Uniform and Fluorescent Poly(urea-siloxane) Microspheres: Preparation and Recyclable Use as Sensors for Cr⁶⁺ and Fe³⁺ Detection

Poly(urea-siloxane) (PUSi) microspheres are synthesized through step-growth and precipitation polymerization of isophorone diisocyanate (IPDI) and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (APDS) in a ternary solvent of acetonitrile/acetone/water (AN/AT/H2O). The chemical structure of the polymer is fully analyzed by nuclear magnetic resonance. Highly uniform PUSi microspheres are achieved with a monomer concentration of up to 20 wt % under optimized conditions, i.e., IPDI/APDS at 7.2/2.8 (mol) in the ternary solvent AN/AT/H2O at 65/20/15 (wt). Under UV lamp irradiation, these PUSi microspheres exhibit blue fluorescence, and the emission intensity is obviously enhanced with decreased IPDI/APDS ratio under excitation of 335 nm. In addition, PUSi microsphere dispersion in water shows concentration-enhanced emission. PUSi emission is ascribed to the flexible siloxane segments, which, combined with the urea groups on PUSi chains, make the polymer chains easy to form clusters through H-bonding, in accordance with the cluster-triggered emission. PUSi microsphere dispersion in water is used as a fluorescent probe for Cr6+ and Fe3+ detection, with high sensitivity and selectivity among fifteen competitive metal ions tested. More interesting and significant, a great feature of this probe is that, in contrast to the common fluorescent probes which are mostly disposable, it is reusable after quite easy separation of the microspheres from a previous testing, a high merit of this probe. Taking into account the easy preparation, the hazardous material free process, and the facile reusability, particularly, one can conclude that this work provides a simple, green, low-cost, and truly reusable fluorescent probe for Cr6+ and Fe3+ detection.