Interfacial Organization in Dipolar Dye-Based Organic Nanoparticles Probed by Second-Harmonic Scattering

Abstract

We have undertaken polarization-resolved SHS experiments to investigate a series of two fluorescent organic nanoparticles (FONs) made from dipolar dyes bearing a triphenylamine-donating end-group and a slightly hydrophilic acceptor end-group. The FONs, which show very large negative surface potentials, responsible for their good colloidal stability in water, are very bright SHS emitters in water, attractive for single-particle tracking and bioimaging. With the polarization-resolved SHS technique, we come up with a new and noninvasive method to gain insight into the origin of the surface potential of the FONs, as well as into their evolution upon change of ionic strength by following their multipolar response when adding salt. In pure water, the polarization-resolved SHS spectroscopy reveals the presence of a region of local polar order close to the surface (i.e., orientation of the dipolar dyes), leading to a strong dipolar contribution. Quite interestingly, when adding KCl salt, the initial two-lobe SHS polar plot transforms into a four-lobe pattern with a net decrease of the SHS intensity, revealing a lower surface potential. Second, we have quantified the decrease of the SHS signal when adding salt with the goal to estimate the change of the NPs' colloidal stability in salty media. From a simple equilibrium model that we propose, we have extracted a lower bound free energy for the two FONs, which are thermally accessible and in good accordance with their respective colloidal stability. I n the last decades, nanoparticles have attracted increased interest in biology and optoelectronics fields, mainly due to their unique properties. Among them, luminescent metal-and semiconductor-based nanoparticles have attracted overwhelming attention. A number of them however raise toxicity and biodegradability issues, which is critical with regard to biomedical applications and environmental concerns. In that perspective, molecular-based fluorescent organic nanoparticles (FONs) 1 emerged as a promising alternative toward optical nanomaterials and fluorescent probes for acute biomaging applications. 2 Among them, FONs made from specific dyes, i.e., dipolar (i.e., push−pull or D−π−A compound) chromophores, offer attractive features as tunable luminescent nanomaterials 3 of interest for cellular as well as in vivo bioimaging purposes 4 or as nanotools for single-particle tracking. 5 Interestingly, the colloidal stability of these nanoparticles in water was observed to strongly depend on the structure of the push−pull compound system. 4,6 Furthermore, investigation of the fluorescence properties (in particular fluorescence emission spectra and lifetimes) reveals different environments, pointing to possible nanostructuration of dipolar dyes within the FONs. Conjecturing that this could be ascribed to a specific organization (and local ordering) of the dye subunits near the surface of the spherically shaped FONs (i.e., at the interface with water), we aimed at investigating this hypothesis by using hyper-Rayleigh scattering (HRS),...

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“…Indeed, replacing H 2 O with D 2 O caused a 50 and 37% increase in the emission quantum yield Φ f for p‐BDZ and c‐BDZ FONs, respectively, confirmed by a similar increase in the intensity averaged lifetimes of the excited states (33 and 22%, respectively) (Figure S4 and Table S2, Supporting Information). We can thus conclude on strong cohesion of a water shell around the protic FONs and suspect that similar results would apply to any kind of polar FONs, since local organization of water molecules has recently been evidenced by nonlinear hyper‐Rayleigh measurements …”

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“…[32][33][34][35] Recently, it was reported that triphenylmethane-based nanoparticles are excellent candidates for single particle tracking and bioimaging due to their enhanced fluorescence and second harmonic signal. [36][37][38] Molecular-based nanomaterials such as nanoGUMBOS offer a new route for designing nonlinear optical probes by combining the inherent molecular nonlinear susceptibilities with the ability to control the resulting nanoparticle size as well as molecular interactions and structural ordering.…”

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“…Moreover, we have investigated the effect of the introduction of bromine (series II ) or tert -butylphenyl (series III ) substituent at the available para positions of the two terminal phenyl groups. This was aimed at both tuning the photophysical properties (by slight modulations of the donating strength) and modulating the colloidal and structural of the subsequent FONs by controlling interchromophoric interactions [12]. Concerning the acceptor moiety, three different acceptors of increasing electron-withdrawing strength have been used: formyl (CHO), dicyanovinyl (DCV) and 1,3-diethyl-2-thiobarbiturate (DETB).…”

“…All FONs made from push-pull derivatives I – IIIa – c display a highly negative surface potential, down to −85 mV for FONs made from dye IIa . The highly negative surface of the FONs suggests that the dipolar dyes show specific arrangement and short distance order near the surface [12], with most probably electron-withdrawing groups (which bear O or N atoms having lone pairs and liable to be H-bond acceptors) pointing towards the water interface. These highly negative surface potential values are favorable features in view of the colloidal stability.…”

“…In particular, molecular engineering of the molecular chromophoric subunits not only allows to tune the optical properties of the FONs but also is effective in tuning their surface properties and colloidal stability [12,13]. Among various criteria, the structural (and colloidal stability of FONs is of crucial importance for use as fluorescent nanoprobes for bioimaging.…”