UV illumination-enhanced ultrasensitive ammonia gas sensor based on (001)TiO2/MXene heterostructure for food spoilage detection.

Ammonia has been used as an important marker to indicate the extent of food spoilage. However, current gas sensors for ammonia suffer from either insufficient sensitivity and selectivity or unsatisfactory levels of automation, impeding their practical application for on-site and real-time monitoring of food quality. To overcome these limitations, we propose here the design of a sensing material by in-situ growing (001)TiO2 onto a two-dimensional transition-metal carbide (Ti3C2Tx, MXene). In this design, TiO2 with a highly active (001) crystal plane provides efficient photogeneration under UV irradiation, while Ti3C2Tx can store holes through Schottky junction formed at the interface with TiO2, which greatly promotes the separation of electron-hole pairs, thereby enhancing ammonia sensing performance. By further introducing UV light for electron excitation, the (001)TiO2/Ti3C2Tx based sensor shows 34 times higher sensitivity for ammonia (30 ppm) than that of Ti3C2Tx. The density functional theory further revealed that the (001) plane of TiO2 and Ti3C2Tx composite configuration exhibited the highest adsorption affinity towards ammonia. Finally, an integrated circuit alarm system including near-field communication and a micro-controller system was designed to detect the decay process of fresh pork, fish, and shrimp. We believe such a sensing technology holds great promise in food quality monitoring.

In-situ grafting temperature-responsive hydrogel as a bifunctional solid-phase microextraction coating for tunable extraction of biomacromolecules.

A temperature-responsive solid-phase microextraction (SPME) coating was prepared via in-situ atom transfer radical polymerization (ATRP) method. By controlling the temperature of solution below and above the lower critical solution temperature (LCST) of the coating, it can switch between hydrophilic and hydrophobic, thus providing a convenient approach for the selective extraction of analytes with different polarities. The average extraction amount of temperature-responsive coating for polar analytes is about 1.5-fold to that of non-polar ones below LCST, and vice versa. Effective extraction of three biomacromolecules was also obtained by controlling the temperature below or above LCST. The adsorption capacity of the coating for the hydrophilic biomacromolecules at 15 °C is 1.5-2 folds that of 50 °C, whereas the adsorption capacity of the coating to BSA at 50 °C is about 3 folds that of 15 °C. This approach holds great promise for SPME because it provides a simple strategy to prepare bifunctional coatings for various applications.

Green light-driven enhanced ammonia sensing at room temperature based on seed-mediated growth of gold-ferrosoferric oxide dumbbell-like heteronanostructures.

Since there is excellent synergy between heterostructures and noble metals due to their unique electro-optical and catalytic properties, the introduction of noble metals into metal oxide semiconductors has substantially improved the performance of gas sensors. However, most of the reported noble metal-metal oxide composites are generally prepared as simple hybrids; hence, there is lack of control over their structure, morphology and dimension. Herein, we report a seed-mediated growth of dumbbell-like Au-Fe3O4 heteronanostructured gas sensors for ammonia detection under green light illumination, in which the particle sizes of Au and Fe3O4 were readily tuned in a wide range. The ammonia gas-sensing performances of Au-Fe3O4 heteronanostructures were greatly improved at room temperature by regulating their dimensions. In particular, the sensitivity improved by 30% while the response and recovery time shortened by 20 s and 50 s for the 7.5 nm Au-loaded Fe3O4-based sensor toward 5 ppm ammonia under 520 nm green light illumination as compared to that in the absence of light. This can be ascribed to the localized surface plasmon effect of Au and the Schottky junction formed at the interface between Au and Fe3O4. Interestingly, the Au-Fe3O4 heteronanostructure exhibits a unique p-type to n-type reversible transition for ammonia detection due to the nature of Fe3O4 NPs related to the trade-off between oxygen vacancies and electron transfer caused by ammonia adsorption. In addition, the calculation based on first-principle theory reveals enhanced adsorption capacities of Fe3O4 for ammonia after Au-doping.

Electro-enhanced solid-phase microextraction of bisphenol A from thermal papers using a three-dimensional graphene coated fiber.

Three-dimensional (3-D) graphene was synthesized by the assembly of graphene oxide with phenolic resin, followed by carbonization in argon. The as-synthesized 3-D graphene has excellent conductivity, good thermal stability, large specific surface area (1511 m² g-1) and pore volume (0.90 cm3 g-1). By immobilizing the 3-D graphene onto the stainless steel wire (SSW), we obtained a 3-D graphene coated fiber that was then used as a working electrode for electro-enhanced SPME, which shows a 3.2-fold improvement of extraction efficiency for bisphenol A (BPA) over that of traditional SPME. Coupled to gas chromatography, the method was developed to the determination of BPA with good linearity (R2 = 0.9935) in the range of 0.1-10 µg mL-1. The limit of detection was calculated to be 0.006 µg mL-1 based on the signal-to-noise of 3. The proposed method was applied for the analysis of three kinds of thermal papers with BPA being detected in all samples (0.696-3.78 mg g-1). Recovery tests were performed to validate the reliability of the method, and the recoveries were found between 81.9% and 119% with relative standard deviations lower than 4.8%.