English

• 0.   Introduction

  Heavy metal ions (HMIs), such as cadmium (Cd²⁺) and lead (Pb²⁺), pose significant health risks even at trace levels^[1-3]. In daily life, chemicals containing HMIs are widely utilized in industrial and agricultural processes, and the resulting wastewater is often discharged into the environment without adequate treatment^[4-9]. Due to their non-biodegradable nature, HMIs tend to bioaccumulate in the food chain, posing severe risks to human health^[10-15]. For instance, Cd²⁺ adversely affects renal tubular function, potentially leading toosteoporosis and fractures upon prolonged exposure^[16-18]. Similarly, Pb²⁺ significantly impacts the nervous system, particularly in children, resulting indevelopmental delays and attention deficits^[19-21]. Thus, the trace detection of HMIs in environmental samples is particularly important.

  Square wave anodic stripping voltammetry (SWASV) is a highly sensitive electrochemical analytical method that measures current intensity to determine analyte concentrations^[22-25]. Compared to other techniques, SWASV provides superior analytical performance by offering high sensitivity, rapid analysis capabilities, and the ability to resolve adjacent redox peaks, making it particularly effective for analyzing multi-component samples^[26-28]. This feature is especially crucial for complex sample analysis. In recent years, SWASV has garnered increasing research interest. However, the preparation of electrode materials with high sensitivity, selectivity, and resolution for SWASV curves remains a challenging issue^[29-31]. The adsorption and catalytic properties of electrode materials towards HMIs can significantly enhance electrochemical signals, making structural design a critical factor in optimizing performance. In designing electrode materials, macroporous architectures enhance transport and interfacial adsorption of HMIs. Additionally, incorporating some functional groups or heteroatoms (e.g., N, S, O) can effectively capture HMIs. This approach enables efficient detection of HMIs through the synergistic effects of adsorption and catalysis.

  Metal-organic frameworks (MOFs) demonstrate significant potential for electrochemical detection of HMIs owing to their synergistic multifunctional structure^[32-34]. On one hand, the porous architecture of MOFs enables effective ion transport pathways, thereby improving the accessibility of HMIs to active sites and simultaneously facilitating the localized accumulation of metal ions^[35]. On the other hand, the valence state transitions of the core metal species could enhance the electron transfer efficiency during the electroreductive deposition of HMIs^[36]. Moreover, MOF materials enriched with aromatic systems and heteroatoms (N, O, S) could effectively adsorb HMIs through the cation···π interaction^[37]. Therefore, the development of MOF materials enriched with aromatic rings and functionalized with open N-, O-, and S-based groups represents a promising strategy for achieving efficient detection of HMIs.

  In this regard, a 3D MOF, designated as [Co₃(L)₂(1, 4-bib)₄]·4H₂O (Co-MOF), was prepared using Co(Ⅱ) cation, 5-[(4-carboxyphenoxy)-methyl]isophthalic acid (H₃L) and 1, 4-bi(1H-imidazol-1-yl)benzene (1, 4-bib), as detailed in the literature (Scheme 1)^[38]. Co-MOF was first reported in our prior work for its topological magnetism^[38]. Herein, we revisit its coordination structure to establish a new structure-function correlation to electrochemical sensing. The resulting structure exhibited abundant nitrogen, oxygen functionalities, and aromatic benzene rings. Owing to these features, Co-MOF demonstrated a strong ability to trap HMIs via cation…π interactions. In this study, the Co-MOF@GCE sensor was fabricated by immobilizing Co-MOF onto a glassy carbon electrode (GCE), enabling both individual and simultaneous detection of HMIs (Scheme 2). The Co-MOF@GCE sensor exhibited an impressive wide linear range and ultra-low detection limit (LOD) for the target analytes. Furthermore, its practical utility was validated through successful quantification of HMIs in various aqueous matrices, including tap water, commercial bottled water, and lake reservoirs. Insight into the adsorption mechanisms was gained through complementary density functional theory (DFT)-based computational analyses, which highlighted the critical role of nitrogen and oxygen atoms, as well as the central cobalt ion, in the adsorption process, supported by electron density distribution simulations.

  Scheme 1

  

  Scheme 1.  Synthetic route of Co-MOF

  下载: 全尺寸图片 幻灯片

  Scheme 2

  

  Scheme 2.  Visualized illustration for the detection of Cd²⁺ and Pb²⁺ using Co-MOF@GCE

  下载: 全尺寸图片 幻灯片

  1.   Experimental

  1.1   Materials and physical measurement

  The solutions of metal ions were obtained by diluting the corresponding standard stock solution (1 mmol·L^-1) of CdCl₂·2.5H₂O and Pb(Ac)₂·3H₂O, respectively. The acetate buffer solution (ABS) was formulated by combining quantitative sodium acetate with acetic acid to achieve the buffering capacity. The lake water was sourced from the Fen River in Shanxi Province. Before the detection, a membrane filtration system with a 220 nm pore size was employed for water purification. Tap water was obtained from our laboratory. The other chemical reagents were purchased.

  A series of analytical techniques was employed to characterize the Co-MOF material. FTIR spectrum was acquired using a Nicolet 6700 spectrometer with potassium bromide pellet preparation. The thermogravimetric (TG) curve was evaluated on a Perkin ElmerTG-7 instrument. Crystalline phase identification was carried out via powder X-ray diffraction (PXRD) measurements conducted on a Rigaku Dmax 2000 diffractometer equipped with Cu Kα radiation (λ=0.154 18 nm, U=45 kV, I=40 mA, 2θ=5°-50°). Electrochemical properties were examined using a CHI660E electrochemical workstation.

  Single-crystal X-ray diffraction data of Co-MOF were collected on a Bruker D8 VENTURE diffractometer utilizing Mo Kα radiation (λ=0.071 07 nm) with φ-ω scan mode and an APEX-Ⅱ CCD detector at 298(2) K. The crystal was synthesized via a solvothermal method. A suitable crystal with dimensions 0.25 mm×0.30 mm×0.25 mm was mounted on a glass capillary using high-vacuum grease. The multi-scan method was applied for absorption corrections. The crystal structure was solved by direct methods using SHELXS-2018/3 and refined anisotropically for all non-hydrogen atoms via full-matrix least-squares techniques with SHELXL-2018/3 within the WINGX environment^[39-41]. All hydrogenatoms were computationally generated based on geometric considerations. Final refinement converged toR₁=0.051 [I > 2σ(I)] and wR₂=0.131 (all data).

  1.2   Synthesis of Co-MOF

  Synthesis of Co-MOF followed the previous literature^[38]. Element analysis (%) Calcd. for C₈₀H₆₆N₁₆ O₁₈Co₃(%): C 55.93, H 3.84, N 13.05; Found(%): C 55.32, H 3.79, N 13.14. IR data (KBr, cm^-1): 3 587(w), 3 025(w), 2 932(w), 1 653(m), 1 595(w), 1 568(w), 1 495(m), 1 443(m), 1 382(s), 1 239(m), 1 157(w), 1 121(m), 1 104(s), 1 072(m), 1 006(s), 846(w), 769(w), 736(w), 680(m), 650(m), 563(s), 498(s).

  1.3   Electrochemical measurements

  A standard three-electrode configuration was utilized for electrochemical analyses, comprising a glassy carbon working electrode (GCE, d=3 mm), Ag/AgCl reference electrode, and Pt counter electrode. A 5.0 mmol·L^-1 [Fe(CN)₆]^3-/[Fe(CN)₆]^4- redox couple in 0.1 mol·L^-1 KCl electrolyte was employed for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. HMIs quantification was performed via SWASV under optimized parameters: 0.025 V amplitude, 15 Hz frequency, 2 s quiet time, and 4 mV step voltage. The analytical process involved a 360 s deposition at -1.0 V followed by a stripping sweep from -1.0 to 0.6 V.

  1.4   Preparation of the Co-MOF@GCE

  The 3 mg Co-MOF crystals were ground in an agate mortar. Then, the finely ground material was dispersed in a 1 mL Nafion dispersion (0.5%) serving as a polymeric binder and sonicated for 20 min to achieve a homogeneous suspension. Concurrently, the GCE surface was mechanically polished with alumina powder, followed by ultrasonic cleaning in ethanol. Finally, 3, 4, 5, and 6 μL of the prepared suspension were drop-cast onto the GCE, respectively.

  1.5   Computational details

  The ion adsorption and electronic transfer behavior at the Co-MOF surface interface were computed and analyzed using the DFT DMol3 module. For the geometric structure optimization, the Perdew-Burke-Ernzerhof (PBE) functional, based on the generalized gradient approximation (GGA), was employed as the exchange-correlation potential. The basis set used was the double numerical basis plus polarization (DNP) function. The special k-point sampling scheme for the Brillouin zone was determined using the Monkhorst-Pack method, with a 2×2×1 k-point mesh. Spin- unrestricted calculations and appropriate multiplicities were used to describe the electronic states of the system in different configurations, allowing for the calculation of changes in the total electronic density. To ensure high accuracy, the energy convergence limit, maximum force, and maximum displacement were set to be less than 1.0×10^-6 Hartree, 0.02 Hartree·nm^-1, and 0.000 5 nm, respectively. Additionally, the real-space truncation sphere radius was set to 0.48 nm, and the occupation orbital tail effect was defined as 0.005 Hartree.

  2.   Results and discussion

  2.1   Crystal structure of Co-MOF

  Crystallographic data were initially reported in the previous literature^[38]. The following refinement focuses exclusively on bond metrics and coordination-environment analysis critical to HMI adsorption and charge distribution, underpinning the DFT mechanism proposed herein. The metal center Co1 is six-coordinated, forming an octahedral mode, with four N atoms from four different 1, 4-bib ligands and two O atoms from two L^3- ligands. The average bond lengths are 0.209 8(11) nm for Co1—O and 0.218 0(13) nm for Co1—N. Similarly, the Co2 is also six-coordinated with two N atoms from two 1, 4-bib ligands and four O atoms from four L^3- ligands. The average bond lengths are 0.209 8(11) nm for Co2—O and 0.212 6(14) nm for Co2—N (Fig.1a). The Co(Ⅱ) ions are interconnected through 1, 4-bib ligands, forming a layered structure (Fig.1b). These polymeric layers are further extended into a 3D framework through coordination with L^3-ligands (Fig.1c and 1d).

  Figure 1

  

  Figure 1.  (a) Coordination sphere of Co(Ⅱ) centers of Co-MOF; (b) 2D schematic diagram of Co-MOF; (c, d) Multi-view of 3D network structure of Co-MOF

  Symmetry codes: #1: 1+x, 1+y, -1+z; #2: -x, 1-y, -z; #3: -x, -y-1, 1-z.

  下载: 全尺寸图片 幻灯片

  2.2   Characterization of Co-MOF

  The FTIR spectrum of the Co-MOF is presented in Fig.2a. Vibrational signatures in the FTIR spectrum demonstrate structural evolution: 2 931 cm^-1 absorption band (aliphatic C—H bonds in alkyl moieties)^[42], carboxylate asymmetric vibration ($\nu_{\mathrm{COO}^{-}}$) at 1 652 cm^-1, and symmetrical stretching vibration ($ \nu_{\mathrm{COO}^{-}} $) at 1 443 cm^{-1 [43]}. The peak at 650 cm^-1 corresponds to the Co—O stretching vibration, and the ν_Co—N vibrational signature detected at 563 cm^-1 provides evidence for coordination bond formation of the Co-MOF^{[21, 44]}.

  Figure 2

  

  Figure 2.  (a) FTIR spectrum, (b) TG curve, and (c) PXRD patterns of Co-MOF

  下载: 全尺寸图片 幻灯片

  The TG curve is shown in Fig.2b. The weight loss of approximately 4.25% at around 294.71 ℃ is likely attributed to the removal of four uncoordinated H₂O molecules from the complex. Upon further heating, the framework of the Co-MOF underwent collapse.

  The experimental and simulated PXRD patterns are shown in Fig.2c. Evidently, the peak positions in the experimental data match well with the simulated ones, indicating the phase purity of the bulk sample.

  2.3   Electrochemical behavior of Co-MOF@GCE

  CV, EIS, and SWASV were employed to evaluate the electrochemical performance of Co-MOF@GCE and bare GCE. EIS serves as an effective method for characterizing electron transfer and impedance change. The EIS spectrum consists of two components: a semicircle and a linear segment, corresponding to electron transfer resistance (R_ct) and diffusion resistance, respectively. R_ct, a key parameter controlling electron transfer kinetics at the electrode surface, reflects the efficiency of surface reactions. As shown in Fig.3a, compared to the bare GCE, Co-MOF@GCE displayed a larger, well-defined semicircle, indicating an enhanced electron transfer resistance at the Co-MOF functionalized interface relative to the bare GCE.

  Figure 3

  

  Figure 3.  (a) EIS spectra and (b) CV curves of bare GCE and Co-MOF@GCE in 0.1 mol·L^-1 KCl containing 5 mmol·L^-1 [Fe(CN)₆]^3-/[Fe(CN)₆]^4-; (c) SWASV curves of Co-MOF@GCE and GCE in the mixed solution of Cd²⁺ (5 μmol·L^-1) and Pb²⁺ (5 μmol·L^-1) in 0.1 mol·L^-1 ABS (pH=5.0)

  下载: 全尺寸图片 幻灯片

  Fig. 3b displays the CV curves of the bare GCE and Co-MOF@GCE within a potential window of-0.2 to 0.8 V. The distinct redox peaks were observed in the CV curve of the bare GCE. In contrast, the redox peaks for Co-MOF@GCE were significantly reduced, which may be due to the high R_ct of Co-MOF@GCE. The modified compounds block the surface electron transfer, and this finding exhibits good concordance with the EIS results.

  To further validate the HMI detection capability of Co-MOF@GCE, SWASV was conducted in 0.1 mol·L^-1 ABS (pH=5.0) for Cd²⁺ and Pb²⁺. Fig.3c resolves two well-separated anodic stripping signals at -0.75 V (Cd²⁺) and -0.58 V (Pb²⁺), with peak potential spacing (ΔE_p=170 mV) enabling simultaneous quantification. The results demonstrate that Co-MOF@GCE exhibited significantly enhanced current responses compared to the bare GCE. The observed enhancement likely originates from the preferential adsorption between Co-MOF and HMIs, which facilitates their preconcentration at the electrode surface. Consequently, despite its elevated R_ct, Co-MOF@GCE achieves good electrochemical sensitivity for the detection of HMIs.

  The electrochemical behavior of Co-MOF@GCE was investigated based on the Randles-Sevcik equation (Fig.4)^[45]:

  $ I_\text{p} = 2.69 × 10^{5}n^{3/2}AcD^{1/2}v^{1/2} $

  Figure 4

  

  Figure 4.  (a, c) CV profiles of GCE and Co-MOF@GCE under scan rate gradients (10-200 mV·s^-1) in 0.1 mol·L^-1 KCl electrolyte containing 5 mmol·L^-1 [Fe(CN)₆]^3-/[Fe(CN)₆]^4- redox probe; (b, d) Linear correlation between I_p and v^1/2

  下载: 全尺寸图片 幻灯片

  where n is the number of electrons transferred, A denotes the electrochemically active surface area (cm²), c is the analyte concentration (mol·cm^-3), D represents the diffusion coefficient (cm²·s^-1), v is the scan rate (V·s^-1), and I_p represents the peak current (A). The ECSA (electrochemical active surface area) values of GCE and Co-MOF@GCE were calculated as 1.80×10^-2 and 4.15×10^-3 cm², respectively.

  2.4   Optimization condition testing

  To achieve optimal detection performance for HMIs, systematic optimization of key parameters—including pH, deposition potential, deposition time, and modifier amount was performed. Initially, the pH was varied from 3.0 to 6.0. As shown in Fig.S1a (Supporting information), the stripping currents of all target ions gradually increased as pH rose from 3.0 to 5.0, reaching a maximum value at pH 5.0. However, further increasing pH beyond 5.0 resulted in diminished current signals, likely due to hydrolysis of metal ions inhibiting their deposition. As a result, pH 5.0 was determined to be the optimal condition.

  The deposition process is a critical step for enhancing the sensitivity of HMIs. In this study, the influence of deposition potential was investigated, and the result was presented in Fig.S1b. The peak currents intensified as the potential shifted from -1.0 to -1.2 V, but declined at more negative potentials. The observed phenomenon originates from HER (hydrogen evolution reaction) initiation during cathodic polarization near its thermodynamic threshold, thereby suppressing the electrodeposition of metal ions at the electrode interface. Consequently, -1.2 V was chosen for the following experiments.

  The effect of deposition time was systematically examined within the range of 50-300 s. As illustrated in Fig.S1c, the peak currents for all target metal ions steadily increased as the deposition time was extended, reaching their highest values at 300 s. Therefore, subsequent experiments adopted a deposition time of 300 s.

  The amount of Co-MOF suspension cast on GCE was optimized. As shown in Fig.S1d, the electrochemical signals tended to reach a maximum value withincreasing Co-MOF suspension (3 mg·mL^-1) volume from 3 to 4 μL. However, a significant decline in current responses was observed at higher volumes of more than 4 μL, which was attributed to a reduced electroactive surface area caused by the excessive aggregation of Co-MOF. Hence, 4 μL was identified as the optimal amount of the modifier. Finally, the optimized detection parameters were identified as pH 5.0, a deposition potential of -1.2 V, a deposition time of 300 s, and a modifier loading of 4 μL.

  2.5   Individual detection of HMIs

  Under the above optimized conditions, SWASV was employed to detect Cd²⁺ and Pb²⁺ in a 0.1 mol·L^-1 ABS at pH 5.0. As shown in Fig.5a and 5b, the oxidation potentials for individual Cd²⁺ and Pb²⁺ were observed at -0.84 and -0.58 V, respectively. The stripping currents increased proportionally with rising metal ion concentrations, and the corresponding calibration curves have been summarized in Table S1.

  Figure 5

  

  Figure 5.  SWASV responses to (a) Cd²⁺ (1.0-16.0 μmol·L^-1) and (b) Pb²⁺ (0.5-10.0 μmol·L^-1), respectively, on Co-MOF@GCE in 0.1 mol·L^-1 ABS (pH 5.0); Linear fitting plots of I vs c for (c) Cd²⁺ and (d) Pb²⁺

  下载: 全尺寸图片 幻灯片

  The Co-MOF@GCE sensor demonstrated good sensing capabilities with established linear ranges of 1.0-16.0 μmol·L^-1 (Cd²⁺) and 0.5-10.0 μmol·L^-1 (Pb²⁺), validating its efficacy in HMIs quantification (Fig.5c and 5d). The LOD was calculated based on the equation LOD=3σ/k, where σ represents the standard deviation of the background current (obtained from twenty SWASV measurements in blank ABS) and k represents the slope of the calibration curve. The LOD values were determined to be 4.609 nmol·L^-1 (Cd²⁺) and 1.307 nmol·L^-1 (Pb²⁺). These results showed that the LOD values of Cd²⁺ and Pb²⁺ were significantly lower than the maximum permissible limits of 26.8 nmol·L^-1 (Cd²⁺) and 48.3 nmol·L^-1 (Pb²⁺) in drinking water, as set by the World Health Organization (WHO)^[46-47]. This indicates the potential of Co-MOF@GCE as a promising sensor for future water quality monitoring applications. Notably, the use of a sole complex for the detection of HMIs is rarely reported. The performance ofCo-MOF@GCE was compared with previously reported sensors for HMI detection (Table S2). The competitive LOD values and wide linear ranges achieved in this study suggest that Co-MOF is a highly effective sensing material for trace-level detection of HMIs.

  2.6   Simultaneous detection of HMIs

  The ability to simultaneously detect multiple HMIs is also crucial for electrochemical sensing materials. As shown in Fig.6a, well-separated and non-overlapping peaks were observed at -0.82 V for Cd²⁺ and -0.60 V for Pb²⁺, respectively. The peak current intensities increased linearly with rising metal ion concentrations, demonstrating excellent linearity within the range of 0.5-7.0 μmol·L^-1. The LOD values were calculated to be 0.47 and 0.008 nmol·L^-1 for Cd²⁺ and Pb²⁺, respectively (Fig.6b). Notably, both LOD values were lower than the concentration set by WHO for drinking water^[48]. The results reveal that Co-MOF@GCE serves as an efficient sensing platform for simultaneous HMIs detection.

  Figure 6

  

  Figure 6.  (a) SWASV responses to Cd²⁺ and Pb²⁺ measured simultaneously with Co-MOF@GCE in 0.1 mol·L^-1 ABS (pH 5.0); (b) Linear fitting plots of I vs c for Cd²⁺ and Pb²⁺

  下载: 全尺寸图片 幻灯片

  2.7   Detection of environmental water samples

  To evaluate the practical utility of Co-MOF@ GCE, it was employed for the detection of environmental water samples. The environmental samples were mixed with a buffer solution at a volume ratio of 1∶9. Then, the amount of metals in the environment wasdetected using SWASV. The known concentrations of metal ions were added to the water samples. Analytical recovery, determined through a validated standard addition method, is quantitatively summarized in Table 1. The recovery ranged between 95% and 105% based on the calibration-curve-derived quantitation. These results demonstrate that the proposed electrochemical sensor is suitable for detecting HMIs in real watersamples.

  Table 1

  

  Table 1.  Co-MOF@GCE for simultaneous detection of Cd²⁺ and Pb²⁺ in environmental water samples

  下载: 导出CSV

  Analyte	Sample	c / (μmol·L-1)		Recovery / %	Relative error / %
  		Added	Found
  Cd2+	Tap water	0	—	—	—
  		4	4.04	100.92	0.92
  		8	8.14	101.75	1.75
  	Mineral water	0	—	—	—
  		4	4.02	100.46	0.46
  		8	7.92	99.03	0.97
  	River water	0	—	—	—
  		4	4.13	103.23	3.23
  		8	7.89	98.56	1.44
  Pb2+	Tap water	0	—	—	—
  		4	4.13	103.19	3.19
  		8	8.07	99.12	0.88
  	Mineral water	0	—	—	—
  		4	3.80	95.16	4.84
  		8	8.14	101.76	1.76
  	River water	0	—	—	—
  		4	4.17	104.23	4.23
  		8	7.68	96.03	3.97

  2.8   Mechanism investigation

  We have speculated a possible electrochemical detection mechanism for the detection of Cd²⁺ and Pb²⁺ by Co-MOF@GCE (Scheme 3)^{[19, 21, 24]}. The phenyl rings and O, N atoms of Co-MOF can effectivelyadsorb HMIs, facilitating the pre-deposition process. The adsorbed M(Ⅱ) species undergoes electrochemicalreduction to its elemental state M(0) at the electrodeinterface. During the anodic stripping step in SWASV, the metallic M(0) is re-oxidized to M(Ⅱ), yielding a characteristic current response used for quantitative analysis. The greater the quantity of adsorbed metal ions, the higher the stripping current. Concurrently, the electrocatalytic mechanism involves dynamic valence state interconversion of Co centers: electrochemical oxidation of Co(Ⅱ) to Co(Ⅲ) during deposition initiates electron transfer, with subsequent reductive regeneration to Co(Ⅱ) during stripping, completing the catalytic cycle. This electron-shuttling mediation between the electrode and M(Ⅱ) species enables significant electrocatalytic signal amplification through continuous redox cycling.

  Scheme 3

  

  Scheme 3.  Co(Ⅲ)/Co(Ⅱ) cycle mechanism for sensing M(Ⅱ) (Cd²⁺ and Pb²⁺)

  下载: 全尺寸图片 幻灯片

  The electrons near the Fermi level, being the most active group of electrons in the system, play a crucial role in analyzing the electrochemical properties of the system^[49]. By thoroughly investigating the electronic characteristics through the band structure and partial density of states (PDOS), the electrochemical behavior of the system can be explored. The band structure calculations revealed that the band gaps of Co-MOF, Pb²⁺@Co-MOF, and Cd²⁺@Co-MOF were 0.13, 0.11, and 0.03 eV, respectively. This indicates that the system exhibits excellent electroactivity both before and after ion adsorption. Further analysis of the PDOSrevealed that there is a suitable energy alignment and significant orbital overlap between the O, N atoms on Co-MOF and Pb²⁺, Cd²⁺, which provides favorable conditions for electron transfer and promotes strong interactions between them. Notably, the increase in hybridization not only increases the number of electron transfer channels but also enhances the electron delocalization. Therefore, the electron transfer between Co-MOF and Pb²⁺, Cd²⁺ effectively drives the transition ofCo-MOF to a metallic state, thereby significantlyreducing contact resistance and enhancing the electroactivity of the system. As shown in Fig.7, near the Fermi level, the atomic orbitals of O and N atoms are largely occupied by electrons, which strongly demonstrates the critical role of Co-MOF in enhancing the overall electroactivity of the system.

  Figure 7

  

  Figure 7.  Band gaps of (a) Co-MOF, (b) Pb²⁺@Co-MOF, and (c) Cd²⁺@Co-MOF; (d, e) PDOS of Cd, Pb, O, and N atoms

  下载: 全尺寸图片 幻灯片

  Additionally, based on molecular orbital theory, the regions of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) where charge density is most concentrated are typically the active sites where chemical reactions are most likely to occur. The distribution of the Co-MOF molecule′s LUMO and HOMO wavefunctions, shown in Fig.8, reflects the charge density distribution of each atom. A detailed analysis revealed that the HOMO is almost entirely localized on the O, N, and Co atoms, while the LUMO is also primarily concentrated in the central regions of these atoms. This phenomenon indicates that the reactive activity of the system is predominantly centered on the O, N, and Co atoms. Therefore, it could be inferred that the atoms contributing most to the LUMO and HOMO are the positions of higher reactivity within the molecule. Furthermore, when metal ions are adsorbed, these high-activity positions, namely, the O, N, and Co atoms, will become the regions where redox reactions are most likely to occur. This conclusion provides an important theoretical foundation for further understanding the chemical reaction mechanism of the Co-MOF system.

  Figure 8

  

  Figure 8.  (a) HOMOs and (b) LUMOs for Co-MOF; (c, d) Structures of the computational model and the optimized Pb²⁺/Cd²⁺-O/N binding distances

  下载: 全尺寸图片 幻灯片

  A detailed investigation of the PDOS reveals significant resonance peaks near the Fermi level, which are located between the Pb, Cd orbitals and the O2p, N2p orbitals. This phenomenon indicates the formation of M—O (M=Pb, Cd) and M—N (M=Pb, Cd) covalent bonds, revealing the prominent electronic interactions between the metal ions and the O and N atoms. Furthermore, it was observed that the formation of these covalent bonds is closely related to the bond lengths between M (M=Pb, Cd) and the surface atoms of Co-MOF^[19]. As shown in the bond length analysis in Fig.8, after adsorption, the average bond lengths of M—O and M—N are 0.220 8/0.263 9 nm, 0.206 1/0.246 9 nm, respectively. These values are nearly identical to those found in the crystal, further confirming that the metal ions find stable adsorption sites around the electronegative N and O atoms on the Co-MOF surface. This finding not only demonstrates the formation of strong chemical covalent bonds between the metal ions and the O and N atoms but also provides important structural evidence for understanding the adsorption behavior of metal ions on the Co-MOF surface. The formation of these covalent bonds not only strengthens the interaction between the metal ions and the Co-MOF surface but also offers new insights for exploring the electrochemical properties of this system^[50].

  Furthermore, through precise analysis of theMulliken charge data, it was found that after the adsorption of metal ions (Pb²⁺, Cd²⁺), the Co center atom exhibits an electronic gain, with the specific gain amounts being 0.174 and 0.162, respectively. This trend of electronic gain indicates that the Co centralatom undergoes a reduction process. Notably, in addition to forming stable covalent bonds with the N and O atoms, the reduced Co atom also exhibits an additional adsorption driving force. This driving force arises from the changes in the electronic structure of the Co atom upon reduction, which provide more favorable conditions for the adsorption of HMIs.

  In summary, the reduction of the Co central atom not only enhances its covalent bond interaction with the N and O atoms but also further improves the system′s adsorption capacity for metal ions, providing new insights for a deeper understanding of the electrochemical adsorption properties of this system.

  3.   Conclusions

  In summary, a cobalt-based metal-organic framework (Co-MOF) functionalized with nitrogen (N), oxygen (O), and aromatic rings was synthesized and utilized for the detection of Cd²⁺ and Pb²⁺. The Co-MOF was strategically designed to be assembled onto a glassy carbon electrode (GCE), forming an advanced electrochemical sensing platform, denoted as Co-MOF@GCE. This system demonstrates remarkable synergistic functionality through integrated adsorption-enrichment and catalytic conversion processes. As a result, it achieved ultralow detection limits of 4.609 nmol·L^-1 for Cd²⁺ and 1.307 nmol·L^-1 for Pb²⁺, along with extended linear response ranges. Comprehensive density functional theory (DFT) calculations revealed the critical cooperative interactions between the Co center and coordinated N/O donors. These findings establish a fundamental mechanism underlying the enhanced ion capture efficiency and electron transfer kinetics. Furthermore, the practical utility of the Co-MOF@GCE sensor was validated through the analysis of real lake water samples. Satisfactory recoveries were achieved for both Cd²⁺ and Pb²⁺ in the lake water samples. This work represents a versatile and user-friendly approach to detecting heavy metal ions (HMIs), effectively bridging the gap between laboratory research and field applications.

  
  Acknowledgments: The current work was supported by the Fundamental Research Program of Shanxi Province (Grant No.202303021222226), the Science and Technology Innovative Foundation of Universities in Shanxi Province (Grant No.2023L380), the Natural Science Foundation of Taiyuan University (Grants No.24TYYB04, 23TYQN22), the Teaching Reform and Innovation Project of Shanxi Province (Grant No.PX-62364), and the Doctoral Starting Research Foundation ofTaiyuan University (Grant No.24TYKY208). All authors claimed no competing interests.
  Conflicts of interest
  Supporting information is available at http://www.wjhxxb.cn
  
• 1. [1]

     SHEN Y Z, GAO X, LU H J, NIE C, WANG J L. Electrochemiluminescence-based innovative sensors for monitoring the residual levels of heavy metal ions in environment-related matrices[J]. Coord. Chem. Rev., 2023, 476: 214927 doi: 10.1016/j.ccr.2022.214927

  2. [2]

     MAO D P, DUAN P H, PIAO Y X. Acid phosphate-activated glassy carbon electrode for simultaneous detection of cadmium and lead[J]. J. Electroanal. Chem., 2022, 925(15): 116898

  3. [3]

     LIU Y Y, KE Y L, SHANG Q G, YANG X F, WANG D S, LIAO G Y. Fabrication of multifunctional biomass-based aerogel with 3D hierarchical porous structure from waste reed for the synergetic adsorption of dyes and heavy metal ions[J]. Chem. Eng. J., 2023, 451: 138934 doi: 10.1016/j.cej.2022.138934

  4. [4]

     WANG B J, ZHU Y, BAI Z S, LUQUE R, XUAN J. Functionalized chitosan biosorbents with ultra-high performance, mechanical strength and tunable selectivity for heavy metals in wastewater treatment[J]. Chem. Eng. J., 2017, 325: 350-359 doi: 10.1016/j.cej.2017.05.065

  5. [5]

     LI L B, BI X Y, ZHEN M Y, REN Y, ZHANG L, YOU T Y. Recent advances in analytical sensing detection of heavy metal ions based on covalent organic frameworks nanocomposites[J]. Trends Anal. Chem., 2024, 171: 117488 doi: 10.1016/j.trac.2023.117488

  6. [6]

     JIANG L, SUN H j, PENG T j, DING W j, LIU B, LIU Q. Comprehensive evaluation of environmental availability, pollution level and leaching heavy metals behavior in non-ferrous metal tailings[J]. J. Environ. Manage., 2021, 290(15): 112639

  7. [7]

     TANG K, CHEN Y T, ZHAO Y X. Exploiting halide perovskites for heavy metal ion detection[J]. Chem. Commun., 2024, 60(34): 4511-4520 doi: 10.1039/D4CC00619D

  8. [8]

     MA L R, WANG Z Z, LIU X, XU F, ABDIRYIM T. Molecules, high sensitivity and selectivity of PEDOT/carbon sphere composites for Pb²⁺ detection[J]. Molecules, 2025, 30(4): 798 doi: 10.3390/molecules30040798

  9. [9]

     ZHAO H R, SUN J C, KUMAR S, LI P H, THALLURI S M, WANG Z M, THUMU U. Recent advances in metal halide perovskite based photocatalysts for artificial photosynthesis and organic transformations[J]. Chem. Commun., 2024, 60(46): 5890-5911 doi: 10.1039/D4CC01949K

  10. [10]

      AZIMI S S, NARGES O S, ZEINAB D, ZAHRA H, ARVIN A, EHSAN S, MOEIN B. A comprehensive image of environmental toxic heavy metals in red meat: A global systematic review and meta-analysis and risk assessment study[J]. Sci. Total Environ., 2023, 889: 164100 doi: 10.1016/j.scitotenv.2023.164100

  11. [11]

      SCUTARAŞU C E, TRINCă C L. Heavy metals in foods and beverages: Global situation, health risks and reduction methods[J]. Foods, 2023, 12(18): 3340 doi: 10.3390/foods12183340

  12. [12]

      DEY P, KAUR M, KHAjURIA A, D KAUR, SINGH M, ALAjANGI H K, SINGLA N, SINGH G, BARNWAL R P. Heavy metal ion detection with nano-engineered materials: Scaling down for precision[J]. Microchem. J., 2024, 196: 109672 doi: 10.1016/j.microc.2023.109672

  13. [13]

      SHI Y K, DONG F J, GONZALEZ R A, WANG G X, YANG L F, CHEN S C, ZHENG H B, WANG S L. Simultaneous detection of heavy metal ions in food samples using a hypersensitive electrochemical sensor based on APTES-incubated MXene-NH₂@CeFe-MOF-NH₂[J]. Food Chem., 2025, 475(30): 143362

  14. [14]

      SHAO Z Y, DI K Z, DING L J, YOU F H, FAN C H, WANG K. Amino-enriched Zn-MOFs with self-reduction for energy-free simultaneous removal and electrochemical detection of heavy metal ions in the aquatic environment[J]. Anal. Chim. Acta, 2025, 1333(2): 343408

  15. [15]

      ZHANG L, PU Y S, XU W L, PENG J Y, LIU Y Q, DU H Z. A ratiometric electrochemical sensor for simultaneous detection of multiple heavy metal ions in water and herbal medicines based on methylene blue-functionalized metal-organic framework[J]. Microchem. J., 2024, 201: 110542 doi: 10.1016/j.microc.2024.110542

  16. [16]

      SELVARAj B, PRIYA G L, BALASUBRAMANIAN S. Harnessing the UiO-67 metal-organic framework for advanced detection of cadmium ions in water bodies[J]. RSC Adv., 2024, 14(48): 35618-35627 doi: 10.1039/D4RA06811D

  17. [17]

      LI W T, ZHANG X A, HU X T, SHI Y Q, LI Z H, HUANG X W, ZHANG W, ZHANG D, ZOU X B, SHI J Y. A smartphone-integrated ratiometric fluorescence sensor for visual detection of cadmium ions[J]. J. Hazard. Mater., 2020, 408(15): 124872

  18. [18]

      MA L, PEI Y W, YANG J, MA J F. A new thiacalix[4]arene-based metal-organic framework as an efficient electrochemical sensor for trace detection of Cd²⁺ and Pb²⁺[J]. Food Chem., 2024, 441(30): 138352

  19. [19]

      WANG F F, LIU C, YANG J, XU H L, PEI W Y, MA J F. A sulfur-containing capsule-based metal-organic electrochemical sensor for super-sensitive capture and detection of multiple heavy-metal ions[J]. Chem. Eng. J., 2022, 438(15): 135639

  20. [20]

      GUO X T, FENG S Y, PENG Y, LI B, ZHAO J W, XU H Y, MENG X R, ZHAI W W, PANG H. Emerging insights into the application of metal-organic framework (MOF)-based materials for electrochemical heavy metal ion detection[J]. Food Chem., 2025, 463(15): 141387

  21. [21]

      GUO T T, HUA B L, GUO Z Y, ZHANG M Q, WANG J R, AN Y Y, LI X N, YAN J Z. A copper(Ⅱ)-based metal-organic framework: Electrochemical sensing of Cd(Ⅱ) and Pb(Ⅱ) and adsorption of organic dyes[J]. Dalton Trans., 2025, 54(4): 1393-1401 doi: 10.1039/D4DT02374A

  22. [22]

      NIU X, PEI W Y, MA J C, YANG J, MA J F. Simultaneous electrochemical detection of gallic acid and uric acid with p-tert-butylcalix[4]arene-based coordination polymer/mesoporous carbon composite[J]. Microchim. Acta, 2022, 189: 93 doi: 10.1007/s00604-022-05201-z

  23. [23]

      JEONG J Y, DO J Y, HONG C A. Target DNA- and pH-responsive DNA hydrogel-based capillary assay for the optical detection of short SARS-CoV-2 cDNA[J]. Microchim. Acta, 2022, 189: 34 doi: 10.1007/s00604-021-05138-9

  24. [24]

      GUO T T, CAO X Y, AN Y Y, ZHANG X L, YAN J Z. Sulfur-bridged Co(Ⅱ)-thiacalix[4]arene metal-organic framework as an electrochemical sensor for the determination of toxic heavy metals[J]. Inorg. Chem., 2023, 62(11): 4485-4494 doi: 10.1021/acs.inorgchem.2c04197

  25. [25]

      SUN Y F, JIAN W, LI P H, YANG M, HUANG X J. Highly sensitive electrochemical detection of Pb(Ⅱ) based on excellent adsorption and surface Ni(Ⅱ)/Ni(Ⅲ) cycle of porous flower-like NiO/rGO nanocomposite[J]. Sens. Actuator B‒Chem., 2019, 292: 136-147 doi: 10.1016/j.snb.2019.04.131

  26. [26]

      MO Y T, SHEN Y. Electrochemical detection of heavy metals in rice, milk and tap water using free-standing carbon felt electrodes[J]. Food Chem., 2024, 460: 140450 doi: 10.1016/j.foodchem.2024.140450

  27. [27]

      CHEN Y Y, LIU Y Y, ZHAO P, LIANG Y, MA Y, LIU H, HOU J Z, HOU C, HUO D Q. Electrochemical detection of heavy metals in rice, milk and tap water using free-standing carbon felt electrodes[J]. Food Chem., 2024, 446(15): 138770

  28. [28]

      KITTE S A, LI S P, NSABIMANA A, GAO W Y, LAI J P, LIU Z Y, XU G B. Stainless steel electrode for simultaneous stripping analysis of Cd(Ⅱ), Pb(Ⅱ), Cu(Ⅱ) and Hg(Ⅱ)[J]. Talanta, 2019, 191, 485-490 doi: 10.1016/j.talanta.2018.08.066

  29. [29]

      HUANG H K, WANG J H, ZHENG Y Q, BAI W D, MA Y, ZHAO X J. Synthesis and application of bismuth nanoparticle-loaded longan porous carbon for simultaneous electrochemical determination of Pb(Ⅱ) and Cd(Ⅱ) in seafoods[J]. Food chem., 2024, 452(15): 139572

  30. [30]

      ONG S C, NG H Q, AHMAD L A, LOW S C. Enhancement of electrode surface hydrophilicity and selectivity with Nafion-PSS composite for trace heavy metal sensing in electrochemical sensors[J]. Anal. Chim. Acta, 2025, 1335(15): 343423

  31. [31]

      DOAN D T, PHAM Y H T, LUONG D D, THI N H, OANH H T, LE T T, NGUYEN H T, HOANG T K D, HOANG M H. A highly sensitive electrochemical sensor for the detection of lead(Ⅱ) ions utilizing rice-shaped bimetallic MOFs incorporated reduced graphene oxide[J]. RSC Adv., 2025, 15(7): 5356-5368 doi: 10.1039/D4RA08952A

  32. [32]

      YANG H L, PENG C W, HAN J J, SONG Y H, WANG L. Three- dimensional macroporous carbon/Zr-2, 5-dimercaptoterephthalic acid metal-organic frameworks nanocomposites for removal and detection of Hg(Ⅱ)[J]. Sens. Actuator B‒Chem., 2020, 320: 128447 doi: 10.1016/j.snb.2020.128447

  33. [33]

      HU Z, DEIBERT B J, LI J. Luminescent metal-organic frameworks for chemical sensing and explosive detection[J]. Chem. Soc. Rev., 2014, 43(16): 5815-5840 doi: 10.1039/C4CS00010B

  34. [34]

      WANG Y, WANG L, HUANG W, ZHANG T, HU X, PERMAN J A, MA S Q. A metal-organic framework and conducting polymer based electrochemical sensor for high performance cadmium ion detection[J]. J. Mater. Chem. A, 2017, 5(18): 8385-8393 doi: 10.1039/C7TA01066D

  35. [35]

      WANG X X, QI Y X, SHEN Y, YUAN Y, ZHANG L D, ZHANG C Y, SUN Y H. A ratiometric electrochemical sensor for simultaneous detection of multiple heavy metal ions based on ferrocene-functionalized metal-organic framework[J]. Sens. Actuator B‒Chem., 2020, 310: 127756 doi: 10.1016/j.snb.2020.127756

  36. [36]

      LI X T, NIU X, YANG J, PEI W Y, MA J F. Thiacalix[4]arene-based complex with Co(Ⅱ) ions as electrode modifier for simultaneous electrochemical determination of Cd(Ⅱ), Pb(Ⅱ), and Cu(Ⅱ)[J]. Microchim. Acta, 2022, 189: 344 doi: 10.1007/s00604-022-05456-6

  37. [37]

      PAN Y C, HU X Y, GUO D S. Biomedical applications of calixarenes: State of the art and perspectives[J]. Angew. Chem. ‒Int. Edit., 2021, 60(6): 2768-2794 doi: 10.1002/anie.201916380

  38. [38]

      AN Y Y, GUO T T, BIAN J H, YAN J Z. A 3D compound based on semi-rigid tricarboxylato-bridged Co(Ⅱ): Synthesis, crystal structure and magnetism[J]. J. Chem. Crystallogr., 2023, 53: 540-546 doi: 10.1007/s10870-023-00993-1

  39. [39]

      SHELDRICK G M. SHELXS-2018, Programs for X-ray crystal structure solution[CP]. University of Göttingen, Germany, 2018.

  40. [40]

      FARRUGIA L J. WINGX: A Windows program for crystal structure analysis[CP]. University of Glasgow, UK, 1988.

  41. [41]

      SHELDRICK G M. SHELXTL-2018, Programs for X-ray crystal structure refinement[CP]. University of Göttingen, Germany, 2018.

  42. [42]

      WANG L, WANG X Y, SHI G S, PENG C, DING Y H. Thiacalixarene covalently functionalized multiwalled carbon nanotubes as chemically modified electrode material for detection of ultratrace Pb²⁺ ions[J]. Anal. Chem., 2012, 84(24): 10560-10567 doi: 10.1021/ac302747f

  43. [43]

      SUTTON C C, FRANKS G V, DA SILVA G. Modeling the antisymmetric and symmetric stretching vibrational modes of aqueous carboxylate anions[J]. Spectroc. Acta Pt. A‒Molec. Biomolec. Spectr., 2015, 134(5): 535-542

  44. [44]

      SHUKLA S N, GAUR P, BAGRI S S, MEHROTRA R, CHAURASIA B, RAIDAS M L. Pd(Ⅱ) complexes with ONN pincer ligand: Tailored synthesis, characterization, DFT, and catalytic activity toward the Suzuki-Miyaura reaction[J]. J. Mol. Struct., 2021, 1225(5): 129071

  45. [45]

      SRIRAM B, BABY J N, HSU Y F, WANG S F, GEORGE M. Toward the development of disposable electrodes based on holmium orthovanadate/f-boron nitride: Impacts and electrochemical performances of emerging inorganic contaminants. [J]. Inorg. Chem., 2021, 60(16): 12425-12435 doi: 10.1021/acs.inorgchem.1c01678

  46. [46]

      YE C, XU F, ULLAH F, WANG M Q. CdS/Ti₃C₂ heterostructure-based photoelectrochemical platform for sensitive and selective detection of trace amount of Cu²⁺[J]. Anal. Bioanal. Chem., 2022, 414(12): 3571-3580 doi: 10.1007/s00216-021-03870-y

  47. [47]

      NGOENSAWAT U, PISUCHPEN T, SRITANA-ANANT Y, RODTHONGKUM N, HOVEN V P. Conductive electrospun composite fibers based on solid-state polymerized poly(3, 4-ethylenedioxythiophene) for simultaneous electrochemical detection of metal ions[J]. Talanta, 2022, 241: 123253 doi: 10.1016/j.talanta.2022.123253

  48. [48]

      REHMAN A U, FAYAZ M, LV H, LIU Y, ZHANG J, WANG Y, DU L J, WANG R H, SHI K Y. Controllable synthesis of a porous PEI-functionalized Co₃O₄/rGO nanocomposite as an electrochemical sensor for simultaneous as well as individual detection of heavy metal ions[J]. ACS Omega, 2023, 8(26): 24125-24126 doi: 10.1021/acsomega.3c02938

  49. [49]

      SEGALL M D, SHAH R, PICKARD C J, PAYNE M C. Population analysis of plane-wave electronic structure calculations of bulk materials[J]. Phys. Rev. B: Condens. Matter, 1996, 54(23): 16317-16320 doi: 10.1103/PhysRevB.54.16317

  50. [50]

      BRIONES-JURADO C, AGACINO-VALDÉS E. On the possible removal of nitrogen monoxide and carbon monoxide on copper ion-exchanged montmorillonite: A DFT study[J]. Int. J. Quantum Chem., 2008, 108(10): 1802-1809 doi: 10.1002/qua.21640

• 

  Scheme 1  Synthetic route of Co-MOF

  下载: 全尺寸图片 幻灯片
  

  Scheme 2  Visualized illustration for the detection of Cd²⁺ and Pb²⁺ using Co-MOF@GCE

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) Coordination sphere of Co(Ⅱ) centers of Co-MOF; (b) 2D schematic diagram of Co-MOF; (c, d) Multi-view of 3D network structure of Co-MOF

  Symmetry codes: #1: 1+x, 1+y, -1+z; #2: -x, 1-y, -z; #3: -x, -y-1, 1-z.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) FTIR spectrum, (b) TG curve, and (c) PXRD patterns of Co-MOF

  下载: 全尺寸图片 幻灯片
  

  Figure 3  (a) EIS spectra and (b) CV curves of bare GCE and Co-MOF@GCE in 0.1 mol·L^-1 KCl containing 5 mmol·L^-1 [Fe(CN)₆]^3-/[Fe(CN)₆]^4-; (c) SWASV curves of Co-MOF@GCE and GCE in the mixed solution of Cd²⁺ (5 μmol·L^-1) and Pb²⁺ (5 μmol·L^-1) in 0.1 mol·L^-1 ABS (pH=5.0)

  下载: 全尺寸图片 幻灯片
  

  Figure 4  (a, c) CV profiles of GCE and Co-MOF@GCE under scan rate gradients (10-200 mV·s^-1) in 0.1 mol·L^-1 KCl electrolyte containing 5 mmol·L^-1 [Fe(CN)₆]^3-/[Fe(CN)₆]^4- redox probe; (b, d) Linear correlation between I_p and v^1/2

  下载: 全尺寸图片 幻灯片
  

  Figure 5  SWASV responses to (a) Cd²⁺ (1.0-16.0 μmol·L^-1) and (b) Pb²⁺ (0.5-10.0 μmol·L^-1), respectively, on Co-MOF@GCE in 0.1 mol·L^-1 ABS (pH 5.0); Linear fitting plots of I vs c for (c) Cd²⁺ and (d) Pb²⁺

  下载: 全尺寸图片 幻灯片
  

  Figure 6  (a) SWASV responses to Cd²⁺ and Pb²⁺ measured simultaneously with Co-MOF@GCE in 0.1 mol·L^-1 ABS (pH 5.0); (b) Linear fitting plots of I vs c for Cd²⁺ and Pb²⁺

  下载: 全尺寸图片 幻灯片
  

  Scheme 3  Co(Ⅲ)/Co(Ⅱ) cycle mechanism for sensing M(Ⅱ) (Cd²⁺ and Pb²⁺)

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Band gaps of (a) Co-MOF, (b) Pb²⁺@Co-MOF, and (c) Cd²⁺@Co-MOF; (d, e) PDOS of Cd, Pb, O, and N atoms

  下载: 全尺寸图片 幻灯片
  

  Figure 8  (a) HOMOs and (b) LUMOs for Co-MOF; (c, d) Structures of the computational model and the optimized Pb²⁺/Cd²⁺-O/N binding distances

  下载: 全尺寸图片 幻灯片

  Table 1.  Co-MOF@GCE for simultaneous detection of Cd²⁺ and Pb²⁺ in environmental water samples

  Analyte	Sample	c / (μmol·L-1)		Recovery / %	Relative error / %
  		Added	Found
  Cd2+	Tap water	0	—	—	—
  		4	4.04	100.92	0.92
  		8	8.14	101.75	1.75
  	Mineral water	0	—	—	—
  		4	4.02	100.46	0.46
  		8	7.92	99.03	0.97
  	River water	0	—	—	—
  		4	4.13	103.23	3.23
  		8	7.89	98.56	1.44
  Pb2+	Tap water	0	—	—	—
  		4	4.13	103.19	3.19
  		8	8.07	99.12	0.88
  	Mineral water	0	—	—	—
  		4	3.80	95.16	4.84
  		8	8.14	101.76	1.76
  	River water	0	—	—	—
  		4	4.17	104.23	4.23
  		8	7.68	96.03	3.97

  下载: 导出CSV
•