Aggregation-induced emission (AIE), which is opposite to the ubiquitous aggregation-caused quenching (ACQ) effect, refers to a photophysical phenomenon shown by a group of luminogenic materials that are non-emissive when they are dissolved in good solvents (e.g. acetonitrile) as isolated molecules but become highly emissive when they are agglomerated in poor solvents (e.g. water) or solid state (Figure 1).^[1-3] Unlike conventional disc-like planar structured luminophores (e.g. perylene), the AIE materials (e.g. Hexaphenylsilole (HPS)) have a propeller-shaped non-planar structure and emit fluoresce  in aggregate formation, due to the restriction of the intramolecular rotation (RIR) of the multiple phenyl rotors against the silole stator (Scheme 1).^[1]

Superabsorbent polymers (SAPs) are three-dimensional (3D), cross-linked hydrophilic networks of polymer chains, which has the superior abilities to absorb and retain ultra high amount (1,000 – 100,000 %) of water in comparison with their own mass.^[4] Due to their low-cost, non-toxicity, environmentally friendly and biocompatibility, SAPs with different natural and synthesised fillers has been widely used in diverse applications, such as personal hygiene products, wound healing materials and water absorption/purification products.

In our research, we found that the synthesised SAPs absorb various amount of water at different swelling rate as a function of pH in the solution, which inspires us to use AIE materials as the nano-fillers in the SAPs systhesis process. The AIE luminophores incorporated in solid SAPs are in aggregated formation with their light emissions turned ‘on’ (Figure 2). Then these AIE molecules start to isolate from each other and become non-emissive, due to the swelling of SAPs structure in aqueous solution (Figure 3). The diminished intervals of the AIE’s fluorescence are found to be related to the pH of the solution (Figure 3). In our future work, the synthesis parameters of SAPs-AIE material and experimental setup for the fluorescence detection will be adjusted, and we are aiming to develop the novel, portable and cost-effective pH sensors based on AIE and SAPs materials.

 

Scheme 1 (A) Planar luminophoric molecules tend to aggregate as discs pile up, which commonly turns ‘off’ light emission. (B) Non-planar luminogenic molecules behave oppositely, with their light emissions turned ‘on’ by aggregate formation.^[1]

 

 

Figure 1 Hexaphenylsilole (HPS) solutions in acetonitrile (good solvent)–water (poor solvent) mixtures containing different volume fractions of water under illumination of a UV lamp.^[2]

 

 

Figure 2 Photo images of the SAPs-AIE sample as prepared (left)without, (right)with 365nm UV light illumination.

 

 

Figure 3 Photo images of the SAPs-AIE samples at different pH solution as a function of timewith 365nm UV light illumination.

 

Article by Dr Yang Yu, Supervisor Dr Youhong Tang.

 

References

[1] Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev., 2011,40, 5361–5388.

[2] Y. Hong, J. W. Y. Lam , B. Z. Tang, Chem. Comm., 2009, 4332-4353.

[3] J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang,  B. Z. Tang,  Adv.  Mater., 2014, DOI: 10.1002/adma.201401356

[4] Q. Wang, J. L. Mynar, M. Yoshida, E.  Lee, M. Lee, K. Okuro, K. Kinbara, T. Aida, Nature 2010, 463 (7279), 339-343.