Sampath Srinivasan

Attogram Sensing of Trinitrotoluene with a Self-Assembled Molecular Gelator

Detection of explosives is of utmost importance due to the threat to human security as a result of illegal transport and terrorist activities. Trinitrotoluene (TNT) is a widely used explosive in landmines and military operations that contaminates the environment and groundwater, posing a threat to human health. Achieving the detection of explosives at a sub-femtogram level using a molecular sensor is a challenge. Herein we demonstrate that a fluorescent organogelator exhibits superior detection capability for TNT in the gel form when compared to that in the solution state. The gel when coated on disposable paper strips detects TNT at a record attogram (ag, 10(-18) g) level (∼12 ag/cm(2)) with a detection limit of 0.23 ppq. This is a simple and low-cost method for the detection of TNT on surfaces or in aqueous solutions in a contact mode, taking advantage of the unique molecular packing of an organogelator and the associated photophysical properties.

Bioinspired Superhydrophobic Coatings of Carbon Nanotubes and Linear π Systems Based on the “Bottom-up” Self-Assembly Approach†

Rough and clean: The physical interaction and self-assembly of oligo(p-phenylenevinylene)s (OPVs) on carbon nanotubes (CNTs) allow the dispersion of the latter in organic solvents. This well-dispersed nanocomposite can be coated on to glass, metal, and mica surfaces to give superhydrophobic self-cleaning surfaces with water contact angles of 165-170° and a sliding angle of less than 2° (see picture).

Reversible self-assembly of entrapped fluorescent gelators in polymerized styrene gel matrix: erasable thermal imaging via recreation of supramolecular architectures.

The reversible shift of emission in fluorescent molecular gelators has been explored for the preparation of a composite polymer film useful for erasable thermal imaging and secret documentation. A gelation-assisted photopolymerization of styrene allowed the entrapment of the fluorescent gelator molecules within a polystyrene matrix with a weak green fluorescence, which upon heating above the T(g) of the polymer resulted in high-contrast fluorescence images due to the strong blue fluorescence of the individual molecules. The blue emission from the disassembled oligo(p-phenylenevinylene) molecules (OPVs) could be reversed to the green emission of the self-assembled OPVs by exposing the polymer film to chloroform vapors. The thermally written images are visible only under UV light and cannot be photocopied. A solvent-vapor-controlled recreation of the self-assembly of a fluorescent organogelator within a polymer matrix and its application in erasable secret documentation has not been reported previously.

Carbon Nanotube Triggered Self‐Assembly of Oligo(p‐phenylene vinylene)s to Stable Hybrid π‐Gels

Gel self-assembly: Addition of small amounts of carbon nanotubes (CNTs) to a solution of oligo(p-phenylene vinylene) OPV1 in toluene triggers their self-assembly to form a composite gel (see picture) with higher stability and better rheological properties than the OPV1 gel. Strong physical interactions between the species allow the encapsulation of CNTs into self-assembled tapes of OPV1, reinforcing the 3D gel network.

Hybrid thermoreversible gels from covalent polymers and organogels

This paper reports on experiments intended for investigating the feasibility of preparing hybrid thermoreversible gels from covalent polymers and noncovalent self-assembling pi-conjugated molecules. The formation and the degree of dispersion of these hybrid gels have been studied with polystyrenes of various tacticities and oligo(p-phenylenevinylene) molecules (OPV) in different nonpolar organic solvents. Detailed investigations of the systems have been carried out by DSC, SAXS, and AFM. It is shown that no liquid-liquid phase separation is involved, indicating that the systems are highly compatible, and that the growth of one type of gel does not interfere with the other. These studies reveal that the resultant hybrid gels are composed of the intermingled fibrillar architectures of both gels.

Solvent-mediated fiber growth in organogels

We have investigated the decisive role of the solvent type in the gelation of an oligo(p-phenylene vinylene) molecule with OH functions on the terminal groups (OPVOH). We show that gels of “hub-like” morphology are obtained with benzyl alcohol, a solvent that mimics the chemical structure of the terminal groups, whereas a “randomly dispersed” morphology is obtained with trans-decahydronaphthalene and with benzyl methyl ether, a solvent very similar to benzyl alcohol except that the hydrogen of the OH function has been replaced by a CH3 group. Also, the thermodynamic behaviour, gel formation and gel melting are similar for benzyl methyl ether and trans-decahydronaphthalene but are significantly different in benzyl alcohol: the gel melting point is higher, and so is the melting enthalpy. Extrapolation of this parameter to 100% OPVOH gives a value significantly higher than that directly measured on pure OPVOH. Molecular organization as studied by X-ray diffraction and fluorescence properties follow the same trend. The occurrence of a molecular compound between OPVOH and benzyl alcohol is contemplated in an attempt to account for these outcomes.

Insight into the gelation habit of oligo(para-phenylene vinylene) derivatives: effect of end-groups

This paper reports a study on the gelation habits of organogels prepared in benzyl alcohol from derivatives of oligo(p-phenylene vinylene) (OPV) bearing C16H33 aliphatic wings and a series of end groups on the backbone. Significant differences are observed based on whether or not these end groups can establish hydrogen bonds with adjacent OPV molecules or with the solvent. It is particularly shown that the fibril structure, the morphology and the molecular organization not only depend upon the aliphatic wings and/or the π–π interactions of the backbones but equally on the nature of these end groups. Evidence highlighting a homogeneous nucleation process for gel formation are also presented and discussed. Discrepancies with thermoreversible gels of covalent polymers are also stressed.