A ratiometric electrochemical strategy for sensitive determination of Furin activity based on dual signal amplification and antifouling nanosurfaces

Biomimetic mesoporous carbon-silica/AAO asymmetric nanochannel array for electrochemical sensing of K+ in rat brain microdialysates and serum

Acquirement of chemical expression in practical brain system is vital to understand the molecular mechanism involved in physiological and pathological processes in brain. Though nanochannels have been demonstrated to be promising platform for electrochemical sensor, it is a great challenge for nanochannels to be employed in practical brain biofluid. In this work, we rationally designed and created the biomimetic asymmetric nanochannels for sensing of K+ through integrating in situ modification of a two-component mesoporous carbon-silica (MCS) thin film with a pore size of ∼3.6 nm at anodic alumina nanochannel array (AAO) with the ∼40 nm pores (denoted as MCS/AAO). Apparent rectification phenomenon in such functionalized nanochannel array was achieved based on diode-like ion transport. Then, 4'-aminobenzeno-18-crown-6 (SP) was selected to be chemically decorated at MCS/AAO as the specific recognition for K+ (SP/MCS/AAO). The developed SP/MCS/AAO exhibited good selectivity towards K+ detection against the coexisting interferences in brain, and possessed a good linear response to K+ concentration in the range of 0.5-10 mM with a detection limit of 0.1 mM. Combined with microdialysis technique, the variation of K+ was successfully determined in rat brain microdialysates and serums. Compared with normal rats, the concentration of K+ was found to be greatly decreased in the cerebral microdialysates and serum of rats with hypertensive model (SHR). This work unveiled a powerful platform for K+, and promised to be extended to design new strategy for detecting other chemical species, in particular non-electroactive species in biofluid related to physiological and pathological events.

Gold Microbeads Enabled Proximity Electrochemiluminescence for Highly Sensitive and Size-Encoded Multiplex Immunoassays

Developing highly sensitive multiplex immunoassays is urgently needed to guide medical research and improve clinical diagnosis. Here, we report the proximity electrochemiluminescence (ECL) generation enabled by gold microbeads (GMBs) for improving the detection sensitivity and multiplexing capacity of ECL immunoassays (ECLIAs). As demonstrated by microscopy and finite element simulation, GMBs can function as spherical ultramicroelectrodes for triggering ECL reactions in solutions. Employing GMBs as solid carriers in the bead-based ECLIA, the electrochemical oxidation of a coreactant can occur at both the GMB surface and the substrate electrode, allowing the coreactant radicals to diffuse only a short distance of ∼100 nm to react with ECL luminophores that are labeled on the GMB surface. The ECL generation via this proximity low oxidation potential (LOP) route results in a 21.7-fold increase in the turnover frequency of ECL generation compared with the non-conductive microbeads that rely exclusively on the conventional LOP route. Moreover, the proximity ECL generation is not restricted by the diffusion distance of short-lived coreactant radicals, which enables the simultaneous determination of multiple acute myocardial infarction biomarkers using size-encoded GMB-based multiplex ECLIAs. This work brings new insight into the understanding of ECL mechanisms and may advance the practical use of multiplex ECLIAs.

Fluorescence and ratiometric photoacoustic imaging of endogenous furin activity via peptide functionalized MoS2 nanosheets

Furin is an important cellular endoprotease, which is expressed at high levels in various cancer cells. Accurate and real-time detection of endogenous furin with high sensitivity and selectivity is significant for the diagnosis of cancer. Herein an activatable nanoprobe (MoS₂@PDA-PEG/peptide, MPPF) with dual-mode near-infrared fluorescence (NIRF)/ratiometric photoacoustic (PA) imaging of endogenous furin activity has been developed. The MPPF nanoprobes were constructed by the covalent functionalization of polydopamine (PDA) coated MoS₂ nanosheets (NSs) with Cy7-labeled furin substrate peptides. Upon cleavage of the peptides by furin, Cy7 molecules are released from MPPF nanoprobes and recover their fluorescence, realizing furin activity detection with the limit of detection (LOD) down to 3.73 × 10^-4 U mL^-1. Meanwhile, the ratio of the PA signal at 768 nm to that at 900 nm (PA₇₆₈/PA₉₀₀) decreases over time due to the destruction of fluorescence resonance energy transfer effect from Cy7 to MoS₂ NSs and the rapid clearance of small Cy7 molecules from tissues. Thus, the simultaneous change in NIRF and ratiometric PA signals enables the imaging of endogenous furin activity in real time, and with high sensitivity, and high selectivity in both tumor cells and tumor-bearing mice.

AIEgen-Based Lifetime-Probes for Precise Furin Quantification and Identification of Cell Subtypes

Biochemical sensing probes based on aggregation-induced-emission luminogens (AIEgens) are widely used in biological imaging and therapy, chemical sensing, and material sciences. However, it is still a great challenge to quantify the targets through fluorescence intensity of AIEgen probes due to their undesirable aggregations. Here, a PyTPA-ZGO probe with three lifetime signals for precise quantification of furin is constructed: the lifetime signal 1 and signal 2 comes from AIEgen PyTPA-P (τ_Pn ) and inorganic nanoparticles Zn₂ GeO₄ :Mn²⁺ -NH₂ (τ_Zn ), respectively, while the lifetime signal 3 is marked as the composite dual-lifetime signal (CDLS_n , C D L S n = τ Z n τ P n ). In contrast, the fluorescence intensity signal of PyTPA-P shows defectively quantitative performance. Furthermore, it is found that the CDLS_n exhibits higher significant differences than the two other lifetime signals (τ_Pn and τ_Zn ) thanks to its wide range between the maximum and minimum signal values and small standard deviation. Therefore, CDLS_n is further used to accurately identify cell subtypes based on the specific concentration of furin in each subtype. The lifetime criterion can realize precise quantification, and it should be a promising direction of AIEgen-based quantitative analysis in the future.

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.

Nonenzymatic Electrochemical Sensor with Ratiometric Signal Output for Selective Determination of Superoxide Anion in Rat Brain

There is still an urgent need to develop reliable analytical methods of O₂^•- in vivo for deeply elucidating the roles of O₂^•- playing in the brain. Herein, a nonenzymatic electrochemical sensor with ratiometric signal output was developed for an in vivo analysis of O₂^•- in the rat brain. Diphenylphosphonate-2-naphthol ester (ND) was designed and synthesized as a specific recognition molecule for the selective determination of O₂^•-. An anodic peak ascribed to the oxidation of 2-naphthol was generated via the nucleophilic substitution between ND and O₂^•- and was increased with the increasing concentration of O₂^•-. Meanwhile, the inner reference of methylene blue (MB) was co-assembled at the electrode surface to enhance the determination accuracy of O₂^•-. The anodic peak current ratio between 2-naphthol and MB exhibited a good linear relationship with the concentration of O₂^•- from 2 to 200 μM. Because of the stable molecule character of ND and its specific reaction with O₂^•-, the developed electrochemical sensor demonstrated excellent selectivity toward various potential interferences in the brain and good stability even after storage for 7 days. Accordingly, the present electrochemical sensor with high selectivity, high stability, and high accuracy was successfully exploited in monitoring the levels of O₂^•- in the rat brain and that of the diabetic model followed by cerebral ischemia.

Ratiometric electrochemical sensor for accurate detection of salicylic acid in leaves of living plants

Detection of signal molecules in living plants is of great relevance for precision farming. In this work, to establish a more effective method for monitoring salicylic acid (SA) in the leaves of living plants, a ratiometric electrochemical sensor was fabricated based on a Cu metal-organic framework (Cu-MOF) and carbon black (CB) composite. The Cu-MOF and CB composite was used to catalyze SA oxidation. Ratiometric oxidation current peak intensities I SA/I Cu-MOFs were used as the response signal for SA. I SA/I Cu-MOFs linearly enhanced with the increase of SA concentration, together with low limits of detection (12.50 μM). Moreover, our sensor is fabricated on a screen-printed electrode (SPE), which is especially suitable for applying to the flat leaves of plants. Using this sensor, the SA level in the leaves of cucumber seedlings was monitored in vivo under salt stress. The proposed sensor is accurate, reliable and practical. This is the first report for developing a ratiometric electrochemical sensor for detecting SA in living plants. Our work can also provide a strategy for in vivo studies on the leaves of plants.

Ratiometric Strategy for Electrochemical Sensing of Carbaryl Residue in Water and Vegetable Samples

Accurate analysis of pesticide residue in real samples is essential for food safety and environmental protection. However, a traditional electrochemical sensor based on single-signal output is easily affected by background noise, environmental conditions, electrode diversity, and a complex matrix of samples, leading to extremely low accuracy. Hence, in this paper, a ratiometric strategy based on dual-signal output was adopted to build inner correction for sensing of widely-used carbaryl (CBL) for the first time. By comparison, Nile blue A (NB) was selected as reference probe, due to its well-defined peak, few effects on the target peak of CBL, and excellent stability. The effects of a derivatization method, technique mode, and pH were also investigated. Then the performance of the proposed ratiometric sensor was assessed in terms of three aspects including the elimination of system noise, electrode deviation and matrix effect. Compared with traditional single-signal sensor, the ratiometric sensor showed a much better linear correlation coefficient (r > 0.99), reproducibility (RSD < 10%), and limit of detection (LOD = 1.0 μM). The results indicated the introduction of proper reference probe could ensure the interdependence of target and reference signal on the same sensing environment, thus inner correction was fulfilled, which provided a promising tool for accurate analysis.

A multifunctional ratiometric electrochemical sensor for combined determination of indole-3-acetic acid and salicylic acid

For the first time, a multifunctional ratiometric electrochemical sensor was developed for quantifying IAA and SA simultaneously.

Antifouling strategies in advanced electrochemical sensors and biosensors

A review presented recent development of antifouling strategies in electrochemical sensors and biosensors based on the modification methods.

Nanotechnology and nanomaterial-based no-wash electrochemical biosensors: from design to application

Nanotechnology and nanomaterial based electrochemical biosensors (ECBs) have achieved great development in many fields, such as clinical diagnosis, food analysis, and environmental monitoring. Nowadays, the single-handed pursuit of sensitivity and accuracy cannot meet the demands of detection in many in situ and point-of-care (POC) circumstances. More and more attention has been focused on simplifying the operation procedure and reducing detection time, and thus no-wash assay has become one of the most effective ways for the continuous development of ECBs. However, there are many challenges to realize no-wash detection in the real analysis, such as redox interferences, multiple impurities, non-conducting protein macromolecules, etc. Furthermore, the complex detection circumstance in different application fields makes the realization of no-wash ECBs more complicated and difficult. Thanks to the updated nanotechnology and nanomaterials, in-depth analysis of the obstacles in the detection process and various methods for fabricating no-wash ECBs, most issues have been largely resolved. In this review, we have systematically analyzed the nanomaterial based design strategy of the state-of-the-art no-wash ECBs in the past few years. Following that, we summarized the challenges in the detection process of no-wash ECBs and their applications in different fields. Finally, based on the summary and analysis in this review, we also evaluated and discussed future prospects from the design to the application of ECBs.

A ratiometric electrochemical sensor for multiplex detection of cancer biomarkers using bismuth as an internal reference and metal sulfide nanoparticles as signal tags

Ratiometric electrochemical sensors can provide a relatively accurate analysis of target analytes due to their self-calibration function. Herein, we report a simple ratiometric strategy for achieving the electrochemical detection of Cd(ii), Hg(ii), Pb(ii) and Zn(ii), as well as multiple cancer biomarkers by using metal sulfide nanoparticles as signal tags. A conductive polymer film of poly(2-amino terephthalic acid) (ATA) was electrochemically produced on a glassy carbon electrode (GCE) and doped with carbon nanotubes (CNTs) and mercaptosuccinic acid (MSA). Using Bi(iii) as an enhancer and internal reference in anodic stripping voltammetry, the MSA-CNT-ATA/GCE exhibited sensitive and distinguishable voltammetric responses to Cd(ii), Hg(ii), Pb(ii) and Zn(ii), with detection limits of 0.13, 0.49, 0.16 and 0.089 μg L^-1, respectively. By using CdS, HgS, PbS and ZnS labeled secondary antibodies as the signal tags, alpha-fetoprotein, carbohydrate antigen 19-9, carbohydrate antigen 125, and carcinoembryonic antigen were determined simultaneously according to the amounts of metal sulfide in the sandwich-type complexes, with detection limits of 0.11 pg mL^-1, 0.68 mU mL^-1, 1.4 mU mL^-1 and 0.23 pg mL^-1, respectively. This ratiometric approach has a wide scope in the electrochemical detection of heavy metal ions as well as immunoassays with metal ions serving as signal tags.

A semisynthetic fluorescent protein assembly-based FRET probe for real-time profiling of cell membrane protease functions in situ

A semisynthetic fluorescent protein assembly-based FRET probe (sFPAP) was proposed for cell membrane protease function assay.