Ti3C2Tx/laser-induced graphene-based micro-droplet electrochemical sensing platform for rapid and sensitive detection of benomyl.

The design and fabrication of high-performance electrode devices are highly important for the practical application of electrochemical sensors. In this study, flexible three-dimensional porous graphene electrode devices were first facilely fabricated using common laser ablation technique at room temperature. After then, hydrophilic two-dimensional MXene (Ti3C2Tx) nanosheet was decorated on the surface of the laser-induced graphene (LIG), resulting in disposable Ti3C2Tx/LIG electrode devices. After introducing Ti3C2Tx nanosheet, the electrochemical active area, electron transfer ability of LIG electrode device and its adsorption efficiency toward organic pesticide benomyl was significantly boosted. As a result, the fabricated Ti3C2Tx/LIG electrode device exhibited significantly enhanced electrocatalytic activity toward benomyl oxidation. Based on this, a novel and ultra-sensitive electrochemical platform for micro-droplet detection of benomyl was achieved in the range of 10 nM-6000 nM with detection sensitivity of 169.9 µA µM-1 cm-2 and detection limit of 5.8 nM. Considering the low-cost Ti3C2Tx/LIG electrode devices are rarely used for electrochemical analysis, we believed this research work will contribute to exploring the broader application of MXene/LIG electrode devices in the field of electrochemical sensing.

Machine Learning-Assisted Automatically Electrochemical Addressable Cytosensing Arrays for Anticancer Drug Screening.

The high-throughput and accurate screening of anticancer drugs is crucial to the preclinical assessment of candidate drugs and remains challenging. Herein, an automatically electrochemical addressable cytosensor (AEAC) for the efficient screening of anticancer drugs is reported. This sensor consists of sectionalized laser-induced graphene arrays decorated by the rhombohedral TiO2 and spherical Pt nanoparticles (LIG-TiO2-Pt) with high electrocatalytic activity for H2O2 and a homemade Ag/Pt electrode couple fixed onto the robot arm. The immobilization of laminin on the surface of LIG-TiO2-Pt can promote its biocompatibility for the growth and proliferation of various tumor cells, which empowers the in situ monitoring of H2O2 directly released from these live cells for drug screening. A machine learning (ML) algorithm is employed to eliminate the possible random or systematic errors of AEAC, realizing rapid, high-throughput, and accurate prediction of different types of anticancer drugs. This ML-assisted AEAC provides a powerful approach to accelerate the evolution of sensing-served tumor therapy.

Co-based metal-organic frameworks synthesized from poly(ethylene terephthalate) waste plastics for rapid detection of p-phenylenediamine.

The environmental issues and health problems of waste plastics have attracted remarkable attention. It is quite important to convert waste plastics into high value-added electrochemical materials. Herein, four kinds of Co-based metal-organic frameworks (CoMOFs) were synthesized from poly(ethylene terephthalate) plastic, and their electrochemical applications were examined. A mixture of N,N-dimethylformamide (DMF) and H2O was used as the solvent, and hydrothermal reaction was employed. It is found that the surface area and porous structure of CoMOFs are closely related to the volume ratio of DMF/H2O. As a result, the prepared CoMOFs exhibit different catalytic enhancement activities toward the oxidation of p-phenylenediamine (PPD). Based on the solvent-controlled sensing performance of CoMOFs, a highly sensitive and rapid detection method has been developed for PPD, with a linear range from 0.05 to 8.0 µM. The detection limit was 45 nM, and the practical application in hair dye samples was successfully demonstrated.

Cobalt-based conjugated coordination polymers with tunable dimensions for electrochemical sensing of p-nitrophenol.

Using planar π-conjugated 2,5-diamino-1,4-benzenedithiol as organic ligand, Co-based conjugated coordination polymers (CoCCPs) with different morphology were prepared through controlling the injection rate of Co2+. When the injection rate decreases from 1.00 to 0.25 mL min-1, the obtained CoCCPs change from 2D nanosheets to quasi-1D nanorods. It is found that the different-shaped CoCCPs exhibit varying electrochemical sensing performance. The prepared CoCCPs-1 with quasi-1D nanowires and porous network structure possesses larger active area, faster electron transfer and higher accumulation ability. Moreover, the CoCCPs-1 is more active for the oxidation of p-nitrophenol (PNP), and greatly enhances its oxidation signal. Based on the morphology-tuned sensing performance of CoCCPs, a highly-sensitive electrochemical sensor has been developed for PNP, with detection limit of 0.00986 µM (9.86 nM). It was used in the analysis of wastewater samples, and the results is validated by other instrumental method.

Graphene/metal-organic framework nano-sandwiches derived N, P-codoped porous carbon nanosheets as robust material for electrochemical analysis.

Construction of novel two-dimensional porous carbon nanosheets with superior electrochemical activity is of great challenge. Here, graphene/ZIF-8 nano-sandwiches derived N, P-codoped porous carbon nanosheets (N, P-codoped PCN) was easily obtained by sequential room temperature self-assembly and high-temperature carbonization method. Relative to the widely used physically exfoliated graphene nanosheets (GN) and graphene/ZIF-8 derived N-doped porous carbon nanosheets (N-doped PCN), N, P-codoped PCN displayed larger active surface, faster electron transport ability and stronger physical adsorption ability, which can be ascribed to the dual doping effect of heteroatoms N and P. As a result, N, P-codoped PCN exhibited remarkable oxidation signal enhancement for tumor marker (8-hydroxy-2'-deoxyguanosine), analgesic and antipyretic drug (acetaminophen) and organic pesticide (benomyl). Besides, the limits of detection were measured as low as 1.58 nM, 7.50 nM and 2.10 nM with sensitivity of 270.00 µA µM-1 cm-2, 757.14 µA µM-1 cm-2 and 272.86 µA µM-1 cm-2 for 8-hydroxy-2'-deoxyguanosine, acetaminophen and benomyl, respectively. Basing on this, a novel and highly sensitive electrochemical sensing platform was developed. It is believed that the reported two-dimensional N, P-codoped PCN with unique structure and composition is highly valuable for the development of carbon-based electrochemical sensors.

Development of Nafion/single-walled carbon nanotube integrated arrays for the rapid detection of salbutamol doping.

Salbutamol (SAL), a drug originally intended for the treatment of bronchial and pulmonary diseases, has repeatedly been used for doping in competitive sports. Herein, an integrated array (NFCNT array) that prepared by template-assisted scalable filtration using Nafion-coated single-walled carbon nanotube (SWCNT) is presented for the rapid field detection of SAL. Spectroscopic and microscopic measurements were used to confirm the introduction of Nafion onto the surface of the array and to analyze the resulting morphological changes. The effects of Nafion addition on the resistance and electrochemical properties of the arrays (e.g., the electrochemically active area, charge-transfer resistance, and adsorption charge) are also discussed in depth. With an electrolyte/Nafion/SWCNT interface and moderate resistance, the NFCNT-4 array prepared containing 0.04 wt% Nafion suspension exhibits the greatest voltammetric response to SAL. Subsequently, a possible mechanism for the oxidation of SAL was proposed, and a calibration curve in the range of 0.1-15 µM was established. Finally, the NFCNT-4 arrays were applied to the detection of SAL in human urine samples with satisfactory recoveries.

Size-Dependent Electrochemistry of Oxygenated Ti3 C2 Tx MXenes.

2D MXenes are widely proved to be potential electrode materials, although the size effect on their electrochemistry is not fully understood. In this work, Ti3 C2 Tx nanoflakes are prepared through acidic etching of Ti3 AlC2 powders, followed by the intercalation treatment with tetrapropylammonium hydroxide. Such a method produces large-scale delaminated and oxygenated nanoflakes. With aid of centrifugation, the nanoflakes with varied lateral sizes and thicknesses are collected, where electrochemical response of charged redox probes and polar phenol molecules is varied. Density functional theory and energy dispersive spectroscopy confirm such electrochemical response is dependent on the size and thickness of used nanoflakes, more exactly the oxygen content on their surface. Taking the nanoflakes obtained using a centrifugal speed of 5000 rpm (MX-TPA0.2 ) as an example, they feature good dispersibility, a high oxygen content, a small size, and a thin thickness. On these nanoflakes electrochemical response of polar p-substituted phenols is pronounced, stemming from a strong electron-withdrawing interaction of their oxygenated termination with the Ar-OH. A sensitive electrochemical sensor is further constructed for the detection of p-nitrophenol. This work thus provides an approach to synthesize MXenes with different sizes and thicknesses as well as further to reveal size-dependent electrochemistry of MXenes.

Electrochemically functionalized graphene for highly sensitive detection of nitrofurazone.

Graphene nanosheets (GS) were prepared by ultrasonic exfoliation of bulk graphite in N-methyl-2-pyrrolidone with the assistance of sodium pyrophosphate. The obtained GS suspension was modified on a glassy carbon electrode (GS/GCE), and then functionalized at different voltages (e.g. 1.0, 1.4 and 1.6 V) for 2 min in pH 7.0 phosphate buffer. The electrochemically functionalized GS/GCE (i.e. EGS/GCE) possesses more oxygen-containing groups and a higher defect level. More importantly, the active response area, electron transfer ability and interface adsorption capacity of the EGS/GCE enhanced remarkably. The possible mechanism of the performance enhancement is discussed, and the sensing application of the EGS/GCE in the detection of nitrofurazone (NFZ) is investigated. Compared with the GS/GCE, the EGS/GCE is much more active for NFZ oxidation and greatly increases the detection sensitivity. As a result, a highly sensitive electrochemical detection method has been developed for NFZ, with a detection limit of 2.1 nM. The practical application of the EGS/GCE was tested in fish meat samples, showing good accuracy and feasibility.

Metal Centers and Organic Ligands Determine Electrochemistry of Metal-Organic Frameworks.

The properties and applications of metal-organic frameworks (MOFs) can be tuned by their metal centers and organic ligands. To reveal experimentally and theoretically the influence of metal centers and ligands on electrochemical performance of MOFs, three MOFs with copper or zinc centers and organic ligands of 2-methylimidazole (2MI) or 1,3,5-benzenetricarboxylic acid (H3 BTC) are synthesized and characterized in this study. 2D and porous Cu-2MI exhibits a larger active area, faster electron transfer capability, and stronger adsorption capacity than bulk Cu-BTC and dodecahedron Zn-2MI. Density functional theory calculations of adsorption ability of three MOFs toward xanthine (XA), hypoxanthine (HXA), and malachite green (MG) prove that 2D Cu-2MI has the strongest adsorption energies to three targets. Rotating disk electrode measurements reveal that 2D Cu-2MI features the biggest intrinsic heterogeneous rate constant toward three analytes. On 2D Cu-2MI sensitive and selective monitoring of XA, HXA, and MG is then achieved using differential pulse voltammetry. Their monitoring in real samples on 2D Cu-2MI is accurate and comparable with that using high-performance liquid chromatography. In summary, regulation of electrochemical sensing features of MOFs is realized through defining selected metal centers and organic ligands.

Two-Dimensional Red Phosphorus Nanosheets: Morphology Tuning and Electrochemical Sensing of Aromatic Amines.

Synthesis of 2D materials with different morphologies is of significance to reveal their morphology-dependent properties and further explore their morphology-dependent applications. This work reports the synthesis of 2D red phosphorus nanosheets (RPNSs) with different thicknesses by means of a phosphorus-amine method together with regulated electrophilicity of the solution. With graphene as the support, the RPNSs produced in 0.01 m HCl feature a unique 2D nanostructure, uniform distribution, and excellent electrochemical performance. Originating from the reacted characteristics between phosphorus and amine, the interaction of aniline with the RPNSs is clarified theoretically and experimentally by means of density functional theory, X-ray photoelectron spectroscopy, and voltammetry. A covalent bond is confirmed to be formed during the deprotonation of aniline and its binding energy reaches -2.31 eV. Stemming from such an adsorption process, different aromatic amines (e.g., p-nitroaniline, p-phenylenediamine) are sensitively and selectively monitored on the RPNSs. Consequently, this work provides a new way to clarify morphology-dependent properties and applications of novel 2D materials.

Simultaneous detection of 4-chlorophenol and 4-nitrophenol using a Ti3C2Tx MXene based electrochemical sensor.

Developing a sensitive and rapid detection method for 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) is very important due to their high toxicity. In this work, bulk Ti3AlC2 powder was etched to Ti3C2Tx for the first time through a hydrothermal reaction in NaF/HCl solution. After ultrasonication in N-methylpyrrolidone (NMP), Ti3C2Tx powder was successfully exfoliated into multilayered Ti3C2Tx nanosheets (i.e. Ti3C2Tx MXene). The prepared Ti3C2Tx MXene not only has a large electrochemical surface area for the oxidation of 4-CP and 4-NP, but also lowers their electron transfer resistance. As a result, the oxidation signals of 4-CP and 4-NP are significantly improved on the surface of the Ti3C2Tx MXene. Based on the remarkable signal amplification of the Ti3C2Tx MXene, a sensitive and rapid method was developed for the simultaneous detection of 4-CP and 4-NP. The linear range is from 0.1 to 20.0 µM for 4-CP, and from 0.5 to 25.0 µM for 4-NP, with detection limits of 0.062 µM (4-CP) and 0.11 µM (4-NP). This method was used in wastewater samples, and the accuracy was confirmed to be good by high-performance liquid chromatography.

Monodispersed Ni active sites anchored on N-doped porous carbon nanosheets as high-efficiency electrocatalyst for hydrogen peroxide sensing.

Metal active species combined with N-doped porous carbon nanosheets usually own excellent electrochemical activity and sensing performance owing to its unique microstructure and composition. In this work, monodispersed Ni active sites anchored on N-doped porous carbon nanosheets (Ni@N-PCN) were facilely prepared via rational metal-organic frameworks (MOFs) route. Firstly, zeolitic imidazolate frameworks-8 (ZIF-8) was in situ grown on physically-exfoliated graphene nanosheets (GN) with homogeneous sandwich-like structure (ZIF-8@GN). Secondly, nickel bonded ZIF-8@GN hybrids (Ni/ZIF-8@GN) were obtained by ionic exchange reaction, and then transformed into Ni@N-PCN by high-temperature pyrolysis. Benefiting from the monodispersed Ni active sites and highly reactive N-doped porous carbon nanosheets (N-PCN), the as-prepared Ni@N-PCN hybrids displayed superior catalytic performance toward hydrogen peroxide (H2O2) sensing. As a result, a highly sensitive electrochemical sensing platform for H2O2 was fabricated with low detection limit (0.032 µM), wide detection linearity (0.2-2332.8 µM), and high sensitivity (6085 µA cm-2 mM-1). Besides, the as-developed electrochemical sensing platform was successfully applied to detect H2O2 contents in biological medicine and food specimens with satisfied results. This study will provide effective guidance for the preparation of novel metal/N-doped carbon nanomaterials and establishment of high-performance electrochemical sensors.

Polyvinylpyrrolidone-assisted solvent exfoliation of black phosphorus nanosheets and electrochemical sensing of p-nitrophenol.

Few-layer black phosphorus (BP) has been considered as a rising star of 2D materials, and however, the poor stability heavily limits its application in electrochemical sensing. In this work, a series of BP nanosheets (BPNS) are simply prepared through ultrasonic exfoliation of bulk BP with the assistance of polyvinylpyrrolidone (PVP) in different solvents, including isopropanol (IPA), ethanol (EtOH), N-methyl pyrrolidone (NMP) and dimethylformamide (DMF). It is found that the exfoliation efficiency in IPA and EtOH is much higher than that in DMF and NMP, and moreover, IPA is superior than EtOH. More importantly, PVP effectively improves the exfoliation efficiency and ambient ability of BPNS via forming a protective layer to lower the oxidation of phosphorus. The exfoliated BPNS in the presence of IPA and PVP (PVP@BPNS) possess larger active response area, higher electron-transfer rate and stronger enhancement effects toward the oxidation of p-nitrophenol (4-NP). As a result, the response signal and sensing sensitivity of 4-NP are remarkably improved, and a novel electrochemical method has been developed for 4-NP detection, with a linear range of 0.10-5.0 µM and a detection limit of 28 nM. It is used to measure 4-NP in wastewater samples, and the results are validated by high-performance liquid chromatography.

Cu-BTC frameworks based electrochemical sensor for hazardous malachite green in aquaculture.

Malachite green (MG) has been widely used for controlling external fungi and parasites in the aquaculture. However, MG has been proven to be very hazardous, and the detection of MG in aquaculture environment is crucial for determining whether MG has been used within the allowed limit and for protecting the environment. Herein, a kind of copper based metal-organic frameworks (MOFs) was prepared using copper nitrate and 1,3,5-benzenetricarboxylic acid (H3BTC) as raw materials. The prepared Cu-BTC materials provide larger active area and higher accumulation capacity for MG, and meanwhile lower the charge-transfer resistance. As a result, the oxidation signal and detection sensitivity of MG are significantly improved by Cu-BTC frameworks. The linear range is 2-500 nM, and the detection limit is 0.67 nM, which is much lower than the reported values. Moreover, other usually-used aquaculture drugs have no interferences, including erythromycin, chloramphenicol, oxytetracycline, furazolidone and nitrofurazone. This method was applied in the water samples, and the results were consistent with those using high-performance liquid chromatography.

Poly(sulfosalicylic acid)-functionalized gold nanoparticles for the detection of tetrabromobisphenol A at pM concentrations.

Developing a sensitive, simple and fast sensing system for 3,3',5,5'-tetrabromobisphenol A (TBBPA) is important because of its ubiquitousness and high toxicity. In this work, a gold nanoparticles (AuNPs) and poly(sulfosalicylic acid) (PSSA) composite film (AuNPs-PSSA) is fabricated in-situ on an electrode surface via cyclic voltammetry scanning. The characterization via scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) spectroscopy indicate that the PSSA film is homogeneously decorated with AuNPs, and a highly uniform and thin composite film is obtained. Electrochemical tests reveal that the AuNPs-PSSA film exhibits larger active surface area, lower charge transfer resistance and higher accumulation efficiency toward TBBPA than single AuNPs and PSSA film. As a result, the oxidation signals and sensing sensitivity of TBBPA are significantly enhanced on the surface of the AuNPs-PSSA. The developed TBBPA sensing platform using AuNPs-PSSA composite film, with low detection limit (25 pM) and wide linear range (0.1-10 nM), is successfully utilized to measure TBBPA level in wastewater samples. The results are highly consistent with those that obtained from high-performance liquid chromatography. The preparation and reusability of the TBBPA sensor can be automatically achieved through CV scanning, providing a promising on-line monitoring system for wastewater samples.

Triethylamine-controlled Cu-BTC frameworks for electrochemical sensing fish freshness.

The simultaneous determination of xanthine (XA) and hypoxanthine (HXA) has been proved to be a feasible approach for the assessment of fish freshness. In this study, copper(II) nitrate and 1,3,5-benzenetricarboxylic acid (H3BTC) were used as precursors to prepare various Cu-BTC frameworks with the addition of various amounts of triethylamine at room temperature. The characterization of X-ray diffraction, Fourier-transform infrared spectroscopy and Raman spectroscopy testified that the obtained materials are Cu-BTC frameworks. However, the amount of triethylamine had significant effects on the morphology, active response area and electron transfer ability of Cu-BTC frameworks. The oxidation behavior of XA and HXA demonstrated that the prepared Cu-BTC frameworks exhibited higher sensing activity, with greatly-enhanced oxidation signals. More importantly, the amount of triethylamine obviously affected the accumulation capacity and signal enhancement ability of Cu-BTCs toward XA and HXA, as confirmed from double potential step chronocoulometry. Based on the triethylamine-tuned signal amplification strategy of Cu-BTC frameworks, a highly-sensitive and simple electrochemical sensing system was developed for the assessment of fish freshness by simultaneous detection of XA and HXA. The developed sensing method was used in practical samples, and the results were validated by high-performance liquid chromatography.

Strategy for Highly Sensitive Electrochemical Sensing: In Situ Coupling of a Metal-Organic Framework with Ball-Mill-Exfoliated Graphene.

A highly sensitive electrochemical sensing system is developed via in situ integration of Cu-based metal-organic frameworks (Cu-BTC, BTC = 1,3,5-benzenetricarboxylic acid) and high-conductivity ball-mill-exfoliated graphene (Cu-BTC@GS) by a simple method. The as-synthesized Cu-BTC@GS hybrids display remarkably enhanced electrochemical activity due to the synergistic effect resulting from the integration. Compared to those of the pristine GS, the introduction of Cu-BTC nanoparticles leads to significant improvement in the surface area and porosity, as revealed by the nitrogen adsorption-desorption analysis. In addition, the oxidation behavior of nicotinamide adenine dinucleotide studied using the rotating ring disk electrode further reveals a superior electron-transfer rate constant ( k) for the composite, indicating higher catalytic ability. Moreover, double potential step chronocoulometry of biomolecules (xanthine and hypoxanthine) and phenolic pollutants (bisphenol A and p-chlorophenol) reveals that the prepared composite possesses greatly enhanced adsorption properties, resulting in much higher response signals and detection sensitivity. Benefiting from the superior reactivity, a highly sensitive electrochemical sensing platform for wide targets is successfully fabricated. It was used in the analysis of plasma, urine, and receipt and wastewater samples, and the results were highly consistent with those obtained by high-performance liquid chromatography. We believe that this study provides an effective strategy for the construction of high-performance electrochemical sensing systems.

Electrochemical sensing performance of Eu-BTC and Er-BTC frameworks toward Sunset Yellow.

Two lanthanide metal-organic frameworks (Ln-MOFs) using 1,3,5-benzenetricarboxylic acid (H3BTC) and Ln(III) nitrate hydrate (Ln = Eu, Er) as precursors have been prepared through a one-step solvothermal approach. The as-synthesized compounds were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy, confirming that the targeted Ln-MOFs were successfully synthesised, with a straw-sheaf appearance. Their sensing properties were studied using cyclic voltammetry and electrochemical impedance spectroscopy, and the results showed that the prepared Ln-BTCs possessed larger electrochemical-response areas and stronger electron-transport capability, especially the Er-BTC. The oxidation behaviour of Sunset Yellow was investigated, and it was found that the Er-BTC exhibited a superior enhancement effect. Chronocoulometry was used to prove that Er-BTC more strongly adsorbs Sunset Yellow, resulting in a larger electrochemical response signal. The novel Sunset Yellow sensor exhibited a linear response in the range of 0.2-100 nM with a low limit of detection (0.05 nM, S/N = 3), and was applied in drink samples. The results were validated by high-performance liquid chromatography, indicating this sensor is a promising tool for Sunset Yellow detection.

Ball-Mill-Exfoliated Graphene: Tunable Electrochemistry and Phenol Sensing.

A simple wet ball-milling method for exfoliating pristine graphite to graphene nanosheets is proposed. The surfactant of cetyltrimethyl ammonium bromide is utilized to greatly improve the exfoliation efficiency of graphene nanosheets. Variation of the ball-milling time is an efficient way to control the size and thickness of graphene nanosheets, as well as the level of edge defects. With an increase of ball-milling time, superior electrochemical reactivity is imparted owing to enlarged active area and increased catalytic ability. The obtained graphene nanosheets are sensitive for electrochemical oxidation of phenols (e.g., hydroquinone, p-chlorophenol, and p-nitrophenol), and thus qualified for the simultaneous sensing of trace level of phenols. The detection limits of simultaneous monitoring of hydroquinone, p-chlorophenol, and p-nitrophenol are as low as 0.017, 0.024, and 0.42 mg L-1 , respectively. The proposed strategy thus opens up a new way to tune electrochemistry of graphene materials as well as to design their new applications.

Morphology-dependent electrochemical sensing performance of metal (Ni, Co, Zn)-organic frameworks.

To clarify the morphology effect of metal-organic frameworks (MOFs) on their electrochemistry as well as to explore their electrochemical applications, three MOFs with metal centers of nickel, cobalt, and zinc are synthesized. The used organic ligand is only 1,3,5-benzenetricarboxylic acid. Characterizations using Fourier Transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and electrochemical techniques reveal that these MOFs possess similar bonding properties, crystalline structures and phase purity, but various morphology and electrochemical activities, including their own redox behavior, electrochemical response toward redox probes and analytes in solutions. As a case study of analytes, voltammetry of Ponceau 4R is investigated on three MOFs. Its sensitive and selective monitoring is further achieved on nickel MOFs with a linear range from 0.5 to 150 nM and a detection limit of 80 pM. Therefore, the morphology of MOFs determines electrochemistry of MOFs and their electrochemical sensing applications.