Evaluation of the electroanalytical performance of carbon-on-gold films prepared by electron-beam evaporation

Thuy P. Nguyen , Richard L. McCreery and Mark T. McDermott *
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2N4, Canada. E-mail: mmcdermo@ualberta.ca

Received 26th February 2020 , Accepted 11th June 2020

First published on 11th June 2020


Carbon film electrodes can often be used without pretreatment, and their fabrication allows for flexibility in size and shape and for mass production. In this work, we are exploring layered structures comprised of thin films of carbon on gold (eC/Au) prepared by electron-beam evaporation. These extremely flat films are not pyrolyzed and are comprised of mainly amorphous carbon but still exhibit reasonable conductivity due to the underlying gold layer. eC/Au electrodes, without any pretreatment, yield similar heterogeneous electron-transfer rates for benchmark redox systems and significantly lower background current in comparison with polished glassy carbon. Interestingly, they show insignificant adsorption of quinones, which is uncommon for most carbon electrode materials. However, eC/Au is still prone to adsorption of airborne hydrocarbons when exposed to ambient air like most graphitic materials. With reproducibly fast electron transfer kinetics, low background current, negligible adsorption, and ultraflat surface, eC/Au films are a promising candidate for electrochemical and sensor applications.


Introduction

Carbon materials have long been used in various electrochemical applications such as electroanalysis, energy storage, and electrosynthesis with often cited advantages including wide potential window, superior mechanical stability, and low cost.1,2 Recently, disposable electrodes have received greater attention because of the rising need for on-spot, in situ monitoring, and point-of-care testing in biomedical, pharmaceutical, industrial, and environmental fields, where no pretreatment or cleaning between measurements is preferred.3 Of particular interest are screen-printed carbon electrodes, which can be easily and massively produced using thick film technology.3,4 However, these films often yield slow electron-transfer rates and widely varied analytical performance. This is due to the addition of polymeric binders and other additives to the carbon inks that tend to pacify the carbon particles in various ink formulations.5,6

Thin carbon film electrodes with high purity and fast electron kinetics have been developed to overcome the above-mentioned problems. These films are prepared by various fabrication processes such as pyrolysis of organic films,7,8 chemical vapor deposition of organic gases,2,9,10 sputtering,11–15 and electron-beam evaporation.16–19 The bulk and surface chemistry of the films, which dictate their electrochemical performance, depend greatly on the fabrication procedure. For example, pyrolyzed photoresist film (PPF) is a disordered, sp2 hybridized carbon material with very flat surface [∼0.5 nm root-mean-square (rms) roughness] and low surface oxide level compared to polished glassy carbon (GC).8 Boron-doped diamond (BDD) thin films can be made by chemical vapor deposition from a mixture of methane and a source of boron, often B2H6 in hydrogen plasma.2 BDD consists of sp3-hybridized carbon atoms and has rough, polycrystalline morphology with grain boundaries at the surface and a small fraction of nondiamond carbon impurity.2 PPF and BDD electrodes exhibit similar electron-transfer rates for outer-sphere redox systems to polished GC, but they are less reactive to inner-sphere redox systems and have significantly lower capacitance and adsorption.2,8,20

Carbon films prepared by various sputtering methods, such as electron cyclotron resonance (ECR) and unbalanced magnetron sputtering, have been extensively studied by Niwa and coworkers. These carbon films, usually ∼40 nm thick, can be deposited at room temperature with un ultraflat surface and tunable sp3/sp2 ratios by varying deposition conditions; rms roughness ranging from 0.07–0.68 nm and sp3 contents ranging from 13–53% were reported.21,22 They generally exhibit wide potential window, low background current, similar electron-transfer rates to polished GC for Ru(NH3)63+/2+ and Fe(CN)63−/4−, high stability, and lower adsorption.22 Their high S/N resulting from the low background current and suppression of electrode fouling by electroactive species without pretreatment proved advantageous in detecting analytically important molecules such as bisphenol A, alkylphenols, serotonin, and oligonucleotides.12,22–24

Electron-beam evaporation has also been used to produce thin, amorphous carbon films with interesting properties and a wide range of applications.16–19,25 Compared to sputtered films, evaporated films generally have significantly higher purity and consequently higher conductivity due to lower inclusion of residual gaseous impurities.26 Electron beam deposited carbon (eC) was initially investigated for use as transparent carbon electrodes.17,18 Our group has previously reported eC films deposited on highly doped silicon substrates. Very thin eC films (7 nm thick) exhibit a near atomic flat surface (rms roughness ∼0.1 nm) and electron-transfer rates of several benchmark redox systems comparable to polished GC and PPF.16 The recent use of eC to fabricate highly stable carbon-based molecular electronic devices has been reviewed by Sachan et al.27 The inherently low conductivity of eC and resulting ohmic potential losses can be overcome by depositing eC on a thin layer of metal like Au.25 A 10 nm eC layer on a 30 nm thick gold layer decreases the sheet resistance from 104–105 Ω □−1 for eC alone to 1.05 Ω □−1 for the eC/Au film.25 Taking advantage of the extreme flatness and high conductivity of the eC/Au bilayer, Morteza Najarian et al. studied electron-transfer kinetics on a multilayer KCl1000/eC10/TiC3/Au30/Cr3/Si/SiOx300 film electrode (subscripts indicate thicknesses in nm) by scanning electrochemical microscopy (SECM).19 The KCl layer was deposited in vacuo without exposure of the eC/Au film to air, then removed by dissolution in the aqueous electrolyte under study. Heterogeneous electron-transfer rates exceeding 14 cm s−1 and 6.9 cm s−1 were obtained for (ferrocenylmethyl)trimethylammonium (FcTMA+) and Ru(NH3)63+ redox systems, which were and continue to be among the highest rate constants reported for carbon electrodes due partly to the multilayer film structure and to the SECM measurement method.28–35

In this contribution, we report the physical, chemical, and electrochemical characterization of 10 nm eC deposited directly onto an Au film (which will be referred to as eC/Au hereinafter). The direct contact between the eC and Au36,37 and the gold thickness of 42 nm are chosen with an eye towards future surface plasmon resonance (SPR) applications. Carbon-on-gold films have shown promise as SPR substrates. A thin carbon overlayer can provide protection for the easily oxidized but more surface plasmon-active metals than gold, e.g. silver, to be used as SPR substrates.36,37 Additionally, self-assembled monolayers (SAMs) of thiols have dominantly been the surface chemistry on gold, but the gold–sulfur bond is susceptible to oxidation and photodecomposition.38 The well-developed carbon-based chemistry, notably electrochemical grafting, is a powerful and robust alternative to SAMs.39,40 The differences in substrate layer composition and film thicknesses relative to that examined by Morteza Najarian et al.19 motivated us to thoroughly explore our eC/Au films. This study aims to characterize the electroanalytical performance of eC/Au electrodes with more commonly used electrochemical methods and to compare this performance to polished GC. Approaches that are common to characterize carbon electrodes were applied here to examine the electrochemical properties of eC/Au: surface properties, electron-transfer kinetics, adsorption, and stability were investigated. Benchmark redox systems, which have been previously classified according to the sensitivity of their electron transfer kinetics to the surface chemistry of a carbon electrode,1 were used here to enable comparison between different carbon electrodes. Understanding of the physicochemical and electrochemical properties of eC/Au should promote its use as a disposable electroanalytical material and electrochemical SPR substrate, as well as broaden the surface chemistry for SPR substrates via electrochemical modifications.

Experimental

Chemicals and materials

Concentrated sulfuric acid (95% to 98%, Caledon); potassium chloride (ACS reagent, 99.0–100.5%, Aldrich); potassium ferricyanide (certified reagent, Caledon); dopamine hydrochloride (Aldrich); hexaammineruthenium(III) chloride (98%, Aldrich); ferrocene (98%, Aldrich); perchloric acid (70%, ACS reagent, Fisher); tetrabutylammonium tetrafluoroborate (TBABF4) (99%, Aldrich); 2-propanol (certified ACS, Fisher); acetonitrile (for HPLC, gradient grade, ≥99.9%, Aldrich); acetone (ACS reagent, ≥99.5%, Aldrich); 9,10-phenanthrenequinone (PQ) (95%, Aldrich); sodium anthraquinone-2-sulfonate (AQMS) (HPLC, ≥98%, Aldrich); activated carbon (Caledon); and lead(II) oxide (certified, Fisher). All were used as received, except PQ was recrystallized from ethyl alcohol. Solutions were prepared fresh daily and purged with nitrogen gas for 10 min before use. Aqueous solutions were prepared with deionized water (DI) purified and deionized through a Barnstead E-Pure system (18.2 MΩ cm, ThermoFisher).

Film preparation

P-type, (100) oriented silicon wafers were diced into 1.3 × 1.85 cm2 chips to serve as substrates. The substrates were cleaned by sonication in acetone, isopropyl alcohol, and DI water for 15 min each, dried with nitrogen gas, and loaded into the evaporation vacuum chamber (Kurt Lesker PVD 75). The chamber was pumped to a typical pressure of below 5 × 10−6 Torr and remained between 10−6 and 10−5 Torr during deposition. Layers shown in Fig. 1A were deposited without breaking vacuum with thicknesses and evaporation rates as follows: Cr adhesion layer (2 nm, 0.03 nm s−1, Au (42 nm, 0.03 nm s−1), and eC (10 nm, 0.01 nm s−1). The target for eC deposition was spectroscopically pure graphite rods (SPI Supplies, PA). Evaporation rates and thicknesses were controlled with a quartz crystal microbalance.
image file: d0an00409j-f1.tif
Fig. 1 (A) Illustration of an eC/Au film with indicated thicknesses of deposited layers on a Si substrate. (B) Representative static contact angle image of a 4 μL drop of DI water resting on a freshly prepared eC/Au film (∼10 min of ambient air exposure).

Surface characterization

Raman spectroscopy. Raman spectra were collected by a Renishaw InVia Raman microscope using an Ar excitation laser (514.5 nm) with 30 mW power through a 50× objective. The instrument was calibrated with a Si standard prior to use.
X-ray photoelectron spectroscopy (XPS). XPS spectra were collected at room-temperature using a Kratos Axis (Ultra) spectrometer with monochromatized Al Kα (1486.71 eV) at nanoFAB. The spectrometer was calibrated by Au 4f7/2 binding energy (84.0 eV) with reference to the Fermi level. The analysis chamber pressure is lower than 5 × 10−10 Torr. CasaXPS software was used for atomic ratio calculations and component analysis.
Water contact angle. Water contact angles were measured with a 4 μL droplet using a Ráme-Hart goniometer (model 590) equipped with DROPimage advanced software. The reported values were averages of two measurements on different areas of each sample (N = 3).
Atomic force microscopy (AFM). Surface roughness was characterized by AFM using a Veeco/Digital Instruments Multi-Mode NanoScope IV and commercial Si3N4 cantilevers (Nanosensors) in tapping mode. The scan rate was 2 Hz. Several images of sizes 500 nm × 500 nm, 1 μm × 1 μm, 2 μm × 2 μm, and 4 μm × 4 μm were taken on different areas of each sample (N = 6). The root-mean-square (rms) roughness values were determined and averaged from these images using Gwyddion software.

Electrochemical characterization

All electrochemical measurements were conducted with a three-electrode cell and a bipotentiostat (model AFCBP1, Pine Instruments) on at least three samples. Home-made Ag/AgCl/KCl (3.5 M) and Ag/Ag+ (0.2 M Ag+ in acetonitrile containing 0.1 M TBABF4) reference electrodes were used for aqueous solutions and acetonitrile solutions, respectively. A Pt mesh was used as a counter electrode, and eC/Au and GC were used as working electrodes. GC (Tokai GC 20) plates (2.5 cm × 6 cm) were manually polished successively in 1, 0.3, and 0.05 μm alumina slurries on Microcloth polishing cloth (Buehler), followed by sonication in DI water for 15 min prior to use. The geometrical working electrode area was defined by a viton O-ring. Chronoamperometry in 1 mM Fe(CN)63− in 1 M KCl yielded an electrode area of 0.319 ± 0.005 cm2 (N = 3).
Stripping of underpotentially deposited lead. Lead from 1 mM PbO in 1 M HClO4 solution was deposited onto the working electrode by stepping the potential from open circuit to −0.4 V for 5 s. The underpotentially deposited lead was subsequently stripped oxidatively at a scan rate of 0.5 V s−1.
Electrode kinetics. The redox systems under investigation were as follows: 1 mM Fe(CN)63− in 1 M KCl, 1 mM Ru(NH3)63+ in 1 M KCl, 1 mM dopamine in 0.1 M H2SO4, and 1 mM ferrocene in acetonitrile containing 0.1 M TBABF4. Heterogenous electron-transfer rate constants were calculated for simple one-electron redox system using the method of Nicholson41 as detailed previously.16
Adsorption measurements. The electrodes were immersed in 10 μM AQMS in 1 M HClO4 and 10 μM PQ in 1 M HClO4 solutions for 10 min and rinsed well with DI water. Cyclic voltammograms were then obtained in 1 M HClO4 at a scan rate of 0.1 V s−1. Surface coverage of quinones was quantified by measuring the area under the voltammetric reduction wave using the method described by Brown and Anson.42

Results and discussion

The relationship between the physicochemical properties of carbon materials and their electroanalytical performance has been a long-term interest to researchers and is explored here for eC/Au films. Material and surface properties, such as carbon microstructure, sp3/sp2 carbon ratio, O/C ratio, roughness, and wettability, were probed for these films. Electrochemical properties of eC/Au have been briefly assessed by cyclic voltammetry19 and are further investigated here, including potential window, capacitance, electron-transfer rates for four benchmark redox species, adsorption, and stability of electrochemical response. In a previous report, a TiC adhesion layer between eC and Au was needed to prevent delamination of the eC film during electrochemical experiments.19 We did not observe delamination of the eC layer for our films, indicating good adhesion of the film to the substrate.

Surface characterization

Carbon microstructure. Raman spectroscopy, regularly used to characterize carbon materials, was used here to investigate the microstructure of the eC/Au films. The Raman spectrum for a 10 nm thick eC layer on Au (Fig. S1) contains a broad, featureless band between 1200–1600 cm−1. This is similar to that observed for eC deposited onto Si16 and the multilayer KCl1000/eC10/TiC3/Au30/Cr3/Si/SiOx300 film (subscripts indicate thicknesses in nm) previously reported.19 The spectrum suggests that the eC/Au film is comprised of disordered amorphous carbon containing both sp2- and sp3-hybrids. This spectral feature is also observed for microcrystalline graphite.43 Deconvolution of the Raman spectrum into the G (graphitic) and D (disordered) bands observed for graphitic materials and subsequent analysis resulted in ∼30% sp3 hybridized carbon, or an sp3/sp2 ratio of ∼0.43 (Fig. S1).
O/C and sp3/sp2 ratios. Oxide groups on the surface of carbon materials are known to influence their electrochemical properties. The amount of surface oxides is typically quantitated with XPS, which was employed to determine O/C ratio and as another measure of the sp3/sp2 ratio of eC/Au films. Survey spectra (Fig. S2) do not contain an Au peak, consistent with uniform coverage of the Au layer by eC and low pinhole density. Analysis of the XPS spectrum shows that eC/Au surfaces exhibit an O/C ratio of 5.6–6% after brief exposure to air during sample transfer. This is less than the typical O/C of 8–15% of polished GC.44 Analysis of the high resolution C 1s peak results in an sp3/sp2 ratio of ∼0.38 (Fig. S3). This is in good agreement with Raman results and indicates that the majority of carbon atoms are sp2 hybridized. Besold et al. proposed that electron beam evaporated carbon films on SiO2 are comprised of graphitic clusters (sp2-hybridized carbon) arranged within a diamond-like framework (sp3-hybdridized carbon).43 This description is consistent with our results.
Wettability. The wettability of the films was studied by measuring water contact angle on the surface. The water contact angle on eC/Au films is 44 ± 2° (Fig. 1B), indicative of a hydrophilic surface. This value is similar to amorphous carbon films prepared by magnetron sputtering (44°),45 and is lower than diamond-like carbon films fabricated by plasma immersion ion implantation (68.4°),46 filtered cathodic vacuum arc technique (77.6°),47 laser arc process (60°),48 and chemical vapor deposition (75°).49 The contact angle of water on freshly polished GC is 37–45°.50
Roughness. Both the carbon film and the underlying Cr/Au film were examined to understand the roughness of the eC layer. Fig. S4 shows AFM images (A) and line scan profiles (B) of Cr2/Au42 and Cr2/Au42/eC10 films deposited on Si substrate (the subscripts indicate thicknesses in nm). Island features are observed on the gold film with a rms roughness (Rq) of 0.89 nm. A 10 nm thick eC layer deposited on top of this relatively rough Au film exhibits a smoother surface with a rms roughness of 0.72 nm. The decreased roughness during eC deposition may be due to the amorphous and non-diffusing nature of eC, in addition to more rapid C–C bond formation in depressions in the Au surface. eC/Au surface is rougher than PPF (rms roughness 0.5 nm),8 eC/Si (rms roughness 0.07–0.11 nm),16 and ECR-sputtered carbon/Si (average roughness 0.07 nm).12 Compared to polished GC (rms roughness 4.1 nm),51 however, eC/Au has a very smooth surface.

Electrochemical characterization

The presence of pinholes, or incomplete coverage of the 10 nm eC film thus exposing the underlying Au, was explored here by stripping of underpotentially deposited (UPD) lead. A monolayer of lead adatoms can be deposited on Au surfaces at potentials slightly more positive than for bulk deposition of lead.52–57 This lead monolayer can be subsequently stripped oxidatively from the gold surface. Fig. 2 contains stripping voltammograms at Au and eC/Au surfaces. Here, an Au film (Si/Cr2/Au42) served as a control. A sharp oxidative stripping wave at −0.2 V can be observed for the control gold film, which is characteristic of lead stripping from Au(111) crystal plane.52 This wave is absent for eC/Au film, implying that no lead was deposited and that pinholes are either absent or present in undetectably low density. A previous investigation of similar eC films used the electrochemical oxidation of the underlying metal (Cu and Ni) as a probe for pinholes and reported a similar finding.19
image file: d0an00409j-f2.tif
Fig. 2 Linear sweep voltammograms for the stripping of lead UPD from Au, eC/Au, and sonicated eC/Au surfaces (ν = 0.5 V s−1).

Adsorbed organic impurities are known to influence the electrochemical reactivity of carbon electrodes, and a treatment involving ultrasonication in a mixture of activated carbon and 2-propanol has been shown to significantly reduce the amount of adsorbed impurities.1 The ability to mass produce eC/Au film electrodes and store them may prompt researchers to potentially use this procedure to clean the surface following storage. We thus monitored the stability of our eC films following ultrasonic treatment. eC/Au films were sonicated in 2-propanol/acetonitrile (50[thin space (1/6-em)]:[thin space (1/6-em)]50 v/v) with an equal volume of activated carbon for 10 min, followed by sonication in DI water for 10 min. Fig. 2 also contains the UPD lead stripping result following this treatment. Exposure of the gold layer is confirmed by the sharp oxidative wave observed at −0.2 V, similar to that of the control gold film. Thus, it is clear that the ultrasonic treatment caused observable delamination of the carbon layer and exposure of the gold layer. This electrode cleaning procedure will not be applicable to eC/Au films. However, commercial production of hundreds of eC/Au electrodes on Si wafers with protection by KCl or other dissolvable films without eC exposure to air may provide an effective alternative to sonication.58

Potential window and electrode capacitance. Carbon electrodes in general enjoy the advantage of a wide potential window in most solvents. Voltammetric scans in 0.05 M H2SO4 reveal similar potential limits for polished GC (2.5 V) and eC/Au (2 V) (Fig. S5). Compared to BDD (potential window of 3.5 V)2 and ECR-sputtered carbon films (potential window of 3.4–3.7 V),22 the potential window of eC/Au is not as wide. However, it is still wider than most metal electrodes and thus applicable for a wide range of redox analytes. Also, the background current is less than 50 μA cm−2 between −0.6 and 1.4 V at eC/Au, which is much lower than polished GC (typically 200 μA cm−2). This suggests that the surface is composed of a low level of electroactive and ionizable surface carbon–oxygen functionalities, consistent with the XPS results described above.

A more quantitative measure of background current is electrode capacitance. Capacitance values were obtained from cyclic voltammograms in 1 M KCl and 1 M HClO4 over a range of scan rates (0.05–1 V s−1) (Fig. S6 and S7) and are reported in Table 1. Capacitance values for eC/Au in 1 M KCl and in 1 M HClO4 are less than 50% of those for polished GC. The electrode capacitance of eC/Au in 1 M KCl is comparable with sputtered carbon films (11.1–11.6 μF cm−2) and slightly higher than BDD and PPF (4–8 and 9.2 μF cm−2, respectively).2,8,23 The low capacitance of eC/Au can be attributed to reduced electroactive or ionizable surface carbon–oxygen functionalities, smoother surface, as well as space-charge effects as is observed at the basal plane of HOPG.59,60 We attribute the higher capacitance in 1 M HClO4 compared to 1 M KCl observed on both eC/Au and polished GC to a mild electrochemical pretreatment effect. Electrochemical pretreatment has been reported to increase roughness and oxygen-containing groups for GC and sputtered carbon electrodes, which is assumed to contribute to the increased background current.23 Clearly, eC/Au offers reasonably wide potential window and low background current and capacitance, both of which are attractive for improved signal-to-background in electrochemical measurements.

Table 1 Electrochemical results for eC/Au and GC
Electrode k 0[thin space (1/6-em)]a (cm s−1) ΔEp (mV) ν = 0.1 V s−1 C 0 (μF cm−2) Γ (pmol cm−2)
Fe(CN)63−/4− (1 M KCl) Ru(NH3)63+/2+ (1 M KCl) Ferrocene+/0 (0.1 M TBABF4) Dopamine (0.1 M H2SO4) 1 M KCl 1 M HClO4 AQMS (1 M HClO4) PQ (1 M HClO4)
a All k0 values were determined from ΔEp values corrected for iRu error and from 5 different scan rates per sample. b Values in parentheses are the relative standard deviation of the mean for at least 3 different eC/Au electrodes.
eC/Au 7.4 × 10−3 (4.5%)b 2.5 × 10−2 (13%) 3.9 × 10−3 (7.5%) 226 ± 7 12 ± 1 19 ± 2 ND 1.6 ± 0.1
GC 1.2 × 10−2 (9.2%) 1.4 × 10−2 (10%) 3.4 × 10−3 (9.1%) 138 ± 8 31 ± 2 45 ± 5 136 ± 50 105 ± 30


Electrode kinetics. As noted above, very high electron-transfer (ET) rates have been measured at similar eC electrodes by SECM.19 There is a large body of literature that employs cyclic voltammetry (CV) to measure the ET kinetics of carbon electrodes;1 thus, for a more complete comparison, we used CV to measure the ET rate constants of several aqueous as well as non-aqueous based benchmark redox systems at eC/Au electrodes and compared with polished GC. These redox systems were chosen based on their different sensitivity to carbon surface structure. Fig. 3 contains cyclic voltammograms of Fe(CN)63− (A), Ru(NH3)63+ (B), dopamine (C) and ferrocene (D) at eC/Au and polished GC. eC/Au electrodes show well-defined, symmetric voltammograms, which are qualitatively comparable to GC for all redox systems under study. In addition, the peak current is linear with ν1/2 over a range of scan rates (0.05–1 V s−1), and the cathodic to anodic peak currents ratio is nearly 1 for all scan rates, consistent with currents controlled by semi-infinite linear diffusion and quasi-reversible ET kinetics. Heterogeneous ET rate constants were calculated for simple one-electron redox systems using the Nicholson method and are summarized in Table 1. All reported k0 values were corrected for sample resistance by the method detailed previously.8,16 The uncompensated cell resistance Ru was determined to be 47.0 ± 0.5 Ω (Fig. S8).
image file: d0an00409j-f3.tif
Fig. 3 Cyclic voltammograms of 4 benchmark redox systems at eC/Au and polished GC. (A) 1 mM Fe(CN)63− (1 M KCl, ν = 0.05 V s−1). (B) 1 mM Ru(NH3)63+ (1 M KCl, ν = 0.2 V s−1). (C) 1 mM dopamine (0.1 M H2SO4, ν = 0.2 V s−1). (D) 1 mM ferrocene (0.1 M tetrabutylammonium tetrafluoroborate in acetonitrile, ν = 0.1 V s−1). The black arrows indicate the scan direction.

It is now known that Fe(CN)63−/4− is not a simple outer-sphere system, and its ET rate depends on the electrode surface condition. The rate is influenced by the fraction of edge plane as well as surface cleanliness, but is relatively insensitive to surface oxides as long as anionic oxide groups are absent.44 The ET rate is also influenced by adsorbed monolayers, the extent of which depends on the type and coverage of the adsorbate.44 Here, eC/Au yields a somewhat lower rate constant for Fe(CN)63−/4− compared to polished GC, which can be ascribed to the reduced sp2 content of eC/Au in comparison with GC.

Ru(NH3)63+/2+ and ferrocene+/0 in acetonitrile are known to be insensitive to surface chemistry or adsorbed monolayers and are thus considered to be simple outer-sphere redox systems.1 For these systems, the ET rate depends on the electronic properties of the electrode and the self-exchange rate of the redox systems but not on the interaction with a surface site or functional group.51 The cyclic voltammetric determined rate constants at eC/Au electrodes are slightly higher than those at a polished GC surface. These results suggest that the density of electronic states of eC/Au is sufficient to support rapid ET for outer-sphere redox systems.

eC/Au exhibits much larger peak separation for dopamine, reflecting slower ET rate compared to polished GC (Table 1). However, this value is still among the smallest reported for thin carbon film electrodes (Table 2). Slow ET rate for dopamine was also observed on PPF,8 sputtered carbon films,22 and BDD.20 Dopamine oxidation is known to require adsorption sites.20 Thus, dopamine ET kinetics slow significantly when such interactions are unfavorable. The slow kinetics of dopamine at eC/Au can be attributed to a number of factors such as decreased ionizable surface carbon–oxygen functionalities which facilitate proton transfer, low roughness, and a low density of edge plane adsorption sites. The sluggish kinetics resulting from weak electrocatalysis for dopamine is consistent with weak quinone adsorption discussed below.

Table 2 Comparison of electron-transfer kinetics at different carbon film electrodes
Electrode k 0 for Ru(NH3)63+/2+ (×10−2 cm s−1) k 0 for Fe(CN)63−/4− (×10−2 cm s−1) ΔEp of dopamine (mV)
ECR-7522 Reversible 1.22 472
ECR-2022 Reversible 0.24 464
BDD (NRL)20 1.2 1.7 480
BDD (USU)20 1.7 1.9 509
Pyrolyzed 7 nm eC/Si16 4.6 1.4 119
7 nm eC/Si16 1.9 0.27 227
Pyrolyzed 200 nm eC/Si16 2.7 2.9 130
200 nm eC/Si16 4.3 0.57 243
PPF8 2 1.2 287
eC/Au (this work) 2.5 0.74 226


Table 2 compares ET kinetics of Ru(NH3)63+/2+, Fe(CN)63−/4−, and dopamine at different carbon film electrodes. Overall, eC/Au exhibits fast electron kinetics for these three redox systems and comparable to ECR-sputtered carbon films, BDD, and PPF. Interestingly, eC behaviors most resemble non-pyrolyzed 200 nm eC/Si, implying similar surface and bulk properties. The voltammetric results in Table 2 indicate that eC/Au is electrochemically active and yields reproducible response without any pretreatment. The most favorable electrochemical results for GC, BDD, and PPF are often accompanied by a pretreatment procedure. There have been numerous pretreatment procedures reported for GC, including mechanical polishing, sonication in solvent suspended activated carbon, vacuum heat treatment, and laser activation.1 The results for BDD in Table 2 correspond to electrodes that were treated with acid and hydrogen plasma to remove nondiamond carbon impurities and produce a hydrogen terminated surface.20 PPF8 and eC/Si films from our previous work16 were also sonicated in activated carbon/isopropanol followed by deionized water prior to use. In summary, eC/Au electrodes provide high electrochemical reactivity without surface pretreatment procedures.

Adsorption. It is known that carbon electrodes are susceptible to passivation via the adsorption of organic molecules. Typically, quinone-type molecules are used as model systems to study adsorption to carbon, since adsorbed quinones are electroactive and their coverage can be easily measured. The adsorption of two quinones from a low concentration (10 μM) on eC/Au was investigated using CV. Fig. 4 contains cyclic voltammograms of 9,10-phenanthrenequinone (PQ) and anthraquinone-2-sulfonate (AQMS) at eC/Au and polished GC. Both of these species are known to adsorb strongly to carbon electrodes and undergo a reversible two-proton, two-electron redox reaction.42,61Fig. 4A shows the voltammograms of PQ on both electrodes, and Fig. 4B shows the voltammogram of PQ on eC/Au with a reduced current scale. Parts C and D are similar data for AQMS. For both quinones, pairs of symmetric peaks are observed at GC, which is characteristic of surface-confined redox species. In contrast, PQ yields very little current on eC/Au, and adsorption current for AQMS is negligible. These observations indicate very weak adsorption for both of these quinones on eC/Au. Integration of the voltammetric redox waves was used to evaluate surface coverage, and the results are presented in Table 1. eC/Au shows negligible adsorption of PQ (∼2 pmol cm−2) compared to GC (∼105 pmol cm−2), and the adsorption of AQMS is below the voltammetric detection limit. This negligible adsorption indicates that eC/Au is potentially less subjective to electrode fouling, leading to improved stability and reproducibility in electrochemical measurements.
image file: d0an00409j-f4.tif
Fig. 4 Cyclic voltammograms of PQ (A) and AQMS (C) at eC/Au and polished GC. Cyclic voltammograms of PQ (B) and AQMS (D) at eC/Au with a reduced current scale. The concentration of the quinones was 10 μM in 1 M HClO4. The scan rate was 0.1 V s−1. The black arrows indicate the scan direction.

The low measurable adsorption of PQ and AQMS on eC/Au is due to either low coverage or adsorption in an electro-inactive configuration. We believe the former factor is more relevant here. PQ and AQMS have been shown to strongly physisorb at monolayer coverage to disordered carbon surfaces like pyrolytic graphite and GC.42,62 However, low coverage of quinones has been measured at other carbon electrodes, such as the basal plane of highly oriented pyrolytic graphite (HOPG) (<1 pmol cm−2),61 BDD (∼3 pmol cm−2),63 PPF (∼20 pmol cm−2),8 and non-pyrolyzed eC/Si (28–30 pmol cm−2).16 In addition, weak adsorption of alkylphenols and bisphenol A was observed at sputtered carbon films.12,22 Adsorption of species like quinones can be promoted by two electrode properties: surface chemistry and electronic effects. For the basal plane of HOPG, the adsorption mechanism of quinones is thought to depend on an electrostatic attraction between the adsorbate and partial surface charges.61 Xu et al. compared the adsorption of anthraquinone-2,6-disulfonate at GC, hydrogenated GC, HOPG, and BDD and proposed that surface chemistry plays a more important role in the adsorption than do electronic effects.63 Weak adsorption on PPF was explained partly by surface effects such as low roughness factor and O/C ratio.8 Niwa et al. also attributed the weak interactions between the adsorbates and sputtered carbon films to extreme flatness of the surface and low concentration of oxygen-containing groups.12,22 In our previous study, we observed an increase in quinone adsorption after pyrolyzing eC/Si films and thus increasing their graphitic content. The increased surface roughness and adsorption sites followed pyrolysis lead to increased quinone adsorption.16 In light of the above-mentioned studies, we attribute the insignificant adsorption of quinones at eC/Au results in part to a reduced density of adsorption sites due to lower sp2 content (∼70% vs. 100% of GC). In addition, the ultraflat surface and low carbon–oxygen functionalities of eC/Au likely contribute. The minimal adsorption of quinone molecules points to a beneficial property of eC/Au electrodes with regard to electrode stability due to fouling.

Stability

The shelf-life stability of eC/Au electrodes was evaluated using several observations. The O/C ratio of eC/Au films increased from 5.6–6% to 8.0–8.4% after one week. Aging in air for 15 days increased water contact angle from 44 ± 2° to 71 ± 3° (Fig. 5A), suggesting that the surface becomes more hydrophobic with air exposure. This phenomenon was also observed on polished GC,50 HOPG,64 and graphene,65 and was ascribed to the adsorption of hydrocarbons or similar hydrophobic impurities present in ambient air. Unlike HOPG64 and graphene65 whose water contact angles quickly increased by ∼40% within 15–20 minutes of air exposure, eC/Au films show a much slower increase in water contact angles with time (∼16% increase after 1 day, Fig. 5B). Stability of electrochemical performance was investigated by tracking voltammetric peak separations of two redox systems, Ru(NH3)63+/2+ and dopamine, over time (Fig. 5C and D). Similar trends were observed for both eC/Au and GC electrodes. For an outer-sphere redox system like Ru(NH3)63+/2+, the peak separation and thus electrochemical reactivity did not change significantly after 2 weeks (two-tailed t-test at 95%, N = 3). For the inner-sphere redox system dopamine, the peak separation increased significantly after 2 weeks (two-tailed t-test at 95%, N = 3). We attribute the slower ET rate of dopamine to reduced adsorption sites resulting from the adsorption of airborne impurities. Adsorption of airborne contaminants have been previously reported to have minor effect on ET rate of Ru(NH3)63+ but decelerate ET rates of Fe(CN)64− at aged HOPG66 and of dopamine at aged GC.50 BDD shows weak adsorption of quinone and good electrochemical reactivity for Fe(CN)63−/4− and Ru(NH3)63+/2+ after exposure to ambient air for two weeks.2 Although eC/Au exhibits negligible quinone adsorption, it is still prone to air oxidation and adsorption of airborne hydrocarbons like most graphitic carbon electrodes, albeit slower than that reported for graphene and HOPG. Graphitic carbon materials are subject to surface oxidation and fouling because they tend to react with oxygen and water to form various oxides such as phenols, lactones, carbonyls, ethers, and carboxylates.67 eC/Au possesses significant sp2 carbon (∼70%), thus behaving similarly to GC in this respect. Nonpolar, hydrogen-terminated surfaces like BDD and hydrogenated GC are less subject to deactivation because of a lower affinity for polar impurities in ambient air and a stable chemical composition due to slow reaction with oxygen/water.67,68 As noted above, a KCl protective layer has been shown to improve the shelf-life of eC/Au films for storage and shipment purposes and may enable commercial production of disposable eC/Au electrodes.19
image file: d0an00409j-f5.tif
Fig. 5 (A) Representative static contact angle image of a 4 μL drop of DI water resting on an eC/Au film after 15 days of ambient air exposure. (B) Bar graph of water contact angle (WCA) measured on eC/Au over time. (C) Bar graph of ΔEp of 1 mM Ru(NH3)63+ (1 M KCl, ν = 0.5 V s−1) measured on eC/Au and GC over time. (D) Bar graph of ΔEp of 1 mM dopamine (0.1 M H2SO4, ν = 0.2 V s−1) measured on eC/Au and GC over time. Error bars are ± 1 standard deviation.

Conclusions

Electron-beam deposited carbon on a gold layer provides an ultraflat surface and a disordered amorphous structure consisting of ∼30% sp3 hybridized carbon. The electrochemical reactivity of eC/Au is generally similar to polished GC for outer-sphere redox systems but yields slower electron-transfer rates for systems that require surface interaction like dopamine. eC/Au differs from polished GC in that it has lower background current and abnormally weak adsorption of quinones, which are attractive for electroanalytical applications. eC/Au is still subject to surface oxidation and adsorption of airborne impurities similar to graphitic materials, which slowed the electron-transfer kinetics of dopamine over time but did not impact Ru(NH3)63+ significantly. The differences in electrochemical reactivity between eC/Au and polished GC might stem from surface properties (low roughness factor, low oxygen functionalities, and lower fraction of edge plane) and possibly bulk electronic properties. eC/Au can be deposited on cheap, non-conducting substrates like plastic to mass-produce disposable electrodes with higher purity than carbon paste or screen-printed carbon electrodes. The electrochemical reactivity of eC/Au can make it useful as electrochemical SPR substrates. Also, eC/Au should be more widely applicable as an SPR substrate compared to Au by providing access to a richer pool of attachment chemistries available via electrochemical modifications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Department of Chemistry, University of Alberta (Assoc. Chair Research Funds to MTM) and the Natural Science and Engineering Research Council (Discovery Grant to MTM). TPN greatly acknowledges Alberta Innovates for fellowship support during the course of the reported research. The authors thank Amin Morteza Najarian, Bryan Szeto, and Tate Hauger for useful conversations. The authors also thank Dr Jillian Buriak and Michael Serpe for giving access to the AFM and contact angle instruments.

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