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Communication

Electrochemical Detection of Microplastics in Water Using Ultramicroelectrodes

by
Changhui Lee
,
Sangwon Han
and
Jun Hui Park
*
Department of Chemistry, Chungbuk National University, Cheongju 28644, Republic of Korea
*
Author to whom correspondence should be addressed.
Chemosensors 2024, 12(12), 278; https://doi.org/10.3390/chemosensors12120278
Submission received: 26 November 2024 / Revised: 17 December 2024 / Accepted: 20 December 2024 / Published: 23 December 2024
(This article belongs to the Section Electrochemical Devices and Sensors)
Browse Figures
Figure 1
<p>Cyclic voltammogram of 10 μm Pt UME in (<b>a</b>) 100 mM potassium ferrocyanide aqueous solution and (<b>b</b>) 100 mM potassium ferricyanide aqueous solution.</p> "> Figure 2
<p>Amperometric i–t curves of solution containing 100 mM potassium ferrocyanide and microplastics produced by grinding (<b>a</b>) disposable storage containers (PS) and (<b>b</b>) plastic cups (PP), using a 10 µm Pt UME at +0.5 V (vs. Ag/AgCl). The inset images provide an enlarged view of the region outlined in the red box, with arrows indicating the step current generated by microplastic collisions.</p> "> Figure 3
<p>Amperometric i–t curves of solution containing (<b>a</b>) 100 mM potassium ferrocyanide with polystyrene beads at +0.5 V (vs. Ag/AgCl) and (<b>b</b>) 100 mM potassium ferricyanide with polystyrene beads at −0.1 V (vs. Ag/AgCl).</p> "> Figure 4
<p>(<b>a</b>) Geometry of COMSOL Multiphysics simulation model to calculate degree of step current; (<b>b</b>) step current as a function of microplastic size collided with a 10 µm-diameter electrode.</p> "> Figure 5
<p>Comparison of the size distribution of microplastics obtained through simulation and the size distribution measured through DLS for (<b>a</b>) PS and (<b>b</b>) PP microplastics. SEM images of (<b>c</b>) PS microplastics and (<b>d</b>) PP microplastics.</p> "> Scheme 1
<p>Schematic of the current change caused by microplastics colliding with the electrode surface.</p> ">
Versions Notes

Abstract

:
Herein, a method for detecting microplastics in water using single-entity electrochemistry is presented, with a focus on the interaction between microplastics in aqueous solution and the surface of an ultramicroelectrode (UME). Polystyrene and polypropylene, two commonly used plastics that were ground and dispersed in aqueous solution, served as the detection target materials. The collisional contact of microplastics with the UME was transduced into a discernible signal. To detect microplastics in solution using an UME, redox species (e.g., ferrocyanide) were continuously oxidized at the electrode, and the resulting steady-state current was monitored. Collisional contact followed by adsorption of microplastics on the UME disturbed the diffusional flux of redox species, resulting in an immediate change in the steady-state current. Detection sensitivity was further enhanced by optimizing the electrolyte composition to induce a migration effect. COMSOL Multiphysics simulations were employed to analyze the magnitude of the current changes as a function of microplastic size. The size distribution obtained from the simulations closely matched measurements from dynamic light scattering (DLS).

1. Introduction

Microplastics have emerged as widespread environmental pollutants in aquatic environments such as oceans and rivers [1,2,3,4,5]. The amount of plastic entering the ocean is increasing owing to rising plastic usage, and plastic can persist in water for very long periods with continuous degradation [3,6]. Plastics fragment due to mechanical and photochemical processes, forming microplastics [7,8,9]. Plastics can also be manufactured to be small from the outset, such as with microbeads in personal care products like toothpaste and shampoo, which are ingested by aquatic organisms ranging from algae to fish [10,11,12,13,14]. In addition to microplastics found in aquatic organisms, microplastics caused by drinking beverages from disposable plastic bottles are also a rising problem to be monitored [15,16,17]. There is growing concern about the potential health effects on humans of ingesting microplastics directly in beverages [18]. Given these risks, there is an increasing need to develop effective methods for detecting microplastics in potable liquids such as bottled water.
Current methods for detecting microplastics include visual inspection using a microscope, thermal analysis where microplastics are thermally decomposed into a gaseous state and detected using a GC-MS instrument, and spectroscopic methods such as Raman spectroscopy or FTIR spectroscopy [19,20,21,22]. However, these methods require sample pretreatment before the experiment, making the analysis both time-consuming and costly. In this context, electrochemical detection is emerging as a promising approach owing to its high sensitivity, specificity, and potential for real-time analysis. Compared to conventional methods, electrochemical detection has advantages such as speed, low cost, and minimized sample pretreatment. Notably, silica and polymer-bead detection methods using single-entity electrochemistry (SEE) have shown promise in determining the size and concentration of spherical particles by measuring the current changes when single-bead particles collide with the surface of an electrode [23].
Recently, SEE has been actively investigated because of its ability to provide detailed information on individual particles such as metal nanoparticles, polymer particles, liquid droplets and cells [24,25,26,27,28,29]. The technique is effective not only for detecting the electrochemical activity of individual nanoparticles but also for analyzing nanoparticle concentration and size distribution [30,31,32,33]. SEE offers significant advantages for observing the characteristics of individual particles, avoiding the averaging effects typically seen in conventional ensemble measurements. These advantages enable precise quantification and real-time monitoring of the dynamic behavior of single entities [34,35,36]. In biomedical research, this approach facilitates individual cell and biomolecule study, which is essential for diagnostics and disease monitoring, as in, for example, detecting macrocytic red blood cells for anemia diagnosis [37,38,39]. Additionally, SEE has been employed to determine the phase-transfer kinetic constants of electroactive species across liquid/liquid interfaces using water-in-oil (W/O) emulsions and to detect water droplets in organic solvents by observing electrode shape changes caused by oxidation upon emulsion collision [40,41]. The versatility of SEE extends to environmental monitoring, where it facilitates contaminant detection in liquid samples.
This technique enables the monitoring of contaminants by measuring fluctuations in steady-state current induced by adsorption on the electrode surface. Previously, the detection of single entities through partial electrode blockage was limited to standard materials (e.g., silica beads and polystyrene beads) [23]. Little research has focused on detecting the naturally occurring, irregularly shaped microplastics commonly found in the environment. Therefore, in this study, we demonstrate an electrochemical method for detecting microplastics produced by grinding disposable storage containers and plastic cups, simulating microplastics typically present in the natural environment. We also estimated the size distribution of these microplastics through COMSOL Multiphysics simulations based on the observed current changes when they collide with the electrode surface. This method allows us to detect realistic microplastic samples in water and has potential applications in monitoring ocean contamination levels.

2. Materials and Methods

2.1. Reagents and Materials

All aqueous solutions were prepared using Millipore water (>18.2 MΩ cm) (Burlington, MA, USA). Potassium ferrocyanide trihydrate (K4Fe(CN)6·3H2O, ≥99.0%) was obtained from Sigma-Aldrich (St. Louis, MO, USA). Potassium ferricyanide (K3Fe(CN)6, 98+%) was obtained from Alfa Aesar (Ward Hill, MA, USA). Polybead® Carboxylate Microspheres (diameter: 2.022 µm) were obtained from Polysciences, Inc. (Warrington, PA, USA). A Pt wire (diameter: 10 µm) was obtained from Goodfellow (Devon, PA, USA). Borosilicate glass capillaries (1.5 mm outer diameter × 0.75 mm inner diameter) were obtained from Sutter Instrument (Novato, CA, USA).

2.2. Instruments

Electrochemical experiments were performed using a CHI 601E potentiostat (CHI Instruments, Austin, TX, USA) with three-electrode systems in a Faradaic cage. A Pt ultramicroelectrode (UME, diameter: 10 µm) and Ag/AgCl electrode (in saturated KCl) were used as the working and reference electrodes, respectively. A Pt wire (diameter: 1 mm) was used as the counter electrode. Scanning electron microscopy (SEM) images were obtained using a Gemini SEM 560 (Zeiss Co., Ltd., Oberkochen, Germany) field emission scanning electron microscope at 3 kV. A CF-10 Centrifuge (DAIHAN Scientific Co., Ltd., Wonju, Republic of Korea) was used to collect microplastic. The microplastic zeta potentials were measured using a Zetasizer Nano ZS (Malvern Panalytical., Ltd., Malvern, UK). Dynamic light scattering (DLS) was used to measure microplastic size using a NanoBrook 90Plus particle size analyzer (Brookhaven Instruments Corporation, Holtsville, NY, USA).

2.3. Preparation of Pt UME

The UME was prepared by following the procedure developed in our laboratory. Briefly, Pt wire (diameter: 10 µm) was sealed in a borosilicate glass capillary washed with hexane, toluene, IPA, ethanol, and water. The electrode was polished using 400, 1200, 2400, and 4000-grit silicon carbide abrasive sandpaper (R&B Co., Ltd., Daejeon, Republic of Korea) until a mirror-like surface was observed. Before each electrochemical experiment, all UMEs were polished using 4000-grit silicon carbide (SiC) sandpaper.

2.4. Preparation of Microplastics

Microplastics were produced by grinding plastic items easily found in everyday environments with sandpaper. Disposable storage containers (polystyrene, PS) and plastic cups (polypropylene, PP) were used for microplastic production. The plastic particles abraded by sandpaper were collected in a conical tube using distilled water. The high-density sandpaper particles were allowed to settle to facilitate separation from microplastics. The floating plastic particles were then isolated and collected using a centrifuge. The shape and size of the collected microplastics were analyzed using SEM and DLS. The zeta potential of the microplastics was measured by dispersing the microplastics in distilled water.

3. Results and Discussion

Monitoring fluctuation in the flux of redox species at the UME enables the detection of microplastics through SEE. An UME has a tiny electrode area that facilitates radial diffusion of redox species to the electrode surface. Therefore, UMEs can obtain steady-state-current response at constant potential conditions in which redox species upon the electrode surface are consumed by electrode reaction. However, in the presence of nonconductive, suspended particles, the diffusive flux of redox species can be partly obstructed when a particle collides with and adheres to the UME. Microplastics adsorbed on the electrode surface can cause an instant decrease in steady-state current by establishing a new steady-state diffusion flux. As microplastics collide multiple times, each collision progressively obstructs the mass transfer to the electrode surface, resulting in stair-step-current responses.
Using this principle, the UME steady-state current can be monitored to detect microplastics dispersed in water. Microplastics in water are randomly moved by Brownian motion. However, owing to the tiny electroactive area of an UME, the possibility of microplastics colliding with the UME by Brownian motion is very low. Therefore, to detect low concentrations of microplastics in water, the collision probability should be increased. In this regard, the migration effect can be leveraged in microplastic detection if the particles carry a charge in an aqueous solution [23].
The microplastics used in this study were derived from ground-up disposable storage containers and plastic cups. The storage containers, composed of polystyrene, are referred to as PS microplastics, while the plastic cups, made of polypropylene, are referred to as PP microplastics. As shown in Table 1, the zeta potential of both PS and PP microplastics are negative. Given this negative surface charge, the frequency of microplastic collisions with the electrode surface will increase if the redox species consumed at the UME surface also carry a negative charge. Consequently, the redox species and electrolyte composition were carefully selected to enhance the migration effect.
In this experiment, potassium ferrocyanide was oxidized to induce a migration effect, thereby increasing the microplastic collision frequency. Cyclic voltammetry was performed to determine the optimal potential for the complete oxidation of ferrocyanide, as shown in Figure 1a. Similarly, the optimal potential for ferricyanide reduction was investigated as a control, as shown in Figure 1b. Since the oxidation of ferrocyanide ions was observed above 0.2 V, microplastic detection was conducted at a potential of +0.5 V to achieve a steady-state current. When a potential of 0.5 V is applied to Pt UME in a potassium ferrocyanide solution, the concentration of ferricyanide ions, which have a more positive charge than ferrocyanide ions, increases near the electrode surface, where the ferrocyanide ions are oxidized to ferricyanide. Additionally, the migration of negatively charged substances toward the positively charged region near the electrode is initiated by the depletion of negatively charged ions at the electrode surface, which maintains charge neutrality. Consequently, negatively charged microplastics are drawn toward the electrode surface as the electrode reaction proceeds. Scheme 1 illustrates the collision of negatively charged microplastics with the UME, leading to a change in the steady-state current of ferrocyanide.
Ground PS and PP microplastics were detected by chronoamperometry in a 100 mM aqueous potassium ferrocyanide solution using a 10 µm Pt UME. As shown in Figure 2, step currents were observed in the presence of microplastics. This step current can be attributed to the collision of microplastics with the electrode surface, where they partially block the electrode surface, reducing mass transfer to the electrode surface and thereby decreasing the oxidation current of ferrocyanide. In the absence of microplastics, no step current was observed when a potential of 0.5 V was applied to the Pt UME in the 100 mM potassium ferrocyanide solution. The average step currents caused by the collision of PS and PP microplastics were 134 and 75 pA, respectively. The difference in step-current magnitudes is attributed to variation in particle size and shape, as PS and PP microplastics were prepared under identical conditions yet differ in size due to their physicochemical properties. Additionally, the collision frequency was higher for PS microplastics than for PP microplastics because PS microplastics are more negatively charged.
To confirm that the step-current response originated from microplastic collisions, standard polystyrene beads were analyzed in the same electrolyte solutions. Polystyrene beads, which have a homogeneous size distribution and a negative charge, serve as an ideal comparison system for microplastic detection. Figure 3a shows the i–t curve for 100 mM potassium ferrocyanide containing polystyrene beads, showing a characteristic staircase-current response. For comparison, an i–t curve for 100 mM potassium ferricyanide containing polystyrene beads was also conducted at −0.1 V. This potential, where the ferricyanide is sufficiently reduced to ferrocyanide, is evident from the CV in Figure 1b. As a result, no staircase current was observed, as shown in Figure 3b.
When potassium ferricyanide was used as the redox species, the concentration of ferrocyanide ions near the electrode surface increased owing to the reduction of ferricyanide ions to ferricyanide ions, temporarily creating a more negatively charged environment in the vicinity of the electrode surface.
Consequently, the negatively charged polystyrene beads were not attracted to the electrode, and no collision events were observed. This confirms that the collision of polystyrene beads with the electrode is driven by the migration effect resulting from the oxidation of ferrocyanide ions.
The magnitude of the step current resulting from microplastic collisions with the electrode surface was simulated using COMSOL Multiphysics to estimate the size of the collided microplastics. Figure 4a illustrates the geometry of the simulation model expressed in two-dimensional axisymmetric mode. The simulation assumed that spherical particles collided at the center of an electrode in solution, with particle radius ranging from 0.1 to 2 µm. The magnitude of the staircase-current response due to particle collisions was calculated by subtracting the steady-state current measured with and without microplastics. The step-current magnitude as a function of the spherical particle size, derived from the simulation, is shown in Figure 4b. The magnitude of the step current increased with the size of the microplastic colliding with the electrode, as larger microplastics hinder the mass transfer of the redox species to the electrode surface more significantly. To estimate the size of the collided microplastics based on the step-current magnitude obtained from electrochemical detection, a calibration curve was established, as shown in Figure 4b. The calibration curve is best represented by two distinct equations: one for microplastics larger than 1 µm and another for those smaller than 1 µm. The equations for the calibration curve are expressed as follows:
y = (3 × 10−12)e3.4x − (3 × 10−12)    x ≤ 1.0 µm
y = (4.3 × 10−12)e0.61x − (7.05 × 10−12)    x > 1.0 µm
Equation (1) corresponds to microplastics smaller than 1 µm in diameter, and Equation (2) corresponds to microplastics larger than 1 µm in diameter. x and y have units of nA and μm, respectively.
Figure 5a,b show the size distribution of microplastics estimated from electrochemical data with help from the COMSOL Multiphysics simulation, compared with the measurement obtained using DLS. For the data collection, step currents below 20 pA were excluded from the calculation because they were difficult to distinguish from noise. The average size of PS microplastics obtained from the calibration curve was 1.06 µm, and the average size for PP microplastics was 0.91 µm. These values closely matched the average sizes measured by DLS, which were 1.02 µm for PS microplastics and 0.94 µm for PP microplastics.
Although the size distribution of the microplastics calculated using the calibration curve closely matches the distribution measured by DLS, a perfect match in individual measurements may not be achieved owing to certain assumptions made during the simulation. The simulation assumed that the microplastics collide at the electrode center, whereas in actual experiments, collisions can occur at various points, including the electrode edges. The mass transfer blockage depends on the microplastic landing positions on the disk electrode, leading to variations in the step current. Furthermore, the simulation assumed that the microplastics are perfectly spherical, while real microplastics exhibit irregular shapes. Figure 5c,d show SEM images of PS and PP microplastics, illustrating their random shapes.
Despite these limitations, the close agreement between the size distributions estimated by electrochemical measurement and those obtained by DLS confirms the validity of this method for monitoring dispersed microplastics in water and providing their approximate size information.

4. Conclusions

Microplastics in potable water pose a potential risk that necessitates monitoring. This study proposes an electrochemical method for the real-time detection of microplastics in water. Specifically, we demonstrate the detection of realistic microplastics, prepared by hand-grinding disposable plastic containers to mimic naturally occurring microplastics in aquatic environments. By leveraging the surface charge properties of polystyrene and polypropylene microplastics, we attracted these particles to the electrode surface through a migration effect induced by the continuous oxidation of potassium ferrocyanide. Upon the microplastic collision with the UME surface, an abrupt change in steady-state current signified successful microplastic detection. Additionally, COMSOL Multiphysics simulations enabled us to estimate the size of the microplastics colliding with the electrode. The size distribution of the microplastics obtained from the simulation closely matched that measured by DLS, suggesting that our simulation model can accurately predict the microplastics sizes in water based on current magnitude changes.

Author Contributions

Conceptualization, J.H.P.; methodology, C.L.; validation, S.H.; formal analysis, C.L.; investigation, S.H.; resources, J.H.P.; data curation, S.H.; writing—original draft preparation, C.L.; writing—review and editing, J.H.P.; visualization, C.L.; supervision, J.H.P.; project administration, J.H.P.; funding acquisition, J.H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Chungbuk National University Korea National University Development Project (2023).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

Special thanks to Seonmin Kim and Suyeon Jin for their assistance with sample preparation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. Schematic of the current change caused by microplastics colliding with the electrode surface.
Scheme 1. Schematic of the current change caused by microplastics colliding with the electrode surface.
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Figure 1. Cyclic voltammogram of 10 μm Pt UME in (a) 100 mM potassium ferrocyanide aqueous solution and (b) 100 mM potassium ferricyanide aqueous solution.
Figure 1. Cyclic voltammogram of 10 μm Pt UME in (a) 100 mM potassium ferrocyanide aqueous solution and (b) 100 mM potassium ferricyanide aqueous solution.
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Figure 2. Amperometric i–t curves of solution containing 100 mM potassium ferrocyanide and microplastics produced by grinding (a) disposable storage containers (PS) and (b) plastic cups (PP), using a 10 µm Pt UME at +0.5 V (vs. Ag/AgCl). The inset images provide an enlarged view of the region outlined in the red box, with arrows indicating the step current generated by microplastic collisions.
Figure 2. Amperometric i–t curves of solution containing 100 mM potassium ferrocyanide and microplastics produced by grinding (a) disposable storage containers (PS) and (b) plastic cups (PP), using a 10 µm Pt UME at +0.5 V (vs. Ag/AgCl). The inset images provide an enlarged view of the region outlined in the red box, with arrows indicating the step current generated by microplastic collisions.
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Figure 3. Amperometric i–t curves of solution containing (a) 100 mM potassium ferrocyanide with polystyrene beads at +0.5 V (vs. Ag/AgCl) and (b) 100 mM potassium ferricyanide with polystyrene beads at −0.1 V (vs. Ag/AgCl).
Figure 3. Amperometric i–t curves of solution containing (a) 100 mM potassium ferrocyanide with polystyrene beads at +0.5 V (vs. Ag/AgCl) and (b) 100 mM potassium ferricyanide with polystyrene beads at −0.1 V (vs. Ag/AgCl).
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Figure 4. (a) Geometry of COMSOL Multiphysics simulation model to calculate degree of step current; (b) step current as a function of microplastic size collided with a 10 µm-diameter electrode.
Figure 4. (a) Geometry of COMSOL Multiphysics simulation model to calculate degree of step current; (b) step current as a function of microplastic size collided with a 10 µm-diameter electrode.
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Figure 5. Comparison of the size distribution of microplastics obtained through simulation and the size distribution measured through DLS for (a) PS and (b) PP microplastics. SEM images of (c) PS microplastics and (d) PP microplastics.
Figure 5. Comparison of the size distribution of microplastics obtained through simulation and the size distribution measured through DLS for (a) PS and (b) PP microplastics. SEM images of (c) PS microplastics and (d) PP microplastics.
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Table 1. Zeta potential values of microplastics derived from grinding up disposable storage containers (PS) and plastic cups (PP).
Table 1. Zeta potential values of microplastics derived from grinding up disposable storage containers (PS) and plastic cups (PP).
ItemPlastic TypeZeta Potential (mV)
Disposable storage containerPolystyrene−40.4
Plastic cupPolypropylene−24.0
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Lee, C.; Han, S.; Park, J.H. Electrochemical Detection of Microplastics in Water Using Ultramicroelectrodes. Chemosensors 2024, 12, 278. https://doi.org/10.3390/chemosensors12120278

AMA Style

Lee C, Han S, Park JH. Electrochemical Detection of Microplastics in Water Using Ultramicroelectrodes. Chemosensors. 2024; 12(12):278. https://doi.org/10.3390/chemosensors12120278

Chicago/Turabian Style

Lee, Changhui, Sangwon Han, and Jun Hui Park. 2024. "Electrochemical Detection of Microplastics in Water Using Ultramicroelectrodes" Chemosensors 12, no. 12: 278. https://doi.org/10.3390/chemosensors12120278

APA Style

Lee, C., Han, S., & Park, J. H. (2024). Electrochemical Detection of Microplastics in Water Using Ultramicroelectrodes. Chemosensors, 12(12), 278. https://doi.org/10.3390/chemosensors12120278

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