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Functional Nanostructured Materials (including low-D carbon)

Direct writing and characterization of 3D conducting polymer PEDOT arrays Peikai Zhang, Nihan Aydemir, Maan Alkaisi, David Edward Williams, and Jadranka Travas-Sejdic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02289 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018

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ACS Applied Materials & Interfaces

Direct writing and characterization of 3D conducting polymer PEDOT arrays Peikai Zhang,†,‡ Nihan Aydemir,†,‡ Maan Alkaisi,‡,§ David E. Williams,†,‡ Jadranka Travas-Sejdic,†,‡* †

School of Chemical Sciences, The University of Auckland, Auckland, New Zealand

‡

MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand

§

Electrical and Computer Engineering, College of Engineering, University of Canterbury, Christchurch,

New Zealand

ABSTRACT: Direct writing is an effective and versatile technique for 3D fabrication of conducting polymer (CP) structures. It is precisely localized and highly controllable; thus provides great opportunities for incorporating CPs into micro-electronic array devices. Herein we demonstrate 3D writing and characterization of poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) pillars in an array format, by using an in-house constructed variant of Scanning Ion Conductance Microscopy (SICM). CP pillars with different aspect ratios were successfully fabricated by optimising the writing parameters: pulling speed, pulling time, concentration of the polymer solution and the micropipette tip diameter. Especially, super high aspect ratio pillars of around 7 µm in diameter and 5000 µm in height were fabricated, indicating a good capability of this direct writing technique. Addition of an organic solvent and a cross-linking agent contribute to a significantly enhanced water-stability of the pillars, critical if the arrays were to be used in biologically relevant applications. Surface morphologies and structural analysis of CP pillars were characterized by SEM and Raman spectroscopy, respectively. Electrochemical properties of the individual pillars of different heights were examined by cyclic voltammetry using a double barrel micropipette as an electrochemical cell. Exceptional mechanical properties of the pillars, such as high flexiblility and robustness, were observed when bended by applying a force. The 3D pillar arrays are expected to provide versatile substrate for functionalised and integrated biological sensing and electrically addressable array devices. Keywords: Direct writing, Conducting polymer, SICM, Micro pillar arrays, Cyclic voltammetry fabricate micro/nano structured CPs for advanced applications,

INTRODUCTION

including

soft

lithography,19

dip-pen

Bioelectronic devices often employ small, biocompatible and

electrodeposition13

functional elements, such as electrode arrays. The success of

techniques are limited mainly to 2D fabrication. Inkjet printing,22

these devices depends on the availability of suitable materials for

PDMS transfer printing12,

and

electrospinning.21

lithography.20

15

However,

these

and traditional photolithography17

the functional elements and cost-effective fabrication methods.

approaches are used to fabricate 3D structures, but are capable of

Conducting polymers (CPs) have emerged as one of the ideal

producing mainly low aspect ratio structures.

candidate materials for remarkable

properties,

bioelectronics,1-4 including

owing to their

electrical

To fabricate high aspect ratio 3D CP structures a new technique

conductivity,

of direct writing has recently been developed. The technique has

biocompatibility, mechanical flexibility and possibility of added

a capability of fabricating very high aspect ratio CP micro

functionality by chemical synthesis.5-6 Therefore CPs have been

structures in a precisely localized and highly controlled fashion 10,

5,

22-27

widely utilized in biological applications such as DNA sensors, 7-8

stretchable interconnects for tissue engineering,

9-11

.

selective

Recent studies have been focused on the direct writing of

capture12-14 or release15-16 of circulating tumor cells and

conducting micro-wires and their possible applications as

mechanical17 and electrical stimulation18 to neural stem cells.

electronic devices, such as stretchable PEDOT:PSS electrodes10

Various techniques have been developed in the last decades to

and flow sensor28, PbS quantum dots-poly(3-hexylthiopehene) 1

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(P3HT) hybrid nanowire photodetectors,11, poly(methyl

methacrylate)/polypyrrole 29-31

reduced graphene oxide gas sensors.

27

or PEDOT:PSS,

(PMMA/PPy)

Apparatus, 2.0 mm OD × 0.30 mm Wall × 0.17 mm Septum ×

and

100 mm L).

However, little work has

been done towards the use of high aspect ratio CP structures in

METHODS

bioelectronics applications. Compared with previously reported

Direct writing

electrode cell-sensing platforms, such as in-plane 2D Au

The mixed polymer ‘ink’ solutions were sonicated for 5 mins

electrodes,32 semiconducting wires,33 PDMS pillars34-35 or low

before the use to ensure a good dispersion of the polymer. The

aspect ratio structures,12 there is a promising prospect for high

solutions were then injected by a MicroFil (a micropipette filling

aspect ratio CP probes in such applications, as they provide

capillary, World Precision Instruments) into the barrel of

advantages of high surface area, intrinsic electrical conductivity,

micropipettes.

biocompatibility, electrochemical activity as well as mechanical

A 0.125 mm diameter platinum (Pt) wire that served as a

flexibility. Therefore, 3D CP structures could be used as ‘soft’

counter electrode was inserted into the micropipette. The

local electrochemical probes for biological sensing. . For

micropipette was then immobilized to the piezo actuator (25 mm

example, while the bending of non-conducting PDMS pillars

compact motorized translation stage, Thorlabs) (Figure 1A) that

have been recently used for the force measurement from small

controlled movement of the pipette. The writing process was

biological organism,34-36 3D CP pillars would allow for a

monitored by a high magnification zoom lens (MVL12X3Z,

combined electrical and mechanical force sensing. To that aim,

Thorlabs) connected USB CMOS camera (DCC1645C, Thorlabs)

understanding of the properties of single CP micro structures is

and controlled by a home-written Labview program. Au/Ti (~40

needed, particularly the relationship between the 3D structure

nm/50 nm) coated slides (Deposition Research Lab, Inc.) were

size, shape and composition and their electrochemical and

used as substrates for direct writing. Constant potential of 0.5 V

mechanical properties.

between substrates and the counter electrode was applied by a

Herein, we report on direct writing of high aspect ratios CP

potentiostat. This direct current signal was used to detect

pillars into an array format, made of modified PEDOT:PSS ‘ink’,

contact/separation status of the micropipette from the substrate.

and on localized characterization of the pillars, both performed by

After the direct writing of the pillars, CP arrays were annealed at

the use of a homemade variant of Scanning Ion Conductance

80 oC in air for 1 h, at atmospheric pressure.

Microscopy (SICM) system.

24, 37-39

A highly improved stability of

the pillars in aqueous systems was achieved by modifying the

Characterization

PEDOT:PSS ‘ink’. The fabricated 3D arrayed structures possess

Scanning Electron Microscopy (SEM) and Raman spectroscopy.

promising electrochemical, electrical and mechanical properties

JCM-6000 SEM (15 kV electron beam, high vacuum mode) was

making these arrays attractive platform substrates for various

used to characterize the surface morphology of the pillars. The

bioelectronics applications.

substrates were tilted to facilitate imaging. Renishaw Raman System 1000 Spectrometer (785 nm excitation) was used for

MATERIALS

structural analysis of the pillars. The direct writing of 100 µm CP

Micropipettes were fabricated from single barrel borosilicate

pillars of four different formulations was performed onto Au

capillaries (Harvard Apparatus, 1.0 mm OD × 0.58 mm ID × 100

coated glass slides. The slides were then mounted on the Raman

mm L) using a laser puller P-2000 (Sutter Instrument). The inner

spectrometer and the laser was focused on the ‘head’ of each

diameter of their tips could be controlled to range from a few

pillar during the scanning.

hundred nanometers to around twenty micrometers by adjusting pulling parameters. Generally, the pipettes with 2 -20 µm inner

Electrical conductivity. The electrical conductivities of CPs are

diameter (ID) tips provided robust and reproducible writing

measured by Jandel RM2 four point probes on drop casted films.

results. As the writing ink, PEDOT:PSS (conductive grade, 1.3

The thickness of the films are measured by a high accuracy

wt% dispersion in H2O), ethylene glycol (EG), dimethyl

digimatic indicator (Mitutoyo ID-H0530E).

sulfoxide (DMSO) and (3-glycidyloxypropyl)trimethoxysilane (GOPS) were purchased from Sigma-Aldrich. For localized

Cyclic voltammetry (CV). For the localized CV measurements,

cyclic voltammetry (CV) measurements, a three-electrode set up

both barrels of the double barrel pipettes were filled with a 0.2

with double barrels pipettes was employed, where the pipettes

wt% PSS water solution, followed by insertion of a Pt wire as the

were fabricated from double barrels capillaries (Harvard

counter electrode into one barrel, and a Pd-H240 wire as reference

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electrode into the other barrel. The pipette was moved by the

switch to a fast pulling speed (usually 200 µm/s) terminated the

piezo actuator to touch the top of each pillar for the individual

writing process (Figure 2C). The last step leaves a small droplet

CV characterization. CHI700D (CH Instruments, Inc) potentiostat

of the solution on the top of the pillar that dries and shrinks to a

was employed for the CV experiments. Their CVs were recorded

small ‘cap’ on top of the pillar. The pipette then moves to the next

from -0.2 V to 1.0 V with a scan rate of 0.1 V/s.

spot for writing of the subsequent pillars (Figure 2 D).

Mechanical properties. Mechanical properties were derived qualitatively by observing the deformation process and recovery of the deformation of the CP pillars upon applying forces onto the pillars by an empty micropipette.

RESULTS AND DISCUSSION The SICM system and a schematic of the direct writing process is shown in Figure 1. The working mechanism is based on the evaporation of water during the pulling process leaving ‘written’ 3D pillars behind. The high surface area of CP pillars, due to their small diameter, warranted a sufficient evaporation rate of water Figure 2. Optical photographs taken by a CMOS camera of 3D direct

under room temperature and atmospheric pressure conditions (20-

writing process: (A) SICM establishes a contact between the micropipette

23 oC, humidity: 55 - 65 %) to form mechanically stable pillars;

and the substrate. (B) Pulling process at pre-determined pulling speed.

thus no heating or reduced pressure was needed.

Water evaporates during the pulling, a polymer pillar fabricated. (C) Fast

The direct writing process by the SICM system was initiated

pulling upwards of the micropipette terminates the fabrication of the

by moving the micropipette, which was pre-filled with the CP

pillar. (D) Pipette moves to a next position. Both the height of the pillars

solution, down to slowly approach the substrate. A constant

and the distance between two pillars are 100 µm.

potential of 0.5 V was applied between the Pt wire inside the pipette and the gold electrode substrate. Once the meniscus of

We have fabricated arrays of CP pillars of different aspect

polymer solution at the tip of the pipette was in a contact with the

ratios. There are three main factors that determine the diameter of

substrate (Figure 2A), the circuit was closed and a significant

the CP pillars: the micropipette tip diameter, the pulling speed,

current signal was detected. This signal was used to confirm the

and the evaporation rate. Therefore, to have a repeatable

contact, complemented by the USB CMOS camera.

fabrication process of the pillars with desired diameter, an appropriate combination between these three factors is required. The micropipette tip’s inner diameter is the most critical parameter in determining the ‘written’ pillars’ diameter. We produced the micropipettes of ranging IDs, from a few hundred nanometers to around twenty micrometers. However, as thinner tips suffer from the blockage, only micropipettes of larger than 2 µm ID tips provided robust and reproducible results. The largest IDs tips micropipettes we used for writing had ID of ~20 µm, as limited by the laser puller, and still they were very effective for

Figure 1. The SICM system (A) showing the micropipette, holder, piezo

writing.

actuator, Au electrodes and the connection. (B) is a schematic of the direct

The effect of micropipette pulling speed on the diameter and

writing set up.

height of CP pillars is shown in Figure 3. The pillars set to be 100 Upon establishing the contact, the micropipette was pulled up

µm high were written with different pulling speeds ranged from

slowly with a pulling speed ranged from 1 to 8 µm/s, depending

0.5 µm/s to 8 µm/s, using the same micropipette (~10 µm ID).

on the concentration of the polymer solution, temperature,

Their diameter was found to be 9.3 µm and 4.2 µm and their

humidity and the micropipette tip diameter. Commonly, 1-2 µm/s

height 92 µm to 55 µm, respectively. When the speed is too low

provided a high pillar writing success rate and repeatability.

(lower than 0.5 µm/s in this case), the pipette blockage is risked, while if the pulling speed is too high (higher than 8 µm/s in this

When the CP pillars reached the desired height (Figure 2B), a



Page 4 of 20

case), there is not enough time for water to evaporate and pillars

either EG or DMSO, were added into the ‘ink’ to decrease the

to solidify. In the latter case, not yet dried pillars could be pulled

evaporation rate and stabilize the ‘written’ structures. A

broken and damaged (e.g. for pulling speed 8.5 and 9.0 µm/s, as

crosslinking agent, GOPS, was shown previously to ‘lock’

shown in Figure 3).

PEDOT:PSS chains via hydrolysis and condensation of silane groups, thus improving the PEDOT:PSS physical stability upon contact with water.18, 44, 47-48 Based on those studies, GOPS was also added to our ‘ink’ to further stabilize the printed structures. Different combinations of EG, DMSO and GOPS additives to the PEDOT:PSS dispersion were investigated to produce an optimized ‘ink’ formulation in terms of the ‘writing’ capability and stability of the ‘written’ structures in deionized water. The investigated formulations were divided into four groups: PEDOT:PSS + DMSO (noted as ‘PEDOTD’); PEDOT:PSS + DMSO + GOPS (‘PEDOTD-G’); PEDOT:PSS + EG (‘PEDOTE’) and PEDOT:PSS + EG + GOPS (‘PEDOTE-G’). In each group, different ratios of PEDOT:PSS to the organic solvent and GOPS were tested for ‘writing’ of the inks (see supporting information Table S1 for details). The four formulations (one from each

Figure 3. CP pillar arrays written by different pulling speed. All these

group), that have produced the best writing effectiveness, are

pillars were set to be 100 µm high and were written using a same

presented in Table 1, along with the used pipette diameter (ID)

micropipette. The pulling speed ranged from 0.5 µm/s to 9.0 µm/s. (A)

and pulling speed optimized for each formulation (see Supporting

SEM image; (B) and (C) are optical photographs of the same array in (A),

Information, Table S2).

taken by the CMOS camera. They are the top row and bottom row respectively

Table 1 Four optimised ‘ink’ formulations, the used pipettes’ inner diameters and pulling speeds

Therefore, the faster the pulling speed the thinner and shorter

PEDO

DMSO

EG

GOPS

IDTip

the pillars are produced, and vice versa, slow pulling leads to

T:PSS

(µL)

(µL)

(µL)

(µm)

thicker and higher pillars. In this work, pulling speeds of 1-2

(µL)

Pulling Speed (µm/s)

µm/s demonstrated high repeatability and a good control over the

PEDOTD

500

150

-

-

5

1

pillars size, thus, for the majority of work discussed below, 1-2

PEDOTD-G

500

175

-

10

20

2

µm/s pulling speed was used. Apart from the pipette tip diameter and pipette pulling speed, the ‘ink’ formulation is another major optimization parameter, as

PEDOTE

500

-

50

-

10

1

PEDOTE-G

500

-

30

10

20

1.5

SEM images of the fabricated PEDOT pillars of different

it not only determines the evaporation rate of solvents, but

aspect ratios, prepared using the formulations presented in Table

importantly, the subsequent stability of the CP pillars in an

1, are shown in Figure 4. The pillars in images (A), (G) and (H)

aqueous environment as well. A high evaporation rate is usually

are fabricated from formulation PEDOTE-G; in (B) from

accompanied with a high chance of blockage, while too slow

PEDOTD-G; in (C) and (D) from PEDOTD; in (E) and (F) from

evaporation would be difficult for the written structure to stabilize

PEDOTE.

and to mechanically support continuous writing process. It was observed that the commercially obtained PEDOT:PSS aqueous solutions have higher evaporation rate than desirable, thus making the writing process difficult. A critical problem related to the use of PEDOT:PSS in aqueous solutions is its water instability: PEDOT:PSS re-disperse homogeneously in hot water41. Different organic solvents have been reported to have a stabilising effect on PEDOT,:PSS and to increase its conductivity and electrochemical activity.41-46 Taking the above points into consideration, organic solvents –


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and 7 µm diameter) PEDOT pillar. Image (A) shows the whole pillar. (B) its base standing on a Au substrate and (C) the enlarged part of the pillar.

The stability of the fabricated pillars using these different formulations was qualitatively tested by simply immersing the pillars in deionized water for different lengths of time (see supporting information, Section 3). With EG or DMSO added into the ‘ink’, the written PEDOT pillars do not dissolve, but can be easily collapsed by the flow. With the addition of GOPS to the formulations, the majority of the pillars left standing even when immersed in water for weeks. Comparing the investigated formulations, the PEDOTE-G pillars demonstrated the supreme stability. The PEDOTE-G pillars were stable in water for weeks without any noticeable sign of dissolution. As Raman spectrum reflects the conformation of PEDOT chains and its oxidation state,41, 51 Raman spectra of the pillars were taken in order to gain an insight into the excellent stability of PEDOTE-G pillars. The spectra of the four investigated ‘ink’ formulations were compared in Figure 6. The most prominent Raman peak observed between 1400 and 1500 cm-1 corresponds to the stretching vibration of Cα=Cβ on the five member PEDOT ring51. That peak position for PEDOT:PSS pillars with just EG or DMSO added to the formulation were similar (with that for Figure 4. SEM images of fabricated 3D PEDOT arrays. (A), (G) and (H)

PEDOTD a bit wider), while there was a shift to lower

are PEDOTE-G; (B) is PEDOTD-G; (C) and (D) are PEDOTD; (E)

wavenumbers when GOPS was added. The peak position for

and (F) are PEDOTE. (D), (F) and (H) are enlarged single pillar images

PEDOTD and PEDOTE is around 1430 cm-1, while for PEDOTD-G

of (C), (E) and (G), respectively.

and PEDOTE-G are at 1420 cm-1 and 1413 cm-1 respectively. The shift to lower wavenumbers indicates more expanded PEDOT

The surfaces morphology of the pillars appears smooth without

chains, thus leading to stronger intermolecular interactions,9, 41, 46,

much roughness or protruding parts, consistent with previously

52-53

reported PEDOT:PSS microstructures.10, 49-50 The diameter of the

and in turn, to an enhanced water stability of the pillars.

The electrical conductivities of these four formulations were

pillars in Figure 4 ranged from 5 µm (Figure 4D) to 20 µm

measured on drop casted films (see Experimental section), and

(Figure 4H). However, pillars of really high aspect ratio were

found to be ca. 35.5 S/cm of PEDOTD, 14.9 S/cm of PEDOTD-G,

possible to be obtained. For example, we fabricated freestanding

50.4 S/cm of PEDOTE and 14.0 S/cm of PEDOTE-G. That

PEDOTD-G pillars with around 7 µm in diameter and 5000 µm in

compares with 4.8 S/cm of pristine PEDOT:PSS.

height (Figure 5), without reaching the limitation. It is to note, however, that with such extremely high aspect ratios, the pillars may start to bend under their own weights (Figure 5A).

Figure 6. Raman spectra (785 nm excitation) of the pillars made using the four optimized writing formulations: PEDOTD, PEDOTD-G, PEDOTE,

PEDOTE-G. Figure 5. SEM images of a freestanding high aspect ratio (5000 µm high



Page 6 of 20

Electrochemical activity of the individual PEDOTE-G pillars in the array was investigated in a three-electrode setup by means of localized cyclic voltammetry (CV) (see Experimental for details), as schematically shown in Figure 7. Localized CV measurements were carried out by lowering the double-barrel pipette on top of the pillar until a contact between the electrolyte drop at the bottom of the pipette and the pillar was secured.

Figure 7. Localized cyclic voltammetry characterization of the PEDOTE-G pillars of different height. (A): Schematics of CV characterization using the three-electrodes setup. (B) An optical photograph taken during the CV process. Figure 8. Cyclic voltamogramms of the PEDOT pillars of different heights. (A): 50 µm, (B): 100 µm, (C): 150 µm, (D): 200 µm, (E): 250

The cyclic voltamogramms of the pillars of different heights

µm, (F): 300 µm. The scan rate was 0.1 V/s. For each height, three CV

(50, 100, 150, 200, 250 and 300 µm) (Figure 7B) were compared.

cycles were obtained on three pillars (marked as 1, 2 and 3 and

For each pillar height, at least three CV cycles were collected on

corresponding to red, blue and green curves, respectively).

at least three pillars of the same height (marked red, blue and green), and presented in Figure 8. The same double-barrels

The excellent electrochemical stability of the pillars was

pipette was used to measure all of the CVs shown in Figure 8,

demonstrated by performing one hundred cyclic voltammograms

employing 0.1 V/s scan rate. While all of the pillars demonstrated good

electrochemical

activity,

some

difference

in

on a 100 µm and a 150 µm high pillars (Figure 9). The stability of

the

the PEDOT:PSS pillars can be attributed to the added EG and

voltamogramms can be seen for the pillars of different heights.

GOPS. 41, 46, 54.

For the pillars of height below 150 µm there are two oxidation peaks observed, around 0.05 V and 0.7 V (vs. Ag/AgCl), respectively (Figure 8 A-C), while for the higher pillars the first oxidation peak (0.05 V) diminished or disappeared (Figure 8 DF) with no significant difference observed for the second oxidation peak (0.7 V) and reduction peak (seen at 0.65 V). The former difference between pillars of different height might be Figure 9. (A) 100 CV cycles of a 100 µm high pillar. (B) 100 CV cycles

caused by the increased electrical resistance for the longer pillars.

of a 150 µm high pillar.

Mechanical properties

of

the PEDOTE-G

pillars

were

qualitatively tested by applying a force onto the pillars with a micropipette. The pipette was moved around by the x, y-stages of the SICM setup to push and bend a 300 µm high pillar. We optically monitored recovery of the pillar position, its flexibility and adherence to the substrate. The pillar demonstrated exceptional resilience to the applied force, flexibility and high elasticity. The pillar was ease to bend (Figure 10 B-D), but recovered vertical position immediately after the force was removed (Figure 10 E). Forces from different directions were applied and even if the pillar was significantly bent, it still flexed


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back.

these properties bring promising prospects for the applications of the written 3D conducting polymer micro structures as soft functional elements in bioelectronics.

ASSOCIATED CONTENT: Supporting information The formulations of writing ‘ink’, their water stability test and the effects of operational parameters on the shape of PEDOT pillars. Figure 10. Mechanical properties of PEDOTE-G pillars. The height of the

AUTHOR INFORMATION

bended pillar is 300 µm. (A): A pillar before bending. (B), (C) and (D): bending of the pillar. (E): Recovery of the pillar position after removing

Corresponding author

the bending force.

*Email: [email protected]

ACKNOLOGMENTS:

For example, the 300 µm pillar was pushed to about half of its original height and bended significantly (Figure 10B), but it

This project is funded by MacDiarmid Institute for Advanced

sprang back when the pipette was removed. In the horizontal

Materials and Nanotechnology.

direction, the ‘head’ of this pillar was pushed away to its far left (Figure 10C) or right (Figure 10D). Once the pipette moved out

REFERENCES:

of the range, the pillar immediately swung back to its initial

1. Sassolas, A.; Leca-Bouvier, B. D.; Blum, L. J., DNA biosensors and microarrays. Chemical Reviews 2008, 108 (1), 109-139. 2. Guimard, N. K.; Gomez, N.; Schmidt, C. E., Conducting polymers in biomedical engineering. Progress in Polymer Science 2007, 32 (8-9), 876-921. 3. Gerard, M.; Chaubey, A.; Malhotra, B. D., Application of conducting polymers to biosensors. Biosensors & Bioelectronics 2002, 17 (5), 345-359. 4. Adhikari, B.; Majumdar, S., Polymers in sensor applications. Progress in Polymer Science 2004, 29 (7), 699-766. 5. Travas-Sejdic, J.; Aydemir, N.; Kannan, B.; Williams, D. E.; Malmstroem, J., Intrinsically conducting polymer nanowires for biosensing. Journal of Materials Chemistry B 2014, 2 (29), 4593-4609. 6. Min, S. Y.; Kim, T. S.; Lee, Y.; Cho, H.; Xu, W.; Lee, T. W., Organic Nanowire Fabrication and Device Applications. Small 2015, 11 (1), 45-62. 7. Aydemir, N.; McArdle, H.; Patel, S.; Whitford, W.; Evans, C. W.; Travas-Sejdic, J.; Williams, D. E., A Label-Free, Sensitive, Real-Time, Semiquantitative Electrochemical Measurement Method for DNA Polymerase Amplification (ePCR). Analytical Chemistry 2015, 87 (10), 5189-5197. 8. Zhu, B.; Booth, M. A.; Shepherd, P.; Sheppard, A.; Travas-Sejdic, J., Distinguishing cytosine methylation using electrochemical, label-free detection of DNA hybridization and ds-targets. Biosensors & Bioelectronics 2015, 64, 74-80. 9. Kim, J. H.; Chang, W. S.; Kim, D.; Cho, S. H.; Seol, S. K., Conductivity enhancement of stretchable PEDOT:PSS nanowire interconnect fabricated by fountain-pen lithography. Materials Chemistry and Physics 2014, 147 (3), 1171-1174. 10. Kim, J. T.; Pyo, J.; Rho, J.; Ahn, J. H.; Je, J. H.; Margaritondo, G., Three-Dimensional Writing of Highly Stretchable Organic Nanowires. Acs Macro Letters 2012, 1 (3), 375-379. 11. Yoo, J.; Jeong, S.; Kim, S.; Je, J. H., A Stretchable Nanowire UV-Vis-NIR Photodetector with High Performance. Advanced Materials 2015, 27 (10), 1712-1717. 12. Hsiao, Y. S.; Luo, S. C.; Hou, S.; Zhu, B.; Sekine, J.; Kuo, C. W.; Chueh, D. Y.; Yu, H. H.; Tseng, H. R.; Chen, P. L., 3D Bioelectronic Interface: Capturing Circulating Tumor Cells

position (upright) (Figure 10E). The above steps were repeated several times with no sign of breakage or separation from the substrate observed, showing robustness of the pillars and good adhesion to the Au substrate. Such robustness of the pillars can be attributed to the chemisorption of PEDOT onto the gold substrate through the Au-S bond.55-56 In addition, a very high aspect ratio PEDOTE-G pillar, of 5000 µm height and 7 µm diameter (aspect ratio of ca. 714), was strong enough to hold its own weight without collapsing (Figure 5A). These outstanding electrochemical and mechanical properties of the fabricated PEDOT pillars promise great prospects for PEDOT arrays in a range of bioelectronics applications, such as sensors and soft electrochemical probes to study biological cells and tissues in a 3D matrix formats.57-59

CONCLUSIONS In conclusion, high aspect ratio and water-stable PEDOT/PSS micro pillar arrays can be effectively fabricated by a direct writing technique using a micropipette. This technique has been proven to be versatile and highly controllable in regards to fabrication of different aspect ratio pillars using different formulations. With the addition of ethylene glycol and the crosslinking agent GOPS, the stability of the PEDOD/PSS pillars, as well as electrochemical activity, were highly improved. Furthermore,

these

pillars

possess

excellent

mechanical

properties: they exhibit robustness, elasticity and flexibility. All



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TOC



Table of Contents/Abstract Graphic 193x73mm (150 x 150 DPI)


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Figure 1. The SICM system (A) showing the micropipette, holder, piezo actuator, Au electrodes and the connection. (B) is a schematic of the direct writing set up. 184x86mm (150 x 150 DPI)



Figure 2. Optical photographs taken by a CMOS camera of 3D direct writing process: (A) SICM establishes a contact between the micropipette and the substrate. (B) Pulling process at pre-determined pulling speed. Water evaporates during the pulling, a polymer pillar fabricated. (C) Fast pulling upwards of the micropipette terminates the fabrication of the pillar. (D) Pipette moves to a next position. Both the height of the pillars and the distance between two pillars are 100 µm. 202x160mm (150 x 150 DPI)


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Figure 3. CP pillar arrays written by different pulling speed. All these pillars were set to be 100 µm high and were written using a same micropipette. The pulling speed ranged from 0.5 µm/s to 9.0 µm/s. (A) SEM image; (B) and (C) are optical photographs of the same array in (A), taken by the CMOS camera. They are the top row and bottom row respectively. 83x85mm (150 x 150 DPI)



Figure 4. SEM images of fabricated 3D PEDOT arrays. (A), (G) and (H) are PEDOTE-G; (B) is PEDOTD-G; (C) and (D) are PEDOTD; (E) and (F) are PEDOTE. (D), (F) and (H) are enlarged single pillar images of (C), (E) and (G), respectively. 166x247mm (150 x 150 DPI)


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Figure 5. SEM images of a freestanding high aspect ratio (5000 µm high and 7 µm diameter) PEDOT pillar. Image (A) shows the whole pillar. (B) its base standing on a Au substrate and (C) the enlarged part of the pillar. 251x141mm (96 x 96 DPI)



Figure 6. Raman spectra (785 nm excitation) of the pillars made using the four optimized writing formulations: PEDOTD, PEDOTD-G, PEDOTE, PEDOTE-G. 500x221mm (150 x 150 DPI)


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Figure 7. Localized cyclic voltammetry characterization of the PEDOTE-G pillars of different height. (A): Schematics of CV characterization using the three-electrodes setup. (B) An optical photograph taken during the CV process. 235x86mm (150 x 150 DPI)



Figure 8. Cyclic voltamogramms of the PEDOT pillars of different heights. (A): 50 µm, (B): 100 µm, (C): 150 µm, (D): 200 µm, (E): 250 µm, (F): 300 µm. The scan rate was 0.1 V/s. For each height, three CV cycles were obtained on three pillars (marked as 1, 2 and 3 and corresponding to red, blue and green curves, respectively). 119x139mm (150 x 150 DPI)


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Figure 9. (A) 100 CV cycles of a 100 µm high pillar. (B) 100 CV cycles of a 150 µm high pillar. 375x142mm (96 x 96 DPI)



Figure 10. Mechanical properties of PEDOTE-G pillars. The height of the bended pillar is 300 µm. 390x206mm (150 x 150 DPI)


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Direct Writing and Characterization of Three ... - ACS Publications

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