An innovative study aimed at understanding the influence of the particle size of ZnO (from the microscale down to the nanoscale) on its antibacterial effect is reported herein. The antibacterial activity of ZnO has been found to be due to a reaction of the ZnO surface with water. Electron‐spin resonance measurements reveal that aqueous suspensions of small nanoparticles of ZnO produce increased levels of reactive oxygen species, namely hydroxyl radicals. Interestingly, a remarkable enhancement of the oxidative stress, beyond the level yielded by the ZnO itself, is detected following the antibacterial treatment. Likewise, an exposure of bacteria to the small ZnO nanoparticles results in an increased cellular internalization of the nanoparticles and bacterial cell damage. An examination of the antibacterial effect is performed on two bacterial species: Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive). The nanocrystalline particles of ZnO are synthesized using ultrasonic irradiation, and the particle sizes are controlled using different solvents during the sonication process. Taken as a whole, it is apparent that the unique properties (i.e., small size and corresponding large specific surface area) of small nanometer‐scale ZnO particles impose several effects that govern its antibacterial action. These effects are size dependent and do not exist in the range of microscale particles.

One of the reasons for the huge interest in nanomaterials originated because of the prohibitive price that commercial companies have to pay for introducing new materials into the market. Nanotechnology enables these companies to obtain new properties using old and recognized materials by just reducing their particle size. For these known materials no government approval has to be obtained. Thus, the interest in nanomaterials has led to the development of many synthetic methods for their fabrication. Sonochemistry is one of the earliest techniques used to prepare nanosized compounds. Suslick, in his original work, sonicated Fe(CO)5 either as a neat liquid or in a decalin solution and obtained 10-20 nm size amorphous iron nanoparticles. A literature search that was conducted by crossing Sono* and Nanop* has found that this area is expanding almost exponentially. It started with two papers published in 1994, two in 1995, and increased to 59 papers in 2002. A few authors have already reviewed the fields of Sono and Nano. It should be mentioned that in 1996, Suslick et al. published an early review on the nanostructured materials generated by ultrasound radiation. Suslick and Price have also reviewed the application of ultrasound to materials science. This review dealt with nanomaterials, but was not directed specifically to this topic. The review concentrated only on the sonochemistry of transition metal carbonyls and catalytic reactions that involve the nanoparticles resulting from their sonochemical decomposition. Grieser and Ashokkumar have also written a review on a similar topic. A former coworker, Zhu, has recently submitted for publication a review article entitled "Novel Methods for Chemical Preparation of Metal Chalcogenide Nanoparticles" in which he reviews three synthetic methods (sonochemistry, sonoelectrochemistry, and microwave heating) and their application in the synthesis of nanosized metal chalcogenides. Although still unpublished, I myself have recently written a review discussing novel methods (sonochemistry, microwave heating, and sonoelectrochemistry) for making nanosized materials. The current review will: (1) Present the four main advantages that sonochemistry has over other methods related to materials science and nanochemistry; (2) concentrate on the more recent (2003) literature that was not reviewed in the previously-mentioned reviews, and (3) focus on a specific question, such as what is the typical shape of products obtained in sonochemistry? This review will not survey the literature related to sonoelectrochemistry.

The work reported here describes interactions between nanoscale Au colloids and two main types of organic functional groups, viz., alkanethiols and amino acids. The surface chemistry of particulate Au is dominated by electrodynamic factors related to its (negative) surface charge. Generalized multiparticle Mie calculations were used to model the optical absorption characteristics of Au particles, existing either singly or in varying degrees of aggregation. Experiments with standard (monodisperse) Au colloids confirm the theoretical prediction of a new peak appearing at longer wavelength that intensifies and shifts further from the original peak with increasing particle size, increasing aggregate size, or shorter interparticle spacing. Control of aggregation degree in alkanethiols is achieved by judicious selection of terminal group composition (single-or doubleended), alkyl chain length, and the presence of pH sensitive groups such as carboxylates. In amino acids, the reactivity of the R-amine (adjacent to -COOH) is found to be pH-dependent. Linking via the R-amine is activated at low pH but suppressed at intermediate and high pH due to electrostatic repulsive forces between the Au surface and the charged carboxylate group or even the (formally neutral) polar carbonyl group in amides. However, dibasic amino acids can still be used to cross-link Au colloids at high pH. The pH insensitive (remote) amine binds amino acids to each particle, leaving protruding pairs of R-amines that can be bridged by a symmetrical linker molecule like glutaraldehyde (via its electrophilic centers). This offers a new way to organize Au nanoparticles into extended architectures and functional materials over a wide range of pH. The potential of Au colloids to recognize and determine dibasic amino acids based on optical absorption changes is briefly assessed. A higher detection limit for cysteine (1.2 µg/mL) was found for larger (40 nm) Au particles.

Nanoscale particles of metallic copper clusters have been prepared by two methods, namely the thermal reduction and sonochemical reduction of copper(II) hydrazine carboxylate Cu-(N 2 H 3 COO) 2 ‚2H 2 O complex in an aqueous medium. Both reduction processes take place under an argon atmosphere over a period of 2-3 h. The FT-IR, powder X-ray diffraction, and UV-visible studies support the reduction products of Cu 2+ ions as metallic copper nanocrystallites. The powder X-ray analysis of the thermally derived products show the formation of pure metallic copper, while the sonochemical method yields a mixture of metallic copper and copper oxide (Cu 2 O). The formation of Cu 2 O along with the copper nanoparticles in the sonochemical process can be attributed to the partial oxidation of copper by in situ generated H 2 O 2 under the sonochemical conditions. However, the presence of a mixture of an argon/hydrogen (95:5) atmosphere yields pure copper metallic nanoparticles, which could be due to the scavenging action of the hydrogen towards the OH • radicals that are produced in solution during ultrasonic irradiation. The synthesized copper nanoparticles exhibit a distinct absorption peak in the region of 550-650 nm. The transmission electron microscopy studies of the thermally derived copper show the presence of irregularly shaped particles (200-250 nm) having sharp edges and facets. On the other hand, the sonochemically derived copper powder shows the presence of porous aggregates (50-70 nm) that contain an irregular network of small nanoparticles. The copper nanoparticles are catalytically active toward an "Ullmann reaction"sthat is, the condensation of aryl halides to an extent of 80-90% conversion. The time course of catalysis was studied for condensation of iodobenzene at 200 °C for a period of 1-5 h. The catalytic ability of copper nanoparticles produced by the thermal and sonochemical methods was compared with that of commercial copper powders.

Nanocrystalline particles of MgO were synthesized using microwave radiation in an ethylene glycol solution. The antibacterial activities of the MgO nanoparticles were tested by treating Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive) cultures with 1 mg mL–1 of the nanoparticles. We have examined the importance of the size effect, pH, and the form of the active MgO species as a bactericidal agent. A clear size dependence of the nanoparticles is observed where the amount of eradicated bacteria was strongly dependent on the particle size.

To date, there is still a lack of definite knowledge regarding the interaction of CuO nanoparticles with bacteria and the possible permeation of the nanoparticles into bacterial cells. This study was aimed at shedding light on the size-dependent (from the microscale down to the small nanoscale) antibacterial activity of CuO. The potent antibacterial activity of CuO nanoparticles was found to be due to ROS-generation by the nanoparticles attached to the bacterial cells, which in turn provoked an enhancement of the intracellular oxidative stress. This paradigm was confirmed by several assays such as lipid peroxidation and reporter strains of oxidative stress. Furthermore, electron microscopy indicated that the small nanoparticles of CuO penetrated the cells. Collectively, the results reported herein may reconcile conflicting concepts in the literature concerning the antibacterial mechanism of CuO nanoparticles, as well as highlight the potential for developing sustainable CuO nanoparticles-based devices for inhibiting bacterial infections.

Metal oxide nanoparticles have marked antibacterial activity. The toxic effect of these nanoparticles, such as those comprised of ZnO, has been found to occur due to an interaction of the nanoparticle surface with water, and to increase with a decrease in particle size. In the present study, we tested the ability of ZnO nanoparticles to affect the viability of the pathogenic yeast, Candida albicans (C. albicans). A concentration-dependent effect of ZnO on the viability of C. albicans was observed. The minimal fungicidal concentration of ZnO was found to be 0.1 mg ml(-1) ZnO; this concentration caused an inhibition of over 95% in the growth of C. albicans. ZnO nanoparticles also inhibited the growth of C. albicans when it was added at the logarithmic phase of growth. Addition of histidine (a quencher of hydroxyl radicals and singlet oxygen) caused reduction in the effect of ZnO on C. albicans depending on its concentration. An almost complete elimination of the antimycotic effect was achieved following addition of 5 mM of histidine. Exciting the ZnO by visible light increased the yeast cell death. The effects of histidine suggest the involvement of reactive oxygen species, including hydroxyl radicals and singlet oxygen, in cell death. In light of the above results it appears that metal oxide nanoparticles may provide a novel family of fungicidal compounds.

Silver nanoparticles with an average size of ∼5 nm were deposited on the surface of preformed silica submicrospheres with the aid of power ultrasound. Ultrasound irradiation of a slurry of silica submicrospheres, silver nitrate, and ammonia in an aqueous medium for 90 min under an atmosphere of argon to hydrogen (95:5) yielded a silver−silica nanocomposite. By controlling the atmospheric and reaction conditions, we could achieve the deposition of metallic silver on the surface of the silica spheres. The resulting silver-deposited silica submicrosphere samples were characterized with X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, energy-dispersive X-ray analysis, high-resolution transmission electron microscopy, high-resolution scanning electron microscopy, photoacoustic spectroscopy, and Fourier transform infrared, UV−visible, and X-ray photoelectron spectroscopy.