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G Model
JIEC 4105 No. of Pages 8
Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry
journal homepage: www.elsevier.com/locate/jiec
DNA and DNA–CTMA composite thin films embedded with carboxyl
group-modified multi-walled carbon nanotubes
Sreekantha Reddy Dugasania,1, Bramaramba Gnapareddya,1,
Mallikarjuna Reddy Kesamaa , Tai Hwan Hab,c,** , Sung Ha Parka,*
a
b
c
Department of Physics and Sungkyunkwan Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea
Hazards Monitoring BNT Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon 34141, Republic of Korea
Department of Nanobiotechnology, KRIBB School of Biotechnology, Korea University of Science and Technology (UST), Daejeon 34113, Republic of Korea
A R T I C L E I N F O
Article history:
Received 29 March 2018
Received in revised form 1 July 2018
Accepted 25 July 2018
Available online xxx
Keywords:
DNA
MWCNT
Composite thin film
X-ray photoelectron spectroscopy
Absorption
A B S T R A C T
Although the intrinsic characteristics of DNA molecules and carbon nanotubes (CNT) are well known,
fabrication methods and physical characteristics of CNT-embedded DNA thin films are rarely
investigated. We report the construction and characterization of carboxyl (–COOH) group-modified
multi-walled carbon nanotube (MWCNT–COOH)-embedded DNA and cetyltrimethyl-ammonium
chloride-modified DNA (DNA–CTMA) composite thin films. Here, we examine the structural,
compositional, chemical, spectroscopic, and electrical characteristics of DNA and DNA–CTMA thin
films consisting of various concentrations of MWCNT–COOH. The MWCNT–COOH-embedded DNA and
DNA–CTMA composite thin films may offer a platform for developing novel optoelectronics, energy
harvesting, and sensing applications in physical, chemical, and biological sciences.
© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights
reserved.
Introduction
For the last few decades, carbon nanotubes (CNT) have received
special attention as one of the most fascinating materials in
physical, chemical, and biological applications with multifunctional characteristics [1–3]. According to their configuration, CNTs
are classified as single-walled CNTs (SWCNT) or multi-walled CNTs
(MWCNT). From the perspectives of chemical stability, stiffness/
tensile strength, surface area, and electrical conductivity, MWCNTs
are very useful [4,5]. Furthermore, MWCNTs composed of multiple
graphene layers exhibit similar optical response as bulk structures
which provide stable optical absorption in the full spectral range
[6]. Recently, researchers demonstrated that arrays of MWCNTs
can be used as stable and durable real-time detectors of neurotransmitters for recording from cells [7–9]. However, the MWCNTs
* Corresponding author.
** Corresponding author at: Hazards Monitoring BNT Research Center, Korea
Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon 34141,
Republic of Korea.
E-mail addresses: taihwan@kribb.re.kr (T.H. Ha), sunghapark@skku.edu
(S.H. Park).
1
These authors contributed equally to this work.
are non-dispersible and insoluble in aqueous solvents due to their
hydrophobic nature. To utilize MWCNTs in various solvents
without losing their functionalities, solubility of MWCNTs in
aqueous and organic solvents must be increased. Efficient ways to
enhance solubility of MWCNTs include physical manipulation
(chopping and wrapping) and chemical (oxidation, OH and
COOH attachment) modification. Among these approaches,
wrapping of MWCNTs with DNA molecules and MWCNTs
functionalized with carboxyl (–COOH) groups have received
considerable attention due to their applications in fields such as
materials engineering, energy harvesting, biotechnology, and
detection systems.
DNA molecules – one of the naturally available biopolymers –
dissolved either in water or organic solvent have been considered
efficient template or scaffold material which can be used as a part
or whole in various devices and sensors [10–16]. DNA can be easily
embedded for novel functionalities (e.g., electromagnetism,
fluorescence, thermal stability, and dynamic mechanical characteristics) with various types of nanomaterials (e.g., metallic/
magnetic/semiconducting/insulating nanoparticles, nanorods,
fluorescent dyes, drugs, proteins, tri/divalent ions as well as
carbon-based materials) [17–20]. Among the functionalized nanomaterials, CNTs as DNA dopants have been used for specific
https://doi.org/10.1016/j.jiec.2018.07.031
1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: S.R. Dugasani, et al.. J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.07.031
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applications such as development of corrosion resistant biocompatible composites, construction of efficient carrier monitors, and
detection of Hg(II), Cd(II), and Pb(II) ions [21–25].
For optimizing the potential advantages of DNA having
MWCNTs as a biocomposite, a systematic study of MWCNTembedded DNA in both aqueous and organic solvents must be
undertaken. In addition, DNA has to be modified to dissolve in
organic solvent through the addition of cetyltrimethyl-ammonium
chloride (CTMA). Although properties of DNA and MWCNTs have
been intensively studied, fabrication methodology, physical
characteristics, and solvent effects of DNA with MWCNTs have
not yet been studied systematically. Due to the specificity in
optoelectronic characteristics, MWCNT-embedded DNA composite
will be very useful in future prospective applications. The
methodology adopted for the development of MWCNT-embedded
DNA composite thin films and observed results will probably find
numerous applications not only in biology or medicine, but also in
materials science and engineering.
In this paper, we develop fabrication methods for DNA and
DNA–CTMA solutions with –COOH-modified MWCNTs dissolved in
water and 1-butanol solvents. We present a comparative study of
DNA and DNA–CTMA composite thin films constructed by the
drop-casting method with varying concentrations of MWCNT–
COOH. The prepared composite thin films are characterized for
structural and elemental composition by X-ray diffraction (XRD)
and energy dispersive X-rays spectroscopy (EDS), respectively. In
addition, surface chemistry such as chemical states, binding
energies, functional groups, and charge transfer mechanisms are
analyzed by X-ray photoelectron spectroscopy (XPS); vibrational/
rotational stretching modes by Raman spectroscopy; and specific
chemical states and chemical bond interactions by Fourier
transform infrared (FTIR) spectroscopy. Lastly, the optical absorption and electrical measurements are performed using ultraviolet–
visible–infrared (UV–vis–NIR) spectroscopy and semiconductor
parameter analyzer, respectively. Our approach probably paves a
new route to the design of highly uniform bioinorganic composite
thin films for use in various practical applications.
Experimental section
Preparation of DNA and cetyltrimethylammonium chloride (CTMA)modified DNA (DNA–CTMA) thin films
To prepare DNA solution, 0.05 g of DNA extracted from salmon
(GEM Corporation, Shiga, Japan) is dissolved in 5 mL of de-ionized
(DI) water followed by magnetic stirring (1000 rpm for 24 h at
room temperature) to result in a 1.0 wt% DNA solution.
CTMA-modified DNA powder is obtained by the following. 3.0 g
of DNA are dissolved in 1000 mL of DI water followed by magnetic
stirring. 6 mL of standard CTMA solution (Sigma-Aldrich, Seoul,
Korea) is dissolved in 1000 mL of DI water in another beaker
followed by magnetic stirring. Then CTMA solution is added slowly
dropwise into DNA solution while stirring. This process allows the
attachment of CTMA surfactant onto the DNA and forms a DNA–
CTMA precipitate. The precipitate solution is stirred for another
5 h, then filtered and washed with excess DI water to remove the
residues of CTMA and NaCl. Finally, DNA–CTMA powder is obtained
from the washed precipitate followed by drying for 2 days at
temperature of 40 C.
For DNA–CTMA solution, 0.05 g of DNA–CTMA is dissolved in
5 mL of 1-butanol followed by magnetic stirring (1000 rpm for 24 h
at room temperature) to result in a 1.0 wt% DNA–CTMA solution.
To construct a thin film, 20 mL of the DNA (DNA–CTMA) solution
is drop-cast on O2 plasma-treated substrates followed by 24 h of
drying at room temperature. Glass (for XRD, EDS, XPS, Raman, FTIR,
and current measurements) and fused silica (for absorption
measurement) substrates of size 5 5 mm2 are treated with O2
plasma before deposition of DNA (DNA–CTMA) solution. Final
thicknesses of both DNA and DNA–CTMA thin films are 2 mm.
Fabrication of DNA and DNA–CTMA composite thin films with
carboxyl (–COOH) group-modified multi-walled carbon nanotubes
(MWCNT–COOH)
We prepared a stock solution of 1 wt% MWCNT–COOH (the
contents of the –COOH group are 4.0 wt% of MWCNTs, inner and
outer diameters are 2 and 8 nm, respectively, length is in the
range of 5–20 mm, and electrical conductivity is 100 S/cm,
Skyspring Nanomaterials, Inc., USA) dissolved in DI water for
DNA (1-butanol for DNA–CTMA) followed by magnetic stirring at
1000 rpm for 2 days at room temperature. To prepare the DNA
(DNA–CTMA) solutions with various concentrations of MWCNT–
COOH ([MWCNT–COOH]), i.e., 0, 0.05, 0.10, 0.15, 0.20, and 0.30 wt%,
0.05 g of DNA (DNA–CTMA) and the appropriate amount of
MWCNT–COOH are mixed in 5 mL of DI water (1-butanol) followed
by magnetic stirring at 1000 rpm for 2 days at room temperature to
achieve a homogeneous mixture of MWCNT–COOH-embedded
DNA (DNA–CTMA) solutions of final [DNA] ([DNA–CTMA]) of 1.0 wt
%. A 20 mL aliquot of the MWCNT–COOH-embedded DNA (DNA–
CTMA) solution is drop-cast onto an O2 plasma-treated substrate
followed by incubation for 24 h at room temperature (Fig. 1a).
Fig. 1. Schematic diagram, structural, and elemental characteristics of DNA and CTMA-modified DNA (DNA–CTMA) composite thin films with carboxyl (–COOH) groupmodified MWCNT (MWCNT–COOH). (a) Schematic diagram of the MWCNT–COOH-embedded DNA composite thin film. (b) Typical X-ray diffraction patterns of DNA and
DNA–CTMA composite thin films without and with MWCNT–COOH (0.3 wt%). (c) Distinctive energy dispersive X-ray spectroscopy (EDS) analysis of MWCNT–COOH powder
and DNA composite thin films without and with MWCNT–COOH.
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X-ray diffraction (XRD)
Fourier transform infrared (FTIR) spectroscopy
The structural characterization of MWCNT–COOH-embedded
DNA and DNA–CTMA composite thin films is performed using
X-ray diffractometer (D8 Advance, Bruker Corporation, USA) with
Cu-Kα radiation of wavelength 0.15405 nm operated in the
diffraction angle range of 10 –50 (Fig. 1b).
The chemical bonds and interactions of the MWCNT–COOHembedded DNA and DNA–CTMA composite thin films in the range
of 600–3700 cm1 are recorded by a TENSOR 27 spectrometer
(Detector: MIR_ATR (ZnSe), Bruker Inc., USA). 32 scans are coadded and averaged with a resolution of 4 cm1. The data in the
FTIR spectra used in the present study are analyzed by subtracting
the background spectrum produced by bare glass (Fig. 4).
Energy dispersive X-ray spectroscopy (EDS)
Elemental composition in the MWCNT–COOH-embedded DNA
thin films is estimated by EDS attached to a field emission scanning
electron microscopy (JSM-7600F, JEOL USA, Inc.) (Fig. 1c).
Ultraviolet–visible–near infrared (UV–vis–NIR) spectroscopy
The chemical states and composition of the MWCNT–COOHembedded DNA composite thin films are obtained from the XPS
(ESCALAB 250Xi, Thermo Scientific, UK) spectrum in binding
energies from 0 to 1350 eV. We fixed the excitation source
(monochromatic Al Kα X-ray source of energy 1486.6 eV) and data
acquisition spot size (650 mm) on all sample surfaces (Fig. 2).
The absorption spectra of the MWCNT–COOH-embedded DNA
and DNA–CTMA composite thin films are obtained by a UV–vis–
NIR spectrophotometer (Cary 5G, Varian, CA, USA) in the
wavelength range between 190 and 2700 nm. The spectrophotometer is equipped with two light sources (a deuterium arc lamp for
UV and a quartz W-halogen lamp for near-infrared and visible) and
two detectors (a cooled PbS detector for near-infrared and a
photomultiplier tube for visible and UV). The spectrophotometer
measured the frequency-dependent light intensities of the sample
(Fig. 5).
Raman spectroscopy
Electrical measurement
Raman modes and chemical bindings of the MWCNT–COOHembedded DNA and DNA–CTMA composite thin films are studied
by Raman spectrum in the frequency range from 200 to 3200 cm1
as obtained by confocal Raman microscope with laser wavelength
of 532 nm (Alpha 300 R, WITec, Germany) (Fig. 3).
The electrical properties of the MWCNT–COOH-embedded DNA
and DNA–CTMA composite thin films are determined using a
semiconductor parameter analyzer (4200-SCS, Keithley Instruments Inc., USA) in the applied bias voltage range between 0 and
5 V (Fig. 6).
X-ray photoelectron spectroscopy (XPS)
Fig. 2. The X-ray photoelectron spectroscopy (XPS) survey spectra and high resolution XPS spectra of the DNA composite thin films. (a) Correlative full scan XPS survey spectra
of MWCNT–COOH powder and DNA composite thin films without and with MWCNT–COOH (0.3 wt%). (b–f) Comparative high resolution XPS spectra of C 1s, O 1s, N 1s, P 2p,
and Na 1s regions in DNA composite thin films without and with MWCNT–COOH. Core spectra of MWCNT–COOH powder peaked only in C 1s and O 1s regions.
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Fig. 3. Representative Raman spectra of the MWCNT–COOH-embedded DNA and DNA–CTMA composite thin films. Comparative Raman spectra of (a) DNA and (b) DNA–
CTMA composite thin films with various [MWCNT–COOH] of 0, 0.05, 0.1, 0.15, 0.2, and 0.3 wt% are depicted. Intensity ratios of D-band to G-band (ID/IG) and D-band to G0 -band
(ID/IG0 ) as a function of [MWCNT–COOH] in DNA and DNA–CTMA composite thin films are shown in insets of (a) and (b), respectively.
Fig. 4. Fourier transform infrared (FTIR) spectra of the MWCNT–COOH-embedded DNA and DNA–CTMA composite thin films. (a, b) FTIR spectra of (a) DNA and (b) DNA–
CTMA composite thin films with various [MWCNT–COOH] are displayed. Due to the presence of –COOH group in MWCNT, the C¼O stretching mode is visible in the range of
1700–2000 cm1 and is marked with a dotted circle.
Results and discussion
Structural and elemental characteristics of the DNA composite
thin films with carboxyl (–COOH) group-modified MWCNT
(MWCNT–COOH) were verified by X-ray diffraction (XRD) and
energy dispersive X-ray spectroscopy (EDS), respectively (Fig. 1).
Fig. 1a shows a schematic diagram of the MWCNT–COOHembedded DNA composite thin film fabricated by the simple
drop-casting method on a given substrate. As shown in Fig. 1b, the
XRD patterns of DNA (DNA–CTMA) thin films and MWCNT–COOHembedded DNA (DNA–CTMA) composite thin films display the
characteristic DNA (DNA–CTMA) and MWCNT–COOH peaks at
around 22.5 and 25 , respectively, which agree well with previous
XRD reports of DNA and MWCNT [26,27]. Fig. 1c shows the typical
EDS spectra of DNA, MWCNT–COOH-embedded DNA composite
thin film as well as MWCNT–COOH powder. From the EDS spectra,
we observe the carbon (C), nitrogen (N), oxygen (O), phosphorus
(P), and sodium (Na) elements in DNA as well as in MWCNT–
COOH-embedded DNA thin film, while C and O were observed in
MWCNT–COOH powder. The atomic weight percentages of C in
DNA, MWCNT–COOH-embedded DNA, and MWCNT–COOH powder were 43.9, 65.0, and 92.8%, respectively. As expected, MWCNT–
COOH powder at a given volume shows the relatively greater
proportion of C as compared to the other samples. In contrast,
atomic weight percentages of the N (P) elements in DNA and
MWCNT–COOH embedded DNA thin films were 16.6 (4.6) and
10.1% (2.2%) but no N and no P elements were observed in
MWCNT–COOH powder.
X-ray photoelectron spectroscopy (XPS) of MWCNT–COOHembedded DNA composite thin films assesses chemical elements
and their quantification in addition to chemical states associated to
binding energies. Representative XPS full survey spectra of the
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Fig. 5. UV–vis–NIR absorption spectra of the MWCNT–COOH-embedded DNA and DNA–CTMA composite thin films. (a) Comparative absorption spectra of MWCNT–COOH
powder, DNA and DNA–CTMA thin films. (b, c) Comparative absorption spectra of DNA and DNA–CTMA composite thin films with various [MWCNT–COOH] of 0, 0.05, 0.1, 0.15,
0.2, and 0.3 wt%, respectively. (d) The variation of absorption as a function of the [MWCNT–COOH] at a fixed wavelength of 1200 nm.
Fig. 6. Current–voltage (I–V) characteristics of the MWCNT–COOH-embedded DNA and DNA–CTMA composite thin films. (a, b) I–V characteristics of the DNA and DNA–CTMA
composite thin films with various [MWCNT–COOH] of 0, 0.05, 0.1, 0.15, 0.2, and 0.3 wt%, respectively. (c) The variation of resistance as a function of the [MWCNT–COOH] at a
fixed applied bias voltage of 5 V.
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DNA composite thin films without and with MWCNT–COOH
(0.3 wt%) are shown in Fig. 2a. The XPS survey spectra showed C, O,
N, P, and Na elements at binding energies of around 284.8, 532.1,
399.2, 133.3 and 1071.2 eV for DNA and 284.8, 530.9, 398.5, 131.7,
and 1069.6 eV for MWCNT–COOH-embedded DNA thin films. The
corresponding chemical states in each peak are assigned to C 1s, O
1s, N 1s, P 2p, and Na 1s [28–31]. Similarly, the XPS survey
spectrum of MWCNT–COOH powder showed C at 284.8 eV and O at
532.5 eV. We observed slightly lower binding energies for
MWCNT–COOH-embedded DNA composite thin film as compared
with the pristine samples. The decrease of binding energies (i.e.,
minute peak shift to lower energy) caused by the interaction
between MWCNT–COOH and DNA molecules could shift the
valence electrons from MWCNT–COOH to DNA.
The de-convoluted XPS characteristic orbital peaks of C 1s, O 1s,
N 1s, P 2p, and Na 1s in DNA, MWCNT–COOH-embedded DNA
composite thin films, and MWCNT–COOH powder are shown in
Fig. 2b–f. For C 1s, the peaks of DNA were placed at binding
energies of 284.8, 286.3, 288.0, and 290.0 eV which correspond to
the C
C/C¼C/C
H, C
O/C
N/NC¼N/N
C
N, N CO/
N
C¼N/N
C¼O and NC(¼O) N, respectively. MWCNT–
COOH powder shows the chemical configurations of sp2, C
O,
and p–p* at 284.8, 285.9, and 291.0 eV, respectively. Consequently,
the peaks of MWCNT–COOH-embedded DNA composite thin film
are obtained at 283.1, 284.8, and 286.6 eV which correspond to the
sp2, C
C/C¼C/C
H, and C
N/C¼N/C
O/NC
O/N
C¼C, respectively (Fig. 2b). Similarly, orbital peaks of O 1s are found at
531.1, 532.6, and 536.2 eV (corresponding chemical bonds are
COC/C¼O/P¼O, C¼O/P¼O, and P
O, respectively) for DNA
thin film and 531.6 and 533.5 eV (C¼O/P¼O and C
O
P/C
O
C,
respectively) for MWCNT–COOH powder. The peaks of MWCNT–
COOH-embedded DNA composite thin film obtained at 529.2,
531.0, and 533.7 eV were assigned to OH
ion, COC/C¼O/P¼O,
and C
C
C/C
C
P, respectively (Fig. 2c). For N 1s, DNA thin
films showed three de-convoluted XPS characteristic peaks at
399.0, 400.4, and 405.1 eV which are assigned to C
NH2/C¼N
C/
N¼C, NC
O/N
C¼O, and N
H, respectively. Similarly, we
recognize the peaks of MWCNT–COOH-embedded DNA composite
thin film at binding energies of 397.3, 398.8, and 401.3 eV as
corresponding to the N
HC, C
NH2/C¼N
C/N¼C, and NC,
respectively (Fig. 2d). For P 2p, the DNA thin film revealed two deconvoluted XPS characteristic peaks at 133.4 and 139.8 eV
associated with chemical assignments of P 2p3/2/PO4/P
O and
P¼O/P
O, respectively, and the MWCNT–COOH-embedded DNA
composite thin film showed a single peak at 131.8 eV related to the
chemical assignments of P 2p3/2/PO4/P
O (Fig. 2e). Lastly, the Na
1s peaks in DNA exhibited at binding energies of 1071.3 and
1076.5 eV correspond to the NaO/Na+O and Na
H
PO4,
respectively. The peaks of MWCNT–COOH-embedded DNA composite thin film obtained at 1069.6 and 1072.9 eV correspond to
NaO/Na+
O and Na+
PO4, respectively (Fig. 2f). Na did not
belong to DNA molecules, but it helps make DNA duplexes stable.
When we add MWCNT–COOH into DNA, the chemical groups of
N
CO/N C¼N/NC¼O and N
C(¼O)
N are significantly
suppressed, while the newly added sp2 group contributes to the
chemical state of C 1s. Similarly, P
O is suppressed while
enhancing OH
ion due to the carboxyl group in MWCNTs for O
1s. N
C
O/N
C¼O and N
H are reduced while N
H
C and
N
C enhance contribution to N 1s. P¼O/P
O are absent in
MWCNT–COOH-embedded DNA thin films in P 2p, as expected. For
the chemical state of Na 1s, the chemical group of Na
H
PO4
+
disappeared while the additional Na PO4 group was considerably enhanced. Although there were several chemical groups
commonly enhanced in both DNA and MWCNT–COOH-embedded
DNA composite thin films, we observe notable negative shifts in
binding energies owing to the interaction between DNA and
MWCNT–COOH. The negative shift in binding energies with the
addition of MWCNT–COOH into DNA indicates charge transfer
from MWCNT–COOH to DNA molecules. Additionally, we observed
appreciable changes in peak intensity, area, and full-width at half
maximum of DNA thin films without and with MWCNT–COOH due
to changes in elemental composition and the distribution of the
chemical groups.
The Raman spectra of DNA and DNA–CTMA thin films
embedded with various concentrations of MWCNT–COOH
([MWCNT–COOH], i.e., 0, 0.05, 0.1, 0.15, 0.2, and 0.3 wt%) are
shown in Fig. 3. We chose up to 0.3 wt% of [MWCNT–COOH] which
guaranteed well mixed and thoroughly dissolved MWCNT–COOH
in DNA and DNA–CTMA solutions ensuring uniform embedding in
DNA and DNA–CTMA composite thin films. We observed characteristic DNA Raman peaks centered at around 485, 678, 732, 786,
890, 1012, 1095, 1248, 1306, 1336, 1375, 1420, 1486, 1576, 1666,
2160, and 2960 cm1, in agreement with previous reports [32–36].
The typical contributions of DNA nucleotides were segregated into
distinct sections such as, 400–800 cm1 for nucleobases/sugar and
phosphate backbone, 800–1200 cm1 for sugar and phosphate
backbone groups, and 1200–2000 cm1 for nucleobases, and
2100–3200 cm1 mainly for hydroxyl groups. The Raman modes
of the DNA bases at 678, 732, 786, 1248, 1306, 1336, 1375, 1486,
1576, and 1666 cm1 were assigned to the ring breathing
vibrational modes of guanine and thymine, ring breathing mode
of adenine, vibrational modes of cytosine and thymine, vibrational
modes of cytosine and adenine, ring vibrational modes of adenine
and cytosine, ring modes of guanine and adenine, ring modes of
thymine and adenine, N7 vibration mode of guanine and ring mode
of adenine, in-plane ring vibrational modes of adenine and
guanine, and the C¼O stretching vibrational modes of thymine
and cytosine, respectively. The Raman bands of DNA sugar and
phosphate backbone groups at 480, 786, 890, 1012, 1095, and
1420 cm1 were assigned to phosphoionic bond (PO2) scissor,
symmetric stretching vibrational modes of PO2, deoxyribose ring
vibrational modes, C
O stretching vibrational modes of deoxyribose, phosphodiester stretching vibrational modes, and the
deoxyribose sugar moieties, respectively. The Raman bands at
2160 and 2965 cm1 are due to the OH and C
H stretching
modes. In addition to DNA Raman bands, the DNA–CTMA thin film
exhibited bands at 2854, 2890, 2937 cm1 due to various modes of
CH2 and CH3 groups in the alkyl chains of CTMA.
Interestingly, DNA and DNA–CTMA thin films embedded with
various [MWCNT–COOH] showed significant strong characteristic
Raman bands of MWCNT–COOH at 1328, 1570, and 2690 cm1 and
weak bands at 225 and 1465 cm1 in addition to the DNA Raman
bands. The MWCNT–COOH-embedded DNA and DNA–CTMA
composite thin films have a radial breathing mode, D-band (from
sp3 carbon), a strain-induced band, a G-band (sp2 carbon), and a G0 band approximately at 225, 1328, 1465, 1570, and 2690 cm1,
respectively [37]. Most of the spectra exhibited stronger G- and Dband than G0 -band. Intensity ratios of D-band to G-band (ID/IG) and
D-band to G0 -band (ID/IG0 ) as a function of [MWCNT–COOH] in DNA
and DNA–CTMA composite thin films are shown in insets of Fig. 3a,
b. With increasing [MWCNT–COOH], Raman bands characteristic
of DNA and DNA–CTMA molecules gradually decrease and
suppress, as expected. By contrast, increasing [MWCNT–COOH]
in DNA and DNA–CTMA thin films increases the characteristic
Raman band intensities of MWCNT–COOH without noticeable
peak shifts. Additionally, the stretching vibrational modes of sugar
and phosphate backbone groups in DNA and DNA–CTMA were
affected by the addition of MWCNT–COOH. Electrostatic interactions and physical adsorption between the MWCNT–COOH and
DNA molecules may occur due to carboxyl groups in MWCNTs.
FTIR spectroscopy is extensively employed for analyzing
molecular interaction and chemical bonds of organic and inorganic
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materials. The FTIR-attenuated reflection spectra of the MWCNT–
COOH-embedded DNA and DNA–CTMA composite thin films are
shown in Fig. 4. The characteristic absorption bands of DNA thin
film without MWCNT–COOH exhibit at around 3360, 1700, 1653,
1603, 1488, 1420, 1370, 1220, 1082, 1050, 1015, 963, 835, and
780 cm1 [38,39]. In addition to DNA absorption bands, the DNA–
CTMA thin film exhibited additional absorption bands at 2850 and
2920 cm1 indicating the presence of the CTMA molecules. The
absorption bands of DNA and DNA–CTMA showed three characteristic spectral ranges of 3600–2500 cm1 for NH, C
H and
OH stretching modes, 1800–1300 cm1 for DNA bases, and 1250–
600 cm1 for the sugar and phosphate backbone groups. Absorption bands due to CH2 and CH3 groups in the alkyl chains of CTMA
in DNA–CTMA thin films appeared at 2950–2850 cm1. The
detailed vibrational and stretching mode assignments of nucleotides have been discussed previously [38,39]. The FTIR spectra of
MWCNT–COOH-embedded DNA and DNA–CTMA composite thin
films show an additional band in the spectral range of 1700–
2000 cm1 revealing the C¼O stretching mode from COOH groupmodified MWCNT as highlighted in the dotted circle in Fig. 4.
Noticeable changes of band positions and intensities with varying
[MWCNT–COOH] in DNA and DNA–CTMA composite thin films
originate from alteration of vibrational and stretching modes
produced by DNA and DNA–CTMA in the presence of MWCNT–
COOH which facilitates hydrophilic and van der Waals interactions.
We took absorption spectra in the UV–vis–NIR wavelength
range of 190–2700 nm to study the optical absorption of MWCNT–
COOH-embedded DNA and DNA–CTMA composite thin films
(Fig. 5). For control, the comparative wide range optical absorption
spectra of the DNA and DNA–CTMA thin films and MWCNT–COOH
powder were measured (Fig. 5a). Characteristic absorption peaks
were observed at wavelengths of 210 and 260 nm for DNA (DNA–
CTMA) and in the range of 850–2600 nm for MWCNT–COOH
[40,41]. By increasing [MWCNT–COOH], DNA (DNA–CTMA) absorption peak intensities at 210 and 260 nm were reduced but
peaks for MWCNT–COOH, around 850–2100 (broad peak) and
2350 nm, were gradually enhanced (Fig. 5b, c), as expected.
The variation of absorption intensities at a given wavelength of
1200 nm as a function of [MWCNT–COOH] in DNA and DNA–CTMA
composite thin films are shown in Fig. 5d. The absorption
intensities monotonically increased with increasing [MWCNT–
COOH] which might be due to hydrophilic interactions, electrostatic interactions, and physical adsorption between DNA and
MWCNT–COOH. Because the –COOH-modified MWCNTs had
relatively stronger interfacial bonding to DNA (DNA–CTMA) and
better dispersion in given solvents than pristine MWCNTs,
MWCNT–COOH was well distributed in DNA (DNA–CTMA) thin
films. Based on absorption results, MWCNT–COOH-embedded
DNA and DNA–CTMA composite thin films have certain advantages
for optical and sensor applications because of the wide spectral
range spanning ultraviolet, visible, telecommunications window to
far-infrared regions.
Finally, we performed electrical measurements to analyze the
charge transfer mechanism in MWCNT–COOH-embedded DNA
and DNA–CTMA composite thin films. The current–voltage (I–V)
characteristics of the DNA and DNA–CTMA thin films embedded
with various [MWCNT–COOH] are shown in Fig. 6. Although I
through DNA (DNA–CTMA) thin films without MWCNT–COOH
obtained in the range of 10 nA (1 nA), the MWCNT–COOHembedded DNA (DNA–CTMA) composite thin films obtained from
10 nA to 1 mA (10 nA to 1 mA) by increasing the [MWCNT–
COOH] from 0.05 to 0.3 wt% at a fixed V of 5 V. The electrical
characteristics of MWCNT–COOH-embedded DNA (DNA-CTMA)
composite thin films were in the semiconductor range
and approaching the metallic range. Both the composite
thin films exhibited monotonic increases of I with increased
7
[MWCNT–COOH] due to the configuration changes of the MWCNT–
COOH in DNA (DNA–CTMA) composite thin films. The varied
resistance of MWCNT–COOH-embedded DNA and DNA–CTMA thin
films as a function of the [MWCNT–COOH] measured at a fixed
applied bias voltage of 5 V are shown in Fig. 6c. We recognize that
the differences in resistance for both composite thin films were
larger at small [MWCNT–COOH] but they became smaller with
increasing [MWCNT–COOH]. This is attributable to different
interconnections caused by MWCNT–COOH. The resistances of
MWCNT–COOH-embedded DNA (DNA–CTMA) composite thin
films varied from 280 MV to 7 MV (from 1.5 GV to 8 kV)
while increasing the [MWCNT–COOH] from 0.05 to 0.3 wt%.
Conclusions
We constructed carboxyl group-modified MWCNT-embedded
DNA and DNA–CTMA composite thin films with varying [MWCNT–
COOH] by the drop-casting method. In order to verify the structural
and elemental analysis, XRD pattern and EDS spectra were
investigated. From the EDS spectra, we observed C, N, O, P, and
Na in DNA as well as MWCNT–COOH-embedded DNA thin film,
while C and O were observed in pristine MWCNT–COOH. Knowing
the chemical states and functional groups in composite thin films
was essential, so XPS spectra were evaluated. The negative shift in
binding energies with the addition of MWCNT–COOH in DNA
revealed charge transfer from MWCNT–COOH to DNA molecules
and appreciable changes in peak intensities indicating changes in
elemental composition and distribution of the chemical groups.
Raman and FTIR spectra provided significant information on
chemical interactions between MWCNT–COOH and the DNA
molecules. The stretching and vibrational modes of sugar and
phosphate backbone groups in DNA and DNA–CTMA were affected
by the addition of MWCNT–COOH due to the electrostatic
interaction and physical adsorption in composite thin films.
Finally, we discussed optical absorption and electrical characteristics. The absorption intensities increased with increasing
[MWCNT–COOH] and current measurements revealed semiconducting and metallic characteristics of the samples. From our
method, it may be possible to manipulate important physical
quantities and chemical characteristics through controlling
concentration of functionalized nanomaterials into DNA and
DNA–CTMA composite thin films.
Acknowledgements
This research was supported by grants from the Korea Research
Institute of Bioscience and Biotechnology (KRIBB) Research
Initiative Program and R&D Convergence Program of the National
Research Council of Science and Technology (NST) of Korea (CAP14-3-KRISS). In addition, the National Research Foundation (NRF)
of Korea supported this project (2016R1D1A1B03933768,
2017R1A2B4010955, and 2018R1A2B6008094).
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