Heptamethine Cyanine Dyes in the Design of Photoactive Carbon
Nanomaterials
Laura Rodríguez-Peŕez,† Carmen Villegas,† M. Ángeles Herranz,*,† Juan Luis Delgado,*,‡,§,∥

and Nazario Martín*,†,⊥

†Departamento de Química Orgańica, Facultad de Química, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040
Madrid, Spain
‡POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018 San Sebastian, Spain
§Faculty of Chemistry, University of the Basque Country UPV/EHU, P. Manuel Lardizabal 3, 20018 San Sebastian, Spain
∥Ikerbasque, Basque Foundation for Science, Maria Diaz de Haro 3, 6 solairua, 48013 Bilbao, Spain
⊥IMDEA-Nanociencia, c/Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain

*S Supporting Information

ABSTRACT: Near-infrared (NIR) absorbing nanomaterials,
built from anionic heptamethine cyanine dyes and single-walled
carbon nanotubes or few-layer graphene, are presented. The
covalent linkage, using 1,3-dipolar cycloaddition reactions, results
in nanoconjugates that synchronize the properties of both
materials, as demonstrated by an in-depth characterization study
carried out by transmission electron microscopy, atomic force
microscopy, thermogravimetric analysis, Fourier transform infra-
red spectroscopy, and X-ray photoelectron spectroscopy. UV−
vis−NIR and Raman spectroscopies further confirmed the unique
electronic structure of the novel photoactive nanomaterials.

■ INTRODUCTION

The field of carbon nanostructures has attracted tremendous
research interest over the last few years because of the
fascinating properties and potential applications of these
materials, which have stimulated scientists of different areas
of knowledge.1 In particular, fullerenes, carbon nanotubes, and
graphene have become extraordinarily popular nanomaterials
because of their unique mechanical, electronic, and thermal
properties.2 However, it is often desirable to further modify
their properties, and, in this sense, the use of a great variety of
chemical methodologies has allowed the fine-tuning of carbon
nanomaterial performance.3

The chemical modification of carbon nanotubes and
graphene is critical to improve their processability for any
practical application. Functionalization schemes that confer an
additional element of control over the nanomaterial properties
are particularly attractive, and, in this regard, the combination
of carbon nanostructures with photo- and electroactive
molecules or polymers attracts considerable attention for
their potential as electron-donor/-acceptor systems for artificial
photosynthesis,4 in photovoltaics,5 or as optical sensors.6

Interactions of carbon nanotubes and graphene with several
photo- and electroactive molecules/macromolecules that
operate in ultraviolet/visible regions of the spectrum, such as
anthracene,7 pyrene,8 tetrathiafulvalenes,9 polymers,10 and
other chromophores,11 have been studied. However, the near-
infrared (NIR) region, which constitutes a significant portion of

the solar spectrum, is very promising for fluorescence detection
and imaging and has been largely unexplored. In this regard,
only a few examples combine carbon nanotubes and graphene
with quantum dots12 and metalomacrocycles such as
porphyrins and phthalocyanines.13 In particular, porphyrins
are characterized by remarkably high extinction coefficients and
tailor-made carbon nanohybrids endowed with porphyrins offer
valuable new systems in the field of nano-optoelectronic devices
for energy conversion, sensing, and biological applications.14

Anionic heptamethine cyanines represent another interesting
class of ionic light harvester dyes showing an intense NIR
absorption that spans from 750 to 900 nm.15 Stimulated by the
aforementioned results on porphyrinoid systems, we succeeded
in preparing heptamethine cyanine molecular nanoconjugates
1−3 as NIR light harvesters in donor−acceptor systems (Figure
1).16−18 Here, we extend the methodology effectively used in
the functionalization of [60] and [70]fullerenes16 with
heptamethine cyanines to single-walled carbon nanotubes
(SWCNTs) and few-layer graphene (FLG) in an effort to
increase the light-absorption capabilities of these carbon
nanomaterials in the NIR region.

Received: October 6, 2017
Accepted: November 23, 2017
Published: December 22, 2017

Article

Cite This: ACS Omega 2017, 2, 9164−9170

© 2017 American Chemical Society 9164 DOI: 10.1021/acsomega.7b01499
ACS Omega 2017, 2, 9164−9170

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■ RESULTS AND DISCUSSION
Synthesis of Cyanine-Based Nanomaterials. The syn-

thesis of the SWCNT−cyanine (5) and FLG−cyanine (6)
nanoconjugates is described in Scheme 1. Formylcyanine 4 was

obtained following a previously reported procedure16a and
subsequently used in the 1,3-dipolar cycloaddition reaction19

with SWCNTs or FLG, and N-octylglycineamino acid
selected in an attempt to increase the solubility of these
nanocarbons. The commercial SWCNTs were used without
any treatment in the reaction, whereas FLG was obtained
through graphite exfoliation in ortho-dichlorobenzene (oDCB)
following Coleman’s procedure.20 Such a FLG suspension in
oDCB was immediately reacted.
The 1,3-dipolar cycloaddition reaction was carried out under

microwave irradiation in a closed quartz tube at 160 °C. After
cooling to room temperature, the solid was washed by filtration
over a 0.2 μm poly(tetrafluoroethylene) (PTFE) membrane

several times with oDCB, CH2Cl2, and MeOH (sonicated,
centrifuged, and filtered) until the filtrate solution remained
colorless, thus affording functionalized nanocarbons 5 and 6.
The obtained nanoconjugates were studied by a number of

characterization techniques, including thermogravimetric anal-
ysis (TGA), Fourier transform infrared (FTIR) spectroscopy,
Raman spectroscopy, X-ray photoelectron spectroscopy (XPS),
atomic force microscopy (AFM), transmission electron
microscopy (TEM), and UV−vis−NIR spectroscopy, to obtain
fully detailed information about the structural, electronic, and
chemical properties of the functionalized SWCNTs and FLG.

Structural Characterization. A first indication of the
covalent attachment of the heptamethine cyanine to SWCNTs
and FLG was obtained from TGA under an inert atmosphere
(Figures 2 and S1, respectively). As anticipated, considering the

different curvature and reactivity of the carbon nanostructures
investigated,21 a slightly higher degree of functionalization was
observed for SWCNTs (34%) as compared to that for FLG
(31%) considering the mass loss of the materials at 650 °C.
From this data, we estimated 1 functional group per 140 carbon
atoms in the case of nanoconjugate 5 and 1 functional group
per 171 carbon atoms in the case of nanoconjugate 6. As
expected, 5 and 6 maintain the decomposition pattern of the
heptamethine cyanine, although with a small increase of the
temperature decomposition maxima (see derivate curves in
Figures 2 and S1), which indicates a thermal stabilization of the
whole system due to the covalent linkage of heptamethine
cyanine to SWCNTs or FLG.
Additional support of the 1,3-dipolar cycloaddition reaction

taking place on SWCNTs and FLG was obtained by FTIR
(Figures 3 and S2). The appearance of the stretching vibrations
of the cyano groups at 2221 cm−1 for 5 and at 2218 cm−1 for 6
corroborates the existence of these groups on the surfaces of
the nanoconjugates formed. Furthermore, the existence of
other characteristic bands of heptamethine cyanine, such as
those at ca. 1100−1096 cm−1, confirms the presence of
heptamethine cyanine molecules in nanomaterials 5 and 6.
XPS provided a semiquantitative analysis of the elements

found on the surface of 5 and 6, giving, in addition, information
about the relative abundance of those elements within the
material surface (Figures 4 and S3).22 Interestingly, in the XPS
survey of nanocarbons 5 and 6, in addition to the core-level
contributions of C 1s at 284.6 eV and O 1s at 532.6 eV,

Figure 1. Cationic heptamethine cyanine-[60]fullerene 1,16b anionic
heptamethine cyanine-[60]fullerene 2,16a and panchromatic light
harvester 3.18

Scheme 1. Synthetic Approach Used for the Preparation of
Carbon Nanomaterials 5 and 6

Figure 2. TGA analysis and first derivate curves of 5 (blue), 4 (green),
and HiPco SWCNTs (black), recorded under nitrogen atmosphere.

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photoelectrons collected from the N 1s core level are also
observed at 399.5 eV.
The XPS C 1s core-level spectrum of 6 can be deconvoluted

into five peak components with binding energies of 284.4,
285.1, 285.8, 287.2, and 289.5 eV, which, similar to previous
reports on covalently functionalized carbon nanomaterials,23

are assignable to CC, C−C, C−N, C−O, and CO bonds,
respectively (Figure 4). For 5, the same contributions to the C
1s core-level spectrum are found (Figure S3).
The analysis of the high-resolution N 1s core-level spectra of

5 and 6 furnished three different contributions. The first
deconvoluted peak, observed at 404.3−403.5 eV, could be
attributed to the quaternary ammonium salts, and the other two
contributions found at lower binding energies could be
attributed to the different types of cyano groups of the
heptamethine cyanine core, which have slightly different
binding energies, in the range from 399.5 to 402.4 eV, because
they are differently involved in the stabilization of the negative
charge density of the dye.24

Considered as a whole, XPS analysis of nanomaterials 5 and
6 demonstrates the presence of different nitrogenated func-
tional groups (cyano, ammonium salts, and pyrrolidines) and,

ultimately, the covalent linkage of heptamethine cyanine to
SWCNTs and FLG.
To complement the characterization of nanoconjugates 5

and 6, AFM and TEM investigations were employed to study
their morphologies.25

From TEM, the disaggregation and dispersion of SWCNTs
induced through covalent functionalization in nanoconjugates
5, when compared with a sample of commercial HiPco
SWCNTs, are clearly observed (Figure S4). Furthermore,
AFM reveals topographic images of commercial HiPco
SWCNTs that present most of the SWCNTs stacked in
aggregates of diameters up to 16 nm, whereas in nanomaterial
5, smaller aggregates, with diameters of 2−6 nm and typical
lengths that range from 500 nm to 2 μm, are observed (Figure
5).

Similar to previous results,26 graphite exfoliation in oDCB
showed FLG nanosheets of less than five layers stacked with
smaller flakes on their surface. The reaggregation of graphene
flakes prior to functionalization was, to some extent, prevented
by keeping the sample in solution because TEM analysis of
nanoconjugate 6 reveals a nanomaterial that is uniformly
disintegrated and with regular flakes that are randomly stacked
onto each other (Figure 6).

Electronic Properties. Raman spectroscopy provides
evidence of the covalent functionalization of SWCNTs and
FLG at the time that allows to obtain valuable information
about the structural and electronic characteristics of the
nanomaterials.27

The Raman spectrum of nanoconjugate 5 at 785 nm
excitation is characteristic of a SWCNT-based nanomaterial,
showing radial breathing modes (RBMs) between 170 and 310
cm−1, the D band, attributed to the disorder of the carbon
hexagonal lattice on the SWCNT sidewalls (sp3 carbons) at
1293 cm−1 and, the G mode or manual torque-tangential mode,
which corresponds to the stretching mode in the graphite (sp2

carbons), at 1593 cm−1 (Figure 7a). On the basis of the
comparison with the starting SWCNTs, several important
conclusions are made: (i) RBM resonance signals reveal a
pattern similar to that of the reference material and thus the
reaction proceeds without preference for metallic or semi-
conducting tubes;28 (ii) the D band increases its intensity, ID/IG
= 0.1 for 5 versus ID/IG = 0.06 for the starting SWCNTs, which

Figure 3. FTIR spectra of 6 (blue) relative to 4 (green) and FLG
(black).

Figure 4. XPS analysis of FLG−cyanine nanoconjugates 6. The inserts
show the C 1s and N 1s core-level spectra deconvolution.

Figure 5. AFM images of: (a) commercial HiPco SWCNTs and (b, c)
nanoconjugate 5. On the right, the height profile of each sample is
displayed.

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proves that the functionalization occurs through covalent
bonding with the sidewall carbon atoms, which convert some
sp2 bonding into sp3;29 and (iii) no appreciable shifts are
observed in the G or G′ modes of nanoconjugate 5 versus the
starting SWCNTs and therefore a negligible electronic

interaction seems to be operating in the ground state between
heptamethine cyanine and SWCNTs.30

For nanoconjugate 6, the typical Raman bands of exfoliated
graphite, namely, the D band at 1347 cm−1, the G band at 1578
cm−1, and the 2D band at 2694 cm−1, are observed by
excitation at 532 nm.31 Upon the formation of nanoconjugate
6, the intensity of the D band increases compared to that for
the exfoliated FLG because of the rehybridization of the carbon
atoms from sp2 to sp3 (Figure 7b). Thus, the ID/IG ratio
increases from 0.25 for FLG to 0.46 for 6, indicating the
covalent anchoring of the heptamethine cyanine moiety to
FLG. The 2D bands of both the exfoliated FLG and
nanoconjugate 6 are highly symmetric and could be fitted by
a single Lorentzian, which allows to determine the presence of
FLG of around three sheets in the Raman study.32 Moreover,
the G band, which is very sensitive to doping effects, does not
experience any shift from exfoliated FLG to nanoconjugate 6,
which, similar to the Raman analysis of 5, indicates a weak or
inexistent ground-state interaction between FLG and the dye.
Finally, the UV−vis−NIR analysis of 5 and 6, as well as that

of formylcyanine 4, permits to corroborate the NIR absorption
increase of FLG and SWCNTs with the covalent anchoring of
heptamethine cyanine (Figures 8 and S5). All of the materials
display the characteristic absorption peak of the dye at ca. 896
nm. For nanoconjugate 5, the absorption spectrum reflects the
loss of the van Hove singularities of SWCNTs, an observation

Figure 6. TEM images of nanoconjugate 6. (a) Scale bar 200 nm and (b) scale bar 10 nm.

Figure 7. (a) Raman spectra of commercial HiPco SWCNTs (black)
and 5 (blue) recorded under 785 nm excitation wavelength. The inset
shows a comparison of D band intensities. (b) Raman spectra of 6
(blue) and exfoliated FLG (black) in oDCB under 532 nm excitation
wavelength. The inset shows the deconvolution of the two-
dimensional (2D) FLG band.

Figure 8. UV−vis−NIR spectra of 5 (blue) relative to 4 (green) and
commercial HiPco SWCNTs (black) in oDCB.

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that once again supports the covalent attachment of the
heptamethine cyanine dye to the sidewalls of SWCNTs.33

■ CONCLUSIONS

In summary, we have carried out the synthesis of a new type of
NIR absorbing nanomaterials, anionic heptamethine cyanine-
based SWCNTs (5) and FLG (6) nanoconjugates, using 1,3-
dipolar cycloaddition reactions. The systematic and meticulous
analyses through thermogravimetric analysis (TGA), FTIR
spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic
force microscopy (AFM), and transmission electron micros-
copy (TEM) unambiguously confirmed the covalent attach-
ment of the heptamethine cyanine moiety to the nanocarbons.
Furthermore, the electronic characteristics of the nano-
conjugates have been investigated considering Raman and
UV−vis−NIR spectroscopies, which also confirmed the
covalent linkage between the electroactive species. The new
hybrid nanomaterials reveal appealing absorbance in the NIR
region, thus paving the way to a variety of optoelectronic
applications.

■ EXPERIMENTAL SECTION

Materials. HiPco SWCNTs were purchased from Carbon
Nanotechnologies (lot: P0261, purity > 82, <18% remaining
iron particles, length = 100−1000 nm, diameter = 0.8−1.4 nm)
and used without any further purification treatment. Graphite
from TIMCAL (TIMREX SFG15, ρ = 2.26 g/cm3, particle size
= 8.80 μm, specific surface = 9.50 m2/g, ashes ≤ 0.100%,
interlamellar distance = 3.354−3.358 Å) was used for the
synthesis of few-layer graphene (FLG). Graphite flakes (200
mg) were dispersed in anhydrous oDCB (200 mL) and
sonicated in a low-power sonication bath for 150 min under an
inert atmosphere to obtain FLG. The dispersion was
centrifuged at 500 rpm for 45 min, and the supernatant was
decanted and stored in solution.
Organic solvents and reagents used in this work were

purchased from commercial suppliers and used as received,
unless stated otherwise. Formylcyanine 4 (Scheme 1) was
synthesized following the previously reported method.16a

Instruments. TGA analyses were carried out under air and
nitrogen in a TA-TGA-Q500 apparatus. The sample (∼0.5 mg)
was introduced inside a platinum crucible and equilibrated at
100 °C, followed by a 10 °C min−1 ramp between 100 and
1000 °C and by an isotherm of 30 min. FTIR spectra were
recorded in Bruker TENSOR 27 using a spectral range of
4000−400 cm−1, with a resolution of 1 cm−1, and in pellets of
dispersed samples of the corresponding materials in dried KBr.
Raman spectra were recorded on Renishaw inVia (SWCNTs)
or NT-MDT (FLG) microscopes at room temperature and at
785 or 532 nm wavelength, respectively. XPS analyses were
carried out using a SPECS GmbH (PHOIBOS 150 9MCD)
spectrometer operating in the constant analyzer energy mode.
A nonmonochromatic aluminum X-ray source (1486.61 eV)
was used with a power of 200 W and voltage of 12 kV. Pass
energies of 75 and 25 eV were used for acquiring both survey
and high-resolution spectra, respectively. Survey data were
acquired from kinetic energies of 1487−400 eV with an energy
step of 1 eV and 100 ms dwell time per point. The high-
resolution scans were taken around the emission lines of
interest with 0.1 eV steps and 100 ms dwell time per point.
SpecsLab version 2.48 software was used for spectrometer
control and data handling. The semiquantitative analysis was

performed from the C 1s (284.3 eV) signal. The samples were
introduced as pellets of 8 mm diameter. UV−vis−NIR spectra
were recorded in a UV-3600 Shimadzu spectrophotometer.
TEM micrographs were obtained using a JEOL 2100
microscope operating at 200 kV. The samples were dispersed
in oDCB and dropped onto a holey carbon copper grid (200
mesh), and the solvent was removed in a vacuum oven for 48 h.
AFM was performed on an SPM Nanoscope IIIa multimode
microscope working in tapping mode with an RTESPSS tip
(Veeco) at a working frequency of ∼235 kHz. The samples
were prepared by spin-coating on mica.

Synthesis of SWCNT−Cyanine Nanoconjugate 5.
Formylcyanine 4 (20 mg) and N-octylglycine (20 mg) were
added to pristine SWCNTs (20 mg) suspended in oDCB (10
mL). The mixture was sonicated for 10 min and heated under
microwave irradiation at 160 °C for 1 h. The final product was
separated from the reaction mixture by filtration over a
poly(tetrafluoroethylene) (PTFE) membrane (0.2 μm). The
black solid was purified by successive washing in membrane
with oDCB, CH2Cl2, and MeOH. FTIR (KBr): ν = 2922, 2854,
2221, 1718, 1659, 1601, 1422 (broad), 1260, 1096, 1021, and
800 cm−1. TGA: weight loss and temperature desorption
(organic anchoring groups): 34.3%, 650 °C. Raman: ID/IG =
0.1. XPS: % atomic: C (284.6 eV) = 93.4, O (532.6 eV) = 5.5,
and N (399.5 eV) = 1.1. UV−vis−NIR (oDCB) λmax: 896 nm.

Synthesis of FLG−Cyanine Nanoconjugates 6. The
exfoliated FLG in dry oDCB was utilized as produced for
further covalent modification. To a suspension of 25 mL of
FLG in oDCB were added formylcyanine 4 (5 mg) and N-
octylglycine (5 mg). The reaction mixture was submitted for 12
microwave cycles following the sequence specified in Table 1.

After cooling to room temperature, the resulting modified FLG
was separated from the reaction mixture by filtration over a
PTFE membrane (0.2 μm). The black solid was purified by
subsequent washing in membrane with oDCB, CH2Cl2, and
MeOH. FTIR (KBr): ν = 2924, 2854, 2218, 1726, 1641, 1581,
1460, 1433, 1157, 1100, and 800 cm−1. TGA: weight loss and
temperature desorption (organic anchoring groups): 20.3%,
400 °C; 10.8%, 650 °C. Raman: ID/IG = 0.45. XPS: % atomic:
C (284.6 eV) = 81.0, O (532.6 eV) = 18.0, and N (399.5 eV) =
1.0. UV−vis−NIR (oDCB) λmax: 896 nm.

■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsomega.7b01499.

Additional TGA and first derivative curves; FTIR
spectra; XPS analysis; TEM images; and UV−vis−NIR
spectra (PDF)

■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: maherran@ucm.es (M.Á.H.).
*E-mail: juanluis.delgado@polymat.eu (J.L.D.).

Table 1. Irradiation Steps Used with the Microwave Reactor

step T (°C) hold time

1 130 1
2 160 5
3 50 1

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mailto:maherran@ucm.es
mailto:juanluis.delgado@polymat.eu
http://dx.doi.org/10.1021/acsomega.7b01499


*E-mail: nazmar@ucm.es (N.M.).
ORCID
M. Ángeles Herranz: 0000-0001-9155-134X
Juan Luis Delgado: 0000-0002-6948-8062
Nazario Martín: 0000-0002-5355-1477
Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
Financial support from the European Research Council (ERC-
320441-Chiralcarbon), the Ministerio de Economiá y Com-
petitividad (MINECO) of Spain (Projects CTQ2014-52045-R,
CTQ2015-71936-REDT, and CTQ2015-70921-P), and the
CAM (PHOTOCARBON project S2013/MIT-2841) is
acknowledged. J.L.D. thanks Ikerbasque, the Basque Founda-
tion for Science, for an “Ikerbasque Research Fellow” contract,
Polymat Foundation, MINECO of Spain, for IEDI-2015-00666
grant and Iberdrola Foundation for financial support.

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ACS Omega Article

DOI: 10.1021/acsomega.7b01499
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http://orcid.org/0000-0001-9155-134X
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http://dx.doi.org/10.1021/acsomega.7b01499


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ACS Omega Article

DOI: 10.1021/acsomega.7b01499
ACS Omega 2017, 2, 9164−9170

9170

http://dx.doi.org/10.1021/acsomega.7b01499