Emergence of Non-Hexagonal Crystal Packing of Deswollen and Deformed Ultra-Soft Microgels under Osmotic Pressure Control.

Highly solvent swollen poly(N-isopropylacrylamide-co-acrylic acid) microgels are synthesized without exogenous crosslinker, making them extremely soft and deformable. These ultralow crosslinked microgels (ULC) are incubated under controlled osmotic pressure to provide a slow (and presumably thermodynamically controlled) approach to higher packing densities. It is found that ULC microgels show stable colloidal packing over a very wide range of osmotic pressures and thus packing densities. Surprising observation of co-existence between hexagonal and square lattices is also made over the lower range of studied osmotic pressures, with microgels apparently changing shape from spheres to cubes in defects or grain boundaries. It is proposed that the unusual packing behavior observed for ULC microgels is due to the extreme softness of these particles, where deswelling causes deformation and shrinking of the particles that result in unique packing states governed by contributions to the entropy at the colloidal system, single particle and ionic levels. These observations further suggest that more detailed experimental and theoretical studies of ultra-soft microgels are required to obtain a complete understanding of their behavior in packed and confined geometries.

Deswelling Induced Morphological Changes in Dual pH and Temperature Responsive Ultra-Low Crosslinked Poly (N-isopropyl acrylamide)-co-Acrylic Acid Microgels.

Poly(N-isopropylacrylamide) microgels prepared without exogenous crosslinker are extremely "soft" as a result of their very low crosslinking density, with network connectivity arising only from the self-crosslinking of pNIPAm chains. As a result of this extreme softness, our group and others have taken interest in using these materials in a variety of bioengineering applications, while also pursuing studies of their fundamental properties. Here, we report deswelling triggered structural changes in poly (N-isopropylacrylamide-co-acrylic acid) (ULC10AAc) microgels prepared by precipitation polymerization. Dynamic light scattering suggests that the deswelling of these particles not only depends on the collapse of the pNIPAm chains but is also influenced by the ionization state of the acrylic acid moieties present in the copolymer. The ULC10AAc microgel behaves like a traditional crosslinked pNIPAm microgel at pH 3.5, showing a sharp decrease in the hydrodynamic diameter around the lower critical solution temperature (LCST) of pNIPAm. As the pH is increased to 4.5 we observe multiple transitions in the deswelling curve, suggesting inhomogeneity in the structure and/or composition of the microgels. At pH 6.5 the microgels cease to be thermoresponsive over the studied temperature range due to increased charge repulsion between the fully deprotonated AAc groups and an increase in gel osmotic pressure due to solvated counterion ingress. Atomic force microscopy images of particles deposited at different temperatures reveal a temperature induced morphological change, with punctate structures forming inside microgels at pH 4.5 and 6.5 and temperature above the gel volume phase transition temperature (VPTT).

Poly (N-Isopropylacrylamide) microgel-based optical devices for humidity sensing.

Optical sensors for environmental humidity have been constructed from poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels. The devices were constructed by first depositing a monolithic layer of pNIPAm-co-AAc microgels on a Au-coated glass substrate followed by the addition of another Au layer on top. The resultant assembly showed visual color, and exhibited multipeak reflectance spectra. We found that the thickness of the device's microgel layer depended on environmental humidity, which corresponded to a change in the device's optical properties. Specifically, at low humidity the microgel layer was collapsed, while it absorbed water from the atmosphere (and swelled) as the humidity increased. Additionally, we investigated how the deposition of the hygroscopic polymer poly (diallyldimethylammonium chloride) (pDADMAC) onto the microgel layer (prior to final Au layer deposition) influenced the devices humidity response. We found that the devices were more sensitive to humidity as the number of pDADMAC layers in the device increased. Finally, we evaluated the device performance at various temperatures, and found that the sensitivity was enhanced at low temperature, although the response was more linear at elevated temperature.

Photothermally Induced Optical Property Changes of Poly(N-isopropylacrylamide) Microgel-Based Etalons.

Poly(N-isopropylacrylamide) microgel-based optical devices were designed such that they can be stimulated to change their optical properties in response to light produced by a light-emitting diode (LED). The devices were fabricated by sandwiching the synthesized microgels between two Cr/Au layers all supported on a glass coverslip with gold nanoparticles (AuNPs) deposited. Here, we found that these devices can be stimulated to change their optical properties when exposed to green LED light, which excites the AuNPs and increases the local temperature, causing the thermoresponsive microgels to decrease in diameter, resulting in a change in the devices' optical properties. We also found that the sensitivity of the devices to light was more pronounced as the environmental temperature approached the lower critical solution temperature (LCST) for the microgels, although the sensitivity of the devices to light exposure dropped off dramatically as the environmental temperature was increased above the LCST. This was a direct result of the microgels already being in their collapsed state and therefore unable to decrease in diameter any further due to light exposure. Finally, we found that the sensitivity of the devices to light exposure increased with increasing number of AuNP layers in the devices. We anticipate that these devices could be used for drug delivery applications; by using light to stimulate microgel collapse, the microgel-based devices can be stimulated to release small molecules on demand.

A novel label-free colorimetric assay for DNA concentration in solution.

Optical devices were fabricated by sandwiching a "monolithic" poly(N-isopropylacrylamide-co-N-(3-aminopropyl) methacrylamide hydrochloride) (pNIPAm-co-APMAH) microgel layer between two semitransparent Au layers. These devices, referred to as etalons, exhibit characteristic multipeak reflectance spectra, and the position of the peaks in the spectra primarily depends on the distance between the Au surfaces mediated by the microgel layer thickness. Here, we show that the positively charged microgel layer can collapse in the presence of negatively charged single stranded DNA (ssDNA) due to ssDNA induced microgel crosslinking. The collapse results in a change in the etalon's optical properties, which can be used to detect target DNA in a complex mixture.

Polymer-based devices for the label-free detection of DNA in solution: low DNA concentrations yield large signals.

Poly (N-isopropylacrylamide-co-N-(3-aminopropyl) methacrylamide hydrochloride) microgel-based optical devices (etalons) have been shown to change their optical properties in the presence of single-stranded DNA. We hypothesize that this is due to the negatively charged DNA penetrating through the Au overlayer of the etalon, resulting in cross-linking and collapse of the positively charged microgels. We have shown that this technology is capable of detecting micromolar concentrations of target DNA in solutions containing two and four base pair mismatch sequences without the use of labels. Furthermore, the device's response increases as the concentration of DNA decreases, which is unique for sensing strategies. We point out that coupling this transduction mechanism to DNA amplification strategies could result in extremely low detection limits.

Poly (N-isopropylacrylamide) microgel-based optical devices for sensing and biosensing.

Responsive polymer-based materials have found numerous applications due to their ease of synthesis and the variety of stimuli that they can be made responsive to. In this review, we highlight the group's efforts utilizing thermoresponsive poly (N-isopropylacrylamide) (pNIPAm) microgel-based optical devices for various sensing and biosensing applications.

Responsive polymers for biosensing and protein delivery.

In this feature article, we review some recent advances in the field of materials chemistry for biosensing, and protein/drug delivery. Our recent work on the development of responsive polymer-based platforms for biosensing and drug delivery will also be highlighted. This feature article is meant to outline the breadth of the utility of polymer-based materials for select applications, as well as their enormous potential impact on future technologies.

Self-assembly of a set of hydrophilic-solvophobic-hydrophobic coil-rod-coil molecules based on perylene diimide.

The self-assembly and the resulting morphology of a set of asymmetrically substituted perylene diimide is discussed. We synthesized perylene diimides with hydrophilic Jeffamine® (PEO/PPO co-oligomer) attached to the imide nitrogen on one side and (hydrophobic) alkyl chains of different lengths on the other. Although studies on asymmetrically substituted perylene diimides have been reported by various authors, both side chains in this work are linear and we discuss the effect of the length of the hydrophobic alkyl side chain on the self-assembly in water and aqueous mixtures and the resulting morphology. We find that self-assembly occurs in water in a range of concentrations (from 10(-7) to 10(-4) M), and the mode of packing of the H-stacked molecules changes with concentration for the propyl substitution. Although self-assembly does not occur in non-aqueous solvents such as acetone, it does upon addition of water. While other authors observed nano-belts and nano-fibers with asymmetrically substituted perylene diimides, the morphology of drop-cast films in our work consists of folded sheets and hollow tubes, of a few microns in diameter. Such folding in a non-chiral system such as the present case is attributed to predominant asymmetry of interactions between the molecules in the three directions, as we discussed in our previous work on an hydrogen bonded system (S. Khanna, M. K. Khan and P. Sundararajan, Langmuir, 2009, 25, 13183-13193).

Unexpected cononsolvency behavior of poly (N-isopropylacrylamide)-based microgels.

Poly (N-isopropylacrylamide) (pNIPAm)-based microgels undergo a transition from fully water swollen (solvated) to deswollen (desolvated) as the temperature of the water they are dissolved in is increased >32 °C. In this submission, we examine how the temperature of this transition, i.e., the volume phase transition temperature (VPTT), depends on the concentration of methanol (MeOH) in water the microgels are dissolved in. To accomplish this, pNIPAm-based etalons are utilized, and it is shown that the VPTT for the microgels is much less than that previously observed for linear pNIPAm and pNIPAm-based microgels. Furthermore, and most interestingly, it is determined that the microgels can collapse in solutions containing high MeOH (>∼65% MeOH) concentration. This is in contrast to previous studies, which show that no VPTT is observed for pNIPAm in aqueous solutions containing >∼65% MeOH.

Microgel-based etalon coated quartz crystal microbalances for detecting solution pH: The effect of Au overlayer thickness.

Poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels were "painted" on the Au electrode of a quartz crystal microbalance (QCM). Another Au layer (overlayer) was subsequently deposited on the microgel layer. This structure is known as a microgel-based etalon. These devices have been shown to exhibit optical properties (i.e., color) that depend on solution pH and temperature, among other things. Previously, we measured QCM frequency shifts that are a result of solution pH changes; the frequency shifts are a direct result of the pH dependent solvation state of the microgels that make up the etalon. In fact, the shifts observed for the etalons were much greater in magnitude than just a microgel layer immobilized on the QCM crystal without the Au overlayer. We reasoned that the Au overlayer lead to an enhancement of the observed frequency change due to its mass. In this submission we investigate how the Au overlayer thickness (mass) affects the observed sensitivity to solution pH. We found that the change in QCM resonant frequency depended dramatically on the mass of the Au overlayer.

Responsive polymers for analytical applications: a review.

Stimuli-responsive polymers are capable of translating changes in their local environment to changes in their chemical and/or physical properties. This ability allows stimuli-responsive polymers to be used for a wide range of applications. In this review, we highlight the analytical applications of stimuli-responsive polymers that have been published over the past few years with a focus on their applications in sensing/biosensing and separations. From this review, we hope to make clear that while the history of using stimuli-responsive polymers for analytical applications is rich, there are still a number of directions to explore and exciting advancements to be made in this flourishing field of research.

Label-free detection of low protein concentration in solution using a novel colorimetric assay.

Dual pH and temperature sensitive microgel-based etalons were fabricated by sandwiching a "monolithic" microgel layer between two semitransparent, Au layers. The devices exhibit visual color and multipeak reflectance spectra, both of which primarily depend on the distance between the Au surfaces mediated by the microgel diameter. We found that a polycationic polyelectrolyte can penetrate through the Au overlayer to interact with negatively charged microgel confined between Au overlayers. In this submission we report that biotinylated polycationic polymer can penetrate through the Au overlayer of a poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel-based etalon and cause the microgel layer to collapse. The collapse results in a shift in the spectral peaks of the reflectance spectra. We found that the extent of peak shift depends on the amount of biotinylated polycation added to the etalon, which can subsequently be used to determine the concentration of streptavidin in solution at pM concentrations.

Polyelectrolyte mediated intra and intermolecular crosslinking in microgel-based etalons for sensing protein concentration in solution.

Biotin modified polycationic polymers are capable of penetrating the Au overlayer of poly(N-isopropylacrylamide)-co-acrylic acid microgel-based etalons. Once penetrated, the polycations crosslink the polyanionic microgels, causing them to collapse, resulting in a concomitant blue shift of the spectral peaks in the reflectance spectrum. We show that the magnitude of the blue shift depends on the concentration of the biotinylated polycation solution exposed to the etalon. This behavior is subsequently used for biosensing applications.

Tubular or subsurface morphology of octabutoxyphthalocyanine upon self-assembly in polymer matrices: effect of the casting solvent.

Spontaneous phase-separated, controlled aggregate structures of photo- and electroactive molecules in polymer matrices are of interest for device fabrication. We show that the self-assembly of octabutoxyphthalocyanine (Pc) in polymer matrices leads to tubular morphology of Pc when the film is prepared with tetrachloroethane (TCE) and subsurface droplet morphology with chloroform. The same morphology is seen with both bisphenol A polycarbonate (BPAPC) and poly(methyl methacrylate) (PMMA) as the matrix. The subsurface morphology results from the rapid association of Pc in the polymer matrix, as the film forms. With the tubular morphology in the films prepared with TCE, percolation threshold is reached with a concentration of Pc as low as 3% (wt) in the polymer. Such phase-separated self-assembly occurs, without any annealing of the films. Even in the absence of the polymer, Pc crystallized from TCE also shows tubular morphology, whereas it exhibits a columnar morphology with chloroform. X-ray diffraction of Pc crystallized from either solvent shows the columnar stacking of the Pc molecules. However, the morphology is tubular when TCE is used. We attribute the difference in the morphology to the higher viscosity of TCE and the diffusion-limited growth, which causes the tubular morphology, whereas the instantaneous self-assembly in less-viscous chloroform leads to droplets. The solvent effect observed here could be used to tailor the morphology of such photoconductive molecules in polymer matrices.

Deterioration of water quality of Surma river.

Surma River is polluted day by day by human activities, poor structured sewerage and drainage system, discharging industrial and household wastes. The charas (natural channels) are responsible for surface runoff conveyance from its urban catchments to the receiving Surma River. Water samples have been collected from a part of Surma River along different points and analyzed for various water quality parameters during dry and monsoon periods. Effects of industrial wastes, municipal sewage, and agricultural runoff on river water quality have been investigated. The study was conducted within the Chattak to Sunamganj portion of Surma River, which is significant due to the presence of two major industries--a paper mill and a cement factory. The other significant feature is the conveyors that travel from India to Chattak. The river was found to be highly turbid in the monsoon season. But BOD and fecal coliform concentration was found higher in the dry season. The water was found slightly acidic. The mean values of parameters were Conductivity 84-805 micros; DO: dry-5.52 mg/l, monsoon-5.72 mg/l; BOD: dry-1mg/l, monsoon-0.878 mg/l; Total Solid: dry-149.4 mg/l, monsoon-145.7 mg/l. In this study, an effort has been taken to investigate the status of concentration of phosphate (PO(-4)) and ammonia-nitrogen (NH4-N) at four entrance points of Malnichara to the city, Guali chara, Gaviar khal and Bolramer khal. Data has been collected from March-April and September-October of 2004. Concentrations have been measured using UV Spectrophotometer. Although the phosphate concentration has been found within the limit set by DOE for fishing, irrigation and recreational purposes, however ammonia-nitrogen has been found to exceed the limit.