Chuan Liang Feng et al- Functional Quantum-Dot/Dendrimer Nanotubes for Sensitive Detection of DNA...

8/3/2019 Chuan Liang Feng et al- Functional Quantum-Dot/Dendrimer Nanotubes for Sensitive Detection of DNA Hybridization

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Biosensors

DOI: 10.1002/smll.200700453

Functional Quantum-Dot/Dendrimer Nanotubesfor Sensitive Detection of DNA Hybridization**

Chuan Liang Feng, Xin Hua Zhong, Martin Steinhart, Anne-Marie Caminade, Jean Pierre Majoral, and

Wolfgang Knoll*

The functionalization of nanotubes (NTs) is an effective

strategy towards design of new hybrid materials combining

customized properties and anisotropy.[1–2] Such materials have

attracted considerable interest for applications such as

biocatalysts, biosensors, and as platforms for biosepara-

tion.[3–5] For example, quantum dots (QDs) exhibiting narrow

emission bandwidth, photochemical stability, and high

quantum yield have been incorporated into the walls of

NTs.[6] However, the strategies for producing QD-modified

NTs[7–8] reported up to now suffer from low efficiency of

chemical functionalization and a lack of control over the

spatial assembly of the QDs. Layer-by-layer (LBL) deposi-

tion,[9] which involves the successive deposition of oppositely

charged polyelectrolytes, allows generation of functional

multilayer systems with high precision, even onto complex

substrates such as nanoparticles[10] and nanoporous

matrices.[11–12] Nanoparticles bearing charged ligands can

easily be incorporated into multilayer systems,[13] and the

rational assembly of different-sized QDs in LBLstructures can

yield so-called ‘‘nanorainbows’’ that emit white light. [14] By

controlling the distance between the layers of different-sized

QDs funnel-like bandgap profiles can be realized,[15–16] which

can show rapid and efficient fluorescence resonance energy

transfer (FRET) along the bandgap gradient.[17]

A configuration that would be particularly advantageous

for sensing consists of functionalized nanotubes aligned within

the pores of a porous membrane, such as self-ordered

nanoporous alumina (anodic aluminum oxide, AAO).[18]

Whereas the synthesis of tubular nanostructures using

such porous materials as shape-defining molds is well-

established,[19] the rational generation of complex functional-

wall architectures has remained a challenge. Here, a strategy

for the design and the fabrication of QD/dendrimer composite

NTs inside AAO membranes with a pore diameter of 400nm,

a lattice constant of 500 nm, and a pore depth of 100 mm

is reported. The arrays of aligned QD/dendrimer composite

NTs enabled the detection of DNA hybridization with

significantly enhanced sensitivity. The high specific surface

area of the pore walls of the AAO membranes (which amounts

to about 70000mm2 per 10mm2 of membrane surface)

combined with graded-bandgap architectures permits efficient

energy transfer to the inner surfaces of the NTs, onto which

single-stranded probe DNA is grafted. Emission from

dye-labeled target DNAcan thus be probed with exceptionally

high sensitivity and selectivity after its hybridization to the

probe DNA.

Globular, fourth-generation N,N -disubstituted hydrazinephosphorus-containing dendrimers[20] having 96 terminal

groups with either cationic [G4(NHþEt2ClÀ)96] (G4þ) or

anionic [G4(CH–COOÀNaþ)96] (G4À) end groups were used

as the matrix forming the walls of the NTs. [21] As demon-

strated previously, tubular nanostructures can easily be

obtained by successively depositing oppositely charged

dendrimers into AAO templates.[22] The thickness of bilayers

of linear polyelectrolytes deposited into nanoporous hosts is

known to significantly exceed that obtained on smooth

substrates and is difficult to control.[11] The well-defined

molecular architecture of dendrimeric polyelectrolytes and

their globular shape allows us to overcome these drawbacks,

and hence, dendrimers are ideal components with which tobuild compartmentalized NT walls.[23]

LBL assembly of QD/dendrimer multilayer systems was

monitored in situ by surface plasmon resonance (SPR)

spectroscopy, using planar Au model substrates coated with

3-mercaptopropionic acid (3-MPA). We consecutively depos-

ited multilayers of G4þ dendrimers and QDs with an emission

wavelength l¼ 614 nm (QDs614). The resulting kinetic mode

SPR curves are shown in Figure 1a. The addition of each layer

led to a regular increase of the reflectivity. The accumulated

shift of the resonance angles was converted into the geometric

film thickness by assuming a refractive index of n% 1.5 for the

dendrimers[24] and of n%2.7 for the QDs[25] (Figure 1b). The

mean increase in the thickness of the multilayer system wasdetermined to be Dd% 2 nm for an additional dendrimer layer

and Dd%5.5 nm for an additional QD614 layer. The formation

of multilayers containing a gradient assembly consisting of

QDs emitting at l¼561 nm (QDs561), QDs emitting at l¼ 594

nm (QDs594), and of QDs614 on 3-MPA-coated Au substrates

was also monitored by SPR. The accumulated shift of the

resonance angles associated with the deposition of additional

layers of the different QD species is shown in Figure 1c. The

thickness of the multilayer system increased by Dd% 2 nm for

every additional dendrimer layer deposited, by Dd% 6.5 nm

for every additional QD561 layer, by Dd% 6.1 nm for every

additional QD594 layer, and by Dd% 5.5 nm for every QD614

communications

[Ã] Prof. W. Knoll, Dr. C. L. Feng

Max Planck Institute for Polymer Research

Ackermannweg 10, 55128 Mainz (Germany)

E-mail: [email protected]

Prof. X. H. Zhong

Department of Chemistry

East China University of Science & Technology

200237 Shanghai, (P.R. China)

Dr. M. SteinhartMax Planck Institute of Microstructure Physics

Weinberg 2, 06120 Halle (Germany)

Prof. J. P. Majoral, Dr. A. M. Caminade

Laboratoire de Chimie de Coordination

Centre National de la Recherche Scientifique

205 Route de Narbonne, 31077 Toulouse Cedex 4 (France)

[ÃÃ] Helpful discussions with Prof. T. Basche, technical support by K.Sklarek, and funding by the German Research Foundation (SFB625: ‘‘Von einzelnen Molekulen zu nanoskopisch strukturiertenMaterialien’’; SPP 1165 (STE 1127/6-3); CERC3 (Mu 334/22-2)) aregratefully acknowledged.

: Supporting Information is available on the WWW under http://www.small-journal.com or from the author.

566 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 5, 566–571



layer (Figure 1d). These values are in good agreement with

previously reported results.[26] Hence, QDs and dendrimeric

polyelectrolytes bearing opposite charges are well-suited com-

ponents for the rational arrangement of different QD species

in functional multilayer systems deposited in porous matrices.To fabricate functional QD/dendrimer multilayer systems

in AAO templates, the template pore walls were modified with

3-aminopropyl dimethylethoxysilane (3-APS). Three G4À/G4

þ

bilayers were assembled before the deposition of the first QD

layer to achieve an optimized fluorescence signal,[23] resulting

in a pore wall/QD spacing of about 12 nm (Figure 2). This

distance corresponds to about two Fo ¨ rster radii of the

water-soluble Zn x

Cd(1À x)Se alloy QDs coated with mercap-

toundecanoic acid (MUA) ligands[27] used in this work. G4þ

dendrimers and negatively charged QDs with a specific

diameter were deposited alternately inside the modified AAO

hosts. The deposition cycles were repeated until desired

number of G4

þ

/QD bilayers was assembled (Figure 2). For themultilayer systems thus obtained, it was hoped that the loss in

fluorescence intensity caused by fluorescence quenching could

be reduced to a few percent.[28] The inner surface of the QD/

dendrimer composite NTs always consisted of a negatively

charged QD layer. The QD/dendrimer composite NTs could

be released by selectively etching the AAO matrix with a 30%

aqueous KOH solution at room temperature.

The incorporation of QDs614 into the walls of the NTs was

evidenced by photoluminescence (PL) spectroscopy. Upon

excitation at l¼ 460 nm NTs containing QDs614 emitted light

at l¼ 614 nm, as expected. Increase in PL intensity was found

to be proportional to the number of QD614 layers (Figure 3a).

No spectral overlap with AAO is

observed in the relevant spectral

range.[29] After dissolution of the

AAO template, single, free NTs

were obtained (Figure 3b). Their

outer diameter of approximately

400 nm corresponds well to the

pore diameter of the AAO tem-

plates used.

Using the same strategy, NTs

with graded bandgap assemblies of

the three Zn x

Cd(1À x)Se alloy QDs

(QDs561,QDs594, and QDs614,

assemblies referred to hereafter

as NTs2) were prepared. The as-

sembly was started with deposition

of QDs561 (green) closest to the

pore walls of the AAO template.

Then, QDs594 (orange) and

QDs614 (red) were deposited.

The inner surface of the QD/dendrimer NTs thus obtained

consisted of a layer of QDs614

(Figures 1 and 3c). The arrange-

ment of the different QD species

in the NTs is associated with a

decrease of the bandgap energy

from the outer to the inner surface

of the NT walls. Therefore, excita-

tion energy transfer from the higher bandgap QDs (the

light-harvesting QDs561 and QDs594) to those with lower

bandgap energy (QDs614) on the exposed inner surface of the

NTs will take place.[30] Moreover, the architecture of the QD

assembly ensures sufficient spectral overlap between thedifferent QD species, which is required for efficient FRET.[31]

As a result, only one amplified emission peak, centered at

l¼ 614 nm, is observed (Figure 3d (2)). We also investigated

NTs prepared by the deposition of QDs561 and subsequent

deposition of QDs614, which did not contain intermediate

QDs594 layers (NTs1). The PL spectrum of the NTs1 sample

shown in Figure 3d (1) exhibits two PL emission peaks

originating from QDs561 and QDs614. However, the peak at

l¼ 614 nm in the NTs2 spectrum has approximately 3.6 times

higher intensity than that in the spectrum of the NTs1. We

attribute the differences between the spectra to the presence

or absence of the QDs594. In NTs1, the poor spectral overlap

between QDs

561

and QDs

614

(see Supporting InformationFigure 1) apparently prevents efficient energy transfer through

the walls of the NTs.

Taking advantage of the wall architecture of the NTs2,

which allows FRET directed to the inner surfaces of the NTs,

the hybridization of DNA was investigated (Figure 4a). Amino

end-group-derivatized probe DNA strands with 30, 50, or 80

bases were covalently coupled to the N -hydroxysuccinimide

(NHS)-activated carboxyl groups of the QD614 layer forming

the inner surface of the walls of the NTs2. A fully comple-

mentary 15-mer DNA oligonucleotide sequence labeled with

the dye Cy5 was selected as the target. The immobilization of

30-mer probe DNA on a planar MUA-coated Au substrate

Figure 1. Deposition of G4þ/QD multilayers on smooth model substrates. a) Kinetic mode SPR

spectroscopy monitoring the successive deposition of G4þ and negatively charged QDs614;

b) geometrical thickness of the G4þ/QD multilayer system determined from the accumulated angular

shift obtained from the corresponding scan mode SPR curves; c) shift of SPR curves upon successive

deposition of G4þ and QD561, QD594, and QDs614 layers; d) geometrical thickness converted from the

accumulated angular shift obtained from the corresponding angular SPR scans in (c).

small 2008, 4, No. 5, 566–571 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 567



and the hybridization of the targets to the probe DNA was

studied by SPR and surface plasmon enhanced fluorescence

spectroscopy (SPFS) (Figure 4b), giving clear evidence of the

successful immobilization of the probe DNA and hybridiza-

tion of the complementary target DNA with the probe

DNA.[32]

The DNAhybridization in the NTs2 was detected using PL

spectroscopy (Figure 4c). A strong PL peak with a maximum

near l¼670 nm originating from Cy5 is observed when the

multilayer is excited at l¼ 460 nm (Figure 4c (2)). For

comparison, the Cy5 PL emission spectra of the same sample

excited at l¼ 630 nm, the wavelength usually used for the

excitation of Cy5, was studied (Figure 4c (4)). A very weak

emission at l¼670 nm, barely stronger than that from the

NTs2 obtained using the same excitation wavelength

(Figure 4c (3)), was observed. By comparing the PL intensity

obtained at l¼ 670 nm for an

excitation wavelength of 460 nm(Figure 4c, (2)) to that recorded

with an excitation wavelength of

630 nm (Figure 4c, (4)), an

increase of the sensitivity for

the detection of DNA hybridiza-

tion by a factor of $15, caused by

the FRET cascade, was esti-

mated. Excitons excited at

l¼ 460 nm are transferred from

QDs with larger bandgaps to

QDs with smaller bandgaps,

eventually reaching the QDs614

located at the inner surface of theNTs, onto which the probe DNA

was covalently immobilized.

Hence, the enhanced PL

emission intensity of the Cy5

dyes must be the result of FRET

from the QD cascade structure.

However, no excitation of the

QDs occurs at an excitation

wavelength of l¼630 nm and

consequently no FRET is possi-

ble. Consequently, no significant

amplification of the detection

communications

Figure 2. Schematic diagram of the preparation of QD/dendrimer composite NTs. The QDs emit at l¼561 nm (QDs561 ), 594 nm (QDs594 ), and

614 nm (QDs614; see Supporting Information Figure 1). The AAO templates were modified with 3-aminopropyl dimethylethoxysilane (3-APS). Three

bilayers consisting of positively charged (G4þ ) and negatively charged dendrimers were alternately deposited inside AAO templates.

Subsequently, G4þ and negatively charged QDs were deposited until the desired number of bilayers was obtained. The fabrication of NTs

exclusively containing QDs614

and of NTs containing a graded bandgap assembly of QDs561

, QDs594

, and QDs614

is displayed. First QDs561

(green;next to the pore walls), then QDs594 (orange, intermediate layer), and finally QDs614 (red, at the inner surface of the NTs) were deposited.

Figure 3. Characterization of QD/dendrimer composite NTs. a) PL spectra of NTs containing 1) one, 2)

three, and 3) five QD614 layers. b) SEM imageof an individual NT containing 15 G4þ/QD614 bilayers; c)

schematic diagram of an NT with a graded bandgap structure formed by assembling three different

sizes of QDs (QDs561,QDs594, QDs614 ) inside an AAO template; d) PL spectra of two different types of

NTs: 1) NTs1 containing five QD561/G4þ and five QD614/G4

þ bilayers. 2) NTs2 containing five QD561/

G4þ, five QDs594/G4

þ and five QDs614/G4þ bilayers. The QDs were excited at l¼ 460 nm. The PL

spectra were normalized to the absorption maximum of the NTs at l¼ 460 nm (Supporting

Information, Figure 2).

568 www.small-journal.com ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 5, 566–571



sensitivity resulting from enhanced Cy5 emission is observed.

The detection limit for DNA hybridization inside the NTs2

was studied by varying the concentration of Cy5-labeled targetDNA. Concentrations ranging from 100 nM to 100 f M were

applied and PL spectra were recorded with an excitation

wavelength of l¼ 460 nm (Figure 4d). A detection limit as low

as 100 f M (Figure 4d (5)) was found, and even for these low

concentrations the emission of the Cy5 could clearly be

separated from the background signal (Figure 4d (6)). To

evaluate the selectivity of the NTs2 functionalized with probe

DNA, a solution containing mismatch 2 (MM2) Cy5-labeled

target DNA was also studied. No hybridization or

nonspecific adsorption occurred, as evidenced by the absence

of Cy5 emission in the corresponding PL spectra (Supporting

Information, Figure 3). Therefore, NTs2 allow the detection of

DNA hybridization with both

significantly enhanced sensitivity

and high selectivity.

Taking into account the strong

dependence of the energy-transfer

efficiency on the distance between

donor and acceptor,[33] the chain

length of the probe DNA is a key

to successful sensing of the hybri-

dization with Cy5 labeled target

DNA. Figure 4e shows the PL

spectra after hybridization to

probe DNA of different chain

lengths. The PL intensity of the

Cy5 signal significantly decreases

as the distance between the chro-

mophores and the QD614 layer at

the inner surface of the NTs2

increases with an increased num-

ber of nucleotides. The strongest

PL intensity of Cy5 is observed forthe 30-mer probe DNA (Figure 4e

(4)). For the 50-mer probe DNA

with 20 additional thymines incor-

porated as spacers (between

the NH2 group grafted onto the

NTs2 and the 15-mer recognition

sequenceattheoppositeendofthe

probe), a much weaker PL inten-

sity of Cy5 is registered (Figure 4e

(3)). The lowest PL intensity is

detected with probe DNA having

80 nucleotides (with a total of 65

thymines spacers) in the strand(Figure 4e (2)).

For the detection of DNA

hybridization, NTs containing

cascaded energy transfer archi-

tectures have two major advan-

tages. Firstly, a much larger signal

amplification of the Cy5 emission

was observed as compared to

one-color systems based on single

QDs.[34] Secondly, although the

detection sensitivity of the NTs2 is not better than that

reported for other sensor systems, for example, nanoparticle-

based assays,

[35]

it should be possible to further optimize theperformance of QD/dendrimer NTs with graded bandgap

architectures by tuning the QD emission, by the incorporation

of additional QD species, and by adjusting the distance

between the QDs and organic dyes.

In conclusion, an efficient approach to the rational

assembly of different QD species in the walls of QD/

dendrimer composite NTs by LBL deposition has been

reported. Directed FRET through the graded bandgap

structure resulted in significantly enhanced detection of

DNA hybridization in the NTs combined with high selectivity.

Arrays of QD/dendrimer nanotubes aligned in AAO

membranes are particularly suitable for sensing because the

Figure 4. Detection of DNA hybridization. a) Schematic diagram displaying the direction of the energy

transfer from the QDs to the chromophore Cy5 attached to the target DNA after hybridization in the

NTs2. b) SPR kinetic scan during the immobilization of 30-mer probe DNA on smooth model

substrates. The inset shows the hybridization of Cy5-labeled complementary target DNA (15-mer) withprobe DNAinvestigated by SPFS. c) Normalized PL spectra of 1) NTs2 beforehybridization and 2) NTs2

after hybridization with Cy5-labeled target DNA using an excitation wavelength of l¼460 nm,

3) normalized PL spectra of the NTs2 before hybridization, and 4) after hybridization with Cy5-labeled

target DNA using an excitation wavelength of l¼ 630 nm. The same excitation intensity was used for

both excitation wavelengths. d) NormalizedPL spectra of the NTs2 after hybridization with Cy5-labeled

targetDNA usingsolutions with concentration of 1) 100 nM, 2 ) 1 0 nM, 3 ) 1 nM,4)100pM,and5)100f M.

e) Normalized PL spectra of the NTs2 1) before and 2-4) after hybridization of Cy5-labelled target DNA

to probe DNA with different strand lengths: 2) 80-mer; 3) 50-mer, and 4) 30-mer (excitation

wavelength l¼460 nm).




NT assembly has a surface about two orders of magnitude

larger area than that of thin-film configurations. Moreover,

such functionalized membranes can easily be integrated into

device architectures so that they may find broad use in

biomedical applications and biosensing. The generic approach

reported here should be applicable to a variety of dye-labeled

DNA, RNA, and protein targets. In addition, NTs containing

cascaded energy-transfer architectures can potentially be used

to spatially control the deposition of a variety of biomolecules,

as required in the fabrication of sensitive high-throughput

genomic arrays and libraries for combinatorial screening.

Finally, nanoparticles with functionalities different from those

of the QDs used here, for example magnetic nanoparticles, can

also be depositedinto thewalls of dendrimer NTs, which might

be applicable in the field of bioseparation.

Experimental Section

Materials: 3-APS, MUA, and 3-MPA were purchased from

Aldrich and used as received. Probe DNA and the complementary

target DNA labeled with Cy5 were purchased from MWG-biotech

AG, Ebersberg, Germany. The nucleotide sequence of the 30-mer

probe DNA was 5(-(TTT)5 TGT ACA TCA CAA CTA-3(; that of the

50-mer probe DNA was: 5(-(TTT)11 TT TGT ACA TCA CAA CTA-3(; and

that of the 80-mer probe DNA was: 5(-(TTT)21 TT TGT ACA TCA CAA

CTA-3(. The nucleotide sequence of the complementary target DNA

was 5(-Cy5-TAG TTG TGA TGT ACA-3( and that of the mismatch 2

target DNA was 5(-Cy5-TAG TTG TCA CGT ACA-3(.

Multilayer preparation on smooth Au substrates: The sub-

strates were prepared by thermal evaporation of 50 nm Au on top

of a 2 nm Cr layer (99.9%, Balzers Materials, Liechtenstein) onto

cleaned glass substrates in an evaporating chamber (Balzers,

Model BAE 250). The Au-coated glass substrates were immersed

in an aqueous 3 mM 3-MPA or MUA solution for 2 h. Solutions

containing 1 mg mLS1 G4R in deionized (Milli-Q) water were used

for the deposition of G4R, and $10S7 M QD solutions in Milli-Q

water were used for the deposition of the QDs. First, a G4R layer

was deposited onto the modified substrates, followed by the

deposition of the first negatively charged QD layer. The deposition

of G4R/QDs bilayers was repeated until the desired number of

bilayers was obtained. Between the deposition steps the samples

were rinsed with water.

Preparation of QD/dendrimer composite NTs: The pore walls of

the AAO templates (pore diameter: 400nm; lattice constant:

500 nm; pore depth: 100mm) were coated with 3-APS by placing the samples in a closed glass vessel containing 3-APS at 135 -C

for 3 h. The 3-APS layer provides a positively charged surface, onto

which the first G4S layer was deposited, followed by the first G4

R

layer. Then, alternating deposition of G4R layers and negatively

charged QD layers was repeated until the desired number of

multilayers was obtained. The inner surfaces of the walls of the

NTs always consisted of a negatively charged QD layer.

DNA immobilization and hybridization inside NTs: The COOH

groups of the QD layer forming the inner surface of the walls of the

NTs were activated by immersing them in an aqueous solution of

1-ethyl-3-(dimethylamino)-propylcarbodiimide (EDC; c(EDC)¼

1 mol/L) and N -hydroxysuccinimide (NHS; c(NHS)¼0.2 mol/l) for

30 min. The amino-functionalized single-stranded DNA was then

immobilized on the inner walls of the NTs from a 1mM aqueous

solution (phosphate buffered saline (PBS), pH 7.4) through

covalent attachment by amide bond formation. The aqueous

solution containing Cy5 labeled target DNA (pH 7.4; 100 nM ) was

applied to the NTs for 30 min at room temperature. Afterwards, the

AAO templates containing the NTs were rinsed three times with

Milli-Q water in order to remove residual labeled target DNA.

Spectroscopic characterization: The principles of the SPR and

SPFS spectroscopy and the setup are described elsewhere.[36] The

Kretschmann configuration[37] was used with an Au film evapo-

rated onto a glass substrate (LaSFN9), which was then optically

matched to the base of a 90- LaSFN9 glass prism ( n¼1.85 at

l¼ 632.8 nm). Absorption spectra were collected using a

diode-array Perkin–Elmer Lambda 900 UV-vis spectrometer.

Transmission, absorbance, and reflection were recorded from

190 to 3200 nm. PL emission spectra were collected using a

FL3095SL spectrometer (J&M TIDAS 9.5, Germany).

Keywords:

fluorescence resonance energy transfer .layer-by-layer assembly . nanotubes . quantum dots .

phosphorous dendrimers

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Received: June 27, 2007Revised: December 14, 2007

Published online: April 2, 2008


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