No abstract

No abstract

We report here the first example of abiotic resistive-pulse sensing of a molecular (as opposed to a particle or macromolecular) analyte. This was accomplished by using a conically shaped nanopore prepared by the track-etch method as the sensing element. It is possible to sense the molecular analyte because the small diameter opening of the conical nanopore (approximately 4.5 nm) is comparable to the diameter of the analyte molecule (approximately 2 nm).

Single-component monolayers of dendrimers and two-component monolayers consisting of dendrimers and n-alkanethiols immobilized on Au substrates are described. Single-component monolayers are prepared by exposing an Au substrate to ethanolic solutions of amine- or hydroxy-terminated polyamidoamine (PAMAM) dendrimers. The resulting monolayers are highly stable and nearly close-packed for dendrimer generations ranging from 4 to 8 (G4−G8). Electrochemical ac-impedance measurements indicate that the dendrimer surface is very porous toward the electroactive redox couple Fe(CN)6 3-/4-. Ferrocene-terminated dendrimer monolayers have also been investigated. Exposure of higher-generation dendrimer monolayers to ethanolic solutions of hexadecanethiol (C16SH) results in a dramatic compression of the dendrimers, and causes them to reorient on the surface from an oblate to prolate configuration. The dendrimers originally present on the surface do not desorb as a consequence of this configurational change. Comparison of the extent of adsorption of C16SH in different media (vapor-phase N2, hexane, and ethanol) shows that solvation of the dendrimers is the primary driving force for the structural change. Finally, the reactivity and stability of the dendrimer monolayers is investigated by on-surface functionalization of the dendrimer monolayer with 4-(trifluoromethyl)benzoyl chloride. The physical and chemical properties of the single- and two-component monolayers are evaluated by using reflection infrared spectroscopy, ellipsometry, contact-angle measurements, ac-impedance spectroscopy, cyclic voltammetry, and surface acoustic wave (SAW)-based analyte-dosing experiments.

Scanning ion conductance microscopy (SICM) is a versatile type of scanning probe microscopy for studies in molecular biology and materials science. Recent advances in feedback and probe fabrication have greatly increased the resolution, stability, and speed of imaging. Noncontact imaging and the ability to deliver materials to localized areas have made SICM especially fruitful for studies of molecular biology, and many examples of such use have been reported. In this review, we highlight new developments in the operation of SICM and describe some of the most exciting recent studies from this growing field.

There is increasing interest in using nanopores in synthetic membranes as resistive-pulse sensors for molecular and macromolecule analytes. In general, this method entails measuring current pulses associated with translocation of the analyte through the nanopore sensor element. A key challenge for this sensing paradigm is building selectivity into the protocol so that the current pulses for the target analyte can be distinguished from current pulses for other species that might be present in the sample. We show here that this can be accomplished with a protein analyte by adding to the solution an antibody that selectively binds the protein. We demonstrate this concept using bovine serum albumin (BSA) and a Fab fragment from a BSA-binding polyclonal antibody. Because the complex formed upon binding of the Fab to BSA is larger than the free BSA molecule, the current-pulse signature for the BSA/Fab complex can be easily distinguished from the free BSA. Furthermore, the BSA/Fab pulses can be easily distinguished from the pulses obtained for the free Fab and from pulses obtained for a control protein that does not bind to the Fab. Finally, we also show that the current-pulse signature for the BSA/Fab complex can provide information about the size and stoichiometry of the complex.

Au colloids in the 2-3-nm size regime were prepared by in situ reduction of HAuCl(4) in the presence of poly(amidoamine) dendrimers. The dendrimers encapsulate the colloids, imparting stability to the aqueous colloidal solutions. The nanocomposite materials can be isolated by precipitation. The dendrimer generation used in the synthesis controls the size of the resultant colloids:  lower-generation dendrimers give rise to larger colloids. The materials were characterized by infrared and UV-vis spectroscopy and transmission electron microscopy.

We report an atomic force microscopy (AFM) investigation of generation 4 and 8 (G4, G8) polyamidoamine (PAMAM) starburst dendrimers adsorbed on Au (111) surfaces. Heights measured for isolated, adsorbed dendrimers indicate they are substantially more oblate than expected from their spherical shapes in solution. By controlling dendrimer concentration and exposure time during adsorption, modified surfaces ranging from isolated molecules to near-monolayer coverages were obtained. Exposure of surfaces bearing adsorbed isolated dendrimers to hexadecanethiol solutions changed their conformation from oblate to prolate as more stable thiol-Au bonds replaced some of the amine-Au bonds. For surfaces of near-monolayer coverage, exposure to hexadecanethiol caused the dendrimers to gradually agglomerate, forming dendrimer "pillars" up to 30-nm high.There is a vast literature pertaining to dendrimers, 1 but only a few reports of individual dendrimer visualization using TEM, 2 STM, 3 and AFM, 4,5 or showing the grainy structure of a dendrimer monolayer using AFM. 6 For our study, dendrimers 7 were adsorbed onto atomically flat Au (111) facets by dipping the substrate 8 into either a 10 -7 M ethanolic solution (for monolayers) or a 10 -9 M solution (for isolated molecules) for 45 s. Au substrates were then alternately rinsed with ethanol and water. To alter the shape of isolated dendrimers, samples were soaked for 4 h in a 1 mM ethanolic solution of hexadecanethiol and then rinsed as described above. To induce agglomeration, dendrimer monolayers were soaked in hexadecanethiol solution for 24-110 h. Measurements made in a minimum of three different areas within each of five well-separated 10 × 10 µm sites on each of two identically prepared Au substrates (30 areas altogether) yielded consistent results. Tapping-mode AFM measurements (topographical data only) in air were performed using a Nanoscope III STM with an E-type scanner. 9 We investigated two different sizes of dendrimers: the soft and deformable G4 (ideal sphere diameter, 4.5 nm) and the larger G8 having a harder exterior (ideal sphere diameter, 9.7 nm). 7 The results for G8 are displayed in parts a-c of Figure 1. Figure 1a shows the topography of the Au surface covered with isolated G8 dendrimers. In the upper right corner, an Au step edge is visible, its height (0.24 nm) providing a vertical reference scale. The measured diameter of the adsorbed G8 dendrimer is approximately 20 nm, but as discussed below, the lateral dimensions are convoluted with the AFM tip shape. Nevertheless, the height data in Figure 1b are reliable. Within experimental error (0.1-0.2 nm) and on the basis of data from more than 100 single dendrimers, the height of the G8 dendrimers on a naked Au surface ranges from 3.5 to 4.0 nm, or about 60% less than the ideal-sphere diameter of 9.7 nm. The variation in height and lateral size of the surface-confined dendrimers may arise from a distribution of molecular sizes resulting from the synthesis, 2b tipinduced deformation of the dendrimers, 6,10-12 ...