New concept of spectroscopic detection of the cationic porphyrin via its spontaneous aggregation at Au@thermoresponsive polymer core@shell nanocomposites

Spectroscopic detection represents a big part of biomedical, biological, material, and environmental research. Here, the formation of J-aggregates of a tetra-cationic porphyrin (TMPyP), serving as a model adsorbate and known as a sensitizing part of antitumor and therapeutically-active agents, has been evidenced by several spectroscopic methods such as: UV-vis absorption spectroscopy, steady-state fluorescence, resonance Raman scattering (RRS) and surface-enhanced resonance Raman scattering (SERRS). It should be noticed that the tetra-cationic porphyrin does not form aggregates at low concentrations in water spontaneously, nor in the presence of low-molecular-weight species enveloping Au nanoparticles (AuNPs, prepared as references by citrate- and/or borohydride-induced reductions); indeed, rather an aggregation of AuNPs with adsorbed TMPyP were observed in these systems. On the contrary, at the surface of core@shell nanocomposites containing plasmonic AuNPs as a core (of three different sizes: 20, 60, and 100 nm) and selected polymers (two thermoresponsive polymers: poly-N-isopropyl-acrylamide and poly-2-(2-methoxy-ethoxy)ethyl-methacrylate, and polystyrene as a reference) as a shell, J-aggregates of TMPyP were detected by UV-vis absorption, fluorescence, RRS and SERRS for the first time in the present work. The mechanism of spontaneous J-aggregates formation of TMPyP at the nanocomposite surface (in TMPyP concentrations falling well below and/or being slightly above monolayer surface-coverage values) is suggested based on (i) coulombic attraction and (ii) amphiphilicity of the polymeric shells. Slightly elevated temperature (32 °C) and prolonged time of ageing (1 month) of the AuNPs@thermoresponsive polymer nanocomposites with adsorbed porphyrin resulted in a further increase of TMPyP J-aggregates formation. This can be regarded as the second aggregation step of the tetra-cationic porphyrin molecules stuck into polymeric shells that is beneficial for its spectroscopic detection. It can be envisaged that the concept of concentrate formation (aggregation) and spectroscopic detection of low concentrations of any species (even extremely charged and well dissolved in aqueous solution) via its spontaneous aggregation at the surface of AuNPs@thermoresponsive polymer core@shell nanocomposites, more pronounced at slightly elevated temperatures, and concomitant aggregation-induced fluorescence quenching of fluorophores could be exploited in biomedical, biological, material, and environmental research domains.