SCREEN-PRINTED MOLYBDENUM DISULFIDE ELECTRODES FOR ELECTROCHEMICAL SENSING OF DOPAMINE

Two-dimensional (2D) molybdenum disulphide (MoS2) belongs to a class of materials called transition metal dichalcogenides (TMDCs). Due to its large surface area, high biocompatibility, non-zero band-gap, structural versatility and mechanical flexibility, MoS2 can be used for next-generation sensing devices. In this work, printable high viscosity inks containing MoS2 in various ratios and ethyl cellulose (EC) as a binder were prepared. The effect of different MoS2 concentrations was investigated by measuring inks rheological properties. Structural and chemical properties of electrodes were studied by optical microscopy and SEM/EDX analysis. In addition, prepared MoS2/EC inks were screen-printed onto fluorine-doped tin oxide (FTO) coated glass substrates to develop working electrodes for electrochemical sensing of dopamine. Finally, differential pulse voltammetry measurements by applying FTO/MoS2 sensors showed that dopamine's limit of detection (LOD) was below the micromolar level. This work marks to the potential for fast, simple and mass printing production of MoS2 sensors to detect further target analytes.


INTRODUCTION
Molybdenum disulfide (MoS2) is a typical layered transition metal sulfide. Its structure consists of three stacked atom layers (S-Mo-S) held together by weak van der Waals forces [1]. MoS2 has unique electronic, optical properties, and due to biocompatibility, high electrochemical catalytic activity, electron transfer ability and specific surface area is suitable for sensing and biosensing applications [2]. Therefore, MoS2 can be used for electrochemical detection of dopamine, glucose, tyrosine and other target analyses [2][3][4].
Dopamine (DA) is a neurotransmitter regulating the function of the central nervous and cardiovascular systems in the human brain. Deficiency of DA can lead to various neurological diseases, e.g. schizophrenia, Alzheimer´s and Parkinson´s disease. Hence, the determination of the DA concentrations is beneficial for disease diagnosis [5]. Standard sensors for detecting various analyses are prepared by modifying commercial carbon screen-printed electrodes (SPE). This step is usually performed by the drop-casting method [2,4,6]. A drop-casting method is defined as the process of dispersing an electrocatalytic material in a suitable solvent followed by the pipetting of the dispersion onto a working carbon electrode. The solvent evaporates, and electrocatalytic material is immobilized on the electrode surface. Although drop-casting is a simple method, it has several disadvantages, e.g. low reproducibility or uncontrollable distribution of the deposited material [7]. In contrast, printing techniques (screen-printing) have high reproducibility and the possibility to prepare various fine structures for a wide range of layer thicknesses. Rowley-Neale et al. prepared screen-printable carbon/MoS2 inks by the modification of commercial carbon-graphite ink with MoS2 particles. In their work, screen-printed carbon/MoS2 working electrodes were used for electrochemical evaluation of oxygen reduction reaction (ORR) [7]. This work aims at the fast and simple preparation of printable high viscosity MoS2 inks, which can be used to fabricate screen-printed FTO/MoS2 sensors for dopamine detection at submicromolar level.

MoS2 working electrode preparation
The first step was the preparation of screen-printable MoS2 inks. The polymeric binder was prepared by dissolution 8 wt% of ethylcellulose in terpineol. Subsequently, different concentrations of MoS2 powder (25, 45 and 60 wt%; samples Mo-25, 45 and 60) were homogenized with a polymeric binder in a hand-held mixing unit. The second step was the screen-printing process, where prepared MoS2 inks were printed on 15×20 mm FTO substrates and used as working electrodes (Figure 1). Before printing, FTO substrates were gradually cleaned in an ultrasonic bath using detergent, acetone and isopropyl alcohol (IPA) for 20 minutes in every solvent. Substrates were also UV-C pretreated for 20 minutes. The yellow high modulus polyester yarn mesh, with mesh count 71 cm -1 and 48 µm thread diameter (PME 71-48 Y, SEFAR), was used in the screen-printing process. The deposition was performed on the manual screen-printing machine (Drucktech). After printing, MoS2 layers were left for 10 minutes for levelling and then dried in a laboratory oven at 120 °C for 30 minutes.

RESULTS AND DISCUSSION
In the first step, we analyzed the rheological behavior of prepared viscous and screen-printable MoS2 inks with different concentrations of MoS2. Figure 2 shows flow curve (a) and viscosity curve for typical pseudoplastic behaviour. This behaviour is characterized by decreasing dynamic viscosity with increasing shear rates and is crucial for the screen-printing process. It is also obvious that the structural breakdown of more concentrated MoS2 ink (Mo-60) occurs at a lower shear rate (~140 s -1 ) compared to ink with 25 wt% of MoS2 (~1170 s -1 ). Dependence of shear stress on shear rate was measured from 0 to 1200 s -1 , which corresponds to the printing process when ink is in the rest (low  ) and when is pressed through the screen mesh (high  ). Values of dynamic viscosity for selected shear rates: 10, 100 and 400 s -1 are listed in Table1.  To investigate inks behaviour during the screen-printing process, time-dependent flow curves were measured (Figure 3). The first interval of the curve simulates behaviour at rest at a preset low shear rate (0.5 s -1 ). The second interval simulates the structural breakdown of the ink during the screen-printing process at a preset high shear rate (2000 s -1 ). The third interval simulates structural regeneration at rest after application using the same preset low shear rate as in the first interval. The recovery rates of samples Mo-25 and Mo-45 were 74 and 90 %, respectively. In the case of the sample Mo-45, viscosity regeneration was rapid from the beginning and then increased linearly (Figure 3 (a), (b)). Figure 3 (c) shows that structure of sample Mo-60 was totally destroyed after applying high shear rates, and thus viscosity regeneration could not be evaluated. The structural and chemical properties of screen-printed MoS2 layers were analyzed. The thickness of screenprinted MoS2 layers was measured using an optical microscope, by a 3D image sequential recording method.
The recorded values confirmed the influence of particle concentration. The thickness of layers increased with the amount of MoS2 particles ( Table 2).  Figure 4 shows results of SEM/EDX analysis where it can be clearly seen that the layer of sample Mo-25 is not as homogenous as thicker layers of samples with higher MoS2 content. The atomic representation of elements (atom %) was further investigated through EDX and confirmed the presence of Sn, which is a component of FTO substrates. The atom% of C, which belongs to the binder, decrease with increasing MoS2 concentration ( Table 3).   The electrochemical behaviour of screen-printed FTO/MoS2 electrodes was assessed by DPV experiment. The DPV curves showing electrochemical oxidation of dopamine in 0.1 M phosphate buffer (pH 7.0) as a supporting electrolyte at above-mentioned manufactured FTO/MoS2 electrodes can be clearly seen in Figure 5. The linear dependence of ip (oxidation peak current) vs. cDA (concentration of dopamine) was investigated in the range of 10 -300 µM for the measured analyte. The limit of detection (LOD) was calculated according to a signal-to-noise ratio (S/N) = 3. The best LOD of 0.342 µM (R 2 = 0.995) was evaluated for Mo-45 sample. LODs of 2.1 µM (R 2 = 0.996) and 1.2 µM (R 2 = 0.982) were achieved for Mo-25 and Mo-60 samples, respectively.

CONCLUSION
In this work, printable inks were prepared containing different concentrations of MoS2 and ethylcellulose as a binder. After rheological behaviour analysis, MoS2 inks were screen-printed on substrates with a conductive FTO layer and were used as a working electrode for the electrochemical determination of dopamine. Prepared MoS2 inks exhibit pseudoplastic flow behaviour. The printable ink containing 45 wt% MoS2 showed the ideal rheological properties because 90 % of viscosity recovered, which is important in the screen-printing process. SEM/EDX analysis confirmed that the layer of sample Mo-45 is homogenous, and the Sn from FTO substrate is not present. Finally, FTO/MoS2 working electrodes for the detection of dopamine were successfully screenprinted. The differential pulse voltammetry measurements showed that the best limit of detection of 0.342 µM was evaluated for the working electrode printed from ink containing 45 wt% MoS2.This work confirmed that it is possible to prepare MoS2 sensors by simple screen-printing technique and that the sensors show great potential towards detection of further target analytes.