Construction of Electrochemical Sensor Modified with Molecularly Imprinted Polymer and rGO-Fe3O4-ZnO Nanocomposite for Determination of Bisphenol A in Polymers and Water Samples

BPA imprinted monolithic pre-column to online solid-phase extraction    for   HPLC.    They   determined    successfully phenolic  compounds  in  river  water  [7].  Haginaka  et  al. develop  a  new  method  by  the  combination   of  isotope imprinting and LC-mass spectrometry for the determination of BPA and its  halogenated  derivatives  in river water [8]. Park et al.  extract  BPA directly  and determine  it by gas chromatographic-mass  spectrometric method [9].

Spectrophotometric Method

Xu et al. applied a sensitive spectrophotometric method using a diazotization-coupling  reaction to the measurement of  BPA.  The  parameters   which  can   affect  the  signal such  as  reagent  concentration   and  pH  were  optimized. The  obtained  data  were  compared  with  HPLC,  and  the presented  method  was  applied  to  the  determination   of BPA in real samples include milk and water bottle samples [10]. Sun et al. present a new method base on fluorescence spectrophotometry     to    direct    measurement    of    BPA in  aqueous   solution   and   pH  =  8   at  λex    =   266   and λem    =  304   nm  [11].   Kum   et  al.   developed   a  rapid spectrophotometric     detection     method     for    BPA    in environments   base  on  the  blue  color  formation  of  the BPA/ferric chloride/ferrocyanide complex [12].

Although   chromatographic    methods   are   commonly used  for  the  determination  of some  molecules like  BPA, some   features   such   as   time-consuming   analysis,   costly instrumentations, complex sample pretreatment process and dependence on the operator for spiking of the sample, led to developing new methods for measuring this molecule [13]. In    recent   decades,   sensor-based    methods   have   been developed  for  the  determination   of  molecules  and  ions. Among  these  methods,  the  electrochemical  sensors  have better   advantages   such   as   low-cost   instruments,   easy operation, reliable data, high accuracy, fast response, simple real sample pretreatment  [14-17].  So, in this research, the new    electrochemical    sensor    was    designed    for    the determination of the analyte.

Nonspecific-binding,      low     selectivity     and     poor regeneration   are   the   problems   of  direct   measurement electrochemical methods [18]. There are some methods and materials  to  overcome  these  disadvantages.  Molecularly imprinted  polymers  (MIPs)  are  one  of the  best  artificial materials for preparing chemical sensors  methods  in  order

to increase  the  selectivity  and  specific-binding  [19].  The high  physical/chemical   and  mechanical  stability  such  as high pressure,  high temperature,  stability versus  acid/base or organic solvents, low-cost preparation,  supreme binding affinity, and economic production are the specific properties of MIPs [19,20]. The electrochemical sensor base on MIPs can  determine  a  wide  variety  of  materials  include  food additives, metals ions, microbial cells, and drugs [21-26].

Nowadays,  some  nanomaterials  such  as  carbon  base

NPs (e.g. graphene)  and metal oxide NPs (e.g. ZnO  and Fe3O4)  are  used  as  a  catalyst  for  improving  the  sensor performance   [27,28].   ZnO  NPs  as  an   environmentally friendly  semiconductor  (band-gap  ~  3.37  eV)  has  been applied  in  electrochemical  devices  because  of their  non- toxicity,  proper  sensing  behavior,  physical  and  chemical stability [29]. Fe3O4  NPs as a superparamagnetic  material, due  to  biocompatibility,   low  toxicity,  catalytic  activity, large surface areas, and simple preparation commonly used in different industries and especially in sensing applications in  order  to  provide  a  maximum  signal  [30].  Graphene nanosheet    (Gr)   as   a   zero    band-gap    semiconductor has  unique   properties   include   large   surface   area,  and high    electrical     conductivity     because     of    abundant electrochemically  desirable  edge carbons  per  mass  of Gr which  comfort  electron  transfer  between  analytes  and the surface of the sensor by a low over-potential [31]. So, they are good candidates  for use in electrochemistry.  In recent years,  metal oxide loading  or doping  on Gr was used to promote the catalytic property of NPs due to the synergetic effect between NPs into nanocomposites [32].

These unique properties have opened a new window of possibilities for developing new analytical methods. In this way,  an innovative  way to synthesize  novel  MIPs  is the incorporation   of  nanoparticles   into  their   structure.   The combination    of   these    two    materials    (polymer    and nanoparticles/nanocomposites)   gives   rise   to   a   hybrid material with potential and new properties. In general, MIPs are  classified  into  two  types  according  to  whether  they are  obtained  as  a  single  continuous   and  porous  piece (molecularly-imprinted  monoliths,  MIMs) or as  individual nano/microparticles     (MIP    micro/nanoparticles).     Some nanoparticles,  such  as  metal  oxide,  carbon  nanoparticles, or  molecular   sieves,  can  be  acted  as  main  monomers or scaffolds of the monolithic structure.  In  contrast,  in  the

synthesis  of MIP nanoparticles,  the role of  nanoparticles, generally, is to act as the core or support of the imprinted polymer film. On the other hand, nanoparticles can increase the surface of the sensor and also the electrochemical site of reaction, so the use of nanoparticles can improve the signal- to-noise ratio, the performance of the sensor, and sensitivity and   detection   limit  of  the  presented   electrode   to  the determination of BPA.

In this study, the use of the rGO-Fe3O4-ZnO@MIP  as a

sensing  layer,  which  was  mixed  with  the  carbon  paste matrix for the voltammetric determination of BPA in spiked and  real  samples  is  described.   Good   conductivity   and selectivity  of  the  proposed  sensor   on  BPA  were  well observed which lead to obtaining a wide linear range (LR) with a low detection limit (DL).

EXPERIMENTAL Chemicals

BPA,    methacrylic     acid,    GO,    ferrous     chloride tetrahydrate,      ferric     chloride     hexahydrate,      sodium hydroxide,   oleic   acid,   ethylene   glycol   dimethacrylate, polyvinylpyrrolidone,   azobisisobutyronitrile,   zinc  acetate dihydrate, graphite, paraffin oil, sodium borohydride, were provided    from    Sigma-Aldrich    and   Merck    Company (Darmstadt, Germany). All the solutions and materials used were of analytical grade. Deionized distilled water (DDW) was  used  for  the  preparation  of  solutions.  The  Britton- Robinson  buffer  solution  (B-R)  applied  to adjust  the  pH value and as a supporting electrolyte.

Apparatus

Electrochemical     experiments    were    performed    at ambient   temperature   (about  25  °C)  using   a   Behpajoh potentiostat/galvanostat   system  (model  BHP-2065,  Iran). The    electrochemical    cell    was    congregated    with    a conventional  three-electrode  system by an Ag/AgCl  (Azar electrode,  Iran),  platinum  wire  and  unmodified/modified carbon  paste  electrodes  (CPE)  as the reference  electrode, auxiliary  electrode  and  working  electrodes,  respectively. The pH value measurements  were done by a Metrohm pH meter  (model  713,  Herisau,  Switzerland).  The  structure and  morphology of the synthesized nanomaterials were investigated by scanning electron microscope  (SEM) SEM-

EDX, Philips, XL30 (Netherland), X-ray powder diffraction (XRD) 38066 Riva, d/G. Via M. Misone, 11/D (TN) Italy and   Fourier   transform   infrared   (FTIR)  Perkin   Elmer, spectrum 100 (USA).

Preparation of the Materials

Fe3O4   NPs. The coprecipitation  method  was used  for the  synthesis  of  Fe3O4NPs.  In  the  first  step,  0.01  and

0.02 mol ferrous  chloride  tetrahydrate  and  ferric  chloride

hexahydrate, respectively, were added to 100 ml DDW in a

250 ml three-necked  flask. The N2  was purged  while  the solution was stirred continuously,  and the temperature was increased  to  80  °C.  In  the  next  step,  sodium  hydroxide solution (40 ml, 2 M) was added to the heated solution and mixed for 1 h. The synthesized magnetic precipitates were collected by the external magnetic field when the solution temperature  reached  about  25  °C.  Finally,  the  obtained Fe3O4  NPs were washed by DDW five times and dried at

70 °C in the oven.

ZnO  NPs.  In  order  to  synthesis  ZnO  NPs,  1  g  zinc acetate dihydrate was added to 250 ml  three-necked  flask containing 100 ml ethanol solution. The mixture was stirred for 20 min then 0.25 M of sodium hydroxide in ethanol was added  slowly until the  pH reached  8.0. The mixture  was stirred for 4 h then filtered and washed by DDW five times and dried at 70 °C in the oven.

Reduced   graphene   oxide   (rGO).   The   rGO   was

synthesis  by  the  addition  of  sodium  borohydride   as   a chemical   reduction   reagent   to  purchased   GO.   Firstly,

150 mg purchased GO was dispersed into 150 ml DDW and sonicated  for 20 min after that 2 ml sodium  borohydride was added to the mixture and the temperature increased to

100   °C  and   kept  for  10  h.  The  obtained   rGO   was centrifuged, washed by DDW five times and dried at 80 °C in the oven.

rGO-Fe3O4     nanocomposite.   For   this   goal,   0.1   g

synthesized  rGO  was  added  to  50  ml  of  DDW:ethanol (1:1 V/V) and sonicated  for 25 min to obtain the uniform suspension.  After  that 0.4 g Fe3O4  NPs was added to the suspension  and  mixed  for  1  h.  The  temperature  of  the solution was increased to 105 °C and kept for 24 h in order to do a hydrothermal reaction. The obtained