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Selective UV-filter detection with sensors based on stainless steel electrodesmodified with polyaniline doped with metal tetrasulfonated phthalocyaninefilms

Luiz Fernando Moreira,a Marcos Roberto de Vasconcelos Lanza,a Auro Atsushi Tanakab

and Maria Del Pilar Taboada Sotomayor*c

Received 3rd February 2009, Accepted 17th April 2009

First published as an Advance Article on the web 28th April 2009

DOI: 10.1039/b902273b

This work describes the construction and application of two amperometric sensors for sensitive

UV-filter determination. The sensors were prepared using stainless steel electrodes in which polyaniline

(PANI) was electrochemically polymerized in the presence of nickel (NiPcTS) or iron (FePcTS)

tetrasulfonated phthalocyanines. The sensor surface characterizations were carried out using atomic

force microscopy (AFM). The PANI/NiPcTS sensor was selective for the chemical UV-filter

p-aminobenzoic acid (PABA) and the PANI/FePcTS sensor was selective for octyldimethyl-PABA

(ODP), both in a mixture of tetrahydrofuran (THF) and 0.1 mol L�1 H2SO4 at a volume ratio of 30 : 70,

and with an applied potential of 0.0 mV vs. Ag|AgCl. A detailed investigation of the selectivity was

carried out for both sensors, in order to determine their responses for ten different UV filters. Finally,

each sensor was successfully applied to PABA or ODP quantification in sunscreen formulations and

water from swimming pools.

Introduction

The growing body of information concerning the damaging

effects of UV radiation, the hole in the ozone layer, and

protection against UV radiation, has been received with great

public interest, due to concerns surrounding skin cancer and skin

ageing. Protection against UV radiation essentially requires

a UV-filter to filter out UVB rays (290–320 nm), which are

responsible for sunburn, or UVA rays (320–400 nm), which cause

skin ageing.

Chemical UV-filters are lipophilic compounds that absorb

dangerous UV light, thereby decreasing the amount of solar

radiation reaching the skin. The efficacy of a cosmetic formula-

tion can be estimated using the sun protection factor (SPF),

which depends on the UV-filter’s content of the formulation.

However, although UV-filters have beneficial effects for the skin,

a number of dermatological reactions and estrogenic effects have

been described.1–3 On the other hand, the recent increase in the

number of products containing UV-filters, the extension of the

spectrum of their usage from sunscreens to cosmetics, and their

applications as additives in plastics, carpets and washing

powders, has resulted in the possibility of aquatic contamination.

However, the toxicology and estrogenic effects of these

compounds within the ecosphere are hitherto unknown or

unpublished.

Various analytical methods for UV-filter quantification have

been reported in the literature, most of which are based on the

aUniversidade Sao Francisco, Braganca Paulista, SP, BrazilbCentro de Ciencias Exatas e Tecnologia, Universidade Federal doMaranhao, Sao Luıs, MA, BrazilcDepartamento de Quımica Analıtica, Universidade Estadual Paulista‘‘Julio de Mesquita Filho’’, 14801-970 Araraquara, SP, Brazil. E-mail:mpilar@iq.unesp.br; Tel: +55-16-33016620

This journal is ª The Royal Society of Chemistry 2009

HPLC technique.4–10 However, the procedures for isolation of

sunscreen agents from cosmetic matrices prior to chromato-

graphic analysis require several sample manipulations, which are

laborious and time-consuming, and therefore not suitable for

routine analyses in the cosmetic industry or for environmental

monitoring. Moreover, large volumes of hazardous solvents

must normally be handled using chromatographic techniques,

which may subsequently be released into the environment.

Additionally, there are no official methods of analysis for many

of these UV-filters, chemicals that include octyldimethyl-PABA

(ODP) and 4-methylbenzylidene camphor (4-MBC).

The need for reliable analytical methods to determine the

active ingredients of pharmaceuticals and cosmetics in the envi-

ronment has recently been highlighted.1–3,11–13 The identification

and development of standard analytical procedures for these new

environmental contaminants will be extremely important for

their successful quantification. It is not only necessary to identify

more reliable, rapid, sensitive and selective methodologies, but

also to develop analytical methods that do not require expensive

equipment, and which at the same time are portable. The

chemical sensor is an analytical tool that incorporates all of these

characteristics.

In the last two decades, conducting polymers have attracted

the attention of researchers from different fields of chemistry and

physics.14–28 Of these polymers, polyaniline (PANI) stands out

due to its interesting characteristics such as easy chemical and

electrochemical synthesis, high electrical conductivity, and high

stability.29–33 For these reasons, it has been widely employed in

sensor and biosensor construction,34–42 amongst other applica-

tions. The possibility of preparing PANI films doped with metal

tetrasulfonated phthalocyanines (MPcTS), which can be effi-

ciently used as counter-anions to neutralize the polymer back-

bone, has been suggested by some researchers for use in battery

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cathodes and other important industrial applications.43–46 The

use of PANI/MPcTS films in sensor construction offers advan-

tages, due to the known catalytic characteristics of the metal

phthalocyanines and analogous compounds,45,47–71 and provides

an alternative and novel method for the selective and sensitive

quantification of important analytes.

To aid in the development of an alternative clean, rapid,

sensitive and selective methodology for UV-filter quantification,

the present work describes the development and application of

sensors constructed with stainless steel electrodes modified with

PANI/MPcTS films. For many of these materials, there is still no

official method of analysis, nor is there any regulation concern-

ing their maximum permitted concentrations in aquatic envi-

ronments.

Experimental

Reagents and solutions

All chemicals used were of analytical or HPLC grade when

necessary. Nickel(II) phthalocyanine-tetrasulfonic acid tetraso-

dium salt, iron(III) phthalocyanine-4,40,400,4%-tetrasulfonic

acid, 4-aminobenzoic acid (PABA), octyldimethyl-PABA (ODP)

[2-ethylhexyl-4-(dimethylamino)benzoate], 4-methylbenzylidene

camphor (4-MBC) [3-(4-methylbenzylidene)-bornan-2-one],

isoamyl p-methoxycinnamate (IMC) [isoamyl-4-(dimethylamino)-

benzoate], methyl anthranilate (MA) [methyl-2-aminobenzoate]

and 2-phenyl-5-benzimidazolesulfonic acid (PBS) were

acquired from Aldrich. Benzophenone-3 (BENZ-3) [2-hydroxy-

4-methoxybenzophenone], octyl-methoxycinnamate (OMC)

[2-ethylhexyl-4-methoxycinnamate], octyl salicylate (OSA)

[2-ethylhexyl salicylate] and avobenzone (AVB) [1-(4-methoxy-

phenyl)-3-(4-tert-butylphenyl)-1,3-propanedione] were pur-

chased from Fluka. Aniline was obtained from Acros Organics�,

and was vacuum distilled before use. Tetrahydrofuran (THF),

acetonitrile (MeCN), and sulfuric acid were obtained from Synth-

Brazil.

Buffer solutions were prepared using water purified in a Mil-

lipore Milli-Q system, and the pH was determined using a Met-

rohm� Model 781 pH/Ion meter.

The ODP and PABA standard solutions for the electroana-

lytical experiments were prepared in THF and 25 mmol L�1

H2SO4, respectively. For the HPLC experiments, these standard

solutions were prepared in the respective mobile phases.

Sensor construction

Purpose-built 304 stainless steel electrodes with a geometrical

area of 0.502 cm2 were used for sensor construction, since

stainless steel is a low cost electrodic material. Initially, the

surfaces of the electrodes were cleaned as previously described in

the literature,72–74 with some modifications. Briefly, the surface of

the stainless steel electrode was manually polished with 0.5 and

0.3 mm alumina suspensions, then washed with distilled water,

ultrasonicated for 30 s in deionized water, followed by a further

30 s ultrasonification in ethanol to eliminate any alumina parti-

cles from the surface. Verification of the electrode surface was

carried out by means of cyclic voltammetry in 0.1 mol L�1 H2SO4

under anaerobic conditions. A characteristic and reproducible

profile was obtained for the clean stainless steel electrode.

1454 | Analyst, 2009, 134, 1453–1461

After cleaning, the electrodes were modified following litera-

ture procedures43,75 with PANI, PANI/FePcTS or PANI/NiPcTS

films. For the PANI/FePcTS sensor, 10.0 mL of deoxygenated

0.5 mol L�1 aqueous sulfuric acid solution, containing 0.12 mol

L�1 aniline or aniline plus 5.0 � 10�3 mol L�1 MPcTS, was used

for the electrochemical synthesis of polyaniline and polyaniline

doped with the respective metal tetrasulfonated phthalocyanine

(MPcTS). The electropolymerization was performed potentio-

dynamically in the potential range of �0.2 to 1.6 V vs. Ag|AgCl,

at a scan rate of 50 mV s�1, over 10 cycles for PANI/FePcTS and

15 cycles for PANI/NiPcTS. The difference in the cycles used for

the sensors’ construction can be explained considering their

performance in the detection of the UV-filters. In addition to the

optimization of the film synthesis cycles, the aniline and MPcTS

concentrations, as well as the potential range for the electro-

synthesis, were also optimized to achieve the best performance

for each sensor in their respective UV-filter determinations. For

comparison, two stainless steel electrodes coated with PANI

films were prepared using 10 and 15 voltammetric cycles.

Sensor surface characterization

The AFM images were collected in contact mode using a Nano-

scope IIIa Multimode Microscope (Digital Instruments�, USA).

Film thickness estimation was carried out in a surface profiler

(Model Dektak 3, Veeco/Sloan Instruments Inc.).

Electrochemical measurements

The electrochemical experiments were performed using

a PGSTAT 30 potentiostat (EcoChemie–Autolab�), interfaced

with a personal computer for data acquisition and potential

control. All electrochemical measurements were carried out in

a conventional three-electrode cell at room temperature, with an

Ag|AgCl electrode as the reference, and a platinum wire as the

counter electrode. The sensors were the working electrodes.

All cyclic voltammetry and amperometric measurements for

ODP and PABA determination were carried out in 7.0 mL of

a mixture of 0.1 mol L�1 H2SO4 and THF in a volume ratio of 70

: 30 (the electrolyte solution). This mixture was previously opti-

mized to yield the best results. For the amperometric measure-

ments, the current obtained at 0.0 mV vs Ag|AgCl (previously

optimized) was, initially, continuously monitored until a steady

state was reached (after approximately three minutes), to obtain

the baseline. After that, 50 mL of 2.5 � 10�2 mol L�1 UV-filter

standard solution was added to the electrolyte, and the mixture

was stirred for 10 seconds to homogenize the solution. The

current was then monitored in the quiescent solution for 10 s,

until the current reached a new steady state. Successive additions

of the respective UV-filter standard solutions were made, in

order to obtain the analytical curve.

HPLC analysis

The chromatographic analyses were performed using a Shi-

madzu� Model 20A liquid chromatograph fitted with an UV/Vis

detector (SPD-20A), an autosampler (SIL–20A) and a degasser

(DGU–20A5), and coupled to a personal computer. A C18

column (250 � 4.6 mm, Shim–Pack CLC–ODS) was positioned

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inside an oven (Shimadzu� CTO–10AS) to maintain a constant

temperature.

For the ODP quantification, the method proposed by

Ribeiro76 was used, due to the lack of any official analytical

method. For this, the mobile phase was prepared by mixing

acetonitrile and water (pH ¼ 2.7) with H3PO4 in a ratio of 93 : 7

(v/v). The flow rate was 1.0 mL min�1 and the sample injection

volume was 20 mL. The column temperature was maintained at

30 �C. The measurement wavelength was 309 nm and the analysis

time was 11 minutes for each standard and sample.

For the PABA quantification, the method proposed by

Richter et al. was used.77 The mobile phase was phosphate buffer

(0.0075 mol L�1, pH 7.3)/acetonitrile in a ratio of 90 : 10 (v/v).

The flow rate was 0.8 mL min�1 and the sample injection volume

was 50 mL. The column temperature was maintained at 40 �C.

The measurement wavelength was 280 nm and the analysis time

was 6 minutes for each standard and sample.

Sunscreen analysis

Sunscreens based on two different compositions containing ODP

(8% (w/w), declared value) or PABA (15% (w/w), declared value)

were tested. Anionic gel-cream samples were dissolved in the

same solvents in which the standard solutions of ODP and

PABA were prepared, while for the HPLC quantification they

were dissolved in their respective mobile phases. Non-ionic

samples were prepared for sensor application in the same way as

the anionic samples, and were then filtered in order to separate

out the remainder of the solid. For the HPLC application, non-

ionic samples were pre-treated following a procedure established

by ANVISA (Agencia Nacional de Vigilancia Sanit�aria do Bra-

zil-National Agency of Sanitary Monitoring).78 For this, 2.0 g of

the gel-cream containing ODP or PABA was mixed with 4.0 mL

of methanol and 250 mL of 1.0 mmol L�1 H2SO4, the mixture was

then warmed to 40 �C for 10 minutes, and finally ultrasonicated

for 3 minutes. After this pre-treatment, the solution was filtered

and injected into the chromatograph for analysis.

Aquatic sample analysis

Two water samples from private swimming pools were enriched

with ODP or PABA and then analyzed using the proposed

sensors in order to evaluate the matrix effect, using the recovery

values.

In addition, solid phase extractions (SPE) were optimized for

ODP and PABA recovery, using these aquatic samples, for later

application with the sensors using the TOC (total organic

carbon) technique. The TOC technique was preferred to the

HPLC methodology because the sample compositions were well

known, so a more rapid analysis could be carried out without the

generation of residues or the consumption of reagents, making

this a ‘‘clean’’ procedure (although it is not selective for the

monitoring of the optimization of the SPE technique). The TOC

equipment used was a Shimadzu Model TOC-VCP analyzer.

To obtain the best conditions for the analyte extraction using

the SPE technique, the cartridge type and the pre-concentration

flow were previously optimized for each of the analytes. For this,

the organic matter in the aqueous sample containing a known

amount of UV-filter was analyzed before and after having passed

This journal is ª The Royal Society of Chemistry 2009

through the cartridge. The extent of the removal of organic

material (corresponding to the analyte in the aquatic sample)

after passage through the cartridge indicated the efficiency of the

extraction procedures. The flow rate allowing total retention

(extraction) in the cartridges was 1 mL min�1 for both analytes.

The most efficient ODP retention was obtained using an ODS-

C18 cartridge, while a cyan-modified silica cartridge provided the

optimum retention for PABA. Both cartridges were obtained

from Agilent Technologies (AccuBOND�, 6 mL, 1000 mg, 50 mm

particle size).

The analyte pre-concentration steps used in this work were as

follows: (1) the cartridge was conditioned (50 mL methanol, 50

mL Milli-Q water), and the pre-concentration flow rate adjusted

(1 mL min�1); (2) the analyte was pre-concentrated (200.0 mL

sample); (3) interferents were removed (50 mL Milli-Q water); (4)

analytes were eluted. For the electroanalytical measurements, the

elution employed 10.0 mL of either THF (for ODP), or 0.025 mol

L�1 H2SO4 (for PABA). For the comparative method (HPLC),

the elution was carried out using the respective mobile phases for

each analyte; (5) the cartridge was cleaned (50 mL methanol).

Results and discussion

Stainless steel electrodes modified with PANI/MPcTS

Cyclic voltammetry was used for PANI and PANI/MPcTS film

preparation by the electro-polymerization of aniline in the

absence or presence of the corresponding metal tetrasulfonated

phthalocyanine. Fig. 1 shows the cyclic voltammetry profiles

recorded for the stainless steel electrode and the PANI and

PANI/MPcTS films on stainless steel electrodes, in a de-aerated

0.1 mol L�1 H2SO4 solution. These profiles were always repro-

ducible for the unmodified and for the PANI and PANI/MPcTS

modified electrodes. It can be observed that for the unmodified

stainless steel electrode, there was one oxidation peak at 1.20 V

vs. Ag|AgCl whose current decreased when PANI was deposited

on the surface, probably due to the presence of the non-

conductive pernigraniline form of the PANI,36 implying a lower

electroactivity of the film at this potential.79 In the presence of

PANI and PANI/MPcTS, three peaks were observed. For the

PANI/MPcTS film, the oxidation at 0.60 V appeared as a weak

shoulder, and another oxidation peak appeared at 1.2 V. At this

last potential, the current was higher for the PANI and the

stainless steel electrodes, meaning that the presence of MPcTS,

the PANI counter-anion, increased the conductivity of the films.

This phenomenon could be explained by an increase in protonic

transport during the redox processes compared with undoped

polyaniline films, as has been previously described by Damos

et al.43

It is worth emphasizing that the electro-polymerization of the

PANI/MPcTS was carried out in different potential ranges.

Some of these were: �0.2 to 0.8; �0.2 to 1.0; 0.0 to 1.0; �0.2 to

1.6 V vs. Ag|AgCl (always with a scan rate of 50 mV s�1). The

PANI/MPcTS electrosynthesis was carried out in solutions of 0.1

and 0.5 mol L�1 H2SO4, HClO4, HNO3 or HCl, and in mixtures

of concentrated H2SO4/dimethyl formamide (DMFA) and 0.1

mol L�1 H2SO4/DMFA in a volume ratio of 95 : 5. All different

conditions were based on procedures described in the literature.

Only the sensor prepared using a �0.2 to 1.6 V potential range in

Analyst, 2009, 134, 1453–1461 | 1455

Fig. 1 Cyclic voltammograms for unmodified stainless steel electrode

(dashed line); and for stainless steel electrode modified with PANI

(dotted line) or with PANI/MPcTS (solid line) in de-aerated 0.1 mol L�1

H2SO4 at a scan rate of 50 mV s�1. (a) PANI/FePcTS and (b) PANI/

NiPcTS.

Fig. 2 AFM images of: (a) 304 stainless steel surface; (b) stainless steel

modified with a PANI film; (c) stainless steel modified with a PANI/

FePcTS film; (d) stainless steel modified with a PANI/NiPcTS film.

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0.5 mol L�1 aqueous sulfuric acid solution gave promising results

for UV-filter detection, so that these conditions were selected for

further experiments. For the other conditions, the sensors did not

show any variation in the cyclic voltammograms or the chro-

noamperometry signals for any of the UV-filters studied.

Sensor surface characterization

AFM images were used to obtain a visual understanding of the

modification of the stainless steel electrodes with PANI and

PANI/MPcTS films. Fig. 2 shows the AFM images obtained for

(a) stainless steel, (b) PANI, (c) PANI/FePcTs and (d) PANI/

NiPcTS film surfaces. In these images, small imperfections on the

stainless steel surface can be observed, despite its previous pol-

ishing (Fig. 2a). However, Fig. 2b clearly shows the change in the

electrode surface following aniline polymerization, with the

apparent formation of polymeric networks covering the stainless

steel surface.

In the third and fourth images (Fig. 2c and 2d), abrupt changes

in the morphology of the stainless steel surfaces can be observed.

In the case of the PANI/FePcTS electrode (Fig. 2c), a rough

surface appears due to the formation of FePcTS agglomerates.

In the case of the PANI/NiPcTS film (Fig. 2d), a more

1456 | Analyst, 2009, 134, 1453–1461

homogeneous distribution can be observed, compared to the

PANI/FePcTS film. Energy dispersive X-ray emission graphs

were used to determine the amount of the metal complexes

immobilized on the films, and indicated that in each film the mass

percentages were 73% and 82% for iron and nickel, respectively.

The PANI/FePcTS and PANI/NiPcTS films showed thick-

nesses of 10 mm and 20 mm, respectively. This difference in the

thicknesses obtained for the two films may be simply explained

by the different cycles used for the sensors construction, which

were based on the optimum performances observed for each

sensor. In addition, it can be seen from Fig. 3 that the greater

thickness of PANI/NiPcTS resulted in surfaces that are more

conductive.

Analytical characteristics of the sensors

The voltammetry profiles of the sensors in the electrolyte solu-

tion consisting of a mixture of H2SO4 and tetrahydrofuran

(THF), in a previously optimized volume ratio of 70 : 30, are

shown in Fig. 3. Profiles typical of doped conductive polymer

films in aqueous/organic media80 can be observed. On the other

hand, two well-defined peaks can be seen for the sensors in the

electrolyte solution. In addition, the anodic and cathodic

currents for PANI/NiPcTS were higher than those for the PANI/

FePcTS sensor, as was suggested by the energy dispersive X-ray

and thickness studies.

The experiments showed that in 7.0 mL of the 0.1 mol L�1

H2SO4/THF 70 : 30 (v/v) solution, at least ten successive addi-

tions of 500 mL of the UV-filter standard solution (the majority

of which was soluble in organic media) could be carried out

without phase separation. Hence, this solution was chosen for

use in all analyses with the proposed sensors.

In a first stage, cyclic voltammetry experiments used the

stainless steel electrode and the PANI sensor in order to explore

the electrochemical behavior of the UV-filters. No changes in the

voltammogram profiles (data not shown) were observed for the

compounds evaluated using these two electrodes.

This journal is ª The Royal Society of Chemistry 2009

Fig. 4 Relative response (%) obtained with PANI/FePcTS (a) and

PANI/NiPcTS (b) sensors for ten different UV-filters (ODP, PABA,

OMC, IMC, AVB, 4-MBC, PBS, BENZ-3, MA and OSA). The

parameter was calculated considering the sensor response of PANI/

FePcTS for ODP or PANI/NiPcTS for PABA as 100%, respectively.

Fig. 3 Cyclic voltammograms for stainless steel electrode modified with

PANI (dotted line) or with PANI/MPcTS (solid line) in the electrolyte

solution (0.1 mol L�1 H2SO4/THF 70 : 30 (v/v) ratio) at a scan rate of 50

mV s�1. (a) PANI/FePcTS and (b) PANI/NiPcTS.

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Next, in order to investigate for which of the UV-filters the

sensors gave better results, cyclic voltammetry was performed

using the electrolyte solution in the presence of 1.8 � 10�4 mol

L�1 of each of the ten UV-filters studied for the PANI/FePcTS

and PANI/NiPcTS sensors. The results obtained in these

experiments are shown in Fig. 4 as the response (as current

variation), at the adequate potential of 0.0 V vs. Ag|AgCl, of each

proposed sensor for the ten compounds evaluated. This potential

was used in these experiments because it is a potential free of

other electrochemical interferences. The PANI/FePcTS sensor

showed a higher response for ODP and the PANI/NiPcTS sensor

showed a higher response for PABA. Therefore, each of the UV-

filters was analyzed by the corresponding PANI/MPcTS sensor.

Fig. 5 and 6 show the voltammetric and chronoamperometric

profiles and their corresponding analytical curves, obtained at

0.0 V vs. Ag|AgCl, for ODP and PABA, respectively. This

applied potential was previously optimized (data not shown) for

each analyte and sensor. It is important to emphasize that, in the

sensors field, potentials close to 0.0 V are preferred in order to

minimize possible interferences from other electrochemically

active compounds.

All of the analytical parameters obtained for the ODP and

PABA quantifications are summarized in Table 1. For the ODP

determination, the PANI/FePcTS sensor showed a linear range

between 1.8 and 10.5 � 10�4 mol L�1, a very high sensitivity and

low detection and quantification limits, which were calculated as

This journal is ª The Royal Society of Chemistry 2009

suggested by the IUPAC recommendations.81 The PANI/

NiPcTS sensor possessed a linear range for PABA between 0.2

and 1.4 � 10�3 mol L�1, a lower sensitivity and higher detection

and quantification limits than the PANI/FePcTS sensor.

Parameters such as response time, repeatability and repro-

ducibility were very similar and can be attributed to the film

preparation procedures, which were practically the same in each

case. The repeatability of the measurements was estimated by

plotting seven successive analytical curves in the corresponding

linear range for each analyte and sensor. In this case, the

repeatability was defined in terms of the relative standard devi-

ation (RSD) of the seven sensitivities obtained. The reproduc-

ibility of each PANI/MPcTS sensor was estimated as the RSD of

the sensitivities obtained using four different sensors constructed

on different days.

Application

Firstly, the proposed sensors were used for analyses of the UV-

filters in different sunscreen formulations (A, B, C and D). The

concentrations of the UV-filters were calculated using the

external calibration method, and the results compared to the

chromatographic method using the paired t-test, which showed

Analyst, 2009, 134, 1453–1461 | 1457

Fig. 6 Typical profile of the PANI/NiPcTS sensor for PABA under

optimized conditions. (a) Cyclic voltammetry plot. (b) Chronoampero-

metric measurements for successive additions of PABA. (c) Analytical

curve.

Fig. 5 Typical profile of the PANI/FePcTS sensor for ODP under

optimized conditions. (a) Cyclic voltammetry plot. (b) Chronoampero-

metric measurements for successive additions of the analyte. (c)

Analytical curve.

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that the results were statistically the same at a confidence level of

95% (Table 2). It should be stressed that no sample pre-treatment

had to be carried out using the sensors for sunscreen analyses in

order to obtain results similar to those obtained by the HPLC

method. Both methods gave similar relative standard deviation

values, suggesting an excellent precision of the proposed sensors.

All these results suggest that the proposed methodology is a more

efficient and rapid alternative for sunscreen analysis.

In order to confirm that the sensors could be applied to aquatic

environments, ODP and PABA recovery studies were performed

using additions of known quantities of the analytes to samples of

1458 | Analyst, 2009, 134, 1453–1461

water from swimming pools (Table 3). Good recoveries were

obtained, with values close to 100% for both UV-filters, indi-

cating that the sensors can be successfully applied to the analysis

of these kinds of samples without influence from the matrix.

Finally, to increase sensitivity, the solid phase extraction (SPE)

method was optimized for each analyte using the swimming pool

water samples. The results obtained after SPE extraction were

compared with the HPLC method (Table 4). The results obtained

using the sensors were statistically the same, using the paired

t-test at a confidence level of 99%, as the results obtained by the

chromatographic method. Such increase in sensitivity may be

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Table 1 Analytical parameters obtained for ODP and PABA determination using the proposed sensors based on the PANI/MPcTS films

Parameter

Sensor

PANI/FePcTS for ODP PANI/NiPcTS for PABA

Linear ranges/mol L�1 1.8 to 10.5 � 10�4 0.2 to 1.4 � 10�3

Sensitivity/mA L mol�1 24093 � 405 6487 � 189Correlation coefficients 0.997 0.9992Detection limits/mmol L�1 8.0 37.5Quantification limits/mmol L�1 25 125Response times/s 0.5 0.5Measurement repeatability (RSD, n ¼ 7) 1.7 2.1Sensor reproducibility (RSD, n ¼ 4) 3.2 2.9

Table 2 Determination of ODP and PABA in sunscreen samples using the sensors based on PANI/MPcTS films

Sensor Sample (composition)

UV-filter concentration in % (w/w)

Student t-valuescDeclared value Comparative method (HPLC)a Proposed sensorb

PANI/FePcTS for ODP A (non-ionic gel) 8 7.94 � 0.06 8.00 � 0.03 3.4B (anionic gel) 8 8.01 � 0.01 8.04 � 0.04 1.3

PANI/NiPcTS for PABA C (non-ionic gel) 15 14.5 � 0.5 14.9 � 0.3 2.3D (anionic gel) 15 14.9 � 0.1 15.0 � 0.3 0.6

a Standard deviation of two duplicates. b Standard deviation for three replicates. tcrit ¼ 4.30 for 2 degrees of freedom. c Statistical values at a confidencelevel of 95%.

Table 3 Recovery values in water from swimming pools determined by the proposed sensors for each of their corresponding analytes

Sensor Sample Added UV-filter/mol L�1 Found UV-filter/mg L�1 Recoverya (%)

PANI/FePcTS for ODP E 1.80 � 10�4 1.77 � 10�4 98.3 � 1.4PANI/NiPcTS for PABA F 7.30 � 10�4 7.18 � 10�4 98.3 � 0.4

a Standard deviation for three replicates.

Table 4 Determination of ODP and PABA in aquatic samples applying the optimized SPE method using the proposed sensors, and the comparativemethod

UV-filter Sample

UV-filter concentration/mg L�1 Student t-valuee

Real value Comparative method (HPLC)a Proposed sensorb HPLC Sensor

ODPc E 50.0 47.6 � 1.3 48.9 � 0.7 2.6 2.7PABAd F 100.0 98.4 � 0.14 98.3 � 0.4 16.2 7.4

a Standard deviation of two duplicates. b Standard deviation for three replicates. tcrit ¼ 63.7 and 9.92 for 1 and 2 degrees of freedom, respectively.c PANI/FePcTS sensor. d PANI/NiPcTS sensor. e Statistical values at a confidence level of 99%.

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useful in order to extend the application of the sensors to a wider

range of environmental media (such as wastewater from treat-

ment plants, surface waters, or sewage effluents).

Conclusions

This work describes an alternative and novel methodology for

the determination of chemical UV-filters based on two highly

sensitive and selective amperometric sensors constructed using

stainless steel electrodes electrochemically modified with PANI/

MPhTS. The sensors showed high sensitivity and selectivity for

the determination of PABA and ODP, and were satisfactorily

applied to the analysis of sunscreens in water from swimming

This journal is ª The Royal Society of Chemistry 2009

pools, thus, opening up the possibility of the quantification of

these chemicals in other aquatic environments. The use of

stainless steel electrodes modified with PANI/FePcTS or PANI/

NiPcTS films is a promising alternative procedure which is clean,

rapid, sensitive and robust, for UV-filter determinations in

sunscreen formulations and aquatic samples.

Acknowledgements

The authors gratefully acknowledge financial support from

CNPq (Proc. 470025/2006-9), ABN/Banco Real and FAPESP.

LFM is indebted to FAPESP for a fellowship. We thank Prof. Dr

Gilberto Medeiros Ribeiro and Mr Vinicius do Lago

Analyst, 2009, 134, 1453–1461 | 1459

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Pimentel (LNLS, Campinas, SP) for assistance in the use of the

AFM microscope.

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