Photophysical and photochemical properties of a novel thiol terminated low symmetry zinc phthalocyanine complex and its gold nanoparticles conjugate

Article abstract of DOI:10.1039/c3dt32698e

A novel thiol functionalized zinc phthalocyanine complex (ZnPcSH) is reported in this work. This complex was conjugated to gold nanoparticles. The photophysical and photochemical properties of the complex and the conjugate were investigated. Upon conjugation a blue shift was observed from the UV-Vis spectra. The conjugate showed a decrease in the fluorescence quantum yield and lifetime. An increase in the triplet quantum yield and lifetime was observed for ZnPcSH following conjugation to gold nanoparticles. Both ZnPcSH and its conjugate with gold nanoparticles showed high singlet oxygen quantum yields with the conjugate being higher than the Pc alone.

Full text of DOI:10.1039/c3dt32698e

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Photophysical and photochemical properties of a novel
thiol terminated low symmetry zinc phthalocyanine
complex and its gold nanoparticles conjugate
Cite this: Dalton Trans., 2013, 42, 4922
Thandekile P. Mthethwa,a Sinem Tuncel,b Mahmut Durmuşb and Tebello Nyokong*a
A novel thiol functionalized zinc phthalocyanine complex (ZnPcSH) is reported in this work. This complex
was conjugated to gold nanoparticles. The photophysical and photochemical properties of the complex
and the conjugate were investigated. Upon conjugation a blue shift was observed from the UV-Vis
spectra. The conjugate showed a decrease in the fluorescence quantum yield and lifetime. An increase in
the triplet quantum yield and lifetime was observed for ZnPcSH following conjugation to gold nanoparti-
cles. Both ZnPcSH and its conjugate with gold nanoparticles showed high singlet oxygen quantum yields
with the conjugate being higher than the Pc alone.
Received 12th November 2012,
Accepted 22nd January 2013
DOI: 10.1039/c3dt32698e
www.rsc.org/dalton
This work reports on the use of an ethoxy group as a linker to
the SH group.
Introduction
Metallophthalocyanines (MPcs) have attracted
a
lot of
The ethoxy groups were chosen because ethoxy substituted
interest lately as photosensitizers for photodynamic therapy Pc compounds have shown a very high singlet oxygen
(PDT). PDT involves the use of light to excite the photosensiti- quantum yield.12 The other three positions in the tetrasubsti-
zer to its triplet state, this allows interaction with molecular tuted ring contain mercapto pyridyl groups for enhanced solu-
oxygen and production of the cytotoxic singlet oxygen that bility and red shifting on the Q band, both are requirements
destroys the tumor cells.1 MPcs have been a subject of interest for use of Pcs in PDT. The mercapto pyridyl groups could also
because of their applications in other areas such as sensors,2 provide another point of attachment of the AuNPs. The
photocatalysis3 and optical limiting.4 The introduction of sub- complex reported in this work is: 1,8(11),15(18)-tri-(2-mercapto-
stituents onto the peripheral and non-peripheral positions of pyridine)-23(24)-4-(2-{2-[2-(2-mercaptoethoxy)-ethoxy]ethoxy}-
the ring improves the solubility of the Pcs and allows for ethoxy) phthalocyaninato zinc(II) (7, abbreviated as ZnPcSH,
further conjugation. The metallophthalocyanine complexes Scheme 1).
containing alkylthio or arylthio ring substituents have been
The use of gold nanoparticles in delivery applications is
the most explored.5–8 These phthalocyanine complexes show a based on the possible use of the conjugates in enhanced per-
red shifted Q band which makes them good candidates for meability and retention (EPR) effect in which the conjugates
PDT. Functionalized phthalocyanine complexes containing can be taken up by tumor cells and be retained.13 Gold nano-
terminal SH groups have been less studied due to the difficulty particles have been studied extensively for biomedical appli-
of the synthesis.9–12 These complexes readily form S–S bonds cations owing to their interesting plasmonic properties and
in air.
non-toxic nature. Their chemical and physical properties have
Conjugates of phthalocyanines and Au nanoparticles made them very useful in diagnostics,14 biosensing15 and drug
(AuNPs) have been reported for drug delivery applications.9,10 delivery.16 The position of the plasmonic peak is greatly depen-
The conjugates reported in the literature used phthalocyanines dent on the size and morphology of AuNPs; therefore studies
which are substituted at the periphery with thiol groups. We have been directed to the synthesis of monodispersed gold
have also reported on the conjugation of AuNPs using a Pc nanoparticles with controlled sizes.17–20 The fabrication of
containing one SH or tetrasubstituted with SH groups.11 The gold nanoparticles has been extended to the modification of
reported complexes contained an alkyl linker to the SH group. their surfaces with different ligands to allow for
bioconjugation.21–24 In this study, the photophysical properties
of the conjugates of ZnPcSH with AuNPs are reported. Singlet
oxygen quantum yield, which is the main requirement for
PDT, is reported in this work for the first time for any Pc–
AuNP conjugate.
aDepartment of Chemistry, Rhodes University, Grahamstown 6140, South Africa.
E-mail: t.nyokong@ru.ac.za
bGebze Institute of Technology, Department of Chemistry, Gebze 41400, Turkey
4922 | Dalton Trans., 2013, 42, 4922–4930
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Scheme 1 Synthesis route to phthalocyanine 7 (ZnPcSH). i: Tetraethylene glycol, K2CO3, DMF; ii: 2-Mercaptopyridine, K2CO3, DMF; iii: Zn(OAc)2, pentanol, DBU; iv:
Methane sulfonyl chloride, triethylamine, dichloromethane; v: (1) Thiourea, ethanol, THF, (2) NaOH, (3) H+.
spectrometer with an attenuated total reflection (ATR) acces-
sory featuring a zinc selenide (ZnSe) crystal. The mass spectra
were recorded on an LCQ-ion trap (Thermo Finnigan, San Jose,
Experimental
Materials
Trisodium citrate (99%), gold(III) chloride trihydrate, sodium CA, USA), equipped with an ES (Electrospray) source and an
borohydride, tetraoctylammonium bromide (TAOBr), zinc MALDI (matrix assisted laser desorption ionization) BRUKER
phthalocyanine (ZnPc) and 1,3-diphenylisobenzofuran (DPBF) Microflex LT using 2,5-dihydroxybenzoic acid as a matrix. 1H
were purchased from Aldrich. 1,8-Diazabicyclo[5,4,0]undec-7- NMR spectra were recorded in DMSO-d6 solutions on a Varian
ene (DBU) was obtained from Fluka. Triethylamine, methane 500 MHz spectrometer.
sulfonyl chloride and thiourea were obtained from Sigma-
Aldrich.
Ground state electronic absorption spectra were performed
on a Shimadzu UV-2550 spectrophotometer. Fluorescence exci-
4-Nitrophthalonitrile (1)25 3-nitrophthalonitrile (2),26 4-(2-{2- tation and emission spectra were recorded on a Varian Eclipse
[2-(2-hydroxyethoxy)ethoxy]ethoxy}-ethoxy)phthalonitrile (3)27–29 spectrofluorimeter. Fluorescence lifetimes were measured
and 3-(2-mercaptopyridine) phthalonitrile (4)30 were synthesized using a time correlated single photon counting (TCSPC) setup
and purified according to literature procedures. All solvents (FluoTime 200, Picoquant GmbH). The excitation source was a
were dried and purified as described by Perrin and Armarego31 diode laser (LDH-P-670 driven by PDL 800-B, 670 nm, 20 MHz
before use. All other reagents were obtained from commercial repetition rate, 44 ps pulse width, Picoquant GmbH). Fluore-
suppliers and used without further purification.
scence was detected under the magic angle with a peltier
cooled photomultiplier tube (PMT) (PMA-C 192-N-M, Pico-
quant) and integrated electronics (PicoHarp 300E, Picoquant
Equipment
The infrared (IR) spectra were recorded between 4000 and GmbH). A monochromator with a spectral width of about
650 cm−1 using
Perkin Elmer Spectrum 100 FT-IR 8 nm was used to select the required emission wavelength
a
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band. The response function of the system, which was 14.04; Found: C 60.42; H 3.81; N 14.20. MS (MALDI-TOF), m/z
measured with a scattering Ludox solution (DuPont), had a (%): Calc. 1097.59, Found 1098.42 [M + H]+ (100).
full width at half-maximum (FWHM) of about 300 ps. The
ratio of stop to start pulses was kept low (below 0.05) to ensure methanesulfonyloxyethoxy)-ethoxy]ethoxy}ethoxy)
1,8(11),15(18)-Tri-(2-mercaptopyridine)-23(24)-4-(2-{2-[2-(2-
phthalo-
good statistics. All luminescence decay curves were measured cyaninato zinc(II) (6). Compound 5 (30 mg, 0.027 mmol) was
at the maximum of the emission peak. The data were analysed dissolved in dichloromethane (10 mL) in the presence of tri-
with the program FluoFit (Picoquant).
ethylamine (20 drops, excess) and cooled down to ∼0 °C in an
The triplet decay kinetics data were collected on a laser ice bath. Methane sulfonyl chloride (10 drops, excess) was
flash photolysis system. The light pulses used in excitation added drop-wise to the solution. The mixture was allowed to
were produced by a Quanta-Ray Nd:YAG laser (1.5 J/9 ns), warm to room temperature and stirred for 30 min. The reac-
pumping a Lambda Physik FL 3002 dye laser (Pyridin 1 in tion was followed by thin-layer chromatography (TLC) using a
methanol). The analyzing beam source was from a Thermo 25 : 1 dichloromethane : ethanol solvent mixture as an eluent.
Oriel 66902 xenon arc lamp, and a photomultiplier tube was When the starting compound 5 was completely consumed, the
used as a detector. Signals were recorded with a two channel, resulting solution was extracted with water and dichloro-
300 MHz digital real time oscilloscope (Tektronix TDS 3032 C). methane phase was dried with anhydrous sodium sulphate.
The samples were placed in a 1 cm path length fluorescence Dichloromethane was removed and the crude product (6) was
spectrophotometric cell, degassed by bubbling argon for purified by preparative TLC (silica gel) using
a 25 : 1
30 min and irradiated at the Q band maxima. The triplet life- dichloromethane : ethanol solvent system as an eluent. Yield:
times were determined by exponential fitting of the kinetic 30 mg (94%). IR [(ATR) νmax/cm−1]: 3061 (Ar C–H), 2919–2851
curves using the Origin Pro 8 software.
(C–H), 1608 (CvN), 1571 (CvC), 1449, 1417, 1386, 1329, 1238,
Transmission electron microscope (TEM) images were 1172 (SvO), 1096 (C–O–C), 963, 896, 804, 758, 740. UV-Vis
1
obtained using a JEOL TEM 1210 transmission electron micro- (DMSO): λmax nm (log ε) 693 (5.14), 630 (4.61), 343 (4.82). H
scope at 100 kV accelerating voltage. TEM samples were pre- NMR (500 MHz, DMSO-d6): δ, ppm = 8.89–6.99 (m, 24H, Pc-H
pared by placing a drop of conjugates or nanoparticle solution and Pyridyl-H), 4.55–3.37 (m, 16H, CH2), 3.15 (s, 3H, CH3).
on the sample grid and allowing it to dry before measure- Calc. for C56H43N11O7S4Zn: C 57.21; H 3.69; N 13.11; Found: C
ments. X-ray powder diffraction patterns were recorded on a 57.54; H 3.88; N 13.45. MS (MALDI-TOF), m/z (%): Calc.
Bruker D8 Discover equipped with a LynxEye detector, using 1175.68, Found 1176.97 [M + H]+ (100).
CuKα radiation (λ = 1.5405 Å, nickel filter). Data were collected
1,8(11),15(18)-Tri-(2-mercaptopyridine)-23(24)-4-(2-{2-[2-(2-mer-
in the range from 2θ = 5° to 100°, scanning at 1° min−1 with a captoethoxy)-ethoxy]ethoxy}ethoxy) phthalocyaninato zinc(II) (7)
filter time-constant of 2.5 s per step and a slit width of (ZnPcSH). A tetrahydrofuran (THF) (10 mL) and ethanol
6.0 mm. Samples were placed on a zero background silicon (3 mL) mixture was deoxygenated by sonication for 30 min
wafer slide. The X-ray diffraction data were treated using Eva using an ultrasonic bath. Compound 6 (20 mg, 0.018 mmol)
(evaluation curve fitting) software. Baseline correction was per- was dissolved in this previously prepared solvent mixture
formed on each diffraction pattern.
and the solution was refluxed under an argon atmosphere in
the dark. Then, thiourea (10 mg, excess) was added during
reflux and the reaction was followed using TLC. After the start-
ing compound 6 was consumed, argon was bubbled through
Synthesis
1,8(11),15(18)-Tri-(2-mercaptopyridine)-23(24)-4-(2-{2-[2-(2- the reaction mixture. Aqueous sodium hydroxide solution
hydroxyethoxy)-ethoxy]ethoxy}ethoxy) phthalocyaninato zinc(II) (20%, 4 mL) which was previously deoxygenated by sonication
(5). 4-(2-{2-[2-(2-Hydroxyethoxy)ethoxy]ethoxy}-ethoxy)phthalo-
for 30 min was added and the mixture was refluxed for 2 h.
nitrile (3) (100 mg, 0.31 mmol), 3-(2-mercaptopyridine) phtha- When the reaction was complete (monitoring with TLC), the
lonitrile (4) (222.2 mg, 0.94 mmol), DBU (0.1 mL, 0.63 mmol) resulting mixture was poured into a mixture of dilute hydro-
and Zn(OAc)2 (114.4 mg, 0.63 mmol) were refluxed in dry chloride acid and ice and extracted with dichloromethane. The
pentanol (3 mL) for 24 h under an argon atmosphere. Then, organic phase was extracted and dried with anhydrous sodium
the reaction mixture was cooled to room temperature and sulphate. The crude product (7, ZnPcSH) was purified by pre-
poured into n-hexane. The green solid product was precipi- parative TLC (silica gel) using a 10 : 1 dichloromethane :
tated and collected by filtration. The crude product (5) was ethanol solvent system as an eluent. Yield: 12 mg (63%). IR
purified over a silica gel column using firstly 25 : 1 and then a [(ATR) νmax/cm−1]: 3079 (Ar C–H), 2919–2867 (C–H), 1603
10 : 1 dichloromethane : ethanol solvent mixture as an eluent. (CvN), 1524 (CvC), 1481, 1449, 1386, 1323, 1279, 1238, 1109,
Yield: 50 mg (15%). IR [(ATR) νmax/cm−1]: 3400 (O–H), 3053 (Ar 1081, 944, 895, 762, 741. UV-Vis (DMSO): λmax nm (log ε) 693
C–H), 2921–2864 (C–H), 1607 (CvN), 1571 (CvC), 1482, 1448, (4.53), 643 (4.16), 343 (4.27). 1H NMR (500 MHz, DMSO-d6):
1418, 1386, 1331, 1315, 1240, 1095 (C–O–C), 962, 896, 731. δ, ppm = 9.48–7.48 (m, 24H, Pc-H and Pyridyl-H), 4.65–3.28
UV-Vis (DMSO): λmax nm (log ε) 693 (4.81), 624 (4.14), 346 (m, 16H, CH2), 7.09 (s, 1H, S–H). Calc. for C55H41N11O4S4Zn:
1
(4.43). H NMR (500 MHz, DMSO-d6): δ, ppm = 8.72–7.01 (m, C 59.32; H 3.71; N 13.83; Found: C 59.61; H 3.94; N 13.78.
24H, Pc-H and Pyridyl-H), 4.22–3.12 (m, 16H, CH2), 7.00 (s, MS (ES-MS), m/z (%): Calc. 1113.66, Found 1136.17 [M + Na]+
1H, O–H). Calc. for C55H41N11O5S3Zn: C 60.19; H 3.77; N (100).
4924 | Dalton Trans., 2013, 42, 4922–4930
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Conjugation of ZnPcSH to gold nanoparticles
where (ΦSΔtd) is the singlet oxygen quantum yield for the stan-
dard (ZnPc, ΦΔStd = 0.67 in DMSO).38 RSample and RStd are the
DPBF photobleaching rates in the presence of the respective
sample under investigation and the standard respectively.
Isample and IStd are the rates of light absorption by the sample
and the standard respectively.
Solutions of the complexes (ZnPcSH and ZnPcSH–AuNPs)
with an absorbance of ∼0.3 at the irradiation wavelength were
prepared in the dark and irradiated at the Q-band. DPBF
absorption at 417 nm was monitored with photolysis time.
Synthesis of TAOBr stabilized AuNPs has been reported
before.32–34 ZnPcSH was conjugated using the method
employed by Kotiaho et al. with modifications.32 The toluene
solution of gold nanoparticles was evaporated to about 1.5 mL
and 1.5 mg (0.0013 mmol) of ZnPcSH was added. The solution
was stirred for 24 h to allow ligand exchange. The Pc–AuNPs
conjugate was purified using Bio-Beads and toluene as an
eluent.
Photophysical and photochemical parameters
Fluorescence quantum yields (ΦF) were determined by a com-
Results and discussion
Synthesis and characterization
parative method35 using eqn (1):
FAstdn2
ZnPcSH complex. The synthesis of phthalonitrile derivatives
(1 to 4, Scheme 1) was previously reported.25–30 The synthesis
ΦF ¼ ΦFðstdÞ
¼
ð1Þ
2
FstdAnstd
where F and Fstd are areas under fluorescence emission curves of unsymmetrical phthalocyanines is complicated compared to
of the samples (ZnPcSH or ZnPcSH–AuNPs) and the standard symmetrically substituted Pcs as they often require extensive
respectively. A and Astd are the absorbances of the samples and purification methods to obtain the desired products. Scheme 1
the standard at the excitation wavelength and n and nstd are shows the chemical structure and the synthetic route of com-
refractive indices of solvents used for the samples and the pound 7. The statistical condensation method of two phthalo-
standard, respectively. ZnPc in DMSO was used as a standard, nitriles 3 and 4 was employed in this work. The synthesis of
ΦF = 0.20 for the determination of fluorescence quantum complex 7 starts with the synthesis of 5. The reaction for the
yields.36 The sample and the standard were both excited at the formation of 5 was performed by a statistical method using a
same relevant wavelength.
3 : 1 ratio of phthalonitrile 3 (A) : phthalonitrile 4 (B). As a
result, isomers were obtained in different amounts during the
synthesis of compound 5. We focused on the AB3 type isomer
(compound 5) in this study and this Pc derivative was separ-
ated by using column chromatography from the isomeric
mixture. We also aimed at the synthesis of compound 7 which
contains only one SH group from compound 5 for attachment
of this compound to Au nanoparticles via an SH group.
The terminal hydroxyl group on complex 5 was converted
into its mesylate counterparts using triethylamine (TEA) fol-
lowed by the addition of methanesulfonyl chloride to give the
substituted phthalocyanine 6. Finally, the terminal thiol func-
tional group was obtained under an inert atmosphere in the
absence of light, in a previously degassed THF–ethanol solvent
mixture containing complex 6. The resultant product was then
hydrolysed using 20% NaOH (also previously degassed). The
inert atmosphere inhibits the formation of disulfides via
aerobic oxidations and ensures the formation of complex 7.
Characterization of complexes of 5–7 was achieved using
infra-red (IR), ultraviolet-visible (UV-Vis), matrix-assisted laser
desorption/ionization-time of flight (MALDI-TOF) and proton
nuclear magnetic resonance (1H NMR) spectroscopies as well
as elemental analyses. The 1H NMR spectrum of the target
complex 7 showed aromatic protons between 7.48 and 9.48
(integrating for 24 protons), the CH2 groups between 3.28 and
4.65 (integrating for 16 protons) and the SH group at 7.09
(integrating for 1 SH proton). The mass spectrum of the complex
showed a molecular ion peak at 1136.17 amu as an M + Na
peak.
The triplet quantum yields were determined by eqn (2).
ΔATεSTtd
ΦT ¼ ΦSTtd
ð2Þ
ΔASTtdεT
where ΔAT and ΔASTtd are the changes in the triplet state absor-
bances of the samples and the standard respectively. εT and
εSTtd are the triplet state molar extinction coefficients for the
samples and the standard respectively. ΦSTtd is the triplet state
quantum yield for the unsubstituted ZnPc standard ΦSTtd = 0.65
for ZnPc in DMSO).37
εT and εsTtd were calculated from the molar extinction coeffi-
cients of their respective ground singlet state (εS and εsStd), the
change in absorbances of the ground states (ΔAS and ΔASStd
and the change in the triplet state absorptions (ΔAT and ΔATStd
using eqn (3a) and (3b).
)
)
ΔAT
ΔAS
ΔASTtd
ΔASStd
εT ¼ εS
ð3aÞ
ð3bÞ
εSTtd ¼ εSStd
Singlet oxygen quantum yield
Singlet oxygen quantum yield (ΦΔ) values were determined in
air using the relative method with DPBF as a singlet oxygen
quencher in DMSO. Eqn (4) was used for calculating singlet
oxygen quantum yields:
RSampleIStd
RStdISample
The UV-Vis spectrum of ZnPcSH is shown in Fig. 1A. It
shows broadening typical of aggregation with a broad peak
ΦΔ ¼ ΦSΔtd
ð4Þ
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Table 1 Spectral, photophysical and photochemical parameters of ZnPcSH and
ZnPcSH–AuNPs in DMSO
Parametera
ZnPcSH
ZnPcSH–AuNP
λabs
694
692
λexc
695
694
λemm
ΦF
708
0.06
3.08 0.02
706
0.04
τF (ns)b
2.38 (0.65) 0.02
1.54 (0.35) 0.01
τ0 (ns)
ΦT
τT (μs)
ΦΔ
51.33
0.63
217
78.75c
0.84
455
0.45
0.68
a λabs = absorption wavelength, λexc = excitation wavelength, λemm
=
emission wavelength. b Amplitudes in brackets. c Average lifetime used.
Au–S bond due to the SH group terminating the ethoxy ligand.
However, it is possible that the sulphur atoms of the mercapto
group also coordinate to the AuNPs. It is expected that the
stronger bond will be that of the SH group, resulting in a
mainly perpendicular arrangement on the AuNPs.
The surface plasmon resonance (SPR) peak was observed at
around 510 nm for both gold nanoparticles (AuNPS) and
ZnPcSH–AuNPs conjugate (Fig. 1B). A blue shift of about 2 nm
was observed in the absorption spectrum of the ZnPcSH–
AuNPs conjugate from that of the ZnPcSH. A blue shift usually
implies that the sulphur groups (which result in red shifting in
phthalocyanines) are engaged with AuNPs. However, the blue
shift in Fig. 1B is very small. Such small blue shifts have been
observed on conjugation of Pcs to AuNPs.11 The spectrum of
ZnPcSH in the presence of AuNPs is broad due to the absorp-
tion of the latter.
Fig. 2 shows the absorption, excitation and emission
spectra of the ZnPcSH–AuNPs conjugate. The absorption is
broadened due to the SPR band of AuNPs. This band is not
observed on the excitation or emission spectra since AuNPs do
not fluoresce. The excitation spectrum is the mirror image of
the emission spectrum for ZnPcAH–AuNPs as was the case for
ZnPcSH alone.
Fig. 1 (A) (i) Absorption, (ii) excitation and (iii) emission spectra of ZnPcSH and
(B) absorption spectra of (i) ZnPcSH, (ii) ZnPcSH–AuNPs and (iii) AuNPs complex
in DMSO.
near 630 nm39 and a more intense and sharper peak at
694 nm due to the monomer. However ZnPcSH obeys Beer’s
law at concentrations lower than 2 × 10−6 M in DMSO. The
spectrum of ZnPcSH is red shifted (at 694 nm) compared to
unsubstituted ZnPc (at 672 nm in DMSO), due to the presence
of sulphur groups and non-peripheral substitution.
The average size of the AuNPs and its conjugate was deter-
mined by powdered XRD of AuNPs and ZnPcSH–AuNPs using
the Scherrer eqn (5):40
The absorption, excitation and emission of the ZnPcSH
complex are shown in Fig. 1 (and Table 1). A slight red shift of
the excitation compared to absorption spectra could be a
result of different types of equipment used. The excitation
spectrum is narrower than the absorption spectrum due to
aggregation in the latter. Aggregates do not fluoresce. The exci-
tation spectrum is the mirror image of the emission spectrum.
kλ
β cosθ
dðÅÞ ¼
ð5Þ
where k is an empirical constant equal to 0.9, λ is the wave-
length of the X-ray source (1.5405 Å), β is the full width at half
maximum of the diffraction peak, and θ is the angular position
of the peak. The XRD patterns of TAOBr–AuNPs and ZnPcSH–
AuNPs were both indexed to a face centred cubic structure of
gold nanoparticles (Fig. 3). The sharper peaks observed for
AuNPs and ZnPcSH–AuNP conjugates
Phthalocyanine-functionalised gold nanoparticles were syn- AuNPs suggest crystallinity which becomes less on conjugation
thesized using a ligand exchange process, where the loosely of the AuNPs to ZnPcSH, Fig. 3B. The estimated average size of
bound TOABr ligands were partially exchanged by Pcs that the TAOBr–AuNPs and Pc–AuNPs was 5.3 nm and 17 nm,
bind covalently to the nanoparticle surface. The ZnPcSH respectively, using the peak at 37.58° (corresponding to gold).
complex is most likely attached to the AuNPs surface via an The large size of ZnPcSH–AuNPs suggests aggregation.
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TEM micrographs of the AuNPs and ZnPcSH–AuNPs are
shown in Fig. 4. Fig. 4A shows monodispersed spherical
AuNPs with an average particle size of 5.5 nm, which is similar
to the one determined using XRD. Upon conjugation with a
ZnPcSH complex the particles show slight aggregation
(Fig. 4B), with an estimated average size of 16.2 nm corre-
sponding to the values from XRD.
Fluorescence quantum yields and lifetimes
Fluorescence quantum yields and lifetimes are important
characteristics of a photosensitizer since molecules with high
fluorescence quantum yields usually have correspondingly low
triplet quantum yields. A slight decrease in the fluorescence
quantum yield was observed for ZnPcSH after conjugation
with AuNPs (Table 1). The fluorescence quantum yield for
ZnPcSH is lower than typical of Pc complexes3 and much lower
than that of the ZnPc standard.36 The decrease in fluorescence
in the presence of heavy metals is often associated with the
population of the triplet state of the molecule.41 This decrease
of the fluorescence quantum yield has been observed pre-
viously for gold nanoparticles–fluorophore systems,7,33,34,42
due to the heavy atom effect of the AuNPs. The fluorescence
lifetimes of both complexes as determined by the photon
counting method are also reported in Table 1 and the decay
curve is shown in Fig. 5. The measured lifetime of the ZnPcSH
complex was 3.08 ns whereas a ZnPcSH–AuNPs conjugate
showed two lifetimes of 2.38 and 1.54 ns. A plausible expla-
nation for two lifetimes is that there may be two different con-
jugation systems due to different orientation and packing of
Pc molecules on the AuNP surfaces. This is due to reports that
when single peripheral tethers were used (as is the case in this
work), the macrocycle rings were found to be tilted at various
angles to the substrate surface,8 with flat orientation being
achieved in some cases, but often, various degrees of tilt are
obtained. This is opposed to when octasubstituted rings are
employed where the so called “octopus” orientation is
observed.8
Fig. 2 (i) Absorption, (ii) excitation and (iii) emission spectra of a ZnPcSH–
AuNPs complex in DMSO.
The quenching of lifetimes may be explained using fluore-
scence radiative lifetime (τ0), which measures the lifetime of
Fig. 3 XRD patterns of (A) TAOBr–AuNPs and (B) ZnPcSH–AuNPs.
Fig. 4 TEM micrographs of (A) TAOBr–AuNPs and (B) ZnPcSH–AuNPs.
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the molecule in the absence of the non-radiative processes. reflected field.46,47 Therefore the observed increase in τ0 when
This lifetime is directly connected to absorption coefficients the phthalocyanine was attached to the gold surface could
and excited state lifetimes and hence can be calculated from suggest an uncorrelated field.
the measurement of fluorescence quantum yield (ΦF) and life-
time (τF) using eqn (6).43
Triplet quantum yields and lifetimes
Triplet quantum yields and lifetimes determine the ability of
the photosensitizer to produce singlet oxygen. Triplet
τ0 ¼ τF=ΦF
ð6Þ
The τ0 values were higher than the measured fluorescence quantum yield values are shown in Table 1 (decay curves
lifetimes (τF) (Table 1). The large values of τ0 compared to τF
shown in Fig. 6). Upon conjugation with AuNPs the triplet
are not surprising because non-radiative processes cause faster
quantum yield increased from 0.63 to 0.84. This effect shows
fluorescence decay. The larger values of τ0 have also been that the presence of gold nanoparticles enhances the triplet
reported previously for various phthalocyanines and in similar
state population. This enhancement of triplet quantum yield
could also be the contribution of the bromide ion present in
studies.44,45 A ZnPcSH–AuNPs conjugate was found to have a
higher τ0 as compared to a ZnPcSH complex. The radiative life- the nanoparticles from the tetraoctylammonium bromide used
in the synthesis.9 The triplet lifetime of a ZnPcSH–AuNPs con-
time (τ0) in the presence of metal particles can either increase
or decrease depending on the molecular dipole and the dis-
jugate (455 µs) was larger than that of the ZnPcSH complex
tance between the fluorophore and the metal. According to the alone (217 µs). This further shows that the triplet state was
studies done previously by Lakowicz,46,47 the radiative rate will
favoured in the presence of the AuNPs. The observation of
increase when the oscillating dipole is not in line with the
increased triplet quantum yield and lifetimes in the presence
of nanoparticles is not in agreement with what is expected,
where increase in triplet yields is expected to be accompanied
by a decrease in lifetime. The increase in the triplet lifetimes
in the presence of Au nanoparticles has been regularly
observed33 and it could be a result of the protection of the Pc
from the environment by the former.
Singlet oxygen quantum yields
Singlet oxygen is produced when photosensitizers in the
excited triplet state transfer energy to the molecular oxygen.
Thus singlet oxygen quantum yield (ΦΔ) is the measure
of the efficiency of singlet oxygen production. The photo-
degradation of the singlet oxygen quencher (DPBF) suggests
the ability of the complexes (ZnPcSH and ZnPcSH–AuNPs)
to produce singlet oxygen. Fig. 7 (A and B) shows time
dependent degradation of DPBF upon irradiation. A higher
singlet oxygen quantum yield of 0.68 was observed
for ZnPcSH–AuNPs as compared to 0.45 of ZnPcSH. The
Fig. 5 Fluorescence decay curves of (i) ZnPcSH and (ii) ZnPcSH–AuNPs in
DMSO.
Fig. 6 Triplet decay curves of (A) ZnPcSH and (B) ZnPcSH–AuNPs in DMSO.
4928 | Dalton Trans., 2013, 42, 4922–4930
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Fig. 7 Photodegradation of DPBF in the presence of (A) ZnPcSH and (B) ZnPcSH–AuNPs in DMSO.
Q-band of the phthalocyanine remained unchanged in both
complexes.
3 T. Nyokong and E. Antunes, in The Handbook of Porphyrin
Science, ed. K. M. Kadish, K. M. Smith and R. Guilard, Aca-
demic Press, New York, vol. 7, 2010, ch. 34, pp. 247–349,
World Scientific, Singapore.
4 M. Hanack, T. Schneider, M. Bartheb, J. S. Shirk, S. R. Flom
and R. G. S. Pong, Coord. Chem. Rev., 2001, 219–221, 235–
258.
Conclusions
A
novel zinc phthalocyanine complex which contains a
free thiol group was successfully synthesized and charac-
terized. The complex was successfully conjugated to gold
nanoparticles via ligand exchange. The surface plasmon peak
of gold nanoparticles was observed on the UV-Vis spectrum of
the conjugate and a blue shift showed the formation of
the conjugate. The fluorescence quantum yield and lifetime
decreased on conjugation. There was an increase in the triple
quantum yield and lifetime upon conjugation. The increase in
the triplet quantum yield was followed by an increase in the
singlet oxygen quantum yield upon conjugation.
5 K. I. Ozoemena and T. Nyokong, Inorg. Chem. Commun.,
2003, 6, 1192–1195.
6 M. C. G. Vior, L. E. Dicellio and J. Awruch, Dyes Pigm.,
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8 Z. Li and M. Lieberman, Langmuir, 2001, 17, 4887–4894.
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Acknowledgements
This work was supported by the Department of Science
and Technology (DST)/Nanotechnology (NIC) and National
Research Foundation (NRF) of South Africa through DST/
NRF South African Research Chairs Initiative for Professor of
Medicinal Chemistry and Nanotechnology and Rhodes
University.
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