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Dec 21, 2017 - Standard Normalization for Accurate Determination of Cd(II) in Soil. Hui Wang 1,2, Guo Zhao 1,2, Yuan Yin 1,2, Zhiqiang Wang 3 and Gang Liu ...

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Screen-Printed Electrode Modified by Bismuth /Fe3O4 Nanoparticle/Ionic Liquid Composite Using Internal Standard Normalization for Accurate Determination of Cd(II) in Soil Hui Wang 1,2 , Guo Zhao 1,2 , Yuan Yin 1,2 , Zhiqiang Wang 3 and Gang Liu 1,2, * 1

2 3

*

Key Lab of Modern Precision Agriculture System Integration Research, Ministry of Education of China, China Agricultural University, Beijing 100083, China; [email protected] (H.W.); [email protected] (G.Z.); [email protected] (Y.Y.) Key Lab of Agricultural Information Acquisition Technology, Ministry of Agricultural of China, China Agricultural University, Beijing 100083, China College of Computer Science and Technology, Shandong University of Technology, Zibo 255049, China; [email protected] Correspondence: [email protected]; Tel.: +86-10-6273-6741

Received: 12 October 2017; Accepted: 18 December 2017; Published: 21 December 2017

Abstract: The quality and safety of agricultural products are threatened by heavy metal ions in soil, which can be absorbed by the crops, and then accumulated in the human body through the food chain. In this paper, we report a low-cost and easy-to-use screen-printed electrode (SPE) for cadmium ion (Cd(II)) detection based on differential pulse voltammetry (DPV), which decorated with ionic liquid (IL), magnetite nanoparticle (Fe3 O4 ), and deposited a bismuth film (Bi). The characteristics of Bi/Fe3 O4 /ILSPE were investigated using scanning electron microscopy, cyclic voltammetry, impedance spectroscopy, and linear sweep voltammetry. We found that the sensitivity of SPE was improved dramatically after functionalized with Bi/Fe3 O4 /IL. Under optimized conditions, the concentrations of Cd(II) are linear with current responses in a range from 0.5 to 40 µg/L with the lowest detection limit of 0.05 µg/L (S/N = 3). Additionally, the internal standard normalization (ISN) was used to process the response signals of Bi/Fe3 O4 /ILSPE and established a new linear equation. For detecting three different Cd(II) concentrations, the root-mean-square error using ISN (0.25) is lower than linear method (0.36). Finally, the proposed electrode was applied to trace Cd(II) in soil samples with the recovery in the range from 91.77 to 107.83%. Keywords: electrochemistry; screen-printed electrode; bismuth; Fe3 O4 nanoparticle; cadmium

1. Introduction Rice is one of the most popular staple food for large parts of Asia, especially in the south of China [1]. It is widely cultivated in these areas, which has the potential to improve the food production [2,3]. Due to unreasonable use of chemical fertilizer and sewage irrigation, cadmium, one of the most toxic heavy metals, has been deposited and exceeded the safety level in soil. Cadmium ions (Cd(II)) in soil are easily absorbed by the paddy [4], and then enter the human body through the digestive system [5–8]. Cd(II) can cause some serious diseases, such as carcinogenic issues, lung damage, kidney failure [7,9–11]. Therefore, quick and accurate identification of Cd(II) concentrations is urgent to prevent soil pollution and ensure food safety. Up to now, the regular methods have been employed for Cd(II) determination, such as X fluorescence spectrometry (XRF) [12], Atomic Absorption Spectroscopy (AAS) [13], Atomic Emission Spectrometry(AES) [14], Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) [15], Inductively Coupled Plasma-Mass Sensors 2018, 18, 6; doi:10.3390/s18010006

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Spectrometry (ICP-MS) [16], Atomic Fluorescence Spectrometry (AFS) [17], biological technique [18] and electrochemical method [19]. However, the spectrographic methods are not suitable for in-situ analysis because of complex operation, large volume, and expensive instruments. The application of biological technique for metal ions detection was limited because the properties of biological materials hard to keep in natural conditions [20]. Compared to approaches mentioned above, the electrochemical method owns some unique merits such as relative portability, inexpensive instrumentation and real analysis [21]. It has been recognized as an effective way for heavy metal ions determination. Screen-printed electrode (SPE) is low-cost and easy-to-use, which is much suited for mass production and application. Nevertheless, bare SPE is hard to trace Cd(II) at low concentrations. Many researches show that nanomaterials have merits to improve the performance of SPE. Magnetic nanoparticles (Fe3 O4 ) [22–24] have broad applications in catalysis, biotechnology as well as analytical chemistry [25] due to the large surface area, quantum size effect, high adsorption ability, excellent biocompatibility, its strong superparamagnetic property, low toxicity and easy preparation. Srivastava et al. [26] developed an electrode coated ionic liquids and Fe3 O4 nanoparticle, applied for the simultaneous determination of DNA bases. Chitosan (CHT) is a natural polymer with abundant primary amino groups and hydroxyl groups. It has excellent adhesivity, hydrophilicity, gel-forming ability, doping feasibility, excellent mechanical stability, permeability, cost-effectiveness, and availability of reactive functional groups for chemical modifications. CHT has been widely employed in the construction of electrochemical sensors and biosensors to improve performances. It also can adsorb metal ions and various organic compounds at the same time. To exploit a cheap, easy to use and sensitive electrode, we have developed a new electrode. It used a mixed solution of Fe3 O4 and CHT to coat on the ion liquid based SPE (ILSPE), and then deposited a bismuth film (Bi) in situ. The new electrode can combine the advantages of Fe3 O4 , CHT, and SPE successfully. Additionally, Bi/Fe3 O4 /ILSPE was applied to determine Cd(II), and used internal standard normalization (ISN) to amendment the linear regression equation to improve the accuracy. At last, a new electroanalytical electrode for Cd(II) determination in soil was created. 2. Experimental 2.1. Chemicals and Reagents All electrochemical measurements were collected using a CHI660D electrochemical workstation (CHI Instrument Company, Shanghai, China) with a three-electrode system. The modified screen-printed electrode was used as the working electrode, a platinum column as an auxiliary electrode and an Ag/AgCl electrode as a reference electrode. Scanning electron microscope photos were achieved by using the JEOLJEM-2011 device. All electrochemical measurements were recorded at room temperature. Ionic liquid(IL), n-octylpyridinium hexafluorophosphate (OPFP), was purchased from Shanghai Chengjie Co., Ltd. (Shanghai, China). Graphite powder (size < 30 µm, 20019126) was obtained from Sinopham Chemical Reagent Co., Ltd. (Shanghai, China). SPE was bought from Suzhou delta system biological cross scientific research institute co. Ltd. (Shanghai, China). Chitosan and nano-Fe3 O4 were obtained from Sigma-Aldrich Corporation (Shanghai, China). Standard solutions of Bi(III) and Cd(II) (1000 mg/L) were provided by National Standard Reference Materials Center of China and diluted as required. Phosphate buffer solution (0.2 M, pH 5.0) was selected as the supporting electrolyte for experiments. The rest of chemicals not mentioned here were of analytical reagent grade and were used as received. Millipore-Q (18.2 MΩ) water was used for all experiments. 2.2. Preparation of Fe3 O4 /ILSPE The ILSPE was fabricated by the following steps in Figure 1. First, 0.05 g cellulose acetate was dissolved in a solution containing 2.5 mL cyclohexanone and 2.5 mL acetone. Second, 0.5 g OPPF and 2.0 g graphite powder were added and mixed to form a homogeneous and viscous ink achieved

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throughultrasonic ultrasonicshaking. shaking. The The modified modified process process was was executed through executed by by using usingaapipette pipetteto todirect directwrite writethe the ◦ compositeonto onto the the surface surface of of SPE. SPE. Third, Third, the composite the prepared prepared electrode electrode was wasannealed annealedatat80 80°CCfor for3030min. min. 0.2ggchitosan chitosan flakes flakes were were weighed weighed and and dissolved dissolved in 0.2 in aqueous aqueous solution solutionofof100 100mL mL1.0% 1.0%acetic aceticacid. acid. Fourth, weighing 0.1 mg Fe 3O4 nanoparticle was dispersed into 10 mL 0.2 wt % (weight/solution Fourth, weighing 0.1 mg Fe3 O4 nanoparticle was dispersed into 10 mL 0.2 wt % (weight/solution volume== 0.2 0.2g/100 g/100 mL) mL) CHT L dispersion volume CHT using using an an ultrasonic ultrasonic treatment treatment for for 22 h. h. 2.0 2.0µµL dispersionliquid liquidwas was dropped on the ILSPE surface and dried at 50 °C for 30 min. For Bi(III) deposition, the 400 Bi(III) was ◦ dropped on the ILSPE surface and dried at 50 C for 30 min. For Bi(III) deposition, the 400 Bi(III) added intointo thethe solution containing Cd(II), 4/ILSPE at −1.2 V, and then was added solution containing Cd(II),electrodeposited electrodepositedononFe Fe3O 3 O4 /ILSPE at −1.2 V, and then reduced to Cd and Bi subsequently by DPV. reduced to Cd and Bi subsequently by DPV.

Figure and modification modification process. process. Figure 1. 1. Fabrication Fabrication and

2.3. 2.3.Sample SamplePreparation Preparation Soil paddy field field in in Beijing, Beijing, China, China,and andpre-treated pre-treated Soilsamples samples were were collected collected from from an an agriculture paddy ◦ C for 30 min in an oven to remove with withthe thefollowing followingsteps. steps. Briefly, Briefly,soil soil samples samples were heated at 200 °C for 30 min in an oven to remove the and mortar, mortar, and and sieved sievedby byaa200 200μm µmsieve. sieve.44ggsoil soil thewater. water.The Thedried driedsoil soil was was ground ground using pestle and sample and extracted extractedCd(II) Cd(II)by byultrasonic ultrasonicprocessing processing samplewas wasadded addedto to40 40 mL mL of of 0.1 0.1 M hydrochloric acid and for for60 60min minatatroom roomtemperature. temperature.Finally, Finally,the thesupernatant supernatantwas wasfiltered filteredby bya amembrane membraneand andadjusted adjustedto pH 5.0 using H3 PO and NaH PO solution. to pH 5.0 using H3PO 4 and NaH 2 PO 4 solution. 4 2 4 2.4. 2.4.Electrochemical ElectrochemicalMeasurement Measurement Procedure Procedure Differential applied to to the the detection detection of of Cd(II) Cd(II) under underoptimized optimized Differential pulse pulse voltammetry voltammetry (DPV) was applied conditions. Cd(II) was performed in 0.2 mol/L phosphate buffer solution in the presence of 400 g/L g/L conditions. Cd(II) was performed in 0.2 mol/L phosphate buffer solution in the presence of 400 Bi(III). the surface surface of ofthe theworking workingelectrode electrodeatatthe the Bi(III).Under Understirring stirringconditions, conditions, Cd(II) Cd(II) was deposited on the potential 1.2 V for 240 s. After 20 s equilibration equilibration period, period, DPV DPV was was carried carried out out from from −1.2 −1.2VVtoto00VV potentialofof−−1.2 with amplitude, 25 25mV; mV;pulse pulsewidth, width,0.2 0.2s;s;sample samplewidth, width, withoptimized optimizedparameters parameters (increase E, 0.01 V; amplitude, 0.02 0.02s;s;pulse pulseperiod, period,0.5 0.5s). s). 3.3.Results Results 3.1. 3.1.Characteristics Characteristics The of the the modified modifiedscreen-printed screen-printedelectrode electrode Themorphological morphological and and electrochemical electrochemical characteristics characteristics of were wereinvestigated investigatedby bysome someeffective effectiveapproaches. approaches. 3−/4− and Figure 2 shows the CV responses 5 mM [Fe(CN) 6 ] 6]3−/4− and Figure 2 shows the CV responsesofofdifferent differentelectrodes electrodesinina amixture mixtureofof 5 mM [Fe(CN) 0.1 (curve a) a) hadhad a pair of weak oxidation peakspeaks with with a peak peak of 380 mV. 0.1MMKCl. KCl.SPE SPE (curve a pair of weak oxidation a to peak toseparation peak separation of This indicated the electron transfer rate of SPE was sluggish due to the poor conductivity of commercial 380 mV. This indicated the electron transfer rate of SPE was sluggish due to the poor conductivity of ink. While onink. ILSPE (curve the redox currents in view of IL. After modifying Fe3 O4 commercial While on b), ILSPE (curvepeak b), the redoxenhanced peak currents enhanced in view of IL. After nanoparticle c), the peak currents considerably, that nanomaterial modifying Fe(curve 3O4 nanoparticle (curve c), theincreased peak currents increased demonstrating considerably, demonstrating that membrane covered on SPE can improve transfer [27].transfer One reason for these results may nanomaterial membrane covered on SPE the can charge improve the charge [27]. One reason for these be the high specific surface area and excellent conductivity of Fe O nanoparticle. Another reason results may be the high specific surface area and excellent conductivity of Fe3O4 nanoparticle. 3 4 that the Fereason redox substances including ferrousincluding and ferricferrous iron. Well-defined Another that the Feprovides 3O4 nanoparticle provides redox substances and ferric 3 O4 nanoparticle

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reduction peaks were observed at the Fe3O4/ILSPE, whereas the peak potential 4 of 13 separation reduced to 120 mV and the redox currents were considerably higher than those at the Sensors 2018, 18, 6 4 of 13 SPE, Fe3O4/SPE, and ILSPE due to the synergistic effects from ion liquid and Fe 3O4 nanoparticles. reduction peaks were observed the Fe /ILSPE, whereas peak potential separation reduced to iron. Well-defined reduction at peaks were at the Fe3the O4/ILSPE, whereas the peak potential 3 O4observed separation to 120 mV and considerably the redox currents were considerably higher at theand SPE,ILSPE 120 mV and thereduced redox currents were higher than those at the SPE,than Fe3those O4 /SPE, Fe 3 O 4 /SPE, and ILSPE due to the synergistic effects from ion liquid and Fe 3 O 4 nanoparticles. due to the synergistic effects from ion liquid and Fe3 O4 nanoparticles.

Figure 2. Cyclic voltammograms of different electrodes in 5 mM/L [Fe(CN)6]3−/4− and 0.1 mol/L KCl, 3−/4 − mol/L Figure 2. Cyclic voltammogramsofofdifferent different electrodes [Fe(CN) 6]3−/4− and 0.1 KCl, Figure 2.(b)Fe Cyclic voltammograms electrodesinin5 5mM/L mM/L [Fe(CN) and 0.1 mol/L 6] (a) SPE, 3O4/SPE, (c)ILSPE, (d)Fe3O4/ILSPE. Scan rate: 50 mV/s. (a) SPE, 3O4O /SPE, (c)ILSPE, (d)Fe3O 4/ILSPE. Scan rate: Scan 50 mV/s. KCl, (a) SPE,(b)Fe (b) Fe /SPE, (c) ILSPE, (d) Fe O /ILSPE. rate: 50 mV/s. 3 4 3 4

Figure 3 shows the electron transfer kinetics of different electrodes were investigated by using Figure 3 shows the electron transfer kinetics of different electrodes were investigated by using EIS measurement. model, the value value electron-transfer resistance (RctEIS ) is Figure 3 showsAccording theAccording electrontotransfer kinetics of different electrodes were investigated by(Rusing EIS measurement. tothe theRandle Randle model, the ofofelectron-transfer resistance ct) is equalequal to the membrane resistance coupled theofcontact contact resistance. As shown in measurement. According to the Randle model, thewith value electron-transfer resistance is equal to ct ) Figure to bulk the bulk membrane resistance coupled with the resistance. As shown in(R Figure 3b, 3b, ct ofR SPE waswas gotresistance as 18 KΩ, indicating bare hadpoor poor conductivity that went against electronthe R bulk membrane coupled with theSPE contact resistance. As shown in Figure 3b, the Rct of the ct of SPE got as 18 KΩ, indicating bare SPE had conductivity that went against electrontransfer. When SPE modified OPFP andpoor nanoparticle respectively, the the electron transfer transfer. When SPE waswas modified bybyOPFP and Fe3O O44conductivity nanoparticle respectively, electron transfer SPE was got as 18 KΩ, indicating bare SPE had that went against electron-transfer. rate was improved, and impedance was decreased. The values of R ct are 1025 Ω and 250 Ω forΩwas rate was and by impedance The respectively, values of Rctthe areelectron 1025 Ωtransfer and 250 for When SPEimproved, was modified OPFP andwas Fe3 Odecreased. rate 4 nanoparticle Fe 3O4/SPE and ILSPE. After SPE modified OPFP and Fe3O4 nanoparticle at the same time, an Fe3O4/SPE and impedance ILSPE. After SPE modifiedThe OPFP andofFe 4 nanoparticle same an improved, was decreased. values R3ctOare 1025 Ω and at 250the Ω for Fe3time, O4 /SPE obviously reduced impedance with the R ct of 100 Ω was observed due to the synergistic effect of obviously impedance with the ct 3of Ω was observed due time, to theansynergistic of and ILSPE.reduced After SPE modified OPFP andRFe O4100 nanoparticle at the same obviously effect reduced OPFP and Fe3O4 nanoparticles. These results are in agreement with the conclusion obtained from the OPFP and Fewith 3O4 nanoparticles. results are indue agreement with the conclusion from impedance the Rct of 100These Ω was observed to the synergistic effect of obtained OPFP and Fe3the O4 CV. CV. nanoparticles. These results are in agreement with the conclusion obtained from the CV.

Figure 3. Electrochemical impedance spectra of different electrode in 5.0 mmol/L K4[Fe(CN)6] + 0.1 mol/L KCl solution with the frequencies from 100,000 to 1 Hz, Ω (a) SPE, (b)Fe3O4/SPE, (c) ILSPE, (d) Fe3O4/ILSPE.

Figure 3. 3. Electrochemical Electrochemicalimpedance impedance spectra of different electrode 5.0 mmol/L K4[Fe(CN) 6] + 0.1 Figure spectra of different electrode in 5.0 in mmol/L K4[Fe(CN) 6 ] + 0.1 mol/L mol/L KCl solution with the frequencies from 100,000 to 1 Hz, Ω (a) SPE, (b)Fe 3 O 4 /SPE, (c) ILSPE, (d) KCl solution with the frequencies from 100,000 to 1 Hz, Ω (a) SPE, (b) Fe3O4/SPE, (c) ILSPE, (d) Fe3O4/ILSPE. Fe3O4/ILSPE.

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The hydrogen evolution on the electrode surface diminishes the accuracy of the stripping Sensors 2018, 18, 6 5 of 13 analysis significantly. The linear sweep voltammetry (LSV) responses of different electrodes in The hydrogen evolution on the electrode surface diminishes the accuracy of the stripping analysis 0.2 mol/LThe phosphate buffer solution (pHelectrode 5.0) weresurface showndiminishes in Figure the 4, respectively. The Fe3O4/ILSPE evolution on the accuracy of the significantly. hydrogen The linear sweep voltammetry (LSV) responses of different electrodes stripping in 0.2 mol/L analysis significantly. The linear sweep voltammetry (LSV) responses of different electrodes in V). without bismuth film exhibited a relatively positive hydrogen overvoltage potential (about −1.1 phosphate buffer solution (pH 5.0) were shown in Figure 4, respectively. The Fe3 O4 /ILSPE without 0.2 mol/L phosphate buffer solution (pH 5.0) were shown in Figure 4, respectively. The Fe 3 O 4 /ILSPE While on Bi/Fe3O4/ILSPE, a more negative hydrogen evolution potential (about −1.2 V) due to the bismuth filmbismuth exhibited a relatively positive hydrogen overvoltage potential (about(about −1.1 V). While on without film exhibited a relatively positive hydrogenelectrodes overvoltageare potential V). formation of bismuth film. It is reported that the bismuth less prone to−1.1 hydrogen Bi/FeWhile O /ILSPE, a more negative hydrogen evolution potential (about − 1.2 V) due to the formation 3 4 4/ILSPE, a more negative hydrogen evolution potential (about −1.2 V) due to the evolution, on dueBi/Fe to 3O the unique crystal plane structure of bismuth. This indicated that modified of bismuth film.ofItbismuth is reported bismuth are electrodes less proneare to hydrogen due to formation film.that It isthe reported thatelectrodes the bismuth less prone evolution, to hydrogen electrode has a sufficient potential window for the stripping analysis of Cd(II). the unique crystal of bismuth. indicated that modified electrode a sufficient evolution, dueplane to thestructure unique crystal plane This structure of bismuth. This indicated thathas modified electrode has afor sufficient potentialanalysis windowof forCd(II). the stripping analysis of Cd(II). potential window the stripping

10 μA 10 μA

a

Current(μA)

Current(μA)

a

b

b

/ILSPE 4 FeFe O3O/ILSPE 3 4

Bi/FeO3O /ILSPE Bi/Fe /ILSPE 3 4 4

-1.6 -1.6

-1.4 -1.4

-1.2 -1.2

-1.0 -1.0

-0.8 -0.8

-0.6 -0.6

-0.4-0.4

Potential (V) Potential (V) vs.Ag/AgCl vs.Ag/AgCl Figure 4. Linear sweep voltammograms in0.2 0.2 mol/L phosphate phosphate buffer solution (a) Bi/Fe 3O4/ILSPE, Figure 4. Figure 4. Linear Linear sweep sweep voltammograms voltammogramsin in 0.2mol/L mol/L phosphatebuffer buffersolution solution(a) (a)Bi/Fe Bi/Fe33O O44/ILSPE, /ILSPE, (b) Fe 3O4/ILSPE. (b) Fe O /ILSPE. 3 4 (b) Fe3O4/ILSPE.

Scanning electron microscope (SEM) was used to research the surface morphological structures

morphological structures Scanning electron microscope used to research surface morphological structures of prepared electrodes. As seen in(SEM) Figurewas 5a, the surface of SPE the shown a disordered distribution of Asblocks, seen in in Figure shown a disordered of prepared electrodes. As seen Figure 5a, the surface ofthe SPE gaps between graphite which affirmatively affected conductivity of electrode.distribution While on of ILSPE,graphite the surface was compact and homogeneous (Figure 5b), indicating of theelectrode. binder OPFP between blocks, which affected thethe conductivity While on the gaps the between graphite blocks, whichaffirmatively affirmatively affected conductivity ofsolid electrode. While on hasthe excellent adhesiveness toand graphite particles in(Figure the(Figure view theindicating low melting of OPFP. ILSPE, surface waswas compact homogeneous 5b),of5b), indicating the solid binder OPFP has the ILSPE, the surface compact and homogeneous thepoint solid binder OPFP Figure 5c adhesiveness exhibited to thegraphite typical SEM image 3O4/ILSPE; it was observed that some ball-like excellent adhesiveness particles inof theFein view the low melting point of OPFP. Figure 5c has excellent to graphite particles theofview of the low melting point of OPFP. structures covering the ILSPE’s surface, showing between structures 3O4 exhibited typicalthe SEM imageSEM of Fe itgood was interfacial observed that some that ball-like Figure 5c the exhibited typical image of Fe3O 4/ILSPE; it was interactions observed someFeball-like 3 O4 /ILSPE; nanoparticles and ILSPE’s surface. covering the ILSPE’s surface, showing good showing interfacialgood interactions between Fe3 O4 nanoparticles structures covering the ILSPE’s surface, interfacial interactions between Feand 3O4 ILSPE’s surface. nanoparticles and ILSPE’s surface. (a) (a)

Figure 5. Cont.

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(b)

(c) (c)

Figure 5. SEM images of (a) SPE, (b) ILSPE and (c) Fe3O4/ILSPE. Figure 5. SEM images of (a) SPE, (b) ILSPE and (c) Fe3 O4 /ILSPE. Figure SPE, (b) ILSPE and (c)are Fe3Oshown 4/ILSPE. The DPV responses of 405.µSEM g/L images Cd(II) of on(a)different electrodes in Figure 6 to illustrate

DPV responses of 40 µg/L Cd(II) on current differentwas electrodes are on shown Figure 6 to illustrate theThe sensitive improvement. A little peak observed the in SPE. After using Fe3O4the The DPV responses of 40 µ g/L Cd(II) was on different electrodes are shown in Figure 6Otonanoparticles, illustrate sensitive improvement. A little peak current observed on the SPE. After using Fe 3 4 The main nanoparticles, Fe3O4/SPE exhibited higher stripping response toward Cd(II) determination. sensitive improvement. A little peak current was observed on the SPE. After using Fe3O4 Fe3reason O4the /SPE exhibited stripping responseoftoward Cd(II) The main might might be thathigher the superior properties Fe3O4 such asdetermination. huge specific surface areareason and good nanoparticles, Fe3O4/SPE exhibited higher stripping response toward Cd(II) determination. The main be conductivity that the superior properties of Fe3 O4transfer such asbarrier, huge specific surface area and good conductivity decreased the electron and accelerated the rate of metal ions reason might be that the superior properties of Fe3O4 such as huge specific surface area and good decreased the electron transfer barrier, and accelerated rate the of metal ions preconcentration on the preconcentration on the electrode surface. While on thethe ILSPE, striping currents Cd(II) ions is also conductivity decreased the electron transfer barrier, and accelerated the rate ofof metal electrode surface. on the ILSPE,surface. the striping of Cd(II) is also enhanced in thestripping view enhanced in the While view the excellent conductivity and adhesion of ionic liquid. The of highest preconcentration onofthe electrode While currents on the ILSPE, the striping currents Cd(II) is alsoof the peak was observed on the Fe 3 O 4 /ILSPE, which might be attributed to IL and Fe 3 O 4 composite film excellent conductivity and adhesion of ionic liquid. The highest stripping peak was observed on the enhanced in the view of the excellent conductivity and adhesion of ionic liquid. The highest stripping modification. This composite membrane not only enhanced the conductivity of the electrode but also Fe3 O4peak /ILSPE, might be Fe attributed towhich IL andmight Fe3 Obe film This composite was which observed on the 3O4/ILSPE, attributed to IL modification. and Fe3O4 composite film 4 composite offered abundant anchor sitesthe for the detection ofthe Cd(II). modification. This composite membrane not only enhanced the conductivity of theabundant electrode but also sites membrane not only enhanced conductivity of electrode but also offered anchor offered abundant anchor sites for the detection of Cd(II). for the detection of Cd(II).

Figure DPV responses µµg/L Cd(II) and 400 µµ g/L Bi(III) inin0.2 0.2 mol/L phosphate buffer solution Figure 6. DPV responses of4040 g/LCd(II) Cd(II)and and400 400 µg/L g/L Bi(III) mol/L phosphate buffer solution Figure 6. 6. DPV responses ofof40 µg/L Bi(III)in 0.2 mol/L phosphate buffer solution (pH 5.0)5.0) onon thethe(a)(a)SPE, (b) FeFe3O 4/SPE, (c) ILSPE, and (d) Fe Fe33OO4/ILSPE. 4/ILSPE.Deposition Depositiontime: time:240240 (pH SPE, (b) 3 O 4 /SPE, (c) ILSPE, and (d) s. s. (pH 5.0) on the (a) SPE, (b) Fe3 O4 /SPE, (c) ILSPE, and (d) Fe3 O4 /ILSPE. Deposition time: 240 s. Deposition Deposition potential: −1.2 V.V. Deposition potential: −1.2 potential: −1.2 V.

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3.2. Optimization of Experimental Parameters 3.2. Optimization Optimization of of Experimental Experimental Parameters Parameters 3.2. The detecting parameters were optimized in 0.2 M phosphate buffer solution containing 35 µ g/L The detecting detecting parameters parameters were wereoptimized optimizedin in0.2 0.2M M phosphate phosphatebuffer buffersolution solutioncontaining containing35 35µg/L µ g/L The of Cd(II). The relationship between the peak current of Bi/Fe 3O4/ILSPE and deposition potential is of Cd(II). Cd(II). The The relationship relationship between the the peak peak current current of of Bi/Fe Bi/Fe 33OO4/ILSPE and deposition potential is of potential described in Figure 7. Withbetween the deposition potential shifting from4 /ILSPE −0.8 V toand −1.2deposition V, the peak current described in Figure 7. With the deposition potential shifting from −0.8 V to −1.2 V, the peak current is described in Figure 7.which Withthe themaximum depositionvalue potential shifting from −0.8 V to may −1.2be V, the thelower peak increased significantly, was achieved at −1.2 V. This increased significantly, which the maximum value was achieved at −1.2 at V.−This may bemay the be lower current increased significantly, which the maximum value was achieved 1.2 V. This potential can offer more energy to improve the efficiency of accumulation. However, when the the potential can offer moremore energy to improve the efficiency ofofaccumulation. However, when the lower potential can offer to improve accumulation. However, whenthis the deposition potential is rangingenergy from −1.2 V to −1.6the V,efficiency the peak currents decreased. We attribute deposition potential is ranging from −1.2 V to −1.6 V, the peak currents decreased. We attribute this deposition potential ispotential ranging from −1.2 V to −1.6 the peak currents We attribute this to the electrochemical stability window andV,interference from Hdecreased. 2 evolution on the deposited to the the electrochemical electrochemical potential potential stability stability window windowand and interference interferencefrom fromH H22 evolution evolution on on the the deposited deposited to Bi/Fe3O4/ILSPE surface. Thus, a deposition potential of −1.2 V was determined as optimum and Bi/Fe33OO4/ILSPE surface. Thus, optimum and and Bi/Fe surface. Thus,a adeposition depositionpotential potentialof of −1.2 −1.2VV was was determined determined as as optimum 4 /ILSPE employed in further experiments. employed in further experiments. employed in further experiments.

Figure 7. 7. Effects of of deposition potential potential on the the stripping peaks peaks currentof of 35µg/L µ g/L Cd(II). Figure Figure 7. Effects Effects of deposition deposition potential on on the stripping stripping peakscurrent current of35 35 µ g/L Cd(II). Cd(II).

The peak current of Bi/Fe3O4/ILSPE changed along with deposition time is shown in Figure 8. The peak peak current current of ofBi/Fe Bi/Fe33O O44/ILSPE /ILSPEchanged changedalong alongwith withdeposition depositiontime timeisis shown shown in in Figure Figure 8. The peak current increased proportionally with the prolonging of deposition time from 20 to 240 s. The peak peak current current increased proportionally with the prolonging of deposition The deposition time time from from 20 20 to to 240 240 s. s. While over 240 s, the stripping current of the peak is not any more linear with deposition time, which Whileover over240 240s,s,the thestripping strippingcurrent currentofofthe thepeak peakisisnot notany anymore morelinear linear with deposition time, which While with deposition time, which is is increasing slowly. In view both of sensitivity and determination time, a deposition time of 240 s is increasing slowly. In view both of sensitivity determination time, a deposition of s240 increasing slowly. In view both of sensitivity andand determination time, a deposition timetime of 240 wass was selected for further studies. was selected for further studies. selected for further studies.

Figure Cd(II). Figure 8. 8. Effects Effects of of deposition deposition time time on on the the stripping stripping peaks peaks current current of of 35 35 µg/L µ g/L Cd(II). Figure 8. Effects of deposition time on the stripping peaks current of 35 µ g/L Cd(II).

Figure 9 shows the stripping response of Cd(II) on the Bi/Fe 3O4/ILSPE affected with changing Figure 9 shows the stripping response of Cd(II) on the Bi/Fe 3O4/ILSPE affected with changing pH value. The peak current increased gradually with decreasing pH from 2.0 to 5.0, while the pH value. The peak current increased gradually with decreasing pH from 2.0 to 5.0, while the

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the stripping response of Cd(II) on the Bi/Fe3 O4 /ILSPE affected with changing 8 of 13 pH value. The peak current increased gradually with decreasing pH from 2.0 to 5.0, while the stripping response disappeared completely completely at high pH values. peak current was found at pHwas 5.0. stripping response disappeared at highThe pHmaximum values. The maximum peak current At excessively circumstances, hydrogen evolution will damage quality of the found at pH 5.0.acidic At excessively acidic enhanced circumstances, enhanced hydrogen evolutionthe will damage Bi(III) film. If Bi(III) the pHfilm. is much Bi(III)higher, ions might susceptible hydrolysistoinhydrolysis neutral and quality of the If thehigher, pH is much Bi(III)beions might beto susceptible in alkalineand media. Thus,media. a pH of 5.0 was be suitable neutral alkaline Thus, a pHfound of 5.0towas found tofor be experiments. suitable for experiments.

of pH pH value value on on the the stripping stripping peaks peaks current current of of 35 35 µg/L Figure 9. 9. Effects of µ g/L Cd(II). Figure Cd(II).

Figure 10 shows the stripping response of Cd(II) on the Bi/Fe 33O44/ILSPE affected by the Bi(III) Figure 10 shows the stripping response of Cd(II) on the Bi/Fe3 O4 /ILSPE affected by the Bi(III) concentration. The peak current of Cd(II) was positively correlated with different levels of Bi(III) from concentration. The peak current of Cd(II) was positively correlated with different levels of Bi(III) from 100 to 400 µ g/L and negatively correlated when the level of Bi(III) exceeded 400 µ g/L. This may be 100 to 400 µg/L and negatively correlated when the level of Bi(III) exceeded 400 µg/L. This may be attributed to the formation of thick bismuth layer on the electrode surface, which is not favorable for attributed to the formation of thick bismuth layer on the electrode surface, which is not favorable for Cd(II) diffusing out. Consequently, 400 µ g/L was selected as the optimal Bi(III) concentration. Cd(II) diffusing out. Consequently, 400 µg/L was selected as the optimal Bi(III) concentration.

Figure Effects of Bi(III) concentration Cd(II). Figure 10. 10. Effects of Bi(III) concentration on on the the stripping stripping peaks peaks current current of of 35 35 µg/L µ g/L Cd(II).

The sensitivity of DPV is much much higher higher than than cyclic cyclic voltammograms. voltammograms. Figure Figure 11 11 shows shows DPV responses of Bi/Fe Bi/Fe333OO44/ILSPE for fordifferent differentCd(II) Cd(II)concentrations concentrationsinin0.2 0.2M Mphosphate phosphatebuffer buffer(pH (pH5.0). 5.0). 4 /ILSPE The deposition potential was kept at − −1.2 V for the stripping analyses. Figure 11a shows different 1.2 V for Figure 11a sharp stripping curve inin Figure 11a, a stripping peaks peaks on onthe themodified modifiedelectrode. electrode.As Asseen seenfrom fromthe thecalibration calibration curve Figure 11a, linear a linearrelationship relationshipwas wasobtained obtainedbetween betweenthe thepeak peakcurrent currentand andCd(II) Cd(II)concentration concentration in in the the range range from from 0.5 µ g/L to 40 µ g/L with the detection limit as 0.05 µ g/L (S/N = 3). The linear regression equation was I(µ A) = 0.810 × C(µ g/L) + 0.028 (r = 0.996), where I is the peak current and C is the concentration of Cd(II).

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0.5 µg/L to 40 µg/L with the detection limit as 0.05 µg/L (S/N = 3). The linear regression equation was I(µA) = 0.810 × C(µg/L) + 0.028 (r = 0.996), where I is the peak current and C is the concentration Sensors 2018, 18, 6 9 of 13 of Cd(II).

Figure (a) Differential Differential pulse for additions additions of of 0, 0, 0.5, 0.5, 1, 1, 5, 5, 10, 10, 15, 15, Figure 11. 11. (a) pulse voltammograms voltammogramsof ofBi/Fe3O4/ILSPE Bi/Fe3O4/ILSPE for 20, 25, 30, 35, 40 g/L Cd(II); (b) The right part shows the calibration curves. Error bar: n = 3. 20, 25, 30, 35, 40 g/L Cd(II); (b) The right part shows the calibration curves. Error bar: n = 3.

There differences in sensitivity amongamong Bi/Fe3 O which which will influence the accuracy Thereare aresome some differences in sensitivity Bi/Fe 3O4/ILSPEs, will influence the 4 /ILSPEs, of the results. conquer deficiencies, the internal normalization (ISN) was used to accuracy of theTo results. To these conquer these deficiencies, the standard internal standard normalization (ISN) was used to calibrate linear regression thiswe method, can get a new linear calibrate the linearthe regression equations.equations. FollowingFollowing this method, can getwe a new linear regression equation asequation N = 0.907as×NC(µg/L) 0.032(r N iswhere the ratio ofthe 10 × I/I0of , I010is×the regression = 0.907 + × C(µ g/L)=+0.996), 0.032(rwhere = 0.996), N is ratio I/I0response , I0 is the response 10 µCg/L, and C is the concentration Cd(II). To demonstrate the improvement current of current 10 µg/L,ofand is the concentration of Cd(II). Toof demonstrate the improvement of measuring accuracy usingaccuracy this newusing method, selected three to detect the different concentration of of measuring thiswe new method, weelectrodes selected three electrodes to detect the different concentration of Cd(II) obtained theofdetection value I0 0.87 (E1: µA; 0.930.98 µ A;µA). E2: 0.87 µ A; 0.98 A). As Cd(II) and obtained the and detection value I0 (E1: 0.93 µA;ofE2: As shown inµTable 1, shown in Table the method results using ISN method more therelative real value, error the results using1,ISN are more closed toare the real closed value, to and errorand arerelative much lower. are much lower. Meanwhile, the root-mean-square errorsand of linear method and0.25, ISN respectively. are 0.36 and 0.25, Meanwhile, the root-mean-square errors of linear method ISN are 0.36 and This respectively. indicates thatcan theconquer ISN method conquer parts of difference between electrodes indicates thatThis the ISN method partscan of difference between electrodes and improve the and improve the measuring precision.error root-mean-square is lower(0.36). than linear method measuring precision. root-mean-square (0.257) is lowererror than (0.257) linear method (0.36). Table 1. Determination results of different Cd(II) concentration. Cd(II) Cd(II) Concentration Concentration (µg/L) (µg/L)

2 2

5

Table 1. Determination results of different Cd(II) concentration. Linear Method ISN Method Linear Method ISN Method Current Electrode Detection Detection Electrode Current Detection Relative Detection Relative Relative (µA) Number Relative Concentration Concentration Number (µA) Concentration Error Concentration Error (%) Error Error(%) (%) (µg/L) (µg/L) (µg/L) (%) (µg/L) E1 E1 E2 E3 E2

± 1.781.78 ± 0.05 0.05 1.63 ± 0.03 ± 1.731.63 ± 0.05

1.93 0.05 1.93 ± ± 0.05 1.76 ± 0.03 1.87 0.05 1.76 ± ± 0.03

3.52 3.52 11.89 6.49 11.89

2.09 0.06 2.09± ± 0.06 2.03 ± 0.04 1.92 0.06 2.03± ± 0.04

4.48 4.48 1.65 3.92 1.65

E1 E2 E3 E3

4.331.73 ± 0.19 ± 4.220.05 ± 0.09 4.474.33 ± 0.12 ±

4.73 ± 0.22 1.87 ± ± 0.05 4.61 0.10 4.89 ± 0.14

6.49 7.74

5.32

2.23

5.13 ± 0.26 1.92± ± 0.06 5.34 0.12 4.99 ± 0.16

2.60 3.92 6.80 0.24

E1 E2 E2 E3

8.720.19 ± 0.29 ± 8.534.22 ± 0.19 0.09 9.73 ± 0.37 4.47 ± 0.12 8.72 ± 0.29 8.53 ± 0.19 9.73 ± 0.37

9.57 ± 0.31 9.25 0.21 4.61 ± ± 0.10 10.68±0.40

3.00 6.40 6.80 2.20

E1

10 5

E3 E1 10

E2 E3

0.03

4.73 ± 0.22

5.32

4.30 7.49 7.74 6.83

10.30 ± 0.37 10.64 ±0.12 0.27 5.34 ± 9.78 ± 0.46

5.13 ± 0.26

2.60

4.89 ± 0.14

2.23

4.99 ± 0.16

0.24

9.57 ± 0.31

4.30

10.30 ± 0.37

3.00

9.25 ± 0.21

7.49

10.64 ± 0.27

6.40

10.68±0.40

6.83

9.78 ± 0.46

2.20

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Compared with some published electrodes for Cd(II) detection [28–31], some properties are Compared with some published electrodes for Cd(II) detection [28–31], some properties are summarized in Table 2. The Bi/Fe3O4/ILSPE has a relatively wide linear detection range and low summarized in Table 2. The Bi/Fe3 O4 /ILSPE has a relatively wide linear detection range and low detection limit. Otherwise, the accumulation time is much shorter than others, which is far more detection limit. Otherwise, the accumulation time is much shorter than others, which is far more suitable for rapid detection. suitable for rapid detection. Table 2. Comparison with other electrodes for Cd(II) detection. Table 2. Comparison with other electrodes for Cd(II) detection. Modifier

Accumulation Time (s)

Modifier Bi/SG/SPE

Accumulation Time (s) 240

G/PANI/PS/SPE Bi/SG/SPE G/PANI/PS/SPE SbSPCE SbSPCE ILGPE ILGPE Bi/Fe 4/ILSPE Bi/Fe3 O34O /ILSPE

240180 180120 120 300 300 240240

Concentration Range (µg/L) Concentration Range (µg/L) 0.95–16.9

10–500 0.95–16.9 10–500 11.5–72.4 11.5–72.4 0.1–3.2 0.1–3.2 0.5–40 0.5–40

Detection Limit (µg/L) Detection Limit 1.4 (µg/L) 1.44.43 4.433.4 3.4 0.01 0.01 0.05 0.05

Reference Reference [28]

[29] [28] [29] [30] [30] [31] [31] The present The present

3.3. Interference Interference Effects Effects 3.3. The interference interference effected effected from from various various common-existed common-existed cations cations and and anions anions in in soil soil were were aa vital vital The property for the electrochemical electrode in Figure 12. The effects of different interferences on the property for the electrochemical electrode in Figure 12. The effects of different interferences on the Bi/Fe 3O4/ILSPE were estimated by DPV in phosphate buffer solution (pH 5.0) containing 40 µ g/L of Bi/Fe3 O4 /ILSPE were estimated by DPV in phosphate buffer solution (pH 5.0) containing 40 µg/L of Cd(II) in in the the absence absence and and presence of interfering and comparison Cd(II) presence aa specified specified concentration concentration of interfering substances substances and comparison of the peak currents. Under the optimized conditions, it was found that 50-fold concentrations of of the peak currents. Under the optimized conditions, it was found that 50-fold concentrations of Na(I), Na(I), Hg(II), Ca(II), Mg(II), Fe(III), Al(III), Ni(II), Cu(II), Pb(II), Zn(II) Cl(I), NO 3(I), did not interfere Hg(II), Ca(II), Mg(II), Fe(III), Al(III), Ni(II), Cu(II), Pb(II), Zn(II) Cl(I), NO3 (I), did not interfere with with Cd(II) determination (the peak current change < 10%). Cd(II) determination (the peak current change < 10%).

Cd(II) concentration (μg/L)

50

40

30

20

10

0 Blank Na(I) Hg(II) Ca(II) Mg(II) Fe(III) Al(III) Ni(II) Cu(II) Pb(II) Zn(II) Cl(I) NO3(I)

Different ions Figure The 40 40 µ µg/L ofCd(II) Cd(II)before beforeand andafter afteradding adding 50-fold 50-fold concentrations concentrations of of Na(I), Na(I), Hg(II), Hg(II), Ca(II), Ca(II), Figure 12. 12. The g/L of Mg(II), Fe(III), Al(III), Ni(II), Cu(II), Pb(II), Zn(II) Cl(I), NO (I). The error bars represent standard Mg(II), Fe(III), Al(III), Ni(II), Cu(II), Pb(II), Zn(II) Cl(I), NO33(I). The error bars represent standard deviations deviations based based on on three three independent independent measurements. measurements.

3.4. 3.4. The Reproducibility The /ILSPEwas wasevaluated evaluatedusing usingthe the two two electrodes electrodes to determinate The reproducibility reproducibilityof ofthe theBi/Fe Bi/Fe33O44/ILSPE 10 10 µg/L µ g/L Cd(II). The electrodes were stored at ambient conditions, which was used to measure the Cd(II) Cd(II) every every seven seven days. days. The relative standard deviation (RSD) was 5.79% for Cd(II) Cd(II) in in three-time three-time measurements. original responses after itsits storage at measurements. The Theelectrode electrodelost lostless lessthan than5.29% 5.29%for forCd(II) Cd(II)ofofitsits original responses after storage ambient conditions forfor 14 14 days. at ambient conditions days.

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3.5. Application to Real Sample Analysis To evaluate the practical performance, Bi/Fe3 O4 /ILSPE was applied to detect Cd(II) in the soil samples. Each extract solution undergoes three parallel determinations in Table 3. Cd(II) in soil samples could be satisfactorily detected with the recovery in the range from 91.77 to 107.83%. For comparing the results of the suggested method with those of FAAS, the t-test was applied. It can be concluded that there is no significant difference between the results obtained by the two methods for p > 0.05. All results indicated that the present Bi/Fe3 O4 /ILSPE electrode could be used for the accurate detection of Cd(II) in soil samples. Table 3. Determination results of Cd(II) in soil samples. Sample

Added (µg/L)

Found (µg/L)

Found by FAAS (µg/L)

Recovery (%)

1

– 5 10 – 5 10

1.45 ± 0.49 6.88 ± 0.26 11.48 ± 0.45 2.85 ± 0.37 7.36 ± 0.39 12.66 ± 0.19

1.38 ± 0.11

– 107.83 100.87 – 91.77 97.23

2

3.02 ± 0.08

4. Conclusions In summary, a low-cost and simple method was used to prepare bismuth film/Fe3 O4 /IL composite modified screen-printed electrode, which was further investigated by CV, EIS, SEM, LSV, and DPV. Compared with Bi/SPE, Bi/IL/SPE, and Bi/Fe3 O4 /SPE, the electrode exhibited some good properties that include excel ent electrochemical activity and accelerated charge transfer kinetics, which owed to the presence of conductive Fe3 O4 /IL composite and the co-deposits ability with heavy metals of bismuth film. The electrode was used internal standard normalization (ISN) to conquer differences between electrodes, which was further confirmed by the real samples analysis with a good accuracy and satisfactory recovery results. Acknowledgments: This work was supported by General Program of National Natural Science Foundation of China (No. 31671578), National High Technology Research and Development Program of China (No. 2013AA102302), the Fundamental Research Funds for the Central Universities (No. 2016 XD001) and the Shandong Provincial Natural Science Foundation of China (No. ZR2015CM016). Author Contributions: Hui Wang and Gang Liu conceived and designed the experiments; Hui Wang and Guo Zhao performed the experiments; Hui Wang and Yuan Yin analyzed the data; Zhiqiang Wang contributed reagents/materials/analysis tools; Hui Wang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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