Article pubs.acs.org/Langmuir
Light-Induced Organic Monolayer Modification of Iodinated Carbon Electrodes Callie Fairman,† Muthukumar Chockalingam,† Guozhen Liu,† Alexander H. Soeriyadi,†,‡ and J. Justin Gooding*,†,‡ †
School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia Australian Centre for NanoMedicine, The University of New South Wales, Sydney, NSW 2052, Australia
‡
S Supporting Information *
ABSTRACT: We report the modification of carbon electrodes formed from pyrolyzed photoresist films (PPF) via plasma iodination followed by the organic monolayer modification of these surfaces. The iodinated surfaces were characterized using cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy to enable the optimization of the iodination while preserving the stability and smoothness of the carbon surface. Subsequently, the C−I surface was further modified with molecules that possess an alkene or alkyne at one end through light activation with low energy (visible range λ 514 nm). The versatility of the modification reaction of the C−I surfaces is shown by reactions with undecylenic acid, 1,8-nonadiyne, and S-undec-10-enyl-2,2,2-trifluoroethanethioate (C11-S-TFA). Modification with 1,8-nonadiyne allows further modification via “click” chemistry with azido-terminated oligo(ethylene oxide) molecules demonstrated briefly to alter the hydrophilicity of the surface after attachment of ethylene oxide moieties. Furthermore, patterning of C11-S-TFA was demonstrated using a simple photolithography technique. Deprotection of the C11-S-TFA gave a free thiol allowed patterning of gold nanoparticles on the surface as verified using scanning electron microscopy (SEM). These results demonstrate that plasma iodination to form C−I is a versatile, simple, and modular approach to functionalize the carbon surface.
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INTRODUCTION Modified carbon electrodes are being extensively explored for a range of applications including electroanalysis,1,2 biosensing,2−4 energy storage devices,5,6 and electrocatalysis.7,8 The most popular method of organic modification of carbon electrodes is via the use of aryl diazonium salts1,9−11 first described by Pinson and co-workers.12 The power of this surface chemistry is (1) it is incredibly stable, forming a C−C bond,13,14 (2) it is relatively simple to synthesize a large range of aryl diazonium salt species1,2,15,16 and aniline derivatives can be converted into aryl diazonium salts and bound to a surface in one step,17 and (3) mixed layers derived from multiple aryl diazonium salts can be prepared.18−21 The drawback of this surface chemistry is really only that, with very few exceptions,22−25 the surface chemistry is prone to the formation of multilayers.1,9 That is, as stable as the layers are, the control is not great enough for all applications. Highly stable monolayers have been formed on silicon surfaces via the reaction of hydrogen-terminated silicon with molecules bearing a terminal alkene or alkyne26−28 that can even allow modified silicon electrodes to be used in aqueous solution without oxidation.29 Similar chemistry can also be used to modify hydrogenated boron-doped diamond with alkenes via ultraviolet (UV) light.30 The modification of the diamond surface is thought to be from photoelectron ejection from the surface to the alkene, forming a radical on the surface that then binds to an alkene.31−33 The surface of the boron-doped diamond is hydrogenated using a hydrogen plasma which also serves to roughen the diamond surface.34 The hydrogen−carbon bond in hydrogenated− © 2013 American Chemical Society
amorphous carbon is very stable. The modification layer therefore requires high energy input to break these bonds; hence the need for UV light. As carbon−halogen bonds have significantly lower bond strength than carbon−hydrogen bonds, the purpose of this paper is to explore the use of iodine plasma for the formation of iodineterminated carbon electrode surfaces followed by the further modification with alkenes and alkynes. The aim is to develop a method of forming monolayers on carbon electrodes such that molecular assemblies35 can be formed on carbon electrodes.36−38 The carbon surfaces to be used are pyrolyzed photoresist films (PPF) first described by Kinoshita and co-workers,39,40 which were shown to be close to atomically flat,41 can be modified using aryl diazonium salts,42 and for which we have optimized the fabrication protocol.43 We show that the iodine plasma can roughen the surface, but smooth surfaces can still be achieved and further modification can be achieved with both alkenes and alkynes upon exposure to visible light. The surface constructs explored to demonstrate the scope of the new carbon electrode modification scheme are shown in Scheme 1.
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EXPERIMENTAL METHODS
Carbon Surface Preparation. Pyrolized photoresist film (PPF) was prepared as follows: A 3 in. ⟨100⟩ n-type silicon wafer was cut into Received: September 24, 2013 Revised: December 7, 2013 Published: December 16, 2013 332
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Scheme 1. Various Surface Construct Strategy That Will Be Explored in This Worka
a Firstly, PPF surface is iodinated by plasma exposure with iodine. Iodinated surface could then be reacted with different alkene and alkyne compounds induced by light (A). Further surface modification could then be achieved via “click” reaction (B) or attachment of nanoparticle (C).
1.4 × 1.4 cm2 pieces using a diamond scribe. The wafer pieces were cleaned by sonication in successive baths of acetone, methanol, and isopropanol. These samples were then dried with nitrogen. Under yellow light, positive photoresist AZ 4620 was spin-coated (3−4 drops/ 0.3−0.5 mL) onto the wafer pieces at 3000 rpm for 30 s. The wafer pieces were heated at 95 °C for 20 min in a temperature-controlled oven (Haraeus Instruments) to remove excess solvent. The samples were kept overnight at 45 °C. The photoresist-coated silicon pieces were then placed in ceramic boats which were then placed in the center of a tube furnace. The following heating program was run: T1= 550 °C, t1 = 20 min; T2 = 680 °C, t2 = 15 min; T3 = 1100 °C, t3 = 60 min with 15 min between each step to allow for the temperature increase. This heating program was conducted under a reductive atmosphere of 5% hydrogen in 95% nitrogen. The pyrolyzed samples were allowed to cool in this atmosphere before being stored in vacuum desiccators. Iodination of PPF. Iodination of the PPF surfaces was performed using a radio-frequency (RF) plasma generator in a reaction chamber (Figure S1). To investigate the effect of the iodine plasma on the PPF, samples were exposed for different time and power settings as outlined in Table S1. For each power setting and time, three repeats were run (Table S1). The iodination time and power settings are denoted as X minutes Y watts, where X is the time exposed to the plasma and Y is the power of the plasma. Solid iodine was put in the bottom of the chamber in a small beaker. The samples were positioned in the middle of the chamber, which was then sealed and placed under vacuum. The chamber was evacuated down to 0.8 mbar and left for 5 min. The RF generator was turned on and the frequency set to 116.5 kHz. The power was increased to the set wattage, and the samples were left in the plasma for the required time. Once finished, the power was turned off and the vacuum slowly released; the samples were removed and rinsed with ethanol. The samples were subsequently dried with nitrogen.
Alkene and Alkyne Addition to an Iodinated PPF Surface. A thin layer of neat liquid alkenes or alkynes was placed on the iodinated surface and placed in a quartz tube, which was then flushed with argon gas (Figure S2). Two different wavelengths of light irradiation were used: 254 and 514 nm. Three modifying molecules were explored: undecylenic acid, nonadiyne, and S-undec-10-enyl-2,2,2-trifluoroethanethioate. The surfaces were exposed to the required alkene or alkyne for 12−16 h under light exposure. The surfaces were washed with dichloromethane and ethanol and dried under nitrogen. In the case of undecylenic acid, the surfaces were also rinsed with water and then hot acetic acid, followed by water and then ethanol before being dried with nitrogen. Synthesis of S-Undec-10-enyl-2,2,2-trifluoroethanethioate (C11-S-TFA). Trifluoroacetyl (TFA)-protected undecenethiol (C11-STFA, Scheme S1) was synthesized in three steps from bromoundecene as shown in our previous publications.44,45 First, bromoundecene was reacted with potassium thioacetate to give acetyl-protected undecenethiol followed by hydrolysis of the acetyl group under acidic conditions. The free undecenethiol was then reacted with trifluoroacetic anhydride to give C11-S-TFA. The compound was purified by vacuum distillation and stored under an inert atmosphere. “Click” Chemistry. PPF surfaces were iodinated at 30 W for 30 min. The surfaces were then modified with nonadiyne with 514 nm light. The surfaces were then reacted with tetra(ethylene glycol) azide (40 mg), in an aqueous solution of copper sulfate (1 mol % relative to the azide) with ascorbic acid (25 mol % relative to the azide) in ethanol (1 mL), and N,N,N′,N″-tetramethylethane-1,2-diamine (1 mol % relative to the azide) as the ligand overnight. The surfaces were removed and rinsed with water and ethanol and were dried with nitrogen. This experiment was repeated to give a sample set of four. Synthesis of Gold Nanoparticles. All glassware was cleaned first with aqua regia (1:1 hydrochloric acid and nitric acid) followed by 333
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piranha (1:2 hydrogen peroxide to sulfuric acid). Note: piranha and aqua regia solutions are highly corrosive and must be treated with extreme caution. After acid cleaning to remove all organic and inorganic contaminants, the glassware was rinsed thoroughly with Milli-Q water. A solution of gold chloride (40 mg) in Milli-Q (150 mL) water was brought to boiling. A solution of sodium citrate (20 mg) was added, and the solution turned red. The solution was allowed to boil until the volume was reduced by half. Subsequently, the reaction was cooled to room temperature and transferred to a 100 mL volumetric flask and made to the mark with Milli-Q water.46,47 This procedure produces gold nanoparticles with a diameter of 67 ± 11 nm measured via transmission electron microscopy (TEM). Surface Patterning of the Iodinated PPF Surface and Gold Nanoparticles Attachment. Three PPF surfaces were iodinated at a power setting of 30 W for 30 min. The surfaces were rinsed with ethanol and dried under nitrogen. Neat S-undec-10-enyl-2,2,2-trifluoroethanethioate (C11-S-TFA) (10 μL) was placed on the surface, and a quartz coverslip was placed over the solution. On top of the quartz coverslip a TEM grid was placed (Figure 1). The TEM grid used was copper grid
measured using the Young equation and results from the interface/ surface tensions between the water and solid surface surrounded by air. Electrochemical Characterization. Electrochemical measurements to probe the surface modification of PPF were conducted as follows. An Ag|AgCl|3.0 M NaCl reference electrode and a platinum wire counter electrode were placed in the top of a custom-made electrochemical cell. The PPF is sandwiched in the base of the electrochemical cell which gives a controlled area of PPF by allowing only a disk with a 3 mm diameter to be exposed, which gives a working electrode area of 7.1 mm2. The redox systems used was 10 mM Fe(CN)63− in 1.0 M KCl from K3Fe(III)(CN)6 and Ru(NH3)63+ in 50 mM phosphate buffer with 1.0 M KCl from [Ru(III)(NH3)6]Cl3. All solutions were degassed with dry nitrogen for 15 min prior to use. For each redox probe, three CV cycles were run at 100 mV s−1. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 220iXL. Monochromatic Al Kα X-rays (1486.6 eV) incident at 58° to the analyzer lens were used to excite electrons from the sample. Emitted photoelectrons were collected on a hemispherical analyzer with multichannel detector at a takeoff angle of 90° from the plane of the sample surface. The analyzing chamber operated below 10−9 mbar, and the spot size was approximately 1 mm2. The resolution of the spectrometer was ∼0.6 eV. All energies are reported as binding energies in electron volts and referenced to the C 1s signal (corrected to 285.0 eV).48,49 Survey scans were carried out, selecting 100 ms dwell time and analyzer pass energy of 100 eV. High-resolution scans were run with 0.1 eV step size, dwell time of 100 ms, and the analyzer pass energy set to 20 eV. After background subtraction using the Shirley routine, spectra were fitted with a convolution of Lorentzian and Gaussian profiles. Atomic Force Microscopy (AFM). Atomic force microscopy (AFM) images were taken using a Digital Instruments Dimension 3000 scanning probe microscope. All images were acquired in tapping mode using commercial Si cantilevers/tips (Olympus) used at their fundamental resonance frequencies, which typically varied between 275 and 320 kHz. AFM images were performed on PPF substrates.
Figure 1. Patterning setup with iodinated PPF and C11-S-TFA. with size 100 mesh × 200 μm pitch. The surface was exposed to 514 nm light for 12 h. The surfaces were removed and rinsed with dichloromethane then ethanol and dried with nitrogen. To deprotect the amine of C11-S-TFA, the surfaces were left in a 10% ammonium hydroxide solution (10% ammonia in water) for 10 min before being removed and rinsed with water and then ethanol and dried with nitrogen. The now thiol-terminated surfaces were exposed to gold nanoparticle solution for 2.5 h before being removed and washed with water and then ethanol and dried with nitrogen. Scanning Electron Microscope (SEM) Imaging. SEM was conducted using a Hitachi S-900 SEM (Berkshire, England). The samples were mounted on a brass sample base with conducting carbon tape to ensure conducting and stability of the sample on the brass mounting stage. Samples were chromium coated before being mounted using Emitech K575x chromium sputter coater. Contact Angle. All contact angles were measured on a Ramé-Hart standard contact angle goniometer, and data acquisition was collected on a personal computer. The angles were processed with DROPimage. The contact angle can be measured by producing a drop of liquid on a solid surface. For these studies only Milli-Q is used. The contact angle is
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RESULTS AND DISCUSSION The initial stages of this work involved the characterization and optimization of the iodination process using plasma. Subsequently, the iodinated surfaces were modified using simple alkenes and alkynes via light activation followed by further chemical modification including “click” chemistry,50,51 and patterning is shown on these surfaces. Optimization of Iodination Conditions. To achieve our aim of forming monolayers on carbon electrodes from unsaturated hydrocarbons after iodination of the carbon surface, it is desirable that complete iodination of the surface of the PPF is
Figure 2. Typical XPS spectrum of iodinated PPF: (A) wide scan of the sample showing two I 3d peaks around 615 eV and one C 1s around 285 eV; (B) narrow scan of the I 3d region showing physisorbed iodine (618.1 and 629.9 eV) and iodine bound to carbon (620.7 and 632.3 eV) showing successful iodination process. 334
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Figure 3. AFM image of PPF surface: (A) bare untreated PPF, (B) PPF surface iodinated for 30 min under 60 W plasma, and (C) PPF surface iodinated for 30 min under 60 W plasma.
Figure 6. Contact angle of water on (A) a nonadiyne-derived iodinated PPF surface and (B) a nonadiyne-derived iodinated PPF surface “clicked” with tetra(ethylene glycol) azide.
achieved while maintaining the smoothness and electrochemical properties. Different power settings between 0 and 100 W were used, and the PPF surfaces were exposed to each setting for different amounts of time between 1 and 60 min (Table S1). The amount of iodination of the PPF was quantified using XPS, while the effect of the iodine on the electrochemical properties of the PPF electrodes was measured using surface-sensitive redox probes. The effect on the iodination on the topography of the surfaces was investigated with AFM, to observe any increase in roughness, and by using a contact angle goniometer. Upon iodination of PPF, an XPS wide scan was performed, and peaks were observed corresponding to carbon, oxygen, and iodine (Figure 2A). In the narrow scan of the I 3d region two pairs of iodine peaks were observed: one pair at 618.1 and 620.7 eV for the I 3d5 and a corresponding pair for the I 3d3 at 629.9 and 632.3 eV (Figure 2B). It was observed that upon rinsing with ethanol, the peak at 618.1 eV diminished and is attributed to physisorbed iodine,52 suggesting the peak at 620.7 eV is attributed to iodine bound to carbon.53 Surfaces left under vacuum with iodine in the chamber, but without plasma formation, only presented iodine peaks at 618.1 eV, which also supports the hypothesis that the peak at 618.1 eV was due to physisorbed iodine while the 620.7 eV was attributed to iodine bound to carbon.54,55 As the power of the plasma increased, it was observed that the amount of surface bound iodine increased relative to the intensity of the C 1s peak at 285 eV (Figure S3). It was also noted that the longer the PPF was left in the plasma, the greater the amount of iodine chemically bound to the surface (Figure S4). For the higher plasma power settings it was noted that the amount of iodine was found to be greater than that expected for a monolayer, but the presence of multilayers of iodine was not supported as there was no increase in the peak at 618.1 eV for the iodine bound to iodine, but the iodine bound to carbon does increase. The increase in the amount of surface bound iodine
Figure 4. Cyclic voltammograms of 5 mM Ru(NH3)62+/3+ in 50 mM phosphate buffer (pH 7) and 1 M KCl scanned at 100 mV/s for PPF exposed to iodine plasma for different times and powers. X min Y W refers to time exposed to plasma and power of plasma, and bare PPF refers to unreacted PPF.
Figure 5. Cyclic voltammograms of 10 mM Ru(NH3)62+/3+ in 50 mM phosphate buffer (pH 7) and 1 M KCl scanned at 100 mV/s on iodinated PPF exposed to iodine plasma for 30 min at 30 W. One set of surfaces was exposed to undecylenic acid and light (λ = 514 nm) (30 min 30 W undecylenic acid). The other set of surfaces were exposed to undecylenic acid but without light (λ = 514 nm) (30 min 30 W undecylenic acid, no light). Both were compared to untreated PPF.
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Figure 7. XPS of C11-S-TFA-derived iodinated PPF surface, flourine 1s, iodine 3d, and sulfur 2p. (A) Exposure to C11-S-TFA without light. (B) Before deprotection of the thiol with ratio of oxidized S to S-CR is 1:1.3. (C) After deprotection of the thiol showing the steps of attachment of C11-S-TFA and further deprotection to reveal thiol functional surfaces with ratio of oxidized S to S−H is 1:2.2.
plasma power and time (Figure S8); the iodine on the surface provides a barrier to electron transfer. As the iodine plasma power and/or the exposure time increased, the surface became more passivated toward Fe(CN)63−/4−. This observation is in good agreement with the XPS analysis where the amount of iodine increase as the plasma power and time increases. It can be seen from Figure S8 that iodination at 30 W for 30 min is needed to completely passivated the surface, which also suggests at least complete coverage of the surface with iodine. Additionally, ruthenium hexylamine Ru(NH3)62+/3+ probe was used as it is an ideal outer-sphere probe that is not surface sensitive. Figure 4 shows that iodination of the PPF does not have a major influence on the behavior of Ru(NH3)62+/3+. While iodination to higher power for a similar time (30 min) also provided excellent passivation, iodination at 30 W for 30 min was deemed sufficient and was used for further investigations in this paper. Modification of Iodinated PPF Using Light. Iodination of silicon has previously been demonstrated by Hamers and coworkers56 followed by subsequent modification with alkenes. Hamers and co-workers showed that due to the iodine−silicon bond being weaker than the hydrogen−silicon bond, the energy needed to perform reaction on the surface with alkenes was less. In this work, controlled surface modification of iodinated PPF using visible light (λ = 514 nm) will be used for light activated modification. First, alkene molecules such as undecylenic acid were reacted with the iodine-modified PPF upon illumination at wavelength 514 nm to form a monolayer and compared with
with increased plasma power was attributed to a roughening of the surface although it is important to note that there was considerable variability between the amounts of iodine measured on surfaces prepared under nominally the same conditions (Figure S4). Using AFM to ascertain the surface roughness, roughening of the surface was observed as the plasma power, and the length of exposure time to the plasma, was increased (Figure S5). As PPF exposed to iodine plasma at 100 W was shown to dramatically increase the surface roughness (Figure S6), it was not investigated further. For both the 30 and 60 W plasma powers roughness, root-mean-square (rms) of under 1 nm was typically observed, which is still significantly smoother than most glassy carbon surfaces (Figure 3). Cyclic voltammetry (CV) was conducted using the iodinated PPF surfaces as electrodes. First, CVs were obtained in 50 mM potassium phosphate buffer (pH = 7) from mixture of K2HPO4 and KH2PO4 (Figure S7). The CV of the iodinated PPF in phosphate buffer revealed that the iodinated exhibited no redox activity between −0.5 and +0.5 V due to the iodine on the surface with the redox peak beginning at −0.54 V being attributed to some residual I2 being reduced to I−. Potassium ferricyanide was then used as it is sensitive to adventitious species adsorbed onto the surface. By using this redox-active species, the affect of iodinating the surface on the electrochemical behavior of PPF can be investigated while any passivation of the surface due to the addition of iodine can be observed. The electrochemical behavior of Fe(CN)63−/4− can be seen for iodine-modified PPF at different 336
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glycol) could produce hydrophilic surface which could have further application such as controlling surface adhesion of biomolecules.58 First, iodinated PPF was exposed to 1,8nonadiyne and visible light (λ = 514 nm). Contact angles were measured on the nonadiyne modified surface (Figure 6) which exhibited a relatively hydrophobic surface of 80.9 ± 2.9°. This contact angle is comparable to those observed by Ciampi et al. on silicon surfaces modified with nonadiyne give a contact angle of 87 ± 3°.29,57 Upon reaction of the nonadiyne derived surface with tetra(ethylene glyol) azide, an angle of 53.4 ± 8.1° was recorded (Figure 6). This decrease of the contact angle after the “clicking” of tetra(ethylene glycol) azide derivative compares to literature values as Ciampi et al. have shown that these surfaces provide contact angles of 54 ± 4°. This is expected as the ethylene glycol derived surface is more hydrophilic due to the presence of the ethers from the glycol chain and the hydroxyl termination, in comparison to the alkyl chain of the nonadiyne. The passivation of the surface was also followed with Fe(CN)63−/4− and Ru(NH3)62+/3+ probes. Passivation of the surface toward Fe(CN)63−/4− was observed (Figures S11 and S12) for both 30 min 30 W iodinated PPF after modification with nonadiyne via light exposure (λ = 514 nm). This passivation of the modified iodinated surfaces with light could be attributed to the surfaces providing a densely packed layer. Further modification with tetra(ethylene glycol) azide derivative also results in passivation of the PPF electrode. Patterning of Iodinated PPF. The patterning of iodinated PPF was conducted using C11-S-TFA The use of S-undec-10enyl-2,2,2-trifluoroethanethioate (C11-S-TFA) to modify a surface has been shown by Gooding and co-workers.59,60 The deprotection of this modified layer reveals a thiol moiety which can be employed to couple to further functionality such as noble metal colloids as shown by Gooding and co-workers. The fluorine group in the TFA protecting group serves as a good XPS marker, and the distal thiol produced after deprotection can also be used to bind to gold nanoparticles (Scheme 1C). Exposing 30 min 30 W iodinated PPF to neat C11-S-TFA and only exposing the outer area of the surface to visible light at (λ = 514 nm) while protecting the middle area with a TEM grid, the surface may be able to be patterned with a thiol-terminated alkene. Modification of the surface is confirmed by XPS with narrow scans shows peaks in the S 2p region as well as the F 1s and I 3d. The narrow scan of the S 2p region indicates sulfur bound to carbon at 164 eV (Figure 7). After deprotection of the thiol, the narrow scan of the S 2p region, sulfur bound to carbon at 164 eV is still observed. The narrow scan of the F 1s region, however, shows the fluorine bound to carbon at 689 eV is lost (Figure 7C). When the iodinated PPF surfaces are exposed to C11-S-TFA but not the visible light, no S 2p or F 1s is seen but in the peaks corresponding to the I 3d region at 620 eV are still present (Figure 7A), indicating there is still iodine bound to carbon. By patterning a thiol-terminated alkene onto the surface and exposing the patterned surface to a solution of gold nanoparticles, the surface can be imaged using SEM.61 Iodinated PPF was exposed to gold nanoparticles, but no nanoparticles were observed on the surface (Figure 8A), while thiol-terminated monolayers attached to the iodinated PPF exposed to gold nanoparticles showed that they had attached to the surface (Figure 8B). For the patterned surface it was seen that the area that was not exposed to light did not have nanoparticles while the area exposed to light was modified with gold nanoparticles (Figure 8C). It can be observed that the nanoparticles are well dispersed on the surface on the modified areas of the PPF surface,
Figure 8. SEM images of patterned C11-S-TFA derived iodinated PPF surfaces. (A) Iodinated PPF not exposed to light, C11-S-TFA, and gold nanoparticles. (B) Iodinated PPF exposed to light, C11-S-TFA, and gold nanoparticles. (C) Patterned iodinated PPF with light, C11-S-TFA, and gold nanoparticles. (D) Boundary between modified and unmodified areas; the line indicates the boundary between the two areas.
iodinated PPF exposed to undecylenic acid without illumination. Passivation of the surface toward Ru(NH3)62+/3+ was observed for the iodinated surface with visible light (λ = 514 nm), while the iodinated surface not exposed to light (λ = 514 nm) did not show significant passivation of the surface toward Ru(NH3)62+/3+ (Figure 5) as expected. More significant passivation was observed through ferricyanide probe (Figure S9) where surface reacted with undecylenic acid (in the presence of light) completely passivated the surface. On the other hand, Figure S10 shows that the untreated surfaces do not react with undecylenic acid either: under visible light (λ = 514 nm) or without visible light, confirming that the reaction only proceeds to the iodinated surface. The ability of the iodinated PPF surfaces to be modified with alkenes makes them amenable to further modification via the attachment of other chemistry, biochemistry, or nanomaterials. Furthermore, using light to modify the surfaces with alkenes makes the surface modification chemistry highly compatible with the formation of patterns using standard photolithographical strategies. Another possible surface chemistry modification of ionidated surface is through reaction with 1,8-nonadiyne which could be further modified via “click” chemistry (Scheme 1B). Gooding and co-workers have showed the use of 1,8-nonadiyne on silicon surfaces. Once the 1,8-nonadiyne is attached to the surface via reaction with the alkyne at one end, the remaining alkyne is available to undergo further reactions.29,57 The use of “click” chemistry to attach an azide derivative of tetra(ethylene 337
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as the nanoparticles were formed in a citrate solution they are citrate capped. The dispersal of citrate-capped gold nanoparticles on surfaces, rather than the formation of aggregates, has been previously noted by Liu et al.,62 where both gold and carbon surfaces have been modified with either amine-terminated thiols in the case of gold surfaces or sulfur- or amine-terminated aryl diazonium derived layers in the case of carbon surfaces.
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CONCLUSION The iodination of PPF can be carried out using RF generated plasma, producing an iodinated surface. The iodinated surface produced maintains its electrochemical attributes as well as its physical attribute of smoothness, providing it is performed at the right plasma power and time settings plasma. Modification of the iodinated PPF surface with alkenes can be conducted with visible light (λ = 514 nm). The use of visible light (514 nm) allows the modification of iodinated PPF with both alkenes and alkynes in patterns where areas not exposed to light were not modified. The self-assembled iodine layers formed were able to be further modified. The use of nonadiyne enables further modification via “click” chemistry while using C 11 -S-TFA facilitates the investigation of the layer on the surface with XPS due to fluorine being an excellent XPS marker. Deprotection of the thiol on C11S-TFA allows for the attachment of gold nanoparticles which could be imaged via SEM. The layers formation and stability has been characterized via cyclic voltammetry with Fe(CN)63−/4− species. We envisaged this surface modification technique could have potential in various sensing application such as formation of molecular wires, DNA patterning, and cells adhesion.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S12. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.J.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Guillaume Le Saux for synthesizing the C11-S-TFA. C.F. thanks the APA and UNSW for a scholarship. J.J.G. thanks the Australian Research Council (DP1094564) and the University of New South Wales for financial support for different aspects of this work.
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REFERENCES
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