Biomedical subjects

Alexander D Q Li

Publications and source records attributed to Alexander D Q Li.

9 recordsLinked to original sources

PubMed2006

Spiropyran-based photochromic polymer nanoparticles with optically switchable luminescence.

Polymer nanoparticles of 40-400 nm diameter with spiropyran-merocyanine dyes incorporated into their hydrophobic cavities have been prepared; in contrast to their virtually nonfluorescent character in most environments, the merocyanine forms of the encapsulated dyes are highly fluorescent. Spiro-mero photoisomerization is reversible, allowing the fluorescence to be switched "on" and "off" by alternating UV and visible light. Immobilizing the dye inside hydrophobic pockets of nanoparticles also improves its photostability, rendering it more resistant than the same dyes in solution to fatigue effects arising from photochemical switching. The photophysical characteristics of the encapsulated fluorophores differ dramatically from those of the same species in solution, making nanoparticle-protected hydrophobic fluorophores attractive materials for potential applications such as optical data storage and switching and biological fluorescent labeling. To evaluate the potential for biological tagging, these optically addressable nanoparticles have been delivered into living cells and imaged with a liquid nitrogen-cooled CCD.

Acrylamides↗

PubMed2006

Folding and unfolding of chromophoric foldamers show unusual colorful single molecule spectral dynamics.

A series of well-defined linear multichromophoric foldamers with a specific sequence of alternating rigid perylene chromophores and flexible ethylene glycol chains were studied by single molecule fluorescence spectroscopy. Monomer showed minor spectral fluctuations compared to trimer and hexamer, which showed unusual and colorful spectral dynamics attributed to a stochastic photoinduced unfolding/folding phenomenon. The range of spectral shapes observed indicates varying degrees of pi-pi interaction between adjacent chromophores, with vibronically resolved green emission indicating completely unfolded structures and broad red emission indicating highly coupled and extensively folded pi-stacks. The rate of switching between different spectral shapes in the spectral trajectories suggests the existence of multiple pathways between the folded and unfolded states.

Biomimetic Materials↗

PubMed2005

Foldamers with hybrid biological and synthetic sequences as selective DNA fluorescent probes.

Foldable polymers with alternating single-strand deoxyribonucleic acid (ssDNA) and planar fluorescent organic chromophores can self-organize into folded nanostructures and hence are hybrid foldamers with biological sequences and synthetic properties. The biological sequence provides highly specific molecular recognition properties, while the physical properties of synthetic chromophores offer sensitive fluorescence detection. In this paper, we describe that rational designed hybrid foldamers exhibit potential in the detection of polynucleotides. Under strictly controlled laboratory conditions, fluorescence measurements indicate that configuration change due to binding of polynucleotides with one or two mismatched bases can be readily distinguished. These results shed light on the design and construction of nanostructured foldamers with actuator and sensory properties, which may find important applications as biological probes.

Base Pair Mismatch↗

PubMed2005

Light-controlled molecular switches modulate nanocrystal fluorescence.

Spiropyran dyes were attached to fluorescent core-shell CdSe/ZnS nanocrystals via thiol-containing linkers. Photoisomerization of the dye to its merocyanine form by UV irradiation caused a dramatic loss in the intrinsic nanoparticle fluorescence, which was regained upon reversing the isomerization with visible light. The fluorescence quenching efficiency increased with increasing spectral overlap of fluorescence emission and merocyanine adsorption bands, consistent with FRET as the quenching mechanism. Typically, complete quenching required at least 80 bound dye molecules per particle.

Benzopyrans↗

PubMed2004

Thermosensitive gold nanoparticles.

Thermosensitive gold nanoparticles were fabricated by conjugating Au with a thiol-terminated poly(N-isopropylacrylamide) or PPA; this polymer stabilizer exhibits a temperature transition while undergoing a hydrophilic to hydrophobic transformation. The introduction of PPA onto gold nanoparticles has sensitized Au nanoparticles with unique temperature dependence. At low temperature (25 degrees C), the solutions containing PPA-functionalized gold nanoparticles are transparent, whereas higher temperatures (30 degrees C) lead to opaque suspensions. The thermosensitive property of PPA-functionalized Au nanoparticles is reversible, and the clear-opaque suspensions can be repeated many times.

Acrylic Resins↗

PubMed2003

Folding versus self-assembling.

Model foldable polymers with sequences of rigid hydrophobic chromophores and flexible hydrophilic tetra(ethylene glycol) were synthesized and used as a paradigm for studying molecular-folding and self-assembly phenomena. Our results demonstrate that intramolecular association or folding prevails over intermolecular interaction or self-assembling in the concentration region from 1 microM to 0.1 M. Importantly, folded polymeric nanostructures have absorption and fluorescence properties that are distinct from those of unfolded polymers or free monomers. We hypothesize that the origins of folding and self-assembly come from interactions between molecular units, and that the key parameter that regulates the on-and-off of such interactions is the distance R separating the two molecular units. Each molecular unit produces a characteristic force field, and when another molecular unit enters this field, the probability that the two units will interact increases significantly. A preliminary estimate of the radius of such a force field for the perylene tetracarboxylic diimide chromophore is about 90-120 A. As a result, phenomena associated with folding or self-assembly of molecular species are observed when these conditions are met in solution.

Ethylene Glycol↗

PubMed2003

Alternating DNA and pi-conjugated sequences. Thermophilic foldable polymers.

Foldable polymers with alternating single-strand deoxyribonucleic acid and planar conjugated organic perylene tetracarboxylic diimide units were found to self-organize into loosely folded nanostructures. Upon heating, the loosely folded structures become more ordered as evidenced by pi-stacking in the perylene segments. The folding and unfolding processes driven by the molecular interactions of adjacent perylenes were monitored in both aqueous and organic solutions. Heat-promoted folding, or inverse temperature behavior, which originates from positive enthalpy changes, was only observed in water. Therefore, we attributed this inverse temperature dependence to hydrophobic effects rather than pi-pi molecular orbital overlap between the perylene planes. These findings shed light on the design of new thermophiles in protein engineering as well as the construction of macromolecular-based nanodevices with actuator and sensory properties.

Base Sequence↗

PubMed2003

To fold or to assemble?

We introduce a new class of foldable oligomers consisting of alternating rigid and flexible regions. The rigid segments overlap to give pi-stacked folded conformers whose formation is driven mostly by pi-pi molecular orbital overlaps. As the oligomer concentration increases, the folded molecular structures further self-assemble into larger nanostructures. The dynamic processes of folding and self-organization are monitored with absorption, fluorescence, and NMR spectroscopies. Our results show that folding dominates at low concentrations (< approximately 1 mM) and precedes self-assembly, which occurs over the initial concentration range of approximately 1-100 mM.

Biomimetic Materials↗

PubMed2002

Multi-temporal self-organizations in 1,3-O,O'-bis(dodecyl)calix[4]arene.

After heat erases the order in 1,3-O,O'-bis(dodecyl)calix[4]arene, it self-organizes the phenyl units into a rigid calix bowl first at T2 = 111 degrees C, followed by the formation of all-trans alkyl chains at T1 = 62 degrees C, which eventually yields a stable cone-shaped structure at room temperature.

Calixarenes↗