Photopatterning the Mechanical Properties of Polysaccharide-Containing Gels Using Fe³⁺ coordination

Photopatterning the Mechanical Properties of Polysaccharide-Containing Gels Using Fe³⁺ coordinatio

Light-Responsive Iron(III)–Polysaccharide Coordination Hydrogels for Controlled Delivery

Visible-light responsive gels were prepared from two plant-origin polyuronic acids (PUAs), alginate and pectate, coordinated to Fe­(III) ions. Comparative quantitative studies of the photochemistry of these systems revealed unexpected differences in the photoreactivity of the materials, depending on the polysaccharide and its composition. The roles that different functional groups play on the photochemistry of these biomolecules were also examined. Mannuronic-rich alginates were more photoreactive than guluronic acid-rich alginate and than pectate. The microstructure of alginates with different mannuronate-to-guluronate ratios changed with polysaccharide composition. This influenced the gel morphology and the photoreactivity. Coordination hydrogel beads were prepared from both Fe–alginate and Fe–pectate. The beads were stable carriers of molecules as diverse as the dye Congo Red, the vitamin folic acid, and the antibiotic chloramphenicol. The photoreactivity of the hydrogel beads mirrored the photoreactivity of the polysaccharides in solution, where beads prepared with alginate released their cargo faster than beads prepared with pectate. These results indicate important structure–function relationships in these systems and create guidelines for the design of biocompatible polysaccharide-based materials where photoreactivity and controlled release can be tuned on the basis of the type of polysaccharide used and the metal coordination environment

Photoresponsive Polysaccharide-Based Hydrogels with Tunable Mechanical Properties for Cartilage Tissue Engineering

Photoresponsive hydrogels were obtained by coordination of alginate–acrylamide hybrid gels (AlgAam) with ferric ions. The photochemistry of Fe­(III)-alginate was used to tune the chemical composition, mechanical properties, and microstructure of the materials upon visible light irradiation. The photochemical treatment also induced changes in the swelling properties and transport mechanism in the gels due to the changes in material composition and microstructure. The AlgAam gels were biocompatible and could easily be dried and rehydrated with no change in mechanical properties. These gels showed promise as scaffolds for cartilage tissue engineering, where the photochemical treatment could be used to tune the properties of the material and ultimately change the growth and extracellular matrix production of chondrogenic cells. ATDC5 cells cultured on the hydrogels showed a greater than 2-fold increase in the production of sulfated glycosaminoglycans (sGAG) in the gels irradiated for 90 min compared to the dark controls. Our method provides a simple photochemical tool to postsynthetically control and adjust the chemical and mechanical environment in these gels, as well as the pore microstructure and transport properties. By changing these properties, we could easily access different levels of performance of these materials as substrates for tissue engineering

Restricted Photoinduced Conformational Change in the Cu(I) Complex for Sensing Mechanical Properties

When designing photoresponsive materials, the impact of a polymer host matrix on the photophysical and photochemical properties of chromophores can be dramatic and advantageous for correlating macromolecular properties. Some compounds possess changes in their photophysical response with variation in the surrounding media (e.g., crystalline glass vs solution). This study demonstrates how changes in the excited state dynamics of [Cu­(dmp)₂]⁺, where dmp = 2,9-dimethyl-1,10-phenanthroline, are used to quantitatively probe the viscosity of the surrounding polymer matrix. A correlation of both excited state lifetime and photoluminescence emission wavelength on viscosity was observed in different supramolecular materials containing [Cu­(dmp)₂]⁺. These effects were attributed to restricted photoinduced structural distortion of the Cu­(I) complex as the polymer matrix hardened. This photoluminescence sensor features a greater dynamic range for viscosity sensing (6 orders of magnitude) and displayed larger changes in lifetime response with respect to typical organometallic mechanosensitive probes

Changing Mechanical Strength in Cr(III)- Metallosupramolecular Polymers with Ligand Groups and Light Irradiation

We have demonstrated the ability to control the mechanical properties of metallosupramolecular materials via choice of ligand binding group, as well as with external light irradiation. These photoresponsive Cr­(III)-based materials were prepared from a series of modified hydrogenated poly­(ethylene-<i>co</i>-butylene) polymers linked through metal–ligand interactions between a Cr­(III) metal center and pyridyl ligand termini of the polymers. The introduction of these Cr­(III)-pyridine bonds gave rise to new mechanical and optical properties of the polymer materials. Depending on the type of pyridyl ligand, density functional theory calculations revealed changes in coordination to the Cr­(III), which ultimately led to materials with significantly different mechanical properties. Electronic excitation of the Cr­(III) materials with 450 and 655 nm CW lasers (800 mW/cm²) resulted in generation of excited state photophysical processes which led to temporary softening of the materials up to 143 kPa (41.5%) in storage modulus (<i>G</i>′) magnitude. The initial mechanical strength of the materials was recovered when the light stimulus was removed, and no change in mechanical properties was observed with light irradiation where there was no absorbance by the Cr­(III) moiety. These materials demonstrate that introduction of metal–ligand bonding interactions into polymers enables the design and synthesis of photoresponsive materials with tunable optical-mechanical properties not seen in traditional polymeric materials

Mössbauer Spectroscopic Characterization of Iron(III)–Polysaccharide Coordination Complexes: Photochemistry, Biological, and Photoresponsive Materials Implications

While polycarboxylates and hydroxyl-acid complexes have long been known to be photoactive, simple carboxylate complexes which lack a significant LMCT band are not typically strongly photoactive. Hence, it was somewhat surprising that a series of reports demonstrated that materials synthesized from iron­(III) and polysaccharides such as alginate (poly­[guluronan-<i>co</i>-mannuronan]) or pectate (poly­[galacturonan]) formed photoresponsive materials that convert from hydrogels to sols under the influence of visible light. These materials have numerous potential applications in areas such as photopatternable materials, materials for controlled drug delivery, and tissue engineering. Despite the near-identity of the functional units in the polysaccharide ligands, the reactivity of iron­(III) hydrogels can depend on the configuration of some chiral centers in the sugar units and in the case of alginate the guluronate to mannuronate block composition, as well as pH. Here, using temperature- and field-dependent transmission Mössbauer spectroscopy, we show that the dominant iron compound detected for both the alginate and pectate gels displays features typical of a polymeric (Fe³⁺O₆) system. The Mössbauer spectra of such systems are strongly dependent on temperature, field, size, and crystallinity, indicative of superparamagnetic relaxation of magnetically ordered nanoparticles. Pectate and alginate hydrogels differ in the size distribution of the iron oxyhydroxy nanoparticles, suggesting that in general smaller nanoparticles are more reactive. Potential biological implications of these results are also discussed

Quantum Dot Photoluminescence Quenching by Cr(III) Complexes. Photosensitized Reactions and Evidence for a FRET Mechanism

Reported are quantitative studies of the energy transfer from water-soluble CdSe/ZnS and CdSeS/ZnS core/shell quantum dots (QDs) to the Cr­(III) complexes <i>trans</i>-Cr­(N₄)­(X)₂⁺ (N₄ is a tetraazamacrocycle ligand, X^– is CN^–, Cl^–, or <u>O</u>NO^–) in aqueous solution. Variation of N₄, of X^–, and of the QD size and composition allows one to probe the relationship between the emission/absorption overlap integral parameter and the efficiency of the quenching of the QD photoluminescence (PL) by the chromium­(III) complexes. Steady-state studies of the QD PL in the presence of different concentrations of <i>trans</i>-Cr­(N₄)­(X)₂⁺ indicate a clear correlation between quenching efficiency and the overlap integral largely consistent with the predicted behavior of a Förster resonance energy transfer (FRET)-type mechanism. PL lifetimes show analogous correlations, and these results demonstrate that spectral overlap is an important consideration when designing supramolecular systems that incorporate QDs as photosensitizers. In the latter context, we extend earlier studies demonstrating that the water-soluble CdSe/ZnS and CdSeS/ZnS QDs photosensitize nitric oxide release from the <i>trans</i>-Cr­(cyclam)­(ONO)₂⁺ cation (cyclam = 1,4,8,11-tetraazacyclotetradecane) and report the efficiency (quantum yield) for this process. An improved synthesis of ternary CdSeS core/shell QDs is also described

Plasmonic Nanocrystal Solar Cells Utilizing Strongly Confined Radiation

The ability of metal nanoparticles to concentrate light <i>via</i> the plasmon resonance represents a unique opportunity for funneling the solar energy in photovoltaic devices. The absorption enhancement in plasmonic solar cells is predicted to be particularly prominent when the size of metal features falls below 20 nm, causing the strong confinement of radiation modes. Unfortunately, the ultrashort lifetime of such near-field radiation makes harvesting the plasmon energy in small-diameter nanoparticles a challenging task. Here, we develop plasmonic solar cells that harness the near-field emission of 5 nm Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The interfaces of Au and PbS domains were designed to support a rapid energy transfer at rates that outpace the thermal dephasing of plasmon modes. We demonstrate that central to the device operation is the inorganic passivation of Au nanoparticles with a wide gap semiconductor, which reduces carrier scattering and simultaneously improves the stability of heat-prone plasmonic films. The contribution of the Au near-field emission toward the charge carrier generation was manifested through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, as measured relative to PbS-only devices

Controlled Synthesis and Single-Particle Imaging of Bright, Sub-10 nm Lanthanide-Doped Upconverting Nanocrystals

Phosphorescent nanocrystals that upconvert near-infrared light to emit at higher energies in the visible have shown promise as photostable, nonblinking, and background-free probes for biological imaging. However, synthetic control over upconverting nanocrystal size has been difficult, particularly for the brightest system, Yb³⁺- and Er³⁺-doped β-phase NaYF₄, for which there have been no reports of methods capable of producing sub-10 nm nanocrystals. Here we describe conditions for the controlled synthesis of protein-sized β-phase NaYF₄: 20% Yb³⁺, 2% Er³⁺ nanocrystals, from 4.5 to 15 nm in diameter. The size of the nanocrystals was modulated by varying the concentration of basic surfactants, Y³⁺:F^– ratio, and reaction temperature, variables that also affected their crystalline phase. Increased reaction times favor formation of the desired β-phase nanocrystals while having only a modest effect on nanocrystal size. Core/shell β-phase NaYF₄: 20% Yb³⁺, 2% Er³⁺/NaYF₄ nanoparticles less than 10 nm in total diameter exhibit higher luminescence quantum yields than comparable >25 nm diameter core nanoparticles. Single-particle imaging of 9 nm core/shell nanoparticles also demonstrates that they exhibit no measurable photobleaching or blinking. These results establish that small lanthanide-doped upconverting nanoparticles can be synthesized without sacrificing brightness or stability, and these sub-10 nm nanoparticles are ideally suited for single-particle imaging

Plasmon-Induced Energy Transfer: When the Game Is Worth the Candle

The superior optical extinction characteristics of noble metal nanoparticles have long been considered for enhancing the solar energy absorption in light-harvesting devices. The energy captured through a plasmon resonance mechanism can potentially be transferred to a surrounding semiconductor matrix in the form of excitons or charge carriers, offering a promising light-sensitization strategy. Of particular interest is the plasmon near-field energy conversion, which is predicted to yield substantial gains in the photocarrier generation. Such a short-range interaction, however, is often inhibited by processes of backward electron and energy transfer, which obscure its net benefit. Here, we employ sample-transmitted excitation photoluminescence spectroscopy to determine the quantum efficiency for the plasmon-induced energy transfer (ET) in assemblies of Au nanoparticles and CdSe nanocrystals. The present technique distinguishes the Au-to-CdSe ET contribution from metal-induced quenching processes, thus enabling accurate estimates of the photon-to-exciton conversion efficiency. We show that in the case of 9.1 nm Au nanoparticles only 1–2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0 nm Au, the photon-to-exciton conversion efficiency increases to 29.5%. The results of the present measurements were used to develop an empirical model for estimating the maximum gain in plasmon-induced carriers versus the mass fraction of Au in a film