Kalathil K. Kartha

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.

Detection of Nitroaromatic Explosives with Fluorescent Molecular Assemblies and π-Gels

Molecular assemblies and gels made up of fluorescent π-systems through noncovalent interactions are fascinating materials with a wide range of properties and applications. Fluorescence is an extremely sensitive property, which gets perturbed upon molecular self-assembly and gelation. Further manipulation of fluorescence in such materials is possible with external stimuli, such as stress, temperature, or with different analytes. Explosives are a class of analytes that respond to certain fluorescent molecular systems; thus allowing their sensing in a required environment. In recent times, this research has become a topic of great demand, resulting in a large number of publications, due to their relevance in safety and security issues. In this account, we record some of the major developments in the field of explosive sensing with fluorescent molecular assemblies and gels.

Solvent‐Directed Self‐Assembly of π Gelators to Hierarchical Macroporous Structures and Aligned Fiber Bundles

Morphology variation: The Boc-alanine linked OPV exhibits an unprecedented formation of periodic macroporous honeycomb structures in chloroform and aligned fiber bundles in toluene (see SEM images). This represents a unique example for a distinct morphology change of an organogelator from macroporous honeycomb to aligned fiber bundles upon changing the solvent.

Supramolecular gels and functional materials research in India.

Supramolecular gels are a class of soft materials made up of small molecules held together through non-covalent interactions. They have reversible properties and a wide range of applications. Chromophore-based gels are of particular interest due to their inherent electronic properties such as emission and charge transport useful for organic electronic device fabrication. Significant contributions have been made by Indian researchers in this area, which are highlighted in this mini review.

Effect of the Bulkiness of the End Functional Amide Groups on the Optical, Gelation, and Morphological Properties of Oligo(p-phenylenevinylene) π-Gelators

Herein, we describe the role of end functional groups in the self-assembly of amide-functionalized oligo(p-phenylenevinylene) (OPV) gelators with different end-groups. The interplay between hydrogen-bonding and π-stacking interactions was controlled by the bulkiness of the end functional groups, thereby resulting in aggregates of different types, which led to the gelation of a wide range of solvents. The variable-temperature UV/Vis absorption and fluorescence spectroscopic features of gelators with small end-groups revealed the formation of 1D H-type aggregates in CHCl(3). However, under fast cooling in toluene, 1D H-type aggregates were formed, whereas slow cooling resulted in 2D H-type aggregates. OPV amide with bulky dendritic end-group formed hydrogen-bonded random aggregates in toluene and a morphology transition from vesicles into fibrous aggregates was observed in THF. Interestingly, the presence of bulky end-group enhanced fluorescence in the xerogel state and aggregation in polar solvents. The difference between the aggregation properties of OPV amides with small and bulky end-groups allowed the preparation of self-assembled structures with distinct morphological and optical features.

Pyridyl‐Amides as a Multimode Self‐Assembly Driver for the Design of a Stimuli‐Responsive π‐Gelator

An oligo(p-phenylenevinylene) (OPV) derivative connected to pyridyl end groups through an amide linkage (OPV-Py) resulted in a multistimuli-responsive π-gelator. When compared to the corresponding OPV π-gelator terminated by a phenyl-amide (OPV-Ph), the aggregation properties of OPV-Py were found to be significantly different, leading to multistimuli gelation and other morphological properties. The pyridyl moiety in OPV-Py initially interferes with the amide H-bonded assembly and gelation, however, protonation of the pyridyl moiety with trifluoroacetic acid (TFA) facilitated the formation of amide H-bonded assembly leading to gelation, which is reversible by the addition of N,N-diisopropyethylamine (DiPEA). Interestingly, addition of Ag(+) ions to a solution of OPV-Py facilitated the formation of a metallo-supramolecular assembly leading to gelation. Surprisingly, ultrasound-induced gelation was observed when OPV-Py was mixed with a dicarboxylic acid (A1). A detailed study using different spectroscopic and microscopic experimental techniques revealed the difference in the mode of assembly in the two molecules and the multistimuli-responsive nature of the OPV-Py gelation.

Self-Assembly in Sensor Nanotechnology

Self-assembled fluorescent molecules are powerful materials for wide-ranging applications, particularly for analyte sensing and optical imaging. Much research has gone into the development of new fluorogenic molecular probes; however, supramolecular architectures of molecular assemblies have not been fully explored. Fluorescent molecules and functional dyes are the ideal candidates for the design of supramolecular sensors since their fluorescence can be significantly modulated by self-assembly and analyte interaction. Various strategies such as molecular recognition and sensing without altering the self-assembly, and analyte-induced disassembly and morphology variations have been used for the generation of a measurable signal, resulting in the sensing and quantification of a required analyte. Herein, we discuss the recent developments on fluorescent supramolecular assemblies in sensor nanotechnology and highlight their potential application.