High‐resolution quantification by charge‐dominant electrophoretic mobility shift of quantum dots

Conjugation of biomolecules to colloidal nanoparticles, such as quantum dots (QDs), often leads to change in mobility. We discover that linking DNA molecules to quantum dots alters their surface charge density without significantly increasing the hydrodynamic radius, causing a prominent shift in electrophoretic mobility. In this study, a high‐resolution molecular quantification method named quantification by QDs electrophoretic mobility shift (qQEMS) is developed based on the charge‐dominant transformation that closely associates DNA quantity to QDs electrophoretic mobility. The versatility of qQEMS is demonstrated by a number of quantification assays in which DNA molecules functioned as enzyme substrates, target‐specific probes, and competitive charge carriers. qQEMS shows a great potential as a generic and versatile quantification platform for a wide range of applications.     

Capillary electrophoretic separation and characterizations of CdSe quantum dots

We have developed a capillary electrophoresis method to characterize the QD surface ligand interactions with various surfactant systems. The method was demonstrated with 2–5 nm CdSe nanoparticles surface-passivated with trioctylphosphine oxide (TOPO). Water solubility was accomplished by surfactant-assisted phase transfer via an oil-in-water microemulsion using either cationic, anionic, or non-ionic surfactants. Interaction between the QD surface ligand (TOPO) and the alkyl chain of the surfactant molecule produces a complex and dynamic surface coating that can be characterized through manipulation of CE separation buffer composition and capillary surface modification. Additional characterization of the QD surface ligand interactions with surfactants was accomplished by UV-VIS spectroscopy, photoluminescence, and TEM. It is anticipated that studies such as these will elucidate the dynamics of QD surface ligand modifications for use in sensors.      

Biomimetic polymersomes as carriers for hydrophilic quantum dots

For polymersomes to achieve their potential as effective delivery vehicles, they must efficiently encapsulate therapeutic agents into either the aqueous interior or the hydrophobic membrane. In this study, cell membrane-mimetic polymersomes were prepared from amphiphilic poly(D,L-lactide)-b-poly(2-methacryloyloxyethylphosphorylcholine) (PLA-b-PMPC) diblock copolymers and were used as encapsulation devices for water-soluble molecules. Thioalkylated zwitterionic phosphorylcholine protected quantum dots (PC@QDs) were chosen as hydrophilic model substrates and successfully encapsulated into the aqueous polymersome interior, as evidenced by transmission electron microscopy (TEM) and flow cytometry. In addition, we also found a fraction of the PC@QDs were bound to both the external and internal surfaces of the polymersome. This interesting immobilization might be due to the ion-pair interactions between the phosphorylcholine groups on the PC@QDs and polymersomes. The experimental encapsulation results support a mechanism of PLA-b-PMPC polymersome formation in which PLA-b-PMPC copolymer chains first form spherical micelles, then worm-like micelles, and finally disk-like micelles which close up to form polymersomes.     

Synthesis of carbon quantum dots for DNA labeling and its electrochemical,fluorescent and electrophoretic characterization

Nanoparticles as a progressively developing branch offer a tool for studying the interaction of carbon quantum dots (CQDs) with DNA. In this study, fluorescent CQDs were synthesized using citric acid covered with polyethylene glycol (PEG) as the source of carbon precursors. Furthermore, interactions between CQDs and DNA (double-stranded DNA and single-stranded DNA) were investigated by spectral methods, gel electrophoresis, and electrochemical analysis. Primarily, the fluorescent behavior of CQDs in the presence of DNA was monitored and major differences in the interaction of CQDs with tested single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) were observed at different amounts of CQDs (µg mL^?1: 25, 50, 100, 250, 500). It was found that the interaction of ssDNA with CQDs had no significant influence on the CQDs fluorescence intensity measured at the excitation wavelengths of 280 nm, 350 nm, and 400 nm. However, in the presence of dsDNA, the fluorescence intensity of CQDs was significantly increased. Our results provide basic understanding of the interaction between CQDs and DNA. Such fabricated CQDs-DNA might be of great benefit for the emerging nanomaterials based biosensing methods.     

Flexible quantum dot sensitized solar cell by electrophoretic deposition of CdSe quantum dots on ZnO nanorods

We report on a flexible quantum dot-sensitized solar cell (QDSSC) based on ZnO nanorods with a length of 2 μm. Due to the good coverage of CdSe QDs on ZnO by the electrophoretic deposition method, a maximum power conversion efficiency of ～1% is achieved for the flexible QDSSC.     

One-bath synthesis of hydrophilic molecularly imprinted quantum dots for selective recognition of chlorophenol

A simple one-bath strategy has been developed to synthesize a novel CdTe@SiO₂@MIP（molecularly imprinted and silica-functionalized CdTe quantum dots,MISFQDs）,in which a silica shell was coated on the surface of CdTe quantum dots （CdTe@SiO₂ QDs） and then a polymer for selective recognition of 4-chlorophenol（4-CP） was constructed on the surface of CdTe@SiO₂ QDs using mercaptoacetic acid as stabilizer,3-aminopropyl-trimethoxysilane（APTES） as functional monomers and tetraethoxysilane（TEOS） as crosslink agent.The structures of CdTe@SiO₂@MIP were analyzed by ultraviolet-visible absorption. Fluorescence,FT-IR spectrum and powder X-ray diffraction.The application and characterization of the CdTe@SiO₂@MIP were investigated by experiments.All results indicated that the CdTe@SiO₂@MIP can selectively recognize 4-chlorophenol.     

Photoinduced enhancement in the luminescence of hydrophilic quantum dots coated with photocleavable ligands

In search of strategies to photoactivate the luminescence of semiconductor quantum dots, we devised a synthetic approach to attach photocleavable 2-nitrobenzyl groups to CdSe-ZnS core-shell quantum dots coated with hydrophilic polymeric ligands. The emission intensity of the resulting nanostructured constructs increases by more than 60% with the photolysis of the 2-nitrobenzyl appendages. Indeed, the photoinduced separation of the organic chromophores from the inorganic nanoparticles suppresses an electron-transfer pathway from the latter to the former and is mostly responsible for the luminescence enhancement. However, the thiol groups anchoring the polymeric envelope to the ZnS shell also contribute to the photoinduced emission increase. Presumably, their photooxidation eliminates defects on the nanoparticle surface and promotes the radiative deactivation of the excited quantum dots. This effect is fully reversible but its magnitude is only a fraction of the change caused by the photocleavage of the 2-nitrobenzyl groups. In addition, these particular quantum dots can cross the membrane of model cells and their luminescence increases by ~80% after the intracellular photocleavage of the 2-nitrobenzyl quenchers. Thus, photoswitchable luminescent constructs with biocompatible character can be assembled combining the established photochemistry of the 2-nitrobenzyl photocage with the outstanding photophysical properties of semiconductor quantum dots and the hydrophilic character of appropriate polymeric ligands.     

Capillary electrophoretic studies on quantum dots and histidine appended peptides self‐assembly

Herein, we designed four peptides appended with different numbers of histidine (His_n‐peptide). We launched a systematic investigation on quantum dots (QDs) and His_n‐peptide self‐assembly in solution using fluorescence coupled CE (CE‐FL). The results indicated that CE‐FL was a powerful method to probe how ligands interaction on the surface of nanoparticles. The self‐assembly of QDs and peptide was determined by the numbers of histidine. We also observed that longer polyhistidine tags (n ≤ 6) could improve the self‐assembly efficiency. Furthermore, the formation and separation of QD‐peptide assembly were also studied by CE‐FL inside a capillary. The total time for the mixing, self‐assembly, separation, and detection was less than 10 min. Our method greatly expands the application of CE‐FL in QDs‐based biolabeling and bioanalysis.     

Capillary electrophoretic studies on displacement and proteolytic cleavage of surface bound oligohistidine peptide on quantum dots

Subtle changes in the chemical structure or the composition of surface bound ligands on quantum dots (QDs) remain difficult to detect. Here we describe a facile setup for fluorescence detection coupled capillary electrophoresis (CE-FL) and its application in monitoring ligand displacement on QDs through metal-affinity driven assembly. We also describe the use of CE-FL to monitor amide bond cleavage by a specific protease, based on Förster resonance energy transfer (FRET) between Cy5 and QDs spaced by a hexahistidine peptide (H6–Cy5). CE-FL allowed separation of unbound QDs and ligand bound QDs and also revealed an ordered assembly of H6–Cy5 on QDs. In a ligand displacement experiment, unlabeled hexahistidine peptide gradually displaced surface bound H6–Cy5 until finally reaching equilibrium. The displacement intermediates were clearly separated on CE-FL. Proteolytic cleavage of surface bound H6–Cy5 by thrombin was monitored by CE-FL through mobility shift, peak broadening, and FRET changes. Enzymatic parameters thus obtained were comparable with those measured by fluorescence spectroscopy.     

Enhancing the stability and biological functionalities of quantum dots via compact multifunctional ligands

We have designed and synthesized a series of modular ligands based on poly(ethylene glycol) (PEG) coupled with functional terminal groups to promote water-solubility and biocompatibility of quantum dots (QDs). Each ligand is comprised of three modules: a PEG single chain to promote hydrophilicity, a dihydrolipoic acid (DHLA) unit connected to one end of the PEG chain for strong anchoring onto the QD surface, and a potential biological functional group (biotin, carboxyl, and amine) at the other end of the PEG. Water-soluble QDs capped with these functional ligands were prepared via cap exchange with the native hydrophobic caps. Homogeneous QD solutions that are stable over extended periods of time and over a broad pH range were prepared. Surface binding assays and cellular internalization and imaging showed that QDs capped with DHLA-PEG-biotin strongly interacted with either NeutrAvidin immobilized on surfaces or streptavidin coupled to proteins which were subsequently taken up by live cells. EDC coupling in aqueous buffer solutions was also demonstrated using resonance energy transfer between DHLA-PEG-COOH-functionalized QDs and an amine-terminated dye. The new functional surface ligands described here provide not only stable and highly water-soluble QDs but also simple and easy access to various biological entities.     

Measurement of electrophoretic mobility of cardiomyocytes

The electrophoretic mobility (EPM) of rat cardiomyocytes with or without the treatment of neuraminidase was studied by cell electrophoresis. The EPM was found to change over a range from 0 to 8.67 μm s^?1/V cm^?1, depending on ionic strength, transmembrane potential, pH value, and/or surface charges. It is interesting that zero EPM was observed but reverse of the mobility was not. These results suggested that the negative charges carried on the cardiomyocyte surface might comprehensively consist of surface sialic acid, plasmalemma proteins, phospholipids, and transmembrane potential. The aberrant electrical double layer formed between the carried negative charges and adions had a big adsorption layer and a diffusion layer whose sizes changed circularly, making only negative charges be carried on the surface of living cardiomyocytes. The special structures on the surface of cardiomyocytes probably play a considerable role in the process of cardiac electrical activity.     

Comparative examination of the stability of semiconductor quantum dots in various biochemical buffers

Due to their greater photostability compared to established organic fluorescence markers, semiconductor quantum dots provide an attractive alternative for the biolabeling of living cells. On the basis of a comparative investigation using differently sized thiol-stabilized CdTe nanocrystals in a variety of commonly used biological buffers, a method is developed to quantify the stability of such a multicomponent system. Above a certain critical size, the intensity of the photoluminescence of the nanocrystals is found to diminish with pseudo-zero-order kinetics, whereas for specific combinations of particle size, ligand, and buffer there appears to be no decay below this critical particle size, pointing out the necessity for thorough investigations of this kind in the view of prospect applications of semiconductor nanocrystals in the area of biolabeling.     

Bio-safety assessment of carbon quantum dots,N-doped and folic acid modified carbon quantum dots: A systemic comparison

The carbon quantum dots(CQDs) and their functionalized materials are promising in biomedical field because of their unique properties;meanwhile,a growing concern has been raised about the potential toxicity of these modified materials in biosystem.In this study,we synthesized original CQDs and two common functionalized CQDs including N-doped CQDs(NCQDs) and folic acid-modified CQDs(FACQDs),and compared the toxicity and biocompatibility with each other in vitro and in vivo.L929,C6 and normal cell MDCK were selected to detect the adverse reaction of these materials in vitro.No acute toxicity or obvious changes were noted from in vitro cytotoxicity studies with the dose of these CQD materials increasing to a high concentration at 1 mg/mL.Among these materials,the FA-CQDs show a much lower toxicity.Moreover,in vivo toxicity studies were performed on the nude mice for 15 days.The experimental animals in 10 or 15 mg/kg groups were similar with animals treated by phosphate buffer solution(PBS) after 15 days.The results of the multifa rious biochemical parameters also suggest that the functionalized products of CQDs do not influence the biological indicators at feasible concentration.Our findings in vitro and in vivo through toxicity tests demonstrate that CQDs and their modified materials are safe for future biological applications.     

Determination of the electrophoretic mobility of chromosomes by free flow electrophoresis. I. Morphology and stability

Isolated metaphase chromosomes of several fibroblastoid cell lines (Chinese hamster, Chinese hamster x human hybrid) were subjected to free flow electrophoresis (FFE) to study their electrophoretic mobility (EM). The morphology and stability of the chromosomes were unaffected by FFE as examined by cytogenetic methods and flow cytometry. The chromosomes of the complement all showed similar EM under most of the conditions applied. At neutral pH the EM of the chromosomes had the same sign as free DNA and about 2/3 of its magnitude. The variation of EM with buffer parameters such as ionic strength, valence of counterions, buffer capacity and dielectric constant of the solvent were investigated. Thermal denaturation increased the EM of the chromosomes by 20%. Partial denaturation might offer a possibility to separate or enrich large amounts of chromosomes by FFE.     

Theory of electrophoretic mobility of polyelectrolyte chains

An explicit formula is derived for the electrophoretic mobility of a linear polyelectrolyte chain in terms of the molecular weight, Debye length and solvent quality. The nature of the crossover between different regimes as the salt concentration and molecular weight are varied is presented. The effect of chain statistics intermediate between the Gaussian and rodlike limits on the electrophoretic mobility is analysed for the first time.     

Surface-plasmon-coupled emission of quantum dots

We studied surface plasmon-coupled emission (SPCE) of semiconductor quantum dots (QDs). These QDs are water-soluble ZnS-capped CdSe nanoparticles stabilized using lysine cross-linked mercaptoundecanoic acid. The QDs were spin-coated from 0.75% PVA solution on a glass slide covered with 50 nm of silver and a 5-nm protective SiO(2) layer. Excited QDs induced surface plasmons in a thin silver layer. Surface plasmons emitted a hollow cone of radiation into an attached hemispherical glass prism at a narrow angle of 48.5 degrees. This directional radiation (SPCE) preserves the spectral properties of QD emission and is highly p-polarized irrespective of the excitation polarization. The SPCE spectrum depends on the observation angle because of the intrinsic dispersive properties of SPCE phenomenon. The remarkable photostability can make QDs superior to organic fluorophores when long exposure to the intense excitation is needed. The nanosize QDs also introduce a roughness near the metal layer, which results in a many-fold increase of the coupling of the incident light to the surface plasmons. This scattered incident illumination transformed into directional, polarized radiation can be used simultaneously with SPCE to develop devices based on both quantum dot emission and light scattered from surface plasmons on a rough surface.     

Fabrication of hydrophilic luminescent zinc oxide quantum dots for selective detection of copper ions and efficient inhibition of harmful fungi

In this study, we have successfully prepared surface modified zinc oxide quantum dots (M-ZnO QDs) with ultra-stable fluorescence and excellent hydrophilicity through introducing (3-aminopropyl)triethoxysilane (APTES). The as-prepared M-ZnO QDs under the optimum condition presented strong yellow fluorescence emission under 355 nm excitation and showed satisfied reproducibility. Physical and chemical properties of the synthesized ZnO QDs were further studied by various characterization techniques. Transmission electron microscopy showed homogeneous distribution of spherical M-ZnO QDs with the average particle size of 4.03 nm. According to the characteristic that metal ions can quench fluorescence, M-ZnO QDs-based fluorescence sensor for the detection of Cu²⁺ in aqueous solution is developed in this work, which has the advantages of excellent selectivity, good sensitivity and a wide linear range. The limit of detection was 0.51 μM and the linear detection range was 1–200 μM for Cu²⁺ determination. The practicability of the fluorescent probe is further validated in the lake water and the satisfactory spiked recoveries of Cu²⁺ ranges from 99.1 % to 108.8 %. Besides, M-ZnO QDs displayed concentration inhibition effect and strain effect on the growth of fungi. Thus, the as-prepared M-ZnO QDs are demonstrated to be promising for Cu²⁺ determination and anti-fungal applications.     

Synthesis of surface-modified quantum dots

The review discusses the main methods used to obtain surface-modified quantum dots, specifically silicon, heavy metal chalcogenide and pnictide semiconductor nanoparticles. Examples of transformation processes of the grafted layer are considered. The importance of surface modification of A^IIB^VI- and A^IIIB^V-type semiconductor nanoparticles for the practical application of quantum dots is shown. It was determined that the most promising areas of their practical application are biology, medicine, and pharmacology. Special attention is paid to the hydrophilization of quantum dots, because only these materials can be used in biomedical applications. Modification of the quantum dot surface with amino acids is considered.

     

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.     

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.