*3.6. Novel 2D Nanomaterials*

One of the critical advantages of nanomaterial printing is the opportunity to tune material properties to address various application needs finely. The development of novel 2D materials is essential in the high throughput printing of advanced biosensors and bioelectronics [141]. For instance, two-dimensional transition metal dichalcogenides (TMDs), such as WSe2, WS2, MoSe2, and MoS2, are direct bandgap monolayers with high flexibility that can be used alone or in combination with graphene to create various flexible sensors [141–144]. TMDs have several exceptional material properties that make them highly suited for many electronics applications. They contain no inversion center, which allows the k-valley index to be manifest as a new degree of freedom charge carrier [144]. Strong spin-orbit coupling leads to spin-orbit splitting, making them well suited to spin transport electronics applications, commonly termed spintronics [145]. Printable TMD inks can be synthesized from bulked cellular samples through liquid-phase exfoliation (LPE). This has been demonstrated for applications such as screen-printed oxygen sensing electrodes [42] and wearable heterostructure photodetectors [143]. Despite recent advances in LPE processes by optimizing dispersion agent concentrations, polymers, stabilizers, and binders, TMDs can be challenging to disperse in printable inks. Many LPE processes

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still rely on toxic and hazardous materials that do not demonstrate biocompatibility for wearable applications [146]. Lee et al. developed a zwitterion-assisted LPE process to synthesize TMDs in water to address this concern, allowing for the development of highlybiocompatible TMD inks [146].

**Figure 9.** Carbon nanotubes for high throughput bioelectronics printing. (**a**) Armchair, zigzag, and chiral CNT geometries, each of which exhibits unique material properties. (reprinted with permission from *Physica E Low Dimens. Syst. Nanostruct*. (2014), 59:186–191. Copyright 2014, Elsevier) (**b**) Schematic representation of the CVD process for CNT synthesis, with illustrations of the base growth (bottom left) and tip growth (bottom right) CNT synthesis methods. (reprinted with permission from *Chem. Biol. Technol. Agric.* (2016), 3(17). Copyright 2016, Springer Nature) (**c**) AFM images of printed MWCNTs at different magnifications. (reprinted with permission from *RSC Adv.* (2017), 7, 44076–44081. Copyright 2017, RSC). (**d**) TEM images of SWCNTs in an SDS solution after sonication for 4 h (top) and 6 h (bottom). (reprinted with permission from *J. Surf. Eng. Mater. Adv. Technol.* (2013), 3, 6–12. (**e**) Image of pixels in a roll-to-roll gravure printed TFT-active matrix with 10 PPI resolution (left) and cross-sectional FIB-SEM of printed SWCNTs on the printed dielectric (right). (reprinted with permission from *Adv. Electron. Mater.* (2020), 6, 1901431, Copyright 2020, Wiley).

Additionally, hexagonal boron nitride (h-BN) is a high bandgap, biocompatible, nanomaterial isostructural to graphene that is highly suited for nanophotonics. It is a natural hyperbolic material in the mid-IR range [147], attractive for use as a substrate for graphene transistors because of its atomic-scale smoothness [32], advantageous for electrochemical sensing [148], of great interest as a capacitive dielectric [130], and potentially suited for the in-situ formation of 1D conducting channels [57]. Printable h-BN monolayers may be synthesized through top-down approaches, such as mechanical and chemical exfoliation, or bottom-down approaches, such as PVD and CVD [148]. Because h-BN has strong in-plane

covalent bonds and weak inter-plane van der Waals forces compared to graphene, h-BN is an attractive 2D material for printable inks. Although h-BN has long been of interest, its potential for high-throughput fabrication via screen and inkjet printing has just recently been appreciated [148]. For instance, h-BN is now well understood for capacitive, dielectric, and transistor substrate applications. Still, new investigations into printable optic devices and electrochemical sensors will be needed to unlock this material's full potential. In one recent work, Desai et al. optimized h-BN nanoplatelet geometries synthesized through exfoliation and deposition thicknesses to yield a printed photo-capacitor with excellent thermal stability ranging from 6–350 K [130]. Additionally, Angizi et al. used edge functionalized h-BN dispersed in ethanol for screen printed Vitamin C detection in a flexible biosensor [149].
