The Surface Modification Of Graphene And The Relevant Application In Sensors

Sensors are always compared to human sense organs.For instance,photoelectric sensor,gas sensor,chemical sensor,pressure/strain sensor and sound sensor can realize the artificial vision,olfaction,gustation,tactile,and hearing,respectively.With the rapid and continuous development of sensor technology,human is able to realize detection with the high tolerance for the extreme environment?i.e.high temperature,pressure,and strong magnetic field?.Sensors have surpassed human`s own sense organs in sensitivity and detection range.Therefore,equipped with sensors,human is enabled to receive abundance information,which is hardly collected by our natural senses.Sensors can be regarded as the extention of human senses.Compared with traditional sensing materials,graphene shows lots of advantages,such as high Young`s modulus?¹ TPa?,high surface area(2630 m² g^-1),good thermal conductivity(⁵300 W m^-1 K^-1),ultra high electron mobility(15,000 cm²·V^-1·s^-1)in room temperature,etc.Hence,graphene shows great potential in the sensors.For example,the high surface area can increase the contact area for gas molecues,biological molecues,and ions.The relevant adsorbents will induce electrical changes in graphene.In addition,the unique structure of graphene will result in the low thermal noise,which can improve the signal-to-noise ratio.For photodetection,the gapless spectrum absorption behavior enables graphene to be a sensing material for broad spectrum detector.So far,the reported graphene based photodetector can response the signal ranging from visible light to mid-infrared light.Along with the broad detection range,these photodetectors can provide high responsitivity as well.Graphene can be regarded as a surface material,which means the electrical properties can be tailored by modifying the structure of graphene surface.Based on this concept,researcher can design the graphene surface as well as the relevant sensor structure,accordingly.This research mainly focuses on different graphene synthesis including:the modified Hummers method,and the electrochemical exfoliation.The as-preapared graphene can be controlable assembled into different structure by changing reaction parameters like temperature and the concentration or density of precursor,to design and fabricate three kinds of sensors:the humidity sensor,pressure sensor,and strain sensor,respectively.These graphene based sensors can realize high sensitivity in low humidity regime,high sensitivity and good limit of detection for pressure,and high sensitivity for strain as well as fully-skin conformal contact.The relevant detailed information is as follows:1.Graphene based humidity sensor:By adopting the modified Hummers method,the graphene can be functionalized with oxygen-containing groups.The modified graphene?also known as graphene oxide?is facilitated with the ability to sense water vapor.For device fabrication,dip coating was used to deposite the sensing film on interdigital electrodes and the impedance of the device was the function of humidity change.The as-prepared humidity sensor exhibited novel characteristics in a range of10%⁹5%RH?room temperature?,such as rapid response behavior?0.68 s?and good stability.Especially,the high sensitivity(5.28 RH^-1)in low humidity regime?<40%?has surpassed those of previous reported sensors based on Grotthuss chain mechanism,which may ascribe to the electron hopping between adjacent local states.2.Graphene based pressure sensor:By adopting freezing-drying technique,a 3D graphene oxide sponge can be synthesized,which shows a lower modulus compared with organic elastomers?i.e.PDMS?.Based on this sponge,a capacitive flexible pressure sensor can be fabrication by sandwiching the sponge between two PET substrate.The as-prepared sensor shows novel sensing characteristics,such as high sensitivity(0.8 kPa^-1),rapid response and recovery behavior?¹00 ms?,and extreme limit of detection?0.24 Pa?,which means the sensor can detect the placement of a tiny petal in 1 cm² sensing area.Moreover,a good spatial resolution for pressure detection can also be realized by the assembly of sensor array.3.Graphene based strain sensor:By adopting electrochemical exfoliation,a relative ordered and intact graphene sheets can be synthesized.A facile strategy named double transfer technique was used to fabricate a wearable strain sensor with ultrahigh skin-conformity by using a commercial make-up.This sensor,based on piezoresivive effect,can achieve a gauge factor of⁵13 and a response time of 54 ms.What`s more,the sensor shows a stretchability?<sup>3</sup>5%?comparable to that of the human skin?<sup>3</sup>0%-40%?,resulting in a high signal-to-noise ratio?<sup>1</sup>4.2?.These characteristics are attractive for precisely monitoring human vital signals?i.e.throat vibration,pulse wave and JVPs?and in appliances with increased human-machine interaction?Retro snake?.In addition,the sensor is biodegradable,being made from a non-toxic water soluble polymer.It`s most likely that biodegradable materials will be increasingly attractive due to the potential environmental problem caused by traditional non-biodegradable polymers?i.e.PDMS,and PI?.