Rapid detection of Cr(VI) ions based on cobalt(II)-doped carbon dots

Rapidly and simply detecting Cr (VI) in aqueous media via a diketopyrrolopyrrole-based chemosensor with both high selectivity and low LOD

BACKGROUND

The high toxicity of hexavalent chromium [Cr (VI)] could not only cause harmful effects on humans, including carcinogenicity, respiratory issues, genetic damage, and skin irritation, but also contaminate drinking water sources, aquatic ecosystems, and soil, impairing the reproductive capacity, growth, and survival of organisms. Due to these harmful effects, detecting toxic Cr (VI) is of great significance. However, the rapid, simple, and efficient detection at a low Cr (VI) concentration is extremely challenging, especially in an acidic condition (existing as HCrO4-) due to its low adsorption free energy.

RESULTS

A diketopyrrolopyrrole-based small molecule (DPPT-PhSMe) is designed and characterized to act as a chemosensor, which allows a high selectivity to Cr (VI) at an acidic condition with a low limit of detection to 10-8 M that is two orders of magnitude lower than the cut of limit (1 μM) recommended by World Health Organization (WHO). Mechanism study indicates that the rich sulfur atoms enhance the affinity to HCrO4-. Combining with favorable features of diketopyrrolopyrrole, DPPT-PhSMe not only allows dual-mode detection (colorimetric and spectroscopic) to Cr (VI), but also enables disposable paper-based sensor for naked-eye detection to Cr (VI) from fully aqueous media. The investigation of DPPT-PhSMe chemosensor for the quantification of Cr (VI) in real life samples demonstrates a high reliability and accuracy with an average percentage recovery of 102.1 % ± 4 (n = 3).

SIGNIFICANCE

DPPT-PhSMe represents the first diketopyrrolopyrrole-derived chemosensor for efficient detection to toxic Cr (VI), not only providing a targeted solution to the bottleneck of Cr (VI) detection in acidic conditions (existing as HCrO4-) caused by its low adsorption free energy, but also opening a new scenario for simple, selective, and efficient Cr (VI) detection with conjugated dye molecules.

Hydrothermal route upcycling surgical masks into dual-emitting carbon dots as ratiometric fluorescent probe for Cr (VI) and corrosion inhibitor in saline solution

Exploration effective route to convert plastic waste into valuable carbon dots with bifunction of metal fluorescence monitoring and corrosion protection in seawater is promising. Herein, using "white-pollution" polypropylene surgical masks as a single precursor, dual-emitting carbon dots (CDs) with excellent ratiometric fluorescent sensitivity and corrosion inhibitor efficiency were fabricated with high yield (∼100 %) by a one-pot in situ acid oxidation hydrothermal strategy without post-treatment and organic solvents. Chemical, structural, morphological, optical properties and the Cr (VI) detection and Cu inhibition mechanism of the synthesized CDs had been systematically studied. Furthermore, a dual-response-OFF proportional fluorescent probe had been developed for the detection of the analyte Cr (VI) with a low detection limit of 24 nM. Additionally, the corrosion inhibition efficiency of the prepared CDs reached approximately 94.01 % for Cu substrate in 3.5 wt% NaCl electrolyte under a CDs concentration of 200 mg/L, which is higher than that of most previous reports.

High-efficient nickel-doped lignin carbon dots as a fluorescent and smartphone-assisted sensing platform for sequential detection of Cr(VI) and ascorbic acid

Using lignin as a raw material to prepare fluorescent nanomaterials represents a significant pathway toward the high-value utilization of waste biomass. In this study, Ni-doped lignin carbon dots (Ni-LCDs) were rapidly synthesized with a yield of 63.22 % and a quantum yield of 8.25 % using a green and simple hydrothermal method. Exploiting the inner filter effect (IFE), Cr(VI) effectively quenched the fluorescence of the Ni-LCDs, while the potent reducing agent ascorbic acid (AA) restored the quenched fluorescence, thus establishing a highly sensitive fluorescence switch sensor platform for the sequential detection of Cr(VI) and AA. Importantly, the integration of a smartphone facilitated the portability of Cr(VI) and AA detection, enabling on-site, in-situ, and real-time monitoring. Ultimately, the developed fluorescence and smartphone-assisted sensing platform was successfully applied to detect Cr(VI) in actual water samples and AA in various fruits. This study not only presents an efficient method for the conversion and utilization of waste lignin but also broadens the application scope of the CDs in the field of smart sensors.

Carbon dots with enhanced red emission for ratiometric sensing and encryption applications

The excitation-dependent emission properties of carbon dots (Cdots) are extensively reported, but their red emission is often weak, limiting their wider application. Here we introduce ethidium bromide, as a functional precursor with red emission, to enhance the red emission for Cdots, with comparable intensity at a broad wavelength range to multi-emission Cdots (M-Cdots). We found that Cdots prepared with ethidium bromide/ethylenediamine exhibited strong blue and red emission at 440 and 615 nm, with optimal excitation at 360 and 470 nm as M-Cdots, respectively, but the Cdots from single ethidium bromide (EB-Cdots) possessed weak red emission. M-Cdots exhibited a broad absorption band at 478 nm, but a band blue-shifted to 425 nm was observed for EB-Cdots, while no absorption was observed at 478-425 nm for the Cdots prepared with citric acid and ethylenediamine. Thus, we proposed that C=O and C=N formed a π-conjugation structure as the absorption band at 478 nm for the red emission of M-Cdots, as also confirmed with the excitation at 470 nm. Moreover, the π-conjugation structure is fragile and sensitive to harsh conditions, so red emission was difficult to observe for the Cdots prepared with citric acid/ethylenediamine or single ethidium bromide. M-Cdots possess two centers for blue and red emission with different structures. The dual emission was therefore used for ratiometric sensing with dichromate (Cr2O72-) and formaldehyde (HCHO) as the targets using the intensity ratio of the emissions at 615 and 440 nm. Due to the comparable intensity at a broad wavelength range, we designed encryption codes with five excitations at 360, 400, 420, 450, and 470 nm as the inputs, and the emission colors were used for information decoding. Thus, we determined why red emission was difficult to realize for Cdots, and our results could motivate the design of red-emission Cdots for extensive applications.

Synergistic impact of bioavailable PHEs and alkalinity on microbial diversity and traits in agricultural soil adjacent to chromium-asbestos mines

Soil microbial communities undergo constant fluctuations, particularly in response to environmental factors. Although the deposition of toxic mine waste is recognized for introducing potentially hazardous elements (PHEs) into the soil, its specific impacts on microbial communities remain unclear. This study aims to explore the combined effects of soil alkalinity and bioavailable PHEs on microbial diversity and traits in agricultural soil adjacent to a chromium-asbestos mining area. By employing a comprehensive analysis, this study indicated that microbiological attributes were reduced in contaminated areas (zone 1), whereas both the levels of bioavailable PHEs (CrWs: 31.08 mg/kg, NiWs: 13.90 mg/kg) and alkalinity indices (CROSS, MCAR, MH) were significantly higher. The spatial distribution of soil alkalinity and bioavailable PHEs, primarily originating from chromium-asbestos mines, has been determined. This study also elucidates the negative relationship between soil stressors (Alkalinity and PHEs) and microbial activities (soil enzymatic activity, microbial respiration, and biomass carbon). The vector's length exhibited a notable difference between zone 1 (0.51) and zone 2 (0.32), indicating a substantial limitation on carbon (C). Also, the investigation of soil bacterial diversity unveiled notable disparities in the prevalence of microbial populations inside zone 1. Proteobacteria constituted 57.18% of the total population indicating a noteworthy prevalence in the contaminated soils. Finally, the random forest (RF) algorithm from machine learning was selected and proven to be a robust choice in Taylor diagrams for predicting the causative stressors responsible for the deterioration of soil microbial health. Therefore, this research offers insights into the health and resilience of soil microbial communities under synergistic stress conditions, which will aid environmentalists in planning future interventions and improving sustainable farming techniques.

The characterization of plant derived-carbon dots and its responses on chlorophyll a fluorescence kinetics, radical accumulation in guard cells, cellular redox state and antioxidant system in chromium stressed-Lactuca sativa

Based on their chemical structure and catalytic features, carbon dots (CDs) demonstrate great advantages for agricultural systems. The improvements in growth, photosynthesis, nutrient assimilation and resistance are provided by CDs treatments under control or adverse conditions. However, there is no data on how CDs can enhance the tolerance against chromium toxicity on gas exchange, photosynthetic machinery and ROS-based membrane functionality. The present study was conducted to evaluate the impacts of the different concentrations of orange peel derived-carbon dots (50-100-200-500 mg L-1 CD) on growth, chlorophyll fluorescence, phenomenological fluxes between photosystems, photosynthetic performance, ROS accumulation and antioxidant system under chromium stress (Cr, 100 μM chromium (VI) oxide) in Lactuca sativa. CDs removed the Cr-reduced changes in growth (RGR), water content (RWC) and proline (Pro) content. Compared to stress, CD exposures caused an alleviation in carbon assimilation rate, stomatal conductance, transpiration rate, carboxylation efficiency, chlorophyll fluorescence (Fv/Fm) and potential photochemical efficiency (Fv/Fo). Cr toxicity disrupted the energy fluxes (ABS/RC, TRo/RC, ETo/RC and DIo/RC), quantum yields and, efficiency (ΨEo and φRo), dissipation of energy (DIo/RC) and performance index (PIABS and PItotal). An amelioration in these parameters was provided by CD addition to Cr-applied plants. Stressed plants had high activities of superoxide dismutase (SOD), peroxidase (POX) and ascorbate peroxidase (APX), which could not prevent the increase of H2O2 and lipid peroxidation (TBARS content). While all CDs induced SOD and catalase (CAT) in response to stress, POX and enzyme/non-enzymes related to ascorbate-glutathione (AsA-GSH) cycle (APX, monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), the contents of AsA and, GSH) were activated by 50-100-200 mg L-1 CD. CDs were able to protect the AsA regeneration, GSH/GSSG and GSH redox status. The decreases in H2O2 content might be attributed to the increased activity of glutathione peroxidase (GPX). Therefore, all CD applications minimized the Cr stress-based disturbances (TBARS content) by controlling ROS accumulation, antioxidant system and photosynthetic machinery. In conclusion, CDs have the potential to be used as a biocompatible inducer in removing the adverse effects of Cr stress in lettuce plants.

Carbon dots-cadmium sulfide quantum dots nanocomposite for 'on-off' fluorescence sensing of chromium(vi) ions

This work involves fluorescent probe which is composed of carbon dots (CD) and cadmium sulfide quantum dots (CdS QD) for the sensitive and selective fluorescence detection of chromium(vi) ions. The blue fluorescent carbon dots were synthesized by hydrothermal method from natural precursor apricot. The carbon dots-cadmium sulfide quantum dots (CD-CdS QD) nanocomposite was synthesized and all as-synthesized products were characterized using different characterization techniques. It showed white fluorescence under UV light which was quenched selectively in the presence of chromium(vi) ions due to the inner filter effect (IFE). The linear decrease in the white fluorescence was observed in the concentration range 2-120 μM of chromium(vi) ions with the limit of detection 2.07 μM. This is novel probe for the sensitive, selective and rapid detection of chromium(vi) ions.

A "turn off-on" fluorescent sensor for detection of Cr(Ⅵ) based on upconversion nanoparticles and nanoporphyrin

Hexavalent chromium (Cr(Ⅵ)) is a significant environmental pollutant because of its toxic and carcinogenic properties and wide use in various industries. Hence, there is an urgent need to develop accurate and selective approaches to detect the concentration of Cr(Ⅵ) in agricultural and aquaculture products to help humans avoid potential hazards of indirectly taking in Cr(Ⅵ). In this work, we report a "turn off-on" fluorescent sensor based on citric acid coated, 808 nm-excited core-shell upconversion nanoparticles (CA-UCNPs) and self-assembled copper porphyrin nanoparticles (nano CuTPyP) for sensitive and specific detection of Cr(Ⅵ). Nano copper 5, 10, 15, 20-tetra(4-pyridyl)-21H-23H- porphine obtained by acid-base neutralization micelle-confined self-assembly method function as an effective quencher due to its excellent optical property and water solubility. Through electrostatic interactions, positively charged nano CuTPyP are attracted to the surface of negatively charged CA-UCNPs, which can almost completely quench the fluorescence emission. In the presence of Cr(Ⅵ), nano CuTPyP can discriminatively interact with Cr(Ⅵ) and form nano CuTPyP/Cr(Ⅵ) complex, which separates nano CuTPyP from CA-UCNPs and restores the fluorescence. The sensing system exhibits a good linear response to Cr(Ⅵ) concentration in the range from 0.5 to 400 µM with a detection limit of 0.36 µM. The sensing method also displays high selectivity against other common ions including trivalent chromium and is applied to the analysis of Cr(Ⅵ) in actual rice and fish samples with satisfactory results.

Fe-doping green fluorescent carbon dots via co-electrolysis of chrysoidine G and potassium ferrocyanide for sensitive Cr(VI) detection

In this study, we aimed to synthesis of Fe-doping green fluorescent carbon dots (G-CDs) through the co-electrolysis of chrysoidine G and potassium ferrocyanide for Cr(VI) detection. The use of potassium ferrocyanide improves the quantum yield and sensing performance of G-CDs toward Cr(VI). The G-CDs have a maximum excitation wavelength of 308 nm and an emission wavelength of 510 nm. Comprehensive analyses including Raman, FT-IR, and XPS provided insights into the chemical structure and composition of the G-CDs. Under optimal conditions, G-CDs demonstrated concentration-dependent quenching upon interaction with Cr(VI). A linear relationship within the range of 0.25-100 µM was established with a calibration equation of ΔF/F0 = 0.005 + 0.015CCr(VI), yielding an R2 value of 0.996 and a limit of detection of 0.15 μM. The applicability of the G-CDs method was demonstrated by successful Cr(VI) detection in water samples with recovery rates ranging from 98.8 % to 100.1 % and relative standard deviation within 3.0 %. The fluorescence lifetime and Zeta potential measurements confirmed that the mechanism was via a static quenching process, while redox reaction, nanoparticle aggregation, and surface charge variation also played significant roles.

Ratiometric fluorometric and colorimetric dual-signal sensing platform for rapid analyzing Cr(VI), Ag(I) and HCHO in food and environmental samples based on N-doped carbon nanodots and o-phenylenediamine

Nitrogen-doped carbon nanodots (N-CNDs) were synthesized simply and efficiently using glutathione. The fluorescence emission of N-CNDs at 430 nm was effectively quenched by the fluorophore 2,3-diaminophenazine (DAP), produced through the oxidation of o-phenylenediamine (OPD) under the catalysis of Cr(VI)/Ag(I). This quenching was attributed to the fluorescence resonance energy transfer effect, while a new fluorescence emission at 560 nm was observed. Furthermore, the redox and chromogenic reaction of Cr(VI) and OPD at pH 5.4 could be effectively inhibited by formaldehyde (HCHO), resulting in the activation of N-CNDs fluorescence and the quenching of DAP fluorescence. Consequently, dual-signal sensing platforms for the rapid analysis of Cr(VI) and Ag(I) using N-CNDs/OPD and HCHO using N-CNDs/OPD/Cr(VI) were successfully constructed. By incorporating a masking reagent such as H2O2 for Cr(VI) and Cl- for Ag(I), the established sensing platform exhibited excellent selectivity and practical applicability for detecting Cr(VI), Ag(I), and HCHO in food and environmental samples.

Development of a highly sensitive fluorescent probe using Delonix regia (Gulmohar) tree pod shell for precise sarcosine detection in human urine samples: advancing prostate cancer diagnosis

We designed a highly sensitive fluorescent sensor for the early detection of sarcosine, a potential biomarker for prostate cancer. This sensor was based on surface-cobalt-doped fluorescent carbon quantum dots (Co-CD) using a FRET-based photoluminescent sensing platform. Blue luminescent carbon quantum dots (CQD) were synthesised through a hydrothermal approach, utilizing Delonix regia tree pod shells. Cobalt was employed to functionalize the CQD, enhancing the quantum-entrapped effects and minimizing surface flaws. To optimize Co-CD preparation, we employed a Box-Behnken design (BBD), and response surface methodology (RSM) based on single-factor experiments. The Co-CD was then used as a fluorescent probe for selective Cu2+ detection, with Cu2+ quenching Co-CD fluorescence through an energy transfer process, referred to as 'turn-off'. When sarcosine was introduced, the fluorescence intensity of Co-CD was restored, creating a 'turn-on' response. The sensor exhibited a Cu2+ detection limit (LOD) of 2.4 µM with a linear range of 0 μM to 10 µM. The sarcosine detection in phosphate buffer saline (PBS, pH 7.4) resulted in an LOD of 1.54 μM and a linear range of 0 to 10 µM. Importantly, the sensor demonstrated its suitability for clinical analysis by detecting sarcosine in human urine. In summary, our rapid and highly sensitive sensor offers a novel approach for the detection of sarcosine in real samples, facilitating early prostate cancer diagnosis.Communicated by Ramaswamy H. Sarma.

Bridging Component Strategy in Phosphomolybdate-Based Sensors for Electrochemical Determination of Trace Cr(VI)

Rapid and sensitive electrochemical determination of trace carcinogenic Cr(VI) pollutants remains an urgent and important task, which requires the development of active sensing materials. Herein, four cases of reduced phosphomolybdates with formulas of the (H2bib)3[Zn(H2PO4)]2{Mn[P4Mo6O31H7]2}·6H2O (1), (H2bib)2[Na(H2O)]2[Mn(H2O)]2{Mn[P4Mo6O31H6]2}·5H2O (2), (H2bib)3[Mo2(μ2-O)2(H2O)4]2{Ni[P4Mo6O31H2]2}·4H2O (3), and (H2bib)2{Ni[P4Mo6O31H9]2}·9H2O (4) (bib = 4,4'-bis(1-imidazolyl)-biphenyl) were hydrothermally synthesized under the guidance of a bridging component strategy, which function as effective electrochemical sensors to detect trace Cr(VI). The difference of hybrids 1-4 is in the inorganic moiety, in which the reduced phosphomolybdates {M[P4MoV6O31]2} (M{P4Mo6}2) exhibited different arrangements bridged by different cationic components ({Zn(H2PO4)} subunit for 1, [Mn2(H2O)2]4+ dimer for 2, and [MoV2(μ2-O)2(H2O)4]6+ for 3). As a result, hybrids 1 and 3 display noticeable Cr(VI) detection activity with low detection limits of 14.3 nM (1.48 ppb) for 1 and 6.61 nM (0.69 ppb) for 3 and high sensitivities of 97.3 and 95.3 μA·mM-1, respectively, which are much beyond the World Health Organization's detection threshold (0.05 ppm) and superior to those of the contrast samples (inorganic Mn{P4Mo6}2 salt and hybrid 4), even the most reported noble-metal catalysts. This work supplies a prospective pathway to build effective electrochemical sensors based on phosphomolybdates for environmental pollutant treatment.

Simultaneous Detection and Decontamination of Dichromate Ions: The Fluorescence Response and Photocatalysis of Thiadiazole-Modified Zr-Metal-Organic Frameworks

The simultaneous analysis and removal of highly toxic hexavalent chromium (Cr (VI)) in contaminated water via an easy method remain a serious task. Based on the guidance of bibliometric analysis, a thiadiazole ligand-modified zirconium metal-organic framework (Zr-MOF) heralds a new and simple approach to Cr (VI) treatment. The concentration can be determined by fluorescence quenching with a low detection limit of 1.4 μM and a high quenching constant of 6.88 × 103 M-1. For the sensing mechanism, the fluorescence intensity of the Zr-MOF decreased rapidly due to the competition of Cr (VI) with the Zr-MOF for absorption excitation energy and the induction of Zr-MOF aggregation. The analysis system also displayed satisfactory stability and applicability. Apart from sensing application, Zr-MOF can convert Cr (VI) to Cr (III), and the reduction rate constant was 0.004 min-1 under irradiation. Therefore, the bifunctional Zr-MOF provided a potential application method for controlling the pollution caused by Cr (VI) in wastewater.

Luminescence of carbon quantum dots and their application in biochemistry

Similar to fullerenes, carbon nanotubes and graphene, carbon dots (CDs) are causing a lot of research work in their own right. CDs are a type of surface-passivated quantum dot that contain carbon atoms. Their distinctive characteristics, such as luminescent emission that varies with size and wavelength, resistance to photobleaching, easy biological binding, lack of toxicity, and economical production without the need for intricate synthetic processes, have led to a noteworthy surge in attention within the research community. Different techniques can be utilized to create these CDs, spanning from basic candle burning to laser ablation. This review article delves into the principles of fluorescence technology, providing insights into how different synthesis methods of quantum dots impact their luminescent properties. Additionally, it highlights the latest applications of quantum dots in catalysis and biomedical fields, with special emphasis on the current status of luminescent properties in biology and chemistry. Towards the end, the article discusses the limitations of quantum dots in current practical applications, pointing out that CDs hold promising potential for future applications.

Ultrafast colorimetric detection of Cr(VI) based on competition of 8-HQ to Cr(VI) and TMB oxides using GO/AuNPs nanocomposites as peroxidase mimic

Rapid detection of ultra-trace heavy metal chromium is very important for ecological environment. Herein, a rapid colorimetric assay was constructed for detecting hexavalent chromium (Cr(VI)) in environment water through the strong peroxidase mimicking activity of graphene oxide/gold nanoparticles (GO/AuNPs) nanocomposites and competition of Cr(VI) to 3,3',5,5'-tetramethylbenzidine (TMB) oxides and 8-hydroxyquinoline (8-HQ). Cr(VI) could effectively prevent the reaction between 8-HQ and TMB oxides to restore the blue color of the system. The detection limit for Cr(VI) was as low as0.018 µM by spectroscopic absorption. Paper-based colorimetric analysis had the detection limit of0.153 µM. The high sensitivity was basically due to the strong peroxidase mimicking activity of GO/AuNPs nanocomposite from synergistic coupling action and the firm chelation between 8-HQ and Cr(VI) from inner-sphere surface complexation. The detection results for real water sample showed that the analysis had feasibility in practical application. It is worth mentioning that the assay is performed by one-step mixing mode at room temperature, and a single test can be completed in half a minute. Indeed, this work not only provided an extremely easy method for real-time detecting Cr(VI) in the environment, but also verified the vitality of colorimetric strategy based on the strong peroxidase mimicking activity and competitive reaction.

Advances in Fluorescent Sensing Carbon Dots: An Account of Food Analysis

Illuminating the use of nanomaterials, carbon quantum dots (CQDs) have transfigured the food safety arena because of their bright luminescence, optical properties, low toxicity, and enhanced biocompatibility. Therefore, fluorescent resonance energy transfer, photoinduced electron transfer, and an internal filtering effect mechanism allow precise detection of food additives, heavy metal ions, pathogenic bacteria, veterinary drug residues, and food nutrients. In this review, we describe the primal mechanism of CQD-based fluorescence sensors for food safety inspection. This is an abridged description of the nanodesign and future perspectives of more advanced CQD-based sensors for food safety analysis.

Carbon Nanotubes Decorated with Coordination Polymers for Fluorescence Detection of Heavy-Metal Ions and Nitroaromatic Chemicals

Herein, [Nd(NO₃)₃(H₂pzdca)] _n (MA-1) was synthesized from a reaction of 2,3-pyrazinedicarboxylic acid [H₂Pzdca] as an organic linker with salt of Nd(III) under solvothermal conditions. The detailed structural analysis for crystals was performed utilizing single-crystal X-ray diffraction (SCXRD). After that, the neodymium-based coordination polymer (MA-1) crystal was directly generated upon the surface of functionalized carbon nanotubes (F-CNTs) through bonds or affinity between F-CNTs and MA-1 via the solvothermal approach. Meanwhile, the existence of F-CNTs does not affect the production of MA-1 crystals. FT-IR, PXRD, SEM, TEM, and SCXRD studies were used to characterize the crystalline material, MA-1 and MA-1@CNT. To investigate the MA-1@CNT sensing properties, Pb(II), As(III), Cr(VI), and nitrobenzene (NB) were utilized as analytes. It is worth mentioning that MA-1@CNT developed as a susceptible sensor exhibits a fluorescence "turn-on" response for Pb(II) and As(III) ions, while a fluorescence "turn-off" response in the case of Cr(VI) and NB with significantly low limit of detection (LOD) values of 15.9 for Pb(II), 16.0 for As(III), 76.9 for Cr(VI), and 21.1 nM for NB, which are comparable with the lowest LOD available in the literature. Furthermore, MA-1@CNT could be conveniently regenerated and reused for at least three cycles by simply filtering and washing with water several times. The sensing mechanism is ascribed to the inner filter effect owing to the overlap between the emission and/or excitation bands of MA-1@CNT with the absorption bands of Cr(VI) and NB. In contrast, the fluorescence enhancement in the case of Pb(II) and As(III) could be correlated to the chelation-enhanced fluorescence phenomenon. These results indicate that MA-1@CNT is an ideal sensor for Pb(II), As(III), Cr(VI), and NB recognition.

Metal-doped carbon dots as robust nanomaterials for the monitoring and degradation of water pollutants

The contamination of the environment by domestic and industrial discharges is a relevant and persistent problem that needs novel solutions. Innovations in the detection, adsorption, and removal or in-situ degradation of toxic components are urgently required. Various effective techniques and materials have been proposed to address this problem, in which carbon dots (CDs) stand out because of their unique properties and low-cost and abundant nature. Their combination with different metals results in the enhancement of their innate properties. Metal-doped CDs have shown excellent results and competitive advantages in recent times. Considering the above useful critiques and CDs notable potentialities, this review discusses different approaches in detail to sense, adsorb, and photodegrade different pollutants in water samples. It was found that altering the electronic structure of CDs via metal doping has a great potential to enhance the optical, electrical, chemical, and magnetic capabilities of CDs, which in turn is beneficial for wastewater treatment.

A High Catalytic Activity Nanozyme Based on Cobalt-Doped Carbon Dots for Biosensor and Anticancer Cell Effect

Nanozyme technology as an emerging field has been successfully applied to chemical sensing, biomedicine, and environmental monitoring. It is very significant for the advance of this field to construct nanozymes with high catalytic activity by a simple method and to develop their multifunctional applications. Here, a new type of cobalt-doped carbon dots (Co-CDs) nanozymes was designed using vitamin B₁₂ and citric acid as the precursors. The homogeneous cobalt doping at carbon nuclear led the Co-CDs to show significant peroxidase-like activity resembling natural metalloenzymes. Based on the high affinity of Co-CDs to H₂O₂ (K_m = 0.0598 mM), a colorimetric sensor for glucose detection was constructed by combining Co-CDs with glucose oxidase. On account of the high catalytic activity of nanozymes and the cascade strategy, a good linear relationship was obtained from 0.500 to 200 μM, with a detection limit of 0.145 μM. The biosensor has realized the accurate detection of glucose in human serum samples. Moreover, Co-CDs could specifically catalyze H₂O₂ in cancer cells to generate a variety of reactive oxygen species, leading to the death of cancer cells, which has useful application potential in tumor catalytic therapy. In this work, the catalytic activity of Co-CDs has been adequately exploited, which extends the application of carbon dots in multiple biotechnologies, including biosensing, disease diagnosis, and treatment.

Recent Progress of Carbon Dots for Air Pollutants Detection and Photocatalytic Removal: Synthesis, Modifications, and Applications

Rapid industrialization has inevitably led to serious air pollution problems, thus it is urgent to develop detection and treatment technologies for qualitative and quantitative analysis and efficient removal of harmful pollutants. Notably, the employment of functional nanomaterials, in sensing and photocatalytic technologies, is promising to achieve efficient in situ detection and removal of gaseous pollutants. Among them, carbon dots (CDs) have shown significant potential due to their superior properties, such as controllable structures, easy surface modification, adjustable energy band, and excellent electron-transfer capacities. Moreover, their environmentally friendly preparation and efficient capture of solar energy provide a green option for sustainably addressing environmental problems. Here, recent advances in the rational design of CDs-based sensors and photocatalysts are highlighted. An overview of their applications in air pollutants detection and photocatalytic removal is presented, especially the diverse sensing and photocatalytic mechanisms of CDs are discussed. Finally, the challenges and perspectives are also provided, emphasizing the importance of synthetic mechanism investigation and rational design of structures.

Emerging metal doped carbon dots for promising theranostic applications

As a bridge between organic fluorophores and inorganic quantum dots, carbon dots (CDs) have been recognized as emerging nanotheranostics for biomedical applications owing to their distinctive merits such as superior optical properties, flexible modification, adjustable functionalities, and remarkable photoactive therapeutic outcome, etc. Compared to metal free CDs, the introduction of metal ion in CDs endowed metal-doped CDs (MCDs) with tunable optical properties and new intrinsic properties, thereby illustrating its different capabilities from metal-free CDs for bioimaging and therapy. This review aims to summarize the recent progress of photonic MCDs as emerging nanoagent for theranostic application such as disease-related diagnostic (involving biosensing and bioimaging) and cancer therapy. The challenges and potential development of MCDs in nanotheranostic fields are also discussed.

Exhausted Cr(VI) Sensing/Removal Aerogels Are Recycled for Water Purification and Solar-Thermal Energy Generation

Heavy metal pollution has resulted in numerous environmental challenges. However, classic approaches, involving the use of solid adsorbents are subject to limitations, including the high energy consumption required for processing before and after use. Accordingly, strategies that facilitate the use of metal capture media that extends beyond waste remediation are attractive. Herein, a porous fluorescent aerogel (CPC aerogel) is constructed by immersing amino-based carbon dots (CDs-NH₂ ) into a polyethyleneimine (PEI)/carboxymethylated cellulose (CMC) aerogel network for the simultaneous detection and adsorption of Cr(VI). Adsorption experiments confirm that the CMC/PEI containing CDs-NH₂ aerogel (CPC aerogel) exhibits good Cr(VI) extraction capacity, and can reach a level that conforms with industrial water safety standards. In addition, the CPC aerogel can continuously detect and remove Cr(VI) at high flux. Following Cr(VI) absorption, the CPC aerogel may be vulcanized (MS_x -CPC gel) and used for solar thermoelectric generation resulting in power generation. Additionally, the MS_x -CPC gel can be used for solar steam generation and exhibits excellent evaporation rates of ≈1.31 kg m^-2 h^-1 under one sun irradiation. The results serve to underscore how materials designed for metal ion recognition and adsorption once exhausted can be exploited to provide materials for solar thermoelectric power generation and seawater desalination.

Synthesis of intrinsic dual-emission type N,S-doped carbon dots for ratiometric fluorescence detection of Cr (VI) and application in cellular imaging

In this paper, intrinsic dual-emission fluorescent carbon dots (CDs) doped with N and S atoms have been firstly fabricated. The characterization results show that CDs are successfully synthesized with two separate fluorescence emissions at 468 nm and 628 nm, respectively. The strong and selective interaction of Cr (VI) ions with CDs lead to obvious fluorescence decrease of CDs at 468 nm, which is caused by a mixed quenching mechanism. At the same time, the fluorescence at 628 nm increase. Interestingly, the CDs solution show obvious color change under the daylight and UV light, so visualization detection of Cr (VI) can be realized in water samples. Based on the data of the emission intensity ratios of F₄₆₈/F₆₂₈, Cr (VI) can be detected from 3.8 to 38.9 μM combined with the linear correlation coefficient of 0.998, and the lowest detection concentration is 47.2 nM. The platform is satisfactorily applied to the detection of Cr (VI) ions in water samples. In addition, the CDs could be applied as fluorescent probes for cell imaging with dual fluorescent emission.

Construction of Co,N-Coordinated Carbon Dots for Efficient Oxygen Reduction Reaction

For the sake of the oxygen reduction reaction (ORR) catalytic performance, carbon dots (CDs) doped with metal atoms have accelerated their local electron flow for the past few years. However, the influence of CDs doped with metal atoms on binding sites and formation mechanisms is still uncertain. Herein, Co,N-doped CDs were facilely prepared by the low-temperature polymerization-solvent extraction strategy from EDTA-Co. The influence of Co doping on the catalytic performance of Co-CDs was explored, mainly in the following aspects: first, the pyridinic N atom content of Co-CDs significantly increased from 4.2 to 11.27 at% compared with the CDs, which indicates that the Co element in the precursor is advantageous in forming more pyridinic-N-active sites for boosting the ORR performance. Second, Co-CDs are uniformly distributed on the surface of carbon black (CB) to form Co-CDs@CB by the facile hydrothermal route, which can expose more active sites than the aggregation status. Third, the highest graphite N content of Co-CDs@CB was found, by limiting the current density of the catalyst towards the ORR. Composite nanomaterials formed by Co and CB are also used as air electrodes to manufacture high-performance zinc-air batteries. The battery has good cycle stability and realizes stable charges and discharges under different current densities. The outstanding catalytic activity of Co-CDs@CB is attributed to the Co,N synergistic effect induced by Co doping, which pioneer a new metal doping mechanism for gaining high-performance electrocatalysts.

Fluorescent nanocellulose-based hydrogel incorporating titanate nanofibers for sorption and detection of Cr(VI)

Chromium pollution is a major environmental concern; thus, effective and multifunctional adsorbents for removing the Cr(VI) ion are urgently needed. A fluorescent nanocellulose-based hydrogel (FNH) incorporating titanate nanofibers (TNs) was developed for the sorption and detection of Cr(VI) ion. The chemical and physical structures of the hydrogels, as well as their sorption and detection properties, were studied. The predicted maximum adsorption capacity and the lowest detection limit of FNH were 648.4 mg/g and 0.039 μg/L, respectively. Furthermore, the sorption and detection mechanisms of FNH were discussed in detail. These results showed that the excellent sorption and detection might be mainly attributed to the three-dimensional (3D) porous structure constructed by TNs and cellulose nanocrystals modified with carbon dots, which improved the sorption ability and provided the rapid visual response to Cr(VI). Furthermore, cost analysis showed that FNH was cheaper than activated carbon in removing the Cr(VI) ion. This work established a facile technique in developing low-cost and multifunctional adsorbents.

In-situ generation of highly active and four-in-one CoFe2O4/H2PPOP nanozyme: Mechanism and its application for fast colorimetric detection of Cr (VI)

Nanozymes have been widely utilized in colorimetric sensors and developing nanomaterials with multienzyme functions have more application prospects due to their cascaded catalytic efficiency. Here, a unique organic-inorganic nanocomposite CoFe₂O₄/H₂PPOP was synthesized by depositing CoFe₂O₄ nanocubes on a fully conjugated porphyrin-based porous organic polymer (H₂PPOP) in situ. CoFe₂O₄/H₂PPOP revealed outstanding tetra-enzyme-like activities, namely oxidase-like, peroxidase-like, catalase-like and superoxide dismutase-like activities. Compared with pure CoFe₂O₄ nanocubes, the catalytic activities of CoFe₂O₄/H₂PPOP were significantly boosted because of the large surface area and extended conjugated structure of H₂PPOP, abundant active substances (CoFe₂O₄) on the surface and the effective electronic transfer between CoFe₂O₄ and H₂PPOP. Based on the oxidase-like activity of CoFe₂O₄/H₂PPOP, a colorimetric platform was constructed for Cr (VI) with a wide linear range (0.6-100 μM) and a low detection limit (26 nΜ). Further utilizing the double oxidase-like and peroxidase-like activities, a more sensitive colorimetric platform with a faster detection speed for Cr (VI) was realized with the LOD as low as 2 nΜ. This work opens up a new way to prepare multi-enzyme active nanozyme and excavates its potential for detecting environmental pollutants.

Recent Advances in Synthesis, Modification, Characterization, and Applications of Carbon Dots

Although there is significant progress in the research of carbon dots (CDs), some challenges such as difficulty in large-scale synthesis, complicated purification, low quantum yield, ambiguity in structure-property correlation, electronic structures, and photophysics are still major obstacles that hinder the commercial use of CDs. Recent advances in synthesis, modification, characterization, and applications of CDs are summarized in this review. We illustrate some examples to correlate process parameters, structures, compositions, properties, and performances of CDs-based materials. The advances in the synthesis approach, purification methods, and modification/doping methods for the synthesis of CDs are also presented. Moreover, some examples of the kilogram-scale fabrication of CDs are given. The properties and performance of CDs can be tuned by some synthesis parameters, such as the incubation time and precursor ratio, the laser pulse width, and the average molar mass of the polymeric precursor. Surface passivation also has a significant influence on the particle sizes of CDs. Moreover, some factors affect the properties and performance of CDs, such as the polarity-sensitive fluorescence effect and concentration-dependent multicolor luminescence, together with the size and surface states of CDs. The synchrotron near-edge X-ray absorption fine structure (NEXAFS) test has been proved to be a useful tool to explore the correlation among structural features, photophysics, and emission performance of CDs. Recent advances of CDs in bioimaging, sensing, therapy, energy, fertilizer, separation, security authentication, food packing, flame retardant, and co-catalyst for environmental remediation applications were reviewed in this article. Furthermore, the roles of CDs, doped CDs, and their composites in these applications were also demonstrated.

An Overview of Synthetic Methods and Applications of Photoluminescent Properties of Carbon Quantum Dots

Carbon quantum dots (CQDs) are promising carbonaceous nanomaterials fortuitously discovered in 2004. CQDs are the rising stars in the nanotechnology ensemble because of their unique properties and widespread applications in sensing, imaging, medicine, catalysis, and optoelectronics. CQDs are notable for their excellent solubility and effective luminescence, and as a result, they are also known as carbon nanolights. Many strategies are used for the efficient and economical preparation of CQDs; however, CQDs prepared from waste or green sustainable methods have greater requirements due to their safety and ease of synthesis. Sustainable chemical strategies for CQDs have been developed, emphasizing green synthetic methodologies based on "top-down" and "bottom-up" approaches. This review summarizes many such studies relevant to the development of sustainable methods for photoluminescent CQDs. Furthermore, we have emphasized recent advances in CQDs' photoluminescent applications in chemical and biological fields. Finally, a brief overview of synthetic processes utilizing the green source and their associated applications are tabulated, providing a clear understanding of the new optoelectronic materials.

Nitrogen-rich carbon quantum dots (N-CQDs) based on natural deep eutectic solvents: Simultaneous detection and treatment of trace Co2+ under saline conditions

Trace Co²⁺, when present in large quantities, is harmful to the environment and therefore cannot be ignored. Inductively coupled plasma mass spectrometry (ICP-MS) is a standard method used to detect metal ions, however, detecting trace Co²⁺ under high saline conditions can be challenging. Similarly, existing Co²⁺ treatment methods are prone to secondary pollution and have high energy consumption. Therefore, it is necessary to find an efficient and non-polluting method for Co²⁺ detection and treatment. This study successfully synthesized nitrogen-rich carbon quantum dots (N-CQDs) based on natural deep eutectic solvents (NADES) using a one-step solvothermal method. The prepared N-CQDs exhibited excellent fluorescence and high salt tolerance. The simultaneous detection and treatment of trace Co²⁺ in water under high salinity conditions were achieved for the first time. The response of the N-CQDs to Co²⁺ under saline condition was linear in the range of 5-250 μM with a limit of detection (LOD) of 1.2269 μM. Feasibility of practical application was assessed by quantitative detection of Co²⁺ in real water samples. Furthermore, the N-CQDs can treat Co²⁺, and the removal rate was 99.98%.

Triple-emission nitrogen and boron co-doped carbon quantum dots from lignin: Highly fluorescent sensing platform for detection of hexavalent chromium ions

Considering that hexavalent chromium ions (Cr⁶⁺) with high toxicity poses a huge threat to human health and the ecological environment, constructing a rapid and accurate sensing platform is of great significance in detecting the toxic substance. The novel nitrogen and boron co-doped carbon quantum dots (N, B-CQDs) from lignin are synthesized as fluorescent sensors for the detection of Cr⁶⁺. The synthetic processes involve the acid hydrolysis step followed by the hydrothermal treatment step. Lignin is firstly depolymerized by cleaving ether bonds in the acidolysis, and N, B-CQDs are consequently formed by the aromatic re-fusion of lignin nanoparticles in the hydrothermal process. The lignin-derived N, B-CQDs show triple emission of purple, blue and green fluorescence under the excitation of 300, 330, and 490 nm, respectively. The triple-emission N, B-CQDs are applied for the triple-channel detection of Cr⁶⁺, which exhibit highly sensitive and selective fluorescence quenching for Cr⁶⁺ with good linearity (R² ≤ 0.996) and very low limit of detection as 0.054, 0.049, and 0.077 μM under the excitation of 300, 330 and 490 nm, respectively. The utilization of renewable lignin as CQDs-based fluorescent sensors opens a new avenue for the rapid and accurate detection of Cr⁶⁺ through a multichannel sensing platform.

Microwave-assisted synthesis of nitrogen-doped carbon dots using prickly pear as the carbon source and its application as a highly selective sensor for Cr(VI) and as a patterning agent

Preparation of water-dispersible carbon dots from inexpensive natural carbon precursors and its application for purposes such as sensing, bio-imaging and patterning agents is showing growing interest in recent years. In this study, we have reported the preparation of nitrogen-doped carbon dots (N-CDs) using prickly pear as the carbon source and m-xylylenediamine as the nitrogen source using a one-step microwave-assisted synthetic process. The N-CDs prepared were characterized on the basis of elemental analysis, XPS, powder-XRD, FT-IR, Raman, TEM, UV-vis and fluorescence spectroscopy. Doping of nitrogen in the N-CDs made them highly fluorescent and the study on their ion-recognition property revealed that they detect highly toxic Cr(VI) with high selectivity and sensitivity (LOD, 0.04 μM) and without interference from the other ions used in this study. By immobilizing these N-CDs onto filter paper, sensor strips were prepared for on-site monitoring/field applications and they were successfully used for the detection of Cr(VI) in water. Detailed spectral analysis revealed that the mechanism of Cr(VI) sensing involved a phenomenon called the "inner filter effect" and analysis of the fluorescence lifetime data suggested the "static quenching" of fluorescence intensity. These N-CDs were used to prepare fluorescent carbon ink and were successfully used as patterning agents.

Ultrasonic-Assisted Synthesis of N-Doped, Multicolor Carbon Dots toward Fluorescent Inks, Fluorescence Sensors, and Logic Gate Operations

Over past decades, the multicolor carbon dots (M-CDs) have attracted enormous attentions due to their tunable photoluminescence and versatile applications. Herein, the nitrogen-doped (N-doped) M-CDs including green, chartreuse, and pink emissive CDs are successfully synthesized by ultrasonic treatment of kiwifruit juice with different additive reagents such as ethanol, ethylenediamine, and acetone. Owing to their strong fluorescence upon irradiation with 365 nm UV light, the highly water-soluble M-CDs present great potential in the anticounterfeit field as fluorescent inks. Particularly, the resulting green emission CDs (G-CDs) with excellent fluorescence and stability are applied as a label-free probe model for “on–off” detection of Fe3+. The fluorescence of G-CDs is significantly quenched by Fe3+ through static quenching. The nanoprobe demonstrates good selectivity and sensitivity toward Fe3+ with a detection limit of ~0.11 μM. Besides, the quenched fluorescence of G-CDs by Fe3+ can be recovered by the addition of PO43− or ascorbic acid (AA) into the CDs/Fe3+ system to realize the “off–on” fluorescent process. Furthermore, NOT and IMPLICATION logic gates are constructed based on the selection of Fe3+ and PO43− or AA as the inputs, which makes the G-CD-based sensors utilized as various logic gates at molecular level. Therefore, the N-doped M-CDs hold promising prospects as competitive candidates in monitoring the trace species, applications in food chemistry, anticounterfeit uses, and beyond.

Carbon dot composites for bioapplications: a review

Recent advancements in the synthesis of carbon dot composites and their applications in biomedical fields (bioimaging, drug delivery and biosensing) have been carefully summarized. The current challenges and future trends of CD composites in this field have also been discussed.

Smartphone-assisted ratiometric fluorescence sensing platform and logical device based on polydopamine nanoparticles and carbonized polymer dots for visual and point-of-care testing of glutathione

As a crucial biothiol, glutathione (GSH) plays a key role in the organisms. Monitoring GSH level is of great significance for disease diagnosis and biomedical research. In this work, polydopamine (PDA) nanoparticles-red fluorescent carbonized polymer dots (r-CPDs) based ratiometric fluorescence sensing platform was constructed and employed for GSH assay. Dopamine (DA) could be oxidized by cobalt oxyhydroxide (CoOOH) nanosheets and further polymerized into PDA nanoparticles with green fluorescence. However, in the presence of GSH, CoOOH nanosheets were reduced and decomposed, which prevented the production of PDA nanoparticles. In the sensing system, green-emitting PDA nanoparticles were employed as a response unit and r-CPDs were used as an internal reference unit. With the addition of GSH, the green fluorescence of PDA nanoparticles decreased as well as the red fluorescence of system remained relatively stable. Importantly, a distinct fluorescence color evolution from green to red was presented with a serious of GSH concentrations. Based on this, a portable smartphone-assisted ratiometric chromaticity analytical method was developed to achieve the on-site visual detection of GSH. Both the established ratiometric fluorescence and ratiometric chromaticity sensing methods for GSH assay have the merits of wide linear range, high sensitivity and excellent accuracy, which are suitable for the determination of GSH in human serum and exhibit great application potential in rapid and accurate monitoring of the GSH levels in clinical. Moreover, an ingenious logical device reflecting GSH levels was designed based on the two different fluorescence signals, which provided a new strategy for the intelligent online detection of GSH in complex biological matrices.

A Simple and Novel Sensor for the Determination of Acetamiprid Based on Its Reducing Effect on the Chemiluminescence of S, N-CQDs in CH3CN-H2O2 System

A simple and novel method for the determination of acetamiprid in water samples is suggested. The method is based on the reducing effect of acetamiprid on the chemiluminescence intensity of new sulfur and nitrogen co-doped carbon dots (S, N-CQDs) in an acetonitrile-hydrogen peroxide (CH₃CN-H₂O₂) system. The possible mechanism was investigated, and it was found that S, N-CQDs react with (¹O₂)₂*, produced from the CH₃CN-H₂O₂ reaction, leading to excited state S, N-CQDs, which deactivate to the ground state by photon emission. Acetamiprid diminishes the chemiluminescence (CL) intensity by competing with S, N-CQDs. The CL intensity reduction is proportional to the concentration of acetamiprid. S, N-CQDs were easily prepared by a hydrothermal method. Under the optimal conditions, a linear range of 2.5 - 25.0 μg L^-1 with a detection limit (3σ) of 0.4 μg L^-1 was obtained. This method was successfully applied to the determination of trace amounts of residual pesticides in water samples.

Synthesis of 4-dimethylaminobenzyl chrysin ester-Zn fluorescent chemical sensor for the determination of Cr(VI) in water

Cr(VI) is a type of dangerous effluent that has caused great harm to human health and the environment. Recognition and perception of Cr(VI) by artificial receptors has attracted extensive attention. A novel fluorescent chemical sensor based on the 5,7-dihydroxyflavone skeleton was designed and synthesized for the selective recognition of Cr(VI). As confirmed by fluorescence technology, the fluorescent probe 4-dimethylaminobenzyl chrysin ester-Zn (DBC-Zn) showed high sensitivity and selectivity for dichromate and a fast response (less than 30 sec) recognition. The fluorescence intensity of DBC-Zn varies linearly with the concentration of Cr(VI) in the range 0.1-1 μM. The detection limit of Cr₂ O₇ ^2- by DBC-Zn is 2.3 nM, which is far lower than the national safe drinking water standard stipulated by the US Environmental Protection Agency (1.9 μM). The quenching mechanism of the probe can be attributed to the interaction of the dynamic quenching effect and the fluorescence internal filtration effect. In addition, the probe has good stability in both neutral and alkaline environments, and the accuracy of quantitative analysis of Cr₂ O₇ ^2- in lake water or tap water is more than 80%. The test paper based on DBC-Zn can effectively detect Cr₂ O₇ ^2- at the concentration of 100 ppb. This shows that the probe has a certain practical application value.

Multifunctional MOF-based probes for efficient detection and discrimination of Pb2+, Fe3+ and Cr2O72-/CrO42

Developing metal-organic framework (MOF)-based fluorescent probes for efficient detection and discrimination of polluting ions in groundwater is vital for environmental protection and human health. In this paper, we prepared two luminescence-active transition MOFs, namely, Zn-MOF and Cd-MOF, and conducted sensing experiments. The results show that they both exhibit multiple-target detection for Fe³⁺, Pb²⁺ and Cr(VI) with high sensitivity, good anti-interference ability and good recyclability even with different frameworks. In addition, Eu³⁺-incorporated samples, Eu³⁺@MOFs, with dual-emission have been fabricated via efficient encapsulation of Eu³⁺ ions into the MOF host. As expected, Eu³⁺@MOF hybrids also act as multi-target and self-calibrated probes to selectively detect Fe³⁺ and Cr(VI) ions. However, the quenching efficiencies of the original MOFs towards Fe³⁺ are higher than those of Eu³⁺@MOFs. Thus, we could differentiate Fe³⁺, Pb²⁺ and Cr(VI) ions by comparing the changes of fluorescence emission between Eu³⁺@MOFs and the original MOFs. The recognition mechanism may be attributed to the competitive energy absorption between MOFs or Eu³⁺@MOFs and the analytes.

Selective adsorption and ultrafast fluorescent detection of Cr(VI) in wastewater using neodymium doped polyaniline supported layered double hydroxide nanocomposite

Neodymium-doped polyaniline supported Zn-Al layered double hydroxide (PANI@Nd-LDH) nanocomposite has been prepared via an ex-situ oxidative polymerization process. The as-prepared nanocomposite shows selective fluorescence detection and adsorption of hexavalent chromium Cr(VI) within a short period. The fluorescence intensity of PANI@Nd-LDH decreases linearly with Cr(VI) concentrations ranging from 200 ppb to 1000 ppb with a limit of detection (LOD) of 1.5 nM and a limit of quantification (LOQ) of 96 nM. The sensing mechanism can be ascribed by the inner filter effect of Cr(VI), the intercalation of Cr(VI) within the intergallery region of LDH, and the synergistic affinity of metal ions along with the polymer chain for Cr(VI). The adsorption performance of PANI@Nd-LDH nanocomposite is evaluated for Cr(VI) from wastewaters, which displayed high removal capacity towards Cr(VI) (219 mg/g) as compared on bare Nd-LDH (123 mg/g) and LDH (88 mg/g) respectively. The adsorption of Cr(VI) on PANI@Nd-LDH depends on the pH of the aqueous solution. The adsorption isotherm and kinetics are supported by the Langmuir model and pseudo-second-order model, respectively. Owing to the highly sensitive detection and adsorption of Cr(VI) from aqueous water samples demonstrated the potential application of PANI@Nd-LDH as an excellent environmental probe can be exploited.

Fluorescent carbon dots embedded in mesoporous silica nanospheres: A simple platform for Cr(VI) detection in environmental water

In this work, amino-functionalized mesoporous silica nanospheres (NH₂-mSiO₂) anchored with carbon dots (CDs) have been designed to construct an outstanding fluorescent sensor for heavy metal detection. Uniform mSiO₂ was chosen to provide an optically transparent scaffold for immobilizing CDs. With the help of amino group modification on the surface of silica, benzene-1,4-diboronic acid (BA) was used as raw material to load CDs in the pores of mSiO₂ by one-step solvothermal method. The proposed nanohybrid can solve the problem of aggregation-induced fluorescence quenching, leading to bright blue emission at 450 nm. Meanwhile, the fluorescence of NH₂-mSiO₂@CDs showed high sensitivity to Cr(VI) in acetic acid buffer solution (pH = 4) with detection limit as low as 5 nM by inner filter effect (IFE) and electrostatic interaction (EI). The proposed method can also be extended to other CDs-based detection systems for chemical/biological sensors.