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Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode
Analytica Chimica Acta xxx (xxxx) xxx
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Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode b, Qingyun Liu a, Jing Li a, Wenjin Yang a, Xinglai Zhang a, Cai Zhang a, Christophe Labbe Xavier Portier b, Fei Liu c, Jinlei Yao d, Baodan Liu a, * a
Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72, Wenhua Road, Shenhe District, Shenyang, 110016, China CIMAP CNRS/CEA/ENSICAEN/Normandie University, 6 Bd Mar echal Juin, 14050, Caen Cedex 4, France c State Key Laboratory of Optoelectronic Materials and Technologies and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou, 510275, PR China d Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, 215009, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Highly aligned GaN micropillars have been homoepitaxially grown on GaN substrate. The GaN working electrode provides sufficient active sites for ion detection. The GaN micropillars exhibit enhanced response in trace Agþ and Cu2þ ion detection. The GaN micropillars show lower detection limit than GaN films.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2019 Received in revised form 5 November 2019 Accepted 7 November 2019 Available online xxx
Driven by the motivation to quantitively control and monitor trace metal ions in water, the development of environmental-friendly electrodes with superior detection sensitivity is extremely important. In this work, we report the design of a stable, ultrasensitive and biocompatible electrode for the detection of trace Agþ and Cu2þ ions by growing n-type GaN micropillars on conductive p-type GaN substrate. The electrochemical measurement based on cyclic voltammetry indicates that the GaN micropillars exhibit quasi-reversible and mass-controlled reaction in redox probe solution. In the application of trace Agþ and Cu2þ determination, the GaN micropillars show superior sensitivity and excellent conductivity by presenting a detection limit of 3.3 ppb for Agþ and 3.3 ppb for Cu2þ. Comparative studies on the electrochemical response of GaN micropillars and GaN film in the simultaneous Agþ and Cu2þ detection reveal that GaN micropillars show three orders of magnitude higher stripping peak current than GaN film. It is assumed that the microarray morphology with large active area and the hydrophilia nature of GaN micropillars are responsible for the excellent sensitivity. This work will open up some opportunities for GaN nanostructure electrodes in the application of trace metal ions detection. © 2019 Elsevier B.V. All rights reserved.
Keywords: GaN micropillar arrays Homoepitaxial growth Electrochemistry Metal ion detection
* Corresponding author. Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72, Wenhua Road, Shenhe District, Shenyang, 110016, China. E-mail address: [email protected] (B. Liu). https://doi.org/10.1016/j.aca.2019.11.010 0003-2670/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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1. Introduction The rapid economic development inevitably leads to increased wastewater discharge in the environment. The heavy metal pollution is gradually recognized to threaten human health and environment [1e3]. The exposed ions in drinking water, such as silver ion (Agþ) and copper ion (Cu2þ), are non-naturally degradable. The bioaccumulation of Agþ and Cu2þ in human body will cause series of function disorders at least or severe diseases at worst [4]. For instance, long-term Cu2þ-rich drinking water would be able to trigger heart diseases, arteriosclerosis and argyria [5]. The toxicity of Agþ mainly relies on the ability to bind with metabolites and inactivate sulfhydryl enzyme. By destroying cell membrane through extracellular mechanisms, the biological enrichment of Agþ can induce skin irritation, stomach distress and organ edema [6,7]. In the past years, considerable efforts have been put forward to quantitatively monitor and control metal ions in the water environment. Various analytic techniques such as anodic stripping voltammetry (ASV) [8], solid-phase extraction (SPE) [9], atomic adsorption spectroscopy (AAS) [10] and inductively couple plasma mass spectrometry (ICP-MS) [11] have been developed towards sensitive detection of ions. Compared with other detection methods, ASV is one of the most extensively adopted approaches in heavy metal detection due to its remarkable sensitivity, easy operation and low cost. In ASV procedures, hanging mercury drop electrodes or mercury film modified electrodes are most commonly employed because of the high signal-to-noise ratio and reproducibility [12,13]. The disadvantages of mercury-based electrodes include high toxicity, potential window limitation (not relevant in the case of Agþ detection) and restrictions on mercury disposal [14]. The development of alternative materials with good electrochemical performance and environment friendly characteristics is essential. Various materials including bismuth [15], gold [16], platinum [17] and screen-printed carbon [18] electrodes have been proposed to replace mercury electrodes. However, the bismuth electrodes are encountered with similar limitations in cation ions detection (for instance, Agþ). The gold and platinum electrodes have high background currents in trace metal detections owe to the excellent catalytic activity for hydrogen revolution. The screenprinted carbon electrodes also suffer from high background current and poor repeatability in practical applications. Recently, gallium nitride (GaN) with chemical inertness, large potential window, excellent biocompatibility and low background current has attracted many attentions in the field of electrochemical sensor [19e24]. For instance, Jiang et al. loaded bismuth on GaN electrode in trace cadmium detection and hydrogen peroxide detection [22,23]. The bismuth-modified GaN showed ultra-sensitivity by showing limit of detection (LOD) of 0.3 mg/L and 5 mM for cadmium detection and hydrogen peroxide detection, respectively. Our recent studies have also demonstrated that GaN electrode showed high sensitivity in trace Agþ detection [24]. However, the influences of surface morphology of GaN electrode have rarely been investigated and compared in electrochemical analysis of trace metal ions. Since nanomaterials have charge carriers limited or close to the electrode/electrolyte interface, the sensitivity of the GaN electrode could be influenced significantly by the electrode surface condition. Our previous studies have pointed out that the surface-to-volume ratio and interfacial charge transfer rate are important parameters in sensitivity improvement of GaN electrodes [25]. Therefore, the sensitivity of the GaN electrode
could be influenced significantly by the electrode surface condition. The sensitivity enhancement could be expected by modulating the surface morphologies of GaN electrodes. Herein, the sensitivity of highly aligned GaN micropillars in trace Agþ and Cu2þ detection using anodic stripping voltammetry is investigated. Scanning electron microscopy (SEM) measurement confirms that the GaN micropillars grown by conventional chemical vapor deposition are highly-aligned on GaN substrate. Using ASV approach, ultrasensitive response to Agþ and Cu2þ ions with low detection limits of 3.3 ppb and 3.3 ppb has been successfully realized on GaN micropillar electrode. The electrochemical performance in trace Agþ and Cu2þ detection between GaN micropillars and GaN film has been compared and discussed. It is found that the large surface-to-volume ratio and hydrophilia nature are the key factors contributing to the ultra-sensitivity of GaN micropillars. The construction of highly sensitive chemical sensors based on homoepitaxially grown GaN micropillars will pave a solid way towards the trace metal ion analysis in biosensing applications. 2. Experimental section 2.1. Material synthesis and characterization GaN micropillars homoepitaxially grown on p-type GaN substrate were obtained through a simple chemical vapor deposition (CVD) process, as described in our previous work [26,27]. Crystalline GaN film etched with inverted-triangles pits grown on [0001]orientated sapphire substrate was obtained by hydrogen vapor phase epitaxy (HVPE) method [28]. The morphology and electrical resistance of GaN film were reported in our previous work [24]. The morphology and composition purity of the as-grown single crystalline GaN micropillars were characterized by a field-emission scanning electron microscopy operating at 20 kV (FESEM Hitachi, SU70) attached X-ray diffraction spectroscopy (EDS). 2.2. Electrochemical behavior characterization A standard three-electrode configuration was adopted for the electrochemical characterization of GaN electrodes. To conduct the measurement, GaN micropillars or GaN film was adopted as working electrode, platinum wire as counter electrode and Ag/AgCl (0.1 M KCl) as reference electrode. The area of the reactive working electrode is 0.053 cm2 (Fig. 1a). Electrochemical behaviors of the GaN micropillars were examined in 1 mM [Fe(CN)6]3-/4- and 1 mM [Ru(NH)3]2þ/3þ, respectively, all dissolved in 0.1 M KCl aqueous solution. Electrochemical potential window of the GaN micropillars was measured in 0.1 M KCl. Analyte solutions containing AgNO3 and Cu(NO3)2 of different concentrations were freshly prepared using Milli-Q (Millipore Direct-Q 8 system) water with a resistivity of 18.2 U cm. 2.3. ASV analysis ASV trace metal ions analysis was performed on an Autolab PGSTAT302 N electrochemical workstation. Standard buffer solutions containing Agþ and Cu2þ were used as model analyte system. The buffer solutions were 0.1 M acetate with pH ¼ 5.2 and contained 30 ppm Ca2þ and 5 ppm Mg2þ, imitating the drinking water system. The Agþ and Cu2þ ions were accumulated on the working electrode for 3 min at optimized voltages. After pre-concentration, differential pulse voltammetry was applied to “strip” the
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 1. (a) The schematic image of the general setup of the three-electrode system, the inset shows the photo of the electrochemical cell. (b) 10 -tilted and (c) cross-sectional SEM images of GaN micropillars. The inset in (b) and (c) is the magnified SEM images showing the morphology of an individual GaN micropillar. The scale bars in (b) and (c) are 20 mm and 8 mm respectively, and the scale bar in all insets is 1 mm.
deposited metals back into the buffer solution. After each measurement, the electrode was directly used for the repeated experiment for three times. Prior to the next cycle, the electrode was “cleaned” at 0.4 V for 5 min after measurement. All the electrochemical experiments were carried out at room temperature. The real sample of tap water was added with the same standard buffer agents before the measurements.
3. Results and discussions The GaN micropillars used as electrode were homoepitaxially grown on crystalline GaN substrate using a CVD method. The typical morphology of the as-grown GaN micropillars is observed using SEM characterization. Fig. 1b and c shows the 10 -tilted and cross-sectional SEM images of GaN micropillars and one can see that a large scale of vertically-aligned GaN micropillars can be observed on [0001]-oriented p-type GaN film substrate. The magnified SEM images inserted in Fig. 1b show that GaN micropillars are featured with hexagonal cross section, which is a typical indicator of [0001]-oriented GaN nanorods [29]. The GaN micropillars are also featured with pyramidal tips on top without the presence of any catalyst. Contrast to the commonly observed smooth side facets of GaN arrays [30,31], the conventional CVD grown GaN micropillars have a rough and corrugated surface, as pointed out by yellow arrows in the inset in Fig. 1c. It is assumed that the periodic corrugated morphology can be regarded as a combined effect of the oscillation in nanowires’ radial growth and the competing result of electrostatic interaction energy in the concave and convex planes, as demonstrated in our previous work [26]. Obviously, the periodic concave and convex surfaces can provide more active sites for ion detection in comparison with conventional round or hexagonal GaN nanowires/nanorods. Additionally, the direct homoepitaxy of GaN micropillars on highly conductive p-GaN substrate also guarantee the good electrical transportation with enhanced signals. The electrochemical behavior characterizations of the GaN micropillars are carried out in the redox probe of [Fe(CN)6]3-/4- and [Ru(NH3)6]2þ/3þ in 0.1 M KCl aqueous solution. Fig. 2a shows the cyclic voltammograms of GaN micropillar arrays in [Fe(CN)6]3-/4redox probe. Well-defined redox waves are observed at scanning rates from 10 mV/s to 200 mV/s, with DEp and IPox/IPred of 113 ± 1 mV and 1.12 ± 0.05 at the scanning rate of 10 mV/s. The anodic and cathodic peak currents show a linear dependence on the square root of the scanning rate (Fig. 2b). Similarly, the electrochemical response of GaN micropillar in [Ru(NH3)6]2þ/3þ redox proves identical tendency at scanning rate from 10 mV/s to 200 mV/s, with DEp and IPox/IPred of 106 ± 1 mV and 0.89±0.05 at scanning rate of 10 mV/s (Fig. 2c and d). The identical tendency in both redox probes
indicates the metal reduction and oxidation reaction on GaN micropillars are mass-transfer controlled processes. Notably, the peak difference between anodic and cathodic current curves is much smaller than that of GaN film (122 mV) [24] and comparable to high quality boron-doped diamond (BDD) electrode (71 mV) [32]. The aforementioned results indicate that quasi-reversible redox activities occur on GaN micropillar surface, making it a suitable electrode candidate for electrochemical analysis. From the cyclic voltammogram of the GaN micropillar in 0.1 M KCl shown in Fig. 3a, the potential window can be deduced to be 1.9 V, ranging from 1.5 V in positive potential to 0.4 V in negative potential (based on the definition of a current density threshold of 0.1 mA/cm2) [33]. This wide potential window bestows GaN micropillar to be capable of multiple types of heavy metal ion detection, such as Cu2þ and Agþ etc. Fig. 3b and c shows the optimized deposition potential for Agþ and Cu2þ detection. According to the previous work, the testing range of deposition potentials are set between 0.3 and 0.1 V for Ag deposition and between 0.5 and 0.3 V for Cu deposition [8,34]. The maximized peaks for anodic stripping voltammetry of Agþ and Cu2þ are reached at 0.1 V and 0.4 V, respectively. Fig. 3d shows the stripping voltammetry in standard buffer solution containing 1000 ppb Agþ under optimal deposition condition. A predominant stripping peak of Agþ is observed at 0.277 ± 0.002 V vs Ag/AgCl, indicating the successful deposition of Ag(I) on the electrode surface (black curve). The full width at half maximum (FWHM) is measured to be 80 ± 1 mV, which is slightly larger than those of Hg (30e50 mV) and BBD (50e60 mV) but is slightly smaller than that of GaN film electrode [24]. Notably, the well-resolved stripping peak shows a slight asymmetry, with a slower current increase on low potential side and a sharp current decrease on the other side. The asymmetry could be assigned to the diverse electron-transfer kinetics across the electrode surface. The exposed side facets of GaN micropillar exhibit dissimilar electrochemical activities, so the Ag(I) nanoparticles deposit on the electrode surface in different sizes and shapes. In this way, the inconsistency in electron transport rate at the electrode surface occur. The surface renewability is examined by performing in-situ electrochemical “cleaning” process. The stripping process is followed to ensure no residual Ag(I) nanoparticles on the electrode surface. As is clearly seen in Fig. 3d, no peak is observed, indicating the full removal of accumulated Ag(I) nanoparticles. Fig. 4a shows the anodic stripping voltammetry of Agþ carried out in the standard buffer solution. Single ASV stripping peak is obtained in standard buffer solution containing Agþ with different concentrations. An obvious stripping peak can still be observed in the case of Agþ concentration as low as 10 ppb, proving the ultrasensitive detection capability of GaN micropillar electrode. The
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 2. Cyclic voltammogram in (a) 1 mM Fe [(CN)6]3-/4- in 0.1 M KCl and (c) 1 mM Ru [(NH)3]2þ/3þ in 0.1 M KCl. Corresponding cathodic peak currents (red curve) and anodic peak currents (black curve) versus the square root of the scan rate from 10 to 200 mV/s are shown in (b) and (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. (a) Cyclic voltammograms of the crystalline GaN micropillar at a scan rate of 100 mV s1. Optimization of the deposition potential of (b) Agþ in the range from 0.3 V to 0.1 V and (c) Cu2þ in the range from 0.3 V to 0.5 V. The deposition time is set at 3 min for all experiments. (d) Anodic stripping voltammograms in standard buffer solution containing 1000 ppb Ag(I) (black curve), the red curve represents the anodic sweeping curve after ‘cleaning’ at þ0.4 V for 5 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
stripping peaks shift positively with the incremental Agþ concentration. This can be explained by Nernst equation, in which the equilibrium potentials of Agþ/Ag redox couple would be increased with ion concentrations. The corresponding calibration curve
shows a good linear relationship between the stripping peak and the examined Agþ concentration with a correlation coefficient of R2 ¼ 0.981. The linear relationship is established to be
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 4. (a) Anodic stripping voltammograms of GaN micropillar after pre-concentration in standard solutions containing Agþ with concentrations from 10 ppb to 1000 ppb; (b) Corresponding calibration curve for Agþ detection, the error bars represent the standard deviation obtained from three measurements (n ¼ 3).
I(mA) ¼ 2.162E-8þ5.226E-10*C (ppb)
(1)
over a wide Agþ concentration range from 10 ppb to 1000 ppb. The limit of detection (LOD) is estimated to be 3.3 ppb at the signal-tonoise ratio of 3 (S/N ¼ 3), which is comparable to the results reported using different techniques (Table 1) and on common electrodes using ASV method (Table 2). The above-mentioned results in trace Agþ detection demonstrate the feasibility and superior sensitivity of GaN micropillar electrode. Apparently, GaN micropillars exhibit lower LOD and better electrochemical behavior than previously reported GaN film [24]. In order to facilitate the application of GaN nanostructures in heavy metal ion detections, more heavy metal ions detection using GaN electrode is expected. Compared with Agþ, the bivalent metal ions (for instance Cu2þ, Cd2þ and Hg2þ) require much more additional energy during reduction process in pre-concentration, and the electrode is required to be more sensitive for trace detection. Hereinafter, Cu2þ is selected as a model target to further demonstrate the sensitivity and feasibility of GaN micropillar electrode. To further demonstrate the sensitivity of GaN micropillars, GaN film reported in our previous work is adopted for comparison [24]. Fig. 5a shows the stripping voltammetry of GaN micropillars and GaN film after pre-concentration in standard buffer solution containing 100 ppb Cu2þ at 0.4 V. Anodic stripping peaks are observed at 0.039 ± 0.002 V and 0.043 ± 0.002 V for GaN micropillars and GaN film, respectively. These two predominant peaks located around 0.1 V suggest the successful deposition of Cu(II) on the electrode surface. Interestingly, GaN micropillars show stronger current response in Cu2þ detection, with six times magnitude higher than GaN film. A model aqueous solution
Table 2 The comparison table of detection limits in trace heavy metal detection. Electrode Materials
Detection limit (ppb)
GaN micropillars Diamond/Graphite BDD Hg Screen-printed Au Bismuth film Mercury film modified screen-printed carbon
Agþ
Cu2þ
References
3.3 5.8 1.0 e e e e
3.3 5.6 10 12 1 10 5
This work [34] [32] [47] [48] [49] [50]
containing 1000 ppb Cu2þ is then employed to further demonstrate the electrochemical behavior of GaN micropillars. As is shown in Fig. 5b, a conspicuous anodic peak appears around 0.001 V vs Ag/ AgCl, with a FWHM of 131 ± 1 mV (black curve). This is slightly larger than those of BBD (60 ± 3 mV) and Hg (32 ± 1) mV [32] electrodes, but it is still not excessively broad to interfere in the multi detection for metal ions. A slight asymmetry is also observed in the stripping peak, showing the evidence for inconsistency in electron transfer rate. After the ‘cleaning’ process, no stripping peak is observed (red curve), proving the excellent renewability of GaN micropillars in trace Cu2þ detection. Fig. 6a and c shows the anodic stripping voltammetry of GaN micropillars and GaN film in trace Cu2þ detection. Predominant anodic peaks of Cu2þ are observed in the vicinity of 0.1 V and shift positively with incremental Cu2þconcentration, which is also frequently observed in the ASV detection of metal ions on other electrodes [8,32,34]. GaN micropillars show excellent linear relationship of
Table 1 The comparison table of detection limits in Ag(I) and Cu (II) detection using different techniques. Metal ion
Techniques
CLOD (ppb)
References
Ag
Direct current plasma Flame atomic absorption spectroscopy Real-time fluorescence Chromogenic probe Inductively coupled plasma atomic emission spectroscopy Inductively coupled plasma-mass spectroscopy Anodic stripping voltammetry Direct current plasma Flame atomic absorption spectroscopy Real-time fluorescence Chromogenic probe Inductively coupled plasma atomic emission spectroscopy Inductively coupled plasma-mass spectroscopy Anodic stripping voltammetry
38 473 200 14 0.3 0.036 3.3 2 40 6.4 1.92 0.006 0.1 3.3
[36] [36] [37] [38] [39] [40] This work [41] [42] [43] [44] [45] [46] This work
Cu
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 5. (a) Anodic stripping voltammograms of GaN micropillar (black) and GaN film (red) in standard buffer solution containing 100 ppb Cu(II); (b) Anodic stripping voltammograms in standard buffer solution containing 1000 ppb Cu(II) (black curve), the red curve represents the anodic sweeping curve after ‘cleaning’ at þ0.4 V for 5 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. (a) Anodic stripping voltammograms of GaN micropillars after pre-concentration in standard solutions containing different Cu(II) concentrations ranging from 10 ppb to 1000 ppb and (b) the corresponding calibration curve for anodic peak current as a dependence of Cu(II) concentration; (c) Anodic stripping voltammograms of GaN film after preconcentration in standard solutions containing different Cu(II) concentrations ranging from 10 ppb to 1000 ppb and (d) the corresponding calibration curve for anodic peak current as a dependence of Cu(II) concentration. The error bars in (b) and (d) represent the standard deviation obtained from three measurements (n ¼ 3). (e) The simultaneous detection of 1 ppm Ag(I) and 1 ppm Cu(II) in standard buffer solution using GaN micropillar (black) and GaN film (red).
I(mA) ¼ 6.326E-8þ5.895E-10*C(ppb)
(2)
with a correlation coefficient of R2 ¼ 0.994 over Cu2þ concentration range from 50 ppb to 1000 ppb (Fig. 6b). The linear relationship of
GaN film is established to be I(mA) ¼ -1.672E-9þ2.012E-10*C(ppb)
(3)
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Table 3 The comparison table of detection limits of GaN micropillars and GaN film in Ag(I) and Cu(II) detection. Parameters
Detection limit (ppb) Sensitivity (nA/ppb) Correlation coefficient Linear range (ppb)
Agþ
Cu2þ
GaN micropillar
GaN film [24]
GaN micropillar
GaN film
3.3 0.5226 ± 0.02 0.981 10e1000
7 0.2044 ± 0.01 0.991 10e1000
3.3 0.5895 ± 0.02 0.994 50e1000
3.3 0.2012 ± 0.01 0.964 10e500
with a correlation coefficient of R2 ¼ 0.964 in Cu2þ concentration range from 10 ppb to 500 ppb (Fig. 6d). GaN micropillars show better sensitivity by exhibiting higher current responses (0.5895 ± 0.02 nA/ppb) in trace ion detections. The sensitivity of GaN micropillars is comparable to the results reported using different techniques (Table 1) and on extensively adopted electrodes using ASV method (Table 2). The possibility of simultaneous detection of multiple metal ions in the standard buffer solutions is further explored and examined. Fig. 6e shows the stripping curves after pre-concentration in the solution concurrently containing 1000 ppb Cu2þ and 1000 ppb Agþ. Conspicuous and well-resolved peaks corresponding to Cu(II) and Ag(I) can be observed around 0.03 ± 0.01 V vs Ag/AgCl and 0.23 ± 0.01 V vs Ag/AgCl, respectively. The GaN micropillar electrodes exhibit three times higher current response than GaN film electrode, indicating a more sensitive electrochemical response to trace metal ion detection. Compared with individual ion detection, the anodic peak of Cu2þ shifts to more negative potential while the anodic peak of Agþ moves to more positive potential in multi ions detection. It is assumed that the interactions between the two metal ions during pre-concentration and stripping process are responsible for anodic peak deviation [8]. Additionally, it should be noted that the GaN micropillar electrode owns the simultaneous detection capability of Cu2þ and Agþ in aqueous solution with lower concentration as shown in Fig. S4 and can be tentatively used for the trace detection of Agþ and Cu2þ in tap water. However, the tap water only shows a weak Agþ signal (Fig. S5), while the Cu2þ peak is not observed, possibly because that the content of Cu2þ in tap water is lower than the detection limit of our electrode. Generally, the surface condition of the electrode plays a vital role in the heavy metal ion detection. It is assumed that due to the large surface-to-volume ratio, GaN micropillars present better sensitivity and lower detection limit in the detection process. As demonstrated in Randles-Sevcik equation, the main factors influencing the magnitudes of anodic stripping peaks of metal ions are as follows [35]: I ¼ 2.69E5 n3/2AD1/2v1/3c
(4)
where I represent the anodic peak value, n the numbers of electrons in reaction, A the electrode area immersed in the solution, D the diffusion coefficient of metal ions, v the scanning rate and c is the solution concentration. It can be clearly implied that when the solution concentration c is identical, the anodic peak intensity is closely related to the numbers of electrons in reaction and the electrode area immersed in the solution when the same metal ion is detected. Table 3 summarizes the important parameters of GaN micropillars and GaN film in trace Agþ and Cu2þ detection. It can be seen that GaN micropillars exhibit lower detection limit and higher sensitivity. Compared with GaN film, the vertically-aligned GaN microarrays offer more deposition sites for metal ions and involve more electrons in the detection process, thus contribute to the conductivity improvement. Moreover, the hydrophilia property of
Fig. 7. The contact angle of GaN micropillar (left) and GaN film (right). The solution used in the experiment is deionized (DI) water.
GaN micropillars help to expand electrode area immersed in the buffer solution. As shown in Fig. 7, the contact angle (the angle where a liquid-vapor interface meets a solid surface) of GaN micropillars is 59.7 ±1.8 , while the GaN film has a contact angle of 100.5 ±0.2 . Therefore, GaN micropillars show obvious hydrophilia nature. This helps it to increase the peak current intensity, resulting in sensitivity promotion in trace Agþ and Cu2þ detection. As a result, the promising application of GaN micropillars can be expected due to their superior electrochemical properties and excellent chemical inertness. 4. Conclusion In summary, GaN micropillars with uniform alignment along the [0001] direction are used in trace Agþ and Cu2þ detection. The asgrown GaN micropillars without further surface modification have shown ultrasensitive detection capability in trace Agþ and Cu2þ detections, by showing detection limits of 3.3 ppb and 3.3 ppb, respectively. Comparison made on the electrochemical responses of GaN micropillars and GaN film demonstrates that GaN micropillars surpass GaN film in conductivity. The GaN micropillars show six orders of magnitude higher in individual Cu2þ detection and three orders of magnitude higher in simultaneous Agþ and Cu2þ detection. It is assumed that the large surface area of GaN microarrays, the interfacial p-n junction and the hydrophilia nature of GaN micropillars contribute to the significant conductivity improvement. As a result, GaN micropillars prove to be a competitive candidate for trace metal ions detection. Further work focusing on real water sample analysis is necessary in order to promote the bioapplication of GaN electrodes in trace metal ions detection and such work is already in progress. Author contributions B. Liu designed the experiment; Q. Liu, J. Li, W. Yang and C. Zhang
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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synthesized the samples and did the measurements; X. Zhang, B. Liu, C. Labbe, X. Portier, F. Liu and J. Yao discussed the results; Q. Liu, J. Li and B. Liu wrote the paper. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was partially supported by the National Natural Science Foundation of China (No.51702326,51872296), the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University) with grant No. OEMT-2017-KF02, and the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No.2017RP25). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2019.11.010. References [1] Y. Lu, X. Liang, C. Niyungeko, J. Zhou, J. Xu, G. Tian, A review of the identification and detection of heavy metal ions in the environment by voltammetry, Talanta 178 (2018) 324e338. [2] Z. Rasouli, Z. Hassanzadeh, R. Ghavami, Application of a new version of GARBF neural network for simultaneous spectrophotometric determination of Zn(II), Fe(II), Co(II) and Cu(II) in real samples: an exploratory study of their complexation abilities toward MTB, Talanta 160 (2016) 86e98. [3] G. Baccolo, M. Clemenza, B. Delmonte, N. Maffezzoli, M. Nastasi, E. Previtali, M. Prata, A. Salvini, V. Maggi, A new method based on low background instrumental neutron activation analysis for major, trace and ultra-trace element determination in atmospheric mineral dust from polar ice cores, Anal. Chim. Acta 922 (2016) 11e18. [4] K. Jomova, M. Valko, Advances in metal-induced oxidative stress and human disease, Toxicology 283 (2011) 65e87. [5] K.J. Barnham, A.I. Bush, Metals in Alzheimer’s and Parkinson’s diseases, Curr. Opin. Chem. Biol. 12 (2008) 222e228. [6] C.M. Wood, C. Hogstrand, F. Galvez, R.S. Munger, The physiology of waterborne silver toxicity in freshwater rainbow trout (Oncorhynchus mykiss): 1. The effects of ionic Agþ, Aquat. Toxicol. (Amst.) 35 (1996) 93e109. [7] P. Miao, Y. Tang, L. Wang, DNA modified Fe3O4@Au magnetic nanoparticles as selective probes for simultaneous detection of heavy metal ions, ACS Appl. Mater. Interfaces 9 (2017) 3940e3947. [8] H. Zhuang, C. Wang, N. Huang, X. Jiang, Cubic SiC for trace heavy metal ion analysis, Electrochem. Commun. 41 (2014) 5e7. [9] J.H. Wang, E.H. Hansen, FI/SI on-line solvent extraction/back extraction preconcentration coupled to direct injection nebulization inductively coupled plasma mass spectrometry for determination of copper and lead, J. Anal. Atomic Spectrom. 17 (2002) 1284e1289. [10] F.Z. Xie, X.C. Lin, X.P. Wu, Z.H. Xie, Solid phase extraction of lead (II), copper (II), cadmium (II) and nickel (II) using gallic acid-modified silica gel prior to determination by flame atomic absorption spectrometry, Talanta 74 (2008) 836e843. [11] C.-X. Yang, H.-B. Ren, X.-P. Yan, Fluorescent metal organic framework MIL53(Al) for highly selective and sensitive detection of Fe3þ in aqueous solution, Anal. Chem. 85 (2013) 7441e7446. [12] J. Wang, J.M. Lu, U. Anik, S.B. Hocevar, B. Ogorevc, Insights into the anodic stripping voltammetric behavior of bismuth film electrodes, Anal. Chim. Acta 434 (2001) 29e34. [13] A. Economou, P.R. Fielden, Mercury film electrodes: developments, trends and potentialities for electroanalysis, Analyst 128 (2003) 205e212. [14] M.B. Gumpu, S. Sethuraman, U.M. Krishnan, J.B.B. Rayappan, A review on detection of heavy metal ions in water - an electrochemical approach, Sens. Actuators B Chem. 213 (2015) 515e533. [15] B. Bas, K. Wegiel, K. Jedlinska, The renewable bismuth bulk annular band working electrode: fabrication and application in the adsorptive stripping voltammetric determination of nickel(II) and cobalt(II), Anal. Chim. Acta 881 (2015) 44e53. [16] D.X.O. Jaramillo, A. Sukeri, L.P.H. Saravia, P.J. Espinoza-Montero, M. Bertotti, Nanoporous gold microelectrode: a novel sensing platform for highly sensitive and selective determination of arsenic (III) using anodic stripping voltammetry, Electroanalysis 29 (2017) 2316e2322.
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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode b, Qingyun Liu a, Jing Li a, Wenjin Yang a, Xinglai Zhang a, Cai Zhang a, Christophe Labbe Xavier Portier b, Fei Liu c, Jinlei Yao d, Baodan Liu a, * a
Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72, Wenhua Road, Shenhe District, Shenyang, 110016, China CIMAP CNRS/CEA/ENSICAEN/Normandie University, 6 Bd Mar echal Juin, 14050, Caen Cedex 4, France c State Key Laboratory of Optoelectronic Materials and Technologies and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou, 510275, PR China d Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application, School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, 215009, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Highly aligned GaN micropillars have been homoepitaxially grown on GaN substrate. The GaN working electrode provides sufficient active sites for ion detection. The GaN micropillars exhibit enhanced response in trace Agþ and Cu2þ ion detection. The GaN micropillars show lower detection limit than GaN films.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2019 Received in revised form 5 November 2019 Accepted 7 November 2019 Available online xxx
Driven by the motivation to quantitively control and monitor trace metal ions in water, the development of environmental-friendly electrodes with superior detection sensitivity is extremely important. In this work, we report the design of a stable, ultrasensitive and biocompatible electrode for the detection of trace Agþ and Cu2þ ions by growing n-type GaN micropillars on conductive p-type GaN substrate. The electrochemical measurement based on cyclic voltammetry indicates that the GaN micropillars exhibit quasi-reversible and mass-controlled reaction in redox probe solution. In the application of trace Agþ and Cu2þ determination, the GaN micropillars show superior sensitivity and excellent conductivity by presenting a detection limit of 3.3 ppb for Agþ and 3.3 ppb for Cu2þ. Comparative studies on the electrochemical response of GaN micropillars and GaN film in the simultaneous Agþ and Cu2þ detection reveal that GaN micropillars show three orders of magnitude higher stripping peak current than GaN film. It is assumed that the microarray morphology with large active area and the hydrophilia nature of GaN micropillars are responsible for the excellent sensitivity. This work will open up some opportunities for GaN nanostructure electrodes in the application of trace metal ions detection. © 2019 Elsevier B.V. All rights reserved.
Keywords: GaN micropillar arrays Homoepitaxial growth Electrochemistry Metal ion detection
* Corresponding author. Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), No. 72, Wenhua Road, Shenhe District, Shenyang, 110016, China. E-mail address: [email protected] (B. Liu). https://doi.org/10.1016/j.aca.2019.11.010 0003-2670/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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1. Introduction The rapid economic development inevitably leads to increased wastewater discharge in the environment. The heavy metal pollution is gradually recognized to threaten human health and environment [1e3]. The exposed ions in drinking water, such as silver ion (Agþ) and copper ion (Cu2þ), are non-naturally degradable. The bioaccumulation of Agþ and Cu2þ in human body will cause series of function disorders at least or severe diseases at worst [4]. For instance, long-term Cu2þ-rich drinking water would be able to trigger heart diseases, arteriosclerosis and argyria [5]. The toxicity of Agþ mainly relies on the ability to bind with metabolites and inactivate sulfhydryl enzyme. By destroying cell membrane through extracellular mechanisms, the biological enrichment of Agþ can induce skin irritation, stomach distress and organ edema [6,7]. In the past years, considerable efforts have been put forward to quantitatively monitor and control metal ions in the water environment. Various analytic techniques such as anodic stripping voltammetry (ASV) [8], solid-phase extraction (SPE) [9], atomic adsorption spectroscopy (AAS) [10] and inductively couple plasma mass spectrometry (ICP-MS) [11] have been developed towards sensitive detection of ions. Compared with other detection methods, ASV is one of the most extensively adopted approaches in heavy metal detection due to its remarkable sensitivity, easy operation and low cost. In ASV procedures, hanging mercury drop electrodes or mercury film modified electrodes are most commonly employed because of the high signal-to-noise ratio and reproducibility [12,13]. The disadvantages of mercury-based electrodes include high toxicity, potential window limitation (not relevant in the case of Agþ detection) and restrictions on mercury disposal [14]. The development of alternative materials with good electrochemical performance and environment friendly characteristics is essential. Various materials including bismuth [15], gold [16], platinum [17] and screen-printed carbon [18] electrodes have been proposed to replace mercury electrodes. However, the bismuth electrodes are encountered with similar limitations in cation ions detection (for instance, Agþ). The gold and platinum electrodes have high background currents in trace metal detections owe to the excellent catalytic activity for hydrogen revolution. The screenprinted carbon electrodes also suffer from high background current and poor repeatability in practical applications. Recently, gallium nitride (GaN) with chemical inertness, large potential window, excellent biocompatibility and low background current has attracted many attentions in the field of electrochemical sensor [19e24]. For instance, Jiang et al. loaded bismuth on GaN electrode in trace cadmium detection and hydrogen peroxide detection [22,23]. The bismuth-modified GaN showed ultra-sensitivity by showing limit of detection (LOD) of 0.3 mg/L and 5 mM for cadmium detection and hydrogen peroxide detection, respectively. Our recent studies have also demonstrated that GaN electrode showed high sensitivity in trace Agþ detection [24]. However, the influences of surface morphology of GaN electrode have rarely been investigated and compared in electrochemical analysis of trace metal ions. Since nanomaterials have charge carriers limited or close to the electrode/electrolyte interface, the sensitivity of the GaN electrode could be influenced significantly by the electrode surface condition. Our previous studies have pointed out that the surface-to-volume ratio and interfacial charge transfer rate are important parameters in sensitivity improvement of GaN electrodes [25]. Therefore, the sensitivity of the GaN electrode
could be influenced significantly by the electrode surface condition. The sensitivity enhancement could be expected by modulating the surface morphologies of GaN electrodes. Herein, the sensitivity of highly aligned GaN micropillars in trace Agþ and Cu2þ detection using anodic stripping voltammetry is investigated. Scanning electron microscopy (SEM) measurement confirms that the GaN micropillars grown by conventional chemical vapor deposition are highly-aligned on GaN substrate. Using ASV approach, ultrasensitive response to Agþ and Cu2þ ions with low detection limits of 3.3 ppb and 3.3 ppb has been successfully realized on GaN micropillar electrode. The electrochemical performance in trace Agþ and Cu2þ detection between GaN micropillars and GaN film has been compared and discussed. It is found that the large surface-to-volume ratio and hydrophilia nature are the key factors contributing to the ultra-sensitivity of GaN micropillars. The construction of highly sensitive chemical sensors based on homoepitaxially grown GaN micropillars will pave a solid way towards the trace metal ion analysis in biosensing applications. 2. Experimental section 2.1. Material synthesis and characterization GaN micropillars homoepitaxially grown on p-type GaN substrate were obtained through a simple chemical vapor deposition (CVD) process, as described in our previous work [26,27]. Crystalline GaN film etched with inverted-triangles pits grown on [0001]orientated sapphire substrate was obtained by hydrogen vapor phase epitaxy (HVPE) method [28]. The morphology and electrical resistance of GaN film were reported in our previous work [24]. The morphology and composition purity of the as-grown single crystalline GaN micropillars were characterized by a field-emission scanning electron microscopy operating at 20 kV (FESEM Hitachi, SU70) attached X-ray diffraction spectroscopy (EDS). 2.2. Electrochemical behavior characterization A standard three-electrode configuration was adopted for the electrochemical characterization of GaN electrodes. To conduct the measurement, GaN micropillars or GaN film was adopted as working electrode, platinum wire as counter electrode and Ag/AgCl (0.1 M KCl) as reference electrode. The area of the reactive working electrode is 0.053 cm2 (Fig. 1a). Electrochemical behaviors of the GaN micropillars were examined in 1 mM [Fe(CN)6]3-/4- and 1 mM [Ru(NH)3]2þ/3þ, respectively, all dissolved in 0.1 M KCl aqueous solution. Electrochemical potential window of the GaN micropillars was measured in 0.1 M KCl. Analyte solutions containing AgNO3 and Cu(NO3)2 of different concentrations were freshly prepared using Milli-Q (Millipore Direct-Q 8 system) water with a resistivity of 18.2 U cm. 2.3. ASV analysis ASV trace metal ions analysis was performed on an Autolab PGSTAT302 N electrochemical workstation. Standard buffer solutions containing Agþ and Cu2þ were used as model analyte system. The buffer solutions were 0.1 M acetate with pH ¼ 5.2 and contained 30 ppm Ca2þ and 5 ppm Mg2þ, imitating the drinking water system. The Agþ and Cu2þ ions were accumulated on the working electrode for 3 min at optimized voltages. After pre-concentration, differential pulse voltammetry was applied to “strip” the
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 1. (a) The schematic image of the general setup of the three-electrode system, the inset shows the photo of the electrochemical cell. (b) 10 -tilted and (c) cross-sectional SEM images of GaN micropillars. The inset in (b) and (c) is the magnified SEM images showing the morphology of an individual GaN micropillar. The scale bars in (b) and (c) are 20 mm and 8 mm respectively, and the scale bar in all insets is 1 mm.
deposited metals back into the buffer solution. After each measurement, the electrode was directly used for the repeated experiment for three times. Prior to the next cycle, the electrode was “cleaned” at 0.4 V for 5 min after measurement. All the electrochemical experiments were carried out at room temperature. The real sample of tap water was added with the same standard buffer agents before the measurements.
3. Results and discussions The GaN micropillars used as electrode were homoepitaxially grown on crystalline GaN substrate using a CVD method. The typical morphology of the as-grown GaN micropillars is observed using SEM characterization. Fig. 1b and c shows the 10 -tilted and cross-sectional SEM images of GaN micropillars and one can see that a large scale of vertically-aligned GaN micropillars can be observed on [0001]-oriented p-type GaN film substrate. The magnified SEM images inserted in Fig. 1b show that GaN micropillars are featured with hexagonal cross section, which is a typical indicator of [0001]-oriented GaN nanorods [29]. The GaN micropillars are also featured with pyramidal tips on top without the presence of any catalyst. Contrast to the commonly observed smooth side facets of GaN arrays [30,31], the conventional CVD grown GaN micropillars have a rough and corrugated surface, as pointed out by yellow arrows in the inset in Fig. 1c. It is assumed that the periodic corrugated morphology can be regarded as a combined effect of the oscillation in nanowires’ radial growth and the competing result of electrostatic interaction energy in the concave and convex planes, as demonstrated in our previous work [26]. Obviously, the periodic concave and convex surfaces can provide more active sites for ion detection in comparison with conventional round or hexagonal GaN nanowires/nanorods. Additionally, the direct homoepitaxy of GaN micropillars on highly conductive p-GaN substrate also guarantee the good electrical transportation with enhanced signals. The electrochemical behavior characterizations of the GaN micropillars are carried out in the redox probe of [Fe(CN)6]3-/4- and [Ru(NH3)6]2þ/3þ in 0.1 M KCl aqueous solution. Fig. 2a shows the cyclic voltammograms of GaN micropillar arrays in [Fe(CN)6]3-/4redox probe. Well-defined redox waves are observed at scanning rates from 10 mV/s to 200 mV/s, with DEp and IPox/IPred of 113 ± 1 mV and 1.12 ± 0.05 at the scanning rate of 10 mV/s. The anodic and cathodic peak currents show a linear dependence on the square root of the scanning rate (Fig. 2b). Similarly, the electrochemical response of GaN micropillar in [Ru(NH3)6]2þ/3þ redox proves identical tendency at scanning rate from 10 mV/s to 200 mV/s, with DEp and IPox/IPred of 106 ± 1 mV and 0.89±0.05 at scanning rate of 10 mV/s (Fig. 2c and d). The identical tendency in both redox probes
indicates the metal reduction and oxidation reaction on GaN micropillars are mass-transfer controlled processes. Notably, the peak difference between anodic and cathodic current curves is much smaller than that of GaN film (122 mV) [24] and comparable to high quality boron-doped diamond (BDD) electrode (71 mV) [32]. The aforementioned results indicate that quasi-reversible redox activities occur on GaN micropillar surface, making it a suitable electrode candidate for electrochemical analysis. From the cyclic voltammogram of the GaN micropillar in 0.1 M KCl shown in Fig. 3a, the potential window can be deduced to be 1.9 V, ranging from 1.5 V in positive potential to 0.4 V in negative potential (based on the definition of a current density threshold of 0.1 mA/cm2) [33]. This wide potential window bestows GaN micropillar to be capable of multiple types of heavy metal ion detection, such as Cu2þ and Agþ etc. Fig. 3b and c shows the optimized deposition potential for Agþ and Cu2þ detection. According to the previous work, the testing range of deposition potentials are set between 0.3 and 0.1 V for Ag deposition and between 0.5 and 0.3 V for Cu deposition [8,34]. The maximized peaks for anodic stripping voltammetry of Agþ and Cu2þ are reached at 0.1 V and 0.4 V, respectively. Fig. 3d shows the stripping voltammetry in standard buffer solution containing 1000 ppb Agþ under optimal deposition condition. A predominant stripping peak of Agþ is observed at 0.277 ± 0.002 V vs Ag/AgCl, indicating the successful deposition of Ag(I) on the electrode surface (black curve). The full width at half maximum (FWHM) is measured to be 80 ± 1 mV, which is slightly larger than those of Hg (30e50 mV) and BBD (50e60 mV) but is slightly smaller than that of GaN film electrode [24]. Notably, the well-resolved stripping peak shows a slight asymmetry, with a slower current increase on low potential side and a sharp current decrease on the other side. The asymmetry could be assigned to the diverse electron-transfer kinetics across the electrode surface. The exposed side facets of GaN micropillar exhibit dissimilar electrochemical activities, so the Ag(I) nanoparticles deposit on the electrode surface in different sizes and shapes. In this way, the inconsistency in electron transport rate at the electrode surface occur. The surface renewability is examined by performing in-situ electrochemical “cleaning” process. The stripping process is followed to ensure no residual Ag(I) nanoparticles on the electrode surface. As is clearly seen in Fig. 3d, no peak is observed, indicating the full removal of accumulated Ag(I) nanoparticles. Fig. 4a shows the anodic stripping voltammetry of Agþ carried out in the standard buffer solution. Single ASV stripping peak is obtained in standard buffer solution containing Agþ with different concentrations. An obvious stripping peak can still be observed in the case of Agþ concentration as low as 10 ppb, proving the ultrasensitive detection capability of GaN micropillar electrode. The
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 2. Cyclic voltammogram in (a) 1 mM Fe [(CN)6]3-/4- in 0.1 M KCl and (c) 1 mM Ru [(NH)3]2þ/3þ in 0.1 M KCl. Corresponding cathodic peak currents (red curve) and anodic peak currents (black curve) versus the square root of the scan rate from 10 to 200 mV/s are shown in (b) and (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. (a) Cyclic voltammograms of the crystalline GaN micropillar at a scan rate of 100 mV s1. Optimization of the deposition potential of (b) Agþ in the range from 0.3 V to 0.1 V and (c) Cu2þ in the range from 0.3 V to 0.5 V. The deposition time is set at 3 min for all experiments. (d) Anodic stripping voltammograms in standard buffer solution containing 1000 ppb Ag(I) (black curve), the red curve represents the anodic sweeping curve after ‘cleaning’ at þ0.4 V for 5 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
stripping peaks shift positively with the incremental Agþ concentration. This can be explained by Nernst equation, in which the equilibrium potentials of Agþ/Ag redox couple would be increased with ion concentrations. The corresponding calibration curve
shows a good linear relationship between the stripping peak and the examined Agþ concentration with a correlation coefficient of R2 ¼ 0.981. The linear relationship is established to be
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 4. (a) Anodic stripping voltammograms of GaN micropillar after pre-concentration in standard solutions containing Agþ with concentrations from 10 ppb to 1000 ppb; (b) Corresponding calibration curve for Agþ detection, the error bars represent the standard deviation obtained from three measurements (n ¼ 3).
I(mA) ¼ 2.162E-8þ5.226E-10*C (ppb)
(1)
over a wide Agþ concentration range from 10 ppb to 1000 ppb. The limit of detection (LOD) is estimated to be 3.3 ppb at the signal-tonoise ratio of 3 (S/N ¼ 3), which is comparable to the results reported using different techniques (Table 1) and on common electrodes using ASV method (Table 2). The above-mentioned results in trace Agþ detection demonstrate the feasibility and superior sensitivity of GaN micropillar electrode. Apparently, GaN micropillars exhibit lower LOD and better electrochemical behavior than previously reported GaN film [24]. In order to facilitate the application of GaN nanostructures in heavy metal ion detections, more heavy metal ions detection using GaN electrode is expected. Compared with Agþ, the bivalent metal ions (for instance Cu2þ, Cd2þ and Hg2þ) require much more additional energy during reduction process in pre-concentration, and the electrode is required to be more sensitive for trace detection. Hereinafter, Cu2þ is selected as a model target to further demonstrate the sensitivity and feasibility of GaN micropillar electrode. To further demonstrate the sensitivity of GaN micropillars, GaN film reported in our previous work is adopted for comparison [24]. Fig. 5a shows the stripping voltammetry of GaN micropillars and GaN film after pre-concentration in standard buffer solution containing 100 ppb Cu2þ at 0.4 V. Anodic stripping peaks are observed at 0.039 ± 0.002 V and 0.043 ± 0.002 V for GaN micropillars and GaN film, respectively. These two predominant peaks located around 0.1 V suggest the successful deposition of Cu(II) on the electrode surface. Interestingly, GaN micropillars show stronger current response in Cu2þ detection, with six times magnitude higher than GaN film. A model aqueous solution
Table 2 The comparison table of detection limits in trace heavy metal detection. Electrode Materials
Detection limit (ppb)
GaN micropillars Diamond/Graphite BDD Hg Screen-printed Au Bismuth film Mercury film modified screen-printed carbon
Agþ
Cu2þ
References
3.3 5.8 1.0 e e e e
3.3 5.6 10 12 1 10 5
This work [34] [32] [47] [48] [49] [50]
containing 1000 ppb Cu2þ is then employed to further demonstrate the electrochemical behavior of GaN micropillars. As is shown in Fig. 5b, a conspicuous anodic peak appears around 0.001 V vs Ag/ AgCl, with a FWHM of 131 ± 1 mV (black curve). This is slightly larger than those of BBD (60 ± 3 mV) and Hg (32 ± 1) mV [32] electrodes, but it is still not excessively broad to interfere in the multi detection for metal ions. A slight asymmetry is also observed in the stripping peak, showing the evidence for inconsistency in electron transfer rate. After the ‘cleaning’ process, no stripping peak is observed (red curve), proving the excellent renewability of GaN micropillars in trace Cu2þ detection. Fig. 6a and c shows the anodic stripping voltammetry of GaN micropillars and GaN film in trace Cu2þ detection. Predominant anodic peaks of Cu2þ are observed in the vicinity of 0.1 V and shift positively with incremental Cu2þconcentration, which is also frequently observed in the ASV detection of metal ions on other electrodes [8,32,34]. GaN micropillars show excellent linear relationship of
Table 1 The comparison table of detection limits in Ag(I) and Cu (II) detection using different techniques. Metal ion
Techniques
CLOD (ppb)
References
Ag
Direct current plasma Flame atomic absorption spectroscopy Real-time fluorescence Chromogenic probe Inductively coupled plasma atomic emission spectroscopy Inductively coupled plasma-mass spectroscopy Anodic stripping voltammetry Direct current plasma Flame atomic absorption spectroscopy Real-time fluorescence Chromogenic probe Inductively coupled plasma atomic emission spectroscopy Inductively coupled plasma-mass spectroscopy Anodic stripping voltammetry
38 473 200 14 0.3 0.036 3.3 2 40 6.4 1.92 0.006 0.1 3.3
[36] [36] [37] [38] [39] [40] This work [41] [42] [43] [44] [45] [46] This work
Cu
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Fig. 5. (a) Anodic stripping voltammograms of GaN micropillar (black) and GaN film (red) in standard buffer solution containing 100 ppb Cu(II); (b) Anodic stripping voltammograms in standard buffer solution containing 1000 ppb Cu(II) (black curve), the red curve represents the anodic sweeping curve after ‘cleaning’ at þ0.4 V for 5 min. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. (a) Anodic stripping voltammograms of GaN micropillars after pre-concentration in standard solutions containing different Cu(II) concentrations ranging from 10 ppb to 1000 ppb and (b) the corresponding calibration curve for anodic peak current as a dependence of Cu(II) concentration; (c) Anodic stripping voltammograms of GaN film after preconcentration in standard solutions containing different Cu(II) concentrations ranging from 10 ppb to 1000 ppb and (d) the corresponding calibration curve for anodic peak current as a dependence of Cu(II) concentration. The error bars in (b) and (d) represent the standard deviation obtained from three measurements (n ¼ 3). (e) The simultaneous detection of 1 ppm Ag(I) and 1 ppm Cu(II) in standard buffer solution using GaN micropillar (black) and GaN film (red).
I(mA) ¼ 6.326E-8þ5.895E-10*C(ppb)
(2)
with a correlation coefficient of R2 ¼ 0.994 over Cu2þ concentration range from 50 ppb to 1000 ppb (Fig. 6b). The linear relationship of
GaN film is established to be I(mA) ¼ -1.672E-9þ2.012E-10*C(ppb)
(3)
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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Table 3 The comparison table of detection limits of GaN micropillars and GaN film in Ag(I) and Cu(II) detection. Parameters
Detection limit (ppb) Sensitivity (nA/ppb) Correlation coefficient Linear range (ppb)
Agþ
Cu2þ
GaN micropillar
GaN film [24]
GaN micropillar
GaN film
3.3 0.5226 ± 0.02 0.981 10e1000
7 0.2044 ± 0.01 0.991 10e1000
3.3 0.5895 ± 0.02 0.994 50e1000
3.3 0.2012 ± 0.01 0.964 10e500
with a correlation coefficient of R2 ¼ 0.964 in Cu2þ concentration range from 10 ppb to 500 ppb (Fig. 6d). GaN micropillars show better sensitivity by exhibiting higher current responses (0.5895 ± 0.02 nA/ppb) in trace ion detections. The sensitivity of GaN micropillars is comparable to the results reported using different techniques (Table 1) and on extensively adopted electrodes using ASV method (Table 2). The possibility of simultaneous detection of multiple metal ions in the standard buffer solutions is further explored and examined. Fig. 6e shows the stripping curves after pre-concentration in the solution concurrently containing 1000 ppb Cu2þ and 1000 ppb Agþ. Conspicuous and well-resolved peaks corresponding to Cu(II) and Ag(I) can be observed around 0.03 ± 0.01 V vs Ag/AgCl and 0.23 ± 0.01 V vs Ag/AgCl, respectively. The GaN micropillar electrodes exhibit three times higher current response than GaN film electrode, indicating a more sensitive electrochemical response to trace metal ion detection. Compared with individual ion detection, the anodic peak of Cu2þ shifts to more negative potential while the anodic peak of Agþ moves to more positive potential in multi ions detection. It is assumed that the interactions between the two metal ions during pre-concentration and stripping process are responsible for anodic peak deviation [8]. Additionally, it should be noted that the GaN micropillar electrode owns the simultaneous detection capability of Cu2þ and Agþ in aqueous solution with lower concentration as shown in Fig. S4 and can be tentatively used for the trace detection of Agþ and Cu2þ in tap water. However, the tap water only shows a weak Agþ signal (Fig. S5), while the Cu2þ peak is not observed, possibly because that the content of Cu2þ in tap water is lower than the detection limit of our electrode. Generally, the surface condition of the electrode plays a vital role in the heavy metal ion detection. It is assumed that due to the large surface-to-volume ratio, GaN micropillars present better sensitivity and lower detection limit in the detection process. As demonstrated in Randles-Sevcik equation, the main factors influencing the magnitudes of anodic stripping peaks of metal ions are as follows [35]: I ¼ 2.69E5 n3/2AD1/2v1/3c
(4)
where I represent the anodic peak value, n the numbers of electrons in reaction, A the electrode area immersed in the solution, D the diffusion coefficient of metal ions, v the scanning rate and c is the solution concentration. It can be clearly implied that when the solution concentration c is identical, the anodic peak intensity is closely related to the numbers of electrons in reaction and the electrode area immersed in the solution when the same metal ion is detected. Table 3 summarizes the important parameters of GaN micropillars and GaN film in trace Agþ and Cu2þ detection. It can be seen that GaN micropillars exhibit lower detection limit and higher sensitivity. Compared with GaN film, the vertically-aligned GaN microarrays offer more deposition sites for metal ions and involve more electrons in the detection process, thus contribute to the conductivity improvement. Moreover, the hydrophilia property of
Fig. 7. The contact angle of GaN micropillar (left) and GaN film (right). The solution used in the experiment is deionized (DI) water.
GaN micropillars help to expand electrode area immersed in the buffer solution. As shown in Fig. 7, the contact angle (the angle where a liquid-vapor interface meets a solid surface) of GaN micropillars is 59.7 ±1.8 , while the GaN film has a contact angle of 100.5 ±0.2 . Therefore, GaN micropillars show obvious hydrophilia nature. This helps it to increase the peak current intensity, resulting in sensitivity promotion in trace Agþ and Cu2þ detection. As a result, the promising application of GaN micropillars can be expected due to their superior electrochemical properties and excellent chemical inertness. 4. Conclusion In summary, GaN micropillars with uniform alignment along the [0001] direction are used in trace Agþ and Cu2þ detection. The asgrown GaN micropillars without further surface modification have shown ultrasensitive detection capability in trace Agþ and Cu2þ detections, by showing detection limits of 3.3 ppb and 3.3 ppb, respectively. Comparison made on the electrochemical responses of GaN micropillars and GaN film demonstrates that GaN micropillars surpass GaN film in conductivity. The GaN micropillars show six orders of magnitude higher in individual Cu2þ detection and three orders of magnitude higher in simultaneous Agþ and Cu2þ detection. It is assumed that the large surface area of GaN microarrays, the interfacial p-n junction and the hydrophilia nature of GaN micropillars contribute to the significant conductivity improvement. As a result, GaN micropillars prove to be a competitive candidate for trace metal ions detection. Further work focusing on real water sample analysis is necessary in order to promote the bioapplication of GaN electrodes in trace metal ions detection and such work is already in progress. Author contributions B. Liu designed the experiment; Q. Liu, J. Li, W. Yang and C. Zhang
Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010
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synthesized the samples and did the measurements; X. Zhang, B. Liu, C. Labbe, X. Portier, F. Liu and J. Yao discussed the results; Q. Liu, J. Li and B. Liu wrote the paper. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was partially supported by the National Natural Science Foundation of China (No.51702326,51872296), the Open Fund of the State Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-sen University) with grant No. OEMT-2017-KF02, and the Basic Science Innovation Program of Shenyang National Laboratory for Materials Science (Grant No.2017RP25). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2019.11.010. References [1] Y. Lu, X. Liang, C. Niyungeko, J. Zhou, J. Xu, G. Tian, A review of the identification and detection of heavy metal ions in the environment by voltammetry, Talanta 178 (2018) 324e338. [2] Z. Rasouli, Z. Hassanzadeh, R. Ghavami, Application of a new version of GARBF neural network for simultaneous spectrophotometric determination of Zn(II), Fe(II), Co(II) and Cu(II) in real samples: an exploratory study of their complexation abilities toward MTB, Talanta 160 (2016) 86e98. [3] G. Baccolo, M. Clemenza, B. Delmonte, N. Maffezzoli, M. Nastasi, E. Previtali, M. Prata, A. Salvini, V. Maggi, A new method based on low background instrumental neutron activation analysis for major, trace and ultra-trace element determination in atmospheric mineral dust from polar ice cores, Anal. Chim. Acta 922 (2016) 11e18. [4] K. Jomova, M. Valko, Advances in metal-induced oxidative stress and human disease, Toxicology 283 (2011) 65e87. [5] K.J. Barnham, A.I. Bush, Metals in Alzheimer’s and Parkinson’s diseases, Curr. Opin. Chem. Biol. 12 (2008) 222e228. [6] C.M. Wood, C. Hogstrand, F. Galvez, R.S. Munger, The physiology of waterborne silver toxicity in freshwater rainbow trout (Oncorhynchus mykiss): 1. The effects of ionic Agþ, Aquat. Toxicol. (Amst.) 35 (1996) 93e109. [7] P. Miao, Y. Tang, L. Wang, DNA modified Fe3O4@Au magnetic nanoparticles as selective probes for simultaneous detection of heavy metal ions, ACS Appl. Mater. Interfaces 9 (2017) 3940e3947. [8] H. Zhuang, C. Wang, N. Huang, X. Jiang, Cubic SiC for trace heavy metal ion analysis, Electrochem. Commun. 41 (2014) 5e7. [9] J.H. Wang, E.H. Hansen, FI/SI on-line solvent extraction/back extraction preconcentration coupled to direct injection nebulization inductively coupled plasma mass spectrometry for determination of copper and lead, J. Anal. Atomic Spectrom. 17 (2002) 1284e1289. [10] F.Z. Xie, X.C. Lin, X.P. Wu, Z.H. Xie, Solid phase extraction of lead (II), copper (II), cadmium (II) and nickel (II) using gallic acid-modified silica gel prior to determination by flame atomic absorption spectrometry, Talanta 74 (2008) 836e843. [11] C.-X. Yang, H.-B. Ren, X.-P. Yan, Fluorescent metal organic framework MIL53(Al) for highly selective and sensitive detection of Fe3þ in aqueous solution, Anal. Chem. 85 (2013) 7441e7446. [12] J. Wang, J.M. Lu, U. Anik, S.B. Hocevar, B. Ogorevc, Insights into the anodic stripping voltammetric behavior of bismuth film electrodes, Anal. Chim. Acta 434 (2001) 29e34. [13] A. Economou, P.R. Fielden, Mercury film electrodes: developments, trends and potentialities for electroanalysis, Analyst 128 (2003) 205e212. [14] M.B. Gumpu, S. Sethuraman, U.M. Krishnan, J.B.B. Rayappan, A review on detection of heavy metal ions in water - an electrochemical approach, Sens. Actuators B Chem. 213 (2015) 515e533. [15] B. Bas, K. Wegiel, K. Jedlinska, The renewable bismuth bulk annular band working electrode: fabrication and application in the adsorptive stripping voltammetric determination of nickel(II) and cobalt(II), Anal. Chim. Acta 881 (2015) 44e53. [16] D.X.O. Jaramillo, A. Sukeri, L.P.H. Saravia, P.J. Espinoza-Montero, M. Bertotti, Nanoporous gold microelectrode: a novel sensing platform for highly sensitive and selective determination of arsenic (III) using anodic stripping voltammetry, Electroanalysis 29 (2017) 2316e2322.
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Please cite this article as: Q. Liu et al., Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.010