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PAPER IN FOREFRONT

Micro-solid phase equilibrium extraction with highly ordered

TiO2 nanotube arrays: a new approach for the enrichment

and measurement of organochlorine pesticides at trace level

in environmental water samples

Qingxiang Zhou & Yunrui Huang & Junping Xiao &

Guohong Xie

Received: 29 November 2010 /Revised: 4 February 2011 /Accepted: 7 February 2011 /Published online: 23 February 2011# Springer-Verlag 2011

Abstract Ordered TiO2 nanotube arrays have been widely

used in many fields such as photocatalysis, self-cleaning,

solar cells, gas sensing, and catalysis. This present study

exploited a new functional application of the ordered TiO2

nanotube arrays. A micro-solid phase equilibrium extrac-

tion using ordered TiO2 nanotube arrays was developed for 

the enrichment and measurement of organochlorine pesti-

cides prior to gas chromatography-electron capture detec-

tion. Ordered TiO2 nanotube arrays exhibited excellent 

merits on the pre-concentration of organochlorine pesticides

and lower detection limits of 0.10, 0.10, 0.10, 0.098,

0.0076, 0.0097, 0.016, and 0.023 μ g L−1 for  α -HCH, β-

HCH, γ-HCH, δ-HCH, p, p’-DDE, p, p’-DDD, o, p’-DDT,

and p, p’-DDT, respectively, were achieved. Four real water 

samples were used for validation, and the spiked recoveries

were in the range of 78 – 102.8%. These results demonstrat-

ed that the developed micro-solid phase equilibrium

extraction using ordered TiO2 nanotube arrays would be

very constructive and have a great beginning with a brand

new prospect in the analysis of environmental pollutants.

Keywords Ordered TiO2 nanotube array. Organochlorine

 pesticides . Micro-solid phase equilibrium extraction . Gas

chromatography-electron capture detection

Introduction

TiO2 has been widely used in many applications ranging

from anticorrosion, self-cleaning coatings, paints to solar 

cells, and photocatalysts due to its high photocatalytic

activity, chemical stability, non-toxicity, and relatively low

cost [1 – 4]. Recently, TiO2 nanotubes have received

considerable attention because of their higher surface area,

 better adsorption ability, and higher photocatalytic activity

in comparison with TiO2 powders [5 – 8]. However, it was

difficult to separate and reuse the TiO2 nanotubes from the

solution. Therefore, the aim became to align the TiO2

nanotubes on a substrate or template. Zwelling and cow-

orkers [9] made a first attempt and successfully achieved

ordered TiO2 nanotubes arrays through a simple anodiza-

tion process. Grimes and coworkers firstly utilized the

Electronic supplementary material The online version of this article

(doi:10.1007/s00216-011-4788-7) contains supplementary material,which is available to authorized users.

Q. Zhou (*) : Y. Huang

Henan Key Laboratory for Environmental Pollution Control,

Key Laboratory for Yellow River and Huaihe River Water 

Environment and Pollution Control, Ministry of Education,

School of Chemistry and Environmental Sciences,

Henan Normal University,

Xinxiang 453007, China

e-mail: zhouqx@henannu.edu.cn

e-mail: zhouqx@cup.edu.cn

Q. Zhou

State Laboratory of Petroleum Resource and Prospecting, Key

Laboratory of Earth Prospecting and Information Technology,College of Geosciences, China University of Petroleum Beijing,

Beijing 102249, China

J. Xiao

Department of Chemistry,

University of Science and Technology Beijing,

Beijing 100083, China

G. Xie

College of Resources and Environment,

Henan Institute of Science and Technology,

Xinxiang 453003, China

Anal Bioanal Chem (2011) 400:205 – 212

DOI 10.1007/s00216-011-4788-7

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highly ordered TiO2 nanotube array film to fabricate the

solar cell [10]. Afterwards, many related preparation

methods and applications of TiO2 nanotubes arrays were

reported, which made it easy to obtain needed TiO2 nano-

tubes arrays for many important engineering applications.

 Nowadays, TiO2 nanotubes arrays have been employed in

many fields including photoelectrochemical hydrogen gen-

eration [11 – 15], solar cells [16 – 18], hydrogen storage [19],gas sensing [20 – 22], templates for growth of compound

semiconductor nanowires for radiation sensing [23], sub-

strate for high interfacial bond strength hydroxyl apatite

coating in implants [24, 25], biomedical applications [26],

and catalyst supports [27]. In environmental field, TiO2

nanotubes arrays have also been reported for organic

contaminant degradation based on its photoelectrocatalytic

activity [28 – 32]. However, to the best of our knowledge,

there has been no report using TiO2 nanotube array for the

goal of enrichment of environmental pollutants.

Organochlorine pesticides (OCPs) have been widely

used for the control of pests in agriculture and cities because of its low cost and effectiveness all over the world.

A huge amount of organochlorinated compounds is

continuously being released into the environment with the

extensive use of OCPs and herbicides, and discharge of 

wastewater from bleaching of pulp and municipal waste-

water treatment. Environmental contamination by OCPs has

 been a major concern over the past several decades in the

world because of their persistence, long-distance transport,

 biological effects, and bioaccumulation along the food

webs. Nowadays, OCPs are known as one of the most 

 persistent organic pollutants in the environment and have

absorbed much attention from most of the governments all

over the world. To date, most of the developed and

developing countries have already banned or restricted the

 production and usage of OCPs, such as dichlorodi-phenyl-

trichloroethanes (DDTs), hexachlorocyclohexanes (HCHs),

hexachlorobenzene, polychlorinated biphenyls, etc. The US

Environmental Protection Agency (EPA) has identified 12

 priority persistent bioaccumulative and toxic compounds of 

special interest [33], which includes DDT and its break-

down products, DDE and DDD. Some of them were listed

in the Stockholm Convention on Persistent Organic

Pollutants. However, recent studies have reported the

 presence of organochlorine pesticides in different sites

[34 – 36]. Therefore, it is necessary to monitor the amounts

and distribution of OCPs and evaluate their effects on the

environment. Thus, simple and highly sensitive analytical

techniques are required to detect and quantify OCPs at trace

levels. Usually, to achieve the necessary level of sensitivity,

an enrichment step is needed before analysis. A variety of 

enrichment steps had been established for separation and

 preconcentration of OCPs such as solvent cooling-assisted

dynamic hollow-fiber-supported headspace liquid-phase

microextraction [37], dispersive liquid – liquid microextrac-

tion [38, 39], solid phase extraction [40], solid phase

microextraction [41, 42], etc. Solid phase extraction has

distinguished from many extraction techniques because of 

its advantages of lower cost, higher enrichment factor, and

less consumption of organic solvents.

The aim of present work was to establish a novel,

effective, reusable, simple, and sensitive determinationmethod for OCPs with TiO2 nanotube arrays. Used as

adsorbents, TiO2 nanotube arrays would avoid the low flow

rate of the conventional solid phase extraction with TiO2

nanotubes cartridge and the difficulty in separation from

dispersion in solution and make it easy to reuse. This new

 protocol is to establish a convenient micro-solid phase

equilibrium extraction (μ SPEE) with TiO2 nanotube arrays

for the enrichment of OCPs and to enlarge the application

field of TiO2 nanotube arrays.

Experimental

Reagents and materials

A working stock solution (10.00 mg L−1) of OCPs was

 prepared in HPLC-grade methanol with mixed standards

containing α -hexachlorocyclohexane (α -HCH), β-

hexachlorocyclohexane (β-HCH), γ-hexachlorocyclohexane

( γ-HCH), δ-hexachlorocyclohexane (δ-HCH), dichlorodi-

 phenyltrichloroethane (o, p’-DDT, p, p’-DDT), dichlorodi-

 p h e n y l d i c h l o r o e t h y l e n e ( p , p ’ - D D E ) , a n d

dichlorodiphenyldichloroethane ( p, p’-DDD) from Beijing

Yingtianyi Chemical Science and Technology Co., Ltd.

(Beijing, China). All the standard solutions were stored at 

4 °C in the refrigerator and protected from light. The

aqueous solutions were prepared daily by diluting the

standard mixture with ultrapure water. HPLC-grade meth-

anol and acetonitrile were obtained from Jiangsu Guoda

Chemical Reagent Co., Ltd. (Huaian, China). Ultrapure

water was prepared in the laboratory using a Millipore

(Billerica, MA, USA) water generator system, and all the

other solvents were of analytical reagent grade unless

stated. One percent sodium hydroxide and 1 mol L−1

hydrochloric acid were used for adjusting the pH value of 

the water samples.

Titanium sheets (99.6% purity) from Beijing Hengli

Taiye Co., Ltd. (Beijing, China), Pt electrode was obtained

from Shanghai Ruosull Technology Co., Ltd., 30 V

 potentiostat (JWY-30 G, Shijiazhuang, China)

Preparation and identification of TiO2 nanotube array

Titanium sheets (0.2 mm thick, 10×20 mm size) with

99.6% purity (Beijing, China) were polished with metallo-

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graphic abrasive paper and were then degreased by

sonicating in acetone, isopropanol, and methanol, respec-

tively. The sheets were air-dried after rinsing with ultrapure

water. The anodic oxidation was accomplished by using

titanium sheet as anode and platinum as cathode. The

distance between two electrodes was 3 cm in all experi-

ments. The electrolyte was composed of 0.14 M NaF and

0.5 M H3PO4 [43]. The anodic oxidation was carried out at 20 V for 1 h. After electrolysis, a titanium sheet was rinsed

with ultrapure water and then air-dried. The TiO2 nanotube

arrays prepared were then analyzed with a field emission

scanning electron microscope (S-4800 FESEM, Hitachi,

Japan; see Fig. 1).

μ SPEE procedure

The TiO2 nanotube array sheet was directly immersed in

10 mL solution with a constant depth and then sealed in

the sample vial. The extraction conditions were the same

as that of the optimized conditions. The stirring rate of the magnetic stirrer was set at 500 rpm. After the

equilibrium between adsorption and desorption was

 basically reached, the TiO2 nanotube array sheet was

taken out and rinsed with distilled water in order to

remove co-adsorbed matrix substances, then air-dried and

eluted for the desorption of analytes. TiO2 nanotube array

sheet was directly immersed in a small amount of 

dichloromethane for complete desorption in an interval

of 7 min. After that, the TiO2 nanotube array sheet wasremoved, and the dichloromethane was dried with mild

stream of nitrogen gas. Then, the residue was dissolved in

100 μ L methanol. Finally, 1 μ L of the solution was

injected for gas chromatography (GC) analysis.

GC analysis

GC analyses were performed on an Agilent 7890A gas-

chromatographic system, equipped with an electron capture

detector (ECD). Separations were carried out using a HP-5

column (30 m× 0.32 mm×0.25μ m). Nitrogen (99.999%) was

employed as the carrier gas (0.6 mL min−1). The 1.0 μ L of astandard solution or sample solution was injected in the

splitless mode at 250 °C using the following program of 

80 °C (held 1 min), then 25 °C min−1 ramp to 200 °C held

for 2 min, and then 10 °C min−1 ramp to 300 °C held for 

5 min. Total run time was 22.8 min. The ECD was

maintained at 300 °C.

Water samples

In this work, four environmental water samples such as tap

water, melted snow water, lake water, and reservoir water 

were collected for validating the proposed method. Tap water 

sample was collected from our own laboratory after it was

flowing for 10 min at a very fast rate. A snow sample was

collected from Henan Normal University, Xinxiang, Henan

Province, 12 Nov 2009. Lake water sample was collected

from Shouxihu Lake, Yangzhou, Jiangsu Province, China.

Reservoir water sample was taken from Xiaoshangzhuang

reservoir, Xinxiang, Henan Province, China. All the collect-

ed water samples were filtered through 0.45-μ m micropore

membranes after sampling and were maintained in glass

containers, and then stored at 4 °C.

Results and discussion

The effect of equilibrium between sample solution

and TiO2 nanotube array

In this μ SPEE procedure, the extraction is affected by

several factors. The analytes, which exist as molecular form

in the solution, would be enriched, and the enriched amount 

of analytes was related to the amount of analytes existing asFig. 1 FESEM images of TiO2 nanotube arrays. a Top view; b

cross-view

Micro-solid phase equilibrium extraction 207

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molecular form in the solution. However, two procedures

will occur in the extraction process and influence the

amount of molecular analytes in the solution, further 

influencing the enrichment performance. First, the analytes

would migrate between gas and liquid phases due to the

vapor pressure for the volatile or half volatile compounds.

This procedure will reach equilibrium. This equilibrium

obeys Henry’s law when the concentrations of analytes arevery low,

 pB ¼ kH xB ð1Þ

where pB is the partial pressure of the (analytes) in the gas

above the solution; xB is the concentration of the (analytes),

and kH  is known as the Henry's law constant, which depends

on the solute (analytes), the solvent, and the temperature.

The second procedure is the adsorption of analytes onto the

adsorbents, and it is also a reversible procedure. Some of the

HCHs and DDTs may adsorb on the surface of the adsorbents,

and some of them will be desorbed at the same time. Finally,

they will reach equilibrium. This equilibrium obeys the

Freundlich equation (Freundlich adsorption isotherm):

lg Q ¼ lg K F þ 1=nð Þ lg xB ð4Þ

Where Q is the weight adsorbed per unit area of TiO2

nanotubes array sheet, k F and (1/ n) are constants for a given

adsorbate and adsorbent at a particular temperature, and pBis the partial pressure of the (analytes) in the gas above the

solution. We substituted Eq. 1 into Eq. 4:

lgQ ¼ lg K F À 1=nð Þ lg kH  þ 1=nð Þ lg pB ð5Þ

In general, these three steps will reach equilibrium, and

the best enrichment will be achieved (see Fig. 2).

On the other hand, if the concentrations of analytes are

larger, which does not make them obey Henry’s law,

suppose the amount of volatile part as C v, so:

lg Q ¼ lg K F þ 1=nð Þ lg C 0 À C vð Þ ð6Þ

The primary experiments have proved that it was

right that the peak areas of HCHs were very small due

to the relatively high vapor pressure or lower compet-

itive adsorption. As the nonvolatile compounds are

concerned, the adsorption obeys the Eq. 4. Beside these,

the parameters including the kind of organic solvents,

sample pH, extraction time, and desorption time would

give rise to influence the enrichment efficiency. In order 

to obtain appropriate extraction efficiency, a procedure

for optimization was necessary. Hence, we performed a

series of experiments for obtaining optimal enrichment 

conditions.

The effect of other factors on μ SPEE

 Effect of the kind organic solvents

In a μ SPEE procedure, different desorption efficiency

would be obtained when different solvents are used due to

the different physical and chemical properties of the organic

solvents and characteristics of the target analytes. In this

experiment, there are five types of solvents such as

methanol, acetonitrile, acetone, hexane, and dichlorome-

Fig. 2 Adsorption equilibrium

of TiO2 nanotube arrays as

adsorbents for analytes

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thane which were used for desorption of OCP pesticides. It 

was found that dichloromethane was the most effective

solvent of the eight analytes, so it was used as the solvent 

for desorption.

 Effect of sample pH 

Sample pH plays an important role in the extraction procedure because pH value determines the existing form

of the analytes, and then the pH of the sample solution

affects the extraction efficiency. In this experiment, the

effect of sample pH on the enrichment of OCPs was

evaluated in a range of pH 3.0~ 8.0 (see Fig. 3). As can be

seen, the peak area of four HCHs decreased with the

increase of pH value, and the peak area of DDTs, DDD, and

DDE reached the maximum at the pH 6.0. The extent of 

 peak area decrease of HCHs was very small. Hence, pH 6

was used in the following experiments.

 Effect of salting-out effect 

Salting-out effect is often an important factor in the extraction

 procedure. In these experiments, it was optimized in the range

of 0~30% (w/ v ). The peak areas of the HCHs increased a little

with the increase of NaCl concentration and the peak areas of 

DDTs, DDD, and DDE decrease significantly when the NaCl

concentration is in the range of 0~30%. The reason may be

that addition of salt suppresses the thickness of electrical

adsorption layer at the TiO2 solution interface, which leads to

the decrease of mixed hemimicelles formed on the TiO2

surface. Based on these results, NaCl was not added.

 Effect of equilibrium time

Equilibrium time is an important parameter in the μ SPEE

 procedure, which determines the enrichment performance

 better or not. In order to achieve a reasonable extraction

time, a series of experiments were designed for investiga-

tion of the effects of extraction time in the range of 20~ 

90 min. The results were shown in Fig. 4. From the figure,

we can see that the adsorption of HCHs reached the

equilibrium rapidly and DDT, DDD, and DDE needed more

time. So, the peak areas of DDT, DDD, and DDE increased

straightly in the time interval. However, the extraction

efficiency improved significantly with increasing equilibri-um time up to 40 min; after that, the extraction efficiency

increased slightly. Hence, for time saving in subsequent 

experiments, 40 min was used as the equilibrium time.

 Effect of desorption time

In the μ SPEE procedure, two steps are very important; one

is the adsorption of analytes on the surface of adsorbents,

and the other is desorption of analytes from the adsorbents.

Ideally, these two procedures are very rapid. In fact, the

 procedure may be various due to different conditions.

Desorption of OCPs from the TiO2 nanotube array sheet with the dichloromethane is controlled by the time. In order 

to achieve complete desorption of OCPs, the desorption

time was investigated in the range of 1~9 min. The results

indicated that the desorption of HCHs was rapid and had no

significant differences in the time interval, but the DDT and

DDD reached the best desorption level in 7 min; DDE

needed longer time for desorption. However, there were

very limited peak area increases with the time over 7 min.

So, 7 min was optioned in subsequent experiments.

Analytical performance

In this experiment, some important quantitative parameters

of the proposed method such as linear range, correlation

coefficients, limits of detection (LODs), and relative

standard deviation (RSD) were evaluated by enriching

10 mL standard solutions, and the results were listed in

Fig. 3 Effect of sample pH equilibrium time, 40 min; NaCl, 0%;

desorption time, 5 min; spiked sample concentration, 1 μ g L−1 for 

each compound. (■) α -HCH ( ) β-HCH ( ) γ-HCH ( ) δ-HCH

( ) p,p’-DDE ( ) p,p’-DDD ( ) o,p’-DDT ( ) p,p’-DDT

Fig. 4 Effect of equilibrium time. pH, 6; desorption time, 5 min;

spiked sample concentration, 1 μ g L−1 for each compound (■) α -HCH

( )β-HCH ( ) γ-HCH ( ) δ-HCH ( ) p,p’-DDE ( ) p,p’-DDD

( ) o,p’-DDT ( ) p,p’-DDT

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Table 1. Calibration curves were performed using 10 mL

ultrapure water spiked with 0.1~100 μ g L−1 each of the

OCPs. Each analyte exhibited good linearity with correla-

tion coefficient ( R2)>0.99 in the studied range. The limits

of detection, calculated on the basis of signal-to-noise ratio

of 3 (S/N=3) were in the range of 0.062~0.212 μ g L−1.

The precision of analytical method was investigated usingsix replicate experiments with 10 mL standard solution

containing each of the OCPs at 1.0 μ g L−1, and the RSD of 

 below 10.0% was obtained.

Analysis of real water samples

To demonstrate the applicability of the TiO2 nanotube array

sheet as μ SPEE adsorbents, the proposed procedure has

 been carried out on four real environmental water samples,

and the results were shown in Table 2. The results indicated

that no OCPs were found in water samples; maybe, it was

due to the limited sample volume which the too-lowconcentration of OCPs could not provide enough amount 

for detection. So, these samples were then spiked with

OCPs at three different concentration levels to investigate

the effect of sample matrices. The spiked recoveries were

satisfied in the range of 78~102.8%. The typical chromato-

grams of real water sample were demonstrated (see Fig. S1,

Electronic Supplementary Material).

Comparison with SPE

In order to validate the proposed method, comparisons with

conventional solid-phase equilibrium (SPE) and dispersive

liquid-liquid microextraction (DLLME) were performed. The

four environmental water samples were preconcentrated with

SPE using C18 and TiO2 nanotubes as the adsorbents, and

their amounts were 200 and 100 mg, respectively. The

enrichment conditions were as that reported in reference

[44]. Sample volume is 50 mL. Dispersive liquid – liquid

microextraction was carried out under the conditions as

reported in [45], and the sample volume is 10 mL. Theresults (see Table S1, Electronic Supplementary Material)

Compound Linear range (μ gL−1) R2 Precision (RSD%, n=6) LOD (μ gL−1)

α -HCH 0.1 – 40 0.9947 9.12 0.10

β-HCH 0.1 – 40 0.9915 7.91 0.10

 γ-HCH 0.1 – 40 0.9973 7.32 0.10

δ-HCH 0.1 – 40 0.9989 8.94 0.098

 p, p’-DDE 0.1 – 100 0.9950 8.01 0.0076

 p, p’-DDD 0.1 – 100 0.9951 7.00 0.0097

o, p’-DDT 0.1 – 100 0.9966 9.88 0.016

 p, p’-DDT 0.1 – 100 0.9952 9.58 0.023

Table 1 Linear range, correla-

tion coefficient, precision, and

detection limits (S/N=3)

Table 2 Recoveries of real water samples spiked at three concentration levels with proposed method

Water samples Blank Added levels

(μ gL−1)

α -HCH β-HCH γ-HCH δ-HCH p, p’-DDE p, p’-DDD o, p’-DDT p, p’-DDT

Tap water ND 0.2 79.2 ±3.4 80.2 ±2.9 82.1 ±4.8 80.5 ±3.4 95.8 ±6.1 95.8 ±1.8 95.6 ±2.8 100.2 ±5.2

 ND 1 78.0± 2.5 78.6±6.1 83.5±5.9 89.6± 2.9 96.2±4.3 99.8± 3.8 96.9±8.1 98.9±2.1

 ND 5 88.2± 5.1 80.1±4.2 86.7±2.8 87.6± 5.6 98.6±6.1 102.5± 2.1 99.2±2.8 99.8±4.9

Snow water ND 0.2 80.1 ±1.9 82.5 ±5.4 79.8 ±2.1 88.2 ±6.2 92.8 ±2.9 98.5 ±8.1 99.8 ±2.9 99.6 ±6.4

 ND 1 82.3± 5.5 83.6±2.9 82.6±5.6 86.7± 3.2 96.5±4.2 92.6± 6.1 100.2±5.2 98.5±5.3

 ND 5 80.5± 3.1 80.9±2.8 81.8±5.6 89.5± 4.6 98.5±3.3 96.2± 2.8 99.8±1.8 101.5±8.1

Shouxihu Lake water ND 0.2 80.2 ± 2.3 82.3 ± 3.2 85.6 ± 4.2 88.9 ± 6.1 95.8 ± 5.2 98.2 ± 4.6 99.8 ± 5.2 99.8 ± 5.6

 ND 1 79.8± 4.1 81.4±3.7 87.8±4.6 89.1± 2.9 98.6±3.2 97.8± 4.1 98.6±4.6 98.6±6.8

 ND 5 81.5± 3.2 82.3±4.8 86.6±2.7 90.1± 1.9 99.4±6.2 101.2± 7.1 102.8±2.5 102.3±2.2

Xiaoshangzhuang

wastewater 

 ND 0.2 81.5± 2.9 81.2±3.1 80.6±5.2 82.5± 2.8 98.6±6.1 98.2± 2.6 99.8±5.1 99.8±3.8

 ND 1 79.2± 3.5 80.6±5.5 79.8±7.1 83.2± 2.9 99.2±6.4 97.8± 4.1 101.2±5.2 98.9±3.7

 ND 5 81.4± 2.9 80.9±3.1 81.5±4.1 85.1± 2.8 97.9±5.1 98.6± 5.2 100.1±3.7 101.2±4.2

 ND not detected

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showed that the enrichment performance was different for 

different target analytes, different sample matrices, and

different initial concentrations of the analytes. In contrast,

the spiked recoveries of the target analytes in wastewater and

lake water were better than that in tap water and snow water;

this was due to the high ionic strength in these two water 

matrices. The detection limits of these target analytes in real

water samples (see Table S2, Electronic SupplementaryMaterial) exhibited that the sensitivity of proposed method

was the same as that of DLLME and was lower than that 

obtained with C18 and TiO2 nanotubes SPE. The results were

 predicted because of two reasons. One is the sample volume,

which was five times higher than that of proposed method.

This accounted for a part of the results. The other is the

different amount of the adsorbents. The effective amount of 

TiO2 nanotube arrays for μ SPEE was far lower than that for 

SPE. Because the TiO2 nanotube array sheet was 400 mg,

and the thickness was 200 μ m. The length of TiO2 nanotube

was about 400 – 500 nm, and there had one layer each side on

the titanium sheet. Hence, the thickness of TiO2 nanotubearrays were about 1 μ m, and the amount of TiO2 nanotube

arrays was about 0.5% of the amount of titanium sheet.

Based on the LODs in Table S2 (see S2, Electronic

Supplementary Material), we can predict that the practice

sensitivity of proposed method will be much higher than that 

of SPE if the sample volume, and the amount of adsorbent 

were as the same. All these results indicated that the

developed μ SPEE provided relative stable and excellent 

results and was a new, robust, and good mode for the

determination of such compounds.

Conclusions

TiO2 nanotube array as a novel nanomaterial has gained

much more attention in many fields. However, its merits

have not been utilized completely. This work demonstrated

for the first time that TiO2 nanotube array could be used as

effective μ SPEE materials for the enrichment of OCPs in

four different environmental water samples. The proposed

method provided good linear range, reproducibility, and

detection limits. Based on the experimental results, a simple

and reliable determination method for OCPs was developed

 based on the μ SPEE. According to the physical and

chemical properties of TiO2 nanotube array and the

different preparation method, different TiO2 nanotube

arrays with good structure would be achieved easily. It is

expected that they would have better enrichment capacity

for selected analytes. TiO2 nanotube array will give a much

higher enrichment performance with the same effective

amount of adsorbents than the established methods. TiO2

nanotube array can be potentially applied to the enrichment 

and determination of many other pollutants.

Acknowledgements This work was supported by the Natural

Science Foundation of China (20877022), the Natural Science

Foundation of Henan Province (082102350022), and the Personal

Innovation Foundation of Universities in Henan Province ([2005]126).

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