Ketjen black/ferrocene dual-doped MOFs and aptamer-coupling gold nanoparticles used as a novel ratiometric electrochemical aptasensor for vanillin detection

Magnetic metal-organic frameworks as sensitive aptasensors for coronavirus spike protein

Electrochemical biosensors, known for their low cost, sensitivity, selectivity, and miniaturization capabilities, are ideal for point-of-care devices. The magnetic metal-organic framework (MMOF), synthesized using the in-situ growth method, consists of ferric salt, magnetic nanoparticles, histidine, and benzene tetracarboxylic acid. MMOF was sequentially modified with aptamer-biotin and streptavidin-horseradish peroxidase, serving as a detector for spike protein and a transducer converting electrochemical signals using H2O2-hydroquinone on a screen-printed electrode. MMOF facilitates easy washing and homogeneous deposition on the working electrode with a magnet, enhancing sensitivity and reducing noise. The physical and electrochemical properties of the modified MMOFs were thoroughly characterized using various analytical techniques. The aptasensors' performance achieved a detection limit of 6 pM for voltammetry and 5.12 pM for impedance spectroscopy in human serum samples. This cost-effective, portable MMOF platform is suitable for rapid point-of-care testing for SARS-CoV-2 spike proteins.

Hemin/G-quadruplex and AuNPs-MoS2 based novel dual signal amplification strategy for ultrasensitively sandwich-type electrochemical thrombin aptasensor

In this work, a novel sandwich-type electrochemical aptasensor based on the dual signal amplification strategy of hemin/G-quadruplex and AuNPs-MoS2 was designed and constructed, which realized the highly sensitive and specific detection of thrombin (TB). In this aptasensor, the 15-mer TB-binding aptamer (TBA-1) modified with thiol group was immobilized on the surface of AuNPs modified glassy carbon electrode (AuNPs/GCE) as capturing elements. Another thiol-modified 29-mer TB-binding aptamer (TBA-2) sequence containing G-quadruplex structure for hemin immobilization was designed. The formed hemin/G-quadruplex/TBA-2 sequence was further combined to the AuNPs decorated flower-like molybdenum disulfide (AuNPs-MoS2) composite surface via Au-S bonds, acting the role of reporter probe. In presence of the target TB, the sandwich-type electrochemical aptamer detection system could be formed properly. With the assistance of the dual signal amplification of AuNPs-MoS2 and hemin/G-quadruplex toward H2O2 reduction, the sandwich-type electrochemical aptasensor was successfully constructed for sensitive detection of TB. The results demonstrate that the fabricated aptasensor displays a wide linear range of 1.0 × 10-6 ∼ 10.0 nM with a low detection limit of 0.34 fM. This proposed aptasensor shows potential application in the detection of TB content in real biological samples with high sensitivity, selectivity, and reliability.

Portable self-powered electrochemical aptasensing platform for ratiometric detection of mycotoxins based on multichannel photofuel cell

Self-powered electrochemical sensors based on photofuel cells have attracted considerable research interest because their unique advantage of not requiring an external electric source, but their application in portable and multiplexed targets assay is limited by the inherent mechanism. In this work, a portable self-powered sensor constructed with multichannel photofuel cells was developed for the ratiometric detection of mycotoxins, namely ochratoxin A (OTA) and patulin (PAT). The spatially resolved CdS/Bi2S3-modified photoanodes and a shared Prussian Blue cathode were integrated on an etched indium-tin oxide slide to fabricate the multichannel photofuel cell. The aptamers of OTA and PAT were covalently bonded to individual photoanode regions to build sensitive interfaces, and the specific recognition of analytes impaired the output performance of constructed PFC. Accordingly, ratiometric sensing of OTA and PAT was achieved by utilizing the output performance of a control PFC as a reference signal. This approach effectively eliminates the impact of light intensity on the accuracy of the detection. Under the optimal conditions, the proposed sensing chip exhibited linear ranges of 2.0-1000 nM and 5.0-500 nM for OTA and PAT, respectively. The detection limits (3 S/N) were determined to be 0.25 nM for OTA and 0.27 nM for PAT. The developed ratiometric sensing method demonstrated good selectivity and stability in the simultaneous detection of OTA and PAT. It was successfully utilized for the analysis of OTA and PAT real samples. This work provides a new perspective for construction of portable and ratiometric self-powered sensing platform.

Immobilization of ferrocene and its derivatives within metal-organic frameworks with high loadings toward efficient oxygen evolution reaction

The use of an appropriate preparation route is the key to immobilize active molecules into a host matrix with high loadings and stability. Herein, we demonstrate a simple and general strategy to immobilize ferrocene and its derivatives into ZIF-8 with high loadings of up to 4.3% Fe content. The unique host pore structure allows for the stabilization of guest molecules and effectively prevents their leaching. As a result, the obtained electrocatalysts exhibit competitive oxygen evolution reaction (OER) catalytic performance. Optimized Fc-CHO/ZIF-8 requires only a low overpotential of 238 mV to achieve 10 mA cm-2, along with a relatively small Tafel slope of 44.4 mV dec-1. This performance is superior to that of commercial IrO2, suggesting its potential application in electrochemical energy conversion.

A CG@MXene nanocomposite-driven E-CRISPR biosensor for the rapid and sensitive detection of Salmonella Typhimurium in food

In this study, we developed a novel electrochemical biosensor based on CRISPR/Cas12a (E-CRISPR) for the rapid and sensitive detection of Salmonella Typhimurium (S. Typhimurium). The CRISPR/Cas12a system was applied to identify S. Typhimurium gene and induce signal changes in electrochemical measurement. The colloidal gold and MXene (CG@MXene) nanocomposites were synthesized and immobilized to improve the performance of the biosensor by decreasing the background noise. The formation process of CG@MXene was well characterized, and experiment conditions were fully optimized. Under the optimal conditions, the proposed E-CRISPR biosensor exhibited excellent sensitivity for S. Typhimurium, with a limit of detection (LOD) of 160 CFU/mL, and great specificity against other common foodborne pathogens. Furthermore, the feasibility of the E-CRISPR biosensor was evaluated by analyzing S. Typhimurium-spiked chicken samples, with a recovery rate ranging from 100.46% to 106.37%. In summary, this research proposed a novel E-CRISPR biosensor from a new perspective to detect S. Typhimurium which can be an alternative approach for bacterial detection in the food supply chain.

Label-free and ultrasensitive electrochemical transferrin detection biosensor based on a glassy carbon electrode and gold nanoparticles

In this study, we developed a label-free and ultrasensitive electrochemical biosensor for the detection of transferrin (Tf), an important serum biomarker of atransferrinemia. The biosensor was fabricated by using glassy carbon electrode (GCE) and modified with gold nanoparticles (AuNPs) via electroless deposition. The electrochemical characteristics of the GCE-AuNPs biosensors were characterized using cyclic voltammetry and electrochemical impedance spectroscopy analysis. Differential pulse voltammetry was used for quantitative evaluation of the Tf-antigen by recording the increase in the anodic peak current of GCE-AuNPs biosensor. The GCE-AuNPs biosensor demonstrates superior sensing performance for Tf-antigen fortified in buffer, with a wide linear range of 0.1 to 5000 μg/mL and a limit of detection of 0.18 μg/mL. The studied GCE-AuNPs biosensor showed excellent sensitivity, selectivity, long-term storage stability and simple sensing steps without pretreatment of clinical samples. This GCE-AuNPs biosensor indicates great potential for developing a Tf detection platform, which would be helpful in the early diagnosis of atransferrinemia. The developed GCE-AuNPs biosensor holds great potential in biomedical research related to point of care for the early diagnosis and monitoring of diseases associated with aberrant serum transferrin levels. These findings suggest that the GCE-AuNPs biosensor has great potential for detecting other serum biomarkers.

A novel magneto-mediated electrochemical biosensor integrated DNAzyme motor and hollow nanobox-like Pt@Ni-Co electrocatalyst as dual signal amplifiers for vanilla detection

Herein, a novel magneto-mediated electrochemical aptasensor using the signal amplification technologies of DNAzyme motor and electrocatalyst for vanilla (VAN) detection was fabricated. The D/B duplex, formed by the DNAzyme motor that was each silenced by a blocker, and hairpin DNA1 (H1) containing adenosine ribonucleotide (rA) site were tethered on the sites of the gold nanoparticles@hollow porphyrinic-Metal-organic framework/polyethyleneimine-reduced graphene oxide (AuHPCN-222/PEI-rGO)-modified gold electrode (AuE). Then, after homogeneous and specific recognition in the presence of the VAN, trigger DNA was released and enriched by magnetic separation technique and introduced to the sensing platform to activate the DNAzyme motor, which efficiently improved target recognition capability and avoided the obstacle of multiple DNA strands tangling. More interestingly, the activated DNAzyme motor could repeatedly bind to and cleave H1 in the presence of Mg2+, leading to the exposure of a plethora of capture probes. The thionine (Thi) functionalized hairpin DNA2 (H2)-Pt@Ni-Co as signal probes could hybridize with capture probes. Additionally, the Pt@Ni-Co electrocatalysts presented catalytic activity towards Thi to obtain stronger electrochemical signals. VAN with concentrations ranging from 1 × 10-6 to 10 μM was determined and a detection limit was down to 0.15 pM. The designed electrochemical sensor was highly selective with specificity, stability, reproducibility, and reliable capability for monitoring the VAN in real samples.

Signal-off electrochemical sensor for matrix metalloproteinase 9 detection based on sacrificial FeMOF and host-guest strategy

Matrix metalloproteinase-9 (MMP-9) has been implicated in various tumor cell invasions and metastases. In light of the limitations of traditional methods for MMP-9 detection, we have constructed a novel biosensor depending on cucurbit[8]uril (CB[8]) -mediated host-guest interactions and a sacrificial iron metal-organic framework (FeMOF). Herein, MMP9-specific peptides modified on the gold bare electrode are bonded to the FeMOF@AuNPs@peptide complex through CB[8] addition. The connection between MMP9-specific peptides and signal peptides via CB[8] provides stability as well as enables the immobilization of FeMOF on the electrode surface. When Fe3+ from the FeMOF interacts with electrochemical buffer K4Fe(CN)6, Prussian blue will be generated on the gold electrode surface, and a significantly enlarged current response can be detected. However, in the presence of MMP-9, their peptide substrates are specifically cleaved at the site between serine (S) and Leucine (L), which causes an abrupt decrease in the electrochemical signal. The change of signal can reflect MMP-9 concentration. This sensor can reach an ultrahigh sensitivity with a wide detection range of 0.5 pg⋅mL-1 to 500 ng⋅mL-1 and a low detection limit of 1.30 pg⋅mL-1. Importantly, this sensor is very simple, relying solely on self-sacrificial label of FeMOF, rather than complex functional materials. Additionally, it has been well used in serum samples, showing attractive potential for practical applications.

Aptamer-modified metal organic frameworks for measurement of food contaminants: a review

The measurement of food contaminants faces a great challenge owing to the increasing demand for safe food, increasing consumption of fast food, and rapidly changing patterns of human consumption. As different types of contaminants in food products can pose different levels of threat to human health, it is desirable to develop specific and rapid methods for their identification and quantification. During the past few years, metal-organic framework (MOF)-based materials have been extensively explored in the development of food safety sensors. MOFs are porous crystalline materials with tunable composition, dynamic porosity, and facile surface functionalization. The construction of high-performance biosensors for a range of applications (e.g., food safety, environmental monitoring, and biochemical diagnostics) can thus be promoted through the synergistic combination of MOFs with aptamers. Accordingly, this review article delineates recent innovations achieved for the aptamer-functionalized MOFs toward the detection of food contaminants. First, we describe the basic concepts involved in the detection of food contaminants in terms of the advantages and disadvantages of the commonly used analytical methods (e.g., DNA-based methods (PCR/real-time PCR/multiplex PCR/digital PCR) and protein-based methods (enzyme-linked immunosorbent assay/immunochromatography assay/immunosensor/mass spectrometry). Afterward, the progress in aptamer-functionalized MOF biosensors is discussed with respect to the sensing mechanisms (e.g., the role of MOFs as signal probes and carriers for loading signal probes) along with their performance evaluation (e.g., in terms of sensitivity). We finally discuss challenges and opportunities associated with the development of aptamer-functionalized MOFs for the measurement of food contaminants.

Chiral MOFs encapsulated by polymers with poly-metallic coordination as chiral biosensors

Chiral materials have drawn the widespread attention for their its chiral recognition ability. The design and synthesis of chiral material are of importance owing to the unpredictability in controlling chirality during the synthesis process. To circumvent problems, a chiral MOF (D-His-ZIF-8) was synthesized by ligand exchange of 2-methylimidazole (Hmim) on ZIF-8 by D-histidine (D-His), which can be treated as chiral host to distinguish amino acid enantiomers. The obtained D-His-ZIF-8 can provide chiral nanochannels for amino acid guests. Meanwhile, polynary transition-metal ion (Co²⁺ and Fe³⁺) coordinating with polydopamine (PDA) wrapped on the surface of D-His-ZIF-8 can increase the active sites. The electrochemical chiral recognition behavior showed that D-His-ZIF-8@CoFe-PDA exhibited good recognition of the tryptophan enantiomer (L/D-Trp) (working potential of -0.2 V vs. Hg/HgCl₂). The LOD and LOQ of L-Trp were 0.066 mM and 0.22 mM, respectively, while the LOD and LOQ of D-Trp were 0.15 mM and 0.50 mM, respectively. Finally, the usefulness of D-His-ZIF-8@CoFe-PDA/GCE was evaluated with a recovery of 94.4-103%. The analysis of real  samples shows that D-His-ZIF-8@CoFe-PDA/GCE is a feasible sensing platform for the detection of L-Trp and D-Trp.

Development of fluorescence sensors with copper-based nanoclusters via Förster resonance energy transfer and the quenching effect for vanillin detection

Two kinds of copper-based metal fluorescent nanoclusters were successfully prepared by the chemical reduction method; one of them (CuNCs) was synthesized by direct reduction of copper sulfate, and the other (CuAuNCs) was synthesized by the stepwise addition of copper salt and chloroauric acid. CuNCs were used to establish the fluorescence resonance energy transfer (FRET) system with neutral red (NR) due to the supramolecular effect of β-cyclodextrin (β-CD) modified on the surface of CuNCs. NR could enter the hydrophobic cavity of β-CD and narrow the distance between CuNCs and NR, which could lead to FRET. Fluorescence was transferred from CuNCs to NR, resulting in amplification of the NR fluorescence signal, which could be used to detect vanillin. In addition, CuAuNCs with strong fluorescence were used as fluorescent probes to detect vanillin through the quenching mechanism. By comparison, the simplicity of CuNC synthesis and the high selectivity of β-CD made the FRET method more practical, which may provide a new strategy for assaying vanillin.

Synthesis and study of CuNiTiO3 as an ORR electrocatalyst to enhance microbial fuel cell efficiency

Microbial fuel cells (MFCs) have the capability of simultaneous sewage treatment and electricity generation. Modifying the cathode electrode enhances their efficiency. In this study, NiTiO₃ and CuNiTiO₃ were synthesized for practical application as cathode catalysts in a dual-chamber MFC and the performance of the modified cathodes was evaluated against a bare graphite electrode. SEM images showed that the particle sizes were mostly in the range of 40-120 and 20-80 nm for NiTiO₃ and CuNiTiO₃, respectively. According to AFM results, CuNiTiO₃ presented a higher surface roughness than NiTiO₃. MFC using CuNiTiO₃/G electrode with a reduction potential value of -0.27 V (vs. SCE) and a power density of 62.18 mW m^-2 showed better oxygen reduction reaction (ORR) activity compared with NiTiO₃/G and the bare graphite. MFC using CuNiTiO₃ cathode also showed the highest values in terms of chemical oxygen demand (COD) removal (75%) and the calculated coulombic efficiency (CE, 10%). The results obtained in this study, introduce CuNiTiO₃ as a promising electrocatalyst for further improvement of the cathodic reactions in MFC applications.

Nanopore-based aptasensor for label-free and sensitive vanillin determination in food samples

Dielectric breakdown technique was utlised to fabricate 5-6 nm nanopores for vanillin detection in various food samples. A highly selective aptamer (Van_74) with high binding affinity towards vanillin was used as capture probe. Under optimal conditions, aptamer/vanillin complex translocation induced deeper events than the bare aptamer. As a result, the proposed nanopore aptasensor exhibits a linear range from 0.5 to 5 nM (R² = 0.972) and a low detection limit of 500 pM, which is significantly better than conventional platforms. Furthermore, our aptasensor showed excellent immunity against different interferons and was used to detect vanillin in different food samples. The food sample measurements were confirmed with an additional UV-Vis assay, the results of the two techniques were statistically evaluated and showed no statistically significant difference. Hence, this work represents a proof-of-concept involving the design and testing of aptamer/nanopore sensors for small molecules detection, which plays a critical role in food safety.

Electrochemical determination of vanillin in cookies at mediated AuNPs/GR nanocomposites modified glassy carbon electrode

Background:

Currently, carbon nanomaterials and carbon nanomaterials-based electrodes have illustrated significant electrocatalytic abilities.

Methods:

An electrochemical sensor was developed for vanillin using graphene (GR) decorated with gold nanoparticles (AuNPs) on a glassy carbon electrode (GCE) with two steps. AuNPs/GR/GCE, as the electrochemical sensor for determination of vanillin, included dropping GR onto the electrode and then electrodepositing AuNPs on GR/GCE. The structure and morphology of the synthesized nanocomposites (AuNPs/GR) on the electrode were confirmed by scanning electron microscopy (SEM).

Results:

Electrochemical studies revealed that modification of the electrode surface with AuNPs/GR nanocomposites significantly increases the oxidation peak currents of vanillin. The peak currents in differential pulse voltammetry (DPV) of vanillin increased linearly with their concentration in the range of 5-120 µM. The limit of detection was found to be 1.7 µM for vanillin. Also, the effect of some interfering compounds, such as NaCl, KCl, glucose, alanine, phenylalanine, glycine, and others, on the determination of vanillin was evaluated, and none of them had a significant effect on the assay recovery

Conclusions:

A new electrochemical biosensor was fabricated with AuNPs/GR nanocomposites. The sensor was successfully used to detect vanillin in cookie samples.

Covalent organic framework modified carbon cloth for ratiometric electrochemical sensing of bisphenol A and S

A novel ratiometric electrochemical sensor was developed based on a carbon cloth electrodeposited with silver nanoparticles and drop-coated by covalent organic framework (COF-LZU1) for simultaneous determination of bisphenol A (BPA) and bisphenol S (BPS). Carbon cloth exhibited a significantly larger electrochemical active area than common glassy carbon electrodes (27.5 times). Silver nanoparticles not only provided a stable reference signal but also enhanced electroactivity for the oxidation of BPA and BPS. COF-LZU1 with good adsorption performance and large periodic π-arrays promoted the enrichment of BPA and BPS to further increase the current response. Compared with the traditional single-signal electrochemical sensor, the developed ratiometric sensor exhibited better reproducibility and a wider linear range for BPA and BPS from 0.5 to 100 μM with a limit of detection of 0.15 μM. Furthermore, the developed sensor showed excellent stability and superior anti-interference ability. The real sample analysis for BPA and BPS has been successfully carried out in mineral water, electrolyte drink, tea, juice, and beer with recoveries of 88.3-111.7%. The developed ratiometric sensor is expected to be a candidate for the preparation of other electrochemical sensors and the analysis of additional practical samples.

Specifically triggered dissociation based ratiometric electrochemical sensor for H2O2 measurement in food samples

A novel ratiometric strategy based electrochemical sensor was developed to quantitative assay of H₂O₂ in different food samples. 4-aminophenylboronic acid pinacol ester (ABAPE) dissociation was specifically triggered by H₂O₂ to generate electro-active 4-aminophenol (4-AP), which not only can be oxidized to indirectly indicate the concentration of H₂O₂, but also endowed the sensor with high selectivity. Meanwhile, a reference probe of poly(thionine) (TH) was modified with ketjen black (KB) and gold nanoparticles (AuNPs) on electrode surface. KB and AuNPs displayed high electrocatalytic activity to 4-AP. A current ratio between 4-AP and TH (i/i_TH) showed a good linear relationship with the concentration of H₂O₂ in a range of 3.0 × 10^-7 - 1.0 × 10^-4 mol/L (0.010 ppm - 3.40 ppm) with a limit of detection of 2.6 × 10^-7 mol/L (0.009 ppm) (S/N = 3). Moreover, the ratiometric strategy based sensor possessed good accuracy, reliability, and stability, and successfully determined H₂O₂ in food samples with satisfactory results.

Ratiometric electrochemical sensor for bisphenol A detection using a glassy carbon electrode modified with a poly(toluidine blue)/gold nanoparticle composite

A ratiometric electrochemical sensor for bisphenol A (BPA) detection is developed using a glassy carbon electrode modified with a poly(toluidine blue)/gold nanoparticle composite (PTB/AuNP/GCE). The ratiometric signal, namely, the oxidation peak current ratio of BPA to PTB, increases linearly with BPA concentration in the 0.2-5.0 μM range, with a detection limit of 0.15 μM. The electrochemical mechanism of BPA is studied at the PTB/AuNP/GCE, and the results show that BPA undergoes an electrooxidation process of two electrons and two protons at the PTB/AuNP/GCE. The proposed sensor has high sensitivity, high stability and good selectivity. The application of BPA in water samples is successfully verified using the proposed ratiometric electrochemical sensor.

A petal-shaped MOF assembled with a gold nanocage and urate oxidase used as an artificial enzyme nanohybrid for tandem catalysis and dual-channel biosensing

A facile one-pot precipitation method was employed to prepare a petal-shaped hybrid under mild conditions. The hybrid is composed of urate oxidase (UOx) encapsulated into a zeolite-like metal-organic framework (MOF) with the doping of a hollow gold nanocage (AuNC). As one of the MOF-enzyme composites, a UOx@MOF(AuNC) hybrid with the features of artificial nanoenzymes was developed as a novel dual-channel biosensing platform for fluorescence (FL) and electrochemical detection of uric acid (UA). As for FL biosensing, enzymatic catalysis of the hybrid in the presence of UA triggered tandem catalysis and oxidation reactions to cause FL quenching. UA was linearly detected in the 0.1-10 μM and 10-300 μM ranges, with the limit of detection (LOD) of 20 nM. As for electrochemical biosensing, the hybrid was dropped on a glassy carbon electrode (GCE) surface to construct a hybrid/GCE platform. Based on the redox reaction of UA on the platform surface, UA was linearly detected in the 0.05-55 μM range, with a LOD of 15 nM. Experimental results confirmed that the hybrid-based dual-channel biosensing platform enabled selective and sensitive responses to UA over potential interferents. The platform has an excellent detection capability in physiological samples. The dual-channel biosensing platform facilitates the exploration of new bioanalysis techniques for early clinical diagnosis of diseases.

MXene@Ag-based ratiometric electrochemical sensing strategy for effective detection of carbendazim in vegetable samples

In this paper, a novel ratiometric electrochemical sensor for carbendazim (CBZ) detection was constructed by a composite of MXene@Ag nanoclusters and amino-functionalized multi-walled carbon nanotubes (MXene@AgNCs/NH₂-MWCNTs). The Ag nanoclusters (AgNCs) embedded in the MXene not only could inhibit the aggregation of MXene flakes and enhance the electrocatalytic ability, but also serve as an internal reference probe for the ratiometric electrochemical detection. Moreover, the introduction of NH₂-MWCNTs can further improve the electrochemical signals of CBZ and Ag, resulting in the enhanced signal amplification and higher sensitivity. Based on these characteristics of the MXene@AgNCs/NH₂-MWCNTs composite, the proposed sensor exhibits a favorable linear relationship between I_CBZ/I_AgNCs and the concentration of CBZ ranging from 0.3 nM to 10 μM and a low limit of detection of 0.1 nM. Moreover, the proposed ratiometric electrochemical sensing platform also demonstrates high selectivity, good reproducibility, secular stability, and satisfactory applicability in vegetable samples.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Facile construction of ratiometric electrochemical immunosensor using hierarchical PtCoIr nanowires and porous SiO2@Ag nanoparticles for accurate detection of septicemia biomarker

Procalcitonin (PCT) is a sensitive and specific biomarker for sepsis diagnosis. In this study, a novel ratio-typed electrochemical immunosensor was constructed for reliable and sensitive assay of PCT based on hierarchical PtCoIr nanowires/polyethylene polyamine-grafted-ferrocene (PtCoIr HNWs/PEPA-Fc) and porous SiO₂@Ag nanoparticles-toluidine blue (porous SiO₂@Ag NPs-TB). Importantly, the PtCoIr HNWs/PEPA-Fc was first modified on the sensing interface, which harvested stable and strong electrochemical signals for readout of Fc due to the enriched anchoring sites created by the PtCoIr HNWs. Meanwhile, porous SiO₂@Ag NPs-TB behaved as the label to conjugate with secondary antibody (Ab₂), which also provided another strong detection signals originated from TB confined in such porous structures. The resulting immunosensor displayed a measurable output of procalcitonin (PCT) in the dynamic scope of 0.001 ~ 100 ng mL^-1 with a low limit of detection (LOD) of 0.46 pg mL^-1 (S/N = 3). Moreover, we exploited this strategy for PCT assay in a diluted human serum sample with acceptable results, exhibiting promising applications in the clinical analysis.

Electropolymerization of poly(methylene blue) on flower-like nickel-based MOFs used for ratiometric electrochemical sensing of total polyphenolic content in chrysanthemum tea

A ratiometric electrochemical sensor for caffeic acid (CAE) detection was constructed using a glassy carbon electrode modified with poly(methylene blue) and flower-like nickel-based metal organic frameworks (PMB@Ni-TPA/GCE). The electrochemical behavior of CAE was investigated at the PMB@Ni-TPA/GCE, and was found to follow a two-electron, two-proton electrooxidation process. PMB was used as the internal reference probe, and Ni-TPA can enhance the electrochemical signals of both CAE and PMB. As the CAE concentration increases, the oxidation peak current of CAE is enhanced but that of PMB keeps almost unchanged. The oxidation peak current ratio between CAE and PMB recorded by differential pulse voltammetry changes linearly with CAE concentration over the range of 0.25-15.0 μM, with a detection limit of 0.2 μM. The proposed sensor was successfully employed to evaluate the total polyphenolic content as CAE equivalent in chrysanthemum tea, and the results were comparable with those given by the reference Folin-Ciocalteu spectrophotometry.

Mn-Doping-induced hierarchical petal growth of a flower-like 3D MOF assembled with black phosphorous nanosheets as an electrochemical aptasensor of human stress-induced phosphoprotein 1

Herein, we report the preparation of Mn-doped Ni-based metal-organic frameworks (Mn-MOF) with 3D hierarchical flower-like superstructures through solvothermal synthesis. The Mn-MOF was assembled with 2D black phosphorous nanosheets (BPNSs) to achieve novel 2D/3D BPNSs/Mn-MOF nanocomposites, followed by the direct coupling of methylene blue (MB)-labeled DNA aptamer on the interface of the nanocomposites-modified glassy carbon electrode (GCE). The aptamer/BPNSs/Mn-MOF/GCE platform was utilized for the capture and efficient detection of stress-induced phosphoprotein 1 (STIP1). Experimental results confirmed that Mn-doping-induced the hierarchical petal growth of the flower-like 3D MOF and its assembly with BPNSs. GCE surface modifications with various components were studied by measuring electrochemical curves. The morphologies, microstructures and spectra of products were characterized. The optimal conditions used for electrochemical measurements were assessed. A smart aptasensor was explored by the aptamer/BPNSs/Mn-MOF/GCE that had multiple attractive merits, including synergistic effects of components, porous superstructures of hierarchical flower-like 3D Mn-MOF and specific aptamer-target recognition. The merits endowed this aptasensor with selective and sensitive signal responses to STIP1 over interferences. This aptasensor enabled the efficient detection of STIP1 in a broad range of 2 × 10^-3-1 × 10⁴ ng mL^-1, accompanied by a low limit of detection of 1 pg mL^-1. This aptasensor realized the successful determination of STIP1 in practical samples, exhibiting high reliability and practicability.

Ultrasensitive detection of amoxicillin by TiO2-g-C3N4@AuNPs impedimetric aptasensor: Fabrication, optimization, and mechanism

The trace amount of antibiotics in water can be enriched in the human body through the food chain, leading to extremely harmful effects on people's health. Therefore, it is urgent to develop new methods to detect trace pollutants in various aquatic phase. An analytical method utilizing the synergistic effect between the sensing strategy and catalytic material with high electron transfer capacity can be used to detect trace antibiotics. In this paper, an ultrasensitive impedimetric aptasensor was fabricated by the synergy between functionalized materials (TiO₂-g-C₃N₄) and gold nanoparticles (Au NPs). Due to the formation of the 'Au-S' bond between the thiol-aptamer and Au NPs, amoxicillin and the aptamer can be specifically recognized on the modified glassy carbon electrode (GCE), and the impedance signal increased rapidly. Meanwhile, the Box-Behnken Design (BBD) strategy was used to reduce the random error of the experiment, so that the prepared aptasensor has the highest sensitivity to the detection of amoxicillin. Under optimized conditions, the sensor successfully achieved the detection of amoxicillin in the ultra-low detection range (0.5-3 nM) and reached the ultra-low detection limit (0.2 nM). The detection strategy has good selectivity, reproducibility, and stability, and thus has good potential to detect amoxicillin in actual wastewater.

Electrochemical aptasensors based on the gold nanostructures

Electrochemical aptasensors as novel diagnostic tools have attracted sufficient research interest in biomedical sciences. In this review, recent leading trends about gold (Au) nanostructures based electrochemical aptasensors have been collected, reviewed, and compared. Here, the considered electrochemical aptasensors were categorized based on the analytes and diagnostic techniques. Pharmaceutical analytes and biomolecules were reviewed in a separate section consisting of a variety of antibiotics, analgesics, and other biomolecules. Various aptasensors have also measured toxins, ions, and hazardous chemicals, and the findings of them have also been reviewed. Many aptasensors have been designed to detect different disease biomarkers that will play an essential role in the future of early diagnosis of diseases. Pathogen microorganisms have been considered as the analyte in several designed electrochemical aptasensors in recent researches, and their results have been reviewed and discussed as another section. Important aspects considered in the review of the mentioned aptasensors were the type of analyte, features of the aptamer as the biorecognition element, type of Au nanostructures, diagnostic technique, diagnostic mechanism, detection range and the limit of detection (LOD). In the last section, an in-depth analysis has been provided based on the crucial features of all included aptasensors.

Assembly of Black Phosphorus Nanosheets and MOF to Form Functional Hybrid Thin-Film for Precise Protein Capture, Dual-Signal and Intrinsic Self-Calibration Sensing of Specific Cancer-Derived Exosomes

As the emerging and noninvasive biomarkers, exosomes play an important role in cancer screening, cancer-related immune response, and the physiological process. The sensitive, specific, and efficient detection of cancer cell-derived exosomes is of significance for early cancer diagnosis of patients. This work developed a novel dual-signal and intrinsic self-calibration aptasensor of exosomes based on a functional hybrid thin-film platform. This platform was constructed via facile assembly of black phosphorus nanosheets (BPNSs) and ferrocene (Fc)-doped metal-organic frameworks (ZIF-67) on indium tin oxide (ITO) slice, followed by combining methylene blue (MB)-labeled single- strand DNA aptamer on ITO slice. The resultant aptamer-BPNSs/Fc/ZIF-67/ITO platform had dual redox-signal responses of MB (labeled on aptamer) and Fc (doped into ZIF-67). In the presence of specific cancer cell-derived exosomes, the redox current of MB regularly reduced and that of Fc (as reference) hardly changed. An intrinsic self-calibration aptasensor was achieved and enabled sensitive detection of exosomes, showing a limit of detection down to 100 particles mL^-1. The aptasensor with a capability of precise protein capture can efficiently determine specific cancer cell-derived exosomes in practical human serum and plasma samples from healthy individuals and breast cancer patients. In light of excellent performances, this aptasensor can be expanded to multiple biomarkers from cell line exosomes and is beneficial for exploring advanced techniques for high-performance detection of exosomes derived from different types of cancer cells. This work promotes the development of current techniques for early cancer screening and clinical diagnosis.