extfieldsofstudy: Medicine Chemistry

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Two New Fluorinated Phenol Derivatives Pyridine Schiff Bases: Synthesis, Spectral, Theoretical Characterization, Inclusion in Epichlorohydrin-β-Cyclodextrin Polymer, and Antifungal Effect.

It has been reported that the structure of the Schiff bases is fundamental for their function in biomedical applications. Pyridine Schiff bases are characterized by the presence of a pyridine and a phenolic ring, connected by an azomethine group. In this case, the nitrogen present in the pyridine is responsible for antifungal effects, where the phenolic ring may be also participating in this bioactivity. In this study, we synthesized two new pyridine Schiff Bases: (E)-2-[(3-Amino-pyridin-4-ylimino)-methyl]-4,6-difluoro-phenol (F1) and (E)- 2-[(3-Amino-pyridin-4-ylimino)-methyl]-6-fluoro-phenol (F2), which only differ in the fluorine substitutions in the phenolic ring. We fully characterized both F1 and F2 by FTIR, UV-vis, 1H; 13C; 19F-NMR, DEPT, HHCOSY, TOCSY, and cyclic voltammetry, as well as by computational studies (DFT), and NBO analysis. In addition, we assessed the antifungal activity of both F1 (two fluorine substitution at positions 4 and 6 in the phenolic ring) and F2 (one fluorine substitution at position 6 in the phenolic ring) against yeasts. We found that only F1 exerted a clear antifungal activity, showing that, for these kind of Schiff bases, the phenolic ring substitutions can modulate biological properties. In addition, we included F1 and F2 into in epichlorohydrin-β-cyclodextrin polymer (βCD), where the Schiff bases remained inside the βCD as determined by the ki , TGA, DSC, and SBET. We found that the inclusion in βCD improved the solubility in aqueous media and the antifungal activity of both F1 and F2, revealing antimicrobial effects normally hidden by the presence of common solvents (e.g., DMSO) with some cellular inhibitory activity. The study of structural prerequisites for antimicrobial activity, and the inclusion in polymers to improve solubility, is important for the design of new drugs.

INTRODUCTION
Invasive fungal diseases are associated with high mortality and morbidity, especially in immunocompromised patients (Edmond et al., 1999;Minari et al., 2002). Complicated fungal infections are often produced by Candida albicans and Cryptococcus spp., two unicellular fungi (i.e., yeasts; Boral et al., 2017). At present, the most used antifungal agents include triazoles, which exhibit good antifungal activity and broad spectrum of action (Sheehan et al., 1999). Triazoles works by inhibiting the lanosterol 14α-demethylase, a member of the CYP51 class of cytochrome P450 enzymes involved in ergosterol biosynthesis in fungi (Lepesheva and Waterman, 2011;Sagatova et al., 2015). Inhibition of 14α-demethylase leads to the depletion of ergosterol (affecting membrane fluidity) and accumulation of toxic metabolites (e.g., 14α-methyl-3,6-diol; Watson et al., 1989). Triazoles, and some other nitrogen-containing heteroaromatic compounds (e.g., pyridine), have been reported to inhibit CYP51 enzymes by direct coordination of nitrogen with the heme iron (type II ligands) (Hitchcock et al., 1990;Lepesheva et al., 2008;Carreño et al., 2018). The combined effects of ergosterol depletion and toxic metabolite accumulation are fungistatic for many pathogenic fungi, including C. albicans and Cryptococcus spp. (Mazu et al., 2016). Unfortunately, the excessive use of azoles has led to development of severe resistance, which significantly reduced their efficacy (Hoffman et al., 2000;Casalinuovo et al., 2004), remarking the need of new, efficient antifungal agents.
Schiff bases with different substituents around the azomethine generates a wide variety of organic compounds exhibiting several interesting properties in diverse areas (Jana et al., 2012;Yu et al., 2016), including applications as antimicrobial compounds (Jarrahpour et al., 2007;Justin Dhanaraj and Sivasankaran Nair, 2009).
To improve drugs and other compounds, it has been observed that fluorine substitution can alter chemical properties, disposition, and biological activity of compounds by affecting lipophilicity (Park et al., 2001). Changes in lipophilicity influence partitioning of compounds into membranes, potentially modulating hydrophobic interactions with either receptors or enzymes (Park et al., 2001;Luzina and Popov, 2013). On the other hand, position of the fluorine atom in the aromatic ring can also determine receptor selectivity (Kirk et al., 1986). Thus, both the presence of fluorine and its position in the aromatic moiety could be relevant in the design of new effective antifungal compounds. In this sense, several triazoles share a 2,4-di-fluorine phenyl substituent (Pore et al., 2015), suggesting that this moiety could contribute to the bioactivity of these antifungal agents.
In this study, we focused our attention on the structurebioactivity relationship of two new pyridine Schiff bases harboring either one or two fluorine substituents in the phenolic ring. These pyridine Schiff bases, (E)-2-[(3-Aminopyridin-4-ylimino)-methyl]-4,6-difluoro-phenol (F1) and (E)-2-[(3-Amino-pyridin-4-ylimino)-methyl]-6-fluoro-phenol (F2) (Figure 1), were analyzed regarding the structural, optical, and electronic properties, complemented by theoretical calculations (DFT level of theory). Furthermore, we assessed the antifungal properties of F1 and F2 against C. albicans and Cryptococcus spp., two yeasts involved in opportunistic infections in humans, potentially with death risk (Taylor-Smith and May, 2016;Boral et al., 2017). To improve biocompatibility, and considering that usually these kind of Schiff bases are prepared in dimethyl sulfoxide (DMSO) for biological tests (Karthikeyan et al., 2006), we also performed inclusion in epichlorohydrinβ-cyclodextrin polymer (βCD) (Gidwani and Vyas, 2014). Cyclodextrins (CDs) are cyclic oligosaccharides closed in a ring that allows the formation of inclusion compounds. Versatility and bioadaptability of CDs are useful to increase the solubility in water to improve delivery of drug molecules (Gidwani and Vyas, 2014). In this context, we also performed studies with F1 and F2 included in βCD. In this work, we found that there is a structure-bioactivity relationship, where the antifungal activity depends on the presence of two fluorine substitutions in the phenolic ring, since only F1 exhibited a clear antifungal effect comparable to that of fluconazole. Finally, we found that the inclusion of either F1 or F2 in βCD improved their solubility in aqueous media and enhanced their antifungal activity, allowing the detection of antimicrobial effects normally hidden by the presence of solvents with some cellular inhibitory activity, such as DMSO.

Physical Measurements
The purity of each new compounds (F1 and F2) was checked by TLC using glass plates pre-coated with SiliaPlate TLC Aluminum Backed TLC supplied by Silicycle as stationary phase and a suitable solvent system as mobile phase (ethyl acetate). The spots were visualized with short wave ultraviolet light (λ = 254 nm) using Spectroline LongLife TM Filter. Melting points were determined on a Stuart Scientific melting point apparatus SMP3 (UK) in open capillary tubes. The molecular ion peak obtained from the experimental m/z data were recorded on SCIEX QTRAP Systems instrument. The 1 HNMR, 13 CNMR, DEPT, HHCOSY, and TOCSY spectra were recorded on a Bruker AVANCE 400 spectrometer operating at 400 MHz, at 25 • C. Samples were dissolved in deuterated dimethyl sulfoxide (DMSO-d6 ), using tetramethylsilane as internal standard. Chemical shifts are expressed in δ (ppm) units; J-values for 1 H-1 H coupling constant are given in Hertz (Hz) and s, d, dd, br refer to singlet, doublet, doublet of doublets, and broad, respectively. FTIR techniques were recorded in an UATR spectrum Two Perkin Elmer. UV-vis spectra were performed using a Shimadzu Model UV-3101 PC UV-vis-NIR scanning spectrophotometer.
Thermal studies were carried out using a Star System 1 thermogravimetric analyzer (TGA) at a heating rate of 10 • C/min. DSC measurements were performed using a Mettler Toledo Star System 822e used to determine the glass transition temperature (T g ) of the ligands, polymer, and inclusion complexes. The T g was measured at a heating rate of 10 • C min −1 . TGA and DSC experiments were conducted under a nitrogen atmosphere. Samples of 2-3 ± 0.1 mg were used for each experiment. Nitrogen adsorption-desorption analysis was measured on a Micromeritics Gemini (Gemini VII 2390, Micromeritics, America).

Voltammetry Methods
For the electrochemical experiments, the working solution contained 10 −3 mol L −1 of the respective compound (F1 and F2) with 10 −1 mol L −1 tetrabutylammonium hexafluorophosphate (TBAPF 6 , supporting electrolyte) in anhydrous CH 3 CN. Prior to each experiment, the working solution was purged with high purity argon, and an argon atmosphere was maintained during the whole experiment, as previously reported (del Valle et al., 2009). A polycrystalline non-annealed platinum disc (2 mm diameter) was used as the working electrode. A platinum gauze of a large geometrical area, separated from the cell's main compartment by a fine sintered glass, was used as the counter electrode. All potentials quoted in this paper are referred to an Ag/AgCl electrode in tetramethylammonium chloride to match the potential of a saturated calomel electrode (SCE) at room temperature. All electrochemical experiments were performed at room temperature on a CHI900B bipotentiostat interfaced to a PC running the CHI 9.12 software that allowed experimental control and data acquisition.

Polymer Loading and A L -Phase Solubility Diagram
Polymer loading was achieved by adding 100 mg of epichlorohydrin-β-cyclodextrin polymer (βCD) polymer to a saturated solution of Schiff bases (10 mg of either F1 or F2 in 5 mL of ethanol) and stirred for 24 h, obtaining a 9.09% w/w of the corresponding Schiff base/cyclodextrin. The resulting solution were concentrated to 1 mL in a rotary evaporator and then lyophilized. Precipitation of either F1 or F2 was not observed during the concentration process, which indicates that both F1 and F2 were included in βCD. Saturated solutions of either F1 or F2 were prepared in an aqueous solution of βCD (1 mL in the 0-100 mg mass range) and stirred at 30 • C until the equilibrium was reached (24 h). Solutions were centrifuged, and the concentration was determined spectrophotometrically at 365 nm for both F1 and F2. The apparent inclusion constant of the Schiff Base-βCD complex (i.e., βF1 and βF2) were determined from the A L -phase solubility diagram according to Where [D] t corresponded to the total concentration of the guest (mol/L); [βCD] t corresponded to the total concentration of cyclodextrin in the polymer (mol/L), S 0 (mol/L) was the intrinsic solubility of the guest in water. Value of k i was determined from the slope (m) of the A L -phase solubility diagram (Equation 2).

DFT Calculations
All structural and electronic properties were obtained using the Amsterdam Density Functional (ADF) code (Te Velde et al., 2001). All molecular structures were fully optimized by an analytical energy gradient method as implemented by Verluis and Ziegler using the hybrid B3LYP functional and the standard Slater-type-orbital (STO) basis set with triple-z quality double plus polarization functions (TZ2P) for all the atoms (Stephens et al., 1994;Ramírez-Tagle et al., 2010;Cohen et al., 2012;Alvarado-Soto and Ramirez-Tagle, 2015;Bjorgaard et al., 2015). Frequency analyses were performed after geometry optimization to corroborate the minimum and to compare with experimental infrared spectra. Time-dependent density functional theory (TDDFT) was used at the same level of theory to calculate excitation energy with the conductor-like screening model for realistic solvents (COSMO) (Sinnecker et al., 2006;Mosquera and Wasserman, 2015;Simpson et al., 2015;Yamin et al., 2016). Three different media were considered for calculations (dichloromethane, acetonitrile, and DMSO) to estimate the hydrogen bond (IHB) stability and to visualize the possible conformational changes due to the solvent polarity. Additionally, calculations were also performed in the gas phase. Natural bond orbital (NBOs) analysis was used to characterize energy of the IHB (Avilés- Moreno et al., 2017;Guajardo Maturana et al., 2017).

Antimicrobial Activity Against Yeasts
F1 and F2, as well as the corresponding compounds included in βCD (i.e., βF1 and βF2) were evaluated for their in vitro growth inhibitory activity against the clinical yeasts Cryptococcus spp. and C. albicans, obtained from the Hospital Clínico of the Universidad de Chile, Santiago, Chile. Minimum inhibitory concentration (MIC) was obtained by broth dilution as described (Cuenca-Estrella et al., 2003). The MIC is defined as the lowest concentration of the tested compounds at which no growth of the strain was observed after the incubation (Cuenca-Estrella et al., 2003). Cryptococcus spp. and C. albicans were previously cultured in Sabouraud agar (Bacto peptone, 10 g/L; glucose, 40 g/L; agar, 15 g/L; pH 5.6) at 28 • C. Further dilutions of microorganisms (0.5 McFarland) were performed with Bacto Tryptic Soy broth (pancreatic digest casein 17.0 g/L, papaic digest of soybean 3.0 g/L, dextrose 2.5 g/L, sodium chloride 5.0 g/L, dipotassium phosphate 2.5 g/L). Stock solutions of the tested compounds were prepared in dimethyl sulfoxide (DMSO) for F1, F2, and fluconazole (K1, commercial antifungal compound used as control); or in water for βF1 and βF2. The concentration range of the compounds tested was between 1.56 and 200 µg/mL; and 15.63-2,000 µg/mL for βCD alone. The inoculated wells were then incubated at 28 • C for 24, 48, and 72 h. As control, DMSO alone or βCD alone were used when necessary. The MIC values of the tested compounds were obtained as µg/mL. All the experiments were performed in biological triplicate, each in technical triplicate.

Statistical Analysis
All values of analyzed data are presented as mean standard error (SE) from three replications. Statistical analysis included was an unpaired t-test. Differences among groups were considered statistically significant when the p < 0.05.

Synthesis and Characterizations
The chemical structure of F1 and F2 is shown in Figure 1. Characteristic constants (molecular weight, yield, melting point, solid color, and Rf are shown in Table S1 in the Supplementary Materials). Purity of both F1 and F2 was confirmed by mass spectra (Figures S2, S3 in the Supplementary Materials) and thin layer chromatography (TLC) (Table S1 in the Supplementary Materials). FTIR for F1 presented bands at 3,483 and 3,298 cm −1 assigned as the stretch modes of the -OH group; and the doublet signals at 3,134 and 3,077 cm −1 for characteristic stretch modes of the -NH 2 group (Carreño et al., 2014;Yilmaz et al., 2017). In the case of F2, the FTIR spectrum showed signals at 3,483 cm −1 (νOH); 3,288 cm −1 (νOH); 3,153 and 3,057 cm −1 (νNH 2 ) and at 2,959 cm −1 , assigned to the stretch modes of -CH groups (Bukowska, 1979;Singh, 2008;Greve et al., 2012) (Figures S4, S5 in the Supplementary Materials). In both cases, peaks assigned as the azomethine -C=N-frequencies (Giannicchi et al., 2013;Vasanthi and Ravikumar, 2013;Senol and Kaya, 2017), and -C=C-stretching were observed at 1,640 and 1,591 cm −1 for F1, and at 1,628 and 1,581 cm −1 for F2, respectively.
The structures of the compounds (F1 and F2) were confirmed by 1D and 2D NMR obtained in DMSO-d6 solutions (see Figures S6, S7 in the Supplementary Materials for numbering of protons and carbons, respectively). The 1 HNMR spectrum for F1 ( Figures S8, S9 in the Supplementary Materials) exhibits the amino proton at 6.15 ppm, whereas the amino proton was observed at 6.11 ppm for F2 (Figures S10, S11 in the Supplementary Materials), effects that can be explained by the nature of fluorine in the phenolic ring (Carreño et al., 2015).In both F1 and F2, the amino proton signal disappeared from the spectrum after D 2 O exchange (Figures S11, S13 in the Supplementary Materials), confirming the assignation.
HHCOSY was also performed (Figures S14, S15 in the Supplementary Materials). Signals assigned to the aromatic protons of both pyridine and phenolic rings appeared at 7.70-6.50 and 7.99-9.00 ppm for F1 and F2, respectively. In the case of F1, for the assignment of H5 and H6 (phenolic ring, see Figure S9 in the Supplementary Materials), we obtained a multiplet (m) between 7.44 and 7.48 ppm. In this context, we performed 1D TOCSY spectrum to complement our analyses and to identify resonances of all the protons in both the pyridine and phenolic rings. Regarding F1, when irradiating H5 ( Figure S16 in the Supplementary Materials), an enhancement was observed for H6 and, similarly, when H6 was irradiated ( Figure S17 in the Supplementary Materials), an enhancement was observed for H5. In the case of protons located in the pyridine ring, when H1 or H2 were irradiated, a spectrum showing enhancement for H3, and the corresponding H1 or H2, were obtained (pyridine ring, see Figures S18, S19 in the Supplementary Materials). All these results indicate that H1, H2, and H3 are located in the pyridine ring (See Figure S6 in the Supplementary Materials). With respect to F2, when H5 was irradiated (phenolic ring), enhancements in H6 and H7 were observed (see Figure S20 in the Supplementary Materials). A similar result was obtained when H7 was irradiated (see Figure S21 in the Supplementary Materials), confirming the proton assignment in the phenolic ring. On the other hand, when H2 was irradiated (pyridine ring), it was possible to observe H1 and H3 signals (see Figure S22 in the Supplementary Materials). These experiments allowed us assigning all signals to their respective protons in the proposed structures (Figure 1).
To complement our analyses, 13 CNMR broad-band decoupled spectrum in DMSO-d6 solution was performed to a total peak assignment for F1 and F2, showing a total agreement with the corresponding inferred structures (see  F1 and F2, respectively, as previously assigned for other Schiff bases (Aranha et al., 2007;Alsaygh et al., 2014). 19 FNMR spectra showed two signals for F1 at −122.64 and −132.75 ppm, corresponding to the fluorine atoms in the 4 and 6 positions in the phenolic ring, respectively (see Figure S27 in the Supplementary Materials). In the case of F2, only one signal, corresponding to the fluorine at the position 6 in the phenolic ring, appeared at −125.07 ppm (see Figure S28 in the Supplementary Materials). Altogether, these results confirmed the structures proposed for both F1 and F2.

UV-Vis Studies
The fluorinated pyridine Schiff bases F1 and F2 are water insoluble at room temperature. Nevertheless, they presented low solubility in dichloromethane and acetonitrile, and high solubility in DMSO at room temperature. In this context, we recorded UV-vis in dichloromethane (ε = 8.93), acetonitrile (ε = 37.5), and DMSO (ε = 46.7) at room temperature, three organic solvents exhibiting increasing polarity (Liu et al., 2010;Płowaś et al., 2013) to determine possible changes in spectra. For both F1 and F2, the UV-vis spectra showed a shoulder and a band for both dichloromethane and DMSO (see Figures S29-S34, and Table S2 in the Supplementary Materials). The shoulder, centered approximately at 260 nm, was assigned to n→π * (-C=N-); whereas the band (approximately at 370 nm) was assigned to π→π * transitions, according to previous reports showing other similar compounds (Reddy et al., 2009;Zoubi and Kandil, 2012;Carreño et al., 2015). All these results were summarized in Table S2 in the Supplementary Materials. These results show that, in both apolar (dichloromethane) and polar (acetonitrile) solvents, and independently on the concentration of the respective Schiff base, the UV-vis exhibits a similar pattern, strongly suggesting that the compounds show no significant interaction with the solvents. In this sense, the π→π * transition (related to the IHB, as will be discussed later in the computational studies), remains essentially unchanged, strongly suggesting the high stability of the IHB. On the other hand, we also tested the UV-vis in presence of DMSO, a solvent that could form intermolecular hydrogen bonds with the solutes (Charisiadis et al., 2014;Sigalov et al., 2017), to corroborate the stability of the IHB under this condition. In this case, we observed a better definition the band assigned to n→π * (-C=N-) (∼260 nm). Furthermore, the band assigned to π→π

DFT and TD-DFT Studies
To better understand the electronic and optical properties of both F1 and F2, we performed DFT calculations. In the first place, geometry optimization of the neutral compounds was carried out with the hybrid B3LYP functional (Orio et al., 2009;Cohen et al., 2012) and the standard Slater-type-orbital (STO) basis, set with triple-z quality double plus polarization functions (TZ2P) for all the atoms (Cohen et al., 2012;Lashgari et al., 2016). Since both F1 and F2 are new compounds, X-ray crystal data is not available. Nevertheless, geometry optimization analyses showed that the main atom distances of both F1 and F2 are in good agreement with the experimental crystallographic data of a previously reported pyridine Schiff base L1 ((E)-2-{[(2aminopyridin-3-yl)imino]-methyl}-4,6-di-tert-butyl-phenol (see Figure S2 in the Supplementary Materials) (Carreno et al., 2012).
Regarding the IHB, F1 and F2 present an OH···N distance of 1.772 and 1.773 Å, respectively (for optimized geometrical parameters, see Table S3 in the Supplementary Materials). With respect to the conformational structure, both F1 and F2 adopt a non-coplanar conformation, with a dihedral angle between the pyridine and the phenolic ring of 40.1 and 43.2 • , respectively. Figure 2 shows the geometry optimizations of F1 and F2, including the energy of the frontier orbital HOMO-LUMO in eV (E HOMO−LUMO = 3.87 eV for F1, and 3.89 eV for F2). The calculated frequencies are congruent with the values observed in the FTIR spectra for the most important functional groups. All the frequencies were positive, which means that the minimum was found. In general, there is no difference between the frequencies of both compounds except for the azomethine vibration, which appears shifted in 30 cm −1 for F1 (see Figures S35, S36, and Table S4 in the Supplementary Materials) (Carreño et al., 2015).
To further study the IHB of both F1 and F2, we evaluated the second-order interaction energy by means of Natural Bond Orbitals (NBOs) calculations (Deshmukh et al., 2006;Weinhold, 2012). The interaction energy is related to stabilizing donoracceptor interactions due to electron delocalization with respect to the zeroth-order natural Lewis structure (Purser, 1999;Jesus et al., 2008). In addition, NBO analysis provides an efficient method for studying participation of hydrogen bond in the stability of N-harboring compounds (Avilés-Moreno et al., 2017). Our analyses indicated that, for both F1 and F2, the IHB acts providing stability between the pyridine and phenolic rings. Based on the energy of the IHB, it is possible to sort the molecules as: F1 (26.7 kcal/mol) > F2 (23.5 kcal/mol). It is important to remark that these energies are found in the typical range for this kind of hydrogen bonds (Avilés-Moreno et al., 2017).
On the other hand, and to further elucidate the UV-vis transitions, we conducted time dependent density functional theory (TD-DFT) calculations for both F1 and F2, in the same solvents used for the Figures S29-S34 (see Supplementary Materials). As shown in the Table S5 in the Supplementary Materials, the most important bands were associated to n→π * and π→π * , in turn related to azomethine group and to the IHB, respectively (see Figure 2 to complement the analysis). It is remarkable that these results were similar in the three solvents used for analysis and even with the gas phase, with no significant shifting in the expected UV-vis spectra in any case, complementing the experimental data and corroborating the high stability of the IHB.

Electrochemical Studies
Electrochemical properties of compounds were studied by observation of redox processes through cyclic voltammetry experiments (Figure 3). Cyclic voltammograms of compounds F1 and F2 (straight lines), performed in an anhydrous CH 3 CN under anoxic conditions, showed several signals, different from blank experiments (dashed lines). Nevertheless, it is possible that some of these signals corresponds to secondary processes of by-products formed after the first reduction/oxidation of compounds and/or solvent-molecules at the working window limits, as it has been observed on past studies for other Schiff bases (Carreño et al., 2014(Carreño et al., , 2015(Carreño et al., , 2018. Therefore, in order to discriminate which signal corresponds to a compoundoriginal process, it is recommended to perform a workingwindow potential study (see Figure S37 in the Supplementary Materials). For instance, F1 displays up to three redox processes, two irreversible oxidations at 0.48 and 1.07, along with an apparent-irreversible reduction at 0.89 V (Figure 3). However, in the working-window study ( Figure S37), the signal at 0.48 is only visible when the working window limit gets near its negative limit; thus, it does not correspond to a compoundoriginal process. Moreover, the apparent-irreversible reduction at 0.89 becomes reversible ( Figure S37), indicating that the excessive current output of the complete-working window cyclic voltammogram (Figure 3) disturb the correct description of electrochemical processes. After all these analyses and data processing, signal description, potential values, reversible character, and mass-transport control (diffusional) were obtained and summarized on Table 1. Both F1 and F2 exhibited secondary signals (around +0.3 V vs. SCE) that were only visible when the working-window reached the most negative potential limit; thus, the electrode surface must and was cleansed between each measurement to assure the obtainment of trustful results.
At first sight, the signals described on Table 1 describing redox processes might seem too subtle in the Figure 3. Nevertheless, if the scan-rate study is revised ( Figure S38), these signals become more pronounced at lower scan-rates, due to negligible effect from double-layer capacitance at slow scans (Brett and Brett, 1993). As reported for similar compounds (Carreño et al., 2014(Carreño et al., , 2015(Carreño et al., , 2018, an irreversible oxidation process is always observed. The nature of the irreversible oxidations observed on the compounds is supported by theoretical calculations; the HOMO position determines that this process is typically ascribed to an oxidation taking place somewhere at the pyridine ring, most likely at the -NH 2 moiety. Reductions, on the other hand, are ascribed for an intramolecular reductive coupling of the azomethine group, which involves a self-protonation reaction reported for similar compounds (Zolezzi et al., 2002;Carreño et al., 2014), which also explains the quasi-and irreversible character of this process. The mass-transport control of the process is defined by the scan rate study results (see Figure S38 and Table S6 in the Supplementary Materials), suggesting that in the experimental conditions all electrochemical processes, except for F2 oxidation, are controlled by diffusion of species from bulk solution.

Solubility Studies
As stated, inclusion of Schiff bases in epichlorohydrin-βcyclodextrine polymer (βCD) could improve biocompatibility by increasing solubility of these compounds in aqueous systems (Gidwani and Vyas, 2014). In this context, we included F1 or F2 in βCD. As the first step, the host-guest interactions were studied in terms of the apparent inclusion constants (k i ) of F1 and F2 within (or inter) βCD cavity. Due to the basic nature of both the pyridine and the amino group present in  Slope × 10 2 2.3 ± 0.8* 7.6 ± 0.5 S 0 ·10 4 (mol L −1 ) 5.1 ± 0.9 7.5 ± 0.9 k i 45 ± 13 101 ± 10 *Standard deviation.
F1 and F2, we speculated that the formation of the inclusion complexes will preferentially occurs between the phenol moiety of the Schiff bases and the βCD cavity. Figure 4 shows A Ltype phase-solubility diagrams of F1 + βCD (βF1) and F2 + βCD (βF2). The A L -Phase solubility diagram shows a linear dependence between total concentration of the βCD with [D] t -S 0 . This linearity indicates a 1:1 (guest:host) stoichiometry; albeit this not necessarily implies that all guests (F1 or F2) are included in the βCD cavity. In this sense, the micro voids found between the crosslinked chains of the βCD (in particular, -OH groups) could form hydrogen bonds with the guests by interactions with the amino groups of the pyridine moiety, contributing to increase the solubility of either F1 or F2 (see Figure 6). Thus, we hypothesize that Schiff bases are included inside βCD cavity (see below). Inclusion constants are shown in Table 2.
As can be noted, the k i value determined for βF2 is approximately the double than ki determined for βF1 ( Table 2). This difference can be explained by the structure of F1 and F2. F1 has two fluorine atoms as substituents (4-and 6-positions) in the phenolic ring, whereas F2 has only one fluorine atom (6position; see Figure 1). In the case of F1, the fluorine found at the 6-position could restrict the accommodation of the phenolic  ring in the βCD cavity, explaining the decreased value of ki in this case.
These results show that it is possible to render F1 and F2 water-soluble by means of inclusion with βCD.

Thermal Studies by TGA and DSC, and Nitrogen Adsorption-Desorption Isotherm
To elucidate how F1 and F2 are interacting with βCD, we performed a physicochemical approach. For this purpose, thermogravimetric analysis (TGA) was used to compare the thermal decomposition temperature (TDT) of the Schiff bases alone (i.e., F1 or F2) and Schiff bases included into βCD (i.e., βF1 or βF2). As control, we also tested βCD alone. The TGA curves exhibited five decomposition curves in one-step corresponding to F1, F2, βCD, βF1, and βF2 ( Figure 5A). For F1 and F2, a weight loss, observed approximately at 220 • C, can be attributed to a decomposition of the Schiff if we considered their melting points (see Table S1 in the Supplementary Materials). In the case of βF1 and βF2, a different pattern was found, where ∼12.5% of weight loss was observed at 300 • C. The βF1 and βF2 presented an extrapolated TDT between 330 and 345 • C at which point the decomposition of the backbone chain occurs ( Figure 5A). As expected, βCD alone exhibited a completely different behavior ( Figure 5A). Table 3 shows a summary of the main data obtained for TDT.
Differential Scanning Calorimetry (DSC) measurement is a good technique to analyzing the interaction between host and guest molecules during the complex inclusion formation (Karoyo et al., 2011). When guest molecules are embedded in the βCD cavity, their melting points usually shift to a different temperature or disappear (El-Maradny et al., 2008;Sapte and Pore, 2016). The DSC thermograms of F1, F2, βCD, βF1, and βF2 are shown in Figure 5B. The DSC curve of F1 and F2 exhibited a weakly discernible glass transition temperature (T g ) at 46.52 and 53.28 • C, respectively. For both compounds, two similar phenomena were observed, a broad exothermic peak with an onset of 110.3 • C for F1 and 118.4 • C for F2, and an endothermic peak with a start of 183.6 • C for F1 and 167.1 • C for F2, assigned to a recrystallization and fusion, respectively. These results are in agreement with those obtained in the melting point measurements (see Table S1 in the Supplementary Materials). In the case of βCD, DSC showed a weight loss attributed to loss of water from βCD cavity ( Figure 5B). On the other hand, the inclusion complexes (i.e., βF1 and βF2) showed appearance of broad peaks at 93.76 and 110.84 • C, respectively, assignable to water loss and disappearance of T g of F1 and F2 (Figure 5B). This result suggests that both F1 and F2 are trapped inside βCD cavity, with replacement of water molecules. The peaks of F1 and F2 also disappeared when compared with βF1 and βF2, respectively, indicating their involvement in the complexation process. These results suggest an existence of strong physical interaction between either F1 or F2 with βCD, indicating the formation of a stable inclusion complex in solid state. To complement these results and further understand the inclusion phenomenon, we performed a nitrogen adsorption-desorption analysis. This analysis is based on the adsorption of nitrogen on the surface of the tested compounds, as a technique for the measurement of the specific surface area (S BET ). The analysis of the nitrogen adsorption isotherm profiles for βF1 and βF2 at 77 K showed an isotherm of type II (see Figure S39 in the Supplementary Materials) that involves physical adsorption of nitrogen gas by non-porous solids, with a negligible microporous volume and low mesoporous volume. Both βF1 and βF2 present a mesoporous volume (pore size) ranging 2-50 nm, according to the IUPAC classification ( Table 4).
The incorporation of F1 and F2 into the βCD cavity (i.e., βF1 and βF2) caused a significant increase in nitrogen adsorption accompanied with a decrease in the pore size, compared with βCD alone (Table 4). These results show that either F1 or F2 incorporation into βCD likely occurs inside the pore channels, and not on the βCD surface. The guest inside the host improved nitrogen retention due to a steric effect and pore blockage. By contrast, if the guest (i.e., F1 or F2) were been adsorbed on the host surface (i.e., βCD surface), we were observed a decrease in the S BET value due to a hindrance produced by the guest, hampering nitrogen adsorption. Frontiers in Chemistry | www.frontiersin.org All these results together confirmed that the guests are found inside βCD, as previously proposed in the solubility studies (Figure 6).
Biological Activity: Antimicrobial Activity of the Schiff Bases (F1 and F2) As stated, structure of the Schiff bases is fundamental for their biological applications (Carreño et al., 2018). In fact, little changes regarding substituents can produce big effect with respect to the antimicrobial properties of pyridine Schiff bases (Carreño et al., 2015(Carreño et al., , 2018. In non-pyridine Schiff bases, the use of halogens in the phenyl moiety can positively increase the antifungal activity in some cases (Karthikeyan et al., 2006). In the case of pyridine Schiff bases, it has been reported that the biological activity against fungi relied on the nitrogen atom in the pyridine ring, and this activity can be modulated by different substituents (i.e., chlorine) in the phenolic ring at the 4 and 6 positions (Carreño et al., 2018). In this sense, the use of fluorine as substituent in different drugs has shown to modulate both chemical and biological properties, including membrane partitioning or interaction with their respective targets (Kirk et al., 1986;Kirk and Filler, 1996;Park et al., 2001;Luzina and Popov, 2013). Thus, we tested both F1 and F2 regarding their antifungal properties against C. albicans and Cryptococcus spp., two yeasts that produce systemic infections in immunocompromised patients (Taylor-Smith and May, 2016;Boral et al., 2017). For that, we determined the minimal inhibitory concentration (MIC); compounds with low MIC are considered better antifungal agents. We found that F1 exerted an antifungal activity against Cryptococcus ssp., which was undistinguishable from the activity observed for the known, commercial antifungal fluconazole, at all the times tested (24 h: p = 0.73; 48 h: p = 0.59; 72 h: p = 0.44; Table 5). On the other hand, we found that neither F1 nor F2 exerted an antifungal effect against a clinical, fluconazoleresistant strain of C. albicans ( Table 5), suggesting that the mechanisms involved in fluconazole resistance could be also participating in the F1 resistance (e.g., efflux pumps; Monapathi et al., 2018). To demonstrate that the presence of fluorine substituents is important for the antifungal activity, we compared the antimicrobial effect of two analogous Schiff bases (L3 and L4, see Figure S1 in the Supplementary Materials) (Carreño et al., 2018) harboring chlorine substituents at the same positions of the fluorine substituents found in F1 and F2, respectively. We found that only L3 exerted a noticeable antifungal activity against Cryptococcus spp. under the tested conditions, whereas L4 exhibited an effect that was undistinguishable from the effect produced by DMSO alone. These results reinforce the fact that, to obtain efficient antifungal activity, it is necessary the presence of two substitutions in the phenolic ring at positions 4 and 6 (Carreño et al., 2018). Nevertheless, not any substitution is suitable, since L4 (chlorine) exerts only a fungistatic activity that disappears after 72 h, whereas F1 (fluorine) seems to produce a fungicide effect, similar to that produced by fluconazole ( Table 5). All these results argue for the relative advantage of fluorinated pyridine Schiff bases over other very similar compounds. Considering that F1 and F2 only differ in one substituent (the fluorine at position 6 in the phenyl moiety is absent from F2), little structural changes can greatly affect the antimicrobial properties of this kind of pyridine Schiff bases. In other words, only one fluorine can make the difference between a fluconazole-like antifungal activity and the lack of antifungal activity. Previously, it has been proposed that, in the case of pyridine Schiff bases, the biological activity depends on the presence of the nitrogen atom in the pyridine ring, and this activity can be modulated by different substituents in the phenolic ring, suggesting that these two rings exert a cooperative effect (Carreño et al., 2015(Carreño et al., , 2018. Our results underline the importance of the substitutions in the phenolic rings, showing that only F1 (i.e., two fluorine at position 4 and 6 in the phenolic rings), and not F2, exerts an important antifungal effect. It is remarkable that synthetic imidazole antifungal drugs, such as fluconazole, present a nitrogen-containing heteroaromatic ring (e.g., imidazole) as well as a phenyl moiety substituted with two fluorines.
In order to better understand the effect of both F1 and F2 in more physiological conditions, we included these compounds in epichlorohydrin-β-cyclodextrin polymer (βCD) (see above) (Gidwani and Vyas, 2014). Cyclodextrins (CDs) are cyclic oligosaccharides formed by monomers of D-glucopyranose bound together by α-1,4-glucosidic linkages. These structures form a rigid, truncated, cone-shaped structure with an internal cavity (5-8 Å) that allows the inclusion of molecules, as stated above. Depending on the number of monomers, CDs may be classified into α (six glucose units), β (seven glucose units), or γ (eight glucose units) (Gidwani and Vyas, 2014;Duchêne and Bochot, 2016). CDs are useful for a broad of applications, especially in drug delivery, in part because they enhance biocompatibility by increasing water solubility (Gidwani and Vyas, 2014). In this context, we also performed studies with F1 and F2 included in βCD (called βF1 and βF2, respectively). Unlike F1 and F2, which were prepared in DMSO, βF1 and βF2 were dissolved in water due to their improved solubility in aqueous solvents (Figure 4). As expected, βF1 exerted an antifungal effect against Cryptococcus spp. Interestingly, the inclusion of F1 in βCD revealed an antimicrobial, previously undetected effect against C. albicans ( Table 6), suggesting that the water solubility contribute to its biological activity. On the other hand, we also observed a consistent fungistatic effect against both Cryptococcus spp. and C. albicans produced by βF2. As stated above, F2 exerted an antifungal effect that was undistinguishable from DMSO alone ( Table 6). Although DMSO is extensively used in pharmaceutical, chemical, and biomedical applications, this solvent exerts an inhibitory effect against eukaryotic cells, including yeasts, by impairing cell replication or increasing cell apoptosis (Cho et al., 2014;Kakolyri et al., 2016). In this sense, DMSO could be screening the antimicrobial effect of F2, which was revealed with βF2 dissolved in water. Accordingly, F1 and DMSO alone were undistinguishable regarding the antimicrobial effect against C. albicans (Table 5). By contrast, βF1 clearly exhibited an antifungal effect against C. albicans, as shown in the Table 6. Thus, the results obtained with βF1 and βF2 show that the inclusion in βCD can reveal antimicrobial effects normally hidden by the presence of solvents with some inhibitory activity, such as DMSO.
Interestingly, when we tested F1, F2, βF1, or βF2 against Gram-negative bacteria (i.e., Salmonella enterica) or Grampositive bacteria (i.e., Staphylococcus aureus), we only observed an effect of βF1 against S. aureus (see Table S7 in the Supplementary Materials). This result suggests that this kind of pyridine Schiff bases could also be useful against bacteria, and the inclusion in βCD could be a mean to reveal its activity. Nevertheless, it is necessary more experimentation to elucidate the mechanism of action of these kind Schiff bases against bacteria.

CONCLUSION
We synthesized and fully characterized two new pyridine Schiff bases (F1 and F2), and we compared them regarding their antifungal effect against yeasts. In addition, we include F1 and F2 into βCD in order to improve their solubility in aqueous media. We found that the phenolic ring is crucial for this kind of Schiff bases regarding the antifungal activity, where the positions 4 and 6 are critical. Furthermore, we found that the inclusion in βCD improves the solubility in aqueous media and can reveal antimicrobial effects normally hidden by the presence of solvents with some inhibitory activity, such as DMSO.
The study of structural prerequisites for antimicrobial activity, and the inclusion in polymers to improve solubility, is important in the design of new drugs.

AUTHOR CONTRIBUTIONS
AC: design, synthesis and characterization of Schiff bases, discussion of all the experiments, theoretical calculations and paper writing; LR: contribution in inclusión of Schiff bases into cyclodextrin; DP-H: computational calculations; RM-T: synthesis of cyclodextrin polymer, inclusion of Schiff bases into cyclodextrin; CZ: contribution in UV-vis studies, melting point and TLC; DO: TGA and DSC; MG: electrochemical studies; ES: SBET studies; RA-P: discussion of theoretical calculations; JF: design of Schiff bases, biological assays, discussion of all the experiments, paper writing.


extfieldsofstudy:
Medicine
Materials Science

s2fieldsofstudy: Chemistry Physics

text:
Dissociative Chemisorption of O2 on Al(111): Dynamics on a Correlated Wave-Function-Based Potential Energy Surface

Dissociative chemisorption of O2 on the Al(111) surface represents an extensively studied prototype for understanding the interaction between O2 and metal surfaces. It is well known that the experimentally observed activation barrier for O2 dissociation is not captured by conventional density functional theory. The interpretation of this barrier as a result of spin transitions along the reaction path has been challenged by recent embedded correlated wave function (ECW) calculations that naturally yield an adiabatic barrier. However, the ECW calculations have been limited to a static analysis of the reaction pathways and have not yet been tested by dynamics simulations. We present a global six-dimensional potential energy surface (PES) for this system parametrized with ECW data points. This new PES provides a reasonable description of the site-specific and orientation-dependent activation barriers. Quasi-classical trajectory calculations on this PES semiquantitatively reproduce both the observed translational energy dependence of the sticking probability and steric effects with aligned O2 molecules.

(S1) More details of the ECW theory and convergence tests can be found in Refs 1, 2 and 3. Because ECW calculations are much more expensive than conventional DFT, the number of points is limited. As a result, two widely applied approaches, i.e., neural networks (NNs) [6][7][8][9] and the corrugation reducing procedure (CRP) 10

B. Potential Energy Surface
where the U i (Q i ) represent the Coulomb (exchange) integrals for two-body interactions ( where L is the nearest-neighbor distance between two surface atoms, and the expansion coefficients P 0 , P 1 , P 2 are obtained by solving a set of linear equations once we find the Morse parameters for three high symmetry sites (top, bridge, and hollow). Following Martin-Gondre et al., 12,13 we include a dependence on the bond length r i in α i to better describe the one- where the two additional parameters 0 and 1 are also optimized.
The choice of the Sato parameters is of essential importance in representing the global PES.
In the FPLEPS function, the periodicity is also incorporated into the Sato parameters, thus offering more flexibility: the site-dependent Sato parameters at the top, bridge, and hollow sites differ and the Fourier expansion in Eq. (S5) again yields the correct periodicity. In addition, there is a shallow physisorption well in the entrance channel in this system, which is not intrinsically described in the LEPS framework. We therefore add a negative Gaussian function with respect to the height of the molecule in the final form of the PES, 12, 13 where A, σ, and z 0 are the amplitude, width, and center of the Gaussian, which represent the well depth, shape and location, respectively. Note that the Gaussian function is also site-dependent just like the adsorption well. These parameters shaping the FPLEPS function are all optimized using the non-linear Levenberg-Marquardt algorithm 18 which minimizes the discrepancies between the energy values predicted by the FPLEPS function and the original CW data. The optimized parameters are reported in Table S1.

C. Dynamics
To validate the FPLEPS PES, extensive quasi-classical trajectory (QCT) calculations have been performed to simulate the dissociative chemisorption dynamics of O 2 on Al(111). The trajectories were initiated at 6.0 Å above the surface with the molecular center randomly chosen to cover the 1×1 unit cell. The diatomic molecule is treated as a rotating oscillator, whose In all calculations, a trajectory is terminated when the molecular bond length increases to 2.5 Å (dissociation), or when the molecule-surface separation increases beyond 6.0 Å (reflection). Note that the influence of substrate motion is neglected here within the frozen surface approximation.

S7
This is partially justified by a very weak dependence of the sticking probability on the surface temperature observed in experiment. 20 A small number of ab initio molecular dynamics trajectories also revealed that surface movements only become prominent when the O 2 molecule is quite close to the surface after dissociation. 22 In addition, low energy electron-hole pair excitations are also neglected here, which only play a minor role in the activated dissociation of molecules on metal surfaces with significant barriers. 23, 24

II. Additional results
We further investigate the anisotropy of the PES at the hollow site in both the entrance and product channels (see Figure S2). Following Cheng et al., 3  In the product channel, the PES cuts are plotted as a function of θ and r at the fcc site for // 3 geometry (φ=-30°) (see Figures S2e-f). Interestingly, the adsorbate tends to rotate from the parallel to a tilted geometry after overcoming the lowest barrier, as evidenced by the local minima θ=~70° in Figures S2e-f with the extended O-O distance. As suggested by some of the S8 current authors, 3 this offers the possibility of an abstraction channel, which yields a single adsorbed O atom and an oxygen atom emitted to the gas phase. 25 Overall, our FPLEPS PES 2D cuts demonstrate a strong dependence of the barrier on the angular coordinates, which are in general accord with the earlier analyses based on the original ECW data. 3 The preference of certain parallel orientations over the perpendicular ones could serve as the origin of the experimentally observed steric effect, as discussed in the main text.


s2fieldsofstudy: Chemistry Biology Engineering

text:
Nanocatalytic Tumor Therapy by Biomimetic Dual Inorganic Nanozyme‐Catalyzed Cascade Reaction

Abstract Emerging nanocatalytic tumor therapies based on nontoxic but catalytically active inorganic nanoparticles (NPs) for intratumoral production of high‐toxic reactive oxygen species have inspired great research interest in the scientific community. Nanozymes exhibiting natural enzyme‐mimicking catalytic activities have been extensively explored in biomedicine, mostly in biomolecular detection, yet much fewer researches are available on specific nanocatalytic tumor therapy. This study reports on the construction of an efficient biomimetic dual inorganic nanozyme‐based nanoplatform, which triggers cascade catalytic reactions for tumor microenvironment responsive nanocatalytic tumor therapy based on ultrasmall Au and Fe3O4 NPs coloaded dendritic mesoporous silica NPs. Au NPs as the unique glucose oxidase‐mimic nanozyme specifically catalyze β‐D‐glucose oxidation into gluconic acid and H2O2, while the as produced H2O2 is subsequently catalyzed by the peroxidase‐mimic Fe3O4 NPs to liberate high‐toxic hydroxyl radicals for inducing tumor‐cell death by the typical Fenton‐based catalytic reaction. Extensive in vitro and in vivo evaluations have demonstrated high nanocatalytic‐therapeutic efficacy with a desirable tumor‐suppression rate (69.08%) based on these biocompatible composite nanocatalysts. Therefore, this work paves a way for nanocatalytic tumor therapy by rationally designing inorganic nanozymes with multienzymatic activities for achieving high therapeutic efficacy and excellent biosafety simultaneously.

DOI: 10.1002/advs.201801733
Tumor microenvironment (TME) is generally known to feature abundant unique characteristics such as acidity, [1] hypoxia, [2] inflammation, [3] and overproduced hydrogen peroxide. [4] Based on the unique tumor physiological microenvironment and the rising nanotechnology, a number of promising nanotheranostic strategies of designing TME-specific/responsive nanoplatforms for efficient tumor therapy and precise bioimaging diagnosis have been extensively explored and developed. [5] Tumor cells produce large amounts of hydrogen peroxide (H 2 O 2 ) by the overexpressed superoxide dismutase (SOD) through a catalytic process by superoxide ions as generated from mitochondria. [6] As a typical feature of TME, the overexpressed H 2 O 2 has thus been exploited for triggering responsive drug-releasing in chemotherapy or producing endogenous O 2 for oxygen-favored cancer therapy such as photodynamic therapy, [7] sonodynamic therapy, [8] or radiotherapy. [9] As an emerging efficient cancertherapeutic modality, the proof-of-concept nanocatalytic tumor therapy as first proposed by our group [10] utilizes intratumoral in situ catalytic chemical reactions to produce oxidative stress for inducing cancer-cell death by damaging intracellular biomolecule substances such as proteins, lipids, and DNA. [11] A typical strategy of the proposed nanocatalytic tumor therapy is the representative chemodynamic therapy, [12] which converts less reactive endogenous H 2 O 2 into the most harmful reactive oxygen species (ROS), hydroxyl radicals (·OH) under mildly acidic TME via an intratumoral Fenton or Fenton-like reaction by metal ions. [13] Such an endogenous but direct chemical energy conversion strategy uses no external energy input such as laser, ultrasound, or magnetic field, thus avoiding the limitations of low tissue-penetration depth and nonspecificity of these external triggers on inducing cancer-cell death.
Based on the fact that the intratumoral H 2 O 2 concentration, generally believed to be 50-100 × 10 −6 m, [4b] is too low to produce desirable and sufficient amount of hydroxyl radicals for inducing satisfactory nanocatalytic-therapeutic efficacy via Fenton or Fenton-like reactions, natural glucose oxidase (GOx) was introduced to elevate the intratumoral H 2 O 2 concentration through natural catalytic oxidation of intratumoral glucose to H 2 O 2 and gluconic acid. [14] However, natural GOx possesses several intrinsic drawbacks such as high cost in preparation and purification, and relatively low operational stability, which will undoubtedly hamper its practical biomedical application under complicated and harsh physiological environments. [15] Therefore, exploring and developing natural GOx alternatives with much-enhanced stability and lowered cost is highly desirable and necessary.
Recently, the merging of biology with nanotechnology has motivated extensive research fever for designing functional nanoplatforms that exhibit unique natural enzyme-mimic catalytic activities for a broad range of biomedical applications. [16] As the promising alternatives for natural enzymes, catalytically active nanomaterials, known as the so-called "artificial enzymes" or "nanozymes," have demonstrated numbers of merits over natural enzymes, such as facile fabrication, low cost, and robust stability against severe conditions. [17] So far, a variety of nanomaterials, such as carbon-based nanoparticles (NPs), [18] metal NPs, [19] and metal oxides NPs [16a,20] have been discovered to possess unique enzyme-mimic catalytic activities, which are extensively used in numerous fields, including biomolecular detection, [21] biosensor, [22] antibacterial applications, [23] immunoassays, [24] cancer diagnostics and therapy, [25] and environmental monitoring. [26] Till now, an exceedingly large number of reports on peroxidase (POD)-mimic nanozymes have been reported, while other kinds of enzyme-mimic NPs, such as GOx-mimic nanozymes, glutathione peroxidase-mimic nanozymes, or SOD-mimic nanozymes, have been rarely explored. Fortunately, Rossi and co-workers discovered that Au NPs could catalyze the oxidation of glucose to H 2 O 2 and glucono delta-lactone (GDL) under the presence of dissolved oxygen, which was very similar to the reaction as catalyzed by GOx, suggesting that Au NPs could serve as a mimic for GOx. [27] Since then, owing to the unique GOx-mimic activity, Au NPs have been explored for diverse applications, most of which involve biomolecule (DNA, glucose, etc.) detection, while leaving many hidden applications to be discovered.
Bearing the unique GOx-mimicking catalytic activity of Au NPs and POD-mimicking catalytic performance of Fe 3 O 4 NPs, we herein report, for the first time, on a biomimetic dual inorganic nanozyme-triggered TME-responsive cascade catalytic reaction for efficient nanocatalytic tumor therapy based on an "all-inorganic biocompatible nanosystem", without employing any toxic chemical drug (Scheme 1). Ultrasmall and highly dispersed Au NPs and Fe 3 O 4 NPs were successively integrated into the large pore channels of dendritic mesoporous silica NPs Adv. Sci. 2019, 6, 1801733 Scheme 1. Schematic illustration of "toxic-drug-free" nanocatalytic tumor therapy by biomimetic inorganic nanomedicine-triggered cascade catalytic reaction. The composite nanoplatform with dual inorganic nanozyme activity was fabricated by the sequential loading of Au NPs as GOx-mimic nanozyme and Fe 3 O 4 NPs as POD-mimic nanozyme into the large mesopores of DMSN NPs followed by PEGylation. The therapeutic process by cascade catalytic chemical reaction includes the initial GOx-mimicking Au nanozyme-mediated catalytic oxidation reaction of glucose into H 2 O 2 , which is further utilized as the reactant for the POD-mimicking Fe 3 O 4 nanozyme-based catalytic Fenton reaction to produce highly toxic ·OH and subsequently induce tumor-cell apoptosis.
(DMSN NPs) to construct the composite nanoplatform, i.e., DMSN-Au-Fe 3 O 4 NPs. After further modification with polyethylene glycol (PEG) molecules for improved biocompatibility and physiological stability, DMSN-Au-Fe 3 O 4 NPs could accumulate into tumor tissue via the typical enhanced permeability and retention (EPR) effect, triggering the intratumoral TMEresponsive cascade catalytic reaction.
Acquiring an unusual anaerobic glycolytic behavior to support their metabolism and proliferation, tumor cells demand nutrients, essentially glucose, in a hysterical manner. [28] Therefore, in TME, masses of glucose molecules are transported and accumulated, which provides the possibility of initiating catalytic reaction by utilizing glucose molecules. The intratumoral cascade catalytic reaction is initially triggered by Au NPs, the GOx mimic inorganic nanozyme which catalyzes the intratumoral glucose oxidation, generating large amounts of H 2 O 2 molecules to serve as the substrate for the subsequent catalytic reaction where H 2 O 2 is disproportionated by the coloaded ultrasmall Fe 3 O 4 NPs via the typical Fenton catalytic reaction, liberating high-toxic hydroxyl radicals to effectively induce tumor-cell death. DMSN-Au-Fe 3 O 4 NPs as the potent nanoplatforms are capable of realizing the endogenous cascade reaction for TME-specific and effective nanocatalytic tumor therapy, in a noninvasive and "toxic-drug-free" way, benefiting from the robust biomimetic nanozymes. It is noted that such a cascade reaction with TME acidity-responsiveness would not be triggered under the neutral condition in normal tissue microenvironment, guaranteeing the high tumor-specificity and therapeutic biosafety.
DMSN NPs with unique central-radial pore structures, serving as the robust nanosupports for small NPs, were synthesized by an anion-assisted approach based on the Stöber mechanism and sol-gel chemistry. [29] A high density of highly dispersed and ultrasmall particle-sized Au NPs was confined into the large mesopores of DMSN NPs via an in situ reduction reaction of HAuCl 4 to produce DMSN-Au NPs, followed by the collection and integration of ultrasmall  pore volume of DMSN NPs were measured to be 197.5 m 2 g −1 and 0.9 cm 3 g −1 , respectively, with an average pore diameter of 23.3 nm according to the nitrogen adsorption-desorption isotherm and corresponding pore-size distribution ( Figure S3, Supporting Information). Such a unique branched structure with large pore size and highly accessible surface area of DMSN NPs is highly in favor of the subsequent growth of ultrasmall Au NPs and deposition of Fe 3 O 4 NPs. After functionalized with amine groups by aminopropyltriethoxysilane (APTES) to provide anchoring sites for Au NPs, the mesopores' surface of DMSN NPs was grown with Au NPs via an in situ reduction of auric ions by NaBH 4. TEM and SEM images reveal that ultrasmall Au NPs of ≈1.5 nm in diameter have been well dispersed and immobilized within the mesopores channels of DMSN NPs ( A typical colorimetric method based on 3,3′,5,5′-tetramethylbenzidine (TMB) was introduced as a substrate to test the POD-like catalytic activity of DMSN-Fe 3 O 4 NPs. In the presence of H 2 O 2 , DMSN-Fe 3 O 4 NPs catalyzed the oxidation of TMB to form the oxidized and therefore blue-colored TMB (oxTMB) featuring characteristic absorbances at 370 and 652 nm (Figure 2a). The possible reaction mechanism involves two steps where the OO bond in H 2 O 2 molecule was broken into ·OH followed by TMB oxidation by ·OH to form oxTMB. UV-vis absorption spectroscopy was used to monitor the production of the colorimetric product oxTMB. Negligible absorbance can be observed in the absence of DMSN-Fe 3 O 4 NPs (Figure 2b), suggesting that no oxidation reaction has occurred in the mixture of TMB and H 2 O 2 . However, an apparently blue-colored solution can be obtained after the addition of DMSN-Fe 3 O 4 NPs into TMB-H 2 O 2 mixture solution (pH 6.5) for 10 min with two major absorbance peaks at 370 and 652 nm attributable to oxTMB, which confirms the production of ·OH by DMSN-Fe 3 O 4 NPs and H 2 O 2 .
To further evaluate the catalytic activity of DMSN-Fe 3 O 4 nanomedicine, the steady-state catalytic kinetics was investigated at room temperature in a reaction system containing DMSN-Fe 3 O 4 NPs, TMB, and H 2 O 2 of varied concentrations (5,10,20,30,40, and 50 × 10 −3 m) in Na 2 HPO 4 -citric acid buffer solution (pH 6.5). The time-dependent absorbance variation of the reaction solution was monitored in time-scan mode at 652 nm using a microplate reader (Figure 2c). At each H 2 O 2 concentration, the concentration-changing rate (v) of oxTMB was calculated from the absorbance-changing rate via the Beer-Lambert law, A = εlc (A is the absorbance, ε is the molar absorbance coefficient, l is the path length, and c is the molar concentration) with l = 10 mm and ε of 39 000 m −1 cm −1 for oxTMB. Furthermore, the concentration change rates of TMB were plotted against corresponding H 2 O 2 concentrations, which follows the Michaelis-Menten equation (Figure 2d), known as where v 0 is the initial velocity of the reaction, V max is the maximal velocity of reaction, After confirming the catalytic activity of POD-mimic Fe 3 O 4 nanozyme in generating highly toxic ·OH from H 2 O 2 molecules, the catalytic activity of Au NPs as the GOxmimic nanozyme and the cascade catalytic performance were investigated. As a GOx mimic, Au NPs catalyze the oxidation of glucose in the presence of oxygen, producing gluconic acid and H 2 O 2 (Figure 3a). Therefore, the production of H 2 O 2 and gluconic acid, and the consumption of O 2 were assessed.
First, the reaction product H 2 O 2 generated from glucose catalyzed by Au NPs was qualitatively detected based on the H 2 O 2 -mediated size-enlargement of Au NPs in the presence of HAuCl 4 . [30] DMSN-Au NPs first catalyze the glucose oxidation reaction, and the in situ generated H 2 O 2 reduces HAuCl 4 to Au 0 following the equation The resulting Au species can further deposit on the surface of the initial Au NPs as seeds, causing the enlarged size of initially deposited Au NPs and correspondingly enhanced UV-vis absorbance intensity. According to the UV-vis results, in the presence of glucose and HAuCl 4 , the absorption of Au NPs at 505 nm increased over time (Figure 3b) and the absorption peak slightly red-shifted from 505 to 515 nm in 1 h of reaction (Figure 3c  The other product of glucose oxidation reaction, gluconic acid, was detected using a gluconic acid-specific colorimetric assay based on the reaction between gluconic acid, hydroxylamine, and FeCl 3 , which leads to the formation of a red compound hydroxamate-Fe 3+ with a typical absorbance peak at 505 nm. [31] Specifically, DMSN-Au NPs were incubated with glucose of different concentrations at pH 6.5 for 30 min. After the addition of hydroxylamine and Fe 3+ , the resulting solution turns into red with the major absorbance peak of the reaction solution being at 505 nm and increases with the elevated concentrations of glucose (Figure 3f), which confirms the production of gluconic acid in this Au NPs-catalyzed reaction and implies the catalytic activity of DMSN-Au NPs on glucose oxidation. In order to monitor the level of dissolved oxygen in the reaction system, a dissolved oxygen meter was used. After adding DMSN-Au NPs into the buffer solution of glucose of different  concentrations, the oxygen level in reaction solution declined rapidly owing to the consumption of dissolved oxygen for the glucose oxidation as catalyzed by the loaded Au NPs (Figure 3g).
Based on TMB colorimetric assay (Figure 3a), it has been verified that the self-organized enzymatic cascade reaction can be achieved by DMSN-Au-Fe 3 O 4 NPs without the aid of any natural enzyme. In detail, DMSN-Au-Fe 3 O 4 NPs and glucose solution were mixed and incubated in a TMB solution for 1 h. The reaction solution then turned blue with the absorbance peak at 370 and 652 nm, indicating the production of ·OH originating from the cascade catalysis of Au and Fe 3 O 4 NPs (Figure 3h). To be specific, the Au NPs immobilized within the mesopores of DMSN NPs initially catalyze the oxidation of glucose into H 2 O 2 , which is further utilized as the reactant for Fe 3 O 4 NPsbased catalytic Fenton reaction to produce hydroxyl radicals. Finally, the generated hydroxyl radicals oxidize the colorless TMB into the blue-colored oxTMB. Therefore, DMSN-Au-Fe 3 O 4 NP is indeed a robust composite nanozyme with dual enzymatic functionalities capable of catalyzing the cascade reaction without the assistance of any natural enzyme, i.e., initially catalyzing glucose oxidation to yield gluconic acid and H 2 O 2 , and then catalyzing H 2 O 2 into high-toxic ·OH. Especially, the catalytic performances of DMSN-Au-Fe 3 O 4 NPs in buffer solutions at pH 5.0, 6.5, 7.4, and 8.0 were conducted to investigate the influence of pH. It has been found that the catalytic activity of DMSN-Au-Fe 3 O 4 NPs is pH-dependent, exhibiting better catalytic activity in acid condition than in neutral or alkaline condition ( Figure S10, Supporting Information).
Before assessing the in vitro nanocatalytic therapeutic efficiency, the surface of DMSN-Au-Fe 3 O 4 NPs was modified with methoxy PEG-thiol (mPEG-SH) molecules for improved stability in physiological microenvironment. The Fourier transform infrared spectroscopy (FTIR) was conducted for the characterization of PEGylation. The FTIR spectrum of DMSN-Au-Fe 3 O 4 -PEG NPs shows the stretch of the COC band at 1060, 1110, 1150, 1250, and 1280 cm −1 , as well as the stretch band of CH 2 and CH 3 at 2890 and 2740 cm −1 , respectively, indicating the successful modification of mPEG-SH molecules ( Figure S11, Supporting Information). Attributing to the PEG modification, the DMSN-Au-Fe 3 O 4 -PEG NPs could be well dispersed in water, phosphate buffer solution, simulated body buffer, saline, dulbecco's modified eagle medium (DMEM), and fetal bovine serum, indicating their excellent stability ( Figure  S12, Supporting Information).
Initially, the cytotoxicities of DMSN-Au-Fe 3 O 4 NPs were tested on murine breast cancer 4T1 cells, as well as two normal cell lines, namely brain capillary endothelial cells and human umbilical vein endothelial cells by the standard cell-counting kit 8 assay. The results show that the DMSN-Au-Fe 3 O 4 NPs exhibit negligible effects on the proliferation of the two kinds of normal cells, at the elevated concentration up to 200 µg mL −1 within 12 and 24 h, indicating their high biosafety and biocompatibility for further in vitro and in vivo therapeutic applications ( Figure S13, Supporting Information). In addition, it has been found that DMSN-Au NPs and DMSN-Fe 3 O 4 NPs exhibit no significant inhibition on 4T1 cancer cells proliferation even at the elevated concentration up to 200 µg mL −1 after incubation for 12 and 24 h, at pH 6.5 (Figure 4a,b) and 7.4 ( Figure S14, Supporting Information). Importantly, when incubated with DMSN-Au-Fe 3 O 4 NPs, the cell viabilities exhibit a marked decline in a dose-dependent manner with an inhibition rate on 4T1 tumor cell viability being higher than 75% at 200 µg mL −1 (Figure 4c). This result indicates that the cytotoxicity of DMSN-Au-Fe 3 O 4 NPs should be attributed to the cascade catalytic reactions by Au and Fe 3 O 4 NPs, both of which are indispensable to trigger the generation of sufficiently high amount of toxic ·OH to induce the apoptosis of 4T1 tumor cells. In detail on the related therapeutic mechanism, H 2 O 2 , as produced by the reaction between glucose and oxygen under the catalysis by GOx mimic-Au NPs in cell culture medium, is further catalyzed by Fe 3 O 4 -based Fenton nanocatalysts to disproportionate, producing ·OH for killing the cancer cells. To evaluate the intracellular ·OH generation, a ROS fluorescence probe 2′,7′-dichlorofluorescin diacetate (DCFH-DA), which turns into 2′,7′-dichlorofluorescein (DCF) with strong green fluorescence in the presence of ROS, was introduced to stain cancer cells in order to reveal the intracellular ROS production. At pH 6.5, 4T1 cancer cells exhibit negligible fluorescence when coincubated only with high doses of glucose in DMEM or with DMSN-Au-Fe 3 O 4 NPs in glucose-free DMEM for 1 h. Comparatively, strong green fluorescence of 4T1 cancer cells can be observed after coincubation with DMSN-Au-Fe 3 O 4 NPs in the presence of glucose (Figure 4d), implying the massive intracellular ROS production by DMSN-Au-Fe 3 O 4 NPs and glucose under weakly acidic condition. Furthermore, it has been found that the DCF fluorescence intensity is dependent on the pH value of cell culture medium and incubation duration. As the incubation duration is prolonged from 15 min to 1 h, the DCF fluorescence intensity of cells increases accordingly when incubated with DMSN-Au-Fe 3 O 4 NPs and glucose under the weakly acidic condition (pH 6.5). However, in the neutral condition (pH 7.4), much weaker DCF fluorescence intensity is observed even after prolonged incubation (Figure 4e). The DCF fluorescence can be further monitored by flow cytometry (Figure 4f), which demonstrates that the DCF fluorescence increases by prolonging the incubation duration of DMSN-Au-Fe 3 O 4 NPs. The CLSM and flow cytometry results demonstrate that the intracellular ROS production as catalyzed by DMSN-Au-Fe 3 O 4 NPs has been substantially promoted in the mildly acidic environment as compared to that in the neutral condition.
Especially, the intracellular nanocatalytic therapeutic mechanism of DMSN-Au-Fe 3 O 4 nanoplatform was investigated by the typical flow cytometric apoptosis assay after stained with FITC-labeled Annexin V and propidium iodide (PI). According   (Figure 5a). [31] Importantly, after i.v. administration, DMSN-Au-Fe 3 O 4 NPs efficiently accumulated into the tumor tissue with the passive accumulation efficacy of 3.49% ID g −1 in 2 h and 1.80% ID g −1 in 12 h (Figure 5b) based on the typical EPR effect for tumor-passive targeting and accumulation. [32] Encouraged by the high efficiency of in vitro nanocatalytic therapy for killing cancer cells, the prolonged blood circulation, and potent tumor accumulation effect, the in vivo nanocatalytic therapeutic efficiency of DMSN-Au-Fe 3 O 4 NPs was evaluated against the 4T1 breast tumor xenograft on nude mice. Eighteen 4T1 tumor-bearing mice (tumor volume ≈ 50 mm 3 ) were randomly separated into three groups (n = 6 per group). Saline (control) and DMSN-Au-Fe 3 O 4 NPs at different doses (10 and 20 mg kg −1 ) were i.v. administrated to investigate the therapeutic performance and related in vivo mechanism. During a 15 day therapeutic period, the body weights of mice in two therapeutic groups show no significant difference from those of mice in the control group (Figure 5c). The tumor volumes of mice in each group were recorded using a digital caliper, and the digital photos of mice were taken every 2 days after the i.v. injection( Figure S19, Supporting Information). It has been found that the i.v. administration of DMSN-Au-Fe 3 O 4 NPs shows a dose-dependent tumor-growth inhibition effect during the therapeutic period (Figure 5d,e), with the inhibition rates of 45.96% and 69.08% at the doses of 10 and 20 mg kg −1 , respectively. This substantial tumor inhibition effect is attributed to the high-toxic hydroxyl radicals as produced by the endogenous cascade reaction triggered by the biomimetic Au and Fe 3 O 4 coloaded nanoplatforms under the mildly acidic TME, effectively inducing tumor cell death. It should be noted that the constructed nanocomposites are highly biocompatible without any toxic substance used, and the toxic effect can only be triggered under the mildly acidic TME, which means that this DMSN-Au-Fe 3 O 4 -based nanocatalytic tumor therapy features high tumor specificity and excellent therapeutic biosafety. The neutral condition of normal tissue will not trigger such a cascade catalytic reaction, therefore no toxic effect will be induced to cause noticeable damages to normal cells/tissues.
In order to reveal the detailed therapeutic mechanism by tumor-pathological analysis, hematoxylin and eosin (H&E), terminal deoxynucleotidyl transferase mediated dUTP nickend labeling (TUNEL) and Ki-67 antibody staining of dissected tumor tissues from each group were conducted. H&E and TUNEL staining results exhibit severe damage and necrosis of tumor cells in two therapeutic groups in comparison to the control group (Figure 5f). Ki-67 antibody staining results reveal the suppressed proliferative activities of tumors cells in two therapeutic groups while there is almost no significant adverse effect on cell proliferation in the control group. Furthermore, H&E staining of the major organs (heart, liver, spleen, lung, and kidney) conducted after the therapeutic period shows no noticeable pathological side effects on major organs of mice in therapeutic groups ( Figure S20, Supporting Information), indicating the high biocompatibility and therapeutic biosafety of DMSN-Au-Fe 3 O 4 NPs as nanocatalytic agents.
Additionally, the rapid clearance of therapeutic nanomaterials is highly favorable for the clinical translation, which can avoid In summary, this work reports on the effective TME-responsive catalytic cascade reactions for nanocatalytic tumor-specific therapy with marked therapeutic efficacy and biosafety based on an inorganic biomimetic DMSN-Au-Fe 3 O 4 composite nanoplatforms. The in situ grown Au NPs within the large mesopores of DMSN NPs as a GOx-mimic nanozyme specifically catalyze β-D-glucose oxidation into gluconic acid and H 2 O 2 under aerobic conditions, while the produced H 2 O 2 can subsequently be catalyzed by the coloaded Fe 3 O 4 -based Fenton nanocatalysts to produce high-toxic hydroxyl radicals which substantially induce tumor-cell death afterward. Both the comprehensive in vitro cell-level and in vivo tumor-bearing mice xenograft evaluations have demonstrated the efficient nanocatalytic tumor

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Adv. Sci. 2019, 6, 1801733 therapy on killing the cancer cells and suppressing the tumor growth by as high as 69.08% of inhibition rate without any toxic substance being used, in highly TME-responsive and tumorspecific manners. The high biocompatibility, high therapeutic biosafety, and easy excretion of these DMSN-Au-Fe 3 O 4 composite nanoplatforms have also been verified which guarantees their further clinical translation. Therefore, the present biomimetic dual inorganic nanocomposite-triggered cascade reaction strategy for TME-responsive and effective nanocatalytic tumor therapy not only establishes a paradigm of "toxic-drug-free" endogenous and noninvasive nanocatalytic biomedicine, but also takes an important step forward in developing biomimetic nanoplatforms with multienzyme mimicking catalytic activities for tumor-specific therapies.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.