Preparation Of "Janus" Microfluidic Paper-Based Analytical Devices And Applications In Paper Spray Mass Spectrometry Analysis

�Janus�material has been widely used in daily life and industry in recent years because of the significantly different wettability between the front and the back.As a kind of natural cellulose material,paper has strong plasticity and is easy to modify,therefore it is favored by researchers to prepare�Janus�material.Microfluidic paper-based analytical devices(?PADs),paper chips for short,can be used for sample pre-processing,detection and other analytical functions on a single paper.It is small in size and easy to carry,making it ideal for rapid in-field analysis.The hydrophilic channel of the traditional paper chip is usually permeable among two sides,and fluid is easily lost and contaminated on the back during the flow process.The�Janus�paper chip has a hydrophilic and hydrophobic channel network on the front side and remains hydrophobic on the back side,which can effectively solve the pollution problem for the traditional paper chip.In addition,the hydrophilic and hydrophobic channels can also be constructed on the front and back of a piece of paper to achieve online filtration and separation.The main preparation methods of�Janus�paper chips include ultraviolet degradation,plasma technology,corona processing and inkjet printing,etc.In this work,Octadecyl trichlorosilane(OTS)monolayers self-assembled on the surface of filter paper to obtain hydrophobic paper on both sides(OTS paper).The OTS paper was then treated with low temperature plasma emitted by Dielectric barrier discharge ionization(DBDI)to prepare�Janus�paper chips.Two kinds of�Janus�paper chips were prepared by the above method.Combined with paper spray mass spectrometry(PS-MS)analysis,a method for glucose determination in cerebrospinal fluid and a method for the determination of active ingredients in the Chinese medicine,Zhishi,were established.Main contents in this work are as follows:(1)Rapid preparation of�Janus�paper chips by DBDI was established.The infiltration depth,infiltration rate,channel dynamic resolution,cleaning solvent type,patterning effect and stability of�Janus�paper chips were investigated.The obtained�Janus�paper chips were characterized by static water Contact Angle(CA),Infrared spectroscopy(IR)and X-ray photoelectron spectroscopy(XPS).The results showed that�Janus�paper chips with hydrophilic and hydrophobic channels on the front side and hydrophobic on the back side can be prepared quickly by DBDI.The water Contact Angle in the hydrophilic region on the front side is 0�,while the water Contact Angle on the back side is still as high as 137.96�1.13�(n=3).The maximum infiltration depth of the fluid on the paper accounts for about 3/5 of the thickness of the whole paper;the infiltration rate is up to 0.2 cm/s;the minimum width of hydrophilic channel and the minimum width of hydrophobic dam are both 1�0.1 mm,and the channel boundary is clearly identified.The hydrophobic dam prepared by this method can also block high proportion of organic reagents(Me OH:H₂O v/v=9:1).The water Contact Angle of OTS paper was still as high as 138.74�1.11�(n=3)after 90 d at room temperature,and the change was no more than 5�.The water Contact Angle of the area that recovered hydrophilic after DBDI irradiation was stable at about 30�after 90 d,showing a good stability.(2)�Janus�paper chips with hydrophilic micro-area on the tip was established by DBDI,and was especially used for analyzing samples in nanoliter.The sample loading,the size of the hydrophilic micro-area on the tip and the conditions of mass spectrometry were investigated.The results showed that the paper chip combined with PS-MS has higher sensitivity in the determination of glucose standard solution than ordinary paper,and the background noise is greatly reduced.When measuring actual cerebrospinal fluid samples,the mass spectrum information was more abundant in the low mass-to-charge ratio range(m/z 100-200 Da).The glucose in cerebrospinal fluid had a good linear relationship in the range of 167-1000?g/m L(r=0.9985)with the relative standard deviation(RSD)6.5-15%,the limit of detection(LOD)100?g/m L,and the average recoveries 105-115%.The contents of glucose in 10 groups of cerebrospinal fluid samples were measured.It was found that there was no significant difference between the results obtained by our method and by method in the hospital.Therefore,it indicated that�Janus�paper chips with hydrophilic micro-area on the tip has great application value and potential for the analysis of nanoliter samples.(3)Online filtering�Janus�paper chip-PS-MS analysis platform was established,which integrates extraction,filtration,separation,enrichment and detection.The length and width of hydrophilic channel,the proportion of extraction solvent,extraction times of the paper chip and mass spectrum conditions were investigated.The Chinese medicine Zhishi is selected as the analysis model.The results showed that the paper chip combined with PS-MS could quickly obtain the mass spectrum profile of Zhishi,and the authenticity is distinguished with the common fake Qingpi.At the same time,the response intensity of the analyte in the mass spectrum was 1.8-2.1 times higher than the traditional ultrasonic extraction added to the ordinary paper.The whole process eliminates the complicated pre-treatment steps such as ultrasonic extraction,centrifuge and filtration,which is of great significance for on-site rapid analysis in remote areas,and also provides a good application platform for integrated analysis in the future.