Sorting fluorescent nanocrystals with DNA

Nanoparticle sensor for label free detection of swine DNA in mixed biological samples

We used 40 ± 5 nm gold nanoparticles (GNPs) as colorimetric sensor to visually detect swine-specific conserved sequence and nucleotide mismatch in PCR-amplified and non-amplified mitochondrial DNA mixtures to authenticate species. Colloidal GNPs changed color from pinkish-red to gray-purple in 2 mM PBS. Visually observed results were clearly reflected by the dramatic reduction of surface plasmon resonance peak at 530 nm and the appearance of new features in the 620-800 nm regions in their absorption spectra. The particles were stabilized against salt-induced aggregation upon the adsorption of single-stranded DNA. The PCR products, without any additional processing, were hybridized with a 17-base probe prior to exposure to GNPs. At a critical annealing temperature (55 °C) that differentiated matched and mismatched base pairing, the probe was hybridized to pig PCR product and dehybridized from the deer product. The dehybridized probe stuck to GNPs to prevent them from salt-induced aggregation and retained their characteristic red color. Hybridization of a 27-nucleotide probe to swine mitochondrial DNA identified them in pork-venison, pork-shad and venison-shad binary admixtures, eliminating the need of PCR amplification. Thus the assay was applied to authenticate species both in PCR-amplified and non-amplified heterogeneous biological samples. The results were determined visually and validated by absorption spectroscopy. The entire assay (hybridization plus visual detection) was performed in less than 10 min. The LOD (for genomic DNA) of the assay was 6 µg ml(-1) swine DNA in mixed meat samples. We believe the assay can be applied for species assignment in food analysis, mismatch detection in genetic screening and homology studies between closely related species.

Biocompatible quantum dots for biological applications

Semiconductor quantum dots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantum dots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantum dots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantum dot approaches. The article is concluded with a look at the future direction of quantum dots.

Biotin-4-fluorescein based fluorescence quenching assay for determination of biotin binding capacity of streptavidin conjugated quantum dots

The valency of quantum dot nanoparticles conjugated with biomolecules is closely related to their performance in cell tagging, tracking, and imaging experiments. Commercially available streptavidin conjugates (SAv QDs) are the most commonly used tool for preparing QD−biomolecule conjugates. The fluorescence quenching of biotin-4-fluorscein (B4F) provides a straightforward assay to quantify the number of biotin binding sites per SAv QD. The utility of this method was demonstrated by quantitatively characterizing the biotin binding capacity of commercially available amphiphilic poly(acrylic acid) Qdot ITK SAv conjugates and poly(ethylene glycol) modified Qdot PEG SAv conjugates with emission wavelengths of 525, 545, 565, 585, 605, 625, 655, 705, and 800 nm. Results showed that 5- to 30-fold more biotin binding sites are available on ITK SAv QDs compared to PEG SAv QDs of the same color with no systematic variation of biotin binding capacity with size.

DNA-based applications in nanobiotechnology

Biological molecules such as deoxyribonucleic acid (DNA) have shown great potential in fabrication and construction of nanostructures and devices. The very properties that make DNA so effective as genetic material also make it a very suitable molecule for programmed self-assembly. The use of DNA to assemble metals or semiconducting particles has been extended to construct metallic nanowires and functionalized nanotubes. This paper highlights some important aspects of conjugating the unique physical properties of dots or wires with the remarkable recognition capabilities of DNA which could lead to miniaturizing biological electronics and optical devices, including biosensors and probes. Attempts to use DNA-based nanocarriers for gene delivery are discussed. In addition, the ecological advantages and risks of nanotechnology including DNA-based nanobiotechnology are evaluated.

Imaging GABAc receptors with ligand-conjugated quantum dots

We report a methodology for labeling the GABAc receptor on the surface membrane of intact cells. This work builds upon our earlier work with serotonin-conjugated quantum dots and our studies with PEGylated quantum dots to reduce nonspecific binding. In the current approach, a PEGylated derivative of muscimol was synthesized and attached via an amide linkage to quantum dots coated in an amphiphilic polymer derivative of a modified polyacrylamide. These conjugates were used to image GABA_C receptors heterologously expressed in Xenopus laevis oocytes.

Semisynthetic DNA-protein conjugates for biosensing and nanofabrication

Conjugation with artificial nucleic acids allows proteins to be modified with a synthetically accessible, robust tag. This attachment is addressable in a highly specific manner by means of molecular recognition events, such as Watson-Crick hybridization. Such DNA-protein conjugates, with their combined properties, have a broad range of applications, such as in high-performance biomedical diagnostic assays, fundamental research on molecular recognition, and the synthesis of DNA nanostructures. This Review surveys current approaches to generate DNA-protein conjugates as well as recent advances in their applications. For example, DNA-protein conjugates have been assembled into model systems for the investigation of catalytic cascade reactions and light-harvesting devices. Such hybrid conjugates are also used for the biofunctionalization of planar surfaces for micro- and nanoarrays, and for decorating inorganic nanoparticles to enable applications in sensing, materials science, and catalysis.

Nanotechnology and molecular cytogenetics: the future has not yet arrived

Quantum dots (QDs) are a novel class of inorganic fluorochromes composed of nanometer-scale crystals made of a semiconductor material. They are resistant to photo-bleaching, have narrow excitation and emission wavelengths that can be controlled by particle size and thus have the potential for multiplexing experiments. Given the remarkable optical properties that quantum dots possess, they have been proposed as an ideal material for use in molecular cytogenetics, specifically the technique of fluorescent in situ hybridisation (FISH). In this review, we provide an account of the current QD-FISH literature, and speculate as to why QDs are not yet optimised for FISH in their current form.

Immobilization of lactobionic acid on the surface of cadmium sulfide nanoparticles and their interaction with hepatocytes

In the current study, beta-galactose-carrying lactobionic acid (LA) was conjugated on the surface of mercaptoacetic acid-coated cadmium sulfide nanoparticles (CSNPs) to ensure specific recognition of liver cells (hepatocytes) and to enhance biocompatibility. Maltotrionic acid-coated CSNPs (MCSNPs) were also prepared for use as a control. The results showed that LA-immobilized CSNPs (LCSNPs) were selectively and rapidly internalized into hepatocytes and emitted more intense fluorescence images as well as demonstrated increased biocompatible behavior in vitro than those of CSNPs and MCSNPs. Furthermore, the uptake amount of LCSNPs into hepatocytes was higher than that of CSNPs and MCSNPs. All these results indicate that LCSNPs may find ever-growing applications in biological labels and detection or contrast agents in life science and medical diagnostics.

Addressable microfluidic polymer chip for DNA-directed immobilization of oligonucleotide-tagged compounds

A microfluidic polymer chip for the self-assembly of DNA conjugates through DNA-directed immobilization is developed. The chip is fabricated from two parts, one of which contains a microfluidic channel produced from poly(dimethylsiloxane) (PDMS) by replica-casting technique using a mold prepared by photolithographic techniques. The microfluidic part is sealed by covalent bonding with a chemically activated glass slide containing a DNA oligonucleotide microarray. The dimension of the PDMS-glass microfluidic chip is equivalent to standard microscope slides (76 x 26 mm(2)). The DNA microarray surface inside the microfluidic channels is configured through conventional spotting, and the resulting DNA patches can be conveniently addressed with compounds containing complementary DNA tags. To demonstrate the utility of the addressable surface within the microfluidic channel, DNA-directed immobilization (DDI) of DNA-modified gold nanoparticles (AuNPs) and DNA-conjugates of the enzymes glucose oxidase (GOx) and horseradish peroxidase (HRP) are carried out. DDI of AuNPs is used to demonstrate site selectivity and reversibility of the surface-modification process. In the case of the DNA-enzyme conjugates, the patterned assembly of the two enzymes allows the establishment and investigation of the coupled reaction of GOx and HRP, with particular emphasis on surface coverage and lateral flow rates. The results demonstrate that this addressable chip is well suited for the generation of fluidically coupled multi-enzyme microreactors.

Nanoscale labels: nanoparticles and liposomes in the development of high-performance biosensors

Technology for the detection of biological species has generated considerable interest in a variety of fields including healthcare, defense, food and environmental monitoring. In a biosensor, labeled specific binding partners are used to emit a detectable signal. Owing to their unique properties, nanomaterials have been proposed as a novel label category and have led to the development of new assays and new transduction mechanisms. In this article, the role of three major types of nanoscale labels (metallic, semiconductor and liposome nanoparticles) in the development of a new generation of optical, electrochemical or gravimetric biosensors will be presented. The underlying transduction principles will be briefly explained and assay strategies relying on the use of these ‘nanolabels’ will be described. The contribution to increased assay performance and sensitivity will be highlighted. Approaches towards simple, cost efficient and sensitive assays are essential to meet the demands of a growing number of applications.

Visualizing human prostate cancer cells in mouse skeleton using bioconjugated near-infrared fluorescent quantum dots

OBJECTIVES

To visualize human prostate cancer cells in mouse bone with bioconjugated near-infrared quantum dot (QD) probes. Near-infrared fluorescent probes using QDs can visualize tumors in deep tissues in vivo.

METHODS

Human prostate cancer C4-2B xenografts grown in mouse tibia were detected by prostate-specific membrane antigen antibody conjugated with QDs emitting light at the near-infrared range of 800 nm (QD800). Images in culture and in vivo were acquired using the IVIS Imaging System.

RESULTS

As few as 5000 cells can be detected subcutaneously when tagged with QD800 conjugate and injected directly into mice. QD800 conjugate injected intravenously in mice harboring C4-2B tumors in tibia detected signals from a minimum of 500 000 cells. The maximal light emission was detected 30 minutes after intravenous injection of QD800 conjugate in mice with established C4-2B tumors.

CONCLUSIONS

Bioconjugated near-infrared QD probes are highly sensitive molecular imaging tools for human prostate cancer micrometastases in mice.

Quantum dots for live cell and in vivo imaging

In the past few decades, technology has made immeasurable strides to enable visualization, identification, and quantitation in biological systems. Many of these technological advancements are occurring on the nanometer scale, where multiple scientific disciplines are combining to create new materials with enhanced properties. The integration of inorganic synthetic methods with a size reduction to the nano-scale has lead to the creation of a new class of optical reporters, called quantum dots. These semiconductor quantum dot nanocrystals have emerged as an alternative to organic dyes and fluorescent proteins, and are brighter and more stable against photobleaching than standard fluorescent indicators. Quantum dots have tunable optical properties that have proved useful in a wide range of applications from multiplexed analysis such as DNA detection and cell sorting and tracking, to most recently demonstrating promise for in vivo imaging and diagnostics. This review provides an in-depth discussion of past, present, and future trends in quantum dot use with an emphasis on in vivo imaging and its related applications.

Effect of poly(ethylene glycol) length on the in vivo behavior of coated quantum dots

The use of nanoparticles, either for the delivery of drugs or for imaging contrast agents, or a combination of both (theranostics), is very appealing in biological and biomedical research. The design of high-quality NIR-emitting quantum dots (QDs), with outstanding optical properties in comparison to that of organic dyes, should lead to novel contrast agents with improved performance for optical and multimodal imaging. Moreover, these nanocrystals could also be used for exploring therapeutic applications, such as drug delivery or phototherapy. In this article, we report the coating of commercial ITK705-amino QDs with methoxy-terminated poly(ethylene glycol) (PEG) of different chain lengths. Homogeneous QD solutions that are stable over extended periods of time were prepared. The impact of the particle coating on their in vivo fate after tail i.v. injection was studied by fluorescence imaging. The speed of the first pass extraction of the coated QDs toward the liver decreased with the PEG length, whereas the hydrodynamic diameter of the particles was increased.

PAMAM dendrimer-enhanced DNA biosensors based on electrochemical impedance spectroscopy

A novel, simple and sensitive DNA biosensor based on DNA-poly(amidoamine) (PAMAM) dendrimer nanoconjugates was developed by using the electrochemical impedance spectroscopy (EIS) technique. In this context, the assay relies on the hybridization of the single-stranded DNA (ssDNA) probe covalently conjugated on a mercaptoacetic acid self-assembled monolayer on gold electrodes, with the generation 4.5 (G-4.5) PAMAM-target DNA complex in solution. Once the double-stranded DNA (dsDNA) formed on the gold electrodes, G-4.5 PAMAM bearing carboxyls on the periphery was anchored on the hybrids; the changes of interfacial electron-transfer resistance (R(et)) of the electrodes were measured using an Fe(CN)(6)(3-/4-) redox probe by electrochemical impedance spectroscopy. The results showed that only a complementary sequence could form a dsDNA-PAMAM with the DNA-PAMAM probe and give an obviously enlarged R(et) value. The non-complementary and three-base mismatched sequence exhibited negligible impedance change compared with the blank measurement (the blank measurement means: ssDNA probe-modified gold electrode was directly measured by EIS). The unique spherical structure combining with more negative charges on the G-4.5 PAMAM periphery anchored on the hybrids could significantly amplify the hybridization signal (R(et) value), and the detection limit for measuring the full complementary sequence is down to pM level.

Quantum dots for live cell and in vivo imaging

In the past few decades, technology has made immeasurable strides to enable visualization, identification, and quantitation in biological systems. Many of these technological advancements are occurring on the nanometer scale, where multiple scientific disciplines are combining to create new materials with enhanced properties. The integration of inorganic synthetic methods with a size reduction to the nano-scale has lead to the creation of a new class of optical reporters, called quantum dots. These semiconductor quantum dot nanocrystals have emerged as an alternative to organic dyes and fluorescent proteins, and are brighter and more stable against photobleaching than standard fluorescent indicators. Quantum dots have tunable optical properties that have proved useful in a wide range of applications from multiplexed analysis such as DNA detection and cell sorting and tracking, to most recently demonstrating promise for in vivo imaging and diagnostics. This review provides an in-depth discussion of past, present, and future trends in quantum dot use with an emphasis on in vivo imaging and its related applications.

Quantum dots for live cell and in vivo imaging

In the past few decades, technology has made immeasurable strides to enable visualization, identification, and quantitation in biological systems. Many of these technological advancements are occurring on the nanometer scale, where multiple scientific disciplines are combining to create new materials with enhanced properties. The integration of inorganic synthetic methods with a size reduction to the nano-scale has lead to the creation of a new class of optical reporters, called quantum dots. These semiconductor quantum dot nanocrystals have emerged as an alternative to organic dyes and fluorescent proteins, and are brighter and more stable against photobleaching than standard fluorescent indicators. Quantum dots have tunable optical properties that have proved useful in a wide range of applications from multiplexed analysis such as DNA detection and cell sorting and tracking, to most recently demonstrating promise for in vivo imaging and diagnostics. This review provides an in-depth discussion of past, present, and future trends in quantum dot use with an emphasis on in vivo imaging and its related applications.

Nanoscale patterns of oligonucleotides formed by electrohydrodynamic jet printing with applications in biosensing and nanomaterials assembly

The widespread use of DNA in microarrays for applications in biotechnology, combined with its promise in programmed nanomaterials assembly, unusual electronic devices, and other areas has created interest in methods for patterning DNA with high spatial resolution. Techniques based on thermal or piezoelectric inkjet printing are attractive due to their noncontacting nature and their compatibility with diverse materials and substrate types; their modest resolution (i.e., 10-20 microm) represents a major limitation for certain systems. Here we demonstrate the use of an operationally similar printing approach that exploits electrohydrodynamic forces, rather than thermal or acoustic energy, to eject DNA inks through fine nozzles, in a controlled fashion. This DNA printer is capable of resolution approaching 100 nm. A range of experiments on patterns of DNA formed with this printer demonstrates its key features. Example applications in DNA-directed nanoparticle assembly and DNA aptamer-based biosensing illustrate two representative uses of the patterns that can be formed.

Nanoparticle self-assembly on a DNA-scaffold written by single-molecule cut-and-paste

Self-assembly guided by molecular recognition has in the past been employed to assemble nanoparticle superstructures like hypercrystals or nanoparticle molecules. An alternative approach, the direct molecule-by-molecule assembly of nanoscale superstructures, was demonstrated recently. Here we present a hybrid approach where we first assemble a pattern of binding sites one-by-one at a surface and then allow different nanoparticles to attach by self-assembly. For this approach, biotin bearing DNA oligomers were picked up from a depot using a cDNA strand bound to an AFM tip. These units were deposited in the target area by hybridization, forming a recognition pattern on this surface. Fluorescent semiconductor nanoparticles conjugated with streptavidin were allowed to assemble on this scaffold and to form the final nanoparticle superstructures.

A New Bioimaging Carrier for Fluorescent Quantum Dots: Phospholipid Nanoemulsion Mimicking Natural Lipoprotein Core

Fluorescent quantum dots (semiconductor nanocrystals) have the potential to revolutionize biological imaging, but their use has been limited by difficulties in obtaining quantum dots that are water soluble and biocompatible. The objectives of our research were to develop a methodology for encapsulation of cadnium-selenium (CdSe) quantum dots (QDs) in phospholipid nanoemulsion that mimics the natural lipoprotein core and to study their interactions with cultured non-small cell lung cancer cells (NSCLC). We found that CdSe QDs can be efficiently encapsulated in the phospholipid nanoemulsion. The QD nanoemulsion has a particle size approximately 80 nm and appears physically stable. The QD nanoemulsion interacts well with cells. The intensity of cellular fluorescence imaging increases with the cell incubation time, indicating more QDs were taken up by the cells, respectively. Two types of fluorescence microscopies confirm that QDs are primarily localized in the cytoplasm but not in the nucleus of the cells.

Correlation analysis of TEM images of nanocrystal molecules

Quantitative characterization of images of nanocrystals and nanostructures is a challenging but important task. The development and optimization of methods for the construction of complex nanostructures rely on imaging techniques. Computer programs were developed to quantify TEM images of nanocrystal/DNA nanostructures, and results are presented for heterodimers and trimers of gold nanocrystals. The programs presented here have also been used to analyze more complex trimers and tetramers linked by branched DNA, as well as for structures made from attaching gold nanocrystals to CdSe/ZnS core-shell quantum dots. This work has the additional goal of enabling others to quickly and easily adapt the methods for their own use.

Experimental determination of quantum dot size distributions, ligand packing densities, and bioconjugation using analytical ultracentrifugation

Analytical ultracentrifugation (AUC) was used to characterize the size distribution and surface chemistry of quantum dots (QDs). AUC was found to be highly sensitive to nanocrystal size, resolving nanocrystal sizes that differ by a single lattice plane. Sedimentation velocity data were used to calculate the ligand packing density at the crystal surface for different sized nanocrystals. Dihydrolipoic acid poly(ethylene glycol) was found to bind between 66 and 60% of the surface cadmium atoms for CdSe nanocrystals in the 1.54-2.59 nm radius size regime. The surface ligand chemistry was found to affect QD sedimentation, with larger ligands decreasing the sedimentation rate through an increase in particle volume and increase in frictional coefficient. Finally, AUC was used to detect and analyze protein association to QDs. Addition of bovine serum albumin (BSA) to the QD sample resulted in a reduced sedimentation rate, which may be attributed to an associated frictional drag. We calculated that one to two BSA molecules bind per QD with an associated frictional ratio of 1.2.

Silica-based multimodal/multifunctional nanoparticles for bioimaging and biosensing applications

In the last decade, the field of nanoparticle (NP) technology has attracted immense interest in bioimaging and biosensing research. This technology has demonstrated its capability in obtaining sensitive data in a noninvasive manner, promising a breakthrough in early-stage cancer diagnosis, stem cell tracking, drug delivery, pathogen detection and gene delivery in the near future. However, successful and wide application of this technology relies greatly on robust NP engineering and synthesis methodologies. The NP development steps involve design, synthesis, surface modification and bioconjugation. Each of these steps is critical in determining the overall performance of NPs. It is desirable to obtain NPs that are highly sensitive, stable, imageable, biocompatible and targetable. It is also desirable to obtain multimodal/multifunctional NPs that will enable imaging/sensing of the target using multiple imaging/sensing modalities. In this review, we focus on silica NPs that have been developed for biosensing applications and silica-based multimodal/multifunctional NPs for bioimaging applications.

Synthesis and bio-imaging application of highly luminescent mercaptosuccinic acid-coated CdTe nanocrystals

Here we present a facile one-pot method to prepare high-quality CdTe nanocrystals in aqueous phase. In contrast to the use of oxygen-sensitive NaHTe or H₂Te as Te source in the current synthetic methods, we employ more stable sodium tellurite as the Te source for preparing highly luminescent CdTe nanocrystals in aqueous solution. By selecting mercaptosuccinic acid (MSA) as capping agent and providing the borate-citrate acid buffering solution, CdTe nanocrystals with high quantum yield (QY >70% at pH range 5.0–8.0) can be conveniently prepared by this method. The influence of parameters such as the pH value of the precursor solution and the molar ratio of Cd²⁺ to Na₂TeO₃ on the QY of CdTe nanocrystals was systematically investigated in our experiments. Under optimal conditions, the QY of CdTe nanocrystals is even high up to 83%. The biological application of luminescent MSA-CdTe to HEK 293 cell imaging was also illustrated.

Targeting the human serotonin transporter (hSERT) with quantum dots

In this paper we report our work on the development of a human serotonin transporter (hSERT) antagonist that can be conjugated to quantum dots. This approach has been used to target and visualize the human serotonin transporter protein (hSERT). We demonstrate that labeling is blocked by the addition of high affinity hSERT antagonists such as paroxetine. This approach may be useful for the development of fluorescent assays to study the location and temporal dynamics of biogenic amine transporters and also holds promise for the development of plate-based high throughput assays used to identify novel transporter antagonists.

Fluorescence in situ hybridization (FISH) on maize metaphase chromosomes with quantum dot-labeled DNA conjugates

Semiconductor nanocrystals, also called quantum dots (QDs), are novel inorganic fluorophores which are brighter and more photostable than organic fluorophores. In the present study, highly dispersive QD-labeled oligonucleotide (TAG)(8) (QD-deoxyribonucleic acid [DNA]) conjugates were constructed via the metal-thiol bond, which can be used as fluorescence in situ hybridization (FISH) probes. FISH analysis of maize metaphase chromosomes using the QD-DNA probes showed that the probes could penetrate maize chromosomes and nuclei and solely hybridized to complementary target DNAs. Compared with the conventional organic dyes such as Cy3 and fluorescein isothiocyanate, this class of luminescent labels bound with oligonucleotides is brighter and more stable against photobleaching on the chromosomes after FISH. These results suggest that QD fluorophores may be a more stable and useful fluorescent label for FISH applications in plant chromosome mapping considering their size-tunable luminescence spectra.

Controlling surface reactions of CdS nanocrystals: photoluminescence activation, photoetching and photostability under light irradiation

Photoluminescence enhancement, photoetching and photostability of CdS nanocrystals were investigated under light irradiation. Strongly photoluminescent nanocrystals were obtained when the nanocrystal was weakly photoexcited in an aqueous solution at pH = 11 in the presence of oxygen. With the support of XPS measurements, the following photoactivation mechanism is proposed: Cd(2+) ions are released from the CdS surface owing to slow photocorrosion in the presence of oxygen, and Cd-OH bond formation occurs on the CdS surface under the alkaline conditions, removing the surface trap states. The wavelength of the irradiating light and the pH of the solution were determined as key parameters for nanocrystal surface modification. For the stability measurements the nanocrystals were extracted with an ammonium salt in a non-polar solvent. The photoluminescence quantum yield for the nanocrystals in the non-polar phase reached approximately 30%. The extracted nanocrystals were remarkably stable even under UV light irradiation, and the photoluminescence intensity was maintained for several months.

Adsorption of CdSe nanoparticles to thiolated TiO2 surfaces: influence of intralayer disulfide formation on CdSe surface coverage

Mixed monolayers of hexadecanoic acid (HDA) and 16-mercaptohexadecanoic acid (MHDA) were adsorbed to nanocrystalline TiO2 films, and CdSe nanoparticles were attached to the mixed monolayer functionalized surfaces. IR absorption spectroscopy was used to characterize the equilibrium binding of HDA and MHDA to TiO2. Surface adduct formation constants (Kad) of (4+/-2)x10(3) M(-1) and (6+/-4)x10(3) M(-1) were measured for HDA and MHDA, respectively. CdSe nanoparticles were adsorbed to the terminal thiol groups of MHDA. The surface coverage of CdSe was greater on mixed monolayers, consisting of approximately 12% MHDA and 88% HDA, than on pure MHDA monolayers. A mechanism is proposed wherein intralayer disulfide formation between MHDA thiol groups causes decreased reactivity toward CdSe nanoparticles. Disulfide formation is less significant at low fractional surface coverages of MHDA. The mechanism is supported by an increase of CdSe adsorption upon chemical reduction of surface disulfides to thiols. Our findings highlight the effect of intermolecular interactions on the affinity of nanoparticles for monolayer-functionalized surfaces.

Fluorescent magnetic nanocrystals by sequential addition of reagents in a one-pot reaction: a simple preparation for multifunctional nanostructures

Core-shell nanostructures consisting of FePt magnetic nanoparticles as the core and semiconducting chalcogenides as the shell were synthesized by a series of reactions in a one-pot procedure. Adding Cd(acac)2 as the cadmium precursor to a reaction mixture containing FePt nanoparticles afforded FePt@CdO core-shell intermediates. The subsequent addition of chalcogens yielded FePt@CdX core-shell nanocrystals (where X was S or Se). The reverse sequence of addition, i.e., adding X before Cd, resulted in spongelike nanostructures because the chalcogens readily formed nanowires in the solution. Transmission electron microscopy, energy-dispersive X-ray spectrometry, selected area electron diffraction, fluorescence spectroscopy, and SQUID were used to characterize the nanostructures. These core-shell nanostructures displayed superparamagnetism at room temperature and exhibited fluorescence with quantum yields of 2.3-9.7%. The flexibility in the sequence of addition of reagents, combined with the compatibility of the lattices of the different materials, provides a powerful yet convenient strategy for generating sophisticated, multifunctional nanostructures.