聚酰胺酸盐稳定的金纳米催化剂用于羧酸的绿色合成

使用一锅法成功制备了水溶性聚酰胺酸盐稳定的金纳米催化剂(AuNPs-PAAS),将该催化剂用于伯醇的催化氧化.利用紫外-可见分光光度计,X射线衍射仪(XRD),透射电子显微镜(TEM)等表征方法对催化剂进行了表征.结果表明,金纳米粒子在聚酰胺酸溶液中处于均匀分散状态,金纳米尺寸约为5 nm.将制备的纳米金催化剂用于伯醇的氧化,评价了其在伯醇氧化成羧酸反应中的催化性能,结果显示,在空气为氧化剂,水为溶剂的条件下,AuNPs-PAAS对伯醇的催化氧化为高效的准均相催化过程,高选择性得到羧酸产物,通过调节溶液的pH值,可以很容易的实现产物与反应体系分离和催化剂的回收和循环利用.     

多元醇法形貌可控制备氧化亚铜微纳米颗粒

以硝酸铜、乙酸铜、乙酰丙酮铜为原料,采用多元醇还原法合成制备了氧化亚铜立方体、微球、空心球、核壳结构等微纳米颗粒。 利用X射线衍射仪(XRD)、场发射扫描电子显微镜(FE-SEM)、透射电子显微镜(TEM)、高分辨透射电子显微镜(HRTEM)和紫外可见吸收光谱(UV-Vis)等测试手段对样品的物相、形貌、元素组分及吸光性能进行了表征。 同时考察了铜源、反应时间、和多元醇类型对氧化亚铜微纳米材料形貌的影响,对产物形成机理进行了初步的探讨。 对产物形成机理进行了初步的探讨。 采用简单低廉的多元醇合成法,可以控制合成不同相貌的氧化亚铜微纳米结构。 对制备形貌可控的氧化物具有一定的指导意义。     

Green Synthesis of Nickel Oxide Nanoparticles from Berberis balochistanica Stem for Investigating Bioactivities

Green synthesis of nanomaterials is advancing due to its ease of synthesis, inexpensiveness, nontoxicity and renewability. In the present study, an eco-friendly biogenic method was developed for the green synthesis of nickel oxide nanoparticles (NiONPs) using phytochemically rich Berberis balochistanica stem (BBS) extract. The BBS extract was rich in phenolics, flavonoids and berberine. These phytochemicals successfully reduced and stabilised the NiNO₃ (green) into NiONPs (greenish-gray). BBS-NiONPs were confirmed by using UV-visible spectroscopy (peak at 305 nm), X-ray diffraction (size of 31.44 nm), Fourier transform infrared spectroscopy (identified -OH group and Ni-O formation), energy dispersive spectroscopy (showed specified elemental nature) and scanning electron microscopy (showed rhombohedral agglomerated shape). BBS-NiONPs were exposed to multiple in vitro bioactivities to ascertain their beneficial biological applications. They exhibited strong antioxidant activities: total antioxidant capacity (64.77%) and 2, 2-diphenyl-1-picrylhydrazyl (71.48%); and cytotoxic potential: Brine shrimp cytotoxicity assay with IC₅₀ (10.40 µg/mL). BBS-NiONPs restricted the bacterial and fungal pathogenic growths at 1000, 500 and 100 µg/mL. Additionally, BBS-NiONPs showed stimulatory efficacy by enhancing seed germination rate and seedling growth at 31.25 and 62.5 µg/mL. In aggregate, BBS extract has a potent antioxidant activity which makes the green biosynthesis of NiONPs easy, economical and safe. The biochemical potential of BBS-NiONPs can be useful in various biomedical and agricultural fields.     

Green Synthesis and Characterization of Cobalt Oxide Nanoparticles Using Psidium guajava Leaves Extracts and Their Photocatalytic and Biological Activities

The advanced technology for synthesizing nanoparticles utilizes natural resources in an environmentally friendly manner. Additionally, green synthesis is preferred to chemical and physical synthesis because it takes less time and effort. The green synthesis of cobalt oxide nanoparticles has recently risen due to its physico-chemical properties. In this study, many functional groups present in Psidium guajava leaf extracts are used to stabilize the synthesis of cobalt oxide nanoparticles. The biosynthesized cobalt oxide nanoparticles were investigated using UV-visible spectroscopic analysis. Additionally, Fourier-transform infrared spectroscopy revealed the presence of carboxylic acids, hydroxyl groups, aromatic amines, alcohols and phenolic groups. The X-ray diffraction analysis showed various peaks ranging from 32.35 to 67.35°, and the highest intensity showed at 36.69°. The particle size ranged from 26 to 40 nm and confirmed the average particle size is 30.9 nm. The green synthesized P. guajava cobalt oxide nanoparticles contain cobalt as the major abundant element, with 42.26 wt% and 18.75 at% confirmed by the EDAX techniques. SEM images of green synthesized P. guajava cobalt oxide nanoparticles showed agglomerated and non-uniform spherical particles. The anti-bacterial activity of green synthesized P. guajava cobalt oxide nanoparticles was evaluated against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli with a 7 to 18 mm inhibitory zone. The photocatalytic activity was evaluated using green synthesized P. guajava cobalt oxide nanoparticles and observed 79% of dye degradation. The MTT assay of P. guajava cobalt oxide nanoparticles showed an excellent cytotoxic effect against MCF 7 and HCT 116 cells compared to normal cells. The percentage of cell viability of P. guajava cobalt oxide nanoparticles was observed as 90, 83, 77, 68, 61, 58 and 52% for MCF-7 cells and 82, 70, 63, 51, 43, 40, and 37% for HCT 116 cells at the concentration of 1.53, 3.06, 6.12, 12.24, 24.48, 50, and 100 μg/mL compared to control cells. These results confirmed that green synthesized P. guajava cobalt oxide nanoparticles have a potential photocatalytic and anti-bacterial activity and also reduced cell viability against MCF-7 breast cancer and HCT 116 colorectal cancer cells.     

Green Synthesis of Silver Nanoparticles Using Extract of Cilembu Sweet Potatoes (Ipomoea batatas L var. Rancing) as Potential Filler for 3D Printed Electroactive and Anti-Infection Scaffolds

Electroactive biomaterials are fascinating for tissue engineering applications because of their ability to deliver electrical stimulation directly to cells, tissue, and organs. One particularly attractive conductive filler for electroactive biomaterials is silver nanoparticles (AgNPs) because of their high conductivity, antibacterial activity, and ability to promote bone healing. However, production of AgNPs involves a toxic reducing agent which would inhibit biological scaffold performance. This work explores facile and green synthesis of AgNPs using extract of Cilembu sweet potato and studies the effect of baking and precursor concentrations (1, 10 and 100 mM) on AgNPs’ properties. Transmission electron microscope (TEM) results revealed that the smallest particle size of AgNPs (9.95 ± 3.69 nm) with nodular morphology was obtained by utilization of baked extract and ten mM AgNO₃. Polycaprolactone (PCL)/AgNPs scaffolds exhibited several enhancements compared to PCL scaffolds. Compressive strength was six times greater (3.88 ± 0.42 MPa), more hydrophilic (contact angle of 76.8 ± 1.7°), conductive (2.3 ± 0.5 × 10⁻³ S/cm) and exhibited anti-bacterial properties against Staphylococcus aureus ATCC3658 (99.5% reduction of surviving bacteria). Despite the promising results, further investigation on biological assessment is required to obtain comprehensive study of this scaffold. This green synthesis approach together with the use of 3D printing opens a new route to manufacture AgNPs-based electroactive with improved anti-bacterial properties without utilization of any toxic organic solvents.     

Green Synthesis of ZnO Nanoparticles Using Plant Extract as a Template

Plant-based nanoparticles (NPs) have found great interest among various scientist in the present era and used in various sector including medicine, agriculture, and food industry. The various chemical constituents of plants aid in the bioreduction of metal ions to a nanoscale. Among the various NPs synthesized, zinc oxide (ZnO) NPs hold a premier position. ZnO NPs have use in textile, cosmetic, diagnostics, optoelectronics, photocatalysis, diodes, and many other areas. NPs synthesized through green synthesis have a potentially greater role in treating clinical pathogens. Present investigations show a simple eco-friendly method for the synthesis of ZnO NPs from the husk of sunflower seeds. Sunflower is an economically important crop, for the formation of edible oil. The husk is considered to be a waste, product in oil industry, however, the biomolecules present in sunflower husk can be used to produce ZnO NPs. Present investigations reveal formation of ZnO NPs and investigations of their structure through scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Their optical properties have been studied by ultraviolet–visible spectrophotometer (UV–Vis) and fluorophotometer. ZnO NPs have also been investigated for their potential phytoremedial properties.     

纳米氧化铁的电化学合成

采用金属铁为"牺牲"阳极,不锈钢片为阴极,在无隔膜电解槽中,用电化学法合成纳米氧化铁.通过XRD、FTIR、TG DSC及粒径分布等测试方法对所得的纳米粒子进行了表征和分析.实验表明:离心后得到的胶体放置于40℃的真空干燥箱中干燥后,得到无定型纳米氧化铁粒子;经320℃煅烧3h后,粒子转化为γ Fe2O3,平均粒径为22.0nm;进一步提高煅烧温度,在540℃煅烧3h后,可得到平均粒径为35.2nm的α Fe2O3.     

Green Synthesis of Silver Nanoparticles Using Arachis hypogaea (Ground Nut) Root Extract for Antibacterial and Clinical Applications

The present study focused on the green synthesis of silver nanoparticles (AgNPs) using Arachis hypogaea (ground nut) root extract for the antibacterial and clinical application. The presence of major phytochemical compounds are found to be 2H-Pyaran,2,5-diethenyltetrahydro, Didodecyl phthalate, Decanoic acid, Tetradecanoic acid, Bis(2-ethylhexyl) phthalate, Dodecanoic acid, Phosphonic acid, 2-(4-Methoxyphenyl)-5-(4-methoxynaphthyl) thiophene and Methyl 2-(N-Benzylimino)-4-chloro-3,3-dimethylbutanoate by GC–MS. Nanoparticles synthesis is confirmed by UV–Vis analysis by observing the maximum absorption spectrum at 450 nm. XRD and SEM–EDX results reveals the synthesized nanoparticles are cubic crystalline with agglomerated particles of silver oxide with biomolecules present around it. TEM images clearly shows that the biosynthesized nanoparticles are mostly spherical and irregular shaped with an average particles size of 30 nm. Highest susceptibility pattern of silver nanoparticle against Enterococcus sp. (35 ± 0.4 mm) which followed by Pseudomonas sp. (33 mm) and Staphylococcus aureus (29 mm). Green synthesized nanoparticles are coated over the commercially available clinical band aid cloth by dip coating method. Silver nanoparticle incorporated band aid cloth showed good antibacterial activity against the harmful bacteria which usually cause infection and interfere during wound healing. Our findings revealed that green nanoparticle has a good antibacterial action against harmful bacteria and showed good response for efficient clinical application.     

Green Synthesis of Silver Nanoparticles Using Natural Extracts with Proven Antioxidant Activity

Natural extracts are a rich source of biomolecules that are useful not only as antioxidant drugs or diet supplements but also as complex reagents for the biogenic synthesis of metallic nanoparticles. The natural product components can act as strong reducing and capping substrates guaranteeing the stability of formed NPs. The current work demonstrates the suitability of extracts of Camellia sinensis, Ilex paraguariensis, Salvia officinalis, Tilia cordata, Levisticum officinale, Aegopodium podagraria, Urtica dioica, Capsicum baccatum, Viscum album, and marine algae Porphyra Yezoensis for green synthesis of AgNPs. The antioxidant power of methanolic extracts was estimated at the beginning according to their free radical scavenging activity by the DPPH method and reducing power activity by CUPRAC and SNPAC (silver nanoparticle antioxidant capacity) assays. The results obtained by the CUPRAC and SNAPC methods exhibited excellent agreement (R²~0.9). The synthesized AgNPs were characterized by UV-vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), dynamic light scattering (DLS) particle size, and zeta potential. The UV-vis absorption spectra showed a peak at 423 nm confirming the presence of AgNPs. The shapes of extract-mediated AgNPs were mainly spherical, spheroid, rod-shaped, agglomerated crystalline structures. The NPs exhibited a high negative zeta potential value in the range from −49.8 mV to −56.1 mV, proving the existence of electrostatic stabilization. FTIR measurements indicated peaks corresponding to different functional groups such as carboxylic acids, alcohol, phenol, esters, ethers, aldehydes, alkanes, and proteins, which were involved in the synthesis and stabilization of AgNPs. Among the examined extracts, green tea showed the highest activity in all antioxidant tests and enabled the synthesis of the smallest nanoparticles, namely 62.51, 61.19, and 53.55 nm, depending on storage times of 30 min, 24 h, and 72 h, respectively. In turn, the Capsicum baccatum extract was distinguished by the lowest zeta potential, decreasing with storage time from −66.0 up to −88.6 mM.     

Ecofriendly Green Synthesis of Copper (II) Oxide Nanoparticles Using Corchorus olitorus Leaves (Molokhaia) Extract and Their Application for the Environmental Remediation of Direct Violet Dye via Advanced Oxidation Process

In this research, copper (II) oxide nanoparticles were prepared by an ecofriendly green method using the extract of corchorus olitorus leaves (Molokhaia) as a surfactant, capping and anti-agglomeration agent. The ecofriendly green CuO NPs were characterized using different chemical and physical techniques and the results confirmed the formation of monoclinic tenorite CuO nanoparticles with an average particle size of 12 nm and BET surface area of 11.1 m²/g. The eco-friendly green CuO NPs were used in environmental remediation for the efficient catalytic degradation of direct violet dye via advanced oxidation process (AOP) in presence of H₂O₂. The impact of AOP environmental parameters affecting the degradation process was investigated. Moreover, the catalytic degradation of the direct violet dye using the ecofriendly green CuO NPs was studied kinetically and thermodynamically and the results showed that the catalytic degradation process agreed well with the pseudo-second-order kinetic model and the process was spontaneous and endothermic in nature. Finally, high catalytic degradation of the direct violet dye was observed when the eco-friendly prepared green CuO NPs were placed in real water samples.     

Green Synthesis of Zinc Oxide Nanoparticles Using Pomegranate Fruit Peel and Solid Coffee Grounds vs. Chemical Method of Synthesis,with Their Biocompatibility and Antibacterial Properties Investigation

This research aims to investigate the synthesis, characterization, and evaluation of the biocompatibility and antibacterial activity of novel zinc oxide (ZnO) nanoparticles (NPs) prepared by Punica granatum peel and coffee ground extracts as the reducing and capping agents. Chemically synthesized ZnONPs were prepared using zinc acetate dihydrate and sodium hydroxide as reducing precursors. ZnONPs were characterized using an ultraviolet-visible spectrophotometer (UV-VIS), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and Fourier transform infrared (FTIR) spectroscopy. Peaks of UV spectra were 300 nm for ZnONPs_PPE, 320 nm (ZnONPs_CE), 290 nm, and 440 nm (ZnONP_Chem), thereby confirming ZnONPs formation. The X-ray diffractograms revealed their hexagonal structure. TEM micrographs of the biosynthesized ZnONPs revealed their hexagonal pattern and nanorod shape for ZnONPs_Chem with particle sizes of 118.6 nm, 115.7 nm, and 111.2 nm, respectively. The FTIR analysis demonstrated the presence of proteins, carboxyl, and hydroxyl groups on ZnONPs surfaces that act as reducing and stabilizing agents. ZnONP_Chem shows the antibacterial effect on Staphylococcus aureus, Enterobacter aerogenes, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Punica peel and coffee ground extracts are effective reducing agents for green ZnONPs synthesis with a lower cytotoxic effect on Vero cells than ZnONPs_Chem with IC50 = 111, 103, and 93 μg/mL, respectively.     

Sonochemical Synthesis of Highly Luminescent Zinc Oxide Nanoparticles Doped with Magnesium(II)

Mut zur Lücke : Mg/ZnO‐Nanopartikel mit heller, stabiler Photolumineszenz in kolloidalen Dispersionen wie im Festkörper bilden sich durch Dotierung von Mg²⁺‐Ionen in ZnO‐Nanopartikel durch sonochemische Synthese. Ihre Bandlücken und Lumineszenzeigenschaften verändern sich mit der Defektkonzentration in den ZnO‐Nanopartikeln. Diese Konzentration hängt vom molaren Mg/Zn‐Verhältnis ab (siehe Bild).

     

Synthesis and Characterization of Nickel Oxide Nanoparticles from Ni(salen) as Precursor

Nickel oxide nanoparticles have been synthesized by thermal treatment of N,N′-(bis(salicylidene)-ethylene-1,2-diamine)Nickel(II); [Ni(salen)]; as precursor which has been synthesized via two methods: [Ni(salen)] were obtained by solid state reaction in absence solvent and co-precipitation reaction in presence of propanol as solvent, respectively. Nickel oxide nanoparticles were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Fourier transform infrared spectroscopy.     

Biogenic Synthesis of Rod Shaped ZnO Nanoparticles Using Red Paprika (Capsicum annuum L. var. grossum (L.) Sendt) and Their in Vitro Evaluation

Journal of Cluster Science - Metal oxide nanoparticles (NPs) have gained attention in biomedicine due to their broad spectrum of applications, such as targeted drug delivery, their use as...     

Green Synthesis and Characterization of Zinc Oxide Using Carica papaya Leaf Extract

Environmental methodologies are gaining recognition in this modern world. Environmental nanotechnology plays a major role in improving modern fields of environmental engineering and science. Metal oxide nanoparticles have exceptional properties due to their small size, including quantum confinement, surface-to-volume ratio, plasmon excitation, high biocompatibility, and surface modifiability. The biosynthesis of nanoparticles using fungi, bacteria, and plants through various biotechnological techniques is currently a new paradigm for environmental protection. Synthesis of nanoparticles through plant extract is good because it eliminates the dangers of toxic chemicals, it is environmentally friendly, simpler, and safer as the reaction time is reduced and it can also be increased in size for higher operation. The present study is based on the development of zinc oxide nanoparticles from papaya leaf extract where zinc nitrate is used as a precursor. The biosynthesized nanoparticles are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, energy-dispersive X-ray analysis, UV-visible spectroscopy, and dynamic light scattering analysis. The crystalline phase determination of the zinc oxide nanoparticles is analyzed by X-ray diffraction and the formation of polycrystalline zinc oxide nanoparticles is confirmed. FT-IR spectrum reveals the main functional groups and chemical information in zinc oxide nanostructures. Morphological analysis is performed using SEM at different magnification levels. EDAX analysis shows the purity of the composite samples. Optical characterization is performed using a UV–vis spectrophotometer. DLS analysis shows that the nanoparticles formed have a relatively well-defined dimension.     

Green Synthesis of Silver Nanoparticles Using Acacia farnesiana (Sweet Acacia) Seed Extract Under Microwave Irradiation and Their Biological Assessment

In the present study, Acacia farnesiana (Sweet acacia) seed extract is used to reduce Ag⁺ → Ag⁰ under microwave irradiation. The formation of silver nanoparticles (AgNPs) is monitored by recording the UV–Vis absorption spectra for surface plasmon resonance (SPR) peak at ~450 nm. The absorbance of SPR increases linearly with increasing temperature of the reaction mixture. Rapid reduction of silver ions occurred to form AgNPs, 80–90 % yield in about 150 s. A marginal decrease in pH and increase in solution potential (E) of the reaction mixture during the formation of AgNPs are in agreement with the proposed mechanism. XRD pattern of the AgNPs agree with the fcc structure of Ag metal, and the calculated crystallite size is ~17 nm. FT-IR and solid-state ¹³C NMR spectra indicate the functional groups of flavonones and terpenoids (biomolecules from plant extract) which are adsorbed on AgNPs, thereby the present method led to in situ biofunctionalization/bio-capping of AgNPs. TG analysis shows the thermal decomposition of these plant residues present on AgNPs at about 250 °C. The spherical shape of the particles with a diameter (?) in the range of ~15–20 nm is evident from FE-SEM image. Elemental analysis by EDX analysis confirms the presence of Ag as the only major element. The in vitro antibacterial screening of AgNPs shows that these bio-capped AgNPs have higher inhibitory action for E. coli and S. aureus followed by B. subtilis and P. aeruginosa. In addition, AgNPs show very good antioxidant property.     

A Green Synthesis of Substituted Coumarins Using Nano Graphene Oxide as Recyclable Catalyst

One‐pot synthesis of some pyranocoumarins and biscoumarins using graphene oxide nanosheets as efficient catalyst has been investigated. The green methods offer appealing attributes such as excellent yields, short reactions times, use of a safe and recyclable catalyst, and a simple workup procedure.     

Characterization and Mechanism Elucidation of Dye Adsorption Using Cuprous Selenide Nanoparticles from Aqueous Solutions

The removal of cationic dyes, methylene blue(MB) and rhodamine B(RB), and anionic dyes, methyl or-ange(MO) and eosin Y(EY), from aqueous solutions by adsorption using Cu₂Se nanoparticles(Cu₂SeNPs) was studied. The effects of the initial pH values, adsorbent doses, contact time, initial dye concentrations, salt concentrations, and operation temperatures on the adsorption capacities were investigated. The adsorption process was better fitted the Langmuir equation and pseudo-second-order kinetic model, and was spontaneous and endothermic as well. The adsorption mechanism was probably based on the electrostatic interactions and π-π interactions between Cu₂SeNPs and dyes. For an adsorbent of 0.4 g/L of Cu₂SeNPs, the adsorption capacities of 23.1(MB), 22.9(RB) and 23.9(EY) mg/g were achieved, respectively, with an initial dye concentration of 10 mg/g(pH=8 for MB and pH=4 for RB and EY) and a contact time of 120 min. The removal rate of MB was still 70.4% for Cu₂SeNPs being reused in the 5th cycle. Furthermore, the recycled Cu₂SeNPs produced from selenium nanoparticles adsorbing copper were also an effective adsorbent for the removal of dyes. Cu₂SeNPs showed great potential as a new adsorbent for dyes removal due to its good stability, functionalization and reusability.     

Green Synthesis of Gold and Silver Nanoparticles Using Leaf Extract of Capsicum chinense Plant

So far, several studies have focused on the synthesis of metallic nanoparticles making use of extracts from the fruit of the plants from the genus Capsicum. However, as the fruit is the edible, and highly commercial, part of the plant, in this work we focused on the leaves, a part of the plant that is considered agro-industrial waste. The biological synthesis of gold (AuNPs) and silver (AgNPs) nanoparticles using aqueous extracts of root, stem and leaf of Capsicum chinense was evaluated, obtaining the best results with the leaf extract. Gold and silver nanoparticles synthesized using leaf extract (AuNPs-leaf and AgNPs-leaf, respectively) were characterized by UV-visible spectrophotometry (UV-Vis), Fourier Transform Infrared Spectroscopy with Attenuated Total Reflection (FTIR-ATR), X-ray Photoelectron Spectroscopy (XPS), Ultra Hight Resolution Scanning Electron Microscopy coupled to Energy-Dispersive X-ray spectroscopy (UHR-SEM-EDX) and Transmission Electron Microscopy (TEM), and tested for their antioxidant and antimicrobial activities. In addition, different metabolites involved in the synthesis of nanoparticles were analyzed. We found that by the use of extracts derived from the leaf, we could generate stable and easy to synthesize AuNPs and AgNPs. The AuNPs-leaf were synthesized using microwave radiation, while the AgNPs-leaf were synthesized using UV light radiation. The antioxidant activity of the extract, determined by ABTS, showed a decrease of 44.7% and 60.7% after the synthesis of the AuNPs-leaf and AgNPs-leaf, respectively. After the AgNPs-leaf synthesis, the concentration of polyphenols, reducing sugars and amino acids decreased by 15.4%, 38.7% and 46.8% in the leaf extract, respectively, while after the AuNPs-leaf synthesis only reducing sugars decreased by 67.7%. These results suggest that these groups of molecules are implicated in the reduction/stabilization of the nanoparticles. Although the contribution of these compounds in the synthesis of the AuNPs-leaf and the AgNPs-leaf was different. Finally, the AgNPs-leaf inhibited the growth of S. aureus, E. coli, S. marcescens and E. faecalis. All of them are bacterial strains of clinical importance due to their fast antibiotic resistance development.