Session H7: Focus Session: Carbon Nanotube Sensing

Selective, Reversible Near-Infrared Fluorescence Quenching of Boronic Acid--Single-Walled Carbon Nanotube Complexes in Response to Glucose

We present the high throughput screening of a library of 30 boronic acid derivatives to form complexes with sodium cholate suspended single-walled carbon nanotubes (SWNTs) to screen for their ability to reversibly report glucose binding via a change in SWNT fluorescence. The screening identifies 4-cyanophenylboronic acid which uniquely causes a reversible wavelength red-shift in SWNT emission. The results also identify 4-chlorophenylboronic acid which demonstrates a turn-on fluorescence response when complexed with SWNTs upon glucose binding in the physiological range of glucose concentration (0 to 30 mM). The mechanism of fluorescence modulation in both of these cases is revealed to be a photo-induced excited-state electron transfer that can be disrupted by boronate ion formation upon glucose binding. This ``turn-on'' sensing scheme that uses the reversible fluorescence quenching and wavelength shift of the BA--SWNT complex offers a new approach for nIR optical sensing of glucose.   

The transduction mechanism of carbon nanotube transistors monitoring single molecule protein dynamics

Recently we have demonstrated high resolution, real-time monitoring of single molecule chemistry using molecules attached to a single-walled carbon nanotube (SWCNT) transistor. The transduction mechanism of SWCNT sensing is often claimed to be due to charge transfer, but here we clearly show the entire effect to be electrostatic. In this study, the chemical system of interest is lysozyme and its enzymatic processing of its binding substrate, peptidoglycan. We investigate the interaction of a lysozyme molecule with the SWCNT conductivity by building devices out of eight different lysozyme variants synthesized by mutagenesis. Each lysozyme variant has a different sequence of surface charges near the SWCNT attachment site, providing a calibrated method of looking at the electrostatic interactions. We observe that positively- and negatively-charged amino acids induce signals of opposite magnitude, while quasi-neutral amino acids like alanine induce little signal at all. The results indicate that careful consideration and manipulation of the charge environment near the attachment site may enhance the signal-to-noise of SWCNT sensors for studying single molecule interactions.   

Electrical Detection of Resonance in a Helically Coiled Carbon Nanowire

Helically coiled Carbon Nanowires (HCNWs) are promising elements, both for their promise as components for NEMS devices as well as for fundamental research. This is due primarily to their exotic geometry. We present here the electrostatic excitation of a HCNW cantilever to resonance and an entirely electrical measurement of the same using harmonic detection of resonance (HDR). The correlation between the directly observed resonance and the electrical signal is shown and, in addition to calculating a lateral spring constant from the observed resonance frequency, we examine the nonlinear behavior of the HCNW when driven to large amplitudes of vibration. Specifically, elliptical oscillation is visually evident and we have measured the electrical response of the corresponding combination mode.   

Arrays of Versatile DNA-Carbon Nanotube Hybrid Chemical Sensor for Vapor Analyte Detection

Large arrays of nanoscale sensors were fabricated for the detection of small molecules based on single-stranded DNA (ssDNA) for chemical recognition and single-walled carbon nanotube field effect transistors (SWNT FETs) for electronic read-out. These versatile sensors are capable of detecting very low concentrations of molecules ranging from volatile organic compounds, whose detection could provide a method for detection or identification of individuals, to noxious compounds designed to harm them. In this work, we deposited enriched semiconducting SWNTs on Si/SiO2 with an APTES adhesion layer. Photolithographically defined contacts resulted in high yield, high performance arrays of SWNT FETs, which were then individually coated in different ssDNA. The arrays of devices were then simultaneously exposed to analytes down to ppb concentrations. The sensing response of a single device is both analyte and ssDNA sequence dependant. The response and recovery to baseline are both fast (seconds) and repeatable without need for refreshing. By using large arrays of differently functionalized devices, we distinguished similar analytes and established electronic signatures indicative of their presence, paving the way for incorporation of ssDNA/SWNT FET arrays in ``electronic nose''-type systems.   

Environmental Charge Noise in Suspended Carbon Nanotube Biosensors

Carbon nanotube field effect transistors (CNT FETs) are a promising platform for probing biological systems at nanometer length scales. The sensitivity of a CNT FET sensor is ultimately limited by intrinsic fluctuations in the conductance of the sensor. Our work aims to understand the mechanisms responsible for these conductance fluctuations, and therefore understand the fundamental detection limits of charge-sensitive biosensors. We have measured conductance fluctuations in both surface-bound CNTs and suspended CNTs. Experiments are performed in physiological buffers -- the typical environment for real-time biosensing. We compare our measurements to a charge noise model and the Hooge model. We find good agreement with the charge noise model, and find that charge noise is reduced by 10 --fold when CNTs are suspended rather than surface-bound. By measuring conductance fluctuations in a variety of liquid buffers we uncover new clues about the origins of charge noise in electrolyte environments.   

Generalized Protein Attachment Chemistry for Highly Sensitive Carbon Nanotube-Based Biosensors

We developed a label free covalent functionalization procedure for attaching proteins to carbon nanotube field effect transistors (CNTFETs). Biomarker proteins are becoming increasingly useful for early diagnosis of disease, ranging from cancer to arthritis to stress. Current clinical immunoassays for measuring patient protein levels are costly and require significant processing time. Using diazonium salts followed by stabilization of carboxylic acid groups, we can attach a variety of proteins to carbon nanotubes as confirmed by atomic force microscopy. Proteins maintain the integrity of their epitope and bind to their corresponding complementary proteins. Carbon nanotube transistors are superior readout elements for such protein binding events due to their speed and comparable scale. Resulting changes in the electronic transport properties of CNTFETs demonstrate a concentration-dependent response. Binding of osteopontin (OPN), a biomarker for prostate cancer, to its complementary single chain variable fragment (scFv) can be detected down to 1 pg/mL with these methods. Moreover, these devices exhibit selectivity for OPN. Such high sensitivity biosensors could be used in parallel to test a single small volume patient sample for any number of potentially ominous biomarker proteins.   

Detection of carbon nanotubes in plant roots through microwave-induced heating

We demonstrate a novel technique for quantitative detection of CNTs in biological samples by utilizing the thermal response of CNTs under microwave irradiation. In particular, rapid heating of CNTs due to microwave absorption was employed to quantify the amount of CNTs present in alfalfa plant roots. Alfalfa roots were prepared by injecting a known amount of CNTs (single walled and multi walled) and exposed to 30-50 W microwave power to generate calibration curves (temperature rise vs. CNT mass). These calibration curves serve as a characterization tool to determine the unknown amount of CNTs absorbed by alfalfa plant roots grown in CNT-laden soil with superior accuracy and sensitivity. Moreover, the threshold for detectable CNT concentration is much lower than common analytical methods of detecting nanomaterials, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Considering the lack of effective detection methods for CNT uptake in plants, this method is not only unique but also practical, as it addresses a major problem in the field of nanotoxicology risk assessment.   

Adsorption of noble gases on individual suspended single-walled carbon nanotubes

Suspended single-walled carbon nanotubes can act as nanomechanical resonators, offering the ability to detect an adsorbed substance with very high sensitivity via the frequency shift due to the adsorbed mass. By measuring the resonance frequency electrically in the presence of vapors at controlled temperature and pressure we have obtained isotherms for 4He, Ar, Kr and Xe, on multiple nanotubes. The behavior resembles that on graphite but with notable differences, including weaker binding energies. The lower binding allows access to behavior at lower 2D chemical potential than on conventional substrates. For 4He the binding energy is reduced by as much as a factor of two. For Ar the derived two-dimensional phase diagram is similar to that on conventional substrates. For Kr there is variation between nanotubes which may be related to commensurability and area per site on the surface of a cylinder. Work supported by NSF DMR 0907690.   

Conductance isotherms for adsorption of noble gases on individual single-walled carbon nanotubes

Using transistors made from suspended carbon nanotubes allows one to probe the interaction of adsorbed atoms and molecules with the carbon substrate electrons. We have studied the effects of adsorbing He, Ne, Ar, Kr, Xe, and other gases on the electrical properties of individual suspended single-walled nanotubes, as a function of pressure and temperature. The conductance changes measurably, and sometimes dramatically, as a monolayer forms and undergoes phase transitions. It yields complementary information to the coverage, which is obtained from the mass shift in the natural vibrational frequency of the nanotube. For example, measurements below the 2D critical point show nonmonotonic features and fluctuations heralding the first-order phase transition. Conductance changes can be measured on a timescale of milliseconds, permitting studies of the dynamics of the monolayer. In the nonlinear regime we observe features in the I-V characteristics as phase transitions are induced by the current and nonequilibrium stationary states occur.   

Dependence of adsorption kinetics on the geometry of substrate

We report on the results of an adsorption kinetics study of linear alkanes on planar graphite and on aligned multiwalled carbon nanotubes. The kinetics study was performed by monitoring the equilibration times for methane, butane and pentane adsorbed on these two substrates as a function of fractional coverage.~ For methane, the time required to reach equilibration was found to decrease as the surface coverage increased. However, for butane and pentane, a systematic increase in the equilibration time with increasing fractional coverage was observed. Although similar results were found for both substrates, the waiting times for longer alkanes adsorbed on multiwalled carbon nanotubes, were found to be longer than the ones obtained for planar graphite. We speculate that the observed increase in the equilibration time with coverage for the longer alkanes is due to the rearrangement of the adsorbed molecules. Results of the current study will also be compared to our previous study of adsorption kinetics of linear alkanes on single-walled carbon nanotubes.   

New concepts in molecular and energy transport within carbon nanotubes: thermopower waves and stochastically resonant ion channels

Our laboratory has been interested in how carbon nanotubes can be utilized to illustrate new concepts in molecular and energy transfer. In the first example, we predict and demonstrate the concept of thermopower waves for energy generation [1]. Coupling an exothermic chemical reaction with a thermally conductive CNT creates a self-propagating reactive wave driven along its length. We realize such waves in MWNT and show that they produce concomitant electrical pulses of high specific power $>$7 kW/kg. Such waves of high power density may find uses as unique energy sources. In the second system, we fabricate and study SWNT ion channels for the first time [2] and show that the longest, highest aspect ratio, and smallest diameter synthetic nanopore examined to date, a 500 $\mu $m SWNT, demonstrates oscillations in electro-osmotic current at specific ranges of electric field, that are the signatures of coherence resonance, yielding self-generated rhythmic and frequency locked transport. The observed oscillations in the current occur due to a coupling between stochastic pore blocking and a diffusion limitation that develops at the pore mouth during proton transport. \\[4pt] [1] Choi W, Hong S, Abrahamson JT, Han JH, Song C, Nair N, Baik S, \textbf{Strano MS}: Chemically driven carbon-nanotube-guided thermopower waves. NATURE MATERIALS, 9 (2010) 423-429.\\[0pt] [2] Lee, CY, Choi W, Han, JH, \textbf{Strano MS}: Coherence Resonance in a Single-Walled Carbon Nanotube Ion Channel. SCIENCE, 239   

Neon adsorption on oxidized single-walled carbon nanohorns

We will present the results of a study of neon adsorption on oxidized single-walled carbon nanohorns. Our adsorption isotherm measurements were conducted at temperatures below 24.5 K, the triple point for Ne. Results for the effective specific surface area and for the effective pore volume of the nanohorn aggregates will be presented. We will also report on the sorbent-loading dependence of the isosteric heat of neon on the nanohorns, and on the binding energy. Our results for this system will be compared with those obtained for Ne on a sample of dahlia-like nanohorns annealed at 520 K.   

Carbon dioxide adsorption on open single walled carbon nanohorn aggregates

The adsorption of carbon dioxide was measured on aggregates of single-walled carbon nanohorns treated with H2O2. Volumetric adsorption isotherms were conducted at several temperatures between 162 and 212 K (below the triple point for CO2). Results on adsorption equilibration times, on the substrate-loading dependence of the isosteric heat, and on the binding energy will be presented. The effective specific surface area values obtained in this study will be compared with the ones obtained for other carbon nanohorn samples.