A series of peptides with a long fatty acyl chain covalently attached to the C-terminal part and a free amine (-NH) group at the N-terminus have been designed so that these molecules can be assembled in aqueous medium by using various noncovalent interactions. Five different peptide amphiphiles with a general chemical formula [HN-(CH)CONH-Phe-CONHC (n = 1-5, C = dodecylamine)] have been synthesized, characterized, and examined for self-assembly and hydrogelation. All of these molecules [P1 (n = 1), P2 (n = 2), P3 (n = 3), P4 (n = 4), P5 (n = 5)] form thermoresponsive hydrogels in water (pH 6.6) with a nanofibrillar network structure. Interestingly, the hydrogels obtained from compounds P4 and P5 exhibit potential antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis) and Gram-negative bacteria (Escherichia coli). Dose-dependent cell-viability studies using MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) by taking human lung carcinoma (A549) cells vividly demonstrates the noncytotoxic nature of these gelator molecules in vitro. Hemolytic studies show nonsignificant or little hemolysis of human erythrocyte cells at the minimum inhibitory concentration (MIC) of these tested bacteria. Interestingly, it has been found that these antibacterial noncytotoxic hydrogels exhibit proteolytic resistance toward the enzymes proteinase K and chymotrypsin. Moreover, the gel strength and gel recovery time have been successfully modulated by varying the alkyl chain length of the N-terminally located amino acid residues. Similarly, the thermal stability of these hydrogels has been nicely tuned by altering the alkyl chain length of the N-terminally located amino acid residues. In the era of antibiotic-resistant strains of bacteria, the discovery of this new class of peptide-based antibacterial, proteolytically stable, injectable, and noncytotoxic soft materials holds future promise for the development of new antibiotics.

A histidine based amphiphile containing a C14 fatty acyl chain, N-histidyl N-myristry ethyl amine (AM1, 14.7mM) forms hydrogels in the presence of Fe 3+ (within the range 1.47 to 4.41mM) and Hg 2+ (within the range 3.67 to 11.02 mM) ions in aqueous dispersions at pH 6.6 (27C). The imidazole ring of the histidine residue plays a vital role to interact with these metalions. The thermal and mechanical stability of these metallo-hydrogels can be tuned by changing the proportion of amphiphile to metal ion ratio (1:0.1 to 1:0.3 for Fe 3+ containing gel and 1:0.25 to 1:0.75 for Hg 2+ containing gel). The metallo-hydrogels were characterized by different spectroscopic and microscopic techniques, low-and wide-angle powder X-ray diffraction and small-angle X-ray scattering studies. FT-IR and NMR spectroscopic studies indicate the participation of the imidazole ring in metal-ion binding. Low-and wide-angle powder X-ray diffraction and small-angle X-ray scattering data are in favour of a layered structure of the supramolecular assembly of the AM1 in presence of metal-ions. Both, the amphiphiles and the metal ion induced hydrogels reveal catalytic activity of p-nitrophenyl esters hydrolysis for the acetyl, n-butyl and n-octyl esters. Ferric ion containing metallo-hydrogel exhibits higher catalytic activity than the corresponding AM1 aggregate in the absence of metal-ions.

Two isomeric amino-acid containing naphthalene diimide based molecules differ in their respective assembly and the intervening amide groups plays an important role in gelation, optical and semiconducting nature of these molecules.

A peptide appended perylene bisimide (PBI)based new hydrogel has been discovered in phosphate buffer medium having the pH ranging from 7.00 to 9.00. This peptide appended PBI-derivative shows interesting photoswitching property in the aggregated state. This gel is thoroughly characterized by UV−visible absorption and fluorescence spectroscopy, field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and rheological studies. TiO 2 nanoparticles are also prepared by using a protein amino acid, glutamic acid, in water medium. The as-synthesized TiO 2 nanoparticles exhibit usual photoswitching behavior. An organic−inorganic hybrid nanomaterial is prepared by incorporating the as-synthesized TiO 2 nanoparticles into the fibrillar gel network of the native gel. This TiO 2 −PBI-based hybrid soft material shows a characteristic of nanofiber and nanoparticle combination in their nanoscale coassembled state as it is evident from the respective TEM images of the hybrid material. Interestingly, this nanohybrid shows improved photoswitching properties (photocurrent gain) compared to that of its individual constituents (TiO 2 nanoparticles and PBI-based xerogel). The increase in photoswitching property holds a future promise for making a new organic−inorganic hybrid material for the optoelectronic device application with a high photocurrent conversion efficiency.

This is a unique example of fluorescent carbon dot-induced hydrogelation of an amino acid-based amphiphile. The carbon dot-to-amphiphile ratio dictates the gel stiffness. Moreover, this hydrogel can be used as a prominent fluorescent ink and the dried gel shows a remarkable, unusual green fluorescence in the solid state.