Ramesh Jagannathan

Viologen-Based Conjugated Covalent Organic Networks via Zincke Reaction

Morphology influences the functionality of covalent organic networks and determines potential applications. Here, we report for the first time the use of Zincke reaction to fabricate, under either solvothermal or microwave conditions, a viologen-linked covalent organic network in the form of hollow particles or nanosheets. The synthesized materials are stable in acidic, neutral, and basic aqueous solutions. Under basic conditions, the neutral network assumes radical cationic character without decomposing or changing structure. Solvent polarity and heating method determine product morphology. Depending upon solvent polarity, the resulting polymeric network forms either uniform self-templated hollow spheres (HS) or hollow tubes (HT). The spheres develop via an inside-out Ostwald ripening mechanism. Interestingly, microwave conditions and certain solvent polarities result in the formation of a robust covalent organic gel framework (COGF) that is organized in nanosheets stacked several layers thick. In the gel phase, the nanosheets are crystalline and form honeycomb lattices. The use of the Zincke reaction has previously been limited to the synthesis of small viologen molecules and conjugated viologen oligomers. Its application here expands the repertoire of tools for the fabrication of covalent organic networks (which are usually prepared by dynamic covalent chemistry) and for the synthesis of viologen-based materials. All three materials-HT, HS, and COGF-serve as efficient adsorbents of iodine due to the presence of the cationic viologen linker and, in the cases of HT and HS, permanent porosity.

Fabrication of Hierarchically Porous Materials and Nanowires through Coffee Ring Effect

We report a versatile method for the fabrication of nanowires and hierarchical porous materials from a wide variety of ceramic materials such as CaCO3, ZnO, CuO, Co3O4, Co-doped ZnO, and Ag2O. The method consists of evaporation of CO2-enriched water microdroplets (diameter ∼3 μm) deposited from an aerosol onto heated substrates (T = 120 °C). A variety of porous scaffolds with 1-3 μm sized pores can be generated by tuning the process conditions. Subsequent sintering of the scaffolds is shown to generate nanosized pores in the walls of the porous scaffold creating a dual hierarchy of pore sizes (∼50 nm and 1-3 μm). We propose a mechanism for the formation of scaffolds based on the coffee-ring effect during the evaporation of microdroplets. Ostwald-ripening of CaCO3 scaffolds prepared without sintering yields scaffold structures consisting of two-dimensional crystals of CaCO3 that are one unit cell thick. The favorable application of CaCO3 scaffolds for the enhancement of bone healing around titanium implants with improved biocompatibility is also demonstrated.

A Chemical Template for Synthesis of Molecular Sheets of Calcium Carbonate

Inspired by the discovery of graphene and its unique properties, we focused our research to develop a scheme to create nacre like lamellar structures of molecular sheets of CaCO3 interleaved with an organic material, namely carbon. We developed a facile, chemical template technique, using a formulation of poly(acrylic) acid (PAA) and calcium acetate to create lamellar stacks of single crystal sheets of CaCO3, with a nominal thickness of 17 Å, the same as a unit-cell dimension for calcite (c–axis = 17.062 Å), interleaved with amorphous carbon with a nominal thickness of 8 Å. The strong binding affinity between carboxylate anions and calcium cations in the formulation was used as a molecular template to guide CaCO3 crystallization. Computational modeling of the FTIR spectra showed good agreement with experimental data and confirmed that calcium ions are bridged between polymer chains, resulting in a net-like polymer structure. The process readily lends itself to explore the feasibility of creating molecular sheets of other important inorganic materials and potentially find applications in many fields such as super capacitors and “low k di-electric” systems.

Single-Cell Characterization of Microalgal Lipid Contents with Confocal Raman Microscopy

The environmental impacts from consumption of fossil fuels have raised interest in finding renewable energy resources throughout the globe. Much focus has been placed on optimizing microalgae to efficiently produce compounds that can substitute for fossil fuels. However, the path to achieving economical feasibility of this substitution is likely to require strain optimization through mutagenesis screens as well as other available approaches and tools. Rapid characterization of the type of fatty acid expressed at a single-cell level can help identify screened cells with the desired lipid characteristics such as chain length and saturation status. Confocal Raman microscopy is a powerful tool for physicochemical characterization of biological samples. It enables single-cell, in vivo monitoring of various cellular components in a rapid, quantitative, label-free, and nondestructive manner. In this chapter, we describe recent advances in this method, which have resulted in remarkable enhancements in the sensitivity, specificity, and spatiotemporal resolution of the technique. We utilize this technique for analyzing lipid content of algal isolates obtained through a mutagenesis screen of the green alga, Chlamydomonas reinhardtii, for increased lipid production at the single-cell level. Our results demonstrate cell-to-cell variation in structural features of expressed lipids among the screened C. reinhardtii mutants, while clonal isolates show little to no variability in expressed lipids. The lack of stochasticity in expression of lipids in clonal populations of C. reinhardtii is a desired feature when accompanied by expression of fatty acids suitable for use as biofuel feedstock.

Redox‐Responsive Covalent Organic Nanosheets from Viologens and Calix[4]arene for Iodine and Toxic Dye Capture

Owing to their chemical and thermal stabilities, high uptake capacities, and easy recyclability, covalent organic polymers (COPs) have shown promise as pollutant sponges. Herein, we describe the use of diazo coupling to synthesize two cationic COPs, COP1++ and COP2++ , that incorporate a viologen-based molecular switch and an organic macrocycle, calix[4]arene. The COPs form nanosheets that have height profiles of 6.00 nm and 8.00 nm, respectively, based on AFM measurements. The sheets remain morphologically intact upon one- or two-electron reductions of their viologen subunits. MD simulations of the COPs containing dicationic viologens indicate that the calix[4]arenes adopt a partial cone conformation and that, in height, the individual 2D polymer layers are 5.48 Å in COP1++ and 5.65 Å in COP2++ , which, together with the AFM measurements, suggests that the nanosheets are composed of 11 and 14 layers, respectively. Whether their viologens are in dicationic, radical cationic, or neutral form, the COPs exhibit high affinity for iodine, reaching up to 200 % mass increase when exposed to iodine vapor at 70 °C, which makes the materials among the best-performing nanosheets for iodine capture reported in the literature. In addition, the COPs effectively remove Congo red from solution in the pH range of 2-10, reaching nearly 100 % removal within 15 minutes at acidic pH.

Calix[4]arene-Based Porous Organic Nanosheets

Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated in structurally well-defined covalent 2D materials. Such a task is challenging, especially without a template, because of the nonplanar configuration and conformational flexibility of the calixarene ring. Here, we report the first-of-a-kind solvothermal synthesis of a calix[4]arene-based 2D polymer (CX4-NS) that is porous, covalent, and isolated as few-layer thick (3.52 nm) nanosheets. Experimental and theoretical characterization of the nanosheets is presented. Atomic force microscopy and transmission electron microscopy results are consistent with the calculated lowest energy state of the polymer. In the lowest energy state, parallel layers are tightly packed, and the calixarenes adopt the 1,2-alternate conformation, which gives rise to a two-dimensional pattern and a rhombic unit cell. We tested the materials ability to adsorb I2 vapor and observed a maximum capacity of 114 wt %. Molecular simulations extended to model I2 capture showed excellent agreement with experiments. Furthermore, the material was easily regenerated by mild ethanol washings and could be reused with minimal loss of efficiency.