Silver compounds and silver ions are used extensively in medical devices because of their wide-spectrum antimicrobial activity. In particular, nanoparticles of silver and silver (I) oxide show great promise for widespread usage in medical polymers and nanodrugs. Here, we demonstrate that a crystalline powder and a saturated aqueous solution of silver (III) oxide clathrate show much stronger antimicrobial activities and oxidative activities than silver (I) oxide.
Silver compounds and silver ions exhibit antimicrobial properties [1–5]. They have a toxic effect on bacteria, viruses, and fungi, which is typical of heavy metals such as mercury, cadmium, and lead. However, in humans, they do not show the high levels of toxicity that are usually associated with other heavy metals. Since World War I, silver compounds have been used to prevent infection. For example, silver sulfadiazine cream has broad antimicrobial activity and is commonly used for burn wounds [2, 3].
Although the use of silver compounds reduced after the introduction of antibiotics, the evolution of antibiotic-tolerant bacteria, such as multi-drug resistant bacteria, has prompted a need to re-evaluate treatment strategies. In addition to the development of new medical devices, there is widespread use of silver alloy-coated urinary catheters, endotracheal breathing tubes and glass products coated with silver, silver compounds or silver-containing nanoparticles and glasses [6–10].
There are three oxidative forms of silver oxide: silver (I), Ag2O; silver (II or I, III), AgO; and silver (III), Ag2O3·Ag2O (I) is the most common oxide and it is used in the production of certain medical devices. AgO is part of the manufacture of silver oxide-zinc alkaline batteries and is formulated as Ag2O·Ag2O3. It has been reported that Ag2O3 can be isolated by electrolysis of NaClO4 and AgClO4 . Although AgClO4 is a useful source of Ag+ ions, the presence of perchlorate represents human health and explosion risks. Pure Ag2O3 may be difficult to obtain commercially and industrially, and its therapeutic characteristics remain poorly understood.
In this study, we have identified a silver crystal which was produced by anodic oxidation of silver salts in aqueous solutions including AgNO3. We also performed to analyze the crystal with X-ray diffraction (XRD), and evaluate its antimicrobial activity.
Materials and methods
Preparation and identification of Ag2O3 clathrate
To obtain Ag2O3 clathrate, electrolysis of 100 mL of 1 M AgNO3 were carried out in 100-mL beaker with platinum electrodes, one of which was installed into a 35-mm film plastic case with two slits (40 × 5 mm2) as anode. Ag2O3 clathrate and pure silver were deposited on the anode and cathode, respectively. The plastic case prevents electrical short circuit caused by connection of either product on the electrodes. Approximately, 380 mg Ag2O3 clathrate was obtained in the plastic case of the electrolysis at 5 V dc for 15 min. XRD analysis of the Ag2O3 clathrate was performed with MO3XHF22 (MAC Science: X-rays Cu Kα: λ = 1.5406 Å, 40 kV, 30 mA, Ni filter and scan area: 10°≦ 2θ ≦ 70°, 2θ = 0.02).
Measurements of antimicrobial activity
Escherichia coli K-12 wild-type W3110 strain was used for measurement of antimicrobial activity of silver compounds. To suppress the precipitation of AgCl, we used a bacterial culture broth LB excluding NaCl for the bioassay. Radius of inhibitory zone of bacteria growth and area of compounds were measured by a stereomicroscopy with digital camera (Olympus) and Image J software (National Institute of Health, USA).
Results and discussion
In an earlier report, Ag2O3 crystal could be produced by anodic oxidation of silver salts in aqueous solutions including AgNO3 . The crystal shows a cubic face centered oxide phase of the “ideal and stable composition” but it also contains Ag3+ and Ag+ ions in various proportions, as Ag2O3 clathrate .
In conclusion, these results clearly indicate that Ag2O3 clathrate may serve as a useful and potent antimicrobial agent. It can be used as both a solid and in a saturated aqueous solution. Since the antimicrobial activity of Ag2O3 clathrate is greater than that of Ag2O, it may be expected to replace the use of Ag2O in certain medical equipment. It may be particularly suited to applications that require the reduction of biofilm formation by methicillin-resistant Staphylococcus aureus . In addition, the procedure described for the isolation of Ag2O3 clathrate crystals is simple and inexpensive, which should enable its use to become widespread.
We thank the Technical Division, School of Engineering, Tohoku University for XRD analysis and Ms. M. Hasegawa and Ms. Y. Sakamoto, Miyagi prefectural Sendai Daini Senior High School for helpful discussions. This study was supported by a joint program for the ‘Exploring Germination and Growth’ program for young Scientists (EGGS) at Tohoku University.
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