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.

Enhanced Room-Temperature Thermoelectric Performance of 2D-SnSe Alloys via Electric-Current-Assisted Sintering.

Single-crystalline tin-selenide (SnSe) has emerged as a high-performance and eco-friendly alternative to the lead-chalcogens often used in mid-temperature thermoelectric (TE) generators. At high temperature >800 K, the phase transition from Pnma to Cmcm causes a significant rise in the TE figure-of-merit (zT) curve. Conversely, the SnSe TE requires a booster at low temperatures, which allows broader applicability from a device perspective. Herein, a synergy of Cu alloy and Ag-coating is realized through a sequential multi-step synthesis, designed to combine different metal deposition effects. Single-crystalline (Cu2Se)x(SnSe)1−x alloys grown by the Bridgman method were then coated with a thin Ag layer by radio frequency (RF) sputtering, and the interlayer epitaxial film was observed via electric-current assisted sintering (ECAS). Consequently, the thin Ag-coating improves the electrical conductivity (σ) and reduces the thermal conductivity (κ) for (Cu2Se)0.005(SnSe)0.995+Ag alloy, increasing the zT curve at close to room temperature (373 K). The incorporation of multistep addition by ECAS enables tuning of the overall solubility of the alloy, which opens a new avenue to optimize TE performance in anisotropic 2D materials.

Latent Redox Reporter of 4-Methoxyphenol as Electrochemical Signal Proxy for Real-Time Profiling of Endogenous H2O2 in Living Cells.

Hydrogen peroxide (H2O2) plays a persuasive role in the human cell physiology. Developing an efficient assay platform and a highly sensitive tracking and quantification of H2O2 in a physiological system is crucial to understand the neoplastic changes and/or redox homeostasis of cells. In this study, a novel turn-on latent electrochemical redox probe coupled with electrocatalytic signal amplification strategy is proposed. A custom-made readily available turn-on latent electrochemical probe 4-methoxyphenylboronic acid pinacol ester (4-MPBP) have been designed for the selective detection of endogenous H2O2 in live cells. The electrochemical probe composed of a latent electrochemical reporter (4-methoxy phenol, 4-MP) bearing a recognition unit (boronic acid pinacol ester) for H2O2 sensing. The selective analyte-triggered chemical transformation releases free electrochemical reporter 4-MP. The amount of H2O2 was evaluated electrochemically at glassy carbon electrode (GCE) with a broad detection range of 0.5 µM-1 mM. An amplified signal response of released 4-MP to build a highly sensitive assay tool has been achieved via replacing the GCE transducer electrode with polydopamine@carbonnanotube-molebtinumdisulfie hybrid modified GCE as it delivered an exceptional dynamic detection range of 0.01-100 µM. The innovative blend of electrochemical molecular probe strategy, with electrocatalytic signal amplification technique has delivered outstanding assay performance at trace level sensing of H2O2. Next, we set up a platform for real-time in vivo monitoring of the endogenously produced H2O2 in Caco-2 and MCF-7 cells through spermine-polyamine analogue and phorbol 12-myristate 13-acetate induction in SSAT/PAO gene and protein kinase C, respectively. As expected, the 4-MPBP latent probe coupled with electrocatalytic signal amplification strategy delivered outstanding performance for in situ H2O2 release and tracking over time.

Multifunctional Regulation of 3D Cell-Laden Microsphere Culture on an Integrated Microfluidic Device.

Three-dimensional (3D) hydrogel microspheres have aroused increasing attention as an in vitro cell culture model. Yet the preservation of cells' original biological properties has been overlooked during model construction. Here we present an integrated microfluidic device to accomplish the overall process including cell-laden microsphere generation, online extraction, and dynamic-culture. The method extends the noninvasive and nonsuppression capabilities of the droplet preparation system and provides a constant microenvironment, which reduces intracellular oxidative stress damage and the accumulation of mitochondria. Compared to the conventional preparation method, the coculture model of tumor-endothelial construction on an integrated platform displays high-level angiogenic protein expression. We believe that this versatile and biocompatible platform will provide a more reliable analysis tool for tissue engineering and cancer therapy.

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.

A Sensitive Ratiometric Long-Wavelength Fluorescent Probe for Selective Determination of Cysteine/Homocysteine.

The development of sensitive fluorescence probes to detect biothiols such as cysteine and homocysteine has attracted great attention in recent times. Herein, we described the design and synthesis of coumarin based long-wavelength fluorescence probe, Bromo-2-benzothiazolyl-3-cyano-7-hydroxy coumarin (BBCH, 2) for selective detections of cysteine and homocysteine. The probe is rationally designed in such a way that both sulfhydryl and adjacent amino groups of thiols are involved in sensing process. Only cysteine/homocysteine able to react with BBCH to release fluorescence reporter (BCH, 1); while, glutathione and other amino acids unable to react with BBCH due to the absence of adjacent amino groups. In presence of cysteine, the color of BBCH is turns from colorless to red and thus BBCH is a naked eye fluorescence indicator for cysteine. Besides, BBCH can discriminate cysteine and homocysteine based on color changes and different reaction rates. The described sensing platform showed good sensing performances to detect cysteine and homocysteine with detection limits of 0.87 and 0.19 µM, respectively. Practical applicability was verified in biological and pharmaceutical samples.

A new electrochemical substrate for rapid and sensitive in vivo monitoring of ß-galactosidase gene expressions.

A 4-Methoxyphenyl-ß-galactopyranoside (4-MPGal) substrate incorporating 4-methoxy phenol (4-MP) as an electrochemical reporter is described for the monitoring of ß-Galactosidase (ß-Gal) gene expressions. ß-Gal derived from Escherichia coli (E. coli) and Aspergillus oryzae (A. oryzae) were investigated, while a graphene oxide film modified electrode was employed as the transducer. The electrochemical signal of 4-MPG within 4-MPGal was masked by protecting their hydroxyl group with galactose. The externally added ß-Gal triggered the deprotection through specific enzymatic hydrolysis with concomitant release of 4-MP. The apparent Km and Vmax values of 4-MPGal are determined to be 0.21 mM and 0.51 µM min(-1) mg of ß-Gal(-1) (E. coli), which is consistent with the previous reports. To detect ß-Gal derived from E. coli, cyclic voltammetry (CV) provides linear ranges of 12-1200 ng mL(-1) and 1.2-12 µg mL(-1) with a limit of detection (LOD) of 5 ng mL(-1), while differential pulse voltammetry (DPV) shows a linear range of 1.2-120 ng mL(-1) and LOD of 1 ng mL(-1). To detect ß-Gal derived from A. oryzae, CV provides linear ranges of 0.1-100 ng mL(-1) and 0.1-1 µg mL(-1) with a LOD of 0.06 ng mL(-1), while DPV shows a linear range of 10 pg mL(-1)-10 ng mL(-1) with a LOD of 8 pg mL(-1). Moreover, we set up a platform for the real-time in vivo monitoring of ß-Gal gene expressions in E. coli cultivated through microbiological culture. The developed sensing platform using 4-MPGal as a substrate is simple, rapid, sensitive, specific and advantageous over its laborious optical analogues.