This article presents a quantitative analysis of the role played by poly(vinyl pyrrolidone) (PVP) in seed-mediated growth of Ag nanocrystals. Starting from Ag nanocubes encased by {100} facets as the seeds, the resultant nanocrystals could take different shapes depending on the concentration of PVP in the solution. If the concentration was above a critical value, the seeds simply grew into larger cubes still enclosed by {100} facets. When the concentration fell below a critical value, the seeds would evolve into cuboctahedrons enclosed by a mix of {100} and {111} facets and eventually octahedrons completely covered by {111} facets. We derived the coverage density of PVP on Ag(100) surface by combining the results from two measurements: i) cubic seeds were followed to grow at a fixed initial concentration of PVP to find out when {111} facets started to appear on the surface; and ii) cubic seeds were allowed to grow at reduced initial concentrations of PVP to see at which concentration {111} facets started to appear from the very beginning. We could calculate the coverage density of PVP from the differences in PVP concentration and the total surface area of Ag nanocubes between these two samples. The coverage density was found to be 140 and 30 repeating units per nm2 for PVP of 55,000 and 10,000 g/mol in molecular weight, respectively, for cubic seeds of 40 nm in edge length. These values dropped slightly to 100 and 20 repeating units per nm2, respectively, when 100-nm Ag cubes were used as the seeds.

The coverage density of poly(ethylene glycol) (PEG) is a key parameter in determining the efficiency of PEGylation, a process pivotal to in vivo delivery and targeting of nanomaterials. Here we report four complementary methods for quantifying the coverage density of PEG chains on various types of Au nanostructures by using a model system based on HS-PEG-NH2 with different molecular weights. Specifically, the methods involve reactions with fluorescamine and ninhydrin, as well as labeling with fluorescein isothiocyanate (FITC) and Cu2+ ions. The first two methods use conventional amine assays to measure the number of unreacted HS-PEG-NH2 molecules left behind in the solution after incubation with the Au nanostructures. The other two methods involve coupling between the terminal –NH2 groups of adsorbed -S-PEG-NH2 chains and FITC or a ligand for Cu2+ ion, and thus pertain to the “active” –NH2 groups on the surface of a Au nanostructure. We found that the coverage density decreased as the length of PEG chains increased. A stronger binding affinity of the initial capping ligand to the Au surface tended to reduce the PEGylation efficiency by slowing down the ligand exchange process. For the Au nanostructures and capping ligands we have tested, the PEGylation efficiency decreased in the order of citrate-capped nanoparticles > PVP-capped nanocages ≈ CTAC-capped nanoparticles ≫ CTAB-capped nanorods, where PVP, CTAC, and CTAB stand for poly(vinyl pyrrolidone), cetyltrimethylammonium chloride, and cetyltrimethylammonium bromide, respectively.

The ability to diagnose cancer based on the detection of rare cancer cells in blood or other bodily fluids is a significant challenge. To address this challenge, we have developed a microfluidic device that can simultaneously sort, enrich, and then detect multiple types of cancer cells from a complex sample. The device, which is made from poly(dimethylsiloxane) (PDMS), implements cell-affinity chromatography based on the selective cell-capture of immobilized DNA-aptamers and yields a 135-fold enrichment of rare cells in a single run. This enrichment is achieved because the height of the channel is on the order of a cell diameter. The sorted cells grow at the comparable rate as cultured cells and are 96% pure based on flow cytometry determination. Thus, by using our aptamer based device, cell capture is achieved simply and inexpensively, with no sample pretreatment before cell analysis. Enrichment and detection of multiple rare cancer cells can be used to detect cancers at the early stages, diagnose metastatic relapse, stratify patients for therapeutic purposes, monitor response to drugs and therapies, track tumor progression, and gain a deeper understanding of the biology of circulating tumor cells (CTCs).

If the cap fits: The epitaxial growth of Ag on Ag nanoplates could be directed to preferentially occur along the lateral or vertical direction by using sodium citrate or poly(vinyl pyrrolidone) (PVP) as capping agents (see picture). The edge length and thickness of the Ag nanoplates could be increased by 100 and 40 times, respectively. Raman scattering signals of Ag nanospheres on the Ag nanoplates were enhanced as the nanoplates became thicker.

Shaped and dimpled: Silver nanocrystals enclosed by concave surfaces and thus high-index facets have been prepared by simply controlling the growth habit of Ag cubic seeds. Four types of concave nanocrystals, including octahedron, cube, octapod, and trisoctahedron, were obtained (see picture).