Hydrogelation of bile acid–peptide conjugates and in situ synthesis of silver and gold nanoparticles in the hydrogel matrix

Abstract

Fabricating supramolecular hydrogels with embedded metal nanostructures is important for the design of novel hybrid nanocomposite materials for diverse applications such as biosensing and chemosensing platforms, catalytic and antibacterial functional materials etc. Supramolecular self-assembly of bile acid–dipeptide conjugates has led to the formation of new supramolecular hydrogels. Gelation of these molecules depends strongly on the hydrophobic character of the bile acids. The possibility of in situ fabrication of Ag and Au NPs in these supramolecular hydrogels by incorporating Ag⁺ and Au³⁺ salts was investigated via photoreduction. Chemical reductions of Ag⁺ and Au³⁺ salts in the hydrogels were performed without adding any external stabilizing agents. In this report we have shown that the color, size and shape of silver nanoparticles formed by photoreduction depend on the amino acid residue of the side chain.

---

Introductions

The self-assembly of small organic molecules forming entangled three dimensional fibrous networks can hold solvent molecules, forming organogels or hydrogels. The gel network is typically made up of nanofibres which are held together by non covalent interactions such as hydrogen bonding, hydrophobic interactions, π–π stacking, etc. A wide variety of molecules of diverse structural classes, including steroids, peptides, carbohydrates etc., are well known low molecular weight gelators (LMWG).¹ Peptide based hydrogels have attracted chemists and biologists due to their non-toxicity, biocompatibility and stimuli responsive nature.² These systems have been used for the synthesis and stabilization of nanoparticles^3a–e and for enzymatic assays and cell culture.^2b,4 The synthesis of facially amphiphilic bile thiols were reported from this group and used for stabilization of gold and silver nanoparticles,^3f and formation of gel–nanoparticles hybrids.^3g

There are only a few hybrid bile acid–amino acid conjugates known in the literature which are potent organogelators. The first report of organogelation based on N-cholyl amino acid derivatives dates back to 2001 by Sudholter et al.⁵ Later, Noponen and co-workers reported the synthesis, organogelation and NMR spectroscopic studies of bile acid–methionine ester conjugates.⁶ Several other bile acid–peptide conjugates are known in the literature,⁷ but the gelation properties of these derivatives were not studied.

The present work aims at the synthesis of bile acid–peptide conjugates 1–10 (Chart 1) and a systematic study of their hydrogelation properties. Since many phe-based peptide gelators are known in the literature reference we decided to use phe as one of the amino acids in compounds 1–8. Chirality of the amino acids in the peptide based gelators is known to have marked influences on gelation and on the gel morphology.⁸ We have also made a number of interesting observations with our systems, including the dependence of color, size and shape of the in situ formed silver nanoparticles on the nature of the amino acid residue in the gelator. To the best of our knowledge this is the first report highlighting such observations. We have also prepared gold nanoparticles in the gel matrix.

Results and discussions

The synthesis of bile acid–peptide conjugates‡ 1–10 were carried out through standard peptide coupling protocols (see ESI†), by varying the bile acid unit and the dipeptide moiety (Chart 1, Table 1). Initial gelation studies were done by the dropwise addition of 0.5 M NaOH solution to the aqueous suspension of the bile acid–peptide conjugates, followed by heating and cooling. Interestingly, only the lithocholyl derivatives formed hydrogels while the other derivatives 5 (deoxycholyl) and 6 (cholyl) gave clear solutions. Diastereomers LC-G-^LF (1) and LC-G-^DF (2) formed more transparent gels (final pH ∼ 11) compared to the other derivatives under these conditions (Fig. 1).

Chart 1

Table 1 Gelation test results for 1% (w/v) concentration^a

Compounds	In NaOH (pH 11)	Phosphate buffer (pH 7)	Phosphate buffer (pH 8)
a G = transparent gel, TG = translucent gel, P = precipitate or suspensions, S = clear solution, WG = weak gel, *compounds other than 1 & 2 formed gels after heating followed by sonication.
LC-G-^LF (1)	G	G	G
LC-G-^DF (2)	G	P	G
LC-^LF-G (3)	G	TG	TG
LC-^DF-G (4)	G	TG	TG
DC-G-^LF (5)	S	S	S
C-G-^LF (6)	S	S	S
LC-^LF-^LF (7)	WG	TG	TG
LC-^DF-^DF (8)	G	P	TG
LC-^LA-^LA (9)	G	TG	TG
LC-G-^DA (10)	G	TG	TG

---

Fig. 1 Images of LC-G-^LF (1) hydrogel in the presence of NaOH at different wt%.

The fact that deoxycholyl and cholyl derivatives did not form gels under the same conditions (5 and 6 vs. 1) suggests that the increase in the number of hydroxyl groups decreases the hydrophobicity of the deoxycholyl and cholyl derivatives which in turn affects aggregation.

Gelation of both LC-G-^LF (1) and LC-G-^DF (2) were then checked in phosphate buffers of pH 7 and 8 (100 mM, P_i, 8.6 mM gelator). Compound 1 formed transparent and strong gels in both the buffers (Fig. 1). Gelation studies of 2 in buffer solutions resulted in some unexpected findings. Even after prolonged heating and sonication, this compound remained as a milky white suspension in pH 7 buffer but a strong, transparent gel was formed in pH 8 buffer (Fig. 2). Gels (1 and 2) were thermoreversible, thixotropic and also injectable in nature (Fig. 3). Also, the xerogel prepared from the gel in pH 8 phosphate buffer re-formed the hydrogel by the addition of water followed by heating and then cooling to room temperature (Fig. 3c, 1 wt% gel).

Fig. 2 (A) Suspension of 2 in pH 7 (B) gel formed in pH 8 buffer.

Fig. 3 Responsive nature of gel 1 in pH 8 buffer (a), demonstrates the injectability nature of AgNPs containing gel 1 (b), xerogel to gel formation of 1 (c).

On the other hand, both LC-^LF-G (3) and LC-^DF-G (4) formed translucent gels in pH 7 and 8 buffers after heating followed by sonication. Also, for LC-^LF-^LF (7) and LC-^DF-^DF (8), 7 formed gels in both pH 7 and 8 phosphate buffers where as 8 formed a gel only in pH 8 but precipitated out in pH 7. LC-G-^DA (10) formed gel at both pH. Therefore, gelation properties of the bile acid–peptide conjugates were very much dependent on the nature, location and chirality of the peptide residue. While the reasons are not entirely clear to us at this time, we believe that it results from a complex interplay of solubility, ionization state and hydrophobicity.

Microscopic structures

Xerogels derived from 1–4 showed liquid crystalline schlieren textures⁹ when viewed in a polarizing optical microscope. Fanlike, birefringent structures were observed in the optical micrograph of the gels (Fig. 4).

Fig. 4 POM images of the xerogels of (a) 1, (b) 2, (c) 3 and (d) 4.

SEM (Fig. 5) and AFM (Fig. 6) images showed that the gels were composed of a three dimensional network of fibres. These SEM images did not show much difference in the morphology of the gels with the chirality of the amino acid. But the SEM images do reveal short or broken fibres for 3 and long flat ribbons for 1. The average diameter of the fibres was 80–120 nm.

Fig. 5 SEM images of the xerogels of (a) 1, (b) 2, (c) 3 and (d) 4.

Fig. 6 AFM images of the xerogels of (a) 1, (b) 2, (c) 3 and (d) 4.

Atomic force microscopy of the gels also revealed the fibrous morphology of the gels. The fiber diameter was found to be comparable to the SEM data.

Single-crystal X-ray crystallography

Single crystals of LC-G-^LF (1) obtained by small scale recrystallization from MeOH/H₂O mixture were used for structure determination by X-ray diffraction. CCDC 1028308 (LC-G-^LF, 1) contains the crystallographic data for this paper.† The compound crystallized in the monoclinic space group (P2₁) (Fig. 7). Additional X-ray data are available in the ESI.†

Fig. 7 Molecular structure of LC-G-^LF, 3(O) (1).

Fig. 8 shows the crystal packing of compound 1 from two views. The steroidal skeletons showed geometric parameters very typical of bile acids. Although packing in the crystal and in the gel aggregates may or may not be similar, crystal structure of a gelator molecule can give insights on the possible modes of molecular association in the gel aggregates. In the crystal, a bilayered structure is observed with the molecules in each layer arranged in a head to tail fashion. These layers run anti-parallel to one another. The α-face of the steroid backbone interacted with each other via van der Waals interactions. Extensive H-bonded network with neighbours and also through water molecules are observed in the crystal structure. Groups involved in H-bonding interaction include the amide N–H, C=O, COOH and hydroxyl group of C-3. The gel structure might also involve H-bonded network similar to that existing in the crystal and gelation might be a consequence of a higher propensity of the growth of aggregates along the molecular length (guided by H-bonding), leading to long fibrillar assemblies.

Fig. 8 Crystal packing of compound 1.

Synthesis of metal nanoparticles in gel matrix

Metal nanoparticles, owing to their unique size-dependent optical, electronic and magnetic properties, have found many applications in catalysis, electronics, optics etc.¹⁰ Antimicrobial activity of AgNPs and AuNPs have been studied in detail as they can inhibit enzymatic systems and alter DNA synthesis.¹¹

There are several methods for the synthesis of silver and gold nanoparticles, viz. chemical reduction, enzymatic reduction, photo reduction etc. Banerjee et al. used Fmoc-Val-Asp-OH based hydrogels as a media for the preparation of fluorescent silver nanoclusters in the presence of sunlight.¹² Earlier report from our lab also detailed the preparation of AgNPs and AuNPs in a bile salt derived gel matrix.¹³

Preparation of AgNPs by chemical reduction

Gels (0.5 wt%) prepared in phosphate buffer (100 mM, pH 8) containing 0.4 mM AgNO₃ were chosen as the scaffold for the preparation of AgNPs. The Ag⁺ doped gels were translucent compared to the undoped gels. Aqueous NaBH₃CN (5 mM) was placed on the top of the gel and was allowed to diffuse in the dark.

With 1 (LC-G-^LF) a yellow colour appeared at the interface within 15 minutes and the colour started diffusing in. But with 2, (LC-G-^DF) a pink colour was observed and the colour change was very slow. After 12 h the excess borohydride solution on the top of the gels was decanted and the absorption spectra of the gels were recorded. The yellow gel showed an absorption maximum at 423 nm while the pink gel showed a broad peak around 500 nm (Fig. 9). With the formation of the nanoparticles the gel became more transparent.

Fig. 9 UV-visible spectra of the AgNPs formed in the hydrogels of (a) 1 and (b) 2.

Interestingly, upon storage, the intensity of the pink colour (from LC-G-^DF, 2 gel) increased and after 3 weeks the colour completely changed to yellow. Absorption spectra also showed changes consistent with the visual appearance. The broad peak at 550 nm underwent blue shift and finally the yellow gel showed absorption maximum around 417 nm (Fig. 10), similar to the system derived from 1. Silver nanoparticles were also prepared in LC-^LF-G (3) and LC-^DF-G (4) hydrogel by the same procedure as followed for 1 and 2 (see ESI†).

Fig. 10 Time dependent UV-visible spectra of the AgNPs synthesized in the gel matrix of 2.

It is, therefore, interesting to note that while the reasons are not yet clear, a subtle change of the molecular structure of the gelator can lead to the formation of silver nanoparticles of different colour and therefore, sizes (vide infra).

Preparation of AgNPs by photoreduction

Photoreduction is a convenient and well accepted method for the preparation of nanoparticles as the absence of an added reagent keeps the system clear. As a preliminary experiment we have exposed Ag⁺ doped gels of both 1 and 2 to sunlight. To our surprise we observed that silver doped LC-G-^DF (2) gel turned pink within a minute and the colour started changing to deep pink, brownish pink, brownish yellow and finally to yellow in 1 h on continued exposure. The silver doped LC-G-^LF (1) gel did not show any colour change even after 2 h. But the addition of NaBH₃CN on the top of this gel showed the development of yellow colour within 15 minutes. This suggested that the reduction of Ag⁺ in the LC-G-^LF (1) gel medium was prevented in the presence of sunlight.

More systematic studies were done by irradiating the gels by light from a 500 W tungsten lamp (keeping the sample at a fixed distance of 12 cm from the light source). Absorbance was recorded at regular time intervals and Fig. 11 shows the changes in the absorption spectrum with time.

Fig. 11 Irradiation time dependent absorption spectral changes of AgNPs synthesized in LC-G-^DF (2) gel (cf. Fig. 10).

The pink gel derived from 2 showed a broad peak in the absorption spectrum centred around 520 nm, with a barely visible shoulder at 385 nm. With increase in irradiation time the peaks gained intensity unsymmetrically and after 1 h 15 min the peaks merged. At this stage the gel was brownish pink and the colour finally changed to yellow after 6 h with a single, less broader peak at 413 nm. The changes in the absorption spectrum were similar to that observed with chemical reduction (Fig. 10), although the time scales were different.

The silver doped LC-G-^LF (1) gel was irradiated for 15 h, but there was neither any change in the colour nor in the absorption spectrum, indicating the stability of silver ions in the system during photo-irradiation. The development of the yellow colour indicating the formation of AgNPs was observed only when NaBH₃CN was added on the top of the same sample. This confirmed that the silver ions in the LC-G-^LF (1) gel were not susceptible to photoreduction.

Similar observations were made during the photoreduction of silver ions in the gels of LC-^LF-G (3) and LC-^DF-G (4). AgNPs were formed only in the gel of 4 and the colour change was from pink to brown (ESI†), where the gel matrix of 3 was not a suitable medium for photoreduction (Table 2).

Table 2 Results for silver nanoparticles synthesis in gel matrices

Compounds	Chemical reduction	Photoreduction
LC-G-^LF (1)	Yellow	No reaction
LC-G-^DF (2)	Pink to yellow	Pink to yellow
LC-^LF-G (3)	Yellow	No reaction
LC-^DF-G (4)	Pink to brown	Pink to brown

---

These observations suggest that Ag⁺ is more strongly associated with gelators 2 and 4, but not with 1 and 3. While photoreduction of Ag⁺ in the presence of peptides is known, the mechanistic details are not clear. We speculate that a stronger interaction of Ag⁺ with a potentially oxidizable ligand would facilitate photoreduction,¹⁴ and slow down chemical reduction. FT-IR data suggest a stronger interaction of Ag⁺ with LC-G-^DF (2) than LC-G-^LF (1) in the xerogel state (see ESI†).

Electron microscope images of AgNPs

TEM studies were carried out to observe the size and shapes of the AgNPs synthesized in gel matrix. The pink and yellow colour AgNPs were prepared in LC-G-^DF (2) gel by photoirradiation for 45 min and 6 h.

The TEM images of the pink gel showed fibres with spherical AgNPs arranged on them (Fig. 12). The average size of the nanoparticles was about 25 nm. In addition, clusters of AgNPs were also seen in the sample. As reported in the literature, clusters and larger particles are responsible for the pink colour.^12,15

Fig. 12 TEM images of the pink AgNPs synthesized in LC-G-^DF (2) gel.

The yellow gel of 2 prepared by prolonged photoirradiation of the pink gel showed that the sample contained spherical nanoparticles of uniform 5–10 nm size (Fig. 13), again consistent with literature data (colour vs. size).

Fig. 13 TEM images of the yellow AgNPs in LC-G-^DF (2) gel.

TEM analysis of the yellow AgNPs prepared in the LC-G-^LF (1) gel matrix by chemical reduction showed that the spherical nanoparticles formed were of smaller size (around 5–10 nm) and no clusters were seen in this case (Fig. 14).

Fig. 14 Spherical AgNPs formed in the LC-G-^LF (1) gel matrix by chemical reduction.

We believe that perhaps for the first time our studies reveal that variable interaction of Ag⁺ with diastereomeric molecules could affect chemical and photochemical reduction of Ag⁺.

Preparation of AuNPs by chemical reduction

Gels prepared in phosphate buffer (100 mM, pH 8) were selected as the media for the synthesis of AuNPs. Gels (0.5 wt%) containing AuCl₃ (0.4 mM) were prepared and aqueous NaBH₃CN (400 μL of 5 mM) was placed on the top of the gel and allowed to diffuse in the dark. In both cases (1 and 2) a pink colour appeared at the interface within an hour. After 4 h, the gels become wine red. The excess borohydride solution on the top of the gels was decanted and absorption spectra were recorded. Unlike AgNPs synthesis, the AuNPs did not show any variation between 1 and 2. It has been reported that the AgNPs structure is more dependent on the composition of organic stabilizing agent than AuNPs due to higher cohesive energy of gold.¹⁶ The wine red gels showed broad absorption bands (Fig. 15) with λ_max at 515 nm for gel (1) and 505 nm with another 650 nm peak for gel (2) as reported for the AuNPs.¹⁷

Fig. 15 Absorption spectra of AuNPs formed in the hydrogels of (a) 1 and (b) 2 by chemical reduction.

TEM images of AuNPs showed that almost spherical uniform nanoparticles were formed by chemical reduction. The average sizes of the nanoparticles were around 20 nm for LC-G-^LF (1) and 5–15 nm for LC-G-^DF (2) gels (Fig. 16).

Fig. 16 AuNPs formed by chemical reduction in the LC-G-^LF (1) gel (a–c) and LC-G-^DF (2) gel matrix (d–f).

Preparation of AuNPs by photoreduction

We have also exposed Au³⁺ doped gels of both 1 and 2 to light from a 500 watt tungsten lamp. Both gels showed pink AuNPs formation with absorbance maximum around 530 nm (Fig. 17).

Fig. 17 Absorption spectra of the AuNPs formed in the hydrogels of (a) 1 and (b) 2 by photoirradiation.

TEM images of the photo reduced AuNPs showed that almost spherical nanoparticles were formed by photoreduction. The average size of the Au nanoparticles were around 4–8 nm for LC-G-^LF (1) and 6–10 nm for LC-G-^DF (2) gels (Fig. 18). Nanoparticle containing hybrid materials were stable for more than 3 months at room temperature in the dark.

Fig. 18 TEM images of AuNPs formed in the (a and b) LC-G-^LF (1) and (c and d) LC-G-^DF (2) gel matrix by photoreduction.

Summary and future outlook

Our work suggests a general structural motif for the lithocholyl–dipeptide conjugate hydrogelators. These results indicate that the hydrophobicity of the bile acid unit as well as the nature of the amino acid residues play important roles in gelation. The bile acid–peptide gel matrices were used for the in situ preparation of silver and gold nanoparticles. The prepared AgNPs showed different colours based on the gelator molecules. In addition, photoreduction of silver ions showed dependence on the peptide residue which signify coupling in their nanoscale constituents. Preliminary studies on the AgNPs/hydrogel nanocomposite indicated pronounced antimicrobial activity. This is being examined in detail and will be reported elsewhere.¹⁸

Acknowledgements

We are grateful to the DST (grant no. SR/S1/OC-68/2011), New Delhi for the support of this work. UM also thanks the DST for the award of a J. C. Bose Fellowship. MM thanks the CSIR for a research fellowship. VSS thanks UGC, New Delhi for a D. S. Kothari postdoctoral fellowship. The Chemical divisional TEM facilities and the AFMM Centre are thanked for the TEM and SEM imaging analysis, respectively. We thank Mr Amar Hosamoni (SSCU, IISc, Bangalore) for help with the X-ray structure analysis.

Notes and references

1. (a) N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821; (b) H. Svobodová, V. Noponen, E. Kolehmainen and E. Sievänen, RSC Adv., 2012, 2, 4985; (c) E. K. Johnson, D. J. Adams and P. J. Cameron, J. Mater. Chem., 2011, 21, 2024; (d) S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem. Rev., 2014, 114, 1973; (e) V. S. Sajisha and U. Maitra, RSC Adv., 2014, 4, 43167; (f) U. Maitra and A. Chakrabarty, Beilstein J. Org. Chem., 2011, 7, 304.
2. (a) Y. Loo, S. Zhang and C. A. E. Hauser, Biotechnol. Adv., 2012, 30, 593; (b) V. Jayawarna, M. Ali, T. A. Jowitt, A. F. Miller, A. Saiani, J. E. Gough and R. V. Ulijn, Adv. Mater., 2006, 18, 611; (c) A. Dasgupta, J. H. Mondal and D. Das, RSC Adv., 2013, 3, 9117; (d) C. Tomasini and N. Castellucci, Chem. Soc. Rev., 2013, 42, 156.
3. (a) G. Palui, J. Nanda, S. Ray and A. Banerjee, Chem.–Eur. J., 2009, 15, 6902; (b) S. Bhowmik, T. Gorai and U. Maitra, J. Mater. Chem. C, 2014, 2, 1597; (c) A. Biswas and A. Banerjee, Chem.–Asian J., 2014, 9, 3451; (d) F. Delbecq, Adv. Colloid Interface Sci., 2014, 209, 98; (e) J. S. Shen, Y. L. Chen, Q. P. Wang, T. Yu, X. Y. Huang, Y. Yanga and H. W. Zhang, J. Mater. Chem. C, 2013, 1, 2092; (f) S. Bhat and U. Maitra, J. Chem. Sci., 2008, 120, 507; (g) S. Bhat and U. Maitra, Chem. Mater., 2006, 18, 4224.
4. (a) Y. Gao, Z. Yang, Y. Kuang, M. L. Ma, J. Li, F. Zhao and B. Xu, Biopolymers, 2010, 94, 19; (b) Y. Gao, F. Zhao, Q. Wang, Y. Zhang and B. Xu, Chem. Soc. Rev., 2010, 39, 3425; (c) M. Zhou, A. M. Smith, A. K. Das, N. W. Hodson, R. F. Collins, R. V. Ulijn and J. E. Gough, Biomaterials, 2009, 30, 2523.
5. H. M. Willemen, T. Vermonden, A. T. M. Marcelis and E. J. R. Sudhölter, Eur. J. Org. Chem., 2001, 2329.
6. V. Noponen, Nonappa, M. Lahtinen, A. Valkonen, H. Salo, E. Kolehmainen and E. Sievänen, Soft Matter, 2010, 6, 3789.
7. (a) C. O. Mills, G. H. Martin and S. Elias, Biochim. Biophys. Acta, 1986, 876, 677; (b) P. W. Swaan, K. M. Hillgren, F. C. Szoka and S. Øie, Bioconjugate Chem., 1997, 8, 520; (c) D. G. Rivera, O. Pando, R. Bosch and L. A. Wessjohann, J. Org. Chem., 2008, 73, 6229.
8. (a) S. Marchesan, C. D. Easton, K. E. Styan, L. J. Waddington, F. Kushkaki, L. Goodall, K. M. McLean, J. S. Forsythe and P. G. Hartley, Nanoscale, 2014, 6, 5172; (b) L. Frkanec and M. Žinic, Chem. Commun., 2010, 46, 522.
9. (a) N. M. Sangeetha, S. Bhat, A. R. Choudhury, U. Maitra and P. Terech, J. Phys. Chem. B, 2004, 108, 16056; (b) N. Filipovic-Vincekovic, V. Babic-Ivancic and I. Smit, J. Colloid Interface Sci., 1996, 177, 646; (c) Molecular Gels, Materials with Self-Assembled Fibrillar Networks, ed. R. G. Weiss and P. Terech, Springer, 2006, pp. 751–752.
10. (a) E. Bisselier and D. Astruc, Chem. Soc. Rev., 2009, 38, 1759; (b) L. Vigderman, B. P. Khanal and E. R. Zubarev, Adv. Mater., 2012, 24, 4811; (c) N. Li, P. Zhao and D. Astruc, Angew. Chem., Int. Ed., 2014, 53, 1756; (d) M. Maity and U. Maitra, J. Mater. Chem. A, 2014, 2, 18952.
11. (a) A. S. Lanje, S. J. Sharma and R. B. Pode, J. Chem. Pharm. Res., 2010, 2, 478; (b) D. Das, T. Kar and P. K. Das, Soft Matter, 2012, 8, 2348; (c) L. Dykman and N. Khlebtsov, Chem. Soc. Rev., 2012, 41, 2256; (d) A. Rai, A. Prabhune and C. Perry, J. Mater. Chem., 2010, 20, 6789; (e) A. Shome, S. Dutta, S. Maiti and P. K. Das, Soft Matter, 2011, 7, 3011; (f) S. Eckhardt, P. S. Brunetto, J. Gagnon, M. Priebe, B. Giese and K. M. Fromm, Chem. Rev., 2013, 113, 4708; (g) V. K. Sharma, R. A. Yngard and Y. Lin, Adv. Colloid Interface Sci., 2009, 145, 83; (h) G. J. Hutchings, M. Brust and H. Schmidbaur, Chem. Soc. Rev., 2008, 37, 1759; (i) M. Rai, A. Yadav and A. Gade, Biotechnol. Adv., 2009, 27, 76.
12. (a) B. Adhikari and A. Banerjee, Chem.–Eur. J., 2010, 16, 13698; (b) A. Biswas and A. Banerjee, Soft Matter, 2015, 11, 4226.
13. (a) A. Chakrabarty, U. Maitra and A. D. Das, J. Mater. Chem., 2012, 22, 18268; (b) A. Chakrabarty and U. Maitra, J. Phys. Chem. B, 2013, 117, 8039.
14. (a) M. Annadhasan, V. R. SankarBabu, R. Naresh, K. Umamaheswari and N. Rajendiran, Colloids Surf., B, 2012, 96, 14; (b) S. Roy and A. Banerjee, Soft Matter, 2011, 7, 5300; (c) I. Chakraborty, T. Udayabhaskararao, G. K. Deepesh and T. Pradeep, J. Mater. Chem. B, 2013, 1, 4059.
15. B. Tang, L. Suna, J. Li, M. Zhang and X. Wang, Chem. Eng. J., 2015, 260, 99.
16. C. Gautier and T. Bürgi, ChemPhysChem, 2009, 10, 483.
17. (a) Y. H. Chien, C. C. Huang, S. W. Wang and C. S. Yeh, Green Chem., 2011, 13, 1162; (b) R. N. Mitra and P. K. Das, J. Phys. Chem. C, 2008, 112, 8159; (c) J. S. Shen, Y. L. Chen, J. L. Huang, J. D. Chen, C. Zhao, Y. Q. Zheng, T. Yu, Y. Yang and H. W. Zhang, Soft Matter, 2013, 9, 2017; (d) Y. Khalavka, J. Becker and C. Sönnichsen, J. Am. Chem. Soc., 2009, 131, 1871; (e) Y. Qiao, H. Chen, Y. Lin and J. Huang, Langmuir, 2011, 27, 11090; (f) J. Liu and Y. Lu, J. Am. Chem. Soc., 2003, 125, 6642.
18. Work in progress in collaboration with Dr Jayanta Halder's group at JNCASR, Bangalore.

---

Footnotes

† Electronic supplementary information (ESI) available. CCDC 1028308. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra17917c

‡ LC-lithocholic acid (X=Y=H), DC-deoxycholic acid (X=H, Y=OH), C-cholic acid (X=Y=OH).

---