Sayanti Brahmachari

Biocompatible nanocrystalline natural bonelike carbonated hydroxyapatite synthesized by mechanical alloying in a record minimum time.

Single phase nanocrystalline biocompatible A-type carbonated hydroxyapatite (A-cHAp) powder has been synthesized by mechanical alloying the stoichiometric mixture of CaCO3 and CaHPO4.2H2O powders in open air at room temperature within 2h of milling. The A-type carbonation in HAp is confirmed by FTIR analysis. Structural and microstructure parameters of as-milled powders are obtained from both Rietvelds powder structure refinement analysis and transmission electron microscopy. Size and lattice strain of nanocrystalline HAp particles are found to be anisotropic in nature. Mechanical alloying causes amorphization of a part of crystalline A-cHAp which is analogous to native bone mineral. Some primary bond lengths of as-milled samples are critically measured. MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay test reveals high percentage of cell viability and hence confirms the biocompatibility of the sample. The overall results indicate that the processed A-cHAp has a chemical composition very close to that of biological apatite.

Refining hydrogelator design: soft materials with improved gelation ability, biocompatibility and matrix for in situ synthesis of specific shaped GNP

Despite the continuous surge in the development of new supramolecular gels, the prediction of a gelators structure still remains elusive. It is also imperative to consolidate the existing inventory of gelators and devise ways to make the gels functional. In the present work, L-phenylalanine based poor (C-16) or non-gelating (C-12 tail) amphiphiles were converted to excellent gelators with the simple incorporation of N-terminal protected amino acid/dipeptide at the end of the alkyl tail. More than 6-fold enhancement in gelation efficiency was observed for amino acid/dipeptides incorporated at the tail of amphiphile in comparison to the corresponding unmodified alkyl tail. Interestingly, amphiphile with the tertiary butyloxycarbonyl (Boc) protected amino acid at the tail had better gelation ability than the amphiphile with the aromatic Fmoc (N-fluorenyl-9-methoxycarbonyl) protecting group. Spectroscopic investigations (XRD and FTIR) revealed that the modification at the tail compels the amphiphiles to take a different course of self-assembly than that adopted by their predecessors (alkyl tailed gelator, C-16). For example, in the case of the amphiphile having a dipeptide at the tail, formation of β-sheet structure through anti-parallel arrangement between the molecules results in notable improvement in its gelation ability. Most importantly, these tail modified amphiphiles were capable of in situ synthesis of gold nanoparticles (GNPs) of specific shape without the help of any external reducing agents in the newly developed soft materials. The biocompatibility of hydrogels is also crucial for their prolific biomedicinal functions. MTT assay showed dramatic improvement in the biocompatibility of the tail modified hydrogelators towards mammalian cells in comparison to the amphiphiles having no amino acid at the tail.

Pyridinium based amphiphilic hydrogelators as potential antibacterial agents

Summary The numerous applications of hydrogelators have led to rapid expansion of this field. In the present work we report the facile synthesis of amphiphilic hydrogelators having a quaternary pyridinium unit coupled to a hydrophobic long alkyl chain through an amide bond. Different amphiphiles with various hydrophobic chain length and polar head groups were rationally designed and synthesized to develop a structure-property relation. A judicious combination of hydrophilic and hydrophobic segments led to the development of pyridinium based amphiphilic hydrogelators having a minimum gelation concentration of 1.7%, w/v. Field emission scanning electronic microscopy (FESEM), atomic force microscopy (AFM), photoluminescence, FTIR studies, X-ray diffraction (XRD) and 2D NOESY experiments were carried out to elucidate the different non-covalent interactions responsible for the self-assembled gelation. The formation of three-dimensional supramolecular aggregates originates from the interdigitated bilayer packing of the amphiphile leading to the development of an efficient hydrogel. Interestingly, the presence of the pyridinium scaffold along with the long alkyl chain render these amphiphiles inherently antibacterial. The amphiphilic hydrogelators exhibited high antibacterial activity against both Gram-positive and Gram-negative bacteria with minimum inhibitory concentration (MIC) values as low as 0.4 μg/mL. Cytotoxicity tests using MTT assay showed 50% NIH3T3 cell viability with hydrogelating amphiphile 2 up to 100 μg/mL.

Biotinylated amphiphile-single walled carbon nanotube conjugate for target-specific delivery to cancer cells

The present work reports the specific targeting of cancerous cells using a non-covalently water dispersed nanoconjugate of biotinylated amphiphile-single walled carbon nanotube (SWNT). The fundamental approach involves incorporation of the biotin into the architecture of the carbon nanotube (CNT) dispersing agent to develop a multifaceted delivery vehicle having a high colloidal stability, substantial cell viability and targeted specificity towards cancer cells. A three way functionalization strategy was employed to introduce a C-16 hydrophobic segment, polyethylene glycol hydrophilic fragment and biotin as the target-specific unit at the -OH, -COOH and -NH2 terminals of l-tyrosine, respectively. The newly developed neutral amphiphile exhibited an efficient SWNT dispersion (72%) in water, significant viability of different mammalian cells (Hela, HepG2, CHO and HEK-293) up to 48 h and also media stability. Most importantly, the biotinylated amphiphile-SWNT dispersion successfully transported the fluorescently labelled Cy3-oligoneucleotide (loaded on the surface of CNT) inside the cancerous Hela, HepG2 cells after 3 h of incubation, in contrast to CHO and HEK-293 cells (devoid of overexpressed biotin receptors). The presence of the biotin moiety in the cellular transporters facilitated the internalization of cargo due to the overexpressed biotin receptors in the cancer cells. Importantly, this nanohybrid was also capable of specifically transporting the anticancer drug doxorubicin to cancer cells, which led to the significant killing of Hela cells compared to the normal CHO cells. Thus, the receptor-mediated specific transportation of cargo into cancer cells was possible only due to the biotinylated CNT dispersing agent. To the best of our knowledge this is the first reported amino acid based biotinylated small amphiphilic molecule that non-covalently dispersed SWNTs and the corresponding nanoconjugate showed excellent cell viability, antibiofouling properties and the desired target-specific drug delivery.

pH-Responsive single walled carbon nanotube dispersion for target specific release of doxorubicin to cancer cells.

Cholesterol-based dipeptide carboxylates were synthesized and exploited for the pH responsive reversible dispersion and precipitation of single walled carbon nanotube (SWCNT) in water specifically at tumorogenic environmental pH (6.0-6.5). The nanohybrid showed excellent pH responsive drug release between pH range of 6.0 and 6.5. The exfoliation of SWCNTs was characterized by microscopic and spectroscopic studies. Importantly, drug-loaded SWCNT dispersions showed better killing efficiency compared to that of the native drug and were highly efficient in selective killing of cancer cells (Hela, HepG2) with 2.5-fold higher efficacy compared to that of normal cells (CHO, NIH3T3).

pH-responsive reversible dispersion of biocompatible SWNT/graphene–amphiphile hybrids

The present work reports synthesis of cholesterol based peptide carboxylates as efficient dispersing agents for single walled carbon nanotubes (SWNTs) as well as graphene in water. Variation of the amino acids within the peptide moiety exhibited interesting changes in their SWNTs dispersion efficacy. The dipeptide carboxylate comprising of two alanine residues showed 80% SWNTs dispersion which is ∼2 fold higher than that obtained by using the common surfactant, SDBS. The dipeptide amphiphiles also efficiently dispersed the 2D-allotrope of carbon, graphene, in water. As to our objective, the terminal carboxylate moiety in these cholesterol based carboxylates exhibited pH-sensitivity towards the reversible solubilization and precipitation of the nanohybrids. Acidification of the nanohybrids with HCl converted the carboxylates to the water insoluble carboxylic acids leading to the precipitation of carbon nanomaterials. Most importantly, addition of an equivalent amount of NaOH resulted in the restoration of stable aqueous dispersion of SWNT/graphene. This reversible precipitation–dispersion cycle was performed time and again. The conversion of the carboxylate salt to the corresponding acid and vice versa is the main reason for such reversible switching between precipitation and dissolution under acidic and basic pH. Indeed the presence of the amino acid/peptide moiety in the structure of cholesterol carboxylates was found to be indispensable for efficient dispersion of carbon nanomaterials. Significant stability of these SWNT dispersions was observed in the presence of high salt and protein concentration. Moreover, the nanohybrids were highly biocompatible with mammalian cells, which increases their future prospects in biomedicine.

Fabrication of SWCNT-Ag Nanoparticle Hybrid Included Self-Assemblies for Antibacterial Applications

The present article reports the development of soft nanohybrids comprising of single walled carbon nanotube (SWCNT) included silver nanoparticles (AgNPs) having superior antibacterial property. In this regard aqueous dispersing agent of carbon nanotube (CNT) containing a silver ion reducing unit was synthesised by the inclusion of tryptophan and tyrosine within the backbone of the amphiphile. The dispersions were characterized spectroscopically and microscopically using TEM, AFM and Raman spectroscopy. The nanotube-nanoparticle conjugates were prepared by the in situ photoreduction of AgNO3. The phenolate residue and the indole moieties of tyrosine and tryptophan, respectively reduces the sliver ion as well as acts as stabilizing agents for the synthesized AgNPs. The nanohybrids were characterized using TEM and AFM. The antibacterial activity of the nanohybrids was studied against Gram-positive (Bacillus subtilis and Micrococcus luteus) and Gram-negative bacteria (Escherichia coli and Klebsiella aerogenes). The SWCNT dispersions showed moderate killing ability (40–60%) against Gram-positive bacteria however no antibacterial activity was observed against the Gram negative ones. Interestingly, the developed SWCNT-amphiphile-AgNP nanohybrids exhibited significant killing ability (∼90%) against all bacteria. Importantly, the cell viability of these newly developed self-assemblies was checked towards chinese hamster ovarian cells and high cell viability was observed after 24 h of incubation. This specific killing of bacterial cells may have been achieved due to the presence of higher –SH containing proteins in the cell walls of the bacteria. The developed nanohybrids were subsequently infused into tissue engineering scaffold agar-gelatin films and the films similarly showed bactericidal activity towards both kinds of bacterial strains while allowing normal growth of eukaryotic cells on the surface of the films.

GNP confinement at the interface of cationic reverse micelles: influence in improving the lipase activity

The present work reports thiol-assisted confinement of gold nanoparticles (GNPs) at the interface of reverse micelles with the aim to enhance the interfacial area and thereby the efficiency of surface-active Chromobacterium viscosum lipase. The strong gold capping ability of optimally hydrophobic thiols (1-dodecanethiol and 1,6-hexanedithiol) was aptly utilized to pull GNPs (∼3–5 nm) from the water pool to the oil/water interface of cetyltrimethylammonium bromide (CTAB) reverse micelles. These small sized GNPs were fitted at the microscopic interface of CTAB reverse micelles possibly because of the comparable thickness of the interface (∼1–2 nm) to that of the GNP diameter. Lipase solubilized within this augmented interface enjoys a flexible conformation, which resulted in the improvement of its activity (∼2.5 fold) with respect to only CTAB microemulsion. The activity of lipase within CTAB reverse micelles was thoroughly studied in the presence of mono and dithiols with varying chain length, where a greater improvement in activity was observed with dithiols. Bidentate ligand property of dithiols led to firm localization of higher number of GNPs at the interface which enhanced the total space in vicinity of enzyme at the interfacial domain. Fitting fusion of small sized GNPs within CTAB reverse micellar interface was confirmed by microscopic and spectroscopic studies. Smooth localization of lipase at the enhanced interface was also confirmed from the improvement in its secondary structure (α-helical content) in circular dichroism spectroscopic analysis. Interestingly, large sized GNPs (∼8 and 20 nm) were found to be well fitted at the interface of bigger head group-containing surfactants, cetyltriethylammonium bromide (CTEAB) and cetyltripropylammonium bromide (CTPAB). The hydrolytic efficiency of lipase in 1,6-hexanedithiol included GNP (∼20–25 nm)-doped CTPAB reverse micelles improved by ∼3.4 fold compared to that observed in only CTAB.