Functionalization Of Carbon Materials And Its Application In Electrochemical Sensing

Chemicals are involved in the transmission of neural signal in the brain,so it is of great importance to realize the measurement of these active substances with high sensitivity and selectivity for understanding the chemical nature of neurophysiology from molecular level.Electrochemical methods have been widely used in the field of in vivo analysis because of its advantages of simple instrument,easy operation,and flexible design of electrochemical interface to achieve detect with selectivity and stability.In this paper,the electrochemical detection of neuroactive substances in rat brain was realized based on carbon materials through the rational design and functionalization of the material.We conducted a systematic study on the electrochemical oxidation behavior of ascorbic acid(AA)on different carbon-based electrodes,including carbon allotropes and their oxides.We found that carbon nanotubes(CNTs)and graphene have the good behavior for the electrochemical oxidation of AA among the five materials of CNTs,graphene,graphene oxide(GO),graphdiyne(GDY)and Graphdiyne oxide(GDYO).And CNTs with different diameters and lengths all have good electrochemical oxidation behavior.In addition,CNTs have the fastest electron transfer rate constant towards AA in all carbon materials.Through a series of characterization of contact angles,density of state(DOS),X-ray photoelectron spectroscopy(XPS)and Raman spectroscopy,we think that the electrochemical oxidation of AA is related to the disorder or defect and density of state of carbon materials,and independent of the hydrophilicity and oxygen-containing functional groups of carbon materials.Based on this,we realized the in-situ electrochemical detection of AA with the CNTs-modified carbon fiber electrode(CFE)during electrical stimulation in the rat brain.We developed a potentiometric method with ion-selective electrode(ISE)by using hollow carbon nanoparticles(HCNs)as the transducing layer and solid contact of Ca²⁺-selective electrode(Ca²⁺-ISE)to effectively promote ion-to-electron transduction between ion-selective membrane(ISM)and the conductive substrate.We found that the use of HCNs reduced the potential drift and improved the stability of the signal response of SS-ISE.And the Ca²⁺-ISE with three-shelled HCNs(3s-HCNs)as the transduction layer has a relatively good stability.Therefore,we fabricated a solid-state Ca²⁺-ISE by forming Ca²⁺-selective membrane(Ca²⁺-ISM)onto 3s-HCNs modified CFE.The electrode showed a 28.1m V/decade Nernst response toward Ca²⁺(concentration range from 10^-5 M to 0.05 M)and was not disturbed by endogenous species in the central nervous system with good selectivity.Therefore,the electrode can be used to real-time detect the dynamics of extracellular Ca²⁺during electrical stimulation.This study provides a new platform for the development of solid-state ISE(SS-ISE),which is useful for in vivo measurements of metal ions and p H in the brains of living rats.GDYO-manganese dioxide(MnO₂)composite,GDYO-MnO₂,was prepared by one-step oxidation of GDY and used as the transducing layer of the K⁺-selective electrode(K⁺-ISE)to facilitate the ion-to-electron transduction between the conductive substrate and ISM interface.This electrode exhibits a lower potential drift compared with K⁺-ISE without a transduction layer,and the electrochemical performance of K⁺-ISE is different between K⁺-ISE constructed with GDYO-MnO₂ with different manganese content.Among them,the performance of K⁺-ISE with GDYO-MnO₂ with 9%manganese content as the transduction layer has the best performance.In addition,the K⁺-ISE based on GDYO-MnO₂ has higher potential stability than that of GO-manganese oxide(MnO_x)composite(GO-MnO_x)with the same manganese content.More importantly,we found that the intrinsic structure and hydrophobicity of GDYO-MnO₂ could stabilize and hinder the formation of water layer at the interface between ion-selective membrane and solid contact.The micro-sized K⁺-ISE with GDYO-MnO₂ as the solid contact shows good stability and high selectivity,therefore,it can achieve reliable sensing of K⁺at the animal level.Moreover,the GDYO-based strategy can be generalized to other SS-ISE without complicated steps.Based on the discovery that GDYO-MnO₂ can reduce and stabilize the water layer as a solid contact material,we believe that the combination of carbon nanomaterials that reduce the water layer and capacitive materials with rapid ion-electron transduction can improve the potential stability and reproducibility of SS-ISE.Therefore,the combination of GDYO-MnO₂and poly(3,4-ethylene dioxothiophene)(PEDOT)(abbreviated as GDYO(Mn)-PEDOT)was used as a novel solid-state contact material to develop a stable,reproducible,wireless ion sensing system.The results show that the micro-sized SS-ISE we designed has good stability in vitro and small potential drift that the potential drift within one week was±1.1 m V.The wireless transmission system based on SS-ISE can realize long-term,real-time and stable monitoring of Ca²⁺in the brain of freely moving animals.Our wireless SS-ISE sensing system is an early demonstration of ion wireless sensing using the uncalibrated ISE in awake animals.It would greatly facilitate research into the design of smart devices that can yield highly reliable physiologically relevant results that will contribute to new findings in brain research.