(Kobs = 800 sÀ1) improvement in its activity (B1.7-fold higher
than hydrogel 3, Fig. 4a). Similarly, 95-fold (Kobs = 625 sÀ1
)
activation was observed for cyt c entrapped within SWNT-4. A
marked improvement in the activity of cyt c immobilized within the
nanocomposites can be attributed to the presence of amphiphobic
SWNTs in amphiphilic networks and increased contact area
between cyt c and substrate in small hydrogel particles.
Hydrophobicity of SWNTs within SAFIN also facilitated mass
transfer efficiently across the interface. Presumably, hydrophobic
purpurogallin rapidly diffused out to bulk toluene from the
hydrophilic domain resulting in higher activation of cyt c
(Fig. 4b). The FESEM image shows entrapped cyt c within an
amphiphilic network of the composite (Fig. 2d). A similar spectral
pattern and position of the soret band (390–420 nm) of cyt c in
water, as well as entrapped in 3 and SWNT-3 hybrid indicated no
structural change in the heme moiety (Fig. S8, ESIw).8
Fig. 3 Plot of storage modulus (G|) and loss modulus (GJ) of 3,
SWNT-3 hybrid gel as a function of angular frequency.
In summary, pristine SWNTs were effectively infused within
a LMW hydrogel without compromising the intrinsic features of
either of the components. Notably, the mechanically reinforced
SWNT–hydrogel composites were found to be a scaffold for
hosting superior biomolecular catalysis in toluene.
We are grateful to DST, India (SR/S1/OC-25/2011) for
financial assistance and CSIR for fellowship to TK and SKM.
Fig. 4 (a) Variation of cyt c activity entrapped in the hydrogel and
SWNT–gel composite in toluene. [Pyrogallol] = 10 mM; [H2O2] =
30 mM; [cyt c] = 5 mg mLÀ1. Experimental errors are within Æ5–10%.
(b) A schematic depiction of the SWNT–hydrogel network as a
scaffold for biomolecular catalysis in toluene.
Notes and references
1 C. Backes, C. D. Schmidt, K. Rosenlehner, J. N. Coleman and
A. Hirsch, Adv. Mater., 2010, 22, 788; J. Lee and T. Aida, Chem.
Commun., 2011, 47, 6757; S. Srinivasan, V. K. Praveen, R. Philip
and A. Ajayaghosh, Angew. Chem., Int. Ed., 2008, 47, 5675;
C. Backes, C. D. Schmidt, F. Hauke, C. Bottcher and A. Hirsch,
J. Am. Chem. Soc., 2009, 131, 2172; Y. Li, B. G. Cousins,
R. V. Ulijn and I. A. Kinloch, Langmuir, 2009, 25, 11760; D. Das
and P. K. Das, Langmuir, 2009, 25, 4421.
2 S. K. Mandal, T. Kar, D. Das and P. K. Das, Chem. Commun.,
2012, 48, 1814; S. Srinivasan, S. S. Babu, V. K. Praveen and
A. Ajayaghosh, Angew. Chem., Int. Ed., 2008, 47, 5746;
S. K. Samanta, A. Pal, S. Bhattacharya and C. N. R. Rao,
J. Mater. Chem., 2010, 20, 6881; A. Kumar, P. K. Vemula,
P. M. Ajayan and G. John, Nat. Mater., 2008, 7, 236; Z. Tan,
S. Ohara, M. Naito and H. Abe, Adv. Mater., 2011, 23, 4053.
3 A. M. Bieser and J. C. Tiller, Supramol. Chem., 2008, 20, 363;
S. R. Jadav, P. K. Vemula, R. Kumar, S. R. Raghavan and G. John,
Angew. Chem., Int. Ed., 2010, 49, 7695; S. R. Jadhav, B. S. Chiou,
D. F. Wood, G. D. -Hoffman, G. M. Glenn and G. John, Soft Matter,
2011, 7, 864; J. C. Tiller, Angew. Chem., Int. Ed., 2003, 42, 3072;
C. Tang, R. V. Ulijn and A. Saiani, Langmuir, 2011, 27, 14438.
4 C. Huang, H. Bai, C. Li and G. Shi, Chem. Commun., 2011,
47, 4962; S. R. Shin, H. Bae, J. M. Cha, J. Y. Mun, Y.-C. Chen,
H. Tekin, H. Shin, S. Farshchi, S. Tang and A. Khademhosseini,
ACS Nano, 2012, 6, 362; A. K. Das, R. Collins and R. V. Ulijn,
Small, 2008, 4, 279.
5 S. Brahmachari, D. Das and P. K. Das, Chem. Commun., 2010,
46, 8386; S. Brahmachari, D. Das, A. Shome and P. K. Das, Angew.
Chem., Int. Ed., 2011, 50, 11243; Z. Sun, D. Rickard, S. D. Bergin,
D. Aherne and J. N. Coleman, J. Phys. Chem. C, 2008, 112, 10692.
6 T. Kar, S. Debnath, D. Das, A. Shome and P. K. Das, Langmuir,
2009, 25, 8639; T. Kar, S. K. Mandal and P. K. Das, Chem. –Eur. J.,
2011, 17, 14952; R. Basu, C. Rosenblat and and R. P. Lemieux,
Liquid Crystals, 2012, 39, 199; X. Lu, C. Sun, F. Li and
H.-M. Cheng, Chem. Phys. Lett., 2008, 454, 305.
7 A. Pal, B. S. Chhikara, A. Govindaraj, S. Bhattacharya and
C. N. R. Rao, J. Mater. Chem., 2008, 18, 2593; S. Bhattacharya,
A. Srivastava and A. Pal, Angew. Chem., Int. Ed., 2006, 45, 2934.
8 L. Dai and A. M. Klibanov, Proc. Natl. Acad. Sci. U. S. A., 1999,
96, 9475; A. M. Klibanov, Nature, 2001, 409, 241; N. Bruns and
J. C. Tiller, Nano Lett., 2005, 5, 45; D. Das, S. Roy, S. Debnath and
P. K. Das, Chem. –Eur. J., 2010, 16, 4911.
densely packed interconnecting fibers of 40–50 nm dimensions
(Fig. S7d, ESIw).
Further we investigated the mechanical properties of the
developed nanohybrids by rheology.2,7 In a typical oscillatory
frequency sweep experiment for hydrogel 3 (at 0.8% w/v) at a
fixed strain (g, 0.01%), the storage modulus (G|) was higher
than the loss modulus (GJ) over the entire angular frequency
(o) range (0.1–200 rad secÀ1, Fig. 3). Interestingly, B4 and 10-fold
increases in G| were observed across the same o range in the
presence of 0.05% and 0.1% w/v SWNT within the hydrogel
of 3 at MGC, denoting the mechanical reinforcement of the
nanocomposite. In both cases, G| and GJ (G| 4 GJ) showed a
plateau region (Fig. 3). The SWNT-3, at two times lower
concentration compared to its MGC (0.4% w/v) and SWNT
(0.05% w/v) showed similar plateau region which is a characteristic
fearure of viscoelastic materials. Both G| and GJ were also higher
compared to that of native hydrogel 3 at 0.8% w/v. Thus, the poor
rigidity of the molecular hydrogel was significantly strengthened
by physical cross-linking of pristine SWNTs with the SAFIN of
the hydrogel.
Development of SWNT–gel hybrids would be further justified if
the nanocomposites offer superior applications with respect to its
native constituents. Previously, amphiphilic networks of molecular
gels have been explored to increase the enzyme activity in organic
solvents.8 Accordingly, the activity of cyt c immobilized in
hydrogels of 3, 4 and composites of SWNT-3, SWNT-4 was
studied following the oxidation of pyrogallol by H2O2 in
toluene. The immobilized cyt c in the hydrogel of 3 and 4
exhibited 70- (Kobs = 455 sÀ1) and 58-fold (Kobs = 375 sÀ1
)
improvement in its activity, respectively, compared to the
activity of cyt c in water (Kobs = 6.5 sÀ1) (Fig. 4a). Interestingly,
cyt c immobilized within SWNT-3 composite exhibited 120-fold
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8389–8391 8391