S. Mohajerzadeh

Silver nanoparticles within vertically aligned multi-wall carbon nanotubes with open tips for antibacterial purposes

Vertically aligned multi-wall carbon nanotube (CNT) arrays were fabricated in tip-growth mode on Ni/Si substrates using plasma enhanced chemical vapor deposition. In a purification process including hydrogenation and acid washing of the Ni/CNTs, the oxygen-containing functional groups were substantially reduced and a wide hollow core at the tip of the CNTs was formed by removing the Ni seeds. Sol–gel silver nanoparticles were deposited on the surface of the unpurified Ni/CNTs, while they could also be embedded within the hollow core of the Ni-removed CNTs. The persistency of the silver ions in the Ni-removed Ag–CNTs in comparison to the release of the silver ions from the Ag–Ni/CNTs in a dilute HNO3 solution confirmed the effective embedding of the Ag nanoparticles within the hollow core of the Ni-removed CNTs. The Ni-removed Ag–CNTs showed a strong antibacterial activity against Escherichia coli bacteria in the dark (inactivation of 99.8 ± 0.1% of the bacteria as compared to the Ni-removed CNTs and the Ag–Ni/CNTs with 62 ± 11 and 47 ± 18% inactivation in 120 min), while the bacteria even exhibited some proliferation on the surface of the unpurified Ni/CNT arrays. In the context of cell membrane rupture of the bacteria through a direct contact mechanism, it was proposed that the Ni seeds acted as obstacles for an effective contact between the bacteria and the tips of the CNT arrays.

Cerium oxide/SnO2-based semiconductor gas sensors with improved sensitivity to CO

SnO2-based semiconductor gas sensors have been successfully fabricated and tested for detecting carbon monoxide and methane. The sensitivity and selectivity of the sensors are tailored by incorporation of different additives such as platinum and cerium oxide. While platinum enhances the sensor response to CH4, ceria suppresses its sensitivity in favor of carbon monoxide. The effect of operating temperature on the performance of sensors is reported. Addition of 10% cerium oxide in the SnO2 sample leads to an insignificant response to methane even at an elevated temperature of 450°C, while its response to CO remains intact.

Fabrication of Sensitive Glutamate Biosensor Based on Vertically Aligned CNT Nanoelectrode Array and Investigating the Effect of CNTs density on the electrode performance

In this report, the fabrication of vertically aligned carbon nanotube nanoelectrode array (VACNT-NEA) by photolithography method is presented. Electrochemical impedance spectroscopy as well as cyclic voltammetry was performed to characterize the arrays with respect to different diffusion regimes. The fabricated array illustrated sigmoidal cyclic voltammogram with steady state current dominated by radial diffusion. The fabricated VACNT-NEA and high density VACNTs were employed as electrochemical glutamate biosensors. Glutamate dehydrogenase is covalently attached to the tip of CNTs. The voltammetric biosensor, based on high density VACNTs, exhibits a sensitivity of 0.976 mA mM(-1) cm(-2) in the range of 0.1-20 μM and 0.182 mA mM(-1) cm(-2) in the range of 20-300 μM glutamate with a low detection limit of 57 nM. Using the fabricated VACNT-NEA, the sensitivity increases approximately to a value of 2.2 Am M(-1) cm(-2) in the range of 0.01 to 20 μM and to 0.1 A mM(-1) cm(-2) in the range of 20-300 μM glutamate. Using this electrode, a record of low detection limit of 10 nM was achieved for glutamate. The results prove the efficacy of the fabricated NEA for low cost and highly sensitive enzymatic biosensor with high sensitivity well suited for voltammetric detection of a wide range of clinically important biomarkers.

Single-cell resolution diagnosis of cancer cells by carbon nanotube electrical spectroscopy.

We report the use of vertically aligned carbon nanotubes (VACNTs) as electrical endoscopes (biosensors) for cancer metastatic diagnosis at single-cell resolution. The device is based on direct signal extraction by means of vertically aligned conductive carbon nanotubes from a live cell membrane, which has been disrupted during carcinogenesis at its primary and progressive stages. The value of this electrical disruption depends on the cancer metastatic grade. In addition, the electrical resonance behavior of the cell, halted during cancer progression, could be monitored as a new cancer diagnostic profile. By taking a second derivative of the cell impedance with respect to applied frequency, we have arrived at a new spectroscopy tool for distinguishing cancerous stages of colon and breast carcinoma cells.

Three-dimensional etching of silicon substrates using a modified deep reactive ion etching technique

We report realization of highly featured three-dimensional micro- and nano-structures on silicon substrates with a single masking layer using a hydrogen-assisted deep reactive ion etching process. Three gases of oxygen, hydrogen and SF6 are used in a sequential passivation and etching process to achieve high aspect ratio features. By controlling the flows of these gases and the power and timing of each subsequence, it is possible to achieve desired deep vertical etching with controlled underetching and recovery, yielding three-dimensional features directly on silicon substrates. Etch rates up to 0.75 µm min−1 have been obtained with a low plasma power density of 1 W cm−2. Also features with a controllable underetching with more than 8 µm in sidewall recession have been achieved. The three-dimensional structures can be used as a mold for polymers as well as a holding substrate for projection display applications where an electro-chromic material (WO3) has been used.

Nanograss and nanostructure formation on silicon using a modified deep reactive ion etching

Silicon nanograss and nanostructures are realized using a modified deep reactive ion etching technique on both plane and vertical surfaces of a silicon substrate. The etching process is based on a sequential passivation and etching cycle, and it can be adjusted to achieve grassless high aspect ratio features as well as grass-full surfaces. The incorporation of nanostructures onto vertically placed parallel fingers of an interdigital capacitive accelerometer increases the total capacitance from 0.45 to 30 pF. Vertical structures with features below 100 nm have been realized.

Carbon nanostructures on silicon substrates suitable for nanolithography

We report the application of vertically grown carbon nanotubes (CNTs) for submicron and nanolithography. The growth of CNTs is performed on silicon substrates using a nickel-seeded plasma-enhanced chemical vapor deposition method at a temperature of 650 °C and with a mixture of C2H2 and H2. The grown CNTs are encapsulated by a titanium-dioxide film and then mechanically polished to expose the buried nanotubes, and a plasma ashing step finalizes the process. The emission of electrons from the encapsulated nanotubes is used to write patterns on a resist-coated substrate placed opposite to the main CNT holding one. Scanning electron microscope has been used to investigate the nanotubes and the formation of nano-metric lines. Also a novel approach is presented to create isolated nanotubes from a previously patterned cluster growth.

Polyphenols attached graphene nanosheets for high efficiency NIR mediated photodestruction of cancer cells

Green tea-reduced graphene oxide (GT-rGO) sheets have been exploited for high efficiency near infrared (NIR) photothermal therapy of HT29 and SW48 colon cancer cells. The biocompatibility of GT-rGO sheets was investigated by means of MTT assays. The polyphenol constituents of GT-rGO act as effective targeting ligands for the attachment of rGO to the surface of cancer cells, as confirmed by the cell granularity test in flow cytometry assays and also by scanning electron microscopy. The photo-thermal destruction of higher metastatic cancer cells (SW48) is found to be more than 20% higher than that of the lower metastatic one (HT29). The photo-destruction efficiency factor of the GT-rGO is found to be at least two orders of magnitude higher than other carbon-based nano-materials. Such excellent cancer cell destruction efficiency provided application of a low concentration of rGO (3 mg/L) and NIR laser power density (0.25 W/cm(2)) in our photo-thermal therapy of cancer cells.

High sensitivity interdigited capacitive sensors using branched treelike carbon nanotubes on silicon membranes

Branched treelike carbon nanotubes on silicon substrate have been exploited for the realization of high sensitivity interdigital capacitive pressure sensors. The interdigital structure has been realized using a micromachining technique on silicon membranes, whereas the growth of nanotubes has been achieved using a direct-current plasma enhanced chemical vapor deposition method. A sequential growth and hydrogenation has led to the formation of multiple branched structures of nanotubes. The growth in an interdigital manner results in a high overlap between neighboring fingers and consequently a magnified response to mechanical variations in the membrane as a result of applying an external pressure is observed. An oscillatory behavior has been observed which may be attributed to the vibration of nanotubes on thinned membranes.

Mediator-less highly sensitive voltammetric detection of glutamate using glutamate dehydrogenase/vertically aligned CNTs grown on silicon substrate.

A sensitive glutamate biosensor is prepared based on glutamate dehydrogenase/vertically aligned carbon nanotubes (GLDH, VACNTs). Vertically aligned carbon nanotubes were grown on a silicon substrate by direct current plasma enhanced chemical vapor deposition (DC-PECVD) method. The electrochemical behavior of the synthesized VACNTs was investigated by cyclic voltammetry and electrochemical impedance spectroscopic methods. Glutamate dehydrogenase covalently attached on tip of VACNTs. The electrochemical performance of the electrode for detection of glutamate was investigated by cyclic and differential pulse voltammetry. Differential pulse voltammetric determinations of glutamate are performed in mediator-less condition and also, in the presence of 1 and 5 μM thionine as electron mediator. The linear calibration curve of the concentration of glutamate versus peak current is investigated in a wide range of 0.1-500 μM. The mediator-less biosensor has a low detection limit of 57 nM and two linear ranges of 0.1-20 μM with a sensitivity of 0.976 mA mM(-1) cm(-2) and 20-300 μM with a sensitivity of 0.182 mA mM(-1) cm(-2). In the presence of 1 μM thionine as an electron mediator, the prepared biosensor shows a low detection limit of 68 nM and two linear ranges of 0.1-20 with a calibration sensitivity of 1.17 mA mM(-1) cm(-2) and 20-500 μM with a sensitivity of 0.153 mA mM(-1) cm(-2). The effects of the other biological compounds on the voltammetric behavior of the prepared biosensor and its response stability are investigated. The results are demonstrated that the GLDH/VACNTs electrode even without electron mediator is a suitable basic electrode for detection of glutamate.