Article pubs.acs.org/JPCB
Differential Photoluminescent and Electrochemiluminescent Behavior for Resonance Energy Transfer Processes in Thermoresponsive Microgels Florent Pinaud,† Romain Millereux,† Pierre Vialar-Trarieux,† Bogdan Catargi,‡ Sandra Pinet,† Isabelle Gosse,† Neso Sojic,*,† and Valérie Ravaine*,† †
Université de Bordeaux, Institut des Sciences Moléculaires, ENSCBP, 16 Avenue Pey Berland, 33607 Pessac Cedex, France CBMN UMR 5248, Université de Bordeaux, Allée de Saint-Hilaire, 33600 Pessac, France
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S Supporting Information *
ABSTRACT: Stimuli-responsive microgels with redox and luminescent resonance energy transfer (LRET) properties are reported. Poly(Nisopropylacrylamide) microgels are functionalized simultaneously with two models dyes: a derivative of tris(bipyridine) ruthenium complex and cyanine 5. Both moieties are chosen as a pair of luminophores with a spectral overlap for resonance energy transfer, where the ruthenium complex acts as a donor and the cyanine an acceptor. The effect of the temperature on the efficiency of the LRET of the microgels has been investigated and compared using either photoluminescence (PL) or electrochemiluminescence (ECL) as the excitation process. In PL, the synthesized microgels exhibit resonance energy transfer regardless of the swelling degree of the microgels. The transfer efficiency is a function of the donor−acceptor distance and can be tuned either by the swell−collapse phase transition or by the dye content in the microgel network. In ECL, the microgels emit light only at the wavelength of the ruthenium complex because the resonance energy transfer does not occur. Indeed, even within the microgel matrix, the cyanine dye is oxidized at the potential required for ECL generation, which impairs its emitting properties. Thus, both excitation channels (i.e., PL and ECL) show differential behavior for the resonance energy transfer processes.
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INTRODUCTION Over the past decade, there has been considerable interest in the development of sensing materials based on stimuliresponsive polymers. In particular, responsive microgels that can swell or shrink in response to changes in their environmental conditions (e.g., temperature, pH, biomolecular recognition, light, etc.) have been the basis for the development of new types of sensors1 based on different transducing methods. Fluorescent micro- and nanogels offer an interesting opportunity to sense local changes at the micro- and nanoscale level in complex environment. Thanks to their high hydrophilicity, these objects exhibit high stability, biocompatibility, and softness. Thus, they have been successfully used to measure the intracellular temperature2,3 or pH4 via the sensing of the fluorescence intensity related to the microgel volume. Transducing volume variations into fluorescence-intensity changes can occur according to various mechanisms, including polaritysensitive fluorophores,5,6 fluorescence resonance energy transfer (FRET),7−12 or fluorescence quenching of quantum dots (QD).13−15 Recently, electrochemiluminescent (ECL) microgels have been reported.16 In this case, the process of light emission results from the excited state of a luminophore provoked by an initial electrochemical reaction at the electrode surface.17 Because ECL does not use any external excitation light source, the background is almost solely due to the dark current of the © XXXX American Chemical Society
light detector; thus, ECL offers excellent sensitivity. In addition, nonspecific signals that may occur in fluorescence-based measurements can be minimized. These microgels were functionalized with ruthenium complexes as redox ECL emitters and derived from the chemistry of thermoresponsive poly(N-alkylacrylamides), among which was poly(N-isopropylacrylamide (pNIPAM). They exhibited volume-phase transitions from the swollen to the collapsed state upon heating. Their ECL signal was found to be correlated with their volumephase transition. Interestingly, it was enhanced up to 2 orders of magnitude at the swell−collapse transition, giving rise to an unusual turn-on signal upon temperature increase. This unexpected behavior was explained by the decrease of the average distance between adjacent redox sites, which favors both charge diffusion via electron-hopping in the microgels and the ECL annihilation mechanism.16 Using stimuli-responsive polymers is also a way to modulate the distance between adjacent fluorophores.18 It has been successfully exploited to promote FRET between lightabsorbing donors and light-emitting acceptor, which typically occurs over distances in the nanometer scale (
