Enhanced Phototherapy Activity by Employing a Nanosilica-Coumarin–Acifluorfen Conjugate as the Supplementary Light Source Generator

Photosensitizers need to be activated by external light to achieve phototherapy. However, unsatisfactory tissue penetration of light and uncontrollable light intensity limit their application. To overcome the limitations, a novel fluorescent conjugate was developed by conjugating coumarin-functionalized nanosilica with acifluorfen to emit fluorescence as the supplementary light to activate protoporphyrin IX (PpIX, a photosensitizer in plant cell that can be induced by acifluorfen) for enhancing phototherapy activity. The results showed that the obtained conjugate could obviously increase the singlet oxygen yield of PpIX and exhibited remarkable photodynamic herbicidal activity against Amaranthus retroflexus. Under low light intensity, its photodynamic herbicidal activity was five to six times higher than that of acifluorfen sodium salt. The conjugate could move upward and downward freely in vascular system of Brassica juncea seedlings and significantly reduced the cytotoxicity of acifluorfen. Moreover, the conjugate was proved to be safe to soybean at concentrations of 200–800 mg/L of acifluorfen. This study gives a novel drug delivery system with the potential of reducing light-dependent drug dosage and harm to nontarget organisms and offers a way to use fluorescent emission as the supplementary light to enhance the phototherapy activity of photosensitizers.