Fluorophoric Conjugate of N-Alkyl Naphthalimide in Sodium Dodecyl Sulfate As a Tunable and Sustainable Sensing System: Differential Sensing of Zn2+ and Al3+ and the Application of Its Zn2+ Complex in Detecting Dipicolinic Acid, a Component of Anthrax Bacterial Endospores

Abstract

A naphthalimide-based fluorophore has been conjugated by an octyl chain on one side and an ethylenediamine (“en”) moiety on the other to result in a conjugate (L1) that was adapted to exhibit selective ion sensing. Since L1 is not a receptor for metal ions in water or in organic solvents, this has been introduced into the self-assembly formed by the surfactant, i.e., sodium dodecyl sulfate (SDS), and its ion sensing was studied by varying the concentration of the SDS since it is known to form premicellar as well as micellar assemblies. Thus, the L1 was successfully converted to a specific, selective, and sustainable chemo-receptor system for Al3+ and Zn2+ with the support of such assemblies of SDS. The selectivity was among three trivalent and eight divalent ions studied. The ion recognition ability was dependent on the nature of the assembly of SDS formed and is efficient in the premicellar media. Both the Al3+ and Zn2+ were differentially recognized just by tuning the SDS concentration. The results were ratified by comparing with several control experiments. The excited-state lifetime measurements and the size of these assemblies measured by dynamic light scattering (DLS) reveal that at >4 mM SDS concentration (critical micellar concentration is 8 mM) the probe L1 is not accessible for any of the metal ions studied. The two other derivatives possessing a shorter (propyl, L2) and a longer (dodecyl, L3) alkyl chain showed that the L1 with an octyl chain performs much better at all the premicellar SDS concentrations. The Zn2+-bound assembly of L1 in 4 mM SDS was demonstrated for sensing the dipicolinic acid (DPA) which happens to be an endospore component of anthrax bacteria. This study was extended to several other carboxylic acid derivatives which would act as control molecular systems. Even in the DPA sensing, the assemblies possessing an octyl derivative stand out when compared to the propyl or the dodecyl derivatives.