Pradip K. Sukul

Assemblies of perylene diimide derivatives with melamine into luminescent hydrogels

We report unique and spontaneous formation of hydrogels of perylene derivatives with melamine. The luminescent gel network is formed by H-type aggregation of the perylene core, supramolecularly cross-linked by melamine units. As a result of controlled aggregation in the extended nanofibers, strong exciton fluorescence emission is observed.

Self healing hydrogels composed of amyloid nano fibrils for cell culture and stem cell differentiation

Amyloids are highly ordered protein/peptide aggregates associated with human diseases as well as various native biological functions. Given the diverse range of physiochemical properties of amyloids, we hypothesized that higher order amyloid self-assembly could be used for fabricating novel hydrogels for biomaterial applications. For proof of concept, we designed a series of peptides based on the high aggregation prone C-terminus of Aβ42, which is associated with Alzheimers disease. These Fmoc protected peptides self assemble to β sheet rich nanofibrils, forming hydrogels that are thermoreversible, non-toxic and thixotropic. Mechanistic studies indicate that while hydrophobic, π-π interactions and hydrogen bonding drive amyloid network formation to form supramolecular gel structure, the exposed hydrophobic surface of amyloid fibrils may render thixotropicity to these gels. We have demonstrated the utility of these hydrogels in supporting cell attachment and spreading across a diverse range of cell types. Finally, by tuning the stiffness of these gels through modulation of peptide concentration and salt concentration these hydrogels could be used as scaffolds that can drive differentiation of mesenchymal stem cells. Taken together, our results indicate that small size, ease of custom synthesis, thixotropic nature makes these amyloid-based hydrogels ideally suited for biomaterial/nanotechnology applications.

Aggregation induced chirality in a self assembled perylene based hydrogel: application of the intracellular pH measurement

With the aim of controlling helicity of self-assembled fibers, a pair of water soluble perylene derivatives has been designed and synthesized. Intermolecular hydrogen bonding interactions and π-π stacking, combined with the effect of molecular chirality, have been found to guide the aggregation that eventually led to gelation in water upon decreasing pH. This work has demonstrated a new paradigm of simple and non-toxic perylene derivatives for biological applications.

Supramolecular hydrogels of adenine: morphological, structural and rheological investigations

Hydrogelation of adenine has been tested here with three isomers of benzene tricarboxylic acid. Upon cooling the homogeneous solution, adenine formed an instantaneous gel with one isomer while the other two did not. Hydrogels were thermoreversible in nature and were characterized with the help of 1H-NMR, FE-SEM, TEM, AFM, PL, XRD, FT-IR and rheological analyses. 1H-NMR studies showed the formation of molecular interaction between adenine and 1,3,5-benzenetricarboxylic acid (1,3,5-B) to form a supramolecular complex that led to the production of gel with a large volume of water. The morphological investigation of dried gel revealed the network of nanofibers that assembled further into belt-like thick fibers. FT-IR and X-ray studies showed that in the gel adenine and 1,3,5-B were mainly assembled through hydrogen bonding interactions into a layered structure, and the interlayer distance was 1.52 nm. The gel network formed as a result of controlled aggregation between adenine and 1,3,5-B was stable even after large deformation under rheological investigations.

Charge-Transfer-Induced Fluorescence Quenching of Anthracene Derivatives and Selective Detection of Picric Acid.

2,6-Divinylpyridine-appended anthracene derivatives flanked by two alkyl chains at the 9,10-position of the core have been designed, synthesized, and characterized by NMR, MALDI-TOF, FTIR, and single-crystal XRD. These anthracene derivatives are able to recognize picric acid (2,4,6-trinitrophenol, PA) selectively down to parts per billion (ppb) level in aqueous as well as nonaqueous medium. Fluorescence emission of these derivatives in solution is significantly quenched by adding trace amounts of PA, even in the presence of other competing analogues, such as 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (NP), nitrobenzene (NB), benzoic acid (BA), and phenol (PH). The high sensitivity of these derivatives toward PA is considered as a combined effect of the proton-induced intramolecular charge transfer (ICT) as well as electron transfer from the electron-rich anthracene to the electron-deficient PA. Moreover, visual detection of PA has been successfully demonstrated in the solid state by using different substrates.

Identification of a robust and reproducible noncluster-type SBU: effect of coexistent groups on network topologies, helicity, and properties

The present work is part of our ongoing quest for a robust and reproducible noncluster-type secondary building unit (SBU) using pyrazole based ditopic ligands and benzene polycarboxylic acids. We report here the construction of a series of Zn(II) coordination polymers using a tailor-made SBU, designed with the proper utilization of the hydrogen bonds of a pyrazole-based ligand 4,4′-methylene-bispyrazole (H2MBP) and isophthalic acid (H2IPA), and its derivatives (R-H2IPA; R = OH, OCH3, CH3, tBu, Br, I, NH2, N3). The results demonstrate that the substituent effect of R-isophthalates does play a decisive role in directing the structural assemblies of this system. One of the promising features of the 2D networks of this series is the presence of SBU-based helical motifs, which by and large remain elusive despite the plethora of reports on various SBU based networks. Throughout the series, hydrogen bonds, π–π interactions and other intermolecular interactions play a crucial role in stabilizing the resulting networks. Furthermore, some of these complexes exhibit some interesting photoluminescent properties in the solid state.

Removal of toxic dyes from aqueous medium using adenine based bicomponent hydrogel

By utilizing hydrogen bonding and π–π stacking interactions, we have demonstrated the construction of three dimensional adenine based gel networks due to the self assembly with complementary tricarboxylic acid derivatives which were designed and unambiguously characterized with the help of NMR, HRMS, and FTIR. Upon cooling the homogeneous aqueous solution of adenine and tricarboxylic acid, it formed hydrogels which were thermoreversible in nature and characterized by various instrumental techniques such as OM, FESEM, TEM, AFM, FL, XRD, FT-IR, rheology etc. Networks of belts in the hydrogel were clearly observed and the dimension of belt depended on the tricarboxylic acid used. The intermolecular hydrogen bonds which were considered to be the driving force for the formation of stable gel were confirmed by FT-IR studies. In spite of the absence of symmetry either in bpca or adenine, these two moieties surprisingly produced gels and it was due to the symmetrical position of complementary interaction sites between adenine and tricarboxylic acids. The mechanical strength of the hydrogel network as revealed by rheological study depended on the tricarboxylic acid used in the two-component systems and also on the composition of fixed pair. These kind of hydrogels have potential to be utilized as inexpensive materials for the treatment of waste water containing organic dyes (methylene blue, rhodamine 6G and crystal violet) that are widely used in textile as well as dye industries.

Aromatic bi-, tri- and tetracarboxylic acid doped polyaniline nanotubes: effect on morphologies and electrical transport properties

Polyaniline nanotubes have been prepared in a facile manner via the chemical polymerization of aniline in an aqueous medium using ammonium persulphate (APS) and ten different dopant acids. The formation of PANI in the composite was confirmed using 1H-NMR, UV-Vis and FTIR spectroscopic studies. An intensive FESEM investigation indicated that the dopant acid plays an important role in the formation of the PANI nanotubes. In particular, the symmetrical positioning of –COOH groups in the aromatic dopant acid is key for forming PANI nanotubes. Symmetrical BTA-1 produces PANI nanotubes, whereas BTA-2 and BTA-3 only generate granular aggregates of PANI. The diameter of the observed nanotubes varies with the molar ratio of BTA-1 to aniline and the diameter of the nanotubes increases with the increasing number of –COOH groups in dopant acids that have the same central core. A comparative study with other tricarboxylic acids shows that the outer diameter (OD) of the PANI nanotubes depends on the size of the dopant under identical reaction conditions and follows the order: ODTPCA > ODBPCA > ODBTA-1. Low temperature charge transport in these composites mainly follows a three dimensional variable range hopping mechanism.

Proton induced aggregation of water soluble isophthalic acid appended arylene diimides: justification with perylene derivative

We report the self-assembly behaviour of five water soluble arylene diimides based on benzene, naphthalene and perylene moieties, by utilizing the essentiality of two major reversible supramolecular interactions, hydrophobic π-stacking and hydrophilic hydrogen bonding. With modification of their π-core, improved molecules have been designed as well as ensuring their water solubility under mild basic conditions. Experimentally, chaotic and randomly oriented free molecules are induced to assemble at just below pH 7 upon the imposition of suitable external acidic stimuli. Photophysically, with change in the nature of the interacting medium from pH-9 to pH-4, molecules that have benzene or naphthalene core, have revealed the predominant edge on J-type aggregation whereas unsubstituted perylene is showing a dominant as well as rotationally displaced H-type interaction because of its higher π-surface. Self assembly at a higher concentration is manifested in the formation of weak hydrogels. However, a large twist angle (36°) in the bay tetra-chlorinated perylene core hardly shows intentional assembly. Monitoring the changes of the morphologies with respect to major moieties concerned, perylene derivatives are responsible for forming short, super entangled network instead of long fibers and it is a key reason of the decrease in the rheological strength of self assemblies. Correlation of rheological and morphological changes with the mode of assembly in our study successfully points out the fact that in our system, co-facial H-type assembly for unsubstituted perylene diimide tends to disfavor long network formation, as well as the rheological strength of the self-aggregation compared to other small π-aromatic systems like benzene or naphthalene.

Water soluble perylene bisimide and its turn off/on fluorescence are used to detect cysteine and homocysteine

Perylene derivatives (P) in the presence of melamine (M) are envisioned as novel fluorescent probe for mercury ions and form a selective complex (PMHg) with mercury ions in neutral medium (pH = 7). This chemo-sensing ensemble probe exhibits high selectivity and sensitivity for thiol-containing amino acids and detects cysteine and homocysteine from healthy to abnormal levels under physiologically-relevant conditions.