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Supramolecular-directed synthesis of RNA-mediated CdS/ZnS
nanotubesw
Anil Kumar*ab and Vinit Kumara
Received (in Cambridge, UK) 14th April 2009, Accepted 7th July 2009
First published as an Advance Article on the web 3rd August 2009
DOI: 10.1039/b907283g
Supramolecular interactions of colloidal CdS/ZnS with RNA in
the presence of excess Zn2+ offer a convenient means for
the production of quantum confined semiconducting tubular
nanostructures in an aqueous medium.
Proteins, DNA and RNA like polymeric biomolecules, have
diverse functionalities and highly specific inter- and intra-
molecular interactions, provide powerful tools to synthesize
tunable self-assembled nanomaterials.1–3 These molecules
have well-defined structures and a high water solubility,
making them important templates for the fabrication of new
organized materials for use in biology and medicine.4 In this
context, biopolymer-templated colloidal semiconductors have
drawn considerable attention for the fabrication of tailored
nanostructures and nanodevices.2,4c,5 Several attempts have
recently focused on developing biomolecule-templated tubular
nanostructures,6 which are expected to find important applications
in nanoelectronics, fluorescence imaging, biolabeling, biosensing
and as a material carrier for drug delivery4a,b because of their
unique optical and electronic properties.4c
Among biopolymers, RNA is least explored, but has distinct
advantages over other biopolymers in terms of its structure
and properties.2,7,8 In contrast to DNA, RNA generally has a
single-stranded structure, and is more prone to fold and
promote the formation of self-assembly in the presence of
bivalent metal ions through intermolecular interactions.9 This
aspect is particularly interesting to nanotechnologists for
producing self-assembled materials from zero-dimensional
to complex three-dimensional structures. The present work
reports for the first time a novel method to synthesize
RNA-templated CdS/ZnS nanotubes at about 15 1C in an
aqueous medium.
The absorption and emission spectra of mixed CdS/ZnS
colloids in an RNA matrix under optimised experimental
conditions ([RNA] = 0.015 g/100 ml, pH = 9.2,
[Cd2+] = 2 � 10�4 mol dm�3, [Zn2+] = 2 � 10�4 mol dm�3,
[HS�] = 2.5 � 10�4 mol dm�3, [Zn2+] = 7 � 10�4 mol dm�3
(added after the preparation of colloids), temperatureB15 1C)
are presented in the ESIw (Fig. S1). Details of the optimisation
are described in the ESIw (Fig. S1a and S1b). These colloids
exhibit an onset of absorption at 400 nm (3.1 eV) and an
emission maximum at 509 nm (2.44 eV). Interestingly, an
increase in the energy of excitation from 400 nm (3.1 eV) to
340 nm (3.6 eV) shifts the emission band to a higher energy
from 509 nm (2.44 eV) to 485 nm (2.56 eV). The intensity of
the emission at 485 nm is, however, reduced by a factor of
about 2.4 compared to that at 509 nm.
The morphology of colloidal nanostructures SP1 (consisting
of CdS/ZnS with excess Zn2+ (7 � 10�4 mol dm�3)) comprises
the formation of nanotubes of micrometer length (r1 mm),
with a height ranging from 7 to 20 nm (Fig. S2, ESIw). Thethree-dimensional AFM image of one such isolated tubular
structure is presented in Fig. 1 and exhibits a tube height of
18 nm, with a surface roughness distribution ranging from
10 to 25 nm. The FESEM image of SP1 (Fig. 2) also depicts
the formation of entangled tubular nanostructure(s), in which
nanotubes are manifested in the form of bundles of varied
dimensions, with a homogenous distribution of Cd, Zn and S
over the entirety of each tube (Fig. S3, ESIw).TEM images of SP1 (Fig. 3) reveal the formation of
networks of nanotubes with an average diameter of 18 nm,
consisting of an inner diameter of 10 nm and a wall thickness
of about 4 nm. Selected area electron diffraction patterns of
SP1 exhibit a ring structure. The XRD pattern of SP1 exhibits
peaks corresponding to CdS, ZnS and Zn(OH)2, formed in
hexagonal, wurtzite and orthorhombic structures, respectively
(Fig. S4, ESIw).The IR spectrum of SP1 is markedly different in regard to
the shape and prominence of the various peaks, and causes a
further shift in the energy of the absorption band due to A, U,
G, C, PO2� and 20-OH in RNA, compared to those of pure
RNA (R1)/RNA in the presence of Zn2+/Cd2+ (R2) (Fig. S5,
panels A, B and C, ESIw), clearly indicating the interaction of
Fig. 1 A three-dimensional AFM image of an isolated nanotube.
Inset: Histogram of the surface roughness distribution of the
nanotube.
aDepartment of Chemistry, Indian Institute of Technology Roorkee,Roorkee 247667, India. E-mail: [email protected];Fax: +91 1332-273560; Tel: +91 1332-285799
bCentre of Nanotechnology, Indian Institute of Technology Roorkee,Roorkee 247667, India
w Electronic supplementary information (ESI): Methodology, electronicand emission spectra, AFM, EDX, XRD, SAED, FTIR spectra,1H NMR, fluorescence lifetime, anisotropy and TEM images.
This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 5433–5435 | 5433
COMMUNICATION www.rsc.org/chemcomm | ChemComm
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CdS/ZnS with different functionalities of RNA upon the
formation of nanotubes. In order to further probe the inter-
action of RNA with CdS/ZnS upon the formation of tubular
structures, 1H NMR spectra of R1, R2 and SP1 were recorded
under identical conditions (Fig. S6, ESIw). A comparison of
the NMR spectra of SP1 with R1 and R2 shows a high field
shift in the resonance absorption of protons corresponding to
purine, pyrimidine and sugars. Besides, the protons of the
sugar, H20, H3
0, H40, H5
0, H500 and 20-OH, are now better
resolved, clearly indicating the involvement of these functional
groups in the formation of the tubular structure.
The relaxation kinetics of colloidal nanostructures SP1 and
SP2 (RNA-mediated CdS/ZnS without excess Zn2+) under
different experimental conditions were analysed by exciting
these samples at 405 nm (3.06 eV) and measuring their
emission at 509 nm (2.44 eV). In all of the cases, fluorescence
decay followed three-exponential kinetics (Fig. S7a, ESIw).For SP1, the average lifetime and quantum efficiency of the
emission was significantly increased compared to that of SP2
(Table S2, ESIw), which also manifested itself by an increase in
the emission intensity associated with the blue shift of the
emission maximum in the steady state fluorescence measurement.
The enhancement of the fluorescence intensity along with the
lifetime suggests that a change in morphology from spherical
particles in SP2 to nanotubes in SP1 causes the passivation of
its surface, such that it reduces non-radiative recombination
involving both the shallow and deeper charge carriers in
CdS/ZnS colloids, such that the emission maxima is slightly
blue-shifted. In an unusual observation, the excitation of SP1
at a higher energy (340 nm) resulted in a significant decrease in
the emission lifetime from 70 to 43 ns.
A comparison of the anisotropy data for SP1 and SP2
(Fig. S7b, ESIw) also indicates that for SP1, the value of the
fluorescence anisotropy is increased to 0.3, compared to
that of 0.24 for SP2. This finding was further analyzed by
measuring the rotational correlation time for both the
samples. In each case, the anisotropy followed bi-exponential
decay, in which the first component (y1) was very similar and
exhibited a value of about 2 ns, and for the second component
(y2) the correlation time for SP1 was significantly higher
(52 ns) compared to that for SP2 (25 ns). The similarity in
the value of y1 for both samples suggests that it is contributed-to
by the spherical nanoparticles, as a smaller portion of these
may be present, even at a higher concentration of Zn2+
(7 � 10�4 mol dm�3), whereas the enhancement in y2for SP1 might be attributable to the formation of tubular
structures, which will obviously rotate more slowly compared
to quantum dots.
The mechanism of evolution of the tubular morphology was
further probed by designing a series of control experiments
using TEMwith various precursors and their different possible
combinations, viz. pure RNA, RNA–Cd2+, RNA–Zn2+and
RNA–Cd2+/Zn2+ (Fig. S8, ESIw). In none of these cases was
the tubular morphology exhibited. From a previous report, it
is known that RNA-mediated CdS, both in the absence and
presence of Cd2+/Zn2+, results in the formation of spherical
nanoparticles,10 whereas RNA-capped ZnS in the presence of
excess Zn2+ (7 � 10�4 mol dm�3) results in the formation of
multiple layers of nanowires (Fig. S8, ESIw). Thus, it could
be the interaction of the combined semiconducting system
(CdS/ZnS) with Zn2+ that contributes to the formation of the
tubular morphology. This aspect was analyzed by performing
another set of control TEM experiments using RNA-mediated
CdS/ZnS without excess Zn2+ (SP2) and with a smaller
amount of excess Zn2+ (3 � 10�4 mol dm�3) (SP3). The sample
without excess Zn2+ produced quantum dots with an average
size of 1.6 nm, whereas in the presence of 3 � 10�4 mol dm�3
of excess Zn2+, these particles remained spherical, but
their size almost doubled to about 3 nm (Fig. S8, ESIw). Theseexperiments evidently indicate that the formation of tubular
structures is induced only in the presence of much higher Zn2+
concentrations (7 � 10�4 mol dm�3), thereby, suggesting the
specific role of Zn2+ in the transformation of quantum dots to
nanotubes. Based on above observations, the formation of the
self-assembly is illustrated in Scheme S1 (ESIw).The addition of excess Zn2+ caused growth of the colloidal
nanostructures in SP1 by binding through phosphate to a
certain optimum length. It also induced the folding of the
RNA-capped CdS/ZnS nanostructures, such that the weak
supramolecular interactions, viz. electrostatic, H-bonding and
p–p stacking involving PO2�, A, U, G, C and 20-OH, eventually
resulted in the formation of nanotubes (Scheme 1). Structural
changes associated with these interactions were evidenced by
IR and NMR spectroscopy: the change in the shape and
prominence of the absorption bands corresponding to
PO2� (1242 cm�1), nucleic bases (1647 cm�1) and 20-OH
Fig. 2 An FESEM image of SP1 showing bundles of entangled
nanotubes.
Fig. 3 A TEM image of SP1 depicting the formation of a network of
nanotubes. Inset: SAED of the nanotubes.
5434 | Chem. Commun., 2009, 5433–5435 This journal is �c The Royal Society of Chemistry 2009
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(1414 cm�1), respectively, in the IR spectrum of SP1
compared to those of R1 and R2, and the observed high-field
chemical shift associated with the improved resolution of
NMR spectra corresponding to the nucleic bases (7.7–8.4 to
7.6–8.3 ppm) and 20-OH (1.8–2.0 to 1.7–1.9 ppm) (Fig. S5 and
S6, ESIw).The above observations suggest that the formation of
nanotubes in the present system takes place by involving
specific interactions of different moieties/functional groups
of the RNA template itself, along with that of CdS and ZnS
upon folding (Scheme 1). These interactions can be described
as Watson–Crick nucleic base pairings, involving –NH2(6)
and –N(1) of A, –CQO(4) and –NH(3) of U, –CQO(6), –NH(1)
and –NH2(2) of G, and –CQO(2), –N(3) and �NH2(4) of C,
through H-bonding and relatively strong hydrophobic inter-
actions (p-stacking) between the aromatic moieties of these
bases. Meanwhile, CdS and ZnS, on different sides of the
folded nanostructure(s), are linked through van der Waals
interactions and H-bonding, consisting of nucleic bases and
Zn(OH)2, to construct the tubular structure.
The above morphological and structural changes also affect
the optical, fluorescence and anisotropic properties of SP1.
The optical spectrum of the nanotubes exhibits a higher
absorption coefficient for the excitonic peak, and the
fluorescence band becomes about four times more intense.
Besides the enhancement of the fluorescence intensity, it is
accompanied by an increase in the emission lifetime by a factor
of about 1.7. These enhancements in the optical properties can
be assigned to the two-dimensional confinement of the charge
carriers along the nanotube, which would lead to an increase
in the density of states in the conduction and valence bands,
and would, therefore, result in an improved excitonic
absorption and emission intensity compared to those of
spherical quantum dots. The decrease in the emission intensity
associated with the decrease in the emission lifetime at higher
energies of excitation is understood due to multiple exciton
generation under these conditions, which might undergo
non-radiative recombination or the annihilation of charge
carriers and thereby reduce the fluorescence intensity and
lifetime.
Anisotropic measurements also evidently support a
change in morphology from spherical particles to nanotubes,
as indicated by the values of anisotropy and rotational
correlation time, which are increased by factors of about
1.25 and 2.1, respectively. An enhancement in y2 is obvious
by the transformation of nanoparticles to tubular structures.
In summary, the present system utilizes the multi-functionality
of RNA to fabricate novel tubular nanostructures through
self-organization in a colloidal CdS/ZnS semiconducting
system. Supramolecular interactions of various functionalities
of RNA with CdS, ZnS and Zn2+ ions perform this
bottom-up synthesis to yield a thermodynamically-stable
arrangement. The presence of excess Zn2+ induces spontaneous
folding of these nanostructures, which subsequently assemble
into a tubular morphology. The participation of Zn2+ in the
formation of the tubular morphology was analyzed. The
enhanced properties of this system, viz. optical, fluorescence,
anisotropy and rotational correlation times, could be utilized
in biosensing, fluorescence imaging and nanoelectronics.
V. K. acknowledges CSIR, New Delhi for the award of
SRF. Thanks are also due to the Head of IIC, IITR, Roorkee
for providing us with the facilities of NMR, TEM, FESEM,
XRD and a single photon counter.
Notes and references
1 (a) C. M. Niemeyer, Angew. Chem., Int. Ed., 2001, 40, 4128–4158;(b) N. Ma, E. H. Sargent and S. O. Kelley, J. Mater. Chem., 2008,18, 954–964; (c) J. J. Storhoff and C. A. Mirkin, Chem. Rev., 1999,99, 1849–1862.
2 L. Jaeger and A. Chworos, Curr. Opin. Struct. Biol., 2006, 16,531–543.
3 (a) M. B. Dickerson, K. H. Sandhage and R. R. Naik, Chem. Rev.,2008, 108, 4935–4978; (b) H. Yan, Science, 2004, 306,2048–2049.
4 (a) P. Guo, J. Nanosci. Nanotechnol., 2005, 5, 1964–1982;(b) C.-C. Chen, Y.-C. Liu, C.-H. Wu, C.-C. Yeh, M.-T. Su andY.-C. Wu, Adv. Mater., 2005, 17, 404–407; (c) N. Ma,E. H. Sargent and S. O. Kelly,Nat. Nanotechnol., 2008, 4, 121–125.
5 (a) R. Baron, B. Willner and I. Willner, Chem. Commun., 2007,323–332; (b) D. L. Feldheim and B. E. Eaton, ACS Nano, 2007, 1,154–159; (c) L. Berti and G. A. Burely, Nat. Nanotechnol., 2008, 3,81.
6 (a) S. Hou, J. Wang and C. R. Martin, J. Am. Chem. Soc., 2005,127, 8586–8587; (b) J. C. Mitchell, J. Robin, J. Malo, J. Bath andA. J. Turberfield, J. Am. Chem. Soc., 2004, 126, 16342–16343.
7 D. Shu, W.-D. Moll, Z. Deng, C. Mao and P. Guo, Nano Lett.,2004, 4, 1717–1723.
8 L. Nasalean, S. Baudrey, N. B. Leontis and L. Jaeger, NucleicAcids Res., 2006, 34, 1381–1392.
9 S. A. Woodson, Curr. Opin. Chem. Biol., 2005, 9, 104–109.10 A. Kumar and V. Kumar, J. Phys. Chem. C, 2008, 112, 3633–3640.
Scheme 1
This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 5433–5435 | 5435
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