One-pot synthesis of nuclear targeting carbon dots with high photoluminescence

Carbon dots (CDs) are novel fluorescent nanomaterials with good water solubility, high resistance to photobleaching and low toxicity. While, there are few studies elaborate on the relationship among reaction conditions, properties and applications of CDs. In this study, a series of CDs are synthesized through a one-pot hydrothermal method, and different reaction conditions are carried out to study the influencing factors of CDs properties. As a result, with the increase of temperature and reaction time, the particle size and zeta potential of CDs increased, the maximum emission wavelength red-shifted and the fluorescence quantum yield (QY) improved. Among them, CD₃₀₀₆ has good water solubility and highest QY of 81.4%, which is beneficial for its applications in bioimaging and ion detection. CD₃₀₀₆ is almost nontoxic in cells at a concentration of 500 µg/mL. In addition, the positive charged CD₃₀₀₆ shows nuclear targeting potential because of its combination with DNA through electrostatic interaction in nucleus. The properties of CDs can be greatly enhanced by controlling reaction conditions, and it provides great application prospects.     

Easy-to-perform organic-solvent-free synthesis of carbon dots with strong green photoluminescence

Carbon dots(CDs) have been extensively studied owing to their fascinating optical properties and wide potential applications. Here, we report an easy-to-perform and organic-solvent-free synthesis strategy for green-emissive CDs(G-CDs) possessing high photoluminescence(PL) quantum yield(QY). The G-CDs are synthesized by heating the homogeneous precursors of citric acid and cyanamide in an open vessel, circumventing the use of organic solvents, complex operations, high-pressure reactors, and expen...     

Quenching of photoluminescence in conjugates of quantum dots and single-walled carbon nanotube

Development of quantum dot (QD) based device components requires controlled integration of QDs into different photonic and electronic materials. In this regard, introduction of methods for regular arrangement of QDs and investigation of properties of QD-based assemblies are important. In the current work we report (1) controlled conjugation of CdSe-ZnS QDs to sidewall-functionalized single-walled carbon nanotube (SWCNT) templates (2) and the effect of conjugation of QDs to SWCNT on the photoluminescence (PL) properties of QDs. We identified that PL intensity and lifetime of QDs are considerably reduced after conjugation to SWCNT. The origin of the quenching of the PL intensity and lifetime was discussed in terms of F?rster resonance energy transfer (FRET). FRET involves nonradiative transfer of energy from a photoexcited QD (energy donor) to a nearby SWCNT (energy acceptor) in the ground state. This was examined by varying the density of QDs on SWCNT and conjugating smaller and bigger QDs to the same SWCNT. We estimated the FRET efficiency in QD-SWCNT conjugates from the quenching of the PL intensity and lifetime and identified that FRET is independent of the density and type of QDs on SWCNT but inherent to QD-SWCNT conjugates.     

One-pot synthesis of concentration and excitation dual-dependency truly full-color photoluminescence carbon dots

The paper describes a kind of truly full-color photoluminescence (PL) CDs. The CDs were prepared by using one-pot hydrothermally heating citric acid and formamide at 200 °C for 2 h. The CDs have three fluorescent centers at blue, green, and red light region. Their color was regulated through two means, including changing excitation wavelengths or CDs concentrations. The emission maxima changed from blue to red with the increase of excitation wavelengths or CDs concentrations. The full-color PL behavior of the CDs was inherited and conserved in the solid polymer matrix, giving multicolor CDs/polymer films and light emitting diodes (LEDs). White-light LED (WLED) with the CIE coordinate approaching to (0.31, 0.32) were also achieved.     

The polymeric characteristics and photoluminescence mechanism in polymer carbon dots: A review

A large amount of emerging research on carbon dots (CDs) has been gradually improving the understanding of their structures, properties and emission mechanism. Distinct from the dominating status of quantum confinement effect in quantum dots, CDs always suffer from the complicated optical properties, deriving from the large differences in raw materials and synthesis methods. The diverse concepts and species puzzle researchers and hinder the further study. Thus, there is an urgent need to unify the definition and clarify the confused relation of CDs. Herein, we classify the raw materials of CDs synthesis into small molecules and polymers, and discuss CDs from the aspects of raw materials. We believe that the polymer-like structures reserved in CDs are universal no matter from the condensation of small molecules or the direct inheritance of polymers. Moreover, many similarities are concluded between CDs and polymers through serious comparisons and enough evidences. The formation processes of CDs are mostly polymerization and the obtained CDs always possess polymeric characteristics, such as abundant reactive functional groups, polydispersity of products, highly crosslinked network structure and other similar properties to non-conjugated fluorescent polymers. Therefore, the new concept, polymer carbon dots (PCDs), is put forward to generalize all kinds of CDs based on the summary of related reports. Besides, the complicated influence factors of photoluminescence (PL) are discussed and mainly classified as molecule state, carbon core state, surface state and crosslink enhanced emission (CEE) effect. In general, this review puts forward PCDs as a unified definition of reported CDs, and summarizes the polymeric characteristics of PCDs from formation process and product properties, as well as simultaneously illustrates the PL mechanism.     

Temperature-modulated photoluminescence of quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) grafted with thermoresponsive poly( N-isopropylacrylamide) chains have been prepared. As the temperature increases, PNIPAM chains shrink and aggregate so that the QDs exhibit enhanced fluorescence emission. At a temperature around the lower critical solution temperature (LCST) of PNIPAM, the fluorescence exhibits a maximum intensity. Our experiments reveal that the fluorescence emission is determined by the interactions between QDs as a function of the interdot distance. The optical interdot distance for the maximum luminescence intensity is approximately 10 nm. The chain length of PNIPAM also has an effect on the luminescence. Short PNIPAM chains are difficult to associate, leading to a large interdot distance, so that the luminescence intensity changes slightly with temperature.     

Mechanisms of photoluminescence in the molecular state of carbon dots prepared from o-phenylenediamine

Sun's group from Beijing University of Technology, China, prepared red-emission CDs in high yield by a solvent-free method based on o-phenylenediamine and analyzed the origin of the red emission and the formation of CDs. 5,14-Dihydroquinoxalino[2,3-b]phenazine (DHQP) was successfully isolated from the system and identified as the fluorophore of the CDs. These findings provide insight into the PL mechanism of this type of CDs and may guide the further development of CDs that can be tuned to obtain tailored properties.

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Temperature-dependent photoluminescence of water-soluble quantum dots for a bioprobe

The photoluminescence of water-soluble CdSe/ZnS core/shell quantum dots is found to be temperature-dependent: as temperature arising from 280 K to 351 K, the photoluminescence declines with emission peak shifting towards the red at a rate of ∼0.11 nm K⁻¹. And the studies show that the photoluminescence of water-soluble CdSe/ZnS quantum dots with core capped by a thinner ZnS shell is more sensitive to temperature than that of ones with core capped by a thicker one. That is, with 50% decrement of the quantum yield the temperature of the former need to arise from 280 K to 295 K, while the latter requires much higher temperature (315.6 K), which means that the integrality of shell coverage is a very important factor on temperature-sensitivity to for the photoluminescence of water-soluble CdSe/ZnS quantum dots. Moreover, it is found that the water-soluble CdSe quantum dots with different core sizes, whose cores are capped by thicker ZnS shells, possess almost the same sensitivity to the temperature. All of the studies about photoluminescence temperature-dependence of water-soluble CdSe/ZnS core/shell quantum dots show an indispensable proof for their applications in life science.     

Synthesis of carbon dots with a tunable photoluminescence and their applications for the detection of acetone and hydrogen peroxide

Carbon dots(CDs) with multi-color emissive properties and a high photoluminescent quantum yield(PLQY) have attracted great attention recently due to their potential applications in chemical,environmental,biological and photo-electronic fields.Solvent-dependent effect in photoluminescence provides a facial and effective approach to tune the emission of CDs.In this study,green emissive nitrogen-doped carbon dots(N-CDs) are synthesized from p-hydroquinone and ethylenediamine through a simple hydrothermal method.The as-prepared N-CDs possess a robust excitation-independent green luminescence and a high PLQY of up to 15.9%.Further spectroscopic characterization indicates that the high PLQY is achieved by the balance of nitrogen doping states and the surface passivation extent in CDs.The N-CDs also exhibit solvent-dependent multi-color emissive property and distinct PLQY in different solvents(the maximum can reach up to 25.3%).Furthermore,the as-prepared N-CDs are applied as fluorescence probes to detect acetone and H2O2 in water.This method has exhibited a low detection limit of acetone(less than 0.1 %) and a quick and linear response to the H_2O_2 with the concentration from 0 to 120 μmol/L.This work broadens the knowledge of applying CDs as probes in the bio and chemical sensing fields.     

Barium charge transferred doped carbon dots with ultra-high quantum yield photoluminescence of 99.6% and applications

Long-emission carbon dots(CDs) is triggering immense enthusiasm on account of their intrinsic merits of high chemical stability and excellent optical properties.In this study,a facile and rapid approach was developed for the preparation of barium-doped carbon dots(Ba-CDs) with yellow fluo rescence emission and high quantum yields.Surface chemistry and the chemical architecture of the Ba-CDs was revealed under various spectroscopic methods.This work provides more insights into the effects of charge transfer caused by Ba heteroatoms,which is considered as the most challenging step in the investigation on luminescence mechanism.Remarkably,the prepared Ba-CDs were successfully applied as fluorescent probes in the detection of trace water in organic solvents(ethanol,isopropanol,acetone,tetrahydrofuran).Comparing with traditional fluorescent probes for water detection in organic solvents,Ba-CDs detection provides a more sensitive,much faster and more economical approach.     

Preparation of carbon quantum dots with tunable photoluminescence by rapid laser passivation in ordinary organic solvents

A simple approach to prepare carbon quantum dots is presented in this communication by laser rapid passivation of nano carbon particles in ordinary organic solvent. The as-prepared carbon dots exhibited visible, tunable and stable photoluminescence (PL). XPS analysis showed that the increased oxygen concentration might be concerned with the origin of PL.     

Small nanoparticles bring big prospect: The synthesis,modification, photoluminescence and sensing applications of carbon dots

As a new type of carbon-based fluorescent nanomaterials, carbon dots(CDs) are provided with the advantages of small size, excellent photoluminescence(PL) property, easy surface modification, robust stability,good water solubility and biocompatibility, which endow them with great potential in sensing. In this review, we first describe the preparation of CDs from different starting materials via various techniques, and pre-/post-modification strategies to modulate their PL properties. Second, we o...     

Core/shell quantum dots with high relaxivity and photoluminescence for multimodality imaging

A series of core/shell CdSe/Zn1-xMnxS nanoparticles were synthesized for use in dual-mode optical and magnetic resonance (MR) imaging techniques. Mn2+ content was in the range of 0.6-6.2% and varies with the thickness of the shell or amount of Mn2+ introduced to the reaction. These materials showed high quantum yield (QY), reaching 60% in organic solvent. Water-soluble nanoparticles were obtained by capping the core/shell particles with amphiphilic polymer, and the QY values in water reached 21%. These materials also demonstrated high relaxivity with r1 values in the range of 11-18 mM-1 s-1 (at room temperature, 7 T). Both optical and MR imaging were performed on nanoparticles in aqueous solution and applied to cells in culture. The results showed that the QY and manganese concentration in the particles was sufficient to produce contrast for both modalities at relatively low concentrations of nanoparticles.     

Surface-tunable photoluminescence from block copolymer-stabilized cadmium sulfide quantum dots

The static and time-resolved photoluminescence properties of polystyrene-b-poly(acrylic acid) (PS-b-PAA)-stabilized cadmium sulfide quantum dots (CdS QDs) have been characterized for the first time, demonstrating tunable emission spectra and quantum yields via different chemical treatments of the PAA layer. Samples with the PAA layer in its cadmium carboxylate form showed more-intense band-edge emission and relatively high quantum yields compared with samples in which the PAA layer was in its acid form. This activation effect is explained in terms of passivation of trap sites on the QD surface by specific interactions between the QD and the cadmium-neutralized PAA layer. Lifetimes of band-edge and trap state emission for the various samples ranged from 40 to 61 ns and 244 to 360 ns, respectively. Impressive long-term stability was also shown for a sample of cadmium-neutralized PS-b-PAA-stabilized QDs dispersed in toluene, which maintained 90% of its photoluminescence over 57 days aging under ambient conditions. It is also shown that Cd2+ activation of photoluminescence does not occur when Mg2+ ions are added to similar QD solutions, indicating potential of these block copolymer-stabilized QDs as Cd2+-selective sensors. Irrespective of chemical treatment of the PAA layer, the external PS brush layer effectively stabilized all samples in various organic solvents, resulting in clear CdS colloids with no observed precipitation over several months. Dynamic light scattering and gel permeation chromatography revealed differences in the aggregation numbers and hydrodynamic radii of colloidal QDs for different treatments of the PAA layer, attributed to the lower solubility of the poly(cadmium acrylate) blocks compared to the PAA blocks in the acid form. Finally, it was demonstrated that the PS-b-PAA-stabilized QDs could be well dispersed in PS homopolymer, producing optically transparent photoluminescent films which retained the emission features of the colloidal QDs. Stable and surface-tunable optical properties via the PAA layer and polymer solubility and processability via the PS layer make these PS-b-PAA-stabilized CdS QDs exciting "building blocks" for the bottom-up assembly of functional hierarchical materials for photonics, sensors, and bio-labeling applications.     

Water-dispersed quantum dots of coordination polymers with strong photoluminescence

Coordination polymers PZn quantum dots with a uniform diameter of 3 ± 0.5 nm were successfully prepared. The PZn QDs exhibit excellent water dispersibility, high photoluminescence, outstanding photostability and remarkable biocompatibility. The results of cellular experiments show that the PZn QDs are highly suitable for long-term cell imaging.     

Temperature-dependent photoluminescence of highly luminescent water-soluble CdTe quantum dots

Highly luminescent water-soluble CdTe quantum dots(QDs) have been synthesized with an electrogenerated precursor.The obtained CdTe QDs can possess good crystallizability,high quantum yield(QY) and favorable stability.Furthermore,a detection system is designed firstly for the investigation of the temperature-dependent PL of the QDs.     

Metal-enhanced photoluminescence from carbon nanodots

In the last couple of years, carbon dots have emerged as a new novel luminescent particle for applications in fluorescence and microscopy in some ways analogous to quantum dots and silicon nanocrystals/particles. As with any fluorescent label or tag, absolute fluorescence intensity, brightness, and particle photostability are a primary concern. In this communication we subsequently show that similar to classical fluorophores, carbon dots located in the near-field, near to Plasmon supporting materials, show enhanced intensities and improved photostabilities.     

Studies on optical absorption and photoluminescence of thioglycerol-stabilized CdS quantum dots

Nanoparticles of CdS were prepared at 303 K by aqueous precipitation method using CdSO4 and (NH4)2S in presence of the stabilizing agent thioglycerol. Adjustment of the thioglycerol (T) to ammonium sulphide (A) ratio (T:A) from 1:25 to 1:3.3 was done during synthesis and nanoparticles of different size were obtained. The prepared colloids were characterized by UV-vis and photoluminescence (PL) spectroscopic studies. Prominent first and second excitonic transitions are observed in the UV-vis spectrum of the colloid prepared with a T:A ratio of 1:3.3. Particle size analysis was done using XRD, high resolution TEM and dynamic light scattering and found to be approximately 3 nm. UV-vis and PL spectral features also agree with this particle size in colloid with T:A of 1:3.3. The band gap of CdS quantum dots has increased from the bulk value 2.4-2.9 eV. PL spectra show quantum size effect and the peak is shifted from 628 to 556 nm when the ratio of T:A was changed from 1:25 to 1:3.3. Doping of CdS with Zn2+ and Cu2+ is found to enhance the PL intensity. PL band shows blue-shift and red-shift on doping with Zn2+ and Cu2+, respectively. UV and PL spectral features of the CdS/Au hybrid nanoparticles obtained by a physical mixing of CdS and Au nanoclusters in various volume ratios is also discussed. Au red-shifts and rapidly quenches the PL of CdS. An additional low energy band approximately 650 nm is observed in the UV visible spectrum of the hybrid nanoparticles.     

Graphene and carbon quantum dots electrochemistry

Graphene and carbon quantum (QDs) dots exhibit interesting and well-defined properties owing to their quantum confinement. In this work, graphene QDs (G-QDs) and carbon QDs of size ~ 6 nm and ~ 2 nm, respectively, were prepared and their potential uses in electrochemistry and electrochemical sensing were subsequently investigated. It was discovered that the C-QDs surface displayed a faster electron transfer rate compared to the G-QDs following analyses with the ferro/ferricyanide redox probe. Studies were also carried out with redox biomarkers such as uric acid (UA) and ascorbic acid (AA), and it was found that while the C-QDs displayed electrocatalytic properties toward the oxidation of both UA and AA, the G-QDs seemed to only have an impact on AA, from the decrease in the oxidation peak potential. This work provides direct electrochemical comparison of the two latest frontiers of carbon nanomaterials and opens the way for their electrochemical sensing applications.     

Structural engineering design of carbon dots for lubrication

Carbon dots (CDs) have opened up a new field of carbon nanomaterials and successively attracted increasing attention since their discovery in 2004. Owing to their ultrasmall size, tunable surface functional groups, excellent dispersibility, attractive stability, low toxicity, environmental friendliness, facile synthesis and low-cost precursors, CDs have been developed as green and promising friction-reducing and anti-wear materials in lubrication science, applied to energy conservation and extension of mechanical service life in recent years. However, there are few reviews focusing on the application of CDs in the important field of lubrication. In this review, we comprehensively summarize the development of CDs in lubrication for the first time. Firstly, three strategies for structural engineering design of CDs to improve their tribological characteristics are fully analyzed, in terms of size and shape control, surface modification and heteroatom doping. Secondly, the advance in lubrication application of CDs, including CDs as additives for lubricants, greases, gel and magnetorheological fluids as well as CDs as lubricating coatings, is systematically highlighted. Thirdly, the lubricating mechanisms of CDs as additives are introduced in detail. Furthermore, the remaining major challenges and opportunities for CDs in lubrication field are discussed and outlined.