gold nanoparticles hosted in a water-soluble silsesquioxane polymer applied as a catalytic material...

Accepted Manuscript

Title: Gold Nanoparticles Hosted in a Water-solubleSilsesquioxane Polymer Applied as a Catalytic Material ontoan Electrochemical Sensor for Detection of NitrophenolIsomers

Author: Paulo Sergio da Silva Bianca C. Gasparini Herica A.Magosso Almir Spinelli

PII: S0304-3894(14)00218-0DOI: http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.032Reference: HAZMAT 15805

To appear in: Journal of Hazardous Materials

Received date: 16-12-2013Revised date: 6-3-2014Accepted date: 16-3-2014

Please cite this article as: P.S. Silva, B.C. Gasparini, H.A. Magosso, A. Spinelli, GoldNanoparticles Hosted in a Water-soluble Silsesquioxane Polymer Applied as a CatalyticMaterial onto an Electrochemical Sensor for Detection of Nitrophenol Isomers, Journalof Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.03.032

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http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.032

http://dx.doi.org/10.1016/j.jhazmat.2014.03.032

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Gold Nanoparticles Hosted in a Water-soluble Silsesquioxane Polymer Applied as a

Catalytic Material onto an Electrochemical Sensor for Detection of Nitrophenol Isomers

Paulo Sérgio da Silva, Bianca C. Gasparini, Hérica A. Magosso, Almir Spinelli*

Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis-

SC, Brazil

*Corresponding author

Telephone number: +55-48-3721-3606

Email address: [email protected]

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Abstract

The water-soluble 3-n-propyl-4-picolinium silsesquioxane chloride (Si4Pic+Cl-) polymer was

prepared, characterized and used as a stabilizing agent for the synthesis of gold nanoparticles

(nAu). The ability of Si4Pic+Cl- to adsorb anionic metal complexes such as AuCl4- ions allowed

well-dispersed nAu to be obtained with an average particle size of 4.5 nm. The liquid suspension

of nAu-Si4Pic+Cl- was deposited by the drop coating method onto a glassy carbon electrode

(GCE) surface to build a sensor (nAu-Si4Pic+Cl-/GCE) which was used for the detection of o-

nitrophenol (o-NP) and p-nitrophenol (p-NP). Under optimized experimental conditions the

reduction peak current increased with increasing concentrations of both nitrophenol isomers in

the range of 0.1 to 1.5 µmol L-1. The detection limits were 46 nmol L-1 and 55 nmol L-1 for o-NP

and p-NP, respectively. These findings indicate that the nAu-Si4Pic+Cl- material is a very

promising candidate to assemble electrochemical sensors for practical applications in the field of

analytical chemistry.

Keywords: Gold nanoparticles; silsesquioxane; electrochemical sensor; simultaneous detection;

nitrophenol isomers.

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1. Introduction

Organic-inorganic hybrid materials, such as silsesquioxanes, have been the subject of

increasing interest due to their great potential for technological applications in multiple fields [1].

Among the characteristics of the silsesquioxane-based materials is the capacity to combine the

physical properties of glass, such as thermal stability and rigidity, with the exchange properties of

the organofunctional group [2]. In addition, they can be synthesized in water-insoluble or water-

soluble forms by simply adjusting in the proportion of the reactants used in the synthesis [3].

These remarkable properties make silsesquioxanes suitable for application as catalysts, protective

coatings and base-materials for the adsorption of metal ions and to build modified electrodes [1-

5]. Recently, some authors [6-8] have reported that soluble charged silsesquioxanes represent an

excellent alternative to support and stabilize metallic nanoparticles of gold (nAu) in aqueous

medium. This behavior was attributed to the ability of the silsesquioxane polymer to allow the

adsorption of the metal complex AuCl4-, resulting in the synthesis of small and well-dispersed

nAu. Furthermore, silsesquioxanes have the capacity to form stable thin films on several surfaces

[6]. The presence of charged groups and counter ions in their chemical structure provides good

electrical conductivity and high stability to the film formed [7]. This property, which is still

underexplored, can be used for the construction of chemically-modified electrodes and

electrochemical sensors. In addition, nanoparticles obtained from noble metals have attracted

increasing scientific interest in several fields [9-14]. The properties of these nanomaterials

strongly depend on their size, shape, composition and structure [11]. Superstructure nanocrystals,

for example, have unique collective properties that differ from those of bulk and individual

nanocrystals. Besides, particles lower than 2 nm can exhibit higher electrocatalytic activity than

the bulk material and higher particles [13,14]. Within this context, when applied to the

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development of electrochemical sensors, nAu lead to an enhancement in the analytical

performance, more specifically, it increase the active surface area and improve the electron

transfer, the sensitivity and the signal/noise ratio [6]. Some authors [15-17] have shown that nAu

can be very efficient in the reduction of aromatic nitrocompounds to their corresponding aromatic

amines. This reduction reaction may be useful for the electrochemical determination of aromatic

nitrocompounds using electrodes modified with nAu.

Aromatic nitrocompounds such as o-NP and p-NP are widely used in the chemical

synthesis of pesticides, explosives, pharmaceuticals, dyestuffs, etc. [17-21] and they are therefore

widely distributed in soils and, in particularly, in aquatic environments. Even at very low

concentrations, the nitrophenol isomers are toxic and hazardous. In the case of plants, for

instance, irrigating crops with water which contains nitrophenol isomers in concentrations over

0.7 mmol L-1 will lead to reduced production [22]. With regard to human and animals, the

ingestion of nitrophenols can lead to severe health problems, such as methemoglobinemia,

fervescence and liver and kidney damage. For these reasons, nitrophenols are classified as

hazardous wastes and priority toxic pollutants by the US Environmental Protection Agency

[22,23]. The purification of wastewater polluted by nitrophenols is a very hard task because of

the high stability of these molecules in terms of chemical and biological degradation [22]. Thus,

the monitoring and detection of nitrophenols is of great importance in environmental surveys.

Recently, we have demonstrated that the insoluble form of Si4Pic+Cl- immobilized in a

carbon paste electrode shows excellent performance for the simultaneous electrochemical

detection of dihydroxybenzene isomers [24]. Now, this paper describes the development of an

electrochemical sensor based on nAu hosted in the water-soluble Si4Pic+Cl- polymer as a

stabilizing agent. The liquid dispersion containing nAu-Si4Pic+Cl- was deposited onto a glassy

carbon electrode surface by the drop coating method in order to build a sensor for nitrophenols.

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The proposed sensor is designated as nAu-Si4Pic+Cl-/GCE. As will be demonstrated, it serves for

the individual as well as simultaneous detection of o-NP and p-NP.

2. Experimental section

2.1. Chemicals, reagents and solutions

All chemicals were of analytical grade and used without further purification. The aqueous

solutions were prepared with water purified in a Milli-Q Millipore (Bedford, MA, USA) system.

Buffer solutions of 0.1 mol L-1 Britton-Robinson (B-R) were used as the supporting electrolyte in

the electrochemical experiments, adjusting the pH with 1.0 mol L-1 NaOH or HCl when

necessary. Stock solutions of 20 mmol L-1 o-NP and p-NP were prepared in ethanol.

2.2. Synthesis of Si4Pic+Cl-

The ion exchange Si4Pic+Cl- polymer was prepared according to a procedure described

elsewhere [25] with some modifications. Briefly, 32 mL of a 1.0 mol L-1 HCl solution and 150

mL of ethanol were added to 0.36 mol of tetraethoxysilane in a round-bottomed flask. The

mixture was stirred for 2.5 h at room temperature (25 ± 2 ºC). In the next step, 0.56 mol of 3-

chloropropyltrimethoxysilane was added and the mixture was stirred for a further 2 h at room

temperature. The temperature was then raised to 55 ºC (328 K) which was maintained for 60 h,

that is, until the initiation of the gelation process. After this stage, the solvent was evaporated and

a xerogel product was obtained, which was washed with ethanol, filtered and dried under

vacuum. Around 150 mL of toluene and 50 mL of pyridine were added to 30 g of the previously

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obtained xerogel. The resulting mixture was refluxed for 8 h. The desired final product

(Si4Pic+Cl- polymer) was filtered, washed with ethanol and dried at 70 ºC (343 K) for 2 h.

2.3. Synthesis of nAu

Gold nanoparticles stabilized in the silsesquioxane polymer (nAu-Si4Pic+Cl-) were

prepared as follows: 5 µL of 0.1 mol L-1 chloroauric acid were added to 2.5 mL of an aqueous

solution of Si4Pic+Cl- (2 g L-1). The mixture was stirred for 5 min at room temperature and 200

µL of a freshly prepared 20 mmol L-1 sodium borohydride solution were quickly added under

continuous stirring. After 60 s the solution changed from colorless to red, indicating the

formation of nAu. The solution obtained was stored at 4 ºC.

2.4. Preparation of the nAu-Si4Pic+Cl-/GCE sensor

The simple, facile drop coating method was used to prepare the nAu-Si4Pic+Cl-/GCE

sensor, as follows: a bare GCE (diameter of 2 mm) was carefully polished with 3 µm alumina

powder, thoroughly rinsed and sonicated for 3 min in purified water. An aliquot of 3 µL of nAu-

Si4Pic+Cl- solution was then pipetted onto the surface of the GCE and allowed to dry in a stove at

40 ºC for 10 min.

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2.5. Apparatus

To characterize the Si4Pic+Cl- polymer prepared elemental analysis was carried out in

triplicate using a Perkin-Elmer model 2400 CNH analyzer. In addition, FT-IR spectra were

obtained at between 4000 and 400 cm-1 from pressed KBr pellets containing around 1.5 wt% of

the polymer on a Bomen Hartmann & Braun (MB-Series) spectrophotometer operating with a

resolution of 4 cm−1. Also, solid-state nuclear magnetic resonance spectroscopy for 13C and 29Si

(CP/MAS NMR) was performed on a Bruker AC300/P spectrometer. 13C CP/MAS spectra were

obtained using pulse sequences with 4 ms contact time, an interval between pulses of 2 s and an

acquisition time of 133 ms. The 29Si spectra were obtained by using pulse sequences with 2.5 ms

contact time at an interval between pulses of 1 s and an acquisition time of 114 ms. Finally,

thermogravimetric analysis of the Si4Pic+Cl- polymer was carried out on a thermoanalyzer

(Shimadzu, Model TGA – 50). The measurements were performed under oxidant atmosphere

with a heating rate of 10 K min−1 in the range of 25 to 700 oC. To characterize nAu-Si4Pic+Cl-,

the plasmonic band of nAu was obtained by ultraviolet-visible (UV–vis) spectroscopy carried out

on a Shimadzu UV 1601PC instrument. The spectra were collected from 300 to 800 nm at room

temperature. Images of nAu-Si4Pic+Cl- were obtained by transmission electron microscopy

(TEM) using a JEOL JEM-2100 microscope operating at 100 kV. For preparation of the samples,

a drop of the solution containing the dispersed nAu-Si4Pic+Cl- was placed on a carbon-coated

copper grid and then dried at room temperature for 24 h. The nanoparticle diameter distribution

was obtained using ImageJ software. The morphology of the nAu-Si4Pic+Cl- immobilized onto

the glassy carbon surface was analyzed by scanning electron microscopy (SEM) using a JEOL

JSM-6701F microscope. The electrochemical experiments were carried out on a PalmSens

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potentiostat (Palms Instruments BV) coupled to a personal computer with the PSTrace 2.52

software installed. A standard one-compartment three-electrode glass cell was used. A bare GCE

or modified GCE served as the working electrode, a platinum sheet as the auxiliary electrode and

an Ag/AgCl (KCl saturated) as the reference electrode. Cyclic voltammetry (CV) and differential

pulse voltammetry (DPV) were recorded at between -0.4 and -1.2 V. DPV measurements were

recorded with the following optimized parameters: E step = 5.0 mV; E pulse = 80 mV; t pulse =

0.01 s and scan rate = 30 mV s-1. All experiments were carried out at room temperature (25 ±

2ºC).

3. Results and discussion

3.1. Characterization of Si4Pic+Cl- polymer

Elemental analysis and spectroscopic data allowed the chemical structure shown in Figure

1 to be proposed for the synthesized Si4Pic+Cl- polymer. The results for the elemental analysis

carried out to determine the amount of functional groups incorporated into the final product

showed that the material analyzed contained 26.2% of C and 2.6% of N, which corresponds to

21.8 and 1.9 mmol of C and N, respectively, per gram of the analyzed material. Each N atom

corresponds to one functional group. As can be seen in Figure 1, some n-propyl groups did not

reacted with the 4-picoline molecules. On comparing this new polymer with that previously

synthesized [25] the most notable difference relates to the amount of functional groups

immobilized. This characteristic is very important since it makes the new material a water-

soluble polymer, while the previously synthesized polymer was insoluble in water.

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FT-IR and NMR data were collected in order to obtain detailed information on the

chemical structure of the prepared material. The FT-IR spectrum is shown in Figure 2. The band

observed at 1650 cm−1 corresponds to the pyridinium ring mode of the polymer and shows that

the synthesis was successful. The band between 1000 and 1250 cm−1 is assigned to the Si-O

stretching mode, νSi–O, of the Si–OH and Si–O–Si bonds. The band observed at ca. 2900 cm−1 is

characteristic of the –CH3 stretching mode and the large band at ca. 3400 cm−1 is due to the νOH

of the silanol group and to the adsorbed water [26].

Table 1 lists the observed chemical shifts and the corresponding assignments of the

CP/MAS 13C and 29Si NMR spectral data obtained for the prepared SiPic+Cl− polymer. The

assignments were made on the basis of reported data [25,27-30]. The 29Si NMR spectrum shows

two different peak regions: the first region around -60 ppm assigned to T species, which

corresponds to Si atoms bonded to C atoms of organic groups, and a second region assigned to Q

species at more negative values, attributed to Si atoms of silanol and siloxane groups. In the first

region, the first peak at -49 ppm, referred as T1, may be assigned to C─Si(OR)2(SiO≡), where R

can be ─H or ─CH3. The second peak, at -58 ppm, is assigned to C─Si(OR)(SiO≡)2 and

corresponds to the T2 signal. Finally, the third peak, at -67 ppm, is assigned to C─Si(SiO≡)3 and

corresponds to the T3 signal. In the second region, three peaks can be observed: at -91 ppm, -101

ppm and -110 ppm, whose are assigned to Q2 [(≡SiO)2Si(OH)2], Q3 [(≡SiO)3Si(OH)] and Q4

[(≡SiO)4Si], respectively. The proposed chemical structure for the polyelectrolyte as well as the

C and Si atom numbering are illustrated in Figure 1.

Information on the thermal stability of the water-soluble polymer was also obtained.

Figure 3 shows the profile of the thermogravimetric curve obtained. The initial 0.7% mass loss at

approximately 55 oC is attributed to water physically adsorbed onto the compound. A second

stage of mass loss, corresponding to 35%, starts at 156 oC and is related to the loss of picoline

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molecules, indicating the beginning of the decomposition of the compound. Finally, a 12% mass

loss, which starts at around 341 oC, is due to the decomposition of the n-propyl groups covalently

bonded on the silica surface.

3.2. Characterization of nAu-Si4Pic+Cl-

Figure 4a shows the UV-vis spectrum obtained for the characterization of the nAu-

Si4Pic+Cl- dispersion, specifically to verify the presence of nAu hosted in the Si4Pic+Cl- polymer

matrix. Figure 4a clearly shows the surface plasma resonance absorption peak at 518 nm, which

corresponds to an average particle diameter of less than 30 nm for the nAu. Additionally, Figures

4b and 4c show the TEM image of the nAu-Si4Pic+Cl- and the corresponding histogram of the

particle size distribution, respectively. The TEM image shows that the synthesized nAu were

spherical in shape with an average diameter of 4.5 nm. A good distribution of nAu in the

Si4Pic+Cl- polymer was also attained, although some clustering of the gold nanoparticles can be

seen in Figure 4b. These results corroborate the results of the UV-vis analysis. Finally, Figure 4d

shows the SEM image of the GCE modified with nAu-Si4Pic+Cl-. Many small nanoparticles,

with sizes of <10 nm, appeared to be well-dispersed on the electrode surface. Previous studies [6-

8] have shown that some charged silsesquioxanes can support and stabilize gold nanoparticles

due to their water solubility and very efficient ability to adsorb anionic metal complexes. Thus, it

is possible that some Cl- counter-ions of the Si4Pic+Cl- polymer were replaced by AuCl4- ions

allowing the obtaining of small and well-dispersed nAu [7].

3.3. Voltammetric behavior of p-NP at nAu-Si4Pic+Cl-/GCE sensor

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The electrocatalytic performance of the nAu-Si4Pic+Cl-/GCE sensor for the reduction of

p-NP was firstly examined by CV. CV curves for 50 µmol L-1 p-NP in B-R buffer pH 7.0 at the

bare GCE, Si4Pic+Cl-/GCE and nAu-Si4Pic+Cl-/GCE sensors were obtained from -0.4 to -1.2 V

followed by a return to -0.4 V at 50 mV s-1, as shown in Figure 5. At the bare GCE (Figure 5a),

the reduction peak for p-NP was observed at -0.834 V with a current of around -1.0 µA.

According to data published elsewhere [17-19] the reduction peak can be attributed to the

reduction of the nitro group of the p-NP to p-(hydroxylamino)phenol via an irreversible 4e-/4H+

electrochemical reduction reaction. At the Si4Pic+Cl-/GCE (Figure 5b), the reduction peak for p-

NP shifted slightly to -0.820 V and a small increase in the current peak to -1.3 µA was observed.

Lastly, at the nAu-Si4Pic+Cl-/GCE sensor (Figure 5c) the reduction peak shifted to -0.809 V and

the current increased to -2.65 µA. These results clearly demonstrate that gold nanoparticles

hosted in the silsesquioxane polymer increased the catalytic activity of the sensor, improving the

kinetics and the thermodynamics of the electrochemical process involved in the p-NP reduction.

Figure 6a shows the cyclic voltammetric response to p-NP at the nAu-Si4Pic+Cl-/GCE

sensor for different potential scan rates (v). In parallel, Figure 6b shows that the reduction peak

currents increased linearly with the square root of the scan rate (v1/2), indicating that the reduction

of p-NP is a diffusion-controlled reaction process [31]. In addition, the effect of the solution pH

on the electrochemical response of p-NP at the nAu-Si4Pic+Cl-/GCE sensor was also investigated

in the pH range of 2.0 to 10.0 in B-R buffer (data not shown). The current increased from pH 2.0

to 7.0 and then decreased. Therefore, pH 7.0 was chosen for the subsequent analytical

experiments. These findings are in agreement with previously published data on the reduction of

p-NP [18]. The nAu-Si4Pic+Cl-/GCE sensor was then tested for the individual and simultaneous

detection of the nitrophenol isomers.

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3.4. Electroanalytical application of nAu-Si4Pic+Cl-/GCE sensor for individual and simultaneous

detection of nitrophenol isomers

DPV was used with the optimized experimental parameters described in the Experimental

section for the electroanalytical application of the nAu-Si4Pic+Cl-/GCE sensor. Figure 7 shows

the results obtained for p-NP alone. As can be seen, the reduction peak current at -0.752 V

increased linearly with the p-NP concentration in the range of 0.1 to 1.5 µmol L-1. The linear

regression equation obtained for the calibration curve shown in the inset was ip/µA = 0.019 +

0.34 [p-NP]/(µmol L−1) (R = 0.9990). The limit of detection (LOD) was calculated according

to the equation: LOD = 3.3 Sb/B, where Sb is the standard deviation of the linear

coefficient and B is the slope of the curve. The calculated LOD was 46 nmol L-1, which is

almost ten times lower than that recommend by the United States EPA for drinking water (430

nmol L-1) [23].

The electrochemical response of the nAu-Si4Pic+Cl-/GCE sensor for the reduction of p-

NP in the presence of 0.5 µmol L-1 o-NP was also evaluated. Figure 8 clearly shows well-defined

and separated reduction peaks. The first peak at -0.608 V relates to the reduction of o-NP and the

second at -0.752 V to the reduction of p-NP. As expected, the current in the former case remained

constant while in the latter case it increased linearly with the p-NP concentration. The linear

regression equation (inset of Figure 8) was ip/µA= 0.027 + 0.31 [p-NP]/(µmol L-1) (R=0.9986)

and is very close to that obtained for the calibration curve of p-NP alone (Figure 7). Thus, it can

be concluded that o-NP does not interfere significantly in the determination of p-NP. The

separation between the reduction peaks was 144 mV, which is sufficient for the simultaneous or

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selective detection of both nitrophenol isomers using the nAu-Si4Pic+Cl-/GCE sensor. Therefore,

we also recorded the DPV response to o-NP in the range 0.1 to 1.5 µmol L-1 at the nAu-

Si4Pic+Cl-/GCE sensor in the absence and presence of p-NP, as well as that for the two species

simultaneously. Figure 9 shows a linear relationship between the reduction peak current at -0.605

V and the o-NP concentration. The linear regression equation obtained was ip/µA= 0.07 + 0.27

[o-NP]/(µmol L-1) (R=0.9986) with a LOD of 55 nmol L-1.

In the presence of 0.5 µmol L-1 p-NP (Figure 10), the peak current for o-NP also increased

linearly with the o-NP concentration, while the current peak for p-NP remained unchanged. The

linear regression equation was ip/µA= 0.04 + 0.33 [o-NP]/(µmol L-1) (R=0.9991), which is very

close to that for o-NP alone. Thus, it can likewise be concluded that p-NP does not interfere

significantly in the determination of o-NP.

Finally, we recorded the DPV voltammograms for simultaneously increasing

concentrations of the two nitrophenol isomers in the range of 0.1 to 1.5 µmol L-1 at the nAu-

Si4Pic+Cl-/GCE sensor. As shown in Figure 11a, a linear increase for both reduction peak

currents was observed for increasing concentrations of the two isomers. The linear regression

equations obtained for o-NP (Fig.11b) and p-NP (Fig. 11c) were ip/µA= 0.024 + 0.35 [o-

NP]/(µmol L-1) (R=0.9959) and ip/µA= 0.0005 + 0.30 [p-NP]/(µmol L-1) (R=0.9985),

respectively. This result indicates that the sensitivity of the nAu-Si4Pic+Cl-/GCE sensor did not

change significantly during the simultaneous detection of o-NP and p-NP when compared with

the detection of the same compounds individually.

3.5. Repeatability and stability

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The repeatability of the nAu-Si4Pic+Cl−/GCE sensor was estimated in B-R buffer (pH 7.0)

containing 0.5 μmol L−1 p-NP. The relative standard deviation (RSD) of five successive

measurements was 2,12%, indicating the good repeatability of the results.

The number of measurement carried out with the same electrode was limited to ten in order to

avoid loss of the initial signal, probably due to desorption of the nitrophenols. Thus, the surface

of the electrode was cleaned and a new film was prepared after ten measurements.

The nAu-Si4Pic+Cl− solution was stored at 4 ºC in refrigerator for about 3 mouths. During

this time new electrodes were prepared using the same procedure. The difference for the peak

current (analytical response) of p-NP reduction from each new electrode was lower than 6%,

indicating good stability of nAu-Si4Pic+Cl−/GCE sensor. Thus, we concluded that the nAu-

Si4Pic+Cl-/GCE sensor can be used for the individual as well as the simultaneous or selective

detection of o-NP and p-NP.

4.0. Conclusions

The water-soluble Si4Pic+Cl- polymer was synthesized and used as a stabilizing agent for

the chemical reduction of nAu. The high efficiencies in relation to the adsorption of metal

complexes, due to the presence of charged groups in this silsesquioxane, allowed gold

nanoparticles with spherical shape, good distribution and average size of 4.5 nm to be obtained.

The nAu-Si4Pic+Cl- material was used for the construction of a selective sensor for the individual

and simultaneous determination of o-NP and p-NP. The sensor was built using the simple and

facile drop coating method. The sensitivity of the nAu-Si4Pic+Cl-/GCE sensor was significantly

higher than that of the bare GCE. The limit of detection achieved for both nitrophenols was in the

order of nmol L-1. For the simultaneous detection, the separation between the reduction peaks for

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o-NP and p-NP was 144 mV, indicating that the selective recognition of both nitrophenol isomers

is feasible. Thus, nAu-Si4Pic+Cl- represents a very promising material for the construction of

sensors for the determination of nitrophenol isomers in water.

Acknowledgements

The authors are grateful to the Brazilian government agencies CAPES (Coordenação de

Aperfeiçoamento de Pessoal de Nível Superior) and CNPq (Conselho Nacional de

Desenvolvimento Cientifico e Tecnológico) for scholarships and financial support.

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FIGURE CAPTIONS

Fig. 1. Chemical structure proposed for the synthesized Si4Pic+Cl- polymer.

Fig. 2. FT-IR spectrum for the synthesized Si4Pic+Cl- polymer.

Fig. 3. Thermogravimetric curve for the synthesized Si4Pic+Cl- polymer.

Fig. 4. UV-Vis spectrum (a), TEM image (b), particle size distribution (c) and SEM image (d) for

nAu-Si4Pic+Cl-.

Fig. 5. Cyclic voltammograms for 50 µmol L-1 p-NP in B-R buffer pH 7.0 at the GCE (a),

Si4Pic+Cl-/GCE (b) and nAu-Si4Pic+Cl-/GCE (c) sensors, v = 50 mV s-1.

Fig. 6. Cyclic voltammograms for 50 µmol L-1 p-NP in B-R buffer pH 7.0 at the nAu-Si4Pic+Cl-

/GCE sensor, v = 10 to 200 mV s-1 (a), plot ip vs. v1/2 (b).

Fig. 7. Differential pulse voltammograms for p-NP (a-i: 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5

µmol L-1) in B-R buffer pH 7.0 at the nAu-Si4Pic+Cl-/GCE sensor. Inset: the calibration curve.

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Fig. 8. Differential pulse voltammograms for p-NP (a-i: 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5

µmol L-1) in the presence of 0.5 µmol L-1 o-NP in B-R buffer pH 7.0 at the nAu-Si4Pic+Cl-/GCE

sensor. Inset: the calibration curve.

Fig. 9. Differential pulse voltammograms for o-NP (a-i: 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5

µmol L-1) in B-R buffer pH 7.0 at the nAu-Si4Pic+Cl-/GCE sensor. Inset: the calibration curve.

Fig. 10. Differential pulse voltammograms for o-NP (a-i: 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5

µmol L-1) in the presence of 0.5 µmol L-1 p-NP in B-R buffer pH 7.0 at the nAu-Si4Pic+Cl-/GCE

sensor. Inset: the calibration curve.

Fig. 11. Differential pulse voltammograms for o-NP and p-NP ((A) a-i: 0.0, 0.1, 0.3, 0.5, 0.7, 0.9,

1.1, 1.3, 1.5 µmol L-1) in B-R buffer pH 7.0 at the nAu-Si4Pic+Cl-/GCE sensor. The calibration

curve for o-NP (B) and p-NP (C).

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Table 1. CP/MAS 13C and 29Si NMR Spectra for the Synthesized Si4Pic+Cl− Polymer

Assignments Chemical Shift / ppm % Si atoms C1 11 C2 23 C3 26 C4 49 C5 130 C6 and C7 144 T1 RSi(OH)2(SiO≡) -49 1,42 T2 RSi(OH)(OSi≡)2 -58 22,53 T3 RSi(OSi≡)3 -67 35,11 Q2 (≡SiO)2Si(OH)2 -91 3,63 Q3 (≡SiO)3Si(OH) -101 25,37 Q4 (≡SiO)4Si -110 11,93

Assignments based on Refs. [27-30]

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Highlights

A silsesquioxane polymer was used as a stabilizer for synthesis of gold nanoparticles.

Gold nanoparticles were obtained with an average particle size of 4.5 nm.

A sensor was built for individual and simultaneous detection of nitrophenol isomers.

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Figure 1

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Figure 2

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figure 3.

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figure 4b

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figure 4c

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figure 4d

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figure 5

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figure 6a

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11a

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figure 11b

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graphical abstract

gold nanoparticles hosted in a water-soluble silsesquioxane polymer applied as a catalytic material...

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