Dual-Colored Carbon Dot Ratiometric Fluorescent Test Paper Based on a Specific Spectral Energy Transfer for Semiquantitative Assay of Copper Ions

Abstract

Classical pH test papers are widely used to measure the acid-base degree of media in a qualitative or semiquantitative manner. However, the extension of portable and inexpensive methods to a wide range of analytes so as to eliminate the tediousness of instrumental assays remains unsuccessful. Here, we report a novel kind of dual-colored carbon dot (CD) ratiometric fluorescent test paper for the semiquantitative assay of copper ions (Cu) by a dose-sensitive color evolution. The preparation of the test paper is based on the following two interesting findings: on the one hand, residual p-phenylenediamine at the surface of as-synthesized red CDs (r-CDs) efficiently binds Cu ions to produce a strong visible absorption that overlaps the emission of blue CDs (b-CDs); on the other hand, the Cu ions render the adsorption of small b-CDs onto the surface of larger r-CDs through their dual-coordinating interactions with the surface ligands of both r-CDs and b-CDs. These two mechanisms lead to a specific spectral energy transfer to quench the fluorescence of b-CDs with a sensitive detection limit of 8.82 nM Cu, whereas the red fluorescence of r-CDs is unaffected as a stable internal standard. Ratiometric fluorescent test papers have been prepared using a mixture of r-CDs and b-CDs (1:7) as ink by jetprinting on a piece of paper. With the addition of Cu ions, the blue test paper produces a consecutive wide-colored evolution from blue to orange-red, with a dose-discerning ability as low as 25 nM.

Synthesis and Applications of Red‐Emissive Carbon Dots

“…Besides gas sensing, most CDs have been used to detection various heavy metal ions in solution. Zhao and Liu groups reported a novel kind of dual‐colored CD ratiometric fluorescent test paper for the semiquantitative assay of copper ions (Cu 2+ ) by a dose‐sensitive color evolution . Herein, two colored CDs, blue CDs and red CDs, were first mixed to form an aqueous homogeneous solution, “an ink”, which were printed on the filter paper through an inkjet printer connected to a computer.…”

Fluorescent Nanomaterials for Color‐Multiplexing Test Papers toward Qualitative/Quantitative Assays

“…i) QDs have high quantum yield, large extinction coefficient, broad absorption, size‐/composition‐tunable emission and large Stokes shift (Figure A) . ii) In comparison, fluorescent CDs have other unique advantages including low cost, low toxicity, high chemical stability, rich raw material in nature, and a large number of carboxyl, hydroxyl, or amide groups at their surfaces for further chemical modification (Figure B) . iii) Similarly, 2D GOs have a large π‐conjugated plane bearing numerous phenol hydroxyl, epoxy, and carboxylic groups to specifically bind chemical/biological species: based on this we have synthesized fluorescent GOs by chemical modification of the surface for the first time (Figure C) .…”

“…Additionally, the ratiometric probe also has been applied to make paper‐based nanosensors for the visual fluorescent detection of Cu 2+ (the right image in Figure B). Recently, we developed a dual‐colored carbon‐dot ratiometric fluorescent test paper for the semiquantitative assay of copper ions by a dose‐sensitive color evolution . With the addition of Cu 2+ ions, the blue test paper produces a consecutive wide‐colored evolution from blue to orange‐red, with a dose‐discerning ability as low as 25 × 10 −9 m .…”

Nanomaterial-functionalized Cellulose: Design, Characterization and Analytical Applications

“…91 Both of these materials provide a measurable response directly from the intrinsic fluorescent properties of the material without the need of an exterior fluorescent agent. Quantum/carbon dot hybrids 85 or two color carbon dot hybrids 92 can also be combined or covalently attached to achieve a more sensitive fluorescent response. By quenching one dot with the target analyte, a broad range of fluorescent response is produced with increasing analyte concentration (Fig.…”