The growing utility of semiconductor quantum dots (QDs) in biochemical and cellular research necessitates, in turn, continuous development of surface functionalizing ligands to optimize their performance for ever more challenging and diverse biological applications. Here, we describe a new class of multifunctional polymeric ligands as a stable, compact and high affinity alternative to multidentate thiolated molecules. The polymeric ligands are designed with a poly(acrylic acid) backbone where pyridines are used as anchoring groups that are not sensitive to degradation by air and light, along with short poly(ethylene glycol) (PEG) pendant groups which are coincorporated for aqueous solubility, biocompatibility and colloidal stability. The percentages of each of the latter functional groups are controlled during initial synthesis along with incorporation of carboxyl groups which serve as chemical handles for subsequent covalent modification of the QD surface. A detailed physicochemical characterization indicates that the multiple pyridine groups are efficiently bound on the QD surface since they provide for relatively small overall hydrodynamic sizes along with good colloidal stability and strong fluorescence over a wide pH range, under high salt concentration and in extremely dilute conditions at room temperature under room light over extended timeframes. Covalent conjugation of dyes and metal-affinity coordination with functional enzymes to the QD surfaces were also demonstrated. Biocompatibility and long-term stability of the pyridine polymer coated QDs were then confirmed in a battery of relevant assays including cellular delivery by both microinjection and peptide facilitated uptake along with intracellular single QD tracking studies and cytotoxicity testing. Cumulatively, these results suggest this QD functionalization strategy is a viable alternative that provides some desirable properties of both compact, discrete ligands and large amphiphilic polymers.

We demonstrate the ability to tune the current rectification in metal–molecule–metal junctions through control of the interaction strength of one of the two metal–molecule contacts. Current–voltage characteristics of thiolate bound molecular wires with a nitro or pyridine termination show that the extent of current rectification in a molecular junction correlates well with the extent of coupling between the chemical linker and metal electrode.

The second-order nonlinear optical response of amine and phenol/ether derivatives (1−8) has been evaluated using experimental and theoretical techniques. Electric-field-induced second-harmonic generation (EFISH) measurements establish that N-phenyl substitution of 4-nitroaniline (1) produces a greater increase in molecular hyperpolarizability than N-methyl substitution. In contrast, O-phenyl substitution of 4-nitrophenol (6) produces a smaller increase in hyperpolarizability than O-methyl substitution. Neither the enhancement of hyperpolarizability upon N-phenyl substitution nor the differential substituent effect is anticipated on the basis of qualitative arguments. Careful theoretical analysis using semiempirical sum-over-states and finite field calculations provide explanations for both observed effects.

Supramolecular structures have the potential to provide macromolecular behavior using relatively low molar mass building blocks. We present here data on the self-assembly of triblock rodcoil molecules which contain a rigid biphenyl ester segment covalently linked to structurally diverse oligomeric segments. These molecules form supramolecular aggregates with molar masses in the range 105−106 Da, and our experiments probe how supramolecular structure can be manipulated by varying the volume fraction of the coillike flexible segments with respect to that of rod segments. The oligostyrene−oligoisoprene diblock coils were synthesized via anionic polymerization and varied in average length from (6sty, 6iso) to (30sty, 30iso). Small-angle X-ray scattering scans revealed layer spacings corresponding to monolayers that increase in size as the coil's molar mass increases. We observed that an increase in coil volume fraction reduces the thermal stability of the supramolecular structure, but a corresponding increase in rod segment length can counteract this effect. Finally the self-organized nanostructures seem to pack into a superlattice based on evidence obtained by X-ray scattering and transmission electron microscopy.

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