Visualizing hypochlorous acid production by human neutrophils with fluorescent graphene quantum dots.

In living organisms, redox reactions play a crucial role in the progression of disorders accompanied by the overproduction of reactive oxygen and reactive chlorine species, such as hydrogen peroxide and hypochlorous acid, respectively. We demonstrate that green fluorescence graphene quantum dots (GQDs) can be employed for revealing the presence of the hypochlorous acid in aqueous solutions and cellular systems. Hypochlorous acid modifies the oxygen-containing groups of the GQD, predominantly opens epoxide ring C-O-C, forms excessive C=O bonds and damages the carbonic core of GQDs. These changes, which depend on the concentration of the hypochlorous acid and exposure time, manifest themselves in the absorbance and fluorescence spectra of the GQD, and in the fluorescence lifetime. We also show that the GQD fluorescence is not affected by hydrogen peroxide. This finding makes GQDs a promising sensing agent for selective detecting reactive chlorine species produced by neutrophils. Neutrophils actively accumulate GQDs allowing to visualize cells and to examine the redox processes via GQDs fluorescence. At high concentrations GQDs induce neutrophil activation and myeloperoxidase release, leading to the disruption of GQD structure by the produced hypochlorous acid. This makes the GQDs a biodegradable material suitable for various biomedical applications.

Photostability and phototoxicity of graphene quantum dots interacting with red blood cells.

Here we discuss fluorescent properties of graphene quantum dots (GQDs) interacting with the membranes of red blood cells. We report the results of spectroscopic, microscopic, and photon-counting measurements of the GQDs in different surroundings for uncovering specific features of the GQD fluorescence, and describe two observed phenomena important for implementation of the GQDs as fluorescent labels and agents for drug delivery. Firstly, the GQDs can suffer from photodegradation but also can be stabilized in the presence of antioxidants (reduced glutathione, N-acetylcysteine, or 1,4-hydroquinone). Secondly, GQDs can accumulate in red blood cell membranes without compromising the viability of the cells but also can induce hemolysis in the presence of visible light. We discuss mechanisms and regimes of the photodegradation, stabilization, interaction of the GQDs with red blood cell membranes, and hemolysis. Notably, photohemolysis for the case is dependent on the light dose and GQD concentration but not caused by the production of reactive oxygen species.

Hysteresis and Stochastic Fluorescence by Aggregated Ensembles of Graphene Quantum Dots.

"Blinking" behavior of fluorophores, being harmful for the majority of super-resolved techniques, turns into a key property for stochastic optical fluctuation imaging and its modifications, allowing one to look at the fluorophores already used in conventional microscopy, such as graphene quantum dots, from a completely new perspective. Here we discuss fluorescence of aggregated ensembles of graphene quantum dots structured at submicron scale. We study temperature dependence and stochastic character of emission. We show that considered quantum dots ensembles demonstrate rather complicated temperature-dependent intermittent emission, that is, "blinking" with a tendency to shorten "blinking" times with the increase of temperature. We verify "blinking" mechanism demonstrating hysteresis of the optical response under pulsed excitation timed to expected rates of dots transition to "dark" nonemitting states. Experimental results are well fitted by a simple qualitative model of transitions to the "dark" states. The obtained results suggest that this type of standardized quantum dots and even their submicron-size agglomerations can be useful as controlled fluorophores for super-resolution microscopy and, particularly, for SOFI-like microscopy.

Integrated Source of Path-Entangled Photon Pairs with Efficient Pump Self-Rejection.

We present a theoretical proposal for an integrated four-wave mixing source of narrow-band path-entangled photon pairs with efficient spatial pump self-rejection. The scheme is based on correlated loss in a system of waveguides in Kerr nonlinear media. We calculate that this setup gives the possibility for upwards of 100 dB pump rejection, without additional filtering. The effect is reached by driving the symmetric collective mode that is strongly attenuated by an engineered dissipation, while photon pairs are born in the antisymmetric mode. A similar set-up can additionally be realized for the generation of two-photon NOON states, also with pump self-rejection. We discuss the implementation of the scheme by means of the coherent diffusive photonics, and demostrate its feasibility in both glass (such as fused silica-glass and IG2) and planar semiconductor waveguide structures in indium phosphide (InP) and in silicon.

Dissipatively coupled waveguide networks for coherent diffusive photonics.

A photonic circuit is generally described as a structure in which light propagates by unitary exchange and transfers reversibly between channels. In contrast, the term 'diffusive' is more akin to a chaotic propagation in scattering media, where light is driven out of coherence towards a thermal mixture. Based on the dynamics of open quantum systems, the combination of these two opposites can result in novel techniques for coherent light control. The crucial feature of these photonic structures is dissipative coupling between modes, via an interaction with a common reservoir. Here, we demonstrate experimentally that such systems can perform optical equalisation to smooth multimode light, or act as a distributor, guiding it into selected channels. Quantum thermodynamically, these systems can act as catalytic coherent reservoirs by performing perfect non-Landauer erasure. For lattice structures, localised stationary states can be supported in the continuum, similar to compacton-like states in conventional flat-band lattices.

Crystallizing highly-likely subspaces that contain an unknown quantum state of light.

In continuous-variable tomography, with finite data and limited computation resources, reconstruction of a quantum state of light is performed on a finite-dimensional subspace. In principle, the data themselves encode all information about the relevant subspace that physically contains the state. We provide a straightforward and numerically feasible procedure to uniquely determine the appropriate reconstruction subspace by extracting this information directly from the data for any given unknown quantum state of light and measurement scheme. This procedure makes use of the celebrated statistical principle of maximum likelihood, along with other validation tools, to grow an appropriate seed subspace into the optimal reconstruction subspace, much like the nucleation of a seed into a crystal. Apart from using the available measurement data, no other assumptions about the source or preconceived parametric model subspaces are invoked. This ensures that no spurious reconstruction artifacts are present in state reconstruction as a result of inappropriate choices of the reconstruction subspace. The procedure can be understood as the maximum-likelihood reconstruction for quantum subspaces, which is an analog to, and fully compatible with that for quantum states.