Carbon dots: synthesis, formation mechanism, fluorescence origin and sensing applications

The emergence of carbon dots (CDs) has opened up an exciting new field in the science and technology of carbon nanomaterials and has attracted increasing interest in recent years. Due to their diverse physicochemical properties and favourable attributes, such as quantum confinement effects and abundant surface defects, CDs and their derived hybrids have shown exciting and indispensable prospects in the energy conversion and storage fields. Considering the latest developments, in this review, we comprehensively summarize the classification and structure of CDs. Three strategies for structural engineering of CDs are presented and analyzed, in terms of the tuning of size and crystallinity, and the methodologies for surface modification and heteroatom doping, with a focus on the relationship among the synthesis methods, structure and properties of the concerned CDs. More importantly, the recent advances in energy-oriented applications of CDs, including photo- and electro-catalysis, light-emitting diodes, photovoltaic cells, lithium/sodium ion batteries and supercapacitors, will be systematically highlighted. Finally, we discuss and outline the remaining major challenges and opportunities for CDs in the future.

Design and fabrication of carbon dots for energy conversion and storage

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.

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

This article discusses three aspects of inner filter effect (IFE) in fluorescence spectroscopy. (i) First, IFE as undesirable in fluorescence measurements: IFE results in non-linear fluorescence response of the analyte under study and it has been verified that IFE cannot be eliminated; it can either be minimized or corrected for intensity loss. Over the years, researchers have proposed many intensity correction methods to avoid IFE related issues. Often analysts using fluorescence spectroscopy, knowingly or unknowingly, ignore IFE or use an inappropriate intensity correction method. Herein, we have highlighted the basis and significances of various correction models that are proposed since 1970s to till date. (ii) Second, IFE mediated concentration dependent red shift (CDRS) as an analytical tool: the conventional fluorescence measurements and IFE correction strategies cannot be applied in analysis of optically dense multi-fluorophoric samples like oils, petrochemicals, biological samples and food samples etc. The strategy for exclusive inclusion of IFE and IFE induced CDRS as characteristics of the system for development of fluorescence based assay, towards maximizing fluorescence sensitivity of optically dense multi-fluorophoric systems, have been discussed. (iii) Third, IFE based sensing: when the sample contains chromophores, which absorb either at the excitation or at the emission wavelength range of the fluorophore, then the chromophores act as a filter. Thus tuning either the absorber or fluorophore concentration will lead to development of fluorescence based assay for a selective analyte. Principles and protocols are described to identify whether a sensing event is due to IFE or any other fluorescence mechanism. Additionally, a brief description is given on advanced findings and progresses made in sensing of for various classes of analytes in the recent past, using IFE concept. The second and third aspect combined together serve as a tool towards enhancing sensitivity of fluorescence measurement.

Inner filter effect in fluorescence spectroscopy: As a problem and as a solution

In the most recent decade, carbon dots have drawn intensive attention and triggered substantial investigation. Carbon dots manifest superior merits, including excellent biocompatibility both in vitro and in vivo, resistance to photobleaching, easy surface functionalization and bio-conjugation, outstanding colloidal stability, eco-friendly synthesis, and low cost. All of these endow them with the great potential to replace conventional unsatisfactory fluorescent heavy metal-containing semiconductor quantum dots or organic dyes. Even though the understanding of their photoluminescence mechanism is still controversial, carbon dots have already exhibited many versatile applications. In this article, we summarize and review the recent progress achieved in the field of carbon dots, and provide a comprehensive summary and discussion on their synthesis methods and emission mechanisms. We also present the applications of carbon dots in bioimaging, drug delivery, microfluidics, light emitting diode (LED), sensing, logic gates, and chiral photonics, etc. Some unaddressed issues, challenges, and future prospects of carbon dots are also discussed. We envision that carbon dots will eventually have great commercial utilization and will become a strong competitor to some currently used fluorescent materials. It is our hope that this review will provide insights into both the fundamental research and practical applications of carbon dots.

/pdf/novel-properties-and-applications-of-carbon-nanodots-3j3pr0gdwx.pdf

Novel properties and applications of carbon nanodots

Color-tunable carbon dots have been synthesized via a one pot hydrothermal synthesis and re-dispersed in dimethylformamide solution after purification. Structural and optical property characterizations indicate that the concentration-dependent photoluminescent properties can be ascribed to the existence of multi-emissive centers in the carbon dots from core states, edge states and surface states. Therefore, a multi-center fluorescent mechanism for the carbon dots has been proposed according to the preservation effect and inductive effect of the solvent. The emission wavelength of the carbon dots with different concentrations can be tuned from 585 to 514 nm under the fixed excitation of 420 nm blue light, and a warm white light-emitting diode with a CIE color coordinate at (0.42, 0.35) is fabricated on a InGaN blue chip emitting 420 nm blue light.

Concentration-induced multi-colored emissions in carbon dots: origination from triple fluorescent centers

Single‐phase white luminescence phosphors are emerging for potential phosphor‐converted white light‐emitting diodes (pc‐WLEDs) and alternatives to trichromatic phosphor blends. In this work, Ba2ZnWO6:Bi3+ phosphors are prepared by a high‐temperature solid‐state reaction method and the luminescent properties are investigated. The broadband excitation from 250 to 400 nm is perfectly matched with the emission of nearly ultraviolet (n‐UV) InGaN chip, which ultrabroadband emission band (full width at half maximum = 217 nm) can cover a wider luminescence region. Li+/Na+/K+ codoping is more desirable for the ultrabroadband emission of Ba2ZnWO6:0.006Bi3+ due to the charge compensation and rationally designed lattice distortion while improving the thermal stability of the phosphor. Furthermore, tunable emissions can be achieved either by changing the content of doped ions or the excitation wavelength due to the different occupation of Ba2+ and Zn2+ sites by Bi3+ ions. Finally, single‐phase warm WLED ((x, y) = (0.4170, 0.4035)) devices with a correlated color temperature of 3329 K and color rendering index of Ra = 75.3 are successfully prepared using Ba2ZnWO6:0.006Bi3+, 0.03Na+ phosphor, and 365 nm InGaN chips. These results indicate that Ba2ZnWO6:Bi3+ has excellent potential as phosphor for n‐UV pc‐WLEDs and provides new ideas for the application of ultrabroadband emitting phosphors in WLEDs.

Single Bi3+ Ultrabroadband White Luminescence in Double Perovskite via Crystal Lattice Engineering toward Light‐Emitting Diode Applications

On-line transfer learning for multi-fidelity data fusion with ensemble of deep neural networks

Near‐infrared phosphor‐converted light‐emitting diodes (NIR pc‐LEDs) are promising for many applications in non‐destructive testing, bio‐imaging, and modern agriculture. However, developing self‐activated NIR phosphors with high efficiency and excellent thermal stability is a great challenge for current research. In this work, a self‐activated far‐red to NIR emitting NaLaMgWO6 phosphor is prepared by the high‐temperature solid‐state reaction method, whose luminescence properties are investigated in detail. In addition, the introduction of Bi3+ ions as sensitizers enhances the emission intensity of far‐red to NIR light (650‐850 nm) through a Bi3+ → WO66– energy transfer design strategy, resulting in an increased internal quantum efficiency (IQE) from 56.4% to 64.8% and thermal stability from 54% to 72% (420 K). Finally, the combination of NaLaMgWO6:Bi3+ with 365/460 nm commercial LED chip is prepared into NIR pc‐LED devices, demonstrating satisfactory performance in the field of night vision detection and other wide range of practical applications.

Self‐Activated Tungstate Phosphor for Near‐Infrared Light‐Emitting Diodes

Single Tb3+ ion doped ratiometric mechanoluminescence for tunable stress visualization

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