The invention discloses Fe < 3 + > visual sensing test
paper based on colorimetric / fluorescent bimodal detection as well as a preparation method and application of the Fe < 3 + > visual sensing test paper. The test paper adopts a three-layer
composite structure design: a
filter paper substrate layer, a PEG intermediate layer and a bimodal sensing layer, and the sensing layer is loaded with 4 '-hydroxy-2, 4-diaminoazobenzene (L) and
lignin blue fluorescent carbon dots (LCDs) in a partitioned manner through a template method. L is synthesized through
diazo coupling reaction, and obvious yellow-purple color
gradation change is generated when L is combined with Fe < 3 + > to form a 1: 1 complex; the LCDs are synthesized through a two-step hydrothermal method, and blue
fluorescence (
lambda max =-440 nm) of the LCDs can be quenched by Fe < 3 + > concentration (1.0-1000 M). In the double-
signal cooperative detection mechanism, the colorimetric part is realized by combining RGB
color analysis with smartphone
image analysis, meanwhile, semi-quantitative
visual discrimination is supported, and the distinguishable concentration of
naked eye colorimetric reaches 10M; and the
fluorescence part realizes Fe < 3 + > detection (the detection range is 10-20 mM) based on a
quenching effect. The test paper can be operated through a sample dropping or dipping method, and false positive is effectively avoided through double-
signal cross validation. The composite test paper has excellent selectivity on Fe < 3 + >, and a novel portable solution is provided for detection of
metal ions in an actual environment.