Application Of Coffee-Ring Effect To Biomedical Detection

Currently,the main methods of biomedical detection are electrochemical and optical methods?colorimetry,fluorescence method,etc.?.Although electrochemical and fluorescence methods have high sensitivity,good repeatability and the accuracy of the test results,these methods need complex and expensive instruments and equipment,professional and technical personnel to operate instruments.The device increases the complexity and cost of testing,and limits its application and promotion.Color-based detection has long been utilized for rapid and on-site chemical analysis in biomedical because of its simple operation and low cost.However,color is a complex signal,which is characterized by hue,value,and saturation.The perception of color by human eyes is influenced by a range of subjective and objective factors?e.g.,background light and individual difference in color perception?that may introduce uncertainty to the test results.Moreover,a range of parameters of camera and the background of sample have an effect on tests.Therefore,it is important to develop an effective signal transduction method that can be used for point-of-care testing?POCT?in biomedical detection.A coffee-ring is a pattern left by drying a drop of coffee on a substrate.Because the liquid near the edge of a droplet on the substrate evaporates faster than that near the center,a capillary flow arises which brings solutes from the center of the droplet to the edge.The flow leads to the formation of a ring-shaped stain on the substrate after complete evaporation of the liquid.Because of its unique properties,coffee-ring effect has been widely used in nano-assembly,sample enrichment etc.Here we demonstrate that the transduction of color-based test result into length can be simply accomplished based on the well-known coffee-ring effect.The color intensity signal which needs the professional instrument to be detected is converted into the width of coffee-ring which can be measured by the naked eye,so that quantitative detection by naked eyes and this method is applied to detection of small molecules,protein and nucleic acid.The main contents are as follows:?1?We report a quantitative analytical method using the naked eye.The method is based on the coffee-ring effect on paper,which converts a colored solution to stains on the paper.It is found that the width of the colored stains is correlated to the concentration of analyte so that quantitative analysis can be accomplished using a ruler.The influence of the shape and lamination of the paper reservoir to the test results is studied to find the optimal one for unambiguous analysis.It is also found that the sample color does not cause interference to the test.The applicability of the method to quantitative detection of lactate.?2?We report a method for enzyme-linked immunosorbent assay?ELISA?based on the coffee-ring effect.As a demonstration,human IgG is quantitatively analyzed.The human IgG was immobilized on the paper chip,and then add blocking buffer and glucose oxidase labelled the Rabbit anti-human IgG,finally the substrate was added to the reactionan and a coffee-ring effect is formed.The influence of the shape and lamination of the starch-iodide paper strip on the test results is studied.It is found that the width of the colored stains correlates with the concentration of the analyte which can be measured for quantitative analysis.Therefore,converting the colorimetric results of an enzyme-linked immunosorbent assay?ELISA?into length based on the coffee-ring effect,so that the quantitative detection of analytes can be carried out simply using a ruler.The method is promising for quantitative point-of-care detection of biomarkers under resource-limited settings.?3?We report a method for a homogeneous coffee-ring assay for visualized quantification of nucleic acid or thrombin.Two single-stranded DNA probes?A and B?are added to sample to selectively bind to target DNA or Thrombin by using NH₄⁺and hemin induced the formation of a hemin/G-quadruplex-based DNAzyme that is peroxidase mimic.The enzyme can catalyze a certain concentration of hydrogen peroxide and iodine ions of the starch-iodide papers to form a purple complex,after complete reaction and drying of the solution on the paper,a coffee-ring is formed.It is found that the width of the colored stains correlates with the concentration of the analyte which can be measured for quantitative analysis.The influence of the entry of the paper reservoir to the test results is also studied to improve the retest.?4?The effect of photonic crystal substrate on the coffee-ring effect was studied,We report a method for visualized quantification of DNA based on loop-mediated isothermal amplification?LAMP?.We used monodisperse SiO₂ nanoparticles as template by self-assemble to form a photonic crystal having close-packed hexagonal structure.The solution for loop-mediated isothermal amplification?LAMP?of DNA is dropcast on a colloid-crystal surface.A coffee-ring containing magnesium pyrophosphate is formed,and surprisingly we find that the width of the coffee ring is linearly correlated to the logarithm of the DNA concentration before the isothermal amplification.Importantly,compared with other substrates,we found that the colloidal-crystal substrate is an appropriate substrate for carrying out the assay.The assay,which only requires 0.50?L sample,is completed within 5.0 min.Semi-quantitative detection of trace DNA is performed using naked eyes.With the use of a smartphone and App,the DNA in a sample can be quantitatively detected with a limit-of-detection of 20 copies.