Paper-supported nanostructured ultrathin gold film electrodes – Characterization and functionalization

Applied Surface Science 329 (2015) 321–329

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Paper-supported nanostructured ultrathin gold film electrodes – Characterization and functionalization Petri Ihalainen a,∗ , Anni Määttänen a , Markus Pesonen b , Pia Sjöberg a , Jawad Sarfraz a , Ronald Österbacka b , Jouko Peltonen a a b

Laboratory of Physical Chemistry, Department of Natural Sciences, Åbo Akademi University, Turku, Finland Physics, Department of Natural Sciences, Åbo Akademi University, Turku, Finland

a r t i c l e

i n f o

Article history: Received 27 October 2014 Received in revised form 17 December 2014 Accepted 22 December 2014 Available online 30 December 2014 Keywords: Paper substrate Ultrathin gold film Self-assembly monolayer PEDOT–PSS Electrochemical characterization

a b s t r a c t Ultrathin gold films (UTGFs) were fabricated on a nanostructured latex-coated paper substrate by physical vapour deposition (PVD) with the aim to provide low-cost and flexible conductive electrodes in paper-based electronics. Morphological, electric and optical properties of UTGFs were dependent on the deposited film thickness. In addition, UTGFs were functionalized with insulating and hydrophobic 1-octadecanethiol self-assembled monolayer and inkjet-printed conductive and hydrophilic poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT–PSS) layer and their electrochemical properties were examined. Results showed that sufficient mechanical stability and adhesion of UTGFs deposited on latex-coated paper was achieved without the need on any additional adhesive layers, enabling a more robust fabrication process of the electrodes. UTGF electrodes tolerated extensive bending without adverse effects and conductivity comparable to the bulk gold was obtained already with the film thickness of 6 nm. Although not been fabricated with the high-throughput method like printing, a very low material consumption (∼12 ␮g/cm2 ) together with a high conductivity (resistivity < 3 × 10−6  cm) makes the UTGFs electrodes potential candidates low-cost components in flexible electronics. In addition, the excellent stability of the UTGF electrodes in electrochemical experiments enables their application in the development of paper-based electrochemical platforms, e.g. for biosensing purposes. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Paper electronics has over the last decade emerged as a new and exciting research topic that holds great scientific and technological interest worldwide. Great efforts are currently being put in the development of paper-based electronic devices and electrochemical platforms [1–14]. The aim is not to replace conventional electronics, but to create a completely new generation of applications, especially in large area electronics, with low-cost, disposable, flexible and robust design. The motivation to use paper as substrate for electronics does not arise only from its low-cost, recyclable and flexible characteristics but also from the fact that physicochemical properties like nano- and microscale topography, surface chemistry and optical transparency can be modified quite conveniently by various coating materials and post-processing methods [2,8,15–17].

∗ Corresponding author. Tel.: +358 2 215 4256. E-mail address: petri.ihalainen@abo.fi (P. Ihalainen). http://dx.doi.org/10.1016/j.apsusc.2014.12.156 0169-4332/© 2014 Elsevier B.V. All rights reserved.

Several different paper grades, including porous cellulose-based chromatography type papers, pigment-coated papers and plasticlaminated photo papers have been used as supporting matrices, deposition substrates or active components in paper electronics [7,8,10–12,16,18–20]. In addition, various deposition methods have been considered for the fabrication of the electric components, including sputtering, evaporation, catalytic growth and printing [6,7,9–11,18–20,25]. Printing has intuitively been the main choice for fabrication of paper-based electronic devices and gadgets. In particular, inkjet printing has emerged as one of the most promising highresolution and cost-effective techniques to fabricate conductive metal thin films due to its low material consumption and highly customized processing [1,8,21]. Among printable conducting materials, gold has been considered as an important material especially in bio-electronics, because of its chemical inertness. Gold also has excellent resistance to oxidation and acids and it is biocompatible [22–24]. For example, gold films with thicknesses as low as 200–300 nm and sufficient conductivity (∼1/7th of conductivity of bulk gold) have been produced and utilized in electrochemical applications [14,25–28]. The material costs for such inkjet-printed

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electrodes in lab-scale were estimated to be around 0.04 D /cm2 . Still, from the applicability and cost issue point of view, there could be a need for further reduce film thickness down to the ultrathin film range. Ultrathin metal films, defined as being under 10 nm thick, play an important role in several fields of materials science and nanotechnology, finding applications in electronic, magnetic and electro-optical devices [29]. Especially, ultrathin gold films (UTGFs) are often used in microfabricated devices due to their high electrical conductivity, optical reflectively and chemical inertness [30–35]. The existing techniques for fabrication of UTGFs are essentially based on a physical vapour deposition (PVD) process in high or ultrahigh vacuum conditions, the most common ones being sputtering, thermal evaporation and laser ablation [36]. Both crystalline and polymeric materials can be used as substrates [37–41]. One particular research area is metallized polymers and their use as electrical contacts on active polymeric layers in organic electronic devices, such as transistors, diodes and solar cells [42–44]. In addition, UTGF electrodes could find applications in analytical and bioanalytical fields. Ultrathin metal films have been reported to result in an enhanced sensitivity in electrochemical immunosensors [45]. One of the main challenges regarding the applicability of UTGFs in microfabrication technology is their comparable weak adhesion to inert and commonly used substrate materials such as glass and silica even when using adhesion promoting processes such as the use of a thin (5–10 nm) oxidative metal underlayer [46,47]. Furthermore, these adhesive metal oxides tend to diffuse to the surface and significantly affect the morphological, optical and electrical properties of the gold films [48,49]. Alternative strategies such as precoating the substrate with a self-assembled layer of, e.g. mercapto- or amine silane have been reported for enhancing the adhesion [50,51]. On the other hand, an excellent adhesion of UTGFs on polymeric substrates has been achieved without the need of an adhesive layer [39,41,52]. This study demonstrates that mechanically stable UTGFs can be fabricated on paper substrate. A nanostructured latex-coated paper was used as a substrate for the vacuum deposited UTGFs with varying thicknesses. The UTGF electrodes were successfully functionalized with hydrophobic alkyl thiol self-assembled monolayers (SAM) and inkjet-printed conductive polymer layer and their electrochemical properties were compared to bare electrodes. Topographical, chemical, electrical and electrochemical characterization of the UTGFs was conducted using atomic force microscopy, contact angle goniometry, X-ray photoelectron spectroscopy, hot-probe technique, cyclic voltammetry and electrochemical impedance spectroscopy. In addition, the mechanical stability and optical properties of the UTGFs were qualitatively examined.

2. Materials and methods 2.1. Paper substrate A multi-layer curtain coated paper was used as a substrate for the nanostructured two-component latex coating (see details in Appendix Fig A.1). The used paper substrate was developed for printed electronics and has excellent barrier properties against water and solvent penetration and contains components normally used in paper making [53]. The two-component latex blend was applied on the multi-layer coated paper by rod-coating. Details of the fabrication and application of nanostructured latex-coated papers have been described elsewhere [54–56]. In brief, a blend of two latex components with different glass transition temperatures (Tg ) was used. The low-Tg (soft) component was an emulsion polymerized carboxylated styrene butadiene acrylonitrile copolymer with Tg = 8–10 ◦ C and an average particle size of 140 nm (DL920,

DOW Chemicals). The high-Tg (hard) component was modified polystyrene with Tg > 90 ◦ C and an average particle size of 140 nm (DPP3710, DOW Chemicals). The hard latex particles provide blocking resistance, mechanical strength and integrity to the film, while the soft latex particles act as a film-forming component. The twocomponent latex blend was prepared by mixing the low-Tg and high-Tg components so that the final weight ratio in the blend was 3:2. The nanostructured surface texture was created by irradiating the latex-coated paper with a short-wavelength infrared (IR) heater for 60 s (IRT systems, Hedson Technologies AB, Sweden). The schematic presentation of the two-phase nanostructure and the relevant surface properties are given in Appendix Fig. A.2. 2.2. Preparation and functionalization of the UTGF electrodes PVD with electrically resistive heating was used to fabricate gold films. A shadow mask was used for electrode patterning. The evaporation was done under high vacuum (10−6 mbar) during two separate runs utilizing a heated aluminium-coated tungsten basket. The evaporation rate was set to 12 nm/min. A deposition monitor (XTM/2, Inficon) was used for gravimetric determination of the amount of evaporated gold on the latex surface. Gold film with nominal thickness of 10 nm (UTGF-10 nm), 20 nm (UTGF-20 nm) and 40 nm (UTGF-40 nm) were fabricated. The functionalization of UTGFs with alkyl thiol SAMs were conducted using 1-octadecanethiol (ODT, Fluka Chemika). Before the thiolation, the evaporated UTGF electrodes were cleaned with plasma (air) flow (PDC-326, Harrick) for 2 min, rinsed with absolute ethanol and dried with nitrogen gas. The paper-supported UTGFs were sealed between two silicon rings in a custom built liquid flow cell (FIAlab Instruments, Inc., USA) and exposed to a solution of ODT (250 ␮L, 5 mM) for 16 h at room temperature in the dark under a cap. After SAM formation, the electrodes were removed from the solution, rinsed immediately with absolute ethanol, and dried with nitrogen gas. Two consecutive print layers of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT–PSS) ink (OrgaconTM IJ-1005, Afga) were inkjet–printed on the working electrode using a drop spacing of 30 ␮m and firing voltage of 23 V. To ensure optimal jetting conditions and good film quality, the ink cartridge temperature was set to 28 ◦ C and the printing plate temperature was set to 48 ◦ C. After printing, the PEDOT–PSS film was dried at RT at least 12 h prior to further analyses. 2.3. Contact angle measurements and surface energy determination A CAM 200 contact angle goniometer (KSV Instruments Ltd., Finland) was used for the determination of the static contact angle of the samples. Purified water (MilliQ), diiodomethane (DIM, Sigma) and ethylene glycol (EG, Sigma–Aldrich) were used as probe liquids with drop volume of 1 ␮L. Contact angles were measured in air in ambient conditions (RH = 25 ± 5%, T = 20 ± 3 ◦ C) and obtained using the software supplied with the instrument, which utilizes a Laplace fit to the projected drop curvature. For surface energy calculations, the Owens–Wendt method [57,58] was applied using the probe liquid surface tension component values suggested by Della Volpe and Siboni [59]. 2.4. Atomic force microscopy An NTEGRA Prima (NT-MDT, Russia) atomic force microscope (AFM) was used to analyze the topography of the samples in intermittent-contact mode. The images (1024 × 1024 pixels) were captured in ambient conditions (RH = 20 ± 2%, T = 26 ± 1 ◦ C) using silicon cantilevers with a nominal tip radius of 10 nm (Model:

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NSG10, NT-MDT, Russia). The scanning rate and the damping ratio were 0.39 Hz and 0.6–0.7, respectively. Tip-sample adhesion of the samples was determined using Peakforce Quantitative Nanomechanical Mapping (Peakforce QNM) [60]. The Peakforce QNM measurements were carried out at ambient conditions (RH = 20 ± 2%, T = 26 ± 1 ◦ C) with a Nanoscope V (MultimodeTM series, Bruker) AFM. The AFM microscope was placed on an active vibration damping table (MOD-1M, JRS Scientific Instruments, Switzerland) standing on a stone table to eliminate external vibrational noise. All the images were obtained using peak force cantilevers (TAP525A, Bruker). Each cantilever used in the analysis was calibrated (spring constant and radius of curvature of the tip) following the calibration procedures given in the Peakforce QNM user guide [60]. Image analysis was conducted using commercial image analysis software (SPIPTM , Image Metrology, Denmark). The root mean square (RMS) roughness values reported here were calculated using 200 nm × 200 nm topographical images. 2.5. X-ray photoelectron spectroscopy The X-ray photoelectron spectroscopy (XPS) spectra were obtained with a PHI Quantum 2000 scanning spectrometer, using monochromatic Al K␣ (1486.6 eV) excitation and charge neutralization by using an electron filament and an electron gun. The photoelectrons were collected at an angle of 45◦ in relation to the sample surface with a hemispherical analyser. The analysis depth was approximately 5–10 nm. The pass energy was 117.4 eV and the acquisition time was 10 min. Three parallel measurement points were conducted for each sample. 2.6. Electrical measurements The conductivity of the films was measured with 4-point probe measurements, in a linear configuration, having a tip spacing of 1.82 mm. A suitable bias current of 1–2 mA was applied over the film, depending on the film resistance. A corresponding voltage was measured until a repeatable and stable value was obtained. The conductivity of the samples was then calculated using finitesize corrections [61]. All the measurements were carried out in ambient conditions. Spring-loaded gold probes with a tip diameter of 60 ␮m were used in the 4-point conductivity setup. The measurements were performed manually, beginning with the lowest possible applied current which was incrementally increased by one order of magnitude per step until a stable voltage was observed. The errors associated with the 4-point conductivity measurements are mainly due to the inaccuracy of measuring the probe tip spacing and sample thickness, both which play a major part in determining the correction factor. The error has been estimated to be no more than 10% and any inaccuracy due to fluctuations in the applied current or measured voltage fits well within the given 10% error. To determine whether the UTGF electrodes were metallic or semiconducting at room temperature, their resistance vs temperature behaviour was studied. Samples (cooled down to ∼0 ◦ C) were coupled to a current–voltage metre (Agilent 4142) and heated to roughly 50 ◦ C using hot air, while the metre measured the resistance of the film. The corresponding increase or decrease indicates whether the film is metallic or semiconducting, respectively. The films which showed semiconducting behaviour were further studied using a Hot-Probe technique. The UTGF electrodes were measured with the help of two thermo electric magnetic field (thermo-e.m.f.) probes. A heating coil was attached to the hot probe and the other probe was kept at room temperature. A temperature gradient causes a thermo-e.m.f. in the films and the sign of voltage indicates the semiconducting type (p- or n-type) of the film.

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In addition, 2-point measurement was used to determine the resistance of the films showing metallic behaviour. 2.7. Electrochemical characterization Cyclic voltammetry measurements were performed using an Autolab General Purpose Electrochemical System (AUT30.FRA2Autolab, Eco Chemie, B.V. (The Netherlands). The three-electrode system studied here consisted of an evaporated gold working electrode (WE) and a counter electrode (CE) both with an active gold area of 0.1 cm2 and a conventional Ag/AgCl (3 M KCl) reference electrode (RE) (Metrohm). To conduct electrochemical experiments, the paper-supported UTGFs were sealed between two silicon rings in a custom built liquid cell (FIAlab Instruments, Inc., USA) and the electrode side was exposed to electrolyte solution. The contacts were outside the cell and were wired to the Autolab instrument. The reference electrode was then lowered into the reaction cell. A solution of potassium ferri(III)cyanide/ferro(II)cyanide (K3 /K4 [Fe(CN)6 ]) was used as a model redox couple solution with a concentration of 1 mM with respect to both forms of the redox couple. A 0.1 M KCl solution was used as the supporting electrolyte. Five cyclic voltammogram (CV) cycles were recorded at two different scan rates (50 mV/s and 100 mV/s). The potential was cycled between two different intervals (from −0.5 V to +0.5 V and from −0.3 V to +0.6 V) and the redox peaks were displayed both in the forward and reverse scans. Impedance measurements were performed using the same Autolab instrument as described above. The measurements were performed in 0.1 M KCl electrolyte at the frequency range from 100 kHz to 10 mHz in the same reaction setup as described above. The measurements were recorded at a dc-potential of 0.2 V by using a sinusoidal AC excitation signal of 10 mV. 3. Results and discussion 3.1. Surface characterization of UTGFs Fig. 1 shows AFM topographical images of a nanostructured latex substrate before and after evaporated UTGFs with different nominal thicknesses. The nanostructured latex surface consists of two distinct height levels with a hydrophobic (protruding areas) and a hydrophilic (lower sections) phase (Fig. 1A and Appendix Fig. A.2). The presence of a UTGF on the latex surface could be identified as randomly distributed and non-uniformly shaped nano-islands covering the latex surface, i.e. a polycrystalline grain structure commonly observed for vapour-deposited UTGFs (Fig. 1B–D) [37,39,62]. Although individual grains can be clearly identified in the AFM topograph, the grains seem to form a continuous, interconnected island network on the surface (Appendix Fig. A.3). In addition, scanning electron microscope images captured for UTGF-10 nm further showed that the interconnect network of grains was present even in the thinnest film (Appendix Fig. A.4). The XPS survey spectrum provided proof for the presence of gold on the surface through the observed Au4f peak (Table 1), confirming that the grains observed in the AFM images were indeed gold. In addition, the changes in the tip-sample adhesion maps showed that the UTGF completely covered both the hydrophilic and hydrophobic phases of the latex surface and decreased the tip-surface adhesion (Appendix Figs. A.5 and A.6). Morphological characteristics of the grains varied with the varying nominal thickness of the UTGFs (Fig. 2, Table 2). However, the average height of the grains was only about 20% of the nominal thickness values. The average diameter (measured as top width of a grain to minimize the effect of tip convolution, Fig. 2A) also increased with the increasing nominal thickness (Table 2). On the

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Fig. 1. (A) AFM topographical image (1 ␮m × 1 ␮m) of nanostructured latex substrate and (B–D) evaporated ultrathin gold films with the nominal thickness of (B) 10 nm (C) 20 nm and (D) 40 nm. The height scale is (A) 60 nm, (B) 40 nm, (C) 20 nm and (D) 15 nm.

Table 1 XPS elemental composition and atomic percentage (at%) of different samples. Sample

C1s [at%]

O1s [at%]

Au4f [at%]

Bare latex surface UTGF-20 nm UTGF-20 nm with ODT SAM

84.8 ± 3.0 47.5 ± 0.6 70.9 ± 1.1

10.7 ± 2.2 8.9 ± 0.3 3.4 ± 0.5

– 43.7 ± 0.3 24.4 ± 1.2

other hand, the RMS roughness (measured from 0.2 ␮m × 0.2 ␮m images to exclude the roughness background due to the nanostructured latex) was not affected by the thickness of UTGFs although it was higher compared to the bare latex surface (Table 2). The growth of vapour-deposited gold films on polymeric surfaces commonly occurs via the formation of three-dimensional islands or grains [41,63]. This is due to the low adhesion of gold towards organic surfaces caused by a large surface energy difference between gold (∼1500 mN/m [64]) and polymeric surface

S2p [at%] – – 1.4 ± 0.4

(e.g. latex ∼43–45 mN/m, Appendix Fig. A.2B). Other parameters involved in the growth process include surface morphology, polymer chain mobility and deposition conditions. The adsorptive growth of evaporated gold clusters on a polymer surface has been suggested to involve four major stages, i.e. nucleation, lateral growth, coarsening, and vertical growth stage [41]. In the initial stage of film growth, metal atoms can easily diffuse into the polymer and form embedded clusters if the deposition temperature is close to or above the glass transition temperature

Fig. 2. Line profiles over individual grains of UTGF with nominal thickness of (A) 10 nm, (B) 20 nm and (C) 40 nm.

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Table 2 Average grain height, grain top width and RMS roughness of ultrathin gold films (UTGFs) with different nominal thicknesses and coatings. Sample

Average grain height [nm]

Bare latex substrate UTGF-10 nm UTGF-20 nm UTGF-20 nm with ODT SAM UTGF-20 nm with PEDOT:PSS film UTGF-40 nm

– 2.1 ± 0.4 6.2 ± 0.3 – – 10.3 ± 0.6

of the polymer [65,66]. The interdiffusion at the interface has been shown to promote the adhesion of metal on polymeric surfaces [67]. Considerably lower height values for grains were observed from AFM topographs compared to the nominal thickness of the films (Table 2). Similar kind of divergence between the grain height and nominal thickness has been previously reported for evaporated gold films on polymeric surfaces [41]. This has been mainly attributed to the reduced coefficient of condensation of gold particles on the polymeric surfaces. The gold atoms have low adhesion towards polymer chains and can easily desorb. This results to a direct re-evaporation of gold atoms, reducing the condensation coefficient below 1. In addition, during evaporating, a small amount of gold (< 0.1 monolayer) has been shown to diffuse inside the polymeric substrate and form an enrichment layer at the interface [41]. Furthermore, a nanostructured latex surface has about 5–10% more surface area compared to a perfectly smooth surface which together with the diffusion of gold into the polymer surface, may explain the smaller grain height values. Finally, AFM tip-sample convolution has been shown to slightly affect the measured height values of the grains [62]. In addition to the topographical variances, clear optical differences between the samples were observed (Fig. 3). The thinnest gold film (UTGF-10 nm) appeared greenish in colour without a typical reflection of gold whereas the thicker films were clearly reflective and golden yellow in colour. A bulk gold film absorbs light in blue wavelength region and thus appears as yellow whereas ultrathin gold films absorb green light and appear red [68]. On the other hand, when viewed in transmitted light (e.g. UTGFs on a transparent glass substrate), UTGFs could appear blue and green [69]. However, the latex substrate is practically opaque and thus the observed green colour is not expected to be due to the transmission of light. The optical differences observed in the UTGF samples arise most probably from the combined effect of nanostructure topography and thickness. The shape and height profile of the nanopatterns have been shown to determine how light

Grain top width [nm] – 5–15 10–20 – – 16–25

RMS roughness [nm] 0.8 1.4 1.4 1.2 0.9 1.5

± ± ± ± ± ±

0.1 0.2 0.1 0.2 0.1 0.1

reflects back from the metal surface [70]. Metal films patterned with sub-wavelength surface features have been shown to result in spectral selectivity by balancing the transmission and reflection characteristics of the surface [70]. By engineering the surface structure, perceived colour of metal has been controlled without employing any form of chemical modification, thin-film coating or diffraction effects. For example, green gold films have been previously produced using surface patterning by embossing [70]. 3.2. Electrical characterization of the UTGF electrodes Electrical characterization of the gold films showed that UTGF10 nm behaved as a semiconductor (n-type) at room temperature whereas UTGF-20 nm showed a metallic behaviour. The resistivity values were calculated using the average grain height as a thickness value (Table 2). The resistivity of UTGF-10 nm varied between 1.8 × 10−4 and 7.1 × 10−5  cm. On the other hand, UTGF20 nm showed a reproducible resistivity value of 2.6 × 10−6  cm, which is very comparable with the bulk resistivity value of gold (2.4 × 10−6  cm). It should also be noted that extensive bending and repeated bending-unbending cycles of a paper substrate with evaporated UTGF electrodes did not have an adverse effect on the resistivity (Fig. 3). The higher resistivity of thin metal films has been explained by the thickness dependence of the transport properties occurring when the bulk mean free path of current carries becomes comparable with the film thickness [71]. Small clusters of metallic material have been shown to exhibit both metallic and semiconductor characteristics. This due to temperature dependent evolution of the band gap and density of electron states in systems containing a low number of atoms [68]. In addition, it has been shown that the temperature-dependent semiconductor characteristics of UTGFs sputtered on glass substrates are dependent on the layer thickness [40]. UTGFs with thicknesses below 5 nm show semiconductor characteristics both at room and low (liquid nitrogen) temperatures. Structures with thicknesses between 5 and 10 nm exhibit semiconductor characteristics at low temperatures but show metallic conductivity at room temperature. Layers with the thickness over 10 nm show metallic characteristics with only marginal temperature-dependence. 3.3. Surface functionalization and electrochemical characterization of the UTGF electrodes

Fig. 3. Photographs of ultrathin gold film (UTGF) with the nominal thickness of 10 nm (green) and the UTGF electrode with the nominal thickness of 20 nm (golden) on the nanostructured latex coated paper. The gap width in the electrode is 20 ␮m. Also shown is a fully bended electrode (upper left corner). (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

The functionalization of the paper-based UTGFs was tested by coating the UTGF-20 nm electrodes with a self-assembled monolayer (SAM) of ODT and an inkjet-printed PEDOT–PSS layer. The first indication of a successful formation of a high quality ODT SAM was observed by water contact angle measurements. The formation of the ODT SAM changed the initially hydrophilic (contact angle 73◦ ) electrodes to clearly hydrophobic (119◦ ). The observed contact angle value is slightly higher than that reported for high quality ODT SAMs on a smooth gold surface (∼110◦ ) [72]. This can be attributed to the increased surface area caused by both

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Fig. 4. (A) Cyclic voltammograms (CVs) of paper-based UTGF electrodes in 0.1 M KCl. Also shown are CVs after coating with (A) ODT SAM and (B) inkjet-printed PEDOT:PSS film. The scan rate was 100 mV/s.

the nanostructured surface of latex and the grain-like morphology of the UTGF. XPS elemental analysis gave further proof for successful surface functionalization (Table 2). After the formation of the ODT SAM, the relative amount of carbon (C1s) increased at the expense of gold (Au4f). Also sulphur (S2p) was detected on the surface. The deposition of the ODT SAM had a negligible effect on the RMS roughness (Table 2) and topography (Appendix Fig. A.4) of the UTGF. However, the tip-surface adhesion was reduced compared to the bare UTGF (Appendix Figs. A.6 and A.7). The AFM topograph (Appendix Fig A.8) of an inkjet-printed PEDOT–PSS film revealed that the polymer film appeared as a randomly tangled nanofiber assembly fully covering the electrode surface. In addition, the introduction of the PEDOT–PSS film decreased the contact angle to 20◦ and reduced the tip-sample adhesion compared to the bare UTGF (Appendix Figs. A.6 and A.8). The contact angle value matches the previously reported value for a PEDOT–PSS film blended with EG [73]. The electrochemical properties of the bare and coated UTGF electrodes were characterized by cyclic voltammetry (Fig. 4). Within the measured potential window (−0.3 to 0.6 V vs Ag/AgCl), very modest oxidation peaks at 0.35 V and 0.60 V and a reduction peak at 0.24 V were observed for the bare UTGF electrode (Fig. 4A). The oxidation peaks most probably arose from the formation of hydrated AuOH and AuO during the positive potential scan whereas the reduction peak was due to the reverse reaction [74]. In addition, there was a constant region (i.e. electrical double layer region) between −0.1 V and 0 V where no faradaic processes occur and non-faradaic (capacitive) current dominates due to the orientation

of charges on both sides of the interface. A sharp increase of the reduction current at potentials below −0.2 V was due to the reduction of the electrolyte producing hydrogen gas [74]. The hysteresis between the anodic and cathodic peaks arose from the electrochemical irreversibility of the oxide layer formation/dissolution. For the electrodes coated with the ODT SAM, the redox processes were found to be completely inhibited and current flow was negligible, indicating a dense and sufficiently defect free monolayer coverage (Fig. 4A). This result is consistent with the contact angle values. The cyclic voltammogram of UTGF electrode coated with the inkjet-printed PEDOT–PSS film showed only one redox state with the reduction peak at 0.14 V and the oxidation peak at 0.32 V (Fig. 4B). A single redox state has been previously observed also for a thick (3.5 ␮m) PEDOT–PSS film drop-casted on screenprinted carbon electrodes [75]. The commercial ink used here was a dispersion of the oxidized (p-doped) PEDOT containing excess of PSS as counter-ion. The excess negative charge of PSS with respect to the positively charged chains of oxidized PEDOT makes the colloidal particles stable in the aqueous media [76]. During reduction, the positive charge carriers of PEDOT are depleted and undoped PEDOT turns insulating [77]. In addition, potassium (K+ ) cations present in electrolyte will migrate into the film to maintain local electroneutrality, and following reduction half-reaction can be presented: PEDOT+ ·PSS− + e− + K+ → PEDOT0 + K+ ·PSS− . This reaction is reversible and PEDOT can be doped via the associated oxidation. The electron transfer properties of the UTGF electrode after ODT SAM modification were studied using the K3 /K4 [Fe(CN)6 ] redox couple. Fig. 5A shows typical cyclic voltammograms for

Fig. 5. (A) Cylic voltammograms for paper-based UTGF electrodes with 1 mM K3 /K4 [Fe(CN)6 ] in 0.1 M KCl before and after the application of the ODT SAM. The scan rate was 100 mV/s. (B) Capacitance vs frequency log–log curves for the bare and coated UTGF electrodes.

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paper-based UTGF electrodes before and after the application of the ODT SAM. A quasi-reversible oxidation and reduction of the redox couple (ferrocyanide/ferricyanide ion) at 0.29 V and 0.16 V (vs Ag/AgCl) was obtained for the bare UTGF electrode. As expected, the insulating ODT SAM completely inhibited the electron transfer between the electroactive species and the electrode surface. Fig. 5B shows log–log curves of capacitance as a function of frequency for the bare and coated UTGF electrodes. In all cases, the interfacial capacitance (capacitance dispersion) was found to be dependent on the frequency, although dispersion was greatly reduced in the lower frequency region after the application of the coatings. The origin of the capacitance dispersion observed on solid electrodes has been shown to be due to surface roughness, variations occurring in the double layer by the presence of atomic scale inhomogeneities of the electrode surface, and the presence of some kinetic processes, most probably related to specific adsorption of anions [78,79]. The capacitance of the bare UTGF electrodes varied between 15 and 75 ␮F/cm2 in the 1000–0.01 Hz region. The capacitance values are consistent with the double layer capacitance usually reported for a bare metal immersed in an electrolyte [80–82]. The capacitance showed a decrease after application of the ODT SAM. It has been previously shown that alkyl thiol SAMs act as good electronic insulators and decrease the capacitance of metal electrodes [83,84]. The decrease in capacitance is due to the lower dielectric constant and increased thickness of the SAM compared to water molecules on a bare electrode. The capacitance value of ∼1 ␮F/cm2 is consistent with the expected capacitance of 1–2 ␮F/cm2 of a dense and defect free alkyl thiol monolayer [85]. Opposite to the insulating ODT SAM, the PEDOT–PSS coated UTGF electrodes showed a clearly higher capacitance (420 ␮F/cm2 ) compared to the bare electrode. The PEDOT–PSS film have been shown to increase the electrode capacitance via its pseudo-capacitance effect, i.e. faradaic charge transfer originating from a very fast sequence of reversible redox, electrosorption or intercalation processes [86]. 4. Conclusions This study demonstrates that mechanically stable and smooth paper-based UTGFs electrodes with conductivity comparable to the bulk gold can be fabricated by PVD on the nanostructured latex coated paper substrate. PVD is not considered to be a high-throughput (HTP) manufacturing method like, e.g. printing. However, one could still find several benefits of using PVD manufactured paper-based UTGF electrodes presented here. First, additional adhesive layers are not needed for the formation of mechanical stable UTGF on the nanostructured latex surface. In addition, PVD method does not require the synthesis of nanoparticles (like gold) and development of stable ink dispersion with optimized rheological properties needed for repeatable printing. This makes PVD fabrication of electrodes very straightforward. In addition, the UTGF electrodes produced without any additives results to relative clean gold surfaces and the risk of surface contamination (other than from ambient air) of components migrating from adhesive layers or residual surfactant components of the inks is removed. This ensures the better long-term stability of surface chemistry, which is important considering surface functionalization and electrochemical properties. Finally, a very high conductivity (near the conductivity of bulk gold) can be achieved with a very low material consumption. Using the current price (∼31.5 D /g) and the density (19.3 g/cm3 ) of gold, the material costs for highly conductive UTGF electrodes with thickness of 6 nm can be estimated to be around 0.0004 D /cm2 . This is a hundred times lower unit price compared to the gold electrodes fabricated by inkjet-printing, which can be considered to be the current state-of-the-art HTP method for the

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fabrication of low-cost and highly conductive paper-based thin film gold electrodes. The above listed benefits could off-set the lack of HTP fabrication possibilities presented by PVD and open up the possibility to robust fabrication of truly low-cost gold conductive electrodes for paper electronic applications. Acknowledgements The SalWe IMO programme (Project number: 648/10) funded by the Finnish Funding Agency for Technology and Innovation (TEKES) and the Academy of Finland, the Center of Excellence Programme (project number 141115) are acknowledged for financial support. In addition, the authors want to thank the laboratory of analytical chemistry at Åbo Akademi University for providing access to their facilities to perform the electrochemical experiments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2014.12.156. References [1] D. Tobjörk, R. Österbacka, Paper electronics, Adv. Mater. 23 (2011) 1935–1961. [2] P. Ihalainen, A. Määttänen, J. Järnström, D. Tobjörk, R. Österbacka, J. Peltonen, Influence of surface properties of coated papers on printed electronics, Ind. Eng. Chem. Res. 51 (2012) 6025–6036. [3] J.L. Delaney, C.F. Hogan, J. Tian, W. Shen, Electrochemical detection for paperbased microfluidics, Anal. Chem. 81 (2009) 5821–5826. [4] A. Apilux, W. Dungchai, W. Siangproh, N. Praphairaksit, C.S. Henry, O. Chailapakul, Lab-on-paper with dual electrochemical/colorimetric detection for simultaneous determination of gold and iron, Anal. Chem. 82 (2010) 1727–1732. [5] Z. Nie, C.A. Nijhuis, J. Gong, X. Chen, A. Kumachev, A.W. Martinez, M. Narovlyansky, G.M. Whitesides, Electrochemical sensing in paper-based microfluidic devices, Lab Chip 20 (2010) 477–483. [6] L. Ge, J. Yan, X. Song, M. Yan, S. Ge, J. Xu, Three-dimensional paper-based electrochemiluminescene immunodevice for multiplexed measurement of biomarkers and point-of-care testing, Biomaterials 33 (2012) 1024–1031. [7] Y. Wu, P. Xue, Y. Kang, K.M. Hui, Paper-based microfluidic electrochemical immunodevice intergrated with nanobioprobes onto graphene film for ultrasensitive multiplexed detection of cancer biomarkers, Anal. Chem. 85 (2013) 8661–8668. [8] J. Lessing, A.C. Glavan, S.B. Walker, C. Keplinger, J.A. Lewis, G.M. Whitesides, Inkjet printing of conductive inks with high lateral resolution on omniphobic “RF paper” for paper-based electronics and MEMS, Adv. Mater. 26 (2014) 4677–4682. [9] R.F.P. Martins, A. Ahnood, N. Correia, L.M.N.P. Pereira, R. Barros, P.M.C.B. Barquinha, R. Costa, I.M.M. Ferreira, A. Nathan, E.E.M.C. Fortunato, Recyclable, flexible, low-power oxide electronics, Adv. Funct. Mater. 23 (2013) 2153–2161. [10] A.D. Mazzeo, W.B. Kalb, L. Chan, M.G. Killian, J.-F. Bloch, B.A. Mazzeo, G.M. Whitesides, Paper-based, capacitive touch pads, Adv. Mater. 24 (2012) 2850–2856. [11] R. Martins, A. Nathan, R. Barros, L. Pereira, P. Barquinha, N. Correia, R. Costa, A. Ahnood, I. Ferreira, E. Fortunato, Complementary metal oxide semiconductor technology with and on paper, Adv. Mater. 23 (2011) 4491–4496. [12] L. Pereira, D. Gaspar, D. Guerin, A. Delattre, E. Fortunato, R. Martins, The influence of fibril composition and dimension on the performance of paper gated oxide transistors, Nanotechnology 25 (2014), 094007-1–094007-11. [13] R. Martins, I. Ferreira, F. Fortunato, Electronics with and on paper, Phys. Status Solidi RRL 5 (2011) 332–335. [14] A. Määttänen, U. Vanamo, P. Ihalainen, P. Pulkkinen, H. Tenhu, J. Bobacka, J. Peltonen, Low-cost paper-based inkjet-printed platform for electrochemical analysis, Sens. Actuators B 177 (2013) 153–162. [15] A. Määttänen, P. Ihalainen, R. Bollström, S. Wang, M. Toivakka, J. Peltonen, Enhanced surface wetting of pigment coated paper by UVC irradiation, Ind. Eng. Chem. Res. 49 (2010) 11351–11356. [16] D. Gasper, S.N. Fernandes, A.G. de Oliveira, J.G. Fernandes, P. Grey, R.V. Pontes, L. Pereira, R. Martins, M.H. Godinho, E. Fortunato, Nanocrystalline cellulose applied simultaneously as the gate dielectric and the substrate in flexible field effect transistors, Nanotechonolgy 25 (2014), 094008-1–094008-11. [17] H. Koga, T. Saito, T. Kitaoka, M. Nogi, K. Suganuma, A. Isogai, Transparent, conductive, and printable composites consisting of TEMPO-oxidized nanocellulose and carbon nanotube, Biomacromolecules 14 (2013) 1160–1165. [18] L.Y. Shiroma, M. Santhiago, A.L. Gobbi, L.T. Kubota, Separation and electrochemical detection of paracetamol and 4-aminophenol in a paper-based microfluidic device, Anal. Chim. Acta 725 (2012) 44–50.

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