Pierre Karam

Postmetalated Zirconium Metal Organic Frameworks as a Highly Potent Bactericide

Metal-organic frameworks (MOFs) have emerged as an important class of hybrid organic-inorganic materials. One of the reasons they have gained remarkable attention is attributed to the possibility of altering them by postsynthetic modification, thereby providing access to new and novel advanced materials. MOFs have been applied in catalysis, gas storage, gas separation, chemical sensing, and drug delivery. However, their bactericidal use has rarely been explored. Herein, we developed a two-step process for the synthesis of zirconium-based MOFs metalated with silver cations as a potent antibacterial agent. The obtained products were thoroughly characterized by powder X-ray diffraction, scanning electron microscopy, UV-visible, IR, thermogravimetric, and Brunauer-Emmett-Teller analyses. Their potency was evaluated against E. coli with a reported minimal inhibitory concentration and minimal bactericidal concentration of as low as 6.5 μg/mL of silver content. Besides the novelty of the system, the advantage of this strategy is that the MOFs could be potentially regenerated and remetalated after each antibacterial test, unlike previously reported frameworks, which involved the destruction of the framework.

Tunable nanothermometer based on short poly(phenylene ethynylene)

We report a self-referencing ratiometric nanothermometer based on short conjugated polyelectrolytes (CPEs). The probe is prepared by complexing a phenylene-based polymer with polyvinylpyrrolidone (PVP), an amphiphilic macromolecule that destabilizes CPE π–π stacking. This makes it possible to shift the equilibrium between the less emissive aggregated state of the CPE (520 nm) and its more emissive single chains (450 nm) within a useful temperature range (15.0–70.0 °C). By testing different PVP molecular weights, we managed to tune the probe thermal sensitivity and response time. Given its potential wide applications, the probe was tested under different pHs and using divalent and monovalent cations. We believe the reported nanothermometer will prove instrumental for ongoing efforts to accurately map and investigate heat production and dissipation at the nanoscale level.

Temperature Mapping in Hydrogel Matrices Using Unmodified Digital Camera

We report a simple, generally applicable, and noninvasive fluorescent method for mapping thermal fluctuations in hydrogel matrices using an unmodified commercially available digital single-lens reflex camera (DSLR). The nanothermometer is based on the complexation of short conjugated polyelectrolytes, poly(phenylene ethynylene) carboxylate, with an amphiphilic polymer, polyvinylpyrrolidone, which is in turn trapped within the porous network of a gel matrix. Changes in the temperature lead to a fluorescent ratiometric response with a maximum relative sensitivity of 2.0% and 1.9% at 45.0 °C for 0.5% agarose and agar, respectively. The response was reversible with no observed hysteresis when samples were cycled between 20 and 40 °C. As a proof of concept, the change in fluorescent signal/color was captured using a digital camera. The images were then dissected into their red-green-blue (RGB) components using a Matlab routine. A linear correlation was observed between the hydrogel temperature and the green and blue intensity channels. The reported sensor has the potential to provide a wealth of information when thermal fluctuations mapped in soft gels matrices are correlated with chemical or physical processes.

The Role of Hydrophobicity in the Cellular Uptake of Negatively Charged Macromolecules

It is generally accepted that positively charged molecules are the gold standard to by-pass the negatively charged cell membrane. Here, it is shown that cellular uptake is also possible for polymers with negatively charged side chains and hydrophobic backbones. Specifically, poly[5-methoxy-2-(3-sulfopropoxy)-1,4-phenylenevinylene], a conjugated polyelectrolyte with sulfonate, as water-soluble functional groups, is shown to accumulate in the intracellular region. When the polymer hydrophobic backbone is dissolved using polyvinylpyrrolidone, an amphiphilic macromolecule, the cellular uptake is dramatically reduced. The report sheds light on the fine balance between negatively charged side groups and the hydrophobicity of polymers to either enhance or reduce cellular uptake. As a result, these findings will have important ramifications on the future design of targeted cellular delivery nanocarriers for imaging and therapeutic applications.

Enhancing porphyrin photostability when locked in metal–organic frameworks

Porphyrins have been widely used in many optical devices given their unique photochemical properties. Their poor photostability has, however, limited their wide applications in bioimaging and biosensing schemes. Herein, we report the remarkable photostability enhancement of the porphyrin, carboxyphenyl porphyrin (TCPP-H2) when locked in a zirconium based metal-organic framework (MOF-525). Steady-state ensemble fluorescence spectroscopy experiments showed minimal changes (2%) in the recorded signal when MOF-525 was continuously illuminated as compared to a 16% decrease for free porphyrins. Single particle fluorescence imaging revealed bright microparticles with exceptional photostability and no-blinking within the experiment window. This study highlights the use of metal-organic frameworks for preparing photostable microstructures by leveraging on their unique self-assembly properties.

Nanosilver loaded GelMA hydrogel for antimicrobial coating of biomedical implants

Bacterial adhesion to the surface of implants in surgical procedures represents a major problem in surgeries, as it incurs high medical costs and could lead to postoperative infections. Different strategies have been developed to decrease the incidence of bacterial infections related medical devices failure. One approach is the modification of the surface of the devices using antibacterial coatings designed to be non-fouling, thus minimizing microbial adhesion. The ability of silver nanoparticles to destroy infectious micro-organisms makes them an attractive candidate for use against “super-bugs” resistant to antibiotics. In this work, we develop a silver nanoparticles loaded methacrylated gelatin (GelMA) hydrogel for antimicrobial coating of biomedical implants. Silver nanoparticles of different sizes and concentrations were synthesized using citrate and ascorbic acid reduction in glycerol water mixtures which were incorporated in an Ultraviolet (UV)-photocrosslinkable GelMA hydrogel. Analysis and characterization of the obtained hydrogels were performed through scanning electron microscopy (SEM), and UV-visible spectrophotometry. The release of silver nanoparticles from the crosslinked hydrogel was quantified using UV spectroscopy. The cell viability was investigated on Rat Aortic Smooth Muscle Cells (RASMC) using different concentrations and sizes of silver nanoparticle loaded GelMA hydrogel. The antibacterial activities of the newly developed hydrogel coating was tested on two different types of bacteria, gram positive Staphylococcus aureus and gram negative Escherichia coli by analyzing the growth of the bacterial cells following treatment with different concentration and size of silver nanoparticles. Experimental results revealed that incorporation of silver nanoparticles in GelMA hydrogel was successfully performed and the release of silver nanoparticles over time from the developed hydrogel to the pathogenic environment was successfully achieved which reduced bacterial growth without any negative effect on cells.