Published papers

Green synthesis approaches for making nanosized ceria using starch from cassava as template molecules to control the particle size are reported. The results of the green synthesis of ceria with an optimum calcination temperature of 800 °C shows a size distribution of each particle of less than 30 nm with an average size of 9.68 nm, while the ratio of Ce3+ to Ce4+ was 25.6%. The green-synthesized nanoceria are applied to increase the sensitivity and attach biomolecules to the electrode surface of the electrochemical aptasensor system for coronavirus disease (COVID-19). The response of the aptasensor to the receptor binding domain of the virus was determined with the potassium ferricyanide redox system. The screen-printed carbon electrode that has been modified with green-synthesized nanoceria shows 1.43 times higher conductivity than the bare electrode, while those modified with commercial ceria increase only 1.18 times. Using an optimized parameter for preparing the aptasensors, the detection and quantification limits were 1.94 and 5.87 ng·mL-1, and the accuracy and precision values were 98.5 and 89.1%. These results show that green-synthesized ceria could be a promising approach for fabricating an electrochemical aptasensor.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), or coronavirus disease 2019 (COVID-19), is still spreading worldwide; therefore, the need for rapid and accurate detection methods remains relevant to maintain the spread of this infectious disease. Electrochemical immunosensors are an alternative method for the rapid detection of the SARS-CoV-2 virus. Herein, we report the development of a screen-printed carbon electrode immunosensor using a hydroxyapatite-gold nanocomposite (SPCE/HA-Au) directly spray-coated with the immobilization receptor binding domain (RBD) Spike to increase the conductivity and surface electrode area. The HA-Au composite synthesis was optimized using the Box−Behnken method, and the resulting composite was characterized by UV−vis spectrophotometry, TEM-EDX, and XRD analysis. The specific interaction of RBD Spike with immunoglobulin G (IgG) antibodies was evaluated by differential pulse voltammetry and electrochemical impedance spectroscopy methods in a [Fe(CN)6]4−/3− solution redox system. The IgG was detected with a detection limit of 0.0561 pg mL−1, and the immunosensor had selectivity and stability of 103−122% and was stable until week 7 with the influence of storage conditions. Also, the immunosensor was tested using real samples from human serum, where the results were confirmed using the chemiluminescent microparticle immunoassay (CMIA) method and showed satisfactory results. Therefore, the developed electrochemical immunosensor can rapidly and accurately detect SARS-CoV-2 antibodies.

Free chlorine is widely used to disinfect water used for drinking and food processing. This requires highly sensitive, simple, and capable continuous-measurement sensors to enable the concentration of free chlorine in water to be monitored and controlled. Free chlorine sensors based on solution-gated graphene field-effect transistors (GFETs) are a suitable platform for highly sensitive and continuous measurements. However, their sensing mechanisms require further elucidation to improve their performance. In this study, we focused on the gate electrode and investigated its influence on the sensing performance. Using the free chlorine sensor based on the solution-gate GFET, we showed that the Dirac point voltage in the transfer curve changed significantly as the free chlorine concentration changed, and the electric double-layer capacitance of the gate electrode decreased. Furthermore, we demonstrated that a solution-gated GFET using graphene or boron-doped diamond as the gate electrode could be used to detect changes in the free chlorine concentration in the concentration range of tap water. The sensing performance in the low concentration range benefits from the wide potential window of carbon-based electrodes, which do not have electrochemically active sites. Using these carbon-based materials as gate electrodes, GFETs have the potential to be used as durable sensors that are resistant to surface fouling and oxidation.

The lipid bilayer membrane of microorganisms exhibits capacitance that can be used to assess the proliferation, reduction, or transformation of these microorganisms by evaluating the permittivity of their lipid bilay-er membrane. Traditionally, the effects of pharmacological substances on viruses have been measured indirectly by observing cytopathic effects on cultured cells. Nevertheless, the novel approach that depends on permittivity is antic-ipated to provide an innovative assay for preliminary screening of potential drugs against microorganisms. In an ef-fort to quantify directly the pharmacological effects of compounds on microorganisms, we devised a method that avoids the indirect pharmacological effects on cultured cells. Instead, this method measures the direct effect by as-sessing the permittivity induced by the lipid bilayer membrane itself. To validate the effectiveness of our approach, and to demonstrate the feasibility of permittivity measurements in microorganisms, we designed an electrode-re-placeable measuring cell using 3D printing technology. Using this measuring cell, we measured permittivity of egg phosphatidylcholine liposomes and yeast cells, both of which exhibit lipid bilayers and are suitable for experimental purposes. We measured the permittivity of a KCl solution, as well as suspension of liposomes or yeast cells in KCl solution. We observed an increase in permittivity within a frequency range of 104–108 Hz, which can be ascribed to the lipid bilayers of the liposomes and yeast cells. Following lysis of liposomes by adding Triton-X 100, we observed a reduction in the enhanced permittivity. Furthermore, when the yeast cell count was reduced by dilution, there was a corresponding decrease in the enhanced permittivity. A decrease in permittivity was also noted in a suspension of yeast cells in KCl solution following heat and enzyme treatments. These results suggest that the permittivity of lipid bilayer membrane can be measured and harnessed to estimate the concentration of microorganisms in a solution. Consequently, we anticipate that this method will be developed as a pioneering assay for early drug screening, given its capacity to evaluate the pharmacological effects of compounds on viruses, without the necessity for cultured cells.

A luminol electrochemiluminescence (ECL)-based sensor for reactive oxygen species (ROS), including H2O2 and hypochlorite was developed using gold particles-modified on polyaniline‑boron doped diamond electrode (Au-PANI/BDD). The gold particles were incorporated onto the surface of boron-doped diamond (BDD) electrode to enhance the electrochemiluminescence of luminol, while the polyaniline was used to provide better attachment of the gold particles on the surface of BDD electrode. The uniform distribution of gold particles on the PANI/BDD surface was verified through FE-SEM EDX analysis with an approximately weight percentage of 45.6 % (w/w). The modified electrode exhibited a remarkable five-fold increase in ECL intensity compared to the unmodified BDD, resulting in higher sensitivity. Furthermore, the sensor's performance was successfully demonstrated for H2O2 concentrations ranged linearly from 1 to 100 μM (R2 ≥ 0.99) with a low detection limit of 0.81 μM, and for hypochlorite concentrations from 1 to 20 μM (R2 ≥ 0.99) with a detection limit as low as 0.51 μM. Additionally, excellent repeatability and stability of the signals was also observed over 7 days. Furthermore, successful application in detecting H2O2 and hypochlorite in real sample highlights its promising potential as an ROS sensor.

Screen-printed carbon electrodes (SPCEs) were modified with gold particles to improve luminol electrochemiluminescence (ECL) in the presence of H2O2 as the co-reactant. Gold particles with sizes ranging from 10 to 50 nm were successfully deposited by the electrochemical technique with around 3.4% (w/w) gold on the carbon surface. The gold-modified SPCE (Au-SPCE) increases the ECL intensity of luminol around 2.5 times more than that using the unmodified SPCE. These signals were also found to be linearly correlated to H2O2 concentration. At an optimum pH of 9, the ECL signals of 1 mM luminol in 0.1 M phosphate buffer solution using the Au-SPCE are linear in the H2O2 concentration range from 0.5 to 200 µM (R2 = 0.99) with an estimated detection limit of 4.78 µM. The developed sensor also showed the excellent repeatability and reproducibility of the ECL signals. Moreover, H2O2 detection in milk and tap water samples was also successfully demonstrated, indicating that the developed sensor is promising for H2O2 detection applications.

In this study, molybdenum-modified boron-doped diamond (MoBDD) was prepared by electrodeposition from a molybdate precursor, and the molybdenum oxide valence state was investigated by XPS analysis. The molybdenum oxide was used as an electrocatalyst in electrochemical nitrogen reduction for ammonia synthesis at room temperature and atmospheric pressure. MoBDD could produce ammonia at 1.26 nmol h−1 cm−2 with a faradaic efficiency of 9.15 % which was higher than a pure Mo plate set as a comparison. The XRD spectroscopy analysis suggests that high state Mo oxides are responsible for the electrocatalytic activity.

Boron-doped diamond (BDD) electrodes have achieved excellent selectivity and switchable product formation for electrochemical CO2 reduction reaction (eCO2RR), although the current should be improved. Surface morphology modulation is a promising approach for improving it by exposing a larger electroactive area. Herein, we fabricated a porous 1 % BDD film (p-BDD) and a rough-surfaced 1 % BDD film (r-BDD) by modifying the surface of a flat 1 % BDD film (f-BDD). Fundamental electrochemical properties, including potential window, differential capacitance, and active area were examined, followed by eCO2RR activity evaluation. The synthetic r-BDD electrode enabled approximately 1.7-fold increases in current density and yield rate (17.6 μmol h−1) toward CO production compared to f-BDD (10.3 μmol h−1). This improvement in eCO2RR is ascribed to the larger electroactive area with a comparable kinetic performance, in comparison to f-BDD. On the other hand, p-BDD possesses the largest real area, but showed the lowest activity for eCO2RR (1.2 μmol h−1 for CO), which could result from hindered diffusion of CO2 inside the pores. X-rays photoelectron spectroscopy and electrochemical impedance spectroscopy were employed to disentangle the morphological structure effect from the chemical reactivity of the electrode surface.

The primary treatment for glaucoma, the most common cause of intermediate vision impairment, involves administering ocular hypotensive drugs in the form of topical eye drops. Observing real-time changes in the drugs that pass through the cornea and reach the anterior chamber of the eye is crucial for improving and developing safe, reliable, and effective medical treatments. Traditional methods for measuring temporal changes in drug concentrations in the aqueous humor employ separation analyzers such as LC-MS/MS. However, this technique requires multiple measurements on the eyes of various test subjects to track changes over time with a high temporal resolution. To address this issue, we have developed a measurement method that employs boron-doped diamond (BDD) microelectrodes to monitor real-time drug concentrations in the anterior chamber of the eye. First, we confirmed the electrochemical reactivity of 13 antiglaucoma drugs in a phosphate buffer solution with a pH of 7.4. Next, we optimized the method for continuous measurement of timolol maleate (TIM), a sympathetic beta-receptor antagonist, and generated calibration curves for each BDD microelectrode using aqueous humor collected from enucleated porcine eyes. We successfully demonstrated the continuous ex vivo monitoring of TIM concentrations in the anterior chambers of these enucleated porcine eyes. The results indicate that changes in intracameral TIM concentrations can be monitored through electrochemical measurements using BDD microelectrodes. This technique holds promise for future advancements in optimizing glaucoma treatment and drug administration strategies.

Although oxidation of benzylic C(sp3)−H bonds is an important process for synthesizing biologically active compounds, the reactions generically require hazardous reagents, catalysts, and harsh conditions. Since electro-organic synthesis utilizes electricity itself as a reagent, reactions can proceed well even under ambient conditions. Herein, we report a selective oxidation of cumene in an electrochemical flow reactor. Cumyl alcohol was selectively obtained under optimum amount of charge and current density. The selective oxidation of cumene to cumyl alcohol was achieved because overoxidation of cumyl alcohol into acetophenone was suppressed by the electrochemical flow reactor.

Boron-doped diamond (BDD) has tremendous potential for use as an electrode material with outstanding characteristics. The substrate material of BDD can affect the electrochemical properties of BDD electrodes due to the different junction structures of BDD and the substrate materials. However, the BDD/substrate interfacial properties have not been clarified. In this study, the electrochemical behavior of BDD electrodes with different boron-doping levels (0.1% and 1.0% B/C ratios) synthesized on Si, W, Nb, and Mo substrates was investigated. Potential band diagrams of the BDD/substrate interface were proposed to explain different junction structures and electrochemical behaviors. Oxygen-terminated BDD with moderate boron-doping levels exhibited sluggish electron transfer induced by the large capacitance generated at the BDD/Si interface. These findings provide a fundamental understanding of diamond electrochemistry and insight into the selection of suitable substrate materials for practical applications of BDD electrodes.

Due to its unmatched corrosion stability and overall versatility, boron-doped diamond (BDD) has superb potential as an electrode material for the production of valuable chemicals via electrochemical reduction of carbon dioxide (CO2). To advance its practical application, it is necessary to explore ways to achieve cost-effective fabrication of BDD electrodes, which can enable significant selectivity for product formation during CO2 electroreduction. Here, BDD electrodes of nanocrystalline form were fabricated employing two distinct chemical vapor deposition (CVD) techniques, which allow for substantial upscaling of electrode dimensions to match industrial/wafer-size standards. Thin (<400 nm) nanocrystalline BDD layers were fabricated at various synthesis temperatures to investigate its effect on the electrical, structural, and electrochemical characteristics. Furthermore, modification of the BDD layer crystallinity and morphology has been performed to significantly alter the overall electrode structure and sp3/sp2-carbon phase-composition. Finally, when employed as cathodes in electrochemical CO2 reduction, nanocrystalline BDD electrodes of modified structure demonstrated enhanced values of faradaic efficiency (FE) towards production of formic acid (HCOOH), surpassing FEHCOOH values observed for microcrystalline form of BDD cathodes. Reported findings present a basis for further development of BDD cathodes, including their design, structure, and composition, which leverages the advantages of scalable and cost-effective synthesis of BDD electrodes.

This study investigates the effects of boron concentration on boron-doped diamond (BDD) electrodes for electrochemical sensors of ciprofloxacin. The effects of boron concentration, scan rate, and pH of BDD electrodes with boron concentrations of 0.1, 0.5, and 1% were examined to determine the optimal conditions. Furthermore, square wave voltammetry (SWV) in phosphate buffer pH 7 was used to analyze the electrochemical behavior of ciprofloxacin. The results revealed a linear calibration curve in the concentration range of 30–100 μM with a recovery of 85–110%. Meanwhile, BDD electrode with the highest boron concentration in this experiment (1%) showed a very low limit of detection of 0.17 μM, meaning that 1% BDD gave a highly sensitive and significant measurement result for the electrochemical sensor of ciprofloxacin. With the results given, this study provides new insights for controlling boron concentrations in diamond electrodes for the electrochemical sensors of quinolone antibiotics.

Urinalysis is attracting interest in personal healthcare management as part of a general move to improve quality of life. Urine contains various metabolites and the protein level in urine is an indicator of kidney function. In this study, a novel electrochemical sensing system based on boron-doped diamond (BDD) electrodes was developed for the detection of protein concentrations in human urine. BDD electrodes have the advantages of a wide electrochemical potential window and low non-specific adsorption, making them ideal for simple, rapid, and compact devices for home detection of bio-relevant substances. Coomassie brilliant blue (CBB), a dye that selectively and strongly binds to urine proteins, was found to be a redox-active indicator to show a decrease in its redox currents in relation to the concentration of protein in urine samples. Our detailed studies of BDD electrodes showed their limit of detection to be 2.57 μg mL−1 and that they have a linear response that ranges from 0 to 400 μg mL−1 in urine samples. We also investigated the detection of urine protein in different urine samples. Our results agreed with those obtained using conventional colorimetric analysis. We believe this to be the first study of electrochemical detection of urine protein in urine samples on BDD electrodes, which is of great significance to be able to obtain results with electrical signals rapidly compared to conventional colorimetric analysis. This CBB-BDD technique has the potential to assist healthcare management in the form of a rapid daily diagnostic test to judge whether a more detailed examination is needed.

The electrochemical reaction of carbon dioxide (CO2) in aqueous electrolyte solutions is attracting increasing attention for sustainable chemical production. Boron-doped diamond (BDD) electrodes have been previously shown to be very effective for the stable electrochemical production of formic acid from CO2. Typically, the electrochemical production of formic acid by CO2 reduction (CO2R) reaction is performed with a dual-compartment flow reactor equipped with a membrane separator. The problems caused by the membrane separator, such as scaling-up, complicated operational control and materials costs can be solved using a membrane free single-compartment reactor. Here we demonstrate anode reaction control for a single-compartment CO2R flow reactor using a surface-activated BDD cathode and achieve a Faradaic efficiency for formic acid production of over 70 %.

We investigated the electrochemical CO2 reduction reaction (CO2RR) properties of boron- and nitrogen-codoped diamond (BNDD) electrodes. The BNDD electrodes exhibited higher CO2RR performance than boron-doped diamond (BDD) electrodes, indicating that the nitrogen doping can promote CO2RR reactivity. In particular, the faradaic efficiencies of the CO2RR products, obtained at a relatively low negative potential (−2.0 V vs Ag/AgCl) in this study, were significantly higher using BNDD electrodes with high nitrogen-doping levels. At this potential, it should be difficult to generate CO2•- as an initial intermediate of CO2RR; therefore, nitrogen doping promoted the generation of the intermediate through another reaction pathway. Linear sweep voltammetry revealed that nitrogen doping could promote the generation of adsorbed hydrogen atoms on the electrode surface by proton reduction. Thus, we concluded that nitrogen doping promoted the initial step of CO2RR through the adsorption of hydrogen atoms on the electrode surface. In addition, electrolysis using various electrolyte solutions confirmed that the influence of dopants on the BNDD electrode surface was significant for CO2RR, particularly at low overpotentials.

We report Cu, Fe, N-doped carbon nanotubes, (Cu,Fe)−N−CNT, as electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. (Cu,Fe)−N−CNT was prepared using a silica coating method in pyrolysis to minimize the formation of carbon-coated metal oxide or carbide nanoparticles, which are known to be inactive for the H2O2 reduction. (Cu,Fe)−N−CNT shows a turnover frequency of 0.66 e− site−1 s−1 at +0.8 V vs. RHE and H2O2 yields of <1 % for the ORR with a utilization factor of active sites of 82 %. Kinetic analysis reveals that 4e− transfer rates for (Cu,Fe)−N−CNT are higher than those of a monometallic counterpart of Fe−N−CNT. In situ X-ray absorption spectroscopy enables us to determine redox potentials: E°’(FeIII/FeII)=0.65 V vs. RHE and E°’(CuII/CuI)=0.45 V for (Cu,Fe)−N−CNT, and E°’(FeIII/FeII)=0.65 V for Fe−N−CNT. These results indicate that bimetallic doping into carbon nanotubes gives the effect on kinetic parameters but not on thermodynamic ones. In other words, there is no direct electronic interactions between the Cu and Fe active sites for (Cu,Fe)-N-CNT because such interactions should modulate their redox potentials.

Drug detection in biological solutions is essential in studying the pharmacokinetics of the body. Electrochemical detection is an accurate and rapid method, but measuring multiple drugs that react at similar potentials is challenging. Herein, we developed an electrochemical sensor using a boron-doped diamond (BDD) electrode modified with a molecularly imprinted polymer (MIP) to provide specificity in drug sensing. The MIP is a polymer material designed to recognize and capture template molecules, enabling the selective detection of target molecules. In this study, we selected the anticancer drug doxorubicin (DOX) as the template molecule. In the electrochemical measurements using an unmodified BDD, the DOX reduction was observed at approximately −0.5 V (vs Ag/AgCl). Other drugs, i.e., mitomycin C or clonazepam (CZP), also underwent a reduction reaction at a similar potential to that of DOX, when using the unmodified BDD, which rendered the accurate quantification of DOX in a mixture challenging. Similar measurements conducted in PBS using the MIP-BDD only resulted in a DOX reduction current, with no reduction reaction observed in the presence of mitomycin C and CZP. These results suggest that the MIP, whose template molecule is DOX, inhibits the reduction of other drugs on the electrode surface. Selective DOX measurement using the MIP-BDD was also possible in human plasma, and the respective limits of detection of DOX in PBS and human plasma were 32.10 and 16.61 nM. The MIP-BDD was durable for use in six repeated measurements, and MIP-BDD may be applicable as an electrochemical sensor for application in therapeutic drug monitoring.

The effect of different substrate materials in the fabrication of an electrode intended for use in the water oxidation reaction is investigated. The electrocatalyst is nickel-iron oxide (NiFeOx) which is deposited by chronoamperometry on nickel, iron, titanium, and stainless steel substrates. The process of electrodeposition is optimized to achieve the lowest overpotential for the water oxidation reaction. The four electrodes are characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and glow-discharge optical emission spectroscopy (GDOES) to describe the effect of the substrate on the nature of the electrocatalyst layer. Electrochemical tests in 1 m NaOH are applied on the four electrodes to assess the stability and the retention of the electrocatalytic properties of the whole electrode. All electrodes show similar overpotential of ≈0.3 V at 10 mA cm−2 implying that the substrate do not affect the electrocatalytic activity of the NiFeOx. After use, the overpotential increases in a range of 30–50 mV for nickel, iron, and titanium substrates, while stainless steel retains the lowest overpotential with an increase of 10 mV. This limited variation can be the effect of smaller NiFeOx nanoparticles compared to other substrates. XPS analysis reveals that after galvanostatic electrolysis, the oxidation state of Fe shifts slightly from Fe(II) to Fe(III), likely Fe3O4 and Fe2O3, while NiO/Ni2O3 change partially to Ni(OH)2.

The electrochemical CO2 reduction reaction (CO2RR) properties of boron-doped diamond powder (BDDP) were investigated. A BDDP-painted electrode was prepared by casting BDDP ink on the surface of a BDD thin-film electrode as a current collector. The CO2RR properties of the BDDP-painted electrode were compared to those of conventional BDD thin-film electrodes, which were prepared directly on a conductive silicon wafer substrate by the microwave plasma-assisted chemical vapor deposition method. The result of the electrolysis at −1.8 V vs. Ag/AgCl in 0.5 M KCl catholyte showed that the BDDP-painted electrode was superior to the BDD thin-film electrode because it produced a large amount of products (formic acid and carbon monoxide) at a low overpotential. The highest performance of the CO2RR in the BDDP-painted electrode was achieved with the BDDP with high boron doping and high sp2 carbon content. We considered that structural defects, boron doping level, and sp2 carbon content influenced the reactivity for CO2 reduction. Therefore, the BDDP-painted electrodes should be useful not only for the production of a large-size electrode but also for efficient CO2RR.