Characterization of cysteamine self assembled on gold functionalized with nitrilotriacetic acid and evaluation of copper(II) binding capacity with adsorption transfer stripping voltammetry

Highlights

• Gold electrode was modified with cysteamine and nitrilotriacetic acid.

• Electrochemistry of modified electrode was studied.

• An AdTSV method for determination of copper was developed.

• Trace levels of copper were determined in urine samples.

Abstract

A self assembled monolayer of cysteamine was prepared on the surface of gold disc electrode and further modified with nitrilotriacetic acid. Modified electrode was characterized with cyclic voltammetry and electrochemical impedance spectroscopy in presence of potassium hexacyanoferrate(II)/(III). Thus prepared electrode was tested for determination of Cu²⁺ with adsorption transfer stripping voltammetry. The binding of Cu²⁺ onto the modified electrode was successfully performed for a wide range of tested concentrations. Electrode response (log I_p) was linearly proportional to −log c(Cu²⁺) with correlation coefficient R² = 0.9759. The detection limit was $1·{10}^{-8}\text{M}$. The influence of interfering substances (Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺ and Pb²⁺) was tested and it was found that Mg²⁺ and Pb²⁺ do not interfere while Mn²⁺, Co²⁺ and Ni²⁺ show slight interference. Real-world testing of copper content in urine samples confirmed practical application of prepared sensor.

Introduction

Significant amounts of heavy metal ions are found in the soil and water due to increased anthropogenic activity. They are circulating, and eventually accumulating, throughout the food chain, thus posing a threat to the ecosystem. Monitoring their level is imperative, considering their toxicity and longevity [1]. Several analytical techniques are used for copper detection. Atomic emission spectrometry [2], inductively coupled plasma mass spectrometry [3] and neutron activation analysis [4] are some of them. These techniques require time consuming manipulation steps and sophisticated instrumentation, so there is a need for fast, reliable and cheap method which can be used in the field. Electrochemical methods, in addition with modified electrodes, meet the above mentioned criteria.

Surface alteration, based on self assembled monolayers (SAMs), has been widely studied over the last 20 years [5], [6]. The approach that is used, when changing electrode surface, depends on the type of the analyte. Modification of metal electrodes, with SAMs, can provide interface with required performance [7] i.e. SAMs can be used as a platform for detecting biologically important molecules such as epinephrine [8], glucose [9], [10], dopamine [11], [12], uric acid [13], proteins [14], cholesterol [15], vitamins [16], antibodies and antigens [17], DNA [18] and pesticides [19]. Possibility of tailoring SAMs properties, with different end groups, enables sensor construction for determination of metal ions and molecules like Zr⁴⁺, Mg²⁺, Ca²⁺, Sr²⁺, La³⁺ and Cd²⁺ [20], [21], [22], [23]. Rubinstein et al. [24] were one of the first to modify electrode surface, with mixed SAM that selectively recognized Cu²⁺ ions. After that, a number of electrochemical techniques for copper measurement, based on SAM modified electrodes were published, such as cyclic voltammetry [25] and adsorptive stripping voltammetry [26] on gold electrode modified with L-cysteine, voltammetric determination on 3-mercaptopropionic SAM [27], differential pulse [28] and anodic stripping voltammetry [29] on meso-2,3-dimercaptosuccinic acid, anodic stripping voltammetry on 2,5-dimercapto-1,3,4-thiadiazol [30], differential pulse voltammetry on cysteamine functionalized with L-lysine [31] and linear sweep anodic stripping voltammetry on 2-mercaptoethanesulfonate [32].

Electrochemical impedance spectroscopy (EIS) is nondestructive, powerful tool used to study and characterize the phenomenon of corrosion, fuel cells and batteries, coatings, and conductive polymers, chemically modified electrodes and adsorption of thin films, modification of electrodes based on SAMs, electron transfer kinetics and the analytical determination of inorganic and organic substances [33]. The basic advantage of EIS is use of an electric model, i.e. equivalent circuit for description of any electrochemical system. In this way we obtain a large number of parameters and gain better understanding of investigated system.

Varieties of electrochemical methods are used in order to determine the analyte and to examine phenomena that occur on the electrode – solution interface. Commonly used techniques, for low concentration analyte measurement, are stripping methods.

Adsorption transfer stripping voltammetry (AdTSV) is analogous to the anodic and cathodic stripping voltammetry apart from accumulation step, which is not electrochemically controlled, but achieved through a process of adsorption. Possible adsorption mechanisms are either physical adsorption on the surface or chemical adsorption by complexation with modified electrode. Adsorption and measurement steps are conducted in different solutions [34]. This represents a comparative advantage over other methods because accumulation could be carried in the field, requiring only modified electrode, and measurement performed later in the laboratory.

In this paper we used nitrilotriacetic acid (NTA) as electrode modification substance. Carboxylic group is known as excellent metal ion chelating agent. NTA has three carboxylic groups and it forms stable complexes with transition metals [35]. Therefore, gold electrode modified with NTA, provides an assembly that allows investigation of complex formation and has potential for Cu²⁺ sensor construction.

The main purpose of this research was to characterize cysteamine (CA) self assembled layer on gold electrode (Au/CA) further functionalized with nitrilotriacetic acid (Au/CA/NTA) and to investigate possibility of thus prepared electrode for Cu²⁺ binding. Au/CA and Au/CA/NTA modifications were investigated with cyclic voltammetry (CV) and EIS using ${[\text{Fe}{(\text{CN})}_6]}^{3-/4-}$ redox pair. Complexation of NTA and Cu²⁺ was tested and optimized with AdTSV.