The origin of photoluminescence in carbon dots has baffled scientists since its discovery. We show that the photoluminescence spectra of carbon dots are inhomogeneously broadened due to the slower relaxation of the solvent molecules around it. This gives rise to excitation-dependent fluorescence that violates the Kasha-Vavilov rule. The time-resolved experiment shows significant energy redistribution, relaxation among the emitting states, and spectral migration of fluorescence spectra in the nanosecond time scale. The excitation-dependent multicolor emission in time-integrated spectra is typically governed by the relative population of these emitting states.

We show that hydrothermal treatment of citric acid produces methylenesuccinic acid, which gives rise to hydrogen-bonded nano-assemblies with CND-like properties.

Herein we unveil the presence of a molecular fluorophore quinoxalino[2,3-b]phenazine-2,3-diamine (QXPDA) in a colossal amount in red emissive CNDs synthesized from o-phenylenediamine, a well-known precursor molecule used for CND synthesis.

We present a method of reversible photoswitching in carbon nanodots with red emission. A mechanism of electron transfer is proposed. The cationic dark state, formed by the exposure of red light, is revived back to the bright state with the very short exposure of blue light. Additionally, the natural on-off state of carbon dot fluorescence was tuned using an electron acceptor molecule. Our observation can make the carbon dots as an excellent candidate for the super-resolution imaging of nanoscale biomolecules within the cell.

Carbon dots are newly discovered bright fluorescent biolabeling probes that nonspecifically bind to multiple cellular structures. Here we report yellow-orange emissive carbon dots that spontaneously localize inside the nucleolus of HeLa cells, specifically binding to the RNA. Single-particle measurements of carbon dots show fluorescence-intensity fluctuations with superior brightness and photostability. These optical properties were used for performing blinking-assisted localization microscopy that shows organization of the nucleolar RNA with improved resolution. Our study opens up the opportunity for single-molecule imaging and super-resolution microscopy applications using fluorescent carbon dots.

In a biological environment, the formation of a protein layer (protein corona) around nanoparticles immensely hampers its targeting capabilities and efficiency of specific delivery. Rational design of a nanoparticle remains one of the biggest challenges due to the lack of in-depth knowledge of the molecular mechanism of this corona formation on the different nanoparticle surfaces. Using computer simulation and experimental study, here, we establish for the first time the role of different surface properties like charge, chain length, architecture, and surface density of different surfactant molecules in the formation of the protein corona. We provide insights into the nanoparticle protein interaction, especially with respect to the structural orientation of a particular protein (human serum albumin) around a gold nanoparticle. We also derived the theoretical optimal conditions to avoid this corona formation. Such an efficient approach can pave the way to engineer smarter nanoparticles, which will avoid the protein absorption to keep their original properties unchanged. SECTION: Physical Processes in Nanomaterials and Nanostructures

Nitrogen-doped, PEGylated carbon dots (C-dots) have been synthesized for the detection of mercury ions (Hg(2+)). The detection limit was found to be 6.8 nM. However, upon functionalization with dithiothreitol (DTT), it reached to as low as 18 pM. The C-dots-Hg(2+) system was also able to efficiently detect biothiols.

The spontaneous protein adsorption on nanomaterial surfaces and the formation of a protein corona around nanoparticles are poorly understood physical phenomena, with high biological relevance. The complexity arises mainly due to the poor knowledge of the structural orientation of the adsorbed proteins onto the nanoparticle surface and difficulties in correlating the protein nanoparticle interaction to the protein corona in real time scale. Here, we provide quantitative insights into the kinetics, number, and binding orientation of a few common blood proteins when they interact with citrate and cetyltriethylammoniumbromide stabilized spherical gold nanoparticles with variable sizes. The kinetics of the protein adsorption was studied experimentally by monitoring the change in hydrodynamic diameter and zeta potential of the nanoparticle-protein complex. To understand the competitive binding of human serum albumin and hemoglobin, time dependent fluorescence quenching was studied using dual fluorophore tags. We have performed molecular docking of three different proteins--human serum albumin, bovine serum albumin, and hemoglobin--on different nanoparticle surfaces to elucidate the possible structural orientation of the adsorbed protein. Our data show that the growth kinetics of a protein corona is exclusively dependent on both protein structure and surface chemistry of the nanoparticles. The study quantitatively suggests that a general physical law of protein adsorption is unlikely to exist as the interaction is unique and specific for a given pair.