Functional nanostructure-loaded three-dimensional graphene foam as a non-enzymatic electrochemical sensor for reagentless glucose detection

Emerging Trends in Nanomedicine: Carbon-Based Nanomaterials for Healthcare

Carbon-based nanomaterials, such as carbon quantum dots (CQDs) and carbon 2D nanosheets (graphene, graphene oxide, and graphdiyne), have shown remarkable potential in various biological applications. CQDs offer tunable photoluminescence and excellent biocompatibility, making them suitable for bioimaging, drug delivery, biosensing, and photodynamic therapy. Additionally, CQDs' unique properties enable bioimaging-guided therapy and targeted imaging of biomolecules. On the other hand, carbon 2D nanosheets exhibit exceptional physicochemical attributes, with graphene excelling in biosensing and bioimaging, also in drug delivery and antimicrobial applications, and graphdiyne in tissue engineering. Their properties, such as tunable porosity and high surface area, contribute to controlled drug release and enhanced tissue regeneration. However, challenges, including long-term biocompatibility and large-scale synthesis, necessitate further research. Potential future directions encompass theranostics, immunomodulation, neural interfaces, bioelectronic medicine, and expanding bioimaging capabilities. In summary, both CQDs and carbon 2D nanosheets hold promise to revolutionize biomedical sciences, offering innovative solutions and improved therapies in diverse biological contexts. Addressing current challenges will unlock their full potential and can shape the future of medicine and biotechnology.

Gold film deposition by infrared laser photothermal treatment on 3D-printed electrodes: electrochemical performance enhancement and application

3D printing has attracted the interest of researchers due to its creative freedom, low cost, and ease of operation. Because of these features, this technology has produced different types of electroanalytical platforms. Despite their popularity, the thermoplastic composites used for electrode fabrication typically have high electrical resistance, resulting in devices with poor electrochemical performance. Herein, we propose a new strategy to improve the electrochemical performance of 3D-printed electrodes and to gain chemical selectivity towards glucose detection. The approach involves synthesising a nanostructured gold film using an infrared laser source directly on the surface of low-contact resistance 3D-printed electrodes. The laser parameters, such as power, focal distance, and beam scan rate, were carefully optimised for the modification steps. Scanning electronic microscopy and energy-dispersive X-ray spectroscopy confirmed the morphology and composition of the nanostructured gold film. After modification, the resulting electrodes were able to selectively detect glucose, encouraging their use for sensing applications. When compared with a gold disc electrode, the gold-modified 3D-printed electrode provided a 44-fold current increase for glucose oxidation. As proof of concept, the devices were utilised for the non-enzymatic catalytic determination of glucose in drink samples, demonstrating the gold film's catalytic nature and confirming the analytical applicability with more precise results than commercial glucometers.

Simple and direct electrochemical detection of rosmarinic acid in food samples based on nanochannel modified carbon electrode

The detection of rosmarinic acid (Ros A) in food samples holds major significance. Simple and convenient electrochemical detection of Ros A with high performance remains a challenge. In this work, a nanochannel array-modified carbon electrode was constructed using a simple and convenient approach to achieve highly sensitive electrochemical detection of Ros A in food samples. Through simple electrochemical pre-activation of a glassy carbon electrode (GCE), oxygen-containing functional groups were introduced on the electrode surface (p-GCE). Vertically-ordered mesoporous silica film (VMSF) was stably grown on p-GCE through electrochemical-assisted self-assembly (EASA) without the introduction of another adhesive layer (VMSF/p-GCE). Transmission electron microscopy (TEM) characterization demonstrated the highly ordered structure of VMSF with a nanochannel diameter around 2.7 nm. Both p-GCE and the nanochannels significantly enhanced the electrochemical signals of Ros A on the electrode, exhibiting dual signal amplification. VMSF/p-GCE demonstrated sensitive detection of Ros A with a linear range of 500 nM to 1 μM and 1 μM to 35 μM. The detection limit (DL) was 26 nM. Combining the good anti-fouling and anti-interference properties of the nanochannels, VMSF/p-GCE can achieve direct electrochemical detection of Ros A in food samples. The sensor can be easily regenerated for repeated use. The simple fabrication, high detection sensitivity and selectivity of the sensor make it a new strategy for rapid preparation of high-performance electrochemical sensors.

Pyrene Derivative Incorporated Ni MOF as an Enzyme Mimic for Noninvasive Salivary Glucose Detection Toward Diagnosis of Diabetes Mellitus

Herein, we demonstrate the detection of glucose in a noninvasive and nonenzymatic manner by utilizing an extended gate field-effect transistor (EGFET) based on the organic molecule pyrene phosphonic acid (PyP4OH8) incorporated nickel metal-organic framework (NiOM-MOF). The prepared electrode responds selectively to glucose instead of sucrose, fructose, maltose, ascorbic acid, and uric acid in a 1× phosphate buffer saline solution. Also, utilizing the scanning Kelvin probe system, the sensing electrode's work function (Φ) is measured to validate the glucose-sensing mechanism. The sensitivity, detection range, response time, limit of detection, and limit of quantification of the electrode are determined to be 24.5 μA mM-1 cm-2, 20 μM to 10 mM, less than 5 s, 2.73 μM, and 8.27 μM, respectively. Most interestingly, the developed electrode follows the Michaelis-Menten kinetics, and the calculated rate constant (km) 0.07 mM indicates a higher affinity of NiOM-MOF toward glucose. The real-time analysis has revealed that the prepared electrode is sensitive to detect glucose in real human saliva, and it can be an alternative device for the noninvasive detection of glucose. Overall, the outcomes of the EGFET studies demonstrate that the prepared electrodes are well-suited for expeditious detection of glucose levels in saliva.

Highly Sensitive Electrochemical Determination of Butylated Hydroxyanisole in Food Samples Using Electrochemical-Pretreated Three-Dimensional Graphene Electrode Modified with Silica Nanochannel Film

The sensitive detection of antioxidants in food is essential for the rational control of their usage and reducing potential health risks. A simple three-dimensional (3D) electrode integrated with an anti-fouling/anti-interference layer possesses great potential for the direct and sensitive electrochemical detection of antioxidants in food samples. In this work, a 3D electrochemical sensor was developed by integrating a 3D graphene electrode (3DG) with vertically ordered mesoporous silica film (VMSF), enabling highly sensitive detection of the common antioxidant, butylated hydroxyanisole (BHA), in food samples. A simple electrochemical polarization was employed to pre-activate the 3DG electrode (p3DG), enhancing its hydrophilicity. Using the p3DG as the supporting electrode, stable modification of VMSF was achieved using the electrochemical assisted self-assembly (EASA) method, without the need for any adhesive agents (VMSF/p3DG). Taking BHA in food as a model analyte, the VMSF/p3DG sensor demonstrated high sensitivity, due to the enrichment by nanochannels, towards BHA. Electrochemical detection of BHA was achieved with a linear range of 0.1 μM to 5 μM and from 5 μM to 150 μM with a low limit of detection (12 nM). Owing to the fouling resistance and anti-interference capabilities of VMSF, the constructed 3D electrochemical sensor can be directly applied for the electrochemical detection of BHA in complex food samples.

Enzyme-free electrochemical sensor platforms based on transition metal nanostructures for clinical diagnostics

The detection of emergent biomarkers is of key significance in numerous clinical, biological, and biomedical fields. Specifically, the design and development of potent electrochemical lactic acid and glucose sensing platforms are especially in great demand in a variety of industries, including those involved in clinical analysis, biomedicine, biological, food, cosmetics, pharmaceuticals, leather, sports, and chemical industries. Nanostructured transition metal-derived materials have opened the door to electrochemical sensors and biosensors due to their advantages of high surface-to-volume ratio, surface reaction activity, catalytic activity, and strong adsorption capability. The primary aim of the present minireview is to highlight the advancement of enzyme-free electrochemical sensor platforms based on transition metal-derived nanostructures with high electrocatalytic activity and sensing performance towards lactic acid and glucose in practical samples. The preparation approaches, structural and composition monitoring, fabrication of sensing electrodes, catalytic activity, sensing performance in real samples, and the exploration of sensing mechanisms are majorly concentrated on in most of our recent research studies. Moreover, state-of-the-art transition metal-derived nanostructure-derived electrochemical sensor platforms, critical comparison of the analytical performance of the sensor platforms, and the future perspectives of the enzyme-free electrochemical sensor for clinical diagnostics are described.

Electrochemical aptasensor fabricated by anchoring recognition aptamers and immobilizing redox probes on bipolar silica nanochannel array for reagentless detection of carbohydrate antigen 15-3

Timely, convenient, and efficient detection of carbohydrate antigen 15-3 (CA15-3) levels in serum holds significant importance in early screening, diagnostic assistance and prognosis prediction of breast cancer. The development of efficient and convenient electrochemical aptasensors with immobilized redox probes for label-free detection of CA15-3 is highly desirable. In this work, a bipolar silica nanochannel array film (bp-SNA) with two distinct functional domains including nanochannels and an outer surface was employed for the immobilization of recognition ligands and electrochemical redox probes, enabling the construction of a probe-integrated aptasensor for reagentless electrochemical detection of CA15-3. Cost-effective and readily available indium tin oxide (ITO) was used as the supporting electrode for sequential growth of a negatively charged inner layer (n-SNA) followed by a positively charged outer layer (p-SNA). The preparation process of bp-SNA is convenient. Functionalization of amino groups on the outer surface of bp-SNA was modified by aldehyde groups for covalent immobilization of recognition aptamers, further establishing the recognition interface. Within the nanochannels of bp-SNA, the electrochemical redox probe, tri (2,2'-dipyridyl) cobalt (II) (Co(bpy)3 2+) was immobilized, which experienced a dual effect of electrostatic attraction from n-SNA and electrostatic repulsion from p-SNA, resulting in high stability of the immobilized probes. The constructed aptasensor allowed for reagentless electrochemical detection of CA15-3 ranged from 0.001 U/mL to 500 U/mL with a low detection limit (DL), 0.13 mU/mL). The application of the constructed aptasensor for CA15-3 detection in fetal bovine serum was also validated. This sensor offers advantages of a simple and readily obtainable supporting electrode, easy bp-SNA fabrication, high probe stability and good stability.

Copper Nanoparticles Confined in a Silica Nanochannel Film for the Electrochemical Detection of Nitrate Ions in Water Samples

The nitrate ion (NO3-) is a typical pollutant in environmental samples, posing a threat to the aquatic ecosystem and human health. Therefore, rapid and accurate detection of NO3- is crucial for both the aquatic sciences and government regulations. Here we report the fabrication of an amino-functionalized, vertically ordered mesoporous silica film (NH2-VMSF) confining localized copper nanoparticles (CuNPs) for the electrochemical detection of NO3-. NH2-VMSF-carrying amino groups possess an ordered perpendicular nanochannel structure and ultrasmall nanopores, enabling the confined growth of CuNPs through the electrodeposition method. The resulting CuNPs/NH2-VMSF-modified indium tin oxide (ITO) electrode (CuNPs/NH2-VMSF/ITO) combines the electrocatalytic reduction ability of CuNPs and the electrostatic attraction capacity of NH2-VMSF towards NO3-. Thus, it is a rapid and sensitive electrochemical method for the determination of NO3- with a wide linear detection range of 5.0-1000 μM and a low detection limit of 2.3 μM. Direct electrochemical detection of NO3- in water samples (tap water, lake water, seawater, and rainwater) with acceptable recoveries ranging from 97.8% to 109% was performed, demonstrating that the proposed CuNPs/NH2-VMSF/ITO sensor has excellent reproducibility, regeneration, and anti-interference abilities.

Homogeneous Electrochemical Aptasensor for Sensitive Detection of Zearalenone Using Nanocomposite Probe and Silica Nanochannel Film

Developing rapid and efficient analytical methods is of great importance for food safety Herein, we present a novel homogeneous electrochemical aptasensor for ultrasensitive quantitative determination of zearalenone (ZEN) based on a nanocomposite probe and silica nanochannel film. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and UV-Vis characterization techniques confirm that graphene oxide (GO) bears an aromatic conjugated structure, along with hydroxyl and carboxyl groups, facilitating the subsequent adsorption of cationic redox hexa-ammine-ruthenium (III) (Ru(NH3)63+) and anionic ZEN aptamer, to form a Ru(NH3)63+-ZEN aptamer-GO nanocomposite probe in a homogeneous solution. Vertically-ordered mesoporous silica films (VMSF) bearing silanol groups can be simply grown on the solid indium tin oxide (ITO) electrode surface and enable the selective preconcentration of Ru(NH3)63+, eventually leading to signal amplification. Since the detachment of Ru(NH3)63+ from the GO surface by the recognized ZEN aptamer in the presence of ZEN, more free Ru(NH3)63+ is released in solution and produces enhanced redox signals at the VMSF modified ITO electrode, allowing quantitative detection of ZEN. On the basis of the above sensing strategy, the proposed homogeneity, due to the assistance of graphene, as well as of the signal amplification and anti-fouling effects of VMSF, accurate analysis of ZEN can be realized in maize and Chinese chestnut samples.

Facile Synthesis of Iron and Nitrogen Co-Doped Carbon Dot Nanozyme as Highly Efficient Peroxidase Mimics for Visualized Detection of Metabolites

Visual detection based on nanozymes has great potential for the rapid detection of metabolites in clinical analysis or home-based health management. In this work, iron and nitrogen co-doped carbon dots (Fe,N-CDs) were conveniently synthesized as a nanozyme for the visual detection of glucose (Glu) or cholesterol (Chol). Using inexpensive and readily available precursors, Fe,N-CDs with peroxidase-like activity were conveniently prepared through a simple hydrothermal method. Co-doping of Fe and N atoms enhanced the catalytic activity of the nanozyme. The nanozyme had a low Michaelis constant (Km) of 0.23 mM when hydrogen peroxide (H2O2) was used as the substrate. Free radical trapping experiments revealed that the reactive oxygen species (ROS) generated in the nanozyme-catalyzed process were superoxide anion radicals (•O2-), which can oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) to generate blue oxidation product (ox-TMB) with characteristics absorbance at 652 nm. Based on this mechanism, a colorimetric sensor was constructed to detect H2O2 ranging from 0.1 μM to 200 μM with a detection limit (DL) of 75 nM. In the presence of glucose oxidase (Gox) or Chol oxidase (Chox), Glu or Chol was oxidized, respectively, and generated H2O2. Based on this, indirect detection of Glu and Chol was realized with linear detection ranges of 5-160 μM and 2-200 μM and DLs of 2.8 μM and 0.8 μM, respectively. A paper-based visual detection platform was fabricated using Fe,N-CDs as nanozyme ink to prepare testing paper by inkjet printing. Using a smartphone to record the RGB values of the testing paper after the reaction, visual detection of Glu and Chol can be achieved with linear detection ranges of 5-160 μM (DL of 3.3 μM) and 2-200 μM (DL of 1.0 μM), respectively.

The Use of Crystalline Carbon-Based Nanomaterials (CBNs) in Various Biomedical Applications

This review study aims to present, in a condensed manner, the significance of the use of crystalline carbon-based nanomaterials in biomedical applications. Crystalline carbon-based nanomaterials, encompassing graphene, graphene oxide, reduced graphene oxide, carbon nanotubes, and graphene quantum dots, have emerged as promising materials for the development of medical devices in various biomedical applications. These materials possess inorganic semiconducting attributes combined with organic π-π stacking features, allowing them to efficiently interact with biomolecules and present enhanced light responses. By harnessing these unique properties, carbon-based nanomaterials offer promising opportunities for future advancements in biomedicine. Recent studies have focused on the development of these nanomaterials for targeted drug delivery, cancer treatment, and biosensors. The conjugation and modification of carbon-based nanomaterials have led to significant advancements in a plethora of therapies and have addressed limitations in preclinical biomedical applications. Furthermore, the wide-ranging therapeutic advantages of carbon nanotubes have been thoroughly examined in the context of biomedical applications.

Homogeneous Electrochemical Aptamer Sensor Based on Two-Dimensional Nanocomposite Probe and Nanochannel Modified Electrode for Sensitive Detection of Carcinoembryonic Antigen

A rapid and convenient homogeneous aptamer sensor with high sensitivity is highly desirable for the electrochemical detection of tumor biomarkers. In this work, a homogeneous electrochemical aptamer sensor is demonstrated based on a two-dimensional (2D) nanocomposite probe and nanochannel modified electrode, which can realize sensitive detection of carcinoembryonic antigen (CEA). Using π-π stacking and electrostatic interaction, CEA aptamer (Apt) and cationic redox probe (hexaammineruthenium(III), Ru(NH₃)₆³⁺) are co-loaded on graphite oxide (GO), leading to a 2D nanocomposite probe (Ru(NH₃)₆³⁺/Apt@GO). Vertically ordered mesoporous silica-nanochannel film (VMSF) is easily grown on the supporting indium tin oxide (ITO) electrode (VMSF/ITO) using the electrochemical assisted self-assembly (EASA) method within 10 s. The ultrasmall nanochannels of VMSF exhibits electrostatic enrichment towards Ru(NH₃)₆³⁺ and size exclusion towards 2D material. When CEA is added in the Ru(NH₃)₆³⁺/Apt@GO solution, DNA aptamer recognizes and binds to CEA and Ru(NH₃)₆³⁺ releases to the solution, which can be enriched and detected by VMSF/ITO electrodes. Based on this mechanism, CEA can be an electrochemical detection ranging from 60 fg/mL to 100 ng/mL with a limit of detection (LOD) of 14 fg/mL. Detection of CEA in human serum is also realized. The constructed homogeneous detection system does not require the fixation of a recognitive aptamer on the electrode surface or magnetic separation before detection, demonstrating potential applications in rapid, convenient and sensitive electrochemical sensing of tumor biomarkers.

Vertically-Ordered Mesoporous Silica Film Based Electrochemical Aptasensor for Highly Sensitive Detection of Alpha-Fetoprotein in Human Serum

Convenient and rapid detection of alpha fetoprotein (AFP) is vital for early diagnosis of hepatocellular carcinoma. In this work, low-cost (0.22 USD for single sensor) and stable (during 6 days) electrochemical aptasensor was developed for highly sensitive and direct detection of AFP in human serum with the assist of vertically-ordered mesoporous silica films (VMSF). VMSF has silanol groups on the surface and regularly ordered nanopores, which could provide binding sites for further functionalization of recognition aptamer and also confer the sensor with excellent anti-biofouling capacity. The sensing mechanism relies on the target AFP-controlled diffusion of Fe(CN)₆^3-/4- redox electrochemical probe through the nanochannels of VMSF. The resulting reduced electrochemical responses are related to the AFP concentration, allowing the linear determination of AFP with a wide dynamic linear range and a low limit of detection. Accuracy and potential of the developed aptasensor were also demonstrated in human serum by standard addition method.

Reagentless Electrochemical Detection of Tumor Biomarker Based on Stable Confinement of Electrochemical Probe in Bipolar Silica Nanochannel Film

The development of simple and probe-integrated aptamer sensors for the electrochemical detection of tumor biomarkers is of great significance for the diagnosis of tumors and evaluation of prognosis. In this work, a probe-integrated aptamer sensor is demonstrated based on the stable confinement of an electrochemical probe in a bipolar nanochannel film, which can realize the reagentless electrochemical detection of the tumor biomarker carcinoembryonic antigen (CEA). To realize the stable immobilization of a large amount of the cationic electrochemical probe methylene blue (MB), a two-layer silica nanochannel array (SNF) with asymmetric charge was grown on the supporting electrode from bipolar SNF (bp-SNF). The inner SNF is negatively charged (n-SNF), and the outer-layer SNF is positively charged (p-SNF). The dual electrostatic interaction including the electrostatic adsorption from n-SNF and the electrostatic repulsion from p-SNF achieve the stable confinement of MB in bp-SNF. The recognitive interface is fabricated by the covalent immobilization of the CEA aptamer on the outer surface of bp-SNF, followed by the blocking of non-specific binding sites. Owing to the stable and abundant immobilized probes and highly specific aptamer interface, the developed aptamer sensor enables the sensitive detection of CEA in the range of 1 pg/mL to 1 μg/mL with a low limit of detection (LOD, 0.22 pg/mL, S/N = 3). Owing to the high selectivity and stability of the developed biosensor, reagentless electrochemical detection of CEA in serum was realized.

Progress on the influence of non-enzymatic electrodes characteristics on the response to glucose detection: a review (2016–2022)

Glucose sensing devices have experienced significant progress in the last years in response to the demand for cost-effective monitoring. Thus, research efforts have been focused on achieving reliable, selective, and sensitive sensors able to monitor the glucose level in different biofluids. The development of enzyme-based devices is challenged by poor stability, time-consuming, and complex purification procedures, facts that have given rise to the synthesis of enzyme-free sensors. Recent advances focus on the use of different components: metal-organic frameworks (MOFs), carbon nanomaterials, or metal oxides. Motivated by this topic, several reviews have been published addressing the sensor materials and synthesis methods, gathering relevant information for the development of new nanostructures. However, the abundant information has not concluded yet in commercial devices and is not useful from an engineering point of view. The dependence of the electrode response on its physico-chemical nature, which would determine the selection and optimization of the materials and synthesis method, remains an open question. Thus, this review aims to critically analyze from an engineering vision the existing information on non-enzymatic glucose electrodes; the analysis is performed linking the response in terms of sensitivity when interferences are present, stability, and response under physiological conditions to the electrode characteristics.

Recent progress and future perspectives of polydopamine nanofilms toward functional electrochemical sensors

Since its discovery in 2007, polydopamine nanofilms have been widely used in many areas for surface functionalization. The simple and low-cost preparation method of the nanofilms with tunable thickness can incorporate amine and oxygen-rich chemical groups in virtually any interface. The remarkable advantages of this route have been successfully used in the field of electrochemical sensors. The self-adhesive properties of polydopamine are used to attach nanomaterials onto the electrode's surface and add chemical groups that can be explored to immobilize recognizing species for the development of biosensors. Thus, the combination of 2D materials, nanoparticles, and other materials with polydopamine has been successfully demonstrated to improve the selectivity and sensitivity of electrochemical sensors. In this review, we highlight some interesting properties of polydopamine and some applications where polydopamine plays an important role in the field of electrochemical sensors.

Equipment of Vertically-Ordered Mesoporous Silica Film on Electrochemically Pretreated Three-Dimensional Graphene Electrodes for Sensitive Detection of Methidazine in Urine

Direct, rapid, and sensitive detection of drugs in complex biological samples is essential for drug abuse control and health risk assessment. In this work, an electrochemical sensor was fabricated based on equipment of vertically-ordered mesoporous silica film (VMSF) on an electrochemically pre-treated three-dimensional graphene electrode (p-3DG), which can achieve direct and sensitive determination of methylthiopyridazine (TR) in urine. Three-dimensional graphene (3DG) with a continuous and interpenetrating graphene network was used as the supporting electrode and simple electrochemical polarization was employed to pre-treat 3DG to improve surface hydrophilicity and electrocatalytic performance. VMSF was easily grown using an electrochemical assisted self-assembly method within 10 s and was stably bound to the p-3DG surface. The nanochannel array on the as-prepared VMSF/p-3DG sensor enriched positively charged TR, leading to significantly improved electrochemical signal. Combined with the high electric activity of p-3DG and the enrichment of nanochannels, VMSF/p-3DG realized sensitive determination of TR ranging from 50 nM to 10 μM with a low detection limit (DL, 30 nM). Owing to the anti-fouling and anti-interference performance of VMSF, the common electroactive molecules including ascorbic acid (AA) and uric acid (UA) did not interfere with the detection. In addition, the detection of TR in buffer and urine exhibited similar sensitivity. Accurate detection of TR in urine was realized.

Disposable Electrochemical Sensors for Highly Sensitive Detection of Chlorpromazine in Human Whole Blood Based on the Silica Nanochannel Array Modified Screen-Printed Carbon Electrode

Rapid and highly sensitive quantitative analysis of chlorpromazine (CPZ) in human whole blood is of great importance for human health. Herein, we utilize the screen-printed carbon electrodes (SPCE) as the electrode substrates for growth of highly electroactive and antifouling nanocomposite materials consisting of vertically ordered mesoporous silica films (VMSF) and electrochemically reduced graphene oxide (ErGO) nanosheets. The preparation of such VMSF/ErGO/SPCE could be performed by using an electrochemical method in a few seconds and the operation is controllable. Inner ErGO layer converted from graphene oxide (GO) in the growth process of VMSF provides oxygen-containing groups and two-dimensional π-conjugated planar structure for stable fabrication of outer VMSF layer. Owing to the π-π enrichment and excellent electrocatalytic abilities of ErGO, electrostatic preconcentration and antifouling capacities of VMSF, and inherent disposable and miniaturized properties of SPCE, the proposed VMSF/ErGO/SPCE sensor could be applied for quantitative determination of CPZ in human whole blood with high accuracy and sensitivity, good stability, and low sample consumption.

Fabrication of a Disposable Electrochemical Immunosensor Based on Nanochannel Array Modified Electrodes and Gated Electrochemical Signals for Sensitive Determination of C-Reactive Protein

Sensitive determination of C-reactive protein (CRP) is of great significance because it is an early indicator of inflammation in cardiovascular disease and acute myocardial infarction. A disposable electrode with an integrated three-electrode system (working, reference, and counter electrodes) has great potential in the detection of biomarkers. In this work, an electrochemical immunosensing platform was fabricated on disposable and integrated screen-printed carbon electrode (SPCE) by introducing nanochannel arrays and gated electrochemical signals, which can achieve the sensitive detection of CRP in serum. To introduce active reactive groups for the fabrication of immuno-recognitive interface, vertically-ordered mesoporous silica-nanochannel film (VMSF) with rich amino groups (NH₂-VMSF) was rapidly grown by electrochemical assisted self-assembly (EASA). The electrochemically reduced graphene oxide (ErGO) synthesized in situ during the growth of NH₂-VMSF was used as a conductive adhesive glue to achieve stable bonding of the nanochannel array (NH₂-VMSF/ErGO/SPCE). After the amino group on the outer surface of NH₂-VMSF reacted with bifunctional glutaraldehyde (GA/NH₂-VMSF/ErGO/SPCE), the converted aldehyde surface was applied for covalent immobilization of the recognitive antibody (Ab) followed with the blocking of the non-specific sites. The fabricated immunosensor, Ab/GA/NH₂-VMSF/ErGO/SPCE, enables sensitive detection of CRP in the range from 10 pg/mL to 100 ng/mL with low limit of detection (LOD, 8 pg/mL, S/N = 3). The immunosensor possessed high selectivity and can realize reliable determination of CRP in human serum.

Highly Sensitive Electrochemical Detection of Paraquat in Environmental Water Samples Using a Vertically Ordered Mesoporous Silica Film and a Nanocarbon Composite

Herein, we demonstrate a sensitive and rapid electrochemical method for the detection of paraquat (PQ) using a glassy carbon electrode (GCE) modified with vertically ordered mesoporous silica films (VMSF) and a nanocarbon composite. The three-dimensional graphene-carbon nanotube (3DG-CNT) nanocarbon composite has a 3D network structure, a large electroactive area and oxygen-containing groups, promoting electron transfer between PQ and the underlying electrode and providing a suitable microenvironment for the stable growth of VMSF. This VMSF/3DG-CNT nanocomposite film could be prepared on the GCE's surface by a two-step electrochemical method with good controllability and convenience. Owing to the synergistic effect of the electrocatalytic ability of 3DG-CNT and the electrostatically enriched capacity of VMSF, the proposed VMSF/3DG-CNT/GCE has superior analytical sensitivity compared with the bare GCE. Furthermore, VMSF has excellent anti-fouling ability that makes the fabricated sensor exhibit satisfactory performance for direct analysis of PQ in environmental water samples.

Probe-Integrated Label-Free Electrochemical Immunosensor Based on Binary Nanocarbon Composites for Detection of CA19-9

Convenient and sensitive detection of tumor biomarkers is crucial for the early diagnosis and treatment of cancer. Herein, we present a probe-integrated and label-free electrochemical immunosensor based on binary nanocarbon composites and surface-immobilized methylene blue (MB) redox probes for detection of carbohydrate antigen 199 (CA19-9), which is closely associated with gastric malignancies. Nanocarbon composites consisting of electrochemically reduced graphene oxides and carbon nanotubes (ErGO-CNT) are electrodeposited onto an indium tin oxide (ITO) electrode surface to form a 3D nanocomposite film, which could provide high surface area to immobilize abundant MB probes, facilitate the electron transfer of MB, and therefore, improve sensitivity. Polydopamine (PDA) served as a bifunctional linker is able to immobilize anti-CA19-9 antibodies and stabilize the inner probe, conferring the sensing interface with specific recognition capacity. Electrochemical detection of CA19-9 is achieved based on the decrease of the redox signal of MB after specific binding of CA19-9 with a wide linear range of 0.1 mU/mL to 100 U/mL and a limit of detection (LOD) of 0.54 nU/mL (S/N = 3). The constructed electrochemical immunosensor has good selectivity, repeatability, reproducibility, and stability. Furthermore, determination of CA19-9 in human serum samples is also realized.

Nanochannel array modified three-dimensional graphene electrode for sensitive electrochemical detection of 2,4,6-trichlorophenol and prochloraz

Convenient, and sensitive detection of pesticides and their metabolites in environmental or food samples is critical for assessing potential environmental and health risks. Here, a three-dimensional (3D) electrochemical sensing platform is proposed based on the integration of nanochannel array on pre-activated 3D graphene (p-3DG) electrodes with no need of additional adhesive layers, which enables sensitive detection of prochloraz and 2,4,6-trichlorophenol (TCP) in environmental and food samples. Through two-step electrochemical polarization, organic phase anodic oxidation, and aqueous phase cathodic reduction, p-3DG electrodes with high active area and excellent electrocatalytic performance were obtained. Vertically-ordered mesoporous silica-nanochannel film (VMSF) can be rapidly grown on the surface of p-3DG by an electrochemical-assisted self-assembly (EASA) method. Taking advantage of the high electrocatalytic activity of p-3DG and the ability of nanochannels to enrich TCP through hydrogen bonding, the VMSF/p-3DG sensor can sensitively detect TCP in the range of 10 nM to 0.1 μM and 0.1–15 μM with a low limit of detection (LOD) of 2.4 nM. Compared with p-3DG and VMSF-modified 2D electrodes, the fabricated sensor has a wide detection linear range and low LOD. The coexistence of model interferents such as protein, surfactant, and humic acid did not affect the electrochemical response of TCP, confirming the high anti-fouling ability of the VMSF/p-3DG sensor. In addition, prochloraz in vegetable and fruit samples was indirectly determined because TCP was the metabolite of prochloraz.

Novel electrochemical and electrochemiluminescence dual-modality sensing platform for sensitive determination of antimicrobial peptides based on probe encapsulated liposome and nanochannel array electrode

With the increasing application of antimicrobial peptides (AMPs) to replace antibiotics in medicine, food and agriculture, it is highly desired to develop a fast, reliable, and convenient strategy for sensitive detection of AMPs. Herein, a novel electrochemical (EC) and electrochemiluminescence (ECL) dual-modality sensing platform was developed based on probe encapsulated liposomes and nanochannel array modified electrodes, which enables sensitive determination of nisin in food samples. The bifunctional probe with both EC and ECL signals, tris(2,2-bipyridyl) dichlororuthenium (II) (Ru(bpy)₃²⁺), was chosen to be easily encapsulated in liposomes (Ru(bpy)₃²⁺@liposome). Based on the unique sterilization mechanism that AMPs can disrupt cell membranes, Ru(bpy)₃²⁺@liposome can be destroyed by nisin and release a large number of Ru(bpy)₃²⁺ probes. Vertically-ordered mesoporous silica-nanochannel film (VMSF) modified ITO electrodes (VMSF/ITO) prepared by electrochemically assisted self-assembly (EASA) method were applied as the sensing electrode. Due to the efficient enrichment of Ru(bpy)₃²⁺ by the negatively charged nanochannel arrays, VMSF/ITO enables detection of the EC/ECL signals of the released Ru(bpy)₃²⁺ probes with ultrahigh sensitivity. In consequence, sensitive dual-modality detection of nisin was achieved by the combination of Ru(bpy)₃²⁺@liposome and VMSF/ITO. The developed sensing system can realize sensitive determination of nisin in ECL mode in the concentration range of 10 ng/ml to 50 μg/ml with a limit of detection (LOD) of 9.3 ng/ml, or in EC mode from 800 ng/ml to 100 μg/ml with a LOD of 70 ng/ml. Combined with the excellent anti-fouling and anti-interference performance of VMSF, rapid and sensitive detection of nisin in milk or egg white was also achieved by the sensor.

A reagentless electrochemical immunosensor for sensitive detection of carcinoembryonic antigen based on the interface with redox probe-modified electron transfer wires and effectively immobilized antibody

Convenient and sensitive detection of tumors marked in serum samples is of great significance for the early diagnosis of cancers. Facile fabrication of reagentless electrochemical immunosensor with efficient sensing interface and high sensitivity is still a challenge. Herein, an electrochemical immunosensor was easily fabricated based on the easy fabrication of immunoassay interface with electron transfer wires, confined redox probes, and conveniently immobilized antibodies, which can achieve sensitive and reagentless determination of the tumor marker, carcinoembryonic antigen (CEA). Carboxyl multi-walled carbon nanotubes (MWCNTs) were firstly modified with an electrochemical redox probe, methylene blue (MB), which has redox potentials distinguished from those of redox molecules commonly existing in biological samples (for example, ascorbic acid and uric acid). After the as-prepared MB-modified MWCNT (MWCNT-MB) was coated on the supporting glassy carbon electrode (GCE), the MWCNT-MB/GCE exhibited improved active area and electron transfer property. Polydopamine (PDA) was then in situ synthesized through simple self-polymerization of dopamine, which acts as the bio-linker to covalently immobilize the anti-CEA antibody (Ab). The developed immunosensor could be applied for electrochemical detection of CEA based on the decrease in the redox signal of MB after specific binding of CEA and immobilized Ab. The fabricated immunosensor can achieve sensitive determination of CEA ranging from 10 pg/ml to 100 ng/ml with a limit of detection (LOD) of 0.6 pg/ml. Determination of CEA in human serum samples was also realized with high accuracy.

Direct and Sensitive Electrochemical Detection of Bisphenol A in Complex Environmental Samples Using a Simple and Convenient Nanochannel-Modified Electrode

Rapid, convenient, and sensitive detection of Bisphenol A (BPA) in complex environmental samples without the need for tedious pre-treatment is crucial for assessing potential health risks. Herein, we present an electrochemical sensing platform using a simple nanochannel-modified electrode, which enables the direct and sensitive detection of BPA in complex samples. A vertically ordered mesoporous silica-nanochannel film (VMSF) with high-density nanochannels is rapidly and stably grown on the surface of a electrochemically activated glassy carbon electrode (p-GCE) by using the electrochemically assisted self-assembly (EASA) method. The high antifouling capability of the VMSF/p-GCE sensor is proven by investigating the electrochemical behavior of BPA in the presence of model coexisting interfering molecules including amylum, protein, surfactant, and humic acid. The VMSF/p-GCE sensor can sensitively detect BPA ranged from 50 to 1.0 μM and 1.0–10.0 μM, with low detection limits (15 nM). Owing to the electrocatalytic performance and high potential resolution of p-GCE, the sensor exhibits high selectivity for BPA detection in the presence of common environmental pollutants, including bisphenol S (BPS), catechol (CC), hydroquinone (HQ), and 4-nitrophenol (4-NP). In combination with the good antifouling property of the VMSF, direct detection of BPA in environmental water samples and soil leaching solution (SLS) is also realized without separation pretreatment. The developed VMSF/p-GCE sensor demonstrated advantages of simple structure, high sensitivity, good antifouling performance, and great potential in direct electroanalysis of endocrine-disrupting compounds in complex samples.

Nanochannel Array on Electrochemically Polarized Screen Printed Carbon Electrode for Rapid and Sensitive Electrochemical Determination of Clozapine in Human Whole Blood

Rapid and highly sensitive determination of clozapine (CLZ), a psychotropic drug for the treatment of refractory schizophrenia, in patients is of great significance to reduce the risk of disease recurrence. However, direct electroanalysis of CLZ in human whole blood remains a great challenge owing to the remarkable fouling that occurs in a complex matrix. In this work, a miniaturized, integrated, disposable electrochemical sensing platform based on the integration of nanochannel arrays on the surface of screen-printed carbon electrodes (SPCE) is demonstrated. The device achieves high determination sensitivity while also offering the electrode anti-fouling and anti-interference capabilities. To enhance the electrochemical performance of SPCE, simple electrochemical polarization including anodic oxidation and cathodic reduction is applied to pretreat SPCE. The electrochemically polarized SPCE (p-SPCE) exhibits an enhanced electrochemical peak signal toward CLZ compared with bare SPCE. An electrochemically assisted self-assembly method (EASA) is utilized to conveniently electrodeposit a vertically ordered mesoporous silica nanomembrane film (VMSF) on the p-SPCE, which could further enrich CLZ through electrostatic interactions. Owing to the dual signal amplification based on the p-SPCE and VMSF nanochannels, the developed VMSF/SPCE sensor enables determination of CLZ in the range from 50 nM to 20 μM with a low limit of detection (LOD) of 28 nM (S/N = 3). Combined with the excellent anti-fouling and anti-interference abilities of VMSF, direct and sensitive determination of CLZ in human blood is also achieved.

Electrochemical Sensor Nanoarchitectonics for Sensitive Detection of Uric Acid in Human Whole Blood Based on Screen-Printed Carbon Electrode Equipped with Vertically-Ordered Mesoporous Silica-Nanochannel Film

Screen-printed carbon electrodes (SPCEs) bear great potential in the detection of biomarker in clinical samples with low sample consumption. However, modification of electrode surfaces to improve the anti-interference ability and sensitivity is highly desirable for direct electroanalysis of whole blood samples. Here, a reliable and miniaturized electrochemical sensor is demonstrated based on SPCE equipped with vertically-ordered mesoporous silica-nanochannel film (VMSF). To achieve stable binding of VMSF and improve the electrocatalytic performance, electrochemically reduced graphene oxide (ErGO) is applied as a conductive adhesion layer, that is in situ reduced from GO nanosheets during fast growth (less than 10 s) of amino groups modified VMSF (NH2-VMSF) using electrochemically assisted self-assembly (EASA). In comparison with bare SPCE, NH2-VMSF/ErGO/SPCE exhibits decreased oxidation potential of uric acid (UA) by 147 mV owing to significant electrocatalytic ability of ErGO. The dual signal amplification based on electrocatalysis of ErGO and enrichment of nanochannels leads to enhanced peak current by 3.9 times. Thus, the developed NH2-VMSF/ErGO/SPCE sensor enables sensitive detection of UA in the range from 0.5 μM to 180 μM with a low limit of detection (LOD, 129 nM, S/N = 3). Owing to good anti-fouling ability and high selectivity of the sensor, direct and rapid detection of UA in human whole blood is realized with very low sample consumption (50 μL).

Facile Pretreatment of Three-Dimensional Graphene through Electrochemical Polarization for Improved Electrocatalytic Performance and Simultaneous Electrochemical Detection of Catechol and Hydroquinone

Three-dimensional graphene (3DG) with macroporous structure has great potential in the field of electroanalysis owing to a large active area, excellent electron mobility and good mass transfer. However, simple and low-cost preparation of 3DG electrodes with high electrocatalytic ability is still a challenge. Here, a fast and convenient electrochemical polarization method is established to pretreat free-standing 3DG (p-3DG) to offer high electrocatalytic ability. 3DG with monolithic and macroporous structure prepared by chemical vapor deposition (CVD) is applied as the starting electrode. Electrochemical polarization is performed using electrochemical oxidation (anodization) at high potential (+6 V) followed with electrochemical reduction (cathodization) at low potential (-1 V), leading to exposure of edge of graphene and introduction of oxygen-containing groups. The as-prepared p-3DG displays increased hydrophilicity and improved electrocatalytic ability. As a proof of concept, p-3DG was used to selective electrochemical detection of two isomers of benzenediol, hydroquinone (p-BD) and catechol (o-BD). In comparison with initial 3DG, p-3DG exhibits increased reversibility of redox reaction, improved peak current and good potential resolution with high potential separation between p-BD and o-BD. Individual or selective determination of p-BD or o-BD in single substance solution or binary mixed solution is realized. Real analysis of pond water is also achieved.

Materials Approaches for Improving Electrochemical Sensor Performance

Electrochemical sensors have emerged as important diagnostic tools in recent years, due to their simplicity and ease of use. Compared to instrumental analysis methods that use complicated experimental and data analysis techniques─such as mass spectrometry, nuclear magnetic resonance (NMR), spectrophotometric methods, and chromatography─electrochemical sensors show promise for use in a wide range of real-time and in situ applications such as pharmaceutical testing, environmental monitoring, and medical diagnostics. In order to identify analytes in complex and/or biological samples, materials used for both the electrode materials and the chemically selective layer have been evolving throughout the years for optimizing the analytical performance of electrochemical sensors to increase sensitivity, selectivity and linear range. In this Perspective, attention will be focused on different types of materials that have been used for electrochemical sensing, including new combinations of well-studied materials as well as novel strategies to enhance the performance of sensing devices. The Perspective will also discuss existing challenges in the field and future strategies for addressing those challenges.

Carbon nanomaterial hybrids via laser writing for high-performance non-enzymatic electrochemical sensors: a critical review

Non-enzymatic electrochemical sensors possess superior stability and affordability in comparison to natural enzyme-based counterparts. A large variety of nanomaterials have been introduced as enzyme mimicking with appreciable sensitivity and detection limit for various analytes of which glucose and H2O2 have been mostly investigated. The nanomaterials made from noble metal, non-noble metal, and metal composites, as well as carbon and their derivatives in various architectures, have been extensively proposed over the past years. Three-dimensional (3D) transducers especially realized from the hybrids of carbon nanomaterials either with metal-based nanocatalysts or heteroatom dopants are favorable owing to low cost, good electrical conductivity, and stability. In this critical review, we evaluate the current strategies to create such nanomaterials to serve as non-enzymatic transducers. Laser writing has emerged as a powerful tool for the next generation of devices owing to their low cost and resultant remarkable performance that are highly attractive to non-enzymatic transducers. So far, only few works have been reported, but in the coming years, more and more research on this topic is foreseeable.

Label-Free and Highly-Sensitive Detection of Ochratoxin A Using One-Pot Synthesized Reduced Graphene Oxide/Gold Nanoparticles-Based Impedimetric Aptasensor

Ochratoxin A (OTA) primarily obtained by the genera aspergillus and penicillium, is one of the toxic substances for different organs and systems of the human body such as the kidney, liver, neurons and the immune system. Moreover, it is considered to cause tumors and fetal malformation even at a very low concentration. Fast and sensitive assay for detection of OTA at ultralow levels in foods and agricultural products has been an increasing demand. In this study, a new label-free electrochemical biosensor based on three-dimensional reduced graphene oxide/gold nanoparticles/aptamer for OTA detection was constructed. The 3D-rGO/Au NPs nanocomposites were firstly synthesized using a one-pot hydrothermal process under optimized experimental conditions. The 3D-rGO/Au NPs with considerable particular surface area and outstanding electrical conductivity was then coated on a glass carbon electrode to provide tremendous binding sites for -SH modified aptamer via the distinctive Au-S linkage. The presence of OTA was specifically captured by aptamer and resulted in electrochemical impedance spectroscopy (EIS) signal response accordingly. The constructed impedimetric aptasensor obtained a broad linear response from 1 pg/mL to 10 ng/mL with an LOD of 0.34 pg/mL toward OTA detection, highlighting the excellent sensitivity. Satisfactory reproducibility was also achieved with the relative standard deviation (RSD) of 1.393%. Moreover, the proposed aptasensor obtained a good recovery of OTA detection in red wine samples within the range of 93.14 to 112.75% along with a low LOD of 0.023 ng/mL, indicating its applicability for OTA detection in real samples along with economical, specific, susceptible, fast, easy, and transportable merits.

2D metal azolate framework as nanozyme for amperometric detection of glucose at physiological pH and alkaline medium

The synthesis of Co-based two-dimensional (2D) metal azolate framework nanosheets (MAF-5-Co^II NS) is described using a simple hydrothermal method. The product was isostructural to MAF-5 (Zn). The as-prepared MAF-5-Co^II NS exhibited high surface area (1155 m²/g), purity, and crystallinity. The MAF-5-Co^II NS-modified screen-printed electrode (MAF-5-Co^II NS/SPE) was used for nonenzymatic detection of glucose in diluted human blood plasma (BP) samples with phosphate buffer saline (PBS, pH 7.4) and NaOH (0.1 M, pH 13.0) solutions. The MAF-5-Co^II NS nanozyme displayed good redox activity in both neutral and alkaline media with the formation of Co^II/Co^III redox pair, which induced the catalytic oxidation of glucose. Under the optimized detection potential, the sensor presented a chronoamperometric current response for the oxidation of glucose with two wide concentration ranges in PBS-diluted (62.80 to 180 μM and 305 to 8055 μM) and NaOH-diluted (58.90 to 117.6 μM and 180 to 10,055 μM) BP samples, which were within the limit of blood glucose levels of diabetic patients before (4.4-7.2 mM) and after (10 mM) meals (recommended by the American Diabetes Association). The sensor has a limit of detection of ca. 0.25 and 0.05 μM, respectively, and maximum sensitivity of ca. 36.55 and 1361.65 mA/cm²/mM, respectively, in PBS- and NaOH-diluted BP samples. The sensor also displayed excellent stability in the neutral and alkaline media due to the existence of hydrophobic linkers (2-ethyl imidazole) in the MAF-5-Co^II NS, good repeatability and reproducibility, and interference-free signals. Thus, MAF-5-Co^II NS is a promising nanozyme for the development of the disposable type of sensor for glucose detection in human body fluids. Graphical abstract.