The Luminescence of Laser-Produced Carbon Nanodots: The Effect of Aggregation in PEI Solution

Carbon nanodots (CNDs) produced in pure water by the ablation of graphite with a nanosecond laser pulse exhibit weak photoluminescence. A small addition of polyethyleneimine (PEI) to the aqueous suspension of CNDs causes a significant increase in emissions. This paper presents experimental and theoretical studies of the emission properties of CND/PEI systems. The obtained CNDs responded to even trace amounts of PEI in solution (~0.014% v/v), resulting in a significant increase in the initial weak blue emission of CNDs and PEI taken separately. Morphology and size measurements showed that particle aggregation occurred in the presence of the polymer. A decrease in the calculated Stokes shift values was observed with increasing PEI content in the solution. This indicates a reduction in the number of non-radiative transitions, which explains the increase in the emission intensity of the CND/PEI systems. These results therefore confirmed that the increase in the emission of CND/PEI systems is caused by particle aggregation. Kinetic studies proved that the process is controlled mainly by diffusion, the initial stage of which has a dominant influence on determining the optical properties of the system.

Synthesis of Multicolor Carbon Dots Catalyzed by Inorganic Salts with Tunable Nonlinear Optical Properties

The nonlinear optical properties of carbon dots have been in the spotlight in recent years. In light of the complexity and diversity of factors affecting the nonlinear optical properties of carbon dots, how to reveal the origin and physical mechanism of the nonlinear optical properties of carbon dots accurately has become a problem. In this work, a template-free method was designed to prepare carbon dots via solid-phase reaction with phloroglucinol as a single carbon source and sodium bisulfate as the catalyst. This method is simple, green, safe, and easy to be prepared on a large scale. Three carbon dots with different luminous colors were obtained by simply adjusting the reaction temperature. The rise of reaction temperature affects the surface functional groups, and then hinders the luminescence of surface states, leading to the change of luminescence properties. The nonlinear optical properties of carbon dots were analyzed by the Z-scan technique. Surprisingly, all carbon dots have nonlinear optical responses, but there are differences in performance. Results prove the increase in sp2 domains may contribute to the significant improvement of the nonlinear optical properties of carbon dots, indicating a direction to improve the nonlinear optical properties of carbon dots.

A Facile Approach to the Hydrothermal Synthesis of Silica Nanoparticle/Carbon Nanostructure Luminescent Composites

Luminescent carbon nanostructures (CNSs) have been intensively researched, but there is still no consensus on a fundamental understanding of their structure and properties that limits their potential applications. In this study, we developed a facile approach to the synthesis of luminescent composite SiO₂ nanoparticles/CNSs by the targeted formation of a molecular fluorophore, as the significant luminescent component of CNSs, on the surface of a silica matrix during a one-stage hydrothermal synthesis. Silica nanoparticles were synthesized by reverse microemulsion and used as a matrix for luminescent composites. The as-prepared silica nanoparticles had a functional surface, a spherical shape, and a narrow size distribution of about 29 nm. One-stage hydrothermal treatment of citric acid and modified silica nanoparticles made it possible to directly form the luminescent composite. The optical properties of composites could be easily controlled by changing the hydrothermal reaction time and temperature. Thus, we successfully synthesized luminescent composites with an emission maximum of 450 nm, a quantum yield (QY) of 65 ± 4%, and an average size of ~26 nm. The synthesis of fluorophore doped composite, in contrast to CNSs, makes it possible to control the shape, size, and surface functionality of particles and allows for avoiding difficult and time-consuming fractionation steps.

Potential Development of N-Doped Carbon Dots and Metal-Oxide Carbon Dot Composites for Chemical and Biosensing

Among carbon-based nanomaterials, carbon dots (CDs) have received a surge of interest in recent years due to their attractive features such as tunable photoluminescence, cost effectiveness, nontoxic renewable resources, quick and direct reactions, chemical and superior water solubility, good cell-membrane permeability, and simple operation. CDs and their composites have a large potential for sensing contaminants present in physical systems such as water resources as well as biological systems. Tuning the properties of CDs is a very important subject. This review discusses in detail heteroatom doping (N-doped CDs, N-CDs) and the formation of metal-based CD nanocomposites using a combination of matrices, such as metals and metal oxides. The properties of N-CDs and metal-based CDs nanocomposites, their syntheses, and applications in both chemical sensing and biosensing are reviewed.

Luminescent Composite Carbon/SiO2 Structures: Synthesis and Applications

Luminescent carbon nanostructures (CNSs) have attracted great interest from the scientific community due to their photoluminescent properties, structural features, low toxicity, and a great variety of possible applications. Unfortunately, a few problems hinder their further development. These include the difficulties of separating a mixture of nanostructures after synthesis and the dependence of their properties on the environment and the aggregate state. The application of a silica matrix to obtain luminescent composite particles minimizes these problems and improves optical properties, reduces photoluminescence quenching, and leads to wider applications. We describe two methods for the formation of silica composites containing CNSs: inclusion of CNSs into silica particles and their grafting onto the silica surface. Moreover, we present approaches to the synthesis of multifunctional particles. They combine the unique properties of silica and fluorescent CNSs, as well as magnetic, photosensitizing, and luminescent properties via the combination of functional nanoparticles such as iron oxide nanoparticles, titanium dioxide nanoparticles, quantum dots (QDs), and gold nanoclusters (AuNCs). Lastly, we discuss the advantages and challenges of these structures and their applications. The novelty of this review involves the detailed description of the approaches for the silica application as a matrix for the CNSs. This will support researchers in solving fundamental and applied problems of this type of carbon-based nanoobjects.

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 (λ_th), 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.

Green synthesis of white light emitting carbon quantum dots: Fabrication of white fluorescent film and optical sensor applications

In this work, we have reported on the facile synthesis of white light-emitting carbon quantum dots (CQD) from corncob by hydrothermal method. This CQD has a broad emission from 380 nm to 650 nm with high photoluminescence intensity even after three months of shelf-life and stable at variable pH conditions. The presence of Si and N impurities in the biomass gives a greater advantage in producing white light emission with high quantum yield (54%) and enhanced lifetime at ambient conditions. The CQD is highly sensitive towards DNA, paracetamol, Pb²⁺, Cu²⁺, Fe³⁺, and Cr³⁺ fluorescence sensing and signifies its application as a multi-modal fluorescence sensor. The results of optical sensitivity calculated from the linear range of 1-10 ng/mL, 0.10-0.30 mg/mL, 2.5446 ng/mL, 0.0694 mg/mL, 0.3103-1.5515 μM/mL, 0.4299-4.7293 μM/mL, 1.3010 μM/mL and 0.05-2.5 μM/mL. The limit of detection is 2.5446 ng/mL, 0.0694 mg/mL, 0.8641 μM/mL, 1.2454 μM/mL, 1.3010 μM/m, 0.8550 μM/mL and 2.8562 μM/mL, respectively. And also, the relative standard deviation values of 2.30%, 4.46%, 1.79%, 1.84%, 0.26%, 1.23% and 0.35% are evidences its possibility of development towards potential optical sensor applications. Flexible white light-emitting sheets were fabricated from the CQD, illuminates uniform brightness, and has good color reproducibility and higher stability under various UV light excitation.

Elucidating the mechanism of dual-fluorescence in carbon dots

Carbon dots have garnered significant attention owing to their versatile and highly tunable optical properties; however, the origins and the underlying mechanism remains a subject of debate especially for dual fluorescent systems. Here, we have prepared carbon dots from glutathione and formamide precursors via a one-pot solvothermal synthesis. Steady state and dynamic techniques indicate that these dual fluorescent dots possess distinct emissive carbon-core and a molecular states, which are responsible for the blue and red optical signatures, respectively. To further glean information into the fluorescence mechanism, electrochemical analysis was used to measure the bandgaps of the two fluorescent states, while femtosecond transient absorption spectroscopy evidenced the two-state model based on the observed heterogeneity and bimodal spectral distribution. Our findings provide novel and fundamental insights on the optical properties of dual fluorescent dots, which can translate to more effective and targeted application development particularly in bioimaging, multiplexed sensing and photocatalysis.

Microanalysis of Two Members of Oxicam Drugs by Quenching the Fluorescence of Newly Isolated Carbonaceous Materials From Incense Ash

For the first time ever, useful fluorescent (FL) carbonaceous materials (CMTS) were isolated from incense ash using facile procedure on two steps; dispersion of the CMTS in water followed by filtration. The CMTS were characterized using the following techniques; dynamic light scattering (DLS), transmission electron microscopy (TEM) and Fourier transform infrared (FT-IR) spectroscopy. The CMTS exhibit excitation wavelength dependent fluorescence emission, so it can be used as a FL probe. The FL probe was employed for sensing and quantitative determination of two members of oxicam family (tenoxicam (TEN) and meloxicam (MEL)) that belongs to non-steroidal anti-inflammatory drugs (NSAIDs). The method is based on the quenching of the FL intensity of the isolated CMTS by inner filter effect mechanism (IFE). The FL intensity decreases in linear relationship with increasing the concentrations of the two cited drugs within the range of 4.0 - 30.0 µg/mL with mean percentage recoveries of 100.04 ± 0.95 and 100.07 ± 1.06 with detection limits of 1.31 µg/mL and 1.06 µg/mL for TEN and MEL, respectively. Finally, the developed sensing system was validated as per ICH guidelines and it was proved to be accurate and precise and applied successfully for quantitative determination of the two cited drugs in their capsule dosage forms with excellent percentage recoveries reaching to 97.66 ± 0.39and 98.19 ± 1.12 for TEN and MEL, respectively.

Micro-photoluminescence of Carbon Dots Deposited on Twisted Double-Layer Graphene Grown by Chemical Vapor Deposition

Carbon-based nanomaterials, such as carbon dots (CDs) and graphene (Gr), feature outstanding optical and electronic properties. Hence, their integration in optoelectronic and photonic devices is easier thanks to their low dimensionality and offers the possibility to reach high-quality performances. In this context, the combination of CDs and Gr into new nanocomposite materials CDs/Gr can further improve their optoelectronic properties and eventually create new ones, paving the way for the development of advanced carbon nanotechnology. In this work, we have thoroughly investigated the structural and emission properties of CDs deposited on single-layer and bilayer graphene lying on a SiO₂/Si substrate. A systematic Raman analysis points out that bilayer (BL) graphene grown by chemical vapor deposition does not always respect the Bernal (AB) stacking, but it is rather a mixture of twisted bilayer (t-BL) featuring domains with different twist angles. Moreover, in-depth micro-photoluminescence measurements, combined with atomic force microscopy (AFM) morphological analysis, show that CD emission efficiency is strongly depleted by the presence of graphene and in particular is dependent on the number of layers as well as on the twist angle of BL graphene. Finally, we propose a model which explains these results on the basis of photoinduced charge-transfer processes, taking into account the energy levels of the hybrid nanosystem formed by coupling CDs with t-BL/SiO₂.

Surface modifications of carbon nanodots reveal the chemical source of their bright fluorescence

Fluorescent carbon nanodots (CNDs) have drawn increasing attention in recent years. These cost-effective and eco-friendly nanomaterials with bright fluorescence have been investigated as promising materials for electrooptic and bioimaging applications. However, the chemical source stimulating their strong fluorescence has not been completely identified to date. Depending on the chemical composition, two absorption peaks are observed in the visible range. In this study, we applied selected chemical modifications to CNDs in order to elucidate the correlation between the chemical structure and optical behavior of CNDs. Varying the amount of acetic acid in the synthesis process resulted in different effects on the absorbance and fluorescence photo-spectra. Specifically, at a low concentration (10%), the fluorescence is dramatically red shifted from 340 to 405 nm. Comprehensive characterization of the chemical modification by FTIR and XPS allows identification of the role of acetic acid in the reaction mechanism leading to the modified photoactivity. The functional group responsible for the 405 nm peak was identified as HPPT. We describe a chemical mechanism involving acetic acid that leads to an increased concentration of HPPT groups on the surface of the CNDs. Applying two additional independent chemical and consequently optical modifications namely solution pH and annealing on the nanodots further supports our proposed explanation. Understanding the molecular origin of CND fluorescence may promote the design and control of effective CND fluorescence in optical applications.

Inner filter effect as a sensitive sensing platform for detection of nitrofurantoin using luminescent drug-based carbon nanodots

In the present paper, a sensitive and selective inner filter effect sensing platform was designed to detect nitrofurantoin (NIT) in pharmaceutical dosage form. Nitrogen and phosphorus co-doped carbon nanodots (CNDs) prepared via solvothermal treatment of folic acid and phosphoric acid. The prepared CNDs exhibit greenish fluorescence at 470 nm when excited at 340 nm with fluorescence quantum yield up to 40%. The CNDs exhibit high stability in various pH, temperature, and ionic strength which adds valuable merits to its pharmaceutical applications. The emission is quenched in the presence of absorber (here NIT) while the fluorophores were not quench by the presence of common pharmaceutical excipients. A fluorometric assay was fabricated to determine NIT in capsules by quenching of the CNDs. The linear response for the proposed method was from 5.0 μM to 90 μM with the detection limit being 1.4 μM. To validate the method, the recovery of NIT in spiked sample was measured which was 96.6% -103.3%. The method was applied to the determination of NIT in pharmaceutical capsule samples, with comparable results of a reference method stated by the British Pharmacopeia (BP). Furthermore, the sub-acute toxicity studies of CNDs were investigated using normal male Balb/c mice forcefully drunk with 3 different dosages of CNDs. Animals did not produce treatment related signs of toxicity or mortality in any of the animals tested during the 28 days observation period. Additionally, no significant (P > 0.05) changes in the body weight, haematological and biochemical parameters compared with the control group were not revealed. Similarly, histopathological examination of the internal vital organs did not show any morphological alterations.

Fluorescence quenching mechanism and the application of green carbon nanodots in the detection of heavy metal ions: a review

This review article highlights the quenching mechanism and applications of green CNDs for the detection of metal ions.

A Systematic Comparative Study of the Toxicity of Semiconductor and Graphitic Carbon-Based Quantum Dots Using In Vitro Cell Models

A comparative, fully parallel study of nanoparticles (NPs) toxicity by in vitro cell viability is shown looking for reliable comparability of nanotoxicological results, a well-recognized bottleneck in the context. This procedure is suitable to compare toxicity of similar NPs, as well as the influence on toxicity of the size, surface, and other characteristics. As a case of study, semiconductor (SQDs) and graphitic-carbon quantum dots (CQDs) with identical surface groups and size were evaluated. All experiments were conducted at same conditions, involving two types of cells (mouse fibroblasts (3T3-L1) and carcinoma human hepatocellular cells (HepG2)) and different extracellular components (in the absence or presence of fetal bovine serum (FBS)). Cell viability demonstrated the excellent biocompatibility of CQDs compared to SQDs, which caused higher percentage of cell death at lower concentrations, as predicted but never clearly demonstrated. However, our comparative studies established that the toxicity of SQDs and CQDs are cellular type-dependent, and the absence or presence of serum proteins reduces the minimal concentration necessary of NPs to produce toxicity.

Highly Luminescent and Biocompatible P and N Co-Doped Passivated Carbon Nanodots for the Sensitive and Selective Determination of Rifampicin Using the Inner Filter Effect

The determination of rifampicin in pharmaceutical dosage forms using a rapid, sensitive, selective, biocompatible, and low-cost method is of vital importance in the pharmaceutical analysis field to ensure its concentration is within the effective range when administered. In this study, nitrogen-and-phosphorous-doped carbon nanodots (CNDs) were prepared using a single-step hydrothermal method with ciprofloxacin as the starting material. The CNDs showed a highly intense blue fluorescence emission centered at 450 nm, with a photoluminescence quantum yield of about 51%. Since the absorption of rifampicin was the same as the excitation spectrum of CNDs, inner filter effect (IFE) quenching occurred and it was used as a successful detection platform for the analysis of rifampicin in capsules. The detection platform showed a dynamic linear range from 1 to 100 μM (R2 = 0.9940) and the limit of detection was 0.06 μM (when S/N = 3). The average spike recovery percentage for rifampicin in the capsule samples was 100.53% (n = 5). Moreover, the sub-chronic cytotoxicity of CNDs was evaluated on healthy male mice (Balb/c) drenched with different amounts of CNDs (10 and 50 mg/kg). During this study period, no mortalities or toxicity signs were recorded in any of the experimental subjects. Based on the cytotoxicity experiment, the proposed nano-probe is considered safe and biocompatible.

Visible-Transparent Luminescent Solar Concentrators Based on Carbon Nanodots in the Siloxane Matrix with Ultrahigh Quantum Yields and Optical Transparency at High-Loading Contents

Visible-transparent luminescent solar concentrators (VT-LSCs) can be integrated with solar cells for designing solar glasses. Recently, rare-earth complexes, semiconductor nanocrystals, and carbon nanodots (CNDs) have been applied in developing VT-LSCs. However, several challenges still existed, such as quantum yields (QYs) at high-loading contents, scattering/reabsorption losses, and stability. Here, highly luminescent and visible-transparent composites based on organosilane-functionalized CNDs (Si-CNDs) cross-linked in the siloxane matrix were prepared. The composites with a high-loading content (∼10 wt %) possess ultrahigh QYs of ∼94% due to surface passivation, cross-linking-enhanced emission, and negligible inter-CND energy transfer. Moreover, they still appear exceptionally transparent and, thus, are suitable for VT-LSCs. Eco-friendly VT-LSCs without colored tinting were fabricated, yielding high internal and external quantum efficiencies of ∼66% and ∼3.9%. Our demonstration would pave a bright way for the utilization of eco-friendly VT-LSCs in solar glasses.

Excitons in Carbonic Nanostructures

Unexpectedly bright photoluminescence emission can be observed in materials incorporating inorganic carbon when their size is reduced from macro–micro to nano. At present, there is no consensus in its understanding, and many suggested explanations are not consistent with the broad range of experimental data. In this Review, I discuss the possible role of collective excitations (excitons) generated by resonance electronic interactions among the chromophore elements within these nanoparticles. The Förster-type resonance energy transfer (FRET) mechanism of energy migration within nanoparticles operates when the composing fluorophores are the localized electronic systems interacting at a distance. Meanwhile, the resonance interactions among closely located fluorophores may lead to delocalization of the excited states over many molecules resulting in Frenkel excitons. The H-aggregate-type quantum coherence originating from strong coupling among the transition dipoles of adjacent chromophores in a co-facial stacking arrangement and exciton transport to emissive traps are the basis of the presented model. It can explain most of the hitherto known experimental observations and must stimulate the progress towards their versatile applications.

On the Emission Properties of Carbon Dots: Reviewing Data and Discussing Models

The emission properties of carbon dots (CDs) have already found many potential applications, from bio-imaging and cell labelling, to optical imaging and drug delivery, and are largely investigated in technological fields, such as lighting and photonics. Besides their high efficiency emission, CDs are also virtually nontoxic and can be prepared through many green chemistry routes. Despite these important features, the very origin of their luminescence is still debated. In this paper, we present an overview of sounding data and the main models proposed to explain the emission properties of CDs and their tunability.

Ultrafast spectroscopic investigation on fluorescent carbon nanodots: the role of passivation

Femtosecond spectroscopy allows to clarify the role of passivation on the fluorescence of carbon nanodots and reveals the lack of interplay between core and surface electronic states.

Carbon Nanodots: A Review—From the Current Understanding of the Fundamental Photophysics to the Full Control of the Optical Response

Carbon dots (CDs) are an emerging family of nanosystems displaying a range of fascinating properties. Broadly speaking, they can be described as small, surface-functionalized carbonaceous nanoparticles characterized by an intense and tunable fluorescence, a marked sensitivity to the environment and a range of interesting photochemical properties. CDs are currently the subject of very intense research, motivated by their possible applications in many fields, including bioimaging, solar energy harvesting, nanosensing, light-emitting devices and photocatalyis. This review covers the latest advancements in the field of CDs, with a focus on the fundamental understanding of their key photophysical behaviour, which is still very debated. The photoluminescence mechanism, the origin of their peculiar fluorescence tunability, and their photo-chemical interactions with coupled systems are discussed in light of the latest developments in the field, such as the most recent results obtained by femtosecond time-resolved experiments, which have led to important steps forward in the fundamental understanding of CDs. The optical response of CDs appears to stem from a very complex interplay between the electronic states related to the core structure and those introduced by surface functionalization. In addition, the structure of CD energy levels and the electronic dynamics triggered by photo-excitation finely depend on the microscopic structure of any specific sub-type of CD. On the other hand, this remarkable variability makes CDs extremely versatile, a key benefit in view of their very wide range of applications.

Resolving the Multiple Emission Centers in Carbon Dots: From Fluorophore Molecular States to Aromatic Domain States and Carbon-Core States

Despite many efforts focused on the emission origin of carbon dots (CDs), it is still a topic of debate. This is mainly due to the complex structure of these nanomaterials. Here, we developed an innovative method to evaluate the number and spectral characterizations of various emission centers in CDs. We monitored the photostability of a series of column-separated CDs under UV irradiation to obtain three-dimensional data sets and resolve them using multivariate decomposition methods. The obtained results clearly revealed the presence of three different types of emission centers in CDs, including molecular states, aromatic domain states, and carbon-core states so that their single or coexisting appearance was found to be deeply dependent on the reaction temperature. Furthermore, density functional theory and time-dependent density functional theory were used to investigate the electronic and optical properties of some different aza-polycyclic and corannulene molecules as possible polycyclic aromatic hydrocarbons responsible for the above-mentioned aromatic domain states.

"Where does the fluorescing moiety reside in a carbon dot?" - Investigations based on fluorescence anisotropy decay and resonance energy transfer dynamics

It has been shown recently that aggregated dyes are responsible for very high fluorescence in a carbon dot (CD). However, what is the location of the fluorescing moiety in CD? Is it inside the CD or attached to the CD's surface? In order to answer these intriguing questions regarding the location of the fluorescing moiety in a CD, we performed rotational anisotropy decay dynamics and resonance energy transfer (RET) dynamics. Rotational correlation time of ∼120 picoseconds nullifies the fact that the whole CD is fluorescing. Instead, we can say that the fluorescing moiety is either embedded inside the CD or attached to the surface of the CD or linked to the CD through covalent bonds. From the fluorescence anisotropy decay dynamics in solvents of different viscosities, we could show that the fluorescing moiety is not attached to the surface of the CD or for that matter, the fluorescing moiety is not in a rigid environment inside the CD. RET dynamical analysis has shown that the time for RET (from CD to acceptor Rh123) is about 5.4 ns and the RET dynamics are independent of the acceptor concentration. Using RET dynamics, we could prove that the fluorescing moiety is not outside the CD; rather, it is inside the CD, but not in a rigid environment. The geometric distance between the fluorescing moiety of the CD and the acceptor (Rh123) has been obtained to be 4.55 nm. Using Förster formulation, the distance between the fluorescing moiety inside the CD and the acceptor Rh123 has been calculated to be 4.24 nm. Thus, we could not only reveal the exact location of the fluorescing moiety in a CD, but we could also demonstrate that unlike for many other nanomaterials, Förster formulation could explain the experimental observables regarding RET involving CD reasonably well.

One-pot synthesis of graphene quantum dots and simultaneous nanostructured self-assembly via a novel microwave-assisted method: impact on triazine removal and efficiency monitoring

Graphene quantum dot (GQDs) assemblies from a one-step microwave reaction as bifunctional materials in remediation of triazines.