pH-Responsive Fluorescence Enhancement in Graphene Oxide-Naphthalimide Nanoconjugates: A Fluorescence Turn-On Sensor for Acetylcholine

Spectroscopic Determination of Acetylcholine (ACh): A Representative Review

Acetylcholine (ACh) is one of the most crucial neurotransmitters of the cholinergic system found in vertebrates and invertebrates and is responsible for many processes in living organisms. Disturbances in ACh transmission are closely related to dementia in Alzheimer's and Parkinson's disease. ACh in biological samples is most often determined using chromatographic techniques, radioenzymatic assays, enzyme-linked immunosorbent assay (ELISA), or potentiometric methods. An alternative way to detect and determine acetylcholine is applying spectroscopic techniques, due to low limits of detection and quantification, which is not possible with the methods mentioned above. In this review article, we described a detailed overview of different spectroscopic methods used to determine ACh with a collection of validation parameters as a perspective tool for routine analysis, especially in basic research on animal models on central nervous system. In addition, there is a discussion of examples of other biological materials from clinical and preclinical studies to give the whole spectrum of spectroscopic methods application. Descriptions of the developed chemical sensors, as well as the use of flow technology, were also presented. It is worth emphasizing the inclusion in the article of multi-component analysis referring to other neurotransmitters, as well as the description of the tested biological samples and extraction procedures. The motivation to use spectroscopic techniques to conduct this type of analysis and future perspectives in this field are briefly discussed.

Photothermal enhanced fluorescence quenching of Tb-norfloxacin for ultrasensitive human epididymal 4 detection

A fluorescence quenching enhanced immunoassay has been developed to achieve ultrasensitive recognition of human epididymal 4 (HE4) modifying the fluorescence quencher. The carboxymethyl cellulose sodium-functionalized Nb₂C MXene nanocomposite (CMC@MXene) was firstly introduced to quench the fluorescence signal of the luminophore Tb-Norfloxacin coordination polymer nanoparticles (Tb-NFX CPNPs). The Nb₂C MXene nanocomposite as fluorescent nanoquencher inhibits the electron transfer between Tb and NFX to quench the fluorescent signal by coordinating the strongly electronegative carboxyl group on CMC with Tb (III) of Tb-NFX complex. Simultaneously, due to the superior photothermal conversion capability of CMC@MXene, the fluorescence signal has been further weakened by the photothermal effect driven non-radiative decay of the excited state under near-infrared laser irradiation. The constructed fluorescent biosensor based on CMC@MXene probe finally realized the enhanced fluorescence quenching effect, and achieved ultra-high sensitivity and selective detection of HE4, exhibiting a wide linear relationship with HE4 concentration on the logarithmic axis in the range of 10^-5 to 10 ng/mL and a low detection limit of 3.3 fg/mL (S/N = 3). This work not only provides an enhanced fluorescent signal quenching method for the detection of HE4, but also provides novel insights for the design of fluorescent sensor toward different biomolecules.

In-situ food spoilage monitoring using a wireless chemical receptor-conjugated graphene electronic nose

Monitoring food spoilage is one of the most effective methods for preventing food poisoning caused by biogenic amines or microbes. Therefore, various analytical techniques have been introduced to detect low concentrations of cadaverine (CV) and putrescine (PT), which are representative biogenic polyamines involved in food spoilage (5-8 ppm at the stage of initial decomposition after storage for 5 days at 5 °C and 17-186 ppm at the stage of advanced decomposition after storage for 7 days at 5 °C). Although previous methods showed selective CV and PT detection even at low concentrations, the use of these methods remains challenging in research areas that require in-situ, real-time, on-site monitoring. In this study, we demonstrated for the first time an in-situ high-performance chemical receptor-conjugated graphene electronic nose (CRGE-nose) whose limits of detection (LODs), 27.04 and 7.29 ppb, for CV and PT are up to 10² times more sensitive than those of conventional biogenic amine sensors. Specifically, the novel chemical receptors 2,7-bis(3-morpholinopropyl)benzo[lmn][3,8] phenanthroline-1,3,6,8(2H,7H)-tetraone (NaPhdiMor (NPM)) and 2,7-bis(2-((3-morpholinopropyl)amino)ethyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NaPhdiEtAmMor (NPEAM)) were designed on the basis of density functional theory (DFT) calculations, and their interaction mechanism was characterized by a DFT 3D simulation. Interestingly, the CRGE-nose was connected on a micro sim chip substrate via wire bonding and then integrated into wireless portable devices, resulting in a cost-effective, high-performance prototype CRGE-nose device capable of on-site detection. The portable CRGE-nose can be used for in-situ monitoring of CV and PT concentration changes as low as 27.04 and 7.29 ppb in real meats such as pork, beef, lamb and chicken.

An overview of recent analysis and detection of acetylcholine

Acetylcholine (ACh), the major neurotransmitter secreted by cholinergic neurons, is widely found in the peripheral and central nervous systems, and its main function is to complete the transmission of neural signals. When cholinergic neurons are impaired, the synthesis and decomposition of ACh are abnormal and the neural signalling transition is blocked. To some extent, the concentration changes of ACh reflects the occurrence and development of many kinds of nervous system diseases, such as Alzheimer's disease, Parkinson's disease, Myasthenia gravis and so on. Thus, researches of the physiological and pathological roles and the tracking of the concentration changes of ACh in vivo are significant to the prevention and treatment of these diseases. In the paper, the pathophysiological functions and the comprehensive research progress on detection methods of ACh are summarized. Specifically, the latest research and related applications of the optical and electrochemical biosensors are described, and the future development directions and challenges are prospected, which provides a reference for the detection and applications of ACh.

Driving force to detect Alzheimer's disease biomarkers: application of a thioflavine T@Er-MOF ratiometric fluorescent sensor for smart detection of presenilin 1, amyloid β-protein and acetylcholine

Currently, the highly sensitive detection of Alzheimer's Disease (AD) biomarkers, namely presenilin 1, amyloid β-protein (Aβ), and acetylcholine (ACh), is vital to helping us prevent and diagnose AD. In this work, a novel metal-organic framework [Er(L)(DMF)_1.27]_n (Er-MOF) (H₃L = terphenyl-3,4'',5-tricarboxylic acid) has been synthesized by solvothermal and ultrasonic methods. Further, through the post-synthesis assembly strategy, the fluorescent dye thioflavine T (ThT) has been introduced into Er-MOF to construct a dual-emission ThT@Er-MOF ratiometric fluorescent sensor. This is the first time that ThT@Er-MOF has been successfully applied in the highly sensitive detection of three main Alzheimer's disease biomarkers in the cerebrospinal fluid through three different low cost and facile detection strategies. Firstly, with the spilted DNA strategy, this is the first time that ThT@Er-MOF can be applied in the label-free detection of SSODN (part of the presenilin 1 gene). Secondly, for the detection of Aβ, because ThT can be specifically combined with Aβ and has an excellent characteristic fluorescence band, the dual-emission ThT@Er-MOF sensor can be selectively applied to detect Aβ over the analog protein, which shows far more sensitivity than other Aβ sensors. Thirdly, through the acetylcholine esterase (AchE) enzymatic cleavage and release strategy, ThT@Er-MOF enhances the detection of acetylcholine (ACh) with a low limit of detection (LOD) value (0.03226 nM). It should be noticed that the three different detection methods are low cost and facile. This study also provides the first example of utilizing laser scanning confocal microscopy (LSCM) to investigate the fluorescence resonance energy transfer (FRET) detection mechanism by ThT@Er-MOF in more detail. The location of FRET occurrence and FRET efficiency can also be investigated by LSCM, which can be helpful to understand the FRET detection process by these unique MOF-based hybrid materials.

Multiplexed determination of intracellular messenger RNA by using a graphene oxide nanoprobe modified with target-recognizing fluorescent oligonucleotides

A multiplexed graphene oxide (GO) fluorescent nanoprobe is described for quantification and imaging of messenger RNAs (mRNAs) in living cells. The recognizing oligonucleotides (with sequences complementary to those of target mRNAs) were labeled with different fluorescent dyes. If adsorbed on GO, the fluorescence of the recognizing oligonucleotides is quenched. After having penetrated living cells, the oligonucleotides bind to target mRNAs and dissociate from GO. This leads to the recovery of fluorescence. Using different fluorescent dyes, various intracellular mRNAs can be simultaneously imaged and quantified by a high content analysis within a short period of time. Actin mRNA acts as the internal control. This GO-based nanoprobe allows mRNA mimics to be determined within an analytical range from 1 to 400 nM and a detection limit as low as 0.26 nM. Up to 3 intracellular mRNAs (C-myc, TK1, and actin) can be detected simultaneously in a single living cell. Hence, this nanoprobe enables specific distinction of intracellular mRNA expression levels in cancerous and normal cells. It can be potentially applied as a tool for detection of cancer progression and diagnosis. Graphical abstract A multiplexed graphene oxide (GO)-based fluorescent nanoprobe is described for quantification and imaging of intracellular messenger RNAs. After penetrating living cells, the recovered fluorescence of the dissociated recognizing oligonucleotides can be analyzed , and this allows for simultaneous detection of up to 3 intracellular messenger RNAs.