Bismuth Nanoclusters/Porous Carbon Composite: A Facile Ratiometric Electrochemical Sensing Platform for Pb2+ Detection with High Sensitivity and Selectivity.

In this work, a ratiometric electrochemical sensor was constructed for the detection of Pb2+ based on a bismuth nanocluster-anchored porous activated biochar (BiNCs@AB) composite. BiNCs with loose structure and AB with abundant oxygen-containing functional groups are favorable for Pb2+ adsorption and preconcentration; meanwhile, porous AB provides more mass transfer pathways and increases electronic and ion diffusion coefficients, realizing high sensitivity for Pb2+ detection. At the same time, BiNCs were proposed as an inner reference for ratiometric electrochemical detection, which could greatly enhance the determination accuracy. Under optimized experimental conditions, the anodic peak current ratio between Pb2+ and BiNCs exhibited a good linear relationship with the concentration from 3.0 ng/L to 1.0 mg/L. The detection limit can be detected down to 1.0 ng/L. Furthermore, the proposed sensor demonstrated good reproducibility, stability, and interference resistance, as well as satisfactory recoveries for the detection of Pb2+ in real samples.

Introduction

Heavy metal ions, especially lead ions (Pb2+), have been rated as the most hazardous environmental pollutants because of their highly toxic and nonbiodegradable nature.[1] The excessive exposure to Pb2+ even at trace levels can trigger serious damage to the nervous system, kidneys, liver, reproductive system, and brain. At the same time, nonbiodegradability means it can be concentrated in living tissues throughout the food chain, posing a serious threat to natural ecosystems and public health.[2] Therefore, it is extremely significant to develop Pb2+ detection technology with high sensitivity and reliability for human health and environmental monitoring.

Until now, some analytical techniques, such as surface-enhanced Raman spectrometry, atomic absorption spectroscopy, UV–vis spectrophotometry, and X-ray fluorescence spectrometry,[3−6] have been employed for Pb2+ detection. However, these techniques require expensive equipment and time-consuming pretreatment steps.[7] By contrast, the electrochemical sensing strategy possesses significant superiority in terms of portable instruments, low cost, simple operation, fast response, high sensitivity, and excellent selectivity.[8,9] Typically, electrochemical sensors track the content of analytes by recording single response signal. Nevertheless, the response signal is susceptible to environmental factors and instrument efficiency, resulting in poor stability and low reproducibility.[10,11,15]

In contrast, the ratiometric electrochemical sensor with two electrochemical signals is a superior alternative to the conventional single signal sensing because of its superior built-in self-calibration ability. For the ratiometric sensing strategy, the ratio of two current signals at different potentials is employed as the detected signal in which the peak current signals usually derive from the analyte itself and the internal reference probe, respectively.[12] The current ratio can provide an intrinsic correction factor to eliminate the contribution from nonspecific interferences,[13] thus greatly improving the detection accuracy and sensitivity. At present, several electroactive materials, such as ferrocene, MOFs, paracetamol, and AuNPs,[14,15] have been exploited as inner references for ratiometric heavy metal ion (HMI) electrochemical sensors. By comparison, metal nanoparticles are attractive since they can provide strong electrochemical signals. Bismuth (Bi) particles are one of the most commonly used nanomaterials in sensing of HMIs due to their low toxicity, easy formation of alloy with HMIs, and insensitivity to the dissolved oxygen in a solution. In addition, good resolution of adjacent peaks from heavy metals and Bi was observed in these studies.[16] These characteristics enable Bi particles to be a promising inner reference probe. However, as far as I am concerned, a Bi-based dual-signal HMI electrochemical sensor has not been reported.

Considering the low electrical conductivity and easy agglomeration of Bi particles, porous activated biochar (AB) with the merits of high electrical conductivity, large surface area, and good chemical stability was chosen as the support material.[17] Additionally, in comparison to other forms of nanomaterials, nanoclusters (NCs) have higher electrocatalytic activity due to a larger number of available electroactive sites to detect analytes.[18] Given these features, herein, through a simple solvothermal method, AB-supported BiNCs (BiNCs@AB) has been synthesized, which was employed as a ratiometric electrochemical sensor for Pb2+ detection (shown in Scheme ). BiNCs can not only form alloy with Pb2+ to promote the adsorption of Pb2+ on the electrode surface but also provide reference signals for the ratiometric electrochemical detection. Moreover, AB with excellent properties of porous structure, good conductivity, and abundant oxygen-containing functional groups could provide more pathways for the rapid electrical/ionic transport and enrichment of Pb2+ on the electrode surface. Benefiting from the synergistic effect of these advantages, the fabricated sensor showed excellent electrocatalytic performances to Pb2+ with the satisfactory linear range and detection limit. The method was also used for the determination of Pb2+ in the paddy water sample.

Scheme 1

Illustrations of the Fabrication Process of the BiNCs@AB Composite Sensor for the Sensing of Pb2+

Results and Discussion

Characterization

The structures and morphologies of AB and BiNCs@AB nanomaterials were evaluated using SEM. KOH activation brought a remarkable change in the material morphology.[19] Focused on the surface of AB derived from Litsea cubeba, lots of dense pores with irregular size could be seen (Figure A). Such a porous structure provides a larger surface area and accelerates mass transfer on the surface of samples. At the same time, it is a substantial platform for the loading of BiNCs due to the presence of abundant functional groups (e.g., carboxyl and amine groups). Through using a reducing agent, the BiNCs were successfully loaded on AB. As can be seen from Figure B,C, a large number of BiNCs are anchored on the AB surface, which confirms the successful synthesis of the BiNCs@AB. The TEM image of BiNCs@AB is shown in Figure C. It could be distinctly seen that BiNCs (dark dots) are well dispersed and anchored on the surface of AB. Figure D displays the particle size distribution of BiNCs on AB. From the bar chart, it is observed that the BiNC size distribution conforms to normal distribution with a mean diameter of 59.6 ± 23.5 nm. Figure E shows the EDS spectrum and corresponding composition information of the BiNCs@AB composite. As listed, the wt % values of C, O, and Bi are 68.46, 20.74, and 10.80%, respectively.

Figure 1

SEM images of (A) AC and (B) BiNCs@AB composite; (C) TEM image of the BiNCs@AB composite; (D) size distribution graph of BiNCs in the BiNCs@AB composite; (E) EDS spectrum of the BiNCs@AB composite.

X-ray diffraction (XRD) patterns of AB, BiNCs, and BiNCs@AB are shown in Figure A. For the AB sample, two distinct diffraction peaks at around 21.1° and 42.7° are observed, which are attributed to (002) and (101) reflections of graphite.[20] In the case of BiNCs, five diffraction peaks are observed at 28.1°, 37.9°, 39.6°, 46.3°, and 51.9°, which agree well with the data for metallic Bi present in the JCPDS card (no. 44-1246).[21] The crystallite size of BiNCs can be calculated from the obtained XRD results by using the Scherer equation (D = Kλ/βcosθ). The achieved crystallite size is 38 nm. In the spectrum of the BiNCs@AB composite, both the characteristic peaks of BiNCs and AB appear, suggesting the successful doping of BiNCs in the porous carbon matrix.

Figure 2

(A) XRD patterns of BiNCs, AB, and BiNCs@AB; XPS spectra of BiNCs@AB: (B) survey scan spectrum, high-resolution spectra of (C) C 1s and (D) Bi 4 f.

XPS was carried out to further analyze the element composition and valence electronic conurbation of the BiNCs@AB composite. In Figure B, the XPS survey spectrum demonstrates the presence of C, Bi, and O elements in the BiNCs@AB. The high-resolution C 1s spectrum of BiNCs@AB depicts four apparent peaks at 284.6, 286.0, 287.4, and 289.1 eV, corresponding to C–C, C–O, C=O, and O–C=O,[20] respectively (Figure C). For the XPS spectra of Bi 4 f (Figure D), two distinct peaks appeared at 164.9 and 159.6 eV are associated with the binding energies of Bi 4 f5/2 and Bi 4 f7/2,[22] respectively. The peak separation between the two peaks is 5.3 eV, indicating the chemical state of Bi3+,[23] which demonstrates that the surface of BiNCs is covered by an oxide layer. The absence of Bi2O3 reflections in the XRD patterns suggests that the Bi2O3 shell is amorphous, probably originating from the spontaneous surface passivation on contact with air.[24]

Figure exhibits the Nyquist diagrams of different modified electrodes in a solution of 0.1 M KCl containing 5.0 mM Fe(CN)63–/4–. A typical Nyquist plot consists of a semicircle at higher frequencies and a linear part at lower frequencies, which are related to the electron-transfer-limited and diffusion-limited process, respectively.[25] The Randle equivalent circuit (the inset in Figure ) was employed to fit the impedance data, where Rs and Rct represent the solution resistance and charge transfer resistance, respectively. Cdl and Zw serve as the double layer capacitance and Warburg constant,[26] respectively. Fitting results reveal that bare GCE displays an Rct value of about 506 Ω. By contrast, AB/GCE exhibits a low Rct value (158.8 Ω). This is because AB with excellent conductivity accelerates the electron transport of [Fe(CN)6]3–/4– on the electrode surface.[27] However, the BiNCs@AB/GCE shows a higher Rct value (276 Ω), which is due to the fact that the introduction of BiNCs reduces the conductivity of the composite electrode.

Figure 3

Nyquist plots of different modified electrodes in 0.1 M KCl solution containing 1.0 mM [Fe(CN)6]3–/4–. Inset in Figure : the corresponding equivalent Randles circuit.

Electrochemical Behaviors of the Modified GCEs

Electrochemical performances of bare GCE, AB/GCE, and BiNCs@AB/GCE were investigated by DPASV in HAc-NaAc buffer (0.1 M, pH 5.0) containing 100 ng·L–1 Pb2+. As shown in Figure , bare GCE (curve a) presents a weak Pb2+ striping peak at −0.54 V, while, at AB/GCE, an increased stripping peak for Pb2+ is observed at AB/GCE (curve b). The functional groups and porous structure of AB facilitate the preconcentration of Pb2+ on the electrode surface. Furthermore, a sharper peak with a remarkably increased current is achieved at BiNCs@AB/GCE (curve c), which is 3.0 and 1.5 times higher than those at bare GCE and AB/GCE, respectively. This is because BiNCs@AB produces a synergistic effect, demonstrating large specific surface area, abundant active sites, and high electrical conductivity, which are beneficial to the loading of Pb2+ and the electron transfer. In addition, BiNCs can form a “fusion alloy” with trace metal ions, further increasing the Pb2+ adsorption capability. In addition, it is worth noting that another anodic peak appears at around −0.17 V originating from the oxidation peak of BiNCs, which makes the construction of the ratiometric Pb2+ electrochemical sensor feasible.

Figure 4

DPASV curves of the bare GCE, AB/GCE, and BiNCs@AB/GCE in 0.1 M HAc-NaAc buffer solution (pH 5.0) containing 100 ng·L–1 Pb2+.

Optimization of Experimental Conditions for Electrochemical Detection

To gain the best performance of the sensing platform, experimental parameters including the amount of modifier (different volumes of the homogenized 1 mg/mL BiNCs@AB suspension), pH, deposition potential, and deposition time were optimized.

The influence of the amount of the BiNCs@AB suspension volume on the current ratio of Pb2+ and BiNCs (IPb2+/IBiNCs) was investigated from 1.0 to 9.0 μL (Figure A). As shown, IPb2+/IBiNCs increases as the modifier volume changes from 1.0 to 5.0 μL. The possible reason is that the content of BiNCs on the modified electrode is enhanced with the increase of the volume of modified materials, which is favorable for the adsorption of Pb2+.[28] However, a decrease in IPb2+/IBiNCs is noticed with a further increase in suspension volume. This might be because a high amount of BiNCs@AB on the electrode surface causes considerable resistance against electron transfer.

Figure 5

Optimization of experimental conditions. Influence of (A) BiNCs@AB dispersion volume, (B) pH value of the detection solution, and (C) deposition potential on the IPb2+/IBiNCs at BiNCs@AB/GCE. Deposition time: 270 s; (D) IPb2+/IBiNCs obtained by DPASV as a function of deposition time.

The effect of electrolyte pH on the stripping peak currents was explored with pH ranging from 3.0 to 7.0. As displayed in Figure B, IPb2+/IBiNCs substantially increases with the increasing pH from 3.0 to 5.0. At highly acidic pH, the hydrophilic groups on the surface of sensing material are protonated, leading to the reduction of the attachment sites of heavy metal ions. However, a decrease in IPb2+/IBiNCs is noticed at pH higher than 5.0. This phenomenon is due to the combination of heavy metal ions and hydroxide ions to form a complex,[29] which inhibits the accumulation of heavy metal ions on the electrode surface. As a result, pH 5.0 is selected for the subsequent electrochemical experiments.

The effect of deposition time on stripping signal was examined in the range of 180–360 s (Figure C). As shown, a significantly enhanced trend of the IPb2+/IBiNCs is observed between 180 and 270 s owing to the increasing amount of Pb2+ accumulated on the surface of BiNCs@AB/GCE. Nevertheless, the IPb2+/IBiNCs remains basically stable after 270 s, which resulted from the saturation of Pb2+ at the BiNCs@AB/GCE surface.[30] Considering the balance between the detection sensitivity and measurement efficiency, a deposition time of 270 s is applied in the study.

The result of deposition potential optimization is shown in Figure D. When the deposition potential is shifted from −0.3 to −0.7 V, the IPb2+/IBiNCs increases remarkably with the complete reduction of Pb2+. However, with the continuous negative shift of deposition potential (−0.7 ∼ −1.0 V), the IPb2+/IBiNCs increases slowly and even decreases, which is caused by the increase of hydrogen evolution interference.[31] Thus, −0.7 V is selected for detection of Pb2+.

Determination of Pb2+ at BiNCs@AB/GCE

Under the optimized conditions, the superior properties of the as-prepared ratiometric electrochemical sensor were evaluated by DPASV. Figure A shows the DPASV responses of different concentrations of Pb2+ on the BiNCs@AB/GCE. The peak current of Pb2+ gradually increases with the increase of Pb2+ concentration, while the current intensity of BiNCs is nearly unchanged. Within the concentration range of 3.0 ng/L to 1.0 mg/L, an excellent linear relationship between IPb2+/IBiNCs and the concentrations of Pb2+ is found, which is expressed as IPb2+/IBiNCs (μA) = 0.0091 + 0.0015c (μg/L) (R2 = 0.9917). Based on the equation of LOD = 3S/q, the limit of detection (LOD) is estimated to be 1.0 ng/L, which is comparable to or even lower than those reported in other studies (Table ). The porous structure and good electrical conductivity of AB and the highly close active sites provided by BiNCs make BiNCs@AB become an ideal material for constructing high-efficiency electrochemical sensors to detect Pb2+.

Figure 6

(A) DPASV curves of BiNCs@AB/GCE response to different concentrations of Pb2+; (B) linear relationship between IPb2+/IBiNCs and concentration of Pb2+.

Table 1

Comparison of the Performance of the Present Sensor with the Reported Bi-Based Sensors for Pb2+ Determination

electrode material	measurement technique	linear range (μg·L–1)	LOD (μg·L–1)	reference
Bi/Au-GN-Cys/GCE	SWASV	0.50–40	0.05	(32)
Hg-Bi-SWNTs/GCE	ASV	0.5–130	0.18	(33)
BiNPs@Ti3C2Tx/GCE	SWASV	12.43–124.3	2.24	(34)
Bi-BiOCl@C/GCE	SWASV	1–60	0.2	(35)
BiNCs@AB/GCE	DPASV	0.003–1000	0.001	this work

Reproducibility, Repeatability, Stability, and Selectivity of the BiNCs@AB/GCE

Five different BiNCs@AB electrodes were fabricated to monitor 100 μg/L Pb2+ with the same procedure to analyze the reproducibility of the sensor. As depicted in Figure A, the relative standard deviation (RSD) of IPb2+/IBiNCs is 2.26%. In addition, the repeatability of BiNCs@AB/GCE was explored by using the same electrode toward the detection of 100 μg/L Pb2+ 15 times, and the corresponding RSD value is 3.90% (Figure B). These results confirm that the electrode has good reproducibility and repeatability.

Figure 7

(A) Reproducibility, (B) repeatability, (C) long-term stability, and (D) selectivity of the BiNCs@AB/GCE.

The electrochemical sensor was stored at room temperature, and the electrode stability during storage was tested every other day. According to Figure C, even after the sensor was stored for 30 days, the IPb2+/IBiNCs still retains 97.53% of the initial value, which suggests that BiNCs@AB/GCE has excellent storage stability.

To explore the selectivity of the BiNCs@AB/GCE, the DPV response of Pb2+ was measured in the presence of interfering substances and without interfering substances. Under the selected optimal experimental conditions, 50 fold concentrations of Hg2+, Na+, Zn2+, NO3–, K+, Cl–, Al3+, Cu2+, SO42–, Co2+, and Cd2+ were added into 0.1 M HAc-NaAc buffer containing 100 μg/L Pb2+. As presented in Figure D, the signal of IPb2+/IBiNCs change does not exceed 5%, which demonstrates that the synthesized sensor has high selectivity for the determination of Pb2+.

Practical Feasibility

To further illustrate the potential application of the proposed ratiometric electrochemical sensor, DPASV technology was applied to detect the Pb2+ in real paddy water. The filtered water samples (by the 0.45 μm membrane) were diluted with a 0.1 M NaAc-HAc buffer (pH = 5.0). Prior to the addition of Pb2+, no obvious current signal is found in diluted paddy water samples, indicating that the concentration of Pb2+ in paddy water samples is less than 1.0 ng/L or there were no target heavy metal ions. A recovery test was performed by adding Pb2+ with the concentrations of 5.0, 50.0, and 100.0 ng/L to diluted water samples. As listed in Table , the detected values of Pb2+ are close to spiked ones, with recovery values of 99.75–101.6% and RSDs of 2.20–2.40%. These results show that this ratiometric strategy-based sensor can efficiently determine Pb2+ in actual water samples, showing admirable reliability and good practicability.

Table 2

Recoveries of Trace Pb2+ in the Local Paddy Water Sample (n = 3)

sample	added (μg·L–1)	found (μg·L–1)	RSD (%, n = 3)	recovery (%)
paddy water	0
	5	5.03 ± 0.12	2.4	100.7
	50	50.80 ± 1.15	2.27	101.6
	100	99.75 ± 2.20	2.2	99.75

Conclusions

In summary, a ratiometric electrochemical platform based on the BiNCs@AB-modified electrode was constructed for the analysis of Pb2+. For design of the sensing material, hierarchical porous AB was employed as the substrate material for in situ growth of BiNCs. The prepared BiNCs@AB composite possesses large active area, fast electron transfer ability, high mass transfer efficiency, and strong enrichment capacity for Pb2+. Additionally, the oxidation signal of BiNCs served as the internal reference, which greatly raises the reproducibility and reliability of the sensor. Benefiting from the synergy of BiNCs and AB, the proposed sensor exhibits excellent sensitivity, good selectivity, and high stability for the detection of Pb2+ and was successfully applied for monitoring Pb2+ in real water samples.

Experimental Section

Chemical Reagent

Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, 99%), ethylene glycol, and other chemicals and reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China, www.sinoreagent.com). The stock standard solution of Pb2+ was prepared by dissolving lead nitrate hexahydrate (PbNO3·5H2O, 99%) in 1% (v/v) nitric acid. NaAc/HAc (pH 5.0) buffer solution was prepared with 0.1 M NaAc and HAc. All chemicals of analytical reagent grade could be used for following experiments directly without further purification.

Apparatus and Characterization

Scanning electron microscopy was conducted with a Hitachi S-300 N scanning electron microscope (Hitachi, Japan). XRD patterns were obtained with a Bruker D8/Advance X-ray diffractometer (D/max-IIIB-40 KV, Japan). Surface chemical analysis of the as-prepared synthesized materials was carried out by X-ray photoelectron spectroscopy (XPS, JPS-9030, JEOL).

Preparation of AB

The AB materials were obtained by the KOH activation method using the Litsea cubeba shell as the carbon precursor. Typically, Litsea cubeba was first washed with deionized water to remove unwanted impurities, which was subsequently crushed by an oil press. In order to increase oxygen-containing functional group moieties and reasonably adsorptive capability, Litsea cubeba was added into KOH solution at a weight ratio of 1:2 and stirred for 24 h. After this, the activated cubeba dregs were put into a tubular furnace and carbonized at 800 °C under an N2 atmosphere for 2 h (heating rate 5 °C·min–1). The product was then washed to pH 7.0 with 1 M HCl and deionized water. Finally, the obtained AB material was put in a ceramic mortar by 30 min grinding to get fine powder.

Synthesis of the BiNCs@AB Composite

In brief, 0.4 g of AB was dissolved ultrasonically into 100 mL of ethanol solution and then mixed with 47.67 mg of NaBH4 under stirring to form solution A. Then, 4.853 g of Bi(NO3)3·5H2O was added into 5 mL of ethylene glycol solution to form solution B. Subsequently, solution B was slowly added into solution A, and the mixture was stirred in an ice bath for 2 h to obtain the BiNCs@AB composite.

Fabrication of the Modified Electrode

First, BiNCs@AB powder was ultrasonically dispersed into ultrapure water to form a homogeneous solution. Prior to modification, the bare glassy carbon electrode (GCE) was carefully polished with alumina slurries and thoroughly washed with deionized water and ethanol. Ultimately, 5 μL of BiNCs@AB suspension was dropped on the surface of the GCE and dried to obtain the BiNCs@AB-modified electrode (BiNCs@AB/GCE).

All electrochemical studies were carried out using a CHI 760B electrochemical analyzer (CH Instruments, Shanghai, China). A GCE (3 mm in diameter) or modified GCE working electrode (Φ = 3 mm), a platinum slide counter electrode, and a saturated calomel reference electrode were used to form the three-electrode system.