Development of an ATP and hydrogen ion image sensor using a patterned apyrase-immobilized membrane

CMOS-Based Redox-Type Label-Free ATP Image Sensor for In Vitro Sensitive Imaging of Extracellular ATP

Adenosine 5'-triphosphate (ATP) plays a crucial role as an extracellular signaling molecule in the central nervous system and is closely related to various nerve diseases. Therefore, label-free imaging of extracellular ATP dynamics and spatiotemporal analysis is crucial for understanding brain function. To decrease the limit of detection (LOD) of imaging extracellular ATP, we fabricated a redox-type label-free ATP image sensor by immobilizing glycerol-kinase (GK), L-α-glycerophosphate oxidase (LGOx), and horseradish peroxidase (HRP) enzymes in a polymer film on a gold electrode-modified potentiometric sensor array with a 37.3 µm-pitch. Hydrogen peroxide (H₂O₂) is generated through the enzymatic reactions from GK to LGOx in the presence of ATP and glycerol, and ATP can be detected as changes in its concentration using an electron mediator. Using this approach, the LOD for ATP was 2.8 µM with a sensitivity of 77 ± 3.8 mV/dec., under 10 mM working buffers at physiological pH, such as in in vitro experiments, and the LOD was great superior 100 times than that of the hydrogen ion detection-based image sensor. This redox-type ATP image sensor may be successfully applied for in vitro sensitive imaging of extracellular ATP dynamics in brain nerve tissue or cells.

Aptamer assisted CRISPR-Cas12a strategy for small molecule diagnostics

Molecular diagnostics are vital for the identification, prevention, and treatment of numerous diseases and are of particular demand in point-of-care (POC) settings. Nevertheless, most reported biosensors based on the CRISPR-Cas system have focused on nucleic-acid targets. Here, we report a versatile diagnostic strategy for small molecules called Molecular Radar (Random Molecular Aptamer-Dependent CRISPR-Assist Reporter), The workflow is simple, convenient, and rapid (conducted at 37 °C in under 25 min), indicating the substantial potential of the proposed assay could be adapted into a biosensor for POC settings and on-site molecular diagnostics. This strategy is based on the CRISPR Cas12a-assisted fluorescence reporter system that consists of Cas12a, CRISPR RNA (crRNA), a single-stranded DNA (ssDNA) probe labeled with a fluorophore at the 5' end and a quencher at the 3' end (F-Q probe), and a single-stranded DNA aptamer for the target molecule. In the presence of a target molecule, the aptamer binds to this small molecule with high specificity and affinity, resulting in a decrease of aptamer hybridized to the crRNA-Cas12a duplex. This decrease in activated Cas12a leads to a significant reduction in fluorescence signal. In this study, adenosine-5'-triphosphate (ATP) was selected as model target molecule and an ATP detect method was developed with high specificity and sensitivity with a linear range from 25 to 500 μM and a detection limit of 104 nM. Moreover, the particular characteristics of CRISPR-Cas12a that we report here for the first time have enriched our understanding of Cas12a and provided guidance for further research on CRISPR-Cas12a-based biosensors.

Extracellular Bio-imaging of Acetylcholine-stimulated PC12 Cells Using a Calcium and Potassium Multi-ion Image Sensor

In biochemistry, Ca²⁺ and K⁺ play essential roles to control signal transduction. Much interest has been focused on ion-imaging, which facilitates understanding of their ion flux dynamics. In this paper, we report a calcium and potassium multi-ion image sensor and its application to living cells (PC12). The multi-ion sensor had two selective plasticized poly(vinyl chloride) membranes containing ionophores. Each region on the sensor responded to only the corresponding ion. The multi-ion sensor has many advantages including not only label-free and real-time measurement but also simultaneous detection of Ca²⁺ and K⁺. Cultured PC12 cells treated with nerve growth factor were prepared, and a practical observation for the cells was conducted with the sensor. After the PC12 cells were stimulated by acetylcholine, only the extracellular Ca²⁺ concentration increased while there was no increase in the extracellular K⁺ concentration. Through the practical observation, we demonstrated that the sensor was helpful for analyzing the cell events with changing Ca²⁺ and/or K⁺ concentration.

A bio-image sensor for simultaneous detection of multi-neurotransmitters

We report here a new bio-image sensor for simultaneous detection of spatial and temporal distribution of multi-neurotransmitters. It consists of multiple enzyme-immobilized membranes on a 128 × 128 pixel array with read-out circuit. Apyrase and acetylcholinesterase (AChE), as selective elements, are used to recognize adenosine 5'-triphosphate (ATP) and acetylcholine (ACh), respectively. To enhance the spatial resolution, hydrogen ion (H⁺) diffusion barrier layers are deposited on top of the bio-image sensor and demonstrated their prevention capability. The results are used to design the space among enzyme-immobilized pixels and the null H⁺ sensor to minimize the undesired signal overlap by H⁺ diffusion. Using this bio-image sensor, we can obtain H⁺ diffusion-independent imaging of concentration gradients of ATP and ACh in real-time. The sensing characteristics, such as sensitivity and detection of limit, are determined experimentally. With the proposed bio-image sensor the possibility exists for customizable monitoring of the activities of various neurochemicals by using different kinds of proton-consuming or generating enzymes.

Use of a small molecule as an initiator for interchain staudinger reaction: A new ATP sensing platform using product fluorescence

We demonstrated that a small molecule induced interchain Staudinger reaction can be employed for highly selective detection of adenosine triphosphate (ATP), an important energy-storage biomolecule. A designed ATP split aptamer (A1) was first functionalized with a weakly fluorescent coumarin derivative due to an azide group (azido-coumarin). The second DNA strand (A2) was covalently linked with triphenylphosphine, which could selectively and efficiently reduce azido to amino group through the Staudinger reaction. The A2 was then hybridized with a half of another designed longer DNA strand (T1). The second half of T1 was a split aptamer and selectively recognized ATP with A1 to form a sandwich structure. The specific interaction between ATP and the aptamers drew the two functionalized DNA strands (A1 and A2) together to initiate the interchain Staudinger reduction at fmol-nmol concentration level, hence produced fluorescent 7-aminocoumarin which could be used as an indicator for the presence of trace ATP. The reaction process had a concentration dependent manner with ATP in a large concentration range. Such a strategy of interchain Staudinger reaction can be extended to construct biosensors for other small functional molecules on the basis of judiciously designed aptamers.