Silver-doped CdS quantum dots capped with mercaptoacetic acid were incorporated into a chitosan-coated filter paper to fabricate a colorimetric test stripe for mercury. Chitosan assisted in the stabilization of CdSAg QDs on cellulose and enhanced the analyte diffusion in the test stripe. Hg(II) ions were captured by mercaptoacetic acid on CdSAg, resulting in a visualized color change from yellow to deep brown. For enhanced sensitivity and quantitation with good reproducibility, the color intensity was recorded, transmitted to a smartphone camera and computed by the ImageJ software to provide a digital readout. The assay was rapid, simple, and selective for mercury as several potential interfering species provoked no signal response. The applicability of the sensing approach was demonstrated for the analysis of spiked Hg(II) ions in cosmetic cream with a detection limit of 124 μM. Graphical abstract Schematic of a colorimetric test stripe consisting of silver-doped CdS quantum dots coated withmercaptoacetic acid and embedded into a chitosan matrix on paper. It enables mercury(II) to be visuallyquantified even in complex matrices such as cosmetics.

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Myoglobin from Asian swamp eel Monopterus albus was purified from fish muscle using salt fractionation followed by column chromatography and molecular filtration. The purified Mb of 0.68 mg/g wet weight of muscle was determined for its molecular mass by MALDI-TOF-MS to be 15,525.18 Da. Using isoelectric focusing technique, the purified Mb showed two derivatives with pI of 6.40 and 7.12. Six peptide fragments of this protein identified by LC-MS/MS were homologous to Mbs of sea raven Hemitripterus americanus, yellowfin tuna Thunnus albacores, blue marlin Makaira nigicans, common carp Cyprinus carpio, and goldfish Carassius auratus. According to the Mb denaturation, the swamp eel Mb had thermal stability higher than walking catfish Clarias batrachus Mb and striped catfish Pangasius hypophthalmus Mb, between 30 and 60 (°)C. For the thermal stability of Mb, the swamp eel Mb showed a biphasic behavior due to the O(2) dissociation and the heme orientation disorder, with the lowest increase in both Kd(f) and Kd(s). The thermal sensitivity of swamp eel Mb was lower than those of the other Mbs for both of fast and slow reaction stages. These results suggest that the swamp eel Mb globin structure is thermally stable, which is consistent with heat-tolerant behavior of the swamp eel particularly in drought habitat.

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Primary structure of myoglobin (Mb) from ordinary muscle of striped snake-head fish (Ophicephalus striatus) was studied. Sequence analysis of cloned cDNA revealed two lengths of nucleotide sequences different in 3´ untranslated regions, MbI gene and MbII gene. In accordance to amino acid coding region, both genes encoded protein with 145 amino acid residues which were different in two amino acid residues at position 114 and 117, indicating Mb isoforms. They are very close to those of various species of fish Mbs and very identical to some marine Mbs. There are amino acid differences among the fish Mbs and other Mbs that occur in the highly conserved regions in fish species. The replacement of alanine with serine 59 in fish species, adjacent to histidine 60 may contribute to the reversible binding of oxygen. Lysine was replaced by asparagine at position 93 which directly binds to the heme. The fish Mbs contain two cysteine residues. One of cysteine residue locates at the terminal of the polypeptide chain which may possibly contribute to internal disulfide interaction and thus forms tightly folded structure. It is implied that the fish Mb might be a novel protein with strong intramolecular bonding.