Int. J. Electrochem. Sci., 13 (2018) 5850 5859 , doi: 10.209642018.06.64 [628000]

Int. J. Electrochem. Sci., 13 (2018) 5850 – 5859 , doi: 10.20964/2018.06.64

International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org

Spectroelectroch emical Studies of Interactions b etween Vitamin
A and Nanocolloidal Silver

Bogdan Tutunaru*, Adriana Samide, Cristian Neamțu, Cristian Tigae
University of Craiova, Faculty of Sciences, Department of Chemistry, Calea București 107i, 200478,
Craiova, Dolj, Romania
*E-mail: [anonimizat]

Rece ived: 28 January 2018 / Accepted: 26 March 2018 / Published: 10 May 2018

The electrochemical stability of retinyl palmitate (RP) known as Vitamin A in mixed water/ethyl
alcohol solutions containing NaCl or NaNO 3, in the absence and presence of silver nanoparticles (nAg)
was investigated by cyclic voltammetry (CV) recorded on platinum electrode. The chemical interaction
between retinyl palmitate and silver nanoparticles was also studied using UV -Vis spectrophotometry
applied on the above mentioned medi a. The experimental results obtained from UV -Vis
spectrophotometry showed that in the presence of NO 3- ions, weak RP -nAg interaction takes place,
while in the presence of chloride ions a significant change in environment composition has been
highlighted du e to the occurrence of instantaneous RP -nAg interaction. The cyclic voltammetry
displayed different shapes of cyclic voltammograms recorded on platinum electrode in water/alcohol
solutions containing RP and RP/nAg in the presence of NO 3- ions compared to t hose recorded in Cl-
presence due to the specific interaction RP -nAg depending on the anion type. Thus, NO 3- addition
leads to mainly electrochemical interaction compared to Cl- presence that favors chemical RP -nAg
interaction. Based on the cyclic voltamme try results obtained in the presence of NO 3- ions, the
electrochemical decomposition mechanism of Vitamin A was proposed.

Keywords: Retinyl palmitate; Silver nanoparticles; Cyclic voltammetry; UV -Vis spectroscopy .

1. INTRODUCTION
Vitamin A, or gener ally referred to as retinoids, is the base compound of the naturally occuring
compounds retinol, retinyl palmitate, retinaldehyde and retinoic acid. These retinoids are biologically
important for the cellular division, differentiation, growth, health and v iability not only in the case of
skin cells [1-3]. Vitamin A (all -trans retinoid) is sensitive to physio -chemical conditions such as heat,
UV-light, Vis -light or oxygen and isomerizes to different cis -isomers with lower bioactivity [ 4]. In
vitro studies sh owed that retinyl palmitate can generate reactive oxygen species under UV -A radiation

Int. J. Electrochem. Sci., Vol. 13, 2018
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and also in these conditions can present photomutagenicity to some human skin cells [ 4]. Clinical in
vitro and in vivo studies of formulations containing octocrylene, oct yl methoxycinnamate,
benzophenone and retinyl palmitate did not indicate any phototoxic potential [ 5].
Photoirradiation of retinyl palmitate generated a series of photodecomposition products and
reactive oxygen species involved in lipid peroxidation [ 6, 7]. Photodecomposition, photomutagenicity
and photocitotoxicity tests indicated that retinyl palmitate is not mutagenic under He -Ne laser
photoirradiation and can be used before photodynamic therapy to enhance its effect [ 8]. In vivo studies
on mice showed t hat retinyl palmitate can be used as a cytoprotectant, especially for damaged cells by
anticancer drugs [ 9].
Scientific Committee on Consumer Safety has established the safe concentration for exposure
to retinyl palmitate and retinyl acetate via body lotio n (0.05 %) and via hand cream, face cream (0.3
%) [10].
The dermatologic use of vitamin A is limited due to its adverse effects and its chemical
stability to oxidation, heat, light, moisture or acids. Solid lipid nanoparticles, nanoemulsions and
liposomes for retinyl palmitate were studied in order to improve the photostability and
biocompatibility [ 11, 12 ].
A reverse -phase high performance liquid chromatography method with ultraviolet detection
was developed for the quantification of retinyl palmitate, ret inol and retinoic acid in cosmetic products
[13]. Many methods have been developed for the analysis of fat soluble vitamins (vitamins A, D, E
and K) in pharmaceutical supplements, infant formula, adult nutritionals [1 4-17].
Endogenous levels of retinoids ( retinyl esthers, retinol, retinal) were quantified in order to
reveal their homeostasis in diseases such as cancer, Alzheimer`s diseas, diabetes or obesity [18-19].
Electrochemical methods have also shown their efficient applicability to the analysis of
biologically active compounds [ 20-26] including retinol [ 27, 28 ].
Nanoparticles exhibits special physico -chemical properties different from their bulk dissolved
forms and have been gaining increasing attention in the applied and fundamental research [ 29-33].
Nanocolloidal/ionic silver presents high antimicrobial activity against fungi, bacteria or microbes and
represents one of the components of pharmaceutical and cosmetic formulations [3 4-37].
The vitamins and colloidal silver can be found as active ingredi ents in different pharmaceutical
or naturist supplements used as tonics, revitalizing and detoxifying as well as in cosmetics. Retinyl
palmitate (RP) is the main storage form of retinoid (vitamin A) i n humans and animals. Due to its
biological effects, it is found in various commercial products such as: drugs, cosmetics and foods.
Moreover, silver nanoparticles (nAg) are also used as adjuvants in different supplements due to its
antiviral activity. The interactions that may occur between vitamins and colloi dal silver nanoparticles
should be known, especially if the commercial products containing these compounds are used in
chloride media or stored for a long time.
This study provides information on specific interactions betw een retinyl palmitate (RP), known
as Vitamin A and silver nanoparticle (nAg). To investigate RP -nAg chemical interaction, the UV -Vis
spectrophotometry was performed for mixed solutions (water/ethylic alcohol) containing Cl- or NO 3-
ions and RP, in the absence and presence of nAg. The c yclic voltammetry recorded on platinum
electrode in NaCl and NaNO 3 alcoholic solution s containing RP, without and with silver nanoparticles

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5852
(nAg) with was used to study t he electrochemical stability of mixtures . The R P electrochemical
decomposition mechanism w as proposed.

2. MATERIALS AND METHODS
2.1. Materials
Pure vitamin A (retinyl palmitate) as yellowish oil was obtained from Merck. Other chemica ls
used in this study e.g. NaCl and NaNO 3 were also supplied by Merk, Germany. A 10-4 mol·L-1 retinyl
palmitate stock alcoholic solution was prepared and stored in a refrigerator (< 10 °C) in dark
environment. 1.0 mol·L-1 NaCl and 1.0 mol·L-1 HNO 3 stock solutions were prepared and used as
supporting electrolyte. The working electrolyte solution (5.0·10-6 mol·L-1 retinyl palmitate, 10-1 mol·L-
1 NaX; X = Cl, NO 3) was pre pared by appropriate dilution. The powder of s ilver nanoparticles was
purchased from Sigma. The working electrolyte solutions were prepared using bidistilled water. The
molecular structure of vitamin A is presented in Fig. 1.

OO
14

Figure 1. Molecular structure of [(2E,4E,6E,8E) -3,7-Dimethyl -9-(2,6,6 -trimethyl -1-
cyclohexenyl)nona -2,4,6,8 -tetraenyl] hexadecanoate; (retinyl palmitate – RP).

Platinum plate were purchased from Merck (purity higher than 99.9 %), cut in the form of
plates with 1×3 cm size. The electrodes had a 2 cm2 active surface. 10-1 mol·L-1 H2SO 4 and 10-1 mol·L-
1 NaOH solutions were used for ultrasonically cleaning of platinum electrodes for 5 min.. An
Ag/AgC l,KCl sat reference electrode was employed in all cyclic voltammetry measurements.

2.2. Methods
The cyclic voltammetry was performed in a standard electrochemical cell with three electrodes:
two id entical platinum electrodes, each having an active area of 1 cm2, were used as working electrode
and auxiliary, respectively . The Ag/AgCl,KCl sat electrode was used as reference electrode .
The cyclic voltammetry me asurements were carried out using a VoltaLab 40 potentiostat with
VoltaMaster 4 software . The cyclic voltammograms were recorded in mixed water/ethylic alcohol
solutions containing NaCl or NaNO 3 and Vitamin A (RP) , in the absence and presence of silver
nanoparticles (nAg) with a potential scan rate of 100 mV·s-1, in a dynamic regime, the stirring rate
being of 300 rot·min-1.
The UV -Vis spectra were recorded by a Varian Cary 50 spectrophotometer (Cary Wi n UV
software) equipped with a spectrophotometric quartz cell having dimensions of 10x10x45 mm .

Int. J. Electrochem. Sci., Vol. 13, 2018
5853
3. RESULTS AND DISCUSSION
3.1. UV-Vis Spectrophotometry results
Figure 2 shows the UV -Vis spectra of RP blank alcoholic solution and RP alcoholic solution
containing Cl- ions or NO 3- without and with nAg . The UV -Vis spectrophotometric scans of the
electrolyte solutions were recorded in the wavelength range of 200 nm and 800 nm.
As shown Figure 2a, two peaks of retinyl palmitate ( RP spectrum ) were identified ; one w ell
defined peak with absor bance maximum around 325 nm and one split peak with high absorbance
value, at 250 nm. There is observ ed a significant effect of chloride ions on RP ( RP_NaCl spectrum),
both peaks becoming very broad with low absorbance maximum values. The UV -Vis spectral scan of
mixture containing RP, Cl- and silver nanoparticles (curve RP_NaCl_nAg) shows a strong interaction
between all the species; the peak recorded at 400 nm corresponding t o silver nanoparticles is split and
shifted to higher absorbance values; the peaks attributed to RP (λ max = 325 nm and λ max = 250 nm)
disappe ar; at wavelength values lower than 300 nm, an absorbance gradual increase was recorded, due
to the formation of intermediate species between RP and silver nanoparticles and/or io nic silver.

012345
200 300 400 500 600absorbance
wavelength /nmaRP
RP_NaCl
RP_NaCl_nAg
012345
200 300 400 500 600Absorbance
wavelength /nmbRP
RP_NaNO3
RP_NaNO3_nAg

012345
200 300 400 500 600absorbance
wavelength /nmcRP
nAg
RP_nAg

Figure 2. UV-Vis spectrophotometric scans of: i) 5·10-6 mol·L-1 RP in alcoholic solution; ii) 5·10-6
mol·L-1 RP, 10-1 mol·L-1 NaNO 3; iii) 5·10-6 mol·L-1 RP, 10-1 mol·L-1 NaNO 3, 500 mg·L-1 nAg;
iv) 5·10-6 mol·L-1 RP, 10-1 mol·L-1 NaCl; v) 5·10-6 mol·L-1 RP, 10-1 mol·L-1 NaCl, 500 mg·L-1
nAg in hydro -alcoholic (9:1) solution.

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5854
In Figure 2b, it can be observed the effect of nitrate io ns and silver nanoparticles on the RP
UV-Vis spectrum. In the presence of nit rate ions ( RP_NaNO 3 spectrum) the two pe aks of RP were
shifted to wavelength lower values: λmax = 300 nm and λ max = 230 nm . After addition of silver
nanoparticles ( RP_NaNO 3_nAg spectrum), other peak at 400 nm corresponding to nAg was registered.
Figure 2c presents the UV -Vis comparative spectra of RP in mixed water/ alcoholic solution,
aqueous suspension of silver nanoparticles and their mixture. The UV -Vis scan of nAg suspension
(nAg spectrum ) shows a peak with absorbance maximum at 400 nm corresponding to silver
nanoparticles and broad absorp tion peak with low intensity, co rresponding to ionic silver [25]. In the
presence of nAg ( RP_nAg spectrum ), the peak at 250 nm completely disappears and two pe aks at 400
nm and 320 nm occur corresponding to nAg and RP-nAg complexes , respectively .
It has been reported that vitamins interact with silver nanoparticles, as an example being the
interaction between vitamin C and silver nanoparticles studied by cyclic voltammetry and UV -Vis
spectrophotometry [25].
In our prev ious study the electrochemical behavior of vitamin C in 10-1 mol·L-1 NaCl solution,
in the absence and presence of silver nanoparticles was investigated [25]. The kinetic s of zero order
reaction was co mputed for Vitamin C degradation in the ab sence and presence of silver nanoparticl es,
the reaction rate constant reaching a double value in the presence of nAg [2 5].
Consequently, nAg has an electrocatalytic effect on the Vitamin C degradation, at constant
current density of 20 mA·cm-2, indicating that afte r electrolysis time of 4.0 min the Vitamin C
electrodegradation reaction is very fast due to the silver ions formati on, that lead s to the vitamin C
rapidly oxidation to ascorbate radical [2 5]. In contrast, the interaction between Vitamin A and silver
nanop articles is instantaneous in Na Cl solution, as shown Figure 2 a through overlapping multiple
interferences highlighted at 400 nm. Consequently, the electrolysis performing is unnecessary, under
these conditions the reaction kinetics is difficult to evaluate and the interaction between nAg and
Vitamin A can be assimilated to an instantaneous chemical transformation reaction in RP_nAg
complexes .

3.2. Electrochemical behavior of RP in the presence of NaNO 3 supporting electrolyte
Figure 3 shows typical cyclic voltammograms of platinum electrode in water/ alcoholic 9:1
(v/v) solutions: i) 10-1 mol·L-1 NaNO 3; ii) 10-1 mol·L-1 NaNO 3, 5·10-6 mol·L-1 RP; iii) 10-1 mol·L-1
NaNO 3, 5·10-6 mol·L-1 RP, 500 mg·L-1 nAg.
On the anodic scan, many oxidation processes are ident ified in working electrolyte solutions
(
,
,
,
). The RP molecules showed an irreversible
behaviour, with successive small anodic peaks of current density on Pt electrode. The anode peaks can
also be attri buted to various electrode processes of nitrate ion (Fig. 3b) at the metal/electrolyte solution
interface. On the cathodic scan, two reduction peaks can be observed (
,
).
A significant decrease in the potential value of 0.2 V was obtained on platinum electrode for
retinyl palmitate under experimental conditions due to the presence of silver nanoparticles. The
relationship between the oxidation currents on working electrode for both electrolytes (presence /
absence of nAg) proves th e interaction of retinyl palmitate with silver nanoparticles and indicates that

Int. J. Electrochem. Sci., Vol. 13, 2018
5855
nAg has an electrocatalytic effect. From the experimental results, it is clearly observed that retinyl
palmitate produces significantly higher voltammetric signals compared to those recorded in the
absence of silver nanoparticles.

-20-10010203040
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2i (mA/cm2)
E (V vs. Ag/AgCl)NaNO3
NaNO3-RP
NaNO3-RP-nAga
NO3-NO2-N2
N2O4N2O
H2N2O2NH3OH+N2H5+
NH4+1.25 V
-0.8 V1.29 V
0.86 V0.49 V1.41 V
0.94 V-0.05 V
1.35 Vb

Figure 3. Cyclic voltammograms of i) 10-1 mol·L-1 NaNO 3; ii) 10-1 mol·L-1 NaNO 3, 5·10-6 mol·L-1 RP;
iii) 10-1 mol·L-1 NaNO 3, 5·10-6 mol·L-1 RP, 500 mg·L-1 nAg, hydr o-alcoholic 9:1 (v/v)
solutions, platinum electrode.

Retinyl palmitate contains five conjugated double bonds (>C=C<) which explains its high
susceptibility to oxidation. Retinyl palmitate undergoes electrochemical transformations at the
electrode surface to highly oxidation state intermediates. According to Scheme 1, the initial ester
molecule cleaves oxidatively with formation of the retinyl cation and the palmitate radical [7].

OO
14-eCH2
OO
14retinyl cation palmitate radical

Scheme 1. Electrochemical degradation of retinyl palmitate.

Also, the retinyl cation may form retinol in the presence of hydroxyl ions. The process of
retinyl cation degradation (Scheme 2) initiates with the formation of cyclohexene derivatives at the
conjugated π system such as epoxide derivati ves. Epoxy structures have also been identified as
intermediates in vitamin A degradation [6, 7].
As the intermediates passed through the double layer, they cleaved to hydroxi -, keto -enol,
aldehyde/ketone, carboxylic acid and/or bifunctional structures of these. The various acid -base
equilibria or the equilibria between the mesomere structures which can form along the electrochemical
degradation mechanism could be influenced by the acidity of the solution, the value of the working
electrode potential and / or the current density. The overall electrochemical mechanism involves
several steps. When hydroalcoholic solution of RP was exposed to higher overpotentials, many
electrochemical degradation products were formed. All the electrodegradation products (ioni c and/or

Int. J. Electrochem. Sci., Vol. 13, 2018
5856
radical) are electrochemicaly unstable. Thus, upon higher polarization, the underproducts further
decompose into inorganic products.

CH2CH2 + H2O
-H+
CH2
OH -e -H+CH2
O
5,6-epoxy intermediate+ H2O
-H+
CH2OH
O-eCH2OH
O
HC CH2OH
+ H2O
-e -H++ 2H2O
-4e -3H+ -CO2
OHOH
OHOH
CH2
OH-e
CH2
OHH3CC
OH
H3CC
OH+ H2O
-H+ H3CC
OHOH
-e -H2OH3CCO+ H2O
-H+ CH3COOH- CO2
-2e -H+
CH3+ H2O
-H+CH3OH+ H2O
-4e -4H+HCOOH-2e -2H+-CO2
+ H2O
-e -H+ H3CC
OHH3C
H2C
CH2OHCH3COOH / HCOOH / CO2 / H2O

Scheme 2. Decomposition products of retinyl cation formed through an electroch emical mechanism.
HC CH2OH
+ H2O+ H2O
+ H2O
+ H2O
+ 2H2O+ H2O + H2O
+ H2O+ H2O+ H2O+ H2O
+ H2O+ H2O+ H2O+ H2O-e
-H+
-H+-H+-H+
-H+-H+-H+
H
CH
C
O O
-2e -2H+
-2e -3H+
-2e -3H+-2e -H+-e -H+-2e -2H+
-4e -4H+ -CO2 -4e -3H+ -CO2-4e -4H+-4e -3H+-4e -3H+
-CO2
C
CH OOH
– CO2- CO2- CO2
H2C C
HO-CO2
HOH2C
-e
-eHOH2CH3C
CHOHCH3
HOH2CH3C
CH HCCHOHCH3OH
HOHOH2CH3C
CHO
OHCCHOHCH3
HOOC CHH3C
CH3CHO / CH3COOH / HCOOH / CO2 / H2OHC
COOHH3C

Scheme 3. Decomposition products of nonatetraene intermediate formed through an electrochemical
mechanism.

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5857
The first cleavage of the carbon chain leads to the generation of a nonatetraene cation
interme diate with high reactivity due to the four double bonds from its structure. The presence of
delocalized π electrons can lead to the electrochemical degradation mechanism through different paths;
a possible degradation pathway is shown in Scheme 3. The palm itate radical can easily stabilize to
palmitic acid. The molecular structure of palmitic acid contains only carbon atoms having sp3
hybridization, its electrochemical degradation requires much higher overvoltages. A greater stability of
palmitic acid can e xplain its accumulation in the system, knowing that is one of the products identified
in the degradation of retinyl palmitate [7].

3.3. Electrochemical behavior of RP in the presence of NaCl supporting electrolyte
Figure 4a shows the cyclic voltammogramms recorded on platinum electrode, between -2.0 V
and +2.0 V at 0.1 V·s-1, in NaCl blank solution and in mixed water/alcohol solutions containing NaCl
and RP, in the presence and in the absence of nAg. Figure 4b displayed the electrochemical processes,
the o xychloride active species that can be formed and the values of the corresponding potentials.
-30-1010305070
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2i (mA/cm2)
E (V vs. Ag/AgClNaCl
NaCl-RP
NaCl-RP-nAga
X-XO-
XO2-XO3-XO4-X2
1.36V
0.89V
0.78V
0.63V0.56V1.63V
1.67V
1.18V
1.20V1.42Vb

Figure 4. Cyclic voltammograms of i) 10-1 mol·L-1 NaCl; ii) 10-1 mol·L-1 NaCl, 5·10-6 mol·L-1 RP; iii)
10-1 mol·L-1 NaCl, 5·10-6 mol·L-1 RP, 500 mg·L-1 nAg, w ater/alcoholic 9:1 (v/v) solutions,
platinum electrode.

An irreversible voltammetric behaviour of platinum electrode in aqueous solution of supporting
electrolyte was observed corresponding to chloride ion oxidation and reduction of oxychloride species
at the electrode surface [25, 26 ]. Once formed in the solution, the hypochlorite ion due to its high
oxidizing action may lead to homogeneous processes such as oxidation of retinyl palmitate in the bulk
electrolyte.
The sharp increase of anodic peak at aroun d -0.9 V when retinyl palmitate is added to
supporting electrolyte solution corresponded to RP adsorption on the electrode surface [28]. In the
presence of silver nanoparticles, this peak becomes very broad and its intensity decreases which prove
the inter action of retinyl palmitate with silver nanoparticles.

Int. J. Electrochem. Sci., Vol. 13, 2018
5858
After adsorption of retinyl palmitate on the working platinum electrode, cyclic voltammogram
shows a well -defined anodic peak at +1.0 V which may correspond to a partial electrochemical
degradation of retinyl palmitate [27].
The cyclic voltammogram shape changes after the silver nanoparticles addition, indicating the
the chemical/electrochemical interaction occurrence of the type RP -nAg.
Both anodic peaks ( -0.9 V and +1.0 V) attributed to retinyl palmit ate adsorption and
electrochemical partial degradation , respectively disappeared in the presence of silver nanopartic les,
while the current density values corresponding to cathodic peaks decreased.

4. CONCLUSIONS
The supporting electrolytes cause chemica l and physical changes on the platinum electrode
surface, and thereby also a different electrochemical response. This study provides information
regarding reactivity and of the reactive intermediates generated by chemical/electrochemical reactions
of retin yl palmitate and silver nanoparticles.
UV-Vis spectrophotometry indicated a strong interaction between retinyl p almitate molecules
and silver nanoparticles , in the presence of Cl- ions and low interaction in the presence of NO 3- ions.
The experimental resu lts obtained from cyclic voltammetry reveal that, the addition of nAg in
solution leads to a considerable d ecrease in the current density , followed by a significant hysteresis
shape modification due to the interaction of Vitamin A with silver nanoparticles .

ACKNOWLEDGMENTS
The funding of this work was supported by the research grants awarded by the University of Craiova,
Romania, in the competition “The Awards of Research Results -ISI Articles”, April 2017.

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