Robust multilayers can be formed on solid surfaces, and subsequently destroyed by changing the environmental conditions, by the layer-by-layer sequential assembly of monomolecular films of a polyacid and polybase from aqueous solution. Interlayer hydrogen bonding produces stable multilayers up to the point where altered pH or other environmental stimulus introduces an unacceptably large electrical charge within them. This is demonstrated for the polyacids poly(acrylic acid), PAA, and poly(methacrylic acid), PMAA, and for the polybases poly(vinylpyrrolidone), PVPON, and poly(ethylene oxide), PEO, in D2O. The adsorption was quantified by Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). The ratio between suppressed ionization of the carboxylic groups within the film and their ionization in solution, as directly measured by FTIR-ATR, was used to estimate the fraction of hydrogen-bonded carboxylic groups; this was ∼0.5 in PVPON/PMAA but only ∼0.1 in the PEO/PMAA system, though the dielectric environments appeared to be similar. The critical pH, at which these hydrogen-bonded layers disintegrated, was controlled by a balance of internal ionization and a fraction of carboxylic groups that formed hydrogen bonds with either PVPON or PEO. The critical point was at pH = 6.9 for the PVPON/PMAA films (relatively strong hydrogen bonding), but pH = 4.6 for the PEO/PMAA films (in which a smaller fraction of segments participated in hydrogen bonding). It was even less, pH = 3.6, in the PEO/PAA system, which contained a larger proportion of ionized groups at a given pH owing to the higher acidity of PAA. As a second avenue to control the stability of these multilayer films, ionic strength was varied systematically. In the PEO/PMAA system, the multilayers were stable up to pH = 4.6 in the environment of 10 mM ions (this ionic strength resulted from the buffer solution to control pH), but the multilayers were stable up to higher pH, pH = 5.15, when 0.4 M NaCl was added. The reason is that a higher ionic strength reduced the intensity of electrostatic repulsion between a given number of ionized groups within the multilayer assembly. A slight weakening of stability with decrease of molecular weight was observed (these experiments concerned the PVPON system) as expected from fewer hydrogen bonds per molecule. Finally, experiments with added rhodamine 6G dye showed that dye or drug molecules can be incorporated into such multilayers and then released as needed at preselected conditionsa feature that may be used in drug release devices.

Hydrogen-bonded multilayers of a neutral polymer (poly(N-vinylpyrrolidone), PVPON) with poly-(methacrylic acid) (PMAA) were used as templates to introduce cross-links between PMAA layers using carbodiimide chemistry and ethylenediamine as a cross-linking agent. Upon exposure to high pH, PVPON is completely released from the hydrogel matrix, producing surface-attached PMAA hydrogels. When such hydrogels are deposited at the surface of silica particles, and the particle core is subsequently dissolved, hollow one-component hydrogel capsules are produced. PMAA hydrogel films and hollow capsules underwent reversible, large (factors of 2 or 3) changes in size in response to changes in solution pH and/or ionic strength. The capsules were used for entrapment and storage of macromolecules such as 500 kDa FITC-dextran by "locking" the capsule wall with an electrostatically associating polycation, poly-N-ethyl-4-vinylpyridinium bromide (QPVP). The release of the encapsulated macromolecules was achieved under high salt concentrations (0.6 M NaCl) when QPVP dissociated from the capsule wall. The pH and salt response of these PMAA hydrogel capsules and the polycation-controlled encapsulation of macromolecules hold promise for applications in biomedicine and biotechnology.

Recent years have seen increasing interest in the construction of nanoscopically layered materials involving aqueous‐based sequential assembly of polymers on solid substrates. In the booming research area of layer‐by‐layer (LbL) assembly of oppositely charged polymers, self‐assembly driven by hydrogen bond formation emerges as a powerful technique. Hydrogen‐bonded (HB) LbL materials open new opportunities for LbL films, which are more difficult to produce than their electrostatically assembled counterparts. Specifically, the new properties associated with HB assembly include: 1) the ease of producing films responsive to environmental pH at mild pH values, 2) numerous possibilities for converting HB films into single‐ or two‐component ultrathin hydrogel materials, and 3) the inclusion of polymers with low glass transition temperatures (e.g., poly(ethylene oxide)) within ultrathin films. These properties can lead to new applications for HB LbL films, such as pH‐ and/or temperature‐responsive drug delivery systems, materials with tunable mechanical properties, release films dissolvable under physiological conditions, and proton‐exchange membranes for fuel cells. In this report, we discuss the recent developments in the synthesis of LbL materials based on HB assembly, the study of their structure–property relationships, and the prospective applications of HB LbL constructs in biotechnology and biomedicine.

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We report on association of tannic acid (TA) with neutral or charged polymers in solution and at surfaces and contrast hydrogen-bonded and electrostatically associated polymer/TA complexes and TA/polymer layer-by-layer (LbL) films as per their stability in the pH scale. The neutral polymers used for hydrogen bonding with TA were poly(N-vinylcaprolactam) (PVCL), poly(N-vinylpyrrolidone) (PVPON), poly(ethylene oxide) (PEO), or poly(N-isopropylacrylamide) (PNIPAM), and the polymer used to explore electrostatic binding with TA was 90% quaternized poly(4-vinylpyridine) (Q90). Association of TA with polymers in solution was explored by measuring the turbidity of solutions. At surfaces, LbL film deposition and pH stability were followed by phasemodulated ellipsometry and in-situ Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR). While electrostatically stabilized films of TA with Q90 could not be deposited at low pH values (pH ) 2), hydrogen-bonded films of TA with PVCL, PVPON, PEO, and PNIPAM could be constructed at pH 2 and did not dissolve until a critical dissolution pH of 9.5, 9, 8.5, and 8 (measured in 0.01 M buffer solutions) for PVCL/TA, PVPON/TA, PEO/TA, and PNIPAM/TA, respectively. In addition, all multilayers could be also constructed at pH 7.5 in solutions with low ionic strength. The high pH stability of these systems as compared to multilayers of the same neutral polymers with polyacrylic (PAA) or polymethacrylic (PMAA) acids is due to higher pK a value of TA of ∼8.5 as estimated in this paper. We also show that multilayers of TA with a copolymer of N-vinylpyrrolidone containing 20 mol % of primary amino groups, PVPON-NH 2 -20, were highly stable in a wide pH range from 1.3 to 11.7 because of combined stabilization through both electrostatic and hydrogenbonding interactions. For all systems, pH windows for deposition and stability of LbL films at surfaces correlated with the phase behavior of TA complexes in solution. High pH stability of hydrogen-bonded films of TA as well as the capability of tuning the critical pH value for film dissolution in the range close to physiological pH values makes such multilayer systems promising candidates for biomedical applications.

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We summarize existing knowledge and present some new results on the relationship between polyelectrolyte multilayer (PEM) growth and phase behavior of polyelectrolyte complexes (PECs) in solution. Detailed understanding of competition between surface and solution as applied to PEMs requires selective labeling of polymers and/or the application of techniques that allow chemically specific monitoring of film components, such as in-situ ATR-FTIR spectroscopy. The trends observed with multilayers directly follow from the properties of PECs in solution. Effects of a number of parameters, such as the type of interacting polyelectrolyte chains, the ratio of their lengths, and ionic strength and pH of deposition solutions, on the likelihood of the multilayer stability or erosion are considered. Polycations with high density of primary amino groups and polyanions with SO 3or SO 4groups show the strongest interpolyelectrolyte binding, resulting in inhibited chain exchange within PECs and/or PEMs. With weakly bound polyelectrolyte pairsspolycations containing quaternary ammonium groups and carboxylate polyanionsswater-soluble PECs are easily formed, often resulting in erosion of PEMs. For the latter case, we report a full phase diagram of polycation/polyanion/NaCl aqueous mixtures and show how ionic strength can be used to tune the deposition of PEMs at surfaces. In addition, we present that the phase behavior of PECs in solution also controls pH response of PEMs at surfaces. Better knowledge of the relationships between the PEMs and PECs allows rational prediction and control of deposition of a wide range of weak or permanently charged polyelectrolytes at surfaces.

We quantitatively studied, using X-ray photoelectron spectroscopy (XPS), oxidation of substrate-immobilized silver nanoparticles (Ag NPs) in a wide range of conditions, including exposure to ambient air and controlled ozone environment under UV irradiation, and we correlated the degree of silver oxidation with surface-enhanced Raman scattering (SERS) enhancement factors (EFs). The SERS activity of pristine and oxidized Ag NPs was assessed by use of trans-1,2-bis(4-pyridyl)ethylene (BPE) and sodium thiocynate as model analytes at the excitation wavelength of 532 nm. Our study showed that the exposure of Ag NPs to parts per million (ppm) level concentrations of ozone led to the formation of Ag(2)O and orders of magnitude reduction in SERS EFs. Such an adverse effect was also notable upon exposure of Ag NPs under ambient conditions where ozone existed at parts per billion (ppb) level. The correlated XPS and SERS studies suggested that formation of just a submonolayer of Ag(2)O was sufficient to decrease markedly the SERS EF of Ag NPs. In addition, studies of changes in plasmon absorption bands pointed to the chemical enhancement as a major reason for deterioration of SERS signals when substrates were pre-exposed to ambient air, and to a combination of changes in chemical and electromagnetic enhancements in the case of substrate pre-exposure to elevated ozone concentrations. Finally, we also found UV irradiation and ozone had a synergistic effect on silver oxidation and thus a detrimental effect on SERS enhancement of Ag NPs and that such oxidation effects were analyte-dependent, as a result of inherent differences in chemical enhancements and molecular binding affinities for various analytes.