1. Jung-Jung Chang and Chih-Hsin Chen*
Agarose Dispersed Liquid Crystals for Real-Time Sensing Mercuric Ions in Water
Jung-Jung Chang and Chih-Hsin Chen*
Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan
Abstract
In this study, agarose was used to disperse liquid crystals (ADLC) for detecting mercuric ion in water. The LC droplets were prepared by a nematic LC, 4-cyano-4’-pentylbiphenyl (5CB), doped with a Hg2+-selective ligand, 5-(pyridine-4-yl)-2-(5-(pyridin-4-yl)thiophen-2-yl)thiazole (ZT). The formation of Hg2+/ZT complexes lead to a distinct radial-to-irregular configurational transition of the LC droplets when Hg2+ solution adding into the ADLC system. The system did not respond to other metal ions such as Al3+, Fe3+, V3+, Cd2+, Zn2+, Cu2+, Pb2+, Mn2+, Ni2+, Co2+, Mg2+, Ca2+, Li+, Na+, and K+. We also demonstrated that the system is insensitive to the change of pH of water and it is capable of detecting Hg2+ in tap water, pond water, river water, and sea water. ADLC system is a sustainable and environmentally-friendly chemistry. The developed method for localized LC droplets can be used a portable sensor device at any location.
Materials and sensing mechanism
The stability of droplets in the system
(a)
Fig. 5. Polarizing images of ZT doped LC droplets in ADLC system immersed in Hg2+ solution after the ADLC was stored for (a) 10 min, (b) 1 day, (c) 1 months, (d) 2 months and (e) 3 months.
radial
The ADLC still responded to Hg2+ after storage for three months.
The reusability of the ADLC
irregular
150 μm
(b)
(c)
(d)
(e)
(f)
Fig. 1. (a) Mechanism illustrations of the LC orientation and the corresponding images for the LC droplets for Hg2+ detection. (b) Schematic illustration of the ADLC holder. The photography of (c) ADLC holder and (d) ADLC film cut from the holder. (e) Top view and (f) lateral view polarized images of ADLCs.
Fig. 6. Polarizing images of of ZT doped LC droplets immersed in 500 μM Hg2+ and then adding 1mM EDTA, repeatedly.
The FTIR spectra of agarose gel
(f)
(g)
(h)
(j)
The ADLC system shows good reusability.
(a)
(b)
Detect the selectivity of the system with other metal ions
Fig. 7. Polarizing images of ZT doped LC droplets in ADLC system immersed in 500 μM of (a) Hg2+, (b) Al3+, (c) Fe3+, (d) V3+, (e) Cd2+, (f) Zn2+, (g) Cu2+, (h) Pb2+, (i) Mn2+, (j) Ni2+, (k) Co2+, (l) Mg2+, (m) Ca2+, (n) Li+ and (o) Na+ (p) K+.
Fig. 2. FTIR spectra of (a) agarose gel and (b) agarose gel dispersed ZT-doped 5CB droplets.
A CN stretching band at 2227 cm-1 in (b) proved the presence of 5CB in agarose gel.
The ADLC system shows good selectivity.
Detection of Hg2+ by using ZT doped LC in agarose
Detect the interference of the system with other metal ions
Fig. 3. Polarized images of (a) ZT-doped LC droplets immersed in 1 mM Hg2+ solution, (b) ZT-doped LC droplets immersed in 0 mM Hg2+ solution, (c) pure LC droplets immersed in 1 mM Hg2+ solution, and (d) pure LC droplets immersed in 0 mM Hg2+ solution.
Irregular configuration of LC droplets showed only when ZT and Hg2+ were both present in ADLC system.
Fig. 8. Polarizing images of ZT doped LC droplets in ADLC system immersed in 500 μM of Hg2+ and 500 μM (a) Al3+, (b) Fe3+, (c) V3+, (d) Cd2+, (e) Zn2+, (f) Cu2+, (g) Pb2+, (h) Mn2+, (i) Ni2+, (j) Co2+, (k) Mg2+, (l) Ca2+, (m) Li+ and (n) Na+ (o) K+.
Limit of detection
The ADLC system for sensing Hg2+ will not be interfered with other metal ions.
Acknowledgement
Fig. 4. Polarizing Images of ZT doped LC droplets in ADLC system immersed in aqueous solution containing (a) 1 mM, (b) 500 μM, (c) 250 μM, (d) 100 μM and (e) 50 μM Hg2+.
Department of Chemistry, Tamkang University
Ministry of Science and Technology, Taiwan
The LOD of Hg2+ for the system is 250 μM.