Recyclable and Flexible Starch-Ag Networks and Its Application in Joint Sensor
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Flexible transparent conductive electrodes are essential component for flexible optoelectronic devices and have been extensively studied in recent years, while most of the researches are focusing on the electrode itself, few topics in material green and recyclability. In this paper, we demonstrate a high-performance transparent conductive electrode (TCE), based on our previous cracking technology, combined with a green and recyclable substrate, a starch film. It not only shows low Rs (less than 1.0 Ω sq−1), high transparency (> 82%, figure of merit ≈ 10,000), but also provides an ultra-smooth morphology and recyclability. Furthermore, a series of biosensors on human joints are demonstrated, showing great sensitivity and mechanical stability.
KeywordsStarch-ANs Recyclability Optoelectronics Ultra-smooth morphology Sensor
Atomic force microscopy
Figure of merit
Indium tin oxide
Scanning electron microscope
Transparent conductive electrode
Currently, electronic devices have been experiencing many new challenges, such as compatibility, mechanical flexibility, and eco-friendly manner [1, 2, 3, 4, 5]. Among those, transparent conductive electrode (TCE) as an important component of those devices is also facing new challenges, like high optical transmittance, low resistance, flexibility, biocompatibility , low-cost , and the recyclability . Currently, indium tin oxide (ITO)  is the widely used TCE, which is a continuous and chemically stable film. However, its fragility induced by the metal oxide and the large expense because of rare metal highly limit its future development. On the other hand, graphene/metal grid [10, 11], for example, metal networks [12, 13] and metal nanowires [14, 15, 16, 17, 18, 19], is facing serious adhesiveness and roughness problems. In addition, their high synthesizing cost and the impossibility to recycle make them detained in the laboratory.
In comparison, a series of TCEs based on crack-nanonetwork (CNN)  have been invented by our group, which presenting brilliant optoelectronic properties, high figure of merit, and the flexibility. With electroplating technology , we further realized the fully wet fabricated CNN based on UV glue with ultra-low sheet resistance (0.13 Ω sq−1) and smooth morphology . Currently, all of the substrates are based on the intrinsic non-degradation polymers, restraining the recycling of precious metal, like Ag and Au. Starch film is a transparent and flexible substrate material, and more importantly, it is an eco-friendly material and could be degraded in water. Jeong et al.  added PVA into a starch film and fabricated a flexible and disposable TCE; thus, it shows great potential of starch film as substrates.
Herein, we took the advantage of water degradability of starch film [24, 25] and fabricated a recyclable TCE, starch-Ag networks (SANs), through embedding our previously reported crack Ag networks in starch film. Via electroplating, we decreased the sheet resistance (Rs) to less than 1.0 Ω sq−1 along with highly optical transparency (> 82%) and high figure of merit (F) of over 10,000. Moreover, due to the peeling off fabrication process and self-supporting network , SAN presents good flexibility, low surface roughness, and the recyclability. Besides, SAN was used to demonstrate its application in biosensors in human joint with good sensitivity and mechanical stability.
Preparation of the Sacrificial Template
Self-cracking materials are a mixture of egg white and deionized water (3:1 by volume). A cracking template is obtained by dip coating above solution on a glass (50 mm × 50 mm), then drying in air about 10 mins, and finally, the self-cracking process occurs.
Ag Seed Layer Deposition
Sputtering (AJA International ATC Orion 8, USA) was used to deposit Ag seed layers (≈ 60 nm) on self-cracking template. Then, the sacrificial layer is removed by rinsing in deionized water.
Electroplating of Ag Networks Based on CNN Layers
One hundred-milliliter Ag electroplate liquid composed of 4 g AgNO3, 22.5 g Na2S2O3·5H2O, and 4 g KHSO3 in deionized water was used for the electroplating deposition. A homemade plating bath is used in the process, with a seed layer as the cathode and a Ag bar (40 mm × 40 mm) as the anode. The current for the electroplating deposition is 10 mA. We changed the thickness of the film by controlling the plating time. Finally, the Ag networks were rinsed in deionized water.
Fabrication of a Starch TCE
The starch solution, composed of 12.5 g corn starch, 1.25 g glycerinum (10 wt%) in 100 ml deionized water, was prepared at 60 °C on a hot plate, with stirring at 500 rpm for 30 min. The bubbles were removed from the starch solution in vacuum environment for 2 h. Four-milliliter starch solution was dip coated on the electroplating TCE and then dried in air for about 20 h under 30–40% RH and 25 °C.
Transfer of Ag Networks
The starch-Ag network film was immersed in DI water under 25 °C for 2 h. Then, the starch layer is dissolved, and finally, the freestanding Ag network was obtained.
The morphologies of samples were conducted by a SEM (ZEISS Gemini 500, Garl Zeiss, Germany), photographic camera, and atomic force microscope (AFM) (Cypher, Asylum Research). The crystallinity and phase information of the metal particles were determined by an X-ray diffraction system (PAN analytical X’Pert-Pro MPD PW 3040/60 XRD with Cu-Kα1 radiation, Netherlands). Optical transmittance was measured using an integrating sphere system (Ocean Optics, USA). Sheet resistance of samples was measured by a van der Pauw method, with four silver paste contacts deposited at the corners of a square sample (20 mm × 20 mm), recording with a Keithley 2400 SourceMeter (Keithley, USA). Two-probe resistance method is conducted in a bending test (Additional file 1).
Results and Discussion
Figure 1b is a schematic figure of the obtained SAN sample, showing a good flexibility and transparency. The SEM image of the metallic network is shown in Fig. 1c, with an average width and height of the Ag networks 2.5 μm and 1 μm respectively, and the inter-thread spacing in the range of 30 to 60 μm. The inset in Fig. 1c clearly displays the detailed morphology of the metal networks. The surface morphology of the SAN film is shown in Fig. 1d, with the inset of cross-sectional image, proving that the Ag networks have been successfully embedded into the starch film and exhibiting a smooth morphology. In addition, the height of Ag networks could be easily modulated by changing the concentration of the electroplating liquid, anode area, and distance between an anode and cathode in the electroplating deposition process , while the width of the networks and the inter-space can be controlled by varying the sacrificial material, concentration, and cracking temperature, as reported in our previous work . The crystallinity of SAN was characterized by X-ray diffraction (XRD) (Fig. 1e), which exhibits the (200), (220), and (311) planes of Ag, and no impurity detected. Atomic force microscopy (AFM) images in Fig. 1f, g confirmed an ultra-smooth surface with an extremely low root-mean-square (RMS) roughness of ~ 0.521 nm.
Optical and Mechanical Performance
Sensing Performance of the SAN
In conclusion, we have developed high-performance recyclable metallic networks, by combining the cracking network with starch substrates. The corresponding figure of merit of the resulting metallic network exceeds 10,000 with the sheet resistance (Rs) to less than 1.0 Ω sq−1 along with highly optical transparency (> 82%). Most importantly, the metallic network presents good flexibility, low surface roughness, and recyclability. Finally, a series of biosensors have been demonstrated showing good performance.
We thank the support from the Guangdong Provincial Engineering Technology Research Center for Transparent Conductive Materials, National Center for International Research on Green Optoelectronics (IrGO), MOE International Laboratory for Optical Information Technologies and the 111 Project.
We thank the financial support from NSFC-Guangdong Joint funding, China (No. U1801256), National Key R&D Program of China (No. 2016YFA0201002), Guangdong Provincial Foundation (2016KQNCX035), NSFC grant (No. 51803064, 51571094, 51431006, 51561135014, U1501244), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40) and Guangdong Innovative Research Team Program (No. 2013C102).
Availability of Data and Materials
All data can be provided on a suitable request.
SL and JWG developed the idea. CC, ZDW, GPD, and ZDF prepared the samples and conducted the experiments. JWG, SL, and JY wrote the paper. All authors discussed the results and commented on the manuscript. JWG directed the research. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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