Gold Nanoparticle-Modified Carbon-Fiber Microelectrodes for the Electrochemical Detection of Cd2+ via Fast-Scan Cyclic Voltammetry

Abstract

Neurotoxic heavy metals, such as Cd²⁺, pose a significant global health concern due to their increased environmental contamination and subsequent detrimental health hazards they pose to human beings. These metal ions can breach the blood-brain barrierblood-brain barrier, leading to severe and often irreversible damage to the central nervous system and other vital organs. Therefore, developing a highly sensitive, robust, and rapid in vivo detection method for these hazardous heavy metal ions is of the utmost importance for early detection, thus initiating timely therapeutics. Detecting ultra-low levels of toxic metal ions in vivo and obtaining accurate speciation information remains a challenge with conventional analytical techniques. In this study, we fabricated a novel carbon carbon-fiber microelectrode (CFM)-based sensor that can detect Cd²⁺ ions using fast-scan cyclic voltammetry by electrodepositing gold nanoparticles (AuNP). We optimized electrochemical parameters that generate a unique cyclic voltammogram (CV) of Cd²⁺ at a temporal resolution of 100 ms with our novel sensor. All our experiments were performed in tris buffer that mimics the artificial cerebellum fluid. We established a calibration curve resulting in a limit of detection (LOD) of 0.01 µM with a corresponding sensitivity of 418.02 nA/ µM. The sensor's selectivity was evaluated in the presence of other metal ions, and it was noteworthy to observe that the sensor retained its ability to produce the distinctive Cd²⁺ CV, even when the concentration of other metal ions was 200 times higher than that of Cd²⁺. We also found that our sensor could detect free Cd²⁺ ions in the presence of complexing agents. Furthermore, we analyzed the solution chemistry of each of those Cd²⁺-ligand solutions using a geochemical model, PHREEQC. The concentrations of free Cd²⁺ ions determined through our electrochemical data align well with geochemical modeling data, thus validating the response of our novel sensor. Furthermore, we reassessed our sensor's LOD in tris buffer based on the concentration of free Cd²⁺ ions determined through PHREEQC analysis, revealing an LOD of 0.00132 µM. We also demonstrated the capability of our sensor to detect Cd²⁺ ions in artificial urine samples, showcasing its potential for application in actual biological samples. To the best of our knowledge, this is the first AuNP-modified, CFM-based Cd²⁺ sensor capable of detecting ultra-low concentrations of free Cd²⁺ ions in different complex matrices, including artificial urine at a temporal resolution of 100 ms, making it an excellent analytical tool for future real-time, in vivo detection, particularly in the brain.

Keywords: cadmium; carbon carbon-fiber microelectrodes; electrodeposition; fast scan cyclic voltammetry; gold nanoparticles; real-time analysis.

Figure 1

Schematic representation of fabrication of AuNP-modified CFMs. (a) Insertion of carbon fiber into a glass capillary. (b) Pulling fiber-filled capillaries into two halves under gravity using a heating coil in a vertical puller. (c) Trimming the exposed length of the carbon fiber manually using a scalpel blade under a microscope. (d) Making an electrochemical connection with Hg and an Ag wire. (e) Electrodeposition of AuNP via cyclic voltammetry (CV) using a three-electrode system using an in-house built Ag/AgCl electrode as the reference electrode (RE) and a Pt wire as the counter electrode (CE). (f) AuNP-modified CFM.

Figure 2

(a) Representative CVs obtained for 0.25 µM Cd²⁺ with AuNP-modified CFMs (orange CV) and bare CFMs (blue CV) in tris buffer. (b) Representative color plot obtained for 0.25 µM Cd²⁺ in tris buffer with AuNP-modified CFM.

Figure 3

(a) Representative CVs obtained for 0.25 µM Cd²⁺ for each scan rate when potential was varied from −0.8 V to −1.4 V and back to −0.8 V. (b) Plot of maximum reduction peak current vs. scan rate. (c) Calibration curve for Cd²⁺ using AuNP-modified CFMs in tris buffer. The potential was cycled from −0.8 V to −1.4V at 400 V/s. Each data point represents the average oxidation current ± standard error of mean obtained for three CFMs with at least 4 replicate measurements for each CFM (minimum of 12 total replicates).

Figure 4

Representative CVs obtained for 0.5 µM Cd²⁺ in the presence 100 µM Cu²⁺ (blue CV), Co²⁺ (orange CV), Mn²⁺ (green CV), Pb²⁺ (yellow CV), and without (purple CV) in tris buffer.

Figure 5

(a) Representative CVs obtained for 0.5 µM Cd²⁺ with the addition of 0.5 µM NTA (blue CV), DTPA (orange CV), DMSA (grey CV), EDTA (yellow CV), and without (purple CV) in tris buffer. (b) Bar graph showing the % [Cd]free in tris buffer (purple bar), with NTA (blue bar), EDTA (yellow bar), DMSA (grey bar), and DTPA (orange bar) acquired from PHREEQC geochemical modeling.

Figure 6

Representative CV (a) and color plot (b) obtained for 0.25 µM Cd²⁺ in artificial urine with 0.1 M KCl using AuNP-modified CFMs. (c) Calibration curve for Cd²⁺ using AuNP-modified CFMs in artificial urine with 0.1 M KCl. The potential was cycled from −0.8 V to −1.4 V at 400 V/s. Each data point represents the average oxidation current ± standard error of mean obtained for three CFMs with at least 4 replicate measurements for each CFM (minimum of 12 total replicates).