Fluorescent Molecular Logic Gates and Pourbaix Sensors in Polyacrylamide Hydrogels.

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.

Molecular engineering of logic gate types by module rearrangement in 'Pourbaix Sensors': the effect of excited-state electric fields.

Two types of fluorescent logic gates are accessed from two different arrangements of the same modular components, one as an AND logic gate (1) and the other as a PASS 0 logic gate (2). The logic gates were designed with an 'electron-donor-spacer1-fluorophore-spacer2-receptor' format and demonstrated in 1 : 1 (v/v) methanol/water. The molecules consist of ferrocene as the electron donor, 4-aminonaphthalimide as the fluorophore and a tertiary alkylamine as the receptor. In the presence of high H+ and Fe3+ levels, regioisomers 1a and 1b switch 'on' as AND logic gates with fluorescence enhancement ratios of 16-fold and 10-fold, respectively, while regioisomers 2a and 2b are functionally dormant, exhibiting no fluorescence switching. The PASS 0 logic of 2a and 2b results from the transfer of an electron from the excited state fluorophore to the ferrocenium unit under oxidising conditions as predicted by DFT calculations. Time-resolved fluorescence spectroscopy provided lifetimes of 8.3 ns and 8.1 ns for 1a and 1b, respectively. The transient signal recovery rate of 1b is ∼10 ps while that of 2b is considerably longer on the nanosecond timescale. The divergent logic attributes of 1 and 2 highlight the importance of field effects and opens up a new approach for regulating logic-based molecules.

Proof of principle of a three-input AND-INHIBIT-OR combinatorial logic gate array.

A designed molecule fluoresces according to a combinatorial logic circuit equivalent to wiring together three-input AND, three-input INHIBIT and two-input OR logic gates.

Fluorescent Photoinduced Electron Transfer (PET) Logic Gates for Acidity (pH) and Redox Potential (pE).

This mini-review highlights the photophysical properties of fluorescent molecular logic gates responsive to acids and oxidants, particularly those developed in our laboratory the past few years. The review pays tribute to earlier developments that lay the foundation for this emerging class of molecules. The logic gates incorporate design concepts based on photoinduced electron transfer from the cross-fertilization of the fluorophore-spacer-receptor and fluorophore-spacer-electron-donor formats. The molecular logic gates explored in detail consist of anthracene and/or naphthalimide fluorophores, while the receptor and electron-donor are typically alkyl amines and ferrocene, respectively. This emerging class of molecule has numerous probable uses, most notably, in corrosion science, geochemistry, molecular cell biology and biomedical diagnostics.

A lab-on-a-molecule with an enhanced fluorescent readout on detection of three chemical species.

A molecular three-input AND logic gate with a naphthalimide fluorophore communicates the presence of a congregation of three biologically relevant cations in aqueous methanol with a 25-fold fluorescence enhancement and ΦFmax of 0.203.