Cotton Thread-microfluidic Devices For Colorimetric Point-of-care Applications

Point-of-care test（POCT）attracts tremendous attention because of its timely on-site detection,simple operation,convenient use and small size.Microfluidic devices is one of the important platforms in POCT.It has the advantages of portability,integration and low cost,and its potential has been widely demonstrated in biomedical diagnostic,food safety supervision and environment monitoring.Most recently,cotton thread has been proposed as a new,economically affordable material for the fabrication of microfluidic circuits for use in fast,low-cost,point-of-care diagnostic testing devices.The current textile industry produces versatile threads with different physical and chemical properties.The easy availability of such thread forms one basis for its use in the fabrication of low-cost analytical tools.Thread,however,normally consists of a waxy layer produced during the thread-fabrication process.This waxy layer would delay the solution soaking as well as decrease the wicking-assisted flow speed.In addition,the possibility of thread as a matrix for molecular separation,metal ions extraction has not been discussed.To explore the potential of cotton thread in point-of-care test applications,this thesis focuses on following part:1.Colorimetric detection of copper in water by cotton thread based pre-concentrationA sensitive and portable on-thread pre-concentration and colorimetric detection method of copper ions was proposed.Commercial cotton thread was proposed as a solid sorbent for effectively extracting copper ions(Cu²⁺)from solution.The adsorbed Cu²⁺was converted to Cu⁺byascorbicacid.TheCu⁺onthethreadswasvisualizedby horseradish-peroxidase-catalyzedcolorchangesofachromogenicoxidized tetramethylbenzidine substrate.The adsorption of Cu²⁺on the plasma-and Na₂CO₃-treated thread was verified by scanning electron microscopy,energy-dispersive spectroscopy,X-ray diffraction,and zeta potential characterization.The Cu⁺-induced dysfunction of HRP on the thread was verified by X-ray photoelectron spectroscopy.With optimized reaction conditions,the concentration-dependent colorimetric response plot was characterized,where the linear detection range was 2–400 nM and the detection limit was about 0.15 nM.Measurements of a group of possible interfering ions and spiked real-water samples demonstrated the good selectivity of the proposed method.The proposed on-thread pre-concentration and colorimetric detection offers a low-cost and effective alternative for detecting of copper ions in real applications.2.Separation and on-thread detection of non-permitted food dyes（Sudan I and Rhodamine-B）Food coloring can improve the color and taste of food.However,industry dyes,such as Sudan I and Rhodamine-B dye have been illegally used as food adulteration,which is a severe threaten to public health.Previous,detection of non-permitted food dyes heavily relies on specialized equipment,such as UV-vis spectrometer,liquid chromatography-tandem mass spectrometry（LC-MS/MS）,etc.Although those methods can precisely measure the existence of non-permitted food dyes,its application in daily food safety supervision is dimmed because they are expensive and normally operated by special trained personnel in central laboratories.Therefore,simple,rapid and reliable analytical methods for the determination of these non-permitted dyes in food stuffs are required.To address the POCT application,we proposed that cotton thread function as a chromatography matrix for separation of Sudan I and Rhodamine-B from mixture.First,the conditions of solvent,sample volume and diameter of cotton thread were optimized to improve the on-thread separation.The characterization of on-thread separated dyes was achieved by a home-made smartphone supported microscope optical spectrometer.It was found that both water soluble Rhodamine-B and fat soluble Sudan I can be well dissolved in 75%ethanol.Loading of the same volume of Rhodamine-B and Sudan I on cotton-thread resulted in colorimetric band with different length on thread.Water soluble Rhodamine-B runs faster fat soluble Sudan I on cotton thread.Then the mixture of Sudan red I and Rhodamine B solution was loaded on the cotton thread.The results showed that distinct color band was formed along the thread.In addition,the pigment spectrum on cotton thread was analyzed by in-house build smartphone-supported microspectrophotometer system.The results confirmed that Rhodamine B can be separated from the mixture because of its faster diffusion speed on thread.In summary,cotton thread can be applied as an economical matrix for separation of illegal-used non-food pigments,while smartphone-supported detection system further strengthen its point-of-test application potential.3.Chitosan functionalization to prolong stable hydrophilicity of cotton thread for thread-based analytical device applicationIn this work,chitosan,a biocompatible polymer,was used to modify cotton thread to improve its fluidic properties for microfluidic applications.Electrostatic-interaction-mediated adsorption of chitosan on alkaline-scoured thread can effectively increase thread’s solution hydrophilicity and wicking rate.Moreover,the proposed process conserves the thread’s hydrophilicity and fluidic properties even when stored for several months.As a microfluidic platform for enzymatic-reaction-associated colorimetric measurement,the suggested chitosan modification results in the accumulation of more analyze in the detection zone because of the improved flow rate,which enhances the colorimetric signal intensity.In this study,a glucose oxidase-mediated colorimetric glucose assay was demonstrated on a chitosan-modified microfluidic thread-based analytical device（μTAD）.A linear range of 1-7.5 mM,a low detection limit of 0.2 mM,and a recovery rate between 93%and 98%was achieved in detecting glucose in artificial urine samples.