SU-8 nanofiber composite

Development of MWCNT/SU-8 nanofiber composite using Electrospinning Technique for Biosensing applications *

Durga Prakash M1, Siva Rama Krishna V1, Asudeb Dutta1, C.S.Sharma2 and Shiv Govind Singh1

Department of Electrical Engineering and Innovation Hub for Nano-X Laboratory, Indian Institute of Technology Hyderabad, Hyderabad 502205, India 2 Department of Chemical Engineering and Innovation Hub for Nano-X Laboratory Indian Institute of Technology Hyderabad, Hyderabad 502205, India [email protected]

Abstract—This paper reports the synthesis and characterization of Multiwall Carbon nanotube (MWCNTs) embedded SU-8 negative photoresist nanofibers. These nanofiber composites were synthesized using electrospinning technique, a simple, robust and low cost technique for synthesizing nanofibers. MWCNTs were dispersed mechanically in SU-8 by high shear mixing using an ultrasonic homogenizer device. The mixture was then electrospun onto a pre-fabricated copper electrodes array on silicon substrate. The parameters of electrospinning are optimized to precisely position nanofibers in between two electrodes of the array. The electrical and morphological analyses of the electrospun MWCNTs embedded SU-8 nanofibers were carried out by current voltage measurements (I-V), scanning electron microscopy (SEM) method respectively. Also interaction between MWCNTs and SU8 in the electrospun nanofibers was studied by XRD spectra and Raman spectra analysis. The maximum conductivity is achieved at 11% loading of carbon nanotubes and is suitable for conductometric based biosensors. Keywords—multiwalled carbon nanotube (MWCNT); SU-8; nanofibers; electrospinning; electrical conductivity

I.

INTRODUCTION

Over the last two decades, polymer nanocomposites based on carbon black, carbon nanofiber, and single/multiwall carbon nanotubes (SW/MWCNTs) have gained enormous interest due to their unique and extraordinary physical and electrical properties which were effectively utilized in various applications [1-2]. . Although carbon nanotubes (CNT) have been widely used in last decade, they applications are limited particularly in the case of biosensors as the cost involved in their production makes their use economically unviable. Furthermore CNTs as synthesized are discontinuous and have relatively low aspect ratio. CNTs further need surface modification for enhanced sensitivity. To overcome these issues with CNT, there are some recent efforts underway to replace CNT by polymer nanofibers [3]. These nanofibers have diameter in the range of some tens of nanometers to submicron and a significantly higher aspect ratio and thus larger surface area as compared to CNTs. However their mechanical and electrical properties are poor. Embedding CNTs in polymer

nanofibers overcomes the drawbacks and are ideal candidates for sensing applications [4]. In this work MWCNTs embedded SU-8 negative photoresist nanofibers were synthesized using electrospinning technique. Electrospinning process is a simple, versatile and widely used low cost method to produce nanofibers at large scale. A large number of polymers have been electrospun into nanofibers [5-9]. SU-8 is an epoxy-based negative photoresist. Which has been commonly used in MEMS applications. It is available in viscosity that can be spun or spread over thickness ranging from below 1µm up to 300 µm and still be processed with standard contact lithography [10-11]. It is also one of the most biocompatible materials known [12] and is often used in bio-MEMS also. SU-8 photoresist based on ultrafine electrospun nanofibers have been recently reported [13-14]. Positioning of a single SU-8 fiber using electrostatic self-assembly has also been demonstrated recently [15]. However to the best of our knowledge, this is first report on electrospinning of MWCNT/SU-8 composite nanofibers with fine control in their morphology and measuring their electrical conductivity. The conductivity for as-spun composite MWCNT/SU-8 nanofibers is measured for six different concentrations of MWCNT in the SU-8 solution. In order to understand the structural changes that are responsible for the increase in conductivity of these nanofibers, Raman spectroscopy, X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) are used. Our results indicate that the crystallinity and electrical conductivity of these composite nanofibers increase with increase in concentration of MWCNTs. An effective strategy for positioning, integration and interrogation of a single nanofiber requires controlled electrospinning as detailed elsewhere [15]. In this study, we also determine the maximum concentration of MWCNTs in SU-8 that allows good electrospinnability of the precursor polymer to carbon nanowires. II.

EXPERIMENTS

Standard multi-walled carbon nanotubes (MWCNTs) with diameter range of 5-20 nm was purchased from Reinste Nano Ventures Pvt. Ltd. (New Delhi, India). SU-8 (2015) was purchased from MicroChem Crop. USA. Aqueous Chloroform

(CHCl3) was purchased from Sigma-Aldrich. For photolithography, positive photoresist S1813 and its corresponding developer purchased from Shipley Inc. USA. All chemicals were analytical pure and used as received without further purification. DI water Millipore system (~18.2 MΩ cm) was used throughout the experiments. A. Electrospinning The equipment includes a syringe with needle, high voltage power supply, collector (Silicon wafer or glass), and flow controller (syringe pump via microcontroller) as shown in “Fig.1”. The collector in this case is the patterned glass substrate. The polymer solution placed in the syringe gets ejected from the syringe pump when a proper bias is applied between syringe needle and collector electrode. The polymer jet follows basically the field lines formed between two electrodes and is spread over the target surface. The electric current in the jet lines also plays an important role in determining the diameter of nanofibers. Typical voltage ranges varies from 5 kV to 20 kV. The distance between the syringe and the collector can be varied from 5 cm to 20 cm. The typical values of flow rate are in the range 1 – 5 µL/min and is usually carried out using a microprocessor controlled motorized syringe pump. In all the experiments, polymer solution was placed into a syringe for electrospinning. A highvoltage DC power supply was connected to a 25-gauge bluntnosed needle attached to the syringe containing the electrospinning solution. The electrospun fibers were collected on a grounded copper plate with attached patterned microelectrodes array of glass substrate. We were able to achieve a single fiber positioned between the two electrodes by applying a voltage of 16 kV, a tip-to-collector distance of 10 cm, and a solution flow rate of 3 µL/min for the deposition time of 3 sec. To uniformly disperse MWCNTs in the SU-8 matrix, MWCNTs samples with different conc. 8, 9, 10, 11, 12, and 13 wt% were dispersed in chloroform (CHCl3) using a probe sonicator operating at 40-60 kHz for 30 min respectively. The resulting dispersions were homogeneous and stable. This solution was then loaded into the syringe and electrospinning was carried out. In order to study the effect of MWCNTs on the conductivity of nanofibers.

B. Fabrication of copper microelectrode arrays In order to perform electrical characterization of nanofibres it is essential that the fibers are positioned in between two conductive regions. Copper electrodes arrays were fabricated essentially for this purpose using standard lift off technique on a glass substrate. Patterning is a way to control the morphology/architecture of the electrospun fibers. The effect of collector geometry, particularly for the alignment of fiber, has been studied earlier [8]. In present study we have used a mask aligner lithography system from SUSS Microtech Lithography GmbH. Germany. A schematic of the patterning process used in this study shown “Fig.2”.

Fig. 2. Schematic process flow representing fabrication of micro-electrode array using lift off process.

C. Characterization Methods Field Emission Scanning Electron Microscopy (FESEM) (Quanta 200, FEI, Frankfurt am Main, Germany; SUPRA 40 VP, Gemini, Carl Zeiss, Oberkochen, Germany) was used to observe the surface morphology of the nonwoven fiber meshes and to determine the range of diameters produced for all of the above studies. The electric conductivity of the single nanofiber was measured by using Cascade two-probe method, the current through the sample was measured with a Keithley's 4200 SCS. The sample was measured four times in different directions by applying the potential of -0.5 to +0.5 V and average value was calculated. XRD studies (X’Pert PRO) using Cu Kα radiation, were performed and Raman studies were carried on a Senterra inVia opus Raman spectrometer (Senterra, Bruker, UK) using 785 nm excitation and the optical power delivered onto the sample 50 mW/cm2 were obtained for pure and composite MWCNTs embedded SU-8 derived fiber in order to change in the graphitic nature of the nanofiber as a function of the CNT weight fraction. III.

Fig.1: Electrospinning equipment and direct-formation fiber process with in situ poling.

RESULTS AND DISCUSSION

A. Optimal parameters of MWCNT/SU-8 electrospun nanofibers Properties of the electrospun nanofiber formations, in particular the fiber diameter and morphology, depend on various parameters that can be divided into three groups: polymer solution properties (solution viscosity, solution concentration, polymer molecular weight, etc.); processing conditions (applied voltage, volume flow rate, etc.); and ambient conditions (temperature, humidity, etc.) In this study, processing and ambient conditions were held constant, in order

to systematically investigate the effect of solution properties on the average fiber diameter. The MWCNT/SU-8 fibers of diameters (200 – 300 nm) and length (l00 µm-200 µm) were spun on collector. The as-spun diameter of MWCNT/SU-8 fibers with different patterns are shown in Fig. 3.

nm. Diameter of a single nanofiber spun on microelectrodes was measured from electron micrographs along its length at several spots and averaged. While calculating the resistance and conductivity for nanofibers, their individual diameters were used in the calculations as observed by FESEM. The conductivity values for different wt% of MWCNT/SU-8 are nanofiber reported in “Fig. 5”. As a result, the I–V curves of the MWCNT/SU-8 fibers were ohmic. Beyond 11% the conductivity starts to decrease which can be attributed to the agglomeration of MWCNTs. This agglomeration causes alternative current paths and increased number of collisions for charged carriers. Thus the effective mobility of the charge carriers decreases resulting in a decreased current.

Fig.3. The SU-8 nanofibers and other patterns; (a) copper microelectrodes array, electrode diameter 100 µm and gap between electrode 50 µm (b) SU-8 nanofibers, (c) magnified view of fig. b diameter 280 nm, and (d) MWCNT/SU-8 nanofiber between copper electodes

B. SEM observation of MWCNT/SU-8 electrospun nanofibers Fig. 4 shows FESEM images of MWCNT/SU-8 nanofibers. Which were aligned sequence and MWCNT aggregated in SU-8 polymer to form the spherical electrospun structures. The effect of fiber by MWCNT is attributed to factor “electrical conductivity” in polymer solution. It is widely known that the addition of MWCNTs increases the electrical conductivity of solution. As can be seen in the ‘Fig. 4”, at higher concentrations of MWCNTs, a higher number beads structures were observed. This could possibly be attributed to non-homogeneity in the composite solution which results in a agglomeration of CNTs at certain locations. However for biosensing applications, conductivity is the key parameter that needs to be optimized. Presence of a few beads is acceptable and can be traded off with the conductivity. The beads can also be avoided by fine tuning the electrospinning parameters.

Fig. 5: Conductivity of single MWCNT/SU-8 nanofiber. The conductivity of 8, 9, 10, 11, 12 and 13 wt% MWCNTs nanofiber are (in S/m × 105); 6.0 ± 0.045, 9.54 ± 0.023, 15.79 ± 0.089, 17.29 ± 0.26, 10.56 ± 0.053 and 9.78 ± 0.047, respectively.

Fig. 6: XRD patterns of the MWCNT/SU-8 fibers concerning wt% MWCNTs (a) 8, (b) 9, (c) 10, (d) 11, (e) 12, (f) 13 and (g) pure MWCNTs

Fig. 4: SEM observations of MWCNT/SU-8 electrospun fibers containing wt% MWCNT: (a) 8, (b) 9, (c) 10, (d) 11, (e) 12 and (f) 13.

C. I-V analysis of MWCNT/SU-8 nanofiber The average diameter for the nanofibers fabricated in bulk while using the same fabrication parameters was ~200 nm while the distribution of diameters ranged from 100 to 300

D. XRD analysis of MWCNT/SU-8 nanofiber In order to verify the in situ electric poling effect during electrospinning process, XRD (X-ray diffraction) pattern for MWCNT/SU-8 electrospun nanofibers was recorded. When a higher voltage of 14 - 18 kV applied in electrospinning process, β-crystalline structure in MWCNT/SU-8 fiber was observed as shown in “Fig. 6 (a-f)”. Since a higher electric field resulted in completely arranging polarity direction of MWCNT/SU-8 fiber and could assist growing-crystalline structure (diffraction peak at 2θ = 26º) in increasing the

conductivity MWCNT/SU-8 fibers. Also the change in intensity of β-phase in pure MWCNTs can be observed in “Fig. 6 (g)”. The results indicated that there was a little difference in β-phase intensity when different wt% concentration of MWCNTs was added in SU-8 solution. The crystalline structure remains unaltered as can be inferred by comparing “Fig 6(a-f)” with “Fig.6 (g)”. The peak positions remain unaltered indicating the intactness of MWCNTs β-crystalline structure.

peak at 2θ = 26º for electrical conductive crystal β phase structure. The Raman analysis reports the greatly increased crystallinity of MWCNT containing SU-8 nanofiber that may have resulted from the nucleation by MWCNTs. This ability to control the electrical properties of the electrospun nanofibers over a wide range, together with their easy integration onto an underlying MEMS structure, should position them as a versatile advanced material for numerous ultrasensitive biosensor applications. REFERENCES [1]

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[5] Fig. 7: Raman spectra of 8, 9, 10, 11, 12 and 13 wt% of MWCNT/SU-8 fiber.

E. Raman analysis of MWCNT/SU-8 Fig.7 shows the representative Raman spectra for the MWCNT/SU-8 nanofiber investigated in our experiments. It is well known that the ratio of D-band to G-band depends on both the degree of graphitization and the alignment of graphitic planes. From the spectra, it was confirmed that the ID/IG value of electrospun nanofibers increased upon increasing the concentrations of MWCNTs up to 11% and a decrease thereafter indicating the possibility of agglomeration at high concentrations of MWCNTs. IV.

CONCLUSION

In this study, electrospinning of MWCNT/SU-8 solution on pre-fabricated copper microelectrodes array platform to fabricate aligned, single suspended electrospun composite nanofibers. These nanowire anchored on microelectrodes have the potential to be used as solid state sensors because their electrical conductivity properties can be directly addressed owing to their integration with the underlying microelectrodes. The The MWCNT/SU-8 solution concentration, chemical treatment of MWCNTs for better dispersion and the electrospinning parameters were optimized to form a sparse and directed network of suspended electrospun nanofibers anchored on the microelectrodes. Formation of mats and entangled nanofibers could be prevented by carefully controlling the electrospinning time. The optimal parameters of SU-8 solution such as weight percentage of SU-8 polymer and MWCNT were discussed. The electrical conductivity of the electrospun MWCNT nanofibers could be turned by two orders magnitudes from 0.6 × 104 S/m to 1.72 × 106 S/m by an addition of 8 – 11 wt% MWCNTs. XRD observation of SU-8 fiber with 8 – 13 wt% MWCNT reveals a high diffraction

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