Hydrothermal synthesis of carbon quantum dots with size tunability via heterogeneous nucleation.

Hydrothermal synthesis has been extensively utilized for fabricating carbon quantum dots (CQDs). Generally, the average sizes of the CQDs are controlled by using specific precursor concentrations, processing temperatures, and reaction times. In our study, the average size of CQDs can simply be controlled by using a different filling volume of sucrose solution in the hydrothermal reactor while keeping the other experimental parameters constant. If homogeneous nucleation plays a major role in the hydrothermal synthesis, the CQDs synthesized by using different filling volumes should have relatively the same size. Nonetheless, we found that the average size of CQDs is inversely correlated with the filling volumes. Particularly, for the hydrothermal syntheses with the filling volumes of 20%, 50%, and 80%, the average size of the CQDs is 15, 13, and 4 nm, respectively. Therefore, the hydrothermal synthesis of CQDs with size-tunability can be achieved by the heterogeneous process associated with the total surface areas between the precursor and reactor.

Reactive argon-plasma activation of screen-printed carbon electrodes for highly selective dopamine determination.

Dopamine (DA) deficiency has been linked to several psychiatric disorders. Electrochemical determination of the level of DA suffers from abundant ascorbic acid (AA) and uric acid (UA) in body fluids. In this work, a facile argon (Ar) plasma treatment was utilized to enhance the electrocatalytic reactivity of screen-printed carbon electrodes (SPCEs) for selective DA detection. Surface characterization of the Ar-treated SCPEs verified that the carbon paste binders were successfully removed and single-bonded oxygenated moieties (-OH and C-O-C) were generated. Interestingly, the sharper D* and D'' Raman interbands were new key evidence of a higher exposure of carbon defect sites. Electrochemical studies further revealed that the Ar-treated SPCEs possessed faster heterogeneous electron-transfer rates, larger electroactive surface areas, and much higher conductivity when compared with untreated electrodes. As a result, the oxidation potentials of AA, DA, and UA in the mixture could be well-resolved and the current responses were significantly increased. The selective determination of DA in the presence of AA and UA by differential pulse voltammetry gave two linear responses with the limit of detection of 0.27 µM (0.15-10 µM range). Moreover, this Ar-treated SPCE had high reproducibility and good storage stability. These results suggest that Ar-plasma treatment could be a promising method to enhance the electrocatalytic properties of SPCEs for the detection of biomolecules.

Molecularly Imprinted Polymer-Amyloid Fibril-Based Electrochemical Biosensor for Ultrasensitive Detection of Tryptophan.

A tryptophan (Trp) sensor was investigated based on electrochemical impedance spectroscopy (EIS) of a molecularly imprinted polymer on a lysozyme amyloid fibril (MIP-AF). The MIP-AF was composed of aniline as a monomer chemically polymerized in the presence of a Trp template molecule onto the AF surface. After extracting the template molecule, the MIP-AF had cavities with a high affinity for the Trp molecules. The obtained MIP-AF demonstrated rapid Trp adsorption and substantial binding capacity (50 µM mg-1). Trp determination was studied using non-Faradaic EIS by drop drying the MIP-AF on the working electrode of a screen-printed electrode. The MIP-AF provided a large linear range (10 pM-80 µM), a low detection limit (8 pM), and high selectivity for Trp determination. Furthermore, the proposed method also indicates that the MIP-AF can be used to determine Trp in real samples such as milk and cancer cell media.

Molecular dynamics study on the effects of charged amino acid distribution under low pH condition to the unfolding of hen egg white lysozyme and formation of beta strands.

Aggregation of unfolded or misfolded proteins into amyloid fibrils can cause various diseases in humans. However, the fibrils synthesized in vitro can be developed toward useful biomaterials under some physicochemical conditions. In this study, atomistic molecular dynamics simulations were performed to address the mechanism of beta-sheet formation of the unfolded hen egg-white lysozyme (HEWL) under a high temperature and low pH. Simulations of the protonated HEWL at pH 2 and the non-protonated HEWL at pH 7 were performed at the highly elevated temperature of 450 K to accelerate the unfolding, followed by the 333 K temperature to emulate some previous in vitro studies. The simulations showed that HEWL unfolded faster, and higher beta-strand contents were observed at pH 2. In addition, one of the simulation replicas at pH 2 showed that the beta-strand forming sequence was consistent with the 'K-peptide', proposed as the core region for amyloidosis in previous experimental studies. Beta-strand formation mechanisms at the earlier stage of amyloidosis were explained in terms of the radial distribution of the amino acids. The separation between groups of positively charged sidechains from the hydrophobic core corresponded to the clustering of the hydrophobic residues and beta-strand formation.

Molecular Mechanisms on the Selectivity Enhancement of Ascorbic Acid, Dopamine, and Uric Acid by Serine Oligomers Decoration on Graphene Oxide: A Molecular Dynamics Study.

The selectivity in the simultaneous detection of ascorbic acid (AA), dopamine (DA), and uric acid (UA) has been an open problem in the biosensing field. Many surface modification methods were carried out for glassy carbon electrodes (GCE), including the use of graphene oxide and amino acids as a selective layer. In this work, molecular dynamics (MD) simulations were performed to investigate the role of serine oligomers on the selectivity of the AA, DA, and UA analytes. Our models consisted of a graphene oxide (GO) sheet under a solvent environment. Serine tetramers were added into the simulation box and were adsorbed on the GO surface. Then, the adsorption of each analyte on the mixed surface was monitored from MD trajectories. It was found that the adsorption of AA was preferred by serine oligomers due to the largest number of hydrogen-bond forming functional groups of AA, causing a 10-fold increase of hydrogen bonds by the tetraserine adsorption layer. UA was the least preferred due to its highest aromaticity. Finally, the role of hydrogen bonds on the electron transfer selectivity of biosensors was discussed with some previous studies. AA radicals received electrons from serine through hydrogen bonds that promoted oxidation reaction and caused the negative shifts and separation of the oxidation potential in experiments, as DA and UA were less affected by serine. Agreement of the in vitro and in silico results could lead to other in silico designs of selective layers to detect other types of analyte molecules.

Blue photoluminescent carbon nanodots from limeade.

Carbon-based photoluminescent nanodot has currently been one of the promising materials for various applications. The remaining challenges are the carbon sources and the simple synthetic processes that enhance the quantum yield, photostability and biocompatibility of the nanodots. In this work, the synthesis of blue photoluminescent carbon nanodots from limeade via a single-step hydrothermal carbonization process is presented. Lime carbon nanodot (L-CnD), whose the quantum yield exceeding 50% for the 490nm emission in gram-scale amounts, has the structure of graphene core functionalized with the oxygen functional groups. The micron-sized flake of the as-prepared L-CnD powder exhibits multicolor emission depending on an excitation wavelength. The L-CnDs are demonstrated for rapidly ferric-ion (Fe(3+)) detection in water compared to Fe(2+), Cu(2+), Co(2+), Zn(2+), Mn(2+) and Ni(2+) ions. The photoluminescence quenching of L-CnD solution under UV light is used to distinguish the Fe(3+) ions from others by naked eyes as low concentration as 100µM. Additionally, L-CnDs provide exceptional photostability and biocompatibility for imaging yeast cell morphology. Changes in morphology of living yeast cells, i.e. cell shape variation, and budding, can be observed in a minute-period until more than an hour without the photoluminescent intensity loss.

Surface porosity and roughness of micrographite film for nucleation of hydroxyapatite.

Heterogeneous nucleation of hydroxyapatite (HAp) can be facilitated by physical and chemical properties of material surface. In this article, we reported how effective surface porosity and roughness are for inducing nucleation of HAp crystal in simulated body fluid. Two types of micrographite film (MGF) prepared from assembly of micrographite flakes were used as seeds to induce HAp crystal: uncompressed (high surface porosity) and compressed (low surface porosity) MGFs. Compressed MGF was prepared by applying mechanical compression to the uncompressed MGF. Uncompressed and compressed MGFs have similar surface wettability with the water contact angles (θ) of 113° and 107°, respectively. The number density of HAp crystals on the uncompressed MGF was higher than that of the compressed MGF by a factor of 6. This result implied that surface porosity and roughness were more effective parameters for inducing HAp crystal than surface wettability. Uncompressed MGF also induced HAp nucleation better than a cover glass although the glass had high wettability (θ = 64°). The effectiveness of uncompressed MGF on inducing HAp crystals was as high as that of the SiO2 -coated Si substrate. Our finding suggests that we do not require to functionalize material surface to be an effective seed; a surface with pores or roughness of the right scale is enough.

Aligned, isotropic and patterned carbon nanotube substrates that control the growth and alignment of Chinese hamster ovary cells.

Here we culture Chinese hamster ovary cells on isotropic, aligned and patterned substrates based on multiwall carbon nanotubes. The nanotubes provide the substrate with nanoscale topography. The cells adhere to and grow on all substrates, and on the aligned substrate, the cells align strongly with the axis of the bundles of the multiwall nanotubes. This control over cell alignment is required for tissue engineering; almost all tissues consist of oriented cells. The aligned substrates are made using straightforward physical chemistry techniques from forests of multiwall nanotubes; no lithography is required to make inexpensive large-scale substrates with highly aligned nanoscale grooves. Interestingly, although the cells strongly align with the nanoscale grooves, only a few also elongate along this axis: alignment of the cells does not require a pronounced change in morphology of the cell. We also pattern the nanotube bundles over length scales comparable to the cell size and show that the cells follow this pattern.

Colloid-Assisted Self-Assembly of Robust, Three-Dimensional Networks of Carbon Nanotubes over Large Areas.

Natural materials, such as bone and spider silk, possess remarkable properties as a result of sophisticated nanoscale structuring. They have inspired the design of synthetic materials whose structure at the nanoscale is carefully engineered or where nanoparticles, such as rods or wires, are self-assembled. Although much work has been done in recent years to create ordered structures using diblock copolymers and template-assisted assembly, no reports describe highly ordered, three-dimensional nanotube arrays within a polymeric material. There are only reports of two-dimensional network structures and structures on micrometer-size scales. Here, we describe an approach that uses plasticized colloidal particles as a template for the self-assembly of carbon nanotubes (CNTs) into ordered, three-dimensional networks. The nanocomposites can be strained by over 200% and still retain high conductivity when relaxed. The method is potentially general and so may find applications in areas such as sensing, photonics, and functional composites.

Carbon-nanotube-based materials for protein crystallization.

We report on the first use of carbon-nanotube-based films to produce crystals of proteins. The crystals nucleate on the surface of the film. The difficulty of crystallizing proteins is a major bottleneck in the determination of the structure and function of biological molecules. The crystallization of two model proteins and two medically relevant proteins was studied. Quantitative data on the crystallization times of the model protein lysozyme are also presented. Two types of nanotube films, one made with the surfactant Triton X-100 (TX-100) and one with gelatin, were tested. Both induce nucleation of the crystal phase at supersaturations at which the protein solution would otherwise remain clear; however, the gelatin-based film induced nucleation down to much lower supersaturations for the two model proteins with which it was used. It appears that the interactions of gelatin with the protein molecules are particularly favorable to nucleation. Crystals of the C1 domain of the human cardiac myosin-binding protein-C that diffracted to a resolution of 1.6 A were obtained on the TX-100 film. This is far superior to the best crystals obtained using standard techniques, which only diffracted to 3.0 A. Thus, both of our nanotube-based films are very promising candidates for future work on crystallizing difficult-to-crystallize target proteins.