Pik Kwan Lo

Modular construction of DNA nanotubes of tunable geometry and single- or double-stranded character

DNA nanotubes can template the growth of nanowires, orient transmembrane proteins for nuclear magnetic resonance determination, and can potentially act as stiff interconnects, tracks for molecular motors and nanoscale drug carriers. Current methods for the construction of DNA nanotubes result in symmetrical and cylindrical assemblies that are entirely double-stranded. Here, we report a modular approach to DNA nanotube synthesis that provides access to geometrically well-defined triangular and square-shaped DNA nanotubes. We also construct the first nanotube assemblies that can exist in double- and single-stranded forms with significantly different stiffness. This approach allows for parameters such as geometry, stiffness, and single- or double-stranded character to be fine-tuned, and could enable the creation of designer nanotubes for a range of applications, including the growth of nanowires of controlled shape, the loading and release of cargo, and the real-time modulation of stiffness and persistence length within DNA interconnects.

Nucleobase-templated polymerization: copying the chain length and polydispersity of living polymers into conjugated polymers.

Conjugated polymers synthesized by step polymerization mechanisms typically suffer from poor molecular weight control and broad molecular weight distributions. We report a new method which uses nucleobase recognition to read out and efficiently copy the controlled chain length and narrow molecular weight distribution of a polymer template generated by living polymerization, into a daughter conjugated polymer. Aligning nucleobase-containing monomers on their complementary parent template using hydrogen-bonding interactions, and subsequently carrying out a Sonogashira polymerization, leads to the templated synthesis of a conjugated polymer. Remarkably, this daughter strand is found to possess a narrow molecular weight distribution and a chain length nearly equivalent to that of the parent template. On the other hand, nontemplated polymerization or polymerization with the incorrect template generates a short conjugated oligomer with a significantly broader molecular weight distribution. Hence, nucleobase-templated polymerization is a useful tool in polymer synthesis, in this case allowing the use of a large number of polymers generated by living methods, such as anionic polymerization, controlled radical polymerizations (NMP, ATRP, and RAFT) and other mechanisms to program the structure, length, and molecular weight distribution of polymers normally generated by step polymerization methods and significantly enhance their properties.

Self-assembly of three-dimensional DNA nanostructures and potential biological applications.

A current challenge in nanoscience is to achieve controlled organization in three-dimensions, to provide tools for biophysics, molecular sensors, enzymatic cascades, drug delivery, tissue engineering, and device fabrication. DNA displays some of the most predictable and programmable interactions of any molecule, natural or synthetic. As a result, 3D-DNA nanostructures have emerged as promising tools for biology and materials science. In this review, strategies for 3D-DNA assembly are discussed. DNA cages, nanotubes, dendritic networks, and crystals are formed, with deliberate variation of their size, shape, persistence length, and porosities. They can exhibit dynamic character, allowing their selective switching with external stimuli. They can encapsulate and position materials into arbitrarily designed patterns, and show promise for numerous biological and materials applications.

Extended Calix[4]arene-Based Receptors for Molecular Recognition and Sensing.

Recent advances in the area of recognition and sensing have shown that artificial receptors derived from extended calix[4]arenes bearing multiple π-conjugated fluorophoric or chromophoric systems have found useful to enhance binding affinity, selectivity and sensitivity for recognition and sensing of a targeted ion or molecule. A comprehensive review of various π-conjugation-extended calix[4]arene-based receptors with the highlight on the design and binding characterization for recognition and sensing is presented.

Chiral Metal–DNA Four‐Arm Junctions and Metalated Nanotubular Structures

DNA is a promising template for the programmable assembly of nanostructures. A number of construction strategies have been developed, such as the weaving together of many DNA strands into “tiles” or the stapling of a long DNA strand into “origami” assemblies. These approaches use DNA as the only information source for organization and result in DNAdense, large, and rigid nanostructures. An interesting alternative is the introduction of synthetic molecules to create DNA assemblies with new structures and functions. The use of organic or inorganic molecules as junctions can eliminate the need to interweave DNA strands for structural definition and thus results in “DNA-economical” structures with smaller sizes and increased dynamic character. Transition-metal junctions are especially useful, because they can impart functional advantages, such as photophysical, redox, catalytic, and magnetic properties, as well as enhanced stability to DNA nanostructures. 6] Because of their diverse coordination geometries, transition metals may also have key structural advantages for DNA nanoconstruction. They can lead to junction architectures that are inaccessible with DNA alone and may be able to mediate the transfer of chirality from the DNA double helix to the junction itself. Such chirality transfer is particularly desirable, as many DNA junctions are intrinsically chiral because of sequence and groove asymmetry, and their assembly can lead to diastereomeric mixtures. Control of the chirality of these junctions can result in stereospecific, higher-yielding DNA-nanostructure syntheses and more specific interactions with other biological molecules. Herein we describe a DNA-templated method for the formation of a chiral metal–DNA junction containing a single copper(I)–bisphenanthroline unit at its central point and four single-stranded DNA arms of different sequences. This structure is the simplest, most compact four-arm junction derived from DNA and would be difficult to access without the mediation of the transition metal. The design, evolution, and optimization of the metal–ligand complex for efficient chirality transfer from DNA is reported. From this structure, we then constructed metal–DNA triangular rungs and formed the first metal–DNA nanotubular structures. In our approach to the construction of chiral junctions with four different arms, we use two DNA strands, D1 and D2, each modified in the middle of the 20-base sequence with a diphenylphenanthroline (dpp) ligand (Figure 1a). A template

Indole-based Cyanine as a Nuclear RNA-Selective Two-Photon Fluorescent Probe for Live Cell Imaging

We have demonstrated that the subcellular targeting properties of the indole-based cyanines can be tuned by the functional substituent attached onto the indole moiety in which the first example of a highly RNA-selective and two-photon active fluorescent light-up probe for high contrast and brightness TPEF images of rRNA in the nucleolus of live cells has been developed. It is important to find that this cyanine binds much stronger toward RNA than DNA in a buffer solution as well as selectively stains and targets to rRNA in the nucleolus. Remarkably, the TPEF brightness (Φσmax) is dramatically increased with 11-fold enhancement in the presence of rRNA, leading to the record high Φσmax of 228 GM for RNA. This probe not only shows good biocompatibility and superior photostability but also offers general applicability to various live cell lines including HeLa, HepG2, MCF-7, and KB cells and excellent counterstaining compatibility with commercially available DNA or protein trackers.

Reorganization of cytoskeleton and transient activation of Ca2+ channels in mesenchymal stem cells cultured on silicon nanowire arrays.

Tissue engineering combines biological cells and synthetic materials containing chemical signaling molecules to form scaffolds for tissue regeneration. Mesenchymal stem cells (MSCs) provide an attractive source for tissue engineering due to their versatility of multipotent differentiation. Recently, it has been recognized that both chemical and mechanical stimulations are essential mediators of adhesion and differentiation of MSCs. While significant progress has been made on the understanding of chemical regulatory factors within the extracellular matrix, the effects of mechanical stimulation exerted by nanomaterials on MSCs and the underlying mechanisms are less well-known. The present study showed that the adhesion, proliferation, and differentiation of MSCs cultured on vertically aligned silicon nanowire (SiNW) arrays were significantly different from those on flat silicon wafer and control substrates. The interactions between MSCs and the SiNW arrays caused the stem cells to preferentially differentiate toward osteocytes and chondrocytes but not adipocytes in the absence of supplementary growth factors. Our study demonstrated that Ca(2+) ion channels were transiently activated in MSCs upon mechanical stimulation, which eventually led to activation of Ras/Raf/MEK/ERK signaling cascades to regulate adhesion, proliferation, and differentiation of MSCs. The stretch-mediated transient Ca(2+) ion channel activation and cytoskeleton reorganization during stem cell-nanowire interaction may be early events of lineage-specific potentiation of MSCs in determining the fates of mesenchymal stem cells cultured on microenvironments with specific mechanical properties.

Ruthenium(II) Complex Incorporated UiO-67 Metal–Organic Framework Nanoparticles for Enhanced Two-Photon Fluorescence Imaging and Photodynamic Cancer Therapy

Ruthenium(II) tris(bipyridyl) cationic complex (Ru(bpy)32+) incorporated UiO-67 (Universitetet i Oslo) nanoscale metal-organic frameworks (NMOFs) with an average diameter of ∼92 nm were developed as theranostic nanoplatform for in vitro two-photon fluorescence imaging and photodynamic therapy. After incorporation into porous UiO-67 nanoparticles, the quantum yield, luminescence lifetime, and two-photon fluorescence intensity of Ru(bpy)32+ guest molecules were much improved owing to the steric confinement effect of MOF pores. Benefiting from these merits, the as-synthesized nanoparticles managed to be internalized by A549 cells while providing excellent red fluorescence in cytoplasm upon excitation with 880 nm irradiation. Photodynamic therapeutic application of the Ru(bpy)32+-incorporated UiO-67 NMOFs was investigated in vitro. The Ru(bpy)32+-incorporated UiO-67 NMOFs exhibited good biocompatibility without irradiation while having good cell-killing rates upon irradiation. In view of these facts, the developed Ru(bpy)32+-incorporated NMOFs give a new potential pathway to achieve enhanced two-photon fluorescence imaging and photodynamic therapy.

Nanoneedle‐Assisted Delivery of Site‐Selective Peptide‐Functionalized DNA Nanocages for Targeting Mitochondria and Nuclei

Peptide-functionalized DNA nano-objects selectively target mitochondria and the nucleus by means of nanoneedle-assisted delivery. This technology preserves the cell viability and structural integrity of nanostructures and assists the nano-objects in escaping degradation by endocytosis. This method opens up a new avenue for further in vitro studies of intracellular behaviors of DNA assemblies and their interactions in specific organelles.

Multifunctional DNA nanomaterials for biomedical applications

The rapidly emerging DNA nanotechnology began with pioneer Seemans hypothesis that DNA not only can carry genetic information but also can be used as molecular organizer to create well-designed and controllable nanomaterials for applications in materials science, nanotechnology, and biology. DNA-based self-assembly represents a versatile system for nanoscale construction due to the well-characterized conformation of DNA and its predictability in the formation of base pairs. The structural features of nucleic acids form the basis of constructing a wide variety of DNA nanoarchitectures with well-defined shapes and sizes, in addition to controllable permeability and flexibility. More importantly, self-assembled DNA nanostructures can be easily functionalized to construct artificial functional systems with nanometer scale precision for multipurposes. Apparently scientists envision artificial DNA-based nanostructures as tool for drug loading and in vivo targeted delivery because of their abilities in selective encapsulation and stimuli-triggered release of cargo. Herein, we summarize the strategies of creating multidimensional self-assembled DNA nanoarchitectures and review studies investigating their stability, toxicity, delivery efficiency, loading, and control release of cargos in addition to their site-specific targeting and delivery of drug or cargo molecules to cellular systems.