Fluorescent Gel from a Self-Assembling New Chromophoric Moiety Containing Azobenzene Based Tetraamide

A new chromophoric low molecular weight (LMW) organic molecule, 1, was synthesized, and it forms gels in various organic solvents including toluene, o-xylene, m-xylene and p-xylene. The resultant gel phase materials exhibit enhanced and red-shifted fluorescence emission in the respective gelling solvents. This gelator molecule is self-assembled using various noncovalent interactions including hydrogen bonding, π−π staking and van der Waals interactions to get the gel phase materials. The molecule 1 is very weakly fluorescent in solution, but its intensity is increased by almost 40 times in their respective gelled state depending on the nature of the gelling solvents. Self-assembly of this molecule in the above-mentioned organic solvents gives an elongated nanofibrillar network that can be visualized through Field Emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM)

Tetrapeptide-Based Hydrogels: for Encapsulation and Slow Release of an Anticancer Drug at Physiological pH

Here, we report two synthetic oligopeptide-based, thermoreversible, pH-sensitive hydrogels. In gel phase, these self-assembling tetrapeptides form a long interconnected nanofibrilar network structure, as is evident from various microscopic techniques, including field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). FTIR, circular dichroism, and wide angle X-ray diffraction (WAXD) favor an antiparallel β-sheet structure of these gelators in the gel state. Finally, these hydrogels have been utilized for entrapment and slow release of an anticancer drug doxorubicin at physiological pH, promising their future application as a drug delivery vehicle

Poly(ethylene glycol)-Based Multidentate Oligomers for Biocompatible Semiconductor and Gold Nanocrystals

We have developed a new set of multifunctional multidentate OligoPEG ligands, each containing a central oligomer on which were laterally grafted several short poly­(ethylene glycol) (PEG) moieties appended with either thioctic acid (TA) or terminally reactive groups. Reduction of the TAs (e.g., in the presence of NaBH₄) provides dihydrolipoic acid (DHLA)-appended oligomers. Here the insertion of PEG segments in the ligand structure promotes water solubility and reduces nonspecific interactions, while TA and DHLA groups provide multidentate anchoring onto Au nanoparticles (AuNPs) and ZnS-overcoated semiconductor quantum dots (QDs), respectively. The synthetic route involves simple coupling chemistry using <i>N</i>,<i>N</i>-dicylohexylcarbodiimide (DCC). Water-soluble QDs and AuNPs capped with these ligands were prepared via cap exchange. As prepared, the nanocrystals dispersions were aggregation-free, homogeneous, and stable for extended periods of time over pH ranging from 2 to 14 and in the presence of excess electrolyte (2 M NaCl). The new OligoPEG ligands also allow easy integration of tunable functional and reactive groups within their structures (e.g., azide or amine), which imparts surface functionalities to the nanocrystals and opens up the possibility of bioconjugation with specific biological molecules. The improved colloidal stability combined with reactivity offer the possibility of using the nanocrystals as biological probes in an array of complex and biologically relevant media

Elucidating the Role of Surface Coating in the Promotion or Prevention of Protein Corona around Quantum Dots

Nonspecific interactions in biological media can lead to the formation of a protein corona around nanocolloids, which tends to alter their behavior and limit their effectiveness when used as probes for imaging or sensing applications. Yet, understanding the corona buildup has been challenging. We hereby investigate these interactions using luminescent quantum dots (QDs) as a model nanocolloid system, where we carefully vary the nature of the hydrophilic block in the surface coating, while maintaining the same dihydrolipoic acid (DHLA) bidentate coordinating motif. We first use agarose gel electrophoresis to track changes in the mobility shift upon exposure of the QDs to protein-rich media. We find that QDs capped with DHLA (which presents a hydrophobic alkyl chain terminated with a carboxyl group) promote corona formation, in a concentration-dependent manner. However, when a polyethylene glycol block or a zwitterion group is appended onto DHLA, it yields a coating that prevents corona buildup. Our results clearly confirm that nonspecific interactions with protein-rich media are strongly dependent on the nature of the hydrophilic motif used. Additional gel experiments using SDS-PAGE have allowed further characterization of the corona protein, and showed that mainly a soft corona forms around the DHLA-capped QDs. These findings will be highly informative when designing nanocolloids that can find potential use in biological applications

Bio-orthogonal Coupling as a Means of Quantifying the Ligand Density on Hydrophilic Quantum Dots

We describe the synthesis of two metal-coordinating ligands that present one or two lipoic acid (LA) anchors, a hydrophilic polyethylene glycol (PEG) segment and a terminal reactive group made of an azide or an aldehyde, two functionalities with great utility in bio-orthogonal coupling techniques. These ligands were introduced onto the QD surfaces using a combination of photochemical ligation and mixed cap exchange strategy, where control over the fraction of azide and aldehyde groups per nanocrystal can be easily achieved: LA-PEG-CHO, LA-PEG-N₃, and bis­(LA)-PEG-CHO. We then demonstrate the application of two novel bio-orthogonal coupling strategies directly on luminescent quantum dot (QD) surfaces that use click chemistry and hydrazone ligation under catalyst-free conditions. We applied the highly efficient hydrazone ligation to couple 2-hydrozinopyridine (2-HP) to aldehyde-functionalized QDs, which produces a stable hydrazone chromophore with a well-defined optical signature. This unique optical feature has enabled us to extract a measure for the ligand density on the QDs for a few distinct sizes and for different ligand architectures, namely mono-LA-PEG and bis­(LA)-PEG. We found that the foot-print-area per ligand was unaffected by the nanocrystal size but strongly depended on the ligand coordination number. Additionally, we showed that when the two bio-orthogonal functionalities (aldehyde and azide) are combined on the same QD platform, the nanocrystal can be specifically reacted with two distinct targets and with great specificity. This design yields QD platforms with distinct chemoselectivities that are greatly promising for use as carriers for in vivo imaging and delivery

Multidentate Zwitterionic Ligands Provide Compact and Highly Biocompatible Quantum Dots

Hydrophilic functional semiconductor nanocrystals that are also compact provide greatly promising platforms for use in bioinspired applications and are thus highly needed. To address this, we designed a set of metal coordinating ligands where we combined two lipoic acid groups, bis­(LA)-ZW, (as a multicoordinating anchor) with a zwitterion group for water compatibility. We further combined this ligand design with a new photoligation strategy, which relies on optical means instead of chemical reduction of the lipoic acid, to promote the transfer of CdSe-ZnS QDs to buffer media. In particular, we found that the QDs photoligated with this zwitterion-terminated bis­(lipoic) acid exhibit great colloidal stability over a wide range of pHs, to an excess of electrolytes, and in the presence of growth media and reducing agents, in addition to preserving their optical and spectroscopic properties. These QDs are also stable at nanomolar concentrations and under ambient conditions (room temperature and white light exposure), a very promising property for fluorescent labeling in biology. In addition, the compact ligands permitted metal–histidine self-assembly between QDs photoligated with bis­(LA)-ZW and two different His-tagged proteins, maltose binding protein and fluorescent mCherry protein. The remarkable stability of QDs capped with these multicoordinating and compact ligands over a broad range of conditions and at very small concentrations, combined with the compatibility with metal–histidine conjugation, could be very useful for a variety of applications, ranging from protein tracking and ligand–receptor binding to intracellular sensing using energy transfer interactions

Understanding the Self-Assembly of Proteins onto Gold Nanoparticles and Quantum Dots Driven by Metal-Histidine Coordination

Coupling of polyhistidine-appended biomolecules to inorganic nanocrystals driven by metal-affinity interactions is a greatly promising strategy to form hybrid bioconjugates. It is simple to implement and can take advantage of the fact that polyhistidine-appended proteins and peptides are routinely prepared using well established molecular engineering techniques. A few groups have shown its effectiveness for coupling proteins onto Zn- or Cd-rich semiconductor quantum dots (QDs). Expanding this conjugation scheme to other metal-rich nanoparticles (NPs) such as AuNPs would be of great interest to researchers actively seeking effective means for interfacing nanostructured materials with biology. In this report, we investigated the metal-affinity driven self-assembly between AuNPs and two engineered proteins, a His₇-appended maltose binding protein (MBP-His) and a fluorescent His₆-terminated mCherry protein. In particular, we investigated the influence of the capping ligand affinity to the nanoparticle surface, its density, and its lateral extension on the AuNP-protein self-assembly. Affinity gel chromatography was used to test the AuNP-MPB-His₇ self-assembly, while NP-to-mCherry-His₆ binding was evaluated using fluorescence measurements. We also assessed the kinetics of the self-assembly between AuNPs and proteins in solution, using time-dependent changes in the energy transfer quenching of mCherry fluorescent proteins as they immobilize onto the AuNP surface. This allowed determination of the dissociation rate constant, <i>K</i>_d^–1 ∼ 1–5 nM. Furthermore, a close comparison of the protein self-assembly onto AuNPs or QDs provided additional insights into which parameters control the interactions between imidazoles and metal ions in these systems

Organogels from Different Self-Assembling New Dendritic Peptides: Morphology, Reheology, and Structural Investigations

Three new peptide based dendrimers with different generations were synthesized, purified, and characterized. Each of these dendrimers form efficient organogels under suitable conditions and these gels were characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), Fourier transformed infrared (FT-IR) spectroscopy, differential scanning calorimetery (DSC) and rheology. It has been observed that gel forming propensity increases from first to second generation dendrimer and it decreases from second to third generation. The hydrogen bonding interaction is the main driving force for the formation of aggregated structure that leads to the formation of a fibrillar network, responsible for gelation. The morphology is network type consisting of taped or twisted fibrils spanning throughout the space. DSC measurements show the thermorevsible first-order phase transition. Rheological studies indicate that flow behavior and segmental motion of these gels are different for different peptidic gels, obtained from various generations of dendritic peptides

Combining Ligand Design with Photoligation to Provide Compact, Colloidally Stable, and Easy to Conjugate Quantum Dots

We describe the design and synthesis of two compact multicoordinating (lipoic acid-appended) zwitterion ligands for the capping of luminescent quantum dots, QDs. This design is combined with a novel and easy to implement photoligation strategy to promote the in situ ligand exchange and transfer of the QDs to buffer media. This method involves the irradiation of the native hydrophobic nanocrystals in the presence of the ligands, which promotes in situ cap exchange and phase transfer of the QDs, eliminating the need for a chemical reduction of the dithiolane groups. Applied to the present LA-zwitterion ligands, this route has provided QDs with high photoluminescence yields and excellent colloidal stability over a broad range of conditions, including acidic and basic pH, in the presence of growth media and excess salt conditions. The small lateral extension of the capping layer allowed easy conjugation of the QDs to globular proteins expressing a terminal polyhistidine tag, where binding is promoted by metal-affinity interactions between the accessible Zn-rich surface and imidazoles in the terminal tag of the proteins. The ability to carry out conjugation in acidic as well as basic conditions opens up the possibility to use such self-assembled QD-protein conjugates in various biological applications