Investigation of plant leaf-derived graphene quantum dot clusters via magnetic force microscopy.

Magnetic force microscopy (MFM) is utilized to characterize the magnetic moment in nanostructured plant leaf-derived graphene quantum dot clusters (GQDCs). The MFM signal reveals that the magnetic response of the GQDCs depends on the height and width of the GQDCs. However, individual GQDs, and smaller clusters with widths of less than 20 nm, have not shown any observable magnetic signal. Importantly, experimental analyses suggest that the magnetic signal of GQDCs distributed in a plane can be effectively detected at room temperature. These results could pave the way for future graphene-based magnetic storage media and spin manipulation quantum devices.

Smart app-based on-field colorimetric quantification of mercury via analyte-induced enhancement of the photocatalytic activity of TiO2-Au nanospheres.

We have devised a unique strategy for highly sensitive, selective, and colorimetric detection of mercury based on analyte-induced enhancement of the photocatalytic activity of TiO2-Au nanospheres (TiO2-Au NSs) toward degradation of methylene blue (MB). Through electrostatic interactions, Au nanoparticles are attached to poly-(sodium 4-styreneulfonate)/poly(diallyldimethylammonium chloride) modified TiO2 nanoparticles, which then form an Au shell on each TiO2 core through reduction of Au3+ with ascorbic acid. Notably, the deposition of Hg species (Hg2+/CH3Hg+) onto TiO2-Au NSs through strong Au-Hg aurophilic interactions speeds up catalytic degradation of MB. The first-order degradation rates of MB by TiO2-Au NSs and TiO2-Au-Hg NSs are 1.4 × 10-2 min-1 and 2.1 × 10-2 min-1, respectively. Using a commercial absorption spectrometer, the TiO2-Au NSs/MB approach provides linearity (R2 = 0.98) for Hg2+ over a concentration range of 10.0 to 100.0 nM, with a limit of detection (LOD) of 1.5 nM. On the other hand, using a low-cost smartphone app that records the color changes (ΔRGB) of MB solution based on the red-blue-green (RGB) component values, the TiO2-Au NSs/MB approach provides an LOD of 2.0 nM for Hg2+ and 5.0 nM for CH3Hg+, respectively. Furthermore, the smartphone app sensing system has been validated for the analyses of various samples, including tap water, lake water, soil, and Dorm II, showing its great potential for on-line analysis of environmental and biological samples. Graphical Abstract ᅟ.

Dirac point induced ultralow-threshold laser and giant optoelectronic quantum oscillations in graphene-based heterojunctions.

The occurrence of zero effective mass of electrons at the vicinity of the Dirac point is expected to create new paradigms for scientific research and technological applications, but the related discoveries are rather limited. Here, we demonstrate that a simple architecture composed of graphene quantum dots sandwiched by graphene layers can exhibit several intriguing features, including the Dirac point induced ultralow-threshold laser, giant peak-to-valley ratio (PVR) with ultra-narrow spectra of negative differential resistance and quantum oscillations of current as well as light emission intensity. In particular, the threshold of only 12.4 nA cm-2 is the lowest value ever reported on electrically driven lasers, and the PVR value of more than 100 also sets the highest record compared with all available reports on graphene-based devices. We show that all these intriguing phenomena can be interpreted based on the unique band structures of graphene quantum dots and graphene as well as resonant quantum tunneling.In graphene, electrons possess zero effective mass in proximity to the Dirac point, an unusual feature that could trigger the development of novel photonic devices. Here, the authors combine graphene quantum dots with two graphene layers and observe laser action with ultralow threshold.

Polymer/reduced graphene oxide functionalized sponges as superabsorbents for oil removal and recovery.

Polyurethane dish-washing (PU-DW) sponges are functionalized sequentially with polyethylenimine (PEI) and graphene oxide (GO) to form PEI/reduced graphene oxide (RGO) PU-DW sponges. The PEI/RGO PU-DW sponge consists of PEI/RGO sheets having numerous pores, with diameters ranging from 236 to 254nm. To further enhance hydrophobicity and absorption capacity of oil, PEI/RGO PU-DW sponge is further coated with 20% phenyltrimethoxysilane (PTMOS). The PTMOS/PEI/RGO PU-DW sponge absorbs various oils within 20s, with maximum absorption capacity values of 880% and 840% for bicycle chain oil and motorcycle engine oil, respectively. The absorbed oils were released completely by squeezing or immersed in hexane. The PTMOS/PEI/RGO PU-DW sponge efficiently separates oil/water mixtures through a flowing system. Having the advantages of faster absorption rate, reusability, and low cost, the PTMOS/PEI/RGO PU-DW sponge holds great potential as a superabsorbent for efficient removal and recovery of oil spills as well as for the separation of oil/water mixtures.

Synthesis, Optical Properties, and Sensing Applications of Gold Nanodots.

In this Personal Account, we briefly address our journey in developing photoluminescent nanomaterials for sensing purposes, with a focus on gold nanodots (Au NDs). Their synthetic strategies, optical properties, and sensing applications are emphasized. The Au NDs can be simply prepared from the etching of small-sized Au nanoparticles (<3 nm in diameter) by thiol compounds such as 11-mercaptoundecanoic acid under alkaline conditions. This simple approach allows the preparation of various functional Au NDs by choosing different thiol compounds as etching agents. Since the optical properties of Au NDs are highly dependent on the core and shell of each Au ND, the selection of etching reagents is important. Over the years we have developed various sensing systems using Au NDs for the detection of metal ions, anions, and proteins, based on analyte-induced photoluminescence quenching/enhancement of Au NDs as a result of changes in their oxidation state, shell composition, and structure.

Photoluminescent graphene quantum dots for in vivo imaging of apoptotic cells.

Apoptosis (programmed cell death) is linked to many incurable neurodegenerative, cardiovascular and cancer causing diseases. Numerous methods have been developed for imaging apoptotic cells in vitro; however, there are few methods available for imaging apoptotic cells in live animals (in vivo). Here we report a novel method utilizing the unique photoluminescence properties of plant leaf-derived graphene quantum dots (GQDs) modified with annexin V antibody (AbA5) to form (AbA5)-modified GQDs (AbA5-GQDs) enabling us to label apoptotic cells in live zebrafish (Danio rerio). The key is that zebrafish shows bright red photoluminescence in the presence of apoptotic cells. The toxicity of the GQDs has also been investigated with the GQDs exhibiting high biocompatibility as they were excreted from the zebrafish's body without affecting its growth significantly at a concentration lower than 2 mg mL(-1) over a period of 4 to 72 hour post fertilization. The GQDs have further been used to image human breast adenocarcinoma cell line (MCF-7 cells), human cervical cancer cell line (HeLa cells), and normal human mammary epithelial cell line (MCF-10A). These results are indispensable to further the advance of graphene-based nanomaterials for biomedical applications.

Synthesis and antimicrobial activity of gold/silver-tellurium nanostructures.

Gold-tellurium nanostructures (Au-Te NSs), silver-tellurium nanostructures (Ag-Te NSs), and gold/silver-tellurium nanostructures (Au/Ag-Te NSs) have been prepared through galvanic reactions of tellurium nanotubes (Te NTs) with Au(3+), Ag(+), and both ions, respectively. Unlike the use of less environmentally friendly hydrazine, fructose as a reducing agent has been used to prepare Te NTs from TeO2 powders under alkaline conditions. The Au/Ag-Te NSs have highly catlaytic activity to convert nonfluorescent Amplex Red to form fluorescent product, revealing their great strength of generating reactive oxygen species (ROS). Au/Ag-Te NSs relative to the other two NSs exhibit greater antimicrobial activity toward the growth of E. coli, S. enteritidis, and S. aureus; the minimal inhibitory concentration (MIC) values of Au/Ag-Te NSs were much lower (>10-fold) than that of Ag-Te NSs and Au-Te NSs. The antibacterial activity of Au/Ag-Te NSs is mainly due to the release of Ag(+) ions and Te-related ions and also may be due to the generated ROS which destroys the bacteria membrane. In vitro cytotoxicity and hemolysis analyses have revealed their low toxicity in selected human cell lines and insignificant hemolysis in red blood cells. In addition, inhibition zone measurements using a Au/Ag-Te NSs-loaded konjac jelly film have suggested that it has great potential in practial application such as wound dressing for reducing bacterial wound infection. Having great antibacterial activitiy and excellent biocompatibility, the low-cost Au/Ag-Te NSs hold great potential as effective antimicrobial drugs.

Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene.

A simple hydrothermal method of preparing highly photocatalytic graphene-ZnO-Au nanocomposites (G-ZnO-Au NCs) has been developed. Zinc acetate and graphene oxide are reduced by catechin to form graphene-zinc oxide nanospheres (G-ZnO NSs; average diameter of (45.3 ± 3.7) nm) in the presence of ethylenediamine (EDA) as a stabilizing agent and gold nanorods (Au NRs) at 300 °C for 2 h. Then Au NRs are deposited onto as-formed G-ZnO NSs to form G-ZnO-Au NCs. Upon ultraviolet light activation, G-ZnO-Au NCs (4 mg mL(-1)) in methanol generates electron-hole pairs. Methanol (hydroxyl group) assists in trapping holes, enabling photogenerated electrons to catalyze reduction of nitrobenzene (NB) to aniline with a yield of 97.8% during a reaction course of 140 min. The efficiency of G-ZnO-Au NCs is 3.5- and 4.5-fold higher than those provided by commercial TiO2 and ZnO NSs, respectively. Surface assisted laser desorption/ionization mass spectrometry has been for the first time applied to detect the intermediates (nitrosobenzene and phenylhydroxylamine) and major product (aniline) of NB through photoelectrocatalytic or photocatalytic reactions. The result reveals that the reduction of NB to aniline is through nitrosobenzene to phenylhydroxylamine in the photoelectrocatalytic reaction, while via nitrosobenzene directly in the photocatalytic reaction. G-ZnO-Au NC photocatalyst holds great potential in removal of organic pollutants like NB and in the production of aniline.

Detection of mercury ions using silver telluride nanoparticles as a substrate and recognition element through surface-enhanced Raman scattering.

In this paper we unveil a new sensing strategy for sensitive and selective detection of Hg(2+) through surface-enhanced Raman scattering (SERS) using Ag2Te nanoparticles (NPs) as a substrate and recognition element and rhodamine 6G (R6G) as a reporter. Ag2Te NPs prepared from tellurium dioxide and silver nitrate and hydrazine in aqueous solution containing sodium dodecyl sulfate at 90°C with an average size of 26.8 ± 4.1 nm (100 counts) have strong SERS activity. The Ag2Te substrate provides strong SERS signals of R6G with an enhancement factor of 3.6 × 10(5) at 1360 cm(-1), which is comparable to Ag NPs. After interaction of Ag2Te NPs with Hg(2+), some HgTe NPs are formed, leading to decreases in the SERS signal of R6G, mainly because HgTe NPs relative to Ag2Te NPs have weaker SERS activity. Under optimum conditions, this SERS approach using Ag2Te as substrates is selective for the detection of Hg(2+), with a limit of detection of 3 nM and linearity over 10-150 nM. The practicality of this approach has been validated for the determination of the concentrations of spiked Hg(2+) in a pond water sample.

Iron telluride nanorods-based system for the detection of total mercury in blood.

We have developed a simple, colorimetric iron telluride (FeTe) nanorods (NRs) based system for the detection of mercury, mainly based on the cation exchange reaction between FeTe NRs and Hg(2+). FeTe NRs (length, 105 ± 21 nm) react with Hg(2+) to form HgTe NRs (length, 112 ± 26 nm) and consequently release Fe(2+) ions that catalyzes the oxidation between a peroxidase substrate 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS) and H(2)O(2). The concentration of Fe(2+) and thereby Hg(2+) can be determined by measuring the absorbance of the ABTS oxidized product at 418 nm. This approach allows the detection of Hg(2+), with a limit of detection of 1.31 nM at a signal-to-noise ratio 3 and a linear range 5-100 nM (R(2)=0.99). The low-cost, simple, sensitive, and reproducible assay has been validated for the detection of Hg(2+) in a blood sample (SRM 955c), with the result being in good agreement with that provided by National Institute of Standards and Technology.

Preparation of photocatalytic Au-Ag2Te nanomaterials.

A facile approach has been developed for the preparation of various morphologies of Au-Ag(2)Te nanomaterials (NMs) that exhibit strong photocatalytic activity. Te NMs (nanowires, nanopencils, and nanorice) were prepared from TeO(2) in the presence of various concentrations (16, 8, and 4 M) of a reducing agent (N(2)H(4)) at different temperatures (25 and 60 °C). These three Te NMs were then used to prepare Au-Ag(2)Te NMs by spontaneous redox reactions with Au(3+) and Ag(+) ions sequentially. The Au-Ag(2)Te nanopencils exhibit the highest activity toward degradation of methylene blue and formation of active hydroxyl radicals on solar irradiation, mainly because they absorb light in the visible region most strongly. All three differently shaped Au-Ag(2)Te NMs (10 µg mL(-1)) provide a death rate of Escherichia coli greater than 80% within 60 min, which is higher than that of 51% for commercial TiO(2) nanoparticles (100 µg mL(-1)). Under light irradiation, the Au NPs in Au-Ag(2)Te NMs enhance the overall photo-oxidation ability of Ag(2)Te NMs through faster charge separation because of good contact between Ag(2)Te and Au segments. With high antibacterial activity and low toxicity toward normal cells, the Au-Ag(2)Te NMs hold great potential for use as efficient antibacterial agents.

Synthesis of enzyme mimics of iron telluride nanorods for the detection of glucose.

Iron telluride nanorods (FeTe NRs, length 45 ± 11 nm) prepared from tellurium nanowires (Te NWs, length 785 ± 170) have been used to detect H(2)O(2) and glucose with a limit of detection (LOD) of 55 nM (linear range = 0.1-5 µM) and 0.38 µM (linear range = 1-100 µM) respectively.

Visual detection of copper(II) ions in blood samples by controlling the leaching of protein-capped gold nanoparticles.

We have developed a simple, low-cost, paper-based probe for the selective colorimetric detection of copper ions (Cu(2+)) in aqueous solutions. The bovine serum albumin (BSA)-modified 13.3-nm Au nanoparticle (BSA-Au NP) probe was designed to detect Cu(2+) ions using lead ions (Pb(2+)) and 2-mercaptoethanol (2-ME) as leaching agents in a glycine-NaOH (pH 12.0) solution. In addition, a nitrocellulose membrane (NCM) was used to trap the BSA-Au NPs, leading to the preparation of a nanocomposite film consisting of a BSA-Au NP-decorated membrane (BSA-Au NPs/NCM). The BSA-Au NPs probe operates on the principle that Cu deposition on the surface of the BSA-Au NPs inhibits their leaching ability, which is accelerated by Pb(2+) ions in the presence of 2-ME. Under optimal solution conditions (5 mM glycine-NaOH (pH 12.0), Pb(2+) (50 µM), and 2-ME (1.0 M)), the Pb(2+)/2-ME-BSA-Au NPs/NCM enabled the detection of Cu(2+) at nanomolar concentrations in aqueous solutions by the naked eye with high selectivity (at least 100-fold over other metal ions). In addition, this cost-effective probe allowed for the rapid and simple determination of Cu(2+) ions in not only natural water samples but also in a complex biological sample (in this case, blood sample).

Quantum dot-sensitized solar cells incorporating nanomaterials.

Quantum dot-sensitized solar cells (QDSSCs) are interesting energy devices because of their (i) impressive ability to harvest sunlight and generate multiple electron/hole pairs, (ii) ease of fabrication, and (iii) low cost. The power conversion efficiencies (η) of most QDSSCs (typically <4%) are, however, less than those (up to 12%) of dye-sensitized solar cells, mainly because of narrow absorption ranges and charge recombination occurring at the QD-electrolyte and TiO(2)-electrolyte interfaces. To further increase the values of η of QDSSCs, it will be necessary to develop new types of working electrodes, sensitizers, counter electrodes and electrolytes. This Feature Article describes the nanomaterials that have been used recently as electronic conductors, sensitizers and counter electrodes in QDSSCs. The nature, size, morphology and quantity of these nanomaterials all play important roles affecting the efficiencies of electron injection and light harvesting. We discuss the behavior of several important types of semiconductor nanomaterials (sensitizers, including CdS, Ag(2)S, CdSe, CdTe, CdHgTe, InAs and PbS) and nanomaterials (notably TiO(2), ZnO and carbon-based species) that have been developed to improve the electron transport efficiency of QDSSCs. We point out the preparation of new generations of nanomaterials for QDSSCs and the types of electrolytes, particularly iodide/triiodide electrolytes (I(-)/I(3)(-)), polysulfide electrolytes (S(2-)/S(x)(2-)), and cobalt redox couples ([Co(o-phen)(3)(2+)/(3+)]), that improve their lifetimes. With advances in nanotechnology, we foresee significant improvements in the efficiency (η > 6%) and durability (>3000 h) of QDSSCs.