Coordinating ligands are widely used to vary the solubility and reactivity of nanoparticles for subsequent bioconjugation. Although long-term colloidal stability is enhanced by using bidentate coordinating ligands over monodentate ones, other properties such as non-specific adsorption of target molecules and ligand exchange have not been quantified. In this study, we modified a near infrared dye to serve as a highly-sensitive reporter for non-specific binding of thiolated target molecules to nanoparticle surfaces that are functionalized with monodentate or bidentate coordinated ligands. Specifically, we analyzed non-specific binding mechanisms to quantum dots (QDs) by fitting the adsorption profiles to the Hill equation and the parameters are used to provide a microscopic picture of how ligand density and lability control non-specific adsorption. Surprisingly, bidentate ligands are worse at inhibiting adsorption to QD surfaces at low target:QD ratios, although they become better as the ratio increases, but only if the nanoparticle surface area is large enough to overcome steric effects. This result highlights that a balance between ligand density and lability depends on the dentate nature of the ligands and controls how molecules in solution can coordinate to the nanoparticle surface. These results will have major implications for a range of applications in nanobiomedicine, bioconjugation, single molecule spectroscopy, self-assembly and nano(photo)catalysis where both non-specific and specific surface interactions play important roles. As an example, we tested the ability of monodentate and bidentate functionalized nanoparticles to resist non-specific adsorption of IgG antibodies that contained free thiol groups at a 1:1 QD:IgG ratio and found that QDs with monodentate ligands did indeed result in lower non-specific adsorption.

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Ligand cross-linking is known to improve the colloidal stability of nanoparticles, particularly in aqueous solutions. However, most cross-linking is performed chemically, in which it is difficult to limit interparticle cross-linking, unless performed at low concentrations. Photochemical cross-linking is a promising approach but usually requires ultraviolet (UV) light to initiate. Using such high-energy photons can be harmful to systems in which the ligand–nanoparticle bond is fairly weak, as is the case for the commonly used semiconductor quantum dots (QDs). Here, we introduce a novel approach to cross-link thiolated ligands on QDs by utilizing the photocatalytic activity of QDs upon absorbing visible light. We show that using visible light leads to better ligand cross-linking by avoiding the problem of ligand dissociation that occurs upon UV light exposure. Once cross-linked, the ligands significantly enhance the colloidal stability of those same QDs that facilitated cross-linking.

We describe the trapping characteristic of hyaluronic acid ͑HA͒ molecules using silicon micromachined nanotweezers having two opposed needle tips ͑3-7 m gap͒ that were coated with Al, Au, and Pt thin films. In an aqueous HA solution, ac voltage ͑20 V pp , sine wave͒ was applied between the nanotweezer tips. In the experiments using the Alcoated nanotweezers, HA molecules were successfully trapped as a thin string bridged between the tips of the nanotweezers when the frequency of applied ac voltage was 500 Hz or higher. The diameter of the trapped string tended to decrease when the frequency of the applied ac voltage was increased. A molecular string with a diameter of 30 nm consisting of a bundle of multiple HA molecules was obtained when the frequency of the applied ac voltage was 3 kHz. In addition to the Al-coated nanotweezers, HA molecules were also successfully trapped as a string using the nanotweezers with tips coated with Au and Pt thin films.

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