Although the pH test paper and pregnancy test paper have been used for decades, it still remains a difficulty to popularize these absorption-based test papers in other detection fields such as environments, medicines, foods, disease diagnosis and public security.

Recently, Prof. ZHANG Zhongping’ s group from Institute of Intelligent Machines (IIM), Chinese Academy of Sciences (CAS), made a significant progress in the preparation principle and visual detection method of nanomaterial-based fluorescent test paper. Prof. ZHANG Zhongping’ s group has focused on developing fluorescent paper to popularize the visualization test paper detection method in the detection fields that require on-site, quick, low-cost and portability detection. They proposed a general strategy to construct a wide-color-varying fluorescent test paper for the dosage-sensitive visualization assays like classical pH test paper. A wide range of consecutive color variations for the visual detection of arsenic ions (As(III)) in water with discernable scale as low as 5 ppb has been achieved. The research was published in Analytical Chemistry (http://pubs.acs.org /doi/pdf/10.1021/acs. analchem.6b01248).

Generally, for a single color the eye is more sensitive to the change of color than brightness. Fluorescent colorimetry test papers can display the presence of target analyte and its amount by the observation of the naked eye on the variations of fluorescence brightness and color under the aid of a UV lamp.

The paper reports a dosage-sensitive fluorescent colorimetry test paper with a very wide/consecutive “from red to cyan” response to the presence and amount of As(III). Red quantum dots (QDs) are modified with glutathione and dithiothreitol as the sensory red probe to obtain the supersensitivity to As(III) by the quenching of red fluorescence through the formation of dispersive QDs aggregates. A small amount of cyan carbon dots (CDs) with spectral blue-green components as the photostable internal standard are unequal-proportional mixed into the QDs solution to produce a composited red fluorescence. When the sensory solution is printed onto a piece of filter paper, surprisingly a serial of color evolution from peach to pink to orange to khaki to yellowish to yellow-green to final cyan with the addition of As(III) is displayed and clearly discerns the dosage scale as low as 5 ppb, which is lower than 10 ppb As(III) in drinking water prescribed by the World Health Organization’s guideline.. The proposed methodology opens a novel pathway toward the real applications of fluorescent test papers.