Anomalous nanoparticle surface diffusion in LCTEM is revealed by deep learning-assisted analysis.

The motion of nanoparticles near surfaces is of fundamental importance in physics, biology, and chemistry. Liquid cell transmission electron microscopy (LCTEM) is a promising technique for studying motion of nanoparticles with high spatial resolution. Yet, the lack of understanding of how the electron beam of the microscope affects the particle motion has held back advancement in using LCTEM for in situ single nanoparticle and macromolecule tracking at interfaces. Here, we experimentally studied the motion of a model system of gold nanoparticles dispersed in water and moving adjacent to the silicon nitride membrane of a commercial LC in a broad range of electron beam dose rates. We find that the nanoparticles exhibit anomalous diffusive behavior modulated by the electron beam dose rate. We characterized the anomalous diffusion of nanoparticles in LCTEM using a convolutional deep neural-network model and canonical statistical tests. The results demonstrate that the nanoparticle motion is governed by fractional Brownian motion at low dose rates, resembling diffusion in a viscoelastic medium, and continuous-time random walk at high dose rates, resembling diffusion on an energy landscape with pinning sites. Both behaviors can be explained by the presence of silanol molecular species on the surface of the silicon nitride membrane and the ionic species in solution formed by radiolysis of water in presence of the electron beam.

Precise Colloidal Plasmonic Photocatalysts Constructed by Multistep Photodepositions.

Natural photosynthesis relies on a sophisticated charge transfer pathway among multiple components with precise spatial, energetic, and temporal organizations in the aqueous environment. It continues to inspire and challenge the design and fabrication of artificial multicomponent colloidal nanostructures for solar-to-fuel conversion. Herein, we introduce a plasmonic photocatalyst synthesized with colloidal methods with five integrated components including cocatalysts installed in orthogonal locations. The precise deposition of individual inorganic components on an Au/TiO2 nanodumbell nanostructure is enabled by photoreduction and photo-oxidation, which selectively occurs at the TiO2 tip sites and Au lateral sites, respectively. Under visible-light irradiation, the photocatalyst exhibited activity of oxygen evolution from water without scavengers. We demonstrate that each component is essential for improving the photocatalytic performance. In addition, mechanistic studies suggest that the photocatalytic reaction requires combining the hot charge carriers derived from exciting both the d-sp interband transition and the localized surface plasmon resonance of Au.

Spontaneous formation of aligned DNA nanowires by capillarity-induced skin folding.

Although DNA nanowires have proven useful as a template for fabricating functional nanomaterials and a platform for genetic analysis, their widespread use is still hindered because of limited control over the size, geometry, and alignment of the nanowires. Here, we document the capillarity-induced folding of an initially wrinkled surface and present an approach to the spontaneous formation of aligned DNA nanowires using a template whose surface morphology dynamically changes in response to liquid. In particular, we exploit the familiar wrinkling phenomenon that results from compression of a thin skin on a soft substrate. Once a droplet of liquid solution containing DNA molecules is placed on the wrinkled surface, the liquid from the droplet enters certain wrinkled channels. The capillary forces deform wrinkles containing liquid into sharp folds, whereas the neighboring empty wrinkles are stretched out. In this way, we obtain a periodic array of folded channels that contain liquid solution with DNA molecules. Such an approach serves as a template for the fabrication of arrays of straight or wrinkled DNA nanowires, where their characteristic scales are robustly tunable with the physical properties of liquid and the mechanical and geometrical properties of the elastic system.

Hybrid Lithographic and DNA-Directed Assembly of a Configurable Plasmonic Metamaterial That Exhibits Electromagnetically Induced Transparency.

Metamaterials are architectures that interact with light in novel ways by virtue of symmetry manipulation, and have opened a window into studying unprecedented light-matter interactions. However, they are commonly fabricated via lithographic methods, are usually static structures, and are limited in how they can react to external stimuli. Here we show that by combining lithographic techniques with DNA-based self-assembly methods, we can construct responsive plasmonic metamaterials that exhibit the plasmonic analog of an effect known as electromagnetically induced transparency (EIT), which can dramatically change their spectra upon motion of their constituent parts. Correlative scanning electron microscopy measurements, scattering dark-field microscopy, and computational simulations are performed on single assemblies to determine the relationship between their structures and spectral responses to a variety of external stimuli. The strength of the EIT-like effect in these assemblies can be tuned by precisely controlling the positioning of the plasmonic nanoparticles in these structures. For example, changing the ionic environment or dehydrating the sample will change the conformation of the DNA linkers and therefore the distance between the nanoparticles. Dark-field spectra of individual assemblies show peak shifts of up to many tens of nanometers upon DNA perturbations. This dynamic metamaterial represents a stepping stone toward state-of-the-art plasmonic sensing platforms and next-generation dynamic metamaterials.

Combination of a Sample Pretreatment Microfluidic Device with a Photoluminescent Graphene Oxide Quantum Dot Sensor for Trace Lead Detection.

A novel trace lead ion (Pb(2+)) detection platform by combining a microfluidic sample pretreatment device with a DNA aptamer linked photoluminescent graphene oxide quantum dot (GOQD) sensor was proposed. The multilayered microdevice included a microchamber which was packed with cation exchange resins for preconcentrating metal ions. The sample loading and recovery were automatically actuated by a peristaltic polydimethylsiloxane micropump with a flow rate of 84 µL/min. Effects of the micropump actuation time, metal ion concentration, pH, and the volumes of the sample and eluent on the metal ion capture and preconcentration efficiency were investigated on a chip. The Pb(2+) samples whose concentrations ranged from 0.48 nM to 1.2 µM were successfully recovered with a preconcentration factor value between 4 and 5. Then, the preconcentrated metal ions were quantitatively analyzed with a DNA aptamer modified GOQD. The DNA aptamer on the GOQD specifically captured the target Pb(2+) which can induce electron transfer from GOQD to Pb(2+) upon UV irradiation, thereby resulting in the fluorescence quenching of the GOQD. The disturbing effect of foreign anions on the Pb(2+) detection and the spiked Pb(2+) real samples were also analyzed. The proposed GOQD metal ion sensor exhibited highly sensitive Pb(2+) detection with a detection limit of 0.64 nM and a dynamic range from 1 to 1000 nM. The on-chip preconcentration of the trace metal ions from a large-volume sample followed by the metal ion detection by the fluorescent GOQD sensor can provide an advanced platform for on-site water pollution screening.

Is the Chain of Oxidation and Reduction Process Reversible in Luminescent Graphene Quantum Dots?

Graphene-based quantum dots (QDs) have received a tremendous amount of attention as a new type of light-emitting materials. However, their luminescence origins remain controversial due to extrinsic states of the impurities and disorder structures. Especially, the function of oxygen-contents should be understood and controlled as a crucial element for tuning the optical properties of graphene-based QDs. Herein, a series of graphene oxide QDs (GOQDs) with different amounts of oxygen-contents are first synthesized via a direct oxidation route of graphite nanoparticle and thoroughly compared with a series of reduced GOQDs (rGOQDs) prepared by the conventional chemical reduction. Irreversible emission and different carrier dynamics are observed between the GOQDs and rGOQDs, although both routes show a similar tendency with regard to the variation of oxygen-functional components. Their luminescence mechanisms are closely associated with different atomic structures. The mechanism for the rGOQDs can be associated with a formation of small sp(2) nanodomains as luminescent centers, whereas those of GOQDs may be composed of oxygen-islands with difference sizes depending on oxidation conditions surrounded by a large area of sp(2) bonding. Important insights for understanding the optical properties of graphene-based QDs and how they are affected by oxygen-functional groups are shown.

Dual role of blue luminescent MoS2 quantum dots in fluorescence resonance energy transfer phenomenon.

Homogeneous blue luminescent MoS2 quantum dots are fabricated by using a lithium intercalation method from MoS2 nanoparticles, and the unique blue photoluminescence property is utilized in the Alexa Fluor 430-dsDNA-MoS2 FRET system, demonstrating the dual function of MoS2 quantum dots as a donor and an acceptor.

Photoluminescent graphene oxide microarray for multiplex heavy metal ion analysis.

An aptamer-linked graphene oxide (GO) microarray is synthesized for multiplex heavy metal ion detection. Fluorescent nanosized GO sheets are micropatterned, and specific aptamers targeting Ag(+) and Hg(2+) are immobilized on the GO array. Upon capture of the target heavy metal ions, electron transfer occurs between the GO (donors) and the heavy metal ions (acceptors), leading to fluorescence quenching of the GO.

AutoDetect-mNP: An Unsupervised Machine Learning Algorithm for Automated Analysis of Transmission Electron Microscope Images of Metal Nanoparticles.

The synthesis quality of artificial inorganic nanocrystals is most often assessed by transmission electron microscopy (TEM) for which high-throughput advances have dramatically increased both the quantity and information richness of metal nanoparticle (mNP) characterization. Existing automated data analysis algorithms of TEM mNP images generally adopt a supervised approach, requiring a significant effort in human preparation of labeled data that reduces objectivity, efficiency, and generalizability. We have developed an unsupervised algorithm AutoDetect-mNP for automated analysis of TEM images that objectively extracts morphological information on convex mNPs from TEM images based on their shape attributes, requiring little to no human input in the process. The performance of AutoDetect-mNP is tested on two data sets of bright field TEM images of Au nanoparticles with different shapes and further extended to palladium nanocubes and cadmium selenide quantum dots, demonstrating that the algorithm is quantitatively reliable and can thus serve as a generalizable measure of the morphology distributions of any mNP synthesis. The AutoDetect-mNP algorithm will aid in future developments of high-throughput characterization of mNPs and the future advent of time-resolved TEM studies that can investigate reaction mechanisms of mNP synthesis and reactivity.

Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence.

Pristine graphene quantum dots and graphene oxide quantum dots are synthesized by chemical exfoliation from the graphite nanoparticles with high uniformity in terms of shape (circle), size (less than 4 nm), and thickness (monolayer). The origin of the blue and green photoluminescence of GQDs and GOQDs is attributed to intrinsic and extrinsic energy states, respectively.