MXene-based Flexible Wearable PH Sensor

Wearable sensors can reflect the physiological information and health status of individuals in real time by continuously monitoring biochemical markers in biological fluids such as sweat,tears and saliva,which is a key technology to realize portable personalized medicine.As one of the most common body fluids,sweat is often used to detect physiological information of human body.The pH value in sweat can affect many biochemical reactions in the human body,which can be used for diagnosis and treatment of human diseases.Therefore,wearable pH sensors that can monitor the pH value of human sweat in real time have attracted wide attention.The performance and application of wearable pH sensors depend to a large extent on the pH sensitive materials used.Currently,the most commonly used pH sensitive materials are hydrogen ionophores,conductive polymers and metal oxides.However,the drawbacks of high price,potential biotoxicity and low sensitivity make pH sensors still face great challenges.Therefore,it is urgent to explore a new pH sensitive material that can overcome the above shortcomings.As a new two-dimensional material,MXene has the advantages of good conductivity,large specific surface area and abundant surface functional groups.This paper emphasizes that functional groups on the surface of MXene（such as-OH,-O）are used as H⁺sensitive sites,and for the first time MXene is directly integrated into pH sensors as a pH sensitive material,and a series of research works are carried out:1.Flexible wearable pH sensor based on Ti₃C₂T_x:The pH of sweat is one of the most important physiological indexes to measure human health,so the real-time monitoring of pH value of sweat is of great significance.A flexible wearable pH sensor based on Ti₃C₂T_x is prepared in this work.Ti₃C₂T_x was prepared by etching precursor Ti₃Al C₂ with Li F/HCl mixture and HF solution,and was directly used as a pH sensitive material.Compared with the precursor Ti₃Al C₂,the pH potential response and selectivity of Ti₃C₂T_x obtained after etching are improved,and the sensitivity of HF-Ti₃C₂T_xwith the best performance is-43.51±0.53 m V pH^-1.By further integrating with solid state Ag/Ag Cl reference electrode,a flexible wearable pH sensor was prepared.The pH sensor showed good sensitivity and resistance to bending,and successfully realized real-time pH monitoring of human sweat.Finally,the response mechanism of Ti₃C₂T_x to H⁺is verified by cyclic voltammetry,which provides theoretical guidance for the follow-up work.However,the sensitivity of the flexible wearable pH sensor based on Ti₃C₂T_x(-43.51±0.53m V pH^-1)is still far from the Nernst response slope(-59.2 m V pH^-1),so it is urgent to explore a kind of MXene material with higher performance.2.Flexible wearable pH sensor based on Nb₂CT_x:Aiming at the low sensitivity of flexible wearable pH sensor based on Ti₃C₂T_x,a flexible wearable pH sensor based on Nb₂CT_x was prepared in this work.The precursor Nb₂Al C was etched with 49%HF to obtain multiple layers of Nb₂CT_x,and then the obtained Nb₂CT_x was stripped and stratified with TMAOH to obtain fewer layers of Nb₂CT_x.A large number of oxygen-containing functional groups（such as-OH and-O）will be generated on the surface of the etched Nb₂CT_x,and hydrogen ions in the solution will exchange protons with-OH on the surface of Nb₂CT_x,resulting in potential difference,which will be used for the detection of pH value in the solution.Subsequently,Nb₂CT_x was coated on the surface of a glassy carbon electrode and a series of electrochemical properties were tested.The results show that the multilayer Nb₂CT_x has the best sensitivity and selectivity for H⁺,and the response slope in B-R buffer solution with pH value of 1-10 is-55.67 m V pH^-1,which is close to the theoretical value of the response slope of Nenst,and significantly improved compared with HF-Ti₃C₂T_x.Finally,Nb₂CT_x was integrated into the flexible wearable pH sensor for online testing of human sweat,realizing the real-time monitoring of the sweat pH value of volunteers during indoor exercise,indicating that MXene is suitable for application as a pH sensitive material in the flexible wearable pH sensor.