Ultrafast Visual Detection of a Trace Amount of Water by Highly Efficient Hybrid Manganese Halides

A quantitative
water detection method is urgently needed in storage
facilities, space exploration, and the chemical industry. Although
numerous physical techniques have been widely utilized to determine
the water content, they still suffer from many disadvantages such
as highly expensive special instruments, complicated analysis processes,
etc. Hence, a convenient, rapid, and sensitive water analysis method
is highly desirable. Herein, we developed a visual fluorescence sensing
technology for water detection based on reversible PL off-on switching
of organic–inorganic hybrid zero-dimensional (0D) manganese
halides. In this work, a family of hybrid manganese halides were synthesized
through a facile solution method, namely, [NH4(18-Crown-6)]2MnBr4, [Ca(18-Crown-6)·3H2O](18-Crown-6)MnBr4, [NH4(dibenzo-18-Crown-6)]2MnBr4, and [Ca(dibenzo-18-Crown-6)·2H2O]MnBr4. Excited by UV light, these highly crystalline manganese
halides exhibit strong green light emissions from the d–d electron
transition of Mn2+ with near-unity photoluminescence quantum
yield and submillisecond lifetime. Benefiting from the dynamic and
weak ionic bonding interactions, these 0D manganese halides display
reversible water-response on/off luminescence switching but fail in
any other aprotic solvents. Therefore, these 0D hybrid manganese halides
can be explored as ultrafast visual fluorescence probes to detect
the trace amount of water in organic solvents with multiple superiorities
of rapid response time (< 2 s), ultralow detection limit (9.71
ppm), excellent repeatability, etc. The reversible water-response
luminescent on/off switching also provides a binary optical gate with
advanced applications in anticounterfeiting and information security,
etc.