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Sensors and Actuators B 139 (2009) 584–591

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

onstruction and characterization of phenol-based sensor derivedrom colloidal chemistry

ajid Bashir, Jingbo L. Liu ∗

epartment of Chemistry, Texas A&M University-Kingsville, MSC 161, 700 University Blvd., Kingsville, TX, 78363, USA

r t i c l e i n f o

rticle history:eceived 30 December 2008eceived in revised form 22 February 2009ccepted 27 February 2009vailable online 24 March 2009

a b s t r a c t

In this study, three main objectives are sought: (a) construction of the biosensor by incorporation of a“capturing element” (consisting of alpha cyclodextrin on a sol-derived substrate) and a “detecting ele-ment” (consisting of 2,6-dichloro-quinonechloroimine and sodium hydroxide as buffer and electrolyte),(b) physical characterization of the sensor using spectroscopy and microscopy, and (c) sensor perfor-

eywords:henol sensororphology

pectroscopy

mance through electrochemical facility. Correspondingly, Raman spectroscopy and UV–vis spectroscopyhave been applied as diagnostic tool to analyze guest–host interaction of the sol-derived phenol sen-sor. The goal of this study is the incorporation of both an electrochemical and colorimetric detectingelements. It is anticipated that this study provides a possibility of use of biosensor for “one chemical”dual-functional detection. We use this bi-functional detector in the identification of phenol. The detec-

ivedand a

abricationharacterization

tor is based upon sol-derdetection is colorimetric

. Introduction

Phenol is an organic compound which is widely used in organicynthesis [1] or in separation science, for example, use of phenolicesins which release phenol to the air and water [2]. Phenol is alsohealth and environmental hazard [3], for example coal tar and

reosote like petroleum products contain phenol and are releasednto atmosphere by burning wood and in vehicle fumes [4]. Degra-ation of organic matter releases phenol, which once it enters theuman body can be metabolized benzene [5]. Phenol can enter theody through the mouth, skin, eyes and respiratory route. Phe-ol can induce a number of side effects, notably carcinogenicity,etrogenicity and mutagenicity as demonstrated in animal models6]. Phenol if ingested can act as a food poisoning agent and mayause mild, sub-acute, acute symptoms and sometimes leading toeath. It causes nausea, vomiting, sweating and diarrhea, excessivealivation, headache, vertigo, and dark colored urine. Chronic expo-ure to phenol or its derivatives produces gastrointestinal irritation,ardiovascular, central nervous system, and respiratory effects [7,8].

The main methodological approaches for detection of phenolave been to use microbial-based sensors coupled to Clark-type

lectrodes [9,10] with electrochemical detection. These devicesperate through mediated electron transfer from microbial sys-ems to electrodes [11]. The advantages for use of amperometriciosensors are substrate selective, lower cost of analysis per

∗ Corresponding author. Tel.: +1 361 593 2919; fax: +1 361 593 3597.E-mail address: kfjll00@tamuk.edu (J.L. Liu).

925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2009.02.072

colloidal chemistry to produce detecting and sensing elements in whichlso electrochemical in nature.

© 2009 Elsevier B.V. All rights reserved.

sample, miniaturization, and ease of training over other chromato-graphic techniques if coupled with the right “sensing element” [12].Non-microbial systems, such as ionization spectroscopy [13], spec-trophotometry [14], and chromatography [15] have also been used,but require greater skills, specialized equipment and have a highersample analysis cost. Examples of enzyme-based assay (using phe-nol hydroxylase was encased in a sol-derived matrix linked to aClark-type oxygen electrode) have shown a limit for detection forphenol of 2.5 �M with poor substrate selectivity and co-addition ofnicotinamide adenine dinucleotide hydrogen (NADH) was required[16]. Other examples include use of the enzyme tyrosinase cova-lently [17] immobilized on the electrode surfaces or with use ofpolymer [18], paste [19] or certain types of gels [20–22] have beenused with varying success. Other organisms used were Trichosporonsp. [23], Pseudomonas putida [24], Bacillus stearothermophilus cellsimmobilized in a hydroxyethyl methacrylate membrane [25], and P.putida GFS-8 immobilized in polyvinyl alcohol cryogel [26]. Screen-printed electrode (SPE) has also been used to detect phenol (usingthe same approach, namely using P. putida DSM 548) [27] andbenzoate and mono-chlorinated phenols at low micromolar con-centrations [28]. Other organics using the same approach havedetected chlorobenzoates (using Pseudomonas-based sensor) [29],chlorinated aromatics using Trichosporon cutaneum-based sensor[30], 2-ethoxyphenol (using Rhodococcus rhodochrous) and other

xenobiotics [31] including on-line monitoring of immobilized P.putida-based sensor [32]. Use of electron transfer agents other thanoxygen (as used in the Clark-type electrode) has a number of advan-tages [33]. Previously both soluble and insoluble mediators suchas p-benzoquinone [34] and ferrocenes [35] have been used in

S. Bashir, J.L. Liu / Sensors and Actu

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by a dip coating procedure [51], where the substrate was immersedinto the sol–gel precursor then withdrawn at a constant rate of1.2–24 cm/min to vary the film thickness. The film was placed ina pre-heated oven at 50 ◦C and this temperature was maintainedfor 30 min to remove methanol. The resulting phenol sensing films

cheme 1. Reaction of Gibbs reagent with phenol anion to yield an indophenolnion, which is stable under neutral conditions. Under acidic conditions, the finalroduct is a benzoquinone derivative, which can be reduced (using a base and alco-ol), to a benzoquinone imine.

iosensor analysis, respectively. The use of colorimetric detections ubiquitous for a number of systems [36] in which a color changeccurs upon action on the designated substrate. The action may beue to an enzyme-linked assay [37] or charge transfer leading to aolor change, the precise color being related to the oxidation statef the sensing element or dye. One coupling agents that has beensed successful in the detection of phenol by Gibbs in 1925 was,6-dichloro- and 2,6-dibromoquinonechloroimines (also known asibbs reagent) [38]. Phenol reacts with N-chloroimine to give the

ndophenol anion, which can be quantified colorimetrically. Thessay can also give a positive value when phenol has a substitutet the para position. It is hypothesized that the reaction of Gibbseagent with phenol anion yields an indophenol anion, which is sta-le under neutral conditions (shown as in Scheme 1) from whichhe color of the final complex is derived.

In our approach the Gibbs molecule is ‘held’ by alphayclodextrin as the ‘trapping element’. Cyclodextrins have beenxtensively used as separation media for drugs [39], as media fornzyme–enzyme or enzyme–substrate reactions to occur [40] or asedia for photo-chemicals reactions to occur [41]. The colorimet-

ic detection can be achieved once the phenol molecule is in closeroximity to the Gibbs reagent. Upon charge transfer, a color changeccurs which indicate that electrons have been transferred betweenhe two molecules, comprising the ‘detecting element’ [42]. Theetecting element, itself is entrapped within a sol-derived typeatrix. The sol–gel method is a wet-chemical technique to fabricate

ubstance starting from a metal alkoxide or metallic inorganic com-ound solutions which react to produce colloidal particles (knowns a sol) [43]. This technique is used primarily for the fabrication ofaterials (typically a metal oxide) starting from a chemical solution

sol) which acts as the precursor for an integrated network (gel). Thearticle size of the sol and gel can be adjusted in the fabricationrocesses to range from 1 nm to 1 �m. The actual composition andize is dependent upon the final usage of the sol–gel system [44].he application of sol–gel is in many fields such as thin films [45],ense ceramic or glass materials [46], optical ceramic fibers [47],owders [48]. These constructs can be used in coatings, construc-ional materials or biomedical applications [49]. The advantagesre the particle size can be controlled precisely, a large specific sur-ace area thereby increasing the potential catalytic activity of thencased catalyst or substance which is acted upon. The sol–gel canlso give rise to a long phase boundary between the different con-titutes (i.e. the interfacial area between the different reactants isarge). Finally through control of synthesis and temperature, theomposition of the sol–gel can be controlled at the molecular levele.g. mono-dispersed particles, pore-size) and uniform distributionf the encased reagents (i.e. high homogeneity) and in some casese of a sol–gel method can be cost effective (compared to other

abrication methods) and finally sol–gel method can be employedithout extensive training (i.e. ease of preparation) [50]. In our

pproach, a metal alkoxide was not used instead a sol-derived filmas formed. The advantages were that the sol-derived films had

lightly higher mobility, which allows us to construct the uniform

ators B 139 (2009) 584–591 585

thin films. Sol-derived films subject to the removal of solvent dis-play greater rigidity than a classical polyacrylamide gel. The aim ofthis work was to analyze the effectiveness of dual detecting elementwith a ‘self-mediator’ and platinum (Pt) electrode as a basis forbiosensor for detection of phenol. The dual-detection method com-bines both colorimetric and cyclic voltammetric (CV) detection withphenol as the self-mediator and Pt electrode as substrate within asol matrix. The detection limit, sensitivity and microstructure of thedeveloped sensors were studied and the effects of working param-eters were reported and discussed. The originality of the sensor isthe combination of two independent detecting channels.

2. Experimental procedure

All chemical reagents, unless otherwise mentioned wereobtained from Sigma–Aldrich (St. Louis, MO), and all solvents andequipment obtained from VWR International Ltd. (West Chester,PA), respectively. This research compasses four major procedures:(a) fabrication of the phenol sensing elements (Sections 2.1 and 2.2),(b) microstructure characterization of the device (Sections 2.3–2.5),(c) electrochemical property measurement of the device (Section2.6), and (d) calibration of the device to optimize with respect tosensitivity, response time and limit of detection (LOD) (Section 2.7).

2.1. Fabrication of the phenol sensing and detecting elements

A cost and time effective sol-method was applied to fabricatethe phenol detector. The sol precursor for the phenol sensor con-tains 2,6-dibromoquinonechloroimines (Gibbs reagent) and alphacyclodextrin (�-CD) which was prepared by dissolving 0.05 mol ofGibbs reagent and 0.05 mol of �-CD in 100 ml mixture of 1:1 (vol-ume ratio) methanol and distilled water, respectively. The solutionwas heat treated and stirred continuously for 30 min under a waterbath, then cooled to room temperature. The above sol precursor washeat treated at 50 ◦C for 2 h in order to evaporate the solvent untilthe optimal viscosity was obtained. Fig. 1 illustrates the componentused to construct the device and the cross-section of the device.

2.2. Fabrication of the thin films onto the platinum-sputteredsubstratum

The sol precursors was deposited on a platinum (Pt) sputteredglass substrate, with surface area ranging between 0.1 and 0.5 cm2,

Fig. 1. Fabrication of �-CD and Gibbs sensor on Pt-sputtered glass slide.

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86 S. Bashir, J.L. Liu / Sensors an

as then equilibrated in 0.1 M sodium hydroxide (NaOH) electrolyteolution for 0.5–1.0 h prior to any electrochemical measurements.

.3. SEM microstructure characterization of the device

The thickness, surface morphology and texture changes of thelm was examined by a scanning electron microscopy (SEM).he SEM was used in the determination of thickness and surfaceorphology of the sol-derived (SD) film using a JEOL JSM-7400F

canning electron microscope (JEOL Ltd., Peabody, MA) scanninglectron microscope. An accelerating voltage of between 5 and0 kV and a high vacuum of 1.9 × 10−6 Torr was employed. The ele-ental composition determination of SD films formed at various

emperatures was obtained using the SEM equipped with an energyispersive spectrometer (EDS).

.4. Phase contrast light microscopy analysis

An Olympus Phase Contrast Optical Microscopy (M021, Olym-us America Incorporation, Irving, TX) was employed to collecthe image of the phenol detector. The images were taken underhree various magnitudes to determine distribution of the �-CDnd Gibbs reagent and the morphology change after introductionf phenol. This experiment was conducted at ambient temperaturend standard atmosphere.

.5. UV–vis and Raman spectroscopy

The absorption spectra of �-CD and Gibbs reagent solsere obtained with a PerkinElmer Lambda 35 ultraviolet–visible

UV–VIS) spectrophotometer (PerkinElmer, Fremont, CA). The oper-ting parameters of the instrument were as follows: slit width,avelength range and scanning speed were 2, 190–900 and 1 nm

rom 900 nm to 960 nm/min, respectively. All experiments werearried out in a quartz cells having a 10 mm path. The concentra-ions of sols used in the UV–vis spectra were all the same (notinghe dilution factor was 500×, colloidal suspension (ml)/distilledater (ml) = 0.20/100). The absorption spectra of �-CD and Gibbs

eagent sols were also collected. The Advantage 200A Raman spec-roscopy (DeltaNu LLC, Laramie, WY). The operating parametersf the instrument were as follows: wavelength range from 200o 2000 cm−1 under ambient temperature and pressure. The soft-are parameters to acquire the spectra used were as the following,

esolution-high, average = off, scanning, 3 time = 30 s, baseline = off,nd polarisation = on and excitation of the sample at 633 nm).

.6. Electrochemical property measurement of the device

Cyclic voltammetry was used to measure the exchange currentensity (io) of the ‘sensing element’ composed of �-CD and Gibbseagent sol-precursor. To determine io, a three-electrode approachhich include the working electrode (WE), counter electrode (CE)

nd reference electrode (RE) was used. For the construction of theE, the Pt-sputtered micro-glass slide was used as a substrate to

upport the ‘sensing element’. The apparent surface area of theensing element was approximately 0.12 cm2. The counter elec-rode (CE) was Pt gauze coated with Pt black. Its apparent surfacerea was approximately 0.35 cm2. Finally, the reversible hydrogenlectrode (RHE) was used as the RE and was separated from theE cell compartment by a closed wet stopcock and a Luggin cap-

llary. All potentials to be determined were in reference to theHE. All io and charge densities was calculated with respect tohe apparent area of the WE electrode. Cyclic voltammetry exper-ments were carried out with 0.1 M NaOH electrolyte with variousolumes (1–1000 �l) of buffered phenol solution. The electrolyte

ators B 139 (2009) 584–591

was deaerated by purging with N2 gas for 30 min before recordingthe CVs.

2.7. Colorimetric-gram calibration of the device

Various concentrations (1 pmol to 1 �mol) of phenol analyteand Gibbs reagent have been applied to identify the colorimetrichistogram. The intensity of the sensor color change depends onthe concentration of both analyte (phenol) and sensing elements(Gibbs). The dependence of intensity on the concentration has beenplotted using maximum intensity as a function of concentration.

3. Results and discussions

3.1. Scanning electron microscopy (SEM)

The scanning electron microscopy with the X-ray energy dis-persive capacity (EDX) was used to determine the morphologyand elemental composition of the Gibbs-based sensor shown as inFig. 2a–c. The cross-section of the film sensor (Fig. 2a) clearly indi-cated that the thin film was uniformly deposited on the substrateand its thickness was estimated as 900 nm. The images (Fig. 2b)were the top view appearance with different magnifications to eval-uate the surface topology and porosity of the film sensor. The topview images indicated that the film was highly porous and the highporosity favored the phenol buffer solution diffusion. In the electro-chemistry study (to be discussed in Section 3.4), it was confirmedthat there is no mass control detected due to the high porosity ofthe film sensor. The elemental composition was determined usingEDX (Fig. 2c) to be C, N and Cl. The element platinum (Pt) was result-ing from the coating layer on the glass substrate which was usedto increase the surface electronic conductivity. The integrity of thefilm was excellent, no cracking or peeling was observed, in factthe integrity was similar to other devises which used a traditionalsol–gel approach [52].

3.2. Optical microscopic image

In order to systematically study the phenol (analyte) detection,the phase contrast optical microscopy was used and the morpholo-gies for the sensing elements (Gibbs reagent, Fig. 3a) and analyte(phenol, Fig. 3b). The phenomena of analyte interaction with thesensing element were displayed in Fig. 3c. It was also observed thatthe uniformity of the sensor composed of the Gibbs reagent beingdistributed within the �-CD aqueous solution (which was used totrap the Gibbs reagent) was achieved using sol-derived method.The fiber-like structure of the Gibbs reagent was clearly detected.Upon addition of phenol (from a volume of 1 �l and a concentra-tion ranging from l to 10 �l/ml), significant change from the fibertexture to the network structure was observed, which also corre-sponded to the observations from the SEM characterization. Thisphenomenal was due to the presumed formation of the indophenolstructure, which is used as a diagnostic for the identification of phe-nol type molecules (see Scheme 1). Analysis of samples of differentconcentrations (1–10 �l/ml with the increment of 1 �l/ml) or stor-age times (from 10 min to 24 h) provided the essentially identicalresults, indicating the reaction was highly reproducible.

3.3. UV–vis and Raman spectroscopy

UV–vis (Fig. 4a) and Raman spectroscopy (Fig. 4b) were used as

complimentary approaches to identify the chemical bonds alter-ation. Fig. 4a is the UV–vis spectra of the sensing element with andwithout addition of phenol. The spectrum indicates that the rotat-ing OH group of phenol was asymmetric with respect to the axisof internal rotational axis of the molecule. The addition of phenol

S. Bashir, J.L. Liu / Sensors and Actuators B 139 (2009) 584–591 587

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ever, our limit of detection was comparable to what was observedwith their approach (1 �M versus 0.1 �M), which used immobi-lizing tyrosinase in a titania sol–gel matrix [53]. Unlike Yu et al.,we also incorporated a colorimetric detector in which addition of

Table 1Summary of sweep rate and measured exchange current density for the electro-chemical detector.

Sample run Sweep rate (mV/s) Exchange current density (mA/cm2)

1 2 3.452 5 3.50

ig. 2. The SEM (at 5 kV) morphology of Gibbs sensing element with addition of phEDX) spectroscopic analysis.

aused the wavelength to shift to blue region. It was reasonable tossume that CD–Gibbs reagent complex bound to phenol to pro-uce the indophenol compound. The Raman spectroscopic analysisFig. 4b) confirmed the formation of the indophenol reacting phenolith Gibbs will cause a change in the Raman shift. This was caused

y the deprotonated alcohol group (R–O−, ∼1050 cm−1) beingeplaced by an imine (R–N R′, ∼550–1350 cm−1, see Scheme 1)roup, giving rise to peak broadening, indicating the formation ofndophenol compounds.

.4. Electrochemical property measurement

In order to better understand the mechanism of Gibbs reagentnd phenol reaction, three-electrode cyclic voltammetry wasmployed using various sweep rates and a constant applied poten-ial, was shown in Fig. 5a–c. In Fig. 5a, the concentration was keptonstant (5.2 �M), the sweep rates were: 2, 5, 10, 20, 50, 100 and00 mV/s (four sweep rates are shown). In Fig. 5b, the concentra-ion was kept constant (2.6 �M), the same sweep rates were usedfour sweep rates are shown). At a sweep rate lower than 20 mV/s,he exchange current densities obtained were essentially identical3.2 mA/cm2). The influence of sweep rate on exchange current den-ity is summarized in Table 1. The independence of reaction kineticsn the sweep rate indicated no mass diffusion was detected; fromhis we can conclude that electron transfer was the rate-limitingtep in the formation of the end product. Upon increasing the sweepate to more than 50 mV/s, the exchange current density decreasedrom 3.2 to 2.4 mA/cm2 which indicates that mass diffusion andlectron transfer are both involved in the formation of the inter-

ediate and end product. In order to ensure that electron transferas the rate-limiting step, lower sweep rates were used, the opti-al sweep rate range was between 10 and 20 mV/s. Lower sweep

ates (1, 2, and 5 mV/s) were also used and gave adequate results,owever, data collection for the experiment was prolonged (i.e. the

(a) cross-section; (b) top-view with low magnification; (c) X-ray energy dispersive

lower the sweep rate, the longer it took for completion of one scan,noting each experiment was on average of two scans, therefore ata sweep rate of 1 mV/s, the experiment took approximately 4 h tocomplete). Fig. 5c, various concentration of phenol buffer solution(1–25 �M) was used, while the sweep rate was kept constant at20 mV/s. The increase in the concentration of the phenol bufferedsolution resulted in an increase in the reaction rate. It was noted thatthe limit of detection for phenol was 1.0 �M; however, for the elec-trochemical detector the LOD was lower. The reproducibility of theaverage current density over the measured concentration rangesand time period was 0.25% R.S.D. The limit of detection comparesvery favorably with other devices used for detection of phenol, sum-marized in Table 3. The reported range varies from 2.5 to 50 �M,which was less sensitive than our device.

Electrochemical study indicated that the electron transfer wasthe limiting reaction step and no mass diffusion was found atslower sweep rate between 2 and 20 mV/s, which is less thanwhat was used by Yu et al., who used a rate of 150 mV/s. How-

3 10 3.254 20 3.255 50 2.456 100 2.207 200 ∼1.85

588 S. Bashir, J.L. Liu / Sensors and Actuators B 139 (2009) 584–591

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nols complex was proportional to the concentrations of both Gibbsand phenol. The limit of phenol detection was found to be similarto the limit obtained from electrochemical analysis (1.0 �M). Thereproducibility of the colorimetric-gram was 2.37% of the average

Table 2The colorimetric detection of phenol using Gibbs-based film sensor.

Volume ofphenol (�l)

Volume of Gibbsreagent (�l)

Total volume(ml)

Color schemeof the sensor

Maximumintensity

10 40 10 394

10 40 5 43210 40 2 45710 20 10 37810 20 5 40010 20 2 42810 10 10 376

ig. 3. The optical images of Gibbs sensing element with addition of phenol: (a) theibbs sensing element captured in �-CD; (b) phenol analyte; (c) addition of phenol

o react with the sensing element.

henol resulted in a change or alteration of color in the sensor. Theimit of detection and linear response was either similar to or betterhan comparable devices, in addition to the ease of construction. Forxample, in our approach, bacteria do not need to be isolated, cul-ured or purified, or even immobilized, resulting in a more robustetector.

.5. Colorimetric-gram calibration

The combination between Gibbs and phenol caused significanthanges in color, which are summarized in Table 2. These color alter-tions (from blue to purple and then to grey) can be tentativelyxplained by the oxidation reaction of product (indophenol) which

Fig. 4. Spectroscopic analysis of sensing element with addition of phenol: (a) UV–visspectra of Gibbs sensing element and addition of phenol to react with the sensingelement; (b) Raman spectra of Gibbs sensing element and addition of phenol to reactwith the sensing element.

is subject to further oxidation. In this study, the colorimetric anal-ysis on phenol and Gibbs concentration was also evaluated by theoptical density measurement shown in Fig. 6a (at three fixed phenolconcentrations) and Fig. 6b (at three fixed Gibbs concentrations).It was found that the increase in the intensity of the indophe-

10 10 5 39210 10 2 41220 10 10 37820 10 5 39420 10 2 448

S. Bashir, J.L. Liu / Sensors and Actuators B 139 (2009) 584–591 589

Fig. 5. Cyclic voltammetry spectra of Gibbs sensing element with addition of phenol: (a) the corresponding chart with different sweep rate was conducted at 5.2 �M; (b)the corresponding chart with different sweep rate was conducted at 2.6 �M; (c) the corresponding chart with different amount of phenol. The sweep rate was 20 mV/s. Theaqueous solution 0.1 M NaOH was used as the electrolyte for all experiments at ambient temperature and pressure.

Table 3Summary of other sensors used for the detection of phenol or its derivatives.

Type/system LOD/mediator/catalyst Linearity range/Eopp Response time/pH Ref.

Amperometric biosensors/eight strains of Pseudomonas, with silverreference and auxiliary. Electrodes immobilized usinggraphite/acetyl cellulose (92 and 4%) composite layer

1 �M/ferrocene, duroquinone anddimethyferrocene

0.005–10 mM/70–100 mV 10–20 s/7 [10]

Two-photon ionization/(–) 0.01 ppm (∼1.06 �M)/(–) (–)/(–) (–)/(–) [13]Amperometric biosensors/Bacillus stearothermophilus loaded onto

silica gel2.5 �M/(–) 2.5 and 400 �M/(–) 10 s/7.6 [14]

Amperometric biosensors/Trichosporon cutaneum covalently boundto AH-Sepharose 4B or to nylon nets

0.5 mg/l (∼0.053 M)/(–) 0.05–5 mg/l/(–) (–)/(–) [15]

Amperometric biosensors 0.1 mg/l (∼1.06 mM)/(–) 0–20 mg/l/(–) (–)/– 800 mVagainst Ag/AgC, pH7.5

[16]

Amperometric biosensors/Pseudomonas putida 0.1 mg/l (∼1.06 mM)/(–) 0–50 �M/(–) (–)/7.5 [23]Amperometric biosensors/Pseudomonas immobilized into agar gel 0.002 mM (biphenyl)/(–) (–)/(–) (–)/4.3 [18]Amperometric biosensors/Pseudomonas putida entrapped in

polyvinyl alcohol (PVA)5 mmol/l (benzoate)/(–) 0–160 �mol/l/(–) (–)/6.8 [24]

Amperometric biosensors/Trichosporon beigelii entrapped inpolyvinyl alcohol

2 �mol/l (4-chlorophenol)/(–) 40 �mol/1/(–) (–)/6.8 [27]

Amperometric biosensors/Rhodococcus rhodochrous 0.05 mM (2-ethoxyphenol)/(–) 0.05 and 0.4 mM/(–) (–)/(–) [29]Amperometric biosensors/Pseudomonas immobilized on glass

particles pretreated with poly (ethylene diamine)10 �M (at a flow rate of 6 ml/min)/(–) (–)/(–) 6–8 min/(–) [30]

590 S. Bashir, J.L. Liu / Sensors and Actu

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ig. 6. Colorimetric-gram of Gibbs sensing element with addition of phenol: (a)ibbs concentration was altered and phenol concentration was kept constant; (b)henol concentration was altered and Gibbs concentration was kept constant.

ntensity over the measured concentration ranges and time periodTable 3).

. Conclusion

Gibbs reagent was distributed in �-CD evenly and the uniformlm sensor was obtained using sol-derived method. Phenol con-act with the sensor resulted in significant changes of the sensoropology using microscopic analysis; the spectroscopic data con-rmed the reaction of Gibbs reagent with Phenol anion to yield an

ndophenol anion. Electrochemical study indicated that the elec-ron transfer was the limiting reaction step and no mass diffusionas found at slower sweep rate between 2 and 20 mV/s. The limitf the detection was 1.0 M� from both electrochemical and colori-etric analysis with the relative standard deviation of less than

.4%. In addition, dual-detection devices are not common, anotherdvantage of our system, in that false positive are less likely.

cknowledgements

The authors are grateful to the technical support at The Chem-stry Department of Texas A&M University-Kingsville (TAMUK). Theraduate student, Chandra S. Padidem is also acknowledged for his

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ators B 139 (2009) 584–591

participation. The electrochemistry data have been collected at theDr. Viola Birss’s laboratory, University of Calgary, Alberta, Canada.The Departmental Grant at TAMUK supported by the Robert A.Welch Foundation (Departmental, AC 0006) is duly acknowledgedfor funding of the research described in this manuscript.

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of-the-art research. She is also coordinating the generalchemistry education and has created new curriculum of

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iographies

Dr. Sajid Bashir is an assistant professor at the Depart-ment of Chemistry, Texas A&M University-Kingsville.He received his Ph.D. degree Analytical Chemistry, atUniversity of Warwick, England, United Kingdom, 2000.He conducted research at Plant Biology Department,Cornell University, from 2001 to 2003. His expertise is on

matrix-assisted laser desorption/ionization time-of-flightmass spectrometry and nanoelectrospray ionization;electro spray ionization coupled to quadruple massanalyzer; nanoelectrospray ionization coupled to hybrid-quadrupole mass time-of-flight analyzer. He successfullyinstructed ten Hispanic students and some of them

ators B 139 (2009) 584–591 591

pursued their Ph.D. education. He was awarded the 2009 Olan Kruse TeachingAward by the Texas A&M University-Kingsville to acknowledge his achievementin teaching. He also collaborated with other faculties to complete NationalInstitute of Health funded project and published more than 30 peer-reviewedpapers.

Dr. Jingbo Louise Liu is an assistant professor atthe Department of Chemistry, Texas A&M University-Kingsville. She obtained her Ph.D. degree in materialsscience and engineering from University of Science andTechnology Beijing, 2001. Her research interests are sen-sors and actuators, alternative energy, water and airpurification, and nanotechnology. She established inter-national collaborations with the faculties from Canada,Mexico, China, and the United Kingdom to conduct state-

Nanoscale Science and Technology in graduate education.She is currently starting a female-owned non-profit company to advance femalefaculties and professionals in the fields of Science, Technology, Engineering andMathematics.