Potentials and pitfalls of fluorescent quantum dots for biological imaging

Recent Applications of Quantum Dots in Pharmaceutical Analysis

Nanotechnology has emerged as one of the most potential areas for pharmaceutical analysis. The need for nanomaterials in pharmaceutical analysis is comprehended in terms of economic challenges, health and safety concerns. Quantum dots (QDs)or colloidal semiconductor nanocrystals are new groups of fluorescent nanoparticles that bind nanotechnology to drug analysis. Because of their special physicochemical characteristics and small size, QDs are thought to be promising candidates for the electrical and luminescent probes development. They were originally developed as luminescent biological labels, but are now discovering new analytical chemistry applications, where their photo-luminescent properties are used in pharmaceutical, clinical analysis, food quality control and environmental monitoring. In this review, we discuss QDs regarding properties and advantages, advances in methods of synthesis and their recent applications in drug analysis in the recent last years.

Simultaneous spectral differentiation of multiple fluorophores in super-resolution imaging using a glass phase plate

By engineering the point-spread function (PSF) of single molecules, different fluorophore species can be imaged simultaneously and distinguished by their unique PSF patterns. Here, we insert a silicon-dioxide phase plate at the Fourier plane of the detection path of a wide-field fluorescence microscope to produce distinguishable PSFs (X-PSFs) at different wavelengths. We demonstrate that the resulting PSFs can be localized spatially and spectrally using a maximum-likelihood estimation algorithm and can be utilized for hyper-spectral super-resolution microscopy of biological samples. We produced superresolution images of fixed U2OS cells using X-PSFs for dSTORM imaging with simultaneous illumination of up to three fluorophore species. The species were distinguished only by the PSF pattern. We achieved ∼21-nm lateral localization precision (FWHM) and ∼17-nm axial precision (FWHM) with an average of 1,800 - 3,500 photons per PSF and a background as high as 130 - 400 photons per pixel. The modified PSF distinguished fluorescent probes with ∼80 nm separation between spectral peaks.

Lateral flow assay applied to pesticides detection: recent trends and progress

Devices based on lateral flow assay (LFA) have been gaining more and more space in the detection market mainly due to their simplicity, speed, and low cost. These devices have excellent sensing format versatility and make these strips an ideal choice for field applications. The COVID-19 pandemic boosted the democratization of this method as a "point of care testing" (POCT), and the trend is that these devices become protagonists for the monitoring of pesticides in the environment. However, designing LFA devices for detecting and monitoring pesticides in the environment is still a challenge. This is because analytes are small molecules and have only one antigenic determinant, which makes it difficult to apply direct immunoassays. Furthermore, most LFA devices provide only qualitative or semi-quantitative results and have a limited number of applications in multi-residue analysis. Here, we present the state of the art on the use of LFA in the environmental monitoring of pesticides. Based on well-documented results, we review all available LFA formats and strategies for pesticide detection, which may have important implications for the future of monitoring pesticides in the environment. The main advances, challenges, and perspectives of these devices for a direction in this field of study are also presented.

Induction of autophagy and ER-phagy caused by CdTe-QDs are protective mechanisms of yeast cell

Quantum dots (QDs), with unique and tunable optical properties, have been are widely used in many fields closely related to our daily lives, such as biomedical application and electronic products. Therefore, the potential toxicity of QDs on the human health should be understood. Autophagy plays an important role in cell survival and death. Endoplasmic reticulum autophagy (ER-phagy), a selective autophagy that degrades ER, responds to the accumulation of misfolded proteins and ER stress. Although many reports have revealed that autophagy can be disturbed by CdTe-QDs and other nanomaterials, there are still lack more detail researches to illustrate the function of autophagy in CdTe-QDs treated cells. And the function of ER-phagy in CdTe-QDs-treated cells remains to be illustrated. On the basis of transcriptome analysis, we explored the effect of CdTe-QDs on Saccharomyces cerevisiae, and firstly illustrated that both of autophagy and ER-phagy were protective mechanisms in CdTe-QDs-treated cells. It was found that CdTe-QDs inhibited the proliferation of yeast cells, disrupted homeostasis of cells, membrane integrity and metabolism process. All of these can be reasons of the reduction of cell viability. The abolish of autophagy and ER-phagy reduce the cell survival, indicating both of them are cell protective mechanisms against CdTe-QDs toxicity in yeast cells. Therefore, our data are significant for the application of CdTe-QDs and provide precious information for understanding of nanomaterials-related ER-phagy.

Research progress in synthesis and biological application of quantum dots

Quantum dots are an excellent choice for biomedical applications due to their special optical properties and quantum confinement effects. This paper reviews the research and application progress of several quantum...

Enhanced Multiplexing of Immunofluorescence Microscopy Using a Long-Stokes-Shift Fluorophore

Immunofluorescence labeling and microscopy offer a highly specific means to visualize proteins or other molecular species in a sample by labeling target antigens with fluorescent probes. These fluorescent probes can then be visualized using a fluorescence microscope, allowing their relative spatial relationships to be determined. Due to spectral overlap of common fluorophores, however, it can be challenging to analyze more than three antigens in a single sample with standard imaging approaches. This article describes multiplexed labeling and imaging of four target antigens through the use of a long-Stokes-shift fluorophore-a fluorophore with an unusually large gap between its excitation and emission maxima-in tandem with three conventional fluorophores. This combination allows for multiplexed imaging of four antigens in a single sample with excellent spectral discrimination suitable for sensitive analyses using standard imaging hardware. Particular advantages of this approach are its flexibility in terms of target antigens and the lack of any specialized procedures, reagents, or equipment beyond the commercially available labeling reagent coupled to the long-Stokes-shift fluorophore. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Four-probe immunofluorescence labeling Basic Protocol 2: Four-probe immunofluorescence imaging.

Gene Expression and Transcriptome Profiling of Changes in a Cancer Cell Line Post-Exposure to Cadmium Telluride Quantum Dots: Possible Implications in Oncogenesis

Cadmium telluride quantum dots (CdTe-QDs) are acquiring great interest in terms of their applications in biomedical sciences. Despite earlier sporadic studies on possible oncogenic roles and anticancer properties of CdTe-QDs, there is limited information regarding the oncogenic potential of CdTe-QDs in cancer progression. Here, we investigated the oncogenic effects of CdTe-QDs on the gene expression profiles of Chang cancer cells. Chang cancer cells were treated with 2 different doses of CdTe-QDs (10 and 25 μg/ml) at different time intervals (6, 12, and 24 h). Functional annotations helped identify the gene expression profile in terms of its biological process, canonical pathways, and gene interaction networks activated. It was found that the gene expression profiles varied in a time and dose-dependent manner. Validation of transcriptional changes of several genes through quantitative PCR showed that several genes upregulated by CdTe-QD exposure were somewhat linked with oncogenesis. CdTe-QD-triggered functional pathways that appear to associate with gene expression, cell proliferation, migration, adhesion, cell-cycle progression, signal transduction, and metabolism. Overall, CdTe-QD exposure led to changes in the gene expression profiles of the Chang cancer cells, highlighting that this nanoparticle can further drive oncogenesis and cancer progression, a finding that indicates the merit of immediate in vivo investigation.

Quantum Dots for Improved Single-Molecule Localization Microscopy

Colloidal semiconductor quantum dots (QDs) have long established their versatility and utility for the visualization of biological interactions. On the single-particle level, QDs have demonstrated superior photophysical properties compared to organic dye molecules or fluorescent proteins, but it remains an open question as to which of these fundamental characteristics are most significant with respect to the performance of QDs for imaging beyond the diffraction limit. Here, we demonstrate significant enhancement in achievable localization precision in QD-labeled neurons compared to neurons labeled with an organic fluorophore. Additionally, we identify key photophysical parameters of QDs responsible for this enhancement and compare these parameters to reported values for commonly used fluorophores for super-resolution imaging.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Stereoselective C-C Oxidative Coupling Reactions Photocatalyzed by Zwitterionic Ligand Capped CsPbBr3 Perovskite Quantum Dots

Semiconductor quantum dots (QDs) have attracted tremendous attention in the field of photocatalysis, owing to their superior optoelectronic properties for photocatalytic reactions, including high absorption coefficients and long photogenerated carrier lifetimes. Herein, by choosing 2-(3,4-dimethoxyphenyl)-3-oxobutanenitrile as a model substrate, we demonstrate that the stereoselective (>99 %) C-C oxidative coupling reaction can be realized with a high product yield (99 %) using zwitterionic ligand capped CsPbBr₃ perovskite QDs under visible light illumination. The reaction can be generalized to different starting materials with various substituents on the phenyl ring and varied functional moieties, producing stereoselective dl-isomers. A radical mediated reaction pathway has been proposed. Our study provides a new way of stereoselective C-C oxidative coupling via a photocatalytic means using specially designed perovskite QDs.

Nanomedicine and Early Cancer Diagnosis: Molecular Imaging using Fluorescence Nanoparticles

Incorporating nanotechnology into fluorescent imaging and magnetic resonance imaging (MRI) has shown promising potential for accurate diagnosis of cancer at an earlier stage than the conventional imaging modalities. Molecular imaging (MI) aims to quantitatively characterize, visualize, and measure the biological processes or living cells at molecular and genetic levels. MI modalities have been exploited in different applications including noninvasive determination and visualization of diseased tissues, cell trafficking visualization, early detection, treatment response monitoring, and in vivo visualization of living cells. High-affinity molecular probe and imaging modality to detect the probe are the two main requirements of MI. Recent advances in nanotechnology and allied modalities have facilitated the use of nanoparticles (NPs) as MI probes. Within the extensive group of NPs, fluorescent NPs play a prominent role in optical molecular imaging. The fluorescent NPs used in molecular and cellular imaging can be categorized into three main groups including quantum dots (QDs), upconversion, and dyedoped NPs. Fluorescent NPs have great potential in targeted theranostics including cancer imaging, immunoassay- based cells, proteins and bacteria detections, imaging-guided surgery, and therapy. Fluorescent NPs have shown promising potentials for drug and gene delivery, detection of the chromosomal abnormalities, labeling of DNA, and visualizing DNA replication dynamics. Multifunctional NPs have been successfully used in a single theranostic modality integrating diagnosis and therapy. The unique characteristics of multifunctional NPs make them potential theranostic agents that can be utilized concurrently for diagnosis and therapy. This review provides the state of the art of the applications of nanotechnologies in early cancer diagnosis focusing on fluorescent NPs, their synthesis methods, and perspectives in clinical theranostics.

Carbon dots based ratiometric fluorescent sensing platform for food safety

Food safety has become a major global concern and the rapid detection of food nutritional ingredients and contaminants has aroused much more attention. Nanomaterials-based fluorescent sensing holds great potential in designing highly sensitive and selective detection strategies for food safety analysis. Carbon dots (CDs) possess tremendous prospects in fluorescent sensing food ingredients and contaminants due to their superior properties of chemical and photostability, highly fluorescence with tunability, and no/low-toxicity. Numerous endeavors are demanded to contribute to overcoming the challenge of lower sensitivity and selectivity of the sensors interfered by various components in intricate food matrices to ensure food safety and human health. Nanohybrid CDs based ratiometric fluorescent sensing with self-calibration is regarded as an efficient strategy for the CDs based sensors for the specific recognition of target analyte in the food matrices. This work is devoted to reviewing the development of nanohybrid CDs based ratiometric fluorescent sensing platform and the perspectives of the platform for food safety. The applications of nanohybrid CDs in sensing are summarized and the sensing mechanisms are briefly discussed.

One-pot synthesis of cerium and praseodymium co-doped carbon quantum dots as enhanced antioxidant for hydroxyl radical scavenging

The antioxidant activity of ceria nanoparticles is tightly regulated by size distribution and heteroatom doping. Inspired by this rule, cerium and praseodymium codoped carbon quantum dots (Ce/Pr-CQDs) were synthesized through the one-pot hydrothermal carbonization method. Taking intrinsic advantage of CQDs, the resultant Ce/Pr-CQDs exhibited uniform and ultra-small morphology with an average size of 2.8 nm, which led to an increased proportion of Ce³⁺. In addition, the doping of Pr into Ce-CQDs improved the redox properties. As we expected, the Ce/Pr-CQDs possessed enhanced hydroxyl radical scavenging properties compared with the cerium-doped carbon quantum dots (Ce-CQDs). Furthermore, Ce/Pr-CQDs with favorable biocompatibility and negligible cytotoxicity are readily internalized into cytoplasm, decreasing the level of reactive oxygen species (ROS). Taken together, the resultant Ce/Pr-CQDs displayed great potential for applications relating to oxidative-stress-associated disease.

Immunochromatographic System for Serodiagnostics of Cattle Brucellosis Using Gold Nanoparticles and Signal Amplification with Quantum Dots

In this article, we describe an immunochromatographic test system developed for rapid serodiagnostics of cattle brucellosis using two markers: Gold nanoparticles (GNPs) and quantum dots (QDs). The test system was compared with immunochromatographic serodiagnostics systems that use only one marker. The approbation of the test system was conducted on samples of cattle sera with low, but diagnostically significant titers of specific antibodies. We show that when two conjugates are used, the intensity of the detectable signal increases by 2–3 times compared with the test system using the QD conjugate and by more than nine times compared with the system using the GNP conjugate.

CdTe QDs based fluorescent sensor for the determination of gallic acid in tea

A new fluorescent light switch method, which based on N‑acetyl‑l‑cysteine capped CdTe QDs (NALC-CdTe QDs), was developed for the detection of gallic acid (GA). The QDs possess a fluorescence emission wavelength at 520nm and with symmetric fluorescence. When KMnO₄ is added, the high fluorescence of QDs could be effectively quenched for the electron transfer process between KMnO₄ and QDs. But with the addition of GA, the fluorescence of KMnO₄-QDs system could recover for the reason that redox reaction of GA and KMnO₄. Therefore, a fluorescent light switch method could be used for GA with a detection range of 0.6-12.6μg·mL^-1 and a detection limit of 0.56ng·mL^-1. Furthermore, the feasibility of the proposed fluorescence biosensor in tea was also studied and satisfactory results were obtained.

Quantum dot conjugated saporin activates microglia and induces selective substantia nigra degeneration

Parkinson's disease (PD) is characterized by profound microglial driven inflammatory processes and the loss of dopamine neurons of the substantia nigra (SNc). Both microglia and dopamine neurons that are affected in the SNc are particularly vulnerable to environmental toxicants and finding more selective ways of targeting these cell types is of importance. Quantum dots (QDs) might be a useful vehicle for selectively delivering toxicants to microglia and owing to their fluorescent capability, they can be microscopically tracked within the cell. Accordingly, we assessed the impact of QDs alone and QDs conjugated to the ribosomal toxin, saporin, upon SNc microglia and dopamine neurons. We found that intra-SNc infused QDs selectively entered microglia and induced morphological changes consistent with an activated state. QDs conjugated to saporin also caused a significant loss of dopamine neurons and motor coordination (on a rotarod test) deficits, along with an increase in the inflammatory microglial actin regulatory factors, WAVE2. These data suggest that QDs might be a viable route for toxicant delivery and also has an added advantage of being fluorescently visible. Ultimately, we found SNc neurons to be exceptionally vulnerable to QD-saporin and suggest that this could be a novel targeted approach to model PD-like inflammatory pathology.

Developments in single-molecule and single-particle fluorescence-based approaches for studying viral envelope glycoprotein dynamics and membrane fusion

Fusion of viral and cellular membranes is an essential step in the entry pathway of all enveloped viruses. This is a dynamic and multistep process, which has been extensively studied, resulting in the endpoints of the reaction being firmly established, and many essential cellular factors identified. What remains is to elucidate the dynamic events that underlie this process, including the order and timing of glycoprotein conformational changes, receptor-binding events, and movement of the glycoprotein on the surface of the virion. Due to the inherently asynchronous nature of these dynamics, there has been an increased focus on the study of single virions and single molecules. These techniques provide researchers the high precision and resolution necessary to bridge the gaps in our understanding of viral membrane fusion. This review highlights the advancement of single-molecule and single-particle fluorescence-based techniques, with a specific focus on how these techniques have been used to study the dynamic nature of the viral fusion pathway.

Quantitative Detection and Real-Time Monitoring of Endogenous mRNA at the Single Live Cell Level Using a Ratiometric Molecular Beacon

Messenger ribonucleic acid (mRNA) plays an important role in various cellular processes. however, traditional techniques cannot realize mRNA detections in live cells as they rely on mRNA purification or cell fixation. To achieve real-time and quantitative mRNA detections at a single live cell level, a single-strand stem-loop-structured ratiometric molecular beacon (RMB) composed of the phosphorothioate-modified loop domain on the 2'-O-methyl RNA backbone with a reporter dye, quencher, and reference dye is proposed to detect the Hsp27 mRNA as a modeled endogenous mRNA. When the RMB hybridizes with the target, the stem-loop structure opens, causing separation of the reporter dye and the quencher and restores the reporter fluorescent signals; therefore, the Hsp27 mRNA can be quantitatively detected according to the ratio of the reporter fluorescent signal to the reference fluorescent signal. Both the phosphorothioate and 2'-O-methyl RNA modifications obviously reduce the nonspecific opening, and the additional reference dye ensures the detection precision using co-localization analysis. Not only does this remove the false-positive signal caused by the nuclease degradation-generated RMB fragment, but it also corrects variations caused by direct measurement of reporter fluorescence intensities at a single cell level owing to inhomogeneity in probe delivery. The designed RMB could detect the Hsp27 mRNA with high signal-to-noise ratio and sensitivity as well as excellent specificity and antidegradation capability proved in vitro and in live cells. Furthermore, it was successfully adopted in subcellular localization, quantitative copy number measurements, and even real-time monitoring of Hsp27 mRNA in live cells, demonstrating that the proposed RMB can be a potential quantitative endogenous mRNA detection tool, especially at a single live cell level.

Study on intracellular delivery of liposome encapsulated quantum dots using advanced fluorescence microscopy

Quantum dots increasingly gain popularity for in vivo applications. However, their delivery and accumulation into cells can be challenging and there is still lack of detailed information. Thereby, the application of advanced fluorescence techniques can expand the portfolio of useful parameters for a more comprehensive evaluation. Here, we encapsulated hydrophilic quantum dots into liposomes for studying cellular uptake of these so-called lipodots into living cells. First, we investigated photophysical properties of free quantum dots and lipodots observing changes in the fluorescence decay time and translational diffusion behaviour. In comparison to empty liposomes, lipodots exhibited an altered zeta potential, whereas their hydrodynamic size did not change. Fluorescence lifetime imaging microscopy (FLIM) and fluorescence correlation spectroscopy (FCS), both combined with two-photon excitation (2P), were used to investigate the interaction behaviour of lipodots with an insect epithelial tissue. In contrast to the application of free quantum dots, their successful delivery into the cytosol of salivary gland duct cells could be observed when applying lipodots. Lipodots with different lipid compositions and surface charges did not result in considerable differences in the intracellular labelling pattern, luminescence decay time and diffusion behaviour. However, quantum dot degradation after intracellular accumulation could be assumed from reduced luminescence decay times and blue-shifted luminescence signals. In addition to single diffusing quantum dots, possible intracellular clustering of quantum dots could be assumed from increased diffusion times. Thus, by using a simple and manageable liposome carrier system, 2P-FLIM and 2P-FCS recording protocols could be tested, which are promising for investigating the fate of quantum dots during cellular interaction.

Designed Synthesis of Dual Emitting Silicon Quantum Dot for Cell Imaging: Direct Labeling of Alpha 2-HS-Glycoprotein

The innocent silicon quantum dots (SQDs) having dual emissive property (blue in VIS and red in NIR), high photostability, and freedom from auto fluorescence are designed and synthesized for the first time using ethylene glycol. A new attempt has been made for direct labeling of Alpha 2-HS-Glycoprotein (Fetuin A) through functionalization of the synthesized dots by EDC coupling. The SQDs were characterized by FTIR, TEM, AFM, XRD, EDX, DLS, and TGA. The chemistry involved in the synthesis and functionalization of dots is elucidated in detail. The synthesized SQDs are suitable for live cell imaging as well as direct labeling of the Fetuin A in the NIR region. The direct labeling technique developed for Fetuin A imaging is robust, more specific, and simple, and reduces the number of incubation and washing steps and produces better quality data compared to the conventional method using Rhodamine B.

Nanoparticles at Biomimetic Interfaces: Combined Experimental and Simulation Study on Charged Gold Nanoparticles/Lipid Bilayer Interfaces

The poor understanding of the interaction of nanomaterials with biologically relevant interfaces is recognized as one of the major issues currently limiting the development of nanomedicine. The central purpose of this study is to compare experimental (confocal microscopy, fluorescence correlation spectroscopy, X-ray reflectivity) and computational (molecular dynamics simulations) results to thoroughly describe the interaction of cationic gold nanoparticles (AuNPs) with mixed zwitterionic/anionic lipid membranes. The adhesion of AuNPs to the lipid membrane is investigated on different length scales from a structural and dynamical point of view; with this approach, a series of complex phenomena, spanning from lipid extraction, localized membrane disruption, lateral phase separation, and slaved diffusion, are characterized and interpreted from a molecular level to macroscopic observations.

Carbon Nanomaterials in Optical Detection

Owing to their unique optical, electronic, mechanical, and chemical properties, flexible chemical modification, large surface coverage and ready cellular uptake, various carbon nanomaterials such as carbon nanotubes (CNTs), graphene and its derivatives, carbon dots (CDs), graphene quantum dots, fullerenes, carbon nanohorns (CNHs) and carbon nano-onions (CNOs), have been widely explored for use in optical detection. Most of them are based on fluorescence changes. In this chapter, we will focus on carbon nanomaterials-based optical detection applications, mainly including fluorescence sensing and bio-imaging. Moreover, perspectives on future exploration of carbon nanomaterials for optical detection are also given.

Silicon Quantum Dot-Based Fluorescent Probe: Synthesis Characterization and Recognition of Thiocyanate in Human Blood

Allylamine-functionalized silicon quantum dots (ASQDs) of high photostability are synthesized by a robust inverse micelle method to use the material as a fluorescent probe for selective recognition of thiocyanate (a biomarker of a smoker and a nonsmoker). The synthesized ASQDs were characterized by absorption, emission, and Fourier transform infrared spectroscopy. Surface morphology is studied by transmission electron microscopy and dynamic light scattering. The synthesized material exhibits desirable fluorescence behavior with a high quantum yield. A selective and accurate (up to 10-10 M) method of sensing of thiocyanate anion is developed based on fluorescence amplification and quenching of ASQDs. The sensing mechanism is investigated and interpreted with a crystal clear mechanistic approach through the modified Stern-Volmer plot. The developed material and the method is applied to recognize the anion in the human blood sample for identification of the degree of smoking. The material deserves high potentiality in the field of bio-medical science.

Stable, small, specific, low-valency quantum dots for single-molecule imaging

We have developed a strategy for synthesizing immediately activable, water-soluble, compact (∼10-12 nm hydrodynamic diameter) quantum dots with a small number of stable and controllable conjugation handles for long distance delivery and subsequent biomolecule conjugation. Upon covalent conjugation with engineered monovalent streptavidin, the sample results in a population consisting of low-valency quantum dots. Alternatively, we have synthesized quantum dots with a small number of biotin molecules that can self-assemble with engineered divalent streptavidin via high-affinity biotin-streptavidin interactions. Being compact, stable and highly specific against biotinylated proteins of interest, these low-valency quantum dots are ideal for labeling and tracking single molecules on the cell surface with high spatiotemporal resolution for different biological systems and applications.

Efficient Quantum-Dot Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence Emitters as Exciton Harvesters

Utilization of triplet excitons plays a key role in obtaining highly efficient quantum-dot light-emitting diodes (QD-LEDs). However, to date, only phosphorescent materials have been implemented to harvest triplet excitons in QD-LEDs. In this work, we introduced a thermally activated delayed fluorescence (TADF) emitter, 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), doped into poly(N-vinylcarbazole) (PVK) as an exciton harvester in red QD-LEDs by solution processing. As a result, electrons leaking to the PVK layer will be trapped by 2CzPN to form long-lifetime TADF excitons in the 2CzPN:PVK layer, and then this harvested exciton energy can be effectively transferred to the adjacent QDs by the Förster resonance energy-transfer process. The fabricated red CdSe/CdS core/shell QD-LEDs show a maximum luminescence efficiency of 17.33 cd/A and longer lifetime. Our results demonstrate that the TADF sensitizer would be a promising candidate to develop highly efficient QD-LEDs.

An intelligent and biocompatible photosensitizer conjugated silicon quantum dots–MnO2 nanosystem for fluorescence imaging-guided efficient photodynamic therapy

We constructed an intelligent and biocompatible BSA–Ce6–Si QDs–MnO₂ nanocomplex as a pH/H₂O₂ responsive photosensitizer nanocarrier for fluorescence imaging-guided photodynamic therapy (PDT).

Morpholine Derivative-Functionalized Carbon Dots-Based Fluorescent Probe for Highly Selective Lysosomal Imaging in Living Cells

The development of a suitable fluorescent probe for the specific labeling and imaging of lysosomes through the direct visual fluorescent signal is extremely important for understanding the dysfunction of lysosomes, which might induce various pathologies, including neurodegenerative diseases, cancer, and Alzheimer's disease. Herein, a new carbon dot-based fluorescent probe (CDs-PEI-ML) was designed and synthesized for highly selective imaging of lysosomes in live cells. In this probe, PEI (polyethylenimine) is introduced to improve water solubility and provide abundant amine groups for the as-prepared CDs-PEI, and the morpholine group (ML) serves as a targeting unit for lysosomes. More importantly, passivation with PEI could dramatically increase the fluorescence quantum yield of CDs-PEI-ML as well as their stability in fluorescence emission under different excitation wavelength. Consequently, experimental data demonstrated that the target probe CDs-PEI-ML has low cytotoxicity and excellent photostability. Additionally, further live cell imaging experiment indicated that CDs-PEI-ML is a highly selective fluorescent probe for lysosomes. We speculate the mechanism for selective staining of lysosomes that CDs-PEI-ML was initially taken up by lysosomes through the endocytic pathway and then accumulated in acidic lysosomes. It is notable that there was less diffusion of CDs-PEI-ML into cytoplasm, which could be ascribed to the presence of lysosome target group morpholine on surface of CDs-PEI-ML. The blue emission wavelength combined with the high photo stability and ability of long-lasting cell imaging makes CDs-PEI-ML become an alternative fluorescent probe for multicolor labeling and long-term tracking of lysosomes in live cells and the potential application in super-resolution imaging. To best of our knowledge, there are still limited carbon dots-based fluorescent probes that have been studied for specific lysosomal imaging in live cells. The concept of surface functionality of carbon dots will also pave a new avenue for developing carbon dots-based fluorescent probes for subcellular labeling.

Simultaneous Gene Delivery and Tracking through Preparation of Photo-Luminescent Nanoparticles Based on Graphene Quantum Dots and Chimeric Peptides

Designing suitable nano-carriers for simultaneous gene delivery and tracking is in the research priorities of the molecular medicine. Non-toxic graphene quantum dots (GQDs) with two different (green and red) emission colors are synthesized by Hummer's method and characterized by UV-Vis, Photoluminescence (PL), Fourier Transform Infrared (FTIR) and Raman spectroscopies, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The GQDs are conjugated with MPG-2H1 chimeric peptide and plasmid DNA (pDNA) by non-covalent interactions. Following conjugation, the average diameter of the prepared GQDs increased from 80 nm to 280 nm in complex structure, and the ζ-potential of the complex increased (from -36.87 to -2.56 mV). High transfection efficiency of the nano-carrier and results of confocal microscopy demonstrated that our construct can be considered as a nontoxic carrier with dual functions for gene delivery and nuclear targeting.

Adhesion of CdTe quantum dots on model membranes and internalization into RBL-2H3 cells

Quantum dots (QDs) have attracted broad attention due to their special optical properties and promising prospect in medical and biological applications. However, the process of QDs on cell membrane is worth further investigations because such process may lead to harmful effects on organisms and also important for QD application. In this study, adhesion of amino- and carboxyl-coated CdTe QDs (A-QDs and C-QDs) on cell membrane and the subsequent internalization are studied using a series of endocytosis-free model membranes, including giant and small unilamellar vesicles, supported lipid bilayers and giant plasma membrane vesicles (GPMVs). The adhered QD amounts on model membranes are quantified by a quartz crystal microbalance. The CdTe QD adhesion on model membranes is governed by electrostatic forces. Positively charged A-QDs adhere on GPMV surface and passively penetrate the plasma membrane via endocytosis-free mechanism, but negatively charged C-QDs cannot. Rat basophilic leukemia (RBL-2H3) cells are exposed to CdTe QDs to monitor the QD internalization process. Both A- and C-QDs are internalized by RBL-2H3 cells mainly via endocytosis. CdTe QDs do not accumulate on the plasma membrane of living cells due to the fast endocytosis and the weakened electrostatic attraction in biological medium, resulting in low chance of passive penetration. The suspended cells after trypsin digestion take more QDs than the adherent cells. A-QDs cause lower cell viability than C-QDs, probably because the approach of positively charged QDs to cells is favored and the smaller aggregates of A-QDs.

A novel detection method of human serum albumin based on CuInZnS quantum dots-Co2+ sensing system

We developed a novel "turn off-on" sensor for human serum albumin (HSA) detection based on CuInZnS quantum dots (CIZS QDs). The photoluminescence (PL) of QDs can be "turned off" by Co(II) first. Because of the strong binding ability of HSA with Co²⁺, Co²⁺ can be removed from CIZS QDs with the addition of HSA. As a result, the PL of CIZS QDs probe can be "turned on" with an increased concentration of HSA over a wide range. The analyte HSA concentration had a proportional linear relationship with the recovered PL intensity of CIZS QDs. The detection limit for HSA was 4.5 × 10^-8 mol L^-1. The results indicated that the CIZS QDs- Co²⁺-BSA sensing system possessed higher sensitivity and better practicability for HSA detection.

PET Cell Tracking Using 18F-FLT is Not Limited by Local Reuptake of Free Radiotracer

Assessing the retention of cell therapies following implantation is vital and often achieved by labelling cells with 2'-[¹⁸F]-fluoro-2'-deoxy-D-glucose (¹⁸F-FDG). However, this approach is limited by local retention of cell-effluxed radiotracer. Here, in a preclinical model of critical limb ischemia, we assessed a novel method of cell tracking using 3'-deoxy-3'-L-[¹⁸F]-fluorothymidine (¹⁸F-FLT); a clinically available radiotracer which we hypothesise will result in minimal local radiotracer reuptake and allow a more accurate estimation of cell retention. Human endothelial cells (HUVECs) were incubated with ¹⁸F-FDG or ¹⁸F-FLT and cell characteristics were evaluated. Dynamic positron emission tomography (PET) images were acquired post-injection of free ¹⁸F-FDG/¹⁸F-FLT or ¹⁸F-FDG/¹⁸F-FLT-labelled HUVECs, following the surgical induction of mouse hind-limb ischemia. In vitro, radiotracer incorporation and efflux was similar with no effect on cell viability, function or proliferation under optimised conditions (5 MBq/mL, 60 min). Injection of free radiotracer demonstrated a faster clearance of ¹⁸F-FLT from the injection site vs. ¹⁸F-FDG (p ≤ 0.001), indicating local cellular uptake. Using ¹⁸F-FLT-labelling, estimation of HUVEC retention within the engraftment site 4 hr post-administration was 24.5 ± 3.2%. PET cell tracking using ¹⁸F-FLT labelling is an improved approach vs. ¹⁸F-FDG as it is not susceptible to local host cell reuptake, resulting in a more accurate estimation of cell retention.

In vivo biodistribution and behavior of CdTe/ZnS quantum dots

The unique features of quantum dots (QDs) make them desirable fluorescent tags for cell and developmental biology applications that require long-term, multitarget, and highly sensitive imaging. In this work, we imaged fluorescent cadmium telluride/zinc sulfide (CdTe/ZnS) QDs in organs, tissues, and cells, and analyzed the mechanism of their lymphatic uptake and cellular distribution. We observed that the fluorescent CdTe/ZnS QDs were internalized by lymph nodes in four cell lines from different tissue sources. We obtained the fluorescence intensity–QD concentrations curve by quantitative analysis. Our results demonstrate that cells containing QDs can complete mitosis normally and that distribution of QDs was uniform across cell types and involved the vesicular transport system, including the endoplasmic reticulum. This capacity for CdTe/ZnS QD targeting provides insights into the applicability and limitations of fluorescent QDs for imaging biological specimens.