Microfluidic paper-based analytical devices (μPADs) are emerging as a new technology for point-of-care diagnosis,food analysis and environmental monitoring.In addition to ease of use and dispose,a key advantage of μPADs is that fluids in μPADs is driven by capillary wicking,therefore,no external pumps are required.Various methods have been reported for fabrication of μPADs via hydrophilic-hydrophobic patterning of paper [1,2].Recently,we reported a novel approach for patterning μPADs via the hydrophobization of filter paper by coupling octadecyltrichlorosilane (OTS) to paper fibers,followed by UV-lithography of the OTS coating [3].However,the UV-lithography is expensive in both equipment and operation costs,time-consuming,and renders the UV-exposed paper yellowish,consequently,leads to poor detectability for on-paper color reaction.Herein we present an alternative method for hydrophilic-patterning of OTS-coated paper by using a hand-held corona generator (as shown in Fig.1) which is portable,low-cost,timesaving and effective for surface treatment.Filter paper was cut to an appropriate size.Then,paper pieces were immersed in an OTS solution at room temperature for 5 min.After silanization,the paper surface became highly hydrophobic (contact angle >125°) due to the formation of self-assembled monolayer of OTS on the paper-fiber surface.The OTS-coated paper was sandwiched between a stenciled plastic mold with desired patterns and a flat plastic sheet and tightly clamped.The to-be-hydrophilized regions of the OTS-coated paper were then exposed to corona plasma by scanning the electrode tip of the generator along the open parts of the stenciled mold.Thus,the corona plasma-treated regions of the paper resumed its native hydrophilicity due to the degradation of the coated OTS,and remained purely white as well.The high wettability and sharp boundary of the corona-patterned paper allowed the transport of aqueous solution within the hydrophilic regions via capillary penetration as shown in Fig.2.The performance of such prepared μPADs was demonstrated by assaying nitrite anion via on–paper Griess reaction (Fig.3).The achieved detection limit was as low as 10 μmol/L,10-fold lower than the reference [3] reported.The NO2 - content in human saliva sample was detected with satisfactory result.