Immobilization of kojic acid in ZnAl-hydrotalcite like compounds

Article abstract of DOI:10.1016/j.jpcs.2011.10.007

Kojic acid (KOJ) is a melanin synthesis inhibitor widely used as skin lightening agent in topical preparations. Unfortunately it is easily susceptible to photo-oxidation, phenomenon responsible for chemical and organoleptic modifications. The aim of this work was the intercalation of KOJ in hydrotalcite-like compounds (HTlc) in order to stabilize KOJ and to reduce its photolability. Hydrotalcite containing Zn and Al (ZnAl-HTlc) was used as host to obtain the final compound ZnAl-HTlc-KOJ. The intercalation was carried out, after many attempts, by ionic exchange mechanism by means of the strong base EtO^- in anhydrous ethanol/dimethylsulfoxide (DMSO) mixture as solvent in order to generate KOJ^- anions. The final product was characterized by the X-ray powder diffraction (XRPD), FT-IR spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and elemental analysis. The intercalated compound was formulated in a siliconic water free self-emulsifying ointment and the in vitro release profile was evaluated. All samples (intercalation compound and its formulation) were submitted also to spectrophotometric assays in order to evaluate the matrix protective effect towards ultraviolet rays.

Full text of DOI:10.1016/j.jpcs.2011.10.007

Journal of Physics and Chemistry of Solids 73 (2012) 94–98
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Immobilization of kojic acid in ZnAl-hydrotalcite like compounds
Valeria Ambrogi a,n, Luana Perioli a, Morena Nocchetti b, Loredana Latterini c, Cinzia Pagano a,
Elena Massetti a, Carlo Rossi a
a
ꢀ
Dipartimento di Chimica e Tecnologia del Farmaco, Universita di Perugia, Via del Liceo 1, 06123 Perugia, Italy
b
c
ꢀ
Dipartimento di Chimica, Universita di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
ꢀ
CEMIN-Dipartimento di Chimica, Universita di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Kojic acid (KOJ) is a melanin synthesis inhibitor widely used as skin lightening agent in topical
preparations. Unfortunately it is easily susceptible to photo-oxidation, phenomenon responsible for
chemical and organoleptic modifications. The aim of this work was the intercalation of KOJ in
hydrotalcite-like compounds (HTlc) in order to stabilize KOJ and to reduce its photolability. Hydro-
talcite containing Zn and Al (ZnAl-HTlc) was used as host to obtain the final compound ZnAl-HTlc-KOJ.
The intercalation was carried out, after many attempts, by ionic exchange mechanism by means of the
strong base EtOꢀ in anhydrous ethanol/dimethylsulfoxide (DMSO) mixture as solvent in order to
generate KOJꢀ anions. The final product was characterized by the X-ray powder diffraction (XRPD), FT-
IR spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and
elemental analysis. The intercalated compound was formulated in a siliconic water free self-emulsify-
ing ointment and the in vitro release profile was evaluated. All samples (intercalation compound and its
formulation) were submitted also to spectrophotometric assays in order to evaluate the matrix
protective effect towards ultraviolet rays.
Received 23 July 2011
Received in revised form
18 September 2011
Accepted 5 October 2011
Available online 12 October 2011
Keywords:
A. Inorganic compounds
B. Chemical synthesis
C. Infrared spectroscopy
C. Thermogravimetric analysis
C. X-ray diffraction
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
as a food additive for preventing enzymatic browning and, as a
cosmetic agent, for skin whitening because of its excellent
Recently a gradual increase in the realization of skin lightening
[1] products has been observed. This is strictly connected to the
growth of skin hyperpigmentation problems. Actually, many
products such as on prescription, over-the-counter (OTC) and
cosmetics, are on the market for the treatment of this kind of skin
conditions. Unfortunately, among the available formulations, it is
not easy to find a topical regimen that fits with the requirements
of safety and efficacy [2].
Hydroquinone is one of the most effective agents in the
hyperpigmentation therapy. Actually, this molecule is not
employed in UE and USA cosmetic market because of its side
effects, namely allergies, irritant dermatitis, acneiform eruptions,
telangectasias and fungal and bacterial infections (folliculitis and
impetiginous lesions). However, the hydroquinone is still used in
Africa, although it is the cause of frequent cutaneous complica-
tions [3,4]. Because of these side effects, studies on alternative
depigmenting agents have been encouraged; one of the most
promising is KOJ (Fig. 1). It is a fungal metabolite produced by
many species of Aspergillus, Acetobacter and Penicillum. It is used
inhibitory activity on the polyphenol oxidase (tyrosinase) [5]. It
is the most popular agent employed for the treatment of melasma
and it can be combined with other skin lighteners. Unfortunately,
it is easily susceptible to oxidation after sunlight exposure, pH
and temperature variations [6,7]. KOJ both in solution and in
cosmetic formulations browns because of its oxidation or it turns
red because of its complexation with metal ions like iron. This
behavior is responsible for its chemical and organoleptic mod-
ifications, while skin-lightening efficacy decreases [6]. Recently,
KOJ has been evaluated also as potential radioprotective agent
that can improve the tolerance of normal tissue to radiation [8].
Hydrotalcite-like compounds [9] have been proposed as
matrices able to increase sunscreen photostability such as p-ami-
nobenzoic acid (PABA), 4-benzophenone and phenylbenzimida-
zole sulfonic acid [10,11]. Therefore, these matrices may
represent a new strategy to improve KOJ stability. Hydrotalcites
are natural anionic lamellar solids with positively charged layers
balanced by exchangeable anions [9]. They have the general
nꢀ
formula [M(II)1ꢀxM(III)x(OH)2]xþ[A
]
xꢀm S, where M(II) is a
x/n
bivalent metal cation, M(III) is a trivalent metal cation (usually
Al), Anꢀ is an exchangeable inorganic or organic anion that
compensates the positive charge of the layer, m are the moles
of solvent S, usually co-intercalated water for mole of compound.
n Corresponding author. Tel.: þ39 0755855135; fax: þ39 0755855135.
E-mail address: valeria.ambrogi@unipg.it (V. Ambrogi).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2011.10.007

V. Ambrogi et al. / Journal of Physics and Chemistry of Solids 73 (2012) 94–98
95
weighted directly into the aluminum pans and thermal analyses
were conducted at a scanning rate of 5 1C/min from 25 to 200 1C.
FT-IR spectra were recorded in KBr dispersion on a Jasco model
FT-IR-410, 420 Herschl series (Jasco Corporation Tokyo, Japan)
using the EasiDiff Diffuse Reflectance Accessory. Samples were
prepared by gently grounding the powder with KBr.
TGA were performed by a Stanton Redcroft STA 781 thermo-
analyzer (Stanton-Redcroft, US) at heating rate of 10 1C minꢀ1 in
Fig. 1. KOJ (5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) chemical structure.
airflow 30 ml minꢀ1
.
An intercalated anion can be replaced by another via ion-
exchange mechanism and for this reason the interlayer region
of a lamellar host can be considered a microvessel where an
anionic molecule may be stored [12,13].
The KOJ content was determined by UV spectrophotometry
(spectrophotomer Agilent 8453) at lmax¼269.0 nm.
H1-NMR spectra were recorded by Brucker AC 200 instrument
(200 MHz) in deuterium chloride.
The aim of this research work was to store the KOJ anions (KOJꢀ)
in the HTlc interlayer region. For this purpose, ZnAl–HTlc was
chosen as lamellar host because for its good photoprotection [11].
Al and Zn are metals largely used both in pharmaceutical and
cosmetic formulations, the first as salt for astringent activity, the
second as oxide (ZnO) for sun photoprotection [14] and derma-
tologic treatments, as zinc ions are known to have a critical and
important role in overall skin health [15].
Metal analyses were performed by Varian 700-ES series
Inductively Coupled Plasma-Optical Emission Spectrometers
(ICP-OES, Palo Alto-CA, USA) using solutions prepared by dissol-
ving the samples in concentrated HNO3 that were then properly
diluted.
Elemental analysis were performed using EA 1108 CHN FISONS
Instruments.
This paper proposes KOJꢀ intercalation into HTlc to improve
its photostability and release.
2.4. Preparation of ZnAl-HTlc-KOJ formulation
A silicone oil ointment (Table 1) containing ZnAl-HTlc-KOJ was
prepared as reported below: cetylstearilic alcohol and cetoma-
crogol 1000 were mixed in a steam bath and then paraffin oil and
silicone oil were added slowly under stirring. The mixture was
stirred until cooling and finally the intercalation product was
added. The final KOJ concentration was 2.5%.
2. Experimental
2.1. Materials
Well crystallized [Zn0.69Al0.31(OH)2](NO3)0.31 ꢁ 0.4H2O was
obtained by co-precipitating ZnCl2 (obtained dissolving ZnO with
HCl 12 M) and AlCl3, from ‘homogeneous solution’ using urea as
precipitant reagent [16]. The obtained Zn(II) and Al(III) hydro-
xycarbonate (HTlc–CO3) was converted into nitrate form by ion
exchange with HNO3 0.1N titration [16].
KOJ, liquid paraffin and silicon oil were provided by Comifarm,
(Perugia, Italy). Hydrochloric acid, nitric acid, AlCl3 ꢁ 6H2O, ZnO,
NaOH and metallic sodium were purchased from Carlo Erba
(Milano, Italy). Cetylstearilic alcohol and cetomacrogol 1000 were
purchased from ACEF (Firenzuola d’Arda Piacenza, Italy). Other
chemicals and solvents were of reagent grade and were used
without further purification.
2.5. In vitro release studies
The formulation was submitted to dissolution test using the
‘‘Petri disk’’ method [10,17] in sink conditions: 200 mg of for-
mulations were spread on the Petri disk (50 mm diameter) with a
spatula to produce a uniform surface of constant dimensions. The
dissolution vessels, closed on the top, were kept in a thermo-
statically controlled water bath at 32 1C. Phosphate buffer pH 5.5
(100 ml) (F.U.XII) was used as dissolution medium and the paddle
rotation speed was kept at 60 rpm [18]. At predetermined times,
2 ml aliquots were removed and analyzed spectrophotometrically
at lmax 269.0 nm. The removed aliquots were replaced with the
same volume of medium. Tests were made in triplicate and the
results were recorded as an average.
2.2. Intercalation of KOJ into HTlc-NO3 (HTlc-KOJ)
Metallic sodium (0.93 g) was slowly added to a mixture of dry
ethanol (200 ml) and DMSO (150 ml) in order to obtain a
stoichiometric amount of ethoxy anions and then KOJ (5.62 g;
molar ratio ethoxyꢀ/KOJꢀ 1/1) was added giving rise to a light
yellow solution. After few minutes, HTlc-NO3 (4.95 g) was added
(molar ratio NO3ꢀ/KOJꢀ 1/1.5) and the resulting suspension was
stirred at room temperature for 24 h. The solid was filtered and
washed three times with a mixture of dry ethanol/DMSO 1/1). The
product was dried at ca. 40 1C under vacuum and was stored in a
desiccator over P2O5 under vacuum.
2.6. Photostability studies
Absorption spectra of the solid samples were recorded by a
Varian (Cary 4000) spectrophotometer, equipped with a 150 mm
integration sphere and a barium sulphate tablet was used as
reference. The spectra were analyzed by the Kubelka-Munk
equation in order to make the comparison possible. Steady state
irradiation experiments were performed by a Xenon lamp as light
2.3. Characterization
Table 1
Silicone oil ointment
XRPD spectra were achieved by a PW 1710 Philips diffract-
ometer (Philips, Almelo, The Netherlands), using the Ni-filtered
Cu Ka radiation.
Ingredient
amount (% w/w)
DSC analyses were carried out using an automatic thermal
analyzer (Mettler Toledo DSC 821e, Italy). Temperature calibrations
were performed with indium as standard. Sealed and holed alumi-
num pans were used for all samples and an empty pan prepared in
the same way was used as a reference. Samples of 3–6 mg were
cetylstearilic alcohol
cetomacrogol 1000
paraffin oil
16.0
4.0
35.0
35.0
10.0
silicone oil
ZnAlHTlc-KOJ

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V. Ambrogi et al. / Journal of Physics and Chemistry of Solids 73 (2012) 94–98
source and by a monochromator selecting the excitation wave-
length (330 nm). Samples were irradiated for a period of 2 h.
1.29 nm relative at the KOJꢀ intercalation. The interlayer distance
of 1.58 nm can be attributed to ethanol and KOJꢀ cointercalation.
Indeed, in the dried ZnAl–HTlc–KOJ XRPD this 003 reflection
greatly decreases whereas the 003 reflection at 1.29 nm increases.
The 003 reflection due to the nitrate anions (0.9 nm) decreases
remarkably and the basal space became smaller than that of the
pristine ZnAl-HTlc–NO3 indicating the presence of nitrate ions
oriented in the interlayer with the plane formed by NO3ꢀ parallel
to the layer [23]. The formula of intercalated product was
obtained through many techniques. The content of Al and Zn
(determined by ICP) in the intercalation compound showed a Al
molar fraction¼0.31. Elemental analysis allowed to evaluate the
amount of nitrogen (relative to nitrate anion not exchanged),
sulphur (relative to DMSO) and hydrogen and carbon. The pre-
sence of DMSO was confirmed by 1H NMR spectrum (data not
reported) performed in deuterium chloride. UV KOJ quantifica-
tion, performed after destruction of the HTlc structure in acidic
medium, showed a loading of 25%. This result was confirmed by
TGA (Fig. 3), which showed the weight loss as function of
temperature and was interpreted as follows: from room tem-
perature to ca. 200 1C the sample loses co-intercalated solvents
(ethanol and DMSO), then from 200 to ca. 1000 1C the weight
losses are attributable to KOJ and residue nitrate decomposition
and to water loss coming from inorganic layers dehydroxylation,
while the pattern of the sample heated at 1000 1C is diagnostic for
the presence of ZnO and ZnAl2O4. Thus the final ZnAl–HTlc–KOJ
3. Results and discussion
3.1. Preparation and characterization of ZnAl-HTlc-KOJ
Synthetic ZnAl–HTlc in the carbonate form, prepared as pre-
viously described, was not suitable for KOJ intercalation because
it is well known that carbonate ions are strongly held in the
interlayer region [19] making very difficult its replacement with
other anions. Therefore, since monovalent anions, as NO3ꢀ, are
more easily replaced than carbonates, ZnAl–HTlc–CO3 was con-
verted into its nitrate form [16,20] and KOJ intercalation was
performed via ionic exchange in water at pH 11.0. Unfortunately
the first attempt failed, since XRPD spectra (data not reported)
showed a peak at 0.76 nm typical of hydroxyl or carbonate forms
and KOJ intercalation did not occur probably because of the acid-
base properties of KOJꢀ anions, which, behaving as relatively
strong Bro¨nsted base (pKa¼7.9 [21]), deprotonate the water
molecules of the solvent by giving rise to OHꢀ intercalation.
Moreover, despite the intercalation process has been performed
using CO2ꢀ free deionized water and the procedure was con-
ducted under nitrogen atmosphere, the complete absence of CO2
could not be assured during all steps. However, the intercalation
product was successfully prepared by working in anhydrous
conditions, generating KOJꢀ through the strong base EtOꢀ in a
mixture of ethanol and DMSO suitable to solve the organic acid. In
this case ZnAl–HTlc–KOJ XRPD showed an increase of the inter-
layer distance as a consequence of KOJꢀ intercalation. The
intercalation product was washed with CO2-free deionized water
to eliminate the inorganic ions and the organic material even-
tually adsorbed (KOJ or its anions). Unfortunately, during this
process KOJꢀ de-intercalation occurred, phenomenon associated
to its basic nature. Thus, in order to prevent KOJꢀ de-intercala-
tion, instead of water, the anhydrous mixture of ethanol and
DMSO was used to wash ZnAl–HTlc–KOJ. Fig. 2 shows the XRPD of
ZnAl–HTlc–KOJ wet (Fig. 2b) and dried at 40 1C under vacuum
compared with that of pristine ZnAl-HTlc–NO3 (Fig. 2a). The
peaks of ZnAl-HTlc–NO3 were indexed by comparing them with
the patterns of ZnAl-HTlc in chloride form, which has already
been indexed based on its structural parameters [22]. The pattern
of the wet ZnAl–HTlc–KOJ shows two 003 reflections at 1.58 and
formula resulted Zn0.69Al0.31(OH)2KOJ0.25(NO3)0.06
ꢂ
0.28EtOH
ꢂ
0.04DMSO (percentages of C, H, N, S: experimental values C
18.1, H 3.6, N 0.7, S 0.9; calculated values C 18.2, H 3.8, N 0.6,
S 0.9).
FT-IR ZnAl-HTlc-NO3 spectrum (Fig. 4) shows a broad band
between 3200 and 3800 cmꢀ1, due to the stretches of hydrogen-
bonded hydroxyl groups of hydroxide layers and interlayer water,
an adsorption peak at ca. 1620 cmꢀ1, relative to the co-inter-
calated water bending and a great peak at 1420 cmꢀ1 due to the
intercalated nitrate. It is worthy that this band almost disap-
peared in ZnAl-HTlc-KOJ spectrum, meaning that nitrate anions
underwent to almost complete exchange by KOJ anions. The very
broad band between 3000 and 3600 cmꢀ1 is diagnostic for
hydrogen-bonded hydroxyl groups of hydroxide layer stretches
with the intercalated compound.
Fig. 5 reports a computer generated model of ZnAl–HTlc–KOJ,
obtained with the Hypherchem program, on the basis of the
structural data of the host and the interlayer distance of the
intercalation compound. The presence in the interlayer region of a
Fig. 2. XRPD patterns of ZnAlHTlc-NO3 (a), wet (b) and dried (c) ZnAlHTlc-KOJ.
Fig. 3. TGA profile of ZnAlHTc-KOJ.

V. Ambrogi et al. / Journal of Physics and Chemistry of Solids 73 (2012) 94–98
97
^
exo
(d)
(c)
(b)
(a)
°c
40
60
80
10
100
15
120
20
140
160
180
30
0
15
25
min
Fig. 6. DSC thermograms of ZnAlHTlc-NO3 (a), ZnAl-HTlc-NO3/KOJ physical
mixture (b), KOJ (c) and ZnAlHTlc-KOJ (d).
Fig. 4. FT-IR spectra of ZnAl-HTlc-NO3 (a), ZnAl-HTlc-NO3/KOJ physical mixture
(b), KOJ (c) and ZnAl-HTlc-KOJ (d).
Fig. 7. KOJ release from the ointment at 32 1C70.1, in phosphate buffer pH 5.5.
(Table 1). This formulation, loaded with ZnAl-HTlc-KOJ containing
2.5% of KOJ, was submitted to release studies.
Fig. 5. Computer generated model of ZnAlHTlc-KOJ (without co-intercalated
solvents).
3.3. In vitro KOJ release from the formulation
KOJꢀ monolayer, with its principal axis almost perpendicular to
the layer plane, was hypothesized. In this model, the anions, after
intercalation, point the 2-hydroxymethyl groups towards the
hydrotalcite sheets allowing, in this way, the formation of
hydrogen bonds.
DSC (Fig. 6) of KOJ shows an endothermic peak at ca. 156 1C
relative to the melting point of pure crystalline substance. The
thermogram relative to ZnAl–HTlc–KOJ does not show the
endothermic peak of KOJ meaning that a new compound was
produced and that KOJꢀ intercalation in HTlc occurred. The
physical mixture shows an endothermic peak at ca. 140 1C super-
imposed with a peak at 156 1C relative to acid fusion. This
indicates that some interactions between KOJ and hydrotalcite
are present in the physical mixture too.
KOJ release from the ointment was performed at pH 5.5 phos-
phate buffer employing the ‘‘Petri disk method’’ [10] (Fig. 7) in
order to reproduce real use conditions. KOJ release resulted
gradual and increased during the test reaching ca. 40% after 4 h
and a maximum of 68% after 8 h. In these conditions, KOJꢀ release
occurred by contact between the buffer and HTlc-KOJ dispersed in
the ointment.
3.4. Photostability studies
The absorption spectra in 200–600 nm region (Fig. 8) were
recorded to investigate the fraction of UV–vis light absorbed by
the samples and the effects of the inorganic matrix in preventing
KOJ photodegradation. The KOJ spectrum (Fig. 8a) has a max-
imum at 300 nm and a shoulder in the 330–400 nm region; ZnAl-
HTlc-KOJ (Fig. 8c) presents a similar spectrum, although the low
energy shoulder is broader and it extends till about 500 nm. The
comparison between the spectrum of the intercalated sample and
the spectra of KOJ in acidic or salt form (Figs. 8a and b,
respectively) revealed evident differences suggesting that the
3.2. Preliminary formulation studies
The ZnAl–HTlc–KOJ formulation was realized taking into
account that the presence of water causes immediate KOJꢀ
release from the intercalation compound. Thus, a water free
self-emulsifying ointment was proposed in order to prevent KOJꢀ
leakage in storage conditions. In this case, the cosmetic ingredient
release can be promoted spreading the formulation on moist skin.
With this aim, a silicone oil self-emulsifying ointment was chosen
inorganic matrix stabilizes arrangements, like
p–p stacking,
which lead to spectral broadening in the Vis region. When the
compounds were dispersed in the cream no significant spectral
changes were observed.

98
V. Ambrogi et al. / Journal of Physics and Chemistry of Solids 73 (2012) 94–98
oil ointment, needs to be further investigated and the occurrence
of new photochemical reactions cannot be excluded at this stage.
4. Conclusions
In the present work, KOJ was successfully intercalated into a
ZnAl-HTlc-like anionic clay by a ion-exchange mechanism per-
formed in anhydrous conditions. The intercalation compound was
then formulated in a water free self-emulsifying ointment designed
to be spread on the moist skin creating an in situ W/O emulsion. The
proposed formulation showed good in vitro KOJ release profiles.
Moreover, photostability studies revealed that KOJ intercalation in
the inorganic matrix reduces photo-induced oxidation and addition
reactions.
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