A graphene oxide-organic dye ionic complex with DNA-sensing and optical-limiting properties

Article abstract of DOI:10.1002/anie.201001004

A charge-transfer complex between graphene oxide (GO) and pyrene dye PNPB has been synthesized by a simple ion-exchange process. Its highly specific interactions with DNA compared to other biomolecules (see scheme) allows selective and rapid detection of DNA in biological mixtures. It also exhibits broadband optical limiting.

Full text of DOI:10.1002/anie.201001004

Angewandte
Chemie
DOI: 10.1002/anie.201001004
Graphene-Based Sensor
A Graphene Oxide–Organic Dye Ionic Complex with DNA-Sensing
and Optical-Limiting Properties**
Janardhan Balapanuru, Jia-Xiang Yang, Si Xiao, Qiaoliang Bao, Maryam Jahan,
Lakshminarayana Polavarapu, Ji Wei, Qing-Hua Xu, and Kian Ping Loh*
Graphene oxide (GO), a nonstoichiometric, two-dimensional
carbon sheet resulting from acid exfoliation of graphite, offers
a new class of solution-dispersible polyaromatic platform for
performing chemistry. Graphene oxide bears covalently
bound epoxide (1,2-ether) and hydroxyl functional groups
on either side of its basal plane, while carboxyl groups are
[1]
located at the edge sites. Due to the presence of aromatic
domains and functional groups, GO undergoes a complex
interplay of ionic and nonionic interactions with different
[
2–6]
molecules in solution.
Graphene oxide can be considered
to be a weak acid cation exchange resin because of the
ionizable carboxyl groups, which allow ion exchange with
metal cations or positively charged organic molecules.
Motivated by the applications of functionalized GO in
[
7]
[8]
biosensing and drug delivery, we report a simple ion-
exchange strategy for electrostatic complexation of GO with a
synthetic dye to form an energy- or charge-transfer com-
[
2,4,9]
plex.
We found that this GO–organic dye charge-transfer
complex exhibits enhanced properties for biosensing and
optical limiting.
Pyrene derivatives are well known for their noncovalent
interactions with molecules having p-electron-rich frame-
[4,9]
works.
To apply them in biosensing, we designed a water-
soluble and positively charged dye, namely, 4-(1-pyrenyl-
vinyl)-N-butylpyridinium bromide (PNPB; for crystal struc-
ture, see Supporting Information, Figure S1). This positively
charged dye can interact with negatively charged GO to form
a fluorescence-quenched charge-transfer complex, named
Scheme 1. a) Synthesis of PNPB. b) Ion-exchange process.
+
À
+
À
hereafter PNP GO (Scheme 1). The FTIR spectrum of
PNP GO sensing platform is based on the unique electro-
static and noncovalent interactions between the analyte and
+
À
PNP GO shows the presence of vibrational features assign-
able to the functional groups in both GO and PNPB
+
PNP , which will “turn on” the quenched fluorescence. The
+
À
(
Supporting Information, S2), and thus attests to successful
selectivity of PNP GO for double-stranded DNA over other
biomolecules was tested with proteins, RNA, carbohydrates,
and surfactants.
coupling between them. Atomic force microscopy imaging of
+
À
PNP GO shows that a monolayer film can be formed
Supporting Information, S3), which reflects the good mono-
(
As shown in Scheme 1b, the carboxyl protons at the edges
+
À
+
layer dispersion of PNP GO . The operating principle of the
of GO first undergo ion exchange with Na , and subsequently
+
+
with PNP . Once PNP is linked to the edges or basal planes
[
*] J. Balapanuru, J.-X. Yang, S. Xiao, Q. L. Bao, M. Jahan, L. Polavarapu,
Prof. Q. H. Xu, Prof. K. P. Loh
Department of Chemistry, National University of Singapore
of GO, it can also undergo p–p interactions with GO via its
pyrene moiety. As a result, the fluorescence of the resultant
[4]
+
À
PNP GO complex is quenched immediately due to strong
3
Science Drive 3, Singapore 117543 (Singapore)
+
[2,4,9]
charge-transfer interactions between PNP and GO.
Figure 1a shows the UV/Vis spectra of PNP GO . The
absorption peaks are slightly broadened and red shifted: from
Fax: (+65)6779-1691
E-mail: chmlohkp@nus.edu.sg
+
À
Prof. J. Wei
+
À
free PNPB to bound PNP GO , the peak shifts from 431 to
Department of Physics, National University of Singapore
Singapore 117543 (Singapore)
4
2
46 nm (Dl = 15 nm), and the absorption peak of GO from
30 to 238 nm (Dl = 8 nm). This is indicative of strong ionic
[
**] This work is supported by NRF-CRP grant “Graphene Related
+
[4]
Materials and Devices”. (R-143-00-360-281).
and p–p interactions between PNP and GO. To reveal the
range of concentrations in which PNP GO exhibits good
dispersion in water, we plotted the absorbance at 238 nm
+
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001004.
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posites of carbon materials such as
[10]
[11]
GO and C .
6
0
+
À
Cation exchange of PNP GO or
+
strong anion–PNP
attraction can
+
release PNP from the bound complex,
and this turn on the fluorescence from
the default off state due to quenching. A
systematic study was performed to
investigate the roles of positively and
negatively charged species in turning on
+
À
the fluorescence of the PNP GO com-
plex. These include the negatively
charged sodium dodecylbenzenesulfo-
nate (SDBS) and sodium dodecylsulfo-
nate (SDS) as well as the positively
charged hexadecyltrimethylammonium
bromide (HDTA) and tetrabutylammo-
nium iodide (TBA; see Supporting
Information S4 for their structures).
The plot of fluorescence intensity
recovery in Figure 1d shows that SDBS
is much more efficient in turning on the
fluorescence than SDS. From this, we
+
À
À1
Figure 1. a) UV/Vis absorption spectra of aqueous solutions of PNP GO (ca. 20 mgL ), PNPB deduce that, besides electrostatic inter-
À6
À1
+
À
À1
(
2ꢀ10 m), and GO (ca. 34 mgL ). b) Fluorescence spectra of PNP GO (ca. 20 mgl ) and
actions, noncovalent interactions (here
due to the p-electron clouds of the
À6
+
À
PNPB (2ꢀ10 m). c) Concentration-dependent UV/Vis absorption spectra of PNP GO from
1.2 to 25.2 mgL (a–h, respectively). The inset shows the plot of optical density at 238 nm
À1
1
À1
versus concentration [mgl ]. d) Compara-
tive fluorescence intensities of PNP GO
10 mgl ) complexed with different sur-
+
À
À1
(
factants at equal concentrations of 1 mm
and fluorescence under UV light (inset).
against
the
concentration
of
+
À
PNP GO (Figure 1c). Beerꢀs law
was used to estimate the effective
extinction coefficient of PNP GO
+
À
from the slope of the linear least-
squares fit, and it was found to be
À1
À1
0
.077 Lmg cm with an R value of
0
.997. The linear relationship
between concentration and absorb-
ance proves homogenous dispersion
+
À
of PNP GO in aqueous media. The
+
À
PNP GO complex is also soluble in
common organic solvents such as o-
dichlorobenzene, ethanol, THF, and
DMF.
The fluorescence spectra of
+
À
PNP GO and PNPB (Figure 1b)
reveal the exited-state interactions
+
of GO and PNP . Almost complete
quenching of fluorescence for
+
À
À1
+
À
Figure 2. a) Fluorescence spectra and b) comparative intensities of PNP GO (10 mgL ) com-
PNP GO
solution is observed,
plexed with DNA, RNA, proteins (BSA, heme, BLP, CTA), and glucose at equal concentrations of
0 mm. c) Selectivity studies with different surfactants and biomolecules: 1) SDBS, 2) SDS,
) HDTA, 4) TBA, 5) NH OH, 6) DNA, 7) RNA, 8) BLP, and 9) glucose and their images under UV
light. d) Image of PNP GO complexed with biological mixture of RNA, BSA, glucose, with and
brought about by photoinduced elec-
tron or energy transfer (see Support-
ing Information, S5), similar to that
2
3
4
+
À
previously reported for polymer com- without DNA, under UV light.
6
550
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Chemie
+
+
À
+
À
benzene ring in SDBS) are also crucial to release PNP from
PNP and GO . An important finding is that the PNP GO
the complex.
complex is selective for DNA over other biomolecules. Even
though RNA is made up of similar nucleotides to DNA
(except for thymine), it is not as efficient as DNA in turning
on the quenched fluorescence. The single-stranded and
supercoiled structure of RNA allows it to undergo p–p
stacking on the GO surface, but its complex-formation energy
The interplay of noncovalent and electrostatic interac-
+
À
tions in PNP GO may impart selectivity towards certain
biomolecules. Hence, the next step was to investigate whether
+
À
PNP GO is selective for DNA, RNA, proteins, and mono/
polysaccharides, which are commonly present in blood serum
or cell extract. The experiments were carried out with the
sodium salt of double-stranded DNA extracted from salmon
testes, RNA from baker’s yeast, bovine serum albumin
+
[7]
with PNP is not sufficiently high to remove it from GO.
+
À
The PNP GO complex can be used to detect traces of
DNA present in a given biological mixture (Figure 2d). A
linear relation was observed between the fluorescence
(
BSA), hemoglobin from bovine blood cells (heme), lens
protein from bovine eye (BLP), a-cymo-trypsinogen-A
CTA), and d-(+)-glucose.
Figure 2a shows fluorescence spectra of PNP GO com-
(
+
À
plexed with different biomolecules at equal concentrations.
The fluorescence-enhancing capability of DNA is eight to ten
times higher than those of RNA, proteins, and glucose
(
Figure 2b).
The main reason for the selectivity of PNP GO for
double-stranded DNA is the higher energy of ion-complex
+
À
+
À
+
À
formation of PNP DNA compared to PNP GO . Being a
large and supercharged molecule, DNA has a greater ionic
attraction for PNP than the weakly ionizable GO (more
+
weakly charged than DNA), and thus ion exchange can
proceed as illustrated in Scheme 2.
+
À
Scheme 2. Sensing by PNP GO . DNA can complex efficiently with
+
+
À
PNP to form ionic complex PNP DNA , and thus switches on the
fluorescence. Other biomolecules undergo p–p stacking on GO but do
+
not remove PNP from GO, and thus fluorescence remains quenched.
+
À
Figure 3. a) Fluorescence spectra of PNP GO complexed with differ-
ent concentrations of DNA ranging from 10 to 400 nm b) Calibration
plot of fluorescence intensity at 580 nm versus concentration of
DNA [nM]. The inset shows the calibration plot at low concentrations
of DNA (up to 50 nm). c) Image of PNP GO complexed with different
concentrations of DNA under UV light.
A control study carried out by adding GO to preformed
+
À
ion complex PNP DNA shows that GO has a minimal
+
À
quenching effect on PNP DNA (Supporting Information,
S4). This provides direct evidence that the electrostatic
+
À
+
À
bonding between PNP and DNA is stronger than that of
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intensity and the amount of DNA (Figure 3). Such quantita-
and PNPB solutions at 1064 nm. When the incident laser
fluence exceeds 2 Jcm , an abrupt drop in transmittance is
À2
tive detection is very useful for determining the amplification
yield of the polymerase chain reaction (PCR) before
+
À
observed for PNP GO solution at a limiting threshold of
[
12]
À2
sequencing and to identify contaminant DNA in recombi-
8.10 Jcm . At 1064 nm the optical-limiting performance of
PNP GO is better than that of carbon nanotubes (10 Jcm )
measured on the same system. Thus such graphene oxide–
organic dye ionic complexes have potential as broadband
optical limiters.
[
13]
+
À
À2
nant protein products. Most importantly, unlike other, toxic
assays for DNA quantification such as ethidium bromide or
PicoGreen, this cost-effective graphene-based powder is
thermally quite stable and safe to handle. Moreover, the
[19]
+
À
specific interactions between DNA and PNP GO made
rapid and selective detection of DNAwith a detection limit of
Figure 4c and d show the dependence of the scattering
signals, collected at an angle of 408 to the propagation axis of
the transmitted laser beam at 532 and 1064 nm, on input laser
fluence. As the input fluence increases, the scattering signals
1
nm possible in this experiment.
Nonlinear optical effects can arise from charge-transfer
+
À
complexes because of the possibility of multiphoton absorp-
tion in the dye, which can give rise to reverse saturable
of PNP GO deviate from linear behavior.
Figure 4e and f show the scattered signals at different
angles (every 58 from 10 to 1608) with respect to the
[
14]
absorption effects in optical limiting. Alternatively, rapid
absorption of laser energy by the dye and efficient energy
transfer to GO can lead to the ioniza-
+
À
transmitted laser beam for PNP GO , GO, and PNPB at an
tion of GO sheets, which forms rapidly
expanding microplasmas. These micro-
plasmas strongly scatter light from the
transmitted beam direction, and this
leads to a decrease in the measured
[
15]
transmitted light energy.
The nonlinear transmittance of sol-
+
À
utions of GO, PNPB, and PNP GO in
deionized water was studied with nano-
second laser pulses (7 ns) at 532 and
1
064 nm. The linear transmittances of
+
À
PNP GO are 63 and 74% at 532 and
064 nm, respectively. Figure 4a shows
1
that, at lower input fluences, there is no
significant change in the transmittance
+
À
of GO, PNPB, and PNP GO , but
when the input fluence is increased
beyond a certain threshold energy, the
+
À
transmittance of PNP GO decreases,
that is, it shows an optical-limiting
property. The nonlinear transmission
+
À
À2
of PNP GO starts at 0.21 Jcm ,
which is much lower than those of
À2
individual GO (1.51 Jcm ) and PNPB
À2
(
1.50 Jcm ) but comparable to that of
graphene–oligothiophene
composite
À2 [16]
(
0.15 Jcm ). At 532 nm, the limiting
threshold value (the input fluence at
which the transmittance falls to 50% of
+
À
the linear transmittance) of PNP GO
À2
is 1.55 Jcm ; this is much improved
from pure GO and better than that of
the benchmark optical liming material
À2
C60 (3.0 Jcm ) measured on the same
[17]
system.
Compared to reported gra-
phene composites, in which the optical
limiting is mainly effective in the visible
+
À
À1
[
10,16,18]
Figure 4. Optical limiting response of aqueous solutions of PNP GO (20 mgL ), GO
region,
one advantage is that the
À1
À6
(
34 mgl ), and PNPB (2ꢀ10 m), measured with 7 ns laser pulses at a) 532 and b) 1064 nm.
Nonlinear scattering response of PNP GO , GO and PNPB solutions at laser pulses of 532 (c
and e) and 1064 nm (d and f), where c) and d) show intensity-dependent scattering signals at
+
À
optical-limiting behavior of PNP GO
+
À
can be extended to the near-infrared
range. Figure 4b shows the optical-lim-
5
32 and 1064 nm, respectively, and e) and f) angle-dependent scattering signals at 532 and
+
À
iting responses of the PNP GO , GO, 1064 nm respectively.
6
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À2
incident intensity of 1.4 Jcm for 532 nm laser pulses and
À2
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+
À
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2
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[
1
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À
limiting behavior of PNP GO is mainly caused by two types
of scattering centers that exist after photoexcitation of
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In conclusion, we have synthesized a graphene oxide–dye
charge-transfer complex by a simple ion-exchange process.
We demonstrate that the GO–dye complex can be used as an
optical sensor for DNA, for which it is selective over a variety
of commonly used surfactants and biomolecules. Unlike other
toxic assays used for DNA quantification, no special precau-
tions were required to handle this thermally stable graphene-
based powder. The GO–dye complex also shows good
broadband optical-limiting properties. Hence, this work
inspires the development of multifunctional GO-based
hybrid materials through a simple ion-exchange process
involving the ionizable functional groups in GO. The concept
can be extended to the creation of strong or weak acid groups
on GO by functionalizing it with other acidic groups, for
example, sulfonate groups. Taking advantage of the high
surface area of GO, potential applications of such GO ionic
complexes also include ion-exchange resins for treating
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Keywords: charge-transfer complexes · graphene oxide ·
.
ion exchange · nonlinear optics · sensors
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6553