Porous boron doped diamond for dopamine sensing: Effect of boron doping level on morphology and electrochemical performance

Journal Pre-proof Porous boron doped diamond for dopamine sensing: Effect of boron doping level on morphology and electrochemical performance Simona Baluchová, Andrew Taylor, Vincent Mortet, Silvia Sedláková, Ladislav Klimša, Jaromír Kopeček, Ondřej Hák, Karolina Schwarzová-Pecková PII:

S0013-4686(19)31896-1

DOI:

https://doi.org/10.1016/j.electacta.2019.135025

Reference:

EA 135025

To appear in:

Electrochimica Acta

Received Date: 20 August 2019 Revised Date:

2 October 2019

Accepted Date: 7 October 2019

Please cite this article as: S. Baluchová, A. Taylor, V. Mortet, S. Sedláková, L. Klimša, Jaromí. Kopeček, Ondř. Hák, K. Schwarzová-Pecková, Porous boron doped diamond for dopamine sensing: Effect of boron doping level on morphology and electrochemical performance, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135025. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Porous boron doped diamond for dopamine sensing: Effect of boron doping level on morphology and electrochemical performance

Simona Baluchováa,b, Andrew Taylorc, Vincent Mortetb,c, Silvia Sedlákovác, Ladislav Klimšac, Jaromír Kopečekc, Ondřej Hákc, and Karolina Schwarzová-Peckováa,b*

a

Charles University, Faculty of Science, Department of Analytical Chemistry, UNESCO

Laboratory of Environmental Electrochemistry, Albertov 6, 128 43 Prague 2, Czech Republic b

Czech Technical University in Prague, Faculty of Biomedical Engineering, Sítná Sq. 3105,

272 01 Kladno, Czech Republic c

FZU - Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21

Prague, Czech Republic

*E-mail: [email protected]

Abstract Complex characterization of as-deposited conventional planar and newly available porous boron doped diamond (BDD) films with various boron doping levels (B/C ratio in the gas phase: 500 ppm, 1000 ppm, 2000 ppm, 4000 ppm, and 8000 ppm) was carried out by scanning electron microscopy, Raman spectroscopy and electrochemistry by performing cyclic voltammetric (CV) experiments with outer- and inner-sphere redox probes [Ru(NH3)6]3+/2+, [Fe(CN)6]3−/4−, and dopamine. The double layer capacitance was estimated from CV and electrochemical impedance spectroscopic measurements. For planar BDD films, a roughly 80% increase in the double layer capacitance, a 100 mV narrower potential window 1

and a 51 mV negative shift of the peak potential of dopamine were observed when the boron doping level increased from 500 ppm to 8000 ppm. Conversely, higher sp2 carbon content confirmed by Raman spectra affects the electrochemical performance of porous samples more significantly than the doping level. For the set of porous BDD electrodes, the double layer capacitance decreased by ca 95% with increasing boron content, peak-to-peak separation values of the studied inorganic redox markers were lower than 59 mV, suggesting their partial adsorption in the bulk of the material, and lower potentials for dopamine oxidation approaching values obtained for other sp2 carbon-based materials were recorded, but no dependency of potential values on boron concentration was revealed. Enhanced electron transfer rate on porous BDD materials resulted in increased sensitivity for dopamine detection using square-wave voltammetry in contrast to the planar set of electrodes. The lowest detection limit of 2×10−7 mol L−1 was estimated in a medium of 0.1 mol L−1 phosphate buffer pH 7.4 on B/C 4000 ppm porous electrodes with increased BDD growth time, without any activation applied between the individual scans. Moreover, for selected porous electrodes, improved selectivity for dopamine sensing in excess of common interfering compounds such as uric acid, ascorbic acid and paracetamol was achieved.

Keywords Boron doping level; Dopamine; Porous boron doped diamond; sp2 carbon content; Voltammetry

Highlights • Planar and porous BDD films deposited at B/C ratios of 500 to 8000 ppm characterized. • Electrochemical behaviour of porous BDD is affected by the presence of sp2 carbon.

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• Following increased growth time of porous layers (“thicker” BDD), behaviour resembles planar electrodes. • For dopamine, the lowest detection limit of 0.21 µmol L−1 achieved on 4000 ppm “thicker” porous electrodes. • Selectivity of “thicker“ porous BDD enhanced for dopamine detection in excess of AA, UA, and paracetamol.

1 Introduction The exceptional and unique properties of boron doped diamond (BDD) include considerable chemical stability and mechanical strength, wide potential window in both aqueous and nonaqueous solvents used in a variety of supporting electrolytes, stability against corrosion even in aggressive media, and inert surface properties. All these will contribute to the resistance to fouling, biocompatibility, very low double-layer capacitance and background currents [1-5], making BDD a more superior material than other common metallic and carbonaceous electrode materials; similarly, novel hybrid carbon-based (nano)materials have been receiving increased attention due to their favourable features comparable to BDD material [6]. However, sp2 carbon and boron doping level, crystallographic orientation, morphological features, such as grain boundaries and point defects, surface termination (H vs. O) are all factors which have a significant impact on characteristics of prepared BDD films, significantly affecting charger transfer kinetics at electrode-electrolyte interface. Amorphous sp2 carbon, formed at the grain boundaries of polycrystalline planar BDD films and by defects during their deposition, causes a higher capacitive current and a narrower working potential window, and decreases corrosion stability [7, 8]. On the other hand, nondiamond sp2 carbon sites act as charge transfer mediators, promoting both outer- and innersphere reactions, and hence substantially higher electron transfer kinetics compared to those at

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diamond (sp3) sites (k0sp2 >> k0sp3) [7, 9]. Additionally, an undesired adsorption of (by)products of redox processes can occur due to the incorporated sp2 carbon impurities, causing BDD electrode surface fouling [10]. Boron content is another crucial factor strongly influencing the final properties of BDD films [10-14]. With increasing boron doping levels the grain size decreases, which is related to the reduced film growth rate caused by the boron addition [11, 15, 16]. Conversely, an increase in both intrinsic stress and sp2 content phases will lead to a deterioration in the rigid diamond lattice. In general, boron concentration governs diamond conductivity. Hence, a high dopant concentration in a BDD will display a lower resistivity. Moreover, the boron content has an impact on the magnitude of the recorded signals and electron transfer kinetics, and thus the potential in recorded voltammograms. Naturally, the electrochemical processes, specifically hydrogen and oxygen evolution reactions, which limit the working potential window are also dependent on boron doping level, as they are associated with adsorption of water molecules/protons or intermediates of limiting reactions (e.g. atomic hydrogen) on boron-rich sites with subsurface substitutional boron defects. Highly boron doped diamond films exhibit narrower working potential windows in both positive and negative potential regions in comparison with lowly doped diamond [12, 17, 18]. Enlarging the electrode surface area has become a prevailing and powerful approach to further improve the already outstanding properties of BDD electrodes, without modifying their surface. By structuring the diamond surface, higher sensitivity, increased selectivity [1922], and higher double-layer capacitance values [23-26] have been achieved, compared with conventional planar BDD films. Thus, porous BDD appears to be a highly perspective material and has been already utilized as a supercapacitor [23-26], a membrane for filtration, extraction, separation [27], ultrafiltration and reverse osmosis [28] processes, for electrochemical decolourization and mineralization of triphenylmethane dyes [29],

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pharmaceuticals [30, 31], and organic pollutants [32], and in electroanalysis of biologically active organic compounds [19-22, 33-35]. Strategies hitherto reported in literature for preparing structured BDD can generally be divided into two main categories, (i) applying an etching process on the diamond surface through various masks, or (ii) deposition of the BDD films on structured templates. Etching techniques have been employed to prepare vertically aligned diamond nanowires [36, 37], BDD nanograss arrays [19], free-standing nanocrystalline diamond membranes [28], porous BDD films [38, 39], and a porous BDD/Ta sensing electrode for the simultaneous determination of dopamine and pyridoxine, with the ability to also respond to serotonin and uric acid [34]. Kondo et al. [40] prepared a porous diamond surface via a two-step thermal treatment with advantageous features, where dense pores were formed on a diamond surface without using a mask or catalyst. The second approach based on diamond coating of structured substrates, such as silicon nanowires [20, 33] or spheres [24, 41], quartz filter [42], fiberglass filters [27], vertically aligned multi-walled carbon nanotubes [21, 23, 35, 43-45], porous titanium substrates [29-32, 46], coral-like polypyrrole films [26], have been widely used for the porous BDD fabrication. With regard to the applicability of porous BDD-based materials to electroanalytical purposes, improved sensitivity and selectivity have already been reported in the sensing of dopamine [19, 21, 22, 34], epinephrine [21], uric acid [19, 47], adenine [20], glucose [33, 48], tryptophan [49, 50], tyrosine [50], catechol [51], pyridoxine [34], and paracetamol [21, 35]. Therefore, porous BDD appears to be a highly useful electrode material for electroanalysis of a wide range of analytes including neurotransmitters. Reliable detection of dopamine is an important task as abnormal levels in physiological fluids are associated with several neurological disorders [52]. An important issue is to achieve sufficient selectivity due to the coexistence of interferents such as ascorbic acid (AA) and uric acid (UA) with dopamine in

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biological matrices. Both interferents are electrochemically easily oxidizable organic compounds commonly occurring in physiological fluids at high concentration levels and are oxidized at similar potential values as dopamine on most non-modified carbon-based electrodes [53-57]. Modification of carbon-based electrodes can lead on one hand, to improved sensitivity and selectivity, but on the other hand, modification procedures are usually time-consuming, can be costly, and frequently result in decreased signal repeatability of electrode-to-electrode measurements or in low long-term stability, especially when enzyme-modified surfaces are considered. Specifically, when a modifier is applied on the surface of BDD electrodes, the ideal characteristics (e.g. chemical and mechanical stability, inert surface properties, biocompatibility) of as-deposited BDD films are significantly reduced and/or completely lost, as obvious from few reports of dopamine sensing on modified BDD electrodes [58-61]. Thus, perspectives of bare electrode materials are still undeniable when considering practical applications, especially when their mechanical and electrochemical properties can be tuned for the specific purposes. Recently, we have developed a porous BDD material deposited on a SiO2 nanofiber template [62, 63]. In this work, the influence of boron doping level (B/C ratio in the gas phase: 500 ppm, 1000 ppm, 2000 ppm, 4000 ppm, and 8000 ppm) on the structural and electrochemical properties of as-deposited porous BDD films was studied for the first time and compared with the as-deposited conventional planar films. Further, the redox behaviour of dopamine was examined and compared with other BDD-based materials. Finally, applicability of porous BDD for dopamine sensing in the presence of AA, UA and widely used analgesic paracetamol was investigated.

2 Experimental Section 2.1 Synthesis of Planar and Porous BDD Films

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Planar BDD layers were deposited on conductive (100) orientated 25 mm2 silicon substrates (ON Semiconductor, Czech Republic). Prior to deposition, the substrates were ultrasonically cleaned in acetone and ethanol, placed in a mixture of 1% HF and 0.1% HCl (to remove the native oxide layer) and seeded using a diluted nanodiamond particle aqueous colloid (NanoAmando) from NanoCarbon Institute (Japan), and then immediately transferred to a deposition chamber that was pumped down to vacuum. BDD growth was then carried out using an ASTeX 5010 (Seki Technotron, Japan) deposition system with the following conditions: 0.5% of methane in hydrogen, 50 mbar gas pressure, 1150 W microwave power, substrate temperature of ~750°C and growth duration of 5 h. Boron doping was obtained by adding trimethylboron in the gas phase as a boron precursor over a B/C ratio range of 500 ppm, 1000 ppm, 2000 ppm, 4000 ppm, and 8000 ppm. For each B/C ratio, half of the BDD layers were then used as a “base” layer for porous BDD layer deposition. To produce porous BDD, a ca 4-5 µm thick porous template consisting of nanodiamond seeded SiO2 nanofibers in a positive tone photoresist solution was spin-coated (at 3000 rpm for 30 s) on top of the BDD base layer and then dried on a hot plate (110 °C / 90 s). Following spin coating and drying, diamond was deposited using the same conditions described above, except the growth duration was reduced to 2.5 h. This process was repeated 3 times in order to obtain homogeneous 15 µm thick porous layers. B/C doping conditions were matched for the base and 3x porous BDD layers for each sample. For a B/C ratio of 4000 ppm, a second set of porous BDD samples (further denoted as “thicker”) were produced using the same conditions and methods except for an increased growth time to 5 h for each layer. Further details of the fabrication methods used to produce the porous BDD template and porous BDD can be found in refs. [62-64]. The surface of the planar and porous BDD samples was hydrogen terminated, i.e. as-deposited.

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Within one deposition procedure, an area of 2.5×2.5 cm2 was covered by BDD which was subsequently cut into squares with an area of 0.5×0.5 cm2, hence in total 25 electrodes were prepared in one-run.

2.2 Chemicals Analytical grade reagents including dopamine hydrochloride, AA, UA, paracetamol, caffeine, hexaammineruthenium chloride (all Sigma-Aldrich, Germany), potassium chloride, potassium hexacyanoferrate trihydrate, sodium dihydrogen phosphate dihydrate (all LachNer, Czech Republic), and sodium hydroxide (Penta, Czech Republic) were used without any further purification. All aqueous solutions were prepared with deionized water (Millipore Mili plus Q system, Billerica, USA) with resistivity of not less than 18.2 MΩ cm.

2.3 Instrumentation and Procedures The surface morphology of deposited layers was examined by scanning electron microscopy (SEM) using a TESCAN FERA3 GM microscope. The thickness of the sample layer was visually estimated from a cross-sectional view in a scanning electron micrograph. Raman spectroscopy was carried out at room temperature using a Renishaw InVia Raman Microscope at a wave-length of 488 nm and a laser power of 6 mW at the sample to assess the quality and diamond layer composition. To perform electroanalytical assays, a conventional three-electrode arrangement was used consisting of a BDD working electrode (7.07 mm2 geometric area) constructed by insulating an as-received BDD film in a Teflon body, a Ag│AgCl reference electrode (3 mol L−1 KCl) and a platinum wire auxiliary electrode (both acquired from Elektrochemické detektory, Czech Republic). Cyclic (CV) and differential pulse voltammetric (DPV) measurements were carried out using a computer controlled Eco-Tribo Polarograph with PolarPro 5.1 software (Eco-

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Trend Plus, Czech Republic). DPV at all BDD films was performed using a pulse height of +50 mV, pulse width of 80 ms followed by 20 ms sampling time, and a scan rate of 20 mV s−1. The effective surface area (Aeff) of BDD samples was estimated by performing CV of 1 mmol L−1 [Fe(CN)6]3−/4− in 1 mol L−1 KCl over the potential scan rate range from 0.01 V s−1 to 0.20 V s−1, based on the Randles-Sevcik equation (1) for a reversible process: Ip = (2.69×105) n2/3 Aeff D1/2 v1/2 c0

(1),

where Ip is peak current (A), n is the number of transferred electrons (1), D is the diffusion coefficient of the used redox marker (7.6 × 10−6 cm2 s−1 [65]), v is the scan rate (V s−1), and c0 is the concentration of the marker (10−6 mol cm−3). Double layer capacitance values (C) were determined for all examined BDD films from CVs recorded in a supporting electrolyte of 1 mol L−1 KCl and by using Equation (2) and Equation (3) below: Cgeom = ∆IAV/Ageom v

(2),

Ceff = ∆IAV/Aeff v

(3),

where ∆IAV is the average difference between the anodic and cathodic currents at 0 V vs. Ag/AgCl (A), v is the scan rate (0.10 V s−1) and Ageom and Aeff correspond to geometric (0.0707 cm2) and effective electrode surface areas, respectively. An Autolab PGSTAT101 controlled by software Nova version 1.11 (Metrohm Autolab B.V., The Netherlands) was used to perform square-wave voltammetric (SWV) experiments. Initially, the influence of SWV parameters, specifically amplitude (A), frequency (f), and step potential of the staircase waveform (∆Es) was examined, and the optimized SW parameters were determined as follows: A = 120 mV, f = 20 Hz, ∆Es = 2 mV, and A = 40 mV, f = 30 Hz, ∆Es = 2 mV for conventional planar BDD and porous BDD films, respectively. Presented SW voltammograms are baseline-corrected. pH measurements were

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performed using a digital pH meter 3510 with a combined glass electrode (Jenway, UK). All electroanalytical experiments were carried out at laboratory temperature (23 ± 1°C). Electrochemical impedance spectroscopy (EIS) was carried out with a Gamry Reference 600+ potentiostat (USA). A three-electrode cell was used with a Ag│AgCl (3 mol L−1 KCl) reference electrode and a platinum counter electrode. The EIS measurements were performed in 1 mol L−1 KCl at 0.0 V and used an AC signal with amplitude of 10 mV in the frequency range from 100 kHz to 0.1 Hz. The concentration dependency of dopamine at different BDD films were constructed from the average of four replicate measurements for each standard solution and evaluated by the least squares linear regression method. Limits of detection (LOD) were calculated as a threefold and limits of quantification (LOQ) as a tenfold of the standard deviation of the intercept, divided by the slope of corresponding calibration plot.

3 Results and Discussion 3.1 Morphological and Structural Characterization Scanning electron micrographs were recorded for all conventional planar (Fig. 1) and porous (Fig. 2) BDD films, deposited at B/C ratios of 500 ppm, 1000 ppm, 2000 ppm, 4000 ppm, and 8000 ppm, and the “thicker” 4000 ppm film. All planar layers show a closed structure with a clearly defined polycrystalline structure; the apparent grain size is typically 250 – 350 nm for 500 – 4000 ppm films, with a higher occurrence of smaller grains of 200 nm for the 8000 ppm film. Porous BDD samples show complete coverage of the SiO2 template with a closed BDD layer up to a B/C ratio of 4000 ppm. At 8000 ppm B/C, coverage of the SiO2 template is not closed, due to the reduced quality and growth rate of BDD films at this high B/C ratio, which is known to lead to poor quality growth [15, 66, 67]. The 4000 ppm “thicker” porous BDD sample grown with an increased growth time shows again complete

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coverage of the SiO2 template but the apparent porosity has been reduced. Similarly, May et al. [22] fabricated porous BDD electrode by deposition of BDD layers on black silicon needles and reported that growth times longer than 2 hours led to non-porous diamond films. Raman spectroscopy was employed to evaluate the presence of non-diamond content and also the boron incorporation in fabricated layers. Fig. 3a shows characteristic Raman spectra of planar BDD layers deposited on Si substrates grown with increasing B/C ratio. This figure shows the evolution of the Raman spectra from that characteristic of nano-crystalline diamond, showing a sharp diamond Raman line at ca 1330 cm−1 to characteristic spectra of highly boron doped diamond with intense bands at ca 480 cm−1 and 1200 cm−1, a red shifted (from ca 1330 cm−1 down to ca 1287 cm−1) Fano shaped diamond Raman line and only a small contribution from non-diamond phase bands at 1530 cm−1 when B/C ratios are above 4000 ppm. In addition, peaks related to the Si substrate are visible at 520 cm−1 and 950 cm−1. The red shift of diamond Raman line with increasing boron doping level is attributed to (i) phonon confinement effect by the addition of a high concentration of boron defects, which disturb the lattice structure, and (ii) the Fano effect with a negative asymmetric coefficient [68, 69]. In our previous study [12], a similar shift from 1332 cm−1 down to 1287 cm−1 for 500 ppm and 8000 ppm BDD films was observed for different excitation wavelength (532 nm). The boron concentration of the layers has been estimated, from the position of the boron related band at the 480 cm−1 peak position based on the empirical relation in [69], to range from ~ 1.3 × 1021 cm−3 for 1000 ppm B/C to ~ 3.8 × 1022 cm−3 for 8000 ppm B/C. Fig. 3b shows Raman spectra taken from the top surface of porous BDD layers deposited on a SiO2 template with increasing B/C ratio. Again, this figure shows the evolution of Raman spectra with characteristic features of nano-crystalline diamond, showing two intense bands at 480 cm−1 and 1200 cm−1, which are characteristic of highly boron doped diamond, and a Fano shaped red shifted (from 1312 cm−1 to 1290 cm−1) diamond Raman line.

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Using the same method as above, the boron concentration of the layers has been estimated to range from ~ 5.7 × 1020 cm−3 for 500 ppm B/C to ~ 2.6 × 1021 cm−3 for 8000 ppm B/C [69], these values compare well with those from the planar BDD samples indicating consistent boron doping conditions. However, the contribution from non-diamond carbon is clearly greater, compared with planar BDD, with peaks at ca 1150 cm−1 and 1500 cm−1 attributed to trans poly-acetylene, graphitic phases with bands at 1340 cm−1 and 1560 cm−1. This increase in non-diamond carbon contribution has been reported by others in porous diamond grown on SiO2 [24, 42, 70] and is related to plasma conditions within the bulk of the porous layer during growth whereby growth species have a lower chance of forming diamond bonds. However, when the growth time of the porous layer is increased, the Raman spectra very closely replicates that of planar BDD with the same B/C doping ratio, which is likely related to the increased Raman signal from the BDD layer rather than the bulk of the porous structure.

3.2 Electrochemical Characterization Electrochemical characterization was performed by assessing CVs recorded in 1 mol L−1 KCl (estimation of anodic potential limits, calculation of double layer capacities) and CVs of outer-sphere [Ru(NH3)6]3+/2+ and inner-sphere [Fe(CN)6]3−/4− redox probes for all tested planar and porous BDD films. They were carefully treated within a potential range avoiding the oxidative decomposition of water, which would inevitably lead to conversion of H-terminated to O-terminated surface. CVs of [Ru(NH3)6]3+/2+ and [Fe(CN)6]3−/4− redox probes exhibiting well-defined pairs of redox peaks are displayed in Fig. 4. As tabulated in Table 1, the corresponding values of anodic and cathodic peak potential difference (∆Ep) and |IpA/IpC| indicate both reversibility and

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the electron-transfer rate of the redox process occurring on the electrode surface for the two redox couples. On planar BDD films, the ∆Ep value was evaluated to be 74 ± 7 mV for [Fe(CN)6]3−/4− and 61 ± 7 mV for [Ru(NH3)6]3+/2+ (where the uncertainty denotes the 95% confidence interval based on a sample size of 5). As the expected ∆Ep of 59 mV for a reversible oneelectron system lies outside the former range but inside the latter range, these results seem to indicate an independence of ∆Ep on the boron content or the presence of sp2 carbon impurity. The ∆Ep values confirm near-reversible behaviour, fast electron transfer kinetics for investigated redox couples and the high quality of all fabricated planar samples with various boron content. For H-terminated BDD electrodes, the independence of ∆Ep for [Fe(CN)6]3−/4− redox system on the doping levels was also reported by Einaga et al. [71] and by Trouillon et al. [72] for the ranges of B/C ratios from 5000 ppm to 50 000 ppm and from 1000 ppm to 50 000 ppm, respectively. Nevertheless, this is valid only for H-terminated surfaces. After oxygenation, the interaction of [Fe(CN)6]3−/4− with carbon-oxygen functionalities results in an increase in ∆Ep [12]. On the other hand, the evaluated ∆Ep in the cyclic voltammogram of [Ru(NH3)6]3+/2+ (30 – 54 mV) and the cyclic voltammogram of [Fe(CN)6]3−/4− (45 – 51 mV) on the whole set of porous BDD films does not include the expected value of 59 mV. The reasons behind such electrochemical behaviour are presumably due to (i) adsorption of the redox markers in the porous material, particularly positively charged [Ru(NH3)6]3+/2+ may interact with partially negatively charged oxygen functionalities formed on sp2 carbon impurities in lower doped films with B/C of 500 ppm and 1000 ppm (corresponding ∆Ep 30 mV and 33 mV, respectively), and (ii) contribution from thin-layer diffusion, as reported in case of other carbon electrode surfaces with porous layers [73-75]. Furthermore, significantly higher background currents especially for films with lower dopant content were recorded on porous

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BDD compared to conventional planar BDD samples, which is associated not only with the larger electrode surface but also with the higher levels of non-diamond carbon content in the porous samples as mentioned above. In addition, an indistinctive signal appeared at a potential of ca +0.10 V in CVs of studied redox markers on porous films with B/C 500 ppm – 4000 ppm, presumably due to the redox reactions of oxygen-containing functionalities (mainly quinone and hydroquinone) formed at sp2-carbon states [76-78]. The electrochemical behaviour of the “thicker” porous film with a B/C ratio of 4000 ppm was tested and ∆Ep values of 54 mV and 69 mV for [Ru(NH3)6]3+/2+ and [Fe(CN)6]3−/4− were recorded, respectively. In the case of surface insensitive redox probe [Ru(NH3)6]3+/2+ the same ∆Ep value was assessed for porous BDD with equal boron doping level (4000 ppm) but different thickness, which is, however, an important factor when reaction kinetics is evaluated, as confirmed by Laurila et al. [79]. Nevertheless, our results suggest that the electron transport through structured sets of BDD electrode materials plays minor role. Contrarily, it is implied that overall reaction kinetics of [Ru(NH3)6]3+/2+ is mainly determined by the electron transfer from the solution species to electrode surface. Therefore, the surface properties of the tested electrodes, especially sp3/sp2 ratio, are held responsible for the observed electrode kinetics. Furthermore, lower background currents when compared with porous set of electrodes and the shape of the voltammetric curves indicate that the performance of “thicker” porous BDD film is more similar to the performance of planar BDD samples. Subsequently, the effective surface area (Aeff) of studied BDD films was estimated by evaluating CVs of 1 mmol L−1 [Fe(CN)6]3−/4− in 1 mol L−1 KCl. Calculated values of Aeff are summarized in Table 1. They were compared with the geometric surface area (Ageom = 7.07 mm2). Lower values of Aeff than Ageom ranging from 3.33 mm2 (500 ppm film) to 6.94 mm2 (4000 ppm) were obtained for all planar films which is in agreement with the heterogeneous

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character of BDD surface possessing non-conductive areas. For porous BDD films, slightly higher Aeff values in comparison with Aeff of planar samples and geometric area Ageom due to the structuring of the surface were obtained. They range from 8.08 mm2 (8000 ppm film) to 11.97 mm2 (2000 ppm film) without any trend in dependence on boron doping level and with relatively small difference between Aeff and Ageom values. This is presumably associated with the limiting rate of diffusion process of the used redox probe to the electrode surface within the bulk of the porous material [21, 37]. The closest value of Aeff (7.16 mm2) to Ageom was obtained at the “thicker” porous BDD with B/C of 4000 ppm. It is a result of two main effects, (i) the reduced porosity resulting from the prolonged growth time during its deposition, and (ii) its heterogeneous character. Moreover, the [Fe(CN)6]3−/4− redox couple is known to be sensitive to the presence of oxygen-containing functionalities on the electrode surface. These are formed on the surface as a result of spontaneous oxidation by atmospheric oxygen [80, 81] or by other species (HCO3−, OH−) contained in a thin layer of water naturally formed on the surface of solids after exposure to air [82, 83]. Hence, CV experiments with this redox marker were repeated after 7 months during which all BDD samples were exposed to air. The most significant difference was observed between lower doped planar BDD films, recorded ∆Ep values considerably increased by 224 mV, 30 mV and 36 mV for the corresponding B/C ratio of 500 ppm, 1000 ppm and 2000 ppm. Conversely, the porous samples with analogous B/C ratios demonstrated resistive behaviour towards spontaneous air oxidation as ∆Ep have remained constant even after exposition to air. These results suggest that the conductivity of porous BDD films is less susceptible to surface oxidation caused by the presence of atmospheric oxygen compared with planar samples, as the presence of sp2 carbon in the former contributes significantly to its conductivity.

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Further, the anodic limit potentials in 1 mol L−1 KCl solution were evaluated (very carefully in order to preserve H-termination) for all tested BDD samples (see Table 1). For planar films, the limiting potentials in anodic region were similar, however slightly higher potentials of +1.50 V were obtained in the case of samples with lower dopant concentration (B/C of 500 – 2000 ppm) which is in accordance with previously published results [10, 13, 17] confirming wider usable anodic region at as-deposited BDD electrodes with lower doping levels. Such observation can be explained by two main facts: (i) the resistivity of BDD electrodes increases with decreasing boron doping level, which is usually manifested by the wider potential windows, and (ii) the rate of limiting reactions is slow due to the lack of boron-rich sites required for the initial reaction step. Interestingly, this phenomenon is more significant on anodically activated BDD electrodes possessing a partially negatively charged oxidized surface [10, 12]. On the contrary, an opposite trend was revealed for porous BDD samples, the widest and narrowest anodic regions were measured on porous films with B/C of 8000 ppm (up to +1.60 V) and 500 ppm (up to +1.20 V), respectively. Comparison of anodic potential limits evaluated for planar and porous films revealed that the difference is the most significant on films with B/C of 500 ppm. This is again in agreement with other studies reporting on reduction of the anodic potential range with increasing amount of non-diamond impurities [10, 71], i.e. sp2 carbon present in porous variant. Apparently, sp2 carbon contributes notably to the acceleration of the oxygen evolution reaction as a result of lower boron density. As the porous BDD samples provided unstable voltammetric responses during the application of highly negative potentials, presumably due to hydrogen evolution in the form of gas bubbles, which can be incorporated into the bulk of porous layers and influence the electrochemical performance of the porous materials, the assessments of the cathodic potential limits (and thus the widths of the working potential windows) were not performed.

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Another feature is the considerable increase of background currents, related to an increase in double layer capacitance (Cdl) when the morphology changes from planar to porous. This difference is because of reduced level of pseudocapacitance on planar electrodes due to the absence of significant amounts of redox-active carbon-oxygen functionalities on the H-BDD surface [84] in comparison with the porous BDD, where such functionalities are frequent due to the presence of sp2 carbon. For comparison purposes, the capacitive behaviour of the double layer was investigated in 1 mol L−1 KCl on all planar and selected porous BDD samples using CV and EIS. Double layer capacities Cgeom and Ceff were calculated from CVs (equations (2) and (3)) using both, geometric Ageom and calculated effective surface Aeff areas. As these differ by a maximum of twice the value, thus causing relatively small differences between Cgeom and Ceff values, the crucial factor estimating the resulting Cdl values seems to be the ratio of sp2 carbon and boron content. In general, larger Cdl values are reported for sp2 carbon electrode materials than for BDD and previously large capacitance values were reported for BDD films with a high content of sp2 carbon impurities, e.g. due to high boron content [85, 86]. For planar BDD films, both Cgeom and Ceff values increase with increasing boron doping level from 16.1 μF cm−2 to 53.0 μF cm−2 and from 34.2 μF cm−2 to 73.9 μF cm−2, respectively (see Table 1). Obviously, boron acts as an internal charge carrier, with contribution of sp2 carbon present in highly doped BDD films as confirmed by Raman spectroscopy for 4000 ppm and 8000 ppm films (band at 1530 cm−1 denoted (6) in Fig. 3a). As a result, an increase in Cdl values with increasing boron content is observed. The obtained values are slightly higher than for planar BDD as reported previously for O-terminated BDD surface [72, 85, 86], including our studies on planar BDD films deposited using similar conditions as in the present study [12, 63, 87], with typical capacitance values of ca 0.2 ‒ 20 μF cm−2 depending on the chemical vapour deposition conditions (e.g., C/H ratio in the carrier

17

gas, boron content, gas flow rate, temperature) and supporting electrolyte and pH used for measurements. Importantly, our study was performed with as-deposited H-terminated BDD surface, without any pre-treatment and carefully avoiding anodic oxidation of the surface. The overall higher Cgeom and Ceff values in comparison to O-terminated BDD confirm a higher local density of states, presumably due to the higher content of sp2 carbon connected to boronrich sites. Anodic oxidation leads to introduction of carbon-oxygen functionalities on one side, but removal of non-diamond (sp2 carbon) impurities on the other side [87], thus resulting in lower Cdl values for O-terminated films, with indistinctive dependency on the boron doping level [12]. Interestingly, an opposite trend was observed for porous BDD samples exhibiting significantly higher capacitance values Cgeom and Ceff declining from 841.8 μF cm−2 to 100.4 μF cm−2 and from 582.4 μF cm−2 to 87.8 μF cm−2, respectively, with increasing boron doping level. In the literature concerning BDD-based porous materials, similar [21, 24, 44] or even higher double-layer capacities reaching the order of mF cm−2 were reported [25, 26, 35, 43]. For our porous films, such high Cdl values can be attributed, on one hand, to larger effective electrode surface area, but on the other hand, to the considerably higher content of sp2 carbon in comparison to planar BDD confirmed also by Raman spectra (see Fig. 3b). Importantly, the ratio of sp2 carbon/boron phase is the crucial factor estimating Cdl values of porous films with a maxima for low-doped films as a result of the large contribution of sp2 carbon phase to the overall capacitance. For highly doped porous films (8000 ppm), Ceff value reaches the minimum of 87.8 μF cm−2, which is comparable with the Ceff value of 8000 ppm planar film 73.9 μF cm−2 and confirms the role of incorporated boron leading to a significant reduction in local density of states in BDD films. Predictably, Cgeom and Ceff values measured on the “thicker” porous BDD sample with reduced porosity (see Fig. 2) and smaller Aeff (see Table 1) are lower than on porous films but 18

are higher than on films with planar structure, demonstrating the impact of chosen deposition conditions on the final properties of the fabricated BDD electrodes. To exclude the effect of pseudocapacitance, EIS was further employed. Experimental data were fitted with Randles equivalent circuit (depicted in the inset of Fig. S1). Such simple model was deliberately selected to provide solely values of capacitance that could be directly compared with the values extracted from the CV measurements (introducing constant phase element in the equivalent circuit would yield to values with different dimensions). EIS measurements were recorded in a frequency range of 100 kHz - 0.1 Hz. In general, smaller values (in contrast to Cgeom and Ceff obtained by CV) of double layer capacitance (CEIS) were assessed for planar and porous BDD electrodes, but confirmed the same trends: For planar BDD electrodes, CEIS values increase from 2.3 µF cm−2 to 17.3 µF cm−2 and for porous BDD, they decrease from 439.0 µF cm−2 to 21.5 µF cm−2 when dopant level increases from 500 ppm to 8000 ppm (see Table 1). The effect of pseudocapacitance is most pronounced for the 500 ppm porous BDD sample, possessing the highest sp2 carbon/boron ratio, and thus exhibiting clear difference for Cdl values estimated from CV and EIS measurements. Further, for this low boron doping level, the large difference in CEIS values for planar and porous BDD evidently confirm substantial contribution of sp2 carbon phase in porous films to the overall capacitance.

19

Table 1 Values of anodic and cathodic peak potential difference ∆Ep and the ratio of their peak heights |IpA/IpC| for redox markers [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ (both of c = 1 mmol L−1) in 1 mol L−1 KCl at a scan rate of 0.10 V s−1 assessed on as-deposited planar and porous BDD films with different B/C ratio. The anodic potential limits and calculated effective surface area (Aeff) of BDD samples along with the double-layer capacitance values estimated from CV using geometric (Cgeom) and effective (Ceff) surface area and estimated from EIS (CEIS) measurements carried out in 1 mol L−1 KCl solution are also reported. B/C ratio 1 mol L−1 KCl [Fe(CN)6]3−/4− [Ru(NH3)6]3+/2+ Aeff (mm2) / ppm Anodic potential limit Cgeom (µF cm−2) Ceff (µF cm−2) CEIS (µF cm−2) ∆Ep (mV) |IpA/IpC| ∆Ep (mV) |IpA/IpC| Planar BDD films 500

+1.50 V

16.1

34.2

2.3

80

0.99

64

0.96

3.33

1000

+1.50 V

26.0

32.8

4.9

75

1.00

54

0.94

5.61

2000

+1.50 V

37.9

51.5

4.6

73

0.99

68

1.00

5.20

4000

+1.40 V

41.4

42.1

7.7

67

1.01

63

1.03

6.94

8000

+1.40 V

53.0

73.9

17.3

76

0.99

56

0.99

5.07

439.0

45

1.06

30

0.76

10.22

45

0.98

33

1.01

9.00

51

1.02

48

1.05

11.97

45

1.01

54

1.12

8.27

48

1.01

51

1.18

8.08

Porous BDD films 500

+1.20 V

841.8

582.4

1000

+1.30 V

464.3

364.7

2000

+1.50 V

182.3

107.7

4000

+1.45 V

104.2

89.1

8000

+1.60 V

100.4

87.8

55.1 21.5

“Thicker” porous BDD film 20

4000

+1.40 V

80.0

79.0

25.3

21

69

1.00

54

1.02

7.16

3.3 Electrochemical Behaviour of Dopamine 3.3.1 Cyclic Voltammetry The electrochemical behaviour of 1 mmol L−1 dopamine in 0.1 mol L−1 phosphate buffer (pH 7.4) was investigated on planar and porous as-deposited BDD samples by CV (Fig. 5). Based on these measurements, the anodic peak potentials Ep,DA and ∆Ep values were determined and compared to those obtained in other studies on BDD films, differing in applied growth conditions and surface pre-treatments. The results are tabulated in Table 2. Dopamine oxidation on planar BDD samples occurred within the potential range from +0.28 V to +0.36 V, depending on the boron doping level (Fig. 5a). For lower doped films (500 ppm and 1000 ppm) more positive (approx. by 0.055 V) oxidation potential of dopamine was observed in comparison with BDD films with B/C of 2000 ppm ‒ 8000 ppm. This indicates lower electron transfer rates and higher energy required for the rate-determining step of the electrode process to take place on the BDD films with lower dopant content. These results are consistent with previous studies reporting on shift of oxidation potentials of 2aminobiphenyl from +0.81 V (B/C 500 ppm; LSV) to +0.72 V (B/C 8000 ppm; LSV) [12], 4chloro-3-methylphenol from +1.25 V (B/C 500 ppm; LSV) to +1.15 V (B/C 8000 ppm; LSV) [88], benzophenone-3 from +0.70 V (B/C 500 ppm; DPV) to +0.64 V (B/C 8000 ppm; DPV) [89] and uric acid from +0.28 V (B/C 3500 ppm; DPV) to +0.17 V (B/C 7500 ppm; DPV) [13] with increasing boron doping level. All obtained potential values and also ∆Ep ranging from 0.294 V to 0.202 V achieved on the tested set of planar BDD electrodes are in agreement with other studies on dopamine on H-terminated BDD films (values of oxidation potentials and ∆Ep are summarized in Table 2), utilized in an as-deposited state or after applying either cathodic pre-treatment or polishing the BDD surface. The other studies reporting higher oxidation potentials ranging between +0.53 V to +0.66 V and higher peak potential differences ∆Ep were performed on O-terminated BDD on which electrode reaction kinetics is 22

obviously hindered due to the presence of oxygen-containing functionalities, thus confirming the “inner-sphere” character of dopamine when considered as a redox marker. Considerably lower dopamine oxidation potential from +0.17 V to +0.23 V and sharp peak shapes were obtained on porous BDD films (Fig. 5b). Additionally, both anodic and cathodic peaks can be clearly distinguished in the CVs of dopamine, and ∆Ep values (0.060 V ‒ 0.102 V) reflect the near-reversible oxidation process for this compound on this porous material. This indicates a substantial influence of non-diamond sp2-carbon areas, contributing significantly to the enhancement of electron transfer rate [7, 9, 90], which is manifested by considerable potential shift to less positive values when compared to the potential values recorded on planar BDD samples containing either none (500 – 2000 ppm) or a very low amount (4000 and 8000 ppm) of sp2 phase. Another effect being considered is that sp2-carbon domains present in porous material can also act as adsorption sites for dopamine [8, 10]. Similar observations for dopamine have been reported in other studies on comparison of sp2 and sp3 carbon-based materials [91, 92]. Moreover, the results on ∆Ep values may be partially influenced by the presence of thin layer mass transport within the porous structure, which is thoroughly discussed in [73-75]. The lower density of pores present in the 4000 ppm “thicker” porous BDD film is responsible for its electrochemical performance: the shape of recorded CV curves for dopamine (Fig. 5b) is comparable to planar samples, but conversely, the values of oxidation potential of +0.24 V and peak potential difference ∆Ep, of 0.14 V approach values recorded on porous BDD samples. Considerable differences in the electrochemical performance of dopamine was also reported in other studies comparing structured and planar BDD films. Fatibello-Filho et al. [21, 35] compared electroanalysis of dopamine, acetaminophen and epinephrine on conventional planar and porous films prepared by deposition of BDD on vertically aligned

23

carbon nanotubes, and the significant decrease in the peak potentials and at least three times higher current densities were achieved on porous discs. Similarly, May et al [22] obtained a sharp increase in the current response of dopamine and a shift of the oxidation peak towards lower potential values on BDD-coated black silicon long-needle electrode in comparison with the planar BDD electrode. Obviously, the dopamine redox reaction is extremely sensitive to the surface chemistry of sp2 and sp3 carbon due to the structuring of the BDD surface, which significantly influences the electron transfer rate and adsorbability of the species involved in the redox reaction.

24

Table 2 An overview of BDD electrodes characterised by performing CV experiments (at a scan rate of 0.10 V s−1 if not stated otherwise) with dopamine in phosphate buffer (PB) or phosphate buffered saline (PBS) of pH in the range of 7.0 – 7.4. Values of anodic oxidation potentials for dopamine EpA,

DA

and peak potential difference between anodic and cathodic peak ∆Ep, DA are summarized. LOD values for dopamine are also

reported along with the electroanalytical technique used for the assessment. Electrode material As-deposited planar BDD films As-deposited H-BDD

As-deposited polycrystalline BDD As-deposited polycrystalline BDD with a NCD layer Cathodically treated HBDD

Supporting electrolyte 0.1 M PB, pH 7.4 PBS, pH 7.2

EpA vs. Ag/AgCl (mV) +361, +355, +317, +284, +310 +360 ± 20

∆Ep (mV) 294, 288, 245, 202, 234 293 ± 30

0.02 M PBS, pH 7

+232

140

0.02 M PBS, pH 7

+215

59

PBS, pH 7.4

+266 ± 4

266 ± 15

+390

322

+340

270

+220

83

+232

140

+355

305

Cathodically treated H- PBS, pH 7.4 BDD Cathodically treated HBDD Cathodically treated H- PBS, pH 7.4 BDD H-BDD with grains of PBS, pH 7.0 several µm H-BDD with submicron

25

Remarks B/C 500, 1000, 2000, 4000, 8000 ppm varies with the used substrate (W, Mo/Re, W/Re); vsc = 300 mV s−1 different surface morphology; DA sensing in the presence of interferents varies with B/C ratio (0.1 %, 1 %, or 5 %); vsc = 50 mV s−1 B/C 0.1 % (1000 ppm) [B] >1020 atoms cm−3 B/C 1000 ppm (Windsor Scientific, UK) different surface morphology different surface

LOD (µmol L−1) 0.35 (SWV;4000 ppm) not reported

Ref.

not reported

[90]

not reported

[72]

not reported

[94]

not reported

[95]

not reported

[96]

not reported

[91]

not reported

[91]

This work [93]

grains Polished BDD Anodically treated OBDD Anodically treated OBDD Anodically treated OBDD Anodically treated OBDD As-deposited porous BDD films As-deposited porous “thicker” BDD film As-deposited porous HBDD As-deposited porous HBDD

0.15 M PBS, pH 7.2 0.07 M PBS, pH 7.0

+370

345

+585

a

+500

530

0.1 M PB, pH 7.4

+600

660

0.07 M PBS, pH 7.0 0.1 M PB, pH 7.4

+524b

514

0.1 M PB, pH 7.4

+196, +196, +225, +180, +174 +240

0.2 M PB, pH 7.0 0.1 M PBS, pH 7.0

morphology [B] ~ 5 × 1020 atoms cm−3 [B] = ~2.1×1021 cm−3 (B/C 104 ppm); vsc = 50 mV s−1 B/C 0.1 % (1000 ppm) [B] >1020 atoms cm−3

not reported

[92]

1.5 (amperometry)

[19]

not reported

[94]

not reported

[95]

1.4 (CV)

[97]

69, 69, 102, 63, 60 140

B/C 10 000 ppm; vsc = 50 mV s−1 B/C 500, 1000, 2000, 4000, 8000 ppm B/C 4000 ppm

0.22 (SWV;4000 ppm) 0.21 (SWV)

+240

90

vsc = 50 mV s−1

not reported

This work This work [21]

+124 (DPV)

-

0.06 (DPV)

[34]

0.8 (amperometry)

[19]

0.27 (DPV)

[22]

determination in the presence of pyridoxine in human serum BDD nanograss array 0.07 M PBS, pH +395 295 [B] = ~2.1×1021 cm−3 (O-terminated) 7.0 (B/C 104 ppm); vsc = 50 mV s−1 BDD-coated black 0.2 M PB, pH 7.0 +290 170 B/C 2000 ppm; silicon long needles vsc = 50 mV s−1 a irreversible voltammetric profile (no cathodic peak); b Ep vs. saturated calomel electrode NCD (undoped nanocrystalline diamond with high graphitic content)

26

3.3.2 Pulse Voltammetric Techniques Electrochemical behaviour of dopamine was evaluated by DPV and SWV in 0.1 mol L−1 phosphate buffer (pH 7.4) on all tested BDD samples. As maximal signal/background current ratio was obtained for the 4000 ppm porous film, this doping level was further preferred for optimization of DPV and SWV for dopamine detection. For planar films obtained using 500 ppm – 8000 ppm boron doping level, larger differences in Ip were observed in DP voltammograms than in the corresponding cyclic voltammograms. Similarly, an increase in Ip and a slight negative shift in the peak potential (Ep) from +0.29 V to +0.20 V were observed with higher boron doping level. Dopamine oxidation on porous BDD films occurred at a potential value of +0.13 V, exhibiting no dependence of peak position on boron content in concordance with CV. More significant differences arise in the comparison of planar and porous films with equivalent B/C ratios, with the largest differences of Ip observed for films prepared using 4000 ppm and 8000 ppm boron doping level. In DPV, porous BDD films with B/C of 4000 ppm and 8000 ppm with a dopamine peak at Ep +0.13 V exhibit 4.4-times and 2.4-times higher Ip than on planar samples (Ep +0.26 V), as a result of larger effective surface areas of porous films and faster electron transfer kinetics due to the presence of sp2 carbon. A smaller increase in the peak heights for 8000 ppm porous samples may be related to incomplete BDD coverage of the template as confirmed by SEM. In SWV experiments based on films prepared using a 4000 ppm boron doping level, the lowest Ip of 0.565 µA was obtained on planar BDD samples, and conversely, a 7.7-time and a 2.6-time higher Ip were obtained on porous and “thicker” porous BDD films for 10 µmol L−1 dopamine solution using optimized SWV parameters (see below). SWV on porous films takes advantage of the small peak potential difference ∆Ep of anodic and cathodic peaks,

27

hence resulting in a higher increase in total Ip of dopamine on porous BDD samples when compared to DPV. Further, relative Ip for ten consecutive DPV scans recorded on each tested BDD film without applying any electrode activation between individual scans were evaluated to assess the repeatability of the provided signals (Fig. 6). In pH 7.4 phosphate buffer, the main product of dopamine oxidation is dopaminequinone, however, subsequent reactions may lead to the formation of other products (e.g. leucodopaminechrome, dopaminechrome, indolequinone and melanin-like compounds [98, 99]), which cause electrode surface passivation that inhibits the electroactive sites as it has already been reported for BDD [92] and other carbon-based electrode materials including glassy carbon, basal/edge-plane and highly oriented pyrolytic graphite [92], tetrahedral amorphous carbon [100], amorphous carbon, functionalized nanodiamonds and pyrolytic carbon [101]. Apparently, the process of polymer film formation is independent of the boron content present in planar BDD electrodes, manifested by a decrease in Ip's of recorded voltammetric signals with an increasing number of scans (Fig. 6a). However, the behaviour of porous BDD samples depends on the dopant concentration. After ten successive scans, Ip values of dopamine are approximately two times higher on porous films with B/C of 500 – 2000 ppm than on 4000 ppm and 8000 ppm films (Fig. 6b). These differences can be attributed to (i) the limited possibility of diffusion of the relatively large molecule of dopamine into the bulk of the porous film as reported in other studies [21], and (ii) current density of dopamine redox reactions, which lead to surface passivation of all investigated films and are associated with boron and sp2 carbon-rich places. As oxidation of dopamine proceeds to a large extent on the external surface of porous BDD films, it is quickly blocked by a polymeric film on porous samples deposited at higher B/C ratios, thus preventing other dopamine molecules from diffusion to the bulk of the porous material. For samples with lower boron doping levels (500 – 2000 ppm), the formation of the fouling films

28

proceeds slower, thus further diffusion of dopamine molecules to the bulk and their subsequent oxidation are enabled and results in enhanced current density. The situation is different in SWV, taking advantage of the quasi-reversible character of the dopamine redox reactions and suppressed formation of a passivating layer during the fast scan. Fig. 7 shows five consecutive SW voltammograms of dopamine recorded on porous and “thicker” porous 4000 ppm BDD films exhibiting stable Ip. Obviously, passivation of the electrode surface was avoided as repeatable signals without using an activation procedure were obtained. The quasi-reversible behaviour of dopamine on porous and “thicker” porous BDD films was confirmed by distinctive signals in forward and backward components of SWV scan (inset in Fig. 7) and it is also reflected in the obtained total current responses. The relative standard deviation (RSD) of the peak heights from ten consecutive scans for 1 µmol L−1 and 10 µmol L−1 dopamine solutions were correspondingly 8.6 % and 0.9 %, 7.1 % and 4.8 %, and 4.7 % and 2.1 % on planar, porous and “thicker” porous BDD films, indicating excellent repeatability of recorded signals. Moreover, the electrode-to-electrode signal reproducibility was estimated on five electrodes of each set of BDD samples (planar, porous, and “thicker” porous) which were prepared under the same conditions, i.e. within one deposition procedure as described in Section 2.1. Expectedly, high reproducibility was confirmed on all types of BDD films, the RSD values of the peak heights for 10 µmol L−1 dopamine were 3.2 %, 5.1 %, and 3.9 % on planar, porous and “thicker” porous BDD films (all B/C 4000 ppm), respectively. Based on these results, SWV experimental parameters including amplitude (A), frequency (f), and step potential of the staircase waveform (∆E), were further optimized using 10 µmol L−1 dopamine in 0.1 mol L−1 phosphate buffer pH 7.4 separately for planar and porous 4000 ppm BDD samples. The investigated ranges were A 10 – 200 mV, f 5 – 100 Hz, and ∆E 1 – 10 mV, and while one parameter was tested, the other two were kept at constant

29

values. The optimized SWV parameters were determined as follows: A = 120 mV, f = 20 Hz, ∆E = 2 mV for planar BDD samples, and A = 40 mV, f = 30 Hz, ∆E = 2 mV for porous BDD films. Concentration dependences of dopamine in the concentration range corresponding to dopamine physiological levels were recorded on planar, porous and “thicker” porous BDD films (see Fig. S2) with a B/C ratio of 4000 ppm to test the applicability of developed methods. The analytical parameters and calculated values of LOD and LOQ are summarized in Table 3. On all investigated 4000 ppm BDD electrodes, linear current responses were achieved for the whole studied range of dopamine concentrations. Clearly, porous and “thicker” porous BDD films outperformed planar sample because both provided significantly higher sensitivity and lower detection limits. For other BDD-based porous electrodes applied for dopamine determination, namely BDD nanograss array electrode [19], porous Ti/BDD electrode [34] and BDD-coated black silicon long-needle electrode [22], LOD values of 0.8 µmol L−1 (by amperometry), 0.06 µmol L−1 (by DPV) and 0.27 µmol L−1 (by DPV) were evaluated, respectively. Amongst the porous BDD electrodes reported in literature, comparable or even lower LOD values of 0.22 µmol L−1 and 0.21 µmol L−1 on porous and “thicker” porous BDD films (both with a B/C of 4000 ppm), respectively, were achieved in this study. Generally, dopamine basal concentration is very low (usually in the nmol L−1 range) [102, 103], nevertheless, higher values ranging between 0.1 – 2 µmol L−1 have been reported for the concentration of dopamine in urine [104] and in the striatum when electrically stimulated [105]. Besides, during real in-vivo experiments performed on model system of adult wild type flies, micromolar concentration levels of dopamine were determined [106]. Moreover, electrodes used in such experiments for neurochemical sensing are calibrated in-vitro. Calibration solutions consist of dopamine, typically in the concentration range from 0.5 to 2 µmol L−1 (or even higher) [107, 108], in buffers of physiological pH, most commonly

30

employed are HEPES buffer, Tris buffer and phosphate buffered saline. Therefore, considering all above-mentioned aspects, such porous and “thicker” porous BDD electrodes seem to be highly promising electrode materials for dopamine sensing with the ability to fulfil the requirements of practical applications.

31

Table 3 Analytical parameters of concentration dependences of dopamine recorded by SWV in 0.1 mol L−1 phosphate buffer pH 7.4, with assessed LOD and LOQ values. BDD material

Linear range

Intercepta

Slopea

(B/C 4000 ppm)

(µmol L−1)

(nA)

(nA µmol−1 L)

planar

1.0 – 10.0

−40 ± 7

60.3 ± 1.2

0.9992

0.35

1.16

porous

0.50 – 10.0

−186 ± 33

449.4 ± 6.4

0.9994

0.22

0.73

“thicker” porous

0.25 – 10.0

−25 ± 11

157.4 ± 2.2

0.9993

0.21

0.70

a

R

All uncertainties denote standard deviations.

32

LOD

LOQ

(µmol L−1) (µmol L−1)

3.3.3 Effects of Interferents The selectivity of the proposed methods was tested under the optimized SWV experimental parameters and instrumental conditions on the planar, porous and “thicker” porous BDD electrodes with B/C 4000 ppm. The influence of the most common tested interferents AA, UA, caffeine and paracetamol was investigated in standard solutions with a fixed dopamine concentration of 10 µmol L−1 for concentration ratios of 1:1 and 1:100 with respect to dopamine. The recorded dopamine oxidation peak currents were compared with those obtained in the absence of each interfering compound. When the SWV signal provided by dopamine changed more than ± 5 %, it was considered as a serious interference. The least problematic compound is caffeine, as is does not affect the voltammetric behaviour of dopamine on planar, porous and “thicker” porous BDD films due to very positive oxidation potential, typically ranging between +1.3 V and +1.5 V vs. Ag/AgCl [109111]. For the other potential interferents, i.e. AA, UA and paracetamol, when the ratio of dopamine : interfering compound was 1:1, all BDD samples provided sufficient selectivity as peaks corresponding to dopamine oxidation were not influenced by their presence. Nevertheless, a different observation was made in the presence of a 100-fold excess of studied interferents. One overlapped peak was recorded in the solutions containing dopamine and AA, UA and paracetamol on planar BDD, but conversely, the “thicker” porous BDD film was able to distinguish signals of dopamine when these interfering substances were present; although in the presence of paracetamol, the peak height of dopamine decreased and the peak potential shifted to less positive values (Fig. S3). It is also worth mentioning that porous BDD with B/C 8000 ppm, which was not fully covered as confirmed by SEM, could differentiate between dopamine and AA, UA or paracetamol voltammetric responses at the concentration ratio of

33

1:100 (Fig. S3). The latter compound provided a clear signal in the presence of dopamine also on porous BDD film with a B/C of 4000 ppm. Apparently, selectivity of porous BDD films can be adjusted by the thickness of the individual layers (therefore, by the porosity of fabricated samples), boron doping level, and undoubtedly, content of sp2 carbon in the BDD films. These factors influence charge transfer kinetics and signal potential positioning and presumably have different effects on particular compounds, depending on their structure and mechanism of the redox reaction. This topic will be more thoroughly investigated in the next part of the research.

4 Conclusions In this work, the influence of boron doping level on the morphology and electrochemical characteristics of as-deposited planar and porous BDD samples was thoroughly investigated. Scanning electron micrographs of all planar layers showed a closed surface with no pinholes and a well-defined polycrystalline morphology while scanning electron micrographs of porous samples confirmed that a 3D template consisting of SiO2 nanofibers was completely covered with BDD, except for B/C 8000 ppm, which showed regions of poor BDD coverage. Raman spectroscopy revealed high levels of sp2 carbon in all porous samples and also its presence in planar samples of higher boron doping level (4000 ppm and 8000 ppm). Electrochemical characterization revealed that the ratio of sp2 carbon / boron content affects significantly the performance of tested BDD films. For planar films, the boron content is the main factor and its increase leads to an increase in double layer capacitance values, narrowing of the widths of the potential windows and a negative shift of the peak potentials of dopamine. On the other hand, the electrochemical performance of porous BDD films is strongly affected by the presence of sp2 carbon. Within the set of porous electrodes, double

34

layer capacitance decreases with increasing boron doping level, peak potential difference values of the outer- and inner- sphere redox markers [Ru(NH3)6]3+/2+ and [Fe(CN)6]3−/4− are lower than 59 mV suggesting their partial adsorption in the bulk of the material, and lower potentials for dopamine oxidation approaching values obtained on other sp2 carbon-based materials were recorded when compared to planar samples. Importantly, no dependency of potential values on boron concentration was revealed. Enhanced electron transfer rates as a result of the presence of sp2 carbon in porous material resulted in higher sensitivity of porous BDD films in comparison to planar BDD for dopamine sensing using differential pulse techniques. SWV performed better than CV and DPV as it provides stable responses with excellent repeatability without applying any activation procedure between the individual scans. It takes advantage of near-reversible character of dopamine redox reaction, resulting in increased sensitivity for porous films in respect to planar films. The lowest limit of detection of 0.21 µmol L−1 obtained for a 4000 ppm “thicker” porous BDD film is sufficiently low for dopamine determination in striatum, urine and neural cell cultures. Moreover, enhanced selectivity for dopamine sensing in the presence of interferents was achieved on this type of porous BDD film, suggesting that selectivity of the porous materials can be altered by adjusting their thickness and/or porosity. The behaviour of this “thicker” porous BDD film with B/C of 4000 ppm exhibiting reduced porosity (confirmed by SEM) due to the prolonged growth time of each porous layer is approaching characteristics obtained for planar BDD electrodes, demonstrating the impact of chosen deposition conditions on the final properties of the fabricated BDD. Evidently, better understanding of factors influencing positioning and stability of voltammetric responses of dopamine and other redox-active species could lead to tuning the properties of BDD films during their deposition, and thus resulting in the design of highly

35

perspective BDD-based porous electrode materials possessing the ability to fulfil the requirements of practical applications on reproducibility, sensitivity, and selectivity. Importantly, reported fabrication procedure enables preparation of identical porous electrodes in one run, thus highly reproducible electrochemical behaviour is ensured which represents a clear advantage over the most of the modified electrodes.

Acknowledgements The research was carried out within the framework of Specific University Research (SVV 260440). The funding provided by the Czech Science Foundation (project 17-15319S), the Technology Agency of the Czech Republic Gama project TG02010056 (sub-project Advanced diamond electrochemical electrodes - ADE2) and by the J. E. Purkyně fellowship awarded to V. Mortet by the Czech Academy of Sciences is gratefully acknowledged. S.B. thanks to the Grant Agency of Charles University (project 390119) for financial support. This work was also supported by the MEYS SAFMAT CZ.02.1.01/0.0/0.0/16_013/0001406, LO1409 and LM2015088 projects (SEM maintenance).

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B/C = 500 ppm

B/C = 1000 ppm

B/C = 2000 ppm

50

B/C = 4000 ppm

B/C = 8000 ppm

51

Fig. 1 Scanning electron micrographs of planar BDD layers grown with increasing B/C (500 up to 8000 ppm).

B/C = 500 ppm

B/C = 1000 ppm

52

B/C = 2000 ppm

B/C = 4000 ppm

53

B/C = 8000 ppm

B/C = 4000 ppm (thicker growth)

54

Fig. 2 Scanning electron micrographs of porous BDD layers grown with increasing B/C ratio (500 up to 8000 ppm).

Fig. 3 488 nm Raman spectroscopy: (a) planar BDD layers on Si substrates showing classical B related features at (1) 480 cm−1, (4) 1200 cm−1 and a (5) red shifted diamond Raman line alongside a small contribution from non-diamond carbon phases with a band at (6) 1530 cm−1. 55

Peaks related to the Si substrate are visible at (2) 520 cm−1 and (3) 950 cm−1; (b) porous BDD layers grown on an SiO2 template showing classical B related features (1) 480 cm−1, (3) 1200 cm−1 and a (4) red shifted diamond Raman alongside typical NCD related Raman features showing non-diamond carbon phases with bands at (5) 1340 cm−1 and (7) 1560 cm−1 and peaks at (2) 1150 cm−1 and (6) 1500 cm−1 attributed to trans poly-acetylene. The assignments have been made based on [68, 69, 112].

20

(a)

(b)

20

10 10

I / µA

I / µA

0

-10

500 ppm 1000 ppm 2000 ppm 4000 ppm 8000 ppm

-20

0

-10

-20 -0.8

-0.4

0.0

-0.3

0.4

E / V vs. Ag/AgCl

15

0.0

0.3

0.6

E / V vs. Ag/AgCl

24

(c)

(d)

0

I / µA

I / µA

12

500 ppm 1000 ppm 2000 ppm 4000 ppm 8000 ppm

-15

-12

-30 -0.4

-0.2

0.0

0.2

0

0.4

E / V vs. Ag/AgCl

-24 -0.4

"thicker" porous 4000 ppm

0.0

0.4

0.8

E / V vs. Ag/AgCl

Fig. 4 Cyclic voltammograms of the redox systems (a, c) [Ru(NH3)6]3+/2+ and (b, d) [Fe(CN)6]3−/4− (both of c = 1 mmol L−1) in 1 mol L−1 KCl recorded on as-deposited (a, b) planar BDD and (c, d) porous BDD films with various boron content at a scan rate of 0.10 V s−1.

56

40 (b)

(a)

40

20

I / µA

I / µA

20

0

0

500 ppm 1000 ppm 2000 ppm 4000 ppm 8000 ppm

-20 -0.4

0.0

0.4

-20 "thicker" porous 4000 ppm -0.4

0.8

0.0

E / V vs. Ag/AgCl

0.4

0.8

E / V vs. Ag/AgCl

Fig. 5 Cyclic voltammograms of 1 mmol L−1 dopamine in 0.1 mol L−1 phosphate buffer pH 7.4 recorded on as-deposited (a) planar BDD and (b) porous BDD films with various B/C ratio.

(a)

100

500 ppm 1000 ppm 2000 ppm 4000 ppm 8000 ppm "thicker" 4000 ppm

75

Ip, rel / %

75

Ip, rel / %

(b)

100

50

25

50

25

porous BDD

planar BDD

0

0 0

2

4

6

8

0

10

2

4

6

8

10

Scan no.

Scan no.

Fig. 6 Relative peak heights of 1 mmol L−1 dopamine in 0.1 mol L−1 phosphate buffer pH 7.4 recorded on as-deposited (a) planar BDD and (b) porous BDD samples with different B/C ratio for ten consecutive DPV scans.

57

40

(a)

1.5

total current

1 µM 10 µM

I / µA

20

forward current

0

3.0

backward current 0.0

0.2

I / µA

0.5

0.0

0.0 0.2

total current

3 forward current 0

backward current

-3 0.0

0.2

0.4

E/V

1.5

0.1

1 µM 10 µM

1.0

0.4

E / V vs. Ag/AgCl

6

I / µA

-20

0.0

(b) I / µA

4.5

0.3

0.1

E / V vs. Ag / AgCl

0.2

0.3

E / V vs. Ag/AgCl

Fig. 7 Five consecutive SW voltammograms of dopamine (of c = 1 µmol L−1 and 10 µmol L−1) in 0.1 mol L−1 phosphate buffer pH 7.4 recorded on as-deposited (a) porous BDD and (b) “thicker” porous BDD films with B/C ratio of 4000 ppm. The inserted graphs show the individual forward and backward components of the total current. Applied SWV parameters were as follows: A = 40 mV, f = 30 Hz and ∆Es = 2 mV.

58

Highlights • Planar and porous BDD films deposited at B/C ratios of 500 to 8000 ppm characterized. • Electrochemical behaviour of porous BDD is affected by the presence of sp2 carbon. • Following increased growth time of porous layers (“thicker” BDD), behaviour resembles planar electrodes. • For dopamine, the lowest detection limit of 0.21 µmol L−1 achieved on 4000 ppm “thicker” porous electrodes. • Selectivity of “thicker“ porous BDD enhanced for dopamine detection in excess of AA, UA, and paracetamol.