University of Pittsburgh Chemistry
Star Group Research
Nanotechnology enabled chemical sensing and energy conversion
Introduction Professor Alexander Star Research Group Members Publications News Nanotechnology Links

Research Interests


Star Group Research Overview

Research on Carbon Nanomaterials

The Star group employs the bottom-up technique of controlled chemical vapor deposition (CVD) to fabricate single-walled carbon nanotubes (SWNTs) and other novel carbon-based nanomaterials. These materials are further modified to impart new properties. Methods of functionalization include: oxidation through enzymatic biodegradation, labeling via covalent linking, decorating with nanoparticles, and the non-covalent attachment of synthetic or natural ligands/polymers. Moreover, the products of both the synthesis and functionalization stages of our research undergo rigorous characterization by a variety of microscopy, electrochemistry, and spectroscopy techniques and solid-state electrical transport measurements. The results of our research have led to positive steps in the development of novel applications, which include the production of chemical and biological sensors, energy conversion devices (fuel cells), and nano-capsules for drug delivery.

Nanotechnology Enabled Chemical and Biological Sensing

Single-walled carbon nanotubes (SWNTs), which are composed of a single cylindrical layer of carbon atoms, have found a prominent position in sensor applications because their electrical conductivity can be modified through interaction with chemical or biological species. The small diameter and relatively long length of SWNTs allows them to probe molecular systems on a local scale by directly "wiring into" individual or small assemblies of molecules. Additionally, as 1-dimensional structures, electrons are confined to the exterior of SWNTs making them extremely sensitive to perturbations in the local charge environment, where analytes may donate or accept electrons, or physisorb acting as charge mobility barriers. As a result of these phenomena, real-time electronic responses in terms of changes in conductance can be witnessed.

Our research group is involved in several carbon nanotube chemical sensor projects, where nanoparticles and polymers are employed to enhance selectivity. For a review of carbon nanotube based gas and vapor sensors see Angew. Chem. Int. Ed., 2008.

Metal Nanoparticle/Carbon Nanotube Chemical Sensors

Single-walled carbon nanotubes (SWNTs) decorated with metal nanoparticles exhibit exquisite selectivity for adsorbing gas molecules. When the SWNT functions as the conduction channel in an electrical circuit, changes in the measured current can be used to detect gas adsorption and/or catalytic reactions at the SWNT surface. This approach has been used to develop an understanding of the interaction between metal nanoparticle decorated carbon nanotubes and a variety of gases, such as nitric oxide (NO), carbon monoxide (CO), hydrogen (H2), ammonia (NH3), nitrogen dioxide (NO2) and hydrogen sulfide (H2S) (refer to J. Phys. Chem. B., 2006 and Nano Lett., 2007).

Polymer/Carbon Nanotube Chemical Sensors

Like metal nanoparticles, SWNTs functionalized with a polymer coating can create gas selective sensors. For example, the polymer, poly (ethylene imine) (PEI), has demonstrated selectivity towards acidic gases (NO2, CO2, SO2). Through the interaction of oxidized NO (NO2), electronic density can be withdrawn from the polymer-nanotube network and a response visualized, even at concentrations as low as parts per billion (ppb) levels (refer to Nanotechnology, 2007).


A bimodal SWNT-based O2 gas sensor was developed from a SWNT network that was decorated with a photoluminescent Eu3+-containing dendtritic polymer. Once illuminated with 365 nm UV light, the device shows reversible and concentration dependent electrical response to O2 gas at room temperature and ambient pressure. This represents the first report of a room temperature and ambient pressure SWNT-based O2 gas sensor that could be used as a personal safety device in confined spaces such as mines, submarines or spacecraft (refer to Nature Chem, 2009).

Polymer/Eu<sup>3+</sup>-Containing Dendtritic Polymer Decorated Carbon Nanotube O<sub>2</sub> gas sensor

Carbon Nanotube Based Biosensor

Carbon Nanotube Based Biological Sensors

There is an explosive interest in 1-dimensional nanostructured materials for biological sensors. Among these nanometer-scale materials, single-walled carbon nanotubes (SWNTs) offer the advantages of possible biocompatibility, size compatibility, and sensitivity towards minute electrical perturbations. In particular, because of these inherent qualities, changes in SWNT conductivity have been explored in order to study the interaction of biomolecules with SWNTs. This change in conductance permits the development of carbon nanotube field-effect transistors (NTFETs) for the detection of biological species. NTFETs show great promise because of their extreme environmental sensitivity, small size, and ultra-low power requirements. NTFETs employ natural polymers such as proteins and single-stranded DNA (ssDNA) to create biosensors for proteomics and genomics (refer to Proc. Natl. Acad. Sci., 2006). For our recent reviews on biosensors, refer to Adv. Mater., 2007 and Chem. Soc. Rev., 2008.



Synthesis and Exploration of Nanocups

Growth and Characterization

Using simple thermal reactions such as chemical vapor deposition (CVD), nitrogen-doped carbon nanotube cups (NCNCs) are synthesized in much the same way as carbon nanotubes. However, introduction of a nitrogen source during the synthesis perpetuates the growth of stacked, cup-like structures capable of being separated through mechanical grinding. Synthesized in stacked conformation, NCNCs are initially comprised of 1 µm long fibers, with diameters ranging from 12-40 nm depending upon catalyst particle size as characterized by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Because the graphitic lattice does not grow parallel to the longitudinal axis of the fibers, grinding with a mortar and pestle or ball mill separates the fibers into individual “cups”. These cups are then opened to a variety of chemical reactivities due to intrinsic nitrogen functionalities concentrated primarily, though not exclusively, on the open basal plane of the structures (refer to ACS Nano, 2008).


Employing Nanocups for Oxygen Reduction

Electrochemical Nano-Catalyst

Nitrogen doped carbon nanocups (NCNCs) show good catalytic properties to Oxygen Reduction Reaction (ORR), which makes them an attractive substitute for noble metals such as Pt or Ru in fuel cell cathodes. Their unique nanocup structure and the nitrogen atom doping, which involves the conjugation of nitrogen lone-pair electrons with graphene π-system, may provide us materials with tailored electronic and mechanical properties. Besides low fabrication costs, NCNCs demonstrate resistance to toxic gases, which represents a major advantage compared to Pt. Further catalytic applications of NCNCs for energy conversion (O2 reduction in basic and acidic conditions and fuel cell prototyping) and electrochemical sensing (such as the detection of H2O2 and glucose) are currently being explored by our group (refer to J. Am. Chem. Soc., 2009).


Nanocapsules

Incorporated nitrogen functionalities may include primary and secondary amines, pyridine-like nitrogen, and sp2 hybridized nitrogen (where a single nitrogen atom replaces carbon). Of particular note, is their ability to self-assemble into nanocapsules by the addition of a cross-linking agent. Moreover, by incubating a desired “cargo” with NCNCs prior to linkage, they are able to take up material and encapsulate it within their hollow interior cavities. We have demonstrated the ability to encapsulate gold nanoparticles (GNPs) of ~ 5 nm in diameter by incubating GNPs with NCNCs prior to the addition of glutaraldehyde as a linking agent. Additionally, to validate the general applicability of this system, terbium-doped zinc sulfide (Tb:ZnS) particles were also taken up and encapsulated by NCNCs using an identical procedure (refer to Adv. Mater., 2009). This type of nanomaterial also contains nitrogen defects that enhance its reactivity and make it more readily biodegradable, thus offering a “green” biocompatible nanomaterial that has potential impacts in materials and biomedical research for materials storage, nanoreactors, drug delivery vehicles, and sensors. Further investigations into their intrinsic chemical reactivities may promote a variety of linkage schemes and permit controlled sensitivity to external stimuli (for nanocapsule opening) such as light, pH, and chemical agents.

Nanocapsules


Recognition and Biodegradation of Carbon Nanomaterials

TEM image of biodegradated SWNTs

Single-walled Carbon Nanotubes

Currently, it is known that carbon nanotubes can cause negative effects such as inflammation, oxidative stress, and cell death. As such, a means to alleviate these effects through breakdown of such a material is necessary. Up until this point, methods for degrading nanotubes, or “cutting” nanotubes, involved the use of harsh solvents consisting of H2SO4 and high concentrations of H2O2. When dealing with environmental issues it is important not to introduce any contaminants harsher than what is being “cleaned.” In our work, we have shown the natural biodegradation of single-walled carbon nanotubes through enzymatic catalysis. By incubating carbon nanotubes with a common enzyme, horseradish peroxidase, and low levels of H2O2 (40 µM) under static conditions, these nanomaterials are oxidized (refer to Nano Lett., 2008 and J. Am. Chem. Soc., 2009). The following animation depicts the enzymatic degradation of carbon nanotubes.


Last Modified: 01-19-10

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