Hydrochromic and piezochromic dual-responsive optical film derived from poloxamer and ethyl cellulose for visual fingerprints identification.

Developing new materials that could identify fingerprint using the naked eye and observe the level 3 microscopic details is challenging. Here, we designed a novel hydrochromic and piezochromic dual-responsive optical film, which achieved the visual transparency transition. The performances of hydrochromic and piezochromic responses from high transparency to opaque whiteness were attributed to the introduction of poloxamer. The hygroscopic swelling of the disordered micelles led to light scattering, causing the hydrochromic response. The piezochromic response may be ascribed to the microcracks in the fragments of poloxamer crystals, which changed the refractive index of light. The fascinating combination of hydrochromic and piezochromic response was effectively applied in fingerprint identification. Hydrochromic response accurately recognized sweat pores, and piezochromic response could gradually reveal the ridges and valleys according to the different color of imprinted fingerprints. The film could identify fake fingerprints based on the differences in sweat pores between fake fingerprints and living fingers. More importantly, the film could easily detected not only the clear ridges but also the detailed sweat pores using the naked eye, indicating that the film has profound research significance in fingerprint analysis and liveness fingerprint detection.

Concealed fluorescent anti-counterfeiting paper prepared by loading perovskite and lead-metal-organic framework on cellulose fibers.

Counterfeiting has caused great concern all over the world. What's more, the fluorescent materials play an important role in technological research and development for high-security. In this work, lead-metal-organic framework (Pb-MOF) and perovskite (MAPbBr3) were used in papers to achieving fluorescence counterfeiting. Pb-MOF, as the template or precursor of MAPbBr3, were in-situ generated on the surface of cellulose fibers (CFs) to preparing into hand sheets (Pb-MOF@CFs). Through the analysis of experimental results, it was found that ligands, reaction systems, addition sequences of drugs, time, etc. would affect the deposition of Pb-MOF on the surface of CFs. Using CH3NH3Br (MABr) as the anti-counterfeiting ink to write on Pb-MOF@CFs, the orange writing leaped across the paper, which caused by Pb in Pb-MOF chemically reacting with MABr forming MAPbBr3. The orange writing displayed green fluorescence under 365 nm ultraviolet lamp excitation. The orange writing with green fluorescence could be extinguished and reconstructed, which had promise for reuse. In addition, fluorescent security papers (MAPbBr3@Pb-MOFs@CFs) were prepared by immersing Pb-MOF@CFs in MABr solution. The fluorescence of MAPbBr3@Pb-MOFs@CFs opened when the surface of it was scraped under 365 nm ultraviolet lamp. This unique fluorescence property was very important in improving the security of products. Consequently, the ongoing research on perovskite and MOFs materials is of great significance.

Visual fluorescence detection of ciprofloxacin by Zn-metal-organic framework@nanocellulose transparent films based on aggregation-induced emission.

The invention and production of Ciprofloxacin (CIP) have a positive impact on medical treatment, but the overuse of CIP is also harmful to the environment. In this paper, we prepared a novel film material for detection of CIP by in situ synthesis of zinc-based metal-organic framework (Zn-BDC) on TEMPO-oxidized cellulose nanofibers (TOCNF). The nanoscale Zn-BDC were uniformly distributed on the TOCNF that was beneficial to realize the transparency and functionality of Zn-BDC@TOCNF whose transparency was up to 87 %. Zn-BDC@TOCNF showed no fluorescence itself while showed bright fluorescence upon the contact of CIP, which was proposed as the aggregation-induced emission (AIE) of CIP that defused and assembled in the Zn-BDC@TOCNF. There was a certain linear relationship between fluorescence intensity and concentration of CIP (R2 = 0.994, LOD = 0.083 µM). In the detection process, CIP could still fluoresce in Zn-BDC@TOCNF even if it was interfered by other ions and small biological molecules, and the weak acid environment was conducive to AIE of CIP. Generally, it was of great significance to establish a rapid and effective monitoring mechanism for CIP in water for environmental protection and ecological balance.

Mechanochromic Self-Healing Materials with Good Stretchability, Shape Memory Behavior, Cyclability, and Reversibility Based on Multiple Hydrogen Bonds.

It is challenging to develop materials with room-temperature self-healing ability and mechanochromic response from mechanical stimuli to optical signals by a facile and simple preparation process. Herein, novel mechanochromic self-healing materials were designed by a simple synthesis procedure, balancing the mechanical properties, self-healing, stretchability, and mechanochromic response. Moreover, we designed and prepared the mechanochromic self-healing materials with different soft and hard segments by introducing multiple hydrogen bonds into the network, improving the mechanical properties and self-healing efficiency. In addition, the optimized sample exhibited good shape memory behavior (shape recovery ratio of 94.4%), self-healing properties (healed by pressing during stretching process), high tensile strength (17.6 MPa), superior stretchability (893%), fast mechanochromic response (strain of 272%), and great cyclic stretching-relaxing properties (higher than 10 times at strain of 300%). Above all, mechanochromic self-healing materials have promising potential in various fields, such as stress sensing, inkless writing, damage warning, deformation detection, and damage distribution.

Fast Response and Visual Transparency Switching Hydrochromic Film Based on the Rational Structure of Cellulose/Poloxamer Copolymers Design for Smart Window.

The authors are motivated to develop a series of hydrochromic copolymers with fast response, reversibility, repeatability, and visual transparency transition. The hydrochromic block copolymers are based on the rational ratio of hydrophilic segments of poloxamer block and hydrophobic segments of ethyl cellulose according to the preparation method of polyurethane. By tuning the ratio of hydrophilic segments or adding hygroscopic salts, the hydrochromic polymer is endowed with the ability to visualize the transparency in response to the relative humidity. Especially, the response time of the polymer is extremely shortened, up to 1 s for the optimized sample. Within the moisture stimulation, the hygroscopic swelling increases the film thickness, leading to a reversible transparency switching from a highly transparent state (82%) to an opaque white state (20.5%).

Nano-MIL-88A(Fe) Enabled Clear Cellulose Films with Excellent UV-Shielding Performance and Robust Environment Resistance.

While tremendous efforts have been dedicated to developing cellulose-based ultraviolet (UV)-blocking films, challenges still remain in simultaneously achieving high transparency, low haze and excellent UV shielding properties via simple and green strategy. Here, we present a facile and eco-friendly route to fabricate flexible, biodegradable and clear UV-shielding nano-MIL-88A(Fe)@carboxymethylated cellulose films (M(Fe)CCFs) via in situ synthesis of nano-MIL-88A(Fe) in carboxymethylated cellulose hydrogel followed by natural drying. The carboxymethylated cellulose film has high transmittance (93.2%) and low haze (1.8%). The introduction of nano-MIL-88A(Fe) endowed M(Fe)CCFs superior UV-shielding ability, while retaining high transmittance (81.5-85.3%) and low haze (2.5-4.9%). Moreover, M(Fe)CCFs showed stable UV blocking performance under UV irradiation, high temperature, acidic or alkaline conditions. Quite encouragingly, the UV-shielding ability of M(Fe)CCFs did not deteriorate, even after 30 days of immersion in aqueous solution, providing films with a long-term use capacity. Thus, M(Fe)CCFs show high potential in the UV protection field. Overall, these UV-blocking films with outstanding performances are a promising candidate to replace conventional film materials made from synthetic polymers in fields such as packaging and flexible electronics.

Tb-coordination polymer-anchored nanocellulose composite film for selective and sensitive detection of ciprofloxacin.

In recent years, along with the extensive application of ciprofloxacin (CIP), it has gradually become one of key environmental issues to be solved urgently. A novel fluorescent responsive nanocellulose composite film was successfully prepared by combining TEMPO-oxidized cellulose nanofibers (TOCNF) and terbium coordination polymer (Tb-AMP), in aqueous medium at room temperature via in-situ synthesis to detect CIP. CIP could supply energy for terbium ion through antenna effect to achieve the green fluorescence of Tb-AMP@TOCNF under 365 nm UV lamp. The transparency of the Tb-AMP@TOCNF was 88% when the deposition ratio of Tb-AMP on TOCNF was 7.2% and the environmental stability was good, which was conducive to fluorescent detection. As CIP concentration increased, the fluorescence intensity of Tb-AMP@TOCNF increased, and fluorescence intensity had a good linear relationship with CIP concentration in the range of 1-8 µM (y = 4.57 + 3.17x, R2 = 0.999, LOD = 0.0392 µM). It was a new way to realize future quantitative colorimetric analysis of pollutants.

Methyl Orange-Doped Polypyrrole Promoting Growth of ZIF-8 on Cellulose Fiber with Tunable Tribopolarity for Triboelectric Nanogenerator.

Cellulose fiber (CelF) is a biodegradable and renewable material with excellent performance but negligible triboelectric polarizability. Methods to enhance and rationally tune the triboelectric properties of CelF are needed to further its application for energy harvesting. In this work, methyl-orange-doped polypyrrole (MO-PPy) was in situ coated on CelF as a mediating layer to promote the growth of metal-organic framework ZIF-8 and to construct a cellulose-based triboelectric nanogenerator (TENG). The results showed that a small amount of MO-PPy generated in situ significantly promoted the growth of ZIF-8 on CelF, and the ZIF-8 deposition ratio was able to increase from 7.8% (ZIF-8/CelF) to 31.8% (ZIF-8/MO-PPy@CelF). ZIF-8/MO-PPy@CelF remained electrically conductive and became triboelectrically positive, and the triboelectricity's positivity was improved with the increase in the ZIF-8 deposition ratio. The cellulose-based TENG constructed with ZIF-8/MO-PPy@CelF (31.8% ZIF-8 deposition ratio) and polytetrafluoroethylene (PTFE) could generate a transfer charge of 47.4 nC, open-circuit voltage of 129 V and short-circuit current of 6.8 µA-about 4 times higher than those of ZIF-8/CelF (7.8% ZIF-8 deposition ratio)-and had excellent cycling stability (open-circuit voltage remained almost constant after 10,000 cycles). MO-PPy not only greatly facilitated the growth of ZIF-8 on CelF, but also acted as an electrode active phase for TENG. The novel TENG based on ZIF-8/MO-PPy@CelF composite has cheerful prospects in many applications, such as self-powered supercapacitors, sensors and monitors, smart pianos, ping-pong tables, floor mats, etc.

De-Doped Polyaniline as a Mediating Layer Promoting In-Situ Growth of Metal-Organic Frameworks on Cellulose Fiber and Enhancing Adsorptive-Photocatalytic Removal of Ciprofloxacin.

New kinds of inorganic-organic hybrid porous materials, metal-organic frameworks (MOFs), have shown great application potential in various fields, but their powdery nature limits their application to a certain extent. As a green and renewable biomass material in nature, cellulose fiber (CelF) has the advantages of biodegradability, recyclability and easy processing, and can be used as an excellent flexible substrate for MOFs. However, the efficient deposition of MOFs on CelF is still a great challenge for the development of this new material. Herein, polyaniline (PANI) and de-doped PANI (DPANI) with rich functional groups as a mediating layer was proposed to promote the in-situ growth and immobilization of some MOFs on CelF. The PANI (especially DPANI) layer greatly promoted the deposition of the four MOFs, and more encouragingly, significantly promoted the in-situ growth and nanocrystallization of MIL-100(Fe). MIL-100(Fe)@DPANI@CelF was selected as an adsorbent-photocatalyst to be used for the adsorptive-photocatalytic removal of ciprofloxacin (CIP) in water. The removal efficiency of CIP by MIL-100(Fe)@DPANI@CelF reached 82.78%, and the removal capacity of CIP was as high as 105.96 mg g-1. The study found that DPANI had a synergistic effect on both the in-situ growth of MIL-100(Fe) on CelF and the adsorption-photocatalysis of CIP in water. The universal platform of PANI-mediated in-situ growth and immobilization of MOFs on CelF constructed in this study widens the road for the development of MOF@CelF composites.

Fire retardant, UV and blue light double-blocking super clear Carboxymethylated cellulose bioplastics enabled by metal organic framework.

It is still a challenge to realize super clear cellulose-based film materials with different functional combinations. This study presents a novel concept of fabricating flame-retardant, mechanically strong, UV and blue light double-blocking carboxymethylated cellulose-based nanocomposite bioplastics enabled by nano-metal organic framework (MIL-125(Ti)-NH2). Carboxymethylated cellulose gel with porous structure acts as nanoreactor and carboxyl groups as reactive sites to facilitate the growth and anchorage of nano-MIL-125(Ti)-NH2. Super clear bioplastics were obtained through hot-pressing. The results show that the neat carboxymethylated cellulose bioplastic possesses high transmittance (94.1% at 600 nm) and low haze (2.0% at 600 nm). The incorporation of nano-MIL-125(Ti)-NH2 enabled nanocomposite bioplastics to obtain UV and blue light double-shielding capability meanwhile retaining high transmittance (79-92.8%) and low haze (2.6-7.2%). Moreover, the incorporation of nano-MIL-125(Ti)-NH2 was found to significantly improve the mechanical strength and decrease the flammability of nanocomposite bioplastics. This facile strategy would direct nanocomposite bioplastics toward diversified applications.

Nano-Metal Organic Framework for Enhanced Mechanical, Flame Retardant and Ultraviolet-Blue Light Shielding Properties of Transparent Cellulose-Based Bioplastics.

From the perspective of sustainable development and practical applications, there has been a great need for the design of multifunctional transparent cellulose-based composite films. We herein propose a novel concept of improving the mechanical, fire-resistant and ultraviolet (UV)-blue light shielding properties of cellulose-based composite bioplastic films though in situ embedding nano-metal organic framework (MIL-125(Ti)-NH2) into regenerated cellulose gel. Regenerated cellulose hydrogel (CH) with a porous structure acts as a nanoreactor and stabilizer to facilitate the growth and anchorage of MIL-125(Ti)-NH2 nanoparticles (MNPs). Subsequently, hot-pressing induces the formation of transparent MIL-125(Ti)-NH2@cellulose bioplastics (MNP@CBPs). As expected, the MNP@CBPs exhibit exceptional UV-blue light shielding capability, while retaining satisfactory optical transmittance. Meanwhile, with the incorporation of MNPs, the mechanical strength of MNP@CBPs is increased by 6.5~25.9%. In addition, MNPs enhance the flame retardant effect of the MNP@CBPs. The limited oxygen index (LOI) of the MNP@CBPs increased from 21.95 to 27.01%. The hot-pressing process improves the resistance of the MNP@CBPs to the penetration of water/non-aqueous liquids. This simple strategy would direct sustainable multifunctional MNP@CBPs toward diversified applications: food containers or packaging materials that can reduce or eliminate food spoilage, screen protectors for blocking harmful light, and promising candidates for protective plastic products, among others.

Mechanically Strong, Liquid-Resistant Photothermal Bioplastic Constructed from Cellulose and Metal-Organic Framework for Light-Driven Mechanical Motion.

The development of photothermal materials with a high light-to-heat conversion capability is essential for the utilization of clean solar energy. In this work, we demonstrate the use of a novel and sustainable concept involving cellulose liquefaction, rapid gelation, in situ synthesis and hot-press drying to convert cellulose and metal-organic framework (Prussian blue) into a stable photothermal bioplastic that can harvest sunlight and convert it into mechanical motion. As expected, the obtained Prussian blue@cellulose bioplastic (PCBP) can effectively absorb sunlight and the surface can be heated up to 70.3 °C under one sun irradiation (100 mW cm-2). As a demonstration of the practicality of PCBP, it was successfully used to drive a Stirling engine motion. Meanwhile, hot-pressing promotes the densification of the structure of PCBP and, therefore, improves the resistance to the penetration of water/non-aqueous liquids. Moreover, PCBP shows good mechanical properties and thermal stability. Given the excellent photothermal performance and environmentally friendly features of photothermal conversion bioplastic, we envisage this sustainable plastic film could play important roles toward diversified applications: a photothermal layer for thermoelectric generator, agricultural films for soil mulching and photothermal antibacterial activity, among others.

Integrated Conductive Hybrid Electrode Materials Based on PPy@ZIF-67-Derived Oxyhydroxide@CFs Composites for Energy Storage.

Due to excellent flexibility and hydrophilicity, cellulose fibers (CFs) have become one of the most potential substrate materials in flexible and wearable electronics. In previous work, we prepared cobalt oxyhydroxide with crystal defects modified polypyrrole (PPy)@CFs composites with good electrochemical performance. In this work, we redesigned the crystalline and nanoscale cobalt oxyhydroxide with zeolitic imidazolate frameworks-67 (ZIF-67) as precursor. The results showed that the PPy@ZIF-67 derived cobalt oxyhydroxide@CFs (PZCC) hybrid electrode materials possess far better capacitance of 696.65 F·g-1 than those of PPy@CFs (308.75 F·g-1) and previous PPy@cobalt oxyhydroxide@CFs (571.3 F·g-1) at a current density of 0.2 A·g-1. The PZCC delivers an excellent cyclic stability (capacitance retention of 92.56%). Moreover, the PZCC-supercapacitors (SCs) can provide an energy density of 45.51 mWh cm-3 at a power density of 174.67 mWh·cm-3, suggesting the potential application in energy storage area.

Phytic Acid Doped Polypyrrole as a Mediating Layer Promoting Growth of Prussian Blue on Cotton Fibers for Solar-Driven Interfacial Water Evaporation.

Phytic acid doped polypyrrole (PPy) as a mediating layer was in-situ coated on cotton fibers (CFs) to promote the growth of Prussian blue (PB) and construct the PB/PPy@CFs composite. The results showed that the proper amounts of PA doped PPy in-situ generated significantly promoted the growth of PB on CFs, the PB deposition ratio increased from 12.29% (PB@CFs) to 32.4% (PB/PPy@CFs), and the growth of PB on PPy@CFs could be completed in 4 h. Scanning electron microscopy (SEM) showed that the PB particles with perfect nano cubic structure were formed in the composite. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) showed that both PB and PPy were successfully deposited on CFs. The PB/PPy@CFs composite had excellent light absorption, hydrophilicity, wettability, and photothermal property, and the surface could be heated up to 81.5 °C under one sun illumination. The PB/PPy@CFs composite as a photothermal conversion material was used for solar-driven interfacial water evaporation, the water evaporation rate was 1.36 kg·m-2·h-1 at the optical concentration of 1 kW·m2, and the corresponding photothermal conversion efficiency increased from 81.69% (PB@CFs) to 90.96% (PB/PPy@CFs).

Design and fabrication of high performance supercapacitor with cellulosic paper electrode and plant-derived redox active molecules.

As a promising substrate, cellulose fibers were widely investigated in supercapacitors for their low cost and sustainability. However, the low performance created great barrier for the future applications of the cellulosic paper-based supercapacitors. The performance of paper-based supercapaciors may be improved by the addition of redox active molecule. As a plant derived redox active molecule, Alizarin red S was used to improve the performance of PEDOT paper-based electrode via a simple post-treatment process. By combination of the treated paper electrode and the redox electrolyte, a symmetric paper-based supercapacitor with a superior performance of 2191.3 m F/cm2 (at 5 mA/cm2) and 4.87 mW h/cm3 (at power density of 36 mW/cm3) were fabricated. The charge and mass transfer mechanisms of paper electrode were detailed discussed. The simple and efficient strategy developed in this work opens up new doors for the development of other cellulose related high performance energy storage devices.

Eu-metal organic framework@TEMPO-oxidized cellulose nanofibrils photoluminescence film for detecting copper ions.

Metal-organic frameworks (MOFs) are emerging highly crystallized three-dimensional network complex formed by self-assembling metal ions and organic ligands. However, all MOFs are nanoscale and micro scale powder materials, which greatly impedes their further applications. In this study, a transparent Eu-MOF@TEMPO-oxidized cellulose nanofibrils (Eu-MOF@TOCNF) photoluminescence material for specifically detecting copper ions was fabricated via in-situ synthesis in hydroalcoholic medium. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, Fluorescence spectrometer and other equipment were applied to characterize functionalized TOCNF film samples, and the results confirmed the successful fabrication of the functionalized TOCNF film with stable fluorescence properties. The film performed a high selectivity toward copper ion in the presence of other interfering metal ions. The fluorescence intensity of the film decreased gradually with the increase of copper ion concentration, and I0/I-1 developed a good linear relationship with [Cu2+], which made the film a promising material for detecting Cu2+ in water body.

High mass loading polyaniline layer anchored cellulose fibers: Enhanced interface junction for high conductivity and flame retardancy.

Novel cellulose fibers-based composite consisted of zirconium oxyhydroxide and phytic acid doped polyaniline was prepared via a two-step method of simple chemical precipitation and followed by in situ polymerization process. Cellulose fibers were firstly modified with zirconium oxyhydroxide to enhance the binding of phytic acid doped polyaniline to the surface. A compact coating of phytic doped polyaniline was developed on zirconium oxyhydroxide modified cellulose fibers through the chelating of zirconium ions to phytic acid. The resulting composite possessed a controllable mass loading of polyaniline, which could significantly improve the conductivity, flame retardancy and electrochemical stability. Therefore, the expected chelating between zirconium ions on cellulose fibers and phytic acid doped in polyaniline supported the excellent properties of the composite paper. Notably, the developed strategy is efficient, low-cost and environmental friendly, and the work opens up new doors to the development of other cellulose fibers-related interface enhancement applications.

Construction of flexible electrodes based on ternary polypyrrole@cobalt oxyhydroxide/cellulose fiber composite for supercapacitor.

With the development of flexible electronic devices, flexible energy storage systems have been research hotpot. Conductive polymers is potential pseudocapacitor materials in energy storage field. Meanwhile, cellulose fiber with natural, degradable, renewable and flexible properties is one of tremendous promising alternatives to the flexible substrates. Hence, a polypyrrole@cobalt oxyhydroxide/cellulose fiber composite electrode is prepared via "liquid phase reduction" strategy in open system at room temperature. The composite electrode exhibits excellent electrochemical properties, which has a high specific capacitance and capacitance retention. The highest specific capacitance of 571.3 F g-1 at 0.2 A g-1 is obtained. Besides, the specific capacitance of the composite electrode has no significant loss, showing high cycle stability (93.02% after 1000 cycles). The excellent electrochemical properties can be ascribed to the introduction of cobalt oxyhydroxide, which restrains the volumetric change of polypyrrole in the electrochemical redox process, and promotes the rapid migration of electrons.

Alkyne functionalized cellulose fibers: A versatile "clickable" platform for antibacterial materials.

We report a facile and effective method to fabricate clickable alkyne-functionalized cellulose fibers (ACFs) through in situ chemical oxidation copolymerization of 3-ethynylaniline and aniline under acidic aqueous solution. The effects of process variables on copolymer deposition onto CFs were investigated and suitable preparation conditions were identified. It was found that aniline significantly facilitated the polymerization of 3-ethynylaniline and shortened the preparation time of ACFs from 48 to 6 h. Antibacterial-modified cellulose fibers were prepared by binding ß-cyclodextrin (ß-CD) to cellulose fibers via click chemistry, followed by the inclusion of ciprofloxacin hydrochloride (CipHCl). The loading and releasing behaviors of CipHCl into/from click product (ACFs@Azide-ß-CD) were also revealed. The load amount of CipHCl into ACFs@Azide-ß-CD increased remarkably, and the release of CipHCl from ACFs@Azide-ß-CD was prolonged. The ACFs@Azide-ß-CD loaded with CipHCl exhibited higher and longer-term antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureu) compared with CFs and ACFs.

Integration of graft copolymerization and ring-opening reaction: A mild and effective preparation strategy for "clickable" cellulose fibers.

A mild and effective strategy to prepare alkynyl-functionalized cellulose fibers (A-CFs) and azido-functionalized cellulose fibers (N3-CFs) was presented. Epoxy-cellulose fibers (Epoxy-CFs), graft copolymerization products of cellulose fibers (CFs) and glycidyl methacrylate (GMA), were prepared using cerium ammonium nitrate (CAN) as initiator. Epoxy groups content of Epoxy-CFs were as high as 2.5 mmol/g. Introduction of alkynyl/azido groups into Epoxy-CFs were achieved through ring-opening reactions of epoxy groups with propargylamine (PgAm) and sodium azide (NaN3), respectively. Under appropriate conditions, A-CFs with alkynyl groups of 0.57 mmol/g and N3-CFs with azido groups of 0.35 mmol/g were obtained. Click reactivities of A-CFs, N3-CFs and Epoxy-CFs were verified by Cu(I) catalyzed alkyne-azido cycloaddition (CuAAC) and thiol-epoxy click reactions. A-CFs, N3-CFs, Epoxy-CFs and their clicked products were characterized by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS).