Fabrication Of Optical Thin Films Based On Two-dimensional Colloidal Crystals And Their Sensing Properties

Colloidal crystal can display distinct color because of its highly ordered structure,which has the advantages of non-toxic and fadeless compare with traditional dyes,and the color of colloidal crystal can be flexibly changed by adjusting the particle size of the structure unit.Among them,monolayer colloidal crystal(MCC)has the advantages of simple preparation technology and good repeatability,which is the preferred material for the fabrication of optical thin film,so it has been widely studied by researchers.In addition,it has been a hot topic to use MCC to fabricate sensing materials that can visually respond to the analyte.However,since MCC itself has no sensing properties to external stimuli,researchers are committed to compound it with other functional materials to solve this problem.As is known to all,responsive polymer gels capable of undergoing a reversible volume phase transition(VPT)in response to an external stimulus are promising active materials for chemical sensors.In addition,metal-organic framework(MOF)as a new type of porous crystalline material has the advantages of high porosity,large specific surface area,high stability,adjustable channel size,etc.,and has been widely studied in gas adsorption,separation and storage.Because of its different adsorption properties to different gases,various sensing materials based MOF has been synthesized.Therefore,responsive polymer gels and MOF has become the preferred materials to fabricate sensors with colloidal crystalsBased on the research of responsive polymer gels or MOF composite with colloidal crystals,a lot of work has been done to fabricate composite materials with various sensing capabilities.However,improving the sensing performance of optical sensors has always been a concern of researchers.At present,it is not enough to study the effect of inherent properties of functional materials(such as MOF)on sensing,which also limits the development of sensing materials.Based on the above background,we constructed MCC/ultrathin polymer gel optical films and MOF optical films,which have visual responses to the pH and vapor,respectively.We have studied its controllable preparation and sensing performance,and also conducted in-depth analysis on the sensing mechanism of the film,in addition,we also discussed the effects of MOF crystal plane and grain boundary on the sensing performance.1.Fabrication of monolayer colloidal crystals/ultrathin polymer gel films and their pH sensing propertiesA novel ultrathin polymer gel optical thin film(PG-MCC)was prepared by infiltrating the poly2-vinyl pyridine(P2VP)into the MCC skeleton.In response to different solutions,the swelling and deswelling of P2VP caused by pH change resulted in obvious changes in the thickness of the film,which shifted the position of the reflection peak and led to changes in the color of the PG-MCC,the visual response to pH was achieved.It is worth noting that,because of the supporting effect of MCC,the location of the reflective peak of PG-MCC can still maintain a good linear relationship with the pH in the strongly acidic solution(pH=2),and the structure of film will not collapse due to the excessive degree of swelling,thus broadening the sensing range of the sensor and improving the stability of the composite film.In addition,the sensor also has the advantages of fast response speed(10 s),high cycle stability and small response hysteresis.This study provides an effective method to improve the stability and sensing range of visual sensors.2.Fabrication of non-close-packed monolayer colloidal crystals/ultrathin polymer gel films and their pH sensing propertiesAlthough the responsive polymer gel sensors that could visually respond to pH was fabricated in the previous work,however,it is still a challenge to fabricate a sensor with the advantages of high sensitivity,fast response,good linearity,high stability and small response hysteresis.Here,we propose a method to improve the sensing performance of polymer gel optical thin film by using non-close-packed monolayer colloidal crystals(ncp MCC)as the scaffold.P2VP was infiltrated into ncp MCC to fabricate ultrathin photonic polymer gel film(UPPGF).Compared with the composite film prepared with MCC as the scaffold,the sensitivity and linear index of UPPGF to pH were significantly improved.We found that the sensitivity and linearity of sensor depends on the volume and filling factor of a responsive polymer gel.Therefore,the sensor with the optimal sensing property was obtained by controlling the non-close-packed degree of ncp MCC.3.Fabrication of ZIF-8 monolayer colloidal crystal films and their vapor sensing propertiesMOF has important applications in the field of chemical sensing due to its flexible structure and excellent adsorption properties.Here,the two-dimensional colloidal crystal optical film was directly fabricated with ZIF-8(MOF constructed by Zn2+ and 2-methylamine)as the structure unit.Because of size-exclusion effect of ZIF-8,the visual distinguish of straight-chain gas molecules and branched-chain gas molecules can be realized.The ZIF-8 with different crystal plane was used to fabricate two-dimensional colloidal crystal films respectively,and tested the responsiveness of different linear alcohols,by comparison,we found that different crystal plane of ZIF-8 has different size-exclusion effect.Compared with polycrystalline ZIF-8 thin films,the prepared ZIF-8 films eliminate the interference of grain boundaries on adsorption of aromatic compounds.Based on the above characteristics,the ZIF-8 film can respond visually to vapor with different concentrations and simulated gasoline with different octane values.Our research on vapor sensing of different crystal plane and grain boundary of ZIF-8 has guiding effect on the preparation of sensing materials based MOF.In conclusion,the responsive polymer gel film and MOF film based on two-dimensional colloidal crystals was prepared in this paper,which could generate visual responses to the pH of solutions and corresponding gases,respectively.We have not only achieved the controllable preparation of the sensor,but also conducted in-depth research on how to improve the sensor's sensing performance(sensitivity,stability,linear relationship).In addition,we also discussed and analyzed the effects of different crystal plane and grain boundary of MOF on gas sensing.Therefore,our work has some guidance on how to construct high-performance optical sensors.