Electrochemical detection of dopamine using negatively charged ordered mesoporous carbon (CMK-3)†

Junhee Yu, Hyo Chan Lee, Hyun Ju Yang, Sunyeong Hong and Je Hyun Bae*
Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daejeon 34134, Republic of Korea. E-mail: jehyunbae@cnu.ac.kr

First published on 29th July 2025

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Introduction

Dopamine (DA) is widely used in neuroscience and medicine owing to its critical role as a neurotransmitter. DA regulates various physiological functions, including metabolism and the central nervous, cardiovascular, renal, and hormonal systems.¹ Abnormal DA levels are closely linked to neurological disorders and depression,² with deficiencies associated with severe neurological conditions such as schizophrenia and muscle rigidity, as well as Huntington's and Parkinson's diseases. Conversely, excessive DA levels can lead to cardiotoxicity that manifests as a rapid heart rate, hypertension, heart failure, and a higher risk of drug addiction. Hence, the ability to precisely detect DA is crucial for accurate disease diagnosis given its significant physiological impact. Biosensors capable of reliably measuring DA concentrations are essential tools for use in medical and clinical scenarios. DA sensors are broadly classified into two categories: those that use specific recognizers and those that do not. While recognizer-based sensors, which rely on enzymes, antibodies, or aptamers, are highly selective and sensitive,³ they require complex fabrication processes and are often of limited stability and durability.⁴ In contrast, non-recognizer-based sensors are simpler and easier to manufacture but are generally less selective and sensitive. DA sensors are categorized as either electrochemical or nonelectrochemical based on the transduction method. While spectroscopic techniques, such as those that rely on fluorescence, color, or surface plasmon resonance, have been used to detect DA,^5–10 electrochemical methods are more commonly used owing to their inherent electrochemical activities.¹¹ Electrochemical sensors offer several advantages, including high sensitivities with low detection limits, rapid response times, the ability to directly measure signals, cost-effectiveness, and miniaturization and portability potential. Accordingly, electrochemical sensors are ideal platforms for the rapid, accurate, and cost-efficient detection of dopamine. Electrochemical sensors based on nanoporous electrodes are highly valued owing to their exceptionally large surface areas per unit volume that facilitate numerous reactions in compact spaces. The high proportion of material exposed at the surface also leads to cost savings because electrochemical reactions primarily occur at surfaces. Early research mainly focused on exploiting the higher surface areas of nanoporous electrodes that enable a large number of recognizers or catalysts to be immobilized in a manner that enhances sensitivity.^12,13 Moreover, the higher catalytic activity of a nanoporous electrode endows it with superior sensitivity compared to that of a traditional flat electrode.^14–18 Moreover, in some cases, sensors devoid of enzymes that are normally used to detect slow-reacting species have been developed.¹⁹ Interest in exploiting the unique structural features of nanoporous electrodes for use in sensor applications has recently been growing.^20–24 Since the confined structure of a nanoporous material limits mass transfer, sensor performance can be enhanced by imparting selectivity based on factors such as the size,^25,26 charge,^27–29 hydrophobicity,^30–32 reaction rate,^19,33,34 and the adsorption characteristics of the reactants.^17,35 While several studies have explored using nanoporous gold or carbon electrodes to detect DA,^36–40 limited research that fully exploits the structural features of nanoporous electrodes appears not to exist. Some studies have demonstrated that carbon nanopipettes or nanoporous gold electrodes can be used to enhance DA-sensor sensitivity by leveraging dopamine adsorption within their nanopores.^41,42 Additionally, structural defects in nanoporous gold electrodes have been used to endow them with selectivity for DA and ascorbic acid (AA) oxidation.³⁹ The surface charge of the nanoporous electrode structure also plays a crucial role in determining sensitivity for DA. For example, negatively charged multiwalled carbon nanotubes have been used to electrochemically detect nanomolar DA concentrations in the presence of interfering substances.⁴³ Moreover, DA has been selectively detected in the presence of AA within negatively charged nanopore electrode arrays at low ionic strengths.²⁷

In this study, we investigated how the surface charge on a nanoporous conducting electrode affects its ability to selectively detect DA and AA. Electric double layer (EDL) within nanopores may overlap given that a pore is comparable in size to EDL,^44–47 which enhances the effect of the surface charges of nanopores that are only a few nanometers in size when compared to those of macroporous electrodes.²⁸ Moreover, nanoporous electrodes are expected to be more advantageous in terms of signal amplification and suppression owing to the influence of surface charge, particularly when compared to electrodes with charged but non-conductive pores, as the charged portion of the pore is also the electrode surface. The potential applications of electrochemical sensors that exploit EDL overlap and surface-charge effects at well-defined nanoporous conducting electrodes have not been thoroughly explored. Negatively charged ordered mesoporous carbons have been used to enhance the surface-charge effect, thereby enabling ultra-trace-level detection and sensitive analysis of DA, even in the presence of interfering species such as AA.

Experimental

Reagents

All chemicals, including sodium phosphate monobasic monohydrate (ACS reagent, ≥98%, Sigma), phosphate buffered saline tablet (Sigma), sodium phosphate dibasic heptahydrate (ACS reagent, 98.0–102.0%, Sigma), potassium chloride (ACS reagent, 99.0–100.5%, Sigma), dopamine hydrochloride (Sigma), L-ascorbic acid (99+%, ACS reagent, >99%, Sigma), potassium hexacyanoferrate(II) trihydrate (ACS reagent, 98.5–102.0%, Sigma), hexaammineruthenium(III) chloride (98%, Acros Organics), ferrocenemethanol (97%, Sigma), hexaammonium persulfate (98%, Daejung), sulfuric acid (97%, Matsuneon), serum-free media (SFM): RPMI 1640 medium (HyClone, with L-glutamine, Cat. No. SH30027.01) supplemented with 1% antibiotic-antimycotic solution (Gibco, Thermo Fisher Scientific, Cat. No. 15240062), N, N-dimethylformamide (99.5%, Junsei), and CMK-3 type B (ACS Material) were used as received. CMK-3 is a type of ordered mesoporous carbon (carbon mesostructured from Korea-3). All aqueous solutions were prepared using ultrapure deionized water from Millipore (ZMQS5V001, Milli-Q Gradient).

Preparing and characterizing mesoporous carbon

The surface of the mesoporous carbon was negatively charged by mixing ordered mesoporous carbon (OMC, CMK-3, 0.2 g) with 1.0 M APS (ammonium persulfate in 2 M H₂SO₄, 12 g) followed by stirring at room temperature for 9 h. The solution containing the modified CMK-3 was filtered through deionized water, and the modified CMK-3 was washed thoroughly to remove H₂SO₄ and APS, after which it was dried at 50 °C for 12 h. Homogeneous catalyst ink was prepared by ultrasonically dispersing 3 mg of the synthesized catalyst in 1 mL of DMF for 15 min. A 3 mm-diameter glassy carbon electrode (GCE, CHI104; CH Instruments) was polished using a 0.05 μm slurry on a microcloth pad (Buehler), after which it was ultrasonically cleaned in deionized water for 5 min. A 2 μL aliquot of the ink was drop-cast onto the GCE, after which it was dried at room temperature for 1 d. The roughness factor (f_R = electrochemical surface area/apparent surface area) was determined by comparing the capacitance of the OMC to that of the GCE. Scanning electron microscopy (SEM; Hitachi S-4300) and transmission electron microscopy (TEM; JEOL JEM-ARM200F NEOARM) were used to analyze the microstructural properties of the mesoporous carbon materials. The Brunauer–Emmett–Teller (BET) method and nonlocal density functional theory (NLDFT) were applied to the adsorption branch using a MicrotracBEL (BELSORP-max) instrument at 77 K with N₂ as the adsorbate. Zeta potentials were measured in deionized water using a Nano ZSP/ZEN5602 analyzer (Malvern Instruments, UK).

Electrochemical measurements

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed in a three-electrode electrochemical cell using a Reference 600+ electrochemical analyzer (Gamry Instruments). Ag/AgCl (3 M NaCl, BASi) and Pt wire (ϕ 0.5 mm, Sigma) were used as the reference and counter electrodes, respectively. Bare GCE and CMK-3 were used as flat working electrodes for comparison with the negatively charged modified CMK-3 (COOH-CMK-3). The solutions were purged with argon gas (99.999%) for 10 min prior to use and all experiments were performed under argon at room temperature.

Results and discussion

Fig. S1† shows SEM and TEM images of CMK-3, which reveal uniformly sized pores approximately 5 nm in diameter. Fig. 1A shows Fourier-transform infrared (FT-IR) spectra of CMK-3 and COOH-CMK-3. Carboxyl groups were generated in the nanoporous structure of CMK-3, which endowed it with a negatively charged surface. Compared to CMK-3, COOH-CMK-3 exhibits a characteristic O–H stretching band at approximately 3340 cm⁻¹, a CO stretching band at around 1720 cm⁻¹, and a C–O stretching band in the 1100–1300 cm⁻¹ range. These spectral features confirm that carboxyl groups had been successfully incorporated in the CMK-3 framework.⁴⁸ The stretching band observed at 1600 cm⁻¹ reveals that the fundamental CC band associated with CMK-3 is retained by COOH-CMK-3. Fig. 1B compares the adsorption volumes of COOH-CMK-3 and CMK-3 using N₂ as the adsorbate at 77 K, which provided insight into their microstructural properties and enabled us to evaluate structural differences following surface modification. Fig. 1B presents N₂ adsorption–desorption isotherms acquired at 77 K for both samples, which exhibited typical type IV isotherms with H1-type hysteresis loops, characteristic of materials with uniform cylindrical mesopores.⁴⁹ The BET surface area of CMK-3 was determined to be 814.6 m² g⁻¹. Carboxyl functionalization led to a significant decrease in surface area to 494.7 m² g⁻¹ (Table S1†). In a similar manner, a significantly lower total pore volume (1.14 vs. 1.92 to cm³ g⁻¹) was observed following introduction of the carboxyl groups, suggestive of partial pore blockage. Fig. 1C displays Barrett–Joyner–Halenda (BJH) pore-size distributions that reveal that both CMK-3 and COOH-CMK-3 have mesopore-centered distributions. Furthermore, both materials exhibited similar average pore diameters (4.85 vs. 5.15 nm), which indicates that the overall meso structure is well preserved even following surface modification. The pore-size distribution results are consistent with the TEM data.

The surface-charge properties of CMK-3 before and after functionalization were evaluated by measuring their zeta potentials in deionized water. Table S2† reveals that pristine CMK-3 exhibited a zeta potential of −2.96 mV, whereas COOH-CMK-3 exhibited a significantly more negative value (−8.74 mV). This notable shift is consistent with the successful incorporation of carboxyl groups that increase the density of negatively charged surface functionalities.⁵⁰ A similar trend was observed by Teng et al., who reported that the zeta potential of mesoporous carbon (MC-COOH) decreased from −18 to −27 mV at pH 6 upon carboxyl-functionalization, which corresponds to a 9 mV increase in surface negativity.⁵¹ These results confirm that carboxyl functionalization enhances the negative charge of the surface, in good agreement with our observation for COOH-CMK-3.

Fig. 2 compares CV traces obtained in 1 M phosphate buffer solution using the bare GCE and mesoporous carbon electrodes. The CV traces are similar in shape, which suggests that no special surface reactions occur on nanoporous carbons bearing COOH functional groups compared to their bare counterparts, and that they exhibit similar current behavior during charging. However, COOH-CMK-3 exhibited a significantly higher charging current. The slopes of the relationships between charge current and scan rate were determined, which revealed that COOH-CMK-3 has an areal capacitance of 0.36952 F cm⁻², which is 310-times higher than that (0.00119 F cm⁻²) recorded for the GCE, implying that the electrochemical surface area of the COOH-CMK-3 electrode is approximately 300-times higher than that of the GCE, and that the mesoporous electrode had been well formed. The synthesized mesoporous material immobilized on the electrode was sufficient for electrochemical experiment as it was stable in solution and did not fall off the GCE easily.

Relationships between EDL overlap, surface charge, and electrolyte concentration in the mesoporous electrode were examined by subjecting negatively charged Fe(CN)₆⁴⁻ in 1 mM and 1 M KCl solutions to CV. The flat GCE exhibited a peak current that remained almost unchanged as the electrolyte concentration was varied; however, a more reversible CV shape was observed in the 1 M electrolyte owing to a lower ohmic drop (Fig. 3A). On the other hand, pristine CMK-3 showed CV traces that were independent of electrolyte concentration (Fig. 3B). In contrast, the electrochemical reaction of Fe(CN)₆⁴⁻ at COOH-CMK-3 was significantly suppressed at low electrolyte concentrations, with no observable peak current because the negatively charged surface of the nanoporous electrode is insufficiently screened at low ionic strength, which hinders electron transfer. However, surface-charge effects are effectively masked at high electrolyte concentrations, which facilitated the appearance of oxidation and reduction peaks.

We further confirmed the effect of surface charge by conducting experiments with uncharged or positively charged reactants. The CV current did not differ significantly with electrolyte concentration in the case of uncharged ferrocenemethanol (FcMeOH), as shown in Fig. S2.† However, positively charged Ru(NH₃)₆³⁺ exhibited a peak current whose magnitude increased with decreasing electrolyte concentration owing to the migration effect of positively charged reactive species in response to the negative charge on the electrode surface as the electrolyte was diluted. These results confirm that the mesoporous carbon electrode maintains a strong negative surface charge. Furthermore, the mesoporous electrode exhibited a more reversible CV trace in 1 M electrolyte than the flat electrode. This behavior is attributable to a shift in mass transport from semi-infinite planar diffusion toward the macro-electrode surface to thin-layer diffusion within the mesopores.⁵² Additionally, the mesoporous electrode exhibited a lower ratio of faradaic to capacitive current because Fe(CN)₆⁴⁻ was unable to fully penetrate its porous structure owing to fast reactions.⁵³

Current densities were normalized against geometric surface area (GSA) in all electrochemical figures presented in this study. The COOH-CMK-3 and CMK-3 electrodes exhibited faradaic current densities several to tens of times higher than that of a GCE with the same GSA, which is attributable to their porosity-associated higher surface areas. However, the flat electrode exhibited a higher current density when normalized against the electrochemically active surface area (ECSA) owing to relatively fast electrochemical reactions that limit reactant access to inner pore surfaces, thereby reducing effective use of the porous surface.³⁴

Accordingly, COOH-CMK-3 can be used to selectively detect DA and AA. DA (pK_a1 = 8.86, pK_a2 = 10.6) is predominantly positively charged at pH 7, whereas AA (pK_a1 = 4.04, pK_a2 = 11.7) is mostly negatively charged; consequently, they are expected to interact differently with the COOH-CMK-3. Fig. 4 shows CV traces acquired for 1 μM DA using GCE, CMK-3, and COOH-CMK-3 in various phosphate buffer and KCl solutions. DA exhibited similar peak-current magnitudes across various electrolyte concentrations on GCE and CMK-3. However, COOH-CMK-3 exhibited a more intense peak current in the dilute electrolyte than in the concentrated one. This behavior is attributable to EDL overlap and the surface-charge effect, which are electrolyte-concentration dependent. The positively charged DA migrates toward the negatively charged carbon surface under low-electrolyte conditions, leading to an accumulation of ions and a stronger current signal. Additionally, a more-reversible CV trace was acquired for DA at the mesoporous electrode in 1 M electrolyte than for the flat GCE electrode. These results are consistent with the CV results for Fe(CN)₆⁴⁻ displayed in Fig. 3, which revealed more-reversible traces for the porous electrodes. The more irreversible CV trace observed at lower electrolyte concentrations compared to that at higher ones is attributable to a higher ohmic drop.

Experiments involving DA were also conducted under acidic conditions. The surface of the COOH-CMK-3 electrode is not negatively charged at pH values below the pK_a (4–5) of the carboxylic acid groups on its carbon surface. Consequently, a less-intense signal is expected compared to that obtained at a neutral pH, where the surface is negatively charged. The experimental results shown in Fig. S3† confirm this expectation; a lower CV current was measured at pH 3 than that at pH 7. The oxidation peak potential of DA was observed to shift positively with decreasing pH, which indicates that protons are involved in the reaction at the electrode. These findings show that a negatively charged carbon electrode surface enhances the DA signal, which is likely due to electrostatic attraction with the positively charged DA, especially for dilute electrolytes.

The electrochemical behavior of AA was examined under various solution conditions. Fig. 5 shows CV traces acquired for 10 μM AA in various phosphate buffer and KCl solutions, using the GCE, CMK-3, and COOH-CMK-3. GCE and CMK-3 exhibited oxidation currents that remained steady, irrespective of electrolyte concentration. However, the negatively charged carbon surface of COOH-CMK-3 repels the negatively charged AA at low electrolyte concentrations, thereby reducing AA penetration into the nanopores of the electrode, resulting in ion depletion and a less-intense current signal owing to the effects of EDL overlap and the surface charge. In contrast, this suppression was less pronounced at higher electrolyte concentrations, leading to a distinct oxidation peak at 0.2 V. The earlier appearance of this oxidation peak using the mesoporous electrode is attributable to enhanced catalytic activity^53–55 and thin-layer diffusion, both of which are facilitated by the mesoporous structure.

AA can interfere with DA detection because they both often coexist in biological samples. Fig. 6 shows DA-detection results acquired in the presence of excess AA. The oxidation peaks of DA and AA overlap when the GCE is used, regardless of electrolyte concentration; consequently, distinguishing DA from AA becomes impossible. In addition, the CV profiles do not change significantly with electrolyte concentration. In contrast, the oxidation currents for DA and AA are clearly distinct in the case of CMK-3, with oxidation peak potentials of 0.021 and 0.237 V recorded for AA for DA, respectively, at higher electrolyte concentrations. The peak potentials for DA and AA acquired in 1 M electrolyte using the mesoporous electrode are consistent with those reported in other studies.^36,37,56 The separation of the overlapping peaks is attributable to bell-shaped CV behavior, which is ascribable to enhanced catalysis at the nanoporous electrode and the lack of reactant species within the nanopores. The CV current intensities did not differ significantly with electrolyte concentration. COOH-CMK-3 exhibited a similar oxidation peak potential to that of CMK-3 at higher electrolyte concentrations. On the other hand, AA oxidation was effectively suppressed by the surface charge effect in dilute electrolytes; consequently, only the DA peak was observed. DA exhibited a significantly higher peak current than AA at high electrolyte concentrations. These findings reveal that COOH-CMK-3 facilitates selective DA detection even in the presence of excess AA, particularly under dilute electrolyte conditions.

Human serum has a basal DA concentration of approximately 1 nM, while it is 10 nM in the brain. In contrast, AA levels in the body range between 0.2 and 0.5 mM.⁵⁷ We used differential pulse voltammetry (DPV) to assess the feasibility of detecting trace amounts of DA and to evaluate the ability of COOH-CMK-3 to simultaneously detect both DA and AA. Phosphate-buffered saline (PBS; 0.137 M NaCl, 0.01 M phosphate buffer, and 0.0027 M KCl) was used to simulate a more realistic biological environment. Experiments using DA were conducted across a range of concentrations, while a constant AA concentration was maintained. Fig. 7A and B reveal that COOH-CMK-3 exhibits a lower AA peak current (at approximately 0 V) than CMK-3. In contrast, COOH-CMK-3 exhibits a higher DA peak current (at approximately 0.2 V) than CMK-3, which is attributable to the surface-charge effect and EDL overlap. The anodic peak currents (j_p) exhibit linear relationships with DA concentration (Fig. 7C and D). A linear calibration curve was obtained for DA in the 5–100 nM concentration range, with a slope of 0.00526 mA nM⁻¹ cm⁻² and a square of correlation coefficient (R²) of 0.9519. A similar curve was obtained for COOH-CMK-3 at concentrations ranging from 0.5 to 100 nM, with a slope of 0.00684 mA nM⁻¹ cm⁻² and R² = 0.97508. Detection (S/N = 3) and quantification (S/N = 10) limits (LODs and LOQs, respectively) of 2.9 and 9.6 nM, respectively, were determined for CMK-3, while COOH-CMK-3 exhibited significantly improved values of 0.72 and 2.4 nM, respectively, which represents a four-fold enhancement. DA detection sensitivity was enhanced in the presence of interfering species such as AA by the introduction of a negative charge onto the CMK-3 surface. The LOD of 0.72 nM is lower than the in vivo basal DA level; hence, COOH-CMK-3 can be used to detect DA with results comparable to those obtained using carbon nanopipettes.⁴¹ Interestingly, the surface charges within the nanopores were not fully screened despite the relatively high electrolyte concentration (approximately 0.1 M in this study) owing to the smallness of the nanopores that enables surface charge effects to influence the results. A higher sensitivity might be observed if the experiment were to be conducted using a more dilute electrolyte. Furthermore, COOH-CMK-3 exhibited a substantially greater relative improvement in sensitivity owing to inherent limitations associated with flat electrodes for enabling simultaneous detection. These findings confirm that COOH-CMK-3 is capable of detecting and analyzing ultra-trace amounts of DA in the presence of interfering species owing to EDL overlap and the surface-charge effect. Furthermore, the linear range and LOD achieved in this study are superior to those reported for other carbon-material-based DA sensors, as summarized in Table 1.

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Sensor stability was evaluated by repeatedly measuring 1 μM DA samples using the same modified electrode over five consecutive days (Fig. 8). The day-to-day DPV responses of the COOH-CMK-3 electrode toward 1 μM DA revealed relative standard deviation (RSD) of 4.034%, indicative of excellent sensor stability.

The negatively charged CMK-3 material was used to detect DA in cell-culture media that more closely resemble real biological samples. The culture medium was spiked with a DA standard solution and then subjected to electrochemical detection. A linear calibration curve was established over the 50–500 nM DA concentration range, with a slope of 0.00026 mA nM⁻¹ cm⁻², an R² value of 0.9886, and a LOD of 4.8 nM and a LOQ of 16 nM. Somewhat lower performance was observed here compared with that observed for PBS owing to interfering substances in the cell culture medium, such as amino acids, vitamins, and D-glucose, in addition to inorganic salts that may foul the electrode surface. Nevertheless, the obtained results are comparable to those reported previously for the detection of nanomolar DA concentrations using high-performance liquid chromatography with electrochemical detection (HPLC-ECD).⁵⁸ Table S3† reveals that spiked recovery values ranged between 96% and 108% (except at 100 nM), further validating electrode reliability. However, surface-modification strategies for mitigating electrode fouling are necessary for use in practical applications involving complex biological matrices (Fig. 9).

Conclusions

We carboxyl-functionalized CMK-3 via persulfate oxidation. FT-IR spectroscopy, BET analyses, and zeta potential measurements confirmed that the CMK-3 surface had been successfully functionalized with carboxyl group, while its mesopores retained their original size. The negatively charged ordered mesoporous carbon delivered a stronger dopamine (DA) signal owing to its positive charge, while suppressing the signal for negatively charged ascorbic acid (AA) through electrostatic interactions; this charge effect was further amplified by EDL overlap. The overlapping DA/AA oxidation-peak issue observed for the flat glassy carbon electrode was effectively addressed using nanoporous electrodes, in which the higher catalytic activity and fewer reactants within the nanopores resulted in thin-layer diffusion that led to bell-shaped cyclic voltammograms. Additionally, differential pulse voltammetry in standard PBS enabled ultra-trace-level DA analysis with a higher DA signal and suppressed AA signal compared to those produced by the nanoporous electrode devoid of surface modification. DA was sensitively detected even in the presence of excess AA, with a detection limit (LOD) of 0.72 nM determined in PBS. This performance is highly impressive for a DA sensor without a specific recognizer. A LOD of 4.8 nM was obtained for a solution with a composition closer to that of an actual sample; this performance is inferior to that observed for PBS and is ascribable to electrode contamination by the matrix. Hence, additional surface modification is required for the analysis of more-complex real samples. These findings show great potential for improving the performance of nanoporous electrode-based sensors in a manner that enables dopamine signals to be tracked in real-time in the human body.

Author contributions

J. Y.: conceptualization, methodology, data acquisition, formal analysis, investigation, validation, writing – original draft, writing – review & editing. H. C. L.: methodology, data acquisition, formal analysis, writing – original draft. H. J. Y.: formal analysis, investigation, validation. S. H.: formal analysis, investigation, validation J. H. B.: conceptualization, writing – original draft, writing – review & editing, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data used in this paper are held by the primary author, who will share the data upon reasonable request.

Acknowledgements

This work was supported by a research fund from Chungnam National University (No. 2023-0613-01). The authors gratefully acknowledge Professor Jae-Young Kim and Mr. Sung-Jin Kim for their assistance in preparing real samples.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5an00579e

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