Photoluminescence Of Carbon Dots Research Articles

Hydrogen bonding effect on pH-sensing mechanism of carbon dots

Recently, carbon dots (CDs) have received huge attention from scientists around the globe due to their unique properties, including excellent optical properties, good photobleaching, good biocompatibility, and ease of production. Notably, CDs exhibit novel results in pH sensing applications in water and intracellular environments due to their excellent anti-interfering ability and photostability. Herein, we successfully synthesized CDs by hydrothermal method using glucose and citric acid as precursors. The formation and breaking of hydrogen bonding play an important role in pH-sensing mechanisms of CDs. By using Raman signal of the hydrogen bonding in the photoluminescence (PL) spectra of CDs, we investigate here the effect of hydrogen bonding on the pH-sensing of CDs. It is expected that the mechanism of pH-sensing on the PL signal could be unraveled. As the results, the hydrogen bonding significantly reduced the PL intensity of CDs in a pH range of 7–11 but it did not influence the PL of CDs in higher pH conditions. Based on the interaction between hydrogen bonding and CDs, a new model for the pH-sensing mechanism of CDs was proposed. This work sheds new light on the mechanism of pH-sensing on the PL signal and suggests a new application of CDs in moisture and water sensors for air, soil, food, and commercial chemicals.

Development of Fe (Iii) Sensor System Using Carbon Nanodots Derived From <i>Plectranthus amboinicus</i>

Carbon Dots (CDs) are a course of carbon nanomaterials just under 10 nm in dimension endowed with signature optical and electronic properties finding applications in sensors, photocatalysis, biomedical as well as optoelectronics. Single stroke hydrothermal synthesis method seems to have been adopted as the generation of nanocarbon dots from the Indian medicinal plant, Plectranthus amboinicus. Advanced characterisation methods such as UV- Visible absorption spectroscopy, fluorescence spectroscopy, Fourier transform infrared spectroscopy and HR TEM study have been adopted to confirm the structure of carbon nanoparticles. The dependence on the excitation of photoluminescence emission behaviour of CDs have been confirmed using PL spectroscopy. The reaction between the many metal ions with the photoluminescence of CDs are studied and found a striking interaction with Fe (III) ions. The equation from Stern-Volmer is used to study the mechanism of extinction involved in the sensing action of carbon dots and the threshold for recognition is found to be 0.30 μM. The existence of surface functional groups leading to the complexation with Fe (III) ions can primarily be the reason for the observed sensing application. The design and development of eco-friendly sensor systems for Iron metal which is also considered as an essential mineral for human health for its application in biomedical and environmental applications is discussed in this paper.

Poria cocos-derived carbon dots for cellular imaging, free radical scavenging and pH sensing

Green, economical preparation of carbon dots (CDs) from traditional Chinese herbal medicine Poria cocos via a one-step hydrothermal treatment was reported. The CDs exhibited excellent aqueous dispersibility, superior photoluminescence (PL) property and low cytotoxicity. Taking advantage of these properties, CDs were demonstrated as excellent multicolor bioimaging agents for the cellular imaging of MDA-MB-435S cells, with main distributions in the cell membranes and cytoplasm. Moreover, the CDs also exhibited good radical scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) thanks to their inheritance of characteristic functionalities from Poria cocos. Lastly, our study also shown that the PL of CDs was reversibly sensitive to pH in the range of 4.0 and 9.0, and a highly sensitive pH-sensing assay was developed exploiting this phenomenon. In summary, the study opened up new development paths for the modernization and vitalization of traditional Chinese medicine in biomedical research.

Влияние рН среды на свойства углеродных точек с различной функционализацией поверхности: размеры и квантовый выход фотолюминесценции

In this study carbon dots synthesized by the hydrothermal method and subsequent functionalization of the surface with carboxyl and hydroxyl groups were explored. As a result of studying the optical and colloidal properties of monofunctionalized carbon dots, a significant effect of the acidity of the nanoparticle environment on them was found. It has been found that for the photoluminescence quantum yield of carbon dots the greatest changes occur in the pH range from 2 to 5 for carbon dots with carboxyl surface groups and from 8 to 12 for carbon dots with hydroxyl groups. The mechanisms of the influence of surface functional groups on the photoluminescence of carbon dots with a change in the pH of the suspension are proposed.

Dual emissive carbon dots: Synthesis strategies, properties and its ratiometric sensing applications

Carbon dots (CDs) have received much attention in the field of sensing as fluorescent nanomaterials. They possess simple preparation, excellent optical properties, low toxicity, and brilliant biocompatibility. Photoluminescence in CDs is caused by chemical features, such as graphitic conjugated cores, molecular fluorophores and surface defect states. The emission wavelength of fabricated CDs primarily accumulates single bands in the blue, green or red region. Meanwhile, the structure of CDs and the relationship between their structure and optical properties are still being debated. Apart from that, the existence of single emission bands reduces the efficiency of CDs for sensing multiple analytes in a single process. To overcome this issue, dual conjugated emitters have been designed using carbon dots, quantum dots and metal–organic frameworks. Their fabrication is time-consuming, requires purification and has unequal photostability. It is critical to design and produce nanoprobes with no labeling and intrinsic dual emission. Thus, this review demonstrates the fundamentals to enlighten the PL mechanism for the multi-emissive response in CDs through ultrafast time-resolved and DFT techniques. An attempt is made to provide an in-depth understanding of diversified paths, such as modulation in their interparticle distance, integration of rare-earth metals and metallic to elucidate the duality in their emission spectra. We have provided insights into implementing intrinsic dual emissive CDs in different ratiometric models. Current states, portable challenges, the significance of fabricating CDs with dual emission peaks and their applications are rationally discussed. The main focus of the review is to discuss various methods and parameters to induce different emissions in CDs with single or dual bands rather than the sensing mechanisms. The goal of this review is to describe the theoretical justification for the sensing response of CDs by varying different parameters, such as pH, precursor, etc.

Photoluminescence Mechanism of Carbon Dots: Triggering Multiple Color Emissions through Controlling the Degree of Protonation.

Carbon dots (CDs) have excellent optical properties, low toxicity and easy preparation, which have led to them being widely used in biomedicine, sensing and optical devices. However, although great progress has been made in the preparation of CDs, the detailed exploration of their photoluminescence (PL) mechanism is still under debate due to their complex structures and surface functionalities. Here, we proposed a single change in the pH of the synthesis condition, which had no effect on the CDs intrinsic core states and avoided the mutual influence of multiple PL origins. The m-phenylenediamine (m–PD) served as a carbon source, whose protonation degree determined the surface state of the resulting CDs and the accompanying fluorescence characteristics. The as-obtained CDs materials can be applied in the chemical sensor and anti-counterfeiting fields in a targeted manner. Therefore, our work not only contributes to the explanation of the CDs PL mechanism, but also obtains a series of CDs materials with controllable PL properties.

Ultrafast Interface Charge Separation in Carbon Nanodot-Nanotube Hybrids.

Carbon dots are an emerging family of zero-dimensional nanocarbons behaving as tunable light harvesters and photoactivated charge donors. Coupling them to carbon nanotubes, which are well-known electron acceptors with excellent charge transport capabilities, is very promising for several applications. Here, we first devised a route to achieve the stable electrostatic binding of carbon dots to multi- or single-walled carbon nanotubes, as confirmed by several experimental observations. The photoluminescence of carbon dots is strongly quenched when they contact either semiconductive or conductive nanotubes, indicating a strong electronic coupling to both. Theoretical simulations predict a favorable energy level alignment within these complexes, suggesting a photoinduced electron transfer from dots to nanotubes, which is a process of high functional interest. Femtosecond transient absorption confirms indeed an ultrafast (<100 fs) electron transfer independent of nanotubes being conductive or semiconductive in nature, followed by a much slower back electron transfer (≈60 ps) from the nanotube to the carbon dots. The high degree of charge separation and delocalization achieved in these nanohybrids entails significant photocatalytic properties, as we demonstrate by the reduction of silver ions in solution. The results are very promising in view of using these “all-carbon” nanohybrids as efficient light harvesters for applications in artificial photocatalysis and photosynthesis.

Defects coordination triggers red-shifted photoluminescence in carbon dots and their application in ratiometric Cr(VI) sensing

Hexavalent chromium ion (Cr(VI)) is one of the most severe environmental nonbiodegradable pollutants. Considering its high biological toxicity, it is of great significance to establish a method for Cr(VI) analysis with high sensitivity and reliability. Therefore, a simple and sensitive ratiometric sensor is developed according to the coordination between Cr(VI) and carbon dots (CDs). In the presence of Cr(VI), the original CDs photoluminescence (PL) emission at 445 nm was significantly shifted to 565 nm and gradually intensified. The coordination mechanism was investigated, the results indicated that the dual-amino groups at ortho-position of benzene ring is critical for this coordination. Under the optimal condition, a good linear relationship between the concentration of Cr(VI) and the PL intensity ratio I565/I445 was obtained from 120 nM to 200 µM, with a low detection limit of 37 nM. Moreover, this method is applied into real water samples analysis with a high quality.

Excitation dependence and independence of photoluminescence in carbon dots and graphene quantum dots: insights into the mechanism of emission.

Excitation-dependent, multicolor emission from different varieties of 0D carbon systems has attracted immense research attention. It is generally accepted that some variants of 0D carbon exhibit excitation dependent emission, while other variants do not. A third variant exhibits both excitation dependent as well as excitation independent emission. In this work we investigate the structure, composition, steady-state emission-excitation and photoluminescence decay dynamics of three distinctly different variants of 0D carbon - amorphous carbon dots (aCDs), graphene quantum dots (GQDs) and nitrogen-doped GQDs (NGQDs). We find that despite significant differences in the structure and composition there is a striking similarity in the excitation energy dependence of the emission characteristics of these three different dots. All of them exhibit excitation energy independent emission below some threshold wavelength (λth), and above this threshold the emission becomes excitation dependent. We also demonstrate that a similar trend is apparent for nearly all variants of 0D carbon reported in the literature. The threshold wavelength correlates well with the excitation wavelength for the most intense emission and the photoluminescence excitation peaks, suggesting a common origin of light emission in these carbon dots. The findings provide important clues for developing a unified general picture for understanding the light emission mechanism in 0D carbon nanostructures.

The influence of the type of ions hydration on photoluminescence of carbon dots in aqueous suspensions

This manuscript is devoted to the study of the influence of the type of ions hydration on photoluminescence (PHL) of carbon dots (CDs) in aqueous suspensions. It is shown that ions with different hydration properties affect differently on CDs PHL. Positively hydrated cations increase the intensity of CDs PHL, and negatively hydrated cations weaken CDs PHL. The effect of the anions hydration type on CDs PHL is mediated through their effect on the strength of cations hydrate shells. Due to the fact that the hydration properties of individual ions are well studied, the proposed hypothesis allows to ‘anticipate’ the PHL response of CDs under the addition of certain salts, which can serve as the basis for creating CDs-based fluorescent sensors.

Surface states of carbon dots and their influences on luminescence

Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescent mechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders the development of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization between the carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorial paper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects and heteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derived luminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are introduced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states.

Signal-on photoluminescent detection of dopamine with carbon dots-MnO2 nanosheets platform based on inner filter effect

Development of new methods for dopamine (DA) assay has attracted continuous attention because it serves a vital role in clinical treatment of neurological diseases such as depression, schizophrenia, and Parkinson's disease. Herein, we design a carbon dots (CDs)-MnO2 nanosheets platform for rapid and sensitive detection of DA in pharmaceutical products. The photoluminescence of CDs can be quenched by MnO2 nanosheets via inner filter effect (IFE) because the absorption of MnO2 nanosheets fully covered the excitation and emission bands of CDs. However, DA can transform MnO2 nanosheets into Mn2+ ions, resulting in the elimination of the IFE and recovery of the photoluminescent signal. This sensing platform shows a selective and sensitive response to DA in the range of 1–30 μM, with a detection limit of 100 nM. The presented IFE-based strategy gives a new insight on development of label-free, selective and sensitive optical methods for drug assay.

Investigation on the Relationship Between Carbon Cores and Fluorescence Moieties by Measurement of Fluorescence Anisotropy of CDs with Different Sizes

Carbon dots(CDs) have been considered as a marvellous photoluminescence(PL) material, and their PL mechanism remains debatable. The carbon core, as an essential part of CDs, apparently plays an intricate role in the PL of CDs. However, the influence of the core on the PL and the relationship between the core and fluorescence moiety are still unclear. Here, we investigated the influence of carbon cores with different sizes on the rotational motion of fluorescence moieties to determine the relationship between carbon cores and fluorescence moieties. CDs with different size distributions were synthesized by controlling carbonization time. The core sizes and rotational correlation time(RCT) of the CD samples were measured by transmission electron microscopy(TEM) and fluorescence anisotropy measurement, respectively. And the rotating unit radius were calculated from the RCT. The experimental results show that the rotational motion of the fluorescence moiety is independent of the carbon core sizes and it possesses total rotational freedom. This work is helpful for understanding the connection between the carbon core and fluorescence moiety and its influence on the PL properties of CDs.

Utilization of carbon dots from jackfruit for real-time sensing of acetone vapor and understanding the electronic and interfacial interactions using density functional theory

Carbon dots are emerging carbon-based nanomaterials because of their unique optical properties, high surface area, and surface chelating functional groups. In this work, carbon dots were prepared from jackfruit using a one-step hydrothermal method and used as a sensing layer in an optical electronic nose for the real-time detection of acetone vapor at room temperature. The carbon dots showed blue photoluminescence with excitation-dependent fluorescence emission, excellent photostability, and a quantum yield of 5.2%. Using principal component analysis, the carbon dot-integrated electronic nose was able to distinguish acetone from hexane, ethanol, methanol, and water and between different concentrations of acetone in ethanol and aqueous solutions. From the time-dependent density functional theory calculations, an increase in carbon dot's extinction coefficients in bulk solvents was in a good agreement with the optical electronic nose results. The calculations of interaction energy using density functional theory method illustrated the electronic coupling and interfacial interactions between carbon dots and acetone and other volatile organic compounds. Interestingly, the unique ambipolar properties of carbon dots were computationally demonstrated. Furthermore, the photoluminescence of carbon dots was also exploited for the detection of acetone in aqueous solutions. Based on this work, our jackfruit-derived carbon dots were demonstrated to be versatile sensing materials for acetone in vapor and solution, and the computational methods highlighted the importance of interfacial electronic coupling towards unique sensing properties of carbon dots.

Photoluminescence Enhancement of Carbon Dots by Surfactants at Room Temperature.

During the past decade, increasing attention has been paid to fluorescent carbon dots (CDs) due to their unique photoluminescence (PL) properties. As synthetic methods gradually develop, many post-functionalization strategies have been developed to enhance the PL of CDs. However, in most cases, the PL enhancement was less than 10-fold with multistep modification processes. In this work, a facile and efficient post-functionalization strategy was successfully applied to enhance the PL intensity of CDs dramatically up to 69 times by surfactants at room temperature for the first time. Furthermore, the mechanism of surfactant-induced PL enhancement of CDs was investigated and in vivo bioimaging was performed. The results demonstrated that electrostatic/non-electrostatic interactions between CDs and surfactants could effectively lower the vibration and rotation of CDs, increase radiative decay processes and, thus, enhance the PL of CDs. This finding may provide new insights into the strategies for enhancing the PL of CDs, further broadening their potential applications.

Modulation of the photoluminescence in carbon dots through surface modification: from mechanism to white light-emitting diodes

Carbon dots (CDs) have emerged as a new type of fluorescent material because of their unique optical advantages, such as high photoluminescence quantum yields (QYs), excellent photo-stability, excitation-dependent emissions, and low toxicity. However, the photoluminescence mechanism for CDs remains unclear, which limits their further practical application. Here, CDs were synthesized via a solvothermal route from citric acid and urea. Through the oxidation and reduction treatment of pristine CDs, the origin of the photoluminescence and the involved mechanism were revealed. We found that the blue/green/red emissions originated from three diverse emitting states, i.e. the intrinsic state, and C=O- and C=N-related surface states, respectively. Based on the as-prepared CDs, a pH sensor depending on the radiometric luminescence detection was developed. Furthermore, we constructed CD/PVP (PVP, polyvinylpyrrolidone) composite films, which exhibited white light emission with photoluminescence QYs of 15.3%. The white light emission with different correlated color temperatures (CCTs), from 4807 K to 3319 K, was obtained by simply changing the amount of PVP solution. Benefiting from the white light-emitting solid-state films, single-component white light-emitting diodes were fabricated with an average color rendering index value (Ra) of 80.0, luminous efficiency of 10.2 lm W−1, and good working stability, thus indicating a promising potential for practical lighting applications.

Time-resolved spectroscopy of the ensembled photoluminescence of nitrogen- and boron/nitrogen-doped carbon dots.

Carbon dots (CDs) have potential applications in various fields such as energy, catalysis, and bioimaging due to their strong and tuneable photoluminescence (PL), low toxicity, and robust chemical inertness. Although several PL mechanisms have been proposed, the origin of PL in CDs is still in debate because of the ensembled nature of the heterogeneous luminophores present in the CDs. To unravel the origin of PL in CDs, we performed time-resolved spectroscopy on two types of CDs: nitrogen-doped (N-CD) and boron-nitrogen co-doped (BN-CD). The PL decays were fitted by stretched exponential functions to estimate the distribution of the decay kinetics in the CDs, which have different PL lifetime distributions. Both CDs displayed main, blue emission decaying in 15 ns, which originates from the dominant molecular state. The analysis of the non-exponential PL decay using stretched exponential fits revealed that the functional surface luminophores are of less variety but of more environmental heterogeneity and have much lower populations in BN-CD than in N-CD.

Mechanistic Insight into the Carbon Dots: Protonation induced Photoluminescence

Although several theories have been proposed, the mechanism of complex photoluminescence in carbon dots (CNDs) is a central quest till date. This report presents pH dependent steady state and time resolved spectroscopy study which identifies a possible origin of the complex photoluminescence in CNDs. The multiple emissive species created by the excited state protonation-deprotonation reaction at certain pH gives rise to inhomogeneous broadening and consequently excitation dependent multicolour emission. The origin of the excited state dynamics is attributed to the significant change of the proton dissociation between ground and excited state. We present a new model on protonation dynamics and show how it affects the emissive states in CNDs.

Investigation of phosphorous doping effects on polymeric carbon dots: Fluorescence, photostability, and environmental impact

Carbon dots have arisen as a potential alternative to traditional quantum dots since they fluoresce but are synthesized from sustainably sourced green chemicals. Herein, fluorescent nitrogen-doped polymeric carbon dots (CDs) were synthesized by using citric acid (CA) or malic acid (MA) as carbon precursors and ethylenediamine as the nitrogen precursor. Additionally, phosphoric acid was used as a doping agent for each type of CDs to evaluate the impact of incorporating phosphorus into the nanoparticles. Thus, four kinds of doped CDs (N-doped or N, P co-doped) were obtained and named as CACDs, CA-P-CDs, MACDs, and MA-P-CDs. Quantum yield and fluorescence lifetime analysis indicate that phosphorus doping of up to c.a. 10 wt% does not induce a remarkable influence on CD photoluminescence. The photostability of the N, P co-doped MACDs (MA-P-CDs), however, was observed to increase compared to the N-doped MACDs under 350 nm UV (UV-B) exposure. Lastly, to assess the impact of this emerging nanoparticle on prokaryotes, the bacterial toxicity of these CDs was tested using Shewanella oneidensis MR-1 as a model microorganism. Interestingly, the CDs exhibited no toxicity in most cases, and in fact facilitated bacteria growth. Hence, this work suggests that CDs are potentially eco-friendly fluorescent materials.

Excitation-Dependent Photoluminescence from Single-Carbon Dots.

Carbon dots (CDs) are carbon-based fluorescent nanoparticles that can exhibit excitation-dependent photoluminescence (PL) "tunable" throughout the entire visible range, interesting for optoelectronic and imaging applications. The mechanism underlying this tunable emission remains largely debated, most prominently being ascribed to dot-to-dot variations that ultimately lead to excitation-dependent ensemble properties. Here, single-dot spectroscopy is used to elucidate the origin of the excitation-dependent PL of CDs. It is demonstrated that already single CDs exhibit excitation-dependent PL spectra, similar to those of the CD ensemble. The single dots, produced by a facile one-step synthesis from chloroform and diethylamine, exhibit emission spectra with several characteristic peaks differing in emission peak position and spectral width and shape, indicating the presence of distinct emission sites on the CDs. Based on previous work, these emission sites are related to the sp2 subregions in the carbon core, as well as the functional groups on the surface. These results confirm that it is possible to integrate and engineer different types of electronic transitions at the nanoscale on a single CD, making these CDs even more versatile than organic dyes or inorganic quantum dots and opening up new routes toward light-emission engineering.