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Preparation, characterization and electrochemistry of Layer-by-Layer films of silver nanoparticles and silsesquioxane polymer
Accepted Manuscript Title: Preparation, characterization and electrochemistry of Layer-by-Layer films of silver nanoparticles and silsesquioxane polymer Author: Rafaela D.de Oliveira Giselle N. Calac¸a Cleverson S. Santos Sergio T. Fujiwara Christiana A. Pessˆoa PII: DOI: Reference:
S0927-7757(16)30811-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.09.061 COLSUA 21039
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
6-7-2016 10-9-2016 15-9-2016
Please cite this article as: Rafaela D.de Oliveira, Giselle N.Calac¸a, Cleverson S.Santos, Sergio T.Fujiwara, Christiana A.Pessˆoa, Preparation, characterization and electrochemistry of Layer-by-Layer films of silver nanoparticles and silsesquioxane polymer, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.09.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation, characterization and electrochemistry of Layer-by-Layer films of silver nanoparticles and silsesquioxane polymer
Rafaela D. de Oliveiraa, Giselle N. Calaçaa, Cleverson S. Santosa, Sergio T. Fujiwaraa and Christiana A. Pessôaa,*
a
Universidade Estadual de Ponta Grossa (UEPG), Department of Chemistry, 84030-000,
Ponta Grossa, PR, Brazil.
*
Corresponding Author
E-mail Address: [email protected] Tel: 55-42-32203731; fax: 55-42-32203042.
Graphical abstract
Highlights
TEM images showed that AgNPs are formed inside of starch chains (≈5nm) and outside of starch chains (≈80nm).
Starch stabilized AgNPs and 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-) formed stable LbL films;
AFM studies suggest that the LbL films formed are homogeneous.
The (SiPy+Cl-/AgNPs-St)5 LbL film exhibited higher electrocatalytic activity for triiodide due to the synergistic effect between the film components.
Abstract This paper describes the synthesis of starch-stabilized silver nanoparticles (AgNPs-St) for the development of electrochemical sensors. Layer-by-Layer (LbL) films were evaluated with the AgNPs-St, as polyanion, alternating with 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-), as polycation, obtaining the (SiPy+Cl-/AgNPs-St)n where n is the bilayers number. UV-Vis characterization of (SiPy+Cl-/AgNPs-St)n LbL films showed a linear increase of the absorbance of surface plasmon resonance band at 400 nm with bilayers number.
Atomic
force
microscopy
revealed
an
increase
in
thickness
of
the
(SiPy+Cl-/AgNPs-St)n LbL films from n=2 to n=10. These LbL films were employed for the electroanalytical determination of triiodide using the differential pulse voltammetry (DPV). The DPV parameters were optimized by 23 factorial design. Two linear ranges were found with detection limits of 5.56 x 10-6 and 1.51 x 10-5 mol L-1 and quantification limits 1.85 x 10-5 and 5.04 x 10-5 mol L-1.
Keywords: silver nanoparticles, Layer-by-Layer films, 3-n-propylpyridinium-silsesquioxane, iodine, electrochemical sensors.
1.
Introduction
Metal nanoparticles (MNPs) have received much attention of the scientific community due to their wide applications in various fields such as sensors,[1,2] biomedical,[3,4] catalysis[5] and optical.[6] These applications are possible because new properties are acquired when the size of the material is reduced.[7] However, the main drawback of such nanoparticles (NPs) is a strong tendency to coalesce due to a high surface energy leading to key process to the application of nanoparticles in terms of both stability and robustness.[8,9] To overcome this issue and improve the dispersion of metal nanoparticles, the anchoring process on some matrix is the ultimate strategy to avoid coalescence.[10] Some materials can be used as matrix such as graphene,[11,12] as studied by Zhang et al., where Li2S nanoparticles where anchored in graphene nanosheets for application in lithium-ion batteries as a superior cathode material alternative to sulfur.[13] Also conducting polymers can be used to stabilize metallic nanoparticles[14–16], as reported by Ghanbari which electrodeposited silver nanoparticles (AgNPs) on polypyrrole to construct an electrochemical sensor for hydrazine[17]. In the same way, Jayaraman et al. demonstrated a simple method to synthesize nanoscale polythiophene-gold nanoparticles hybrid systems assembled by the Langmuir-Blodgett method.[18] Starch is one of the most abundant biopolymer in nature and presents some characteristics like biodegradability, low-cost and non-toxic, allowing its use as an excellent candidate in a fabrication of eco-friendly devices in a modern claimed green chemistry research.[19,20]
Considering this, biopolymers, such as starch[21] and chitosan[22,23] for
example, are a good alternative that can be used as a matrix material for anchoring and stabilize metal nanoparticles. The extensive network of hydrogen bonds in these stabilizers provides surface passivation or protection against nanoparticle aggregation. Also, the presence of starch or chitosan in the reaction mixture avoids the use of relatively toxic organic solvents. Therefore, the use of biologically compatible materials for nanoparticles stabilization will play a crucial role. Khan et al. prepared silver nanoparticles by a simple chemical reduction method using ascorbic acid and starch as reducing and stabilizing agents, respectively. The authors concluded that starch plays an important rule acting as stabilizer, shape-directing and capping agent during the growth of the metal nanoparticles process.[24]
Kaur et al. proposed a simple method to synthesize biodegradable starch films with improved mechanical properties due to incorporation of gold and cadmium sulfide nanoparticles.[25] Although the synthesis and characterization of the metal nanoparticles is well established in the literature, the electrode modification by them is still the subject of discussion.[26–28] In this context, the Layer-by-Layer deposition (LbL) became an important methodology used by the electrochemists to modify electrodes, which basically consists of polyelectrolytes layers of opposite charges sandwiched.[29] Moreover, the simplicity of the electrostatic attraction governing the LbL methodology enables it as a powerful tool in different fields.[30–32] Despite of the simplicity of the LbL method, sandwiching an insulating polyelectrolyte between electroactive layers, such as starch and chitosan, could lead to an undesirable loss of electrochemical properties.[33] Therefore, the immobilization of metallic nanoparticles on thin films provides an increase of superficial area and, consequently,
its
conductivity
and
catalytic
efficiency.[34]
The
polycation
3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-), used in this paper, is a water-soluble polymer with good properties for formation of films and a strong anionic exchange capacity. SiPy+Cl- have been applied in LbL films for the construction of modified electrodes for the development of new electrochemical sensors.[35,36] Iodine is important for the organism due to its contribution in biological mechanisms, such as thyroid gland activity and development neurological. Many iodine compounds are used in pharmaceuticals products as antiseptics, disinfectants or products for treat diseases caused by iodine deficiency. In addition, the iodine is also applied in chemical analysis, organic synthesis, dietary supplements and pesticides.[37,38] The iodine can be indirectly determinate through triiodide in aqueous medium. Most of the analytical methods described in the literature for iodine detection are based on potentiometric methodologies. As the triiodide ions present well defined voltammetric response, voltammetric sensors can also be used for indirect detection of iodine.[37–39] Since the interaction between the Ag nanoparticles and iodine/ triiodide is very strong [40], the main goal of this work is to describe a detailed electrochemical study of starch-stabilized silver nanoparticles/silsesquioxane modified electrodes, designed as (SiPy+Cl-/AgNPs-St)n, by the LbL method and its application for triiodide determination.
2. 2.1.
Material and Methods Material and reagents
All stock solutions were prepared using distilled water. Sodium borohydride (VETEC, NaBH4, 98%), silver nitrate (VETEC, AgNO3, 99.5% ), potassium iodide (VETEC, KI, P.A. 99%), soluble starch from potato (C6H10O5)n (Sigma-Aldrich) , ammonium hydroxide (Merck, NH4OH, P.A. 28-30%), sodium phosphate monobasic (Nuclear, NaH2PO4, P.A. 98%) , sodium phosphate dibasic (Synth, Na2HPO4, P.A. 99%), isopropyl alcohol (Neon, C3H8O, P.A. 99.5%), hydrogen peroxide solution (Neon, H2O2, P.A. 35%), sodium nitrate (Cinética Química Ltda, NaNO3, P.A. 99%) and iodine (VETEC, P.A. 99.8%). All reagents were of analytical grade and they were used as received without further purification. The 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-) polyelectrolyte was synthesized by a method previous described in the literature.[41]
2.2.
Synthesis of silver nanoparticles and characterization
The starch was solubilized in distilled water with magnetic stirring and heat (70 °C) for 30 minutes. Then, AgNO3 solution was added in starch solution under magnetic stirring and ice bath (5 °C). After 10 minutes, NaBH4 solution was added with speed of 1 mL min-1. The magnetic stirring was kept for 30 minutes and the final solutions concentrations were 3.6 x 10-3 mol L-1 NaBH4, 9 x 10-4 mol L-1 AgNO3 and 0.6% (w/v) starch solution. The size of nanoparticles was evaluated using UV-Vis spectroscopy (Varian Cary 50 Bio) and dynamic light scattering (DLS). Zeta potential measurements (Zetasizer Nano Series ZS90) were performed to verify the AgNPs stability. The starch-stabilized silver nanoparticles (AgNPs-St) were stored in a dark bottle at room temperature (approximately 25 °C). DLS measurements (Zetasizer Nano Series ZS90) were performed in triplicate during a period of 115 days to check AgNPs agglomeration. The structure of AgNPs-St was characterized using X-ray diffractometer (XRD) Rigaku Ultima IV with Cu-Kα (λ=1,54 Å) incident radiation. The diffracted intensities were recorded from 25º to 85º 2θ angles. Transmission electron microscopy (TEM) images were recorded by a JEOL JEM 1200EX-II transmission electron microscope operated at 120kV.
2.3.
LbL assembly of (SiPy+Cl-/AgNPs-St)
The fluorine-doped tin oxide (FTO) substrates were previously cleaned by immersion in solution of NH4OH, H2O2 and water (1:1:5) and heated (70 ºC) for 10 minutes. Then, the substrates were immersed in isopropyl alcohol under heat for few minutes. Finally, the substrates were dried at room temperature. The LbL films were carried out by immersion of FTO alternately in SiPy+Cl- (2.0 mg mL-1 and pH 6.5), used as polycation, and AgNPs-St (9.0 x 10-4 mol L-1 and pH 9.0), as polyanion. The immersion time for both polyelectrolytes was 5 minutes. After FTO immersion in each polyelectrolyte solution, the films were washed in distilled water (10 seconds) and dried in air. This process was repeated until the desired bilayers number was obtained. The films produced were denoted as (SiPy+Cl-/AgNPs-St)n where n is the bilayers number.
2.4. Characterization of (SiPy+Cl-/AgNPs-St)n LbL film The kinetic study of deposition of polyelectrolytes was evaluated by UV-Vis spectroscopy using films containing from 1 to 15 bilayers. For this study, the (SiPy+Cl-/AgNPs-St)n LbL films (n= number of bilayers) were deposited onto quartz substrates. Fourier transform infrared spectroscopy (FTIR) measurements were carried out for the SiPy+Cl- and AgNPs-St drop-coated films and (SiPy+Cl-/AgNPs-St)5 LbL film deposited onto FTO substrates using a PerkinElmer® Frontier FTIR spectrometer coupled with universal attenuated total reflectance (ATR) sampling accessory. Spectra were collected in the transmission mode in the range from 660 to 4500 cm-1 at 4 cm-1 resolution, with 128 scans. For the starch, FTIR measurements were obtained in a Shimadzu IR Prestige 21 FTIR spectrometer, using KBr pellets, in the transmission mode in the range from 400 to 4000 cm-1. The spectral resolution was set at 4 cm-1 and with 64 scans. Film morphology analyses were obtained with an Atomic force microscope (AFM) Shimadzu SPM 9600, in non-contact mode with length 1 µm and width 1 µm, using (SiPy+Cl-/AgNPs-St)n LbL films with n = 2, 4, 5, 6, 8 and 10 bilayers. The thickness measurements were based on isolating a small area of FTO substrate in order to get a “step”
to detect the height difference between the LbL film and substrate.
2.5. Electrochemical studies Electrochemical measurements were carried out in three-electrode electrochemical cell: Ag/AgCl electrode was used as the reference electrode (RE), platinum wire as counter electrode (CE) and the working electrode (0.5 cm2 of geometric area) was the (SiPy+Cl-/AgNPs-St) LbL films. Preliminary studies were conducted by cyclic voltammetry (CV) in the potential range from 0.0 V to -1.0 V at 50 mV s-1 using supporting electrolyte 0.1 mol L-1 NaNO3 and 0.1 mol L-1 phosphate buffered saline (PBS), both with pH 7.0. Differential pulse voltammetry (DPV) were performed for studies of triiodide determination. Before the measurements, the supporting electrolyte was purged with nitrogen for 5 min. It was used triiodide solution (I2/KI) standardized. Studies with different supporting electrolytes were
performed:
Phosphate
Buffered
Saline
(PBS),
NaH2PO4,
Na2HPO4
and
Britton-Robinson (BR) buffer, in a concentration of 0.1 mol L-1. The measurements were carried out in the potential range from 0.0 to -1.0 V using scan rate of 50 mV s-1, pulse potential (Epulse) of 0.05 V, pulse time (tpulse) of 0.05 s and triiodide concentration of 1.0 x 10-3 mol L-1. These same voltammetric conditions were used to verify the interaction between the films components and for the study of bilayers number in 0.1 mol L-1 PBS with pH 7.0. The instrumental parameters - pulse potential (Epulse), the pulse time (tpulse) and the scan rate (v) - for triiodide determination by DPV were optimized by 23 full factorial design, including a central point assayed in quadruplicate to calculate standard deviation. The factors and levels, which were selected from previous studies, were: Epulse = 50 mV (−1) and 70 mV (+1); tpulse = 0.03 s (−1) and 0.05 s (+1); v = 20 mV s-1 (−1) and 40 mV s-1 (+1). The DPV measurements were performed with (SiPy+Cl-/AgNPs-St)5 LbL film, concentration of triiodide of 1.0 x 10-3 mol L-1 in the potential range from 0.0 V to -1.0 V and step potential of 0.005 V. Analytical curves were obtained from a stock solution of triiodide (1.0 x 10-3 mol L-1). The detection limit (DL) and the quantification limit (QL) were calculated by the standard deviation of the mean current for blank signal in pure electrolytes (Sb) and the slope of the analytical curves (s), using DL = 3Sb/s and QL = 10Sb/s according IUPAC.[42]
3.
Results and Discussion
3.1. Characterization of AgNPs-St The formation of AgNPs-St was monitored by UV-Vis spectroscopy (Fig. 1a). The absorbance band observed at 400 nm corresponds to the characteristic surface plasmon resonance band of AgNPs, which appears usually near to 420 nm.[43,44] The DLS results (Fig. 1b) show two populations of AgNPs-St in nanometric scale with average sizes of 21 and 77 nm. Zeta Potential measurements suggest stability and negative charge for the AgNPs-St, with value of -113 mV. The AgNPs-St was monitored by DLS for 30 days to check for NPs agglomeration. Any significant change in size distribution of the AgNPs-St was noticed during 115 days of storage of the AgNPs-St, indicating low agglomeration (Fig. 1c). It can be observed in Fig. 1c, the AgNPs-St shows two populations (similar to Fig. 1b), however the percentage of distribution is higher for larger particles sizes. Nevertheless, the nanoparticle sizes do not exceed 150 nm similar to the DLS results obtained in the first day. Fig. 2 provides TEM images of the AgNPs-St indicating that the AgNPs formed inside of starch chains are smaller, approximately 4-5 nm (Fig. 2a), while the AgNPs formed outside of starch chains presented larger sizes (approximately 80 nm), as shown in Fig. 2b. These results confirm the size distribution found by DLS. Furthermore, it is observed that the shape of the AgNPs-St is predominantly spherical.
XRD analysis was carried out to investigate the crystal structure of AgNPs-St formed. XRD patterns of lyophilized AgNPs-St (Fig. 3a) exhibits characteristic peaks at 2θ of 38.0, 44.1, 64.3 and 77.2, indexed to crystallographic planes (111), (200), (220) and (311), respectively. Similar values were described in the literature for metallic silver.[43,45] These diffraction peaks are referring to a face centered cubic (FCC) structure typically presented for Ag. Fig. 3b shows XRD patterns of AgNPs-St lyophilized and calcined (500 °C for 1 hour) to remove the high noise originated from the starch. In this XRD spectrum, it was observed the peaks at scattering angles (2θ) of 38.0, 44.2, 64.3 and 77.3, similar to AgNPs-St, however with higher intensity and sharper than the non-calcined AgNPs-St. This behavior was expected due
to the removal of noisy background and the growth of crystallites (Fig. 3b). This is due to the removal of noisy background and the growth of crystallites.[43]
3.2. Growth and characterization of (SiPy+Cl-/AgNPs-St)n Layer-by-Layer films The negative charge of AgNPs-St allows it to be used as polyanion in the growth of LbL films combined with SiPy+Cl- as polycation. The growth of the bilayers was monitored by UV-Vis spectroscopy. Initially, the spectra of precursors were carried out to observe the characteristic absorption band of the components used for LbL films growth (Fig. 4a). The AgNPs-St spectrum exhibits an absorption band at 400 nm, which corresponds to surface plasmon resonance band of AgNPs.[43] The starch solution spectrum does not exhibit any absorption bands in UV-Vis spectra in the range of 320 to 500 nm and the SiPy+Cl- exhibited one absorption band at 258 nm attributed to the 𝜋 → 𝜋 ∗ transition of the pyridine groups in SiPy+Cl-, as related in the literature.[35] As shown in Fig. 4a, (SiPy+Cl-/AgNPs-St)5 LbL film on quartz showed absorption band at 425 nm attributed to AgNPs. The band shift observed in relation to the AgNPs-St spectra can be indicative of the electrostatic interactions between AgNPs-St and SiPy+Cl- solution. In a similar study, the interaction between polyelectrolytes (pyridinium ion of SiPy+Cl- with platinum nanoparticles incorporated, Pt-SiPy+Cl- and sulphonatephenylenevinylene, PVS) in the LbL film was confirmed by the shifts of the bands of UV-Vis and FTIR spectra. [36] The LbL film growth was monitored by UV-Vis spectroscopy observing the increase in absorbance intensities at 425 nm (Fig. 4b). A linear increase in absorbance with bilayers number deposited (n = 1 to 15) indicates LbL film growth with R = 0.993. The kinetic study of the deposition was performed by varying the immersion time (60 – 300 seconds) of the quartz substrate in the polyelectrolytes solution and the intensity of the band was monitored at 425 nm. Fig. 4c showed linear growth in the absorbance band as a function of the immersion time between 60 to 180 seconds (R = 0.9992) and maximum absorbance at 240 seconds. For the subsequent studies, it was fixed an immersion time of 4 minutes (240 s) because in this time occurs the saturation of the LbL surface, since any significant increase in the absorbance is observed.
Fig. 5 shows the FTIR spectra obtained for starch, drop-coated film of AgNPs-St and SiPy+Cl- and (SiPy+Cl-/AgNPs-St)5 LbL film. The analysis of the spectrum of starch (Fig. 5a) presented bands at 993 and 1171 cm-1 for the C-O-C symmetric stretching and other at 2928 cm-1 attributed to the C-H stretching (not shown). In the FTIR spectrum of drop-coated film of AgNPs-St (Fig. 5b), it can be seen two main bands at 1078 and 1005 cm-1 assigned to symmetric stretching of the C-O-C groups of the starch. Once the starch was used as stabilizing agent, these small shifts of the C-O-C bands are indicative of possible interactions between starch and AgNPs. The drop-coated film of SiPy+Cl- spectrum (Fig. 5c) shows a band at 775 cm-1 related to the Si-O-Si group symmetric stretching, other in 833 cm-1 attributed to different vibrational modes with large contribution of bands related to angular deformation of the Si-C-H group and stretching of Si-C. In addition, the SiPy+Cl- spectra also presented a broad and intense band at 1100 cm-1 assigned to the Si-O-Si groups asymmetric stretching. The characteristic bands of the pyridinium ring were of observed at 1634 and 1489 cm-1.[35,36]
However, in the (SiPy+Cl-/AgNPs-St)5 LbL film spectrum, the C-O-C symmetric stretching of AgNPs-St appeared at 1005 cm-1 and 1009 cm-1. The band of SiPy+Cl- at 775 cm-1 related to the Si-O-Si group symmetric stretching appeared at 764 cm-1 in the (SiPy+Cl-/AgNPs-St)5 LbL film. The band related to pyridinium ring appeared at 1634 and 1489 cm-1 for SiPy+Cl- and at 1638 and 1490 cm-1 for the (SiPy+Cl-/AgNPs-St)5 LbL film. Also, it can be noted that the absorption band at 833 cm-1 for SiPy+Cl-, assigned to angular deformation of the Si-C-H group and stretching of the Si-C, was shifted for (SiPy+Cl-/AgNPs-St)5 LbL film at 863 cm-1. The broad band related to Si-O-Si asymmetric stretching of the cage structure at 1100 cm-1 and the bands related to C-O-C symmetric stretching of AgNPs-St at 1005 cm-1 and 1009 cm-1 occur in the same region of the spectrum. Therefore, due to overlapping of these peaks in this region of the spectrum for (SiPy+Cl-/AgNPs-St)5 LbL film, the assignment of the bands cannot be straightforward. In general, the spectra of the (SiPy+Cl-/AgNPs-St)5 LbL film is similar to those obtained for the polyelectrolytes in drop-coated films, which confirms the interaction
between the AgNPs-St and SiPy+Cl- in the LbL films.[35,36] Fig. 6 shows AFM images for the (SiPy+Cl-/AgNPs-St)n LbL films (n=0, 2, 4 and 10). The LbL films were deposited on FTO and AFM analysis were carried out in three different regions to minimize errors. The root-mean-square (RMS) of the (SiPy+Cl-/AgNPs-St)n LbL films (n=0, 2, 4, 6, 8 and 10), listed in Table 1, revealed a decrease in the roughness of the film from FTO (31.148 ± 2.217 nm) until 8 bilayers (22.321 ± 0.980 nm). This behavior can be attributed to the random deposition of the molecules covering the irregularities of the substrate, thus decreasing the roughness. It is important to note that the films with 8 bilayers (22.321 ± 0.980 nm) and 10 bilayers (22.878 ± 2.956 nm), considering the standard deviation, practically presented no variation of roughness. This suggests homogeneity in the deposition of the film LbL from 8 bilayers. The literature shows similar results for others LbL films.[35,46]
The thickness of the LbL films was obtained by AFM technique (Fig. 7). The LbL films exhibited thickness in the range of 254.32 to 1130 nm. Furthermore, the results showed that the increase in the thickness of the films is linear from 4 to 10 bilayers. This indicates that the same amount of materials was deposited at each bilayer.
3.3. Electrochemical detection of triiodide with (SiPy+Cl-/AgNPs-St)n LbL films Differential pulse voltammetry (DPV) studies were performed to evaluate the electrochemical sensing properties of LbL films in the presence of triiodide (Fig. 8). The voltammetric determination of iodine can be evaluated through triiodide as demonstrated by equations to follow:[47] 𝐼2 (𝑠) + 𝐼 − (𝑎𝑞) ⇌ 𝐼3− (𝑎𝑞) 𝐼3− (𝑎𝑞) + 2𝑒 − ⇌ 3𝐼 − (𝑎𝑞)
A comparative study was also carried out with the (SiPy+Cl-/AgNPs-St)5 LbL film, the SiPy+Cl- drop-coated film, the starch drop-coated film, the AgNPs-St drop-coated film and the FTO substrate. For this study a fixed concentration of 1.0 x 10-3 mol L-1 triiodide in 0.1 mol L-1 PBS at pH 7.0 was used, as shown in Fig. 8. Comparing the results obtained, it can be observed
that (SiPy+Cl-/AgNPs-St)5 LbL film presented higher current values (Ipc=-90.86 μA) and less negative reduction potential (Epc=- 0.26 V), in relation to the others modified electrodes. It can be noted that drop-coated film of starch showed lower current (Ipc=-15.34 μA, Epc=-0.535), while the FTO substrate exhibited current of Ipc=−48.62 μA and Epc=-0.555. These results confirmed that the (SiPy+Cl-/AgNPs-St)5 LbL film exhibited higher electrocatalytic activity due to the synergistic effect between the film components.
Fig. 9 shows the spectra for AgNPs-St in presence of different concentrations of triiodide. UV-Vis spectroscopy revealed that the increasing of triiodide concentration reduces the intensity of characteristic band of AgNPs (~420nm) indicating that triiodide firstly interacts with AgNPs-St. This interaction can be responsible for the increase in the intensity of the cathodic current for the (SiPy+Cl-/AgNPs-St)5 LbL film. Only at higher concentrations of triiodide it starts to react with starch. These results are in agreement with Vigneshwaran et al which proved that previously occur the interaction between the AgNPs and triiodide. Then, the triiodide tends to react with starch forming the complex with amylose, as reported in the literature.[40] The dependence of the electrochemical response of (SiPy+Cl-/AgNPs-St)5 LbL film was studied as a function of bilayers number. For these studies, LbL films were prepared containing from 1 to 9 bilayers with SiPy+Cl-/AgNPs-St architecture and voltammetric response evaluated in presence of 1.0 x 10-3 mol L-1 triiodide (Fig. 10). Five-bilayer SiPy+Cl-/AgNPs-St LbL film was chosen for the subsequent studies because it presented higher current values, similar to those previously reported in the literature.[33,36] The current tends to be lower with the increase of bilayers due to the film resistance, which inhibits the electron transfer process.
3.4. Optimization of DPV parameters by factorial design The instrumental parameters of DPV were studied in order to improve the response of peak current for triiodide determination. DPV parameters were optimized using a 23 full
factorial design, thus the pulse potential (Epulse), the pulse time (tpulse) and the scan rate (v) were simultaneously evaluated, making it possible to obtain the best analytical signal in terms of current intensity. The results are shown in Table 2.
According to statistical analysis of the data, the main effects (Epulse =
6.36, tpulse =
4.03 and v = 6.98), second-order effects (Epulse vs tpulse = -6.44, Epulse vs v = -3.20, tpulse vs v = -5.97), as well as the third-order effect (-1.03) were significant at 95% confidence levels (0.14 x 3.18 t95%, v=3 = 0.45). The geometric representation for interaction of three factors (Fig. 11) shows that the highest current intensity (-47.273 µA) is obtained at the -++ vertex for the factors Epulse, tpulse, and v respectively. Hence, the optimal DPV parameters were: pulse potential (Epulse) of 50 mV, pulse time (tpulse) of 0.05 s and scan rate (v) of 40 mV s-1.
3.5. Analytical curves Analytical curves were obtained in triplicate and the results presented in Fig. 12.Under the optimized conditions, the analytical curves were obtained in the range from 4.34 x 10 -5 to 3.47 x 10-4 mol L-1 (Fig. 12a) and from 4.4 x 10-4 to 4.24 x 10-3 mol L-1 (Fig. 12b) of triiodide. The detection limit (DL) and quantification limit (QL) were calculated as recommended by IUPAC.[42] From Fig. 12a, it was found a linear equation: I/µA = – 0.747 – 3.05 x 104 [triiodide] (R = 0.9936), DL = 5.56 x 10-6 mol L-1 and QL = 1.85 x 10-5 mol L-1. From Fig. 12b, it was found a linear equation: I/µA = – 15.949 – 2.51 x 104 [triiodide] (R = 0.9938), DL =1.51 x 10-5 mol L-1 and QL= 5.04 x 10-5 mol L-1.
In order to evaluate the efficiency of the proposed methodology, recovery tests were performed in pure electrolyte (0.1 mol L-1 PBS, pH 7.0). Three determinations were carried out for each analytical curve. All data were obtained in triplicate and the standard deviations were calculated. As shown in Table 3, the recovery values for Fig. 12a and Fig. 12b ranged from
100.60% to 103.26% and from 91.55% to 95.88%, respectively. The percentages of recovery in these samples proved to be suitable for analytical purposes.
Table 4 presents the response characteristics of different electrodes including, electrodes composition, linear range, quantification and detection limit. It is clear that response characteristics vary with the modified electrodes. From Table 4, it can be seen that the LbL film electrode showed in this work has detection limit comparable with electrodes previously reported in the literature. In addition, considering the linear range, the performance of the (SiPy+Cl-/AgNPs-St) LbL film with two linear detection ranges also allow determination in wide concentration range (in the order of 10-5 to 10-3 mol L-1).
4.
Conclusions
In this paper, TEM images of starch-stabilized silver nanoparticles indicated that AgNPs formed inside of starch chains were smaller (≈5nm) than AgNPs formed outside of starch chains (≈80nm). These silver nanoparticles were combined with SiPy+Cl- in LbL film and provide interesting electrochemical properties. The (SiPy+Cl-/AgNPs-St) LbL film was applied as modified electrode for triiodide determination as triiodide due to its interaction with AgNPs-St. This interaction provides an increase in the intensity of the cathodic current for the (SiPy+Cl-/AgNPs-St)5 LbL film when compared with the use of the LbL film components separately. The electrochemical measurements showed linear response in two concentrations ranges with low QL and DL in the order of 10-6 mol L-1 to 10-5 mol L-1 and recovery tests presented values over 90%. The concentration range of triiodide determination is larger than other values reported in the literature. These electrochemical response obtained in our studies open up the possibility of an alternative method for triiodide determination.
Acknowledgments The authors would like to thank Brazilian funding agencies (Fundação Araucária and CAPES) for their financial support of this research work.
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0.7
10
(b)
(a) 8
0.5
Distribution (%)
Absorbance (a.u.)
0.6
0.4 0.3 0.2
6 4 2 0
0.1 0.0 200
300
400
500
600
700
-2 0
20
40
60
80
100
120
140
160
Particle size (nm)
Wavelength (nm) (c) 10
Distribution (%)
8
6
4
2
0 20
40
60
80
100
120
140
Particle size (nm)
Fig. 1. (a) UV-Vis spectra of AgNPs-St, (b) the particle size histogram by DLS 1 day and (c) after 115 days.
(a)
(b)
Fig. 2. TEM images of AgNPs-St dispersion: (a) inside and (b) outside of starch chains.
700
2000
(a)
Ag (111)
600
(b)
Ag (111)
1800 1600 1400
400
Intensity
Intensity
500
Ag (200)
300
1200 1000
Ag (200) 800 600
Ag (220)
Ag (220)
200
Ag (311)
Ag (311)
400 200
100
0
25
0 25
30
35
40
45
50
55
60
2 (degree)
65
70
75
80
85
30
35
40
45
50
55
60
65
70
75
80
2 (degrees)
Fig. 3. XRD spectra of (a) lyophilized AgNPs-St and (b) samples obtained after carbonization at 500 °C for 1 h.
0.7
0.30
AgNPs-St Starch + (SiPy Cl /AgNPs)5
(a)
0.5
(b) 0.25
0.4 Absorbance (a.u.)
Absorbance (a.u.)
0.6
0.3 0.2 0.1 0.0
0.15
0.10
0.05
-0.1 -0.2 320
0.20
340
360
380
400
420
440
460
480
500
0.00 3
5
Wavelength (nm)
Absorbance (a.u.)
0.55
7
9
11
13
15
Bilayers number
(c)
0.50
0.45
0.40
30
60
90
120
150
180
210
240
270
300
330
Immersion time (s)
Fig. 4. (a) UV-Vis spectra of AgNPs-St (diluted in distilled water 1:20), starch 1% (w/v) solution and (SiPy+Cl-/AgNPs-St)5 LbL film (b) Relationship between the absorbance at 425 nm and bilayers number. (c) Adsorption kinetic of the (SiPy+Cl-/AgNPs-St)5 LbL film.
(a) 993
1171
Intensity (a.u.)
(b) 1078
1005
(c) 833
1634
775 1489
(d)
1100 1490 1638
863 1151
764
1078 1009
1700 1600 1500 1400 1300 1200 1100 1000 900
800
700
-1
Wavenumber (cm )
Fig. 5. FTIR spectra for films of (a) starch, (b) drop-coated film of AgNPs-St, (c) drop-coated film of SiPy+Cl- and (d) (SiPy+Cl-/AgNPs-St)5 LbL film.
(a)
(b)
(d)
(c)
Fig. 6. - AFM images of (a) FTO, (b) (SiPy+Cl-/AgNPs-St)2, (c) (SiPy+Cl-/AgNPs-St)6 and (d) (SiPy+Cl-/AgNPs-St)10 LbL films. Scan window of 1 x 1 µm.
1200
Thickness (nm)
1000
800
600
400
200
0 4
5
6
7
8 +
9
10
-
Bilayers number (SiPy Cl /AgNPs-St)
Fig. 7. Relationship between thickness and bilayers number for (SiPy+Cl-/AgNPs-St)n LbL films with n = 4, 5, 6, 8 and 10 bilayers.
-120
3-
-3
[I ]=1 x 10 mol.L
-1
FTO + (SiPy Cl /AgNPs-St)5
-100
+
-80
I (A)
-
SiPy Cl AgNPs-St Starch
-60 -40 -20 0 0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1
E (V) vs Ag/AgCl PBS 0.1 mol L pH 7 Fig. 8. Differential pulse voltammograms in presence of 1.0 x 10-3 mol L-1 of triiodide in 0.1 mol L-1 PBS, pH 7.0 for (SiPy+Cl-/AgNPs-St)5 LbL film, SiPy+Cl- drop-coated film, starch drop-coated film, AgNPs-St drop-coated film and FTO substrate. Potential range from 0.0 to -1.0 V vs. Ag/AgCl, Estep = 0.005 V, = 50mV s-1, Epulse = 0.05 V and tpulse = 0.05 s.
1.0
[Iodine] -1 mol L 2.4 5.7 10.0 15.0 55.0 69.0 89.0
Absorbance (a.u.)
0.8
0.6
0.4
0.2
0.0 300
400
500
600
700
800
Wavelength (nm)
Fig. 9. UV-Vis spectra for different concentrations of triiodide.
-120
-100
I (A)
-80
-60
-40
-20
0 0
2
4
6
8
10
Bylayers number
Fig. 10. Current obtained by DPV as a function of bilayers number for (SiPy+Cl-/AgNPs-St)n. n = 1 to 9 bilayers. Parameters: 0.1 mol L-1 PBS, pH 7.0, iodine 1.0 x 10-3 mol L-1. Estep = 0.005 V, v = 50 mV s-1, Epulse = 0.05 V and tpulse = 0.05 s.
Fig. 11. Geometric representation of interaction: pulse potential vs scan rate vs pulse time.
-14 -120
(a)
(b)
-12 -100
-10
-80
I (A)
I (A)
-8 -6 -4
-60 -40
-2
-20
0 0.00
0.05
0.10
0.15
0.20
0.25
[triiodide] mmol L
0.30 -1
0.35
0.40
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
[triiodide] mmol L
3.5
4.0
4.5
-1
Fig. 12. Analytical curves of the electrochemical sensor (SiPy+Cl-/AgNPs-St)5 as a function of triiodide concentration (a) 4.34 x 10-5 to 3.47 x 10-4 mol L-1 and (b) 4.4 x 10-4 to 4.24 x 10-3 mol L-1, in 0.1 mol L-1 PBS, pH 7.0, scan range from 0.0 to -1.0 V vs. Ag/AgCl. Estep = 0.005 V, v = 40 mV s-1, Epulse = 0.05 V and tpulse = 0.05 s.
Table 1 – Root-mean-square (RMS) roughness for (SiPy+Cl-/AgNPs-St)n LbL films Bilayers number RMS roughness (nm) (1 x 1 μm) 0
31.148 ± 2.217
2
29.499 ± 3.494
4
25.290 ± 2.751
6
23.939 ± 3.071
8
22.321 ± 0.980
10
22.878 ± 2.956
Table 2. Factors, levels, and response values for the 23 full factorial design for instrumental parameters of DPV. Factors Runs
Response
Epulse
v
tpulse
I (µA)
01
-
-
-
−26.63
02
+
-
-
−41.60
03
-
+
-
−29.80
04
+
+
-
−40.43
05
-
-
+
−30.09
06
+
-
+
−34.25
07
-
+
+
−47.27
08
+
+
+
−42.96
09
0
0
0
−33.48
10
0
0
0
−33.64
11
0
0
0
−33.78
12
0
0
0
−33.50
Table 3. Recovery results for triiodide in electrolyte using (SiPy+Cl-/AgNPs-St)5 LbL film in 0.1 mol L-1 PBS, pH 7.0. Estep = 0.005 V, v = 40 mV s-1, Epulse = 0.05 V and tpulse = 0.05 s. Added Analytical curve
1
2
Found
Recovery
(mmol L-1) (mmol L-1) (%) 0.08
0.08 ± 0.02 103.26
0.08
0.08 ± 0.02 100.60
0.15
0.15 ± 0.01 102.76
1.28
1.21 ± 0.06 94.53
1.70
1.63 ± 0.02 95.88
2.84
2.60 ± 0.03 91.55
Table 4. Construction and performance of several LbL film electrodes. Type
of
Sensor
Voltammetric
Electrodes
(mol
LbL
4.34 x 10-5
(SiPy+Cl-/AgNPs-St)5
to 3.47 x 10-4
LbL
4.40 x 10-4
(SiPy+Cl-/AgNPs-St)5
to 4.24 x 10-3
DL (mol
QL L-1)
(mol L-1)
5.56 x 10-6
1.85 x 10-5
10-5
10-5
1.51 x
5.04 x
Kormosh
3.9 x 10−7
N
1 x 10-6 to 1 x 10-1
5 x 10−7
N
PMeT
1 x 10-6 to 1 x 10-1
N
N
(KCH2)(I3)2
2 x 10-3 to 7 x 10-6
3 x 10-6
N
AuNPs
1 x 10-8 to 6 x 10-7
1 x 10-8
N
Membrane ZnO nanotubes
Reference
This paper
1 x 10-6 to 1 x 10-1
Rhodamine B
Colorimetric
L-1)
composition
Membrane
Potentiometric
Linear range
et al [38] Ibupoto et al [37] Karagozler et al [39] Farhadi et al [48] Chen et al [49]
Note: N, not reported; LbL, Layer-by-Layer; SiPy+Cl-, 3-n-propylpyridinium silsesquioxane; AgNPs-St, starch-stabilized silver nanoparticles; ZnO, zinc oxide; PMeT, poly(3-methylthiophene; (KCH2)(I3)2, deprotonated ketoconazole-triiodide; AuNPs, gold nanoparticles.
S0927-7757(16)30811-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2016.09.061 COLSUA 21039
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
6-7-2016 10-9-2016 15-9-2016
Please cite this article as: Rafaela D.de Oliveira, Giselle N.Calac¸a, Cleverson S.Santos, Sergio T.Fujiwara, Christiana A.Pessˆoa, Preparation, characterization and electrochemistry of Layer-by-Layer films of silver nanoparticles and silsesquioxane polymer, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.09.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation, characterization and electrochemistry of Layer-by-Layer films of silver nanoparticles and silsesquioxane polymer
Rafaela D. de Oliveiraa, Giselle N. Calaçaa, Cleverson S. Santosa, Sergio T. Fujiwaraa and Christiana A. Pessôaa,*
a
Universidade Estadual de Ponta Grossa (UEPG), Department of Chemistry, 84030-000,
Ponta Grossa, PR, Brazil.
*
Corresponding Author
E-mail Address: [email protected] Tel: 55-42-32203731; fax: 55-42-32203042.
Graphical abstract
Highlights
TEM images showed that AgNPs are formed inside of starch chains (≈5nm) and outside of starch chains (≈80nm).
Starch stabilized AgNPs and 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-) formed stable LbL films;
AFM studies suggest that the LbL films formed are homogeneous.
The (SiPy+Cl-/AgNPs-St)5 LbL film exhibited higher electrocatalytic activity for triiodide due to the synergistic effect between the film components.
Abstract This paper describes the synthesis of starch-stabilized silver nanoparticles (AgNPs-St) for the development of electrochemical sensors. Layer-by-Layer (LbL) films were evaluated with the AgNPs-St, as polyanion, alternating with 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-), as polycation, obtaining the (SiPy+Cl-/AgNPs-St)n where n is the bilayers number. UV-Vis characterization of (SiPy+Cl-/AgNPs-St)n LbL films showed a linear increase of the absorbance of surface plasmon resonance band at 400 nm with bilayers number.
Atomic
force
microscopy
revealed
an
increase
in
thickness
of
the
(SiPy+Cl-/AgNPs-St)n LbL films from n=2 to n=10. These LbL films were employed for the electroanalytical determination of triiodide using the differential pulse voltammetry (DPV). The DPV parameters were optimized by 23 factorial design. Two linear ranges were found with detection limits of 5.56 x 10-6 and 1.51 x 10-5 mol L-1 and quantification limits 1.85 x 10-5 and 5.04 x 10-5 mol L-1.
Keywords: silver nanoparticles, Layer-by-Layer films, 3-n-propylpyridinium-silsesquioxane, iodine, electrochemical sensors.
1.
Introduction
Metal nanoparticles (MNPs) have received much attention of the scientific community due to their wide applications in various fields such as sensors,[1,2] biomedical,[3,4] catalysis[5] and optical.[6] These applications are possible because new properties are acquired when the size of the material is reduced.[7] However, the main drawback of such nanoparticles (NPs) is a strong tendency to coalesce due to a high surface energy leading to key process to the application of nanoparticles in terms of both stability and robustness.[8,9] To overcome this issue and improve the dispersion of metal nanoparticles, the anchoring process on some matrix is the ultimate strategy to avoid coalescence.[10] Some materials can be used as matrix such as graphene,[11,12] as studied by Zhang et al., where Li2S nanoparticles where anchored in graphene nanosheets for application in lithium-ion batteries as a superior cathode material alternative to sulfur.[13] Also conducting polymers can be used to stabilize metallic nanoparticles[14–16], as reported by Ghanbari which electrodeposited silver nanoparticles (AgNPs) on polypyrrole to construct an electrochemical sensor for hydrazine[17]. In the same way, Jayaraman et al. demonstrated a simple method to synthesize nanoscale polythiophene-gold nanoparticles hybrid systems assembled by the Langmuir-Blodgett method.[18] Starch is one of the most abundant biopolymer in nature and presents some characteristics like biodegradability, low-cost and non-toxic, allowing its use as an excellent candidate in a fabrication of eco-friendly devices in a modern claimed green chemistry research.[19,20]
Considering this, biopolymers, such as starch[21] and chitosan[22,23] for
example, are a good alternative that can be used as a matrix material for anchoring and stabilize metal nanoparticles. The extensive network of hydrogen bonds in these stabilizers provides surface passivation or protection against nanoparticle aggregation. Also, the presence of starch or chitosan in the reaction mixture avoids the use of relatively toxic organic solvents. Therefore, the use of biologically compatible materials for nanoparticles stabilization will play a crucial role. Khan et al. prepared silver nanoparticles by a simple chemical reduction method using ascorbic acid and starch as reducing and stabilizing agents, respectively. The authors concluded that starch plays an important rule acting as stabilizer, shape-directing and capping agent during the growth of the metal nanoparticles process.[24]
Kaur et al. proposed a simple method to synthesize biodegradable starch films with improved mechanical properties due to incorporation of gold and cadmium sulfide nanoparticles.[25] Although the synthesis and characterization of the metal nanoparticles is well established in the literature, the electrode modification by them is still the subject of discussion.[26–28] In this context, the Layer-by-Layer deposition (LbL) became an important methodology used by the electrochemists to modify electrodes, which basically consists of polyelectrolytes layers of opposite charges sandwiched.[29] Moreover, the simplicity of the electrostatic attraction governing the LbL methodology enables it as a powerful tool in different fields.[30–32] Despite of the simplicity of the LbL method, sandwiching an insulating polyelectrolyte between electroactive layers, such as starch and chitosan, could lead to an undesirable loss of electrochemical properties.[33] Therefore, the immobilization of metallic nanoparticles on thin films provides an increase of superficial area and, consequently,
its
conductivity
and
catalytic
efficiency.[34]
The
polycation
3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-), used in this paper, is a water-soluble polymer with good properties for formation of films and a strong anionic exchange capacity. SiPy+Cl- have been applied in LbL films for the construction of modified electrodes for the development of new electrochemical sensors.[35,36] Iodine is important for the organism due to its contribution in biological mechanisms, such as thyroid gland activity and development neurological. Many iodine compounds are used in pharmaceuticals products as antiseptics, disinfectants or products for treat diseases caused by iodine deficiency. In addition, the iodine is also applied in chemical analysis, organic synthesis, dietary supplements and pesticides.[37,38] The iodine can be indirectly determinate through triiodide in aqueous medium. Most of the analytical methods described in the literature for iodine detection are based on potentiometric methodologies. As the triiodide ions present well defined voltammetric response, voltammetric sensors can also be used for indirect detection of iodine.[37–39] Since the interaction between the Ag nanoparticles and iodine/ triiodide is very strong [40], the main goal of this work is to describe a detailed electrochemical study of starch-stabilized silver nanoparticles/silsesquioxane modified electrodes, designed as (SiPy+Cl-/AgNPs-St)n, by the LbL method and its application for triiodide determination.
2. 2.1.
Material and Methods Material and reagents
All stock solutions were prepared using distilled water. Sodium borohydride (VETEC, NaBH4, 98%), silver nitrate (VETEC, AgNO3, 99.5% ), potassium iodide (VETEC, KI, P.A. 99%), soluble starch from potato (C6H10O5)n (Sigma-Aldrich) , ammonium hydroxide (Merck, NH4OH, P.A. 28-30%), sodium phosphate monobasic (Nuclear, NaH2PO4, P.A. 98%) , sodium phosphate dibasic (Synth, Na2HPO4, P.A. 99%), isopropyl alcohol (Neon, C3H8O, P.A. 99.5%), hydrogen peroxide solution (Neon, H2O2, P.A. 35%), sodium nitrate (Cinética Química Ltda, NaNO3, P.A. 99%) and iodine (VETEC, P.A. 99.8%). All reagents were of analytical grade and they were used as received without further purification. The 3-n-propylpyridinium silsesquioxane chloride (SiPy+Cl-) polyelectrolyte was synthesized by a method previous described in the literature.[41]
2.2.
Synthesis of silver nanoparticles and characterization
The starch was solubilized in distilled water with magnetic stirring and heat (70 °C) for 30 minutes. Then, AgNO3 solution was added in starch solution under magnetic stirring and ice bath (5 °C). After 10 minutes, NaBH4 solution was added with speed of 1 mL min-1. The magnetic stirring was kept for 30 minutes and the final solutions concentrations were 3.6 x 10-3 mol L-1 NaBH4, 9 x 10-4 mol L-1 AgNO3 and 0.6% (w/v) starch solution. The size of nanoparticles was evaluated using UV-Vis spectroscopy (Varian Cary 50 Bio) and dynamic light scattering (DLS). Zeta potential measurements (Zetasizer Nano Series ZS90) were performed to verify the AgNPs stability. The starch-stabilized silver nanoparticles (AgNPs-St) were stored in a dark bottle at room temperature (approximately 25 °C). DLS measurements (Zetasizer Nano Series ZS90) were performed in triplicate during a period of 115 days to check AgNPs agglomeration. The structure of AgNPs-St was characterized using X-ray diffractometer (XRD) Rigaku Ultima IV with Cu-Kα (λ=1,54 Å) incident radiation. The diffracted intensities were recorded from 25º to 85º 2θ angles. Transmission electron microscopy (TEM) images were recorded by a JEOL JEM 1200EX-II transmission electron microscope operated at 120kV.
2.3.
LbL assembly of (SiPy+Cl-/AgNPs-St)
The fluorine-doped tin oxide (FTO) substrates were previously cleaned by immersion in solution of NH4OH, H2O2 and water (1:1:5) and heated (70 ºC) for 10 minutes. Then, the substrates were immersed in isopropyl alcohol under heat for few minutes. Finally, the substrates were dried at room temperature. The LbL films were carried out by immersion of FTO alternately in SiPy+Cl- (2.0 mg mL-1 and pH 6.5), used as polycation, and AgNPs-St (9.0 x 10-4 mol L-1 and pH 9.0), as polyanion. The immersion time for both polyelectrolytes was 5 minutes. After FTO immersion in each polyelectrolyte solution, the films were washed in distilled water (10 seconds) and dried in air. This process was repeated until the desired bilayers number was obtained. The films produced were denoted as (SiPy+Cl-/AgNPs-St)n where n is the bilayers number.
2.4. Characterization of (SiPy+Cl-/AgNPs-St)n LbL film The kinetic study of deposition of polyelectrolytes was evaluated by UV-Vis spectroscopy using films containing from 1 to 15 bilayers. For this study, the (SiPy+Cl-/AgNPs-St)n LbL films (n= number of bilayers) were deposited onto quartz substrates. Fourier transform infrared spectroscopy (FTIR) measurements were carried out for the SiPy+Cl- and AgNPs-St drop-coated films and (SiPy+Cl-/AgNPs-St)5 LbL film deposited onto FTO substrates using a PerkinElmer® Frontier FTIR spectrometer coupled with universal attenuated total reflectance (ATR) sampling accessory. Spectra were collected in the transmission mode in the range from 660 to 4500 cm-1 at 4 cm-1 resolution, with 128 scans. For the starch, FTIR measurements were obtained in a Shimadzu IR Prestige 21 FTIR spectrometer, using KBr pellets, in the transmission mode in the range from 400 to 4000 cm-1. The spectral resolution was set at 4 cm-1 and with 64 scans. Film morphology analyses were obtained with an Atomic force microscope (AFM) Shimadzu SPM 9600, in non-contact mode with length 1 µm and width 1 µm, using (SiPy+Cl-/AgNPs-St)n LbL films with n = 2, 4, 5, 6, 8 and 10 bilayers. The thickness measurements were based on isolating a small area of FTO substrate in order to get a “step”
to detect the height difference between the LbL film and substrate.
2.5. Electrochemical studies Electrochemical measurements were carried out in three-electrode electrochemical cell: Ag/AgCl electrode was used as the reference electrode (RE), platinum wire as counter electrode (CE) and the working electrode (0.5 cm2 of geometric area) was the (SiPy+Cl-/AgNPs-St) LbL films. Preliminary studies were conducted by cyclic voltammetry (CV) in the potential range from 0.0 V to -1.0 V at 50 mV s-1 using supporting electrolyte 0.1 mol L-1 NaNO3 and 0.1 mol L-1 phosphate buffered saline (PBS), both with pH 7.0. Differential pulse voltammetry (DPV) were performed for studies of triiodide determination. Before the measurements, the supporting electrolyte was purged with nitrogen for 5 min. It was used triiodide solution (I2/KI) standardized. Studies with different supporting electrolytes were
performed:
Phosphate
Buffered
Saline
(PBS),
NaH2PO4,
Na2HPO4
and
Britton-Robinson (BR) buffer, in a concentration of 0.1 mol L-1. The measurements were carried out in the potential range from 0.0 to -1.0 V using scan rate of 50 mV s-1, pulse potential (Epulse) of 0.05 V, pulse time (tpulse) of 0.05 s and triiodide concentration of 1.0 x 10-3 mol L-1. These same voltammetric conditions were used to verify the interaction between the films components and for the study of bilayers number in 0.1 mol L-1 PBS with pH 7.0. The instrumental parameters - pulse potential (Epulse), the pulse time (tpulse) and the scan rate (v) - for triiodide determination by DPV were optimized by 23 full factorial design, including a central point assayed in quadruplicate to calculate standard deviation. The factors and levels, which were selected from previous studies, were: Epulse = 50 mV (−1) and 70 mV (+1); tpulse = 0.03 s (−1) and 0.05 s (+1); v = 20 mV s-1 (−1) and 40 mV s-1 (+1). The DPV measurements were performed with (SiPy+Cl-/AgNPs-St)5 LbL film, concentration of triiodide of 1.0 x 10-3 mol L-1 in the potential range from 0.0 V to -1.0 V and step potential of 0.005 V. Analytical curves were obtained from a stock solution of triiodide (1.0 x 10-3 mol L-1). The detection limit (DL) and the quantification limit (QL) were calculated by the standard deviation of the mean current for blank signal in pure electrolytes (Sb) and the slope of the analytical curves (s), using DL = 3Sb/s and QL = 10Sb/s according IUPAC.[42]
3.
Results and Discussion
3.1. Characterization of AgNPs-St The formation of AgNPs-St was monitored by UV-Vis spectroscopy (Fig. 1a). The absorbance band observed at 400 nm corresponds to the characteristic surface plasmon resonance band of AgNPs, which appears usually near to 420 nm.[43,44] The DLS results (Fig. 1b) show two populations of AgNPs-St in nanometric scale with average sizes of 21 and 77 nm. Zeta Potential measurements suggest stability and negative charge for the AgNPs-St, with value of -113 mV. The AgNPs-St was monitored by DLS for 30 days to check for NPs agglomeration. Any significant change in size distribution of the AgNPs-St was noticed during 115 days of storage of the AgNPs-St, indicating low agglomeration (Fig. 1c). It can be observed in Fig. 1c, the AgNPs-St shows two populations (similar to Fig. 1b), however the percentage of distribution is higher for larger particles sizes. Nevertheless, the nanoparticle sizes do not exceed 150 nm similar to the DLS results obtained in the first day. Fig. 2 provides TEM images of the AgNPs-St indicating that the AgNPs formed inside of starch chains are smaller, approximately 4-5 nm (Fig. 2a), while the AgNPs formed outside of starch chains presented larger sizes (approximately 80 nm), as shown in Fig. 2b. These results confirm the size distribution found by DLS. Furthermore, it is observed that the shape of the AgNPs-St is predominantly spherical.
XRD analysis was carried out to investigate the crystal structure of AgNPs-St formed. XRD patterns of lyophilized AgNPs-St (Fig. 3a) exhibits characteristic peaks at 2θ of 38.0, 44.1, 64.3 and 77.2, indexed to crystallographic planes (111), (200), (220) and (311), respectively. Similar values were described in the literature for metallic silver.[43,45] These diffraction peaks are referring to a face centered cubic (FCC) structure typically presented for Ag. Fig. 3b shows XRD patterns of AgNPs-St lyophilized and calcined (500 °C for 1 hour) to remove the high noise originated from the starch. In this XRD spectrum, it was observed the peaks at scattering angles (2θ) of 38.0, 44.2, 64.3 and 77.3, similar to AgNPs-St, however with higher intensity and sharper than the non-calcined AgNPs-St. This behavior was expected due
to the removal of noisy background and the growth of crystallites (Fig. 3b). This is due to the removal of noisy background and the growth of crystallites.[43]
3.2. Growth and characterization of (SiPy+Cl-/AgNPs-St)n Layer-by-Layer films The negative charge of AgNPs-St allows it to be used as polyanion in the growth of LbL films combined with SiPy+Cl- as polycation. The growth of the bilayers was monitored by UV-Vis spectroscopy. Initially, the spectra of precursors were carried out to observe the characteristic absorption band of the components used for LbL films growth (Fig. 4a). The AgNPs-St spectrum exhibits an absorption band at 400 nm, which corresponds to surface plasmon resonance band of AgNPs.[43] The starch solution spectrum does not exhibit any absorption bands in UV-Vis spectra in the range of 320 to 500 nm and the SiPy+Cl- exhibited one absorption band at 258 nm attributed to the 𝜋 → 𝜋 ∗ transition of the pyridine groups in SiPy+Cl-, as related in the literature.[35] As shown in Fig. 4a, (SiPy+Cl-/AgNPs-St)5 LbL film on quartz showed absorption band at 425 nm attributed to AgNPs. The band shift observed in relation to the AgNPs-St spectra can be indicative of the electrostatic interactions between AgNPs-St and SiPy+Cl- solution. In a similar study, the interaction between polyelectrolytes (pyridinium ion of SiPy+Cl- with platinum nanoparticles incorporated, Pt-SiPy+Cl- and sulphonatephenylenevinylene, PVS) in the LbL film was confirmed by the shifts of the bands of UV-Vis and FTIR spectra. [36] The LbL film growth was monitored by UV-Vis spectroscopy observing the increase in absorbance intensities at 425 nm (Fig. 4b). A linear increase in absorbance with bilayers number deposited (n = 1 to 15) indicates LbL film growth with R = 0.993. The kinetic study of the deposition was performed by varying the immersion time (60 – 300 seconds) of the quartz substrate in the polyelectrolytes solution and the intensity of the band was monitored at 425 nm. Fig. 4c showed linear growth in the absorbance band as a function of the immersion time between 60 to 180 seconds (R = 0.9992) and maximum absorbance at 240 seconds. For the subsequent studies, it was fixed an immersion time of 4 minutes (240 s) because in this time occurs the saturation of the LbL surface, since any significant increase in the absorbance is observed.
Fig. 5 shows the FTIR spectra obtained for starch, drop-coated film of AgNPs-St and SiPy+Cl- and (SiPy+Cl-/AgNPs-St)5 LbL film. The analysis of the spectrum of starch (Fig. 5a) presented bands at 993 and 1171 cm-1 for the C-O-C symmetric stretching and other at 2928 cm-1 attributed to the C-H stretching (not shown). In the FTIR spectrum of drop-coated film of AgNPs-St (Fig. 5b), it can be seen two main bands at 1078 and 1005 cm-1 assigned to symmetric stretching of the C-O-C groups of the starch. Once the starch was used as stabilizing agent, these small shifts of the C-O-C bands are indicative of possible interactions between starch and AgNPs. The drop-coated film of SiPy+Cl- spectrum (Fig. 5c) shows a band at 775 cm-1 related to the Si-O-Si group symmetric stretching, other in 833 cm-1 attributed to different vibrational modes with large contribution of bands related to angular deformation of the Si-C-H group and stretching of Si-C. In addition, the SiPy+Cl- spectra also presented a broad and intense band at 1100 cm-1 assigned to the Si-O-Si groups asymmetric stretching. The characteristic bands of the pyridinium ring were of observed at 1634 and 1489 cm-1.[35,36]
However, in the (SiPy+Cl-/AgNPs-St)5 LbL film spectrum, the C-O-C symmetric stretching of AgNPs-St appeared at 1005 cm-1 and 1009 cm-1. The band of SiPy+Cl- at 775 cm-1 related to the Si-O-Si group symmetric stretching appeared at 764 cm-1 in the (SiPy+Cl-/AgNPs-St)5 LbL film. The band related to pyridinium ring appeared at 1634 and 1489 cm-1 for SiPy+Cl- and at 1638 and 1490 cm-1 for the (SiPy+Cl-/AgNPs-St)5 LbL film. Also, it can be noted that the absorption band at 833 cm-1 for SiPy+Cl-, assigned to angular deformation of the Si-C-H group and stretching of the Si-C, was shifted for (SiPy+Cl-/AgNPs-St)5 LbL film at 863 cm-1. The broad band related to Si-O-Si asymmetric stretching of the cage structure at 1100 cm-1 and the bands related to C-O-C symmetric stretching of AgNPs-St at 1005 cm-1 and 1009 cm-1 occur in the same region of the spectrum. Therefore, due to overlapping of these peaks in this region of the spectrum for (SiPy+Cl-/AgNPs-St)5 LbL film, the assignment of the bands cannot be straightforward. In general, the spectra of the (SiPy+Cl-/AgNPs-St)5 LbL film is similar to those obtained for the polyelectrolytes in drop-coated films, which confirms the interaction
between the AgNPs-St and SiPy+Cl- in the LbL films.[35,36] Fig. 6 shows AFM images for the (SiPy+Cl-/AgNPs-St)n LbL films (n=0, 2, 4 and 10). The LbL films were deposited on FTO and AFM analysis were carried out in three different regions to minimize errors. The root-mean-square (RMS) of the (SiPy+Cl-/AgNPs-St)n LbL films (n=0, 2, 4, 6, 8 and 10), listed in Table 1, revealed a decrease in the roughness of the film from FTO (31.148 ± 2.217 nm) until 8 bilayers (22.321 ± 0.980 nm). This behavior can be attributed to the random deposition of the molecules covering the irregularities of the substrate, thus decreasing the roughness. It is important to note that the films with 8 bilayers (22.321 ± 0.980 nm) and 10 bilayers (22.878 ± 2.956 nm), considering the standard deviation, practically presented no variation of roughness. This suggests homogeneity in the deposition of the film LbL from 8 bilayers. The literature shows similar results for others LbL films.[35,46]
The thickness of the LbL films was obtained by AFM technique (Fig. 7). The LbL films exhibited thickness in the range of 254.32 to 1130 nm. Furthermore, the results showed that the increase in the thickness of the films is linear from 4 to 10 bilayers. This indicates that the same amount of materials was deposited at each bilayer.
3.3. Electrochemical detection of triiodide with (SiPy+Cl-/AgNPs-St)n LbL films Differential pulse voltammetry (DPV) studies were performed to evaluate the electrochemical sensing properties of LbL films in the presence of triiodide (Fig. 8). The voltammetric determination of iodine can be evaluated through triiodide as demonstrated by equations to follow:[47] 𝐼2 (𝑠) + 𝐼 − (𝑎𝑞) ⇌ 𝐼3− (𝑎𝑞) 𝐼3− (𝑎𝑞) + 2𝑒 − ⇌ 3𝐼 − (𝑎𝑞)
A comparative study was also carried out with the (SiPy+Cl-/AgNPs-St)5 LbL film, the SiPy+Cl- drop-coated film, the starch drop-coated film, the AgNPs-St drop-coated film and the FTO substrate. For this study a fixed concentration of 1.0 x 10-3 mol L-1 triiodide in 0.1 mol L-1 PBS at pH 7.0 was used, as shown in Fig. 8. Comparing the results obtained, it can be observed
that (SiPy+Cl-/AgNPs-St)5 LbL film presented higher current values (Ipc=-90.86 μA) and less negative reduction potential (Epc=- 0.26 V), in relation to the others modified electrodes. It can be noted that drop-coated film of starch showed lower current (Ipc=-15.34 μA, Epc=-0.535), while the FTO substrate exhibited current of Ipc=−48.62 μA and Epc=-0.555. These results confirmed that the (SiPy+Cl-/AgNPs-St)5 LbL film exhibited higher electrocatalytic activity due to the synergistic effect between the film components.
Fig. 9 shows the spectra for AgNPs-St in presence of different concentrations of triiodide. UV-Vis spectroscopy revealed that the increasing of triiodide concentration reduces the intensity of characteristic band of AgNPs (~420nm) indicating that triiodide firstly interacts with AgNPs-St. This interaction can be responsible for the increase in the intensity of the cathodic current for the (SiPy+Cl-/AgNPs-St)5 LbL film. Only at higher concentrations of triiodide it starts to react with starch. These results are in agreement with Vigneshwaran et al which proved that previously occur the interaction between the AgNPs and triiodide. Then, the triiodide tends to react with starch forming the complex with amylose, as reported in the literature.[40] The dependence of the electrochemical response of (SiPy+Cl-/AgNPs-St)5 LbL film was studied as a function of bilayers number. For these studies, LbL films were prepared containing from 1 to 9 bilayers with SiPy+Cl-/AgNPs-St architecture and voltammetric response evaluated in presence of 1.0 x 10-3 mol L-1 triiodide (Fig. 10). Five-bilayer SiPy+Cl-/AgNPs-St LbL film was chosen for the subsequent studies because it presented higher current values, similar to those previously reported in the literature.[33,36] The current tends to be lower with the increase of bilayers due to the film resistance, which inhibits the electron transfer process.
3.4. Optimization of DPV parameters by factorial design The instrumental parameters of DPV were studied in order to improve the response of peak current for triiodide determination. DPV parameters were optimized using a 23 full
factorial design, thus the pulse potential (Epulse), the pulse time (tpulse) and the scan rate (v) were simultaneously evaluated, making it possible to obtain the best analytical signal in terms of current intensity. The results are shown in Table 2.
According to statistical analysis of the data, the main effects (Epulse =
6.36, tpulse =
4.03 and v = 6.98), second-order effects (Epulse vs tpulse = -6.44, Epulse vs v = -3.20, tpulse vs v = -5.97), as well as the third-order effect (-1.03) were significant at 95% confidence levels (0.14 x 3.18 t95%, v=3 = 0.45). The geometric representation for interaction of three factors (Fig. 11) shows that the highest current intensity (-47.273 µA) is obtained at the -++ vertex for the factors Epulse, tpulse, and v respectively. Hence, the optimal DPV parameters were: pulse potential (Epulse) of 50 mV, pulse time (tpulse) of 0.05 s and scan rate (v) of 40 mV s-1.
3.5. Analytical curves Analytical curves were obtained in triplicate and the results presented in Fig. 12.Under the optimized conditions, the analytical curves were obtained in the range from 4.34 x 10 -5 to 3.47 x 10-4 mol L-1 (Fig. 12a) and from 4.4 x 10-4 to 4.24 x 10-3 mol L-1 (Fig. 12b) of triiodide. The detection limit (DL) and quantification limit (QL) were calculated as recommended by IUPAC.[42] From Fig. 12a, it was found a linear equation: I/µA = – 0.747 – 3.05 x 104 [triiodide] (R = 0.9936), DL = 5.56 x 10-6 mol L-1 and QL = 1.85 x 10-5 mol L-1. From Fig. 12b, it was found a linear equation: I/µA = – 15.949 – 2.51 x 104 [triiodide] (R = 0.9938), DL =1.51 x 10-5 mol L-1 and QL= 5.04 x 10-5 mol L-1.
In order to evaluate the efficiency of the proposed methodology, recovery tests were performed in pure electrolyte (0.1 mol L-1 PBS, pH 7.0). Three determinations were carried out for each analytical curve. All data were obtained in triplicate and the standard deviations were calculated. As shown in Table 3, the recovery values for Fig. 12a and Fig. 12b ranged from
100.60% to 103.26% and from 91.55% to 95.88%, respectively. The percentages of recovery in these samples proved to be suitable for analytical purposes.
Table 4 presents the response characteristics of different electrodes including, electrodes composition, linear range, quantification and detection limit. It is clear that response characteristics vary with the modified electrodes. From Table 4, it can be seen that the LbL film electrode showed in this work has detection limit comparable with electrodes previously reported in the literature. In addition, considering the linear range, the performance of the (SiPy+Cl-/AgNPs-St) LbL film with two linear detection ranges also allow determination in wide concentration range (in the order of 10-5 to 10-3 mol L-1).
4.
Conclusions
In this paper, TEM images of starch-stabilized silver nanoparticles indicated that AgNPs formed inside of starch chains were smaller (≈5nm) than AgNPs formed outside of starch chains (≈80nm). These silver nanoparticles were combined with SiPy+Cl- in LbL film and provide interesting electrochemical properties. The (SiPy+Cl-/AgNPs-St) LbL film was applied as modified electrode for triiodide determination as triiodide due to its interaction with AgNPs-St. This interaction provides an increase in the intensity of the cathodic current for the (SiPy+Cl-/AgNPs-St)5 LbL film when compared with the use of the LbL film components separately. The electrochemical measurements showed linear response in two concentrations ranges with low QL and DL in the order of 10-6 mol L-1 to 10-5 mol L-1 and recovery tests presented values over 90%. The concentration range of triiodide determination is larger than other values reported in the literature. These electrochemical response obtained in our studies open up the possibility of an alternative method for triiodide determination.
Acknowledgments The authors would like to thank Brazilian funding agencies (Fundação Araucária and CAPES) for their financial support of this research work.
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0.7
10
(b)
(a) 8
0.5
Distribution (%)
Absorbance (a.u.)
0.6
0.4 0.3 0.2
6 4 2 0
0.1 0.0 200
300
400
500
600
700
-2 0
20
40
60
80
100
120
140
160
Particle size (nm)
Wavelength (nm) (c) 10
Distribution (%)
8
6
4
2
0 20
40
60
80
100
120
140
Particle size (nm)
Fig. 1. (a) UV-Vis spectra of AgNPs-St, (b) the particle size histogram by DLS 1 day and (c) after 115 days.
(a)
(b)
Fig. 2. TEM images of AgNPs-St dispersion: (a) inside and (b) outside of starch chains.
700
2000
(a)
Ag (111)
600
(b)
Ag (111)
1800 1600 1400
400
Intensity
Intensity
500
Ag (200)
300
1200 1000
Ag (200) 800 600
Ag (220)
Ag (220)
200
Ag (311)
Ag (311)
400 200
100
0
25
0 25
30
35
40
45
50
55
60
2 (degree)
65
70
75
80
85
30
35
40
45
50
55
60
65
70
75
80
2 (degrees)
Fig. 3. XRD spectra of (a) lyophilized AgNPs-St and (b) samples obtained after carbonization at 500 °C for 1 h.
0.7
0.30
AgNPs-St Starch + (SiPy Cl /AgNPs)5
(a)
0.5
(b) 0.25
0.4 Absorbance (a.u.)
Absorbance (a.u.)
0.6
0.3 0.2 0.1 0.0
0.15
0.10
0.05
-0.1 -0.2 320
0.20
340
360
380
400
420
440
460
480
500
0.00 3
5
Wavelength (nm)
Absorbance (a.u.)
0.55
7
9
11
13
15
Bilayers number
(c)
0.50
0.45
0.40
30
60
90
120
150
180
210
240
270
300
330
Immersion time (s)
Fig. 4. (a) UV-Vis spectra of AgNPs-St (diluted in distilled water 1:20), starch 1% (w/v) solution and (SiPy+Cl-/AgNPs-St)5 LbL film (b) Relationship between the absorbance at 425 nm and bilayers number. (c) Adsorption kinetic of the (SiPy+Cl-/AgNPs-St)5 LbL film.
(a) 993
1171
Intensity (a.u.)
(b) 1078
1005
(c) 833
1634
775 1489
(d)
1100 1490 1638
863 1151
764
1078 1009
1700 1600 1500 1400 1300 1200 1100 1000 900
800
700
-1
Wavenumber (cm )
Fig. 5. FTIR spectra for films of (a) starch, (b) drop-coated film of AgNPs-St, (c) drop-coated film of SiPy+Cl- and (d) (SiPy+Cl-/AgNPs-St)5 LbL film.
(a)
(b)
(d)
(c)
Fig. 6. - AFM images of (a) FTO, (b) (SiPy+Cl-/AgNPs-St)2, (c) (SiPy+Cl-/AgNPs-St)6 and (d) (SiPy+Cl-/AgNPs-St)10 LbL films. Scan window of 1 x 1 µm.
1200
Thickness (nm)
1000
800
600
400
200
0 4
5
6
7
8 +
9
10
-
Bilayers number (SiPy Cl /AgNPs-St)
Fig. 7. Relationship between thickness and bilayers number for (SiPy+Cl-/AgNPs-St)n LbL films with n = 4, 5, 6, 8 and 10 bilayers.
-120
3-
-3
[I ]=1 x 10 mol.L
-1
FTO + (SiPy Cl /AgNPs-St)5
-100
+
-80
I (A)
-
SiPy Cl AgNPs-St Starch
-60 -40 -20 0 0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1
E (V) vs Ag/AgCl PBS 0.1 mol L pH 7 Fig. 8. Differential pulse voltammograms in presence of 1.0 x 10-3 mol L-1 of triiodide in 0.1 mol L-1 PBS, pH 7.0 for (SiPy+Cl-/AgNPs-St)5 LbL film, SiPy+Cl- drop-coated film, starch drop-coated film, AgNPs-St drop-coated film and FTO substrate. Potential range from 0.0 to -1.0 V vs. Ag/AgCl, Estep = 0.005 V, = 50mV s-1, Epulse = 0.05 V and tpulse = 0.05 s.
1.0
[Iodine] -1 mol L 2.4 5.7 10.0 15.0 55.0 69.0 89.0
Absorbance (a.u.)
0.8
0.6
0.4
0.2
0.0 300
400
500
600
700
800
Wavelength (nm)
Fig. 9. UV-Vis spectra for different concentrations of triiodide.
-120
-100
I (A)
-80
-60
-40
-20
0 0
2
4
6
8
10
Bylayers number
Fig. 10. Current obtained by DPV as a function of bilayers number for (SiPy+Cl-/AgNPs-St)n. n = 1 to 9 bilayers. Parameters: 0.1 mol L-1 PBS, pH 7.0, iodine 1.0 x 10-3 mol L-1. Estep = 0.005 V, v = 50 mV s-1, Epulse = 0.05 V and tpulse = 0.05 s.
Fig. 11. Geometric representation of interaction: pulse potential vs scan rate vs pulse time.
-14 -120
(a)
(b)
-12 -100
-10
-80
I (A)
I (A)
-8 -6 -4
-60 -40
-2
-20
0 0.00
0.05
0.10
0.15
0.20
0.25
[triiodide] mmol L
0.30 -1
0.35
0.40
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
[triiodide] mmol L
3.5
4.0
4.5
-1
Fig. 12. Analytical curves of the electrochemical sensor (SiPy+Cl-/AgNPs-St)5 as a function of triiodide concentration (a) 4.34 x 10-5 to 3.47 x 10-4 mol L-1 and (b) 4.4 x 10-4 to 4.24 x 10-3 mol L-1, in 0.1 mol L-1 PBS, pH 7.0, scan range from 0.0 to -1.0 V vs. Ag/AgCl. Estep = 0.005 V, v = 40 mV s-1, Epulse = 0.05 V and tpulse = 0.05 s.
Table 1 – Root-mean-square (RMS) roughness for (SiPy+Cl-/AgNPs-St)n LbL films Bilayers number RMS roughness (nm) (1 x 1 μm) 0
31.148 ± 2.217
2
29.499 ± 3.494
4
25.290 ± 2.751
6
23.939 ± 3.071
8
22.321 ± 0.980
10
22.878 ± 2.956
Table 2. Factors, levels, and response values for the 23 full factorial design for instrumental parameters of DPV. Factors Runs
Response
Epulse
v
tpulse
I (µA)
01
-
-
-
−26.63
02
+
-
-
−41.60
03
-
+
-
−29.80
04
+
+
-
−40.43
05
-
-
+
−30.09
06
+
-
+
−34.25
07
-
+
+
−47.27
08
+
+
+
−42.96
09
0
0
0
−33.48
10
0
0
0
−33.64
11
0
0
0
−33.78
12
0
0
0
−33.50
Table 3. Recovery results for triiodide in electrolyte using (SiPy+Cl-/AgNPs-St)5 LbL film in 0.1 mol L-1 PBS, pH 7.0. Estep = 0.005 V, v = 40 mV s-1, Epulse = 0.05 V and tpulse = 0.05 s. Added Analytical curve
1
2
Found
Recovery
(mmol L-1) (mmol L-1) (%) 0.08
0.08 ± 0.02 103.26
0.08
0.08 ± 0.02 100.60
0.15
0.15 ± 0.01 102.76
1.28
1.21 ± 0.06 94.53
1.70
1.63 ± 0.02 95.88
2.84
2.60 ± 0.03 91.55
Table 4. Construction and performance of several LbL film electrodes. Type
of
Sensor
Voltammetric
Electrodes
(mol
LbL
4.34 x 10-5
(SiPy+Cl-/AgNPs-St)5
to 3.47 x 10-4
LbL
4.40 x 10-4
(SiPy+Cl-/AgNPs-St)5
to 4.24 x 10-3
DL (mol
QL L-1)
(mol L-1)
5.56 x 10-6
1.85 x 10-5
10-5
10-5
1.51 x
5.04 x
Kormosh
3.9 x 10−7
N
1 x 10-6 to 1 x 10-1
5 x 10−7
N
PMeT
1 x 10-6 to 1 x 10-1
N
N
(KCH2)(I3)2
2 x 10-3 to 7 x 10-6
3 x 10-6
N
AuNPs
1 x 10-8 to 6 x 10-7
1 x 10-8
N
Membrane ZnO nanotubes
Reference
This paper
1 x 10-6 to 1 x 10-1
Rhodamine B
Colorimetric
L-1)
composition
Membrane
Potentiometric
Linear range
et al [38] Ibupoto et al [37] Karagozler et al [39] Farhadi et al [48] Chen et al [49]
Note: N, not reported; LbL, Layer-by-Layer; SiPy+Cl-, 3-n-propylpyridinium silsesquioxane; AgNPs-St, starch-stabilized silver nanoparticles; ZnO, zinc oxide; PMeT, poly(3-methylthiophene; (KCH2)(I3)2, deprotonated ketoconazole-triiodide; AuNPs, gold nanoparticles.