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Surface characterization of dinitrophenyl-diaminophenyl nanoplatformon glassy carbon
Haslet Ekşi a, Vinod Kumar Gupta b,c,⁎, Zafer Üstündağ d, Necip Atar d,M. Oğuzhan Çağlayan e, Ali Osman Solak a,f
a Ankara University, Faculty of Science, Department of Chemistry, Ankara, Turkeyb Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Indiac Vice Chancellor, Dr. R M L Avadh University Faizabad, UP 224001, Indiad Dumlupınar University, Faculty of Arts and Science, Department of Chemistry, Kütahya, Turkeye Cumhuriyet University, Faculty of Engineering, Department of Chemical Eng., Sivas, Turkeyf Kyrgyz-Turk Manas University, Faculty of Eng., Dept. of Chem. Eng., Bishkek, Kyrgyzstan
⁎ Corresponding author at: Department of Chemistry,Roorkee, Roorkee 247667, India. Tel.: +91 5278 24622
2,4-dinitrophenol (DNP)was covalently bonded to glassy carbon substrate by cyclic voltammetry (CV) technique.2, 4-diaminophenol film (DAPO)was acquired with electrochemical reduction of 2,4-dinitrophenol film (DNPO).Prepared films were characterized by X-ray photoelectron spectroscopy (XPS) and germanium attenuated totalreflection infrared (GATR-FT-IR) spectroscopy. A hemispherical germanium ATR element used with p-polarizedlight at 65° incidence angle yielded high signal/noise IR spectra for monolayer coverage of molecules on glassycarbon substrate. Layer thickness offilmswasmeasuredusing ellipsometer. Nitro and amine groups involved sur-face were commented on mono–multi layer film thickness with modified cyclic number effects.
Modification of carbon and metal surface is an important researcharea in material science, electrochemistry, molecular electronics,nanotechnology and sensor applications [1–12]. Chemically modifiedsurface has unique properties that can be used in biosensors, lubricantsfor low-level electrical contacts and junctions for molecular electronics[13–15]. Among the possible substrates the carbon surfaces have specificfeatures such as compactness,mechanical stability, chemical resistance tocorrosion and low permeability to fluids and gases [16]. Due to these fea-tures, glassy carbon is commonly used as electrodes for electrochemicalstudies [17,18].
Glassy carbon surface can bemodifiedby various techniques includingelectrochemical oxidation, electrochemical reduction and other chemicaland physical modifications [19,20]. Electrochemical oxidation includesvarious methods such as electropolymerization [21], amine oxidation[22–24], alcohol oxidation [25], decarboxylation [26] and nucleophilicreaction [27]. He et al. studied quercetinmodified carbon substrate devel-oped by electrochemical oxidation methods. They reported that thehydroxyl groups on the anodized carbon surface behave as nucleophiles
Indian Institute of Technology3; fax: +91 5278 246330.
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leading to bond formation via nucleophilic attack [27]. Various studies in-clude applications of modified electrode such as molecular electronicsand sensor studies [28–30]. The last decade has seen a rise in the studyof determination of heavy metals and organic pollutants in waters onelectrochemical and various techniques [31–36].
The modified surface is characterized by electrochemical, spectro-scopic, microscopic (e.g. electron, probe) and optical methods. Cyclicvoltammetry (CV)— an electrochemical method, is an effective methodused in modified electrode surface characterization [37]. The methodreveals the differences between bare and modified electrodes byimplementing a redox probe (e.g. dopamine, ferrocene, ferricyanide,etc.). Another method is the electrochemical impedance spectroscopy(EIS) in which dielectric and transport properties of porous electrodeand passive surfaces are determined [38]. In addition to electrochemicalmethods, characterization can be achieved by spectroscopic methodse.g. electron spectroscopy for chemical analysis (ESCA), IR, Raman, elec-tron spin resonance (ESR), etc. The characterization technique is basedon examination of the elemental distribution of surface [18,39–42].Rahman et al. have characterized EDTA bonded conducting polymermodified electrode by ESCA. They reported the nitrogen of secondaryand primary amines and carbon of carbonyl as evidence for the aminesand carbonyl groups [43]. Other substantial spectroscopic characteriza-tion techniques are Infrared and Raman. These powerful techniqueshave the advantage of providing structural information about the
Fig. 1. Cyclic voltammogram for the modification of GC surface with 1 mM DNP in0.01 M HCl vs. Ag/AgCl/KClsat, scan rate is 200 mV s−1.
O
HO
NO2
NO2
+ 12 e-0.01 M HCl
O
HO
NH2
NH2
GC GCDNPO-GC DAPO-GC
Scheme 1. Electrochemical reduction of DNPO-GC Surface.
50 H. Ekşi et al. / Journal of Molecular Liquids 187 (2013) 49–53
nanofilm on the substrate. McCreery and his research group haveshown molecular orientation of azobenzene and biphenyl derivativeson carbon surface by Raman and ATR-FT-IR infrared spectroscopy[44–47].
Ultra thin films on substrate can be characterized by ellipsometrywhich is a powerful and sensitive technique. By an indirect method,one can get information on surface uniformity and thickness fromthe reflection properties (e.g. polarization and intensity) of incidencelight. According to its principle, the sensitivity of the ellipsometry forthickness measurement of thin films can be as low as 0.1 nm [48].
Present work represents the derivatization of 2,4-dinitrophenol(DNP) on the glassy carbon surface by electrochemical oxidation.2,4-Dinitrophenol was used as the grafting material because it waseasily bonded onto the carbon surface via electrochemical oxidation.Surface confined DNP was reduced to DAP which possesses a veryuseful functional junction including two amino groups for various stud-ies such as molecular electronics and sensor applications. 2-Hydroxy-3,5-dinitrophenoxy-glassy carbon (DNPO-GC) surface was character-ized by cyclic voltammetry, ESCA and GATR-FT-IR and ellipsometricmethods. DAPO-GC surface was synthesized by means of electrochem-ical reduction of DNPO-GC to obtain amino ligands which possess po-tential binding capacity for metals and further organic linkages suchas amidization. This surface was also characterized as in the case ofDNPO-GC. Thickness (i.e. monolayer–multi-layer alteration) of surfacefilm vs. cyclic number of CV was monitored by ellipsometry.
Fig. 2. Reducing voltammogram of DNPO film on GC surface in 0.01 M HCl by cyclicvoltammetry (CV) vs. Ag/AgCl/KClsat, scan rate is 200 mV s−1.
2. Materials and methods
2.1. Reagents and chemicals
2,4-Dinitrophenol (DNP, Merck, Germany) solutions were pre-pared at the concentration of 1 mM in 0.01 M HCl (Merck, Germany)solution. The solution was thoroughly deoxygenated by purgingwith high purity argon gas (99.999%) for 10 min before the electro-chemical measurements. Argon blanket was alwaysmaintained overthe solutions to supply an inert atmosphere during electrochemicalmeasurements. All cyclic voltammograms were recorded at a tem-perature of (25 ± 1) °C. Ultra pure water with a specific resistanceof 18.3 MΩ cm (Human Power 1+ purification system) was usedfor preparing all of the aqueous solutions, cleaning the glasswareand polishing the electrodes. All chemicals were of the highest puri-ty available from Merck, Fluka or Riedel chemical companies andwere used as received.
2.2. Electrodes and instrumentation
In the CV modification and reduction (electrochemical character-ization) studies, BAS MF-2012 glassy carbon electrodes were used asworking electrode. A traditional three-electrode cell system wasused in all electrochemical experiments. Ag/AgCl/KCl(sat) referenceelectrode was used in aqueous solutions. Pt wire was used as counterelectrode. Cyclic voltammetry technique was performed using BASCV-50W (Bioanalytical Systems, West Lafayette, IN, USA) and PCI4/300 model potentiostat/galvanostat (Gamry Instruments, PA, USA)equipped with C3 cell stand. For spectroscopic and ellipsometriccharacterizations Tokai GC-20 glassy carbon electrodes were usedas working electrode.
Fig. 3. ESCA (XPS) Spectra of N1s for nitro and amine involved DNPO-GC and DAPO-GCsurface.
Fig. 5. Fitting procedure to distinguish NH2 scissors and C_C stretching band range forDAPO film.
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2.3. Procedure
2.3.1. Preparation of glassy carbon surfacesAll GC electrodes were cleaned by polishing with fine wet emery
papers (Buehler with grain size of 4000) and then with 0.3 μm and0.05 μm alumina slurry made from dry Buehler alumina and HumanPower 1+ (Ultrapure) water (18.3 MΩ cm) on Buehler polishingmicrocloth to remove the oxide and other functionalities. PolishedGC electrodes were sonicated in ultra pure water twice and then oncewith amixture of 1:1 (v/v) isopropyl alcohol/acetonitrile (IPA + MeCN)(Riedel) treated with activated carbon. Sonication time interval for eachstep was 10 min. Alumina impurity on the bare GC is cleaned by usingan ultrasonic bath.
2.3.2. Modification of glassy carbon surfaceThe electrodes were thenmodified by cyclic voltammetry over a po-
tential range from 1.0 V to 1.6 V at 200 mV s−1 scan rate between 1 and10 scans in 1 mM DNP (in 0.01 M HCl) solution. After the preparation,modified electrode was rinsed with a stream of acetonitrile to removeany physisorbed and unreacted materials from the electrode surface.DNPO-modified glassy carbon electrode was stored in atmosphere.DAPO-GC was prepared in 0.01 M HCl with reduction of DNPO film onglassy carbon in the potential range from 0.0 V to −1.5 V at 200 V s−1
scan rate for 10 scans. The DAPO modified GC electrode was stored inopen atmosphere like DNPOmodified GC, since it is stable against atmo-spheric oxygen.
2.3.3. Electron spectroscopy for chemical analysis (ESCA, XPS)XPS spectra were obtained from “Specs XPS X-ray Photoelectron
Spectroscope” with Mg–Kα-X-rays (1253.6 eV, 10 mA) and wereused for the characterization of the DNPO-GC and DAPO-GC surfaces.
Fig. 4. Infrared spectra of DNPO and DAPO films were obtained by subtracting spec-trum of bare GC from modified GC.
2.3.4. Infrared spectroscopyInfrared spectra of the DNPO-GC, DAPO-GC and bare GC were col-
lected using a Bruker Tensor 27 FT-IR spectrometer with a Ge attenu-ated total reflectance accessory (GATR; 65° incident angle relative tosurface normal, Harrick Scientific) and a liquid N2-cooled MCT detec-tor averaging 256 scans. Infrared measurements were performed onthe Tokai GC-20 glassy carbon electrode surface.
2.3.5. Ellipsometric measurementsDNPO nano-film thickness was determined with high precision
discrete wavelength ellipsometer (ELX-02C/01R). The wavelengthwas 532 nm for all experiments. The thickness values of DNPOfilms on the glassy carbon surface were determined from the averageof the measurements using incidence angle of 70° (near Brewsterangle of the surface). In the calculation of film thickness, a mathe-matical fitting was performed for a four-phase model consisting ofgraphite/glassy carbon/DNPO film/air. Refractive indices are 3.0841for graphite (substrate), 1.900 for glassy carbon (70 nm), 1.460 fornanofilm organic (i.e. DNPO) layer and 1.000 for air (ambient), as-suming, thickness and refractive indices are reasonably correlatedfor all films. Extinction coefficients (k) are 1.782 for graphite, 0.810for glassy carbon and 0.000 for organic film and air.
3. Results and discussion
3.1. DNPO/DAPO nanofilms on glassy carbon surface
DNPO-GC surface was prepared using 1 mM (DNP) solution in0.01 M HCl by CV over a potential difference range from 1.0 V to 1.6 Vat 200 mV s−1 scan rate between 1 and 10 scans. The voltammogramis shown in Fig. 1. In Fig. 1, during the first cycle a major part of glassycarbon surface was coated. During the 2nd–4th cycles, pinholes onthe modified glassy carbon were filled, meanwhile, branching and
Table 2Thickness of DNPO film and cyclic number for DNPO film onglassy carbon by electrochemical oxidation by CV method.
Fig. 6. Graphic of cycle number for DNPO film on glassy carbon by electrochemicaloxidation by CV method — thickness measurement changes.
52 H. Ekşi et al. / Journal of Molecular Liquids 187 (2013) 49–53
multilayer formation [49] commenced. 10 cycles were preferred forelectrochemical and spectroscopic characterizations.
DAPO-GC was synthesized from the reduction of nitro group [50]on the DNPO film on GC surface in the potential range from 0.0 V to−1.5 V at 200 V s−1 scan rate for 2 scans in 0.01 M HCl solution(Fig. 2). The reaction peak of nitro groups on DNPO-GC was very in-tense and sharp. Equation for the electrochemical reduction of nitrogroups on DNPO-GC surface is shown in Scheme 1. It is well knownthat the electrochemical reduction reaction for each nitro group in-volves six electron transfers sequentially.
3.2. Characterization of nanofilms by ESCA (XPS)
Both modified surface and bare GC contain C and O atoms. Accord-ingly, an XPS spectrum of GC substrate was not taken as a result ofconsideration in terms of C and O peaks. Since DNPO and DAPOfilms contain N atoms, N1s peak around 400–410 eV was consideredfor characterization (Fig. 3).
N1s peak at 406 eV occurs due to N of nitro group of DNPO film. In398 eV, N1s peak is obtained due to N of amine group of DAPO film[51,52].
According to ESCA spectra, nitro groups can be considered as evi-dence of DNPO film formation. DNPO film was reduced to DAPO filmby electrochemical method, as agreed by N1s peak observed 398 eV.
3.3. Characterization of nanofilms by GATR-FT-IR
The IR spectra data of DAPO and DNPO films were collected in thewavenumbers of 600–4000 cm−1 range. Above 2000 cm−1, there wasno characteristic band, so 600–2000 cm−1 range of the spectra was an-alyzed. The spectra of films were obtained by subtracting spectrum ofbare GC from modified GC. Infrared band frequencies for these filmsare listed in Table 1 with their assignments. In Fig. 4, the spectra ofDNPO film exhibit strong bands at 845 cm−1, 1342 cm−1 and1530 cm−1 belonging to NO2 groups of DNPO on GC surface. In contrastto DNPO film, DAPO film shows only NH2 scissors at 1610 cm−1 and thenitro group bands disappeared in mentioned band range. This is a goodproof of reducing NO2 groups to NH2 groups on GC surface. The band at1610 cm−1 was found by fitting, because the C_C stretching bandwasoverlapped with NH2 scissors in 1500–1700 cm−1 range (Fig. 5). Byfitting algorithm, C_C stretching band at 1577 cm−1was distinguishedfrom band of NH2 scissors. The bands in the range of 1054–1066 cm−1
were observed for both surfaces which indicate that an asymmetricC\O\C stretching exists between DNPO group and GC. The band at1231–1262 cm−1 range in both films can be due to asymmetricC\C\O stretch of phenol. The out of plane C\H bands of DNPO andDAPO were observed in the range of 643–669 cm−1. In DAPO filmthere is a ring breathing vibration relating to the benzene groups on
GCwhile at 832 cm−1 in DNPO there is a NO2 scissors with ring breath-ing [47,53].
3.4. Thickness measurements of DNPO nanofilms on GC surface byellipsometry
Theoretical thickness of DNPO monolayer films is about 7.5 Å. Cy-clic number for DNPO film on glassy carbon by electrochemical oxida-tion by CV method and thickness measurement was given in Table 2.The graphical presentation of thickness-cyclic number changes wasshown in Fig. 6.
Cyclic number vs. thickness data of DNPO film (determined byellipsometry) synthesis by electrochemical oxidation with CVmethodwas shown in Table 2. By comparing the theoretical thickness ofDNPO monolayer, it can be suggested that a monolayer formationwas achieved during the first cycle. After the first cycle, film thicknessincreased abruptly due to multi-layer formation.
As shown in Fig. 6, multi-layer formation over 1 cycle is expectedas mentioned in the literature [20,49,54–70].
4. Conclusions
In this study, GC surface was covalently attached with DNP usingelectrochemical oxidation technique by CV. DNPO film was also char-acterized by several methods including CV, ESCA (XPS) GATR-FT-IRand ellipsometry. NH2 and NO2 groups existed in molecular junctionand are preferred in molecular electronics. DNPO-GC surface wasmodified by electrochemical reduction into the DAPO-GC surface.The existence of NO2 and NH2 groups was proved by ESCA (XPS)and IR spectroscopy.
Acknowledgments
Authors would like to thank TUBITAK for the supporting the pro-jects, project no: 106T622. The author thanks METU Central Laborato-ry for acquiring the XPS spectra data.
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