Assembling Nanoparticle Clusters by Kinetic Control Using Weakly Interacting DNA

In this paper, we describe the use of weakly interacting DNA linkages to assemble nanoparticles into defined clusters. Gold nanoparticles (AuNPs) were synthesized and functionalized with thiol modified single-stranded DNA (ssDNA) and hybridized with ssDNA linkers of a defined length (L). The self-assembly kinetics were altered by manipulating interparticle energetics through changes to linker length, rigidity, and sequence. The linker length regulated the hybridization energy between complementary AuNPs, were longer L increased adhesion, resulting in classical uncontrollable aggregation. In contrast, L of six complementary bases decreased adhesion and resulting in slower nucleation that promoted small cluster formation, the growth of which was studied at two assembly temperatures. Results indicated that a decrease in temperature to 15 oC increased cluster yield with L6 as compared to 25 oC. Finally, the clusters were separated from unassembled AuNPs by sucrose gradient ultracentrifugation (UC) and studied via UV-visible spectrophotometry (UV-vis), dynamic light scattering (DLS) and transmission electron microscopy (TEM)

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-<i>co</i>-pAAm polymers, with transition temperatures (<i>T</i>_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 <i>T</i> < <i>T</i>_C the polymers were hydrophilic and extended between particles. In contrast, at <i>T</i> > <i>T</i>_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 ¹H NMR. The thermal response was probed by UV–vis and DLS. The structure of the assembled aggregates at <i>T</i> > <i>T</i>_C was studied via in situ small-angle X-ray scattering, which revealed interparticle distances defined by polymer conformation

Probing Resonance Energy Transfer and Inner Filter Effects in Quantum Dot–Large Metal Nanoparticle Clusters using a DNA-Mediated Quench and Release Mechanism

The energy transfer between DNA-linked CdSe/ZnS quantum dots (qdots) and gold nanoparticles (AuNPs) is described. The assembly produced qdot–AuNP clusters with satellite-like morphology. Owing to the programmability of the DNA linkage, both assembly as well as disassembly were used as a tool to probe quenching efficiency. Upon assembly, resonance energy transfer between the qdot donor and AuNP acceptor was measured as photoluminescence (PL) quenching. The magnitude of the quenching was approximated upon measurement of PL recovery once the cluster was disassembled by addition of a ssDNA fuel strand, which effectively displaced the qdot-to-AuNP dsDNA linkage. This controllable assembly/disassembly behavior was then used as a morphological tool to separate PL quenching from an inner filter effect originating from the AuNP’s high surface plasmon resonance (SPR) extinction. This corrected quenching value was observed from steady state PL measurements, which were then substantiated by PL decay measurements. Finally, the quenching efficiency was related to cluster spatial properties via use of the nanometal surface resonance energy transfer (NSET) method. The AuNP interface to qdot core distance was estimated at ≈8 nm, which was close to the distances visualized by TEM

Direct Attachment of Oligonucleotides to Quantum Dot Interfaces

A straightforward functionalization strategy for the direct attachment of single-stranded oligonucleotides (ssDNA) to quantum dot (qdot) interfaces is described. The approach takes advantage of a histidine-mediated phase transfer protocol that results in qdots with high colloidal stability in aqueous buffers. The weakly bound histidine encapsulation facilitates monolayer exchanged with both thiolated ssDNA and polyhistidine-tagged proteins. The successful biomodification at the qdot interface was probed by FRET analysis. The modest FRET efficiencies measured suggest the DNA to be in an extended conformation that is the result of high surface coverage that the direct attachment provides

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, <i>T</i>_m, hydrodynamic diameter, <i>D</i>_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 <i>D</i>_h. The equilibrium binding constant, <i>K</i>, and dissociation rate constant, β, were determined by dialysis. Results indicate that the high negative electrostatic potential within the DNA shell of the particle significantly decreases β and enhances <i>K</i> for DOX but has little effect on <i>K</i> and β for ActD. The cytotoxicity of the vehicles was studied, with IC₅₀ = 5.6 ± 1.1 μM and 46.4 ± 9.3 nM for DOX-DNA-AuNP and IC₅₀ = 0.12 ± 0.07 μM and 0.76 ± 0.46 nM for ActD-DNA-AuNP, in terms of drug and particle concentrations, respectively

Using 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 blue-shifts ( due to CsPbBrxI3-x intermediate concentrations, the wavelengths of which are proportional to concentrations. An assay response factor, C, relates 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 and morphology, and concentration, are discussed

Alloying and Phase Transformation of Fe/FeNi Core/Alloy Nanoparticles at High Temperatures

This work explores how to form and tailor the alloy composition of Fe/FexNi1-x core/alloy nanoparticles by annealing a pre-formed particle at elevated temperatures between 180 – 325 oC. This annealing allowed for a systematic FeNi alloying at a nanoparticle whose compositions and structure began as a alpha-Fe rich core, and a thin gamma-Ni rich shell, into mixed phases resembling gamma-FeNi3 and gamma-Fe3Ni2. This was possible in part by controlling surface diffusion via annealing temperature, and the enhanced diffusion at the many grain boundaries of the nanoparticle. Lattice expansion and phase change was characterized by powder X-ray diffraction (XRD), and composition was monitored by X-ray photoelectron spectroscopy (XPS). Of interest is that no phase precipitation was observed (i.e., heterostructure formation) in this system and the XRD results suggest that alloying composition or alloy gradient is uniform. This uniform alloying was considered using calculations of bulk diffusion and grain boundary diffusion for Fe and Ni self-diffusion, as well as Fe-Ni impurity diffusion is provided. In addition, alloying was further considered by calculations for Fe-Ni mixing enthalpy (Hmix) and phase segregation enthalpy (HSeg) using the Miedema model, which allowed for the consideration of alloying favorability or core-shell segregation in the alloying, respectively. Of particular interest is the formation of stable metal carbides compositions, which suggest that the typically inert organic self-assembled monolayer encapsulation can also be internalized