Transport of citrate and polymer coated gold nanoparticles (AuNPs) in porous media: Effect of surface property and Darcy velocity.

With the release of nanoparticles (NPs) into the subsurface, it is imperative to better understand the fate and transport of NPs in porous media. Three types of stable AuNPs were used as model NPs to investigate the impact of surface coatings (type and coverage) and water velocity on the NP transport in a porous media (column studies). The NPs were electrostatic stabilized citrate AuNPs and sterically stabilized AuNPs with amphiphilic block co-polymer (PVA-COOH) in two particle/polymer ratios (weak vs. strong stabilization). The citrate AuNPs transport was sensitive to ionic changes in the mixing front of the plume, where destabilization occurred, and will therefore depend on the size/type of release. Blocking of deposition sites by aggregates was seen to facilitate transport, whereby a higher flow velocity (larger shadow zone) also resulted in better transport. The polymeric surface coating had great impact with steric repulsion as a main force contributing to the transport of NPs in the porous media. Sufficient polymer coating was crucial to obtain highly unfavorable attachment conditions (very low α) where the enhanced NP mobility was independent of the water velocity (comparable to solute tracer). Without sufficient steric stabilization, the transport and recovery was significantly reduced compared to the solute tracer, but increased with increasing water velocity. This highlights the importance of sufficient surface coating to achieve enhanced mobility, but also the increased risk of spreading to down-gradient receptors. For the (weakly) sterically stabilized NPs, the loss of polymer through ligand exchange with the porous media negates transport.

Micro-flow-injection analysis (µFIA) immunoassay of herbicide residue 2,6-dichlorobenzamide ­ towards automated at-line monitoring using modular microfluidics.

As a part of developing new systems for continuously monitoring the presence of pesticides in groundwater, a microfluidic amperometric immunosensor was developed for detecting the herbicide residue 2,6-dichlorobenzamide (BAM) in water. A competitive immunosorbent assay served as the sensing mechanism and amperometry was applied for detection. Both the immunoreaction chip (IRC) and detection (D) unit are integrated on a modular microfluidic platform with in-built micro-flow-injection analysis (µFIA) function. The immunosorbent, immobilized in the channel of the IRC, was found to have high long-term stability and withstand many regeneration cycles, both of which are key requirements for systems utilized in continuous monitoring. The IRC was regenerated during 51 cycles in a heterogeneous competitive assay out of which 27 were without the analyte (the highest possible signal level) in order to assess the regeneration capability of the immunosorbent. Detection of BAM standard solutions was performed in the concentration range from 62.5 µg L(-1) to 0.0008 µg L(-1). Non-linear regression of the data using the four-parameter logistic equation generated a sigmoidal standard curve showing an IC50 value (concentration that reduces the signal by 50%) of 0.25 µg L(-1). The strongest signal variation is observed in the concentration range between 0.02 and 2.5 µg L(-1), which includes the 0.1 µg L(-1) threshold limit set by the European Commission for BAM in drinking water. The presented results demonstrate the potential of the constructed µFIA immunosensor as an at-line monitoring system for controlling the quality of ground water supply.

Mobility of electrostatically and sterically stabilized gold nanoparticles (AuNPs) in saturated porous media.

The stability of gold nanoparticles (AuNPs) stabilized electrostatically with citrate or (electro)sterically by commercially available amphiphilic block copolymers (PVP-VA or PVA-COOH) was studied under various physicochemical conditions. Subsequently, the mobility of the AuNPs in porous media (sand) was investigated in column studies under environmental relevant physicochemical conditions. Electrostatically stabilized AuNPs were unstable under most physicochemical conditions due to the compression of the electrical double layer. Consequently, aggregation and deposition rapidly immobilized the AuNPs. Sterically stabilized AuNPs showed significantly less sensitivity towards changes in the physicochemical conditions with high stability, high mobility with negligible retardation, and particle deposition rate coefficients ranging an order of magnitude (1.5 × 10-3 to 1.5 × 10-2 min-1) depending on the type and amount of stabilizer, and thereby the surface coverage and attachment affinity. The transport of sterically stabilized AuNPs is facilitated by reversible deposition in shallow energy minima with continuous reentrainment and blocking of available attachment sites by deposited AuNPs. The stability and mobility of NPs in the environment will thereby be highly dependent on the specific stabilizing agent and variations in the coverage on the NP. Under the given experimental conditions, transport distances of the most mobile AuNPs of up to 20 m is expected. Due to their size-specific plasmonic properties, the easily detectable AuNPs are proposed as potential model or tracer particles for studying transport of various stabilized NPs under environmental conditions.

In situ SU-8 silver nanocomposites.

Nanocomposite materials containing metal nanoparticles are of considerable interest in photonics and optoelectronics applications. However, device fabrication of such materials always encounters the challenge of incorporation of preformed nanoparticles into photoresist materials. As a solution to this problem, an easy new method of fabricating silver nanocomposites by an in situ reduction of precursors within the epoxy-based photoresist SU-8 has been developed. AgNO3 dissolved in acetonitrile and mixed with the epoxy-based photoresist SU-8 forms silver nanoparticles primarily during the pre- and post-exposure soft bake steps at 95 °C. A further high-temperature treatment at 300 °C resulted in the formation of densely homogeneously distributed silver nanoparticles in the photoresist matrix. No particle growth or agglomeration of nanoparticles is observed at this point. The reported new in situ silver nanocomposite materials can be spin coated as homogeneous thin films and structured by using UV lithography. A resolution of 5 µm is achieved in the lithographic process. The UV exposure time is found to be independent of the nanoparticle concentration. The fabricated silver nanocomposites exhibit high plasmonic responses suitable for the development of new optoelectronic and optical sensing devices.

Optimization of an immunoassay of 2,6-dichlorobenzamide (BAM) and development of regenerative surfaces by immunosorbent modification with newly synthesised BAM hapten library.

Dichlobenil is an extensively used herbicide worldwide which is transformed to the mobile 2,6-dichlorobenzamide (BAM) in soil. BAM has been found in many European groundwater resources that are exploited for drinking water. Currently, immunoassay based monitoring technique (plate based ELISA) is being employed to quantitatively detect BAM in water samples. In this work, as a starting step of developing immunoassay based on-site monitoring systems for pesticide analysis, the heterogeneous BAM immunoassay is optimised in terms of surface (polymer) regeneration. We have synthesised a small library of BAM haptens which are slightly different in chemical structures, immobilised them on surfaces and compared the affinity constants of the monoclonal antibody HYB 273 towards them. By using ELISA technology, we also have checked the regeneration potentials of the haptens, correlated these results to the affinity constants and found that BAM hapten with an intermediate affinity has better regeneration potential.