Molecularly Imprinted Polymers (MIPs) in Sensors for Environmental and Biomedical Applications: A Review.

Molecular imprinted polymers are custom made materials with specific recognition sites for a target molecule. Their specificity and the variety of materials and physical shapes in which they can be fabricated make them ideal components for sensing platforms. Despite their excellent properties, MIP-based sensors have rarely left the academic laboratory environment. This work presents a comprehensive review of recent reports in the environmental and biomedical fields, with a focus on electrochemical and optical signaling mechanisms. The discussion aims to identify knowledge gaps that hinder the translation of MIP-based technology from research laboratories to commercialization.

Enhancement of the Fouling Resistance of Zwitterion Coated Ceramic Membranes.

Ceramic membranes suffer from rapid permeability loss during filtration of organic matter due to their fouling propensity. To address this problem, iron oxide ultrafiltration membranes were coated with poly(sulfobetaine methacrylate) (polySBMA), a superhydrophilic zwitterionic polymer. The ceramic-organic hybrid membrane was characterized by scanning electron microscopy (SEM) and optical profilometry (OP). Membranes with and without polySBMA coating were subjected to fouling with bovine serum albumin solution. Hydraulic cleaning was significantly more effective for the coated membrane than for the non-coated one, as 56%, 66%, and 100% of the fouling was removed for the first, second, and third filtration cycle, respectively. Therefore, we can highlight the improved cleaning due to an increased fouling reversibility. Although some loss of polymer during operation was detected, it did not affect the improved behavior of the tested membranes.

Influence of pH, ionic strength and natural organic matter concentration on a MIP-Fluorescent sensor for the quantification of DNT in water.

The effect of sample water chemistry on a carbon dot labeled molecularly imprinted polymer (AC-MIP) sensor for the detection of 2,4-dinitrotoluene (DNT) was investigated. Hydrogel MIP films were fabricated and tested in DNT solutions in various matrices, representative of natural water conditions, to assess applicability of the sensors to real water samples. The effect of pH, natural organic matter (NOM), ionic strength and cation type on the swelling of the hydrogel and fluorescence quenching was investigated. An increase in ionic strength from 1 mM to 100 mM produced a quenching amount of MIPs decreased of about 19% and 30% with NaCl and CaCl2 respectively. In the range of pH tested, from 4 to 9, quenching was higher at basic environment for both MIPs and non-imprinted polymers (NIPs) due to increased hydrogel swelling. NOM contributed to the background quenching, but the effect could be addressed by an adjusted calibration equation. In both lake and tap water, DNT concentrations read by the sensors were close to the values measured by HPLC, within 72%-105% of true values. The AC-MIP films fabricated in this work are promising materials for the detection of water contamination in the field and the quantitative analysis of DNT concentration.

Abatement of 2,4-D by H2O2 solar photolysis and solar photo-Fenton-like process with minute Fe(III) concentrations.

The Photo-Fenton-like (PF-like) process with minute Fe(III) concentrations and the Hydrogen Peroxide Photolysis (HPP), using Xe-lamp or solar light as sources of irradiation, were efficiently applied to eliminate the herbicide 2,4-D from water. PF-like experiments concerning ferric and H2O2 concentrations of 0.6 mg L-1 and 20 mg L-1 respectively, using Xenon lamps (Xe-lamps) as a source of irradiation and 2,4-D concentrations of 10 mg L-1 at pH 3.6, exhibited complete 2,4-D degradation and 77% dissolved organic carbon (DOC) removal after 30 min and 6 h of irradiation respectively whereas HPP (in absence of ferric ions) experiments showed a 2,4-D reduction and DOC removal of 90% and 7% respectively after 6 h of irradiation. At pH 7.0, HPP process achieved a 2,4-D abatement of approximately 75% and a DOC removal of 4% after 6 h. PF-like exhibited slightly improved 2,4-D and DOC removals (80% and 12% respectively) after the same irradiation time probably due to the low pH reduction (from 7.0 to 5.6). Several chlorinated-aromatic intermediates were identified by HPLC-MS. These by-products were efficiently removed by PF at pH 3.6, whereas at neutral PF-like and acid or neutral HPP, they were not efficiently degraded. With natural solar light irradiation, 10 and 1 mg L-1 of 2,4-D were abated using minor H2O2 concentrations (3, 6, 10 and 20 mg L-1) and iron at 0.6 mg L-1 in Milli-Q water. Similar results to Xe-lamp experiments were obtained, where solar UV-B + A light H2O2 photolysis (HPSP) and solar photo-Fenton-like (SPF-like) played an important role and even at low H2O2 and ferric concentrations of 3 and 0.6 mg L-1 respectively, 2,4-D was efficiently removed at pH 3.6. Simulated surface water at pH 3.6 containing 1 mg L-1 2,4-D, 20 mg L-1 H2O2 and 0.6 mg L-1 Fe(III) under natural sunlight irradiation efficiently removed the herbicide and its main metabolite 2,4-DCP after 30 min of treatment while at neutral pH, 40% of herbicide degradation was achieved. In the case of very low iron concentrations (0.05 mg L-1) at acid pH, 150 min of solar treatment was required to remove 2,4-D.

The Effects of Carbon Dots on Immune System Biomarkers, Using the Murine Macrophage Cell Line RAW 264.7 and Human Whole Blood Cell Cultures.

Carbon dots (CDs) are engineered nanoparticles that are used in a number of bioapplications such as bioimaging, drug delivery and theranostics. The effects of CDs on the immune system have not been evaluated. The effects of CDs on the immune system were assessed by using RAW 264.7 cells and whole blood cell cultures. RAW cells were exposed to CD concentrations under basal conditions. Whole blood cell cultures were exposed to CD concentrations under basal conditions or in the presence of the mitogens, lipopolysaccharide (LPS) or phytohaemmagglutinin (PHA). After exposure, a number of parameters were assessed, such as cell viability, biomarkers of inflammation, cytokine biomarkers of the acquired immune system and a proteome profile analysis. CDs were cytotoxic to RAW and whole blood cell cultures at 62.5, 250 and 500 µg/mL, respectively. Biomarkers associated with inflammation were induced by CD concentrations ≥250 and 500 µg/mL under basal conditions for both RAW and whole blood cell cultures, respectively. The humoral immune cytokine interleukin (IL)-10 was increased at 500 µg/mL CD under both basal and PHA activated whole blood cell culture conditions. Proteome analysis supported the inflammatory data as upregulated proteins identified are associated with inflammation. The upregulated proteins provide potential biomarkers of risk that can be assessed upon CD exposure.

Effect of water chemistry on the aggregation and photoluminescence behavior of carbon dots.

Carbon dots are rapidly emerging carbon-based nanomaterials that, due to their growing applications, will inevitable find their way to natural waters; however, their environmental fate is mostly unknown. Carbon dots with different surface functionality were fabricated and characterized by TEM and FT-IR. Their surface charge, given by the zeta potential, and their hydrodynamic diameter in suspension were investigated under a variety of environmentally relevant conditions. The effect of ionic strength was studied in the presence of monovalent (NaCl) and divalent (CaCl2) cations, for pH levels from 3 to 11; humic acid was used as a model for dissolved natural organic matter. Total potential energies of interactions were modeled by classical DLVO theory. The experimental results showed that water chemistry altered the surface charge of the nanomaterials, but their hydrodynamic size could not be correlated to those changes. Evidence of specific interactions was found for the amino functionalized particles in most cases, as well as the plain carbon dots in the presence of Ca2+ and humic acid. Nanoparticles remained largely stable in suspension, with some exception at the highest ionic strength considered. DLVO theory did not adequately capture the aggregation behavior of the system. Moreover, cation and/or humic acid adsorption negatively affected the emission intensity of the particles, suggesting limitations to their use in natural water sensing applications. The particular stability shown by the carbon dots results in exposure to organisms in the water column and the possibility of contamination transported to significant distances from their source.

Photonic Molecularly Imprinted Polymer Film for the Detection of Testosterone in Aqueous Samples.

The detection of testosterone in aqueous solutions is a difficult task due to the low concentration levels that are relevant in environmental and physiological samples. Current analytical methods are expensive and/or complex. To address this issue, we fabricated a molecularly imprinted polymer (MIP) photonic film for the detection of testosterone in water. The films were obtained using colloidal crystals as templates for the pore morphology. Monodispersed silica particles with an average diameter 330 nm were used to obtain the colloidal crystal by vertical deposition. A solution of acrylic acid with testosterone as the imprinted template was infiltrated in the colloidal crystal and polymerized via bulk polymerization; the particles were then removed by acid etching and the testosterone eluted by a suitable solvent. The material was characterized by FTIR, swelling experiments and microscopy; MIPs were investigated by equilibrium rebinding, kinetics and reuse experiments. The results showed that the MIPs exhibited selectivity to the template, a 30-min equilibration time and stability after at least six cycles of use and regeneration. After incubation, the reflectance spectra of the films showed a shift of the Bragg diffraction peak that correlated with testosterone concentration in the 5⁻100 ppb range.

Colloidal crystal templated molecular imprinted polymer for the detection of 2-butoxyethanol in water contaminated by hydraulic fracturing.

The authors describe a molecularly imprinted polymer (MIP) that enables detection of 2-butoxyethanol (2BE), a pollutant associated with hydraulic fracturing contamination. Detection is based on a combination of a colloidal crystal templating and a molecular imprinting. The MIPs are shown to display higher binding capacity for 2BE compared to non-imprinted films (NIPs), with imprinting efficiencies of ∼ 2. The tests rely on the optical effects that are displayed by the uniformly ordered porous structure of the material. The reflectance spectra of the polymer films have characteristic Bragg peaks whose location varies with the concentration of 2BE. Peaks undergo longwave red shifts up to 50 nm on exposure of the MIP to 2BE in concentrations in the range from 1 ppb to 100 ppm. This allows for quantitative estimates of the 2BE concentrations present in aqueous solutions. The material is intended for use in the early detection of contamination at hydraulic fracturing sites. Graphical abstract Molecularly imprinted polymers (MIPs) sensor with the sensing ability on reflectance spectra responding to the presence of 2-butoxyethanol (2BE) for early detection of hydraulic fracking contamination.

Enhanced retention of bacteria by TiO2 nanoparticles in saturated porous media.

The simultaneous transport of TiO2 nanoparticles and bacteria Pseudomonas aeruginosa in saturated porous media was investigated. Nanoparticle and bacterium size and surface charge were measured as a function of electrolyte concentration. Sand column breakthrough curves were obtained for single and combined suspensions, at four different ionic strengths. DLVO and classical filtration theories were employed to model the interactions between particles and between particles and sand grains. Attachment of TiO2 to the sand was explained by electrostatic forces and these nanoparticles acted as bonds between the bacteria and the sand, leading to retention. Presence of TiO2 significantly increased the retention of bacteria in the sand bed, but microorganisms were released when nanomaterial influx ceased. The inclusion of nanomaterials in saturated porous media may have implications for the design and operation of sand filters in water treatment.

An experimental study on the aggregation of TiO2 nanoparticles under environmentally relevant conditions.

The eventual future scenario of a release of nanomaterials into the environment makes it necessary to assess the risk involved in their use by studying their behavior in natural waters. NanoTiO2 is one of the most commonly employed nanomaterials. In the present work we studied the aggregation rates, aggregate size and aggregate morphology of NanoTiO2 under the presence of inert electrolytes, divalent cations, and these two combined with natural organic matter, in an effort to provide a comprehensive investigation of the phenomena of interaction of nanomaterials and natural waters and elucidate some of the conflicting information reported in the literature. The stability of nanoparticles could be explained in all cases, at least qualitatively, in terms of classical DLVO interactions (Electrical Double Layer, Van der Waals). Divalent cations were adsorbed to the surface of the nanoparticles, neutralizing the negative charge at pH values greater than the point of zero charge and inducing aggregation. Natural organic matter (NOM) adsorbed to the particles and made their zeta potential more negative, hence stabilizing them by lowering the pH of maximum aggregation. Divalent cations partially neutralized the adsorbed NOM, and at high concentrations aggregation was observed with Ca(2+) but not Mg(2+), suggesting the presence of specific Ca(2+)-NOM bridges. SEM images visually revealed a fractal-like morphology of the aggregates formed under unfavorable conditions.