No abstract

Although it is hypothesized that surface (like surface charge) and physical characteristics (like particle size) play important roles in cellular interactions of nanoparticles (NPs), a systematic study probing this issue is missing. Hence, a comparative cytotoxicity study quantifying nine different cellular endpoints, was performed with a broad series of monodisperse, well characterized silicon (Si) and germanium (Ge) NPs with various surface functionalizations. Human colonic adenocarcinoma Caco-2 and rat alveolar macrophage NR8383 cells were used, to clarify the toxicity of this series of NPs. The surface coatings on the NPs appeared to dominate the cytotoxicity: the cationic NPs exhibited cytotoxicity, whereas the carboxylic acid-terminated and hydrophilic PEG- or dextran-terminated NPs did not. Within the cationic Si NPs, smaller Si NPs were more toxic than bigger ones. Manganese-doped (1 % Mn) Si NPs did not show any added toxicity, which favors their further development for bioimaging. Iron-doped (1 % Fe) Si NPs showed some added toxicity, which may be due to the leaching of Fe3+ ions from the core. A silica coating seemed to impart toxicity, in line with the reported toxicity of silica. Intracellular mitochondria seem to be a target organ for the toxic NPs since a dose-, surface charge- and size-dependent imbalance of the mitochondrial membrane potential was observed. Such imbalance led to a series of other cellular events for cationic NPs, like decreased mitochondrial membrane potential (ΔΨm) and ATP production, induction of ROS generation, increased cytoplasmic Ca2+ content, production of TNF-α and enhanced caspase-3 activity. Taken together, the results explain the toxicity of Si NPs/Ge NPs largely by their surface characteristics, provide insight in the mode of action underlying the observed cytotoxicity, and give directions on synthesizing biocompatible Si and Ge NPs, as this is crucial for bioimaging and other applications in for example the field of medicine.

We present an investigation into the influence of nanocrystal size on the reactivity of silicon nanocrystals (Si-NCs) in near-UV photochemical hydrosilylation. The size-dependent reactivity of Si-NCs with photoluminescence (PL) in the visible and near-infrared regions was evaluated using PL and Fourier-transform infrared (FTIR) spectroscopy, and small-angle X-ray scattering (SAXS). Under near-UV excitation, Si-NCs with PL in the visible spectral region react faster than Si-NCs with near-IR PL, allowing partial separation of a mixture of Si-NC sizes through hydrosilylation. This is attributed to quantum size effects in the exciton-mediated mechanisms proposed for this reaction.

Stimuli-responsive photonic materials are useful in sensors, optical filters, inks, displays, and other technologies. [1, 2] In particular, photonic hydrogels can show large color changes in response to variations in osmotic pressure, and have a wide range of tunable functionality through selection of suitable hydrogel monomer(s). [3][4][5] Chiral nematic liquid crystals (LCs) can have unique photonic properties that could be valuable in the form of a hydrogel.[6] Although there are some reports of photonic hydrogels with chiral nematic structures prepared mostly through polymerization of functionalized molecular LCs, [7] a general approach to achieve the broad range of responsive functionalities found in conventional hydrogels (for example, temperature, pH, solvent) is still lacking. Herein we report novel nanocomposite hydrogels prepared by self-assembly of nanocrystalline cellulose (NCC) with various hydrogel monomers. These new chiral nematic hydrogels have tunable colors, respond to various stimuli, and have interesting mechanical and swelling behavior. We demonstrate that the hydrogels can be prepared as large, freestanding films and can be photopatterned. Furthermore, the embedded NCC offers new potential for modification to produce functional materials. Our findings will be important for developing sensors and other chiral optoelectronic devices.Materials with chiral nematic order show selective reflection of light with circular polarization. They currently have applications such as distributed feedback lasing and polarized photoluminescence.[6] To construct a photonic hydrogel with chiral nematic order, we turned to cellulose, a widely-available, inexpensive, and biocompatible material. Cellulose-based hydrogels are used commercially in wound dressings, superabsorbent materials, and other applications. [8] The surface of cellulose and its derivatives can be tailored to produce hydrogels that respond to external stimuli such as temperature or pH. [9] Nanocrystalline cellulose (NCC), prepared by acid hydrolysis of cellulose, [10][11][12] has been used to reinforce hydrogels to improve their typically poor mechanical properties. [13][14][15][16] NCC forms a chiral nematic LC phase in water, where the NCC rods are organized in a left-handed helical assembly. These lyotropic phases exhibit photonic color when the helical pitch of the assembly is on the order of the wavelength of visible light, selectively reflecting left-handed circularly polarized light. Tatsumi et al. recently created a hydrogel with liquid crystalline order using NCC, but it did not show any photonic properties. [15] A main criterion to successfully prepare photonic hydrogels using NCC was to find suitable conditions for polymerization of hydrogel precursors while retaining the formation of the chiral nematic phase of NCC, which is very sensitive to changes in pH and ionic strength. [17] In our experiments, we used sulfuric acid hydrolysis to prepare acidic dispersions of NCC. Mixing NCC (3 wt %, pH 2.4) with nonionic hydrogel precursors ...

We demonstrate that silicon nanocrystals (Si-NCs) exhibiting relatively high near-IR photoluminescent quantum yields also exhibit a notable photothermal (PT) response. The PT effect has been quantified as a function of NC size, defect concentration, and irradiating energy, suggesting that the origin of the PT response is a combination of carrier thermalization and defect-mediated heating. The PT effect observed under NIR irradiation suggests that Si-NCs could find use in combined in vivo PL imaging and PT therapy.