Grantee Research Project Results

The overall objective of this work is to develop nanocavity sensor (category II) arrays for the isolation, detection and quantitation of engineered nanoparticles (ENPs) and distinguish these from naturally occurring nanomaterials in complex environmental matrices. Currently, the number of ENPs on the market is large and is expected to increase with advances in synthetic and technological developments. While nanomaterials have numerous applied uses and the benefits of nanotechnology are widely publicized, the discussion of their potential effects on human health and the environment is just beginning. Thus there is a greater need to develop sensors that can provide information on the presence and toxicity of nanomaterials.

 

Nanosensors can be classified under two main categories:(i) Nanotechnology-enabled sensors or sensors that are themselves nanoscale or have nanoscale materials or components, and (ii) Nanoproperty-quantifiable sensors or sensors that are used to measure nanoscale properties. The first category can eventually result in lower material cost, reduced weight and energy consumption. The second category can enhance our understanding of the potential toxic effects of emerging pollutants from nanomaterials including fullerenes, dendrimers, and carbon nanotubes. Despite the enormous literatures and reviews on Category I sensors, there are few sensors to measure nanoscale properties or sensors belonging to Category II. This class of nanosensors is an area of critical interest to nanotoxicology, detection and risk assessment, as well as for monitoring of environmental and/or biological exposure.

This project has led to the development of novel category 2 nanosensors and phase-inverted poly (amic) acid PAA nanofilter membranes. The membranes have been fitted onto an optical transducer that was previously developed in our lab and the possibility of ultrasensitive detection of the nanomaterials has been explored. In addition, we have tested the sensors for analysis in complex environmental samples including soil, sediments and water. Finally, we have determined the analytical performance of the nanosensor for continuous and in–situ sensing. These have been compared to existing technology. During the final segment of the grant activities, we explored the use of the membranes to sense different types of engineered nanomaterials and validated the sensors at a waste water treatment plant (Figure 1). Overall, this project resulted in 15 peer-reviewed publications, two patent disclosures, 16 invited and/or keynote presentations and 21 conference abstracts. The PI also started the first Gordon Conference on Environmental Nanotechnology and held the meeting in NH from May 29-June 3, 2011 (Table 1).

 

The following section highlights the salient aspects of the project:

 

Size-exclusive Nanosensor for Quantitative Analysis of Fullerene C₆₀: A Concept Paper: This paper presents the first development of a mass-sensitive nanosensor for the isolation and quantitative analyses of engineered fullerene (C₆₀) nanoparticles, while excluding mixtures of structurally similar fullerenes. Amino-modified beta cyclodextrin (β-CD-NH₂) was synthesized and confirmed by ¹HNMR as the host molecule to isolate the desired fullerene C₆₀. This was subsequently assembled onto the surfaces of gold-coated quartz crystal microbalance (QCM) electrodes using N-Dicyclohexylcarbodiimide/N-hydroxysuccinimide (DCC/NHS) surface immobilization chemistry to create a selective molecular configuration described as (Au)-S– (CH₂)₂-CONH-beta–CD sensor. The mass change on the sensor configuration on the QCM was monitored for selective quantitative analysis of fullerene C₆₀ from a C₆₀/C₇₀ mixture and soil samples. About ~10¹⁴-10¹⁶ C₆₀ particles/cm² were successfully quantified by QCM measurements. Continuous spike of 200 µl of 0.14 mg C₆₀ /ml produced changes in frequency (-∆f) that varied exponentially with concentration. FESEM and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) confirmed the validity of sensor surface chemistry before and after exposure to fullerene C₆₀. The utility of this sensor for spiked real-world soil samples has been demonstrated. Comparable sensitivity was obtained using both the soil and purified toluene samples. This work demonstrates that the sensor has potential application in complex environmental matrices.

 

Catalytic Reduction of Hexavalent Chromium using Flexible Nanostructured Poly(amic acids): Conducting polymers can be tuned by manipulating the delocalized π electron system for chemical and electrocatalytic applications. We hereby describe the reduction of Cr(VI) to Cr(III) by flexible nanostructured conducting poly(amic acid) (PAA) in both solution phase and as a thin film on gold electrode. Sodium borohydride was used as a reducing agent to prepare different sizes (3-20 nm) of palladium nanoparticles (PdNPs). The effects of experimental parameters such as particle size, temperature and Cr(VI) concentration on the kinetics and efficiency of reduction process were investigated. Results show that in PAA solution, Cr(VI) was efficiently reduced by 85.9% within a concentration range of 1.0 x 10^-1 – 1.0 x 10² mM. In the presence of PdNPs and heat (40^oC), the reduction efficiency increased to 96.6% and 99.9% respectively. When employed on a solid electrode, PAA undergoes a quasi-reversible electrochemistry in acidic media with reduction efficiency for Cr(VI) at 72.84%. The method was validated using both colorimetric and Electron Paramagnetic Resonance techniques, which confirmed the formation of Cr(III) as the product of catalytic reduction. Additional characterization conducted using TEM, XRD and XPS confirmed that there was no significant change in Pd particle size and distributions after dispersion in PAA whereas its phase and oxidation state remained unchanged. Electrochemical characterization showed the reversible and recyclable features of PAA thus confirming its dual role as catalyst stabilizer and reducing agent. This approach provides a significant advantage over conventional methods such as bioremediation which typically require longer time for complete reduction.

 

Environmental Applications of Poly(amic acid)-based Nanomaterials: Nanoscale materials offer new possibilities for the development of novel remediation and environmental monitoring technologies. Different nanoscale materials have been exploited for preventing environmental degradation and pollutant transformation. However, the rapid self-aggregation of nanoparticles or their association with suspended solids or sediments where they could bioaccumulate supports the need for polymeric coatings to improve mobility, allows faster site cleanups and reduces remediation cost. The ideal material must be able to coordinate different nanomaterials functionalities and exhibit the potential for reusability. We hereby describe two novel environmental applications of nanostructured poly (amic acid)-based (nPAA) materials. In the first application, nPAA was used as both reductant and stabilizer during the in-situ chemical reduction of Chromium (VI) to Chromium (III). Results showed that Cr (VI) species were rapidly reduced within the concentration range of 1.0 - 10^-1 x 10² mM with efficiency of 99.9% at 40ºC in water samples and 90% at 40^oC in soil samples respectively. Furthermore, the presence of PdNPs on the PAA-Au electrode was found to significantly enhance the rate of reduction. In the second application, nPAA membranes were tested as filters to capture, isolate and detect nanosilver. Preliminary results demonstrate the capability of the nPAA membranes to quantitatively capture nanoparticles from suspension and quantify their abundance on the membranes. Silver nanoparticles detection at concentrations near the toxic threshold of silver was also demonstrated.

 

Accomplishments

• The PI started the first Gordon Conference on Environmental Nanotechnology and chaired the first meeting, which was held at the Waterville Valley Resort, Waterville, NH from May 29-June 3, 2011 (Conference program, Table 1).
• This grant has led to the development of the mass-sensitive nanosensor for the isolation and quantitative analyses of engineered fullerene (C₆₀) nanoparticles, while excluding mixtures of structurally similar fullerenes.
• PAA membranes were successfully fabricated as flexible, free-standing membranes using phase inversion techniques. These membranes show well-defined and uniform filtration properties. The surface pore sizes were controlled by the synthesis conditions, method of desolvation and the concentrations of the polymer composition. The synthetic approach affords well controlled membranes leading to various desired sizes from less than 5nm to greater than 100nm.
• Preliminary results showed that these PAA membranes can filter quantum dots (smaller than 5nm) directly from aqueous solution at 96.78% efficiency. The standard deviation could be significantly improved at much lower concentrations of the nanoparticles (5nm).
• PAA membranes were placed on a 13mm Millipore stainless steel swinny filter holder to create a filter paper-type device. This is followed by the addition of the environmental (aqueous) samples to be filtered: Specified volume (mL) of aqueous food supplements and environmental samples containing the AgNPs or specified volume (ml) standard AgNPs stocking solutions. The AgNPs contained in the aqueous samples were trapped on the PAA membranes. The captured AgNPs were imaged and characterized by SEM and EDX techniques.
• PAA have been tested as nanofilters to isolate and remove silver nanoparticles, quantum dots and titanium dioxide particles in food supplements and environmental samples. Filtration efficiency of over 99% was recorded.
• PAA membranes were also used as a sensor for silver nanoparticles and other synthetic metal oxides. Results showed that concentration relationship in low concentration range (0.1ppm to 10ppm) with detection limit of ~50ppb.
• Filtration characteristics of the PAA membranes using the sandwich two chamber cell showed that size-dependent isolation of engineered nanoparticles of varying sizes in water and food supplements samples could be achieved based on redox current from the silver nanoparticles. The approach was tested to remove silver nanoparticles from commercial food supplements of varying particle sizes with > 99.9% removal efficiency. PAA showed excellent performance not only for isolation at sub-nanometer size range but also as a platform for detection of engineered nanoparticles.
• PAA have been tested for application in environmental matrices with significant improvements recorded over the conventional approaches for Chromium (VI), silver nanoparticles and fullerenes (Table 2).

Sadik O. A., Zhou A. L., Kikandi S., Du N., Wang Q., Sensors as tools for quantitation, nanotoxicity and nanomonitoring assessment of engineered nanomaterials, Journal of Environmental Monitoring (Critical Review), 2009, 11, 1782-1800.

 

Du N., Wong C., Feurstein, M., Sadik O., Umbach C., Sammakia B., Flexible Conducting Polymers: Effects of Chemical Composition on Structural, Electrochemical and Mechanical Properties, Langmuir, 2010. DOI: 10.1021/la101314j

 

Andreescu, D., Wanekaya A., O.A. Sadik, Wang J., “Nanostructured Polyamic Membranes as Electrode Material,” Langmuir, 21(15), 6891-6899, 2005

 

Breimer M., Yevgheny E., Sadik O. A., “Incorporation of Metal Particles in Polymerized Organic Conducting Polymers - A Mechanistic Insight,” Nano Letters, 1 (6), 305, 2001.