A pH‐Responsive Gating Membrane System with Pumping Effects for Improved Controlled Release

In this study, we report on a novel composite membrane system for pH‐responsive controlled release, which is composed of a porous membrane with linear grafted, positively pH‐responsive polymeric gates acting as functional valves, and a crosslinked, negatively pH‐responsive hydrogel inside the reservoir working as a functional pumping element. The proposed system features a large responsive release rate that goes effectively beyond the limit of concentration‐driven diffusion due to the pumping effects of the negatively pH‐responsive hydrogel inside the reservoir. The pH‐responsive gating membranes were prepared by grafting poly(methacrylic acid) (PMAA) linear chains onto porous polyvinylidene fluoride (PVDF) membrane substrates using a plasma‐graft pore‐filling polymerization, and the crosslinked poly(N,N‐dimethylaminoethyl methacrylate) (PDM) hydrogels were synthesized by free radical polymerization. The volume phase‐transition characteristics of PMAA and PDM were opposite. The proposed system opens new doors for pH‐responsive “smart” or “intelligent” controlled‐release systems, which are highly attractive for drug‐delivery systems, chemical carriers, sensors, and so on.     

Multifunctional Up‐Converting Nanocomposites with Smart Polymer Brushes Gated Mesopores for Cell Imaging and Thermo/pH Dual‐Responsive Drug Controlled Release

Multifunctional nanocarriers based on the up‐conversion luminescent nanoparticles of NaYF₄:Yb³⁺/Er³⁺ core (UCNPs) and thermo/pH‐coupling sensitive polymer poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] (P(NIPAm‐co‐MAA)) gated mesoporous silica shell are reported for cancer theranostics, including fluorescence imaging, and for controlled drug release for therapy. The as‐synthesized hybrid nanospheres UCNPs@mSiO₂‐P(NIPAm‐co‐MAA) show bright green up‐conversion fluorescence under 980 nm laser excitation and the thermo/pH‐sensitive polymer is active as a “valve” to moderate the diffusion of the embedded drugs in‐and‐out of the pore channels of the silica container. The anticancer drug doxorubicin hydrochloride (DOX) can be absorbed into UCNPs@mSiO₂‐P(NIPAm‐co‐MAA) nanospheres and the composite drug delivery system (DDS) shows a low level of leakage at low temperature/high pH values but significantly enhanced release at higher temperature/lower pH values, exhibiting an apparent thermo/pH controlled “on‐off” drug release pattern. The as‐prepared UCNPs@mSiO₂‐P(NIPAm‐co‐MAA) hybrid nanospheres can be used as bioimaging agents and biomonitors to track the extent of drug release. The reported multifunctional nanocarriers represent a novel and versatile class of platform for simultaneous imaging and stimuli‐responsive controlled drug delivery.     

Positively Thermo‐Sensitive Monodisperse Core–Shell Microspheres

In this paper, we report on a novel family of monodisperse thermo‐sensitive core–shell hydrogel microspheres that is featured with high monodispersity and positively thermo‐responsive volume phase transition characteristics with tunable swelling kinetics, i.e., the particle swelling is induced by an increase rather than a decrease in temperature. The microspheres were fabricated in a three‐step process. In the first step, monodisperse poly(acrylamide‐co‐styrene) seeds were prepared by emulsifier‐free emulsion polymerization. In the second step, poly(acrylamide) or poly[acrylamide‐co‐(butyl methacrylate)] shells were fabricated on the microsphere seeds by free radical polymerization. In the third step, the core–shell microspheres with poly‐ (acrylamide)/poly(acrylic acid) based interpenetrating polymer network (IPN) shells were finished by a method of sequential IPN synthesis. The proposed monodisperse core–shell microspheres provide a new mode of the phase transition behavior for thermo‐sensitive “smart” or “intelligent” monodisperse micro‐actuators that is highly attractive for targeting drug delivery systems, chemical separations, sensors, and so on.     

Colorimetric Logic Gates Based on Poly(2‐alkyl‐2‐oxazoline)‐Coated Gold Nanoparticles

A straightforward end‐capping strategy is applied to synthesize xanthate‐functional poly(2‐alkyl‐2‐oxazoline)s (PAOx) that enable gold nanoparticle functionalization by a direct “grafting to” approach with citrate‐stabilized gold nanoparticles (AuNPs). Owing to the presence of remaining citrate groups, the obtained PAOx@AuNPs exhibit dual stabilization by repulsive electrostatic and steric interactions giving access to water soluble molecular AND logic gates, wherein environmental temperature and ionic strength constitute the input signals, and the solution color the output signal. The temperature input value could be tuned by variation of the PAOx polymer composition, from 22 °C for poly(2‐ⁿpropyl‐2‐oxazoline)@AuNPs to 85 °C for poly(2‐ethyl‐2‐oxazoline)@AuNPs. Besides, advancing the fascinating field of molecular logic gates, the present research offers a facile strategy for the synthesis of PAOx@AuNPs of interest in fields spanning nanotechnology and biomedical sciences. In addition, the functionalization of PAOx with xanthate offers straightforward access to thiol‐functional PAOx of high interest in polymer science.     

Glucose‐Responsive Bioinorganic Nanohybrid Membrane for Self‐Regulated Insulin Release

A bioinorganic nanohybrid glucose‐responsive membrane is developed for self‐regulated insulin delivery analogous to a healthy human pancreas. The application of MnO₂ nanoparticles as a multifunctional component in a glucose‐responsive, protein‐based membrane with embedded pH‐responsive hydrogel nanoparticles is proposed. The bio‐nanohybrid membrane is prepared by crosslinking bovine serum albumin (BSA)–MnO₂ nanoparticle conjugates with glucose oxidase and catalase in the presence of poly(N‐isopropyl acrylamide‐co‐methacrylic acid) nanoparticles. The preparation and performance of this new nanocomposite material for a glucose‐responsive insulin release system is presented. The activity and stability of immobilized glucose oxidase and the morphology and mechanical properties of the membrane are investigated. The enzymatic activity is well preserved in the membranes. The use of MnO₂ nanoparticles not only reinforces the mechanical strength and the porous structure of the BSA‐based membrane, but enhances the long‐term stability of the enzymes. The in vitro release of insulin across the membrane is modulated by changes in glucose concentration mimicking possible fluctuations of blood‐glucose level in diabetic patients. A four‐fold increase in insulin permeation is observed when the glucose concentration is increased from normal to hyperglycemic levels, which returns to the baseline level when the glucose concentration is reduced to a normal level.     

Color Tunable Poly (N‐Isopropylacrylamide)‐co‐Acrylic Acid Microgel–Au Hybrid Assemblies

A new class of materials that are capable of color tunability over 300 nm with a 15 °C temperature change is introduced. The materials are assembled from thermoresponsive poly (N‐isopropylacrylamide)‐co‐acrylic acid (pNIPAm‐co‐AAc) microgels, which are deposited on Au coated glass substrates. The films are also pH responsive; the temperature‐induced color change was suppressed at high pH and is consistent with the behavior of a solution of suspended microgels. The mechanism proposed to account for the observed optical properties suggests that they result from the two Au layers being separated from each other by the “monolithic” microgel film, much like a Fabry‐Pérot etalon or interferometer. It is the modulation of the distance between these two layers, facilitated by the microgel collapse transition at high temperature, that allows the color to be tuned. The sensitivity of the system presented here will be used for future sensing and biosensing applications, as well as for light filtering applications.     

Original Fuel‐Cell Membranes from Crosslinked Terpolymers via a “Sol–gel” Strategy

Hybrid organic/inorganic membranes that include a functionalized (‐SO₃H), interconnected silica network, a non‐porogenic organic matrix, and a ‐SO₃H‐functionalized terpolymer are synthesized through a sol–gel‐based strategy. The use of a novel crosslinkable poly(vinylidene fluoride‐ter‐perfluoro(4‐methyl‐3,6‐dioxaoct‐7‐ene sulfonyl fluoride)‐ter‐vinyltriethoxysilane) (poly(VDF‐ter‐PFSVE‐ter‐VTEOS)) terpolymer allows a multiple tuning of the different interfaces to produce original hybrid membranes with improved properties. The synthesized terpolymer and the composite membranes are characterized, and the proton conductivity of a hybrid membrane in the absence of the terpolymer is promising, since 8 mS cm^?1 is reached at room temperature, immersed in water, with an experimental ion‐exchange‐capacity (IEC_exp) value of 0.4 meq g^?1. Furthermore, when the composite membranes contain the interfaced terpolymer, they exhibit both a higher proton conductivity (43 mS cm^?1 at 65 °C under 100% relative humidity) and better stability than the standard hybrid membrane, arising from the occurrence of a better interface between the inorganic silica and the poly[(vinylidene fluoride)‐co‐hexafluoropropylene] (poly(VDF‐co‐HFP)) copolymer network. Accordingly, the hybrid SiO₂‐SO₃H/terpolymer/poly(VDF‐co‐HFP) copolymer membrane has potential use as an electrolyte in a polymer‐electrolyte‐membrane fuel cell operating at intermediate temperatures.     

Chain Extension of Stimuli‐Responsive Polymer Brushes: A General Strategy to Overcome the Drawbacks of the “Grafting‐To” Approach

Stimuli‐responsive polymer brushes are smart materials for the design of bio‐interactive and responsive interfaces. The “grafting‐to” approach is a convenient preparation procedure that allows the modification of surfaces with preformed and most notably well‐defined functionalized macromolecules. However, the shortcoming of this approach is an intrinsic limitation of the grafting density, which in turn affects the stimuli‐responsive properties of the brush system. Here, a general strategy to overcome this limitation and to simultaneously improve the switching behavior of a temperature‐responsive poly(N‐isopropylacrylamide) (PNiPAAm) brush is reported. A technically simple processing step is used in combination with the thermal azide–alkyne cycloaddition to perform the chain extension of alkyne‐functionalized PNiPAAm brushes with azide‐functionalized PNiPAAm molecules.     

Gated Mesoporous SiO2 Nanoparticles Using K+‐Stabilized G‐Quadruplexes

The K⁺‐induced formation of G‐quadruplexes provides a versatile motif to lock or unlock substrates trapped in the pores of mesoporous SiO₂ nanoparticles, MP‐SiO₂ NPs. In one system, the substrate is locked in the MP‐SiO₂ NPs by K⁺‐ion‐stabilized G‐quadruplex units, and the pores are unlocked by the elimination of K⁺ ions using Kryptofix [2.2.2] (KP) or 18‐crown‐6‐ether (CE) from the G‐quadruplexes. In the second system, the substrate is locked in the pores by means of K⁺‐stabilized aptameric G‐quadruplex/thrombin units. Unlocking of the pores is triggered by the dissociation of the aptamer/thrombin complexes through the KP‐ or CE‐mediated elimination of the stabilizing K⁺ ions. In the third system, duplex DNA units lock the pores of MP‐SiO₂ NPs, and the release of the entrapped substrate is stimulated by the K⁺‐ion‐induced dissociation of the duplex caps through the formation of the K⁺‐stabilized G‐quadruplexes. The latter system is further implemented to release the anti‐cancer drug, doxorubicin, in the presence of K⁺ ions, from the MP‐SiO₂ NPs. Preliminary intracellular experiments reveal that doxorubicin‐loaded MP‐SiO₂ NPs lead to effective death of breast cancer cells.     

Double Stimuli‐Responsive Isoporous Membranes via Post‐Modification of pH‐Sensitive Self‐Assembled Diblock Copolymer Membranes

Double stimuli‐responsive membranes are prepared by modification of pH‐sensitive integral asymmetric polystyrene‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer membranes with temperature‐responsive poly(N‐isopropylacrylamide) (pNIPAM) by a surface linking reaction. PS‐b‐P4VP membranes are first functionalized with a mild mussel‐inspired polydopamine coating and then reacted via Michael addition with an amine‐terminated pNIPAM‐NH₂ under slightly basic conditions. The membranes are thoroughly characterized by nuclear magnetic resonance (¹H‐NMR), Fourier transform infrared spectroscopy and X‐ray‐induced photoelectron spectroscopy. Additionally dynamic contact angle measurements are performed comparing the sinking rate of water droplets at different temperatures. The pH‐ and thermo‐double sensitivities of the modified membranes are proven by determining the water flux under different temperature and pH conditions.     

Thermogelling,ABC Triblock Copolymer Platform for Resorbable Hydrogels with Tunable,Degradation‐Mediated Drug Release

Clinical application of injectable, thermoresponsive hydrogels is hindered by lack of degradability and controlled drug release. To overcome these challenges, a family of thermoresponsive, ABC triblock polymer‐based hydrogels has been engineered to degrade and release drug cargo through either oxidative or hydrolytic/enzymatic mechanisms dictated by the “A” block composition. Three ABC triblock copolymers are synthesized with varying “A” blocks, including oxidation‐sensitive poly(propylene sulfide), slow hydrolytically/enzymatically degradable poly(ε‐caprolactone), and fast hydrolytically/enzymatically degradable poly(d ,l ‐lactide‐co‐glycolide), forming the respective formulations PPS₁₃₅‐b‐PDMA₁₅₂‐b‐PNIPAAM₂₂₅ (PDN), PCL₈₅‐b‐PDMA₁₅₀‐b‐PNIPAAM₁₅₀ (CDN), and PLGA₆₀‐b‐PDMA₁₄₈‐b‐PNIPAAM₁₅₂ (LGDN). For all three polymers, hydrophilic poly(N,N‐dimethylacrylamide) and thermally responsive poly(N‐isopropylacrylamide) comprise the “B” and “C” blocks, respectively. These copolymers form micelles in aqueous solutions at ambient temperature that can be preloaded with small molecule drugs. These solutions quickly transition into hydrogels upon heating to 37 °C, forming a supra‐assembly of physically crosslinked, drug‐loaded micelles. PDN hydrogels are selectively degraded under oxidative conditions while CDN and LGDN hydrogels are inert to oxidation but show differential rates of hydrolytic/enzymatic decomposition. All three hydrogels are cytocompatible in vitro and in vivo, and drug‐loaded hydrogels demonstrate differential release kinetics in vivo corresponding with their specific degradation mechanism. These collective data highlight the potential cell and drug delivery use of this tunable class of ABC triblock polymer thermogels.     

Confinement of Thermoresponsive Hydrogels in Nanostructured Porous Silicon Dioxide Templates

A thermoresponsive hydrogel, poly(N‐isopropylacrylamide) (poly(NIPAM)), is synthesized in situ within an oxidized porous Si template, and the nanocomposite material is characterized. Infiltration of the hydrogel into the interconnecting nanoscale pores of the porous SiO₂ host is confirmed by scanning electron microscopy. The optical reflectivity spectrum of the nanocomposite hybrid displays Fabry–Pérot fringes characteristic of thin film interference, enabling direct, real‐time observation of the volume phase transition of the confined poly(NIPAM) hydrogel. Reversible optical reflectivity changes are observed to correlate with the temperature‐dependent volume phase transition of the hydrogel, providing a new means of studying nanoscale confinement of responsive hydrogels. The confined hydrogel displays a swelling and shrinking response to changes in temperature that is significantly faster than that of the bulk hydrogel. The porosity and pore size of the SiO₂ template, which are precisely controlled by the electrochemical synthesis parameters, strongly influence the extent and rate of changes in the reflectivity spectrum of the nanocomposite. The observed optical response is ascribed to changes in both the mechanical and the dielectric properties of the nanocomposite.     

Controlled Design of Well‐Dispersed Ultrathin MoS2 Nanosheets inside Hollow Carbon Skeleton: Toward Fast Potassium Storage by Constructing Spacious “Houses” for K Ions

The large volume expansion induced by K⁺ intercalation is always a big challenge for designing high‐performance electrode materials in potassium‐ion storage system. Based on the idea that large‐sized ions should accommodate big “houses,” a facile‐induced growth strategy is proposed to achieve the self‐loading of MoS₂ clusters inside a hollow tubular carbon skeleton (HTCS). Meantime, a step‐by‐step intercalation technology is employed to tune the interlayer distance and the layer number of MoS₂. Based on the above, the ED‐MoS₂@CT hybrids are achieved by self‐loading and anchoring the well‐dispersed ultrathin MoS₂ nanosheets on the inner surface of HTCSs. This unique compositing model not only alleviates the mechanical strain efficiently, but also provides spacious “roads” (hollow tubular carbon skeleton) and “houses” (interlayer expanded ultrathin MoS₂ sheets) for fast K⁺ transition and storage. As an anode of potassium‐ion batteries, the resultant ED‐MoS₂@CT electrode delivers a high specific capacity of 148.5 mAh g^?1 at 2 A g^?1 after 10 000 cycles with only 0.002% fading per cycle. The assembled ED‐MoS₂@CT//PC potassium‐ion hybrid supercapacitor device shows a high energy density of 148 Wh kg^?1 at a power density of 965 W kg^?1, which is comparable to that of lithium‐ion hybrid supercapacitors.     

Ultra‐Thin Self‐Assembled Protein‐Polymer Membranes: A New Pore Forming Strategy

Self‐assembled membranes offer a promising alternative for conventional membrane fabrication, especially in the field of ultrafiltration. Here, a new pore‐making strategy is introduced involving stimuli responsive protein‐polymer conjugates self‐assembled across a large surface area using drying‐mediated interfacial self‐assembly. The membrane is flexible and assembled on porous supports. The protein used is the cage protein ferritin and resides within the polymer matrix. Upon denaturation of ferritin, a pore is formed which intrinsically is determined by the size of the protein and how it resides in the matrix. Due to the self‐assembly at interfaces, the membrane constitutes of only one layer resulting in a membrane thickness of 7 nm on average in the dry state. The membrane is stable up to at least 50 mbar transmembrane pressure, operating at a flux of about 21 000–25 000 L m^?2 h^?1 bar^?1 and displayed a preferred size selectivity of particles below 20 nm. This approach diversifies membrane technology generating a platform for “smart” self‐assembled membranes.     

Field‐Induced n‐Doping of Black Phosphorus for CMOS Compatible 2D Logic Electronics with High Electron Mobility

Black phosphorus (BP) has been considered as a promising two‐dimensional (2D) semiconductor beyond graphene owning to its tunable direct bandgap and high carrier mobility. However, the hole‐transport‐dominated characteristic limits the application of BP in versatile electronics. Here, we report a stable and complementary metal oxide semiconductor (COMS) compatible electron doping method for BP, which is realized with the strong field‐induced effect from the K⁺ center of the silicon nitride (Si_xN_y). An obvious change from pristine p‐type BP to n type is observed after the deposit of the Si_xN_y on the BP surface. This electron doping can be kept stable for over 1 month and capable of improving the electron mobility of BP towards as high as ~176 cm² V^–1 s^–1. Moreover, high‐performance in‐plane BP p‐n diode and further logic inverter were realized by utilizing the n‐doping approach. The BP p‐n diode exhibits a high rectifying ratio of ~10⁴. And, a successful transfer of the output voltage from “High” to “Low” with very few voltage loss at various working frequencies were also demonstrated with the constructed BP inverter. Our findings paves the way for the success of COMS compatible technique for BP‐based nanoelectronics.     

Preparation and Characterization of a pH‐ and Thermally Responsive Poly(N‐isopropylacrylamide‐co‐acrylic acid)/Porous SiO2 Hybrid

A multifunctional nanohybrid composed of a pH‐ and thermoresponsive hydrogel, poly(N‐isopropylacrylamide‐co‐acrylic acid) [poly(NIPAM‐co‐AAc)], is synthesized in situ within the mesopores of an oxidized porous Si template. The hybrid is characterized by electron microscopy and by thin film optical interference spectroscopy. The optical reflectivity spectrum of the hybrid displays Fabry–Pérot fringes characteristic of thin film optical interference, enabling direct, real‐time observation of the pH‐induced swelling, and volume phase transitions associated with the confined poly(NIPAM‐co‐AAc) hydrogel. The optical response correlates to the percentage of AAc contained within the hydrogel, with a maximum change observed for samples containing 20% AAc. The swelling kinetics of the hydrogel are significantly altered due to the nanoscale confinement, displaying a more rapid response to pH or heating stimuli relative to bulk polymer films. The inclusion of AAc dramatically alters the thermoresponsiveness of the hybrid at pH 7, effectively eliminating the lower critical solution temperature (LCST). The observed changes in the optical reflectivity spectrum are interpreted in terms of changes in the dielectric composition and morphology of the hybrids.     

Integrating Ionic Gate and Rectifier Within One Solid‐State Nanopore via Modification with Dual‐Responsive Copolymer Brushes

A dual‐functional nanofluidic device is demonstrated that integrates the ionic gate and the ionic rectifier within one solid‐state nanopore. The functionalities are realized by fabricating temperature‐ and pH‐responsive poly(N‐isopropyl acrylamide‐co‐acrylic acid) brushes onto the wall of a cone‐shaped nanopore. At ca. 25 °C, the nanopore works on a low ion conducting state. When the temperature is raised to ca. 40 °C, the nanopore switches to a high ion conducting state. The closing/opening of the nanopore results from the temperature‐triggered conformational transition of the attached copolymer brushes. Independently, in neutral and basic solutions, the conical nanopore rectifies the ionic current. While in acid solutions, no ion rectifying properties can be found. The charge properties of the copolymer brushes, combined with the asymmetrical pore geometry, render the nanopore a pH‐tunable ionic rectifier. The chemical modification strategy could be applied to incorporate other stimuli‐responsive materials for designing smart multi‐functional nanofluidic systems resembling the “live” creatures in nature.     

Thermoresponsive Graphene Membranes with Reversible Gating Regularity for Smart Fluid Control

Precisely regulating water and molecule permeation through membranes is of crucial significance in broad domains such as water filtration and smart reactors. Comparing with routine stiff membranes, stimuli‐response polymers endow porous membranes with various gating properties, but most of these membranes have only one‐way gating performance, that is, either positive or negative. Here poly(N‐isopropylacrylamide) (PNIPAM) grafted graphene oxide (GO) membranes with reversible positive/negative gating regularity are constructed by simply tuning the molecule grafting density. The water and small molecule permeance of the membranes can be regulated by adjusting environment temperature. Based on this tunable thermoresponsive gating regularity, a bidirectional fluidic controlling system is designed by integrating a positive membrane and a negative membrane, which can be employed as a self‐adaptive gating reactor. This strategy provides an insight into constructing smart gating membranes with extraordinary properties, showing promising applications in micro/nanofluidic valves and temperature sensitive biochemical reactors.     

Polymer‐Modified Mesoporous Silica Thin Films for Redox‐Mediated Selective Membrane Gating

Controlling structure and function to switch ionic transport through synthetic membranes is a major challenge in the fabrication of functional nanodevices. Here we describe the combination of mesoporous silica thin films as structural unit, functionalized with two different redox‐responsive ferrocene‐containing polymers, polyvinylferrocene (PVFc) and poly(2‐(methacryloyloxy)ethyl ferrocenecarboxylate) (PFcMA), by using either a grafting to, or a grafting from approach. Both mesoporous film functionalization strategies are investigated in terms of polymer effect on ionic permselectivity. A significantly different ionic permselective behavior can be observed. This is attributed to different polymer location within the mesoporous film, depending on the functionalization strategies used. Additionally, the influence of chemical oxidation on the ionic permselective behavior is studied by cyclic voltammetry showing a redox‐controlled membrane gating as function of polymer location and the pH value. This study is a first step of combining redox‐responsive ferrocene‐containing polymers and mesoporous membranes, and thus towards redox‐controlled ionic transport through nanopores.     

Red‐ to NIR‐Emitting,BODIPY‐Based,K+‐Selective Fluoroionophores and Sensing Materials

Optical sensing materials for the selective measurement of potassium ions (K⁺) in water are presented. The indicator dyes are based on an aza‐crown ether as a receptor and borondipyrromethenes (BODIPY) dyes as fluorophores. Fluorescence enhancement is caused by the reduction of photoinduced electron transfer (PET) upon complexation with K⁺ ions. The family of new indicators possesses tuneable optical properties (green to red excitation, red to NIR emission) and PET efficiencies. They exhibit high brightness with quantum yields between 0.20 and 0.47 in the “on” state and a molar absorption coefficient between 30 000 and 290 000 m ^?1 cm^?1. The new indicator dyes are immobilized in biocompatible hydrogel matrices to obtain stable nonleaching and fast responding (t₉₀ ≈ 10 s) sensing materials for continuous measurements of potassium. They are realized in various formats such as planar optodes, fiber‐optic sensors, and water‐dispersible polymer‐based nanoparticles. Apart from fluorescence intensity measurements, self‐referenced read‐out of fluorescence decay time is demonstrated. All sensor materials display a high K⁺/Na⁺ selectivity and are not influenced by pH within the physiologically relevant range. Practical applicability of the materials is emphasized by application of a fiber‐optic sensor to quantification of K⁺ in serum, which shows excellent correlation with the reference measurements.