1. important function in the metabolism of iron and

1. Introduction

Detection of metal ions has been an
interesting field of research because of their widespread roles in chemical,
biological, and environmental processes 1–3. As the second most abundant transition metal and
essential trace element in the human body 4–6, zinc plays a significant role in
gene transcription, DNA binding, and apoptosis 7–9. However, excessive amounts of zinc
in vivo cause many diseases such as Alzheimer’s disease 9–11. To date, various chemosensors for zinc ions
have been developed, but it’s still a huge challenge to distinguish Zn2+
from Cd2+ because they have similar properties 12–14. Therefore, it is important to
develop sensors capable of discriminating Zn2+ from Cd2+.

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Indium is well known as one
of the heavy metals and its salts have
attracted great attentions in biological chemistry due to their “low
toxicities” 15. However, some of studies recently suggested
that indium interferes with iron metabolism at sites of absorption,
transportation and storage in cells. In addition, some animal studies have
reported that acute intravenous administration of In3+ is severely
pernicious to the liver and kidney 16. For
these reasons, it needs developing chemosensor for indium ion.

Copper is the third most abundant
trace element in the body, which is associated with various biological processes 17–19. Copper
ion dependent enzymes act as catalysts to help many body functions. The enzymes
provide energy for biochemical reactions, assist in the transformation of melanin
for pigmentation, and repair the connective tissues 20–22. On
the other hand, the excessive amount of copper ions can cause negative and
serious diseases, such as
Alzheimer’s and Wilson’s diseases 23–25. Therefore,
detecting copper ion becomes important and necessary.

Cobalt is
extensively used in various materials and plays an important function in the
metabolism of iron and synthesis of hemoglobin 26–28. However,
excess intake of Co2+ can cause various diseases, such as heart
disease, pulmonary edema, skin disorders, thyroid enlargement, asthma, decreased
cardiac output and vasodilation 29–31. Meanwhile, cobalt deficiency in the human body could
induce retarded growth, loss of appetite and anaemia 32–34. Thus, designing practical, selective, and
competent sensors for cobalt ion have become in demand.

Multifunctional
sensors detecting metal ions through fluorescence or color change have been actively
considered because they have various benefits such as simple analysis technique and lower costs than
sensors for a single ion
35–37. Some sensors for multiple targets
have been developed such as Al3+/Zn2+/Cd2+ 38, Zn2+/Cd2+/Pi
39, Al3+/Fe3+/Cr3+
40 and Zn2+/Al3+/Fe2+/Fe3+
35,37, but they are still relatively rare.

Quinoline moieties have been used as
a great fluorophore and chromophore in chemosensor since they have distinct
spectral properties and good photostability 41–45. In addition, furan moieties have been generally used as a
binding site for metal ions 46–48. In this regard, we took into consideration combining
quinoline and furan moieties to develop a novel chemosensor for sensing metal
ions with unique optical properties.

Herein,
we report the development and application of a multiple-target fluorescent and
colorimetric chemosensor 1. Sensor 1 detected Zn2+ and In3+
by fluorescence enhancement and Cu2+ and Co2+ by color
changes from colorless to pale yellow in an aqueous environment. Moreover,
sensor 1 could quantify Zn2+
and Co2+ in water samples. Based
on the Job plot, various spectroscopic experiments and theoretical calculations,
their binding structures and sensing mechanisms were proposed.