Talanta 62 (2004) 912–917
Flow injection analysis of zinc pyrithione in hair care products on a
cobalt phthalocyanine modified screen-printed carbon electrode
Ying Shiha,∗, Jyh-Myng Zenb,∗, Annamalai Senthil Kumarb, Pei-Yen Chenb
aDepartment of Applied Cosmetology, Hung Kuang University, Taichung 433, Taiwan, ROC
bDepartment of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 40217, Taiwan, ROC
Received 6 August 2003; received in revised form 21 October 2003; accepted 21 October 2003
Zinc pyrithione (ZPT) is an antibacterial and antifungal reagent that is often utilized for the antidandruff activity in hair-care shampoos
with a composition level up to 1% in the formulation. It has some adverse effects to human and animal if consumed orally. A disposable type
of cobalt phthalocyanide modified screen-printed carbon electrode (CoPc/SPE) in couple with flow injection analysis (FIA) was developed
for easy and selective analysis of ZPT in commercial hair-care products. Under the optimized FIA conditions, the CoPc/SPE yielded a linear
calibration plot in the window of 6–576?M with sensitivity and detection limit of 1.65nA?M−1and 0.9?M (i.e. 1.42pg in 5?l sample
loop), respectively, in 0.1M KOH solution at an applied potential of 0.3V versus Ag/AgCl. Since the approach is simple, easy, selective, and
inexpensive, it offers a potential application of daily ZPT analysis in hair-care products.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Zinc pyrithione; Cobalt phthalocyanine; Screen-printed carbon electrode; Flow-injection analysis
Zinc pyrithione (ZPT, zinc bis(2-pyridylthio)-N-oxide)
(Scheme 1) is an active ingredient in most of daily hair care
products (shampoos, conditioner, hair-rinses, etc.). Its ef-
fective bactericide, fungicide and algaecide functions were
often utilized for the antidandruff and antifouling activities
in the products [1–5]. The recommended maximum amount
of ZPT in hair care items is 1% (with preservatives), ac-
cording to the scientific committee on cosmetic products
and non-food products intended for consumer (SCCNFP)
. However, the toxicity of ZPT remains uncertain. The
adverse effect and toxicity of ZPT in the hair-care products
have been investigated for many decades . A lethal dosage
of ZPT in the range of 1–100 and 25mgkg−1per day,
respectively, for dogs and monkeys would result in emesis,
pupil dilation, paralysis, blindness, and diarrhea, etc [6–9].
Teratologic studies with rat and rabbits with an oral dosage
of 15mgkg−1per day of ZPT show increased incidence of
fused or foked ribs [6,8]. It was also found ZPT producing
∗Corresponding authors. Fax: +886-422-862547.
E-mail addresses: email@example.com,
firstname.lastname@example.org (Y. Shih).
strong inhibition of cell growth in vitro in rabbit corneal
cells and human fibroblasts . In addition, throw away of
ZPT-containing hair-care products to water and/or soil may
have significant impact on the environment and, in turn, to
food chain contamination. Recent embrotoxicological study
postulates the uptake of ZPT by fish samples . All the
above data indicate the requirement for rapid detection and
controlling analysis of ZPT in the hair-care formulations.
Several analytical methods have been reported previously,
including thin-layer chromatography (TLC)  and high
performance liquid chromatography (HPLC) [13–17], titra-
tion with Ti(III) , and electrochemical approach for the
ZPT assays . Even though the titrimetric methods ap-
ply a simple way for ZPT determination , poor sensitiv-
ity appears to be less attractive for practical analysis; while
the HPLC and TLC methods are too cumbersome. Owing
to the interaction of Zn2+with stainless steel components,
HPLC is not a very appropriate procedure for the direct ZPT
analysis [18,19]. In some cases the strong interaction can
even damage the stainless-steel body. In addition, due to the
(e.g. N-dansylaziridine)  were always required for sen-
sitive analysis. Alternatively, ZPT in commercial products
can be determined by trans-chelation to the Cu2+complex
0039-9140/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
Y. Shih et al./Talanta 62 (2004) 912–917
Scheme 1. Structure of zinc pyrithione.
using the normal-phase HPLC . In that process, the cen-
tral metal ion Zn2+in the ZPT complex was exchanged to
Cu2+ions as CPT complex and was further used for detec-
tion purpose (the CPT’s absorption coefficient is about three
times higher over ZPT) [20–22]. This clearly stresses the
search for an easy, simple, and less complicated analytical
assay for ZPT in real samples.
Electrochemical methods offer convenient, easy, and di-
rect quantification methods in analytical chemistry .
However, so far there are very few reports regarding ZPT
analysis. Recently metal oxide based carbon paste electrode
comparative differential pulse voltammetric studies (DPV)
with SnO2, CeO2, Cr2O3, PbO, CdO, and CuO systems re-
sulted in the best performance to SnO2/CPE in pH 12.5
(0.1M tetrabutylammonium hydroxide) with a detection an-
odic peak potential of ca. 0.6V versus SCE. Even though the
method is useful for the practical analysis, the preparation,
cost, and time factors made it less appreciable. Moreover,
the practical analysis requires extensive off-line sample pre-
treatment procedures. For example, the real samples were
first treated with 10ml of 99% methanol to separate the
ZPT from the hair-care shampoos, followed by dissolving
in dimethylformamide (DMF) for the quantification assays
trochemical systems showed profound oxidation behavior to
methanol in alkaline medium, leading to unreliable data.
In this work, a disposable type cobalt phthalocya-
nine (CoPc) modified screen-printed carbon electrodes
(CoPc/SPE) was developed as a simple ZPT analytical as-
say without any sample pretreatment procedures by flow
injection analysis (FIA) in 0.1M KOH solution at an ap-
plied potential of 0.3V versus Ag/AgCl. The CoPc/SPE
based FIA shows several advantages for the ZPT detec-
tion over the SnO2/CPE based DPV assays, such as low
over-potential and low working volume, etc. Note that the
CoPc based screen-printed carbon electrodes have already
been successively used for the application of some thiols
containing molecules in alkaline pHs [24,25]. Meanwhile,
a recent review by Honeychurch and Hart describes many
useful metal-ions analysis by SPEs . The high stability,
low cost, and easy to be prepared nature of the CoPc/SPE
is an ultimate success of the present approach for FIA. In
addition, the CoPc/SPE based working systems also shows
good selectivity for the ZPT containing hair-care products.
Typical real sample analysis was successfully verified with
the present approach with less operation time and without
any sample pretreatment procedures.
Cobalt phthalocyanine (?-form, Aldrich, Milwaukee, WI,
USA), potassium hydroxide (Showa, Japan), zinc pyrithione
(Sigma, USA) and all other compounds (ACS-certified
reagent grade) were used without any further purification.
Aqueous solutions are prepared with doubly distilled deion-
ized water by reverse osmosis technique. A fresh 0.1M
KOH stock solution was prepared daily.
Voltammetric measurements and FIA coupled flow-
through experiments were carried out with a CHI 621
electrochemical workstation (Austin, TX, USA). A three-
electrode cell assembled with a bare SPE or the CoPc/SPE
working, an Ag/AgCl reference (BAS RE-5), and platinum
disc auxiliary electrodes were used. Since oxygen did not
interfere the analysis at the detection potentials, no deaera-
tion was performed in this study. The FIA system consisted
of a carrier reservoir, a Cole Parmer Masterflex micropro-
cessor pump drive, a Rehodyne 7725 sample injection valve
(5?l loop), interconnecting Teflon tubing, and a BAS CC-5
thin layer electrochemical detector with a BAS electro-
chemical detector. The BAS flow-through system consists
of the working SPE in-between the gasket and the closing
holder as reported earlier [27,28]. The SPE with a working
area of 0.196cm2and a conductive track radius of 2.5mm
was purchased from Zensor R&D (Taichung, Taiwan). The
measured average resistance was 85.64 ± 2.10?cm−1.
The CoPc/SPE was prepared by drop-coating with 3?l
(90%:10% (w/w)) of carbon ink and CoPc mixed solution,
prepared in 5:5 volume ratio of dichloromethane and ace-
tonitrile. It was then dried in an oven at 60 ± 2◦C for
2–3min. Net loading of CoPc on the working SPE is about
1.5mgcm−2. The CoPc/SPE was equilibrated in a carrier
solution at 0.3V versus Ag/AgCl until the current becomes
constant in FIA, which normally took less than 4min. The
quantification of ZPT was achieved by measuring the ox-
idation peak current in FIA at room temperature (25◦C).
After experiments, the electrodes were washed thoroughly
with distilled water and stored in a close container at normal
Branded commercial shampoo samples with (Type 1;
#1–4) and without ZPT (Type 2; $1) were purchased from
a local supermarket. The Type 1 products are formulated
with ammonium laureth sulfate, sodium chloride, glycol
distearate, dimethicone, cetyl alcohol, hydrogenated poly-
decene, sodium citrate, sodium benzoate, citric acid, and
benzyl alcohol in addition with 1% ZPT. The ingredients
are almost the same except without ZPT for Type 2 sham-
poos. Each sample were weighed carefully in the range of
Y. Shih et al./Talanta 62 (2004) 912–917
0.460–0.480g and then dissolved in 250ml, 0.1M KOH.
All solutions were filtered before regular FIA injection.
3. Results and discussion
3.1. Basic electrochemical studies
Cyclic voltammetric (CV) response of the CoPc/SPE and
bare SPE with and without 144?M of ZPT in 0.1M KOH
solution at a scan rate of 50mVs−1were shown in Fig. 1. As
can be seen, no obvious faradic response to ZPT in the en-
tire potential range was observed at the bare SPE (Fig. 1A).
On the other hand, the CoPc/SPE showed profound oxida-
tion characteristic with a feeble anodic peak at about 0.3V
versus Ag/AgCl (Fig. 1B). This observation clearly demon-
strates the electrocatalytic mediation of the CoPc towards
the ZPT oxidation reaction. Previous study towards the H2S
gas oxidative sensing reaction at CoPc in alkaline medium
also resulted in a similar feeble peak behavior at ∼0.4V
versus Ag/AgCl . The observation further supports the
CoPc redox mediation effect in the present work with the
following type of reaction scheme:
xCo1+Pc/SPE ↔ xCo2+Pc/SPE
xCo2+Pc/SPE+yZPT → xCo1+Pc/SPE+yZPTO
In the above scheme, the higher oxidation state of Co2+(i.e.
Co2+Pc/SPE) was reversibly electrogenerated at 0.1–0.5V
and was directly participated in the irreversible ZPT oxida-
tion to ZPTO (oxidized form of the complex). Further exper-
iments with increasing scan rate (v) from 5 to 500mVs−1
showed a regular increase in the ZPT oxidation current. A
double logarithmic plot of anodic peak current (ipa) ver-
sus v was further adopted to understand the exact nature of
Fig. 1. Cyclic voltammetric response of SPE (A) and the CoPc/SPE (B)
with and without 144?M of zinc pyrithione (ZPT) in 0.1M KOH at a
scan rate of 50mVs−1.
the oxidation mechanism based on the following simplified
equations.For the diffusion controlled process [ ipa∝ v1/2]:
log(ipa) = constant +1
For the adsorption controlled mechanism [ ipa∝ v]:
log(ipa) = constant + log(v);
The reaction mechanism can usually understand from the
simple plots of either ipa versus v1/2or ipa versus v for
the process of diffusion or adsorption controlled reaction,
respectively [29,30]. However, this approach is much dif-
ficult for the case of mixed adsorption and diffusion con-
trolled system. On the other hand, the double logarithmic
plot of ipaversus v gives exact information about the nature
and extend of the reaction process, where the slope (m =
∂log(ipa)/∂log(v)) of 0.5 or 1.0 corresponding to the dif-
fusion and adsorption controlled processes [30,31]. For the
case of mixed adsorption and diffusion controlled mecha-
nism the m value lies in the window of 0.5 < m > 1.0. In-
terestingly, a m value of 0.7 was obtained from the log(ipa)
versus log(v) plot in this study, which further reminiscent
the ZPT oxidation as mixed adsorption and diffusion con-
trolled reaction mechanisms on the CoPc/SPE.
3.2. Flow injection analysis
Initial FIA studies are focused to the optimization of
solution phase and experimental conditions for the ZPT
analysis. Fig. 2 shows the typical FIA response of 60?M
of ZPT (5?l loop) under increasing applied potential (Eapp)
and flow rate (Hf). As can been seen in Fig. 2A, the ZPT
detection currents increase with Eapp. It is important to
notice that the stability of the experiments decreases dra-
matically as Eapp> 0.3V (insert figures in Fig. 2A). It is
expected that oxygen gas evolution reaction at Eapp> 0.3V
may complicate the ZPT detection scheme in the FIA. To
achieve a stable ZPT analysis, Eapp= 0.3V was chosen as
optimal for the rest of the analysis. Fig. 2B shows a peak
like trend FIA response of 60?M ZPT at different Hfat an
Eappof 0.3V. The Hfat 0.4mlmin−1was found to have a
maximum FIA detecting current signal. It is expected that
both the adsorption and diffusion mechanisms can compete
with each other under the working Hfwindow.
Under the optimized FIA condition, three different types
of alkaline base electrolytes of KOH, NaOH, and NaOH +
KCl of the ionic strength 0.1M was tested for 60?M ZPT in
FIA. The obtained respective current signals values are 99.7,
41.3 and 42.3nA. It is obvious that KOH solution found
can influence the current enhancement. Exact mechanism
for such effect is unknown for us now.
Y. Shih et al./Talanta 62 (2004) 912–917
Fig. 2. Effects of applied potential (A) and flow rate (B) on the detection
of 60?M of ZPT at the CoPc/SPE by flow injection analysis using 0.1M
KOH carrier solution.
Fig. 3 shows the FIA calibration response for ZPT on the
CoPc/SPE. The FIA responses are systematically increased
with [ZPT]. A linear range of 6–576?M with sensitivity and
regression coefficient of 1.65nA?M−1and 0.9989, respec-
Fig. 3. Calibration response (A) and graph (B) for the ZPT by flow
injection analysis at an applied potential of 0.3V vs. Ag/AgCl using
0.1M KOH carrier solution. Flow rate: 0.4mlmin−1. Sample loop: 5?l.
Fig. 4. Continuous flow injection response on the detection of 60 and
576?M of ZPT at the CoPc/SPE. Other conditions are the same as Fig. 3.
tively, was obtained (Fig. 3B). The detection limit (S/N =
3) is 0.9?M (i.e. 1.42pg per 5?l loop), which is lower
than those reported earlier by HPLC and SnO2/CPE systems
[19,22]. Continuous injections (n = 13) of 60 and 576?M
Fig. 5. Flow injection analysis of the ZPT assays in two kinds of shampoo
products (Types 1 and 2) at the CoPc/SPE. All the standard ZPT samples
were prepared as a mixture with real samples. Other conditions are the
same as Fig. 3.
Y. Shih et al./Talanta 62 (2004) 912–917
Results for the assay of ZPT in commercial shampoos by FIA
Weight mass (mg)Original value (?M) Injectb(?M)Detected value after spike (?M) Recovery (%)
#1 46755.51 ± 0.79 (0.97%) 60112.86 ± 1.84
176.74 ± 0.27
293.92 ± 0.67
126.94 ± 0.51
184.7 ± 0.88
305.19 ± 4.95
121.23 ± 0.54
182.32 ± 0.75
301.23 ± 0.43
126.32 ± 0.60
183.24 ± 0.45
306.21 ± 1.02
60.35 ± 0.32
120.12 ± 0.20
239.27 ± 0.77
95.58 ± 2.00
101.02 ± 0.83
99.33 ± 1.03
102.03 ± 0.60
99.18 ± 0.94
99.78 ± 4.96
103.30 ± 0.70
102.55 ± 0.87
100.82 ± 0.62
101.31 ± 0.81
98.09 ± 0.71
100.28 ± 1.16
100.58 ± 0.32
100.10 ± 0.20
99.69 ± 0.77
#246065.72 ± 0.33 (0.991%)60
#3 48059.25 ± 0.45 (0.994%)60
#4 458 65.53 ± 0.55 (1.02%)60
aSamples #1–4 and $1 are Types 1 and 2, respectively.
bInjected samples mixture as composed of standard ZPT with real sample in 0.1M KOH.
ZPT shows low R.S.D. values of 3.87 and 3.54%, respec-
tively, indicating the excellent stability and reproducibility
is disposable, it can offer for bulk preparation and routine
Finally, the real sample assays were demonstrated for two
different types of shampoo products (Types 1 and 2). Fig. 5
shows typical FIA real sample analysis by standard addition
ture with the real samples. As can be seen, the Type 2 sham-
poo (in absence of ZPT) did not show any detectable current
signal in FIA (Fig. 5B), while the Type 1 sample (with ZPT)
showed good FIA signals (Fig. 5A). This particular experi-
ment validates the present approach to be practical. In addi-
tion, there is no interference of the matrix effects from other
ingredients in the hair-car products. Table 1 showed quan-
titative data based on the present approach. The obtained
values are very close to the labeled values, which further
verified the quality of the commercial hair-care products.
The CoPc modified screen-printed carbon electrode was
effective and selective for the flow injection analysis of zinc
pyrithione (ZPT) in hair care products. The hydrodynamic
flow injection parameters are systematically optimized and
the obtained analytical performance with the CoPc/SPE is
good. The approach allows the ZPT real sample analysis
without any tedious pretreatment procedures as seen in con-
ventional techniques. Since the approach is less expensive
and easy to operate, it can be easily extended to a convenient
portable type of ZPT analyzer. The work is now in progress
in our laboratory.
The authors gratefully acknowledge financial supports
from the National Science Council of Republic of China.
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