Two-dimensional Boron Carbon Nitride (BCN) is a complex ternary system that has recently attracted great attention due to its ability to be tuned over a range of chemical, optical and electrical properties. In the last decade, BCN structures have been extensively researched for many energy-related applications, from supercapacitors and lithium ion batteries to electrocatalysts and sensors. However, the stoichiometry dependent properties of BCN as well as the difficult-to-control domain distribution of boron, carbon, and nitrogen atoms throughout the planes result in challenges for the fabrication of devices with reproducible performance. This review starts by discussing the fundamental properties of BCN as compared to its parent compounds (hexagonal boron nitride and graphene). Then the fabrication methods are comprehensively reviewed, analyzing each method’s advantages and shortcomings. This is followed by an explanation of BCN characteristics while particular attention is given to the surface chemistry and engineering of nanosheets. Applications of two dimensional BCN will also be reviewed to illustrate its significance over the last decade. Lastly, future trends and prospects of BCN structures will be reviewed, indicating on-going areas of research and the possible integration of BCN in semiconductor and energy-related applications.

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The presence of various ions is an important factor in evaluating water quality. Nitrate, nitrite and ammonium in water can cause health issues and pose environmental concerns1,2. Some sources of the nitrogen species are from natural water and some of them arise from industrial and agricultural activities. Currently commercially available sensors for measuring nitrogen compounds in water are based on colorimetric techniques and potentiometric methods incorporating ion selective electrodes. Progress still needs to be made towards fabricating devices with less instrumentation costs, less complexity, less maintenance and without need for any reagents to facilitate measurement of species1. Chemiresistive sensors are solid-state devices which can be simply fabricated from two contacts and a conductive sensing material affixed on a suitable substrate. They operate by detecting modulations in resistance of the conducting film due to surface charge transfer as a result of interactions with the analyte(s)3. Here, we demonstrate chemiresistive devices capable of quantifying aqueous nitrate, nitrite and ammonium ions selectively. Due to challenges in the aqueous phase such as ionic strength effect, probability of side reactions, non-specific bonding on the surface, low interaction energy between analyte and surface, chemiresistive technology has not been developed extensively in water quality sensors4,5. In this study, we have overcome the issues in water by coating the chemiresistive devices with selective membranes. If the fabricated sensors are used as an array, the total nitrogen concentration in water can be measured online which is a significant advance since nitrate, nitrite and ammonium may interconvert and a single nitrate, nitrite or ammonium sensor by itself cannot give the total amount of nitrogen in a sample. For device fabrication, p-doped carbon nanotubes were selected as a sensitive conductive layer which were modified with selective membranes to improve the sensing performance. Nitrite sensors worked over a dynamic range of 67 ppb to 67 ppm with a 27.6% response at 67 ppm. Nitrate showed 13.2% response from 2.2 ppm to 220 ppm. Ammonium devices operated over a dynamic range of 10 ppb to 100 ppm with a 23.6% response at 100 ppm. The proposed response mechanism involves both an electrostatic gating effect and surface charge transfer. Compared with paper-based colorimetric sensors, the proposed devices perform better with a lower detection limit and the ability to perform continuous online measurements. Moreover, the chemiresistive responses of the devices were compared with their potentiometric responses and found to be equally sensitive but more selective. Unlike ion-selective electrodes, the resulting devices do not require the use of reference electrodes and are therefore potentially more robust for use in continuous water analyzers and in resource-poor settings. The chemiresistive devices showed low interferences and good reversibility. They were also tested in river water samples and showed satisfactory results. The fabricated devices are an advanced proof of concept and have the potential to replace the current technology. Nuñez, L., Cetó, X., Pividori, M. I., Zanoni, M. V. B. & del Valle, M. Development and application of an electronic tongue for detection and monitoring of nitrate, nitrite and ammonium levels in waters. Microchem. J. 110, 273–279 (2013). Li, D., Xu, X., Li, Z., Wang, T. & Wang, C. Detection methods of ammonia nitrogen in water: A review. TrAC - Trends Anal. Chem. 127, 115890 (2020). Choi, S. J. & Kim, I. D. Recent Developments in 2D Nanomaterials for Chemiresistive-Type Gas Sensors. Electronic Materials Letters vol. 14 (The Korean Institute of Metals and Materials, 2018). Kruse, P. Review on water quality sensors. J. Phys. D. Appl. Phys. 51, (2018). Dalmieda, J., Zubiarrain-Laserna, A., Saha, D., Selvaganapathy, P. R. & Kruse, P. Impact of Surface Adsorption on Metal-Ligand Binding of Phenanthrolines. J. Phys. Chem. C 125, 21112–21123 (2021). Figure 1

Molybdenum disulfide (MoS2) is a promising material for applications in sensors, energy storage, energy conversion devices, solar cells, and fuel cells. Because many of those applications require conductive materials, we recently developed a method for preparing a conductive form of MoS2 (c-MoS2) using dilute aqueous hydrogen peroxide in a simple and safe way. Here, we investigate modulating the chemical and mechanical surface properties of c-MoS2 thin films using diazonium chemistry. In addition to a direct passivation strategy of c-MoS2 with diazonium salts for electron-withdrawing groups, we also propose a novel in situ synthetic pathway for modification with electron-donating groups. The obtained results are examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The degree of surface passivation of pristine and functionalized c-MoS2 films was tested by exposing them to aqueous solutions of different metal cations (Fe2+, Zn2+, Cu2+, and Co2+) and detecting the chemiresistive response. While pristine films were found to interact with several of the cations, modified films did not. We propose that a surface charge transfer mechanism is responsible for the chemiresistive response of the pristine films, while both modification routes succeeded at complete surface passivation. Functionalization was also found to lower the coefficient of friction for semiconducting 2H-MoS2, while all conductive materials (modified or not) also had lower coefficients of friction. This opens up a pathway to a palette of dry lubricant materials with improved chemical stability and tunable conductivity. Thus, both in situ and direct diazonium chemistries are powerful tools for tuning chemical and mechanical properties of conductive MoS2 for new devices and lubricants based on conductive MoS2.