Lee J. Hubble

Gold nanoparticle chemiresistor sensor array that differentiates between hydrocarbon fuels dissolved in artificial seawater.

Gold nanoparticle films (Au(NPF)) functionalized with a range of hydrophobic and hydrophilic thiols were assembled in chemiresistor sensor arrays that were used to differentiate between complex mixtures of analytes in the aqueous phase. A chemiresistor array sampled a simple system of linear alcohols (methanol, ethanol, propan-1-ol, and butan-1-ol) dissolved in water over a range of concentrations. Discriminant analysis confirmed that the response patterns of the array could be used to successfully distinguish between the different alcohol solutions at concentrations above 20 mM for all of the alcohols except methanol, which was distinguished at concentrations above 200 mM. Alcohol solutions more dilute than these concentrations had response patterns that were not consistently recognizable and failed cross validation testing. This defined the approximate limit of discrimination for the system, which was close to the limits of detection for the majority of the individual sensors. Another Au(NPF) chemiresistor array was exposed to, and successfully identified crude oil, diesel, and three varieties of gasoline dissolved in artificial seawater at a fixed concentration. This work is a demonstration that the pattern of responses from an array of differently functionalized Au(NPF) sensors can be used to distinguish analytes in the aqueous phase.

Multifunctional water-soluble molecular capsules based on p-phosphonic acid calix[5]arene

p-Phosphonic acid calix[5]arene forms molecular capsules in water based on two of the molecules, which can be loaded with carboplatin using intense shearing, and attached to single wall carbon nano-tubes. Spin coating of the capsules onto a substrate affords 2 nm fibres of stacked calixarenes, with the self-assembly understood using molecular modelling.

Selective diameter uptake of single-walled carbon nanotubes in water using phosphonated calixarenes and ‘extended arm’ sulfonated calixarenes

Single-walled carbon nanotubes (SWCNTs) have been successfully solubilized using water-soluble p-phosphonated calix[n]arenes (n = 4, 6, 8) and ‘extended arm’ upper rim functionalized (benzyl, phenyl) p-sulfonated calix[8]arenes. Selective SWCNT diameter solubilization has been demonstrated and subsequent preferential enrichment of SWCNTs with semiconducting or metallic electronic properties has been achieved. These water-soluble supramolecular systems can be incorporated into post-growth purification protocols with direct implications in areas such as nano-electronics and device fabrication.

High-throughput fabrication and screening improves gold nanoparticle chemiresistor sensor performance.

Chemiresistor sensor arrays are a promising technology to replace current laboratory-based analysis instrumentation, with the advantage of facile integration into portable, low-cost devices for in-field use. To increase the performance of chemiresistor sensor arrays a high-throughput fabrication and screening methodology was developed to assess different organothiol-functionalized gold nanoparticle chemiresistors. This high-throughput fabrication and testing methodology was implemented to screen a library consisting of 132 different organothiol compounds as capping agents for functionalized gold nanoparticle chemiresistor sensors. The methodology utilized an automated liquid handling workstation for the in situ functionalization of gold nanoparticle films and subsequent automated analyte testing of sensor arrays using a flow-injection analysis system. To test the methodology we focused on the discrimination and quantitation of benzene, toluene, ethylbenzene, p-xylene, and naphthalene (BTEXN) mixtures in water at low microgram per liter concentration levels. The high-throughput methodology identified a sensor array configuration consisting of a subset of organothiol-functionalized chemiresistors which in combination with random forests analysis was able to predict individual analyte concentrations with overall root-mean-square errors ranging between 8-17 μg/L for mixtures of BTEXN in water at the 100 μg/L concentration. The ability to use a simple sensor array system to quantitate BTEXN mixtures in water at the low μg/L concentration range has direct and significant implications to future environmental monitoring and reporting strategies. In addition, these results demonstrate the advantages of high-throughput screening to improve the performance of gold nanoparticle based chemiresistors for both new and existing applications.

Dynamic response of gold nanoparticle chemiresistors to organic analytes in aqueous solution

We investigate the response dynamics of 1-hexanethiol-functionalized gold nanoparticle chemiresistors exposed to the analyte octane in aqueous solution. The dynamic response is studied as a function of the analyte-water flow velocity, the thickness of the gold nanoparticle film and the analyte concentration. A theoretical model for analyte limited mass-transport is used to model the analyte diffusion into the film, the partitioning of the analyte into the 1-hexanethiol capping layers and the subsequent swelling of the film. The degree of swelling is then used to calculate the increase of the electron tunnel resistance between adjacent nanoparticles which determines the resistance change of the film. In particular, the effect of the nonlinear relationship between resistance and swelling on the dynamic response is investigated at high analyte concentration. Good agreement between experiment and the theoretical model is achieved.

Electrical noise in gold nanoparticle chemiresistors: Effects of measurement environment and organic linker properties

This study was conducted to assess and characterize the intrinsic electrical noise associated with thiolate functionalized gold nanoparticle films utilized as chemiresistor sensors. Recently, this sensor type has been demonstrated to have liquid-based sensing capabilities. Differences in the thermal noise component of the different thiolate functionalized nanoparticle films in ambient air and in deionized water are detailed. Comparable values are found between real impedance values and resistance values derived from the Nyquist equation, in the frequency domain, for differing chemical measurement environments. Voltage-biased studies in aqueous media demonstrate the devices retention of the intrinsic 1/f noise, known to exist in gold nanoparticle chemiresistors operating in ambient air. These results have implications on the ultimate sensitivity of devices utilized for liquid-based analyte detection.

A water-soluble fluoroionophore: p-(4-sulfonatophenyl)calix[8]arene

The conformational flexibility of water-soluble p-(4-sulfonatophenyl)calix[8]arene leads to pH dependent autofluorescence. Solution-based and solid-state techniques are consistent with the spontaneous formation of intra-molecular excimers upon solvation, with molecular simulations revealing that this occurs through π-stacking. Time-based fluorescence intensity studies provide insight into the solution dynamics, and solvated shelf-life, to significantly enhance the analytical protocol. Conductivity techniques support the formation of micelles, with no evidence for a direct link between higher-order structures and autofluorescence. Disruption of this excimer by divalent metal cations provides scope for its utility as a fluoroionophore, which is also consistent with molecular simulations, without the requirement of incorporating additional fluorophoric and water solubilising sub-units.

Determination of alkanes in aqueous solution using gold nanoparticle chemiresistors: Dynamic response characteristics

The electrical impedance change and response time of chemiresistor sensors based on gold nanoparticles capped with 1-hexanethiolate ligands were investigated for several organic analytes dissolved in aqueous solution. Analytes that partitioned strongly into the nanoparticle film resulted in greater impedance changes and longer response times than analytes which had a smaller partition coefficient.

Detecting and identifying aqueous solutions of hydrocarbons with a gold nanoparticle chemiresistor sensor array

A sensor array of gold nanoparticle chemiresistors, functionalized with a variety of thiolate ligands, was used to detect different hydrocarbons dissolved in water. Discriminant analysis of the sensor responses showed that the diluted saturated solutions of kerosene, turpentine, two types of oil and three types of petrol could be clearly identified across a range of concentrations.