Determination of energetics and kinetics from single-particle intermittency and ensemble-averaged fluorescence intensity decay of quantum dots

Recent Progress of Fluorescent Carbon Dots and Graphene Quantum Dots for Biosensors: Synthesis of Solution Methods and their Medical Applications

In the fields of health and biology, fluorescent nanomaterials have emerged as highly potential and very useful candidates for use in biosensor applications. These typical highly powerful nanomaterials are carbon dots (CDs) and graphene quantum dots (GQDs) among many other metallic nanomaterials. In the context of medical biosensors, this review article investigates the techniques of synthesis, and many uses of these nanomaterials, the obstacles that they face, and the potential for their future. We cover the significance of fluorescent nanomaterials, their use in the medical field, as well as the several techniques of synthesis for CDs and GQDs, including ultrasonication, hydrothermal, electrochemical method, surface modification, and solvothermal. In addition, we also discuss their biomedical applications, which include biomolecule detection, disease diagnosis and examine the obstacles and prospective possibilities for development of ultra-bright, ultra-sensitive, and selective biosensors for use in in-vivo research.Fluorescent carbon dots and graphene quantum dots is synthesized by using several types of raw material and methods. These Carbon dots and graphene quantum dots are used in the medical field includes detection of biomaterials, detection of cancer, virus and mutation in DNA.

Dual-QDs ratios fluorescent probe for sensitive and selective detection of silver ions contamination in real sample

Silver ions, as a commonly used industrial heavy metal, tends to deposit in the body and induce many diseases. In this work, modified CdTe QDs with green and red emission were synthesized to assemble dual-QDs, which could be efficient and selective utilized for Ag⁺ determination through the electron transfer progress between surface functional group of dual-QDs and Ag⁺, and the aggregation of Ag⁺ on the surface of dual-QDs. Under the appropriate pH value and volume ratio, the interaction between the surface functional groups of assembled dual-QDs reduce the affinity of Hg²⁺ in this system. The fluorescent signal of dual-QDs simultaneously attenuation or enhancement in the same proportion remove the interference of Cu²⁺ and other metal ions. Therefore, this method can selectively detect Ag⁺ without any masking agents. The linear region of detection was from 0 to 800 nmol/L (R² > 0.998), and low of detection (LOD) was 7.7 nmol/L, which could meet the corresponding standards of World Health Organization (WHO) and Environmental Protection Agency (EPA). This effective proposed dual-QDs ratios fluorescent probe has been applied to detect Ag⁺ in real environment water, tea and Citri Reticulatae Pericarpium (CRP) water.

Luminescent, low-toxic and stable gradient-alloyed Fe:ZnSe(S)@ZnSe(S) core:shell quantum dots as a sensitive fluorescent sensor for lead ions

In this paper, an aqueous-based approach is introduced for facile, fast, and green synthesis of gradient-alloyed Fe-doped ZnSe(S)@ZnSe(S) core:shell quantum dots (QDs) with intense and stable emission. Co-utilization of co-nucleation and growth doping strategies, along with systematic optimization of emission intensity, provide a well-controllable/general method to achieve internally doped QDs (d-dots) with intense emission. Results indicate that the alloyed ZnSe(S)@ZnSe(S) core:shell QDs have a gradient structure that consists of a Se-rich core and a S-rich shell. This gradient structure cannot only passivate the core d-dots by means of the wider band gap S-rich shell, but also minimizes the lattice mismatch between alloyed core-shell structures. Using this novel strategy and utilizing the wider band gap S-rich shell can obviously increase the cyan emission intensity and also drastically improve the emission stability against chemical and optical corrosion. Furthermore, the cytotoxicity experiments indicate that the obtained d-dots are nontoxic nanomaterials, and thus they can be considered as a promising alternative to conventional Cd-based QDs for fluorescent probes in biological fields. Finally, it is demonstrated that the present low-toxicity and gradient-alloyed core:shell d-dots can be used as sensitive chemical detectors for Pb²⁺ ions with excellent selectivity, small detection limit, and rapid response time.

Enhanced Luminescent Stability through Particle Interactions in Silicon Nanocrystal Aggregates

Close-packed assemblies of ligand-passivated colloidal nanocrystals can exhibit enhanced photoluminescent stability, but the origin of this effect is unclear. Here, we use experiment, simulation, and ab initio computation to examine the influence of interparticle interactions on the photoluminescent stability of silicon nanocrystal aggregates. The time-dependent photoluminescence emitted by structures ranging in size from a single quantum dot to agglomerates of more than a thousand is compared with Monte Carlo simulations of noninteracting ensembles using measured single-particle blinking data as input. In contrast to the behavior typically exhibited by the metal chalcogenides, the measured photoluminescent stability shows an enhancement with respect to the noninteracting scenario with increasing aggregate size. We model this behavior using time-dependent density functional theory calculations of energy transfer between neighboring nanocrystals as a function of nanocrystal size, separation, and the presence of charge and/or surface-passivation defects. Our results suggest that rapid exciton transfer from "bright" nanocrystals to surface trap states in nearest-neighbors can efficiently fill such traps and enhance the stability of emission by promoting the radiative recombination of slowly diffusing excited electrons.

Carbon dots preparation as a fluorescent sensing platform for highly efficient detection of Fe(III) ions in biological systems

Water-soluble carbon dots (CDs) were prepared, using a facile hydrothermal oxidation route of cyclic oligosaccharide α-CD, as carbon sources, and alkali as additives. The successful synthesis of CDs was confirmed by scanning electron microscopy (SEM), dynamic light scattering (DLS), FTIR, UV-visible absorption, and emission fluorescence. The characterizations showed that the prepared CDs are spherical and well-dispersed in water with average diameters of approximately 2 nm. These water-soluble CDs have excellent photo stability towards photo bleaching during 30 days. The obtained CDs showed a strong emission at the wavelength of 450 nm, with an optimum excitation of 360 nm. The fluorescence quenching of CDs in the presence of Fe(III) ions was used as fluorescent probes for quantifying Fe(III) ions in aqueous solution. Under optimum condition, the fluorescence intensity versus Fe(III) concentration gave a linear response, according to Stern-Volmer equation. The linearity range of the calibration curve and the limit of detection were 1.60×10(-5) to 16.6×10(-5) mol L(-1), and 6.05×10(-6) mol L(-1), respectively, which was in the range for serum analysis of Fe(III). It was concluded that the prepared CDs had a great potential as fluorescent probes for applications in analysis of Fe(III) ions in the blood serum samples, which is hardly interfered by other ions.

CdTe quantum dot as a fluorescence probe for vitamin B(12) in dosage form

We here report the CdTe quantum dot (CdTe QDs)-based sensor for probing vitamin B12 derivatives in aqueous solution. In this paper, simple and sensitive fluorescence quenching measurements has been employed. The Stern-Volmer constant (KSV), quenching rate constant (kq) and binding constant (K) were rationalized from fluorescence quenching measurement. Furthermore, the fluorescence resonance energy transfer (FRET) mechanism was discussed. This method was applicable over the concentration ranging from 1 to 14μg/mL (VB12) with correlation coefficient of 0.993. The limit of detection (LOD) of VB12 was found to be 0.15μg/mL. Moreover, the present approach opens a simple pathway for developing cost-effective, sensitive and selective QD-based fluorescence sensors/probes for biologically significant VB12 in pharmaceutical sample with mean recoveries in the range of 100-102.1%.

Unraveling the exciton quenching mechanism of quantum dots on antimony-doped SnO₂ films by transient absorption and single dot fluorescence spectroscopy

Integrating quantum dots (QDs) into modern optoelectronic devices requires an understanding of how a transparent conducting substrate affects the properties of QDs, especially their excited-state dynamics. Here, the exciton quenching dynamics of core/multishell (CdSe/CdS(3ML)ZnCdS(2ML)ZnS(2ML)) quantum dots deposited on glass, tin oxide (SnO₂), and antimony (Sb)-doped tin oxide (ATO) films are studied by transient absorption and single QD fluorescence spectroscopic methods. By comparing ensemble-averaged fluorescence decay and transient absorption kinetics, we show that, for QDs on SnO₂, the exciton is quenched by electron transfer from the QD to SnO₂. At the QD-ATO interface, much faster exciton quenching rates are observed and attributed to fast Auger recombination in charged QDs formed by Fermi level equilibration between the QD and n-doped ATO. Single QDs on SnO₂ and ATO show similar blinking dynamics with correlated fluctuations of emission intensities and lifetimes. Compared to QDs on SnO₂, QDs on ATO films show larger variation of average exciton quenching rates, which is attributed to a broad distribution of the number of charges and nature of charging sites on the QD surface.

Synthesis of positively charged CdTe quantum dots and detection for uric acid

The CdTe dots (QDs) coated with 2-Mercaptoethylamine was prepared in aqueous solution and characterized with fluorescence spectroscopy, UV-Vis absorption spectra, high-resolution transmission electron microscopy and infrared spectroscopy. When the λex=350 nm, the fluorescence peak of positively charged CdTe quantum dots is at 592 nm. The uric acid is able to quench their fluorescence. Under optimum conditions, the change of fluorescence intensity is linearly proportional to the concentration of uric acid in the range 0.4000-3.600 μmol L(-1), and the limit of detection calculated according to IUPAC definitions is 0.1030 μmol L(-1). Compared with routine method, the present method determines uric acid in human serum with satisfactory results. The mechanism of this strategy is due to the interaction of the tautomeric keto/hydroxyl group of uric acid and the amino group coated at the CdTe QDs.

A novel method for ranitidine hydrochloride determination in aqueous solution based on fluorescence quenching of functionalised CdS QDs through photoinduced charge transfer process: spectroscopic approach

A novel method for the quantitative determination of ranitidine hydrochloride (RNH) based on the fluorescence quenching of functionalised CdS quantum dots (QDs) by RNH in aqueous solution was proposed. The method is simple, rapid, specific and highly sensitive with good precision. The thioglycolic acid (TGA)-capped CdS QDs were synthesized from cadmium nitrate and sodium sulfide in alkaline solution. Under the optimal conditions, the Stern-Volmer calibration plot of F(0)/F against concentration of RNH was linear in the range of 0.50-15.0 μg mL(-1) with a correlation coefficient of 0.996. The limit of detection (LOD) was 0.38 μg mL(-1). The method was satisfactorily applied to the direct determination of RNH in pharmaceutical formulations with no significant interference from excipients. The results were found to be in good agreement with those obtained by the reference method and the claimed value. The accuracy and reliability of the method were further ascertained by recovery studies via the standard-addition method, with percentage recoveries in the range of 98.47 to 102.30%. The possible fluorescence quenching mechanism for the reaction was also discussed.

Poisson-distributed electron-transfer dynamics from single quantum dots to C60 molecules

Functional quantum dot (QD)-based nanostructures are often constructed through the self-assembly of QDs with binding partners (molecules or other nanoparticles), a process that leads to a statistical distribution of the number of binding partners. Using single QD fluorescence spectroscopy, we probe this distribution and its effect on the function (electron-transfer dynamics) in QD-C60 complexes. Ensemble-averaged transient absorption and fluorescence decay as well as single QD fluorescence decay measurements show that the QD exciton emission was quenched by electron transfer from the QD to C60 molecules and the electron-transfer rate increases with the C60-to-QD ratio. The electron-transfer rate of single QD-C60 complexes fluctuates with time and varies among different QDs. The standard deviation increases linearly with the average of electron-transfer rates of single QD-C60 complexes, and the distributions of both quantities obey Poisson statistics. The observed distributions of single QD-C60 complexes and ensemble-averaged fluorescence decay kinetics can be described by a model that assumes a Poisson distribution of the number of adsorbed C60 molecules per QD. Our findings suggest that, in self-assembled QD nanostructures, the statistical distribution of the number of adsorbed partners can dominate the distributions of the averages and standard deviation of their interfacial dynamical properties.

Correlated Single Quantum Dot Blinking and Interfacial Electron Transfer Dynamics

The electron transfer (ET) dynamics from core/multi-shell (CdSe/CdS(3ML)ZnCdS(2ML)ZnS(2ML)) quantum dots (QDs) to adsorbed Fluorescein (F27) molecules have been studied by single particle spectroscopy to probe the relationship between single QD interfacial electron transfer and blinking dynamics. Electron transfer from the QD to F27 and the subsequent recombination were directly observed by ensemble-averaged transient absorption spectroscopy. Single QD-F27 complexes show correlated fluctuation of fluorescence intensity and lifetime, similar to those observed in free QDs. With increasing ET rate (controlled by F27-to-QD ratio), the lifetime of on states decreases and relative contribution of off states increases. It was shown that ET is active for QDs in on states, the excited state lifetime of which reflects the ET rate, whereas in the off state QD excitons decay by Auger relaxation and ET is not a competitive quenching pathway. Thus, the blinking dynamics of single QDs modulate their interfacial ET activity. Furthermore, interfacial ET provides an additional pathway for generating off states, leading to correlated single QD interfacial ET and blinking dynamics in QD-acceptor complexes. Because blinking is a general phenomenon of single QDs, it appears that the correlated interfacial ET and blinking and the resulting intermittent ET activity are general phenomena for single QDs.

Interaction of theory and experiment: examples from single molecule studies of nanoparticles

This article is in part the author's perspective on the revolution that has occurred in theoretical chemistry during the past half-century. In this period much of theoretical chemistry has moved from its initial emphasis on analytic treatments, resulting in equations for physical chemical and chemical phenomena, to the detailed computation of many different systems and processes. In the best sense the old and the new are complementary and their coexistence can benefit both. Experiment too has seen major developments. One of the newer types of experiment is that of single molecule studies. They range from those on small inorganic and organic nanoparticles to large biological species. We illustrate some of the issues that arise, using the topic of 'quantum dots' (QDs), and choosing a particular inorganic nanoparticle, CdSe, the most studied of these systems. Its study reflects the problems that arise in experiment and in theories in this field. The complementary nature of the conventional ensemble experiments and the new single molecule experiments is described and is illustrated by trajectories for the two types of experiments. The research in the QD field is both experimentally and theoretically a currently ongoing process, for which the answers are not fully known in spite of the large body of research. The detailed role of surface states is part of the problem. The field continues to yield new and unexpected results. In a sense this part of the article is an interim report that illustrates one analytic approach to the topic and where computer calculations and simulations can be expected to provide added insight.

Fluorescence intermittency of silicon nanocrystals and other quantum dots: a unified two-dimensional diffusion-controlled reaction model

We present a unified model involving two-dimensional diffusion-controlled reactions of both slow and fast reaction coordinates to elucidate the dynamic origin of fluorescence intermittency observed not just in quantum dots but also in organic chromphores and biomolecules. This improved model also solves the puzzling behavior of Si nanocrystals which display an unusually large m (exceeding 2) for the power-law decay of t(-m) and provides remedies for the deficiencies in existing models.

Blinking suppression and intensity recurrences in single CdSe-oligo(phenylene vinylene) nanostructures: experiment and kinetic model

We report time-resolved single molecule fluorescence imaging of individual CdSe quantum dots that are functionalized with oligomeric conjugated organic ligands. The fluorescence intensity trajectories from these composite nanostructures display both a strong degree of blinking suppression and intensity fluctuations with characteristic recurrence times on the order of 10-60 s. In addition, fluorescence decay rate measurements of individual hybrid nanostructures indicate significantly modified non-radiative quantum dot decay rates relative to conventional ZnS-capped CdSe quantum dots. We show that a modified diffusive reaction coordinate model with slow fluctuations in quantum dot electron energies (1S(e), 1P(e)) can reproduce the experimentally observed behaviour.

Size Effects and Breakdown of the Power-Law Blinking Statistics of CdSe Nanorods

In this study, the dependence of sample size and light intensity on the fluorescence intermittency of semiconductor nanorods is investigated. We present a model with diffusion-controlled electron-transfer reactions involving anomalous diffusion in energy configuration space. This model leads to a general formula t(-m) exp[-(Gammat)n] for the temporal behavior of blinking statistics, where m and n are related to the time dependence of the spectral diffusion. We reanalyze the experimental data of the long-time bending tail of CdSe nanorods and elucidate the size effects of the bending rates and activation energy.