Generation and mixing of subfemtoliter aqueous droplets on demand.

We describe a novel method of generating monodisperse subfemtoliter aqueous droplets on demand by means of piezoelectric injection. Droplets with volumes down to 200 aL are generated by this technique. The droplets are injected into a low refractive index perfluorocarbon so that they can be optically trapped. We demonstrate the use of optical tweezers to manipulate and mix droplets. For example, using optical tweezers we bring two droplets, one containing a calcium sensitive dye and the other calcium chloride, into contact. The droplets coalesce with a resulting reaction time of about 1 ms. The monodispersity, manipulability, repeatability, small size, and fast mixing afforded by this system offer many opportunities for nanochemistry and observation of chemical reactions on a molecule-by-molecule basis.

Green fluorescent protein in inertially injected aqueous nanodroplets.

We inertially inject and study the contents of optically trappable aqueous nanodroplets (hydrosomes) emulsified in a perfluorinated matrix. A new piezoelectric actuated device for production of single hydrosomes on demand is introduced. Hydrosomes containing enhanced green fluorescent protein (EGFP) were injected, optically trapped, and held at the focus of an excitation laser in a confocal microscope, and single-molecule photobleaching events were observed. The rotational diffusion time of EGFP in trapped hydrosomes was measured using time-resolved fluorescence anisotropy. In free solution, the mean rotational diffusion time was determined to be 13.8 +/- 0.1 ns at 3 microM and 14.0 +/- 0.2 ns at 10 microM. In hydrosomes, the mean rotational diffusion time was similar and determined to be 12.6 +/- 1.0 ns at 3 microM and 15.5 +/- 1.6 ns at 10 microM. We conclude that the rotational motion inside the nanodroplets is consistent with rotation in free solution and that the protein therefore does not aggregate at the water-oil interface. Protein can be confined in hydrosomes with high efficiency using this technique, which provides an alternative to surface attachment or lipid encapsulation and opens up new avenues of research using single molecules contained in fluid nanovolumes.

Application of scanning probe microscopy to the characterization and fabrication of hybrid nanomaterials.

Scanning probe microscopy (SPM) is a widely used experimental technique for characterizing and fabricating nanostructures on surfaces. In particular, due to its ability to spatially map variations in materials properties with nanometer spatial resolution, SPM is particularly well suited to probe the subcomponents and interfaces of hybrid nanomaterials, i.e., materials that are made up of distinct nanometer scale components with distinguishable properties. In addition, the interaction of the SPM tip with materials can be intentionally tuned such that local surface modification is achieved. In this manner, hybrid nanostructures can also be fabricated on solid substrates using SPM. This report reviews recent developments in the characterization and fabrication of hybrid nanomaterials with SPM. Specific attention is given to nanomaterials that consist of both organic and inorganic components including individual biomolecules mounted on inorganic substrates. SPM techniques that are particularly well suited for characterizing the mechanical and electrical properties of such hybrid systems in atmospheric pressure environments are highlighted, and specific illustrative examples are provided. This review concludes with a brief discussion of the remaining challenges and promising future prospects for this field.