Elsevier

Analytica Chimica Acta

Volume 502, Issue 2, 30 January 2004, Pages 207-212
Analytica Chimica Acta

Simultaneous detection of cadmium, copper, and lead using a carbon paste electrode modified with carbamoylphosphonic acid self-assembled monolayer on mesoporous silica (SAMMS)

https://doi.org/10.1016/j.aca.2003.10.001Get rights and content

Abstract

A new sensor was developed for simultaneous detection of cadmium (Cd2+), copper (Cu2+), and lead (Pb2+), based on the voltammetric response at a carbon paste electrode modified with carbamoylphosphonic acid (acetamide phosphonic acid) self-assembled monolayer (SAM) on mesoporous silica (Ac-Phos SAMMS). The adsorptive stripping voltammetry (AdSV) technique involves preconcentration of the metal ions onto Ac-Phos SAMMS under an open circuit, then electrolysis of the preconcentrated species, followed by a square wave potential sweep towards positive values. Factors affecting the preconcentration process were investigated. The voltammetric responses increased linearly with the preconcentration time from 1 to 30 min or with metal ion concentrations ranging from 10 to 200 ppb. The responses also evolved in the same fashion as adsorption isotherm in the pH range of 2–6. The metal detection limits were 10 ppb after 2 min preconcentration and improved to 0.5 ppb after 20 min preconcentration.

Introduction

Currently, quantification of heavy metals at subsurface hazardous waste sites relies upon collection of liquid discrete samples for subsequent laboratory analysis using techniques such as ICP-MS and AAS. Analysis by off-site governmental or commercial laboratories often results in lengthy turnaround times. Sensors for obtaining real-time ppb-level heavy metal concentrations would reduce time and costs associated with the characterization and treatments of hazardous waste sites. Electrochemical sensors based on stripping voltammetry appear to be a promising technique for determining aqueous heavy metal concentrations. These sensors are usually sensitive, compact, low cost, and easily integrated into field-deployable units [1], [2], [3], [4].

Stripping voltammetry for trace metal ions usually involves preconcentration of metal ions at an electrode surface, followed by quantification of the accumulated species by voltammetric methods. Preconcentration of metal species at mercury drop or mercury film electrodes have been available for decades. However, mercury drop electrodes have the disadvantage of being mechanically unstable during various steps of the assay procedure, thus they are less desirable than solid-state sensors in routine field applications [5], [6]. In addition, mercury-based electrodes have issues related to the use and disposal of toxic mercury. On the other hand, in adsorptive stripping voltammetry (AdSV), preconcentration at an electrode modified with functional ligands uses the specific binding properties of the ligand (towards the target metal ions) to accumulate the metal ions via ion exchange or chelation onto the electrode without applying any potential. In addition to being solid-sate and mercury-free, this technique has several advantages: (1) it can preconcentrate metal ions that cannot be reductively accumulated, (2) electrolytes are not required in the preconcentration solution, thus reduces the risk of introduction of contaminants or competing ligands, and (3) with the appropriate ligand, the overall selectivity of the analysis for the targeted metal ions may increase.

Electrodes for AdSV for trace metal ions can be fashioned by adsorption of monolayers of host molecules [7] (e.g., functionalized self-assembled monolayers (SAMs) and polymeric films) on the electrode surfaces, or by embedding suitable functional ligands in a conductive porous matrix [8], [9], [10], [11], [12], [13], [14], [15], [16]. The number of functional groups on SAM thin films may be limited and the stability and durability of the SAM films may be poor. Although, the polymeric films have superior stability and durability over SAM thin films, the detection may be affected by shrinking and swelling of the polymeric films caused by the changes in solution pH or electrolyte concentration, leading to slow and reduced electrode responses [17], [22].

Carbon paste electrodes modified with functional ligands have been more widely used to preconcentrate and quantify trace metal ions [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, the ligands in these sensors are in loose association or physical contact with the electrode. Thus, to avoid degradation of the sensor over time as a result of depletion of ligand-bearing material, the modifiers must be insoluble in the solvents employed [8]. Alternatively, the ligands may be first covalently bonded to high surface area substrates (such as porous silica) before being embedded into the carbon graphite matrix, which allows chemical functionality to be retained despite diffusion or abrasive wear. Polysiloxane-immobilized amine ligands [18] and aminopropyl-grafted silica gel [19] have been used successfully by Walcarius et al. as electrode modifiers in order to study the uptake of copper(II) from an aqueous solution. In the preconcentration stage, the diffusion of the target analytes to the binding sites located inside of amorphous materials (i.e., silica gel) having a tortuous structure can be slow. Because such diffusion is normally the rate-determining step in the voltammetric analyses that rely on porous materials as electrode modifiers [20], having easy accessibility to the binding sites is an important feature of the electrode modifiers. With that in mind, well-ordered, hexagonal porous silicas (MCM-41) have been recently exploited as a substrate for immobilization of simple functional groups like amine and thiol [20]. However, such study is still limited to material aspects of functionalized amorphous silicas versus functionalized MCM-41 as electrode modifiers by using mercury as a representative metal ion and not the metrological aspects of adsorptive stripping voltammetry. This work reports the application of the MCM-41 modified with a self-assemble monolayer functionality as electrode modifier for simultaneous detection of toxic metal ions.

For a number of years, PNNL has been a leader in the development of a new class of nanoengineered sorbents, the self-assembled monolayer on mesoporous supports (SAMMS) [21], [22], [23], [24], [25]. These nanoporous hybrid materials are highly efficient sorbents and their interfacial chemistry can be fine-tuned to selectively sequester any specific target species. One successful material is the acetamide phosphonic acid self-assembled monolayer on mesoporous silica (Ac-Phos SAMMS) [24], [25]. Fig. 1a shows the schematic of SAMMS with acetamide phosphonic acids as terminal monolayers (Fig. 1b). The mesoporous silica (MCM-41) had a surface area of 989 m2/g and a nominal pore size of 5.0 nm. The high surface area of MCM-41 and the monolayer assembly technique afforded a high functional group density on the Ac-Phos SAMMS (i.e., 2.0 mmol acetamide phosphonic acid per gram of Ac-Phos SAMMS). In addition to the suitable pore structures and high number of binding sites, Ac-Phos SAMMS has been demonstrated to be highly selective for cadmium, copper, and lead in aqueous solutions without significant interference from other common cations like Na+, which were present at much higher concentrations [25]. For example, in the presence of 5 mg/l (each) of Cd2+, Co2+, Cr3+, Cu2+, Mn2+, Pb2+, Zn2+, and Ni2+ and 2,300 mg/l of Na+ (at pH 5.5 and the solution per solid ratio of 200) the mass-weight distribution coefficients (Kd) of cadmium, copper and lead are in the order of 104. This selectivity is a result of the molecular recognition properties of acetamide phosphonic acid ligands, which prefer heavy metal ions over alkaline and alkaline earth metals.

This work investigated the use of a carbon paste electrode modified with Ac-Phos SAMMS for simultaneous detection of cadmium(II), copper(II), and lead(II) after preconcentration at an open circuit. Factors affecting the preconcentration process, including preconcentration time, metal ion concentrations, and the pH of the solution, were investigated. Then the optimal operating parameters and the electrode application/regeneration are reported.

Section snippets

Apparatus

Square wave voltammetry (SWV) experiments were performed on an electrochemical detector, model CHI660A (CH Instruments Inc.), equipped with a three electrode system: a self-made carbon paste electrode modified with Ac-Phos SAMMS as the working electrode, a platinum wire as the auxiliary electrode, and a KCl saturated Ag/AgCl electrode as the reference electrode. All measurements were made at room temperature and under an atmospheric environment. Square wave voltammetry was operated at a

Results and discussion

Because SAMMS is an electronic insulator, desorption of previously accumulated metal ions from the surface of SAMMS to the electrode/solution interface must occur, usually by immersing the electrode in an acidic solution, for the voltammetric detection to be accomplished. Concurrent to the desorption process, a thorough electrolysis is performed by applying a negative potential to reduce the desorbed metal ions (M2+) to metal elements (M(0)), followed by the quantification of metal ions via

Conclusions

The self-assembled monolayer chemistry enables ready installation of a wide variety of functional interfaces on mesoporous MCM-41 silica, leading to excellent specificity and selectivity for any desired analyte when the materials are used as modifiers in electrochemical sensors. The high functional density of the monolayers on Ac-Phos SAMMS, resulting from the combined effects of an extremely large surface area of MCM-41 and the self-assemble chemistry used in the installation of the functional

Acknowledgements

This work was supported by the SERDP Program, US Department of Defense and the EMSP Program, US Department of Energy (DOE). Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the US DOE. The research described in this paper was performed in part at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. The authors thank

References (26)

  • M.F Mousavi et al.

    Talanta

    (2001)
  • T Molina-Holgado et al.

    Anal. Chim. Acta

    (1995)
  • B Ogorevc et al.

    Anal. Chim. Acta

    (1995)
  • A Walcarius et al.

    Electrochim. Acta

    (1999)
  • M Etienne et al.

    Sens. Actuators B: Chem.

    (2001)
  • J Wang et al.

    Analyst

    (1993)
  • J. Wang, Analytical Electrochemistry, VCH, New York,...
  • Y Lin et al.

    Proc. SPIE

    (1999)
  • Y Lin et al.

    Biomed. Microdevices

    (2001)
  • J Wang et al.

    Anal. Chem.

    (1997)
  • J Wang et al.

    Electroanalysis

    (1995)
  • I Turyan et al.

    Electroanalysis

    (2001)
  • S.V Prabhu et al.

    Anal. Chem.

    (1987)
  • Cited by (0)

    View full text