Curcumin-derived one-and two-component organogelators and their performance as template for the synthesis of silver nanoparticles

2.1 Instruments

All chemicals were purchased from Aldrich Chemicals and were used without further purification. All solvents were purified before use. Fourier transform infrared (FT-IR) spectra were recorded using a Jasco VALOR III Fourier transform infrared spectrophotometer. Nuclear magnetic resonance (NMR) spectra were obtained using a Bruker AMX-400 (Darmstadt, Germany) high-resolution NMR spectrometer, and the chemical shifts were reported in ppm with tetramethylsilane (TMS) as an internal standard. The sample morphologies were characterized using field-emission scanning electron microscopy (FE-SEM, JSM-7001, JEOL, Japan). The xerogel for SEM analysis was prepared by vacuum freeze-drying of the gel formed in the solvent at the critical gelation concentration (CGC) for 12–24 h. The dried samples held on glass substrates were attached to a copper holder for SEM by conductive adhesive tape and were coated with platinum. The X-ray diffraction (XRD) measurements of the xerogel were monitored by Rigaku RINT2000 with a scan rate of 1°/min.

2.2 Gelation test

A pre-calculated weight of gelators (3 mg) and a measured volume of solvent (100 μL) were placed in a glass vial, and the system was heated until all solid materials were dissolved. The solution was then slowly cooled to room temperature, and the test tube was inverted to investigate gel formation. Gel was denoted by G and weak gel was denoted by WG. Systems in which only solution remained throughout the tests were denoted by solution (S). The systems in which the gelator could not be dissolved even at the boiling point of the solvent were considered to be insoluble systems (I). Systems where heating resulted in the dissolution of the gelators and cooling resulted in their precipitation were denoted by P. Unless otherwise mentioned, two-component gels were prepared by mixing 2 mg of BCC, 3 mg of sodium stearate and dissolved in DMF. Thus obtained mixture was heated to dissolve, followed by cooling to form gel. For single gel system, 2 mg of BCC was added to 100 μL of ED, followed by heating and cooling process leads to form gel. These gels were used to study for the effect of external stimuli such pH, UV, metal ion etc.

2.3 Preparation of Gel-Emulsion

Gel-emulsions were prepared according to the reported procedure¹⁷. In a typical preparation, 2 mg of the compound BCC (and 3 mg of SS) was added to a mixture of 0.2 mL of styrene and 0.8 mL of water at room temperature, and the resulting mixture was stirred and slightly heated if necessary to dissolve the compound. Then, the mixture was vigorously shaken by hand for 2 min; finally, a viscous and light yellow emulsion was produced. Formation of the gel-emulsion was confirmed by inverting the test tube to observe whether the mixture inside could still flow. A positive result was obtained when the mixture inside showed no fluidity.

3.1 Gelation properties

The ability of BCC to self-assemble was first tested by the formation of organogels in solvents of different polarity, and the results are summarized in Table 1.

The design of BCC is structurally similar to A(LS)₂ type gelators which consists of cholesteryl (S), benzene (A) and ester groups (L). In the case of A(LS)₂ type molecules, the cholesteryl group at the terminal positions have a profound effect on gelation. Therefore, it was expected that BCC would exhibit gelation in a variety of solvents. However, BCC failed to form gel in any of the organic solvents tested except ethylene diamine (Table 1). Interestingly, BCC was found to form gel in a number of solvents in the presence of the surfactant sodium stearate (SS) in the ratio of 1:1.5 (BCC:SS) and 1:2. If the ratio of BCC:SS increased over 1:3, it leads to precipitation. The compound SS is a well-known hydrogelator (Zhang and Weiss, 2016). The addition of SS to BCC enables gelation in almost all of the solvents in which BCC was dissolved (Table 1). Therefore, the structure property relationships of BCC-ED and BCC-SS-solvents were studied separately and compared.

3.2 BCC-ED gel system

BCC was found to be easily soluble in all amines such as hexyl amine (HA), trioctyl amine (TOA), 1,3-diaminopropane (DAPr), 1,5-diamino pentane (DAP) and diethyl amine (DEA) except ED. However, BCC was found to be soluble in ED upon heating. And, gelation was observed while cooling to room temperature. The critical gelation concentration of BCC-ED was found to be 20 gL^-1. Repeated heating and cooling cycles does not affect the sol-to-gel process. Moreover, BCC failed to form gel in other amines due to the high solubility of BCC at room temperature. This indicates that BCC can act as a sensor for the selective detection of ED by selective gelation. The addition of amines to BCC leads to colour change from brilliant yellow to reddish brown (Fig. 1). This colour change is due to the keto-enol tautomerism of the curcumin unit in BCC. In acidic solution (pH < 7), BCC exist in diketone form whereas in basic solution (pH > 7) it exists in enolate form (Fig. S3) (Supharoek et al., 2018).

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Fig. 1

The digital images of gel-sol process of BCC-ED gel.

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To gain visual insights regarding the morphology of molecular aggregation modes, BCC-ED gel was subjected to SEM analysis, and the results are depicted in Fig. 2.

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Fig. 2

Representative SEM images of xerogel of BCC-ED.

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As observed in SEM images of the xerogels, BCC-ED existed as self-assembled fibre-like aggregates with a length of several micrometres and diameter of approximately 100–150 nm. The self-assembly of uniform sized nanoparticles and fibrous textures can be observed for BCC-ED at CGC (Fig. 2a). However, when increasing the BCC concentration (above CGC), mostly entangled fibrous textures were observed (Fig. 2b). These morphological images revealed that the nanoparticles are interconnected to form fibrous structure at low concentration, which further entangled to form interconnected fibrous structures at higher concentration. The gel-sol transition temperatures (T_gel) of the gelators were determined and the concentration dependence T_gel values showed that T_gel increases with increase in the concentration of gelators (Fig. S4 and Table S1) (Haldar and Karmakar, 2015).

Generally, in LMWGs gel system, temperature-dependent ¹H NMR will be recorded in the solvent in which gelators form gel in order to determine the driving forces for the aggregation phenomena as well as the thermo-reversibility of the gels. Since the gelling solvent (ED) could not be used as NMR solvent in this case, solid state NMR was performed for BCC-ED at various temperatures and the results are merged in Fig. 3.

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Fig. 3

Temperature-dependent SSNMR spectra of BCC-ED gel.

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From the spectra, it was observed that the peak at 195 ppm corresponding to carbonyl group (—C⚌O; carbon “a” in the figure) of the curcumin unit is broad at 303 K. However, with increasing temperature from 303 K to 323 K, the peak was found to be sharp and sharpness increased when the temperature was further increased to 333 K. This indicates that BCC-ED remained in gel state at low temperature (303 K) and converted to solution state when increasing temperature to 333 K. This revealed the occurrence of a gel-to-sol phase transition with increase in temperature. It may be due to the breaking of secondary forces, which in this case is the hydrogen bonding between amine of ethylene diamine and diketo group of curcumin unit. Similar sharpness of the peaks was observed along with slight shifting of peaks in almost all of the peaks with increase in temperature. For example, the peak at 165.3 ppm corresponding to —O—C(⚌O)—O—Ch* was shifted to 164.6 when increasing the temperature from 303 K to 333 K, which indicated that carbonyl (in ester) group is involved in gel formation. Similarly, the peaks at 142.6 ppm corresponding to —C⚌CH— in curcumin unit, 134.7 ppm corresponding to ArC—O—C⚌O, 113.1 ppm corresponding to ArC and 56.7 ppm corresponding to methoxy carbon were shifted to 142.2, 134.4, 113.5 and 58.1 ppm, respectively suggesting that these functional groups are involved in the self- assembly of BCC in the presence of ethylene diamine. Moreover, the peak at 75.1 ppm corresponding to cholesteryl carbon attached to oxygen shifted to 74.6 ppm and then to 73.6 ppm while increasing the temperature to 323 K and 333 K respectively. These results indicated that the cholesteryl unit is involved in gel formation. Based on these evidences, it is understood that the aromatic carbons, cholesteryl carbons and hydrogen bonding between curcumin diketo groups with ethylene diamine are responsible for the gelation properties of BCC. Furthermore, it suggested that the cholesteryl-cholesteryl interaction (van der Waals forces), π-π stacking as well as hydrogen bonding between the amino groups of ED with the carbonyl group of curcumin unit played an important role for the formation of gel. Instead of diketo group of curcumin, there is another possibility of hydrogen bonding between carbonyl group (ester; linker) and amine. However, the structure and length of the molecule by XRD analysis and simulation studies matches with the structure which undergo hydrogen bonding through diketo group of curcumin. Based on these results, the mechanism of gel formation of BCC with ethylene diamine was proposed and the schematic representation of the proposed mechanism was shown in Fig. 4.

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Fig. 4

Schematic representations of aggregation of BCC in ED.

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To gain visual insights into the molecular packing, the xerogel of BCC-ED was subjected to XRD analysis, and the results are depicted in Fig. 5. According to the Bragg lattice diffraction formula, nλ = 2d sinθ, where λ is the wavelength and n = 1. XRD was operated with a copper excitation wavelength of 0.154 nm to determine the incident angle θ and the layer spacing d. Using the data, the formula produced an obvious peak for BCC at 2θ = 1.81 and d = 4.88 nm. BCC-ED exhibited peak at 2θ = 2.16, and d = 4.08 nm. These interlayer distances were found to coincide with the results obtained from the molecular simulation software (Fig. 6).

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Fig. 5

XRD diffraction patterns of (a) BCC and (b) BCC-ED.

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Fig. 6

The simulated molecular models of BCC and BCC-ED.

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3.3 BCC-SS gel system in different solvent

The compound BCC in the presence of SS formed gel in polar solvents such as DMF, DMSO, m-cresol and low polar solvents such as xylene, toluene, dichlorobenzene, THF etc. (Table 1). Amines such as ethylene diamine, hexyl amine, 1,3-diaminopropane, 1,5-diaminopentane and trioctyl amine also form gel with BCC-SS. Unlike BCC, BCC-SS can form gel in all the amines studied, indicating that BCC-SS cannot act as a sensor for ED. The gel colour varied from pale yellow to dark yellow and reddish brown or brown depending on the solvents. The colour of the gels had no apparent change during repeated heating and cooling cycles (Fig. 7). Unlike other amines, trioctyl amine shows dark yellow colour, whereas all other amines exhibited dark reddish brown. All the gels were found to be semi-transparent except THF and trioctyl amine based gels, which were opaque. All the gels exhibited excellent thermo-reversibility, which means that the compounds could be brought into solution by heating and the hot solution could be converted back to the gels upon cooling to room temperature. This cycle could be repeated several times.

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Fig. 7

Representative digital images of gels of BCC-SS in (1) hexylamine; (2) 1,5-diamino pentane; (3) 1,3-diamino propane; (4) m-cresol, (5) Trioctyl amine; (6) Toluene, (7) Chlorobenzene; (8) xylene; (9) THF; (10) Acetone and (11) chloroform.

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From the Table 1, it was observed that BCC-SS did not form gel in alcohols, oleylamine and in monomers, such as tertiary butyl methacrylate (t-BMA) and methyl methacrylate (MMA). In addition to that, BCC-SS form gel emulsion in water and styrene mixture after heating and cooling process; whereas BCC forms precipitate in styrene. To investigate the effect of the water content on the phase behaviour of the gel-emulsions, BCC-SS/water/styrene with different water contents, including 0, 20, 40, 60, 70, 80, 90 and 100% (v/v) was studied. The systems with >50% water did not form gel, whereas all other proportions of < 50% water formed gels. In addition to that, styrene alone (100% styrene) did not form gel in the absence of water (Fig. S5). This could be due to the insoluble nature of BCC in water.

3.4 Morphological studies

To gain visual insights regarding the morphologies of the molecular aggregation modes, all the gelators were subjected to SEM analysis, and the representative images are depicted in Fig. 8 and Fig. S6. As observed in the SEM images of the xerogels, the BCC-SS gels in various solvents exhibited self-assembled, one-dimensional, fibre-like aggregates with length of several micrometres and diameter of approximately 100–200 nm. These xerogels undergo further entanglement to form bundles of fibrous aggregates. The nanostructures of the xerogels in different solvents are significantly different, and the morphology of the aggregates vary from fibre and rod to wrinkled texture with the change of solvents. This morphology is structurally similar to our previously reported morphology for bis-cholesteryl appended isosorbide based gelator systems (Balamurugan et al., 2016). It may be due to the fact that both are structurally A(LS)₂ type, in which only A is different.

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Fig. 8

Representative SEM images of BCC-SS in (a) Xylene; (b) 1,2-dichlorobenzene; (c) Chloroform; (d) 1,5-diaminopentane; (e) BCC + Styrene/H₂O(1:1); (f) sodium thiocyanate (g) SS in dilute water (h) BCC-SS-styrene xerogel.

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For example, the xerogel of BCC in xylene and 1,2-dichlorobenzene exhibited long range fibrous structures whereas chloroform and 1,5-diamino pentane exhibited short range dense fibrous structures (Fig. 8a-d). The structure of xerogel of BCC in DMF resembles the structure of xylene (Fig. S6c). In the case of 1,2-dichlorobenzene gel system, the self-assembly of uniform spherical spheres connected to each other in a linear fashion and formed a fibrous structure. The arrangement of spherical spheres can be seen clearly in Fig. 8b. The size of each sphere is roughly measured as 20 nm, and therefore nearly 4 to 6 nanoparticle join in a linear way forming 100 nm length fibre as observed in Fig. 8b (Fig. S8-d). Fig. 8e and 8 h corresponds to the gelation ability of BCC and BCC-SS in styrene/water (1:1; v/v), respectively. BCC fail to form gel or gel-emulsion in styrene/water (1:1; v/v), and precipitation was observed. This was further supported by morphological analysis of the same by SEM, which exhibited uniform size spherical nano-spheres as shown in Fig. 8e. However, in the presence of SS, BCC can form gel-emulsion in styrene/water (1:1; v/v), and resulted in randomly arranged short-range dense fibrous structure as seen in SEM images (Fig. 8h). Hence, increasing water content more than 50% in styrene/water mixture leads to turbidity, which may be due to the non-aqueous solubility of BCC.

In order to determine the efficiency of sodium stearate, it was replaced by other sources of sodium as well as stearate and the gelation abilities with BCC was analysed. Replacing sodium stearate by other stearates such as stearic acid and iron stearate leads to precipitate formation. Similarly replacing sodium stearate by other sodium sources such as sodium oxalate, sodium thiocyanate, sodium lauryl sulphate (SLS) and sodium acetate were performed in DMF. Among these, sodium thiocyanate forms gel (Fig. 8f and Fig. S7) whereas others failed to form gel. However, sodium thiocyanate did not form gel in the absence of BCC, which indicates that in the BCC-SS gel system in DMF, the BCC is not just bound by SS. The BCC and SS cooperatively combined and formed gel.

In addition, the ratio of SS also affects the aggregation mode of BCC-SS gel system. The gel system containing BCC-SS (1:2) in xylene exhibited fibrous structure (Fig. 8a). However, when increasing the concentration of SS in BCC-SS (1:3 wt%) in xylene exhibited sheet like structure (Fig. S6e).

Heating of gel samples is known to result in an increase of gelator ¹H NMR signals due to disaggregation of the gel network into smaller, dissolvable aggregates observable by NMR (Džolić et al., 2006). To study the formation and the thermos-reversibility of the gels, the temperature-dependency of the ¹H NMR spectra was investigated for BCC-SS (1:1.5; wt %) in DMSO‑d₆ and the results are presented in Fig. S8. As shown in the spectra, when increasing the temperature from 303 K to 363 K, a significant up field shifting occurred for the proton signals at 7.3 ppm, corresponding to aromatic protons (curcumin unit in BCC), which revealed that aromatic protons are involved in aggregation.

Generally, in two component system, H-bonding plays an important role in gel formation (Willemen et al., 2002; Hirst and Smith, 2004). However, H-bonding is not commonly seen in the case of sodium stearate (SS). SS [CH₃(CH₂)₁₆COONa] is an ionic compound and its organic part contains polar groups. Therefore, dipole-dipole, ion-dipole interactions as well as van der Waals forces are the possible/available driving forces for gel formation. So it is assumed that there is additional possibility of ion-pole interactions between sodium (SS) and carbonyl group (—C⚌O) of the curcumin unit. Therefore, the existence of π-π interaction as well as ion-dipole interaction might be the base for gel formation at room temperature (303 K). However, the exact mechanism of gel formation is unclear since the non-covalent interactions between molecules are subtle and sometimes hard to understand (Raeburn and Adams, 2015). When increasing the temperature from 303 K to 363 K, the gel → sol transition occurred, which may have been due to the breaking of ion-dipole interaction, which in turn disrupted the gel state. Increasing the temperature caused disruptions in the supramolecular arrangement and decreased the effects of the secondary forces (the disruption of intermolecular forces between the two molecules).

3.5 Effect of external stimuli towards the stability of the gels

The stability of the gels over external stimuli such as pH (acid-base reaction), sonication and UV irradiation were studied. BCC/BCC-SS failed to form gel in any solvents in the presence of sonication (without heating) as well as UV irradiation. Similarly, the gel formed by heating and cooling process was not affected by sonication/UV irradiation; instead, the gel became stronger after sonication than freshly prepared gels. This result indicated that the phase transitions could not be tuned by sonication. However, it improved the stability of the gel to some extent. The addition of acid (acetic acid) to the BCC-ED or BCC-SS gel in DMF turned the colour of the gel to yellow with precipitation. However, addition of base (TEA) resulted in colour change to brown, but failed to form gel. These results indicate that gel-to-sol can be tuned by acid; hence gel cannot be retained by addition of base.

Since curcumin forms complexes via its β-diketone moiety with almost all the metal ions (Wanninger et al., 2015; Călinescu et al., 2019), the influence of the metal ions for example Cu²⁺, Fe³⁺, Hg²⁺, Pd²⁺, Ni²⁺ and Zn²⁺ towards the stability of BCC-ED/BCC-SS (in DMF) gels were tested. With progressive addition of approximately 10 equivalents of these cations to the gels resulted in precipitation after heating and cooling process. This may be due to the selective complexation of these metal ions with the diketone group of BCC, separating/breaking the forces between BCC and ED or BCC and SS, which leads to precipitation (Fig. S9).

3.6 Potential application of BCC based gelators as a template for silver nanoparticles (AgNPs) synthesis

In general, the organo/hydro gelators can be used as a template or support for the synthesis of nanoparticles such as silver nanoparticle (AgNP), gold nanoparticles (AuNPs) and iron nanoparticles (FeNPs). Since the gelator possess 3D-network structure, a large number of functional groups can provide space for the nucleation and growth of the noble metal nanoparticles (Wu et al., 2015; Zhu et al., 2019). Therefore, we attempted to use our gelators as template/structure directing agent for the synthesis of AgNPs.

Here, we used three different gel system as templates for the synthesis of AgNPs as follows: (1) BCC-ED gel; (ii) BCC-SS-DMF gel and (iii) SS-DMF gel. Once the above gels were prepared, an equal amount of AgNO₃ was added directly into the gel in each case and the resulting mixture was heated again to 80 °C for 20–30 min and cooled to room temperature. In all cases, gel state was retained. However, the colour of the gels turned to dark brown/ brownish black along with formation of silver mirror (Zaier et al., 2017) as shown in Fig. 9.

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Fig. 9

Digital images of gels obtained from (a) BCC-SS gel; (b) BCC-ED + AgNO₃ and (c) SS-DMF-AgNO_3.

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From the Fig. 9, the deposition of Ag can be seen at the bottom of the vials due to the presence of sodium metal (Na⁺) which undergo reduction reaction with AgNO₃ (Fig. 9). However, the stability of the gel was not affected by metal ions (Fig. S10). The appearance of silver formation can be seen in the vial by naked eye (Fig. 9). The resulting gels were diluted by adding excess DMF and centrifuged. The DMF was decanted and the deposits were washed by using additional acetone and centrifuged. The acetone was decanted and the resulting powder was allowed to dry at vacuum oven at room temperature. AgNPs can be separated from the template by dissolving template in chloroform/dichloromethane. The samples were subjected to XRD analysis and the results are merged in Fig. 10.

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Fig. 10

The XRD patterns of synthesized AgNPs via different template.

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The XRD patterns for the synthesized AgNPs by different templates are merged in Fig. 10. When comparing the XRD pattern of BCC-ED, the templates containing SS such as BCC-SS-DMF gel as well as SS-DMF gel system exhibited peaks at the 2θ value of <35°. These peaks are attributed to the formation of silver stearate as shown in Fig. 11. The formation of silver stearate was further verified by the morphological analysis by SEM, optical microscope and FT-IR analysis. The FT-IR spectra of AgNPs synthesized by different templates are shown in Fig. S11 (see supporting information). From the spectra, it was observed that the peaks around 1515 and 1415 cm⁻¹ corresponding to υ_as (COO) and υ_s(COO) vibration peaks for the ester present in silver stearate (Mao et al., 2019). The morphological analysis of silver stearate by SEM exhibited layered structures as shown in Fig. 12, which resembles other reported structure (Lee et al., 2003). The silver stearate exhibited platelet crystals of layer textures under optical microscope (Fig. S12), which is in accordance with the reported textures (Lin et al., 2004). AgNPs can be seen clearly from the top view of the sample BCC-ED-Ag placed over SEM substrate/carbon tape (Fig. S12d) The synthesis of AgNPs from silver stearate was reported previously (Mao et al., 2019), in which silver stearate was subjected to sintering at 523k leading to partial decomposition of silver stearate, producing silver stearate coated AgNPs. We can apply the same protocol for the production of AgNPs, if necessary. This means that, silver stearate, which is obtained from gels containing sodium stearate should undergo high temperature sintering and AgNPs thus obtained will be coated with silver stearate (Mao et al., 2019). Therefore, obtaining pure AgNPs is a tedious process and novel methods/reagents are in demand for the efficient production of pure NPs.

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Fig. 11

The formation of silver stearate.

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Fig. 12

SEM images of silver stearate.

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Interestingly, without addition of sodium stearate and without forming silver stearate, AgNPs were successfully synthesized by using BCC-ED gel system, especially at very low temperature (80 °C). The XRD pattern of thus obtained AgNPs exhibited peaks at 2θ values of 38°, 44°, 64° and 77°, respectively corresponding to (1 1 1), (2 0 0), 220 and (3 1 1) planes (Fig. 10). This XRD pattern is in accordance with the structure of Ag(0) reported in literature (Mu et al., 2013; Anandalakshmi et al., 2016; Mao et al., 2019). However, the gel system without ED (BCC-SS-DMF gel) and without BCC (SS-DMF gel) failed to form well resolved XRD pattern of Ag(0). These results indicated the importance of BCC as well as ED for the successful formation of AgNPs.

The morphological analysis of thus obtained AgNPs exhibited spherical nano-spheres with the size of 60–100 nm (Fig. 13). The morphology of AgNPs were found to be in accordance with the reported literature (Mu et al., 2013; Anandalakshmi et al., 2016). These results suggested that the template based on BCC and ED (BCC-ED gel) played an important role in the formation of AgNPs. The silver nitrate appears to be reduced by BCC-ED gel resulting in AgNPs without adding any other reagents, indicating that BCC-ED act as reducing agent as well as capping/stabilizing agent for the synthesis of AgNPs. It is acceptable since, curcumin was reported as reducing agent (Tc̵nnesen and Greenhill, 1992; Safavi et al., 2017) for the reduction of Fe³⁺ to Fe²⁺(Tc̵nnesen and Greenhill, 1992) as well as for AgNO₃ (Yang et al., 2016). The presence of the diketo-moiety in BCC structure seems to be essential both in redox reactions and in the scavenging of oxygen radicals (Tc̵nnesen and Greenhill, 1992).

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Fig. 13

The representative SEM images of AgNPs obtained from BCC-ED gel (different magnifications).

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Although BCC-SS-DMF gel contains the reducing agent BCC, the reduction of AgNO₃ by sodium from SS takes place predominantly over the reduction by curcumin unit of BCC. Therefore, BCC-SS-DMF gel failed to produce AgNPs at this stage. In order to produce AgNPs, it should undergo further reaction such as sintering at high temperature and purification steps are needed. These results revealed that BCC produce AgNPs in the presence of ED. In addition, the reduction reaction by BCC-ED used very low temperature (80 °C) and the template can be effortlessly removed by non-gelling solvent such as dichloromethane or chloroform and can be reused. These results revealed that BCC-ED acts as the best reducing agent/template for the synthesis of AgNPs from AgNO₃. Hence, BCC-ED gel system is a highly potential platform for the efficient synthesis of AgNPs.