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creen-printed Prussian Blue modified electrode for simultaneousetection of hydroquinone and catechol
ihaela Buleandraa, Andreea Alexandra Rabincaa, Constantin Mihailciucb,driana Balanc, Cornelia Nichitac, Ioan Stamatinc, Anton Alexandru Ciucua,∗
University of Bucharest, Faculty of Chemistry, Department of Analytical Chemistry, 90-92 Panduri Avenue, 050663 Bucharest, RomaniaUniversity of Bucharest, Faculty of Chemistry, Department of Physical Chemistry, 4-12 Regina Elisabeta, 030018 Bucharest, RomaniaUniversity of Bucharest, Faculty of Physics, 3Nano-SAE Research Centre, MG38-Magurele, Bucharest, Romania
r t i c l e i n f o
rticle history:eceived 1 May 2014eceived in revised form 2 July 2014ccepted 11 July 2014vailable online 21 July 2014
A simply and high selectively electrochemical method for simultaneous determination of hydroquinone(HQ) and catechol (CC) has been developed at an electrochemically activated screen-printed carbonelectrode (SPCE) modified with Prussian Blue (PB). The PB acted as a mediator and thereby enhancedthe rate of electron transfer in chemical reaction. Various optimization studies such as the pH of themeasuring solution, linear range of response, sensitivity and detection limit, were conducted to obtainmaximum amperometric responses for analytes measurement. Differential pulse voltammetry (DPV) wasused for the simultaneous determination of HQ and CC in their mixture, and the peak-to-peak separationfor HQ and CC was about 0.11 V. The two corresponding well-defined oxidation peaks of HQ and CCat activated Prussian Blue-modified screen-printed carbon electrode (PB-SPCE) occur at −0.012 V and+0.094 V, respectively. Under the optimized condition in DPV, the oxidation peak current of HQ and ofCC is linear over a range from 4.0 × 10−6 M to 9.0 × 10−5 M HQ and from 1.0 × 10−6 M to 9.0 × 10−5 M CC.
−7 −7
The obtained detection limit for HQ and CC was 1.17 × 10 M and 4.28 × 10 M, respectively. DPV canbe used for individual or simultaneous determination of HQ and CC. The proposed activated PB-SPCEwas successfully applied to the simultaneous determination of HQ and CC in spiked tap water. Using thestandard addition method, the average recovery of the proposed method based on activated PB-SPCE was99.03% and 95.87% for HQ and CC, respectively.
. Introduction
Hydroquinone (HQ, 1,4-dihydroxybenzene) and catechol (1,2-ihydroxybenzene, CC) are two important isomers of phenolicompounds, which are often used in cosmetics, pesticides, flavoringgents, antioxidant, secondary coloring matters, and photographyhemicals [1–3]. During the application and manufacturing pro-ess of these compounds, some of them are inevitably releasednto the environment and contaminate rivers and ground waters.hus, in industrial effluents and sanitary wastewater exists aarge number of dihydroxybenzene isomers [3–5]. Meanwhile,
ost of them are highly toxic to both environment and humanven at very low concentrations [6]. High concentration of HQ
an lead to fatigue, headache and tachycardia in humans [7]; itan also lead to cancer such as acute myeloidleukemia [8]. Thebsorption of HQ or CC from the gastrointestinal tract can induce
some disease such as renal tube degeneration and liver functiondecrease [9]. Because of their high toxicity and low degradabil-ity in the ecological environment, HQ, and CC are consideredas environmental pollutants by the US Environmental ProtectionAgency (EPA) and the European Union (EU) [10]. They have beenextensively studied due to their biological and environmentalimportance.
Because of their similar structures and properties, they usuallycoexist in products and environmental samples and it is a chal-lenge to directly and simultaneously determinate the isomers [11].Therefore, reliable analytical procedures are required for simulta-neous determination of HQ and CC in various matrices with highsensitivity.
Up to now, various analytical methods have been exploitedfor the determination of the dihydroxybenzenes, such as highperformance liquid chromatography (HPLC) [12], spectropho-
The established methods for the determination of HQ and CCre commonly performed after pretreatment and separation [4].ompared with the chromatographic and optical methods, elec-rochemical methods [18–27] are preferable and attractive for theimultaneous detection of such phenolic compounds due to thedvantages of fast response, cheap instrument, low cost, simpleperation, time saving, high sensitivity, and excellent selectivity3] being also more feasible for miniaturization of analysis.
Since HQ and CC have a basic quinone structures they areoth electrochemically active and thus can be determined withlectrochemical techniques [28] such as cyclic voltammetry (CV),ifferential pulse voltammetry (DPV) and square wave voltam-etry (SWV). The major difficulty for the sensitive, selective and
imultaneous detection of HQ and CC is that the voltammetriceaks corresponding to oxidation/reduction of two phenol iso-ers are, in many cases, highly overlapped for most conventional
olid electrodes. Moreover, the competition of the phenolic isomersy electrode surface makes the relationship between the voltam-etric response and the isomers concentrations, in the mixtures,
on-linear [29].In order to address the above problems, many materials have
een used to fabricate chemically modified electrodes (CMEs) tochieve simultaneous voltammetric determination of dihydroxy-enzene isomers [2,3,11,22–43]. A CME is an excellent approacho address the signal separation problem by introducing a modi-er with which the extent of the interaction differs significantly
rom analyte to analyte [18,22]. Hence, some carbon [23,33] or car-on nanotubes (CNTs) modified electrodes [21,22] were used forhe determination of dihydroxybenzene isomers. For instance, Yut al. [23] reported that CC and HQ can be high sensitive simulta-eously determined at mesoporous carbon CMK-3 electrode withhe peak-to-peak separation of the oxidation potential (�Epa) of25 mV. The simultaneous determination of HQ and CC at a glassyarbon electrode modified with multiwall carbon nanotubes haseen also proposed with potential wave separations of 102 mVetween the oxidation peaks of HQ and CC [20]; consequently, theuthors used multi-electrode array modified with MWCNTs andowered the detection limits of dihydroxybenzene isomers [21]. Lit al. reported that a disposable screen-printed electrode which wasodified by multiwalled carbon nanotubes and gold nanoparticles
28] can separate oxidation peaks of HQ and CC. Another method forhe simultaneous determine HQ and CC was investigated by Wangnd co-workers [22], using modified electrode like covalent mod-fication of glassy carbon electrode with PASA/MWNTs compositelm. Polymer-modified electrodes prepared by electropolymeriza-ion were also used for the simultaneous determination of CC andQ isomers [2,24,33,37,44–48]. Other methods for the simulta-eous determination of HQ and CC have been investigated by Wangt al. with the GCEs modified by penicillamine [11], and asparticcid [49]. Li et al. [27] developed a sensitive and simultaneous HQnd CC determination method in the presence of resorcinol with
Zn/Al layered double hydroxide modified GCE and de Carvalhot al. [29] reported another electrochemical method for the simulta-eous determination of phenol isomers using carbon fiber electrodeodified by titanium oxide.Nevertheless, it is still interesting to investigate novel elec-
rode material for the simultaneous determination of HQ and CC.he aim of this work was to develop an amperometric sensor forimultaneous hydroquinone and catechol measurement based onrussian Blue (PB) modified screen-printed carbon electrode (SPCE)nd which should possesses properties such as low cost, goodtability, rapid response time, low detection limit and good selec-
ivity toward some possible interferents. To our best knowledge,he simultaneous determination of the mentioned compounds atn activated PB-SPCE has not been reported. CV and DPV resultshow that the isomers can be detected selectively and sensitively at
uators B 203 (2014) 824–832 825
PB-SPCE with peak-to-peak separation of about 0.11 V. The pro-posed method has been applied to simultaneous determination ofHQ and CC in a water sample with simplicity and high selectivity.
2. Experimental
2.1. Reagents and solutions
Hydroquinone and catechol were provided by Sigma–Aldrich.All other chemicals employed were of analytical reagent grade.Double distilled water was used throughout this work. For studyof pH effect phosphate buffer solutions (PBS) of pH 5.91–8.04 wereused, whereas for all other experiments, PBS of pH 6.64 was used.Stock solutions 1.0 × 10−2 M of HQ and CC were daily prepared indouble distilled water. The working solutions were prepared justbefore use by diluting the stock solutions with 1/15 M PBS of pH6.64.
2.2. Instrumentation
All voltammetric measurements were performed with an AUTO-LAB electrochemical analyzer (PGSTAT 128N Ecochemie B.V.,Netherlands). The terminals of the working (WE), reference andcounter electrodes of the AUTOLAB electrochemical analyzer wereconnected to the respective terminals of the disposable SPE systemvia standard connectors and all data processing and experimentalcontrols were driven through the Nova 1.8 software installed on acomputer interfaced with the electrochemical analyzer.
Commercially available screen-printed Prussian Blue/carbonelectrodes (PB-SPCEs) (ref. 710) and screen-printed carbon (SPC)electrodes (ref. 110) obtained from Dropsens were used. The diam-eter of the working electrodes was 4 mm, which resulted in anapparent geometric area of 0.125 cm2. Disposable SPE systems hada carbon paste or a carbon paste modified with PB WE, carbonCE and silver RE. Their dimensions are: 3.4 cm × 1.0 cm × 0.05 cm(Length × Width × Height). A single disposable SPE system is forone-time use only.
AFM images were recorded with the Integrated Platform SPM-NTegra, model Prima microscope (NT-MDT trade mark). Themorphology of the PB-SPCEs and SPCEs was investigated with AFMin noncontact mode using a cantilever with silicon tip of 1.2 nmradius (SG-10); 1 × 1 �m surface areas were scanned at differentscale of magnification down to 100 × 100 nm.
2.3. Methodology
To enhance the electron transfer rate between substrates andelectrodes, PB is acted as a mediator which increases the selec-tivity of the sensor also. Firstly, the PB acts as an electron transfermediator to accelerate electron transfer rate for oxidation of HQ andCC. This whole redox reaction involves electron transfer at the WEsurface and thus, the response can be observed by amperometricmeasurements. The schematic diagram of the detection principleis shown in Fig. 1.
Such a detection scheme was proposed by others also [50,51] indescribing their electrodes for pesticide and glucose-6-phosphatedetection. All the responses can be attributed to the oxidation ofdihydroxybenzene to produce quinone and vice versa. The pro-posed electrode mechanism is presented in Section 3.3.
2.4. Optimization study
Various optimization studies, such as pH and scan rate, wereperformed by using cyclic voltammetry (CV) for the electrochem-ical behavior of HQ and CC. Differential pulse voltammetry (DPV)
826 M. Buleandra et al. / Sensors and Actuators B 203 (2014) 824–832
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Fig. 2. Cyclic voltammograms of a mixture of 5.0 × 10−4 M HQ and 5.0 × 10−4 MCC in phosphate buffer pH 6.64 obtained at PB-modified SPCE (a) before and (c)
ig. 1. Schematic diagram of HQ/CC and PB mediated redox reaction at the WEurface.
as used for the direct determination of HQ and CC and for thenterference studies also.
.5. Electrode pretreatment
Initially the unmodified and PB-SPCEs were pretreated by per-orming 10 cyclic voltammetric scans from −0.2 V to 3.0 V at a scanate of 500 mV s−1, in phosphate buffer solution pH 6.64, this stepssuring the electrode activation [52]. All the applied potentialsentioned in the paper are referred to the internal Ag pseudo-
eference electrode of the SPCEs.
.6. Electrochemical measurement of HQ and CC
The unmodified- and PB-SPCEs were connected to the respectiveerminals of the AUTOLAB electrochemical analyzer for measuringoltammetric response. In all such experiments, 50 �L drop of solu-ion (in 1/15 M PBS of pH 6.64) was dispensed on the SPCE coveringhe three electrodes to connect the electrochemical cell.
Cyclic voltammetric experiments were performed in the poten-ial range −0.4 V to +0.6 V with a scan rate of 100 mV s−1 unlesstherwise stated.
Differential pulse voltammograms were recorded between −0.4nd +0.6 V for different concentrations of HQ and CC solutions pre-ared in phosphate buffer, pH 6.64; the optimized instrumentalarameters of DPV were as follows: scan rate of 10 mV s−1, pulsemplitude 25 mV, sampling width 17 ms, pulse width 100 ms, pulseeriod 500 ms. The oxidation peak of HQ at −0.012 V and of CC at0.094 V were used for quantification of HQ and CC, respectively.
To demonstrate the usefulness of the proposed activated PB-PCEs, tap water samples were spiked with known concentrationf HQ or CC (1 × 10−6 M) and analyzed.
. Results and discussion
.1. Pretreatment procedure
Good electrical conductivity of the electrodes is an impor-ant factor. The type of carbon-based and the using pretreatment
ethod have a highly effective on the electrode surface [53]. Its reported that electrochemical pretreatments of SPEs is used toncrease the electron transfer rate on the carbon electrode and can
mprove their electrochemical behavior [54].
Initially the relevance of the electrochemical pretreatment ofnmodified- and PB-SPCEs was examined. Fig. 2 shows the CVsbtained for a solution containing an equimolar mixture of HQ
after electrochemical pretreatment and at unmodified SPCE (b) before and (d) afterelectrochemical pretreatment; scan rate: 100 mV s−1. Pretreatment parameters: 10CV scans, potential range −0.2 V to +3.0 V, scan rate 500 mV s−1.
and CC at both untreated and pretreated PB-SPCE in 1/15 M PBSof pH 6.64. At untreated PB-SPCE (Fig. 2, curve a) the oxidation andreduction peaks of HQ and CC merge into large peaks, the cathodicone negatively shifted. This indicates that HQ and CC cannot besimultaneously determined at the untreated PB-SPCE. For the bareuntreated SPC electrode (Fig. 2, curve b) only one single peak wasobtained for both anodic and cathodic scan of HQ and CC. It is clearthat the two isomers cannot be separated, as in case of untreatedPB-modified SPCE, and the peak-to-peak separation was signifi-cantly higher (about of 0.330 V). At the activated PB-SPCE (Fig. 2,curve c), two anodic peaks are observed at −0.012 V and +0.094 V,which were attributed to the electrochemical oxidation of HQ andCC, respectively. The electrochemical reduction of HQ and CC areobserved at −0.109 V and +0.022 V, respectively. The peak poten-tial separation is about 0.11 V, calculated from half-wave potentials(−0.031 V for HQ and +0.053 for CC, respectively). The peak-to-peak separation is 0.097 V for HQ and 0.072 V for CC, respectively.Therefore the first redox couple is less reversible than the secondone. Similar results were obtained at pretreated un-modified SPCE(Fig. 2, curve d). The degree of reversibility for these redox sys-tems remained, more or less, the same after the electrochemicalpretreatment procedure as indicated by the sharper oxidation andreduction waves observed at the conditioned surfaces. Comparedwith pretreated SPCEs, the peak currents of pretreated PB-SPCEs areincreased, indicating that the PB-SPCEs retain significant electricalconductivity and can be used for sensor development to detect HQand CC (as shown in Fig. 2, curves b and c).
Electrochemical pretreatment process contributed also to theelectrochemical separation of the two peaks of the phenolic com-pounds (CC and HQ) which made possible their determination inpresence. Moreover, as can be seen from Fig. 2, both peaks appearin the range of the formal oxido-reduction potential of the redoxcouple PBred/PBox.
These results indicate that the PB-SPCEs cannot only identify theCC and HQ, but also enhance the detection sensitivity. In all furtherstudies pretreated PB-SPCEs were used.
3.2. AFM studies
Un-activated/activated PB-SPCEs and activated PB-SPCE after its
usage in electrochemical detection of HQ and CC were character-ized using AFM. A comparison of Fig. 3a–c reveals a significantmorphological difference among all three SPC electrode sur-faces. In case of un-activated PB-SPCE (Fig. 3a) the average grain
M. Buleandra et al. / Sensors and Actuators B 203 (2014) 824–832 827
Fig. 3. AFM topography images of un-activated PB-modified SPCE (a), activated PB-modified SPCE (b) and activated PB-modified SPCE after its usage in electrochemicaldetection of HQ and CC (c).
sS3S5rts
ize of 319 nm and an average roughness 96 nm were obtained.ize distribution has two peaks centered at 230–240 nm and40–350 nm and a small peak at 90 nm. In case of activated PB-PCE (Fig. 3b) the average grain size of the aggregates reaches
00 nm and roughness of 153 nm. During the activation step aeducing in size of each nanoparticle takes place with a simul-aneously aggregation. A detailed analysis in aggregates showsmall particles centered at approx. 170 nm and 220 nm (see inset
Fig. 3b) respectively at 50 nm (shoulder in inset). Finally, in caseof activated PB-SPCE after its usage in electrochemical detectionof HQ and CC (Fig. 3c) the average roughness decreases down to52 nm with several peaks centered on 70–100 nm, 160 nm, 220 nm
and 270 nm, and average grain size of 160 nm could observed.After deposition of HQ and CC, due to intermolecular interactions,particles are breaking apart increasing the specific area on theSPCE.
828 M. Buleandra et al. / Sensors and Actuators B 203 (2014) 824–832
ig. 4. The effect of pH on the peak current of (a) 5.0 × 10−4 M HQ and (b).0 × 10−4 M CC in phosphate buffer at PB-modified SPCE; scan rate 100 mV s−1.
.3. Influence of pH
The pH of the buffer is essential to determine the sensitivity ofhe sensors since the stability of PB is dependent on the pH of theystem [55] and the two electrode reactions involving the HQ andC are pH-dependent also [31,56].
The effect of pH value on the electrochemical behavior of HQn presence of CC was carefully investigated by CV in the pH rangef 5.91–8.04 using 1/15 M PBS. From Fig. 4, it can be seen that thexidation peak current of HQ and CC increase with increasing pHalue until it reaches 6.64, and then the oxidation peak currentsecrease when the pH increases further. This could be related to theact that proton took part in the electrochemical reaction. When theH value was high, the shortage of proton prevented the oxidationnd reduction of CC and HQ, leading to the decrease of current peakntensity. On the other hand, the diphenol, as it is well known, tendso form anions and make the peak current decrease [18,30,32].
In addition, in the selected pH range (5.91–8.04) the peak poten-ials shifts negatively with the increase of pH (Fig. 5) for bothQ and CC, indicating that the protons are directly involved in
he electrochemical redox process also [31,56]. The four regres-ion lines were almost parallel, implying that the anodic orathodic peak potential separation between CC and HQ is con-tant at different pH solutions [56]. For HQ, two linear relationshipsetween peak potential and solution pH were obtained in the
nvestigated pH range and having the regression equations of Epa
V) = −0.0575 pH + 0.3237 (R2 = 0.9930) for the oxidation processnd Epc (V) = −0.0569 pH + 0.2675 (R2 = 0.9970) for the reduc-ion process. Similar to HQ, for CC two linear relationships werebtained for the oxidation and reduction processes, the regression
quations being Epa (V) = −0.0551 pH + 0.4503 (R2 = 0.9969) andpc (V) = −0.0535 pH + 0.3664 (R2 = 0.9963) for the oxidation andeduction process, respectively. These linear regression equations
Fig. 5. The effect of pH on the peak potential of a mixture of 5.0 × 10−4 M HQ and5.0 × 10−4 M CC in phosphate buffer at PB-modified SPCE; scan rate 100 mV s−1.
showed that the uptake of electrons is accompanied by an equaluptake of protons for both HQ and CC [31,45,47,57].
The slopes of Ep vs. pH are close to the theory value of58.5 mV pH−1 for two electrons and two protons process [58], sug-gesting that the redox reaction of HQ and CC at PB-SPCE should bea two electrons and two protons process as it was also reported ina number of previously published papers [20,30,56,59].
According to the following formula [60,61]:dEp/dpH = 2.303mRT/nF in which, m is the number of proton,n is the number of electron, m/n was calculated to be 0.98 and0.99 for the oxidation and reduction process of HQ, respectively.Similarly, m/n = 0.95 and 0.95 for the oxidation and reductionprocess of CC. It indicates that the number of proton and electroninvolved in the electrochemical redox process of HQ and CC isequal as reported by others also [31,32]. Thus, the electrochemicaloxidation of hydroquinone or catechol at the PB-SPCE should be atwo-electron and two-proton process, as shown previously in theliterature [20,38,42].
Based on obtained results which indicate a 2H+, 2e− mechanismfor CC and HQ, a mediated electrode mechanism for the oxidationof CC and HQ by PB can be expected:
where PBred and PBox are the reduced and oxidized forms of themediator, Ph is the phenolic compound and Q is correspondingoxidized form, the mediator being regenerated (Eq. (2)). This mech-anism is in good agreement with that proposed by Palleschi et al.in case of thiol detection [50] and pesticide biosensor [62]. There-fore, in order to achieve high sensitivity, pH 6.64 was selected asthe optimum pH for the simultaneous electrochemical detection ofHQ and CC at the PB-SPCE.
3.4. Influence of scan rate
In order to investigate the electrochemical behaviors of HQand CC at activated PB-SPCE, the effect of the scan rate wasstudied by cyclic voltammetry (Fig. 6). For each dihydroxy-benzene isomer, a pair of redox peak is observed for scanrates (v) from 10 to 700 mV s−1. Good linear relationshipsbetween the anodic (ipa) and cathodic (ipc) peak currents of thevoltammograms for HQ and CC and the square root of scanrate were obtained. The corresponding linear regression equa-
M. Buleandra et al. / Sensors and Actuators B 203 (2014) 824–832 829
Fig. 6. Cyclic voltammograms of 5.0 × 10−4 M HQ and 5.0 × 10−4 M CC in phos-p(4
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hate buffer pH 6.64 at PB-modified SPCE at different scan rates: (a) 10 mV s−1;b) 25 mV s−1; (c) 50 mV s−1; (d) 100 mV s−1; (e) 200 mV s−1; (f) 300 mV s−1; (g)00 mV s−1; (h) 500 mV s−1; (i) 600 mV s−1 and (j) 700 mV s−1.
ndicates diffusional electroactive species taking part in the twolectrode reactions [31,38,62].
Furthermore, the influence of scan rate on the redox peak poten-ial was also investigated for calculating the kinetic parameters.
ith the increase of scan rate, the oxidation peak potential (Epa)hifted positively and the reduction peak potential (Epc) shiftedegatively and at scan rates greater than 300 mV s−1 the anodic andathodic peak potential showed linear relationship with the loga-ithm of scan rate as demonstrated by others also [32]. For HQ twoinear regression equation: Epa (V) = 177.52 log v + 182.18 (V s−1,2 = 0.9966) and Epc (V) = −164.63 log v − 146.56 (V s−1, R2 = 0.9860)ere obtained. In case of CC, the linear relationship equationere: Epa (V) = 240.49 log v − 324.79 (V s−1, R2 = 0.9901) and Epc
V) = −114.83 log v + 11.208 (V s−1, R2 = 0.9695). According to Lav-ron’s model [60,61], the plot of the peak potential vs. decimalogarithm of scan rate yields two straight lines with slopes of2.3RT/˛nF and 2.3RT/(1 − ˛)nF for the cathodic and anodic peaks,
espectively, and ̨ (charge transfer coefficient) can be estimatedrom the slope of the straight lines. Therefore, for HQ, the electronransfer coefficient (˛) and electron transfer number (n) were cal-ulated as 0.53 and 2, respectively. Similarly, ̨ equals 0.66 and nquals 2 were obtained for CC. The results are in good agreementsith those presented in the literature [32].
The apparent heterogeneous electron transfer rate constant (ks)an be obtained according to reference [60] based on equation:
og ks = ̨ log(1 − ˛) + (1 − ˛) log ̨ − logRT
nFv− ˛(1 − ˛)nF�Ep
2.3RT
here n is the number on electrons involved in the reaction, �Ep
s the peak potential separation (Epa − Epc), ̨ is the charge transferoefficient, v is the scan rate, R is the gas constant, T is the absoluteemperature and F is the Faraday constant. As the number of elec-rons involved in electrochemical redox process of HQ and CC is 2,he values of ks are 0.31 s−1 for HQ and 0.55 s−1 for CC. The smalleralues of ks indicate that the HQ and CC on PB-modified SPCE sur-ace show a quasi-reversibility of the electron transfer process.
.5. Simultaneous measurement of HQ and CC
Once the optimal conditions were established, the simultaneousnd quantitative determination of the two isomers was carried out
Fig. 7. Differential pulse voltammogram of solution containing 5 × 10−6 M HQ and5 × 10−6 M CC in presence of 5 × 10−6 M uric acid obtained at PB-modified SPCE.
by differential pulse voltammetry because of its higher current sen-sitivity and better resolution. The determination of HQ and CC intheir mixture was performed from 1 × 10−6 M to 1 × 10−4 M con-centration range. The results showed that anodic peak current of HQat −0.012 V is proportional to the concentration from 4 × 10−6 Mto 9 × 10−5 M. The regression equation is ipa = 0.3016c + 0.1045(ipa: �A, c: �M) (R = 0.9982) and the detection limit (LOD) of1.17 × 10−7 M and the quantification limit (LOQ) of 3.54 × 10−7 Mwere calculated from linear regression analysis. Similarly, theanodic peak current of CC at +0.094 V increases linearly with con-centration from 1 × 10−6 M to 9 × 10−5 M. The regression equationis ipa = 0.2857c + 0.1434 (ipa: �A, c: �M) (R = 0.9974) and obtaineddetection limit and quantification limit were 4.28 × 10−7 M and1.30 × 10−6 M, respectively.
3.6. Reproducibility
The reproducibility of the proposed activated PB-SPCE wasinvestigated by comparing the DPV peak current of a 5 × 10−6 MHQ and CC in a mixed solution for 3 electrodes. This was evaluatedin terms of relative standard deviation (% RSD) and was found to be4.07% in case of HQ and 4.45% in case of CC for three independentsets of experiments.
3.7. Interference study
The interference study was performed to assess the selectivityof the proposed sensor. The selectivity of the PB-SPCE was investi-gated in 1/15 M PBS of pH 6.64 containing 5.0 × 10−6 M HQ and CCby DPV method. The results showed that 10-fold of ascorbic acid,phenol, resorcinol and nitrophenol do not interfere with the deter-mination of HQ and CC (signals change below 5%). In addition, forthe uric acid a separate peak at +0.234 V was obtained (Fig. 7). Thiscould be used for simultaneous determination of hydroquinone,catechol and uric acid.
All the results imply that the PB-SPCE has good selectivitytoward the mentioned interferents.
3.8. Analytical applications – analysis of HQ and CC in tap water
To investigate the possible application of the proposed methodin direct simultaneous determinations of HQ and CC local tapwater samples were tested. The determination of HQ and CC in the
830 M. Buleandra et al. / Sensors and Actuators B 203 (2014) 824–832
Table 1Recovery results for HQ and CC in tap water samples using PB-modified SPC electrode (n = 3).
Tap water sample no. HQ and CC concentrationAdded (�M)
amples was carried out using DPV at the PB-SPCE in 1/15 M PBSpH 6.64). Since the amounts of HQ and CC were under the detec-ion limit of the method, the recovery experiments were performedor the samples with known added concentration of HQ and CC. Theesults are listed in Table 1. Using the standard addition method, theverage recovery of the proposed method was 99.03% and 95.87%or HQ and CC, respectively. The feasibility of the PB-SPCE in theirect simultaneous determination of HQ and CC is evident.
.9. Comparison of results
The performance of the activated PB-SPCE was compared withther HQ and CC sensing systems reported in literature in lightf technology, limit of detection and detection range (Table 2). Itas observed that most of the reported HQ and CC amperometric
sensors suffer few major inherited drawbacks, e.g. time consumingmodification procedures whereas in the proposed sensor modifi-cation procedure (electrochemical activation) took less than 1 min.The HQ and CC sensor also showed a broad detection range andlower detection limit compared to several other reported sensors.Also, the PB-SPCE exhibited excellent anti-interference property.Thus, it is clear that the present method could overcome manydisadvantages of the reported ones.
4. Conclusions
This research has developed a cheap, sensitive, and rapidmethod for the electrochemical determination of HQ and CCin aqueous pH 6.64 PBS solution without previous separation.By employing both electrochemically pretreated screen-printed
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arbon electrodes modified with Prussian Blue and DPV tech-ique, a direct and simultaneous determination of the twoositional isomers was achieved. The oxidation peak potentialsor HQ and CC were completely separated at the activated PB-PCE, exhibiting well-defined and quasireversible redox peaksnd showing greatly enhanced activity. Under optimized condi-ions, the linear calibration curves for HQ and CC were in theanges of 4.0 × 10−6–9.0 × 10−5 M and 1.0 × 10−6–9.0 × 10−5 M,ith detection limits of 1.17 × 10−7 M and 4.28 × 10−7 M, respec-
ively calculated by linear regression analysis. This method waspplied to the direct determination of HQ and CC in tap water withatisfactory recovery results. A successful elimination of the foulingffect by the oxidized product of HQ on the response of CC has beenchieved at the activated PB-SPCE. Moreover the PB-SPCE exhib-ted good selectivity toward ascorbic acid, phenol and uric acid.he response of PB-SPCE was remarkably high compared to theegligible current responses of the studied interferents.
cknowledgements
The financial support of Romanian Grants PN-II-ID-PCE-2011-3-784 – contract no 251/2011 and Bilateral project 667/2013 fundedy UEFISCDI are acknowledge.
eferences
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Biographies
Mihaela Buleandra is assistant professor at University of Bucharest, Faculty ofChemistry, Department of Analytical Chemistry. Her major interest is studying theanalytical properties of selective chemical sensors and improving their properties.She has good experience working with chemically modified and screenprinted elec-trodes as well as with electroanalytical techniques.
Andreea Alexandra Rabinca is a second year graduate student working on her M.Sc.at the University of Bucharest. Her major interest is developing selective electro-chemical sensors and using them amperometric detectors in electroanalysis.
Constantin Mihailciuc is associate professor at University of Bucharest, Faculty ofChemistry, Department of Physical Chemistry. His major interest is in analyticalelectrochemistry and bioelectrochemistry.
Adriana Balan is a researcher at University of Bucharest, Faculty of Physics,3-Nanoscience Center. Her main research interest included functional surface char-acterizations of carbon-based materials and nanostructures for applications insensors development.
Cornelia Nichita is a researcher at University of Bucharest, Faculty of Physics, 3-Nanoscience Center. Her main research interest included surface characterizationsof carbon-based materials for chemical sensors development.
Ioan Stamatin is full professor at University of Bucharest, Faculty of Physics. Hisresearch activity is mainly focused on material science: synthesis, post-synthesistreatments, chemical–physical processing and nanostructures for applications inelectronics, optoelectronics and sensoristics.
Anton Alexandru Ciucu is full professor of Analytical Biochemistry at University ofBucharest, Faculty of Chemistry, Department of Analytical Chemistry. His researchwork interest is in analytical biochemistry focused on chemically modified elec-trodes and biosensors development for their applications in bioanalysis.
