The nucleation and growth of solids from solutions impacts many natural processes and is fundamental to applications in materials engineering and medicine. For a crystalline solid, the nucleus is a nanoscale cluster of ordered atoms that forms through mechanisms still poorly understood. In particular, it is unclear whether a nucleus forms spontaneously from solution via a single- or multiple-step process. Here, using in situ electron microscopy, we show how gold and silver nanocrystals nucleate from supersaturated aqueous solutions in three distinct steps: spinodal decomposition into solute-rich and solute-poor liquid phases, nucleation of amorphous nanoclusters within the metal-rich liquid phase, followed by crystallization of these amorphous clusters. Our ab initio calculations on gold nucleation suggest that these steps might be associated with strong gold-gold atom coupling and water-mediated metastable gold complexes. The understanding of intermediate steps in nuclei formation has important implications for the formation and growth of both crystalline and amorphous materials.
Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope is presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned
Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature.
We have discovered a voltage threshold for permeation through a synthetic nanopore of dsDNA bound to a restriction enzyme that depends on the sequence. Molecular Dynamic simulations reveal that the threshold is associated with a nanoNewton force required to rupture the DNA-protein complex. A single mutation in the recognition site for the restriction enzyme, i.e. a single nucleotide polymorphism (SNP), can easily be detected as a change in the threshold voltage. Consequently, by measuring the threshold voltage in a synthetic nanopore, it may be possible to discriminate between two variants of the same gene (alleles) that differ in one base.Restriction enzymes are used prevalently in recombinant DNA technology for cleaving doublehelical DNA segments containing a specific target sequence. Another use is genotyping. Because the binding to the target is extraordinarily sequence specific, restriction enzymes can be used to identify single nucleotide polymorphisms (SNPs) that occur when variants of the same gene (alleles) differ in one base.We have discovered a method for discriminating between alleles that uses a synthetic nanopore to measure the binding of a restriction enzyme to DNA. When a voltage is applied across a membrane containing a nanopore, polyanionic DNA immersed in electrolyte at the cathode diffuses toward the anode and is driven across the membrane by the electric field in the pore. The force due to the field acting on the strand during the translocation impels DNA to bend and stretch within the pore. 1-4 At low fields ℰ < 500mV/10nm, double-stranded DNA (dsDNA) easily permeates pores with diameters ≥2.4nm because the double helix (~2nm diameter) is smaller than the pore. 5 But the permeability of DNA through the pore changes dramatically if it is bound to a restriction enzyme.To study the binding of a restriction enzyme like EcoRI to DNA, we introduced an excess of the enzyme in solution with DNA without the Mg +2 cofactor that is required for cleaving the nucleic acid. Under these conditions, EcoRI is thought to bind and diffuse along DNA. 6,7 The diffusive motion along the strand is arrested at the cognate site, i.e. -GAATTC-for EcoRI. Bulk measurements of the binding at the cognate site indicate a free energy of formation ΔG =−15.2kcal/mol. 6-9 However, the introduction of any mutation among the cognate sites produces a position-dependent reduction in the binding energy that ranges from 6-13kcal/mol. 8,9 Site-specific DNA-binding proteins also have an affinity for nonspecific DNA. In contrast with site-specific binding or binding to a non-cognate site with a single nucleotide mutation, a nonspecifically bound complex is not localized to a particular site. For EcoRI, sites that differ from the cognate sequence by two or more base-pairs(bps) are considered nonspecific since they are not cleaved and show low binding constants. For a nonspecifically bound EcoRI-DNA complex, the free energy of formation is reduced to −4.8kcal/mol. 8,9We measured the permeability of dsDNA in solution with EcoRI ...
Nanoparticles (NPs) confined in thin layers of liquid within liquid cells used for in situ transmission electron microscopy (TEM) move very slowly, in contrast to free particles in bulk liquid. The reason is still poorly understood. Here, we tracked gold NPs moving in water at the liquid−solid interface with in situ TEM at rates of 100 frames per second. The recorded motion exhibited three key features: (1) it was made up of sustained sequences of "sticky" motion where NPs only moved a few nanometers each time; (2) sporadic long "flights" where the NPs traveled tens to hundreds of nanometers between frames; and (3) "flights" are accompanied by intermittent, fast pivoted rotations. Trajectory analysis shows that the displacements follow a truncated Levy distribution, pointing to desorptionmediated motion of NPs at the liquid−solid interface. We further associate pivoted rotations with a transient "weakly adsorbed" state between desorption and adsorption of NPs. The frequency of desorption was also controlled by electron flux and solution chemistry. We propose that the pattern of motion is the result of an inhomogeneous distribution of surface charges on silicon nitride (SiN x ). Such insight into the interactions between NPs and solid surfaces in liquids is useful for understanding dynamics at liquid−solid interfaces and has general implications for microfluidics, nanotribology, sensing, and self-assembly.
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