Lower Critical Solution Temperature Of PNIPAM Research Articles

Preparation and characterization of bovine serum albumin surface-imprinted thermosensitive magnetic polymer microsphere and its application for protein recognition

A novel bovine serum albumin surface-imprinted thermosensitive magnetic composite microsphere was successfully prepared by the surface grafting copolymerization method in the presence of temperature-sensitive monomer N-isopropylacrylamide (NIPAM), functional monomer methacrylic acid (MAA) and cross-linking agent N,N′-methylenebisacrylamide (MBA). The structure and component of the thermosensitive magnetic molecularly imprinted microsphere were investigated by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), vibrating sample magnetometer (VSM) and thermogravimetric analysis (TGA). The results of thermosensitivity, adsorption capacity, selectivity and reusability showed the formation of a thermosensitivity grafting polymer layer P(NIPAM–MAA–MBA) on the surface of Fe3O4@SiO2 and the good adsorption capacity and specific recognition for template protein. When the adsorption temperature was higher than the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM), shape memory effect of imprinted cavities would be more effective. In other words, it was more conducive to capture template molecules under this condition and the imprinting factor would be higher. On the other hand, when the desorption temperature was lower than LCST of PNIPAM, the decrease of shape memory effect between imprinted cavities and template molecules would facilitate the release of template molecules from the imprinted cavities. Based on this property, the adsorption and desorption of template molecules could be regulated by system temperature indirectly which benefited from the existence of thermosensitivity imprinting layer.

Spontaneous Temperature Control Using Reversible Spectroscopic Responses of PNIPAM-coated Gold Nanorods

Abstract Poly(N-isopropylacrylamide) (PNIPAM) was coated on gold nanorods (NRs) without forming aggregates. The PNIPAM-coated NRs (PNIPAM-NRs) formed aggregates at higher temperature than the lower critical solution temperature (LCST) of the PNIPAM layers. The aggregation showed reversible spectroscopic responses under repeated temperature cycles. When a PNIPAM-NR solution was irradiated with near-infrared (near-IR) light, the temperature increased and aggregates formed showing broad and weak surface plasmon bands. The spectroscopic responses in near-IR work as a negative feedback for heating a photoirradiated nanorod solution. The feedback offers spontaneous temperature control of the photoirradiated solution at the LCST of PNIPAM.

Pulsatile Releasing Platform of Nanocontainers Equipped with Thermally Responsive Polymeric Nanovalves

Polymer-gated reservoirs built of an ordered porous anodic aluminum oxide (AAO) platform equipped with poly(N-isopropylacrylamide) (PNIPAM) brushes grafted from the surface using atom transfer radical polymerization were fabricated and studied. The brushes of different lengths were grafted from the AAO surface with pores having diameters of 30, 50, and 80 nm, respectively. Polymer brushes served as thermally responsive valves that can immediately open and close the pores just by crossing the PNIPAM lower critical solution temperature. Calcein was used as a model fluorescent molecule to follow a release process from the platform of the gated nanocontainers. The studied systems enable the burst release of the loaded substance, followed by diffusion-controlled release that depends on the pores’ diameter. These platforms are characterized by a high loading capacity in comparison to polymer films and can be loaded repeatedly. Importantly, the opening/closing of the nanocontainers is reversible, and a pulsatile...

Structural properties of thermoresponsive poly(N-isopropylacrylamide)-poly(ethyleneglycol) microgels

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.

Atomic scale characterization of the conformational dynamics of a thermo-sensitive and a non-thermo-sensitive oligomer using vibrational spectra obtained from molecular dynamics

Molecular dynamics simulations (MD) are used to calculate the vibrational spectra of a thermo-sensitive oligomer, namely, poly(N-isopropylacrylamide) (PNIPAM) and a non-thermo-sensitive oligomer, namely, poly(acrylamide) (PAAM) and characterize the atomic scale conformations. Despite the structural similarity between the two polymers, the response of PNIPAM and PAAM to a thermal stimulus is widely different; a coil-to-globule transition is observed for PNIPAM above a lower critical solution temperature (LCST) of 305 K whereas the same is absent in PAAM. Simulations in both the cases are performed above and below the LCST of PNIPAM, namely at 278 K and 310 K, to evaluate the effect of temperature on the polymer conformations. The vibrational spectra of bonds involving atoms from the polymer backbone and the various side-groups (amide I, amide II, and isopropyl group of PNIPAM and amide I and amide II group of PAAM) of the polymers were analyzed to study the conformational changes in the polymer. The differences in the vibrational spectra are used to understand the dynamics of conformational transitions in the two polymers and identify the changes in the relative interactions between various atoms in the backbone and in the side groups of the polymer with water at two different temperatures, namely at 278 K and 310 K. The systematic trends in the observed peak intensities and frequency shifts at the low, medium, and high frequency end of the spectrum for the various atoms in the two polymers are rationalized on the basis of bond-lengths, local coordination, strength of hydrogen bonding, and neighboring solvation environment. The analysis of the vibrational spectra for amide I and amide II regions of PNIPAM suggests a coil-to-globule transition in going from 278 K to 310 K. The differences are evaluated in terms of the strength, stability, and structure of the hydrogen-bond network between polymer and polymer and between polymer and water. Comparison of the vibrational spectra of isopropyl groups in PNIPAM at 278 K and 310 K suggests dehydration of the isopropyl moieties at 310 K. In the case of PNIPAM, we observe that polymer-water interactions are dominant below the LCST whereas polymer–polymer interactions dominate above the LCST. On the other hand, the vibrational spectra of amide I and amide II group of PAAM, at 278 K and 310 K, do not show any significant difference in terms of the interactions between polymer and polymer and interactions between polymer and water. Analysis of the peak intensities, of the amide II stretching band, observed in the frequency range 3500–3700 cm−1 suggests that the fraction of bonded and non-bonded hydrogen atoms are similar at both 278 K and 310 K. This indicates that the interactions between polymer and polymer and between polymer and water are similar at both the temperatures. The interactions between PAAM and its surrounding environment are found to be unaffected as the temperature is raised from 278 K to 310 K. Comparisons with experimental studies are made where possible. Our study provides useful insights into the nature of various inter-molecular interactions and their role in influencing the atomic scale conformational dynamics in oligomers.

UCST-driven self-assembly and crosslinking of diblock copolymer micelles

The upper critical solution temperature (UCST) behaviour of poly[oligo(ethylene glycol) monomethylether methacrylate] (POEGMA) in 1-octanol was exploited to self-assemble and crosslink micellar aggregates. Four diblock copolymers, POEGMA-block-poly(N-isopropylacrylamide) (POEGMA-b-PNIPAM), POEGMA-b-poly(N,N-diethylacrylamide) (POEGMA-b-PDEAM), and two examples of POEGMA-b-[PNIPAM-co-poly(pentafluorophenyl acrylate)] (POEGMA-b-(PNIPAM-co-PPFPA)), containing different amounts of activated PFP esters, were found to reversibly self-assemble into well-defined spherical micelles upon cooling in 1-octanol, as evidenced by dynamic light scattering (DLS) and electron microscopy. Transition temperatures, measured by turbidity and DLS, were around room temperature and the PNIPAM and PDEAM blocks did not significantly influence the critical temperature of the POEGMA block compared to homo POEGMA. The aggregates exhibited an inverted morphology compared to PNIPAM core–POEGMA shell micelles accessible through a thermally triggered self-assembly in water exploiting the lower critical solution temperature (LCST) behaviour of PNIPAM. Inverted micelles with PNIPAM-co-PPFPA coronae in 1-octanol were shell crosslinked using a diamine and subsequently transferred into water. This procedure yielded cage-like structures with swollen POEGMA cores and crosslinked PNIPAM shells. The shells reversibly collapsed onto the cores when heated above the LCST of PNIPAM, providing particles with novel architecture and the potential to host and release guest molecules by a thermal trigger.

Multiresponsive Clay-Containing Layer-by-Layer Films

We report on polymer/clay layer-by-layer films responsive to multiple stimuli. Temperature- and salt-responsive films were constructed using assembly of poly(N-isopropylacrylamide) (PNIPAM) and montmorillonite clay nanosheets. An additional pH response was achieved by depositing and cross-linking hybrid, dual-network PNIPAM/clay/PNIPAM/poly(methacrylic acid) (PMAA) multilayers. Both types of films remained stable in a wide pH range and were highly swollen. For example, PNIPAM/clay films swelled up to ~14.5 times their dry film thickness in low-salt solutions at 25 °C, as shown by laser scanning confocal microscopy. At temperatures higher than PNIPAM's lower critical solution temperature (LCST) of 32 °C, or in 0.3 M Na(2)SO(4) solutions at room temperature, both PNIPAM/clay and PNIPAM/clay/PNIPAM/PMAA films reversibly deswelled as a result of collapse of PNIPAM chains. Films of both types showed a decrease in permeability to fluorescein-tagged dextrans of various molecular weights. Importantly, film permeability to dextrans was decreased at temperatures above PNIPAM's LCST, and the effect could be reversed by lowering the temperature. Inclusion of PMAA within multilayers provided an additional pH response to film swelling and permeability. Hybrid PNIPAM/clay/PNIPAM/PMAA films showed drastic deswelling at low pH values due to the onset of hydrogen bonding between PNIPAM and PMAA, and the diffusion of 70 kDa dextran through multilayers at acidic pH was completely blocked. These multiresponse features of clay-containing films make them promising candidates for applications in sensing, actuation, and controlled delivery.

Micellization and Micellar Aggregation of Poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) Triblock Terpolymers in Water

Three poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEP−PEO−PNIPAm, “PON”) triblock terpolymers were synthesized using a combination of anionic and reversible addition−fragmentation chain transfer polymerization, and their micellization and micellar aggregation properties in dilute aqueous solution were studied by dynamic light scattering (DLS) and cryo-TEM. The PEP and PEO blocks had molecular weights of 3000 and 25 000, respectively, while the PNIPAm block was varied with molecular weights equal to 4000, 10 000, and 21 000. In dilute aqueous solution the terpolymers formed well-defined micelles with hydrophobic PEP cores surrounded by hydrophilic PEO−PNIPAm coronas at temperatures below the lower critical solution temperature (LCST) of PNIPAm. DLS revealed that the PON micelles prepared by a dialysis technique were significantly smaller than those prepared by a thin-film hydration technique. In either case, at temperatures above the LCST of PNIPAm, the micelles assoc...

Thermosensitive Hydrogels Composed of Hyaluronic Acid and Gelatin as Carriers for the Intravesical Administration of Cisplatin

The aim of this work was to evaluate the use of thermosensitive hydrogels for intravesical cisplatin delivery into the bladder. Poly(N-isopropylacrylamide) (PNIPAM) was grafted onto hyaluronic acid (HA) to synthesize an HPN copolymer, which was further grafted with gelatin to form an HPNG copolymer. A 3% concentration of HPN and HPNG was sufficient to exert a thermosensitive response, whereas a concentration of 8% was needed for PNIPAM to form the hydrogel. The physicochemical and drug delivery properties were examined by scanning electron microscopy (SEM), the lower critical solution temperature (LCST), hydration ratio, and in vitro cisplatin release. The incorporation of HA and gelatin produced a different microstructure compared to the parent PNIPAM hydrogel. Gelatin conjugation increased the fibrous structure in the matrix. The LCSTs of PNIPAM, HPN, and HPNG were 32.3, 32.0, and 30.7°C, respectively. The copolymers showed an eightfold increase in the hydration capacity compared to PNIPAM, with no significant difference in values between HPN and HPNG. The release of cisplatin from an aqueous solution (control) was nearly complete after 8 h, compared to 85, 80, and 52% release from PNIPAM, HPN, and HPNG, respectively. In vivo evaluation of cisplatin levels in bladder tissues was performed following intravesical instillation in rats. When the dwell time was extended to 6 h, PNIPAM showed a sevenfold enhancement in the drug concentration in the bladder wall. HPNG also showed a twofold increase in the drug concentration. The administration of cisplatin by the HPN carrier did not change the drug accumulation compared to the control. Confocal laser scanning microscopic results confirmed the trend of drug absorption from various systems. A histological examination showed no adverse change in the urothelium with HPN or HPNG application. PNIPAM caused partial desquamation of umbrella cells. The thermosensitive hydrogels prepared in this study may be promising carriers for targeted drug delivery to the bladder.

Flower micelles of poly(N-isopropylacrylamide) with azobenzene moieties regularly inserted into the main chain

Hydrophobically modified poly(N-isopropylacrylamide) (PNIPAM) with an azobenzene-containing short segment repeatedly inserted into the main chain was synthesized. The reversible trans–cisphotoisomerization of azobenzene was found to exert little effect on the lower critical solution temperature (LCST) or the cloud point of PNIPAM in aqueous solution. The characterization results of variable-temperature 1H NMR, TEM and DLS indicate the formation of flower micelles in cold water with hydrated PNIPAM. The segregation of azobenzene moieties in the micelle core from PNIPAM chains in micelle corona may explain the absence of an effect of photoisomerization of the chromophore on the LCST of PNIPAM. While the micelle core could react during the photoisomerization by swelling (with azobenzene in the cis form) or contracting (in the trans form) due to a polarity change, PNIPAM undergoes the hydration–dehydration transition according to its LCST. Another interesting finding is that the flower micelles resulting from this type of hydrophobically modified PNIPAM exhibit a much reduced propensity for inter-micellar association and could remain well dispersed at T > LCST of PNIAM, in sharp contrast with flower micelles formed by α,ω-telechelic PNIPAM. The multiple intrachain folding arising from this new PNIPAM structure appears to be the origin of the enhanced dispersion stability. With azobenzene groups stacked together forming the micelle core, the multiple loops for individual PNIPAM chains could reduce the number of inter-micellar bridge chains and restrict the chain entanglements upon dehydration of PNIPAM, which, consequently, reduce the propensity for micellar aggregation.

Surface grafting of thermoresponsive microgel nanoparticles

A monolayer of thermoresponsive microgel nanoparticles, containing poly(N-isopropylacrylamide) (PNIPAM), has been anchored to the surface of silicon wafers, glass slides, polyvinylidene fluoride (PVDF) fibers, and tungsten wires using a “grafting to” approach. The behavior of the synthesized grafted layers is compared with the behavior of the PNIPAM brushes (densely end-grafted layers). The comparison demonstrates that in many aspects the microgel grafted layer is comparable to PNIPAM brushes with respect to its thermoresponsive properties. Indeed, the grafted monolayer swells and collapses reversibly at temperatures below and above the lower critical solution temperature (LCST) of PNIPAM. For the flat silicon substrate, a wettability study of the grafted layer shows an approximately 20° increase in the advancing contact angle of water upon heating above the LCST of PNIPAM. Wettability data obtained for the tungsten wires indicate that the grafted microgel layer retains its ability to undergo morphological changes when exposed to external temperature variations on complex curved surfaces. Therefore, the microgel-grafted layer can be considered as a system capable of competing with the PNIPAM brushes.

Amphiphilic Asymmetric Comb Copolymer with Pendant Pyrene Groups and PNIPAM Side Chains: Synthesis, Photophysical Properties, and Self-Assembly

An amphiphilic asymmetric comb polymer with pendant pyrene groups and poly(N-isopropylacrylamide) (PNIPAM) side chains was synthesized based on click chemistry and reversible addition-fragmentation chain transfer polymerization. Gel permeation chromatography, FTIR, and (1)H NMR results all indicated successful synthesis of a well-defined comb polymer. The photophysical properties and self-assembly of the polymer in solution were studied by UV-vis spectroscopy, fluorescence technique, and transmission electron microscopy. The intensity ratio of the excimer peak (I(E)) to the monomer peak (I(M)) of the comb polymer in THF was used to monitor the formation of inter- or intramolecular excimers. At low polymer concentration, the value of I(E)/I(M) kept unchanged, indicating the formation of intramolecular excimer; at high polymer concentration, the value increased rapidly with concentration because of the formation of intermolecular excimer. The change of the intensity ratio of the first to the third vibronic band (I(1)/I(3)) on the monomer emission of the comb polymer also proved the association of the pendant pyrene groups in THF at high polymer concentration. In aqueous solution, the comb polymer chains self-assembled into vesicles with pyrene groups in the walls and PNIPAM side chains in the coronae. The value of the critical aggregation concentration of the polymer was determined by fluorescence technique. Temperature exerted a significant effect on the size and morphology of the vesicles. At a temperature above the lower critical solution temperature (LCST) of PNIPAM, PNIPAM brushes in the coronae of vesicles collapsed on the surface of the structures forming nanosized domains, and vesicles with smaller size were obtained. Fluorescence quenching experiments indicated that the collapsed PNIPAM chains protected a part of pyrene groups from being quenched by nitromethane at a temperature above the LCST of PNIPAM.

Parallel Effects of Cations on PNIPAM Graft Wettability and PNIPAM Solubility

Stimuli-responsive surfaces grafted with thermoresponsive polymers switch from hydrophilic to hydrophobic thermally, making these surfaces attractive in applications such as in microfluidics devices, as antifouling surfaces, and in cell culture and tissue engineering. These materials exhibit changes in wettability as the polymer undergoes a phase transition above its lower critical solution temperature (LCST). Because the presence of salts affects LCSTs in accordance to the Hofmeister series, salt effects on the wettability of these thermoresponsive surfaces will dramatically impact device performance. Prior studies of such effects have focused on the influence of anions. Detailed studies of the effects of cations have not been carried out. Here, the influence of varying cation identity in a series of mono-, di-, and trivalent sulfate salts on the wettability of a stimuli-responsive grafted surface was investigated by measuring advancing water contact angle (Theta(a)) changes. The cation-induced changes in Theta(a) were correlated with corresponding changes in surface morphology examined by AFM. The results showed that the effects of varying cations on surface wettability are as large as the effects of varying anion identity and concentration (i.e., Theta(a) changes of up to 90 degrees). Parallel studies of the effects of varying the cation identity and concentration for these same cation sulfate salts in solution show that cation variation also has a large effect on the LCST of PNIPAM, the stimuli responsive polymer component of the nanocomposite grafts that were studied. Moreover, analyses of the Theta(a) and LCST data using activity showed that the Theta(a) or LCST versus cation activity/concentration could be readily grouped by charge. Such differences are not seen in similar studies where anion identity, charge, and concentration are changed.

Thermal-responsive hydrogel surface: tunable wettability and adhesion to oil at the water/solid interface

We present a thermal-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel, which can responsively and reversibly control the wettability and adhesion to oil at water/solid interface. The hydrogel surface can switch from a superoleophobic and low adhesive state below the PNIPAM lower critical solution temperature (LCST, ∼32 °C) to an oleophobic and high adhesive state above its LCST. Moreover, the thermal-responsive wettability and adhesion to oil at the water/solid interface can be tailored by designing and constructing ordered micro-structures on the surface. We demonstrated the thermal-responsive changing of the topography on PNIPAM hydrogel surface by atomic force microscopy and proposed a mechanism of inter-molecular to intra-molecular hydrogen binding transitions. We believe the reported material has promising utilizations in micro fluidics, control of bio-relevant adhesion and under-water antifouling materials.

Gating Characteristics of Thermo‐Responsive Membranes with Grafted Linear and Crosslinked Poly(N‐isopropylacrylamide) Gates

AbstractThermo‐responsive porous membranes with grafted linear and crosslinked poly(N‐isopropylacrylamide) (PNIPAM) gates are successfully prepared at temperatures above and below the lower critical solution temperature (LCST) of PNIPAM by using a plasma‐induced grafting polymerization method, and the effects of operation pressure and grafting temperature on the thermo‐responsive gating characteristics of the prepared membranes are investigated systematically. The fluxes of water through the grafted membranes increase simply with increasing the operation pressure no matter whether the environmental temperature is 40 °C or 25 °C. Under high operation pressure (e.g., higher than 0.14 MPa), the grafted linear PNIPAM gates deform to a certain extent, whereas the grafted crosslinked PNIPAM gates do not deform. For both membranes with grafted linear and crosslinked PNIPAM gates, the membranes prepared at 25 °C (below the LCST of PNIPAM) show larger thermo‐responsive gating coefficients than those prepared at 40 °C (above the LCST of PNIPAM), which results from different distributions of grafted PNIPAM gates in the membrane pores. When the PNIPAM gates are grafted at 25 °C, the grafted layer near the membrane surface is much thicker than that inside the membrane pores; on the other hand, when the PNIPAM gates are grafted at 40 °C, the grafted layer is homogeneously formed throughout the whole pore length. Both linear and crosslinked grafted PNIPAM gates in the membrane pores exhibit stable and repeatable thermo‐responsive “open‐close” switch performances under the operation pressure of 0.26 MPa. The results in this study provide valuable guidance for designing, fabricating, and operating thermo‐responsive gating membranes with desirable performances.

Responsive Polymer‐Protein Bioconjugates Prepared by RAFT Polymerization and Copper‐Catalyzed Azide‐Alkyne Click Chemistry

AbstractResponsive polymer‐protein conjugates were synthesized by combination of reversible addition‐fragmentation chain transfer (RAFT) polymerization and a grafting‐to approach by a highly efficient “click chemistry” strategy. A model protein, bovine serum albumin (BSA), was functionalized with an alkyne moiety by reaction of its free cysteine residue with propargyl maleimide. Azido‐terminated poly(N‐isopropylacrylamide) (PNIPAM‐N3) was prepared via RAFT, and polymer‐protein coupling was accomplished by copper‐catalyzed azide‐alkyne cycloaddition. In aqueous solution, the conjugates were observed by dynamic light scattering to be larger than the free polymer or the unmodified protein. Upon heating above the PNIPAM lower critical solution temperature (LCST), the PNIPAM‐BSA bioconjugates formed stable nanoparticles composed of dehydrated polymer and hydrophilic protein.magnified image

Effects of Hofmeister Anions on the LCST of PNIPAM as a Function of Molecular Weight

The effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated as a function of molecular weight and polymer concentration with a temperature gradient microfluidic device under a dark-field microscope. In solutions containing sufficient concentrations of kosmotropic anions, the phase transition of PNIPAM was resolved into two separate steps for higher molecular weight samples. The first step of this two step transition was found to be sensitive to the polymer's molecular weight and solution concentration, while the second step was not. Moreover, the binding of chaotropic anions to the polymer was also influenced by molecular weight. Both sets of results could be explained by the formation of intramolecular and intermolecular hydrogen-bonding between polymer chains. By contrast, the hydrophobic hydration of the isopropyl moieties and polymer backbone was found to be unaffected by either the polymer's molecular weight or solution concentration.

Thermosensitive polymer coated nanomagnetic particles for separation of bio-molecules

Core–shell type thermosensitive magnetic particles were prepared via seed polymerization process. Double layer surfactant coated magnetic particles were first synthesized and then a rich poly-( N-isopropylacrylamide) (PNIPAM) shell layer was attached using double layer magnetic particles as the seeds. Thiodiglycolic acid was used as the primary surfactant and 4-vinylaniline as the secondary surfactant. Carboxylated thermosensitive microspheres were prepared by adding methacrylic acid (MAA) in the polymerization process. PNIPAM has a lower critical solution temperature (LCST) of 32 °C in water, and changes from hydrophilic below the LCST to hydrophobic above it. The size of these thermosensitive polymer coated magnetic particles was measured by using transmission electron microscopy (TEM). TEM results show that magnetic particles were nanosized and the calculated mean diameter of the particles was about 12 nm. Bovine serum albumin (BSA) was selected as a model protein for the separation study. Adsorption of BSA on the thermosensitive magnetic particles was mainly dependent on the properties of the particles’ surface. By increasing the temperature above the LCST of PNIPAM, the particles shrank and were able to adsorb larger quantity of proteins, which was subsequently desorbed at lower temperature. It was believed that carboxylated thermosensitive particles adsorb proteins through hydrogen bonding. When the two extremes of hydrophobic interaction and hydrogen bonding were compared, it was found that more proteins are adsorbed using the later interaction.

Characterization of Cyanine Dye-Labeled Poly(N-isopropylacrylamide) Core/Shell Microgels Using Fluorescence Resonance Energy Transfer

The swelling behavior of the core component in poly(N-isopropylacrylamide) (pNIPAm) core/shell microgels is investigated via fluorescence resonance energy transfer (FRET). Photon correlation spectroscopy (PCS) data show that the cross-linked polymer network in the core is hindered from expanding to its maximum volume in the presence of the added shell. Covalent attachment of the sulfoindocyanine donor/acceptor pair of Cy5/Cy5.5 to the core component allows investigation of the core swelling behavior with FRET. In the absence of the shell, only a small degree of energy transfer is observed in the core when it is swollen to its maximum volume below the pNIPAm lower critical solution temperature (LCST) of 31 °C. Addition of a pNIPAm shell produces a significant degree of FRET under the same solution conditions, illustrating that the polymer chains in the core adopt a constrained conformation relative to their fully swollen state. The volume phase transition behavior of the fluorescently labeled core and core/shell particles was interrogated with PCS and correlates well with FRET analysis over the same temperature range, providing evidence that donor and acceptor molecules are homogeneously distributed throughout the core. The structure−function relationship between the core and shell is explained in terms of a radially distributed cross-linker density gradient created in the core and shell components during the two-stage particle synthesis. Furthermore, these analyses allow for calculation of the actual shell thickness, as the apparent shell thickness obtained from PCS measurements underestimates the actual thickness by an amount equivalent to the compression-induced core radius decrease.

Shell-Restricted Swelling and Core Compression in Poly(N-isopropylacrylamide) Core−Shell Microgels

We report on an investigation of the structural relationship between the core and shell components in stimuli-sensitive core/shell microgels. Photon correlation spectroscopy measurements indicate that the presence of the shell alters the extent of core swelling. Core particles composed of poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-AAc) undergo volume changes as a function of temperature and pH, where deprotonation of the acidic sites results in a volume increase (particle swelling). The addition of a poly(N-isopropylacrylamide) (pNIPAm) shell restricts the core from swelling to its native volume both above and below the pKa of acrylic acid. Furthermore, the pH responsive core undergoes only small temperature-induced volume changes above the characteristic lower critical solution temperature (LCST) of pNIPAm (31 °C) at pH values where the acid groups are fully deprotonated. The addition of a pNIPAm shell results in compression of the core and induces a large volume change at the pNIPAm LCST. Th...