Nanoprecipitation-Enhanced Sensitivity in Enzymatic Nanofluidic Biosensors

Single nanochannels show unique transport properties due to nanoconfinement. It has been demonstrated that at submillimolar concentrations of divalent cations, a nanoprecipitation reaction can occur in nanochannels. Although several reports have shown, described, and modeled the nanoprecipitation process, no further advantages have been taken from this phenomenon. Here, we show that the nanoprecipitation reaction can be incorporated into enzyme-modified nanochannels to enhance the performance of small-molecule biosensors via in situ amplification reactions. Contrary to the working principle of previous enzymatic nanofluidic biosensors, the nanofluidic biosensor described in this work operates on the basis of concerted functions: pH-shifting enzymatic activity and nanoprecipitation. We show that the simple addition of Ca2+ and Mg2+ ions in the working analyte solution containing urea can lower the detection limit from the nanometer to the subnanometer regime and modulate the dynamic linear range. This approach enables the implementation of more sensitive real-time nanofluidic detection methods without increasing the complexity of the nanofluidic platform or the sensing approach. We envision that the integration of concerted functions in nanofluidic architectures will play a key role in expanding the use of these nanoscale devices for analytical purposes.

Polyelectrolyte-multivalent molecule complexes: physicochemical properties and applications

The complexation of polyelectrolytes with other oppositely charged structures gives rise to a great variety of functional materials with potential applications in a wide spectrum of technological fields. Depending on the assembly conditions, polyelectrolyte complexes can acquire different macroscopic configurations such as dense precipitates, nanosized colloids and liquid coacervates. In the past 50 years, much progress has been achieved to understand the principles behind the phase separation induced by the interaction of two oppositely charged polyelectrolytes in aqueous solutions, especially for symmetric systems (systems in which both polyions have similar molecular weight and concentration). However, in recent years, the complexation of polyelectrolytes with alternative building blocks such as small charged molecules (multivalent inorganic species, oligopeptides, and oligoamines, among others) has gained attention in different areas. In this review, we discuss the physicochemical characteristics of the complexes formed by polyelectrolytes and multivalent small molecules, putting a special emphasis on their similarities with the well-known polycation-polyanion complexes. In addition, we analyze the potential of these complexes to act as versatile functional platforms in various technological fields, such as biomedicine and advanced materials engineering.

Highly sensitive acetylcholine biosensing via chemical amplification of enzymatic processes in nanochannels

Acetylcholinesterase-modified nanochannels are proposed as reliable and reproducible nanofluidic sensors for highly sensitive detection of acetylcholine. The operation mechanism relies on the use of weak polyelectrolytes as "chemical amplifiers" that adjust/reconfigure the nanochannel surface charge in the presence of local pH changes induced by the enzymatic reaction. Experimental results show that the presence of acetylcholine can be transduced into measurable ionic signals with a low limit of detection.

Borate-driven ionic rectifiers based on sugar-bearing single nanochannels

Recently, much scientific effort has been centered on the control of the ionic transport properties of solid state nanochannels and the rational design and integration of chemical systems to induce changes in the ionic transport by means of interactions with selected target molecules. Here, we report the fabrication of a novel nanofluidic device based on solid-state nanochannels, which combines silane chemistry with both track-etched and atomic layer deposition (ALD) technologies. Nanodevice construction involves the coating of bullet-shaped single-pore nanochannels with silica (SiO₂) by ALD and subsequent surface modification by reaction between silanol groups exposed on pore walls and N-(3-triethoxysilylpropyl)-gluconamide, in order to create a gluconamide-decorated nanochannel surface. The formation of a boroester derivative resulting from the selective reaction of borate with the appended saccharides leads to important changes in the surface charge density and, concomitantly, in the iontronic properties of the nanochannel. Furthermore, we propose a binding model to rationalize the specific interaction saccharide-borate in the surface. Besides, this unique nanodevice exhibits a highly selective and reversible response towards borate/fructose exposure. On the basis of the surface charge variation resulting from borate binding, the nanochannel can reversibly switch between "ON" and "OFF" states in the presence of borate and fructose, respectively. In addition, this work describes the first report of the functionalization of PET/SiO₂ nanochannels by the ALD technique. We believe that this work provides a promising framework for the development of new nanochannel-based platforms suitable for multiple applications, such as water quality monitoring or directed molecular transport and separation.

Enzymes hosted in redox-active ionically cross-linked polyelectrolyte networks enable more efficient biofuel cells

Redox mediators are pivotal players in the electron transfer process between enzymes and electrodes. We present an alternative approach for redox mediation based on branched polyethyleneimine (BPEI) modified with an osmium complex. This redox polyelectrolyte is crosslinked with phosphate to produce colloidal particles with a diameter of ca. 1 μm, which, combined with glucose oxidase (GOx), can form electroactive assemblies through either layer by layer assembly (LbL) or one-pot drop-casting (OPDC). The addition of NaCl to these colloidal systems induces the formation of films that otherwise poorly grow, presenting an outstanding catalytic current. The system was tested as a bioanode delivering a power output of 148 μW per nmol of mediator. These results are explained in terms of the interactions of the ions with the polyelectrolyte and represent a new route for the development of bioelectrochemical devices involving redox mediators and enzymes.

High-sensitivity detection of dopamine by biomimetic nanofluidic diodes derivatized with poly(3-aminobenzylamine)

During the last few years, much scientific effort has been devoted to the control of ionic transport properties of solid state nanochannels and the rational integration of chemical systems to induce changes in the ionic transport by interaction with selected target molecules for (bio)sensing purposes. In this work, we present the construction and functional evaluation of a highly sensitive dopamine-responsive iontronic device by functionalization of bullet-shaped track-etched single nanochannels in PET membranes with poly(3-aminobenzylamine) (PABA). The variety of basic groups in this amino-appended polyaniline derivative allows programming of the ion selectivity of the channel by setting the pH conditions. On the other hand, the amino-pendant groups of PABA become suitable binding sites for the selective chemical reaction with dopamine, leading to a change in the nanochannel surface charge. Thus, the exposure of the PABA-modified nanochannel to dopamine solutions selectively produces changes in the iontronic response. By rationally selecting the conditions for both the dopamine binding step and the iontronic reading, we obtained a correlation between the rectification efficiency and dopamine concentration down to the nanomolar range, which was also successfully interpreted in terms of a simple binding model.

Redox-active polyamine-salt aggregates as multistimuli-responsive soft nanoparticles

Polyamine-salt aggregates have become promising soft materials in nanotechnology due to their easy preparation process and pH-responsiveness. Here, we report the use of hexacyanoferrate(ii) and hexacyanoferrate(iii) as electroactive crosslinking agents for the formation of nanometer-sized redox-active polyamine-redox-salt aggregates (rPSA) in bulk suspension. This nanoplatform can be selectively assembled or disassembled under different stimuli such as redox environment, pH and ionic strength. By changing the charge of the building blocks, external triggers allow switching the system between two phase states: aggregate-free solution or colloidal rPSA dispersion. The stimuli-activated modulation of the assembly/disassembly processes opens a path to exploit rPSA in technologies based on smart nanomaterials.

Electrochemically addressable nanofluidic devices based on PET nanochannels modified with electropolymerized poly-o-aminophenol films

Nanofluidic field-effect transistors (nFETs) have attracted attention from the scientific community due to their remarkable level of control over ionic transport. Particularly, the combination of nanofluidic systems and electroactive polymers has demonstrated to be an interesting approach to achieve an electrochemically addressable device. In this work, the development of nFETs based on the integration of electropolymerized poly-o-aminophenol (POAP) films into track-etched nanochannels is proposed. The electropolymerization of POAP on the tip side of Au-sputtered asymmetric PET nanochannels not only allowed having a programmable tip diameter but also offered a precise and very rapid control of ionic transport by switching an external bias voltage. Moreover, the system exhibited a reversible behaviour between non-selective and anion-selective states. We believe that this work provides new tools and concepts to design and build high-performance nanofluidic field-effect transistors working under electrochemically controlled conditions.

A study of the complex interaction between poly allylamine hydrochloride and negatively charged poly(N-isopropylacrylamide-co-methacrylic acid) microgels

Negatively charged poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAm-co-MAA)) microgels undergo size changes in response to changes in temperature and pH. Complexation of these microgels with positively charged polyelectrolytes can greatly affect their physical properties and their capacity for encapsulating active molecules. Here we study the interaction between (P(NIPAm-co-MAA)) microgels and a model positively charged polyelectrolyte, poly allylamine hydrochloride (PAH), with different molecular weights. Experiments were conducted at temperatures below and above the lower critical solution temperature (LCST) of the microgel (30-32 °C), at 20 and 40 °C, respectively, and for PAH at molecular weights of 15, 50, and 140 kDa. Below the LCST, dynamic light scattering and zeta potential measurements with molecular simulation show that for the 15 kDa PAH there is preferential accumulation of PAH inside the microgel, whereas for the higher molecular weight PAH, the polyelectrolyte deposits mainly on the microgel surface. Above the LCST, PAH is preferentially located on the surface of the microgels for all molecular weights studied as a result of charge segregation in the hydrogels. Confocal scanning laser microscopy and flow cytometry were used to quantify rhodamine labelled PAH associated with the microgel. Isothermal titration calorimetry studies give insight into the thermodynamics of the interaction of PAH with the hydrogels, and how this interaction is affected by the molecular weight of PAH. Finally, microgels with encapsulated doxorubicin were exposed to PAH, revealing that the drug is displaced from the microgel by the PAH chains.

Multitasking polyamine/ferrioxalate nano-sized assemblies: thermo-, photo-, and redox-responsive soft materials made easy

Responsive nanomaterials have emerged as key components in materials sciences. Herein, we report the one-step preparation of multi-stimuli responsive polyamine-salt aggregates (PSA) by ionically crosslinking polyethylenimine with potassium ferrioxalate (FeOx). The unique properties of FeOx enables a novel class of soft nanomaterial that disassembles by exposure to light, reducing environments and temperature.

Continuous assembly of supramolecular polyamine-phosphate networks on surfaces: preparation and permeability properties of nanofilms

Supramolecular self-assembly of molecular building blocks represents a powerful "nanoarchitectonic" tool to create new functional materials with molecular-level feature control. Here, we propose a simple method to create tunable phosphate/polyamine-based films on surfaces by successive assembly of poly(allylamine hydrochloride) (PAH)/phosphate anions (Pi) supramolecular networks. The growth of the films showed a great linearity and regularity with the number of steps. The coating thickness can be easily modulated by the bulk concentration of PAH and the deposition cycles. The PAH/Pi networks showed chemical stability between pH 4 and 10. The transport properties of the surface assemblies formed from different deposition cycles were evaluated electrochemically by using different redox probes in aqueous solution. The results revealed that either highly permeable films or efficient anion transport selectivity can be created by simply varying the concentration of PAH. This experimental evidence indicates that this new strategy of supramolecular self-assembly can be useful for the rational construction of single polyelectrolyte nanoarchitectures with multiple functionalities.

Practical use of polymer brushes in sustainable energy applications: interfacial nanoarchitectonics for high-efficiency devices

The discovery and development of novel approaches, materials and manufacturing processes in the field of energy are compelling increasing recognition as a major challenge for contemporary societies. The performance and lifetime of energy devices are critically dependent on nanoscale interfacial phenomena. From the viewpoint of materials design, the improvement of current technologies inevitably relies on gaining control over the complex interface between dissimilar materials. In this sense, interfacial nanoarchitectonics with polymer brushes has seen growing interest due to its potential to overcome many of the limitations of energy storage and conversion devices. Polymer brushes offer a broad variety of resources to manipulate interfacial properties and gain molecular control over the synergistic combination of materials. Many recent examples show that the rational integration of polymer brushes in hybrid nanoarchitectures greatly improves the performance of energy devices in terms of power density, lifetime and stability. Seen in this light, polymer brushes provide a new perspective from which to consider the development of hybrid materials and devices with improved functionalities. The aim of this review is therefore to focus on what polymer brush-based solutions can offer and to show how the practical use of surface-grafted polymer layers can improve the performance and efficiency of fuel cells, lithium-ion batteries, organic radical batteries, supercapacitors, photoelectrochemical cells and photovoltaic devices.

Self-assembled phosphate-polyamine networks as biocompatible supramolecular platforms to modulate cell adhesion

The modulation of cell adhesion via biologically inspired materials plays a key role in the development of realistic platforms to envisage not only mechanistic descriptions of many physiological and pathological processes but also new biointerfacial designs compatible with the requirements of biomedical devices. In this work, we show that the cell adhesion and proliferation of three different cell lines can be easily manipulated by using a novel biologically inspired supramolecular coating generated via dip coating of the working substrates in an aqueous solution of polyallylamine in the presence of phosphate anions-a simple one-step modification procedure. Our results reveal that selective cell adhesion can be controlled by varying the deposition time of the coating. Cell proliferation experiments showed a cell type-dependent quasi-exponential growth demonstrating the nontoxic properties of the supramolecular platform. After reaching a certain surface coverage, the supramolecular films based on phosphate-polyamine networks displayed antiadhesive activity towards cells, irrespective of the cell type. However and most interestingly, these antiadherent substrates developed strong adhesive properties after thermal annealing at 37 °C for 3 days. These results were interpreted based on the changes in the coating hydrophilicity, topography and stiffness, with the latter being assessed by atomic force microscopy imaging and indentation experiments. The reported approach is simple, robust and flexible, and would offer opportunities for the development of tunable, biocompatible interfacial architectures to control cell attachment for various biomedical applications.

Highly Sensitive Biosensing with Solid-State Nanopores Displaying Enzymatically Reconfigurable Rectification Properties

Molecular design of biosensors based on enzymatic processes taking place in nanofluidic elements is receiving increasing attention by the scientific community. In this work, we describe the construction of novel ultrasensitive enzymatic nanopore biosensors employing "reactive signal amplifiers" as key elements coupled to the transduction mechanism. The proposed framework offers innovative design concepts not only to amplify the detected ionic signal and develop ultrasensitive nanopore-based sensors but also to construct nanofluidic diodes displaying specific chemo-reversible rectification properties. The integrated approach is demonstrated by electrostatically assembling poly(allylamine) on the anionic pore walls followed by the assembly of urease. We show that the cationic weak polyelectrolyte acts as a "reactive signal amplifier" in the presence of local pH changes induced by the enzymatic reaction. These bioinduced variations in proton concentration ultimately alter the protonation degree of the polyamine resulting in amplifiable, controlled, and reproducible changes in the surface charge of the pore walls, and consequently on the generated ionic signals. The "iontronic" response of the as-obtained devices is fully reversible, and nanopores are reused and assayed with different urea concentrations, thus ensuring reliable design. The limit of detection (LOD) was 1 nM. To the best of our knowledge, this value is the lowest LOD reported to date for enzymatic urea detection. In this context, we envision that this approach based on the use of "reactive signal amplifiers" into solid-state nanochannels will provide new alternatives for the molecular design of highly sensitive nanopore biosensors as well as (bio)chemically addressable nanofluidic elements.

Layer-by-Layer Assembled Microgels Can Combine Conflicting Properties: Switchable Stiffness and Wettability without Affecting Permeability

Responsive interfacial architectures of practical interest commonly require the combination of conflicting properties in terms of their demand upon material structure. Switchable stiffness, wettability, and permeability, key features for tissue engineering applications, are in fact known to be exclusively interdependent. Here, we present a nanoarchitectonic approach that decouples these divergent properties by the use of thermoresponsive microgels as building blocks for the construction of three-dimensional arrays of interconnected pores. Layer-by-layer assembled poly( N-isopropylacrylamide- co-methacrylic acid) microgel films were found to exhibit an increase in hydrophobicity, stiffness, and adhesion properties upon switching the temperature from below to above the lower critical solution temperature, whereas the permeability of redox probes through the film remained unchanged. Our findings indicate that the switch in hydrophilicity and nanomechanical properties undergone by the microgels does not compromise the porosity of the film, thus allowing the free diffusion of redox probes through the polymer-free volume of the submicrometer pores. This novel approach for decoupling conflicting properties provides a strategic route for creating tailorable scaffolds with unforeseen functionalities.

Highly-organized stacked multilayers via layer-by-layer assembly of lipid-like surfactants and polyelectrolytes. Stratified supramolecular structures for (bio)electrochemical nanoarchitectonics

Supramolecular self-assembly is of paramount importance for the development of novel functional materials with molecular-level feature control. In particular, the interest in creating well-defined stratified multilayers through simple methods using readily available building blocks is motivated by a multitude of research activities in the field of "nanoarchitectonics" as well as evolving technological applications. Herein, we report on the facile preparation and application of highly organized stacked multilayers via layer-by-layer assembly of lipid-like surfactants and polyelectrolytes. Polyelectrolyte multilayers with high degree of stratification of the internal structure were constructed through consecutive assembly of polyallylamine and dodecyl phosphate, a lipid-like surfactant that act as a structure-directing agent. We show that multilayers form well-defined lamellar hydrophilic/hydrophobic domains oriented parallel to the substrate. More important, X-ray reflectivity characterization conclusively revealed the presence of Bragg peaks up to fourth order, evidencing the highly stratified structure of the multilayer. Additionally, hydrophobic lamellar domains were used as hosts for ferrocene in order to create an electrochemically active film displaying spatially-addressed redox units. Stacked multilayers were then assembled integrating redox-tagged polyallylamine and glucose oxidase into the stratified hydrophilic domains. Bioelectrocatalysis and "redox wiring" in the presence of glucose was demonstrated to occur inside the stratified multilayer.

Thermo-responsive PNIPAm nanopillars displaying amplified responsiveness through the incorporation of nanoparticles

The possibility of combining more than one stimulus-responsive property into a single material holds interesting potential for the creation of adaptive devices to be used in diverse fields such as drug delivery, nanomedicine and tissue engineering. This paper describes a novel material based on thermo-responsive PNIPAm nanopillars with amplified surface properties through the incorporation of Fe₃O₄ nanoparticles. The incorporation of magnetic nanoparticles into the nanopillars, prepared via surface-initiated atom-transfer radical polymerization in anodized aluminum oxide templates, sharply increased their stiffness and hydrophobicity when increasing the temperature above the volume phase transition temperature. Furthermore, their magnetic response turned out to be proportional to the amount of the incorporated nanoparticles. The possibility of sharply increasing the stiffness with a temperature variation close to the human body temperature paves the way to the application of these substrates as "smart" scaffolds for cell culture. Additionally, the presence of superparamagnetic nanoparticles in the nanopillars offers the possibility of using these nanostructured systems for magnetic hyperthermia.

Solvent Effects on the Structure-Property Relationship of Redox-Active Self-Assembled Nanoparticle-Polyelectrolyte-Surfactant Composite Thin Films: Implications for the Generation of Bioelectrocatalytic Signals in Enzyme-Containing Assemblies

The search for strategies to improve the performance of bioelectrochemical platforms based on supramolecular materials has received increasing attention within the materials science community, where the main objective is to develop low-cost and flexible routes using self-assembly as a key enabling process. Important contributions to the performance of such bioelectrochemical devices have been made based on the integration and supramolecular organization of redox-active polyelectrolyte-surfactant complexes on electrode supports. Here, we examine the influence of the processing solvent on the interplay between the supramolecular mesoorganization and the bioelectrochemical properties of redox-active self-assembled nanoparticle-polyelectrolyte-surfactant nanocomposite thin films. Our studies reveal that the solvent used in processing the supramolecular films and the presence of metal nanoparticles not only have a substantial influence in determining the mesoscale organization and morphological characteristics of the film but also have a strong influence on the efficiency and performance of the bioelectrochemical system. In particular, a higher bioelectrochemical response is observed when nanocomposite supramolecular films were cast from aqueous solutions. These observations seem to be associated with the fact that the use of aqueous solvents increases the hydrophilicity of the film, thus favoring the access of glucose, particularly at low concentrations. We believe that these results improve our current understanding of supramolecular nanocomposite materials generated via polyelectrolyte-surfactant complexes, in order to use the processing conditions as a variable to improve the performance of bioelectrochemical devices.

Effect of gold nanoparticles on the structure and electron-transfer characteristics of glucose oxidase redox polyelectrolyte-surfactant complexes

Efficient electrical communication between redox proteins and electrodes is a critical issue in the operation and development of amperometric biosensors. The present study explores the advantages of a nanostructured redox-active polyelectrolyte-surfactant complex containing [Os(bpy)2Clpy](2+) (bpy=2,2'-bipyridine, py= pyridine) as the redox centers and gold nanoparticles (AuNPs) as nanodomains for boosting the electron-transfer propagation throughout the assembled film in the presence of glucose oxidase (GOx). Film structure was characterized by grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM), GOx incorporation was followed by surface plasmon resonance (SPR) and quartz-crystal microbalance with dissipation (QCM-D), whereas Raman spectroelectrochemistry and electrochemical studies confirmed the ability of the entrapped gold nanoparticles to enhance the electron-transfer processes between the enzyme and the electrode surface. Our results show that nanocomposite films exhibit five-fold increase in current response to glucose compared with analogous supramolecular AuNP-free films. The introduction of colloidal gold promotes drastic mesostructural changes in the film, which in turn leads to a rigid, amorphous interfacial architecture where nanoparticles, redox centers, and GOx remain in close proximity, thus improving the electron-transfer process.