Synthesis and photophysical studies of CdTe quantum dot-monosubstituted zinc phthalocyanine conjugates

Article abstract of DOI:10.1016/j.ica.2010.12.027

The linkage of unsymmetrically monosubstituted 4-aminophenoxy zinc phthalocyanine (ZnAPPc, 5) to CdTe quantum dots capped with mercaptopropionic acid (MPA), l-cysteine (l-cys) or thioglycolic acid (TGA) has been achieved using the coupling agents ethyl-N(

Full text of DOI:10.1016/j.ica.2010.12.027

Inorganica Chimica Acta 367 (2011) 173–181
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and photophysical studies of CdTe quantum dot-monosubstituted
zinc phthalocyanine conjugates
⇑
Sarah D’ Souza, Edith Antunes, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 March 2010
Received in revised form 25 November 2010
Accepted 10 December 2010
Available online 16 December 2010
The linkage of unsymmetrically monosubstituted 4-aminophenoxy zinc phthalocyanine (ZnAPPc, 5) to
CdTe quantum dots capped with mercaptopropionic acid (MPA),
L-cysteine (L-cys) or thioglycolic acid
(TGA) has been achieved using the coupling agents ethyl-N(3-dimethylaminopropyl) carbodiimide and
N-hydroxy succinimide, which facilitate formation of an amide bond to form the QD–ZnAPPc-linked con-
jugate. The formation of the amide bond was confirmed using Raman and IR spectroscopies. Atomic force
microscopy (AFM) and UV–Vis spectroscopy were used further to characterise the conjugate. Förster
resonance energy transfer (FRET) resulted in stimulated emission of ZnAPPc in both the linked (QD–
Keywords:
Quantum dots
Aminophenoxy phthalocyanine
Fluorescence quantum yields
Förster resonance energy transfer
ZnAPPc-linked) and mixed (QD:ZnAPPc-mixed) conjugates. The linked
L-cys and TGA QDs conjugates
(QD–ZnAPPc-linked) gave the largest FRET efficiencies hence showing the advantages of covalent linking.
Fluorescence quantum yields of QDs were decreased in QD:ZnAPPc-mixed and QD:ZnAPPc-linked.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Metallophthalocyanines (MPcs) have potential for application
Quantum dots (QDs) are semiconducting nanoparticles, approx-
imately 2-10 nm in diameter (10–50 atoms) [11], and their unique
physical properties can be tuned by changing their size [12]. QDs
have high photostability, broad absorption spectra and a narrow
emission range, such that light of the correct wavelength causes
simultaneous excitation (fluorescence) of various sizes of quantum
dots [12,13].
in many areas such as in electrophotography, chemical sensors,
liquid crystals, non-linear optics, optical data storage and as carrier
generation materials in near-IR devices [1–5]. Phthalocyanines
have also been used as photosensitisers in photodynamic therapy
(
PDT), due to their strong absorption in the red (Q-band) which
overlaps with the region of maximum light penetration in tissues
6].
MPcs possess high thermal stability due to both the interlinking
nitrogen atoms [7] and an extended electron conjugated system
8]. In recent years, there has been increasing interest in low sym-
Energy transfer from QDs to different phthalocyanine photosen-
sitizers has been demonstrated in a number of studies [14–17]
where the two components are mixed together, through a process
called Förster resonance energy transfer (FRET). In most reported
FRET studies, quantum dots were not covalently bound to phthal-
ocyanines, but were mixed. Covalent linking of QDs to the Pc ring
has not received much attention, apart from a report on the conju-
gates of SiPc with CdSe core QDs through axial ligation [18] and our
recent reports of covalent linking of QDs to symmetrically substi-
tuted Zn (ZnTAPc) and In (InTAPc) tetraamino phthalocyanine
[19,20]. However, in the case of ZnTAPc and InTAPc the linking
point of the QDs to Pcs could be through some or all of the four
amino groups. In the current work only one amino group is
available, thus this should result in a more defined covalent link
between QDs and ZnAPPc. The linking agents N-ethyl-N(3-dimeth-
ylaminopropyl) carbodiimide (EDC) and N-hydroxy succinimide
(NHS) were employed for catalysing the formation of the amide
bond between the carboxylic acid of QDs and the amino group of
monosubstituted 4-aminophenoxy zinc phthanocyanine (ZnAPPc,
[
p
[
metry MPc derivatives, which can provide the unique properties
required for use in specific applications. The design of these low
symmetry compounds has become the focus of intense interest
amongst many phthalocyanine researchers [9]. However synthesis
of low symmetry MPc compounds through statistical condensation
results in a wide variety of derivatives which require lengthy sep-
aration. In this work we report on a new zinc phthalocyanine mon-
osubstituted with a 4-aminophenoxy group (ZnAPPc, compound 5,
Scheme 1). The use of aminophenoxy as a substituent in MPc com-
pounds has been reported [10], but their use in low symmetry
compounds is reported here for the first time. The new molecule
is covalently linked to thiol capped CdTe quantum dots, Scheme 2.
5). The use of two coupling agents is due to the fact that Jiang et
⇑
Corresponding author. Tel.: +27 46 6038260; fax: +27 46 6225109.
E-mail address: t.nyokong@ru.ac.za (T. Nyokong).
al. [21] have estimated that when using a mixture of EDC and
NHS, about 60% of carboxylic acid groups are NHS-activated and
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.12.027

1
74
S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
Reacting ratio of 1.2: 1
NH2
CN
CN
NC
NC
NH2
K CO , Ar, DMSO
2
3
+
room temperature, 24hrs
HO
O2N
O
1
2
3
Reacting ratio of 1: 3
N
NC
NH2 NC
N
N
N
N
Zn(OAc), DBU
N
Zn
N
+
1
50 °C, pentanol, overnight
NC
O
NC
N
O
H2N
Scheme 1. Synthesis of ZnAPPc (5) derivative.
O
O
O
NH2
S
QD
N
H
N
N
N
N
EDC/NHS
N
N
N
N
O
N
QD
S
N
Zn
N
N
N
Zn
N
N
OH
N
Capping agents for CdTe QDs:
O
O
O
OH
HS
HS
NH2
OH
OH
HS
L-cysteine
MPA
TGA
Scheme 2. Representation of the link formation of QDs to compound 5 and molecular structure of the capping agents for QDs: L-cysteine, MPA (mercaptopropionic acid) and
thioglycolic acid (TGA).
3
0% EDC-activated leaving only 10% not activated. There is less
G25 column were employed for chromatographic separations.
The rest of the reagents were obtained from commercial supplies
and used as received.
activation if either EDC or NHS is employed separately.
2
. Experimental
2.2. Equipment
2.1. Materials
Fluorescence emission and excitation spectra were recorded on
Varian Eclipse spectrofluorimeter. Ground-state electronic
Tellurium powder (200 mesh),
L-cysteine, 3-mercaptopropionic
a
acid (MPA), thioglycolic acid (TGA), dimethylsulphoxide (DMSO)
and anhydrous potassium carbonate were obtained from SAAR-
CHEM. Ultra pure water was obtained from a Milli-Q Water System
absorption spectra were recorded on a Varian Cary 500 UV–
Vis–NIR spectrophotometer. The mass spectra were acquired on a
Bruker Daltonics (Bremen, Germany) Autoflex III MALDITOF mass
spectrometer. H NMR spectra were obtained using a Bruker
AMX 400 MHz and a Bruker Avance II+ 600 MHz NMR spectrome-
ter. X-ray powder diffraction patterns were recorded on a Bruker
1
(
1
Millipore Corp., Bedford, MA, USA). Trifluoroacetic acid (TFA),
,8-diazabicyclo[5.4.0]undec-7-ene (DBU), N-ethyl-N(3-dimethyl-
aminopropyl) carbodiimide (EDC), N-hydroxy succinimide (NHS),
aminophenol and 4-nitrophthalonitrile were purchased from
Fluka. Zinc acetate dihydrate was obtained from British Drug
D8 Discover equipped with a proportional counter, using Cu K
a
radiation (k = 1.5405 Å, nickel filter). Data were collected in the
ꢀ1
House (BDH) Chemicals. DMSO-d
6
was obtained from Aldrich. C18
range from 2h = 5° to 60°, scanning at 1° min
with a filter
Phenomenex reverse phase C18 Sep-Pak columns and sephadex
time-constant of 2.5 s per step and a slit width of 6.0 mm. Samples

S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
175
were placed on a zero background silicon wafer slide. The X-ray
diffraction (XRD) data were treated using Eva (evaluation curve fit-
ting) software. Baseline correction was performed on each diffrac-
tion pattern by subtracting a spline fitted to the curved background
and the full-width at half-maximum values used in this study were
obtained from the fitted curves. Elemental analysis was carried out
using a VARIO ELEMENTAR EL III CHNS instrument. FT-IR spectra
the fluorescence intensity of the QD alone at the same excitation
wavelength.
2.3.2. FRET parameters
FRET efficiency (Eff) is determined experimentally from the
fluorescence quantum yields of the donor in the absence (UF(QD)
)
Mix
and presence (
U
) of the acceptor using Eq. (3) [25–27]:
QD
(
KBr pellets) were recorded on a Perkin–Elmer spectrum 2000
Mix
ð
ð
U
U
Þ
Þ
FT-IR spectrometer. Raman spectral data were collected with Bru-
ker RAM II spectrometer (equipped with a 1064 nm Nd:YAG laser
and a liquid nitrogen cooled germanium detector). Liquid samples
FðQDÞ
Eff ¼ 1 ꢀ
ð3Þ
FðQDÞ
FRET efficiency (Eff) is related to r (Å) by Eq. (4) [25,28]:
2
in DMSO:H O as the solvent were employed. Atomic force micros-
copy (AFM) images were recorded in the non-contact mode in air
with a CP-11 Scanning Probe Microscope from Veeco Instruments
6
R
0
Eff ¼
ð4Þ
6
6
R þ r
0
(
Carl Zeiss, South Africa) at a scan rate of 1 Hz. Samples for AFM
were prepared by spin coating samples of QDs in NaOH or DMSO:-
water (10:1) solvent mixture (the latter in the absence or presence
of compound 5).
Fluorescence lifetimes were measured using a time correlated
single photon counting setup (TCSPC) (FluoTime 200, Picoquant
GmbH). The excitation sources were a light emitting diode (LED,
PLS-500, 497 nm, 10 MHz repetition rate, Picoquant GmbH) with
a linear polariser and a diode laser (LDH-P-C-485, 480 nm,
where r is the centre-to-centre separation distance (in Å) between
donor and acceptor, R (the Förster distance, Å) is the critical dis-
tance between the donor and the acceptor molecules at which the
efficiency of energy transfer is 50% and depends on the quantum
yield of the donor Eq. (5) [25,28]:
0
6
0
23
2nꢀ4
R
¼ 8:8 ꢂ 10
j
U
FðQDÞ
J
ð5Þ
2
where
the medium,
j
is the dipole orientation factor, n the refractive index of
the fluorescence quantum yield of the donor in
1
0 MHz repetition rate, Picoquant GmbH). The response function
U
F
of the system, which was measured with a scattering Ludox solu-
tion (DuPont), had a full width at half-maximum (FWHM) of about
the absence of the acceptor, and J is the Förster overlap integral,
defined by Eq. (6):
9
50 ps. The ratio of stop to start pulses was kept low (below 0.05)
to ensure good statistics. All luminescence decay curves were mea-
sured at the maximum of the emission peak. The data were ana-
lysed with the program FluoFit (Picoquant GmbH). The support
plane approach was used to estimate the errors of the decay times.
4
J ¼
UQDðkÞ
e
ZnPcðkÞk dk
ð6Þ
where fQD is the normalised QD emission spectrum and
e
ZnPc is the
molar extinction coefficient of ZnPc derivatives, k is the wavelength
2
of the acceptor, at the Q-band. In this case, it is assumed that
j is
2
/3 for mixed QDs with compound 5; such assumption is often
2
2
.3. Fluorescence studies
made for donor–acceptor pairs in a liquid medium, since their
dipole moments are considered to be isotropically oriented during
the excited state lifetimes. Although electrostatic interactions are
anticipated between QDs and the sterically demanding phthalocya-
nines in mixed QDs, the exact orientation of the ZnPc derivative
with regard to the QD is not known with certainty. Various confor-
mations can be attained by the phthalocyanine compound on the
surface of QDs when the two are mixed. For the perpendicular
orientation of the ZnPc derivative on the QD some degree of move-
ment of the donor/acceptor species is expected. As a result the iso-
tropic dynamical average (
static isotropic average (
pair is not rigid. The
.3.1. Fluorescence quantum yields
Fluorescence quantum yields (
F
U ) of the QDs or compound 5
were determined by the comparative method [22] (Eq. (1)),
2
F ꢁ AStd ꢁ n
U
F
¼
U
FðStdÞ
ð1Þ
2
Std
F
Std ꢁ A ꢁ n
where F and FStd are the areas under the fluorescence curves of the
QDs (or compound 5) and the reference, respectively. A and AStd are
the absorbances of the sample and reference at the excitation wave-
length respectively, and n and nStd are the refractive indices of sol-
vents used for the sample and reference, respectively. ZnPc in DMSO
was employed as a standard,
rhodamine 6G in ethanol with
dard for the quantum dots [24,25]. Both the sample and reference
were excited at the same relevant wavelength (610 nm for com-
pound 5 and 500 nm for QDs). The absorbance of the ZnPc standard
2
j
= 2/3) is more appropriate than the
2
j
= 0.476) because the donor–acceptor
2
j = 2/3 was also employed for the linked
U
U
F
= 0.2 [23] for compound 5, whilst
F
= 0.94 was employed as the stan-
QDs–ZnAPPc. FRET parameters were computed using the program
PHOTOCHEMCAD [29].
2.4. Synthesis of CdTe quantum dots
(
or compound 5) or of QDs at the excitation wavelength ranged be-
The preparation of thiol capped QD was via a modified method
tween 0.04 and 0.05. The determined fluorescence quantum yield
values of the QDs were employed in determining their fluorescence
quantum yields in the mixture with compound 5 (
U
adopted from literature [15,30]. Briefly, 2.35 mmol of CdCl
was dissolved in 125 ml of water and 5.7 mmol of the respective
carboxylic acid thiol { -cysteine ( -cys), 3-mercaptopropionic acid
MPA) and thioglycolic acid (TGA), Scheme 2} was added under
2 2
ꢁH O
Mix
FðQDÞ
U
) or linked
linked
FðQDÞ
L
L
using Eq. (2):
(
Mix
F
stirring. The solution was adjusted to a pH between 11 and 12 with
1 M NaOH. Nitrogen gas was bubbled through the solution for
about 1 h. The aqueous solution was reacted with H Te gas, which
2
Mix
QD
U
¼
¼
U
FðQDÞ
ð2aÞ
FðQDÞ
F
QD
linked
QD
was generated by the reaction of NaBH4 with Te powder in the
presence of 0.5 M H SO under a flow of nitrogen gas. A change
F
linked
FðQDÞ
U
U
FðQDÞ
ð2bÞ
2
4
F
QD
2
of colour of the solution containing CdCl and the thiol was ob-
where
U
F
(QD) is the fluorescence quantum yield of the QDs alone
served on addition of H Te gas. The solution was then refluxed un-
2
Mix
linked
and was used as standard, FQD (or FQD ) is the fluorescence inten-
sity of QDs, in the mixture (or linked) with compound 5 when ex-
cited at the excitation wavelength of the QDs (500 nm) and FQD is
der air at 100 °C for different times to control the size of the CdTe
QDs. On cooling, the QDs were precipitated out from solution using
excess ethanol, the solutions were then centrifuged to harvest the

1
76
S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
QDs. Sizes (D) of the synthesized QDs were estimated using the
polynomial fitting function [31] Eq. (7),
D ¼ ð9:8127 ꢂ 10ꢀ Þk ꢀ ð1:7147 ꢂ 10 Þk þ ð1:0064Þk
7
3
ꢀ3
2
ꢀ
ð194:84Þ
ð7Þ
where k is the absorption maxima of the QDs. The fitting function is
not valid for sizes of quantum dots outside the size range 1–9 nm
[
31]. The size was also determined using XRD. The particle diame-
ter, d, for XRD data was determined using the Scherrer Eq. (8),
kk
bcosh
dðÅÞ ¼
:
ð8Þ
where k is an empirical constant equal to 0.9, k is the wavelength of
the X-ray source (1.5405 Å), b is the full width at half maximum of
the diffraction peak, and h is the angular position of the peak.
Fig. 1. Absorbance (i), excitation (ii) and emission (iii) spectra of compound 5
ZnAPPc) in DMSO:water (10:1) solvent mixture (excitation wavelength = 671 nm),
5 ꢂ 10 M.
(
ꢀ
4
2
.5. Synthesis of monosubstituted 4-aminophenoxy zinc
soluble in 100% water). The sample was then run through a Sepha-
dex column to separate any residual impurities such as the un-
linked ZnAPPc from the linked which elutes first whilst the
remaining bands are discarded.
phthalocyanine (5)
2.5.1. 4-Aminophenoxyphthalonitrile (3, Scheme 1)
To a suspension of 4-aminophenol {(1) 0.76 g, 6.94 mmol}, 4-
nitrophthalonitrile {(2) 1 g, 5.78 mmol} in 25 mL of dry DMSO
was added anhydrous K CO (1.64 g, 11.56 mmol). The mixture
was left to reflux under argon overnight with stirring. A further
.8 g (5.48 mmol) of K CO was added after 24 h to the suspension.
After 48 h, the resulting product was poured into 1 M HCl
260 mL), forming a precipitate that was recrystallized from meth-
anol/water (1:1) to yield a dark brown product. Yield: 0.613 g
2
3
3
. Results and discussion
0
2
3
3.1. Synthesis and characterisation
(
3.1.1. Monosubstituted 4-aminophenoxy zinc phthalocyanine (ZnAPPc,
5)
1
(
7
45.1%). H NMR (DMSO-d
.68–7.73 (1H, m, Ar–H), 8.0–8.7 (1H, m, Ar–H), 7.13–7.48 (4H,
m, Ar–H).
6
): d, ppm 8.25–8.31 (1H, m, Ar–H),
The synthesis of the dinitrile precursor (3) was adapted from lit-
erature [32] for similar compounds with moderate yields. The syn-
thetic procedure outlined in Scheme 1 shows the statistical
condensation approach used for the synthesis of compound 5. This
method is based on the reaction of two differently substituted pht-
halonitriles in a ratio of 3:1. Following extensive purification, the
target compound was obtained in low yields (29%), with the Q
band at 673 nm in DMSO, Fig. 1. The elemental analyses and mass
and H NMR spectral data for compound 5 were consistent with its
structure. Beer’s law was observed for compound 5 for concentra-
tions less than 5 ꢂ 10 M. The compound is unsymmetrically
2.5.2. Monosubstituted 4-aminophenoxy zinc phthalocyanine (ZnAPPc,
5)
To a suspension of 1,2 dicyanobenzene {(4) 0.11 g, 0.84 mmol},
-aminophenoxyphthalonitrile {(3) 0.066 g,0.28 mmol} and zinc
4
acetate (0.0671 g, 0.306 mmol) in 5 mL of pentanol, 2 mL of DBU
was added. The mixture was left to reflux under argon overnight
with stirring. The resulting product was cooled before the addition
of methanol, precipitating compound 5 from the solution. The pre-
cipitate was collected using centrifugation and further purified by
centrifuging with a 1:1 mixture of chloroform and dichlorometh-
ane. This choloroform/dichloromethane mixture was then re-
moved using a rotary evaporation. The product was further
purified using a Phenomenex C18 Sep-Pak column with 0.05% TFA
in methanol as the solvent. Yield: 0.055 g (29%). UV–Vis (DMSO),
1
ꢀ5
substituted and it would be expected that there is some Q band
splitting due to loss of symmetry, however, this is not clear from
the absorption spectra in Fig. 1. Fig. 1 shows broadening on the
excitation spectrum when compared to the absorption spectrum,
suggesting loss of symmetry on excitation. The loss of symmetry,
due to unsymmetrical nature of the molecule, is even more evident
in the fluorescence spectrum (Fig. 1(iii)). A Stokes shift of 15 nm
was observed and is typical of MPc compounds [23].
k
max (nm) (log
e
): 673 (4.6), 608 (3.8), 346 (4.1). IR [(KBr)
max
m /
ꢀ
1
cm ]: 3414 (NH ), 2924 (C@C), 1647–1596 (NH bend), 1484
2
1
(
(
C@C), 1327 (C–O–C), 1114–1060 (CN), 731 (Zn–N). H NMR
DMSO-d ): d, ppm 9.68–9.71 (2H, br s, Ar–H), 9.43–9.45 (6H, m,
3.1.2. Quantum dots
6
QDs grow through the Ostwald ripening process during the
Pc–H), 8.25–8.29 (6H, m, Pc–H), 7.9–7.96 (3H, m, Pc–H), 7.82–
course of heating. As they grow, both the absorbance and the emis-
sion spectra shift to longer wavelengths. The QDs emission wave-
lengths chosen for FRET work are shown in Table 1.
7
21 9
.84 (4H, m, Ph–H). Anal. Calc. for C38H N OZn: C, 66.57; H,
3
.09; N, 18.41. Found: C, 67.32; H, 3.29; N, 18.10%. MALDI-TOF
+
MS m/z calc. 685.04 amu, found 684.14 amu (M) .
2.5.3. Synthesis of linked QDs ZnAPPc conjugate
Table 1
For the formation of the amide linked QD–ZnAPPc, a mixture
Emission spectral data and size determination of CdTe core QDs quantum dots using
different methods. For the polynomial fitting water was employed as the solvent.
containing 2 mM NHS, 5 mM EDC, CdTe QDs (0.001 g/mL) and
ꢀ4
ZnAPPc (compound 5, 1 ꢂ 10 M) in DMSO was allowed to react
for 1 h. Experiments, where the ZnAPPc were mixed with QDs,
without covalent linking, resulting in mixed QD:ZnAPPc were also
performed, using the same ratio of QDs to compound 5 as was used
for the formation of the linked conjugate. The linked QDs–ZnAPPc
conjugate was purified by first washing with water to remove ex-
cess NHS, EDC and unlinked QDs (as the linked conjugates are not
FWHMa
Thiol capping
k (emission) (nm)
Size (nm)
Polynomial (Eq. (7))
XRD
MPA
TGA
621
614
601
3.5
3.2
3.5
3.9
3.4
4.1
60
59
73
L-cysteine
a
FWHM = full width at half maximum.

S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
177
Fig. 2. XRD plot for CdTe–MPA capped QDs.
glass surface coating from a 0.1 M NaOH, DMSO:water solvent
mixture and in the presence of compound 5. In 0.1 M NaOH solu-
tion only, the CdTe QDs show a size distribution from 4 to 27 nm
(Fig. 4E) however populations with size distributions from 12 nm
and below in the section analysed occurred more frequently sug-
gesting that CdTe QDs are not totally monodisperse even in basic
media. In DMSO:water the QDs show a size distribution of from
2
to up to 60 nm (Fig. 4D) which is a very wide distribution, sug-
gesting that aggregation tendencies are worsened in this solvent
system, and also showing the reported solvent dependency [34].
The same aggregation behaviour was observed in the presence of
compound 5. However, in the presence of compound 5, the AFM
histograms showed that clusters of a size distribution of 11 nm
and less were more predominant indicating that the more aggre-
gated clusters occur less frequently Fig. 4F. The AFM images in
the presence of compound 5, Fig. 4C, shows more enhanced
clusters as opposed to smaller dots observed for QDs alone in
DMSO:water, Fig. 4A.
Fig. 3. Absorbance (a) and fluorescence (b) spectra of MPA coated CdTe in
DMSO:water (9:1).
The X-ray diffraction pattern of CdTe MPA QDs employed in this
work is shown in Fig. 2. Although the diffraction pattern is rather
broad, it corresponds well with the three characteristic peaks for
bulk CdTe structure. X-ray powder diffraction can provide impor-
tant details about the crystal structure and properties of the QDs
and so this technique was employed to determine the size of the
CdTe QDs. According to the estimate obtained using Eq. (7), the size
of the MPA coated CdTe core QDs chosen for this work is 3.5 nm
whilst the XRD calculation gives the size to be 3.9 nm (Table 1)
3.1.3. Linked QDs–ZnAPPc
The capping agent located on the surface of CdTe core QDs were
linked to ZnAPPc (compound 5) by coupling the carboxylic group of
the capping agent to the amino group on 5 using EDC/NHS mixture
as a linking agent. The resulting conjugate is represented as QD–
ZnAPPc-linked. EDC and NHS were employed as linking agents, cat-
alysing the formation of the amide bond between the carboxylic
acid of QDs and the amine group of ZnAPPc.
Raman spectra were collected in order to characterise the new
compound and conjugates, Fig. 5. The main difference between
the linked (QD–ZnAPPc-linked, Fig. 5a) and mixed (QD:ZnAPPc-
mixed, Fig. 5b) conjugates was the position of the main peaks
attributed to the phthalocyanine structure (Fig. 5c) at 2916 and
using Fig. 2. The sizes for CdTe TGA QDs and CdTe L-cys QDs are
also slightly higher when using XRD as opposed to the polynomial.
Since the polynomial used is only an estimate, the sizes deter-
mined by XRD will be employed in this work. Fig. 3 shows the
absorption and fluorescence emission spectra of the synthesized
QDs in DMSO:water (9:1), using CdTe MPA QDs as an example. This
solvent mixture allows for solubilisation of the quantum dots
whilst maintaining the phthalocyanine in its monomeric form,
hence QDs alone were also studied in this mixture. The full width
at half maximum (FWHM, Table 1) is an indication of the quality of
the QDs and should be ꢃ70 nm or less. The synthesized QDs show
the FWHM to be in this region.
CdTe QDs capped with thiols are known to aggregate in acidic
conditions due to detachment of surface ligands [33]. Aggregation
of QDs results in red shifting in the emission spectra accompanied
by broadening [34]. Solvents also have an effect on the aggregation
nature of CdTe QDs [34]. There was no change in FWHM of the QDs
recorded from direct synthesis conditions in NaOH or when re-
corded using DMSO:water mixture. The emission spectra shifted
from 627 nm in 0.1 M NaOH to 622 nm in DMSO:water, hence
showing red shifting in NaOH.
ꢀ1
3004 cm . For the QD:ZnAPPc-mixed, there was no peak in this
region as was the case with QDs alone. The peak for the QD–
ꢀ1
ZnAPPc-linked conjugates was shifted from 2916 cm (for ZnAPPc
ꢀ1
alone) to 2923 cm . The shifts (or absence) of the peaks in this re-
gion confirm the formation of QD–ZnAPPc-linked conjugate due to
changes in the molecular structure as a result of bond formation
between the QDs and ZnAPPc.
In the IR spectrum of the linked QD–ZnAPPc, Fig. 6a, there was
an indication of amide bond formation by the presence of a band
characteristic of the amides at 1664 cm whilst it was not ob-
served in the mixture of QDs with ZnAPPc, Fig. 6b. The differences
between the mixed and the linked confirm the linking.
Absorption spectra were used for further characterisation of the
mixed and linked species in the DMSO:water solvent mixture
(Fig. 7). Interestingly there was a shift in the Q band from
673 nm (of ZnAPPc alone) to 678 nm upon formation of the QD–
ZnAPPc-linked, showing changes in the environment. The mixed
Q band gave a Q band very close to that of ZnAPPc alone at
674 nm. The spectra of the QD–ZnAPPc-linked does not show
ꢀ
1
Atomic force microscopy data (Fig. 4) provided the information
about surface morphology of CdTe QDs on a cross section of the

1
78
S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
Fig. 4. AFM images of MPA CdTe QDs deposited on a glass surface from (A) 0.1 M DMSO:water (9:1), (B) 0.1 M NaOH and (C) QDs and ZnAPPc in DMSO:water (9:1).
Corresponding histograms (D, E and F).
Fig. 5. Raman spectra of (a) ZnAPPc–CdTe–MPA QD linked, (b) ZnAPPc–CdTe–MPA
QD mixed, (c) ZnAPPc alone and (d) CdTe MPA QDs alone in DMSO:water.
Fig. 6. IR spectra of (a) ZnAPPc – CdTe–MPA linked, (b) ZnAPPc + CdTe–MPA mixed,
(c) ZnAPPc (compound 5) and (d) MPA capped QDs.

S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
179
Fig. 8. Electronic spectrum showing the overlap between the absorption spectra of
the ZnAPPc and the photoemission spectra of the respective QDs. Solvent: water.
Fig. 7. Ground state electronic absorption spectra and Q band position of (a) CdTe
MPA QDs alone, (b) ZnAPPc alone (673 nm), (c) QD:ZnAPPc-mixed (674 nm) and (d)
QD–ZnAPPc-linked (678 nm) in 9:1 v/v DMSO:water solution.
Table 2
Fluorescence quantum yield parameters for ZnAPPc:CdTe core QD interaction in
DMSO:water (9:1) mixture for both the linked and mixed conjugates.
Thiol cappinga
U
F(QD)
Refs.
Mix
FðQDÞ
Linked
FðQDÞ
U
U
MPA (3.9)
TGA (3.4)
LCys (4.1)
MPA (3.7)
TGA (3.6)
LCys (3.5)
0.57
0.57
0.04
0.59
0.62
0.09
0.21
0.48
0.03
–
–
–
0.04
0.03
0.004
–
–
–
This work
This work
This work
[15]b
[15]
[15]
a
QD size in brackets.
Data in pH 7.4 buffer.
b
aggregation. Aggregation in MPc compounds is characterised by
the formation of a blue shifted band due to the aggregate. This is
not observed in Fig. 7. Small shifts in phthalocyanine spectra
may be attributed to changes in the environment such as axial liga-
tion. This difference in spectra between the linked and mixed is an
indirect way of confirming the linkage. There was an increase in
absorption in the 500 nm region for QD–ZnAPPc-linked and
QD:ZnAPPc-mixed due to the presence of QDs (using MPA capped
QDs as an example). The QD–ZnAPPc-linked shows a lower absorp-
tion in the 500 nm region than QD:ZnAPPc-mixed, due to different
amounts of QDs in the two.
3.2. Fluorescence quantum yields
F
The fluorescence quantum yield (U ) value for compound 5 was
calculated to be 0.33. This value is within the range for MPc com-
pounds [23]. Fluorescence quantum yield ( ) values for the CdTe
core QDs in DMSO:H O (9:1) are listed in Table 2. The QDs show
U
F
2
slightly lower quantum yields compared to the values obtained in
pH 7.4 buffer for similar sized QDs [15], Table 2. When QDs were
Mix
FðQDÞ
mixed with compound 5, the
U
were even lower due to the
known quenching effects of MPc compounds on QDs [15]. This is a
regular occurrence for QDs in the presence of phthalocyanine units
[
15] and the quenching has been attributed to the transfer of energy
from donor QDs to phthalocyanine acceptor molecules. This results
in a lowering of QD fluorescence intensity, in either a QD:ZnAPPc-
mixed or QD–ZnAPPc-linked species, and therefore a reduction in
fluorescence quantum yields of the QDs. Non-radiative (NR) decay
Fig. 9. Emission of QDs alone, ZnAPPc, QDs in the mixture with the ZnAPPc or
ZnAPPc linked to QDs. (a) TGA, (b)
L-cys and (c) MPA capped QDs. (kexcitation =
processes may also be used to account for the decline in
F
U values.
500 nm, in (9:1) DMSO:water solvent mixture).
3
.3. Förster resonance energy transer (FRET)
excitation energy to an acceptor molecule (compound 5) non-rad-
iatively such that the acceptor is raised to a higher energy state.
The acceptor may or may not be fluorescent.
FRET is a photophysical process through which an electronically
excited fluorescent donor molecule (QDs in this case) transfers its

1
80
S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
In order for FRET to occur, there should be an overlap between
Table 3
Fluorescence lifetimes of MPA, L-cys and TGA capped QDs in the absence and presence
of compound 5.
the fluorescence spectra of QD with the absorption spectrum of
ZnAPPc and this is observed in Fig. 8. On mixing solutions of
ZnAPPc with QDs and monitoring the emission spectra of the
QDs, a decrease in the fluorescence emission (on exciting at
a
F2 (ns) a (±1.2)
s
F3 (ns) a (±0.2)
Compound
s
F1 (ns) (±2.7)
s
MPA QDs alone
ZnAPPc–MPA QDs mixed 14.1 (0.25)
TGA QDs alone
ZnAPPc
19.0 (0.23)
4.8 (0.33)
3.8 (0.37)
4.4 (0.27)
2.9 (0.24)
0.9 (0.45)
0.8 (0.38)
0.8 (0.45)
0.4(0.57)
5
00 nm where QDs absorb and compound 5 does not) of the latter
23.6 (0.28)
18.5 (0.19)
was observed, Fig. 9. These changes are due to energy transfer from
the QDs to ZnAPPc. There is no clear stimulated emission of ZnAPPc
by TGA (Fig. 9a) and L-cys (Fig. 9b) capped QDs. For MPA capped
TGA QDs mixed
L
-cys alone
10.9 (0.03)
2.8 (0.21)
2.9 (0.21)
0.6 (0.76)
0.6 (0.76)
QDs there is clear (but weak) stimulated emission at ꢃ688 nm
where ZnAPPc emits, for both mixed and linked QDs, Fig. 9c. It is
important to note that the relative amounts of ZnAPPc and QDs
will be different in the mixed and linked QD–ZnAPPc making com-
parison between the two difficult. The lack of clear stimulated
emission for TGA capped QDS could be a result of less overlap be-
tween the emission spectrum of these QDs and the absorption
spectrum of compound 5. The overlap between absorption spec-
ZnAPPc–L-cys QDs linked 11.0 (0.03)
a
Relative abundance in brackets.
3.4. Determination of FRET parameters
In FRET, the acceptor may or may not be fluorescent, hence FRET
parameters are determined in section. FRET facilitates the non-
radiative transfer of energy from the donor to the acceptor mole-
cule. The efficiency of FRET is known to be dependent on a number
of parameters such as the spectral overlap term (J) estimated by
overlapping QD emission with the absorbance of ZnPc derivatives
shown in Fig. 8. This extent of overlap has varied units and in this
trum of compound 5 and emission spectrum of L-cys capped QDs
is similar to that of MPA capped QDs, yet the latter show more
stimulated emission, emphasising that the differences in the nat-
ure of capping agents can influence the quenching of the QDs
emission.
Fig. 9 shows that for mixed QDs–ZnAPPc (where the ratio of QDs
to ZnAPPc were kept the same for all the three types of QDs), the
emission is quenched more for the MPA capped QDs compared to
6
work the units used were in cm [25]. The PhotochemCAD program
6
ꢀ1
ꢀ1
gives J units as cm following the use of
e
0
ZnPc in M cm and the
values in this work were
wavelength k in nm in Eq. (6). The J and R
either L-cys or TGA capped QDs. In order study the differences be-
computed using PhotochemCAD [29] whilst the r values were cal-
tween the quenching of the different QDs by ZnAPPc for mixed
conjugates, time resolved fluorescence decay curves were re-
corded. A sample of the fluorescence decay curve is shown in
Fig. 10. The fluorescence lifetimes are listed in Table 3. The pres-
ence of three lifetimes is common occurrence for QDs. Though
there is lack of agreement in literature. However, the longer life-
culated using Eq. (4) and are listed in Table 4.
ꢀ14
cm6 for porphyrin
J values are generally of the order of 10
based molecules and the values obtained in this work were of
ꢀ13
6
the order 10
cm in Table 4, thus confirming good spectral over-
lap, which is desirable since a greater value of J indicates good
spectral overlap between the emission spectrum of the donor
and the absorption spectrum of the acceptor giving an estimation
of a good donor–acceptor oscillator match and hence a greater
probability for FRET. This large J value would probably enhance
the efficiency of energy transfer (FRET). The efficiency of FRET
time (
involvement of surface states in the carrier recombination process
35]. The shorter lifetime ( in Table 3) component may be attrib-
uted to the intrinsic recombination of initially populated core
states [36–38] and the shortest lifetime ( in Table 3) is attributed
1
s in Table 3) component is usually associated with the
[
s
2
s
3
(Eff) values calculated using Eq. (3) from the QD to the ZnPc deriv-
to radiative depopulation due to band edge recombination at the
surface [39]. Table 3 shows that MPA capped QDs gave the largest
ative are shown in Table 3.
0
The values of r were smaller than for R for MPA capped QDs
decreased (ꢃ26%) in the longest lifetime (
1
s ) in the presence of
compound 5 compared to the other two QDs hence there is more
(
for both the linked and mixed systems), showing that Eff will be
greater than 50% as observed in Table 4. For TGA and -cys capped
for the QD–ZnAPPc-linked but not for QD:ZnAPPc-
L
quenching of fluorescence in Fig. 9 for MPA capped QDs.
QDs, r < R
0
6000
4000
2000
0
6
4
2
0
-
-
-
-
2
4
6
8
0
20
40
60
80
Time (ns)
Fig. 10. Fluorescence decay curves of CdTe QDs (MPA capped) in the presence of ZnAPPc.

S. D’ Souza et al. / Inorganica Chimica Acta 367 (2011) 173–181
181
Table 4
Energy transfer parameters for ZnAPPc–CdTe: thiol QD interactions (in DMSO:H
9:1) for linked and mixed conjugates).
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2
O
(
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[
[
The unsymmetrically monosubstituted 4-aminophenoxy zinc
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namic therapy. The MPA linked ZnAPPc had a high fluorescence
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This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF), South
Africa through DST/NRF South African Research Chairs Initiative
for Professor of Medicinal Chemistry and Nanotechnology and
Rhodes University, and by DST/Mintek Nanotechnology Innovation
centre. Edith Antunes thanks CSIR/Swiss JRF of South Africa for
Post-Doctoral funding. S.D. thanks the African Laser Centre for a
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