The emerging properties of noble metal nanoparticles are attracting huge interest from the translational scientific community. In this feature article, we highlight recent advances in the adaptation of noble metal nanomaterials and their biomedical applications in therapeutics, diagnostics and sensing.

Understanding the regulatory factors of self-assembly processes is a necessity in order to modulate the nano-structures and their properties. Here, the self-assembly mechanism of a peptide-perylenediimide (P-1) conjugate in mixed solvent systems of THF/water is studied and the semiconducting properties are correlated with the morphology. In THF, right handed helical fibers are formed while in 10% THF-water, the morphology changes to nano-rings along with a switch in the helicity to left-handed orientation. Experimental results combined with DFT calculations reveal the critical role of thermodynamic and kinetic factors to control these differential self-assembly processes. In THF, P-1 forms right handed helical fibers in a kinetically controlled fashion. In case of 10% THF-water, the initial nucleation of the aggregate is controlled kinetically. Due to differential solubility of the molecule in these two solvents, elongation of the nuclei into fibers is restricted after a critical length leading to the formation of nano-rings which is governed by the thermodynamics. The helical fibers show superior semi-conducting property to the nano-rings as confirmed by conducting-AFM and conventional I-V characteristics.

A water insoluble peptide-hydrogel that shows unique compartmentalization by not allowing any exchange to and from the hydrogel and can protect enzymes from denaturation.

Cation−π and charge-transfer (CT) interactions are ubiquitous in nature and involved in several biological processes. Although the origin of both the interactions in isolated pairs has extensively been studied, CT interactions are more prominent in supramolecular chemistry. Involvement of cation−π interactions in the preparation of advanced functional soft materials is uncommon. Moreover, a combination of these two interactions within a pair of electron donor (D) and acceptor (A) is uncharted. Here, we present a rational design to incorporate a combination of these two interactions within a D−A pair. A pyrene− peptide conjugate exhibits a combination of cation−π and CT interactions with a cationic naphthalenediimide (NDI) molecule in water. Nuclear Overhauser effect spectroscopy NMR along with other techniques and density functional theory calculations reveal the involvement of these interactions. The π-planes of pyrene and NDI adopt an angle of 56°to satisfy both the interactions, whereas β-sheet formation by the peptide sequence facilitates self-assembly. Notably, the binary system forms a self-supporting hydrogel at a higher concentration. The hydrogel shows efficient self-healing and injectable property. The hydrogel retains its thixotropic nature even at an elevated temperature. Broadly, we demonstrate a pathway that should prove pertinent to various areas, ranging from understanding biological assembly to peptide-based functional soft materials.

A water insoluble hydrogel that expels 50% of the water upon irradiation with UV-light.

Owing to the worldwide threats from the terrorist activities, easy and convenient sensing of explosives is extremely important. Picric acid (PA) is a well-known explosive that is commonly used in military operations. Such uses pollute the environment, resulting in a menace to human health. Development of a sensor for PA at a femtogram scale detection is a challenging task. In this work, we demonstrate a small peptide-based gelator capable of sensing PA selectively in solution-, gel-, and a gel-coated paper-based system. In solution, detection of PA is achieved through quenching of the monomeric emission of the gelator, while in the gel- or paper-based system, the sensing mechanism relies entirely on the decrease in excimer emission in the presence of PA. In the solution and gel state, the detection limits were found to be 115.24 ppt and 22.91 ppb, respectively. The gel, when coated on paper strips, could detect PA at femtogram scale (11.45 × 10–15 g cm–2) with a detection limit of 0.229 ppt. Notably, in spite of a very low vapor pressure of PA, the newly developed paper strip is able to sense PA vapor with a detection limit between ppt and ppb. DFT calculation revealed that energetically favorable complex formation between the pyrene of the gelator and PA is behind the quenching. The self-assembly of the peptide and its photophysical properties were used to create this simple, convenient, and economic method of PA detection on surfaces in a contact mode, and the paper-strip-based method holds promise for efficient practical uses.

In the last two decades, several Schiff bases have been reported as AIEgens which remain nonemissive in organic solvents but show strong fluorescence in the presence of water. A methodical analysis involving 21 Schiff bases, including some of the reported molecules, shows that in the presence of water, the Schiff bases hydrolyze to yield the corresponding starting aldehydes and amines. The observed emission in the presence of water is found to be originated from the aggregation of the fluorogenic aldehydes and not of the original molecules. Thus, while the aggregation-induced emission (AIE) effect is valid for these systems, certainly, these Schiff bases cannot be termed as AIEgens. Notably, the observation that these aldehydes can act as AIEgens through their excimer emission is an important phenomenon with respect to the current understanding of AIEgens.

Molecular organization of electron-deficient aromatic systems like perylenediimides (PDI) is extremely appealing, as they are potential candidates for organic electronics. The performance of these molecules in such applications primarily depends on the self-organization of the molecules. However, any correlation between the morphology of these self-assembled semiconducting molecules and their electrical performances has not yet been formulated. Herein, for the first time, we have made an effort to find such a correlation by studying the self-assembly, morphology, and their conducting properties for a peptide-PDI conjugate. The PDI conjugate formed fiber-like morphology in relatively nonpolar solvents (THF and CHCl) while in more polar solvents (HFIP, MeOH, ACN, and acetone), spherical morphology could be found. Interestingly, the self-assembly and the morphologies showed a clear dependence on the solvent polarity. In polar solvents, the conjugate aggregates more efficiently than in the nonpolar solvents, and with decrease in solvent polarity, the dimension of the nanostructures increased. However, in all the tested solvents, irrespective of their polarity, the PDI-peptide conjugate adopts a right-handed helicity. To find a correlation between the morphologies with the conducting property, detailed electrical characterization of these nanostructures was carried out. While no significant change could be observed for the dc conductivities of these nanostructures, the ac conductivities show prominent difference at the low-frequency region. A dispersion of conductivity was observed for the nanospheres due to the polarization effect. A critical correlation between the nanostructures and the activation energy was observed as with decrease in radii of curvature of the aggregates the activation energy increases with an exception in the case of MeOH. The observed results suggest that the long-range transport of charge carriers is less favorable when the aggregates are small and closely packed.