Amperometric assay for aldolase activity: antibody-catalyzed ferrocenylamine formation

Self-Immolative Electrochemical Redox Substrates: Emerging Artificial Receptors in Sensing and Biosensing

The development of artificial receptors has great significance in measurement science and technology. The need for a robust version of natural receptors is getting increased attention because the cost of natural receptors is still high along with storage difficulties. Aptamers, imprinted polymers, and nanozymes are some of the matured artificial receptors in analytical chemistry. Recently, a new direction has been discovered by organic chemists, who can synthesize robust, activity-based, self-immolative organic molecules that have artificial receptor properties for the targeted analytes. Specifically designed trigger moieties implant selectivity and sensitivity. These latent electrochemical redox substrates are highly stable, mass-producible, inexpensive, and eco-friendly. Combining redox substrates with the merits of electrochemical techniques is a good opportunity to establish a new direction in artificial receptors. This Review provides an overview of electrochemical redox substrate design, anatomy, benefits, and biosensing potential. A proper understanding of molecular design can lead to the development of a library of novel self-immolative redox molecules that would have huge implications for measurement science and technology.

Development of an activity-based ratiometric electrochemical probe of the tumor biomarker γ-glutamyl transpeptidase: Rapid and convenient sensing in whole blood, urine and live-cell samples

γ-Glutamyl transpeptidase (GGT) is a key biomarker for cancer diagnosis and post-treatment surveillance. Currently available methods for sensing GGT show high potential, but face certain challenges including an inability to be used to directly sense analytes in turbid biofluid samples such as whole blood without tedious sample pretreatment. To overcome this issue, activity-based electrochemical probes (GTLP and GTLPOH) were herein developed for a convenient and specific direct targeting of GGT activity in turbid biosamples. Both probes were designed to have GGT catalyze the hydrolysis of the gamma-glutamyl amide moiety of the probe, and result in a self-immolative reaction and concomitant ejection of the masked amino ferrocene reporter. The GTLPOH probe, delivered distinctive key results including high sensitivity, high affinity, a wide detection range of 2-100 U/L, and low LOD of 0.38 U/L against GGT. This probe delivered a precise target for sensing GGT and was free of interference from other electroactive biological species. Furthermore, the GTLPOH probe was employed to monitor and quantify the activity of GGT on the surfaces of tumor cells. The designed sensing method was also validated by the direct quantitative measurement of GGT activity in whole blood and urine samples, and the results were found to be consistent with those of the standard fluorometric assay kit. Thus, GTLPOH is of great significance for its promise as a point-of-care tool for early-stage cancer diagnosis as well as a new drug screening method.

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.

A Robust and Cost-Effective Luminescent-Based High-Throughput Assay for Fructose-1,6-Bisphosphate Aldolase A

Hypoxic solid tumors induce the stabilization of hypoxia-inducible factor 1 alpha (HIF1α), which stimulates the expression of many glycolytic enzymes and hypoxia-responsive genes. A high rate of glycolysis supports the energetic and material needs for tumors to grow. Fructose-1,6-bisphosphate aldolase A (ALDOA) is an enzyme in the glycolytic pathway that promotes the expression of HIF1α. Therefore, inhibition of ALDOA activity represents a potential therapeutic approach for a range of cancers by blocking two critical cancer survival mechanisms. Here, we present a luminescence-based strategy to determine ALDOA activity. The assay platform was developed by integrating a previously established ALDOA activity assay with a commercial NAD/NADH detection kit, resulting in a significant (>12-fold) improvement in signal/background (S/B) compared with previous assay platforms. A screening campaign using a mixture-based compound library exhibited excellent statistical parameters of Z' (>0.8) and S/B (~20), confirming its robustness and readiness for high-throughput screening (HTS) application. This assay platform provides a cost-effective method for identifying ALDOA inhibitors using a large-scale HTS campaign.

Ratiometric electrochemical detection of β-galactosidase

A novel ferrocene-based substrate for the ratiometric electrochemical detection of β-galactosidase was designed and synthesised. It was demonstrated to be an excellent electrochemical substrate for β-Gal detection with sensitivity as low as 0.1 U mL^-1.

Electrochemical latent redox ratiometric probes for real-time tracking and quantification of endogenous hydrogen sulfide production in living cells

Hydrogen sulfide (H₂S) was discovered as a third gasotransmitter in biological systems and recent years have seen a growing interest to understand its physiological and pathological functions. However, one major limiting factor is the lack of robust sensors to quantitatively track its production in real-time. We described a facile electrochemical assay based on latent redox probe approach for highly specific and sensitive quantification in living cells. Two chemical probes, Azido Benzyl ferrocene carbamate (ABFC) and N-alkyl Azido Benzyl ferrocene carbamate (NABFC) composed of azide trigger group were designed. H₂S molecules specifically triggered the release of reporters from probes and the current response was monitored using graphene oxide film modified electrode as transducer. The detection limits are 0.32µM (ABFC) and 0.076µM (NABFC) which are comparable to those of current sensitive methods. The probes are successful in the determination of H₂S spiked in whole human blood, fetal bovine serum, and E. coli. The continuous monitoring and quantification of endogenous H₂S production in E. coli were successfully accomplished. This work lays first step stone towards real-time electrochemical quantification of endogenous H₂S in living cells, thus hold great promise in the analytical aspects of H₂S.

Ratiometric electrochemical detection of hydrogen peroxide and glucose

Hydrogen peroxide (H₂O₂) detection is of high importance as it is a versatile (bio)marker whose detection can indicate the presence of explosives, enzyme activity and cell signalling pathways. Herein, we demonstrate the rapid and accurate ratiometric electrochemical detection of H₂O₂ using disposable screen-printed electrodes through a reaction-based indicator assay. Ferrocene derivatives equipped with self-immolative linkers and boronic acid ester moieties were synthesised and tested, and, through a thorough assay optimisation, the optimum probe showed good stability, sensitivity and selectivity towards H₂O₂. The optimised conditions were then applied to the indirect detection of glucose via an enzymatic assay, capable of distinguishing 10 μM from the background within minutes.

A switchable electrochemical redox ratiometric substrate based on ferrocene for highly selective and sensitive fluoride detection

A ferrocene based electrochemical redox substrate for highly selective and sensitive fluoride detection in non-transparent solutions.

Ratiometric electrochemical detection of alkaline phosphatase

A novel ferrocene-based substrate incorporating a phosphate trigger is validated for the ratiometric electrochemical detection of alkaline phosphatase activity.

Molecular probe for enzymatic activity with dual output

A novel molecular probe for enzymatic activity with a dual output detection-mode has been developed. The probe effectively detected the presence of the bacterial protease penicillin-G-amidase; a single cleavage by the enzyme initiated the fragmentation of a self immolative dendritic platform to release two reporter units. The signals of the free reporters were detected by two different spectroscopic techniques, fluorescence and UV-vis. This is the first reported molecular probe with two different chromogenic reporter units activated by a specific stimulus.

Ferrocene-based derivatization in analytical chemistry

Ferrocene-based derivatization has raised considerable interest in many fields of analytical chemistry. This is due to the well-established chemistry of ferrocenes, which allows rapid and easy access to a large number of reagents and derivatives. Furthermore, the electrochemical properties of ferrocenes are attractive with respect to their detection. This paper summarizes the available reagents, the reaction conditions and the different approaches for detection. While electrochemical detection is still most widely used to detect ferrocene derivatives, e.g., in the field of DNA analysis, the emerging combination of analytical separation methods with electrochemistry, mass spectrometry and atomic spectroscopy allows ferrocenes to be applied more universally and in novel applications where strongly improved selectivity and limits of detection are required.

A new visual screening assay for catalytic antibodies with retro-aldol retro-Michael activity

Fast and convenient methods are required for the detection of novel catalysts. We have developed a new assay to allow direct visualization of retro-aldol retro-Michael catalytic activity and have demonstrated it with catalytic antibody 38C2. The assay is based on a catalytic cleavage of a physiologically stable substrate to release 3,4-cyclohexeneoesculetin. The latter then reacts with iron(III) to generate a non-soluble complex that precipitates in the form of a black dye. This assay may be used for screening new catalysts for retro-aldol retro-Michael activity with improved efficiency for specific prodrug activation.