An Electrochemical Biosensor with Dual Signal Outputs: Toward Simultaneous Quantification of pH and O2 in the Brain upon Ischemia and in a Tumor during Cancer Starvation Therapy

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Raman Fiber Photometry for Understanding Mitochondrial Superoxide Burst and Extracellular Calcium Ion Influx upon Acute Hypoxia in the Brain of Freely Moving Animals

Developing a novel tool capable of real-time monitoring and simultaneous quantitation of multiple molecules in mitochondria across the whole brain of freely moving animals is the key bottleneck for understanding the physiological and pathological roles that mitochondria play in the brain events. Here we built a Raman fiber photometry, and created a highly selective non-metallic Raman probe based on the triple-recognition strategies of chemical reaction, charge transfer, and characteristic fingerprint peaks, for tracking and simultaneous quantitation of mitochondrial O₂ ^.- , Ca²⁺ and pH at the same location in six brain regions of free-moving animal upon hypoxia. It was found that mitochondrial O₂ ^.- , Ca²⁺ and pH changed from superficial to deep brain regions upon hypoxia. It was discovered that hypoxia-induced mitochondrial O₂ ^.- burst was regulated by ASIC1a, leading to mitochondrial Ca²⁺ overload and acidification. Furthermore, we found the overload of mitochondrial Ca²⁺ was mostly attributed to the influx of extracellular Ca²⁺ .

Real-time Tracking and Sensing of Cu+ and Cu2+ with a Single SERS Probe in the Live Brain: Toward Understanding Why Copper Ions Were Increased upon Ischemia

The imbalance of Cu⁺ and Cu²⁺ in the brain is closely related to neurodegenerative diseases. However, it still lacks of effective analytical methods for simultaneously determining the concentrations of Cu⁺ and Cu²⁺ . Herein, we created a novel SERS probe (CuSP) to real-time track and accurately quantify extracellular concentrations of Cu⁺ and Cu²⁺ in the live brain. The present CuSP probe demonstrated specific ability for recognition of Cu⁺ and Cu²⁺ in a dual-recognition mode. Then, a microarray consisting of 8 CuSP probes with high tempo-spatial resolution and good accuracy was constructed for tracking and simultaneously biosensing of Cu⁺ and Cu²⁺ in the cerebral cortex of living brain. Using our powerful tool, it was found that that the concentrations of Cu²⁺ and Cu⁺ were increased by ≈4.26 and ≈1.80 times upon ischemia, respectively. Three routes were first discovered for understanding the mechanisms of the increased concentrations of Cu⁺ and Cu²⁺ during ischemia.

Long-Term Tracking and Dynamically Quantifying of Reversible Changes of Extracellular Ca2+ in Multiple Brain Regions of Freely Moving Animals

Understanding physiological and pathological processes in the brain requires tracking the reversible changes in chemical signals with long-term stability. We developed a new anti-biofouling microfiber array to real-time quantify extracellular Ca²⁺ concentrations together with neuron activity across many regions in the mammalian brain for 60 days, in which the signal degradation was < ca. 8 %. The microarray with high tempo-spatial resolution (ca. 10 μm, ca. 1.3 s) was implanted into 7 brain regions of free-moving mice to monitor reversible changes of extracellular Ca²⁺ upon ischemia-reperfusion processes. The changing sequence and rate of Ca²⁺ in 7 brain regions were different during the stroke. ROS scavenger could protect Ca²⁺ influx and neuronal activity after stroke, suggesting the significant influence of ROS on Ca²⁺ overload and neuron death. We demonstrated this microarray is a versatile tool for investigating brain dynamic during pathological processes and drug treatment.

A Robust Au-C≡C Functionalized Surface: Toward Real-Time Mapping and Accurate Quantification of Fe2+ in the Brains of Live AD Mouse Models

Described here is that Au-C≡C bonds showed the highest stability under biological conditions, with abundant thiols, and the best electrochemical performance compared to Au-S and Au-Se bonds. The new finding was also confirmed by theorical calculations. Based on this finding, a specific molecule for recognition of Fe²⁺ was designed and synthesized, and used to create a selective and accurate electrochemical sensor for the quantification of Fe²⁺ . The present ratiometric strategy demonstrates high spatial resolution for real-time tracking of Fe²⁺ in a dynamic range of 0.2-120 μM. Finally, a microelectrode array with good biocompatibility was applied in imaging and biosensing of Fe²⁺ in the different regions of live mouse brains. Using this tool, it was discovered that the uptake of extracellular Fe²⁺ into the cortex and striatum was largely mediated by cyclic adenosine monophosphate (cAMP) through the CREB-related pathway in the brain of a mouse with Alzheimer's disease.

Rational Design of Specific Recognition Molecules for Simultaneously Monitoring of Endogenous Polysulfide and Hydrogen Sulfide in the Mouse Brain

A biosensor was created for the simultaneous monitoring of endogenous H₂ S_n and H₂ S in mouse brains and exploring their roles in activation of the TRPA1 channel under two types of brain disease models: ischemia and Alzheimer's disease (AD). Based on DFT calculations and electrochemical measurements, two probes, 3,4-bis((2-fluoro-5-nitrobenzoyl)oxy)-benzoic acid (M_PS-1 ) and N-(4-(2,5-dinitrophenoxy) phenyl)-5-(1, 2-dithiolan-3-yl)pentanamide (M_HS-1 ), were synthesized for specific recognition of H₂ S_n and H₂ S. Through co-assembly of the two probes at the mesoporous gold film with good anti-biofouling ability and electrocatalytic activity, this microsensor showed high selectivity for H₂ S_n and H₂ S against potential biological interferences. The biosensor can simultaneously determine the concentration of H₂ S_n from 0.2 to 50 μm, as well as that of H₂ S from 0.2 to 40 μm. The expression of TRPA1 protein positively correlated with levels of H₂ S_n under both ischemia and AD.

The Rise of Fiber Electronics

As a new direction in applied chemistry, fiber electronics allow device configuration to evolve from three to two dimensions and then to one dimension. The reduction in dimension brings unique properties, such as ultraflexibility, tissue adaptability, and weavability, enabling their use in a variety of applications, particularly in various emerging fields related to implantable devices and wearable systems. The different types of fiber electrode materials are summarized based on the one-dimensional configuration and their distinctive interfaces, various devices, and promising applications. The remaining challenges and future directions are finally highlighted.

A SERS Optophysiological Probe for the Real-Time Mapping and Simultaneous Determination of the Carbonate Concentration and pH Value in a Live Mouse Brain

To have a profound understanding of the physiological and pathological processes in a brain, both chemical and electrical signals need to be recorded, but this is still very challenging. Herein, micrometer- to nanometer-sized SERS optophysiological probes were created to determine both the CO₃ ^2- concentration and the pH in live brains and neurons because both species play important roles in regulating the acid-base balance in the brain. A ratiometric SERS microarray of eight microprobes with tip sizes of 5 μm was established and used for the first time for real-time mapping and simultaneous quantification of CO₃ ^2- and pH in a live brain. We found that both the CO₃ ^2- concentration and the pH value dramatically decreased under ischemic conditions. The present SERS technique can be combined with electrophysiology without cross-talk to record both electrical and chemical signals in brains. To deepen our understanding of the mechanism of ischemia on the single-cell level, a SERS nanoprobe with a tip size of 200 nm was developed for use in a single neuron.

Real-time monitoring of peroxynitrite (ONOO−) in the rat brain by developing a ratiometric electrochemical biosensor

As a reactive oxygen species (ROS), peroxynitrite (ONOO⁻) generated by nitric oxide (NO) and superoxide anion (O₂˙⁻) plays important roles in physiological and pathological processes in the brain.