Fluorescent Molecular Logic Gates and Pourbaix Sensors in Polyacrylamide Hydrogels

Abstract

Polyacrylamide hydrogels formed by free radical polymerisation were formed by entrapping anthracene and 4-amino-1,8-naphthalimide fluorescent logic gates based on photoinduced electron transfer (PET) and/or internal charge transfer (ICT). The non-covalent immobilisation of the molecules in the hydrogels resulted in semi-solid YES, NOT, and AND logic gates. Two molecular AND gates, examples of Pourbaix sensors, were tested in acidic aqueous methanol with ammonium persulfate, a strong oxidant, and displayed greater fluorescence quantum yields than previously reported. The logic hydrogels were exposed to aqueous solutions with chemical inputs, and the fluorescence output response was viewed under 365 nm UV light. All of the molecular logic gates diffuse out of the hydrogels to some extent when placed in solution, particularly those with secondary basic amines. The study exemplifies an effort of taking molecular logic gates from homogeneous solutions into the realm of solid-solution environments. We demonstrate the use of Pourbaix sensors as pE-pH indicators for monitoring oxidative and acidic conditions, notably for excess ammonium persulfate, a reagent used in the polymerisation of SDS-polyacrylamide gels.

Keywords: fluorescence; hydrogel; internal charge transfer; molecular logic gate; photoinduced electron transfer; polyacrylamide.

Figure 1

The concept of a sensor (chemosensor, logic gate) encapsulated within a pore of a cross-linked water-compatible polymer matrix (top). The polymer is synthesised from acrylamide (monomer) and N,N′-methylene-bis-acrylamide (cross-bridge) using APS and TEMED to form the polyacrylamide hydrogel (bottom).

Figure 2

The molecules studied in cross-linked polyacrylamide hydrogels.

Figure 3

Emission spectra of 6 in 1:9 (v/v) MeOH/H₂O excited at λ_ex = 368 nm. The numbers in parentheses are the binary input conditions, as given in Table 2.

Figure 4

(a) Hydrogel pellets A and B (1.8 cm diameter). Sample A is a blank polyacrylamide control (PASS 0 logic gate). Sample B is embedded with 3 and exposed to a 0.1 M NaOH solution. (b) Sample B after exposure to 0.1 M HCl (beaker, right side). A yellow fluorescence is observed due to protonation of the amine receptor, preventing PET. The acid solution is also fluorescent due to diffusion of 3 from the hydrogel. The samples are irradiated with 365 nm UV light.

Figure 5

Two polyacrylamide pellets with 60 μM 4 irradiated with 365 nm light after 1 day of exposure to aqueous solutions. (Left) pellet of 4 after exposure to deionised water and (right) after exposure to aqueous 1 M NaCl solution. Na⁺-driven on–off (NOT) logic is observed. The supernatant in the beaker (right) emits blue emission due to leaching of 4 and incomplete saturation of the benzocrown receptor.

Figure 6

Vials D-F irradiated with 365 nm UV light in a dark cabinet. Vial D contains a hydrated polyacrylamide hydrogel embedded with 5 in the presence of APS. Vial E contains a hydrated hydrogel with 5, APS and 0.5 M NaOH solution, which turns off the fluorescence. Vial F contains a dried fragment of solid polyacrylamide with 5 emitting a green fluorescence.