- Email: [email protected]
Development of a novel highly conductive and flexible cotton yarn for wearable pH sensor technology
Accepted Manuscript Title: Development of a novel highly conductive and flexible cotton yarn for wearable pH sensor technology Authors: Rachel E. Smith, Stella Totti, Eirini Velliou, Paola Campagnolo, Suzanne M. Hingley-Wilson, Neil I. Ward, John R. Varcoe, Carol Crean PII: DOI: Reference:
S0925-4005(19)30114-5 https://doi.org/10.1016/j.snb.2019.01.088 SNB 26014
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4 October 2018 20 December 2018 17 January 2019
Please cite this article as: Smith RE, Totti S, Velliou E, Campagnolo P, Hingley-Wilson SM, Ward NI, Varcoe JR, Crean C, Development of a novel highly conductive and flexible cotton yarn for wearable pH sensor technology, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.01.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Development of a novel highly conductive and flexible cotton yarn for wearable pH sensor technology
Rachel E. Smith a, Stella Tottib, Eirini Vellioub, Paola Campagnolo c, Suzanne M. HingleyWilsond, Neil I. Warda, John R. Varcoea and Carol Crean*a a
Department of Chemistry, University of Surrey, GU2 7XH, United Kingdom.
b
IP T
Department of Chemical and Process Engineering, University of Surrey, GU2 7XH, United Kingdom. c
d
SC R
Department of Biosciences and Medicine, University of Surrey, Guildford, GU2 7XH, United Kingdom. Department of Microbial Sciences, University of Surrey, GU2 7XH, United Kingdom.
U
Email: [email protected]
M
A
pH sensor fabricated from conductive cotton yarns and polyaniline Highly conductive, flexible cotton yarns from PEDOT:PSS and carbon nanotubes Rapid and selective in artificial sweat with fabricated quasi-reference electrode Antibacterial and biocompatible properties Feasibility of sensing platform for real-time wound and skin analysis.
TE D
N
Highlights
Abstract
The simple and effective approach of “dipping and drying” cotton yarn in a dispersion of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) and multi-walled
EP
carbon nanotubes (MWCNT) resulted in the development of a highly conductive and flexible cotton fibres. Subsequent polyaniline (PANi) deposition yielded electrodes with significant
CC
biocompatible and antibacterial properties that could be fabricated (alongside quasi-reference electrodes) into solid-state wearable pH sensors, which achieve rapid, selective, and Nernstian responses (-61 ± 2 mV pH-1) over a wide pH range (2.0 – 12.0), even in a pH-
A
adjusted artificial sweat matrix. This development represents an important progression towards the realisation of real-time, on-body, wearable sensors. Keywords: wearable sensor, pH, PEDOT:PSS, carbon nanotubes, conductive cotton, quasireference electrode
1. Introduction 1
The growing interest in flexible and wearable electronics has inspired the development of fibre-based electrodes.1 These are lightweight and allow easy integration into textiles, presenting a promising platform for wearable devices. 2 Typical applications for fibre-based electrodes include the following wearable technologies: antennas, photovoltaic cells, batteries, supercapacitors and sensors. 3 Conducting fibres can be made using wet-spinning, dry-spinning and fibre extrusion techniques.4 The coating of cotton yarns with conductive materials is an emergent and effective method for making
IP T
fibre-based electrodes. Cotton is a natural, sustainable yarn that is popular for the fabrication of fabrics.1 Conducting materials can be deposited onto cotton yarns using
a variety of methods including dip-coating, drop casting, inkjet and screen printing, and
SC R
chemical vapour deposition.5,6
Only a few studies have reported the use of a conductive cotton substrate in sensing
platforms.
Poly(3,4-ethylenedioxythiophene)-poly(styrene
sulfonate)-
(PEDOT:PSS)-coated cotton was used for stress-strain monitoring7 and acetone
U
vapour detection,4 whilst carbon nanotube-(CNT)-modified cotton was applied to pH,
N
K+ and NH4+ sensing,8 and to optical-based immunoassays.9,10 To the best of our knowledge, few examples report on conducting cotton threads with both low enough
A
electrical resistances that they can undergo electrodeposition, and the necessary
M
flexibility required for wearable applications.11,12
Here we describe highly conductive cotton coatings that permit deposition of pH sensitive polyaniline (PANi), enabling the real-time monitoring of skin pH. Epidermal
TE D
pH gives an indication of the physiological condition of the skin and the state of wound healing. A wearable skin pH-sensor targeted at skin and wound monitoring would both aid and lower the cost of treatment.13 PANi is a pH sensitive polymer that has been used in many pH sensors.14,15 We simplified pH sensor fabrication by modifying cotton
EP
yarns using a straightforward “dipping and drying” process: three conductive dispersions were studied alongside optimisation of key variables (e.g. the number of
CC
coating cycles used). The combination of the flexibility of cotton yarns with the conductive properties of multi-walled carbon nanotubes (MWCNT) and PEDOT:PSS yielded
conductive,
flexible
cotton
threads.
After
successful
PANi
A
electropolymerisation, cotton-based sensors were created that were effective in solidstate pH analysis. In this study, we reveal a wearable pH sensor that can be applied over a wide pH range (2.0 - 12.0 vs. a wearable Ag/AgCl quasi-reference electrode), even when applied to a pH-adjusted artificial sweat matrix.16 Additionally, these biocompatible composite yarns show evidence of significant antibacterial properties.
2
2. Experimental 2.1 Materials PEDOT:PSS dispersion (Clevios PH 1000, 1 wt.%) was procured from Heraeus (Germany). Analytical grade of ethylene glycol (EG), dimethyl sulfoxide (DMSO), boric acid, phosphoric acid, acetic acid, sodium hydroxide, ammonium chloride, magnesium chloride, pyruvic acid, glucose, urea, calcium carbonate, potassium nitrate and sodium nitrate, Escherichia coli, and nutrient broth were all purchased from Sigma Aldrich (UK).
IP T
The gauze fabric (100% cotton) and cotton yarns (100%) were purchased from a local pharmacy. Thin MWCNTs (95% purity) were purchased from Nanocyl (Belgium).
Ag/AgCl paste (C2130809D5) was obtained from Sun Chemicals ® (Parsippany, NJ).
SC R
An E. coli wild-type strain constitutively expressing yellow fluorescent protein (YFP;
Balaban et al.17) was used in these experiments. Epofix resin was purchased from Struers® (Denmark). Alamar Blue® was purchased from ThermoFisher Scientific (UK).
U
2.2 Instrumentation
N
Electrical resistance was measured across 1 cm of each fibre using a multi-meter. Scanning electron microscopy (SEM) images were obtained using a JEOL USA JSM-
A
7100F field emission electron microscope. Dynamic mechanical analysis (DMA) was
M
carried out on a TA Instruments Q800. Fluorescence intensity measurements during antibacterial tests were performed on an A1+ confocal Nikon microscope (Nikon A1M
used
for
the
measurements.
TE D
on Eclipse Ti-E). An EDAQ potentiostat (EA163) combined with an e-corder (401) was PANi
electropolymerisation
and
potentiometric
(open
circuit)
Raman spectroscopy was performed using a Renishaw inVia confocal Raman
EP
microscope. The following conditions were used for the single spectra acquisitions: 532 nm laser (50 mW total power), 1 s of sample photobleaching followed by application of 1% laser power for 5 s; 16 acquisitions were collected and combined. The following
CC
conditions were used for the Raman mapping: 785 nm laser (300 mW total power), 1% laser power for 5 s for 1 acquisition. A sample stage step size of 6 µm by 6 µm was
A
used with 3,538 spectra in total being collected across the cross-sectional area. The PANi-coated PEDOT-MWCNT cross-sections were prepared by mounting the yarns in Epofix resin (Struers®) and polishing to a 1 µm diamond finish using a semi-automatic polishing system (MetPrep Saphir 520, Coventry, UK).
2.3 Fabrication of conductive cotton fibres 3
The cotton yarns were dip coated in three different dispersions by straightforward “dipping and drying”: Dispersion 1: The cotton yarns were submerged in PEDOT:PSS ink (2 wt.%) for 1 min before being dried at room temperature for 30 min. This constitutes one dip cycle, which was repeated up to 12 times. The resulting dip-coated PEDOT:PSS-cotton yarns were then treated in either: (1) DMSO, (2) EG, or (3) DMSO directly followed by EG. For solvent treatment (1) and (2), the yarns were immersed in the solvent for 2 h
IP T
at room temperature with subsequent drying at 140 °C for 1 h. For solvent treatment (3), the PEDOT:PSS-cotton yarns were treated in DMSO for 2 h, dried at 140 °C for 1
h, treated in EG for 2 h, and finally were dried at 140 °C for 1 h. Treatment times of 2 h
SC R
were adopted due to previous research investigation the solvent treatment of wet-spun
PEDOT:PSS fibres (20-25 m), in which 5 min was the optimal treatment time. 18 The coated cotton yarns are thicker (700-750 m) and to ensure optimal electrical properties, a longer treatment time was chosen to remove PSS.
MWCNT to an aqueous solution of 10 mg ml-1 sodium dodecylbenzenesulfonate
N
1
U
Dispersion 2: MWCNT dispersion (0.3 wt.%) was prepared by adding 3 mg ml(SDBS) followed by tip-sonication for 30 min (500 W, f = 20 kHz, 30% amplitude).8,19
A
The cotton yarns were then submerged in the MWCNT dispersion for 20 s, dried at
M
room temperature for 15 min, washed with distilled water, and then dried again. This constitutes a one dip cycle, which was performed up to 70 times. Dispersion 3: A mixed PEDOT-MWCNT dispersion containing 0.5 wt.%
TE D
PEDOT:PSS and 1.5 wt.% MWCNT was prepared using tip sonication (conditions as before).20 The cotton yarns were submerged in the PEDOT-MWCNT dispersion for 1 min and were then dried at room temperature for 15 min. This constitutes a one dip
EP
cycle, which was performed up to 10 times.
2.4 Antibacterial testing
CC
The bacterial cells were picked from a Luria Bertani agar plate, then grown in liquid culture with agitation at 300 rpm for 16 h at 37 °C. The cells were diluted to obtain a final cell density of 1.28 x 108 cells ml-1. A 300 µl aliquot of the cell-containing media
A
(pH 6.8 - 7.2) was added to 5 mm lengths of each of the coated cotton yarn types in a 96-well plate. The well plate was incubated and shaken at 150 rpm at 37 °C for 4 h. After this period, each “biofouled” fibre was transferred to a clean well (containing no solution or bacilli), and images on a Nikon confocal microscope using the Fluorescein isothiocyanate (FITC) filter at 488 nm and using a 20× air objective lens with a
4
numerical aperture of 0.8. The mean intensity of fluorescence was measured across a fixed area of 0.217 mm2. 2.5 Cell viability testing The PEDOT-MWCNT-cotton yarns (5 mm lengths, bare and PANi-coated) were sterilised via dipping in ethanol for 3 h, air-drying and 10 min in a UV/ozone generator (BioForce Nanosciences, USA). The human keratinocyte cells (HaCaT, 300493, CLS,
IP T
Germany) were seeded at a density of 104 cells per well in a 96-well plate and incubated in a humidified atmosphere of 5% CO2 and 20% O2 at 37 C for 24 h to allow adherence
to the well surface. After this period, the fibres were then placed on top of the cells. The
SC R
samples were incubated with Alamar Blue® dye (diluted in the cell culture medium) for 1.5 h at different culture time points after fibre administration on the cells: 1, 3 and 5 d. The absorbance was measured with a spectrophotometer (Synergy, BioTek, USA) at 570 nm and at 600 nm (reference wavelength). Variance analysis (two-way ANOVA)
U
was performed using Graph Pad Prism® software with a p-value threshold 0.05 to
N
evaluate whether there was a statistical difference between the experimental
A
conditions.
M
2.6 Fabrication and testing of the pH-sensitive fibre electrodes Initial studies depositing Polyaniline (PANi) onto fibre electrodes by dip-coating (dispersed nanofibers) did not produce stable electrodes. During analysis the PANi-
TE D
coating delaminated producing inaccurate readings. For this reason electrochemical deposition was used to coat fibre electrodes with PANi. PANi was coated onto fibre electrodes using potentiostatic polymerisation in a solution of aniline (0.1 M) in HNO3 (1.0 M). A three-electrode set-up was used: a Pt-mesh counter electrode (1 cm2), a
EP
Ag/AgCl-reference electrode (RE, 3.0 M KCl internal solution, BASi, USA) and the coated cotton yarn fibre under test as the working electrode. A potential of 0.80 V (vs.
CC
Ag/AgCl RE) was applied for either 180 s, 300 s or 600 s duration. Potentiometry for pH analysis measured the potential of 1 cm of the coated
cotton fibre electrodes vs. a Ag/AgCl double junction reference electrode (3.0 M NaCl
A
internal solution, BASi, USA). The interference studies of the PANi-coated PEDOTMWCNT-cotton yarns were performed using the method reported by Nyein et al.: aqueous solutions of 1 mM NH4Cl, 1 mM MgCl2, 1 mM CaCO3, 8 mM KNO3 and 20 mM NaNO3 were subsequently added to a pH 4 Britton-Robinson buffer solution.16 On addition of an interference solution, a 20 s waiting period was applied before the voltage was measured. At the end of the study, the solution was changed to a pH 5 buffer 5
solution before the voltage was re-measured. Artificial sweat matrix was prepared using the same method as reported by Parilla et al.21 A wearable Ag/AgCl quasi-reference electrode comprised of 0.2 g of Ag/AgCl paste that was applied to both sides of 1 cm2 of gauze fabric followed by curing (80 °C for 20 min). A two-electrode set-up was used with the potential of the PANi-coated PEDOT-MWCNT-cotton sensing electrode recorded against the Ag/AgCl quasireference electrode. During analysis, a 200 l drop of pH buffer (12.0 to 2.0; maximum
3. Results and discussion
Dispersion 1: Conductive PEDOT:PSS Coated Cotton
SC R
3.1 Development and characterisation of conductive cotton
IP T
range of Britton-Robinson buffering system) was placed between the two electrodes.
Previous studies have reported that treating PEDOT:PSS with solvents, such as, ethylene glycol (EG) and/or DMSO can reduce resistance, 14 due to the removal of
U
insulating PSS material from the PEDOT:PSS coating and re-ordering of the polymer
N
chains.22,23 Sankar et al. soaked cotton fibres in a DMSO solution containing PEDOT:PSS to give a resistance of 77 Ω cm -1.11 Alamer et al. used a similar solution
A
and repeated drop casting (20 coats) onto the cotton to achieve a very low resistance
M
of 1.58 Ω sq-1.24,25While these resistances are some of the lowest reported, such high PEDOT concentrations typically lead to brittle materials (note the mechanical properties
TE D
were not reported). The method below presents a novel yet simple approach of coating cotton yarns with PEDOT:PSS solution (dispersion 1). Measurements showed a significant reduction in resistance with each dip cycle (Fig. 1(a)). The lowest resistance achieved without solvent post-treatment was 150 ± 34 Ω cm-1 after the maximum 12
EP
dip cycles tested.
The three solvent post-treatments (EG, DMSO, and the DMSO-EG combination) succeeded in reducing the resistance of PEDOT:PSS coated cotton. The EG treatment
CC
led to lower resistances than the DMSO treatment, while the combined DMSO-EG treatment yielded the lowest resistances (Fig. 1(a)). Single solvent treatment of the
A
fibres made using 12 dip cycles had a lower effect on resistance than when using fibres made with lower number of dip cycles: the fibres made using 12 dip cycles showed only a minor change on DMSO treatment (150 ± 34 Ω cm-1 to 144 ± 35 Ω cm-1), while EG treatment yielded resistance values of 105 ± 32 Ω cm-1. In contrast, the combined DMSO-EG treatment lowered the resistance of the 12-dip coated cotton to 23 ± 10 Ω cm-1. As the resistance values for the 11 dip cycle fibres did not differ significantly after
6
with combined DMSO-EG treatment (21 ± 13 Ω cm-1), 11 dip cycles was used in the further analyses (as the percolation threshold has been reached). Dispersion 2: Conductive MWCNT-Coated Cotton Fig. 1(b) shows the resistances of the MWCNT-coated cotton fibres after each dip cycle, which rapidly decrease up to dip cycle 36 (101 ± 15 Ω cm-1); the resistances decrease more slowly (ca. 5 Ω cm-1 per dip) after this. The lowest resistance achieved
IP T
(55 ± 5 Ω cm-1) was after the maximum 70 dips tested, which is low enough to facilitate PANi electrodeposition. However, the use of 70 dip cycles is not industrially useful in
SC R
terms of material synthesis. Dispersion 3: Conductive PEDOT:PSS:MWCNT-Coated Cotton
The PEDOT:PSS-only cotton fibres (made from dispersion 1) had a poor tensile strength (see below), which is an issue that needs to be overcome prior to use in
U
wearable sensors. In contrast, the MWCNT-modified cotton fibres (dispersion 2) was
N
flexible and strong but required a very high number of dip cycles (70) in order to achieve low enough resistances. In response to these findings, a mixed dispersion containing
A
PEDOT:PSS and MWCNT was used to coat the cotton yarns to overcome these
M
individual limitations. The PEDOT-MWCNT-coated cotton fibres exhibited impressive flexibilities (vide infra) and high conductivities. As shown in Fig. 1(c), a resistance of 55 ± 28 Ω cm-1 was achieved after only 10 dip cycles (cf. MWCNT-only system yielding 55
TE D
± 5 Ω cm-1 after 70 dip cycles).
Scanning Electron Microscopy (SEM) SEM shows how coating cotton threads with the conductive inks alters their surface
EP
morphologies (Fig. 2). The PEDOT:PSS-cotton (dispersion 1) clearly shows the presence of a polymer layer around the cotton thread (Fig. 2(b) and 2(c)). Single solvent
CC
(EG and DMSO) treatment yielded PEDOT:PSS-cotton fibres with more ridges than the untreated PEDOT:PSS-cotton fibres. This was due to PSS removal, which leads to a change in surface morphology as previously reported (Fig. 2(d) and 2(e)).26,27,28The
A
PEDOT:PSS-cotton fibres treated with combined DMSO-EG displayed the most ridged surface (Fig. 2(f)) due to the highest level of PSS removal. These observations correlate to findings by Kim et al. who reported a significant decrease in thickness of PEDOT:PSS films after EG treatment.29 The PEDOT-MWCNT-cotton fibres (dispersion 3) have smooth surfaces (Fig. 2(h)). In contrast, the MWCNT-cotton fibres (dispersion 2) have rougher surfaces where 7
the individual cotton fibres in the yarn can still be seen (Fig. 2(g)). This was due to a combination of the large number of drying and washing steps (70 dips) and because the MWCNT were absorbed into the cotton fibrils as opposed to forming a surface coating.
Mechanical Strength Dynamic mechanical analysis (DMA) was performed on the conductive cottons as an
IP T
initial assessment of their ability to withstand general wear-and-tear (resulting data is presented in Table S1 and Fig. S1). The MWCNT-modified cotton yarns (70 dips) achieved the highest ultimate tensile strengths (219 ± 35 MPa, 13 ± 3% strain at break),
SC R
as anticipated.5 The final PEDOT:PSS-cotton yarns (11 dips, combined DMSO-EG
treated) showed the lowest ultimate tensile strengths (62 ± 12 MPa, 7 ± 2% strain at break). The PEDOT-MWCNT-cotton yarns showed intermediate values (130 ± 25 MPa,
U
10 ± 2% strain at break) whilst maintaining flexibility.
N
Raman Characterisation
The spectra of the PEDOT:PSS-cotton yarns showed notable changes after the
A
different solvent treatments (Fig. 3(a)). The peaks at 1580 and 1600 cm-1 (PSS-derived
M
aromatic ring chain vibrations)30,31 were less defined in the spectra of the solvent treated fibres compared to the untreated benchmark, suggesting that the PSS content had been reduced. The peaks at 1259 and 1367 cm -1 are attributed to Cα-Cα inter-ring
TE D
stretching and Cβ-Cβ stretching vibrations, respectively (PEDOT component).32 The large peak between 1400 and 1500 cm-1 is due to the symmetric stretching of the fivemembered thiophene ring (PEDOT component). This ring can be present as either the benzoid (Cα=Cβ) or quinoid (Cα-Cβ) resonant structure depending on the doping level
EP
of the polymer. This PEDOT-diagnostic peak shifts from 1436 cm-1 in the untreated fibres to 1425 cm-1 after solvent treatment, a shift that has been attributed to a
CC
conformational change of the PEDOT chains (from a coiled benzoid structure to a more linear quinoid structure), which leads to an enhancement in the mobility of the charge carriers in the PEDOT:PSS coating.33 A similar shift has been previously observed for
A
reduced dopant concentration in PEDOT:PSS,34 suggesting that DMSO reduces the PSS content leading to the observed decrease in resistance (Fig. 1(a)). For the PEDOT-MWCNT yarn, this PEDOT-diagnostic peak shifted to 1445 cm1
(Fig. 3(b)), indicating that the PEDOT is in the benzoid (Cα=Cβ) form (recall this yarn
was not subjected to solvent post-treatment). As expected, the spectrum of the PEDOT-MWCNT-cotton fibres was similar to that of the PEDOT:PSS-cotton, but with 8
the addition of the MWCNT-derived D and G bands. The spectra of the MWCNT-cotton and PEDOT-MWCNT yarns both show a D band at 1350 cm-1, whilst the G band appears at 1588 cm-1 for MWCNT-cotton and 1583 cm-1 for PEDOT-MWCNT-cotton. For the PEDOT-MWCNT-cotton fibres, the PEDOT-derived Cβ-Cβ symmetrical peak at 1368 cm-1 is hidden behind the MWCNT-derived D band (1350 cm-1) and the Cα-Cα inter-ring stretching peak (1259 cm-1) is not observed above the strong Raman scattering of the CNTs (similarly observed by Mannayil et al. when using a high CNT
IP T
density in PEDOT:PSS-CNT dispersions)35.
3.2 Development of pH Sensor
SC R
PANi was deposited onto each of the three different conductive cotton yarns by electropolymerisation to make the fibres pH sensitive. The deposition of PANi was
confirmed by SEM, Raman spectroscopy and cyclic voltammetry (Figs. S2, S3 and S4, respectively). The SEM micrograph in Fig S2 clearly shows the edge of the PANi film
U
which has been electrodeposited onto the fibre. The Raman spectra of the PANi-coated cotton substrates exhibit typical PANi bands at 1617, 1593, 1482, 1336, 1221 and 1163
N
cm-1, which have been assigned to C-C ring stretching (benzenoid segment), C=C ring
A
stretching (quinone segment), C=N and CH=CH stretching (quinone segment), C-N+.
M
stretching (semiquinone radical cation segment), C-N stretching (benzenoid), and C-H in-plane bending (quinone segment), respectively.36 Slight variations in Raman peak positions of PANi are observed depending on which conductive cotton was coated.
TE D
Raman mapping was performed on a cross-section of a PANi-coated PEDOTMWCNT-cotton yarn to identify whether the coating was solely on the outside of the thread, or whether the PANi had infiltrated into the core of the yarn. The spectra collected with the 785 nm laser showed a stronger intensity 1446 cm-1 peak (PEDOT
EP
Cα=Cβ stretch) than with the use of the 532 nm laser (Fig. 4(a)), and was strong enough to map PEDOT. The intensity of the peaks at 1162 cm-1 (PANi: C-H in-plane bending
CC
of the quinoid rings) and 1446 cm-1, were used to map the location of PANi and PEDOT in the cross-section (Fig. 4(b)). While some PANi is evident in the core of the fibre, the majority of the PANi is located as a coating on the outside of the yarn. The map of the
A
1446 cm-1 peak shows a clear coating of PEDOT on the outer part of the thread along with large zones of PEDOT within the cotton core. This suggests that the PEDOTMWCNT ink, which has strong interactions with the cellulose and natural polysaccharides in the cotton, has penetrated into the yarn in addition to providing an outer coating. As a result, the inside of the cotton yarn was conductive with electropolymerised PANi coating individual fibrils inside the yarn core. 9
Cyclic voltammograms (CVs) of the PANi-coated conductive cottons show variations in the PANi redox peak potentials depending on which fibre electrode was coated (Fig. S4). Oxidation peaks at 0.35 V, 0.60 V and 0.87 V vs. Ag/AgCl were observed for PANi coated on (DMSO-EG solvent treated) PEDOT:PSS-cotton, the most conductive of the coated cotton fibres. The first oxidation at 0.35 V vs. Ag/AgCl corresponds to the conversion from the leucoemeraldine base to emeraldine salt. The peak at 0.60 V vs. Ag/AgCl has been attributed to the dimer intermediate, p-
IP T
aminodiphenylamine, which is found at ~0.70 V vs. Ag/AgCl.37 The third peak at 0.87 V relates to the oxidation of emeraldine salt to pernigraniline salt. The CVs for PANiMWCNT-cotton and PANi-PEDOT-MWCNT-cotton are dominated by the capacitive
SC R
properties of the MWCNT. Two oxidation peaks are observed above the capacitive
background at 0.21 V and 0.61 V vs. Ag/AgCl for PANi-MWCNT-cotton, and a single oxidation peak at 0.56 V vs. Ag/AgCl for the PANi-PEDOT-MWCNT-cotton, which are similar to previously observed redox couples recorded with PANi-CNTs.38,39These two
U
fibre electrodes are more resistive than PEDOT:PSS-cotton and may result in less
N
conductive polyaniline being formed on the fibre electrodes. This also correlates with the Raman spectra (Fig. S3) which exhibit peaks typical of those previously reported
A
for oligoanilines.40 The peak at 1328 cm-1 in PANi-PEDOT:PSS-cotton is typical for
M
normal PANi and corresponds to the C-N+. vibration of delocalised polaronic structures. This peak is blue shifted when looking at the spectra for PANi-PEDOT-MWCNT-cotton and merges with the D-band of MWCNT (1346 cm-1) for PANi-MWCNT-cotton (Fig. S3),
TE D
suggesting a relatively lower extent of polaron delocalisation in PANI on these electrodes and is in accordance with a lower conductivity of PANI.40 The pH sensitivity of the PANi-coated conductive cotton materials was assessed against a double-junction Ag/AgCl RE (Fig. 5). The PEDOT:PSS-cotton substrate gave
EP
the poorest pH sensitivity (-22 ± 9 mV pH-1), suspected to be due to swelling of the hydrophilic PSS affecting the resistance of the electrode and resulting in a poor quality
CC
PANi film. The PANi-coated MWCNT-cotton and PEDOT-MWCNT-cotton electrodes showed improved pH sensitivities with Nernstian responses (-61 ± 4 and -61 ± 2 mV pH-1, respectively) with little variation between measurements on repeated electrode
A
fabrications (Fig. 5, n = 3). The PANi-coated PEDOT-MWCNT-cotton-based electrode achieved low electrical resistance values, maintained flexibility, and yielded a Nernstian response to pH (-61 ± 2 mV pH-1) (Table 1); this superior electrode was taken forward for integration in a wearable pH sensor. The selectivity of the PANi-PEDOT-MWCNT-cotton sensor was studied by subsequently adding interfering ions (NH4+, Mg2+, Ca2+, K+ and Na+) at physiologically 10
relevant concentrations to a pH 4 buffer solution and monitoring the change in voltage (Fig. 6(a)). The voltage changes observed were negligible after each addition of the interfering ions and were significantly lower in magnitude to the voltage changes observed due to pH variations (Fig. 6(a)). This indicates that this electrode selectively measures solution pH in the presence of common physiological interferences. The PANi-PEDOT-MWCNT cotton yarn working electrode had a pH sensitivity of -61 ± 2 mV pH-1 (across the pH range 2.0 – 12.0) when tested in combination of a
IP T
wearable Ag/AgCl quasi-reference RE in a solid-state configuration (Fig. 6(b)). This is comparable to a previously developed pH sensor (electropolymerised PANi onto a CNT
wire) that gave a Nernstian response was over the pH range 1.0 - 13.0.41 A Nernstian
SC R
response was maintained with analyte volumes as small as 200 µl (Fig. 6(b)). The shift
in potential for this solid-state configuration is due to the change in RE (cf. the Ag/AgCl REs used in the solution tests). Response times of 60 ± 20 s were achieved, demonstrating the electrode’s capability for real-time wearable monitoring. The pH
U
response of the pH fibre sensor vs. the solid quasi-reference electrode was also tested
N
with pH-adjusted artificial sweat matrices (200 l) containing common sweat interferences (Fig. 6(b)): a response of -63 ± 3 mV pH-1 was observed. The y-intercept
A
shifted from 402 ± 42 mV to 467 ± 30 mV (n=3) with pH-adjusted buffered solutions
M
and artificial sweat matrices, respectively. The potential of the quasi-reference electrode (Ag/AgCl paste) is dependent on the Cl- concentration, thus we hypothesise that the high Cl- content in the artificial sweat matrices influenced the y-intercept. This
TE D
may be improved with the addition of a membrane (e.g. PVB) to protect the quasireference electrode.
Successful operation (a sensitive linear response) across the pH range 4.0 - 9.0 is essential for application in a pH-based wound monitoring system. Chronic wounds
EP
have a maximum pH of 9.0 and healthy, while unwounded skin has a minimum pH of 4.0.42,43 For application as an epidermal patch, this sensor needs to monitor skin pH
CC
over the critical range of 4.0 - 7.0.44
Biocompatibility and antibacterial properties
A
An initial cytotoxicity test was performed on the pH-sensitive fibres by exposure to cultured human keratinocytes cells. The fibres were placed on top of the skin cells and the cell viability was measured at days 1, 3 and 5 post fibre administration. A significant decrease (p < 0.05) was observed in the cell viability of the PEDOT-MWCNT fibres after 24 h: 63 ± 5% compared to 100 ± 10% for the control (seeded cells without fibres) (Fig. 7(a)). This would suggest potential cytotoxicity of the bare PEDOT-MWCNT fibres. 11
The toxicity of pristine CNT is well reported in the literature because it interacts with cell proteins, interferes with protein structure, and results in cell death.45 In contrast, the PANi-PEDOT-MWCNT-cotton fibres demonstrated no significant change in cell viability throughout the time of the test. As discussed, SEM and Raman imaging indicated that the PANi had enveloped the PEDOT-MWCNT-cotton fibres (Fig. S2 and Fig. 4(a)), acting as a protective casing, which maintains the viability of any cells in contact with the yarns. This is supported by previous research that has shown PANi to be
IP T
biocompatible.46,47 It is important that materials used to monitor wound pH do not encourage
bacterial growth. The antibacterial properties of the coated cotton yarns were evaluated
SC R
by using wild type E. coli expressing yellow fluorescent protein (YFP) as a model system.17 A suspension of bacterial cells was incubated for 4 h with the coated cotton
yarns after which the intensity of each yarn’s fluorescence (FITC, 488 nm) was measured. The mean fluorescence intensity was used as a quantified representation
U
of the extent of bacterial growth on the coated cotton yarns. Images of the fluorescent
N
bacteria grown on coated cotton yarns are shown in Fig. S5.
The PEDOT:PSS-cotton fibres (dual DMSO-EG treated) showed moderate
A
antibacterial activity (intensity = 259 ± 67 a.u.), compared to the bare cotton yarn, which
M
gave a benchmark intensity of 694 ± 93 a.u. (Fig. S6). The fluorescence images of the cotton yarns (Fig. S5) show that the bacterial cells were growing within the individual yarn fibres. The PEDOT:PSS surface coating on the cotton discouraged the growth or
TE D
attachment of bacteria. MWCNT-cotton fibres exhibited a lower mean fluorescence intensity (161 ± 4 a.u., Fig. S6) demonstrating higher antibacterial activity. This was expected as Kang et al. previously reported that MWCNTs are toxic to E. coli as they initiate the production of stress-related gene products.48 The lowest mean fluorescence
EP
intensity was achieved by the PEDOT-MWCNT-cotton fibres (64 ± 13 a.u.), which clearly discourages bacterial growth.48 PANi-PEDOT-MWCNT demonstrated a similar
CC
mean fluorescence intensity of 70 ± 7 a.u. (Fig. 7(b)), which shows that although PANi is biocompatible with skin cells, it does not diminish the antibacterial property of the PEDOT-MWCNT fibres. This highlights the potential for use of PANi-PEDOT-MWCNT-
A
cotton fibres in pH sensing wound dressings.
4. Conclusion A new pH sensing conductive flexible cotton-based electrode concept has been developed using a dispersion containing both PEDOT:PSS and MWCNT. These flexible PEDOT-MWCNT-cotton fibres showed low resistances (55 ± 28 Ω cm-1) 12
alongside significant antibacterial properties when coated with pH sensitive PANi. As proof-of-concept investigation, these PANi-coated PEDOT-MWCNT-cotton fibre electrodes were used as pH indicator electrodes in a solid-state sensor alongside using a Ag/AgCl quasi-reference electrode. This configuration is analogous to smart dressings for wound monitoring or an epidermal patch. This sensor achieved a rapid, selective, and Nernstian response (-61 ± 2 mV pH-1) over a wide pH range (2.0 – 12.0). After the necessary optimisation and validation, this sensor concept will help facilitate
IP T
development of a high performance, wearable platform for the future real-time monitoring of skin and wound pH.
SC R
5. Conflicts of interest There are no conflicts of interest to declare. 6. Acknowledgements
U
The authors thank the University of Surrey for providing funding for a studentship for
N
RES and the EPSRC for providing the capital funding that funded the Raman
A
CC
EP
TE D
M
A
microscope used in this work (EP/M022749/1).
13
7. Figures U n tre a te d
1000 -1
DM SO
R e s is ta n c e / c m
800
EG D M S O -E G
600
400
IP T
200
0 9
10
11
12
D ip c y c le
SC R
(a)
U
300
N
200
100
0 30
40
50
-1
800
70
(b)
EP
600
400
200
CC
R e s is ta n c e / c m
60
TE D
D ip c y c le
M
20
A
R e s is ta n c e / c m
-1
400
0
A
0
Fig.1
2
4
6
8
10
D ip c y c le
(c)
Resistance measurements on cotton yarns dip coated in (a) aqueous PEDOT:PSS
dispersions (2 wt.%) with no solvent post-treatment, and post-treatment in either ethylene glycol (EG), dimethyl sulfoxide (DMSO), and DMSO followed by EG in combination (DMSO-EG), (b) MWCNT, and (c) PEDOT-MWCNT dispersions. The error bars represent the standard deviation of measurements on n = 3 replicate yarns.
14
a.
b.
100 m
c.
SC R
IP T
100 m
d.
10 m
TE D
M
A
N
U
10 m
10 m
f.
10 m
100 m
h.
10 m
A
CC
EP
e.
g.
Fig. 2
Scanning electron microscopy (SEM) images of: (a) uncoated cotton; (b and c) PEDOT:PSS-
cotton (11 dips in dispersion 1) before solvent post-treatment; (d) DMSO-treated PEDOT:PSS-cotton; (e) EG-treated PEDOT:PSS-cotton; (f) dual DMSO-EG-treated PEDOT:PSS-cotton; (g) MWCNT-cotton (70 dips in dispersion 2), and (h) PEDOT-MWCNT-cotton (10 dips in dispersion 3).
15
C=C / C -C C -C
C -C
In te n s ity
U n tre a te d DMSO
IP T
EG D M S O -E G 1400
R a m a n s h ift / c m
1600 -1
SC R
1200
(a)
D
U
G & D'
N
MW CNT
A
In te n s ity
C -C
C -C
TE D
C -C
M
P E D O T -M W C N T
1200
1400
1600
-1
(b)
EP
R a m a n s h ift / c m
P E D O T :P S S
A
CC
Fig. 3 Raman spectra using a 532 nm laser of: (a) PEDOT:PSS-coated cottons after the different solvent post-treatments; (b) MWCNT-, PEDOT-MWCNT-, and PEDOT:PSS-cottons.
16
C -H
1163
C = C r in g ( Q )
C=N
(S Q R )
& C H = C H (Q )
C - C r in g ( B )
+.
C -N
C -N
(B )
(S Q R )
1593
C -C
1617
1482
1336 1418
P A N i- P E D O T - M W C N T 1417
1162 1221
1593
1610
532 nm
1331
P A N i- P E D O T - M W C N T
1446
785 nm 1400
1600
R a m a n s h ift / c m
-1
SC R
1200
IP T
In te n s ity
1221
(a)
Intensity of the peak at 1162 cm-1
(b)
Raman spectra of (a) polyaniline-coated PEDOT-MWCNT-cotton (using lasers 532 and
EP
Fig. 4
TE D
M
A
N
U
Intensity of the peak at 1446 cm-1
785 nm). (b) Raman cross-sectional interpolated maps using a 785 nm laser of PANi-coated PEDOT:MWCNT-cotton. The colour scale represents the intensity at the peaks at 1162 cm-1
A
CC
(PANi semi-quinone radical) and 1446 cm -1 (PEDOT Cα=Cβ).
17
PANi-MWCNT-cotton
-61 ± 4 mV pH-1
400
-1 PANi-PEDOT-MWCNT-cotton -61± 2 mV pH
300
PANi-PEDOT:PSS-cotton
-22 ± 9 mV pH-1
200 100 0
3
4
5
6
7
8
IP T
Voltage / mV vs. Ag/AgCl
500
pH
pH sensitivity of the PANi-coated conductive cotton yarns. The error bars represent the
A
CC
EP
TE D
M
A
N
U
standard deviation of measurement on n = 3 replicate fibres.
SC R
Fig. 5
18
+ +
+ 20 m M Na
2+
+ 8 mM K
+ 1 mM Ca
+ 1 mM Mg
260
2+
+
+ 1 m M NH4
pH 4
240 220
pH 5
200
IP T
V o lta g e /m V v s . A g /A g C l
280
180 0
5
10
15
20
25
30
35
T im e / m in
S k in
SC R
(a) W ounds
-1
-6 1 2 m V p H
-1
-6 3 3 m V p H
-1
N
200
-6 0 4 m V p H
U
400
0
A
-2 0 0
-4 0 0 2
4
6
8
pH
M
V o lta g e / m V v s . R E
600
10
12
TE D
2 0 .0 m L o f p H b u ffe re d s o lu tio n s ( v s . A g /A g C l D J E )
S o lid -s ta te 2 0 0 L o f p H b u ffe r e d s o lu tio n s ( v s . q u a s i- A g / A C l)
S o lid -s ta te 2 0 0 L o f p H -a d ju s te d a rtific ia l s w e a t m a trix
Fig. 6
EP
( v s . q u a s i- A g /A g C l)
(b)
Potentiometric (open circuit) response of pH analysis of PANi-PEDOT-MWCNT-cotton
CC
electrodes: (a) Selectivity when various sweat-related interferences (NH4+, Mg2+, Ca2+, K+ and Na+) were added to the pH 4 aqueous samples (20 ml). (b) Response in buffered solution to varying pH levels (vs. Ag/AgCl double junction electrode (DJE) and Ag/AgCl quasi-reference electrode) and in pH-
A
adjusted artificial sweat matrix (200 µl) (vs. Ag/AgCl quasi-reference electrode). The error bars represent the standard deviation of measurement on n = 3 replicate yarns
19
C o n tr o l P E D O T - M W C N T - c o tto n P A N i- P E D O T - M W C N T
100
75
50
25
IP T
C e ll v ia b ility / %
125
0 2
2
1
7
2
4
0
T im e / h r
SC R
1000
B a re C o tto n Y a rn P E D O T -M W C N T
800
P A N i- P E D O T - M W C N T
U
600
N
400
200
0
M
C o a te d C o tto n Y a rn s
A
F lu o r e s c e n c e In te n s ity
(a)
(a) Cell viability (measured absorbance, 570 nm) compared to control taken at three time
TE D
Fig. 7
(b)
points: 1, 3 and 5 d post sensor administration. Wells without sensors were used as controls and were considered 100% viable. Error bars represent the standard errors of three independent experiments. (b) Mean intensity of fluorescence (FITC, 488 nm) across a fixed area (0.217 mm 2) of bare cotton yarn,
EP
bare and PANi-coated PEDOT-MWCNT-cotton and Ag/AgCl-gauze biofouled with E. coli wild type strain expressing yellow fluorescent protein (YFP). Error bars represent the standard errors of three
A
CC
independent experiments.
20
Table 1 Summary data (mean ± sample std. dev.) for PEDOT:PSS-, MWCNT-, PEDOT-MWCNTtype cotton-based electrodes (n = 3). Solvent post-
Cotton yarn
treatment
DMSO and
PEDOT:PSS
MWCNT
Ultimate tensile strength
Mean Intensity of
pH sensitivity after
cycles
/ Ω cm
/ MPa
fluorescent bacteria
polyaniline coated
/ a.u.
/ mV pH-1
-1
21 ± 13
52 ± 5
259 ± 67
-22 ± 9
-
70
55 ± 5
226 ± 16
161 ± 4
- 61 ± 4
-
10
55 ± 28
134 ± 15
64 ± 13
SC R
PEDOT-
Resistance
11
EG
MWCNT
Dip
IP T
Coating on
References 1
- 61 ± 2
Y. Zhang and Y. Cui, Flexible and Printed Electronics, 2017, 2: e:042001. R. A. Street, A. C. Arias, Stretchable Electronics, Wiley-VCH Verlag GmbH & Co. KGaA, 2012, 355-378. 3 W. Zeng, L. Shu, Q. Li, S. Chen, F. Wang and X. M. Tao, Adv. Mater., 2014, 26, 5310-5336. 4 A. Mirabedini, J. Foroughi, S. Farajikah and G. G. Wallace, RSC Adv., 2016, 6, 44687-44716. 5 D. P. Hansora, N. G. Shimpi and S. Mishra, RSC Adv., 2015, 5, 107716-107770. 6 X. H. Zhong, Y. L. Li, J. M. Feng, Y. R. Kang and S. S. Han, Nanoscale, 2012, 4, 5614-5618. 7 Y. Q. Li, P. Huang, W. B. Zhu, S. Y. Fu, N. Hu and K. Liao, Sci. Rep., 2017, 7:e45013. 8 T. Guinovart, M. Parrilla, G. A. Crespo, F. X. Rius and F. J. Andrade, Analyst, 2013, 138, 52085215. 9 X. Jia, T. Song, Y. Liu, L. Meng and X. Mao, Anal. Chim. Acta., 2017, 969, 57-62. 10 L. L. Meng, T. T. Sing and X. Mao, Talanta, 2017, 167, 379-384. 11 A. Sankar and Mohanta, AIP Conf. Proc., 2016, 1731, 120019. 12 N. Liu, W. Ma, J. Tao, X. Zhang, J. Su, L. Lo, C. Yang, Y. Gao, D. Golberg and Y. Bando, Adv. Mater., 2013, 25, 4925-4931. 13 T. R. Dargaville, B. L. Farrugia, J. A. Broadbent, S. Pace, Z. Upton and N. H. Voelcker, Biosens. Bioelectron., 2013, 41, 30-42. 14 T. Guinovart, G. Valdés-Ramírez, J. R. Windmiller, F. J. Andrade and J. Wang, Electroanal., 2014, 26, 1345-1353. 15 A. Vacca, M. Mascia, S. Rizzardini, S. Corgiolu, S. Palmas, M. Demelas, A. Bonfiglio and P. C. Ricci, RSC Adv., 2015, 5, 79600-79606. 16 H. Y. Y. Nyein, W. Gao, Z. Shahpar, S. Emaminejad, S. Challa, K. Chen, H. M. Fahad, L. C. Tai, H. Ota, R. Davis and A. Javey, ACS Nano, 2016, 10, 7216-7224. 17 N. Q. Balaban, J. Merrin, R. Chait, L. Kowalik and S. Leibler, Science, 2004, 305, 1622-1625. 18 D. O. Reid, PhD thesis., University of Surrey, 2017. 19 L. Hu, M. Pasta, F. Mantia, L. Cui, S. Jeong and H. D. Deshazer, Nano Lett., 2010, 10, 708-714. 20 D. J. Yun, K. Hong, S. Kim, W. M. Yun, J. Jang, W. Kwon, C. Park and S. W. Rhee, ACS Appl. Mater. Interfaces, 2011, 3, 43-49. 21 M. Parrilla, J. Ferré, T. Guinovart and F. J. Andrade, Electroanal., 2016, 28, 1267-1275. 22 M. Jikei, T. Yamaya, S. Uramoto and K. Matsumoto, Int. J. Soc. Mater. Eng. Resour., 2014, 20, 158-162. 23 S. Lee, J. Nanosci. Nanotechnol., 2016, 16, 2880-2882. 24 F. A. Alamer, J. Alloys Compd., 2017, 702, 266-273. 25 F. A. Alamer (2015), Patent No. US20160258110, 2015, Mecca, SA: United States. 26 J. Ouyang, C. W. Chu, F. C. Chen, Q. Xu and Y. Yang, Adv. Funct. Mater., 2005, 15, 203-208. 27 Q. Wei, M. Mukaida, Y. Naitoh and T. Ishida, Adv. Mater., 2013, 25, 2831-2836. 28 J. Gasiorowski, R. Menon, K. Hingerl, M. Dachev and N. S. Sariciftci, Thin Solid Films, 536, 211-215. 29 D. J. Yun, J. H. Kim, S. H. Kim, M. Seol, D. Yu, H. Kwon, Y. Ham, J. Chung, Y. Kim and S. Heo, Nanotechnology, 2016, 27:e165706 30 T. R. Chou, S. H. Chen, Y. T. Chiang, Y. T. Lin and C. Y, Chao, J. Mater. Chem. C, 2015, 3, 3760-3766.
A
CC
EP
TE D
M
A
N
U
2
21
31
A
CC
EP
TE D
M
A
N
U
SC R
IP T
S. H. Chang, C.H. Chiang, F. S. Kao, C. L. Tien and C. G. Wu, IEEE Photonics Journal, 2014, 6, 8400307. 32 D. Yoo, J. Kim, S. H. Lee, W. Cho, H. H. Choi, F. S. Kim and J. H. Kim, J. Mater. Chem. A., 2015, 3, 6526-6533. 33 L. Zhang, H. Deng, S. Liu, Q. Zhang, F. Chen and Q. Fu, RSC Adv., 2015, 5, 105592-105599. 34 W. W. Chiu, J, Travas-Sejdic, R. P. Cooney, G. A Bowmaker, J. Raman Spectrosc, 2006, 37, 13541361. 35 J. Mannayil, S.M. Raman, J. Sankaran, R. Raman, J. M. K. Ezhuthachan, Phys. Status Solidi A, 2018, 215, 1701003. 36 C. Liu, J. Zhang, G. Shi and F. Chen, J. Appl. Polym. Sci, 2004, 92, 171-177. 37 G. A. Planes, J. L. Rodríguez, M. C. Miras, G. García, E. Pastor and C. A. Barbero, Phys. Chem. Chem. Phys., 2010, 12, 10584-10593. 38 L. Pilan, M. Raicopol, A. Pruna and V. Branzoi, Surf. Interface Anal., 2012, 44, 1198-1202. 39 L. Jin, Y. Jiang, M. Zhang, H. Li, L. Xiao, M. Li and Y. Ao, Sci. Rep., 2018, 8, 6268. 40 G. ́Ciric-Marjanovic,M. Trchov, J. Stejskal, J. Raman Spectrosc.2008, 39, 1375 – 1387. 41 N. Ferrer-Anglada, M. Kaempgen and S. Roth, Phys. Stat. Sol., 2006, 13, 3519-3523. 42 L. R. Bennison, C. N. Miller, R. J. Summers, A. M. B. Minnis, G. Sussman and W. McGuiness, Wounds Practise and Research, 2017, 25, 63-69. 43 C. R. Kruse, K. Nuutila, C. C. Lee, E. Kiwanuka, M. Singh, E. J. Caterson, E. Eriksson and J. A. Sørensen, Wound Repair Regen., 2015, 23, 459-464. 44 H. Lambers, S. Piessens, A. Bloem, H. Pronk and P. Finkel, Int. J. Cosmet. Sci., 2006, 28, 359370. 45 S. Y. Madani and A. M. Seifalian, Nano. Rev., 2013, 4, 1-14. 46 A. J. Bandodkar, V. W. S. Hung, W. Jia, G. Valdés-Ramírez, J. R. Windmiller, A. G. Martinez, J. Ramírez, G. Chan, K. Kerman and J. Wang, Analyst, 2013, 138, 123-128. 47 H. Lee, T. K. Choi, Y. B. Lee, H. R. Cho, R. Ghaffari, L. Wang, H. J. Choi, T. D. Chung, N. Lu, T. Hyeon, S. H. Choi and D. H. Kim, Nature Nanotechnol., 2016, 11, 566-572. 48 S. Kang, M. Herzberg, D. F. Rodrigues and M. Elimelech, Langmuir, 2008, 24, 6409-6413.
22
S0925-4005(19)30114-5 https://doi.org/10.1016/j.snb.2019.01.088 SNB 26014
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
4 October 2018 20 December 2018 17 January 2019
Please cite this article as: Smith RE, Totti S, Velliou E, Campagnolo P, Hingley-Wilson SM, Ward NI, Varcoe JR, Crean C, Development of a novel highly conductive and flexible cotton yarn for wearable pH sensor technology, Sensors and amp; Actuators: B. Chemical (2019), https://doi.org/10.1016/j.snb.2019.01.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Development of a novel highly conductive and flexible cotton yarn for wearable pH sensor technology
Rachel E. Smith a, Stella Tottib, Eirini Vellioub, Paola Campagnolo c, Suzanne M. HingleyWilsond, Neil I. Warda, John R. Varcoea and Carol Crean*a a
Department of Chemistry, University of Surrey, GU2 7XH, United Kingdom.
b
IP T
Department of Chemical and Process Engineering, University of Surrey, GU2 7XH, United Kingdom. c
d
SC R
Department of Biosciences and Medicine, University of Surrey, Guildford, GU2 7XH, United Kingdom. Department of Microbial Sciences, University of Surrey, GU2 7XH, United Kingdom.
U
Email: [email protected]
M
A
pH sensor fabricated from conductive cotton yarns and polyaniline Highly conductive, flexible cotton yarns from PEDOT:PSS and carbon nanotubes Rapid and selective in artificial sweat with fabricated quasi-reference electrode Antibacterial and biocompatible properties Feasibility of sensing platform for real-time wound and skin analysis.
TE D
N
Highlights
Abstract
The simple and effective approach of “dipping and drying” cotton yarn in a dispersion of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) and multi-walled
EP
carbon nanotubes (MWCNT) resulted in the development of a highly conductive and flexible cotton fibres. Subsequent polyaniline (PANi) deposition yielded electrodes with significant
CC
biocompatible and antibacterial properties that could be fabricated (alongside quasi-reference electrodes) into solid-state wearable pH sensors, which achieve rapid, selective, and Nernstian responses (-61 ± 2 mV pH-1) over a wide pH range (2.0 – 12.0), even in a pH-
A
adjusted artificial sweat matrix. This development represents an important progression towards the realisation of real-time, on-body, wearable sensors. Keywords: wearable sensor, pH, PEDOT:PSS, carbon nanotubes, conductive cotton, quasireference electrode
1. Introduction 1
The growing interest in flexible and wearable electronics has inspired the development of fibre-based electrodes.1 These are lightweight and allow easy integration into textiles, presenting a promising platform for wearable devices. 2 Typical applications for fibre-based electrodes include the following wearable technologies: antennas, photovoltaic cells, batteries, supercapacitors and sensors. 3 Conducting fibres can be made using wet-spinning, dry-spinning and fibre extrusion techniques.4 The coating of cotton yarns with conductive materials is an emergent and effective method for making
IP T
fibre-based electrodes. Cotton is a natural, sustainable yarn that is popular for the fabrication of fabrics.1 Conducting materials can be deposited onto cotton yarns using
a variety of methods including dip-coating, drop casting, inkjet and screen printing, and
SC R
chemical vapour deposition.5,6
Only a few studies have reported the use of a conductive cotton substrate in sensing
platforms.
Poly(3,4-ethylenedioxythiophene)-poly(styrene
sulfonate)-
(PEDOT:PSS)-coated cotton was used for stress-strain monitoring7 and acetone
U
vapour detection,4 whilst carbon nanotube-(CNT)-modified cotton was applied to pH,
N
K+ and NH4+ sensing,8 and to optical-based immunoassays.9,10 To the best of our knowledge, few examples report on conducting cotton threads with both low enough
A
electrical resistances that they can undergo electrodeposition, and the necessary
M
flexibility required for wearable applications.11,12
Here we describe highly conductive cotton coatings that permit deposition of pH sensitive polyaniline (PANi), enabling the real-time monitoring of skin pH. Epidermal
TE D
pH gives an indication of the physiological condition of the skin and the state of wound healing. A wearable skin pH-sensor targeted at skin and wound monitoring would both aid and lower the cost of treatment.13 PANi is a pH sensitive polymer that has been used in many pH sensors.14,15 We simplified pH sensor fabrication by modifying cotton
EP
yarns using a straightforward “dipping and drying” process: three conductive dispersions were studied alongside optimisation of key variables (e.g. the number of
CC
coating cycles used). The combination of the flexibility of cotton yarns with the conductive properties of multi-walled carbon nanotubes (MWCNT) and PEDOT:PSS yielded
conductive,
flexible
cotton
threads.
After
successful
PANi
A
electropolymerisation, cotton-based sensors were created that were effective in solidstate pH analysis. In this study, we reveal a wearable pH sensor that can be applied over a wide pH range (2.0 - 12.0 vs. a wearable Ag/AgCl quasi-reference electrode), even when applied to a pH-adjusted artificial sweat matrix.16 Additionally, these biocompatible composite yarns show evidence of significant antibacterial properties.
2
2. Experimental 2.1 Materials PEDOT:PSS dispersion (Clevios PH 1000, 1 wt.%) was procured from Heraeus (Germany). Analytical grade of ethylene glycol (EG), dimethyl sulfoxide (DMSO), boric acid, phosphoric acid, acetic acid, sodium hydroxide, ammonium chloride, magnesium chloride, pyruvic acid, glucose, urea, calcium carbonate, potassium nitrate and sodium nitrate, Escherichia coli, and nutrient broth were all purchased from Sigma Aldrich (UK).
IP T
The gauze fabric (100% cotton) and cotton yarns (100%) were purchased from a local pharmacy. Thin MWCNTs (95% purity) were purchased from Nanocyl (Belgium).
Ag/AgCl paste (C2130809D5) was obtained from Sun Chemicals ® (Parsippany, NJ).
SC R
An E. coli wild-type strain constitutively expressing yellow fluorescent protein (YFP;
Balaban et al.17) was used in these experiments. Epofix resin was purchased from Struers® (Denmark). Alamar Blue® was purchased from ThermoFisher Scientific (UK).
U
2.2 Instrumentation
N
Electrical resistance was measured across 1 cm of each fibre using a multi-meter. Scanning electron microscopy (SEM) images were obtained using a JEOL USA JSM-
A
7100F field emission electron microscope. Dynamic mechanical analysis (DMA) was
M
carried out on a TA Instruments Q800. Fluorescence intensity measurements during antibacterial tests were performed on an A1+ confocal Nikon microscope (Nikon A1M
used
for
the
measurements.
TE D
on Eclipse Ti-E). An EDAQ potentiostat (EA163) combined with an e-corder (401) was PANi
electropolymerisation
and
potentiometric
(open
circuit)
Raman spectroscopy was performed using a Renishaw inVia confocal Raman
EP
microscope. The following conditions were used for the single spectra acquisitions: 532 nm laser (50 mW total power), 1 s of sample photobleaching followed by application of 1% laser power for 5 s; 16 acquisitions were collected and combined. The following
CC
conditions were used for the Raman mapping: 785 nm laser (300 mW total power), 1% laser power for 5 s for 1 acquisition. A sample stage step size of 6 µm by 6 µm was
A
used with 3,538 spectra in total being collected across the cross-sectional area. The PANi-coated PEDOT-MWCNT cross-sections were prepared by mounting the yarns in Epofix resin (Struers®) and polishing to a 1 µm diamond finish using a semi-automatic polishing system (MetPrep Saphir 520, Coventry, UK).
2.3 Fabrication of conductive cotton fibres 3
The cotton yarns were dip coated in three different dispersions by straightforward “dipping and drying”: Dispersion 1: The cotton yarns were submerged in PEDOT:PSS ink (2 wt.%) for 1 min before being dried at room temperature for 30 min. This constitutes one dip cycle, which was repeated up to 12 times. The resulting dip-coated PEDOT:PSS-cotton yarns were then treated in either: (1) DMSO, (2) EG, or (3) DMSO directly followed by EG. For solvent treatment (1) and (2), the yarns were immersed in the solvent for 2 h
IP T
at room temperature with subsequent drying at 140 °C for 1 h. For solvent treatment (3), the PEDOT:PSS-cotton yarns were treated in DMSO for 2 h, dried at 140 °C for 1
h, treated in EG for 2 h, and finally were dried at 140 °C for 1 h. Treatment times of 2 h
SC R
were adopted due to previous research investigation the solvent treatment of wet-spun
PEDOT:PSS fibres (20-25 m), in which 5 min was the optimal treatment time. 18 The coated cotton yarns are thicker (700-750 m) and to ensure optimal electrical properties, a longer treatment time was chosen to remove PSS.
MWCNT to an aqueous solution of 10 mg ml-1 sodium dodecylbenzenesulfonate
N
1
U
Dispersion 2: MWCNT dispersion (0.3 wt.%) was prepared by adding 3 mg ml(SDBS) followed by tip-sonication for 30 min (500 W, f = 20 kHz, 30% amplitude).8,19
A
The cotton yarns were then submerged in the MWCNT dispersion for 20 s, dried at
M
room temperature for 15 min, washed with distilled water, and then dried again. This constitutes a one dip cycle, which was performed up to 70 times. Dispersion 3: A mixed PEDOT-MWCNT dispersion containing 0.5 wt.%
TE D
PEDOT:PSS and 1.5 wt.% MWCNT was prepared using tip sonication (conditions as before).20 The cotton yarns were submerged in the PEDOT-MWCNT dispersion for 1 min and were then dried at room temperature for 15 min. This constitutes a one dip
EP
cycle, which was performed up to 10 times.
2.4 Antibacterial testing
CC
The bacterial cells were picked from a Luria Bertani agar plate, then grown in liquid culture with agitation at 300 rpm for 16 h at 37 °C. The cells were diluted to obtain a final cell density of 1.28 x 108 cells ml-1. A 300 µl aliquot of the cell-containing media
A
(pH 6.8 - 7.2) was added to 5 mm lengths of each of the coated cotton yarn types in a 96-well plate. The well plate was incubated and shaken at 150 rpm at 37 °C for 4 h. After this period, each “biofouled” fibre was transferred to a clean well (containing no solution or bacilli), and images on a Nikon confocal microscope using the Fluorescein isothiocyanate (FITC) filter at 488 nm and using a 20× air objective lens with a
4
numerical aperture of 0.8. The mean intensity of fluorescence was measured across a fixed area of 0.217 mm2. 2.5 Cell viability testing The PEDOT-MWCNT-cotton yarns (5 mm lengths, bare and PANi-coated) were sterilised via dipping in ethanol for 3 h, air-drying and 10 min in a UV/ozone generator (BioForce Nanosciences, USA). The human keratinocyte cells (HaCaT, 300493, CLS,
IP T
Germany) were seeded at a density of 104 cells per well in a 96-well plate and incubated in a humidified atmosphere of 5% CO2 and 20% O2 at 37 C for 24 h to allow adherence
to the well surface. After this period, the fibres were then placed on top of the cells. The
SC R
samples were incubated with Alamar Blue® dye (diluted in the cell culture medium) for 1.5 h at different culture time points after fibre administration on the cells: 1, 3 and 5 d. The absorbance was measured with a spectrophotometer (Synergy, BioTek, USA) at 570 nm and at 600 nm (reference wavelength). Variance analysis (two-way ANOVA)
U
was performed using Graph Pad Prism® software with a p-value threshold 0.05 to
N
evaluate whether there was a statistical difference between the experimental
A
conditions.
M
2.6 Fabrication and testing of the pH-sensitive fibre electrodes Initial studies depositing Polyaniline (PANi) onto fibre electrodes by dip-coating (dispersed nanofibers) did not produce stable electrodes. During analysis the PANi-
TE D
coating delaminated producing inaccurate readings. For this reason electrochemical deposition was used to coat fibre electrodes with PANi. PANi was coated onto fibre electrodes using potentiostatic polymerisation in a solution of aniline (0.1 M) in HNO3 (1.0 M). A three-electrode set-up was used: a Pt-mesh counter electrode (1 cm2), a
EP
Ag/AgCl-reference electrode (RE, 3.0 M KCl internal solution, BASi, USA) and the coated cotton yarn fibre under test as the working electrode. A potential of 0.80 V (vs.
CC
Ag/AgCl RE) was applied for either 180 s, 300 s or 600 s duration. Potentiometry for pH analysis measured the potential of 1 cm of the coated
cotton fibre electrodes vs. a Ag/AgCl double junction reference electrode (3.0 M NaCl
A
internal solution, BASi, USA). The interference studies of the PANi-coated PEDOTMWCNT-cotton yarns were performed using the method reported by Nyein et al.: aqueous solutions of 1 mM NH4Cl, 1 mM MgCl2, 1 mM CaCO3, 8 mM KNO3 and 20 mM NaNO3 were subsequently added to a pH 4 Britton-Robinson buffer solution.16 On addition of an interference solution, a 20 s waiting period was applied before the voltage was measured. At the end of the study, the solution was changed to a pH 5 buffer 5
solution before the voltage was re-measured. Artificial sweat matrix was prepared using the same method as reported by Parilla et al.21 A wearable Ag/AgCl quasi-reference electrode comprised of 0.2 g of Ag/AgCl paste that was applied to both sides of 1 cm2 of gauze fabric followed by curing (80 °C for 20 min). A two-electrode set-up was used with the potential of the PANi-coated PEDOT-MWCNT-cotton sensing electrode recorded against the Ag/AgCl quasireference electrode. During analysis, a 200 l drop of pH buffer (12.0 to 2.0; maximum
3. Results and discussion
Dispersion 1: Conductive PEDOT:PSS Coated Cotton
SC R
3.1 Development and characterisation of conductive cotton
IP T
range of Britton-Robinson buffering system) was placed between the two electrodes.
Previous studies have reported that treating PEDOT:PSS with solvents, such as, ethylene glycol (EG) and/or DMSO can reduce resistance, 14 due to the removal of
U
insulating PSS material from the PEDOT:PSS coating and re-ordering of the polymer
N
chains.22,23 Sankar et al. soaked cotton fibres in a DMSO solution containing PEDOT:PSS to give a resistance of 77 Ω cm -1.11 Alamer et al. used a similar solution
A
and repeated drop casting (20 coats) onto the cotton to achieve a very low resistance
M
of 1.58 Ω sq-1.24,25While these resistances are some of the lowest reported, such high PEDOT concentrations typically lead to brittle materials (note the mechanical properties
TE D
were not reported). The method below presents a novel yet simple approach of coating cotton yarns with PEDOT:PSS solution (dispersion 1). Measurements showed a significant reduction in resistance with each dip cycle (Fig. 1(a)). The lowest resistance achieved without solvent post-treatment was 150 ± 34 Ω cm-1 after the maximum 12
EP
dip cycles tested.
The three solvent post-treatments (EG, DMSO, and the DMSO-EG combination) succeeded in reducing the resistance of PEDOT:PSS coated cotton. The EG treatment
CC
led to lower resistances than the DMSO treatment, while the combined DMSO-EG treatment yielded the lowest resistances (Fig. 1(a)). Single solvent treatment of the
A
fibres made using 12 dip cycles had a lower effect on resistance than when using fibres made with lower number of dip cycles: the fibres made using 12 dip cycles showed only a minor change on DMSO treatment (150 ± 34 Ω cm-1 to 144 ± 35 Ω cm-1), while EG treatment yielded resistance values of 105 ± 32 Ω cm-1. In contrast, the combined DMSO-EG treatment lowered the resistance of the 12-dip coated cotton to 23 ± 10 Ω cm-1. As the resistance values for the 11 dip cycle fibres did not differ significantly after
6
with combined DMSO-EG treatment (21 ± 13 Ω cm-1), 11 dip cycles was used in the further analyses (as the percolation threshold has been reached). Dispersion 2: Conductive MWCNT-Coated Cotton Fig. 1(b) shows the resistances of the MWCNT-coated cotton fibres after each dip cycle, which rapidly decrease up to dip cycle 36 (101 ± 15 Ω cm-1); the resistances decrease more slowly (ca. 5 Ω cm-1 per dip) after this. The lowest resistance achieved
IP T
(55 ± 5 Ω cm-1) was after the maximum 70 dips tested, which is low enough to facilitate PANi electrodeposition. However, the use of 70 dip cycles is not industrially useful in
SC R
terms of material synthesis. Dispersion 3: Conductive PEDOT:PSS:MWCNT-Coated Cotton
The PEDOT:PSS-only cotton fibres (made from dispersion 1) had a poor tensile strength (see below), which is an issue that needs to be overcome prior to use in
U
wearable sensors. In contrast, the MWCNT-modified cotton fibres (dispersion 2) was
N
flexible and strong but required a very high number of dip cycles (70) in order to achieve low enough resistances. In response to these findings, a mixed dispersion containing
A
PEDOT:PSS and MWCNT was used to coat the cotton yarns to overcome these
M
individual limitations. The PEDOT-MWCNT-coated cotton fibres exhibited impressive flexibilities (vide infra) and high conductivities. As shown in Fig. 1(c), a resistance of 55 ± 28 Ω cm-1 was achieved after only 10 dip cycles (cf. MWCNT-only system yielding 55
TE D
± 5 Ω cm-1 after 70 dip cycles).
Scanning Electron Microscopy (SEM) SEM shows how coating cotton threads with the conductive inks alters their surface
EP
morphologies (Fig. 2). The PEDOT:PSS-cotton (dispersion 1) clearly shows the presence of a polymer layer around the cotton thread (Fig. 2(b) and 2(c)). Single solvent
CC
(EG and DMSO) treatment yielded PEDOT:PSS-cotton fibres with more ridges than the untreated PEDOT:PSS-cotton fibres. This was due to PSS removal, which leads to a change in surface morphology as previously reported (Fig. 2(d) and 2(e)).26,27,28The
A
PEDOT:PSS-cotton fibres treated with combined DMSO-EG displayed the most ridged surface (Fig. 2(f)) due to the highest level of PSS removal. These observations correlate to findings by Kim et al. who reported a significant decrease in thickness of PEDOT:PSS films after EG treatment.29 The PEDOT-MWCNT-cotton fibres (dispersion 3) have smooth surfaces (Fig. 2(h)). In contrast, the MWCNT-cotton fibres (dispersion 2) have rougher surfaces where 7
the individual cotton fibres in the yarn can still be seen (Fig. 2(g)). This was due to a combination of the large number of drying and washing steps (70 dips) and because the MWCNT were absorbed into the cotton fibrils as opposed to forming a surface coating.
Mechanical Strength Dynamic mechanical analysis (DMA) was performed on the conductive cottons as an
IP T
initial assessment of their ability to withstand general wear-and-tear (resulting data is presented in Table S1 and Fig. S1). The MWCNT-modified cotton yarns (70 dips) achieved the highest ultimate tensile strengths (219 ± 35 MPa, 13 ± 3% strain at break),
SC R
as anticipated.5 The final PEDOT:PSS-cotton yarns (11 dips, combined DMSO-EG
treated) showed the lowest ultimate tensile strengths (62 ± 12 MPa, 7 ± 2% strain at break). The PEDOT-MWCNT-cotton yarns showed intermediate values (130 ± 25 MPa,
U
10 ± 2% strain at break) whilst maintaining flexibility.
N
Raman Characterisation
The spectra of the PEDOT:PSS-cotton yarns showed notable changes after the
A
different solvent treatments (Fig. 3(a)). The peaks at 1580 and 1600 cm-1 (PSS-derived
M
aromatic ring chain vibrations)30,31 were less defined in the spectra of the solvent treated fibres compared to the untreated benchmark, suggesting that the PSS content had been reduced. The peaks at 1259 and 1367 cm -1 are attributed to Cα-Cα inter-ring
TE D
stretching and Cβ-Cβ stretching vibrations, respectively (PEDOT component).32 The large peak between 1400 and 1500 cm-1 is due to the symmetric stretching of the fivemembered thiophene ring (PEDOT component). This ring can be present as either the benzoid (Cα=Cβ) or quinoid (Cα-Cβ) resonant structure depending on the doping level
EP
of the polymer. This PEDOT-diagnostic peak shifts from 1436 cm-1 in the untreated fibres to 1425 cm-1 after solvent treatment, a shift that has been attributed to a
CC
conformational change of the PEDOT chains (from a coiled benzoid structure to a more linear quinoid structure), which leads to an enhancement in the mobility of the charge carriers in the PEDOT:PSS coating.33 A similar shift has been previously observed for
A
reduced dopant concentration in PEDOT:PSS,34 suggesting that DMSO reduces the PSS content leading to the observed decrease in resistance (Fig. 1(a)). For the PEDOT-MWCNT yarn, this PEDOT-diagnostic peak shifted to 1445 cm1
(Fig. 3(b)), indicating that the PEDOT is in the benzoid (Cα=Cβ) form (recall this yarn
was not subjected to solvent post-treatment). As expected, the spectrum of the PEDOT-MWCNT-cotton fibres was similar to that of the PEDOT:PSS-cotton, but with 8
the addition of the MWCNT-derived D and G bands. The spectra of the MWCNT-cotton and PEDOT-MWCNT yarns both show a D band at 1350 cm-1, whilst the G band appears at 1588 cm-1 for MWCNT-cotton and 1583 cm-1 for PEDOT-MWCNT-cotton. For the PEDOT-MWCNT-cotton fibres, the PEDOT-derived Cβ-Cβ symmetrical peak at 1368 cm-1 is hidden behind the MWCNT-derived D band (1350 cm-1) and the Cα-Cα inter-ring stretching peak (1259 cm-1) is not observed above the strong Raman scattering of the CNTs (similarly observed by Mannayil et al. when using a high CNT
IP T
density in PEDOT:PSS-CNT dispersions)35.
3.2 Development of pH Sensor
SC R
PANi was deposited onto each of the three different conductive cotton yarns by electropolymerisation to make the fibres pH sensitive. The deposition of PANi was
confirmed by SEM, Raman spectroscopy and cyclic voltammetry (Figs. S2, S3 and S4, respectively). The SEM micrograph in Fig S2 clearly shows the edge of the PANi film
U
which has been electrodeposited onto the fibre. The Raman spectra of the PANi-coated cotton substrates exhibit typical PANi bands at 1617, 1593, 1482, 1336, 1221 and 1163
N
cm-1, which have been assigned to C-C ring stretching (benzenoid segment), C=C ring
A
stretching (quinone segment), C=N and CH=CH stretching (quinone segment), C-N+.
M
stretching (semiquinone radical cation segment), C-N stretching (benzenoid), and C-H in-plane bending (quinone segment), respectively.36 Slight variations in Raman peak positions of PANi are observed depending on which conductive cotton was coated.
TE D
Raman mapping was performed on a cross-section of a PANi-coated PEDOTMWCNT-cotton yarn to identify whether the coating was solely on the outside of the thread, or whether the PANi had infiltrated into the core of the yarn. The spectra collected with the 785 nm laser showed a stronger intensity 1446 cm-1 peak (PEDOT
EP
Cα=Cβ stretch) than with the use of the 532 nm laser (Fig. 4(a)), and was strong enough to map PEDOT. The intensity of the peaks at 1162 cm-1 (PANi: C-H in-plane bending
CC
of the quinoid rings) and 1446 cm-1, were used to map the location of PANi and PEDOT in the cross-section (Fig. 4(b)). While some PANi is evident in the core of the fibre, the majority of the PANi is located as a coating on the outside of the yarn. The map of the
A
1446 cm-1 peak shows a clear coating of PEDOT on the outer part of the thread along with large zones of PEDOT within the cotton core. This suggests that the PEDOTMWCNT ink, which has strong interactions with the cellulose and natural polysaccharides in the cotton, has penetrated into the yarn in addition to providing an outer coating. As a result, the inside of the cotton yarn was conductive with electropolymerised PANi coating individual fibrils inside the yarn core. 9
Cyclic voltammograms (CVs) of the PANi-coated conductive cottons show variations in the PANi redox peak potentials depending on which fibre electrode was coated (Fig. S4). Oxidation peaks at 0.35 V, 0.60 V and 0.87 V vs. Ag/AgCl were observed for PANi coated on (DMSO-EG solvent treated) PEDOT:PSS-cotton, the most conductive of the coated cotton fibres. The first oxidation at 0.35 V vs. Ag/AgCl corresponds to the conversion from the leucoemeraldine base to emeraldine salt. The peak at 0.60 V vs. Ag/AgCl has been attributed to the dimer intermediate, p-
IP T
aminodiphenylamine, which is found at ~0.70 V vs. Ag/AgCl.37 The third peak at 0.87 V relates to the oxidation of emeraldine salt to pernigraniline salt. The CVs for PANiMWCNT-cotton and PANi-PEDOT-MWCNT-cotton are dominated by the capacitive
SC R
properties of the MWCNT. Two oxidation peaks are observed above the capacitive
background at 0.21 V and 0.61 V vs. Ag/AgCl for PANi-MWCNT-cotton, and a single oxidation peak at 0.56 V vs. Ag/AgCl for the PANi-PEDOT-MWCNT-cotton, which are similar to previously observed redox couples recorded with PANi-CNTs.38,39These two
U
fibre electrodes are more resistive than PEDOT:PSS-cotton and may result in less
N
conductive polyaniline being formed on the fibre electrodes. This also correlates with the Raman spectra (Fig. S3) which exhibit peaks typical of those previously reported
A
for oligoanilines.40 The peak at 1328 cm-1 in PANi-PEDOT:PSS-cotton is typical for
M
normal PANi and corresponds to the C-N+. vibration of delocalised polaronic structures. This peak is blue shifted when looking at the spectra for PANi-PEDOT-MWCNT-cotton and merges with the D-band of MWCNT (1346 cm-1) for PANi-MWCNT-cotton (Fig. S3),
TE D
suggesting a relatively lower extent of polaron delocalisation in PANI on these electrodes and is in accordance with a lower conductivity of PANI.40 The pH sensitivity of the PANi-coated conductive cotton materials was assessed against a double-junction Ag/AgCl RE (Fig. 5). The PEDOT:PSS-cotton substrate gave
EP
the poorest pH sensitivity (-22 ± 9 mV pH-1), suspected to be due to swelling of the hydrophilic PSS affecting the resistance of the electrode and resulting in a poor quality
CC
PANi film. The PANi-coated MWCNT-cotton and PEDOT-MWCNT-cotton electrodes showed improved pH sensitivities with Nernstian responses (-61 ± 4 and -61 ± 2 mV pH-1, respectively) with little variation between measurements on repeated electrode
A
fabrications (Fig. 5, n = 3). The PANi-coated PEDOT-MWCNT-cotton-based electrode achieved low electrical resistance values, maintained flexibility, and yielded a Nernstian response to pH (-61 ± 2 mV pH-1) (Table 1); this superior electrode was taken forward for integration in a wearable pH sensor. The selectivity of the PANi-PEDOT-MWCNT-cotton sensor was studied by subsequently adding interfering ions (NH4+, Mg2+, Ca2+, K+ and Na+) at physiologically 10
relevant concentrations to a pH 4 buffer solution and monitoring the change in voltage (Fig. 6(a)). The voltage changes observed were negligible after each addition of the interfering ions and were significantly lower in magnitude to the voltage changes observed due to pH variations (Fig. 6(a)). This indicates that this electrode selectively measures solution pH in the presence of common physiological interferences. The PANi-PEDOT-MWCNT cotton yarn working electrode had a pH sensitivity of -61 ± 2 mV pH-1 (across the pH range 2.0 – 12.0) when tested in combination of a
IP T
wearable Ag/AgCl quasi-reference RE in a solid-state configuration (Fig. 6(b)). This is comparable to a previously developed pH sensor (electropolymerised PANi onto a CNT
wire) that gave a Nernstian response was over the pH range 1.0 - 13.0.41 A Nernstian
SC R
response was maintained with analyte volumes as small as 200 µl (Fig. 6(b)). The shift
in potential for this solid-state configuration is due to the change in RE (cf. the Ag/AgCl REs used in the solution tests). Response times of 60 ± 20 s were achieved, demonstrating the electrode’s capability for real-time wearable monitoring. The pH
U
response of the pH fibre sensor vs. the solid quasi-reference electrode was also tested
N
with pH-adjusted artificial sweat matrices (200 l) containing common sweat interferences (Fig. 6(b)): a response of -63 ± 3 mV pH-1 was observed. The y-intercept
A
shifted from 402 ± 42 mV to 467 ± 30 mV (n=3) with pH-adjusted buffered solutions
M
and artificial sweat matrices, respectively. The potential of the quasi-reference electrode (Ag/AgCl paste) is dependent on the Cl- concentration, thus we hypothesise that the high Cl- content in the artificial sweat matrices influenced the y-intercept. This
TE D
may be improved with the addition of a membrane (e.g. PVB) to protect the quasireference electrode.
Successful operation (a sensitive linear response) across the pH range 4.0 - 9.0 is essential for application in a pH-based wound monitoring system. Chronic wounds
EP
have a maximum pH of 9.0 and healthy, while unwounded skin has a minimum pH of 4.0.42,43 For application as an epidermal patch, this sensor needs to monitor skin pH
CC
over the critical range of 4.0 - 7.0.44
Biocompatibility and antibacterial properties
A
An initial cytotoxicity test was performed on the pH-sensitive fibres by exposure to cultured human keratinocytes cells. The fibres were placed on top of the skin cells and the cell viability was measured at days 1, 3 and 5 post fibre administration. A significant decrease (p < 0.05) was observed in the cell viability of the PEDOT-MWCNT fibres after 24 h: 63 ± 5% compared to 100 ± 10% for the control (seeded cells without fibres) (Fig. 7(a)). This would suggest potential cytotoxicity of the bare PEDOT-MWCNT fibres. 11
The toxicity of pristine CNT is well reported in the literature because it interacts with cell proteins, interferes with protein structure, and results in cell death.45 In contrast, the PANi-PEDOT-MWCNT-cotton fibres demonstrated no significant change in cell viability throughout the time of the test. As discussed, SEM and Raman imaging indicated that the PANi had enveloped the PEDOT-MWCNT-cotton fibres (Fig. S2 and Fig. 4(a)), acting as a protective casing, which maintains the viability of any cells in contact with the yarns. This is supported by previous research that has shown PANi to be
IP T
biocompatible.46,47 It is important that materials used to monitor wound pH do not encourage
bacterial growth. The antibacterial properties of the coated cotton yarns were evaluated
SC R
by using wild type E. coli expressing yellow fluorescent protein (YFP) as a model system.17 A suspension of bacterial cells was incubated for 4 h with the coated cotton
yarns after which the intensity of each yarn’s fluorescence (FITC, 488 nm) was measured. The mean fluorescence intensity was used as a quantified representation
U
of the extent of bacterial growth on the coated cotton yarns. Images of the fluorescent
N
bacteria grown on coated cotton yarns are shown in Fig. S5.
The PEDOT:PSS-cotton fibres (dual DMSO-EG treated) showed moderate
A
antibacterial activity (intensity = 259 ± 67 a.u.), compared to the bare cotton yarn, which
M
gave a benchmark intensity of 694 ± 93 a.u. (Fig. S6). The fluorescence images of the cotton yarns (Fig. S5) show that the bacterial cells were growing within the individual yarn fibres. The PEDOT:PSS surface coating on the cotton discouraged the growth or
TE D
attachment of bacteria. MWCNT-cotton fibres exhibited a lower mean fluorescence intensity (161 ± 4 a.u., Fig. S6) demonstrating higher antibacterial activity. This was expected as Kang et al. previously reported that MWCNTs are toxic to E. coli as they initiate the production of stress-related gene products.48 The lowest mean fluorescence
EP
intensity was achieved by the PEDOT-MWCNT-cotton fibres (64 ± 13 a.u.), which clearly discourages bacterial growth.48 PANi-PEDOT-MWCNT demonstrated a similar
CC
mean fluorescence intensity of 70 ± 7 a.u. (Fig. 7(b)), which shows that although PANi is biocompatible with skin cells, it does not diminish the antibacterial property of the PEDOT-MWCNT fibres. This highlights the potential for use of PANi-PEDOT-MWCNT-
A
cotton fibres in pH sensing wound dressings.
4. Conclusion A new pH sensing conductive flexible cotton-based electrode concept has been developed using a dispersion containing both PEDOT:PSS and MWCNT. These flexible PEDOT-MWCNT-cotton fibres showed low resistances (55 ± 28 Ω cm-1) 12
alongside significant antibacterial properties when coated with pH sensitive PANi. As proof-of-concept investigation, these PANi-coated PEDOT-MWCNT-cotton fibre electrodes were used as pH indicator electrodes in a solid-state sensor alongside using a Ag/AgCl quasi-reference electrode. This configuration is analogous to smart dressings for wound monitoring or an epidermal patch. This sensor achieved a rapid, selective, and Nernstian response (-61 ± 2 mV pH-1) over a wide pH range (2.0 – 12.0). After the necessary optimisation and validation, this sensor concept will help facilitate
IP T
development of a high performance, wearable platform for the future real-time monitoring of skin and wound pH.
SC R
5. Conflicts of interest There are no conflicts of interest to declare. 6. Acknowledgements
U
The authors thank the University of Surrey for providing funding for a studentship for
N
RES and the EPSRC for providing the capital funding that funded the Raman
A
CC
EP
TE D
M
A
microscope used in this work (EP/M022749/1).
13
7. Figures U n tre a te d
1000 -1
DM SO
R e s is ta n c e / c m
800
EG D M S O -E G
600
400
IP T
200
0 9
10
11
12
D ip c y c le
SC R
(a)
U
300
N
200
100
0 30
40
50
-1
800
70
(b)
EP
600
400
200
CC
R e s is ta n c e / c m
60
TE D
D ip c y c le
M
20
A
R e s is ta n c e / c m
-1
400
0
A
0
Fig.1
2
4
6
8
10
D ip c y c le
(c)
Resistance measurements on cotton yarns dip coated in (a) aqueous PEDOT:PSS
dispersions (2 wt.%) with no solvent post-treatment, and post-treatment in either ethylene glycol (EG), dimethyl sulfoxide (DMSO), and DMSO followed by EG in combination (DMSO-EG), (b) MWCNT, and (c) PEDOT-MWCNT dispersions. The error bars represent the standard deviation of measurements on n = 3 replicate yarns.
14
a.
b.
100 m
c.
SC R
IP T
100 m
d.
10 m
TE D
M
A
N
U
10 m
10 m
f.
10 m
100 m
h.
10 m
A
CC
EP
e.
g.
Fig. 2
Scanning electron microscopy (SEM) images of: (a) uncoated cotton; (b and c) PEDOT:PSS-
cotton (11 dips in dispersion 1) before solvent post-treatment; (d) DMSO-treated PEDOT:PSS-cotton; (e) EG-treated PEDOT:PSS-cotton; (f) dual DMSO-EG-treated PEDOT:PSS-cotton; (g) MWCNT-cotton (70 dips in dispersion 2), and (h) PEDOT-MWCNT-cotton (10 dips in dispersion 3).
15
C=C / C -C C -C
C -C
In te n s ity
U n tre a te d DMSO
IP T
EG D M S O -E G 1400
R a m a n s h ift / c m
1600 -1
SC R
1200
(a)
D
U
G & D'
N
MW CNT
A
In te n s ity
C -C
C -C
TE D
C -C
M
P E D O T -M W C N T
1200
1400
1600
-1
(b)
EP
R a m a n s h ift / c m
P E D O T :P S S
A
CC
Fig. 3 Raman spectra using a 532 nm laser of: (a) PEDOT:PSS-coated cottons after the different solvent post-treatments; (b) MWCNT-, PEDOT-MWCNT-, and PEDOT:PSS-cottons.
16
C -H
1163
C = C r in g ( Q )
C=N
(S Q R )
& C H = C H (Q )
C - C r in g ( B )
+.
C -N
C -N
(B )
(S Q R )
1593
C -C
1617
1482
1336 1418
P A N i- P E D O T - M W C N T 1417
1162 1221
1593
1610
532 nm
1331
P A N i- P E D O T - M W C N T
1446
785 nm 1400
1600
R a m a n s h ift / c m
-1
SC R
1200
IP T
In te n s ity
1221
(a)
Intensity of the peak at 1162 cm-1
(b)
Raman spectra of (a) polyaniline-coated PEDOT-MWCNT-cotton (using lasers 532 and
EP
Fig. 4
TE D
M
A
N
U
Intensity of the peak at 1446 cm-1
785 nm). (b) Raman cross-sectional interpolated maps using a 785 nm laser of PANi-coated PEDOT:MWCNT-cotton. The colour scale represents the intensity at the peaks at 1162 cm-1
A
CC
(PANi semi-quinone radical) and 1446 cm -1 (PEDOT Cα=Cβ).
17
PANi-MWCNT-cotton
-61 ± 4 mV pH-1
400
-1 PANi-PEDOT-MWCNT-cotton -61± 2 mV pH
300
PANi-PEDOT:PSS-cotton
-22 ± 9 mV pH-1
200 100 0
3
4
5
6
7
8
IP T
Voltage / mV vs. Ag/AgCl
500
pH
pH sensitivity of the PANi-coated conductive cotton yarns. The error bars represent the
A
CC
EP
TE D
M
A
N
U
standard deviation of measurement on n = 3 replicate fibres.
SC R
Fig. 5
18
+ +
+ 20 m M Na
2+
+ 8 mM K
+ 1 mM Ca
+ 1 mM Mg
260
2+
+
+ 1 m M NH4
pH 4
240 220
pH 5
200
IP T
V o lta g e /m V v s . A g /A g C l
280
180 0
5
10
15
20
25
30
35
T im e / m in
S k in
SC R
(a) W ounds
-1
-6 1 2 m V p H
-1
-6 3 3 m V p H
-1
N
200
-6 0 4 m V p H
U
400
0
A
-2 0 0
-4 0 0 2
4
6
8
pH
M
V o lta g e / m V v s . R E
600
10
12
TE D
2 0 .0 m L o f p H b u ffe re d s o lu tio n s ( v s . A g /A g C l D J E )
S o lid -s ta te 2 0 0 L o f p H b u ffe r e d s o lu tio n s ( v s . q u a s i- A g / A C l)
S o lid -s ta te 2 0 0 L o f p H -a d ju s te d a rtific ia l s w e a t m a trix
Fig. 6
EP
( v s . q u a s i- A g /A g C l)
(b)
Potentiometric (open circuit) response of pH analysis of PANi-PEDOT-MWCNT-cotton
CC
electrodes: (a) Selectivity when various sweat-related interferences (NH4+, Mg2+, Ca2+, K+ and Na+) were added to the pH 4 aqueous samples (20 ml). (b) Response in buffered solution to varying pH levels (vs. Ag/AgCl double junction electrode (DJE) and Ag/AgCl quasi-reference electrode) and in pH-
A
adjusted artificial sweat matrix (200 µl) (vs. Ag/AgCl quasi-reference electrode). The error bars represent the standard deviation of measurement on n = 3 replicate yarns
19
C o n tr o l P E D O T - M W C N T - c o tto n P A N i- P E D O T - M W C N T
100
75
50
25
IP T
C e ll v ia b ility / %
125
0 2
2
1
7
2
4
0
T im e / h r
SC R
1000
B a re C o tto n Y a rn P E D O T -M W C N T
800
P A N i- P E D O T - M W C N T
U
600
N
400
200
0
M
C o a te d C o tto n Y a rn s
A
F lu o r e s c e n c e In te n s ity
(a)
(a) Cell viability (measured absorbance, 570 nm) compared to control taken at three time
TE D
Fig. 7
(b)
points: 1, 3 and 5 d post sensor administration. Wells without sensors were used as controls and were considered 100% viable. Error bars represent the standard errors of three independent experiments. (b) Mean intensity of fluorescence (FITC, 488 nm) across a fixed area (0.217 mm 2) of bare cotton yarn,
EP
bare and PANi-coated PEDOT-MWCNT-cotton and Ag/AgCl-gauze biofouled with E. coli wild type strain expressing yellow fluorescent protein (YFP). Error bars represent the standard errors of three
A
CC
independent experiments.
20
Table 1 Summary data (mean ± sample std. dev.) for PEDOT:PSS-, MWCNT-, PEDOT-MWCNTtype cotton-based electrodes (n = 3). Solvent post-
Cotton yarn
treatment
DMSO and
PEDOT:PSS
MWCNT
Ultimate tensile strength
Mean Intensity of
pH sensitivity after
cycles
/ Ω cm
/ MPa
fluorescent bacteria
polyaniline coated
/ a.u.
/ mV pH-1
-1
21 ± 13
52 ± 5
259 ± 67
-22 ± 9
-
70
55 ± 5
226 ± 16
161 ± 4
- 61 ± 4
-
10
55 ± 28
134 ± 15
64 ± 13
SC R
PEDOT-
Resistance
11
EG
MWCNT
Dip
IP T
Coating on
References 1
- 61 ± 2
Y. Zhang and Y. Cui, Flexible and Printed Electronics, 2017, 2: e:042001. R. A. Street, A. C. Arias, Stretchable Electronics, Wiley-VCH Verlag GmbH & Co. KGaA, 2012, 355-378. 3 W. Zeng, L. Shu, Q. Li, S. Chen, F. Wang and X. M. Tao, Adv. Mater., 2014, 26, 5310-5336. 4 A. Mirabedini, J. Foroughi, S. Farajikah and G. G. Wallace, RSC Adv., 2016, 6, 44687-44716. 5 D. P. Hansora, N. G. Shimpi and S. Mishra, RSC Adv., 2015, 5, 107716-107770. 6 X. H. Zhong, Y. L. Li, J. M. Feng, Y. R. Kang and S. S. Han, Nanoscale, 2012, 4, 5614-5618. 7 Y. Q. Li, P. Huang, W. B. Zhu, S. Y. Fu, N. Hu and K. Liao, Sci. Rep., 2017, 7:e45013. 8 T. Guinovart, M. Parrilla, G. A. Crespo, F. X. Rius and F. J. Andrade, Analyst, 2013, 138, 52085215. 9 X. Jia, T. Song, Y. Liu, L. Meng and X. Mao, Anal. Chim. Acta., 2017, 969, 57-62. 10 L. L. Meng, T. T. Sing and X. Mao, Talanta, 2017, 167, 379-384. 11 A. Sankar and Mohanta, AIP Conf. Proc., 2016, 1731, 120019. 12 N. Liu, W. Ma, J. Tao, X. Zhang, J. Su, L. Lo, C. Yang, Y. Gao, D. Golberg and Y. Bando, Adv. Mater., 2013, 25, 4925-4931. 13 T. R. Dargaville, B. L. Farrugia, J. A. Broadbent, S. Pace, Z. Upton and N. H. Voelcker, Biosens. Bioelectron., 2013, 41, 30-42. 14 T. Guinovart, G. Valdés-Ramírez, J. R. Windmiller, F. J. Andrade and J. Wang, Electroanal., 2014, 26, 1345-1353. 15 A. Vacca, M. Mascia, S. Rizzardini, S. Corgiolu, S. Palmas, M. Demelas, A. Bonfiglio and P. C. Ricci, RSC Adv., 2015, 5, 79600-79606. 16 H. Y. Y. Nyein, W. Gao, Z. Shahpar, S. Emaminejad, S. Challa, K. Chen, H. M. Fahad, L. C. Tai, H. Ota, R. Davis and A. Javey, ACS Nano, 2016, 10, 7216-7224. 17 N. Q. Balaban, J. Merrin, R. Chait, L. Kowalik and S. Leibler, Science, 2004, 305, 1622-1625. 18 D. O. Reid, PhD thesis., University of Surrey, 2017. 19 L. Hu, M. Pasta, F. Mantia, L. Cui, S. Jeong and H. D. Deshazer, Nano Lett., 2010, 10, 708-714. 20 D. J. Yun, K. Hong, S. Kim, W. M. Yun, J. Jang, W. Kwon, C. Park and S. W. Rhee, ACS Appl. Mater. Interfaces, 2011, 3, 43-49. 21 M. Parrilla, J. Ferré, T. Guinovart and F. J. Andrade, Electroanal., 2016, 28, 1267-1275. 22 M. Jikei, T. Yamaya, S. Uramoto and K. Matsumoto, Int. J. Soc. Mater. Eng. Resour., 2014, 20, 158-162. 23 S. Lee, J. Nanosci. Nanotechnol., 2016, 16, 2880-2882. 24 F. A. Alamer, J. Alloys Compd., 2017, 702, 266-273. 25 F. A. Alamer (2015), Patent No. US20160258110, 2015, Mecca, SA: United States. 26 J. Ouyang, C. W. Chu, F. C. Chen, Q. Xu and Y. Yang, Adv. Funct. Mater., 2005, 15, 203-208. 27 Q. Wei, M. Mukaida, Y. Naitoh and T. Ishida, Adv. Mater., 2013, 25, 2831-2836. 28 J. Gasiorowski, R. Menon, K. Hingerl, M. Dachev and N. S. Sariciftci, Thin Solid Films, 536, 211-215. 29 D. J. Yun, J. H. Kim, S. H. Kim, M. Seol, D. Yu, H. Kwon, Y. Ham, J. Chung, Y. Kim and S. Heo, Nanotechnology, 2016, 27:e165706 30 T. R. Chou, S. H. Chen, Y. T. Chiang, Y. T. Lin and C. Y, Chao, J. Mater. Chem. C, 2015, 3, 3760-3766.
A
CC
EP
TE D
M
A
N
U
2
21
31
A
CC
EP
TE D
M
A
N
U
SC R
IP T
S. H. Chang, C.H. Chiang, F. S. Kao, C. L. Tien and C. G. Wu, IEEE Photonics Journal, 2014, 6, 8400307. 32 D. Yoo, J. Kim, S. H. Lee, W. Cho, H. H. Choi, F. S. Kim and J. H. Kim, J. Mater. Chem. A., 2015, 3, 6526-6533. 33 L. Zhang, H. Deng, S. Liu, Q. Zhang, F. Chen and Q. Fu, RSC Adv., 2015, 5, 105592-105599. 34 W. W. Chiu, J, Travas-Sejdic, R. P. Cooney, G. A Bowmaker, J. Raman Spectrosc, 2006, 37, 13541361. 35 J. Mannayil, S.M. Raman, J. Sankaran, R. Raman, J. M. K. Ezhuthachan, Phys. Status Solidi A, 2018, 215, 1701003. 36 C. Liu, J. Zhang, G. Shi and F. Chen, J. Appl. Polym. Sci, 2004, 92, 171-177. 37 G. A. Planes, J. L. Rodríguez, M. C. Miras, G. García, E. Pastor and C. A. Barbero, Phys. Chem. Chem. Phys., 2010, 12, 10584-10593. 38 L. Pilan, M. Raicopol, A. Pruna and V. Branzoi, Surf. Interface Anal., 2012, 44, 1198-1202. 39 L. Jin, Y. Jiang, M. Zhang, H. Li, L. Xiao, M. Li and Y. Ao, Sci. Rep., 2018, 8, 6268. 40 G. ́Ciric-Marjanovic,M. Trchov, J. Stejskal, J. Raman Spectrosc.2008, 39, 1375 – 1387. 41 N. Ferrer-Anglada, M. Kaempgen and S. Roth, Phys. Stat. Sol., 2006, 13, 3519-3523. 42 L. R. Bennison, C. N. Miller, R. J. Summers, A. M. B. Minnis, G. Sussman and W. McGuiness, Wounds Practise and Research, 2017, 25, 63-69. 43 C. R. Kruse, K. Nuutila, C. C. Lee, E. Kiwanuka, M. Singh, E. J. Caterson, E. Eriksson and J. A. Sørensen, Wound Repair Regen., 2015, 23, 459-464. 44 H. Lambers, S. Piessens, A. Bloem, H. Pronk and P. Finkel, Int. J. Cosmet. Sci., 2006, 28, 359370. 45 S. Y. Madani and A. M. Seifalian, Nano. Rev., 2013, 4, 1-14. 46 A. J. Bandodkar, V. W. S. Hung, W. Jia, G. Valdés-Ramírez, J. R. Windmiller, A. G. Martinez, J. Ramírez, G. Chan, K. Kerman and J. Wang, Analyst, 2013, 138, 123-128. 47 H. Lee, T. K. Choi, Y. B. Lee, H. R. Cho, R. Ghaffari, L. Wang, H. J. Choi, T. D. Chung, N. Lu, T. Hyeon, S. H. Choi and D. H. Kim, Nature Nanotechnol., 2016, 11, 566-572. 48 S. Kang, M. Herzberg, D. F. Rodrigues and M. Elimelech, Langmuir, 2008, 24, 6409-6413.
22