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: anilkfcy@iitr.ernet.in;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

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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

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