Light harvesting nanomaterials for single molecule detection – LHnanoMat

Efficient light energy collection, transport and transfer to a functional « acceptor » is a fundamental process at the origin of natural and artificial photosynthesis, photocatalysis, (organic) photovoltaic energy conversion, as well as single molecule detection e.g. in bio-medical sciences. The nature and function of the acceptor is very different in all these processes: while the photosynthetic reaction center uses this energy to perform a multistep electron transfer, a fluorophore reemits light after collecting the energy from the antenna. The latter is of key importance for amplifying fluorescence signal, which enables detection of single molecules at low excitation power and designing ultrasensitive biosensing assays. On the other hand, the function of the so-called “antennas” for collection and transport is fundamentally the same in all cases: to harvest excitation light and efficiently transfer to the acceptor. Currently the field of optical nanoantennas is dominated by plasmonics, which employs collective excitation of the conduction electrons in metallic nanoparticles or surfaces. It was successfully used to transport excitation energy along nanostructured metallic surfaces or locally enhance the antenna-acceptor coupling and transfer efficiency. It has been widely applied for single molecule detection for biosensing, and also finds applications in enhancing solar cell harvesting efficiency. However, achieving high amplification efficiency from plasmonic nanoantenna requires very precise control of positioning of functional acceptor in the hot spots. Thus, in case of single-molecule detection, the fluorophore should be placed between two metallic beads using sophisticated approaches.
We recently achieved a breakthrough in the field of single molecule detection with the demonstration of single-molecule detection under ambient light illumination using organic nanoparticles (ONPs) as giant light-harvesting nanoantennas. An extremely efficient excitation energy transfer is achieved with cationic rhodamine dyes encapsulated in a poly(methyl methacrylate) (PMMA) matrix using bulky fluorinated counterions to avoid p-p stacking and self-quenching. With 60 nm-sized biocompatible ONPs, we achieved a 1000-fold amplification of the effective brightness of a single energy acceptor (Cy5) located within the nanoparticle. Remarkably, this antenna effect is 3-fold higher than that of the best plasmonic nanoantennas. In parallel, we also observed that a large number of rhodamine dyes (>500), encapsulated in poly(D,L-lactide-co-glycolide) (PLGA) 30 nm ONPs, behaves as a single emitter displaying ON-OFF blinking with nearly 100% contrast, further supporting the presence of an extremely fast energy transport/delocalization over the entire nanoparticle. The blinking occurs because a single energy trap is able to efficiently quench the emission from the dye ensemble as it was already demonstrated for conjugated polymers. However, this quenching process limits the length, over which the energy can be transported and limits the use of these systems in organic photonics.
Here, we propose to design synthetic light-harvesting nanomaterials (LHN’s) made of chromophore-doped organic nanoparticles (0D) and nanowires (1D). We will apply these LHN’s to single molecule detection under ambient light excitation via Förster resonance energy transfer to single acceptor dyes. This application will also serve as a benchmark to demonstrate the performances of our LHN’s for light collection, transport and transfer to any kind of acceptor paving the route for numerous applications.