Fabrication of multifunctional g-C3N4-modified Au/Ag NRs arrays for ultrasensitive and recyclable SERS detection of bisphenol A residues

Monolayer g-C3N4-modified Au/Ag nanorods (g-C3N4/Au/Ag NRs) array is fabricated as a dual-function platform with high surface-enhanced Raman scattering (SERS) response and excellent photocatalytic degradation ability for bisphenol A (BPA) residues. FDTD simulation results of Au/Ag NRs proves that the electromagnetic field intensity is significantly enhanced at the gap of Ag NRs and Au NPs and the protrusion of Au NPs, which endows the arrays with excellent SERS activity. The arrays exhibit high sensitivity for rhodamine 6G (R6G) (LOD = 1.1 × 10-11 mol/L) and high SERS enhancement (EF = 9.2 × 107). In addition, the g-C3N4/Au/Ag NRs could degrade ˃90% of BPA adsorbed on the substrate surface within 140 min under visible light irradiation, and maintains its SERS activity after repeated use for 4 times. The dual-function platform with high SERS response and excellent recycling capability is proved to be reliable and is very promising for monitoring of BPA residues in food.

Reduced Glutathione-Modified Electrode for the Detection of Hydroxyl Free Radicals

Hydroxyl radicals (•OH) are known as essential chemicals for cells to maintain their normal functions and defensive responses. However, a high concentration of •OH may cause oxidative stress-related diseases, such as cancer, inflammation, and cardiovascular disorders. Therefore, •OH can be used as a biomarker to detect the onset of these disorders at an early stage. Reduced glutathione (GSH), a well-known tripeptide for its antioxidant capacity against reactive oxygen species (ROS), was immobilized on a screen-printed carbon electrode (SPCE) to develop a real-time detection sensor with a high selectivity towards •OH. The signals produced by the interaction of the GSH-modified sensor and •OH were characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV curve of the GSH-modified sensor in the Fenton reagent exhibited a pair of well-defined peaks, demonstrating the redox reaction of the electrochemical sensor and •OH. The sensor showed a linear relationship between the redox response and the concentration of •OH with a limit of detection (LOD) of 49 µM. Furthermore, using EIS studies, the proposed sensor demonstrated the capability of differentiating •OH from hydrogen peroxide (H2O2), a similar oxidizing chemical. After being immersed in the Fenton solution for 1 hr, redox peaks in the CV curve of the GSH-modified electrode disappeared, revealing that the immobilized GSH on the electrode was oxidized and turned to glutathione disulfide (GSSG). However, it was demonstrated that the oxidized GSH surface could be reversed back to the reduced state by reacting with a solution of glutathione reductase (GR) and nicotinamide adenine dinucleotide phosphate (NADPH), and possibly reused for •OH detection.

In Vivo Electrochemical Biosensors: Recent Advances in Molecular Design, Electrode Materials, and Electrochemical Devices

Electrochemical biosensors provide powerful tools for dissecting the dynamically changing neurochemical signals in the living brain, which contribute to the insight into the physiological and pathological processes of the brain, due to their high spatial and temporal resolutions. Recent advances in the integration of in vivo electrochemical sensors with cross-disciplinary advances have reinvigorated the development of in vivo sensors with even better performance. In this Review, we summarize the recent advances in molecular design, electrode materials, and electrochemical devices for in vivo electrochemical sensors from molecular to macroscopic dimensions, highlighting the methods to obtain high performance for fulfilling the requirements for determination in the complex brain through flexible and smart design of molecules, materials, and devices. Also, we look forward to the development of next-generation in vivo electrochemical biosensors.

Electrochemical Sensors for the Detection of Reactive Oxygen Species in Biological Systems: A Critical Review

Reactive oxygen species (ROS) involving superoxide anion, hydrogen peroxide and hydroxyl radical play important role in human health. ROS are known to be the markers of oxidative stress associated with different pathologies including neurodegenerative and cardiovascular diseases, as well as cancer. Accordingly, ROS level detection in biological systems is an essential problem for biomedical and analytical research. Electrochemical methods seem to have promising prospects in ROS determination due to their high sensitivity, rapidity, and simple equipment. This review demonstrates application of modern electrochemical sensors for ROS detection in biological objects (e.g., cell lines and body fluids) over a decade between 2011 and 2021. Particular attention is paid to sensors materials and various types of modifiers for ROS selective detection. Moreover, the sensors comparative characteristics, their main advantages, disadvantages and their possibilities and limitations are discussed.

Indolenine-Based Derivatives as Customizable Two-Photon Fluorescent Probes for pH Bioimaging in Living Cells

Novel pH probes based on 2-(6-methoxynaphthalen-2-yl)-3,3-dimethyl-3H-indole have been synthesized and characterized. These compounds display excellent "off-on" fluorescence responses to acidic pH especially under two-photon (TP) excitation conditions as well as strong selectivity and sensitivity toward H⁺. These features are supported by fluorescence quantum yields over 35%, TP cross sections ∼60 GM, and good resistance to photodegradation under acidic conditions. The synthetic versatility of this model allows subcellular targets to be tuned through minor scaffold modifications without affecting its optical characteristics. The effectiveness of the probes' innate photophysical properties and the structural modifications for different pH-related applications are demonstrated in mouse embryonic fibroblast cells.

A highly selective and sensitive upconversion nanoprobe for monitoring hydroxyl radicals in living cells and the liver

Excessive reactive oxygen species (ROS) would attack living cells and cause a series of oxidative stress related diseases, such as liver damage. Hydroxyl radicals (·OH) are currently known as one of the most toxic and harmful free radicals to organisms. Therefore, studies involving hydroxyl radicals have become important research topics in the fields of biology, biochemistry, and biomedicine. In addition, imaging of analytes using upconversion nanoparticles (UCNPs) possesses significant advantages over that using general fluorescent dyes or nanoparticles due to its high spatial resolution, reduced photodamage, and deep tissue penetration properties. Herein, we designed a highly selective and sensitive hydroxyl radical nanoprobe based on the luminescence resonance energy transfer between upconversion nanoparticles and methylene blue (MB). The concentration of ·OH could be determined by the fluorescence recovery of the UCNPs due to the oxidative damage of MB. Using this nanoprobe, the ·OH in living cells or in liver tissues could be monitored with high sensitivity and selectivity.

In situ detection of hydroxyl radicals in mitochondrial oxidative stress with a nanopipette electrode

A nanopipette sensor was designed for the in situ detection of ˙OH around mitochondria with high selectivity and sensitivity.

An endoplasmic reticulum-targeting fluorescent probe for imaging ˙OH in living cells

A new ER-targeting fluorescent probe for ˙OH is developed and applied to imaging ˙OH generation as well as lipid droplet formation in ER stress.

A novel electrochemical sensor for determination of hydroxyl radicals in living cells by coupling nanoporous gold layer with self-assembled 6-(Ferrocenyl) hexanethiol

The detection of hydroxyl radicals (•OH) in live cells is significant to study its physiological and pathological roles, while it is full of challenge due to the extremely low concentration and short lifetime of •OH. Herein, we have developed a novel electrochemical sensor based on 6-(Ferrocenyl) hexanethiol (6-FcHT) self-assembled nanoporous gold layer (NPGL) modified GE (6-FcHT/NPGL/GE), which can detect the release of •OH from living cells with high sensitivity and selectivity. The superior sensitivity can stem from the unique porous architecture of NPGL, which enlarged electrode surface area and expedited electron transportation during electrochemical reactions. Additionally, NPGL provides more active binding sites for the assembly of capture agent (6-FcHT) of •OH, thus ensuring high selectivity. For comparison, 6-FcHT/GE was applied to detect •OH, and the obtained sensitivity was 0.0305 mA nM^-1 and detection limit was 0.133 nM in the linear range of 0.4 nM-70 nM. After modification of NPGL, the sensitivity of 6-FcHT/NPGL/GE to the •OH response was increased to 0.1364 mA nM^-1, detection limit was reduced to 0.316 pM and the linear range was extended from 1 pM to 100 nM. It is worth mentioning that a plenty of extra merits has also been validated like reproducibility, repeatability and stability, enabling to direct electrochemical detection of •OH in HepG2 cells.

Double signal amplification through a functionalized nanoporous Au–Ag alloy microwire and Au nanoparticles: development of an electrochemical ˙OH sensor based on a self-assembled layer of 6-(ferrocenyl)hexanethiol

Novel electrochemical sensors were developed based on a FcHT functionalized NPAMW and AuNPs for the analysis of ˙OH released from live cells.

Ratiometric SERS imaging and selective biosensing of nitric oxide in live cells based on trisoctahedral gold nanostructures

Herein, a ratiometric SERS probe was created for monitoring nitric oxide (NO) by designing a novel molecule, 3,4-diaminobenzene-thiol, and immobilizing this molecule onto trisoctahedral gold nanostructures with superior SERS capability. The established probe possessed good selectivity and biocompatibility, high sensitivity and accuracy, thus enabling imaging and biosensing of NO in live cells.

In vivo monitoring of local pH values in a live rat brain based on the design of a specific electroactive molecule for H+

We have developed a two-channel electrochemical ratiometric biosensor for local pH determination in different regions of the rat brain with remarkable selectivity and accuracy, and report the accurate pH values.

A Rationally Designed Upconversion Nanoprobe for in Vivo Detection of Hydroxyl Radical

The detection of •OH in live organisms is crucial to the understanding of its physiological and pathological roles; while this is too challenging because of the extremely low concentration and high reactivity of the species in the body. Herein, we report the rational design and fabrication of an NIR-light excited luminescence resonance energy transfer-based nanoprobe, which for the first time realizes the in vivo detection of •OH. The nanoprobe is composed of two moieties: upconversion nanoparticles with sandwich structure and bared surface as the energy donor; and mOG, a modified azo dye with tunable light absorption, as both the energy acceptor and the •OH recognizing ligand. The as-constructed nanoprobe exhibited ultrahigh sensitivity (with the quantification limit down to 1.2 femtomolar, several orders of magnitude lower than that of most previous •OH probes), good biocompatibility, and specificity. It was successfully used for monitoring [•OH] levels in live cells and tissues.

Ratiometric fluorescence probe for monitoring hydroxyl radical in live cells based on gold nanoclusters

Determination of hydroxyl radical ((•)OH) with high sensitivity and accuracy in live cells is a challenge for evaluating the role that (•)OH plays in the physiological and pathological processes. In this work, a ratiometric fluorescence biosensor for (•)OH was developed, in which gold nanocluster (AuNC) protected by bovine serum albumin was employed as a reference fluorophore and the organic molecule 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (HPF) acted as both the response signal and specific recognition element for (•)OH. In the absence of (•)OH, only one emission peak at 637 nm ascribed to AuNCs was observed, because HPF was almost nonfluorescent. However, fluorescence emission at 515 nm attributed to the HPF product after reaction with (•)OH--dianionic fluorescein--gradually increased with the continuous addition of (•)OH, while the emission at 637 nm stays constant, resulting in a ratiometric determination of (•)OH. The developed fluorescent sensor exhibited high selectivity for (•)OH over other reactive oxygen species (ROS), reactive nitrogen species (RNS), metal ions, and other biological species, as well as high accuracy and sensitivity with low detection limit to ∼0.68 μM, which fulfills the requirements for detection of (•)OH in a biological system. In addition, the AuNC-based inorganic-organic probe showed long-term stability against light illumination and pH, good cell permeability, and low cytotoxicity. As a result, the present ratiometric sensor was successfully used for bioimaging and monitoring of (•)OH changes in live cells upon oxidative stress.