Photoluminescence of a freely suspended monolayer of quantum dots encapsulated into layer-by-layer films

A single monolayer of CdSe/ZnS quantum dots (QDs) has been encapsulated into a 60 nm free-suspended layer-by-layer (LbL) film. The QD monolayer showed a low light-emission within this film in contact with supporting solid substrates, but the manifold increase of photoluminescencence intensity was observed when the film was lifted and freely suspended over the microfabricated cylindrical cavities. This phenomenon was discussed in relationship with the effect of the elimination of the surface quenching enhanced by optical reflection from highly reflective silicon cavities. We suggest that a significant increase of the photoluminescence intensity of QD monolayers suspended over the microfabricated array can be interesting for future diagnostic and sensing applications.     

Temperature-sensitive photoluminescence of CdSe quantum dot clusters

The formation of narrow size dispersed and nanometer size aggregates (clusters) of cadmium selenide (CdSe) quantum dots (QDs) and their temperature-sensitive photoluminescence (PL) spectral properties close to room temperature (298 K) are discussed. CdSe QDs formed stable clusters with an average diameter of approximately 27 nm in the absence of coordinating solvents. Using transmission electron microscopy (TEM) imaging, we identified the association of individual QDs with 2-5 nm diameters into clusters of uniform size. A suspension of these clusters in different solvents exhibited reversible PL intensity changes and PL spectral shifts which were correlated with temperature. Although the PL intensity of CdSe QDs encapsulated in host matrixes and the solid state showed a response to temperature under cryogenic conditions, the current work identified for the first time QD clusters showing temperature-sensitive PL intensity variations and spectral shifts at moderate temperatures above room temperature. Temperature-sensitive reversible PL changes of clusters are discussed with respect to reversible thermal trapping of electrons at inter-QD interfaces and dipole-dipole interactions in clusters. Reversible luminescence intensity variations and spectral shifts of QD clusters show the potential for developing sensors based on QD nanoscale assemblies.     

Tunable metallization by assembly of metal nanoparticles in polymer thin films by photo- or electron beam lithography

The technique of patterning of surfaces with metal-rich structures on micro- or nanoscales was developed by assembling metal nanoparticles into a thin film of polymer in a controllable way. Palladium (Pd) nanoparticles were incorporated into a thin film of poly(methyl methacrylate) (PMMA) using palladium (II) bis(acetylacetonato), Pd(acac)(2), as a precursor vaporized in a nitrogen atmosphere. Depending upon its dose, the irradiation of a PMMA film by UV light or an electron beam (EB) enhances its reducing capability against Pd(acac)(2). This dependency on dose can be used to control the formation and assembly of Pd nanoparticles. Using this technique, binary patterns consisting of metal-rich and metal-poor regions in the polymer film can be created simply by irradiating the surface of the polymer through a binary photomask. Besides the creation of binary patterns, it is also possible to create grayscale patterns where the density of Pd nanoparticles can be tuned to provide shades of gray by the use of light with continuously modulated intensity. Because the electron beam also enhances the reducing power of PMMA against Pd(acac)(2), it is thus possible to obtain highly metallized films with nanoscale pattern features. The PMMA film can be selectively removed by oxygen plasma treatment or by pyrolysis. Thus, highly metallized surfaces with binary or grayscale patterns can be obtained by selective removal of the PMMA films. The metallized regions possess relatively high resistivity against CF(4) plasma compared to the bare silicon surface; therefore, the metallized surface patterns can be transferred onto the underlying silicon substrate by CF(4) plasma treatment. Because of the nanosize effect of metal nanoparticles, the thermal treatment at 900 degrees C, which is significantly lower than the melting temperature of the bulk Pd, yields continuous metallic features by binding the assembled nanoparticles.     

Controlled self-assembly of hydrophobic quantum dots through silanization

We demonstrate the formation of one-, two-, and three-dimensional nanocomposites through the self-assembly of silanized CdSe/ZnS quantum dots (QDs) by using a controlled sol-gel process. The self-assembly behavior of the QDs was created when partially hydrolyzed silicon alkoxide monomers replaced hydrophobic ligands on the QDs. We examined systematically self-assembly conditions such as solvent components and QD sizes in order to elucidate the formation mechanism of various QD nanocomposites. The QD nanocomposites were assembled in water phase or on the interface of water and oil phase in emulsions. The partially hydrolyzed silicon alkoxides act as intermolecules to assemble the QDs. The QD nanocomposites with well-defined solid or hollow spherical, fiber-like, sheet-like, and pearl-like morphologies were prepared by adjusting the experimental conditions. The high photoluminescence efficiency of the prepared QD nanocomposites suggests partially hydrolyzed silicon alkoxides reduced the surface deterioration of QDs during self-assembly. These techniques are applicable to other hydrophobic QDs for fabricating complex QD nanocomposites.     

In vivo NIR imaging with CdTe/CdSe quantum dots entrapped in PLGA nanospheres

Luminescent near-infrared (NIR) CdTe/CdSe QDs were synthesized and encapsulated in poly(lactic-co-glycolic acid) (PLGA) nanospheres to prepare stable and biocompatible QDs-loaded nanospheres for in vivo imaging. QDs were encapsulated with PLGA nanospheres by a solid dispersion method and optimized to have high fluorescence intensity for in vivo imaging detection. The resultant QDs-loaded PLGA nanospheres were characterized by various analytical techniques such as UV-Vis measurement, dynamic light scattering (DLS), fluorescence spectroscopy, and transmission electron microscopy (TEM). Finally, we evaluated toxicity and body distribution of QDs loaded in PLGA nanospheres in vitro and in vivo, respectively. From the results, the QDs loaded in PLGA nanospheres were spherical and showed a diameter range of 135.0-162.3 nm in size. The QD nanospheres increased their stability against photooxidation and photobleaching, which have the high potential for applications in biomedical imaging. We have also attained non-invasive in vivo imaging with light photons, representing an intriguing avenue for obtaining biological information by the use of NIR light.     

Photochemical instability of thiol-capped CdTe quantum dots in aqueous solution and living cells: process and mechanism

The process and mechanism of photochemical instability of thiol-capped CdTe quantum dots (QDs) in aqueous solution were experimentally studied. After laser irradiation, the corresponding Raman bands of the Cd-S bond decreased obviously, indicating bond breaking and thiol detachment from the QD surfaces. Meanwhile, a photoinduced aggregation of QDs occurred with the hydrodynamic diameter increased to hundreds of nanometers from an initial 20 nm, as detected with dynamic light scattering measurements. The bleaching of the photoluminescence of QDs under laser irradiation could be attributed to the enhanced nonradiative transfer in excited QDs caused by increased surface defects due to the losing of thiol ligands. Singlet oxygen (1O2) was involved in the photooxidation of QDs, as revealed by the inhibiting effects of 1O2 quenchers of histidine or sodium azide (NaN3) on the photobleaching of QDs. The linear relationship in Stern-Volmer measurements between the terminal product and the concentration of NaN3 demonstrated that 1O2 was the main pathway of the photobleaching in QD solutions. By comparing the photostability of QDs in C2C12 cells with and without NaN3 treatment, the photooxidation effect of 1O2 on photobleaching of cellular QDs was confirmed.     

Copolymer nanosphere encapsulated CdS quantum dots prepared by RAFT copolymerization: synthesis,characterization and mechanism of formation

Cadmium sulfide (CdS) quantum dots (QDs) encapsulated in block copolymer spheres were synthesized by an aqueous emulsion polymerization process. First, stable dispersions of CdS QDs in water were prepared using a polymer dispersant, either poly(acrylic acid) or a random copolymer having an average of ten acrylic acid and five butyl acrylate units. These polymer dispersants were prepared by reversible addition-fragmentation chain transfer polymerization. Then, the CdS QDs dispersed in water were encapsulated in a polystyrene shell using an emulsion polymerization process. Spectroscopic and microscopic techniques were used to characterize the resulting nanocomposites. Optical properties of QDs in polymer microspheres were investigated by UV-vis and fluorescence spectroscopic studies. Particle sizes of all CdS QD samples were calculated from absorption edges using Henglein's empirical curve. Transmission electron microscopy was used to determine the size and morphology of CdS QD samples. These observations were used to elucidate the mechanism of formation of the resulting well-defined polymer-encapsulated CdS nanoparticles.     

Elastomeric flexible free-standing hydrogen-bonded nanoscale assemblies

Poly(ethylene oxide) (PEO) is a key material in solid polymer electrolytes, biomaterials, drug delivery devices, and sensors. Through the use of hydrogen bonds, layer-by-layer (LBL) assemblies allow for the incorporation of PEO in a controllable tunable thin film, but little is known about the bulk properties of LBL thin films because they are often tightly bound to the substrate of assembly. The construction technique involves alternately exposing a substrate to a hydrogen-bond-donating polymer (poly(acrylic acid)) and a hydrogen-bond-accepting polymer (PEO) in solution, producing mechanically stable interdigitated layers of PEO and poly(acrylic acid) (PAA). Here, we introduce a new method of LBL film isolation using low-energy surfaces that facilitate the removal of substantial mass and area of the film, allowing, for the first time, the thermal and mechanical characterization that was previously difficult or impossible to perform. To further understand the morphology of the nanoscale blend, the glass transition is measured as a function of assembly pH via differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The resulting trends give clues as to how the morphology and composition of a hydrogen-bonded composite film evolve as a function of pH. We also demonstrate that LBL films of PEO and PAA behave as flexible elastomeric blends at ambient conditions and allow for nanoscale control of thickness and film composition. Furthermore, we show that the crystallization of PEO is fully suppressed in these composite assemblies, a fact that proves advantageous for applications such as ultrathin hydrogels, membranes, and solid-state polymer electrolytes.     

Hybrid fluorescent nanoparticles from quantum dots coupled to cellulose nanocrystals

Carboxylated cellulose nanocrystals (CNCs) were decorated with CdSe/ZnS quantum dots (QDs) using a carbodiimide chemistry coupling approach. The one-step covalent modification was supported by nanoscale imaging, which showed QDs clustered on and around the CNCs after coupling. The QD–CNC hybrid nanoparticles remained colloidally stable in aqueous suspension and were fluorescent, exhibiting the broad excitation and narrow emission profile characteristic of the QDs. QD–CNCs in nanocomposite films imparted strong fluorescence within CNC-compatible matrices at relatively low loadings (0.15 nmol QDs/g of dry film), without altering the overall physical properties or self-assembly of the CNCs. The hybrid QD–CNCs may find applications in nanoparticle tracking, bio-imaging, optical/sensing devices, and anti-counterfeit technologies.     

Time-dependent photoluminescence blue shift of the quantum dots in living cells: effect of oxidation by singlet oxygen

Time-dependent photoluminescence (PL) enhancement, blue shift, and photobleach were observed from the thiol-capped CdTe quantum dots (QDs) ingested in mouse myoblast cells and human primary liver cancer cells. It was revealed that the PL blue shift resulted from the photooxidation of the QD core by singlet oxygen molecules formed on the QD core surface.     

Facile synthesis of fluorescent quantum dot‐polymer nanocomposites via frontal polymerization

We report an available approach for quickly fabricating CdS QD‐polymer nanocomposites via frontal polymerization (FP). First, we synthesized (3‐mercaptopropyl)‐1‐trimethoxysilane (MPS)‐capped CdS quantum dots (QDs). With these MPS‐capped CdS QDs containing mercapto groups, MPS‐capped CdS QDs can be easily incorporated into a poly(N‐methylolacrylamide) (PNMA) matrix via FP. A variety of features for preparing QD‐polymer nanocomposites, such as initiator concentration and CdS concentration, were thoroughly investigated. The fluorescence properties of QD‐polymer nanocomposites prepared via FP are comparatively investigated on the basis of ultraviolet–visible (UV–vis) spectra and photoluminescence (PL) spectra. Results show that the PL intensity of QD‐polymer nanocomposites prepared via the FP method is superior to that obtained by the traditional batch polymerization (BP) method. In addition, by measuring the changes of PL intensity of the samples immersed in different concentrations of copper acetate solution, we found the QD‐polymer nanocomposites can be ultrasensitive to copper ions. This FP process can be exploited as a facile and rapid way for synthesis QD‐polymer nanocomposites on a large scale, avoiding the fluorescence quenching of nanocrystals during incorporation nanocrystals into polymer matrices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2170–2177, 2010     

Assembly kinetics of nanocrystals via peptide hybridization

The assembly kinetics of colloidal semiconductor quantum dots (QDs) on solid inorganic surfaces is of fundamental importance for implementation of their solid-state devices. Herein an inorganic binding peptide, silica binding QBP1, was utilized for the self-assembly of nanocrystal quantum dots on silica surface as a smart molecular linker. The QD binding kinetics was studied comparatively in three different cases: first, QD adsorption with no functionalization of substrate or QD surface; second, QD adsorption on QBP1-modified surface; and, finally, adsorption of QBP1-functionalized QD on silica surface. The surface modification of QDs with QBP1 enabled 79.3-fold enhancement in QD binding affinity, while modification of a silica surface with QBP1 led to only 3.3-fold enhancement. The fluorescence microscopy images also supported a coherent assembly with correspondingly increased binding affinity. Decoration of QDs with inorganic peptides was shown to increase the amount of surface-bound QDs dramatically compared to the conventional methods. These results offer new opportunities for the assembly of QDs on solid surfaces for future device applications.     

Highly photoluminescent multilayer QD-glass films prepared by LbL self-assembly

A novel and facile preparation method for layer-by-layer (LbL) self-assembled films incorporating quantum dots (QDs) and having intense photoluminescence (PL) from blue to red is presented. Functional sol-gel-derived glass layers prepared by the hydrolysis of 3-aminopropyltrimethoxysilane (APS) or 3-mercaptopropyltrimethoxysilane (MPS) have been used as a linkage between QD layers. Absorption, PL spectroscopy, transmission electron microscopy, and atomic force microscopy were employed for characterization, which revealed that the QDs in the prepared films had a nearly close-packed coverage and were not aggregated. The PL efficiencies of the QDs (CdTe or ZnSe, both are thioglycolic acid-stabilized) dispersed in the films were roughly half that of the initial colloidal solutions but reached 24% before a refractive index correction. The thickness of the red-emitting film with 10 CdTe QD layers was approximately 50 nm. The concentration of QDs in the film derived from the first absorption peak was approximately 0.01 M. Because the PL starts to show a red shift, the obtained concentration is practically the ultimate one in the glass matrix. The mercapto, amino, and carboxyl groups play important roles in LbL self-assembling processes.     

基于粒子辅助水滴模板法制备仿生复眼结构的研究

将微粒“皮克林乳化效应”(Pickering emulsions)和水滴模板法(breath figure method)有机结合,探索通过建立粒子辅助的水滴模板法,实现纳米粒子在蜂窝状多孔膜内壁的自组装复合,构建微纳复合的多级仿生结构.并进一步利用聚二甲基硅氧烷(PDMS)复制转移技术,获得类似于复眼结构的多级微纳复合界面仿生结构.     

Photoinduced fluorescence intensity oscillation in a reaction-diffusion cell containing a colloidal quantum dot dispersion

The nonlinear spontaneous oscillation of photoluminescence (PL) intensity in an ensemble of semiconductor quantum dots (QDs), which differs from the fluorescence intermittency of a single QD, is investigated. The PL intensity in a QD dispersion slowly oscillates with time under continuous illumination. The oscillatory behavior is found to vary with changing QD concentration, solvent viscosity, volume fraction of irradiated region, and irradiation intensity. On the basis of the Gray-Scott model [Chemical Oscillation and Instabilities: Non-linear Chemical Kinetics (Clarendon, Oxford, 1994); J. Phys. Chem. 89, 22 (1985); Chem. Eng. Sci. 42, 307 (1987)], and its comparison with the experimental results, it is revealed that the following processes are important for PL oscillation: (1) mass transfer of QDs between the illuminated and dark regions, (2) autocatalytic formation of vacant sites on QD surfaces via photodesorption of ligand molecules, and (3) passivation of vacant sites via photoadsorption of water molecules.     

Single molecular stamping of a sub-10-nm colloidal quantum dot array

We introduce a nanoscale stamping technique of sub-10-nm colloidal quantum dot (QD) arrays to highly localized areas of three-dimensional nanostructures using a quartz tuning fork employed as the stamp pad (the "Nano Stamp"). CdSe/ZnS core-shell nanoparticles with diameters of 9.8 nm were deposited on microfabricated silicon probe tips. The number of transferred QDs, which ranged from several thousands down to single molecular order (less than 10), was precisely controlled by adjusting the stamping depths and angles. The stamping areas were varied from 1.2 microm x 1.2 microm down to 30 nm x 30 nm. Using the Nano Stamp, QDs can be transferred to a variety of protruding nanostructures. The amount of particles transferred to the tip was assessed by fluorescence intensity measurements, and the number of particles was estimated by direct transmission electron microscopy (TEM) observation. Correlation between the fluorescence intensity and the observed stamping depth and the approaching angle of the tip was found, demonstrating the efficacy of our Nano Stamp technique.     

Plasmonic resonance instigated enhanced photoluminescence in quantum dot dispersed nematic liquid crystal

In the present investigation, the optical property of the nematic sample p-methoxybenzylidene p-decylaniline, dispersed with SiO₂ quantum dot (QD), has been reported. Enhanced luminescence has been observed from nematic-QD composites. Surface plasmonic effect along with QD exciton has been highlighted to discuss the observed intensification in photoluminescence. The intensified photoemission from the nematic composites can also be inferred from improved orientational behaviour of the liquid crystal molecules due to dispersion of QDs. Variation in the intensity of photoabsorption can be harnessed for development of luminescent display devices and optical parameter-driven scientific applications.     

A hybrid quantum dot-antibody fragment fluorescence resonance energy transfer-based TNT sensor

We demonstrate the use of luminescent QDs conjugated to antibody fragments to develop solution-phase nanoscale sensing assemblies, based on fluorescence resonance energy transfer (FRET) for the specific detection of the explosive 2,4,6-trinitrotoluene (TNT) in aqueous environments. The hybrid sensor consists of anti-TNT specific antibody fragments attached to a hydrophilic QD via metal-affinity coordination. A dye-labeled TNT analogue prebound in the antibody binding site quenches the QD photoluminescence via proximity-induced FRET. Analysis of the data collected at increasing dye-labeled analogue to QD ratios provided an insight into understanding how the antibody fragments self-assemble on the QD. Addition of soluble TNT displaces the dye-labeled analogue, eliminating FRET and resulting in a concentration-dependent recovery of QD photoluminescence. Sensor performance and specificity were evaluated.     

Highly crosslinked poly(dimethylsiloxane) microbeads with uniformly dispersed quantum dot nanocrystals

This study demonstrates how luminescent semiconductor nanocrystals (quantum dots or QDs) can be dispersed uniformly in a poly(dimethylsiloxane) (PDMS) matrix by polymerizing a mixture of the prepolymer oligomers and the nanocrystals with a relatively large concentration of crosslinking molecules. A microfluidic device is used to fabricate PDMS microbeads embedded with the QDs by using flow focusing to first form monodisperse droplets of the prepolymer/crosslinker/nanocrystal mixture in a continuous aqueous phase. The droplets are subsequently collected, and heated to polymerize them into solid microbead composites. The degree of aggregation of the nanocrystals in the matrix is studied by measuring the nonradiative resonance energy transfer (RET) between the nanocrystals. For this purpose, two quantum dots are used with maxima in their luminescence emission spectrum at 560 nm and 620 nm. When the nanocrystals are within the F?rster radius (approximately 10 nm) of each other, exciton energy cascades from the QDs which emit at the shorter wavelength to the QDs which emit at the longer wavelength. This energy transfer is quantified, for two concentration ratios of the prepolmer to the crosslinker, by measuring the deviation of the microbead luminescence spectrum from a reference spectrum obtained by dispersing the QD mixture in a solvent (toluene) in which the nanocrystals do not aggregate. For a low concentration of crosslinking molecules relative to the prepolymer (5:1 by weight prepolymer to crosslinker), strong RET is observed as the emission of the 620 nm QDs is increased and the 560 nm QDs is decreased relative to the reference. In the emission spectrum for a higher concentration of crosslinkers (2:1 by weight prepolymer to crosslinker), the resonance energy transfer is less relative to the case of the low concentration of crosslinkers, and the spectrum more closely resembles the reference. This result indicates that the increase in the crosslinker concentration has reduced the nanocrystal aggregation in the cured polymer. The use of crosslinking can serve as a general paradigm for forming, from a prepolymer/nanoparticle mixture, a composite in which the particles are not aggregated. Under the usual conditions the entropic cost to a linearly growing polymer chain of surrounding nanoparticles forces them to aggregate; crosslinking kinetically entraps the particles and circumvents this aggregation driving force. The QD/polymer composite microbeads fabricated in this study find applications in bead-based platforms for high-throughput, multiplexed screening, where the emission spectrum of the QD luminescence can be used as a spectral barcode to label the beads. For microbeads in which the nanocrystals are uniformly dispersed, this barcode is undistorted by energy transfer, and is easily read.     

A hierarchical self-assembly route to three-dimensional polymer-quantum dot photonic arrays

We demonstrate a new hierarchical self-assembly strategy for the formation of photonic arrays containing quantum dots (QDs), in which sequential self-assembly steps introduce organization on progressively longer length scales, ranging from the nanoscale to the microscale regimes. The first step in this approach is the self-assembly of diblock copolymers to form block ionomer reverse micelles (SA1); within each micelle core, a single CdS QD is synthesized to yield the hybrid building block BC-QD. Once SA1 is completed, the hydrophobic BD-QD building blocks are blended with amphiphilic block copolymer stabilizing chains in an organic solvent; water addition induces secondary self-assembly (SA2) to form quantum dot compound micelles (QDCMs). Finally, aqueous dispersions of QDCMs are slowly evaporated to induce the formation of three-dimensional (3D) close-packed arrays in a tertiary self-assembly step (SA3). The resulting hierarchical assemblies, consisting of a periodic array of hybrid spheres each containing multiple CdS QDs, exhibit the collective property of a photonic stop band, along with photoluminescence arising from the constituent QDs. A high degree of structural control is possible at each level of organization by judicious selection of experimental variables, allowing various parameters governing the collective optical properties, including QD size, nanoparticle spacing, and mesocale periodicity, to be independently tuned. The resulting control over optical properties via successive self-assembly steps should provide new opportunities for hierarchical materials for QD lasers and all-optical switching.