Magnetite nanoparticles (NPs) are one of the most investigated nanomaterials so far and modern synthesis methods currently provide an exceptional control of their size, shape, crystallinity and surface functionalization. These advances have enabled their use in different fields ranging from environmental applications to biomedicine. However, several studies have shown that the precise composition and crystal structure of magnetite NPs depend on their redox phase transformations, which have a profound impact on their physicochemical properties and, ultimately, on their technological applications. Although the physical mechanisms behind such chemical transformations in bulk materials have been known for a long time, experiments on NPs with large surface-to-volume ratios have revealed intriguing results. This article is focused on reviewing the current status of the field. Following an introduction on the fundamental properties of magnetite and other related iron oxides (including maghemite and wüstite), some basic concepts on the chemical routes to prepare iron oxide nanomaterials are presented. The key experimental techniques available to study phase transformations in iron oxides, their advantages and drawbacks to the study of nanomaterials are then discussed. The major section of this work is devoted to the topotactic oxidation of magnetite NPs and, in this regard, the cation diffusion model that accounts for the experimental results on the kinetics of the process is critically examined. Since many synthesis routes rely on the formation of monodisperse magnetite NPs via oxidation of wüstite counterparts, the modulation of their physical properties by crystal defects arising from the oxidation process is also described. Finally, the importance of a precise control of the composition and structure of magnetite-based NPs is discussed and its role in their biomedical applications is highlighted.

Perovskites have showed significant potential for the application in photodetectors due to their outstanding electrical and optical properties. Integrating two-dimensional (2D) materials with perovskites can make full use of the high carrier mobility of 2D materials and strong light absorption of perovskite to realize excellent optoelectrical properties. Here, we demonstrate a photodetector based on the WTe₂/CsPbI₃ heterostructure. The quenching and the shortened lifetime of photoluminescence (PL) for CsPbI₃ perovskite confirms the efficient charge transfer at the WTe₂/CsPbI₃ heterojunction. After coupled with WTe₂, the photoresponsivity of the CsPbI₃ photodetector is improved by almost two orders of magnitude due to the high-gain photogating effect. The WTe₂/CsPbI₃ heterojunction photodetector reveals a large responsivity of 1157 A W⁻¹ and a high detectivity of 2.1 × 10¹³ Jones. The results pave the way for the development of high-performance optoelectronic devices based on 2D materials/perovskite heterojunctions.

Ultrasonic therapy has drawn increasing attention due to its noninvasiveness, great sensitivity and strong penetration capabilities. However, most of traditional rigid ultrasonic probes cannot achieve a solid interfacial contact with irregular nonplanar surfaces, which leads to unstable therapeutic effects and limitations of widespread use in practical applications. In this paper, a new flexible ultrasonic patch based on carbon nanotube (CNT) films is designed and fabricated to achieve a potential application in ultrasonic therapy. This patch is composed of a CNT film, a thermal protective layer and a heat sinking layer, and has the advantages of simple structure, soft, ultrathin and completely conforming to the treatment area. Theoretical and experimental studies are performed to investigate the acoustic and temperature fields before and after deformation. Effects of key design parameters of the patch on acoustic performances and temperature distributions are revealed. Numerical results indicate that the CNT film patch can produce ultrasounds over a wide frequency range and temperatures under the threshold of burn injury whether it is bent or not. Furthermore, it is also noted that the sound waves emitted from the bending patch are focused at the center of the bending patch, which demonstrates that the target treatment area can be controlled.

Self-powered wearable sensing systems have attracted great attention for their application in continuous health monitoring, which can reveal real-time physiological information on the body. Here, an innovative self-powered sound-driven humidity sensor for wearable intelligent dehydration monitoring system has been proposed. The sensor is primarily comprised of PTFE membrane, ZnO nanoarrays and Ti thin film. The piezoelectric/triboelectric effect of ZnO nanoarrays/PTFE membrane is coupled with the humidity sensing process. Sound wave can drive PTFE membrane to vibrate, and the contact and separation between PTFE and ZnO can generate electrical signals through piezoelectric/triboelectric effect. At the same time, the surface of the nanostructures can absorb the water molecules, which will influence the electrical output of the device. The device can convert sound energy into electrical output without any external electricity power supply, and the outputting voltage decreases with increasing relative humidity, acting as the sensing signal. The sensor has been integrated with data processing unit and wireless transmission module to form a self-powered wearable intelligent dehydration monitoring system, which can actively monitor the humidity of exhaled breath and transmit the information to the mobile phone. The results can open a possible new direction for the development of sound-driven gas sensors and will further expand the scope for self-powered nanosystems.

Two-dimensional materials have potential applications for flexible thermoelectric materials because of their excellent mechanical and unique electronic transport properties. Here we present a functionalization method by a Lewis acid-base reaction to modulate atomic structure and electronic properties at surface of the MoS₂ nanosheets. By AlCl₃ solution doping, the lone pair electronics from S atoms would enter into the empty orbitals of Al³⁺ ions, which made the Fermi level of the 1T phase MoS₂ move towards valence band, achieving a 1.8-fold enhancement of the thermoelectric power factor. Meanwhile, benefiting from the chemical welding effect of Al³⁺ ions, the mechanical flexibility of the nanosheets restacking has been improved. We fabricate a wearable thermoelectric wristband based on this improved MoS₂ nanosheets and achieved 5 mV voltage output when contacting with human body. We think this method makes most of the transition metal chalcogenides have great potential to harvest human body heat for supplying wearable electronic devices due to their similar molecular structure.

Sensing lower molecular weight in a diluted solution using a label-free biosensor is challenging and requires a miniaturized plasmonic structure, e.g. a vertical Au nanorod (AuNR) array-based metamaterials. The sensitivity of a sensor mainly depends on transducer properties and hence for instance, the AuNR array geometry requires optimization. Physical vapour deposition methods (e.g. sputtering and e-beam evaporation) require a vacuum environment to deposit Au, which is costly, time-consuming, and thickness-limited. On the other hand, chemical deposition, i.e. electroplating deposit higher thickness in less time and at lower cost, becomes an alternative method for Au deposition. In this work, we present a detailed optimization for the electroplating-based fabrication of these metamaterials. We find that slightly acidic (6.0 < pH < 7.0) gold sulfite solution supports immersion deposition, which should be minimized to avoid uncontrolled Au deposition. Immersion deposition leads to plate-like (for smaller radius AuNR) or capped-like, i.e. mushroom (for higher radius AuNR) structure formation. The electroplating time and DC supply are the tuning parameters that decide the geometry of the vertically aligned AuNR array in area-dependent electroplating deposition. This work will have implications for developing plasmonic metamaterial-based sensors.

This work investigates the effect of plasma treatment on the morphology and composition of 15 × 15 mm² silver nanoparticle (70–80 nm) thin films. The silver nanoparticles are deposited onto thermal silica (SiO₂/Si) substrates by spin-coating, then they are treated by an open-to-air microwave argon plasma jet characterized by a neutral gas temperature of 2200 ± 200 K. Scanning electron microscopy analysis reveals that the number of isolated nanoparticles in the film sample decreases after exposure to multiple jet passes, and that polygonal structures with sharp corners and edges are produced. Similar structures with much rounder edges are obtained after conventional thermal annealing at temperatures up to 1300 K. Based on localized surface plasmon resonance analysis in the range of 350–800 nm, the main extinction band of silver nanoparticles experiences a redshift after treatment with the plasma jet or with thermal annealing. Moreover, both treatments induce surface oxidation of the nanoparticles, as evidenced by x-ray photoelectron spectroscopy. However, only the plasma-exposed samples exhibit a significant rise in the surface-enhanced Raman scattering (SERS) signal of oxidized silver at 960 cm⁻¹. 29 $\times$ 29 μm² mappings of hyperspectral Raman IMAging (RIMA) and multivariate curve resolution analysis by log-likelihood maximization demonstrate that the SERS signal is controlled by large-scale micrometer domains that exhibit sharp corners and edges.

A rapid, clean plasma-chemical technique is demonstrated here, for cost-effective, synthesis of surface vacancy engineered, 2D, molybdenum-oxide nanomaterials, during a one-step, integrated synthesis-hydrogenation process for biomedical applications. A laminar plasma beam populated with O and H radicals impinges on a molybdenum target, out of which molybdenum-oxide nanomaterials are very rapidly generated with controlled surface O vacancies. 2D, dark-blue coloured, nano-flake/ribbon like MoO_3−x is produced maximum up to 194 g h⁻¹, the core of which still remains as stoichiometric molybdenum-oxide. These nanomaterials can get heated-up by absorbing energy from a near-infrared (NIR) laser, which enable them as photothermal therapy (PTT) candidate material for the invasive precision therapy of cancer. The surface defects endows the products with robust ferromagnetism at room temperature conditions (maximum saturation-magnetization: 6.58 emu g⁻¹), which is order of magnitude stronger than most other vacancy engineered nanomaterials. These nanometric metal-oxides are observed to be perfectly compatible in animal physiological environment and easily dispersed in an aqueous solution even without any pre-treatment. The MoO_3−x nanomaterials are stable against further oxidation even under prolonged atmospheric exposure. In vitro experiments confirm that they have ideal efficacy for photothermal ablation of human and murine melanoma cancer at relatively lower dose. During in vivo PTT treatments, they may be manipulated with a simple external magnetic field for targeted delivery at the malignant tumours. It is demonstrated that commensurate to the neutralization of the malignant cells, the nanomaterials themselves get self-degraded, which should get easily excreted out of the body.

Quantum dots (QD) are rapidly making their way into several application sectors including optoelectronics, and there is a strong need to focus on non-toxic QDs. In this work, we have synthesized graphene QDs in the size range of 1.4–4.2 nm from inexpensive graphite by oxidative cleavage using a sulphuric and nitric acid mixture. A subsequent hydrogen peroxide oxidation step, investigated using two thermal budgets, has resulted in QDs with excellent photoluminescence (PL) intensity. Prolonged, higher temperature oxidation results in smaller size GQDs. X-ray photoelectron spectroscopy analysis confirmed the role of ·OH radicals in the oxidation process and Raman analysis revealed that the higher thermal budget oxidation results in lower defect density. To overcome the challenges in device adaptability due to the inherent acidity in the QDs, a post-synthesis neutralization process was devised. The neutralized GQDs were formed into a film to be used as the active layer in a photodetector device. Fluorescence decay analysis showed there is no significant change in lifetime because of the film formation process. The fabricated GQD photodetector device exhibited high photocurrent under ultraviolet illumination with an ON/OFF ratio of 400% at an applied bias of ±1 V. The device performance underlines the high potential for the non-toxic, top-down synthesized GQDs for application in optoelectronic devices.

Ag₂S quantum dot (QD) photodetectors (PDs) have attracted a lot of attention in the field of imaging system and optical communication. However, the current Ag₂S PDs mainly works in the near-infrared band, and its detection ability in the visible band remains to be strengthened. In this paper, we used poly(3-hexylthiophene) (P3HT) with high carrier mobility and Ag₂S QDs to construct heterojunction PD. Stronger absorption in blends with polymer P3HT compared to single Ag₂S QDs. The optical absorption spectra show that the Ag₂S/P3HT has strong light absorption peak at 394 and 598 nm. The results show that P3HT significantly enhances the absorption of Ag₂S QDs from the visible to near-infrared band. The output characteristics, transfer characteristics and fast switching capability of the device at 405 nm, 532 nm and 808 nm were tested. The device has the responsivity of 6.05 A W⁻¹, 83.72 A W⁻¹ and 37.31 A W⁻¹ under 405 nm, 532 nm and 808 nm laser irradiation. This work plays an important role in improving the detection performance of Ag₂S QDs and broadening its applications in photoelectric devices for weak light and wide spectrum detection.