Construction Of Electrochemistry And Electrochemiluminescence Acetylcholine Biosensor Based On Carbon Dots

Carbon dots (CDs) are carbon nanocrystals with a diameter of less than 10 nm. CDs, as a rising star in the nanocarbon family, have been attracting considerable attention because of their fascinating merits, including low cytotoxicity, good biocompatibility, excellent solubility, good chemical inertness, resistance to photobleaching and tunable band gap. In particular, the unique optical and electrochemical properties of CDs make them promising in sensor, optronics, biomedicine and catalysis.Acetylcholine (ACh) is a widely studied neurotransmitter, and is involved in neurotransmission for the central and peripheral nervous systems. ACh is important in many biological processes, such as learning, memory, emotions, and movement. Detecting ACh in biological samples is therefore important in neuronal cholinergic system research. Moreover, ACh exist not only in nervous systems, but also in almost all living cells, including epithelial cells, endothelial cells and immune cells, have the ability to synthesize ACh, which belongs to the non-neuronal cholinergic system. In the non-neuronal cholinergic system, ACh can regulate the basic and frequently used nerve-independent cell functions like proliferation, differentiation, organization of the cytoskeleton, locomotion, secretion, ciliary activity and local release of mediators. However, low concentrations of ACh (pM level) exist in the non-neuronal cholinergic system, leading to the detection of ACh for nerve-independent cell functions much important and challenging.In this research paper, we developed a new method for CDs preparation and utilized CD/layered double hydroxide and CD/cationic dipeptide nanovesicle composites for construction ACh electrochemistry and electrochemiluminescence (ECL) biosensors, respectively. The specific works are as follows:1. CDs prepared by high-energy ball millingCDs with multiple surface states were prepared by high-energy ball milling a mixture of activated carbon and KOH, followed by ultrafiltration purification. The as-prepared CDs exhibit dual-wavelength photoluminescence emission peaks. Furthermore, the first emission peak is independent of excitation wavelength (λex) and the λex-independent peak is mainly related to the surface states of the CDs controlled by the oxygen-containing functional groups; the second emission peak is dependent on λex and red shifts monotonically with increasing λex and the λex-dependent peak is speculated to originate from the carbogenic core, which is influenced by the particle size of the CDs. Meanwhile, the as-prepared CDs also present dual-wavelength ECL property.2. Constrction of ACh electrochemisty biosensor based on CDs decorated on flower-like NiAl layered double hydroxidesThe flower-like NiAl layered double hydroxides (NiAl-LDHs) with a cross-linking nanosheet structure and high specific surface area were hydrothermally fabricated. Then CDs with negative surface charge were decorated on the NiAl-LDHs to obtain NiAl-LDH/CD composites. Compared with the pristine NiAl-LDHs, the NiAl-LDH/CD composites exhibit enhanced electroconductivity and electrocatalytic performance for ACh oxidation. The NiAl-LDH/CD composites modified glassy carbon electrode has a wide linear response range of 5-6885μmol L-1,a high sensitivity of 133.20±0.03 mA M-1 cm-2 and a low limit of detection of 1.7 μmol L-1. The biosensor also exhibit good durability and long-term stability, and excellent selectivity for ACh detection. The good performances in the non-enzymatic detection of ACh are attributed to the synergistic effect between NiAl-LDHs and CDs. First, compared to monodisperse nanoflakes, the cross-linking nanosheet structure and high surface area-to-volume ratio of flower-liked NiAl-LDHs effectively increases the electrode-electrolyte contact area leading to fast reaction kinetics. Second, the negatively surface charged CDs decorating on positively charged brucite-like layers of NiAl-LDHs, improve the electroconductivity, biocompatibility and affinity for ACh.3. Constrction of ACh electrochemiluminescence biosensor based on cationic dipeptide nanovesicle/CD compositesMonodispersed CDs embedded within cationic dipeptide nanovesicles (CDPNVs) have been designed and self-assembled via a micro structure conversion from peptide nanotubes to peptide nanovesicles. The CDPNV/CD composites exhibit a high biocompatibility and stable structure and also show a stable and strong ECL signal due to the synergetic effect between the two ECL luminophores. In addition, the CDPNV/CD composite-modified electrode can ultrasensitively detect ACh with an ultra-low detection limit of 2.4 p mol L-1 (S/N=3) and good selectivity as well as long-term stability. The good performances in the ECL detection of ACh are attributed to the synergistic effect between CDPNVs and CDs. First, CDs with anionic surface charge can embed into the CDPNVs to form a stable CDPNV/CD composite structure. Second, the CDPNV/CD composites can add the ECL routes. Third, CDPNVs and CDs both have good selectivity for ACh detection. The nanocomposites with synergetic effect have wide potential applications in biosensors, bioimaging, biomedicine and bioelectronics as well as in biocatalysis and bioenergy.