Constructed MXene matrix composites as sensing material and applications thereof: A review.

Most studies on MXene matrix composites for sensor development have primarily focused on synthesis and application. Nevertheless, there is currently a lack of research on how the introduction of different materials affects the sensing properties of these composites. The rapid development of MXene has raised intriguing questions about improving sensor performance by combining MXene with other materials such as polymers, metals and inorganic non-metals. This review will concentrate on the construction of MXene-based composites and explore ways to enhance their sensor applications. Specifically, this review describes why the introduction of materials to the system brings the advantage of low concentration and high sensitivity assays, as well as the MXene-based frameworks that have been recently investigated. Lastly, in order to capture the current trend of MXene-based composites in sensor applications and identify promising research directions, this review will critically evaluate the potential applications of newly developed MXene systems.

Localized electron-accepted yellow-emission carbon dots encapsulated in UiO-66 for efficient visible-light driven photocatalytic activity.

Metal organic frameworks (MOFs) possess a large surface area, inherent porosity and high crystallinity. Nevertheless, they lack electron acceptors, which limit the exploitation of their photocatalytic properties. Carbon dots (CDs) known for excellent optical properties can serve as localized electron acceptors. As a novel hybrid nanomaterial, the structure of CDs@MOFs effectively facilitates charge separation and carrier transfer, bring about a marked improvement of photocatalytic activity. In this study, yellow-emission carbon dots (YCDs) were encapsulated within zirconium-based metal organic framework (UiO-66) via a dynamic adsorption method. Compared with blue carbon dots (BCDs), the YCDs@UiO-66 exhibited superior degradation performance. It demonstrates that incorporation of YCDs broadens the UV absorption range of UiO-66, thereby enhancing light utilization. The degradation efficiency of YCDs@UiO-66 was 92.6%, whereas UiO-66 alone achieved only 63.1%. Notably, the results of the radical quenching experiment and electron paramagnetic resonance (EPR) revealed that h+ and •O2- played a prominent role in the photodegradation of tetracycline hydrochloride (TCH). This study highlights that the introducing YCDs in MOFs-mediated photocatalytic reactions is a viable strategy to improve catalytic efficiency.

Carbon dots modified/prepared by supramolecular host molecules and their potential applications: A review.

Supramolecular host molecules are used as tools in the design of multifunctional nanoparticles for sensors, catalysts, biometric elements, etc. Combining with carbon dots (CDs) has excellent host-guest recognition properties and fluorescence characteristics, which can precisely capture and identify target analytes. Consequently, supramolecular host molecules-based CDs can significantly improve the detection performance of ions and molecules with different structures or intrinsic chemical properties. This currently responds to a wide range of analytes including metal cations, anions, organic compounds and other biomolecules, yielding fascinating achievements in the field of chemistry. Therefore, the present review summarizes outstanding supramolecular host molecules-based CDs reported in the past ten years. The focus is on elucidating the mechanisms, methodologies, advantages and disadvantages of modifying or preparing CDs with supramolecular host molecules. Current challenges encountered and outlooks are also be discussed.

Carbon dots as a promising therapeutic approach for combating cancer.

With advances in nanotechnology and the development of emerging therapeutic modalities, a surge in research on the use of nanomedicines for biomedical applications has occurred over the past three decades. Carbon dots (CDs) are new members of carbon-based nanomaterials that can be used in cancer treatment to reduce side effects of drugs and improve treatment efficiency, offering new medical opportunities for further research. Urged as such, we are encouraged to classify CDs-based cancer treatments, including photodynamic therapy (PDT), photothermal therapy (PTT), sonodynamic therapy (SDT), chemodynamic therapy (CDT), chemotherapy (CT), and synergistic therapy. The advantages of these approaches are summarized, as well as ways to improve certain shortcomings. Meanwhile, this review highlights the feasible practice of CDs in cancer treatment, which is further developed and widely used in the biomedical field.

Sensing performance and mechanism of carbon dots encapsulated into metal-organic frameworks.

Metal-organic frameworks (MOFs) can be combined with nanomaterials and the combined composites have excellent optical properties. Carbon dots (CDs) with tiny particle size, non-toxic and rich surface functional groups are novel fluorescent materials. Carbon dots@metal-organic frameworks (CDs@MOFs) are synthesized by encapsulating CDs into MOFs. CDs@MOFs are promising composites for the preparation of a new generation of fluorescence sensors, which combine the hybrid properties of MOFs and the special optical properties of CDs. Urged as such, we are encouraged to categorize according to the sensing mechanisms. These include fluorescence resonance energy transfer (FRET), aggregation-caused quenching (ACQ), static quenching, dynamic quenching, photo-induced electron transfer (PET), inner filter effect (IFE) and so on. Based on the above mechanisms, CDs@MOFs can specifically interact with target analytes to generate fluorescence quenching. This review covers the research progress of CDs@MOFs in recent five years (with 103 refs), synthetic design of CDs@MOFs and introduces the sensing mechanism. The current challenges and future research directions are discussed briefly. The sensing mechanism and applications of CDs@MOFs.

Self-quenching-resistant solid-state carbon dots for mechanism and applications.

Solid-state carbon dots (SCDs) have been widely investigated by scholars owing to their stability, environmental friendliness, and their good optical properties. The current studies on carbon dots (CDs) are mainly focused on the solutions of CDs, while the researches on SCDs are relatively few in comparison. Nowadays, the fabrication and design of high-performance SCDs have attracted much interest. However, due to resonance energy transfer and π-π interactions, CDs undergo aggregation-induced quenching (ACQ) phenomena. This poses an obstacle to the acquisition of SCDs and affects their luminescence performance. Publications of the past 5 years are reviewed on how to suppress the ACQ phenomenon and improve the fluorescence and phosphorescence emission of CDs (Ref. 87) and about the mechanism of achieving the luminescence of SCDs. Then, the applications of SCDs in the fields of luminescent devices, anti-counterfeiting, and detection are outlined. The concluding section analyzes the current challenges faced by SCDs and provides an outlook. Mechanism of photoluminescence from solid state carbon dots.

A fluorescein-coumarin based ratiometric fluorescent probe for detecting hydrazine and its real applications in cells imaging.

According to the mechanism of hydrazine to ester bond elimination, a novel ratiometric fluorescent probe (FCP) based on the fluorescein-coumarin structure is designed and synthesized for detecting hydrazine. The obvious red shift in the absorption and fluorescence spectrum is caused by the hydrolysis of the ester bond of FCP by upon mixing with hydrazine. The proposed FCP probe is shown to have linear detection ranges from 0 to 250 nM for hydrazine, and LOD is 0.364 nM. In addition, this promising probe possesses high sensitivity and selectivity for monitoring the intracellular hydrazine, thus, has great potential to be applied in early diagnosis of disease. Meanwhile, it has been successfully applied to the detection and cell imaging of hydrazine in actual water samples.

Designing enhanced and ratiometric probes for detecting OCl- based on substituents influencing the fluorescence of HBT and their application in strips and bioimaging.

Nitro groups with a strong electron-withdrawing effect can powerfully influence the fluorescence of fluorophores. In this work, through adjusting the nitro group at the HBT fluorophore to construct a phenylhydrazone structure, two probes (HBTN and HBTH) were synthesized to detect OCl-. Consequently, HBTN with the nitro group quenched the fluorescence of HBT and HBTH without the nitro group causing a redshift of the fluorescence, which resulted in enhanced and ratiometric fluorescence signal changes during the detection process. Among them, HBTN shows good ability to selectively detect OCl- in the concentration range of 0-14 µM with the detection limit of 2.06 nM. Based on HBTN, a portable test strip for detecting OCl- was made for the convenient quantification of the OCl- concentration with a spectrophotometer, and exogenous and endogenous OCl- was successfully imaged in cells.

WS2 quantum dots-MnO2 nanosheet system for use in ratiometric fluorometric/scattered light detection of glutathione.

Based on WS2 quantum dots (QDs) as fluorescent signals and MnO2 nanosheets as second-order scattering (SOS) signals, a combination of fluorescence and scattered light was used to construct a ratio sensing platform for glutathione (GSH) detection. When MnO2 nanosheets are added to WS2 QDs, the fluorescence of WS2 QDs is quenched by MnO2 nanosheets through IFE. Large-sized MnO2 nanosheets increase the SOS of the system and gradually approach the fluorescence. After adding GSH to WS2 QDs-MnO2, the MnO2 nanosheets were decomposed into Mn2+. The disappearance of the characteristic absorption peak of the MnO2 nanosheets suppressed the IFE to WS2 QDs, resulting in the fluorescence recovery of WS2 QDs. The reduction in size of MnO2 nanosheets after decomposition results in a decrease in the SOS of the system. Therefore, the ratio detection of GSH is obtained through the fluorescence and SOS dual signal response. Under optimal experimental conditions, the value of F406/S648 is linearly related to the GSH concentration in the range 0 to 60 µM, and the limit of detection (LOD) of GSH is 0.12 µM. In addition, the system is also used for the determination of GSH in real water samples and human serum, with good analytical results. Graphical abstract Schematic principle of fluorescence/scattered light system based on WS2 QDs-MnO2 for GSH ratiometric detection.

Ratiometric fluorescent nanoprobes based on Resonance Rayleigh Scattering and inner filter effect for detecting alizarin red and Pb2.

A new ratiometric fluorescent strategy for detection of alizarin red (ARS) was designed based on the fluorescence of CDs and scattered light of scatterer. The CDs-ARS system can be used to detect Pb2+ based on that the complexation between ARS and Pb2+. With the addition of ARS, the fluorescence of CDs was apparently quenched via inner filter effect (IFE). Resonance Rayleigh Scattering (RRS) at 350 nm was enhanced by an increase in the number of scatterer. The value of ln(I350/I425) was linearly correlated with ARS concentration in the range of 0-80 µM, and the detection limit for ARS was calculated to be 68.1 nM. When Pb2+ was added to the CDs-ARS system, the complexation of ARS with Pb2+ increased the size of the scatterer, resulting in the increase of the RRS intensity at 350 nm. Due to the affinity between ARS and Pb2+, the overlap of the emission spectra of CDs and the absorption spectra of ARS was reduced, resulting in the IFE effect was inhibited and the recovery of the fluorescence of CDs. The value of I350/I425 linearly increased with the addition of Pb2+ within the range of 10-50 µM, the limit of detection was 36.8 nM. As for practical application, CDs and CDs-ARS were applied to detect ARS and Pb2+ in tap water and poor water, respectively. The recovery values were obtained to be 95.4-98.8% and 93.4-101.7%. Furthermore, the system of CDs-ARS has been successfully applied to H1299 cell imaging.

4-aminoantipyrine modified carbon dots and their analytical applications through response surface methodology.

A sensitive and selective nanoprobe for detection of hypochlorite (OCl-) based on 4-aminoantipyrine (AAP) modified carbon dots (CDs-AAP) has been prepared. The CDs-AAP exhibit an emission peak at 484 nm when the excitation wavelength is 370 nm, accompanying 36 nm red shift compare with the pristine CDs. The addition of OCl- lead to the AAP on the surface of CDs experience a process of hydrazide hydrolysis and double bond addition, causing the singlet and triplet electrons of the excited state more closer in energy (ie, the energy difference between the two is reduced), eventually quenching the fluorescence of CDs due to heavy atomic effects. Central composite design (CCD) and response surface method (RSM) were used to optimize the detection variables of pH, incubation time and temperature. The designed model study indicated that the optimum detection conditions was pH 7.0, temperature 30 °C and incubation time 20 min, respectively. Under optimal conditions, the fluorescent intensity of the nanoprobe linearly responded to the OCl- concentration from 3 µM to 36 µM and the limit of detection was 40 nM. The proposed nanoprobe was successfully used to the detection of OCl- in tap water and pool water, and the recovery were in the range of 94% - 103%. In addition, the nanoprobe was also applied in imaging of VMSCs cells and labeling E.coli.

Novel fluorescein- and pyridine-conjugated schiff base probes for the recyclable real-time determination of Ce3+ and F.

The effect of aminopyridines substituted at different positions on the fluorescence properties deserves to be studied. Since 2-aminopyridyl-based probes have been reported, the effects of 3-aminopyridine and 4-aminopyridine on the performance of fluorescein probes were discussed in here. Two Schiff base fluorescein probes FN-1, FN-2 were designed and synthesized. Among them, since the ligand shows a highly selective and sensitive response to metal charge transfer (LMCT), the fluorescence of FN-1 can be quenched by Ce3+ ions in PBS buffer. At the same time, a specific precipitation reaction between Ce3+ and F- releases the free probe to restore the fluorescence of FN-1. Therefore, FN-1 can be used for the recyclable 'ON-OFF-ON' detection of Ce3+and F-. The detection limits for Ce3+and F- are 4.48 µM and 11.58 µM in concentration range of 0-50 µM and 0-150 µM. However, due to the para position of N and amino groups on 4-aminopyridine, the spatial structure of FN-2 cannot be complexed with ions, resulting in poor selectivity. Polyvinylidene fluoride (PVDF) membrane containing FN-1 were prepared for the real-time qualitative detection of Ce3+and F- in real water samples. FN-1 exhibits high water solubility and biocompatibility and has been successfully applied to biological imaging in vascular smooth muscle cells (VSMCs).

The fluorescence mechanism of carbon dots, and methods for tuning their emission color: a review.

Carbon dots (CDs) display tunable photoluminescence and excitation-wavelength dependent emission. The color of fluorescence is affected by electronic bandgap transitions of conjugated π-domains, surface defect states, local fluorophores and element doping. In this review (with 145 refs.), the studies performed in the past 5 years on the relationship between the fluorescence mechanism and modes for modulating the emission color of CDs are summarized. The applications of such CDs in sensors and assays are then outlined. A concluding section then gives an outlook and describes current challenges in the design of CDs with different emission colors. Graphical abstract Schematic representation of the relationship between the color-emitting (blue, green, yellow, red and multicolor) modulation of carbon dots and fluorescence mechanism including bandgap transitions of conjugated π-domains and surface defect states.

Enhanced fluorescence probes based on Schiff base for recognizing Cu2+ and effect of different substituents on spectra.

Three enhanced fluorescence probes based on Rhodamine B-Schiff base structure were synthesized for detecting Cu2+. The corresponding detection limits were found to be 0.25 µM, 0.15 µM and 0.18 µM. Binding ratio and binding sites were determined by Job's and nuclear magnetic titration experiments. The binding constants obtained by the Benesi-Hildebrand equation to be 341.0 M-0.5,1.8 × 104 M-1, and 265.4 M-0.5, respectively. As isomers, the different effects of probes on Cu2+ detection were researched. By adjusting the position and the size of the substituent group, the effects of binding sites and steric hindrance on the complexation ratio, response time and detection limit were discussed. Optimal spatial combination structure with Cu2+ was obtained through energy calculation. Detection mechanism of Rhodamine B ring opening based on the complex of the Schiff base with Cu2+ was confirmed. E. coli staining and detection of real water samples had expanded their applications.

Synthesis and spectral analysis of fluorescent probes for Ce4+ and OCl- ions based on fluorescein Schiff base with amino or hydrazine structure: Application in actual water samples and biological imaging.

Two Schiff base fluorescein probes (FDA, FDH) based on fluorescein-aldehyde and nitroaniline derivatives were synthesized. The effects of amino and hydrazine substituents in fluorescein backbones were examined via fluorescence and absorbance spectra. In the presence of Ce4+, the fluorescence of FDA was quenched due to the ligand to metal charge transfer (LMCT). Hypochloric acid can react with the CN bond, and blocking the photo induced electron transfer (PET) of FDH leads to enhancement of the fluorescence. FDA showed detection limits for Ce4+ and OCl- as low as 63 nM in concentration range of 0-4 µM. FDH showed detection limits for OCl- as low as 0.8 µM in concentration rang 0-100 µM. Polyvinylidene fluoride (PVDF) membrane containing the probes was prepared for the real-time qualitative detection of Ce4+ and OCl- in real water samples. The probes were successfully applied to biological imaging in vascular smooth muscle cells (VSMCs) and are expected to find applications in biosensing.

Yellow-emissive carbon dots with a large Stokes shift are viable fluorescent probes for detection and cellular imaging of silver ions and glutathione.

Yellow-emissive carbon dots (Y-CDs) were prepared by a solvothermal method using anhydrous citric acid and 2,3-phenazinediamine as the starting materials. The Y-CDs display a 24% fluorescence quantum yield, a 188-nm Stokes' shift and excellent stability. They are shown here to be excellent fluorescent probes for the determination of Ag(I) ion and glutathione (GSH). If exposed to Ag(I) ions, they are bound by the carboxy groups of the Y-CDs, and this causes quenching of fluorescence (with excitation/emission maxima at 380/568 nm) via a static quenching mechanism. This effect was used to design a fluorometric assay for Ag(I). The quenched fluorescence of the Y-CDs can be restored by adding GSH due to the high affinity of GSH for Ag(I). The calibration plot for Ag(I) is linear in the 1-4 µM Ag(I) concentration range, and the limit of detection is 31 nM. The respective values for GSH are 5-32 µM, and 76 nM, respectively. The method was applied to the detection of Ag(I) in spiked environmental water samples and gave recoveries ranging from 93 to 107%. It was also applied to the determination of GSH in tomatoes and purple grapes and gave satisfactory recoveries. The Y-CDs display low cytotoxicity and were successfully used to image Ag(I) and GSH in H1299 cells. Graphical abstract Schematic presentation of the mechanism of yellow fluorescent CDs for the detection of Ag+ and glutathione.

Surface modification and chemical functionalization of carbon dots: a review.

Surface functional groups strongly affect the properties of carbon dots (CDs). Amino, carboxy, and hydroxy groups are most commonly encountered in CDs, and they can be introduced via covalent and noncovalent modification. This article (with 116 refs.) reviews the progress made in the past few years. Following an introduction into the field, a large section covers methods for covalent modification (via amide coupling reactions, silylation, and other reactions including esterification, sulfonylation and copolymerization). Next section reviews methods for noncovalent modifications (π interactions, complexation/chelation, and electrostatic interactions). The resulting modified CDs are powerful nanomaterials for targeting and extracting analytes, and in drug release. The modification of the surface also affects fluorescence quantum yields, complexation capacity, the color of fluorescence, and their quenching capability. Current challenges are critically assessed in the concluding section. Graphical abstract The modification methods of carbon dots (CDs) includes covalent and noncovalent. Covalent modifications include amidation, silylation, esterification, sulfonylation and copolymerization reaction. Noncovalent modifications include electrostatic interactions, complexation and π interactions.

Fluorescent probes for detecting cysteine.

Cysteine plays a crucial role in physiological processes. Therefore, it is necessary to develop a method for detecting cysteine. Fluorimetry has the advantages of convenient detection, short response time, high sensitivity and good selectivity. In this review, fluorescent probes that detect cysteine over the past three years are summarized based on structural features of fluorophores such as coumarin, BODIPY, rhodamine, fluorescein, CDs, QDs, etc and reaction groups including acrylate, aldehyde, halogen, 7-nitrobenzofurazan, etc. Then, effects of different combinations between fluorophores and response groups on probe properties and detection performances are discussed.

Dual-channel fluorescence detection of mercuric (II) and glutathione by down- and up-conversion fluorescence carbon dots.

The fluorescent carbon dots (CDs) with high fluorescent quantum yield (φf = 62%) and down- and up-conversion fluorescence properties were synthesized by one-pot hydrothermal treatment of citric acid and tris(hydroxymethyl)methyl aminomethane. The CDs displayed the capability to absorb excitation wavelength at 660 nm and 330 nm with fluorescence emission wavelength at 398 nm and 399 nm, respectively. The CDs showed high selectivity towards Hg2+ against various metal ions. Around 70% fluorescence was quenched by 40 µM Hg2+ through dynamic and static quenching mechanisms. Because of stronger affinity between the thiol and Hg2+, over 90% fluorescence was recovered by adding 40 µM glutathione to CDs-Hg2+ system. The calibration curves exhibited wide linear region for Hg2+ (0-4 µM) and glutathione (0-30 µM). The limits of detection with down- and up-conversion for Hg2+ were calculated to be 0.23 µM and 0.25 µM, and for glutathione were 0.28 µM and 0.29 µM, respectively. Inspired by the sensing results, logic gates with Hg2+ and glutathione as inputs were also established. Most importantly, this method was applied to detect Hg2+ and glutathione in tap water and lake water, and the recovery values were obtained to be 96.2%-110.4% and 93.4%-96.9%.

A dual spectroscopic fluorescence probe based on carbon dots for detection of 2,4,6-trinitrophenol/Fe (III) ion by fluorescence and frequency doubling scattering spectra and its analytical applications.

A convenient, highly sensitive and reliable assay for 2,4,6­trinitrophenol (TNP) and Fe (III) ion (Fe3+) in the dual spectroscopic manner is developed based on novel carbon dots (CDs). The CDs with highly blue emitting fluorescent were easily prepared via the one-step potassium hydroxide-assisted reflux method from dextrin. The as-synthesized CDs exhibited the high crystalline quality, the excellent fluorescence characteristics with a high quantum yield of ~13.1%, and the narrow size distribution with an average diameter of 6.3±0.5nm. Fluorescence and frequency doubling scattering (FDS) spectra of CDs show the unique changes in the presence of TNP/Fe3+ by different mechanism. The fluorescence of CDs decreased apparently in the presence of TNP via electron-transfer. Thus, after the experimental conditions were optimized, the linear range for detection TNP is 0-50µM, the detection limit was 19.1nM. With the addition of Fe3+, the FDS of CDs appeared to be highly sensitive with a quick response to Fe3+ as a result of the change concentration of the scattering particle. The emission peak for FDS at 450nm was enhanced under the excitation wavelength at 900nm. The fluorescence response changes linearly with Fe3+ concentration in the range of 8-40µM, the detection limits were determined to be 44.1nM. The applications of CDs were extended for the detection of TNP, Fe3+ in real water samples with a high recovery. The results reported here may become the potential tools for the fast response of TNP and Fe3+ in the analysis of environmental pollutants.