Additive‐free Aqueous Dispersions of Two‐Dimensional Materials with Glial Cell Compatibility and Enzymatic Degradability

Biological and environmental degradation of two-dimensional materials

As the use of two-dimensional materials continues to grow, so too does the need to understand the environmental and biological impact of such materials. Degradation is a critical step in the life cycle of any material, but the majority of such knowledge is obtained from test tube and in vitro studies. Therefore, there remains a gap in understanding the degradability of two-dimensional materials in complex systems (in vivo) and in different ambient environments. In this Review, we highlight the need for more data-driven studies on the degradation of two-dimensional materials, including their kinetics, by-products, stability and possible downstream effects. Although challenging, building an understanding of the degradation profiles of different advanced materials in various environments at the chemical and molecular level is essential.

Water as Solvent for the Dispersion of 2D Nanostructured Materials

The development of large number of two-dimensional (2D) nanostructured materials that followed the success of graphene and the need for their handling and manipulation e. g., in inks, brought to the fore the study of solvents and substances that contribute to the stabilization of 2D nanomaterials in the liquid phase. The successful dispersion of 2D materials in solvents is combined with one of the most widespread preparation methods, that of liquid phase exfoliation. In this article, a review for the role of water in the preparation of different 2D nanostructures and their stable dispersions in the liquid phase is discussed. The use of water as a solvent or dispersant is instrumental in promoting materials with an ecological footprint, low cost, and sustainability.

A review of biocompatible polymer-functionalized two-dimensional materials: Emerging contenders for biosensors and bioelectronics applications

Bioelectronics, a field pivotal in monitoring and stimulating biological processes, demands innovative nanomaterials as detection platforms. Two-dimensional (2D) materials, with their thin structures and exceptional physicochemical properties, have emerged as critical substances in this research. However, these materials face challenges in biomedical applications due to issues related to their biological compatibility, adaptability, functionality, and nano-bio surface characteristics. This review examines surface modifications using covalent and non-covalent-based polymer-functionalization strategies to overcome these limitations by enhancing the biological compatibility, adaptability, and functionality of 2D nanomaterials. These surface modifications aim to create stable and long-lasting therapeutic effects, significantly paving the way for the practical application of polymer-functionalized 2D materials in biosensors and bioelectronics. The review paper critically summarizes the surface functionalization of 2D nanomaterials with biocompatible polymers, including g-C3N4, graphene family, MXene, BP, MOF, and TMDCs, highlighting their current state, physicochemical structures, synthesis methods, material characteristics, and applications in biosensors and bioelectronics. The paper concludes with a discussion of prospects, challenges, and numerous opportunities in the evolving field of bioelectronics.

Biocompatible 2D Materials via Liquid Phase Exfoliation

2D materials (2DMs), such as graphene, transition metal dichalcogenides (TMDs), and black phosphorus (BP), have been proposed for different types of bioapplications, owing to their unique physicochemical, electrical, optical, and mechanical properties. Liquid phase exfoliation (LPE), as one of the most effective up-scalable and size-controllable methods, is becoming the standard process to produce high quantities of various 2DM types as it can benefit from the use of green and biocompatible conditions. The resulting exfoliated layered materials have garnered significant attention because of their biocompatibility and their potential use in biomedicine as new multimodal therapeutics, antimicrobials, and biosensors. This review focuses on the production of LPE-assisted 2DMs in aqueous solutions with or without the aid of surfactants, bioactive, or non-natural molecules, providing insights into the possibilities of applications of such materials in the biological and biomedical fields.

Spontaneous Decoration of Ultrasmall Pt Nanoparticles on Size-Separated MoS2 Nanosheets

Liquid exfoliation can be considered as a viable approach for the scalable production of 2D materials due to its various benefits, although the polydispersity in the obtained nanosheet size hinders their straightforward incorporation. Size-separation can help alleviate these concerns, however a correlation between nanosheet size and property needs to be established to bring about size-specific applicability. Herein, size-selected aqueous nanosheet dispersions have been obtained via centrifugation-based protocols, and their chemical activity in the spontaneous reduction of chloroplatinic acid is investigated. Growth of ultrasmall Pt nanoparticles was achieved on nanosheet surfaces without a need for reducing agents, and stark differences in the nanoparticle coverage were observed as a function of nanosheet size. Defects in the nanosheets were probed via Raman spectroscopy, and correlated to the observed size-activity. Additionally, the effect of reaction temperature during synthesis was investigated. The electrochemical activity of the ultrasmall Pt nanoparticle decorated MoS2 nanosheets was evaluated for the hydrogen evolution reaction, and enhancement in performance was observed with nanosheet size, and nanoparticle decoration density. These findings shine light on the significance of nanosheet size in controlling spontaneous reduction reactions, and provide a deeper insight to intrinsic properties of liquid exfoliated nanosheets.

Nanomechanical study of aqueous-processed h-BN reinforced PVA composites

Hexagonal boron nitride (h-BN) as a filler has significantly improved the mechanical properties of various polymers composites. Among them, polyvinyl alcohol (PVA) is particularly important for its wide range of industrial applications and biocompatibility nature. However, preparing a homogenous composite of h-BN and PVA in water is troublesome as the aqueous processing of h-BN without any additives is challenging. In this context, a pre-processing technique is used to produce an additive-free aqueous dispersion of h-BN. The uniformly dispersed composites are then prepared with different concentrations of h-BN. Free-standing thin films are fabricated using the doctor blade technique, and nanoindentation is employed to understand their deformation behaviour at smaller length scale for better understanding of micro-mechanism involved. Reduced elastic modulus and hardness of 10 wt% h-BN/PVA composite film are enhanced by ∼93% and ∼159%, respectively, compared to pristine PVA. Frequency sweep dynamic mechanical analysis is performed between 1 and 50 Hz, and the elastic properties of composite materials are found to improve significantly upon addition of h-BN nanosheets. Besides, the impact of h-BN incorporation in stress relaxation behaviour and hardness depth profiling are also investigated. The observed improvement in mechanical properties of the composites may be attributed to the uniform distribution of the nanosheets and the strong interfacial interaction between h-BN and PVA, which ensures efficient mechanical stress transfer at the interface.

Spontaneous formation of gold nanoparticles on MoS2 nanosheets and its impact on solution-processed optoelectronic devices

Understanding size-dependent properties of 2D materials is crucial for their optimized performance when incorporated through solution routes. In this work, the chemical nature of MoS₂ as a function of nanosheet size is investigated through the spontaneous reduction of chloroauric acid. Microscopy studies suggest higher gold nanoparticle decoration density in smaller nanosheet sizes, resulting from higher extent of reduction. Further corroboration through surface-enhanced Raman scattering using the gold-decorated MoS₂ nanosheets as substrates exhibited an enhancement factor of 1.55 × 10⁶ for smaller nanosheets which is 7-fold higher as compared to larger nanosheets. These plasmonic-semiconductor hybrids are utilized for photodetection, where decoration is found to impact the photoresponse of smaller nanosheets the most, and is optimized to achieve responsivity of 367.5 mAW^-1 and response times of ∼17 ms. The simplistic modification via solution routes and its impact on optoelectronic properties provides an enabling platform for 2D materials-based applications.

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Reducing agent-free Au nanoparticle decoration on aqueously dispersed 2H-MoS₂.

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Control on Au nanoparticle decoration density through nanosheet size-selection.

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SERS as a probe for determining the decoration density along with microscopy.

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Enhanced photodetection by spontaneous modification with Au on MoS₂ films.

Solution Processing of Topochemically Converted Layered WO3 for Multifunctional Applications

Solution processing of nanomaterials is a promising technique for use in various applications owing to its simplicity and scalability. However, the studies on liquid-phase exfoliation (LPE) of tungsten oxide (WO₃ ) are limited, unlike others, by a lack of commercial availability of bulk WO₃ with layered structures. Herein, a one-step topochemical synthesis approach to obtain bulk layered WO₃ from commercially available layered tungsten disulfide (WS₂ ) by optimizing various parameters like reaction time and temperature is reported. Detailed microscopic and spectroscopic techniques confirmed the conversion process. Further, LPE was carried out on topochemically converted bulk layered WO₃ in 22 different solvents; among the solvents studied, the propan-2-ol/water (1 : 1) co-solvent system appeared to be the best. This indicates that the possible values of surface tension and Hansen solubility parameters for bulk WO₃ could be close to that of the co-solvent system. The obtained WO₃ dispersions in a low-boiling-point solvent enable thin films of various thickness to be fabricated by using spray coating. The obtained thin films were used as active materials in supercapacitors without any conductive additives/binders and exhibited an areal capacitance of 31.7 mF cm^-2 at 5 mV s^-1 . Photo-electrochemical measurements revealed that these thin films can also be used as photoanodes for photo-electrochemical water oxidation.