Ingestible sensors are potentially a powerful tool for monitoring human health. Sensors have been developed that can, for example, provide pH and pressure readings or monitor medication, but capsules that can provide key information about the chemical composition of the gut are still not available. Here we report a human pilot trial of an ingestible electronic capsule that can sense oxygen, hydrogen, and carbon dioxide. The capsule uses a combination of thermal conductivity and semiconducting sensors, and their selectivity and sensitivity to different gases is controlled by adjusting the heating elements of the sensors. Gas profiles of the subjects were obtained while modulating gut microbial fermentative activities by altering their intake of dietary fibre. Ultrasound imaging confirmed that the oxygen-equivalent concentration profile could be used as an accurate marker for the location of the capsule. In a crossover study, variations of fibre intake were found to be associated with differing small intestinal and colonic transit times, and gut fermentation. Regional fermentation patterns could be defined via hydrogen gas profiles. Our gas capsule offers an accurate and safe tool for monitoring the effects of diet of individuals, and has the potential to be used as a diagnostic tool for the gut. NATuRE ELECTRONiCS

Two-dimensional piezotronics will benefit from the emergence of new crystals featuring high piezoelectric coefficients. Gallium phosphate (GaPO4) is an archetypal piezoelectric material, which does not naturally crystallise in a stratified structure and hence cannot be exfoliated using conventional methods. Here, we report a low-temperature liquid metal-based two-dimensional printing and synthesis strategy to achieve this goal. We exfoliate and surface print the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide bandgap two-dimensional (2D) GaPO4 nanosheets of unit cell thickness, while featuring lateral dimensions reaching centimetres. The unit cell thick nanosheets present a large effective out-of-plane piezoelectric coefficient of 7.5 ± 0.8 pm V−1. The developed printing process is also suitable for the synthesis of free standing GaPO4 nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials.

COMMUNICATION (1 of 8)deployed in the form of van der Waals (vdW) heterostructures that enable fascinating coupled properties from stacked individual layers of 2D sheets which can be exploited in several applications, [2,6,7] including tunneling transistors, [8] quantum hall systems, [9] electrochemical hydrogen evolution reaction, [10] optoelectronics, [4,11] and electronics. [3,12] The p-n junctions are the building blocks of the semiconductor industry, in which the p-n junction heterostructures made from ultrathin materials are of great interest in specialized electronics, optoelectronics, and photonics due to their intriguing coupled properties of the different crystals. [5][6][7]13] Several methods for exfoliation and/or deposition exist such as chemical vapor deposition (CVD), [7] pulsed laser deposition (PLD), [14] molecular beam epitaxy (MBE), [15] pick-and-lift vdW technique, [16] and mechanical exfoliation. [17] These conventional approaches are time consuming and require complicated fabrication processes, [18] yet resulting in devices with small effective areas. [19] Liquid metals are emerging materials which can be used in microfluidics components, [20] sensors, [21] electrodes, [22] phototransistors, [23] flexible and stretchable devices, [24] disease treatment, [22] biomedical field, [22] and in synthesis of low-dimensional materials. [25] Liquid metals have been shown to form a naturally occurring atomically thin layer of oxide at their interface with air, [26][27][28][29] and using liquid metal as a reaction solvent can give access to a sizable portion of oxide elements including oxides which are intrinsically nonlayered crystals. [26] The exfoliated oxides can be converted to sulfides and phosphates. [30] Combination of these atomically thin layers should provide a vast number of vdW heterostructures that are yet to be explored.In this work, atomically thin oxide skin of low melting point liquid metals of tin and indium including p-type tin oxide (SnO) [27] and n-type indium oxide (In 2 O 3 ) [31] are stacked to produce large-area heterostructures with a high degree of homogeneity. Indeed, the p-n vdW heterojunctions feature current rectification properties with exceptionally fast photoresponse times and high sensitivity for UV light. The demonstrated liquid metal synthesis framework offers the possibility of synthesizing and exploring a range of tailored heterostructures for applications in next-generation optoelectronic and photodetection devices.

Intriguing physical and chemical properties of atomically thin semiconductors provide avenues for the development of the next-generation electronics, optoelectronics, and sensing applications. However, many materials are intrinsically nonlayered and therefore difficult to obtain in two dimensions (2D) due to the presence of strong in-plane bonds. Here, we adopted liquid metal synthetic strategies to produce 2D gallium sulfide (Ga2S3), which is an intrinsically nonlayered material. The obtained monoclinic α-Ga2S3 has a relatively high field-effect mobility of 3.5 cm2 V–1 s–1 and features a p-type material with a bandgap of 2.1 eV. Photodetectors that are made based on these synthesized 2D Ga2S3 exhibit relatively strong photodetectivity of 1010 jones and photoresponsivity of 240 A W–1 in visible wavelengths. The 2D Ga2S3 is also found to be suitable for sensing of nitrogen dioxide (NO2) gas at low evaluated temperatures. Excellent electronic, optical, and gas sensing performance demonstrated in this work offers great promises for synthesizing high quality 2D materials based on the liquid metal framework.

that offers novel electrical, electronic, and optical properties that are distinct from their bulk counterparts along with mechanical flexibility and high compatibility with state-of-the-art silicon-based platform. [2][3][4] Among prominently studied materials in the last few decades is zinc oxide (ZnO), a highly versatile tunable material. [5] The optoelectronic properties of ZnO in various morphologies have been investigated widely. [6] Due to its strong absorption in the UV region, ZnO is an attractive candidate for visible-blind photodetectors. [7] With a wide bandgap of 3.39 eV, exciton binding energy as large as 60 meV at room temperature, and the ability to undergo a strong quantum confinement effect, atomically thin ZnO promises an excellent platform for optoelectronic applications. [8] Most importantly, oxygen adsorbed onto the surface of ZnO provides low electron densities that can enable low dark current which is ideal for low energy applications. [9] Though thin nanosheets (<20 nm) ZnO has been used as an active layer for various applications, the lack of a reliable and controllable synthesis technique to obtain large area few atoms thin ZnO has prevented the miniaturization of ZnO based optoelectronic devices slowly making the material less competitive compared to other emerging systems relying on atomically thin functional layers. [10][11][12] Also, ZnO and other planar metal-semiconductormetal (MSM) based UV photodetectors developed to date have Atomically thin 2D materials are highly sought for high-performance electronic and optoelectronic devices. Despite being a widely recognized functional material for a plethora of applications, ultra-thin nanosheets of zinc oxide (ZnO) at a millimeter-scale for developing high-performance electronic/optoelectronic devices have not been reported. This has prevented the exploration of electronic and optical properties of ZnO when it is only a few atoms thick. Here, a liquid metal exfoliation technique is used that takes advantage of the van der Waals forces between the interfacial oxide and the chosen substrate to obtain ZnO nanosheets with lateral dimensions in the millimeter scale and thickness down to 5 nm. Their suitability for applications is shown by demonstrating a visible-blind photodetector with high figures of merit as compared to other ZnO morphologies. At extremely low operating bias of 50 mV and low optical intensity of 0.5 mW cm −2 , the ZnO photodetector demonstrates an external quantum efficiency (EQE), responsivity (R), and detectivity (D*) of 4.3 × 10 3 %, 12.64 A W −1 , and 5.81 × 10 15 Jones at a wavelength of 365 nm. The trap-mediated photoresponse in the ZnO nanosheets is further utilized to demonstrate optoelectronic synapses. Versatile synaptic functions of the nervous systems are optically emulated with the ultra-thin ZnO nanosheets.