Xiaoyun He

Adsorption and desorption of methylene blue on porous carbon monoliths and nanocrystalline cellulose

The dynamic batch adsorption of methylene blue (MB), a widely used and toxic dye, onto nanocrystalline cellulose (NCC) and crushed powder of carbon monolith (CM) was investigated using the pseudo-first- and -second-order kinetics. CM outperformed NCC with a maximum capacity of 127 mg/g compared to 101 mg/g for NCC. The Langmuir isotherm model was applicable for describing the binding data for MB on CM and NCC, indicating the homogeneous surface of these two materials. The Gibbs free energy of -15.22 kJ/mol estimated for CM unravelled the spontaneous nature of this adsorbent for MB, appreciably faster than the use of NCC (-4.47 kJ/mol). Both pH and temperature exhibited only a modest effect on the adsorption of MB onto CM. The desorption of MB from CM using acetonitrile was very effective with more than 94 % of MB desorbed from CM within 10 min to allow the reusability of this porous carbon material. In contrast, acetonitrile was less effective than ethanol in desorbing MB from NCC. The two solvents were incapable of completely desorbing MB on commercial granular coal-derived activated carbon.

Porous Graphitized Carbon Monolith as an Electrode Material for Probing Direct Bioelectrochemistry and Selective Detection of Hydrogen Peroxide

For the first time, graphitized carbon particles with a high surface area have been prepared and evaluated as a new material for probing direct electrochemistry of hemoglobin (Hb). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging revealed that the carbon monolithic skeleton was constructed by a series of mesopores with irregular shapes and an average pore diameter of ~5.6 nm. With a surface area of 239.6 m(2)/g, carbon particles exhibited three major Raman peaks as commonly observed for carbon nanotubes and other carbon materials, i.e., the sp(3) and sp(2) carbon phases coexisted in the sample. A glassy carbon electrode modified with carbon monoliths and didodecyldimethylammonium bromide exhibited direct electron transfer between Hb molecules and the underlying electrode with a transfer rate constant of 6.87 s(-1). The enzyme electrode displayed a pair of quasi-reversible reduction-oxidation peaks at -0.128 and -0.180 V, reflecting the well-known feature of the heme [Fe(3+)/Fe(2+)] redox couple: a surface-controlled electrochemical process with one electron transfer. This reagentless biosensing approach was capable of detecting H(2)O(2), a simple molecule but plays an important role in analytical and biological chemistry, as low as 0.1 μM with linearity of 0.1-60 μM and a response time of <0.8 s, comparing favorably with other carbon based electrodes (5 s).

Fabrication and Characterization of Nanotemplated Carbon Monolithic Material

A novel hierarchical nanotemplated carbon monolithic rod (NTCM) was prepared using a novel facile nanotemplating approach. The NTCM was obtained using C60-fullerene modified silica gels as hard templates, which were embedded in a phenolic resin containing a metal catalyst for localized graphitization, followed by bulk carbonization, and template and catalyst removal. TEM, SEM, and BET measurements revealed that NTCM possessed an integrated open hierarchical porous structure, with a trimodal pore distribution. This porous material also possessed a high mesopore volume and narrow mesopore size distribution. During the course of carbonization, the C60 conjugated to aminated silica was partly decomposed, leading to the formation of micropores. The Raman signature of NTCM was very similar to that of multiwalled carbon nanotubes as exemplified by three major peaks as commonly observed for other carbon materials, i.e., the sp3 and sp2 carbon phases coexisted in the sample. Surface area measurements were obtained using both nitrogen adsorption/desorption isotherms (BET) and with a methylene blue binding assay, with BET results showing the NTCM material possessed an average specific surface area of 435 m2 g(-1), compared to an area of 372 m2 g(-1) obtained using the methylene blue assay. Electrochemical studies using NTCM modified glassy carbon or boron doped diamond (BDD) electrodes displayed quasi-reversible oxidation/reduction with ferricyanide. In addition, the BDD electrode modified with NTCM was able to detect hydrogen peroxide with a detection limit of below 300 nM, whereas the pristine BDD electrode was not responsive to this target compound.

Thermally controlled growth of carbon onions within porous graphitic carbon-detonation nanodiamond monolithic composites

Unique porous carbon monoliths containing thermally annealed carbon onions, were prepared from a resorcinol formaldehyde precursor rod, containing silica gel acting as a hard template, detonation nanodiamond, and Fe3+ as a graphitisation catalyst. Detonation nanodiamond was converted to carbon onions during controlled pyrolysis under N2, where the temperature cycle reached a maximum of 1250 °C. Thermal characterisation and high resolution electron microscopy have confirmed the graphitisation of nanodiamond, and revealed the resulting quasi-spherical carbon onions with an average particle size of 5.24 nm. The bimodal porous composite contains both macropores (5 μm) and mesopores (10 nm), with a BET surface area of 214 m2 g−1 for a nanodiamond prepared monolith (0.012 wt% nanodiamond in the precursor mixture), approximately twice that of blank monoliths, formed without the addition of nanodiamond, thus providing a new approach to increase surface area of such porous carbon rods. Raman spectroscopy and X-ray photoelectron spectroscopy also confirmed an enhanced graphitisation of the monolithic carbon skeleton resulting from the elevated thermal conductivity of the added nanodiamond. TEM imaging has confirmed the nanodiamond remains intact following pyrolysis at temperatures up to 900 °C.

New strategies for stationary phase integration within centrifugal microfluidic platforms for applications in sample preparation and pre-concentration

New approaches for fabrication of centrifugal microfluidic platforms (μCDs) for sample micro-extraction and pre-concentration in bioanalytical and environmental applications are presented. The integration of both octadecylsilica (C18) micro-particulate and porous carbon monolithic stationary phases was demonstrated and on-disc extractions of analytes in samples of different nature were performed. A novel strategy based on the packing of micro-particulate stationary phases using porous organic polymer monoliths as column frits was demonstrated through the in situ photo-polymerisation of monolithic frits in a specific area of the micro-channel, thereby greatly facilitating stationary phase packing within μCD platforms. An enrichment factor of 3.7 was obtained for vitamin B12 following on-disc pre-concentration on the octadecylsilica columns. UV-Vis absorbance measurements were also performed in the outlet reservoir permitting quasi-on-line analysis of the small volume fractions collected after extraction, with limits of detection (LODs) found for vitamin B12 (LOD = 43 μM) being rather similar to those found with a commercially available spectrophotometer (LOD = 37 μM). Furthermore, the first integration of carbon monoliths within microfluidic channels is reported. Carbon monoliths were fabricated as rods and cut into discs for their integration within the microfluidic network, offering a highly porous bimodal structure with low flow-through back-pressures, excellent chemical stability as well as adequate mechanical stability. The carbon monolith-based μCD platform was evaluated as a rapid semi-automated pre-concentration approach suitable for in-field use prior to in-lab HPLC quantitation of pollutants at low concentration levels. Calculated mean recoveries for phenol from tap water spiked-samples by using this on-disc pre-concentration method were 68 ± 4% (n = 4, RSD = 5%).