Low-cost 3D-printing system for the deposition of reduced graphene oxide (rGO) thin films by dip-coating with application for electrode fabrication

Abstract A low-cost 3D-printing system is reported for the deposition of thin films by a dip-coating technique. The structure was constructed using 3D-printed pieces of polylactic acid (PLA) joined by simple snap-fit. This structure assembly and design simplifies the construction and replicability of dip-coating equipment. The components used are affordable and accessible, reducing the cost of implementation. An Arduino controls the system through a C++ program that varies the pulling and dipping speed from 0.5 to 20 mm/s. The pulling and dipping process uses a servomotor motion transformed into linear by a rack and pinion mechanism. The performance of the presented system was validated by comparison of thin-film reduced graphene oxide (rGO) deposition onto gold. The rGO thin films obtained were homogeneous and smooth, capable of being used as electrodes in biosensors.


Introduction
There are various techniques for obtaining thin films such as chemical deposition in liquid phase and sol-gel. Among these techniques, sol-gel deposition is simple, flexible, and economical for obtaining high quality films. The main feature of sol-gel techniques is that they easily allow the deposition of complex structures obtained in solution on substrates. [1] Deposition of the thin film may make a material versatile for various applications that may include electronic components and displays, solid surface coating, electrochemical biosensors, [2,3] optical coatings, and optical data storage devices. [4] Sol-gel thin films are formed by gravitational or centrifugal draining which provides an easy-to-apply technique. The sol-gel deposition techniques provide an alternative to chemical vapor deposition in the formation of films or particles. Initially, little effort had been made to understand the physical and chemical fundamentals involved in the sol-gel thin film formation until its widespread use. Today, it is gaining more popularity not only in research and development laboratories but also in industrial production. The dip coating process is easy to scale up and offers good thickness control, in addition to other advantages, although its full potential has not yet been explored. [5] The deposition thickness is controlled by the pulling speed and by the concentration and viscosity of the coating liquid. There is normally an operational range that allows the preparation of smooth and homogeneous films. The common pulling speeds used in dip-coating deposition are between 1 and 15 mm/s but are dependent upon the solvent and precursor system employed in the solution. [6] Several open-source equipment has been reported with successful performance and results, [7][8][9] but on the other hand, they remain difficult to implement due to the materials used in the construction of the deposition system structure. In this present work, a low-cost 3D-printing system is reported for the deposition of thin-films by dip-coating constructed using 3D-printed pieces and controlled by an Arduino board and a servomotor.

Experimental
The dip-coating system was constructed using 3D-printed pieces and assembly by simple snap-fit. An Arduino microcontroller is employed to drive a servomotor. A switch and a potentiometer were incorporated in order to include a manual mode for sample positioning. A 5 V DC power supply was used to source all components of the electronics.

Mechanical design and construction
The dip-coating system was constructed by 3D-printed pieces using polylactic acid (PLA) in a Flashforge Guider IIs 3D printer. The computeraided design (CAD) was developed in SolidWorks 3D CAD software [10] and the printing was done using FlashPrint slicing software. [11] The design shown in Figure 1 was based on the rack and pinion mechanism. The mechanism advantages are its response to steering inputs, its simplicity, and its robustness. [12] This mechanism easily translates a rotational motion of the servomotor used into a linear motion. Due to the servomotor rotation angle limitation, a maximum linear path length was defined. For the presented design and using a MG996R TowerPro servomotor with a limiting angle of 180 , the maximum path length for the linear motion is 45 mm.
After 3D printing, each part was joined together by a simple snap-fit following the design shown in Figure 1.

Wiring to the Arduino board
Arduino UNO is a microcontroller board based on the ATmega328p. Due to its robust design, it is one of the most used options to begin in electronics and coding. The Arduino board has 14 digital input/output pins, 6 analog inputs, and a USB connection. Of the 14 digital pins, 6 may be used as the pulse width modulation (PWM) output. The board also has a 5 V and 3.3 V voltage supply for low current applications.   [13] In the breadboard circuit view, four components are connected to the Arduino board. These components are a servomotor, a switch, a potentiometer, and a 2-way terminal connector designated in Figure 3 by J1, S2, U1, and TRM1, respectively.
The servomotor is an MG996R TowerPro with 3 pinouts (þ, pulse, -). The pulse pin is connected to a PWM output of the Arduino board, for example, D10 as shown in Figure 3.
A single-pole single-throw switch is used in order to shift between the two working modes (manual and programmed mode) of the dip coating system. This shift between modes is recognized by the digital input pin on the Arduino board, D5, as shown in Figure 3.
The potentiometer is used to control the rotation direction of the servomotor in the manual mode. Clockwise and counterclockwise direction of the motor is defined by the voltage of the potentiometer middle pin and read by the analog input pin A0 of the Arduino board.
The 2-way terminal connector is used to power the Arduino board and the servomotor by a parallel circuit using a 5 V DC (output current greater than 1.5 A, more details in the supplementary material) power supply. Terminal pin 2 is connected to the Arduino board's VIN and 5 V DC positive output, and terminal pin 1 is connected to the Arduino board's GND and 5 V DC ground.

Other components wiring
This section describes the connections on the switch, potentiometer, and resistor designated by S2, U1, and R1 that do not involve digital or analog pins from the Arduino board. The two remaining pins at the sides of the potentiometer are connected to 5 V voltage supply and GND pin of the Arduino board. Due to the low supply current limit of the Arduino pins, the potentiometer U1 value must be at least 1 kX for proper operation.
The single-pole single-throw switch S2 is connected to Arduino's 5 V voltage by pin 1 and pin 3 is not connected. Switch pin 2 is wired to resistor R1 and the resistor is also connected to Arduino's ground. Thus, R1 works as a pull-down resistor holding switch pin 2 connected to Arduino's ground and linked to 5 V when the switch is toggled. This switch toggle shifts the dip-coating working mode from manual to programmed.

Arduino code installation
The open-source Arduino Software (IDE) makes it easy to write code and upload it to the board. This software may be used to program and upload any Arduino board. The software that controls the equipment is written in Arduino Programming Language, a framework built on top of Cþþ, and is provided and described in the supplementary material. Specific information about functions, values (variables and constants), and structures are available in Arduino reference documentation.

Results and discussion
Dip coating system assembly All parts that conform to the system's structure were made by 3D printing. As mentioned in the previous section, due to the design and model of the servomotor, the limit of the trajectory traveled by the rack and pinion mechanism is 45 mm.
The system may operate in the manual and programmed modes. In manual mode, the direction of rotation of the motor at constant speed is controlled by the position of the potentiometer and can be programmed. In programmed mode, the system runs the full path at a constant speed that was present in the Arduino code.   operates as a working electrode in an electrochemical biosensor. The thinfilm structure and morphology characterization were performed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and electrochemical techniques demonstrating the high quality of the films obtained with the equipment described in this work, detailed by Zuñiga et al. [2] Dip coating deposition was carried out for 5 minutes with 2.25 mm/s of pulling and dipping speed. Figure 5 shows an example of an electrochemical biosensor developed using the equipment described in this work.

Conclusions
Low-cost and easy-to-build instrumentation for dip-coating deposition has been reported. The device was constructed employing 3D-printed components using common affordable electronics components such as MG996R servomotor and Arduino UNO board. The structure presented reduced complexity in construction in comparison to similar reported equipment. The control of the pulling and dipping speed by a Cþþ program in Arduino IDE software is easy to customize. The two operational modes include a programmed mode that controls pulling and dipping speed along the entire path. The range of speeds in the manual mode is normally used to prepare the substrate and the solution initiating the dip-coating deposition.
This system is operating since 2020 with the same design mechanism and model, but with enhancements in the Arduino code over the years. The pulling and dipping speed may be set to operate from 0.5 to 20 mm/s. This range includes the common speeds used in the dip-coating deposition. Figure 5. rGO thin-film is used in a biosensor application as the working electrode using the low-cost 3D-printing system.
The thin-film depositions obtained using this instrumentation have demonstrated homogeneous properties for biosensor applications and potentially other applications. These results verify its performance with similar results to commercial dip-coating devices.