Two‐Photon Excitation of a Plasmonic Nanoswitch Monitored by Single‐Molecule Fluorescence Microscopy

Abstract

Visible-light excitation of the surface plasmon band of silver nanoplates can effectively localize and concentrate the incident electromagnetic field enhancing the photochemical performance of organic molecules. Herein, the first single-molecule study of the plasmon-assisted isomerization of a photochrome-fluorophore dyad, designed to switch between a nonfluorescent and a fluorescent state in response to the photochromic transformation, is reported. The photochemistry of the switchable assembly, consisting of a photochromic benzooxazine chemically conjugated to a coumarin moiety, is examined in real time with total internal reflection fluorescence microscopy in the presence of silver nanoplates excited with a 633 nm laser. The metallic nanostructures significantly enhance the visible light-induced performance of the photoconversion, which normally requires ultraviolet excitation. The resulting ring-open isomer is strongly fluorescent and can also be excited at 633 nm. These stochastic emission events are used to monitor photochromic activation and show quadratic dependence on incident power. The utilization of a single laser wavelength for both photochromic activation and excitation effectively mimics a pseudo two-colours system.

Electrical detection of plasmon-induced isomerization in molecule–nanoparticle network devices

“…This mechanism was observed by fluorescence experiments on Ag nanoparticles decorated with benzooxazine ring derivatives and diarylethene. [46][47][48] Here, the LSP excitation at 590 nm requires a significant molecular absorption at around 295 nm. For the AzBT molecules, the absorption at this wavelength is lower (about 1/3) than at the peak of maximum absorption (at 336 nm) but not zero.…”

“…[40][41] For example, the conductivity of an organic semiconducting thin film is increased when strongly coupled to the plasmonic modes of a nanostructured metallic layer, 42 and tunneling charge transfer plasmon (tCTP), resulting from quantum interactions between tunneling electrons and plasmonic nanostructures, was recently demonstrated in molecular tunnel junctions of various molecules connected between two NPs. [43][44] Albeit the isomerization of molecules attached onto a NPs induced by LSP excitation (here referred to as PII: plasmon-induced isomerization) has been observed from purely optical methods (Raman or fluorescence spectroscopies), [45][46][47][48] the modulation of the electrical conductance of a molecularly functionalized NP network induced by PII of the molecules decorating the NPs has not been reported. Here, we combine NPSANs and molecular plasmonics to demonstrate the electrical detection (conductance variation) of PII of azobenzene derivatives (azobenzene bithiophene: AzBT).…”

Noncontact Metal–Spiropyran–Metal Nanostructured Substrates with Ag and Au@SiO2 Nanoparticles Deposited in Nanohole Arrays for Surface-Enhanced Fluorescence and Trace Detection of Metal Ions

“…However, the conformational constraint of the polymer networks for spiropyran will extend the ring-opening time up to 300 s, and reduce the service efficiency. , The other method is to assemble spiropyran onto the surface of a noble metal. The enhancement of a local electromagnetic (EM) field on the metal surface not only improves the fluorescence intensity but also avoids the excessively long response time of the ring-opening reaction. − For instance, Wang et al assembled the PMMA-SP composite film on the metal substrate composed of 60 nm Au nanospheres. The EF of fluorescence is up to 32 due to the strong EM coupling between the gaps of Au nanospheres, and the ring-opening reaction of spiropyran is completed within 60 s, which is suggested to be used for biosensing .…”

A Multi‐Stimuli‐Responsive Oxazine Molecular Switch: A Strategy for the Design of Electrochromic Materials

“…[34][35][36][37][38] Of these, pH-sensitive molec-ular switches are the most abundant and have wide applications in pH indicators,b iosensing, [39] and information communication. [40,41] More importantly, the colors of pH-sensitive molecular switches exhibit wide variety,h ighp urity,a nd ideal molar absorption coefficients. However,t he application of pHsensitivem olecular switches in ECM is hindered by the lack of an electrical response ability.…”

“…Molecular switches represent a kind of molecule with properties such as optical properties, polarity, and solubility that can be changed when the structure of the molecule undergoes transformation under external stimuli, such as light, heat, pH, force, and electric fields . Of these, pH‐sensitive molecular switches are the most abundant and have wide applications in pH indicators, biosensing, and information communication . More importantly, the colors of pH‐sensitive molecular switches exhibit wide variety, high purity, and ideal molar absorption coefficients.…”