Faecal matter is considered as primary source of contaminant in water. The enhancement of
fluorescence of faecal pigments (FPs) like urobilin (UB) and stercobilin (SB) by Zn(II) is often used
for the detection of FPs in aqueous media (Schlesinger's test).1 Detailed photophysical study shows
that in aqueous media, FPs have the intriguing presence of multiple emitting species which indicate
the coexistence of monomers (λex= 525 nm, λem= 540 nm), lower-order H-aggregates (dimers) (λex=
506 nm, λem= 516 nm), and higher-order H-aggregates (λex= 480 nm, λem= 500 nm) even at a very low
concentration.2 This work presents an in-depth study of the nature of the FP-Zn(II) complexation
using spectroscopic, thermodynamic and computational studies. It is found that FP-Zn(II) complexes
have a lower solubility in water that results in the formation of molecular aggregates of the
complexes. This is the primary region of time-dependent loss of FP-Zn(II) fluorescence which
adversely affects the sensitivity of FPs detection in the aqueous medium. Both spectroscopic and
computational evidence have corroborated well with each other. The study was further extended by
utilizing this aggregation-induced emission phenomenon of FPs by using a series of primary alcohols
with increasing viscosity. It was found that FP-Zn(II) has stable and maximum fluorescence intensity
in 1-Hexanol. Furthermore, 1-Hexanol can also be used as an efficient extraction medium for FPs,
which can enhance fluorescence as well as minimization of matrix components interference for high
sensitivity detection (picomolar and sub picomolar concentration). These molecular-level
understanding of the fluorescence behaviour of FPs and FP-Zn(II) is expected to aid in the
development of aqueous phase analytical strategy towards the fluorescence-based analysis of the
faecal matter, an essential component of water quality analysis.
References
1. Bixler, J. N.; Cone, M. T.; Hokr, B. H.; Mason, J. D.; Figueroa, E.; Fry, E. S.; Yakovlev, V. V.; Scully, M. O.,
Ultrasensitive detection of waste products in water using fluorescence emission cavity-enhanced spectroscopy. Proceedings
of the National Academy of Sciences of the United States of America 2014, 111 (20), 7208-11. DOI:
10.1073/pnas.1403175111
2. Prakash, S.; Mishra, A. K., Stercobilin and Urobilin in Aqueous Media: Existence of Specific H-Aggregates and
Nonspecific Higher Aggregates at Different Concentrations. The Journal of Physical Chemistry A 2020.
DOI:10.1021/acs.jpca.0c08642