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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 chemical analysis is demonstrated by detection of glucose and lactate.

Toward Quantitative Chemical Analysis Using a Ruler on Paper: An Approach to Transduce Color to Length Based on Coffee-Ring Effect