Acrylic Acid Films Research Articles

Yolk-Shell Encapsulation of Cells by Biomimetic Mineralization and Visible Light-Induced Surface Graft Polymerization.

The pursuit of low-cytotoxicity modification strategies represents a prominent avenue in cell coating research, holding immense significance for the advancement of practical living cell-related technologies. Here, we presented a novel method to fabricate encapsulated yeast cells with a yolk-shell structure by biomimetic mineralization and visible-light-induced surface graft polymerization. In this approach, an amorphous calcium carbonate (ACC) shell was first deposited on the surface of a yeast cell (cell@ACC) modified with 4 layers of self-assembled poly(diallyl dimethylammonium chloride) (PDADMAC)/poly(acrylic acid) (PAA) film using a biomimetic mineralization technique. Subsequently, polyethylenimine (PEI) was absorbed on the surface of cell@ACC by electrostatic interaction. Then, a cross-linked shell was introduced by surface-initiated graft polymerization of poly(ethylene glycol) diacrylate (PEGDA) on cell@ACC under irradiation of visible light using thioxanthone catechol-O,O'-diacetic acid as the photosensitizer. After the removal of the inner ACC shell, the yolk-shell-structured yeast cells (cell@PHS) were obtained. Due to the mild conditions of the strategy, the cell@PHS could retain 98.81% of its original viability. The introduction of the shell layer significantly prolonged the lag phase of yeast cells, which could be tuned between 5 and 25 h by regulating the thickness of the shell. Moreover, the cell@PHS showed improved resistance against lyticase due to the presence of a protective shell. After 30 days of storage, the viability of cell@PHS was 81.09%, which is significantly higher than the 19.89% viability of native yeast cells.

Wrinkle Clamp Down on Structure Crack Strain Sensor Based on High Poisson's Ratio Material for Home Health Monitoring and Human-Machine Interaction.

Flexible wearable crack strain sensors are currently receiving significant attention because they can be used in a wide range of physiological signal monitoring and human-machine interaction applications. However, sensors with high sensitivity, great repeatability, and wide sensing range remain challenging. Herein, a tunable wrinkle clamp down structure (WCDS) crack strain sensor based on high Poisson's ratio material with high sensitivity, high stability, and wide strain range is proposed. Based on the high Poisson's ratio of the acrylic acid film, the WCDS was prepared by a prestretching process. The wrinkle structures can clamp down the crack to improve the cyclic stability of the crack strain sensor while maintaining its high sensitivity. Moreover, the tensile properties of the crack strain sensor are improved by introducing wrinkles in the bridge-like gold stripes connecting each separated gold flake. Owing to this structure, the sensitivity of the sensor can reach 3627, stable operation over 10 000 cycles is achieved, and the strain range can reach about 9%. In addition, the sensor exhibits low dynamic response and good frequency characteristics. Because of its demonstrated excellent performance, the strain sensor can be used in pulse wave and heart rate monitoring, as well as posture recognition and game control.

Rapid self-healing capability as a metric for flexible spacing coating toward macroscopic supramolecular assembly of rigid building blocks

Macroscopic supramolecular assembly (MSA) refers to rapid molecular recognition between macroscopic components (>10 μm). Normally, MSA of rigid materials relies on a so-called “flexible spacing coating” beneath surface groups to provide high molecular mobility to realize multivalent binding. However, design principle of such coatings remains elusive. Here, we report an intuitive metric based on self-healing phenomena to judge whether a film can be a “flexible spacing coating” and apply this metric to extend film systems for MSA of rigid materials. The correlation between MSA and self-healing based on poly(ethylenimine)/poly(acrylic acid) films is revealed by similar dependence of assembly ratios and healing performance on film rigidity. Films capable of self-healing within 30 s can be used as flexible spacing coating for MSA. This metric is applied to three different film systems and proven effective to design coatings for MSA of rigid materials, which is applied in advanced manufacture of heterogeneous structure. • Macroscopic self-assembly (MSA) has been correlated with intrinsic self-healing • Rapid self-healing film can be used as flexible coating to facilitate MSA • Design principle of flexible coatings has proven versatile to several film systems Rapid molecular recognition between macroscopic components is realized by highly mobile surface groups based on a flexible coating. However, the design principle of such coatings remains elusive. Zhang et al. demonstrate an intuitive metric of self-healing performance to judge whether coatings on rigid materials can accelerate macroscopic self-assembly.

Effects of layered double hydroxides on poly(vinyl alcohol)/poly(acrylic acid) films for green food packaging applications

Here, a poly(vinyl alcohol) (PVA)/poly(acrylic acid) (PAA)/layered double hydroxide (LDH) nanocomposite film exhibiting improved gas barrier and mechanical properties was synthesized by incorporating LDH into a PVA/PAA blend film via a relatively facile method. To investigate the effect of the LDH on the nanocomposites, their characteristics, as well as a gas barrier, mechanical, thermal, and optical properties, were evaluated based on their LDH contents (0%, 1%, 2%, 3%, and 5%). The Fourier-transform infrared (FTIR) and X-ray diffraction (XRD) results revealed that LDH could interact with the PVA/PAA matrix. The scanning electron microscopy (SEM) results revealed that the aggregation occurred at an LDH concentration of ≥3%. Further, the barrier and mechanical properties increased up to an LDH content of 2% and decreased at >3%. Particularly, the oxygen permeability decreased by nearly 50%, following the incorporation of the LDH. Moreover, the thermal stability and melting point increased slightly, and the ultraviolet (UV)-blocking properties were improved while the transparency in the visible light region was maintained. These results demonstrated that the most optimal properties were exhibited at an LDH concentration of 2%. The PVA/PAA/LDH nanocomposites were largely nontoxic and ecofriendly; they exhibited high gas barrier and mechanical properties. Therefore, the PVA/PAA/LDH nanocomposite was proposed as a green packaging material with potential applications in the food and medicine fields.

Wafer scale synthesis of organic semiconductor nanosheets for van der Waals heterojunction devices

Organic semiconductors (OSC) are widely used for consumer electronic products owing to their attractive properties such as flexibility and low production cost. Atomically thin transition metal dichalcogenides (TMDs) are another class of emerging materials with superior electronic and optical properties. Integrating them into van der Waals (vdW) heterostructures provides an opportunity to harness the advantages of both material systems. However, building such heterojunctions by conventional physical vapor deposition (PVD) of OSCs is challenging, since the growth is disrupted due to limited diffusion of the molecules on the TMD surface. Here we report wafer-scale (3-inch) fabrication of transferable OSC nanosheets with thickness down to 15 nm, which enable the realization of heterojunction devices. By controlled dissolution of a poly(acrylic acid) film, on which the OSC films were grown by PVD, they can be released and transferred onto arbitrary substrates. OSC crystal quality and optical anisotropy are preserved during the transfer process. By transferring OSC nanosheets (p-type) onto prefabricated electrodes and TMD monolayers (n-type), we fabricate and characterize various electronic devices including unipolar, ambipolar and antiambipolar field-effect transistors. Such vdW p-n heterojunction devices open up a wide range of possible applications ranging from ultrafast photodetectors to conformal electronics.

A robust affinity chromatography system based on ceramic monoliths coated with poly(amino acid)‐based polymeric constructs

AbstractTraditional chromatographic separation systems are disadvantaged by low flow rates, a high pressure drop across the column, low capacity and poor reusability. Searching for more efficient separation systems we introduced the use of a ceramic monolith as robust support in bioseparations. A coating consisting of l‐asparagine as ligand, poly(l‐lysine) as spacer arm and a commercial poly(ethylene acrylic acid) film forming copolymer network (Michem 4983‐40R) was developed as a coating for these ceramic monoliths. Poly(l‐lysine) was synthesized by ring‐opening polymerization of ε‐trifluoroacetyl‐l‐lysine N‐carboxyanhydride and coupled to a commercial film‐forming poly(ethylene acrylic acid) network. This construct was then ‘decorated’ with l‐asparagine via the terminal amino functional groups of poly(L‐lysine) and coated onto the ceramic monolith to selectively bind l‐asparaginase. Adsorption/elution experiments showed reversible binding between l‐asparagine and l‐asparaginase, and the subsequent release of l‐asparaginase, and between 83% and 94% of the active enzyme was recovered by elution with d‐asparagine and NaCl solutions. The functional activity of the eluted l‐asparaginase was verified by a Nessler's assay. While traditional separation processes (adsorption and elution) using gel bead packings take many hours, the ceramic monolith system achieves the same of level of separation in about 1 h. This new system served as a proof of concept for its application in protein separation and purification. This work paves the way to a better understanding of the use of ceramic monoliths as stationary phase coated with a stable polymer construct for more robust and efficient supports in affinity chromatography. © 2020 Society of Industrial Chemistry

Design and characterization of a plasma chamber for improved radial and axial film uniformity

AbstractFilm uniformity is an issue in many deposition processes, especially in substrates with elongated lateral dimensions such as fibers. In this study, a plasma chamber was designed with dual electrodes to improve the uniformity of plasma polymer films. Plasma‐polymerized acrylic acid films were deposited via single‐electrode or double‐electrode configurations onto silicon wafer pieces placed at the center of the chamber to mimic the potential position of a fiber. A radial and spatial map of film chemistry and thickness throughout the chamber was determined which demonstrated improved radial uniformity and film stability using the dual‐electrode configuration, thus highlighting the importance of spatial characterization when designing a reactor for the coating of three‐dimensional objects such as fibers.

Study on self‐healing behavior of the layer‐by‐layer assembled polyethylenimine/poly(acrylic acid) film

AbstractMany studies have demonstrated that intrinsic self‐healing polyelectrolyte coatings can be fabricated by exponentially growing Layer‐by‐Layer (LbL) assembled polyelectrolyte multilayers. However, due to the lack of controlled self‐healing evaluation methods, the researches on self‐healing properties have remained at some simple qualitative characterization. In our study, a self‐healing branched polyethylenimine (bPEI)/poly(acrylic acid) (PAA) films is successfully prepared by LbL and a new systematical self‐healing evaluation method is developed. The cuts with controllable depth, width, and period can be easily fabricated on bPEI/PAA films through the colloidal lithography and photolithography. Colloidal lithography, as a facile, low‐cost, and versatile unconventional technique, can realize precise and repetitive fabrication of cuts at the same sample and in the same area, which will be beneficial to the systematic study and evaluate self‐healing behavior. The self‐healing behaviors on the different cuts are observed by scanning electron microscope (SEM). The SEM results reveal that bPEI/PAA film could realize damage/healing process for multiple times at the same sample and in the same area. In addition, the self‐healing abilities of the LbL films are related to the damage number, depth, and width of cuts. With the increase of damage number, depth, and width of cuts, the loss of polyelectrolyte was also increased, which lead to the flow of remaining polyelectrolytes were not enough to heal the cuts. This work provides a new route to systematically characterize the self‐healing ability of bPEI/PAA films as well as other polyelectrolyte films.

Exploiting Reactor Geometry to Manipulate the Properties of Plasma Polymerized Acrylic Acid Films.

A number of different reactor geometries can be used to deposit plasma polymer films containing specific functional groups and result in films with differing properties. Plasma polymerization was carried out in a low-pressure custom-built stainless steel T-shaped reactor using a radio frequency generator. The internal aluminium disk electrode was positioned in two different geometries: parallel and perpendicular to the samples at varying distances to demonstrate the effect of varying the electrode position and distance from the electrode on the properties of plasma polymerized acrylic acid (ppAAc) films. The surface chemistry and film thickness before and after aqueous immersion were analysed via X-ray photoelectron spectroscopy and spectroscopic ellipsometry, respectively. For a perpendicular electrode, the ppAAc film thicknesses and aqueous stability decreased while the COOH/R group concentrations increased as the distance from the electrode increased due to decreased fragmentation. For films deposited at similar distances from the electrode, those deposited with the parallel electrode were thicker, had lower COOH/R group concentrations and greater aqueous stability. These results demonstrate the necessity of having a well characterized plasma reactor to enable the deposition of films with specific properties and how reactor geometry can be exploited to tailor film properties.

Converting waste Allium sativum peel to nitrogen and sulphur co-doped photoluminescence carbon dots for solar conversion, cell labeling, and photobleaching diligences: A path from discarded waste to value-added products

Proper waste utilization in order to promote value added product is a promising scientific practice in recent era. Inspiring from the recurring trend, we propose a single step oxidative pyrolysis derived fluorescent carbon dots (C-dots) from Allium sativum peel, which is a natural, nontoxic, and waste raw material. Because of its excellent optical properties, and photostability this C-dots have been used in versatile area of applications. Due to its immediate water dispersing character, C-dots reinforced Poly(acrylic acid) (PAA) films revealed improvement in uniaxial stretching behavior and can be used as transparent sunlight conversion film. The nanocomposite film has been tested against rigorous simulated sunlight which proved almost identical sunlight conversion behavior with no photo-bleachable character which is definitely added an extra quality of transparent polymer films. Moreover, the C-dots dispersion has been used as in vitro biomarker for living cells owing to its ease in solubility, biocompatibility, non-cytotoxicity and bright fluorescence even in subcutaneous environment. For this case, adipose derived mesenchymal stem cells (ADMSCs) have been chosen and injected to rabbit ear skin to perform two-photon imaging experiment. The present work opens a new avenue towards the large-scale synthesis of bio-waste based fluorescent C-dots, paving the way for their versatile applications.

Layer-by-Layer Assembly of Three-Dimensional Optical Functional Nanostructures.

Nanotransfer printing (nTP) technology can generate highly functional three-dimensional (3D) nanostructures in a low-cost and high-throughput fashion. Nevertheless, the fabrication yield and quality of the transferred nanostructures are often limited by the merging of the surface patterns of replica stamps during transfer printing. Here, an nTP technology was developed to fabricate large-area and crack-free 3D multilayer nanostructures. Instead of directly depositing materials on the patterned flexible stamp in conventional nTPs, we transferred the nanostructures straightforwardly onto an attached polydimethylsiloxane slab by removing a sacrificial water-soluble poly(acrylic acid) film, which can avoid the cracking of metal film and the failures of printing nanostructures onto target substrates. Based on this approach, subwavelength-thick polarization rotators working at infrared wavelengths were fabricated. Excellent performance of linear polarization rotation over a broadband was realized. This nTP approach could complement existing fabrication techniques and benefit the development of various functional nanostructures with complex multilayer hierarchies.

Patterned Slippery Surface through Dynamically Controlling Surface Structures for Droplet Microarray

Patterned slippery surfaces based on the technique of slippery liquid-infused porous surface (SLIPS) have broad technological applications in biofouling, liquid handling, biomolecule collection, and droplet microarrays. However, methods to generate patterned SLIPS with flexibility and practical potential remain a technical roadblock. Here, we report an approach for generating patterned SLIPS, and by utilizing which, as a proof-of-technique application, the droplet microarrays have been presented. Our approach is based on a photocross-linkable polyelectrolyte poly(ethyleneimine)/azide-poly(acrylic acid) (PEI/PAA-N3) film, whose surface structures can be dynamically controlled. We have prepared the surface-patterned PEI/PAA-N3 film containing microstructured and flat regions. We then demonstrate that, because of much higher lubricant retaining capability of the microstructured regions, a patterned SLIPS can be simply generated through infusing the lubricant onto the surface and applying a shear spinning pro...

Multitechnique investigation into the aqueous behavior of plasma polymers.

Plasma polymers are often used in applications requiring aqueous immersion; therefore, it is important to understand how this exposure affects the physical and chemical properties of the films. Three different plasma polymer films were deposited at different distances from the electrode, and the film properties were characterized using contact angle, ellipsometry, and x-ray photoelectron spectroscopy. The film behaviors in aqueous solutions were studied via quartz crystal microbalance with dissipation (QCM-D). Exposure to buffer solutions produced significant swelling of the plasma polymerized acrylic acid films, with swelling increasing with distance from the powered electrode, results that could be correlated with changes in film chemistry. Plasma polymerized octadiene and allylamine exhibited little swelling. These films exhibited changes in thickness and contact angle with respect to distance from the electrode, but this had little influence on their behavior in aqueous solution. By combining QCM-D with the more traditional surface chemical analysis techniques, the authors have been able to explore both swelling behavior and the effect that sample position and thus deposition parameters have on film properties and aqueous behavior. This approach gives the authors the basis to define deposition parameters to assist the engineering of thin films for applications such as biosensing and tissue engineering applications where specific chemistries and film behaviors are desired.

(Invited) Electro/Electroless Grafting of Organic Dielectric Film on Silicon Surface and Its Application on through Silicon Via

With the rapid development of electronic industry, the packaging technology requires higher density and integration, which is a great challenge for semiconductor manufacturing. In order to continue the acknowledged “Moore's law”, the three-dimensional through silicon via (TSV) packaging of stacked chips has become the focus of attention, providing fresh ideas for the industry. The preparation of insulators is one of the key technologies for TSV. However it is difficult for conventional methods to prepare SiO2 insulators with high coverage in TSV with small size and high aspect ratio. In addition, higher performance of TSV proposes a big challenge for SiO2. Organic polymers can replace SiO2 used as an insulator for TSV, because their lower dielectric constant can greatly improve the properties of TSV. Electrografting is a novel technology for polymer film preparation, which has the advantages of being simple, low cost and does not required special equipment.. In recent years, the modification of the Si surface with the electrochemical reduction of diazonium salts has become attractive because it can be performed in aqueous media, which is more acceptable for industrial applications. In the best understood reaction, a diazonium salt with an electrolyte such as tetrabutylammonium cations is reduced by an externally applied potential, typically at around −1 V versus the saturated calomel electrode. In our study, the electrografting technology is used to graft polymer film onto a Si surface, and its mechanism and the effects of pulsed voltage and composition of the solution are investigated. In the process of electrografting, 4-nitrobenzene diazonium (NBD) tetrafluoroborate is reduced by electrons provided by an electric source, forming nitrophenyl radicals. These radicals covalently bond to the Si surface to form a polynitrophenyl (PNP) primer layer, and initiate the polymerization of vinyl monomers. The radical-terminated polyvinyl chains formed in the solution and nitrophenyl radicals are then added to the aromatic rings of the primer layer to form the expected polyvinyl film. Consequently the grafted film is composed of PNP and the polymer of monomers. Furthermore, we propose an easier chemical grafting method based on the finding that diazonium salt can be spontaneously reduced at the Si surface when HF is present in the aqueous solution even without an externally applied potential., and study the influence factors of structure, morphology and component of the polymer film. This process is conducted in acidic aqueous media containing HF. From the mechanistic point of view, Si atoms at the Si surface are oxidized by HF, providing electrons for the reduction of NBD cations at the open circuit potential. The mechanism of the grafting of the polymer of monomers is the same as that of an electrografting process. The polymer films prepared by either the electrografting or chemical grafting methods have good adhesion, dielectric properties and thermal properties. Then the polymer film is applied in TSV. As far as we know, insufficient film thickness and low coverage are still serious problems for plasma-enhanced chemical vapor deposition methods. However, both electrografting and chemical grafting overcome these problems. In addition, the narrow diameter and high aspect ratio (≥1:10) are not limitations for grafting methods. Poly(methyl methacrylate) film has a coverage of 76%, while poly(acrylic acid) film has a coverage as high as 100%.

Deposition of polyacrylic acid films on PDMS substrate in dielectric barrier corona discharge at atmospheric pressure

This paper reports on deposition of acrylic acid films polymerized by an efficient and cost‐effective technique of dielectric barrier corona discharge at atmospheric pressure. The liquid acrylic acid was vaporized and carried by argon gas into plasma to deposit polyacrylic acid films on polydimethylsiloxane substrate. A nonthermal corona discharge was generated in a pyrex flask using a steel tube‐to‐plate asymmetric electrode configuration. The plasma was excited using an in‐house developed power supply operating with continuous wave signals of 10‐kHz frequency. The emission spectra of plasma species were recorded to know their contribution during deposition process. The deposited surfaces were characterized using contact angle measurements, atomic force microscopy, Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy and film thickness measurements. A maximum film growth rate of 363 nm/min was achieved under optimal condition of discharge. The results suggest that this plasma technique is capable of depositing organic coatings with a high concentration of carboxylic functional groups that could be potentially used for biomedical and microfluidic applications.

Switchable Hydrogel-Gated Organic Field-Effect Transistors

Stimuli-responsive hydrogels represent a class of materials capable of reversibly switching their morphological and physicochemical characteristics. An ultrathin poly(acrylic acid) film (ca. 6 nm) grafted onto the gate of a p-type EGOFET is studied, and the correlation between the swelling state of the hydrogel and the transistor output characteristics is presented. The hydrogel-related swelling process occurring in basic medium causes an increase in threshold voltage due to the abrupt and intense increase of the negative charge density on the gate electrode. The variation of the drain current during the in situ modification of the pH electrolyte allows a quantitative analysis of the hydrogel switching kinetics. This work shows not only the relevance of EGOFET as an analytical tool in the broad sense, i.e., able to follow in real time phase transition processes of stimuli-responsive materials, but also the relevance of using a hydrogel for field-effect-based (bio)detection according to the ability of such material to overcome the well-known Debye length problematics.

A stability study of plasma polymerized acrylic acid films

A medium pressure parallel-plate DBD set-up was used for the deposition of carboxylic acid-rich films using acrylic acid as precursor. A variety of discharge powers (21–33W) and monomer flow rates (0.25–0.5g/h) were applied to develop a range of coatings from highly unstable to completely stable in aqueous solutions. FT-IR and XPS were used to identify which coatings were stable under dynamic incubation conditions and what influence plasma operating parameters had on functional group preservation. White light interferometry was also used to calculate deposition rates (66–126nm/min) and to determine the deposition regime (monomer deficient). Human foreskin fibroblasts were seeded onto the complete range of investigated samples and the effects of coating (in)stability on cell adhesion, viability and morphology were systematically studied. Results showed that the 27 W-0.25g/h treatment conditions resulted in the most stable coatings. Although a concentration of only 0.7% of carboxylic acid groups was found under these experimental conditions, the positive effects of a highly acid-rich surface were still maintained as cell-material interactions were significantly enhanced.

Process for patterning features in poly(acrylic acid) for microelectronic applications

A method for patterning micro-scale features in a poly(acrylic acid) (PAA) film for engineering applications has been developed. Because PAA is a water-soluble polymer, careful attention has to be given during the development portion of the photolithographic process. To obtain well-defined patterns, development time was reduced by half following a regular photolithography exposure step. The remaining photoresist, and the PAA underneath it, were removed using a plasma ash process. After stripping the photoresist, polygonal windows such as triangles, rectangles, squares, pentagons, hexagons, heptagons, and octagons were created. This plasma ash process for patterning micro-scale features in PAA holds potential for fabrication of polymer microstructures, sacrificial layer micromolding, and patterned substrate micromolding. As a proof of concept, we applied these patterns to a solder-based self-assembly process to form 3D polyhedra.

Controlling wettability of PECVD-deposited dual organosilicon/carboxylic acid films to influence DNA hybridisation assay efficiency.

Herein, plasma polymerisation of a dual-layer tetraethyl orthosilicate (TEOS) and acrylic acid (AA) film under a specific recipe is performed. Newly deposited films are found to retain a weakly-bound soft layer of partially polymerised acrylic acid (wbAA), which can be ultimately removed by washing. However, when on the surface, this soft layer is shown to be influential in manipulating the properties of a robust covalently-bound AA (cbAA) underlayer when treated appropriately. Specifically, treatment of the as-deposited dual-layer TEOS/AA films via timed incubation in a humidity-controlled environment results in changes in the water contact angle (WCA) of the cbAA, and ultimately the surface of the TEOS/AA, enabling tuning of the wettability of the acrylic acid layer. Through the use of a controlled incubation environment of the TEOS/AA, followed by washing, we have demonstrated that carboxylic-acid containing surfaces with a WCA between 85° and 10° can be routinely generated, using basic apparatus and simple methodology. Moreover, these surfaces not only retain their AA functionality, demonstrated by covalent-linking of amine-terminated single-stranded DNA, but also strongly inhibit non-specific binding of the DNA strands. The efficiency of these surfaces to be used in DNA direct-binding hybridisation assays has been demonstrated, with limits of detection of 1.11 and 1.66 nM being measured.

Electroanalytical Application of Screen-printed Carbon Electrode Modified with Conductive Graphene Oxide–Poly(acrylic acid) Film for Label-free Detection of Human Immunoglobulin G

A facile and cost-effective modification of screen-printed carbon electrode employing a graphene oxide–poly(acrylic acid) film allows both good electrochemical reactivity and electrode surface functionalization with antibody. The analytical performances of the modified electrode for detection of human immunoglobulin G (IgG) were satisfied with a linear concentration range of 1.0–100 ng mL−1, sensitivity of 1.2318 µA cm−2 mL ng−1, and a limit of detection of 0.54 ng mL−1. The proposed electrode is promising in sensitive and selective clinical analysis to detect IgG at physiological concentration.