Molecular recognition at the liquid-liquid interface of colloidal microcapsules

Article abstract of DOI:10.1039/b906458c

Dithiocarbamate chemistry is used as a crosslinking tool to fabricate FePt colloidal microcapsules which provide a versatile scaffold for "host-guest" recognition at the liquid-liquid interface.

Full text of DOI:10.1039/b906458c

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Molecular recognition at the liquid–liquid interface of colloidal
microcapsulesw
Debabrata Patra,a Chiara Pagliuca,a Chandramouleeswaran Subramani,a
Bappaditya Samanta,a Sarit S. Agasti,a Nada Zainalabdeen,b Stuart T. Caldwell,b
Graeme Cooke*b and Vincent M. Rotello*a
Received (in Austin, TX, USA) 1st April 2009, Accepted 22nd May 2009
First published as an Advance Article on the web 9th June 2009
DOI: 10.1039/b906458c
Dithiocarbamate chemistry is used as a crosslinking tool to
fabricate FePt colloidal microcapsules which provide a versatile
scaffold for ‘‘host–guest’’ recognition at the liquid–liquid interface.
Initially oleic acid and oleyl amine coated FePt NPs
(B7 nm) were synthesized following a reported procedure.12
Functionalization of the NPs was accomplished by place
exchange reaction with HS–C11–TEG–OH and dopamine–
C8–TEI ligands, respectively (Scheme 1, see also ESIw). The
modified NPs were dissolved in milliQ water and pH of
the solution was adjusted to B9 by adding 0.1 M NaOH
solution.
Molecular recognition mediated by hydrogen bonding serves as
an underlying mechanism to understand biological functions
and provides a powerful toolkit to design novel functional
materials.1 Unlike biological systems, the recognition between
the ‘‘host–guest’’ pair is mostly effective in nonaqueous media
due to the high dielectric nature of water.2 To enhance the
effectiveness of hydrogen bonding in water, recent efforts have
been made to create hydrophobic environment of different
dimensions in the vicinity of the ‘‘host–guest’’ functional pair.3
Oil–water emulsions offer a unique microenvironment4
where the larger interfacial area of emulsions can act as a
suitable platform to localize the ‘‘host–guest’’ pair. To encap-
sulate the ‘‘host’’ molecule for further recognition at the inter-
face, a stable emulsion is highly desired. Microcapsules such as
polymeric microcapsules,5 vesicles,6 colloidal microcapsules7
can act as a suitable candidate due to their structural stability.
Amongst these, colloidal microcapsules feature great advan-
tages due to their easy fabrication, enhanced mechanical
stability and controlled permeability.8
Herein, we introduce an approach to fabricate stable
colloidal microcapsules that are compatible with most host–
guest systems. In this strategy, orthogonally functionalized
FePt nanoparticles (NPs) are self-assembled at the oil–water
interface and crosslinked via dithiocarbamate (DTC) chemistry.9
Host–guest chemistry was demonstrated by encapsulation of
the flavin polymer10 (‘‘host’’) inside the microcapsules to
obtain three-point hydrogen bonding interaction at the inter-
face with a complementary diaminopyridine (DAP, ‘‘guest’’)
amphiphile. Host–guest interactions at the interface were
then monitored by fluorescence quenching of the flavin
fluorophore.11
For our studies we used triethylene imine (TEI) func-
tionalized FePt NPs to fabricate colloidal microcapsules.
a Department of Chemistry, University of Massachusetts Amherst,
710 North Pleasant Street, Amherst, MA 01003, USA.
E-mail: rotello@chem.umass.edu; Fax: +1 413-5452058
b Glasgow Centre for Physical Organic Chemistry, WestCHEM,
Department of Chemistry, University of Glasgow, Joseph Black
Building, Glasgow, UK G12 8QQ
w Electronic supplementary information (ESI) available: Synthesis of
(i) TEI ligand, (ii) TEI functionalized FePt NPs, (iii) DAP amphiphile
and (iv) colloidal MCs. See DOI: 10.1039/b906458c
Scheme 1 (a) Orthogonal functionalization of FePt NPs. (b) Mecha-
nism of cross-linking of FePt NPs via DTC chemistry to fabricate
microcapsule. (c) Flavin encapsulated microcapsule and ‘‘host–guest’’
recognition at oil–water interface. (d) Three-point hydrogen bonding
interaction between flavin and DAP.
ꢀc
This journal is The Royal Society of Chemistry 2009
4248 | Chem. Commun., 2009, 4248–4250

View Article Online
In the next step, 250 mL of the NPs solution were transferred
to an Eppendorf tube and 10 mL of 0.5 M CS2 in 1,2,4-
trichlorobenzene (TCB) were added to it. Vigorous shaking
for 10 s gave stable emulsions and emulsions were kept for
30 min prior to analysis. Excess NPs were washed two times by
milliQ water and emulsions were visualized by an optical
microscope (OM) (Fig. 1a, see ESIw for size distribution).
This assembly strategy was further investigated by using
different nonpolar solvents such as dichloromethane and
hexane. Low boiling organic solvents resulted in the formation
of crumbled microcapsules due to rapid evaporation of the
solvent. (e.g. dichloromethane, inset of Fig. 1a). Micro-
capsules were drop casted on carbon coated copper grid
for transmission electron microscope (TEM) analysis. Low-
magnification image of the dried capsules shows (Fig. 1b) thin
film like texture, while the high-magnification image (inset of
Fig. 1b) reveals that the capsule is composed of densely packed
crosslinked FePt NPs.
Fig. 2 Fluorescence microscopy images of (a) flavin encapsulated
microcapsule (b) after addition of excess DAP. Similarly, fluores-
cence images of (c) N-methyl flavin encapsulated micro capsule
(d) after addition of excess DAP. All scale bars represent 40 mm of
length.
Successful encapsulation of the host flavin polymer inside
the microcapsules during their formation was confirmed by
fluorescence of the microcapsules in aqueous medium
(Fig. 2a). To study the recognition process inside the micro-
capsule, an excess of the guest DAP amphiphile was added to
the aqueous solution. Concomitant quenching of fluorescence
(Fig. 2b) from the microcapsule was observed due to three-
point hydrogen bonding interaction between ‘‘host–guest’’ at
the oil–water interface. Semiquantitative assessment of the
binding constant between flavin and complementary DAP
was obtained by monitoring the fluorescence intensity of a
single microcapsule. Fluorescence titration of flavin in the
presence of increasing DAP equivalents yielded a Ka E 250 MÀ1
(Fig. 3) which is comparable to their solution study. Titration
of excess DAP amphiphile into microcapsules containing the
fluorescent N(3)-methylated polymer (Fig. 2c) failed to display
significant quenching (Fig. 2d and 3), suggesting that three-
point hydrogen bonding interactions were responsible for
fluorescence quenching.
The fact that DAP–flavin recognition quenched fluores-
cence prevented direct determination of the localization of
the host–guest complexes within the microsphere. Localization
can be inferred, however, from the experiments performed
using a planar interface. In this experiment the fluorescence of
flavin polymer was recorded in TCB. In the next step, an
aqueous solution of excess DAP was added to the organic
phase. No fluorescence quenching was observed after agita-
tion, indicating that DAP and flavin polymers are soluble only
in water and oil, respectively (for details see ESIw Fig. S1),
and recognition can occur only at the interface, i.e. in the
proximity of the particle shell.
Fig. 1 OM micrographs of (a) FePt microcapsules at oil-in-water
interface using DTC crosslinking strategy. Inset shows crumbled micro-
capsules. (b) Low-magnification TEM images of dried microcapsules
showing film like texture. Inset represents higher-magnification TEM
image.
Fig. 3 Binding plot shows dramatic quenching of flavin polymer
fluorescence upon addition of DAP (Ka E 250 MÀ1) whereas control
N-methylated flavin does not show any fluorescence quenching.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4248–4250 | 4249

View Article Online
In summary, we have developed a simple and mild way to
fabricate stable colloidal microcapsules at the oil–water inter-
face based on orthogonal functionalization of FePt NPs
followed by DTC mediated crosslinking. Furthermore, we
have shown using fluorescence experiments that these capsules
provide a suitable macroscopic platform to confine host–guest
pairs at their liquid–liquid interface.
Angew. Chem., Int. Ed., 2007, 46, 7378; (c) P. Broz, S. Driamov,
J. Ziegler, N. Ben-Haim, S. Marsch, W. Meier and P. Hunziker,
Nano Lett., 2006, 6, 2349.
6 (a) J. Hotz and W. Meier, Langmuir, 1998, 14, 1031;
(b) R. Stoenescua and W. Meier, Chem. Commun., 2002, 3016.
7 (a) P. Pieranski, Phys. Rev. Lett., 1980, 45, 569; (b) D. Rautaray,
A. Banpurkar, S. R. Sainkar, A. V. Limaye, S. Ogale and
M. Sastry, Cryst. Growth Des., 2003, 3, 449; (c) D. Wang,
H. Duan and H. Mohwald, Soft Matter, 2005, 1, 412;
This research was supported by the NSF: (CHE-0808945,
VR), MRSEC facilities, and the Center for Hierarchical
Manufacturing (DMI-0531171).
(d) A. Boker, J. He, T. Emrick and T. P. Russell, Soft Matter,
¨
2007, 3, 1231; (e) H. Skaff, Y. Lin, R. Tangirala, K. Breitenkamp,
A. Boker, T. P. Russell and T. Emrick, Adv. Mater., 2005, 17, 2082;
¨
(f) P. Arumugam, D. Patra, B. Samanta, S. S. Agasti,
C. Subramani and V. M. Rotello, J. Am. Chem. Soc., 2008, 130,
10046; (g) E. Glogowski, R. Tangirala, J. He, T. P. Russell and
T. Emrick, Nano Lett., 2007, 7, 389; (h) H. Duan, D. Wang,
N. S. Sobal, M. Giersig, D. G. Kurth and H. Mohwald, Nano
Lett., 2005, 5, 949; (i) B. Samanta, D. Patra, C. Subramani, Y. Ofir,
G. Yesilbag, A. Sanyal and V. M. Rotello, Small, 2009, 5, 685.
8 B. Wang, M. Wang, H. Zhang, N. S. Sobal, W. Tong, C. Gao,
Y. Wang, M. Giersig, D. Wang and H. Mohwald, Phys. Chem.
Chem. Phys., 2007, 9, 6313.
9 (a) J. Sharma, R. Chhabra, H. Yan and Y. Liu, Chem. Commun.,
2008, 2140; (b) F. Dubois, B. Mahler, B. Dubertret, E. Doris and
Charles Mioskowski, J. Am. Chem. Soc., 2007, 129, 482;
(c) M.-H. Park, Y. Ofir, B. Samanta, P. Arumugam,
O. R. Miranda and V. M. Rotello, Adv. Mater., 2008, 20, 4185.
10 J. B. Carroll, B. J. Jordan, H. Xu, B. Erdogan, L. Lee, L. Cheng,
C. Tiernan, G. Cooke and Vincent M. Rotello, Org. Lett., 2005, 7,
2551.
11 S. T. Caldwell, G. Cooke, S. G. Hewage, S. Mabruk, G. Rabani,
V. M. Rotello, B. O. Smith, C. Subramani and P. Woisel, Chem.
Commun., 2008, 4126.
12 (a) S. Srivastava, B. Samanta, B. J. Jordan, R. Hong, Q. Xiao,
M. T. Tuominen and V. M. Rotello, J. Am. Chem. Soc., 2007, 129,
11776; (b) Y. Ofir, B. Samanta, P. Arumugam and V. M. Rotello,
Adv. Mater., 2007, 19, 4075; (c) M. Chen, J. P. Liu and S. Sun,
J. Am. Chem. Soc., 2004, 126, 8394.
Notes and references
1 (a) D. J. Cram, Science, 1988, 240, 760; (b) D. J. Cram, Angew.
Chem., Int. Ed. Engl., 1988, 27, 1009; (c) M. M. Conn and
J. Rebek, Jr., Chem. Rev., 1997, 5, 1647.
2 E. Fan, S. A. Van Arman, S. Kincaid and A. D. Hamilton, J. Am.
Chem. Soc., 1993, 115, 369.
3 (a) V. M. Rotello, E. A. Viani, G. Deslongchamps, B. A. Murray and
J. Rebek Jr., J. Am. Chem. Soc., 1993, 115, 797; (b) R. P. Bonar-Law,
J. Am. Chem. Soc., 1995, 117, 12397; (c) K. Ariga and T. Kunitake,
Acc. Chem. Res., 1998, 31, 378; (d) D. Y. Sasaki, K. Kurihara and
T. Kunitake, J. Am. Chem. Soc., 1991, 113, 9685; (e) Y. Ikeura,
K. Kurihara and T. Kunitake, J. Am. Chem. Soc., 1991, 113, 7342;
(f) X. Cha, K. Ariga, M. Onda and T. Kunitake, J. Am. Chem. Soc.,
1995, 117, 11833; (g) Y. Oishi, T. Kato, M. Kuramori, K. Suehiro,
K. Ariga, A. Kamino, H. Koyano and T. Kunitake, Chem.
Commun., 1997, 1357; (h) J. S. Nowick, T. Cao and G. Noronha,
J. Am. Chem. Soc., 1994, 116, 3285; (i) Y. Aoyama, Y. Tanaka and
S. Sugahara, J. Am. Chem. Soc., 1989, 111, 5397.
4 S. Ishizaka, S. Kinoshita, Y. Nishijima and N. Kitamura, Anal.
Chem., 2003, 75, 6035.
5 (a) J. A. Opsteen, R. P. Brinkhuis, R. L. M. Teeuwen, D. W. P.
M. Lowik and J. C. M. van Hest, Chem. Commun., 2007, 3136;
¨
(b) D. M. Vriezema, P. M. L. Garcia, N. S. Oltra, N. S. Hatzakis,
S. M. Kuiper, R. M. Nolte, A. E. Rowan and J. C. M. van Hest,
ꢀc
This journal is The Royal Society of Chemistry 2009
4250 | Chem. Commun., 2009, 4248–4250