Analyzing Spin Selectivity in DNA-Mediated Charge Transfer <i>via</i> Fluorescence Microscopy

Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the significance of electron spin in biological processes and to assessing the potential of chiral assemblies for organic spintronics applications. Here, we use fluorescence microscopy to visualize the effects of spin-dependent charge transport in self-assembled monolayers of double-stranded DNA on ferromagnetic substrates. Patterned DNA arrays provide background regions for every measurement to enable quantification of substrate magnetization-dependent fluorescence due to the chiral-induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and the efficiency of DNA-mediated charge transfer to the surface. The latter process is modulated using an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. We discuss our results in the context of the current literature on the chiral-induced spin selectivity effect across various systems

Halide Anions as Shape-Directing Agents for Obtaining High-Quality Anisotropic Gold Nanostructures

The fundamental role of halide anions in the seed-mediated synthesis of anisotropic noble metal nanostructures has been a subject of debate within the nanomaterials community. Herein, we systematically investigate the roles of chloride, bromide and iodide anions in mediating the growth of anisotropic Au nanostructures. A high-purity surfactant solution of hexadecyltrimethylammonium bromide (CTABr) is used to reliably probe the role of each halide anion without interference from impurities. Our investigation reveals that bromide anions are required for the formation of Au nanorods, while the controlled combination of both bromide and iodide anions are necessary for the production of high-quality Au nanoprisms. Chloride anions, however, are ineffective at promoting anisotropic architectures and are detrimental to nanorod and/or nanoprism growth at high concentrations. We examine the seed structure and propose a growth model based on facet-selective adsorption on low-index Au facets to rationalize the nanostructures obtained by these methods. Our approach provides a facile synthesis of anisotropic Au nanostructures by way of a single growth solution and yields the desired morphologies with high purity. These results demonstrate that appropriate combinations of halide anions provide a versatile paradigm for manipulating the morphological distribution of Au nanostructures

Small-Molecule Patterning via Prefunctionalized Alkanethiols

Interactions between small molecules and biomolecules are important physiologically and for biosensing, diagnostic, and therapeutic applications. To investigate these interactions, small molecules can be tethered to substrates through standard coupling chemistries. While convenient, these approaches co-opt one or more of the few small-molecule functional groups needed for biorecognition. Moreover, for multiplexing, individual probes require different surface functionalization chemistries, conditions, and/or protection/deprotection strategies. Thus, when placing multiple small molecules on surfaces, orthogonal chemistries are needed that preserve all functional groups and are sequentially compatible. Alternately, we approach high-fidelity small-molecule patterning by coupling small-molecule neurotransmitter precursors, as examples, to monodisperse asymmetric oligo­(ethylene glycol)­alkanethiols during synthesis and prior to self-assembly on Au substrates. We use chemical lift-off lithography to singly and doubly pattern substrates. Selective antibody recognition of prefunctionalized thiols was comparable to or better than recognition of small molecules functionalized to alkanethiols after surface assembly. These findings demonstrate that synthesis and patterning approaches that circumvent sequential surface conjugation chemistries enable biomolecule recognition and afford gateways to multiplexed small-molecule functionalized substrates