Low temperature scanning tunneling microscope images and spectroscopic data have been obtained on subnanometer size Pb clusters fabricated using the technique of buffer layer assisted growth. Discrete energy levels were resolved in current-voltage characteristics as current peaks rather than current steps. Distributions of peak voltage spacings and peak current heights were consistent with Wigner-Dyson and Porter-Thomas distributions, respectively, suggesting the relevance of random matrix theory to the description of the electronic eigenstates of the clusters. The observation of peaks rather than steps in the current-voltage characteristics is attributed to a resonant tunneling process involving the discrete energy levels of the cluster, the tip, and the states at the interface between the cluster and the substrate surface.
We develop a new strategy to prepare quantum dot (QD) barcode particles by polymerizing double emulsion droplets prepared in capillary microfluidic devices. The resultant barcode particles are composed of stable QDtagged core particles surrounded by hydrogel shells. These particles exhibit uniform spectral characteristics and excellent coding capability, as confirmed by photoluminescence analyses. By using double emulsion droplets with two inner droplets of distinct phases as templates, we have also fabricated anisotropic magnetic barcode particles with two separate cores or with a Janus core. These particles enable optical encoding and magnetic separation, thus making them excellent functional barcode particles in biomedical applications.The increasing use of high-throughput assays in biomedical applications, including drug discovery and clinical diagnostics, 1,2 demands effective strategies for multiplexing. One promising strategy is to use barcode particles, which are particles that encode information about their specific compositions and enable simple identification. Many encoding strategies have been proposed for these barcode particles; these include incorporation of segmented nanorods, 3 photo-patterning, 4-6 as well as the use of photonic crystals, 7,8 fluorescent silica colloids, 9 and semiconductor quantum dots (QDs). [10][11][12][13][14][15][16] In particular, the semiconductor QDs hold immense promise as barcode elements because of their excellent optical properties such as simultaneous excitation of multiple wavelength-and-intensity with a single light source, minimal spectral width, and remarkable photo-stability. In particular, the semiconductor QDs hold immense promise as barcode elements because of their excellent optical properties such as minimal spectral width, and remarkable photo-stability. In addition, by mixing QDs with different emission wavelengths at different concentrations, significantly larger combinations can be generated with a single excitation wavelength. To generate barcodes, QDs can be incorporated into the particles before their polymerization or during their swelling, 10,14 The QDs can also be applied as a coating on the surface of the particles. 11,12 However, QD barcodes generated by this approach often suffer from leakage of QDs; this significantly affects the performance and stability of the barcode particles. In addition, traditional processes used for generating such particles provide little control over the characteristics of the result particles, such as particle sizes and QDs number and distribution within each particles; thus the resultant QD barcodes often have high variability of fluorescence intensities. [13][14][15][16] This is exacerbated by the uncontrolled motion of these barcode particles, which reduces the sensitivity of the assay 6 and demands more complicated procedures for enriching the barcode particles during assay 17 . These, together with the debatable biocompatibility of such particles, have limited their applications. 16 Thus, novel appr...
Microcapsules with core-shell structures are excellent vehicles for the encapsulation of active ingredients; however, the actives often leak out of these structures over time, without observable damage to them. We present a novel approach to enhancing the encapsulation of active ingredients inside microcapsules. We use two components that can form solid precipitates upon mixing and add one each to the microcapsule core and to the continuous phase. The components diffuse through the shell in the same manner as the actives, but upon meeting, they precipitate to form solid particles within the shell; this significantly reduces leakage through the shell of the microcapsules. We show that the reduction in the leakage of actives is due to the blockage of channels or pores that exist in the shell of the capsules by the solid precipitates.
We present a strategy for preparing size-controlled gas-filled microparticles using two aqueous components that chemically react to produce the gas. We use a dual-bore microfluidic device to isolate the reactants of two gas-producing reactions until they are encapsulated in the outer droplet. The reactants in the monodisperse droplets merge and produce the gas bubbles, which are stabilized with a surfactant and form the core of the microparticles. The number and size of the generated gas bubbles are governed by the gas-forming reaction used. Our versatile strategy can be applied to a wide range of gas-producing reactions.
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