Layer-by-Layer Films of Graphene Nanoplatelets and Gold

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Layer-By-Layer Films of Graphene Nanoplatelets and Gold Nanoparticles for Methyl Parathion Sensing Gustavo Rodrigues, Celina Massumi Miyazaki, Rafael Jesus Gonçalves Rubira, Carlos Jose Leopoldo Constantino, and Marystela Ferreira ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00007 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Nano Materials

Layer-By-Layer Films of Graphene Nanoplatelets and Gold Nanoparticles for Methyl Parathion Sensing

Gustavo H. S. Rodrigues†, Celina M. Miyazaki†, Rafael J. G. Rubira‡, Carlos J. L. Constantino‡, Marystela Ferreira†*

†Universidade

Federal de São Carlos, CCTS, Sorocaba, SP, CEP 18052-780 – Brazil

‡Universidade

Estadual Paulista, Presidente Prudente – SP, CEP 19060-900 – Brazil

*[email protected]

KEYWORDS layer-by-layer film, polymer stabilized graphene, gold nanoparticles, electrochemical sensor, methyl parathion

ABSTRACT The wide use of pesticides in agriculture makes necessary the development of cost-effective, easy and rapid methods aiming for environmental and health protection. We propose the fabrication and investigation of Layer-by-Layer (LbL) films composed of reduced graphene oxide stabilized in polyelectrolytes in the presence and absence of gold nanoparticles (AuNPs) towards methyl parathion (MP) detection.

ACS Paragon Plus Environment

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Besides the film assembly and sensing application, we performed a systematic investigation of the influence of MP distribution on the limit of detection and linear range of the sensor. Indium tin oxide (ITO) electrodes modified with the LbL films were applied by differential pulse voltammetry achieving a LOD and a linear range of 0.226 ppm and 0.25–40 ppm in the absence of AuNPs, while in the their presence, a wider linear range was achieved (0.5–60 ppm, with LOD of 0.770 ppm). Real samples analysis indicated potential application in water, ground, and vegetables, with recovery varying from 92 to 113 %.

Introduction Organophosphorus pesticides are widely used to control agricultural pests,1 being frequently found in food crops, ground water, soil and atmosphere.1 Methyl parathion (MP) [O,O-dimethyl-O-(4-nitrophenyl) phosphorothioate] is considered extremely toxic by the World Health Organization,2 and related to a number of health problems. These include malfunction in the central nervous system due to acetylcholinesterase (AChE) inhibition,3,4 genotoxic and mutagenic effects in cells, chromosome aberrations,5,6 type 2


2

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diabetes induction,1 and infertility due to sperm damage.7 Because of its toxicity, MP was banned from the European Union and by the Brazilian Regulation Agency (ANVISA). However, many developing countries still utilize it due to its high efficiency in the control of weeds. Therefore, methods are required to monitor MP already released in the environment in a fast, effective way, in addition to identifying its illicit use. High-performance liquid chromatography (HPLC) allied to spectrophotometry techniques are commonly used in detecting pesticides, but they are unsuitable for routine operations. Indeed, these methods are time-consuming, require expensive instrumentation, and demand sample preparation steps and specialized professionals. Electrochemical methods are promising alternatives for their low-cost, high sensitivity and specificity8,9, being also simple and fast. Many electrochemical sensors for MP are based on the AChE inhibition,4,10–12 but the use of such proteins increases the cost of fabrication since its immobilization on the electrodes demands special care in the control of temperature and pH, and for transport and storage of the sensing unit.13 Robust

nonenzymatic

electrochemical

sensors

may

be

made

with

catalytic

nanostructured materials, with which faster electron transfer kinetics, lower detection


3


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potentials and high sensitivity have been obtained.14 Tian et al. developed a nonenzymatic sensor for MP made with CuO-TiO2 modified glassy carbon electrode (GCE), whose limit of detection (LOD) was 1.21 ppb,15 while Kumaravel and Chandrasekaran16 fabricated a nanosilver/Nafion composite electrochemically deposited on GCE electrodes with a LOD of 0.08 μmol·L-1. Electrode modification with nanomaterials can be performed with the Layer-by-Layer (LbL) technique, which is a versatile, low-cost method to functionalize surfaces via spontaneous adsorption of charged materials on solid surfaces by electrostatic attraction, Van der Waals interactions, or hydrogen bonding.17,18 It has been proven effective to modify solid electrodes giving the desired functionality to the sensing units by nanostructuring multilayers, allowing synergy among distinct materials.19–21 Despite these advantages, the LbL technique has been little explored for MP sensor fabrication. Graphene-based materials have been applied in sensors and biosensors due to their efficient electron transfer and for promoting direct electron transfer in enzymatic sensors.22 GCE modified with graphene and inorganic materials were applied in MP electrochemical detection.23–25 Because of its hydrophobicity, stable aqueous


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dispersions

of

graphene

nanoplatelets

were

stabilized

with

polymers22,26

or

functionalized by polar groups.27,28 Metal nanoparticles have also had their catalytic and optical properties exploited in sensing,29 leading to high-performance electrochemical sensors in terms of high sensitivity and selectivity, i.e. less interference by lowering the redox potential (less negative potential for reduction, or less positive in oxidation).30–33 In this study, we report the fabrication of LbL films made with functionalized graphene nanoplatelets and gold nanoparticles to detect MP in a non-enzymatic sensor. The novelty here is related to the deep investigation of the interactions between the nanostructured materials on the film architecture and also the systematic study of the distribution of the analyte on the surface of each sensor to elucidate how the MP interacts with the film materials. Differential pulse voltammetry (DPV) was used to detect MP in standard solutions and in samples of tap water, soil and cabbage.

Experimental Section

Chemicals


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Graphite powder, H2SO4, KMnO4, K2S2O3, P2O5, H2O2, H6N2O4S, were all analytical grade and used in the graphite oxidation processes. Gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9%), sodium borohydride, poly(styrene sulfonate) sodium salt - PSS (Mw = ~ 70000), poly(diallyldimethylammonium chloride) - PDDA (20 wt.% in water, Mw =100,000 –200,000), and poly(allylamine hydrochloride) - PAH (Mw ~15,000 ) were purchased from Sigma-Aldrich and used as received. Ultrapure water was obtained from a Sartorius water purification system (18.2 MΩ cm at 25 °C).

GPDDA, GPSS, and AuNP synthesis Graphite was pre-oxidized as described in 34,35. Briefly, 5 g of graphite was added to a mixture of 7.5 mL H2SO4, 2.5 g K2S2O8 and 2.5 g P2O5 (at 80 °C) and cooled down until room temperature. After dilution with 500 mL of ultrapure water, the solid was vacuum filtrated, washed until neutral pH and dried. In the following oxidation step, 5 g of preoxidized graphite was added to the 115 mL concentrated H2SO4 cooled down to 0°C. Subsequently, the Hummers’ method was applied for oxidation.36 15 g of KMnO4 was slowly added, keep stirring for 2 h and the temperature less than 20 °C. 230 mL of


6

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ultrapure water was added, allowing the reaction to take place for 15 minutes, after which 700 mL of ultrapure water and 12.5 mL of H2O2 were poured. At this point, the color changed from dark brown to yellow. The solid GO was filtrated and washed with 1.25 L of 1:10 HCl. Graphene nanoplatelets stabilized in PDDA (GPDDA) were synthesized following the procedures adopted by Mascagni et al.22. Briefly, 20 mg of GO was suspended in 200 mL of ultrapure water, sonicated for 2 h and heated up to 40 °C. The mixture was dripped in a PDDA suspension (30 g·L-1) under vigorous stirring. Hydrazine sulfate (0.05 g·L-1) was added and kept under reflux at 90 °C for 12 h. The PSS stabilized graphene nanoplatelets (GPSS) were prepared as in Miyazaki et al,22,35 with 250 mL of 1.0 mg·mL-1 GO aqueous suspension being sonicated for 30 min. and 2.5 g of PSS was added under vigorous stirring. After addition of 0.3255 g of hydrazine the mixture was kept at 90 °C under reflux for 12 h. Both GPDDA and GPSS were vacuum filtrated and dried in a vacuum oven at 90 ºC. Gold nanoparticles (AuNP) were synthesized by salt reduction and with a polymeric stabilizing agent. A 1.0 mmol·L-1 PAH (cationic polyelectrolyte) suspension was added


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to a 2.0 mmol·L-1 HAuCl4·3H2O solution, followed by addition of 10 mmol·L-1 sodium borohydride in a proportion of 1:1:1 (v/v/v). The resulting suspension was used without any purification step.

Layer-by-Layer film assembly and characterization Positively charged GPDDA and negatively charged GPSS aqueous suspensions were prepared at 0.1 mg·mL-1 by sonication bath for 30 min, while the AuNP suspension was used as synthesized without any dilution or additional procedures. Scheme 1 illustrates the procedures for fabrication of two LbL architectures: (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10. For (GPDDA/GPSS)10, the substrate was first immersed into the positively charged GPDDA suspension because the substrates (quartz slides for UV-vis spectroscopy or ITO-covered glass for electrochemistry) exhibit negatively charged oxygen species

37,38.

After 15 min, the substrate was submitted to a washing step by

immersion in ultrapure water for 30 s to remove weakly adsorbed material and avoid cross-contamination. Then, the GPDDA-coated substrate was immersed in a GPSS suspension for 15 min, followed by another washing step. These steps can be repeated


8

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to control the number of layers, and for assembly of the (GPDDA/GPSS)10 film, they were

repeated

10

times.

A

similar

procedure

was

adopted

for

the

(GPDDA/GPSS)1(AuNP/GPSS)10 film. For the AuNP suspension, immersion was made for 6 min (except for the first immersion performed for 2 h).

Scheme 1. Procedures to modify ITO electrodes with (GPPDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 LbL films (assembly). The immersion time was 15 min for both GPDDA and GPSS, and 6 min for AuNP.


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A Thermo Scientific model Genesys 10 spectrophotometer was used to monitor the synthesis and the LbL film assembly. FTIR spectra were collected with a Thermo Nicolet Nexus 470 with a spectral resolution of 4 cm-1 in the transmittance mode, and under N2 purging to avoid interference from CO2 and water vapor. Use was made of a micro-Raman Renishaw, model in-Via, equipped with a 514 nm laser and coupled to a Leica microscope allowing a spatial resolution of ca. 1 μm2 using an objective of 50× magnification. Area Raman mappings were recorded using the streamline accessory. Quartz plates for UV-vis spectroscopy and Si slide for FTIR and Raman spectroscopies were cleaned with a mixture of 5:1:1 (v/v/v) H2O:H2O2:NH4OH at 80 °C for 10 min. The FE-SEM images of LbL films deposited on glass slides were obtained in a Zeiss Sigma microscope (Jena, Germany) with surface chemical analysis from dispersive spectroscopy (EDS).

Electrochemical experiments


10

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The LbL films were deposited on ITO-coated glass slides (Delta Technologies, USA) that had been cleaned by immersion in chloroform, washing with isopropyl alcohol in an ultrasonic bath for 30 min, rinsed with ultrapure water and dried. The electrochemical measurements were performed in an Autolab model PGSTAT 30 (Eco Chemie, Netherlands) controlled by the NOVA 1.11.1 software, using a 3-electrode cell with 10 mL of supporting electrolyte of pH 6.3 phosphate buffer. Platinum foil (1.0 cm2) and saturated calomel electrode (SCE) were used as counter-electrode and reference electrode, respectively, while the working electrode was the LbL modified-ITO electrodes. Cyclic voltammogram (CV) measurements for electrochemical behavior mapping were performed from -1.0 V to +0.8 V vs SCE, with a scan rate of 100 mV·s-1. Before MP detection, each sensor was cycled 30 times for signal stabilization. DPV analysis was performed from -0.2 to 0.0 V vs. SCE, with an amplitude of 5 mV and a modulation time of 1 s.

Results and Discussion


11


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Synthesis and LbL film assembly Figure 1A shows the UV-vis spectra of the suspensions used in LbL film fabrication. Before reduction, GO displayed bands at 229 and 282 nm, characteristic of

π–π*

transition of aromatic C–C bond, and n–π* transition of C=O bonds, respectively.39,40 After chemical reduction, both GPDDA and GPSS suspensions presented a band at 270 nm due to π–π* transition (shifted to higher wavelengths), suggesting that the electronic sp2 conjugation was restored.39 The PSS benzene group absorption band at 227 nm41 was present in GPSS and pristine PSS spectra. Figure 1B shows the UV-vis spectra before and after reduction of gold suspension. Before reduction with borohydride, bands were observed at ~230 and 290 nm assigned to the [AuCl4]- ion,42– 45

which disappeared after reduction. Then, a band due to the collective plasmon

resonance was observed at 540 nm.46,47 Figure 1C and 1D show that the amount of material increases linearly with the number of bilayers in (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 LbL films, respectively. The correlation coefficient was R2 = 0.971 taking the band at 270 nm for (GPDDA/GPSS)10, while for the (GPDDA/GPSS)1(AuNP/GPSS)10 film, R2 = 0.995 at 270 nm and 0.996 at 540 nm.


12

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Figure 1. UV-vis spectra of (A) suspensions of GPDDA and GPSS, precursor GO, and the neat stabilizing polymers, PSS and PDDA; (B) suspensions of gold salt and nanoparticles;

LbL

assembly

of

(C)

(GPDDA/GPSS)10

and

(D)

(GPDDA/GPSS)1(AuNP/GPSS)10. The insets in C and D show the absorbance vs. number

of

bilayers

for

(GPDDA/GPSS)10

(R2

=

0.971)

and

(GPDDA/GPSS)1(AuNP/GPSS)10 (R2 = 0.995 at 270 nm and 0.996 at 540 nm) films.


13


Page 14 of 46

LbL film characterization Figure

2

depicts

the

FTIR

spectra

of

(GPDDA/GPSS)10

and

(GPDDA/GPSS)1(AuNP/GPSS)10 LbL films, and for drop cast films of neat materials. GPDDA and GPSS had typical bands of graphene-based materials at ~1630 cm-1 (C=C stretching) and 1720 cm-1 (C=O stretching).48 The GPSS spectrum also showed characteristic PSS bands at 1570 cm-1 (C=C stretching in the benzene ring), 1182 and 1036 cm-1 due to the asymmetric and symmetric S=O stretching, respectively, 1128 cm1

(in-plane skeleton vibration of benzene ring), and at 1009 cm-1 (in-plane deformation of

benzene ring). The GPDDA spectrum had additional bands arising from the polymer at 1120 (C–N stretching), 1467 (ring stretching) and 1574 cm-1 (C–N stretching).49 The S=O bands (1182 to 1195 cm-1 and 1036 to 1032 cm-1) and C–N bands (1574 to 1577 cm-1) for the (GPDDA/GPSS)10 film were shifted, suggesting that film assembly was governed by electrostatic forces between PSS and PDDA. Contributions of π – π stacking between the graphene layers may also occur, however, shifts in the C=C stretching band (~1630 cm-1) were not clearly observed because of the superposition of other vibration modes.


14

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The AuNPs spectrum had broad bands at ~3400 and ~2900 cm-1 due to the stretching modes of N–H (NH3+) and C–H from PAH (stabilizing AuNP), respectively.50 Additional PAH bands at 1606, 1514 and 1463 cm-1 are assigned to asymmetric and symmetric deformations of NH3+, and –CH2– deformation, respectively.50 The sulfonic group band from PSS at 1036 cm-1 is red-shifted to 1034 cm-1, while the C–N vibrations from PAH shifted from 1574 to 1578 cm-1 probably because these chemical groups are involved in the physical interaction responsible for the (GPDDA/GPSS)1(AuNP/GPSS)10 film formation.


15


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Figure 2. FTIR spectra for cast films of neat materials and for LbL film of (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10.

The surface morphology as probed with SEM images with two levels of magnification in Figure 3(A,C) appears with wrinkled sheets adsorbed on the surface for (GPDDA/GPSS)10. For the (GPDDA/GPSS)1(AuNP/GPSS)10 film, spherical particles are seen uniformly distributed through the graphene sheet surfaces in Figure 3(B,D). EDS analysis (Supp. Inf. – Fig. S1) indicated characteristic peaks of Au, C and O, while peaks of Sn, In and Si are related to the ITO-coated glass substrate.


16

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Figure

3.

SEM

images

of

LbL

(A,C)

(GPDDA/GPSS)10

and

(B,D)

(GPDDA/GPSS)1(AuNP/GPSS)10 in two different magnifications.

Figure 4 depicts the optical images and Raman spectra of the two LbL films. For comparison, neat GPDDA, GPSS, and AuNP spectra are also shown. Except for AuNP, all the spectra presented the characteristics G band at ~ 1590 cm-1 from the E2g degenerate mode zone vibration51 of the sp2 carbon network, and the D band at ~ 1350 cm-1 attributed to defects in the sp2 carbon lattice52,53 (Fig. 4B,E). However, only the LbL film spectrum showed a shoulder at ~1610 cm-1 (D’ band), which is also related to


17


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structural disorder54 (Fig. 4C,F). The D’ band is absent in the powder samples because it is assigned to vibrations involving surface graphene layers, i.e. layers which are not sandwiched between two other graphene layers.54 Therefore, in powder samples, this can be negligible while in the LbL films, fabricated from exfoliated sheets, the amount of surface graphene layers is relatively high. As expected for chemically reduced graphene oxide samples, bands at large wavenumbers (the second-order bands) appeared as bumps between 2700 and 3220 cm-1 because of the disorder and large number of corrugations or wrinkles on the surface.55 The G peak is shifted to higher wavenumbers (~12 cm-1) after the LbL assembly for both architectures, owing to electron transfer from graphene to an electron-acceptor group. In this case, electron transfer takes place from graphene to the amine groups from GPDDA or from PAH-stabilized AuNP.48,56,57 The D band position had negligible changes, which would be an indication of non-covalent interactions.58 The optical micrograph of (GPDDA/GPSS)10 film in Fig. 4A showed large dark aggregates. However, both dark and clear regions showed the G and D bands, indicating a complete coverage of the substrate, with large aggregates randomly distributed on the LbL film surface. The (GPDDA/GPSS)1(AuNP/GPSS)10 film also


18

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showed the typical D and G bands for all regions of the surface, but with smaller aggregates (see Fig. 4D).

Figure 4. Optical images and micro-Raman spectra of (GPDDA/GPSS)10 (A, B and C) and (GPDDA/GPSS)1(AuNP/GPSS)10 (D, E and F) LbL films. (a), (b) and (c) in spectra panels indicate the three different spots where the spectra were collected. The microRaman spectra for neat GPDDA, GPSS, and AuNP are also presented for comparison. Laser line at 514.5 nm.

Methyl parathion – LbL film interactions


19


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The interaction between LbL films and MP was probed with micro-Raman analysis. The Raman spectrum of MP powder (spectrum b – Figure 5) shows bands at 769 (C–S stretching), 861 (P–O stretching), 1004 (C–N stretching), 1216 (C–O stretching), 1348 (C–H bending), 1377 (N–O stretching) and 1590 (phenyl stretching) cm-1.59 In addition, bands at 2454 (P(=S)H stretching), 2850 (C–H symmetric stretching of the CH3 groups) and 2959 cm-1 (C–H antisymmetric stretching)60 were identified. The (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 LbL films were prepared on silicon substrates and 20 μL of MP (0.7 mg mL-1) were dropped cast and left to dry. In the LbL films spectra, the MP characteristic bands had a slight shift (~2 cm-1) suggesting a physical interaction with the film materials (physisorption). Basically, the spectra of LbL films exposed to MP in Figures 5A and 5B are a superposition of the LbL film spectrum (before exposing to MP) and MP powder spectrum. A significant signal amplification was observed with graphene on the LbL film (even in the absence of AuNP). The effect of surfaceenhanced Raman scattering (SERS) by graphene has been referred to as GERS.61–63 Unlike the amplification owing to colloidal nanoparticles, which is based on the electromagnetic mechanism, GERS arises from a chemical mechanism governed by


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charge transfer between the target molecule and the substrate. In such a charge transfer process, the positive and negative charges in the molecule become further separated, increasing the polarizability of the molecule and therefore the Raman scattering.61 Usually, the effect is easily verified in single-layer graphene but can be low or negligible in multilayered graphene.61,62 The (GPDDA/GPSS)1(AuNP/GPSS)10 LbL film also exhibited SERS amplification. The band at 2686 cm-1 is related to NH+ stretching from PAH

60

(Figure 5B). These LbL films are therefore promising for

providing SERS and/or GERS amplification, while so far only systems involving colloidal suspensions or/and hybrid graphene-nanoparticles have been used for this purpose.64– 66

The distribution of MP on the LbL films was investigated by mapping the intensity of MP band (1348 cm-1), shown as red dots. On the (GPDDA/GPSS)10 film, MP molecules interact with graphene aggregates, probably via hydrophobic interactions (π–π stacking), as shown by the coincidence of the red dots on (or in the vicinity) the dark graphene aggregates. The mapping in the (GPDDA/GPSS)1(AuNP/GPSS)10 film shows


21


MP molecules distributed uniformly, forming homogeneously distributed “bubbles” on the surface.

A (GPDDA/GPSS)10

2959

1348 cm-1 Mapped band

3090

3080

a

1526

1377

b

1230

861

769

Si

a 1216

c) (GPDDA/GPSS)10 5000

1110

a) (GPDDA/GPSS)10 + MP b) MP powder

10000

methyl parathion (MP) 2850

D’ 1611

20 µm

Intensity (a.u.)

1590

15000

1104

Intensity (counts)

20000

c

b

c

0 600

800

1000

1200

1400

2500

1600

Wavenumber (cm-1)

2700

2900

3300

3100

Wavenumber (cm-1)

B (GPDDA/GPSS)1(AuNP/GPSS)10 1348 cm-1 Mapped band

15000

3080

2850

3182

1590

2686

a b

1526

1377

1110

1230

a

Si

1216

769

5000

1104

861

a) (GPDDA/GPSS)1(AuNP/GPSS)10 + MP b) MP powder c) (GPDDA/GPSS)1(AuNP/GPSS)10

10000

2454

Intensity (a.u.)

methyl parathion (MP) 20 µm

3090

2959

20000

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 46

c

b c

0 600

800

1000

1200

1400

2500

1600

Figure

5.

Raman

spectra

2700

2900

3100

3300

Wavenumber (cm-1)

Wavenumber (cm-1)

for:

(A)

(GPDDA/GPSS)10

and

(B)

(GPDDA/GPSS)1(AuNP/GPSS)10 LbL films after interaction with MP (curve a), neat MP powder (curve b) and pristine LbL films (curve c). The insets show 2D area Raman mappings with the intensity distribution of MP band (at 1348 cm-1, collected with a step


22

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of 2 μm for an area of 90 m x 90 m, leading to 2,025 spectra, and 12 min and 15 s time

of

acquisition

of

each

mapping)

in

the

(GPDDA/GPSS)10

and

(GPDDA/GPSS)1(AuNP/GPSS)10 films. Laser line at 514.5 nm. Scale bar: 20 m (optical image).

Cyclic voltammograms of ITO electrodes modified with (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 LbL films in phosphate buffer solution had no evident redox peaks (black line in Figure 6). The electrochemical reactions governing the MP detection were proposed previously.67,68 The presence of 100 μg·mL-1 MP (red line) leads to an irreversible reduction peak of MP nitro group (–NO2) to hydroxylamine (– NH–OH) via a four-electron reduction process (Pc2 in Fig. 6A and B, and reaction 1 in Fig. 6C). This peak appears at EPc2 = -0.75 V and -0.86 V for ITO/(GPDDA/GPSS)10 and ITO/(GPDDA/GPSS)1(AuNP/GPSS)10, respectively. Well-defined redox peaks (Pa1 and

Pc1 in Fig. 6A and B, and reaction 2 and 3 in Fig. 6C) with EPa1 = 0.02 and 0.14 V for (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 LbL films, respectively, are related to hydroxylamine oxidation.67,68 The corresponding reduction occurred at EPc1 = -


23


Page 24 of 46

0.17 and -0.32 V. These redox potentials are close to reported values in the literature.67,68 Any of these processes of oxidation (reaction 2) and reduction (reaction 1 and 3) can be used to identify the MP, however, the best result was obtained using the oxidation potential related to the reaction 2 (EPa1 = -0.1 V in the DPV curves, as discussed hereafter), which was selected to next analytical experiments. The sensing performance of each sensor depends on the charge transfer capacity of the sensing surface. Graphene-based materials are known for their fast, facilitated charge transfer reactions with adsorbed molecules.69 Likewise, we believe that our sensor is based on the conducting properties of graphene. The LbL film without AuNPs has lower redox potentials, which means that stabilization with the insulating PAH can hamper electron transfer.


24

Page 25 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure

6.

Cyclic

voltammograms

of

the

ITO

electrodes

modified

with

(A)

(GPDDA/GPSS)10 and (B) (GPDDA/GPSS)1(AuNP/GPSS)10 in pH 6.3 phosphate buffer (black line) and in 100 μg·mL-1 MP (red line). Scan rate: 100 mV·s-1. (C) Electrochemical reaction mechanism proposed.67,68

Methyl parathion detection Preliminary tests indicated the best results for DPV over SWV and so the MP quantification was performed by obtaining DPV curves in a range of MP concentrations


25


Page 26 of 46

in phosphate buffer at pH 6.3 using the modified-ITO electrodes. The current peaked at -0.10V for the ITO/(GPDDA/GPSS)10 electrode in Figure 7A, whose value increased linearly with MP concentration from 0.25 to 40 ppm, with a correlation of R2 = 0.982 and sensitivity of 7.06×10-2 (μA·cm-2)·ppm-1. The ITO/(GPDDA/GPSS)1(AuNP/GPSS)10 electrode exhibited a peak at -0.11 V, with a wider linear range, 0.5 – 60 ppm, with R2 = 0.995 and sensitivity of 3.74×10-2 (μA·cm-2)·ppm-1. LOD was calculated according to IUPAC recommendation as 3 × SD/b, where SD is the standard deviation of 10 blank measurements, and b is the sensitivity, determined as the slope of the calibration curve.70 LOD was 0.226 (0.859 μmol·L-1) and 0.770 ppm (2.93 μmol·L-1) for (GPDDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 electrodes, respectively. The lower LOD in the absence of AuNPs ((GPDDA/GPSS)10 film) is possibly related to the non-conducting PAH used to stabilize the AuNPs, which may hamper charge transfer. The wider linear range for the LbL architecture containing AuNPs, on the other hand, is related to the more homogeneous distribution of MP molecules, as indicated in the micro-Raman analysis of Figure 5. In the absence of AuNPs, MP molecules interact by


26

Page 27 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


π – π stacking with graphene aggregates (see Raman mapping in Figure 5A), with adsorption sites on the sensor surface saturating at lower MP concentrations.

Figure 7. DPV profiles (A) for MP from 0 to 120 ppm using the ITO/(GPDDA/GPSS)10 electrode

and

(B)

from

0

to

110

ppm

MP

using

the

ITO/(GPDDA/GPSS)1(AuNP/GPSS)10 electrode. (C, D) anodic peak current vs. MP concentration for ITO/(GPDDA/GPSS)10 and ITO/(GPDDA/GPSS)1(AuNP/GPSS)10


27


Page 28 of 46

electrodes, respectively. The inset in C and D show the linear fitting with R2 = 0.982 and 0.995, respectively.

A comparison of the performance of non-enzymatic sensors for MP is shown in Table 1. Some sensors reported in the literature had lower LOD than the sensors made with LbL films here, but most of the highly sensitive units were more sophisticated to make, with high fabrication costs that make them unsuitable for routine applications. Furthermore, the sensing units presented in this work had a larger detection range than all of the reported sensors but one.


28

Page 29 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 1. Comparison of performances of different non-enzymatic sensors for methyl parathion.

Electrode

LOD (μmol·L-1)

Linear range (μmol·L-1)

Method

Ref

CuO-TiO2 decorated GCE

0.0046

0–7.60

DPV

15

Nanosilver/nafion modified GCE

0.2815

0.2–1.0

Amperometry

16

0.0874

0.3–1.44

DPV

0.0032

0.01–1900

Amperometry

23

0.5–120

SWV

67

MoS2 and graphene nanocomposite

AuNP electrochemically deposited on Nafion-coated 0.1 GCE Graphene-Co3O4 functionalized porphyrin

0.011

0.4–20

DPV

71

MWCNT-chitosan modified GCE

0.019

0.19–6.82

SWV

72

Stearic acid modified GCE

0.95

2.07–12.42

Amperometry

73

0.66

2 – 25

DPV

ZrO2NPs-GNs-modified GCEa

0.002

0.0068–3.10

SWV

74

Microbial-SPCEb

0.5

2–80

CV

75

CFME-NiTsPc/Nafionc

0.34

3.4–34

SWV

76

pSC6-Ag NPs/GCE d

0.004

0.01–80

DPV

77

ITO/(GPDDA/GPSS)10

0.859

0.95–152

DPV

This work

(0.226 ppm)

(0.25-40 ppm)

2.930

1.90–228

DPV

This work

ITO/(GPDDA/GPSS)1(AuNP/GPSS)10


29

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

(0.770 ppm) a

(0.5-60 ppm)

Electrochemically synthesized zirconia nanoparticles decorated graphene nanosheets;

immobilized on the screen printed carbon electrode (SPCE);

c

Page 30 of 46

b

whole cells of recombinant Escherichia coli

carbon fiber microelectrode (CFME) obtained by tetrasulfonated phtalocyanine

(NiTsPc) electrodeposition on carbon surface combined with Nafion; d para-sulfonatocalix[6]arene (pSC6)-modified silver nanoparticles electrodeposited on GCE


30

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Real sample analysis The LbL-modified ITO electrodes were applied to determine MP in tap water, soil, and cabbage. Phosphate was added to the tap water to allow for ionic conduction, while the soil and cabbage samples were suspended and blended with phosphate buffer, respectively. The mixtures were filtrated and the measurements were taken with the supernatant. Standard additions of MP shown in Table 2 gave recovery values ranging from 92 to 113 %.

Table 2. Real sample analysis performed with LbL film-modified electrodes for methyl parathion detection.

(GPDDA/GPSS)10 Sample Added (ppm) Found* (ppm) SD* (ppm)

Groun

Cabbag

d

e

15.00

15.00

15.00

13.75

16.91

17.02

0.45

0.81

0.92

Tap water


31


Recovery (%)

91.67

112.7

Page 32 of 46

113.4

(GPDDA/GPSS)1(AuNP/GPSS) 10

Sample Added (ppm) Found* (ppm) SD* (ppm) Recovery (%) *Average

Groun

Cabbag

d

e

15.00

15.00

15.00

16.52

15.94

16.53

0.77

0.34

0.68

110.1

106.3

110.2

Tap water

concentration and SD were calculated from 3 measurements.

Conclusions We have successfully assembled LbL films of oppositely charged polymer-stabilized graphene layers as (GPDDA/GPSS)10, also including gold nanoparticles in another architecture ((GPDDA/GPSS)1(AuNP/GPSS)10). Based on a structural characterization, we infer that MP interacts differently with the LbL films, being adsorbed on graphene aggregates in the film without AuNPs, while for (GPDDA/GPSS)1(AuNP/GPSS)10 the MP molecules are distributed homogeneously on the film. These differences cause a


32

Page 33 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lower

LOD

(0.226

ppm,

ITO/(GPDDA/GPSS)10

and

with

linear

wider

range

linear

from

range

0.25 for

to MP

40

ppm)

for

detection

in

ITO/(GPDDA/GPSS)1(AuNP/GPSS)10 (0.5–60 ppm, with LOD of 0.770 ppm). The sensors made with LbL films were capable of detecting MP in real samples, e.g. tap water, soil and cabbage with recovery values ranging from 92 – 113%. The dependence on the LbL film architecture suggests a possible way to tune the sensing properties by seeking synergy among the various materials that can be explored with the LbL technology.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. SEM and EDS analysis of the (GPDDA/GPSS)1(AuNP/GPSS)10 film (PDF)

AUTHOR INFORMATION

Corresponding Author *[email protected]


33


Page 34 of 46

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources FAPESP (2014/15093-7, 2016/19387-0,2017/03523-5, 2013/14262/7, 2016/09634-0).

ACKNOWLEDGEMENT

We thank Prof. Osvaldo N. Oliveira Jr. for the English revision. This work was supported by the São Paulo Research Foundation (FAPESP) (2014/15093-7, 2016/19387-0,2017/03523-5, 2013/14262/7, 2016/09634-0), and by the Brazilian National Council for Scientific and Technological Development (CNPq) grant 465572/2014-6, and the projects of the National Institute for Science and Technology on Organic Electronics (INEO) [FAPESP 2014/50869-6 and CAPES (Education Ministry) 23038.000776/201754].

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LbL film sensor assembly for electrochemical detection of Methyl Parathion 83x35mm (300 x 300 DPI)


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Layer-by-Layer Films of Graphene Nanoplatelets and Gold

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