Photophysical properties of zinc phthalocyanine-uridine single walled carbon nanotube - Conjugates

Article abstract of DOI:10.1016/j.saa.2015.04.040

The photophysical properties of the conjugate of uridine and zinc mono carboxy phenoxy phthalocyanine (ZnMCPPc-uridine, 4) are reported in this work. The conjugate was also adsorbed onto single walled carbon nanotubes (ZnMCPPc-uridine-SWCNT, 5). The X-ray photoelectron spectroscopy of 4 showed three N 1s peaks while that of 5 showed four N 1s peak, a new peak at 399.4 eV of 5 was assigned to pyrrolidonic nitrogen, due to the interaction of the pyrrolic nitrogen of 4 with the oxygen moiety of SWCNT-COOH in 5. The triplet lifetime, triplet and singlet oxygen quantum yields of the zinc mono carboxy phenoxy phthalocyanine increased by over 40% in the presence of uridine. SWCNTs resulted in only a small quenching of the triplet state parameters of 4.

Full text of DOI:10.1016/j.saa.2015.04.040

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical properties of zinc phthalocyanine–uridine single walled
carbon nanotube – conjugates
⇑
Racheal O. Ogbodu, Edith K. Amuhaya, Philani Mashazi, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The conjugate between uridine and zinc mono carboxy phenoxy phthalocyanine showed improved triplet
lifetime, triplet and singlet oxygen quantum yields compared to the phthalocyanine alone, and these val-
ues were not drastically reduced by adsorption onto single walled carbon nanotubes.
ꢀ
The photophysical properties of
uridine–zinc mono phenoxy carboxy
phthalocyanine are reported.
ꢀ
ꢀ
The conjugate was also adsorbed onto
single walled carbon nanotubes.
The triplet lifetime, triplet and singlet
oxygen quantum yields increased by
over 40% in the presence of uridine.
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical properties of the conjugate of uridine and zinc mono carboxy phenoxy phthalocyanine
(ZnMCPPc–uridine, 4) are reported in this work. The conjugate was also adsorbed onto single walled
carbon nanotubes (ZnMCPPc–uridine–SWCNT, 5). The X-ray photoelectron spectroscopy of 4 showed
three N 1s peaks while that of 5 showed four N 1s peak, a new peak at 399.4 eV of 5 was assigned to
pyrrolidonic nitrogen, due to the interaction of the pyrrolic nitrogen of 4 with the oxygen moiety of
SWCNT–COOH in 5. The triplet lifetime, triplet and singlet oxygen quantum yields of the zinc mono
carboxy phenoxy phthalocyanine increased by over 40% in the presence of uridine. SWCNTs resulted in
only a small quenching of the triplet state parameters of 4.
Received 2 December 2014
Received in revised form 14 April 2015
Accepted 20 April 2015
Available online 24 April 2015
Keywords:
Uridine
Carboxy phenoxy phthalocyanine
Triplet quantum yields
Singlet oxygen quantum yields and triplet
lifetime
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
which is the chief cytotoxic species responsible for cancer cell
death [1–4]. There are several MPcs that are currently in different
Metallophthalocyanines (MPcs) are good photosensitizers for
photodynamic therapy (PDT); this is in part due to their low
toxicity in the dark, as well as excellent photophysical properties,
especially their efficiency in the generation of singlet oxygen,
stages of clinical trials [1,5] for PDT. However, selectivity and
specificity of the photosensitizers to the tumor cells remain a
challenge. One of the developing strategies for PDT is to conjugate
photosensitizers to biomolecules (such as nucleosides) for biocom-
patibility and cancer specificity. A silicon(IV) phthalocyanine con-
jugated axially with different nucleoside moieties has been
reported [6]. The complex had high singlet oxygen quantum yields
⇑
Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
http://dx.doi.org/10.1016/j.saa.2015.04.040
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

232
R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
and it exhibited a high in vitro photodynamic activity [6]. A zinc
phthalocyanine ring substituted with nucleoside (adenine
nanotubes (SWCNTs). SWCNTs are nanomaterials with a hollow
and cage like interior through which they can anchor drugs.
SWCNTs have high aspect ratio and large surface area [14–16].
Functionalized SWCNTs have the advantage of moving across cel-
lular membranes with less cytotoxicity when compared to the
unfunctionalized SWCNTs [16]. Pcs are electron-donating com-
plexes due to their strong aromatic system while SWCNTs have
the ability to accept electrons when interfaced with an electron
donor via functionalization [17]. This is expected to result in
quenching of the photophysical properties. However, work done
recently by our group indicated that the triplet and singlet oxygen
quantum yields of MPcs increases or decreases in the presence of
SWCNTs depending on the ring substituent [18,19]. The effects of
SWCNTs on the photophysical properties of MPc–uridine conjugate
are examined. Conjugate 4 was adsorbed onto SWCNTs repre-
sented as ZnMCPPc–uridine–SWCNT (5). The current study shows
a
derivative) was reported by Li and Ng [7], but no photochemical
or photophysical behavior was reported. Herein, we report for
the first time on the photophysical properties: triplet lifetimes,
and triplet and singlet oxygen quantum yields of the conjugate of
uridine (a nucleoside) with zinc mono carboxy phenoxy phthalo-
cyanine (represented as ZnMCPPc–uridine, 4, Scheme 1). ZnPc
was chosen because its derivatives are in clinical trials [1].
Uridine has been shown to have anticancer properties [8–12].
Studies by Haakensen et al. [13] showed that uridine 50-diphos-
pho-glucuronosyltransferase is over expressed in breast cancer.
Thus, the linking of a uridine to phthalocyanine will provide speci-
ficity and selectivity of MPc to cancer cells.
The PDT effects of MPcs can be further improved by adsorption
onto drug delivery agents [14,15], such as single walled carbon
(A)
(B)
Scheme 1. Synthetic routes for the formation of conjugate 4 (A) and 5 (B).

R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
233
improved triplet lifetimes, and triplet and singlet oxygen quantum
yields when uridine is linked to ZnMCPPc. The triplet lifetimes, and
triplet and singlet oxygen quantum yields of ZnMCPPc–uridine
complex are reported alone and when immobilized onto SWCNT
(Scheme 1B).
of all the complexes in DMSO, details have been described before
[24]. The singlet oxygen phosphorescence signal was compared
with that of ZnPc standard.
Fluorescence lifetimes were measured with a FluoTime 300
‘EasyTau’ spectrometer (PicoQuant GmbH) using a time correlated
single photon counting (TCSPC) technique. The samples were
excited at 670 nm with a diode laser (LDH-P-670, 20 MHz repeti-
tion rate, 44 ps pulse width, PicoQuant GmbH). The detector
employed was a Peltier cooled Photomultiplier (PMA-C 192-M,
PicoQuant GmbH).
Experimental
Materials
Dimethyl sulfoxide (DMSO), chloroform, hexane, N,N0-dicyclo-
hexylcarbodiimide (DCC), dimethylformamide (DMF) and tetrahy-
drofuran (THF) were obtained from SAARCHEM. Zinc
phthalocyanine (ZnPc), uridine, dimethylaminopyridine (DMAP),
trifluoroacetic acid (TFA), triethylamine and Sephadex LH-20
microbeads were obtained from Sigma Aldrich. Single-walled car-
Synthesis
The synthesis of 20,30-O-isopropylideneuridine (2, Scheme 1A)
has been reported before [25].
bon nanotubes (SWCNT–COOH, 1–5 nm in diameter and 1–5
l
m
Synthesis of ZnMCPPc–uridine (4) Scheme 1A
in length) were obtained from Nanolab. ZnMCPPc (1) was synthe-
sized according to literature methods [20].
A mixture of ZnMCPPc (1) (0.2 g, 0.28 mmol) and DCC (0.06 g
0.59 mmol) dissolved in 4 ml of DMF was stirred for 24 h under
argon at room temperature. DMAP (0.01 g, 0.08 mmol) was then
added and the reaction mixture was allowed to stir for 24 h.
20,30-O-isopropylideneuridine (2) (0.07 g, 0.30 mmol) was added
and the mixture was further stirred for 72 h. The reaction mixture
Equipment
Thermal gravimetric analyses (TGA) were recorded on
a
Shimadzu DTG-TG 60H with a gas flow of 120 ml/min and operated
under a nitrogen atmosphere at 10 °C /min.
X-ray powder diffraction (XRD) patterns were recorded on a
Bruker D8 Discover XRD equipped with a Lynx Eye Detector, using
Cu K radiation (=1.5405 Å, nickel filter). Details have been previ-
a
ously reported [21,22]. Raman spectra were obtained with a
Bruker Vertex 70
– Ram II spectrometer (equipped with a
1064 nm Nd:YAG laser and liquid nitrogen cooled germanium
detector). Solid samples diluted with KBr were used.
Transmission electron microscopy (TEM) images were obtained
using a Zeiss Libra TEM 120 model operated at 90 kV. Infrared
spectra were recorded on
a Perkin–Elmer Universal ATR
Sampling accessory spectrum 100 FT-IR spectrometer. The elemen-
tal micro analysis was measured using
Microcube ELIII.
a Vario-Elementar
Mass spectral data were collected with a Bruker AutoFLEX III
Smartbeam TOF/TOF mass spectrometer. The instrument was oper-
ated in positive ion mode using a m/z range of 400–3000 amu, as
described before [22], using alphacyano-4-hydroxycinnamic acid
as the MALDI matrix.
X-ray photoelectron spectroscopy (XPS) was done using a
Kratos Axis Ultra DLD, using an Al (monochromatic) anode,
equipped with charge neutralizer and the operating pressure kept
below 5 ꢁ 10ꢂ9 torr. For wide/survey XPS scans, the following
parameters were used: emission current was kept at 10 mA and
the anode (HT) voltage at 15 kV as described in detail before [23].
Absorption spectra were recorded on a Shimadzu UV–Vis 2550
spectrophotometer and fluorescence emission and excitation
spectra on Varian Eclipse spectrofluorimeter using
a 360–
1100 nm filter. Laser flash photolysis experiments were performed
to determine the triplet decay kinetics. The excitation pulses were
produced by a tunable laser system consisting of a Nd:YAG laser
(355 nm, 135 mJ/4–6 ns) pumping an optical parametric oscillator
(OPO, 30 mJ/3–5 ns) with a wavelength range of 420–2300 nm
(NT-342B, Ekspla). The details have been reported [22]. Solutions
for triplet state studies were de-aerated with argon for 15 min
before measurement. The absorbance of the solution at the Q-band
was 1.5 for these studies. Triplet lifetimes were determined by
exponential fitting of the kinetic curves using Origin Pro 8
software.
Fig. 1. FTIR Spectra of (A); 20,30-O-isopropylideneuridine, 2 (a), ZnMCPPc–20,30-O-
isopropylideneuridine, 3 (b), ZnMCPPc–uridine, 4 (c) and ZnMCPPc, 1 (d) and (B);
ZnMCPPc–uridine, 4 (a), ZnMCPPc–uridine–SWCNT, 5 (b) and SWCNTs (c).
The time resolved phosphorescence of singlet oxygen at
1270 nm was used to determine the singlet oxygen quantum yields

234
R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
Fig. 3. TGA profile of ZnMCPPc–uridine, 4 (a), SWCNT–COOH (b) and ZnMCPPc–
uridine–SWCNT, 5 (c).
Fig. 2. Absorption spectra of ZnMCPPc–uridine–SWCNT, 5 (a), SWCNT–COOH (b),
ZnMCPPc, 1 (c) and ZnMCPPc–uridine, 4 (d) in DMSO.
8.00 (m, 7H, Ar-H), 7.75–7.73 (m, 8H, Ar-H), 7.49 (d, 2H, Ar-H),
7.20 (d, 1H, H-3), 5.79 (d, 1H, H-2), 5.64 (d, 1H, H-10) 5.39 (d, 1H,
H-20), 5.12 (d, 1H, H-30), 4.14 (d, 1H, H-40), 3.96 (m, 2H, H-50).
was poured into 40 ml of distilled water and extracted with 40 ml
of chloroform three times. The organic layers were combined and
dried in vacuo. The solid product (ZnMCPPc–20,30-O-isopropyli-
deneuridine (3)) was purified by silica gel column chromatography
using the solvent mixture of THF and hexane (95:5 v/v) as eluent.
ZnMCPPc–20,30-O-isopropylideneuridine (3): Yield 51% (0.14 g);
Anal. Calc. for C51H34N10O8Zn: C, 62.49; H, 3.50; N, 14.29. Found: C,
62.00; H, 3.15; N, 14.04%. MALDI-TOFMS (m/z): 980 g/mol, Found:
981 [M+H]+; IR [(ATR) vmax/cmꢂ1]: 3322 (OH str.), 2927 and 2851
(CH str.), 1773, 1713 and 1625 (C@O), 1228 (OH bend), 1088 and
1052 (CAOAC); 1H NMR (600 MHz, DMSO-d6): 11.30 (s, 1H, NH),
8.15 (d, 2H, Ar-H), 8.10–8.00 (m, 7H, Ar-H), 7.75–7.73 (m, 8H, Ar-
H), 7.49 (d, 2H, Ar-H), 7.15 (d, 1H, H-3), 5.58 (d, 1H, H-2), 5.57
(d, 1H, H-10) 4.84 (d, 1H, H-20), 4.69 (d, 1H, H-30), 4.01 (d, 1H, H-
40), 3.52 (m, 2H, H-50), 1.43 (s, 3H, CH3), 1.23 (s, 3H, CH3).
To de-protect complex 3, 0.1 g (0.1 mmol) was dispersed in 2 ml
of distilled water; this was followed by the slow addition of 4 ml of
50% aqueous trifluoroacetic acid (TFA) at 0 °C. The reaction mixture
was allowed to warm up to room temperature and stirred for 2 h as
described in literature [25]. The de-protected complex was
obtained by extracting the reaction mixture using 4 ml of chloro-
form three times. The organic fractions were combined and dried
in vacuo to afford complex 4. The solid product was purified by sil-
ica gel column chromatography using the solvent mixture of THF
and hexane (95:5 v/v) as eluent.
Adsorption of 4 onto SWCNT to form ZnMCPPc–uridine–SWCNT (5)
ZnMCPPc–uridine (4) was adsorbed on SWCNT–COOH accord-
ing to literature methods for adsorbing of other MPc [26] with
slight modification as follows: 10 mg of SWCNT–COOH were ultra-
sonicated for 30 min in 3 ml of DMF to give a brown colored col-
loidal solution. ZnMCPPc–uridine (4) (0.02 g, 0.02 mmol) was
then added to the solution to give a blue suspension and the result-
ing mixture was stirred for 5 days to give a dark blue colloidal solu-
tion indicating the adsorption of 4 onto SWCNT–COOH. The
reaction mixture was poured into 20 ml of distilled water and
was extracted using chloroform three times. The chloroform frac-
tion was allowed to dry in vacuo, and was washed with methanol
and the solid product was obtained by centrifugation. The final
product was purified with size exclusion chromatography using
the solvent mixture of THF and hexane (95:5 v/v). The hybrid
of ZnMCPPc–uridine–SWCNT (5) was larger than
4
and
ZnMCPPc–uridine (4): Yield 73% (0.07 g), UV–Vis (DMSO) kmax
(nm) (log
e): 674 (5.5), 608 (4.7), 349 (5.0). Anal. Calc. for
C48H30N10O8Zn: C, 61.32; H, 3.22; N, 14.90. Found: C, 61.53; H,
3.32; N, 13.32%. MALDI-TOFMS (m/z): 940 g/mol, Found: 940
[M]+. IR [(ATR) vmax/cmꢂ1]; 3058–2918 (OH str.), 1687 and 1596
(C@O), 1224 (OH bend), 1085 and 1048 (CAOAC); 1H NMR
(600 MHz, DMSO-d6): 11.30 (s, 1H, NH), 8.15 (d, 2H, Ar-H), 8.10–
Table 1
Photophysical parameters of complex 1, 4 and 5 in DMSO.
b
Compound
akabs kEm UD sT UT
S
D
UF
sF
(ns)
(nm) (nm) 0.01
(l
s) 0.02
0.01 0.01
ZnMCPPc, 1
ZnMCPPc–uridine, 4
675 685 0.20 202 0.35 0.57 0.18 3.00
674 683 0.49 447 0.62 0.79 0.21 3.04
ZnMCPPc–uridine–SWCNT 674 684 0.44 347 0.50 0.88 0.19 3.11
adsorbed, 5
a
kQ = absorption maxima.
kEm = emission maxima.
Fig. 4. XRD pattern of SWCNT–COOH (a), ZnMCPPc–uridine–SWCNT, 5 (b) and
ZnMCPPc–uridine, 4 (c).
b

R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
235
SWCNT–COOH only, and was excluded first followed by un-bonded
SWCNT–COOH and 4.
ZnMCPPc–uridine–SWCNT (5): UV–Vis (DMSO): kmax nm 674,
IR [(ATR) vmax/cmꢂ1]; 3397–3165 (OH str.), 1597 (C@O), 1230
(OH bend), 1087 and 1045 (CAOAC); [Raman vmax/cmꢂ1]: 2551
(G⁄), 1589 (G), 1284 (D).
Photophysical and photochemical parameters
Triplet (UT) and fluorescence (UF) quantum yields for conjugate
4 and 5 were determined using the comparative methods as
described before [27–30]. ZnPc in DMSO was employed as a stan-
dard for triplet quantum yield (UT = 0.65 [27] and fluorescence
quantum yield (UF = 0.20 [30]). Singlet oxygen (UD) was deter-
mined using the time resolved phosphorescence decay curve at
1270 nm as described before [24,31] using ZnPc as standard for
singlet oxygen quantum yield (UD = 0.67 [32]).
Results and discussion
Fig. 6. Wide-scan X-ray photoelectron spectra of ZnMCPPc–uridine–SWCNT, 5 (a),
ZnMCPPc–uridine, 4 (b) and SWCNT–COOH (c).
Synthesis and characterization
Conjugate 4 was synthesized by first protecting uridine. This
was done to ensure that only one hydroxyl group was available
for reaction. Uridine was treated with anhydrous acetone and
H2SO4 to give the protected group (20,30-O-isopropylideneuridine
(2)). An esterification reaction between 2 and 1 (in the presence
of DCC in order to activate the carboxyl group of 1), afforded 3,
which was confirmed by FTIR, elemental analysis, 1H NMR and
mass spectroscopy. The de-protection of 3 was achieved by treat-
ing the conjugate with aqueous TFA to give 4, Scheme 1A. There
was a general shift in the 1H NMR peaks of uridine to lower fields
due to de-shielding effect of the aromatic ring of the MPcs.
Scheme 1B shows the route for the formation of ZnMCPPc–
Table 2
Surface composition for conjugate 4, 5 and SWCNT–COOH as determined by X-ray
photoelectron spectroscopy. Binding energy position in brackets.
Compound
Atomic concentration (%)
C (1s)
O (1s)
N (1s)
Zn (2p)
ZnMCPPc–uridine, 4
81.83
(282)
83.89
(283)
84.23
(284)
6.67
(530)
5.86
(531)
15.77
(532)
10.43
(396)
9.26
(397)
Not found Not found
1.01
(1019)
0.99
ZnMCPPc–uridine–SWCNT
adsorbed, 5
SWCNT–COOH
(1019)
uridine–SWCNT (5) by adsorption (via
p–p stacking interactions)
of ZnMCPPc–uridine (4) onto SWCNT–COOH. Pcs are highly aro-
matic in nature and are known [33] to interact strongly with the
FTIR spectra
carbon nanotubes (CNTs) by p–p stacking, hence this type of inter-
The presence of ester bond in 3 was confirmed using FTIR.
Fig. 1A (b) shows the spectrum of 3 which exhibited an intense
CH stretch at 2927 and 2851 cmꢂ1, C@O peak at 1713 and
1625 cmꢂ1 and CAOAC stretch at 1088 and 1052 cmꢂ1. These
peaks were at different wave-numbers compared to either 1 or 2
(Fig. 1A (d) and (a) respectively). The decrease in the intensity of
CH stretch of 3 at 2927 and 2851 cmꢂ1 (Fig. 1A (b)), the shift in
the broad OH peaks at 3322 cmꢂ1 to 3058 and 2918 cmꢂ1 for 4
(Fig. 1A (c)) and the appearance of the C@O peaks at different
wave-numbers of 1687 and 1596 cmꢂ1 in 4 (Fig. 1A (c)) compared
to 3, confirm the de-protection of the two hydroxyl groups of 3 to
form conjugate 4.
action is expected in this work. There was a color change from blue
to dark-blue upon adsorption, and the presence of SWCNTs was
confirmed using combination of techniques such as, FTIR, XRD,
XPS, TEM, UV–Vis spectrophotometer and Raman spectroscopy.
The adsorption of 4 to SWCNT–COOH was also confirmed using
FTIR, Fig. 1B (b) shows the FTIR spectra of ZnMCPPc–uridine–
SWCNTs 5, the spectra exhibited both the characteristics peaks of
4 and SWCNT–COOH.
UV–Vis spectra
The UV–Vis absorption spectra of the conjugates (4,5) in DMSO
are represented in Fig. 2 and compared to MPc alone (1). There was
negligible change in the Q-band maxima of 1 in the presence of uri-
dine, Table 1. However, on adsorption of 4 onto SWCNTs (Fig. 2a) to
form 5, there was a broadening of the Q-band and an enhancement
in the B-band intensity due to absorption of SWCNTs. Several
reports have shown that when Pcs or porphyrins are adsorbed onto
Fig. 5. Raman spectra of ZnMCPPc–uridine–SWCNT, 5 (a) and SWCNT–COOH (b). G⁄
is the non-dispersive phonon mode, G is the tangential vibrational mode (sp2) and D
is the breathing mode; disorder band (sp3).

236
R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
Thermogravimetric analysis (TGA)
A
Thermogravimetric analysis was used to determine the thermal
decay profiles of the complexes which points to different structural
material. The thermal decomposition profiles were obtained under
a steady flow of N2, at a heating rate of 10 °C minꢂ1. The thermo-
gravimetric plots for 4, 5 and SWCNT–COOH are shown in Fig. 3.
Conjugate 4, SWCNT–COOH and 5 showed a weight loss of 18%,
58%, and 90% respectively at 500 °C, Fig. 3. Conjugate 5 (Fig. 3c)
and 4 (Fig. 3a) showed similar thermogram pattern up to 160 °C
indicating the presence of ZnMCPPc–uridine in the hybrid
(ZnMCPPc–uridine–SWCNT). Conjugate 5 was more thermally
stable compared to SWCNT–COOH up to 160 °C. The different ther-
mal stability of 5 compared to SWCNTs and 4 shows alteration in
its physical properties. Conjugate 5 was less stable compared to
4 and SWCNT–COOH, showing a weight loss of 90%, this pattern
is similar to what has been reported before for MPc–SWCNT
hybrids, showing less stability compared to the Pc or SWCNTs only
[33,35].
B
X-ray diffractometry (XRD) characterization
The X-ray diffractometry patterns of conjugate 4,
5 and
SWCNT–COOH are shown in Fig. 4. SWCNTs show characteristic
XRD pattern, Fig. 4a, with two main peaks at 2h values of 25.2°
and 43.5° with d-spacing at 3.5 and 2.1 respectively, corresponding
to the (002) [36,37] and (111) [38] reflections of carbon.
Conjugate 4 (Fig. 4c) exhibited typical XRD pattern of phthalocya-
nine [39], showing a broad peak at 2h value of 18.9° corresponding
to a d-spacing value of 4.67. Conjugate 5 (Fig. 4b) exhibited a broad
peak at 2h value of 23.8° corresponding to d-spacing value of 3.74,
this signal overlaps the 2h peak at 25.2° for the reflection of carbon
(002) in SWCNTs and the typical broad signal of 4. We also
observed disappearance of the XRD peak at 2h value of 43.5° for
the reflection of carbon (111) in SWCNTs. The changes observed
in the XRD pattern of conjugate 5 as compared to 4 indicate suc-
cessful adsorption of 4 onto SWCNT–COOH.
Fig. 7. High-resolution X-ray photoelectron N 1s spectra of ZnMCPPc–uridine, 4 (A)
and ZnMCPPc–uridine–SWCNT, 5 (B).
CNTs, there is no shift in the absorption bands of the former
[33,34], only broadening of the peaks is observed. Broadening of
the spectra is observed in this work on adsorption of complex 4
Raman spectroscopy
The extent of functionalization of a material can be determined
using Raman spectroscopy. SWCNTs are known to exhibit charac-
teristic Raman peaks which are centered at 2500 cmꢂ1 (G⁄),
onto SWCNTs in Fig. 2, in accordance with literature [33] for
interaction of the Pc and CNTs.
p–p
Fig. 8. TEM Image of SWCNT–COOH (A) and ZnMCPPc–uridine–SWCNT, 5 (B).

R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
237
Fig. 10. Absorption (a) Excitation (b) and Emission (c) spectra of ZnMCPPc–uridine,
4
(excitation wavelength = 610 nm) in DMSO. The concentration 4 or 5 is
Fig. 9. Fluorescence decay curve of ZnMCPPc–uridine, 4 and ZnMCPPc–uridine–
ꢃ6.5 ꢁ 10ꢂ6 M.
SWCNT,
5 in DMSO. Excitation wavelength = 683 nm. The concentration =
3.2 ꢁ 10ꢂ7 M.
atom; this interaction was possible due to the presence of the oxy-
gen moiety from the SWCNT–COOH backbone. The last peak at
402.4 eV is assigned graphitic nitrogen, as a result of nitrogen atom
from the Pc interacting with the carbon atom in SWCNTs sheet
[45,46]. The change of the pyrrolic nitrogen of 4 at 400.5 eV to gra-
phitic nitrogen for 5 at 402.4 eV and the additional pyrrolidonic
nitrogen peak for 5 indicates successful adsorption of ZnMCPPc–
uridine (4) onto SWCNT–COOH to form ZnMCPPc–uridine–
1590 cmꢂ1 (G-band (tangential mode); sp2)) [40,41] and
1270 cmꢂ1 (D (disorder band, (breathing mode); sp3), these peaks
were observed in conjugate 5 (Fig. 5a), indicating the presence of
SWCNTs in ZnMCPPc–uridine–SWCNT, 5. The D:G ratio is generally
considered as a good parameter for measuring the extent of func-
tionalization of carbon nanotubes. The intensity of the D band is
often enhanced on functionalization [42,43], resulting in an
increase in the D:G ratio. This is because the G-band is not affected
by defects, whereas the D-band is enhanced by the presence of
defects due to conversion of sp2 to sp3. This is often observed on
direct chemical linking of materials to SWCNTs. SWCNTs used in
this work are already functionalized with COOH. An increase in
the D band is not expected in this work since the MPc compound
is adsorbed onto SWCNTs and not chemically linked.
SWCNT (5) and also shows that beyond
p–p interaction, the
pyrrolic nitrogen atom of the Pc-ring interacted with both the car-
bon and the oxygen moiety of the SWCNTs.
Transmission electron microscopy (TEM)
Conjugate 5 was further characterized using transmission elec-
tron microscopy. Conjugate 5 (Fig. 8B) and SWCNT–COOH (Fig. 8A)
were dispersed through ultrasonication in DMF. The images
obtained for the two are clearly different. The black particles on
the surface of the SWCNT–COOH (Fig. 8B) show that there is an
interaction between 4 and SWCNT–COOH. Fig. 8B shows that the
hybrid was aggregated on adsorption of 4 onto SWCNT–COOH,
From our experiments, we observed an increase in the intensity
of the G-band of 5 (Fig. 5a) and calculations reveal that the D:G
ratio of 0.13 for SWCNT–COOH (Fig. 5b) decreased upon adsorption
of 4 onto SWCNTs to give 5. Increases in the G band are associated
with removal of some amorphous carbon from the nanotubes [44].
X-ray photoelectron spectroscopy (XPS)
which is due to
p–p stacking often observed for phthalocyanines.
X-ray photoelectron spectroscopy was further use to confirm
the surface atomic concentration and the chemical interactions of
the complexes. From the wide survey XPS spectra, conjugate 5
(Fig. 6a) showed four main peaks at 1019, 531, 397 and 283 eV
(Table 2) which were assigned to Zn 2p, O 1s, N 1s and C 1s respec-
tively. These peaks were also observed for 4 at slightly different
binding energies compared to conjugate 5, due to the interaction
of 4 with SWCNTs for 5.
The high resolution spectra obtained for the N 1s component of
conjugate 4 (Fig. 7A) and 5 (Fig. 7B) shows that they are clearly dif-
ferent. Upon the deconvolution of the N 1s spectra of conjugate 4
(Fig. 7A), three N 1s peaks at 396.1, 397.5 and 400.5 eV were
observed. The peaks at 396.1 and 397.5 eV are assigned to pyridinic
nitrogen while the peak at 400.5 eV is assigned to pyrrolic nitrogen
[45–47]. The deconvoluted N 1s spectra of conjugate 5 (Fig. 7B),
showed four N 1s peaks instead of three observed for 4. All the N
1s peaks for 5 shifted to higher binding energies, indicating differ-
ent environment of 4 in the presence of SWCNT–COOH. The first
two peaks for 5 at 396.6 and 397.7 eV are assigned to pyridinic
nitrogen. The new peak at 399.4 eV is assigned to pyrrolidonic
nitrogen [47], which are pyrrolic nitrogen interacting with oxygen
Fig. 11. Representative triplet decay curve of ZnMCPPc–uridine, 4 and ZnMCPPc–
uridine–SWCNT, 5 in DMSO. Concentration = 4.7 ꢁ 10ꢂ6 M.

238
R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
parameters may occur depending on the relative rates of deactiva-
tion of the excited states by non-radiative processes from the sin-
glet and triplet states [49].
The adsorption of 4 onto SWCNTs to form 5 resulted in decrease
in the UT. The decrease in triplet quantum yield in the presence of
SWCNTs is due to electron transfer mediated process between the
MPc and the SWCNTs. SWCNTs deactivates the photo-excited MPc
and in some cases resulting in a lack of observation of the triplet
decay curve [42]. The triplet lifetimes also decreased on adsorption
of 4 onto SWCNTs to give 5, this is evident in the triplet decay
curve of 5 as compared to 4 (Fig. 11). This indicates that SWCNTs
quenched the photo-excited triplet state of the Pc more than the
singlet state.
Singlet oxygen quantum yield UD
The amount of singlet oxygen generated was determined by
measuring its phosphorescence emission. Representative singlet
oxygen phosphorescence decay profile for conjugate 4 and 5 are
shown in Fig. 12. The presence of uridine increases the singlet oxy-
gen of complex 1 (UD = 0.20) to 0.49 and 0.44 for 4 and 5 respec-
tively (Table 1). The increase in UD in the presence of uridine is
due to the ability of nucleoside to generate singlet oxygen at
excited state [50]. Slight decrease in UD was observed on adsorb-
ing 4 onto SWCNTs to form 5, this corresponds to the decrease in
Fig. 12. Representative singlet oxygen phosphorescence decay profile of ZnMCPPc–
uridine,
4
and ZnMCPPc–uridine–SWCNT,
5
in DMSO. Concentration =
4.7 ꢁ 10ꢂ6 M.
This indicates successful adsorption of complex 4 on SWCNT–
COOH to form 5.
triplet quantum yield. The values of S
= UD/UT represent the effi-
D
ciency of energy transfer between the triplet state of the phthalo-
cyanine and ground state triplet oxygen to form excited singlet
Photophysical parameters
state oxygen. The S was near unity for 4 and 5.
Fluorescence lifetimes (
sf) and fluorescence quantum yield (UF
)
D
Fluorescence lifetimes (
sf) were determined using the time
correlated single photon counting (TCSPC) method. A typical
fluorescence decay curve for 4 and 5 is shown in Fig. 9. A mono-
exponential decay curve indicating one lifetime was obtained for
Conclusions
The synthesis of a conjugate of uridine with zinc mono carboxy
phenoxy phthalocyanine (ZnMCPPc–uridine, 4) is reported for the
first time. The conjugate was adsorbed onto SWCNTs (ZnMCPPc–
uridine–SWCNT, 5). The conjugates were characterized by different
spectroscopic methods, the X-ray photoelectron spectroscopy of 4
showed three N 1s peaks while that of 5 showed four N1s peaks,
the new peak at 399.4 eV of 5 was assigned to pyrrolidonic nitro-
gen, due to the interaction of the pyrrollic nitrogen of 4 with the
1,
4 and 5. The fluorescence lifetimes remained relatively
unchanged on the conjugation of ZnMCPPc to uridine to form 4
and on adsorption of 4 onto SWCNTs to form 5, Table 1. The
changes are too insignificant to show the photo-induced electron
transfer (PET) from the photo-excited phthalocyanine to the
SWCNTs. The lack of clear deactivation of the Pcs’ singlet state by
CNTs through PET (using time resolved fluorescence experiments)
has been reported [48], and was attributed to time resolution of
the experiments, and this could be the case in this work.
The ground state absorption spectrum (Fig. 10a) and the excita-
tion spectrum (Fig. 10b) were found to be similar and both were
mirror image of the emission spectrum for 4 (Fig. 10c), indicating
that the molecule emitting in the excited state is the same as the
one absorbing in the ground state.
oxygen moiety of SWCNT–COOH in 5 showing that beyond
p–p
interaction, the nitrogen atom of the Pc-ring interacted with the
SWCNT–COOH. There was an increase of over 40% in the triplet
and singlet oxygen quantum yields of ZnMCPPc in the presence
of uridine, implying that uridine improved the photophysical prop-
erties of ZnMCPPc. There was a slight decrease of the photophysical
parameters of 4 in the presence of SWCNTs in 5. Thus, SWCNTs
resulted in only a small quenching of the triplet state parameters
of 4 and the reported conjugates are potential bi-functional agents
for photodynamic therapy.
The fluorescence quantum yields (UF) were calculated on exci-
tation at 610 nm at an absorbance of 0.05. There was no significant
change in the UF of complex 1 (UF = 0.18) on conjugation to uridine
(4, UF = 0.21) and on adsorption of 4 onto SWCNTs (conjugate 5
(UF = 0.19)) Table 2.
Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF) of
South Africa through DST/NRF South African Research Chairs
Triplet quantum yield (UT) and life time (sT
)
The triplet decay curve for complex 1, 4 and 5 were measured at
490 nm. A typical triplet decay curve for 4 and 5 is shown in Fig. 11.
A mono-exponential curve was obtained for 1, 4 and 5. The pres-
ence of uridine (conjugate 4) increased the triplet quantum yield
and lifetime of ZnMCPPc, 1 (Table 1). Increases in triplet quantum
yields have also been observed on linking of mono substituted
phthalocyanines to pyrene and folic acid [18,19]. Thus, uridine
increases both the triplet yields and lifetimes, showing its impor-
tance when linked to photosensitizers for PDT. Increases in UT
are expected to be associated with decreases in UF especially in
the presence of a heavy atom. However, excited state properties
will also be affected by non-radiative decay processes [49].
Hence, an increase in both fluorescence and triplet state
Initiative
for
Professor
of
Medicinal
Chemistry
and
Nanotechnology (UID 62620) as well as Rhodes University and
DST/Mintek Nanotechnology Innovation Centre (NIC) – Sensors,
South Africa. RO thanks Organization for Women in Science in
the Developing World (OWSD) for a graduate bursary.
References
[1] R. Bonnett, Chemical Aspects of Photodynamic Therapy, Gordon and Breach
Science Publishers, Amsterdam, 2000.
[2] P. Zimcik, M. Miletin, J. Ponec, M. Kostka, Z. Fiedler, J. Photochem. Photobiol., A
155 (2003) 127–131.

R.O. Ogbodu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 231–239
239
[3] I. Okura, Photosensitization of Porphyrins and Phthalocyanines, Gordon and
[27] T.H. Tran-Thi, C. Desforge, C. Thiec, J. Phys. Chem. 93 (1989) 1226–1233.
[28] P. Kubat, J. Mosinger, J. Photochem. Photobiol., A 96 (1996) 93–97.
[29] S. Frey-Forgues, D. Lavabre, J. Chem. Educ. 76 (1999) 1260–1264.
[30] A. Ogunsipe, D. Maree, T. Nyokong, J. Mol. Struct. 650 (2003) 131–140.
[31] M.S. Patterson, S.J. Mdsen, R. Wilson, J. Photochem. Photobiol., B 5 (1990) 69–
84.
[32] N. Kuznetsova, E.A. Makarova, S.N. Dashkevich, N.S. Gretsova, E.A. Kalmykova,
V.M. Negrimovsky, O.L. Kaliy, E.A. Lukyanets, Russ. J. Gen. Chem. 70 (2000)
140–148.
Breach publishers, Germany, 2001.
[4] T. Nyokong, E. Antunes, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The
Handbook of Porphyrin Science, vol. 7, Academic Press, New York, 2010, pp.
247–349 (Chapter 34, World Scientific, Singapore).
[5] M.E. Kenny, Y. Liu. Photodynamic Therapy with Phthalocyanine and Radical
Sources. US 2012/0323164 A1.
[6] X. Shen, B. Zheng, X. Huang, L. Wang, J. Huang, Dalton Trans. 42 (2013) 10398–
10403.
[7] X. Li, D.K.P. Ng, Tetrahedron Lett. 42 (2001) 305–309.
[8] J. Pliml, F. Sorm, C. Czech, Chem. Commun. 29 (1964) 2576–2577.
[9] F. Sorm, J. Vesely, Neoplasma 15 (1968) 339–343.
[33] X. Wang, Y. Liu, W. Qiu, D. Zhu, J. Mater. Chem. 12 (2002) 1636–1639.
[34] H. Murakamia, G. Nakamurab, T. Nomuraa, T. Miyamoto, N. Nakashima, J.
Porphyrins Phthalocyanines 11 (2007) 418–427.
[10] K.L. Grove, Y.-C. Chene, Cancer Res. 56 (1996) 4187–4191.
[11] S. Nagar, R.P. Remmel, Oncogene 25 (2006) 1659–1672.
[12] C. Guillemette, R.C. Millikan, B. Newman, D.E. Housman, Cancer Res. 60 (2000)
950–956.
[13] V.D. Haakensen, M. Biong, O.C. Lingjærde, M.M. Holmen, J.O. Frantzen, Y. Chen,
D. Navjord, L. Romundstad, T. Lüders, I.K. Bukholm, H.K. Solvang, V.N.
Kristensen, G. Ursin, A.-L. Børresen-Dale, Å. Helland, Breast Cancer Res. 12
(R5) (2010) 1–11.
[14] S. Hampel, D. Kunze, D. Haase, Int. J. Nanomed. 3 (2008) 175–182.
[15] Y. Lin, L.F. Allard, Y.P. Sun, J. Phys. Chem. B 108 (2004) 3760–3764.
[16] A. Bianco, K. Kostarelos, C.D. Partidos, M. Prato, Chem. Commun. 5 (2005) 571–
577.
[35] M. Zhang, T. Murakami, K. Ajima, K. Tsuchida, A.S.D. Sandanayaka, O. Ito, S.
Iijima, M. Yudasaka, PNAS 105 (2008) 14773–14778.
[36] S. Sapra, D.D. Sarma, Pramana 65 (2005) 565–570.
[37] I. Stamatina, A. Morozan, A. Dumitru, V. Ciupina, G. Prodan, J. Niewolski, H.
Figiel, Physica E 37 (2007) 44–48.
[38] Y. Zhang, X. Sun, L. Pan, H. Li, Z. Sun, C. Sun, B.K. Tay, J. Alloys Compd. 480
(2009) L17–L19.
[39] A.W. Snow, J.R. Griffith, N.P. Marullo, Macromolecules 17 (1984) 1614–1624.
[40] B. Ballesteros, G. de la Torre, C. Ehli, G.M. Aminur Rahman, F. Agullo-Rueda,
D.M. Guldi, T. Torres, J. Am. Chem. Soc. 129 (2007) 5061–5068.
[41] M.S. Dresselhaus, G. Dresselhaus, A. Jorio, J. Phys. Chem. C 111 (2007) 17887–
17893.
[17] B. Ballesteros, S. Campidelli, G. de la Torre, C. Ehli, D.M. Guldi, M. Prato, T.
Torres, Chem. Commun. (2007) 2950–2952.
[18] R.O. Ogbodu, E. Antunes, T. Nyokong, Polyhedron 60 (2013) 59–67.
[19] R.O. Ogbodu, E. Antunes, T. Nyokong, Dalton Trans. 42 (2013) 10769–10777.
[20] R. Gabrio, D. Donata, M. Filippis, D. Paola, F. Lia, N. Daniele, European patent
application No. 2441386C, Filed on 21/03/2001.
[21] S. D’Souza, E. Antunes, C. Litwinski, T. Nyokong, J. Photochem. Photobiol., A 220
(2011) 11–19.
[22] N. Masilela, T. Nyokong, J. Photochem. Photobiol., A 223 (2011) 124–131.
[23] A. Fashina, E. Antunes, T. Nyokong, Polyhedron 53 (2013) 278–285.
[24] P. Modisha, E. Antunes, J. Mack, T. Nyokong, Int. J. Nanosci. 12 (2013) 1350010
(10).
[42] W. Chidawanyika, T. Nyokong, Carbon 48 (2010) 2831–2838.
[43] E.P. Dillon, C.A. Crouse, A.R. Barron, NANO 2 (2008) 156–164.
[44] K. Yang, M. Gu, Y. Guo, X. Pan, G. Mu, Carbon 47 (2009) 1723–1737.
[45] R. Sen, B.C. Satishkumar, S. Govindaraj, K.R. Harikumar, M.K. Renganathan,
C.N.R. Rao, J. Mater. Chem. 7 (1997) 2335–2337.
[46] J. Casanovas, J.M. Ricart, J. Rubio, F. Illas, J.M. Jimenez-Mateos, J. Am. Chem. 118
(1996) 8071–8076.
[47] H. Schmiersa, J. Friebel, P. Streubel, R. Hesse, R. Köpsela, Carbon 37 (1999)
1965–1978.
[48] B. Ballesteros, G. de la Torre, C. Ehli, G.M.A. Rahman, F. Agulló-Rueda, D.M.
Guldi, T. Torres, J. Am. Chem. Soc. 129 (2007) 5061–5068.
[49] A. Beeby, A.W. Parker, M.S.C. Simpson, D. Phillips, J. Photochem. Photobiol. B:
Biol. 16 (1992) 73–81.
[50] X. Shu, S. Diego, R.Y. Tsien, L. Jolla, United States patent application No. US
2013/0330718 A1, Filed on 12/12/2013.
[25] A.M. Bello, E. Poduch, M. Fujihashi, M. Amani, Y. Li, I. Crandall, R. Hui, P.I. Lee,
K.C. Kain, E.F. Pai, L.P. Kotra, J. Med. Chem. 50 (2007) 915–921.
[26] N. Nakashima, Y. Tomonari, H. Murakami, Chem. Lett. 31 (2002) 638–639.