Tailoring CsPbBr3 Growth via Non-Polar Solvent Choice and Heating Methods.

This study describes an investigation of the role of non-polar solvents on the growth of cesium lead halide (CsPbX3 X = Br and I) nanoplatelets. We employed two solvents, benzyl ether (BE) and 1-octadecene (ODE), as well as two nucleation and growth mechanisms, one-pot, facilitated by microwave irradiation (MWI)-based heating, and hot-injection, using convection. Using BE and MWI, large mesoscale CsPbBr3 nanoplatelets were produced, whereas use of ODE produced small crystallites. Differences between the products were observed by optical spectroscopies, which showed first band edge absorptions consistent with thicknesses of ∼9 nm [∼15 monolayer (ML)] for the BE-CsPbBr3 and ∼5 nm (∼9 ML) for ODE-CsPbBr3. Both products had orthorhombic crystal structures, with the BE-CsPbBr3 revealing significant preferred orientation diffraction signals consistent with the asymmetric and two-dimensional platelet morphology. The differences in the final morphology were also observed for products formed via hot injection, with BE-CsPbBr3 showing thinner square platelets with thicknesses of ∼2 ML and ODE-CsPbBr3 showing similar morphologies and small crystallite sizes. To understand the role solvent plays in crystal growth, we studied lead plumbate precursor (PbBrn2-n) formation in both solvents, as well as solvent plus ligand solutions. The findings suggest that BE dissolves PbBr2 salts to a higher degree than ODE, and that this BE to precursor affinity persists during growth.

Understanding the Surface Properties of Halide Exchanged Cesium Lead Halide Nanoparticles.

This report describes a characterization study of the surfaces of CsPbBr3 and CsPbBr3- xI x perovskite nanoparticles (NPs) obtained via a simultaneous purification and halide exchange (HE) postsynthetic processing technique. We studied composition-dependent NP-ligand interactions via diffusion ordered NMR (DOSY) and quantified resulting photoluminescence quantum yield (QY) as a function of halide exchange as well as ligand exchange. Importantly, ligand binding strength and QY were found to decrease when successive purification and/or halide/ligand exchange steps were taken without careful concurrent additions of acid and base ligands. This suggests that ligands added during postsynthetic processing steps are localized at the surface of the NP, passivating open surface sites. Further, we show that CsPbBr3- xI x with increasing CsPbI3 character, obtained via the same method, have decreasing ligand density, from 6.4 to 1.4 to 0.2 nm-2, indicating the composition-dependence of surface ligand binding, which also has consequences on the QY of the resulting mixed-halide NPs. These results shed further light on the importance of ion-ligand moiety additions during purification and halide exchange of highly emissive CsPbBr3 NPs to maintain their as-synthesized properties, as well as the intrinsic differences in surfaces binding and photostability between near-unity QY CsPbBr3 and mixed-halide CsPbBr3- xI x NPs.

Stepwise Assembly and Characterization of DNA Linked Two-Color Quantum Dot Clusters.

The DNA-mediated self-assembly of multicolor quantum dot (QD) clusters via a stepwise approach is described. The CdSe/ZnS QDs were synthesized and functionalized with an amphiphilic copolymer, followed by ssDNA conjugation. At each functionalization step, the QDs were purified via gradient ultracentrifugation, which was found to remove excess polymer and QD aggregates, allowing for improved conjugation yields and assembly reactivity. The QDs were then assembled and disassembled in a stepwise manner at a ssDNA functionalized magnetic colloid, which provided a convenient way to remove unreacted QDs and ssDNA impurities. After assembly/disassembly, the clusters' optical characteristics were studied by fluorescence spectroscopy and the assembly morphology and stoichiometry was imaged via electron microscopy. The results indicate that a significant amount of QD-to-QD energy transfer occurred in the clusters, which was studied as a function of increasing acceptor-to-donor ratios, resulting in increased QD acceptor emission intensities compared to controls.

Void coalescence in core/alloy nanoparticles with stainless interfaces.

The oxidation properties of nanoparticles with core/alloy microstructure and stainless steel like interfaces is described. In particular, 15-nm Fe/FeCr nanoparticles with a stainless steel like interface are prepared. These particles show a unique morphological transformation that is induced by surface oxidation, oxide passivation, and vacancy coalescence. This Kirkendall diffusion results in a tailorable oxide layer thickness, Fe-core size, as well as void size and symmetry. Much like the interface of bulk stainless steel, the interfacial FeCr oxide passivates oxidation, resulting in self-limited diffusion. Because of this, a highly uniform and stable core-void-shell morphology is observed.

Multifunctional DNA-gold nanoparticles for targeted doxorubicin delivery.

In this report we describe the synthesis, characterization, and cytotoxic properties of DNA-capped gold nanoparticles having attached folic acid (FA), a thermoresponsive polymer (p), and/or poly(ethylene glycol) (PEG) oligomers that could be used to deliver the anticancer drug doxorubicin (DOX) in chemotherapy. The FA-DNA oligomer used in the construction of the delivery vehicle was synthesized through the reaction of the isolated folic acid N-hydroxysuccinimide ester with the amino-DNA and the conjugated DNA product was purified using high performance liquid chromatography (HPLC). This approach ultimately allowed control of the amount of FA attached to the surface of the delivery vehicle. Cytotoxicity studies using SK-N-SH neuroblastoma cells with drug loaded delivery vehicles were carried out using a variety of exposure times (1-48 h) and recovery times (1-72 h), and in order to access the effects of varying amounts of attached FA, in culture media deficient in FA. DOX loaded delivery vehicles having 50% of the DNA strands with attached FA were more cytotoxic than when all of the strands contained FA. Since FA stimulates cell growth, the reduced cytotoxicity of vehicles fully covered with FA suggests that the stimulatory effects of FA can more than compensate for the cytotoxic effects of the drug on the cell population. While attachment of hexa-ethylene glycol PEG(18) to the surface of the delivery vehicle had no effect on cytotoxicity, 100% FA plus the thermoresponsive polymer resulted in IC50 = 0.48 ± 0.01 for an exposure time of 24 h and a recovery time of 1 h, which is an order of magnitude more cytotoxic than free DOX. Confocal microscopic studies using fluorescence detection showed that SK-N-SH neuroblastoma cells exposed to DOX-loaded vehicles have drug accumulation inside the cell and, in the case of vehicles with attached FA and thermoresponsive polymer, the drug appears more concentrated. Since the biological target of DOX is DNA, the latter observation is consistent with the high cytotoxicity of vehicles having both FA and the thermoresponsive polymer. The study highlights the potential of DNA-capped gold nanoparticles as delivery vehicles for doxorubicin in cancer chemotherapy.

Discrete dipole approximation analysis of plasmonic core/alloy nanoparticles.

The surface plasmon resonance (SPR) properties of Au/Au(x)Ag(1-x) core/alloy nanoparticles (NPs) have been investigated by means of the discrete dipole approximation. The core/alloy microstructure was varied by changing the shell alloy composition x, its thickness t(S), and the shell thickness to core radius ratio (t(S)/r(C)) in the range of 0.05-1.0. These changes resulted in a novel tuning of SPR shape, frequency, and extinction. These models were compared with experimental results for Au/Au(x)Ag(1-x) NPs prepared by a microwave-mediated hydrothermal processing method, which produces core/alloy NPs with SPR signatures closely resembling those of the models.

Near infrared bioluminescence resonance energy transfer from firefly luciferase--quantum dot bionanoconjugates.

The bioluminescence resonance energy transfer (BRET) between firefly luciferase enzymes and semiconductive quantum dots (QDs) with near infrared emission is described. The QD were phase transferred to aqueous buffers using a histidine mediated phase transfer route, and incubated with a hexahistidine tagged, green emitting variant of firefly luciferase from Photinus pyralis (PPyGRTS). The PPyGRTS were bound to the QD interface via the hexahistidine tag, which effectively displaces the histidine layer and binds directly to the QD interfaces, allowing for short donor-acceptor distances (∼5.5 nm). Due to this, high BRET efficiency ratios of ∼5 were obtained. These PPyGRTS-QD bio-nano conjugates were characterized by transmission electron microscopy, thermal gravimetric analysis, Fourier transform infrared spectroscopy and BRET emission studies. The final optimized conjugate was easily observable by night vision imaging, demonstrating the potential of these materials in imaging and signaling/sensing applications.

DNA-guided crystallization of colloidal nanoparticles.

Many nanometre-sized building blocks will readily assemble into macroscopic structures. If the process is accompanied by effective control over the interactions between the blocks and all entropic effects, then the resultant structures will be ordered with a precision hard to achieve with other fabrication methods. But it remains challenging to use self-assembly to design systems comprised of different types of building blocks-to realize novel magnetic, plasmonic and photonic metamaterials, for example. A conceptually simple idea for overcoming this problem is the use of 'encodable' interactions between building blocks; this can in principle be straightforwardly implemented using biomolecules. Strategies that use DNA programmability to control the placement of nanoparticles in one and two dimensions have indeed been demonstrated. However, our theoretical understanding of how to extend this approach to three dimensions is limited, and most experiments have yielded amorphous aggregates and only occasionally crystallites of close-packed micrometre-sized particles. Here, we report the formation of three-dimensional crystalline assemblies of gold nanoparticles mediated by interactions between complementary DNA molecules attached to the nanoparticles' surface. We find that the nanoparticle crystals form reversibly during heating and cooling cycles. Moreover, the body-centred-cubic lattice structure is temperature-tuneable and structurally open, with particles occupying only approximately 4% of the unit cell volume. We expect that our DNA-mediated crystallization approach, and the insight into DNA design requirements it has provided, will facilitate both the creation of new classes of ordered multicomponent metamaterials and the exploration of the phase behaviour of hybrid systems with addressable interactions.

Thermal aggregation properties of nanoparticles modified with temperature sensitive copolymers.

In this paper, we describe the use of a temperature responsive polymer to reversibly assemble gold nanoparticles of various sizes. Temperature responsive, low critical solution temperature (LCST) pNIPAAm-co-pAAm polymers, with transition temperatures (T(C)) of 51 and 65 °C, were synthesized with a thiol modification, and grafted to the surface of 11 and 51 nm gold nanoparticles (AuNPs). The thermal-responsive behavior of the polymer allowed for the reversible aggregation of the nanoparticles, where at T < T(C) the polymers were hydrophilic and extended between particles. In contrast, at T > T(C), the polymer shell undergoes a hydrophilic to hydrophobic phase transition and collapses, decreasing interparticle distances between particles, allowing aggregation to occur. The AuNP morphology and polymer conjugation were probed by TEM, FTIR, and (1)H NMR. The thermal response was probed by UV-vis and DLS. The structure of the assembled aggregates at T > T(C) was studied via in situ small-angle X-ray scattering, which revealed interparticle distances defined by polymer conformation.

Designing quantum rods for optimized energy transfer with firefly luciferase enzymes.

The bioluminescence resonance energy transfer (BRET) between firefly luciferase from Photinus pyralis (Ppy) with core/shell semiconductive quantum rods (QRs) has been studied as a function of QR aspect ratio and internal microstructure. The QRs were found to be ideal energy acceptors, and Ppy-to-core distances were optimized using rod-in-rod microstructures that were achieved by the synthetic control of rod morphology, surface chemistry, and Ppy:QR loading. The BRET ratios of >44 measured are the highest efficiencies to date.

Using the Photoluminescence Color Change in Cesium Lead Iodide Nanoparticles to Monitor the Kinetics of an External Organohalide Chemical Reaction by Halide Exchange.

In this work, we demonstrate a photoluminescence-based method to monitor the kinetics of an organohalide reaction by way of detecting released bromide ions at cesium lead halide nanoparticles. Small aliquots of the reaction are added to an assay with known concentrations of CsPbI3, and the resulting Br-to-I halide exchange (HE) results in rapid and sensitive wavelength blueshifts (Δλ) due to CsPbBrxI3-x intermediate concentrations, the wavelengths of which are proportional to concentrations. An assay response factor, C, relates Δλ to Br- concentration as a function of CsPbI3 concentration. The observed kinetics, as well as calculated rate constants, equilibrium, and activation energy of the solvolysis reaction tested correspond closely to synthetic literature values, validating the assay. Factors that influence the sensitivity and performance of the assay, such as CsPbI3 size, morphology, and concentration, are discussed.

Investigation of the drug binding properties and cytotoxicity of DNA-capped nanoparticles designed as delivery vehicles for the anticancer agents doxorubicin and actinomycin D.

Oligonucleotide-functionalized gold nanoparticles (AuNP) were designed and synthesized to be delivery vehicles for the clinically used anticancer drugs doxorubicin (DOX) and actinomycin D (ActD). Each vehicle contains a tailorable number of DNA duplexes, each possessing three high-affinity sequences for the intercalation of either DOX or ActD, thus allowing for control of drug loading. Drug binding was evaluated by measuring changes to DNA melting temperature, T(m), hydrodynamic diameter, D(h), and surface plasmon resonance wavelength, λ(spr), with drug loading. These studies indicate that DOX intercalates at its high-affinity sequence bound at the AuNP, and that ActD exhibits relatively weaker binding to its preferred sequence. Agarose gel electrophoresis further confirmed drug binding and revealed that particle mobilities inversely correlate with D(h). The equilibrium binding constant, K, and dissociation rate constant, ß, were determined by dialysis. Results indicate that the high negative electrostatic potential within the DNA shell of the particle significantly decreases ß and enhances K for DOX but has little effect on K and ß for ActD. The cytotoxicity of the vehicles was studied, with IC(50) = 5.6 ± 1.1 µM and 46.4 ± 9.3 nM for DOX-DNA-AuNP and IC(50) = 0.12 ± 0.07 µM and 0.76 ± 0.46 nM for ActD-DNA-AuNP, in terms of drug and particle concentrations, respectively.

Probing the quenching of CdSe/ZnS qdots by Au, Au/Ag, and Au/Pd nanoparticles.

The resonance energy transfer between CdSe/ZnS quantum dots (qdots) and three metallic nanoparticles (NPs) with different surface plasmon resonance (SPR) characteristics were studied. Gold, gold/silver and gold/palladium NPs were used as energy acceptors for qdots with donor emission at 570 nm. Due to the different spectral overlaps between the SPR signatures and qdot emission, varied energy transfer characteristics were observed. The energy transfer was quantified via the Stern-Volmer relationship, since in this study the energy transfer was collision based. The Au/Ag and Au/Pd NPs in particular showed high K(SV) values, while the Au NPs showed much lower energy transfer efficiency. Since the NPs used in this study were relatively large (d ~ 15-20 nm), the experimental system was also influenced by the NP extinction coefficients of ≈10(8) M(-1) cm(-1). To address this potential inner filter effect, the quenching profiles were normalized by SPR transmittance. The results are important to the field, as many of these classes of nanomaterials are being employed in energy transfer based studies, as well as in colorimetric sensing.

Layer-by-layer processing and optical properties of core/alloy nanostructures.

A novel hydrothermal layer-by-layer processing method for the fabrication of core/alloy nanoparticles with highly tunable surface plasmon resonance is described. For a model system of Au/Au(x)Ag(1-x), the processing temperature, alloy composition, and alloy thickness resulted in unique and tailorable plasmonic signatures. The discrete dipole approximation and selective alloy etching were used to correlate this optical response with the particle morphology and alloy phase ultrastructure.

A modular phase transfer and ligand exchange protocol for quantum dots.

In this paper, we describe a quantum dot (qdot) phase transfer protocol using ligand exchange and the amino acid histidine. The phase transfer from nonpolar solvents to aqueous buffers is homogeneous, and no appreciable precipitation occurs. The molecule histidine was chosen in order to first displace the organic encapsulation and second to provide a weakly chemisorbing intermediate at the qdot ionic interface. This allows the histidine to act as an intermediate shell upon which further direct ligand exchange can occur. Since this intermediate encapsulation is easily displaced by an assortment of different molecules while in aqueous buffers, we refer to this approach as modular. Characterization via FTIR and NMR revealed the extent of ligand exchange, and provides insights into the interfacial binding mechanism. The colloidal stability and photostability of the qdots was probed via UV-vis and steady state fluorescence, which revealed promising quantum yield stability of greater than 1 year. The qdots have hydrodynamic diameters of <12 nm and surface charges dependent upon ligand type and coverage. The modularity of this approach is shown by tailoring the qdot surface charge via sequential ligand exchange using mixed monolayers of carboxylic acid and poly(ethylene glycol)-terminated thiols.

Stepwise surface encoding for high-throughput assembly of nanoclusters.

Self-assembly offers a promising method to organize functional nanoscale objects into two-dimensional (2D) and 3D superstructures for exploiting their collective effects. On the other hand, many unique phenomena emerge after arranging a few nanoscale objects into clusters, the so-called artificial molecules. The strategy of using biomolecular linkers between nanoparticles has proven especially useful for construction of such nanoclusters. However, conventional solution-based reactions typically yield a broad population of multimers or isomers of clusters; furthermore, the efficiency of fabrication is often limited. Here, we describe a novel high-throughput method for designing and fabricating clusters using DNA-encoded nanoparticles assembled on a solid support in a stepwise manner. This method efficiently imparts particles with anisotropy during their assembly and disassembly at a surface, generating remarkably high yields of well-defined dimer clusters and Janus (two-faced) nanoparticles. The method is scalable and modular, assuring large quantities of clusters of designated sizes and compositions.

Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution.

The ability of cesium lead halide (CsPbX3; X = Cl(-), Br(-), I(-)) perovskite nanoparticles (P-NPs) to participate in halide exchange reactions, to catalyze Finkelstein organohalide substitution reactions, and to colorimetrically monitor chemical reactions and detect anions in real time is described. With the use of tetraoctylammonium halide salts as a starting point, halide exchange with the P-NPs was performed to calibrate reactivity, stability, and extent of ion exchange. The exchange of CsPbI3 with Cl(-) or Br(-) causes a significant blue-shift in absorption and photoluminescence, whereas reacting I(-) with CsPbBr3 causes a red-shift of similar magnitudes. With the high local halide concentrations and the facile nature of halide exchange in mind, we then explored the ability of P-NPs to catalyze organohalide exchange in Finkelstein like reactions. Results indicate that the P-NPs serve as excellent halide reservoirs for substitution of organohalides in nonpolar media, leading to not only different organohalide products, but also a complementary color change over the course of the reaction, which can be used to monitor kinetics in a precise manner. The merits of using P-NP as spectrochemical probes for real time assaying is then expanded to other anions which can react with, or result in unique, classes of perovskites.

Probing Bioluminescence Resonance Energy Transfer in Quantum Rod-Luciferase Nanoconjugates.

We describe the necessary design criteria to create highly efficient energy transfer conjugates containing luciferase enzymes derived from Photinus pyralis (Ppy) and semiconductor quantum rods (QRs) with rod-in-rod (r/r) microstructure. By fine-tuning the synthetic conditions, CdSe/CdS r/r-QRs were prepared with two different emission colors and three different aspect ratios (l/w) each. These were hybridized with blue, green, and red emitting Ppy, leading to a number of new BRET nanoconjugates. Measurements of the emission BRET ratio (BR) indicate that the resulting energy transfer is highly dependent on QR energy accepting properties, which include absorption, quantum yield, and optical anisotropy, as well as its morphological and topological properties, such as aspect ratio and defect concentration. The highest BR was found using r/r-QRs with lower l/w that were conjugated with red Ppy, which may be activating one of the anisotropic CdSe core energy levels. The role QR surface defects play on Ppy binding, and energy transfer was studied by growth of gold nanoparticles at the defects, which indicated that each QR set has different sites. The Ppy binding at those sites is suggested by the observed BRET red-shift as a function of Ppy-to-QR loading (L), where the lowest L results in highest efficiency and furthest shift.

Kinetic and thermodynamic assessments of the mediator-template assembly of nanoparticles.

The understanding of kinetic and thermodynamic factors governing the assembly of nanoparticles is important for the design and control of functional nanostructures. This paper describes a study of the kinetic and thermodynamic factors governing the mediator-template assembly of gold nanoparticles into spherical assemblies in solutions. The study is based on spectrophotometric measurements of the surface plasmon (SP) resonance optical property. Gold nanoparticle cores ( approximately 5 nm) encapsulated with tetraoctylammonium bromide shells were studied as a model system. The mediator-template assembly involves a thioether-based multidentate ligand (e.g., MeSi(CH2SMe)3) which functions as a mediator, whereas the tetraoctylammonium bromide capping molecules function as template agents. On the basis of the temperature dependence of the SP optical property in the mediator-template assembly process, the kinetic and thermodynamic parameters such as the reaction rate constant and reaction enthalpy have been determined. The results led to two important findings. First, the mediator-template assembly of nanoparticles is an enthalpy-driven process. Second, the enthalpy change (-1.3 kcal/mol) is close to the magnitude of the van der Waals interaction energy for alkyl chains and the condensation energy of hydrocarbons. Implications of the findings to the understanding of the interparticle interactions have also been discussed.

Functionalization of quantum rods with oligonucleotides for programmable assembly with DNA origami.

The DNA-mediated self-assembly of CdSe/CdS quantum rods (QRs) onto DNA origami is described. Two QR types with unique optical emission and high polarization were synthesized, and then functionalized with oligonucleotides (ssDNA) using a novel protection-deprotection approach, which harnessed ssDNA's tailorable rigidity and denaturation temperature to increase DNA coverage by reducing non-specific coordination and wrapping. The QR assembly was programmable, and occurred at two different assembly zones that had capture strands in parallel alignment. QRs with different optical properties were assembled, opening up future studies on orientation dependent QR FRET. The QR-origami conjugates could be purified via gel electrophoresis and sucrose gradient ultracentrifugation. Assembly yields, QR stoichiometry and orientation, as well as energy transfer implications were studied in light of QR distances, origami flexibility, and conditions.