data by a tip in the present molecular system. This may lead to a molecular device for memory. Experimental Section
Figure 2. Illustrations of the substrate structures that consist of bulk glass, 50 nm ITO layer, 5 nm titanium (not shown), 60 nm gold film, and the SAM with encapsulated ferrocenium guest molecules (a), as well as the supramolecular structure of the immobilized capsule (1 · 2) on a gold surface (b). Please note that the sulfur function was attached to the upper part of capsules to prevent their dissociation within the monolayer.
potential and a lower apparent standard rate transfer of heterogeneous electron transfer than uncomplexed Fc. We have shown the blockage of electron transfer by calix[4]arene monomer (1) and the heterodimer SAM (1 · 2) on a gold electrode in comparison with that on a bare gold substrate.7,9 We have investigated the electrochemistry of the same dimers freely moving in solution and immobilized in SAMs, which was compared with that of the free Fc+ at a bare gold electrode.7 The encapsulated Fc+ is easier to reduce than the free Fc+,7 which agrees well with the results obtained by Mendoza et al.10 Furthermore, the reduction reaction for encapsulated Fc+ is quasireversible. On the basis of the study described above, the encapsulated Fc+ in a SAM on a gold surface can possibly be reduced by an electrode biased with a proper potential in the vicinity of the surface. A scanning electrochemical microscopy (SECM) electrode (tip) on the micrometer scale is envisaged to be a good probe for this purpose.
Chemicals. Potassium chloride (KCl, ACS reagent, 99+%, Aldrich, Mississauga, ON, Canada) was used to prepare the electrolyte solution with a concentration of 0.1 mol/L, which was utilized for probe approach curves (PACs) in SECM. A solution of 0.9 mmol/L ferrocenemethanol (FcMeOH, 97%, Aldrich) with 0.1 mol/L potassium chloride as the supporting electrolyte was used for determining the SECM probe radius. Deionized water (F > 18 MΩ cm, Milli-Q, Millipore, Mississauga, ON, Canada) was used to prepare all aqueous solutions. Ethanol (anhydrous ethyl alcohol, 100% pure) was obtained from Commercial Alcohols Inc. (Toronto, ON, Canada). Tetrahydrofuran (THF, g99% pure) and dichloromethane (CH2Cl2, g99.8%) were purchased from Caledon Laboratories Ltd. Ferrocenium hexafluorophosphate (Fc+PF6-) and tetrabutylammonium hexafluorophosphate (TBA+PF6-, electrochemical grade, g99.0%) were obtained from Sigma-Aldrich. All reagents and solvents were used without further purification. Formation of Encapsuled Ferrocenium with Dialkylsulfide Feet. Dialkylsulfide substituted tetraarylurea calix[4]arene (1) and tetratosylurea calix[4]arene (2) were synthesized as reported by our group.9 The host-guest system where the ferrocenium is in the capsule (calix[4]arene (1) and tetratosylurea calix[4]arene (2)) can be prepared by mixing the ferrocenium and two calix[4]arenes in CH2Cl2 solution. The ferrocenium encapsulation processes were reported by Bo¨hmer et al.35,36 Briefly, SAMs of the capsule filled with dichloromethane were formed in a dichloromethane solution with 0.1 mM capsule (tetratosylurea calix[4]arene (2) was 11% in excess) for 18 h. SAMs of the Fc+ filled capsules were formed in a dichloromethane solution with 1 mM capsule with Fc+ (tetratosylurea calix[4]arene (2) was 9.6% more, and ferrocenium hexafluorophosphate was 20% in excess) for 18 h. Au Substrate Preparation. Gold (99.99%, the Royal Canadian Mint., Ottawa, ON, Canada) was used for gold thin film preparation. Gold films of 60 nm were deposited by e-beam evaporation deposition at a pressure of 10-6 Torr on a microscopy slide (Fisher Scientific Co., Ottawa, ON, Canada).
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Two kinds of gold samples were prepared. One was a round gold disk with a diameter of 10 mm; another was a gold dot pattern covering a surface area of 5 × 5 mm2, with 0.5 µm diameter dots and 3 µm distance between two adjacent dot centers. Before gold deposition all glass slides were cleaned by the following procedures: sonication in 2% Hallmanex solution (HELLMA GmbH & Co. KG, Berlin, Germany) for 15 min and rinsing with Milli-Q-water for 20 times (10 mL each time), after that, sonicating in Milli-Q-water for 15 min, then rinsing with the 100% pure ethanol 7 times (10 mL each time). Finally a sonication in ethanol for 15 min was carried out and the samples were dried under nitrogen (99.998%, Praxair Canada Inc. Mississauga, ON, Canada). Preparation of Self-Assembly Monolayer (SAM) on Gold. Gold films on glass slides were immersed in SAM forming solutions with minimal delay after the metal deposition. All samples were then cleaned by rinsing extensively with ethanol and dried with N2. Ultramicroelectrodes (UMEs). The 1.1-µm-radius Pt UMEs with a glass sheath-to-electrode radius ratio of 6 (RG ) 6) used in the experiments were all homemade. The detailed procedure was published elsewhere.20,21 Before each experiment, the UME tip surface was well-polished and characterized by three methods: optical microscopy, cyclic voltammetry (CV), and probe approach curves (PACs).20 The UME radius can be estimated by the following equation:
a)
iT,∞ 4nFDc0
(1)
where iT,∞ is the steady state limiting current of the UME in the bulk FcMeOH solution, n is the number of electrons transferred in the tip reaction (here n ) 1 for FcMeOH), F is the Faraday constant, and D and c0 are the diffusion coefficient and concentration of FcMeOH, respectively. The diffusion coefficient here was taken as 7.8 × 10-6 cm2/s from a previous report.37 Probe Approach Curves. These PAC experiments were carried out by driving a biased tip toward a substrate and recording the tip current versus tip-to-substrate distance, based on our first home-built SECM instrument, which was depicted in detail elsewhere.20 SAM samples were clamped in the SECM cell and a substrate area of 0.5 cm2 was exposed to the solution as described elsewhere.19-21,38 A normalized PAC is obtained by plotting the normalized current (the actual tip current divided by the tip current in the bulk solution) versus normalized distance (the gap distance divided by the radius of the tip). All the experiments were performed at room temperature. To determine the distance between the tip and the substrate, oxygen in the electrolyte solution was used as the redox mediator. A potential of -0.70 V versus a Ag/AgCl reference electrode was applied to the tip to reduce the oxygen.21 The steady-state current was obtained at the tip because of the hemispheric diffusion of O2 from the bulk solution to the tip surface. When the biased tip was driven to the substrate, the substrate blocked the O2 diffusion to the tip surface. As a result, the tip current decreased with the decrease of the gap distance between the tip and the substrate (negative feedback). Once the tip current reached a certain value that was set in the instrument control software, the instrument stops tip approaching automatically. By superposing the experimental PAC with the theoretical one, the gap distance between the tip and the substrate can be determined.20,21 After the coarse approaching described above, the tip was lifted by 5 to 7 µm, and the potential on the tip was changed to
the desired value (e.g., -0.10 V) at which only the encapsulated Fc+ can be reduced. The biased tip was allowed to approach the substrate again at a very low speed of 20 nm/s to read the oxidation state of encapsulated Fc+ moieties at various substrate potentials. Substrate/Tip Cyclic Voltammetry (S/T CV). This part of the experiment was carried out with our second SECM instrument, which is modified based on R-SNOM (Wissenschaftliche Instrumente und Technologie GmbH, Germany).18,39 One precisely designed homemade SECM tip holder was used to replace one of the objective lenses on the turret of the microscope. Because the tip is located almost in the same position of the objective lens’ center, the tip lateral position can be labeled as the center of an image when the objective lens is used at the beginning. The sample is fixed on a scanning stage right below the objective lens. Thus, a desired substrate spot can be easily found with an objective lens first and then relocated under the electrode by turning the objective turret for switching between the objective lens and the UME. The reference electrode (Ag/AgCl wire) was also fixed on the tip holder. A bipotentiostat (CHI 832A, CH Instruments, Austin TX) was used to measure the tip and/or substrate currents, as well as applied potentials to both the UME and substrate if necessary. The freshly prepared sample was connected with one wire in order to get a potential applied and fixed in a Petri dish. The electrolyte solution was the same KCl solution. The 1.1-µmradius Pt UME was used as for the SECM probe. One homemade Ag/AgCl wire was used as the reference electrode. The potential reported here is calibrated to the saturated Ag/ AgCl scale by using FcMeOH as the standard redox species. The SECM tip was allowed to approach the substrate surface and stopped at a tip-to-substrate distance less than 0.05 µm. The tip was biased at a certain potential (reduction potential of the encapsulated Fc+ for instance), and the potential on the substrate varied in a certain range. The currents at both the tip and the substrate were recorded vs potential applied on the substrate. Open Circuit Potential (OCP)-Time Curves. A carefully prepared new SAM sample with encapsulated Fc+ (a round gold film with 10 mm diameter) was fixed in the SECM electrolyte cell and the 0.1 M KCl electrolyte solution was used in this experiment. A potential of 0.40 V was biased on this sample first for 10 min to make sure all immobilized encapsulated Fc+ turned to their oxidation states. Then the open circuit potential (OCP) was monitored for 250 s. After this experiment, a potential of -0.20 V was applied to the sample for 10 min, at which point the encapsulated Fc+ was reduced to Fc completely. The OCP was monitored again for 250 s. Results and Discussion Probe Approach to the SAM Surface. A freshly prepared SAM sample with encapsulated Fc+ immobilized on a 0.5-µmdiameter gold dot array was clamped in the SECM cell and kept at the OCP. The 0.1 mol/L KCl solution was used as the medium. The UME was biased at -0.70 V so that the oxygen dissolved in the solution (air saturated) was reduced and the reduction current reached the diffusion-controlled steady-state current.21 The tip was driven to the SAM surface at a fast speed (4.0-5.0 µm/s) at first and the tip stopped automatically when the normalized current reached 0.8. As explained in the Experimental Section, this negative feedback is expected in the coarse approach and can be used to roughly position the tip relative to the substrate. Then the tip was withdrawn 40 µm
Reading Oxidation States of Encapsulated Ferrocenium
Figure 3. Probe approach curves (PACs) to sample surfaces at the open circuit potential (OPC). (a) A typical PAC of a SECM probe biased at -0.70 V toward the SAM with encapsulated Fc+ on a 0.5 µm diameter gold dot. Oxygen was used as the redox mediator to roughly position the probe to the substrate. The end tip-to-substrate distance was larger than 1 µm and the approach speed was at 1 µm/s. (b) A PAC for the same tip biased at -0.10 V to approach the same SAM spot as in panel a. The approach speed was set at 20 nm/s. The gold dot array substrate was clamped in the SECM cell with a surface area of 0.5 cm2 exposed to the KCl solution for both panels a and b. The radius of the SECM probe is 1.1 µm and the RG (the ratio of the total radius of the tip to the radius of conductive wire) is 6.
and allowed to precisely approach the substrate surface with a low speed (1.0 µm/s) until the normalized tip current reached 0.38. A typical PAC to SAM with encapsulated Fc+ is shown in Figure 3a. The experimental data overlapped well with the theoretical curve to an infinite insulating substrate.40,41 The ideal substrate structure that encapsulated Fc+ is illustrated in Figure 2. The substrate fabrication was discussed in detail in our recent publication.7 Briefly, on the cleaned glass slide, one layer of ITO was deposited followed a layer of gold. The gold was covered by the SAM of the encapsulated Fc+, whose structure is illustrated in Figure 2b. From the PAC, the gap distance can be determined (directly read from the PAC). The tip-to-substrate distance from the experimental PAC in Figure 3a is found to be 0.7 µm corresponding to the normalized current of 0.38.11,20 We did not observe any positive feedback in this situation although encapsulated Fc+ can be reduced at the tip. This observation suggests that the SAM was tightly assembled on the gold surface and the tip did not reach the Fc+ moieties at this distance. As a result, a typical negative feedback was observed in the experiments (Figure 3a). Reading Oxidation States of Encapsulated Fc+. The tip at the tip-to-substrate distance of 0.7 µm was withdrawn about 5 µm, following the above experiment. The potential biased on the tip was changed to -0.10 V and the substrate was still kept at the OCP. At this tip potential, the O2 reduction is so weak that the tip current was close to 0 at a tip-to-substrate distance of 5 µm (Figure 3b). When this biased tip approached the vicinity of the SAM sample at a speed of 20 nm/s, the tip current increased suddenly and then the tip current decreased dramatically. As mentioned above, the substrate surface was covered by the SAM with encapsulated Fc+. When the tip approached
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15565 it so closely, the encapsulated Fc+ can be reduced to Fc (reaction 2) at the tip. The sudden increased Faradaic current is therefore expected. The Faradaic current at the SECM tip was only observed when it is close enough to the Fc+ moieties. The current disappeared abruptly afterward. There are at least two factors accounting for the abrupt current drop: (1) the depletion of the encapsulated Fc+ because of the reduction at the tip when the tip is in the vicinity of the Fc+ moieties and (2) the missing of the Fc+ by the tip when it passed the Fc+ layer and moved further toward the substrate. It is impossible for the Fc+ to diffuse to the tip electrode. Note that the Fc cannot be converted back to the Fc+ (at least the conversion kinetics is very slow) at the OCP on the substrate and the tip was driven very slowly (20 nm/s) in Figure 3b. The current drop in Figure 3b is probably due to factor 1, the depletion of the encapsulated Fc+. Figure 4 shows the tip’s maximum current when the tip approached the same substrate as in Figure 3b but biased at different potentials. When the applied potential on the substrate is high enough, e.g., 0.40 V versus Ag/AgCl as shown in Figure 4a, the SECM tip can reduce the fixed encapsulated Fc+, and the reduction product Fc can be reoxidized immediately by the substrate.7 Therefore, a strong positive feedback was observed in the experiments (Figure 4a). The principle of this current enhancement is very similar to that for the detection of a single molecule trapped in a small volume gap between a nanometer electrode and a conductive substrate and undergoing repeated oxidation and reduction at the two electrodes.42,43 Indeed, the product, encapsulated Fc, can be reoxidized by the substrate if a proper potential for Fc oxidation is applied to the substrate (reaction 3). Electrons transfer from the tip to the substrate via the medium of the fixed encapsulated Fc+ in this way. The electron transfer rate to the substrate from the tip is determined by two reactions:
Fc+ + e f Fc +
Fc f Fc + e
(2) (3)
If both the potentials on the tip and substrate are appropriate, a very high electron transfer rate from the tip to encapsulated Fc+ and then to the substrate can be reached and a tip Faradaic current enhancement can be observed as demonstrated in Figure 4a. In our experiments, the potential at the tip is always biased at -0.1 V, which is enough to keep reaction 2 occurring very quickly. If the potential on the substrate is not high enough to reoxidize the Fc quickly and completely, the electron transfer rate depends on that of reaction 3. The tip Faradaic current should rely on the substrate potential, i.e., electrochemical kinetic control on the substrate. The result shown in Figure 4b confirmed this prediction. When the potential at the substrate was about 0.30 V, the normalized maximum tip current was about 5.8, which is lower than the normalized tip current in Figure 4a (more than 14). The current drop observed in panels a and b of Figure 4 is because of factor 2 as discussed in Figure 3b at the OCP, i.e., the electrode was driven too far and could not reach the Fc+ moieties any longer. Figure 4c shows the result at a substrate potential of 0.00 V. No positive feedback was observed. At this substrate potential, the fixed encapsulated Fc+ was reduced to Fc so that no Fc+ existed on the substrate anymore. Consequently, there appeared no Faradaic current at the tip when it approached closely to the substrate. Figure 4d shows the summarized data of the maximum normalized current observed at the tip at various substrate potentials. It is evident that the maximum normalized current is dependent on the substrate potentials in the range of 0.00 to
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Figure 4. The probe approach curves (PACs) when the same tip approached the SAM with Fc+ at various substrate potentials of 0.40 V (a), 0.30 V (b), 0.00 V (c), and the plot of the maximum tip current vs the potentials biased on substrate (d). All other experimental conditions are the same as those in Figure 3b.
0.40 V. All these results confirm that the tip current change observed in these experiments is due to reaction 2. The tip can read the oxidation state of encapsulated Fc+ in the SAM on the substrate. It should be emphasized that our SAM and UME system is relatively easy to make and keep running, as described above. Substrate/Tip Cyclic Voltammograms (S/T CVs). The SECM tip was allowed to approach a SAM sample with dichloromethane as a guest molecule on a 10 mm diameter gold disk that immersed in the KCl solution. There is no encapsulated Fc+ existed in the SAM. Without the encapsulated Fc+ in the SAM and therefore without any electrochemically active species, this gold disk covered with the SAM behaved as an insulating substrate. The gap distance chosen was less than 50 nm, which was estimated according to the PACs. The tip was biased at 0.40 V and the potential on the substrate was varied between -0.20 and 0.50 V. The results are shown in panels a and b of Figure 5. One can see clearly that there is no Faradaic current in Figure 5a, because there was no encapsulated Fc+ in the SAM on substrate. Reaction 1 cannot happen in the case of Figure 5a even when the substrate potential varied. As mentioned above, the capacitive current at the substrate electrode indicates that the gold surface covered with the SAM with encapsulated dichloromethane molecules displayed the insulating property, which agrees with the results in which the SAM blocked electron transfer from a redox species in a solution to the electrode covered with the SAM.9 There is no change in tip current in this situation (Figure 5b). A micropatterned SAM sample with encapsulated Fc+ (SAMs were located on all gold dots with 0.5 µm diameter and 3 µm distance between two adjacent dots, as shown in Figure 5g) was used to run the same experiments as in panels a and b of Figure 5. The results are shown in panels c and d of Figure 5 when the tip was located to be above a gold dot with immobilized encapsulated Fc+ and in panels e and f of Figure 5 when the tip was located to be above the space between two
gold dots. All the CVs of substrate are in the same scale and so are the tip response current plots. Panels c and e of Figure 5 are the CVs for the substrate, and panels d and f of Figure 5f are the plots of the tip current vs substrate potential. In panels c and e of Figure 5, the cathodic peak for the reduction of encapsulated Fc+ (reaction 2 at the substrate) did not appear clearly in the potential range. However, the current increased quickly when the potential went to the negative end. One anodic peak can be observed clearly at about 0.20 V. This peak can be assigned to the oxidation reaction of encapsulated Fc to encapsulated Fc+. The very large difference between the anodic and cathodic peak potentials is due to the large surface area of the substrate electrode and uncompensated iR drop. There is no significant difference between panels c and e of Figure 5, which were run on the same sample at different time, indicating a reasonable stability of the encapsulated Fc+ in the aqueous solution. The tip current response was very clear when the tip was above the patterned SAM dot(s) with encapsulated Fc+, as shown in Figure 5d. When the substrate potential was altered to the negative side, the encapsulated Fc+ started to be reduced to Fc when the potential was more negative than 0.0 V. Because the tip was biased at 0.40 V, which is high enough to oxidize the encapsulated Fc to Fc+ on the substrate, the electrons were transferred from the substrate to tip through the intermediate, encapsulated Fc+ SAM. Therefore, the encapsulated Fc+ in SAM plays an important role in the tip response current increase. While the potential on substrate shifts from the negative to the positive side, the tip response current decreased dramatically in the potential range of -0.1 to 0.15 V. In this process, the encapsulated Fc was oxidized to Fc+ by both the tip and the substrate. The oxidation of the encapsulated Fc can be confirmed on CVs on the substrate (Figure 5c). The encapsulated Fc is consumed quickly on the substrate and as a result the tip current drops quickly in this potential range. After the potential on the substrate becomes larger than 0.15 V, the encapsulated Fc+
Reading Oxidation States of Encapsulated Ferrocenium
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15567
Figure 5. Cyclic voltammograms (CVs) at the substrate (a, c, and e) and tip (b, d, and f), and a scanning electron microscopy image of the gold dot array (g). The substrate for CVs in panels a and b is the SAM (1 · 2) without encapsulated Fc+, which is immobilized on a 10 mm diameter gold disk, while that for CVs in panels c-f is the SAM encapsulated with Fc+, which is immobilized on a gold dot array covering a surface area of 5 × 5 mm2, with 0.5 µm diameter dots and 3 µm distance between two adjacent dot centers. The tip in panel d was positioned above a dot covered with the SAM with encapsulated Fc+ while that in panel f was located above a non-SAM-coated region, a space between two adjacent SAM spots. The tip-to-substrate distance chosen was less than 50 nm in panels a-f. The two types of substrate were immersed in the KCl solution. The tip was biased at 0.40 V.
cannot be reduced on the substrate anymore, so there is no Faradaic current appearing on the tip response current plot. Although the potential difference between the substrate (0.15 V) and the tip (0.4 V) still exists, there is no electron transfer occurring between these two electrodes because the encapsulated Fc+ cannot transfer electrons in this situation, as explained above (both at the oxidation potential of encapsulated Fc+). The two cycles are plotted in panels c and d of Figure 5: both are overlapped very well, indicating that the SAMs with encapsulated Fc+ are quite stable during this time period. Panels e and f of Figure 5 demonstrate the results when the tip was above the non-SAM surface, a space between two gold dots where no encapsulated Fc+ exists. Although the substrate CV (Figure 5e) is very similar to that in Figure 5c, the tip
response current in Figure 5f is almost negligible in comparison with Figure 5d. One can consider that there is no response current appearing on the tip when the potential is varied on the substrate in this situation. The electron cannot transfer between the tip and substrate without the encapsulated Fc+ in the entire scan potential range (-0.20 to 0.50 V). In addition, the oxygen reduction current at the tip biased at -0.1 V is very small compared to the current appearing on the SAM-gold surface with encapsulated Fc+. Open Circuit Potentials of the Two Oxidation States of Encapsulated Fc+. The open circuit potentials of the two states of encapsulated Fc+ on the substrate were also monitored. A freshly prepared SAM sample on a 10 mm diameter gold disk was clamped in the SECM cell, and was biased at 0.40 V for
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Zhu et al. exist for the encapsulated Fc+ and Fc on this substrate immersed in the aqueous solution, indicating that both the oxidation states can be constantly stabilized in SAMs on gold surfaces under the experimental conditions. Acknowledgment. We appreciate the financial support of NSERC (New Discovery and Equipment Grants), OPC through ORDCF, CIPI, CFI, OIT, PREA, UWO (Academic Development Fund and a Start-up Fund), as well as the Deutsche Forschungsgemeinschaft (SFB 625). Technical assistance from John Vanstone, Jon Aukima, Joseph Chan, Sherrie McPhee, Mary Lou Hart, and Marty Scheiring is gratefully acknowledged.
Figure 6. The open circuit potential-time curves recorded in the 0.1 mol/L KCl solution for the SAM with encapsulated Fc and Fc+, respectively. The black curve was recorded after a potential of 0.40 V was applied to the SAM sample for 10 min. The red one was recorded after a substrate potential of -0.20 V was biased on the SAM sample for 10 min.
10 min in order to make sure that all the encapsulated Fc+ was in the oxidized state. The OCP was monitored for 250 s, and the OCP curve was plotted in Figure 6 (black curve). Although the OCP was not very stable at the beginning, the OCP tended to be stable at 80 mV. This confirms that the encapsulated Fc+ can exist in a stable form in the SAM. The sample was then biased at -0.20 V for 10 min to reduce all the encapsulated Fc+ to Fc, and the OCP was recorded for 250 s. The result was also plotted in Figure 6 (red curve). The OCP quickly stabilized at -36 mV after 200 s. The OCP in this case of encapsulated Fc on the substrate is much more negative than that in the case of the encapsulated Fc+ on substrate. There is a difference of 116 mV between two OCP values. This result also confirms that the encapsulated Fc can exist in the SAM. The OCP of encapsulated-Fc substrate easily reached a stable value, indicating that the Fc in the SAM is more stable than Fc+ in the SAM in aqueous solution. The relative instability of Fc+ in the SAM can probably be attributed to the interaction of the hydrophilic Fc+ with water.44 Conclusions A homemade 1.1-µm-radius UME biased at -0.10 V was used to probe the oxidation states of the encapsulated Fc+ in calix[4]arene heterodimer immobilized on gold surfaces without adding additional redox media in the solution for the first time. The probe approach curves showed that the tip current increased suddenly at one point where the tip was very close to the ferrocenium moieties. The tip current was confirmed to be the reduction current of the encapsulated Fc+ in the SAM, and dependent on the substrate potential. These experiments indicate that a SECM tip can be used to read the oxidation states of the immobilized encapsulated redox species in the SAM. When a tip biased at 0.40 V is engaged to the substrate with an applied potential scanned in the range of -0.20 to 0.50 V, cyclic voltammetry was carried out on both the substrate and the tip. The tip response current showed that the two oxidation states of the encapsulated Fc+ alternated with the varying substrate potential. Comparing the tip response current above the SAM with the encapsulated Fc+ and that above the SAM without encapsulated Fc+, the tip response current is due to the oxidation of encapsulated Fc in the SAM near the tip. Two stable OCPs
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