Ti 3 C 2 T x MXene with an organ-like structure was synthesized from Ti 3 AlC 2 (MAX phase) through the typical hydrofluoric (HF) acid etching method. Ti 3 C 2 T x MXene was further alkaline-treated with a sodium hydroxide solution to obtain alkalized Ti 3 C 2 T x . Room-temperature planar-type gas-and humidity-sensing devices were also fabricated by utilizing Ti 3 C 2 T x MXene and alkalized Ti 3 C 2 T x sensing material based on the dip coating method, respectively. The intercalation of the alkali metal ion (Na + ) and the increase of the surface terminal oxygen−fluorine ratio ([O]/[F]) in Ti 3 C 2 T x can effectively improve humidity-and gas-sensing properties at room temperature. The developed alkalized Ti 3 C 2 T x sensor exhibited excellent humidity-sensing characteristics (approximately 60 times response signal change) in the relative humidity (RH) with a range of 11−95% and considerable NH 3 sensing performance (28.87% response value to 100 ppm of NH 3 ) at room temperature. The improvement of NH 3 and humidity-sensing properties indicated that alkalized Ti 3 C 2 T x has great potential in chemical sensors, especially in NH 3 and humidity sensors. KEYWORDS: MXene, organ-like structure, alkalized Ti 3 C 2 T x , NH 3 and humidity sensing, room temperature

This review summarized recent advances in CDs-based bioimaging including in vitro imaging in delivering CDs into different types of cells and in vivo applications in distribution and uptake of CDs, imaging-guild drug delivery and tumor therapeutics.

Carbon dots (CDs) combined with a nanomaterial-based quencher has created an innovative way for designing promising sensors. Herein, a novel fluorescent-sensing platform was designed for sensitive detection of organophosphorus pesticides (OPs). The preparation of CDs was based on one-step hydrothermal reaction of 3-aminobenzeneboronic acid. The fluorescence of CDs can be quenched by manganese dioxide (MnO) nanosheets via the Förster resonance energy transfer (FRET). In the presence of butyrylcholinesterase (BChE) and acetylthiocholine, the enzymatic hydrolysate (thiocholine) can efficiently trigger the decomposition of MnO nanosheets, resulting in the recovery of CDs fluorescence. OPs as inhibitors for BChE activity can prevent the generation of thiocholine and decomposition of MnO nanosheets, accompanying the fluorescence "turn-off" of the system. So the BChE-ATCh-MnO-CDs system can be utilized to detect OPs quantitatively based on the fluorescence turn "on-off". Under the optimum conditions, the present FRET-based approach can detect paraoxon ranging from 0.05 to 5 ng mL with a detection limit of 0.015 ng mL. Meanwhile, the present strategy also showed a visual color change in a concentration-dependent manner. Thus, the proposed assay can potentially be a candidate for OPs detection.

Glutathione (GSH) monitoring has attracted extensive attention because it serves a vital role in human pathologies. Herein, a convenient fluorescence "turn off-on" nanosensor based on graphene quantum dots (GQDs)-manganese dioxide (MnO2) nanosheet has been designed for selective detection of GSH in living cells. The fluorescence intensity of GQDs can be quenched by MnO2 nanosheets via a fluorescence resonance energy transfer. However, GSH can reduce MnO2 nanosheets to Mn(2+) cations and release GQDs, causing sufficient recovery of fluorescent signal. The MnO2 nanosheets serve as both fluorescence nanoquencher and GSH recognizer in the sensing platform. The sensing platform displayed a sensitive response to GSH in the range of 0.5-10 μmol L(-1), with a detection limit of 150 nmol L(-1). Furthermore, the chemical response of the GQDs-MnO2 nanoprobe exhibits high selectivity toward GSH over other electrolytes and biomolecules. Most importantly, the promising platform was successfully applied in monitoring the intracellular GSH in living cells, indicating its great potential to be used in disease diagnosis. Meanwhile, this GQDs-MnO2 platform is also generalizable and can be easily expanded to the detection and imaging of other reactive species in living cells.

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In the present study, a novel colorimetric sensing platform was constructed for quantitative detection of acetylcholinesterase (AChE) activity and its inhibitor. Manganese dioxide (MnO) nanosheets as an oxidase-mimicking nanomaterial could directly oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxTMB without the need for horseradish peroxidase and HO. When AChE was introduced, acetylthiocholine could be catalytically hydrolyzed to produce thiocholine, which easily triggers the decomposition of MnO nanosheets, causing the decrease of solution absorbance. Owing to the inhibition effect of organophosphorus pesticides, the enzymatic activity was suppressed, preventing the decomposition of MnO and resulting in the increase of absorbance. Under optimal conditions, the colorimetric platform shows sensitive responses to AChE and paraoxon in the range of 0.1-15 mU mL and 0.001-0.1 μg mL, respectively. The detection limits of AChE and paraoxon reached 35 μU mL and 1.0 ng mL, respectively. Furthermore, the MnO-TMB platform has been used to fabricate test strips for rapid and convenient visual detection of AChE and its inhibitor with highly promising performance.

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Early diagnosis and monitoring of SARS-CoV-2 virus is essential to control COVID-19 outbreak. In this study, we propose a promising surface enhanced Raman scattering (SERS)-based COVID-19 biosensor for ultrasensitive detection of SARS-CoV-2 virus in untreated saliva. The SERS-immune substrate was fabricated by a novel oil/water/oil (O/W/O) three-phase liquid-liquid interfaces self-assembly method, forming two layers of dense and uniform gold nanoparticle films to ensure the reproducibility and sensitivity of SERS immunoassay. The detection was performed by an immunoreaction between the SARS-CoV-2 spike antibody modified SERS-immune substrate, spike antigen protein and Raman reporter-labeled immuno-Ag nanoparticles. This SERS-based biosensor was able to detect the SARS-CoV-2 spike protein at concentrations of 0.77 fg mL -1 in phosphate-buffered saline and 6.07 fg mL -1 in untreated saliva. The designed SERS-based biosensor exhibited excellent specificity and sensitivity for SARS-CoV-2 virus without any sample pretreatment, providing a potential choice for the early diagnosis of COVID-19.