Electronic Supplementary Material
A glassy carbon electrode modified with bismuth nanotubes in a silsesquioxane
framework for sensing of trace lead and cadmium by stripping voltammetry
Yusong Li a, b · Guangqiang Sun b · Yuehua Zhang b · Cunwang Ge*b · Ning Bao c · Yihong
Wang *a
a
Department of Chemistry and Chemical Engineering, Southeast University, Nanjing 210092,
Jiangsu, P. R. China
b
School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019,
Jiangsu, P. R. China
c
School of public health, Nantong University, Nantong 226019, Jiangsu, P. R. China
Experimental
Reagents and chemicals
Bismuth nitrates and Nafion perfluorinated polymer (5 wt % solution) were purchased
from
Sigma-Aldrich
Co.
(St.
Louis,
USA
http://www.sigmaaldrich.com/).
Octa(3-aminopropyl) silsesquioxane (POSS-NH3+) was obtained from Hybrid Plastics Co.
(Hattiesburg, USA, http:// www.hybridplastics.com/). The standard solution of lead in water
(GBW(E)080581) was purchased from National Research Centre for Certified Reference
Material. And all other reagents were of analytical grade and purchased from Shanghai
Chemical Reagent Company (http://www.reagent.com.cn/). The water (resistivity 18 MΩ·cm)
used throughout all experiments was purified by a Milli-Q plus system (Millipore, Bedford,
MA, USA, http://www.millipore.com/). Supporting electrolyte used in the experiments was a
0.2 M acetate buffer solution (pH 4.0). All electrolytes were thoroughly deaerated by purging
with high purity nitrogen before each electrochemical experiment. Unless otherwise stated, all
experiments were conducted at ambient temperature. Toy sample consisting of Pb and Cd was
obtained from Nantong Bureau of Import and Export Inspection and Quarantine.
Apparatus
The morphology and nanostructure of the as-prepared sw-BiNTs were characterized
with field emission HRTEM (Tecnai G2 F20 S-Twin, operated at 200 kV, FEI Co. USA)
coupled with electron diffraction and energy-dispersive X-ray spectroscopy (EDS) (GENESIS
2000 XMS，EDAX Co. USA). The analysis for HRTEM was performed with the software
Gatan Digital Micrograph™ version for GMS 1.8.0. Samples of sw-BiNTs for HRTEM were
obtained by dispersing in ethanol with ultrasonic and dripping a droplet of suspension on a
carbon-coated holey film on a Cu grid, followed by drying the copper grid at room
temperature. The stripping voltammetry measurements were performed with a CHI 660C
workstation (Shanghai Chenhua Apparatus Co., China) using a conventional three-electrode
system composed of a glassy carbon electrode (GCE) modified by sw-BiNTs (GCE, 3.0 mm
in diameter, CHI 104, Shanghai Chenhua Apparatus Co., China) as the working electrode, a
platinum gauze as the counter electrode, and a KCl-saturated calomel electrode (SCE)
(+0.242 V vs. NHE) as the reference electrode, respectively. All potentials in this paper were
referenced to SCE. The working electrode was polished and ultrasonically cleaned before
each voltammogram in order to ensure a clean uniform surface for each scan. The
concentration of Cd and Pb migrated from leatherwear was determined for reference using
graphite furnace atomic absorption spectrophotometer (AAS) with Zeeman background
correction (Perkin-Elmer, Analyst 800, with, Norwalk, CT, USA).
Reactivation of sw-BiNTsGCE
After a series of measurements, the sw-BiNTsGCE must be reactivated by a
polarization at a suitable potential, which was more negative to the oxidation potential of
bismuth (0.074 V) and more positive than the oxidation potential of metal ions (-0.571 V for
Pb and -0.837 V for Cd). Here, we selected the reactivated potential as -0.3 V for 180 s in pH
4.0 acetate buffer electrolyte under purging nitrogen.
Fig.S1 TEM photographs (a) and energy-dispersive X-ray spectroscopy(b) of sw-BiNTs.
Details of TEM and EDS of sw-BiNTsGCE
The TEM images obtained from sw-BiNTs has displayed in main text. And the
single-walled feature of sw-BiNTs has revealed in Fig 1. Herein, we reveal its single-walled
feature in another aspect. As shown in Fig.S1a, we can observe the tubular structure of
sw-BiNTs, which shows the single-walled structure again (marked by the arrow in Figure
S1a). The composition of sw-BiNTs was characterized by energy-dispersive X-ray spectrum
(EDS) as shown in Fig. S1b. The silicon and bismuth elements were present in the sample.
Optimization of experimental variables for analysis of Pb(II) and Cd(II) at
sw-BiNTsGCE
To obtain the best sensitivities for the simultaneous analysis of Pb(II) and Cd(II) in
samples, the key parameters including the sw-BiNTs content, the pH of the acetate buffer
solution, the deposition potential and time were optimized by varying the examined parameter
while holding the others fixed. As mentioned above, the pre-concentration potential and the
time were optimized to be -1.1 V and 30 s, respectively. The influence of the sw-BiNTs
content, the pH of the acetate buffer solution upon the resulting stripping responses were
investigated and shown in Fig. S2.
The dependence of stripping peak current of Pb(II) and Cd(II) on the volume of
sw-BiNTs on the GCE was studied in the range of 2~20 μL for a 0.2 M acetate buffer solution
(pH 4.0) containing 5 μM Pb(II) and Cd(II) (Fig. S2). As illustrated in Fig. S2a, both the
stripping peak currents of Pb(II) and Cd(II) are increasing rapidly upon raising the sw-BiNTs
volume from 2 to 5 μL. However, as the volume of sw-BiNTs was more than 5 μL, stripping
peak currents were decreased sharply, which could attribute to the thick sw-BiNTs film
coated on the electrode. As we all know, the solution of sw-BiNTs can form a film structure
on the GCE after drying. And the film thickness, which was relative to the volume of
sw-BiNTs mixture slurry, could affect the stripping signal, as reported on bismuth film [S1]
and antimony film [S2], in which thin films would be saturated by the metal ion depositing
and diffusing into the film and thick films would pose mass transfer limitations to metal ion
diffusing out of the film during the stripping step [S1]. Therefore, the optimized volume of
sw-BiNTs for modifying GCE was 5 μL to analyze both of heavy metals.
The effect of the pH value of the buffer solution on the stripping peak current of Pb(II)
and Cd(II) was examined in the range of 3.5~5.5 for a 0.2 M acetate buffer solution
containing 5 μM each of Pb(II) and Cd(II). As clearly illustrated in Fig. S2b, the stripping
peak current increased with increasing the pH value of buffer as pH below 4.0, and the
highest stripping peak current was achieved at pH 4.0. Afterward, the decrease of stripping
voltammetric signals for both target metals was observed as the pH value of solution above
4.0. However, the traditional bismuth film electrode was commonly used in solution at pH 4.5
[S3]. Due to its active surface, sw-BiNTs were alert to the hydroxyl ion. It is most probably
due to a prevailing effect of sw-BiNTs hydrolysis in high pH solution, and the stripping signal
was faint and prone to errors. Also, at the higher pH, the high hydroxyl concentration will
result in precipitation of Pb(II) and Cd(II) as Pb(OH)2 and Cd(OH)2, as Dai reported [S4].
Therefore, pH 4.0 was selected as optimal in the following experiments.
Fig.S2 The dependence of stripping peak current of Pb(II) and Cd(II) on the volume of
sw-BiNTs mixture slurry on the GCE (a) and pH value (b) in a 0.2 M acetate buffer
solution(pH 4.0) containing 5 μM each of Pb(II) and Cd(II). SWV stripping scan condition
was the same as in Fig. 2.
Fig.S3 The dependence of stripping peak current of Pb(II) and Cd(II) on the deposition time
(a) and deposition potential (b) in a 0.2 M acetate buffer solution(pH 4.0) containing 5 μM
each of Pb(II) and Cd(II). SWV stripping scan condition was the same as in Fig. 2.
The detection limits for Cd and Pb
Fig.S4 The detection limits for Cd (a) with 5 nM and Pb (b) with 1 nM at the sw-BiNTs
modified electrode in a 0.2 M acetate buffer solution(pH 4.0). SWV stripping scan condition
was the same as in Fig. 2.
Reference
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Bi(III) concentration on the stripping voltammetric response of in situ bismuth-coated
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antimony and poly(p-aminobenzene sulfonic acid) for sensing lead(II) by square wave
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