In this work, a facile and interesting fabrication of the Förster resonance energy transfer (FRET) system is presented for the first time by introducing an independent donor and acceptor into a confined space, which is different from the typical “donor–linker–acceptor” FRET system. Human serum albumin (HSA), as a proof-of-concept, is selected to load a pair of spectra-matchable but independent fluorescent dyes (ES1 and R1) in its different binding sites. Furthermore, this space-confined FRET system could act as a ratiometric sensor for the quantitative recognition of Ag+ in the water samples with good selectivity and high sensitivity.

A gold nanoparticle-based colorimetric sensor for the determination of creatinine was developed as an important index for early diagnosis of kidney function and corresponding renal diseases. Because of the unique synergistic coordination capability of adenosine and creatinine with Ag+ on a particle surface, our system exhibits an excellent selectivity to creatinine among various ions and biomolecules. There are good linear relationships of absorption changes (A630 nm/520 nm) over creatinine concentrations, so both colorimetric qualitative detection by the naked eye and quantitative determination by UV–vis spectrometer could be realized with an excellent limit of detection compared with that of other methods. Finally, by testing creatinine in practical samples, such as urine mimic and bovine serum, good recoveries were obtained with proper relative standard deviations.

In this work, an N-T/gold nanoparticle system was assembled for the convenient and colorimetric identification of Cr based on its coordination chemistry with the thymine derivative (N-T). In addition to the visible colorimetric changes for the qualitative Cr identification, the A630 nm/520 nm values changed linearly with Cr concentration over a range of 0.5–2.5 μM and the detection limits reached 0.03 μM and 0.07 μM for Cr3+ and Cr2O72−, respectively, both of which satisfied the Cr water quality standards of the U.S. EPA, WHO and China. Furthermore, a more convenient method was developed for quantitative determination of Cr concentrations in practical samples based on the Eyedropper function in Microsoft's PowerPoint software, wherein the RGB (red, green and blue) value of the sample could be easily screened in the colorimetric card for a more accurate result with good recovery, avoiding complicated instrumentation.

•A fast, convenient, and portable colorimetric probe for arylamine (e.g. aniline) determination is developed based on functional AuNPs.•Our probe shows a very good selectivity to aniline than nearly all other tested ions and biomolecules.•Our probe shows a high sensitivity with a LOD of 0.5 mM, which could compete with even better than some of instrument-based methods.•It realizes qualitative and quantitative detection of aniline in water by naked eyes or absorbance change without professional operation.Aniline, a typical example of arylamines, is one of most important raw materials in many industries. However, arylamine-related pollution in waters threatens the public health and environment along with their wide applications. In this work, a simple, fast, and convenient colorimetric sensing system was presented for the arylamine (e.g. aniline) recognition in aqueous media. The system was presented using diazonium salt (DS) decorated gold nanoparticles (AuNPs), based on the classic coupling reaction of arylamine with diazonium salt. DS-AuNPs showed an excellent response and a good sensitivity to aniline, and the limit of detection reached 0.5 μM for aniline in this work. There was a good linear relationship (R2 = 0.997) between the values of A640 nm/520 nm and aniline concentrations over a range of 5–60 μM, which covered the Chinese standard of the maximum permissible pollutant emission concentration for aniline. The introduction of arylamine (e.g. aniline) caused a rapid and obvious red-to-blue color change, so the arylamine-polluted water could be qualitatively recognized by our naked-eyes and quantitatively measured by absorbance changes.

A series of microenvironment-sensitive fluorescent dyes, SA1–4, have been presented, which can light up human serum albumin (HSA) in aqueous media and solid state with colorful emissions as well as dramatic fluorescence enhancements respectively, based on twisted intramolecular charge transfer and molecular rotor strategy. These microenvironment-sensitive SA1–4 exhibited excellent fluorescent capabilities in the fast, convenient, selective, and sensitive recognition of HSA, especially in the quantitative albumin assay in human urine for assessment of kidney function and diagnosis of renal disease. Moreover, SA1–4/HSA complexes could be applied in fluorescence imaging in living cells.