A simple, instrument-free, paper-based analytical device with dual-emission carbon dots (CDs) (blue CDs and red CDs) was developed for the semiquantitative, visual, and sensitive speciation analysis of lead ions in a real sample with a sensitive detection limit of 2.89 nM. When a paper strip was immersed into the sample solution, the blue fluorescence was quenched by Pb 2+ in solution, while the red fluorescence served as a background reference without color change, and significant color evolutions from blue to red were observed under the ultraviolet lamp, resulting in a semiquantitative visual detection. Furthermore, a smartphone was used in the visual detection of lead ions by identifying the RGB value of the fluorescent probe solution and corresponding paper strip. The application of smartphones and fluorescent paper strips has greatly shortened the detection time and reduced the cost of detection, providing a new strategy for the on-site and semiquantitative detection of heavy-metal ions in water samples.

The white backlight in displays is generated by optimizing the proportions of individual emitters with different wavelengths by variations in materials composition, phase, and structure. Color pixels usually result from the separation of white light or the excitation with multiwavelength or multipulse sources. However, it is a challenge to develop a material that comprises a single structure and emits over the full visible spectrum, but where the emission wavelengths can be controlled by a simple excitation source. Herein, we report an upconversion nanostructure that incorporates several lanthanide ions in the same core@shell@shell structure. The combination of multiple narrow spectral bands results in the emission of white light. The emission colors can be tuned by changing the excitation power density, which manipulates the photon transfer pathways. Applications such as flat-panel displays and imaging have been demonstrated.

Instrument-free, portable, and direct read-out minidevices have wider application prospects in various fields, especially for real-time/on-site sensing. Herein, combined with a paper strip, a smartphone sensing platform integrated with a UV lamp and dark cavity by 3D-printing technology has been developed for the rapid, sensitive, instrument-free, and visual quantitative analysis in real-time/on-site conditions. The platform proved the feasibility for visual quantitative detection of pesticide via a fluorescence "on−off−on" response with a single dualemissive ratiometric paper strip. Red-emitting CdTe quantum dots (rQDs) were embedded into the silica nanoparticles (SiO 2 NPs) as an internal reference, while blue-emitting carbon dots (bCDs) as a signal report unit were covalently linked to the outer surface of SiO 2 NPs. The blue fluorescence could be quenched by gold nanoparticles (Au NPs) and then recovered with pesticide. The red (R), green (G), and blue (B) channel values of the generated images were determined by a color recognizer application (APP) installed in the smartphone, and the R/B values could be used for pesticide quantification with a sensitive detection limit (LOD) of 59 nM. The smartphone sensing platform based on 3D printing might provide a general strategy for visual quantitative detection in a variety of fields including environments, diagnosis, and safety monitoring.

A simple and effective ratiometric fluorescence nanosensor for the selective detection of Cu(2+) has been developed by covalently connecting the carboxyl-modified red fluorescent cadmium telluride (CdTe) quantum dots (QDs) to the amino-functionalized blue fluorescent carbon nanodots (CDs). The sensor exhibits the dual-emissions peaked at 437 and 654 nm, under a single excitation wavelength of 340 nm. The red fluorescence can be selectively quenched by Cu(2+), while the blue fluorescence is a internal reference, resulting in a distinguishable fluorescence color change from pink to blue under a UV lamp. The detection limit of this highly sensitive ratiometric probe is as low as 0.36 nM, which is lower than the U.S. Environmental Protection Agency (EPA) defined limit (20 μM). Moreover, a paper-based sensor has been prepared by printing the hybrid carbon dots-quantum dots probe on a microporous membrane, which provides a convenient and simple approach for the visual detection of Cu(2+). Therefore, the as-synthesized probe shows great potential application for the determination of Cu(2+) in real samples.

To achieve selectivity in direct chemiluminescence (CL) detection is very significant and a great challenge as well. Here, we report a novel concept of developing intrinsically selective CL switching at the surface of Fe(3)O(4) nanoparticles for the sensitive detection and simultaneous determination of various pesticides. Fe(3)O(4) nanoparticles have peroxidase-like catalytic activity and catalyze the decomposition of dissolved oxygen to generate superoxide anions, so that the CL intensity of luminol was amplified by at least 20 times. The CL signals can be quenched by the addition of ethanol because ethanol readily reacts with superoxide anions as a radical scavenger. However, the quenching effect can be inhibited through the specific binding of target molecules on Fe(3)O(4) nanoparticles, leading to CL "turn-on" in the presence of ethanol. The novel CL "switching-on" concept demonstrated unique advantages in the detection of pesticide residues. Using the surface coordinative reactions, nonredox pesticide ethoprophos were sensitively detected with a detection limit of 0.1 nM and had a very wide detection range of 0.1 nM to 100 μM. More importantly, the selectivity of CL switching is tunable through the special surface modification of Fe(3)O(4) nanoparticles, and these Fe(3)O(4) nanoparticles with different surface groups can generate unique CL response pattern for the simultaneous determination of various pesticides. Meanwhile, the superparamagnetic properties of Fe(3)O(4) nanoparticles provide a simple magnetic separation approach to attain interference-free measurement for real detection. The very facile and versatile strategy reported here should open a new window to exploration of selective CL molecular switching and application of magnetic nanoparticles for chemo/biodetection.

Graphene oxide has widely been employed in various fields, but its structure and composition has still not been fully understood. Here we report that freshly prepared graphene oxide exhibits a large number of π-conjugated carbon radicals at its π-network plane, which result from the addition reaction of hydroxyl radicals from H2O2 onto the conjugated double bonds of graphene oxide. The π-conjugated carbon radicals can directly initiate the long-lasting visible chemiluminescence of luminol, which is even stronger than that obtained when horseradish peroxidase and H2O2 are used. Previously, graphene oxide was mainly reported to be a quencher of chemiluminescence instead. Remarkably, the reacted radicals can be regenerated, thereby enabling the repetitive initiation of chemiluminescence by re-treatment of graphene oxide. The results reported here provide a new understanding of the structure, properties, and applications of graphene oxide.

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