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Fully Written Flexible Potentiometric Sensor Using Two-dimensional Nanomaterial-based Conductive Ink Chengmei Jiang, Xunjia Li, Yao Yao, Yibin Ying, and Jianfeng Ping Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04334 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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Analytical Chemistry

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Fully Written Flexible Potentiometric Sensor Using Two-dimensional

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Nanomaterial-based Conductive Ink

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Chengmei Jiang, Xunjia Li, Yao Yao, Yibin Ying, Jianfeng Ping*

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School of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang Road,

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Hangzhou 310058, P.R. China

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Corresponding author: Prof. Jianfeng Ping

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E-mail: [email protected]

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ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract: The emerging demand for flexible, portable, easily accessible, and cost-effective electronic

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fabrication has motivated to develop novel techniques to manufacture electronic components and

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devices. Inspired by daily hand-writing, an all-written potentiometric sensor was developed by using a

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Chinese brush pen-based writing technique. A writing ink made from graphene nanosheet (GN) as a

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conductive component, Triton x-100 as a stabilizer, and xanthan gum as a binder, was used to obtain

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flexible electrode substrate. Results demonstrate GN ink-based writing electrode (GN-WE) possesses

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good conductivity, fast electron transfer kinetics, considerable stability, and favorable flexibility. By

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further writing cadmium ion selective membrane (Cd2+-ISM) and photo-polymerised reference

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membrane (RM) on the surface of GN-WE, an all-solid-state potentiometric sensor for cadmium ion

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was constructed. A large bulk capacitance (41.67 μF) and excellent potential stability (drift of 0.156 mV

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h–1) was achieved at the developed all-written potentiometric sensor, which is much superior to the

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solid-contact potentiometric sensor using GCE as electrode substrate. Furthermore, real sample analysis

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reveals that our GN ink-based potentiometric sensor could be used as a reliable and stable sensor for

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cadmium ion detection in food and environment.

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Keywords: Graphene ink, Chinese writing brush, Ion selective electrode, Potentiometry, Heavy metal

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INTRODUCTION

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Inspired by routine hand writing, multiple writing tools, such as pencil, brush pen, ball pen, and

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fountain pen have been recently utilized to directly write electronics. As a traditional Chinese writing

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and painting instrument and shaped like the painbrush in oil painting in the western countries, the brush

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pen is invented in China long time ago before Christ, and has been utilized as the main writing tool in

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eastern countries for thousands of years.1 A typical brush pen is constructed through a combination of a

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bunch of filaments (e.g., bristle, nylon fibrils) with a head of handle (e.g., bamboo, wood). During the

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process of writing, the brush pen is initially soaked in an ink container, following handwriting pressure,

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the ink is transferred onto a substrate. Specifically, the capillary shear and stress force from the brush

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deliver the ink onto the substrate to form continuous pattern. This writing technique possesses

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tremendous merits, i.e. it is capable of achieving writing on various substrates of both soft and rigid, for

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example, some flexible substrates like polyethylene terephthalate (PET) membrane, even utilizing

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human skin as a substrate to real-time monitor human physiological indicators.2 Nowadays there is a

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great interest in developing flexible electronics, since the wearable functional devices, medical

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monitoring systems, and flexible energy storage and conversion devices are appearing to be the research

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hotspots, which hold great promise to empower doctors to timely monitor vital signs of patients simply

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and efficiently.3-4 Moreover, the convenient writing fabrication strategy can construct electronics with

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decreased cost, increased performance, rapidity, and easy accessibility.5

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As for the writing ink, graphite powder (GP) is always used as the main component of the

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traditional ink, and its extensive use is restricted due to its poor conductivity. Thus, tremendous efforts

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have been conducted to improve the conductivity of writing ink. For this, diverse nanomaterials, such

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as the Ag nanoparticles,6 Cu nanoparticles,7 and carbon nanotubes,8 have been introduced to fabricate

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the conductive ink for writing electronics, since their remarkable characteristics including high surface

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area to volume ratio and superior electrical conductivity.9 Graphene nanosheet (GN), as the most notable

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two-dimensional nanomaterial, has received considerable attention due to its outstanding electrical,

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mechanical, and thermal properties.10-12 In the past decades, GN has exhibited great promising in various

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fields, such as electronics, chemical and biological sensors, energy-storage devices, catalysts, and etc.13 3



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Meanwhile, the high carrier mobility at room temperature, high Young’s modulus, and excellent thermal

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conductivity of GN motivates researchers to explore GN-based conductive ink. To date, GN-based

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conductive ink has been produced successfully for various applications in electronic components and

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circuits.14 Extensively, a multitude of additional materials are incorporated within GN-based inks to

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apply in sensors, conductive patterns, and supercapacitors. For example, Karim et al. reported an ink

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made from functionalized organic nanoparticles and GN for wearable e-textile applications.15 Hassan et

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al. demonstrated an inkjet-printed humidity sensor based on graphene and zinc oxide nanocomposite

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ink.16 Wu et al. constructed an ammonia gas sensor utilizing a hybrid ink made by doping commercial

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nano-silver ink with GN.17 All these demonstrate that the GN-based conductive ink is a promising

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substitute for traditional metallic ink. However, the combination of writing technique and GN-based

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conductive ink to explore flexible sensing devices has not been reported yet.

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Potentiometry, as one of the most portable and common electrochemical techniques, has

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traditionally exhibited an unrivalled simplicity of instrumentation and operation.18-19 Therefore, it is still

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one of the workhorses in clinical laboratories and almost the universal method to measure pH in various

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practical conditions.20 Ion selective electrodes (ISEs) as one kind of typical potentiometric sensors are

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widely used in environmental monitoring, industrial analysis, and medical diagnosis on account of their

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tremendous advantages including simple analytical procedure, rapid response, and relatively low cost.4,

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21-22

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casting the electrode substrate with membrane solution, bringing great improvements to the detection

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sensitivity, and extending the application range, as well as simplifying the structure, operating mode,

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and maintaining of sensors.23 Notwithstanding, the poor ion-to-electron transduction between the

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electronic conductor and the ionically conducting ion-selective membrane (ISM) along with the

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formation of water layer at the interface may cause the potential instability and irreproducibility. To

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solve these problems, diversified intermediate solid contacts are introduced, such as conducting

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polymers (polypyrrole and polyaniline),24-26 carbon nanomaterials (carbon nanotubes, GN, fullerene,

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and ordered macroporous carbon),27-30 metal nanomaterials (gold nanoclusters, platinum nanoparticles,

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and silver nanoparticles),31-34 and etc. Under continuous efforts, the state-of-art of ASS-ISEs have

Particularly, the rapid development of all-solid-state ISEs (ASS-ISEs) in recent years by directly

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reached superior performances with improved internal stability, outstanding resistance to redox

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interferents, light, as well as soluble oxygen. However, most of ASS-ISEs are made from glassy carbon

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electrode (GCE) and metallic electrode as electrode substrates, which are lack of flexibility, generally

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incapable to meet the emerging demand for flexible electronics. Therefore, the search of flexible

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electrode substrates with effective ion-to-electron transduction and easy fabrication for potentiometric

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sensors is still a big challenge.

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In this work, we combined the writing technique and GN-based conductive ink to fabricate a

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flexible all-written potentiometric sensor. The GN-based ink was mainly composed of GN as conductive

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component, a non-ionic surfactant of Triton x-100 as a stabilizer to avoid disrupting the electrostatic

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stabilization of GN, and a water-soluble polysaccharide of xanthan gum as a binder to increase the

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viscosity of the ink.35 The fabrication of GN ink-based writing electrode (denoted as GN-WE) was

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performed using a brush pen. In order to fabricate a potentiometric sensor, ion selective membrane (ISM)

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and reference membrane (RM) was then coated on the surface of two flexible GN-WEs (Scheme 1)

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successively through writing with a brush pen. Here, we selected cadmium ion selective membrane

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(denoted as Cd2+-ISM) as a model, since Cd2+ has undesirable effects on the kidney, liver, skin, bone,

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and teeth, it can accumulate and store in living organisms that may lead to cancer.36-38 Results

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demonstrate the obtained flexible all-written potentiometric sensor, i.e. Cd2+-ISM coated GN-WE

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(denoted as GN-WE/Cd2+-ISM) and RM coated GN-WE (denoted as GN-WE/RM), possesses good

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potential stability, high sensitivity, and short response time, which is superior to the traditional solid-

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contact potentiometric sensor (i.e. saturated Ag/AgCl electrode and Cd2+-ISM coated GCE, denoted as

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GCE/Cd2+-ISM) and GP ink-based potentiometric sensor (i.e. saturated Ag/AgCl electrode and Cd2+-

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ISM coated GP ink-based writing electrode, denoted as GP-WE/Cd2+-ISM).

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MATERIALS AND METHODS

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Reagents. All the chemicals were of analytical grade and were used as received. Graphene water

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slurry (thickness of 3-10 nm, size of 5-10 µm) with its solid content of 5.0 wt% was purchased from

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XFNANO Materials Tech Co., Ltd. (Nanjing, China). Triton x-100, xanthan gum, potassium

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tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB), poly(vinyl chloride) (PVC), cadmium 5



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ionophore I (N,N,N′,N′-tetrabutyl-3,6-dioxaoctane-dithioamide, ETH 1062),39 tetrahydrofuran (THF),

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2-nitrophenyl octyl ether (NPOE), n-butyl arylate (monomer), 2,2-dimethoxy-2-phenylacetophenone

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(photoinitiator), 1,6-hexanediol diacrylate (cross-linker), and ETH 500 were obtained from Sigma-

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Aldrich. Graphite powder with a ≤30 µm particle size (≥99.85%), KCl as well as nitrites including

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Pb(NO3)2, Ca(NO3)2, Cd(NO3)2, Zn(NO3)2, Mg(NO3)2, Ni(NO3)2, Cu(NO3)2, Co(NO3)2, and AgNO3,

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were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The deionized water

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was employed to prepare all the aqueous solution.

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Preparation of GN-based Ink and GP-based Ink. The Triton x-100, a non-ionic surfactant, was

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chosen to stabilize the GN or GP suspensions. The xanthan gum, a water-soluble polysaccharide, was

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selected to act as a binder that can be utilized to increase the viscosity of the ink.35 The GN-based ink

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consisted of 1:1.7:133 Triton x-100:xanthan gum:GN by mass was prepared by ultrasonication for half

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an hour. The preparation of GP-based ink was the same as the used method as above.

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Fabrication of All-written Potentiometric Sensor. As illustrated in Scheme 1, for preparation of

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the writing electrode, firstly, the brush pen was soaked into the GN-based or GP-based ink before writing

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on the PET membrane. There is a shear stress applied on the solution existing between the two

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boundaries, i.e. the solution-substrate and the solution-brush interfaces during writing process. After the

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ink dried at room temperature, a slight pressing is applied on the GN-based electrode or GP-based

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electrode. In addition, the thickness of GN or GP electrodes can be adjusted by the number of writing

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cycles (writing once with the brush pen soaked in ink is defined as one-cycle writing, once again to dip

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the brush pen in ink and then repeat the writing on the original handwriting trace, then achieving two-

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cycles writing, note that each cycle writing uses the same force), it is optimized to three writing cycles

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(the thickness of GN-WE is about 100 µm). The size of electrode is approximately 6 mm × 20 mm, a

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sensing part with a size of 6 mm × 5 mm and an electrical conducting area with a scale of 6 mm × 5 mm

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were obtained at both ends of electrode, the middle part is covered with plastic mask in order to prohibit

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contacting with water during test.

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To prepare the Cd2+-ISM, 1.0 mg of cadmium ionophore ETH 1062, 65.8 mg of NPOE, 0.2 mg of

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KTFPB, and 33.0 mg of PVC were added into 1.0 mL of organic solvent (THF). The obtained mixture 6


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was then sonicated for 30 min. For RM, it consists of 13 mg of AgCl (obtained by mixing 3.0 M AgNO3

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and 3.0 M KCl solutions, vacuum drying oven evaporates moisture), 152.4 mg of 2,2-dimethoxy-2-

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phenylacetophenone (as photoinitiator), 20 µL of cross-linking mixture (cross-linking mixture made up

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of 14.2 mg of 1,6-hexanediol diacrylate (as cross-linker) and 1.1 mL of n-butyl acrylate (monomer)),

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34 mg of KCl, 180 µL of n-butyl acrylate, and 5 mg of ETH 500.4 To fabricate the flexible cadmium

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ion selective electrode (Cd2+-ISE), the Cd2+-ISM solution was directly written on the surface of sensing

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part of GN or GP based-electrode utilizing a brush pen, and the GCE/Cd2+-ISM was prepared by drop-

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casting the Cd2+-ISM solution onto the bare GCE. As for the reference electrode (RE, i.e. GN-WE/RM),

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we also used writing method to cover over the GN-WE with RM, then an ultraviolet lamp (~4 W,

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spanning wavelengths: 320-440 nm) was used for photopolymerization for 5 min under argon flow.

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After that, all the fabricated Cd2+-ISEs were left to evaporate the organic solvent overnight at 25 ± 1ºC.

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Prior to the test in sample solutions, these Cd2+-ISEs should be soaked in 10–3 M Cd(NO3)2 aqueous

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solution for 12 h and finally conditioned for about 24 h by soaking in an aqueous solution containing

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10–4 M Cd(NO3)2, 10–2 M NaNO3, and 10–3 M Na2EDTA, and for the GN-WE/RM, we conditioned it

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in 0.1 M KCl aqueous solution for more than 24 h.

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Characterization. Scanning electron microscopy (SEM) images were collected on a Zeiss Ultra-

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55 field emission scanning electron microscope (Carl Zeiss Microscopy, Germany). Raman spectra was

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carried out by a LabRAM HR Evolution Raman microscope system (Horiba Jobin Yvon). A He-Ne

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laser (632.8 nm) was used for excitation. The film resistance was optimized on a ST2263 Double testing

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digital four-probe tester using a four-point-probe method.

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Measurement of Electromotive Force. The potentiometry tests were conducted with a portable

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PalmSens instrument (Palm Instrument BV, Houten) which connects to a palmtop PC by means of an

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embedded wireless bluetooth module. A two-electrode system was used for the measurements of

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electromotive force (EMF), whereas the fabricated Cd2+-ISE was employed as working electrode, the

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GN-WE/RM was used as RE coupled with GN-WE/Cd2+-ISM to compose an all-written potentiometric

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sensor. For GP-WE/Cd2+-ISM and GCE/Cd2+-ISM, the RE of a saturated Ag/AgCl electrode (3.3 M

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KCl solution) was adopted. All the potentiometric tests were operated at room temperature (25 ± 1ºC). 7



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Electrochemical Impedance Spectroscopy and Chronopotentiometry Measurements. A

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Solartron Analytical model 1260 Impedance-Gain-Phase Analyzer coordinated with a model 1287

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Electrochemical Interface (Solartron Analytical, Farnborough, UK) were used to perform the

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electrochemical impedance spectroscopy (EIS) experiments. The parameters of EIS measurements were

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as follows: DC potential is 0 mV, AC amplitude is 50 mV, and scan frequency ranges from 900 kHz to

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1.0 Hz. Chronopotentiometric studies were performed with a CHI 760E Electrochemical Workstation

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(CH Instruments, USA). A constant current of +1 nA was applied on the Cd2+-ISEs last for 100 s and

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subsequently a constant current of -1 nA was immediately applied on the Cd2+-ISEs with 100 s, the

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potential was recorded as a function of time. Both of the two experiments were performed with the

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traditional three-electrode system, in which the reference electrode was served by GN-WE/RM (for GN-

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WE/Cd2+-ISM) or a saturated Ag/AgCl electrode (3.3 M KCl solution) (for GCE/Cd2+-ISM or GP-

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WE/Cd2+-ISM), the fabricated Cd2+-ISE was used as working electrode, and a platinum wire was

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employed as counter electrode.

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Real Sample Application. Lake water is used as a typical representative of water for sample test.

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The collected samples were filtered using a disposable needle filter (Sigma-Aldrich), in which the pore

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size is 0.45 μm and the diameter of filter is 13 mm. The recovery test was used to estimate the practical

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applicability of the fabricated GN ink-based all-written potentiometric sensor in real samples. In

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addition, the Cd2+ levels in lake water samples were also detected by inductively coupled plasma mass

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spectrometry (ICP-MS) machine (ELAN DRC-e, PekinElmer, USA).

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RESULTS AND DISCUSSION

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Characterization of GN-WE. The fabricated GN-WE was firstly characterized by SEM and

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Raman spectra. The SEM images were applied to observe the morphology of the surface and cross-

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section of our prepared electrode. As shown in Figure 1A, a flat surface is observed resulting from the

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applied pressing during the electrode fabrication process, and the cross-section image of GN-WE

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(Figure 1B) reveals that GN-WE has a clearly layer-by-layer structure, demonstrating that the pressure

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applied on the electrode did not destroy the layered structure of GN. It is notable that the thickness of

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writing electrode can be adjusted by altering the writing cycles. Furthermore, the crystal structure of the 8


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as-prepared GN-WE was investigated by Raman spectra measurement. As shown in Figure 1C, the

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obtained Raman spectra of different positions on electrode are almost identical, suggesting a uniform

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structure of GN-WE. Moreover, two prominent peaks at 1335 and 1581 cm–1 are observed in the

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spectrum of GN-WE, corresponding to the well-documented D and G bands, respectively. This Raman

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spectrum is the same as that of GN,4 indicating that the writing and pressing processes did not break the

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crystal lattice of GN sheets.

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To optimize the writing cycle of GN-WE, we adopted five GN-WEs of different thickness with

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writing one-cycle, two-cycles, three-cycles, four-cycles, and five-cycles separately. As shown in Figure

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S1 (Supporting Information), the film thickness increased from 31 μm to 182 μm with the increase of

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writing cycles. While the square resistance reduces with the increase of writing cycles (Figure 1D),

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indicating the conductivity of GN-WE can be enhanced by increasing the writing cycles. In view of the

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synthesized factors including flexibility and conductivity (proper thickness will improve the

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conductivity, but too thick will affect its flexibility), three-cycles was selected as the ultimate writing

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condition, and the thickness (~100 µm) of GN-WE obtained from three-cycles writing condition was

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confirmed by the entire cross-section image of SEM (Figure S1).

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In order to demonstrate the mechanical flexibility of GN-WE, we measured the electrical resistance

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of GN-WE before and after various bending angles (30°, 60°, and 90°) and cycles (10, 20, 50, and 100)

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utilizing the four-point-probe method. As shown in Figure 2A, there is no obvious fluctuation of the

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square resistance values after a series of bending cycles, indicating a favorable flexibility of GN-WE.

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To further investigate the conductivity and flexibility of the GN-based conductive ink writing trace on

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different substrates, we used the light-emitting diode (LED) to demonstrate. As illustrated in Figure

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2C, the handwriting of word ‘ZJU’ on a cellulose paper can form an electrical pathway. Moreover, the

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brightness of LED was not influenced by the mechanical stress when using PET as substrate (Figure

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2D). The ability withstand mechanical stress like bending is an attractive property that allows longer

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lifetime of the sensor.

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EIS was applied to investigate the electron transfer kinetics of a redox probe at the developed GN-

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WE, GCE, and GP-WE. As illustrated in Figure S2 (Supporting Information), a high-frequency 9



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semicircle was observed in all EIS curves of these three electrodes. The charge transfer resistance (Rct)

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value for the [Fe(CN)6]3–/4– redox probe is computed by measuring the diameter of the high frequency

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semicircle in the Nyquist plots. By fitting the data using the equivalent circuit (inset of Figure S2), the

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value of Rct (~131 ± 10.8 Ω) for GN-WE is similar to that (~133 ± 2.1 Ω) obtained at GCE, which is

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much smaller than that of GP-WE (~380 ± 4.7 Ω).

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Potentiometric Performance of GN Ink-based All-written Potentiometric Sensor. The

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potentiometric characterizations of GN-WE/Cd2+-ISM coupled with GN-WE/RM were performed by

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recording the EMF value as a function of time with gradually increased concentrations of Cd2+, for

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comparison, the GP-WE/Cd2+-ISM and GCE/Cd2+-ISM were measured as well. The sensitivity and

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linear range of GN-WE/Cd2+-ISM were shown in the dynamic curve (Figure 3A) of EMF value

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recorded for increasing the concentration of Cd2+ in the solution and the corresponding calibration

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curves (Figure 3B). After each addition, no obvious perturbations or random noise can be seen,

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indicating the Cd2+ sensor works well. The response of GN-WE/Cd2+-ISM was almost Nernstian

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exhibiting a slope of 28.57 mV/decade (standard deviation was 0.18 mV/decade, R2 = 0.9992) with a

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linear range from 10−6 to 10−2.5 M (Figure 3B), the detection limit of GN-WE/Cd2+-ISM was about

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10−6.5 M. In contrast, the control groups of GP-WE/Cd2+-ISM (Figure S3, Supporting Information)

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and GCE/Cd2+-ISM (Figure S4, Supporting Information) exhibited linear ranges of 10−5.5-10−3 M and

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10−6-10−2 M of Cd2+, respectively, and the slopes of calibration curves were calculated to be 27.26 ±

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0.96 mV/decade (R2 = 0.9950) for GP-WE/Cd2+-ISM and 25.95 ± 0.65 mV/decade (R2 =0.99504) for

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GCE/Cd2+-ISM. The detection limits of GP-WE/Cd2+-ISM and GCE/Cd2+-ISM were 10−6.0 and 10−6.2

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M, respectively, which were calculated as the intersection of two slope lines in Figure S3B and Figure

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S4B. Furthermore, the response time of GN-WE/Cd2+-ISM was about 10 s at low concentrations (inset

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in Figure 3A), which was faster than that of GP-WE/Cd2+-ISM (inset in Figure S3A) (~26 s) and

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GCE/Cd2+-ISM (inset in Figure S4A) (~22 s). All above indicate that GN-WE/Cd2+-ISM owns a better

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analytical performance, which is ascribed to the higher ionic signal transfer efficiency of GN substrate

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than GP and GC substrates. In addition, the reproducibility of the GN ink-based potentiometric sensor

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was evaluated by measuring the Nernstian slope and the EMF value in Cd(NO3)2 aqueous solution (10–5 10


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M) of five sensors fabricated under the same conditions. As illustrated in Figure S5 (Supporting

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Information), the Nernstian slope and potential value of a certain concentration almost maintain steady.

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Since selectivity is a crucial factor when developing systems for the analysis of Cd2+ in real samples,

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herein the potentiometric selectivity coefficients defined by its relative response for the primary ion over

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the other ions present in the solution were calculated for five divalent interferences (Ni2+, Ca2+, Co2+,

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Zn2+, and Mg2+) using the separate solution method (SSM). As summarized in Table 1, the GN-

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WE/Cd2+-ISM exhibits large value of selectivity coefficients towards various interferential metal ions.

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These results are in good agreement with those obtained at the ISE using GCE as substrate (Table 1).

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All these demonstrate that the selectivity of these prepared Cd2+-ISEs are solely dependent on the Cd2+-

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ISM while show no relationship with the supporting substrates. To further investigate the flexibility of

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the fabricated all-written potentiometric sensor, the EMF values of GN-WE/Cd2+-ISM before and after

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bending were observed. As shown in Figure 2A, the EMF values of GN-WE/Cd2+-ISM keep constant

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on the whole bending processes, and the bending angle of GN-based all-written potentiometric sensor

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can keep around 90° for different bending cycles (Figure 2B).

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To further evaluate the performance of the GN ink-based sensor, chronopotentiometry, water layer

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test, and EIS were successively studied in detail. Chronopotentiometry and water layer test were

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employed to evaluate short-time and long-term stability of GN ink-based writing potentiometric sensor.

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Typical chronopotentiograms obtained at GN-WE/Cd2+-ISM and GCE/Cd2+-ISM were presented in

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Figure 4A, that is, the potential response in Cd(NO3)2 aqueous solution (10‒3 M) was recorded as a

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function of time with applying a constant current of +1 nA and subsequently a reverse current at ‒1 nA

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lasting for a same interval of 100 s. When a reversed current was applying, the potential was jumping

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immediately, according to Ohm’s law R = E/I, the total resistance of Cd2+-ISE can be calculated on basis

263

of the potential jump. For GN-WE/Cd2+-ISM (Figure 4A), the estimated overall resistance was 1.07

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MΩ, and its short-term stability of 0.024 mV s–1 can be derived from the ratio ΔE/Δt, which is much

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lower than the calculated potential drift value (0.54 mV s–1) of GCE/Cd2+-ISM over the running time.

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Furthermore, according to the fundamental capacitor equation, i.e. I = C×dE/dt, the capacitance (C)

267

value was calculated as 41.67 μF for GN-WE/Cd2+-ISM, which is much higher than that (1.85 μF) 11



268

obtained at the GCE/Cd2+-ISM.

269

The water layer test was performed to distinguish the existence of hypothetical thin water layer,

270

which could produce a severe effect on the potentiometric response of ASS-ISEs. The water layer test

271

conducted on electrochemical workstation is consist of measuring the corresponding potential value of

272

the electrode as an initial solution of the primary ion (10‒3 M Cd2+) altered for another solution of the

273

interfering ion (10‒3 M Na+), and then changed again for the initial solution (10‒3 M Cd2+). As illustrated

274

in Figure 4B, there is no obvious potential shift as the electrode is placed in order in 10‒3 M Cd2+, 10‒3

275

M Na+, and 10‒3 M Cd2+ solution for GN-WE/Cd2+-ISM, indicating the absence of water layer, which

276

can be ascribed to the hydrophobicity of the GN. Nevertheless, a slightly upward drift is observed for

277

GCE/Cd2+-ISM, as a result of the presence of water layer between the Cd2+-ISM and GCE substrate.

278

Additionally, we used the response potential measured at the last part of the water layer experiment to

279

evaluate long-term stability of GN-WE/Cd2+-ISM and GCE/Cd2+-ISM. As expected, a low drift of 0.156

280

mV h‒1 was obtained for the potential signal of GN-WE/Cd2+-ISM, which is much lower than that

281

obtained from GCE/Cd2+-ISM (0.715 mV h‒1), indicating the superior long-term stability of GN ink-

282

based all-written potentiometric sensor.

283

Moreover, impedance measurements of GCE/Cd2+-ISM and GN-WE/Cd2+-ISM were also

284

conducted and the obtained results are shown in Figure 5. The combination of bulk membrane resistance

285

and the contact resistance between the electrode substrate and the coated membrane equals the diameter

286

value of the semicircle in the high-frequency part. According to the fitted data in Figure 5, we found

287

that the integrated resistance value of GN-WE/Cd2+-ISM (~1.0 MΩ) is much smaller than that obtained

288

at GCE/Cd2+-ISM (~11.2 MΩ). The potential reason for the small resistance value at GN-WE/Cd2+-ISM

289

may be attributed to the rapid charge transfer kinetics, resulting from the excellent electronic conductive

290

and larger double-layer capacitive characteristics of the GN substrate that facilitate the charge transport

291

across the interfaces. All these demonstrate that the GN-WE can serve as an effective ion-to-electron

292

transducer that transfers ion signal of membrane into electron signal of electrode.

293

Real Sample Detection. To estimate the practical applicability of the fabricated GN ink-based all-

294

written potentiometric sensor, we adopted three river water samples for recovery test. The test was 12


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295

performed as following: the negligible Cd2+ concentration of 1.357 nM, 0.4402 nM, and 0.1642 nM for

296

three original lake water samples were firstly confirmed by ICP-MS. Then, 10 mL of the above water

297

samples were added with 10 μL of 1.0×10−2 M, 1.0×10−1 M, and 1.0 M Cd(NO3)2 aqueous solutions to

298

give final Cd2+ concentrations of 1.0×10−5 M, 1.0×10−4 M, and 1.0×10−3 M, respectively, which were

299

used for the further potentiometric measurement performed on our fabricated GN-WE/Cd2+-ISM.

300

Finally, the standard curve was constructed by recording the potential response as a function of the Cd2+

301

concentrations, which was utilized for the following analysis. For the purpose of evaluating the recovery

302

of the added concentration, 10 mL of water samples with a Cd2+ concentration of 10−5 M (added 10 μL

303

of 1.0×10−2 M Cd(NO3)2 aqueous solution) were adopted to detect the potential response. As

304

summarized in Table 2, the GN ink-based writing potentiometric sensor possesses comparable recovery

305

result with an average recovery value of 98.13%. All these demonstrate that our GN ink-based all-

306

written potentiometric sensor provides a promising alternative for rapid detection of Cd2+ in real samples.

307

CONCLUSIONS

308

In this work, a low-cost but high-performance flexible all-written potentiometric sensing device

309

was fabricated by utilizing GN-based conductive ink and brush pen-based writing technique. Results

310

show that our all-written potentiometric sensor exhibits a Nernstian response to Cd2+ with a low

311

detection limit of 10−6.5 M, fast response time (~10 s), and relative stability, which is much superior to

312

the solid-contact potentiometric sensor using GCE as electrode substrate and GP ink-based writing

313

potentiometric sensor. Furthermore, our developed all-written potentiometric sensing device can be a

314

reliable and stable Cd2+ sensor in real sample analysis. This work provides a useful avenue for using

315

brush pen-based writing technique to directly paint functional materials on flexible substrate as a new

316

generation of flexible electrode for potentiometric sensors in heavy metal ion sensing, which possesses

317

broad prospects for implementing electrochemical sensing devices into the area of agriculture and food

318

safety detection.

319

ASSOCIATED CONTENT

320

Supporting Information

321

Five figures showing the relationship between writing cycle and thickness of GN-WE, impedance 13



322

plots for GCE, GN-WE, and GP-WE in 0.1 M KCl aqueous solution containing 5.0 mM

323

K3[Fe(CN)6]/K4[Fe(CN)6] (molar ratio is 1:1), potentiometric performance of GP-WE/Cd2+-ISM,

324

potentiometric performance of GCE/Cd2+-ISM, and potentiometric response of a certain

325

concentration (10–5 M) and Nernstian slope for five GN ink-based all-written potentiometric sensors.

326

AUTHOR INFORMATION

327

Corresponding Author

328

*E-mail: [email protected].

329

ORCID

330

Jianfeng Ping: 0000-0002-0579-9830

331

Notes

332

The authors declare no competing financial interest.

333

ACKNOWLEDGEMENT

334

This research was supported by the National Natural Science Foundation of China (No. 31301468).

335

REFERENCES

336

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393


Table 1. Potentiometric selectivity coefficient values, logKpotCdM. Mn+

logKpotCdM (GN-WE/Cd2+-ISM)

logKpotCdM (GCE/Cd2+-ISM)

Mg2+

3.1×10–3

3.4×10–3

Zn2+

1.8×10–4

2.0×10–4

Ni2+

8.0×10–5

5.6×10–5

Ca+

8.0×10–5

5.5×10–5

Co2+

1.1×10–5

1.1×10–5

394

17



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Page 18 of 26

Table 2. Recovery test of Cd2+ in three kind of lake water samples. Recovery test Sample

Found (10‒6 M)

Added (10‒6 M)

Recovery (%)

Lake water 1

10.0

9.97 ± 0.41

99.7

Lake water 2

10.0

9.41 ± 0.55

94.1

Lake water 3

10.0

10.06 ± 0.77

100.6

18


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398

Figure captions:

399

Scheme 1. Schematic illustration of the fabrication process of GN ink-based all-written potentiometric

400

sensor.

401 402

Figure 1. Characterization of GN-WE. (A, B) SEM images of surface (A) and cross-section (B). (C)

403

Raman spectra for different positions of GN-WE. (D) The effect of thickness on the conductivity of

404

GN-WE.

405 406

Figure 2. Mechanical flexibility. (A) EMF value (recorded in 10–4 M Cd(NO3)2 aqueous solution) and

407

conductivity value after a series of bend cycles (10, 20, 50, and 100) with different bending angles (30°,

408

60°, and 90°). (B) Photograph of GN-WE/Cd2+-ISM bent outwards by 90°, inset: direct handwriting

409

using brush pen on PET substrate. (C) Photograph of conductive GN ink-based handwriting on paper.

410

(D) Photograph of GN ink-based handwriting on PET substrate before and after force applied.

411 412

Figure 3. Potentiometric performance of /Cd2+-ISM. (A) Dynamic curve of potentiometric response

413

recorded for increasing the concentration of Cd2+ in the solution, inset: expansion of the selected range.

414

(B) The corresponding calibration curve.

415 416

Figure 4. Chronopotentiograms and water layer test. (A) Chronopotentiograms for GCE/Cd2+-ISM and

417

GN-WE/Cd2+-ISM recorded in 10–3 M Cd(NO3)2 aqueous solution. The applied current is +1 nA for

418

100 s and −1 nA for 100 s. (B) Water layer test for GCE/Cd2+-ISM and GN-WE/Cd2+-ISM recorded in

419

10–3 M Cd(NO3)2 aqueous solution, the measurements were switched between 10–3 M Cd(NO3)2

420

aqueous solution and 10–3 M NaCl aqueous solution.

421 422

Figure 5. Impedance plots for GCE/Cd2+-ISM and GN-WE/Cd2+-ISM in 10–3 M Cd(NO3)2 aqueous

423

solution. Frequency range, from 900 kHz to 1.0 Hz; Edc, 0 V; Eac, 50 mV.

19



425

Scheme 1.

426

20


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Figure 1.

429

21



431

Figure 2.

432 433

22


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Figure 3.

435

23



437

Figure 4.

438

24


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Figure 5.

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TOC

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Fully Written Flexible Potentiometric Sensor Using ... - ACS Publications

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