Towards a scalable and controllable preparation of highly-uniform surface-enhanced Raman scattering substrates: Defect-free nanofilms as templates.

The uniformity and reproducibility of substrates highly determine the applicability of surface-enhanced Raman scattering (SERS). Production of them, however, remains a challenge. Herein, we report a template-based strategy for the strictly controllable and handily scalable preparation of a very uniform SERS substrate, Ag nanoparticles (AgNPs)/nanofilm, where the template used is a flexible, transparent, self-standing, defect-free and robust nanofilm. Importantly, the obtained AgNPs/nanofilm is self-adhesive to surfaces of different properties and morphologies, ensuring in-situ and at real-time SERS detection. The enhancement factor (EF) of the substrate for rhodamine 6G (R6G) could reach 5.8 × 1010 with a detection limit (DL) of 1.0 × 10-15 mol L-1. Moreover, 500 bending tests and one-month storage showed no observable performance degradation, and up to 50.0 cm2 scaled-up preparation depicted negligible effect upon the structure and the sensing performance. The real-life applicability of AgNPs/nanofilm was demonstrated by the sensitive detection of tetramethylthiuram disulfide on cherry tomato and fentanyl in methanol with a routine handheld Raman spectrometer. This work thus provides a reliable strategy for large area wet-chemical preparation of high-quality SERS substrates.

Imine Bond-Based Fluorescent Nanofilms toward High-Performance Detection and Efficient Removal of HCl and NH3.

A new kind of imine bond-based fluorescent nanofilm was developed as multifunctional materials for high-performance detection and efficient removal of hydrogen chloride (HCl) and ammonia (NH3). The flexible, uniform, and photochemically stable nanofilms as prepared showed fast (<1 and <0.5 s), sensitive (<150 ppb and <1.5 ppm), and selective response to HCl and NH3, respectively, and the removal efficiencies to HCl and NH3 are 187.5 and 37.5% (w/w), respectively. A reversible earthy-red to green fluorescence color change upon adsorption of NH3 or HCl enabled visualized monitoring of the two gases in the air. Mechanism studies revealed that the adsorption of HCl is a result of hydrogen bond formation between the analyte and the imine groups. Adsorption of NH3, however, is a result of chemical reaction with the pre-adsorbed HCl. The applicability of the detection and removal strategies as developed was further verified by conducting the tests on real-life or simulated scenarios.

Real-time and rapid prediction of TVB-N of livestock and poultry meat at three depths for freshness evaluation using a portable fluorescent film sensor.

Traditional meat freshness evaluation methods are cumbersome and time consuming. In this study, the freshness of goat and duck meat at -1, 4, 10, and 25 °C was monitored by the fluorescent film sensor, and the content of total volatile basic nitrogen (TVB-N) and biogenic amines (BAs) of the samples was also determined using traditional methods. Correlation and partial least squares (PLS) regression analyses were performed between sensor response intensity (RI) and freshness indices. The results showed that the RI, TVB-N, and BAs contents of goat and duck meat at a subcutaneous sampling depth of 0-1 cm were highly correlated. Moreover, the regression coefficients (R2) of the PLS model of TVB-N were all higher than 0.8. Notably, the R2 of duck meat at 25 °C in the PLS model was 1. This study accurately predicted TVB-N values in livestock and poultry meats by the fluorescent film sensor for the first time, which is real-time, and rapid, with great potential for meat freshness evaluation in future production.

Dual-Phase Emission AIEgen with ICT Properties for VOC Chromic Sensing.

In a film-based fluorescence sensor, luminogens are of vital importance since they play the role of probes or indicators. Traditional organic luminogens like pyrene show high luminescence quantum yields in dilute solutions, but their applications are usually limited by the aggregation-caused quenching (ACQ) effect and bad photochemical stability. Thus, this paper reports a novel aggregation-induced emission luminogen (AIEgen) containing both pyrene and o-carborane (CB-PY), which possesses unique dual-phase emission both in solution and solid state and intramolecular charge transfer (ICT) properties, fulfilling the gap between ACQ and AIE compounds. Importantly, the fluorophore presents extraordinary stability that there was almost no attenuation in the emission intensity of CB-PY in the solid state after 4 months of exposure at ambient conditions. It is these merits that make CB-PY exhibit outstanding sensing performances for volatile organic compounds (VOCs), where the fluorescence test strip shows fast, reversible, and visual discrimination of four organic solvents with varied polarities. Moreover, 92#, 95#, and 98# gasolines could be discriminated with CB-PY, showing different colors under UV illumination.

Robust and Large-Area Calix[4]pyrrole-Based Nanofilms Enabled by Air/DMSO Interfacial Self-Assembly-Confined Synthesis.

The modular construction of defect-free nanofilms with a large area remains a challenge. Herein, we present a scalable strategy for the preparation of calix[4]pyrrole (C[4]P)-based nanofilms through acryl hydrazone reaction conducted in a tetrahydrazide calix[4]pyrrole (CPTH)-based self-assembled layer at the air/DMSO interface. With this strategy, robust, regenerable, and defect-free nanofilms with an exceptionally large area (∼750 cm2) were constructed. The thickness and permeability of the film systems can be fine-tuned by varying the precursor concentration or by changing another building block. A typical nanofilm (C[4]P-TFB, ∼67 nm) depicted high water flux (39.9 L m-2 h-1 under 1 M Na2SO4), narrow molecular weight cut-off value (∼200 Da), and promising antifouling properties in the forward osmosis (FO) process. In addition, the nanofilms are stable over a wide pH range and tolerable to different organic solvents. Interestingly, the introduction of C[4]P endowed the nanofilms with both outstanding mechanical properties and unique group-selective separation capability, laying the foundation for wastewater treatment and pharmaceutical concentration.

pH-Responsive dopamine-based nanoparticles assembled via Schiff base bonds for synergistic anticancer therapy.

The dynamic Schiff base bond is exploited to construct monodisperse dopamine-based nanoparticles with autofluorescence and pH-sensitivity. This allows facile monitoring and pH-responsive drug release in the acidic tumor microenvironment. Anticancer drugs doxorubicin and a photosensitizer chlorin e6 are further loaded into the nanoparticles and synergistic anticancer efficacy is achieved.

One-Step Synthesis of Hydrophobic Multicompartment Organosilica Microspheres with Highly Interconnected Macro-mesopores for the Stabilization of Liquid Marbles with Excellent Catalysis.

The combination of an emulsion template with polymerization is a very convenient approach to the one-step realization of both simple control porous structures via a change in emulsion formulation and easy functionalization via the concomitant choice of an on-demand monomer. A major challenge of this approach is the inherent instability of the oil/water interface in emulsions, especially the occurrence of chemical reactions in oil or aqueous phases. This study reports the pioneering preparation of highly interconnected macro-mesopores and multicompartment (HIMC) vinyl organosilica microspheres with hydrophobicity by the one-step formation of W/O/W emulsions acting as a template. The emulsion system consists of acidified deionized water, a stabilizer, and vinyltriethoxysilane (VTEO) in which VTEO can be used to produce an organosilica skeleton of the resultant microsphere by a sol-gel process. The study demonstrated that the marvelous stability of W/O/W emulsions aids the formation of multicompartment organosilica microspheres with highly interconnected macro-mesopores by emulsion droplets rather than single-compartment (SC) microspheres. Meanwhile, the internal porous structure and surface morphology of as-prepared organosilica microspheres could be largely tuned by a simple variation of the pH value, the volume fraction of the water phase, and the stabilizer concentration in the initiating multiemulsions. Benefiting from such a well-orchestrated structure and the existence of numerous vinyl groups on the surface, HIMC organosilica microspheres exhibit very high hydrophobicity (with a water contact angle larger than 160°), which allows them to stabilize liquid marbles with excellent stability and high mechanical robustness. Because of its strong catalyst, Ag nanoparticles within HIMC organosilica microspheres enable Ag/HIMC-vinyl organosilica microsphere-based liquid marbles to be an efficient catalytic microreactor, realizing the complete degradation of MB to leuco methylene blue by NaBH4 in 10 min. The result of this work could provide some guidance for the easy, low-cost, benign preparation of HIMC microspheres having the potential to be excellent supporter of metal nanoparticles or other functionalized compounds for applications in sensing, optoelectronics, and catalysis.

Molecular Gels as Intermediates in the Synthesis of Porous Materials and Fluorescent Films: Concepts and Applications.

Low-molecular-mass organic gelator (LMOG)-based molecular gels are known as one of the most attractive soft materials and have received great attention since the early 1990s. In the last few decades, many LMOGs have been synthesized, and a series of theories have been proposed to better understand molecular gels. However, only limited applications of LMOGs have been realized for a variety of reasons, such as their lack of stability compared to chemical gels. Therefore, efforts to explore the applications of these materials are especially meaningful. As an example, this feature article mainly introduces studies on the application of LMOGs as intermediates in porous materials and fluorescent sensing films. Particular attention will be paid to gelator design, LMOG emulsion preparation, solid surface modification, and the practical application of the obtained materials. Concepts that are related to these studies, such as organic gel-water interface equilibria and molecular gel strategies, will be comprehensively illustrated. Finally, we will conclude with a study of LMOG-based intermediates. Some challenges and future perspectives related to these research areas will also be presented. It is anticipated that this feature article will not only contribute to the further understanding of LMOG-based intermediates but also will help to promote the practical application of molecular gels and facilitate development in related research areas.

Smart magnetic ionic liquid-based Pickering emulsions stabilized by amphiphilic Fe3O4 nanoparticles: Highly efficient extraction systems for water purification.

This study presents the general method to formulate magnetically responsive ionic liquid (IL)-based Pickering emulsions that are stabilized by amphiphilic Fe3O4 nanoparticles. The magnetic nanoparticle stabilizer (MN-CHOL) was synthesized using the surface-initiated ATRP method with further modification that uses a specially designed cholesteryl derivative, and characterized by FT-IR, XPS, TGA, and magnetization measurements. It is confirmed that the resulting MN-CHOL shows a stronger interfacial activity, efficiently emulsifying C4mim [PF6] and water, and resultantly forming stable Pickering emulsions without the help of any co-surfactant. Due to its super paramagnetism and high saturation magnetization, MN-CHOL attached on the IL interface enables droplets of IL to be moved very conveniently on their target for as many times by an external magnetic field without demulsification, indicating high controllability and excellent stability. The resulting Pickering emulsion is a good extraction system that efficiently separates chlorobenzene, phenol, and methyl orange from aqueous solution. Subsequently, the simple magnetic separation was applied, to produce purified water due as a result of the rapid removal of organic pollutants from contaminated water.

Creation of reduced graphene oxide based field effect transistors and their utilization in the detection and discrimination of nucleoside triphosphates.

Two low-cost, micropatterned, solution-gated field effect transistors (modified FET and unmodified FET) based on reduced graphene oxide (RGO) were developed and used for detection and discrimination of nucleoside triphosphates (NTPs). The modified FET was realized by simple deposition of a positively charged bis-pyrenyl derivative, py-diIM-py, onto the conducting RGO strips of the unmodified FET. The electrical properties and sensing behaviors of the as-prepared devices were studied comprehensively. Electrical transfer property tests revealed that both of the two FETs exhibit V-shaped ambipolar field effect behavior from p-type region to n-type region. Sensing performance studies demonstrated that modification of the native FET with py-diIM-py improves its sensing ability to NTPs-GTP and ATP in particular. The detection limit of GTP and ATP was as low as 400 nM, which is the lowest value for graphene-based electronic sensors reported so far. Furthermore, based on the cross-reactive responses of the two devices to NTPs, NTPs can be conveniently distinguished via combining use of the two devices. The enhancement of the modifier (py-diIM-py) to the sensing performance of the FET is tentatively attributed to its possible mediation role in sticking onto RGO strips and accumulating analytes by electrostatic association with the relevant species. Because they are sensitive and fast in response, simple and low-cost in preparation, and possibly useful in sensor-array fabrication, the developed sensors show great potential in real-life application.

Towards a new FRET system via combination of pyrene and perylene bisimide: synthesis, self-assembly and fluorescence behavior.

A new fluorescent derivative of cholesterol, N,N'-(N-(2-(3ß-cholest-5-en-3yl-formamido)ethyl) pyrene-1-sulfonamido)ethyl perylene-3,4:9,10-tetracarboxylic acid bisimide (CPPBI), was designed and synthesized. In the design, pyrene (Py) and perylene bisimide (PBI) were specially chosen as the energy donor and the acceptor, respectively. Fluorescence studies revealed that (1) CPPBI shows a strong tendency to form supra-molecular assemblies, (2) the assemblies possess a high efficiency of fluorescence resonance energy transfer (FRET) via intermolecular interactions, and (3) the profile and position of its fluorescence emission are highly dependent upon the nature of its medium, but the medium shows little effect on the efficiency of the energy transfer, suggesting that the chromophores including both Py and PBI units enjoy some rotational and/or translational mobility in the aggregated state of the compound. Temperature- and concentration-dependent (1)H NMR spectroscopy studies revealed that both hydrogen-bonding and π-π stacking play a great role in stabilizing the assemblies of the compound, and confirmed the existence of π-π stacking between the Py moieties and between the PBI residues of the compound, of which the donor and the acceptor may have arranged in an appropriate orientation and at a suitable distance which are the key factors to determine the FRET efficiency. Moreover, the CPPBI-based film possesses unusual photochemical stability, and its emission is sensitive to the presence of some organic vapors, in particular aniline.

Synergetic effect based gel-emulsions and their utilization for the template preparation of porous polymeric monoliths.

A polymerizable cholesteryl derivative (COA) was synthesized and used as a stabilizer for creating gel-emulsions with water in polymerizable monomers, of which they are styrene (ST), tert-butyl methacrylate (t-BMA), ethylene glycol dimethyl acrylate (EGDMA), and methyl methacrylate (MMA), etc. Interestingly, in addition to COA, the presence of a small amount of Span-80 is a necessity for the formation of the monomers containing gel-emulsions. Unlike conventional ones, the volume fraction of the dispersed phase in the gel-emulsions as created could be much lower than 74%, a critical value for routine gel-emulsions. Stabilization of these gel-emulsions as created has been attributed to the synergetic effect between COA, a typical low-molecular-mass gelator (LMMG), and Span-80, a surfactant, of which the former gels the continuous phase and the latter minimizes the interfacial energy of the continuous phase and the dispersed phase. SEM observation confirmed the network structures of COA in the gel-emulsions. Rheological tests demonstrated that the storage modulus, G', and the yield stress of the gel-emulsions decrease along with increasing the volume fraction of the dispersed phase, water, provided it is not greater than 74%-a result inconsistent with the theory explaining formation of routine gel-emulsions and in support of the conclusion that the systems under study follow a different mechanism. Furthermore, unlike LMMG-based stabilizers reported earlier, the gelator, COA, created in the present study has been functioning not only as a stabilizer but also a monomer. To illustrate the conceptual advantages, the gel-emulsions of water in ST/DVB/AIBN were polymerized. As expected, the densities and internal structures of the monoliths as prepared are highly adjustable, functionalization of the materials with cholesterol has been realized, and at the same time the problem of stabilizer leaking has been avoided. A preliminary test for gas adsorption demonstrated that the monoliths as prepared are good adsorbents for some volatile organic compounds (VOCs), in particular benzene, toluene, ethylbenzene, and xylene-the famous and toxic BTEX. It is believed that the findings reported in the present work provide not only a new strategy for creating novel gel-emulsions but also a new route for functionalizing porous polymeric monoliths.

"Yin and Yang" tuned fluorescence sensing behavior of branched 1,4-bis(phenylethynyl)benzene.

Achieving high sensing performance and good photostability of fluorescent films based on adlayer construction represents a significant challenge in the area of functional fluorescent film research. A solution may be offered by "Yin and Yang", a balance idea from Chinese philosophy, for the design of a fluorophore and the relevant assembly. Accordingly, a 1,4-bis(phenylethynyl)benzene (BPEB) derivative (C2) with two cholesteryl residues in the side chains and two glucono units in the head and tail positions was designed and synthesized. As a control, compound C1 was also prepared. The only difference between C1 and C2 is that the hydroxyl groups in the glucono residues of C1 are fully acetylated. Studies of the fluorescence behaviors of the two compounds in solution revealed that both the profile and the intensity of the fluorescence emission of the compounds, in particular C2, are dependent on their concentration and on the nature of solvents employed. Presence of HCl also alters the emission of the compounds in solution. On the basis of the studies, three fluorescent films were prepared, and their sensing performances to HCl in vapor state were studied. Specifically, Film 1 and Film 3 were fabricated via physical coating, separately, of C2 and C1 on glass plate surfaces. As another comparison, Film 2 was also fabricated with C2 as a fluorophore but at a much lower concentration if compared to that for the preparation of Film 1. As revealed by SEM and fluorescent microscopy studies, Film 1 and Film 2 exhibit well-defined microstructures, which are spherical particles and spherical pores, respectively, while Film 3 is characterized by irregular aggregates of C1. Fluorescence measurements demonstrated that Film 1 and Film 3 both display an aggregation emission, of which the emission from Film 1 is supersensitive to the presence of HCl vapor (detection limit: 0.4 ppb, a lowest value reported in the literatures). For Film 3, however, its emission is insensitive to the presence of the vapor. Similarly, the emission from the nonaggregated state of C2, a characteristic emission of Film 2, is also insensitive to the presence of the vapor. Furthermore, the sensing process of Film 1 to the vapor is highly selective and fully reversible, which lays foundation for its real-life uses. As for C2, the results from solution studies and those from film studies demonstrate clearly that introduction of auxiliary structures with opposite properties onto a typical fluorophore is a good strategy to develop fluorescent supramolecular motifs with rich assembly properties and great potential of applications.

Water-in-oil gel emulsions from a cholesterol derivative: structure and unusual properties.

As the first example of cholesterol derivatives, butane-1,4-dicarboxamide of di-cholesteryl L-alaninate has been shown to form water-in-oil type gel emulsions. The oils can be n-alkanes (7 < or = n < or = 10) and commercial fuels. Importantly, the preparation of the gel emulsions is a simple agitation process at room temperature, heating, cooling, addition of a co-solvent, or other additional procedures is not necessary. SEM, optical and con-focal laser scanning fluorescence microscopy measurements revealed the foam-like structures of the gel emulsions. Rheological studies demonstrated that these gel emulsions are mechanically stable, and exhibit typical viscoelastic properties. Surprisingly, the storage modulus, G' and the yield stress of the gel emulsions decrease along with increasing the volume ratio of the dispersed phase, water, a property different from the gel emulsions reported in the literatures. From the view point of application, the present gel emulsions are superior to others because of the simplicity in preparation and the non-ionic nature of the emulsifier, which will definitely benefit the practical applications.

Novel dimeric cholesteryl-based A(LS)2 low-molecular-mass gelators with a benzene ring in the linker.

Three novel dimeric cholesteryl-based A(LS)(2) low-molecular-mass organic gelators (LMOGs) with phthaloyl, isophthaloyl, or terephthaloyl moieties in the linkers were designed and prepared. According to the linker structures, the compounds are denoted as 1 (o-), 2 (m-), and 3 (p-), respectively. Gelation tests revealed that the difference of relative positions of two cholesterol moieties in the benzene ring can produce a dramatic change in the gelation behaviors of the compounds. Importantly, 2 and 3 are more efficient gelators than 1, and their self-assembly behaviors are also very different from each other as revealed by scanning electron microscopy (SEM) measurements. Very interestingly, 2 gels xylene spontaneously at room temperature, and the sol-gel phase transition of the system is mechanically controllable. FTIR and (1)H NMR spectroscopy studies revealed that hydrogen bonding and pi-pi interactions between the molecules of the gelators play an important role in the formation and maintenance of the gels. The X-ray diffraction (XRD) analysis revealed that in the gel of 2/benzene, 2 aggregated into a layered structure with an interlayer distance of 3.54 nm, which is just the length of 2.

Supramolecular gels based on organic diacid monoamides of cholesteryl glycinate.

Three diacid monoamides of cholesteryl glycinate (1, 2, and 3) were designed and synthesized. The gelation behaviors of these compounds and their corresponding ammonium salts (1('), 2('), and 3(')) were tested in 28 solvents. It was found that 1, 2, and 3 were weak gelators, but their neutralization with ammonia enhanced their gelling ability significantly. In particular, 1(') behaves as an "ambidextrous gelator." It gelatinizes both apolar solvents and water. More interestingly, some of the alkyl alcohols and water can be gelatinized at room temperature by simply bubbling ammonia into the system, which contained a suitable amount of 1. Several techniques, such as polarizing optical microscopy and scanning electron microscopy (SEM), revealed that 1(') aggregates into spherulite microparticles in 1-pentanol, at the dilution or the gel state. Moreover, SEM monitoring studies revealed that 1(') first aggregated into microparticles and then these particles aggregated into fiber-like structures and finally into a three-dimensional (3D) network structure. Attenuated total reflection Fourier infrared transform spectroscopy and salt effect studies demonstrated respectively that hydrogen bonding formation between NH and CO and electrostatic interaction are two of the main driving forces for the formation of the gels.

New dicholesteryl-based gelators: chirality and spacer length effect.

Eight new diacid amides of dicholesteryl L(D)-alaninates were designed and prepared. The compounds with spacers containing three, four, five, or six carbon atoms and L-alanine residues are denoted as 1a, 2a, 3a, and 4a, respectively, and those containing D-alanine residues are denoted as 1b, 2b, 3b, and 4b, respectively. A gelation test revealed that a subtle change in the length of the spacer and an inverse in the chirality of the amino acid residue can produce a dramatic change in the gelation behavior of the compounds and the microstructures of the gels, as revealed by SEM, XRD, and CD measurements. Importantly, for the compounds 1 and 2, those containing d-alanine residues (1b, 2b) are more efficient gelators than their analogues with opposite chirality (1a, 2a). For the compounds of longer spacers (3, 4), however, those containing l-alanine residues (3a, 4a) are superior to the corresponding ones with d-alanine residues (3b, 4b). Very interestingly, of the 139 gel systems studied, at least 11 of them gel spontaneously at room temperature. Studies of the rheological properties of the example systems of these gels demonstrated that change in the spacer lengths of the gelators has a great effect upon the mechanical properties of the corresponding gels, and the studies also revealed the thixotropic properties of the gels. Furthermore, it was observed that 4a forms water-in-oil gel emulsions with some organic solvents by simple agitating the systems at room temperature.

A novel low-molecular-mass gelator with a redox active ferrocenyl group: tuning gel formation by oxidation.

A novel low-molecular-mass gelator containing a redox-active ferrocenyl group, cholesteryl glycinate ferrocenoylamide (CGF), was intentionally designed and prepared. It was demonstrated that the gelator gels 13 out of the 45 solvents tested. Scanning electron microscopy (SEM) measurements revealed that the gelator self-assembled into different supramolecular network structures in different gels. Chemical oxidation of the ferrocenyl residue resulted in phase transition of the gel from gel state to solution state. FTIR and (1)H NMR spectroscopy studies revealed that hydrogen bonding between the gelator molecules in the gel was one of the main driving forces for the formation of the gels.

Amino acid derivatives of cholesterol as "latent" organogelators with hydrogen chloride as a protonation reagent.

A series of low molecular weight organic gelator (LMOG) gel systems sensitive to alkaline/acidic stimuli was established by employing amino acid derivatives of cholesterol as "latent" gelators, which are cholesteryl glycinate (1), cholesteryl L-alaninate, cholesteryl D-alaninate, cholesteryl L-phenyl alaninate, and cholesteryl D-phenyl alaninate. The hydrochloric salts are denoted as 2, 3, 4, 5, and 6, respectively. For the 18 solvents tested, one proved to be a weak gelator and gels only two of the solvents. Its gelation ability, however, was greatly improved by bubbling HCl gas, which was produced by reaction of concentrated sulfuric acid with NaCl, through its solution owing to protonation of its amino group. It was demonstrated that the protonated form of it gelled 14 of the solvents tested. Further investigation revealed that the gels changed into solution with addition of any of the amines, including triethylamine (TEA), diethylamine, ethylenediamine, and NH3. The phase transition could be reversed by further introduction of the acidic gas. SEM measurements showed that 1 self-assembled into different supramolecular structures in different gels. Salt effect studies proved that electrostatic interaction is one of the driving forces for formation of the gels.

Preparation of silver-poly(acrylamide-co-methacrylic acid) composite microspheres with patterned surface structures.

Acrylamide (AM) and methacrylic acid (MAA) copolymer microgels were prepared by a reverse suspension polymerization technique. The microgels were used as templates for the preparation of silver-poly(acrylamide-co-methacrylic acid) [Ag-P(AM-co-MAA)] composite microspheres. The surface structures of the microspheres prepared in this way are characterized by zigzag-like structures. It was found that the composition of the microgels, the nature and dosage of surfactants, the quantity of the metal, and even the reduction methods employed have a significant effect upon the surface structures of the microspheres. X-ray diffraction analysis confirmed that Ag formed during the process is in a crystal state of a face-centered cubic structure.