Ultrafast excited-state dynamics at interfaces: fluorescent DNA probes at the dodecane/water interface

Abstract

Although the interfaces between two isotropic media are of primary importance in many areas of science and technology, their properties are only partially understood. Our strategy to obtain an insight into these properties is to investigate the ultrafast excited-state dynamics of environment-sensitive molecular probes at liquid interfaces using time-resolved surface second harmonic generation, and to compare it with the dynamics of the same molecules in bulk solutions. Additionally, this approach gives rich information on how the chemical reactivity may change when going from the bulk phase to the interface. This is illustrated by an investigation performed with a series of fluorescent DNA probes at the dodecane/water interface without and with the presence of DNA in the aqueous phase. Substantial differences in the conformation of these cyanine dyes (aggregated or not) and in the excited-state dynamics are observed when going from bulk solutions to the interface. Moreover, the presence of double-stranded DNA in the aqueous phase induces some chirality at the interface.

Exploring Fluorescent Dyes at Biomimetic Interfaces with Second Harmonic Generation and Molecular Dynamics

“…The SSHG setup has been described in detail previously. , The probe pulses (∼100 fs, ∼0.7 μJ) centered at 1020 nm were generated by a collinear optical parametric amplifier (TOPAS-C, Light Conversion) pumped by the output of a Ti:sapphire amplifier (Solstice, Spectra-Physics). This wavelength is close to the maximum in a SSHG band of 1 , which originates from two-photon resonance with the S 1 ← S 0 transition . The probe pulses were focused on the sample by a 400 mm lens and hit the sample under the total internal reflection condition with an angle of incidence of about 70°.…”

“…This wavelength is close to the maximum in a SSHG band of 1, which originates from two-photon resonance with the S 1 ← S 0 transition. 32 The probe pulses were focused on the sample by a 400 mm lens and hit the sample under the total internal reflection condition with an angle of incidence of about 70°. The quadratic dependence of the signal was checked to ensure the absence of higher-order processes (Figure S13, Supporting Information).…”

“…33 ■ RESULTS AND DISCUSSION Affinity of the Dye for the Interfaces. In previous studies, 32,33 the high interfacial affinity of YO DNA probes was mostly inferred from the high intensity of the SSHG signal as well as from various observations, such as the disruption of aggregates upon adsorption at the interface. Direct information is obtained here by measuring the adsorption isotherm of 1 at liquid/liquid interfaces by SSHG.…”

“…As a dye, we chose cyanine dye 1 (Chart ), which belongs to the thiazole orange (TO) family of fluorescent DNA probes. This dye is also closely related to the yellow oxazole (YO) DNA-probe family, , several of which have recently been shown to have a high affinity for the dodecane/water interface. , This affinity can be understood by considering that these DNA probes are soluble in water but have a high binding constant to DNA upon intercalation into the hydrophobic base-pair stack. The selected phospholipids are DPPC and DPPG (Chart ), which have zwitterionic and anionic heads, respectively, whereas 1 bears a single positive charge.…”