Graphene oxide nanoparticles having long wavelength absorbing chlorins for highly-enhanced photodynamic therapy with reduced dark toxicity

Article abstract of DOI:10.3390/ijms20184344

The long wavelength absorbing photosensitizer (PS) is important in allowing deeper penetration of near-infrared light into tumor tissue for photodynamic therapy (PDT). A suitable drug delivery vehicle is important to attain a sufficient concentration of PS at the tumor site. Presently, we developed graphene oxide (GO) nanoparticles containing long wavelength absorbing PS in the form of the chlorin derivative purpurin-18-N-ethylamine (maximum absorption wavelength [λmax] 707 nm). The GO-PS complexes comprised a delivery system in which PS was loaded by covalent and noncovalent bonding on the GO nanosheet. The two GO-PS complexes were fully characterized and compared concerning their synthesis, stability, cell viability, and dark toxicity. The GO-PS complexes produced significantly-enhanced PDT activity based on excellent drug delivery effect of GO compared with PS alone. In addition, the noncovalent GO-PS complex displayed higher photoactivity, corresponding with the pH-induced release of noncovalently-bound PS from the GO complex in the acidic environment of the cells. Furthermore, the noncovalently bound GO-PS complex had no dark toxicity, as their highly organized structure prevented GO toxicity. We describe an excellent GO complex-based delivery system with significantly enhanced PDT with long wavelength absorbing PS, as well as reduced dark toxicity as a promising cancer treatment.

Full text of DOI:10.3390/ijms20184344

International Journal of
Molecular Sciences
Article
Graphene Oxide Nanoparticles Having Long
Wavelength Absorbing Chlorins for Highly-Enhanced
Photodynamic Therapy with Reduced Dark Toxicity
Eun Seon Kang, Tae Heon Lee, Yang Liu, Ki-Ho Han , Woo Kyoung Lee * and Il Yoon *
Center for Nano Manufacturing and Department of Nanoscience and Engineering, Inje University,
Gimhae 50834, Korea
* Correspondence: wlee@inje.ac.kr (W.K.L.); yoonil71@inje.ac.kr (I.Y.); Tel.: +82-55-320-3875 (W.K.L.);
+82-55-320-3871 (I.Y.); Fax: +82-55-320-3875 (W.K.L.); +82-55-321-7034 (I.Y.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 31 July 2019; Accepted: 2 September 2019; Published: 5 September 2019
Abstract: The long wavelength absorbing photosensitizer (PS) is important in allowing deeper
penetration of near-infrared light into tumor tissue for photodynamic therapy (PDT). A suitable drug
delivery vehicle is important to attain a sufficient concentration of PS at the tumor site. Presently,
we developed graphene oxide (GO) nanoparticles containing long wavelength absorbing PS in the
form of the chlorin derivative purpurin-18-N-ethylamine (maximum absorption wavelength [
λ
]
max
707 nm). The GO–PS complexes comprised a delivery system in which PS was loaded by covalent
and noncovalent bonding on the GO nanosheet. The two GO–PS complexes were fully characterized
and compared concerning their synthesis, stability, cell viability, and dark toxicity. The GO–PS
complexes produced significantly-enhanced PDT activity based on excellent drug delivery effect
of GO compared with PS alone. In addition, the noncovalent GO–PS complex displayed higher
photoactivity, corresponding with the pH-induced release of noncovalently-bound PS from the GO
complex in the acidic environment of the cells. Furthermore, the noncovalently bound GO-PS complex
had no dark toxicity, as their highly organized structure prevented GO toxicity. We describe an
excellent GO complex-based delivery system with significantly enhanced PDT with long wavelength
absorbing PS, as well as reduced dark toxicity as a promising cancer treatment.
Keywords: photodynamic therapy; graphene oxide; chlorins; covalent and noncovalent bond;
singlet oxygen
1. Introduction
Graphene, an sp2-bonded two-dimensional carbon nanosheet in the form of a honeycomb lattice, is
a nanomaterial that has received much attention for diverse applications, such as nanoelectronics, energy
storage and conversion, nanocomposite materials, and biomedicine, on the basis of its extraordinary
physicochemical properties [
extensively studied in recent years, especially for drug delivery [
photothermal therapy (PTT) [13 17]. Furthermore, GO can be easily modified by biomolecules with
abundant functional groups (epoxy, hydroxyl, and carboxylic acid units) on the GO nanosheet, resulting
in improved properties, such as stability, solubility, biocompatibility, and targeting ability [18 21].
In addition, the toxicity of GO strongly depends on several factors, such as concentration, and the
size, shape, and type of dispersants, among others [22 25]. Therefore, surface modification of GO
1–
5]. Among the graphene derivatives, graphene oxide (GO) has been
6–
10], biological imaging [11 12], and
,
–
–
–
significantly affects the toxicity of GO, and can result in reduced or no toxicity [26–28].
Photodynamic therapy (PDT), which is a non-invasive and patient-specific cancer treatment, uses
a photosensitizer (PS) to absorb light of an appropriate wavelength and transfer photon energy to the
Int. J. Mol. Sci. 2019, 20, 4344; doi:10.3390/ijms20184344
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surrounding oxygen, which generates highly toxic singlet oxygen (1O2) [29
–
35]. Effective nanocarriers
exhibiting high cellular uptake into tumor cells have recently been developed by the authors of this
study using gold nanoparticles (sphere and rod types) [31 32] and polyoxometalate complexes [33].
,
GO has been studied for use as a PDT, as well as a combination PDT/PTT [29,36–38]. However, these
studies typically used only commercially available PSs that do not absorb at long wavelengths; these
include chlorin e6 (Ce6,
λmax 660 nm) [29,30], zinc (II) phthalocyanine (λmax 670 nm) [37], porphyrin
derivative [38], and 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a (HPPH, Photochlor;
λ
max
665 nm) [39]. Therefore, the development of an effective GO-based PDT system incorporating a PS with
long wavelength absorption remains a great challenge. Furthermore, the toxicity of GO is important as
a potential delivery vehicle.
In this study, we tried to reduce the toxicity of GO in the formation of PS-containing GO
complexes. We used a synthetic chlorin derivative (purpurinimide) as a long wavelength-absorbing
PS (
λ
max
707 nm) to permit deeper light penetration at tumor sites. In addition, we fabricated two
GO complexes using the same PS by two different synthetic methods: covalent and noncovalent
bonding. We compared both GO complexes in terms of their synthesis, stability, cell viability, and dark
toxicity. Both GO complexes showed significantly enhanced PDT effects compared to those of free PS
(chlorin), which was attributed to the excellent delivery effects of GO. The two GO complexes exhibited
comparable phototoxicity, whereas the noncovalent GO complex showed lower dark toxicity than that
of the covalent GO complex. To the best of our knowledge, this is the first example of covalent and
noncovalent GO–PS complexes that use synthetic chlorin molecules with long wavelength absorption.
2. Results and Discussion
2.1. Characterizations of Prepared Photosensitizers and GO–PS Complexes
The synthesis of the purpurinimide derivative as a PS was straightforward (Scheme 1a). Methyl
pheophorbide-a (MPa) was obtained by extraction of chlorophyll-a paste followed by column separation.
MPa was converted to purpurin-18 (P18), and esterification of P18 afforded purpurin-18 methyl ester
(P18ME). Finally, purpurin-18-N-ethylamine (P18NEA, PS
(tert-butyloxycarbonyl) group from Boc-protected purpurinimide (P18NEAB). GO was prepared by a
modified Hummer’s method [40], and was conjugated to PS using amide formation with the terminal
stacking
39 45 46],
1) was formed after removal of the Boc
1
amine unit of the PS for covalent bonding [36,41–44] and supramolecular hydrophobic/π-π
interactions on the surface of sp2-bonded nanocarbons in GO for noncovalent bonding [29
,37
,
,
,
resulting in two GO complexes: GO–PS 2 and GO–PS 3, respectively (Scheme 1b).

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Scheme 1. (a) Synthetic scheme of purpurin-18-N-ethylamine, photosensitizer (PS) 1 b) Preparation
. (
of graphene oxide–photosensitizer (GO–PS) complexes via covalent (GO–PS
2) and non-covalent bonds
(GO–PS 3).
All reaction steps were monitored by thin-layer-chromatography (TLC) and UV-VIS absorption
spectroscopy. The structures of , and
were characterized by 1H-NMR, UV-VIS, fluorescence, FT-IR,
1,
2
3
Raman spectroscopy, thermogravimetric analysis (TGA), high resolution fast atom bombardment mass
spectrometry (HRFABMS) spectrometry, and transmission electron microscopy (TEM) (Figure 1 and
Figures S1–S13, see the supplementary information). The 1H-NMR spectrum of
1 contained two key
triplet signals at 4.56 and 3.30 ppm, corresponding to two aliphatic CH2 protons between two nitrogen
atoms, and a broad singlet signal at 4.87, corresponding to the NH2 terminus in the formation of the

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purpurinimide (Figure S5 in the supplementary information). HRFABMS analysis of PS
1
confirmed
the formation of PS
information). The UV-VIS absorption spectra of
in Figure 1a. The absorption spectra of and
The values were progressively red shifted for MPa (668 nm), P18 (700 nm), P18ME (700 nm), and
1
(calculated [M]+ = 621.3189; found = 621.3191, Figure S8 in the supplementary
1–3
and GO in dimethyl sulfoxide (DMSO) are shown
2
3
included the characteristic absorption features of 1.
λ
max
PS
the PS
binding of PS
1
(707 nm) (Figures S2, S4 and S6 in the supplementary information). In
1
2
and
core remained unchanged. The UV-VIS peak at 707 nm was used to confirm the successful
in GO–PS complexes and (Figure 1b), in which the absorbance (after subtraction of
absorbance contributed by GO only) was consistent with the concentration of PS in each sample.
The successful functionalization of GO–PS was confirmed by FT-IR spectroscopy (see Figure 1c and
3
, the
λ
values of
max
1
2
3
1
2
Figure S9 in the supplementary information). The FT-IR spectra (Table 1) of GO included peaks at
1723 cm−1 (vibrational C = O stretching frequency of surface carboxylic groups), 1623 cm−1 (skeletal
vibrations of C = C) and 1400–1000 cm−1 (C–O stretching and O–H deformations of the hydroxyl
and epoxy groups). Upon successful functionalization of GO with the terminal NH2 unit from PS
the amide bond C = O stretching frequency appeared at 1629 cm−1 (Figure S9 in the supplementary
information) [47]. The fluorescence emission spectra (excitation wavelength, 530 nm) of showed
good fluorescence activity of the PS in (Figure 1d), which is very useful for monitoring intracellular
uptake of the PS. Fluorescence intensity was lower in and than in 1, because of a fluorescence
quenching effect by GO as an electron acceptor. Notably, the fluorescence intensity of (noncovalently
bonded PS, quenching 61%) was decreased compared to (covalently bound, quenching 16%) due to
the better stacking interactions of . The fluorescence emission peak ( max) of was blue shifted
(739 nm), and the peak of was red shifted (779 nm) from that of (772 nm). Raman spectra of GO,
and
1,
1–3
1–3
2
3
3
2
π
–π
3
λ
2
3
1
2,
3
revealed changes in the characteristic D (the symmetry A1g mode) and G (the E2g mode of sp2
carbon atoms) bands at 1343.9 and 1586.6, 1352.5 and 1595.1, and 1352.9 and 1586.9 cm−1, respectively
(Figure 1e) [47]. Importantly, to determine the exact amount of organic content (PS) in the complexes,
TGA (Figure 1f) of GO,
27.2% and 77.7%, respectively. Complex
800 ◦C in N2. The GO–PS complexes
2
, and
3
was performed. The weight loss of GO and
and had a weight loss of 55.8% and 42.3%, respectively, at
and were determined to contain approximately 43.4 wt %
1
at 800 ◦C in N2 was
2
3
2
3
of GO and 56.6 wt % of PS, and 70.1 wt % of GO and 29.9 wt % of PS, respectively, according to the
following equations (x and y are the weight percentage of GO and PS, respectively) [8].
0.272x + 0.777y = 0.558
for
for
2
3
(1)
0.272x + 0.777y = 0.423
x + y = 1
The content was used to calculate the constant concentration of the PS in each sample for the cell
studies that included 912 nmol/mg in complex and 482 nmol/mg in complex . Therefore, complex
(covalently bound) was determined to contain approximately two times (ratio of 1.89) more PS than
complex (noncovalently bound). The TEM of GO, and are shown in Figure 1g. The average size
of GO and GO–PS complexes and were 96.1 47.7 nm (107.0 5.0 nm by dynamic light scattering,
Figure S10 in the supplementary information), 104.2 27.1 nm, and 187.2 40.1 nm based on TEM,
respectively (Figures S11–S13 in the supplementary information).
2
3
2
3
2
3
2
3
±
±
±
±

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Figure 1. Characterisation of PS
1
, and GO–PS complexes
) UV-VIS absorbance standard curve of
and GO (KBr); ( ) Fluorescence emission spectra of
) Raman spectra of , and GO; ( ) thermogravimetric analysis
) transmission electron microscopy (TEM) images of GO, 2, and (scale
bar 100 nm for GO, and 200 nm for 2 and 3).
2
and
3
: (
a
) UV-VIS absorbance spectra of
1
,
2
,
3
and GO (dimethyl sulfoxide (DMSO)); (
(DMSO), R2 = 0.99903; (
) FT-IR spectra of
excitation wavelength was 530 nm; (
(TGA) graph of , and GO; (
b
1
at 707 nm
c
1–
3
d
1–3,
e
2,
3
f
2,
3
g
3

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Table 1. Representative FT-IR vibrational frequency (wavenumber, cm−1) of important functional
groups for GO, PS 1, GO–PS 2, and GO–PS 3.
Compound
GO
1
2
3
C = O stretching
C = C
1723
1623
1735
1681, 1645
1732, 1629 (amide bond)
1677
1736
1679, 1644
C–O/O–H
1400–1000
Importantly, the covalent bonding-assisted reaction of GO–PS complex
weak acidic condition (pH 4–6) to prevent noncovalent binding of the PS
the synthesis. However, GO–PS complex was formed at neutral pH to maintain the noncovalently
bound PS on the GO nanosheet. Figure 2 shows the results of the release test of PS from both GO–PS
complexes and (solid samples in water and methylene chloride (MC) layers), in which PS was easily
released from GO–PS complex (noncovalently bound) in the acidic condition (pH 4.2), which resulted
in a pink colour of the MC layer (Figure 2b). This observation indicated that PS was successfully
noncovalently bound on the GO nanosheet in GO–PS complex . GO–PS complex displayed no
release of PS at the same pH because of covalent binding alone, which resulted in a colourless MC
layer (Figure 2b), which clearly indicated that complex contained only covalently-bound PS without
noncovalently bound PS. In GO–PS complex , PS was released at 43% for 1 h, 58% for 3 h, and 63% for
24 h from the pH 4.2 aqueous layer containing acetone into the MC layer, in which delivery ability of
for 3 h corresponded with 92% of 24 h. This observation exhibited that had a fast intracellular PS
2
was carried out under a
1
on the GO nanosheet during
3
1
1
2
3
1
3
1
3
2
2
3
3
3
release effect. The pH-induced release of the noncovalently-bound PS (purpurin-18 methyl ester) from
the GO surface has been described previously, where the acidic environment (pH 5.4) easily released
noncovalently-bound PS from the GO surface [48].
Figure 2. pH-induced release test of PS
1
in GO–PS
2
and
3
b
at the pH 4.2 aqueous layer into the
) No release of PS from (colourless
methylene chloride (MC) layer. ( ) A schematic presentation. (
a
1
2
MC layer) and release of PS 1 from 3 (pink coloured MC layer).
2.2. In Vitro Photocytotoxicity and Dark Toxicity
The successful cellular uptake and localization of
were confirmed by fluorescence images of the PS in each compound using confocal laser scanning
microscopy (CLSM, Figure 3). CLSM images revealed that and delivered markedly more PS into
the cells than did (PS alone without GO), which was due to the efficacy of GO as a nanocarrier. Most
1–3 in A549 (human lung adenocarcinoma) cells
2
3
1

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of the PSs were located in the cell cytoplasm [33]. Interestingly, there was no significant difference
in cellular uptake results of complexes and , indicating that the difference in loading methods
2
3
had negligible effects. The cellular uptake mechanisms of GO-based materials are not yet clearly
known. However, recent reports have described several mechanisms, such as endocytosis [47], direct
penetration of cell membranes [49], and micropinocytosis [50]. Among them, endocytosis is the most
common mechanism for the uptake of GO nanosheet complexes, whereas free PS (as an organic
molecule) can be internalized into cells by passive diffusion [51].
Figure 3. Uptake of
confocal laser scanning microscopy (CLSM). The concentration of PS was 1
4‘,6-Diamidino-2-phenylindole (DAPI) nuclear dye (blue), PS (chlorins, red) is shown in
1
,
2
, and
3
by A549 cells before irradiation after 24 h incubation assessed using
M in each sample.
or and in
µ
1
,
2
3
merged images. Bars, 100 µm.
The photocytotoxicity and dark toxicity of
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay at concentration ranges of
1.0–20.0 M for and 1.4–14.0 M for and . A549 cells (10
104 cells/well) were incubated with the
PSs for 24 h and photoirradiated for the photocytotoxicity test using a BioSpec LED with a band-pass
filter of 610–710 nm (total light dose 2 J
cm−2, irradiation time 15 min; Figure 4). After the cells were
incubated for 3, 12, and 24 h, their viability (%) was estimated on the basis of the mitochondrial activity
1–3 and GO for A549 cells were evaluated using the
µ
1
µ
2
3
×
·

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of NADH (reduced form of nicotinamide adenine dinucleotide) dehydrogenase (summarized in Tables
S1–S3 in the supplementary information).
Figure 4. Cell viability against A549 cells for photocytotoxicity (light, total light dose 2 J
irradiation time 15 min) and dark cytotoxicity (dark) after 3, 12, and 24 h incubation times of (
concentrations of 1.0–20.0 M, ( and at concentrations of 1.4–14.0 M, and ( ) photocytotoxicity
(total light dose 2 J and at low concentrations of 0.14–1.05 M. The percentage of cell
cm−2) of
·
cm−2
at
;
a
)
1
µ
b
)
2
3
µ
c
·
2
3
µ
viability was determined by an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)
assay. Error bars represent the standard deviation of three replicate experiments.
Upon photoirradiation, cell viability decreased for all compounds, consistent with the increased
concentration as well as incubation time. PS
h incubation with half maximal inhibitory concentration (IC50) values of 12.61, 5.49, and 1.44
respectively (Figure 4a, Table 2). However, on the basis of the excellent delivery effects of the GO
1
displayed low photocytotoxicity at 3, 12, and 24
µM,

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nanocarrier, GO complexes
2
and
and
M, Figure 4c). IC50 values of
and 0.21, and 0.31 and 0.20 M, respectively. The values exceeded those of PS
11 and 26 times, and 5 and 7 times, respectively (Table 2). To the best of our knowledge, this is the best
result in recent PDT research with such a low light dose (total light dose of 2 J
cm−2). Interestingly, the
IC50 results for and (Table 2) showed that the IC50 values of decreased, while remained constant.
This suggests a difference in the delivery effect, and therefore the time-dependent intracellular uptake
using CLSM at 30 min, 3 h, and 24 h was performed. As shown in Figure 5, had a faster delivery
compared with at all times. Therefore, the lower and constant IC50 values of may be because of the
faster delivery effect of 3 based on rapid intracellular release of PS from the GO complex.
3
exhibited significantly-enhanced photocytotoxicity (Figure 4b).
To evaluate the IC50 values of
(0.14–1.05
2
3
, cell viability was determined at a low concentration range
and at 3, 12, and 24 h incubation were 0.69 and 0.22, 0.48
were 18 and 57 times,
µ
2
3
µ
1
·
2
3
2
3
3
3
2
Table 2. IC50 values (
µM) of PS
1, GO–PS
2, and GO–PS 3 at 3 h, 12 h, and 24 h incubation times
after photoirradiation.
Compound/Incubation Time
1
2
3
3 h
12 h
24 h
12.61
5.49
1.44
0.69
0.48
0.31
0.22
0.21
0.20
Figure 5. Time-dependent intracellular uptake of
3 h, and 24 h incubation assessed using CLSM. The concentration of PS was 2
Merged images are shown with DAPI (cell nuclei, blue) and PS (chlorins, red) in 2 or 3. Bars, 50 µm.
2
and
3
by A549 cells without irradiation after 30 min,
µM in each sample.
Complexes
similar to the toxicity of free GO [37]. Notably, complex
have been due to the extensive covering of the GO nanosheet in complex
chlorin molecules, which formed a highly organized structure of PS that enabled good contact with the
flat GO surface through strong stacking. The resulting hydrophobic interactions mitigated the
toxicity exhibited by free GO [37].
To evaluate the effect of GO nanosheets in GO complexes and 3, we used GO alone (with no PS).
1
and
3
exhibited negligible dark toxicity, whereas
had less dark toxicity than did
with noncovalently-bound
2
had dose-dependent dark toxicity
3
2
. This might
3
π-π
2
GO had concentration-dependent dark toxicity and no phototoxicity effect (Tables S1 and S2 in the
supplementary information).
2.3. Live/Dead Cell Imaging and Singlet Oxygen Photogeneration
The cell viability results after photoirradiation were confirmed by fluorescence imaging of live
(green)/dead (red) cells stained with calcein acetoxymethyl (AM)/propidium iodide (PI) (Figure 6a).
Almost all cells treated with GO complexes
number of dead cells treated with , indicating that the PS loaded on GO nanosheets provided more
effective photodynamic activity than that of the PS alone.
2 and 3 were dead (red colour) compared with the smaller
1

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Figure 6.
(AM)/propidium iodide (PI) after photoirradiation followed by 3 h incubation with
The concentration of each PS was 1 M. Bars, 100 m. ) 1,3-Diphenylisobenzofuran (DPBF)
absorbance decay (%, 50 M in DMSO) at 418 nm after photoirradiation (total light dose 2 J
cm−2
irradiation time 15 min) in the absence (control) and presence of 1 , GO, and methylene blue
(
a) Fluorescence images of live (green)/dead (red) cells stained with calcein acetoxymethyl
1,
2
, and 3.
µ
µ
(b
µ
·
;
µM 1–3
(MB) (with no tumor cells). Error bars represent the standard deviation of three replicate experiments.
To quantify the relative photodynamic effects of all compounds in the absence of tumor cells,
1O2 photogeneration was measured by 1,3-diphenylisobenzofuran (DPBF) as a selective 1O2 acceptor
(Figure 6b) [43
,
52]. GO complex
was almost comparable to methylene blue (MB), a standard 1O2 sensitizer.
2
exhibited higher 1O2 photogeneration compared with that of the free
PS . Notably, compound
1
2
It is interesting that even though the fluorescence intensity of
(Figure 1d), 1O2 photogeneration of 2 was higher than 1.
2 was lower (16% quenching) than 1
Interestingly,
(Figure 1d), whereas
. Complex
showed lower 1O2 photogeneration compared with
activity (Figure 4) based on the delivery effect of GO, as well as the release effect of the PS from
3
displayed markedly decreased (61% quenching) fluorescence intensity compared
presented slightly lower (6.03%) 1O2 photogeneration compared with
, whereas exhibited better PDT
in the
with
1
1
3
3
1
3
3
acidic environment of the cells (Figure 2). GO alone displayed no 1O2 photogeneration. These results
proved that the significantly enhanced photodynamic activity of the complexes was attributable to
their higher induction of 1O2 photogeneration by better intracellular penetration and localization of PS

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molecules than those of free PS. The complexes achieved this better penetration via an endocytosis
approach based on the excellent nanocarrier properties of GO.
3. Materials and Methods
3.1. Materials
Acetone, methylene chloride (CH2Cl2, MC), methanol (MeOH), and dimethyl ether
were purchased from SK Chemical. Potassium hydroxide (KOH), potassium bromide (KBr),
anhydrous sodium sulphate, dimethyl sulfoxide (DMSO), sodium bicarbonate, hydrogen
chloride (HCl), and sulfuric acid (H2SO4) were purchased from Samchun Chemical.
N-(tert-Butoxycarbonyl)-1,2-diaminoethane(N-boc-ethylenediamine), 1,3-diphenylisobenzofuran
(DPBF), and methylene blue (MB) were purchased from TCI Chemical.
1-Propanol and
pyridine were purchased from Burdick and Jacson Chemical. Trifluoro acetic acid (TFA),
1-ethyl-3-(3-dimethylamino-propyl)carbodiimide (EDC), and N-hydroxysuccinimide (NHS) were
purchased from Sigma-Aldrich Chemical. RPMI-1640 medium, penicillin-streptomycin, and fetal
bovine serum (FBS) were obtained from Gibco Life Technologies, Inc. (Grand Island, NY, USA).
3.2. General
All reactions were monitored by thin-layer-chromatography (TLC) using Merck 60 silica gel
F254 pre-coated (0.2 mm thickness) glass-backed sheets. Silica gel 60A (230–400 mesh, Merck) was
used for column chromatography. UV-VIS absorption spectra were recorded on a SCINCO S-3100
UV-VIS spectrophotometer using a 1 cm quartz cuvette. 1H-NMR spectra were obtained using a
Varian spectrometer (500 MHz) at the Biohealth Product Research Center of Inje University. High
resolution fast atom bombardment mass spectrometry (HRFABMS) analysis was conducted using a
Jeol JMS700 high resolution mass spectrometer at the Daegu center of the Korea Basic Science Institute
(KBSI), Kyungpook National University, Korea. Transmission electron microscopy (TEM) images
were obtained using an H-7600 microscope (Hitachi) at Pusan National University. Fluorescence
spectra were obtained using a model F7000 fluorescence spectrophotometer (Hitachi) at Gyeongsang
National University. Raman spectra were obtained using a LabRAM HR800UV Raman spectrometer
at Gyeongsang National University. Thermogravimetric analysis (TGA) was conducted using a TA
Instrument Q600, PH470 at the Pusan center of KBSI.
3.3. Synthesis of Photosensitizer
3.3.1. Purpurin-18 (P18)
Methyl pheophorbide-a (MPa) was synthesized from chlorophyll-a paste extracted using acidic
methanol. MPa (1 g) was dissolved in pyridine (5 ml) and diethyl ether (400 ml), followed by addition
of KOH solution (12 g dissolved in 80 ml of 1-propanol). The resulting solution was aerated and stirred
for 3 h. The solution was extracted with water (500 ml) and the pH was adjusted to 2 with aqueous 1
M H2SO4. The acidic solution was separated with CH2Cl2/H2O and the organic layer was evaporated.
The purple residue was purified by column chromatography with an eluent of 5% MeOH/CH2Cl2.
1H-NMR (500 MHz, CDCl3):
δ 9.30, 9.20, and 8.52 (all s and 1H, 10-H, 5-H and 20-H, respectively),
7.78 (dd, J = 17.8, 11.6 Hz, 1H, 31CH = CH2), 6.23 (d, J = 17.8 Hz, 1H, trans-32-H), 6.13 (d, 11.5 Hz, 1H,
cis-32-H), 5.12 (d, J = 8.6 Hz, 1H, 17-H), 4.35 (q, J = 7.3 Hz, 1H, 18-H), 3.57 (s, 3H, 12-CH3), 3.57(s, 3H,
12-CH3), 3.47 (m, 2H, 81-CH2), 3.28 (s, 3H, 2-CH3), 3.05 (s, 3H, 7-CH3), 2.79–2.70 (m, 1H, 172-CH2),
2.56–2.45 (m, 1H, 171-CH2), 1.70 (d, J = 7.3 Hz, 3H, 18-CH3), 1.56 (t, J = 7.6 Hz, 82-CH3),
−0.28 (all brs and 1H, NH).
−
0.04 and

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3.3.2. Pupurin-18 Methyl Ester (P18ME)
P18 (300 mg) was dissolved in CH2Cl2 and stirred at room temperature for 5 min with diazomethane
1
(5 ml, excess). The mixed solution was evaporated and used in the next step without purification. H
NMR (500 MHz, CDCl3):
δ 9.63, 9.40, and 8.59 (all s and 1H, 10-H, 5-H and 20-H, respectively), 7.91
(dd, J = 17.8, 11.5 Hz, 1H, 31CH = CH2), 6.32 (d, J = 17.9 Hz, 1H, trans-32-H), 6.21 (d, J = 11.5 Hz, 1H,
cis-32-H), 5.21 (d, J = 8.7 Hz, 1H, 17-H), 4.41 (q, J = 7.5 Hz, 1H, 18-H), 3.82 (s, 3H, 12-CH3), 3.67 (q,
J = 7.9 Hz, 2H, 81-CH2), 3.61 (s, 3H, 172-OCH3), 3.36 (s, 3H, 2-CH3), 3.19 (s, 3H, 2-CH3), 2.79–2.70 (m,
1H, 172-CH2), 2.54–2.41 (m, 2H, 171-H), 2.07–1.96 (m, 1H, 172-CH2), 1.76 (d, J = 7.4 Hz, 3H, 18-CH3),
1.69 (t, J = 7.7 Hz, 3H, 82-CH3), 0.28 and −0.03 (all brs, 1H, NH).
3.3.3. Purpurin-18-N-Ethylamine, PS 1
P18ME was dissolved in CH2Cl2 (5 ml) and N-(tert-butoxycarbonyl)-1,2-diaminoethane was
added to the solution. The resulting solution was stirred at room temperature for 24 h in the dark.
The color changed from purple to green. KOH solution (KOH 30 mg/MeOH 5 ml) was added to the
solution until the color changed to purple. The solution was then separated with water to remove
KOH and the solvent was evaporated. The mixed residue was dissolved in MC and was poured into
TFA and stirred for 30 min. The color changed to green and the residue was poured into 2 M Na2CO3
and the solvent was evaporated. Column separation using an eluent of 10% MeOH/MC afforded
1
the final compound. H NMR (500 MHz, CDCl3):
δ 9.62, 9.37, and 8.57 (all s and 1H, 10-H, 5-H and
20-H), 7.91 (dd, J = 17.9, 11.5 Hz, 1H, 31CH = CH2), 6.29 (d, J = 17.9 Hz, 1H, trans-32-H), 6.17 (d, J
= 11.5 Hz, 1H, cis-32-H), 5.34 (d, J = 9.1 Hz, 1H, 17-H), 4.87 (brs, 2H, NH2), 4.56 (t, J = 6.5Hz, 1H,
N-CH2-CH2-NH2), 4.35 (q, J = 7.3 Hz, 2H, 18-H), 3.82 (s, 3H, 12-CH3), 3.65 (q, J = 13.3 Hz, 2H, 81-CH2),
3.56 (s, 3H, 172-CH2), 3.30 (t, J = 6.9 Hz, 2H, N-CH2-CH2-NH2), 3.17 (s, 3H, 7-CH3), 2.75–2.64 (m, 1H,
172-CH2), 2.48–2.33 (m, 2H, 171-CH2), 2.03 (d, J = 8.8 Hz, 3H, 18-CH3), 1.67 (t, J = 7.6 Hz, 3H, 82-CH3),
0.07 and
−
0.11 (all brs, 1H, NH). UV-VIS (DMSO)
λ
(log ε): 419 (4.08), 511 (2.90), 551 (3.34), 651
max
(2.96), 707 (3.63) nm. HRFABMS: calculated for C36H41N6O4 (MH +) 621.3189; found 621.3191. IR (KBr)
3450, 2960, 2922, 2852, 1734, 1681, 1645, 1601, 1544, 1526, 1453, 1261, 1238, 1094, 1062, 801, 634 cm−1
.
3.4. Synthesis of GO and GO–PS Complexes
3.4.1. Synthesis of GO
GO was prepared by Hummer’s method [40]. IR (KBr) 3175, 2242, 1961, 1743, 1723, 1698, 1623,
1507, 1393, 1372, 1221, 1050, 846, 586 cm−1
.
3.4.2. Preparation of GO–PS Complex with Covalent Bond, GO–PS 2 (GO:PS = 1:2)
GO (40 mg) was dissolved in DMSO (20 ml) and was sonicated for 5 min. EDC (84 mg) and NHS
(62 mg) were added into the GO solution with sonication (5 min) and adjusted to pH 4–6 using an
aqueous solution of 1 M NaOH and 1 M HCl. Thereafter, the GO solution was stirred with P18NEA
(80 mg) at room temperature for 24 h (dark condition). After the reaction, the mixed solution was
dialyzed using a dialysis membrane with a molecular weight cutoff 10.0 kDa in distilled water in
the dark for 3 days to remove unbound PS
washed with cold MC. Finally, the solution was freeze-dried for 3 days, and a dark-purple powder was
obtained. UV-VIS (DMSO) max (log ): 420 (4.06), 510 (3.40), 551 (3.54), 648 (3.36), 707 (3.69) nm. IR
(KBr): 3206, 2964, 2924, 2853, 1732, 1677, 1629 (carbonyl group on amide bond), 1528, 1242, 1169, 1071,
791, 588 cm−1
1 and DMSO, centrifuged several times with water, and
λ
ε
.
3.4.3. Preparation of GO–PS Complex with Noncovalent Bond, GO–PS 3 (GO:PS = 1:2)
GO (40 mg) was dissolved in DMSO (20 ml) and sonicated for 5 min. Thereafter, PS
1 (80 mg) was
added into the GO solution. The mixed solution was filled with deionized water (20 ml) and stirred

Int. J. Mol. Sci. 2019, 20, 4344
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at room temperature for 24 h in the dark. After the reaction, the mixed solution was dialyzed using
dialysis membrane with a molecular weight cutoff 10.0 kDa in distilled water in the dark for 3 days to
remove unbound PS
1 and DMSO, centrifuged several times with water, and washed with cold MC.
Finally, the solution was freeze-dried for 3 days to obtain a dark-purple powder. UV-VIS (DMSO)
λ
max
(log ε): 419 (4.63), 512 (3.75), 551 (3.99), 650 (3.74), 707 (4.22) nm. IR (KBr): 3340, 2958, 2923, 2853, 1736,
1679, 1644, 1527, 1241, 1063, 792, 586 cm−1
.
3.5. Cell Culture and Photoirradiation
A549 human lung carcinoma cells were obtained from the cell line bank of the Seoul National
University Cancer Research Center. The cells were grown in RPMI-1640 (Sigma-Aldrich) containing
◦
10% fetal bovine serum and 1% penicillin at 37 C in a humidified atmosphere of 5% CO2 in air.
Phosphate-buffered saline (PBS, Sigma-Aldrich), model CK40-32 PH optical microscope (Olympus),
SynergyHT fluorescence multi-detection reader (BioTek), trypsin-ethylenediamine tetraacetic acid
(EDTA) solution, and an incubator (37 ◦C, 5% CO2) were used in the procedures. PDT was conducted
using a BioSpec LED equipped with a band-pass filter of 640–710 nm.
3.6. MTT Assay and Cell Viability
Cells were placed in wells of a 48-well, flat-bottomed microplate, 200
After 24 h, the medium was removed and the plates were washed two times with PBS. PS
, GO–PS , and GO in 200
removed and the plates were washed three times with PBS, followed by the addition of medium (200
The plates were irradiated (2 J
cm−2) for 15 min, followed by the MTT assay to evaluate photodynamic
µL, containing 1
×
104 cells.
, GO–PS
1
2
3
µl of mixed medium were added to wells. After 24 h, the solutions were
µl).
·
activity. MTT solution was added to each well followed by incubation for 1 h and absorbance at 450 nm
was measured using a fluorescence multi-detection reader. Incubation occurred for 3, 12, and 24 h
following photoirradiation. Each group consisted of three replicate wells for each experiment.
3.7. Cellular Accumulation
A549 cells were spread out on confocal dishes (1
24 h, the medium was removed and dishes were washed three times with PBS. Furthermore, 1
PS , GO–PS , and GO–PS were added to the dishes. After 24 h, the dishes were washed three times
×
104 cells/dish) and incubated for 1 day. After
M of
µ
1
2
3
with PBS. Fixation solution was added for 10 min. The fixed cells were washed with PBS and stained
with diamidino-2-phenylindole (DAPI) (200 nM) to identify the nucleus. Cellular uptake images were
shown using CLSM with an LSM 510 META microscope (Carl Zeiss).
3.8. Singlet Oxygen Photogeneration
DPBF was used as a selective singlet oxygen (1O2) acceptor, being bleached upon reaction with
1O2. Seven sample solutions of DPBF in DMSO (50
µ
M) containing DPBF only (50
M), DPBF + GO–PS (1 M), DPBF + GO–PS
M) were prepared in the dark. All samples were put into wells of a 48-well plate and covered
with aluminum foil. The plate was irradiated (2 J
cm−2) for 15 min. The absorbance of each sample
µ
M, control sample),
DPBF + MB (1
+ GO (1
µM), DPBF + PS
1
(1
µ
2
µ
3
(1 M), and DPBF
µ
µ
·
was measured at 418 nm using a fluorescence multi-detection reader.
3.9. Live and Dead Cell Imaging
A549 cells were spread out on a cell culture slide (2
The medium was removed and dishes were washed three times with PBS. Thereafter, 1
GO–PS , and GO–PS were added to the dishes, respectively. After 24 h, the slide was irradiated for
15 min (2 J
cm−2) and incubated for 24 h. The slide was stained for 1 h using a Live/Dead Cell staining
×
104 cells/well) and were incubated for 1 day.
M of PS
µ
1,
2
3
·
Kit (EZ-View, BIOMAX) and examined using fluorescence microscopy at an excitation wavelength of
490 nm.

Int. J. Mol. Sci. 2019, 20, 4344
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4. Conclusions
We successfully prepared two complexes comprising GO nanoparticles and either covalently
and noncovalently bound PS, which is a synthetic and long wavelength-absorbing chlorin derivative
(purpurin-18-N-ethylamine, PS
1). The complexes were fully characterized (Table 3). Cell viability
determinations demonstrated that both GO–PS complexes
2
and had significantly enhanced PDT
3
activity compared to that of free PS. This enhancement corresponded to an increase in intracellular
accumulation via endocytosis based on the excellent delivery features of GO followed by higher 1O2
photogeneration after photoirradiation in tumour cells, which was confirmed by CLSM and live/dead
cell staining (calcein AM/PI) images. The PDT activity of GO–PS complexes
However, the noncovalently bound GO–PS complex had lower dark toxicity than did
complex
displayed lower 1O2 photogeneration compared with free PS, but had better PDT activity
2
and
3
were similar.
3
2. GO–PS
3
based on the delivery effect of GO, as well as the pH-induced release of the PS from the noncovalent
GO–PS complex in the acidic environment in the cells. The results will be useful in developing
third-generation PSs with long wavelength absorption as well as low dark toxicity based on the
excellent delivery system of GO in PDT for cancer.
Table 3. Summary of synthesis, stability, cell viability, and dark toxicity for GO–PS 2 and GO–PS 3.
GO–PS 2
GO–PS 3
noncovalent bond by
hydrophobic/π-π stacking at
neutral pH
covalent bond by amide formation
at pH 4–6 (weak acidic)
Synthesis and synthetic condition
56.6 wt % PS; 43.4 wt % GO (PS:
29.9 wt % PS; 70.1 wt % GO (PS:
Organic content by TGA
Size by TEM
912 nmol/mg)
482 nmol/mg)
104.2 ± 27.1 nm
187.2 ± 40.1 nm
release of PS under weak acidic
condition
Stability in acidic aqueous solution stable under weak acidic condition
Cell viability (IC50) at 3–24 h
0.69–0.31 µM
0.22–0.20 µM
negligible until 14.0 µM
73.11%
incubation times
Dark toxicity
dose-dependent toxicity
1O2 Photogeneration (remaining
of DPBF)
64.32% (comparable with MB,
59.22%)
Supplementary Materials: Supplementary materials can be found at http://www.mdpi.com/1422-0067/20/18/
4344/s1.
Author Contributions: E.S.K. contributed for the experiments containing synthesis of the chlorin derivative, GO
and GO complexes, and in vitro study. T.H.L. and Y.L. contributed to the in vitro study. K.-H.H. gave suggestions.
W.K.L. and I.Y. are corresponding authors and contributed with financial support and in preparation of the paper.
Acknowledgments: This research was supported by a National Research Foundation (NRF) of Korea
grant funded by the Korean government (NRF-2014R1A1A4A01008346, NRF-2015R1D1A1A01057746, and
NRF-2017R1A2B4010615). We would like to thank Editage (www.editage.co.kr) for English language editing.
Conflicts of Interest: The authors declare no conflicts of interest.
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